دانلود رایگان مقاله تشکیل کارآمد لاکتون گلپر در تبدیل فاز بخار اسید لوولینیک

Abstract

Vapor-phase lactonization of levulinic acid to produce angelica lactones, which include α-, β- and γ-form isomers, was performed in fixed-bed down-flow glass reactors over various oxide catalysts. SiO2 and SiO2-Al2O3 showed relatively high catalytic activity. The lactonization of levulinic acid to angelica lactones was found to be an endothermic equilibrium reaction, and the pressure equilibrium constant was calculated to be 0.2 atm at 275 °C. High temperatures and reduced pressures were effective for shifting the equilibrium from levulinic acid to angelica lactones, while the suitable reaction temperature was estimated to be 275 °C because temperatures higher than 275 °C decreased the selectivity to angelica lactones. The highest angelica lactones yield of 87.5% was achieved at a levulinic acid conversion of 95.3% over SiO2 under reduced pressure conditions of ca. 5 kPa at 275 °C. IR, NH3-TPD and TG analyses were performed for characterizing the catalysts used after the reactions together with a silylated SiO2 prepared for studying the active species on SiO2. The silanol groups of SiO2 with weak acidity were proposed to be the active species.

5. Conclusions

Vapor-phase lactonization of LA to ALs was performed in fixedbed down-flow glass reactors over various solid catalysts. SiO2-Q3 showed the highest catalytic performance, especially at high W/FLA values. The weak acidity of SiO2, which is supposed to inhibit the further oligomerization of generated ALs, increases the selectivity to ALs. In the study of trimethylsilylation of surface silanol of SiO2 catalyst, we proposed that the silanol groups of SiO2 were the active species. Slight catalyticdeactivationof SiO2 was observedat a low W/FLA of 0.4 h, which was attributed to the coke accumulation on the catalyst surface, whereas the used catalyst can be regenerated by calcination at 500 ◦C without obvious decrease in the catalytic activity. Lactonization of LA to ALs and water is an equilibrium reaction, and the pressure equilibrium constant is calculated to be ca. 0.2 atm at 275 ◦C. Although high reaction temperatures can improve the conversion of LA, they decrease the selectivity to ALs. Therefore, the suitable temperature for lactonization of LA to AL is estimated to be 275 ◦C. In addition, reduced pressure conditions are efficient for shifting the equilibrium from LA to ALs, and an ALs yield of 87.5% with a LA conversion of 95.3% and an ALs selectivity of 91.8% was achieved over SiO2-Q3 at 275 ◦C.