Reaction Performance And Theoretical Studies On Liquid-phase Selective Hydrogenation Of Cinnamaldehyde Over Pt-Fe Bimetallic Catalysts

The selective hydrogenation ofα,β-unsaturated aldehydes with conjugated C=C and C=O double bonds remains a challenging and unsolved problem to produceα,β-unsaturated alcohols or saturated aldehydes so far.Particularly,α,β-unsaturated alcohols,which are scarce in natural resources but widely needed in the fields of spices,cosmetics and pharmaceutical intermediates,have become indispensable chemicals in fine chemical industry.The selective hydrogenation ofα,β-unsaturated aldehydes to corresponding unsaturated alcohols is of interest to many researchers.However,C=C bond is easier to hydrogenate and more reactive than C=O bond in both thermodynamics and kinetics.Therefore,the selective adsorption and activation of C=O bond put forward higher requirements for the design and preparation of catalysts.The liquid-phase selective hydrogenation of cinnamaldehyde（CAL）,which is a typicalα,β-unsaturated aldehyde,is often used as a model reaction to investigate the relationship between the structure of the catalyst and the reaction performance.Based on literatures and preliminary work,it has been found that Pt-based catalysts gained a foothold in the catalytic hydrogenation of CAL,and the electronic properties of active metal Ptcomponent can be modified by Fe dopant.At the same time,the nature of the support and metal-support interaction will also affect the adsorption and activation of reactant,thus reaction performance is improved.In this paper,the Ptor Pt-Fe bimetallic catalysts were prepared by impregnation method using mesoporous TS-1 microspheres（MTS-1-MS）as support,then applied to the liquid-phase selective hydrogenation of CAL.The results are as follows:Firstly,TS-1 microspheres with mesoporous channels were prepared,and Ptnanoparticles were loaded onto MTS-1-MS by impregnation method to obtain 5 wt%Pt/MTS-1-MS catalyst.After pretreatment at 400℃for 2 h in hydrogen atmosphere,the catalyst was applied to selective hydrogenation of CAL,showing 29.2%CAL conversion and 79.8%COL selectivity.After doped with Fe,part of Fe species entered the Ptlattice and then formed Pt-Fe alloy.The particle size decreased and the dispersion increased.Compared with Pt/MTS-1-MS catalyst,the PtFe_0.25/MTS-1-MS-350 catalyst prepared by calcining at 350℃with Fe/Ptmolar ratio of 0.25 had the best catalytic performance.The characterization results of XRD and XPS proved that Fe species coexisted in the form of PtFe alloy and Fe Ox in the catalyst,the electrons which transferred from Fe species to Ptspecies changed the electronic properties of active components,thereby the catalytic performance was significantly enhanced,showing95.7%CAL conversion and 89.2%COL selectivity.The initial activity can reach 19440h^-1 at 90℃,and the apparent activation was 15.7 kJ/mol.It can be reused at least 9times without obvious loss of activity and selectivity.Further characterization results of H₂-TPR and NH₃-TPD evidenced that the Pt-Fe interaction,Pt/PtFe-support interaction and the Lewis acid site of the support also affected the properties of the catalysts.Therefore,PtFe_0.25/MTS-1-MS-350 catalyst had the best catalytic performance.Secondly,in order to better understand the performance-structure relationship of catalysts,rationalize the preparation of catalysts,and obtain a higher selectivity to COL,it is necessary to adopt density functional theory（DFT）to fully understand the surface phenomena of catalysts.According to the PtFe alloy observed in the experiment,the PtFex（111）surfaces were constructed to simulate the active species surface of the catalyst.Based on the d-band center model,it was found that the Fe dopant increased the width of the d-band,so the d-band centers have to move away from the fermi level to ensure a constant d-band filling.At the same time,the adsorption mode and adsorption energy of CAL on each surface,the desorption energy of each hydrogenation product,the bond length and vibration frequency of C=C/C=O bond were investigated.The adsorption of CAL and the desorption of COL were related to the hydrogenation selectivity of the reaction.When the molar ratio of Fe/Ptincreased to 0.25,CAL tended to be vertically adsorbed on the surface at a certain angle（di-σCO mode）,and COL was more easily desorbed from the surface,so the COL selectivity increased,which explained to some extent the optimal catalytic performance when the molar ration of Fe/Ptwas 0.25.In addition,compared the density of states（DOS）of the CAL,Pt（111）and PtFe_0.25（111）surface before and after adsorption,it was found that Ptd orbitals on the Pt（111）surface are more likely to interact with C p_z orbital of C=C bond,but for the PtFe_0.25（111）surface,Ptd orbitals are more likely to interact with O p_zorbital of C=O bond,so after doped with suitable amount of Fe,activity and COL selectivity were improved.