Theoretical study of the influence of H-SAPO-34 modified with Zn2+on the formation of butadiene

In the process of methanol to olefin (MTO), dienes are the precursors of aromatic species. Experimental study showed that Zn2+ modified H-SAPO-34 (Zn@H-SAPO-34) promoted aromatics formation at the initial stage of MTO reaction. Our recent work reported the reaction mechanism of the aromatization of dienes to low methylbenzene aromatics catalyzed by Zn@H-SAPO-34. Herein, using density functional theory (DFT), the possible reaction pathways of diene formation on H-SAPO-34 with and without Zn doping were systematically investigated with propene as the initial olefin. After Zn2+ modification, the acidic strength of adjacent Bronsted acid sites (BAS) is enhanced, which is conductive to the alkene-mediated butadiene formation pathway catalyzed by B-acid, and the Lewis acid produced by Zn2+ provides catalytic site for the transfer dehydrogenation reaction of methanol with propene, which promotes the formation of formaldehyde and induces the formaldehyde-mediated mechanistic pathway. Further microkinetic calculation reveals that formaldehyde-mediated pathway is the optimal reaction pathway for butadiene formation. This study demonstrates that the synergistic effect of BAS and LAS improves the formation activity of diene on Zn@H-SAPO-34. This work will help to deepen the understanding of the role of Zn2+ in the early stage of MTO reaction.