Efficient Conversion of CO2 to Methanol Catalyzed by Three

May 28, 2006 - Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin Universit...
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Ind. Eng. Chem. Res. 2006, 45, 4567-4573

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Efficient Conversion of CO2 to Methanol Catalyzed by Three Dehydrogenases Co-encapsulated in an Alginate-Silica (ALG-SiO2) Hybrid Gel Song-wei Xu, Yang Lu, Jian Li, Zhong-yi Jiang,* and Hong Wu Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, People’s Republic of China, and State Key Laboratory of Bioreactor Engineering, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China

In this study, the conversion of carbon dioxide to methanol was realized through a novel biochemical approach that was catalyzed by three dehydrogenases: formate dehydrogenase (FateDH), formaldehyde dehydrogenase (FaldDH), and alcohol dehydrogenase (ADH). The dehydrogenases were encapsulated in an alginate-silica (ALG-SiO2) hybrid gel, which was prepared through in situ growth of the silica precursor within an alginate solution, which was followed by Ca2+ cross-linking. Methanol yields that were catalyzed by free dehydrogenases, and by dehydrogenases immobilized in pure alginate (ALG) gel and in ALG-SiO2 hybrid gel, were 98.8%, 71.3%, and 98.1%, respectively. Furthermore, methanol yield that was catalyzed by dehydrogenases in an ALG-SiO2 composite could be retained as high as 76.2% after 60 days storage and as high as 78.5% after 10 times recycling. The significantly improved catalytic properties of the dehydrogenases in the ALG-SiO2 composite were attributed to the creation of the appropriate immobilizing microenvironment: high hydrophilicity, moderate rigidity and flexibility, ideal diffusion characteristics, and optimized cage confinement effect. 1. Introduction The efficient conversion of CO2 has attracted much considerable attention, because of the fact that it is the most abundant C1 compound and is the major greenhouse gas. In addition, the conversion of CO2 to more-useful chemicals is important in recycling carbon species.1-5 However, CO2 is one of the moststable molecules, because of its higher standard Gibbs energy of formation (398.34 kJ/mol). Hence, to obtain high conversion of CO2, excessive energy and severe equipment are essential to chemical approaches. For example, the temperature of reduction of CO2 to methanol via conventional chemical catalysis was as high as 550 K and the pressure was ∼9 MPa. In comparison, the bioconversion of CO2, using entire cells or enzymes as catalysts, has attracted increased attention, because of the high yield and selectivity that can be attained under milder reaction conditions, without pollution.6-8 In 1999, Obert and Dave reported the consecutive reduction approach of CO2 with three dehydrogenases: formate dehydrogenase (FateDH), formaldehyde dehydrogenase (FaldDH), and alcohol dehydrogenase (ADH) as biocatalysis, and reduced nicotinamide adenine dinucleotide (NADH) as a terminal electron donor.8 The three dehydrogenases were co-encapsulated in a sol-gel silica matrix to proceed the entire catalysis. However, the low water solubility and reactivity of the silica precursor typically necessitates cosolvents and catalysts in the sol-gel process. Moreover, alcohol liberated from hydrolysis, co-solvents, and catalysts are deleterious to bioactivity.9,10 To solve these problems, several approaches were attempted to modify the sol-gel technique and stabilize the biological activity, including the introduction of a sonication technique (instead of using alcohol as a co-solvent), the use of novel precursors, and the use of supercritical drying or freeze-drying techniques.11-14 Because of the inherent limitations, these methods could only address some of the intrinsic disadvantages thta have been previously mentioned. * To whom correspondence should be addressed. Fax: +86-2227892143. E-mail address: [email protected].

Recently, natural biosilicate formation has inspired innovative routes to design carriers for bioencapsulation.15,16 Many reports have presented the preparation of alginate-silica composites, through the incorporation of silica particles into alginate gel beads, or through the modification of sol-gel silica with alginate.17-19 In this study, a novel alginate-silica (ALG-SiO2) composite that was prepared through an in situ co-assembly technique has been used for yeast alcohol dehydrogenase (YADH) encapsulation,20 leading to the remarkably enhanced catalytic activity and stability of YADH, because of less enzyme leakage and improved carrier microenvironment. In addition, the sequential conversion of CO2 to methanol catalyzed by three dehydrogenases co-encapsulated in the ALG-SiO2 composite were systematically investigated, and a high yield of methanol has been obtained. 2. Experimental Section 2.1. Chemicals. Formate dehydrogenase (FateDH, 1.4 U/mg, solid), formaldehyde dehydrogenase (FaldDH, 4.4 U/mg, solid), alcohol dehydrogenase (ADH, 303 U/mg, solid), and reduced nicotinamide adenine dinucleotide (98%) were purchased from Sigma (USA). Sodium alginate (average molecular weight of 6.27 × 105) was purchased from Shanghai Tianlian (China). Tetramethoxysilane (TMOS) was purchased from Wuhan University (China). Tris-HCl buffer was prepared using a solution of trizma base and adjusting the pH with 0.1 N HCl. All other chemicals were of analytical grade. 2.2. Preparation of ALG-Enzyme Biocomposites. Sodium alginate was dissolved in deionized water at a final concentration of 2.0% (w/v), unless noted otherwise. To co-encapsulate the three dehydrogenases, a 4-mL aliquot of the alginate solution was mixed with 1 mL of 0.05 M, pH 7.0 tris-HCl buffer containing FateDH (4.5 mg), FaldDH (4.5 mg), and ADH (1.0 mg). This mixture solution was then added dropwise into a gelation medium of 20 mL of a 0.2 M CaCl2 solution, using a 10-mL syringe through a 0.7-mm-diameter

10.1021/ie051407l CCC: $33.50 © 2006 American Chemical Society Published on Web 05/28/2006

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needle under constant stirring at room temperature. The beads thus formed were cured in the gelation medium for 30 min and then were removed. The diameter of ALG gel beads was determined to be 2.73 ( 0.02 mm. To encapsulate the three dehydrogenases separately, a 1.33 mL aliquot of the alginate solution was mixed with 0.33 mL of 0.05 M, pH 7.0 tris-HCl buffer that contained FateDH (4.5 mg). This mixture solution was then added dropwise into 20 mL of a 0.2 M CaCl2 aqueous solution to form biocomposite beads under constant stirring at room temperature. The biocomposite beads were cured in the CaCl2 solution for 30 min and then were removed. FaldDH (4.5 mg) and ADH (1.0 mg) were respectively immobilized by the same procedure. Leakage of each dehydrogenase during the biocomposites formation was determined by measuring the absorbance of the gelation medium solution at 280 nm, using an ultraviolet-visible (UV-Vis) spectrophotometer (model U-2800, Hitachi). 2.3. Preparation of ALG-SiO2-Enzyme Biocomposites. To co-encapsulate the three dehydrogenases in the ALG-SiO2 composite, a 1.47 mL aliquot of TMOS solution was vigorously mixed with 4 mL of alginate solution for 5 min, followed by mixing with 1 mL of 0.05 M, pH 7.0 tris-HCl buffer that contained FateDH (4.5 mg), FaldDH (4.5 mg), and ADH (1.0 mg). This mixture was added dropwise into 20 mL of a 0.2 M CaCl2 solution. The biocomposite beads were cured in the CaCl2 solution for 30 min and then were removed for further use. The diameter of the ALG-SiO2 composite beads was determined to be 2.31 ( 0.01 mm. To encapsulate the three dehydrogenases separately in the ALG-SiO2 composite, 0.49 mL of TMOS solution was vigorously mixed with 1.33 mL of alginate solution for 5 min, followed by mixing with 0.33 mL of 0.05 M, pH 7.0 tris-HCl buffer that contained FateDH (4.5 mg). This mixture was added dropwise into 20 mL of a 0.2 M CaCl2 solution. The biocomposite beads were cured in the CaCl2 solution for 30 min and then were removed. FaldDH (4.5 mg) and ADH (1.0 mg) were respectively immobilized by the same procedure. Leakage of the three dehydrogenases during the formation of the biocomposites was determined. 2.4. Enzymatic Conversion of CO2 to Methanol. NADH was dissolved in 2 mL of 0.05 M, pH 7.0 tris-HCl buffer that contained 4.5 mg of FateDH, 4.5 mg of FaldDH, and 1.0 mg of ADH, with a final NADH concentration of 940 µM. The mixture solution was transferred to a reaction system and then CO2 was added through bubbling, with the pressure being maintained at 0.5 MPa. The reactor was tubular, with an outer diameter of 2.2 cm, an inner diameter of 1.5 cm, and a height of 15.8 cm. The pressure was maintained at 0.5 MPa, and no outward gas flow was permitted during the reaction, to decrease the amount of methanol that was evaporating and leaking out. The enzymatic reaction lasted 8 h for the sufficient production of methanol. Methanol concentration was determined in the liquid phase, and the vaporization of methanol was not taken into account. The concentration of intermediate products, formate and formaldehyde, were too low to be detected. Conversion of CO2 by immobilized dehydrogenases was performed by mixing NADH solution (940 µM) and the biocomposite beads in the reaction system and then bubbling into CO2 for 8 h, with the pressure being maintained at 0.5 MPa. The optimum conversion temperature and pH were determined. Operational stability and the activity during storage of the immobilized enzyme were also determined. The methanol concentration was determined via gas chromatography (GC) measurements (Hewlett-Packard, model HP-6890).

Figure 1. Scanning electron microscopy (SEM) photomicrograph of the internal structure of an alginate gel.

2.5. Characterization. The cross-sectional morphology of the biocomposite beads was observed using scanning electron microscopy (SEM) (model XL-30 ESEM, Philips). The hydrophilicity of the ALG gel and ALG-SiO2 hybrid composite was determined by measuring the water contact angle, using a goniometer (Erma Contact Angle Meter, Japan). 3. Results and Discussion 3.1. Biocomposite Formation. Alginates are anionic, hydroxylated, and hydrogen-bonding biopolymers that are inexpensive, nontoxic, hydrophilic, and biocompatible.21-24 It has been observed that alginate can promote and direct biosilicate formation.25 The monolithic hybrid composite formed within 20 min at natural pH, normal temperature, and normal pressure by simply mixing TMOS with alginate solutions. The promoting effect of alginate on the biocomposite formation would be due to the fact that covalent bonding of the carboxyl of alginate with elemental silicon accelerated the hydrolysis of TMOS, and nucleation of the silica shell around the alginate molecule accelerated the gel formation.15,26 When the TMOS/silica/silanol and alginate mixture solution was added dropwise into a CaCl2 solution, the ALG-SiO2 hybrid composite formed through the gelation of TMOS and the Ca2+ cross-linking with alginate, as well as through the intermolecular interaction of the silanol/ silica with alginate matrix. SEM photomicrographs were used to determine the structure of the biocomposites and the distribution of the silica in the composite. Figure 1 demonstrates an SEM photomicrograph for the structure of an ALG bead. The similar structure of ALG and ALG-SiO2 (Figure 2a) demonstrates the homogeneous distribution of SiO2 in the alginate matrix. Energy-disperive X-ray (EDX) spectroscopy (Figure 2b) showed that the ALGSiO2 composite contained elemental silicon, and the SEM photomicrograph in the inset of Figure 2b demonstrated the association of silica with the alginate matrix, the adsorption of silica on the surface of alginate, and a mixture of silica with the fibrous network of alginate. 3.2. Thermodynamic Analysis of the Bioconversion. The bioconversion process in this study can be simply described in Figure 3, which involves a consequential reduction of CO2 to formate, followed by a reduction to formaldehyde, and finally a reduction to methanol by the three dehydrogenases. Some thermodynamic aspects of the sequential bioconversion of CO2 to methanol were tentatively analyzed through the standard reaction properties (∆rH° and ∆rG°) and the equilibrium constant

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Figure 4. Methanol yields catalyzed by dehydrogenases co-encapsulated in the ALG gel (biocomposite I), encapsulated separately in the ALG gel (biocomposite II), co-encapsulated in the ALG-SiO2 composite (biocomposite III), and encapsulated separately in the ALG-SiO2 composite (biocomposite IV). Figure 2. (a) SEM photomicrograph of the internal structure of the ALGSiO2 composite. (b) Energy-dispersive X-ray (EDX) spectrum of the ALGSiO2 composite.

Figure 3. Enzymatic conversion of carbon dioxide to methanol.

K calculation from standard formation reaction properties (∆fH°i and ∆fG°i) of species, using the relations

∆rH° ) ∆rG° )

∑νi∆fH°i

∑νi∆fG°i ) -RT ln K

(1) (2)

where νi is the stoichiometric number (positive for products and negative for reactants) for species i, and K is the equilibrium constant written in terms of species. For the purpose of comparison, the standard reaction properties (∆rH° and ∆rG°) and the equilibrium constant K of conventional chemical catalysis of CO2 to methanol was calculated. The thermodynamics of the CO2 conversion to methanol via conventional chemical catalysis and biocatalysis, which are shown as reactions 3 and 4, respectively, were analyzed through eqs 1 and 2.

CO2(g) + 3H2(g) T CH3OH(g) + H2O(g)

(3)

CO2(g) + 3NADH + 3H + T CH3OH(l) + 3NAD+ + H2O(l) (4) Table 1 presents the standard formation reaction properties (∆fH°i and ∆fG°i) of the species involved in the two reactions at 25 °C, pH 7.0, 100 kPa, and I ) 0.1.27,28 Thermodynamic analysis indicated that the values of ∆rH° and ∆rG° for conventional chemical catalysis were -49.01 kJ/mol and 3.79 kJ/mol, respectively, and the values of ∆rH° and ∆rG° for biocatalysis in the present study were -39.85 kJ/mol and -67.84 kJ/mol, respectively. Calculated from eq 2, the equilibrium constant for conventional chemical catalysis and bioconversion were 0.217 and 9.590 × 106, respectively. The equilibrium constant of the biocatalysis of CO2 to methanol is much higher than that of conventional chemical catalysis.

Table 1. Standard Formation Reaction Properties (∆fH°i and ∆fG°i) of Species at 25 °C, pH 7.0, 100 KPa, and I ) 0.1 ∆fH°i (kJ/mol)

∆fG°i (kJ/mol)

species

gas (g)

liquid (l)

gas (g)

liquid (l)

CO2 CH3OH H2 H+ H2O NADH NAD+

-393.51 -200.7 0

-238.7

-394.36 -162.0 0

-166.3

-241.82

0.31 -285.83 -30.70 0.31

-228.57

-0.61 -237.13 20.20 -0.61

3.3. Comparison of Methanol Yields of Co-encapsulation and Separate Encapsulation. NADH serves as a limiting reagent in the overall reduction; therefore, efficiency of the reaction and the yield of the methanol production were calculated based on the amount of NADH consumed as shown in eq 5.8,11

methanol yield (%) ) 3 × amount of methanol produced (in moles) × 100 (5) initial amount of NADH (in moles) In addition, the consumption of CO2 was not determined. As shown in Figure 4, methanol yields catalyzed by biocomposite I and biocomposite III (dehydrogenases were coencapsulated) were higher than those of biocomposite II and biocomposite IV (dehydrogenases were encapsulated separately). In biocomposite I and biocomposite III, the three dehydrogenases were co-immobilized in an ALG gel or ALG-SiO2 composite. The three dehydrogenases would be distributed in each biocomposite bead. Therefore, the three dehydrogenases could be immobilized in the same bead. All three dehydrogenases immobilized in biocomposite I and III would be accessible to the intermediate products (formate and formaldehyde) in the same bead, which would shorten the diffusion distance of the intermediate products and elevate the local concentration of the intermediate products (Figure 5). However, for the conversion of CO2 to methanol catalyzed by dehydrogenases encapsulated separately in biocomposite II and IV, the intermediate products must diffuse outward from one bead first and then diffuse into a neighboring bead that contains another type of dehydrogenase for subsequent reduction. 3.4. Comparison of Methanol Yields of Different Carriers. The methanol yields catalyzed by free enzymes, and by enzymes

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Figure 5. Illustration of substrates, intermediate products, and final product: (a) beads where dehydrogenases were co-encapsulated and (b) beads where dehydrogenases were encapsulated separately.

Figure 6. Methanol yields catalyzed by free dehydrogenases and dehydrogenases co-encapsulated in the ALG gel and ALG-SiO2 composite.

immobilized in an ALG gel and ALG-SiO2 biocomposite, were investigated and are shown in Figure 6. The methanol yields catalyzed by free dehydrogenases reached a value of 98.8%. A small decrease in the methanol yield was observed in those catalyzed by the dehydrogenases encapsulated in the ALGSiO2 composite. However, a significant decrease in the methanol yields for those catalyzed by ALG-encapsulated dehydrogenases is shown in Figure 6. At the same time, an alginate-silica matrix without dehydrogenases was also used to convert CO2; no methanol was found. The ALG-SiO2-enzyme biocomposite and ALG-enzyme biocomposite were stored at 4 °C, to investigate the effect of storage on the enzyme activity and methanol yields. Figure 7 shows that methanol yields catalyzed by enzymes immobilized in the two carriers lost ∼10% after the initial 7 days. In addition, the yield curves of methanol catalyzed by dehydrogenases immobilized in ALG-SiO2 composite then reduced slowly in the following 53 days. After storage for 60 days, 76.2% of the methanol yield catalyzed by dehydrogenases immobilized in the ALG-SiO2 composite could be still retained. However, the yield curves of methanol catalyzed by dehydrogenases immobilized in ALG gel were reduced sharply in the following 53 days, and only 28.4% of the methanol yield could be retained after storage for 60 days.

Figure 7. Methanol yields catalyzed by immobilized dehydrogenases during storage.

Figure 8. Effect of cycle number on methanol yields catalyzed by immobilized dehydrogenases.

Meanwhile, the effect of recycling on the catalytic efficacy of immobilized ADH was investigated. As shown in Figure 8, the relative activities of immobilized dehydrogenases and methanol

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Figure 9. Leakage of three dehydrogenases in the ALG gel and ALGSiO2 composites.

yields all decreased as the recycling times increased. After 10 cycles, methanol yield catalyzed by dehydrogenases immobilized in ALG gel decreased almost to zero, whereas a methanol yield of 78.5% was observed for those results catalyzed by dehydrogenases immobilized in the ALG-SiO2 composite. Compared to the dehydrogenases in an ALG gel, the results of dehydrogenases encapsulated in the ALG-SiO2 composite showed high initial enzyme activity retention and significant enzyme stability during storage and reuse. This was mainly due to the different loading efficiency of the dehydrogenases and the microenvironmental differences between the two carriers. Figure 9 showed that the loading efficiency of each dehydrogenase in the ALG-SiO2 composite was remarkably higher than that in the ALG gel. The loading efficiency increase of enzymes in the ALG-SiO2 composite was due to the reduction of water loss and the physical cage confinement for the enzyme encapsulated at the silica/alginate interface.20,29 Generally speaking, the bioactivity of the biomolecules in living nature can be well-retained, because of the abundance of water around the biomolecule, along with appropriate pore structure. Therefore, we attribute the higher catalytic activity and stability retention of the dehydrogenases co-encapsulated in the ALG-SiO2 composite to the improved microenvironment, including high hydrophilicity, moderate rigidity and flexibility, optimized cage confinement effect, and ideal diffusion characteristics. The content of water was >90 wt % in the wet ALGSiO2-enzyme biocomposite. Even when the bicomposite was freeze-dried for 12 h, the biocomposites contained ∼5% water. The measured water contact angles for the ALG gel and ALGSiO2 hybrid composite also indicated the higher hydrophilicity of ALG gel (13°) and the ALG-SiO2 hybrid composite (5°). A sufficient amount of water contained in the ALG-SiO2 composite would effectively maintain the bioactivity for the hydrophilic alginate and silica. When enzymes were immobilized in the sol-gel silica matrix, the rigid silica matrix and too-small pore diameter, along with the unfavorable enzyme-silica interaction (mainly hydrogen bonding of enzyme with Si-OH of silica) would cause the crowded effect to restrict the essential mobility of the enzymes and, thus, decrease the bioactivity of the enzymes.30 When enzymes were immobilized in a negatively charged alginate gel, the enzyme-alginate interaction (mainly electrostatic interaction) would also decrease the conformational motion and dynamics of the enzymes. In addition, although the loose structure of alginate gel arising from the remarkable swelling is favorable for substrate and product diffusion, it will also

Figure 10. Effect of temperature on the methanol yields catalyzed by immobilized dehydrogenases.

unavoidably cause significant enzyme leakage and, thus, worsen the mechanical, storage, and operation stability.21 Being encapsulated in the ALG-SiO2 composite, the biomolecules were assumed to lie at the alginate/silica interface, which is a space that is big enough for the conformational motion and mobility of the enzymes to occur. In addition, appropriate physical cage confinement of the dehydrogenases would increase the stability of the enzyme in the ALG-SiO2 composite. Moreover, the silica-alginate interaction (mainly hydrogen bonding) would significantly decrease the enzyme-alginate interaction and enzyme-silica interaction.20,31 The homogeneous distribution of rigid silica in an alginate matrix can significantly increase the mechanical property and lower the degree of swelling.29 On the other hand, the catalytic activity of immobilized enzymes was usually affected by the diffusion limitations of substrates or products in the carriers. The diffusion coefficient of NADH in the ALG-SiO2 composite was (1.9 ( 0.2) × 10-10 m2/s, as determined via the method mentioned by Jiang et al.; this value is slightly less than that of NADH in the bulk solution ((3.3 ( 0.2) × 10-10 m2/s) but a is slightly greater than that of NADH in ALG gel.32,33 This ensures the rapid diffusion of substrates and intermediate products or final product during the enzymatic reaction. In the conventional sol-gel process for bioencapsulation, the concentration of byproduct alcohol generated from the precursor hydrolysis was relatively high, because the alcohol cannot diffuse or evaporate from the gel within a short time. The accumulation of alcohol can consequently inactivate the enzymes, to a serious extent.9,12 In this study, the alcohol generated would be rapidly liberated into the bulk solution after the dropwise addition of the alginate-TMOS mixture into the CaCl2 solution, which significantly shortens the time for the intimate interaction between the alcohol and the enzyme. 3.5. Effect of Temperature and pH on the Methanol Yields. The effect of temperature on the activity of immobilized dehydrogenases for the reduction of CO2 to methanol was studied at various temperatures at pH 7.0 in 0.05 M tris-HCl buffer, and the results are shown in Figure 10. The optimum temperature of enzymatic reaction catalyzed by the three dehydrogenases was observed at 37 °C (FateDH), 37 °C (FaldDH), and 25 °C (ADH) separately.34-36 As shown in Figure 10, the optimum temperature for dehydrogenases immobilized in the ALG-SiO2 composite and ALG gel were both at 37 °C, which is consistent with that of free FateDH (37 °C) and FaldDH; this

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Figure 11. Effect of pH on the methanol yields catalyzed by immobilized dehydrogenases.

would be due to the fact that the catalytic activity of FateDH was lowest and it affected the entire bioconversion. The effect of pH on the activity of immobilized dehydrogenases for the reduction of CO2 to methanol was studied at various pH values at 37 °C. The optimum pH values of the enzymatic reaction catalyzed by the three dehydrogenases was observed at pH 7.0 (FateDH), 7.0 (FaldDH), and 7.0 (ADH) separately.34-36 Figure 11demonstrates the maximum yields of methanol catalyzed by dehydrogenases, both immobilized in the ALG-SiO2 composite and in the ALG gel, at pH 7.0; this observation also was consistent with that of free dehydrogenases. 4. Conclusions Efficient bioconversion of carbon dioxide to methanol was realized at natural pH and low temperature and pressure, catalyzed by three dehydrogenases that were co-encapsulated in the novel alginate-silica (ALG-SiO2) composite. Methanol yields catalyzed by dehydrogenases immobilized in the ALGSiO2 composite exhibited only a minor decrease, compared with that of free dehydrogenases. Compared to the dehydrogenases in ALG gel, the dehydrogenases that were encapsulated in the ALG-SiO2 composite showed highly initial enzyme activity retention, and significantly improved stability during storage and reuse. This was mainly due to the more comfortable microenvironment that was created: high hydrophilicity, moderate rigidity and flexibility, optimized cage confinement effect, and ideal diffusion characteristics. The maximum methanol yields catalyzed by dehydrogenases immobilized in the ALG gel and ALG-SiO2 composite were set at pH 7.0 and a temperature of 37 °C, which is exactly consistent with those of free dehydrogenases. Acknowledgment Financial support from the Program for Changjiang Scholars and the Innovative Research Team in University (PCSIRT) and financial support from the National Natural Science Foundation of China (Grant Nos. 20176039 and 20576096) are greatly appreciated. Literature Cited (1) Kobayashi, T.; Takahashi, H. Novel CO2 Electrochemical Reduction to Methanol for H2 Storage. Energy Fuels 2004, 18, 285. (2) Traynor, A. J.; Jensen, R. J. Direct Solar Reduction of CO2 to Fuel: First Prototype Results. Ind. Eng. Chem. Res. 2002, 41, 1935.

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ReceiVed for reView December 17, 2005 ReVised manuscript receiVed March 16, 2006 Accepted April 25, 2006 IE051407L