Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

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 Abstract

Conversion of Oxygenates on H-ZSM-5 Zeolites—Effects of Feed Structure and Si/Al Ratio on the Product Quality

“…For these feeds, Figure 6a shows an increased carbon yield for C2 products and a reduced carbon yield for C3 and C4 products. This is related to the reduced reactivity of ethane and ethylene compared to propylene and bigger olefins, as reported by Plessow et al [12]. However, again, by increasing the Al content of the catalyst, these differences disappear (Figure 6a-d The similarity of the product distributions regardless of different zeolite crystal sizes and of the converted feed of experimental series 2 shown in Figure 6a is related to their C7 main chain, which rapidly cracks into C3 and C4 products.…”

“…(3) The reaction types of these follow-up reactions after dehydration are different (3) The reaction types of these follow-up reactions after dehydration are different build-up and breakdown reactions, which may be considered as a chain [11][12][13]. It can be assumed that at a certain number of reaction events, the chain reaches a statistically equal distribution since a product of one reaction can act as an educt for the other reaction.…”

“…Comparison of the conversion of ethanol and methanol as small oxygenates has already revealed a discrepancy in terms of the proposed consecutive reactions of C1 and C2 fragments. Even recent publications do not explain the source of this discrepancy in detail, which promotes a controversial discussion [12]. Exemplarily, another view on the hydrocarbon interconversion mechanism describes a another view on the hydrocarbon interconversion mechanism describes a chain of competing reactions with different probabilities, which determine the final product distribution as a statistical measure [13,14].…”

Theoretical investigation of the side-chain mechanism of the MTO process over H-SSZ-13 using DFT and ab initio calculations

“…This is complicated by the autocatalytic nature of the olefin cycle and the fact that the side-chain mechanism requires the presence of an aromatic, which, in the case of H-SSZ-13, cannot leave the zeolite cavity due the small pore size and will therefore also influence the olefin cycle in the occupied cavity. We therefore stress that the above analysis is only qualitative and that a quantitative analysis requires advanced beyond steady-state kinetics [108][109][110][111][112][113][114] that take the spatial and temporal changes in the reactor into account, including the formation and retention of aromatics. For a complete picture, additionally, the full aromatic cycle including the paring mechanism needs to be considered.…”

Theoretical investigation of the olefin cycle in H-SSZ-13 for the ethanol-to-olefins process using ab initio calculations and kinetic modeling

“…3 The current understanding of the mechanistic details revolves around the concept of the hydrocarbon pool (HCP) comprised of a variety of (unsaturated) hydrocarbons that continuously undergo oligomerization and cracking reactions. [4][5][6][7][8][9][10] While the initial formation of carbon-carbon bonds in the multi-step initiation of the MTO process from MeOH has been attributed to either impurities [11][12][13][14] or direct initiation, [15][16][17][18] the ETO process initiates via EtOH dehydration to ethene. 19 In a majority of experimental studies, the ETO conversion was performed on (modified) H-ZSM-5 where the first stage of EtOH dehydration proceeds at a high rate and is virtually independent of the selectivity of the catalyst towards higher olefins.…”