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Synthesis, characterization and catalytic evaluation of SBA-15 supported 12-tungstophosphoric acid mesoporous materials in the oxidation of benzaldehyde to benzoic acid
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Synthesis, characterization and catalytic evaluation of SBA-15 supported 12-tungstophosphoric acid mesoporous materials in the oxidation of benzaldehyde to benzoic acid Bei-Bei Dong, Bing-Bing Zhang, Hai-Yan Wu, Xi Chen, Ke Zhang, Xiu-Cheng Zheng * College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 December 2012 Received in revised form 25 February 2013 Accepted 7 March 2013 Available online xxx
A series of 12-tungstophosphoric acid catalysts supported on mesoporous molecular sieves SBA-15 were synthesized via wet impregnation method and characterized by various techniques. It has been found that all the catalysts retained the mesopore structure of SBA-15. The surface areas and pore volumes decreased with the increase of HPW loadings, whereas the mean pore diameter exhibited the opposite behavior. In addition, HPW units were highly dispersed on the SBA-15 supports when the 12tungstophosphoric acid loading was less than 20 wt.%. The results of the catalytic oxidation of benzaldehyde to benzoic acid with aqueous hydrogen peroxide, in the absence of any organic solvent and co-catalysts, indicated that SBA-15 supported 12-tungstophosphoric acid was an efficient catalyst. The sample with 20 wt.% loading of 12-tungstophosphoric acid was found to be more active than other catalysts under the reaction conditions. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Chemical synthesis C. Infrared spectroscopy C. X-ray diffraction D. Catalytic properties
1. Introduction Nowadays, benzoic acid is widely used in medicines, veterinary medicines, industrial raw materials, preservatives for food, cosmetics, resin preparations, plasticizers, dyestuffs, synthetic fibers, and intermediates [1,2]. It is mostly produced by the oxidation of toluene both in vapor and liquid phases. However, these methods involve expensive reagents, high temperatures, but give low yields. Meanwhile, toluene is also a representative of aromatic hydrocarbons categorized as hazardous material [3]. Thus, it is urgent to explore a new and efficient catalyst under neutral, mild and practical conditions. It is known that heteropoly acids (HPAs) exhibit unique physicochemical properties with their structural mobility and catalytic multifunctionality. HPAs possess a very strong Brønsted acidity and appropriate redox properties, which can be tuned by varying their chemical composition. Furthermore, they have other advantages, including ease of handling, environmental compatibility, non-toxicity, and experimental simplicity [2,4]. Among the solid heteropoly acids, 12-tungstophosphoric acid (HPW), the strongest HPAs in the Keggin series, has been extensively studied as super acid catalyst for many organic reactions and widely used
* Corresponding author. Tel.: +86 371 67781780. E-mail address: [email protected] (X.-C. Zheng).
in industrial processes [5,6]. However, the use of bulk HPAs is limited by their low thermal stability, low surface area and separation problems, which could be overcome by introducing them into suitable supports [7]. SBA-15, a new type of mesoporous molecular sieves with ordered hexagonal array of uniform tubular channels with high surface area, large pore diameters, thick walls, high thermal stability and hydrothermal stability, has attracted considerable attention in the field of heterogeneous catalysis and nanoscale materials [8]. However, SBA-15 does not exhibit intrinsic catalytic activity. It is necessary to replace part of the silicon of the structure or graft onto the internal surface of the pores with heteroatoms, which are able to make the solids catalytically active. Gagea et al. found that compared to ultrastable Y zeolites, HPW/SBA-15 catalysts showed a broader optimum reaction temperature window for the isomerization and higher ndecane dibranching selectivity [9]. He et al. found that the finely dispersed and chemically bound HPW species and pore size were responsible for the enhanced selectivity in the acetic anhydride catalytically condensed from acetic acid systems [10]. Yang et al. found that the SBA-15 based composite introduced by HPW was very stable in the acidic property and the structural regularity [11]. Lizama et al. found that the W based catalysts prepared from HPW showed better catalytic performance in deep hydrodesulfurization of 4,6dimethyldibenzothiophene than the counterparts prepared from traditionally used W ammonium salts [12]. Rao et al. found that
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HPW/SBA-15 catalysts exhibited 30–70% higher activity than that based on regular silica gel due to the higher surface area and surface concentration of silanols in SBA-15 [13]. Kumar et al. reported that SBA-15 could accommodate discrete phosphotungstic acid due to its large pore volume and high surface area [14]. Among the as-prepared catalysts, 30 wt.% HPW/SBA-15 showed the highest phenol conversion in the vapour phase tert-butylation of phenol. In addition, HPW/SBA-15 catalysts were also used for biodiesel production. For example, Castanheiro et al. reported the esterification of free fatty acids for biodiesel production over HPW immobilized on SBA-15 [15]. Brahmkhatri et al. reported the esterification of oleic acid for biodiesel production over HPW anchored to SBA-15, as well as biodiesel production from waste cooking oil [16]. However, to the best of our knowledge, much less work has been done for the oxidation of benzaldehyde to benzoic acid with aqueous hydrogen peroxide using HPW/SBA-15 catalysts. In this work, HPW/SBA-15 mesoporous materials were synthesized by using a wet impregnation method and characterized by various techniques. The catalytic performance of the HPW/ SBA-15 catalysts for the oxidation of benzaldehyde to benzoic acid with aqueous hydrogen peroxide was studied. The effect of different HPW loadings on the structure and catalytic properties was comparatively investigated. The results will be helpful for the study and potential utility of heteropoly acids.
Nicolet 380 Fourier transform spectrometer using a KBr pellet technique. Diffuse reflectance ultraviolet-visible spectroscopy (DR UV-vis) was recorded in the wavelength range 200–800 nm using a Varian Cary 5000 UV-Vis-NIR spectrophotometer. BaSO4 was used as the reference. Scanning electron microscopy (SEM) image was taken using ZEISS SUPRA55 scanning electron microscope with an accelerating voltage of 15.0 kV. Temperature programmed desorption of ammonia (NH3-TPD) was carried out with the use of Tianjin XQ TP5080 auto-adsorption apparatus. Approximately 50 mg of sample was pretreated in a helium atmosphere (30 ml/min, 500 8C, 1 h), cooled to 20 8C, and then loaded into the reaction cell. The samples were exposed to ammonia at 100 8C for 0.5 h and then purged with helium. NH3-TPD spectra were registered between 100 and 750 8C (temperature ramp: 10 8C/min). The NH3 consumption was monitored by TCD detector. 2.4. Catalytic reaction The oxidation experiment of benzaldehyde with H2O2 was conducted in a 50 ml three-neck round-bottomed flask. 0.5 g of HPW/SBA-15 was dispersed into 17.5 ml of 30 wt.% H2O2 under stirring. Then, 5.0 ml of benzaldehyde was added and reacted at 80 8C for 6 h. The final reaction solution was cooled at 0 8C in a refrigerator and the as-prepared solid was washed with cold H2O (2 8C). The as-prepared solid was dried in air at 80 8C overnight and the yield was calculated according to the Eq. (1).
2. Experimental Yield of benzoic acid ð%Þ ¼
2.1. Materials Pluronic P123 triblock polymer (EO20PO70EO20, Mav = 5800, Aldrich), tetraethyl orthosilicate (TEOS), sodium tungstate (Na2WO42H2O), disodium hydrogen phosphate (Na2HPO412H2O), sulfuric acid, hydrochloric acid, hydrogen peroxide, diethyl ether and benzaldehyde were used as received from Sinopharm Chemical Reagent Co., Ltd. All these chemicals were of A.R. grade. 2.2. Synthesis of catalysts SBA-15 was prepared via a hydrothermal method according to the literature [17]. HPW was synthesized via an improved diethyl ether extraction method. The HPW/SBA-15 mesoporous materials were prepared by a wet impregnation method. 2 g of SBA-15 was impregnated with a calculated aqueous solution of HPW under stirring and the excess water was slowly vaporized at 50 8C. The obtained materials were dried overnight at 100 8C. The prepared catalysts were denoted as x wt.% HPW/SBA-15 (x = 5, 10, 15, 20, 25, 30, 35, 40, and 45, respectively).
mass of the dried solid sample ðgÞ 100 theoretic mass of benzoic acid from 5 ml of benzaldehyde ðgÞ (1)
3. Results and discussion Fig. 1 showed the low-angle XRD patterns of SBA-15 and the typical HPW/SBA-15 catalysts. All samples exhibited XRD patterns with one very intense diffraction peak and two weak peaks at about 0.90 8, 1.54 8 and 1.82 8 indexed to (1 0 0), (1 1 0) and (2 0 0) reflections, which were characteristic of 2-D hexagonal (P6 mm) structure with excellent textural uniformity. The results indicated that the HPW/SBA-15 samples retained the ordered mesopore structure of SBA-15. On the other hand, compared with SBA-15, there was a slightly shift toward higher angles for the peaks of HPW/SBA-15 catalysts with the increase of HPW loading, indicating a decrease of d-spacing (Table 1). This may be due to the restructuring of silica during the step of adsorption of HPW in water solution [11]. (100)
N2 adsorption–desorption experiments were performed on Quantachrome NOVA 1000e surface area and pore size analyzer at 196 8C. Prior to analysis the samples were degassed at 250 8C under vacuum (1.33 Pa) for 2 h. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method and the pore volume was determined by nitrogen adsorption at a relative pressure of 0.98. The micropore areas were estimated using the correlation of t-plot method. Pore size distribution were derived from the desorption profiles of the isotherms using the Barrett– Joyner–Halanda (BJH) method. X-ray powder diffraction (XRD) patterns were collected on Panalytical XPert Pro diffractometer using Cu Ka radiation (l = 1.5418 A´˚ ). The working voltage and current of the X-ray tube were 40 kV and 100 mA. Fourier transform infrared spectroscopy (FT-IR) was recorded on Thermoscientific
Intensity/counts
2.3. Characterization methods (110) (200)
x = 45
x = 25
x=5
SBA-15
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
2 Theta/deg Fig. 1. Low-angle XRD patterns of SBA-15 and the x wt.% HPW/SBA-15 catalysts.
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Table 1 Textural and structural characteristics of SBA-15 and the HPW/SBA-15 catalysts. 0
Sample
SBET a (m2/g)
SM b (m2/g)
SEx c (m2/g)
Vp d (cm3/g)
VM e (cm3/g)
Dv f (nm)
Da g (nm)
d1 0 0 spacing (nm)
a0 h (nm)
d i (nm)
d j (nm)
SBA-15 5 wt.% HPW/SBA-15 10 wt.% HPW/SBA-15 15 wt.% HPW/SBA-15 20 wt.% HPW/SBA-15 25 wt.% HPW/SBA-15 30 wt.% HPW/SBA-15 35 wt.% HPW/SBA-15 40 wt.% HPW/SBA-15 45 wt.% HPW/SBA-15
708.5 651.5 616.6 564.8 548.5 500.0 447.2 431.0 376.9 349.0
183.5 165.5 151.3 135.0 130.1 121.6 108.1 100.0 98.2 83.3
525.0 486.0 465.3 429.8 418.4 378.4 339.1 331.0 278.7 265.7
0.988 0.919 0.878 0.822 0.787 0.716 0.654 0.643 0.561 0.527
0.097 0.088 0.081 0.075 0.071 0.066 0.059 0.053 0.050 0.046
6.629 6.615 6.590 6.608 6.600 6.603 6.612 6.596 6.600 6.597
5.576 5.646 5.697 5.721 5.740 5.729 5.851 5.964 5.949 6.044
9.999 9.981 9.768 9.606 9.580 9.516 9.448 9.308 9.248 9.160
11.546 11.525 11.279 11.092 11.062 10.988 10.910 10.748 10.679 10.577
4.917 4.910 4.689 4.484 4.462 4.385 4.298 4.152 4.079 3.980
5.970 5.879 5.582 5.371 5.322 5.259 5.059 4.784 4.730 4.533
a b c d e f g h i j
Specific surface area calculated by the BET method. Micropore area calculated by the V–t method (t-plot method micropore analysis). External surface area calculated by the V–t method (t-plot method micropore analysis). Total pore volume determined by N2 adsorption at a relative pressure of 0.99. Micropore volume calculated by the V–t method (t-plot method micropore analysis). Micropore diameter corresponding to the maxium of the pore size distribution obtained from the desorption isotherm by the BJH method. Mean pore diameter obtained from the desorption isotherm by the BJH method. pffiffiffi Unit-cell parameter determined from the position of the (1 0 0) diffraction line as a0 ¼ 2d1 0 0 = 3. Pore wall thickness calculated as d = a0 Dv. 0 Pore wall thickness calculated as d = a0 Da.
The high-angle XRD patterns of bulk HPW and the typical HPW/ SBA-15 catalysts were shown in Fig. 2. The characteristic peaks of HPW were absent in the patterns of HPW/SBA-15 when the HPW loading was less than 20 wt.%. Then, very small peaks of HPW appeared and the intensity increased with the increase of HPW loading. However, all the typical characteristic diffraction peaks of crystalline phase of HPW were not observed, even at the highly loaded 45 wt.% HPW/SBA-15 sample, as compared to the XRD pattern of bulk HPW. This indicated that HPW was finely dispersed on the surface, inside the hexagonal channels or incorporated in the pore walls of SBA-15 because of its large pore volume and high surface area to accommodate more discrete HPW species [11,16,18]. The XRD data offered 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/SBA-15 catalysts. However, no information of such molecule structure can be obtained from XRD. The N2 adsorption–desorption isotherms of SBA-15 and the typical HPW/SBA-15 catalysts were depicted in Fig. 3. All the isotherms were the type of IV in nature according to the IUPAC classification and exhibited the H1 hysteresis loops, indicating the characteristic of mesoporous solids [19]. Furthermore, the
adsorption branches of all the isotherms showed a sharp inflection at relative pressures between 0.60 and 0.80 indicating the typical capillary condensation within uniform pores [11]. The pore size distributions of SBA-15 and the typical HPW/SBA-15 catalysts showed that all the samples had narrow pore size distribution, and the sharpness of this step indicated the uniformity of the mesopore size distribution (Fig. 4). It was interesting that all the HPW/SBA15 catalysts exhibited similar mesopore diameter value corresponding to the maximum of the pore size distributions (about 6.60 nm) and the data was only slightly lower that that of SBA-15 (6.62 nm). The results showed that the HPW/SBA-15 samples preserved the mesoporous channels of SBA-15 supports. The physicochemical parameters of SBA-15 and HPW/SBA-15 catalysts were listed in Table 1. It was found that SBA-15 exhibited high surface area (708.5 m2/g) and the surface areas of the HPW/ SBA-15 catalysts were above 345.0 m2/g. The bulk HPW showed very low surface area (0.8 m2/g). In other words, SBA-15 supports provided an opportunity for HPW to be dispersed over its large surface area. Furthermore, as the HPW loading increased, all the structural parameters of HPW/SBA-15 catalysts decreased relative to SBA-15. For example, it was observed the specific surface area
Adsorption Volume/ (cm /g)
x = 45
3
x = 25
Intensity/counts
bulk HPW
x = 45
x = 25
x=5
SBA-15
x=5
0.1 10
15
20
25
30
35
40
45
50
55
60
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
Relative pressure/(P/P )
2 Theta/deg Fig. 2. High-angle XRD patterns of bulk HPW and the x wt.% HPW/SBA-15 catalysts.
Fig. 3. N2 adsorption–desorption isotherms of SBA-15 and the x wt.% HPW/SBA-15 catalysts.
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-1
dV/dD/(cm g nm )
4
x = 25
Absorbance
3 -1
x = 45
x=5
(c) (b)
SBA-15
(a)
4
6
8
10
12
14
16
18
20
Pore diameter (nm)
200
300
400
500
600
700
800
Wavelength/nm
Fig. 4. Pore size distributions of SBA-15 and the x wt.% HPW/SBA-15 catalysts.
Fig. 6. DR UV–vis spectra of SBA-15 (a), 25 wt.% HPW/SBA-15 catalyst (b), and bulk HPW (c).
was 548.5 m2/g with 20 wt.% HPW loading but decreased to 349.0 m2/g with 40 wt.% HPW loading, which was mainly due to the formation of bulk HPW crystals in the pores of SBA-15 support. There were four different kinds of oxygen atoms, in Keggin anion structure [XM12O40]n: oxygen atoms bound to three M atoms and to the X heteroatom (Oa), bridging oxygen atoms (Ob and Oc), and terminal oxygen atoms (Od) [15]. Fig. 5 illustrated the FT-IR spectra of SBA-15, bulk HPW and the 25 wt.% HPW/SBA-15 catalyst. Bulk HPW showed four adsorption bands appeared in the range of 400–1400 cm1: 1082 cm1, 983 cm1, 890 cm1, and 804 cm1, which was attributed to the asymmetry vibrations P–Oa (internal oxygen connecting P and W), W5 5Od (terminal oxygen bonding to W atom), W–Ob (edge-sharing oxygen connecting W) and W–Oc (corner-sharing oxygen connecting W3O13 units) associated with Keggin ion, respectively [20,21]. Meanwhile, SBA-15 exhibited three characteristic bands: 1080 cm1 and 810 cm1 corresponded to characteristic of anti-symmetric vibration non-bridging oxygen atoms of Si–O–H bonds and symmetric stretching vibration of tetrahedral SiO4 structure units [22]; the band at about 459 cm1 corresponded to characteristic of tetrahedral bending of Si-O bonds [23]. In the 25 wt.% HPW/ SBA-15 catalyst, the bands 1082 cm1, 983 cm1, and 804 cm1 characteristic for HPW were masked by matching bands of SBA-15 matrix framework. The weak bands attributed to HPW at
<700 cm1 failed to appear, because of low concentration of HPW due to its dilution by SBA-15. Furthermore, the intensities of HPW bands gradually increased with the increase of HPW loading (not shown here). However, the band at approximately 890 cm1 would not appear until the loading of HPW was more than 25 wt.%. The results suggest that HPW retains but a degree of structural distortion its Keggin structure in the HPW/SBA-15 catalysts. The diffuse reflectance (DR) UV–vis spectra of SBA-15, bulk HPW, and the 25 wt.% HPW/SBA-15 material were recorded to comparatively study the nature of HPW species present in the HPW/SBA-15 samples (Fig. 6). As it can be seen from Fig. 6c, the electronic spectrum of bulk HPW consisted of a broad group of signals assigned to oxygen to metal (O2 ! W6+) charge transfer in PW12O403 anion with two maxima (at about 256 and 350 nm) and an absorption edge at 475 nm, which was agreement with those previous reports [24–26]. The HPW/SAB-15 samples showed one well-defined maximum at 265 nm attributed to the O2 ! W6+ charge transfer. Fig. 7 exhibited the typical SEM images of SBA-15, bulk HPW, and the 25 wt.% HPW/SBA-15 catalyst. The micrograph revealed that the HPW/SBA-15 catalyst retained the typical SBA-15 morphology, showing mainly chains of grain-type SBA-15 particles with 0.3–1.0 mm grain size. Zhao et al. reported that the shape of SBA-15 particles was dependent on the local curvature energy presented at the interface of the inorganic silica and amphiphilic block copolymer species [24]. According to this view, the HPW molecules had no influence on the curvature energy in the synthetic process of HPW/SBA-15 catalysts, which may explain the similar morphologies of SBA-15 and the HPW/SBA-15 catalyst. On the other hand, bulk HPW had a different morphology with irregular macro-structure probably due to its low surface area (0.8 m2/g). The SEM pictures also indicated the SBA-15 supports dispersed the bulk HPW particles. The acid properties including acidic sites amount and acid strength of the typical HPW/SBA-15 mesoporous materials were studied with NH3-TPD (Fig. 8). The NH3-TPD profile of bulk HPW was also comparably shown. No TPD peak of SBA-15 was observed, indicating that SBA-15 material did not have obvious acid sites. The curve of bulk HPW exhibited a broad peak at ca. 170 8C, which was ascribed to its weak strength acid sites. Meanwhile, a sharp peak appeared at ca. 550 8C. The NH3-TPD curves of the three HPW/SBA15 samples displayed a wide peak from 110 to approximately 400 8C, with a maximum desorption rate at about 200 8C corresponded to desorption of weakly held, physisorbed ammonia. Furthermore, the intensity increased with the increase of HPW
Transmittance/%
(c)
(b)
(a)
1400 1300 1200 1100 1000
900
800
700
600
500
400
-1
Wavenumber/cm
Fig. 5. FT-IR spectra of SBA-15 (a), 25 wt.% HPW/SBA-15 catalyst (b), and bulk HPW (c).
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Fig. 7. SEM images of SBA-15 (A), bulk HPW (B), and 25 wt.% HPW/SBA-15 catalyst (C).
loadings. In addition, a weak peak represented desorption of chemisorbed ammonia appeared at about 530 8C on the curve of 25 wt.% HPW/SBA-15 sample and became more obvious on the curve of 45 wt.% HPW/SBA-15 sample. The total concentrations of acid sites obtained by integration of the ammonia desorption amounts was 0.0462 mmol/g, 0.0657 mmol/g, and 0.1193 mmol/g for 5 wt.% HPW-SBA-15, 25 wt.% HPW-SBA-15, and 45 wt.% HPW-SBA-15 sample,
respectively. In other words, the acid site concentration did not increase in proportion to the HPW loading. It could be concluded from these values that not all acid sites provided by the HPW were available for ammonia adsorption due to the agglomeration of HPW when they were introduced in SBA-15 supports via impregnation method. Fig. 9 showed the catalytic activities of the HPW/SBA-15 catalysts for the oxidation of benzaldehyde with H2O2. The white 80 78
bulk HPW x = 45 x = 25
200
300
400
500
600
74 72 70 68 66 64
x=5
100
Yield of benzoic acid/%
Intensity/a.u.
76
62 700
o
Temperature/ C Fig. 8. NH3-TPD spectra recorded with the x wt.% HPW/SBA-15 catalysts and bulk HPW.
60
0
5
10
15
20
25
30
35
40
45
50
x/wt.% Fig. 9. Catalytic activities of the x wt.% HPW/SBA-15 catalysts for benzaldehyde oxidation.
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solid product was approved to be benzoic acid by FT-IR and melting point measurements. The yield was about 62% with the HPW loading at 5 wt.% and reached the maximum value (77.3%) with the HPW loading at 20 wt.%. Then, the yield slightly decreased to 75.3% when the HPW loading increased to 25 wt.% and kept ca. 72% in the 30–45 wt.% HPW loading region. In addition, the as-prepared solid was khaki and mostly melt at 113 8C ascribed to by-product over pure SBA-15. These results indicated that HPW played an important role in the catalytic reaction and the catalytic performance of HPW/SBA-15 catalysts was affected by the loading of HPW, the state of HPW species and the pore textural structure. Comparing with the traditional process of toluene oxidation, such as Refs. [3,25,26], the as-prepared HPW/SBA-15 mesoporous materials exhibited similar or even higher yield of benzoic acid. The results indicated that HPW/SBA-15 was an efficient catalyst for the green synthesis of benzoic acid.
Acknowledgements This work was supported by the National Natural Science Foundation of PR China (No. J1210060) and the Innovation Foundation of Zhengzhou University (No. 2012cxsy083). The authors gratefully acknowledge Dr. T. Li (College of Chemistry and Molecular Engineering, Peking University, China) for conducting the NH3-TPD and Prof. Y. Bai (School of Materials Science and Engineering, University of Science and Technology Beijing, China) for conducting the SEM experiments.
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Synthesis, characterization and catalytic evaluation of SBA-15 supported 12-tungstophosphoric acid mesoporous materials in the oxidation of benzaldehyde to benzoic acid Bei-Bei Dong, Bing-Bing Zhang, Hai-Yan Wu, Xi Chen, Ke Zhang, Xiu-Cheng Zheng * College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 December 2012 Received in revised form 25 February 2013 Accepted 7 March 2013 Available online xxx
A series of 12-tungstophosphoric acid catalysts supported on mesoporous molecular sieves SBA-15 were synthesized via wet impregnation method and characterized by various techniques. It has been found that all the catalysts retained the mesopore structure of SBA-15. The surface areas and pore volumes decreased with the increase of HPW loadings, whereas the mean pore diameter exhibited the opposite behavior. In addition, HPW units were highly dispersed on the SBA-15 supports when the 12tungstophosphoric acid loading was less than 20 wt.%. The results of the catalytic oxidation of benzaldehyde to benzoic acid with aqueous hydrogen peroxide, in the absence of any organic solvent and co-catalysts, indicated that SBA-15 supported 12-tungstophosphoric acid was an efficient catalyst. The sample with 20 wt.% loading of 12-tungstophosphoric acid was found to be more active than other catalysts under the reaction conditions. ß 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Composites B. Chemical synthesis C. Infrared spectroscopy C. X-ray diffraction D. Catalytic properties
1. Introduction Nowadays, benzoic acid is widely used in medicines, veterinary medicines, industrial raw materials, preservatives for food, cosmetics, resin preparations, plasticizers, dyestuffs, synthetic fibers, and intermediates [1,2]. It is mostly produced by the oxidation of toluene both in vapor and liquid phases. However, these methods involve expensive reagents, high temperatures, but give low yields. Meanwhile, toluene is also a representative of aromatic hydrocarbons categorized as hazardous material [3]. Thus, it is urgent to explore a new and efficient catalyst under neutral, mild and practical conditions. It is known that heteropoly acids (HPAs) exhibit unique physicochemical properties with their structural mobility and catalytic multifunctionality. HPAs possess a very strong Brønsted acidity and appropriate redox properties, which can be tuned by varying their chemical composition. Furthermore, they have other advantages, including ease of handling, environmental compatibility, non-toxicity, and experimental simplicity [2,4]. Among the solid heteropoly acids, 12-tungstophosphoric acid (HPW), the strongest HPAs in the Keggin series, has been extensively studied as super acid catalyst for many organic reactions and widely used
* Corresponding author. Tel.: +86 371 67781780. E-mail address: [email protected] (X.-C. Zheng).
in industrial processes [5,6]. However, the use of bulk HPAs is limited by their low thermal stability, low surface area and separation problems, which could be overcome by introducing them into suitable supports [7]. SBA-15, a new type of mesoporous molecular sieves with ordered hexagonal array of uniform tubular channels with high surface area, large pore diameters, thick walls, high thermal stability and hydrothermal stability, has attracted considerable attention in the field of heterogeneous catalysis and nanoscale materials [8]. However, SBA-15 does not exhibit intrinsic catalytic activity. It is necessary to replace part of the silicon of the structure or graft onto the internal surface of the pores with heteroatoms, which are able to make the solids catalytically active. Gagea et al. found that compared to ultrastable Y zeolites, HPW/SBA-15 catalysts showed a broader optimum reaction temperature window for the isomerization and higher ndecane dibranching selectivity [9]. He et al. found that the finely dispersed and chemically bound HPW species and pore size were responsible for the enhanced selectivity in the acetic anhydride catalytically condensed from acetic acid systems [10]. Yang et al. found that the SBA-15 based composite introduced by HPW was very stable in the acidic property and the structural regularity [11]. Lizama et al. found that the W based catalysts prepared from HPW showed better catalytic performance in deep hydrodesulfurization of 4,6dimethyldibenzothiophene than the counterparts prepared from traditionally used W ammonium salts [12]. Rao et al. found that
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HPW/SBA-15 catalysts exhibited 30–70% higher activity than that based on regular silica gel due to the higher surface area and surface concentration of silanols in SBA-15 [13]. Kumar et al. reported that SBA-15 could accommodate discrete phosphotungstic acid due to its large pore volume and high surface area [14]. Among the as-prepared catalysts, 30 wt.% HPW/SBA-15 showed the highest phenol conversion in the vapour phase tert-butylation of phenol. In addition, HPW/SBA-15 catalysts were also used for biodiesel production. For example, Castanheiro et al. reported the esterification of free fatty acids for biodiesel production over HPW immobilized on SBA-15 [15]. Brahmkhatri et al. reported the esterification of oleic acid for biodiesel production over HPW anchored to SBA-15, as well as biodiesel production from waste cooking oil [16]. However, to the best of our knowledge, much less work has been done for the oxidation of benzaldehyde to benzoic acid with aqueous hydrogen peroxide using HPW/SBA-15 catalysts. In this work, HPW/SBA-15 mesoporous materials were synthesized by using a wet impregnation method and characterized by various techniques. The catalytic performance of the HPW/ SBA-15 catalysts for the oxidation of benzaldehyde to benzoic acid with aqueous hydrogen peroxide was studied. The effect of different HPW loadings on the structure and catalytic properties was comparatively investigated. The results will be helpful for the study and potential utility of heteropoly acids.
Nicolet 380 Fourier transform spectrometer using a KBr pellet technique. Diffuse reflectance ultraviolet-visible spectroscopy (DR UV-vis) was recorded in the wavelength range 200–800 nm using a Varian Cary 5000 UV-Vis-NIR spectrophotometer. BaSO4 was used as the reference. Scanning electron microscopy (SEM) image was taken using ZEISS SUPRA55 scanning electron microscope with an accelerating voltage of 15.0 kV. Temperature programmed desorption of ammonia (NH3-TPD) was carried out with the use of Tianjin XQ TP5080 auto-adsorption apparatus. Approximately 50 mg of sample was pretreated in a helium atmosphere (30 ml/min, 500 8C, 1 h), cooled to 20 8C, and then loaded into the reaction cell. The samples were exposed to ammonia at 100 8C for 0.5 h and then purged with helium. NH3-TPD spectra were registered between 100 and 750 8C (temperature ramp: 10 8C/min). The NH3 consumption was monitored by TCD detector. 2.4. Catalytic reaction The oxidation experiment of benzaldehyde with H2O2 was conducted in a 50 ml three-neck round-bottomed flask. 0.5 g of HPW/SBA-15 was dispersed into 17.5 ml of 30 wt.% H2O2 under stirring. Then, 5.0 ml of benzaldehyde was added and reacted at 80 8C for 6 h. The final reaction solution was cooled at 0 8C in a refrigerator and the as-prepared solid was washed with cold H2O (2 8C). The as-prepared solid was dried in air at 80 8C overnight and the yield was calculated according to the Eq. (1).
2. Experimental Yield of benzoic acid ð%Þ ¼
2.1. Materials Pluronic P123 triblock polymer (EO20PO70EO20, Mav = 5800, Aldrich), tetraethyl orthosilicate (TEOS), sodium tungstate (Na2WO42H2O), disodium hydrogen phosphate (Na2HPO412H2O), sulfuric acid, hydrochloric acid, hydrogen peroxide, diethyl ether and benzaldehyde were used as received from Sinopharm Chemical Reagent Co., Ltd. All these chemicals were of A.R. grade. 2.2. Synthesis of catalysts SBA-15 was prepared via a hydrothermal method according to the literature [17]. HPW was synthesized via an improved diethyl ether extraction method. The HPW/SBA-15 mesoporous materials were prepared by a wet impregnation method. 2 g of SBA-15 was impregnated with a calculated aqueous solution of HPW under stirring and the excess water was slowly vaporized at 50 8C. The obtained materials were dried overnight at 100 8C. The prepared catalysts were denoted as x wt.% HPW/SBA-15 (x = 5, 10, 15, 20, 25, 30, 35, 40, and 45, respectively).
mass of the dried solid sample ðgÞ 100 theoretic mass of benzoic acid from 5 ml of benzaldehyde ðgÞ (1)
3. Results and discussion Fig. 1 showed the low-angle XRD patterns of SBA-15 and the typical HPW/SBA-15 catalysts. All samples exhibited XRD patterns with one very intense diffraction peak and two weak peaks at about 0.90 8, 1.54 8 and 1.82 8 indexed to (1 0 0), (1 1 0) and (2 0 0) reflections, which were characteristic of 2-D hexagonal (P6 mm) structure with excellent textural uniformity. The results indicated that the HPW/SBA-15 samples retained the ordered mesopore structure of SBA-15. On the other hand, compared with SBA-15, there was a slightly shift toward higher angles for the peaks of HPW/SBA-15 catalysts with the increase of HPW loading, indicating a decrease of d-spacing (Table 1). This may be due to the restructuring of silica during the step of adsorption of HPW in water solution [11]. (100)
N2 adsorption–desorption experiments were performed on Quantachrome NOVA 1000e surface area and pore size analyzer at 196 8C. Prior to analysis the samples were degassed at 250 8C under vacuum (1.33 Pa) for 2 h. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method and the pore volume was determined by nitrogen adsorption at a relative pressure of 0.98. The micropore areas were estimated using the correlation of t-plot method. Pore size distribution were derived from the desorption profiles of the isotherms using the Barrett– Joyner–Halanda (BJH) method. X-ray powder diffraction (XRD) patterns were collected on Panalytical XPert Pro diffractometer using Cu Ka radiation (l = 1.5418 A´˚ ). The working voltage and current of the X-ray tube were 40 kV and 100 mA. Fourier transform infrared spectroscopy (FT-IR) was recorded on Thermoscientific
Intensity/counts
2.3. Characterization methods (110) (200)
x = 45
x = 25
x=5
SBA-15
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
2 Theta/deg Fig. 1. Low-angle XRD patterns of SBA-15 and the x wt.% HPW/SBA-15 catalysts.
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Table 1 Textural and structural characteristics of SBA-15 and the HPW/SBA-15 catalysts. 0
Sample
SBET a (m2/g)
SM b (m2/g)
SEx c (m2/g)
Vp d (cm3/g)
VM e (cm3/g)
Dv f (nm)
Da g (nm)
d1 0 0 spacing (nm)
a0 h (nm)
d i (nm)
d j (nm)
SBA-15 5 wt.% HPW/SBA-15 10 wt.% HPW/SBA-15 15 wt.% HPW/SBA-15 20 wt.% HPW/SBA-15 25 wt.% HPW/SBA-15 30 wt.% HPW/SBA-15 35 wt.% HPW/SBA-15 40 wt.% HPW/SBA-15 45 wt.% HPW/SBA-15
708.5 651.5 616.6 564.8 548.5 500.0 447.2 431.0 376.9 349.0
183.5 165.5 151.3 135.0 130.1 121.6 108.1 100.0 98.2 83.3
525.0 486.0 465.3 429.8 418.4 378.4 339.1 331.0 278.7 265.7
0.988 0.919 0.878 0.822 0.787 0.716 0.654 0.643 0.561 0.527
0.097 0.088 0.081 0.075 0.071 0.066 0.059 0.053 0.050 0.046
6.629 6.615 6.590 6.608 6.600 6.603 6.612 6.596 6.600 6.597
5.576 5.646 5.697 5.721 5.740 5.729 5.851 5.964 5.949 6.044
9.999 9.981 9.768 9.606 9.580 9.516 9.448 9.308 9.248 9.160
11.546 11.525 11.279 11.092 11.062 10.988 10.910 10.748 10.679 10.577
4.917 4.910 4.689 4.484 4.462 4.385 4.298 4.152 4.079 3.980
5.970 5.879 5.582 5.371 5.322 5.259 5.059 4.784 4.730 4.533
a b c d e f g h i j
Specific surface area calculated by the BET method. Micropore area calculated by the V–t method (t-plot method micropore analysis). External surface area calculated by the V–t method (t-plot method micropore analysis). Total pore volume determined by N2 adsorption at a relative pressure of 0.99. Micropore volume calculated by the V–t method (t-plot method micropore analysis). Micropore diameter corresponding to the maxium of the pore size distribution obtained from the desorption isotherm by the BJH method. Mean pore diameter obtained from the desorption isotherm by the BJH method. pffiffiffi Unit-cell parameter determined from the position of the (1 0 0) diffraction line as a0 ¼ 2d1 0 0 = 3. Pore wall thickness calculated as d = a0 Dv. 0 Pore wall thickness calculated as d = a0 Da.
The high-angle XRD patterns of bulk HPW and the typical HPW/ SBA-15 catalysts were shown in Fig. 2. The characteristic peaks of HPW were absent in the patterns of HPW/SBA-15 when the HPW loading was less than 20 wt.%. Then, very small peaks of HPW appeared and the intensity increased with the increase of HPW loading. However, all the typical characteristic diffraction peaks of crystalline phase of HPW were not observed, even at the highly loaded 45 wt.% HPW/SBA-15 sample, as compared to the XRD pattern of bulk HPW. This indicated that HPW was finely dispersed on the surface, inside the hexagonal channels or incorporated in the pore walls of SBA-15 because of its large pore volume and high surface area to accommodate more discrete HPW species [11,16,18]. The XRD data offered 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/SBA-15 catalysts. However, no information of such molecule structure can be obtained from XRD. The N2 adsorption–desorption isotherms of SBA-15 and the typical HPW/SBA-15 catalysts were depicted in Fig. 3. All the isotherms were the type of IV in nature according to the IUPAC classification and exhibited the H1 hysteresis loops, indicating the characteristic of mesoporous solids [19]. Furthermore, the
adsorption branches of all the isotherms showed a sharp inflection at relative pressures between 0.60 and 0.80 indicating the typical capillary condensation within uniform pores [11]. The pore size distributions of SBA-15 and the typical HPW/SBA-15 catalysts showed that all the samples had narrow pore size distribution, and the sharpness of this step indicated the uniformity of the mesopore size distribution (Fig. 4). It was interesting that all the HPW/SBA15 catalysts exhibited similar mesopore diameter value corresponding to the maximum of the pore size distributions (about 6.60 nm) and the data was only slightly lower that that of SBA-15 (6.62 nm). The results showed that the HPW/SBA-15 samples preserved the mesoporous channels of SBA-15 supports. The physicochemical parameters of SBA-15 and HPW/SBA-15 catalysts were listed in Table 1. It was found that SBA-15 exhibited high surface area (708.5 m2/g) and the surface areas of the HPW/ SBA-15 catalysts were above 345.0 m2/g. The bulk HPW showed very low surface area (0.8 m2/g). In other words, SBA-15 supports provided an opportunity for HPW to be dispersed over its large surface area. Furthermore, as the HPW loading increased, all the structural parameters of HPW/SBA-15 catalysts decreased relative to SBA-15. For example, it was observed the specific surface area
Adsorption Volume/ (cm /g)
x = 45
3
x = 25
Intensity/counts
bulk HPW
x = 45
x = 25
x=5
SBA-15
x=5
0.1 10
15
20
25
30
35
40
45
50
55
60
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
Relative pressure/(P/P )
2 Theta/deg Fig. 2. High-angle XRD patterns of bulk HPW and the x wt.% HPW/SBA-15 catalysts.
Fig. 3. N2 adsorption–desorption isotherms of SBA-15 and the x wt.% HPW/SBA-15 catalysts.
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-1
dV/dD/(cm g nm )
4
x = 25
Absorbance
3 -1
x = 45
x=5
(c) (b)
SBA-15
(a)
4
6
8
10
12
14
16
18
20
Pore diameter (nm)
200
300
400
500
600
700
800
Wavelength/nm
Fig. 4. Pore size distributions of SBA-15 and the x wt.% HPW/SBA-15 catalysts.
Fig. 6. DR UV–vis spectra of SBA-15 (a), 25 wt.% HPW/SBA-15 catalyst (b), and bulk HPW (c).
was 548.5 m2/g with 20 wt.% HPW loading but decreased to 349.0 m2/g with 40 wt.% HPW loading, which was mainly due to the formation of bulk HPW crystals in the pores of SBA-15 support. There were four different kinds of oxygen atoms, in Keggin anion structure [XM12O40]n: oxygen atoms bound to three M atoms and to the X heteroatom (Oa), bridging oxygen atoms (Ob and Oc), and terminal oxygen atoms (Od) [15]. Fig. 5 illustrated the FT-IR spectra of SBA-15, bulk HPW and the 25 wt.% HPW/SBA-15 catalyst. Bulk HPW showed four adsorption bands appeared in the range of 400–1400 cm1: 1082 cm1, 983 cm1, 890 cm1, and 804 cm1, which was attributed to the asymmetry vibrations P–Oa (internal oxygen connecting P and W), W5 5Od (terminal oxygen bonding to W atom), W–Ob (edge-sharing oxygen connecting W) and W–Oc (corner-sharing oxygen connecting W3O13 units) associated with Keggin ion, respectively [20,21]. Meanwhile, SBA-15 exhibited three characteristic bands: 1080 cm1 and 810 cm1 corresponded to characteristic of anti-symmetric vibration non-bridging oxygen atoms of Si–O–H bonds and symmetric stretching vibration of tetrahedral SiO4 structure units [22]; the band at about 459 cm1 corresponded to characteristic of tetrahedral bending of Si-O bonds [23]. In the 25 wt.% HPW/ SBA-15 catalyst, the bands 1082 cm1, 983 cm1, and 804 cm1 characteristic for HPW were masked by matching bands of SBA-15 matrix framework. The weak bands attributed to HPW at
<700 cm1 failed to appear, because of low concentration of HPW due to its dilution by SBA-15. Furthermore, the intensities of HPW bands gradually increased with the increase of HPW loading (not shown here). However, the band at approximately 890 cm1 would not appear until the loading of HPW was more than 25 wt.%. The results suggest that HPW retains but a degree of structural distortion its Keggin structure in the HPW/SBA-15 catalysts. The diffuse reflectance (DR) UV–vis spectra of SBA-15, bulk HPW, and the 25 wt.% HPW/SBA-15 material were recorded to comparatively study the nature of HPW species present in the HPW/SBA-15 samples (Fig. 6). As it can be seen from Fig. 6c, the electronic spectrum of bulk HPW consisted of a broad group of signals assigned to oxygen to metal (O2 ! W6+) charge transfer in PW12O403 anion with two maxima (at about 256 and 350 nm) and an absorption edge at 475 nm, which was agreement with those previous reports [24–26]. The HPW/SAB-15 samples showed one well-defined maximum at 265 nm attributed to the O2 ! W6+ charge transfer. Fig. 7 exhibited the typical SEM images of SBA-15, bulk HPW, and the 25 wt.% HPW/SBA-15 catalyst. The micrograph revealed that the HPW/SBA-15 catalyst retained the typical SBA-15 morphology, showing mainly chains of grain-type SBA-15 particles with 0.3–1.0 mm grain size. Zhao et al. reported that the shape of SBA-15 particles was dependent on the local curvature energy presented at the interface of the inorganic silica and amphiphilic block copolymer species [24]. According to this view, the HPW molecules had no influence on the curvature energy in the synthetic process of HPW/SBA-15 catalysts, which may explain the similar morphologies of SBA-15 and the HPW/SBA-15 catalyst. On the other hand, bulk HPW had a different morphology with irregular macro-structure probably due to its low surface area (0.8 m2/g). The SEM pictures also indicated the SBA-15 supports dispersed the bulk HPW particles. The acid properties including acidic sites amount and acid strength of the typical HPW/SBA-15 mesoporous materials were studied with NH3-TPD (Fig. 8). The NH3-TPD profile of bulk HPW was also comparably shown. No TPD peak of SBA-15 was observed, indicating that SBA-15 material did not have obvious acid sites. The curve of bulk HPW exhibited a broad peak at ca. 170 8C, which was ascribed to its weak strength acid sites. Meanwhile, a sharp peak appeared at ca. 550 8C. The NH3-TPD curves of the three HPW/SBA15 samples displayed a wide peak from 110 to approximately 400 8C, with a maximum desorption rate at about 200 8C corresponded to desorption of weakly held, physisorbed ammonia. Furthermore, the intensity increased with the increase of HPW
Transmittance/%
(c)
(b)
(a)
1400 1300 1200 1100 1000
900
800
700
600
500
400
-1
Wavenumber/cm
Fig. 5. FT-IR spectra of SBA-15 (a), 25 wt.% HPW/SBA-15 catalyst (b), and bulk HPW (c).
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Fig. 7. SEM images of SBA-15 (A), bulk HPW (B), and 25 wt.% HPW/SBA-15 catalyst (C).
loadings. In addition, a weak peak represented desorption of chemisorbed ammonia appeared at about 530 8C on the curve of 25 wt.% HPW/SBA-15 sample and became more obvious on the curve of 45 wt.% HPW/SBA-15 sample. The total concentrations of acid sites obtained by integration of the ammonia desorption amounts was 0.0462 mmol/g, 0.0657 mmol/g, and 0.1193 mmol/g for 5 wt.% HPW-SBA-15, 25 wt.% HPW-SBA-15, and 45 wt.% HPW-SBA-15 sample,
respectively. In other words, the acid site concentration did not increase in proportion to the HPW loading. It could be concluded from these values that not all acid sites provided by the HPW were available for ammonia adsorption due to the agglomeration of HPW when they were introduced in SBA-15 supports via impregnation method. Fig. 9 showed the catalytic activities of the HPW/SBA-15 catalysts for the oxidation of benzaldehyde with H2O2. The white 80 78
bulk HPW x = 45 x = 25
200
300
400
500
600
74 72 70 68 66 64
x=5
100
Yield of benzoic acid/%
Intensity/a.u.
76
62 700
o
Temperature/ C Fig. 8. NH3-TPD spectra recorded with the x wt.% HPW/SBA-15 catalysts and bulk HPW.
60
0
5
10
15
20
25
30
35
40
45
50
x/wt.% Fig. 9. Catalytic activities of the x wt.% HPW/SBA-15 catalysts for benzaldehyde oxidation.
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solid product was approved to be benzoic acid by FT-IR and melting point measurements. The yield was about 62% with the HPW loading at 5 wt.% and reached the maximum value (77.3%) with the HPW loading at 20 wt.%. Then, the yield slightly decreased to 75.3% when the HPW loading increased to 25 wt.% and kept ca. 72% in the 30–45 wt.% HPW loading region. In addition, the as-prepared solid was khaki and mostly melt at 113 8C ascribed to by-product over pure SBA-15. These results indicated that HPW played an important role in the catalytic reaction and the catalytic performance of HPW/SBA-15 catalysts was affected by the loading of HPW, the state of HPW species and the pore textural structure. Comparing with the traditional process of toluene oxidation, such as Refs. [3,25,26], the as-prepared HPW/SBA-15 mesoporous materials exhibited similar or even higher yield of benzoic acid. The results indicated that HPW/SBA-15 was an efficient catalyst for the green synthesis of benzoic acid.
Acknowledgements This work was supported by the National Natural Science Foundation of PR China (No. J1210060) and the Innovation Foundation of Zhengzhou University (No. 2012cxsy083). The authors gratefully acknowledge Dr. T. Li (College of Chemistry and Molecular Engineering, Peking University, China) for conducting the NH3-TPD and Prof. Y. Bai (School of Materials Science and Engineering, University of Science and Technology Beijing, China) for conducting the SEM experiments.
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