Prepartion, Characterization And Catalytic Performance Of Phosphotungstic Acid Supported On Bimodal Mesoporous SiO₂

The development of eco-friendly and high efficient catalyst is one of the mostimportant research fields in green chemistry. Heteropoly acid （HPA） has attractedmuch attention in the applications of catalysis because of its high acidity, favourableredox properties and controllable of its catalytic properties. However, HPA exhibitslow surface area, pore volume and good solubility in aqueous and some polar solvents,which limit the applications in heterogeneous catalytic fields. Thus, the uniformdispersion of HPA on porous solid with high surface area and pore volume, as well aslarge pore size is ideally pursued to improve the performance in many potentialheterogeneous catalytic applications.Mesoporous molecular sieves were widely used as HPW supports for theirincredible features, such as high specific surface area, well-ordered pore structure,uniform pore size distribution and controllable pore sizes. But the existing researchswere concentrate on the supports of mesoporous molecular sieves with single pore,there are few reports about the research that HPW-supported catalyst with acharactersits of hierarchical structures. Bimodal mesoporous materials （BMMs） havethe controlled small pores （about3nm） and large pores （around16nm）. There areabundant-OH groups on the surface of BMMs channels. Compared with the singlechannel of mesoporous sieves, such as MCM-41and SBA-15, bimodal structures ofBMMs are beneficial for large molecular to diffuse inside and helps to enhance thedispersion of HPA.In this thesis, a series of HPW/BMMs catalysts were prepared by supportingHPW on BMMs via impregnation method, and their physicochemical properties werecharacterized by means of the several techniques, such as X-Ray diffraction （XRD）,Fourier transform infrared spectroscopy （FT-IR）, Ultraviolet-visible diffusereflectance spectroscopy（UV-Vis）, Thermogravimetric analysis （TG）,31P MAS NMR,N2adsorption-desorption, Ammonia-Temperature programmed desorption（NH3-TPD）, Scanning electron microscopy （SEM）, Transmission electronmicroscopy （TEM）, and X-Ray fluorescence （XRF）. The esterification performancesof the resultant catalysts were evaulated. The results are as follows:1. HPW/BMMs were prepared by supporting phosphotungstic acid on bimodalmesoporous SiO₂via impregnation method. Their structures and physical-chemical properties were characterized by various methods. The effects of solvents such aswater, ethanol, acetone and acetonitrile, the amount of HPW loading on their structure,dispersion and acidity of supported phosphotungstic acid were systematicallyinvestigated. The results showed that the bimodal mesoporous structures of BMMswere maintained after loading HPW （the mean small pore size is3nm and the meanlarge pore size is16nm）; however, the order degree, surface area and pore volume ofcatalyst decreased with HPW loading. At lower loadings, a new HPW species wasobserved. The relative amount of the species depended on the HPW loading. At higherloadings, the species decreased.In addition, the results showed that the catalyst prepared in the aqueous solutionwas the best when using ethanol, acetone, acetonitrile, and aqueous solution assolvent respectively, indicating the unremarkable influences on the order degree ofcrystalline, and showing the best dispersing properties, while acetonitrile, wasdemonstrated as the worst. Furthermore, the acid density of40HPW/BMMs-WAcalcined at200℃was highest (0.001381mmol·m^-2); the strong acidity and aciddensity of40HPW/BMMs-AC was increased, while its weak acidity decreased. Theinfluences of ethanol and acetone were in-between.2. The effects of the preparation parameters, such as the impregnating solvents,calcination temperature, HPW-loaded capacity and mesoporous structure of BMMs onthe catalytic performance were investigated in details. The results showed that theHPW/BMMs catalysts prepared in aqueous solution and calcined at200℃exhibithigher activity than that others obtained by using acetonitrile, ethanol, and acetone assolvent respectively. Its catalytic activity was up to72.8%at the best conditions:methanol/oil molar ratio of20:1, catalyst amount of5%（wt）, reaction temperature of373K and time of12h.3. Under the optimum conditions aboved, reuse and deactivation of catalystswere also studied. The results showed that the conversion of esterification wasdecreased from72.8%to60.1%after reused4times. Then, the recovery catalystswere systematically characterized by various techniques. The results showed that theKeggin structure of HPW did not change obviously by Uv-Vis, but the order degreeof crystalline was changed markedly by XRD. It is because of the absorption oforganic non-polar molecule. But the main reasons of the deactivation might be thecritical coke deposit on catalyst surface and the loss of HPW （HPW amount wasdecreased from36%to20%after reused4times）. 4. By comparison, MCM-41, SBA-15and SiO₂as support respectively wereinvestigated. The results showed that BMMs has the best dispersing performance andcan promote the adsorbtion and diffusion of reactants inside the channels because ofits bimodal mesoprous structure. Its catalytic activity was around58.4%, higher thanthat of40HPW/MCM-41（42.5%）,40HPW/SBA-15（54.8%）,40HPW/SiO₂（28.1%）,under the following conditions: catalyst amount3%, methanol/oil molar ratio of20:1,reaction temperature of373K and time of12h.