Methanol-to-Olefins Catalysis on HSSZ-13 and HSAPO-34 and Its Relationship to Acid Strength

Catalyst lifetime and product selectivity of methanol-to-olefins (MTO) catalysis on isostructural HSSZ-13 and HSAPO-34, possessing Brønsted acid sites of different acid strengths, are examined and interpreted to elucidate the role of acid strength in MTO catalysis with and without high-pressure H2 co-feeds. MTO catalysis without H2 cofeeds on HSSZ-13 results in a shorter catalyst lifetime and higher paraffins-to-olefins ratio than on HSAPO-34, plausibly due to faster rates of formaldehyde formation and involvement of formaldehyde in subsequent alkylation reactions that transform active chain carriers to inactive polycyclics on Brønsted acid sites of higher acid strength. Higher reactivities of protons of greater acid strength in catalyzing hydrogenation of hydrocarbons and oxygenates result in a higher increment in lifetime, paraffins selectivity, and methane selectivity on HSSZ-13 than on HSAPO-34 when coprocessing high-pressure H2 during MTO catalysis as affirmed in independent kinetic studies and density functional theory (DFT) calculations. These results provide mechanistic insights into the critical role of acid strength in influencing catalyst lifetime and product selectivity during MTO catalysis and rationalize, from a mechanistic and kinetic vantage point, why it may be advantageous to use aluminosilicates such as HSSZ-13 in bifunctional catalytic formulations to upgrade syngas mixtures or during MTO catalysis with cofeeds of molecular hydrogen.