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Highly selective vapor phase Beckmann rearrangement over H-USY zeolites
Applied Catalysis A: General 189 (1999) 237–242
Highly selective vapor phase Beckmann rearrangement over H-USY zeolites Lian-Xin Dai1 , Katsuyuki Koyama, Mitsunori Miyamoto, Takashi Tatsumi ∗ Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan
Abstract The vapor Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam catalyzed by H-USY with different SiO2 / Al2 O3 ratios was studied. The catalytic performance of H-USY zeolites for the highly selective formation of ε-caprolactam was greatly improved by adjusting the SiO2 /Al2 O3 ratio and by using an appropriate diluent. It has been found that H-USY catalysts with SiO2 /Al2 O3 ratios of 27 and 62 exhibited excellent catalytic activity, selectivity and stability when 1-hexanol was used as diluent. The NH3 -TPD results indicate that an appropriate amount of relatively weak acid sites of H-USY zeolites are effective for highly selective Beckmann rearrangement. The interaction of alcohols with H-USY zeolites has been investigated by FT-IR to explain the improving effect of 1-hexanol. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Beckmann rearrangement; Acidity; USY zeolite; Diluent effect; Alcohol
1. Introduction As a starting material for the manufacture of nylon 6, ε-caprolactam is of high industrial importance. The current industrial route for the production of ε-caprolactam is the vapor phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam promoted by highly concentrated sulfuric acid. This process has several disadvantages. The by-production of a large amount of ammonium sulfate formed in the neutralization of the oleum is a serious drawback. Corrosion and safety problems are also posed. ∗ Corresponding author. Present address: Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Yokohama 240-8501, Japan; Tel.: +81-45-339-3393; fax: +81-45-339-3391 E-mail addresses: [email protected] (L.-X. Dai), [email protected] (T. Tatsumi) 1 Present address: Research Institute of Innovative Technology for the Earth (RITE), Kizugawadai, Kyoto 619-0292, Japan.
To solve these problems, strenuous efforts have been devoted to developing alternative heterogeneous catalysts which carry out the rearrangement reaction in a vapor phase process [1]. Compared to amorphous materials, a variety of zeolites give improved catalytic performance; however, they are still insufficient for industrial application. Landis and Venuto [2] reported the catalytic performance of a series of zeolites (Na–X, La–X, H–Y, H-mordenite, etc.). Butler and Poles [3] used H–Y and H–Pd–Y as catalysts and found that highly proton-exchanged zeolites are active and selective for the Beckmann rearrangement and that the Brønsted acidity of the catalyst is responsible for the transformation of oxime into ε-caprolactam whereas Pd causes the formation of nitriles. Corma and his co-workers [4,5] investigated the influence of the level of Na+ exchange in the H–Na–Y samples on the rearrangement reaction and suggested that medium and strong acid sites catalyze the reaction while
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the Na+ ions of zeolite accelerate the formation of nitriles. So far, however, despite the easy availability of Y zeolite, it gives rise to relatively low selectivity for ε-caprolactam and a rapid decay of activity when benzene is used as diluent [2–5]. In this decade, attention has been focused on the effect of diluent on the Beckmann rearrangement over zeolites. Highly siliceous ZSM-5 zeolites have been reported to exhibit high activity and selectivity for ε-caprolactam when methanol is used as diluent [6]. Very recently, the use of ethanol/water mixture has been proved to be effective in retarding the deactivation of the borosilicate of MFI-type zeolite, B-MFI [1,7]. It is pointed out that the weakly acidic B-MFI zeolite is suitable for the Beckmann rearrangement [1,7–9]. We have already reported that zeolites with medium strength acidity [10–12] and also mesoporous molecular sieves [13] exhibit excellent catalytic performance in this reaction when 1-hexanol is used as diluent. In this study, investigation is extended to the use of 1-hexanol as diluent for the enhancement of activity and selectivity of H-USY zeolites in the vapor phase Beckmann rearrangement of cyclohexanone oxime. According to the literature mentioned above, the assumption with respect to acid strength of the zeolites, which is favorable for the Beckmann rearrangement, ranges from strong acid site and to sites with very low acidity or even almost neutral silanol groups. The information obtained from the catalytic reactions, the temperature-programmed desorption of ammonia (NH3 -TPD) and infrared spectroscopy (FT-IR) are combined for characterization of the interaction of alcohols diluent with the active sites of H-USY zeolites. It has been shown that an appropriate amount of relatively weak acid sites of H-USY zeolites are effective for the highly selective Beckmann rearrangement in the presence of 1-hexanol as diluent.
2. Experimental The proton-form of USY zeolites, H-USY(6.3), H-USY(27), H-USY(62), H-USY(97) and H-USY(390) (the number in the parentheses represents the SiO2 /Al2 O3 ratio), which were prepared by the steaming of H–Y followed by washing with HCl, were used in this study. The rearrangement reaction was
carried out using a continuous flow reactor made of stainless steel operated at ambient pressure. The feed cyclohexanone oxime was dissolved in methanol or 1-hexanol and injected with a syringe pump along with N2 as a carrier gas. The reaction conditions were as follows: reaction temperature of 623 K, 0.1 MPa, oxime/diluent/N2 molar ratio of 1/9/5.9 and W/F of 80 gcat h mol−1 oxime . The reaction products collected were analyzed by a gas chromatograph equipped with a 4 m long packed column of silicone SE-52 (5%). Acidity measurements were performed by temperature-programmed desorption of ammonia using a Bel Japan Multitask-TPD analyzer. A sample (50 mg) was pretreated in a helium stream (50 ml min−1 ) at 773 K for 1 h and then exposed to 20 Torr of NH3 for 15 min at 373 K. Subsequently, the sample was outgassed at the same temperature for 1 h in vacuum. The desorption of NH3 was measured on mass number 16 by an Anelva Q-Mass detector from 373 to 873 K at a heating rate of 10 K min−1 . Helium was used as a carrier gas with a flow rate of 50 ml min−1 . The total amount of acid, which was defined as the ammonia desorbed from 373 to 873 K, was calibrated by comparing with the desorption from the standard sample (JRC-25) with a known amount of strong acid sites. IR spectra were recorded at room temperature on a Perkin–Elmer 1600 FT-IR instrument on self-supported H-USY pellets, which had been treated by refluxing in methanol or 1-hexanol at 353 K for 4 h.
3. Results and discussion The results of the reaction of cyclohexanone oxime over H-USY zeolites with varying SiO2 /Al2 O3 ratios are presented in Table 1. The low oxime conversion over H-USY(390) with both methanol and 1-hexanol diluent should be due to the scarcity of acid as will be described below. The low selectivity for ε-caprolactam in the case of methanol diluent was accompanied with the formation of a large amount of by-products, especially 5-cyanopentane/5-cyano-1-pentene and cyclohexanone/cyclo-2-hexenone as well. With decreasing SiO2 /Al2 O3 ratio (increasing acid amount) of H-USY zeolites, oxime conversion increased. The selectivity for ε-caprolactam formation increased initially with decreasing SiO2 /Al2 O3 ratio, and the maximum selectivity for ε-caprolactam was attained at an
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Table 1 The distribution of products in vapor phase Beckmann rearrangement reactiona Catalystb
Acid amount (mmol/g) Diluent
Conversion (%)c Selectivity (%)c
Othersd H-USY(390) n.d.e H-USY(97) 0.28 H-USY(62) 0.50 H-USY(27) 0.92 H-USY(6.3) 3.27 H-USY(390) n.d.e H-USY(97) 0.28 H-USY(62) 0.50 H-USY(27) 0.92 H-USY(6.3) 3.27
methanol methanol methanol methanol methanol 1-hexanol 1-hexanol 1-hexanol 1-hexanol 1-hexanol
35.9→12.8 75.1→21.4 97.1→62.5 98.0→70.1 94.7→78.5 45.8→17.6 89.5→56.5 100.0→99.9 99.9→99.9 99.9→99.8
39.5→39.6 47.8→63.5 81.9→86.5 78.3→85.9 64.5→80.1 64.1→80.3 85.1→87.9 93.7→94.6 88.8→93.8 76.6→86.0
29.8→30.4 28.5→18.6 7.7→6.1 9.1→4.7 3.0→4.4 12.3→7.1 4.3→3.3 2.1→1.7 2.6→2.1 3.6→3.1
19.3→19.1 16.0→12.6 5.9→5.0 6.1→4.5 20.5→8.9 10.2→7.6 3.9→3.8 2.2→2.7 1.9→2.8 3.3→6.9
11.4→10.9 7.7→5.3 4.5→2.4 6.5→4.9 12.0→6.6 13.4→5.0 6.7→5.0 2.0→1.0 6.7→1.3 16.5→4.0
Reaction conditions: 623 K, 0.1 MPa, oxime/diluent/N2 = 1/9/5.9 (molar), W/F = 80 gcat h mol−1 oxime . The number in parentheses represents the SiO2 /Al2 O3 ratio. c Time on stream = 1 h (left) and 6 h (right). d Mostly dimers; octahydrophenazine and tetrahydrocarbazole. e Not detected. a
b
SiO2 /Al2 O3 ratio of 62. With further decrease in the SiO2 /Al2 O3 ratio, the selectivity for ε-caprolactam formation decreased slightly, mainly due to the increased formation of dimers and polymers. The relatively high selectivity for nitriles on H-USY samples with a high SiO2 /Al2 O3 ratio might be related to their hydrophobicity. The Beckmann rearrangement is commonly assumed to take place by way of formation of O-protonated oxime to be followed by a migration from carbon to nitrogen of a substituent accompanied with loss of H2 O:
The recovery of H2 O to the iminium intermediate leads to R1 –C(OH)=NR2 from which the much more favorable tautomer amide is formed. However, the hydrophobic H-USY samples cannot retain sufficient H2 O, and thus, the ring opening of the iminium intermediate may eventually occur as follows:
Significant formation of cyclohexanone/ cyclohexenone was observed for the H-USY catalysts with high SiO2 /Al2 O3 ratio. This is in agreement with the results reported by Aucejo et al. [4]; cyclohexanone yield increased with decreasing acid amount of H–Na–Y. In contrast, H-USY(6.3) also gave rise to considerable formation of cyclohexanone/cyclohexenone. The reason is not clear but it should be noted that the selectivity for these products decreased with processing time. As shown in Table 1, the performance of all the catalysts was improved by the use of 1-hexanol as diluent. The changes in oxime conversion and selectivity for ε-caprolactam formation with the SiO2 /Al2 O3 ratio were similar to those in the case of methanol. As a result, both H-USY(27) and H-USY(62) showed excellent activity, selectivity and stability. The formation of three types of by-products was retarded simultaneously. The activity and selectivity for the formation of ε-caprolactam in the vapor phase Beckmann rearrangement are very much dependent upon the acidic character of zeolites. It has been reported [4,5] that the strong Brønsted acidity of the Y zeolite is responsible for the formation of ε-caprolactam. However, the majority of researchers have suggested that the
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Fig. 1. NH3 -TPD profiles of various zeolites. (A) H-USY(6.3); (B) H-USY(27); (C) H-USY(62); (D) H-USY(97); (E) H-USY (390).
very weak [1,7–9,14] or medium-strength acid sites [15] or even almost neutral silanol groups [6] of the zeolites are favorable for this rearrangement reaction, while the strong acid sites accelerate the formation of by-products [16]. Fig. 1 shows the NH3 -TPD profiles from H-USY zeolites with various SiO2 /Al2 O3 ratios. The total amounts of desorbed ammonia are summarized in Table 1. The H-USY(6.3) sample shows only an intense NH3 desorption peak at 448 K with a tail towards high temperature, indicating the existence of a large amount of weak and medium strength acid sites on this zeolite. For H-USY(27) and H-USY(62),
there are two desorption peaks with maxima at about 436 and 573–603 K, which are referred to as weak and medium strength acid sites, respectively. A mere trace amount of acidity was observed for the zeolites with SiO2 /Al2 O3 ratios of 97 and 390. We speculate that an appropriate amount of relatively weak acid sites of zeolites is effective for highly selective Beckmann rearrangements. This is supported by the NH3 -TPD results of H-USY(27) and H-USY(62) zeolites as shown in Fig. 1. Considerably low oxime conversion over H-USY(390) and H-USY(97) catalysts should be due to a too small amount of acidity. Furthermore, the relatively lower selectivity for ε-caprolactam formation on H-USY(97) could be accounted for by stronger average acidity (no distinctive peak was present for weak acid sites) compared to H-USY(27) and H-USY(62). This result is consistent with the observation by Sato et al., who reported that the strong acid sites of MFI zeolites accelerated the conversion of lactam into nitriles as well as the decomposition of oxime into cyclohexanone [17]. Therefore, we believe that both the high acid density of H-USY(6.3) and the insufficient amount of acid sites on H-USY(390) and H-USY(97) are unfavorable for the highly selective formation of ε-caprolactam. Only a moderate amount of relatively weak acid sites on H-USY(27) and H-USY(62) zeolites seems to be required for the Beckmann rearrangement. Hoelderich et al. [1,7–9] also have suggested recently that the weakly acidic borosilicate of MFI-type zeolite is very suitable for this reaction. Fig. 2 shows FT-IR spectrum for H-USY zeolites treated with alcohols. The infrared bands appeared in the range 3000–2850 cm−1 , which can be assigned to alkoxy groups formed by an esterification reaction of the surface hydroxyl groups of Y zeolites with alcohols: R–OH + HO–Si → R–O–Si + H2 O, and/or an interaction of alcohol molecules with skeletal Brønsted acid sites via strong hydrogen bonds according to the following scheme [18–21]:
For the H-USY zeolites with SiO2 /Al2 O3 ratios of 6.3 and 62, the decrease in the intensities of ν (C–H) bands of methoxy group with increasing outgassing temperature was not so obvious compared to those of
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Fig. 2. FT-IR spectra of various zeolites treated with alcohols. (A) Treated with methanol; (B) Treated with 1-hexanol. (1) SiO2 /Al2 O3 =6.3; (2) SiO2 /A12 O3 =62; (3)SiO2 /Al2 O3 =390. Outgassing temperature: top=273 K; bottom=573 K.
hexoxy group. This indicates that the adsorption of methanol on H-USY with a relatively low SiO2 /Al2 O3 ratio was stronger than that of 1-hexanol (Fig. 2-A1 and 2-B1 ; Fig. 2-A2 and 2-B2 ). In contrast, 1-hexanol adsorbed on H-USY(390) with a high SiO2 /Al2 O3 ratio more strongly than methanol (Fig. 2-A3 and 2-B3 ). It was observed that the adsorption of 1-hexanol on the relatively hydrophilic H-USY(62) zeolite was not too strong and appropriate (Fig. 2-B2 ). Thus, it is conceivable that the relatively strong Brønsted acid sites (>623 K in Fig. 1-C) might be covered preferentially with 1-hexanol; thus, the activity and selectivity for the formation of ε-caprolactam was enhanced effectively by the usage of 1-hexanol. A very small number of acid sites on H-USY(390) would be covered with 1-hexanol due to their stronger interaction (Fig. 2-B3 ), resulting in the lower activity and selectivity for ε-caprolactam compared to those of the other catalysts. The slightly low activity of H-USY(6.3) in the case of methanol diluent compared to the case of 1-hexanol diluent in spite of a high amount of acid may be ascribed to the strong adsorption of methanol on the hydrophilic surface as revealed by FT-IR results (Fig. 2-A1 ). The fact that H-USY(6.3) catalyst exhibited a relatively high selectivity for the formation of dimers at the initial stage of the reaction,
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irrespective of diluents, may be attributed to its high acid density. Another disadvantage of H-USY(6.3) is that the decomposition of 1-hexanol was more than 30% after 1 h to ca. 10% after 6 h of time on stream. However, 1-hexanol decomposition decreased with increasing SiO2 /Al2 O3 ratio, and reduced to lower than 1% for H-USY(62) catalyst after 6 h. For all the catalysts, the polymeric by-products produced in the case of 1-hexanol were dimers, while heavier products were significantly formed when methanol was used. There is no evident difference in the structure of dimers depending on the catalysts. It is noteworthy that the H-USY zeolites used in the presence of 1-hexanol were only slightly brown-colored in contrast to the black-colored catalyst used in methanol diluent, suggesting that the deposition of heavier polymers (coke precursors) on the H-USY zeolites were retarded effectively by use of 1-hexanol as diluent, resulting in the high stability of the catalysts. Recently, Hoelderich et al. compared the properties of amorphous silica, silicalite as well as the silicalite treated with ammonia through examination with FT-IR analysis [1,8]. They proposed that the vicinal (hydrogen bonded) silanols and silanol nesting on the silicalite surface should be suitable for the Beckmann rearrangement. It is well known that silanol groups are formed on the dealumination of Y zeolites [22]. The excellent catalytic performance of the moderately dealuminated USY zeolites as well as FSM-16 and MCM-41 [13], which contain a lot of hydrogen-bonded silanol groups, for this rearrangement, is consistent with the proposition made by Hoelderich et al. However, it is also to be noted that highly dealuminated USY zeolites give rise to poor performance irrespective of diluents, a certain amount of Al being favorable for the activity of Y zeolites for the rearrangement in contrast to the MFI silicalites [6,8].
4. Conclusions In conclusion, the catalytic performance of USY zeolites for the vapor phase Beckmann rearrangement of cyclohexanone oxime was greatly improved by adjusting the SiO2 /Al2 O3 ratio and by using an appropriate diluent. It has been shown that H-USY catalysts
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with SiO2 /Al2 O3 ratios of 27 and 62 exhibited excellent catalytic activity and selectivity when 1-hexanol was used as diluent. References [1] W.F. Hoelderich, J. Roeseler, G. Heitmann, A.T. Liebens, Catal. Today 37 (1997) 353. [2] P.S. Landis, P.B. Venuto, J. Catal. 6 (1966) 245. [3] J.D. Butler, T.C. Poles, J. Chem. Soc. Perkin II (1973) 1262. [4] A. Aucejo, M.C. Burguet, A. Corma, V. Fornes, Appl. Catal. 22 (1986) 187. [5] A. Corma, H. Garcia, J. Primo, Zeolites 11 (1991) 593. [6] H. Sato, K. Hirose, M. Kitamura, Y. Nakamura, Stud. Surf. Sci. Catal. 49 (1989) 1213. [7] J. Roeseler, G. Heitmann, W.F. Hoelderich, Appl. Catal. A 144 (1996) 319. [8] W.F. Hoelderich, G. Heitmann, Catal. Today 38 (1997) 227. [9] P. Albers, K. Seibold, T. Haas, G. Prescher, W.F. Hoelderich, J. Catal. 176 (1998) 561.
[10] L.X. Dai, R. Hayasaka, Y. Iwaki, T. Tatsumi, Chem. Commun. (1996) 1071. [11] L.X. Dai, Y. Iwaki, K. Koyama, T. Tatsumi, Appl. Surf. Sci. 121/122 (1997) 335. [12] T. Tatsumi, L.X. Dai, in: Proceedings of the 12th International Zeolite Conference, July 5–10, 1998, Baltimore, (1999) 1455. [13] L.X. Dai, K. Koyama, T. Tatsumi, Catal. Lett. 53 (1998) 211. [14] T. Yashima, K. Miura, T. Komatsu, Stud. Surf. Sci. Catal. 84 (1994) 1897. [15] P.S. Singh, R. Bandyopadhyay, S.G. Hegde, B.S. Rao, Appl. Catal. A 136 (1996) 249. [16] T. Ushikubo, K. Wada, J. Catal. 148 (1994) 138. [17] H. Sato, S. Hasebe, H. Sakurai, K. Urabe, Y. Izumi, Appl. Catal. 29 (1987) 107. [18] T.R. Forester, R.F. Howe, J. Am. Chem. Soc. 109 (1987) 5076. [19] P. Salvador, W. Kladnig, J. Chem. Soc., Faraday Trans. I 73 (1977) 1153. [20] K. Ludmila, N. Jana, N. Kveta, J. Catal. 124 (1990) 441. [21] X.Z. Jiang, J. Mol. Catal. A: Chem. 121 (1997) 63. [22] J.R. Sohn, S.J. DeCanio, P.O. Fritz, J.H. Lunsford, J. Phys. Chem. 90 (1986) 4847.
Highly selective vapor phase Beckmann rearrangement over H-USY zeolites Lian-Xin Dai1 , Katsuyuki Koyama, Mitsunori Miyamoto, Takashi Tatsumi ∗ Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-Ku, Tokyo 113-8656, Japan
Abstract The vapor Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam catalyzed by H-USY with different SiO2 / Al2 O3 ratios was studied. The catalytic performance of H-USY zeolites for the highly selective formation of ε-caprolactam was greatly improved by adjusting the SiO2 /Al2 O3 ratio and by using an appropriate diluent. It has been found that H-USY catalysts with SiO2 /Al2 O3 ratios of 27 and 62 exhibited excellent catalytic activity, selectivity and stability when 1-hexanol was used as diluent. The NH3 -TPD results indicate that an appropriate amount of relatively weak acid sites of H-USY zeolites are effective for highly selective Beckmann rearrangement. The interaction of alcohols with H-USY zeolites has been investigated by FT-IR to explain the improving effect of 1-hexanol. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Beckmann rearrangement; Acidity; USY zeolite; Diluent effect; Alcohol
1. Introduction As a starting material for the manufacture of nylon 6, ε-caprolactam is of high industrial importance. The current industrial route for the production of ε-caprolactam is the vapor phase Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam promoted by highly concentrated sulfuric acid. This process has several disadvantages. The by-production of a large amount of ammonium sulfate formed in the neutralization of the oleum is a serious drawback. Corrosion and safety problems are also posed. ∗ Corresponding author. Present address: Division of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Yokohama 240-8501, Japan; Tel.: +81-45-339-3393; fax: +81-45-339-3391 E-mail addresses: [email protected] (L.-X. Dai), [email protected] (T. Tatsumi) 1 Present address: Research Institute of Innovative Technology for the Earth (RITE), Kizugawadai, Kyoto 619-0292, Japan.
To solve these problems, strenuous efforts have been devoted to developing alternative heterogeneous catalysts which carry out the rearrangement reaction in a vapor phase process [1]. Compared to amorphous materials, a variety of zeolites give improved catalytic performance; however, they are still insufficient for industrial application. Landis and Venuto [2] reported the catalytic performance of a series of zeolites (Na–X, La–X, H–Y, H-mordenite, etc.). Butler and Poles [3] used H–Y and H–Pd–Y as catalysts and found that highly proton-exchanged zeolites are active and selective for the Beckmann rearrangement and that the Brønsted acidity of the catalyst is responsible for the transformation of oxime into ε-caprolactam whereas Pd causes the formation of nitriles. Corma and his co-workers [4,5] investigated the influence of the level of Na+ exchange in the H–Na–Y samples on the rearrangement reaction and suggested that medium and strong acid sites catalyze the reaction while
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the Na+ ions of zeolite accelerate the formation of nitriles. So far, however, despite the easy availability of Y zeolite, it gives rise to relatively low selectivity for ε-caprolactam and a rapid decay of activity when benzene is used as diluent [2–5]. In this decade, attention has been focused on the effect of diluent on the Beckmann rearrangement over zeolites. Highly siliceous ZSM-5 zeolites have been reported to exhibit high activity and selectivity for ε-caprolactam when methanol is used as diluent [6]. Very recently, the use of ethanol/water mixture has been proved to be effective in retarding the deactivation of the borosilicate of MFI-type zeolite, B-MFI [1,7]. It is pointed out that the weakly acidic B-MFI zeolite is suitable for the Beckmann rearrangement [1,7–9]. We have already reported that zeolites with medium strength acidity [10–12] and also mesoporous molecular sieves [13] exhibit excellent catalytic performance in this reaction when 1-hexanol is used as diluent. In this study, investigation is extended to the use of 1-hexanol as diluent for the enhancement of activity and selectivity of H-USY zeolites in the vapor phase Beckmann rearrangement of cyclohexanone oxime. According to the literature mentioned above, the assumption with respect to acid strength of the zeolites, which is favorable for the Beckmann rearrangement, ranges from strong acid site and to sites with very low acidity or even almost neutral silanol groups. The information obtained from the catalytic reactions, the temperature-programmed desorption of ammonia (NH3 -TPD) and infrared spectroscopy (FT-IR) are combined for characterization of the interaction of alcohols diluent with the active sites of H-USY zeolites. It has been shown that an appropriate amount of relatively weak acid sites of H-USY zeolites are effective for the highly selective Beckmann rearrangement in the presence of 1-hexanol as diluent.
2. Experimental The proton-form of USY zeolites, H-USY(6.3), H-USY(27), H-USY(62), H-USY(97) and H-USY(390) (the number in the parentheses represents the SiO2 /Al2 O3 ratio), which were prepared by the steaming of H–Y followed by washing with HCl, were used in this study. The rearrangement reaction was
carried out using a continuous flow reactor made of stainless steel operated at ambient pressure. The feed cyclohexanone oxime was dissolved in methanol or 1-hexanol and injected with a syringe pump along with N2 as a carrier gas. The reaction conditions were as follows: reaction temperature of 623 K, 0.1 MPa, oxime/diluent/N2 molar ratio of 1/9/5.9 and W/F of 80 gcat h mol−1 oxime . The reaction products collected were analyzed by a gas chromatograph equipped with a 4 m long packed column of silicone SE-52 (5%). Acidity measurements were performed by temperature-programmed desorption of ammonia using a Bel Japan Multitask-TPD analyzer. A sample (50 mg) was pretreated in a helium stream (50 ml min−1 ) at 773 K for 1 h and then exposed to 20 Torr of NH3 for 15 min at 373 K. Subsequently, the sample was outgassed at the same temperature for 1 h in vacuum. The desorption of NH3 was measured on mass number 16 by an Anelva Q-Mass detector from 373 to 873 K at a heating rate of 10 K min−1 . Helium was used as a carrier gas with a flow rate of 50 ml min−1 . The total amount of acid, which was defined as the ammonia desorbed from 373 to 873 K, was calibrated by comparing with the desorption from the standard sample (JRC-25) with a known amount of strong acid sites. IR spectra were recorded at room temperature on a Perkin–Elmer 1600 FT-IR instrument on self-supported H-USY pellets, which had been treated by refluxing in methanol or 1-hexanol at 353 K for 4 h.
3. Results and discussion The results of the reaction of cyclohexanone oxime over H-USY zeolites with varying SiO2 /Al2 O3 ratios are presented in Table 1. The low oxime conversion over H-USY(390) with both methanol and 1-hexanol diluent should be due to the scarcity of acid as will be described below. The low selectivity for ε-caprolactam in the case of methanol diluent was accompanied with the formation of a large amount of by-products, especially 5-cyanopentane/5-cyano-1-pentene and cyclohexanone/cyclo-2-hexenone as well. With decreasing SiO2 /Al2 O3 ratio (increasing acid amount) of H-USY zeolites, oxime conversion increased. The selectivity for ε-caprolactam formation increased initially with decreasing SiO2 /Al2 O3 ratio, and the maximum selectivity for ε-caprolactam was attained at an
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Table 1 The distribution of products in vapor phase Beckmann rearrangement reactiona Catalystb
Acid amount (mmol/g) Diluent
Conversion (%)c Selectivity (%)c
Othersd H-USY(390) n.d.e H-USY(97) 0.28 H-USY(62) 0.50 H-USY(27) 0.92 H-USY(6.3) 3.27 H-USY(390) n.d.e H-USY(97) 0.28 H-USY(62) 0.50 H-USY(27) 0.92 H-USY(6.3) 3.27
methanol methanol methanol methanol methanol 1-hexanol 1-hexanol 1-hexanol 1-hexanol 1-hexanol
35.9→12.8 75.1→21.4 97.1→62.5 98.0→70.1 94.7→78.5 45.8→17.6 89.5→56.5 100.0→99.9 99.9→99.9 99.9→99.8
39.5→39.6 47.8→63.5 81.9→86.5 78.3→85.9 64.5→80.1 64.1→80.3 85.1→87.9 93.7→94.6 88.8→93.8 76.6→86.0
29.8→30.4 28.5→18.6 7.7→6.1 9.1→4.7 3.0→4.4 12.3→7.1 4.3→3.3 2.1→1.7 2.6→2.1 3.6→3.1
19.3→19.1 16.0→12.6 5.9→5.0 6.1→4.5 20.5→8.9 10.2→7.6 3.9→3.8 2.2→2.7 1.9→2.8 3.3→6.9
11.4→10.9 7.7→5.3 4.5→2.4 6.5→4.9 12.0→6.6 13.4→5.0 6.7→5.0 2.0→1.0 6.7→1.3 16.5→4.0
Reaction conditions: 623 K, 0.1 MPa, oxime/diluent/N2 = 1/9/5.9 (molar), W/F = 80 gcat h mol−1 oxime . The number in parentheses represents the SiO2 /Al2 O3 ratio. c Time on stream = 1 h (left) and 6 h (right). d Mostly dimers; octahydrophenazine and tetrahydrocarbazole. e Not detected. a
b
SiO2 /Al2 O3 ratio of 62. With further decrease in the SiO2 /Al2 O3 ratio, the selectivity for ε-caprolactam formation decreased slightly, mainly due to the increased formation of dimers and polymers. The relatively high selectivity for nitriles on H-USY samples with a high SiO2 /Al2 O3 ratio might be related to their hydrophobicity. The Beckmann rearrangement is commonly assumed to take place by way of formation of O-protonated oxime to be followed by a migration from carbon to nitrogen of a substituent accompanied with loss of H2 O:
The recovery of H2 O to the iminium intermediate leads to R1 –C(OH)=NR2 from which the much more favorable tautomer amide is formed. However, the hydrophobic H-USY samples cannot retain sufficient H2 O, and thus, the ring opening of the iminium intermediate may eventually occur as follows:
Significant formation of cyclohexanone/ cyclohexenone was observed for the H-USY catalysts with high SiO2 /Al2 O3 ratio. This is in agreement with the results reported by Aucejo et al. [4]; cyclohexanone yield increased with decreasing acid amount of H–Na–Y. In contrast, H-USY(6.3) also gave rise to considerable formation of cyclohexanone/cyclohexenone. The reason is not clear but it should be noted that the selectivity for these products decreased with processing time. As shown in Table 1, the performance of all the catalysts was improved by the use of 1-hexanol as diluent. The changes in oxime conversion and selectivity for ε-caprolactam formation with the SiO2 /Al2 O3 ratio were similar to those in the case of methanol. As a result, both H-USY(27) and H-USY(62) showed excellent activity, selectivity and stability. The formation of three types of by-products was retarded simultaneously. The activity and selectivity for the formation of ε-caprolactam in the vapor phase Beckmann rearrangement are very much dependent upon the acidic character of zeolites. It has been reported [4,5] that the strong Brønsted acidity of the Y zeolite is responsible for the formation of ε-caprolactam. However, the majority of researchers have suggested that the
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Fig. 1. NH3 -TPD profiles of various zeolites. (A) H-USY(6.3); (B) H-USY(27); (C) H-USY(62); (D) H-USY(97); (E) H-USY (390).
very weak [1,7–9,14] or medium-strength acid sites [15] or even almost neutral silanol groups [6] of the zeolites are favorable for this rearrangement reaction, while the strong acid sites accelerate the formation of by-products [16]. Fig. 1 shows the NH3 -TPD profiles from H-USY zeolites with various SiO2 /Al2 O3 ratios. The total amounts of desorbed ammonia are summarized in Table 1. The H-USY(6.3) sample shows only an intense NH3 desorption peak at 448 K with a tail towards high temperature, indicating the existence of a large amount of weak and medium strength acid sites on this zeolite. For H-USY(27) and H-USY(62),
there are two desorption peaks with maxima at about 436 and 573–603 K, which are referred to as weak and medium strength acid sites, respectively. A mere trace amount of acidity was observed for the zeolites with SiO2 /Al2 O3 ratios of 97 and 390. We speculate that an appropriate amount of relatively weak acid sites of zeolites is effective for highly selective Beckmann rearrangements. This is supported by the NH3 -TPD results of H-USY(27) and H-USY(62) zeolites as shown in Fig. 1. Considerably low oxime conversion over H-USY(390) and H-USY(97) catalysts should be due to a too small amount of acidity. Furthermore, the relatively lower selectivity for ε-caprolactam formation on H-USY(97) could be accounted for by stronger average acidity (no distinctive peak was present for weak acid sites) compared to H-USY(27) and H-USY(62). This result is consistent with the observation by Sato et al., who reported that the strong acid sites of MFI zeolites accelerated the conversion of lactam into nitriles as well as the decomposition of oxime into cyclohexanone [17]. Therefore, we believe that both the high acid density of H-USY(6.3) and the insufficient amount of acid sites on H-USY(390) and H-USY(97) are unfavorable for the highly selective formation of ε-caprolactam. Only a moderate amount of relatively weak acid sites on H-USY(27) and H-USY(62) zeolites seems to be required for the Beckmann rearrangement. Hoelderich et al. [1,7–9] also have suggested recently that the weakly acidic borosilicate of MFI-type zeolite is very suitable for this reaction. Fig. 2 shows FT-IR spectrum for H-USY zeolites treated with alcohols. The infrared bands appeared in the range 3000–2850 cm−1 , which can be assigned to alkoxy groups formed by an esterification reaction of the surface hydroxyl groups of Y zeolites with alcohols: R–OH + HO–Si → R–O–Si + H2 O, and/or an interaction of alcohol molecules with skeletal Brønsted acid sites via strong hydrogen bonds according to the following scheme [18–21]:
For the H-USY zeolites with SiO2 /Al2 O3 ratios of 6.3 and 62, the decrease in the intensities of ν (C–H) bands of methoxy group with increasing outgassing temperature was not so obvious compared to those of
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Fig. 2. FT-IR spectra of various zeolites treated with alcohols. (A) Treated with methanol; (B) Treated with 1-hexanol. (1) SiO2 /Al2 O3 =6.3; (2) SiO2 /A12 O3 =62; (3)SiO2 /Al2 O3 =390. Outgassing temperature: top=273 K; bottom=573 K.
hexoxy group. This indicates that the adsorption of methanol on H-USY with a relatively low SiO2 /Al2 O3 ratio was stronger than that of 1-hexanol (Fig. 2-A1 and 2-B1 ; Fig. 2-A2 and 2-B2 ). In contrast, 1-hexanol adsorbed on H-USY(390) with a high SiO2 /Al2 O3 ratio more strongly than methanol (Fig. 2-A3 and 2-B3 ). It was observed that the adsorption of 1-hexanol on the relatively hydrophilic H-USY(62) zeolite was not too strong and appropriate (Fig. 2-B2 ). Thus, it is conceivable that the relatively strong Brønsted acid sites (>623 K in Fig. 1-C) might be covered preferentially with 1-hexanol; thus, the activity and selectivity for the formation of ε-caprolactam was enhanced effectively by the usage of 1-hexanol. A very small number of acid sites on H-USY(390) would be covered with 1-hexanol due to their stronger interaction (Fig. 2-B3 ), resulting in the lower activity and selectivity for ε-caprolactam compared to those of the other catalysts. The slightly low activity of H-USY(6.3) in the case of methanol diluent compared to the case of 1-hexanol diluent in spite of a high amount of acid may be ascribed to the strong adsorption of methanol on the hydrophilic surface as revealed by FT-IR results (Fig. 2-A1 ). The fact that H-USY(6.3) catalyst exhibited a relatively high selectivity for the formation of dimers at the initial stage of the reaction,
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irrespective of diluents, may be attributed to its high acid density. Another disadvantage of H-USY(6.3) is that the decomposition of 1-hexanol was more than 30% after 1 h to ca. 10% after 6 h of time on stream. However, 1-hexanol decomposition decreased with increasing SiO2 /Al2 O3 ratio, and reduced to lower than 1% for H-USY(62) catalyst after 6 h. For all the catalysts, the polymeric by-products produced in the case of 1-hexanol were dimers, while heavier products were significantly formed when methanol was used. There is no evident difference in the structure of dimers depending on the catalysts. It is noteworthy that the H-USY zeolites used in the presence of 1-hexanol were only slightly brown-colored in contrast to the black-colored catalyst used in methanol diluent, suggesting that the deposition of heavier polymers (coke precursors) on the H-USY zeolites were retarded effectively by use of 1-hexanol as diluent, resulting in the high stability of the catalysts. Recently, Hoelderich et al. compared the properties of amorphous silica, silicalite as well as the silicalite treated with ammonia through examination with FT-IR analysis [1,8]. They proposed that the vicinal (hydrogen bonded) silanols and silanol nesting on the silicalite surface should be suitable for the Beckmann rearrangement. It is well known that silanol groups are formed on the dealumination of Y zeolites [22]. The excellent catalytic performance of the moderately dealuminated USY zeolites as well as FSM-16 and MCM-41 [13], which contain a lot of hydrogen-bonded silanol groups, for this rearrangement, is consistent with the proposition made by Hoelderich et al. However, it is also to be noted that highly dealuminated USY zeolites give rise to poor performance irrespective of diluents, a certain amount of Al being favorable for the activity of Y zeolites for the rearrangement in contrast to the MFI silicalites [6,8].
4. Conclusions In conclusion, the catalytic performance of USY zeolites for the vapor phase Beckmann rearrangement of cyclohexanone oxime was greatly improved by adjusting the SiO2 /Al2 O3 ratio and by using an appropriate diluent. It has been shown that H-USY catalysts
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