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Interaction of Methanol over CsCl- and KCl-Doped η-Alumina and the Attenuation of Dimethyl Ether Formation
The Journal of Physical Chemistry C ( IF 3.7 ) Pub Date : 2022-06-16 , DOI: 10.1021/acs.jpcc.2c02275
Alastair R McInroy 1 , John M Winfield 1 , Christopher C Dudman 2 , Peter Jones 2 , David Lennon 1
Affiliation  

As part of a program to investigate aspects of surface chemistry relevant to methyl chloride synthesis catalysis, the interaction of methanol with η-alumina doped with either CsCl or KCl in the range 0.01–1.0 mmol g(cat)–1 is investigated by a combination of diffuse reflectance infrared Fourier transform spectroscopy and temperature-programed desorption (TPD). Infrared spectra (IR) recorded at 293 K show that increasing the concentration of the group 1 metal chloride progressively decreases the surface concentration of associatively chemisorbed methanol and changes the environment in which the adsorbed methanol resides. For CsCl concentrations of ≥0.6 mmol g(cat)–1, chemisorbed methoxy species dominate the IR spectrum, while TPD studies show that the amount of methanol adsorbed onto the surface, and subsequently desorbed unchanged, changes relatively little. In the TPD experiments, some of the adsorbed methanol reacts to give dimethyl ether (DME) which then desorbs; for dopant concentrations of 1.0 mmol g(cat)–1, DME formation is suppressed to below the limit of detection. Unexpectedly, the presence of formate species generated at 293 K is also observed spectroscopically, characterized by a νasym(COO) mode which exhibits a hypsochromic shift relative to potassium formate; surface concentrations of formate are higher at higher loadings of group 1 metal chloride. Temperature-programed IR spectroscopy shows that the room-temperature formate species desorbs, decomposes, or migrates on warming to 653 K. Thermal ramping of the methanol-saturated surface also results in formate production but one that exhibits an IR profile in agreement with earlier observations and literature values. Increasing the concentrations of the group 1 metal chloride progressively decreases the presence of the thermally induced formate moiety. The study not only reinforces the concept of group 1 metal chloride additives progressively rendering ineffective those Lewis acid sites present at the η-alumina surface which convey discrete reaction characteristics [e.g., (i) dimerization of methanol to form DME and (ii) an activated methoxy → formate transition] but also suggests the generation of reactive sites not present in the undoped alumina.

中文翻译:

甲醇在 CsCl 和 KCl 掺杂的 η-氧化铝上的相互作用和二甲醚形成的减弱

作为研究与氯甲烷合成催化相关的表面化学方面的计划的一部分,甲醇与掺杂有 0.01–1.0 mmol g (cat) –1的 CsCl 或 KCl 的 η-氧化铝的相互作用通过组合研究漫反射红外傅里叶变换光谱和程序升温脱附(TPD)的研究。在 293 K 记录的红外光谱 (IR) 表明,增加第 1 族金属氯化物的浓度会逐渐降低缔合化学吸附甲醇的表面浓度,并改变吸附甲醇所在的环境。对于 ≥0.6 mmol g (cat)的 CsCl 浓度–1,化学吸附的甲氧基物质在 IR 光谱中占主导地位,而 TPD 研究表明,吸附到表面上并随后解吸不变的甲醇量变化相对较小。在 TPD 实验中,部分吸附的甲醇反应生成二甲醚 (DME),然后脱附;对于 1.0 mmol g (cat) –1的掺杂剂浓度,DME 的形成被抑制到检测限以下。出乎意料的是,在 293 K 产生的甲酸盐物质的存在也被光谱观察到,其特征是 ν不对称(COO) 模式,其表现出相对于甲酸钾的低色移;第 1 族金属氯化物的负载量越高,甲酸盐的表面浓度越高。程序升温 IR 光谱显示室温甲酸盐物质在升温至 653 K 时会解吸、分解或迁移。甲醇饱和表面的热升温也会导致甲酸盐的产生,但其 IR 曲线与早期观察结果一致和文学价值。增加第 1 族金属氯化物的浓度会逐渐减少热诱导甲酸部分的存在。该研究不仅强化了第 1 组金属氯化物添加剂的概念,该添加剂逐渐使存在于 η-氧化铝表面的路易斯酸位点失效,这些路易斯酸位点传达了离散的反应特性 [例如,(i) 甲醇二聚形成 DME 和 (ii) 活化的甲氧基→甲酸转变],但也表明产生了未掺杂氧化铝中不存在的反应位点。
更新日期:2022-06-16
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