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SBA-15-SO3H and application for dehydration of xylose to furfural
Journal of Industrial and Engineering Chemistry 19 (2013) 1395–1399
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Preparation of solid acid catalyst packing AAO/SBA-15-SO3H and application for dehydration of xylose to furfural Derun Hua, PanPan Li, Yulong Wu *, Yu Chen, Mingde Yang *, Jie Dang, Quanhua Xie, Ji Liu, Xiao-yin Sun Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 October 2012 Accepted 1 January 2013 Available online 11 January 2013
The solid acid catalyst packing AAO/SBA-15-SO3H was prepared by the co-condensation and grafting method with porous anodic aluminum oxide (AAO) as support. FT-IR, SEM and TEM were applied to characterize the prepared samples. Results showed that catalysts prepared by two methods both contained active centers, and SBA-15 nanorod arrays grow inside a porous alumina membrane AAO and are perpendicular to the substrate. Their catalytic performances were tested for dehydration of xylose to furfural. The conversion of xylose and selectivity of furfural were 90% and 74% on the AAO/SBA-15SO3H(C) catalyst prepared by the co-condensation method, respectively. The deactivation and regeneration of the AAO/SBA-15-SO3H(C) catalyst for the dehydration of xylose were also investigated, the activity of catalyst treated by 30 wt.% H2O2 almost was recovered. Crown Copyright ß 2013 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved.
Keywords: Solid acid catalyst packing AAO/SBA-15-SO3H Xylose Dehydration
1. Introduction Furfural derived from biomass is an important organic material and has been considered as a sustainable intermediate for the preparation of fine chemicals, pharmaceuticals, and furan-based polymers [1]. For energy crisis and environmental pollution, it is important to develop the technology for furfural. Furfural can be produced from agricultural raw (or waste) materials by acidic degradation. The reaction involves hydrolysis of pentosan into pentoses (e.g., xylose) and successive (much slower) dehydration of the latter to form furfural (Scheme 1) [2]. Dehydration of xylose to furfural is an acid-catalyzed reaction. Now, mineral liquid acids, such as concentrated sulfuric acid and hydrochloric acid, are used in the most industrial furfural. However, mineral acids have serious drawbacks in terms of separation and recycling, as well as material corrosion. Recently, solid acids have been of particular interest in the dehydration of xylose [3–7]. Solid acids are used as catalysts and show high catalytic performance, and material corrosion is controlled [8,9]. However, application of some solid catalysts is limited due to poor hydrothermal stability and small pore size. Compared with these catalysts, the sulfonic acid functionalized SBA-15 with a larger pore size has attracted attention because of the good hydrothermal stability [10–12]. Shi et al.
* Corresponding authors. Fax: +86 106 9771464. E-mail addresses: [email protected] (Y. Wu), [email protected] (M. Yang).
investigated the catalytic activity of sulfonic acid functionalized SBA-15 for dehydration of xylose to furfural in the liquid-phase, and the results were very satisfactory [12]. However, process with the pulverous solid acid and the highpressure reactor is single-stage extraction and low efficiency. For the improvement of the process of furfural, it requires not only the good catalyst, but also the efficiency of the mass transfer. Therefore, we prepared solid acid catalyst packing, which is composed of porous anodic aluminum oxide as matrix material, SBA-15 as support and a sulfonic acid group as the active component. Porous anodic aluminum oxide membranes, having ordered and a vertical one-dimensional (1D) channel structure, have attracted a lot of interest in the preparation of monodisperse and ordered 1D nanostructure within their pores [13–16]. Until now, many nanorod arrays have been synthesized using porous alumina membrane as the growth-limiting template. Solid acid catalyst packing makes the even distribution of active components on the matrix, which can reduce byproducts due to excessive local reactions. In addition, the design of packing is also very beneficial for the mass transfer and reducing the coke formation during the dehydration of xylose. In the study, the AAO/SBA-15-SO3H catalyst packing, consisting of a base (AAO), support (SBA-15) and active phase (sulfoacid), was synthesized by co-condensing and grafting methods and applied to xylose to furfural. The packing catalyst synthesized by two methods possesses good mechanical stability and regeneration, a high reaction rate, mass transfer and heterogeneous distribution of the active site as is shown in Fig. 1.
1226-086X/$ – see front matter . Crown Copyright ß 2013 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.01.002
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D. Hua et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1395–1399
Scheme 1. Reaction for hydrolysis of pentosan to furfural.
AAO/SBA-15-SO3H was obtained. Procedures for AAO/SBA-15SO3H catalyst are shown in Scheme 2.
2. Experiments 2.1. Preparation of catalysts
2.2. Characterization of catalysts Co-condensing preparation of AAO/SBA-15-SO3H(C): P123 (1.0 g, MW = 5800) from Aldrich Company was dissolved in 100 ml ethanol solution, and 1 ml TEOS (tetraethoxysilane) and 0.4 g HCl (38 wt.%) were added to the solution, then 0.15 ml MPTMS (3-mercaptopropyltrimethoxysilane) and 1 ml H2O2 (30 wt.%) were added and kept stirring at room temperature for 20 h. AAO membrane (1 g) was added in the fore-prepared solution, and the mixture was treated by rotary evaporator method at 40 8C for 10 h, then AAO membrane was dried at 60 8C for 12 h, washed by ethanol and dried at 60 8C again. Grafting preparation of AAO/SBA-15-SO3H(G): the preparation procedure was similar to that described the above and AAO/SBA-15 was obtained, then the sample was dried and calcined to remove the template in air at 540 8C for 6 h. In a typical grafting procedure, AAO/SBA-15 (1.0 g) and MPTMS (0.15 ml) were added into toluene (40.0 g), and the mixture was kept stirring for 24 h and filtered. The solid was washed by refluxing mixture of dichloromethane (20.0 g) and ethyl acetate (20.0 g) for 24 h for removal of MPTMS. Then the functionalized AAO/SBA-15 was added into the mixture of H2O2 (40.0 g), CH3OH (120.0 g) and H2O (400.0 g), the suspension was stirred at room temperature for 12 h and filtered. After filtration, the precipitate was added into 20 ml H2SO4 (0.5 mol/l), stirred for 12 h and filtered, then washed by ethanol and water, last dried, and
The mid-infrared spectra were collected on the Digilab FTS3000 FT-IR Spectrum Gx FTIR spectrometer (Perkin Elmer Company) by the coaddition of 250 scans at a 4 cm1 resolution, and KBr sample window. The morphology of the catalyst was recorded with field emission scanning electron microscope (FEI QUANTA 200F). The transition electron microscopy (TEM) images were obtained with JEM-2100LaBa (Ishizuka Electronics Corporation). Before examined, the sample was soaked by H3PO4 (7 wt.%) for 4 days. Mechanical stability was tested by ultrasonic irradiation. The sample was in an ultrasonic bath (SB-5200DTDN, 40KHz) at 20 8C for 1 h, then the sample was dried, weight of the sample was compared before and after ultrasonic irradiation. 2.3. Test of catalyst for xylose to furfural Xylose (0.75 g, Sigma–Aldrich, 99%) and catalyst (0.1 g) were mixed with the mixture of H2O (7.5 ml) and toluene (17.5 ml), and the resulting mixture was introduced into an autoclave (50 ml). The autoclave was sealed and heated. The slurry was stirred with a magnetic stirrer at 500 rpm and kept at 160 8C for 4 h. The autoclave was cooled to room temperature. Then the aqueous and organic phases were filtered by a microstrainer. The production
Fig. 1. The scheme of solid acid catalyst packing and dehydration of xylose to furfural.
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Scheme 2. Procedures for AAO/SBA-15-SO3H catalyst.
was analyzed by HPLC (Shimadzu Company) with column (HPX87H) and GC (Agilent) with column (PEG-20M). Xylose conversion and furfural selectivity are calculated as: C xylose ¼
ðmxyloses mxylosee Þ=M xylose mxyloses =M xylose
(1)
Sfurfural ¼
ðmfurfuralw þ mfurfuralt Þ=M furfural ðmxyloses mxylosee Þ=Mxylose
(2)
Y furfural ¼ C xylose Sfurfural
(3)
where C xylose , Sfurfural and Y furfural are denoted as the conversion of xylose, the selectivity of furfural and yield of furfural, respectively. mxyloses and mxylosee are the mass concentration of xylose before and after the reaction, M xylose is the molar mass of xylose, mfurfuralw and mfurfuralt are the mass concentration of the furfural in the aqueous and organic phases, respectively. 3. Results and discussion SEM was used to characterize the morphology feature of catalyst. Fig. 2 is the scanning electron microscopy (SEM) images of the catalysts. As determined from Fig. 2a, AAO substrate has a mean diameter of 180 nm with a size deviation of ca. 10%. Fig. 2b
clearly shows that the pores of AAO membranes are filled with SBA-15. Additionally, it is found that SBA-15 nanorods happen to contract under the drying processes and are perpendicular to AAO membranes. Fig. 2c shows the bundle-shaped SBA-15 after removal of the AAO. The average diameter of SBA-15 is about 180 nm and mainly depends on the pore size of the AAO. TEM technology is employed to further investigate the inner structure of the catalyst. Fig. 3 is the transmission electron microscopy (TEM) images of AAO/SBA-15-SO3H. Fig. 3a shows that AAO/SBA-15-SO3H holds a circular lamellar a circular columnar mesostructure in the central region, Fig. 3b shows that AAO/SBA15-SO3H has 1D alignment of the silica-nanochannels orientating along the long axis of the nanopores of the AAO template. The mesostructure is attributed to the circular columnar mesostructure of the pores of the AAO membrane and is significantly affected by the concentration of P123 [17]. When the concentration of P123 was below 15 mg/ml, nanorods were composed of columnar nanochannels paralleling the long axis of the AAO nanochannel. Otherwise, the columnar nanochannel, circling the axis of the AAO nanochannels, are present. The IR spectra of all solid acid catalysts packing are shown in Fig. 4. All samples exhibit infrared bands at 450, 800 and 1000– 1260. The IR spectra at 450, 800 and 1000–1260 cm1 are assigned to the symmetric and asymmetric stretching of Si–O–Si vibrations
Fig. 2. The SEM images of catalysts.
D. Hua et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1395–1399
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Fig. 3. The TEM picture of AAO/SBA-15-SO3H.
Fig. 4. FT-IR spectra of solid acid catalysts packing (a) AAO/SBA-15, (b) AAO/SBA-15SH, (c) AAO/SBA-15-SO3H(G), and (d) AAO/SBA-15-SO3H(C).
for the tetrahedral SiO4 structure units, and the bands are usually assigned to Vd(Si–O–Si), Vs(Si–O–Si) and Vas(Si–O–Si), respectively[18]. The band at 3400 cm1 is assigned to stretching mode of Si– O–H. In the case of AAO/SBA-15-SH, the 2950–2850 cm1 regions are assigned to methylene stretching bands [19], indicating that the 3-MPTS groups have been anchored in the pore channels of SBA-15 material. The band centered at 2570 cm1 is assigned to the thiol (–SH) stretching vibrations [12]. The above results demonstrate that the mercaptopropyl groups are successfully introduced into the interior mesopore surfaces. For the AAO/SBA15-SO3H(C) and AAO/SBA-15-SO3H(G) samples, the band at 2570 cm1 is absent, but the new band at 1650 cm1 which is assigned to the asymmetric stretching band of the SO2 moieties [20], is present, which confirms the formation of sulfonic acid species after the oxidation reactions. All catalysts are tested in the batch experiments for dehydration of xylose to furfural. Results are summarized in Table 1. It is Table 1 Catalytic performance of the catalysts. Catalysts
Conversion (%)
Selectivity (%)
Yield (%)
None AAO AAO/SBA-15 AAO/SBA-15-SH AAO/SBA-15-SO3H(C) AAO/SBA-15-SO3H(G) H2SO4 (4%) SBA-15-SO3H
17 64 85 80 90 99 92 92
13 45 47 29 74 55 71 74
2 29 40 23 67 49 65 68
found that AAO, AAO/SBA-15 and AAO/SBA-15-SH have catalytic activity due to its Brønsted acid capacity [21], but the activity is lower than that of SBA-15-SO3H, AAO/SBA-15-SO3H(C) and AAO/ SBA-15-SO3H(G), because the acid number of the former is lower than that of the latter; xylose conversion on AAO/SBA-15-SO3H(C) is slightly lower than that on AAO/SBA-15-SO3H(G), but furfural yield and selectivity of the latter are obviously higher than that of the former. In xylose to furfural, if conversion of xylose is above 90%, we may think the reaction ends. In this case, selectivity of the product is evaluation parameters for a catalyst. In according to the results of FT-IR, the AAO/SBA-15-SO3H(C) and AAO/SBA-15SO3H(G) samples both have sulfonic acid groups. However, they have a different catalytic activity for the dehydration of xylose to furfural due to the different distribution of sulfonic acid sites on AAO/SBA-15. AAO/SBA-15-SO3H(C) prepared by the co-condensation method can hold highly homogeneous sulfonic acid site coverage on AAO/SBA-15. In contrast, sulfonic acid sites on the AAO/SBA-15-SO3H(G) irregularly distribute, and most of the sulfonic acid sites aggregate on the surface or near the pore mouth of mesoporous SBA-15, which inhibit the access of reactants to the active acid sites. As a result, the AAO/SBA-15-SO3H(G) sample synthesized by the grafting method has lower catalytic activity than the AAO/SBA-15-SO3H(C) sample. For comparison, the catalytic activity of the liquid acid (H2SO4 4%) catalyst was also tested in the reaction. Although the xylose conversion was similar, the furfural selectivity is lower than that observed on the AAO/ SBA-15-SO3H(C) catalyst. Further, the solid acid AAO/SBA-15-SO3H catalyst has a few advantages against the liquid acid catalyst (H2SO4 4%). The solid catalyst can be separated easily from the reaction mixture and recycled, and has no erosion on equipment. In order to test the mechanical stability of catalyst, quantitative catalyst is treated in the ultrasound for 1 h, and the loss of mass after the ultrasound is less than 5%, indicating that AAO/SBA-15SO3H has a good mechanical stability. The loss of activity of the catalyst results from furfural resinification and furfural condensation, which lead to the formation of coke from depositing on the catalyst surface and covering the acid sites of the catalyst. After the spent catalyst is treated by H2O2, the catalytic activity can be recovered completely. Thus, mesoporous materials functionalized with sulfonic acid are alternative for industrial furfural processes.
4. Conclusions AAO/SBA-15-SO3H catalyst was prepared successfully by the co-condensing and grafting methods, and the performance of the
D. Hua et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1395–1399
catalyst was tested in xylose to furfural. Results show that the performance of the catalyst prepared by the co-condensing method is better than that of the catalyst prepared by the grafting method. The conversion of xylose was above 90%, and the selectivity to furfural was 74%. Reaction to xylose to furfural is related to the amount of SO3H, and SO3H is the active site. Additionally, the cause of the deactivation of the AAO/SBA-15SO3H(C) catalyst is attributed to the formation of by-products resulting from the oligomerisation/polymerization of furfural and condensation of intermediates of the dehydration of xylose. For the spent SBA-15-SO3H(C) catalyst, the catalytic activity is completely recovered after treatment by H2O2. Solid acid catalyst packing can unite catalysis and extraction in a packed extraction column, which is a good chance for the production of furfural. Acknowledgments This project was supported by National Natural Science Foundation of China (No. 21176142), Independent Research Programs of Tsinghua University (No.2011Z08141), and National Key Technology R&D Program (No.2011BAD14B01) and National Basic Research Program of China (973 Program) (No. G2006CB705809).
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Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Preparation of solid acid catalyst packing AAO/SBA-15-SO3H and application for dehydration of xylose to furfural Derun Hua, PanPan Li, Yulong Wu *, Yu Chen, Mingde Yang *, Jie Dang, Quanhua Xie, Ji Liu, Xiao-yin Sun Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 October 2012 Accepted 1 January 2013 Available online 11 January 2013
The solid acid catalyst packing AAO/SBA-15-SO3H was prepared by the co-condensation and grafting method with porous anodic aluminum oxide (AAO) as support. FT-IR, SEM and TEM were applied to characterize the prepared samples. Results showed that catalysts prepared by two methods both contained active centers, and SBA-15 nanorod arrays grow inside a porous alumina membrane AAO and are perpendicular to the substrate. Their catalytic performances were tested for dehydration of xylose to furfural. The conversion of xylose and selectivity of furfural were 90% and 74% on the AAO/SBA-15SO3H(C) catalyst prepared by the co-condensation method, respectively. The deactivation and regeneration of the AAO/SBA-15-SO3H(C) catalyst for the dehydration of xylose were also investigated, the activity of catalyst treated by 30 wt.% H2O2 almost was recovered. Crown Copyright ß 2013 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved.
Keywords: Solid acid catalyst packing AAO/SBA-15-SO3H Xylose Dehydration
1. Introduction Furfural derived from biomass is an important organic material and has been considered as a sustainable intermediate for the preparation of fine chemicals, pharmaceuticals, and furan-based polymers [1]. For energy crisis and environmental pollution, it is important to develop the technology for furfural. Furfural can be produced from agricultural raw (or waste) materials by acidic degradation. The reaction involves hydrolysis of pentosan into pentoses (e.g., xylose) and successive (much slower) dehydration of the latter to form furfural (Scheme 1) [2]. Dehydration of xylose to furfural is an acid-catalyzed reaction. Now, mineral liquid acids, such as concentrated sulfuric acid and hydrochloric acid, are used in the most industrial furfural. However, mineral acids have serious drawbacks in terms of separation and recycling, as well as material corrosion. Recently, solid acids have been of particular interest in the dehydration of xylose [3–7]. Solid acids are used as catalysts and show high catalytic performance, and material corrosion is controlled [8,9]. However, application of some solid catalysts is limited due to poor hydrothermal stability and small pore size. Compared with these catalysts, the sulfonic acid functionalized SBA-15 with a larger pore size has attracted attention because of the good hydrothermal stability [10–12]. Shi et al.
* Corresponding authors. Fax: +86 106 9771464. E-mail addresses: [email protected] (Y. Wu), [email protected] (M. Yang).
investigated the catalytic activity of sulfonic acid functionalized SBA-15 for dehydration of xylose to furfural in the liquid-phase, and the results were very satisfactory [12]. However, process with the pulverous solid acid and the highpressure reactor is single-stage extraction and low efficiency. For the improvement of the process of furfural, it requires not only the good catalyst, but also the efficiency of the mass transfer. Therefore, we prepared solid acid catalyst packing, which is composed of porous anodic aluminum oxide as matrix material, SBA-15 as support and a sulfonic acid group as the active component. Porous anodic aluminum oxide membranes, having ordered and a vertical one-dimensional (1D) channel structure, have attracted a lot of interest in the preparation of monodisperse and ordered 1D nanostructure within their pores [13–16]. Until now, many nanorod arrays have been synthesized using porous alumina membrane as the growth-limiting template. Solid acid catalyst packing makes the even distribution of active components on the matrix, which can reduce byproducts due to excessive local reactions. In addition, the design of packing is also very beneficial for the mass transfer and reducing the coke formation during the dehydration of xylose. In the study, the AAO/SBA-15-SO3H catalyst packing, consisting of a base (AAO), support (SBA-15) and active phase (sulfoacid), was synthesized by co-condensing and grafting methods and applied to xylose to furfural. The packing catalyst synthesized by two methods possesses good mechanical stability and regeneration, a high reaction rate, mass transfer and heterogeneous distribution of the active site as is shown in Fig. 1.
1226-086X/$ – see front matter . Crown Copyright ß 2013 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.01.002
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D. Hua et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1395–1399
Scheme 1. Reaction for hydrolysis of pentosan to furfural.
AAO/SBA-15-SO3H was obtained. Procedures for AAO/SBA-15SO3H catalyst are shown in Scheme 2.
2. Experiments 2.1. Preparation of catalysts
2.2. Characterization of catalysts Co-condensing preparation of AAO/SBA-15-SO3H(C): P123 (1.0 g, MW = 5800) from Aldrich Company was dissolved in 100 ml ethanol solution, and 1 ml TEOS (tetraethoxysilane) and 0.4 g HCl (38 wt.%) were added to the solution, then 0.15 ml MPTMS (3-mercaptopropyltrimethoxysilane) and 1 ml H2O2 (30 wt.%) were added and kept stirring at room temperature for 20 h. AAO membrane (1 g) was added in the fore-prepared solution, and the mixture was treated by rotary evaporator method at 40 8C for 10 h, then AAO membrane was dried at 60 8C for 12 h, washed by ethanol and dried at 60 8C again. Grafting preparation of AAO/SBA-15-SO3H(G): the preparation procedure was similar to that described the above and AAO/SBA-15 was obtained, then the sample was dried and calcined to remove the template in air at 540 8C for 6 h. In a typical grafting procedure, AAO/SBA-15 (1.0 g) and MPTMS (0.15 ml) were added into toluene (40.0 g), and the mixture was kept stirring for 24 h and filtered. The solid was washed by refluxing mixture of dichloromethane (20.0 g) and ethyl acetate (20.0 g) for 24 h for removal of MPTMS. Then the functionalized AAO/SBA-15 was added into the mixture of H2O2 (40.0 g), CH3OH (120.0 g) and H2O (400.0 g), the suspension was stirred at room temperature for 12 h and filtered. After filtration, the precipitate was added into 20 ml H2SO4 (0.5 mol/l), stirred for 12 h and filtered, then washed by ethanol and water, last dried, and
The mid-infrared spectra were collected on the Digilab FTS3000 FT-IR Spectrum Gx FTIR spectrometer (Perkin Elmer Company) by the coaddition of 250 scans at a 4 cm1 resolution, and KBr sample window. The morphology of the catalyst was recorded with field emission scanning electron microscope (FEI QUANTA 200F). The transition electron microscopy (TEM) images were obtained with JEM-2100LaBa (Ishizuka Electronics Corporation). Before examined, the sample was soaked by H3PO4 (7 wt.%) for 4 days. Mechanical stability was tested by ultrasonic irradiation. The sample was in an ultrasonic bath (SB-5200DTDN, 40KHz) at 20 8C for 1 h, then the sample was dried, weight of the sample was compared before and after ultrasonic irradiation. 2.3. Test of catalyst for xylose to furfural Xylose (0.75 g, Sigma–Aldrich, 99%) and catalyst (0.1 g) were mixed with the mixture of H2O (7.5 ml) and toluene (17.5 ml), and the resulting mixture was introduced into an autoclave (50 ml). The autoclave was sealed and heated. The slurry was stirred with a magnetic stirrer at 500 rpm and kept at 160 8C for 4 h. The autoclave was cooled to room temperature. Then the aqueous and organic phases were filtered by a microstrainer. The production
Fig. 1. The scheme of solid acid catalyst packing and dehydration of xylose to furfural.
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Scheme 2. Procedures for AAO/SBA-15-SO3H catalyst.
was analyzed by HPLC (Shimadzu Company) with column (HPX87H) and GC (Agilent) with column (PEG-20M). Xylose conversion and furfural selectivity are calculated as: C xylose ¼
ðmxyloses mxylosee Þ=M xylose mxyloses =M xylose
(1)
Sfurfural ¼
ðmfurfuralw þ mfurfuralt Þ=M furfural ðmxyloses mxylosee Þ=Mxylose
(2)
Y furfural ¼ C xylose Sfurfural
(3)
where C xylose , Sfurfural and Y furfural are denoted as the conversion of xylose, the selectivity of furfural and yield of furfural, respectively. mxyloses and mxylosee are the mass concentration of xylose before and after the reaction, M xylose is the molar mass of xylose, mfurfuralw and mfurfuralt are the mass concentration of the furfural in the aqueous and organic phases, respectively. 3. Results and discussion SEM was used to characterize the morphology feature of catalyst. Fig. 2 is the scanning electron microscopy (SEM) images of the catalysts. As determined from Fig. 2a, AAO substrate has a mean diameter of 180 nm with a size deviation of ca. 10%. Fig. 2b
clearly shows that the pores of AAO membranes are filled with SBA-15. Additionally, it is found that SBA-15 nanorods happen to contract under the drying processes and are perpendicular to AAO membranes. Fig. 2c shows the bundle-shaped SBA-15 after removal of the AAO. The average diameter of SBA-15 is about 180 nm and mainly depends on the pore size of the AAO. TEM technology is employed to further investigate the inner structure of the catalyst. Fig. 3 is the transmission electron microscopy (TEM) images of AAO/SBA-15-SO3H. Fig. 3a shows that AAO/SBA-15-SO3H holds a circular lamellar a circular columnar mesostructure in the central region, Fig. 3b shows that AAO/SBA15-SO3H has 1D alignment of the silica-nanochannels orientating along the long axis of the nanopores of the AAO template. The mesostructure is attributed to the circular columnar mesostructure of the pores of the AAO membrane and is significantly affected by the concentration of P123 [17]. When the concentration of P123 was below 15 mg/ml, nanorods were composed of columnar nanochannels paralleling the long axis of the AAO nanochannel. Otherwise, the columnar nanochannel, circling the axis of the AAO nanochannels, are present. The IR spectra of all solid acid catalysts packing are shown in Fig. 4. All samples exhibit infrared bands at 450, 800 and 1000– 1260. The IR spectra at 450, 800 and 1000–1260 cm1 are assigned to the symmetric and asymmetric stretching of Si–O–Si vibrations
Fig. 2. The SEM images of catalysts.
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Fig. 3. The TEM picture of AAO/SBA-15-SO3H.
Fig. 4. FT-IR spectra of solid acid catalysts packing (a) AAO/SBA-15, (b) AAO/SBA-15SH, (c) AAO/SBA-15-SO3H(G), and (d) AAO/SBA-15-SO3H(C).
for the tetrahedral SiO4 structure units, and the bands are usually assigned to Vd(Si–O–Si), Vs(Si–O–Si) and Vas(Si–O–Si), respectively[18]. The band at 3400 cm1 is assigned to stretching mode of Si– O–H. In the case of AAO/SBA-15-SH, the 2950–2850 cm1 regions are assigned to methylene stretching bands [19], indicating that the 3-MPTS groups have been anchored in the pore channels of SBA-15 material. The band centered at 2570 cm1 is assigned to the thiol (–SH) stretching vibrations [12]. The above results demonstrate that the mercaptopropyl groups are successfully introduced into the interior mesopore surfaces. For the AAO/SBA15-SO3H(C) and AAO/SBA-15-SO3H(G) samples, the band at 2570 cm1 is absent, but the new band at 1650 cm1 which is assigned to the asymmetric stretching band of the SO2 moieties [20], is present, which confirms the formation of sulfonic acid species after the oxidation reactions. All catalysts are tested in the batch experiments for dehydration of xylose to furfural. Results are summarized in Table 1. It is Table 1 Catalytic performance of the catalysts. Catalysts
Conversion (%)
Selectivity (%)
Yield (%)
None AAO AAO/SBA-15 AAO/SBA-15-SH AAO/SBA-15-SO3H(C) AAO/SBA-15-SO3H(G) H2SO4 (4%) SBA-15-SO3H
17 64 85 80 90 99 92 92
13 45 47 29 74 55 71 74
2 29 40 23 67 49 65 68
found that AAO, AAO/SBA-15 and AAO/SBA-15-SH have catalytic activity due to its Brønsted acid capacity [21], but the activity is lower than that of SBA-15-SO3H, AAO/SBA-15-SO3H(C) and AAO/ SBA-15-SO3H(G), because the acid number of the former is lower than that of the latter; xylose conversion on AAO/SBA-15-SO3H(C) is slightly lower than that on AAO/SBA-15-SO3H(G), but furfural yield and selectivity of the latter are obviously higher than that of the former. In xylose to furfural, if conversion of xylose is above 90%, we may think the reaction ends. In this case, selectivity of the product is evaluation parameters for a catalyst. In according to the results of FT-IR, the AAO/SBA-15-SO3H(C) and AAO/SBA-15SO3H(G) samples both have sulfonic acid groups. However, they have a different catalytic activity for the dehydration of xylose to furfural due to the different distribution of sulfonic acid sites on AAO/SBA-15. AAO/SBA-15-SO3H(C) prepared by the co-condensation method can hold highly homogeneous sulfonic acid site coverage on AAO/SBA-15. In contrast, sulfonic acid sites on the AAO/SBA-15-SO3H(G) irregularly distribute, and most of the sulfonic acid sites aggregate on the surface or near the pore mouth of mesoporous SBA-15, which inhibit the access of reactants to the active acid sites. As a result, the AAO/SBA-15-SO3H(G) sample synthesized by the grafting method has lower catalytic activity than the AAO/SBA-15-SO3H(C) sample. For comparison, the catalytic activity of the liquid acid (H2SO4 4%) catalyst was also tested in the reaction. Although the xylose conversion was similar, the furfural selectivity is lower than that observed on the AAO/ SBA-15-SO3H(C) catalyst. Further, the solid acid AAO/SBA-15-SO3H catalyst has a few advantages against the liquid acid catalyst (H2SO4 4%). The solid catalyst can be separated easily from the reaction mixture and recycled, and has no erosion on equipment. In order to test the mechanical stability of catalyst, quantitative catalyst is treated in the ultrasound for 1 h, and the loss of mass after the ultrasound is less than 5%, indicating that AAO/SBA-15SO3H has a good mechanical stability. The loss of activity of the catalyst results from furfural resinification and furfural condensation, which lead to the formation of coke from depositing on the catalyst surface and covering the acid sites of the catalyst. After the spent catalyst is treated by H2O2, the catalytic activity can be recovered completely. Thus, mesoporous materials functionalized with sulfonic acid are alternative for industrial furfural processes.
4. Conclusions AAO/SBA-15-SO3H catalyst was prepared successfully by the co-condensing and grafting methods, and the performance of the
D. Hua et al. / Journal of Industrial and Engineering Chemistry 19 (2013) 1395–1399
catalyst was tested in xylose to furfural. Results show that the performance of the catalyst prepared by the co-condensing method is better than that of the catalyst prepared by the grafting method. The conversion of xylose was above 90%, and the selectivity to furfural was 74%. Reaction to xylose to furfural is related to the amount of SO3H, and SO3H is the active site. Additionally, the cause of the deactivation of the AAO/SBA-15SO3H(C) catalyst is attributed to the formation of by-products resulting from the oligomerisation/polymerization of furfural and condensation of intermediates of the dehydration of xylose. For the spent SBA-15-SO3H(C) catalyst, the catalytic activity is completely recovered after treatment by H2O2. Solid acid catalyst packing can unite catalysis and extraction in a packed extraction column, which is a good chance for the production of furfural. Acknowledgments This project was supported by National Natural Science Foundation of China (No. 21176142), Independent Research Programs of Tsinghua University (No.2011Z08141), and National Key Technology R&D Program (No.2011BAD14B01) and National Basic Research Program of China (973 Program) (No. G2006CB705809).
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