J . Phys. Chem. 1992, 96, 3403-3408 The possible presence of adsorbed contaminants in the films is obviously of great concern. However, despite the fact that their presence leads to a possible explanation of the experimental curves for those liquids and substrates forming thicker layers near po, we do not believe that the contamination is a problem in the present experiments. The overall features are reproducible from experiment to experiment, and since any species that is soluble in pentane would also be soluble in both tetrachloromethane and cyclohexane it is clear that its presence is not sufficient to cause wetting-it may merely determine the extent to which the wetting film grows at a particular vapor pressure. Further, the quantities of solute required to give fits vary widely from experiment to experiment without any qualitative change in the isotherms themselves. The fact that water does not wet mica is a good indication that if any water-soluble material is present the quantity is not enough to seriously affect the results. Finally, it is also important to remark that even if contaminants were present, they would have no more than a very marginal influence on the behavior of the thinner films. Implications. These measurements together with previously published short a c c o u n t ~show ~ , ~ the ~ wealth of information that can be obtained by careful and accurate measurements of adsorption isotherms on well-defined substrate surfaces. The extent of the linear regime at low vapor pressures, the Occurrence of
3403
layering at room temperature, and the definite (but not unexpected) nonagreement with Lifshitz theory in thin films are all features that have not previously been seen or sufficiently welldocumented. A model of adsorption that can successfully account for these features is a challenge to the theoretician. Perhaps the attempt of Mahanty and to incorporate finitesize effects into Lifshitz theory is a first step in the right direction. On the experimental side the field is open. The increase in adsorption of OMCTS with temperature suggests the possibility of a wetting transition at some higher temperature. Studies of solidlike films of substances with melting points well above room temperature should provide further interesting comparison with low-temperature work. A method of protecting the mica from the atmosphere prior to evacuation may solve the problem of (possible) surface contamination.
Acknowledgment. We thank B. W. Ninham, R. M. Pashley, and E. Z. Radlinska for helpful discussions. Registry No. OMCTS, 556-67-2;H20,7732-18-5;Si, 7440-21-3; CCI4, 56-23-5;pentane, 109-66-0;cyclohexane, 110-82-7. (69) Mahanty, J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1974, 70, 637.
Gallosilicate Zeolite Catalysts: Structural Features of [Si,Ga]-ZSM-5 Xinsheng Liu and Jacek Klinowski* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 I E W,U.K. (Received: May 16, 1991; In Final Form: October 28, 1991)
A series of gallosilicates with the ZSM-5 structure was synthesized in an alkaline medium using tetrapropylammonium bromide (TPABr) as a template, and characterized by XRD, IR, TGA, SEM, MAS NMR, and chemical analysis. The results clearly demonstrate that the number of defects, the strength of interaction between the template molecules and the framework, the distribution of particle sizes, and the size of the crystallites, as well as the content of template, water molecules, and sodium cations, are all correlated with the gallium content.
Introduction Isomorphous substitution of Si atoms in zeolitic frameworks by heteroatoms such as Al,’ B,2 Fe,3-7 Ge,* Ti,”2 and Ga’3-30is
of considerable importance in view of the interesting adsorptive and catalytic properties of the products in reactions such as
(1) Keijsper, J. J.; Post, M. F. M. ACS Symp. Ser. 1989, No. 398. (2) Coudurier, G.;Auroux, A.; Vbdrine, J. C.; Farlee, R. D.; Abrams, L.; Shannon, R. D. J . Catal. 1987, 108, 1 and references therein. (3) Szostak, R.; Thomas, T. L. J . Catal. 1986, 100, 555. (4) Marosi, L.;Stabenow, J.; Schwarzmann, M. German Patent N M 2 831 61 1, 1980. ( 5 ) Borade, R. 9.; Halgeri, A. B.; Prasada Rao, T. S.R. In New Developments in Zeolite Science and Technology. Proceedings o f t h e 7th International Zeolite Conference. Murakami, Y . , lijima, A., Ward, J. W., Eds.; Kodansha: Tokyo, 1986;p 851. (6) Meagher, A.; Nair, V.; Szostak, R. Zeolites 1988, 8, 3. (7) Calis, G.; Frenken, P.; de Boer, E.; Swolfs, A.; Hefni, M. A. Zeolites
1987, 106, 287.
1987, 7, 319.
(8) Szostak, R. Molecular Sieves; Van Nostrand Reinhold: New York, 1989. (9) Perego. G.;Belussi, G.; Corno, C.; Taramasso, M.; Buonomo, F.; Esposito A. In ref 5 , p 129. (IO) Guth, J. L.;Kessler, H.; Wey, R. In ref 9, p 121. (11) Reddy, J. S.;Kumar, R.; Ratnasamy, P. Appl. Catal. 1990,58, LI. (12)Thangaraj, A.; Kumar, R.; Ratnasamy, P. Appl. Catol. 1990.57, L1. (13) (a) Inui, T.; Matsuda, H.; Yamase, 0.;Nagata, H.; Fukuda, K.; Ukawa, T.; Miyamato, A. J. Catal. 1986.98.491. (b) Inui, T.; Makino, Y.; Okazami, F.; Miyamoto, A. J . Chem. Soc., Chem. Commun. 1986. 571. (14) (a) Kitagawa, H.;Sendoda, Y.; Ono,Y. J . Catal. 1986,101, 12. (b) Shibata, M.;Kitagawa, H.; Sendoda, Y.; Ono, Y. In ref 5, p 717. (c) Sirokman, G.; Sendoda, Y.; Ono,Y.Zeolites 1986, 6. 299. (15) Ono, Y.;Kanae, K. J. Chem. Soc.. Faraday Trans. 1991, 87, 669.
(16)Simmons, D. K.; Szostak, R.; Agrawal, P. K.; Thomas, T. L.J . Catal.
(17) Scurrell, M. S.Appl. Catal. 1988, 41, 89. (18) la) GneD. N. S.:Dovemet. J. Y.: Guisnet. M. J . Mol. Catal. 1988. 45,’281. ‘ (b) Gnkp, N. S.;Doyemet, J. Y .; Seco, A. M.; Ramoa Ribeiro, F.f Guisnet, M. Appl. Catal. 1988, 43, 155. (19) Petit, L.;Bournonville, J. P.; Raatz, F. In Zeolites: Facts, Figures, Future; Jacobs, P. A.; van Santen, R. A,, Eds.; Elsevier: Amsterdam, 1989; p 1163. (20) Chen, N. Y.;Degnan, T. F.; McCullen, S.B. U S . Patent 4,861,932 (1989). (21) Kanai, J.; Kawata, N. Appl. Catal. 1989, 55, 115. (22)Nemet-Marrodin, M. U S . Patent 4,861,933(1989). (23) (a) Berti, G.; Moore, J. E.; Salusinzky, L.; Seddon, D. Ausf. J. Chem. 1989, 42, 2095. (b) Seddon, D. PCT Int. Appl. WO 89 10,190 (1990). (24) Khodakov, A. Yu.; Kustov, L. M.; Bondarenko, T. N.; Dergachev, A. A.; Kazansky, V. 9.; Minachev, Kh. M.; Borbely, G.; Beyer, H. K. Zeolites 1990, 10, 603. (25) Herbst, J. A.; Owen, H.; Schipper, P. H. U S . Patent 4,929.337 (1990). (26) Le Van Mao, R.; Jao, J.; Sjiariel, B. Catal. Lett. 1990, 6, 23. (27) Price, G. L.; Kanazirev, V. J. Coral. 1990, 126, 267. (28) (a) Petit, L.;Bournonville, J. P.; Guth, J. L.; Raatz, F.; Seive, A. Eur. Pat. 351,311 (1990). (b) Petit, L.; Bournonville, J. P.; Guth, J. L.;Raatz, F.; Kessler, H. Eur. Pat. 351,312 (1990). (c) Seive, A,; Guth, J. L.; Raatz, F.; Petit, L. Eur. Pat. Appl. 342,075 (1990). (29) Klazinga, A. H.;Seelen, K. J. M. E.; Minderhoud, J. K. Eur. Pat. 380,I80 ( 1 990).
0022-3654/92/2096-3403$03.00/00 1992 American Chemical Society
3404 The Journal of Physical Chemistry, Vol. 96, No. 8,1992
Liu and Klinowski
TABLE I: Commition of the Startinn Gels and ISi.G.1-ZSM-5 Products
product no.
NaOH
TPA
1
21.4 21.4 22.5 23.2 23.9 25.3
4.3 4.3 4.3 4.3 4.3 4.3
2 3 4 5 6
starting gel Ga0,- SiO, 1 1 1
I 1 1
94 65 43 30 23
IO
H2SOa H20 8.4 8.4 8.4 8.4 8.4 8.4
1800 1800 1800 1800 1800 1800
structure ZSM-5 ZSM-5 ZSM-5 ZSM-5 ZSM-5
Si/Ga"
Na/Ga"
Ga/ucb
Na/ucb
TPA/ucc
(Na + TPA)/uc
67.6 51.1 30.4 24.8 17.5
0.33 0.26 0.19 0.16 0.18
1.4 1.8 3.1 3.7 5.2
0.5 0.6 0.6 0.6 0.9
3.5 3.2 2.9 2.4 2.0
4.0 3.7 3.5 3 .O 2.9
amorphous
"By atomic absorption spectroscopy. bCalculated on the assumption that all Ga atoms are present in the framework and that there are no framework defects. E By thermogravimetric analysis.
cracking, oxidation, polymerization, isomerization, cyclization, and the conversion of methanol to gasoline. The introduction of gallium into aluminosilicate zeolites results in high selectivity to aromatics in the catalytic conversion of olefins and paraffin^.'^-^^ The Cyclar process, in which C3-C5alkanes are transformed into aromatics, proceeds over zeolite [Si,Ga]-ZSM-5.14+i8 Isomorphous substitution of silicon by heteroatoms in silicalite, a highly siliceous analogue of the aluminosilicate zeolite ZSM-5, has been extensively investigated, particularly the substitution of A1 leading to zeolite [Si,Al]-ZSM-5. Although a critical review of the synthesis and structure of [Si,Al]-ZSM-5 has recently appeared,' there is no detailed study of the structure and adsorptive and catalytic properties of zeolite ZSM-5 containing other heteroatoms. Handreck and Smith31,32 investigated several series of [Si,M]ZSM-5 samples (where M = Al, Ga, or Fe) obtained by direct hydrothermal synthesis and found that when the number of M atoms in the unit cell is above a certain value (>2.1 for Al, > 1.6 for Ga, and >0.7 for Fe), the product is a mixture of silicalite and [Si,M]-ZSM-5. We have examined in detail the synthesis and structural features of a series of [Si,Ga]-ZSM-5 with different Ga contents, prepared by direct hydrothermal synthesis in a basic medium containing TPABr as template, using chemical analysis, powder X-ray diffraction, infrared spectroscopy, magic-anglespinning (MAS) NMR, thermogravimetric analysis, and scanning electron microscopy.
Experimental Section Syntbesii. A series of gallosilicates with the ZSM-5 structure was prepared as follows: 0.15-1.2 g of 99.99% pure Ga203 (Aldrich) was dissolved in a solution consisting of 3.0-3.55 g of NaOH in 25 mL of H 2 0 (solution 1); 4.0 g of TPABr (Fluka) was dissolved in 100 g of H 2 0 and mixed with 3.0 g of H2S04 (solution 2); 22.5 g of 40 wt % colloidal silica (Fisher) was mixed with 25 g of H 2 0 (solution 3). Solution 1 was then mixed with solution 2 and solution 3 added to the mixture while stirring. The gel was transferred into a Teflon-lined autoclave, which was then heated at 170 "C for 6 days without stirring. The product was filtered, washed with distilled water, and dried at 60 O C in air. Characterization, Powder XRD patterns were collected on a Philips automatic diffractometer fitted with a vertical goniometer using Cu K a radiation. Infrared spectra were recorded on a Nicolet 1OX FT-IR instrument using the KBr wafer technique. Thermogravimetric measurements were performed on a PerkinElmer Delta Series TGA7 instrument, in which 3-6 mg of sample was heated to 550 OC at a rate of 10 OC min-' in dry flowing air. Scanning electron micrographs were taken on a Jeol EM-2OOCX electron microscope operating at 200 keV. 29SiMAS NMR spectra were collected at 79.5 MHz on a Bruker MSL-400 spectrometer using 20' pulse angles and 30-s recycle delays. The rotors were spun in air at 4.5 kHz in a double-bearing probehead. For each sample 400 scans were acquired. Chemical shifts are given in ppm from external TMS. (30) (a) Bellaloui, A.; Plee,D.; Mariaudeau, P. Appl. Carol. 1990, 63, L7. (b) Mariaudeau, P.; Naccache. C. J . Mol. Coral. 1990, 59. L31. (31) Handreck, G.P.; Smith, T. D. J . Chem. Soc., Faraday Trans. I 1989, 85, 32 IS. ( 3 2 ) Handreck, G. P.; Smith, T. D. J . Chem. Soc., Faraday Trans. I 1988, 84, 4191.
TABLE 11: Particle Sizes of ISi.Gal-ZSM-5 SamDles no. Si/Ga size, rm no. Si/Ga 1 2 3
67.6 51.1 30.4
11
4
13 32
5
24.8 17.5
size, pm 4-13 1-14
Results and Discussion Table I gives the composition of the synthesis gels and of [Si,Ga]-ZSM-5 products. Since XRD does not detect any amorphous material, SEM shows that all samples are monophasic, and "Ga MAS NMR (spectra not shown) detects only the presence of 4-coordinated Ga, we conclude that all crystalline products contain only framework gallium. Table I shows the following. (1) Pure [Si,Ga]-ZSM-5 is easily obtained when the Si/Ga ratio in the gel is above 20; otherwise an amorphous material is obtained. (2) Si/Ga ratios of the products and the corresponding gels are similar. The higher the Si/Ga ratio of the gel, the higher the Si/Ga ratio of the product. (3) The number of Na+ cations present in the unit cell of [Si,Ga]-ZSM-5 (calculated on the assumption that all Ga atoms are part of the zeolitic framework) decreases as the content of Ga decreases. By contrast, the Na/Ga ratio increases with decreasing Ga content (Table I). The number of TPA+ cations decreases with increasing Ga content. (4)The sum of the numbers of Na+ and TPA+ cations per unit cell is different from the number of Ga atoms. In samples with high Ga content (>3.7 Ga3+/uc), the total is smaller than that of Ga, while for the samples with low Ga content (C3.7 Ga3+/uc), larger than that of Ga. We interpret these results as follows. With increasing Ga content, the framework becomes less hydrophobic and organophilic and increasingly hydrophilic. This is well known to happen in aluminosilicate zeolites with increasing A1 content. As a result, the interactions between the template molecules and the hydrophobic framework become weaker, and the number of TPA+ molecules in the channels decreases. On the other hand, when the Ga content increases, TPA', a large univalent cation, can no longer completely compensate the negative charge of the framework, and smaller cations are required. Interstitial space vacated by the replacement of TPA+ by Na+ or H+ is occupied by water (see the TGA results below). Our samples can be divided into two groups. Those with low Ga contents (samples 1, 2, and 3 with C3.7 Ga3+/uc) contain structural defects which allow the extra Na+ and TPA+ cations to compensate the framework charge.33 By contrast, at high Ga contents (samples 4 and 5 with the number of Ga3+/uc > 3.7), a certain amount of H+ must be present to balance the negative charge of the framework. This is supported by 29SiMAS NMR (see below). Scanning electron micrographs (SEM) of [Si,Ga]-ZSM-5 shown in Figure 1 reveal the following. (1) Each particle in the sample is composed of much smaller microcrystallites of [Si,Gal-ZSM-5. The crystallites change their size as the gallium content changes: the higher the Ga content, the smaller the crystallites. (2) The crystal habit of particles in the same sample is independent of particle size. (3) The size of the particles is around 10 pm and the size distribution is correlated with the Ga (33) Keijsper, J . J.: Post, M .
F. M. Reference
I . p 28.
Gallosilicate Zeolite Catalysts
- 1.
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3405
SI I GS = 67.6
6.6 pm
SI / GS = 51.1
'7 -. SI I Ge = 30.4
SI / GS =24.0
6.6 pm
Si Ge = 17.5
Figure 1. Scanning electron micrographs of [Si,Ga]-ZSM-5 samples with Si/Ga ratios of (a) 67.6; (b) 51.1; (c) 30.4;(d) 24.8; (e) 17.5.
content. At lower Ga contents (samples 1,2, and 3), particle size is uniform (Figure la-). However, when the Ga content is higher (samples 4 and 5), the distribution of particle sizes is inhomogeneous (Figure 1, d and e). Table I1 lists the average particle size of samples with different gallium contents. Crystals of silicalite and [Si,Ga]-ZSM-5 have a different morphology, and SEM detects a trace amount of silicalite in [Si,Ga]-ZSM-5 samples with lower Ga content (
