Factors Affecting Selective Hydrogenation of Acrolein over Supported Silver and Silver Alloy Catalysts
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This work investigates critical factors of catalysts, which play an important role in modifying activity and selectivity of heterogeneous catalysis. One such example is the selective hydrogenation of α,β-unsaturated aldehyde to α,β- unsaturated alcohol (using acrolein as a model reactant), which was used here as a probe reaction. Selective hydrogenation of acrolein to allyl alcohol is particularly challenging as it is difficult to obtain substantial selectivity towards C=O bond hydrogenation both thermodynamically and kinetically. The hydrogenation of C=C bond is about 35 kJ/mole easier than that of C=O bond. This is because acrolein molecule lacks substituents at C=C bond, which makes it especially vulnerable to hydrogenation towards saturated alcohol. Previous research has made great progress in understanding some of the factors (choice of metal, process condition, particle size that may improve selectivity. But there is no reported catalyst that provides high activity and selectivity at the same time. Thus we studied this reaction systematically from two aspects: 1) alloying effect by doping silver with a very dilute amount of hydrogen active metal; 2) support effects. Pd−Ag alloy catalysts with very dilute amounts of Pd were synthesized. Extended X-ray absorption fine structure spectroscopy (EXAFS) results demonstrated that when the concentration of Pd was as low as 0.01 wt %, Pd was completely dispersed as isolated single atoms in Ag nanoparticles. The activity for the hydrogenation of acrolein was improved by the presence of these isolated Pd atoms due to the creation of sites with lower activation energy for H2 dissociation. For the support effect study, a series of supported Ag catalysts using standard supports were prepared (SiO2, Al2O3, TiO2, CeO2, ZrO2, and La2O3) of different particle sizes to study the support effect on selectivity of acrolein hydrogenation towards allyl alcohol. On each support, a similar particle size effect on selectivity (bigger particle size, better selectivity) was observed. We found support reducibility plays an important role in modifying the selectivity towards the unsaturated alcohol as the selectivity towards allyl alcohol was higher for TiO2, CeO2, La2O3 supported catalysts than over SiO2 and Al2O3 supported catalysts at the same particle size and at same conversion. A detailed pathway analysis and kinetics study is done on Ag/TiO2 in an effort to understand the role of the support on catalyst performance. It is observed that the selectivity is as high as 88% at 1% conversion and then decreases to a stable 52% at 10% conversion. The selectivity to allyl alcohol was better than Ag/SiO2 of same size at the same conversion. The reaction orders of both reactants were noticeably different for Ag/TiO2 compared to Ag/SiO2. Formation of Ti3+ sites is believed to favor the adsorption of acrolein through carbonyl bond which is also confirmed by DFT calculations. To conclude, we were able to demonstrate that both alloying and support effects play an important role in modifying both selectivity and activity, of Ag catalysts for selective hydrogenation of acrolein towards allyl alcohol. Another aspect studied, was structure sensitivity for this reaction where we observed Ag(100) is the most favorable surface for adsorption of acrolein (among Ag(221), Ag(111), Ag(100) surfaces) through carbonyl bond and thus shifting the reaction towards unsaturated alcohol(desired product) .