Synthesis, Characterization And Catalytic Performance Of Modified Heteropoly Compounds

Oxidation reactions constitute an important research area because they have awide range of applications in organic synthesis and fine chemical industry. Theselective oxidation of alcohols into their corresponding carbonyl compounds is one ofcrucially important and fundamental transformations from both synthetic andindustrial points of view, due to the wide-ranging utility of these products as versatileintermediates of valuable compounds such as pharmaceuticals, agricultural chemicalsand vitamins. Oxidation of alcohols has always been the research hot spot, in thepursuit of fast reaction rate and high yield, meanwhile, researchers begin to payattention to how to develop environment friendly, mild reaction conditions of alcoholoxidation process. In selective catalytic oxidation of organic compounds, hydrogenperoxide is an attractive oxygen donor as it has a high content of active oxygenspecies, and water is only byproduct. It is also much cheaper and safer to use thanorganic peroxides or peracids, making the process promising for future application.Most examples of alcohol oxidation with hydrogen peroxide employed heteropolycompounds as catalysts have been reported. Heteropoly compounds are a kind of newexcellent acid catalyst and redox catalyst due to their tunable acid and redoxproperties, a certain structure and electrons and protons move/storage capacity,hydrolytic and thermal stability, solubility in various media, and so forth. Heteropolycompounds can also be supported on carriers such as SiO2or carbon, etc, and showhigh catalytic ability and selectivity. The structure and composition of heteropolycompounds can be modified and changed, thus changing their catalytic properties.In this paper, heteropoly compounds were modified by changing theircountercations and amount of substituted metals, introduction of organic ligands orbeing supported on suitable carriers, thus making their application in the oxidation ofalcohols. The heteropoly compounds after modification were characterized by IR,XRD, UV-vis, SEM, BET to study their structural characteristics and physicochemicalproperties such as: stability, morphology, solubility, etc. The influence of modification of heteropoly compounds on their catalytic properties was studied. The relationshipbetween countercations, structural change of heteropoly compounds and catalyticproperties was studied systematically, and the mechanism of the oxidation overmodified heteropoly compounds was studied preliminary.The main experimental results and conclusions are as follows:1. Firstly, Fen-substituted （n=0,1,2,3） Keggin-type silicotungstates TBA4[α-SiW₁2O₄0],TBA₅[α-SiW₁1{Fe（OH₂）}O₃9], TBA₆[γ-SiW₁0{Fe（OH₂）}2O₃8] and TBA7[α-SiW9{Fe（OH₂）}3O37]（TBA=[（C4H9）4N]⁺） were synthesized and characterized by IR, UV-vis and XRD. Theresults illuminated that the amount of Fe did not affect the Keggin-type structure ofsilicotungstates. But TBA4[α-SiW₁2O₄0], TBA₅[α-SiW₁1{Fe（OH₂）}O₃9] and TBA7[α-SiW₉{Fe（OH₂）}3O37] were α-isomer, diiron-substituted TBA₆[γ-SiW₁0{Fe（OH₂）}2O₃8] wasγ-isomer.With H₂O₂as oxidant, the influence of Fe-center on the catalytic oxidation ofcyclohexanol was studied. Non-substituted Keggin anion [α-SiW₁2O₄0]4-was inactive.Only when Fe was on the coordination site, the catalytic activity of silicotungstateswas increased. These results showed that Fe was the active center of catalytic system.The catalyst TBA₆[γ-SiW₁0{Fe（OH₂）}2O₃8] showed the highest catalytic reactivity. Thisresult showed that the skeleton structure of Keggin-type silicotungstates played animportant role in catalytic reactivity, and diiron site of γ-Keggin silicotungstates waseffective for the oxygenation with H₂O₂. With TBA₆[γ-SiW₁0{Fe（OH₂）}2O₃8] as catalyst,the optimal condition: acetonitrile as solvent, temperature at70oC, n（H₂O₂）=160mmol, n （catalyst）=0.3mmol,89.2%conversion of cyclohexanol was achieved. IRand UV-vis spectra of TBA₆[γ-SiW₁0{Fe（OH₂）}₂O₃8] after reaction showed thatγ-Keggin structure was stable and the catalyst was not deactivated under the reactionconditions used.2. The dimeric mono-Fe（III）-substituted phosphotungstates [CTA]₁0[（PW₁1FeO₃9）₂O],[TEA]10[（PW₁1FeO₃9）₂O] and [TMA]₁0[（PW₁1FeO₃9）₂O]（cetytrimethyl ammonium bromide（CTAB）, tetraethylammonium bromide （TEAB） and tetramethy lammonium bromide（TMAB）） were synthesized and characterized by IR, XRD, UV-vis, SEM and BET.The results showed that the longer alkyl chains of the organic countercations was, theless compact the particles showed, thus the specific surface area was bigger and thecatalyst was more accessible to the substrate. The catalytic activities of thesephosphotungstates were tested in the oxidation of cyclohexanol using H₂O₂as oxidantand tert-Butanol as solvent.[CTA]₁0[（PW₁1FeO₃9）₂O] showed to be the most efficient catalyst, the result indicated that longer alkyl chains of the organic countercations wasfavorable to enhancing the catalytic activity of the catalysts.In addition, IR spectra of the recovered catalyst at different reaction temperatureand at different reaction time at70oC, showed that the catalyst [CTA]10[（PW₁1FeO₃9）₂O]was not stable at70oC and decomposed completely with the formation of monomeric[CTA]₄PW₁1Fe（H₂O）O₃9after3h of the reaction. But there was still dimeric[CTA]10[（PW₁1FeO₃9）₂O] partially existing in the catalytic system at the first3h of thereaction. During the first3h of the reaction, the dimeric [CTA]10[（PW₁1FeO₃9）₂O]gradually decomposed into monomeric [CTA]4PW₁1Fe（H₂O）O₃9, this dynamic processfacilitated catalytic oxidation of cyclohexanol.3. SiO₂-supported CTA₁0[（PW₁1FeO₃9）₂O] was synthesized by a co-precipitationmethod. IR, XRD, DRS UV-vis, TG-DTA and BET results proved that CTA₁0[（PW₁1FeO₃9）₂O]could be finely dispersed on the SiO₂, and the dimeric structure was stable in thesupported catalysts. All samples had evenly distributed mesoporous channel.With H₂O₂as oxidant and tert-Butanol as solvent, the catalytic activity ofCTAFe2/SiO₂was tested in the liquid-solid phase oxidation of hexanol. The resultsshowed that SiO₂had no catalytic reactivity. The conversion of hexanol increasedafter [CTA₁0[（PW₁1FeO₃9）₂O] supported on SiO₂. The yield of hexanal and hexanoicacid all increased with the load of CTAFe2/SiO₂increasing. From the selectivity andactivity into consideration, the optimal load was60%. A little leaching of the activespecies occurred during the reaction, but the dimeric structure of CTAFe2/SiO₂was stable.With O₂as oxidant, the catalytic activity of CTAFe2/SiO₂was tested in thegas-solid phase oxidation of hexanol. The results showed that their reactivity wasenhanced after CTA₁0[（PW₁1FeO₃9）₂O] supported on SiO₂. When the load was60%,the catalytic activity of CTAFe2/SiO₂reached highest. IR spectra and XRD patterns ofthe catalysts after reaction showed that the structure of the catalysts changed anddimeric structure disappeared. The structure of CTA₁0[（PW₁1FeO₃9）₂O] changed aftergas-solid phase oxidation of hexanol and rearranged into1:12Keggin anion[PW12O40]3-. That’s because organic cations decomposed from250oC to400oC, whichdrived the release of Fe from the Keggin anion [（PW₁1FeO₃9）₂O]10-yielding Fe ascountercation. The remained lacunary [PW₁1FeO₃9]4-was unstable and rearrangedinto [PW12O40]3-.4. The mono-iron-substituted Keggin-type silicotungstates Rb5SiFe（OH₂）W₁1O₃9·7H₂O and Cs5SiFe（OH₂）W₁1O₃9·6H₂O were synthesized, which were characterized by IR,XRD, UV-vis, SEM, BET, and TG-DTA. The results showed that the preparedsilicotungstates were of Keggin structure and Keggin unit of heteropoly anion[SiFe（OH₂）W₁1O₃9]5-did not change by introducing different countercations. Butcountercations could change the morphology, specific surface area and dissolubility ofthe silicotungstates. The specific surface area increased gradually and the particlesaccumulated loosen gradually and dissolubility in water decreased gradually with theradius of countercations increasing in the order of Rb<Cs.With H₂O₂as oxidant, cyclohexanol was efficiently oxidized to cyclohexanone overthese two silicotungstates. The results showed that the catalyst Rb5SiFe（OH₂）W₁1O₃9·7H₂Oshowed the highest catalytic reactivity. For organic liquid phase oxidation with H₂O₂asoxidant, the catalyst with good dissolubility could form water-oil biphasic reactionsystem, thus making it better catalytic reactivity. The catalyst Rb5SiFe（OH₂）W₁1O₃9·7H₂Ocould dissolved in H₂O₂and presented as liquid state in the reaction process. Thus thecontact area between the catalyst and organic substrate magnified and the conversionof cyclohexanol was increased. Moreover, water-oil biphasic system allowed easyrecovery of catalyst. The catalyst could be reused three times without appreciable lossof activity. With Rb5SiFe（OH₂）W₁1O₃9·7H₂O as catalyst and tert-Butanol as solvent,the optimal conditions: temperature at80oC, n（H₂O₂）/n（cyclohexanol） was1.5,n（catalyst） was0.4mmol,52.1%conversion of cyclohexanol was achieved.5. Introducing organic ligand into heteropoly compound, a new inorganic-organichybrid material Na（ANIH）4ANI[PW₁1Ni（ANI）O₃9]·12H₂O （ANI=aniline） was prepared basedon transition-metal monosubstituted Keggin type anion [PW₁1Ni（H₂O）O₃9]5-as inorganicspecies and aniline as organic ligand by a covalent bonding of Ni-N. The as-preparedhybrid material was characterized by elemental analysis, IR，UV-vis, XRD, TG-DTA,ICP and XPS. The results approved that the Keggin structure of the hybrid materialstill remained and the Ni-N covalent bond is formed between the transition-metal inthe Keggin-type anion [PW₁1Ni（H₂O）O₃9]5-and the nitrogen atom of ANI, whichilluminated that heteropoly anions reacted chemically with organic ligand, andelectrons transferred from the nitrogen atom to the transition metal Ni center. Also theanion [PW₁1Ni（ANI）O₃9]5-was surrounded by five ANI containing species: four ofthem were protonated ANIH^<sup>+as countercations and one was noncoordinated ANImolecule. Photocatalytic degradation of dye RB indicated that the as-prepared hybrid material Na（ANIH）4ANI[PW₁1Ni（ANI）O₃9]·12H₂O showed higher photocatalyticactivity. Therefore, the hybrid material has great potential for application in practicalphotodegradation.