6729
J. Phys. Chem. 1992,96,6729-6737
Characterization of Acid Sites in Beta and ZSM-26 Zeolites RamesbB. Borade and Abraham Clearfield* Department of Chemistry, Texas A & M University, College Station, Texas 77843 (Received: February 1, 1992; I n Final Form: April 10, 1992)
Beta (Si/Al = 5-20) and ZSM-20 zeolites with low Si/AI ratios have been synthesized, using tetraethylammonium hydroxide as an organic template, and characterized by XRD, SEM, TGA, and XPS. The X-ray diffraction study indicated that an analcimelike phase crystallizes along with zeolite Beta when the starting mole ratio Si/Al 5. X-ray photoelectron spectroscoW (XPS) showed that a single-component Gaussian-shaped Al(2p) peak is associated with the framework A1 species present in pure (no other crystalline and/or amorphous phase) Beta, ZSM-20, and HY zeolites. The presence of amorphous Al species produced a shoulder peak to the main Al(2p) peak at low-binding-energy values, whereas the presence of A1 species associated with the other zeolitic structure gave a shoulder peak at high-binding-energy values. The acidic properties of these zeolites were studied by temperature-programmed desorption (TPD) of pyridine, FTIR spectra in the hydroxyl as well as pyridine region, and X-ray photoelectron spectra of chemisorbed pyridine. TPD of pyridine revealed that the chemisorbed pyridine was desorbed at somewhat higher temperature from Beta zeolites as compared to ZSM-20 and/or HY zeolites. The N(1s) XPS spectra of pyridine chemisorbed on these materials indicated that the Beta,ZSM-20, and STY (steam-treated Y) zeolites contain three N(1s) component peaks, whereas the HY zeolite spectrum yielded two N(1s) component peaks. The N( 1s) binding energy data suggested that acid sites present in Beta, ZSM-20, and STY zeolites are stronger than those in HY zeolites. An IR study of zeolite Beta showed only one acidic hydroxyl band at 3602 cm-I, whereas ZSM-20 showed classical high-frequency (3644-cm-I) and low-frequency (3542-cm-I) bands similar to those observed in HY zeolite. The IR spectra obtained at a pyridine desorption temperature of 450 O C revealed that most of the pyridine was desorbed from Bronsted acid sites of HY zeolite but not from the Bronsted acid sites of Beta and ZSM-20 zeolites. From the comparison of these results with those obtained with HY zeolite, the following order for their acidic strength of the framework hydroxyls is derived: Beta > STY > ZSM-20 1 HY. The n-hexane cracking activity, a representative Bransted acid catalyzed reaction, was found to be consistent with the derived acidity order.
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Introduction Many of the commercial cracking catalysts contain a modified form of HY zeolite which possesses 3-dimensional 12-membered-ring pore openings with a faujasitetype structure.I Both, Betaz.' and ZSM-2O"J zeolites also have 12-membered-ring pore openings, but these zeolites have a higher silicon-&aluminum ratio than HY zeolite. The high silicon-to-aluminum ratio as compared to HY and the presence of 3dimensionall2-membered-ringpore openings suggest that these materials may have technological potential similar to the use of HY zeolite as a catalyst in refining and petrochemical proctsses. Zeolite Beta was initially synthesized by Wadlinger et al.6 using tetraethylammonium hydroxide as an organic template. The structure of Beta zeolite was described by Treacy and NewsamZ and Higgins et ale3 Beta zeolite in an intergrowth hybrid of two distinct but closely related str~ctures**~~' which have tetragonal and monoclinic symmetry. In both systems, straight 12-membered-ring channels are present in two crystallographic directions perpendicular to [Ool], while the 12-membered ring in the third direction, parallel to the c axis, is sinusoidal. The sinusoidal channels have circular openings (5.5 A), and the straight channels have elliptical openings. The only difference between the two polymorphs is in the pore dimension of the straight channels. In the tetragonal system, the straight channels have openings of 6.0 X 7.3 A, whereas in the monoclinic system they are 6.8 X 7.3 A. The first synthesis of ZSM-20 was reported by Ciric.* Valyocsik9 later reported an improved method of preparing the same material by replacing the tetramethyl orthosilicate by tetraethyl orthosilicate. Derouane et a1.I0 were able to index the powder pattern of ZSM-20 on the basis of a hexagonal unit cell with a = 17.3 A and c = 28.6 A. Fii16p4 et al. reported that its structure consists of supercages (as in faujasite structures) which are interconnected by circular 12-membered-ring openings to form large straight channels running along the [Ool] direction. The straight channels are interconnected through a two-dimensional pore system running parallel to the a axis. The openings of the latter pores are also formed from 12-membered rings, but (in this direction) with an elliptical shape. According to Newsam et al.,5 To whom all correspondence is to be addressed.
the structure of ZSM-20 is an intergrowth mixture of blocks with the cubic faujasite framework and EMT topology. In faujasite, supercages have 4 12-ring apertures arranged tetrahedrally, interconnecting adjacent supercages in a fawcentered cubic array. Two types of supercagesoccur in ZSM-20. The larger supercages have 5 12-ring apertures, 2 of which are aligned to form straight channels along the [Ool] direction. The smaller supercages have 3 12-ring apertures and provide lateral connections between channels. The shape-selective catalytic properties of zeolite Beta in the cracking of paraffins and in the isomerization of m-xylene have recently been investigated by Martens and Jacobs" and Corma et al.lz Ratnasamy and co-workers" studied m-xylene isomerization and toluene alkylation and disproportionation reactions. Wang et al.14 reported disproportionation and transalkylation reactions over zeolite Beta. One of its potential major applications seems to be in the catalytic hydrodewaxing of petroleum oils wherein it is able to lower the pour point of the oil by hydrodewaxing the *paraffins to branched isomers.lS several gr0upsl4~~~' studied the catalytic properties of ZSM-20 and found that ZSM-20 has high thermal stability and a higher activity for the n-hexane cracking reaction than does HY zeolite. It has been claimed that ZSM-20 can be used as a catalyst for a variety of hydrocarbon conversion reactions.10J620 These studies indicate that zeolite Beta as well as ZSM-20 may be of potential interest as a catalyst for hydrocarbon transformation reactions. Even though these zeolites have 12-membered-ring pore openings as in HY zeolite, they differ from each other in their channel structure and, hence, catalytic properties. Another important factor that controls catalytic behavior is the acidity of the zeolite. A literature survey suggests that, apart from a few report~,~uI tbere is little information concerning the characterization of the acidic properties of these zeolites. In the present work, we have synthesized zeolite Beta with varying Si/Al ratios and ZSM-20 zeolites using tetraethylammonium hydroxide as an organic template. The synthesized zeolites have been characterized by X-ray diffraction, scanning electron microscopy, chemical analysis, and thermogravimetric analysis. The acidic properties were studied by temperatureprogrammed desorption (TPD), IR spectroscopy and X-ray photoeIectron spectroscopy (XPS) using pyridine as a probe
0022-365419212096-6729$03.00/0 0 1992 American Chemical Society
6730 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 molecule, and the catalytic properties were studied using n-hexane cracking as a model reaction. The acidic and catalytic properties were then compared with the HY and steam-treated Y zeolites. The results demonstrate that Beta is the most highly acidic and most active catalyst of those examined for the n-hexane cracking reaction.
Experimental Section I. Syntbesii. zeolite Beta. The synthesis of zeolite Beta22s23 was carried out hydrothermally from the system containing 1Na2O-lAl2O3-xSiO2-27.5TEAOH-yH20. The value of x was varied from 10 to 30 and that of y from 400 to 450. Tetraethylammonium hydroxide (TEAOH, 40% solution in water, Alfa product), sodium aluminate (28.4% Na20, 46.8% A1203,24.8% H20), tetraethyl orthosilicate (TEOS, Alfa product), and demineralized water were used as reagents for the synthesis. In a typical preparation, 199.9 g of tetraethylammonium hydroxide solution and 4.26 g of sodium aluminate were added to a 500-mL beaker, and the mixture was stirred until a clear and viscous solution was formed. Then to this mixture was added 125.8 g of tetraethyl orthosilicate, which was followed by addition of 27.0 g of H20. The whole mixture was stirred vigorously for about an hour, and then the temperature was increased to about 333 K (using a hot plate). The ethanol formed by hydrolysis was allowed to evaporate. The gel which formed was then transferred to a Teflon-lined stainless steel autoclave and kept in an air oven for crystallization at 160-180 OC for 3-4 days. To stop the crystallization process, the autoclave was immersed in cold water. The solid product recovered by filtration was washed repeatedly with demineralized hot water until the pH of the fdtrate was -7.0. Finally, the product was dried at 120 OC in an air oven overnight and then calcined at 540 OC for 15 h to remove the organic material occluded in the zeolite pores. The Na form of zeolite Beta which was obtained was converted to the NH4+ form by repeated ion exchange with 2 M NH4Cl solution at 80 OC until the Na+ content in the zeolite was less than 0.05%. The protonated form was obtained by calcining the NH4+form at 450 OC for 15 h. ZSM-20. zeolite ZSM-20 was also synthesized hydrothermally using a starting chemical composition very similar to that of zeolite Beta and TEAOH as a source of the organic cation. The starting reaction gel composition was 1Na20-lAl2O3-30.9SiO227.5TEAOH-350H20. The crystalliition was carried out at 100 OC for 12-13 days. When the reaction was over, the autoclave was quenched in cold water and the solid product was separated from its mother liquor by filtration. The washing, drying, calcination, and ammonium-exchange procedures were very similar to those of Beta zeolite. ZROlite HY aod Steam-TreatedY. The precursor NH4Y zeolite sample was supplied by Strem Chemicals, Inc. The NH4+-exchange level was 821, and the Si/AI ratio determined by ICP analysis was 2.46. This sample was calcined at 300 OC for 8 h and then repeatedly exchanged with 2 M NH4Cl solution until the residual sodium content was less than 0.05%. A steam-treated Y (STY)sample was obtained by first placing ca. 5 g of N&Y sample in a quartz tube with an internal diameter of 12 mm. The sample was then carefully heated (heating rate 10 K/min) to 550 OC in the presence of a Nz flow (30 cm3/min). When the temperature reached 550 OC, water was passed (using a feed pump) through a heated line at a flow rate of 30 cm3/min for 4 h over the zeolite bed. After this treatment, the water flow was stopped and the sample was allowed to cool to room temperature in the presence of the N2 flow. The sample was then treated with the 0.1 N HCI solution to remove the extraframework material. II. Characterization. X-ray powder diffraction patterns were obtained with a Seifert-Scintag PAD-V automated powder diffractometer using Cu Ka radiation. Surface area measurements were done on an Autosorb-6 sorption system (Quanta Chrome Cow.) at liquid nitrogen temperature. The samples were degassed at 573 K and Torr for 24 h. Scanning electron micrographs were obtained at the Texas A & M Electron Microscopy Center
Borade and Clearfield using a JEOL JSM-25 S-I1 microscope. The samples were coated with a Au-Pd evaporated film. TG and DT analyses were carried out by using a Du Pont (Model 951) thermal analyzer. The sample, ca.30 mg, was placed in a qua@ bucket and heated under a N2atmosphere with a heating rate of 10 OC/min. Total analysis of the zeolite samples was carried out by dissolving the sample in hydrofluoric acid and then diluting with demineralized water to the desired volume. The resulting solution was analyzed for Si, Al, and Na by an inductively-coupled plasma method, using an ARL 3510 ICP spectrometer. IJI. Acidity Mewremeats. Pyridhe AasOrpbioa. All samples were activated in a conventional high-vacuum system (P Torr) at 400 OC for 16 h. The samples were cooled to 100 OC under vacuum and then exposed to pyridine vapors overnight. The excess pyridine was desorbed by evacuating the samples at 200 OC for about 16 h. These samples were used for TPD and XPS experiments. TPD of Pyriaiee, TPD spectra of pyridine from various samples were obtained by using a Du Pont (Model 951) thermal analyzer. The sample, ca. 30 mg, was placed in a quartz bucket, and adsorbed pyridine was desorbed by increasing the sample temperature under a N2 atmosphere with a heating rate of 10 K/min. The TPD spectrum (derivative plot) was obtained from this curve by plotting the rate of pyridine desorption as a function of temperature. XPS Measurements. The sample chemisorbed with pyridine was pressed into a pellet and then mounted on a stainless steel sample holder. The sample was evacuated in the pretreatment chamber to = lo-' Torr before being transferred to the analyzer chamber for XPS measurements. The XPS spectra of all samples were r d e d at room temperature by using an HP 5950A ESCA spectrometer. The X-ray source used was Mg K a (hv = 1253.6 eV). The spectrometer was operated at 20 mA and 15 kV and the analyzer at a constant pass energy mode (29.35 eV). The residual gas pmsure in the spectrometer chamber during the data acquisition was less than Torr. The measurements were performed in the following sequence: Si(2p), Al(2p), N(ls), O(ls), and C( 1s). All binding energy values were corrected for charging by assuming the binding energy of the Si(2p) level to be 103.3 eV. The accuracy of binding energy with respect to this standard value was within f0.2 eV. The intensities of various XPS lines were determined by using nonlinear background subtraction and integration of peak areas. All peaks were deconvoluted into one, two, or three components by keeping the full width at half-maximum (fwhm) constant in a particular spectnun and assuming that each peak has a Gaussian shape. In this deconvolution operation, the fwhm value of 2.0 eV was adopted since all other lines recorded, Si(2p), A1(2p), and O(ls), showed a fwhm value of 2.0 f 0.1 eV for a pure zeolite sample. Infrared Experiments. The IR spectra in the hydroxyl as well as pyridine region were obtained by using the following procedure. The samples were p r d into self-supporting wafers containing 12 mg of material. The wafers were mounted on stainless steel sample holders and introduced into a Pyrex vacuum cell which was designed to accommodate four samples at a time. The cell was connected to the vacuum system (P = Torr), and the samples were degassed at 400 OC for 16 h. Then the cell was closed, the samples were allowed to cool down to room temperature, and their IR spectra were recorded. Afterwards, pyridine vapors were admitted to the cell. Then the temperature of the sample was increased to 100 OC, and pyridine vapor was allowed to react overnight. Finally, excess pyridine was desorbed by evacuating the samples at desired temperatures for 16 h. The samples were cooled down to room temperature, and the spectra were recorded. All the spectra were recorded in the range of 4OOO-1OOOan-'with a 2-cm-I resolution using a Digilab FI'S-40 spectrometer. Cracking Activity. The n-hexane cracking activity was determined by using a quartz reactor with an internal diameter of 5 mm. Forty milligrams of the zeolite sample, in the form of 20-45-mesh-sizechips, was placed in the reactor and the remaining
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Acid Sites in Beta and ZSM-20 Zeolites
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6731 TABLE I: Bulk and surface Properties of Various Zeolites TG wt loss (%) surface due to Si/AI area
zeolite Beta/A Beta/B Beta/C Beta/D
ZSM-20/E ZSM-ZO/F
STY/G HY/H
I
I. 2 tl (deg)
Figure 1. X-ray powder diffraction patterns of various zeolites. Sample designation as in Table I. Asterisks denote peaks of analcime.
part filled by quartz chips, which served as a preheating zone. Prior to the reaction, the zeolite samples were activated carefully under flowing nitrogen (27 mL/min) at 100,200,300, and 400 OC for 40 min at each temperature. After activation, the catalyst temperature was reduced to 350 OC, and then the reactor was bypassed but kept at 350 OC for 30 min while the nitrogen was directed through a hexane saturator at 0 OC. The hexane flow (ca. 27 mL/min total flow) was then passed through the reactor with the catalyst at 350 OC. After 5 min on stream, a gas sample was injected into the gas chromatograph. Nitrogen was used as an internal standard. The activity based on the weight of hydrated zeolite was determined from the weighed sum of the products, including small amounts of c6 isomers. A Carle AGC-111 gas chromatograph with a 3-m Porapack N column (80-100 mesh) operated isothermally at 145 OC was used for the analysis of the cracking products.
Results a d Discussion I. Sample Chprrcteristia. XRD and SEM. The X-ray powder diffraction patterns of as-synthesized zeolite Beta with varying Si/Al ratios are shown in Figure 1. The XRD pattern of Beta is characterized by a combination of sharp and broad reflections. The values of the d spacings and the relative intensities of the reflections of sample A correspond to the values reported in the literature for zeolite Beta.3*14*21-24 However, the XRD pattern of Beta zeolite reported by various authors did not show the presence of a doublet near d = 4.1 15 A. We observed not only a doublet (with d = 4.1 15 and 4.060 A) but also some changes in the relative intensities of these peaks when the Si/AI ratio of the sample was varied. For example, the sample Beta/A shows a doublet with d values of 4.1 15 and 4.060 A. The intensity of the peak at d = 4.060 A in the case of Beta/B was found to be reduced significantly with respect to the peak at d = 4.1 15 A, and sample Beta/C shows nearly an absence of this peak. In contrast, the Beta/D sample that contains an analcimelike material shows the reverse order of intensity of those peaks and some additional
water
org
chem
XPS
(BET)
2.89 3.35 4.17 6.31 8.87 10.60
15.31 15.44 15.20 10.44 15.49 15.00
16.6 12.5 10.0 7.5 6.5 2.4 3.8 2.3
19.7 12.2 10.5 7.2 1.6 4.3 3.6 4.6
514 478 518 342 543 65 1 493 668
reflections which are not related to the analcime phase. The scanning electron micrographs of Beta zeolites showed uniform cuboid-shaped crystals with a crystal size of 0.15-0.35 pm. The change in the Si/Al ratio did not have any appreciable effect on the shape or size of the Beta crystals in the range studied. The Beta samples A-D showed an absence of amorphous matter. The X-ray powder patterns of as-synthesized ZSM-20 shown in Figure 1 (samples E and F) match well with the ZSM-20 samples of Dwyer et a1.16v18 and Derouane et Comparison of the XRD patterns of ZSM-20 with the HY sample shows, in addition to the faujasite reflections, some additional reflections related to the hexagonal system. For ZSM-20, the most intense reflection occurs near 28,5O. The XRD of ZSM-20/E shows a relatively large hump in magnitude in the region 28, 15-30°, as compared to the sample ZSM-ZO/F. This was suspected to result from the presence of some amorphous material in this sample. Scanning electron micrographs of ZSM-20/E indeed showed the presence of amorphous particles which are absent in the ZSM20/F sample. The crystals of ZSM-20 are multiply twined with a mean diameter of 0.2-0.4 pm. Bulk and Surface Characteristics. We have used the TGA weight loss due to the organic template as one of the criteria of purity of the synthesized samples. In agreement with the literature data, TGA curves of our Beta21924 and ZSM-2e6 samples showed four step weight loss curves (not shown). For zeolites, the first step is generally assigned to the desorption of physically adsorbed and/or occluded water. In the case of zeolite Beta, the first step was complete at about 150 OC, whereas for ZSM-20 it continued up to 200 OC. The water content was found to increase with Al content regardless of the zeolite crystal structure. This indicates the more hydrophobic nature of zeolite Beta relative to ZSM-20 zeolite (see water contents in Table I). The third step in the weight loss curves, between 150 and 1000 OC for Beta and 200 and 1000 OC for ZSM-20 zeolites, has been ascribed to the desorption/decompositionof TEAOH species stabilized on the outer surface of the crystallites, the occluded template in the zeolite pores, and TEA cations associated with the aluminum species which act as a framework charge compensating cations, respectively. The total weight loss due to TEA species for pure Beta and ZSM-20 samples (that do not contain extra crystalline and/or amorphous impurities and show the bulk and surface Si/Al ratios to be very close to each other, Table I) amounts to 15.2 0.2% (Table I). Very similar weight losses have been reported for Beta21q24and ZSM-2026 zeolites. The presence of an extra crystalline phase in Beta/D (analcime) and amorphous material in ZSM-ZO/E lowered the total organic content in these as-synthesized materials (when compared with their corresponding pure analogues). This reduction signifks that samples Beta/A, Beta/B, Beta/C, and ZSM-2O/F do not contain amorphous material on the surface or inside the zeolite pores/cavities. Comparison of the bulk and the surface Si/Al ratios of protonated samples, determined by chemical analysis and by the XPS method, respectively, provided another approach to evaluate the homogeneity of the sample. The Beta samples A-C showed the bulk and surface (Si/Al) ratios to be very close to each other, indicating that A1 species, mainly responsible for the acidity of the zeolites, are distributed homogeneously from the bulk to the surface of the zeolite crystallites. The BET surface area of these three Beta samples ( A X ) was within the range of 500 & 22 m2/g
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6732 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 TABLE U: XPS Dah of 7 ZeOllta Si/AI SUP) zeolite chemical XPS' BE* fwhm" BetalA 16.6 19.7 103.3 2.0 12.2 103.3 2.0 12.5 BetajB 103.3 2.0 10.0 10.5 BCta/C 7.5 103.3 2.0 7.5 BCta/D 103.3 2.0 6.5 1.6 ZSM-20JE 4.3 103.3 2.0 2.4 ZSM-ZOJF 3.6 103.3 2.0 3.8 STY/G 103.3 2.0 2.3 4.8 HY/H
Borade and Clearfield
BE
M2P) fwhm
74.2 74.5 74.5 74.8 75.5 75.0 75.1 75.1
"
O(1S)
BE
2.0 2.0 2.0 2.2 2.0 2.1 2.2 2.0
532.6 532.5 532.1 532.6 532.6 532.6 532.5 532.6
fwhm 2.0 2.0 2.0 2.2 2.0 2.2 2.2 2.0
BE 402.2 401.8 401.9 402.1 402.1 402.5 402.2 402.8
fwhm 3.0 3.2 3.1 2.4 4.0 2.4 2.2 3.3
"Binding energy and fwhm values are in eV. of the zeolite. As far as sample Beta/D is concerned, this sample showed uniform Al distribution but the surface area was found to be approximately 30% lower than that of pure Beta zeolite samples (see Table I). The low surface area is associated with the presence of a crystalline but dense analcime phase. The ZSM-ZO/E sample was observed by XPS measurements to have a lower (Si/Al) ratio on the surface than in the bulk, whereas sample ZSM-20/F showed the reverse. The lower BET surface area of ZSM-ZO/E (543 mZ/g) as compared to that of sample ZSM-20/F (641 m2/g) together with the XPS data clearly suggests that this sample contains extraframework Al species. The HY sample also showed a surface deficiency of aluminum. The STY sample displayed a uniform aluminum distribution from bulk to the surface of the zeolite crystallites, but the surface area was found to be considerably lower when compared with the H Y zeolite (see Table I). This signifies that even though the STY sample contains extraframework aluminum species because of the HCl treatment, these species are distributed homogeneously from the bulk to the surface of the crystallites, affecting the surface area and pore volume and not the Si/Al ratio. Thus, it can be concluded that Beta samples A-C are relatively pure materials; the sample Beta/D shows a homogeneous Al distribution but contains an extracrystalline impurity phase, and samples ZSM-20/F and HY/H are relatively pure materials as compared to ZSM-20/E and STY, respectively. II. Acidity Meuwemeats. XPS Study. N( 1s) X-ray photoelectron spectroscopy of chemisorbed pyridine has become a valuable technique for the identification and quantification of Brransted and Lewis acid sites present in various z e ~ l i t e s and ~'~~~ A1P04-5-based molecular sieves.z9 The binding energy (peak maximum) and the fwhm values of the Si(2p). Al(2p), O(ls), and N(1s) peaks are presented in Table 11. A binding energy (BE) of 103.3 eV of Si(2p) level was used as an internal reference. The BE of the O(1s) peak agreed with the reported value of the O(1s) peak for zeolites having Si/Al ratios of 2.3-15.0. The BE of the q l s ) peak of various zeolites under study was found to be 532.5 f 0.1 eV. The most significant changes were observed in the Al(2p) and N(1s) XPS peaks. The BE of the Al(2p) XPS peak of pure Beta zeolites was 74.4 f 0.2 eV. In the case of ZSM-20, HY, and STY zeolites, it was found to be somewhat higher (75.1 eV). This shift in the BE toward higher value with an increasein the Si/Al ratio is consistent of BarP' who m l a t e d this shift in the BE value with the-f with the covalency/ionicity in zeolite bonding. Okamoto et ala3' studied various zeolitesby XPS and concluded that the 4we charge on the silicon is reduced with increasing Al content of the zeolite, while the -ve charge on the oxygen is enriched. The charge density on the aluminum varies with the zeolite composition but increases with an increase in A1 content. Barr30reported that an increase in the Si/Al ratio, i.e., NaANaX-Nay-mordenite, etc., providcp a system in which the natural covalency of Si-O units in the silica is dominant and drives up the relative ionicity of the A l a and Na-O bonds. The relative Br0nsted acidity of these zeolite systems has been shown to follow a similar progression. Thus, at low Si/Al ratio Le., in the case of zeolite HA, the 0-H bond is reported to be covalent and the acidity weak. As the related amount of silica is increased with its covalent S i 4 bond, the ionicity of the 0-H bond increases. Based on this interpretation, the ionicity of the 0-H bond in Beta
C
/\ LL?&e+ 75 5
3
75 5
I
Binding energy (eY1
Figure 2. Al(2p) XP spectra of various zeolites. Sample designation as in Table I.
zeolite should be relatively high, and therefore, one should expect Beta zeolite to be a relatively stronger acid than ZSM-20 or HY zeolites. In addition to the shift in the BE value with Si/Al ratio, the Al(2p) XPS peak of some of the samples displays interesting features (Figure 2). Before discussing the deconvoluted Al(2p) XP spectra, one should realize that all single-component Si(2p), Al(2p), and O( 1s) peaks have fwhm values of 2.0 f 0.1 eV. The presence of extra crystalline and/or amorphous material is manifested as a shoulder to the main Al(2p) peak (Figure 2D and 2E). The interesting results here, however, lie in the observation that both samples exhibit a main peak at 74.7-75.5 eV (particular cam) but the shoulder peak appears at a high binding energy value (76.1 ev) for sample Beta/D and at low binding energy value (73.6 eV) for sample ZSM-20/F. The main Al(2p) peak, -74.7-75.5 eV, corresponds to the framework A1 species of Beta/ZSM-20 structure, whereas the low-binding-energy peak is due to the extraframework Al species present on the surface of the ZSM-20 c r y ~ t a l l i t e s . ~Here, - ~ ~ we assign the high-binding-energy Al(2p) peak (at 76.1 eV), present in sample Beta/D, to the framework Al species but associated with the analcimelike structure, since the XRD of this sample showed a mixture of Beta and analcime. The Si/AI ratio of pure analcime is reported to be 2.0. This is also an example which provides independent supporting evidence for the fact that there is a shift in the BE value with change in the Si/Al ratio of the sample. To the best of our knowledge, this is the first example where XPS has demonstrated its ability to distinguish two kinds of framework A1 species in the sample
Acid Sites in Beta and ZSM-20 Zeolites associated with two different zeolite crystal structures. It is important to mention here that in the STY sample, even though it contains extraframework Al species as indicated by the low BET surface area, the Al(2p) XPS peak does not contain a shoulder peak at a low-binding-energy value (Figure 2, sample G). The Al(2p) peak appears to have a perfect Gaussian shape with fwhm value of 2.0 eV. It is therefore, proposed that the extraframework A1 species present in the STY sample are polycationic oxidic aluminum species of the type Al(OH)2+as proposed by Lunsford et al.34*35 for their steam-treated HY and/or ZSM-20 zeolites. The solid-state NMR of their sample revealed that the extraframework Al was present in at least two forms, an octahedral form having a chemical shift of 0 ppm and a tetrahedral form having a chemical shift near 54 ppm. The species at 54 ppm was believed to reside in the 8 cages as Al(OH)2+ions, with Al being bonded to three framework oxygen atoms. Through inductive effects, the structural framework OH groups that are in the region of this cation become strongly acidic. If these extraframework A1 species interact with at all the zeolite framework, then it may well be that being a part of the zeolite framework in the sense of charge compensating cations, the BE’S of these extraframework tetrahedral A1 species and framework A1 species may not differ considerably. Before discussing the N(1s) XPS spectra, we shall discuss the pyridine-to-aluminum ratios of the zeolites (N/Al), which is a qualitative measurement of the acidic nature of the A1 species (see Table IV). It is well-known that A1 species present in the zeolite framework (in their protonated forms) are responsible for the acidity of zeolites. If we assume that all Al atoms are situated at the lattice positions and are associated with a Brmted or Lewis acid site and each acid site adsorbs one pyridine molecule, then the N/Al ratio should be equal to 1.O. The N/Al ratio for Beta samples A-C was found to be 1.O f 0.1, indicating that all acid sites present in Beta zeolites are accessible to pyridine molecules. This N/Al ratio was decreased to 0.42 for the sample containing analcime even though A1 atoms associated with the analcime structure are expected to be in framework. Analcime33has a three-dimensional pore structure (cubic, a = 13.7 A), with elliptical pore openings having dimensions of 1.6 X 4.2 A. The kinetic diameter of the pyridine molecule is 5.9 A. Therefore, it cannot enter into the channels of analcime. Thus, its presence lowers the N/Al ratio. In the case of HY zeolite, the N/Al ratio significantly lower than one is explained on the basis of zeolite pore structure.2” The low N/Al ratio (0.38) observed for the relatively pure ZSM-20/F sample suggests that the zeolite pore structure plays a similar role as was observed for HY zeolites. This led us to conclude that, similar to HY, the ZSM-20 zeolite framework structure contains small cages whose pore openings are smaller than the kinetic diameter of the pyridine molecule. The further decrease in the N/Al ratio from 0.38 to 0.10 in ZSM-20/E reveals that some of the surface acid sites are not aocessible to the pyridine molecules, probably due to pore blocking. The extraframework A1 species present on the surface of ZSM-2O/E are believed to be acidic in nature because the N( 1s) XP spectrum of this sample showed a N( 1s) component peak at significantly lower BE value (398.7 eV). This BE value corresponds to the nitrogen of pyridine associated with Lewis acid sites present in y-Al2O3 or HY zeolite calcined at 700 OC.zBThe acidic nature of extraframework A1 species present on the surface of the STY sample is indicated by nearly the same N/Al value before and after steam treatment. The significant difference in the BE of the N(1s) Lewis acid component underlines that the nature of the extraframework A1 species present in STY is different than the extraframework A1 species present in ZSM-2O/E. Now, turning our attention to the N( 1s) XPS peaks of pyridine chemisorbed on various zeolites (Table I11 and Figure 3), it becomes evident that (a) N( ls) peaks are substantially broader than their corresponding Al(2p) peaks, (b) some N(1s) peaks clearly showed the presence of a shoulder peak to the main peak at lowor high-binding-energy values, and (c) the BE values determined from the N(1s) peak maximum varied from 401.8 to 402.8 eV. When deconvolution of the N(1s) peak was carried out by keeping
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6733 r
09.0
3 9 4 . 0 409.0
401.5
401.5
394.0
Binding Energy(eV)
Figure 3. N(1s) XP spectra of pyridine chemisorbed on various zeolites recorded at room temperature. Sample designation as in Table I.
TABLE IIk Biding Eaergy and Relative Intensities of N( Is) Components of Pyridine Chemisorbed on Various Zeolites N(1s) binding
energy, eV zeolite Beta/A Beta/B Beta/C Beta/D ZSM-ZO/E ZSM-2OIF
STY/G HY/H
N/AI 0.89 1.17 0.93 0.42 0.10 0.38 0.46 0.42
I 400.4 400.1 400.5 400.2 398.7 400.0 400.5 400.4
I1 402.1 401.8 402.0 402.1 401.3 401.7 402.2 402.5
I11 403.4 403.2 403.4 403.5 403.2 403.2 403.2
re1 intensity I1 I11 23.8 52.0 25.2 21.2 48.0 30.8 25.0 50.0 25.0 5.3 21.3 73.4 21.2 48.0 40.8 18.3 71.5 10.2 25.9 55.0 19.1 14.4 85.6
I
the fwhm value of 2.0 eV and assuming that each component peak has a Gaussian shape, the BE values of each component peak were found to be reproducible and comparable with those of N( 1s) component peaks of other samples of the same structure. The best curve fitting for HY zeolite was achieved with two N(ls) component peaks and for all other zeolites with three N(1s) component peaks. The next important aspect of the present study is the peak assignment. The N(1s) component peak (I) which occurs at low BE values (400.2 f 0.3) is assigned to the nitrogen of pyridine associated with the Lewis acid sites. The second (11) medium-BE (402.2 f 0.2 eV) and the third (111) high-BE peaks (403.2 f 0.2 eV) were assigned to the nitrogen of pyridine associated with relatively weak and strong Brornsted acid sites, respectively. The third (111) peak is not present in HY zeolite. The foregoing peak assignment is further supported by the dehydroxylation study made with the Beta zeolite. In this dehydroxylation experiment, small portions of sample Beta/B were dehydroxylated at different temperatures and then pyridine adsorption/desorption experiments were carried out as mentioned in the Experimental Section. Figure 4 depicts the N( 1s) XPS spectra of pyridine chemisorbed on Beta/B zeolite dehydroxylated at various temperatures, and Table IV compares binding energy and relative intensity data of the N( 1s) component peaks. Kazan~ky’~ studied the dehydroxylation mechanism of various zeolites using IR spectroscopy and showed that the dehydroxylation can proceed by two different mechanisms depending on the Si/Al ratio of zeolite. It is reported that in mechanism 1, destruction of two Brornsted acid sites and generation of one
Borade and Clearfield
6734 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
1
7001100
'
B Y \
409.0
401.5
2
394.0
Binding EnergyW)
Figure 4. Effect of dehydroxylation temperature on the N( 1s) XP spactra of pyridine chemisorbed on Beta/B zeolite. Figures indicate dehydroxylation and pyridine desorption temperature in O C .
TABLE I V Effect of Debydroxylation of Beta/B on the Relative Iotemitica of N(1s) Components of chemisorbed Pyridine (Pyridine DeaomtionTemperature 200 OC) N( 1s) binding dehydroxylation energy, eV re1 intensity temp, OC N/AI I I1 111 I I1 I11 ~
500 600 700 70P a
1.17 0.75 0.42 0.52
400.1 400.1 400.1 399.9
401.9 403.2 25.8 43.7 30.5 401.8 402.9 25.6 63.5 11.9 401.9 29.7 70.3 402.0 38.5 61.5
Pyridine desorption temperature 100 O C .
Lewis acid site are operative for samples having Si/AI ratios 5. Indeed, our XPS data indicated that for ZSM-5 zeolites which have a Si/AI ratio -40.0, mechanism 2 was operative?' The N/Al ratio of the Beta/B sample initially is 1.17. The ratio decreases as a function of dehydroxylation temperature. At a dehydroxylation temperature of 700 OC, the N/Al ratio becomes 0.42. This suggests that in the case of zeolite Beta (Si/Al = 12.5) dehydroxylation mechanism 1 is operative, which contradicts the proposal of K a z a n s k ~ .Recent ~~ studies3' on the thermal stability of zeolite Beta using '*%e NMR indicated that a severe process of dealumination involving the breaking of the 12-membered rings takes place at temperatures 2760 OC. Our dehydroxylation temperature was much lower than this dealumination temperature. Moreover, the XRD pattern of our Beta samples dehydroxylated at 600 and 700 "C showed that the zeolite crystal structure remains intact. Therefore, it is deduced that the dehydroxylation mechanism depends upon not only the Si/Al ratio of the zeolite but also the geometric position of hydroxyl groups in the crystal structure, i.e., the distance between two hydroxyl groups, bond length, bond angle, etca3*A detailed discussion of the dehydroxylationmechanism is beyond the scope of the present paper. Figure 4 clearly shows that upon dehydroxylation, the disappearance of the N(1s) XPS peak associated with the strong
4000
3800
3600
3400 4000
3800
3600
3400
Wavenumber (cm')
Figure 5. FTIR spectra in the hydroxyl stretching region for various zeolites. Sample designation as in Table I. Spectrum (1) after and (2) before pyridine adsorption.
Bronsted acid site is followed by the simultaneous increase in the relative intensity of the Lewis acid component peak. The disappearance of strong Bransted acid sites and generation of Lewis acid sites and the presence of two N( 1s) component peaks with a fwhm value of 2.0 eV independently confirm the presence of two types of Bransted and one type of Lewis acid sites in Beta zeolites. The ZSM-20 sample does not require justification since sample E clearly shows the presence of three N ( 1s) component peaks. Based on the above peak assignment, the (B/L) ratio determined from the relative intensities of the N(1s) component peaks was in good agreement with the one derived from the pyridine IR absorbance ratios for the samples having uniform Al distribution from bulk to the surface of the zeolite crystallites. Having characterized the nature of the various acid sites in these zeolites, an attempt was next made to evaluate the strength of the acid sites by comparing the binding energy of various N(ls) components. The binding energy value of N( 1s) component peaks associated with Lewis acid sites in all zeolites is comparable (400.2 k 0.3 eV). The N(1s) component peak assigned to relatively strong Bransted acid sites is present in all samples except HY zeolite. The BE value of this peak also remains more or less the same (403.2 f 0.2 eV). Thii suggests that all our samples except HY contain additional strong structural Bransted acid sites, including the STY sample. On the basis of the BE value of the N( 1s) component peak, zeolites can be arranged for their acidity as follows: Beta * ZSM-20 STY >> HY. Infrued The acidic nature of the hydroxyl groups present in various zeolites can be compared by studying the IR spectra in the hydroxyl region. Pyridine is often used as a probe molecule. Figure 5 shows the hydroxyl stretching region of the zeolites before and after pyridine adsorption. Before pyridine adsorption, two types of hydroxyl groups, vibrating at 3744 and 3602 cm-', are observed in pure Beta zeolites (samples A X ) . Sample C displays an additional very weak band at 3660 cm-' and another broad band at -3500 cm-I. The intensity of the band
.-
Acid Sites in Beta and ZSM-20 Zeolites
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6735 TABLE V Comparison of B/L Ratios Determined by IR and XPS Metbods (Pyridine Desorption Temwrature 200 O C ) ~~
B/L
B/L zeolite Beta/A Beta/B Beta/C Beta/D a
Wavenumber (cm-')
Figure 6. FTIR spectra in the pyridine region (1575-1425 cm-I) for
various zeolites. Pyridine desorption temperature 200 OC. Sample designation as in Table I. at 3602 an-'was found to increase as a function of the Al content of the Beta samples (A-C), and upon pyridine adsorption, this band showed a drastic reduction in the intensity. Therefore, this band was assigned to the strong structural Bransted acid sites present in Beta zeolites. The band at 3660 cm-' assigned by various authors to OH groups associated with the extraframework Al species3H1is also affected by pyridine adsorption (sample C). The bands at 3744 and -3500 an-'correspond to terminal S i I H groups and hydrogen-bonded O H groups, respectively. Surprisingly, sample D, which is a mixture of Beta and analcime, does not exhibit a sharp hydroxyl band(s) even though the pyridine region showed the presence of Bransted and Lewis acid sites (Figure 6 ) . The IR spectra in the hydroxyl region of pure ZSM-20 zeolite (sample F) consist of two main bands very similar to those observed in HY zeolite (sample H). Therefore, the sharp highfrequency band at 3644 cm-' is assigned to the bridged hydroxyl groups in the large super cages of the faujasite structure. The spectrum of this sample shows that these hydroxyl groups are easily accessible to the pyridine molecule. The low-frequency broader band at 3542 cm-I, due to the bridged hydroxyl groups present in small cages, is not accessible to the pyridine molecule. The IR spectra of ZSM-20/E and STY samples contained new bands at 3660 and 3690 cm-I in addition to those usual bands at 3542, 3644, and 3744 cm-I, which confirmed the presence of extraframework A1 species in these samples. The nature of these ex-
IR
XPS
zeolite
IR
3.1 3.7 3.1 1.7"
3.2 3.7 3.0 3.7
ZSM-ZOJE
3.0 4.9 3.2 9.6
~
ZSM/20/F
STY/G HY/H
XPS ~~
3.7 4.5 2.9 6.0
Lewis acid peak not well defined.
traframework A1 species is a subject of discussion. The band at 3660 an-'associated with the extraframework Al species is visible in the STY sample, which is also affected by pyridine adsorption. Similar observations have been made for STY and ZSM-20 zeolites before and after steam treatment.34*36 BarthomeauP2 related the shift in the OH frequency and the Si/Al ratio of various zeolites. According to Chu and Chang,"' the shift in frequency toward higher values means that the hydroxyl groups are less acidic. Thus, the order of zeolites based on their acid strength as derived from the vibrational frequencies of the OH groups is as follows: Beta >> ZSM-20 = HY. Even though this order is consistent with our XPS results, it should be kept in mind that the relationship between the shift in IR frequency of the hydroxyl group and acidity of a zeolite may not hold good if zeolites of different structures are considered.'* The IR spectra in the region 1575-1425 cm-' are shown in Figure 6. In this region, three sharp bands due to C-C stretching vibrations of pyridine are seen. The strong band at 1490 cm-I is due to the pyridine adsorbed on both Br~rnstedand Lewis acid sites, while bands at 1545 and 1455 cm-' are due to protonation of the pyridine molecule by Bransted acid sites and pyridine adsorbed on Lewis sites, respectively. The relative concentration of Bransted and Lewis acid sites in zeolites may be determined using the relation B/L = ( A B / A L ) ( t L / e B ) , where AB/ALis the absorbance ratio and t L / t B is the extinction coefficient ratio. It is reported4 that the tL/eB value is 1.5 for samples having a Si/Al ratio > 7.5 and 1.15 for samples having a Si/Al ratio
