Theoretical Study on Elementary Reactions in the Methanol-to-olefin Process

Light olefins, which are vital components for the petrochemical industry, are traditionally obtained through the refinement of crude oil. However, the rapidly increasing demand of oil-based chemicals calls for the development of new technologies based on alternative natural sources. Among these processes, the methanol-to-hydrocarbons (MTH) technology is one of the most promising and currently even ready for commercial use. Methanol is made from synthesis gas (CO+H2), which can be formed either by steam reforming of natural gas or by gasification of coal or biomass. Methanol can then be converted to a pre-equilibrium mixture of methanol, dimethyl ether and water, which can be processed catalytically to olefins (methanol-to-olefins, MTO) or gasoline (methanol-togasoline, MTG). Quite remarkably, for more than 30 years the actual mechanism of the MTO process in acidic zeolites has been the source of considerable debate, fueled by the countless diverse and often conflicting propositions. The trickiest step to elucidate has always been the formation of the first carbon-carbon bond, for which more than 20 distinct mechanisms have been proposed. Early speculation mainly centered on mechanisms based on the ‘direct’ formation of small olefins from only methanol, dimethylether and single-carbon derivates. More recent developments suggest, however, that these direct mechanisms might not be responsible for the observed boost in hydrocarbon synthesis after all. A plausible alternative is given by the ‘hydrocarbon pool’ model, in which impurities in the mixture undergo repeated methylation and subsequent olefin elimination. Our goal is to unequivocally determine the underlying mechanism of the MTO process from a theoretical viewpoint, as experimental methods are often impractical for evaluating individual reaction steps. Before looking into alternative hydrocarbon pool proposals, we need to explain why the numerous direct carbon-carbon coupling mechanisms proposed fail in consistent production of ethylene. Furthermore, we need to identify the directly formed species which might interact with the hydrocarbon pool species. In this extended abstract we summarize our approach for this primary investigation and give more detail on some crucial points.