Alpha, Beta-unsaturated Aldehydes To Select The Hydrogenation New Catalyst Design And Research

α,β-unsaturated alcohols are important intermediates in the production of perfumes, flavorings and pharmaceuticals. α,β-unsaturated alcohols are commercially achieved by using NaBH₄ or AlLiH₄ as reductant, but this process involves a lot of pollutants. Development of the alternative green processes for the synthesis of α,β-unsaturated alcohols has attracted great attentions. Heterogeneous catalysis via direct hydrogenation of α,β-unsaturated aldehyde are able to reduce the conjugated C =O double bond in the presence of molecular H₂.This process provides a clean and economic approach to the production of α,β-unsaturated alcohols. However, the manipulation of the selectivity in the hydrogenation of α,β-unsaturated aldehydes is of considerable challenge, as it is thermodynamically more favored to produce saturated aldehyde or saturated alcohol than the unsaturated alcohol on metals. The only feasible way to enhance the selectivity to unsaturated alcohol is to kinetically activate the C=O bond rather than the C=C bond over delicately designed catalysts.It have been documented that lots of metal catalysts mainly provide saturated alcohol or saturated aldehyde and metal cobalt is only one promising non-noble metal for the selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohol. In recent years, gold has attracted growing interest in catalyst research since it has been shown that gold nanoparticles with sizes below 5 nm exhibit enhanced activity in CO oxidation reactions at room temperature, despite the inert character of gold. Gold has been more recently tested again in hydrogenation reaction, and it also seems a very promising catalyst for this type of reaction. Indeed, gold nanoparticles are even more selective in partial selective hydrogenation than Pt metal. In this dissertation, the cobalt and gold was chosen as active species. The Co-based amorphous catalysts and Au catalysts have been investigated systematically for the hydrogenation of crotonaldehyde and cinnamaldehyde. The influence of surface structure, electronic properties and the interaction between metal and support of the catalyst were studied in detail by various analytical and spectroscopic techniques. In addition, by correlation of the hydrogenation reaction results with above results, the active sites were discussed.1. Studies on the metal-promoted amorphous CoB catalysts for the hydrogenation of crotonaldehydeThe promoting effect of tin on the liquid phase hydrogenation of crotonaldehyde over amorphous CoB catalyst has been investigated. SnCl₂ or SnCl₄ was introduced to the reactant before hydrogenation or incorporated to the CoB catalyst during the reduction of CoCl₂ by potassium borohydride. It is found that the preparation approach has a marginal or adverse effect on the formation of crotyl alcohol. CoB catalyst modified by a small amount of tin salts in the reactant is more effective in selective hydrogenation than co-reduced CoSnB catalysts. The overall activity of the CoB catalyst was depressed upon the addition of the tin salts in the reactant. The tin salts in the reactant exert their influence on the selectivity by adsorption on the surface of the CoB catalyst. The tin ions preferentially occupy the active sites relating to C=C bond adsorption, leading to monotonous decrease of r_C=C with increased Sn/Co ratios. On the other hand, the adsorbed tin ions function as Lewis acid sites, the crotonaldehyde molecule being adsorbed via the donation of a lone electron pair from the oxygen of the carbonyl group. This bonding induces the polarization of the C=O bond which is favorable for nucleophilic attack on the carbon atom by hydrogen dissociatively adsorbed on neighboring active sites and, thus, enhances the hydrogenation of this functional group. However, incorporation of tin to the catalyst by co-reduction is mainly against the formation of crotyl alcohol. SnCl₂ causes a faster drop of the reactivity towards the C=O bond rather than the C=C bond. Although SnCl₄ increases both the reactivities of the C=O and C=C bonds, the former increased to a less extent than the latter. Their different catalytic behaviors were tentatively interpreted on the basis of XPS measurements. On the CoSn_â…¡B catalyst, tin is predominantly in its metallic state, which preferentially blocks the active sites for C=O bond saturation. Moreover it is found that the carbonyl selectivity sequence in crotonaldehyde hydrogenation when metal ion was introduced to the reactant before hydrogenation is Fe³⁺ > Sn⁴⁺ ≈ Sn²⁺ ≈ Zn²⁺ > Co²⁺ > Cu²⁺> CoB, while he carbonyl selectivity sequence when metal ion was incorporated to the CoB catalyst during the reduction of CoCl₂ by potassium borohydride is Fe³⁺ > Zn²⁺ > Sn²⁺ > Cu²⁺ > CoB > Sn⁴⁺.2. Studies on the CoFeB catalysts for the hydrogenation of crotonaldehyde and cinnamaldehydeBinary CoB and ternary CoFeB amorphous alloy catalysts with different Fecontent were prepared by the chemical reduction method. In liquid phase hydrogenation of crotonaldehyde, the incorporation of Fe to the CoB catalyst suppressed the overall activity, while effectively improved the selectivity and yield to crotyl alcohol. On the optimum CoFeB-3 catalyst with a nominal Fe/(Co+Fe) ratio of 60 mol%, the initial selectivity amounted to 71.1%, and the yield of crotyl alcohol reached 63.5%, which is one of the best results reported in the open literatures dealing with crotonaldehyde over Co-based catalysts. While in liquid phase hydrogenation of cinnamaldehyde on the CoFeB-1 catalyst with a nominal Fe/(Co+Fe) ratio of 20 mol%, the yield of cinnamyl alcohol reached 95.3%, which is much higher than 75.9% on CoB catalyst. The investigations of the effect of Fe on structures and properties of amorphous catalyst showed that the addition of iron to CoB catalysts led to an improvement of amorphous nanoparticles dispersion and thermal stability of amorphous structure. The electron density of Co did not varied, but the addition of iron resulted in an increase in the ratio of the surface oxidized states in the catalyst. On the other hand the capacity of hydrogen adsorption on CoFeB catalyst was depressed and the ratio of strong relative amount of the weakly and strongly bound hydrogen was adjusted. The relative amount of the strongly bound hydrogen is decreased by 35% over CoFeB-3 catalyst. It is found that the selectivity enhancement was due to less decrement in the intrinsic formation rate of crotyl alcohol than that of butanal instead of increasing the activation of the C=O bond. Based on the characterizations including X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and previous findings, the selectivity enhancement from Fe modification was attributed to the ensemble size effect.3. Preparation, characterization of Au/TiO₂ catalysts and their catalytic properties for the hydrogenation of crotonaldehydeThe nanoparticle Au/TiO₂ catalysts prepared by deposition-precipitation method have been investigated systematically for the hydrogenation of crotonaldehyde. The effects of treatment conditon, metal loading and reduction temperature on the surface structure, electronic properties of the catalyst and on its catalytic performance in hydrogenation of crotonaldehyde were studied by various analytical and spectroscopic techniques. It is found that the partice sizes of gold nanopartilces and the interaction of gold and support are the main reasons on the catalytic performance. Along with theincrease of gold nanoparticles, the catalytic activity first increased and then decreased. For Au/TiO₂(R) and Au/TiO₂(core shell), the optimum hydrogenation rate of crotonaldehyde were obtained when the particle size of gold are 2 nm and 3.4 nm respectively. Moreover, the stronger interaction of metal and support often lead to increased electron density on metallic gold. The increased electron density on metallic gold not only alter the interaction of the active sites with the functional group and facilitate a partial transfer of electron density to the Ï€_CO orbital of the unsaturated bond, but also weaken the binding of the C=C group on the active sites. These effect of electron-rich metallic Co can account for the increased reactivity of the C=O group relative to the C=C group.The effect of supports on the catalytic properties has been studied. It was found that the optimum yield of crotyl alcohol were above 65% on Au/TiO₂(R) and Au/TiO₂(core shell) catalysts, which were much higher than that on Au/TiO₂(P25). From these results it was presumably concluded that different support may influence gold particle size, strain effect and gold structure which also contribute to the catalytic performance.4. Preparation, characterization of mesoporous silica anchored gold catalyst and their catalytic properties for the hydrogenation of crotonaldehydeAu nanoparticles grafted on surface-functionalized mesoporous silica exhibits surprisingly high selectivity and yield to crotyl alcohol, compared with the results from catalysts made with double-solvent method and impregnation method. On Au-apts-MCM-41 catalyst the optimum yield of crotyl alcohol reached 47.3% and corresponding selectivity reached 50.9%, which is one of the best results reported in the open literatures dealing with crotonaldehyde over gold catalyst supported on silica. The higher yield to crotyl alcohol is related to the presence of interaction between gold nanoparticle and amino functional group, while the catalytic activity increases with particle size decreasing. The kinetic study for the hydrogenation of crotonaldehyde over Au-apts-MCM-41 catalyst showed the reaction is zero order with respect to crotonaldehyde and first order with respect to hydrogen at H₂ pressure. The activation energy for this reaction calculated is 63 kJ·mol^-1. In addition, the hydrogenation results of crotonaldehyde over gold nanoparticle anchored overmesoprous molecular sieves (HMS, MCM-41, MCM-48 and SBA-15) were compared. It was found that the optimum yield of crotyl alcohol were comparable, but on Au-apts-SBA-15 the hydrogenation rate were much higher than other three catalysts and the activity sequence is in accordance with the gold particle size.