摘要:

J O U R N A L O F C H E M I S T R Y Materials On sorption and thermal properties of the zirconium phosphate fluoride [(H2en)0.5][Zr2(PO4)2(HPO4)F] H2O Michael Feist,a Martin Wloka,a Matthias Epple,b Ekkehard Postc and Erhard Kemnitz*a aInstitut fu� r Chemie der Humboldt Universita� t zu Berlin, Hessische Str. 1–2, D-10115 Berlin, Germany bInstitut fu� r Anorganische und Angewandte Chemie der Universita� t Hamburg, Germany cNetzsch Gera�tebau GmbH, Selb, Germany The thermal behavior of the title compound (ZrPO-1) has been investigated using conventional thermal analysis techniques as well as coupled TG–MS, TPD and XRD. Owing to the channel structure, ZrPO-1 reveals zeolitic properties in the dehydration range (up to 200 °C) whereas the removal of the organic template above 400 °C results in a complete loss of the sorptive activity.Water may be substituted by ammonia which is then released in two well separated steps at ca. 150 and 300 °C, respectively. The stronger diVerentiation of the water sites by the ammonia sorption allows attribution of the desorption ranges to the channels and the cavities, respectively, that exist in the structure.X-Ray characterization of the product after removal of the template above 400 °C did not yield any known zirconium phosphate or oxide. EXAFS measurements show that no significant changes occur in the short-range order around zirconium neither by dehydration nor by annealing. However, the environment of zirconium becomes increasingly disordered at higher temperatures. Recently, we reported the crystal structure of matically shown in Fig. 2, the cation is disordered between two positions around an inversion center, which results in an [(H2en)0.5][Zr2(PO4)2(HPO4)F] H2O which was designated ZrPO-1.1 It represents the first zirconium homologue to the empty space remaining in the channel, whereas the nitrogen atoms adopt identical positions. They are linked to the fluorine well known microporous aluminium phosphate zeolites, (AlPO4-n).2 The structures of further representatives of the atoms of the ZrO5F octahedra via hydrogen bridges.Fig. 1 further demonstrates that the water molecules occupy two ZrPO-n type which have been obtained with diVerent templates will be described elsewhere.3 crystallographically diVerent sites. For each orientation of the template cation, the remaining empty space is occupied by one The structure of ZrPO-1 contains a three-dimensional arrangement of zirconium octahedra (one ZrO6 and one water molecule, which is disordered as well (occupancy factor 0.5).Another water molecule being situated on the twofold ZrO5F), and phosphate tetrahedra [one PO3(OH) and two PO4] being connected via common oxygen atoms.Both fluor- axis is not disordered (occupancy factor 0.5 as well). Within a small cavity, it is tetrahedrally linked to the inorganic frame ine atoms and OH groups are terminal and, therefore, do not participate in the polyhedra connectivity. The eight-membered via four hydrogen bridges: two times with a donor, and two times with an acceptor function. cyclic arrangement of alternating octahedra and tetrahedra forms holes with a diameter of approximately 6.5 A° .Channels Our study of the thermal behavior of ZrPO-1, assumed to exhibit zeolitic properties, has been directed by the following in the y-direction exist in the structure, that form bottlenecks owing to the eight-membered polyhedra arrangement (Fig. 1). aspects. First, the dehydration and rehydration processes and their reversibility with special regard to the two diVerent The organic template, i.e.the ethylenediammonium cation, occupies these channels between two bottlenecks. As sche- bonding situations of the water; secondly, the possible substitution of water in the channel by other molecules of comparable size, and, thirdly, the chemical and structural changes that are caused by the thermally induced removal of the template.Fig. 1 Crystal structure of [(H2en)0.5][Zr2(PO4)2(HPO4)F] H2O (ZrPO-1). Half of the water molecules are situated in the tetragonal cavities (shadowed circles), the other half in the eight-membered ring Fig. 2 Schematic view of the disordering of the ethylenediammonium channels. Owing to the disordering of the template cations, which are not shown here for better legibility (see Fig. 2), the water molecules in cation in the channels of ZrPO-1 (view in y direction; dotted circles: oxygen positions of the disordered water molecule) the channels are disordered as well (empty circles, both positions shown). J. Mater. Chem., 1998, 8(2), 433–438 433Experimental Synthesis The preparation of ZrPO-1, which is described in more detail elsewhere,1 has been performed by hydrothermal synthesis (180 °C) from an aqueous solution of ZrOCl2·8H2O, ethylenediamine monohydrate, 40% HF and H3PO4.Thermal investigations We have used a conventional DTA–TG apparatus (Netzsch STA 429) as well as a TG–MS coupling (Netzsch STA 409/skimmer system).4 The macro sample holder system with platinum crucibles was used (Pt/PtRh10 thermocouples; a- Al2O3 as reference; 100 ml Ar min-1 for the DTA–TG, 100 ml He min-1 for the TG–MS measurements; b=5 K min-1; sample mass ca. 100 mg). For the enthalpimetric evaluation of the DTA curves (maximal precision of 10–12%) the measuring system was calibrated according to literature recommendations. 5,6 The TPD studies were performed with a Perkin Elmer FTIR spectrometer PE 2000 (TGS detector, resolution 4 cm-1, 70ml N2 min-1).A mixture of N2 : NH3 (ca. 351) was used for the NH3 sorption. In order to avoid additional surface NH3 sorption, every loading was followed by one hour flushing in pure N2. For the X-ray characterization, a powder diVractometer XRD 7 (SeiVert PM, Freiberg, Germany) was used. Fig. 3 STA curves for the dehydration and the template step: (a) of EXAFS measurements non-treated ZrPO-1; (b) of a dehydrated (200 °C) sample followed by NH3 sorption (cf. Table 1); (c) NH3 TPD curve of sample (b) X-Ray absorption fine-structure spectroscopy (EXAFS) was carried out at the Hamburger Synchrotronstrahlungslabor (HASYLAB) at Deutsches Elektronen-Synchrotron (DESY), In accordance with the idea of diVerent water sites (Fig. 1), Hamburg, at beamline X (ROEMO II). The DORIS III the ion current (I.C.) curves for m/z 17 and 18 reveal a storage ring was operated at 4.5 GeV positron energy and discontinuous shape (Fig. 4) whereas the DTA curves do not currents of 50–120 mA. The incoming synchrotron beam was show any shoulder or double peak [Fig. 3(a)]. Additional monochromatized by a Si(311) double-crystal monochromainformation on the dehydration step is obtained from the DTA tor.Experiments were performed at the Zr K-edge (ca. and DTG traces in Fig. 5, which correspond to the manifold 17 999 eV) in transmission mode at room temperature. The sample treatment described in Table 1. The reversibility of the ground samples were thoroughly mixed with boron nitride dehydration as well as the ammonia sorption properties of the and pressed to a pellet.For quantitative data evaluation we dehydrated phase clearly reveal the zeolitic character of ZrPO- used the programs AUTOBK and FEFFIT of the University 1. Upon rehydration, the original bonding situation of water ofWashington package.7 Theoretical standards were computed is recovered. This may be deduced from the X-ray powder with the program FEFF 5.048 using the crystal structure of pattern (see next section) as well as from the identity of signal [(H2en)0.5][Zr2(PO4)2(HPO4)F] H2O.1 All fits were carried shape and area (DdehydH ca. 51 kJ mol-1) for the first to third out with k3-weighted data using a k-range of 2–14 A ° -1 and an dehydration steps (cf.Table 1 for details of the experimental R-range of 0.6–6.6 A° .The amplitude reduction factor S02 was procedure). The slightly enhanced mass gain upon rehydration fixed to 1.22 as this value was obtained from fits of the pure may be explained by the higher sorption activity of the educt compound with constant coordination number. One primarily dehydrated phase. When it is reheated without prezero- rection [E0 =+12.1(6) eV] was chosen for the flushing in argon, this surface adsorbed water is released, first four shells (6 O/F, 5.5 P, 6.125 O, 2.75 O) and another causing a small separate eVect [Fig. 5(b)], whereas the true [E0=-2.8(5) eV] for the zirconium shell at 6.62 A ° . In order dehydration eVect has the same shape and area as for the first to reduce the number of fit parameters, the same Debye– heating.On the contrary, if a rehydrated sample is exposed to Waller factor was chosen for the three oxygen shells. a dry argon flow (20 h, 25 °C), the adsorbed water is released, and the pre-eVect does not occur upon heating [Fig. 5(c)]. Results and Discussion The interpretation of the second endothermal reaction range turned out to be more complicated even with the use of Thermoanalytical and absorption studies TG–MS measurements.The curves in Fig. 4 show that the template removal step is primarily caused by the release of Two well separated endothermal eVects, correlated to mass loss steps, characterize the thermal behavior of ZrPO-1 under carbon-containing species from the ethylenediammonium cation. Taking into account earlier findings on the thermal normal pressure in argon [Fig. 3(a)]. With respect to the above described structural features, the first mass loss is attributed behavior of the related triethylenediammonium compounds, e.g. of chlorometalates like (H2trien)[CoCl4],9 several mass to dehydration (up to 200 °C) where the observed mass loss of 3.5% corresponds well to the value calculated for 1 mol H2O spectral peaks may be readily attributed to fragments of the template cation.However, the whole decomposition process, (3.36%). Consequently, the second step (350–450 °C) is due to the removal of the template but, looking at the low mass loss unfortunately, is rather unspecific because several processes are overlaying each other in this temperature range. This of 3.5% (Dmcalc 9.35% for 0.5 en·2HF, or 5.62% for 0.5 en) and the curve shape (no plateau and not complete at 450 °C), produces the quasi-continuum of higher mass peaks (cf.Fig. 4) and does not allow postulation of any main reaction for it is rather incomplete or, at least, more complicated than a simple evaporation process. the decay. 434 J. Mater. Chem., 1998, 8(2), 433–438Fig. 5 DTA and DTG curves for the dehydration range of ZrPO-1 for repeated dehydration/rehydration cycles corresponding to the stepwise sample treatment described in Table 1: (a) first dehydration (step 1); (b) second dehydration (step 4); (c) third dehydration (step 8) The disappearance of the zeolitic properties of ZrPO-1 is expressed by the fact that neither water nor ammonia may be absorbed by the product phase which is formed during the template removal step (Table 1).The small amount of water taken up at 25 °C subsequent to the template removal step is only surface adsorbed. Therefore, the small mass loss of ca. 1% that appears on reheating has no corresponding DTA signal. An identically treated sample, cooled in ammonia, does Fig. 4 TG–MS curves of ZrPO-1 (ion current curves for m/z not show any mass loss upon reheating under argon. 17,18,19,20 and mass number scan at the beginning of the template step) A completely diVerent situation is found when ZrPO-1 is only dehydrated (200 °C) and is then cooled to room temperature in an ammonia flow. Upon reheating, two well separated The template removal step consists of a first partial step with bigger substance flow (340–450 °C) and a remarkably TG steps [Fig. 3(b),(c)] indicate diVerently bound NH3 molecules which have been absorbed by the host structure retarded following step (up to 700 °C), and is not completely finished even above 900 °C. Under these dynamic conditions, (DmNH3#DmH2O !), possibly onto the positions that were originally occupied by the two diVerent types of water molecules.either the organic template is partially retained by the solid phase or it reacts secondarily with the zirconium phosphate With NH3, the second desorption maximum is observed at a much higher temperature (DT ca. 150 K) than the first one matrix to yield further volatile products. Obviously, no massconstant phase, e.g. related to a zirconium oxide, is formed which is in the dehydration range.This shift is too large for the assumption that only variations in the hydrogen bonding here as might have been expected. Nevertheless, some further qualitative details may be would diVerentiate the water sites, now being substituted by ammonia molecules. The changes caused by the ammonia deduced from the TG–MS data. First, it may be seen that water is released not only in the dehydration step but is also sorption seem to be more profound to rationalize the considerable shift of the second desorption peak.One possible reason formed during the template decomposition. This may be explained by condensation reactions of two neighbouring is the formation of NH4+ ions, which could explain a stronger fixation of ammonia in the solid via additional coulomb PMOH groups.Secondly, the formation of ammonia in the second TG step is observed when considering the I.C. intensity interactions rather than just hydrogen bonding. The formation of ammonium ions is possible if one takes into account the ratio for m/z 17518. The mass loss corresponds to that expected only for water in the first TG step but, owing to a certain terminal OH groups existing in the structure of ZrPO-1.Owing to the greater basicity of ammonia relative to water the OH contribution by ammonia, it is much higher in the second step. Thirdly, HF liberation (m/z 19, 20) takes place predominantly groups can act as proton donors to form NH4+ ions. Thus, the two desorption maxima could be interpreted in in the template removal step.That means that not only the organic component is removed from the channels but the terms of, first, hydrogen bound NH3 molecules, which would be quite analogous to the situation of water in the structure zirconium phosphate matrix is attacked as well and the sorptive activity of the Zr–P–O phase(s) disappears. The small amounts of ZrPO-1. Secondly, NH4+ ions, which are formed via reaction between OH groups and NH3, would be more strongly retained of HF detected in the first TG step were attributed to surface adsorbed HF which is rationalized by the conditions of the in the structure and explain the liberation of NH3 at higher temperatures.That means that we are able to attribute the hydrothermal synthesis. J. Mater. Chem., 1998, 8(2), 433–438 435Table 1 Procedure of sample treatment for three diVerent samples of ZrPO-1a step process temperature programb Dm (%) 1 dehydration (200 °C in Ar -3.6 2 cooling 325 °C in Ar — 3 rehydration 12 h at 25 °C in moist air +4.0 4 dehydration (200 °C in Ar -0.6 and -3.3 5 cooling 325 °C in Ar — 6 rehydration 43 h at 25 °C in moist air +4.0 7 dry gas flow 20 h at 25 °C 0 (!) 8 dehydration and further heating (460 °C in Ar -3.5 (H2O) and -3.5 (template) 9 cooling 325 °C in Ar — 10 rehydration 20 h at 25 °C in moist air +1.0 11 heating (460 °C in Ar -1.0 1 dehydration (200 °C in Ar -3.6 2 NH3 absorption 325 °C in NH3 +2.9 3 desorption (370 °C in Ar (TPD) -1.2 and -1.6 1 dehydration and further heating ( 460 °C in Ar -3.5 (H2O) and -3.5 (template) 2 NH3 absorption 3180 °C in Ar followed by 325 °C in NH3 — 3 heating (200 °C in Ar 0 (!) aSteps 1–11 performed in a single experimental run on the thermobalance, the ammonia sorption in a preparative scale followed by the STA measurement.b(=heating, 3=cooling. first desorption peak to channel sites whereas the second peak (O, N, C) are present. Furthermore, there are zirconium neighbors at 4.85 A ° (N=1), 5.1–5.25 A ° (N=3), 5.45 A ° (N= corresponds to cavity sites. 1.5) and 6.62 A ° (N=4). It was not possible to separate fluorine and oxygen in the Structural investigations first neighboring shell owing to almost identical backscattering The structural changes caused by the dehydration yield powder behavior and distance. Therefore we combined the fluorine diagrams (Fig. 6) which are quite similar to the educt phase, shell and the first oxygen shell into one oxygen shell with N= but not so readily assigned. However it can be deduced that 6 at 1.99–2.10 A ° .The phosphorus atoms were combined into the rehydrated phase is identical with the educt state. a single shell. The next oxygen shells (N=6.125 and 2.75) were Meanwhile, we succeeded in preparing a single crystal of the also fitted.However, it should be kept in mind that higher dehydrated phase for a crystal structure analysis, the evaluation shells are influencing the third oxygen shell. Additionally, of which is underway. The following template step, as already multiple scattering paths come into play above R=3.5 A °. As discussed above, eVects deeper chemical and structural changes an indicator for long-range order we used a distinct shell of of ZrPO-1 resulting in a completely diVerent powder pattern.four zirconium atoms at 6.62 A ° . No other shells were included Our attempts for an indexation or an interpretation in terms into the fit. of known crystalline phases existing in the Zr–O–P system As expected, the main structural framework did not change failed, which led us to investigate the structural changes by upon dehydration.Only small changes are visible even upon EXAFS measurements. annealing at 800 °C. This is seen from the close similarity of Two diVerent crystallographic positions exist for zirconium. EXAFS Fourier transforms as shown by visual inspection Surrounding shells were computed for both positions to derive (Fig. 7 and 8). Consequently, the coordination numbers were the average zirconium environment. Zirconium is surrounded fixed to their theoretical (crystallographic) values during the by 0.5 fluorine atom (1.99 A ° ), 5.5 oxygen atoms (2.02–2.10 A ° ), fits. The results of the fits (average of two EXAFS scans at 5.5 phosphorus atoms (3.34–3.55 A ° ), 6.125 oxygen atoms room temperature) are listed in Table 2.(3.81–4.02 A ° ) and 2.75 oxygen atoms (4.08–4.14 A ° ). Above Two main results can be summarized as follows: all five 4.14 A ° , a number of overlapping shells of light backscatterers shells are present in the four samples and the interatomic distances remain the same within the experimental scatter. Fig. 6 Calculated (a) and experimental powder diVractogram of ZrPO- Fig. 7 k3-weighted Zr K-edge EXAFS functions x(k) for ZrPO-1: (a) untreated, (b) dehydrated at 200 °C, (c) heated to 400 °C, and (d) heated 1 (b) compared with those of a dehydrated (200 °C) (c) and a rehydrated sample (d) to 800 °C, template removed 436 J. Mater. Chem., 1998, 8(2), 433–438thermal behavior of ZrPO-1 diVers from that of typical representatives of the neighbouring AlPO family, e.g.that of the VPI-5 type which represents a wide-pore 18-ring aluminium phosphate.10 Upon dehydration (120 °C !) it rapidly rearranges to another intact AlPO structure, AlPO-8, if the compound is prepared in the presence of dipropylamine. On the other hand, the VPI-5 structure remained stable if the hydrothermal synthesis was performed in the presence of dipentylamine.11 For the known ZrPOs, a template eVect governing the thermal stability of the metal phosphate fluoride framework is not observed with the main structural motif and reversible hydration/dehydration behavior remaining unchanged.Conclusions ZrPO-1 shows zeolite-like properties at low temperatures. This Fig. 8 Fourier-transform magnitudes of ZrPO-1 after diVerent thermal is evidenced by the conservation of the channel structure of treatments (cf.Fig. 6). The coordination of zirconium remains the zirconium phosphate framework upon dehydration, by its unchanged up to R#4 A° ; however, the disorder increases as visible reversibility, and by the possibility of substituting water by from the decreasing height of the Zr–O/F peak at 2.1 A ° . The Zr–Zr ammonia.On the other hand, a complete loss of the sorptive shell at 6.6 A ° almost vanishes upon heating to 800 °C, indicating a activity is observed when the template is thermally removed structural change. The data are not corrected for phase shifts. from the ZrPO-1 structure. The channel structure collapses but the short-range order of the Zr atoms remains nearly This illustrates that the short-range order around zirconium is unchanged which might indicate that the formed structure is not changed by dehydration and annealing.However, the very similar. However, this is not necessarily the case since the structural disorder, as represented by the Debye–Waller factors, short-range arrangement of layered zirconium phosphates is, increases, indicating a slow breakdown of the zeolitic structure. to some degree, similar to the short-range constitution of 3D This change is most obvious in the ZrMZr shell at 6.62 A ° .If ZrPOs.3 It is not clear as yet whether the product after removal the coordination number is fixed to four as in the original of the template represents a single phase system or a mixture structure, the Debye–Waller factor almost quadruples in the of various zirconium phosphates and/or oxides.Therefore, sample heated to 800 °C. If the Debye–Waller factor is kept further structural investigations of the detemplated phase constant at 2.6×10-3 A ° 2 and the coordination number is are required. varied, we obtain N=2.3 (400 °C) and N=0.6 (800 °C). The strong correlation between N and s2 prevents the exact determination of N, but it is quite clear that the environment of The support of Dr.W.-D. Emmerich (Netzsch Gera�tebau GmbH, Germany) who enabled the TG–MS investigations to zirconium becomes increasingly disordered at higher temperature. It was very diYcult to fit the phosphorus shell. The be carried out is gratefully acknowledged. We thank Larc Tro� ger (Hamburg) for experimental assistance, and we are ZrMP distance was found to be too large, and also an unreasonably high Debye–Waller factor was found. We ascribe grateful to HASYLAB at DESY for generous allocation of beamtime.The financial support by the Deutsche these problems to a strong correlation between the ZrMP shell and the neighboring ZrMO shell. Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie (FCI) is also gratefully acknowledged. Despite the structural similarity, the template eVect and the Table 2 Results of EXAFS experiments carried out at the zirconium K-edge for ZrPO-1 after diVerent thermal treatments (coordination numbers fixed to their crystallographic values for hydrated and dehydrated phases) sample treatment 400 °C 800 °C 25 °C 200 °C (beginning of (template (untreated) (dehydrated) template step) removed) 1st shell: 6 O/F Na 6 6 6 6 R/A ° 2.06(2) 2.08(2) 2.08(2) 2.07(2) s2/10-3A ° 2 4.1 5.2 5.4 7.2 2nd shell: 5.5 P Na 5.5 5.5 5.5 5.5 R/A ° 3.72(2) 3.85(2) 3.72(2) 3.66(2) s2/10-3 A ° 2 7.8 95.5 8.9 6.2 3rd shell: 6.125 O Na 6.125 6 6 6 R/A ° 3.74(2) 3.86(2) 3.74(2) 3.66(2) 4th shell: 2.75 O Na 2.75 2.5 2.5 2.5 R/A ° 4.06(2) 4.04(2) 4.05(2) 4.03(2) higher shell: 4 Zr Na 4 4 4 4 R/A ° 6.63(2) 6.62(2) 6.62(2) 6.63(2) s2/10-3A ° 2 2.6 2.9 4.8 10.5 aFixed.J. Mater. Chem., 1998, 8(2), 433–438 4377 E. A. Stern, M. Newville, B. Ravel, Y. Yacoby and D. Haskel, References Physica B, 1995, 208–209, 117. 1 E. Kemnitz, M. Wloka, S. Trojanov and A. Stiewe, Angew. Chem., 8 J. J. Rehr, R. C. Albers and S. I. Zabinsky, Phys. Rev. L ett., 1992, 1996, 108, 2809. 69, 3397. 2 S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and 9 M. Feist, R. Kunze, D. Neubert, K. Witke and E. Kemnitz, E. M. Flanigan, J. Am. Chem. Soc., 1982, 104, 1146. J. T herm. Anal., 1997, 49, 635. 3 M. Wloka, S. I. Troyanov and E. Kemnitz, J. Solid State Chem., 10 M. E. Davis, C. Saldarriaga, C. Montes, J. M. Garces and in press. C. Crowder, Nature (L ondon), 1988, 331, 698. 4 T. Rampke, W. D. Emmerich, E. Post and L. Giersig, J. T herm. 11 J. O. Perez, P. Jen Chu and A. Clearfield, J. Phyhem., 1991, Anal., 1996, 47, 633. 95, 9994. 5 K. Heide, Dynamische T hermische Analysenmethoden, VEB Deutscher Verlag fu� r GrundstoYndustrie, Leipzig, 1982. 6 H. K. Cammenga, W. Eysel, E. Gmelin, W. Hemminger, G. W. Paper 7/05459I; Received 28th July, 1997 H. Ho�hne and S. M. Sarge, T hermochim. Acta , 1993, 219, 333. 438 J. Mater. Chem., 1998, 8(2), 433&ndash

ISSN:0959-9428    

DOI:10.1039/a705459i

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials A new three-dimensional sodium molybdenum(v) hydroxymonophosphate: Na8(Mo2O4OH)3(PO4)3(PO3OH)·12.25H2O A. Leclaire,*a C. Biot,a H. Rebbah,*b M. M. Borela and B. Raveaua aL aboratoire CRISMAT , URA 1318 associe�e au CNRS, ISMRA et Universite� de Caen—Bd du Mare�chal Juin 14050- Caen cedex- France bL aboratoire de cristallographie et cristallogene`se-Institut de Chimie U.S.T .H.B.B.P. 32 El-Alia Bab-Ezzouar Alger—Alge�rie A new sodium molybdenum(V) hydroxymonophosphate, Na8(Mo2O4OH)3(PO4)3(PO3OH)·12.25 H2O has been synthesized. It crystallizes in the monoclinic space group P21/c with a=13.024(4), b=25.936(4), c=13.276(3) A ° , b=111.72(2)°. Its structure involves Na[Mo6P4O27(OH)4]2 clusters similar to those encountered in several other molybdenum(V) hydroxyphosphates.Such clusters built up of two rings of six edge-sharing Mo octahedra interconnected through one NaO6 octahedron diVer from those previously described by the fact that hydrogen is mainly connected to the MoMO bond forming twelve Mo(O5OH) octahedra, whereas among the eight tetrahedra one observes two P(O3OH) tetrahedra and six PO4 tetrahedra.The originality of this structure deals with the fact that such clusters form with several sodium octahedra, Na(H2O)4O2 and Na(H2O)2O4, a threedimensional framework that delimits intersecting tunnels running along a, b and c. The coordination of other sodium cations and H2O molecules that can be considered as intercalated species is also discussed. The studies of reduced molybdenum phosphates and hydroxy- (Prolabo, 98%) Na2MoO4·2H2O (Merck, 99.5%) and finely phosphates performed in the last ten years, have shown that divided metallic molybdenum (Goodfellow, 99.9%).The best these compounds exhibit a large structural diversity. Numerous results for such a synthesis were obtained from a mixture of layer and tunnel structures have been generated which are of 1.442 g of Na2MoO4·2H2O, 0.634 g of NaOH, 0.230 g of Mo, relevance to applications in catalysis owing to their micropo- 1 cm3 of H3PO4 and 7 cm3 H2O highly homogenized in a rous character and also their redox properties (for reviews 25 cm3 Teflon lined autoclave.The reaction vessel was mainsee ref. 1–4). tained at 220 °C and autogenous pressure for 36 h before slow The structural chemistry of the anhydrous molybdenum cooling at 1.6° h-1 to room temperature.phosphates is generally diVerent from that of the hydroxyphos- By this method, well formed orange crystals could be easily phates. In the anhydrous phosphates, a large number of infinite separated from a roseate powder, whose crystallographic three-dimensional frameworks built up from MoO6 octahedra nature was not identified.The orange crystals were washed and PO4 tetrahedra only can be generated. This is not the with water, rinsed with ethanol and finally dried in a desiccator. case for hydroxyphosphates, where most of the time molyb- Microprobe analysis of these crystals evidenced a cationic denum and phosphorus form isolated polyanionic groups with ratio Na5Mo5P of 85654 in accord with the formula oxygen and hydroxy groups, that are held together in a three- Na8(Mo2O4OH)3(PO4)3(PO3OH)·12.25 H2O deduced later dimensional framework by introducing foreign cations such as from the crystal structure determination.transition or post transition cations (e.g. iron, zinc) or univalent Thermogravimetry, performed under argon between 25 and cations (e.g.alkali). Thus, the molybdenum hydroxyphosphates 600 °C, shows a continuous loss of water in good agreement are less stable, involve stabilization by hydrogen bonding only with the presence of 28.5 hydrogen atoms per formula and are generally prepared by hydrothermal techniques. For unit (Dmobs=17%; Dmcalc=18%). No intermediate phase this reason, the number of molybdenum(V) hydroxyphosphates could be detected during the dehydration owing to the poor of alkali cations that do not involve any other transition or crystallization of the products.post transition elements is small compared to anhydrous molybdenum(V) phosphates. The sodium hydroxyphosphates (PPh4)2[(H3O)2NaMo6P4O24(OH)7]·5H2O,5 Na3[Mo2O4- Structure determination (HPO4) (PO4)]·2H2O,6 and (Et4N)6Na2[Na12(H3PO4)- An orange plate like crystal with dimensions 0.097×0.168 x {Mo6O15(HPO4) (H2PO4)3}4]·xH2O7 and the caesium 0.036 mm was selected for the structure determination.The hydroxyphosphate Cs(H3O)[Mo2O2(PO4)2(HPO4)]8 are repcell parameters reported in Table 1 were determined and resentative of the unusual molybdenum(V) alkali hydroxyrefined by diVractometric technique at 294 K with a least monophosphates that do not involve any other transition square refinement based upon 25 reflections with 18h22°.element than molybdenum. Among these hydroxyphosphates, The systematic absences l=2n+1 for h0l and k=2n+1 for only one, Na3[Mo2O4(HPO4) (PO4)],6 does not contain other 0k0 are consistent with the space group P21/c. The data were univalent cations such as phosphonium, ammonium or collected with an Enraf Nonius CAD4 diVractometer, and oxonium.We report on the synthesis and crystal stucture of a parameters are reported in Table 1. Among the 9460 unique new molybdenum(V) hydroxymonophosphate Na8(Mo2O4- measured reflections, 3896 with I3s(I) corrected for Lorentz OH)3(PO4)3(PO3OH)·12.25H2O, with an original structure. and polarization and absorption and secondary extinction eVects were used to solve and refine the structure.The atoms Synthesis procedure were located by the heavy atom method, i.e. the molybdenum atoms were located by the deconvolution of the Patterson Single crystals of the title compound were grown from a hydrothermal reaction of H3PO4 (Prolabo, 75%), NaOH function and the other atoms by subsequent Fourier synthesis.J. Mater. Chem., 1998, 8(2), 439–444 439Table 1 Summary of crystal data, intensity measurements and structure Table 2 Positional parameters and their estimated standard deviationsa refinement parameters for Na8Mo6P4O27(OH)4·12.25H2O atom x y z B/A ° 2 crystal data: space group P21/c Mo(1) 0.6111(1) 0.39072(7) 0.7077(1) 1.00(6) Mo(2) 0.3980(2) 0.39170(7) 0.6374(1) 1.02(6) cell dimensions a=13.024(4) A ° b=25.936(4) A ° b=111.72(2)° Mo(3) 0.2416(1) 0.41456(7) 0.3596(1) 1.15(6) Mo(4) 0.3438(2) 0.43037(7) 0.2277(1) 1.18(6) c=13.276(3) A ° volume/A ° 3 4116.1(7) Mo(5) 0.6362(2) 0.42959(7) 0.3236(1) 1.18(6) Mo(6) 0.7485(2) 0.41246(7) 0.5275(1) 1.14(6) Z 4 Dc/g cm-3 2.51 P(1) 0.4947(5) 0.3531(2) 0.4404(5) 0.96(2) P(2) 0.1888(5) 0.3142(2) 0.4946(5) 1.36(2) intensity measurements: P(3) 0.4798(5) 0.3882(2) 0.0782(5) 1.84(2) l(Mo-Ka) 0.71073 P(4) 0.8126(5) 0.3127(2) 0.7029(5) 1.68(2) scan mode v–2/3h Na(1) 1/2 1/2 1/2 1.20(3) scan width/° 1.5+0.35 tanh Na(2) 0.4936(8) 0.2330(3) 0.2970(7) 2.48(3) slit aperture/mm 1.3+tanh Na(3) 0.483(1) 0.2512(4) 0.067(1) 6.40(6) max h/° 27 Na(4) 0.1152(8) 0.1993(4) 0.3428(8) 3.60(4) standard reflections three measured every 3600 s Na(5) 0.7212(8) 0.2807(4) 0.4589(8) 3.28(4) measured reflections 9460 Na(6) 0.2774(8) 0.2855(4) 0.3108(8) 2.72(3) reflections with I>3s 3896 Na(7) 0.569(1) 0.0232(4) 0.430(1) 6.32(6) h min., max.-16, 16 Na(8) 0.956(1) 0.4686(5) 0.378(1) 5.60(5) k min., max. 0, 33 Na(9)b 0.827(5) 0.141(2) 0.486(4) 6.40(2) l min., max. 0, 16 Na(10)b 0.109(4) 0.037(2) 0.440(4) 6.40(2) m/mm-1 2.11 O(1) 0.638(1) 0.4122(6) 0.835(1) 1.84(5) O(2) 0.503(1) 0.3346(5) 0.683(1) 1.68(5) structure solution and refinement: parameters refined 507 O(3) 0.505(1) 0.4419(5) 0.625(1) 1.12(4) O(4) 0.732(1) 0.3363(5) 0.751(1) 1.68(5) agreement factors R=0.055 Rw=0.066 weighting scheme w=1/s2 O(5) 0.726(1) 0.4341(6) 0.669(1) 1.68(5) O(6) 0.602(1) 0.3689(6) 0.538(1) 0.88(4) D/s max.<0.005 O(7) 0.377(1) 0.4114(6) 0.747(1) 2.32(6) O(8) 0.278(1) 0.4392(5) 0.522(1) 0.80(4) O(9) 0.276(1) 0.3384(5) 0.599(1) 1.28(5) All the calculations were done on a Spark station with the O(10) 0.396(1) 0.3687(5) 0.470(1) 1.20(4) O(11) 0.124(1) 0.4485(6) 0.302(1) 2.00(5) XTAL package.9 Full crystallographic details, g O(12) 0.260(1) 0.3719(5) 0.247(1) 1.44(5) structure factors, have been deposited at the Cambridge O(13) 0.354(1) 0.4639(5) 0.363(1) 1.12(4) Crystallographic Data Centre (CCDC).See Information for O(14) 0.169(1) 0.3522(5) 0.400(1) 1.68(5) Authors, J. Mater. Chem., 1998, Issue 1. Any request to the O(15) 0.257(1) 0.4687(5) 0.133(1) 1.92(5) CCDC for this material should quote the full literature citation O(16) 0.381(1) 0.3836(5) 0.119(1) 1.60(5) and the reference number 1145/71.O(17) 0.491(1) 0.4727(5) 0.247(1) 1.44(5) O(18) 0.491(1) 0.3852(5) 0.342(1) 1.20(4) The molybdenum, phosphorus, oxygen atoms O(1)–O(31) O(19) 0.725(1) 0.4648(6) 0.291(1) 2.32(6) and Na(1) were identified without any ambiguity. The distinc- O(20) 0.589(1) 0.3858(5) 0.184(1) 1.68(5) tion between the sodium atoms [Na(2)–Na(10)] and the water O(21) 0.636(1) 0.4640(5) 0.456(1) 1.36(5) molecules [O(32)–O(46)] was diYcult, owing to the same O(22) 0.722(1) 0.3701(5) 0.399(1) 1.60(5) number of electrons of the two species.The identification of O(23) 0.866(1) 0.4451(5) 0.543(1) 1.68(5) the diVerent species was performed on the basis of the O(24) 0.826(1) 0.3494(5) 0.614(1) 1.44(5) O(25) 0.492(1) 0.2962(5) 0.420(1) 1.28(5) interatomic distances and the coordination of each atom.The O(26) 0.229(1) 0.2645(5) 0.468(1) 2.24(6) refinement of the isotropic thermal factors and the occupancy O(27) 0.079(1) 0.3099(6) 0.515(1) 2.32(6) factors, coupled with the ratios between the height of the O(28) 0.476(1) 0.3402(6) 0.013(1) 2.56(6) Fourier peaks allowed first to distribute 0.25 Na+ in the Na(9) O(29) 0.469(1) 0.4377(6) 0.019(1) 2.72(6) and Na(10) sites, 0.5 H2O in the O(42), O(43), O(44) and O(30) 0.766(1) 0.2612(6) 0.649(1) 2.96(6) O(45) sites and one 0.25 H2O in the O(46) site.Then in the O(31) 0.925(1) 0.3070(8) 0.799(1) 4.08(6) H2O(32) 0.065(1) 0.1346(6) 0.183(1) 2.40(3) following refinement the occupancy factors were fixed.The H2O(33) 0.321(1) 0.1949(6) 0.324(1) 2.64(3) refinement of the atomic coordinates, the isotropic thermal H2O(34) 0.357(1) 0.2788(6) 0.160(1) 2.96(4) factors of the water molecules and incompletely occupied H2O(35) 0.925(1) 0.3772(6) 0.354(1) 3.2(4) sodium sites, and the anisotropic thermal factors of the remain- H2O(36) 0.086(1) 0.0470(6) 0.057(1) 3.12(4) ing atoms led to R=0.055 and Rw=0.066 and to the atomic H2O(37) 0.625(1) 0.2780(6) 0.243(1) 3.04(4) parameters listed in Table 2.As the compound was synthesized H2O(38) 0.890(1) 0.2550(7) 0.447(1) 3.92(4) H2O(39) 0.102(1) 0.2594(7) 0.190(1) 3.44(4) by hydrothermal reaction with an excess of sodium hydroxide, H2O(40) 0.642(1) 0.1943(7) 0.445(1) 3.76(4) hydroxy groups were susceptible to be present in the structure.H2O(41) 0.901(2) 0.319(1) 0.158(2) 8.32(7) Thus, a calculation of the electrostatic bond strength balance H2O(42)c 0.151(2) 0.137(1) 0.478(2) 2.32(6) was performed, using the Brese and O’KeeVe formulation10 for H2O(43)c 0.738(3) 0.433(2) 0.076(3) 4.80(8) MoV, Na and PV species (rij=1.879, 1.80, 1.604, respectively) H2O(44)c 0.266(3) 0.047(2) 0.378(3) 5.60(8) and the Brown curves11 for the hydrogen bonds.A lack of ca. H2O(45)c 0.936(3) 0.069(2) 0.338(3) 4.80(8) H2O(46)b 0.091(4) 0.995(2) 0.308(4) 1.60(8) 0.7 in the electrostatic valence of an oxygen atom is characteristic of an hydroxy group and a lack of ca. 1.4 indicates a aAnisotropically refined atoms are given in the form of the isotropic water molecule. The O(32)–O(46) sites identified as water equivalent displacement parameter defined as B=4/3SiSjaiajbij.Water molecules during the resolution of the structure receive about molecules refined isotropically. 0.262 electrostatic valence from the Mo, P or Na atoms. Five bOccupancy=0.25. oxygen sites O(5), O(8), O(17), O(31) and O(27) receive, cOccupancy=0.5. respectively, 1.150, 1.033, 1.159, 1.380 and 1.180 electrostatic valence from the Mo, P or Na atoms. However, O(27) receives only 0.673 more electrostatic valence from the four water molecules O(32), O(35), O(38) and O(39).Thus it can be concluded that O(5), O(8), O(17) and O(31) are hydroxy 440 J. Mater. Chem., 1998, 8(2), 439–444groups so that three OH groups belong to the MoO6 octahedra phosphate the structure of each cluster consists of two rings of six Mo(O5OH) edge-sharing octahedra; each Mo6 ring and one OH group to the PO4 tetrahedra.The water loss deduced from thermogravimetry is consistent with 28.5 H shares its apices with three PO4 tetrahedra [two tetrahedra P(2) and P(3) at the periphery of the cluster and one central atoms per formula. From these observations this compound can be formulated Na8(Mo2O4OH)3(PO4)3(PO3OH)·12.25 phosphate group P(1)] and one P(O3OH), P(4).The two Mo6 rings are connected through one NaO6 octahedron Na(1) that H2O. The theoretical valency of five for molybdenum is confirmed by the bond strength calculations which lead to valencies of 4.85, 5.03, 4.81, 4.75, 4.95 and 4.93 respectively for Table 3 Distances (A ° ) and angles (°) in the polyhedra the six molybdenum atoms.Mo(1) O(1) O(2) O(3) O(4) O(5) O(6) O(1) 1.69(2) 2.93(2) 2.79(2) 2.76(2) 2.89(2) 3.96(2) Description of the structure and Discussion O(2) 106.6(7) 1.96(1) 2.89(2) 2.77(2) 3.93(2) 2.82(2) O(3) 100.5(6) 95.6(6) 1.94(1) 3.91(2) 2.72(2) 2.76(2) The projection of the structure of this new molybdenum(V) O(4) 95.3(6) 87.8(6) 162.1(7) 2.03(1) 2.75(2) 2.83(2) hydroxymonophosphate along a (Fig. 1) shows that it consists O(5) 99.6(7) 153.1(7) 85.1(6) 84.0(6) 2.08(2) 2.53(2) of centrosymmetric clusters Na[Mo6P4O27(OH)4]2 intercon- O(6) 170.0(6) 82.9(6) 81.1(5) 81.9(5) 70.7(5) 2.29(1) nected through sodium cations and water molecules. Mo(2) O(2) O(3) O(7) O(8) O(9) O(10) Each Na[Mo6P4O27(OH)4]2 cluster (Fig. 2) is very similar O(2) 1.96(1) 2.89(2) 2.90(2) 3.99(2) 2.76(2) 2.79(2) to the Na[Mo12P4O24(OH)7] clusters observed for the O(3) 95.0(6) 1.96(1) 2.83(2) 2.76(2) 3.93(2) 2.78(2) phosphate polymer (H3O)2NaMo6P4O24(OH)7(PPh4)2·5H2O, O(7) 106.7(7) 102.8(7) 1.65(2) 2.87(2) 2.69(2) 3.94(2) whose structure has been described by Haushalter and Lai.5 O(8) 154.5(6) 84.9(5) 98.1(6) 2.13(1) 2.81(2) 2.64(2) Nevertheless the clusters of the two structures diVer from O(9) 87.6(6) 161.8(6) 93.7(7) 85.1(5) 2.03(1) 2.83(2) O(10) 81.7(6) 81.0(5) 170.1(6) 73.1(5) 81.5(6) 2.30(1) each other by the distribution and the number of protons.Haushalter and Lai5 observe fourteen OH groups per cluster, Mo(3) O(8) O(10) O(11) O(12) O(13) O(14) instead of eight in the present compound. Moreover in the O(8) 2.12(1) 2.64(2) 2.88(2) 3.97(2) 2.72(2) 2.83(2) O(10) 72.6(5) 2.33(1) 3.99(2) 2.83(2) 2.79(2) 2.78(2) Haushalter phase all OH groups are assumed to be linked to O(11) 97.4(6) 168.8(7) 1.69(1) 2.93(2) 2.84(2) 2.78(2) the phosphorus forming seven PO3(OH) tetrahedra per cluster, O(12) 154.9(5) 82.4(5) 107.3(6) 1.94(2) 2.86(2) 2.75(2) in contrast with the present structure where six OH groups O(13) 84.0(6) 81.5(5) 102.8(6) 94.8(6) 1.94(1) 3.92(2) per cluster are linked to molybdenum, forming MoO5(OH) O(14) 85.5(6) 78.7(5) 95.9(7) 87.3(6) 159.6(5) 2.04(2) octahedra, and only two OH groups are linked to phosphorus Mo(4) O(12) O(13) O(15) O(16) O(17) O(18) [P(4)] forming PO3(OH) tetrahedra.Then in our hydroxy- O(12) 1.94(2) 2.86(2) 2.93(2) 2.74(2) 3.98(2) 2.82(2) O(13) 94.5(6) 1.95(1) 2.84(2) 3.98(2) 2.76(2) 2.78(2) O(15) 108.1(6) 103.2(6) 1.67(1) 2.78(2) 2.86(2) 3.92(2) O(16) 85.7(6) 160.8(5) 95.0(7) 2.08(2) 2.91(2) 2.77(2) O(17) 155.2(5) 84.6(6) 96.2(6) 87.3(6) 2.14(2) 2.60(2) O(18) 83.4(5) 82.1(5) 166.7(7) 78.9(5) 71.9(5) 2.28(1) Mo(5) O(17) O(18) O(19) O(20) O(21) O(22) O(17) 2.11(1) 2.60(2) 2.88(2) 2.86(2) 2.73(2) 3.96(2) O(18) 71.9(5) 2.31(2) 3.94(2) 2.83(3) 2.82(2) 2.85(2) O(19) 99.7(7) 170.3(6) 1.65(2) 2.74(2) 2.82(3) 2.85(2) O(20) 86.7(5) 80.5(6) 94.3(7) 2.07(1) 3.99(2) 2.77(2) O(21) 84(1) 0 82.2(6) 102.2(7) 162.2(6) 1.97(1) 2.89(2) O(22) 155.4(6) 83.6(6) 104.5(7) 87.2(6) 95.0(6) 1.95(1) Mo(6) O(5) O(6) O(21) O(22) O(23) O(24) O(5) 2.08(2) 2.53(2) 2.74(2) 3.93(2) 2.90(2) 2.78(2) O(6) 71.1(5) 2.26(1) 2.80(2) 2.82(2) 3.93(2) 2.75(2) O(21) 85.6(6) 82.9(6) 1.95(1) 2.89(2) 2.82(2) 3.93(2) O(22) 154.4(6) 83.8(6) 95.9(6) 1.95(1) 2.88(2) 2.72(2) O(23) 100.1(7) 170.0(6) 101.3(6) 104.6(7) 1.69(1) 2.77(2) O(24) 85.0(6) 79.3(5) 161.8(7) 86.1(6) 95.7(6) 2.04(1) P(1) O(6) O(10) O(18) O(25) O(6) 1.57(1) 2.50(2) 2.51(2) 2.53(2) Fig. 1 Projection of the structure of Na8(Mo2O4OH)3(PO4)3- O(10) 107.5(8) 1.53(2) 2.48(2) 2.48(2) (PO3OH) along a O(18) 107.5(8) 107.8(8) 1.54(2) 2.53(2) O(25) 111.2(7) 109.9(9) 112.8(9) 1.50(1) P(2) O(9) O(14) O(26) O(27) O(9) 1.56(1) 2.51(2) 2.51(2) 2.50(2) O(14) 108.1(8) 1.54(2) 2.46(2) 2.49(2) O(26) 110.7(8) 109(1) 1.48(2) 2.55(2) O(27) 106.7(9) 107.7(9) 114(1) 1.55(2) P(3) O(16) O(20) O(28) O(29) O(16) 1.57(2) 2.52(2) 2.46(3) 2.48(2) O(20) 105.9(9) 1.58(1) 2.50(2) 2.55(2) O(28) 106.2(9) 107.7(8) 1.51(2) 2.53(2) O(29) 109(1) 112.4(9) 116(1) 1.48(2) P(4) O(4) O(24) O(30) O(31) O(4) 1.55(2) 2.56(2) 2.51(2) 2.48(2) O(24) 110.4(9) 1.57(2) 2.51(2) 2.56(2) O(30) 109(1) 108.2(9) 1.53(2) 2.57(2) O(31) 106(1) 110(1) 113(1) 1.55(2) Mo(1),Mo(2) 2.581(3). Mo(2),Mo(3) 3.541(3).Mo(3),Mo(4) 2.594(3). Mo(4),Mo(5) 3.541(3). Fig. 2 Na[Mo6P4O27(OH)4]2 cluster Mo(5),Mo(6) 2.595(3).Mo(6),Mo(1) 3.524(3). J. Mater. Chem., 1998, 8(2), 439–444 441Fig. 3 Projection of the structure of Na8(Mo2O4OH)3- (PO4)3(PO3OH)·12.25H2O along c shares three apices with each of them. Thus these clusters can be formulated Na[(Mo2O4OH)3(PO4)3(PO3OH)]2. Similarly to (H3O)2NaMo6P4O24(OH)7(PPh4)2·5H2O, each Mo octahedron has one free apex characteristic of MoV. Note also that in our phase the OH groups of the MO5(OH) octahedra are always shared by two octahedra.The presence of OH groups linked to molybdenum in the Na[Mo6P4O27(OH)4]2 cluster is unusual so making direct comparison with the structures of Haushalter et al. may be of limited value since the hydrogen atoms were not localised but only assumed to be linked to the phosphate groups. Recent work on Na2Cd3(Mo2O4OH) 6 (PO4) 2 (PO3OH)6[N (CH3) 4] 4·10H2O and Cd9(Mo2O4OH)12(PO4)6(PO3OH)10[N(CH3)4]8·15H2O12 Table 4 NaMO distances<3.2 A° a Na(1)MO(3) 2.23(1) Na(6)MO(12) 2.38(2) Na(1)MO(3)i 2.23(1) Na(6)MH2O(33) 2.41(2) Na(1)MO(13) 2.29(1) Na(6)MO(26) 2.46(2) Na(1)MO(13)i 2.29(1) Na(6)MH2O(34) 2.58(2) Na(1)MO(21) 2.27(1) Na(6)MO(25) 2.64(2) Na(1)MO(21)i 2.27(1) Na(6)MO(14) 2.76(2) Na(2)MO(25) 2.32(2) Na(6)MO(10) 3.01(2) Na(2)MO(2)ii 2.35(2) Na(7)MO(29)v 2.29(2) Na(2)MH2O(34) 2.34(2) Na(7)MO(29)iv 2.42(2) Na(2)MH2O(37) 2.39(2) Na(7)MO(1)ii 2.46(2) Na(2)MH2O(40) 2.41(2) Na(7)MO(17)iv 2.55(2) Na(2)MH2O(33) 2.60(2) Na(7)MH2O(43)v 2.59(2) Na(3)MO(25)ii 2.34(2) Na(7)MO(15)iv 3.03(2) Na(3)MO(28) 2.41(2) Na(7)MH2O(44)vi 3.22(2) Na(3)MH2O(34) 2.49(3) Na(7)MO(7)ii 3.25(2) Na(3)MH2O(37) 2.49(2) Na(8)MH2O(36)iv 2.35(2) Na(3)MO(2)ii 2.66(2) Na(8)MH2O(36)x 2.38(2) Na(3)MO(26)ii 3.09(2) Na(8)MH2O(35) 2.40(2) Na(4)MO(31)iii 2.33(2) Na(8)MH2O(46)iv 2.41(2) Na(4)MH2O(42) 2.34(2) Na(8)MO(11)vii 2.78(2) Na(4)MO(26) 2.45(2) Na(8)MO(19) 2.80(2) Na(4)MH2O(39) 2.51(2) Na(8)MO(23) 2.91(2) Na(4)MH2O(32) 2.59(2) Na(8)MO(23)viii 3.12(2) Na(4)MH2O(33) 2.78(2) Na(9)MH2O(41)v 2.36(2) Na(5)MH2O(38) 2.36(2) Na(9)MH2O(40) 2.64(2) Fig. 4 The surrounding of the sodium cations Na(5)MO(30) 2.42(2) Na(9)MH2O(43)v 2.75(2) Na(5)MO(22) 2.45(2) Na(9)MO(1)ii 2.89(2) Na(5)MH2O(40) 2.45(2) Na(9)MO(4)ii 2.96(2) show also the presence of six OH linked to molybdenum Na(5)MO(6) 3.15(2) Na(9)MH2O(38) 3.15(2) in the clusters. Na(5)MH2O(37) 2.67(2) Na(10)MH2O(45)xi 2.31(2) Na(5)MO(24) 2.68(2) Na(10)MH2O(44) 2.48(2) In these condensed polyanions, the geometry of the Na(5)MO(25) 2.86(2) Na(10)MO(15)v 2.58(2) MoO5(OH) octahedra is characteristic of that encountered for Na(6)MH2O(39) 2.35(2) Na(10)MH2O(42) 2.65(2) MoV.As observed in Table 3, abnormally short MoMO bonds ranging from 1.65 to 1.69 A ° are observed corresponding to the aSymmetry codes: i: 1-x, 1-y, 1-z; ii: x, 1/2-y, z-1/2; iii: x-1, free apex, opposite very long MoMO bonds, ranging from 2.26 1/2-y, z-1/2; iv: 1-x, y-1/2. 1/2-z; v: x, 1/2-y, z+1/2; vi: 1-x, to 2.33 A ° , and four intermediate equatorial MoMO bonds, -y, 1-z; vii: 1+x, y, z; viii: 2-x, 1-y, 1-z; ix: 1+x, 3/2-y, 3/2+z; x: 1+x, 1/2-y, 1/2+z; xi: x-1, y, z.ranging from 1.94 to 2.14 A ° . Nevertheless a distinction can be 442 J.Mater. Chem., 1998, 8(2), 439–444made for the equatorial MoMO distances: smaller distances (1.94–1.97 A ° ) correspond to oxygen atoms bridging two octahedra, intermediate distances (2.03–2.08 A ° ) characterize the MoMOMP bonds, whereas larger MoMO bonds (2.08–2.14 A ° ) characterize the hydroxy groups bridging two Mo octahedra, i.e. MoMOHMMo. With PMO distances ranging from 1.48–1.58 A ° (Table 3) the monophosphate groups are less regular than observed in many monophosphates.The shortest PMO distances (1.48–1.53 A ° ) are due to the fact that the corresponding oxygen atom is free, whereas the longer ones correspond either to PMOMH or to PMOMMo bonds. The Na(1) octahedron that ensures the junction between two Mo6 rings exhibits a remarkable regular octahedral coordination with six NaMO distances ranging from 2.23–2.29 A ° (Table 4).The MoMMo distances in the Mo6 clusters (Table 3), ranging from 2.581–2.595 A ° and from 3.524–3.541 A ° are very similar to those observed for (H3O)2NaMo6P4O24(OH)7- (PPh4)2·5H2O, suggesting the existence of some strong MoMMo interactions. The great diVerence between this phase and that described by Haushalter and Lai5 deals with the relative position of the molybdenum clusters and with their connection through Na+ cations. In (H3O)2NaMo6P4O24(OH)7, the molybdenum clusters are linked through Na+ cations, along one direction only forming one dimensional chains,5 whereas in Na8(Mo2O4OH)3(PO4)3(PO3OH)·12.25H2O the Na+ cations form a three-dimensional network with the Na[Mo6P4O27(OH)4]2 clusters.Fig. 1 shows that Na(2) forms indeed one NaMOMP bond with one cluster and one NaMOMMo bond with another cluster, that Na(3) shares two bonds, NaMOMP bond with the same cluster and forms one NaMOMP bond with another cluster, whereas Na(4) forms one NaMOMP bond with one cluster and a second one with another cluster, and Na(7) is connected to two diVerent clusters through two NaMOMMo and two NaMOMP bonds, respectively.In the same way, the projection of the structure along c (Fig. 3) shows that Na(8) ensures the connection between two clusters, forming one NaMOMMo bond with one unit and two NaMOMMo bonds with a second unit. By contrast, atoms Na(5), Na(6), Na(9) and Na(10) form NaMO bonds within the same cluster only so that such ions do not participate to the cohesion of the framework, at least from the viewpoint of strong NaMOMP or NaMOMMo bonds.We can consider this structure as a mixed framework ‘Mo–P–Na–O’ built up only of octahedra and tetrahedra. In such a description, the Na(2), Na(3) and Na(4) cations that, like Na(1), exhibit an octahedral coordination, are suYcient to construct a tridimensional framework with the Na[Mo6P4O27(OH)4]2 clusters.In the latter, the Na+ cations Fig. 6 Tunnels running along a (a), b (b), c (c) Na(5), Na(6), Na(7), Na(8), Na(9) and Na(10) which exhibit a diVerent coordination can then be considered as interpolated cations. The Na(2) and Na(4) atoms form Na(H2O)4O2 octahedra [Fig. 4(a,c)] with NaMO distances ranging from 2.32–2.78 A ° (Table 4) whereas more distorted Na(H2O)2O4 octahedra [Fig. 4(b)] are observed for Na(3) with NaMO distances ranging from 2.34–3.09 A ° (Table 4). Such Na(H2O)4O2 and Na(H2O)2O4 octahedra share their apices and edges to form disconnected chains running along c, at y#0.25 (Fig. 5); the association of the Na(H2O)4O2 and Na(H2O)2O4 octahedra with the Na[Mo6P4O27(OH)4]2 clusters, by sharing corners and edges, leads to the formation of large tunnels running along a [Fig. 6(a)], b [Fig. 6(b)] and c [Fig. 6(c)] so that this hydroxymonophosphate can also be described as an Fig. 5 Chains of NaO6 octahedra running along c at y=1/4 intersecting tunnel structure. J. Mater. Chem., 1998, 8(2), 439–444 443In such an intersecting tunnel structure the Na(5), Na(6) and and the presence of OH groups not only on the P tetrahedra but also on the Mo octahedra.Na(8) cations form bicapped triangular prisms, Na(H2O)3O5 [Fig. 4(d,e)] and Na(H2O)4O4 [Fig. 4(g)] respectively, with Based on these observations, it should be possible to synthesize many other molybdenum(V) hydroxymonophos- seven NaMO distances ranging from 2.35 to 2.91 A° , the eight NaMO bonds being >3 A° (3.01–3.15 A ° ); it is noted that two phates with microporous properties by associating Na+ with larger univalent cations, such as rubidium, caesium or adjacent Na(8) polyhedra share one face.The Na(7) cations exhibit a strongly distorted cubic coordination [Fig. 4( f )] with alkyl/arylammonium ions. two adjacent Na(H2O)2O6 cubes sharing one edge. The Na(9) and Na(10) sites are only partially occupied (Table 2). These References cations that form bonds with only one Na[Mo6P4O27(OH)4]2 cluster have their ligands displayed on the same side with 1 R.C. Haushalter and L. A. Mundi, Chem.Mater. 1992, 4, 31. 2 G. Costentin, A. Leclaire, M-M. Borel, A. Grandin and B. Raveau, respect to Na+; in the polyhedra Na(H2O)4O2 [Fig. 4(h)] and Rev. Inorg. Chem., 1993, 13, 77.Na(H2O)3O [Fig. 4(i )], the interatomic NaMO distances range 3 R. Peascoe and A. Clearfield, J. Solid State Chem., 1991, 95, 289. from 2.31–3.15 A° . 4 K. Kasthuri Rangan and J. Gopalakrishnan, Inorg. Chem., 1996, In conclusion, a new sodium molybdenum hydroxymono- 35, 6080. phosphate has been synthesized. It is to our knowledge 5 R. C. Haushalter and F. W. Lai, Inorg.Chem., 1989, 28, 2904. an unusual molybdenum(V) hydroxyphosphate that contains 6 L. A. Mundi and R. C. Haushalter, Inorg. Chem., 1990, 29, 2879. 7 R. C. Haushalter and F. W. Lai, Angew. Chem., Int. Ed. Engl., 1989, only sodium as a univalent interpolated cation, the only 28, 743. other example being Na3[Mo2O4(HPO4) (PO4)]·2H2O6 which 8 L. A. Mundi, L. Yacullo and R. C. Haushalter, J. Solid State Chem., exhibits a layer structure. The presence of clusters 1991, 95, 283. A[Mo6P4O31Hn]2 seems to be a characteristic of molyb- 9 Xtal3.2 Reference Manual, ed. S. R. Hall, H. D. Flack, denum(V) hydroxyphosphates, since it has previously been J. M. Stewart, Universities of Western Australia, Australia, observed in several compounds with Na or Fe or Zn, viz. Geneva, Switzerland and Maryland. 10 N. E. Brese and M. O’KeeVe, Acta Crystallogr., Sect. B. 1991, (Et4N)6Na2[Na12(H3PO4){Mo6O15(HPO4) (H2PO4)3}]·xH2O,7 47, 192. (PPh4)2[(H3O)2NaMo6P4O24(OH)7]·5H2O,5 [(CH3)4N]2- 11 I. D. Brown, Acta Crystallogr., Sect. A. 1976, 32, 24. (H3O)2[Zn3Mo12O30(HPO4)2(H2PO4)6]·11.5H2O,13 12 A. Guesdon, M. M. Borel, A. Leclaire and B. Raveau, Chem. Eur. [(CH3)4N]2(NH4)2[Fe2Mo12O30(H2PO4)6(HPO4)2]·nH2O,14 J., 1997, 3, 1797. and [(CH3)4N]2Na4[Fe3Mo12O30(HxPO4)8]·nH2O.14 13 L. A. Mundi and R. C. Haushalter, Inorg. Chem., 1992, 31, 3050. Two important features characterize this original structure: 14 L. A. Meyer and R. C. Haushalter, Inorg. Chem., 1993, 32, 1579. its three-dimensional mixed framework built up of Mo and Na octahedra and P tetrahedra that forms intersecting tunnels Paper 7/07568E; Received 20th October, 1997 444 J. Mater. Chem., 1998, 8(2), 439–444

ISSN:0959-9428    

DOI:10.1039/a707568e

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Sonochemical preparation and properties of nanostructured palladium metallic clusters N. Arul Dhas and A. Gedanken* Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900, Israel Nanoscale particles of palladium metallic clusters have been prepared at room temperature by sonochemical reduction of a 152 molar mixture of palladium acetate, Pd(O2CCH3)2, and myristyltrimethylammonium bromide, CH3(CH2)12N(CH3)3Br (NR4X), in tetrahydrofuran (THF) or methanol.Apart from its stabilizing eVect, NR4X acts as a reducing agent, probably due to the decomposition that occurs at the liquid-phase region immediately surrounding the collapsing cavity, and provides reducing radicals. Addition of 0.2 M ethanol–methanol in the THF process enhances the sonochemical reduction of PdII because of its highly volatile nature producing various reducing radicals inside the collapsing bubble.Analyses by UV–VIS spectroscopy indicated the initial formation of a PdII–NR4X complex, which, in turn, reduced to Pd0. Elemental analysis of the resulting solid (sonication residue) shows that the THF process yields NR4X stabilized-palladium clusters, whereas the methanol process shows the formation of pure Pd agglomerates. X-Ray diVraction (XRD) and transmission electron microscopy (TEM) with selected area electron diVraction (SAED) techniques were carried out to ascertain the nature, size and morphology of the Pd clusters.TEM of NR4X stabilized-Pd shows the presence of spherical particles of 10–20 nm in size.Selected area electron diVraction (SAED), along with TEM, reveals that the pure Pd consists of dense agglomerates, whereas NR4X stabilized-Pd exists as thin crystallites. These Pd nanoclusters are catalytically active towards carbon–carbon coupling, or Heck reaction, in the absence of phosphine ligands, to a moderate extent of 30% conversion. Hydrogenation of cyclohexene to cyclohexane has also been studied using sonochemically generated Pd materials.The catalytic ability of these Pd materials was compared with the commercial Pd on carbon material. In the past, a flurry of activity has been directed towards studied. Also, we report the catalytic activity of sonochemically the preparation of nanosized noble metallic clusters due to a prepared Pd nanoparticles, in the hydrogenation of cyclolarge enhancement in the catalytic properties of nanoparticles hexene and the CMC coupling or Heck reaction.compared to the bulk.1–6 The smaller the metal particles, the larger the fraction of the metal atoms that are exposed at Experimental surfaces, where they are accessible to reactant molecules and available for catalysis. The catalytic activity of the particles Materials generally depends on their size,3 shape,4 and stabilizing agents,5 All manipulations for the preparations of the sample were which are controlled by the preparative conditions.Owing to performed in an inert-atmosphere box (nitrogen atm,<10 ppm the diversity of approaches to the design of these nanoscale O2). Tetrahydrofuran (Bio-Lab, HPLC grade) was distilled materials,7–10 a broad spectrum of physicochemical properties over sodium/benzophenone.Methanol and ethanol (Bio-Lab are possible. Synthetic methods such as controlled chem- HPLC grade) were used as received. Ultrasonic irradiation ical reduction,7 photochemical reduction,8 electrochemical was accomplished with a high intensity ultrasonic probe reduction,9 and metal vaporization10 have all been used to (Misonix XL sonicator; 1 cm diameter Ti horn, 20 kHz, improve the catalytic properties of palladium.The change in 100 Wcm-2). the catalytic activity due to the diVerent preparation conditions, though implied, has not been suYciently clarified. Sonochemistry arises from acoustic cavitation phenomenon, Synthesis that is, the formation, growth and implosive collapse of bubbles Ultrasound irradiation of a mixture of 152 molar ratio of in a liquid medium.11 The extremely high temperatures palladium acetate and myristyltrimethylammonium bromide (>5000 K), pressures (>20 MPa), and very high cooling rates (NR4X) in THF or methanol yields nanoparticles of palladium.(>107 K s-1) attained during acoustic cavitation lead to many On the addition of NR4X to a solution of palladium acetate unique properties in the irradiated solution.Using these in THF or methanol, strong colour intensification is observed, extreme conditions, Suslick and his coworkers have prepared which indicates the desired interaction between the Pd salt amorphous Fe 12 and Mo2C13 by using sonochemical and NR4X. This may be due to the formation of a NR4X–PdII decomposition of metal carbonyls in alkane solvent.Adopting complex.7 Typically, a solution of Pd(O2CMe)2 (ca. 72 mg) and a similar technique, we have obtained amorphous Ni by the NR4X (ca. 204 mg) in THF (50 ml ) or methanol (50 ml ) was sonochemical decomposition of nickel tetracarbonyl at ambient sonicated using high intensity ultrasound radiation for three conditions.14 Okitsu et al.15 have prepared Pd nanoparticles hours by employing a direct immersion titanium horn under in the presence of polymeric surfactants.argon at a pressure of roughly 0.2 MPa. A round bottom Recently, we described a novel in situ preparation of carbon- Pyrex glass vessel (total volume 55 ml) was used for the activated palladium nanoparticles by the sonochemical ultrasound irradiation, which had a silicon rubber septum for decomposition of a carbon rich organometallic precursor.16 In gas bubbling or sample extraction without exposing the sample a continuation of our studies on noble metallic nanoparticles, to air.The solutions to be sonicated were purged with argon herein we describe a simple recipe for the preparation of gas, and were kept under argon throughout the experiment.palladium nanoparticles, using the sonochemical reduction The sonication cell was kept immersed in a cold bath contain- process in non-aqueous medium. The role of simple alcohols, ing a dry ice–acetone mixture during the entire sonication. such as ethanol and methanol, on the sonochemical reduction and the choice of solvent on NR4X stabilization of Pd were The resulting black-coloured colloidal solution was taken into J.Mater. Chem., 1998, 8(2), 445–450 445the glove box and carefully transferred into a centrifuge tube. detached and the mixture was centrifuged (10 000 rpm). The products were identified by GC and NMR spectroscopy. The The black powders were recovered by centrifugation (9000 rpm for 30 min), washed thoroughly with THF and ethanol, and conversion (%) was calculated from the GC data.Hydrogenation of cyclohexene using diVerent Pd samples were dried in vacuum. Dried samples were preserved in vials in an inert-atmosphere glove box for further studies. Ultrasound examined under similar conditions. The time course of the catalytic activity experiments of these Pd powders, in the irradiation using a 0.2 M alcoholic (methanol or ethanol) THF medium was also carried out under similar conditions.The hydrogenation of cyclohexene, was studied by using 10 mg of Pd, 0.5 ml of cyclohexene in 10 ml of diethyl ether, under elemental analyses of THF process derived powder shows 26% C, 3.1% H, 1.6% N, 4.1% Br and that of alcoholic THF similar conditions.During each 10 min interval, 1 ml of the reaction mixture was collected using a septum and was immedi- process derived Pd shows a lower stabilizer content in the sonication product (19% C, 1.9% H, 1.1% N, 2.8% Br). The ately centrifuged. The reaction product was analyzed using a Carbosieve G column with a thermal conductivity detector formation of NR4X stabilized-Pd is further confirmed by the IR spectrum of the product, which shows the presence of (TCD).Since diVerent compounds have diVerent TCD responses, correction factors were included in the calculations. characteristic CMH stretching bands between 2800 and 3000 cm-1 of methyl and methylene groups in the ‘high A quartz high pressure reactor was used for the catalytic studies of the carbon–carbon coupling reaction.A mixture of frequency region’. On the other hand methanol process-derived Pd shows a negligible amount of carbon (<1%) without N bromobenzene (0.21 ml), styrene (0.23 ml), potassium acetate (235 mg) and 6 mg of Pd powder was placed in a pressure and Br impurities. Upon drying the material in vacuum, highly pyrophoric material is obtained.vessel and diluted with dimethylformamide (1 ml). The mixture was heated to 125 °C for 20 h and stirred under argon. The Commercially available palladium (10%) supported on carbon (Pd/C, Aldrich) was used as received for the catalytic resulting mixture was cooled and filtered. A known amount of internal standard was added to the filtrate and was dissolved studies. In situ prepared Pd/C nanoparticles were obtained using the procedure described elsewhere.16 In brief, sonication in dichloromethane prior to being analyzed by GC.of a solution of tris-m-(dibenzylideneacetone)dipalladium (300 mg) in mesitylene (50 ml ) under argon for 3 h yields in Results and Discussion situ Pd/C (ca. 55% Pd) nanoparticles. The resulting solid powder was washed thoroughly with THF and ethanol and UV–VIS spectra dried in vacuum prior to the catalytic studies. Sonochemical reduction was considered to be brought about To prepare nanostructured pure Pd particles on silica by the reducing radicals generated by ultrasound pyrolysis of (Pd/SiO2, 10% Pd), preheated Stobers silica was used.Stobers the solvent and/or the substances present in the medium. The silica17 has been prepared by base hydrolysis and condensation sonochemical reduction was followed using the UV–VIS of TEOS in an aqueous ethanol medium containing ammonia. absorbance of the PdII–NR4X complex.The UV–VIS spectrum A slurry of preheated Stobers silica (100 mg), palladium acetate of palladium acetate in THF [Fig. 1(a)] shows a strong (200 mg), and NR4X (400 mg) was irradiated in methanol absorption at 385 nm.Fig. 1( b) shows the absorption spectrum (50 ml ) for 2 h under argon. The resulting solid powder was of the reaction mixture (palladium acetate and NR4X in THF) washed thoroughly with THF and ethanol and dried in vacuum before irradiation and shows a strong absorption maximum prior to the catalytic studies. at lmax=420 nm corresponding to the PdII–NR4X complex. The observed visible absorption band of the PdII–NR4X com- Characterization plex is ascribed to the d–d transition of PdII.The red shift of The sonochemical reduction of PdII was followed by UV–VIS the visible absorption band of the reaction mixture by the spectroscopy with a Varian (model-DMS 100S) spectrophoto- addition of NR4X [with respect to the palladium acetate in meter.The X-ray diVraction patterns of Pd nanoparticles were THF (lmax=385 nm)] indicates the formation of a palladium recorded by employing a Rigaku X-ray diVractometer (Model- acetate–surfactant complex. The intensity of this absorption 2028, Cu-Ka). The size of crystallites was calculated from the peak of the PdII–NR4X complex gradually decreased with peak width at a half maximum of (111) X-ray reflection, using irradiation time.The spectra show a continuous rise in the the Debye–Scherrer equation.18 The lattice parameters were background towards higher energies, which is due to Mie calculated with a least-squares fit using the (111), (200), (220) scattering from the Pd nanoparticles in solution.19 For comand (311) X-ray reflections. The transmission electron micro- parison, Fig. 2 shows the change in absorption with sonication graphs (TEM) and selected area electron diVraction (SAED) patterns were obtained by employing JEOL-JEM 100SX microscopes.Samples for the TEM examination were prepared by suspending dried samples in dioxane. A drop of the sample suspension was allowed to dry on a copper grid (400 mesh, electron microscopy sciences) coated with carbon film. Samples for elemental analysis were submitted in sealed vials without exposure to air.Catalysis In the heterogeneous phase, the activities of diVerent palladium nanoclusters were determined by the hydrogenation of cyclohexene in diethyl ether medium at room temperature. A Pyrex glass reactor was used for the hydrogenation catalytic studies.Typically, 5 mg of Pd was transferred into the reactor in the glove box, without exposing them to air, and were stirred with 5 ml of diethyl ether under high pure N2 atmosphere for Fig. 1 UV–VIS spectra of: (a) palladium acetate in THF; (b) palladium 10 min. Then H2 gas was bubbled for 10 min and 0.2 ml acetate–NR4X complex in THF (before irradiation); (c) after 1 h (ca. 2mM) of cyclohexene was injected through a septum. The irradiation of (b); (d) after 2 h irradiation of (b); (e) after 3 h irradiation reaction mixture was kept under hydrogen atmosphere of (b); (f ) after 2 h irradiation of the palladium acetate–NR4X complex in alcoholic THF medium (ca. 0.2 MPa) with stirring for 30 min; then the H2 supply was 446 J. Mater. Chem., 1998, 8(2), 445–450The acceleration in the rate of sonochemical reduction of PdII in ethanolic–methanolic solution of THF or methanol process may be due to the volatile nature of these alcohols, which can evaporate into the gas-phase region of the collapsing bubble and produce several primary reducing radicals to an even greater extent.22 Primary radicals are generated through the adiabatic heating of the gas contained in the bubble, as the ‘hot spot’ theory for cavitation bubble collapse postulates.11 The reducing radicals generated in the bubble can either recombine or react with the surfactant (NR4X) in the interfacial region and generate secondary radicals.Consequently, the formation of large amounts of reducing radicals (from the solute and surfactant) accelerates the reduction of PdII to Pd0.In general, however, a-hydroxyalkyl radicals are reducing Fig. 2 Plot of absorbance intensity of the 420 nm peak as a function species, that are likely to initiate the reduction process.23 The of sonication time. ($) 0.2 M ethanolic THF medium; (&) THF primary step for the formation of a-hydroxyalkyl radicals from medium. the aliphatic alcohols may be written as, R¾MCH2OH�R¾MVCHOH+HV (3) time, at 420 nm for the complex, in the presence and absence A higher concentration of alcohols does not help to enhance of alcohol.As is clear in both examples, there is a continuous the formation of reducing radicals.22 The sonochemical and linear decrease in the PdII–NR4X complex concentration reduction of a PdII–NR4X complex in the presence of simple during sonication, but the rate of reduction of the complex is alcohols is consistent with the following reaction: enhanced by a factor of 1.5–2 in the presence of alcohol (ethanol or methanol).The linear rate of reduction of PdII Pd(CH3CO2)2 NR4X+2R¾MVCHOH�Pd suggests that a constant number of reducing radicals are +NR4X+2CH3CO2H+2R¾MCHO (R¾=CH3, H) (4) created per unit time, and the enhanced reduction in the presence of alcohol implies that the production of reducing The long chain NR4X salts formed directly at the reduction species is greater in the presence of alcohol.A similar enhanced center in high local concentration act as very eVective protecting reduction of PdII is observed in the methanol process. agents to keep the particles of freshly reduced metal in solution.Therefore, it is evident that the aliphatic alcohols play an The screening of the metal particles by large lipophilic alkyl important role in determining the rate of nucleation of the groups protects the metallic core from agglomeration. It is metal in solution, as recognized earlier.20 worth mentioning that the NR4X stabilized-Pd powders are highly stable and are redispersible without showing any sign of Mechanism of sonochemical reduction Pd powder segregation.The lack of stabilizing action shown by NR4X in methanol process is probably due to its high solubility The sonochemical reduction of PdII can be explained by in methanol and therefore yields metallic Pd precipitates. considering the sonochemical reaction site.In the sonochemical process, there are three diVerent regions where the sonoc Powder X-ray studies ical reaction can occur,11 namely: (i) the gas-phase within the collapsing cavity, where elevated temperatures and high press- Fig. 3 shows X-ray powder diVraction profiles of the studied ures are produced; (ii) the thin liquid layer immediately samples. The XRD pattern of initial stabilized-Pd (THF surrounding the collapsing cavity (interfacial region), where the temperature is lower than in the gas-phase reaction zone, but still high enough for a sonochemical decomposition reaction to occur; (iii) the bulk solution at ambient temperatures where reactions take place between solute molecules and the reducing radicals.The low vapor pressure of the PdII complex eliminates the possibility of the sonochemical reduction taking place in the gas-phase region.The liquid-phase zone and the bulk solution are the regions where the major part of the sonochemical reduction of PdII to nanosized Pd0 occurs. It has been well accepted21 that the long chain surfactants are subjected to pyrolysis by ultrasound irradiation to yield a large variety of reducing radical intermediates such as VH, VCH3, VCH2R, etc.This pyrolysis event also mainly occurs in the liquid-phase region, not in the gas-phase region, owing to its non-volatile nature. Collapsing of the bubble will aid the dispersion of the radicals. According to Okitsu et al.,15 the following elementary reactions are proposed to explain the mechanism of sonochemical reduction.NR4X�VRNR3X+VH (sonolysis) (1) 2 VH+PdII�Pd0+2H* (2) Okitsu et al.15 have observed a considerable increase in the amount and rate of reduction of PdII by using excess surfactant (polymers) in aqueous medium. They claim that this is due to the generation of a large number of secondary reducing species in the interfacial region resulting from the attack of H and Fig. 3 X-Ray diVractograms of: (a) as-formed Pd (THF process); OH radicals (sonochemical products of water) on the (b) and (c) calcined (a) at 400 and 600 °C respectively; (d) as-formed Pd (methanol process) surfactant.J. Mater. Chem., 1998, 8(2), 445–450 447Fig. 4 Transmission electron micrograph and associated SAED pattern (inset) of as-formed Pd: (a) methanol process; (b) THF process; (c) alcoholic THF process process) is X-ray amorphous [Fig. 3(a)]. The absence of an are evolved after heating them at 400 and 600 °C under argon for 4 h [Fig. 3(b) and (c)]. The peak positions are consistent XRD peak indicated that either the particles were crystallographically amorphous in nature or that the crystalline with the metallic Pd. The intensity of the XRD reflections are increased with calcination temperature.The increase in the domains were too small to give rise to crystal reflections in the XRD pattern. However, the nanocrystalline nature of these intensity may be due to crystallization of Pd upon heat treatments at higher temperature. The observed sharpening of powders was further confirmed by the SAED pattern, using TEM (see below). The X-ray diVraction peaks of stabilized-Pd the X-ray peak upon high thermal treatment is due to the 448 J.Mater. Chem., 1998, 8(2), 445–450Table 1 Catalytic yields of hydrogenation and CMC coupling reaction accompanying particle growth during the secondary crystallization process. The X-ray reflections were indexed on the basis conversion (%) of the fcc structure of Pd with space group Fm3m (JCPDS card no. 5-681). The alcoholic THF-derived Pd showed a material hydrogenation CMC coupling similar behaviour upon X-ray diVraction after heat treatment. commercial Pd/C 51 20 The X-ray diVraction pattern [Fig. 3(d)] of methanol processin situ prepared Pd/C 86 30a derived Pd shows the characteristic diVraction peaks corre- NR4X stabilized-Pd (THF 64 25 sponding to Pd0 without thermal treatment.All possible Pd process) peaks are present, indicating a polycrystalline nature. The pure Pd (methanol process) 12 3 calculated lattice constant a=3.8872 A ° is in good agreement pure Pd/SiO2 (methanol 16 9 with literature values. The average size of crystallites from the process) X-ray line broadening is 90 and 70 nm for the initial Pd aRef. 16. (methanol process) and the crystallized Pd (THF process), respectively.It is gratifying to see that the size of the Pd crystallites by THF process is small, even after crystallization. The relatively large size of the initial Pd derived by methanol process is a direct result of agglomeration, owing to the absence of a protecting agent. Microstructure Fig. 4 reveals the TEM microstructure of the palladium nanoparticles. The pure Pd prepared by the methanol process shows [Fig. 4(a)] the presence of irregular particles which are highly agglomerated. The formation of dense agglomerates may be due to the attraction between the nanoparticles, owing to the lack of a stabilizer. The TEM selected area electron diVraction (SAED) pattern of pure Pd nanoparticles shows diVraction Fig. 5 Catalytic activity of diVerent palladium nanoclusters towards spots typical for an agglomerated crystalline material [inset of hydrogenation of cyclohexene as a function of time. (&) Commercial Fig. 4(a)]. The diVraction pattern could be indexed to an fcc carbon activated palladium; (6) NR4X stabilized palladium prepared unit cell of dimension a=3.8892 A ° . The TEM picture by the THF process; (+) in situ prepared carbon activated palladium, [Fig. 4(b)] of the initial NR4X stabilized Pd (THF process) see ref. 16. shows that the particles are spherical in nature, with a mean diameter of roughly 20 nm. The particles are well separated, (THF process) along with commercial Pd/C towards the without forming an aggregate network on the TEM grid. The hydrogenation of cyclohexene.It is very clear that both types SAED pattern of nanoclusters of NR4X stabilized-Pd (THF of sonochemically prepared Pd show a higher conversion than process) apparently consists of a ring pattern with diVraction does the conventional catalyst. It is worth mentioning that the spots [inset of Fig. 4(b)]. The ring pattern is due to diVraction in situ Pd/C shows 100% conversion at as little as 40 min.spots from the thin Pd-crystallites. On the other hand, the The catalytic ability of the sonochemically derived palladium alcoholic THF-derived NR4X-stabilized Pd shows [Fig. 4(c)] nanoparticles was examined in CMC bond forming processes, agglomerates of thin particles. The formation of more aggrebetween bromobenzene and styrene, in the absence of phosgated Pd particles in the alcoholic THF medium may be due phine ligands (Table 1).The catalytic activity of these powders to the smaller stabilizer (NR4X) content that promotes the varied with their nature. The commercial Pd/C shows a 20% formation of agglomerates. The SAED pattern of alcoholic conversion towards the Heck reaction. In situ prepared Pd/C THF-derived Pd again shows its crystalline nature [inset of again shows a maximum conversion (30%).The stabilized-Pd Fig. 4(c)]. From the SAED patterns it is evident that both (THF process) shows a higher activity (25%) as compared stabilized-Pd are nanocrystalline. From the TEM micrographs with pure Pd (3% conversion, methanol process). The pure it is evident that NR4X stabilization of Pd clusters allows for Pd derived from the methanol process shows poor conversion, particles free of agglomeration.Addition of alcohols enhances as expected. Using a higher concentration of NR4X in the the solubility of NR4X, consequently leading to the formation THF process leads to a higher stabilizer content in the resulting of a smaller stabilizer content in stabilized Pd or pure Pd (in powder.However, this poor catalytic activity may be due to methanol process) aggregates. the screening of Pd surfaces by a large number stabilizer molecules. It is remarkable that stabilized Pd clusters catalyze Catalytic activity the Heck reaction of non-activated bromobenzene, which still represents a synthetic challenge in a practical sense. Indeed, in The yields of cyclohexane in the hydrogenation of cyclohexene and the trans-stilbene in the CMC coupling reaction of as- the case of activated bromoaromatics, these are reported to show poor-to-moderate conversion (5&nd30%) with Pd colloids.prepared (before calcination) Pd clusters generated sonochemically are summarized in Table 1. In the hydrogenation reaction, commercial Pd/C shows a 51% conversion of cyclohexene Conclusion to cyclohexane for 30 min under our experimental conditions. In situ prepared Pd/C shows a maximum conversion of ca.Sonochemical reduction of PdII with NR4X in THF and methanol yields stabilized-Pd and pure Pd, respectively, at 86%. The higher catalytic activity towards hydrogenation of cyclohexene may be due to the reactive Pd nanoparticles room temperature.The in situ extraordinarily high temperatures and exceptionally high pressures attained during encapsulated in the amorphous carbon matrix. NR4X stabilized- Pd (THF process) shows a better conversion compared cavitational collapse, combined with high rates of cooling, lead to the formation of large amounts of reducing radicals to that of the commercial one. Pure Pd and Pd/SiO2 show poor conversion. The poor catalytic ability of pure Pd is due which act as reducing agents for the PdII�Pd0 reaction.UV–VIS studies have shown that addition of alcohol in THF to the fact that aggregation under the experimental conditions prompts drastic loss of catalytic activity. Fig. 5 compares the or use of a methanol medium remarkably enhances the sonochemical reduction process.The stabilizing agent, NR4X activity of in situ prepared Pd/C and NR4X stabilized-Pd J. Mater. Chem., 1998, 8(2), 445–450 4492 H. Bonnemann, W. Brijoux and Th. Joussen, Angew. Chem., Int. freshly generated at the reduction center, helps to keep the Ed. Engl., 1990, 29, 273. Pd nanoclusters free from agglomeration during the THF 3 J. S. Bradley, E. W. Hill, S. Behal, C.Klein, B. Chaudret and process. The formation of NR4X stabilized-Pd with a lower A. Duteil, Chem. Mater., 1992, 4, 1234. stabilizer content (alcoholic THF process) and pure Pd 4 S. Ahmadi, Z. L. Wang, T. C. Green, A. Henglein and M. A. El- (methanol process) indicates that the solubility of NR4X Sayed, Science, 1996, 272, 1924. 5 H. Bonnemann and G. A. Braun, Angew. Chem., Int. Ed.Engl., prevents the protective action of NR4X causing agglomer- 1996, 35, 1992. ation. TEM and SAED studies have shown that the stabilized- 6 R. F. Heck, Org. React., 1982, 27, 345; R. F. Heck, Palladium Pd nanoclusters are nanocrystalline and composed of aggre- Reagents in Organic Synthesis, Academic Press, New York, 1985; gates of spherical particles of size 10–20 nm. The particle size Y.Ben-David, M. Portnoy, M. Gozin and D. Milstein, of 70 nm for stabilized-Pd (X-ray line broadening) indicates a Organometallics, 1992, 11, 1995; M. T. Reetz and G. Lohmer, Chem. Commun., 1996, 1921. more than three-fold increase in size upon calcination. TEM 7 H. Bonnemann, W. Brijoux, R. Brinkmann, R. Fretzen, and SAED results, along with XRD studies of pure Pd, reveal Th.Joussen, R. Koppler, B. Korall, P. Neiteler and J. Richter, that they are strongly agglomerated and polycrystalline in J.Mol. Catal., 1994, 86, 129. nature. The formation of crystalline Pd products suggests 8 Y. Yonezawa, T. Sato, S. Kuroda and K. Kuge, J. Chem. Soc., that an interfacial and/or bulk process is involved in the Faraday T rans., 1991, 87, 1905. sonochemical reduction.The catalytic activity of NR4X 9 M. T. Reetz and W. Helbig, J. Am. Chem. Soc., 1994, 116, 7401. 10 G. C. Trivino, K. J. Klabunde and E. B. Dale, L angmuir, 1987, stabilized-Pd (THF process) towards hydrogenation and 3, 986. CMC coupling reaction is better than it is with the commer- 11 K. S. Suslick (editor), Ultrasounds: its Chemical, Physical and cially available Pd/C.From the elemental analyses, TEM Biological EVects, VCH,Weinheim, 1988. images and catalytic studies it is quite evident that the choice 12 K. S. Suslick, S. B. Choe, A. A. Cichowlas and M. W. GrinstaV, of solvent has a direct impact on the chemical composition, Nature (L ondon), 1991, 353, 414. 13 T. Hyeon, F. Fang and K. S. Suslick, J. Am. Chem. Soc., 1996, aggregation, dispersity and consequently the catalytic activity 118, 5492. of the nanoparticles. 14 Y. Koltypin, G. Katabi, R. Prozoroav and A. Gedanken, J. Non- Cryst. Solids, 1996, 201, 159. 15 K. Okitsu, H. Bandow, Y. Maeda and Y. Nagata, Chem. Mater., This research program was supported by Grant no. 94-00230 1996, 8, 315. from the US–Israel Binational Foundation (BSF), Jerusalem. 16 N. Arul Dhas, H. Cohen and A. Gedanken, J. Phys. Chem. B, 1997, The authors are grateful to Prof. M. Deutsch, Department of 101, 6834. Physics, and Prof. Z. Malik, Department of Life Sciences, for 17 W. Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 1968, extending their facilities to us. Also, we wish to thank Prof. 26, 62. 18 H. Klug and L. Alexander, X-Ray DiVraction Procedures, Wiley, D. Milstein and Dr Y. Ben-David of the Department of New York, 1962. Organic Chemistry at the Weizmann Institute of Science for 19 J. A. Creighton and D. G. Eadon, J. Chem. Soc., Faraday T rans., their encouragement. The authors thank Dr Shifra Hochberg 1991, 87, 3881. for editorial assistance. 20 P. Mulvaney, R. Cooper, F. Grieser and D. Meisel, J. Phys. Chem., 1990, 94, 8339. 21 A. E. Alegria, Y. Lion, T. Kondo and P. Riesz, J. Phys. Chem., 1989, 93, 4908. References 22 M. Gutierrez and A. Henglein, J. Phys. Chem., 1988, 92, 2978. 23 J. Z. Sostaric, P. Mulvaney and F. Grieser, J. Chem. Soc., Faraday 1 G. Schmid, Clusters and Colloids: From T heory to Applications, T rans., 1995, 91, 2843. VCH, Weinheim, 1994; Y. Volokitin, J. Sinzig, L. J. de-Jongh, G. Schmid, M. N. Vargaftik and I. I. Moiseev, Science, 1996, 384, 621. Paper 7/06100E; Received 20th August, 1997 450 J. Mater. Chem., 1998, 8(2), 445–4

ISSN:0959-9428    

DOI:10.1039/a706100e

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Hydrogen bonded associates in the Bayer process (in concentrated aluminate lyes): the mechanism of gibbsite nucleation A � gnes Buva�ri-Barcza, Ma�rta Ro�zsahegyi and Lajos Barcza* Institute of Inorganic and Analytical Chemistry, L . Eo� tvo�s University, Budapest-112, P.O.Box 32, 1518 Hungary The highly alkaline aluminate lye, which is produced in the first step of Bayer process, is characterized by low water activity and strong competition for water molecules.The consequence of these eVects is that two (or more) negatively charged species can be brought together and stabilized in the form of anionic hydrogen bonded complexes. Depending on temperature, total concentration and aluminate to hydroxide ratio, aluminate–hydroxide as well as aluminate–aluminate associates can be formed.Among the oligomerized aluminates, the cyclic hexamer seems to play a key role, as it contains octahedrally coordinated aluminate ions and is able to form nuclei in further polymerization for the partial precipitation of Al(OH)3. The interactions have been experimentally investigated using specially developed methods and a detailed computational analysis.Alumina is produced in several million tons per year by the CaO·Al2O3·10H2O 27 points to the existence of polymeric Al6(OH)246- species, containing six edge-linked, octahedrally Bayer process. The first step of this method is the extraction of bauxite using concentrated NaOH at high temperature.1,2 coordinated aluminate ions. Based on solid state 27Al NMR investigations,28–30 more and more data have been accumulated When the bauxite contains ‘monohydrates’ (AlOOH: boehmite and diaspore, minerals from temperate zones) the extraction on concentrated aluminate solutions containing not only tetrahedral but also octahedral species.31–33 Concerning the com- requires higher concentration (20–30% NaOH) and higher temperature (200–250 °C under 3.5 MPa pressure), while baux- position of aluminate lye, similar semi-empirical results can be deduced using molecular modelling.34 ite containing ‘trihydrates’ [Al(OH)3: gibbsite and hydrargillite, minerals from the tropics] can be extracted at lower tempera- According to our earlier work, the viscosity of a solution with strictly constant cation concentration and temperature is ture (120–140 °C) and with less concentrated caustic soda (ca. 15% NaOH).1–3 The highly alkaline solution is metastable at sensitive to any association, and the conductivity (under the same circumstances) is very sensitive to the hydroxide ion any temperature and can be decomposed to form Al(OH)3 precipitate. [The symbol Al(OH)3 represents here the concentration.35 (When either the total concentration or the temperature is changed, no theory is able to accurately predict amorphous�pseudo-boehmite�bayerite�gibbsite series.4] The separated product is always contaminated by some free their eVect on concentrated solutions.) To elucidate the diVerences between the two main versions of the Bayer process, the alkali, which is attributed to the unwashable mother-liquor.All forms of the precipitated Al(OH)34,5 consist of octahedrally investigation of the corresponding systems has been conducted, coordinated aluminiums connected by bridging hydroxide ions combining the experimental methods mentioned above with in a layered structure of six-membered rings6,7 ( like the honey- the most eYcient computer simulation. Two model series were comb structure of the carbon atoms in graphite layers), and chosen which resemble the processes used in practice: (i) a the layers are held together by hydrogen bonds.3 It is generally system with constant 6 M sodium (hydroxide) concentration at known that the production of Al(OH)3 is influenced both 25 °C and (ii ) a system with constant 4 M sodium (hydroxide) qualitatively and quantitatively by the total concentration, the concentration at 65 °C.In both series, the molar ratios (convenalumina/ sodium hydroxide ratio and the temperature of the tionally: r=Na2O/Al2O3) were varied down to rather low aluminate lye.1,2 In spite of the fact that several empirical and values (r#1.35, i.e. the concentration of aluminate in such semi-empirical rules are known concerning alkaline aluminate solutions was very high). solutions, further work is of importance.The interaction between aluminium and hydroxide ions has been mainly investigated in terms of hydrolysis, i.e. in more or Experimental less acidic solutions,10–14 where the main component is the Sodium hydroxide and water were purified to eliminate any octahedral Al(H2O)63+ ion itself, however the relative stability carbonate.Aluminate solutions were prepared by dissolving of a terdecamer cation Al13O4(OH)24(H2O)127+ (containing high purity (99.99%) aluminium metal in sodium hydroxide twelve [AlO6] octahedra joined together by common edges) is solution. Contamination by silicate and carbonate were care- of note.15 fully avoided: the solutions were prepared and stored in The composition of alkaline aluminate solutions and the polythene vessels under carbon dioxide free nitrogen.structure of the aluminate anion have been investigated by To check whether the properties of solutions depend on several workers,9,16–22 the definitive review is that of Eremin their preparation, freshly prepared or aged, heated and/or et al.,23 who analysed UV, IR, Raman and NMR spectra, as mixed and/or diluted solutions of identical composition were well as electrochemical, thermodynamic and kinetic properties. studied.Disregarding some solutions of very low molar ratio Similarly, the formation constants for the Al(OH)3–Al(OH)4- (r<1.3, i.e., with extremely high aluminate concentration), the equilibrium system have been measured by several workresults are very reproducible, which means that the equilibria ers;10–14 the definitive review of earlier work is that of Baes between the diVerent species must be fast and reversible.and Mesmer.12 In solid state, the Al2O(OH)62- unit (built up The viscosities were measured in Ostwald type viscometers from two [AlO4] tetrahedra) was identified in a crystalline constructed of alkali resistant glass and having capillary diam- solid24 (and detected later in concentrated solutions by IR, NMR and Raman methods25,26) while the structure of solid eters of 0.47, 0.53, 0.63 or 0.84 mm.They were calibrated with J. Mater. Chem., 1998, 8(2), 451–455 451tration (r>6 in 6.0 M and r10 in 4.0 M solutions, respectively), while a mixture of monomeric and dimeric aluminate species is assumed if r2 in 6.0 M solutions at 25 °C or r4 in 4.0 M solutions at 65 °C (which means that the aluminium concentrations are<3 or 1 M, respectively).The viscosities of solutions with higher aluminate content can be best approached (in 4.0 M solutions at 65 °C) assuming monomeric, dimeric and hexameric aluminate species, while any combination of polymeric species (including the variation mentioned as the simplest case) fits well for 6.0 M solutions, too.The measured conductivities show a very interesting trend, characteristic for complex formation.35 Namely, when we suppose a single monomeric aluminate component [i.e. Al(OH)4-] at relatively low total aluminate concentration, the equilibrium concentration of monomeric aluminate can be presumed to be equal to the total aluminium concentration (cAl): cAl=[NaAl(OH)4] (1) and we can assume: k=lNaOH [NaOH]+lNa-aluminate [NaAl(OH)4] (2) where k is the measured conductivity, l is the molar conductance of a given species (in the given media), while [ ] denotes equilibrium concentrations [eqn.(2) reflects the well known Fig. 1 Viscosities (×=every fifth data point, —=calculated curve) rule of additivity].According to our experiences, this rule is measured in 6 M solutions and at 25 °C valid also for the conductivity of rather concentrated electrolyte mixtures with constant common cation concentration, e.g. for that of NaOH–NaClO4 solutions with constant 6.0 M sodium ion concentration, or OH–K2CO3 mixtures with constant 5.0 M potassium ion concentration.35 As lNaOH can be measured in pure NaOH solutions (cAl= 0.0 M) very precisely, the ‘free’ sodium hydroxide concentrations and its conductivity [based on eqn.(2), disregarding the conductivity of sodium aluminate] can be calculated in the concentration range under discussion. The calculated, hypothetical values give a straight line, as shown in Fig. 2. The (negative) diVerence between the measured and hypothetical conductivities can not be explained by assuming completely undissociated sodium aluminate ion pairs (since the molar conductance can never be negative) but only by interactions which decrease the hydroxide ion concentration, such as the formation of a hydrogen bonded hydroxide– Fig. 2 Conductivities (×=every tenth data point, —=calculated data, B=the calculated conductivity of the free sodium hydroxide content) aluminate complex Al(OH)4-·OH-. Extremely low water measured in 6 M solution and at 25 °C activity is characteristic for aluminate lyes8,17,23 and therefore strong competition exists for water molecules, which can bring the two negatively charged species together.(Of course, the glycerol solutions of known concentration and viscosity.36 The flow-time varied from 60 to 150 s, measured with an accuracy associate can bind some water molecules, also, but the total quantity of bound water is surely decreased.The role of low of±0.05 s. Conductivity data were recorded with a Radiometer CDM-2d type conductometer using CDC-104 or other, water activity has been previously discussed by Scotford and Glastonbury17 and by Eremin et al.23) specially adapted electrodes.36,37 The reproducibility of measured data was better than ±0.5%.The temperature was kept The oligomerization equilibria of tetrahydroxo aluminate anions and their background (OH- here) can be explained strictly constant (within ±0.02 °C), as was the sodium ion concentration (variation <±0.1%).similarly. Further, both possibilities have been considered. Since the formation constant of the Al(OH)4- ion (b4= The data were measured in diVerent ranges of molar ratios (r>6, r=6–3, r=3–2 and r=2–1.35), i.e. starting with low [Al(OH)4-]/[Al3+][OH-]4) is extremely high (at 25 °C and at ionic strength extrapolated to 0.0: log b43313,14), it must aluminate content and increasing the concentration up to the highest values.The measurements were repeated several times. be regarded as the basic component for all species containing aluminium in alkaline solution: Some of the viscosity data measured in 6.000 M solutions and at 25.0 °C are presented in Fig. 1 and those of conductivities pAl(OH)4-+q OH-=(Al(OH)4-)p(OH-)q (3) in Fig. 2. (The data measured in 4.000 M solutions and at 65.0 °C show no special characteristics, the trends of data The general definition of the formation constant for this equilibrium is as follows: are similar.) bpq=[(Al(OH)4-)p(OH-)q]/[Al(OH)4-]p [OH-]q (4) Results We can define a parameter Y 35,36 (in general form, similarly to the simpler case represented by eqn.(2), at constant cation The measured viscosity data could be fitted in a computer simulation [based on equations similar to eqn.(2) and (5),36,37 concentration and temperature) as with coeYcient(s) characterising the contribution(s) of the Y=.. fpqbpq [Al(OH)4-]p [OH-]q (5) individual species to the viscosity measured] assuming monomeric species alone only at relatively low aluminium concen- where fpq symbolizes the linear contribution of a ( p,q) species 452 J.Mater. Chem., 1998, 8(2), 451–455to the given property. It follows that fpq is a constant valid only for the given electrolyte and temperature (such as molar conductance in the case of conductometric measurements). According to eqn. (5) and using the expressions of mass balances: cAl=.. p bpq [Al(OH)4-]p [OH-]q (6) cOH-=..q bpq [Al(OH)4-]p [OH-]q (7) both the viscosity and conductivity data can be evaluated in parallel and step by step in the whole concentration range investigated. With the aid of eqn. (6) and (7), some relations, used earlier, can be defined more exactly, as cNa+=cOH-+cAl=const (8) Fig. 3 Molar speciation in 6 M solutions at 25 °C as a function of the and total aluminium concentration (ci expressed as molar concentration of Al in the form of the given species; —=Al(OH)4-; – –= r=cNa+/cAl (9) [Al(OH)4]22-;— —=Al6(OH)246-; E=Al(OH)4-·OH-; In systems of low aluminate content (at higher r values: r6 I=[Al2(OH)82-·OH-]) or r10, respectively, for [Na+]=6 or 4 M) the viscosity data can be interpreted by monomeric aluminate species [which can be either Al(OH)4- or Al(OH)4-·OH-; viz.(1,0) and (1,1) species]. In the same concentration range, the conductivity data indicate both (1,0) and (1,1) species and their stability constants can be computed using eqn. (5)–(7). Correlating the viscosity data with these constants using eqn. (5)–(7) leads to a perfect fit of measured and calculated data. (We should mention here that constants calculated in the range of lower aluminate concentrations were kept unchanged for the subsequent computer calculations.) The evaluation of viscosity data in themselves shows the increasing presence of dimeric species with increasing cAl (with decreasing r, between r=6–2 and r=10–4, respectively for [Na+]=6 and 4 M), corresponding to Al2O(OH)62-, indicated here as {Al(OH)4-}2 or (2,0).[However, the formation of an Al(OH)4-·Al(OH)4- species could also be rationalized similarly to the formation of Al(OH)4-·OH- species, and it may Fig. 4 Molar speciation in 4 M solutions at 65 °C as a function of the have a similar Raman spectrum to that of Al2O(OH)62-.34] total aluminium concentration (see Fig. 3 caption for key) Computation of the conductivity data does not give a good fit assuming only (1,0), (1,1) and (2,0) species, and hydroxide reasonable considering the temperatures and the mainly hydroassociate (2,1) has to be introduced.It is remarkable that no gen bonded character of the interactions. The mole fractions (2,2) complex exists:38 repulsion among the four negatively of diVerent species as a function of cAl are presented in Figs. 3 charged ions (two of them being hydroxide ions) seems to be and 4.too strong. It should be mentioned that the existence of tri-, tetra-, Both viscosity and conductivity data can be well fitted down penta-, hepta- and higher oligomeric species can be neglected to r=2.1 (6 M, 25°C, cAl2.86 M) or r=4.3 (4 M, 65°C, as minor components, when we characterize the system cAl0.93 M), respectively, with the series of (1,0), (1,1), (2,0) assuming a minimum number of species present, however, the and (2,1) species.Below these r values (at higher cAl), many possibility of their existence at low concentrations is not ruled combinations were tried but the simplest (i.e. using the lowest out, in contrast to (1,2) Al(OH)63- or (2,-1) Al2 (OH)7- number of constants) and best fit was achieved when a species, which were, among others, also tested. hexameric (6,0) species was added to the series.The calculated stability constants are summarized in Table 1. Discussion It must be emphasized that these values are only valid for the given environment: for the constant cation concentration and The key species in the aluminate lye seems to be the (cyclic) temperature (and may also involve ion pair and other formahexamer, since this is the smallest oligomer where octahedral tion constants).The association constants are much higher in coordination can be fulfilled for every aluminium (Fig. 5). The 6 M NaOH (25 °C) than in 4 M (65 °C), the diVerences being ring contains six octahedrally coordinated aluminiums connected by two bridging hydroxide ions and the aluminium atoms are in a planar arrangement.This structure is identical with Table 1 Main species in aluminate lyes and their formation constants the basic unit of both bayerite and gibbsite3,6,7 determined in stability constant, bpq the solid phase, therefore the olation type polymerization by O2- bridging ligand seems less probable. p,q species 6 M, 25°C 4M, 65°C If we assume that (four) monomeric Al(OH)4- units are able to attach in-plane to the hexameric aluminate (see Fig. 5), 1,0 Al(OH)4- 1 1 0,1 OH- 1 1 a second six-membered ring could be formed (cf. naphthalene 1,1 Al(OH)4-·OH- 3.9±0.2 0.25±0.01 C10H8) with formula Al10(OH)388-. Similarly, a condensed 2,0 [Al(OH)4]22- 1.9±0.2 0.38±0.04 three ring system ( like anthracene or phenanthrene, C14H10): 2,1 [Al(OH)4]22-·OH- 26.7±3.1 1.0±0.1 Al14(OH)5210- could also be formed. The oligomer correspond- 6,0 [Al(OH)4]66- 33.0±4.3 15.0±2.0 ing to coronene (C24H12), consisting of seven rings can also be J.Mater. Chem., 1998, 8(2), 451–455 453range in 6.0 M NaOH solution (at 25 °C). It follows that the important hexameric species is only formed in a rather narrow range of cAl, where [Al(OH)4-] is very low.The consequence is that the on-plane polymerization will be predominant, leading to very small crystallite particles termed ‘mealy’ product industrially (with relatively high unwashable caustic soda content). Lower total concentration and higher temperature (4.0 M and 65 °C) hinder hydrogen bonded associations (Fig. 4): oligomerization is predominant over nearly the whole concentration range, but [Al(OH)4-] is high enough to promote in plane polymerization.The main process is therefore the growth of nuclei, which occurs also at lower aluminate concentration Fig. 5 The conventional atom-and-bond (a) and coordination poly- resulting in the precipitation of larger particles (of ‘sandy’ hedral (b) model (omitting the charges) of the Al6(OH)246- ion texture).Financial support of this work from the Hungarian Research supposed: Al24(OH)8412-. (Theoretically this compound can exist, but it is mentioned only for demonstrating the relative Foundation (OTKA T 019493) is gratefully acknowledged. decrease of charge as a function of the degree of in-plane polymerization.) The water solubility of such types of oligomer References is surely influenced by their size and charge, and associates containing three to four hexameric units are probably at the 1 T.G. Pearson, T he Chemical Background to the Aluminium limit of water-solubility. In-plane polymerization results in Industry, Royal Institute of Chemistry, London, 1955. structures identical with the structure of layers in crystalline 2 S.I. Kuznetsov and V. A. Derevyakin, T he Physical Chemistry of Alumina Production in the Bayer Method, Metallurgizdat, gibbsite.3,6,7 Moscow, 1964. On the other hand, the growth of the hexameric core is 3 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, possible not only in-plane but also on-plane. Two hexamers Pergamon Press, London–New York, 1984, p.273.could attach by hydrogen bonds, since the shape of their 4 N. Dezelic, N. Dikinski and R. H. Wolf, J. Inorg. Nucl. Chem., surfaces and the positions of the appropriate functional groups 1981, 33, 791. fit well, like those of the parallel layers in gibbsite.3 In this 5 H. A. Van Straten and P. L. De Bruyn, J. Colloid Interface Sci., 1984, 102, 260. way polymers with general formula of [Al6(OH)24]n 6n- can be 6 V.R. Rothbauer, F. Zigan and H. O’Daniel, Z. Kristallogr., 1967, formed, which essentially copy the structure of bayerite or 125, 317. gibbsite, where the layers are held together in a similar manner. 7 H. Saalfield and M. Wedde, Z. Kristallogr., 1974, 139, 129. 3,6,7 The solubility limit can be set again as n=3–4, but the 8 J.Za� mbo� , L ight Metals, 1986, 199.further, insoluble species (the precipitate) will have undoubt- 9 J.R. Glastonbury, Chem. Ind. (L ondon), 1969, 121. edly higher charges than the product formed by the in-plane 10 L. G. Sille�n, Quart. Rev., 1959, 13, 146. 11 R. J. Stol, A. K. Van Helden and P. L. De Bruyn, J. Colloid polymerization of the same number of Al atoms. It follows Interface Sci., 1976, 57, 115.that the precipitate shall always contain a charge (compensated 12 C. F. Baes and A. Mesmer, Jr., Hydrolysis of Cations, J. Wiley, New by Na+ ions), which should be manifested, in good agreement York, 1976. with industrial experience, as the ‘free caustic soda’ content of 13 Stability Constants of Metal-ion Complexes, Inorganic L igands, ed. the bayerite. E. Ho� gfeldt, Pergamon, Oxford, 1982.The type of polymerization (in-plane or on-plane or a 14 Critical Stability Constants, ed. R. M. Smith and E. Martell, Plenum, New York, 1976, vol. 4. mixture as the most probable case) depends on both the actual 15 G. Johansson, Acta Chem. Scand., 1960, 14, 771. concentration of the hexaaluminate and its ratio to the concen- 16 K. F. Jahr and I. Pernoll, Ber. Bunsen-Ges. Phys.Chem., 1965, 69, tration of Al(OH)4-. When this picture is considered from the 221, 226. standpoint of precipitation of gibbsite, the equilibria given in 17 R. F. Scotford and J. R. Glastonbury, Can. J. Chem. Eng., 1972, Fig. 6 can be deduced. The sign > indicates irreversible 50, 754. precipitation (via in-plane or/and on-plane polymerization) 18 A. S. Russell, J.D. Edwards and C. S. Taylor, J. Metals, 1955, 7, 1223. while the symbol Al(OH)3 represents the amorphous�pseudo- 19 R. C. Plumb and J. W. Swaine, Jr., J. Phys. Chem., 1964, 68, 2054. boehmite�bayerite�gibbsite series. The main process pro- 20 L. A. Carriera, V. A. Maroni, J. W. Swaine, Jr. and R. C. Plumb, ceeds via monomeric�dimeric�hexameric aluminates, while J. Chem. Phys., 1966, 45, 2216.association with hydroxide ions (as it decreases the concen- 21 J. D. Hem, Adv. Chem. Ser., 1968, 73, 98. tration of important species) only retards the main precipi- 22 K. Wefers,Metall. (Berlin), 1967, 21, 422. tation process. 23 N. I. Eremin, Yu. A. Volokhov and V. E. Mironov, Usp. Khim., 1974, 43, 224. High total concentration and relatively low temperature is 24 G.Johansson, Acta Chem. Scand., 1966, 20, 505. known to promote the formation of hydrogen bridged associ- 25 R. J. Moolenaar, J. G. Evans and L. D. McKeever, J. Phys. Chem., ates as is demonstrated in Fig. 3: the hydroxide+aluminate 1970, 74, 3629. associates are dominant in nearly the whole concentration 26 R. J. Hill, G. V. Gibbs and R. C. Peterson, Aust. J. Chem., 1979, 32, 321. 27 W. Gessner, D. Mu� ller, H. J. Behrens and G. Scheler, Z. Anorg. Allg. Chem., 1981, 486, 193. 28 D. Mu� ller, W. Gessner, A. Samoson, E. Lipmaa and G. Scheler, J. Chem. Soc., Dalton T rans., 1986, 1277. 29 S. F. Dec, G. E. Maciel, and J. J. Fitzgerald, J. Am. Chem. Soc., 1990, 112, 9069. 30 S. M. Bradley, R. A. Kydd and C. A. Fife, Inorg. Chem., 1992, 31, 1181. 31 J. W. Akitt and W. Gessner, J. Chem. Soc., Dalton T rans., 1984, 147. 32 J. W. Akitt, W. Gessner and M. Weinberger, Magn. Reson. Chem., 1988, 26, 1047. Al(OH)4 – Al2(OH)8 2– Al6(OH)24 6– Al(OH)4 – Al(OH)4 – Al2(OH)8 2– • OH– • OH– Al(OH)3 ± ± OH– Fig. 6 The coherence of equilibria in highly alkaline solutions of 33 S. M. Bradley and J. V. Hanna, J. Chem. Soc., Chem. Commun., 1993, 1249. aluminate [>: precipitation of Al(OH)3] 454 J. Mater. Chem., 1998, 8(2), 451–45534 A. R. Gerson, J. Ralston and R. St. C. Smart, Colloids Surf. A: 37 M. Pa�lfalvi-Ro� zsahegyi, Z. G. Szabo� and L. Barcza, Acta Chim. Hung., 1980, 104, 303. Physicochem. Eng. Asp., 1996, 110, 105. 35 M. Pa�lfalvi-Ro�zsahegyi, A� . Buva�ri, L. Barcza and Z. G. Szabo� , 38 L. Barcza and M. Pa�lfalvi-Ro�zsahegyi, Mater. Chem. Phys., 1989, 21, 345. Acta Chim. Hung., 1979, 102, 401. 36 P. W. Atkins, Physical Chemistry, Oxford University Press, Oxford, 4th edn., 1990. Paper 7/05468H; Received 28th July, 1997 J. M

ISSN:0959-9428    

DOI:10.1039/a705468h

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Fluence and current density dependence of silver nanocluster dimensions in ion-implanted fused silica Marta Antonello,a George W. Arnold,b Giancarlo Battaglin,c Renzo Bertoncello,*a Elti Cattaruzza,b Paolo Colombo,d Giovanni Mattei,b Paolo Mazzoldib and Fiorella Trivillina aUnita` I.N.C.M., Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, via L oredan 4, 35131 Padova, Italy bUnita` I.N.F.M., Dipartimento di Fisica, Universita` di Padova, via Marzolo 8, 35131 Padova, Italy cUnita` I.N.F.M., Dipartimento di Chimica Fisica, Dorsoduro 2137, 30123 Venezia, Italy dDipartimento di Ingegneria Meccanica, Sezione Materiali, Universita` di Padova, via Marzolo 9, 35131 Padova, Italy Implantation of suitable metal ions in glass substrates leads to the formation of nanometer-size colloidal particles in a thin surface layer.The non-linear optical properties of such colloids, in particular the enhancement of optical Kerr susceptibility, suggest that the ion implantation technique may play an important role for the production of all-optical switching devices. In spite of the very large interest due to possible applications in device construction, processes governing the chemical and physical interactions between the dielectric host and the implanted metal are far from being completely understood.It is known that the formation of these particles in glasses is governed by the chemical reactivity of the substrates with the implanted ions, by the metal concentration and by its mobility.In this work, where Ag-implanted silica samples are investigated, a particular emphasis is given to the study of the dependence of the silver cluster dimensions on the ion fluence and ion current density. Silver is present in the matrix as metallic nanoclusters and the in-depth distribution of the cluster dimensions is strongly dependent on the ion current and fluence.Higher current densities favour a silver concentration increase close to the depth of maximum radiation damage. In spite of the little diVerences of silver total amount in the four samples, the shapes of optical absorption spectra show peculiar features strongly related to size and concentration of the silver metallic clusters. Composite materials formed by embedding semiconductor or metal.We report in the following on results obtained for silica implanted with silver ions at a fixed energy, varying fluences metal nanoclusters in glass display high non-linear properties and current densities in order to examine the role of these two and attract much attention as promising materials for optoparameters in determining the final chemical and structural electronics, aiming to design all-optical switching devices.1 state of the implanted layer.We examined samples with little In particular, glasses containing metal crystallites show an variations of the implantation parameters because the mobility enhanced third-order susceptibility, whose real part is related of silver atoms in silica can be dramatically influenced by to the intensity-dependent refractive index.1–3 Such composites fluence and ion current density.12 can be obtained following diVerent routes namely, ion implantation, 4 sol–gel processes,5 quenching and heat treatments and processes involving porous glasses.Experimental Among these methods of synthesis, ion implantation has attracted a large interest for the possibility to pattern the Samples materials, to overcome the doping solubility limits, and to Corning 7940 fused silica samples were implanted with silver introduce virtually any element in the glass substrate.The at a fluence of 5×1016 ions cm-2 and current densities of 1.0 interaction between incident ions and substrate causes eVects and 1.5 mA cm-2 (LFLC and LFHC samples, respectively); directly connected to radiation damage, such as mechanical other samples were implanted with silver at a fluence of 6×1016 stresses, density and composition modifications, and conseions cm-2 and at current densities of 1.0 and 1.5 mA cm-2 quent mechanical, optical and chemical durability property (HFLC and HFHC samples, respectively).The meaning of the changes. In addition to these, depending on the choice of the acronyms is: L=lower, H=higher, F=fluence, C=current pair ‘implanted atom–dielectric host’, chemical interactions density.The four implantations were performed at the energy with the formation of particular compounds are possible.6–8 of 270 keV, at room temperature and at a residual pressure of Because of the interest for optoelectronic applications, most of 2×10-4 Pa.The reproducibility of the samples has been the papers which appeared in the recent literature dealt with ascertained. implantation in silica glass of metals with very weak reactivity, i.e. mainly copper, gold and silver: papers which review the Equipment state of the art of the research in this field appeared regularly in the literature.7,9–11 The X-ray photoelectron spectroscopy (XPS) and X-ray- In spite of the large interest, processes governing the inter- excited Auger electron spectroscopy (XE–AES) measurements action between the dielectric host and the implanted metal are were achieved with a Perkin Elmer W 5600ci spectrometer far from being completely understood.Our group is active in using non-monochromatized Mg-Ka radiation (1253.6 eV). the study of the chemical interactions in silica and silicate The working pressure was 10-7 Pa. The spectrometer was glasses implanted with metal ions with the aim at giving a calibrated by assuming the binding energy (Eb) of the Ag 3d5/2 contribution to the understanding of the processes governing line at 368.2 eV with respect to the Fermi level.Depth profiles of the diVerent elements were carried out by cycles of Ar+ the interaction between the dielectric host and the implanted J.Mater. Chem., 1998, 8(2), 457–461 457Table 1 Silver concentration maximum, the corresponding depth and sputtering at an energy of 2.5 keV. Owing to surface charging, retained dose for the four samples, as determined by RBS measure- samples showed a shift of signal energies of 3–5 eV toward ments.The TRIM calculated projected range is 130 nm higher Eb: as internal reference for charging eVects we assumed the O 1s peak of fused silica to be at 532.7 eV of Eb.13 All over LFLC LFHC HFLC HFHC the depth profiles there is no indication of preferential sputtermax. Ag conc. ing induced by Ar+ etching: the O 1s peak shape does not (atom%) 22±1 35±1 26±1 32±1 show any asymmetry and Si 2p Eb values always indicate the depth/nm 93±3 84±3 90±3 83±3 presence of stoichiometric silica.Survey scans (187.85 eV pass retained dose (%) 70±3 72±3 75±3 67±3 energy; 0.4 eV step-1; 0.05 s step-1 dwell time) were obtained in the 0–1100 eV Eb range. Detailed scans were recorded at 58.70 eV pass energy, 0.25 eV step-1, 0.1 s step-1 dwell time the local radiation damage caused by the silver ion current for the Si 2p, Si KLL, Ag 3d, Ag MNN, O 1s and C 1s lines.density and fluence. Higher current densities favour the chemi- After a Shirley-type background subtraction, the raw spectra cal behaviour of the implanted species that in the case of silver were fitted using a non-linear least-square fitting program means aggregation to form larger and larger metallic clusters adopting Gaussian–Lorentzian peak shapes for all the peaks.close to the depth of maximum radiation damage. The maxi- The atomic composition was evaluated using sensitivity factors mum concentration depth depends on the current density and as provided by W V5.4A software. it occurs at a depth always lower than the projected range, Rp Transmission electron microscopy (TEM) measurements (130 nm) calculated through the TRIM code.15 The LC samples were performed at LAMEL Institute of CNR-Bologna on a present the maximum concentration at depths where there is Philips CM30 microscope operating at 300 kV equipped with the maximum of radiation damage (93 nm), while the HC ones an energy dispersive spectroscopic X-ray microanalyzer.Crossshow their maxima at lower depths.This behaviour can be sectional samples were prepared by mechanical prethinning attributed to a migration of silver atoms toward the higherfollowed by a low-angle milling using argon ions with energies damaged region of the samples,12 followed by sample sputtering of 5 keV. To minimize ion damage, samples were cryogenically when irradiated at the higher current density.cooled during ion milling. Rutherford backscattering spectrometry (RBS) analyses were XPS and XE–AES analyses performed at Laboratori Nazionali INFN di Legnaro using a 2.2 MeV 4He+ beam. Spectra were taken with an angle of 55° Silver has a very high chemical stability, so we do not expect any significative reactivity of the implanted silver atoms with between the sample normal and the beam direction in order to enhance the profile depth scale.the host silica matrix. XPS and XE–AES analyses establish the silver oxidation states; in particular it is possible to ident- UV–VIS absorption spectra in the 200–800 nm wavelength region were recorded with a CARY 5E UV–VIS–NIR dual ify the metallic and oxidized species from the position and shape analyses of the XPS and XE–AES signals16,17 together beam spectrometer. X-Ray diVraction (XRD) spectra were acquired with the 2h with the value of the ‘a parameter’, i.e.the sum of the Eb of XPS Ag 3d5/2 peak and the kinetic energy of Ag M4NN angle ranging from 10 to 80° using Cu-Ka radiation with a wavelength l=0.154 nm. XE–AES peak. The analyses of the silver implanted samples do not display significant diVerences among the four samples: so, only the Results and Discussion spectra of one sample (HFHC) will be discussed.At the sample surface the Ag 3d5/2 signal has a very low RBS measurements intensity with an Eb value of 368.2±0.1 eV. Remembering that The concentration depth profiles of the four silver implanted the 3d5/2 peak components for metallic silver and silver oxides samples, as determined by RBS measurements (considering the range close to an Eb value of 368.2 eV, no unambiguous diVerent densities of silver and silica in the Ag+SiO2 system), evidence of the silver oxidation state can be obtained from the are reported in Fig. 1. The relevant result of the four spectra XPS peaks. However the Ag M4NN XE–AES signal for the is the concentration profile of silver: it is very low at the four metallic and oxide species have diVerent kinetic energy and sample surfaces and the in-depth distribution has a very peaked the corresponding a parameter takes values around shape.The RBS data (summarized in Table 1) show that at 726.3±0.2 eV for metallic silver and ranges from 723.5 to the same fluence the higher current density induces a higher 724.4 eV for silver oxides, as obtained from literature data16,17 Ag concentration at the maximum Ag concentration depth: and from our standards.the silver in-depth distribution has a narrower peak shape, in In our samples the a parameter for Ag at the surface takes accordance with literature data.14 the value of 726.5±0.2 eV. Following the depth profile we find These results agree well with a silver mobility dependent on that the a parameter does not change (within the experimental errors) indicating that silver is embedded in the silica matrix in the form of metallic species.Moreover, in the four samples, even the silver a parameter obtained by Ag 3d5/2 and AgM5NN (usually less used than AgM4NN) shows, within the experimental errors, a constant value of ca. 720.6 eV all over the depth profile: this indicates once more the presence of only metallic silver. In spite of this, we observe a very diVerent behaviour of Ag 3d5/2 energy position even after the correction for the sample charging: an increase/decrease of silver concentration corresponds to a decrease/increase of the Eb of Ag 3d5/2 peak (Fig. 2): close to its concentration maximum the Ag 3d5/2 Eb value shifts down by several tenths of eV (367.2 eV for the HFHC sample, 367.9 eV for the LFLC one). This behaviour can be attributed to a silver cluster size dependent charging, i.e. the increase of the silver clusters dimension induces a diVerent electrical charging of silver cluster with respect to the Fig. 1 Concentration depth profiles of the four diVerent samples, as determined by RBS measurements host silica matrix when X-ray irradiated. Even the full width 458 J.Mater. Chem., 1998, 8(2), 457–461Fig. 3 XRD spectra of the four diVerent samples. We also report the indexation of the peaks according to the silver fcc structure. is 6 nm, while in the LFHC and LFLC ones the metallic Fig. 2 XPS Ag 3d5/2 peak positions, at diVerent depths, in the HFHC clusters probably are too small to be detected.sample (&); silver depth concentration profile of silver in the same sample (+) TEM analyses In the light of XRD and XPS analyses we decided to perform at half maximum (FWHM) of the Ag 3d5/2 peak (not reported) TEM measurements with the purpose of directly detecting the throughout the depth profile exhibits a decrease when the Ag presence of Ag nanoprecipitates.We chose for this analysis the concentration increases, indicating a more homogeneous samples LFLC and HFHC, i.e. those giving the most diVerent chemical arrangement of silver atoms (formation of very large XRD spectra. metallic clusters). In Fig. 4 a bright field micrograph of the LFLC silver As far as the O 1s line is concerned, there is no appreciable implanted sample is reported.The figure clearly shows spheri- signal in the 529.0 eV range attributable to silver oxide along cal particles in the implanted region of the silica matrix. the whole profile: the value of 532.7±0.1 eV for O 1s together with a value of 103.6±0.1 eV for the Si 2p line observed in the XPS profile (pertinent to SiO2) are clear indications that there is no chemical interaction between silver and the silica matrix.All these observations suggest that silver is present in the matrix as a metallic species and the Ag 3d5/2 Eb and FWHM depend on the Ag concentration. As already reported6 the silver concentration is related to the cluster dimensions: this originates a narrowing of the silver 3d5/2 peak with increasing silver concentration.In addition, as evidenced in Fig. 2, when silver concentration increases the energy position of Ag 3d5/2 peak shifts toward Eb values lower than the reference one for bulk metallic silver. X-Ray diVraction spectra Usually, ion-implanted samples are not investigated by means of X-ray diVraction. This occurs even if the implanted atoms aggregate to form crystalline clusters: frequently their low concentration inside the analyzed thickness does not allow their observation. Nevertheless we have performed this investigation on our four samples because of its characteristics of easiness and of non-destructivity.The four diVraction spectra are reported in Fig. 3. At low diVraction angles all the spectra show a large signal: this is a characteristic feature of amorphous silica.The HFHC and HFLC samples clearly show the presence of crystalline silver, whereas its characteristic diVraction peaks are hardly observable in the LFHC sample and seem to be absent in the LFLC one. Since the possibility to detect metallic nanoparticles by means of XRD measurements is related to the concentration and the mean dimension of nanocrystallites, we directly deduce from this evidence that the silver precipitation is more and more evident increasing both the ion current density and the fluence.A detailed analysis of the diVraction pattern indicates the presence of silver nanocrystals with a diameter mean value Fig. 4 Cross-section bright field TEM micrograph and in-depth distri- (calculated using the Debye–Scherrer formula18) of 12 nm for bution of the mean cluster dimensions of the LFLC sample.The inset shows the fcc (spotty) pattern of the silver nanoclusters. HFHC sample. In the HFLC sample the diameter mean value J. Mater. Chem., 1998, 8(2), 457–461 459Fig. 6 UV–VIS absorption spectra of the four diVerent samples the LFLC sample is almost colourless, the LFHC sample is yellow and the increase of the silver fluence and current density (HFLC and HFHC samples) induce a more and more intense reddish colouring.19 On the basis of the Mie theory20 (adopting the electric-dipole approximation of optical absorption coeYcient) the band centered at 410 nm is due to the presence of metallic silver clusters whose diameter is in the range 2–20 nm, while the bands centered at 450 and 530 nm should originate from very large metallic silver clusters whose diameter is in the range of 70–90 nm21 (the electric-dipole approximation is no more justified when clusters diameter is larger than approximately 20 nm22).TEM micrographs show that all the clusters are almost spherical and the largest ones (HFHC sample) have a diameter Fig. 5 Cross-section bright field TEM micrograph and in-depth distri- of ca. 30 nm in a very Ag-rich region. Mie theory assumes that bution of the mean cluster dimensions of the HFHC sample. The inset every nanoparticle is an independent scattering point, i.e. the shows the fcc (spotty) pattern of the silver nanoclusters. volume fraction occupied by metal cluster in the implanted layer is small.In our samples silver concentration reaches very high values, then the previous assumption should not be Selected area diVraction (SAD) and microdiVraction with a correct: mutual interactions among spherical nanoparticles convergent beam ca. 15 nm in diameter indicate that the silver may induce several components in absorption spectra. The clusters are crystalline.In the LFLC sample (Fig. 4) the inbands centered at 450 and 530 nm can then be attributed to depth distribution of the cluster dimension is almost symmetric the interaction of neighbouring silver clusters as well as to the with respect to the larger cluster layer (13 nm mean diameter). presence of high order absorption terms (i.e., principally the In Fig. 5 the bright field micrograph of the HFHC silver quadrupole one).22 This phenomenon, as shown in Fig. 6, is implanted sample still shows spherical particles which reach a more manifest increasing the local silver concentration going larger mean diameter (18 nm) at the maximum Ag concenfrom the LFLC up to the HFHC sample. tration depth. The SAD indicates that the nanoparticles are The UV–VIS absorption spectra evidence the same sequence crystalline. The in-depth cluster distribution in this case is no of modifications that we observed in the XRD spectra: LFLC longer symmetric with respect to the larger cluster layer and HFHC show the greatest diVerence.These diVerences (Fig. 5). In particular, coming in from the surface the cluster among the four samples seem induced primarily by the fluence dimensions increase up to the maximum value and then variation, even if a small ion current density increase shows abruptly fall to a diameter close to 2 nm.The distribution of observable eVects too. the cluster dimensions agrees with the diameter mean value All the experimental evidences can be explained considering obtained by XRD measurements. the eVects of silver radiation-enhanced diVusion (RED) during TEM results are in agreement with our previous hypotheses.implantation: silver atoms move toward either the surface or In particular, the diameter mean value of the nanoclusters is the high-damaged region of the sample. The atoms that reach lower in the LFLC sample than in the HFHC one: this explains the surface are there preferentially sputtered during irradiation, the lack of XRD peaks evidenced in the LFLC sample.while those reaching the high-damaged region aggregate to form metallic nanoclusters. In the region between the surface UV–VIS absorption spectra and the Ag maximum concentration depth a certain number of irradiation-induced defects is anyway present: these act as The presence of silver nanoclusters embedded in silica originates an absorption band due to surface plasmon resonance nucleation centers for the formation of (smaller) silver nanoprecipitates, disfavouring in this way a further Ag diVusion.(SPR) centered around a wavelength of 400–410 nm. Optical absorption measurements of the four diVerent Ag-implanted In the deeper implanted layers instead the defect center density is very low.Increasing fluence and/or ion current density, the silica samples are reported in Fig. 6. In the LFLC sample the high wavelength side of SPR band is slightly asymmetric. This eVects of silver RED increase too: more and more Ag atoms move inside the matrix inducing a higher silver concentration behaviour is still more evident in the LFHC and HFLC samples and the UV–VIS spectrum of the HFHC sample in a buried region of limited thickness.Here, silver aggregates to form larger and larger metallic nanoparticles. In addition, clearly shows the superposition of three diVerent bands centered at ca. 410, 450 and 530 nm. the RED induces a considerable depletion of silver in the other implanted regions. When the RED increases, this depletion The SPR phenomenon leads to sample colouring: to the eye 460 J.Mater. Chem., 1998, 8(2), 457–461becomes more important where the density of irradiation- This work was partially supported by Progetto Finalizzato ‘Materiali Speciali per Tecnologie Avanzate’ of the CNR induced defects is lower, i.e. in the deeper implanted layers. Here, silver diVusion is favoured by the low concentration of (Rome).nucleation centers. This argumentation is in accord with previous results,6 even if in the case of LFHC and HFHC References samples a local temperature-increase mechanism related to the variation of implantation current density may be involved. 1 E.M. Vogel, J. Am. Ceram. Soc., 1989, 72, 719. 2 F. Hache, D. Ricard and C. Flytzanis, J. Opt. Soc. Am. B, 1986, 3, 1647. 3 Y. Fainman, J. Ma and S. H. Lee,Mater. Sci. Rep., 1993, 9, 53. Conclusions 4 H. Rissel and I. Ruge, in Ion Implantation, J. Wiley and Sons, 1986. 5 D. Kundu, I. Honma, T. Osawa and H. Komiyama, J. Am. Ceram. Nanometer-size silver clusters may be synthesized in silica Soc., 1994, 77, 1110. glass by ion implantation. The presence and size of such 6 P. Mazzoldi, F. Caccavale, E. Cattaruzza, A.Boscolo–Boscoletto, clusters depend on the fluence and ion current density: little R. Bertoncello, A. Glisenti, G. Battaglin and C. Gerardi, Nucl. Instrum.Methods B, 1992, 65, 367. variations of implantation parameters lead to very diVerent 7 G. Battaglin, Nucl. Instrum.Methods B, 1996, 116, 102. situations. 8 R. Bertoncello, F. Trivillin, E. Cattaruzza, P. Mazzoldi, XRD analyses show an increasing nanocluster concentration G.W. Arnold, G. Battaglin and M. Catalano, J. Appl. Phys., 1995, and mean dimension as the fluence increases. Analogous eVects 77, 1294. take place when the ion current density increases at constant 9 P. Mazzoldi, G. W. Arnold, G. Battaglin, R. Bertoncello and fluence. F. Gonella, Nucl. Instrum.Methods B, 1994, 91, 478. 10 R.F. Haglund, Jr., L. Yang, R. H. Magruder, C. H. White, The UV–VIS optical spectra show the presence of a broad R. A. Zuhr, Lena Yang, R. Dorsinville and R. R. Alfano, Nucl. absorption band in the 300–600 nm wavelength range, with a Instrum.Methods B, 1994, 91, 493. structured shape due to the superposition of three bands 11 P. Mazzoldi, G. W. Arnold, G. Battaglin, F. Gonella and centered at 410, 450 and 530 nm respectively.These bands are R. F. Haglund, Jr., J. Nonlinear Opt. Phys.Mater., 1996, 5, 285. in agreement with the presence of a high-density region of 12 G. W. Arnold, P. Mazzoldi, L. Tramontin, A. Boscolo-Boscoletto silver clusters: the diameter of the nanoprecipitates lies in the and G. Battaglin,Mater. Res. Soc. Symp. Proc., 1993, 279, 285. 13 M. P. Seah and G. C. Smith, in Practical Surface Analysis, ed. range of 2–20 nm. The bands become more and more evident D. Briggs and M. P. Seah, Wiley, Chichester, 2nd edn., 1990, vol. 1, increasing the local silver concentration going from the LFLC appendix 1, pp. 543–544. up to the HFHC sample. 14 N. Matsunami and H. Hosono, Appl. Phys. L ett., 1993, 63, 2050. TEM analyses of the LFLC sample show that the in-depth 15 J.P. Biersack and L. G. Haggmark, Nucl. Instrum. Methods, 1980, cluster mean dimension distribution is almost symmetric with 174, 275. respect to the larger cluster layer, while in the HFHC sample 16 J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bomben, in Handbook of X-Ray Photoelectron Spectroscopy, ed. J. Chastain, the in-depth distribution is no more symmetric.Higher current Perkin Elmer Corp., Eden Prairie, MN, 1992. densities seem to favour the chemical behaviour of the 17 X-Ray Photoelectron Spectroscopy Database, version 1.0, National implanted species that in the case of silver means aggregation Institute of Standards and Technology, Gaithersburg, MD, 1989. to form larger and larger metallic clusters, whose dimension 18 H. P. Klug and L. E. Alexander, in X-Ray DiVraction Procedures increases close to the depth of maximum radiation damage. for Polycrystalline and AmorphousMaterials, J. Wiley & Sons, New The maximum concentration depth depends on the current York, 1974, p. 687. 19 J. Neddersen, G. Chumanov and T. M. Cotton, Appl. Spectrosc., density and it occurs at a depth always lower than the 1992, 47, 1959. calculated projected range. 20 G. Mie, Ann. Phys., 1908, 25, 377. Chemical investigations suggest that, within the sensitivity 21 G. W. Arnold and J. A. Borders, J. Appl. Phys. 1977, 48, 1488. limits of the XPS and XE–AES analyses, silver is present in 22 U. Kreibig and M. Vollmer, in Optical Properties ofMetal Clusters, the matrix only as a metallic species; the Ag 3d5/2 XPS peak Springer Series in Materials Science, Springer-Verlag, Berlin, width and position depend on Ag cluster size. This means that Heidelberg, 1995, vol. 25. in these systems the XPS technique is sensitive to the size of the colloids trapped in the silica matrix. Paper 7/05745H; Received 6th August, 1997 J. Mater. Chem., 1998, 8(2), 457–461 461

ISSN:0959-9428    

DOI:10.1039/a705745h

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Controlled microstructures of amphiphilic cationic azobenzenemontmorillonite intercalation compounds Makoto Ogawa*a,b and Ayako Ishikawab aPRESTO, Japan Science and T echnology Corporation (JST) and bInstitute of Earth Science, Waseda University, Nishiwaseda 1–6-1, Shinjuku-ku, T okyo 169–50, Japan The intercalation of two amphiphilic cationic azobenzene derivatives into the interlayer space of montmorillonite has been conducted by the ion exchange reactions between sodium montmorillonite and p-(v-trimethylammoniopentyloxy)-p¾- (dodecyloxy)azobenzene bromide or p-(v-trimethylammoniodecyloxy)-p¾-(octyloxy)azobenzene bromide.X-Ray diVraction and elemental analysis results indicated that the dye cations were intercalated into the interlayer space of montmorillonite.The spectral properties as well as the X-ray diVraction results have revealed that the adsorbed azo dye cations form so-called J-like aggregates with mono- and bi-layers in the interlayer space of montmorillonite. The orientation of the chromophore was controlled by host– guest and guest–guest interactions. The intercalated azo dyes exhibit reversible trans–cis photoisomerization by UV and visible light irradiation.Intercalation of guest species into layered inorganic solids is a method of producing ordered inorganic–organic assemblies with unique microstructures controlled by host–guest and guest–guest interactions.1,2 Among possible layered solids, the smectite group of layered clay minerals provides attractive features such as large surface area, swelling behavior, and ion exchange properties for organizing organic guest species.3,4 The organization of photoactive species on the surface of smectites has been investigated to probe the surface properties of smectites as well as to construct novel photofunctional supramolecular systems,5,6 since the photoprocesses of the photoactive species are environmentally sensitive.7,8 Along this line, photochromic reactions of organic dyes in the interlayer space of smectites have been reported.9–17 We have been interested in the photochemistry of azobenz- C8H17 O C10H20 N N O N+ CH3 CH3 CH3 C12H25 O C5H10 N N O N+ CH3 CH3 CH3 C8AzoC10N+ C12AzoC5N+ Scheme 1 Molecular structures of the amphiphilic azo dyes used enes in the interlayer spaces of layered silicates.14–17 The construction of photoresponsive supramolecular systems based on the photochemical isomerization of azobenzene has been silicates through electrostatic attractions between the negaintensively studied.18 The photoisomerization of azobenzene tively charged surface of the silicate layer and the cationic dyes in the interlayer space may lead to novel photoresponsive as well as dye–dye interactions.inorganic–organic nanocomposites. The hydrophobic modification of the surface properties of Experimental smectites by the intercalation of surfactants11–16,19,20 has been conducted for the introduction of azobenzenes into the inter- Materials layer spaces of smectites.11–16,19,20 Although the intercalated Sodium montmorillonite (Kunipia F, Kunimine Industries Co.; azobenzenes isomerize eVectively in the hydrophobic interlayer reference clay sample of The Clay Science Society of Japan) space of organoammonium silicates,14–16 it has been diYcult was used as the host material.The cation exchange capacity to evaluate and control the location and the orientation of the (c.e.c.) of the Na-montmorillonite is 119 mequiv./100 g of clay.intercalated azo dyes in the hydrophobic interlayer space. The two amphiphilic azo dyes C12AzoC5N+Br and In order to overcome this limitation, a cationic azobenzene C8AzoC10N+Br were purchased from Sogo Pharmaceutical derivative has been used as the guest species.17 In this paper, Co. and used without further purification. we report the intercalation of two amphiphilic azobenzene derivatives p-(v-trimethylammoniopentyloxy)-p¾-(dodecyloxy)- Sample preparation azobenzene bromide (C12AzoC5N+) and p-(v-trimethylammoniodecyloxy)- p¾-(octyloxy)azobenzene bromide, Intercalation of C12AzoC5N+ and C8AzoC10N+ into mont- (C8AzoC10N+) (molecular structures are shown in Scheme 1) morillonite was carried out by a conventional ion exchange into montmorillonite and the photochemical reactions of the method in which an aqueous suspension of montmorillonite azo dyes in the interlayer spaces are reported.A series of was mixed with an ethanol solution of C12AzoC5N+Br- or amphiphilic azo dyes with variable alkyl chain length have C12AzoC5N+Br- (0.014 M) and the mixture was allowed to been synthesized and the formation of self-assembled structures react for one day at 70 °C.The amount of the added dye was has been observed in aqueous solutions and in films.21–25 In 1.2 times excess of the cation exchange capacity of the clay, the present system, it seems possible to control the orientation since excess amounts of amphiphilic species may be adsorbed as a salt (intersalation). After centrifugation, the resulting of the chromophore in the interlayer space of swelling layered J.Mater. Chem., 1998, 8(2), 463–467 463yellowish solid was washed with ethanol and dried under is ascribable to the diVerence in the size of the dye cations as well as the amounts of the adsorbed dyes. reduced pressure at room temperature. The intercalation compounds were dispersed in toluene with sonication and casted The compositions of the products were determined by elemental analyses as C, 27; N, 3% for the C12AzoC5N+ on quartz substrates, so that thin films were obtained.The thin films are used for the photochemical studies. montmorillonite intercalation compound and C, 26; N, 3% for the C8AzoC10N+ montmorillonite intercalation compound. From the elemental analyses, the amounts of the adsorbed azo Characterization dyes were determined as ca. 110 and 100 mequiv./100 g clay X-Ray powder diVraction patterns of the products were for the C12AzoC5N+ and the C8AzoC10N+ montmorillonite recorded on a Rigaku RAD-IA diVractometer using monochrointercalation compounds, respectively. These values indicate matic Cu-Ka radiation. Absorption spectra of the films were that the cation exchange between sodium ions and recorded on a Shimadzu UV-3100PC spectrophotometer. The C12AzoC5N+ or C8AzoC10N+ ions occurred almost quantitatcomposition of the products were determined by the CHN ively.As observed for the intercalation of long chain alkylamanalysis (Yanaco MT-3). monium ions into smectites, the two amphiphilic azo dyes preferred to occupy the interlayer space of montmorillonite to Photochemical reactions replace the interlayer sodium ions eVectively.From the observed basal spacings and the sizes of The photochemical reaction of the intercalated azobenzene C12AzoC5N+ and C8AzoC10N+ ions, the orientation of the was conducted by UV and visible light irradiation with a intercalated dye cations can be discussed. Supposing that the 500W super high pressure Hg lamp (USHIO USH-500D).A alkyl chains of the two amphiphilic azo dyes were fully band-pass filter, Toshiba UV-D35, with transmittance centered extended, two types of orientation can be expected from the at 350 nm, was used to isolate the UV light. For the cis–trans observed basal spacings. One is an interdigitated monomolecu- reverse reactions, a sharp cut filter, HOYA L42 (cut-oV wavelar layer of the dyes with the alkyl chains inclined to the length 420 nm) was used to obtain visible light.The reactions silicate sheet. The other is a bilayer coverage of the dyes with were monitored by the change in the absorbance of the transtheir alkyl chains inclined to the silicate sheet. Note that the isomer of the azobenzene.A sample film was set in a cryostat tilt angles are diVerent in the two models. with optical windows (Oxford DN-1704), and the photochem- The intercalation of the amphiphilic azo dyes alters the ical reactions were performed at constant temperatures between surface properties of montmorillonite to strongly organophilic 100 and 400 K for a single sample. as has been observed for the long chain alkylammonium smectites.26–28 Consequently, the C12AzoC5N+ and Results and Discussion C8AzoC10N+ montmorillonite intercalation compounds swell in organic solvents such as toluene and chloroform. Thin films In the reaction between C12AzoC5N+Br- and Nawere obtained by casting the suspension in toluene onto a montmorillonite, a yellowish solid was obtained.The XRD quartz substrate.The X-ray diVraction patterns of the films pattern of the product is shown in Fig. 1( b), together with that are shown in Fig. 1(c) and (e) for the C12AzoC5N+ and of Na montmorillonite [Fig. 1(a)]. The basal spacing of the C8AzoC10N+ montmorillonite intercalation compounds, product was 2.4 nm, indicating an interlayer expansion of respectively. The basal spacings of the films (2.4 and 2.5 nm 1.4 nm.(The thickness of the silicate layer of montmorillonite for the C12AzoC5N+ and C8AzoC10N+ montmorillonite inter- is 9.6 A ° .) A yellowish solid was also obtained by the reaction calation compounds, respectively) were same as those observed between C8AzoC10N+Br- and Na-montmorillonite. The XRD for the powdered samples, indicating that the arrangements of pattern of the product [Fig. 1(d)] shows a basal spacing of the intercalated azo dyes did not change during the film 2.5 nm, which indicates an interlayer separation of 1.5 nm. The preparation and the solvents employed for the films prep- diVerence in the basal spacings between the two compounds aration are completely removed. Although the films are slightly turbid, they are still useful for photochemical studies.The visible absorption spectrum of the C12AzoC5N+ montmorillonite intercalation compound film is shown in Fig. 2(a). In the absorption spectrum, a band due to the trans-azobenzene chromophore was observed at ca. 385 nm, which is shifted towards longer wavelength relative to that (355 nm) of monomeric C12AzoC5N+ in a dilute ethanol solution of Fig. 1 The X-ray powder diVraction patterns of (a) sodium montmoril- Fig. 2 The absorption spectra of (a) the C12AzoC5N+ montmorillonite lonite, (b) and (c) C12AzoC5N+ montmorillonite intercalation compound: powder (b) and cast film (c); (d) and (e) C8AzoC10N+ intercalation compound and (b) the C8AzoC10N+ montmorillonite intercalation compound montmorillonite intercalation compound: powder (d) and cast film (e) 464 J.Mater. Chem., 1998, 8(2), 463–467C12AzoC5N+Br. The absorption spectrum of the C8AzoC10N+ montmorillonite intercalation compound film is shown in Fig. 2( b). A broad absorption band centered at 373 nm was observed in the absorption spectrum. Compared to that of the dye solution (the absorption maximum of a dilute C8AzoC10N+Br solution appeared at 355 nm), the absorption band due to the p–p* transition of trans-azobenzene shifted to longer wavelength.It should be noted that the absorption band of the C8AzoC10N+ montmorillonite intercalation compound film was observed at a shorter wavelength than that of the C12AzoC5N+ montmorillonite intercalation compound film. A C12AzoC5N+ montmorillonite intercalation compound with a C12AzoC5N+ loading of 0.1 of the c.e.c.showed a basal spacing of ca. 1.3 nm, indicating the adsorbed dyes arranged parallel to the silicate layers. This intercalation compound was orange, diVerent from that (yellow) of the intercalation compound in which the interlayer cations were replaced almost quantitatively. The adsorbed dyes interact with the surface of silicate layer when the adsorbed amount is low, while the dye–dye interactions are dominant at high loading.In the molecular assembly, the chromophore interacts to give aggregated states and the dye–dye interactions cause both bathochromic and hypsochromic spectral shifts depending on the microstructures. According to Kasha’s molecular exciton theory,29 the observed bathochromic shifts of the absorption bands of the intercalation compounds were ascribable to J-like aggregates of the intercalated C12AzoC5N+ and C8AzoC10N+ ions in the interlayer space of montmorillonite.The spectral shifts reflect the orientation of the dipoles in the aggregates; smaller spectral red shifts are expected for the aggregates with larger tilt angles of the dipoles.29 The diVerence Fig. 3 Proposed microstructures of (a) the C12AzoC5N+ montmoril- in the wavelength of the absorption maxima observed for the lonite intercalation compound and (b) the C8AzoC10N+ montmoril- C12AzoC5N+ and C8AzoC10N+ montmorillonite intercalation lonite intercalation compound.(a¾) Less plausible model for compounds suggests two diVerent orientations of the interca- the C12AzoC5N+ montmorillonite intercalation compound, with the lated azo dyes.For the C12AzoC5N+ montmorillonite system, C12AzoC5N+ arranged as an interdigitated monolayer. Note that the distance between the adjacent chromophores is larger in this model. a greater spectral shift is observed, indicating the smaller tilt angle of the azobenzene dipoles in the intercalated dye aggregates. The absorption band observed for the C8AzoC10N+ montmorillonite system showed the smaller spectral shift com- molecular structures.21–23 C8AzoC10N+ has been reported to form a H-aggregate in cast films as revealed by the X-ray pared to that observed for the C12AzoC5N+ montmorillonite system, suggesting a greater tilt angle of the azobenzene dipoles diVraction and the hypsochromic shift of the absorption band in the visible spectrum.On the contrary, C8AzoC10N+ ions in the interlayer space of montmorillonite. As discussed previously, two possible orientations of the form J aggregates which show a bathochromic shift of the absorption band upon intercalation into the interlayer space intercalated species are proposed from the gallery height and the size of the dye; one is a monomolecular layer and the other of montmorillonite.This observation implies that the states of the dyes in the interlayer spaces are controlled by the electro- is a bimolecular coverage in the interlayer spaces. Since the basal spacings of the two intercalation compounds are similar, static attractions between the negatively charged silicate surfaces and the cationic dyes as well as the dye–dye interactions.the tilt angles in the bilayer assembly must be larger than that in the monolayer aggregate. Consequently, the bilayer structure The photochemical reaction of the intercalated azobenzene has been investigated by UV and visible light irradiation. of the intercalated C12AzoC5N+ [as shown in Fig. 3(a)] seems to be a plausible model to explain the observed spectral shift.Fig. 4 shows the change in the absorption spectra of the C12AzoC5N+ montmorillonite intercalation compound upon On the contrary, the intercalated C8AzoC10N+ ions are thought to form an interdigitating monomolecular layer in the UV and visible light irradiation. After UV irradiation, the band due to the trans-isomer (at ca. 385 nm) decreased [spectra interlayer space of montmorillonite as shown in Fig. 3( b). The diVerences in the microstructures of the C12AzoC5N+ (b) and (c) in Fig. 4 were recorded after UV irradiation for 20 and 50 min, respectively], indicating trans–cis isomerization. and C8AzoC10N+ montmorillonite systems are ascribed to the location of the azobenzene chromophore in the amphiphilic UV irradiation for a longer period did not cause any further spectral change.The absorption band ascribable to the cis- ions. The C8AzoC10N+ ions are able to adopt an interdigitating monolayer without producing void spaces. On the contrary, isomer appeared at 330 nm. Upon visible light irradiation, the absorption spectrum recovered [Fig. 4(d) shows the absorption for the C12AzoC5N+ ions to form an interdigitating monolayer, the distance of adjacent azobenzene chromophores must be spectrum recorded after visible light irradiation for 13 min].This spectral recovery was also observed thermally. Reversible larger than that expected for the interdigitating C8AzoC10N+ monolayer to weaken the dye–dye interactions [as sche- spectral changes were repeatedly observed. The ratio of the cis-isomer formed by UV irradiation at the photostationary matically shown in Fig. 3(a¾)]. As a consequence, the C12AzoC5N+ ions form a bilayer in the interlayer space of state at room temperature was ca. 60% from the change in the absorption band due to the trans-isomer. montmorillonite as shown in Fig. 3(a). Shimomura and co-workers have extensively investigated A similar change in the absorption spectra was observed for the C8AzoC10N+ montmorillonite intercalation compound.the preparation and the organization of a series of amphiphilic azo dyes with variable alkyl chain length and found that the Fig. 5 shows the change in the absorption spectra of the C8AzoC10N+ montmorillonite intercalation compound upon microstructures of the dye aggregates varied depending on the J. Mater. Chem., 1998, 8(2), 463–467 465Fig. 6 The temperature dependence of the fraction of the photochemically formed cis-isomer at the photostationary states for (O) the Fig. 4 The change in the absorption spectra of the C12AzoC5N+ C12AzoC5N+ montmorillonite and (D) the C8AzoC10N+ montmorilmontmorillonite intercalation compound: before (a) and after UV lonite intercalation compounds irradiation for (b) 20 and (c) 50 min; (d) after subsequent visible light irradiation for 13 min ture.Fig. 6 shows the variation of the fraction of the photochemically formed cis-isomer at the photostationary states at diVerent temperatures. These values decreased with decreasing temperature, suggesting that the motion of the azobenzene was restricted at lower temperatures. It has been reported that dialkyldimethylammonium ions in the interlayer space of silicates exhibit a gel-to-liquid crystal phase transition to aVect the photoprocess of the intercalated species and the permeability. 11,16,19,32,33 In the present system, the states of the amphiphilic azo dyes might aVect the observed temperature dependent photochemical reactions. Upon increasing the temperature above 340 K, the fraction of the cis-isomer at the photostationary state decreased as a result of competitive photochemical and thermal processes.Organoammonium-exchanged clays have been utilized as adsorbents for poorly water soluble species with specific selectivity. 34 The amphiphilic azo dye intercalated montmorillonites may find applications as novel adsorbents with photo-controllable selectivity.Fig. 5 The change in the absorption spectra of the C8AzoC10N+ montmorillonite intercalation compound: before (a) and after UV irradiation for (b) 16 min and (c) 32 min; (d) after subsequent visible Conclusions light irradiation for 8 min The intercalation of cationic amphiphilic azo dyes into the interlayer space of montmorillonite has been conducted by a UV and visible light irradiation.After UV irradiation, the conventional ion exchange method. The intercalated azo dyes band due to the trans-isomer (at around 373 nm) decreased formed J-like aggregates in the interlayer space of montmoril- [spectra (b) and (c) in Fig. 5 were recorded after UV irradiation lonite. Although the two dyes gave similar basal spacings, for 16 and 32 min, respectively], indicating trans-cis isomerizabsorption spectra showed diVerences in the microstructures ation.UV irradiation for a longer period did not cause any of the products. Two diVerent models, one a monomolecular further spectral change. Upon visible light irradiation, the the other a bimolecular layer, in the interlayer spaces, have absorption spectrum recovered [Fig. 5(d)].The ratio of the been proposed for the intercalation compounds and this diVercis- isomer formed by UV irradiation at the photostationary ence has been ascribed to the diVerence in the molecular state at room temperature was ca. 60% from the diVerence in structures of the dyes. The intercalated azo dyes exhibited the absorption spectra. photoisomerization at room temperature. It is worth noting that the azobenzene chromophore isomerized eVectively in the interlayer space of montmorillonite, The present work was partially supported by Waseda despite the fact that the azobenzene chromophore aggregates University as a Special Project Research.in the interlayer space. It has been pointed out that the isomerization of the azobenzene chromophore in a molecular assembly was restricted owing to the lack of free volume.In References order for the azobenzene chromophore to isomerize eVectively, 1 Intercalation Chemistry, ed. M. S. Whittingham and A. J. Jacobson, eVorts have been made by means of complexation with a Academic Press, New York, 1982. cyclodextrin cavity30 and with a polyion complex31 to create 2 Progress in Intercalation Research, ed. W.Mu�ller-Warmuth and suYcient room for photoisomerization. R. 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Boyd, J. F. Lee and M. M. Mortland, Nature, 1988, 333, 345; Miner., 1992, 40, 485; (b) M. Ogawa, T. Aono, K. Kuroda and Y. Yan and T. Bein, Chem. Mater., 1993, 5, 905. C. Kato, L angmuir, 1993, 9, 1529; (c) M. Ogawa, T. Wada and K. Kuroda, L angmuir, 1995, 11, 4598. 21 M. Shimomura, R. Ando and T. Kunitake, Ber. Bunsen-Ges. Phys. Chem., 1983, 87, 1134. Paper 7/06507H; Received 5th September, 1997 J. Mater. Chem., 1998, 8(2), 463–4

ISSN:0959-9428    

DOI:10.1039/a706507h

出版商:RSC    

年代:1998

数据来源: RSC