New Routes For Synthesizing Heterogeneous Catalytic Materials

The development of safe, stable, green, and efficient novel catalysts has becomemore and more important, because of the lack of energy and environmentalpollution in the world. Heterogeneous catalysts, which have obvious advantage ofgood recycleability, play an important role in the field of catalysis. However,heterogeneous catalysts usually exhibit lower catalytic activity than thehomogeneous ones, which strongly limited the wide applications of heterogeneouscatalysts. On the other hand, the deep investigation on the reaction themselves isvery helpful for design and synthesis of novel heterogeneous catalysts with superiorcatalytic performances. On this background, we developed various alternativeheterogeneous catalysts, which showed excellent catalytic properties in a series ofreactions.Base on the investigation on the synthesis of heterogeneous catalysts, we havedone these work:（1） Synthesizing Au nanoparticles supported on pyrrolidonemodified SBA-15and Cu species supported on Schiff base modified porouspolydivinylbenzene;（2） Supporting Au or bimetallic Au-Pd nanoparticles onvarious supports;（3） Synthesizing novel solid acid catalysts by adjusting thehydrophobicity and hydrophilicity of the catalysts;（4） Synthesizing catalysts withspecial morphology, which could be helpful for the mass transfer of the reaction, leading to high catalytic activity.At the same time, we investigated the catalytic performances of the synthesizedcatalysts by performing various kinds of reactions, including oxidation, acidcatalyzed reaction, conversion of biomass, and coupling reactions. In addition, wealso investigate the mechanism of the reaction.Homogeneous catalysts show high activity in various reactions, because they aresolved in the reaction solvent and have good mixture with the substrate. Because ofthe solubility of homogeneous catalysts, it is very difficult to separate them from thereaction mixture, which increases the catalyst cost, produces environmentalunfriendly pollution, and produces difficulties to get pure product. In order to solvethese problems, one effective method is using heterogeneous catalysts to replacehomogeneous ones. Compared with homogeneous catalysts, heterogeneous catalystshave much fewer exposed active centers, Hence, the heterogeneous have muchlower activity than homogeneous ones. Hence, the design and synthesis ofheterogeneous catalysts with high catalytic activity is a challenge.In the second chapter, by heterogeneration of homogeneous catalyst, wesupported conventional solvable polyvinylpyrrolidone stabilized Au nanoparticlesand homogeneous Schiff base-Cu catalyst on mesoporous silica （SBA-15） andpolydivinylbenzene, respectively. As a result, heterogeneous catalysts ofAu/SBA-15-Py and PDVB-SB-Cu were obtained. In aerobic oxidation of alcoholsand olefins, Au/SBA-15-Py exhibited very high activity, which is only slight lowerthan that of homogeneous catalysts. The high activity of Au/SBA-15-Py is due tothe interaction between pyrrolidone groups, and the large surface area of SBA-15,which should be helpful for the high dispersion of Au nanoparticles. Compared withthe performances of Au/SBA-15-Py in the oxidation reactions, the activity ofPDVB-SB-Cu is more similar to homogeneous catalysts. This was because theporous organic polymer of PDVB-SB-Cu could support “a organic environment forreaction”, which was helpful for reaction. More importantly, both Au/SBA-15-Pyand PDVB-SB-Cu exhibited good recyclability, while the homogeneous catalystscould not be reused. This investigation should open a new route for preparing alternative heterogeneous catalysts from homogeneous catalysts.Supporting the catalytic active sites on solid carriers in precipitation orion-exchange methods is an important method to synthesize heterogeneous catalysts,which is a good method for wide productions. The catalytic activities of thesecatalysts are depended on the support, morphology and leaching of active sites.Therefore, it is desirable to perform the investigation on synthesizing highly activeheterogeneous catalyst using suitable solid support and method.In the third chapter, we synthesized Au nanoparticles on Mg-Al layered doublehydroxide （Au/LDH） by ion-exchange and reduction method. Au/LDH catalystshows excellent catalytic performances in aerobic oxidation of various alcohols. Thehigh activity of Au/LDH was due to the dehydration ability of LDH for alcohols. Inaddition, we also synthesized Au nanoparticles on Mg-Al mixed oxides, obtainingAu/MAO, which could effectively catalyze the homocoupling of phenylboronic acid.Base on both experimental and theoretical calculation work, a proposed mechanismon Au catalyzed homocoupling of phenylboronic acid was suggested. In order toeffectively control the morphology of the supported metal nanoparticles, we addedPd species into Au/MAO, obtaining Au-Pd/MAO catalyst. This kind of bimetallicAu-Pd nanoparticles could produce rich facet edge and corner sites, exhibiting veryhigh activity and amazing product selectivity. Furthermore, in consideration most ofthe research work on Au catalysts was focused on small-size Au nanoparticles, weperformed the aerobic oxidation of styrene on positively charged bulky Au particles.Interestingly, the bulky Au particles were even more active than the small-size ones.Biomass, which is regarded as a renewable energy, has been widely investigatedin recent years. However, it is still a great challenge to selective transform biomassto valuable fine chemicals. This is because the composition of biomass complex,and the conversion of biomass is accompanied by various side reactions, leading touseless by-product. The dehydration of fructose to5-hydroxyfurfural （HMF） is atypical reaction for the conversion of biomass into valuable chemicals. Inconsideration of the catalyst recyclability, various heterogeneous solid acids have been prepared to catalyze this reaction. However, there is one big problem for thesecatalysts: the HMF product could be further decomposed into levulinic acid andformic acid. As we have introduced above, the surface area, stability, and surfacegroups are very important for heterogeneous catalysts. This is because the largesurface area of support could highly disperse the active centers; the good stabilitycould keep the catalyst structure during the reaction, the rich surface groups couldbe helpful to support and interact with the active metal nanoparticles. However, thehydrophobicity and hydrophilicity and of the solid support are usually neglected.Actually, the hydrophobicity and hydrophilicity are important for the reactions,especially for the hydrations and dehydrations.In the forth chapter, based on the theoretical calculation results on hydration ofHMF, it is suggested that the dehydration could be completely prevented byseparating water molecule from the acid sites. According to this suggestion, wesynthesized superhydrophobic solid acid catalyst （P-SO₃H） by copolymerization. Inthe dehydration of fructose, P-SO₃H gave HMF yield at nearly to100%, because itssuperhydrophobicity could prevent the hydration of HMF, which was agree with thetheoretical calculation result. Furthermore, based on these result, we synthesizedsuperhydrophilic sulfonated carbon materials, which gave higher activity thanconventional heterogeneous solid acid catalyst. This because that thesuperhydrophilic carbon support could lead to the enrichment of water around theactive sites, increasing the reaction rate.Adjusting the morphology of heterogeneous catalyst is also regarded as anefficient method to improve the catalytic activity. The limitation of mass transfer onheterogeneous could lead to lower activity than homogeneous catalysts. It issuggested that the design of a heterogeneous catalyst with little limitation on masstransfer would be favorable for improving the catalytic activity.Another method to obtain efficient catalysts is to adjust the catalyst morphology.The limitation of mass transfer is an important reason for the lower catalytic activityon heterogeneous catalysts than homogeneous ones. If we can decrease thelimitation on mass transfer by synthesizing heterogeneous catalyst with special morphology, the catalytic activity of heterogeneous should be improved.In the fifth chapter, we successfully synthesized hollow sphere carbon materials（HSC）. After sulfonation, the-SO₃H groups were introduces on the surface of HSC,obtaining HSC-SO₃H. HSC-SO₃H exhibited excellent catalytic performances in theacetalisation of glycerol with acetone, which is because of the rich microporicity onthe thin hollow sphere carbon shell. Compared with conventional porousheterogeneous catalyst, the special morphology of HSC-SO₃H catalyst is veryfavorable for the transfer of substrates and products.