Preparation Of Supported Polyoxometalates And Multi-corviponent Metal Oxides Catalysts For Selective Oxidation

Selective oxidation of organic compounds is a very important reaction process,widely used in petroleum chemical industry, fine chemicals, pharmaceutical andother fields. Introducing suitable catalysts into the reaction processes couldsignificantly improve the substrate conversion and products selectivity.Polyoxometalate compounds and multi-component metal oxides are two types ofimportant catalyst systems, which have already shown special catalytic properties fora variety of selective oxidation reaction. Currently, it is still a very interesting subjectto design and to prepare more efficient heterogeneous catalysts for the application inimportant selective oxidation processes.In early literature work, for the liquid phase selective oxidation of olefins andthe thiophene sulfides, it was well known that polyoxometalates (POM) are highlyactive and selective catalysts. However, most of these catalysts are homogeneous inessentially under the reaction conditions, which is unfavorable for the recovery andreuse of the catalysts. Therefore, more attention has been recently drawn ondeveloping efficient heterogeneous POMs catalyst with high stability. And, It wasreported that a few multi-component metal oxides were active catalysts for the gasphase ammoxidation of cyclohexanol. However, the yield to the main product ofadiponitrile is rather low. Therefore, it is highly desirable to develop more active andselective catalysts, and to reveal the nature of active centers.In this thesis, a series of supported POMs and multi-component metal oxidescatalysts were prepared, and their catalytic properties were investigated for theliquid-phase oxidation of olefins or sulfur compounds, and selective ammoxidationof cyclohexanol. Combined various characterization results, the composition, structure, and surface properties of the catalysts were studied, and were correlatedwith their catalytic performance. The nature of active centers and the catalyticmechanism of the catalysts were also discussed. The main experimental results andconclusions are summarized as follows:1. Surfactant-assisted sol–gel synthesis of zirconia supportedphosphotungstates or Ti-substituted phosphotungstates for catalytic oxidationof cyclohexeneKeggin-type POMs including phosphotungstate [PW12O40]3-and Ti-substitutedphosphotungstates [PW11TiO40]5-,[α,α-P2W20Ti4O78]10-were supported on ZrO2matrix by a surfactant-assisted sol-gel copolymerization technique. The resultantZrO2-POMs materials were characterized by a variety of techniques. The resultsshowed that the ZrO2-POMs materials have worm-like disordered mesoporous, theprimary structure of POMs was preserved after the formation of the composite, andthe POM clusters were highly dispersed in ZrO2-POMs composites.The catalytic properties of the ZrO2-POMs were investigated for theliquid-phase oxidation of cyclohexene using H2O2as oxidant. All the POMs-ZrO2materials are catalytically active for the H2O2-mediated oxidation of cyclohexenewith relatively high selectivity to allylic oxidation products. Moreover, thesematerials exhibit high stability against leaching of active species, and can be easilyrecycled without obvious loss of activity and selectivity. The excellent stability andrecyclability of the ZrO2-POMs catalysts should be mainly assigned to the existenceof stable links between the POM units and the ZrO2matrix. By comparing thecatalytic properties of ZrO2-POMs composite to the corresponding TBA salts ofPOMs, it was found that the presence of Ti species in the two Ti-containingZrO2-POMs catalyst does not play significant role in influencing the catalyticactivity and selectivity of the catalysts. This might be attribute to formation ofrelatively strong interaction between Ti species and the ZrO2support (i.e., formationof Ti-O-Zr links) during the preparation process (e.g., calcination) of ZrO2-POMscatalysts. The excellent stability and recyclability of the ZrO2-POMs catalysts should be mainly assigned to the existence of stable links between the POM units and theZrO2matrix.2. Zirconia supported transition metal substituted phosphotungstates forselective oxidation of substratesA series of ZrO2supported transition metal substituted phosphotungstates(ZrO2-PW11M, M=Mn, Fe, Co, Ni, Cu) were prepared by sol-gel method.Characterization results indicated that the PW11M could be successfully supportedon the surface of mesoporous ZrO2. The structure of the PW11M did not changesignificantly during the preparation process. The preparation conditions showedconsiderable effect on the mesoporous structure of the catalysts.The catalytic properties of the ZrO2-PW11M materials were investigated for theoxidative desulfurization reaction of thiophene sulfides and the alkenes selectiveoxidation reaction with hydrogen peroxide as oxidant. It was found that theZrO2-PW11M catalysts are highly active for the selective oxidation of olefin, and theoxidative desulfurization reaction of BT or DBT. Both the types and the substitutedposition of the transition metals could significantly influence the catalytic propertiesof the ZrO2-PW11M catalysts. Among them, the sample ZrO2-PW11Co shows thehighest catalytic activity and product selectivity. Besides, the ZrO2-PW11M catalystsare quite stable, can be easily recycled, which can be mainly assigned to theformation of relative strong interaction between the PW11M units and the ZrO2supports. Compared with the previously studied Ti-containing catalyst, the catalystsof ZrO2-PW11M show quite different product selectivities, implying the formationof different peroxy intermediates over the two catalyst systems. For instance, the Tisubstituted POMs generates-TiOOH or-TiOO-peroxy intermediate in the media ofhydrogen peroxide, while peroxy intermediates like [PO4(WO(O2)2)4)]3-might beformed when hydrogen peroxides react with PW11M species.3. SiO2supported PVSbO catalyst for cyclohexanol ammoxidationA series of SiO2supported VSbO and PVSbO catalysts (VSbO/SiO2andPVSbO/SiO2) were prepared by impregnation method, and were characterized by different means. On the surface of the supported catalyst, different active species like(VO)2P2O7, VSbO4and SbOPO4could be detected, and the particle size of thesespecies increased gradually to form relatively large crystalline particle with theloading amounts of PVSbO. A number of acidic sites, especially Lewis acid, couldbe detected on the surface of catalysts. Moreover, it was found that P species easilyenriched on the surface of the SiO2supported PVSbO catalysts.The catalytic properties of these materials were investigated for theammoxidation of cyclohexanol to adiponitrile. The results showed that theconversion of cyclohexanol increased with the increase of metal oxides loading. Thecatalyst with30%PVSbO gave the highest selectivity of adiponitrile.(VO)2P2O7andVSbO4species should be the main active sites for the ammoxidation of cyclohexanol.Compared with VSbO/SiO2catalysts, the corresponding PVSbO/SiO2catalystsshowed better catalytic performance. According to the characterization results, weproposed that the role of P species may be assigned to the following reasons: first,the enrichment of P could stabilize the formed VⅣspecies, and avoid the deepoxidation of these VⅣspecies, thus being benefit to achieve the catalytic cyclesmoothly. Second, more acidic sites could be produced after the introduction of P tothe catalyst, which is suitable for the activation of ammonia, resulting in theimprovement of catalytic activity.