Chemistries Mediating Deactivation in Methanol-to-Hydrocarbons Conversion and Strategies to Mitigate Them
University of Minnesota
The dehydrative conversion of methanol to hydrocarbons (MTH) on solid acid catalysts is the final step in upgrading any gasifiable carbon feedstock to fuels and chemicals. MTH occurs via an autocatalytic reaction sequence in which a zeolite/zeotype cavity hosts an unsaturated hydrocarbon guest to together constitute the supramolecular chain carrier that engages in a complex network of reactions for chain carrier propagation. This complex network of reactions is summarized by dual catalytic cycles where the olefins-based chemistries of methylation and β-scission are coupled with the aromatics-based chemistries of methylation and dealkylation through hydrogen transfer and cyclization steps. This mechanistic scheme, known as the hydrocarbon pool mechanism, has provided context for rationalizing structure-function relationships in MTH catalysis over the past decade. This description however, is devoid of mechanistic guidance on pathways that mediate catalyst deactivation. Deactivation in MTH catalysis is initiated by unproductive dehydrogenation reactions of methanol to form formaldehyde via methanol disproportionation and olefin transfer hydrogenation. Subsequent alkylation reactions between formaldehyde and active olefinic/aromatic co-catalysts instigate cascades for dehydrocyclization, resulting in the formation of inactive polycyclic aromatic hydrocarbons and termination of the chain carrier.
Addition of a distinct catalytic function that selectively decomposes formaldehyde mitigates chain carrier termination without disrupting the high selectivity to ethylene and propylene in methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes. The efficacy of this bifunctional strategy to prolong catalyst lifetime increases with increasing proximity between the active sites for formaldehyde decomposition and the H+ sites of the zeolite/zeotype. Co-processing sacrificial hydrogen donors mitigates chain carrier termination by intercepting, via saturation, intermediates along dehydrocyclization cascades. This strategy increases in efficacy with increasing concentration of the hydrogen donor and provides opportunity to realize steady-state methanol-to-hydrocarbons catalysis on small-pore zeolites and zeotypes.