J. Phys. Chem. 1992, 96, 3073-3079
3073
Sorption Properties of Titanium Silicate Molecular Sieves S. P.Mirajkar, A. Thangaraj, and V. P. Shirakar* National Chemical Laboratory, Pune 41 1 008 India (Received: February 26, 1991)
Titanium silicates (TS-1) of MFI structure, with different Si/Ti ratio (7-99) including the silicalite analogue, were prepared by hydrothermal crystallization at 443 K. Following the XRD characterization, it was revealed that the unit cell volume of the microporous solids increased with the increase in the framework titanium content. Sorption properties of the protonic forms (obtained via ammonium exchange and subsequent deammoniation) of these samples were studied by BET specific surface areas using low-temperature(78 K) nitrogen sorption and equilibrium sorption capacities for water, n-hexane, cyclohexane, benzene, and n-butylamine (n-BA). Sorption kinetics were obtained for these sorbates at 298 K and PIP, = 0.8 up to 2 h. Specific surface areas were found to decrease with the decrease in the framework titanium content in the solid. Equilibrium sorption capacities for n-hexane and cyclohexane were comparable to MFI aluminosilicate (ZSM-5); however, that for water in titanium silicate was found to be considerably lower than for the corresponding aluminosilicate. Isotherms of n-BA sorption in these titanium silicates over the temperature range 323-523 K yielded useful information about the sorption and acidic centers. Sorption isotherm data were analyzed in terms of Langmuir, BET, Dubinin, Sips, and Freundlich adsorption isotherm models. Sorption affinities (chemical potential) for n-BA sorption decreased sharply with coverage with the increase in titanium content. Isosteric heats of n-BA sorption also showed marginal increase with the increase in the titanium content.
Introduction Novel shape-selective ZSM-5 (MFI) zeolites are now best known for their excellent thermal, hydrothermal, and acid stability and coke resistant ability in acid-catalyzed hydrocarbon conversion reactions1 of industrial importance. Similar to the conventional tectosilicate zeolites, ZSM-5 is formed by isomorphous replacement of silicon in a silica matrix by A13+. In recent years Barrer2 and Szostak3 have reviewed attempts at isomorphous replacement of framework Si4+and/or A13+species of synthetic zeolites, especially of ZSM-5, by trivalent species like Fe3+,Cr3+,Ga3+,La3+, B3+,and Ti4+. Very recently, successful hydrothermal synthesis of the titanium derivative of silicalite-1 (ZSM-5, MFI structure), called TS-1, with framework substitution of Si4+by Ti4+has been reported4-' by several researchers including a group from our laboratory. Whether or not these titanium-containing species are substituted in the framework and the 960-cm-l band in the framework IR spectra represents the Ti-OH bond is not yet fully elucidated. These titanium silicates including TS-1 are found to catalyze oxidation reactions with H202*-10and ammoxidation of ketones and aldehydes to oximes." It is expected that with different degrees of framework titanium insertion, the physicochemical properties of the parent zeolite will be modified to a considerable extent. In fact, the magnitude of the modifications in sorption and acidic properties would be able to provide an estimate, at least qualitatively, of the degree of titanium insertion in the parent zeolite. In view of this, we camed out hydrothermal synthesis of MFI type molecular sieve without any added alumina and with Si/Ti to be in the range of 7-99. It was then thought to be interesting to study modifications brought about by such systematic titanium insertions in the (1) Rabo, J. A. Catal. Reu. Sci. Technol. 1982, 24, 202. (2) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (3) Szostak, R. Principles of Synthesis and Identification; Van Nostrand Reinhold: New York. 1989. (4) Taramasso, M.;Perego, G.; Notari, B. U S . Patent 1983, 4,410,501. (5) Taramasso, M.; Manara, G.; Fattore, V.; Notari, B. US. Patent 1987, 4,666,692. (6) Perego. G.; Bellussi, C.; Corno, C.; Taramasso, M.; Buonomo, F.; Esposito, A. In Proceedings of the International Conference on Zeolites, Tokyo; Murukami, Y., Iijima, A., Ward, J. W., Eds.; Elsevier: Amsterdam, 1986; p 129. (7) Thangaraj, A.; Kumar, R.; Mirajkar, S. P.; Ratnasamy, P. J . Catal. 1991,130, IT (8) Holderich, W.; Messe, M.; Nauman, F. Angew. Chem., Int. Ed. Engl. 1988, 27, 26. (9) Neri, C.; Esposito, A.; Anfossi, B.; Buonomo, F. Eur. Patent 100,119 (1984). (10) Esposito, A.; Taramasso, M.; Neri, C.; Buonomo, F. UK Patent 102,665 (1985). (11) Eur.Pat. Application 1986, 6,109,400. '
0022-3654/92/2096-3073$03.00/0
sorption and acidic properties of the resulting titanium silicates. The results are described in the present communication. Experimental Section Materials. Seven different samples (titanium silicates, including titanium-free silicalite- 1) were synthesized hydrothermally using the procedure described earlier' at 443 K with different Si/Ti ratios. Tetraethyl orthosilicate (TEOS) was used as a source of silica. Titanium tetrabutoxide (Ti(OBu),) in dry 2-propanol was used as a source of titanium, while a 20% aqueous solution of tetrapropylammonium hydroxide was used as the templating agent. In a typical synthesis experiment 45 g of TEOS was added to a 70 g of 20% aqueous solution of TPAOH with stirring. To the resultant clear solution, a solution of 2.2 g of titanium tetrabutoxide in 20 g of dry 2-propanol was added and was stirred for 1 h to complete the hydrolysis of TEOS and Ti(OBu),. Finally, 60 g of water was added to the above clear solution and was stirred at 348-353 K for about 3-6 h to remove alkoxide groups. The mixture was then transferred to a stainless steel autoclave (275 mL capacity) and was crystallized under static conditions at 433 K for 3 days. The crystalline solid thus obtained was filtered, washed with distilled water, dried at 393 K for 4 h, and calcined at 823 K in air for 10 h to decompose the template. The material so obtained was treated twice with 10% ammonium acetate solution at 368 K for 5 h and was filtered, washed, and dried at 393 K. The ammonium form was deammoniated in air at 823 K for 10 h. n-Butylamine, cyclohexane, n-hexane, and benzene used for sorption measurements were of high purity (>99.9%) and were further dried over activated 3 A molecular sieve extrudates and were purified by freeze-thaw techniques. Double-distilled water was used for the sorption measurements and high-purity nitrogen was used for the surface area measurements. Methods. The chemical compositions of the samples were estimated as follows: A known quantity of TS-1 sample was heated at a high (1073 K) temperature in a platinum crucible (in duplicate) for 6 h to a constant weight. The ignited TS-1 powder was treated with hydrofluoric acid and evaporated to dryness. The HF treatment was repeated three times. From the loss in weight, silica was estimated. The residue was treated with concentrated sulfuric acid and dilute hydrogen peroxide. The titanium present in the residue (which forms yellow peroxotitanium complex with H 2 0 2 )was estimated by colorimetry. The Si/Ti ratio was also estimated by XRF (Rigaku, 3070). Percentage crystallinity and crystalline phase purity was obtained using an XRD (Philips PW-1710, Cu K a ) unit and framework (400-1250 cm-I) I R spectroscopy (Perkin Elmer 22 1). Surface area measurements were carried out by low-temperature (77 K) nitrogen sorption using Accusorb-2800 (Micromeritics). The sorption measurements 0 1992 American Chemical Society
3074 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992
Mirajkar et al.
TABLE I: Unit Cell Composition, Parameters, Symmetry, and Crystal Size of TS-1 unit cell parameters sample Si/Ti unit cell composn a, A b, A c. A
were carried out on an all-glass gravimetric apparatus using
vapor pressure and by noting ;he corresponding equilibrium amount sorbed. The sorption isotherm was obtained initially at the lowest (323 K) temperature and then subsequently at the next higher temperature after carrying out the activation procedure. In order to check the reversibility, desorption measurements were carried out. After each isotherm the sample was evacuated at 10” Torr for at least 8 h. The X-ray diffractograms were recorded for each sample before and after the sorption measurements to check the structural stability. Results and Discussion (A) Crystallinity, Crystal Morphology, and Chemical Composition, All the zeolite samples were crystallized at 4 3 3 K with the variation in the crystallization period ranging from 3 to 5 days passing from silicalite to a product with Si/Ti as low as 7.0. Table I lists all the seven samples studied along with the Si/Ti in the product. Unit cell parameters and crystal symmetry from XRD studies are also listed along with the crystal morphology of the products determined by SEM.7 All the crystals exhibited cuboid shape with the decrease in the average crystallite size from 3-2 to 0.5 pm with the increase in titanium incorporation in the crystal. A similar observation has already been reported15J6 in the case (12) Shiralkar, V. P.; Kulkarni, S. B. Z . Phys. Chem. ( h i p r i g ) 1984,265, 313. ( 1 3) Shiralkar, V. P.; Kulkarni, S. B. J . Colloid Interjuce Sci. 1985, 108, I. (14) Rao, G. N.; Joshi, P. N.; Kotasthane, A. N.; Shiralkar, V. P. J . Phys. Chem. 1990, 94, 8589.
(IS) Kulkarni, S.B.; Shiralkar, V. P.; Kotasthane, A. N.; Borade, R. 8 . ; Ratnasamy, P. Zeolites 1982, 2, 313. (16) Kotasthane, A. N.; Shiralkar, V . P.; Hegde, S. G.;Kulkarni, S. B. Zeolires 1986, 6, 253.
vol, A3
svm
crvstal size. um
A
of aluminum and iron(II1) incorporation in the silicalite of MFI structure. The product Si/Ti in all cases is lower than that of the starting gel, indicating that part of the silica remained in the mother liquor. Most of the titanium was found to be incorporated in the solid, as the filtrate did not show an appreciable amount of titanium. It has been shown from an earlier communication7 that sample no. 7 contained part of the titanium as extralattice amorphous Ti02, whereas samples 1-6 did not. This indicates that even with the modified procedure of synthesis there exists some limit (Si/Ti = 10) for the incorporation of titanium into the silica matrix of the MFI structure. This is also evident by the higher crystallization period needed for the crystallization of products with lower Si/Ti ratio. The maximum degree of titanium incorporation (8.73 Ti/uc; uc = unit cell) in the MFI structure obtained in the present studies is considerably higher than that reported6J7J8earlier. Unit cell volumes calculated from the unit cell constants are also found to increase from 5344.8 to 5396.7 A3 with the increase in titanium content in the product except for sample no. 7. This observation is logical on account of the longer T i 4 bond6J7( 1 . 7 9 A) than Si-0 (1.61 A) and thereby supports the incorporation of Ti4+ in the framework lattice. Figure 1 shows the XRD profiles of the typical TS-1 molecular sieve and silicalite-1 sample after calcination at 8 2 3 K. All the samples exhibited the highly crystalline character and confirmed orthorhombic crystal symmetry in the as-synthesized form. It ~~
~~
( 1 7 ) Chen, M. C.; Chu, S. J.; Chang, N . S.; Chen, P. Y.; Chang, T. K.; Chen, L. Y . Cutulysis 1987; Ward, J. W., Ed.; Elsevier: Amsterdam, The Netherlands, 1988; p 253. (1 8) Kraushaar, B. Ph.D. Thesis, The Technical University of Eindhovcn, 1989.
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3075
Titanium Silicate Molecular Sieves
TABLE II: Eauilibrium Sorotion CaDacities for Titanium Silicates (TS-1)" ~~
sample
water"
n-hexane"
cyclohexane"
benzene"
n-butylamine"
1
4.48 (1 .4)c 10.59 (3.3) 13.52 (4.2) 16.86 (5.1) 19.30 (5.9) 26.34 (8.0) 23.71 (7.2) 3.29 (1 .O)
8.37 (12.5) 8.60 ( 1 2.8) 8.90 (1 3.2) 8.96 (13.2) 9.10 (13.3) 9.37 (13.6) 8.68 (1 2.6) 6.61 (9.6)
2.74 (4.0) 4.06 (5.9) 4.69 (6.8) 4.65 (6.7) 5.47 (7.8) 5.92 (8.4) 1.92 (2.7) 0.07 (
