Selective Hydrogenation Of Cinnamaldehyde Over Pt-based Catalysts

The selective hydrogenation of cinnamaldehyde is often regarded as a model reaction to investigate the structure-performance relationship of catalysts because it contains conjugated C=C and C=O bonds in the cinnamaldehyde molecule,so that there are competitive hydrogenations of C=C bond and of C=O bond.The selective hydrogenation products,including cinnamyl alcohol and hydrocinnamaldehyde,are important intermediates for production of fine chemicals such as fragrances and pharmaceuticals.Supported metal catalysts,as an important class of industrial multiphase catalysts,have a strong link between the structure and size morphology of metal nanoparticles and catalytic activity.Pt-based catalysts have been widely investigated in the selective hydrogenation of cinnamaldehyde,so the precise regulation of the morphological size and electronic structure of Pt nanoparticles on the catalysts is an important way to achieve high catalytic activity and hydrogenation selectivity.Additionally,Fe or Co was usually adopted to modify the Pt-based catalysts with obviously improved COL selectivity.The spinel AB₂O₄ material,as an excellent composite oxide material,can realize spital isolation of ions A from ions B and Fe or Co can act as ions A in spinel AB₂O₄.Therefore,the intimate contact between the highly dispersed ions and the Pt particles can be realized,which facilitates the interaction between them and thus affects the morphology and electronic structure of Pt nanoparticles.In the first part of this paper,Pt-based catalysts with different Pt particle sizes（ranging from 1.6 nm to 3.4 nm）were prepared by the impregnation method with a controlled Pt loading,and the catalytic performance and structural sensitivity of Pt/Fe_0.5Co_0.5Al₂O₄ catalysts in the selective hydrogenation of cinnamaldehyde were systematically investigated.It was found that the hydrogenation activity of Pt/Fe_0.5Co_0.5Al₂O₄ catalyst increased with the increasing Pt loading.Meanwhile,the selectivity of the target product cinnamyl alcohol enhanced with the increasing Pt particle size from 68%（1.6 nm）to 93.8%（3.4 nm）.Therefore,cinnamyl alcohol was preferentially obtained on large Pt particles.H₂-TPR and in situ CO-IR characterization successfully revealed that the electronic properties and geometry of Pt nanoparticles varied with the modulation of Pt particle size in the Pt/Fe_0.5Co_0.5Al₂O₄ catalysts.The proportion of electron-rich Pt sites on the surface of large Pt nanoparticles increased and the proportion of low coordination sites decreased compared to small Pt particles,which contributed to the preferential adsorption and activation C=O bond.The in situ CO-IR characterization with the cinnamaldehyde as probe molecule showed that different adsorption modes existed on Pt nanoparticles of different size,and the carbonyl groups of cinnamaldehyde were mainly adsorbed and activated in a tilted mode on the larger size Pt nanoparticles under the electronic and spatial resistance effects,thus higher selectivity of cinnamyl alcohol was obtained.In the second part of this thesis,mesoporous TS-1 material was chosen as the support for Pt-Co bimetallic catalysts.A series of Pt-based catalysts with different Co/Pt molar ratios were prepared by modulating the amount of introduced Co species,and the catalytic performances of the PtCo_x/TS-1 catalysts were investigated in the selective hydrogenation of cinnamaldehyde.It was found that the catalytic activity and the selectivity toward cinnamyl alcohol showed a"volcanic"trend with the increase of Co/Pt molar ratio,and the optimal catalytic performance was exhibited when the Co/Pt molar ratio was 0.5.Therefore,the catalytic performance was improved by adjusting the bimetallic Pt/Co ratio to optimize the synergistic effect between the two metals.The improved catalytic performance results from the formation of Pt₃Co alloy structure within the PtCo_0.5/TS-1 catalyst,and the electronic properties of the active metal Pt nanoparticles on the surface of the catalyst were modified by the Co species,thus promoting the production of cinnamyl alcohol.