(521bk) Kinetics of Hydrocarbon Transformations in Zeolite Pores

Methanol-to-olefins (MTO) conversion provides a potentially sustainable route to small olefins. MTO is also mechanistically interesting, as surface species referred to as “hydrocarbon pool” are believed to be essential for catalytic turnover. Insights into the chemistry of these species may allow addressing key challenges in MTO, such as olefin selectivity and deactivation. While the presence of cyclic species is well documented in the literature, knowing the exact constitution of these species and distinguishing relevant intermediates from spectators remain challenging tasks. Moreover, experimental values for the energy barriers associated with the transformations inside the pores are lacking. Here, measurement and data analysis protocols are established to gain access to such barriers. The ring contraction of 1,3,5,5-tetramethylcyclohexadiene (TMCH) is one of several MTO-relevant model reactions, and mordenite serves as a model catalyst with pores large enough to permit adsorption of this reactant. In situ diffuse reflectance UV-Vis and FTIR spectroscopy are used to monitor the transformation kinetics. Both methods show that TMCH is protonated to cyclohexenyl cations, which are stable enough in the pores to be observable (Ï€ – Ï€* transition at 314 nm and C=C-C⁺ stretch at 1548 cm^-1). These cyclohexenyl cations contract to alkyl-substituted cyclopentenyl cations (292 nm, 1504 cm^-1). Detailed analysis of UV-vis and IR spectra reveals a more complicated reaction network. Cyclopentenyl cations of different constitution are formed, that are distinguished by the substitution pattern of the allylic system (1,2,3 alkyl vs 1,3 alkyl-substituted). Band overlap and background effects present an additional challenge to extracting kinetics, making necessary careful data treatment and analysis. It is demonstrated that true activation energies for such surface transformations from reactant to product state can be determined from measurements at various temperatures, and comparison with reported data from liquid acid catalysis confirms their reasonableness.