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Novel efficient synthesis of β-naphthyl methyl ether using sulfated mesoporous molecular sieve Al-MCM-41
Applied Catalysis A: General 178 (1999) L1±L6
Letter
Novel ef®cient synthesis of b-naphthyl methyl ether using sulfated mesoporous molecular sieve Al-MCM-41 Li-Wen Chen, Chih-Yu Chou, An-Nan Ko* Department of Chemistry, Tunghai University, Taichung, Taiwan Received 13 April 1998; received in revised form 7 July 1998; accepted 29 July 1998
Abstract The mesoporous molecular sieve Al-MCM-41 and sulfated Al-MCM-41 were prepared and characterized with various techniques: XRD, ICP±AES, BET, 27Al and 29Si MAS±NMR, FTIR, TPD of ammonia and N2 adsorption isotherms. Impregnation of Al-MCM-41 with sulfuric acid results in the decrease of crystallinity, surface area, pore volume and the expelling of aluminum from the Al-MCM-41 framework, with concomitant increase of the Lewis acidity. b-Naphthyl methyl ether was synthesized from b-naphthol and methanol at 2008C and 35 bar over Al-MCM-41 and sulfated Al-MCM-41 in a batch autoclave reactor. The catalytic results were compared with those obtained by using sulfuric acid, amorphous silica± alumina, g-alumina, USY and H-ZSM-5 zeolites. The sulfated Al-MCM-41 catalyst forms the exclusive product of b-naphthyl methyl ether and has much higher yields than other catalysts except USY. The non-framework aluminum existing in the intrachannel space of Al-MCM-41 probably causes the enhancement of catalyst activity. The catalytic results are correlated to the acidity, the surface area and the pore size of the catalysts. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous molecular sieve; Methanol; b-Naphthol; b-Naphthyl methyl ether; Sulfated Al-MCM-41
1. Introduction b-Naphthyl methyl ether (b-NME) has been used in perfumery; it is traditionally manufactured from bnaphthol and methanol in the presence of sulfuric acid. However, the drawbacks of such a process include corrosion, safety hazards, separation procedures, and environmental problems due to the use of sulfuric acid. Therefore, the authors attempted to utilize solid acid catalysts to prepare b-NME. A mesoporous molecular sieve Al-MCM-41 was recently developed and was reported to exhibit acidity [1±3]. Liquid phase *Corresponding author. Fax: +886-4-3590426.
alkylation of anthracene, naphthalene and thianthrene to produce 2-tert-butyl plus 2,6-and 2,7-di tert-butyl derivatives were investigated using large pore zeolites and mesoporous molecular sieve MCM-41. The catalyst activity increases from MCM-41 and modernite to b- and Y-zeolites [4]. Isopropylation of benzene and toluene to yield cumene and isopropyltoluene were performed over an MCM-41/g-Al2O3 catalyst. Such catalyst contained mainly strong Lewis acid sites. Such sites were related to the presence of extra-framework aluminum formed during template removing [5,6]. The purposes of this study include the preparation and the characterization of Al-MCM-41 modi®ed
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00271-3
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with sulfuric acid SO2ÿ 4 /Al-MCM-41). Such a catalyst was applied in the synthesis of b-NME. The catalytic results are correlated to the catalyst properties and compared with those of 18 M sulfuric acid and other solid acids such as Al-MCM-41, amorphous silica±alumina, g-alumina, USY and H-ZSM-5 zeolite. 2. Experimental Amorphous silica±alumina (Si/Al5.7, Strem), galumina (Strem), USY (Si/Al27.5, PQ) and H-ZSM5 (Si/Al25.5, PQ) were obtained from commercial sources. These catalysts were calcined at 5008C for 6 h before catalytic reaction. Al-MCM-41 (Si/ Al25.0) was prepared from sodium aluminate (RDH), sodium silicate (Aldrich), sulfuric acid (RDH) and cetyltrimethylammonium bromide following the procedures given in [7]. The sulfated AlMCM-41 (3.7 wt% SO2ÿ 4 /Al-MCM-41) was prepared by impregnating an appropriate amount of 0.5 N sulfuric acid on Al-MCM-41. Then the sample was dried at 1108C for 12 h. The as-synthesized catalysts were calcined at 5408C for 6 h and then characterized by the methods of XRD (Shimadzu XD-5), ICP±AES (Allied Analytical ICAP 9000), 27Al and 29Si MAS± NMR (Bruker MSL 200 MHz), BET analysis (Quantasorb), FTIR (Perkin±Elmer 2000), TPD of ammonia and N2 adsorption isotherm (Micromeritics ASAP 2000). The catalyst (0.50 g) was added into a mixture of methanol (20 ml, RDH) and b-naphthol (7.12 g, RDH), and the reaction was carried out in a stirred batch autoclave reactor (100 ml, Autoclave Engineers) at 2008C and 35 bar for 1 h. The reactor was cooled down to 08C and the solid products which had condensed on the stirrer were washed and dissolved in acetone. Then the whole product was analyzed with a GC (HP 5890 series II) using a ¯ame-ionization detector and a 50 m0.2 mm PONA column (Supelco). 3. Results and discussion ICP±AES analysis of Al-MCM-41 gives a Si/Al mole ratio of 25.0 in the bulk sample. Fig. 1 shows the
Fig. 1. XRD powder pattern of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
powder X-ray diffraction patterns of calcined AlMCM-41 and SO2ÿ 4 /Al-MCM-41 materials. The AlMCM-41 sample (Fig. 1(a)) exhibits a strong peak Ê and two weak peaks at at the d-spacing of 36.2 A Ê Ê those of 21.7 A and 19.0 A. Such well-de®ned pattern is typical of Al-MCM-41 [2,7,8] and can be indexed on a hexagonal lattice with unit cell parameter Ê [9]. Impregnation of Al-MCM-41 with a041.8 A sulfuric acid results in an evident decrease in the (1 1 0) and (2 0 0) peak intensities. The XRD patterns of both samples show a sharp and intense (1 0 0) peak, indicating long-range ordering of the materials. Fig. 2 shows N2 adsorption isotherm of Al-MCM-41 and sulfated Al-MCM-41. Both samples exhibit a hysteresis loop in the region of P/P0 above 0.4; such hysteresis is assigned to the capillary condensation in the mesopores [10]. Fig. 3 indicates the pore size distribution of the two samples. The Ê and a small main peak appears in the range 20±30 A Ê . Sulfation of Al-MCM-41 peak appears at 30±40 A results in a decrease of the pore volume from 1.45 to 0.88 cm3/g. Fig. 4 shows the 27Al MAS±NMR spectra of the calcined samples. For the Al-MCM-41 sample, the peak at 54 ppm is ascribed to tetrahedral coordinate structural aluminum and the peak at 0.7 ppm to the aluminum expelled from the Al-MCM-41 structure upon calcination (Fig. 4(a)) [11]. After impregnation
L.-W. Chen et al. / Applied Catalysis A: General 178 (1999) L1±L6
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Fig. 2. N2 adsorption isotherms of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
Fig. 3. The pore size distribution of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
Fig. 4. 27Al NMR spectra of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
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Fig. 5. 29Si NMR spectra of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
of Al-MCM-41 with sulfuric acid, the tetrahedral aluminum peak at 55 ppm decreases dramatically, with a concomitant increase of octahedral aluminum peak at 0 ppm, indicating a greater extent of aluminum expelled from the Al-MCM-41 framework (Fig. 4(b)). Fig. 5 shows the 29Si MAS±NMR spectra of AlMCM-41 and SO2ÿ 4 /Al-MCM-41. Both samples exhibit very broad peaks which resemble that of amorphous silica. The spectra contain three resonances: at ÿ108, ÿ103 and ÿ92 ppm, corresponding to Si(4Si), Si(OSi)3(OH) and Si(OSi)2(OH)2 environments, respectively [12]. Modi®cation of Al-MCM-41 with sulfuric acid reveals only minor differences of both signal position and intensity in the spectrum, except for a slight decrease of the resonance at ÿ103 ppm.
Fig. 6. TPD of ammonia profile from (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
Fig. 6 illustrates the temperature-programmed desorption pro®les of ammonia from two samples. For Al-MCM-41, the curve shows a gradual rise to a maximum at 2008C and thereafter declines, with a smaller peak appearing at 5408C. The sulfated AlMCM-41 exhibits a similar shape, but a higher amount of strong acid sites as compared to that of Al-MCM41. The broad peaks observed for both samples infer broad acid strength distribution in accordance with the catalyst H-MCM-41 reported in the literature [2,8,12]. According to the FTIR spectra of pyridine adsorbed on Al-MCM-41 and SO2ÿ 4 /Al-MCM-41 at room temperature and subsequent thermal treatment at 200±4008C, both samples contain a larger amount of Lewis acid
Table 1 The physical properties and catalytic results of various catalysts Catalyst
Si/Al mole ratio
BET surface area (m2gÿ1)
b-Naphthol conversion (mol%)a
b-NME selectivity (mol%)a
Al-MCM-41 SO2ÿ 4 =Al-MCM-41 Silica±alumina g-Alumina USY H-ZSM-5 H2SO4
25.0 25.0 5.7 ± 27.5 25.5 ±
1032 674 285 232 665 344 ±
36.4 64.3 18.9 0.0 64.0 5.7 55.4
100 100 100 ± 100 100 100
a
Reaction conditions: methanol/b-naphthol mole ratio 10, 2008C, 35 bar, 1 h and 1000 rpm.
L.-W. Chen et al. / Applied Catalysis A: General 178 (1999) L1±L6
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sites (at 1450 cmÿ1) than that of Brùnsted acid sites (at 1545 cmÿ1). Furthermore, most of the Brùnsted acid sites are lost after evacuation at 4008C, whereas a signi®cant amount of Lewis acid sites are still present, showing the existence of strong Lewis acid sites in the samples. The same was found for the catalyst MCM-41/g-Al2O3 [5]. In addition, SO2ÿ 4 /Al-MCM41 exhibits a notably higher percentage of retention at 4008C for the 1450 cmÿ1 peak than Al-MCM-41, indicating the presence of even stronger Lewis acid sites in the SO2ÿ 4 /Al-MCM-41 samples. Such a result is consistent with that obtained from the 27Al MAS± NMR spectra, which shows the expelling of aluminum from the Al-MCM-41 structure after treatment with sulfuric acid (Fig. 4). Thus the Lewis acidity is enhanced [5,6]. Table 1 compares the physical properties and catalytic results of the various catalysts. Sulfuric acid treatment of Al-MCM-41 sample results in a decrease of surface area from 1032 to 674 m2gÿ1 that is consistent with the measurement of XRD, pore volume and N2 adsorption isotherm. The conversion of b-naphthol is calculated according to the amount of converted b-naphthol. The chemical equation is as follows:
higher than that obtained from sulfuric acid (55.4 mol%). Therefore, sulfuric acid modification of Al-MCM-41 enhances both the catalyst acidity and the catalytic activity. To investigate the reasons for such a result, the sulfated Al-MCM-41 was stirred and washed with de-ionzed water. The filtrate was found via atomic absorption spectroscopy to contain aluminum cation. Furthermore, the resulting catalyst exhibits a significant decrease of b-naphthol conversion from 64.3% to 45.5%. Therefore the non-framework aluminum existing in the intrachannel space is supposed to increase the catalyst activity due to the increase of Lewis acidity [5,6]. Consequently, the sulfated mesoporous molecular sieve Al-MCM-41 appears to be a potential catalyst for manufacturing the fine chemical of b-NME due to its large surface area, uniform mesopores, highly thermal stability and strong acidity.
Both the catalytic activity and the yield of b-NME follow the decreasing order: SO2ÿ 4 /Al-MCM41USY>H2 SO4 >Al-MCM-41>silica±alumina>HZSM-5>g-alumina. Based on the results from FTIR of adsorbed pyridine, TPD of ammonia and those reported elsewhere [2,5,8], the catalyst acid strength also exhibits a similar trend, except for H-ZSM-5 zeolite. As the methylation of b-naphthol is an acid-catalyzed reaction, the conversion of b-naphthol is correlated to the catalyst acid strength. Although the acid strength of H-ZSM-5 is the highest among all solid catalysts, its low catalytic activity is due to the small pore size that hinders both the entrance of bnaphthol and the formation of b-NME. Here a novel catalytic result is observed for SO2ÿ 4 /Al-MCM-41 molecular sieve. When Al-MCM-41 is impregnated with 3.7 wt% H2SO4, the yield of b-NME increases from 36.4 to 64.3 mol%, a value that is even much
of National Tsing Hua University for her assistance in NMR measurements and helpful discussions. We also thank Prof. F.-L. Wang of Providence University for the use of gas sorption analyzer.
Acknowledgements This work was supported in part by the National Science Council of the Republic of China. (NSC 872113-M-029-005). We are grateful to Prof. K.J. Chao
References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] A. Corma, V. Fornes, M.T. Navarro, J. Perez-Pariente, J. Catal. 148 (1994) 569. [3] X.S. Zhao, G.Q. (Max) Lu, G.J. Millar, Ind. Eng. Chem. Res. 35 (1996) 2075. [4] E. Armengol, A. Corma, H. Garcia, J. Primo, Appl. Catal. A 149 (1997) 411. [5] J. Medina-Valtieera, O. Zaldivar, M.A. Sanchez, J.A. Montoya, J. Navarrete, J.A. de los Reyes, Appl. Catal. A 166 (1998) 387.
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[6] J. Medina-Valtieera, M.A. Sanchez, J.A. Montoya, J. Navarrete, J.A. de los Reyes, Appl. Catal. 158 (1997) L1. [7] H.P. Lin, S. Cheng, C.-Y. Mou, J. Chin. Chem. Soc. 43 (1996) 375. [8] C.-Y. Chen, H.-X. Li, M.E. Davis, Microporous Mater. 2 (1993) 17. [9] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, T.U. Chu, D.H. Olsen, E.W. Sheppard,
S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [10] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [11] C.-F. Cheng, J. Klinowski, J. Chem. Soc. Faraday Trans. 92 (1996) 289. [12] A. Liepold, K. Roos, W. Reschetilowski, A.P. Esculcas, J. Rocha, A. Philippou, M.W. Anderson, J. Chem. Soc. Faraday Trans. 92 (1996) 4623.
Letter
Novel ef®cient synthesis of b-naphthyl methyl ether using sulfated mesoporous molecular sieve Al-MCM-41 Li-Wen Chen, Chih-Yu Chou, An-Nan Ko* Department of Chemistry, Tunghai University, Taichung, Taiwan Received 13 April 1998; received in revised form 7 July 1998; accepted 29 July 1998
Abstract The mesoporous molecular sieve Al-MCM-41 and sulfated Al-MCM-41 were prepared and characterized with various techniques: XRD, ICP±AES, BET, 27Al and 29Si MAS±NMR, FTIR, TPD of ammonia and N2 adsorption isotherms. Impregnation of Al-MCM-41 with sulfuric acid results in the decrease of crystallinity, surface area, pore volume and the expelling of aluminum from the Al-MCM-41 framework, with concomitant increase of the Lewis acidity. b-Naphthyl methyl ether was synthesized from b-naphthol and methanol at 2008C and 35 bar over Al-MCM-41 and sulfated Al-MCM-41 in a batch autoclave reactor. The catalytic results were compared with those obtained by using sulfuric acid, amorphous silica± alumina, g-alumina, USY and H-ZSM-5 zeolites. The sulfated Al-MCM-41 catalyst forms the exclusive product of b-naphthyl methyl ether and has much higher yields than other catalysts except USY. The non-framework aluminum existing in the intrachannel space of Al-MCM-41 probably causes the enhancement of catalyst activity. The catalytic results are correlated to the acidity, the surface area and the pore size of the catalysts. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous molecular sieve; Methanol; b-Naphthol; b-Naphthyl methyl ether; Sulfated Al-MCM-41
1. Introduction b-Naphthyl methyl ether (b-NME) has been used in perfumery; it is traditionally manufactured from bnaphthol and methanol in the presence of sulfuric acid. However, the drawbacks of such a process include corrosion, safety hazards, separation procedures, and environmental problems due to the use of sulfuric acid. Therefore, the authors attempted to utilize solid acid catalysts to prepare b-NME. A mesoporous molecular sieve Al-MCM-41 was recently developed and was reported to exhibit acidity [1±3]. Liquid phase *Corresponding author. Fax: +886-4-3590426.
alkylation of anthracene, naphthalene and thianthrene to produce 2-tert-butyl plus 2,6-and 2,7-di tert-butyl derivatives were investigated using large pore zeolites and mesoporous molecular sieve MCM-41. The catalyst activity increases from MCM-41 and modernite to b- and Y-zeolites [4]. Isopropylation of benzene and toluene to yield cumene and isopropyltoluene were performed over an MCM-41/g-Al2O3 catalyst. Such catalyst contained mainly strong Lewis acid sites. Such sites were related to the presence of extra-framework aluminum formed during template removing [5,6]. The purposes of this study include the preparation and the characterization of Al-MCM-41 modi®ed
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00271-3
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with sulfuric acid SO2ÿ 4 /Al-MCM-41). Such a catalyst was applied in the synthesis of b-NME. The catalytic results are correlated to the catalyst properties and compared with those of 18 M sulfuric acid and other solid acids such as Al-MCM-41, amorphous silica±alumina, g-alumina, USY and H-ZSM-5 zeolite. 2. Experimental Amorphous silica±alumina (Si/Al5.7, Strem), galumina (Strem), USY (Si/Al27.5, PQ) and H-ZSM5 (Si/Al25.5, PQ) were obtained from commercial sources. These catalysts were calcined at 5008C for 6 h before catalytic reaction. Al-MCM-41 (Si/ Al25.0) was prepared from sodium aluminate (RDH), sodium silicate (Aldrich), sulfuric acid (RDH) and cetyltrimethylammonium bromide following the procedures given in [7]. The sulfated AlMCM-41 (3.7 wt% SO2ÿ 4 /Al-MCM-41) was prepared by impregnating an appropriate amount of 0.5 N sulfuric acid on Al-MCM-41. Then the sample was dried at 1108C for 12 h. The as-synthesized catalysts were calcined at 5408C for 6 h and then characterized by the methods of XRD (Shimadzu XD-5), ICP±AES (Allied Analytical ICAP 9000), 27Al and 29Si MAS± NMR (Bruker MSL 200 MHz), BET analysis (Quantasorb), FTIR (Perkin±Elmer 2000), TPD of ammonia and N2 adsorption isotherm (Micromeritics ASAP 2000). The catalyst (0.50 g) was added into a mixture of methanol (20 ml, RDH) and b-naphthol (7.12 g, RDH), and the reaction was carried out in a stirred batch autoclave reactor (100 ml, Autoclave Engineers) at 2008C and 35 bar for 1 h. The reactor was cooled down to 08C and the solid products which had condensed on the stirrer were washed and dissolved in acetone. Then the whole product was analyzed with a GC (HP 5890 series II) using a ¯ame-ionization detector and a 50 m0.2 mm PONA column (Supelco). 3. Results and discussion ICP±AES analysis of Al-MCM-41 gives a Si/Al mole ratio of 25.0 in the bulk sample. Fig. 1 shows the
Fig. 1. XRD powder pattern of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
powder X-ray diffraction patterns of calcined AlMCM-41 and SO2ÿ 4 /Al-MCM-41 materials. The AlMCM-41 sample (Fig. 1(a)) exhibits a strong peak Ê and two weak peaks at at the d-spacing of 36.2 A Ê Ê those of 21.7 A and 19.0 A. Such well-de®ned pattern is typical of Al-MCM-41 [2,7,8] and can be indexed on a hexagonal lattice with unit cell parameter Ê [9]. Impregnation of Al-MCM-41 with a041.8 A sulfuric acid results in an evident decrease in the (1 1 0) and (2 0 0) peak intensities. The XRD patterns of both samples show a sharp and intense (1 0 0) peak, indicating long-range ordering of the materials. Fig. 2 shows N2 adsorption isotherm of Al-MCM-41 and sulfated Al-MCM-41. Both samples exhibit a hysteresis loop in the region of P/P0 above 0.4; such hysteresis is assigned to the capillary condensation in the mesopores [10]. Fig. 3 indicates the pore size distribution of the two samples. The Ê and a small main peak appears in the range 20±30 A Ê . Sulfation of Al-MCM-41 peak appears at 30±40 A results in a decrease of the pore volume from 1.45 to 0.88 cm3/g. Fig. 4 shows the 27Al MAS±NMR spectra of the calcined samples. For the Al-MCM-41 sample, the peak at 54 ppm is ascribed to tetrahedral coordinate structural aluminum and the peak at 0.7 ppm to the aluminum expelled from the Al-MCM-41 structure upon calcination (Fig. 4(a)) [11]. After impregnation
L.-W. Chen et al. / Applied Catalysis A: General 178 (1999) L1±L6
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Fig. 2. N2 adsorption isotherms of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
Fig. 3. The pore size distribution of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
Fig. 4. 27Al NMR spectra of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
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Fig. 5. 29Si NMR spectra of calcined (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
of Al-MCM-41 with sulfuric acid, the tetrahedral aluminum peak at 55 ppm decreases dramatically, with a concomitant increase of octahedral aluminum peak at 0 ppm, indicating a greater extent of aluminum expelled from the Al-MCM-41 framework (Fig. 4(b)). Fig. 5 shows the 29Si MAS±NMR spectra of AlMCM-41 and SO2ÿ 4 /Al-MCM-41. Both samples exhibit very broad peaks which resemble that of amorphous silica. The spectra contain three resonances: at ÿ108, ÿ103 and ÿ92 ppm, corresponding to Si(4Si), Si(OSi)3(OH) and Si(OSi)2(OH)2 environments, respectively [12]. Modi®cation of Al-MCM-41 with sulfuric acid reveals only minor differences of both signal position and intensity in the spectrum, except for a slight decrease of the resonance at ÿ103 ppm.
Fig. 6. TPD of ammonia profile from (a) Al-MCM-41 and (b) 3.7% SO2ÿ 4 /Al-MCM-41.
Fig. 6 illustrates the temperature-programmed desorption pro®les of ammonia from two samples. For Al-MCM-41, the curve shows a gradual rise to a maximum at 2008C and thereafter declines, with a smaller peak appearing at 5408C. The sulfated AlMCM-41 exhibits a similar shape, but a higher amount of strong acid sites as compared to that of Al-MCM41. The broad peaks observed for both samples infer broad acid strength distribution in accordance with the catalyst H-MCM-41 reported in the literature [2,8,12]. According to the FTIR spectra of pyridine adsorbed on Al-MCM-41 and SO2ÿ 4 /Al-MCM-41 at room temperature and subsequent thermal treatment at 200±4008C, both samples contain a larger amount of Lewis acid
Table 1 The physical properties and catalytic results of various catalysts Catalyst
Si/Al mole ratio
BET surface area (m2gÿ1)
b-Naphthol conversion (mol%)a
b-NME selectivity (mol%)a
Al-MCM-41 SO2ÿ 4 =Al-MCM-41 Silica±alumina g-Alumina USY H-ZSM-5 H2SO4
25.0 25.0 5.7 ± 27.5 25.5 ±
1032 674 285 232 665 344 ±
36.4 64.3 18.9 0.0 64.0 5.7 55.4
100 100 100 ± 100 100 100
a
Reaction conditions: methanol/b-naphthol mole ratio 10, 2008C, 35 bar, 1 h and 1000 rpm.
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sites (at 1450 cmÿ1) than that of Brùnsted acid sites (at 1545 cmÿ1). Furthermore, most of the Brùnsted acid sites are lost after evacuation at 4008C, whereas a signi®cant amount of Lewis acid sites are still present, showing the existence of strong Lewis acid sites in the samples. The same was found for the catalyst MCM-41/g-Al2O3 [5]. In addition, SO2ÿ 4 /Al-MCM41 exhibits a notably higher percentage of retention at 4008C for the 1450 cmÿ1 peak than Al-MCM-41, indicating the presence of even stronger Lewis acid sites in the SO2ÿ 4 /Al-MCM-41 samples. Such a result is consistent with that obtained from the 27Al MAS± NMR spectra, which shows the expelling of aluminum from the Al-MCM-41 structure after treatment with sulfuric acid (Fig. 4). Thus the Lewis acidity is enhanced [5,6]. Table 1 compares the physical properties and catalytic results of the various catalysts. Sulfuric acid treatment of Al-MCM-41 sample results in a decrease of surface area from 1032 to 674 m2gÿ1 that is consistent with the measurement of XRD, pore volume and N2 adsorption isotherm. The conversion of b-naphthol is calculated according to the amount of converted b-naphthol. The chemical equation is as follows:
higher than that obtained from sulfuric acid (55.4 mol%). Therefore, sulfuric acid modification of Al-MCM-41 enhances both the catalyst acidity and the catalytic activity. To investigate the reasons for such a result, the sulfated Al-MCM-41 was stirred and washed with de-ionzed water. The filtrate was found via atomic absorption spectroscopy to contain aluminum cation. Furthermore, the resulting catalyst exhibits a significant decrease of b-naphthol conversion from 64.3% to 45.5%. Therefore the non-framework aluminum existing in the intrachannel space is supposed to increase the catalyst activity due to the increase of Lewis acidity [5,6]. Consequently, the sulfated mesoporous molecular sieve Al-MCM-41 appears to be a potential catalyst for manufacturing the fine chemical of b-NME due to its large surface area, uniform mesopores, highly thermal stability and strong acidity.
Both the catalytic activity and the yield of b-NME follow the decreasing order: SO2ÿ 4 /Al-MCM41USY>H2 SO4 >Al-MCM-41>silica±alumina>HZSM-5>g-alumina. Based on the results from FTIR of adsorbed pyridine, TPD of ammonia and those reported elsewhere [2,5,8], the catalyst acid strength also exhibits a similar trend, except for H-ZSM-5 zeolite. As the methylation of b-naphthol is an acid-catalyzed reaction, the conversion of b-naphthol is correlated to the catalyst acid strength. Although the acid strength of H-ZSM-5 is the highest among all solid catalysts, its low catalytic activity is due to the small pore size that hinders both the entrance of bnaphthol and the formation of b-NME. Here a novel catalytic result is observed for SO2ÿ 4 /Al-MCM-41 molecular sieve. When Al-MCM-41 is impregnated with 3.7 wt% H2SO4, the yield of b-NME increases from 36.4 to 64.3 mol%, a value that is even much
of National Tsing Hua University for her assistance in NMR measurements and helpful discussions. We also thank Prof. F.-L. Wang of Providence University for the use of gas sorption analyzer.
Acknowledgements This work was supported in part by the National Science Council of the Republic of China. (NSC 872113-M-029-005). We are grateful to Prof. K.J. Chao
References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] A. Corma, V. Fornes, M.T. Navarro, J. Perez-Pariente, J. Catal. 148 (1994) 569. [3] X.S. Zhao, G.Q. (Max) Lu, G.J. Millar, Ind. Eng. Chem. Res. 35 (1996) 2075. [4] E. Armengol, A. Corma, H. Garcia, J. Primo, Appl. Catal. A 149 (1997) 411. [5] J. Medina-Valtieera, O. Zaldivar, M.A. Sanchez, J.A. Montoya, J. Navarrete, J.A. de los Reyes, Appl. Catal. A 166 (1998) 387.
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[6] J. Medina-Valtieera, M.A. Sanchez, J.A. Montoya, J. Navarrete, J.A. de los Reyes, Appl. Catal. 158 (1997) L1. [7] H.P. Lin, S. Cheng, C.-Y. Mou, J. Chin. Chem. Soc. 43 (1996) 375. [8] C.-Y. Chen, H.-X. Li, M.E. Davis, Microporous Mater. 2 (1993) 17. [9] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, T.U. Chu, D.H. Olsen, E.W. Sheppard,
S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [10] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [11] C.-F. Cheng, J. Klinowski, J. Chem. Soc. Faraday Trans. 92 (1996) 289. [12] A. Liepold, K. Roos, W. Reschetilowski, A.P. Esculcas, J. Rocha, A. Philippou, M.W. Anderson, J. Chem. Soc. Faraday Trans. 92 (1996) 4623.