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Hydrolysis of sucrose over composite catalysts
Chemical Engineering Journal 184 (2012) 347–351
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Hydrolysis of sucrose over composite catalysts D.S. Pito a , I.M. Fonseca b , A.M. Ramos b , J. Vital b , J.E. Castanheiro a,∗ a b
Centro de Química de Évora, Departamento de Química, Universidade de Évora, 7000-671 Évora, Portugal REQUIMTE, CQFB, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
a r t i c l e
i n f o
Article history: Received 29 September 2009 Received in revised form 31 December 2011 Accepted 4 January 2012 Keywords: Sucrose Hydrolysis Zeolite PVA PDMS
a b s t r a c t The hydrolysis of sucrose into glucose and fructose was carried out over composite catalysts at 80 ◦ C. These catalysts consisted of USY and Beta zeolites dispersed in polymeric matrices (PDMS and PVA). The swelling degree of polymers increased with the amount of zeolite immobilised in PDMS and PVA. This result can be explained by the increase in the polymeric matrix channelling. Additionally, the effective diffusivity of sucrose increases with the amount of USY and Beta zeolite that was dispersed in PDMS. The catalytic activity increases with the amount of zeolite dispersed in the polymeric matrix. The PVA composites showed higher catalytic activity than the PDMS ones. To study the catalytic stability of the Beta2/PVA catalyst, four consecutive batch runs were carried out with the same catalyst. The Beta2/PVA catalyst was recycled and reused with a negligible loss in its activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Due to environmental pressure and a decrease in fossil fuel sources, alternative fuel sources, such as biomass or renewable feedstock sources, have become increasingly popular. Sugars are the key intermediates between biomass and chemicals. Sucrose occurs widely in the plant kingdom and constitutes the main carbohydrate reserve and energy source in the human diet [1,2]. The main industrial process for sucrose hydrolysis into glucose and fructose (Fig. 1) is carried out using an enzyme as the catalyst. However, the enzymes used in this process suffer from various drawbacks, such as the production of waste, low thermal stability, and problems of separating and recovering the enzyme from the product. One possible solution to overcome these drawbacks is to replace the enzyme catalysts with heterogeneous catalysts [3]. The hydrolysis of sucrose has been carried out using heterogeneous catalysts, such as zeolites [4,5], polystyrene with sulfonic acid groups [6–8], sulfonated mesoporous silica [9], V2 O5 /␥alumina [10] and silica-supported heteropolyacids [11]. Homogenous and heterogeneous catalysts can be immobilised in polymeric matrices to improve their selectivity or activity because of the environment created by the polymeric matrix around the catalyst [12,13]. In addition, the use of composite catalysts can improve the amount of reactants near the active centre by changing the hydrophilic/hydrophobic balance [12]. In a previous study, poly(vinyl alcohol) that was cross-linked with sulfosuc-
∗ Corresponding author. Tel.: +351 266745311; fax: +351 266744971. E-mail address: [email protected] (J.E. Castanheiro). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.033
cinic acid was used as the catalyst in esterification of acetic acid with isoamyl alcohol [14] and in the esterification of fatty acids to biodiesel [15]. In a recent report, the hydrolysis of sucrose was studied using PVA with sulfonic acid groups [16]. In this study, composite catalysts consisting of zeolites (USY and Beta) dispersed in poly(vinyl alcohol) and polydimethylsiloxane were used in the hydrolysis of sucrose. 2. Experimental 2.1. Catalysts preparation Beta zeolite (Si/Al = 40) was synthesised according to Wadlinger and Kerr [17]. Sodium aluminate (3.0 g, 0.0365 mol NaAlO2 , Riedelde-Haën), 110 g of 40% (w/w) tetraethylammonium hydroxide (TEAOH, 0.299 mol, Aldrich) and 291.57 g of silica sol [1.461 mol Ludox LS (30% SiO2 )] were mixed and transferred to a Teflon autoclave. After crystallisation (145 h at 150 ◦ C), the zeolite was collected by filtration, dried at 115 ◦ C, and calcined at 500 ◦ C to remove the organic template. The USY zeolite (CBV 720) was purchased from Zeolyst Int. 2.2. Composite catalyst preparation PDMS composites were prepared according to Vankelecom et al. [18]. Beta and USY zeolites were dispersed ultrasonically in methyl isobutylketone (4 g). The RTV 615B cross-linker (General Electric, 0.33 g) was added to the zeolite suspension, and this mixture was stirred for 2 h. After adding the PDMS pre-polymer (General
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D.S. Pito et al. / Chemical Engineering Journal 184 (2012) 347–351
CH2OH O
CH2OH
O OH
OH O
OH
CH2OH OH
OH
Sucrose (S) + H2O
CH2OH O
CH2OH
OH
O OH
OH OH
OH
CH2OH OH
OH
Glucose (G)
Fructose (F)
Fig. 1. Schematic representation of sucrose hydrolysis to glucose and fructose.
Electric, RTV 615A, 3.3 g), mixing was continued for 1 h. The mixture was cast as a film on a Teflon plate and cured in vacuum at 150 ◦ C. PVA catalysts were prepared by dissolving the PVA (MERCK, average molecular weight: 72,000) in water at 90 ◦ C for 6 h. Next, the appropriate amount of succinic acid was added to the PVA solutions, which were vigorously stirred at room temperature for 24 h. After this period, different amounts of the zeolites (USY and Beta) were added to the PVA solution. The PVA mixtures were then stirred at room temperature for another 24 h. The mixture were poured and cast on a Teflon plate, and the cast polymers were allowed to dry at 60 ◦ C for 24 h. The dried PVA matrix was heated at 120 ◦ C for 2 h. 2.3. Catalyst characterisation The textural characterisation of the catalysts was based on a nitrogen adsorption isotherm, and was determined at 77 K with a Micromeritics ASAP 2010 apparatus. The amount of silica and aluminium in the Beta zeolite was measured by dissolving the catalyst in H2 SO4 /HF 1:1 (v/v) and analysing the obtained solution using inductively coupled plasma analysis (ICP), which was carried out in a Jobin-Yvon ULTIMA instrument. X-ray powder diffraction (XRD) of the zeolite was obtained on a Bruker powder diffractometer. The swelling degree of the catalytic composite was measured by immersing the polymeric matrix samples in water at 80 ◦ C for 24 h. Then, the polymeric samples were taken out, wiped with tissue paper and weighed. The swelling degree, Q, was calculated by the following: Q =
m − m0 m0
(1)
where m is the mass of swollen sample and m0 is the initial mass.
The success of the cross-linking of PVA was evaluated by FTIR spectroscopy. The FTIR spectra were recorded in a Bio-Rad FTS 155 instrument. Scanning electron microscopy (SEM) of the catalysts was carried out using a Hitachi model S-2400. 2.4. Catalytic experiments The catalytic experiments were carried out in a stirred batch reactor at 80 ◦ C. In a typical experiment, the reactor was loaded with 100 cm3 of sucrose (0.6 mol dm−3 ). The reactions were initiated by the addition of the catalyst (0.511 g). In all experiments, the stirrer speed was kept constant at 500 rpm. Stability tests of the Beta2 PVA were carried out by running four consecutive experiments under the same reaction conditions. After the hydrolysis was complete, the Beta2 PVA was separated from the reaction mixture by filtration, washed with water and dried overnight at 100 ◦ C. Portions of the solution were removed at known time intervals, and the rotation of polarised light was noted with the help of a Polax-D type polarimeter. The samples were also analysed by high performance liquid chromatography using a refractive index detector. 3. Results and discussion 3.1. Catalyst characterisation Fig. 2 shows the nitrogen adsorption–desorption isotherm of the USY and Beta zeolites. These materials show a type I isotherm (according to IUPAC nomenclature), which is characteristic of microporous zeolites. The specific surface area was determined using the BET method, and the microporous volume and
D.S. Pito et al. / Chemical Engineering Journal 184 (2012) 347–351
349
Table 1 Physicochemical characterisation of the zeolites. Sample
Si/Al ratioa
Surface areab (m2 /g)
External surface areac (m2 /g)
Microporous volumec (cm3 /g)
Beta USY
40 15
506 680
10 12
0.19 0.29
a b c
ICP. BET. t-method.
250
be explained by the greater hydrophilic nature of the PVA polymer compared with PDMS. The swelling degree of PDMS and PVA increased with the amount of catalyst (Beta and USY zeolite) dispersed in the polymer. This result can be explained by the increases in the free-volume in the polymeric matrix. Similar results were also observed by Kulkarni et al. [20]. The diffusivities of sucrose were calculated assuming a firstorder irreversible reaction. Specifically, the values of the maximum reaction rate were calculated from the slopes of the kinetic curves obtained for two different pellet sizes. Plate geometry was assumed for the pellet, and the following relationships for the Thiele modulus and the effectiveness factor were used:
V (cm3/g)
200
150
100
50
0 0
0,2
0,4
0,6
0,8
1
p/pº
=L×
mesoporous surface area were determined by the t-method, using a standard isotherm proposed by Gregg and Sing [19]. Table 1 shows the values of the BET specific area (SBET ), the microporous volume (Vmicro ) and the external surface area (Sext ). Fig. 3 shows the X-ray diffraction patterns that were obtained for the USY and Beta zeolite catalysts. Changes in the structure of the catalysts were not observed after synthesis and purification procedures. Fig. 4 shows the FTIR spectra of succinic acid (A), PVA (B) and PVA cross-linked with succinic acid (C). The intense band at 1730 cm−1 in spectrum B, which does not overlap with the band present in the spectrum of succinic acid (A), is characteristic of the free carboxylic group (1700 cm−1 ) and suggests that the cross-linking with succinic acid was successful. The characteristics of the polymeric matrices are shown in Table 2. The swelling degree of the composite catalyst was measured for water at 80 ◦ C. The swelling degree of the PDMS composite catalysts was lower than in the PVA catalysts. This behaviour can
(2)
tanh()
=
Fig. 2. N2 adsorption–desorption isotherms of the zeolites. () USY; () Beta.
k × p De
(3)
where k is the intrinsic kinetic constant, the pellet volumic mass, L is the membrane half thickness and De is the effective diffusivity. The effective diffusivity of sucrose increases with the amount of zeolite dispersed in the PDMS. This behaviour can be explained by the increases of composite catalyst channelling with the catalyst loading. Similar results were also obtained by Süer et al. [21] and Wu et al. [22]. Fig. 5 shows the SEM of the USY1/PDMS (Fig. 5A) and USY2/PDMS (Fig. 5B) composite catalysts. The polymeric matrix channelling increases with the increase of the zeolite amount in PDMS. In a previous report, similar results were also observed by Vital et al. [23]. 3.2. Catalytic experiments The hydrolysis of sucrose was carried out over composite catalysts at 80 ◦ C, and its products are glucose and fructose (Fig. 1). The initial activity of the PVA composite catalysts in sucrose hydrolysis was compared in Fig. 6. The initial activity was calculated by the maximum slope of the experimental kinetic curve of sucrose divided by the amount of catalyst. The catalytic activity
B
Transmittance (a.u.)
Intensity (a.u.)
A
B
C
A 5
15
25
35
2 (º) Fig. 3. XRD patterns of (A) USY and (B) Beta zeolites.
45
1800
1600
1400
1200
1000
Wavenumber
800
600
400
(cm-1)
Fig. 4. FTIR spectra of catalysts: (A) succinic acid; (B) PVA; (C) PVA cross-linked with succinic acid.
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Table 2 Characterisation of composite catalysts. Sample
Catalysts loading (%)
Thickness (mm)
Water swellinga (%)
De × 1010 (m2 /h)
Beta1/PDMS Beta2/PDMS USY1/PDMS USY2/PDMS Beta1/PVA Beta2/PVA USY1/PVA USY2/PVA
5 30 5 30 5 30 5 30
0.135 0.124 0.137 0.142 0.128 0.141 0.125 0.135
0.006 0.014 0.011 0.051 0.223 0.535 0.121 0.211
1.4 3.2 1.1 3.5 – – – –
a
Water swelling was measured by immersing dried pieces of polymeric matrixes in water at 80 ◦ C.
Fig. 5. Scanning electron micrograph of (A) USY1/PDMS; (B) USY2/PDMS.
increased with the amount of zeolite dispersed in the PVA matrix. This behaviour can be explained by the increase in the catalyst amount and by the increases of channelling of the polymeric matrix. As reported by Süer et al. [21], the increase in catalytic activity may be ascribed to an increase of the polymeric matrix channelling. The increase in the composite channelling led to an increase of the reagent permeation and, therefore, higher diffusion rates for the catalyst particles. Beta/PVA composites showed higher catalytic activity than USY/PVA composites (Fig. 6). This behaviour can be explained by the hydrophilic/hydrophobic balance of the PVA composite catalysts. In fact, the swelling degree of the Beta/PVA composite catalysts was higher than that of the USY/PVA catalysts (Table 2). Fig. 7 shows the initial activity of PDMS on composite catalysts in the hydrolysis of sucrose. The catalytic activity increases with the amount of zeolite dispersed in the PDMS. This behaviour can be explained by the increase in the catalyst amount immobilised in PDMS and also by the increase in polymeric matrix channelling,
Fig. 6. Hydrolysis of sucrose over Beta and USY zeolites dispersed in poly(vinyl alcohol), at 80 ◦ C. Initial activities were taken as the maximum observed reaction rate and were calculated from the maximum slope of the sucrose kinetic curve.
which can be observed in Fig. 5. An increase of sucrose diffusivity with the amount of zeolite was observed (Table 2). The hydrophobic/hydrophilic balance in the PDMS composites probably varied with the amount of zeolite amount immobilised in the PDMS. Actually, the swelling degree increased with zeolite loading on composite (Table 2). Similar results were also observed by Vital et al. [23]. Additionally, the catalytic activity of PVA composites was higher than with the PDMS composites, which can be explained due to the greater hydrophilicity of the PVA compared with the PDMS. In fact, the swelling degree of PDMS composites is smaller than with the PVA composites (Table 2). The catalytic activity of PVA with sulfonic acid groups (4.96 × 10−2 mol/h gcat ) [16] was smaller than the catalytic activity obtained over Beta2/PVA (8.61 × 10−2 mol/h gcat ). The low activity of PVA with sulfonic acid groups [16] could be due to the decrease in the accessibility of SO3 H groups for sucrose, given the high degree of cross-linking of the PVA.
Fig. 7. Hydrolysis of sucrose over Beta and USY zeolites dispersed in poly(dimethylsiloxane), at 80 ◦ C. Initial activities were taken as the maximum observed reaction rate and were calculated from the maximum slope of the sucrose kinetic curve.
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The catalytic activity of the composite catalysts was found to increase with the amount of zeolite that was dispersed in the polymeric matrix. The catalytic activity of PVA composites was higher than with the PDMS composites. To study the catalytic stability of Beta2/PVA catalyst, four consecutive batch runs with the same catalyst were carried out, and similar values in catalytic activity were observed for each run. References
Fig. 8. Stability studies of the Beta2/PVA catalyst on the hydrolysis of sucrose. The catalytic activity was calculated as the initial reaction rate of the hydrolysis of sucrose in four consecutive experiments.
To study the catalytic stability of the Beta2/PVA, different batch runs were carried out with the same catalyst sample at the same conditions. Fig. 8 shows the relative catalytic activity of the Beta2/PVA catalyst, i.e., the catalytic activity after the different runs divided by the initial activity. After the second batch, a stabilisation of the catalytic activity was observed. The catalytic activity of Beta2/PVA for sucrose hydrolysis was compared with the activity of other catalysts used in this reaction that were reported in literature. The catalytic activity (expressed as mol/h gcat ) of the different solid catalysts was found to increase as follows: 4.74 × 10−4 mol/h gcat (obtained with V2 O5 /Al2 O3 , [10]) < 5.54 × 10−4 mol/h gcat (obtained with HPA/silica, [11]) < 5.51 × 10−3 mol/h gcat (obtained with silica with sulfonic acid groups, [9]) < 3.59 × 10−2 mol/h gcat (obtained with PS-SO3 H, [8]) < 4.87 × 10−2 mol/h gcat (obtained with zeolite, [4]) < 4.96 × 10−2 mol/h gcat , (obtained over PVA 40, [16]) < 8.61 × 10−2 mol/h gcat (obtained over Beta2/PVA, [present work]) < 8.75 × 10−2 mol/h gcat (obtained over polystyrene with sulfonic acid groups, [7]). 4. Conclusions The hydrolysis of sucrose to glucose and fructose was carried out over composite catalysts consisting of USY and Beta zeolites dispersed in PDMS and PVA matrices. The reactions were carried out in a batch reactor at 80 ◦ C. The swelling degree of PDMS and PVA increased with the amount of catalyst (Beta and USY zeolite) dispersed in the polymers, which can be explained by an increase in the polymeric matrix channelling. An increase in the effective diffusivity of sucrose through the composite, with the amount of zeolite dispersed in PDMS, was observed.
[1] A. Corma, S. Iborra, A. Velty, Chemical routes for the transformation of biomass into chemicals, Chem. Rev. 107 (2007) 2411–2502. [2] H.E. Hoydonckx, D.E. De Vos, Suhas, A. Chavan, P.A. Jacobs, Esterification and transesterification of renewable chemicals, Top. Catal. 27 (2004) 83–96. [3] T. Okuhara, Water-tolerant solid acid catalysts, Chem. Rev. 102 (2002) 3641–3666. [4] C. Moreau, R. Durand, F. Aliès, M. Cotillon, T. Frutz, M. Théoleyre, Hydrolysis of sucrose in the presence of H-form zeolites, Ind. Crops Prod. 11 (2000) 237–242. [5] C. Buttersack, D. Laketic, Hydrolysis of sucrose by dealuminated Y-zeolites, J. Mol. Catal. 94 (1994) L283–L290. [6] A. Masroua, A. Revillon, J.C. Martin, A. Guyot, G. Descotes, Hydrolyse d’oligo et polysaccharides en présence de résines échangeuses d’ions et de polyméres hydrosolubles, Bull. Soc. Chim. Fr. 3 (1988) 561–566. [7] T. Mizota, S. Tsuneda, K. Saito, T. Sugo, Hydrolysis of methyl acetate and sucrose in SO3 H-group-containing grafted polymer chains prepared by radiationinduced graft copolymerization, Ind. Eng. Chem. Res. 33 (1994) 2215–2219. [8] M.M. Nasefa, H. Saidia, M.M. Sennab, Hydrolysis of sucrose by radiation grafted sulfonic acid membranes, Chem. Eng. J. 108 (2005) 13–17. [9] P.L. Dhepe, M. Ohashi, S. Inagaki, M. Ichikawa, A. Fukuoka, Hydrolysis of sugars catalyzed by water-tolerant sulfonated mesoporous silicas, Catal. Lett. 102 (2005) 163–169. [10] H. Iloukhani, S. Azizian, N. Samadani, Hydrolysis of sucrose by heterogeneous catalysts, React. Kinet. Catal. Lett. 72 (2001) 239–244. [11] H. Iloukhani, S. Azizian, N. Samadani, Hydrolysis of sucrose by heterogeneous catalysis, Phys. Chem. Liq. 40 (2002) 159–165. [12] I.F.J. Vankelecom, Polymeric membranes in catalytic reactors, Chem. Rev. 102 (2002) 3779–3810. [13] S.S. Ozdemir, M.G. Buonomenna, E. Drioli, Catalytic polymeric membranes: preparation and application, Appl. Catal. A: Gen. 307 (2006) 167–183. [14] J.E. Castanheiro, A.M. Ramos, I. Fonseca, J. Vital, Esterification of acetic acid by isoamylic alcohol over catalytic membranes of poly(vinyl alcohol) containing sulfonic acid groups, Appl. Catal. A: Gen. 311 (2006) 17–23. [15] C. Caetano, L. Guerreiro, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Esterification of fatty acids to biodiesel over polymers with sulfonic acid groups, Appl. Catal. A: Gen. 359 (2009) 41–46. [16] D.S. Pito, I.M. Fonseca, A.M. Ramos, J. Vital, J.E. Castanheiro, Hydrolysis of sucrose using sulfonated poly(vinyl alcohol) as catalyst, Bioresour. Technol. 100 (2009) 4546–4550. [17] R.L. Wandlinger, G.T. Kerr, US Patent Appl. 28341 (1975). [18] I.F.J. Vankelecom, R.F. Parton, M.J.A. Casselman, J.B. Uytterhoeven, P.A. Jacobs, Oxidation of cyclohexane using FePcY-zeozymes occluded in polydimethylsiloxane membranes, J. Catal. 163 (1996) 457–464. [19] S.J. Gregg, K.S.W. Sing, Adsorption Surface Area Porosity, first ed., Academic Press, New York, 1982. [20] S. Kulkarni, S. Tambe, A. Kittur, M.Y. Kariduraganavar, Preparation of novel composite membranes for the pervaporation separation of water–acetic acid mixtures, J. Membr. Sci. 285 (2006) 420–431. [21] M.G. Süer, N. Bac¸, L. Yilmaz, Gas permeation characteristics of polymer–zeolite mixed matrix membranes, J. Membr. Sci. 91 (1994) 77–86. [22] S. Wu, J.E. Gallot, M. Bousmina, C. Bouchard, S. Kaliaguine, Zeolite containing catalytic membranes as interphase contactor, Catal. Today 56 (2000) 113–129. [23] J. Vital, A.M. Ramos, I.F. Silva, H. Valente, J.E. Castanheiro, Hydration of ␣-pinene over zeolites and activated carbons dispersed in polymeric membranes, Catal. Today 56 (2000) 167–172.
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Hydrolysis of sucrose over composite catalysts D.S. Pito a , I.M. Fonseca b , A.M. Ramos b , J. Vital b , J.E. Castanheiro a,∗ a b
Centro de Química de Évora, Departamento de Química, Universidade de Évora, 7000-671 Évora, Portugal REQUIMTE, CQFB, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
a r t i c l e
i n f o
Article history: Received 29 September 2009 Received in revised form 31 December 2011 Accepted 4 January 2012 Keywords: Sucrose Hydrolysis Zeolite PVA PDMS
a b s t r a c t The hydrolysis of sucrose into glucose and fructose was carried out over composite catalysts at 80 ◦ C. These catalysts consisted of USY and Beta zeolites dispersed in polymeric matrices (PDMS and PVA). The swelling degree of polymers increased with the amount of zeolite immobilised in PDMS and PVA. This result can be explained by the increase in the polymeric matrix channelling. Additionally, the effective diffusivity of sucrose increases with the amount of USY and Beta zeolite that was dispersed in PDMS. The catalytic activity increases with the amount of zeolite dispersed in the polymeric matrix. The PVA composites showed higher catalytic activity than the PDMS ones. To study the catalytic stability of the Beta2/PVA catalyst, four consecutive batch runs were carried out with the same catalyst. The Beta2/PVA catalyst was recycled and reused with a negligible loss in its activity. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Due to environmental pressure and a decrease in fossil fuel sources, alternative fuel sources, such as biomass or renewable feedstock sources, have become increasingly popular. Sugars are the key intermediates between biomass and chemicals. Sucrose occurs widely in the plant kingdom and constitutes the main carbohydrate reserve and energy source in the human diet [1,2]. The main industrial process for sucrose hydrolysis into glucose and fructose (Fig. 1) is carried out using an enzyme as the catalyst. However, the enzymes used in this process suffer from various drawbacks, such as the production of waste, low thermal stability, and problems of separating and recovering the enzyme from the product. One possible solution to overcome these drawbacks is to replace the enzyme catalysts with heterogeneous catalysts [3]. The hydrolysis of sucrose has been carried out using heterogeneous catalysts, such as zeolites [4,5], polystyrene with sulfonic acid groups [6–8], sulfonated mesoporous silica [9], V2 O5 /␥alumina [10] and silica-supported heteropolyacids [11]. Homogenous and heterogeneous catalysts can be immobilised in polymeric matrices to improve their selectivity or activity because of the environment created by the polymeric matrix around the catalyst [12,13]. In addition, the use of composite catalysts can improve the amount of reactants near the active centre by changing the hydrophilic/hydrophobic balance [12]. In a previous study, poly(vinyl alcohol) that was cross-linked with sulfosuc-
∗ Corresponding author. Tel.: +351 266745311; fax: +351 266744971. E-mail address: [email protected] (J.E. Castanheiro). 1385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2012.01.033
cinic acid was used as the catalyst in esterification of acetic acid with isoamyl alcohol [14] and in the esterification of fatty acids to biodiesel [15]. In a recent report, the hydrolysis of sucrose was studied using PVA with sulfonic acid groups [16]. In this study, composite catalysts consisting of zeolites (USY and Beta) dispersed in poly(vinyl alcohol) and polydimethylsiloxane were used in the hydrolysis of sucrose. 2. Experimental 2.1. Catalysts preparation Beta zeolite (Si/Al = 40) was synthesised according to Wadlinger and Kerr [17]. Sodium aluminate (3.0 g, 0.0365 mol NaAlO2 , Riedelde-Haën), 110 g of 40% (w/w) tetraethylammonium hydroxide (TEAOH, 0.299 mol, Aldrich) and 291.57 g of silica sol [1.461 mol Ludox LS (30% SiO2 )] were mixed and transferred to a Teflon autoclave. After crystallisation (145 h at 150 ◦ C), the zeolite was collected by filtration, dried at 115 ◦ C, and calcined at 500 ◦ C to remove the organic template. The USY zeolite (CBV 720) was purchased from Zeolyst Int. 2.2. Composite catalyst preparation PDMS composites were prepared according to Vankelecom et al. [18]. Beta and USY zeolites were dispersed ultrasonically in methyl isobutylketone (4 g). The RTV 615B cross-linker (General Electric, 0.33 g) was added to the zeolite suspension, and this mixture was stirred for 2 h. After adding the PDMS pre-polymer (General
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D.S. Pito et al. / Chemical Engineering Journal 184 (2012) 347–351
CH2OH O
CH2OH
O OH
OH O
OH
CH2OH OH
OH
Sucrose (S) + H2O
CH2OH O
CH2OH
OH
O OH
OH OH
OH
CH2OH OH
OH
Glucose (G)
Fructose (F)
Fig. 1. Schematic representation of sucrose hydrolysis to glucose and fructose.
Electric, RTV 615A, 3.3 g), mixing was continued for 1 h. The mixture was cast as a film on a Teflon plate and cured in vacuum at 150 ◦ C. PVA catalysts were prepared by dissolving the PVA (MERCK, average molecular weight: 72,000) in water at 90 ◦ C for 6 h. Next, the appropriate amount of succinic acid was added to the PVA solutions, which were vigorously stirred at room temperature for 24 h. After this period, different amounts of the zeolites (USY and Beta) were added to the PVA solution. The PVA mixtures were then stirred at room temperature for another 24 h. The mixture were poured and cast on a Teflon plate, and the cast polymers were allowed to dry at 60 ◦ C for 24 h. The dried PVA matrix was heated at 120 ◦ C for 2 h. 2.3. Catalyst characterisation The textural characterisation of the catalysts was based on a nitrogen adsorption isotherm, and was determined at 77 K with a Micromeritics ASAP 2010 apparatus. The amount of silica and aluminium in the Beta zeolite was measured by dissolving the catalyst in H2 SO4 /HF 1:1 (v/v) and analysing the obtained solution using inductively coupled plasma analysis (ICP), which was carried out in a Jobin-Yvon ULTIMA instrument. X-ray powder diffraction (XRD) of the zeolite was obtained on a Bruker powder diffractometer. The swelling degree of the catalytic composite was measured by immersing the polymeric matrix samples in water at 80 ◦ C for 24 h. Then, the polymeric samples were taken out, wiped with tissue paper and weighed. The swelling degree, Q, was calculated by the following: Q =
m − m0 m0
(1)
where m is the mass of swollen sample and m0 is the initial mass.
The success of the cross-linking of PVA was evaluated by FTIR spectroscopy. The FTIR spectra were recorded in a Bio-Rad FTS 155 instrument. Scanning electron microscopy (SEM) of the catalysts was carried out using a Hitachi model S-2400. 2.4. Catalytic experiments The catalytic experiments were carried out in a stirred batch reactor at 80 ◦ C. In a typical experiment, the reactor was loaded with 100 cm3 of sucrose (0.6 mol dm−3 ). The reactions were initiated by the addition of the catalyst (0.511 g). In all experiments, the stirrer speed was kept constant at 500 rpm. Stability tests of the Beta2 PVA were carried out by running four consecutive experiments under the same reaction conditions. After the hydrolysis was complete, the Beta2 PVA was separated from the reaction mixture by filtration, washed with water and dried overnight at 100 ◦ C. Portions of the solution were removed at known time intervals, and the rotation of polarised light was noted with the help of a Polax-D type polarimeter. The samples were also analysed by high performance liquid chromatography using a refractive index detector. 3. Results and discussion 3.1. Catalyst characterisation Fig. 2 shows the nitrogen adsorption–desorption isotherm of the USY and Beta zeolites. These materials show a type I isotherm (according to IUPAC nomenclature), which is characteristic of microporous zeolites. The specific surface area was determined using the BET method, and the microporous volume and
D.S. Pito et al. / Chemical Engineering Journal 184 (2012) 347–351
349
Table 1 Physicochemical characterisation of the zeolites. Sample
Si/Al ratioa
Surface areab (m2 /g)
External surface areac (m2 /g)
Microporous volumec (cm3 /g)
Beta USY
40 15
506 680
10 12
0.19 0.29
a b c
ICP. BET. t-method.
250
be explained by the greater hydrophilic nature of the PVA polymer compared with PDMS. The swelling degree of PDMS and PVA increased with the amount of catalyst (Beta and USY zeolite) dispersed in the polymer. This result can be explained by the increases in the free-volume in the polymeric matrix. Similar results were also observed by Kulkarni et al. [20]. The diffusivities of sucrose were calculated assuming a firstorder irreversible reaction. Specifically, the values of the maximum reaction rate were calculated from the slopes of the kinetic curves obtained for two different pellet sizes. Plate geometry was assumed for the pellet, and the following relationships for the Thiele modulus and the effectiveness factor were used:
V (cm3/g)
200
150
100
50
0 0
0,2
0,4
0,6
0,8
1
p/pº
=L×
mesoporous surface area were determined by the t-method, using a standard isotherm proposed by Gregg and Sing [19]. Table 1 shows the values of the BET specific area (SBET ), the microporous volume (Vmicro ) and the external surface area (Sext ). Fig. 3 shows the X-ray diffraction patterns that were obtained for the USY and Beta zeolite catalysts. Changes in the structure of the catalysts were not observed after synthesis and purification procedures. Fig. 4 shows the FTIR spectra of succinic acid (A), PVA (B) and PVA cross-linked with succinic acid (C). The intense band at 1730 cm−1 in spectrum B, which does not overlap with the band present in the spectrum of succinic acid (A), is characteristic of the free carboxylic group (1700 cm−1 ) and suggests that the cross-linking with succinic acid was successful. The characteristics of the polymeric matrices are shown in Table 2. The swelling degree of the composite catalyst was measured for water at 80 ◦ C. The swelling degree of the PDMS composite catalysts was lower than in the PVA catalysts. This behaviour can
(2)
tanh()
=
Fig. 2. N2 adsorption–desorption isotherms of the zeolites. () USY; () Beta.
k × p De
(3)
where k is the intrinsic kinetic constant, the pellet volumic mass, L is the membrane half thickness and De is the effective diffusivity. The effective diffusivity of sucrose increases with the amount of zeolite dispersed in the PDMS. This behaviour can be explained by the increases of composite catalyst channelling with the catalyst loading. Similar results were also obtained by Süer et al. [21] and Wu et al. [22]. Fig. 5 shows the SEM of the USY1/PDMS (Fig. 5A) and USY2/PDMS (Fig. 5B) composite catalysts. The polymeric matrix channelling increases with the increase of the zeolite amount in PDMS. In a previous report, similar results were also observed by Vital et al. [23]. 3.2. Catalytic experiments The hydrolysis of sucrose was carried out over composite catalysts at 80 ◦ C, and its products are glucose and fructose (Fig. 1). The initial activity of the PVA composite catalysts in sucrose hydrolysis was compared in Fig. 6. The initial activity was calculated by the maximum slope of the experimental kinetic curve of sucrose divided by the amount of catalyst. The catalytic activity
B
Transmittance (a.u.)
Intensity (a.u.)
A
B
C
A 5
15
25
35
2 (º) Fig. 3. XRD patterns of (A) USY and (B) Beta zeolites.
45
1800
1600
1400
1200
1000
Wavenumber
800
600
400
(cm-1)
Fig. 4. FTIR spectra of catalysts: (A) succinic acid; (B) PVA; (C) PVA cross-linked with succinic acid.
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Table 2 Characterisation of composite catalysts. Sample
Catalysts loading (%)
Thickness (mm)
Water swellinga (%)
De × 1010 (m2 /h)
Beta1/PDMS Beta2/PDMS USY1/PDMS USY2/PDMS Beta1/PVA Beta2/PVA USY1/PVA USY2/PVA
5 30 5 30 5 30 5 30
0.135 0.124 0.137 0.142 0.128 0.141 0.125 0.135
0.006 0.014 0.011 0.051 0.223 0.535 0.121 0.211
1.4 3.2 1.1 3.5 – – – –
a
Water swelling was measured by immersing dried pieces of polymeric matrixes in water at 80 ◦ C.
Fig. 5. Scanning electron micrograph of (A) USY1/PDMS; (B) USY2/PDMS.
increased with the amount of zeolite dispersed in the PVA matrix. This behaviour can be explained by the increase in the catalyst amount and by the increases of channelling of the polymeric matrix. As reported by Süer et al. [21], the increase in catalytic activity may be ascribed to an increase of the polymeric matrix channelling. The increase in the composite channelling led to an increase of the reagent permeation and, therefore, higher diffusion rates for the catalyst particles. Beta/PVA composites showed higher catalytic activity than USY/PVA composites (Fig. 6). This behaviour can be explained by the hydrophilic/hydrophobic balance of the PVA composite catalysts. In fact, the swelling degree of the Beta/PVA composite catalysts was higher than that of the USY/PVA catalysts (Table 2). Fig. 7 shows the initial activity of PDMS on composite catalysts in the hydrolysis of sucrose. The catalytic activity increases with the amount of zeolite dispersed in the PDMS. This behaviour can be explained by the increase in the catalyst amount immobilised in PDMS and also by the increase in polymeric matrix channelling,
Fig. 6. Hydrolysis of sucrose over Beta and USY zeolites dispersed in poly(vinyl alcohol), at 80 ◦ C. Initial activities were taken as the maximum observed reaction rate and were calculated from the maximum slope of the sucrose kinetic curve.
which can be observed in Fig. 5. An increase of sucrose diffusivity with the amount of zeolite was observed (Table 2). The hydrophobic/hydrophilic balance in the PDMS composites probably varied with the amount of zeolite amount immobilised in the PDMS. Actually, the swelling degree increased with zeolite loading on composite (Table 2). Similar results were also observed by Vital et al. [23]. Additionally, the catalytic activity of PVA composites was higher than with the PDMS composites, which can be explained due to the greater hydrophilicity of the PVA compared with the PDMS. In fact, the swelling degree of PDMS composites is smaller than with the PVA composites (Table 2). The catalytic activity of PVA with sulfonic acid groups (4.96 × 10−2 mol/h gcat ) [16] was smaller than the catalytic activity obtained over Beta2/PVA (8.61 × 10−2 mol/h gcat ). The low activity of PVA with sulfonic acid groups [16] could be due to the decrease in the accessibility of SO3 H groups for sucrose, given the high degree of cross-linking of the PVA.
Fig. 7. Hydrolysis of sucrose over Beta and USY zeolites dispersed in poly(dimethylsiloxane), at 80 ◦ C. Initial activities were taken as the maximum observed reaction rate and were calculated from the maximum slope of the sucrose kinetic curve.
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The catalytic activity of the composite catalysts was found to increase with the amount of zeolite that was dispersed in the polymeric matrix. The catalytic activity of PVA composites was higher than with the PDMS composites. To study the catalytic stability of Beta2/PVA catalyst, four consecutive batch runs with the same catalyst were carried out, and similar values in catalytic activity were observed for each run. References
Fig. 8. Stability studies of the Beta2/PVA catalyst on the hydrolysis of sucrose. The catalytic activity was calculated as the initial reaction rate of the hydrolysis of sucrose in four consecutive experiments.
To study the catalytic stability of the Beta2/PVA, different batch runs were carried out with the same catalyst sample at the same conditions. Fig. 8 shows the relative catalytic activity of the Beta2/PVA catalyst, i.e., the catalytic activity after the different runs divided by the initial activity. After the second batch, a stabilisation of the catalytic activity was observed. The catalytic activity of Beta2/PVA for sucrose hydrolysis was compared with the activity of other catalysts used in this reaction that were reported in literature. The catalytic activity (expressed as mol/h gcat ) of the different solid catalysts was found to increase as follows: 4.74 × 10−4 mol/h gcat (obtained with V2 O5 /Al2 O3 , [10]) < 5.54 × 10−4 mol/h gcat (obtained with HPA/silica, [11]) < 5.51 × 10−3 mol/h gcat (obtained with silica with sulfonic acid groups, [9]) < 3.59 × 10−2 mol/h gcat (obtained with PS-SO3 H, [8]) < 4.87 × 10−2 mol/h gcat (obtained with zeolite, [4]) < 4.96 × 10−2 mol/h gcat , (obtained over PVA 40, [16]) < 8.61 × 10−2 mol/h gcat (obtained over Beta2/PVA, [present work]) < 8.75 × 10−2 mol/h gcat (obtained over polystyrene with sulfonic acid groups, [7]). 4. Conclusions The hydrolysis of sucrose to glucose and fructose was carried out over composite catalysts consisting of USY and Beta zeolites dispersed in PDMS and PVA matrices. The reactions were carried out in a batch reactor at 80 ◦ C. The swelling degree of PDMS and PVA increased with the amount of catalyst (Beta and USY zeolite) dispersed in the polymers, which can be explained by an increase in the polymeric matrix channelling. An increase in the effective diffusivity of sucrose through the composite, with the amount of zeolite dispersed in PDMS, was observed.
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