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A novel method to prepare shape-selective catalysts by complexation–impregnation
Catalysis Communications 29 (2012) 153–157
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
A novel method to prepare shape-selective catalysts by complexation–impregnation Bing Xue, Hui Li, Jie Xu, Ping Liu, Yu Zhang, Yongxin Li ⁎ School of Petrochemical Engineering, Changzhou University, Jiangsu 213164, China
a r t i c l e
i n f o
Article history: Received 7 June 2012 Received in revised form 26 September 2012 Accepted 9 October 2012 Available online 15 October 2012 Keywords: Complexation–impregnation Shape-selective catalysis para-Dialkylbenzene MgO/MCM-22
a b s t r a c t The MgO/MCM-22 catalysts were prepared by complexation–impregnation method and their shape-selective performances in the synthesis of para-dialkylbenzenes by alkylation of alkylbenzenes were investigated. Characterization results indicate that the dispersion of the Mg2+ complex is limited to the external surface of the zeolites during complexation–impregnation process, and this results in the nearly complete coverage of the external Brønsted acid sites. Consequently, the shape-selective catalysts prepared by complexation–impregnation method have improved shape-selectivity and maintain the high catalytic activity. Compared with the conventional method for preparation of shape-selective catalysts, the strategy developed here is simple and efficient. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Shape-selective catalysis is important for production of raw materials in the petrochemical and chemical industries. Shapeselective catalysis can be used to produce para-dialkylbenzene derivatives that are valuable performance chemicals [1–3]. It is well known that ZSM-5 and MCM-22 zeolites are suitable catalysts for shapeselective catalysis because their 10-membered ring channels have pore sizes that are very similar to the molecular dimensions of paradialkylbenzene [4,5]. However, the acidic sites on the external surface of zeolite cause side reactions, for example isomerization of paradialkylbenzene, which decreases the para-selectivity [6]. Therefore, it is necessary to passivate these sites to obtain the desired paraselectivity. This has been achieved by a number of modification techniques, including chemical vapor deposition of silica (SiO2-CVD) [7], chemical liquid deposition of silica (SiO2-CLD) [8,9], pre-coking [10] and impregnation of metal or non-metal compounds [11]. Although SiO2-CVD or SiO2-CLD modification with simple silicon alkoxides can greatly improve the para-selectivity, these processes are complicated. Pre-coking is often difficult to control, and needs to be repeated after the recovered zeolite catalysts are regenerated by calcination. By contrast, covering the external acidic sites of zeolite with metal or non-metal compounds is a convenient method. Metal or non-metal oxides could be used to coat ZSM-5 or MCM-22 and eliminate the external surface acidic sites, but this would also coat the acidic sites in the channels and decrease the catalytic activity [12]. Therefore, a simple and efficient strategy for deactivating the external acid sites of zeolite without influencing the channel acidic sites is required. We have developed a novel method for preparing MgO/MCM-22 shape-selective catalysts using a complexation–impregnation method ⁎ Corresponding author. Tel./fax: +86 519 86330135. E-mail address: [email protected] (Y. Li). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2012.10.006
[13,14]. During the complexation–impregnation process, when the molecular size of the complex is larger than the pore entrance of the MCM-22 zeolites, Mg2+ can be restricted to the external surface of the zeolite. This not only covers the external acidic sites, but also prevents deactivation of the acidic sites in the zeolite channels by the generated MgO. In our previous studies [13,14], citric acid and malic acid were used as ligands for preparing shape-selective catalysts. Although high toluene conversions were achieved over those catalysts prepared using the complexation–impregnation method, the selectivity for p-xylene was low (about 61.2% and 50.1% for citric acid and malic acid, respectively). On the basis of a preliminary study, we suggest that the molecular size of the ligands may affect the selectivity for p-xylene. In the present study, MgO/MCM-22 shape-selective catalysts were prepared by selecting a series of ligands with different molecular sizes and the performances of these catalysts for para-alkylation of toluene and ethylbenzene were investigated. To the best of our knowledge, this method is the most efficient and simple strategy for the preparation of shape-selective catalysts developed to date.
2. Experimental 2.1. Catalyst preparation MCM-22 zeolite with Si/Al = 50 was synthesized according to an established procedure [15]. The MgO/MCM-22 catalysts were prepared as follows: MCM-22 was impregnated with an ethanol solution containing acetylacetone and magnesium nitrate. The mole ratio of acetylacetone to magnesium nitrate is 1:1. The mixture was stirred for 1 h and allowed to stand overnight. The resulting materials were calcined at 823 K for 5 h in an air stream. The obtained catalyst was denoted as x% MgO/MCM-22(CI), where x represented the mass percentage of MgO based on the supports and CI represented complexation–
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Table 1 Shape-selective performances of MgO/MCM-22(CI) catalystsa in alkylation of toluene with DMCb. xMgO (%) Conversion of toluene (%) Selectivity of p-xylene in xylene isomers (%) 0 3 6 9 12 15 a b
43.1 39.8 36.1 33.2 32.2 32.1
24.5 36.1 51.8 64.3 73.9 73.2
Acetylacetone is used as ligand. Conditions: temperature=653 K; feed ratio (toluene:DMC)=4:1; WHSV=1.0 h−1.
impregnation method. Other MgO/MCM-22(CI) catalysts with different ligands were prepared by ligand exchange under the same conditions described above. MgO/MCM-22 catalysts prepared by a general impregnation method were synthesized according to an established procedure [12], denoted as MgO/MCM-22(GI). 2.2. Characterization of catalysts N2 adsorption/desorption analyses were performed at 77 K using a physical adsorption instrument (ASAP 2010, Micromeritics). FT-IR spectroscopy with pyridine and 2,4-dimethylquinoline (2,4-DMQ) adsorption was carried out using a Bruker FT-IR spectrometer (TENSOR 27) with a high temperature vacuum chamber [16]. The scanning range was from 1700 to 1400 cm−1 and the resolution was 4 cm−1. The sample powder was pressed into a self-supporting wafer. Prior to each experiment, the catalysts were evacuated (1 Pa) at 653 K for 3 h, and then cooled at 303 K for 2 h, and then exposed to 4 KPa of pyridine or 2,4-DMQ solubilized in CH2Cl2 (30 μmol for 1 mL solvent) for 5 min, and finally evacuated for 1 h at 303 K. After adsorption of pyridine or 2,4-DMQ the samples were heated to 473 K at 10 K min−1 and the spectra were recorded. 2.3. Catalyst testing Vapor phase alkylation of toluene or ethylbenzene was carried out in a fixed bed continuous down-flow reactor. 3 g of the catalyst (in the form of 20–30 mesh pellets) was loaded in the middle of the
reactor fitted with a thermocouple for temperature measurement. The reactor was heated to the requisite temperature in a tubular furnace controlled by a digital temperature controller/indicator. The reaction mixture of toluene or ethylbenzene with alkylation agent was introduced at the top of the reactor by means of an infusion pump. The products were collected in a water-cooled condenser attached to the end of the reactor and analyzed by gas chromatography (GC-2010, SHIMADZU) using a FFAP capillary column and flame ionization detector (FID). The catalysts were tested for 10 h on stream, and conversions were compared after steady state was attained (6 h). 3. Results and discussion 3.1. Catalytic performance of MgO/MCM-22(CI) in alkylation of toluene Table 1 shows the catalytic performance of MgO/MCM-22(CI) in para-selective alkylation of toluene with dimethyl carbonate (DMC) with acetylacetone as the ligand. Over parent MCM-22, a high toluene conversion (43.1%) was observed, but the selectivity for p-xylene (24.5%) was close to the equilibrium composition of xylene isomers (m-xylene 55.6%, o-xylene 19.3%, p-xylene 25.1%) [17]. After MgO modification by the CI method, the conversion of toluene decreased from 39.8% to 32.2% as the amount-of-substance fraction of MgO (xMgO) increased from 3% to 12%. With a further increase in xMgO from 12% to 15%, no changes in the conversion of toluene were observed. When xMgO was increased from 3% to 12%, the selectivity for p-xylene increased (36.1% to 73.9%), and with further increases in xMgO no obvious changes in the selectivity for p-xylene were detected. Therefore, the highest selectivity for p-xylene (73.9%) was obtained over 12% MgO/MCM-22(CI). Alkylation of toluene with DMC over MgO/MCM-22 catalysts prepared by a general impregnation (GI) method, denoted as MgO/MCM-22(GI), has been reported previously [12]. With MgO/MCM-22(GI) catalysts, the conversion of toluene over MgO/MCM-22(GI) catalysts decreased sharply as xMgO increased from 3% to 15%. The highest selectivity for p-xylene (about 65.9%) was obtained over 15% MgO/MCM-22(GI), but the conversion of toluene (4.4%) was much lower than that over MgO/MCM-22(CI) with the same xMgO. The difference in catalytic performance between MgO/MCM-22(CI) and MgO/MCM-22(GI) is thought to occur as follows. During the GI
Scheme 1. Comparison between GI and CI process in preparation of MgO/MCM-22 catalysts.
B. Xue et al. / Catalysis Communications 29 (2012) 153–157
155
0.08 1647
Scheme 2. Complexation reaction between Mg2+ and acetylacetone.
process, Mg2+ can diffuse freely throughout the channels and over the external surface of the zeolites. As a result, the MgO generated after calcination is dispersed on the external surface of zeolite and in the channels, as shown in Scheme 1. Because the acidic sites in the channels of MCM-22 are covered, the conversion of toluene decreases. Although the acidic sites on the external surface are not completely covered by MgO with the GI method, the para-selectivity improves if xMgO increases. This increase could be ascribed to MgO in the MCM-22 channels reducing the pore diameter. By contrast, the process for the MgO/MCM-22(CI) catalysts is very different. During the CI process, Mg 2+ is present as a complex (Scheme 2). This complex is larger than the pore size of MCM-22 zeolite. Therefore, dispersion of the complex is limited to the external surface of the zeolite. After calcination, the generated MgO is only present on the external surface of zeolite, and the acidic sites in the zeolite channels are not deactivated. Consequently, with the CI method, the para-selectivity can be improved without reducing the catalytic activity of MCM-22. This supposition is supported by the following characteristics of the catalysts. The BET surface area of the MgO/MCM-22(CI) catalyst decreased gradually as xMgO increased from 3% to 12%, as shown in Table 2. Further increases in xMgO resulted in no obvious changes in the BET surface area. A slight decrease in the micropore volume was also observed (Table 2), which indicates that a very small proportion of the micropores were blocked during the complexation–impregnation process. Complexation equilibrium between Mg2+ and acetylacetone results in incomplete complexation of the Mg2+, which leads to diffusion of a very small amount of Mg2+ into the MCM-22 channels and a slight decrease in the micropore volume. With 15% MgO/MCM-22(CI), a micropore area of 237 m 2/g and a micropore volume of 0.106 cm3/g were maintained, which indicates that the micropore system of the zeolite is well preserved during the complexation–impregnation process. In contrast, the BET surface areas and micropore volumes of the MgO/MCM-22(GI) catalysts decreased as xMgO increased from 3% to 15%. For 15% MgO/MCM-22(GI), the micropore area was 99 m2/g and the micropore volume was 0.051 cm3/g [12]. These values are much lower than those for 15% MgO/MCM-22(CI). Obviously, most of MgO is dispersed in the MCM-22 channels during the general impregnation process, which results in blockage of the micropore system of the MCM-22 zeolite. FT-IR with 2,4-dimethylquinoline (2,4-DMQ) adsorption was used to investigate the MgO coverage of the external Brønsted acid sites on MCM-22 in the MgO/MCM-22(CI) and MgO/MCM-22(GI) catalysts, as shown in Fig. 1. For the 15% MgO/MCM-22(CI) catalyst, the peak at
Intensity (a.u.)
0.06 Parent MCM-22
0.04
0.02
MgO/MCM-22(GI)
MgO/MCM-22(CI)
0.00 1700
1650
1600
1550
1500
1450
1400
Wavenumber (cm-1) Fig. 1. Infrared spectra of 2,4-DMQ adsorbed on parent MCM-22 and 15% MgO/MCM-22 catalysts.
1647 cm −1, which corresponds to the adsorbed 2,4-DMQ molecule on the external Brønsted acidic sites of MCM-22 zeolite [18,19], decreased in intensity obviously as compared with parent MCM-22. This indicates that the external Brønsted acid sites on the MCM-22 zeolite have been basically covered after 15% MgO modification by the CI method. The intensity of the peak at 1647 cm −1 on 15% MgO/MCM-22(GI) catalyst is significantly higher than that on 15% MgO/MCM-22(CI) catalyst, which indicates that the amount of the external Brønsted acid sites on MgO/MCM-22(GI) is more than that on MgO/MCM-22(CI) with same xMgO. Consequently, the more external Brønsted acid sites on zeolites result in the low para-selectivity. The acidity values of MgO/MCM-22 catalysts with different MgO loadings were characterized by FT-IR with pyridine adsorption, as shown in Table 3. The concentrations of Brønsted and Lewis acid sites were calculated from the intensities of the bands at 1540 and 1445 cm−1 respectively, by using the values of extinction coefficients: 1.13 amd 1.28 cm/μmol determined by previous reports [20]. For MgO/ MCM-22(CI) catalysts, the number of Brønsted acid sites decreased as xMgO increased from 3% to 12%. With further increases in xMgO from 12% to 15%, the number of Brønsted acid sites did not change. The change in the number of Brønsted acid sites for the MgO/MCM-22(CI) catalysts agrees with the trends for catalytic activity of toluene alkylation. For MgO/MCM-22(GI) catalysts, the number of Brønsted acid sites decreased gradually as xMgO increased from 3% to 15%. These results are also consistent with the observed catalytic activity trends for toluene alkylation. Fig. 2 shows the FT-IR spectra with pyridine adsorption for 15% MgO/MCM-22(CI) and 15% MgO/MCM-22(GI) catalysts. Importantly, the number of Brønsted acid sites on MgO/MCM-22(CI) was higher than that on MgO/MCM-22(GI). Consequently, the more Brønsted acid sites on zeolites results in the high toluene conversion.
Table 3 Results of FT-IR spectra with pyridine adsorption of MgO/MCM-22a.
Table 2 The textural parameters of MgO/MCM-22(CI)a catalysts. xMgO (%)
ABET (m2 g−1)
AM (m2 g−1)
AE (m2 g−1)
VT (cm3 g−1)
VM (cm3 g−1)
0% 3% 6% 9% 12% 15%
358 332 303 277 256 255
290 283 268 252 240 237
68 49 35 25 16 18
0.216 0.210 0.206 0.197 0.190 0.184
0.135 0.131 0.130 0.127 0.119 0.106
ABET — BET surface area; AM — micropore area; AE — external surface area; VT — total pore volume; VM — micropore volume. a Acetylacetone is used as ligand.
xMgO (%)
0 3 6 9 12 15
MgO/MCM-22(CI)b
MgO/MCM-22(GI)
Cpy+ (μmol/g)
CpyL (μmol/g)
Cpy+ (μmol/g)
CpyL (μmol/g)
173 163 155 144 129 128
78 422 373 329 249 232
173 143 119 69 37 15
78 413 382 346 276 192
a Cpy+, CpyL: concentrations of Brønsted and Lewis acid sites able to retain pyridine adsorbed at 473 K. b Acetylacetone is used as ligand.
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0.15
0.04
B+L 1490
Intensity (a.u)
Intensity (a.u.)
0.12 B 1540
0.09
0.06
1647
L 1445
MgO/MCM-22(CI)
0.03 DEM
0.02 Acetylacetone
0.01
0.03
MA
MgO/MCM-22(GI)
0.00 1700
1650
1600
1550
1500
1450
0.00 1700
1400
1650
1600
1550
1500
1450
1400
Wavenumber (cm-1)
Wavenumber (cm-1) Fig. 2. Infrared spectra of pyridine adsorbed on 15% MgO/MCM-22 catalysts.
Fig. 3. Infrared spectra of 2,4-DMQ adsorbed on 15% MgO/MCM-22(CI) catalysts with different ligands.
3.2. Effect of chelating ligands on the shape-selective performance of MgO/MCM-22(CI)
chelating ligands occupy a large surface area during the CI process, which means that many external Brønsted acid sites remain after calcination. The number of external Brønsted acid sites decreases as the ligands decrease in size, which increases the selectivity for p-xylene.
Table 4 shows the catalytic performance of 15% MgO/MCM-22(CI) catalysts with different ligands in shape-selective alkylation of toluene with DMC. Other chelating ligands, including malonic acid (MA), dimethyl malonate, diethyl malonate (DEM), ethyl acetoacetate and methyl acetoacetate, also improved the selectivity for p-xylene and gave high toluene conversion (Table 4, entries 3–7). The effect of chelating ligands on the catalytic performance varied widely. For example, 76.1% selectivity for p-xylene and 31.4% toluene conversion were obtained over MgO modified MCM-22 in the presence of MA during the CI process (Table 4, entry 3). However, only 49.7% selectivity for p-xylene was observed when the chelating ligand was replaced by DEM during the CI process (Table 4, entry 7). FT-IR spectra with 2,4-dimethylquinoline adsorption of 15% MgO/MCM-22(CI) in the presence of MA and DEM are present in Fig. 3. The peak intensity at 1647 cm−1 on MgO/MCM-22(CI) in the presence of DEM was higher than that in the presence of MA, which suggests that the number of the external Brønsted acid sites is higher in the former than in the latter. This result can be ascribed to the difference in the sizes of the chelating ligands, with DEM being larger than MA (as shown in Table 4). Large
Table 4 Shape-selective performances of 15% MgO/MCM-22(CI) catalysts with different ligandsa. Conversion of toluene (%)
Selectivity of p-xylene in xylene isomers (%)
Molecular size of Mg2+ complexes (Å × Å)b
4.4 32.2
65.9 73.9
5.3 ∗ 7.9
3
31.4
76.1
5.3 ∗ 7.4
4
33.9
66.7
5.2 ∗ 8.9
5
34.1
61.6
4.6 ∗ 9.5
6
35.3
54.1
5.4 ∗ 10.1
7
36.0
49.7
5.4 ∗ 12.5
Entry
1 2
a b
Chelating ligand
No
Conditions: temperature=653 K; feed ratio (toluene:DMC)=4:1; WHSV=1.0 h−1. Results were obtained by ab initio method with Gaussian.
3.3. Catalytic performance of MgO/MCM-22(CI) in synthesis of para-dialkylbenzene Once the reaction conditions were established, the scope of the MgO-coated zeolites prepared by CI for catalysis of the alkylation of alkylbenzenes to para-dialkylbenzene was explored. The para-selective alkylation reactions of alkylbenzenes with varying alkyl chain length (C1–C2) all proceeded efficiently (Table 5), giving the corresponding para-dialkylbenzenes as the main products with high activity. Obviously, the conversion of alkylbenzene over the MgO/MCM-22(CI) catalyst was much higher than that over the MgO/MCM-22(GI) catalyst, which can be ascribed to the coverage of the external acidic sites and protection of the inner acidic sites during the CI process. Notably, the toluene conversion in alkylation of toluene with DMC was much higher than that with methanol over the investigated catalysts under the same conditions. The activity of dialkylcarbonate was higher than that of alcohol in alkylation of alkylbenzene. 4. Conclusions In conclusion, we have developed a very simple and efficient strategy for preparation of shape-selective catalysts by CI. This novel Table 5 Catalytic performances of 15% MgO/MCM-22 catalysts in para-alkylation of alkylbenzenea. Entry
Alkylation agents
Alkylbenzene \R
Con. (%)
para-Selectivity in dialkylbenzene isomers (%)
1b 2c 3b 4b 5c 6b 7c 8b 9c 10b
DMC DMC Methanol DMC DMC DECd DEC DEC DEC Ethanol
\CH3 \CH3 \CH3 \C2H5 \C2H5 \CH3 \CH3 \C2H5 \C2H5 \C2H5
32.1 4.4 15.3 27.1 13.4 29.1 11.5 28.9 11.6 15.7
73.2 65.9 71.1 75.7 73.9 74.4 74.1 80.4 78.6 79.1
a Conditions: temperature = 653 K; feed ratio (alkylbenzene:alkylation agent) = 4:1; WHSV = 1.0 h−1. b Catalyst prepared by CI method and acetylacetone is used as ligand. c Catalyst prepared by GI method. d Refer to diethyl carbonate.
B. Xue et al. / Catalysis Communications 29 (2012) 153–157
strategy has considerable advantages when compared with the conventional modification method. Coverage of the external acid sites and protection of the inner acid sites on zeolite could be achieved simultaneously by this method. We believe that this method could be used to prepare other shape-selective catalysts. Acknowledgments The authors thank the financial support from the National Natural Science Foundation of China (21076027), the Natural Science Foundation of Jiangsu Province (BK2011231) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] T.C. Tsai, S.B. Liu, I. Wang, Applied Catalysis A: General 181 (1999) 355. [2] M.A. Hernández, J.A. Velasco, M. Asomoza, S. Solís, F. Rojas, V.H. Lara, Industrial and Engineering Chemistry Research 43 (2004) 1779. [3] Y.J. Ji, B. Zhang, L. Xu, H.H. Wu, H.G. Peng, L. Chen, Y.M. Liu, P. Wu, Journal of Catalysis 283 (2011) 168. [4] B.Z. Liu, Z.X. Yu, Y.T. Meng, L.H. Cui, Z.R. Zhu, Studies in Surface Science and Catalysis 175 (2010) 271.
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[5] M.T. Portilla, F.J. Llopis, S. Valencia, A. Corma, Applied Catalysis A: General 393 (2011) 257. [6] J. Čejka, A. Krejčí, N. Žilková, J. Kotrla, S. Ernst, A. Weber, Microporous and Mesoporous Materials 53 (2002) 121. [7] J.H. Kim, A. Ishida, M. Okajima, M. Niwa, Journal of Catalysis 161 (1996) 387. [8] Y.H. Yue, Y. Tang, Y. Liu, Z. Gao, Industrial and Engineering Chemistry Research 35 (1996) 430. [9] R.W. Weber, K.P. Möller, C.T. O'Connor, Microporous and Mesoporous Materials 35–36 (2000) 533. [10] F. Bauer, W.H. Chen, E. Bilz, A. Freyer, V. Sauerland, S.B. Liu, Journal of Catalysis 251 (2007) 258. [11] Z.R. Zhu, Q.L. Chen, Z.K. Xie, W.M. Yang, D.J. Kong, C. Li, Journal of Molecular Catalysis A: Chemical 248 (2006) 152. [12] B. Xue, Y.X. Li, L.J. Deng, Catalysis Communications 10 (2009) 1609. [13] Y. Zhang, B. Xue, J. Xu, P. Liu, Y.X. Li, Petrochemical Technology (China) 41 (2012) 277. [14] J.L. Zhang, B. Xue, J. Xu, P. Liu, Y.X. Li, Journal of Molecular Catalysis (China) 25 (2011) 514. [15] A. Corma, C. Corell, J. Pérez-Pariente, Zeolites 15 (1995) 2. [16] B. Xue, J. Xu, C. Xu, R. Wu, Y.X. Li, K. Zhang, Catalysis Communications 12 (2010) 95. [17] T. Yashima, H. Ahmad, K. Yamazaki, M. Katsata, N. Hara, Journal of Catalysis 16 (1970) 273. [18] S. Laforge, D. Martin, M. Guisnet, Microporous and Mesoporous Materials 67 (2004) 235. [19] S. Laforge, D. Martin, J.L. Paillaud, M. Guisnet, Journal of Catalysis 220 (2003) 92. [20] M. Gusinet, P. Ayrault, J. Datka, Polish Journal of Chemistry 71 (1997) 1455.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
A novel method to prepare shape-selective catalysts by complexation–impregnation Bing Xue, Hui Li, Jie Xu, Ping Liu, Yu Zhang, Yongxin Li ⁎ School of Petrochemical Engineering, Changzhou University, Jiangsu 213164, China
a r t i c l e
i n f o
Article history: Received 7 June 2012 Received in revised form 26 September 2012 Accepted 9 October 2012 Available online 15 October 2012 Keywords: Complexation–impregnation Shape-selective catalysis para-Dialkylbenzene MgO/MCM-22
a b s t r a c t The MgO/MCM-22 catalysts were prepared by complexation–impregnation method and their shape-selective performances in the synthesis of para-dialkylbenzenes by alkylation of alkylbenzenes were investigated. Characterization results indicate that the dispersion of the Mg2+ complex is limited to the external surface of the zeolites during complexation–impregnation process, and this results in the nearly complete coverage of the external Brønsted acid sites. Consequently, the shape-selective catalysts prepared by complexation–impregnation method have improved shape-selectivity and maintain the high catalytic activity. Compared with the conventional method for preparation of shape-selective catalysts, the strategy developed here is simple and efficient. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Shape-selective catalysis is important for production of raw materials in the petrochemical and chemical industries. Shapeselective catalysis can be used to produce para-dialkylbenzene derivatives that are valuable performance chemicals [1–3]. It is well known that ZSM-5 and MCM-22 zeolites are suitable catalysts for shapeselective catalysis because their 10-membered ring channels have pore sizes that are very similar to the molecular dimensions of paradialkylbenzene [4,5]. However, the acidic sites on the external surface of zeolite cause side reactions, for example isomerization of paradialkylbenzene, which decreases the para-selectivity [6]. Therefore, it is necessary to passivate these sites to obtain the desired paraselectivity. This has been achieved by a number of modification techniques, including chemical vapor deposition of silica (SiO2-CVD) [7], chemical liquid deposition of silica (SiO2-CLD) [8,9], pre-coking [10] and impregnation of metal or non-metal compounds [11]. Although SiO2-CVD or SiO2-CLD modification with simple silicon alkoxides can greatly improve the para-selectivity, these processes are complicated. Pre-coking is often difficult to control, and needs to be repeated after the recovered zeolite catalysts are regenerated by calcination. By contrast, covering the external acidic sites of zeolite with metal or non-metal compounds is a convenient method. Metal or non-metal oxides could be used to coat ZSM-5 or MCM-22 and eliminate the external surface acidic sites, but this would also coat the acidic sites in the channels and decrease the catalytic activity [12]. Therefore, a simple and efficient strategy for deactivating the external acid sites of zeolite without influencing the channel acidic sites is required. We have developed a novel method for preparing MgO/MCM-22 shape-selective catalysts using a complexation–impregnation method ⁎ Corresponding author. Tel./fax: +86 519 86330135. E-mail address: [email protected] (Y. Li). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2012.10.006
[13,14]. During the complexation–impregnation process, when the molecular size of the complex is larger than the pore entrance of the MCM-22 zeolites, Mg2+ can be restricted to the external surface of the zeolite. This not only covers the external acidic sites, but also prevents deactivation of the acidic sites in the zeolite channels by the generated MgO. In our previous studies [13,14], citric acid and malic acid were used as ligands for preparing shape-selective catalysts. Although high toluene conversions were achieved over those catalysts prepared using the complexation–impregnation method, the selectivity for p-xylene was low (about 61.2% and 50.1% for citric acid and malic acid, respectively). On the basis of a preliminary study, we suggest that the molecular size of the ligands may affect the selectivity for p-xylene. In the present study, MgO/MCM-22 shape-selective catalysts were prepared by selecting a series of ligands with different molecular sizes and the performances of these catalysts for para-alkylation of toluene and ethylbenzene were investigated. To the best of our knowledge, this method is the most efficient and simple strategy for the preparation of shape-selective catalysts developed to date.
2. Experimental 2.1. Catalyst preparation MCM-22 zeolite with Si/Al = 50 was synthesized according to an established procedure [15]. The MgO/MCM-22 catalysts were prepared as follows: MCM-22 was impregnated with an ethanol solution containing acetylacetone and magnesium nitrate. The mole ratio of acetylacetone to magnesium nitrate is 1:1. The mixture was stirred for 1 h and allowed to stand overnight. The resulting materials were calcined at 823 K for 5 h in an air stream. The obtained catalyst was denoted as x% MgO/MCM-22(CI), where x represented the mass percentage of MgO based on the supports and CI represented complexation–
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Table 1 Shape-selective performances of MgO/MCM-22(CI) catalystsa in alkylation of toluene with DMCb. xMgO (%) Conversion of toluene (%) Selectivity of p-xylene in xylene isomers (%) 0 3 6 9 12 15 a b
43.1 39.8 36.1 33.2 32.2 32.1
24.5 36.1 51.8 64.3 73.9 73.2
Acetylacetone is used as ligand. Conditions: temperature=653 K; feed ratio (toluene:DMC)=4:1; WHSV=1.0 h−1.
impregnation method. Other MgO/MCM-22(CI) catalysts with different ligands were prepared by ligand exchange under the same conditions described above. MgO/MCM-22 catalysts prepared by a general impregnation method were synthesized according to an established procedure [12], denoted as MgO/MCM-22(GI). 2.2. Characterization of catalysts N2 adsorption/desorption analyses were performed at 77 K using a physical adsorption instrument (ASAP 2010, Micromeritics). FT-IR spectroscopy with pyridine and 2,4-dimethylquinoline (2,4-DMQ) adsorption was carried out using a Bruker FT-IR spectrometer (TENSOR 27) with a high temperature vacuum chamber [16]. The scanning range was from 1700 to 1400 cm−1 and the resolution was 4 cm−1. The sample powder was pressed into a self-supporting wafer. Prior to each experiment, the catalysts were evacuated (1 Pa) at 653 K for 3 h, and then cooled at 303 K for 2 h, and then exposed to 4 KPa of pyridine or 2,4-DMQ solubilized in CH2Cl2 (30 μmol for 1 mL solvent) for 5 min, and finally evacuated for 1 h at 303 K. After adsorption of pyridine or 2,4-DMQ the samples were heated to 473 K at 10 K min−1 and the spectra were recorded. 2.3. Catalyst testing Vapor phase alkylation of toluene or ethylbenzene was carried out in a fixed bed continuous down-flow reactor. 3 g of the catalyst (in the form of 20–30 mesh pellets) was loaded in the middle of the
reactor fitted with a thermocouple for temperature measurement. The reactor was heated to the requisite temperature in a tubular furnace controlled by a digital temperature controller/indicator. The reaction mixture of toluene or ethylbenzene with alkylation agent was introduced at the top of the reactor by means of an infusion pump. The products were collected in a water-cooled condenser attached to the end of the reactor and analyzed by gas chromatography (GC-2010, SHIMADZU) using a FFAP capillary column and flame ionization detector (FID). The catalysts were tested for 10 h on stream, and conversions were compared after steady state was attained (6 h). 3. Results and discussion 3.1. Catalytic performance of MgO/MCM-22(CI) in alkylation of toluene Table 1 shows the catalytic performance of MgO/MCM-22(CI) in para-selective alkylation of toluene with dimethyl carbonate (DMC) with acetylacetone as the ligand. Over parent MCM-22, a high toluene conversion (43.1%) was observed, but the selectivity for p-xylene (24.5%) was close to the equilibrium composition of xylene isomers (m-xylene 55.6%, o-xylene 19.3%, p-xylene 25.1%) [17]. After MgO modification by the CI method, the conversion of toluene decreased from 39.8% to 32.2% as the amount-of-substance fraction of MgO (xMgO) increased from 3% to 12%. With a further increase in xMgO from 12% to 15%, no changes in the conversion of toluene were observed. When xMgO was increased from 3% to 12%, the selectivity for p-xylene increased (36.1% to 73.9%), and with further increases in xMgO no obvious changes in the selectivity for p-xylene were detected. Therefore, the highest selectivity for p-xylene (73.9%) was obtained over 12% MgO/MCM-22(CI). Alkylation of toluene with DMC over MgO/MCM-22 catalysts prepared by a general impregnation (GI) method, denoted as MgO/MCM-22(GI), has been reported previously [12]. With MgO/MCM-22(GI) catalysts, the conversion of toluene over MgO/MCM-22(GI) catalysts decreased sharply as xMgO increased from 3% to 15%. The highest selectivity for p-xylene (about 65.9%) was obtained over 15% MgO/MCM-22(GI), but the conversion of toluene (4.4%) was much lower than that over MgO/MCM-22(CI) with the same xMgO. The difference in catalytic performance between MgO/MCM-22(CI) and MgO/MCM-22(GI) is thought to occur as follows. During the GI
Scheme 1. Comparison between GI and CI process in preparation of MgO/MCM-22 catalysts.
B. Xue et al. / Catalysis Communications 29 (2012) 153–157
155
0.08 1647
Scheme 2. Complexation reaction between Mg2+ and acetylacetone.
process, Mg2+ can diffuse freely throughout the channels and over the external surface of the zeolites. As a result, the MgO generated after calcination is dispersed on the external surface of zeolite and in the channels, as shown in Scheme 1. Because the acidic sites in the channels of MCM-22 are covered, the conversion of toluene decreases. Although the acidic sites on the external surface are not completely covered by MgO with the GI method, the para-selectivity improves if xMgO increases. This increase could be ascribed to MgO in the MCM-22 channels reducing the pore diameter. By contrast, the process for the MgO/MCM-22(CI) catalysts is very different. During the CI process, Mg 2+ is present as a complex (Scheme 2). This complex is larger than the pore size of MCM-22 zeolite. Therefore, dispersion of the complex is limited to the external surface of the zeolite. After calcination, the generated MgO is only present on the external surface of zeolite, and the acidic sites in the zeolite channels are not deactivated. Consequently, with the CI method, the para-selectivity can be improved without reducing the catalytic activity of MCM-22. This supposition is supported by the following characteristics of the catalysts. The BET surface area of the MgO/MCM-22(CI) catalyst decreased gradually as xMgO increased from 3% to 12%, as shown in Table 2. Further increases in xMgO resulted in no obvious changes in the BET surface area. A slight decrease in the micropore volume was also observed (Table 2), which indicates that a very small proportion of the micropores were blocked during the complexation–impregnation process. Complexation equilibrium between Mg2+ and acetylacetone results in incomplete complexation of the Mg2+, which leads to diffusion of a very small amount of Mg2+ into the MCM-22 channels and a slight decrease in the micropore volume. With 15% MgO/MCM-22(CI), a micropore area of 237 m 2/g and a micropore volume of 0.106 cm3/g were maintained, which indicates that the micropore system of the zeolite is well preserved during the complexation–impregnation process. In contrast, the BET surface areas and micropore volumes of the MgO/MCM-22(GI) catalysts decreased as xMgO increased from 3% to 15%. For 15% MgO/MCM-22(GI), the micropore area was 99 m2/g and the micropore volume was 0.051 cm3/g [12]. These values are much lower than those for 15% MgO/MCM-22(CI). Obviously, most of MgO is dispersed in the MCM-22 channels during the general impregnation process, which results in blockage of the micropore system of the MCM-22 zeolite. FT-IR with 2,4-dimethylquinoline (2,4-DMQ) adsorption was used to investigate the MgO coverage of the external Brønsted acid sites on MCM-22 in the MgO/MCM-22(CI) and MgO/MCM-22(GI) catalysts, as shown in Fig. 1. For the 15% MgO/MCM-22(CI) catalyst, the peak at
Intensity (a.u.)
0.06 Parent MCM-22
0.04
0.02
MgO/MCM-22(GI)
MgO/MCM-22(CI)
0.00 1700
1650
1600
1550
1500
1450
1400
Wavenumber (cm-1) Fig. 1. Infrared spectra of 2,4-DMQ adsorbed on parent MCM-22 and 15% MgO/MCM-22 catalysts.
1647 cm −1, which corresponds to the adsorbed 2,4-DMQ molecule on the external Brønsted acidic sites of MCM-22 zeolite [18,19], decreased in intensity obviously as compared with parent MCM-22. This indicates that the external Brønsted acid sites on the MCM-22 zeolite have been basically covered after 15% MgO modification by the CI method. The intensity of the peak at 1647 cm −1 on 15% MgO/MCM-22(GI) catalyst is significantly higher than that on 15% MgO/MCM-22(CI) catalyst, which indicates that the amount of the external Brønsted acid sites on MgO/MCM-22(GI) is more than that on MgO/MCM-22(CI) with same xMgO. Consequently, the more external Brønsted acid sites on zeolites result in the low para-selectivity. The acidity values of MgO/MCM-22 catalysts with different MgO loadings were characterized by FT-IR with pyridine adsorption, as shown in Table 3. The concentrations of Brønsted and Lewis acid sites were calculated from the intensities of the bands at 1540 and 1445 cm−1 respectively, by using the values of extinction coefficients: 1.13 amd 1.28 cm/μmol determined by previous reports [20]. For MgO/ MCM-22(CI) catalysts, the number of Brønsted acid sites decreased as xMgO increased from 3% to 12%. With further increases in xMgO from 12% to 15%, the number of Brønsted acid sites did not change. The change in the number of Brønsted acid sites for the MgO/MCM-22(CI) catalysts agrees with the trends for catalytic activity of toluene alkylation. For MgO/MCM-22(GI) catalysts, the number of Brønsted acid sites decreased gradually as xMgO increased from 3% to 15%. These results are also consistent with the observed catalytic activity trends for toluene alkylation. Fig. 2 shows the FT-IR spectra with pyridine adsorption for 15% MgO/MCM-22(CI) and 15% MgO/MCM-22(GI) catalysts. Importantly, the number of Brønsted acid sites on MgO/MCM-22(CI) was higher than that on MgO/MCM-22(GI). Consequently, the more Brønsted acid sites on zeolites results in the high toluene conversion.
Table 3 Results of FT-IR spectra with pyridine adsorption of MgO/MCM-22a.
Table 2 The textural parameters of MgO/MCM-22(CI)a catalysts. xMgO (%)
ABET (m2 g−1)
AM (m2 g−1)
AE (m2 g−1)
VT (cm3 g−1)
VM (cm3 g−1)
0% 3% 6% 9% 12% 15%
358 332 303 277 256 255
290 283 268 252 240 237
68 49 35 25 16 18
0.216 0.210 0.206 0.197 0.190 0.184
0.135 0.131 0.130 0.127 0.119 0.106
ABET — BET surface area; AM — micropore area; AE — external surface area; VT — total pore volume; VM — micropore volume. a Acetylacetone is used as ligand.
xMgO (%)
0 3 6 9 12 15
MgO/MCM-22(CI)b
MgO/MCM-22(GI)
Cpy+ (μmol/g)
CpyL (μmol/g)
Cpy+ (μmol/g)
CpyL (μmol/g)
173 163 155 144 129 128
78 422 373 329 249 232
173 143 119 69 37 15
78 413 382 346 276 192
a Cpy+, CpyL: concentrations of Brønsted and Lewis acid sites able to retain pyridine adsorbed at 473 K. b Acetylacetone is used as ligand.
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B. Xue et al. / Catalysis Communications 29 (2012) 153–157
0.15
0.04
B+L 1490
Intensity (a.u)
Intensity (a.u.)
0.12 B 1540
0.09
0.06
1647
L 1445
MgO/MCM-22(CI)
0.03 DEM
0.02 Acetylacetone
0.01
0.03
MA
MgO/MCM-22(GI)
0.00 1700
1650
1600
1550
1500
1450
0.00 1700
1400
1650
1600
1550
1500
1450
1400
Wavenumber (cm-1)
Wavenumber (cm-1) Fig. 2. Infrared spectra of pyridine adsorbed on 15% MgO/MCM-22 catalysts.
Fig. 3. Infrared spectra of 2,4-DMQ adsorbed on 15% MgO/MCM-22(CI) catalysts with different ligands.
3.2. Effect of chelating ligands on the shape-selective performance of MgO/MCM-22(CI)
chelating ligands occupy a large surface area during the CI process, which means that many external Brønsted acid sites remain after calcination. The number of external Brønsted acid sites decreases as the ligands decrease in size, which increases the selectivity for p-xylene.
Table 4 shows the catalytic performance of 15% MgO/MCM-22(CI) catalysts with different ligands in shape-selective alkylation of toluene with DMC. Other chelating ligands, including malonic acid (MA), dimethyl malonate, diethyl malonate (DEM), ethyl acetoacetate and methyl acetoacetate, also improved the selectivity for p-xylene and gave high toluene conversion (Table 4, entries 3–7). The effect of chelating ligands on the catalytic performance varied widely. For example, 76.1% selectivity for p-xylene and 31.4% toluene conversion were obtained over MgO modified MCM-22 in the presence of MA during the CI process (Table 4, entry 3). However, only 49.7% selectivity for p-xylene was observed when the chelating ligand was replaced by DEM during the CI process (Table 4, entry 7). FT-IR spectra with 2,4-dimethylquinoline adsorption of 15% MgO/MCM-22(CI) in the presence of MA and DEM are present in Fig. 3. The peak intensity at 1647 cm−1 on MgO/MCM-22(CI) in the presence of DEM was higher than that in the presence of MA, which suggests that the number of the external Brønsted acid sites is higher in the former than in the latter. This result can be ascribed to the difference in the sizes of the chelating ligands, with DEM being larger than MA (as shown in Table 4). Large
Table 4 Shape-selective performances of 15% MgO/MCM-22(CI) catalysts with different ligandsa. Conversion of toluene (%)
Selectivity of p-xylene in xylene isomers (%)
Molecular size of Mg2+ complexes (Å × Å)b
4.4 32.2
65.9 73.9
5.3 ∗ 7.9
3
31.4
76.1
5.3 ∗ 7.4
4
33.9
66.7
5.2 ∗ 8.9
5
34.1
61.6
4.6 ∗ 9.5
6
35.3
54.1
5.4 ∗ 10.1
7
36.0
49.7
5.4 ∗ 12.5
Entry
1 2
a b
Chelating ligand
No
Conditions: temperature=653 K; feed ratio (toluene:DMC)=4:1; WHSV=1.0 h−1. Results were obtained by ab initio method with Gaussian.
3.3. Catalytic performance of MgO/MCM-22(CI) in synthesis of para-dialkylbenzene Once the reaction conditions were established, the scope of the MgO-coated zeolites prepared by CI for catalysis of the alkylation of alkylbenzenes to para-dialkylbenzene was explored. The para-selective alkylation reactions of alkylbenzenes with varying alkyl chain length (C1–C2) all proceeded efficiently (Table 5), giving the corresponding para-dialkylbenzenes as the main products with high activity. Obviously, the conversion of alkylbenzene over the MgO/MCM-22(CI) catalyst was much higher than that over the MgO/MCM-22(GI) catalyst, which can be ascribed to the coverage of the external acidic sites and protection of the inner acidic sites during the CI process. Notably, the toluene conversion in alkylation of toluene with DMC was much higher than that with methanol over the investigated catalysts under the same conditions. The activity of dialkylcarbonate was higher than that of alcohol in alkylation of alkylbenzene. 4. Conclusions In conclusion, we have developed a very simple and efficient strategy for preparation of shape-selective catalysts by CI. This novel Table 5 Catalytic performances of 15% MgO/MCM-22 catalysts in para-alkylation of alkylbenzenea. Entry
Alkylation agents
Alkylbenzene \R
Con. (%)
para-Selectivity in dialkylbenzene isomers (%)
1b 2c 3b 4b 5c 6b 7c 8b 9c 10b
DMC DMC Methanol DMC DMC DECd DEC DEC DEC Ethanol
\CH3 \CH3 \CH3 \C2H5 \C2H5 \CH3 \CH3 \C2H5 \C2H5 \C2H5
32.1 4.4 15.3 27.1 13.4 29.1 11.5 28.9 11.6 15.7
73.2 65.9 71.1 75.7 73.9 74.4 74.1 80.4 78.6 79.1
a Conditions: temperature = 653 K; feed ratio (alkylbenzene:alkylation agent) = 4:1; WHSV = 1.0 h−1. b Catalyst prepared by CI method and acetylacetone is used as ligand. c Catalyst prepared by GI method. d Refer to diethyl carbonate.
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strategy has considerable advantages when compared with the conventional modification method. Coverage of the external acid sites and protection of the inner acid sites on zeolite could be achieved simultaneously by this method. We believe that this method could be used to prepare other shape-selective catalysts. Acknowledgments The authors thank the financial support from the National Natural Science Foundation of China (21076027), the Natural Science Foundation of Jiangsu Province (BK2011231) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] T.C. Tsai, S.B. Liu, I. Wang, Applied Catalysis A: General 181 (1999) 355. [2] M.A. Hernández, J.A. Velasco, M. Asomoza, S. Solís, F. Rojas, V.H. Lara, Industrial and Engineering Chemistry Research 43 (2004) 1779. [3] Y.J. Ji, B. Zhang, L. Xu, H.H. Wu, H.G. Peng, L. Chen, Y.M. Liu, P. Wu, Journal of Catalysis 283 (2011) 168. [4] B.Z. Liu, Z.X. Yu, Y.T. Meng, L.H. Cui, Z.R. Zhu, Studies in Surface Science and Catalysis 175 (2010) 271.
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