| dc.description.abstract | Our research in the past, silver was dispersed on a solid base of Mg2AlO-hydrotalicite using the traditional wet impregnation method via impregnating, calcinating and reducing to preparation Ag/Mg2AlO catalyst for selective hydrogenation of citral. The investigation confirms the activity of Ag/Mg2AlO inferior to gold catalyst, but selectivity as good as gold catalyst. To avoid sintering, in the present study, silver nitrate as precursor salts, and the Mg2AlO-hydrotalcite calcined at 100°C as a support by past experience, using the wet impregnation method to be containing silver nitrate impregnating, drying and fixating on Mg2AlO to preparation of Ag/Mg2AlO catalyst by chemical reduction method with NaBH4. This study found the catalyst drying and fixating at 100~150°C, the color evident changes from white to black, and have a comparable gold catalyst activity and selectivity. The study will take catalyst preparation are divided into two categories, one is Ag*/Mg2AlO catalyst by drying and fixating without NaBH4 reducing, and another is Ag/Mg2AlO catalyst by drying and fixating with NaBH4 reducing.
The basic of Mg2AlO-hydrotalcite as a support plays an important role on 10wt%Ag*/Mg2AlO catalyst, the strong basic sites of Mg2AlO can promote silver nitrate to decompose even at low temperatures 50°C or 25°C as drying and fixating temperature, the 10wt%Ag*/Mg2AlO catalyst still has considerable activity. The color changes from white to black, silver nitrate partially decomposed into Ag0 when the drying and fixating temperature raise, a selection of the best activity and selectivity at 150°C. Drying and fixating temperature greater than 150°C, silver is sintering then activity decreased significantly.
Preparation 10wt%Ag/Mg2AlO catalyst by 10wt%Ag*/Mg2AlO drying and fixating with NaBH4 reducing, the NaBH4 reducing procedures will making silver nitrate decomposed into Ag0, but 10wt%Ag/Mg2AlO catalyst activity is far less 10wt%Ag*/Mg2AlO catalyst without NaBH4 reducing. Drying and fixating temperature at 150°C not enough to be effective fixed silver nitrate on the support, using NaBH4 reducing procedures, silver easily sintering and Na ion residues weakens the activity of silver catalyst. In addition, drying and fixating temperature not enough, a portion of the silver nitrate too late to reduce on the support, it will dissolving to reducing liquid and reducing to the Nano silver in the reducing liquid, the reducing liquid caused higher activity than the catalyst.
Chemical reduction method does not apply to Mg2AlO support, the strong basic sites of Mg2AlO can promote silver nitrate to decompose when drying and fixating. SiO2 and γ-Al2o3 can strengthening silver nitrate fixed on the support by increase drying and fixating temperature, and using NaBH4 reducing procedures to preparation Ag/SiO2 and Ag/γ-Al2O3 catalysts, but the activity remains below Ag/Mg2AlO catalyst, more far less Ag*/Mg2AlO catalyst. This study found that simple method containing silver nitrate impregnating, drying and fixating without NaBH4 reducing procedures, can get a hydrogenation catalyst Ag*/Mg2AlO with high activity and selectivity for selective hydrogenation of α,β- unsaturated aldehyde.
The interaction force of reactant molecule adsorbed on the surface of the silver metal catalyst was similar of the dipole-dipole interaction and dispersion force which was such as physical interaction not as the covalent interaction of gerenal hydrogenation metal catalyst, making the 10wt%Ag*/Mg2AlO catalyst in α,β- unsaturated aldehyde reaction different from general catalytic behavior of metal catalyst. The C=O bond of conjugated C=C/C=O bond was hydrogenated priority, the remained C=C bond from conjugated C=C/C=O bond would not be hydrogenated continuously. The longer fuctional groups of unsaturated aldehyde molecule, the greater catalytic activity. Besides, cinnamaldehyde have the larger steric hindrance of the phenyl group around the conjugated C=C bond, but the selectivity of cinnamyl alcohol from cinnamaldehyde reduction is smaller than that of nerol/geraniol from citral.
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