Unraveling the Reaction Mechanisms Governing Methanol‐to‐Olefins Catalysis by Theory and Experiment

Abstract

The conversion of methanol to olefins (MTO) over a heterogeneous nanoporous catalyst material is a highly complex process involving a cascade of elementary reactions. The elucidation of the reaction mechanisms leading to either the desired production of ethene and/or propene or undesired deactivation has challenged researchers for many decades. Clearly, catalyst choice, in particular topology and acidity, as well as the specific process conditions determine the overall MTO activity and selectivity; however, the subtle balances between these factors remain not fully understood. In this review, an overview of proposed reaction mechanisms for the MTO process is given, focusing on the archetypal MTO catalysts, H-ZSM-5 and H-SAPO-34. The presence of organic species, that is, the so-called hydrocarbon pool, in the inorganic framework forms the starting point for the majority of the mechanistic routes. The combination of theory and experiment enables a detailed description of reaction mechanisms and corresponding reaction intermediates. The identification of such intermediates occurs by different spectroscopic techniques, for which theory and experiment also complement each other. Depending on the catalyst topology, reaction mechanisms proposed thus far involve aromatic or aliphatic intermediates. Ab initio simulations taking into account the zeolitic environment can nowadays be used to obtain reliable reaction barriers and chemical kinetics of individual reactions. As a result, computational chemistry and by extension computational spectroscopy have matured to the level at which reliable theoretical data can be obtained, supplying information that is very hard to acquire experimentally. Special emphasis is given to theoretical developments that open new perspectives and possibilities that aid to unravel a process as complex as methanol conversion over an acidic porous material.

Methanol to olefins: activity and stability of nanosized SAPO-34 molecular sieves and control of selectivity by silicon distribution

“…Therefore, taking into account the experimental results obtained and assuming that the paring scheme of the hydrocarbon pool is expected to be less predominant in SAPO-34 59 , olefins selectivity should be produced through aromatic intermediates by the side-chain methylation mechanism 60 . These aromatic and positively charged transition states could lead to different selectivities depending on differences in acid strength, as suggested recently by Westgard et al 61 .…”

Complex Reaction Environments and Competing Reaction Mechanisms in Zeolite Catalysis: Insights from Advanced Molecular Dynamics

“…[1][2][3] Within the field of heterogeneous catalysis, the conversion of methanol to hydrocarbons (MTH) or olefins (MTO) over acidic zeolites received a lot of attention during the last decades, due to its relevance in the search for alternative processes to produce hydrocarbon products. [4][5][6][7] The MTO process has experimentally been developed in the past 3-4 decades and is currently being industrialized. [5] Methanol can be obtained from coal, natural gas or biomass and is in turn converted into ethene, propene or other hydrocarbons.…”

“…[5,6,26] There is still an ongoing debate about the exact reaction mechanism, for which 6 two possible reaction routes have been proposed. [26] On the one hand, the methylation can occur in a concerted fashion, meaning that protonation of methanol, water formation and the methyl transfer occur simultaneously.…”

Direct Observation of Cyclic Carbenium Ions and Their Role in the Catalytic Cycle of the Methanol‐to‐Olefin Reaction over Chabazite Zeolites

“…Catalysts used in the MTO process are typically microporous solid acids, including zeolites and zeotype molecular sieves, among which the H-ZSM-5 zeolite with framework type MFI and SAPO-34 with framework type CHA deliver the best catalytic performance in the MTO reaction. [1][2][3][4] Despite the tremendous research efforts over the past 30 years, [4][5][6][7][8] the reaction mechanism of the first C À C bond in the MTO process remains elusive. In general, the hydrocarbon pool (HCP) mechanism, [9] that is, that cyclic organic species confined in the zeolite cage or intersection of channels act as co-catalysts, has been generally accepted as a rational explanation for the olefins production from the C 1 reactant, methanol.…”