Microkinetic Simulations of Methanol-to-Olefin Conversion in H-SAPO-34: Dynamic Distribution and Evolution of the Hydrocarbon Pool and Implications for Catalytic Performance

The dominating hydrocarbon pool species (HCPs) in zeolitesformethanol-to-olefin (MTO) conversion have been the subject of intensedebate for decades due to the diversity of structures and the complexityof reaction networks. We performed microkinetic simulations in a three-sitemodel to study the MTO conversion in industrially relevant H-SAPO-34zeolite under a wide range of operating conditions. The energeticsof 229 and 342 elementary reaction steps were employed, respectively,in the aromatic-based and olefin-based cycles. The dynamic distributionand evolution of the retained aromatic or olefinic HCPs and the originof olefin products were revealed with respect to reaction conditions.We corroborate that the olefin-based cycle dominates the MTO conversionin H-SAPO-34 under most reaction conditions, and the contributionof the aromatic-based cycle increase with increasing temperature,decreasing pressure, and/or decreasing water partial pressure. Theparing route, dedicated highly to propene formation, prevails in thearomatic-based cycle; the side chain route, favoring exclusively etheneformation, only prevails at extremely lower temperatures, higher pressures,and higher water contents. The inherent activity of each aromaticHCP via the paring cycle increases, while its population retainedin H-SAPO-34 usually decreases, with the methylation degree remarkablyfrom tetramethylbenzene to hexamethylbenzene. The contents of highermethylbenzenes increase with decreasing water partial pressure, leadingto the enhanced contribution of the aromatic-based cycle. The olefin-basedcycle contributes to the formation of both ethene and other olefins,and the product distribution is drastically sensitive to reactionconditions. Ethene predominantly comes from the cracking of C-5 and C-6 species, and propene comes from C-5 to C-7 species. The olefin-based cycle shifts from theinterconversion of C-2-C-7 olefins towardC(2)-C-5 ones with increasing temperatureand water partial pressure to enhance ethene formation. Under industriallyrelevant conditions, the conversion rate of methanol via the olefin-basedcycle is 40-fold greater than that via the aromatic-based cycle (6.6vs 0.15 s(-1)), and the former agrees unexpectedlywith the experimental value in low-silica AlPO-34 (7.0 s(-1)). This work thus solves the puzzle of dominating HCPs as a functionof reaction conditions in H-SAPO-34 and provides mechanistic insightsinto the kinetic behaviors, which are the basis for the optimizationof the MTO catalyst and process.