合成气制乙醇的铑基催化剂中助剂铁,锂的作用本质研究

催化活性测试表明,助剂Fe具有显著提高乙醇生成选择性及铑催化活性的双重作用;助剂Li具有显著提高乙醇选择性的作用,对铑催化活性影响不大。基于H_2/D_2同位素效应结果及CO化学吸附、IR、XRD、XPS等的表征结果,认为助剂Fe经活化处理后大部分与Rh形成RhFe合金,使Rh分散度显著提高,从而提高了乙醇的选择性;Rh分散度的提高以及小部分以Fe~(2+)(Fe~(3+))形式存在的助剂Fe促进甲酰基的生成及随后的氢解断C-O键反应是助剂Fe促使铑催化活性提高的两个因素。Li的主要作用在于通过与C_2含氧中间体乙烯酮氧端的弱亲合作用,促进了乙醇前驱体的生成,从而使乙醇生成选择性提高。

THE PROMOTION EFFECTS OF IRON AND LITHIUM ON ETHANOL SYNTHESIS FROMSYNGAS OVER RHODIUM-BASED CATALYSTS

The promoting action of iron and lithium in syngas conversion to ethanol over rhodium catalysts and the nature of the action have been studied by means of catalytic activity testing,H_2/D_2 isotope effects, H_2 and CO chemisorption, XRD,IR, and XPS. The results indicated that the addition of iron to Rh/SiO_2 catalysts led to an approximately twofold increase in catalytic activity and 16.8% increase in ethanol selectivity without decreasing acetaldehyde selectivity, indicating that there were no isolated active sites for the hydrogenation of acetaldehyde on the Fe - promoted Rh/SiO_2 catalysts. The result of CO chemisorption showed that Rh dispersion increased from 0.64 to 0.93 with addition of iron and XPS revealed that most of the iron existed in the form of Fe~0 on the reacted Rh Fe (1 : 0.3)/SiO_2 catalysts, along with small amounts of Fe~(2+) and Fe~(3+). From these results, it may be inferred that RhFe alloy was formed on the Fe - promoted Rh/SiO_2 catalysts after reduction with H_2, and this resulted in the noticeable increase in Rh dispersion, which was supposed to be responsible for the increase in ethanol selectivity. Two factors responsible for the promotion of catalytic activity by iron were suggested ro be: the increase in Rh dispersion, and the promotion effects of Fe~(2+) or Fe~(3+) on the formation of formyl intermediate and its subsequent hydrogenolysis.The addition of lithium to Rh/SiO_2 catalysts did not exert a noticeable influence on the catalytic activity but resulted in 11.5% increase in ethanol selectivity. IR and XPS revealed that the adsorptive and electric properties of Rh were almost not influenced by lithium promotion. The increase in ethanol selectivity was attributed to the promotion effects of Li~+ on the formation of ketene, the precursor of ethanol, by partially lowering the potential energy of the C_2-intermediate through the interaction between Li~+ and the oxygen end of ketene intermediate.