- Email: [email protected]
Decomposition Degree of Chlorofluorocarbon (CFC) and CFC Replacements during Recovery with Surface-Modified Activated Carbon
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
177, 329–334 (1996)
0039
Decomposition Degree of Chlorofluorocarbon (CFC) and CFC Replacements during Recovery with Surface-Modified Activated Carbon SEIKI TANADA,* ,1 NAOHITO KAWASAKI,* TAKEO NAKAMURA,*
AND IKUO
ABE†
*Faculty of Pharmaceutical Sciences, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577; and †Osaka Municipal Technical Research Institute, 1-6-50, Morinomiya, Joto-ku, Osaka 536, Japan Received April 7, 1995; accepted June 15, 1995
INTRODUCTION The recovery efficiency of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC113) and three CFC replacements (1,1-dichloro-1-fluoroethane, HCFC141b; 1,3-dichloro-1,1,2,2,3-pentafluoro-propane, HCFC225cb; and 2,2,3,3,3-pentafluoro-1-propanol, 5FP) were investigated on the basis of their degree of decomposition and adsorption isotherms. We prepared activated carbons with various surface polarities to elucidate the recovery efficiency, the amount adsorbed, and the degree of decomposition. A correlation between the physicochemical properties of the activated carbon surface and the amount of CFC or CFC replacements adsorbed was not observed. The amount of CFC113 adsorbed onto untreated activated carbon was the largest of all. That of HCFC225cb adsorbed onto activated carbon treated with hydrogen gas was larger than that adsorbed onto untreated activated carbon and activated carbon treated with 6 N nitric acid. The amount of 5FP and HCFC141b adsorbed on the various activated carbons was not substantial. The degree of decomposition of CFC replacements using the untreated activated carbon except for HCFC225cb was the largest of all. In the case without the activated carbon, that of CFC and the CFC replacements increased in the order 5FP, CFC113 or HCFC225cb, and HCFC141b. These results indicated that the stability of CFC and CFC replacements molecules was controlled by the number of carbon-fluoride groups and/or hydrogen atoms. It is concluded that the recovery of CFC replacements was possible using the surface-modified activated carbons rather than the untreated activated carbon. The degree of decomposition of the CFC replacements during recovery using the activated carbon depends on the relationship between the adsorption site of the surface of the activated carbon and the polarity, hydrophilic site, or hydrophobic site of the CFC replacement molecule. It is assumed that the recovery of CFC replacements using HT-AC decreased the amount of hydrofluoric acid produced. q 1996 Academic Press, Inc.
Key Words: surface modified activated carbon; nitric acid, surface modified activated carbon; hydrogen gas, decomposition degree; Chlorofluorocarbon (CFC), decomposition degree; CFC replacement, fluoride ion; chloride ion.
1
To whom correspondence should be addressed.
One of the most serious global environmental pollution problems is depletion of the ozone layer by chlorofluorocarbons (CFCs). The CFC molecules are comprised of halogens such as chlorine and fluorine. The hydrocarbons which contain a hydrogen atom are decomposed more readily than ones having no hydrogens. Hence, CFC replacements, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), have been developed and are presently produced in large tonnage scale. The CFC replacements are more easily decomposed than CFCs. The oxidation-decomposition of HCFC141b and 5FP were reported by Sato and Nakamura (1) or Edney et al. (2) and Izaki et al. (3), respectively. The ozone depletion potential of the CFC replacements is smaller than that of CFCs, and HFCs contain no chlorine. HCFCs are considerably less harmful toward stratospheric ozone than CFCs. However, HCFCs can transport chlorine atoms into the stratosphere. The global warming potentials of CFC replacements are less than that of CFCs. However, the recovery of CFC replacements, if possible, will lead to substantial energy savings. Activated carbon has been used as an adsorbent in various industrial fields, such as solvent recovery and gas separation, and as a catalyst support material (4). The amount of CFC and CFC replacements adsorbed onto surface-modified activated carbons has been previously reported to occur from the gaseous phase (5). However, the recovered solvent is decomposed to a large extent. The decomposition of chlorocarbons by metal oxidation produces mainly hydrochloric acid and carbon dioxide has been reported (6). Also, many reports on CFC decomposition have noted the oxidation of the compounds over various catalysts including aluminasupported gold (7), metal oxides (6), and so on. Little work on the determination of the degree of decomposition of HCFCs using surface-modified activated carbon has been done so far. In the present study, the decomposition of CFC and CFC replacements is investigated to determine the recovery effi-
329
/ m4585$3914
12-22-95 14:46:47
coida
0021-9797/96 $12.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AP: Colloid
330
TANADA ET AL.
ciencies using surface-modified activated carbons as catalysis and the decomposition mechanism of CFC replacements during recovery.
follows: the 10-ml aliquot was put into a white porcelain dish and 1 ml of potassium chlorate was added and then titrated with prepared 0.1 N aqueous silver nitrate (9). Base or Acid Consumptions
MATERIALS AND METHODS
Materials Untreated activated carbon (U-AC) produced from nutshells, DIAHOPE 006, was commercially obtained from Mitsubishi Chemical Industries, Ltd. The surface of the activated carbon was modified to be hydrophobic and hydrophilic using hydrogen and 6 N nitric acid, respectively. The hydrophilic activated carbon (NT-AC) was soaked for 1 h in boiling 6 N nitric acid and then washed in distilled water. The solvents used included 1,1,2-trichloro-1,2,2-trifluoroethane (CFC113) (Wako Pure Chemical Industries, Ltd., Osaka, Japan), 1,1-dichloro-1-fluoroethane (HCFC141b) (Asahi Glass Co., Ltd., Tokyo, Japan), 1,3-dichloro1,1,2,2,3-pentafluoropropane (HCFC225cb) (Asahi Glass Co., Ltd., Tokyo, Japan), and 2,2,3,3,3-pentafluoro-1-propanol (5FP) (Nakarai Tesque, Inc., Kyoto, Japan). Degree of Decomposition The amount of fluoride and chloride ions produced was measured to elucidate the degree of decomposition of CFC and the CFC replacements. The degree of decomposition of CFC and the CFC replacements was measured as follows: U-AC (approximately 0.2 g), distilled water (20 ml), and CFC or the CFC replacements (5 ml) were put in an autoclave of 125 ml and the autoclave was incubated for 24 h at 1207C. The mixture was allowed to cool to 257C before being filtered through a funnel covered with filter paper (Toyo, No. 2). The fluoride ion was determined by standard methods of analysis for hygienic chemists, that is, the lanthanum alizarin complexone methods (8). A 10-ml aliquot was put into a 50-ml colored tube and 1 ml of alizarine complexone solution and 20 ml of acetone in the 50-ml colored tube for a total volume of exactly 50 ml. The fluoride ion was measured at a maximum wavelength of 420 nm using a Hitachi U-1100 spectrophotometer (Hitachi, Ltd., Tokyo, Japan). The amount of chloride ions was determined as
Base or acid consumptions of the untreated activated carbon and surface-modified activated carbon were determined using the acid–base titration method (10). Base consumption was measured as follows: Activated carbon (approximately 1.0 g) was added to a 100-ml portion of prepared aqueous sodium hydroxide (0.1 N) and the suspension was shaken for 48 h at 257C. Basicity was determined by titrating with prepared hydrochloric acid (0.1 N). Acid consumption was measured as follows: Activated carbon (approximately 1.0 g) was added to a 100-ml portion of prepared hydrochloric acid (0.1 N) and the suspension was shaken for 48 h at 257C. Acidity was determined by titrating with prepared aqueous sodium hydroxide (0.1 N). Adsorption Isotherms of CFC and the CFC Replacements The adsorption isotherms of CFC and CFC replacements were measured using a previously described method (5). RESULTS AND DISCUSSION
Specific Surface Area, Pore Size Distribution, and Polarity of Activated Carbons The specific surface area, the total pore volume, the base or acid consumptions, and the amount of polarity groups per unit specific surface area of the untreated activated carbon (U-AC) and surface-modified activated carbons (HT-AC and NT-AC) are shown in Table 1. The larger surface area of HT-AC compared to U-AC or NT-AC caused strong activation. The base consumption of NT-AC and U-AC was two and nine times larger than that of HT-AC, respectively. It is assumed that the decreasing total pore volume of NT-AC caused the pores to fill with new oxidic groups, while the increasing pore volume of HT-AC decreased the oxidic groups in the pores of the activated carbon. The pore size distribution of U-AC, NT-AC, and HT-AC are shown in
TABLE 1 Specific Surface Area, Total Pore Volume, Base Acid Consumptions, and Amount of Polarity Groups per Unit Surface Area Consumption (mmol/g) Activated carbon
Specific surface area (m2/g)
Total pore volume (ml/g)
Base
Acid
Amount of polarity groups per unit surface area (mmol/cm2)
U-AC NT-AC HT-AC
1035 1052 1122
0.575 0.566 0.616
0.146 0.659 0.080
0.332 0.174 0.432
4.618 7.918 4.563
/ m4585$3914
12-22-95 14:46:47
coida
AP: Colloid
DECOMPOSITION OF CFC REPLACEMENTS
331
FIG. 1. Pore size distribution of surface-modified activated carbons: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
FIG. 3. Adsorption isotherms of HCFC225cb onto surface-modified activated carbons at 207C: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
Fig. 1. The pore size distribution was not changed with surface modification. Therefore, the physical factors of the activated carbon are not significantly changed, while the chemical ones are changed. Base consumption of adsorbents indicated the number of carboxyl and hydroxide groups on the surface of activated carbon (11). The result of base consumption indicated that there are more carboxyl and hydroxide groups on the surface of NT-AC. It is not clear that acid consumption of adsorbents indicate any polarity groups. Acid consumption of activated carbon was larger in the order NT-AC, U-AC, and HT-AC. Therefore, it is assumed that the polarity groups are between the surface of NT-AC and that of HT-AC.
treated activated carbon (U-AC) and the activated carbon treated with nitric acid (NT-AC) or hydrogen gas (HT-AC) are shown in Figs. 2–5, respectively. We already reported the adsorption isotherms of CFC113, HCFC225cb, and 5FP onto the surface-modified activated carbons (5). When considering the adsorption rate at low pressure, the untreated activated carbon and the surface-modified activated carbons were appropriate for the recovery of CFC113 or HCFC225cb and 5FP, respectively. The hydrophobicity of CFC and the hydrophobicity of CFC and the CFC replacement molecules increase in the order 5FP, HCFC141b, HCFC225cb, and CFC113, because the 5FP, HCFC141b or HCFC225cb, and CFC113 molecules contain two hydrogen atoms and one hydroxide group, three hydrogen atoms, and one hydrogen atom, respectively. Hence, it is considered that the amount of CFC113 adsorbed onto HT-AC is the largest. However, the amount of adsorbed CFC113 increased in the order HTAC, NT-AC, and U-AC. HCFC225cb adsorbed less than the equilibrium pressure of 50 Torr in the order NT-AC, U-AC, HT-AC, and NT-AC while HT-AC was the opposite, with more than 50 Torr. The amount of 5FP adsorbed is different between U-AC
Adsorption Isotherms of CFC and CFC Replacements CFC113 has been used as a dry-cleaning detergent, but the Montreal Protocol calls for a CFC113 ban in the 2000 year. CFC113 replacements are currently under development. It is considered that HCFC225cb and 5FP are excellent CFC113 replacements. The adsorption isotherms of CFC113, HCFC225cb, HCFC141b, and 5FP onto the un-
FIG. 2. Adsorption isotherms of CFC113 onto surface-modified activated carbons at 207C: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
/ m4585$3914
12-22-95 14:46:47
coida
FIG. 4. Adsorption isotherms of HCFC141b onto surface-modified activated carbons at 207C: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
AP: Colloid
332
TANADA ET AL.
or HT-AC and NT-AC. It is concluded that the adsorbed amount of CFC and the CFC replacements does not influence the chemical properties of the adsorbents. The recovered CFC and the CFC replacements must be reproduced for energy saving or minimal environmental impact. However, CFC and CFC replacements may be decomposed by the functional groups on the adsorbents’ surface, that is, the phenolic hydroxide and the carboxyl groups of the activated carbon, because only the surface polarity of activated carbon varied. The new modification to the adsorbents’ surface is required for the recovery of CFC and the CFC replacements. Therefore, the degree of decomposition during the recovery of CFC and the CFC replacements was investigated to elucidate their recovery efficiency using U-AC, NT-AC, and HT-AC. Amount of Fluoride and Chloride Ions Produced during Decomposition The atmospheric concentration of CFC and CFC replacements will be extremely small. There are no known adverse environmental impacts associated with these compounds at such low concentrations. The ultimate removal mechanism for all products is incorporation into rain, sea, and cloud water where hydrolysis can take place. We reported (12) that HCFCs used during recovery with the activated carbon decomposed to produce hydrofluoric acid and hydrochloric acid. They were produced owing to the products which corroded stainless steel. Therefore, we considered that the degree of decomposition of the HCFCs have to be elucidated. The C–F bond is very strong compared to the C–Cl and C–H bonds. Therefore, the greater the number of C–F bonds, the more stable the CFC replacements. The number of C–F bonds in CFC113, HCFC225cb, HCFC141b, and 5FP molecules are 3, 5, 1, and 5, respectively. Sato and Nakamura (1) and Edney et al. (2) reported that HCFC141b was decomposed for CClFO, HCl, CO, and CO2 after the
FIG. 5. Adsorption isotherms of 5FP onto surface-modified activated carbons at 207C: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
/ m4585$3914
12-22-95 14:46:47
coida
FIG. 6. Decomposition degree of CFC113 onto surface-modified activated carbons at 1207C: h, fluoride ions; º, chloride ions.
oxidation-decomposition. Izaki et al. (3) reported that 5FP was decomposed to CF2O, CF3CF2CHO, CF3OOOCF3 , HCl, CO, and CO2 . It is considered that HCFC225cb was degraded for CClF2CF2CFO and HCl. However, these decompositions take place under atmospheric conditions. There is no study associated with decomposition during the recovery of the CFC replacements. To elucidate the degree of decomposition during recovery CFC replacements, it is necessary to take into account the relationship between the CFC replacements and the activated carbon, the CFC replacements and water vapor, and the water vapor and the activated carbon. Therefore, we proposed the autoclave method to evaluate the degree of decomposition during the recovery of CFC and CFC replacements. The adsorption of an organic solvent in the liquid phase is closely related to the surface polarity of the activated carbon. Hence, when the CFC replacements, water, and activated carbon were put into an autoclave, the CFC replacements were initially adsorbed on the activated carbon. The CFC replacements then desorbed from the activated carbon with increasing temperature and decomposed. The internal pressure of the 125-ml autoclave during the CFC and CFC replacement decomposition was 5 to 10 kg/ cm2 . Hence, the adsorption of CFC and the CFC replacements onto activated carbon hardly occurred. Since CFC and CFC replacements contain fluoride and chloride atoms, it is assumed that hydrofluoric acid and hydrochloric acid are formed during recovery. The amounts of fluoride and chloride ions produced in the decomposition solution of CFC113, HCFC225cb, HCFC141b, and 5FP are shown in Figs. 6–9, respectively. The degree of decomposition of CFC and the CFC replacements onto U-AC was the largest of all the activated carbons. It is assumed that in the decomposition mechanism of the CFC replacements, when the HCFCs contact the surface of the activated carbon, hydrogen atoms are released, then the produced HCFCs radically decompose at the catalytic sites of the activated carbon. HCFC141b was easier to decompose than HCFC225cb, 5FP, and CFC113, because the number of C-F groups in an HCFC141b mole-
AP: Colloid
DECOMPOSITION OF CFC REPLACEMENTS
333
FIG. 7. Decomposition degree of HCFC225cb onto surface-modified activated carbons at 1207C: h, fluoride ions; º, chloride ions.
FIG. 9. Decomposition degree of 5FP onto surface-modified activated carbons at 1207C: h, fluoride ions.
cule is low. The degree of decomposition of HCFC141b increased in the order of NT-AC, HT-AC, and U-AC. The release of hydrogen atoms was depressed based on the polar groups on the NT-AC surface. Hence, the degree of decomposition of HCFC141b on NT-AC was the smallest of all. That of HCFC225cb and 5FP increased in the order of HTAC, NT-AC, and U-AC. There are many phenolic hydroxide and carboxyl groups on the surface of NT-AC (11). It is considered that since the CFC and CFC replacements did not contact the surface of NT-AC because NT-AC has a hydrophilic surface, the decomposition of CFC and the CFC replacements did not apparently occur. In the case of using U-AC for the recovery, the CFC and CFC replacements tend to decompose to hydrofluoric acid or hydrochloric acid. The use of surface-modified activated carbons was depressed by the decreasing catalysis of activated carbon. The interaction of the NT-AC–CFC or NTAC–CFC replacements was smaller than that of the NTAC–water molecule. Therefore, the recycling of the activated carbon may occur very simply because of the easy desorption. The amount of adsorbed CFC and CFC replacements and the degree of decomposition indicated that U-AC is suitable for the recovery of CFC113, NT-AC is suitable
for that of HCFC141b, and HT-AC is suitable for that of HCFC225cb and 5FP.
FIG. 8. Decomposition degree of HCFC141b onto surface-modified activated carbons at 1207C: h, fluoride ions; º, chloride ions.
/ m4585$3914
12-22-95 14:46:47
coida
Effect of U-AC and HT-AC on the Degree of Decomposition of HCFC225cb HCFC225cb will be used as a detergent in the near future. Hence, the degree of decomposition of HCFC225cb was further investigated to estimate the surface catalysis of the activated carbon. The amount of HCFC225cb adsorbed onto HT-AC was the largest of all the activated carbons and the degree of decomposition of HCFC225cb on HT-AC was the smallest. Therefore, the relationship between the amount of activated carbon (HT-AC and U-AC) and the degree of decomposition was investigated. U-AC or HT-AC (approximately 0.2, 0.4, 0.8, or 1.0 g), distilled water (20 ml), and HCFC225cb (5 ml) were put into an autoclave of 125 ml and the autoclave was incubated for 24 h at 1207C. The mixture was allowed to cool to 257C before being filtered through a funnel covered with filter paper (Toyo, No. 2). The amount of fluoride and chloride ions was determined using the lanthanum alizarin complexone method (8) and the titration method, respectively. This result is shown in
FIG. 10. Effect of U-AC and HT-AC on decomposition degree of HCFC225cb: l, fluoride ions (U-AC); j, chloride ions (U-AC); s, fluoride ions (HT-AC); h, chloride ions (HT-AC).
AP: Colloid
334
TANADA ET AL.
Fig. 10. The amount of chloride ions produced during the decomposition of HCFC225cb on U-AC and HT-AC increased with increasing amounts of activated carbon, while the amount of fluoride ions produced on U-AC decreased. The amount of activated carbon is related to the degree of decomposition. Since the concentration of HCFC225cb is lower in the gaseous phase during recovery, only small portions of HT-AC should be used. It is concluded that: (1) CFC and CFC replacements decomposed during recovery using activated carbon. (2) Judging from the adsorption isotherms and the degree of decomposition of CFC and CFC replacements, the recovery efficiency of CFC113 was the largest with U-AC. That of HCFC225cb or 5FP and HCFC141b was the largest with HT-AC and NT-AC, respectively. (3) It is necessary to take into account the balance between the amount of activated carbon and water vapor during desorption due to the recovery of the CFC replacements. (4) The degree of decomposition of CFC replacements depends on the interaction between the surface of the activated carbon and the CFC replacement molecules.
/ m4585$3914
12-22-95 14:46:47
coida
ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Science Research from the Japan Private School Promotion Foundation and the Environmental Science Program, Environmental Science Institute of Kinki University.
REFERENCES 1. Sato, H., and Nakamura, T., Nippon Kagaku Kaishi, 548 (1991). 2. Edney, E. O., Gay, B. W., and Driscoll, D. J., J. Atomos. Chem. 12, 105 (1991). 3. Izaki, R., Yoshitake, H., and Sato, H., in ‘‘Abstracts of Papers, The Annual Meeting of the Society of Environmental Science, Japan, Tsukuba, November 1992,’’ p. 75. 4. Kinoshita, K., ‘‘Carbon,’’ Chap. 4. Wiley, New York, 1988. 5. Kawasaki, N., Tanada, S., Nakamura, T., and Abe, I., J. Colloid Interface Sci., in press. 6. Aida, T., Higuchi, R., and Niiyama, H., Chem. Lett., 2247 (1990). 7. Okazaki, S., and Kurosaki, A., Chem. Lett., 1901 (1989). 8. Pharmaceutical Society of Japan, ‘‘Eisei Shikenho Tyukai,’’ 4.1.1(21)1, 1990. 9. Pharmaceutical Society of Japan, ‘‘Eisei Shikenho Tyukai,’’ 4.1.1(19)1, 1990. 10. Boehm, H. P., and Voll, M., Carbon 8, 227 (1970). 11. Ogino, K., Adsorption News 4(2), 4 (1990). 12. Tanada, S., Kawasaki, N., Nakamura, T., and Abe, I., Chem. Lett., 143 (1995).
AP: Colloid
177, 329–334 (1996)
0039
Decomposition Degree of Chlorofluorocarbon (CFC) and CFC Replacements during Recovery with Surface-Modified Activated Carbon SEIKI TANADA,* ,1 NAOHITO KAWASAKI,* TAKEO NAKAMURA,*
AND IKUO
ABE†
*Faculty of Pharmaceutical Sciences, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577; and †Osaka Municipal Technical Research Institute, 1-6-50, Morinomiya, Joto-ku, Osaka 536, Japan Received April 7, 1995; accepted June 15, 1995
INTRODUCTION The recovery efficiency of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC113) and three CFC replacements (1,1-dichloro-1-fluoroethane, HCFC141b; 1,3-dichloro-1,1,2,2,3-pentafluoro-propane, HCFC225cb; and 2,2,3,3,3-pentafluoro-1-propanol, 5FP) were investigated on the basis of their degree of decomposition and adsorption isotherms. We prepared activated carbons with various surface polarities to elucidate the recovery efficiency, the amount adsorbed, and the degree of decomposition. A correlation between the physicochemical properties of the activated carbon surface and the amount of CFC or CFC replacements adsorbed was not observed. The amount of CFC113 adsorbed onto untreated activated carbon was the largest of all. That of HCFC225cb adsorbed onto activated carbon treated with hydrogen gas was larger than that adsorbed onto untreated activated carbon and activated carbon treated with 6 N nitric acid. The amount of 5FP and HCFC141b adsorbed on the various activated carbons was not substantial. The degree of decomposition of CFC replacements using the untreated activated carbon except for HCFC225cb was the largest of all. In the case without the activated carbon, that of CFC and the CFC replacements increased in the order 5FP, CFC113 or HCFC225cb, and HCFC141b. These results indicated that the stability of CFC and CFC replacements molecules was controlled by the number of carbon-fluoride groups and/or hydrogen atoms. It is concluded that the recovery of CFC replacements was possible using the surface-modified activated carbons rather than the untreated activated carbon. The degree of decomposition of the CFC replacements during recovery using the activated carbon depends on the relationship between the adsorption site of the surface of the activated carbon and the polarity, hydrophilic site, or hydrophobic site of the CFC replacement molecule. It is assumed that the recovery of CFC replacements using HT-AC decreased the amount of hydrofluoric acid produced. q 1996 Academic Press, Inc.
Key Words: surface modified activated carbon; nitric acid, surface modified activated carbon; hydrogen gas, decomposition degree; Chlorofluorocarbon (CFC), decomposition degree; CFC replacement, fluoride ion; chloride ion.
1
To whom correspondence should be addressed.
One of the most serious global environmental pollution problems is depletion of the ozone layer by chlorofluorocarbons (CFCs). The CFC molecules are comprised of halogens such as chlorine and fluorine. The hydrocarbons which contain a hydrogen atom are decomposed more readily than ones having no hydrogens. Hence, CFC replacements, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), have been developed and are presently produced in large tonnage scale. The CFC replacements are more easily decomposed than CFCs. The oxidation-decomposition of HCFC141b and 5FP were reported by Sato and Nakamura (1) or Edney et al. (2) and Izaki et al. (3), respectively. The ozone depletion potential of the CFC replacements is smaller than that of CFCs, and HFCs contain no chlorine. HCFCs are considerably less harmful toward stratospheric ozone than CFCs. However, HCFCs can transport chlorine atoms into the stratosphere. The global warming potentials of CFC replacements are less than that of CFCs. However, the recovery of CFC replacements, if possible, will lead to substantial energy savings. Activated carbon has been used as an adsorbent in various industrial fields, such as solvent recovery and gas separation, and as a catalyst support material (4). The amount of CFC and CFC replacements adsorbed onto surface-modified activated carbons has been previously reported to occur from the gaseous phase (5). However, the recovered solvent is decomposed to a large extent. The decomposition of chlorocarbons by metal oxidation produces mainly hydrochloric acid and carbon dioxide has been reported (6). Also, many reports on CFC decomposition have noted the oxidation of the compounds over various catalysts including aluminasupported gold (7), metal oxides (6), and so on. Little work on the determination of the degree of decomposition of HCFCs using surface-modified activated carbon has been done so far. In the present study, the decomposition of CFC and CFC replacements is investigated to determine the recovery effi-
329
/ m4585$3914
12-22-95 14:46:47
coida
0021-9797/96 $12.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
AP: Colloid
330
TANADA ET AL.
ciencies using surface-modified activated carbons as catalysis and the decomposition mechanism of CFC replacements during recovery.
follows: the 10-ml aliquot was put into a white porcelain dish and 1 ml of potassium chlorate was added and then titrated with prepared 0.1 N aqueous silver nitrate (9). Base or Acid Consumptions
MATERIALS AND METHODS
Materials Untreated activated carbon (U-AC) produced from nutshells, DIAHOPE 006, was commercially obtained from Mitsubishi Chemical Industries, Ltd. The surface of the activated carbon was modified to be hydrophobic and hydrophilic using hydrogen and 6 N nitric acid, respectively. The hydrophilic activated carbon (NT-AC) was soaked for 1 h in boiling 6 N nitric acid and then washed in distilled water. The solvents used included 1,1,2-trichloro-1,2,2-trifluoroethane (CFC113) (Wako Pure Chemical Industries, Ltd., Osaka, Japan), 1,1-dichloro-1-fluoroethane (HCFC141b) (Asahi Glass Co., Ltd., Tokyo, Japan), 1,3-dichloro1,1,2,2,3-pentafluoropropane (HCFC225cb) (Asahi Glass Co., Ltd., Tokyo, Japan), and 2,2,3,3,3-pentafluoro-1-propanol (5FP) (Nakarai Tesque, Inc., Kyoto, Japan). Degree of Decomposition The amount of fluoride and chloride ions produced was measured to elucidate the degree of decomposition of CFC and the CFC replacements. The degree of decomposition of CFC and the CFC replacements was measured as follows: U-AC (approximately 0.2 g), distilled water (20 ml), and CFC or the CFC replacements (5 ml) were put in an autoclave of 125 ml and the autoclave was incubated for 24 h at 1207C. The mixture was allowed to cool to 257C before being filtered through a funnel covered with filter paper (Toyo, No. 2). The fluoride ion was determined by standard methods of analysis for hygienic chemists, that is, the lanthanum alizarin complexone methods (8). A 10-ml aliquot was put into a 50-ml colored tube and 1 ml of alizarine complexone solution and 20 ml of acetone in the 50-ml colored tube for a total volume of exactly 50 ml. The fluoride ion was measured at a maximum wavelength of 420 nm using a Hitachi U-1100 spectrophotometer (Hitachi, Ltd., Tokyo, Japan). The amount of chloride ions was determined as
Base or acid consumptions of the untreated activated carbon and surface-modified activated carbon were determined using the acid–base titration method (10). Base consumption was measured as follows: Activated carbon (approximately 1.0 g) was added to a 100-ml portion of prepared aqueous sodium hydroxide (0.1 N) and the suspension was shaken for 48 h at 257C. Basicity was determined by titrating with prepared hydrochloric acid (0.1 N). Acid consumption was measured as follows: Activated carbon (approximately 1.0 g) was added to a 100-ml portion of prepared hydrochloric acid (0.1 N) and the suspension was shaken for 48 h at 257C. Acidity was determined by titrating with prepared aqueous sodium hydroxide (0.1 N). Adsorption Isotherms of CFC and the CFC Replacements The adsorption isotherms of CFC and CFC replacements were measured using a previously described method (5). RESULTS AND DISCUSSION
Specific Surface Area, Pore Size Distribution, and Polarity of Activated Carbons The specific surface area, the total pore volume, the base or acid consumptions, and the amount of polarity groups per unit specific surface area of the untreated activated carbon (U-AC) and surface-modified activated carbons (HT-AC and NT-AC) are shown in Table 1. The larger surface area of HT-AC compared to U-AC or NT-AC caused strong activation. The base consumption of NT-AC and U-AC was two and nine times larger than that of HT-AC, respectively. It is assumed that the decreasing total pore volume of NT-AC caused the pores to fill with new oxidic groups, while the increasing pore volume of HT-AC decreased the oxidic groups in the pores of the activated carbon. The pore size distribution of U-AC, NT-AC, and HT-AC are shown in
TABLE 1 Specific Surface Area, Total Pore Volume, Base Acid Consumptions, and Amount of Polarity Groups per Unit Surface Area Consumption (mmol/g) Activated carbon
Specific surface area (m2/g)
Total pore volume (ml/g)
Base
Acid
Amount of polarity groups per unit surface area (mmol/cm2)
U-AC NT-AC HT-AC
1035 1052 1122
0.575 0.566 0.616
0.146 0.659 0.080
0.332 0.174 0.432
4.618 7.918 4.563
/ m4585$3914
12-22-95 14:46:47
coida
AP: Colloid
DECOMPOSITION OF CFC REPLACEMENTS
331
FIG. 1. Pore size distribution of surface-modified activated carbons: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
FIG. 3. Adsorption isotherms of HCFC225cb onto surface-modified activated carbons at 207C: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
Fig. 1. The pore size distribution was not changed with surface modification. Therefore, the physical factors of the activated carbon are not significantly changed, while the chemical ones are changed. Base consumption of adsorbents indicated the number of carboxyl and hydroxide groups on the surface of activated carbon (11). The result of base consumption indicated that there are more carboxyl and hydroxide groups on the surface of NT-AC. It is not clear that acid consumption of adsorbents indicate any polarity groups. Acid consumption of activated carbon was larger in the order NT-AC, U-AC, and HT-AC. Therefore, it is assumed that the polarity groups are between the surface of NT-AC and that of HT-AC.
treated activated carbon (U-AC) and the activated carbon treated with nitric acid (NT-AC) or hydrogen gas (HT-AC) are shown in Figs. 2–5, respectively. We already reported the adsorption isotherms of CFC113, HCFC225cb, and 5FP onto the surface-modified activated carbons (5). When considering the adsorption rate at low pressure, the untreated activated carbon and the surface-modified activated carbons were appropriate for the recovery of CFC113 or HCFC225cb and 5FP, respectively. The hydrophobicity of CFC and the hydrophobicity of CFC and the CFC replacement molecules increase in the order 5FP, HCFC141b, HCFC225cb, and CFC113, because the 5FP, HCFC141b or HCFC225cb, and CFC113 molecules contain two hydrogen atoms and one hydroxide group, three hydrogen atoms, and one hydrogen atom, respectively. Hence, it is considered that the amount of CFC113 adsorbed onto HT-AC is the largest. However, the amount of adsorbed CFC113 increased in the order HTAC, NT-AC, and U-AC. HCFC225cb adsorbed less than the equilibrium pressure of 50 Torr in the order NT-AC, U-AC, HT-AC, and NT-AC while HT-AC was the opposite, with more than 50 Torr. The amount of 5FP adsorbed is different between U-AC
Adsorption Isotherms of CFC and CFC Replacements CFC113 has been used as a dry-cleaning detergent, but the Montreal Protocol calls for a CFC113 ban in the 2000 year. CFC113 replacements are currently under development. It is considered that HCFC225cb and 5FP are excellent CFC113 replacements. The adsorption isotherms of CFC113, HCFC225cb, HCFC141b, and 5FP onto the un-
FIG. 2. Adsorption isotherms of CFC113 onto surface-modified activated carbons at 207C: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
/ m4585$3914
12-22-95 14:46:47
coida
FIG. 4. Adsorption isotherms of HCFC141b onto surface-modified activated carbons at 207C: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
AP: Colloid
332
TANADA ET AL.
or HT-AC and NT-AC. It is concluded that the adsorbed amount of CFC and the CFC replacements does not influence the chemical properties of the adsorbents. The recovered CFC and the CFC replacements must be reproduced for energy saving or minimal environmental impact. However, CFC and CFC replacements may be decomposed by the functional groups on the adsorbents’ surface, that is, the phenolic hydroxide and the carboxyl groups of the activated carbon, because only the surface polarity of activated carbon varied. The new modification to the adsorbents’ surface is required for the recovery of CFC and the CFC replacements. Therefore, the degree of decomposition during the recovery of CFC and the CFC replacements was investigated to elucidate their recovery efficiency using U-AC, NT-AC, and HT-AC. Amount of Fluoride and Chloride Ions Produced during Decomposition The atmospheric concentration of CFC and CFC replacements will be extremely small. There are no known adverse environmental impacts associated with these compounds at such low concentrations. The ultimate removal mechanism for all products is incorporation into rain, sea, and cloud water where hydrolysis can take place. We reported (12) that HCFCs used during recovery with the activated carbon decomposed to produce hydrofluoric acid and hydrochloric acid. They were produced owing to the products which corroded stainless steel. Therefore, we considered that the degree of decomposition of the HCFCs have to be elucidated. The C–F bond is very strong compared to the C–Cl and C–H bonds. Therefore, the greater the number of C–F bonds, the more stable the CFC replacements. The number of C–F bonds in CFC113, HCFC225cb, HCFC141b, and 5FP molecules are 3, 5, 1, and 5, respectively. Sato and Nakamura (1) and Edney et al. (2) reported that HCFC141b was decomposed for CClFO, HCl, CO, and CO2 after the
FIG. 5. Adsorption isotherms of 5FP onto surface-modified activated carbons at 207C: l — l, U-AC; j — j, NT-AC; m — m, HT-AC.
/ m4585$3914
12-22-95 14:46:47
coida
FIG. 6. Decomposition degree of CFC113 onto surface-modified activated carbons at 1207C: h, fluoride ions; º, chloride ions.
oxidation-decomposition. Izaki et al. (3) reported that 5FP was decomposed to CF2O, CF3CF2CHO, CF3OOOCF3 , HCl, CO, and CO2 . It is considered that HCFC225cb was degraded for CClF2CF2CFO and HCl. However, these decompositions take place under atmospheric conditions. There is no study associated with decomposition during the recovery of the CFC replacements. To elucidate the degree of decomposition during recovery CFC replacements, it is necessary to take into account the relationship between the CFC replacements and the activated carbon, the CFC replacements and water vapor, and the water vapor and the activated carbon. Therefore, we proposed the autoclave method to evaluate the degree of decomposition during the recovery of CFC and CFC replacements. The adsorption of an organic solvent in the liquid phase is closely related to the surface polarity of the activated carbon. Hence, when the CFC replacements, water, and activated carbon were put into an autoclave, the CFC replacements were initially adsorbed on the activated carbon. The CFC replacements then desorbed from the activated carbon with increasing temperature and decomposed. The internal pressure of the 125-ml autoclave during the CFC and CFC replacement decomposition was 5 to 10 kg/ cm2 . Hence, the adsorption of CFC and the CFC replacements onto activated carbon hardly occurred. Since CFC and CFC replacements contain fluoride and chloride atoms, it is assumed that hydrofluoric acid and hydrochloric acid are formed during recovery. The amounts of fluoride and chloride ions produced in the decomposition solution of CFC113, HCFC225cb, HCFC141b, and 5FP are shown in Figs. 6–9, respectively. The degree of decomposition of CFC and the CFC replacements onto U-AC was the largest of all the activated carbons. It is assumed that in the decomposition mechanism of the CFC replacements, when the HCFCs contact the surface of the activated carbon, hydrogen atoms are released, then the produced HCFCs radically decompose at the catalytic sites of the activated carbon. HCFC141b was easier to decompose than HCFC225cb, 5FP, and CFC113, because the number of C-F groups in an HCFC141b mole-
AP: Colloid
DECOMPOSITION OF CFC REPLACEMENTS
333
FIG. 7. Decomposition degree of HCFC225cb onto surface-modified activated carbons at 1207C: h, fluoride ions; º, chloride ions.
FIG. 9. Decomposition degree of 5FP onto surface-modified activated carbons at 1207C: h, fluoride ions.
cule is low. The degree of decomposition of HCFC141b increased in the order of NT-AC, HT-AC, and U-AC. The release of hydrogen atoms was depressed based on the polar groups on the NT-AC surface. Hence, the degree of decomposition of HCFC141b on NT-AC was the smallest of all. That of HCFC225cb and 5FP increased in the order of HTAC, NT-AC, and U-AC. There are many phenolic hydroxide and carboxyl groups on the surface of NT-AC (11). It is considered that since the CFC and CFC replacements did not contact the surface of NT-AC because NT-AC has a hydrophilic surface, the decomposition of CFC and the CFC replacements did not apparently occur. In the case of using U-AC for the recovery, the CFC and CFC replacements tend to decompose to hydrofluoric acid or hydrochloric acid. The use of surface-modified activated carbons was depressed by the decreasing catalysis of activated carbon. The interaction of the NT-AC–CFC or NTAC–CFC replacements was smaller than that of the NTAC–water molecule. Therefore, the recycling of the activated carbon may occur very simply because of the easy desorption. The amount of adsorbed CFC and CFC replacements and the degree of decomposition indicated that U-AC is suitable for the recovery of CFC113, NT-AC is suitable
for that of HCFC141b, and HT-AC is suitable for that of HCFC225cb and 5FP.
FIG. 8. Decomposition degree of HCFC141b onto surface-modified activated carbons at 1207C: h, fluoride ions; º, chloride ions.
/ m4585$3914
12-22-95 14:46:47
coida
Effect of U-AC and HT-AC on the Degree of Decomposition of HCFC225cb HCFC225cb will be used as a detergent in the near future. Hence, the degree of decomposition of HCFC225cb was further investigated to estimate the surface catalysis of the activated carbon. The amount of HCFC225cb adsorbed onto HT-AC was the largest of all the activated carbons and the degree of decomposition of HCFC225cb on HT-AC was the smallest. Therefore, the relationship between the amount of activated carbon (HT-AC and U-AC) and the degree of decomposition was investigated. U-AC or HT-AC (approximately 0.2, 0.4, 0.8, or 1.0 g), distilled water (20 ml), and HCFC225cb (5 ml) were put into an autoclave of 125 ml and the autoclave was incubated for 24 h at 1207C. The mixture was allowed to cool to 257C before being filtered through a funnel covered with filter paper (Toyo, No. 2). The amount of fluoride and chloride ions was determined using the lanthanum alizarin complexone method (8) and the titration method, respectively. This result is shown in
FIG. 10. Effect of U-AC and HT-AC on decomposition degree of HCFC225cb: l, fluoride ions (U-AC); j, chloride ions (U-AC); s, fluoride ions (HT-AC); h, chloride ions (HT-AC).
AP: Colloid
334
TANADA ET AL.
Fig. 10. The amount of chloride ions produced during the decomposition of HCFC225cb on U-AC and HT-AC increased with increasing amounts of activated carbon, while the amount of fluoride ions produced on U-AC decreased. The amount of activated carbon is related to the degree of decomposition. Since the concentration of HCFC225cb is lower in the gaseous phase during recovery, only small portions of HT-AC should be used. It is concluded that: (1) CFC and CFC replacements decomposed during recovery using activated carbon. (2) Judging from the adsorption isotherms and the degree of decomposition of CFC and CFC replacements, the recovery efficiency of CFC113 was the largest with U-AC. That of HCFC225cb or 5FP and HCFC141b was the largest with HT-AC and NT-AC, respectively. (3) It is necessary to take into account the balance between the amount of activated carbon and water vapor during desorption due to the recovery of the CFC replacements. (4) The degree of decomposition of CFC replacements depends on the interaction between the surface of the activated carbon and the CFC replacement molecules.
/ m4585$3914
12-22-95 14:46:47
coida
ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Science Research from the Japan Private School Promotion Foundation and the Environmental Science Program, Environmental Science Institute of Kinki University.
REFERENCES 1. Sato, H., and Nakamura, T., Nippon Kagaku Kaishi, 548 (1991). 2. Edney, E. O., Gay, B. W., and Driscoll, D. J., J. Atomos. Chem. 12, 105 (1991). 3. Izaki, R., Yoshitake, H., and Sato, H., in ‘‘Abstracts of Papers, The Annual Meeting of the Society of Environmental Science, Japan, Tsukuba, November 1992,’’ p. 75. 4. Kinoshita, K., ‘‘Carbon,’’ Chap. 4. Wiley, New York, 1988. 5. Kawasaki, N., Tanada, S., Nakamura, T., and Abe, I., J. Colloid Interface Sci., in press. 6. Aida, T., Higuchi, R., and Niiyama, H., Chem. Lett., 2247 (1990). 7. Okazaki, S., and Kurosaki, A., Chem. Lett., 1901 (1989). 8. Pharmaceutical Society of Japan, ‘‘Eisei Shikenho Tyukai,’’ 4.1.1(21)1, 1990. 9. Pharmaceutical Society of Japan, ‘‘Eisei Shikenho Tyukai,’’ 4.1.1(19)1, 1990. 10. Boehm, H. P., and Voll, M., Carbon 8, 227 (1970). 11. Ogino, K., Adsorption News 4(2), 4 (1990). 12. Tanada, S., Kawasaki, N., Nakamura, T., and Abe, I., Chem. Lett., 143 (1995).
AP: Colloid