镁铝复合氧化物负载铜催化剂上甘油选择性氢解合成丙二醇

Selective Hydrogenolysis of Glycerol to Propylene Glycol on MgO-Al_2O_3 Dispersed Cu Catalysts

我们通过乙醇溶液浸渍法合成出了具有高分散度金属Cu的Cu/MgO-Al2O3(Mg/Al原子比=1/1,3/1,4/1)、Cu/MgO和Cu/Al2O3等催化剂.在200℃,6.0 MPa H2和二氧六环溶剂中,这些催化剂高选择性地将甘油氢解为1,2-丙二醇(选择性>90%),而单位表面Cu原子的甘油转化速率则随催化剂表面碱中心与Cu原子比例的提高而增大.N2O化学吸附-H2程序升温还原实验表明Cu粒子的本征氢解能力不随其负载量以及载体中的Mg/Al原子比发生明显改变,加之碱性MgO-Al2O3载体本身不催化甘油的转化,我们推测在甘油氢解反应中金属Cu粒子与载体界面处的碱中心辅助Cu粒子活化甘油分子的α位C—H键,从而加速甘油脱氢为甘油醛步骤以及甘油氢解反应的进行.CO2程序升温脱附实验以及对甘油氢解反应中Cu/MgO-Al2O3催化剂稳定性的考察结果暗示在甘油氢解反应中起主要作用的碱中心是载体表面上与Mg2+键连的羟基基团(即B碱OH-).这些对甘油氢解反应中金属中心与碱性中心协同作用的认识对进一步理性设计高效的甘油或其它多元醇分子氢解催化剂具有重要指导意义.

Cu/MgO-Al2O3（Mg/Al atomic ratio=1/1,3/1,4/1）,Cu/MgO and Cu/Al2O3 catalysts with high Cu dispersions were prepared by an impregnation method using ethanol as solvent instead of water.Compared to water,ethanol as solvent was favorable not only for obtaining the high dispersions of the Cu clusters on the basic oxide supports,but also for retaining the basicity and structures of the supports during the catalyst preparation.The density of the basic sites for the dispersed Cu catalysts increased with increasing the MgO contents in the basic supports.These catalysts catalyzed glycerol hydrogenolysis to propylene glycol with a high selectivity（〉90%）,and showed slight deactivation in dioxane at 200 ℃ and 6.0 MPa H2.Their reaction rates normalized by per exposed Cu atom increased with increasing the ratios of the basic sites to Cu atoms on the catalyst surfaces.The intrinsic hydrogenolysis activity of the Cu atoms were probed by N2O chemisorption-H2 temperature programmed reduction,which did not change essentially with the Cu loadings（2—6 wt%） and the Mg/Al atomic ratios of the MgO-Al2O3 supports.Meanwhile,the basic sites alone on the supports were inactive for the glycerol conversion.Taken together,we propose a synergetic effect that the basic sites at the interfaces between the Cu sites and the supports assist the cleavage of α-C—H bonds of glycerol on the Cu surfaces leading to the faster dehydrogenation of glycerol to glyceraldehyde,a kinetically-relevant step in glycerol hydrogenolysis to propylene glycol.These active basic sites appear to be the Br nsted OH-sites,i.e.the hydroxyl groups bonded to the surface Mg2＋cations on MgO-Al2O3.The understanding on such synergy between the basic sites and Cu sites in glycerol hydrogenolysis provides a basis for the rational design of superior hydrogenolysis catalysts and selective removal of oxygen from biomass-derived feedstocks.