Mechanism Of The Hydrogenation Of α, β-Unsaturated Aldehyde

The synthesis of a large number of fine chemicals, particularly in the field of chemistry, medicine and pharmaceuticals, involves the selective hydrogenation of a, -unsaturated aldehyde. The competitive hydrogenation between the C=G and C=C double bonds is the key to determine the different hydrogenation products. Saturated carbonyls are comparatively easy to achieve because thermodynamic favors the hydrogenation of the C=C bond. Therefore, research efforts were directed at looking for efficient but cheap hydrogenation catalyst and developing its responding hydrogenation mechanism to improve the selectivity to unsaturated alcohols.In the first part of our dissertation, three kinds of catalysts Co/Al2O3, Co-Ce/Al2O3 and Pd/Al2O3 were prepared by impregnancy method. Subsequently, catalyst activity evaluation was proceeded in the hydrogenation reaction of cinnamaldehyde. A series of modern physical tests were also carried out to establish relations between selectivity and activity and changes during the course of reduce and reaction of abvoe catalysts. The reaction behaviors of cinnamaldehyde and crotonaldehyde moleculars on the alumina-supported metal catalysts were investigateded mainly by in-situ FT-IR spectroscopy. Although the reaction mechanism was quite varified over different catalysts, the progressive experiments were proceeded to discover the completed reaction mechanism. Especially, the quantum chemistry calculation software (Gaussian98) was used to help discuss the C=C hydrogenation of crotonaldehyde.The low reaction activity and higher saturated alcohol selectivity in the reaction of cinnamaldehyde on Co/Al2O3 catalyst was attributed to the SMSI effect between Co3O4 and carrier. It is assumed that hydrogenation of carbonyls on Co surfaces occurs via the Horiuti-Polanyi mechanism. In order to explain the little promoting effect of Ce to Co, the possible electronics effect which referred to as "electrophilic C=O activation" was brought up to account for the promoting effect of the second metal according to the experiment results of in-situ FT-IR. Palladium catalyst is very active for C=C bond hydrogenation and gives almost a 100% yield to saturated aldehyde. It is thought there also exsits competition in the formation of saturated aldehyde besides the usual competition betweenC=C and C=O to formate saturated aldehyde and a, p-unsaturated alcohol respectively, namely, except the 3,4-addition of hydrogen gives the saturated aldehyde, two kinds of adsorbed structures C=C=O and C=C=C can also formate the enol, which isomerizes into a saturated aldehyde.For this, in the next part of the thesis, we use transition state theory to investigate the reaction of crotonaldehyde hydrogenation. Using density functional theory, the geometric parameters and vibration frequencies were calculated at B3LYP/6-311G(d) level. The theoretical frequencies are well in agreement with the experimental data. The geometries of reactants, transition states and products have been optimized and verified by frequency analysis. The relative single-point energies of the above structures have been calculated at the same level. The zero-point-energy ( ZPE ) corrections were obtained. The three reaction barriers are 648.99 kJ/mol and 684.71 kJ/mol and 194.73 kJ/mol respectively. By comparing the calculated barriers, it is found that reaction (1) is the main reaction path and the approach of the reaction barriers between reaction (1) and reaction (2) results in the competition of 3,4-addition and 1,4-addition of hydrogen. The whole reaction is exothermic reaction with the energy of 92.3 kJ/mol. The barrier of reaction (3) is responsible for the enol structure observed in the in-situ FT-IR.