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

Abstract

The dominating hydrocarbon pool species (HCPs) in zeolites for methanol-to-olefin (MTO) conversion have been the subject of intense debate for decades due to the diversity of structures and the complexity of reaction networks. We performed microkinetic simulations in a three-site model to study the MTO conversion in industrially relevant H-SAPO-34 zeolite under a wide range of operating conditions. The energetics of 229 and 342 elementary reaction steps were employed, respectively, in the aromatic-based and olefin-based cycles. The dynamic distribution and evolution of the retained aromatic or olefinic HCPs and the origin of olefin products were revealed with respect to reaction conditions. We corroborate that the olefin-based cycle dominates the MTO conversion in H-SAPO-34 under most reaction conditions, and the contribution of the aromatic-based cycle increase with increasing temperature, decreasing pressure, and/or decreasing water partial pressure. The paring route, dedicated highly to propene formation, prevails in the aromatic-based cycle; the side chain route, favoring exclusively ethene formation, only prevails at extremely lower temperatures, higher pressures, and higher water contents. The inherent activity of each aromatic HCP via the paring cycle increases, while its population retained in H-SAPO-34 usually decreases, with the methylation degree remarkably from tetramethylbenzene to hexamethylbenzene. The contents of higher methylbenzenes increase with decreasing water partial pressure, leading to the enhanced contribution of the aromatic-based cycle. The olefin-based cycle contributes to the formation of both ethene and other olefins, and the product distribution is drastically sensitive to reaction conditions. Ethene predominantly comes from the cracking of C5 and C6 species, and propene comes from C5 to C7 species. The olefin-based cycle shifts from the interconversion of C2–C7 olefins toward C2–C5 ones with increasing temperature and water partial pressure to enhance ethene formation. Under industrially relevant conditions, the conversion rate of methanol via the olefin-based cycle is 40-fold greater than that via the aromatic-based cycle (6.6 vs 0.15 s–1), and the former agrees unexpectedly with the experimental value in low-silica AlPO-34 (7.0 s–1). This work thus solves the puzzle of dominating HCPs as a function of reaction conditions in H-SAPO-34 and provides mechanistic insights into the kinetic behaviors, which are the basis for the optimization of the MTO catalyst and process.

Machine Learning Prediction of Transition-State Energies in Small-Pore Zeolites Combining Acidity and Topology Descriptors

“…Alkylation of aromatics with olefins or alcohols is an industrially important reaction in zeolite catalysis and is highly sensitive to zeolite framework topologies. − In addition, alkylation of aromatics like methylbenzene with methanol is one of the elementary reactions in the methanol-to-olefin conversion − and is usually catalyzed by 8-MR small-pore zeolites. , As a result, toluene methylation with methanol to p -xylene was taken as a model reaction to develop the ML models as a proof of concept for the prediction of TS energies in a series of small-pore zeolites (see Figure S1) using readily available descriptors. Toluene methylation is a typical catalytic reaction by the Brønsted acid site of zeolites, and the TS bears the S N -2 structure in different topologies.…”

Microkinetic simulations of ketene conversion to olefins in H-SAPO-34 zeolite for bifunctional catalysis

“…The three-site model established previously for the microkinetic simulations of the methanol conversion to olefins in zeolites was utilized herein for the conversion of ketene to olefins in H-SAPO-34 (Scheme S1†). 42 Similarly, the HP site with both HCPs and Brønsted acid site is the active center for ketene conversion and olefin formation. The HB site with only the Brønsted acid site was introduced to describe the pores that are not occupied by HCPs.…”

“…It is noteworthy that olefins themselves are likely to be the dominating HCPs in H-SAPO-34 for the conversion of methanol, and the contents of aromatics may be much lower under the reaction conditions for bifunctional catalysis than those for the conversion of methanol. 8,42 As a result, only the olefin-based cycle was taken into account. The similarities and differences between the ketene and methanol conversions in the estimated activities, selectivity, and evolution preference were demonstrated.…”

Intergrowth Zeolites, Synthesis, Characterization, and Catalysis

“…For the commercialized SAPO-34, reaction conditions such as temperature and weight hourly space velocity (WHSV) have a significant impact on the MTO reaction (Table , Entries 1–3). In general, raising the reaction temperature remarkably contributes to methanol conversion and light olefin (C 2 –C 3 ) selectivity, while a low WHSV reduces coking and extends catalyst lifetime. , In addition, topology, acidity, and crystal size are crucial factors that influence the MTO performance of molecular sieve catalysts, but there are few reports that specifically study the acidity of intergrowth zeolites. Therefore, in this section, the effects of topology and morphology on MTO activity are separately discussed, and an improved strategy for morphology optimization is proposed based on the elucidation of the coking and deactivation mechanism of intergrowth zeolite.…”