重氧水和合成气与卡宾簇合物的模型反应研究铑催化乙醇合成机理

MODEL REACTIONS OF SUPPORTED CARBENE CLUSTER WITH D_2 O OR D_2^(18)O AND SYNGAS FOR MECHANISM STUDY OF Rh-CATALYZED ETHANOL SYNTHESIS

负载在SiO_2载体上的卡宾簇合物Fe_2(CH_2)(CO)_8被用于模拟负载型铑催化剂上的卡宾吸附物种,在95℃和2.0 MPa的条件下,负载样品与含氘水的合成气作用后,得到主要产物甲烷,並伴有乙酸、乙醛、乙烯和乙烷。其中,乙酸为DCH_2COOD和CH_3COOD的混合物,这表明反应过程是经过乙烯酮和乙酰基两种中间体进行的。用重氧水(D_2~18O)为截取剂,在合成气(CO/H_2=1/2)存在下,可检测到^(18)O在乙醛、乙醇和乙酸甲酯(经乙酸通甲醇吹扫酯化)的产物,说明乙酰基和乙醛与重氧水之间存在可逆的水合交换反应。根据水合醛化机制,可以解释Katzer等观察到的同位素杂组现象。

In the work on the mechanism of ethanol synthesis, Takeuchi and Katzer used labeled carbon monoxide (13C18O and 12C18O)to study the ethanol formation on a Rh/TiO2 catalyst. The catalyzed hydrogena-tion of labeled CO produced eight isotopic isomers of ethanol. Their results enabled them to raise serious doubts on the mechanisms of ethanol synthesis proposed previously and led them to assume a complex mechanism where a M = CH2 species is formed and reacts as 'carbene-ketene ( ethylene oxides)' . Note that ethylene oxide is generally not observed as a product in the CO + H2 synthesis. For this reason, Kiennemann has postulated that adsorbed aldehyde can react with a water molecule, then the reversible addition of water on physisorbed aldehyde can explain the formation of 16O and 18O in ethanol. However, there is no experimental evidence obtained so far. In this work, we further take into account of the reversible hydration of acetyl in the mechanism. Thus, there are two possible water-equilibria: one dealing with acetyl and the other with aldehyde. Support for this mechanism has been obtained from the in situ chemical-trapping with D2O or D218O in the presence of syngas with supported carbene carbonyl cluster Fe2 (CH2 )(CO) 8. Reaction of supported sample with D2O and syngas at 95 ℃ and 2.0 MPa gave CH3COOD and DCH2COOD, accompanying with other byproducts like trapping experiment with syngas and CD3OD. Owing to the high boiling point of acetic acid, methanol was introduced into the microreactor to carry out the acetic acid after reaction. With syngas and D218O as trapping-agent, condensates from the reaction mixtures were analyzed by GC/MS to give the. isotopic repartition of 18O2 in acetaldehyde, ethanol and methyl acetate as 47%, 41% and 24% respectively, when the ratio of CO : H2 : D218O is 10/20/1. The results clearly showed the statistical isotopic distribution for ethanol is also obtained by water-acetyl ( aldehyde ) exchange without the assumption of ethylene oxide ( c.f. Takeuchi and Katzer). Note the combustion heat of ethylene oxide and acetaldehyde is 1.26×103 and 1.17×103kJ /mol respectively. The ethylene oxide is a species with high potential energy, and it is generally not observed as a product in the CO + H2 synthesis. So it is doubtful that equilibrium between ketene and ethylene oxide is present in the ethanol synthesis. Likewise, it could be deduced that an exchange between water and ketene ad-species is not occurred from the change of concentration of 18O in CH3C18OOCH3 (from 24% to 19% with increasing ratio of D218O:H2 from 1/20 to 1/15).