Catalytic Routes for Upgrading C1 Feedstocks… The Hills Molecules Climb

Jan 31, 2024, 4:00 pm5:00 pm



Event Description

C1 molecules, typified by abundant methane and carbon dioxide, are destined to serve as carbon sources for energy carriers and chemicals as we transition to a low-carbon future. We illustrate the kinetic and thermodynamic challenges in directing catalytic routes for the conversion of these critical feedstocks to products of higher value in the context of two case studies.

Strong, apolar C-H bonds in methane confer significant thermodynamic barriers for non-oxidative conversion of methane to aromatics. Medium pore MFI zeolites modified by well-dispersed carbidic molybdenum aggregates (MoCx/ZSM-5) reduce kinetic barriers to methane pyrolysis and catalyze dehydroaromatization (DHA) reactions with high benzene (≳ 70%) and aromatic (≳ 95%) selectivity at conversions near the ~10% equilibrium limit at ~950 K. We combine learnings from thermodynamics, reaction kinetics, and mass transport to clarify mechanistic observations and ascertain the identity of molecular events, species, and catalytic moieties that determine the activity of benchmark DHA catalysts. In doing so, we illustrate the significance of dispersive hydrogen transport at catalyst-bed length scales during DHA catalysis, and we leverage this understanding to develop polyfunctional catalytic formulations that alleviate thermodynamic constraints on maximum single-pass conversion in DHA catalysis.

We resolve persistent mechanistic questions regarding the selectivity to methanol and CO in competing CO2 hydrogenation pathways, the reactive intermediates involved, and the evolution in structure and function of commercial Cu/ZnO/Al2O3 catalyst using the general conceptual framework for analyzing rates and reversibility of catalytic reaction sequences. These kinetic analyses show a dearth of H* species during catalysis, provide thermodynamic constraints precluding sequential reverse water gas shift and CO hydrogenation as the pathway for methanol generation, reveal hydrogen and water as species salient in determining methanol selectivity and yield by impacting both the forward and reverse rates of CO2 hydrogenation on Cu/ZnO/Al2O3, and explicate the fundamentals of novel sorption-enhanced methanol synthesis, which not only alleviates equilibrium constraints but also alters the intrinsic rate at which the system approaches equilibrium.