Studies On Liquid-phase Selective Hydrogenation Of Cinnamaldehyde Over Pt-Fe/HPZSM-5 Catalysts

Due to the insufficient supply of natural resources,the selective hydrogenation of?,?-unsaturated aldehydes to prepare?,?-unsaturated alcohols has met the extensive demand in organic syntheses,fine chemicals,pharmaceuticals and other fields,so it has always been a hot topic of concern.As one of the typical?,?-unsaturated aldehydes,the liquid phase selective hydrogenation of cinnamaldehyde has also been widely studied as a model reaction.On the one hand,its hydrogenation product cinnamyl alcohol has important application in industries;on the other hand,it is an intensively studied model reaction for studying the structure and activity relationship of catalysts.However,because cinnamaldehyde molecules contain a variety of functional groups,including conjugated C=C and C=O bonds,the C=C bonds are more easily to be activated and hydrogenated to form the saturated aldehydes from the thermodynamic point of view.Therefore,it is very challenging to improve the selectivity to cinnamyl alcohol while maintaining the high reactivity simultaneously.In recent years,molecular sieve materials have been widely applied to petrochemical and biomass conversion fields.ZSM-5 zeolite with MFI topology is widely used in petrochemical,fine chemical industry,environmental protection and other industries because of its unique properties,such as adjustable Si/Al ratio,good hydrothermal stability,excellent shape selectivity,three-dimensional cross channel,large specific surface area,high pore volume and porosity.In this thesis,a commercial ZSM-5 with high Si/Al ratio?Si/Al₂=168?was used as raw material to produce the hierarchical porous ZSM-5?HPZSM-5?after part desilication through the alkaline solution treatment.The HPZSM-5 still maintains the original framework of ZSM-5,whereas a hierarchical porous structure with mesopores and micropores is formed,which is very helpful for the mass transfer and diffusion of larger molecules such as cinnamaldehyde.The platinum nanoparticles were loaded on the hierarchical porous molecular sieve HPZSM-5 by a conventional impregnation method.The catalyst precursor was calcined at different temperatures and then reduced in flowing hydrogen to obtain 5 wt%Pt/HPZSM-5 catalyst for liquid-phase selective hydrogenation of cinnamaldehyde.The results showed that compared with 5 wt%Pt/ZSM-5 catalyst,the activity of 5 wt%Pt/HPZSM-5 catalyst and the selectivity to cinnamyl alcohol were improved distinctly.After Fe was doped,the dispersion of Pt nanoparticles was further improved,and the catalytic activity of PtFe_0.25/HPZSM-5 with suitable Fe/Pt molar ratio of 0.25 was further improved.The initial activity could reach 3.41 s^-1,and the selectivity to cinnamyl alcohol of 87.6%was furnished.More importantly,the PtFe_0.25/HPZSM-5 catalyst can be recycled at least 9 times without distinct loss in activity or selectivity to cinnamyl alcohol.After further characterization of Pt/ZSM-5,Pt/HPZSM-5 and PtFe_0.25/HPZSM-5 catalysts,it showed that after partial desilication by alkali treatment,the acidity of ZSM-5 can be remained,which was beneficial to the adsorption of substrates and the desorption of products.After adding Fe promoters with an optimal amount,there is electron donation from Fe speices to Pt active sites so that the electron density of Pt species was increased to facilitate the activation of hydrogen molecules.As a result,the activity of PtFe_0.25/HPZSM-5 catalyst was further improved,so the highest catalytic activity and cinnamyl alcohol selectivity were obtained on the PtFe_0.25/HPZSM-5catalyst.Moreover,we also applied the ZSM-5?Si/Al₂=91?molecular sieves with lower Si/Al ratios as support for Pt or Pt-Fe.After partial desilication with sodium hydroxide,HPZSM-5 was obtained to support Pt-Fe speices.When compared with the PtFe_0.25/ZSM-5 catalyst,the PtFe_0.25/HPZSM-5 catalyst showed inferior results including the conversion of cinnamaldehyde and selectivity to cinnamyl alcohol.Based on the characterization results,the HPZSM-5 had less acidity than its host ZSM-5 with a relatively lower Si/Al atomic ratio,as a result,the PtFe_0.25/HPZSM-5with a lower Si/Al atomic ratio showed lower activity and selectivity to cinnamyl alcohol.