Structure-activity Relationship And Catalytic Mechanism Of Pt-WO_x Catalysts In Glycerol Hydrogenolysis

1,3-Propanediol（1,3-PDO）is an important chemical intermediate,and can be used as the monomer of polypropylene terephthalate（PTT）,which is a new kind of polyester material with excellent function.Glycerol hydrogenolysis to 1,3-PDO is a green route,which can not only relieve the pressure of fossil resources exhausting and environmental change,but also contribute to the development of biodiesel industry chain.Because of the high 1,3-PDO selectivity,WO_x based bifunctional catalysts are widely used in glycerol hydrogenolysis.However,there are still many problems remaining on this system.This dissertation focuses on the role of WO_x based catalysts in glycerol hydrogenolysis,and gives a detail study on the metallic properties,hydrogen spillover effect,nature of active site and reaction mechanism.The effect of metallic properties on the reaction routes was studied on Pt and Ru catalysts with the comparison of their catalytic performances and characterizations.ZrO₂ supported Pt and Ru metals show a less electron density,which favor the C-C cleavage reaction through dehydrogenation and decarbonylation.On the ZrW support,the highly dispersed WO_x species can enrich the electron density of the metals,making them more metallic and suppressing the C-C bond cleavage.The route to1,3-PDO is closely correlated with the activation of hydrogen,and Pt metal is more selective than Ru because of its strong hydrogen activation and spillover capacity.Two routes exist on 1,2-PDO formation,and the dehydrogenation-dehydration-hydrogenation route is thermodynamically easier than the dehydration-hydrogenation route,while the first route is accompanied with the C-C cleavage route and can be inhibited in high hydrogen pressure.The hydrogen spillover effect in glycerol hydrogenolysis was investigated over Pt/ZrW catalysts by changing the tetragonal/monoclinic ZrO₂ composition and thermal treatment conditions of the Pt precursor,and the correlation between metal surface and spillover capacity was also discussed.A linear relationship was observed between the H/Pt ratio and glycerol conversion,which confirms that hydrogen spillover is important for glycerol hydrogenolysis on these WO₃-rich nanocatalysts.Pt（111）terraces was increased by using heterogeneous ZrO₂-based support or calcining Pt precursor before reduction,which enhanced the spillover capacity and improved the hydrogenolysis activity.Additionally,two methods（changing WO_x loading and Mn doping）were used to adjust the WO_x domain size of Pt/WO_x/ZrO₂ catalysts under monolayer dispersion threshold,and their performances were investigated in glycerol hydrogenolysis.The formation of 1,3-propanediol is found to be structurally sensitive to the polymerization degree of WO_x domains,with medium sized WO_x being of benefit to1,3-PDO.WO_x structure and acid properties of these catalysts were comprehensively characterized.It is confirmed that Pt-（WO_x）_n-H is the catalytic active structure,providing super strong Br?nsted acid site and metallic Pt site adjacent to each other.The 2°-OH group of glycerol is strongly adsorbed and activated on Br?nsted acid site at reaction temperature,and the active hydrogen species participated in the reaction is the one that weakly adsorbed on Pt surface,confirmed by infrared spectroscopy.Combined with the in situ infrared study of 2-propanol hydrogenolysis,the mechanism of 1,3-PDO formation is proposed to be involved with steps of secondary carbonium formation and stabilization on super strong Br?nsted acid,and the direct attack of activated hydrogen from Pt surface.Based on the above results,the WO_x/Pt/SiO₂ model catalysts with only Pt-（WO_x）_n-H active site were prepared by loading Pt metal and then WO_x species,and the effects of WO_x domain size and reduction degree of Pt-（WO_x）_n-H on catalytic performance were studied.Pt-（WO_x）_n-H achieved the highest performance when medium polymeric WO_x was presented.However,it can be easily reduced and decrease the formation rate of 1,3-PDO.Characterization combined with DFT studies confirmed that on the formation of strong Br?nsted acid site,Pt and WO_x play the role of electron acceptor and conductor,respectively.Medium polymeric WO_x species can delocalize excess electron to a broader Pt surface space,thus making the Br?nsted acid on Pt-（WO_x）_n-H stronger.But the partial reduction of WO_x can weaken the acidity obviously,resulting in a lower catalytic performance.