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Heterogenization of AlCl3 on mesoporous molecular sieves and its catalytic activity
Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved
673
Heterogenization of AICI3 on mesoporous molecular sieves and its catalytic activity Kyoung-Ku Kang and Hyun-Ku Rhee* School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Kwanak-ku, Seoul, 151-742, Korea. Aluminum containing mesoporous materials were prepared by direct hydrothermal synthesis and AICI3 immobilization. All the samples were characterized by well-established methods. According to the results of XRD and N2 physisorption, all the mesoporous molecular sieves, pure silica and aluminum substituted samples, have a long-range order structure. The catalytic performance of AICI3 immobilized mesoporous materials in the liquid phase alkylation of benzene is compared with those of other aluminum containing mesoporous materials. The AICI3 immobilized mesoporous materials are more active than other materials and the selectivity to the mono-alkylation product increases as the chain length of olefin molecules becomes large or as the pore size decreases. 1. INTRODUCTION Linear alkyl benzenes (LABs), which are used in the production of biodegradable surfactants, are synthesized commercially by benzene alkylation with linear alkenes. This reaction is usually carried out in the liquid phase in the presence of Lewis acid (AICI3 and ZnCh) or using Bronsted acid (HF and H2SO4). However, this reaction system suffers from several disadvantages such as the corrosive nature, potential environmental hazards and difficulties in separation, recycling and disposal of the spent catalysts. To overcome such problems, heterogeneous processing using solid acid catalysts is highly desirable and thus an extensive effort has been directed to the heterogenization of homogeneous catalysts using clay minerals and zeolites as supports. For example, heterogeneous Friedel-Craft catalysts based on AICI3 and ZnCb immobilized on montmorillonite and silica gel have been reported to show a high catalytic activity for the alkylation reaction [1, 2]. The H^ form zeolite beta has also been known to have a good catalytic activity for the liquid phase alkylation of benzene with light olefins [3]. In this study, alkylation of benzene has been carried out with three olefins, which have different chain lengths, using heterogeneous Lewis acid catalysts prepared by modification of Si-MCM-41 and Si-SBA-15 with AICI3. We have also prepared Al-MCM-41 and Al-SBA-15 by the direct synthesis method and compared their catalytic activities with those of the former. •Adress for correspondence: E-mail. hkrhcc(a)snu.ac.kr Fax. +82-2-888-7295 Tel. +82-2-880-7415 ** This work was supported by Grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and also partially by the Brain Korea 21 Program sponsored by the Ministry of Education.
674
2. EXPERIMENTAL 2.1. Preparation of mesoporous materials The Si-MCM-41 was prepared using a cationic surfactant (cetyltrimethyl ammonium bromide), as a template and sodium silicate solution as a silica source and following the synthesis procedure reported elsewhere [4]. The Si-SBA-15 was obtained by hydrothermal synthesis in the presence of PI23 (BASF: triblock copolymer) as template [5]. All the samples were washed, dried at 373 K and calcined in air at 823 K. The direct synthesis of Al-MCM-41 and Al-SBA-15 in aluminosilicate form was realized by applying almost the same procedure as for the pure silica, except for the addition for aluminum source. The remainder of synthesis procedure is the same as the one for pure silica materials. To obtain the HAl-MCM-41 and HAl-SBA-15 catalysts, the calcined Al-MCM-41 and Al-SBA-15 were converted to the H^ form through NH4^ ion exchange and subsequent calcination. 2.2. Immobilization of AICI3 Anhydrous AICI3 was dissolved in dry benzene. The pure silica samples were heated in a flask at 473 K for 24 h under vacuum condition. The dried Si-MCM-41 was cooled to room temperature under dry N2(g). The AICI3 solution and dried benzene were added to the silica samples. The resulting mixture was refluxed under nitrogen for 48 h, the solvent was eliminated by syringe, and the solid was repeatedly washed with dry solvent more than five times. All the immobilization processes was carried out in a glove box under dry N2(g). Finally, AICI3 immobilized MCM-41 and SBA-15 catalysts were dried at 373 K for 24 h. 2.3. Catalyst characterization All the samples were characterized by various analysis techniques. The small-angle X-ray scattering (SAXS) patterns were measured at room temperature using a Bruker GADDS diffractometer. The N2 adsorption isotherm was measured at liquid N2 temperature with a Micromeritics instrument (ASAP 2010). The specific surface area and pore size were calculated by using BET method and BJH algorithm. 2.3. Alkylation The alkylation of benzene was carried out in the liquid phase with magnetic stirring under refluxing condition for 1-3 h. Under the atmosphere of nitrogen, 100 mmmol of each of the alkenes (1-hexene, 1-octene and 1-dodecene) was added over a period of 30 min to a reactor containing 200 mmol of dried benzene and Ig of catalyst. The AICI3 immobilized catalysts were recycled. The conversion of alkene was analyzed by gas chromatography. 3. RESULTS AND DISCUSSION The SAXS patterns of pure silica and aluminum incorporated mesoporous samples exhibited well defined reflections of hexagonal structure as reported [4, 5]. The SAXS patterns for aluminosilicate MCM-41 and SBA-15 prepared by the direct synthesis procedure showed almost the same SAXS pattern and intensity as those of pure silica sample. The AICI3
675
immobilized mesoporous samples exhibit nearly the same SXAS patterns and the intensities remain almost the same as those for their parent pure silica samples as shown in Figures 1 and 2. These results indicate that the incorporation of aluminum has no influence on the hexagonal structure formed during the direct synthesis procedure. i - AICI3-SBA-15(Si/AI=25) AICI3-MCM-4I (Si/AI=25)
\
AI-MCM-41 (Si/AI=25) AI-MCM-41 (Si/AI=50) Si-MCM-41
•AI-SBA-15(Si/AI=25) •AI-SBA-15(Si/AI=50) -Si-SBA-15
(A C 4)
"c
.>0) "(3 0^
J
.1 2 theta
Fig. 1. SAXS patterns of MCM-41
Fig. 2. SAXS patterns of SBA-15
The results of N2 physisorption for all the mesoporous samples registered surface areas over 800 m^/g and narrow pore size distributions, being typical of mesoporous molecular sieves (c/ Table 1). The results of XRD and N2 physisorption analyses confirmed that the structural integrity of the mesoporous materials remained intact after heterogenization with AICI3. All the aluminum containing samples except for HAl-SBA-15 were found to be effective for the liquid phase alkylation of benzene with olefins as may be noticed from Table 1. The conversion of olefins over HAl-SBA-15 synthesized by the direct synthesis method is very low. Especially, the alkylation of benzene did not progress at all over HAl-SBA-15 with Si/Al=50. Since the SBA-15 was synthesized under acidic condition with 1.6 M aqueous HCl solution, the acidic condition caused the elution of aluminum to the reaction mixture. Therefore, the Al-SBA-15 prepared by direct synthesis contains less aluminum than the initial reactant gel and shows a lower activity. The selectivity to the mono-substituted alkyl benzene increased as the chain length of the olefin molecules becomes large or as the pore size decreases. It should also be noted that AICI3 immobilized mesoporous samples exhibited an enhanced catalytic activity in comparison to HAl-MCM-41 and HAl-SBA-15 with the same Si/Al ratio, respectively, and these catalysts could be re-used three times without loss of catalytic activity.
676
Table 1 The structural data and the catalytic reaction results Si/Al
BET surface area (m^/g)
BJH adsorption average pore size (A)
00
1107.1
37.7
-
-
-
Si-MCM-41''
1 -hexene
1-dodecene conversion / selectivity
(%)
50
963.8
39.5
49.7/73.1
39.8/77.1
41.3/95.7
25
1013.1
42.04
29.9/79.9
32.3/79.7
31.1 /93.2
00
891.4
63.2
-
-
-
50
851.1
60.9
25
824.6
58.7
13.1 /62.1
10.3/61.7
11.7/77.3
70.1/76.5
53.8/92.8
67.4/78.3
48.3/94.1
85.0/60.0
73.8/75.9
81.9/63.1
71.9/74.8
Al-MCM-4r Si-SBA-15' Al-SBA-15' 25 853.4 69.7/74.9 35.6 (fresh) AICI3MCM-41 25 62.9/77.0 (recycled) 25 748.3 61.5 83.9/61.3 (fresh) AICI3SBA-15 25 79.8/63.9 (recycled) The alkylation of benzene was carried out under rcfluxing condition for 3 h. " direct synthesis, ^ selectivity to linear alkyl benzene
4. CONCLUSIONS The SAXS patterns of pure silica and aluminum incorporated mesoporous samples exhibited well defmed reflections of hexagonal structure with their surface areas and pore sizes being typical of mesoporous molecular sieves. The results of SAXS and N2 physisorption analyses confirmed that all the samples have well developed hexagonal mesoporous structure. All the aluminum containing MCM-41 and AICI3 immobilized samples were found effective for the liquid phase alkylation of benzene with olefins. Among various samples the AICI3 immobilized catalyst is the most active and the selectivity to mono alkyl benzene increases as the chain length of olefin molecules becomes large or as the pore size decrease.
REFERENCES 1. V. V. Veselosky, A. S. Gybin, A. S. Lozanova, A. M. Moiseenkov, W. A. Smit, and R. Caple, Tetrahedron Lett., 29 (1989) 175. 2. J. H. Clark, A. P. Kybett, D. J. Macquarrie, S. J. Barlow, and P. Landon, J. Chem. Soc, Chem. Commun., 1353 (1989). 3. E. Armengol, A. Corma, H. Garcia, and J. Primo, J. Appl. Catal. A., 149 (1997) 177. 4. K-K. Kang and H-K Rhee, Stud. Surf. Sci. Catal., 141 (2002) 101. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, and G.D. Stucky, Science, 279(1998)548.
673
Heterogenization of AICI3 on mesoporous molecular sieves and its catalytic activity Kyoung-Ku Kang and Hyun-Ku Rhee* School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Kwanak-ku, Seoul, 151-742, Korea. Aluminum containing mesoporous materials were prepared by direct hydrothermal synthesis and AICI3 immobilization. All the samples were characterized by well-established methods. According to the results of XRD and N2 physisorption, all the mesoporous molecular sieves, pure silica and aluminum substituted samples, have a long-range order structure. The catalytic performance of AICI3 immobilized mesoporous materials in the liquid phase alkylation of benzene is compared with those of other aluminum containing mesoporous materials. The AICI3 immobilized mesoporous materials are more active than other materials and the selectivity to the mono-alkylation product increases as the chain length of olefin molecules becomes large or as the pore size decreases. 1. INTRODUCTION Linear alkyl benzenes (LABs), which are used in the production of biodegradable surfactants, are synthesized commercially by benzene alkylation with linear alkenes. This reaction is usually carried out in the liquid phase in the presence of Lewis acid (AICI3 and ZnCh) or using Bronsted acid (HF and H2SO4). However, this reaction system suffers from several disadvantages such as the corrosive nature, potential environmental hazards and difficulties in separation, recycling and disposal of the spent catalysts. To overcome such problems, heterogeneous processing using solid acid catalysts is highly desirable and thus an extensive effort has been directed to the heterogenization of homogeneous catalysts using clay minerals and zeolites as supports. For example, heterogeneous Friedel-Craft catalysts based on AICI3 and ZnCb immobilized on montmorillonite and silica gel have been reported to show a high catalytic activity for the alkylation reaction [1, 2]. The H^ form zeolite beta has also been known to have a good catalytic activity for the liquid phase alkylation of benzene with light olefins [3]. In this study, alkylation of benzene has been carried out with three olefins, which have different chain lengths, using heterogeneous Lewis acid catalysts prepared by modification of Si-MCM-41 and Si-SBA-15 with AICI3. We have also prepared Al-MCM-41 and Al-SBA-15 by the direct synthesis method and compared their catalytic activities with those of the former. •Adress for correspondence: E-mail. hkrhcc(a)snu.ac.kr Fax. +82-2-888-7295 Tel. +82-2-880-7415 ** This work was supported by Grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and also partially by the Brain Korea 21 Program sponsored by the Ministry of Education.
674
2. EXPERIMENTAL 2.1. Preparation of mesoporous materials The Si-MCM-41 was prepared using a cationic surfactant (cetyltrimethyl ammonium bromide), as a template and sodium silicate solution as a silica source and following the synthesis procedure reported elsewhere [4]. The Si-SBA-15 was obtained by hydrothermal synthesis in the presence of PI23 (BASF: triblock copolymer) as template [5]. All the samples were washed, dried at 373 K and calcined in air at 823 K. The direct synthesis of Al-MCM-41 and Al-SBA-15 in aluminosilicate form was realized by applying almost the same procedure as for the pure silica, except for the addition for aluminum source. The remainder of synthesis procedure is the same as the one for pure silica materials. To obtain the HAl-MCM-41 and HAl-SBA-15 catalysts, the calcined Al-MCM-41 and Al-SBA-15 were converted to the H^ form through NH4^ ion exchange and subsequent calcination. 2.2. Immobilization of AICI3 Anhydrous AICI3 was dissolved in dry benzene. The pure silica samples were heated in a flask at 473 K for 24 h under vacuum condition. The dried Si-MCM-41 was cooled to room temperature under dry N2(g). The AICI3 solution and dried benzene were added to the silica samples. The resulting mixture was refluxed under nitrogen for 48 h, the solvent was eliminated by syringe, and the solid was repeatedly washed with dry solvent more than five times. All the immobilization processes was carried out in a glove box under dry N2(g). Finally, AICI3 immobilized MCM-41 and SBA-15 catalysts were dried at 373 K for 24 h. 2.3. Catalyst characterization All the samples were characterized by various analysis techniques. The small-angle X-ray scattering (SAXS) patterns were measured at room temperature using a Bruker GADDS diffractometer. The N2 adsorption isotherm was measured at liquid N2 temperature with a Micromeritics instrument (ASAP 2010). The specific surface area and pore size were calculated by using BET method and BJH algorithm. 2.3. Alkylation The alkylation of benzene was carried out in the liquid phase with magnetic stirring under refluxing condition for 1-3 h. Under the atmosphere of nitrogen, 100 mmmol of each of the alkenes (1-hexene, 1-octene and 1-dodecene) was added over a period of 30 min to a reactor containing 200 mmol of dried benzene and Ig of catalyst. The AICI3 immobilized catalysts were recycled. The conversion of alkene was analyzed by gas chromatography. 3. RESULTS AND DISCUSSION The SAXS patterns of pure silica and aluminum incorporated mesoporous samples exhibited well defined reflections of hexagonal structure as reported [4, 5]. The SAXS patterns for aluminosilicate MCM-41 and SBA-15 prepared by the direct synthesis procedure showed almost the same SAXS pattern and intensity as those of pure silica sample. The AICI3
675
immobilized mesoporous samples exhibit nearly the same SXAS patterns and the intensities remain almost the same as those for their parent pure silica samples as shown in Figures 1 and 2. These results indicate that the incorporation of aluminum has no influence on the hexagonal structure formed during the direct synthesis procedure. i - AICI3-SBA-15(Si/AI=25) AICI3-MCM-4I (Si/AI=25)
\
AI-MCM-41 (Si/AI=25) AI-MCM-41 (Si/AI=50) Si-MCM-41
•AI-SBA-15(Si/AI=25) •AI-SBA-15(Si/AI=50) -Si-SBA-15
(A C 4)
"c
.>0) "(3 0^
J
.1 2 theta
Fig. 1. SAXS patterns of MCM-41
Fig. 2. SAXS patterns of SBA-15
The results of N2 physisorption for all the mesoporous samples registered surface areas over 800 m^/g and narrow pore size distributions, being typical of mesoporous molecular sieves (c/ Table 1). The results of XRD and N2 physisorption analyses confirmed that the structural integrity of the mesoporous materials remained intact after heterogenization with AICI3. All the aluminum containing samples except for HAl-SBA-15 were found to be effective for the liquid phase alkylation of benzene with olefins as may be noticed from Table 1. The conversion of olefins over HAl-SBA-15 synthesized by the direct synthesis method is very low. Especially, the alkylation of benzene did not progress at all over HAl-SBA-15 with Si/Al=50. Since the SBA-15 was synthesized under acidic condition with 1.6 M aqueous HCl solution, the acidic condition caused the elution of aluminum to the reaction mixture. Therefore, the Al-SBA-15 prepared by direct synthesis contains less aluminum than the initial reactant gel and shows a lower activity. The selectivity to the mono-substituted alkyl benzene increased as the chain length of the olefin molecules becomes large or as the pore size decreases. It should also be noted that AICI3 immobilized mesoporous samples exhibited an enhanced catalytic activity in comparison to HAl-MCM-41 and HAl-SBA-15 with the same Si/Al ratio, respectively, and these catalysts could be re-used three times without loss of catalytic activity.
676
Table 1 The structural data and the catalytic reaction results Si/Al
BET surface area (m^/g)
BJH adsorption average pore size (A)
00
1107.1
37.7
-
-
-
Si-MCM-41''
1 -hexene
1-dodecene conversion / selectivity
(%)
50
963.8
39.5
49.7/73.1
39.8/77.1
41.3/95.7
25
1013.1
42.04
29.9/79.9
32.3/79.7
31.1 /93.2
00
891.4
63.2
-
-
-
50
851.1
60.9
25
824.6
58.7
13.1 /62.1
10.3/61.7
11.7/77.3
70.1/76.5
53.8/92.8
67.4/78.3
48.3/94.1
85.0/60.0
73.8/75.9
81.9/63.1
71.9/74.8
Al-MCM-4r Si-SBA-15' Al-SBA-15' 25 853.4 69.7/74.9 35.6 (fresh) AICI3MCM-41 25 62.9/77.0 (recycled) 25 748.3 61.5 83.9/61.3 (fresh) AICI3SBA-15 25 79.8/63.9 (recycled) The alkylation of benzene was carried out under rcfluxing condition for 3 h. " direct synthesis, ^ selectivity to linear alkyl benzene
4. CONCLUSIONS The SAXS patterns of pure silica and aluminum incorporated mesoporous samples exhibited well defmed reflections of hexagonal structure with their surface areas and pore sizes being typical of mesoporous molecular sieves. The results of SAXS and N2 physisorption analyses confirmed that all the samples have well developed hexagonal mesoporous structure. All the aluminum containing MCM-41 and AICI3 immobilized samples were found effective for the liquid phase alkylation of benzene with olefins. Among various samples the AICI3 immobilized catalyst is the most active and the selectivity to mono alkyl benzene increases as the chain length of olefin molecules becomes large or as the pore size decrease.
REFERENCES 1. V. V. Veselosky, A. S. Gybin, A. S. Lozanova, A. M. Moiseenkov, W. A. Smit, and R. Caple, Tetrahedron Lett., 29 (1989) 175. 2. J. H. Clark, A. P. Kybett, D. J. Macquarrie, S. J. Barlow, and P. Landon, J. Chem. Soc, Chem. Commun., 1353 (1989). 3. E. Armengol, A. Corma, H. Garcia, and J. Primo, J. Appl. Catal. A., 149 (1997) 177. 4. K-K. Kang and H-K Rhee, Stud. Surf. Sci. Catal., 141 (2002) 101. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, and G.D. Stucky, Science, 279(1998)548.