We evaluate the effect of the number of methanol molecules per acidic site of H-ZSM-5 on the methoxylation reaction at room temperature by applying operando diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS), which capture the methoxylation reaction by simultaneously probing surface adsorbed species and reaction products, respectively. To this end, the methanol loading in H-ZSM-5 (Si/Al ≈ 25) pores is systematically varied between 32, 16, 8 and 4 molecules per unit cell, which corresponds to 8, 4, 2 and 1 molecules per Brønsted acidic site, respectively. The operando DRIFTS/MS data show that the room temperature methoxylation depends on the methanol loading: the higher the methanol loading, the faster the methoxylation. Accordingly, the reaction is more than an order of magnitude faster with 8 methanol molecules per Brønsted acidic site than that with 2 molecules, as evident from the evolution of the methyl rock band of the methoxy species and of water as a function of time. Significantly, no methoxylation is observed with ≤1 molecule per Brønsted acidic site. However, hydrogen bonded methanol occurs across all loadings studied, but the structure of hydrogen bonded methanol also depends on the loading. Methanol loading of ≤1 molecule per acidic site leads to the formation of hydrogen bonded methanol with no proton transfer (i.e. neutral geometry), while loading ≥2 molecules per acidic site results in a hydrogen bonded methanol with a net positive charge on the adduct (protonated geometry). The infrared vibrational frequencies of methoxy and hydrogen bonded methanol are corroborated by Density Functional Theory (DFT) calculations. Both the experiments and calculations reflect the methoxy bands at around 940, 1180, 2868–2876 and 2980–2973 cm⁻¹ which correspond to ν(C–O), ρ(CH₃), ν_s(C–H) and ν_as(C–H), respectively.

中文翻译：

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沸石H-ZSM-5中依赖甲醇的甲氧基化反应。

我们通过应用操作性漫反射红外傅里叶变换光谱（DRIFTS）和质谱（MS）来捕获H-ZSM-5酸性位点在室温下甲醇分子数量对甲氧基化反应的影响，从而捕获甲氧基化反应通过同时探测表面吸附的物质和反应产物。为此，H-ZSM-5（Si / Al≈25）孔中的甲醇负载在每个晶胞中有系统地在32、16、8和4个分子之间变化，这对应于每个Brønsted8、4、2和1个分子酸性部位。该operandoDRIFTS / MS数据显示，室温下甲氧基化取决于甲醇的负载量：甲醇负载量越高，甲氧基化速度越快。因此，从每个布朗斯台德酸性位点使用8个甲醇分子的反应要比具有2个分子的反应快一个数量级以上，这是从甲氧基物质的甲基岩石带和水随时间的变化而明显看出的。值得注意的是，每个布朗斯台德酸性位点≤1个分子均未观察到甲氧基化。然而，氢键合甲醇在所有研究的负载中均发生，但氢键合甲醇的结构也取决于负载。每个酸性位点的甲醇负载量≤1个分子，导致形成没有质子转移的氢键合甲醇（即中性几何结构），而每个酸性位点负载≥2个分子会导致加氢的甲醇在加合物上带有净正电荷（质子化几何结构）。密度泛函理论（DFT）计算证实了甲氧基和氢键合甲醇的红外振动频率。两个实验和计算反映在大约940，1180，2868年至2876年的甲氧基带和2980至73年厘米^-1对应于ν（C-O），ρ（CH ₃），ν_小号（CH）和ν_分别为（C–H）。