| dc.description.abstract | Silver is dispersed on a solid base of Mg2AlO-hydrotalicite using a wet impregnation method to obtain a effective catalyst for selective hydrogenation of alpha,beta-unsaturated aldehydes.
In the present study, 5wt% Ag/Mg2AlO catalysis on liquid-phase hydrogenation of a citral mode reactant. We disussed the experiment factors such as solvent effect, the whole phenomenon of hydrogenation, the molecular structure of reactants, steric effect and other impact from the experimental changes. The silver catalyst is characteristized by TGA, BET, TEM, XRD, TPR, TPD. In addition, we used γ-Al2O3, MgO, CeO2, SiO2 as support to observe the impact of support effect.
The optimal catalyst, 5wt% Ag/Mg2AlO, was prepared by following process: Mg2AlO calcined at 100 °C as a support , 5wt% Ag/Mg2AlO catalyst calcined at 250 °C, and reducted at 400 oC before the catalysis. 5wt% Ag/Mg2AlO catalyst display effectively chemoselective hydrogenation of citral into unsaturated alcohols (Nerol and Geriol) for selectivity of 97%.
The interaction force of unsaturated aldehyde molecule adsorbed on the surface of the silver metal catalyst, is different from the convalent force of conventional hydrogenation catalyst (Pt, Ni, etc), making the 5wt% Ag/Mg2AlO catalyst in citral reaction from a general catalytic behavior of metal catalysts. The C=O bond of conjugated C=C/C=O bond was hydrogenated prioritily; the remained C=C bond from conjugated C=C/C=O bond would not be hydrogenated continuously. The longer fuctional group of unsaturated aldehyde molecule, the greater catalytic activity.The 5wt% Ag/Mg2AlO has better citral hydrogenation activity in polar solvents than in nonpolar solvent. Higher alcohol carbon number can offer the catalytic better reactivity, but the unsaturated alcohol selectivity is slightly decreased.
In the discussion of support effect, we conclude the activity of silver catalyst is proportional the relations with the base site concentration of support. The particle size of the silver catalyst is not affect the activity obviously. The base sites of support can promote hydrogen dissociated on the surface of silver-based catalyst. Among the supports in study, Mg2AlO-hydrotalcite in α,β-unsaturated aldehyde hydrogenation can be obtained the high potency silver catalyst.
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