Studies On Preparation, Characterization And Catalytic Desulfurization Performance Of Silica Pillared Montmorillonite Composite Incorporated With Phosphotungstic Acid

Heteropoly acid (HPA) is formed by two or more different oxy-acidthrough condensation. Due to its excellent acidity and redox property, it hasbeen widely applied in all sorts of catalytic oxidation and acid catalyzedreaction system. However, HPA suffers from some shortcomings, such as thesmall surface area, low thermal stability and the undesirable reusability.Thesedrawbacks greatly limit their applications in catalytic processes. Thus, it hasgreat significant to load HPA onto appropriate carrier to improve its specificsurface area, catalytic activity and reusability. As a kind of layered silicateclay mineral with interlayer cation exchange property, montmorillonite (MMT)can be introduced large quantities of organic and inorganic ions in theinterlayer, forming porous composite functional materials by ion exchange,intercalation, and pillaring method etc. Former researchers did a lot of workon loading heteropoly acid on MMT or modified MMT, but there are fewreports on heteropoly acid intercalated MMT catalytic composite in the literature. Therefore, assembling the HPA into the silica pillared MMTmaterial can make the HPA firmly stick to the interlayer framework of MMT.It will reduce the loss of HPA in the catalytic reaction and improve thecatalytic performance of the catalyst, which has important academic value andpractical applicationThis paper applied ion exchange, sol-gel, in situ synthesis, and molecularself-assembly methods etc, effectively introduced phosphotungstic acid (HPW)into the interlayer of MMT. Using X-ray powder diffraction (XRD), X-rayfluorescence analysis (XRF), fourier transform infrared spectroscopy (FT-IR),thermogravimetry-differential thermal analysis (TG-DTA), nitrogenisothermal adsorption-desorption (BET), scanning electron microscopy (SEM),transmission electron microscope (TEM), and many other kinds of testmethods characterized the structure, stability of the HPW intercalated MMTcomposite. This paper studied the effect of the different surfactant, differentcontent of HPW and different heteroatoms vacancy phosphotungstic acid tothe interlayer structure, discussed the formation mechanism of the compositematerials, examined the catalytic desulfurization performance of the catalyticmaterials to the model oil and the catalytic cracking FCC diesel oil. The mainresearch work and achievements are as follows.1. Using hexadecyl three methyl ammonium bromide (CTAB) as theinterlayer templates of the silica pillared MMT, different content ofphosphotungstic heteropoly acid intercalated MMT composites (HPW-SPC) were prepared. HPW/MMT samples with the same phosphotungstic acidcontent were also prepared using the imprepnation method. Results showedthat HPW was intercalated into the interlayer framework of the silica pillaredMMT composites, and its original Keggin structure retained. Oxidationdesulphurization experiment results showed that the HPW-SPC samplesexhibited far higher catalytic oxidative activity than the impregnation samples.When the reaction temperature is60oC, H2O2/S=3:1,0.15g catalyst canreduce the sulfur content in25mL of model oil from500ppm to7.1ppm in120min, with sulfur removal of98.6%.2. A series of phosphotungstic acid intercalated MMT desulfurizationcomposites were prepared using surfactants with different molecular lengthand different functional groups as silica pillared MMT interlayer template.HPW was successfully introduced into the interlayer framework of silicapillared MMT composites by ion exchange and sol-gel method. The interlayerspacing and specific surface area of the composite catalytic materials enlargedwith the increase of the carbon length of the templates. The organic functionalgroup has a great influence on the specific surface area, pore distribution andpore volume of the phosphotungstic acid intercalated MMT composite. Whena template with benzyl was introduced into the interlayer of the MMT clay, theformed MMT hybrid material has a large pore volume of1.29cm3.g-1andlager pore size (9.0nm). However, the large pore size affected the stability ofHPW-SPC composite. Catalytic experimental results showed that, although the resulting material prepared using CTAB as the silica pillared MMTtemplate did not exhibit excellent catalytic desulphurization performance asthe materials formed by the template with benzene ring, it possesses a highstability and good reusability.3. The mono-lacunary phosphotungstic acid and transiton metal modifiedlacunary phosphotungstic acid were synthesized under acidic condition. Theselacunary phosphotungstic heteropoly acids were intercalated into the interlayerframework of the silica pillared MMT by sol-gel method and formedmesoporous lacunary phosphotungstic acid intercalated MMT composites(MPW11-SPC). The introduction of the lacunary phosphotungstic acid did notdestroy the lamellar structure of MMT and these composite materials possessuniform pore size. However, the transiton metal modified lacunaryphosphotungstic acid has a strong influence to the morphology of silicapillared MMT composite. The results of the catalytic oxidation desulfurizationshowed that, the mono-lacunary intercalated MMT composites showed bettercatalytic performance than the phosphotungstic acid intercalated MMTcomposites, which can nearly completely catalytic oxidize DBT into thecorresponding sulfone. But, the catalytic performance of the transiton metalmodified lacunary phosphotungstic acid intercalated MMT compositesreduced. This is because the Br nsted acid structure of the heteropoly acidstructure was damaged with the introduction of the impurity atoms, thus thecatalytic performance is not so ideal. 4. In strong acidic medium, different content of phosphotungstic acidintercalated mesoporous MMT composites were prepared using in situsynthesis method. The composites were characterized by various analysismethods. The results showed that tungsten source and phosphorus source weresuccessfully intercalated into the interlayer framework of the silica pillaredMMT, and in-situ synthesized phosphotungstic acid. When high content(25wt.%) of HPW was introduced into the framework of the MMT compositeby in situ synthesis method, the MMT lamellar structure was also wellpreserved. The catalyst showed a good catalytic performance in oxidizationdesulfurization of DBT in the model oil, and its catalytic performanceimproved with the increase of the content of tungsten source and phosphorussource. But the formation of excessive amounts of HPW has an adverse effectto the structure and catalytic activity of the material. When the reactiontemperature is60oC, H2O2/S=3,0.15g catalyst can reduce the sulfur contentin25ml model oil from500ppm to18.5ppm in120min, with thedesulphurization removal of96.3%.5. Discussed the synthesis mechanism of the HPW-SPC materials. TheHPW-SPC materials were synthesized through the interlayer ion exchange andmolecular self-assembly technology with HPW anion introduced into theframework of the pillared MMT clay. Firstly, the cationic template wasintroduced into the interlayer of the MMT through the ion exchangetechnology, forming the organic MMT. Then, through molecular self-assembly technology the HPW anion was attracted into the interlayer of the MMT.Finally, during the interaction process of the introduced silicon source and themolecules in the interlayers of MMT, phosphotungstic acid will bond with thesilicon hydroxyl forming a part of the framework of the silica pillared MMTcomposite.6. Explored the catalytic oxidation desulfurization mechanism. During theoxidation desulfurizaion process, HPW in the interlayer framework of thepillared MMT composite can accept the activce oxygen free radicals from thehydrogen peroxide and form a middle state oxoperoxo species, while the silicapillared MMT materials will transfer the oxoperoxo species to thesulfur-containing substrates, make the oxoperoxo species contact with thesulfur-containing compound, leading the oxidation reactions proceed smoothly.The GC-MS results of the model oil before and after catalytic oxidationreaction showed that the sulfur-containing compounds were oxidized intosulfone with relatively strong polarity, which can be extracted by theacetonitrile solvent, so as to achieve the purpose of desulfurization. And,because the HPW in the HPW-SPC materials was intercalated into theinterlayer framework of the silica pillared MMT, it can be constantly reuse andshow good reusability.