Design,Preparation And Property Of Supported Pd-based Hydrogenation Nanocatalysts

Catalytic hydrogenation reactions have proven to be highly important and fundamental transformations in organic synthesis.They play a crucial role in a large number of fine chemicals,such aspharmaceuticals,pigments,functional polymers and perfumes.The catalytic hydrogenation approach offers an atom-economic and environmentally benign organic reaction with low-cost and high yields.Catalytic hydrogenation display superior performance in the synthesis of products,which are unable to obtain by the traditional route.Actually,many important hydrogenation reactions still rely on the use of homogeneous catalysts.But the heterogeneous catalysts are environmentally more friendly and easier to separate and re-use than their homogeneous counterparts.The rational design of highly active and selective heterogeneous metal supported catalystsremains a great challenging task.This thesis is mainly aimed at the synthesis and property of Pd-based nanocatalysts and its composites as well as their application in the catalytic hydrogenation.The content of this paper is divided into the following four chapters:The first chapter is the introduction.First,the development and application of catalytic hydrogenation are decribed.Then,the selection and desigen principle of supported nanocatalysts for catalytic hydrogenation are briefly summarized.Subsequently,kinetic and mechanistic studies on ?,?-unsaturated aldehydes hydrogenation are introduced.A brief description of the research basis and contents of the dissertation is given to end the chapter.In chapter 2,a simple one-pot wet-chemical approach is developed for the synthesis of well-defined monodisperse Pd nanotetrahedrons on ultrathin MoO3-x nanosheets(Pd/MoO3-x).Novel 2D oxygen-deficient MoO3-x nanosheets are formed in the synthetic process,which can work as robust catalyst support.Furthermore,in situ growth route is beneficial for boosting chemical attachment and electroniccommunication between Pd nanotetrahedrons and MoO3-x nanosheets.Such a composite system that combines the advantages of electronic effect and strong metal-support interaction contributes to excellent performance for selective hydrogenation of ?,?-unsaturated aldehydes with high conversion(97%)and selectivity(96%)to its saturated aldehydes.Catalytic activity of Pd/MoO3-x nanocatalysts is improved through electronic structure regulation of Pd nanocrystals via electron-donating effect of oxygen vacancies in MoO3-x nanosheets.The high catalytic activity of our obtained hybrid catalysts associated with a synergy of electronic effect and strong metal-support interaction inspires the future exploration for other 2D oxides as functional supports to fabricate hybrid nanocatalysts for selective hydrogenation of a,P-unsaturated aldehydes.In chapter 3,we have developed a simple and novel strategy to prepare permanent self-organized colloidal metal-organic superstructure through redox reaction between palladium ions and ethynylferrocene(eFc)capping ligand.The obtained monodisperse and well-separated ultrasmall Pd nanoclusters(about 1.6 nm)are embedded within redox-active organic matrixes(Pd@eFc).Furthermore,polymerized eFc networks serve as robust support providing physical barriers against aggregation of tiny Pd nanoclusters while keeping Pd nanocluster core highly accessible and as a selectivity regulator for hydrogenation reactions through tuning of the electronic structure of Pd nanoclusters.This colloidal metal-organic superstructured Pd@eFc clusters exhibit nearly complete conversion(99%)and excellent selectivity(98%)for aromatic substrate(cinnamaldehyde),and 97%for aliphatic substrate(3-methyl-2-butenal)for selective hydrogenation of a,?-unsaturated aldehydes to saturated aldehydes.The outstanding selectivity of Pd@eFc nanocatalyst is ascribed to strong metal-support interaction effect and strong adsorption ability of redox support for C=C bond facilitated by ?-? interaction.Our findings encourage on-going study to explore controlled hierarchically self-assembled superstructures for a wide range of organic transformations.In chapter 4,we report an interesting result of reversible hydrogenation and oxidative dehydrogenation of a redox dye over Pd-ZnO1-xhybrid nanocatalyst under ambient conditions.Thionine(TH+)is used as a model compound to evaluate the catalytic performance.The reversible color switching of thionine between purple(TH+)and colorless leuco-thionene(LTH)depends upon the reducing or oxidizing environments.Our new developed Pd-ZnO1-x nanocatalyst exhibits a high catalytic activity for the hydrogenation of TH+with turnover frequency(TOF)as high as 397 h-1 under H2(1 bar).The oxidative dehydrogenation of LTH is performed under 1 bar O2 flow in the same reaction system.Pd-ZnO1-x nanocatalyst readily adsorbs and subsequently dissociates O2 to oxidize LTH to original purple color(TH+)with higher efficiency.The abundant oxygen vacancies on ZnO1-x nanorods and strong metal-support interaction(SMSI)promote the adsorption and subsequent dissociation of molecular hydrogen and oxygen leading to high catalytic activity.This novel reversible color switching of organic dyes can perform successively more than 10 cycles in a one pot-fashion by using Pd-ZnO1-x nanocatalyst with a small loss in performance.The highly efficient reversible color switching of TH+/LTH over Pd-ZnO1-x nanocatalyst provides state-of-the-art protocol to find practical applications as printing inks for rewritable paper,sensing and security feature devices.