Selective Hydrogenation Of Citral Over Novel Pt-based Catalysts

The liquid hydrogenation of citral over Pt-based catalysts was investigated in FCZ-1 autoclave reactor.The products distribution was affected by catalyst preparation method and support functionalization.The relationship between the selective hydrogenation activity and morphology of catalyst was also discussed.1.Liquid hydrogenation of citral over Pt-based catalysts with different support was investigated in FCZ-1 autoclave reactor.IM-Pt/SiO₂,IM-Pt/SBA-15,IM-Pt/TiO₂, IM-Pt/γ-Al₂O₃,IM-Pt/MWNTs and IM-Pt/AC catalysts were all prepared by impregnation method.At mild condition(40℃,1.0 MPa),the IM-Pt/MWNTs catalyst exhibited the optimum citral conversion and unsaturated alcohol(UCOL) selectivity,which is 32.4%and 56.0%,respectively.Pt/Fe₂O₃ catalyst was synthesized by co-precipitation method and applied in citral liquid hydrogenation.By increasing the mole ratio of Pt and Fe,the selectivity of nerol was enhanced and the selectivity of geraniol was decreased.Pt/TiO₂ catalyst was synthesized by wetness impregnation,sol-gel and deposition-precipitation method.The catalysts performed different catalytic activity in liquid hydrogenation of citral.The catalyst prepared by sol-gel method showed the highest activity for hydrogenation of citral.At 60℃and 1.0 MPa of H₂ pressure,the citral conversion and UCOL selectivity are 8.2%and 72.7%,respectively.2.Pt/p-MWNTs catalyst was prepared by electroless metal deposition method.Due to the structure and electric property of carbonnanotubes,Pt was dispersed well on p-MWNTs.The catalyst was evaluated at different temperature,H₂ pressure and resident time.It was found that high temperature,low H₂ pressure and long resident time were in favor of C=O hydrogenation.Under optimum reaction condition,80℃and 1.0 MPa of H₂ pressure,the citral conversion is 54.5%and the UCOL selectivity reaches 59.2%.3.The Pt catalysts supported on MWNTs,grafted by polyacrylic acid(PAA) or not, have been successfully prepared by a simple eletroless metal deposition method.The MWNTs supported catalysts show higher activities than the conventional Pt/AC catalyst in citral hydrogenation.The functionalization of the MWNTs can increase the activity of the catalyst and change the reaction route.The catalyst characterization results showed that Pt particles are dispersed more homogeneously on the PAA grafting MWNTs than the other two supports.Meanwhile the PAA grafting treatment can weaken the effect of MWNTs and prevent the formation of Pt(111),which is supposed to change the reaction route.4.Pt/p-MWNTs catalysts were prepared with different ethylene glycol(EG) volume, pH value and concentration of NO₃^- by electroless metal deposition method.The catalytic test results show that the best ratio of EG volume and Pt is 1 mL/mg and the proper pH range is 4 to 7.The addition of NO₃^- in catalyst preparation process can improve the size of Pt particle,and facilitate the growth of Pt(111).As a result,the conversion of citral is decreased,the selectivity of citronellal is increased and the selectivity of UCOL is decreased.Irregular Pt/PAA-g-MWNTs,tetra-,octahedral Pt/PAA-g-MWNTs,tetra-, octapods Pt/PAA-g-MWNTs and nanocube Pt/PAA-g-MWNTs were synthesized by electroless metal deposition method by addition of NO₃^- and Cu²⁺ and using PAA-g-MWNTs as support.The catalyst catalytic properties were tested in citral hydrogenation.The results show that the irregular Pt/PAA-g-MWNTs has the best catalytic activity,the tetra-,octahedral Pt/PAA-g-MWNTs and tetra-,octapods Pt/PAA-g-MWNTs have the similar catalytic activity,and the nanocube tetra-, octapods Pt/PAA-g-MWNTs performed the worst.Meanwhile,the tetra-,octahedral Pt/PAA-g-MWNTs shows the special stereoselectivity of geraniol and nerol in citral hydrogenation.