Theoretical investigation of the paring mechanism of the MTO process in different zeolites
Annika E. Enss, Philipp N. Pleßow, Felix Studt
Abstract
The paring mechanism, which belongs to the aromatic cycle of the methanol-to-olefins process and produces propene, is investi- gated 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 ob- served 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). The lowest barriers are found for H-SAPO-34, which is often the commercial catalyst in methanol-to-propene plants.