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Acidity and 1-butene isomerization of synthesized smectite-type catalysts containing different divalent cations
Applied Catalysis A: General 187 (1999) 141–146
Acidity and 1-butene isomerization of synthesized smectite-type catalysts containing different divalent cations Masayuki Shirai a,∗ , Kuriko Aoki a , Kazuo Torii b , Masahiko Arai a a
Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba, Sendai 980-8577, Japan b Tohoku National Industrial Research Institute, Nigatake, Miyagino, Sendai 983-8551, Japan Received 23 March 1999; received in revised form 18 May 1999; accepted 21 May 1999
Abstract Smectite-type materials which contain different divalent cations (Ni2+ , Co2+ , Mg2+ , and Zn2+ ) in octahedral sheets have been synthesized and characterized by pyridine adsorption and catalytic effectiveness for the isomerization of 1-butene. All smectite-type materials examined have Lewis acid sites; however, the activity values for the isomerization of 1-butene significantly depend on the divalent cation species included. Nickel-containing smectite-type catalysts are stable and the most active among the smectite catalysts. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Synthesized smectite-type catalysts; 1-Butene isomerization; Lewis acid sites
1. Introduction Smectite-type materials have layered structures in which each layer is composed of one octahedral sheet sandwiched by two tetrahedral sheets. The octahedral sheets contain divalent cations which are surrounded by six oxygen atoms in octahedral structure and the tetrahedral sheets contain Si4+ ions which are surrounded by four oxygen atoms in tetrahedral structure. The trilayers are negatively charged and are held together by the electrostatic interaction with exchangeable cations in the interlayer region. There are two types of sites for transition metal cations in smectite-type materials. One is an ion-exchangeable site in the interlayer space and the other is a lattice site within the octahedral sheet. Smectite-type materi∗ Corresponding author. Tel./fax: +81-22-217-5631 E-mail address: [email protected] (M. Shirai)
als containing transition metal ions in the lattice sites are found in nature and have been synthesized [1,2]. We have invented a hydrothermal method to synthesize smectite-type materials [3–8]. It is possible with this method to prepare smectite materials including different divalent cations in the octahedral sheets and also with controlled pore size in meso range. Due to their relatively high surface area and easy introduction of various transition metal cations in the lattice and on ion-exchangeable sites, synthesized smectite-type materials can serve as potentially useful catalysts and supports. Nickel-substituted smectite-materials show high activities for the decomposition of 2-propanol [9], the selective dimerization of ethylene [10], and the hydroisomerization of n-pentene [11] and hydrogenation of acetonitrile [12]. Cobalt-substituted smectite materials show high activity for the hydrodesulfurization of thiophene [13]. The activities for hydrogenolysis of n-butane on plat-
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Table 1 Results of characterization of smectite samples prepared Sample pHa
Compositionb (Si : M : Na) Specific surface areac (cm2 g−1 ) Total pore volumec (cm3 g−1 ) Average pore diameterc (nm)
Ni-481 10.4 8 : 6.03 : 0.90 Ni-359 11.7 8 : 6.16 : 1.86 Co-380 6.1 8 : 5.98 : 0.22 Co-202 9.0 8 : 6.34 : 0.85 Mg-518 9.5 8 : 6.49 : 0.49 Mg-325 11.4 8 : 6.62 : 1.10 Zn-153 6.6 8 : 6.53 : 1.28
481 359 380 202 518 325 153
0.31 0.21 0.34 0.16 0.39 0.22 0.17
2.6 2.4 3.6 3.2 3.0 2.7 4.3
a
pH value for hydrothermal synthesis of smectite materials. Number of atoms in the unit cell determined by X-ray fluorescence method; M : Ni, Co, Mg, or Zn. c Determined by nitrogen adsorption and desorption isotherms. b
inum supported on smectite-type materials depend on substituted divalent cation species in the octahedral sheets [14,15]. Pore-size controlled smectites loaded with palladium show size-selective hydrogenation of butadiene–acrylonitrile rubbers [16]. Cobalt oxide on smectite-type materials shows higher activity for thiophene hydrodesulfurization than Co–Mo/Al2 O3 [17]. In this study we report the evaluation of thermal stability and acidic properties of synthesized smectite-type materials which contain various divalent cations (Ni2+ , Co2+ , Mg2+ , and Zn2+ ) in the lattice sites and their activities for the 1-butene isomerization as a test reaction of acidic catalysts. 2. Experimental 2.1. Sample preparation Smectite-type materials were synthesized with a hydrothermal method described previously [3–8]. The aqueous solution of sodium silicate (SiO2 /Na2 O = 3.22) and sodium hydroxide was mixed with the aqueous solution of metal chloride to precipitate Si–M (M: divalent metal cation, Si : M = 8 : 6) hydroxides. The precipitation pH of Si–M hydroxide was controlled by changing the molar ratio of sodium hydroxide to sodium silicate. After separating and washing of Si–M hydroxide, the slurries were prepared from Si–Mg hydroxide and water. The Si–M slurries were treated hydrothermally in an autoclave at 473 K under autogaseous water vapor pressure for 2 h. The resultant samples were dried at 353 K; then we obtained smectite catalysts. The smectite catalysts are denoted by the divalent species in octahedral sheets and BET surface area, e.g.
Ni-481 for the Ni2+ substituted smectite-type material with a surface area of 481 m2 g−1 . The characteristics of the smectite catalysts prepared are given in Table 1. 2.2. Isomerization of 1-butene The catalytic reactions were carried out in a closed circulating system (volume: 148 ml). After pre-evacuation of a catalyst (200 mg) in the reactor at various temperatures (373–973 K) for 1 h, the reaction was conducted by circulating 1-butene at 323 K. The initial pressure of 1-butene was 13 kPa. Products were analyzed by a gas chromatograph with a flame ionization detector and a column packed with a Sebaconitrile 25% supported Uniport C column. The catalytic activities were evaluated from the formation of main products of trans-2-butene and cis-2-butene. 2.3. Pyridine adsorption To examine the acidic properties, we measured IR spectra of pyridine adsorbed on catalysts. A wafer of smectite catalyst was treated in an IR cell at various temperatures (373–923 K) for 1 h under dynamic evacuation. Then the wafer was exposed to pyridine vapor (1.0 kPa) at 323 K for 0.5 h, followed by evacuation at 433 K for 2 h to remove physisorbed pyridine molecules. 3. Results 3.1. Smectite catalysts Seven kinds of smectite catalysts were used in this study (Table 1). The content of sodium in the sam-
M. Shirai et al. / Applied Catalysis A: General 187 (1999) 141–146
Fig. 1. Initial rates of 1-butene isomerization of the Ni-481 (䊉) and Ni-359 (䊊) catalysts as a function of pre-evacuation temperature.
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Fig. 2. Initial rates of 1-butene isomerization of the Co-380 (N) and Co-202 (4) catalysts as a function of pre-evacuation temperature.
ples depended on pH at the producing of the Si–M hydroxide. The ratios of Si : M (M; divalent cation) of the smectite catalysts were about 8 : 6. XRD patterns showed that all the samples have smectite-type structure [8]. Smectite catalysts prepared at lower pH had fewer sodium ions, higher surface areas, and larger pore volumes for a series of samples including the same divalent cation species in the octahedral sheet. 3.2. Isomerization of 1-butene The Ni-481, Ni-359, and Co-380 catalysts were found to be active for the isomerization. The products were trans-2-butene and cis-2-butene. The other smectites showed no activities for the isomerization of 1-butene under the reaction conditions used. Fig. 1 shows the initial rates at 323 K for the Ni-481 and Ni-359 catalysts pre-evacuated at temperatures from 473 to 973 K. The activities of Ni2+ -substituted smectite catalysts increased with increasing evacuation temperature and showed a maximum after pre-evacuation at 773–873 K; then the activity decreased to almost zero at 973 K. The activities were different for the Ni-481 and the Ni-359 catalysts. After pre-evacuation at 873 K, the activity of the Ni-481 catalyst was 2.1 × 10−4 mol min−1 g−1 cat , which was almost 16 times higher than the maximum activity of the Ni-359 catalyst after pre-evacuation at 773 K. The activity of the Co-380 catalyst also depended on the pre-evacuation temperature in a similar fashion to the activity of the Ni2+ -substituted cata-
Fig. 3. Infrared spectra of pyridine adsorbed on smectites. Sample and evacuation temperature: (a) Ni-481, 573 K; (b) Ni-359, 473 K; (c) Co-380, 573 K; (d) Co-202, 473 K; (e) Mg-518, 473 K; (f) Mg-325, 473 K; (g) Zn-153, 473 K.
lysts; however, the Co-380 catalyst was less active compared to the nickel-substituted smectite catalyst (Fig. 2). The maximum activity of Co-380 was 3.1 × 10−7 mol min−1 g−1 cat , which was as almost one-seven hundredth as that for the Ni-481 catalyst. The Co-202 catalyst did not show the isomerization activities under the reaction conditions used. 3.3. Pyridine adsorption Fig. 3 shows IR spectra of pyridine adsorbed on smectite samples. Absorbances were normalized by the sample weight. Four peaks were observed
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Fig. 4. Intensities of the Lewis acid band as a function of pre-evacuation temperature. (a) nickel smectites (䊉: Ni-481 and 䊊: Ni-359); (b) cobalt smectites (N:Co-380 and 4: Co-202); (c) magnesium smectites (䊏: Mg-518 and 䊐: Mg-325); (d) zinc smectite (H: Zn-153).
at 1446–1454, 1489–1492, 1574–1578, and 1608– 1612 cm−1 corresponding to pyridine molecules coordinated to metal cations with Lewis acid character [18]. No peak was measured for any of the samples used at near 1550 cm−1 , assigned to pyridinium ions formed from pyridine adsorbed on Brönsted acid sites. The peak areas of the band at 1446–1454 cm−1 are plotted as a function of pre-evacuation temperature in Fig. 4. The peak shift was hard to observe with pre-evacuation temperature (IR resolution is 4 cm−1 ). These peak areas are normalized with the sample weights, being proportional to the amount of Lewis acid sites per unit weight of sample. For the Ni2+ -substituted smectite catalysts (Ni-481 and Ni-359), the areas slightly increase up to 773–873 K;
however the areas appreciably decrease after 973 K treatment. The peak area for the Ni-481 catalyst was higher than that for the Ni-359 catalyst at any pre-evacuation temperature. For the Co2+ -substituted smectite samples (Co-380 and Co-202), the areas slightly decreased up to 773 K and then the intensities appreciably decreased with increasing pre-evacuation temperature. The peak area of the IR spectra for coordinated pyridine on the Co-380 catalyst was higher than that for the Co-202 catalyst at any pre-evacuation temperature. For the Mg2+ smectite samples (Mg-518 and Mg-325) and the Zn2+ sample (Zn-153), the areas simply decreased with increasing pre-evacuation temperature. The peak area for Mg-518 was higher than that for Mg-325 at pre-evacuation temperatures examined.
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3.4. Thermal stability of nickel-substituted smectite catalysts The thermal stability values of catalytic active nickel-substituted smectite materials were investigated by measuring the surface area of the samples treated at various temperatures (573–873 K) for 1 h. The surface areas of the samples decreased with increasing pre-treatment temperature. The surface areas of Ni-481 and Ni-359 treated at 873 K were 381 and 184 m2 g−1 cat , respectively. The nickel smectite catalyst prepared at lower pH had thermally stable structure. Fig. 5. Surface areas of the Ni-481 (䊉) and Ni-359 (䊊) catalysts as a function of pre-evacuation temperature.
4. Discussion Only the Ni-481, Ni-359, and Co-380 catalysts showed the isomerization activities. No other smectite materials were active. Moreover, the activity of Ni-481 was about 700 times higher than that of Co-380. These results mean that the isomerization activity of the smectite materials depends on the divalent cation species (M2+ ) in the octahedral sheet; the order was Ni2+ Co2+ Mg2+ = Zn2+ = 0. The selectivities of the isomerization (cis-2-butene/trans-2-butene) on Ni-481 and Ni-359 smectite catalysts were 0.6 and 0.7, respectively, and that on the Co-380 smectite catalyst was 1.5. These selectivities for the isomerization resemble the selectivities on solid acid sites [19]. Pyridine adsorption experiments show that all the smectite materials have Lewis acid sites and do not have Brönsted acid sites. The Lewis acid sites would be located both on tetrahedral sheets and on the edge framework of the trilayer sheets, in consideration of the smectite structure. The active Lewis acid sites would be located on the edge framework, since the activity depends on the divalent species existing in octahedral sheet sandwiched by two tetrahedral sheets. Luca et al. reported that Zn2+ -substituted fluorohectorite catalysts showed higher activities for the alkylation of benzene with benzyl chloride, producing diphenylmethane, than clays-supported zinc divalent cations. They also reported that the active sites for the reaction were non-exchangeable Lewis acid sites in smectite structure [20]. This report supports that nickel divalent cations which exist on the edge framework act as active Lewis acid sites. The synthesized
smectite samples used have large surface areas, and many small fragments with the same smectite structure intercalate in the interlayer region [8]. The high activities of the Ni2+ substituted catalysts are probably derived from Ni2+ Lewis acid sites located on the edge framework. The behavior of the Lewis acid sites in the nickel-substituted smectites does not correspond to the catalytic activities. The amount of Lewis acid sites on the nickel-substituted smectites treated at 573 K increased slightly after 773 K treatment, even though catalytic activity increased rapidly after 773 K treatment. One possible explanation for this discrepancy is that the Lewis acid sites determined by pyridine adsorption include the active edge framework of the trilayer sheets and inactive sites on the tetrahedral sheets, as described above. The quality of Lewis acid sites also accounts for this discrepancy. The surface area of nickel-substituted smectites gradually decreased with increasing treatment temperature (Fig. 5). Strong Lewis acid sites may be produced with high temperature treatment. Ozaki et al. reported that NiO–SiO2 catalysts showed high activity for isomerization of 1-butene and that the catalysts gave two maximum activities at 373 and 873 K on evacuation at elevated temperatures [21]. They concluded that the active sites for the isomerization were Brönsted acid sites after being evacuated at 373 K, while they were Lewis acid sites after being evacuated at 873 K [22]. The synthesized smectite catalysts in this study showed the maximum in the activity at 773–873 K because nickel cations at
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edge sites of the octahedral sheets have only Lewis acid character. The Ni-481 catalyst shows 16 times higher activities than the Ni-359 catalyst, even though the former catalyst has only twice the number of Lewis acid sites and only 1.5 times larger surface area. The Co-380 catalyst shows the isomerization activity, while the Co-202 catalyst is inactive, although both cobalt-substituted smectites have Lewis acid sites. It is possible to say that the pH for the synthesis of smectite influences the isomerization activity. The pH values in synthesizing Ni-481 and Ni-359 catalysts were 10.4 and 11.7, respectively. Although all the samples were washed by distilled water after the hydrothermal treatment, smectite samples synthesized at higher pH have larger amounts of sodium species than the ideal composition of smectite materials (Si : M : Na = 8 : 6 : 0.66) (Table 1). It means that sodium ions exist not only in the interlayer region holding the electronegative layers in smectite structure, but also near Lewis acid sites as NaOH or Na2 O species. Because the excess sodium species act as poison to uncoordinated Ni2+ sites, some of Lewis acid sites would be deactivated by basic sodium species. The same reason would also explain why Co-202 showed no activity for the isomerization, because Co-202 has larger amount of sodium species than Co-380 does. 5. Conclusion 1-butene isomerization activities of newly synthesized divalent cation (Ni2+ , Co2+ , Mg2+ , and Z2+ )-substituted smectite materials were studied. The activity depended on the cation species; Ni2+ -substituted smectite catalysts showed highest isomerization activities. Co2+ -substituted smectite catalysts were also active but showed lower activities for the reaction. Mg2+ and Zn2+ smectite materials did not show any isomerization activities. Ni2+ or Co2+ -substituted smectite catalysts which were synthesized at lower pH have larger surface area and showed higher isomerization activities. The pH at smectite synthesis is crucial for the determination of
the final catalytic activity. The amount of Lewis acid sites determined by pyridine adsorption did not correspond to the 1-butene isomerization activity. The Lewis acid sites exist on the tetrahedral sheets and on the edge framework of the trilayer sheets. The active sites are Lewis acid sites located on edges of the octahedral sheets.
References [1] G.W. Brindley, Crystal structures of clay minerals and their X-ray identification, in: G.W. Brindley, G. Brown (Eds.), Mineralogical Society, London, 1984, Chap. 2. [2] N. Güven, in: S.E. Bailey (Ed.), Hydrous Phyllosilicates, Mineralogical Society of America, Washington, DC, 1988, Chap. 13. [3] K. Torii, T. Iwasaki, Chem. Lett. (1986) 2021. [4] K. Torii, T. Iwasaki, Clay. Sci. 7 (1987) 1. [5] K. Torii, T. Iwasaki, Chem. Lett. (1988) 2045. [6] K. Torii, T. Iwasaki, Y. Onodera, M. Shimada, J. Chem. Soc. Jpn., Chem. Indust. Chem. (1989) 345. [7] K. Torii, Y. Onodera, T. Iwasaki, M. Shirai, M. Arai, Y. Nishiyama, J. Porous Mater. 4 (1997) 261. [8] K. Torii, T. Iwasaki, Y. Onodera, Y. Nishiyama, J. Ceram. Soc. Jpn. 100 (1992) 368. [9] Y. Nishiyama, M. Arai, S.L. Guo, M. Sonehara, N. Naito, K. Torii, Appl. Catal. 95 (1993) 17. [10] K. Urabe, M. Koga, Y. Izumi, J. Chem. Soc., Chem. Commun. (1989) 807. [11] J. Gaaf, R. van Santen, A. Knoester, B. van Wingerden, J. Chem. Soc., Chem. Commun. (1983) 655. [12] M. Arai, M. Kanno, Y. Nishiyama, K. Torii, M. Shirai, J. Catal. 182 (1999) 507. [13] M. Arai, Y. Minato, K. Torii, M. Shirai, Catal. Lett., in press. [14] M. Arai, S.L. Guo, M. Shirai, Y. Nishiyama, K. Torii, J. Catal. 161 (1996) 704. [15] M. Shirai, K. Torii, M. Arai, Jpn. J. Appl. Phys., in press. [16] M. Shirai, N. Suzuki, Y. Nishiyama, K. Torii, M. Arai, Appl. Catal. A. Gen. 177 (1999) 215. [17] E. Hayashi, E. Iwamatsu, M.E. Biswas, S.A. Ali, Y. Yamamoto, Y. Sanada, A.K.K. Lee, H. Hamid, T. Yoneda, Chem. Lett. (1997) 433. [18] E.P. Parry, J. Catal. 2 (1963) 371. [19] J.W. Hightower, W.K. Hall, Chem. Eng. Prog. Symp. Ser. 63 (1967) 122. [20] V. Luca, L. Kevan, C.N. Rhodes, D.R. Brown, Clay Miner. 27 (1992) 515. [21] K. Kimura, H. Ai, A. Ozaki, J. Catal. 18 (1970) 2771. [22] J.R. Sohn, A. Ozaki, J.Catal. 61 (1980) 29.
Acidity and 1-butene isomerization of synthesized smectite-type catalysts containing different divalent cations Masayuki Shirai a,∗ , Kuriko Aoki a , Kazuo Torii b , Masahiko Arai a a
Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba, Sendai 980-8577, Japan b Tohoku National Industrial Research Institute, Nigatake, Miyagino, Sendai 983-8551, Japan Received 23 March 1999; received in revised form 18 May 1999; accepted 21 May 1999
Abstract Smectite-type materials which contain different divalent cations (Ni2+ , Co2+ , Mg2+ , and Zn2+ ) in octahedral sheets have been synthesized and characterized by pyridine adsorption and catalytic effectiveness for the isomerization of 1-butene. All smectite-type materials examined have Lewis acid sites; however, the activity values for the isomerization of 1-butene significantly depend on the divalent cation species included. Nickel-containing smectite-type catalysts are stable and the most active among the smectite catalysts. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Synthesized smectite-type catalysts; 1-Butene isomerization; Lewis acid sites
1. Introduction Smectite-type materials have layered structures in which each layer is composed of one octahedral sheet sandwiched by two tetrahedral sheets. The octahedral sheets contain divalent cations which are surrounded by six oxygen atoms in octahedral structure and the tetrahedral sheets contain Si4+ ions which are surrounded by four oxygen atoms in tetrahedral structure. The trilayers are negatively charged and are held together by the electrostatic interaction with exchangeable cations in the interlayer region. There are two types of sites for transition metal cations in smectite-type materials. One is an ion-exchangeable site in the interlayer space and the other is a lattice site within the octahedral sheet. Smectite-type materi∗ Corresponding author. Tel./fax: +81-22-217-5631 E-mail address: [email protected] (M. Shirai)
als containing transition metal ions in the lattice sites are found in nature and have been synthesized [1,2]. We have invented a hydrothermal method to synthesize smectite-type materials [3–8]. It is possible with this method to prepare smectite materials including different divalent cations in the octahedral sheets and also with controlled pore size in meso range. Due to their relatively high surface area and easy introduction of various transition metal cations in the lattice and on ion-exchangeable sites, synthesized smectite-type materials can serve as potentially useful catalysts and supports. Nickel-substituted smectite-materials show high activities for the decomposition of 2-propanol [9], the selective dimerization of ethylene [10], and the hydroisomerization of n-pentene [11] and hydrogenation of acetonitrile [12]. Cobalt-substituted smectite materials show high activity for the hydrodesulfurization of thiophene [13]. The activities for hydrogenolysis of n-butane on plat-
0926-860X/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 1 9 8 - 2
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Table 1 Results of characterization of smectite samples prepared Sample pHa
Compositionb (Si : M : Na) Specific surface areac (cm2 g−1 ) Total pore volumec (cm3 g−1 ) Average pore diameterc (nm)
Ni-481 10.4 8 : 6.03 : 0.90 Ni-359 11.7 8 : 6.16 : 1.86 Co-380 6.1 8 : 5.98 : 0.22 Co-202 9.0 8 : 6.34 : 0.85 Mg-518 9.5 8 : 6.49 : 0.49 Mg-325 11.4 8 : 6.62 : 1.10 Zn-153 6.6 8 : 6.53 : 1.28
481 359 380 202 518 325 153
0.31 0.21 0.34 0.16 0.39 0.22 0.17
2.6 2.4 3.6 3.2 3.0 2.7 4.3
a
pH value for hydrothermal synthesis of smectite materials. Number of atoms in the unit cell determined by X-ray fluorescence method; M : Ni, Co, Mg, or Zn. c Determined by nitrogen adsorption and desorption isotherms. b
inum supported on smectite-type materials depend on substituted divalent cation species in the octahedral sheets [14,15]. Pore-size controlled smectites loaded with palladium show size-selective hydrogenation of butadiene–acrylonitrile rubbers [16]. Cobalt oxide on smectite-type materials shows higher activity for thiophene hydrodesulfurization than Co–Mo/Al2 O3 [17]. In this study we report the evaluation of thermal stability and acidic properties of synthesized smectite-type materials which contain various divalent cations (Ni2+ , Co2+ , Mg2+ , and Zn2+ ) in the lattice sites and their activities for the 1-butene isomerization as a test reaction of acidic catalysts. 2. Experimental 2.1. Sample preparation Smectite-type materials were synthesized with a hydrothermal method described previously [3–8]. The aqueous solution of sodium silicate (SiO2 /Na2 O = 3.22) and sodium hydroxide was mixed with the aqueous solution of metal chloride to precipitate Si–M (M: divalent metal cation, Si : M = 8 : 6) hydroxides. The precipitation pH of Si–M hydroxide was controlled by changing the molar ratio of sodium hydroxide to sodium silicate. After separating and washing of Si–M hydroxide, the slurries were prepared from Si–Mg hydroxide and water. The Si–M slurries were treated hydrothermally in an autoclave at 473 K under autogaseous water vapor pressure for 2 h. The resultant samples were dried at 353 K; then we obtained smectite catalysts. The smectite catalysts are denoted by the divalent species in octahedral sheets and BET surface area, e.g.
Ni-481 for the Ni2+ substituted smectite-type material with a surface area of 481 m2 g−1 . The characteristics of the smectite catalysts prepared are given in Table 1. 2.2. Isomerization of 1-butene The catalytic reactions were carried out in a closed circulating system (volume: 148 ml). After pre-evacuation of a catalyst (200 mg) in the reactor at various temperatures (373–973 K) for 1 h, the reaction was conducted by circulating 1-butene at 323 K. The initial pressure of 1-butene was 13 kPa. Products were analyzed by a gas chromatograph with a flame ionization detector and a column packed with a Sebaconitrile 25% supported Uniport C column. The catalytic activities were evaluated from the formation of main products of trans-2-butene and cis-2-butene. 2.3. Pyridine adsorption To examine the acidic properties, we measured IR spectra of pyridine adsorbed on catalysts. A wafer of smectite catalyst was treated in an IR cell at various temperatures (373–923 K) for 1 h under dynamic evacuation. Then the wafer was exposed to pyridine vapor (1.0 kPa) at 323 K for 0.5 h, followed by evacuation at 433 K for 2 h to remove physisorbed pyridine molecules. 3. Results 3.1. Smectite catalysts Seven kinds of smectite catalysts were used in this study (Table 1). The content of sodium in the sam-
M. Shirai et al. / Applied Catalysis A: General 187 (1999) 141–146
Fig. 1. Initial rates of 1-butene isomerization of the Ni-481 (䊉) and Ni-359 (䊊) catalysts as a function of pre-evacuation temperature.
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Fig. 2. Initial rates of 1-butene isomerization of the Co-380 (N) and Co-202 (4) catalysts as a function of pre-evacuation temperature.
ples depended on pH at the producing of the Si–M hydroxide. The ratios of Si : M (M; divalent cation) of the smectite catalysts were about 8 : 6. XRD patterns showed that all the samples have smectite-type structure [8]. Smectite catalysts prepared at lower pH had fewer sodium ions, higher surface areas, and larger pore volumes for a series of samples including the same divalent cation species in the octahedral sheet. 3.2. Isomerization of 1-butene The Ni-481, Ni-359, and Co-380 catalysts were found to be active for the isomerization. The products were trans-2-butene and cis-2-butene. The other smectites showed no activities for the isomerization of 1-butene under the reaction conditions used. Fig. 1 shows the initial rates at 323 K for the Ni-481 and Ni-359 catalysts pre-evacuated at temperatures from 473 to 973 K. The activities of Ni2+ -substituted smectite catalysts increased with increasing evacuation temperature and showed a maximum after pre-evacuation at 773–873 K; then the activity decreased to almost zero at 973 K. The activities were different for the Ni-481 and the Ni-359 catalysts. After pre-evacuation at 873 K, the activity of the Ni-481 catalyst was 2.1 × 10−4 mol min−1 g−1 cat , which was almost 16 times higher than the maximum activity of the Ni-359 catalyst after pre-evacuation at 773 K. The activity of the Co-380 catalyst also depended on the pre-evacuation temperature in a similar fashion to the activity of the Ni2+ -substituted cata-
Fig. 3. Infrared spectra of pyridine adsorbed on smectites. Sample and evacuation temperature: (a) Ni-481, 573 K; (b) Ni-359, 473 K; (c) Co-380, 573 K; (d) Co-202, 473 K; (e) Mg-518, 473 K; (f) Mg-325, 473 K; (g) Zn-153, 473 K.
lysts; however, the Co-380 catalyst was less active compared to the nickel-substituted smectite catalyst (Fig. 2). The maximum activity of Co-380 was 3.1 × 10−7 mol min−1 g−1 cat , which was as almost one-seven hundredth as that for the Ni-481 catalyst. The Co-202 catalyst did not show the isomerization activities under the reaction conditions used. 3.3. Pyridine adsorption Fig. 3 shows IR spectra of pyridine adsorbed on smectite samples. Absorbances were normalized by the sample weight. Four peaks were observed
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Fig. 4. Intensities of the Lewis acid band as a function of pre-evacuation temperature. (a) nickel smectites (䊉: Ni-481 and 䊊: Ni-359); (b) cobalt smectites (N:Co-380 and 4: Co-202); (c) magnesium smectites (䊏: Mg-518 and 䊐: Mg-325); (d) zinc smectite (H: Zn-153).
at 1446–1454, 1489–1492, 1574–1578, and 1608– 1612 cm−1 corresponding to pyridine molecules coordinated to metal cations with Lewis acid character [18]. No peak was measured for any of the samples used at near 1550 cm−1 , assigned to pyridinium ions formed from pyridine adsorbed on Brönsted acid sites. The peak areas of the band at 1446–1454 cm−1 are plotted as a function of pre-evacuation temperature in Fig. 4. The peak shift was hard to observe with pre-evacuation temperature (IR resolution is 4 cm−1 ). These peak areas are normalized with the sample weights, being proportional to the amount of Lewis acid sites per unit weight of sample. For the Ni2+ -substituted smectite catalysts (Ni-481 and Ni-359), the areas slightly increase up to 773–873 K;
however the areas appreciably decrease after 973 K treatment. The peak area for the Ni-481 catalyst was higher than that for the Ni-359 catalyst at any pre-evacuation temperature. For the Co2+ -substituted smectite samples (Co-380 and Co-202), the areas slightly decreased up to 773 K and then the intensities appreciably decreased with increasing pre-evacuation temperature. The peak area of the IR spectra for coordinated pyridine on the Co-380 catalyst was higher than that for the Co-202 catalyst at any pre-evacuation temperature. For the Mg2+ smectite samples (Mg-518 and Mg-325) and the Zn2+ sample (Zn-153), the areas simply decreased with increasing pre-evacuation temperature. The peak area for Mg-518 was higher than that for Mg-325 at pre-evacuation temperatures examined.
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3.4. Thermal stability of nickel-substituted smectite catalysts The thermal stability values of catalytic active nickel-substituted smectite materials were investigated by measuring the surface area of the samples treated at various temperatures (573–873 K) for 1 h. The surface areas of the samples decreased with increasing pre-treatment temperature. The surface areas of Ni-481 and Ni-359 treated at 873 K were 381 and 184 m2 g−1 cat , respectively. The nickel smectite catalyst prepared at lower pH had thermally stable structure. Fig. 5. Surface areas of the Ni-481 (䊉) and Ni-359 (䊊) catalysts as a function of pre-evacuation temperature.
4. Discussion Only the Ni-481, Ni-359, and Co-380 catalysts showed the isomerization activities. No other smectite materials were active. Moreover, the activity of Ni-481 was about 700 times higher than that of Co-380. These results mean that the isomerization activity of the smectite materials depends on the divalent cation species (M2+ ) in the octahedral sheet; the order was Ni2+ Co2+ Mg2+ = Zn2+ = 0. The selectivities of the isomerization (cis-2-butene/trans-2-butene) on Ni-481 and Ni-359 smectite catalysts were 0.6 and 0.7, respectively, and that on the Co-380 smectite catalyst was 1.5. These selectivities for the isomerization resemble the selectivities on solid acid sites [19]. Pyridine adsorption experiments show that all the smectite materials have Lewis acid sites and do not have Brönsted acid sites. The Lewis acid sites would be located both on tetrahedral sheets and on the edge framework of the trilayer sheets, in consideration of the smectite structure. The active Lewis acid sites would be located on the edge framework, since the activity depends on the divalent species existing in octahedral sheet sandwiched by two tetrahedral sheets. Luca et al. reported that Zn2+ -substituted fluorohectorite catalysts showed higher activities for the alkylation of benzene with benzyl chloride, producing diphenylmethane, than clays-supported zinc divalent cations. They also reported that the active sites for the reaction were non-exchangeable Lewis acid sites in smectite structure [20]. This report supports that nickel divalent cations which exist on the edge framework act as active Lewis acid sites. The synthesized
smectite samples used have large surface areas, and many small fragments with the same smectite structure intercalate in the interlayer region [8]. The high activities of the Ni2+ substituted catalysts are probably derived from Ni2+ Lewis acid sites located on the edge framework. The behavior of the Lewis acid sites in the nickel-substituted smectites does not correspond to the catalytic activities. The amount of Lewis acid sites on the nickel-substituted smectites treated at 573 K increased slightly after 773 K treatment, even though catalytic activity increased rapidly after 773 K treatment. One possible explanation for this discrepancy is that the Lewis acid sites determined by pyridine adsorption include the active edge framework of the trilayer sheets and inactive sites on the tetrahedral sheets, as described above. The quality of Lewis acid sites also accounts for this discrepancy. The surface area of nickel-substituted smectites gradually decreased with increasing treatment temperature (Fig. 5). Strong Lewis acid sites may be produced with high temperature treatment. Ozaki et al. reported that NiO–SiO2 catalysts showed high activity for isomerization of 1-butene and that the catalysts gave two maximum activities at 373 and 873 K on evacuation at elevated temperatures [21]. They concluded that the active sites for the isomerization were Brönsted acid sites after being evacuated at 373 K, while they were Lewis acid sites after being evacuated at 873 K [22]. The synthesized smectite catalysts in this study showed the maximum in the activity at 773–873 K because nickel cations at
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edge sites of the octahedral sheets have only Lewis acid character. The Ni-481 catalyst shows 16 times higher activities than the Ni-359 catalyst, even though the former catalyst has only twice the number of Lewis acid sites and only 1.5 times larger surface area. The Co-380 catalyst shows the isomerization activity, while the Co-202 catalyst is inactive, although both cobalt-substituted smectites have Lewis acid sites. It is possible to say that the pH for the synthesis of smectite influences the isomerization activity. The pH values in synthesizing Ni-481 and Ni-359 catalysts were 10.4 and 11.7, respectively. Although all the samples were washed by distilled water after the hydrothermal treatment, smectite samples synthesized at higher pH have larger amounts of sodium species than the ideal composition of smectite materials (Si : M : Na = 8 : 6 : 0.66) (Table 1). It means that sodium ions exist not only in the interlayer region holding the electronegative layers in smectite structure, but also near Lewis acid sites as NaOH or Na2 O species. Because the excess sodium species act as poison to uncoordinated Ni2+ sites, some of Lewis acid sites would be deactivated by basic sodium species. The same reason would also explain why Co-202 showed no activity for the isomerization, because Co-202 has larger amount of sodium species than Co-380 does. 5. Conclusion 1-butene isomerization activities of newly synthesized divalent cation (Ni2+ , Co2+ , Mg2+ , and Z2+ )-substituted smectite materials were studied. The activity depended on the cation species; Ni2+ -substituted smectite catalysts showed highest isomerization activities. Co2+ -substituted smectite catalysts were also active but showed lower activities for the reaction. Mg2+ and Zn2+ smectite materials did not show any isomerization activities. Ni2+ or Co2+ -substituted smectite catalysts which were synthesized at lower pH have larger surface area and showed higher isomerization activities. The pH at smectite synthesis is crucial for the determination of
the final catalytic activity. The amount of Lewis acid sites determined by pyridine adsorption did not correspond to the 1-butene isomerization activity. The Lewis acid sites exist on the tetrahedral sheets and on the edge framework of the trilayer sheets. The active sites are Lewis acid sites located on edges of the octahedral sheets.
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