Langmuir 1997, 13, 6249-6254
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Adsorption of Methanol on ZSM-5 Zeolites Bernd Hunger,*,† Silke Matysik,‡ Matthias Heuchel,† and Wolf-Dietrich Einicke‡ Institute of Physical and Theoretical Chemistry, Institute of Technical Chemistry, Faculty of Chemistry and Mineralogy, University of Leipzig, D-04103 Leipzig, Germany Received June 10, 1997. In Final Form: August 25, 1997X
The interaction of methanol with a NaZSM-5 and a HZSM-5 zeolite was investigated by means of temperature-programmed desorption (TPD) and adsorption measurements. For both zeolites the desorption curves are structured, indicating different adsorption sites. In the case of HZSM-5 the desorption at higher temperatures is accompanied with a partial conversion of methanol. It was investigated how the course of the reaction depends on the initial adsorbed amount of methanol. Using a regularization method, desorption energy distribution functions have been calculated. The energy range of these distributions correlates well with the heats of adsorption in the literature. The desorption energy distributions between 50 and 65 kJ mol-1, which can be attributed to a nonspecific interaction, reflect the formation of stronger hydrogen bonds in the methanol clusters at higher loading on HZSM-5 than on NaZSM-5. The shape of the desorption energy distribution of NaZSM-5 in the range 65-100 kJ mol-1, specific for the interaction of methanol with Na+ cations, shows two clearly distinguished maxima, indicating two kinds of Na+ cations. The obtained results for the HZSM-5 zeolite reflect the energetic heterogeneity of the bridging Si-OH-Al groups. Both, the shape and wideness of adsorption energy distributions extracted from measured adsorption isotherms agree well with the determined desorption energy distributions.
Introduction The general interest on information about the interaction of methanol with ZSM-5 zeolites seems strongly to depend on the type of the exchanged cations. While the interaction with HZSM-5 is an intensively studied problem, because of the industrial importance of methanol conversion on these catalysts (e.g., the MTG and MTO processes), the interaction with NaZSM-5 is barely investigated, although interesting applications in the separation of liquid mixtures exist.1,2 Beside investigations devoted to catalytic activity and reaction mechanism on HZSM-5 zeolites (e.g., refs 3-8) much research work was carried out using spectroscopic methods (IR, MAS NMR) to characterize different surface species of methanol as well as to perform in situ studies of reactions on these zeolites (e.g., refs 9-13). Further, several theoretical studies have been carried out (e.g., refs 14-19) in which both the strength of the interaction of methanol with the * To whom correspondence should be addressed. † Institute of Physical and Theoretical Chemistry. ‡ Institute of Technical Chemistry. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Einicke, W.-D.; Heuchel, M.; von Szombathely, M.; Bra¨uer, P.; Scho¨llner, R.; Rademacher, O. J. Chem. Soc., Faraday Trans. 1 1989, 85, 4277. (2) Einicke, W.-D.; Gla¨ser, B.; Lippert, R.; Heuchel, M. J. Chem. Soc., Faraday Trans. 1995, 91, 971. (3) Chang, C. D.; Silvestri, A. J. J. Catal. 1977, 47, 249. (4) Ono, Y.; Mori, T. J. Chem. Soc., Faraday Trans. 1 1981, 77, 2209. (5) Schulz, H.; Zhao, S.; Kusterer, H. Stud. Surf. Sci. Catal. 1991, 60, 281. (6) Schulz, H.; Bo¨hringer, W.; Zhao, S. Proceedings of the 9th International Zeolite Conference, Montreal, 1992; von Ballmoos, R., Higgins, J. B., Treacy, M. M. J., Eds.; Butterworth-Heinemann: Woburn, MA, 1993; Vol. II, p 567. (7) Nova´kova´, J.; Kubelkova´, L.; Habersberger, K.; Dolejsek, Z. J. Chem. Soc., Faraday Trans. 1 1984, 80, 1457. (8) Anderson, J. R.; Foger, K.; Mole, T.; Rajadhyaksha, R. A.; Sanders, J. V. J. Catal. 1979, 58, 114. (9) Forester, T. R.; Howe, R. F. J. Am. Chem. Soc. 1987, 109, 5076. (10) Ernst, H.; Freude, D.; Mildner, T.; Pfeifer, H. Stud. Surf. Sci. Catal. 1994, 84C, 1717. (11) Mirth, G.; Lercher, J. A.; Anderson, M. W.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1990, 86, 3039. (12) Anderson, M. W.; Klinowski, J. J. Am. Chem. Soc. 1990, 112, 10. (13) Hunger, M.; Horvath, T. J. Am. Chem. Soc. 1996, 118, 12302.
S0743-7463(97)00615-X CCC: $14.00
acidic bridging Si-OH-Al groups and the methanol conversion were investigated. At present, the theoretical results for the adsorption energy of methanol on OH groups in acid zeolites differ between 58 and 83 kJ mol-1 (see refs in ref 16), and despite the efforts, there is no agreement about the true value of this important quantity. Haase and Sauer15 determined in an ab initio study for a neutral methanol molecule H-bonded to the zeolite Si-OH-Al group a value of 83 ( 20 kJ mol-1 but claimed with several reasons that this value “is still too small”, while recently Blaszkowski and van Santen17 determined with a density functional theory (DFT) method a lower value of 65 kJ mol-1. In this context it is surprising that the number of experimental determinations of the interaction energy of methanol is small. Haase and Sauer15 compared their result with an experimental value of 110-120 kJ mol-1, given only in a personal communication. This high value seems to agree with a heat of desorption of 120 kJ mol-1 determined for methanol desorption at higher temperarures.20 In contrast, a relatively low value of 62 kJ mol-1, determined some years ago by Messow et al.,21 is often used for comparison (see e.g., refs 16 and 17). But, this value was deduced from experimental heats of immersion of liquid methanol, and therefore it represents a mean interaction of methanol with the zeolite at high coverage, which means it is not very specific for the interaction with the acid OH groups. (14) Sauer, J.; Ko¨lmel, C.; Haase, F.; Ahlrichs, R. Proceedings of the 9th International Zeolite Conference, Montreal, 1992; von Ballmoos, R., Higgins, J. B., Treacy, M. M. J., Eds.; Butterworth-Heinemann: Woburn, MA, 1993; Vol. I, p 679. (15) Haase, F.; Sauer, J. J. Am. Chem. Soc. 1995, 117, 3780. (16) Sinclair, P. E.; Catlow, C. R. A. J. Chem. Soc., Faraday Trans. 1997, 93, 333. (17) Blaszkowski, S. R.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 2292. (18) Nusterer, E.; Blo¨chl, P. E.; Schwarz, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 175. (19) Titiloye, J. O.; Tschaufeser, P.; Parker, S. C. In Spectroscopic and Computational Studies of Supramolecular Systems; Davies, J. E. D., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992; p 137. (20) Jayamurthy, M.; Vasudevan, S. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 1521. (21) Messow, U.; Quitzsch, K.; Herden, H. Zeolites 1984, 4, 255.
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At last, Pope22 has reported differential molar enthalpies on the basis of microcalorimetric measurements. He found at low coverage (up to one methanol molecule per SiOH-Al group) 110-70 kJ mol-1 and at high coverage 50 kJ mol-1. If one summarizes these partly contradictory results, it seems there is a demand for clarification on the basis of further experimental investigations concerning the interaction energy of methanol with HZSM-5. Therefore, the motivation for this work is applying temperatureprogrammed desorption (TPD) to obtain detailed information about the adsorption of methanol on MFI type zeolites (NaZSM-5, HZSM-5). The advantage of TPD is foremost the possibility to get information about interaction strength at low and very low coverage, where other methods (microcalorimetry, isotherm measurements) have problems with the adjustment of equilibrium, because of the very low bulk pressures. A further advantage of TPD is the possibility to investigate simultaneously reactions which occur during desorption. A further purpose of the study was a direct comparison of TPD investigations with the evaluation of adsorption isotherms at 298 K in order to find out the adequacy of the results of both, quite different, methods.
Hunger et al.
Figure 1. Desorption curves for different initial amounts of methanol on NaZSM-5: 1a (methanol, 31 amu) and 1b (dimethyl ether, 45 amu), 2.5 mmol g-1; 2 (methanol, 31 amu), 1.3 mmol g-1; 3 (methanol, 31 amu), 0.5 mmol g-1.
After adsorption of methanol at room temperature, the TPD on NaZSM-5 shows, see curve 1a in Figure 1, a structured desorption profile between 300 and 650 K with
three distinct ranges. Additionally to the desorption of methanol (31 amu), a desorption of small amounts of dimethyl ether (45 amu, curve 1b, Figure 1) was observed starting at about 500 K (peak maximum, 596 K). Because the course of desorption observed with TCD agrees well with the 31 amu response, the conversion of methanol seems to be very small. A further argument for this very small conversion of methanol is the observation that during repeated cycles of adsorption/desorption with one sample there are only small deviations for both the desorbed amount and the course of desorption. The beginning of the methanol conversion on NaZSM-5 zeolites at higher temperatures was also observed with FTIR9 and 13 C MAS NMR10 experiments. The desorbed amount of methanol is 2.5 ( 0.3 mmol g-1, corresponding to about 14-15 molecules per unit cell (uc), i.e., 2.4 molecules per cation. Assuming that during desorption only an insignificant portion of the methanol is reacting, the applied adsorption conditions allow an initial loading of 65% of the saturation capacity at 298 K (3.83 mmol g-1). The determination of the amounts belonging to the single ranges of the desorption curve was carried out by a combination of isothermal and nonisothermal desorption, as described in ref 24. The desorbed amount between 400 and 650 K is about 6-7 molecules per uc (curve 2, Figure 1), and that between 500 and 650 K is about 3 molecules per uc (curve 3, Figure 1). The TPD on the HZSM-5 zeolite shows, after saturation with methanol at room temperature and following isothermal desorption of the excess, a desorption curve with a pronounced maximum at about 400 K and a structured flank between 450 and 600 K (see curve 31 amu in Figure 2). The desorbed amount is 2.6 ( 0.3 mmol g-1, i.e., 2.5 molecules per bridging Si-OH-Al group. However, in contrast to the case of NaZSM-5, the reactivity of this zeolite is, as expected, higher. This follows from the desorption of dimethyl ether (45 amu) starting above 400 K, water desorption (18 amu) beginning at about 450 K, and a fast increasing hydrocarbon desorption (41 amu, mainly propene) above 500 K. The structuring of the flank of methanol desorption above 450 K should also be attributed to this. If the temperature program was stopped at 400 K and the desorption took place isothermally at this temperature, no reaction could be observed. The amount of methanol desorbed at this temperature is about 1.4 mmol g-1, so that afterward there is still approximately one adsorbed molecule per OH group. A stronger bound 1:1 adsorbate complex of methanol on the
(22) Pope, C. G. J. Chem. Soc., Faraday Trans. 1993, 89, 1139. (23) Gla¨ser, B. Thesis, University of Leipzig, 1994.
(24) Hunger, B.; Heuchel, M.; Matysik, S.; Beck, K.; Einicke, W.-D. Thermochim. Acta 1995, 269/270, 599.
Experimental Section Zeolites. The NaZSM-5 zeolite was a commercial material “HS30” (Na6[Al6Si90O192]) supplied by Chemiewerk Bad Koestritz GmbH, Germany. The micropore volume determined by means of nitrogen adsorption at 77 K was 0.155 cm3 g-1.23 The saturation capacity for methanol amounted to 3.83 mmol g-1.23 The H-form, HZSM-5, was prepared by a threefold ion exchange with 0.1 N HNO3 at room temperature. The Si/Al ratio was determined by chemical analysis and by 27Al MAS NMR to be 15. Temperature-Programmed Desorption (TPD). The temperature-programmed desorption (TPD) was carried out in a conventional flow device with helium as carrier gas (3 L h-1). For evolved gas detection both a thermal conductivity detector (TCD) and a quadrupole mass spectrometer (Leybold, Transpector CIS System) with a capillary-coupling system were used. For each experiment 100 mg of the hydrated zeolite was used in a mixture with 1 g of quartz of the same grain size (0.2-0.4 mm). At first the probes were heated with 10 K min-1 in the helium flow up to 673 K and activated at this temperature for 1 h. After this time no water could be detected in the helium stream. Next the zeolite was cooled to room temperature and loaded with methanol by injection of small pulses. Then the sample was flushed with helium till no further desorption could be observed. Afterwards the linear temperature program (10 K min-1) was started. For a kinetic evaluation, experiments were carried out using different heating rates (2-20 K min-1). The gas phase composition was analyzed by means of the mass units for methanol (31 and 32 amu), water (18 amu), dimethyl ether (45 amu), C3-C5 aliphatic hydrocarbons (40-44, 56, 58, 70, and 72 amu), and aromatics (91 amu: specifically benzene, 78 amu; toluene, 92 amu, xylene: 106 amu). The determination of the desorbed methanol amount was carried out by an internal calibration of the TCD signal and the 31 amu response. Adsorption Measurements. The adsorption isotherms of methanol vapor were determined gravimetrically on a McBain balance fitted to a vacuum system (
