Production of Hydrogen from Glucose as a Biomass Simulant

Dec 14, 2007 - Production of Hydrogen from Glucose as a Biomass Simulant: Integrated Biological and Thermochemical Approach. Sadashiv M. ... Hydrogen ...
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Ind. Eng. Chem. Res. 2008, 47, 3645-3651

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Production of Hydrogen from Glucose as a Biomass Simulant: Integrated Biological and Thermochemical Approach Sadashiv M. Swami,† Vaibhav Chaudhari,† Dong-Shik Kim,† Sang Jun Sim,‡ and Martin A. Abraham*,† Department of Chemical and EnVironmental Engineering, UniVersity of Toledo, Toledo, Ohio 43606, and Department of Chemical Engineering, Sungkyunkwan UniVersity, Suwon, 440-746, Korea

Hydrogen production from biomass was investigated using an integrated biological and thermochemical process. Glucose was used as a biomass surrogate and was first converted to ethanol in a fermentation process. The fermentation experiments were carried out using Saccharomyces cereVisiae. The fermentation broth was then used in aqueous phase reforming (APR) over a platinum-based catalyst. An economic analysis of the proposed process demonstrates the economic viability of producing hydrogen from biomass using fermentation combined with APR. The average production yield of hydrogen was ∼25%. The hydrogen obtained from APR of the fermentation broth was compared against the yield from a feed containing 5% ethanol in water. While the catalyst was stable for an extended time on stream during APR of ethanol, very rapid deactivation was observed during APR of fermentation broth. Different catalyst characterization techniques, including XRD, BET surface area, and ICP-AES, were employed to investigate the causes of catalyst deactivation. Although the analysis suggested similar catalyst changes in both cases, and the exact deactivation mechanism could not be concluded, these techniques helped to eliminate some mechanisms while suggesting other possible deactivation routes. Nanofiltration of the fermentation broth was shown to remove the impurities leading to deactivation. Introduction Hydrogen is receiving increased attention as a future energy alternative owing to the recent global efforts to reduce the dependence on fossil fuels and the desire to reduce carbonbased emissions. Hydrogen offers great potential as a clean renewable energy carrier. It has the highest gravimetric energy density of any known fuel, and it can be used in a fuel cell with a high energy conversion efficiency compared to other electrochemical and combustion processes. As fuel cells are becoming more efficient and approaching commercialization, hydrogen will become an important component of the alternative energy profile. Despite its tremendous potential, hydrogen energy has many technical challenges. One of them is the production of sufficient quantities of hydrogen.1 There are many ways to produce hydrogen including steam reforming or thermal cracking of natural gas, coal gasification, electrolysis, and biological production. Currently, nearly 90% of hydrogen is produced by the steam reforming of natural gas and light oil fractions.2 These methods consume fossil fuel, and thus fail to address either the long-term reliance on fossil resources or the carbon emissions issues associated with their consumption. Direct biological hydrogen production can be classified into biophotolysis using algae and cyanobacteria, photodecomposition by photosynthetic bacteria, fermentation, and hybrid systems using photosynthetic decomposition combined with fermentation.3-5 The major problems in the biological hydrogen production are slow reaction rates and the problems associated with scale-up of bioreactors.6 Also, even if most nonphotosynthetic anaerobic bacteria use glucose to produce hydrogen,7 usually the resulting components after hydrolysis of biomass wastes * To whom correspondence should be addressed. Tel.: (419) 5308092. E-mail: [email protected]. † University of Toledo. ‡ Sungkyunkwan University.

consist of various sugars including arabinose and xylose as well as glucose, which are not well utilized by anaerobic bacteria. Because hydrogen is not produced through biological processes in sufficient yield to operate commercial size fuel cells, biohydrogen technologies still need further progress.8 Thus, combinations of other well-established techniques have been tested to overcome their limitations. Direct conversion of biomass to hydrogen has been extensively studied in the past few years. The most common techniques for biomass conversion are gasification and pyrolysis.9 However, the major disadvantage of these processes is the decomposition of the biomass feed stock, leading to char and tar formation.10,11 Overcoming this difficulty, aqueous phase reforming (APR) has been demonstrated as an alternative technique for hydrogen production from sugars.12 In order to reduce the char formation and enhance the biomass conversion, it has been suggested that the biomass feedstock be converted to organic chemicals before reforming. In previous work, researchers first reduced glucose to sucrose through hydrogenation, before APR.13 As an alternative, we have proposed an integrated fermentation and reforming process. APR has been chosen over conventional steam or autothermal reforming so that unconverted biomass from a fermentation process can still be utilized in a reforming process without decomposition. Biomass feedstock will be first converted to ethanol and organic acids in a fermentation process followed by aqueous phase reforming of these converted products to hydrogen. The integrated biological and reforming process concept is depicted in Figure 1, in which conventional conversion processes based on fermentation and catalytic reforming are combined to convert waste biomass into hydrogen. Biomass wastes from food or agricultural processing can be converted, nearly eliminating the cost of raw materials (or even possibly generating revenue by reducing waste disposal related costs) and improving the overall economics of the process. In order to be a practical option for industrial and agricultural interests, an economically

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Figure 1. Process concept for conversion of biomass resources to hydrogen showing integrated biological and catalytic process steps.

viable system must be developed for converting target waste streams to hydrogen. Aqueous phase reforming is conceived as an alternative for steam reforming of biomass-derived compounds. The pressure and temperature in APR are adjusted such that reactants remain in a liquid phase. For steam reforming of ethanol, stoichiometry requires a minimum steam to carbon ratio (S/C) of 1.5,14 which corresponds to an ethanol concentration of 51.92% (vol %). However, this concentration is very high compared to the ethanol concentration in fermentation broth. The aqueous phase reforming reactions can be carried out at very low concentrations of ethanol corresponding to the ethanol concentration in fermentation broth, saving the energy required for concentrating ethanol in distillation processes. Thus, expensive distillation units and energy costs can be avoided from the process. APR process is carried out at lower temperature (∼250 °C) at which the water gas shift (WGS) reaction is thermodynamically favorable.15 The WGS reaction is reversible and exothermic; therefore, the reaction equilibrium shifts to favor H2 formation at low temperature and thus the CO level in product gases obtained from an APR process is less than that obtained from a steam reforming process. Due to this reason, the water gas shift unit is eliminated. Since the APR process has fewer process units than steam reforming (SR), both fixed costs and operating costs associated with APR will be lower than those for the SR process. In the current work, we have evaluated an integrated process for conversion of glucose, as a model for biomass-derived organic species. An analysis of the fermentation process is used to determine the products of fermentation that make up the feed to the APR process. Further analysis of the APR process is then presented in terms of catalyst development, to provide a full description of the overall integrated process for conversion of biomass to hydrogen. Energy Aspects of Biomass Conversion to Hydrogen. Ethanol is being conceived as a renewable energy source. In 2002, NREL published a report on conversion of lignocellulosic biomass to ethanol with a net energy efficiency of approximately 53%.16 We have used the NREL report as a basis and adapted it as the proposed integrated process for the production of hydrogen, as shown in Figure 2. In the integrated scheme, there is no need for concentrating the ethanol from fermentation broth, which was instead directly used in the aqueous phase reformer. Therefore, distillation and evaporator units in the ethanol producing process were replaced by an aqueous phase reformer. In the original NREL analysis, lignocellulosic biomass was pretreated and saccharified followed by the fermentation process, during which most of the sugar was converted to ethanol. However, lignin and other solid residues do not participate in the fermentation process and remain unconverted. This solid

residue would be burned in the boiler section to produce the steam required to operate the plant. The energy balance for the entire ethanol producing plant was estimated using ASPEN plus software, and an overall energy efficiency of 53% was obtained. The major energy losses during ethanol production are shown to be within the distillation and evaporator units (∼20%).16 In the case of the integrated biological and thermochemical approach, the dilute ethanol stream from the fermentation process is directly utilized in the reformer without preconcentration. Therefore, the energy-intensive distillation and evaporator units can be eliminated. The ethanol stream is pretreated through centrifugation to eliminate cell biomass from the fermentation process before the filtered ethanol stream is introduced to the aqueous phase reformer. The energy balance calculations on the reformer section were done using Chemcad software, assuming that the reformer operated at 250 °C and 600 psia. The equilibrium ethanol conversion was estimated by minimizing the total Gibbs free energy. In this way, the energy loss in the reformer section was calculated to be 14%. Consequently, the overall energy efficiency for the integrated process of biomass conversion to hydrogen, with ethanol as an intermediate product, was found to be 59%, about 6% higher than the ethanol production plant described in the NREL process. In addition to the 6% gain in the conversion efficiency, the final product of the integrated process, hydrogen, can be more efficiently utilized in fuel cells (40-45%), whereas ethanol would be burned in a low efficiency internal combustion engine (20-25%).17 Thus, the well-to-wheel efficiencies for hydrogen utilization in fuel cell and ethanol combustion in the Otto engine were reported to be 10.8% and 7%, respectively, representing an almost 54% increase for the conversion of biomass energy to power.18 Methane, which is produced as a byproduct of APR, can also be used as a fuel to drive the hydrogen production process, as needed. Experimental Materials and Methods Fermentation Process. Typically, glucose solution in water, 100 g/L (0.55 M), was used for fermentation. Saccharomyces cereVisiae, commonly known as baker’s yeast, obtained from Sigma and stored at 8 °C in a dry place, was used as the microorganism for carrying out all the fermentations. Fermentations were carried out in a batch reactor comprised of a 500 mL conical flask with a 300 mL working volume. The conical flasks were provided with openings for sampling and pH probe. Fermentation solution containing 100 g/L glucose was prepared in deionized water and then autoclaved at 120 °C for 20 min. The autoclaved solution was then allowed to cool down in the hood and 3 g of S. cereVisiae was added. The pH of the solution was monitored during the entire experiment and was maintained close to 6 through the addition of 1 N sodium hydroxide solution. Fermentation experiments were carried out in a shaker set at 120 rpm with temperature control maintaining the temperature at 30 °C. After the fermentation, the broth was kept in a refrigerator for sedimentation to occur. The supernatant was then carefully decanted without disturbing the cells, and then centrifuged. Finally, the liquid sample was filtered using 0.2 µm nylon membrane filter paper (Whatman). Some further experiments without pH adjustment gave similar results (data not shown). Even if the pH was not maintained, the pH did not fall below 5 under our experimental conditions and the yeast was equally effective as when pH was adjusted. The further fermentations were run on glucose only, without pH adjustments and without any additional nutrients because Na+ and nutrients were suspected to affect the performance of the catalyst during aqueous phase reforming.

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Figure 2. Case study of hydrogen production from lignocellulosic biomass.

Figure 3. Experimental setup for aqueous phase reforming of the 5% ethanol and fermentation sample.

All the fermentation samples were analyzed with HPLC (Shimadzu) equipped with a refractive index detector (RID-10 A), using an Aminex 87H column (Bio-Rad Laboratories, Inc.). The mobile phase was 0.5 mM H2SO4 at a flow rate of 0.6 mL/min and 65 °C. The standard compounds used for calibration were glucose (Sigma, 99.5%), ethanol (200 proof, ACS/USP grade, Pharmco Products Inc.), acetic acid (Fisher Scientific, assay 100%), glycerol (Fisher Scientific, enzyme grade), and succinic acid. Aqueous Phase Reforming. Aqueous phase reforming was carried out in a 1.2 cm o.d. tubular flow reactor, using the experimental setup shown in Figure 3. The APR experiments were carried out at 250 °C, 600 psi. The results obtained during aqueous phase reforming of the fermented samples were compared with those from a solution of 5% ethanol in deionized water, close to the concentration of ethanol in the fermentation sample, which was 49 g/L. The reactor tube was heated using an external heating tape (BIH051-040LD, Thermolyne BriskHeat). The reaction tem-

perature was monitored using a K-type thermocouple inserted into the reactor tube and located immediately downstream of the catalyst bed. The temperature was controlled using an Omega temperature controller (SSRDIN660DC25 and CNi16D44C4EI). Product gases were analyzed using a mass spectrometer (Pfeiffer Vacuum, OmniStar, Model GSD 301 C2). The liquid products were characterized using HPLC (Shimadzu) and TOC (Shimadzu TOC-VCPN). The catalyst used in this study was prepared by the wet impregnation method. Platinum(II) acetylacetonate (Strem Chemicals) was used for the impregnation. Tetrahydrofuran (THF), obtained from Fisher Scientific and used as received, was used as the impregnation solvent. MI-386 support was obtained from Grace-Davison, courtesy of Catacel Corp. After impregnation, the catalyst was calcined in air at 300 °C for 2 h, and then reduced at 300 °C for 2 h using 5% H2 in a N2 mixture. The fresh catalyst sample and the samples used in aqueous phase reforming were characterized using X-ray diffraction (X’pert PRO, PANalytical, Serial Number DY2134) to examine the

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possible surface morphology change. The Pt loading on Al2O3 support was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES; Perkin-Elmer, Plasma II emission spectrometer). Results and Discussion The fermentation of glucose to ethanol can be summarized as

C6H12O6 f 2C2H5OH + 2CO2 One mole of glucose is converted into 2 mol of ethanol and 2 mol of carbon dioxide through several metabolic reaction steps, through an enzymatic process that includes the transfer of energy through adenosine triphosphate (ATP). Although pyruvic acid and acetaldehyde are produced as intermediate products, fermentation is very selective toward ethanol production. Figure 4 shows glucose and ethanol concentrations in the fermentation broth as a function of reaction time. After 24 h of fermentation, an ethanol concentration of 48.6 g/L, or more than 95% of theoretical yield, was observed. During experiments with S. cereVisiae but without addition of yeast extract or pH control, ethanol yield of 45 g/L after 24 h of fermentation was still obtained, indicating that these added components did not substantially enhance the performance of the fermentation step. Therefore, subsequent fermentation experiments were performed without addition of yeast extract and without pH control. HPLC analysis of the final sample indicates that, in addition to ethanol, glucose was converted to small quantities of succinic acid (1.08 g/L), glycerol (4.68 g/L), and other oxygenated hydrocarbons, as expected for nonselective fermentation.19 The final fermentation broth also contained very small amounts of unreacted glucose (0.26 g/L). The product from the fermentation reaction is primarily ethanol, the reforming of which can be written as

C2H5OH + 3H2O f 6H2 + 2CO2 When other oxygenated hydrocarbons are present in the fermentation broth, they can also be converted to hydrogen through a similar reforming reaction. Thus, in the case of the fermentation broth, the overall yield of hydrogen is calculated relative to the glucose feed to the fermenter. Results from APR are provided in terms of H2 yield, defined as

% yield )

Fi (νi)(Fj,0)

× 100

where Fi is the moles of species i produced per minute, νi is the stoichiometric coefficient of species i, and Fj,0 is the molar flow rate of the reactant in moles per minute. For the systems in which glucose was used as a feed, the overall H2 yield (total process, from glucose) and the H2 yield from ethanol measured in the fermentation broth will both be obtained. This measure of yield describes both the conversion of the reactant and the selectivity for the reforming reaction, and is thus believed to be an appropriate measure of the catalyst effectiveness. Aqueous phase reforming was first evaluated for a pure 5% (by weight) ethanol feed, corresponding to 49.3 g/L ethanol; close to 100% theoretical yield when the starting feed for fermentation is 100 g/L glucose. Table 1 shows the distribution of products and byproducts obtained during aqueous phase reforming experiments conducted at 250 °C, 600 psi. Hydrogen was the dominant gas product, with significant amounts of CH 4

Figure 4. Liquid product analysis of fermentation of 100 g/L glucose by yeast. Table 1. Summary of Aqueous Phase Reforming Experiments Conducted at 250 °C and 600 psi Using 5% Pt Supported on Al2O3 product distribution, mol/mol of ethanol reacted experiment ethanol EtOH, pH 10 (NaOH) EtOH, pH 10 (buffer) fermentation

H2 ethanol (mol %) (% X) 46.06 54.26 54.23 52.63

90.81 73.88 50.42 31.02

H2 1.97 1.44 1.46 0.29

CO CO2 CH4

acetic acid

0.08 0.12 0.04 0.05

0.03 0.08 0.07 0.05

0.98 0.45 0.54 0.06

1.23 0.64 0.65 0.04

and CO2 also present. However, formation of methane is unavoidable due to a simultaneous methanation reaction taking place at the catalyst surface. Even under steam reforming conditions and very high temperature (900 °C), CH4 selectivities were reported20 to be in the range of 10-20%. However, CO was only present in a very small amount, a result of the water gas shift reaction that promotes the formation of CO2 at these reaction temperatures. Combining the composition results with conversion data indicates that hydrogen yield was about 2535%, and stable for more than 47 h of reaction time. Additional analysis of the liquid products was obtained using HPLC and revealed the presence of acetic acid as the only other major byproduct. Next, the fermentation sample was tested in the APR. In this experiment, a feed containing 5% ethanol in water was run for the first 20 h, after which the feed was switched to the fermentation sample. The H2 production was stable in the case of the ethanol feed (demonstrated for more than 20 h in different experiments under similar conditions). However, H2 yield decreased rapidly when the feed was switched to fermentation broth, and within 10-12 h of reaction time, the H2 yield became zero, as shown in Figure 5. Even after the feed was switched back to the original pure ethanol feed, the H2 yield remained near zero. In a further effort to determine if the catalyst deactivation was reversible, an attempt was made to regenerate the catalyst by flowing air through the reactor system for 2 h at 300 °C. Although H2 yield after the regeneration was lower than for the fresh catalyst, the yield was substantially enhanced relative to that obtained prior to regeneration, and the yield remained stable for approximately 25 h. This could confirm the fact that the catalyst deactivation in the case of fermentation broth was due to adsorption of fermentation impurities on catalyst surfaces. Once those impurities were removed in the regeneration process, the original catalyst activity was recovered. Although stable conversion of the fermentation sample was not obtained, the products of the reaction were quantified.

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Figure 5. H2 yields obtained during reaction-regeneration-reaction cycle of aqueous phase reforming of 5% ethanol in water and fermentation sample, all at 250 °C and 600 psi using platinum on alumina catalyst.

Figure 7. Effect of pH on hydrogen productivity during APR of pure ethanol and fermentation sample. Squares represent data points for feed in which pH was adjusted to 10, while triangles represent data points for feed condition in which pH adjustment was not done. In both the cases filled symbols represent pure ethanol feed while open symbols represent fermentation sample.

Figure 6. Effect of ionic strength of Na+ ion on hydrogen productivity during APR of ethanol.

Similar gas-phase results were obtained for APR of the fermentation broth and the ethanol feed, with substantial yields of H2, CO2, and CH4, and virtually no production of CO. The exit stream from APR of the fermentation sample did not show any of the components found in the fermentation broth (glycerol, succinic acid, or glucose), but showed unconverted ethanol and a peak corresponding to acetic acid. Analysis of Catalyst Deactivation. Generally, the pH in a fermentation broth is maintained constant by addition of sodium hydroxide, as the performance of the microorganisms is optimal within a specific pH range.21 Therefore, it is pertinent to study the effect of Na+ ion and its ionic strength on the hydrogen production rates from the APR. The effect of ionic strength of Na+ ion on the hydrogen productivity was studied using different buffer combinations at pH 10. The experiments were conducted using 5% ethanol at 250 °C, 600 psi, and 5% platinum on alumina catalyst. The different base combinations used to adjust the pH of the 5% ethanol in water samples were 0.003 M NaOH solution, 0.02 M NaOH/0.04 M NaHCO3, and 0.067 M NaOH/ 0.133 M NaHCO3. The ionic strength, calculated according to

1 I ) ∑(CiZ2) 2 was 0.003, 0.06, and 0.2, respectively. As shown in Figure 6, increasing ionic strength had an adverse effect on the hydrogen production during APR at pH 10 and 250 °C. It was seen that, as the ionic strength of the Na+ ion was increased at the same pH, the H2 yield decreased substantially. The above experiments were done at constant pH 10 with varying ionic strength of Na+ ions. However, further experiments were needed to evaluate the effect of pH on catalyst performance. Thus, an experiment was conducted without addition of Na+ ion, for which the pH of the 5% ethanol-water solution was measured to be around 3.84. Results for these experiments are shown in Figure 7, which indicates higher levels of H2 yield at lower pH, but the same deactivation of the catalyst during APR of the fermentation sample. X-ray diffraction (XRD) analysis of the fresh and used catalysts was done in order to see if there were any changes in

Figure 8. X-ray diffraction analysis of fresh 5% Pt/Al2O3 catalyst and used catalyst. Table 2. BET Surface Area Analysis of the Catalyst Samples Used for the APR Experiments feed

catalyst

reaction time (h)

surf. area (m2/g)

ethanol ethanol ethanol/fermentation ethanol/buffer

support only 5% Pt/Al2O3 5% Pt/Al2O3 5% Pt/Al2O3

24.0 51.5 18.0a/33.5b 39.5a/19.0b

13.75 2.65 10.38 1.01

a Reaction time for simulated sample. b Reaction time for fermentation sample. Total catalyst exposure time under APR condition is addition of reaction times for simulated sample and fermentation sample.

the catalyst support. The “used catalyst” was the catalyst sample used during APR of the fermentation broth and collected at the end of the experiment. It was observed that the oxidation state of the platinum remained the same throughout the reaction. However, the alumina support underwent a significant phase change, as shown in Figure 8, which compares the used catalyst with fresh catalyst and with pure alumina support. The original γ-Al2O3 phase changed to a boehmite phase (AlO(OH)) during the course of the reaction. This phase change of alumina at high temperature and pressure under aqueous environment was unexpected, but had previously been observed by Yoshimira et al.22 However, because this phase change was observed for the catalyst used during APR of both the 5% ethanol feed and the fermentation feed, we could not conclude that this was the reason for the catalyst deactivation in the case of fermentation samples. The Brunauer-Emmett-Teller (BET) surface area of the catalyst samples was measured to provide a quantitative relation between the final surface area of the catalyst, its activity, and the phase change observation; these results are summarized in Table 2. The surface area of fresh catalyst was measured as 160.58 m2/g; thus all materials underwent substantial surface area loss through the reaction process. In one experiment, Al2O3 support without any active platinum was used in place of catalyst, under the reaction conditions used for the APR of pure

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Table 3. Inductively Coupled Plasma Atomic Emission Spectroscopic (ICP-AES) Analysis of the Catalyst Samples Used in Aqueous Phase Reforming Experiments catalyst sample fresh fermentation experimenta 5% ethanol, pH 10 (NaOH)a 5% ethanol, pH 10 (buffer)a a

theor value (%) measd value (%) std dev 5 5 5 5

5.0867 3.4967 3.5033 3.3833

0.1795 0.1358 0.0404 0.0153

Spent catalyst samples used under the reforming conditions reported.

ethanol. The surface area of the used support was seen to decrease similarly to the catalyst samples used in the APR experiments. The final surface area appeared to be primarily a function of the length of exposure time; there was no observable difference in the surface area loss with reaction of fermentation broth or the use of buffer. Thus, we are unable to conclude that deactivation was related to loss of surface area. An alternative mechanism of catalyst deactivation during APR could be leaching of Pt metal from the catalyst surface by the hot aqueous solution. In order to test this hypothesis, the concentration of Pt on the catalyst was determined using ICPAES. A sample of used catalyst was dissolved in aqua regia, and the results are provided in Table 3. The fresh catalyst showed approximately 5% Pt/Al2O3 (same as the theoretical value), which was used for each of these experiments. However, the ICP-AES analysis showed that the Pt concentration of the used catalyst was about ∼3.5%, regardless of reaction conditions or feed. This drop in Pt concentration occurred under all conditions, and may be related to the incorporation of water in the support and the observed phase change. In order to confirm that Pt leaching did not occur, the liquid samples that were collected were analyzed and found to have no measurable Pt content. Thus, our analysis of the catalyst deactivation observed during APR of the fermentation broth indicated that this could not be attributed to fundamental changes in the catalyst, through either phase change, loss of surface area, or reduction in the platinum concentration. However, the fermentation broth is known to contain additional species not present in the 5% ethanol sample, including sulfur, nitrogen, and phosphorus containing compounds. A series of preliminary experiments have been completed which indicate that APR of the fermentation broth can be successfully completed if the broth is first subjected to nanofiltration, eliminating all larger molecules derived from the biological system, and producing an aqueous organic solution containing only low molecular weight species. As a result, we believe that these biologically derived molecules, particularly those containing phosphorus and sulfur, interact with the catalyst to cause the observed deactivation, and we are exploring this in more detail. Conclusions The integrated biological and thermochemical approach for hydrogen production has many potential advantages over either of the processes conducted individually. Preliminary results from aqueous phase reforming combined with fermentation demonstrated that hydrogen can be successfully produced from glucose. The fermentation process has been optimized to yield a product broth that contains mostly ethanol near its theoretical limit, but also additional organic acids. When this broth was fed to the APR reactor operating with a Pt/Al2O3 catalyst, H2 yields of 25-35% are initially obtained. Although current hydrogen yields are low, improved catalyst formulations can be designed or modified reaction conditions can be used to increase the yield.

Unfortunately, the catalyst was observed to deactivate rapidly during APR of the fermentation sample. The Al2O3 support underwent a phase change through reaction with water and a loss of available surface area, but experiments with a pure ethanol feed indicated that these were not the cause of the deactivation. Nanofiltration of the fermentation broth produced a feed that could be reformed without catalyst deactivation, but further work is required to identify the specific components present in the broth that lead to catalyst deactivation. Even with the limitations existing in the current process, an energy analysis of the proposed integrated process for the production of hydrogen from biomass reveals that it can be more efficient than the conversion to ethanol. Modifying the NREL process design to incorporate APR of the fermentation broth demonstrated potential energy efficiency near 60%, greater than the 53% efficiency calculated for the biomass to ethanol process. In addition, analysis of the energy utilization suggests that higher overall efficiencies can be obtained from a process that produces hydrogen for use with a fuel cell than from one that produces ethanol for use with an internal combustion engine. Acknowledgment This research was supported through Grant DE-FG3605GO85025 from the Department of Energy. Additional funding in support of this research was received from the Ohio Department of Development through the Wright Fuel Cell Group. M.A.A. is pleased to be able to participate in this special issue in honor of Prof. Bruce Nauman, his department chair upon his graduation with a B.S. from RPI. Literature Cited (1) Dunn, S. Hydrogen futures: toward a sustainable energy system. Int. J. Hydrogen Energy 2002, 27, 235-264. (2) Das, D.; Veziroglu, T. N. Hydrogen production by biological processes: a survey of literature. Int. J. Hydrogen Energy 2001, 26, 1328. (3) Benemann, J. R. Feasibility analysis of photobiological hydrogen production. Int. J. Hydrogen Energy 1997, 22, 979-987. (4) Kruse, O.; Rupprecht, J.; Mussgnug, J. H.; Dismukes, G. C.; Hankamer, B. Photosynthesis: a blueprint for solar energy capture and biohydrogen production technologies. Photochem. Photobiol. Sci. 2005, 4 (12), 957-970. (5) Zhang, H.; Bruns, M. A.; Logan, B. E. Biological hydrogen production by clostridium acetobutylicum in an unsaturated flow reactor. Water Res. 2006, 40 (4), 728-34. (6) Hallenbeck, P. C. Fundamentals and limiting processes of biological hydrogen production. In Biohydrogen III, Renewable Energy System by Biological solar energy conVersion; Rogner, M., Igarashi, Y., Asada, Y., Jun Miyake, J., Eds.; Elsevier Science: New York, 2004. (ISBN-13: 9780080443560.) (7) Nandi, R.; Sengupta, S. Microbial production of hydrogen: an overview. Crit. ReV. Microbiol. 1998, 24, 61-84. (8) Levin, D. B.; Pitt, L.; Love, M. Biohydrogen production: prospects and limitations to practical application. Int. J. Hydrogen Energy 2004, 29, 173-185. (9) Huber, G. W.; Iborra, S.; Avelino, C. Synthesis of transportation fuels from biomass: Chemistry, Catalysts, and Engineering. Chem. ReV. 2006, 106 (9), 4044-4098. (10) Swami, S. M.; Abraham, M. A. Integrated catalytic process for conversion of biomass to hydrogen. Energy Fuels 2006, 20 (6), 26162622. (11) Evans, R. J.; Milne, T. A. Molecular characterization of the pyrolysis of biomass. Energy Fuels 1987, 1 (2), 123-137. (12) Cortright, R. D.; Davada, R. R.; Dumesic, J. A. Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002, 418 (6901), 964-966. (13) Cortright, R. D. Hydrogen generation from biomass-derived carbohydrates via the aqueous phase reforming (APR) process. DOE hydrogen program reView; Arlington, VA, 2005.

Ind. Eng. Chem. Res., Vol. 47, No. 10, 2008 3651 (14) Breen, J. P.; Burch, R.; Colemn. H. M. Metal-catalysed steam reforming of ethanol in the production of the hydrogen for fuel cell application. Appl. Catal., B: EnViron. 2002, 39 (1), 65-74. (15) Davada, R. R.; Shabakar, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Aqueous-phase reforming of ethylene glycol on silicasupported metal catalysts. Appl. Catal., B: EnViron. 2003, 43 (1), 13-26. (16) Aden, A.; Ruth, M.; Ibsen, K.; Jechura, J.; Neeves, K.; Sheehan, J.; Wallace, B. National Renewable Energy Laboratory, June 2002, NREL/ TP-510-32438. (17) http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Sarah_Carter/efficiency.html (accessed Sept 2007). (18) Ahlvik, P.; Brandberg, A. Well-to-wheel efficiency for alternative fuels from natural gas or biomass. Ecotraffic ERD3 AB; Publication No. 2001:85, 2001. (ISSN 1401-9612.) (19) Eva, A.; Christer, L.; Gunnar, L.; Claes, N.; Lena, G. Continuous estimation of product concentration with calorimetry and gas analysis during anaerobic fermentation of Saccharomyces cerevisiae. Thermochim. Acta 2002, 394 (1-2), 185-190.

(20) Wanat, E. C.; Venkataraman, K.; Schmidt, L. D. Steam reforming and water-gas shift of ethanol on Rh and Rh-Ce catalysts in a catalytic wall reactor. Appl. Catal., A: Gen. 2004, 276 (1-2), 155-162. (21) Nielsen, M. K.; Arneborg, N. The effect of citric acid and pH on growth and metabolism of anaerobic Saccharomyces cereVisiae and Zygosaccharomyces bailii cultures. Food Microbiol. 2007, 24 (1), 101105. (22) Yoshimura, C.; Noguchi, H.; Doi, M. High-temperature hydrothermal treatment of anodic coatings on alumina. Plat. Surf. Finish. 1987, 74 (6), 72-75.

ReceiVed for reView June 30, 2007 ReVised manuscript receiVed October 6, 2007 Accepted October 10, 2007 IE070895P