铑基催化剂上CO+H_2合成乙醇的原位漫反射红外光谱研究

In-situ DR-FTIR Study of Ethanol Synthesis from CO/H_2 over Rhodium-based Catalyst

采用原位漫反射红外光谱法研究高温高压下CO+H2在Rh-Mn-Li/SiO2催化剂上合成乙醇的反应中间体和反应机理,结果显示:2 922 cm-1,2855 cm-1两峰主要是由乙醇合成前体卡宾H2C=M的振动造成的;250℃,3.4 MPa时卡宾能在催化剂表面快速大量形成,反应后吹扫实验显示卡宾在催化剂表面的吸附并不相同,部分吸附较弱易脱附;部分吸附较强,在催化剂表面以相当稳定的形态存在,难以脱附;因此可能存在部分惰性卡宾,其不参与反应却占据了催化剂的部分表面,降低了催化剂的反应活性.250℃时,压力从0.5 MPa升高到3.0 MPa时,产物中CO2和CH4的含量降低,而乙醇的含量增加;反映出压力是影响反应产物分布的重要因素,压力提高能增大乙醇在产物中含量;同时也揭示出在该催化剂上CO2、CH4的生成路径与乙醇的生成路径存在着竞争性,压力是影响反应路径选择的重要因素.实验结果还表明卡宾的形成不是合成乙醇的速控步骤,而乙酰基能否大量形成则决定着乙醇产生的快慢.经过红外光谱分析后认为"CO缔合-卡宾-乙烯酮-乙酰基-乙醇"应是Rh基催化剂上合成乙醇较为合理的机理.

In this paper, in-situ-DR-FHR technique is employed to investigate the reaction intermediates and the mechanism of the CO hydrogenation to ethanol over Rh-Mn-Li/SiOa catalyst. The results show that the two adsorption peaks of 2 922 cm^-1 and 2 855 cm^-1 are mainly caused by carbene which is the synthetic precursor of ethanol. At 250℃ and 3.5 MPa, a lot of carbenes can form quickly after the CO/Ha mixed gas is leaded into the reactor. The sweeping of Na after CO ＋ Ha reaction shows that part of carbenes can adsorb firmly on the surface of Rh-MnLi/SiOa catalyst and cannot been blown off easily, but the other is converse. So part of inert carbenes probably exist which dont take part in reaction but control some of the surface of the catalyst and lead to reduce reactivity of the catalyst. At 250℃, with the pressure from 0.5 MPa to 3.0 MPa, the ethanol content of the products increases while the CO2 and CH4 content decrease which indicates the reaction pressure is an important factor affecting the distribution of the products and higher pressure can produce more ethanol in the products. At the same time, the result of the pressure affecting the distribution of the products also indicates that the reaction paths of CO2 and CH4 are competitive with that of ethanol and the reaction pressure affect the selectivity of the reaction paths. The experimental results also show that the building-up process of carbene is not an rate-determining step and wether ethanol can be produced rapidly or not is decided by the content of acetyl on the surface of the catalyst. At last, utilizing in-situ-DR-FTIR experimental results, we can conclude that ＂CO association-carbene-vinyl ketone-acetyl-ethanol＂ should be an reasonable mechanism of the CO hydrogenation to ethanol over Rh-Mn-Li/SiO2 catalyst.