(10h) True Activation Energies As Anchor Points in Hydrocarbon Pool Kinetics

Small olefins like ethylene and propylene are precursors for important chemicals including ethylene oxide, ethylene glycol, and propylene oxide, and they serve as the monomers for key polymers (PE, PP). Methanol-to-olefins (MTO) conversion catalyzed by zeolites or zeotypes is an alternative, sustainable route to produce olefins, if methanol is synthesized from biomass rather than fossil resources. A characteristic of the reaction mechanism is the presence of long-lived surface intermediates in the pores of the zeolite, which are believed to be essential for olefin formation. Cyclic species, identified as alkylcyclopentenylium and alkylbenzenium ions, constitute a large fraction of the species in the pores and are among the proposed intermediates in the catalytic cycle. However, the pathways for the formation and transformations amongst these intermediates and the associated energetics are still debated. The difficulty lies in determining the true activation energies for the individual elementary reaction steps amongst the multiple reactions occurring in the hydrocarbon pool. Hence, the experimental activation energy values for various transformations such as ring contraction, cyclization, isomerization, alkylation, and cleavage inside the pores are still unknown.

The goal of this work is to observe and determine activation energies of MTO-relevant cation transformations in zeolite and zeotype pores. The reactions investigated are ring contraction and cyclization at moderate temperatures (to eliminate side reactions) within the zeolite frameworks of MFI (Si/Al, Si/Ga, Si/Fe, Si/B), MOR, BEA, and FAU. Zeolites were obtained from Zeolyst; zeotypes were synthesized. As model reactants, 1,3,5,5-tetramethylcyclohexadiene (TMCH) or 2,6-dimethyl-2,4,6-octatriene (OCT) were used. The reacting and the product species are unsaturated carbocations of reasonable longevity inside the zeolite pores, such that the transformation kinetics may be observed by in situ diffuse reflectance IR spectroscopy. For example, protonation of TMCH molecules gave cyclohexenyl cations (1549 cm^-1), which contracted to a mixture of cyclopentenyl cations (1506 cm^-1 and 1489 cm^-1). OCT, a linear molecule, was protonated in the pores to octadienyl cations (1567 cm^-1), which cyclized to cyclopentenyl cations (1491 cm^-1). These key species are identified via their C=C-C⁺ stretching vibration. In situ UV-vis spectroscopy was used as a second method to monitor side reactions and corroborate rate constants. Kinetics, via disappearing and appearing adsorbate bands, were measured isothermally (Figure 1).

Several challenges were encountered. In large pore zeolites, undesired dimerization was preferred at more than 50% coverage. Further, cyclopentenyl cations of different constitution formed and required distinction by the substitution of the allylic system (1,2,3 alkyl vs 1,3 alkyl-substituted). Here we present the most suitable data treatment and kinetic model to demonstrate that reasonable activation energies can be extracted through analysis of UV-vis and IR spectra. The average activation energy for ring contraction in H-MOR was found to be 82 kJ/mol, which is comparable with observations in liquid acids. The activation energy for the cyclization reaction in H-MOR and H-ZSM-5 was similar, suggesting a minor influence of the framework. However, lower acid strength zeotypes (Si/Ga and Si/Fe) required a higher activation energy for the cyclization of octatriene, showcasing the effect of acid strength on the cyclization barrier.

The results demonstrate that spectroscopy can be applied to observe transformations of surface intermediates and measure the kinetics of elementary steps in zeolite pores, which will help answer fundamental questions about the MTO mechanism and other zeolite-catalyzed processes. Experimentally determined, true activation energies will be useful for comparison with microkinetic models or computed energy profiles.

The authors acknowledge the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Catalysis Science, under Award Number DE-SC0021041.