Zn²⁺改性 H-SAPO-34 对甲醇制烯烃反应中烯烃循环影响的理论研究

Theoretical Study of the Influence of H-SAPO-34 Modified with Zn²⁺ on Olefin-based Cycles During Methanol-to-olefins Reaction

摘要

采用密度泛函理论(DFT)计算结合微观动力学模拟, 系统研究了H-SAPO-34 中的Brønsted(B)酸位点与Zn@H-SAPO-34中B酸位点和Lewis(L)酸位点上烯烃循环生成乙烯和丙烯的反应机理, 以揭示Zn²⁺改性对甲醇制烯烃(MTO)催化性能的影响. 结果表明, 在H-SAPO-34催化的烯烃循环中, 丙烯选择性高于乙烯(与文献报道一致). Zn²⁺改性增强了邻近B酸位点的酸性强度, 显著降低了乙烯和丙烯生成路径的总自由能垒, 从而提升了反应活性, 同时缩小了两条路径之间能垒的差异. 在L酸位点上, 乙烯的总自由能垒略低于丙烯, 因而L酸对乙烯的相对选择性更高, 但两者的能垒均远高于B酸位点, 表明L酸位点并非主要活性中心. 微观动力学模拟进一步揭示, Zn²⁺改性后B酸位点仍为主要活性中心, 其总转化频率(TOF)较未改性H-SAPO-34提高2~3个数量级, 且丙烯的选择性优势保持不变, 而乙烯相对选择性随温度升高呈上升趋势. 综上, Zn²⁺改性通过增强B酸位点酸强度和引入L酸位点, 在保持丙烯选择性优势的同时提升了整体反应活性, 并缩小了乙烯与丙烯的选择性差异, 为高效MTO催化剂设计提供了理论依据.

Abstract

The reaction mechanisms of the olefin-based cycles for ethene and propene formation at Brønsted acid sites（BAS） in H-SAPO-34 and at BAS and Lewis acid sites（LAS） in Zn@H-SAPO-34 were systematically investigated by means of density functional theory（DFT） calculations combined with microkinetic simulations， with the aim of elucidating the influence of Zn²⁺ modification on the catalytic performance of the methanol-to-olefins（MTO） reaction. The results demonstrate that the selectivity to propene based on the olefin-based cycle catalyzed by H-SAPO-34 is higher than that to ethene（consistent with the previous reports）. The Zn²⁺ modification enhances the acidic strength of neighbouring BAS， significantly reducing the overall free energy barriers for both the ethene and propene formation pathways and improving activity. It also decreases the difference in their barriers. At LAS， the overall free energy barrier for ethene formation has been found to be slightly lower than that for propene， indicating a relative selectivity advantage toward ethene； however， both barriers are significantly higher than those at BAS， suggesting that LAS are not the dominant active sites. Further microkinetic simulations show that BAS remain the main active sites after Zn²⁺ modification， with total turnover frequencies（TOF） increased by 2—3 orders of magnitude compared to unmodified H-SAPO-34. The propene selectivity advantage is kept and the relative selectivity towards ethene increases with rising temperature. In summary， the enhancement of B-acid strength and the introduction of LAS by Zn²⁺ modification improve the overall catalytic activity while maintaining the propene selectivity advantage and reducing the selectivity difference between ethene and propene. This work provides theoretical guidance for the rational design of high-performance MTO catalysts.

关键词

Zn²⁺改性H-SAPO-34 / 密度泛函理论 / 甲醇制烯烃 / 烯烃循环

Key words

Zn²⁺-modified H-SAPO-34 / Density functional theory / Methanol-to-olefins（MTO） / Olefin-based cycle

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基金资助

山西省高等学校科技创新项目(2024L437)

化工过程强化山西省重点实验室开放课题(2024-CPI10)

山西工学院科研启动费(022015)