Alkylammonium polycations as structure-directing agents in MFI zeolite synthesis1

Microporous and Mesoporous Materials 22 (1998) 107–114

Alkylammonium polycations as structure-directing agents in MFI zeolite synthesis1 Larry W. Beck, Mark E. Davis * Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA Received 16 January 1998; received in revised form 20 February 1998; accepted 20 February 1998

Abstract Silicate and aluminosilicate MFI-type zeolites are synthesized with di- and triquaternary ammonium cations as structure-directing agents (SDA). Solid-state NMR spectra show that polycations are intact in the as-synthesized zeolites. Materials synthesized with polycationic molecules are compared to zeolites synthesized with tetrapropylammonium, the normal SDA for MFI-type zeolite. The zeolite nucleation rates and particle sizes decrease as the net charge of the organic cation increases, i.e. SDAn+ where n=1, 2 or 3. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Diquat; Triquat; Molecular sieve; ZSM-5

1. Introduction An investigation of MFI-type zeolite synthesis using mono-, di-, and triquaternary ammonium cations is presented in order to further explore the mechanism of organic additives as zeolite structure-directing agents (SDAs). Tetrapropylammonium ( TPA) cation is the most common organic additive used to direct the formation of pure silica zeolite MFI. Here, the polycations are also shown to direct the formation of zeolite MFI. By conceptually forming a bond between methyl carbons of two TPA molecules, the diquaternary ammonium cation II can be formed, while the triquaternary cation III is conceptually equivalent to a trimer of TPA. * Corresponding author. Fax: +1 626 568 8743. 1Dedicated to Professor Lovat V.C. Rees in recognition and appreciation of his lifelong devotion to zeolite science and his outstanding achievements in this field. 1387-1811/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 8 ) 0 0 09 6 - 1

Polycations have previously been shown to direct zeolite formation. Polymeric cations based on the DABCO unit (1,4-diazobicyclo[2.2.2]octane) have been reported to direct the formation of GME, MOR, and pure silica MTW zeolites [1,2]. One interest in preparing zeolites via a linear polymer is the desire to control the porosity of the zeolite by reducing the number of stacking faults. For example, zeolite MOR made by this method has fewer stacking faults along the 12-membered ring axis, allowing access to more of the internal pore volume. Jansen and coworkers [3] attempted to synthesize large single crystals of ZSM-5 using diquat II. One motivation for this work was to optimize the zeolite synthesis to give millimetersized, fault-free crystals for crystallographic studies. The resulting particles were actually smaller for II versus I. Diquaternary ammonium structure-directing

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Here a systematic study of zeolite MFI formation using mono-, di-, and triquaternary ammonium cations is presented. Over 20 different molecules or cations have previously been reported to influence the formation of ZSM-5, including tripropylamine (neutral ), tetrapropylammonium (organic cation), sodium (alkali cation) [8]. The goal of the present study is not to increase the list of SDAs capable of forming ZSM-5, but rather to investigate the influence of three similar SDAs on the synthesis mechanism. Compounds I, II, and III have nearly the same hydrophylicity/ hydrophobicity, the same C/N+ [9], and should thus have similar effects on hydrophobic hydration in the synthesis solutions (see hypothesis proposed by Burkett and Davis [10]). Importantly, the length scale of the entities involved in the zeolite assembly process is increased in this series of organic molecules. The cations I, II, and III must presumably span one, two, and three channel intersections in ZSM-5, respectively (vide infra).

2. Experimental section 2.1. Structure-directing agent syntheses

agents have given new, synthetic zeolites and highsilica versions of known aluminosilicates. EUO [4] and NES [5] are examples of novel structure types each with intersecting two-dimensional pore channels that have been synthesized with diquat SDAs. Moini et al. [6 ] reported that a variety of zeolite phases could be synthesized by systematically varying the spacer length (number of methylene carbons) of the diquat (CH ) N+(CH ) N+(CH ) , 33 2n 33 where n=5–16. Pure EUO and NES-type zeolites are synthesized with n=5, 6 and n=10, respectively. Several reports exist for the synthesis of high-silica zeolites using diquats based on 4,4∞trimethylenebis(N-alkyl-N-methylpiperidinium). Recently, Tsuji and Davis [7] showed that by varying the alkyl group of this diquat, either MFI, MTW or BEA-type pure silica materials are formed.

Unless stated otherwise, all chemicals were used as received from Aldrich. Bis-1,6-(tripropylammonium)hexamethylene diiodide (SDA II ) was prepared by exhaustive alkylation of 1,6-diaminohexane with iodopropane. Approximately 300 ml of 2-butanone, 83 g of anhydrous potassium carbonate, and 21.5 g of the diamine were added to a dry three-necked flask equipped with a mechanical stirrer, an addition funnel, and a reflux condenser. The reaction flask was then flushed with purified argon gas and vented through a bubbler. Iodopropane, 100 ml, was quickly transferred to the addition funnel. The mixture was gently heated to reflux, 80°C, under argon as the iodopropane was added dropwise to the stirring mixture. After approximately 8–10 h, the solution was cooled to room temperature and filtered to remove the potassium salts. The butanone was removed by rotary evaporation resulting in an off-white semi-solid. The bis-alkylammonium iodide salt was recrystallized from cold 2-butanone

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and ethyl acetate. 13C NMR d (normalized intensity): 11.0(3), 16.0(3), 22.2(1), 26.4(1), 59.3(1), 61.1(3). A 0.1 M aqueous solution of the iodide salt was eluted over a strong anion exchange resin (Supelco) in the hydroxide form. The basic, structure-directing reagent solution was concentrated to approximately a 30 wt% solution and the OH− exchange efficiency was determined by standard titration with 0.0997 N HCl. Bis-N,N-(tripropylammoniumhexamethylene)diN,N-propylammonium triiodide (SDA III ) was prepared by the exhaustive alkylation of bis-N,N(aminohexamethylene)amine with iodopropane by the same procedure as described above. 13C NMR d (intensity): 11.0(2), 16.0(2), 22.3(1), 26.4(1), 59.3(1), 61.0(2). 2.2. Zeolite syntheses Two basic stoichiometries and aging procedures were followed for the zeolite synthesis described herein; typical procedures and reagents are described below as protocols A and B. Tetrapropylammonium hydroxide, when used, was added as a 40 wt% solution obtained from Alpha. The reaction mixtures were heated in static Teflonlined autoclaves in preheated convection ovens; the products collected by centrifugation and subsequently washed with distilled water to remove nonoccluded cations. (A) Zeolite synthesis mixture SiO :0.24/ 2 n SDAn+(OH−) :0.087 NaOH:18 H O, where n 2 SDA is either I +, II 2+ or III 3+ (see above). SiO is from silicic acid (Malinckrodt) that shows 2 13.7% weight loss below 500°C, presumably from H O. The structure-directing reagent, alkali cation, 2 and remaining water were heated to a boil, loosely covered. The silicic acid powder was added to the boiling solution and heated for approximately 10 min. The hot mixture was filtered through a microfiber filter ( Whatman) and the water lost during boiling was replaced. The still warm solution was quickly transferred to the pressure vessel and heated to the synthesis temperature of 125°C. (B) Zeolite synthesis mixture SiO :0.3/n 2 SDAn+(OH−) :30 H O, where SDA is either n 2 I +, II 2+ or III 3+. SiO is from tetraethylorthosili2

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cate ( TEOS). An aqueous solution was made of the basic reagents and the appropriate amount of water. This solution was added at one drop every 2 s to vigorously stirring TEOS in a pre-weighed round-bottomed flask. After a homogeneous solution was obtained (approximately 1–2 h), the ethanol was removed by rotary evaporation. Water lost during evaporation was replaced and the clear solution mixture was transferred to an autoclave and heated to the synthesis temperature, 125–175°C. Aluminosilicate ZSM-5 was also synthesized using a similar preparation procedure. The stoichiometry of the synthesis mixture was SiO :1/ 2 23 Al O :0.3/n SDAn+(OH−) :0.1 NaOH:30 H O, 2 3 n 2 where Al O was from aluminum hydroxide 2 3 ( Rheis) and SiO was again from TEOS. The 2 aluminum hydroxide was added to the prehydrolyzed, homogeneous TEOS mixture just after rotary evaporation of ethanol; the synthesis protocol was otherwise the same. The synthesis temperature used for aluminum containing solutions was 145°C. 2.3. Analysis The crystallinity of the products was obtained using a Scintag XDS 2000 X-ray powder diffractometer using Cu Ka radiation. Transmission electron micrographs were acquired on a Philips EM420 microscope operating at 120 kV with a tungsten filament. Particles were dispersed in a dilute ethanol suspension and placed on carboncoated copper grids; samples were dried at 100°C prior to mounting on the microscope stage. The amount of organic occluded in the product was determined using a Du Pont 951 thermogravimetric analyzer; 10–15 mg samples were heated in air at rates of 5°C/min. NMR spectra of the SDA reagents (aqueous solution) were run on a Bruker AM500 instrument at 11.74 T (1H, 500.1 MHz; 13C, 125.9 MHz). NMR spectra of the solid samples were collected on a Bruker AM300 instrument at 7.05 T (1H, 300.0 MHz; 13C, 75.54 MHz; 29Si, 59.64 MHz) equipped with high-power amplifiers. All solid-state spectra were acquired with magic-angle spinning (MAS) 7 mm zirconia rotors at 4–5 kHz. Cross-polarization (CP) experiments

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used a 1H 90° pulse length of 5.6 ms, a recycle delay of 2 s, and a cross-polarization period of 2.5 ms. 13C NMR spectral chemical shifts were externally referenced to tetramethylsilane.

3. Results Pure silica ZSM-5 was synthesized following procedure A (above) and using the structuredirecting agents I, II, and III at 125°C. The X-ray powder diffraction ( XRD) patterns are shown in Fig. 1 for the samples collected after 24 h at the synthesis temperature. The zeolite reaction mixtures following procedure A were initially clear solutions. The crystal formation process was monitored in situ by performing the synthesis in glass scintillation vials in an oven with a transparent window. The amount of time required for the clear

Fig. 1. X-ray powder diffraction patterns for as-synthesized zeolite samples prepared with the SDAs noted on the right.

reaction mixtures to turn completely opaque was recorded as a relative estimate of zeolite formation rate. Samples quenched at this point, but not prior to it, show evidence of crystallinity by XRD for all three SDAs. The synthesis mixtures, at 125°C, turned opaque after 1.2 h for SDA I, 2.5 h for II, and 5 h for III. More accurate kinetic information using in situ SAXS and SANS methods will be reported later. The amount of silica converted to zeolite for samples quenched immediately after the mixture became opaque is approximately 50% for all three SDAs studied. After 24 h the yield increased to 70–80%, again independent of which SDA was used. The XRD results show evidence of peak-broadening due to particle size effects. The average crystallite sizes were estimated from the data shown in Fig. 1 according to the Scherrer equation: l=Kl/[(B2−b2)1/2 cos(2h/2)], where l is the average crystallite size, K=0.893, l=0.15405 nm, B is the peak-broadening (radians), and b is the inherent instrumental broadening (radians) determined using the standard NIST-SRM 660 [11]. The Gaussian peak-width at half-maximum for the first well-resolved reflection (102), 2h=13.9°, was used for the calculation. Average particle sizes are 75 nm, 50 nm, and ≤25 nm for materials prepared with I, II, and III, respectively. [Similar results were obtained for the (133), 2h=24.4° reflection.] Schlenker and Peterson [12] recently reported calculated XRD patterns for very small particles of MFI and FAU-type zeolites. The computed XRD pattern for a hypothetical 20×20×13.4 nm3 ZSM-5 crystal (10 unit cells in each dimension) is similar to the experimental pattern shown in Fig. 1 for SDA III. The trend in crystal size suggested by the XRD line-width analysis qualitatively agrees with the sizes observed in the transmission electron micrographs ( TEM ) shown in Fig. 2. The micrograph of ZSM-5 made with I shows dark regions, approximately 70 nm in diameter, in a 250 nm diameter particle. The darker areas probably arise from differences in particle thickness, indicating the outer particle surface is somewhat rough. Dark field images (not shown) show the entire particle has approximately the same crystalline orientation. The particles synthesized with II have rougher

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Fig. 2. Transmission electron micrographs (×260 000) for as-synthesized pure silica MFI samples synthesized with the SDA indicated. Samples collected by centrifugation from 125°C synthesis solution after 2.5 h for I, 3 h for II, and 5.5 h for III.

Fig. 2. (Continued)

external surfaces indicated by the more complicated dark and light striation; the average particle size is approximately 150 nm. The crystal size determined by XRD peak-broadening is 50 nm

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Fig. 2. (Continued)

which is slightly larger than the largest uniform area apparent from the micrograph. For ZSM-5 prepared with III the average particle size is approximately 100 nm and is clearly an aggregate of small needle-like crystals, indicating non-uniform crystal growth planes. The crystal diameter estimated from XRD is #20 nm; that is consistent with an average of the long and short dimensions of the needle-like subparticles. Thermogravimetric analysis ( TGA) of the as-synthesized samples shown in Fig. 1 gives approximately 14% total weight loss independent of which SDA is used in the synthesis. There can be a maximum of four quaternary ammonium cations, four hydroxides, and at least four water molecules per unit cell (OH− and one H O are 2 from the defect sites that balance the cationic charge [13]), giving a theoretical weight loss of 13.3%. Therefore, the pores of the as-synthesized materials are filled with quaternary ammonium cations. 13C−CP MAS NMR spectra of the as-synthesized materials (same as illustrated in Fig. 1) are shown in Fig. 3. For comparison, the 13C NMR spectra of the SDA molecules as aqueous, iodide salts are also shown. Spectral assignments for each numbered peak are made on the

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Fig. 3. 13C NMR spectra of the structure-directing agents. 13C CP MAS spectra (a), (c), and (e) show the intact SDAs occluded in as-synthesized pure silica ZSM-5 collected after heating for 24 h at 125°C. For comparison, spectra (b), (d), and (f ) are for the aqueous SDA iodide salts. Spectral assignments for the labeled peaks are made on the in-text schemes.

schemes shown above. As previously reported for I [10], all methylene resonances are shifted downfield slightly, 1.0–2.0 ppm, for the zeolite occluded molecules; the methyl resonances are shifted slightly upfield, 0.5–1.5 ppm. These spectra clearly show that the SDAs are intact inside the ZSM-5 materials. A synthesis mixture containing both I and II also produces zeolite ZSM-5; the synthesis mixture was SiO :0.153 II 2+(OH−) :0.087 I +OH−:0.087 2 2 NaOH:18 H O. ( The total quaternary ammonium 2 and hydroxide stoichiometries are the same as for the prior syntheses.) The time lapse before the clear solution became opaque was slightly more than 1 h, the same as for the synthesis with I alone. XRD ( Fig. 1) line-width analysis of this material

gives an average particle size of 75 nm, the same as for the material resulting from I. However, the 13C CP MAS NMR spectrum in Fig. 4(a) shows that both cations are occluded in the as-synthesized material. The relative amounts of the two SDAs inside the zeolite are the same as in the initial synthesis solution, approximately 2:1, as determined by spectral subtraction. The 13C CP MAS NMR spectral intensity of I occluded in as-synthesized MFI [same as spectrum 3(a)] was weighted by 0.35, shown in Fig. 4(b), and subtracted from the spectrum 4(a) (intensities reflected similar sample amounts and number of scans). The difference spectrum, labeled 4(a−b), is interpreted as the signal coming from II occluded in the zeolite [compare to Fig. 3(c)] and represents 0.65 of the total spectral intensity in 4(a). Notice the 16.7 ppm methylene peak of I does not completely overlap the 18.5 ppm methylene peak of II, making this analysis possible. Zeolite ZSM-5 synthesized in the presence of both SDAs incorporates the organic cations according to their ratio in the initial synthesis solution. ZSM-5 is also synthesized with TEOS as the silica source instead of silicic acid and without NaOH as a co-mineralizing agent (synthesis procedure B) using the diquat and triquat. The crystal formation rates as monitored by mixture opacity

Fig. 4. 13C CP MAS NMR spectrum (a) shows the occluded SDAs in as-synthesized ZSM-5 synthesized with both I and II in the synthesis solution. Spectrum (b) illustrates only I occluded [same as spectrum 3(a)]. Spectrum (a−b) is the result of the weighted subtraction of spectrum (b) from (a) using the normalized factors shown on the right [note similarity to spectrum 3(c)].

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were slightly slower than for synthesis protocol A for each SDA, respectively. However, the aging conditions and compositions for the two synthesis procedures are very different and prevent a rigorous comparison. ZSM-5 was made using all three SDA molecules at temperatures ranging from 110°C to 170°C and each SDA remained intact according to 13C CP MAS NMR results. The average crystallite sizes, as determined by XRD peak-broadening, for materials resulting from each SDA did not change greatly by increasing the synthesis temperature or overall synthesis time (up to one month). Aluminosilicate ZSM-5 was also synthesized using all three SDAs following the modified procedure B in order to observe whether the presence of aluminum affects the ultimate crystal size; aluminosilicate zeolite particles can be larger than pure silica analogues for some zeolite types. However, the crystallite sizes, determined by XRD peak-broadening, of the aluminosilicate zeolites are similar to those for the pure silica materials. All synthesized zeolites contained approximately the theoretical limit of the respective organic SDA according to TGA. The 13C CP MAS spectra (not shown) of the organic cations inside the as-synthesized zeolites reveal the SDAs are occluded intact; the spectra are indistinguishable from those of the pure silica materials ( Fig. 3). 29Si MAS NMR spectral analysis (spectra not shown) gives a framework silicon to aluminum ratio of 34, 27, and 25 for zeolites made with I, II, and III, respectively; the silicon to aluminum ratio of all the initial synthesis solutions was 23.

4. Discussion Pure silica and aluminosilicate MFI-type materials are shown to be synthesized using II and III and the maximum amount of these organic molecules are occluded in the zeolites according to TGA. The fact that ZSM-5 can be synthesized from a pure silica mixture without any other additives, namely sodium, clearly indicates that II and III are capable of directing the formation of MFI-type materials. The 13C NMR spectra of the zeolite products show that the polycations are

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occluded intact. This is important since it is known that tripropylamine, the likely Hoffman elimination product of the polycations, is an effective SDA for MFI. Tripropylamine [13C d (intensity): 56.4(1), 20.3(1), 12.0(1)] is not observed in any of the as-synthesized zeolites. According to a proposed mechanism of the early stages of zeolite crystallization, negatively charged silicate species begin to organize around the solvated organic cation in solution [10]. Subsequently, the organic cations partially coated with silica aggregate together in an organized manner. By local rearrangements, long-range order resembling the three-dimensional zeolite structure results. Other studies have shown that TPA, I, is located in the void-space at the intersection of the straight and sinusoidal channels of ZSM-5. Assuming the diquat II participates in the same enclathration and self-assembly process, silicon atoms that eventually form two adjacent channel intersections must be pre-assembled to coat SDA II in solution. Intuitively, it must be more difficult to organize the critical number of silicate species around the divalent or trivalent cations than TPA. This may explain the observed increased time prior to nucleation upon changing the SDA. If this is the case, then enclathration of the SDA by silica is the rate limiting step involved in zeolite nucleation. In our view this is the first required event in the proposed assembly mechanism. The next important observation is the effect of the SDA on particle morphology. TEM pictures clearly indicate these materials are not uniform, single-crystals but appear to be aggregates of smaller crystals. The peak-broadening observed by XRD shows that as the SDA charge increases the crystalline domain size decreases. The present data are not sufficient to definitively explain this observed trend. However, invoking the hypothesis described where silica-coated SDA molecules are viewed as precursors to zeolite formation, the TPA/SiO precursor is more symmetrical (approxi2 mating a sphere) than the composites containing polycations II or III; the reduced symmetry of the polycations appears to translate into a reduce symmetry of the as-made zeolites, i.e. the needlelike morphology for zeolite made with III (see Fig. 2). Also, since the zeolite yield is constant

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with respect to the SDA used in the synthesis and the crystallite size (determined by XRD line broadening) decreases as the SDA charge increases, there must be more crystallites and therefore more nucleation events in the polycation syntheses compared to the synthesis with TPA. This seems contradictory to our observation that the apparent nucleation kinetics decrease as the SDA charge increases. In the synthesis with a mixture of organic cations I and II, the resulting product had both SDAs occluded in the same ratio as the initial stoichiometry of the synthesis mixture. This implies that I and II are equally effective as pore-filling molecules. However, I dominates the rate of nucleation and determines the final crystallite size; this is consistent with the observation that nucleation with I is faster. Several computational investigations have estimated the energy of stabilization for organic molecules occluded in zeolite framework models in attempts to ascertain their effect on zeolite synthesis; known and potential structuredirecting agents have been investigated [14,15]. The experimental results presented here suggest that pore-filling alone is not an effective measure of a molecule’s ability to act as a structure-directing agent. Obviously, a highly effective SDA like TPA has additional roles besides filling the void space during the formation of a zeolite, e.g. possibly stabilizing colloidal intermediates.

5. Conclusions Polycations II and III are shown to be structuredirecting agents for pure silica and aluminosilicate ZSM-5. These cations are stable under hydrothermal synthesis conditions and are occluded intact inside the zeolites. However, the alkylammonium polycations have slower nucleation kinetics than SDA I. Additionally, the average crystallite size decreases as the size and charge of the SDA used in the material’s synthesis increases. A synthesis with II and I present shows that while both SDAs fill the internal zeolite pores without preference,

SDA I dominates the rate of nucleation and the ultimate crystal morphology.

Acknowledgement We are pleased to participate in this special issue in honor of Professor Lovat Rees. M.E.D. and family wish to personally thank Lovat and his wife for gracious hospitality over the years. L.W.B. is supported by a National Science Foundation, Postdoctoral Fellowship in Chemistry. Additional financial support for this project was provided by Chevron Corporate Research. We thank P. Wagner for help in acquiring the electron micrographs.

References [1] R.H. Daniels, G.T. Kerr, L.D. Rollmann, J. Am. Chem. Soc. 100 (1978) 3097–3100. [2] M.E. Davis, C. Saldarriaga, J. Chem. Soc., Chem. Commun. (1988) 920–921. [3] J.C. Jansen, in: H. van Bekkum, E.M. Flanigen, J.C. Jansen ( Eds.), Studies in Surface Science and Catalysis, vol. 58, Elsevier, Amsterdam, 1991, pp. 125–127. [4] G.W. Dodwell, R.P. Denkewicz, L.B. Sand, Zeolites 5 (1985) 153–157. [5] M.D. Shannon, J.L. Casci, P.A. Cox, S.J. Andrews, Nature 353 (1991) 417–420. [6 ] A. Moini, K.D. Schmitt, E.W. Valyocsik, R.F. Polomski, Zeolites 14 (1994) 504–511. [7] K. Tsuji, M.E. Davis, Microporous Mater. 11 (1997) 53–64. [8] M.E. Davis, R.F. Lobo, Chem. Mater. 4 (1992) 756–768. [9] Y. Kubota, M.M. Helmkamp, S.I. Zones, M.E. Davis, Microporous Mater. 6 (1996) 213–229. [10] S.L. Burkett, M.E. Davis, J. Phys. Chem. (1994) 4647–4653. [11] M. Tsapatsis, M. Lovallo, T. Okubo, M.E. Davis, M. Sadakata, Chem. Mater. 7 (1995) 1734–1741. [12] J.L. Schlenker, B.K. Peterson, J. Appl. Cryst. 29 (1996) 178–185. [13] H. Koller, R.F. Lobo, S.L. Burkett, M.E. Davis, J. Phys. Chem. 99 (1995) 12588–12596. [14] V. Shen, K. Watanabe, A.T. Bell, J. Phys. Chem. B 101 (1997) 2207–2212. [15] T.V. Harris, S.I. Zones, in: J. Weitkamp, H.G. Karge, H. Pfeifer, W. Ho¨lderich (Eds.), Studies in Surface Science and Catalysis, vol. 84, Elsevier, Amsterdam, 1994, pp. 29–36.