Study Of Alumina-Based Catalysts For The Hydrolysis Of Trifluoromethane

CHF3is the major by-product during the manufacture of CHClF2(R22, HCFC-22) which is the feedstock of various polymers, foams, blowing agents and fire extinguishing agents.CHF3has the second highest global warming potential among all the greenhouse gases.It is a potent and synthetic greenhouse gas with global warming potential of11,700times (for a100-year time horizon) higher than carbon dioxide. It necessitates the research and development of potential routes for the treatment of CHF3and other synthetic greenhouse gases. Compared with the intensive research on the abatement of CFCs and CO2emission, there are relatively limited research focused on the treatment of CHF3.The present work mainly investigates the effects of Al2O3phases, water vapor concentration and zinc promoter on the activity and stability of alumina catalysts for CHF3hydrolysis. The structure, composition and surface groups of fresh and spent catalysts were studied by kinds of analysis characterizations, such as energy dispersive spectrometer, X-ray diffraction, BET and so on. Based on the experimental results, characterization data and thermodynamic analysis, a reaction mechanism is proposed which explains the effect of water vapor and deactivation of catalyst reasonably.This work mainly focuses on the activity and stability of different phases of Al2O3in the decomposition of CHF3. y-Al2O3was prepared by calcination of AlOOH at650℃for4h.δ-Al2O3was prepared by calcination of AlOOH at900℃for4h. Calcination of AlOOH at1200℃for4h resulted in the sample of α-Al2O3. Because of its small amounts of surface hydroxyl and low surface area,α-Al2O3is almost inactive for the targeted reaction although it is stable under conditions studied (with the presence of HF product).δ-Al2O3, γ-Al2O3and AlOOH show high activities at the initial stage of reaction, while they are inevitably fluorinated to AlF3or transform to α-Al2O3following long-time reaction. The results confirm that the high activity and stability of γ-Al2O3and AlOOH are related to the concentration of water vapor, pases structure and doppingof zinc promoter.Catalytic hydrolysis over alumina was carried out at temperatures between350℃and520℃. The concentration of water vapor influences the activity and stability of catalysts, markedly. The results show that the presence of small amounts of water vapor (0.8%) significantly improves the activity of γ-Al2O3, while higher water vapor concentration (>2.8%) does not further enhance the decomposition of CHF3. However, the catalyst stability is enhanced significantly with the introduction of30%vapor concentration in the feed stream.Zn is confirmed to be an efficient promoter for the improvement of activity and stability of alumina catalyst in the present study. The dopping of15%Zn to γ-Al2O3can significantly improve of activity and stability of catalyst. Part of Zn on the surface of alumina transforms into ZnAl2O4during catalyst preparation which functions as a structure modifier, preventing γ-Al2O3from changing to α-Al2O3or A1F3.Based on the experimental results and thermodynamic analysis, a reaction mechanism is proposed which explains the effect of water vapor and deactivation of catalyst reasonably. In the presence of water vapor, especially with high concentrations of water vapor, γ-alumina is fully hydrolyzed with the formation of large amounts of hydroxyls. CHF3interacts with hydroxyls with two fluorine atoms binding to two hydrogen atoms of hydroxyls. Releasing two HF, it transforms to intermediate, CHF. The formation of Al-Osurface-CHF-Osurface-Al protects the alumina surface from fluorinating. The intermediate interacts with H2O and release one HF. The interaction of OH-(Al-O) with CHF3retains the active sites via reaction cycle, while the interaction of OH-(Al-O) with HF results in the gradual deactivation of catalyst by total fluorination with the formation of AlF3.