Methanol-to-Olefins Catalysis in Erionite Molecular Sieves: Towards Enhancing the Ethylene Selectivity

The catalytic conversion of methanol-to-olefins (MTO) is an efficient and commercialized route to obtain light-olefins. SAPO-34, a molecular sieve with a chabazite (CHA) topology, is the commercial catalyst for the MTO process that produces high light-olefins selectivities (>80%) and proceeds via successive methylation and dealkylation/cracking reactions in a dual-cycle mechanism involving organic hydrocarbon-pool (HP) species.

Although ethylene and propylene selectivities are cumulatively high in SAPO-34, promoting the formation of ethylene (i.e., increasing E/P) without deleteriously impacting the overall light-olefins selectivities (>70%) and catalyst lifetime, remains a scientific challenge. ERI-type molecular sieves are promising candidates for shifting the light-olefin selectivities towards ethylene due the size of the ERI cage (Cage-Defining Ring (CDR) size = 6.75 Å for ERI compared to 7.45 Å for CHA) [1]. Here, we synthesize ERI-type materials (ERI-type zeolites (Si/Al = 5-11), SSZ-98 (Si/Al = 4-7), UZM-12 (Si/Al = 4), and SAPO-17 (Si/T = 0.07-0.17)) and investigate their MTO activity (400 ^oC, 1 atm, and WHSV = 1.3 hr^-1).

Our reaction results demonstrate that ERI-type materials have a higher propensity to produce ethylene than propylene when compared to SAPO-34 (E/P=0.7 at Si/T=0.07), as SAPO-17 catalysts yield an E/P=0.8-1.2 (averaged selectivity ratio when methanol conversion is >97%) whereas ERI-type zeolites yield an E/P=1.1-1.7 at the aforementioned reaction conditions. When the Si/T in SAPO-17 is high and the Si/Al is low in the zeolites, the catalytic behavior of the SAPOs approaches that of zeolites. We additionally show that the E/P can be further increased (E/P = 3+) and the lifetime improved (by a factor of 3+) by manipulating the reaction conditions and feed composition.

The improvement in E/P in ERI-type materials over CHA-type molecular sieves is likely due to the alteration of the entrapped polymethylaromatics.

References:

[1] J.H. Kang et al., ACS Catalysis 2019 9(7), 6012-6019.