(300b) In-Situ UV-Vis-NIR Diffuse Reflectance Spectroscopic Investigation of n-Butane Isomerization On H-Mordenite and Pt/H-Mordenite at Various H2 Partial Pressures

Catalytic skeletal isomerization of n-butane is valuable for production of isobutane and isobutene, used in alkylation and methyl tert.-butyl ether production, respectively [1]. Currently used platinum-doped chlorided alumina catalysts are candidates for replacement because of their corrosive nature and resulting potential process hazards. Due to their high acidity, zeolites such as H-Mordenite and solid oxides such as sulfated zirconia are capable of performing this difficult reaction [2,3] and represent a possible alternative. These catalysts have the advantage that they are tolerant to contaminants and are regenerable. However, both catalysts suffer from deactivation and require platinum doping and H2 addition to the feed to stabilize performance, as does chlorided alumina. Complicating the investigation of these catalysts and interpretation of the data is the redox activity of sulfated zirconia and chlorided alumina's sensitivity to traces of water. Although the properties of H-Mordenite may vary, the factors influencing them are well understood, making this material a suitable candidate for a fundamental investigation. H-Mordenite is a one-dimensional zeolite with a 12-membered ring main channel that has been extensively investigated for use in many reactions because of its high acidity and specific pore geometry [4]. As applied to the isomerization of n-butane, there is no consensus in the literature concerning the presence of monomolecular or bimolecular routes of formation. Geometric constrictions in the zeolite pore system have been cited as preventing or severely restricting a bimolecular rearrangement within the zeolite pores. It is unknown whether the acid strength of H-Mordenite is high enough to promote the energetically difficult monomolecular rearrangement [3-8]. To optimize selectivity, clarification of all active reaction paths, including those leading to side products and coke formation, is compulsory. Our strategy is to compare H-Mordenite and Pt/H-Mordenite (0.3-0.4 wt% Pt) in a variety of reaction conditions in both a plug flow reactor an in-situ UV-vis-NIR diffuse reflectance spectroscopic cell, which allows us to monitor the formation of unsaturated surface deposits with time on stream. The influence of the H2 partial pressure on the performance of H-Mordenite and Pt/H-Mordenite was investigated in a plug flow reactor at atmospheric pressure. The n-butane partial pressure was held constant at 0.1 bar while the H2 content in the feed was reduced stepwise from an initial level of 0.9 bar using helium as a replacement. Online product analysis using a gas chromatograph showed a gradual increase in rate for both catalysts as H2 was eliminated from the feed. Surprisingly, deactivation on H-Mordenite did not occur until 0.2 bar of H2 was reached, whereas Pt/H-Mordenite began deactivating at 0.1 bar of H2. The decrease in rate with increasing H2 partial pressure has previously been attributed to either hydrogenation of trace olefins (which serve as precursors for carbenium ions) in the feed or suppression of sec-butyl carbenium ion formation in agreement with LeChatelier's principle [4,8]. For the latter to be the case, it must be true that carbenium ion formation occurs through a short-lived penta-coordinated carbocation intermediate followed by dihydrogen abstraction, resulting in carbenium ion formation. For trace olefins to be hydrogenated on platinum free Mordenite, it must be true that H2 is activated by the zeolites, which has indeed been shown to occur [9]. Previous studies have shown that platinum doping of H-Mordenite enhances the rate of isobutane formation [4]. Our data reveals that platinum addition has little impact on the isobutane formation rate, but does result in decreased isobutane selectivity with increasing H2 concentrations and increasing temperature. A bimolecular route is clearly active on both catalysts as indicated by the presence of propane, pentane, and isopentane products. However, the C3 and C5 products are not always formed in a 1:1 ratio, as expected from the disproportionation reaction of an octyl intermediate. C3/C5 ratios larger than one have been attributed to the formation of a nonyl intermediate (formed from C4 and C5 species) that selectively undergoes disproportionation to three C3 molecules [10]. On platinum, propane may also be formed through hydrogenolysis at high H2 concentrations. The selectivity to propane on H-Mordenite increases at the expense of isobutane as the conversion increases, for example as a result of temperature increase or H2 partial pressure decrease. These observations indicate that the preferred pathway of propane formation depends on the reaction conditions. Because the performance of the H-Mordenite catalysts with and without platinum was stable, activation energies for individual products could be calculated from Arrhenius plots under a variety of conditions. The activation energy for isobutane formation at 0.9 bar H2 was found to be 135 kJ/mol on H-Mordenite and 149 kJ/mol on Pt/H-Mordenite. The activation energy for propane formation was determined to be 188 kJ/mol on H-Mordenite and 116 kJ/mol on Pt/H-Mordenite. Activation energies for C5 formation were slightly lower on Pt/H-Mordenite than on H-Mordenite. It is expected that the monomolecular isomerization mechanism is characterized by a significantly higher activation energy than the bimolecular mechanism because of the primary carbenium ion that must be formed from a cyclopropane intermediate, assuming that this is the most energetically difficult step of the rearrangement. Because of the linearity of the Arrhenius plots it is apparent that the isobutane formation mechanism does not change in the 250-330°C temperature range. Activation energies were also calculated at different partial pressures of H2 on the Pt/H-Mordenite. The data reveals constant isobutane activation energies, indicating no mechanistic change for isobutane formation under varying H2 partial pressures. Propane activation energies increase as H2 partial pressure is decreased, indicating that a different pathway of propane formation becomes dominant. We also investigated the course of deactivation of H-Mordenite during n-butane isomerization. The decline in activity has previously been attributed to coke formation on the catalyst surface, which was proposed to poison acid sites of block pores [3]. We used in-situ UV-vis-NIR diffuse reflectance spectroscopy performed with a Perkin-Elmer Lambda 950 spectrometer to monitor accumulation of unsaturated compounds on the catalyst surface while simultaneously collecting kinetic data with an online gas chromatograph. The resulting data confirms that catalyst deactivation coincides with the accumulation of unsaturated species. Multiple absorption bands suggest the presence of multiple unsaturated species with differing degrees of saturation. Comparison with literature data [11] suggests the species may be allylic cations with a varying number of further conjugated double bonds. To further explore the nature of these deposits, spectra were taken before and after attempts to solubilize the adsorbed species using a soxhlet extractor with multiple solvents (methanol, dichloromethane, heptanes). Species responsible for bands <500 nm could be partially dissolved, while those absorbing at higher wavelength remained. These observations are consistent with the findings of Murzin et al. who identified adsorbed species on Pt-Beta and ZSM-5 during reactions with n-butane to be n-butane tetramers to heptamers [3]. The soluble coke likely consists of these n-butane oligomers, whereas species absorbing >500 nm are likely highly conjugated polyaromatic species. By calculating activation energies for isobutane and propane under varying reaction conditions, we have determined that the route of isobutane formation is independent of H2 partial pressure or temperature, while propane is likely formed through multiple routes. The use of in-situ UV-vis-NIR diffuse reflectance spectroscopy has allowed us to conclusively assert that deactivation is caused by adsorbed allylic cations, which begin to accumulate at low H2 partial pressures. H-Mordenite was surprisingly stable, with deactivation not setting in until a H2 partial pressure of 0.2 bar was reached.

References 1. Asuquo, R. A., Eder-Mirth, G., Lercher, J.A., J. Catal. 155, 376 (1995) 2. Hino, M., Kobayashi, S., Arata, K., J. Am. Chem. Soc. 101, 6439 (1979) 3. Villegas, J.I., Kumar, N., Heikkilä, T., Lehto, V.-P., Salmi, T., Murzin, D. Y., Chem. Eng. J. 120, 83 (2006) 4. Asuquo, R. A., Eder-Mirth, G., Seshan, K., Pieterse, J. A. Z., Lercher, J.A., J. Catal. 168, 292 (1997) 5. Oliveira, A. C., Essayem, N., Tuel, A., Clacens, J.-M., Tâarit, Y. B., J. Mol. Catal. A: Chem. 293, 31 (2008) 6. Krupina, N.N., Proskurnin, A.L., Dorogochinskii, A.Z., React. Kinet. Catal. Lett. 32, 135 (1986) 7. Cañizares, P., de Lucas, A., Dorado, F., Pérez, D., Appl. Catal., A. 190, 223 (2000) 8. Tran, M.-T., Gnep, N.S., Szabo, G., Guisnet, M., J. Catal. 174, 185 (1998) 9. Meusinger, J., Corma, A., J. Catal. 152, 189 (1995) 10. Dumesic, J.A., Rudd, D.F., Apericio, L.M., Rekoske, J.E., and Trevino, A, A., in ?The Microkinetics of Heterogeneous Catalysis,? p. 272. Am. Chem. Soc., Washington, DC, 1993. 11. Demidov, A.V., Davydov, A.A., Mater. Chem. Phys., 39, 13 (1994)