高温沉淀铁基催化剂上费托合成含氧化合物生成机理的研究

采用漫反射原位红外光谱法及化学捕获方法,考察了费托合成过程中高温沉淀铁基催化剂表面吸附物种的变化,并探讨了含氧化合物的生成机理。结果表明,CO在高温沉淀铁基催化剂上有线式和桥式两种吸附态存在,同时,CO的吸附在催化剂表面生成大量含氧化合物前驱体。费托原位实验捕获到了一些较关键的中间产物:表面乙酸盐、表面酰基、甲氧基等。同时发现高温沉淀铁基催化剂表面具有以下反应共性:醇类可与表面羟基结合生成烷氧基团;催化剂表面的吸附分子具有氧化性;一些基础化学物质如OH^-、晶格氧等可以与甲醇或乙醛分子发生反应。对CH₃OH + CO及CH₃I + CO + H₂两个反应的化学捕获实验表明,酰基是C₂⁺含氧化合物的重要中间产物,酰基加氢是形成C₂⁺含氧化合物的关键步骤。根据表面中间产物及反应特性,得到了高温沉淀铁基催化剂上费托合成含氧化合物的生成机理。

Mechanism of oxygenates formation in high temperature Fischer-Tropsch synthesis over the precipitated iron-based catalysts

In-situ DRIFTS and chemical trapping techniques were employed to investigate the adsorbed species over the surface of precipitated iron-based catalysts and the mechanism of oxygenates formation in high temperature Fischer-Tropsch synthesis. The results showed that both linear and bridged CO molecules are present on the catalyst surface, which leads the formation of numerous oxygenated precursors. Some crucial surface intermediates are detected by the in-situ DRIFTS, such as acetate, acetyl and methoxide. The surface of precipitated iron-based catalysts is characterized by following facts: (ⅰ) alcohols are able to react with free surface hydroxyls to form alkoxy species; (ⅱ) surface adsorbed molecules exhibit certain oxidizing ability; (ⅲ) basic sites such as OH- and lattice oxygen may react with CH₃OH or CH₃CHO molecules. By chemical trapping of the CH₃OH + CO and CH₃I + CO + H₂ reactions, it was found that acetyl is an important intermediate for oxygenates and the hydrogenation of acetyl is a crucial step for the formation of oxygenates. On the basis of these observations, the mechanism of oxygenates formation in high temperature Fischer-Tropsch synthesis over the precipitated iron-based catalysts was then proposed.