Research On Modified Carbon-based Palladium Catalyst For Selective Catalytic Hydrogenation Of Crucial Intermediates In The Synthesis Of Vitamin E

Vitamin E,as one of vital nutrients that a human body requires but cannot synthesize in sufficient amounts,plays a significant role in controlling fertility of human beings as an antioxidant.One of prevalent approaches to Vitamin E involves the reaction between isophytol and TMBQ(2,3,5-trimethylhydroquinone).In this route,the semi-hydrogenation of relevant alkynols and reduction process of TMBQ play extremely important parts.In the semi-hydrogenation of alkynols,Lindlar catalyst was utilized traditionally.However,it contains some poisonous elements(such as plumbum)and goes through deactivation even after the first run in aqueous media.While in the conventional reduction process of TMBQ,the use of chemical reactant may lead to the waste of natural resources and endanger the environment.Recently,heterogeneous catalysis,as one of circulative and cheap methods,exhibits its advantages in these processes.Among various catalysts,carbon-based material,especially nitrogen-doped carbon(CN),caught more and more attention with such advantages as cheap and biomassed-derived precursors,peculiar physical and chemical property and efficient catalytic activity.When it comes to the semi-hydrogenation of alkynols,a PdZn/CN@ZnO catalyst was easily obtained from thermal condensation of biomass-derived glucosamine hydrochloride(GAH)and ZnO,followed by a wet impregnation of Pd nanoparticles and subsequent high temperature reduction in H2 atmosphere.Selective hydrogenation of 2-methyl-3-butyn-2-ol(MBY)was employed firstly as a model reaction.Compared with PdZn/ZnO,this catalyst presented six times higher activity while still maintaining high selectivity(92%).Through proper adjustment of reaction conditions and introduction of additives,the final yield of MBE could reach above 95%.Various evidences showed that the substitution effect of corner and edge Pd atoms helped to improve the selectivity.In addition,the presence of CN promoted its hydrophilic property and there existed interaction between PdZn and CN,thus improving the catalytic activity.Within at least nine runs the catalyst exhibited excellent stability.When expanding the substrates to other alkynols,this catalyst still displayed satisfactory performance as well.As for the catalytic hydrogenation of TMBQ to 2,3,5-Trimethylhydroquinone(TMHQ),wasted carbon was employed as the raw material while NaHCO3 as well as(NH4)2C2O2 was utilized to activate it in high temperature in N2 atmosphere.Thus a high-surface activated N-doped activated-carbon material(named ANAC)was obtained.Pd/ANAC was eventually prepared with PdCl2 as the precursor and formic acid as the reductant.When it was applied in the hydrogenation of TMBQ in room temperature,3-fold higher activity was achieved than Pd/AC(palladium on activated carbon)and the selectivity could reach above 99%.Through further research,more parameters of intrinsic kinetics were obtained.The value of reaction order a and b turned out to be 1 and 1.05 respectively.The reaction activation energy was calculated to be 56 kJ/mol.All those could play parts in guiding the practical industrial production of TMHQ.To sum up,through the conversion from green biomass-based precursor and reuse of waste carbon,carbon-based materials were finally prepared and employed as efficient catalysts in the hydrogenation of alkynols and TMBQ.Those catalysts opens up new designs for high efficient synthesis of intermediates for Vitamin E.