Adsorption of water vapor on the AlPO4-based catalysts and reaction mechanism for CFCs decomposition

Applied Catalysis A: General 271 (2004) 55–60

Adsorption of water vapor on the AlPO4 -based catalysts and reaction mechanism for CFCs decomposition Yusaku Takita a,∗ , Jun-Ichi Moriyama a , Yusuke Yoshinaga a , Hiroyasu Nishiguchi a , Tatsumi Ishihara a , Sachio Yasuda b , Yusuke Ueda c , Momoji Kubo c , Akira Miyamoto c a

Department of Applied Chemistry, Faculty of Engineering, Oita University, Showa Denko, Oita Works, Dannoharu, Oita 870-1192, Japan b Department of Environmental and Chemical Engineering, Nippon Bunri University, Ichiki, Oita 870-0316, Japan c Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan Received in revised form 28 January 2004; accepted 1 February 2004

Abstract Reversibly and irreversibly adsorbed water on AlPO4 and CePO4 –AlPO4 (Ce/Al = 1/9) were determined by a TPD method and an electro-balance. The amounts of reversible and irreversible adsorption over AlPO4 were larger than that of CePO4 –AlPO4 . However, the surface concentration of surface hydroxyls on AlPO4 was smaller than that on CePO4 –AlPO4 . Catalytic activity for CCl2 F2 decomposition of AlPO4 is smaller than that of CePO4 –AlPO4 . To explain these results, a surface intermediate, Osurface –CF2 –Osurface , is proposed. The stabilization energy was calculated for the surface species; CCl2 F2 interacting with surface hydroxyls and the O–CX2 –O (X = Cl or F) type surface species. The calculations suggest that P–Osurface H· · · Cl–CF2 –Cl· · · HOsurface –P species and Al–Osurface –CF2 –Osurface –Al species are the most stable. To clarify the reaction mechanism, trace amounts of by-products in the decomposition of CH2 FCF3 which is hydrofluorocarbon with two carbon atoms were analyzed and found the formation of HO–CH2 CF2 –OH which may be derived from the surface intermediate, Osurface –CH2 CF2 –Osurface . This strongly supports the formation of bidentate surface intermediates. © 2004 Elsevier B.V. All rights reserved. Keywords: Adsorption of water vapor; Surface intermediate; Reaction mechanism; Concentration of surface hydroxyls; CFCs decomposition; Aluminum phosphate

1. Introduction So far, it has been found to explore the effective catalysts for CFCs decomposition. A number of catalysts including metal oxides, activated charcoal, various zeolites, and supported noble metal [1–6] have been reported. Special catalysts such as CuO/CuSO4 [7], BPO4 [8–10], and FeCl3 /active charcoal [11], are also reported. Decomposition of CFCs is generally accompanied by the formation of hydrogen halides, which are strong acids. HF is especially a corrosive acid and reacts with the components of catalysts to produce fluorides, which lead to deactivation. In the previous papers [12–15], the authors reported that AlPO4 has high decomposition activity, high CO2 selec-

∗

Corresponding author. Tel.: +81 975 54 7894; fax: +81 975 54 7979. E-mail address: [email protected] (Y. Takita).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.02.046

tivity, and extremely long catalyst life. For example, when the feed gas was composed of 0.5 mol% CCl2 F2 , about 50 mol% water vapor and air, CCl2 F2 is converted completely at 450 ◦ C. Hydrolysis is estimated for the essential reaction from two evidences. Decomposition rate was dependent on the pressure of water vapor but not the oxygen pressure, and the G changes for hydrolysis of CCl2 F2 is negative in contrast to the positive value for oxidative decomposition. Catalytic activity was promoted by the addition of 10 at.% of Ce to AlPO4 . Participation of acidic sites was estimated in many studies, however, the addition of Ce to AlPO4 decreases the acidity. This suggests that a factor other than acidity is important in this reaction. Therefore, authors focus on the hydroxyls on the surface of catalysts. The authors reported on water adsorption behavior of AlPO4 -based catalysts and the reaction mechanism for the decomposition of CCl2 F2 in this paper.

Y. Takita et al. / Applied Catalysis A: General 271 (2004) 55–60

2. Experimental 2.1. Catalyst preparation Aluminum phosphate catalysts were prepared by the precipitation method described below. An aqueous solution of 10 wt.% ammonium hydroxide was slowly added into an aqueous solution composed of stoichiometric amounts of aluminum nitrate (0.5 M) and 85% ortho phosphoric acid with stirring until the pH of the solution reached 4.5. The precipitate obtained was washed well with pure water and filtrated. The powder obtained was pressed into cylindrical form, crushed, and sieved into 14–32 mesh granules and finally it was calcined at 1000 ◦ C for 5 h in air. Ce added AlPO4 catalyst was prepared using a solution containing Ce nitrate together with Al nitrate and phosphoric acid. The catalyst containing Ce was determined to a mixture of CePO4 and AlPO4 from an analysis by XRD (Rigaku RINT-2500HF system). The so formed catalyst was denoted as CePO4 –AlPO4 (Ce/Al = 1/9) in this paper. Specific surface area of the AlPO4 and CePO4 –AlPO4 (Ce/Al = 1/9) are 100 and 42 m2 /g, respectively. 2.2. Measurement of the adsorption of water vapor on the catalysts

3. Results and discussion 3.1. Irreversible and reversible adsorption of water vapor on AlPO4 -based catalysts and the catalytic activities for CCl2 F2 decomposition Usually, irreversibly adsorbed water on the catalysts can be evaluated by the TPD method. TPD of water vapor adsorbed on AlPO4 and CePO4 –AlPO4 (Ce/Al = 1/9) were measured. CePO4 –AlPO4 (Ce/Al = 1/9) is an AlPO4 catalyst promoted by Ce, which is a mixture of CePO4 and AlPO4 . The surface concentrations of Ce and Al of the catalyst are determined to be 1–9 by XPS suggesting that the surface composition is the same as that of the bulk. TPD curves of water vapor from the catalysts are shown in Fig. 1. Both TPD curves are composed of single peak with a peak top at 350 ◦ C and have a tail at higher temperatures. Desorption of water vapor continued to 800 ◦ C, but desorption rate was very small. The irreversibly adsorbed water amounted to 76.0 ␮mol/g for AlPO4 . The amount of hydroxyls remained on the surface of the AlPO4 catalyst at various temperatures can be calculated from the TPD spectrum. The results are shown in Table 1. A high concentration of water vapor (about 55 mol%) is present in the decomposition of CCl2 F2 . So both reversibly and irreversibly adsorbed water should be present on the catalyst surface during the decomposition of CCl2 F2 . Therefore, both reversibly and irreversibly adsorbed water on the AlPO4 -based catalysts was determined using an electro-balance. The results are shown in Fig. 2. The enthalpy of adsorption of water vapor for AlPO4 and CePO4 –AlPO4 are calculated to 11.4 and 16.7 kJ/mol from the data in Fig. 2, respectively. Adsorption of water on AlPO4 is stronger than that on CePO4 –AlPO4 . 60

0

50 -10

(a)

40

(b)

30

-20 -30

20

-40 -50

200

300

(a)

10

(b)

0

400 500 600 Temperature(ºC )

700

800

Rate of desorption ( a.u.)

The total amounts of reversible and irreversible adsorption of water vapor were measured using an electro-balance, Bell FMS-␮-LP250. About 2 g of the samples are used. Liquid water was fed with an evaporator/mixer located at the entrance of the electro-balance using a micro-liquid-pump and mixed with He. The amount of irreversibly adsorbed water vapor was determined by the TPD method. After the evacuation at 800 ◦ C for 0.5 h, the sample was cooled to 200 ◦ C in vacuum and a sufficient amount of a mixture of water vapor and He was introduced with a circulation system to the sample and the gas was circulated for 0.5 h. Then the mixed gas on the sample was flushed with a He stream and TPD was measured in a He flow (40 cm3 /min) with a rate of 10 ◦ C/min after base line became stable. Catalytic reactions were carried out under an atmospheric pressure using a continuous flow reaction system with a fixed-catalyst-bed in the reactor as used by Takita et al. [14]. The reaction conditions are as follows. The 4.50 g of catalyst was used. Feed gas was composed of 0.5 mol% CCl2 F2 , 7.5 mol% O2 , 57.6 mol% H2 O, balance N2 and the feed rate was 40.0 cm3 /min. The gas effluent from the reactor was slightly washed with distilled water to remove HCl and HF. Analysis of the reaction products was carried out using Shimadzu GC-8ATP gas chromatographs (thermal conductivity detector, TCD) with a Porapak Q column (4 mm i.d. × 7 m) and a molecular sieve 5A (4 mm i.d. × 3 m) column and JEOL JMS-AMII 150 Mass Spectrometer. X-ray diffraction (XRD) patterns were obtained by a Rigaku RINT 2500HF system. The specific surface areas

of the fresh and used catalysts were determined by BET method (N2 adsorption) using a Carlo Erba SORPTY-1750 analyzer.

Weight loss (%)

56

900

Fig. 1. TPD curves of water vapor desorbed from AlPO4 and CePO4 – AlPO4 : (a) AlPO4 and (b) CePO4 –AlPO4 .

Y. Takita et al. / Applied Catalysis A: General 271 (2004) 55–60

57

Table 1 Amount of water adsorbed on AlPO4 and CePO4 –AlPO4 (Ce/Al = 1/9)

AlPO4 (105 m2 /g) Total (␮mol/g) Reversible Irreversible Total (␮mol/m2 ) Reversible Irreversible OH coverage

250 ◦ C

300 ◦ C

350 ◦ C

400 ◦ C

450 ◦ C

500 ◦ C

425 349 76.0

280 211 69.5

192 133 58.9

141 91.7 49.6

107 66.5 40.0

78.2 45.5 32.7

4.04 3.32 0.72 46.0

AlPO4 –CePO4 (Ce/Al = 1/9) (42 m2 /g) Total (␮mol/g) 205 Reversible 168 Irreversible 36.5 Total (␮mol/m2 ) Reversible Irreversible OH coverage

4.87 4.00 0.87 55.5

2.67 2.01 0.66 30.4 132 98.8 33.5 3.15 2.35 0.80 35.9

The adsorbed water determined by an electro-balance is the total of the irreversibly and reversibly adsorbed water and the adsorbed amount determined by TPD is the irreversibly adsorbed water. So the amount of reversibly adsorbed water was determined by subtracting the amounts determined by TPD from the amounts determined by an electro-balance. The amount at each temperature are summarized in Table 1. The proportion of the reversible adsorption (82–75% at 250–300 ◦ C) was similar for both AlPO4 and CePO4 –AlPO4 catalysts. It increased with increasing the temperature for both catalysts, however, 67.4–64.8% for CePO4 –AlPO4 is higher than 62.6–58.2% for AlPO4 at 450–500 ◦ C. The amount of adsorbed water on AlPO4 per unit weight is larger than that on CePO4 –AlPO4 in accordance with their surface areas. 3.2. Surface hydroxyl concentration and reaction mechanism for CCl2 F2 decomposition The amounts of the reversible and irreversible adsorption of water vapor on the both catalysts are shown in Table 1.

1.82 1.26 0.56 20.7 101 72.3 28.2 2.39 1.72 0.67 27.2

1.34 0.87 0.47 15.3

1.01 0.63 0.38 11.5

0.74 0.43 0.31 8.43

79.5 55.7 23.8

60.8 41.0 19.8

45.7 29.6 16.1

1.90 1.33 0.57 21.6

1.45 0.98 0.47 16.5

1.08 0.70 0.38 12.3

Total amount of surface hydroxyls on AlPO4 are larger than that on CePO4 –AlPO4 on a weight basis. Catalytic activities for both catalysts are shown in Table 2. Since catalytic reactions were carried out using the same amount (4.50 g) of catalysts, it is clear from the conversion in Table 2 that catalytic activity of Ce promoted catalysts is higher than that of AlPO4 . As reported in the previous paper [14], the absence or presence of water vapor in the gas phase strongly affects the catalytic activity, however, the rate of decomposition is not affected by the concentration of water vapor in the concentration of about 55 mol%. Since the decomposition rate depends on the CCl2 F2 concentration, the rate determining step is adsorption of CCl2 F2 or surface reaction of the surface intermediates derived from CCl2 F2 . If one surface hydroxyl interacts with CCl2 F2 molecule to form a surface intermediate, the reaction rate over AlPO4 should be larger than that over CePO4 –AlPO4 as opposed to the observed reaction rates. The concentration of water vapor on the surface of the catalysts are also shown in Table 1. As can be seen from the table, the concentration of surface hydroxyls of CePO4 –AlPO4 is higher than that of AlPO4 . This

Fig. 2. Adsorption isotherm of water vapor on AlPO4 and CePO4 –AlPO4 : (a) AlPO4 and (b) CePO4 –AlPO4 .

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Y. Takita et al. / Applied Catalysis A: General 271 (2004) 55–60

Table 2 Rates of CCl2 F2 decomposition Temperature (◦ C)

Catalyst

Conversion (%)

Formation rate ␮mol/g min

␮mol/g m2

AlPO4

300 375

0.172 26.4

0.0078 1.19

7.76 × 10−5 0.0119

CePO4 –AlPO4 (Ce/Al = 1/9)

300 375

0.344 86.2

0.0155 3.89

3.69 × 10−4 0.0926

Catalyst: 4.50 g; feed rate: 0.5 cm3 /min CCl2 F2 , 23.0 cm3 /min H2 O(g) , 3.3 cm3 /min O2 , balance N2 .

is well agreed with the adsorption enthalpy of the catalysts. How does the surface concentration of water vapor explain the catalytic activity? If two surface hydroxyls interact with CCl2 F2 molecule to form bidentate surface intermediates, the observed reaction rates can be explained rationally. Authors tentatively propose a reaction mechanism for CCl2 F2 decomposition as shown in Scheme 1. In the initial stage of the reaction, CCl2 F2 molecule would interact with surface hydroxyl groups to form a surface intermediate I (species I). Species I is dehydrohalogenated to form a bidentate surface species II. Then the C–Osurface bond of the bidentate species II is hydrolyzed to form an unidentate intermediate with hydroxyl group, HOCF2 –Osurface . Succeeding hydrolysis and dehydrohalogenation of the species lead to CO2 formation. Unidentate, bidentate, and tridentate surface intermediates can be considered for the intermediate I of the decomposition. The stabilization energies for various intermediates on the surface of ALPO-5 were calculated using DMol program by MSI Co., although the crystal structure is different from

that of AlPO4 . The calculation shows that bidentate intermediates are most stable. The calculated stabilization energies for various bidentate intermediates are shown in Table 3. The data only contains intermediates which are bound to the same kind of hydroxyls, AlOH or POH. As can be seen from Table 3, the intermediates adsorbed on POH are more stabilized than that on AlOH. Therefore, CCl2 F2 molecule may interact with surface hydroxyls of PO4 units. The intermediates that two Cl atoms interacted with the surface hydroxyls seem to be more stable than others. Species I is transformed into species II by releasing hydrogen halogenides. Therefore, the stabilization energies of various types of surface species II were calculated and the results are shown in Table 4. According to the results, CF2 bound to two O atoms attached to Al, Al–Osurface – CF2 –Osurface –Al, is most stable. Therefore, HCl molecules are eliminated from the P–Osurface H· · · Cl–CF2 –Cl· · · HOsurface –P species to form the Al–Osurface –CF2 –Osurface –Al species. The calculations of stabilization energy were car-

Scheme 1. Proposed reaction mechanism for CCl2 F2 decomposition over AlPO4 -based catalysts.

Y. Takita et al. / Applied Catalysis A: General 271 (2004) 55–60

59

Table 3 Calculated stabilization energy for adsorption of CCl2 F2 on ALPO-5

Table 4 Calculated stabilization energy for surface species formed on the ALPO-5

Surface hydroxyl

Surface species

Stabilization energy (kJ/mol)

Partial charge on CCl2 F2

Stabilization energy (kJ/mol)

+349 −13.3

0.03

+397 −13.5

0.01 –

−14.1

0.02

−25.9

0.06

−27.2

+12.5

+49.2 −36.3

−44.8

0.11

0.15

ried out for the species on the ALPO-5, however, the situation for the surface species on the aluminum phosphate catalyst would be similar to those on the ALPO-5. A more detailed calculation is needed to discuss the reaction mechanism, however, it seems to be probable that CCl2 F2 interacts with surface hydroxyls through Cl atoms and then Cl atoms are removed from the CCl2 F2 molecule. The surface species II may successively react with surface hydroxyls and lead to the production of CO2 . 3.3. Detection of intermediate compounds in the decomposition of CH2 FCF3 over CePO4 –AlPO4 In the previous section, authors proposed a reaction mechanism for CCl2 F2 decomposition. Analysis of tracer amounts of by-products in the decomposition is often helpful to explore the reaction mechanism. According to Scheme 1, –Osurface –CF2 –Osurface – species will be formed on the surface in the decomposition of CCl2 F2 . If the

hydrolysis of C–Osurface bonds proceeds, the product will be F2 C(OH)2 . But F2 C(OH)2 is not stable therefore the detection of this compound is difficult. However, if decomposition of CH2 FCF3 is proceeded by a similar reaction mechanism, –Osurface –CH2 –CF2 –Osurface – would be formed as a surface species II, then the stable HO–CH2 –CF2 –OH molecule may be obtained by the hydrolysis of the –Osurface –CH2 –CF2 –Osurface – species. So the decomposition of CH2 FCF3 was carried out and the analysis of tracer amounts of by-products was carried out. The experiment was conducted by a smaller W/F (4.28 × 10−4 g h/cm3 ) than the standard reaction conditions (1.87 × 10−4 g h/cm3 ) for the decomposition of CCl2 F2 . All the products of the decomposition at 375 ◦ C were collected in a cold trap at liquid nitrogen temperature for 14 h. Then the collected sample was warmed to ambient temperature to release gaseous products. The remaining compounds were analyzed with GC–MS. Fig. 3 is a chromatogram of the liquid products. A small peak was observed at 22.7 min and it differs from air, CO2 , and CH2 FCF3 . Mass spectral data for this peak is shown in Fig. 4. The mass numbers of the signal are 81, 47, 96, 44, 40, 77, 82, 80, 110 and 83 in the order of signal intensity. The signals at m/e 82, 81 and 80 are attributed to C2 H4 F2 O, C2 H3 F2 O and C2 H2 F2 O, and the signal at m/e 96 to C2 H2 F2 O2 . The signal at m/e 77 can be identified to C2 H2 FO2 , 69 to CF3 , 67 and 66 to

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Y. Takita et al. / Applied Catalysis A: General 271 (2004) 55–60

ever, only significantly small peaks at m/e 97 and 98 appeared. This may suggest that CF3 COOH, CF3 CHO, CF2 HCOOH, and CF2 HCHO were not formed in the reaction. Small peaks at 91 and 83 may be attributed to C3 HF2 O and C2 H2 F3 . All the intense peaks can be derived from C2 H4 F2 O2 . It can be concluded from these results that HO– CH2 CF2 OH is formed in the decomposition of CH2 FCF3 . This may be a strong support for the proposed reaction mechanism.

References

Fig. 3. GC chromatogram of a collected liquid sample.

Fig. 4. Mass spectral data of a product in the decomposition of CH2 FCF3 over CePO4 –AlPO4 .

CHF2 O and CF2 O, 77 to C2 H2 FO2 , 47 to CFO and 44 to CO2 , respectively. If CF3 COOH is formed, it should give peaks at 114 and 97, similarly 95 and 78 for CF2 HCOOH, 98 and 78 for CF3 CHO, 79 and 78 for CF2 HCHO. How-

[1] S. Okazaki, A. Kurosaki, Chem. Lett. (1989) 1901. [2] T. Aida, R. Higuchi, H. Niiyama, Chem. Lett. (1990) 2247. [3] T. Aida, R. Higuchi, H. Niiyama, Kagaku Kougaku Ronbunsyu 17 (1991) 943. [4] T. Takakura, H. Nagata, K. Mizuno, Y. Tamori, K. Wakabayashi, Proceedings of the Annual Meeting of Chemical Engineering, Japan, Preprint No. P298, 1992. [5] H. Nagata, T. Takakura, S. Tashiro, M. Kishida, K. Mizuno, Y. Tamori, K. Wakabayashi, Sekiyu Gakkaishi 37 (1994) 209. [6] H. Nagata, T. Takakura, S. Kishida, K. Misuno, Y. Tamori, K. Wakabayashi, Chem. Lett. (1993) 1545. [7] E. Jacob, Chem. Abstr. 113 (1990) 196942c. [8] S. Imamura, T. Shiomi, S. Ishihara, K. Utani, Ind. Eng. Chem. Res. 29 (1990) 1758. [9] S. Imamura, K. Imakubo, Y. Fujimura, Nippon Kagaku Kaishi 645 (1991). [10] S. Imamura, T. Ikeda, S. Ishida, Nippon Kagaku Kaishi 139 (1989). [11] D. Miyatani, K. Shinoda, T. Nakamura, M. Ohta, K. Yasuda, Chem. Lett. (1992) 795. [12] Y. Takita, T. Ishihara, Catal. Surv. Jpn. 2 (2) (1998) 165. [13] Y. Takita, G.-L. Li, R. Matsuzaki, H. Wakamatsu, H. Nishiguchi, Y. Moro-oka, T. Ishihara, Chem. Lett. (1997) 13. [14] Y. Takita, M. Ninomiya, R. Matsuzaki, H. Wakamatsu, H. Nishiguchi, T. Ishihara, Phys. Chem. Chem. Phys. 1 (1999) 4501. [15] Y. Takita, H. Wakamatsu, G.-L. Li, Y. Moro-oka, H. Nishiguchi, T. Ishihara, J. Mol. Catal. A 155 (1999) 111.