羰基铑簇合物衍生的多相烯烃氢甲酰化催化剂的性能及红外表征

Properties and IR Characterization of Rhodium Carbonyl Cluster-Derived Heterogeneous Olefin Hydroformylation Catalysts

在常压下,Rh_4(CO)_(12)或Rh_6(CO)_(16)簇合物衍生的Rh/SiO_2对乙烯,丙烯氯甲酰化反应表现出良好的催化活性和选择性,并有利于直线醛的形成.乙烯氢甲酰化体系的表面催化比活性与Rh分散性的关系表明.乙烯氢甲酰化反应具有结构敏感性,而乙烯加氢反应具有结构非敏感性,高分散的金属表面有利于主反应的选择性.通过红外光谱跟踪,发现表面Rh在反应气氛中显示零价.根据接触时间对催化反应的影响,推测多相烯烃氢甲酰化及加氢反应都在Ph^0活性中心上进行.另外,担载羰基铑簇合物的热分解红外研究结果指出,表面羰基化合物金属中心上的配位不饱和性对氢甲酰化催化活性似乎起着重要影响,簇合物只有完全脱羰才能提供高活性的催化中心.在反应气氛和CO气氛中,担载Rh_6(CO)_(16)表现出一致的热稳定性,说明反应气中的CO对稳定羰基物起着主导作用.

Under atmospheric pressure, the hydroformylations of ethylene and propylene were studied under different conditions over Rh4(CO)12-or Rh6(CO)16-derived Rh/SiO2catalysts, which were made with strict exclusion of air. This approach to metal catalysts, led to high catalytic activities and selectivities for oxygenate formation, and to high regioselectivity for n- butanal formation in propylene hydroformylation. The dependency of ethylene hydroformylation system on Rh dispersion was investigated. On the basis of the fact that the specific activity to propanal increases with an increase of Rh dispersion, namely as the fraction of edge and corner Rh atoms increases, ethylene hydroformylation is suggested to be structure sensitive. In contrast, ethylene hydrogenation is regarded as a structure insensitive reaction since the specific activity to ethane is substantially independent of Rh dispersion. Thus ethylene is favorably hydroformylated over a highly dispersed metal catalyst. This relationship also can reveal the characteristics of heterogeneous ethylene hydroformylation and hydrogenation. The influence of contact times on the catalytic performance was examined for ethylene hydroformylation system. It is observed that the formation of propanal is favored by short contact time and that of ethane follows the reverse. This indicates that these two catalytic processes seems to compete with each other on the same kind of active sites rather than different individual ones. The surfaces of Rh/SiO2 catalysts were characterized via CO infrared probing. Despite that low Rh loading surfaces give rise to Rh1(CO)2species under a CO atmosphere, they yield only linear and bridging adsorbed CO species upon treatment by an ethylene hydroformylation atmosphere. This demonstrates that oxidative addition of surface OH- to zerovalent Rh with CO coordination can be quenched in the presence of H2. Meanwhile it is suggested that surface zerovalent Rh may be responsible for both hydroformylation and hydrogenation of olefins. The results of thermal decomposition of Rh6 (CO)16/SiO2 under ethylene hydroformylation conditions and a CO pressure monitored by IR indicate that CO in the reaction gases plays a dominant role in the stabilisation of the surface cluster and such cluster-derived Rh catalysts are able to provide highly active sites only when decarbonyled thoroughly. The coordination unsaturation of metal centre in a carbonyl compound seems to be of importance for catalytic activity in the hydroformylation reaction.