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肉桂醇在香料、香精以及医药等诸多高端领域有着重要应用,常通过肉桂醛加氢法制备.由于热力学上肉桂醛中C=C–C=O官能团的C=C双键加氢比C=O双键更有优势,因此不管是从学术角度还是工业生产角度来看,高选择性还原C=O基团都是一项极具挑战的任务.肉桂醛加氢反应的副产物苯丙醛以及苯丙醇的生成不仅导致肉桂醇收率降低,而且大大增加分离纯化成本,因此设计并制备出有利于C=O官能团高选择性氢化的高效催化剂具有重要意义.Meerwein-Ponndorf-Verley(MPV)反应以异丙醇为氢供体,是一种可选择加氢C=C–C=O中C=O官能团的反应.目前用于MPV还原的催化剂(均相或多相)在实际应用中通常选择性不高,使得目标产物得率低且分离成本高.本文报道了负载于一种特殊氧化铝表面的氧化钛催化剂(记为TiO_x/γ-Al_2O_3-nt),其催化肉桂醛经MPV还原为肉桂醇的催化性能优异,表征结果发现该特殊氧化铝可导致表面氧化钛呈较高比例的低价钛物种高度分散状态,从而成为一种高效的肉桂醛MPV还原催化剂.TEM结果表明,这种特殊氧化铝和普通氧化铝在形貌上有很大差别,具有比较规整的一维纳米粒子形貌.综合XRD,TEM,Raman以及H/D同位素交换表征结果 ,可得到氧化钛高度分散在氧化铝表面的结论 .原位XPS结果表明,TiO_x/γ-Al_2O_3-nt催化剂表面具有较高浓度的Ti(III)物种,而以普通氧化铝为载体的催化剂TiO_x/γ-Al_2O_3-c在相同的还原条件下其表面Ti(III)物种浓度较低,这种差异的来源是具有规整形貌的一维纳米氧化铝提供了更加均匀的表面位点使得表面高度分散的氧化钛容易被还原为低价态.NH_3-TPD结果表明,TiO_x/γ-Al_2O_3-nt催化剂具有高的L酸酸量.肉桂醛MPV还原反应结果显示,表面负载氧化钛的特殊氧化铝(TiO_x/γ-Al_2O_3-nt)是一种非常高效的催化剂,具有很高的目标产物肉桂醇的选择性,几乎观察不到副产物的生成,多次套用实验结果也证实该催化剂具有良好的稳定性.该催化剂的高性能可归纳为以下两个方面的原因:一方面,L酸是MPV还原反应的活性中心,该催化剂具有高的L酸酸量,因此转化率高;另一方面,其表面较高浓度的Ti(III)物种可以使肉桂醛以垂直吸附模式(吸附终端为C=O)在催化剂表面吸附,这种吸附模式可以高选择性地还原为目标产物肉桂醇,因此同时具有很高的选择性.
Cinnamic alcohol in spices, fragrances and medicine and many other high-end areas have an important application, often prepared by the cinnamic aldehyde hydrogenation method due to the thermodynamic cinnamic aldehyde in the C = C-C = O functional group C = C double bond hydrogenation ratio C = O double bond is more advantageous, therefore, high selective reduction C = O group is a challenging task, both from an academic point of view and industrial production.Phenylpropionaldehyde, a by-product of cinnamic aldehyde hydrogenation And the formation of phenylpropanol not only leads to the decrease of cinnamyl alcohol yield but also greatly increases the cost of separation and purification. Therefore, it is of great significance to design and prepare an efficient catalyst for the highly selective hydrogenation of C = O functional groups.Meerwein-Ponndorf-Verley ) Reaction with isopropanol as a hydrogen donor is an alternative reaction for the hydrogenation of C═O functional groups in C = C-C = O. The catalysts currently used for MPV reduction (homogeneous or heterogeneous) Is usually not highly selective, making the yield of the target product low and the separation cost is high.This paper reports the titania catalyst supported on a special alumina surface (denoted as TiO_x / γ-Al_2O_3-nt) MPV reduced to cinnamyl alcohol catalytic performance, characterization results found The special alumina can lead to a high proportion of surface titanium oxide with a high proportion of low-priced titanium species highly dispersed state, thus becoming an efficient cumene aldehyde MPV reduction catalyst.TEM results show that this special alumina and ordinary alumina in morphology The results of in-situ XPS showed that the titanium oxide nanoparticles were highly dispersed on the surface of aluminum oxide by XRD, TEM, Raman and H / D isotope exchange characterization, Ti (III) species on the surface of TiO_x / γ-Al_2O_3-nt catalysts were higher than those on the surface of TiO_x / γ-Al_2O_3-nt catalysts, The reason for this difference is that the one-dimensional nano-alumina with a regular morphology provides a more uniform surface location so that the surface highly dispersed titanium oxide can be easily reduced to a lower valence state.NH 3 -TPD results show that the TiO x / γ-Al_2O_3-nt catalyst has a high amount of L acid.The results of MPV reduction of cinnamaldehyde show that the special alumina (TiO_x / γ-Al_2O_3-nt) supported on surface is a very efficient catalyst with high The target production Cinnamyl alcohol selectivity, almost no by-product formation was observed, and the results of several experiments also proved that the catalyst has good stability.The high performance of the catalyst can be summarized as the following two reasons: on the one hand, L acid Is the active center of MPV reduction reaction, which has a high L acid amount and thus high conversion rate. On the other hand, higher concentrations of Ti (III) species on the surface allow the cinnamaldehyde to adsorb in a vertical adsorption mode C = O) is adsorbed on the surface of the catalyst. This adsorption mode can be highly selectively reduced to the target product cinnamyl alcohol, thus having high selectivity.