Preparation And Properties Of Quaternized Polyhedral Oligomeric Silsesquioxanes Stabilized Palladium Nanohybrid Catalysts

Noble metal nanoparticles(NMNPs)are widely used in the fields of catalytic coupling reactions,degradation of organic dyes,optical sensing and medical diagnosis,due to their unique structural morphology,high specific surface area,excellent chemical activity,good stability,and high catalytic performance.However,one of the main problems in these aspects is that the easy aggregation of metal nanoparticles leads to the inhibition of their catalytic activity,which greatly limits the development prospects of metal nanoparticles.A large number of studies have shown that the metal nanoparticles as heterogeneous catalysts fixed on various support materials can improve their dispersion and catalytic activity,which is considered to be an effective way to solve the above problems.Among the many catalyst carriers,it is known that the recently developed new graphene oxide(GO)is one of the most studied carbon nanomaterials.GO has become the most promising candidate for catalyst carriers due to its unique structure and surface properties.Two-dimensional(2D)GO nanosheets have high specific surface area,excellent electron transport ability,and good chemical stability,which can produce synergistic effect between metal nanoparticles and GO,and further enhance the stability and versatility of nanocomposite catalysts.However,it still remains a great challenge to fine adjust the size of metal nanoparticles and improve their dispersion and catalytic activity.Polyhedral oligomeric silsesquioxane(POSS)is an inorganic-organic hybrid material,nano-sized POSS has many functionalized reaction sites,excellent thermal stability,and chemical stability,and is highly soluble in many organic/ inorganic solvents.These unique properties are favored by many researchers,and POSS has also been proved to be an ideal modifier.Therefore,introducing hydrophilic quaternized POSS(QPOSS)to modify metal nanocatalyst is an effective solution to the above problems.In this thesis,we choose QPOSS and combine with GO nanosheets to stabilize metal nanoparticles to construct nanohybrid catalysts.The quaternary ammonium groups on the synthesized highly hydrophilic QPOSS can well stabilize and disperse the Pd metal nanoparticles generated in situ,thereby achieving high-efficiency degradation of organic dyes and effectively catalyzing the Suzuki coupling reaction.The main research contents include the following two parts:In the first part,hydrophilic quaternized QPOSS was prepared and used to stabilize palladium nanoparticles to obtain Pd NPs@QPOSS nanohybrid catalyst for catalytic reduction of organic dyes and nitrophenols.The size of Pd NPs can be effectively regulated through the molar ratio between QPOSS and Pd precursors.Water uniformly dispersed nano-sized QPOSS stabilized ultra-fine Pd NPs were obtained as the molar ratio of precursors was 1:8.It was found that low-dose Pd NPs@QPOSS nanohybrids as catalysts exhibited outstanding catalytic reduction activity for methylene blue(MB)and nitrophenol(4-NP)with high conversion frequencies(TOF)of 213.7 and 17.8 min-1 respectively,which is superior to commercial Pd/C catalyst.The nanocatalysts we prepared could be stably dispersed in water for one month and there was no significant loss of catalytic activity in four catalytic cycle.The outstanding catalytic activity and good stability can be attributed to QPOSS as a stabilizer having strong coordination interaction with as-generated Pd NPs.In addition,we evaluated the decolorizing properties of the nanocatalysts for other common organic pollutants including methyl orange(MO),congo red(CR),methyl orange(MO),rhodamine 6G(R6G),rhodamine B(Rh B),rhodamine 6G(R6G),and nitrophenol derivatives.The organic-inorganic hybrid nanocatalyst Pd NPs@QPOSS is very effective in catalytic reduction of a series of colored dyes and nitrophenols,which also proves that the nanohybrid as catalyst has potential application value in industrial degradation of organic pollutants in wastewater.In the second part,Pd NPs supported on highly hydrophilic quaternized QPOSS functionalized reduced graphene oxide(r GO)was successfully constructed as a nanohybrid catalyst for efficient heterogeneous catalytic reactions.Octaammonium POSS(OAPOSS)was first covalently grafted to carboxylated GO nanosheets through an amidation reaction,and then the OAPOSS decorated on r GO nanosheets was quaternized with glycidyltrimethylammonium chloride(GDTMAC)via an epoxide ring-opening reaction to obtain a highly functional hydrophilic QPOSS@r GO hybrid.Finally,the Pd NPs were loaded onto QPOSS@r GO nanosheets through an in situ reduction route.The quaternary ammonium groups on QPOSS@r GO can strongly coordinate with Pd NPs,making it particularly advantageous for stabilizing Pd NPs and preventing the leaching of Pd metals.Compared to the Pd NPs@r GO catalyst without modification with hydrophilic QPOSS,the low-dose Pd NPs@QPOSS@r GO nanocatalysts exhibited outstanding catalytic efficiency in the reduction of methylene blue and nitrophenols.In addition,the as-designed nanocatalysts also showed high catalytic activity for the Suzuki coupling reaction when utilizing pure water as a green solvent.It was proved that QPOSS and r GO can synergistically catalyze and promote the diffusion of reactants.The Pd NPs@QPOSS@r GO nanohybrid catalyst showed no significant loss of catalytic activity after reusing it five times.Our results also demonstrated that highly hydrophilic QPOSS endows the resulting Pd NPs@QPOSS@r GO nanohybrid with good dispersibility and stability in water,which remarkably further improves the catalytic activity of the nanocatalyst.