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MCM-41 supported 12-tungstophosphoric acid mesoporous materials: Preparation, characterization, and catalytic activities for benzaldehyde oxidation with H2O2
Solid State Sciences 24 (2013) 21e25
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Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
MCM-41 supported 12-tungstophosphoric acid mesoporous materials: Preparation, characterization, and catalytic activities for benzaldehyde oxidation with H2O2 Ya Chen, Xiao-Li Zhang, Xi Chen, Bei-Bei Dong, Xiu-Cheng Zheng* College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 May 2013 Received in revised form 26 June 2013 Accepted 29 June 2013 Available online 9 July 2013
Mesoporous molecular sieves MCM-41 and bulk 12-tungstophosphoric acid (HPW) were synthesized and employed to prepare 5e45 wt.% HPW/MCM-41 mesoporous materials. Characterization results suggested the good dispersion of HPW within MCM-41 when the loading of HPW was less than 35 wt.% and HPW/MCM-41 retained the typical mesopore structure of the supports. The results of the catalytic oxidation of benzaldehyde to benzoic acid with 30% H2O2, in the absence of any organic solvent and cocatalysts, indicated that HPW/MCM-41 was an efficient catalyst and 30 wt.% HPW/MCM-41 sample exhibited the highest catalytic activity among these materials. Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords: HPW/MCM-41 Mesoporous materials Characterization Benzaldehyde Benzoic acid Hydrogen peroxide
1. Introduction Heteropoly acids have several advantages, such as strong Brønsted acidity, no corrosion, which make them as promising acid, redox and bifunctional catalysts in homogeneous as well as in heterogeneous systems [1,2]. Among the heteropoly acids, 12tungstophosphoric acid (H3PW12O40, HPW), the strongest one in the Keggin series, have been extensively studied as super acid catalysts for many organic reactions and have been found industrial application in several processes [3]. However, bulk HPW has the disadvantages including weak thermal stability, low porosity and specific surface area (<10 m2/g) and separation problems from reaction mixture due to its high solubility in polar reaction media [4]. To overcome these disadvantages, a huge amount of effects have been attempted. One method was dispersion HPW on a support with high surface area structure, such as SiO2 [5,6], mesoporous silica [7e9], TiO2 [10], and active carbon [11]. The supported materials may keep the properties of HPW without the dissolution or elution.
* Corresponding author. Tel.: þ86 371 67781780. E-mail address: [email protected] (X.-C. Zheng). 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.06.017
Owning to the unique properties, such as high specific surface area and pore volume, tunable pore size and narrow pore size distribution, mesoporous materials of the M41S (Mobil Composition of Matter) family have attracted worldwide attention. Among the members, MCM-41 has attracted considerable attention due to its potential use in adsorptive separations and catalytic conversion of large molecules [12,13]. For example, HPW clusters with diameters about 1.2 nm can be introduced inside the MCM-41 pores, thus allowing for a significant increase in the surface area of the HPW. The large pore sizes of MCM-41 play a major role in overcoming the problem of the obstruction of bulky organic molecules resulting from the small pore volume of HPW. Therefore, the HPW/ MCM-41 materials can be used as a potential solid acid catalyst with high activity and thermal stability [14]. As an important organic intermediate, benzoic acid is generally produced by the oxidation of toluene both in vapour or liquid phases. However, these methods involve expensive reagents, high temperatures, and give unsatisfactory yields. Furthermore, toluene is also a representative of aromatic hydrocarbons categorized as hazardous material [15]. In this respect, heterogeneous catalysis can play a key role in the development of environmentally benign processes particularly in the chemical industries. In addition, it is interesting that hydrogen peroxide has been widely used as oxidant in catalytic oxidation of organic compounds because of its unique
22
Y. Chen et al. / Solid State Sciences 24 (2013) 21e25
properties such as inexpensive, environmentally clean, and easy to handle. In this paper, HPW/MCM-41 mesoporous catalysts were synthesized and characterized by various techniques. The effect of various HPW loadings on the structure and catalytic properties for
Yield of benzoic acid ð%Þ ¼
reaction solution was cooled at 0 C in a refrigerator and the solid was washed with cold water (2 C). The as-prepared solid was dried in air at 80 C overnight and the yield was calculated according to Eq. (1).
amount of the dried solid sample ðgÞ 100 theoretic amount of benzoic acid from 5:0 ml of benzaldehyde ðgÞ
the oxidation of benzaldehyde to benzoic acid with 30% H2O2 of these catalysts was comparatively investigated.
(1)
2. Experimental
The stability property of the catalysts was investigated under the same reaction condition except for the amount of catalyst. The filtered catalyst was dried at 100 C overnight and then directly used in the next catalytic reaction.
2.1. Synthesis of HPW/MCM-41 materials
3. Results and discussion
MCM-41 and bulk HPW was prepared via an alkali hydrothermal process [16a] and an improved diethyl ether extraction method [16b], respectively. The HPW/MCM-41 materials were prepared by an impregnation method and the process was as follows: 2 g of MCM-41 was dispersed into a solution of the desired amount of HPW in 60 ml water under vigorous stirring. The excess water was slowly vaporized at 50 C and dried in an oven overnight at 100 C. The as-prepared catalysts were denoted as x wt.% HPW/MCM-41 (x ¼ 5, 10, 15, 20, 25, 30, 35, 40, and 45, respectively).
The low-angle XRD patterns of MCM-41 and the typical x wt.% HPW/MCM-41 materials are shown in Fig. 1. All the patterns illustrate the characteristics of a typical mesoporous MCM-41 structure. Furthermore, as it can be seen from Fig. 1 that HPW has a striking effect on the width and intensity of the main reflection at high d1 0 0 spacing and the peak become broader and weaker as the loading of HPW increase (Table 1). This suggests the mesoporous structure of MCM-41 supports remain almost unchanged upon the HPW loading but the long-range order is decreased noticeably. So far, some explanations regarding this phenomenon have been discussed. Braga et al. [17] reported that two reasons could cause the loss on the long range ordering of MCM-41: support hydrolysis, which could cause a partial destruction on the mesopores structure due to a transformation of MCM-41 siloxane into silanol groups, or a nonhomogenous distribution of anionic species, such as [PW12O40]3, with silanol groups within the walls of MCM-41 pores. Khder et al. [14] assumed that the severe reflection of MCM-41 in presence of HPW gradual hydrolysis in acidic medium and was mainly due to radiation absorption by the tungsten element present in HPW. The high-angle XRD patterns of bulk HPW and the typical x wt.% HPW/MCM-41 materials are shown in Fig. 2. We can see that the characteristic peaks of HPW are absent in the patterns of HPW/ MCM-41 when the HPW loading is not more than 35 wt.%. Since
N2 adsorption-desorption experiments were performed on a Quantachrome NOVA 1000e surface area & pore size analyser at 196 C. Prior to analysis the samples were degassed at 250 C under vacuum (1.33 Pa) for 2 h. Specific surface areas were calculated by the BrunauereEmmetteTeller (BET) method and the pore volume was determined by nitrogen adsorption at a relative pressure of 0.99. The micropore areas were estimated using the correlation of t-plot method. Pore size distributions were derived from the desorption profiles of the isotherms. X-ray powder diffraction (XRD) patterns were collected on a Panalytical XPert Pro diffractometer using Cu Ka radiation (l ¼ 1.5418 Ǻ). The working voltage and current of the X-ray tube were 40 kV and 100 mA. FT-IR spectra of the samples were recorded on a Thermoscientific Nicolet 380 Fourier transform spectrometer using a KBr pellet technique. Diffuse reflectance (DR) UVeVIS spectra were recorded in the wavelength range 200e800 nm using a Varian Cary 5000 UVeVise NIR spectrophotometer. BaSO4 was used as the reference. Temperature programmed desorption of ammonia (NH3-TPD) was carried out with the use of a Tianjin XQ TP5080 auto-adsorption apparatus. Approximately 50 mg sample was pretreated in a helium atmosphere (30 ml/min, 500 C, 1 h), cooled to 20 C, and then loaded into the reaction cell. The samples were exposed to ammonia at 100 C for 0.5 h and then purged with helium. NH3-TPD spectra were registered between 100 and 750 C (temperature ramp: 10 C/min). The NH3 consumption was monitored by TCD detector.
(100) (110)
(200)
x = 45
Intensity/counts
2.2. Characterization methods
x = 25
x=5
2.3. Catalytic reaction The oxidation of benzaldehyde with H2O2 was conducted in a 50 ml three-neck round-bottomed flask. 0.5 g of HPW/MCM-41 was dispersed into 17.5 ml 30 wt.% H2O2 under stirring. Then, 5.0 ml of benzaldehyde was added and reacted at 80 C for 6 h. The final
MCM-41
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
2 Theta/deg Fig. 1. Low-angle XRD patterns of MCM-41 and x wt.% HPW/MCM-41 materials.
Y. Chen et al. / Solid State Sciences 24 (2013) 21e25
23
Table 1 Textural and structural characteristics of MCM-41 and the HPW/MCM-41 materials. x wt.% HPW/MCM-41
SBETa (m2/g)
SMb (m2/g)
SExc (m2/g)
Vpd (cm3/g)
VMe (cm3/g)
Daf (nm)
d100 (nm)
a0g (nm)
dh (nm)
MCM-41 x¼5 x ¼ 10 x ¼ 15 x ¼ 20 x ¼ 25 x ¼ 30 x ¼ 35 x ¼ 40 x ¼ 45
1074 929 805 790 734 650 583 553 393 392
149 125 120 118 84 108 87 95 81 78
925 804 685 672 650 542 496 458 312 314
0.669 0.587 0.515 0.506 0.469 0.417 0.372 0.364 0.282 0.269
0.181 0.161 0.149 0.147 0.110 0.132 0.111 0.118 0.108 0.099
2.492 2.525 2.555 2.564 2.561 2.563 2.557 2.637 2.872 2.741
3.679 3.785 3.732 3.785 3.732 3.785 3.732 3.732 3.732 3.785
4.248 4.371 4.309 4.371 4.309 4.371 4.309 4.309 4.309 4.371
1.756 1.846 1.754 1.807 1.748 1.808 1.752 1.672 1.437 1.630
a b c d e f g h
SBET, specific surface area calculated by the BET method. SM, micropore area calculated by the Vet method (t-plot method micropore analysis). SEx, external surface area calculated by the Vet method (t-plot method micropore analysis). Vp, total pore volume determined by N2 adsorption at a relative pressure of 0.99. VM, micropore volume calculated by the Vet method (t-plot method micropore analysis). Da, mean pore diameter obtained from the desorption isotherm by the BJH method. pffiffiffi a0, unitecell parameter determined from the position of the (100) diffraction line asa0 ¼ 2d100 = 3. d, pore wall thickness calculated as d ¼ a0 e Da.
MCM-41 has a pore size higher than HPW nanocrystals, HPW are expected to be present on the surface, inside the hexagonal channels or incorporated in the pore walls of MCM-41. However, with loadings above 40 wt.%, some reflections from the HPW crystals are observed indicating poor dispersion or agglomeration of the HPW crystals. On the other hand, although the intensity increases with the increase of HPW loading, not all the typical characteristic diffraction peaks attributed to HPW are observed, even at the highly loaded 45 wt.% HPW/MCM-41 sample, as compared to the XRD pattern of bulk HPW. The XRD data offers evidence for the existence of HPW in the form of hydrated surface molecular species or small clusters containing few Keggin units in the supported HPW/MCM-41 materials. The N2 adsorptionedesorption isotherms and pore size distributions of MCM-41 and HPW/MCM-41 samples with different loading percentages of HPW are depicted in Fig. 3. According to the IUPAC classification, these isotherms can be assigned to type IV identified with three well-defined regions: (1) a slow increase in nitrogen uptake at low relative pressure, corresponding to a monolayer-multilayer adsorption on the pore walls; (2) a sharp step at intermediate relative pressures indicative of a capillary condensation within mesopores and (3) a plateau with a slight inclination at high relative pressure associated with multilayer
adsorption on the external surface of the materials [18]. As it can be seen from Fig. 3, these samples show a drop in the adsorption condensation region, at P/Po ¼ 0.2e0.4. Meanwhile, the pore size distributions show a unique peak centred at about 3.8 nm diameters (Fig. 3, inset). Table 1 presents the textural and structural parameters of MCM41 and HPW/MCM-41 materials. These parameters include SBET (specific surface area), SM (micropore area), SEx (external surface area), Vp (total pore volume), VM (micropore volume), Da (mean pore diameter), d100 (the d-spacing of the 1 0 0 diffraction), a0 (unitcell parameter), and d (pore wall thickness). It can be seen that MCM-41 presents the highest surface area (SBET ¼ 1074 m2/g) and pore volume (Vp ¼ 0.669 m3/g). With increasing HPW loading, a reduction in the surface area and pore volume are observed. For example, the data of specific surface area and total pore volume of 5 wt.% HPW/MCM-41 are 929 m2/g and 0.587 m3/g. While the data reduce to 392 m2/g and 0.269 m3/g in the 45 wt.% HPW/MCM-41. This is consistent with the previous results, such as references [19,20]. The reduction in the pore volume and surface area after loading can be due to the fact that the HPW is deposited inside the mesochannels and well dispersed on the surface of the hexagonally ordered mesoporous MCM-41 support [14].
(d) (c)
x = 45
dV /dD(cm / g nm )
bulk HPW
(a)
Volume/(ml/g)
Intensity/counts
(b)
x = 40
x = 35 Pore diam eter/nm
x=5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
10
20
30
40
50
60
70
80
2 Theta/deg Fig. 2. High-angle XRD patterns of bulk HPW and x wt.% HPW/MCM-41 materials.
Relative pressure/(P/P ) Fig. 3. N2 adsorptionedesorption isotherms and pore size distributions (inset) of MCM-41 (a) and the HPW/MCM-41 materials: (b) 5 wt.% HPW/MCM-41, (c) 25 wt.% HPW/MCM-41, and (d) 45 wt.% HPW/MCM-41.
Y. Chen et al. / Solid State Sciences 24 (2013) 21e25
FT-IR spectra are recorded for the supported HPW samples in order to confirm the presence of Keggin anion on MCM-41. The [PW12O40]3 Keggin ion structure consists of a PO4 tetrahedron surrounded by four W3O13 groups formed by edge-sharing octahedra. These groups are connected to each other by corner-sharing oxygen [21]. Fig. 4 illustrates the FT-IR spectra of MCM-41, bulk HPW and the 25 wt.% HPW/MCM-41 samples. Bulk HPW show four strong adsorption bands, at 1082 (PeO, internal oxygen connecting P), 983 (W]O, terminal oxygen bonding to W), 890 (WeOb, edgesharing oxygen connecting W), and 804 cm1 (WeOeW, cornersharing oxygen connecting W3O13 units) and one weak band at 524 cm1 (WeOeP) [22e24]. The framework bands of MCM-41 appeared at 1080 cm1 is assigned to the characteristic of antisymmetric vibration non-bridging oxygen atoms of SieOeSi bonds. The bands at 810 and 460 cm1 correspond to the symmetric stretching vibration and bending vibration of the rocking mode of SieOeSi bonds. In addition, a weak band at 973 cm1 is due to symmetric stretching vibration of SieOH [24]. The bands of 1082, 983, and 804 cm1 characteristic for HPW are masked by matching bands of MCM-41 matrix framework in 25 wt.% HPW/ MCM-41 sample (Fig. 4b). The weak bands attributed to HPW at <700 cm1 fail to appear, because of low concentration of HPW due to its dilution by MCM-41 support. The DR UVeVIS spectra of MCM-41, bulk HPW, and the 25 wt.% HPW/MCM-41 sample are recorded to study comparatively the nature of HPW species present on the MCM-41 supports (Fig. 5). The spectrum of bulk HPW consists of a broad group of signals assigned to oxygen to metal (O2 / W6þ) charge transfer in [PW12O40]3 anion with two maxima (at about 256 and 350 nm), which is agreement with those previous reports [25e27]. The typical HPW/MCM-41 sample shows one well-defined maximum at 265 nm attributed to the O2 / W6þ charge transfer, suggesting the presence of undegraded HPW species on the supports. NH3-TPD can be used to investigate the acid properties including acidic sites amount and acid strength of the acid catalysts. The NH3-TPD profiles of MCM-41, x wt.% HPW/MCM-41 materials (x ¼ 5, 25, 45), and bulk HPW are comparably shown in Fig. 6. No TPD signal can be found for MCM-41, indicating the pure supports have not obvious acid sites. The profile of bulk HPW exhibits a broad peak at ca. 170 C ascribed to desorption of physisorbed ammonia and a sharp peak appears at ca. 550 C, which is ascribed to desorption of chemisorbed ammonia, respectively. The NH3-TPD profiles of the HPW/MCM-41 materials display a wide peak from 125 C to approximately 325 C, with a maximum desorption rate at
Absorbance
24
(c)
(b)
(a)
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength/nm Fig. 5. DR UVeVIS spectra of MCM-41 (a), 25 wt.% HPW/MCM-41 (b), and bulk HPW (c).
about 180 C corresponded to desorption of weakly held, physisorbed ammonia. Furthermore, the intensity increases with the increase of HPW loadings. The total concentrations of acid sites obtained by integration of the ammonia desorption amounts is 0.0089 mmol/g, 0.0494 mmol/g, and 0.0868 mmol/g for 5 wt.% HPW/MCM-41, 25 wt.% HPW/MCM-41, and 45 wt.% HPW/MCM-41 catalyst, respectively. In other words, the acid site concentration does not increase in proportion to the HPW loading. It can be concluded from these values that not all acid sites provided by the HPW are available for ammonia adsorption due to the agglomeration of HPW when they are introduced on MCM-41 supports. Fig. 7 shows the catalytic activities of the HPW/MCM-41 catalysts for the oxidation of benzaldehyde with H2O2. The yield is about 71% with the HPW loading at 5 wt.% and reaches the maximum value (80%) with the HPW loading at 30 wt.%. Then, the yield slightly decreases to 73% when the HPW loading increases to 35 wt.% and keeps ca. 72% over the 40 wt.% and 45 wt.% HPW loading samples. The difference in the catalytic activity may be explained by correlating with the pore textural structure, the total acidity, and the state of HPW species. As the HPW loading increases surface area and pore volume decreases as expected (Table 1). On the other hand, the acidity increases with increase in HPW loading,
(b)
(a)
Intencity/a.u.
Transmittance/%
(c)
bulk HPW x = 45 x = 25 x=5 MCM-41
1400 1300 1200 1100 1000 900
800
700
600
500
400
-1
Wavenumber/cm
Fig. 4. FT-IR spectra of MCM-41 (a), 25 wt.% HPW/MCM-41 (b), and bulk HPW (c).
150 200 250 300 350 400 450 500 550 600 650 700 o
Temperature/ C Fig. 6. NH3-TPD spectra recorded with MCM-41, x wt.% HPW/MCM-41, and bulk HPW.
Y. Chen et al. / Solid State Sciences 24 (2013) 21e25
results indicate that the HPW/MCM-41 material is an efficient catalyst for the green synthesis of benzoic acid.
Yield /%
80
78
4. Conclusion
76
A series of MCM-41 supported 12-tungstophosphoric acid mesoporous materials were synthesized by using a wet impregnation method. The characterization results indicated that the materials retained the mesopore structure of MCM-41 and 12tungstophosphoric acid units were highly dispersed on the MCM41 supports when the HPW loading was less than 35 wt.%. Furthermore, the results of catalytic reaction indicated that MCM41 supported 12-tungstophosphoric acid was an efficient catalyst for the oxidation of benzaldehyde to benzoic acid with aqueous hydrogen peroxide and the sample with thirty weight percent of 12-tungstophosphoric acid was found to be more active than other catalysts under the reaction conditions.
74
72
70
68 0
5
10
15
20
25
30
35
40
45
50
The loading of HPW/wt.% Fig. 7. Catalytic activities of the HPW/MCM-41 materials.
Acknowledgements
which is also expected. The value of total acidity is directly related to the concentration of active species, HPW. Hence, it is obvious that the yield of benzoic acid increases with increase in wt.% loading of HPW to some extent. When the value of HPW loading increases furthermore (x > 30), the surface area and pore volume decreases obviously. Meanwhile, agglomeration of HPW crystals at loading above 35 wt.% (Fig. 2) may block the pore structure. Therefore, the concentration of active species exposed increases. This leads to the decrease of catalytic activity. Fig. 8 shows the typical recycle study of the 30 wt.% HPW/MCM41 catalyst. The weight of the catalyst changes from 0.50 g to 0.33 g due to the partly leaching of HPW species and the after-treatment process including separation and dry etc. Our previous work [16b] showed that the trace by-product was probably peroxybenzoic acid. The yield of benzoic acid decreases from 80.3% to 66.1% in the second recycle. This can be explained by the elution of HPW crystalline which has poor interaction with MCM-41 supports. However, the yield was comparatively constant in the further reactions and the selectivity of benzoic acid was kept above 98.5%. These findings revealed that the HPW/MCM-41 materials exhibited high stability in the test reaction. Comparing with the traditional process of toluene oxidation, such as reference [15,26,27], the HPW/MCM-41 mesoporous materials exhibits similar or even higher yield of benzoic acid. The
Yield Selectivity
100
Percentage/%
25
80
60
40
20
0 1
2
3
Reaction cycles Fig. 8. Recycle study of 30 wt.% HPW/MCM-41 material.
4
This work was supported by the National Natural Science Foundation of P.R. China (No. J1210060) and the Innovation Foundation of Zhengzhou University (No. 2012cxsy083 & No. 2013xjxm022). The authors gratefully acknowledge Dr. T. Li (College of Chemistry and Molecular Engineering, Peking University, China) for conducting the NH3-TPD experiments. References [1] Y. Liu, L. Xu, B.B. Xu, Z.K. Li, L.P. Jia, W.H. Guo, J. Mol. Catal. A 297 (2009) 86e92. [2] Z.H. Yuan, B.Z. Chen, J.S. Zhao, Chem. Eng. Sci. 66 (2011) 5137e5147. [3] A. Heydari, H. Hamadi, M. Pourayoubi, Catal. Commun. 8 (2007) 1224e1226. [4] I. Holclajtner-Antunovi c, U.B. Mio c, M. Todorovi c, Z. Jovanovi c, M. Davidovi c, D. Bajuk-Bogdanovi c, Z. Lausevi&cacute, Mater. Res. Bull. 45 (2010) 1679e 1684. [5] L.R. Pizzio, P.G. Vázquez, M.N. Blanco, Appl. Catal. A Gen. 287 (2005) 1e8. [6] P. Vázquez, L. Pizzio, C. Cáceres, H. Thomas, J. Mol. Catal. A Chem. 161 (2000) 223e232. [7] G. Kamalakar, K. Komura, Y. Kubota, Y. Sugi, J. Chem. Technol. Biotechnol. 81 (2006) 981e988. [8] G. Kamalakar, K. Komura, Y. Sugi, Appl. Catal. A Gen. 310 (2006) 155e163. [9] Q.H. Xia, K. Hidajat, S. Kawi, J. Catal. 209 (2003) 433e444. [10] S. Choi, Y. Wang, J. Liu, Catal. Today 55 (2000) 117e124. [11] I.V. Kozhevnikov, Appl. Catal. A Gen. 256 (2003) 3e18. [12] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppared, J.B. Higgins, S.B. McCullen, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834e10843. [13] Y.C. Kim, J.Y. Jeong, J.Y. Hwang, S.D. Kim, S.C. Yi, W.J. Kim, J. Memb. Sci. 325 (2008) 252e261. [14] A.E.R.S. Khder, H.M.A. Hassan, M.S. El-Shall, Appl. Catal. A Gen. 411e412 (2012) 77e86. [15] M. Ilyas, M. Sadiq, Catal. Lett. 128 (2009) 337e342. [16] (a) X.C. Zheng, C.Y. Yang, J. Wang, M.Y. Li, X.Y. Wang, J. Zhengzhou Univ. (Nat. Sci. Ed.) 40 (2008) 109e112; (b) B.B. Dong, B.B. Zhang, H.Y. Wu, S.D. Li, K. Zhang, X.C. Zheng, Microporous Mesoporous Mater. 176 (2013) 186e193. [17] P.R.S. Braga, A.A. Costa, E.F. de Freitas, R.O. Rocha, J.L. de Macedo, A.S. Araujod, J.A. Dias, S.C.L. Dias, J. Mol. Catal. A Chem. 358 (2012) 99e105. [18] E. Kraleva, M.L. Saladino, A. Spinella, G. Nasillo, E. Caponetti, J. Mater. Sci. 46 (2011) 7114e7120. [19] B.R. Jermy, A. Pandurangan, Appl. Catal. A 295 (2005) 185e192. [20] J.C. Juan, J. Zhang, M.A. Yarmo, J. Mol. Catal. A 267 (2007) 265e271. [21] K.U. Nandhini, B. Arabindoo, M. Palanichamy, V. Murugesan, J. Mol. Catal. A 243 (2006) 183e193. [22] X.L. Yang, W.L. Dai, H. Chen, J.H. Xu, Y. Cao, H.X. Li, K.N. Fan, Appl. Catal. A 283 (2005) 1e8. [23] G.A. Eimer, S.G. Casuscelli, G.E. Ghione, M.E. Crivello, E.R. Herrero, Appl. Catal. A 298 (2006) 232e242. [24] G. Karthikeyan, A. Pandurangan, J. Mol. Catal. A 311 (2009) 36e45. [25] D.Y. Zhao, J.Y. Sun, Q.Z. Li, G.D. Stucky, Chem. Mater. 12 (2000) 275e279. [26] F. Yang, J. Sun, R. Zheng, W.W. Qiu, J. Tang, M.Y. He, Tetrahedron 60 (2004) 1225e1228. [27] X.Q. Li, J. Xu, F. Wang, J. Gao, L.P. Zhou, G.Y. Yang, Catal. Lett. 108 (2006) 137e140.
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MCM-41 supported 12-tungstophosphoric acid mesoporous materials: Preparation, characterization, and catalytic activities for benzaldehyde oxidation with H2O2 Ya Chen, Xiao-Li Zhang, Xi Chen, Bei-Bei Dong, Xiu-Cheng Zheng* College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 May 2013 Received in revised form 26 June 2013 Accepted 29 June 2013 Available online 9 July 2013
Mesoporous molecular sieves MCM-41 and bulk 12-tungstophosphoric acid (HPW) were synthesized and employed to prepare 5e45 wt.% HPW/MCM-41 mesoporous materials. Characterization results suggested the good dispersion of HPW within MCM-41 when the loading of HPW was less than 35 wt.% and HPW/MCM-41 retained the typical mesopore structure of the supports. The results of the catalytic oxidation of benzaldehyde to benzoic acid with 30% H2O2, in the absence of any organic solvent and cocatalysts, indicated that HPW/MCM-41 was an efficient catalyst and 30 wt.% HPW/MCM-41 sample exhibited the highest catalytic activity among these materials. Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords: HPW/MCM-41 Mesoporous materials Characterization Benzaldehyde Benzoic acid Hydrogen peroxide
1. Introduction Heteropoly acids have several advantages, such as strong Brønsted acidity, no corrosion, which make them as promising acid, redox and bifunctional catalysts in homogeneous as well as in heterogeneous systems [1,2]. Among the heteropoly acids, 12tungstophosphoric acid (H3PW12O40, HPW), the strongest one in the Keggin series, have been extensively studied as super acid catalysts for many organic reactions and have been found industrial application in several processes [3]. However, bulk HPW has the disadvantages including weak thermal stability, low porosity and specific surface area (<10 m2/g) and separation problems from reaction mixture due to its high solubility in polar reaction media [4]. To overcome these disadvantages, a huge amount of effects have been attempted. One method was dispersion HPW on a support with high surface area structure, such as SiO2 [5,6], mesoporous silica [7e9], TiO2 [10], and active carbon [11]. The supported materials may keep the properties of HPW without the dissolution or elution.
* Corresponding author. Tel.: þ86 371 67781780. E-mail address: [email protected] (X.-C. Zheng). 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.06.017
Owning to the unique properties, such as high specific surface area and pore volume, tunable pore size and narrow pore size distribution, mesoporous materials of the M41S (Mobil Composition of Matter) family have attracted worldwide attention. Among the members, MCM-41 has attracted considerable attention due to its potential use in adsorptive separations and catalytic conversion of large molecules [12,13]. For example, HPW clusters with diameters about 1.2 nm can be introduced inside the MCM-41 pores, thus allowing for a significant increase in the surface area of the HPW. The large pore sizes of MCM-41 play a major role in overcoming the problem of the obstruction of bulky organic molecules resulting from the small pore volume of HPW. Therefore, the HPW/ MCM-41 materials can be used as a potential solid acid catalyst with high activity and thermal stability [14]. As an important organic intermediate, benzoic acid is generally produced by the oxidation of toluene both in vapour or liquid phases. However, these methods involve expensive reagents, high temperatures, and give unsatisfactory yields. Furthermore, toluene is also a representative of aromatic hydrocarbons categorized as hazardous material [15]. In this respect, heterogeneous catalysis can play a key role in the development of environmentally benign processes particularly in the chemical industries. In addition, it is interesting that hydrogen peroxide has been widely used as oxidant in catalytic oxidation of organic compounds because of its unique
22
Y. Chen et al. / Solid State Sciences 24 (2013) 21e25
properties such as inexpensive, environmentally clean, and easy to handle. In this paper, HPW/MCM-41 mesoporous catalysts were synthesized and characterized by various techniques. The effect of various HPW loadings on the structure and catalytic properties for
Yield of benzoic acid ð%Þ ¼
reaction solution was cooled at 0 C in a refrigerator and the solid was washed with cold water (2 C). The as-prepared solid was dried in air at 80 C overnight and the yield was calculated according to Eq. (1).
amount of the dried solid sample ðgÞ 100 theoretic amount of benzoic acid from 5:0 ml of benzaldehyde ðgÞ
the oxidation of benzaldehyde to benzoic acid with 30% H2O2 of these catalysts was comparatively investigated.
(1)
2. Experimental
The stability property of the catalysts was investigated under the same reaction condition except for the amount of catalyst. The filtered catalyst was dried at 100 C overnight and then directly used in the next catalytic reaction.
2.1. Synthesis of HPW/MCM-41 materials
3. Results and discussion
MCM-41 and bulk HPW was prepared via an alkali hydrothermal process [16a] and an improved diethyl ether extraction method [16b], respectively. The HPW/MCM-41 materials were prepared by an impregnation method and the process was as follows: 2 g of MCM-41 was dispersed into a solution of the desired amount of HPW in 60 ml water under vigorous stirring. The excess water was slowly vaporized at 50 C and dried in an oven overnight at 100 C. The as-prepared catalysts were denoted as x wt.% HPW/MCM-41 (x ¼ 5, 10, 15, 20, 25, 30, 35, 40, and 45, respectively).
The low-angle XRD patterns of MCM-41 and the typical x wt.% HPW/MCM-41 materials are shown in Fig. 1. All the patterns illustrate the characteristics of a typical mesoporous MCM-41 structure. Furthermore, as it can be seen from Fig. 1 that HPW has a striking effect on the width and intensity of the main reflection at high d1 0 0 spacing and the peak become broader and weaker as the loading of HPW increase (Table 1). This suggests the mesoporous structure of MCM-41 supports remain almost unchanged upon the HPW loading but the long-range order is decreased noticeably. So far, some explanations regarding this phenomenon have been discussed. Braga et al. [17] reported that two reasons could cause the loss on the long range ordering of MCM-41: support hydrolysis, which could cause a partial destruction on the mesopores structure due to a transformation of MCM-41 siloxane into silanol groups, or a nonhomogenous distribution of anionic species, such as [PW12O40]3, with silanol groups within the walls of MCM-41 pores. Khder et al. [14] assumed that the severe reflection of MCM-41 in presence of HPW gradual hydrolysis in acidic medium and was mainly due to radiation absorption by the tungsten element present in HPW. The high-angle XRD patterns of bulk HPW and the typical x wt.% HPW/MCM-41 materials are shown in Fig. 2. We can see that the characteristic peaks of HPW are absent in the patterns of HPW/ MCM-41 when the HPW loading is not more than 35 wt.%. Since
N2 adsorption-desorption experiments were performed on a Quantachrome NOVA 1000e surface area & pore size analyser at 196 C. Prior to analysis the samples were degassed at 250 C under vacuum (1.33 Pa) for 2 h. Specific surface areas were calculated by the BrunauereEmmetteTeller (BET) method and the pore volume was determined by nitrogen adsorption at a relative pressure of 0.99. The micropore areas were estimated using the correlation of t-plot method. Pore size distributions were derived from the desorption profiles of the isotherms. X-ray powder diffraction (XRD) patterns were collected on a Panalytical XPert Pro diffractometer using Cu Ka radiation (l ¼ 1.5418 Ǻ). The working voltage and current of the X-ray tube were 40 kV and 100 mA. FT-IR spectra of the samples were recorded on a Thermoscientific Nicolet 380 Fourier transform spectrometer using a KBr pellet technique. Diffuse reflectance (DR) UVeVIS spectra were recorded in the wavelength range 200e800 nm using a Varian Cary 5000 UVeVise NIR spectrophotometer. BaSO4 was used as the reference. Temperature programmed desorption of ammonia (NH3-TPD) was carried out with the use of a Tianjin XQ TP5080 auto-adsorption apparatus. Approximately 50 mg sample was pretreated in a helium atmosphere (30 ml/min, 500 C, 1 h), cooled to 20 C, and then loaded into the reaction cell. The samples were exposed to ammonia at 100 C for 0.5 h and then purged with helium. NH3-TPD spectra were registered between 100 and 750 C (temperature ramp: 10 C/min). The NH3 consumption was monitored by TCD detector.
(100) (110)
(200)
x = 45
Intensity/counts
2.2. Characterization methods
x = 25
x=5
2.3. Catalytic reaction The oxidation of benzaldehyde with H2O2 was conducted in a 50 ml three-neck round-bottomed flask. 0.5 g of HPW/MCM-41 was dispersed into 17.5 ml 30 wt.% H2O2 under stirring. Then, 5.0 ml of benzaldehyde was added and reacted at 80 C for 6 h. The final
MCM-41
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
2 Theta/deg Fig. 1. Low-angle XRD patterns of MCM-41 and x wt.% HPW/MCM-41 materials.
Y. Chen et al. / Solid State Sciences 24 (2013) 21e25
23
Table 1 Textural and structural characteristics of MCM-41 and the HPW/MCM-41 materials. x wt.% HPW/MCM-41
SBETa (m2/g)
SMb (m2/g)
SExc (m2/g)
Vpd (cm3/g)
VMe (cm3/g)
Daf (nm)
d100 (nm)
a0g (nm)
dh (nm)
MCM-41 x¼5 x ¼ 10 x ¼ 15 x ¼ 20 x ¼ 25 x ¼ 30 x ¼ 35 x ¼ 40 x ¼ 45
1074 929 805 790 734 650 583 553 393 392
149 125 120 118 84 108 87 95 81 78
925 804 685 672 650 542 496 458 312 314
0.669 0.587 0.515 0.506 0.469 0.417 0.372 0.364 0.282 0.269
0.181 0.161 0.149 0.147 0.110 0.132 0.111 0.118 0.108 0.099
2.492 2.525 2.555 2.564 2.561 2.563 2.557 2.637 2.872 2.741
3.679 3.785 3.732 3.785 3.732 3.785 3.732 3.732 3.732 3.785
4.248 4.371 4.309 4.371 4.309 4.371 4.309 4.309 4.309 4.371
1.756 1.846 1.754 1.807 1.748 1.808 1.752 1.672 1.437 1.630
a b c d e f g h
SBET, specific surface area calculated by the BET method. SM, micropore area calculated by the Vet method (t-plot method micropore analysis). SEx, external surface area calculated by the Vet method (t-plot method micropore analysis). Vp, total pore volume determined by N2 adsorption at a relative pressure of 0.99. VM, micropore volume calculated by the Vet method (t-plot method micropore analysis). Da, mean pore diameter obtained from the desorption isotherm by the BJH method. pffiffiffi a0, unitecell parameter determined from the position of the (100) diffraction line asa0 ¼ 2d100 = 3. d, pore wall thickness calculated as d ¼ a0 e Da.
MCM-41 has a pore size higher than HPW nanocrystals, HPW are expected to be present on the surface, inside the hexagonal channels or incorporated in the pore walls of MCM-41. However, with loadings above 40 wt.%, some reflections from the HPW crystals are observed indicating poor dispersion or agglomeration of the HPW crystals. On the other hand, although the intensity increases with the increase of HPW loading, not all the typical characteristic diffraction peaks attributed to HPW are observed, even at the highly loaded 45 wt.% HPW/MCM-41 sample, as compared to the XRD pattern of bulk HPW. The XRD data offers evidence for the existence of HPW in the form of hydrated surface molecular species or small clusters containing few Keggin units in the supported HPW/MCM-41 materials. The N2 adsorptionedesorption isotherms and pore size distributions of MCM-41 and HPW/MCM-41 samples with different loading percentages of HPW are depicted in Fig. 3. According to the IUPAC classification, these isotherms can be assigned to type IV identified with three well-defined regions: (1) a slow increase in nitrogen uptake at low relative pressure, corresponding to a monolayer-multilayer adsorption on the pore walls; (2) a sharp step at intermediate relative pressures indicative of a capillary condensation within mesopores and (3) a plateau with a slight inclination at high relative pressure associated with multilayer
adsorption on the external surface of the materials [18]. As it can be seen from Fig. 3, these samples show a drop in the adsorption condensation region, at P/Po ¼ 0.2e0.4. Meanwhile, the pore size distributions show a unique peak centred at about 3.8 nm diameters (Fig. 3, inset). Table 1 presents the textural and structural parameters of MCM41 and HPW/MCM-41 materials. These parameters include SBET (specific surface area), SM (micropore area), SEx (external surface area), Vp (total pore volume), VM (micropore volume), Da (mean pore diameter), d100 (the d-spacing of the 1 0 0 diffraction), a0 (unitcell parameter), and d (pore wall thickness). It can be seen that MCM-41 presents the highest surface area (SBET ¼ 1074 m2/g) and pore volume (Vp ¼ 0.669 m3/g). With increasing HPW loading, a reduction in the surface area and pore volume are observed. For example, the data of specific surface area and total pore volume of 5 wt.% HPW/MCM-41 are 929 m2/g and 0.587 m3/g. While the data reduce to 392 m2/g and 0.269 m3/g in the 45 wt.% HPW/MCM-41. This is consistent with the previous results, such as references [19,20]. The reduction in the pore volume and surface area after loading can be due to the fact that the HPW is deposited inside the mesochannels and well dispersed on the surface of the hexagonally ordered mesoporous MCM-41 support [14].
(d) (c)
x = 45
dV /dD(cm / g nm )
bulk HPW
(a)
Volume/(ml/g)
Intensity/counts
(b)
x = 40
x = 35 Pore diam eter/nm
x=5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
10
20
30
40
50
60
70
80
2 Theta/deg Fig. 2. High-angle XRD patterns of bulk HPW and x wt.% HPW/MCM-41 materials.
Relative pressure/(P/P ) Fig. 3. N2 adsorptionedesorption isotherms and pore size distributions (inset) of MCM-41 (a) and the HPW/MCM-41 materials: (b) 5 wt.% HPW/MCM-41, (c) 25 wt.% HPW/MCM-41, and (d) 45 wt.% HPW/MCM-41.
Y. Chen et al. / Solid State Sciences 24 (2013) 21e25
FT-IR spectra are recorded for the supported HPW samples in order to confirm the presence of Keggin anion on MCM-41. The [PW12O40]3 Keggin ion structure consists of a PO4 tetrahedron surrounded by four W3O13 groups formed by edge-sharing octahedra. These groups are connected to each other by corner-sharing oxygen [21]. Fig. 4 illustrates the FT-IR spectra of MCM-41, bulk HPW and the 25 wt.% HPW/MCM-41 samples. Bulk HPW show four strong adsorption bands, at 1082 (PeO, internal oxygen connecting P), 983 (W]O, terminal oxygen bonding to W), 890 (WeOb, edgesharing oxygen connecting W), and 804 cm1 (WeOeW, cornersharing oxygen connecting W3O13 units) and one weak band at 524 cm1 (WeOeP) [22e24]. The framework bands of MCM-41 appeared at 1080 cm1 is assigned to the characteristic of antisymmetric vibration non-bridging oxygen atoms of SieOeSi bonds. The bands at 810 and 460 cm1 correspond to the symmetric stretching vibration and bending vibration of the rocking mode of SieOeSi bonds. In addition, a weak band at 973 cm1 is due to symmetric stretching vibration of SieOH [24]. The bands of 1082, 983, and 804 cm1 characteristic for HPW are masked by matching bands of MCM-41 matrix framework in 25 wt.% HPW/ MCM-41 sample (Fig. 4b). The weak bands attributed to HPW at <700 cm1 fail to appear, because of low concentration of HPW due to its dilution by MCM-41 support. The DR UVeVIS spectra of MCM-41, bulk HPW, and the 25 wt.% HPW/MCM-41 sample are recorded to study comparatively the nature of HPW species present on the MCM-41 supports (Fig. 5). The spectrum of bulk HPW consists of a broad group of signals assigned to oxygen to metal (O2 / W6þ) charge transfer in [PW12O40]3 anion with two maxima (at about 256 and 350 nm), which is agreement with those previous reports [25e27]. The typical HPW/MCM-41 sample shows one well-defined maximum at 265 nm attributed to the O2 / W6þ charge transfer, suggesting the presence of undegraded HPW species on the supports. NH3-TPD can be used to investigate the acid properties including acidic sites amount and acid strength of the acid catalysts. The NH3-TPD profiles of MCM-41, x wt.% HPW/MCM-41 materials (x ¼ 5, 25, 45), and bulk HPW are comparably shown in Fig. 6. No TPD signal can be found for MCM-41, indicating the pure supports have not obvious acid sites. The profile of bulk HPW exhibits a broad peak at ca. 170 C ascribed to desorption of physisorbed ammonia and a sharp peak appears at ca. 550 C, which is ascribed to desorption of chemisorbed ammonia, respectively. The NH3-TPD profiles of the HPW/MCM-41 materials display a wide peak from 125 C to approximately 325 C, with a maximum desorption rate at
Absorbance
24
(c)
(b)
(a)
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength/nm Fig. 5. DR UVeVIS spectra of MCM-41 (a), 25 wt.% HPW/MCM-41 (b), and bulk HPW (c).
about 180 C corresponded to desorption of weakly held, physisorbed ammonia. Furthermore, the intensity increases with the increase of HPW loadings. The total concentrations of acid sites obtained by integration of the ammonia desorption amounts is 0.0089 mmol/g, 0.0494 mmol/g, and 0.0868 mmol/g for 5 wt.% HPW/MCM-41, 25 wt.% HPW/MCM-41, and 45 wt.% HPW/MCM-41 catalyst, respectively. In other words, the acid site concentration does not increase in proportion to the HPW loading. It can be concluded from these values that not all acid sites provided by the HPW are available for ammonia adsorption due to the agglomeration of HPW when they are introduced on MCM-41 supports. Fig. 7 shows the catalytic activities of the HPW/MCM-41 catalysts for the oxidation of benzaldehyde with H2O2. The yield is about 71% with the HPW loading at 5 wt.% and reaches the maximum value (80%) with the HPW loading at 30 wt.%. Then, the yield slightly decreases to 73% when the HPW loading increases to 35 wt.% and keeps ca. 72% over the 40 wt.% and 45 wt.% HPW loading samples. The difference in the catalytic activity may be explained by correlating with the pore textural structure, the total acidity, and the state of HPW species. As the HPW loading increases surface area and pore volume decreases as expected (Table 1). On the other hand, the acidity increases with increase in HPW loading,
(b)
(a)
Intencity/a.u.
Transmittance/%
(c)
bulk HPW x = 45 x = 25 x=5 MCM-41
1400 1300 1200 1100 1000 900
800
700
600
500
400
-1
Wavenumber/cm
Fig. 4. FT-IR spectra of MCM-41 (a), 25 wt.% HPW/MCM-41 (b), and bulk HPW (c).
150 200 250 300 350 400 450 500 550 600 650 700 o
Temperature/ C Fig. 6. NH3-TPD spectra recorded with MCM-41, x wt.% HPW/MCM-41, and bulk HPW.
Y. Chen et al. / Solid State Sciences 24 (2013) 21e25
results indicate that the HPW/MCM-41 material is an efficient catalyst for the green synthesis of benzoic acid.
Yield /%
80
78
4. Conclusion
76
A series of MCM-41 supported 12-tungstophosphoric acid mesoporous materials were synthesized by using a wet impregnation method. The characterization results indicated that the materials retained the mesopore structure of MCM-41 and 12tungstophosphoric acid units were highly dispersed on the MCM41 supports when the HPW loading was less than 35 wt.%. Furthermore, the results of catalytic reaction indicated that MCM41 supported 12-tungstophosphoric acid was an efficient catalyst for the oxidation of benzaldehyde to benzoic acid with aqueous hydrogen peroxide and the sample with thirty weight percent of 12-tungstophosphoric acid was found to be more active than other catalysts under the reaction conditions.
74
72
70
68 0
5
10
15
20
25
30
35
40
45
50
The loading of HPW/wt.% Fig. 7. Catalytic activities of the HPW/MCM-41 materials.
Acknowledgements
which is also expected. The value of total acidity is directly related to the concentration of active species, HPW. Hence, it is obvious that the yield of benzoic acid increases with increase in wt.% loading of HPW to some extent. When the value of HPW loading increases furthermore (x > 30), the surface area and pore volume decreases obviously. Meanwhile, agglomeration of HPW crystals at loading above 35 wt.% (Fig. 2) may block the pore structure. Therefore, the concentration of active species exposed increases. This leads to the decrease of catalytic activity. Fig. 8 shows the typical recycle study of the 30 wt.% HPW/MCM41 catalyst. The weight of the catalyst changes from 0.50 g to 0.33 g due to the partly leaching of HPW species and the after-treatment process including separation and dry etc. Our previous work [16b] showed that the trace by-product was probably peroxybenzoic acid. The yield of benzoic acid decreases from 80.3% to 66.1% in the second recycle. This can be explained by the elution of HPW crystalline which has poor interaction with MCM-41 supports. However, the yield was comparatively constant in the further reactions and the selectivity of benzoic acid was kept above 98.5%. These findings revealed that the HPW/MCM-41 materials exhibited high stability in the test reaction. Comparing with the traditional process of toluene oxidation, such as reference [15,26,27], the HPW/MCM-41 mesoporous materials exhibits similar or even higher yield of benzoic acid. The
Yield Selectivity
100
Percentage/%
25
80
60
40
20
0 1
2
3
Reaction cycles Fig. 8. Recycle study of 30 wt.% HPW/MCM-41 material.
4
This work was supported by the National Natural Science Foundation of P.R. China (No. J1210060) and the Innovation Foundation of Zhengzhou University (No. 2012cxsy083 & No. 2013xjxm022). The authors gratefully acknowledge Dr. T. Li (College of Chemistry and Molecular Engineering, Peking University, China) for conducting the NH3-TPD experiments. References [1] Y. Liu, L. Xu, B.B. Xu, Z.K. Li, L.P. Jia, W.H. Guo, J. Mol. Catal. A 297 (2009) 86e92. [2] Z.H. Yuan, B.Z. Chen, J.S. Zhao, Chem. Eng. Sci. 66 (2011) 5137e5147. [3] A. Heydari, H. Hamadi, M. Pourayoubi, Catal. Commun. 8 (2007) 1224e1226. [4] I. Holclajtner-Antunovi c, U.B. Mio c, M. Todorovi c, Z. Jovanovi c, M. Davidovi c, D. Bajuk-Bogdanovi c, Z. Lausevi&cacute, Mater. Res. Bull. 45 (2010) 1679e 1684. [5] L.R. Pizzio, P.G. Vázquez, M.N. Blanco, Appl. Catal. A Gen. 287 (2005) 1e8. [6] P. Vázquez, L. Pizzio, C. Cáceres, H. Thomas, J. Mol. Catal. A Chem. 161 (2000) 223e232. [7] G. Kamalakar, K. Komura, Y. Kubota, Y. Sugi, J. Chem. Technol. Biotechnol. 81 (2006) 981e988. [8] G. Kamalakar, K. Komura, Y. Sugi, Appl. Catal. A Gen. 310 (2006) 155e163. [9] Q.H. Xia, K. Hidajat, S. Kawi, J. Catal. 209 (2003) 433e444. [10] S. Choi, Y. Wang, J. Liu, Catal. Today 55 (2000) 117e124. [11] I.V. Kozhevnikov, Appl. Catal. A Gen. 256 (2003) 3e18. [12] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppared, J.B. Higgins, S.B. McCullen, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834e10843. [13] Y.C. Kim, J.Y. Jeong, J.Y. Hwang, S.D. Kim, S.C. Yi, W.J. Kim, J. Memb. Sci. 325 (2008) 252e261. [14] A.E.R.S. Khder, H.M.A. Hassan, M.S. El-Shall, Appl. Catal. A Gen. 411e412 (2012) 77e86. [15] M. Ilyas, M. Sadiq, Catal. Lett. 128 (2009) 337e342. [16] (a) X.C. Zheng, C.Y. Yang, J. Wang, M.Y. Li, X.Y. Wang, J. Zhengzhou Univ. (Nat. Sci. Ed.) 40 (2008) 109e112; (b) B.B. Dong, B.B. Zhang, H.Y. Wu, S.D. Li, K. Zhang, X.C. Zheng, Microporous Mesoporous Mater. 176 (2013) 186e193. [17] P.R.S. Braga, A.A. Costa, E.F. de Freitas, R.O. Rocha, J.L. de Macedo, A.S. Araujod, J.A. Dias, S.C.L. Dias, J. Mol. Catal. A Chem. 358 (2012) 99e105. [18] E. Kraleva, M.L. Saladino, A. Spinella, G. Nasillo, E. Caponetti, J. Mater. Sci. 46 (2011) 7114e7120. [19] B.R. Jermy, A. Pandurangan, Appl. Catal. A 295 (2005) 185e192. [20] J.C. Juan, J. Zhang, M.A. Yarmo, J. Mol. Catal. A 267 (2007) 265e271. [21] K.U. Nandhini, B. Arabindoo, M. Palanichamy, V. Murugesan, J. Mol. Catal. A 243 (2006) 183e193. [22] X.L. Yang, W.L. Dai, H. Chen, J.H. Xu, Y. Cao, H.X. Li, K.N. Fan, Appl. Catal. A 283 (2005) 1e8. [23] G.A. Eimer, S.G. Casuscelli, G.E. Ghione, M.E. Crivello, E.R. Herrero, Appl. Catal. A 298 (2006) 232e242. [24] G. Karthikeyan, A. Pandurangan, J. Mol. Catal. A 311 (2009) 36e45. [25] D.Y. Zhao, J.Y. Sun, Q.Z. Li, G.D. Stucky, Chem. Mater. 12 (2000) 275e279. [26] F. Yang, J. Sun, R. Zheng, W.W. Qiu, J. Tang, M.Y. He, Tetrahedron 60 (2004) 1225e1228. [27] X.Q. Li, J. Xu, F. Wang, J. Gao, L.P. Zhou, G.Y. Yang, Catal. Lett. 108 (2006) 137e140.