Supported 12-Phosphotungstic Acid And Its Salts As Catalysts For The Synthesis Of Fructone And Fructone-B

Heteropoly acid and its salts catalysts supported on various carriers such as amorphous silica (SiO₂), mesoporous molecule sieve (SBA-15), ultra stable Y zeolite(USY) and dealuminated ultra stable Y zeolite (DUSY) were prepared by impregnation. Their physicochemical properties such as dispersion, specific surface area, acidity and so on were characterized by X-ray diffraction (XRD), N₂ adsorption BET, Hammett indicator, infrared spectrum (FT-IR) and scanning electronic microscope (SEM). The leakage of heterpoly compounds on catalysts into water was measured by ultraviolet-visual spectrophotometer. The catalytic activity and selectivity were tested in synthesizing fructone and fructone-B, and the catalytic stability was also evaluated by water treatment. The main results of this thesis are as follows.(1). It was found that the specific surface area of SiO₂ supported 12-phosphotungstic acid (PW) and its salts decreased; but the acidity increased with the increase of loading. XRD results indicated that supported PW catalysts showed a high PW dispersion when its loading reached 30% by weight, while it was true for the supported Cs- and K-salt of PW catalysts when the loading was 20%. The catalytic activity increased with the increase of PW loading until the loading reached 30%. The conversion of ethyl acetoacetate for PW/SiO₂ was 69.8%, for Cs_2.5H_0.5PW/SiO₂ it was 68.6% and for K_2.5H_0.5PW/SiO₂ it was 68.4%, and with the fructone selectivity all beyond 97%. The water treatment test showed that the deactivation of catalyst was correlated with leaching of active heteropoly components; and the sequence of catalytic stability was Cs_2.5H_0.5PW/SiO₂ ≈ K_2.5H_0.5PW/SiO₂ > PW/SiO₂.(2). As for SBA-15 supported PW and its Cs- and K-salt catalysts, the specific surface area was reduced but the acidity increased with increasing the loading. From the XRD pattern, it was found that the structure of SBA-15 remained even the loading was as high as 80%. The acidity of SBA-15 supported catalysts was rather weak in the low and medium loading because of the strong interaction between PW anions and silanol groups on the surface of SBA-15 wall, as shown in the FT-IR spectrum. When the loading of PW on SBA-15 was lower than 30%, or the loadings of Cs- and K-salt of PW on SBA-15 were lower than 20%, the catalysts were inactive. It showed highest catalytic activity when the loading reached 80%, with PW/SBA-15 being 73.2%, Cs2.5H0.5PW/SBA-15 being 67.8% and K2.5H0.5PW/SBA-15 being 61%,respectively; and with the fructone selectivity all beyond 97%. 8O%C82.5Ho.5PW/SBA-15 showed rather low leakage in water treatment and correspondingly gave a high catalytic stability than 80% PW/SBA-15 in the reaction.(3). With regard to DUSY supported PW and its Cs- and K-salt catalysts, it showed the highest catalytic activity when the substitute number of cation was 2.5 at the same PW salt loading. The catalytic activity of 3O%Cs2.5Ho.5PW/DUSY was higher than that of 3O%K2.5Ho.5PW/DUSY. It was also shown that the catalytic activity of pure salt of PW was rather low, and the supported PW catalyst showed lower activity than pure PW. In contrast, the catalytic activity for supported salts of PW was improved remarkably, and it increased up to the loading of 30%. The optimal reaction conditions for synthesizing fructone over 3O%Cs2.sHo5PW/DUSY were as follows. The reaction temperature was 363 K, molar ratio of ethyl acetoacetate to glycol was 1:1.5, reaction time was 90 min, the catalyst concentration was 0.6 wt% and cyclohexane as water removal was 40wt% in the reaction medium. In this case, Conversion of ethyl acetoacetate reached 98.7% with the fructone selectivity beyond 97%. The water treatment test showed that supported PW catalyst exhibited low stability than the supported salts catalyst. It was obvious that deactivation of catalyst was correlated with leaching of active heteropoly components, as indicated by the solubilization test of heteropoly compounds (HPC) during water treatment and by the SEM pictures and FT-IR spectrum before and after water treatment. The sequence of catalytic stability was Cs2.5H0.5PW/ SiO2 K2.5H0.5PW/ SiO2 >PW/ SiO2 in this polar reation system. Compared with USY support, dealuminated ultra stable Y was a more suitable support for heteropoly acid and its salts. DUSY supported catalysts exhibited higher catalytic activity, higher stability and lower leakage of HPC in water treatment.(4). As for synthesizing fructone-B, it was found that the catalyst with a Cs2.5H0.5PW loading of 30%, namely 30%Cs2.5H0.5PW/DUSY, exhibited the highest activity among cesium salts of PW catalysts supported on DUSY And the optimal reaction conditions were as follows. The reaction temperature was 363 K, molar ratio of ethyl acetoacetate to 1,2-propanediol was 1:1.5, and reaction time was 90 min, the catalyst concentration was 0.8 wt% and cyclohexane as water removal was 36 wt% of the reaction medium. In this case, Conversion of ethyl acetoacetate reached 95.3% with the fructone-B selectivity of 100%. The catalytic stability test revealed that the catalyst activity of 30%PW/DUSY obviously reduced by 67.6%, while 30%Cs2.5H0.5PW/DUSY only by 11.4% after five reaction recycles. The leakage of heteropoly compound (HPC) by water leaching showed that the decreasing of activitywas proportional to the leaching amount of active component in the polarity system. The deactivation is thus suggested to mainly ascribe to the lost of heteropoly compound. The satisfied catalytic stability of the supported cesium salts of PW catalyst derived from the rigidity of cesium salts of PW in the polar reaction system.