Preparation Of Supported Nanocatalysts Using Ionic Liquids As Solvents And Their Selective Hydrogenation Properties On α, β-unsaturated Aldehydes

The products ofα,β-unsaturated aldehyde selective hydrogenation are valuable intermediates, which are widely used in perfumes, flavoring, Pharmaceuticals, pesticides and foods. Becauseα,β-unsaturated aldehydes contain both C=C and C=O bonds, their hydrogenation reaction are more complex. High selectivity to the desired product at high conversion level is a very important topic from synthetic point of view. Therefore, it is practically significant to develop effective catalysts with high catalytic activity and selectivity for the hydrogenation ofα,β-unsaturated aldehydes.In this thesis, three series of silica supported nickel, ruthenium and palladium nanocatalysts were prepared from tetraethyl orthosilicate by modified sol-gel method innovatively using imidazole type ionic liquids as solvents. Their catalytic performances for liquid phase selective hydrogenation ofα,β-unsaturated aldehydes were studied as well. The influences of chemical structure of imidazole type ILs, contents of active element, calcination and reduction temperatures, and promoters on their properties were investigated. The catalysts were characterized by means of nitrogen adsorption-desorption (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed reduction of hydrogen (H₂-TPR), temperature programmed desorption of carbon monoxide (CO-TPD), energy dispersive X-ray spectroscopy (EDS), infrared spectroscopy (IR), and thermal gravimetry (TG).The results show that the Ni/SiO₂ catalyst prepared with 1-butyl-3-methyl-imidazole tetrafluoroborate ionic liquid as solvent, with the nickel content of 20wt%, calcinated at 873K, reduced at 823K in hydrogen respectively, and with the addition of 5wt% promoter Sm exhibits excellent catalytic activity and selectivity for the hydrogenation of citral. The conversion of citral and the selectivity to citronellal is 66.3% and 90.8%, respectively, over this catalyst.Under the optimal reaction conditions: 5%Sm-20%Ni/SiO₂ catalyst dosage (based on the mass of used citral) 20%, reaction pressure 1 MPa, temperature 413 K, stirring speed 1200 r/min and reaction time 10 h, the conversion of citral and the selectivity to citronellal can reach 95.1% and 96.7%, respectively. The catalytic performances are stable after other five runs. But the activity decreases by 19.7% and the selectivity to citronellal decreases by 24.2% after recycling for 8 times.The Ru/SiO₂ and the Pd/SiO₂ catalysts, prepared by modified sol-gel method with 1-butyl-3-methyl-imidazole tetrafluoroborate ionic liquid as solvent, both show high catalytic activity and selectivity to citronellal in liquid phase hydrogenation of citral. The conversion of citral is 87.3% and the selectivity to citronellal is 85.1% when the 5%Ru/SiO₂ was used as catalyst. Moreover, with the 5%Pd/SiO₂catalyst, the conversion of citral can reach 98.3%, and the selectivity to citronellal and dihydrocitronellal is 40.9% and 52.5%, respectively.The catalysts 5%Sm-Ni/SiO₂, 5%Ru/SiO₂ and 5%Pd/SiO₂ mentioned above display excellent catalytic performance for the liquid phase selective hydrogenation of otherα,β-unsaturated aldehydes such as cinnamaldehyde, benzaldehyde and acetophenone.