(101c) Unraveling the Complex Relationship between Framework Topology and Acidity on the Light Olefins Selectivities in the Methanol-to-Olefins Reaction

The methanol-to-olefins (MTO) process represents an alternative and attractive route for upgrading natural gas and coal to higher value products and to alleviate the progressively growing demand for ethylene (E) and propylene (P). The catalyst used commercially in the MTO technology is SAPO-34 (CHA), which exhibits exceptional catalytic performance (approximately, 90% selectivity to C₂-C₄ at complete methanol conversion; E/P~1).

In spite of the commercial success of SAPO-34, there is a growing interest in developing strategies that would augment the olefins product distribution to favor the formation of either more ethylene or propylene, depending on demand and/or profitability. To that end, prior reports identified cage size and acidity as parameters that influence product selectivity in MTO. Here, we systematically investigate the MTO behavior of 46 molecular sieves belonging to the following framework types (AEI, CHA, LEV, SWY and ERI) that are synthesized over a wide range of Si/Al=4-31 and Si/(Al+P)=0.04-0.3, to rationalize the effect of cage dimensions on the olefins product distribution as a function of acid site density and strength. We specifically aim to address the key question of why acidity significantly influences the E/P of some frameworks but not others.

Catalysts were characterized using a myriad of techniques and evaluated in MTO for their product distribution. Our results show that changes in acid site density and strength play a secondary role to the dominating influence of cage architecture on product distribution in AEI- and CHA-type molecular sieves. Decreasing the cage size, in going from AEI and CHA to LEV, SWY and ERI, however, results in substantial changes in the E/P as a function of acidity. These results are rationalized based on differences in the hydrocarbon-pool (HP) species that form, particularly in early stages of the reaction.