| Dawson structure phosphotungstic acid is a new type of environment friendly acid catalyst and selective oxidation catalyst. However, pure heteropolyacid (abbreviated as HP As) used as heterogeneous catalysts are hindered by their relatively low surface area (1-10m2/g) and high solubility in polar reaction systems, which results in the difficulties of separation and recycling use. The disadvantage can be overcome by supporting the HP As on adequate porous solids or using their salts with different cations. In addition, Keggin structure HP As have been widely studied and well known to be an efficient catalyst for a variety of organic reactions. Comparatively,the catalytic application of Dawson structure HP As have been largely overlooked. Dawson structure phosphotungstic acid is reported to show more excellent catalytic performance than that of Keggin structure phosphotungstic acid in many reactions. Therefore, it is very important to develop a series of new Dawson structure supported phosphotungstic acid or phosphotungstic acid salt catalyst. In this dissertation, ten kinds of tungstophosphoric compounds catalysts with Dawson structure were prepared, and characterized by means of FT-IR, UV-Vis, TG-DTA, SEM, EDS, CV and XRD. They were applied to the esterilication of acetic acid or p-hydroxy benzoic acid with n-butanol, synthesis of tetrahydrofuran(abbreviated as THF) and prepration of adipic acid(abbreviated as AA). Some satisfied study results were obtainded.1. Study on the preparations characterizations and their catalytic performance of H6P2W18O62·13H2O and H6P2W9Mo9O62·24H2O for synthesis of adipic acidDawson structure phosphotungstic acid and tungstomolybdophosphate acid were prepared through hydrothermal synthesis. The synthesis of adipic acid with cyclo-hexanone and 30 percent hydrogen peroxide under microwave irradiation was carried out as a probe reaction to study their catalytic activity. The main effects of the reaction were optimized with orthogonal experiments. The results showed under the conditions of the mole ratio of C6H10O:H2O2:H2C2O4·2H2O:H6P2W18O62·13H2O =100:500:3.0:0.25, and C6H10O:H2O2:H2C2O4·2H2O:H6P2W9Mo9 O62·24H2O=100: 400:1.25:0.25, the reaction temperature of 100℃, microwave power of 400 W, reaction time of 3.5 h, the yield of adipic acid could reach 91.78% and 87.33%, respectively. The used catalyst could be concentrated by the rotary evaporation and be reused five times by easily separation of the product after reaction, the isolated yield of adipic acid was still above 62.50% and 45.89% respectively. Compared with conventional heating method, microwave irradiation possessed such advantages as simple work-up procedure, shorter reaction time, higher yield and environmental friendliness.2 Study on the preparations, characterizations and catalytic properties of Daw-son structure rare earth phosphotungstates and ammonium phosphomolybdateThree kinds of rare earth phosphotungstates with Dawson structure formulated as RE2P2W18O62 (RE=La,Ce,Y) were prepared and used in catalytic synthesis of tetrahydrofuran from 1,4-butanediol. The influences of rare earth element, catalyst dosage, reaction temperature and reaction time were investigated. The results indicated that La2P2W18O62 showed the best catalytic properties in synthesis of THF among the three rare earth heteropoly salts. Under the optimal condition, i.e. w (catalyst)=1.2%(relative to the dosage of 1,4-butanediol), the reaction temperature (oil bath) of 180℃, the reaction time of 25 min, the average yield of THF could reach 97.6%. After reaction these catalysts could be recycled and still exhibited catalytic activity with a yield of 86.6% after five cycle reactions. The results show that behaved as an excellent heterogeneous catalyst in the synthesis of n-butyl acetate. The optimum synthetic conditions were determined as follows: molar ratio of n-butanol to acetic acid at 2.0:1.0, mass of the catalyst being 1.44% of the total reaction mixture, reaction temperature of 120 ℃ and reaction time of 150 min. Under above conditions, the conversion of acetic acid was above 97.8%. The selectivity of n-butyl acetate based on acetic acid was, in all cases, nearly 100%. The catalysts could be recycled and still exhibited high catalytic activity with 90.4% conversion after five cycles of reaction. It was found by means of TG-DTA and Py-IR that the catalyst deactivation was due to decrease of the Lewis acid sites,which was caused by coking on catalyst surface.Adipic acid and octadecanoic acid were synthesized through catalytic oxidation with 30% hydrogen peroxide over (NH4)3P2Mo18O62. The effects of catalyst amount, 30% H2O2 amount, reaction time were investigated. The main effects of the reaction were optimized with orthogonal experiments and simple fact experiments. The optimal conditions for the synthesis of adipic acid are the mole ratio of C6H10O: H2O2:H2C2O4:(NH4)6P2Mo18O62=100:500:2.0:0.087, the reaction temperature of 100℃, microwave power of 400W, irradiation time of 3.5 h with 72.5% yield of adipic acid. The catalyst could be reused for five times and yield of adipic acid was still above 43.2%. For the synthesis of octadecanoic acid, under the optimal conditions, i.e. catalyst amount of 3.7wt%(relative to the dosage of octadecanol), n(octadecanol):n(30% H2O2) of 1.0:10.0, the reaction temperature of 95℃, the reaction time of 4.0 h, the isolated yield of octadecanoic acid was obtained 95.3%. Catalyst was reused for 5 times and 91.6% yield of octadecanoic acid was remained.According to the literatures and our experimental results, the peroxocoordination ions{PO4[MoO(O2)2]4}3- and{PO4[MoO2o]4}3-,which are synthesized by the coordination between (NH4)6P2Mo18O62 and 30 percent hydrogen peroxide, are presumed to be the probable active structure. The transformation from cyclohexanone to adipic acid is achieved through Baeyer-Villiger oxidation(which produces ε-caprolactone) and hydrolyses.3. Study on the preparations, characterizations and their catalytic properties of supported Dawson structure phosphotungstic acidFive new Dawson structure supported phosphotungstic acid H6P2W18O62/SiO2, H9P2W15V3/C, H6P2W18O62/Diatomite, H6P2W18O62/MWCNTs and H6P2W18O62/ MCM-48 were prepared respectively. Their catalytic performance were studied in the catalytic synthesis of tetrahydrofuran, n-butyl p-hydroxy benzoate,n-butyl acetate or adipic acid. The main effects of the reaction were optimized with orthogonal experiments and simple fact experiments. The optimized conditions were as follows:(1) Under reaction conditions of catalyst amount (30%H6P2W18O62/SiO2) of 6.6%(relative to the dosage of 1,4-butanediol), reaction temperature of 185℃ ～ 190℃ and reaction time of 40 min, the isolated yield of tetrahydrofuran could reach 91.7%. The catalyst could be reused for five times or even more with no evident decline in its catalytic performance. Under the optimum conditions for supported amount of 25%, the mass ratio of H6P2W18O62/SiO2 to total reactant of 1.1%, the molar ratio of n-butyl alcohol to acetic acid of 2.0:1.2, the reaction time of 2.0 h, the reaction temperature of 120 ℃, the yield of n-butyl acetat could reach 95.30%.63.24% yield of n-butyl acetat was remained after the catalyst repeated used for five times.(2)Under the optimum conditions for supported quantity of 30%, the mass ratio of H9P2W15V3/C to total reactant of 8.7%, the molar ratio of n-butyl alcohol to p-hydroxy benzoic acid of 2.0:1.0, the reaction time of 3.0 h, the reaction temperature of 125℃, the yield of n-butyl p-hydroxy benzoate could reach 91.30%. The catalyst could be reused for 5 times with 76.42% yield of n-butyl p-hydroxy benzoate.(3) The H6P2W18O62/Diatomite catalysts showed both Bronsted and Lewis acidity, and the catalyst with 40% H6P2W18O62 loading had the highest total acidity and catalytic activity because of the monolayer coverage of the active species.97.1% yield of tetrahydrofuran could be obtained under the reaction conditions of 180℃ for 45 min, catalyst amount 3.2%(relative to the dosage of 1,4-butanediol). After reaction, these catalysts could be recycled and still exhibited catalytic activity with a yield of 62.9% after five cycle reactions. It was found by means of UV-Vis, TG-DTA and Py-IR that the catalyst deactivation was due to the dissolution of H6P2W18O62 and decrease of the acid sites, which was caused by coking on catalyst surface.(4)Under the optimum conditions for supported quantity of 40%, the mass ratio of H6P2W18O62/Diatomite to total reactant of 2.8%, the molar ratio of n-butyl alcohol to p-hydroxy benzoic acid of 3.0:1.0, the reaction time of 3 h, the reaction temperature of 125℃, the yield of n-butyl p-hydroxy benzoate could reach 90.8%. When the catalyst being used for 5 times the yield of n-butyl p-hydroxy benzoate was still above 65.6%.(5) The results indicate that the catalytic activity of the supported catalyst is higher than that of Dawson structure phosphotungstic acid and multiwalled carbon nanotubes alone. The esterification yield was up to 97.5% under the optimal conditions, namely, the phosphotungstic acid supported quantity was 50%, the molar ratio of acetic acid to n-butyl alcohol of 2:1, the quality ratio for H6P2W18O62 /MWCNTs to total raw materials of 1.1%, the reaction temperature of 120℃, the reaction time of 2.0 h. The catalyst could be used repeatedly 5 times and the isolated yield of n-butyl acetate was still above 60.9%.(6)Under the optimum conditions for supported quantity of 40%, the mass ratio of H6P2W18O62/MCM-48 to the mass of cyclohexanone reactant of 2.8%, the molar ratio of C6H10O:H2O2:H2C2O4·2H2O of 100:450:1.88, the reaction temperature of 95 ℃, microwave power of 200W, irradiation time of 3.5 h, the mean yield of adipic acid was 81.3%. The catalyst could be used repeatedly 5 times by easily separation of the hot filtration after reaction, the product’s yield was 64.6%.Compared with the traditional catalysts, Dawson structure heteropoly compounds have the determination internal structure, a unique pseudo-liquid phase, strong acidity and redox activity. In the organic catalytic reaction, the Dawson structure heteropoly compounds show operational simplicity, environmental friendliness, higher activity and higher selectivity. Moreover, the supported catalyst can be used for many times. |
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