Chlorodifluoromethane (CHClF2, R-22 or HCFC-22) has been generally accepted as the most suitable refrigerant for air conditioners. However, R-22 is a controlled substance under the Montreal protocol because of stratospheric ozone depletion potential. It has to be phased out by 2040 in developing cou...
Chlorodifluoromethane (CHClF2, R-22 or HCFC-22) has been generally accepted as the most suitable refrigerant for air conditioners. However, R-22 is a controlled substance under the Montreal protocol because of stratospheric ozone depletion potential. It has to be phased out by 2040 in developing countries. Also, R-22 is known as the most important raw material for the manufacture of valuable fluorinated compounds such as tetrafluoroethylene (C2F4, TFE), hexafluoropropylene (C3F6, HFP) and octafluorocyclobutane (C4F8, RC-318). Since the pyrolysis of R-22 is carried out at high temperatures (>750℃), there exists some problems such as formation of coke, corrosive HCl and HF. In order to improve relatively poor TFE yield, especially at high reaction temperature, catalysts were introduced and their effects were studied. The catalytic pyrolysis of R-22 over Cu-based catalysts using various supports was investigated and compared to the non-catalytic pyrolysis. In the catalytic pyrolysis of R-22 over activated carbon supported Cu catalysts, the highest TFE yield amounted to 62% at 700℃. The conversion of R-22 in the catalytic pyrolysis was higher than that in the non-catalytic case under the tested conditions. In addition, reduced Cu catalysts were more active than CuO catalysts. Enhanced TFE yield over reduced Cu catalysts in this study can be explained by the increased heat transfer from the heat source (tube outside) to gaseous reactant via the Cu catalyst particles, resulting in increase in the overall rate of pyrolysis reaction. And alteration of reactivity as a function of time was observed. The change of activity with time-on-stream is believed due to the transformation of the catalyst surface from oxide phase to fluoride phase by the attack of the hydrogen fluoride produced during the pyrolysis. In the catalytic pyrolysis of R-22 over metal fluoride based catalysts, the conversions of R-22 over metal fluoride catalyst increased in the following order: AlF3>Mixed>CaF2. And the physical mixture catalysts of AlF3 and CaF2 (Mixed) showed the highest selectivity and yield for TFE. After the reaction, XRD shows that bimetallic fluorides such as CaAlF5 were formed. EPMA shows that bimetallic mixture catalysts exhibited the lowest and negative average amount of adsorbed fluorine atoms, which indicates the formation of substoichiometric metal fluorides such as AlFx (x<3), CaFx (x<2) and CaAlFx (x<5). Generally, the catalyst surface can be modified by the attack of corrosive gases such as HF and HCl produced during the pyrolysis at high reaction temperature. Therefore, enhanced activity of physical mixture catalyst may be interpreted as due to the surface modifications, such as the formation of bimetallic fluoride and the variation of fluorine content on the surface of catalyst, caused by the attack of HF produced during the pyrolysis of R-22. In the catalytic and non-catalytic pyrolysis of R-22, TFE was produced as a main product, and trifluoromethane (CHF3, R-23) and trifluoroethylene (CF2CHF, TrFE) were formed as minor products. In the catalytic pyrolysis of R-22, the selectivity for R-23 decreased with time-on-stream, whereas the selectivity for TFE increased. Cu-promoted aluminum oxide showed higher selectivity for R-23 than aluminum fluoride, especially at initial reaction step. In both catalysts, the selectivities for TFE and R-23 were undergone some fluctuations and then reached a steady state after the reaction for 4 h. It was found that difluorocarbene (:CF2) formed during the pyrolysis plays an important role in the formation of R-23 as well as TFE. Product profiles suggest that TFE is formed via the coupling reaction of :CF2, while R-23 is formed via the secondary reaction of :CF2 with HF mainly produced during the catalytic pyrolysis of R-22. Increasing selectivity to TFE and simultaneously decreasing selectivity to R-23 with time on stream over both catalysts could be explained in terms of a competition between two parallel pathways of the coupling or dimerization of :CF2 and the secondary reaction of :CF2 with HF.
Chlorodifluoromethane (CHClF2, R-22 or HCFC-22) has been generally accepted as the most suitable refrigerant for air conditioners. However, R-22 is a controlled substance under the Montreal protocol because of stratospheric ozone depletion potential. It has to be phased out by 2040 in developing countries. Also, R-22 is known as the most important raw material for the manufacture of valuable fluorinated compounds such as tetrafluoroethylene (C2F4, TFE), hexafluoropropylene (C3F6, HFP) and octafluorocyclobutane (C4F8, RC-318). Since the pyrolysis of R-22 is carried out at high temperatures (>750℃), there exists some problems such as formation of coke, corrosive HCl and HF. In order to improve relatively poor TFE yield, especially at high reaction temperature, catalysts were introduced and their effects were studied. The catalytic pyrolysis of R-22 over Cu-based catalysts using various supports was investigated and compared to the non-catalytic pyrolysis. In the catalytic pyrolysis of R-22 over activated carbon supported Cu catalysts, the highest TFE yield amounted to 62% at 700℃. The conversion of R-22 in the catalytic pyrolysis was higher than that in the non-catalytic case under the tested conditions. In addition, reduced Cu catalysts were more active than CuO catalysts. Enhanced TFE yield over reduced Cu catalysts in this study can be explained by the increased heat transfer from the heat source (tube outside) to gaseous reactant via the Cu catalyst particles, resulting in increase in the overall rate of pyrolysis reaction. And alteration of reactivity as a function of time was observed. The change of activity with time-on-stream is believed due to the transformation of the catalyst surface from oxide phase to fluoride phase by the attack of the hydrogen fluoride produced during the pyrolysis. In the catalytic pyrolysis of R-22 over metal fluoride based catalysts, the conversions of R-22 over metal fluoride catalyst increased in the following order: AlF3>Mixed>CaF2. And the physical mixture catalysts of AlF3 and CaF2 (Mixed) showed the highest selectivity and yield for TFE. After the reaction, XRD shows that bimetallic fluorides such as CaAlF5 were formed. EPMA shows that bimetallic mixture catalysts exhibited the lowest and negative average amount of adsorbed fluorine atoms, which indicates the formation of substoichiometric metal fluorides such as AlFx (x<3), CaFx (x<2) and CaAlFx (x<5). Generally, the catalyst surface can be modified by the attack of corrosive gases such as HF and HCl produced during the pyrolysis at high reaction temperature. Therefore, enhanced activity of physical mixture catalyst may be interpreted as due to the surface modifications, such as the formation of bimetallic fluoride and the variation of fluorine content on the surface of catalyst, caused by the attack of HF produced during the pyrolysis of R-22. In the catalytic and non-catalytic pyrolysis of R-22, TFE was produced as a main product, and trifluoromethane (CHF3, R-23) and trifluoroethylene (CF2CHF, TrFE) were formed as minor products. In the catalytic pyrolysis of R-22, the selectivity for R-23 decreased with time-on-stream, whereas the selectivity for TFE increased. Cu-promoted aluminum oxide showed higher selectivity for R-23 than aluminum fluoride, especially at initial reaction step. In both catalysts, the selectivities for TFE and R-23 were undergone some fluctuations and then reached a steady state after the reaction for 4 h. It was found that difluorocarbene (:CF2) formed during the pyrolysis plays an important role in the formation of R-23 as well as TFE. Product profiles suggest that TFE is formed via the coupling reaction of :CF2, while R-23 is formed via the secondary reaction of :CF2 with HF mainly produced during the catalytic pyrolysis of R-22. Increasing selectivity to TFE and simultaneously decreasing selectivity to R-23 with time on stream over both catalysts could be explained in terms of a competition between two parallel pathways of the coupling or dimerization of :CF2 and the secondary reaction of :CF2 with HF.
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