Elucidating and exploiting the chemistry of Keggin heteropolyacids in the methanol-to-DME conversion: enabling the bulk reaction thanks to operando Raman

Schnee Josefine; Gaigneaux Eric M.

摘要

Operando Raman spectroscopy is used here to enlighten crucial and yet unconsidered aspects of the catalytic behavior of Keggin heteropolyacids (HPAs) in the gas phase dehydration of methanol to dimethylether (DME). On one hand, HPAs are since a long time claimed as being able to absorb methanol into their bulk, but on the other hand this feature is not yet really exploited when it comes to develop/use HPA-based catalysts for the methanol-to-DME process. Actually, the conditions in which the bulk is simultaneously accessible and catalytically active are not yet clearly reported. Clarifying this is precisely the aim of our work, for which we have used H3PW12O40 and H4SiW12O40. We precisely operando-follow the nu(s)(W=O-t) band reflecting whether methanol penetrates or not in the HPAs' bulk. We show that, when the HPAs are used without having been beforehand completely dehydrated, methanol penetrates into their bulk. However, the conversion remains low (<5% at 150 degrees C) due to remaining crystallisation water reducing the availability of the active protons within the bulk, so limiting the activation of methanol. A complete dehydration pretreatment is thus required if one wants to fully benefit of the bulk reactivity. However, we show that this benefit is obtained only if the pre-treatment is done adequately. The most intuitive pre-treatment, namely the HPAs' dehydration at about 300 degrees C and then cooling down to the reaction temperature 150 degrees C at which methanol is fed, is shown to be inefficient. This is due to the inaccessibility of the fully dehydrated bulk to methanol, so limiting the catalytic reaction to only the surface of the HPAs. At the opposite, a pretreatment sequence consisting of dehydrating at 300 degrees C, then cooling down to 25 degrees C, exposing to methanol at this temperature, and then only heating to the reaction temperature, is shown to succeed combining penetration of methanol into the bulk and availability of the active protons. With such sequence, an unprecedented conversion of 40% is obtained thanks to a full exploitation of the HPAs' bulk. These results show that the HPAs' bulk can contribute significantly to convert methanol, provided that the catalysts are properly pre-treated.

机译：这里使用Operando拉曼光谱启发至关重要,而被忽视的方面的催化行为Keggin heteropolyacids(hpa)的甲醇气相脱水dimethylether(测距装置)。很长时间以来声称能够吸收甲醇到他们的大部分,但另一方面这个功能还没有真正利用它开发/使用HPA-based催化剂methanol-to-DME过程。大部分的同时访问和催化地活跃还不清楚报道。我们的工作,我们已经使用H3PW12O40和态导电聚苯胺。ν(s) (W =水果)乐队反映是否甲醇穿透hpa的大部分。当hpa没有被使用事先完全脱水,甲醇渗透到他们的大部分。在150摄氏度转换仍然很低(< 5%)剩余的水结晶减少内活跃的质子的可用性散装,所以限制了甲醇的激活。因此脱水预处理完成如果想充分受益反应性。好处是只有在预处理获得的充分完成。预处理,即hpa的脱水约300度,然后冷却反应温度150摄氏度的甲醇是美联储,效率低下。完全是由于无法理解甲醇脱水散装,所以限制了催化反应的表面下丘脑-垂体-肾上腺轴的。组成的脱水在300摄氏度冷却到25摄氏度,暴露甲醇在这个温度,然后加热到反应温度、显示结合甲醇渗透到成功的体积和可用性活跃的质子。这样的序列,前所未有的转换获得的40%是由于一个完整的开发下丘脑-垂体-肾上腺轴的体积。hpa的散装可以贡献显著转化为甲醇,提供了催化剂适当的预处理。

Elucidating and exploiting the chemistry of Keggin heteropolyacids in the methanol-to-DME conversion: enabling the bulk reaction thanks to operando Raman

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