Mechanistic studies of methanol-to-hydrocarbons conversion on diffusion-free MFI samples

Self-pillared pentasil MFI (similar to 1 nm diffusion length) exhibited low ethene selectivity (1.1%) at <100% conversion for the catalytic reaction of dimethyl ether (DME) at 723 K and similar to 60 kPa DME pressure suggesting that the aromatics-based catalytic cycle is intrinsically suppressed in the pores of MFI under these reaction conditions. Co-feeding toluene or p-xylene with DME increased the number of chain carriers of the aromatics-based cycle, thereby enhancing its propagation and resulting in a 2-3-fold increase in ethene selectivity. Co-feeding propene or 1-hexene, however, did not have an effect on the product distribution, suggesting that the olefins-based hydrocarbon pool is saturated in the pores of MFI. At high temperature (723K) and low DME space velocity (<= 2.5 mol C [mol Al-s](-1)), conditions resulting in complete DME/methanol conversion, the catalyst bed comprises two stages: The first stage performs methanol-to-hydrocarbons chemistry in the presence of DME/methanol; the second stage begins after 100% DME conversion is achieved and is characterized by the absence of DME/methanol. The aromatics-based methylation/cracking cycle is absent in the second stage as methylbenzenes cannot dealkylate in the absence of DME/methanol, and the dominant pathway to ethene formation under these reaction conditions is olefin inter-conversion. (C) 2015 Elsevier Inc. All rights reserved.