低温甲醇合成研究进展

Research developments of low-temperature methanol synthesis

日本学者Tsubaki等开创了一种全新的低温甲醇合成反应路径。该路径以含有二氧化碳的合成气为反应原料,使用单一低碳醇（包括甲醇）同时作为催化剂和溶剂,实现了反应原料一氧化碳在低温（443 K）条件下,一步转化率达到70%～100%。原位红外和多种表征手段证明,该反应能够在低温条件下进行,是由于催化剂上吸附的甲酸盐物种可以和多种低碳醇溶剂在低温条件发生酯化反应,生成相对应的甲酸酯。而生成的甲酸酯很容易在低温条件下,铜基催化剂表面,发生加氢反应,生成甲醇和相应的溶剂醇。该种全新的甲醇合成路径克服了常规甲醇合成过程中,甲酸盐必须在高温条件下才能发生加氢反应的关键步骤。同时,还介绍了适用于低温甲醇合成反应的金属Cu/ZnO催化剂制备方法的研究进展。全新的溶胶-凝胶-燃烧法、固相研磨-燃烧法以及甲酸辅助燃烧法直接制备高活性、纳米尺度、高分散的金属Cu/ZnO催化剂,而不需要额外的还原流程。

Conventional production of methanol is very inefficient since only 10%-15% one-pass conversion is achieved typically at 5.0-10.0 MPa and 523-573 K due to the severe thermodynamic limitations of its exothermal reaction （CO ＋ 2H2 = CH3OH）. A novel route of low-temperature methanol synthesis developed by Tsubaki et al from CO2-containing syngas only by adding alcohols including methanol itself is reviewed. These alcohols act as the homogeneous co-catalysts and solvent, realizing 70%-100% one-pass conversion at only 5.0 MPa and 443 K. The key step is the reaction of the adsorbed formate species with alcohols to yield ester species at low temperatures, followed by the hydrogenation of ester by hydrogen atoms on metallic Cu. This changes the normal reaction path of conventional, high-temperature methanol synthesis from formate via methoxy to methanol. A series of new methods, such as sol-gel auto-combustion, solid-state combustion and formic acid assisted combustion methods, to prepare highly active metallic Cu/ZnO catalysts without further reduction are also reviewed. During the decomposition of metal-contained precursors in an argon atmosphere, H2 and CO are liberated and act in situ as the reducing agents to obtain pure metals and metallic catalysts. The X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive spectroscopy, and temperature-programmed reduction analysis reveal that the as-prepared catalyst without further reduction is converted into metallic CuO and ZnO species.