(532bh) Enhancing the Propylene Selectivity in the Methanol-to-Olefins Reaction over SAT-Type Molecular Sieves

Ethylene (E) and propylene (P) are two of the most important building blocks in the petrochemical industry as they are key in the synthesis of a wide spectrum of polymers and fine chemicals. However, a global propylene shortage is predicted as demand is expected to outstrip supply in upcoming years due to the rapidly growing demand for propylene alongside the low propylene yield that is achieved in naphtha cracking processes. These market dynamics have paved the road for on-purpose production technologies such as, methanol-to-olefins (MTO), to mitigate this shortfall in supply.

Molecular sieves containing Brønsted acid sites are at the nexus of the MTO process, with SAPO-34 (CHA) being the catalyst deployed industrially. In MTO, framework structure and composition (acidity) together with reaction conditions determine the reaction intermediates that form as a part of the dual-cycle mechanism, which ultimately dictate product selectivity. In prior reports, enhancing propylene selectivity was primarily achieved by utilizing (i) 10- or 12-member ring (MB) zeolites (e.g., MFI and *BEA) with high Si/Al (i.e., low acid sites) in order to suppress the aromatics cycle and improve the alkene cycle, or (ii) 8-MR molecular sieves with cages larger than CHA (e.g., AEI and DDR), in order to accommodate higher substituted methylbenzenes.

In this work, we present an alternative approach for enhancing P/E by utilizing a low acidity small-pore catalyst with a cage that is significantly narrower than CHA. We report the synthesis and characterization of several SAT-type molecular sieves (e.g., MgAPO, CoAPO and SAPO), and their MTO behavior over a wide range of reaction conditions. Our results show that the combination of low acidity and unique structural features of the narrow SAT-cage lead to a catalytic pathway and a mechanism that predominantly favors propylene formation (P/E=2-4.2; P=40-50%).