Aromatics-based catalytic cycle Olefins-based catalytic cycle MFI Diffusion free Ethene selectivity Low space velocity High conversion a b s t r a c t Self-pillared pentasil MFI ($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 $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 (723 K) and low DME space velocity (62.5 mol C [mol Al-s] À1 ), 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.
Increasing crystallize size or aluminum content in MFI-type zeolites independently enhances the propagation of the aromatics-based methylation/dealkylation cycle relative to that of the olefinsbased methylation/cracking cycle in methanol-to-hydrocarbons (MTH) conversion and consequentially results in higher ethene selectivity. We report that ethene selectivity increases monotonically with increasing aluminum content for ZSM-5 samples with nearly identical crystallite size as a follow-up to our recent report detailing the effects of crystallite size (J. Catal. 321 (2015) 23) on MTH selectivity. We also propose that the confected effects of crystallite size and acid site density on MTH selectivity can be correlated using a descriptor that represents the average number of acid sites that an olefin-precursor will interact with before elution.
Copper‐oxo clusters exchanged in zeolite mordenite are active in the stoichiometric conversion of methane to methanol at low temperatures. Here, we show an unprecedented methanol yield per Cu of 0.6, with a 90–95 % selectivity, on a MOR solely containing [Cu3(μ‐O)3]2+ active sites. DFT calculations, spectroscopic characterization and kinetic analysis show that increasing the chemical potential of methane enables the utilization of two μ‐oxo bridge oxygen out of the three available in the tricopper‐oxo cluster structure. Methanol and methoxy groups are stabilized in parallel, leading to methanol desorption in the presence of water.
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