Liquid-phase Selective Hydrogenation Of Cinnamaldehyde With Pt Or Ni-based Catalysts Supported On Composite Oxides

The selective hydrogenation of cinnamaldehyde is a kind of atom-economic reaction,and the semi-hydrogenated products can be widely applied in fragrance industry,pharmaceutical intermediates,cosmetics and other fine chemicals.Moreover,the selective hydrogenation of cinnamladehyde can be used as a model reaction to investigate the relationship between catalyst structure and performance.Because the C=C bond energy of cinnamaldehyde is lower than C=O bond energy,and there is also benzene ring in the strcuture of cinnamaldehyde,it is very challengeable to achieve high selectivity towards the preferential hydrogenation of C=C or C=O double bond.Therefore,design of heterogeneous catalysts with high catalytic activity and high selectivity to target products is a research concern.Pt-based catalysts are widely investigated for the selective hydrogenation of cinnamaldehyde due to their excellent catalytic activity.However,monometallic Pt catalysts can only show poor selectivity toward semi-hydrogenated products.To improve the catalytic performance of Pt-based catalysts,the second metal component such as Fe,Co or Ni is usually adopted to modulate the Pt nanoparticles.However,in bimetallic Pt-M catalysts prepared using traditional methods,the second metals usually exist as their oxides aggregates,so that the modulating of Pt with the second metal oxides is difficult to control fully owing to lack of the intimate contact.In recent years,AB₂O₄ spinel and ABO₃ perovskite composite metal oxides with unique structure and controllable composition are regarded as ideal catalytic materials and have been widely used in many catalytic fields.“A”site ions and“B”site transition metal ions(such as Fe,Co,Ni)can be dispersed highly in these composite oxides.Under this circumstance,there will be imtimate contact bewteen Pt nanoparticles and transition metal oxides when composite oxides are used to load Pt nanoparticles.In this work,we synthesized Co_xFe_1-xAl₂O₄₊?and Ni_xFe_1-xAl₂O₄₊?spinel composite oxides using“Co/Fe”or“Ni/Fe”as“A”site ions and“Al”as“B”site ions.We also synthesized La Ni O₃ perovskite composite oxides using“La”as“A”site ions and“Ni”as“B”site ions.The transition metals(Co,Fe or Ni)can be well dispersed in these composite oxides.The Pt-based catalysts were prepared by impregnation method using Co_xFe_1-xAl₂O₄₊?and Ni_xFe_1-xAl₂O₄₊?spinel composite oxides as supports,and the Ni-based catalysts were also obtained by reducing Ni_xFe_1-xAl₂O₄₊?and La Ni O₃ composite oxides in hydrogen atmosphere at high temperatures.Then Pt or Ni-based catalysts were applied to the liquid-phase selective hydrogenation of cinnamaldehyde.The main contents are as follows:(1)Co_xFe_1-xAl₂O₄₊?spinel composite oxides were prepared using a sol-gel method and supported Pt NPs were employed for the liquid-phase selective hydrogenation of cinnamaldehyde.Pt/Co_xFe_1-xAl₂O₄₊?catalysts showed over 90%selectivity to cinnamyl alcohol,and the catalytic activity increased with the increase of x in the support.Upon investigation of kinetic behaviour with Pt/Co_0.5Fe_0.5Al₂O₄₊?catalyst as a model catalyst,the initial activity(TOF)of 4.1s^-1 and the initial MSR of24.5 mol g _Pt^-1h^-1 were furnished.As for the selectivity to cinnamyl alcohol,it almost kept around 95%at whatever cinnamaldehyde conversions despite of the reaction temperatures.Pt/Co_0.5Fe_0.5Al₂O₄₊?catalyst can be recycled for ten times without obvious loss in activity or selectivity to cinnamyl alcohol.This suggests that the active site on Pt/Co_xFe_1-xAl₂O₄₊?catalyst preferentially adsorb cinnamaldehyde via the terminal carbonyl groups.H₂-TPR,CO-IR and XPS studies reveal that the intimate contact of Pt nanoparticles with highly dispersed Fe O_x and Co O_x species plays an important role in determinging the catalytic performance of the Pt/Co_xFe_1-xAl₂O₄₊?catalysts.The Pt^?+/Pt~0 ratio increased with the x value so that the Pt/Co Al₂O₄₊?was the most active among the Pt/Co_xFe_1-xAl₂O₄₊?catalysts(2)Ni_xFe_1-xAl₂O₄₊?spinel composite oxides were prepared using a sol-gel method to support Pt NPs for the liquid-phase selective hydrogenation of cinnamaldehyde.Pt/Ni_xFe_1-xAl₂O₄₊?catalysts showed different selectivity to hydrocinnamaldehyde or cinnamyl alcohol.As a result,92.2%cinnamyl alcohol selectivity and 80.7%hydrocinnamaldehyde selectivity were achieved on Pt/Fe Al₂O₄₊?and Pt/Ni Al₂O₄₊?,respectively.The selectivity switch between semi-hydrogenation of C=C and C=O was achieved by adjusting the Ni/Fe molar ratios in the Ni_xFe_1-xAl₂O₄₊?.Based on the kinetic investigation of further hydrogenation of semi-hydrogenated product,it was found that there was significant difference in the reaction rate of selective hydrogenation of C=C or C=O over Pt-based catalysts with different Ni/Fe molar ratios.The hydrogenation of C=O was preferential over Pt-based catalysts with higher Fe ratio in Ni_xFe_1-xAl₂O₄₊?,while both the hydrogenation of C=O and C=C were hydrogenated over Pt-based catalysts with higher Ni ratio although the rate of C=C hydrogenation was faster.Based on the detailed characterization and kinetic studies,the Pt/Ni_xFe_1-xAl₂O₄₊?catalysts showed different product selectivity mainly due to different Pt active sites and thus different hydrogenation rate for C=C or C=O groups.The Pt/Fe Al₂O₄₊?catalyst had the predominant Pt(111)facets so that the preferential adsorption and activation of C=O bond can be realized to form cinnamyl alcohol.With regards of the Pt/Ni Al₂O₄(10)?catalyst,the low coordinated Pt sites such as step-edges,corners,and defects and two neighboring Pt sites predominated on the surface besides Pt(111),thus both the C=C group and C=O could be adsorbed and activated.Therefore,hydrocinnamaldehyde and a small amount of cinnamyl alcohol were found in the products over Pt/Ni Al₂O₄(10)?catalyst,and the selectivity to hydrocinnamaldehyde was only 80%.Additionally,Pt/Ni_0.5Fe_0.5Al₂O₄₊?catalyst can also be recycled for 8 times without obvious loss in catalytic performance.(3)The supported Ni-based catalysts were obtained by reduction of Ni_xFe_1-xAl₂O₄₊?composite oxides in hydrogen at higher temperatures,and then applied to the liquid-phase selective hydrogenation of cinnamaldehyde.Ni Al₂O₄₊?catalyst showed a certain catalytic activity with the selectivity toward hydrocinnamaldehyde reaching about 80.0%.Cu,Zn and P were adopted to modulate the Ni-based catalysts,but the improvement of catalytic performance was not obvious.(4)The precursor of Ni O-Ln O_x catalyst was prepared using a sol-gel method,and Ni/Ce O₂,Ni/Pr₆O₁₁ and Ni/La₂O₃ catalysts were obtained by reduction in hydrogen flow at high temperatures.When applied to the selective hydrogenation of cinnamaldehyde,the Ni/La₂O₃ catalyst showed higher catalytic activity,affording the90.2%selectivity to hydrocinnamaldehyde,with the initial activity of 2050 h^-1.XRD,TEM,H₂-TPR and CO₂-TPD studies confirm the formation of La Ni O₃ perovskite structure in the catalyst precursor,which enhanced the interaction between Ni species and lanthanum species,and effectively avoided the agglomeration of Ni nanoparticles during the reduction,so that Ni nanoparticles could achieve good dispersion.Moreover,the C=O group in cinnamaldehyde was preferentially adsorbed and activated by basic sites(La³⁺-O^2-)on Ni/La₂O₃ catalyst,so that the chemisorption of C=C bond over active Ni becomes more favorable and the selective hydrogenation of C=C bond occurs.