(79c) Improving Methanol-to-Olefins Conversion Performance of CHA Materials By Seeding the Hydrocarbon Pool

Methanol-to-olefins conversion on zeotype materials proceeds via a mechanism described by a dual-cycle schematic comprising olefin and aromatic methylation/cracking cycles, with methylbenzenes formed during an induction period acting as co-catalytic scaffolds in ethene and propene formation. Decreasing catalyst lifetime with increasing methanol pressure at equivalent methanol weight hourly space velocities and increasing catalyst lifetime obtained when using dimethylether as the feed instead of methanol evidence formaldehyde, formed in hydride transfer steps, as a key intermediate effecting polycylic aromatic hydrocarbon formation leading to catalyst deactivation. Reactions unproductive to C-C bond formation predominate at low turnover numbers, characterized by a low concentration of active chain carriers and high effluent methane selectivities. We reasoned that introduction of active hydrocarbon chain carriers into an otherwise empty pore before the first turnover would lead simultaneously to a reduction in the length of the induction period, as well as a decrease in carbon loss due to the introduction, at low turnover numbers, of reaction pathways competing with methanol dehydrogenation to form formaldehyde.

We report experiments in which seeding the CHA zeotypic materials with acetaldehyde (0.1 kPa CH₃CHO, 673K, 0.02-0.5 C/H⁺) reduces the initial rate of carbon loss by introducing reaction pathways that compete with methanol dehydrogenative transfer events. Increasing seed loadings and inlet methanol pressures lead to lower cumulative carbon loss at early turnovers, consistent with lower formaldehyde formation rates being responsible for the observed reduction in carbon loss. The introduced seed also acts as a co-catalyst, thereby reducing the length of the induction period, leading to higher turnover numbers with increasing seed loadings at equivalent times-on-stream. These studies exemplify a strategy for mitigating the deleterious effects of methanol hydride abstraction events that effect MTO deactivation.