| Bifunctional catalyst is a type of material with two active centers.Acid-base bifunctional catalyst is one of them,referring to the material with both acidic and basic sites.The acid-base property can promote the processes of dehydration and dehydrogenation,respectively,making acid-base bifunctional materials played an important role in industry field.Among various catalytic materials,metal oxides usually have acid,base or bifunctional properties,and can be used as the catalysts or supports,and also have a large modification space as well,which means they can improve their catalytic performance simply by adjusting the morphology,molecular sizes and the exposed active sites.Furthermore,metal-supported heterogeneous catalysts often have electronic and synergistic effects,which enrich the acid/base active sites as well as promote the overall catalytic activity.Hence,a study on metal-based bifunctional materials preparation is necessary.Combined with the effective modulation of acidbase property,we can enhance its catalytic activity and reaction selectivity,which do great help to expand the application fields of bifunctional catalysts.Based on metal oxides,we finally obtain the metal-based bifunctional materials with a high catalytic activity and stability by surface modification,reconstruction as well as the material structure designing.By that,an effective synthesis strategy by using metal oxides to replace organic functional groups as the origin of acid and base sites in dual-functional materials is proposed.With a further study into the acid-base modulation and the active sites distribution control,we can finally reveal the internal relationship between acid-base properties and the catalytic performances,the main works are as follows:First,a series of alumina supported W-Zn bimetallic catalysts were prepared by a simple impregnation method.It was found that the content of Zn affected the active sites distribution,and the calcination temperature would influence the acid-base property as well.Meanwhile,the relationship between catalyst morphology and its activity performance was also verified in the selective oxidation of alcohols in Chapter 2.Catalyzed by 15W-2.3Zn-Al2O3,the conversion of benzyl alcohol reached 97.5%,and the selectivity of benzaldehyde was 95.2%.The experiment results indicated that the proper zinc doping content could adjust the active sites distribution density,which enhance the electronic interaction and give the obtained catalyst excellent catalytic performance.There is no significant decrease in catalytic activity during the 6 runs recycling.Proved by theoretical calculation results,present reaction system features a broad substrate scope,and reveals a sustainable and green method to process oxidative dehydrogenation.Based on the results of the oxidative dehydrogenation,we further studied the catalytic performance of mixed metal oxides in oxidation lactonization of diethylene glycol(DEG).Using layered double metal hydroxide as a precursor,a series of mixed metal oxides(MMO)were obtained by calcination,and we also investigated the morphological changes of them.The experimental results show that prepared ZnOZnCr2O4(Zn-Cr-O)is an efficient catalyst in probe reaction compared with other mixed metal oxides.After screening the reaction conditions,Zn-Cr-O calcined at 450℃exhibited excellent 1,4-dioxane-2-one(PDO)selectivity,within a 4 hours reaction time,the conversion of DEG reached 81.95%and the selectivity of PDO was 96.22%.The acid-base properties of prepared catalysts are measured by CO2-TPD and NH3-TPD,showing that the calcination temperature will influence the acid/base properties,especially the increasing amount of Lewis acid derived from M-O-M bond,which benefit the H2O2 activation.XPS results also indicated that the electron migration between Zn and Cr elements made their connection closer.These findings indicated that the active components and the acid-base properties had significant impacts on the catalytic activity.Meanwhile,the stability of metal oxides and the chemical bond in Zn and Cr species ensured the stability of prepared catalysts.During 10 runs cycles,the catalyst has excellent catalytic performances,and no corresponding active ions leaching observed.To further investigate the contribution of bifunctional catalysts design strategy and their acid-base property in reaction,a titanium silica material with a high specific surface area was prepared firstly using a sol-gel method,and then the metal species anchoring was carried out.The catalytic activity was verified in the deacetalizationKnoevenagel reaction(Chapter 4).By comparing the morphology,specific surface area,synergistic effects and acid-base distribution differences of the Mg(Ca)-Zr/Ti-HMS catalysts,we found that the two-step preparation method was an efficient way to avoid the agglomeration of dual active sites,and the harmonious coexistence as well as the uniform dispersion of acid-base sites made the materials more active.Catalyzed by Mg(Ca)-Zr/Ti-HMS-T,a high benzaldehyde dimethylacetal conversion(>99.6%)and benzylidene malononitrile selectivity(>99.8%)was obtained.Meanwhile,Mg-Zr/TiHMS catalyst has robust stability,its catalytic performance almost unchanged during 10 consecutive cycles.Experiment results verify the importance of coexistence of acidbase sites,for acidic sites promote the adsorption of benzaldehyde dimethylacetal and polarize the C=O bond in benzaldehyde,meanwhile,the basic sites activate α-H of malononitrile to produce carbanion and further formed the target product.Combined with the research progress in this work,we also proposed a new synthesis strategy of synthesizing a controllable acid-base bifunctional mesoporous materials.On the basis of acid-base bifunctional catalysts research,we continue to prepare high-efficient metal-based alkylation catalysts with highly dispersed active sites,and test them in the selective alkylation reaction of naphthalene and anthracene,respectively.Combined with the chemical calculations,the possibility of shapeselective catalysis was further studied.For naphthalene alkylation(Chapter 5),an efficient zirconium-based alkylation catalyst was prepared with the assistance of citric acid modification,and the tert-butanol was used as an alkylation reagent to prepare 2,6di-tert-butylnaphathalene(DTBN).With the alkali treatment,the mesoporous structure introducing and the enhanced synergistic effect,the conversion of naphthalene was increased significantly.This study indicates that the optimized acidity,promotional synergistic effect between Zr and mordenite as well as the optimization of mordenite structure,do great contributions to a high ratio of 2,6-DTBN/2,7-DTBN.Only requires 5.0 equivalents of solvent,3.0 equivalents of tert-butanol and 20 wt.%of catalyst dosage,the ratio of 2,6-DTBN/2,7-DTBN can reach 49,overcoming the selectivity barriers in naphthalene alkylation reaction.This chapter solves the problems such as the low ratio value of 2,6-DTBN/2,7-DTBN,large amounts of catalyst and solvent usage,and short life of catalyst,etc.,having huge industrial application potentials.For the selective anthracene alkylation,the tert-amyl anthracene was smoothly prepared over boron-doped Fe/Zr bimetallic solid acid catalysts.In this chapter,the surface acidity was controlled by boric acid and metal modification.The work shows that when using mesitylene as the solvent,high selectivity(91.4%)and conversion(47.8%)can be achieved over 1.0B-Fe-Zr/MOR catalyst,thereby providing a key guidance for tert-amyl anthracene production directly in anthracene alkylation. |
