Correlation of Brønsted acid strengths and species substituted Si in ZSM-5 and their effects on propylene polymerization in MTA reaction

Methanol to aromatics (MTA) is regarded as an efficient process to generate benzene, toluene, xylene (BTX), and other important chemical raw materials, and the polymerization of alkenes is an important reaction in MTA. In this work, the effect of different Brønsted acid strengths over ZSM-5 on propylene polymerization was studied by using the density functional theory (DFT) method. H-M-ZSM5 with different Brønsted acid strengths are formed by substituting Si atoms in ZSM-5 by Al, Ga, In, Fe, and B atoms, respectively. And three steps including the formation of propoxide, propylene dimerization, and the deprotonation of C6 hydrocarbon are investigated. The i-propoxide is most easily generated by the protonation of propylene. And the polymerization of i-propoxide with propylene is easier compared to n-propoxide in the dimerization step. Subsequently, hexoxy primary carbon dehydrogenation to generate C6 olefins is prone in the C6 hydrocarbon deprotonation step. Therefore, the optimal path is the protonation of propylene to produce i-propoxide, which in turn polymerizes with propylene to produce hexoxy (IM5), followed by a deprotonation step to produce the most favorable product 2-Methyl-pentene. Meanwhile, the enhanced strength of the Brønsted acid significantly promotes the activity of propylene protonation and C6 hydrocarbons deprotonation, while it has little effect on the propylene dimerization. Therefore, the Brønsted acid strength is proposed as an appropriate descriptor to reflect the catalytic activity of H-M-ZSM5 to the propylene protonation and C6 hydrocarbons deprotonation. And H–Al-ZSM5 and H-Ga-ZSM5 with high acid strength are found to have the best performance for the propylene polymerization to C6 olefins.