<br>HZSM-5 中沸石质子反应性的分子调节,Journal of the American Chemical Society

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HZSM-5 中沸石质子反应性的分子调节
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2024-04-04 , DOI: 10.1021/jacs.3c12680
Yaxin Chen ₁ , Xinyou Ma _{2,

3} , John H Hack ₂ , Shuhao Zhang ₁ , Anyang Peng ₁ , James P Dombrowski ₁ , Gregory A Voth _{2,

3} , Andrei Tokmakoff ₂ , Mayfair C Kung ₁ , Harold H Kung ₁

Affiliation

在酸性 HZSM-5 沸石中，甲醇分子与沸石质子相互作用的反应性可以通过共吸附化学计量的电子密度供体E来形成 [( E )(CH ₃ OH)(HZ)] 络合物来改变。该络合物中的甲醇脱水为二甲醚的速率由一系列具有质子亲和力 (PA) 的E确定，范围从 C ₆ F ₆的 659 kJ mol ^–1到 C ₄ H ₈ O 的 825 kJ mol ^–1并被发现遵循表达式：Ln(Rate) – Ln(Rate _{N ₂} ) = β(PA – PA _{N ₂} ) ^γ ，其中E = N ₂是参考值，β 和 γ 是常数。这种趋势可能是由于随着PA的增加，[( E )(CH ₃ OH)(HZ)]络合物中溶剂化质子的稳定性增加。重要的是，当化学计量的E预吸附在沸石上时，在稳态流动反应中也观察到了这一点。如E为D ₂ O所证明的，当E的存在量超过[( E )(CH ₃ OH)(HZ)]络合物时，对甲醇反应性的影响减弱。提出甲醇脱水反应涉及[( E )(CH ₃ OH)(CH ₃ OH)(HZ)]作为过渡态，这得到了CH流动时形成的初始二甲醚的同位素分布的支持。 ₃ OH通过每个沸石质子含有一个CD ₃ OH的ZSM-5。讨论了这对 HZSM-5 催化甲醇脱水机理的影响。

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Molecular Tuning of Reactivity of Zeolite Protons in HZSM-5

In acidic HZSM-5 zeolite, the reactivity of a methanol molecule interacting with the zeolite proton is amenable to modification via coadsorbing a stochiometric amount of an electron density donor E to form the [(E)(CH₃OH)(HZ)] complex. The rate of the methanol in this complex undergoing dehydration to dimethyl ether was determined for a series of E with proton affinity (PA) ranging from 659 kJ mol^–1 for C₆F₆ to 825 kJ mol^–1 for C₄H₈O and was found to follow the expression: Ln(Rate) – Ln(Rate_N₂) = β(PA – PA_N₂)^γ, where E = N₂ is the reference and β and γ are constants. This trend is probably due to the increased stability of the solvated proton in the [(E)(CH₃OH)(HZ)] complex with increasing PA. Importantly, this is also observed in steady-state flow reactions when stoichiometric quantities of E are preadsorbed on the zeolite. As demonstrated with E being D₂O, the effect on methanol reactivity diminishes when E is present in excess of the [(E)(CH₃OH)(HZ)] complex. It is proposed that the methanol dehydration reaction involves [(E)(CH₃OH)(CH₃OH)(HZ)] as the transition state, which is supported by the isotopologue distribution of the initial dimethyl ether formed when a flow of CH₃OH was passed over ZSM-5 containing one CD₃OH per zeolite proton. The implication of this on the mechanism of catalytic methanol dehydration on HZSM-5 is discussed.

更新日期：2024-04-04

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