Abstract
The relevance of a selection of organic impurities for the initiation of the MTO process was quantified in a kinetic model comprising 107 elementary steps with ab initio computed reaction barriers (MP2:DFT). This model includes a representative part of the autocatalytic olefin cycle as well as a direct initiation mechanism starting from methanol through CO-mediated direct C–C bond formation. We find that the effect of different impurities on the olefin evolution varies with the type of impurity and their partial pressures. The reactivity of the considered impurities for initiating the olefin cycle increases in the order formaldehyde < di-methoxy methane < CO < methyl acetate < ethanol < ethene < propene. In our kinetic model, already extremely low quantities of impurities such as ethanol lead to faster initiation than through direct C–C bond formation which only matters in complete absence of impurities. Graphic
Highlights
In a sustainable methanol economy the methanol-to-olefins (MTO) process enables the production of hydrocarbons through the formation of carbon-carbon bonds from potentially renewable feed stocks [1]
The key steps that we have identified for the initiation reaction starting from MeOH or dimethyl ether (DME) using Density functional theory (DFT):MP2 [68] are (1) the oxidation of MeOH to formaldehyde (FA) and subsequent oxidation to carbon monoxide (CO) and (2) the methylation of CO to methyl acetate (MA) forming the first C–C bond [68].Within this reaction sequence, the highest free energy barrier at a reaction temperature of 673 K has been identified for the oxidation of FA to CO ranging from 250 to 275 kJ mol−1, depending on the zeolite employed
Impurities that are commonly considered to play an important role for the formation of the C–C bond during the initiation phase or within the autocatalytic olefin cycle and that we consider in the present study are: propene (C3H6 ), ethene (C2H4 ), ethanol (EtOH), methyl acetate (MA), carbon monoxide (CO), dimethoxy methane (DMM) and formaldehyde (FA)
Summary
In a sustainable methanol economy the methanol-to-olefins (MTO) process enables the production of hydrocarbons through the formation of carbon-carbon bonds from potentially renewable feed stocks [1]. The industrially applied process is operated with porous acidic zeolite catalysts at temperatures between 350 to 400 ◦C [2]. The acidic sites of the catalyst transform methanol (MeOH) and its condensation product dimethyl ether (DME) into a variety of olefins. Extensive experimental and theoretical studies have described a dual-cycle concept comprising the olefin and the aromatic cycle. In the former, olefins are formed by repeated alkene methylation [13,14,15] and cracking [2, 7], whereas the latter comprises the methylation of aromatics producing olefins via either the side chain or pairing mechanism
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