Theoretical investigation of the paring mechanism of the MTO process in different zeolites

The paring mechanism, which belongs to the aromatic cycle of the methanol-to-olefins process and produces propene, is investigated for H-ZSM-5 (MFI structure), H-SSZ-24 (AFI structure) and H-SAPO-34 (CHA structure) with the heptamethylbenzylium cation as a co-catalyst using DFT and ab initio calculations. We focus on the mechanistic pathway of the paring mechanism that we have proposed recently for H-SSZ-13 [Catal. Sci. Technol. 12 (2022) 3516–3523.], in which ring contraction occurs from hexamethylmethylenecyclohexadiene and the resulting five-membered ring keeps an unsaturated methylene group throughout the process, leading to tetramethylfulvene as the intermediate after propene elimination. The highest free energy barriers (at 400 °C) for this mechanism are found to be 139 kJ/mol, 156 kJ/mol and 167 kJ/mol for H-SAPO-34, H-SSZ-24 and H-ZSM-5, respectively, compared to 127 kJ/mol for H-SSZ-13. All these barriers are low enough to be kinetically relevant and can thus explain the observed formation of propene as one of the main products of the MTO-process in these zeolites. The barriers for polymethylbenzene methylation to recover the active species are higher than the barriers for the paring mechanism in all zeolites (164 kJ/mol, 187 kJ/mol and 189 kJ/mol for H-SAPO-34, H-SSZ-24 and H-ZSM-5, compared to 163 kJ/mol for H-SSZ-13). ... mehrThe lowest barriers are found for H-SAPO-34, which is often the commercial catalyst in methanol-to-propene plants.