Mechanism of the Catalytic Conversion of Methanol to Hydrocarbons

Abstract

The discovery of the dual aromatic- and olefin-based catalytic cycles in methanol-to-hydrocarbons (MTH) catalysis on acid zeolites has given a new context for rationalizing structure–function relationships for this complex chemistry. This perspective examines six major chemistries involved in the hydrocarbon pool mechanism for MTHolefin methylation, olefin cracking, hydrogen transfer, cyclization, aromatic methylation, and aromatic dealkylationwith a focus on what is known about the rate and mechanism of these chemistries. The current mechanistic understanding of MTH limits structure–function relationships to the effect of the zeolite framework on the identity of the hydrocarbon pool and the resulting product selectivity. We emphasize the need for assessing the consequences of zeolite structure in MTH in terms of experimentally measured rates and activation barriers for individual reaction steps and in terms of speciation preferences within the dual olefin- and aromatic-catalytic cycles to alter their relative propagation. In the absence of individual reaction rates, we propose using ethene/isobutane selectivity as a measure to describe the relative rates of propagation for the aromatic- and olefin-based cycles.

Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

“…Over ZSM-5 catalysts, the transformation of methanol can be tuned towards light olefin production (MTO) at high temperatures and low pressures [16][17][18][19]. The underlying chemistry involves chain growth and cracking where larger molecules obtained through methylation by surface methoxy groups (CH 3 + Z − ) crack to give a product distribution rich in light olefins [20]. To obtain a detailed understanding of this reaction mechanism, it is important to confirm the origin of these surface methylating species.…”

“…A current lack of knowledge on the primary surface reactant, the source of the surface methylating group, has led to the lumping of methanol and DME in previous experimental and modelling kinetic studies [20][21][22]. This lumping methodology is fraught in its usage as it eludes the fact that both species have different interactions with the sites of the ZSM-5 catalysts and avoids mechanistic descriptions necessary for a microkinetic model.…”

A research into the thermodynamics of methanol to hydrocarbon (MTH): conflictions between simulated product distribution and experimental results

“…3a-c) obtained in each reaction, showing formation of olefins is not thermodynamically favoured under the applied conditions. In the temperature range of 300-500°C, which was indicated to donate large portions of C 2 -C 4 olefins in the previous researches, [3,5,12,19], ethylene selectivity is higher than other two olefin types, and shows a gradually increasing trend. It seems C 4 H 8 (1-butene) is greatly supressed in each case, as reflected by the observed data.…”

“…However, our assumption is based on the 'hydrocarbon-pool' cycle, which has been considered as the origin of series of MTH reactions and gives C 2 -C 4 olefins and methyl-aromatics as the primary products [4]. Further consideration is given to the fact that latter stage hydrogen transfer may convert these primary products into either saturated hydrocarbons (alkanes) or graphite species (deeply dehydrogenated cokes), and these intra-conversions indeed play an important role in deciding the following catalytic process, and the final product yields [3,4,11,12]. This simulation also takes the advantage that all of these 'hydrocarbon-pool' primary products (C 2 -C 4 olefins and C 6 -C 9 aromatics) take a large portion in the product yield of short time (a few minutes to several hours) lab reactions and industrial applications [4,5].…”

“…Methyl groups are added onto the hydrocarbon pool molecules in alkylation steps and light olefins are subsequently formed by the corresponding de-alkylation steps. The initial olefins can be converted into alkanes by occupying H 2 , whereas continuous loss of H 2 , followed by the cyclization steps forms more aromatics [3][4][5]. In a mature MTH system, series of the above reactions proceed in a parallel way and, therefore, bring in a complexity to the products and orders of their formation; however, thermodynamics always say coke (extremely dehydrogenated products, or solid carbons) is the dominating product in MTH and most other hydrocarbon transformations [6].…”