J. Chem. SOC., Faraahy Trans. I , 1984,80, 1367-1376 Properties of Y-type Zeolites with Various SiliconlAluminium Ratios Obtained by Dealumination with Silicon Tetrachloride Distribution of Aluminium and Hydroxyl Groups and Interaction with Ethanol BY LUDMILA KUBELKOV~~,* VLASTIMIL SEIDL,~ JANA NOVLKOVA, S O ~ A BEDNMOVL AND PAVEL JfRe J. Heyrovskg Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Machova 7, 121 38 Prague 2, Czechoslovakia Received 28th March, 1983 Y-type zeolites with %/A1 ratios of 3.420, prepared by the dealumination of NaY (%/A1 = 2.5) with SiCl,, have been characterized using X-ray diffraction analysis, sorption- capacity data and by obtaining infrared spectra of the framework, OH groups and adsorbed pyridine. Dealuminated zeolites exhibit good crystallinity and some aluminium released from the lattice remains in extralattice positions in the zeolite cavities or in an amorphous phase to provide strong electron-acceptor sites.The zeolites were obtained directly in the H form; further exchange with NHZ affected their proton-donor properties only slightly. In addition to weakly acidic or non-acidic hydroxyls (bands at 3740 and 3620 cm-l), strongly acidic framework hydroxyls were present (bands at 3630 and 3560 cm-l) in amounts dependent on the amount and distribution of aluminium. An analysis of substances released during temperature- programmed desorption from zeolites with preadsorbed ethanol revealed the influence of dealumination on the catalytic activity: the maximum of evolved ethylene as a main product was reached on moderately dealuminated zeolites (Si/Al = 3.4-6) with an appreciable number of both strong electron-acceptor and strong proton-donor sites.In 1980 Beyer published a new method for the thermochemical modification of zeolites1 consisting of the substitution of aluminium in the lattice by silicon using the reaction of the zeolite with SiCl,. Studies have so far concerned the struct~rel-~ of the highly dealuminated Y zeolites obtained by this method (Si/Al = 20-50), their selectivity for adsorption of n-hexane, n-butane, benzene, ammonia and water,l a demonstration of the presence of strong proton-donor sites4 and a comparison with HZSM-5 and HZSM- 1 1 with regard to isomerization and hydrocracking of n-decane on Pt-loaded zeolite^.^ The possibility of the modification of mordenites has also been rep~rted.~ In this work we deal with Y zeolites with Si/Al ratios in the range 3-20, taking into account the distribution of aluminium between the lattice and extralattice positions, the electron-acceptor properties and the types and acidity of hydroxyl groups.[This subject was considered very briefly in ref. (6).] The catalytic activity is demonstrated by the data obtained from the temperature-programmed desorption (t.p.d.) of ethanol. t Present address: Institute of Chemical Technology, Department of Mineralogy, Suchbatarova 5, 166 28 Prague 6, Czechoslovakia. 13671368 PROPERTIES OF Y-TYPE ZEOLITES EXPERIMENTAL Dehydrated NaY (provided by the Research Institute for Oil and Hydrocarbon Gases, Bratislava) was dealuminated by SiCl, according to the procedure described in ref.(1) at reaction temperatures of 620-770 K for a period of 1-2 h. After purging with dry nitrogen the product was washed until chloride ions were no longer present in the wash water (AgNO, test) and was dried at 320 K. Letters E-B (in order of decreasing A1 content) were used to designate the dealuminated zeolites (table 1). Some samples were subjected to cationic exchange with a Table 1. Chemical composition and sorption capacity of dealuminated Y zeolites Si/A1 100 Na/A1 capacity zeolite (molar ratio) (molar ratio) /mmol (Ar) g-la NaY NH,Y Y-E Y-E/H Y-D Y-D/H Y-c Y-C/H Y-B Y-B/H Y-Ab 2.55 2.50 3.45 3.40 4.9 5.1 5.7 5.75 19.9 19.1 43.4b 99 27 12 7 6 3 5 2 13 7 - 10.9 - 9.2 9.3 9.2 9.4 9.4 - - - a Per g of dry sample; obtained from H.K. Beyer. 0.5 mol dm-3 NH,NO, solution at 350 K. The usual designation NH,Y has been used for the initial Y zeolite (HY after deammoniation and dehydration) and the terms E/H to B/H for the dealuminated zeolites. Term A is reserved for the sample kindly provided by H. K. Beyer (table 1). Prior to measurements of sorption capacity, infrared spectra of OH groups and of adsorbed pyridine and temperature-programmed desorption of ethanol, the samples were dehydrated and deammoniated at 670 K and lo-, Pa. The sample composition was obtained by chemical analysis, and the sorption capacity was determined by adsorption of Ar at 78 K and 13.3 kPa. X-ray powder patterns were measured on a Geigerflex difractometer, and the unit-cell edge dimension was obtained by averaging the values calculated from the individual diffractions in the range t9 = 245".The mid-infrared (i.r.) spectra of zeolites contained in KBr pellets (0.1 wt %) were recorded on a Nicolet MX-1E Fourier-transform infrared spectrometer. The i.r. spectra of the hydroxyl groups and adsorbed pyridine were measured on a Beckman IR-7 spectrometer using zeolite plates of 5-8 mg cm-2 thickness. Interaction with pyridine was carried out for 30 min at 430 K and pyridine pressure of 0.56 kPa. Then the weakly adsorbed species were removed by evacuation at 520 K for 45 min and the spectrum was recorded after cooling to ambient temperature. The numbers of strong proton-donor and electron-acceptor sites were evaluated from the heights of the 1545 and 1455 cm-l bands of the pyridinium ions and pyridine-aluminium complexes, respectively, with the assumption that the extinction coefficients remain constant. The values used,' cpyH+ = 9.7 x cm2 molecule-l [in good agreement with value reported in ref.(S)] and cpyAl = 30.5 x cm2 molecule-l, were determined by the adsorption of successive doses of pyridine on H,,Na,Y zeolite in its hydroxylated and dehydroxylated forms. The conversion of ethanol was followed in the t.p.d. experiments. 10 pmol of alcohol were adsorbed at ambient temperature onto 0.1 g of zeolite. After adsorption for 1 h the sample wasL. KUBELKOV~, v. SEIDL, J. NOVAKOVA, s. BEDNA~OVA AND P. J ~ R ~ J 1369 heated at a rate of 6 K min-'. The maximum pressure of the desorbed vapours did not exceed 10-1 Pa.The desorbates were removed by the vacuum system of an M 1305 mass spectrometer which was used for their analysis and by auxiliary diffusion and ion vacuum pumps. RESULTS AND DISCUSSION CHEMICAL ANALYSIS AND SORPTION CAPACITY The results of chemical analysis (table 1) demonstrate that the reaction of NaY zeolite with SiCl, and subsequent washing with water are accompanied by a decrease in the mole fraction of aluminium [Al/(Si+Al)] from 28% in NaY to 4% in the dealuminated sample Y-B. This method can thus be successfully used for the preparation of zeolites of both high' and intermediate silica contents. It also follows from table 1 that the Na/Al ratio in the initial zeolite approaches unity, in agreement with the fact that aluminium in synthetic Y zeolites is localized in the lattice and sodium ions compensate the lattice charge.In dealuminated zeolites this ratio is about one order of magnitude lower, and thus sodium does not play an important role in charge compensation. The low ratio shows that the zeolites were obtained directly in the acid form, which was confirmed by the i.r. spectra of hydroxyl groups in the dehydrated samples. Consequently, NH, exchange did not increase the acidity substantially (table 1). The sorption-capacity values correspond to good crystallinity of the dealuminated samples (table 1). A decrease in this value relative to the initial zeolite corresponds to data published for stabilized zeolite^.^^ lo An even lower value was cited in ref.(1 1) for a highly dealuminated Y zeolite prepared by combined hydrothermal treatment and acid leaching E8.6 mmol (Ar) g-l]; sample A, provided by H. K. Beyer, yielded a value comparable with that obtained for our samples (table 1). Although the decrease in sorption capacity can be explained by lattice contraction and filling of the cavities by Al-containing species, the presence of a small amount of amorphous phase cannot be excluded for any of the dealuminated zeolites. X-RAY DIFFRACTION ANALYSIS AND I.R. SPECTRA OF SKELETAL VIBRATIONS X-ray powder patterns with very sharp and well defined peaks confirmed the relatively perfect structural arrangement of the dealuminated Y zeolites. All the diffraction peaks of NaY zeolite were found in the powder patterns of the dealuminated zeolites, with shifts in position corresponding to the respective changes in unit-cell dimensions and with changes in relative intensity in character with the literature data.l Similarly, the i.r.spectra of the skeletal vibrations contained the same type of bands as Nay, as can be seen from fig. 1. It is known that substitution of aluminium in the zeolite lattice by silicon leads to a contraction of the lattice and to a change in the bond orders, appearing as a shift of the bands of most of the skeletal vibrations to higher wavenumbers. A linear dependence has been foundl2?l3 between the position of a given band and the mole fraction of aluminium in the lattice for the faujasite-type synthetic zeolites and Y zeolites dealuminated by chelates. Recently this has been confirmed for stabilized zeolites and for hydrothermally dealuminated Y zeolites through a comparison of the values of the lattice Si/Al obtained from i.r.data and from the results of magic- angle-spinning nuclear magnetic resonance using 29Si.147 l5 In fig. 2 the position of the band arising from the internal asymmetric stretching vibration of the lattice tetrahedral2 is plotted against the mole fraction of aluminium determined by chemical1370 PROPERTIES OF Y -TYPE ZEOLITES 10.3 I I 1 I I L 1100 9 00 700 500 wavenum ber/cm Fig. 1. Mid-i.r. spectra of the zeolites NaY (-) and dealuminated Y-C (---) and Y-B ( . . .). analysis, for X and Y zeolites obtained by direct synthesis and for dealuminated Y zeolites. A reasonable linear relationship is found (fig.2) for data from synthesized zeolites (with Si and A1 in the lattice) and from highly dealuminated Y zeolites, while the data of samples Y-C to Y-E exhibit a marked deviation. This fact can be explained by the suggestion that some A1 in the moderately dealuminated zeolites is located in the extralattice positions. The ratios of the number of Si atoms to the number of A1 atoms in the skeleton (Si/Als), which correspond to the respective band positions according to the above relationship, are listed in table 2. These values indicate that only 70-60% of the total amount of A1 is located in the skeleton of moderately dealuminated zeolites. The same Si/Al, ratios are retained even after NH,+ exchange. This conclusion is further supported by an analysis of the dependence of the unit-cell dimension (a) on A1 content.Breck16 published the following relationship for synthetic zeolites of the faujasite type with an Si/Al ratio of 1-3: a = 0.000868NA,+2.419~; NA1 = 192/(1 +Si/Al,). (1) Application of this relationship to the system studied here is thus an approximation; nonetheless, the experimentally determined unit-cell dimensions of moderately dealu- minated zeolites are systematically appreciably lower than the values calculated from eqn (1) using Si/Al ratios found by chemical analysis (table 2). Consequently, the value of Si/Al, calculated from the experimental values of the unit-cell dimensions is in reasonable agreement with the ratios obtained from i.r. data (table 2). Note that no substantial differences were found either in the positions of the i.r.bands or in the values of the unit-cell dimension of the highly dealuminated zeolites Y-B (Si/Al = 20) and Y-A (Si/Al = 43.5) (see fig. 2 and table 2). The aluminium content is apparently so low that the applicability of the above relationships is dubious.L. KUBELKOVA, V. SEIDL, J. NOVAKOVA, S. BEDNhfiOVA AND P. J i R e 1371 h ? \ ' 0 '\\ \ \ \ \ 0 1 2 3 lOAI/(Si + Al) Fig. 2. Dependence of the position (a), height (b) and product of height and half-bandwidth (c) of the asymmetric stretching vibration band of lattice tetrahedra on the mole fraction of A1 in the following zeolites: 0, synthesized X and Y; 0, NaY and dealuminated Y-E, Y-D, Y-C and Y-B; A, dealuminated Y-A. Table 2. Unit-cell dimensions (a), Si/Al ratios obtained from chemical analysis and Si/Al, ratios obtained from X-ray diffraction and i.r. data %/A1 a/nm Si/Al, (chemical sample analysis) exptl theor.X-ray i.r. NaY 2.5 2.469 1 2.4667 2.35 2.45 Y -E 3.4 2.4456 2.4570 5.3 5.2 Y-D 4.9 2.441 1 2.4473 6.6 7.2 Y-c 5.7 2.4323 2.4440 11.6 9.9 Y-B 19.9 2.4264 2.427 1 21.8 - Y-A" 43. 5" 2.427 1 2.4248 - - a Obtained from H. K. Beyer.1372 PROPERTIES OF Y-TYPE ZEOLITES Fig. 2 also demonstrates that the intensity of the band produced by internal asymmetric vibrations of the lattice tetrahedra increases with decreasing aluminium content. Simultaneously, the half-bandwidth, Av;, decreases ; nonetheless, the product of these two values also increases. These values depend on a change in the dipole moment and thus reflect the ionic content of the skeletal bonds.The trend found is in complete agreement with the results of theoretical calculation^,^^ according to which exchange of aluminium for silicon in the skeleton increases the ionic character of the bonds. HYDROXYL GROUPS The individual types of hydroxyl groups and their proton-donor properties were characterized using i.r. spectra and their interaction with pyridine. Fig. 3 depicts the spectra of hydroxyl groups in the HY zeolite and in the dealuminated Y-E/H to Y-B/H zeolites after dehydration and deammoniation at 670 K (the spectrum of sample Y-A is given for comparison). The participation of OH groups in the formation of stable pyH+ ions is apparent from fig. 4, where changes in the spectra of the HY zeolite and the dealuminated Y-C/H zeolite are shown.The numbers of strong proton-donor and electron-acceptor sites, calculated from the heights of the pyridine-species bands, are compared in table 3 with the numbers of lattice and extralattice aluminium atoms determined from the Si/Al ratio obtained by chemical analysis and from the lattice Si/Al, ratio derived from mid-i.r. data. These results indicate that dealumination has a marked effect on the properties of the Y zeolite and that several types of OH groups appear in dealuminated zeolites. (a) Strongly acidic hydroxyls with bands at 3630 and 3560 cm-l, analogous to the structural OH groups in the HY zeolite (bands at 3645 and 3550 cm-l). These were present in considerably smaller amounts in moderately dealuminated zeolites than in HY zeolite; highly dealuminated samples contained very few of these groups.In the HY zeolite only hydroxyls in large cavities formed stable pyH+ species, while all the structural OH groups of dealuminated zeolites took part in the formation of these species (fig. 4). Nevertheless, the number of stable pyridinum ions was always lower than the number of lattice aluminium atoms not compensated by Na+ cations (table 3). (b) Dealuminated zeolites contain a new type of OH group characterized by a band at ca. 3620 cm-l. The number of these groups decreased with dealumination (fig. 3). They were non- or only weakly acidic or were inaccessible to the pyridine molecule (fig. 4). It can thus be assumed that the appearance of these hydroxyls is closely related to the presence of extralattice aluminium.(c) Non-acidic SiOH hydroxyls with a band at 374Ocm-l were found in all the zeolites in amounts increasing with dealumination (fig. 3). Highly dealuminated zeolites contained a large number of these groups apparently as a result of the formation of structural defects and the presence of an amorphous SiO, or AlSiO phase. Some of them may exhibit acidic properties (fig. 4), probably evoked by aluminium. EXTRALATTICE ALUMINIUM It has been shown recently8~1a-20 that firmly bound pyridine complexes can be formed with both extralattice aluminium species (stabilized Y and AlHY zeolites) and lattice aluminium atoms after the release of the structural hydroxyls by dehydroxyl- ation. The structural hydroxyls of the HY zeolite are, however, stable at a temperature of 670 K: dehydroxylation hardly occurs, so that the zeolite contains only a negligible number of strong electron-acceptor sites (table 3).It has been found that dealuminated zeolites exhibit even greater thermal stability; thus the large number of strong electron-acceptor sites found in moderately dealuminated samples apparentlyL. KUBELKOVA, v . SEIDL, J. NOVAKOVA, s. BEDNAROVA AND P. ~ i ~ f i 1373 I 1 I I I _....... . . . . . . ..- .... ..*...... .....* ........ .' 6 . - . . . . . 1 I I I I 3500 3700 wavenurnberlcm -' Fig. 3. 1.r. spectra of the OH groups of (1) HY, (2) Y-E/H, (3) Y-D/H, (4) Y-C/H, (5) Y-B/H and (6) Y-A. provides evidence of extralattice aluminium (table 3). As the adsorption of pyridine measures the coordinatively unsaturated and accessible aluminium, the number of such sites may be lower than the total number of extralattice aluminium atoms (table 3).The formation of pyAl complexes in highly dealuminated Y-B/H (Si/Al = 20) and Y-A (Si/Al = 43.5) samples confirms the presence of extralattice aluminium species in these substances, in agreement with suggestion of cationic A1 implied from 29Si and 27Al n.m.r. studies., The appearance of extralattice aluminium is most probably caused by hydrolysis of chloride complexes that were not removed from the zeolite during its reaction with SiCl, and subsequent purging with dry nitrogen. The reaction with SiCl, can be formally described by the equation1 Na,(AlO,), (Si02)y + SiCl, +NaCl + AlC1, + Na,-l (A102)z-1 (Si02)u+1.PROPERTIES OF Y-TYPE ZEOLITES I I I I 1 I 1 3500 3600 3700 I I I 3500 3600 3700 wavenumber/cm -' Fig.4. 1.r. spectra of the OH groups of (a) HY and (b) Y-C/H after activation at 670 K in VQCUO (---), after interaction with pyridine at 430 K and evacuation at 520 K (---) and after evacuation at 680 K (. . . .). Table 3. Number of aluminium species (At, total, A,, skeletal and Aex, extralattice) and number of strong proton-donor sites (H), electron-acceptor sites (L) and Na ions ( N ) in the HY zeolite and dealuminated Y zeolites HY 18.8 19.3 0 7.9 0.15 5.1 Y-E/H 17.0 11.1 5.9 4.2 2.2 1.3 Y-D/H 12.4 8.8 3.6 5.4 1.9 0.4 Y-C/H 11.3 6.5 4.8 3.8 2.0 0.2 Y-B/H 3.9 - 0.9 0.75 0.3 Y -A 2.0 - - - - 0.2d 0.25 a Chemical analysis; from mid-i.r. data; from the heights of the 1455 and 1545 cm-l bands; from the height of the 1488 cm-l band.During washing with water, hydroxide complexes of aluminium are formed and transferred into solution, depending on the acidity of the suspension. From this point of view, different pH values of the suspension seemingly account for the absence of a decrease in the extralattice aluminum content with dealumination in our moderately dealuminated zeolites (table 3); for Si/Al = 3.4-5.7 the pH of the first suspension had values of 4.8, 2.5 and 2.8 for zeolites Y-E, Y-D and Y-C, respectively. The X-ray photoelectron spectroscopy data showed that the surface of dealuminated zeolites was enriched in aluminium; however, the surface Si/Al ratio did not exceed twice the bulk value. Extralattice aluminium is thus a bulk phenomenon.Because ofL. KUBELKOVP;, v. SEIDL, J. NOVAKOVA, s. BEDNA~~OVP; AND P. J ~ R ~ J 1375 A B/H C/HD/H E/H HY dealuminatedi J- b . 1 .1 .1 Y W c .C g -E 0.1 0.2 0.3 Al/(Si + Al) Fig. 5. Amounts of ethanol (a) and ethylene (b) released during the temperature-programmed desorption of preadsorbed ethanol on HY and dealuminated Y zeolites plotted as a function of Al/(Si + Al). the relatively low number of strongly acidic hydroxyls, this aluminium apparently helps to compensate the lattice charge by being present as cations or oxide clusters in the zeolite cavities; it may also be present in the amorphous phase. It is important from a catalytic point of view that this aluminium is a source of strong electron-acceptor sites. REACTIVITY The above results indicate that the overall amount of aluminium and its distribution affect both the electron-acceptor and proton-donor properties of the dealuminated zeolites, which are very different from those of the initial HY zeolite.Their influence on the catalytic activity is demonstrated by the transformation of preadsorbed ethanol. Fig. 5 shows the amount of unreacted alcohol and of ethylene, the main reaction product, as a function of the ratio Al/(Si+Al) for the zeolites studied. The amounts of desorption products were obtained from t.p.d. measurements by integrating the areas below the respective curves. Unreacted ethanol was released in the temperature range 250-470 K, ethylene at 470-570 K. From these data it follows that moderately dealuminated zeolites with an appreciable number of strong proton-donor and electron-acceptor sites have the greatest efficiency in ethanol transformation.The HY zeolite, with a large number of proton-donor sites and a negligible number of strong electron-acceptor sites, exhibits much lower activity, in a similar manner to the highly dealuminated zeolites with a low number of both types of site. The amount of extralattice A1 may thus be considered an important factor influencing the activity of the zeolites studied.1376 PROPERTIES OF Y-TYPE ZEOLITES CONCLUSIONS The reaction of NaY zeolite with SiCl, at 620-770 K followed by washing with water yielded Y zeolites exhibiting Si/A1 ratios of 3.4-20 and good crystallinity. Some of the aluminium removed from the lattice remains in extralattice positions in the zeolite cavities where it helps to compensate the skeletal charge.Aluminium may also be contained in the amorphous portion of the system. This phenomenon is apparently a result of hydrolysis of the chloride complexes of aluminium left in the zeolite after reaction with SiCl, and purging with nitrogen. The extralattice aluminium may be in either the cation or oxide form and acts as a strong electron acceptor. The number of A1 atoms and their distribution also affect the number and type of OH groups. In addition to non-acidic SiOH hydroxyls (the band at 3740 cm-l) and hydroxyls probably related to the presence of the extralattice A1 (the band at ca. 3620 cm-l), dealuminated zeolites contain strong proton-donor sites : structural hydroxyls corresponding to bands at 3630 and 3560 cm-l are similar to those present in the HY zeolite.Compared with stabilized Y zeolites, dealuminated zeolites exhibit other acid properties and different hydroxyl compositions, and thus constitute a related system with new properties. Moderately dealuminated zeolites were most active in the interaction with ethanol, resulting in the formation mainly of ethylene; these zeolites contain large numbers of both extralattice aluminium atoms and strong proton-donor sites. We thank Dr H. K. Beyer for providing the highly dealuminated Y-A zeolite and for stimulating discussions. H. K. Beyer and I. Belenykaya, in Catalysis by Zeolites, ed. B. Imelik, C. Naccache, Y. Ben Taarit, J. C. Vedrine, G. Coudurier and H. 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