Preparation And Catalytic Performance Of Hemicelluloses Supported Noble Metal Compounds

C-C coupling reactions are important methodologies for the preparation of organic molecules, perhaps the two most important are the Suzuki and Heck reactions. For the properties of functional-group compatibility, high stereoselectivity and mild reactive conditions, Suzuki and Heck reactions have been widely applied to the synthesis of natural compounds, pharmaceuticals and fine chemicals. Homogeneous palladium-based catalysts have been largely used for Suzuki and Heck coupling reactions, due to their highly catalytic activity. However, homogeneous catalysts have many shortcomings, such as hard recycled, easily agglomerated, easily deactivated, high cost and so on. These problems result in the challenges of catalytic activity, stability and cost for catalysts.Recently, heterogeneous catalysts have attracted much attention, for they can not only overcome the difficulties of homogeneous ones, but also have great potential for industrial application. Hemicelluloses are heteropolysaccharides with various branches, hydroxyl groups and uronic acid, and therefore hemicelluloses can bind and stabilize metal ions. Besides, heteroatoms have high affinity toward various metal nanoparticles, so the chelating ligands containing O, N and S atom result in more stable nanocomposites. Under these backgrounds, the main researches are described as follows:1. Preparation of carboxymethyl hemicelluloses with O ligands supported palladium catalyst and study of its catalytic performance in Heck coupling reactionsCarboxymethyl hemicelluloses which have both carboxyl groups and hydroxyl groups are synthesized by hemicelluloses with monochloroacetic acid. And they have greater ability for palladium stabilization compared to hemicelluloses. The catalyst showed high catalytic activity and versatility towards Heck coupling reactions. The optimal conditions for Heck coupling reactions were as follows, when the catalyst loading was 1.9 mol% Pd, DMF was used as solvent, reaction temperature was 120 oC and the base was triethylamine, the product yield can be up to 99%. The catalyst can be reused through simple filtration at least for 5 times without obvious lose of catalytic activity. The main reason for the decrease of catalytic activity may be due to the agglomeration of palladium particles, which was indicated by the results of TEM and heterogeneity test.2. Preparation of N ligands functionalized hemicelluloses stabilized palladium nanocomposites and study of their catalytic performance in Suzuki coupling reactions1,10-phenanthrolin-5-amine modified carboxymethyl hemicelluloses and ethylenediamine functionalized hemicelluloses are two kinds of hemicelluloses derivatives with N ligands, which have strong ability for metal nanoparticles stabilization. The palladium/ hemicelluloses nanocomposites with N ligands were prepared by in situ reduction and deposition method. The catalysts were applied to the catalysis of Suzuki coupling reactions. The results indicated that the hemicelluloses-based catalyst with flexible ethylenediamine ligand have better catalytic activity compared to rigid ligand. When the amount of catalyst was reduced to 0.26 mol%, the product yield of 100% was maintained. The recyclability test showed the rigid 1,10-phenanthrolin-5-amine ligand modified hemicelluloses supported palladium was more stable than the flexible ligands supported catalyst. After the catalyst being reused for five times, the product yield was just decreased to 88%. Both catalysts had good catalytic activity for aryl iodide at room temperature and had great potential for industrial application.3. Fabrication of S ligands modified xylose hydrochar microspheres supported palladium nanocomposites for Suzuki cross-coupling reactionsMercaptopropyltrimethoxysilane modified xylose hydrochar microspheres were used as supports for palladium particles. And the raspberry catalyst was prepared by the reduction of dioctyl ether. The morphology, composition and catalytic performance of the raspberry catalyst were studied systematically. The results showed the optimal reaction conditions for Suzuki coupling reaction were listed below: when the catalyst loading was 1.4 mol% Pd, ethanol was used as solvent, the base was K2CO3, and reacted under 100 oC, the product yield can be up to 100%. The catalyst could be readily recovered and reused in at least five successive cycles without obvious loss in activity. Heterogeneity test indicated the Pd leaching during catalytic process was trace.4. Preparation of porous thermo sensitive hydrogel immobilized Pd/Cu bimetallic nanoparticles for Suzuki and Heck reactions in aqueous mediaThe paper prepared cationic porous thermo sensitive hydrogel is demonstrated to have two merits. Firstly, the three dimensional network of hydrogel is benefit for the encapsulation of reacted substrates, and the organic synthesis can be performed without surfactant or cosolvent being added. Secondly, the quaternary ammonium groups and hydroxyl groups in hydrogel contribute to the immobilization of [PdCl4]2- and [Cu(NH3)4]2+. The hydrogel stabilized Pd/Cu bimetallic catalyst was prepared by impregnation, and the hydrogel immobilized Pd/Cu bimetallic catalyst was applied to Suzuki and Heck reactions in aqueous media. The reactions can be accelerated due to the enriched concentration of the Pd/Cu catalyst and reactants within the hydrogel by the reversible deswelling/swelling of the intelligent hydrogel. Aryl iodides were more active and when the aryl halide with electron-withdrawing groups, H-Pd/Cu had higher catalytic activity. The catalyst recyclability study and heterogeneity test showed that the catalytic systems in the reaction were stable.