摘要:

Adsorption of polycations on clays: an in situ study using 133Cssolution-phase NMR Christopher Breen; Jolyon 0.Rawsod and Brian E. Mann,b "Materials Research Institute, Shefield Hallam University, Shefield, UK S1 1 WB bChemistry Department, University of Shefield, Shefield, UK S3 7HF 133Cs solution-phase NMR has been evaluated as an in situ probe to study the adsorption of polycations, and other ions, onto clays in aqueous suspensions containing 2.5 mass% low-iron Texas bentonite. In particular, the effectiveness of the polycations FL15, FL16, FL17, of general formula [( Me,NCH,CHOHCH,),]"+, and Magnafloc 1697, [(CH2CHCH2N(Me)2CH2CHCH2),,]n+,at displacing Cs+ ions from the Texas bentonite is compared to Na', K+, Me,N+ (TMA') and paraquat2+. This information has been correlated with that obtained from particle-size and electrophoretic measurements in aqueous solution, together with that obtained from adsorption isotherms, variable temperature X-ray diffraction (XRD) and thermogravimetry (TG) studies using dry powdered samples.FL15, FL17 and 1697 all exhibited high affinity adsorption isotherms on all the cation-exchanged forms of WL, whereas the adsorption of TMA' ions, which represent the cationic portion of the polymers, was of lower affinity. The maximum amount of polymer adsorbed, Qmx, depended on the resident exchange cation, varying as Na' >K+ >Cs+, and the molecular mass of the polymer. Q,, on the Na-clay approached twice the amount of polycation required to fulfil the cation exchange capacity (CEC) of the Texas bentonite.XRD profiles confirmed that the polycations resided between the clay lamellae except at low loadings on the Cs+-clay. In the absence of competing (po1y)cations the 133Cs NMR exhibited a broad (1500 Hz), weak signal. As (po1y)cation was added the signal narrowed, eventually to 15 Hz, and grew considerably in intensity. As the adsorption isotherms suggested, the polycations were extremely effective at displacing the resident Cs+ ions, closely followed by paraquat2+. TMA' ions were considerably less effective and Na+ and K+ ions were ineffective. Magnafloc 1697 was not as effective at displacing Cs' ions as the FL series of polymers, and this was reflected in the zeta potential (c)and particle-size measurements. This data suggested that the FL polycations were able to penetrate the interlayer more effectively than the bulkier 1697, which was only adsorbed up to 68% of the CEC.Clay-polymer interactions are of considerable importance in a wide variety of applications including water treatment,' stabilisation of soil structure,2 paper coating3 and drilling fluids! Moreover, the general perception that clays act only as a low cost, mineral filler in paints, rubbers and plastics is erroneous insofar as their ability to enhance the mechanical and/or optical properties' of the final product also contributes to their selection. Our interest stems from the current endeav- our to formulate ecologically friendly, water-based drilling fluids to address concern over the environmental impact associated with the disposal of whole (oil-based) muds and contaminated c~ttings.~,~ The polymer additives must be able to overcome the natural tendency for clays to swell and/or disperse in the presence of water.This is of particular signifi- cance when a reactive shale formation, which contains a significant proportion of swelling clay, is encountered because the wellbore wall may become unstable and the drilled cuttings may disintegrate, causing a disastrous change in the rheological properties of the mud.6 The adsorption of both anionic and cationic polyelectrolytes on clays is known to be influenced by factors such as the resident cation on the clay,7 the amount of charge on the polymer' and the ionic strength of the exchange medi~m.~ Early work on the adsorption of anionic polymers on clays established that the repulsion between the polymer and the negatively charged clay surface resulted in little adsorption.Stutzmann and Siffert' have shown that the maximum amount of polymer adsorbed on Na-montmorillonite, Q,,,, was 15 mgg-' at high ionic strength, yet only 3mgg-1 in the absence of added salt. Increasing the salinity results in the charges on the polymer being screened from one another, thus allowing the polymer to coil once more and 'collapse' onto the clay surface. The current consensus is that as the degree of hydrolysis increases the chain becomes more extended and less flexible. Under these conditions the polyanions bind to the positive edge sites uia relatively few anionic segment^,^ thus bridging between particles in suspension.Polycations are adsorbed due to the coulombic interactions between the cationic groups and the negatively charged clay surface which results in flocculation as charge neutralisation OCCU~S.~~'~At high loadings a reversal of surface charge may occur and electrostatic stabilisation may be re-established. The influence of molecular mass and density of charge on the interaction of polycations with Na-montmorillonitelo*'l has been extensively studied using cationic copolymers of acryl- amide (AM) and N,N,N-trimethylaminoethylchloride acrylate (CMA): f CH -CH )--(CH,-CH 3 --I I co co + ,CH,I I NH, OCH,CH,N-CH, C1-\ CH, AM CMA Scheme 1 The cationicity, z, of the polymer (PCMA) was increased from 1 to 100% by increasing the number of CMA units in the copolymer.12 Adsorption isotherms for the polycations on Na-montmorillonite showed that Q,,, decreased from 27OOmgg-' for z=1% to 32Omgg-1 for z=IOO%.Flocculation studies suggested that below z = 15% particle bridging was the major adsorption mechanism, whereas at z >25% charge neutralisation became the dominant mechan- ism and particle aggregation occurred via an electrostatic patch rnechani~m.'~-'' Multinuclear NMR has provided a wealth of information concerning the organisation of A1 and Si within the aluminosil- icate la~er,'~?'~ the structure and state of motion of water in the interlayer regi~n'*~~~ and, recently, the interlayer cations have been successfully utilised to provide complementary information about interlayer organisation.20*2' In general, how- J.Muter. Chem., 1996,6(2), 253-260 253 ever, these studies have been limited to dry powdered samples which is far removed from the situation in drilling muds or the processing of clay suspensions. 39Kis the nucleus of choice for the study of clay-polymer interactions in drilling muds because KCl is added to flocculate suspended clay particles. However, despite a natural abundance of 93.1%, 39K, a spin 3/2 nucleus, has very poor sensitivity relative to hydrogen and spectral accumulation times are very long. Nonetheless, useful results have been obtained from powdered samples using solid- state NMR." In contrast 133Cs, a spin 7/2 nucleus, is 100% abundant, has good sensitivity and a very small linewidth factor.Consequently, this nucleus has been used to advantage in solid-state NMR studies of powdered samples of mordenite22 and zeolite-AZ3 and in magic angle spinning (MAS) NMR studies of clay powders and slurries of high solid content^.'^ We have evaluated 133Cs solution-phase NMR, without recourse to MAS, as an in situ probe to study the adsorption of polycations, and other ions, onto clays in aqueous suspen- sions containing 2.5 mass% of a low-iron Texas bentonite. In particular we have studied how effective the polycations FL15, FL16, FL17, of general formula Me NCH CHOHCH2),]"+, and Magnafloc 1697, [(CH, 9-----7HCH2N(Me),CH2 HCH2),]"+ , are at displacing Cs' ions from the Texas bentonite compared to Na', K', Me,N+ and paraquat2+. This information has been correlated with that obtained from particle-size and electrophoretic measurements in aqueous solution, together with that obtained from adsorption isotherms, variable tem- perature X-ray diffraction (XRD) and thermogravimetry (TG) studies using dry powdered samples.The comparison and correlation of in situ information with data obtained from the resulting dry film or powdered product is relevant to a wide range of materials processing. Experimental Adsorbates The polymers used, their structures, molecular masses and approximate lengths are provided in Table 1. The distance between charge ceptres, derived from molecul!r models of the polymers, are 4.8 A for the FL series and 7.4 A for 1697.Sample preparation The clay used for this study was a Texas bentonite, Westone- L (supplied by Southern Clay Products) which contains only 0.75% (m/m) Fe203. The <2 pm fraction was collected using standard sedimentation procedures and then concentrated using 1.0mol dm-3 aqueous NaCl. The resulting slurry was centrifuged for 2 h at 10000 rpm and the exchange procedure repeated twice more. The clay was then repeatedly washed with deionised water and centrifuged until the conductivity of the supernatant was <30 pS. The clay, subsequently referred to as Na+-WL, was then stored as a 50g dm-3 suspension until required. The cation exchange capacity (CEC) of the Na+-WL form, pretreated at 120 "C, was determined using the ammonium acetate method and found to be 0.81 mequiv (g clay)-'.The K and Cs exchanged forms, K+-WL and Cs+-WL, were prepared in a similar manrler employing three contacts with KCl or CsCl, purified to the same conductivity criterion and stored as 50g dm-3 suspensions. XRF analysis (vide infra) confirmed that the exchange procedures resulted in essentially homoionic exchange forms. Adsorption isotherms Samples were prepared by introducing 10cm3 of 50g dm-3 M+-WL (0.5 g clay) into new polypropylene bottles together with a quantity of adsorbate and sufficient deionised water to give a total volume of 20 cm3. The clay-adsorbate suspensions were then shaken at 25 "C for 2 h.The samples were centrifuged and the supernatant decanted before washing the sample once with water to remove excess polymer. The samples were dried at 120 "C, ground and then stored at 120 "C. Nitrogen content was determined using a Gerhardt Vapodest Kjeldahl Autoanalyser. Samples for powder XRD and TG studies were air-dried after centrifugation and ground to (75 pm whereas samples for variable-temperature XRD were deposited as slurries on glass slides and dried in air. NMR spectroscopy Samples for 133Cs NMR were prepared by introducing 2 cm3 of 50g dm-3 Cs+-WL into new polypropylene bottles along with a quantity of adsorbate and sufficient deionised water to give a final clay concentration of 25 g dm-3 in a total volume of 4 cm3.Each sample was shaken as above and allowed to equilibrate overnight. A 3 cm3 portion of each clay-adsorbate suspension was then transferred, after vigorous shaking, into an 8 mm diameter NMR tube which in turn was placed into a 10mm diameter NMR tube containing a quantity of ['HJ acetone as lock. All spectra were obtained using a Bruker WH400 NMR spectrometer, B,=9.4 T, at a frequency of 52.488 MHz for 133Cs. In a typical experiment 800 transients were acquired in 1000 points, with a typical sweep width of 20 kHz, which were transformed with 16000 points. The 133Cs signal in the clay suspensions decayed rapidly after pulsing so all spectra were recorded using the stated sweep-width (SW) of 20 kHz together with a pulse-width (PW) of 10 ps.A line broadening factor of 100 Hz was used. The proportion of Cs+ ions associated with the clay was determined by reference to a 0.01 mol dm-3 CsCl solution saturated with CuSO, to broaden the peak. This approach was adopted to allow for quadrupolar effects. The area under the Cs+ resonance in the Cs+-WL suspensions was then integrated with respect to the broadened 0.01 mol dmP3 CsCl peak and the quantity of Cs+ ion determined. Preliminary experiments established that (i) the equilibrium between polycation and Cs +-WL was established Table 1 Essential features of polycations used in this study FL15/17 1697 number of approximFte number of approximFte MI cationic units length/A MI cationic units length/A FL15 5000 30 150 1697 100000 430 3813 FL17 1ooooo 630 3024 254 J.Mater. Chem., 1996,6(2), 253-260 in less time than that required to mix the clay with the polymer and put the sample in the spectrometer, (ii) that the area under the 133Cs resonance varied linearly with the amount of Cs+-WL in suspension up to 50g dm-3, and (iii) that the 133Cs integral intensity remained constant for at least 20 min. Sample characterisation Samples for XRF analysis were prepared using the LiB407 fusion method. The resulting beads were analysed on a Philips PW2400 XRF spectrometer using calibration software pre- pared from certified reference materials. XRD profiles for pressed powde: samples were recorded using Cu-Ka radiation (A= 1.5418 A) on a Philips PW1830 diffractometer operating at 35 kV and 45 mA at a 28 scan rate of 2" min-'.Variable-temperature XRD data (Co-Ka, R-1.789 A) were obtained on a Phillips PW1050 diffractometer operating at 40 kV and 40 mA using a simple heating stage." Samples were presented to this heating stage as oriented films on glass slides and were heated from 50 to 250 "C, in increments of 50°C. The samples were held at these temperatures for 30 min prior to recording the XRD trace. TG traces were obtained using a Mettler TG50 thermobal- ance equipped with a TClOA processor. Samples (7 mg) were heated from 35 to 800°C at 20"Cmin-' under a flow of 20 cm3 min- ' dry nitrogen carrier gas. The samples were preconditioned in the nitrogen flow for 15 min prior to initiat- ing the heating ramp to remove physisorbed water.The zeta potentials (5) were measured using a Malvern Zetasizer I1 and a Leeds and Northrup Microtrac particle size analyser. Clay-polymer suspensions were prepared as described for NMR experiments (vide supra). Results FL15, FL17 and 1697 all exhibited high affinity adsorption isotherms on all the cation-exchanged forms of WL, whereas the adsorption of TMA+ ions, which represent the cationic portion of the polymers, was of lower affinity (Fig. 1). The maximum amount of polymer adsorbed, Qmax, depended on the resident exchange cation, varying as Na>K>Cs, and on the molecular mass of the polymer. The higher the molecular mass the greater the Q,,, value as anti~ipated.~?" Moreover, the combination of polymer structure and exchange cation was significant insofar as the difference between the Q,,, values for FL17 and 1697 increased as Na<K<Cs.If all the cation exchange sites on the clay surface were satisfied by cationic units on the polymers then saturation would occur at Q,,, values of 83 mg g-' for FL17 and 103 mg g-' for 1697. Clearly, this value was exceeded by the FL polymers on all cation-exchanged forms. In contrast, Qmaxfor 1697 on Cs +-WL was only 68% of that required to satisfy the exchange capacity. Increasing the FL16 loading on Na+-Wk resulted in a change in basal spacing from a value of 12.5 A, characteristic of on? water layer between adjacent aluminosilicate layers, to 15.0 A (Fig.2) which is consistent with one layer of polymer residing between the layers. The presence of two poorly resolved peaks, near 28=7", in the diffraction traces (Fig. 2) suggest that the adsorbed polymer was segregated in different interlayers at low loadings. This was confirmed for both FL17 and 1697 on Na+-WL [Fig. 3(a), (c)] when oriented films of these clay-polymer complexes were heated at elevated tempera- tures. Thermal treatment at 50 "C for 30 min was sufficient to completely dehydrate the water-expanded, 12.5 A, layers resulting in collapse to a 10 A spacing,26 whereas considerably higher temperatures were necessary to denature the polymer and cause complete collapse at low loadings. At higher loadinGs of FL17 on Na+-WL, evidence for a higher spacing (16 A) complFx was observed [Fig.3(b)] which did not deteriorate to a 10A spacing even at 250°C. Higher basal spacings for 144 108 72 F I h -8 0 36 rn v Fola, 0.01 0.02 0.03 0.04 0.05 0.06 n8 100 0 1 s c 5 144 5 108 72 36 0 0 0.01 0.02 0.03 0.04 0.05 0.06 initial nitrogen concentration/mol drn" Fig. 1 Adsorption isotherms for (a)FL series and TMA' and (b)1697 on various cation-exchanged forms of Westone L. .,Cs+-WL/FLlS;A, Cs+-WL/FL17; 0,Cs+-WL/TMA+; 0, Cs+-WL/1697; A, Na+-WL/FL17; 0, Na+-WL/1697; V, K+-WL/FL17; K+-WL/1697. (Points 1-4: see Fig. 5.) 12000-A 10000-36000-C7 Y .-i3 6000-g!.-1 0.0 I0 r 40 ;lo 28/degrees Fig.2 Powder XRD profiles for FL16 on Na+-WL. The numbers on the curves represent the percentage of the CEC occupied by the polycation. polymer-treated clays have been observed, particularly when the polymers are of lowlo or zero ~ationicity.~~Fig. 3(d) illustrates that the Na+-WL/1697 complex behaved in a similar manner to Na+-WL/FL17, even though the broadness of the peaks suggest a less well ordered system. TG studies of J. Muter. Chem., 1996, 6(2), 253-260 255 9000 8000 7000 6000 So00 4000 3000 2000 1000 I I t I I I , I I 1 I 1 I I i 0 I 1 I I I I I I 1 I I-I 1 0) 10000 c4L.- 3500 9000 8000 7000 2500 8000 2000 so00 1500 4000 3000 1000 2000 500 lo00 I I I 1 # I I I TI 1 I I 1 I0 I 4 0 4 6 8 10 12 14 16 18 20 4 6 8 10 12 14 16 18 20 2Bldegrees Fig.3 Temperature dependence of XRD profiles for Na+-WL loaded with (a) 20% CEC FL17; (b) 100% CEC FL17; (c) 20% CEC 1697; (d) 100% CEC 1697.Temperatures are: (i) 25, (ii) 50, (iii) 100, (iv) 150, (v) 200 and (vi) 250°C. the clay-polymer complexes confirmed this thermal stability intensity 7:12:15:16:15:12:7.28It is probable that the observed insofar as the temperatures at which the onset of the major 133Cs signal arises from the central +++-3 transition which weight losses occurred (Fig. 4)coincided with the collapse of contributes 19% to the intensity, and that it is enhanced by the basal spacing. Similar trends were observed in the diffrac- other broadened transitions, especially the +$-+++and the tion traces for both Cs+-and K+-exchanged WL, although -+-+ -$ transitions.The remaining transitions were probably firm evidence of interlamellar polymer only occurred at higher too broad to be distinguished from the background when the loadings on the Cs+-exchanged form. Cs' was associated with the clay. However, once liberated The 133CsNMR spectra obtained for the isotherm points from the surface all seven transitions become degenerate and designated 1, 2, 3 and 4 in Fig. l(u) are presented in Fig. 5. contribute to the observed signal. The resulting peak represents These spectra illustrate how the linewidth of the 133Csreson-a weighted average between Cs' associated with the clay and ance decreased sharply, whilst the intensity increased, as FL17 Cs+ in solution and it increases in intensity as cationic species was added to Cs+-WL.Fig. 6 shows how the decrease in is added and displaces Cs' from the clay. The 133CsNMR linewidth and increase in intensity of the 133CsNMR was linewidth for Cs+-WL in the absence of added polycation was affected by the type of cation added to the system. An aligned extremely broad at 1500 Hz, and the appreciable narrowing as 133Cs spectrum consists of seven equally spaced lines of relative the initial aliquots of polymers were added was unexpected. 256 J. Mater. Chem., 1996, 6(2), 253-260 I I I1 I I I 200 400 600 200 400 600 TPC Fig.4 Derivative thermograms for the desorption of (a) FL17 from Cs+-WL and (b) 1697 from Na+-WL.The numbers on the curves represent the percentage of the CEC occupied by the polycation. 1 2 IA4L 3 4 I loo00 0 -5Ooo freq uenc y/Hz Fig. 5 Effect of FL17 adsorption on the linewidth and intensity of the 133CsNMR resonance at the points marked 1, 2, 3 and 4 in Fig, l(u) Cs+ is known to have a high affinity for the clay surface and there will be a significant number of different sites on each platelet, some in close proximity to octahedral sites containing iron, others not. The Cs+ ions on a particular platelet probably visit each of these sites but are unable to move from one platelet to another. Consequently, the peak for Cs+-WL has a large linewidth because it is composed of a number of smaller linewidth peaks with varying chemical shifts.When polycation is added, Cs+ ions are displaced from the surface of platelets and once in the aqueous phase these ions are free to visit other platelets. Eventually, an averaging of the 133Cs signals over all platelets will occur, the efficiency of which will depend upon I 0.025-(b) 0.020 0 E2 0.015 $ v)2 0.010 0.005 CEC Fig.6 Change in (a) linewidth and (b) intensity of the 133Cs NMR resonance caused by the displacement of Cs+ ions by a range of cations and polycations. Symbols as in Fig. 1, plus: *,Cs+-WL/Paraquat2+; +, Cs+-WL/Na+; 0,Cs+-WL/K+. the quantity of Cs+ ion in solution and ultimately the ability of the displaced cations to diffuse from one platelet to another.It is reasonable to assume that a value of 150 Hz, at which the initial rapid decrease levels off, is a crude estimate of the linewidth for Cs+ ion on an individual clay platelet. Once the line has narrowed to this value then subsequent additions of polycation should result in a steady decrease in linewidth proportional to the amount of polycation added, which is what was observed [Fig. 6(u)].Clearly, the addition of poly-cations caused the most rapid reduction in linewidth. The linewidths in the presence of 0.00192 mol dm-3 of N+ (equival- ent to 10% CEC) of FL17 and 1697 were 196 Hz and 475 Hz, respectively, reinforcing the marked efficiency of FL 17 in displacing Cs+ ions. Both paraquat2+ and K+ were reasonably effective at reducing the linewidth at low concentrations although the influence of K+,like that of Na+ ,decreased with increased concentrations in solution.TMA' exhibited an intermediate influence on the linewidth. The final values of the linewidth recorded in the presence of clay and FL17 were identical to those observed for CsCl/FL17 solutions at rep- resent at ive concentrations confirming that the CsCl/FL 17 interaction caused negligible broadening of the 133Cs signal compared to the effect of the clay. The influence of added cations on the intensity of the 133Cs resonance, determined by reference to a 0.01 mol dm-3 CsCl solution saturated with CuSO,, is shown in Fig. 6(b) and closely parallels the linewidth data. The amount of Cs+ ion visible by NMR in the absence of polycation (top spectrum of Fig.5) was equivalent to 20% of the total Cs+ ion present. Clearly, as the adsorption isotherms (Fig. 1) and the linewidth data [Fig. 6(u)] suggested, the polycations were extremely J. Muter. Chem., 1996, 6(2),253-260 257 effective, closely followed by paraquat", at displacing the resident Cs+ ions. Me,N+ ions were much less effective whereas K+ and Na' were ineffective. XRF analysis of dry powdered samples saturated with FL17 in the plateau region of the isotherm confirmed that 96% of the exchangeable Cs' ions had been displaced by the polycation. When polycations adsorb onto negatively charged clay particles the point of zero charge (PZC) should be reached when the number of cationic units adsorbed matches the number of anionic centres on the clay.If the PZC is reached before the CEC then this indicates a buildup of charge on the clay/polymer particles. In the current context this may be due to (i) inefficient displacement of the exchange cations or (ii) the presence of a significant number of cationic units in loops or trains which are not in contact with the clay surface. Alternative (i) is more likely because highly charged polycations which are smaller than the size of the adsorbent particles, as is the case here, adsorb in a flat configuration." The PZC and the minimum in the particle-size curve for the adsorption of FL15 on Csf-WL [Fig. 7(a)] indicated that the negative charge on the particles was equalised at 60% CEC.Up to this point the particles were aggregating, probably via an electrostatic patch after which the further adsorption of poly- cation resulted in a buildup of positive charge and electrostatic stabilisation was re-established. Note, however, that displace- ment of Cs+ continued upon addition of further polymer which may indicate that charge neutralisation preceded floc formation. The PZC and the point of maximum aggregation was reached when 1697 displaced 24% of the resident Cs+ ions, i.e. the number of Cs+ ions on the external faces of tactoids. The adsorption of 1697 on the external surface of the Cs +-WL tactoids would encourage electrostatic patch floccu- lation to occur more readily. This agrees with the diffraction data where there was little evidence of the penetration of 1697 into the interlamellar regions of Cs+-or K+-WL at low ..................... ................9,..G:'.36 12 -12 -36 -60 > E \ 4a 21 -6 -33 r) FEC 0.000 0.008 0.011 0.024 0.032 0.040 [N]/mol dm3 Fig.7 Change in zeta potential (c)and particle size resulting from the adsorption of (a) FL15 on Cs+-WL (B) and Naf-WL (a) and (b)1697 on Csf-WL (a). loadings. Moreover, the 133Cs NMR data clearly showed that floc formation did not prevent displacement of Cs+ ions and that 1697 displaced much less Cs+ than the FL series of polymers, yet the particle size of the flocs was the same at 8 pm. In contrast, Na+-WL formed flocs with a broad distri- bution around 45 pm.The presence of unsatisfied cationic centres on the Cs +-WL/FL17 complex was confirmed experimentally by adding fresh Cs+-WL to an exhaustively washed sample of FL17 saturated Cs+-WL. The integral intensity of the resulting 133Cs signal suggested that 0.003 mol dm-3 of Cs+ ion (equivalent to 16% CEC) was displaced from the fresh Cs+-WL by the previously unsatisfied cationic centres on the FL17-saturated Cs+-WL. Discussion Polymer loading and location in powdered samples The degree of dispersion of individual clay particles in aqueous suspension depends on the clay concentration, its particle size, the exchange cation and the ionic strength of the medi~m.~~.~' When the exchange ions are Na' or Li' the individual platelets are well dispersed, whereas with Kf ,Cs' and divalent ions the particles are aggregated into tactoids.The number of platelets per tactoid is cation-dependent and Cs +-WL would contain more platelets per tactoid than K+-WL. This degree of platelet dispersion is reflected in the cation dependence of the Q,,, values (Fig. 1) insofar as the amount of FL15, FL17 and 1697 adsorbed on Naf-WL was almost double the CEC and largely independent of polymer structure and molecular mass. This suggests that the polymers were able to access both faces of each individual particle when in suspension. In contrast the amount of polycation adsorbed by Csf-WL was influenced by the molecular structure. The amount of sorbed FL17 exceeded the exchange capacity by 15% whereas the bulkier 1697 occupied only 68% of the exchange sites, indicating that 1697 was excluded from some interlayer regions just as high molecular mass, cationic polysaccharides were excluded from the interlayers of illite.7 The Qmaxvalues for the adsorption of FL17 and 1697 on K+-WL reflected the intermediate nature of the tactoid formation in the K+-exchanged form.30 The XRD profiles (Fig.2 and 3) support the suggested locations of the different polymers in the various cation-exchanged forms of WL. The thermal stability of peaks associ- ated with polymer-expanded interlayers in the XRD profiles (Fig. 3) confirmed that FL17 resided in some interlamellar regions, at loadings of 25% CEC, in both Na+-WL and K+-WL, but was excluded from Cs+-WL. In contrast, 1697 only occupied interlayer sites in Na+-WL at this loading. At higher loadings, the polycations were present in at least some interlamellar regions, irrespective of polymer structure or resi- dent exchange ion. The correlation between the adsorption isotherm and XRD data was anticipated given that the samples underwent almost identical treatment following contact with the polymer.The most significant of these post-contact procedures was the centrifugation step where any unattached cationic groups on the polymers would come into contact with exchange sites on neighbouring clay particles as the water was forced from the sample. Thus the data obtained from dried clay could represent samples in which more of the original exchange cations had been displaced from the clay surface than in the clay-polymer suspension.The mode of interaction of cationic polymers with Na+- exchanged clay depends upon both the ratio of charged to uncharged units in a polymer, i.e. its cationicity, z, and upon its molecular mass.'O,ll Flocculation studies of Na+-montmor- illonite with long, high molecular mass PCMA" indicated that at z <15% the polymer adopts a more extended conformation 258 J. Mater. Chem., 1996,6(2), 253-260 and adsorbs via a particle bridging mechanism whereas at ~>25% the polymer is adsorbed on the clay surface and charge neutralisation is the dominant process. A bridging mechanism results in flocculation of the suspended particles and should lead to an increase in the observed particle size.However, the amount of exchange cation displaced will be low if all the cationic units on the polymer are not bound to an exchange site. In contrast, when the polycation precipitates out on the clay surface the number of displaced cations should reflect the number of cationic units in contact with the clay surface. Very small quantities of FL17 caused considerable flocculation of Na +-WL and electrostatic redispersal was difficult to achieve (Fig. 7). Consequently, it was anticipated that the combination of 133Cs NMR information with the particle-size and electrophoretic data would indicate the way in which the polycation adsorbed on WL in aqueous suspension. Polymer loading and location in suspension The variation in ability of different cationic species to reduce the 133Cs linewidth (Fig.5 and 6) was attributed to the displacement of resident Cs' ions from a wide range of different sites on the clay surface. The evidence in Fig. 6(b) confirms the inability of Na' and K+ to displace Cs' ions from WL and agrees with the accepted replaceability series Cs' >K+ >Na+, which is attributed to the magnitude of the cation hydration energy.31 Nonetheless, it appears that K+ could selectively displace Cs' from a small number of sites and this Cs' goes into aqueous solution causing the linewidth to narrow appreciably. When all the Cs' had been displaced from these particular sites further addition of K+ caused little significant reduction in the linewidth. 133CsMAS NMR spectra of Cs+-exchanged hectorite suspended in 0.1 mol dm-3 CsCl exhibit two resonances which have been attributed to Cs' strongly bound in the Stern layer via interactions with the basal oxygens and a more loosely held Cs' ion in the Gouy layer.24 Perhaps it is the Cs' in the Gouy layer that the K+ can displace.The polycations are not capable of such discrete selectivity because of the close proximity of the charge centres in the polymer. When a cationic unit adsorbs onto the surface adjacent cationic units must also displace Cs+ ions into solution. Consequently, the reduction in linewidth proceeds much more effectively in the presence of the polycations because they release many more Cs' ions, from a range of sites, into the aqueous phase.The variation in ability of the other cationic species studied herein to displace Cs+ from the clay is reflected in the different curves in Fig. 6(a). Fig. 6(b) shows that the amount of Cs' ion displaced by the various cationic species correlated with the reduction in line- width, and the order in affinity of the polycations for Cs+-WL (Fig. 1) correlates with the amount of Cs' ion released into solution when aliquots of polymer were adsorbed. The amount of Cs+ ion replaced by FL15, FL17 and paraquat2+ indicates quantitative displacement at the plateau. In addition, the adsorption isotherm data [Fig. l(b)] suggested that only 68% of the available Cs' ions, equivalent to an aqueous concen- tration of 0.014mol dmP3, were displaced by 1697 which agrees with the amount of Cs' ion observed by NMR.This agreement between the number of Cs' ions displaced and the Q,,, values from Fig. 1 was also observed for TMA'. In contrast, the inorganic ions Na' and K+ were almost com- pletely ineffective at displacing Cs+ from the clay surface. Nonetheless, it is clear that all the cationic species capable of displacing Cs' ion do so via a charge neutralisation mechanism since each subsequent addition of Nf caused an increase in the 133Cs signal. Influence of polymer structure Both polycations are small compared to the clay particles and the charge separation on both polymers is less than that on the clay surface. Under these conditions a mosaic of positive patches would be formed on the clay surface because the surface charge density is not neutralised unif0rm1y.l~ Flocculation occurs when a polymer-coated positive patch comes in contact with a polymer-free negative patch and the liberation of further Cs+ ion when fresh Cs+-WL is added to FL17-saturated Cs +-WL provides evidence for this floccu- lation mechanism.The marked increase in particle size and rapid buildup of charge on Cs+-WL when only small amounts of 1697 were added suggests that this polycation is restricted to the outer surfaces of tactoids whereas FL17 appears more capable of distributing itself throughout the Cs+-WL aggre- gates. It is, however, difficult to accept a model which requires a large polymer bearing hundreds of cationic centres to diffuse between adjacent layers in a Csf-loaded tactoid.A redistri-bution mechanism is more likely. Imagine that a Cs+-tactoid in which the external faces are saturated with FL17 collides with another tactoid which has no polycation on its external faces. Cs+ ions are displaced and the strength of the resulting interaction between the tactoids is sufficient to peel an individ- ual layer off either tactoid, thus exposing two fresh Cs+- saturated surfaces for further interaction with polycation. Clearly, this mechanism must be invoked to explain the eventual liberation of 68% of the exchangeable Cs+ ions by 1697, yet it does not explain the flocculation of Cs+-WL at low loadings of 1697 [Fig.7(b)]. Preliminary 23Na NMR studies indicate that the flocculation of Na+-WL by FL15 occurs via an electrostatic patch mechanism. However, because Na+-WL is dispersed as single sheets there are six times more particles per unit volume compared to Csf-WL and commen- surately more collisions. It is, therefore, not surprising that flocculation occurs at lower loadings, that the flocs contain more particles and that they do not readily disperse. Conclusions The combination of techniques used in this study have shown that the polycations are adsorbed via a charge neutralisation process which leads to flocculation via the electrostatic patch model. The quantity of polycation adsorbed, the particle size and electrophoretic mobility is influenced primarily by the exchange cation on the clay surface via its control over the degree of dispersion of the individual clay platelets.Polymer structure has little influence on the amount adsorbed by Na+-WL, but it appears that the bulkier 1697 polycation is excluded from some regions of the Cs+- and K+-WL tactoids. This is reflected in the expansion of the layers determined by powder X-ray diffraction. References 1 A. Guyot, R. Audebert, R. Botet, B. Cabane, F. Lafuma, R. Julien, E. Pafferkorn, C. Pichot, A. Revillon and R. Varoqui, J. Chim. Phys., 1990,87, 1859. 2 B. Gu and H,E. Donor, Clays Clay Miner., 1992,40,151. 3 M. Falk, L. Odberg, L. Wagberg and G. Risinger, Colloids Su$, 1989,40,115. 4 L. Bailey, M. Keall, A. Audibert and J.Lecourtier, Langmuir, 1994, 10,1544. 5 J. M. Adams, Clay Miner., 1993,28, 509. 6 T. W. Beihoffer, D. S. Dorrough, C. K. Deem, D. D. Schmidt and R. P. Bray, Oil Gas J., 1992,90,47. 7 B. K. G. Theng, Clays Clay Miner., 1982,30, 1. 8 Th. Stutzmann and B. Siffert, Clays Clay Miner., 1977,25, 392. 9 P. Espinasse and B. Siffert, Clays Clay Miner., 1979,27,279. 10 G. Durand-Piana, F. Lafuma and R. Audebert, J. Colloid Interface Sci., 1987, 119,474. 11 R. Denoyel, G. Durand, F. Lafuma and R. Audebert, J. Colloid Interface Sci., 1987,119,474. 12 F. Mabire, R. Audebert and C. Quivoron, Polymer, 1984,25, 1317. 13 J. Gregory, J. Colloid Interface Sci., 1973,42,448. 14 J. Gregory, J. Colloid Interface Sci., 1976,55, 35. J. Muter. Chem., 1996,6(2), 253-260 259 15 16 17 E.Dickinson and L. Eriksson, Adv. Colloid Interface Sci., 1991, 34, 1. C. M. Alma, G. R. Hayes, A. V. Samoson and E. T. Lipmaa, Anal. Chem., 1984,56,729. C. A. Weiss Jr., S. P. Altaner and R. J. Kirkpatrick, Am. Mineral., 1987,72,935. 23 24 25 26 M-K. Ahn and L. E. Iton, J. Phys. Chem., 1989,93,4924. C. A. Weiss Jr., R. J. Kirkpatrick and S. J. Altaner, Geochim. Cosmochim. Acta, 1990,54, 1655. G. Brown, B. Edwards, E. G. Ormerod and A. A. Weir, Clay Miner., 1972,9,407. D. R. Collins, A. N. Fitch and C. R.A. Catlow, J. Mater. Chem., 18 19 J. Hougardy, W. E. E. Stone and J. J. Fripiat, J. Chem. Phys., 1976, 64,3840. M. Lipsicas, C. Straley, P. M. Costanzo and R. F. Giese, J. Colloid 27 28 1992,2,865. R. Levy and T. W. Francis, J. Colloid Interface Sci., 1975,50,442. N. Boden, S. A. Corne, P. Halford-Maw and K. W. Jolley, J.Map. Interface Sci., 1985,107,221. Reson., 1992,98,92. 20 21 V. Laperche, J-F. Lambert, R. Prost and J. J. Fripiat, J. Phys. Chem., 1990,94,8821. J-F. Lambert, R. Prost and M. E. Smith, Clays Clay Miner., 1992, 29 30 P. Bar-On and I. Shainberg,J. Colloid Interface Sci., 1970,33,471. S. L. Swartzen-Allen and E. Matjevic, J. Colloid Inter$ace Sci., 1975,50, 143. 40,253. 31 R. E. Grim, Clay Miner., McGraw-Hill, New York, 1968. 22 P-J. Chu, B. C. Gerstein, J. Nunan and K. Klier, J. Phys. Chem., 1987,91,3588. Paper 5/04175I; Received 28th June, 1995 260 J. Mater. Chem., 1996,6(2), 253-260

ISSN:0959-9428    

DOI:10.1039/JM9960600253

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

"0solid-state NMR examination of La,O, formation Fatima Ali," Mark E. Smith,"" Stefan Steuernagelb and Harold J. Whitfield' "Physics Laboratory, University of Kent, Canterbury, Kent, UK CT2 7NR Bruker Analyt ische Messtec hnik, Silberstreifen, D- 76287 Rheinstetten, Germany 'Department of Applied Physics, Royal Melbourne Institute of Technology, Box 3476 W, Melbourne, Victoria 3001, Australia Solid-state 170magic-angle spinning (MAS) NMR spectroscopy has been used to study the formation of hexagonal La203 from both dehydroxylation of La(OH), and a sol-gel route starting from lanthanum isopropoxide. Both routes proceed via the formation of the oxyhydroxide [LaO(OH)] which is not commonly observed for other simple oxides. 170NMR spectroscopy is very sensitive to structural changes even in amorphous samples and the resonances of specific phases are assigned elucidating the chemical shift ranges of the local oxygen coordinations in these materials.The quadrupolar coupling constant for the OLa, site in La203 is determined by the application of satellite transition NMR spectroscopy. Oxygen-17 is potentially one of the most informative nuclei available for solid-state NMR studies, with its wide chemical shift range and small electric quadrupole moment.'-3 Its major handicap is its low sensitivity, stemming from the low natural abundance (0.037%). Efficient observation of 170NMR spec- tra requires isotopic enrichment and enriched samples have proved to be highly informative, as for example in the study of high temperature superconductors where oxygens in the Cu02 planes are readily distinguished from the other oxygens in the str~cture.''~ There is much interest in sol-gel-formed materials,' and for oxides as the structure evolves through a series of disordered states a local atomic-scale probe such as 170NMR is a powerful approach for characterisation.29Si solid-state NMR has been used extensively for examination of Si0,-containing gels as a result of the established sensitivity of the isotropic 29Si chemical shift to the local SiO, connec-tivity. However, many technologically significant sol-gel- formed oxides contain nuclei that are difficult for conventional pulsed Fourier-transform NMR techniques, so that 170is the most amenable NMR approach, and this has previously been applied to Zr026 and Ti02.7 Even for SiO,-containing gels it has recently been demonstrated that even when 29Si magic- angle spinning (MAS) NMR data is available, 170 from enriched samples can be readily observed and the 170 NMR data is much less ambiguous than 29Si.170 NMR has detected nanoscale phase separation in Ti0,-Si02 gels by distinguishing Si-0-Si, Si-0-Ti and Ti-0-Ti bonds, and the evol- ution of the structure with heat treatment has been Lanthanum oxide has a number of technological appli- cations including as a supported catalyst for oxidative coupling of methane and for hydrogenation. The phases of interest are often amorphous and difficult to characterise by other methods such as conventional X-ray diffraction (XRD).La203 in its applications is often doped and the mechanism and kinetics of oxide production of the individual components should each be understood in detail since they can be very different, and these differences could lead to segregation effects and non- uniform doping. La203 doped with SrO is an importagt catalyst for methane coupling" and it is also a good oxygen- ion conductor at elevated temperatures." La203 can be pre- pared via a variety of routes, including dehydroxylation of La(OH), and a sol-gel method hydrolysing lanthanum iso- propoxide. This paper presents solid-state 170 NMR to exam- ine the structural evolution from the initial phases to hexagonal La203. The intermediate states are largely amorphous and eventually form a nanocrystalline product.Previous 170NMR studies of lanthanum-containing com- pounds have included La203 at both natural abundance and in an enriched sample, where two resonances have been observed at 6 584 and 467.2,12 On the basis of the relative integrated intensities of 2: 1, the resonances have been assigned to OLa, and OLa,, respectively, from the crystal structure.I3 This demon- strates the expected decrease in the chemical shift with increasing coordination number. From the linewidths, upper estimates of C, (C, =e2Qq/h, the quadrupolar coupling constant, where eQ is the nuclear electric quadrupole moment and eq is the maxi- mum component of the electric field gradient) of 2.2 and 1.4 MHz, respectively, were made.2 In this paper, satellite- transition NMR ~pectroscopy'~ on an enriched sample con- strains C, much better for the OLa, site.The OLa, environ- ments in La4Si207N2 and La5Si3012N have been measured at 6 575 and 596, re~pectively.~ Similarly in the ceramic high- temperature superconductor La1.85Sro.15Cu04, the diamagnetic axial site [O(l), which does not lie in the CuO, plane and hence is a pure chemical shift rather than having a conduction electron contribution] resonates at 6 475.'~~This evidence points to oxygen in diamagnetic compounds that is principally coordi- nated by lanthanum resonating in the range 6 600-450, with higher coordination numbers having smaller chemical shifts. Experimenta1 170-enriched lanthanum carbonate [La,(CO,),] was prepared by taking an unenriched sample, sealing it in a Pyrex ampoule with 10 atom% 170-enriched H,O and heating to 105 "C for 48 h.The excess of water was removed under vacuum at room temperature. For the inorganic preparation of La203, an unenriched sample was taken and calcined at 900 "C to ensure the decomposition of any hydroxide or carbonate. The calcined oxide was sealed in a Pyrex ampoule with a slight excess of 170-enriched water compared to that required for complete conversion to La(OH),. The ampoule was heated to 110 "C for 72 h and then dried under vacuum at room temperature. The La(OH), was heated in an Al,O, boat under a stream of dry nitrogen for 90 min at 325 "C, producing LaO(0H). Some of this sample was further heated at 700 "C for 30 min under a dry nitrogen atmosphere, producing well crystallised La203. The identity of all samples was confirmed by powder XRD to be single phase of the desired composition.For the sol-gel preparation, 99.9% lanthanum isopropoxide [La(OC,H,),; Johnson Matthey] was cooled to dry-ice tem- perature (-80 "C). A mixture of 10 atom% l7O-enriched water and ethanol (1:4 mole ratio) was dropped slowly onto the lanthanum isopropoxide, until just enough water was added J. Muter. Chem., 1996, 6(2), 261-264 261 for complete hydrolysis. The resulting cloudy solution was allowed to warm slowly to room temperature over a period of ca. 2 h with continual stirring and kept in a dry glovebox. The gel had any excess water and solvent removed by evacuation at room temperature and the resulting powder was successively heated to different temperatures for 2 h periods, each time quenching back to room temperature and re-crushing the powder.Below 450°C the powder was sealed in a Pyrex tube which had been initially evacuated. Above 450°C the powder was placed in a platinum crucible and heated under a flow of dry nitrogen. XRD was again carried out on all samples. NMR studies were carried out at three magnetic fields of 9.4, 11.7, and 14.1 T using Bruker MSL 400, ASX 500 and Varian VXR 600 spectrometers, where I7O resonates at 54.24, 67.82 and 81.33 MHz, respectively. A single pulse correspond- ing to a tip angle of ca. 20" and recycle delay of 5 s were used with around 5000 acquisitions necessary for an adequate signal-to-noise ratio.An initial deadtime of <10 ps only was necessary at all fields. Spectra were acquired under MAS with a 7 mm double bearing (DB) probe used at 9.4 T, spinning at ca. 4.5 kHz, a 4 mm DB probe at 11.7 T and a 5 mm DB probe at 14.1 T, with spin rates of 11-14 kHz used for the smaller probes. Spectra were referenced externally to H20 at 6 0. Results and Discussion Initially, 170-enriched La,(CO,), was examined, which XRD showed to be single phase and a prominent, sharp resonance close to 6 0 was observed with a single, small accompanying sideband at negative shift [Fig. 1 (a)]. Although enrichment of Fig. 1 "0 MAS NMR spectra at 9.4T from (a) La,(C03)3, (b) La(OH),, (c) LaO(0H) and (d) hexagonal La20, 262 J.Muter. Chem., 1996,6(2), 261-264 the carbonate is certain since on subsequent calcination the oxide was strongly enriched, the prominent peak is not attribu- table to the carbonate. Observation of the 170 resonance from solid carbonates has proved somewhat problematic. In our work, exchange of 170from isotopically enriched water with a variety of carbonates (calcium, strontium, barium and lith- ium) has been performed. Taylor and Urey, using accurate density determination, showed this exchange to be a rapid and complete enrichment method for carbonate^.'^^'^ This has been confirmed by our NMR results, since the oxides produced on subsequent calcination are clearly enriched.However, for all carbonates at 9.4 T spinning at <5 kHz gave spectra that closely resembled Fig. l(a).17 For CaCO, at 11.7 T with MAS of ca. 13 kHz the 170resonance from the carbonate is charac- terised by C, =6.97 MHz, q =1 (quadrupole asymmetry par- ameter) and hi,,, 204 (isotropic chemical shift)," which will not be narrowed under the conditions available at 9.4 T here. Close examination of the spectrum [Fig. l(a)] reveals broad, weak features close to 6 200 which are probably part of the un-narrowed carbonate resonance. Hence the sharp resonance must be some species common to all the carbonates. The exact identity of the species responsible for producing this resonance is at present unclear, but it is likely to be some OH-containing species that is not removed by simple evacuation at room temperature. Such a species would be expected to resonate at close to 6 0, although from empirical correlations the C, would be expected to be larger.lg It could be that the proton is only weakly associated and the line-broadening is dominated by chemical shift anisotropy that is close to axial symmetry, which is narrowed efficiently by MAS and gives rise to the single sideband.It will be extremely interesting to accurately identify the species responsible for this resonance. La(OH), produced a single 170 resonance at 6 3.9, as expected from the crystal structure2' with a width (FWHM) of 1700 Hz [Fig. l(b)]. The peak position is in the region expected for hydroxy species and the local coordination is OLa,H.The width is probably a combination of residual second-order quadrupolar coupling, since for a strongly associ- ated proton C, is expected to be large,lg and unaveraged dipolar coupling, since the homonuclear H-H coupling will be quite large in such a system and no 'H decoupling was used. Single-phase LaO(0H) also gave a well defined 170 spectrum [Fig. l(c)]. There is a prominent resonance at 6 546.5 with a FWHM of 650 Hz flanked by an extensive manifold of spinning sidebands. LaO(0H) has two equally populated oxygen sites with coordinations OLa, and OLa,H.,' The signal at 6 546.5 is consistent with the OLa, resonance and this 'metal only' environment will have a small C, that will narrow readily under MAS.The second site is a hydroxy-like environment so is anticipated to resonate'near 6 0, and close inspection reveals that at ca. 6 40 there is a broad feature close to the baseline. Hence it resonates where expected, but experiences a large coupling constant that is not efficiently narrowed by MAS of 4.5 kHz at 9.4 T and the intensity is spread over a wide frequency range. This highlights the necessity of both fast MAS and high magnetic fields for 170 observation when it has distinct bonds to carbon and/or hydrogen which produces large C, values. Heating LaO(0H) to 700 "C produces hexagonal La,O, and the spectrum is consistent with those previously observed [Fig. 1 (d)] .2,12 Again, an extensive manifold of spinning side- bands for each of the two sites can be observed.The sideband intensities do not follow a monotonic decrease but show a modulation that results from the first-order powder pattern of the non-central transitions, and in particular the first singular- ity of the (&3/2, & 1/2) transition can be seen. For the OLa, site, C, is then 450 kHz, which can be compared to the previous upper estimate of 2.2 MHz. The latter result is simply a reflection that in such ionic systems the centreband linewidth for 170is generally dominated by chemical shift dispersion, even in crystalline systems. This point is reinforced by the fact that the spinning sidebands from the (43/2, f1/2) satellite transition are not noticeably narrower than the (1/2, -1/2) centreband, whereas if second-order quadrupole effects domi- nated the broadening the satellite lines would have only 30% of the width of the central transition.', This also illustrates that for small C,, observation of the spinning sideband mani- fold of the non-central transitions, which has been termed satellite transition spectro~copy,~~ is the best way to deter- mine C,.The gel-formed samples showed quite different I7O NMR spectra initially from the inorganic preparation. The sample heated to 110 "C shows a main peak at 6 132 and a secondary minor broader peak at 6 ca. -40 [Fig. 2(a)]. On further heating to 200 "C for a 2 h period the main peak shifts to 6 144 [Fig. 2( b)] and then at 450 "C is shifted to 6 165 [Fig. 2(c)] with a second major resonance at 6 546.This second peak can be readily identified as LaO(0H) from the shift and is con- firmed by XRD detecting crystalline LaO(0H). The peak at 6 ca. 150 is most likely an intermediate coordination in the amorphous phase [XRD showed the samples of Fig. 2(a) and (b) to be amorphous] which has some carbons closely associ- ated with the oxygen so as to produce the observed shift. On heating to 850"C, the two resonances from hexagonal La203 are seen [Fig. 2(d)]. These 14.1 T peaks have linewidths (FWHM) of 780 Hz (OLa,) and 420 Hz, which can be com- pared to the 9.4T data where the corresponding linewidths are 590 Hz and 320 Hz, respectively [from Fig. l(d)]. Both of n *1 1 I\ Fig. 2 "0 MAS NMR spectra at 11.7 T from lanthanum oxide gels heated to (a) 110, (b) 200, (c) 450 "C and (d) at 14.1 T for a gel heated to 850 "C for successive periods of 2 h (*indicates an instrumental artefact) these peaks are broader at higher field, emphasising that the linewidths are dominated by chemical shift dispersion which increases (in Hz) with the applied magnetic field.The 170NMR results obtained here for the lanthanum oxide gel make an interesting comparison with previous 170 NMR work on Zr026 and Ti0,.7 The atomic scale insight provided by NMR allows subtle differences in the hydrolysis and condensation behaviour of the different alkoxides to be understood, which is vital for improving sol-gel synthesis, especially of multi-component gels. The gels for I7O NMR work (refs.6, 7 and here) have been produced under very similar synthesis conditions, allowing direct comparison. In ZrO, once the excess fluid had been removed, even at room temperature only amorphous regions of progenitor monoclinic and tetragonal zirconia can be seen. In TiOz gels, again the OH groups are lost and extensive condensation has occurred once the excess fluid had been removed, leaving just OTi3 (as in the final TiO,) and OTi, environments, and no species that can be identified with C-0 bonds are seen. Heat treatment causes the OTi, environments to evolve into the OTi, environ- ments of anatase. This La203 gel behaves very differently, as during the early stages the large peak at 6 ca. 150 is probably from some C-0 and/or hydroxylated species that persists to rather high temperatures.Then, prior to conversion to the oxide, the oxhydroxide is formed which was not observed for the other oxides. It is clear that in sol-gel preparation the details of the route towards final oxide formation can be very different for particular metals, proceeding through different intermediate states which must be carefully understood. Conclusion Solid-state 170NMR has been shown to be an extremely powerful atomic-scale probe for the characterisation of oxide materials. Formation of hexagonal La,O, starting from both La(OH), and hydrolysis of lanthanum isopropoxide proceeds via the formation of LaO(0H). The presence of oxyhydroxides has not been previously been observed in NMR studies of other oxide gels.Also, it appears that some oxygen connection to carbon persists in these lanthanum gels up to at least 450"C, very much higher than other gels. C, for the OLa, site in Laz03 has been much more accurately determined than previously by utilising satellite transition NMR spectroscopy at 450 kHz. The authors thank Dr. T. J. Bastow, CSIRO, Melbourne, Australia for his encouragement and interest in this work. F.A. is grateful to KISR (Kuwait) for providing a scholarship to study at Kent. M.E.S. thanks the EPSRC for funding work on 170 NMR characterisation of materials through GR/J23938 and for access to the ultra-high-field NMR facility through GR/K43667. References 1 E. Oldfield, C. Coretsopoulos, S. Yang, L.Reven, H. C. Lee, J. Shore, 0.H. Han, E. Ramli and D. Hinks, Phys. Rev. B, 1989, 40, 6832. 2 T. J. Bastow and S. N. Stuart, Chem. Phys., 1990,143,459. 3 R. K. Harris, M. J. Leach and D. P. Thompson, Chem. Muter., 1992,4,260. 4 M. S. Went and J. A. Reimer, Chem. Muter., 1990,2,389. 5 C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, New York, 1990. 6 T. J. Bastow, M. E. Smith and H. J. Whitfield, J. Muter. Chem., 1992,2,989. 7 T. J. Bastow, A. F. Moodie, M. E. Smith and H. J. Whitfield, J. Muter. Chem., 1993,3,697. 8 M. E. Smith and H. J. Whitfield, J. Chem. Soc., Chem. Commun., 1994,723. 9 P. J. Dirken, M. E. Smith and H. J. Whitfield,J. Phys. Chem., 1995, 99, 395. 10 Z. Kalinek and E. E. Wolf, Cutul. Lett., 1991,9,441. J. Muter. Chem., 1996, 6(2), 261-264 263 11 S. J. Milne, R. J. Brook and Y. S. Zhen, Br. Ceram. Proc., 1989, 41,243. 12 S. Yang, J. Shore and E. Oldfield,J. Magn. Reson., 1992,99,408. 13 W. C. Koehler and E. 0.Wollan, Acta Crystallogr., 1953,6,741. 14 C. Jager, NMR Basic Principles and Progress, eds. B. Bliimich and R. Kosfeld, Springer-Verlag, Berlin, 1994, vol. 3 1, p. 134. 15 T. I. Taylor and H. C. Urey, J. Chem. SOC., 1940,131. 16 T. I. Taylor and H. C. Urey, J. Am. Chem. SOC., 1940,62,2833. 17 M. E. Smith and H. J. Whitfield, unpublished results. 18 M. E. Smith, S. Steuernagel and H. J. Whitfield, Solid State NMR, 1995, 4, 313. 19 S. Schramm and E. Oldfield, J. Am. Chem. SOC., 1984,106,2502. 20 W. H. Zachariasen, Acta Crystallogr., 1948, 1,266. 21 P. V. Klevtsov and L. P. Sheina, Iuest. Akad. Nauk SSSR, Neorg. Muter., 1965,1,2219. Paper 5/03983E; Received 20th June, 1995 264 J. Muter. Chem., 1996, 6(2), 261-264

ISSN:0959-9428    

DOI:10.1039/JM9960600261

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Influence of crystal structure on the luminescence properties of bismuth(m), europium (111) and dysprosium (111) in Y2Si05 Jun Lin," Qiang Su, Shubin Wang and Hongjie Zhang Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China The luminescence properties of Bi3+, Eu3+, Dy3+ and energy transfer from Bi3+ to Dy3+and Eu3+ have been studied in two modifications of Y2Si05 (low-temperature X1 type and high-temperature X2 type) and discussed in relation to their crystal structures. The Bi3+ ion luminesces in the blue region of the spectrum in X1-Y,Si05 but in the UV region in X2-Y2Si0,. Two obviously different luminescent centres have been observed for Bi3+ and Eu3+ in X,-Y,SiO,, but only one has been seen in X,- Y,SiO,.The Stokes shift (9200 cm-') for Bi3+ in X,-Y2Si05 is much larger than that (5000cm-') in X2-Y,Si05. This suggests that the host lattice is more rigid in X2-Y,Si05 than in X1-Y2Si05. As a result, the Bi3+, Eu3+ and Dy3+ ions show higher emission intensity in the former than in the latter type. X,-Y,SiO, is more suitable for Bi3++Eu3+ energy transfer and X,-Y,SiO, is more suitable for Bi3+ -+Dy3+ energy transfer. Rare-earth-metal oxyorthosilicates RE,SiOS (RE =rare-earth metal) are well known to exhibit luminescence. There are two structural modifications: X,-type (or A-type) for species con- taining larger RE3+ (La-Tb) and X,-type (or B-type) for smaller RE3+ (Dy-Lu, Sc).' For Y,SiO, the high-temperature modification belongs to the X,-type and the low-temperature form to the X,-t~pe.,-~ Many papers have been published on the luminescence and energy-transfer properties of rare-earth- metal ions in X,-Y,SiO,.For example, Y,Si05 containing Ce3+ and Tb3+,5-8 Tb3+ and Eu3+,**' and Dy3+ (Pr") and EU3+10 have been reported (all for X,-type modifications). However, little attention has been paid to the luminescence in X,-Y,SiO, .3 Bismuth ion (Bi3+) is a mercury-like ion with a 6s2 configur- ation, whose luminescence properties depend strongly on the composition and crystal structure of the host. Although the Bi3+ ion has been studied in various hosts for many years," no luminescence of Bi3 + in rare-earth-metal oxyorthosilicates has been reported as yet.Because Y,SiO, has two modifications with different structures, it is of great interest to study the dependence of the luminescence properties on the crystal structure of Y,SiO,. For this purpose, we chose Bi3+ and two rare-earth-metal ions, Eu3+ and Dy3+, as activators in Y,Si05. The results reveal that the crystal structures of Y,SiO, (X, or X,) have great influence on the luminescence properties of Bi3+, Eu3+ and Dy3+ and energy transfer from Bi3+ to Eu3+ and Dy3+. Experimental Results Luminescence of Bi3+ As predicted, striking differences were observed between the luminescence properties of Bi3 + in X,-Y,SiO, and X,-Y,SiO,. In X,-Y,Si05, the Bi3+ luminesces in the blue region of the spectrum. The excitation and emission spectra of X,- Y1,994Bi0~006Si05are shown in Fig.1. The emission spectrum under UV excitation (Aex =306 nm) consists of a single broad band with a maximum at 426 nm and a half-width of 68 nm. The excitation spectrum corresponding to this emission con- sists of two bands, a strong one with a maximum at 306 nm and a weak one at 372 nm. Excitation of the latter band (A,,= 372 nm) yields a weak emission band with a maximum around 414 nm, whose shape is rather different from that of the emission band at 426 nm. This suggests that these two exci- tation bands arise from two different Bi3+ luminescent centres. However, in X,-Y2Si05 the Bi3+ shows luminescence in the ultraviolet (UV) region. The emission spectrum of X2-Yl~9g4Bio~oo6Si05has a maximum at 343 nm and a half-width of 43 nm, and the corresponding excitation spectrum has a maximum value at 293 nm, as shown in Fig.2. Neither the emission nor excitation band is symmetrical, indicating the existence of more than one Bi3+ luminescent centre in this All powder samples were prepared by the sol-gel techniq~e.'~.~~ The starting materials used for preparation of the phosphors were Y203 (99.99%), Eu203 (99.99%), Dy203 (99.9%), Bi2o3 (analytical reagent) and Si(OC2H5)4 (chemical purity). The samples were fired at 1050 "C for 10 h to form X1-Y,Si05, and at 1350 "C for 6 h to form X2-Y2Si05. All products were examined using X-ray powder diffraction (XRPD; Rigaku D/max-IIB X-ray diffractometer, Cu-Ka radi- ation) and appeared to be single phase (JCPDS cards 21-1456 and 21-1458).The excitation and emission spectra were meas- ured at room temperature on a SPEX Fluorolog I1 spectro-fluorometer equipped with a 450 W xenon lamp as the excitation source. I \ 07n I 414 \ 450 wavelengthlnm Fig. 1 Excitation (a) and emission (b) spectra for Bi3+ in XI-Yl.994Bio.m6Si05at room temperature. (a) A,, =426 nm; (b) (-) A,, = 306 nm, (---) A,,=372 nm. J. Muter. Chem., 1996,6(2), 265-269 265 250 300 350 400 wavele ng th/nm Fig. 2 Excitation (a) and emission (b) spectra for Bi3+ in X2-Yl,gg4Bio,oo6SiO~ room temperature. (a) ,Iem= 343 nm, (b) ,Iex=at 293 nm. host lattice also. Furthermore, by comparing Fig. 1 and 2, we find that the emission intensity for Bi3+ in X2-Y2Si05 is much stronger than that in X1-Y,Si05.Luminescence of Eu3+ and Dg' Both the red emission of Eu3+, 5D0-7F2, and the yellow emission of Dy3+, 4F9/2-6H13/2, belong to hypersensitive trans- itions (AJ=2, AL=2), which are strongly influenced by the surrounding en~ir0nment.l~Their luminescence properties often show a similar character and so these two ions were selected for investigation together. Upon excitation to the 7Fo-5L6 transition of Eu3+ at 394 nm, Eu3+ exhibits a red emission in both X,-Y,Si05 and X2- Y,SiO,, with the hypersensitive transition 5D0-7F2as the most prominent group of signals, as shown in Fig. 3. The spectral character and emission intensities of the two host lattices are very different.In X,-Y2Si05, the emission spectrum of Eu3+ [Fig. 3(u)] shows two 5Do-7Folines (579,581 nm), five 5D0-7F, lines (584,587,590,593,598 nm) and seven 5D0-7F, lines (606, 612, 615, 622, 625, 628, 632 nm), while in X2-Y,Si05 only one 5D,-7F0 (580 nm), three 5D0-7F,(588, 594, 597 nm) and five 5Do-7F2 (612, 616, 619, 623, 626 nm) are present [Fig. 3(b)]. Furthermore, the spectral lines of Eu3+ in X,-Y,Si05 appear much broader than those in X,-Y,SiO,, and the total emission intensity for Eu3+ in the former modification is weaker than that in the latter. The Dy3+ ion shows similar spectral character to Eu3+ in the two hosts. Upon excitation to the 6H15/2-6P7,2 transition of Dy3+ at 349 nm, the Dy3+ ion shows two emission bands in the blue (B, 460-510 nm) and yellow (Y, 560-600 nm) regions of the spectrum in both host lattices, with the hypersen- sitive transition 4F9/2-6H13/2 yellow emission .as the prominent group, as shown in Fig.4. The spectral lines for Dy3+ in X,-Y,Si05 [Fig. 4(u)] appear broader than those in X2-Y,Si0, [Fig. 4(b)]. In addition, Dy3+ shows a much stronger emission intensity in the X,-type than in the X,-type modification. These results are in good agreement with those for Eu3+. Energy transfer from Bi3+to Df+ and Eu" By co-doping Bi3+ and Dy3+, Bi3+ and Eu3+ in Y2Si0, (X, and X2), we can investigate the interactions between Bi3+ and Dy3+, Bi3+ and Eu3+. Our experimental results reveal that energy transfer from Bi3+ to Dy3+ takes place in both types of Y,Si05.This is demonstrated by the excitation spectra of Dy3 + emission (A,= 576 nm), as shown in Fig. 5. As well as the characteristic excitation lines of the Dy3+ f-f transitions in the longer- wavelength region, broad excitation bands appear in the UV 266 J. Muter. Chem., 1996,6(2), 265-269 6.0--mb 4.0-T-2.0 -1.5 \ 1.0 73 7 0.5 0.0 550 600 650 wavelengthln m Fig. 3 Emission spectra for Eu3+ in X,-Y,,,,EU~.,~S~O~ (a) and Xz- Y,,8,Euo.12Si0, (b)under excitation at 394 nm (7F0-'L, transition of Eu3+) at room temperature. The notation 0-J (J=O, 1, 2) indicates the 'Do-7F, transitions of Eu3+. region, with maxima at 307 and 292 nm for X,-Y,SiO, and X2-Y2Si0,, respectively. According to the excitation spectra of Bi3+ shown in Fig.1 and 2, we know that these broad bands belong to the absorption of Bi3+. This indicates that energy transfer occurs from Bi3+ to Dy3+ in both types of Y,Si05. Excitation of the Bi3+ bands results in both Bi3+ emission and Dy3+ emission. In X,-Y,SiO,, the emission spectrum contains cu. 80% Bi3+ emission and 20% Dy3+ emission, while in X2- Y2Si0, it contains ca. 60% Bi3+ emission and 40% Dy3+ emission. This suggests that the sensitizing effect of Bi3+ on Dy3+ is better in X,-Y,SiO, than in X,-Y,SiO,. The estimated energy-transfer efficiency (vet)from Bi3+ to Dy3 + in X,-Y2Si05 is cu. 52%, according to vet= 1-I/Io, where I. (Aex=293 nm) is the total emission intensity of the donor Bi3+ without the acceptor and I (Ae,=293 nm) the corresponding value in the presence of the acceptor Dy3+.I5 Although this transfer is not complete, it is very useful for improving the Dy3+ emis- sion intensity under short-wavelength UV excitation.For example, under 254 nm excitation (radiation from the low- pressure mercury discharge) yellow light can be seen in the phosphor X2-Y1~934Bio~oo6Dyo~06~i~5,but is not visible in x2-Y1.94Dy0.06Si05, indicating the important role played by Bi3+ . Energy transfer from Bi3+ to Eu3+ was observed only in X,-Y,SiO,, not in X,-Y2Si05. Fig. 6 shows the excitation spec- trum of Eu3+emission (615 nm) in ~,-~,~934~io~oo6~uo~~~i~5. This excitation spectrum consists of f-f transition lines within the Eu3 4f6 configuration in the longer-wavelength region + (350-500 nm) and the absorption band of Bi3+ (AmaX=305 nm) which overlaps with the Eu3+charge-transfer band (Amax = 273 nm) in the short-wavelength region.Excitation of Bi3+ absorption band at 305 nm yields 60% Eu3+ emission and 307 Dy3+:f-f 500 550 600 650 wavelengthlnm Fig. 4 Emission spectra for Dy3+ in Xl-Yl~,,Dyo~,Si05 (a) and X2- Y,,,,Dyo~06Si05(b) under excitation at 349 nm (6H15/2-6P7/2 transition of Dy3+) at room temperature 40% Bi3+ emission. The Bi3+ +Eu3+ energy-transfer efficiency was estimated to be 53%.15 These results imply that Eu3+ is a better acceptor for Bi3+ excitation energy than Dy3+ in Xl-Y2Si05. Discussion In order to explain our experimental results, first let us consider the crystal structure of rare-earth-metal oxyorthosilicates.' The X,-type (type A) RE,SiO, crystallizes in the monoclinic crystal system, with space group P2,/c (Z=4). Two different sites are available for the rare-earth-metal ions.The first site [A,, coordination number (CN)=91 is coordinated by eight oxygens belonging to SiO, tetrahedra and one oxygen not belonging to any SiO, groups (denoted 'free oxygen'). The second site (A,, CN =7) is surrounded by four oxygens belong- ing to the Si04 groups and three free oxygens. The point symmetry for the A, and A, sites is C1.Owing to the existence of free oxygen, there is an appreciable covalent component in the RE-0 bonding.1,12p16 In addition, it can be expected that the degree of covalency on the A, sites is higher than that on the A, site (three free oxygens on A, site and one on A, site).I6 The X,-type (type B) RE2Si05 also crystallizes in the mono- clinic crystal system, but with space group B2/b (2=8).These silicates also contain rare-earth-metal ions in two different sites, but the two sites have a similar coordination. The first site (B,, CN=7) is surrounded by five oxygens belonging to SiO, groups and two free oxygens, whereas for the second site (B,, CN =6) there are four SiO, oxygens and two free oxygens coordinating to RE. Both B, and B, sites have C, point symmetry, as do the Al and A, sites, and the difference between lot n 7.5 5.0 2.5 250 300 350 400 450 500 wavelengthtnm Fig.5 Excitation spectra of Dy3+ emission in X,-Y1.934Bi0.006Dy0.06Si05 (a) and X2-Y1.934Bi0.006D~0.06si05 (b)at room temperature (Aern =576 nm, 4F9/2-6H13/2 of Dy3+) 10 $6 2 250 300 350 400 450 500 wavelentgt hlnm B, and B, sites is very small.For clarity, all of these structural data are collected in Table 1. The Bi3+ ion has a 6s2 ground state configuration and a 6s6p excited state configuration. Its luminescence properties are usually discussed with respect to the energy-level scheme 'So <3P0<3P,<3P, <'P, .ll At room temperature only the allowed 1S04+3P1transitions often occur. Therefore, the exci- tation and emission bands for Bi3+ in X,-Y,SiO, and X2- Y2Si05 are simply ascribed to 1S0-3P, and 3P1-1S0transitions, respectively. Since the two excitation bands for Bi3+ in X1- Y2Si05 correspond to two different emission bands (Fig.l),it is assumed that two different luminescent centres exist for J. Mater. Chem., 1996,6(2), 265-269 267 Table 1 Structural data for RE2Si0,' type crystal system space group z RE site Xl monoclinic P21lC 4 A1 A2 x2 monoclinic B2fb 8 Bl B2 Bi3+. It is well known that the position of the 1S0-3P1transition is closely related to the extent of covalency, i.e., the 1S0-3P, excitation band shifts to lower energy if the extent of covalency of the Bi-0 bond increases.17 As mentioned above the A, site (CN=7) shows higher covalency than the A, site in X1- Y,SiO,; therefore, the excitation band at lower energy (372 nm) can be assigned to Bi3+ on an A, site, while that at higher energy (306 nm) to Bi3+ on an A, site.The Stokes shifts for Bi3+(Al) and Bi3+(A2) luminescences are 9200 and 2700 cm-', respectively. This is in agreement with the general observation that the Stokes shift increases if the coordination number of Bi3+ increases (or space available for Bi3+ increases).18 Note that the emission intensity of Bi3+(A1) is much stronger than that of Bi3+(A2). This is not surprising; because the ionic radius of Bi3+ is larger than that of Y3+ (rBi3+=117 pm, ry3+ = 102 pm, both for CN =8), it is reasonable to assume that Bi3+ fits the nine-coordinate A, site better than the seven-coordinate A, site by replacing the Y3+. As a result the concentration of Bi3+(A1) is higher than that of Bi3+(A2).In view of the low doping concentration (0.3%) of Bi3+, no concentration quench- ing can occur, so that Bi"(A,) shows a much higher emission intensity than Bi3+(A2). Bi"(A,) plays a minor role in the luminescence of X1-Y 1.994Bi0.006Si05 and will be neglected in the following discussion. Because the Bi3+ ion has asymmetrical excitation and emis- sion bands in X2-Y,Si05, we can suppose that the Bi3+ ions have been included into the B, and B, sites simultaneously. These two types of Bi3+ ions cannot be distinguished clearly due to the similarity between the B, and B2 sites. The Stokes shift for the Bi3+ luminescence in X2-Y2Si05 is cu. 5000 cm-', which is far smaller than that (9200 ern-') for Bi3+(A1) in X1- Y,SiO,.The larger the Stokes shift, the less rigid the host lattice." Thus it can be assumed that the host lattice of X1- Y,Si05 is not so rigid as that of X2-Y2Si05. This is related to the lower temperature of formation of the former modification, and its higher coordination number (CN=7,9). According to the model proposed by Verwey and Blasse," the results concerning the emission intensities of Bi3+ can be accounted for. Because the host lattice of X,-Y,SiO, is not so rigid as that of X,-Y2SiOs, the expansion after excitation is less restric- ted by the surroundings of Bi3+ in the former than in the latter. As a result, more non-radiative relaxations take place in X,-Y,SiO, than in X2-Y,Si05, and Bi3+ shows a weaker emission intensity in the former than in the latter.The similar results for Eu3+ and Dy3+can also be explained in this way. In general, the Bi3+ emission is at lower energy if the coordi- nation number or space available for Bi3+ increases.18 This explains why Bi3+ luminesces in the blue region in X,-Y,SiO, (CN=9, 7), but in the UV region in X,-Y,SiO, (CN=6, 7). In agreement with the low local symmetry (C,) in X,-Y2Si0, and X,-Y,SiO,, the hypersensitive transitions of Eu3+ 5Do-7F, (red) and Dy3+ 4F9/2-6H13/2 (yellow) dominate in their emission spectra. The number of lines expected for the 5DO-7F0, 5Do-7F, and 5D0-7F, transitions of Eu3+ are 1, 3 and 5, respectively, under C, symmetry.20 The presence of two 5Do-7Fo lines for Eu3+ in X1-Y2Si05 indicates clearly that the Eu3+ ions occupy the A, and A, sites simultaneously, which agrees well with the results for Bi3+ in X,-Y,SiO,.According to the same principle as that used when explaining the Bi3+ results, the 5Do-7Fo line at higher energy (579 nm) can be assigned to Eu3+(A1), the CN silicon- bonded oxygen free oxygen point symmetry C1 C1 C1 C1 other at lower energy (581 nm) to Eu3+(A2). On the other hand, only one 5Do-7Fo line (580 nm) appears for Eu3+ in X2-Y2Si05, indicating that the two sites (B, and B,) occupied by Eu3+ are very similar. This also agrees well with the structural characteristics, the results for Bi3 and previous observations' + on X2-Y2Si05. Furthermore, because the differences between the A, and A, sites in X1-Y,Si05 are larger than those between the B, and B2 sites in X2-Y2Si05, the spectral lines for Eu3+ and Dy3+ in the former become broader than those in the latter.This is the so-called effect of inhomogeneous broadening of spectral lines. The energy-transfer effect from Bi3+ to Dy3+ and Eu3+ can be accounted for from the viewpoint of spectral overlap in a qualitative way. No matter what the mechanism for the energy transfer from an excited ion S* to another ion A, the probability of energy transfer (PSA)is proportional to the spectral overlap between the emission band of S and excitation (absorption) band/lines of A.21 As far as the rare-earth-metal ions Dy3+ and Eu3+ are concerned, the positions of their excitation lines do not change much with the host lattice, i.e., 300-470 nm for Dy3+ f-f transitions and 320-530 nm for Eu3+ f-f transitions, as well as the charge transfer band (CTB) at 220-320 nm.Thus the position of the Bi3+ emission band plays a dominant role in the energy transfer from Bi3+ to Dy3+ and Eu3+. In X1-Y2Si05, the Bi3+ displays a strong emission from 390 nm to 460 nm, which overlaps with the moderately intense f-f absorption lines of Dy3+ (387, 452 nm) and the strong f-f absorption lines of Eu3+ (394, 465 nm). Thus, energy transfer from Bi3+ to Dy3+ and Eu3+ has been observed, but the transfer effect of Bi3+ is better with Eu3+ than with Dy3+. In X2-Y,Si05, the strong Bi3+ emission is restricted to the range 320-370 nm, which overlaps with the strong Dy3+ f-f absorp-tion lines (324 , 349, 367 nm) and the very weak Eu3+ f-f absorption lines (320, 365 nm).Consequently, more efficient energy transfer occurs from Bi3+ to Dy3+, but no energy transfer from Bi3+ to Eu3+ takes place. Conclusions The crystal structures of Y,SiO, (X, and X, modifications) have a great influence on the luminescence properties of Bi3+, Eu3+ and Dy3+ and on the energy transfer from Bi3+ to Dy3+ and Eu3+. X,-Y2Si05 is a better host for luminescence than X1-Y2Si05. We wish to thank the National Natural Science Foundation of China and National Committee for Science and Technology of China for financial support of this work. References J. Felsche, Struct. Bonding, 1973,13,99. J. Ito and H. Johnson, Am.Miner., 1968,53,1940. E. M. Rabinovich,J. Shmulovich,V. J. Fratello and N. J. Kopylov, Am. Ceram. SOC.Bull., 1987,66,1505. J. Reichardt, M. Stiebler, R. Hirrle and S. Kemmler-Sack, Phys. Status Solidi, A, 1990,119,631. T. E. Peters, J. Electrochem. Soc., 1969, 116,985. A. H. G. de Mesquito and A. Bril, Muter. Res. Bull., 1969,4,643. M. Leskelti and J. Suikkanen, J. Less-Common Met., 1985,112,71. 268 J. Muter. Chem., 1996, 6(2), 265-269 8 J. Shmulovich, G. W. Berkstresser, C. D. Brandle and A. Valentino, J. Electrochem. SOC., 1988,135,3141. 15 16 J. C. Bourcet and F. K. Fong, J. Chem. Phys., 1974,60,34. M. J. J. Lammers and G. Blasse, J. Electrochem. SOC., 1987, 134, 9 10 11 12 13 14 D. Meiss and S. Kemmler-Sack, Phys. Status Solidi, A, 1991, 124, 371. D. Meiss and S. Kemmler-Sack, Muter. Chem. Phys., 1993,35, 114. G. Blasse, Muter. Chem. Phys., 1987,16,201. J. Lin and Q. Su, Muter. Chem. Phys., 1994,38,98. J. Lin and Q. Su, J. Alloys Comp., 1994,210, 159. Q. Su, Z. Pei, L. Chi, H. Zhang, Z. Zhang and F. Zou, J. Alloys Comp., 1993,192,25. 17 18 19 20 21 2068. G. Blasse, J. Solid State Chem., 1972,4, 52. A. Wolfert, E. W. J. L. Oomen and G. Blasse, J. Solid State Chem., 1985,59,280. J. W. M. Verwey and G.Blasse, Muter. Chem. Phys., 1990,25,91. S. Zhang, Chinese J. Lumin., 1983,5, 18. G. Blasse, Philips Res. Reports, 1969,24, 131. Paper 5/03281D; Received 22nd May, 1995 J. Muter. Chem., 1996, 6(2), 265-269 269

ISSN:0959-9428    

DOI:10.1039/JM9960600265

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Synthesis and crystal structure of zeolite W, resembling the mineral merlinoite Anna Bieniok,"" Klaus Bornholdt,b Uwe Brendel" and Werner H. Baur" "Institutfur Kristallographie und Mineralogie, Universitat Frankfurt, Senckenberganlage 30, 0-60054Frankfurt, Germany bInstitutfur Physikalische Chemie, Universitat Hamburg, Bundesstr. 45, 0-20146 Hamburg, Germany Zeolite W, K'0.3 [Si21.7A110.3064]20H20,was synthesized and the influences of excess alkalinity and the K/( K +Na) ratio on the synthesis products were studied. This synthetic zeolite is very similar to the mineral merlinoite. Its crystal structure was refined by the Rietveld method using X-ray powder diffraction data. The crystal structures of merlinoites from two locations and of two rather different synthetic zeolites of MER framework topology are known so far.Even though the chemistry of the pore filling is very different in the four compounds, the geometry and the density of their frameworks are remarkably similar. Zeolite W is a synthetic phase that has the same framework topology as the mineral merlinoite. It was first synthesized in 1953 by Breck (communicated as an unpublished result to Sherman'), 24 years before the natural counterpart was disco- vered in cracks of a kalsilite-melilitite lava in Cupaello, Rieti in Italy., Zeolite W was synthesized by different routes in the system Na,0-K,0-Si02-A1,03 (see Sherman' and references cited therein) but was often misidentified because its X-ray pattern is similar to that of natural phillipsite. The synthesis products mentioned in the literature, namely K-M, K-H or Linde W, are identical with zeolite W.A useful comparison of these phases and related zeolites is given by Sherman.' Milton3 patented the synthesis of a potassium aluminosilicate phase which he called zeolite W, and described its adsorption proper- ties. Recently zeolite W was synthesized with Sr cations in the reactive geL4 Single crystals of a barium aluminosilicate (phase A) have the cation ratio,f= K/( K +Na), on the synthesis products were tested. The syntheses in which rn was varied were conducted at f=O.8, while rn= 1.4 was used for batches with varying f: The Si:A1 ratios were determined by energy dispersive analysis by X-rays (EDAX) (Philips SEM 505/D806 with EDAX- terminal PV9900) and a chemical analysis by inductively coupled plasma-optical emission spectroscopy (ICP-OES) was performed.Thermogravimetry (TG) and differential thermo- gravimetry (DTG) studies (Mettler TG50) were performed in an N, atmosphere with a heating rate of 10K min-' from 303 K to 873 K. In addition the sample was characterized by 29Si and 27Al MAS-NMR spectroscopy (Bruker MSL-400). Data collection and refinement Powder X-ray diffraction (PXRD) data were taken with Cu- Ka radiation on a Siemens D500 diffractometer at room The crystals were of tetragonal symmetry but had the same framework topology as merlinoite, although this was not known until later. This phase A, synthetic zeolite W and merlinoite were assigned the zeolite structure code MER7 when the structure of the natural compound was determined by Galli et d8from single crystal X-ray diffraction data.Additional structural data for a natural K,Ba-merlinoite from the Kola island were published later.' A structural refinement of syn- thetic zeolite W, which is only available as a fine powder, is reported in this study. In addition, the influence of two synthesis parameters, the excess alkalinity and the cation ratio, upon the crystallization has been investigated. Experimental Synthesis Zeolite W was crystallized in batches from gels with the follow- ing composition: xNa,O * yK20* A1,0, * 29Si02 *400H@, with 4.3<x<6.6 and 17.0<y<26.3. The NaOH and KOH pellets were dissolved in distilled water. The Si02 (precipitated silicic acid, SiO, -0.5 H20; Merck) was added slowly and the resulting mixture was shaken for 30 min.Then, aluminate solution [100g solution containing 0.1 mol A1203 and 0.22 mol Na,O (Riedel de Haen, Merck)] was added and the shaking was continued for another 30 min. The resulting gel was transferred into PTFE vessels which were placed in stainless-steel auto- claves. The crystallization was carried out at 423 K for 24 or 29 h. The products were washed to neutral pH and dried overnight at 373 K. The effect of the excess alkalinity, rn =(K +Na -Al)/Si, and temperature. Rietveld-type crystal structure refinements were been obtained in a high temperature/high pressure ~ynthesis.~?~ performed using the program system GSAS." Experimental conditions, crystallographic data and definitions are given in Table 1.The refinement started in space group Irnrnm using the structural parameters determined for natural merlinoite.8 During the first refinement cycles eight geometric observations were used to restrain bond distances within the framework. Table 1 Experimental conditions and crystallographic data for zeolite W radiation CU-K? (Al=1.5406 A, A2 = 1.544 A) monochromated on graph- 20 scan range/degrees ite (002) (secondary) 7- 120 20 step width/degrees 0.02 step scan time/s 10 space group Immm cell constants/A 14.0948( 6) 14.2026( 6) 10.0421(5) cell volume/A3 20 10.3 (2) cell formula formula mass/g mol-' Ki0.3[%i.~Al10.3oa]'20H20 2674 Dc/g Cm -contributing reflections observations 2.24 1815 (a1+ ~2) 5649 profile parameters structural parameters 12 46 Rexp RWP RP 0.076 0.132 0.099 Difference Fourier maps were calculated and the largest peaks were tested as possible cation or water sites.Neutral atomic scattering factors were employed for all atoms. The two tetrahedral positions were refined using the scattering factor for Si, which means that the scattering contribution of the 10.3 A1 atoms per unit cell (out of 32 Si/A1 positions) were modelled solely by the displacement factors of the Si/A1 sites. Preferred orientation, assuming needle-shaped crystals, was corrected for." Subsequent refinement of profile and structural parameters led to a model with nine non-framework atoms and to values of the residuals of R,, =0.132, R =0.099 and GoF =3.00 (for definitions see Table 1).No geometric restraints on the bond lengths were used in the final stages of the refinement. Some of the population factors and the isotropic displacement param- eters (Uiso)of the nine non-framework positions were held constant during the refinement. It is difficult to distinguish between positions occupied by water molecules or by potass- ium atoms from bond length considerations alone, because their distances to framework oxygen atoms are similar (see Table 4 later). Therefore, occupancy factors and short inter- cationic distances were the criteria for assigning chemical elements to specific atomic positions.In this way 10.3 K atoms and 19.5 water oxygen atoms per unit cell were assigned to nine sites within the pores of zeolite W. Positions K(l) and K(2) are nearly fully occupied potassium sites, whereas the population factors of K(3) and K(4) were fixed at 50% occupation because of short distances to neighbouring water oxygen atoms. Thus, 10.3 potassium atoms are located in one unit cell, which happens to match exactly the value expected from the chemical analysis. Positions OW( 1)-OW( 5) must be occupied by water molecules, although some of their popu- lation factors refine to values greater than 1.00. The displace- ment and occupancy factors and the observed bond distances suggest in these cases the possibility of mixed population by water molecules and potassium ions.Splitting of these positions led to too short interatomic distances and a fixing of the population factors to 1.0 caused the framework atom positions to shift by large amounts. Some of the non-framework positions are highly disordered in the channel system of zeolite W, which is indicated by the large isotropic displacement factors of these sites [K(4), OW(l), OW(5)l. Because of the large and unre- liable values of the displacement factors the overpopulations at the sites of the water oxygen atoms, which are strongly correlated with them, cannot be taken at face value. They show us that this Rietveld-type crystal structure refinement is at the limit of what can reasonably be expected from powder diffaction data for a fairly complicated crystal structure.Results Synthesis The effect of increasing excess alkalinity, rn, on the crystalliz- ation products is illustrated in Fig. 1. Zeolite W crystallizes in a range of m between 1.4 and 2.9. At rn< 1.2, zeolite L is formed instead of zeolite W. Zeolite L crystallizes in the range rn =0.7- 1.2. The widest channel in zeolite L (LTL) is a twelve- ring (as opposed to an eight-ring in MER) but it has a slight!y higher framework density than zeolite W (!6.4 T per 1000A3 for LTL as compared to 16.0T per 1000A3 for MER;7 T= tetrahedral cation). An increase in the value of rn yields an aluminosilicate phase called L-20, which is not further ident- ified. The final product L-23 occurs at rn=3.2.This phase is similar to a synthesis product called zeolite M by Breck.12 The influence of the cation ratio,f, is less pronounced than the effects of the variation of the alkalinity; a lowering off from 0.8 to 0.4 in steps of 0.1 resulted in a 40% loss of crystallinity. The Si :A1 ratios of the fully crystalline products are given in Table 2. The Si: A1 ratio of the synthesized 272 J. Muter. Chem., 1996,6(2), 271-275 0 1.o 2.0 3.0 4.0 m Fig. 1 Influence of the excess alkalinity m on the crystallization products: 0,zeolite L; +, zeolite W; 0,L-20; A, L-23 Table 2 Effect of excess alkalinity, m, on the crystallisation products product m Si :A1 ratio zeolite L 0.8 3.0 zeolite W 1.4 2.1 zeolite W 1.8 1.80 zeolite W 2.2 1.68 L-20 2.6 1.39 L-23 4.1 1.16 aluminosilicates diminishes with increasing excess alkalinity.This is in accordance with the model of Lechert and co-worker~,'~.'~which describes the dependence of the Si :A1 ratio of the zeolite on the excess alkalinity of the batch. A chemical analysis of the sample with rn =1.4 gave a unit cell composition of Klo.76Nao.2s [~i21~7~~lo~3~64]nH20,indicating that a nearly pure potassium zeolite W was obtained. The sodium content can be neglected because the obtained value is smaller than the possible inaccuracy of the chemical analysis. The Si:Al ratio is 2.1. The sole incorporation of potassium into the zeolite implies that sodium is not necessary for the crystalliz- ation of zeolite W.The zeolite W crystals are prismatic with a square cross section and a length of 4-5 pm, as shown in Fig. 2. This is a novel morphology and has not been reported before. It resembles the shape of the particles shown in an SEM photo- graph by Belhekar et uL4 (Fig. 2B in ref. 4), which are described as hexagonal in the text, but in fact are clearly tetragonal. The TG and DTG curves (Fig. 3) show three well defined peaks at 323, 427 and 473 K, which are associated with a total mass loss of 15.7%. This corresponds to a water content of approxi- -1 pm Fig. 2 SEM image of zeolite W crystals 2.000mg that the sum of the exchangeable cations must equal exactly L--7--/the number of A1 atoms per unit cell.100-Crystal structure refinement 200-Zeolite W crystallizes in the orthothombic space grqup Immm y 300-with cell copstants a = 14.0948( 6) A, b= 14.2026( 6) A and c = t-10.0421 (5) A. For the final positional and displacement param- eters and occupancy factors of the eight framework and nine 400-extraframework positions, see Table 3; for interatomic distances and angles, see Table 4. The powder diffraction pattern with 500-observed, calculated and difference curves is shown in Fig. 5. There are two crystallographically different tetrahedral pos--0.002 -0.001 0 0.001 0.002 itions in the unit cell. A mean value of 2.5: 1 for the Si:Al mass loss/mg s-1 ratio can be obtained on the basis of the T-0 distances observed for these two tetrahedra.This is slightly in excess of Fig. 3 Thermogravimetry (TG, right) and differential thermogravime- what would be expected from the chemical compositioptry (DTG, left) curves for zeolite W in an N, atmosphere deduced above. The mean T-0 distances of 1.640 and 1.654 A observed for the two aluminosilicate tetrahedra aremately 20 molecules H20 per unit cell. For natural merlinoite a water loss of 15.57% is reported2 occurring in three steps between 293 and 523 K. Compared to merlinoite, water is Table 4 Interatomic distances (A) and angles (degrees) in zeolite W desorbed in zeolite K-W at a lower maximum temperature. The 29Si MAS-NMR spectrum shown in Fig. 4 reveals five T( 1 )-O( 1) 1.641(8) T( 2)-0 ( 2) 1.7OO(8) peaks according to the five possible silicon environments and T(1)-0(3) 1.624( 6) T( 2)-0 ( 4) 1.630(8) gives an Si:Al ratio of 2.3, which is reasonably close to the T( 1)-O( 5) 1.636( 9) T( 2)-0 ( 5) 1.628(8) ratio obtained from the chemical analysis.The 27Al MAS- T(1)-0(6) 1.658( 14) T( 21-0 (6) 1.657( 13) NMR spectrum exhibits one peak at 6-59.5, which means mean 1.640 mean 1.654 that only tetrahedrally coordinated aluminium is present in O(1)-T( 1)-O( 3) 11 1.1 (9) O(2)-T( 2)-O( 4) 114.6(9)) the sample. O(1)-T( 1)-O(5) 111.0(7) 0(2)-T( 2)-0 ( 5) 109.3(7) 0( 1 )-T( 1 )-0(6) 106.1 (7) 0(2)-T( 2)-0 ( 6) 103.2(7)The various results of the thermogravimetry and chemical O(3)-T( 1)-O(5) 1 10.5 ( 6) O(4)-T( 2)-O( 5) 113.2( 6) analyses (TG, DTG, EDAX and ICP-OES) yield a probable O(3)-T( 1)-0(6) 106.5(8) O(4)-T( 2)-O( 6) 104.4(7)chemical composition of the as-synthesized zeolite correspond- O(5)-T( 1)-0(6) 11 1.4(6) 0( 5)-T( 2)-0 (6) 111.7(6) ing to K10.3 [Si21.7Allo.3064] *20H20,whereby it is assumed T( 1)-O( 1)-T( 1) 136.4(9) T( 2)-O( 2)-T( 2) 142.4( 10) -93.1 T( 1)-0(3)-T( 1) 147.5( 12) T(2)-0(4)-T( 2) 144.7( 14) T( 1)-0(5)-T(2) 145.7( 6) T( 1)-0(6)-T(2) 144.7( 6) K( 1)-OW( 3) 2.99(2) 2 x 0W( 1)-OW (4) 2.80( 3) 2 x K(1)-0(4) 3.03(2) 2 x 0W( 2)-0 W ( 3) 2.98(2) 2 x K( 1 )-O( 11 3.06( 1) 2 x 0w(2)-0 ( 2) 3.15( 1) 4 x K(2)-0(2) 2.85(2) 2 x OW (3)-O( 6) 3.03( 1) 4 x K(2)-OW(2) 3.03( 1) OW( 3)-O( 4) 3.11(1) 2 x K(2)-OW( 1) 3.17(2) 2 x 0W( 4)-OW (4) 2.17( 4) K( 3)-OW( 4) 1.71(4) 2 x OW(4)-O( 5) 2.90(2) 2 x K(3)-OW(5) 1.93(4) 2 x OW( 4)-O( 4) 3.1 l(4) I I I I I I 1 I I 1 -60 -80 -1 00 -1 20 -1 40 K( 3)-0( 3) 3.12( 1) 2 x OW(5)-OW(5) l.89(8)s K( 4)-OW (4) 2.29( 3) 4 x OW(5)-OW(4) 3.01(3) 2 x K(4)-OW( 1) 2.654 1) 4 x OW(5)-O( 5) 3.30( 3) 4 xFig.4 29Si MAS NMR spectrum of zeolite W Table 3 Positional, thermal and population parameters for zeolite W" Wyckoff atom X Y z uiso/A2 occup. position 0.1106(3) 0.2471 (3) 0.15 17( 7) 0.021(1) 1.oo 0.2829( 3) 0.1094( 3) 0.1602( 7) 0.027( 1) 1.oo 0.1283( 9) 0.2862( 9) 0.0 0.024( 5) 1.oo 0.3171( 10) 0.1278( 1 1 ) 0.0 0.057( 7) 1.oo 0.0 0.2196( 10) 0.1748 (22) 0.059( 6) 1.oo 0.2884( 8) 0.0 0.2088( 20) 0.014( 5) 1.oo 0.1792(5) 0.1570( 5) 0.1840( 1 1) 0.018( 3) 1.oo 0.3666(6) 0.1633( 7) 0.2475( 17) 0.037( 4) 1.oo 0.1554( 7) 0.5 0.0 0.070( 6) 0.83( 1) 0.5 0.21 35( 9) 0.0 0.168( 6) 1.oo 0.5 0.5 0.332( 3) 0.111(9) 0.5 0.5 0.5 0.0 0.350 0.5 0.3534(9) 0.3827( 10) 0.0 0.200 1.33( 1) 0.0 0.5 0.5 0.030 0.76(4) 0.0 0.5 0.203 (2) 0.067(8) 1.54(3) 0.077 (2) 0.0 0.299( 3) 0.04( 1 ) 0.5 0.067 (4) 0.0 0.0 0.20 0.5 Parameters without standard deviations were kept constant during refinement.J. Muter. Chem., 1996,6(2), 271-275 273 I I I I I I I Ii -2.0 1 20 I 40 I 60 I 80 I 100 I 120 2eldegrees Fig. 5 Observed (+) and calculated profiles and difference plot for zeolite W with tick marks at the positions of the Bragg peaks insufficiently different to prove an ordering of the Si and A1 atoms over the two tetrahedral sites.The 10.3 A1 atoms per unit cell must be statistically distributed among the 32 tetra- hedral sites within the unit cell. The charge on the framework is balanced by 10.3 potassium ions located in four extraframework sites. They fill the cages and pores of the zeolite W channel system together with the 19.5 crystallographically detectable water molecules. Position K( 1) is 83% occupied and is coordinated by four framework oxygen atoms and two oxygen atoms of waaer molecules with distances ranging from 2.99(2) to 3.06(1)A. Site K(2) (fully occupied) is surrounded by twoo framework and three water oxygen atoms [2.85(2)-3.17(2) A]. Positions OW( l), OW(2) and OW(3) are the water oxygen sites, which complete the coordination spheres of these two potassium positions.The partiallyo occupied position K(3) forms only weak bonds of 3.12(1)A to the framework oxygen atoms. Position K(3) alternates with position OW( 5) which is also partly occupied. Position K(4) is a semi-occupied potassium position coordi- nated by water molecules OW(1) and OW(4) and has no short distances to any framework atoms. There are. a few impossibly short distances from cations to water molecules and between water molecules. Either they occur between partly occupied sites and the sum of their occupancy factors does not exceed 1.0 [e.g. OW(5)-OW(5)] or they involve atoms with extremely large displacement factors, which means that apprec- iably larger distances would still be possible within the distri- bution of electron density of these sites [e.g.K(3)-OW(5)]. In the structure refinement of zeolite W, all cations known from the chemical analysis were located in the pores of the framework. Cation positions with strong bonds to the frame- work oxygen atoms are determined with reliable displacement factors. It was more difficult to obtain reasonable positions and displacement factors for non-framework components, which are not or only weakly bonded to the framework. Their distribution in the pores of zeolite W resembles the electrolytic solution described by Baurl' for the cation and water molecules in the large pore of the faujasite crystal structure, but since the pores in zeolite W are clearly smaller the analogy may be tenuous. Discussion The double eight-rings (D8R) located in the origin and in the centre of the body-centred unit cell can be taken as the secondary building units of the tetrahedral framework structure of zeolite W.The D8R units are connected via four-rings. The most symmetric space group possible for this framework is 14/mmm, which can be called its topological symmetry.16 This framework structure can also be described by linking double- crankshaft chains to form a 4.S2 two-dimensional (2D) network with the sequences SSSSSSSS and SCSCSCSC in adjacent 274 J. Muter. Chem., 1996,6(2), 271-275 eight-rings (net no. 17 of 'untwisted' frameworks in ref. 17). Three types of polyhedral cages are found in this flexible framework." The D8R units, which are octagonal prisms (face symbol 4882, label opr), large cages (48448482, pau), which are similiar to gmelinite cages but based on eight-rings instead of six-rings, and small cages (4284, ste), which are open tetragonal prisms.The pau cages (the nomenclature for the cages is taken from ref. 18) are arranged in the c direction alternating with D8R units. There are two pau cages and two D8R units in each unit cell. The ste cages are located between the D8R units and between the pau cages in the a and b directions. There are four of these small cages per unit cell. The access from the pau to the ste cages is controlled by deformed single eight-rings, which are preferentially blocked by cations. These are the locations of 71% of all potassium in zeolite W [K(1) and K(2)].The ste cages are centred by water molecules [OW(2)], their eight-ring openings in the c direction are blocked by OW(3) water molecules. The pau cages are filled by potassium position K(4) and two water molecule sites [OW(l) and OW(4)]. The D8R units are filled with water molecules [OW(5)] and potassium [K(3)], which centres the single eight-rings of the unit. Fig. 6 demonstrates the construction of the pau cages and the D8R units by the combination of the double-crankshaft chains. In zeolite W the D8R units are elliptically deformed with their long axes parallel to each other, thus reducing the topological four-fold symmetry to two-fold symmetry (Fig. 7). This might be due to the influence of the different cation positions at the ends of the long and short elliptical axes of the D8R unit, respectively. A similar deformation is observed in natural Ca-8 and K,Ba-merlin~ite.~ Unlike this, the synthetic Ba-containing phase A6 crystallizes with tetragonal symmetry in space group 14/mmm.The nearly fully occupied potassium positions K(l) and K(2) in zeolite W are also observed in merlinoite' (as the partially occupied K,Ba sites). A position similar to K( 3) is described as a cation position as well, but it is shifted to smaller values in z, thus it is located outside the D8R unit. Table 5 shows similarities and dissimilarities of the structures of both the natural and synthetic phases with framework topology MER. The chemical composition given for phase A studied by Solov'eva et aL6 is different from the approximate formula stated in the original paper, where it is given as Ba2[X] BaC1, [( Si,A1),OI6] [with X either (Si,Al)(OH,O), or (OH,Cl)].The chemical information given in the original paper Fig.6 Projection of the structure of zeolite W along the [lo01direction. Combination of double-crankshaft chains forms D8R units and pau cages, which are filled by potassium ions (large circles) and water molecules (small circles) Table 5 Characteristics of natural and synthetic compounds of framework topology MER ~ merlinoite, Rieti, Italy merlinoite, Kola Island phase A zeolite W composition" Si:Alb framework density' sp?ce group a/+blbCI4 VIA3 K,Ca2 [A19Si23064] *-24H20 NaK,Ba, [Al,2Si,,064] -20H20 Ba,, [AllZSi20064] Cl,(OH),2.44 16.0 lmmm 14.116 14.229 9.946 1997.7 1.58 15.8 lmmm 14.099(5) 14.241(5) 10.08(1) 2023.9 10 As determined by chemical analysis.' 1.75 16.0 14/mmm 14.194( 6) 14.194(6) 9.934( 6) 200 1.40 4 T per lo00 A3. 15.9 lmmm 14.0948 (6) 14.2026( 6) 10.0421(5) 2010.3 9 extraframework sites 13 " Idealised chemical composition. ditions, is much more similar to the natural phase merlin~ite'.~ than the synthetic material described by Solov'eva et al. Programs used: GSAS-General Structure Analysis System;" SADIAN90 (atomic distances and angles);20 POWDER CELL 1.5, Program for representation and vari- ation of crystal structures;21 STRUPLO, polyhedral represen- tation of crystal structures.22 The authors acknowledge Matthias Feuerstein for the NMR results and Doris Budel for carrying out the thermal analyses.References 1 J. D. Sherman, in Molecular Sieves 11, ed. J .R. Katzer, ACS Symp. Ser. 40, American Chemical Society, Washington, 1977, p. 30. 2 E. Passaglia, D. Pongiluppi and R. Rinaldi, N. Jb. Miner. Mh., 1977,355. 3 R. M. Milton, Zeolite W, US Patent, 3.012.853, 1961. 4 A. A. Belhekar, A. J. Chandwadkar and S. G. Hegde, Zeolites, 1995, 15, 535. 5 I. A. Belitskiy and V. S. Pavlyuchenko, Dokl. Akad. Nauk SSSR, 1967,173,654. 6 L. P. Solov'eva, S. V. Borisov and V. V. Bakakin, Sou. Phys. Crystallogr. (Engl. Transl.), 1972, 16/6, 1035. 7 W. M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, Butterworth-Heinemann, London, 1992. 8 E. Galli, G. Gottardi and D. Pongiluppi, N. Jb. Miner. Mh., 1979, 1, 1. 9 S. V. Baturin, Yu. A. Malinovskii and I. B. Runova, Mineral. Zh., 1985,7,67 (in Russian). 10 A. C. Larson and R. B. Von Dreele, GSAS-General Structure Analysis System, LANSCE, Los Alamos National Laboratory, Los Alamos, NM, 1994. 11 W. A. Dollase, J. Appl. Crystallogr., 1986,19,267. 12 D. W. Breck, Zeolite Molecular Sieves, R. E. Krieger Publishing Company, Malabar, FL, 1974. 13 H. Lechert, H. Kacirek and H. Weyda, in Molecular Sieves, ed. M. L. Occelli and H. E. Robson, van Nostrand Reinhold, New York, 1992, p. 494. 14 H. Lechert, P. Staelin and C. Kuntz, Zeolites, 1995, submitted.15 W. H. Baur, Am. Mineral., 1964,49,697. 16 G. Gottardi, Tschermaks Mineral. Petrogr. Mitt., 1979,26,39. 17 J. V. Smith, Am. Mineral. 1978,63,960. 18 J. V. Smith, in Zeolites: Facts, Figures, Future, ed. P. A. Jacobs and R. A. van Santen, Elsevier, Amsterdam, 1989, p. 29. 19 R. J. Donahoe, B. S. Hemingway and J. G. Liou, Am. Miner., 1990, 75, 188. 20 W. H. Baur and D. Kassner, Z. Krist. Suppl. Issue, 1991,3, 15. 21 W. Kraus and G. Nolze, POWDER CELL 1.5, Bundesanstalt f. Materialforschung und -prufung, Berlin, 1994. 22 R. X. Fischer, A. le Lirzin, D. Kassner and B. Rudinger, 2.Krist. Suppl. Issue., 1991,3, 75. Paper 5/03508B; Received 1st June, 1995 I. Fig. 7. Polyhedral representation of the framework of zeolite W with the unit cell outlined is contradictory and inconclusive.In view of the crystal struc- ture determinations performed on the merlinoites and on zeolite W, the interpretation given in Table 5 for the chemical composition of phase A appears justified. Compared to the natural merlinoite samples, which have four or five chemically different cations to balance the net charge of the framework, synthetic zeolite W is a nearly pure potassium form. The upit cell volume of all four phases is extremely close to 2000A3 (maximum deviation 1.3%). If we consider cell constant deter- minations from other Na- and/or K-containing samples of zeolite W, which were reported in the literature,lg the maximum deviation between the cell volumes increases to only 2.6%. The same is true for the deviations of the values of the individual cell constants, which do not differ by more than 1.5% from each other. This similarity is also evident when we compare the frame- work densities of the four samples. This is quite remarkable in view of the fact that the filling of the pores is chemically and geometrically very different in the four compounds. The present crystal structure refinement of zeolite W shows that this material, when synthesized under hydrothermal con- J. Muter. Chem., 1996,6(2), 271-275 275

ISSN:0959-9428    

DOI:10.1039/JM9960600271

出版商:RSC    

年代:1996

数据来源: RSC