Hydrogen from glucose: A combined study of glucose fermentation, bioethanol purification, and catalytic steam reforming

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Hydrogen from glucose: A combined study of glucose fermentation, bioethanol purification, and catalytic steam reforming Nestor Sanchez a, Ruth Yolanda Ruiz b, Bernay Cifuentes a, Martha Cobo a,* a

Energy, Materials and Environmental Group, Department of Chemical Engineering, Universidad de La Sabana,
, Colombia Campus Universitario Puente del Comu´n, Km. 7 Autopista Norte, Bogota b Agroindustrial Process Group, Agroindustrial Process Engineering, Universidad de La Sabana, Campus
, Colombia Universitario Puente del Comu´n, Km. 7 Autopista Norte, Bogota

article info

abstract

Article history:

A technical analysis of hydrogen (H2) production by coupling fermentation, distillation, and

Received 8 November 2015

reforming is reported. A glucose solution (300 g/L glucose) was first fermented using a

Received in revised form

commercial Saccharomyces cerevisiae strain at 30
C. The fermented samples were then

27 January 2016

distilled, giving bioethanol with a steam/ethanol ratio of 3:1. Several synthetic and real

Accepted 28 January 2016

bioethanol samples were subjected to catalytic steam reforming (SR) over Rh0.4Pt0.4/CeO2.

Available online xxx

At 700
C, real bioethanol samples produced the highest H2 yield (2.6 mol H2/mol inlet)

Keywords:

tween ethanol and fermentation byproducts, which increased H2 production. The catalyst

compared to synthetic samples (0.8e1.6 mol H2/mol inlet), due to a synergistic effect beAliphatic compound

was stable over 20 h during the SR of bioethanol at 700
C with no appreciable carbona-

Fermentation

ceous deposition. Overall mass and energy balances confirmed that this process produced

Hydrogen

8.2 mol H2/mol glucose, and could produce 5092 kJ/kg glucose as useful energy work output

Raw bioethanol

in fuel cell applications.

RhPt/CeO2

Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Steam reforming

Introduction Fossil fuels are currently the most abundantly used energy source [1]. However, fossil fuels are non-renewable, expensive, and finite energy source that generate a range of harmful compounds, such COx, NOx, SOx, and methane (CH4). In contrast, when hydrogen (H2) is obtained from renewable sources, its application in fuel cells can be considered a sustainable and renewable energy source, since fuel cells

reserved.

generate a greater power output than internal combustion engines when using the same amount of fuel. Moreover, no environmental issues have been associated with the use of hydrogen in fuel cell applications [2]. According to Fig. 1, syn-gas containing elevated concentrations of H2 (stream 7) could be obtained from biomass by engaging biological and catalytic processes. Generally, biomass feedstock is pretreated by either acidic or enzymatic hydrolysis to breakdown complex sugars (e.g., lignin, hemicellulose, and starch) into simple sugars (i.e., glucose and

* Corresponding author. Tel./fax: þ57 1 861 5555x25207. E-mail addresses: [email protected] (N. Sanchez), [email protected] (R.Y. Ruiz), [email protected] (B. Cifuentes), [email protected] (M. Cobo). http://dx.doi.org/10.1016/j.ijhydene.2016.01.155 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sanchez N, et al., Hydrogen from glucose: A combined study of glucose fermentation, bioethanol purification, and catalytic steam reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.155

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Fig. 1 e H2 production from biomass: combined biological and catalytic process (delimited zone corresponding to the experimental data obtained in this research).

fructose) [3]. Hence, glucose can be selected as a model compound to study this process. As shown in Table 1, sugars are transformed into raw bioethanol by fermentation (Eq. (1)) using different microorganisms (i.e., bacteria and yeast). Saccharomyces cerevisiae is preferred at an industrial level, due to its durability, high ethanol yield, and high ethanol tolerance [3]. S. cerevisiae transforms glucose into ethanol and carbon dioxide by the Embden-Meyerhof-Parnas (EMP) pathway [4]. As previously optimized by Hu et al. [5], the final composition of ethanol after fermentation can exceed 15 vol% when using very high gravity (VHG) fermentation. The produced raw bioethanol (Fig. 1, stream 4) can also contain traces of a range of organic compounds generated by other metabolic pathways involving glucose and amino acids [10]. These pathways can produce a wide variety of compounds [11e13], with previous studies reporting a number of these compounds as being problematic for downstream bioethanol steam reforming (SR) (e.g., 1-propanol, 2-methyl-1propanol, 1-butanol, and 3-methyl-1-butanol, and ethyl acetate [14,15]). Distillation of this raw bioethanol would therefore be necessary to remove water, sugars, nutrients, and heavier fermentation byproducts, before further H2 production by SR. Distillation is preferred over other techniques (e.g., gas stripping, ozonation, pervaporation), as it can be easily applied at an industrial level. Raw bioethanol is frequently purified in a complex distillation process to break down the azeotrope and produce pure ethanol, in a process involving

high energy consumption [8]. In contrast, according to Fig. 1, a steam/ethanol molar ratio of 3:1 (Stream 6, S/E ¼ 3:1, ethanol concentration ¼ 50 vol%) can be obtained in a simple flash distillation. In Fig. 1, we propose the transformation of ethanol into H2 by steam reforming (Table 1, Eq. (2)). A S/E ¼ 3:1 (Stream 6) are sufficient according to the SR stoichiometry. Larger S/E ratios have been reported as positive in the SR of ethanol due to the increase in H2 yield by the dual effect of the SR and WGS reactions (Table 1, Eqs. (2) and (4)) [7,8]. In addition, water excess can promote carbon deposition removal in supports with elevated oxygen storage capacity (OSC), due to water activates the oxygen mobility from the support to let carbon gasification [16]. However, water excess would also increase the energy required to heat the feed (Fig. 1, steam 6) until the SR reactor temperature (usually > 500
C) because of the elevated water heat capacity. Rossetti et al. [17] reported that the higher the S/ E ratio, the lower the process overall electrical efficiency, because the additional energy to heat up the excess of water would come from the fuel cell electrical output. Therefore, we selected a bioethanol feed with the stoichiometric S/E ¼ 3:1 in order to maximize the overall electrical efficiency. Table 1 lists the main equations/reactions for the SR process. Due to its complex nature, several byproducts are produced during the bioethanol SR and travel along with H2 in the stream 7 of Fig. 1 (e.g., carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethylene (C2H4), and acetaldehyde (CH3CHO)) [7,8]. To accomplish the quality requirements

Table 1 e Network of equations for hydrogen production from glucose. Description General fermentation reaction Desirable ethanol steam reforming Undesirable ethanol steam reforming Water gas shift reaction Higher alcohol steam reforming Higher alcohol steam reforming CO-PROX

Reaction

Eq.

Ref.

C6 H12 O6 /2CO2 þ 2C2 H5 OH C2 H5 OH þ 3H2 O/2CO2 þ 6H2 C2 H5 OH þ H2 O/2CO þ 4H2 CO þ H2 O#CO2 þ H2 Cn H2nþ1 OH þ ð2n  1ÞH2 O/nCO2 þ 3nH2 Cn H2nþ1 OH þ ðn  1ÞH2 O/nCO þ 2nH2 2CO þ O2 /2CO2

1 2 3 4 5 6 7

[6] [7,8] [7,8] [7] [8] [8] [9]

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specified by ISO 14687-2 for proton exchange membrane (PEM) fuel cell applications, it is mandatory to produce high H2 concentrations with low CO contents [18]. Consequently, a subsequent purification of the stream 7 is often required. For example, preferential oxidation of CO (CO-PROX, Eq. (8)) has been widely used to eliminate the remaining CO, which can poison the fuel cell [9,18]. In addition, complete CO conversion can be achieved at ~120
C [19,20]. A number of recent studies have focused on H2 production from different feedstocks. Dan et al. [13] studied the production of H2 from fir wood using Ni/La2O3eAl2O3 and Ni/CeO2e Al2O3 as catalysts. Both catalysts were active at 350
C, giving complete ethanol conversion. However, the catalyst suffered a degree of deactivation at lower temperatures due the oxidation of Ni to the inactive NiO. Rass-Hansen et al. [21] studied steam reforming of Danish wheat straw over Ni/MgAl2O4 and Ru/MgAl2O4 catalysts, concluding that bioethanol produced faster catalyst deactivation than ethanol-water mixtures, due to the presence of aliphatic compounds, which lead to carbon deposition on the catalyst surface. Le Valant et al. [15] found that synthetic bioethanol containing 1 vol% of different aliphatic alcohols (methanol, 1-propanol, 1-butanol, 1pentanol, isopropanol, 2-methyl-1-propanol, and 3-methyl1-butanol) significantly affected the Rh/MgAl2O4/Al2O3 catalyst during the SR. A detrimental effect on the H2 yield was observed in the order C5>C4>C3, while methanol exhibited a positive effect on the SR, increasing the H2 yield [14,15]. In contrast, Vargas et al. [22] evaluated H2 production from sugar cane molasses over CeeZreCo oxide catalysts, finding that higher alcohols have no effect on catalytic behavior. However, weak metal-support interactions and carbon filament deposition deactivated the catalyst, thus reducing bioethanol conversion. Therefore, catalyst deactivation and the loss of selectivity over time are of great and continuing concern in terms of H2 production from bioethanol. Regardless of the metal and support used, stability remains a challenge for catalyst design in bioethanol reforming. Noble metals are the preferred catalyst candidates for the SR of bioethanol due to their greater ability to break CeC bonds. The use of bimetallic catalysts is advantageous in allowing modification of the electronic properties of metal surfaces [23]. Among the noble metals, Rh and Pt, respectively, exhibited high activity for ethanol CeC bond breakage, and realizing the catalytic water gas shift (WGS, Eq. (4)) conversion [7,24]. Hence, a particularly resistant catalyst might be expected. However, the catalyst support can also influence the activity and stability of the catalyst [25]. Due to the low metal loading when using noble metals (typically <1%), the direct participation of the support on the reaction mechanism is favored [23]. The OSC is a critical property to favor C removal and to avoid deactivation, especially when lower S/E ratios are employed [25]. Thus, higher oxygen storage capacity and oxygen mobility allow a more effective gasification/oxidation of adsorbed carbon on the surface. Cerium oxides have shown promising results as supports for the SR because of their oxygen mobility, ability to disperse the metal phase, and strong metal-support interactions that can minimize the sintering of metal particles [26]. Previous studies conducted in our laboratory have shown that RhPt/CeO2 with metal loads of 0.4%Rh and 0.4%Pt is a suitable catalyst for the SR of ethanol-water mixtures [27,28].

3

We herein aimed to evaluate the overall process shown in Fig. 1, by performing the tests for the delimited zone, to obtain H2 from glucose, a model sugar molecule. Raw bioethanol was obtained by the fermentation of glucose using a commercial S. cerevisiae strain. The effect of the byproducts produced during fermentation (ethyl acetate, 1-propanol, 2-methyl-1-propanol, 1-butanol, and 3-methyl-1-butanol) on the steam reforming of bioethanol over a bimetallic 0.4%Rh-0.4%Pt/CeO2 catalyst was assessed. Catalytic activity and yield were compared between 400
C and 700
C, with a steam/ethanol ratio 3:1. The stability of the Rh0.4Pt0.4/CeO2 catalyst in the SR of bioethanol samples was evaluated by 20 h time-on-stream (TOS) at 700
C. Thermogravimetric analysis (TGA) of all catalysts was performed to identify carbonaceous deposition. Finally, overall mass and energy balances for the complete process of Fig. 1 allowed the effectiveness of the process to be determined.

Experimental Fermentation tests Yeast activation and inoculum preparation Dry S. cerevisiae yeast cells (Fermentis, Ethanol Red®, France) were rehydrated in distilled water according to supplier instructions. This S. cerevisiae strain is used at an industrial level, due to its stability, high ethanol yields, and high ethanol tolerance. Activation and subsequent inoculum preparation were carried out based on the method reported by JoannisCassan et al. [29] with some modifications. Activation was carried out over 30 min at 30
C. Rehydrated yeast was conserved in Potato Dextrose Agar (PDA, Merck, Germany) until the experiments were carried out. Active yeast was incubated in YPD broth (Yeast extract 10 g/L, peptone 20 g/L, glucose 20 g/L, pH 5.50 ± 0.05) in a shaker (Innova 48®, USA) at 30
C and 200 rpm to give an initial yeast concentration of 107 colony forming units (CFU) per mL.

Bioethanol production Raw bioethanol production was carried out in a 4 L bioreactor at 30
C and 200 rpm over 62 h. An optimized synthetic glucose medium (SGM), previously reported by Hu et al. [5], was used as the nutrient solution (glucose 300 g/L, peptone 21.5 g/L, yeast extract 15 g/L, MgSO4.7H2O 1.5 g/L, Ca3PO4 0.5 g/L). The medium was sterilized at 120
C over 20 min. The bioreactor was then loaded with the nutrient solution (2 L) and adjusted to pH 5.5 and 10 vol% of the inoculum solution. Glucose was quantified using the 3,5-dinitrosalicylic (DNS) method in a Lambda 750 UV/Vis/NIR Spectrophotometer (Perkin Elmer, USA). All measurements were recorded at 540 nm [30]. Biomass was quantified using the dry weight method [31]. A portion (1 mL) of each sample was centrifuged at 12,000 rpm for 6 min at 4
C (Z216 MK, Hermle, Germany). This procedure was repeated twice. Between each centrifugation, the pellets were washed with distilled water. Finally, all pellets were dried at 80
C over 36 h.

Ethanol and aliphatics quantification Ethanol and aliphatic compounds that are detrimental to catalyst performance in bioethanol SR (i.e., 1-propanol, 1-butanol,

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ethyl acetate, 3-methyl-1-butanol, and 2-methyl-1-propanol) [15] were quantified using a Clarus 580 Gas Chromatograph (GC, Perkin Elmer, USA), equipped with an Elite-Wax ETR column (60 m, 0.25 mm ID, Perkin Elmer, USA) connected to a Flame Ionization Detector (FID). Both detector and injector temperatures were adjusted to 200
C. The oven temperature was adjusted to 80
C over 15 min. The limit of detection of the technique was determined as 40 ppm.

Distillation Bioethanol samples were distilled in a rotary evaporator (Heidolph 2, Germany) at 0.2 bar and 70
C for 30 min to give a steam/ethanol ratio of 3:1 (S/E ¼ 3). Both the distillate and the bottoms products were quantified by GC as described in Section Ethanol and aliphatics quantification. A simulation of the flash distillation process was carried out using Aspen Plus® V7.2 (USA) to determine the energy required in the raw bioethanol purification. The non-random two-liquid (NRTL) model was used as the thermodynamic method. Initial conditions were adjusted according to the results obtained during fermentation.

Catalytic tests Catalyst synthesis Catalyst samples were prepared by the incipient wetness coimpregnation method. Active metals were obtained from rhodium(III) chloride hydrate (RhCl3.H2O) (SigmaeAldrich, USA) and chloroplatinic acid hexahydrate (H2PtCl6.6H2O) (SigmaeAldrich, USA) solutions in water. The CeO2 support was obtained from the calcination of cerium nitrate hexahydrate (99.5%, Alfa Aesar, USA), at 700
C in a muffle oven for 2 h. The amounts of active metals were co-impregnated onto the support to obtain 0.4 wt% Rh and 0.4 wt% Pt. We previously reported that this Rh:Pt ratio promotes a higher H2 yield and complete ethanol conversion [27,28]. Co-impregnation was achieved by constant stirring of the metal-support mixture,

drying at 100
C for 24 h and subsequent calcination in air at 700
C for 2 h. In all cases, the calcined solids were sieved using an 80-mesh to ensure a particle size of less than 177 mm.

Steam reforming reactions The catalytic performance of Rh0.4Pt0.4/CeO2 was evaluated in a fixed bed reactor at atmospheric pressure. The catalyst particles (100 mg) were diluted by mixing with quartz particles (200 mg, 80-mesh). The catalyst mixture was loaded into a quartz tube reactor (12 mm ID) with a fused frit equidistant from both ends of the tube. Before the reaction, the catalysts were reduced in 10% H2/N2 (300 mL/min) at 700
C for 1 h. Different samples, each containing a specific concentration of ethanol, water, and other contaminants (see Table 2) were pumped continuously into the heated reaction system using a Simdos 02 metering pump (KNF Neuberger, USA). These samples were prepared according to the results from the fermentation and distillation processes. The vaporized reactants were combined with the incoming N2 (diluent and internal standard) and passed through the catalyst bed. For each test, the EtOH:H2O:N2 molar ratio was adjusted to 1:3:51 (stoichiometric S/E ¼ 3 mol ratio) with a total gas flow of 333 mL/min. Reaction conditions were optimized to obtain a desirable plug flow reactor (PFR) performance when synthetic ethanolwater mixtures were reformed (E-C, Table 2). To achieve this, back mixing and channeling were avoided when a ratio of 45:1 was maintained between the catalyst bed height and catalyst particle size (L/Dp), and when a ratio of 60:1 was maintained between the reactor internal diameter and catalyst particle size (D/Dp) [32,33]. Each catalytic activity test was conducted at a gas hourly space velocity (GHSV) of 70,600 h1 at atmospheric pressure between 400
C and 700
C at 20
C increments (continuous sequence, 25 min at each temperature). In addition, using the conditions described above, time-on-stream (TOS) SR catalytic stability tests were conducted over 20 h at 700
C for D-B-ST and E-C-ST samples listed in Table 2.

Table 2 e Composition of the ethanol liquid streams subjected to SR, and the Rh0.4Pt0.4/CeO2 catalyst TGA results. Sample namea R-Bd D-B D-B-ST E-C E-C-ST E-EA E-1P E-2M1P E-3M1B S-B FC a

b c d

Type of stream

Real Real Real Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Synthetic Fresh catalyst

Concentration (mol%) Ethanol

Water

EA

1P

2M1P

1B

3M1B

4.60 24.58 24.58 25.00 25.00 25.00 25.00 25.00 25.00 25.00 e

82.39 75.39 75.39 75.00 75.00 74.99 74.99 74.99 74.98 74.94 e

0.003 0.011 0.011 e e 0.011 e e e 0.011 e

e 0.006 0.006 e e e 0.006 e e 0.006 e

0.006 0.012 0.012 e e e e 0.012 e 0.012 e

0.004 0.013 0.013 e e e e e e 0.013 e

0.008 0.022 0.022 e e e e e 0.022 0.022 e

Otherb

Weight loss (mg carbon/gcat h)c

12.44 e e e e e e e e e e

e 0.67 0.00 1.42 0.00 0.59 0.12 0.26 0.40 0.24 0.06

Sample names: R-B: Raw bioethanol; D-B: Distilled bioethanol; D-B-ST: Distilled bioethanol used during the stability test; E-C: Ethanol:water mixture used as control; E-C-ST: Ethanol:water mixture used in the stability test; EA: Ethyl acetate; 1P: 1-propanol; 2M1P: 2-methyl-1propanol; 1B: 1-butanol; 3M1B: 3-methyl-1-butanol; S-B: Synthetic bioethanol; and FC: Fresh catalyst. Calculated from mass balance. From TGA. R-B sample was not reformed, only quantified.

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The compounds in the product stream, including H2, CH4, CO, CO2, ethanol, and ethylene, were quantified online using a Clarus 580 Gas Chromatograph (GC, Perkin Elmer, USA), equipped with a Carboxen 1010 plot column (30 m, 0.53 mm ID, Restek, USA) connected to a thermal conductivity detector (TCD), along with an Innowax column (30 m, 0.53 mm ID, Perkin Elmer, USA) connected to an FID. Elemental balances between the inlet ethanol-water feed and the outlet products were measured in all tests. The test was declared effective when the elemental carbon balance was ~100%. The ethanol conversion (XEtOH), H2 yield (YH2) (mol H2/mol inlet), and molar distribution (yi) for each detected product were calculated according to Eqs. (8)-(10) below; where NEtOH,0 is the molar flow (mol/min) of the ethanol inlet, NEtOH,F is the unreacted molar flow (mol/min) of ethanol in the product stream detected by GC, and Nj is the molar flow of the component j in the product stream, which can be CO, CO2, CH4, or C2H4.
XEtOH ¼ ½NEtOH;0  NEtOH;F  NEtOH;0

(8)

YH2 ¼ ½mol H2 out=½mol inlet

(9)

yj ¼ Nj

.X

Nj

5

energy balance was to compare the amount of energy delivered by either bioethanol or by H2 as a fuel. The energy available (DHAV ) from each fuel was first estimated according to Eq. (15). The lower heating value (LHV) for ethanol was 26,870 kJ/kg [39] and for H2 was 120,210 kJ/kg [40]. The net energies ðDHT Þ of both ethanol and H2 were then estimated according to Eqs. (16) and (17), respectively. Furthermore, Eqs. (18)e(24) were used to estimate the net energy of both fuels. The standard enthalpy of reaction for SR and CO-PROX were 173 kJ/mol and 283 kJ/mol, respectively [8]. Finally, the useful energy work output ðDHEW Þ in an engine was calculated according to Eq. (25), where h represents efficiency. Only ~20% of the chemical energy stored in the ethanol could be used as useful mechanical work in an internal combustion engine [38]. In contrast, proton exchange membrane fuel cells (PEMFCs) can reach an electrical efficiency of ~50% [2]. DHAV ¼ mFuel LHVFuel

(15)

DHT;Et ¼ DHAV þ DHS

(16)

DHT ; H2 ¼ DHAV þ DHS þ DHR þ DHC þ DHCOP

(17)

(10) DHR ¼ r2 DHTR þ NSR F

X

  X SR xSR xSR i;F Cpi dT  N0 i;0 Cpi dT

(18)

Catalyst characterization TGA/DTG (thermogravimetric analysis/derivative thermogravimetric analysis) was performed using a thermogravimetric analyzer (TGA1, Mettler Toledo, USA). Each sample (30 mg) was pretreated in pure N2 at 100
C and subsequently heated to 1000
C in air (5
C/min, 100 mL/min flow rate) to burn off the deposits. Weight loss was expressed as mg carbon gcat1 h1 according to the method reported by Osorio-Vargas et al. [34].

Overall mass and energy balances An overall mass balance of the process was carried out to calculate the mol H2/mol fermented glucose as an indicator of the total process yield [35], based on the stability test product distribution. In addition, the total CO2 produced during the process was estimated as shown in Eq. (11). The amount of CO2 produced during fermentation was estimated by solving the equation system shown in Eqs. (1) and (11)e(14) [36]. The yield of ethanol calculated based on the substrate (YP/S) is included in this calculation (Eq. (12)) [37]. In the following equations, n is the mole number, r1 is the reaction progress (mol), YP/S is the yield of ethanol from glucose, nFCO2 is the mol CO2 from fermentation CO-PROX is the estimated estimated using Eqs. (11)e(14), and nCO 2 mol CO2 released in the CO-PROX reaction. F COPROX nTotal CO2 ¼ nCO2 þ nCO2

(11)

 
 in  mGlucose  mout YP=S ¼ mout Ethanol Glucose

(12)

r1 ¼

nout Ethanol 2

2r1 ¼ nFCO2

(13) (14)

The energy balance was performed according to Akande et al. [38], as it is shown in Eqs. (15)e(25). The purpose of the

r2 ¼ XEt NSR Et;0

(19)




DHTR ¼ DHR þ

X

sSR i Cpi dT

X

   xFi Cpi dT þ mW Cpw dT  DHVap ðTrefÞ  X x0i Cpi dT  mSG;0

DHC ¼ mSG;F

(20)

(21)

X  xCOP DHCOP ¼ r3 DHTCOP þ NCOP F i;F Cpi dT X  xCOP  NCOP 0 i;0 Cpi dT

(22)

r3 ¼ XCO NCOP CO;0

(23)




DHTCOP ¼ DHCOP þ

X

sCOP Cpi dT i

DHEW ¼ hDHT

(24) (25)

Results and discussion Production of bioethanol suitable for catalytic steam reforming Bioethanol was produced from glucose fermentation in a 4 L bioreactor, using a medium previously optimized by Hu et al. [5]. Table 2 shows the bioethanol concentration after 62 h fermentation (R-B sample). The obtained ethanol concentration was 4.6 mol% (~15 vol%), which is slightly lower than that reported by Hu et al. [5] (17.1 vol%). The total conversion of glucose measured by the DNS method was 95%, with 72.4% of this glucose being transformed into ethanol, the rest was converted into other components, which include water, CO2,

Please cite this article in press as: Sanchez N, et al., Hydrogen from glucose: A combined study of glucose fermentation, bioethanol purification, and catalytic steam reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.155

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and other aliphatic compounds [41]. The five aliphatic components shown in Table 2 were followed during fermentation and distillation, as they have been reported to have a negative effect on the SR of bioethanol [15]. The concentrations of these components were <300 ppm (Table 2), with the most abundant being 3-methyl-1-butanol followed by 2-methyl-1propanol, 1-butanol, and ethyl acetate. In addition, a wide variety of components have been reported during fermentation, such as acetaldehyde, methanol, acetic acid, lactic acid, citric acid, succinic acid, glycerol, butyric acid, isovaleric acid, hexanoic acid, isoamyl acetate, phenylethyl acetate, ethyl butyrate, ethyl hexanoate, and ethyl octanoate [11e13]. Some of these compounds could be present in concentrations below the limit of detection. From mass balance, it was possible to determine that 12.4 mol% of the R-B stream corresponded to yeast, residual glucose, and salts added to the fermentation medium. Batch distillation using a rotary evaporator at 0.2 bar and 70
C over 30 min led to the desirable S/E ¼ 3:1 (D-B stream, Table 2). Water and fermentation residues (others in R-B stream) were concentrated in the distillation bottoms. Distillate fraction (D-B) concentrates the lightest compounds. Indeed, 1-propanol was not detected in the raw bioethanol sample (R-B), but was observed following distillation, due to an increase in its concentration. The effect of these aliphatic contaminants in the bioethanol SR is analyzed in the following section.

Hydrogen production by bioethanol steam reforming Catalytic activity As batch distillation process do not completely eliminate contaminants from the bioethanol sample, it is important to determine the effect of the compounds that are known to have a detrimental effect on the SR. Thus, each sample listed in Table 2 was subjected to SR from 400
C to 700
C over the Rh0.4Pt0.4/CeO2 catalyst. Fig. 2a shows the temperature vs. ethanol conversion profile of each ethanol-containing sample. Reaction conditions were set up for the synthetic ethanol/

water sample (E-C, Table 2), to obtain a standard lighteoff curve, represented by an S-shaped pattern. This refers to the temperature range at which mass transfer effects are diminished under certain reaction conditions [42,43]. However, the other samples examined did not display this S-shaped curved, suggesting that the contaminants promoted complete ethanol conversion in this temperature range. A similar result was reported by Dou et al. [44], who found that crude glycerol (glycerol, fatty acid methyl esters, and methanol) conversion was higher than pure glycerol conversion during the SR over a NiO/Al2O3 catalyst. Nevertheless, ethanol conversion is related to ethanol SR for H2 production only above 500
C (Fig. 2b). As SR is an endothermic reaction, it is favored at high temperatures [8]. Below 500
C, the presence of aliphatic components in the stream likely promoted alterna
n et al. [45] suggested that tive pathways instead of SR. Remo the presence of impurities in the alcohol solution could lead to reactions between the impurities either prior to or during the SR, thus generating variations in catalytic activity and selectivity [46]. In general, the yield of H2 for all synthetic ethanol streams containing impurities presented a similar behavior over the temperature range studied (Fig. 2b). In addition, aliphatic compounds exhibited a positive effect on the H2 yield at 700
C when compared to the E-C sample. Increased chain length in the aliphatic compounds thermodynamically favored H2 production. Indeed, the highest H2 yield was obtained during the SR of the raw, distilled bioethanol sample (D-B), with a maximum inlet of 2.6 mol H2/mol inlet. The aliphatic compounds in this sample may therefore be responsible for the increase in H2 yield from the bioethanol sample. Similar results were reported by Vargas et al. [22] in the SR of bioethanol from sugar cane molasses. Their sample tended to produce higher H2 yields than both ethanol-water mixtures and synthetic ethanol mixtures (ethanol, 1-propanol, and higher alcohols), due to reforming of the alcohols following the distillation process. Thus, it appeared that the fermentation byproducts present in the real, distilled bioethanol samples (D-B), improved

Fig. 2 e (a) Ethanol conversion, and (b) H2 yields for different ethanol-containing samples (see Table 2) as a function of temperature over the Rh0.4Pt0.4/CeO2 catalyst. Reaction conditions: S/E ¼ 3/1 and GHSV ¼ 70,600 h¡1. Please cite this article in press as: Sanchez N, et al., Hydrogen from glucose: A combined study of glucose fermentation, bioethanol purification, and catalytic steam reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.155

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both ethanol conversion and H2 yield during catalytic SR over Rh0.4Pt0.4/CeO2. Hence, the catalyst stability during the SR of this sample was compared to the ethanol-water mixture (E-C) SR. The results were analyzed as follows.

Stability test The stability of the Rh0.4Pt0.4/CeO2 catalyst was evaluated by 20 h TOS at 700
C for the D-B and E-C streams (Table 2). This temperature was selected to maximize H2 yield and accelerate possible catalyst deactivation. Fig. 3 shows the ethanol conversion and product distribution during the steam reforming of both ethanol-containing samples. During the tests, total ethanol conversion was maintained for both streams with a reaction rate of 4E-5 mol s1 gcat1, showing that catalyst deactivation did not take place. A similar result was achieved by Le Valant et al. [14] when reforming ethanol with 2-methyl1-propanol as an impurity using an RheNi/YeAl2O3 catalyst with 24 h TOS at 675
C and 2 bar. For bioethanol (D-B) SR, the H2 concentration was higher than the thermodynamic limit (dashed line in Fig. 3a), suggesting that the fermentation byproducts may be responsible for the H2 yield increase in the bioethanol sample. During the initial 2 h, an increase in H2 concentration was observed, reaching a maximum molar concentration of 0.78, which remained constant for 20 h. The presence of alcohols, esters, and aldehydes in the D-B sample produced more intermediate reaction products than the reference sample through either dehydrogenation or dehydration of these components [8]. The H2 concentration for the D-B sample (Fig. 3a) was therefore higher and less variable than that of the E-C sample (Fig. 3b). The dehydrogenation of higher alcohols present in the D-B sample could result in its higher H2 concentration. CeO2 support promotes dehydrogenation instead of dehydration [34], and so the ethoxy-species adsorbed on the catalyst can be decomposed into CHx species. Subsequently, the dehydrogenation/hydrogenation of the CHx species produces H2, CO2, and CO. This accounts for the increase in H2 concentration and decreasing CH4 concentration during the 20 h TOS for D-B SR (Fig. 3a). The E-C SR (Fig. 3b) showed a slight increase in CH4

7

content, while the H2 concentration dropped, thus giving higher CO and CO2 concentrations than D-B SR. The observed differences between the two samples appear to be due to CHx species dehydrogenation/hydrogenation, water activation, and the oxidation of *C species [23]. Activation of water is a vital step in SR due to the fundamental role of the *OH species in subsequent oxidation steps. Following cleavage of the CeC bond and water activation at high temperatures, CO adsorbed on the metal is oxidized to give CO2 according to the watergas-shift reaction (WGSR, Eq. (4)). As a result, specific H2, CO2, and CO profiles are obtained for each sample. As more H2 and CO2 were obtained with the D-B sample, the presence of other aliphatic compounds in the sample promote reaction of ethoxy-species adsorbed on the catalyst though the WGSR pathway involving water molecules to produce H2 and CO2. This behavior increased with time. In contrast, the desorption of ethoxy-species upon reforming of the E-C sample favors CH4 and CO production. The latter mechanism can produce additional carbon deposition, as it inhibits oxidation of the *C species [23]. We will discuss this in the following section.

Carbon deposition Carbonaceous deposition on the catalyst surface was evaluated by TGA under air. Table 2 shows the TGA results of both the fresh and used catalysts after activity and stability tests. Among the streams subjected to activity tests, carbon deposition followed the order E-C > D-B > E-EA > E-3M1B > E2M1P > E-1P. The E-C catalyst sample presented a larger weight loss, which suggests the ethoxy-species desorption pathway, as discussed in Section Stability test. Among the contaminated samples, the ethyl acetate contained in E-EA sample was likely degraded into ethoxy and acetyl groups, thus leading some to carbon deposition [47]. Higher alcohols (i.e., 1-propanol, 2-methyl-1-propanol, and 3-methyl-1butanol) were likely degraded by dehydrogenation into their corresponding aldehyde, followed by some aldol condensation to produce heavier components, thus producing carbon deposition [48]. It is well known that the redox properties of ceria supported catalysts can allow for the alcohol

Fig. 3 e Ethanol conversion and product distribution (H2, CO, CO2, and CH4) as a function of time-on-stream (TOS) for the (a) D-B, and (b) E-C samples (see Table 2). Reaction conditions: 700
C, S/E ¼ 3, and GHSV ¼ 70,600 h¡1. Please cite this article in press as: Sanchez N, et al., Hydrogen from glucose: A combined study of glucose fermentation, bioethanol purification, and catalytic steam reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.155

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dehydrogenation and the following aldol condensation [49]. Alcohol dehydrogenation was evidenced by the increasing on H2 yield in the presence of aliphatic alcohols (see Fig. 2), which once transformed into aldehydes, promote H2 yield and ethanol conversion [14,50]. Other carbon deposition mechanisms have been reported in different supports. For instance, Le Valant et al. [15] reported that the presence of aliphatic components allow for the 1%Rh/MgAl2O4/Al2O4 catalyst deactivation after 8 h TOS at 675
C and 2 bar due to the formation of stable carbocations which facilities olefin formation, yielding coke and resulting in deactivation of the catalyst. They proposed this mechanism based on the evidence of H2 yield reduction in the presence of higher alcohols in the bioethanol. Differences in carbon deposition mechanisms could be ascribed to their acidic support nature, which favor dehydration instead dehydrogenation reactions of the adsorbed reactants. S-B catalyst sample presented less carbon deposition than the D-B sample during the activity test, likely due to the presence of other unidentifiable compounds on the D-B sample. However, both H2 yield and ethanol conversion were higher in the D-B sample than the S-B sample. Hence, the presence of additional aliphatic compounds in the D-B sample promotes H2 formation while consequently increasing carbon deposition on the catalyst. Nevertheless, the observed weight loss was lower for the D-B sample than for the E-C sample, likely due to a difference in reaction mechanism through presence of the aliphatic compounds, as discussed in Section Stability test. Overall, carbon deposition was lower than in previous reports. For example, Akande et al. [38] reported a weight loss of 2.5 mg carbon/h over a Ni/Al2O3 catalyst for the bioethanol SR (ethanol 12 vol%, lactic acid 1 vol%, glycerol 1 vol%, maltose 0.001 vol%, water 86 vol%) at 400
C. Low temperature is known to favor the dehydration of alcohols to olefins, which are coke precursors. Thus, Rass-Hansen et al. [21] reported a weight loss of 0.05 mg carbon/h over Ni/ MgAl2O4, 0.041 mg carbon/h over NieK/MgAl2O4, and 0.021 mg carbon/h over Ru/MgAl2O4 at 600
C. These catalysts were employed for the SR of bioethanol (ethanol 44 mol%, ethyl acetate 0.5 mol%, 1,1-diethoxyethane 0.2 mol%, trace constituents: propanol, 2-methyl-1-propanol, 3-methyl-1butanol, and 2-methyl-1-butanol), with similar results reported to our Rh0.4Pt0.4/CeO2 catalyst, where the coke formation rate was 0.020 mg carbon/h. In this way, noble metals have an important incidence on the low carbon formation [23]. Therefore, Rh0.4Pt0.4/CeO2 is a stable material for the steam reforming of bioethanol samples. As this catalyst is a basic material, alcohol dehydration is disfavored, thus avoiding olefin production and further carbonaceous deposition on the catalyst surface [25]. Finally, TGA was performed on both aged catalyst samples to evaluate carbonaceous deposition over the catalyst surface after 20 h TOS at 700
C (D-B-ST and E-C-ST samples, Table 2). No appreciable changes in the weight of either sample were observed. As reported by Zanchet et al. [23], the control of carbon accumulation depends on the availability of surface oxygen required to oxidize the *C species. In contrast to base metals, carbon nucleation is energetically unfavorable on noble metal surfaces, thus reducing the solubility of carbon on the latter materials. The equilibrium between ethanol

activation and carbon removal is therefore key, and was obtained with our Rh0.4Pt0.4/CeO2 catalyst at 700
C. In SR, the presence of reactants with high redox potentials (e.g., H2, CO, and H2O) as well as the metal-support interaction tunes the availability of surface oxygen based on the gas composition and temperature [23]. Hence, it was possible to obtain a more active and resistant catalyst by employing both a lessoxophilic metal (Pt) and a more-oxophilic metal (Rh), thus promoting rapid activation of ethanol and aliphatics by two different pathways, i.e., alcohol dehydrogenation via a-C-H and OeH activation, respectively. The use of CeO2 as a support likely enhanced the oxygen storage capacity and oxygen mobility, allowing a more effective gasification/oxidation of adsorbed carbon on the surface [27]. In addition, the presence of low amounts of aliphatic compounds favored the SR pathway through water activation. Although an active catalyst is mandatory for the development of this technology, overall mass and energy analysis is necessary to elucidate its feasibility. This will be discussed in the following Section Mass and energy balances.

Mass and energy balances Mass balance An overall mass balance to obtain energy from biomass was performed according to Fig. 1. Results are shown in Table 3 for when 1000 g (1 kg) of glucose was employed as the calculus base. In addition, the complete conversion of CO during the CO-PROX purification step was assumed at 120
C [19]. According to these results, 95% glucose was converted into bioethanol. Overall, 8.2 mol H2 is produced for each mole of glucose entering the process (H2 in stream 9/glucose in stream 1), while the 30% hydrogen entering the process (H in glucose þ water) is recovered as H2 for used as fuel. This indicates that combination of the fermentation-distillationreforming processes is an effective technology for H2 production. Hu et al. [47] studied direct glucose steam reforming (Eq. (26)) over Ni/Al2O3, finding that coke deposition is a grave problem during the process, due to glucose instability at elevated temperatures. C6 H12 O6 þ 6H2 O/12H2 þ 6CO2

(26)

Glucose steam reforming leads to a higher carbonaceous deposition rate (50 mg carbon gcat1 h1) compared to the bioethanol SR (0.67 mg carbon gcat1 h1 over Rh0.4Pt0.4/CeO2, Table 2), due to the tendency of glucose to polymerize at elevated temperatures. Therefore, a blockage of the catalyst is achieved after 20 min TOS [47]. To prevent this, a high S/C ratio is required during glucose reforming to reduce carbon deposition and avoid operating issues. However, the presence of larger amounts of water in the feed requires higher energy consumption. In terms of hydrogen yield, glucose SR delivered 0.77 mol H2/mol H2 in glucose [47] using a Ni/Al2O3 catalyst at 700
C, 1 atm, and with a S/C ¼ 6. This is lower than the hydrogen yield reported here (1.37 mol H2/mol H2 in glucose). The role of S. cerevisiae on the degradation of glucose into bioethanol further improves H2 production by catalytic steam reforming. Without this biotechnological step, the catalyst

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Table 3 e Overall mass balance to obtain energy from biomass. Stream names are as outlined in Fig. 1. Stream

1

2

3

4

5

6

7

8

9

10

11

Temperature (
C)

25

25

30

30

93

93

700

92

92

120

120

1000.0 0.0 0.0 0.0 2197.6 123.6 0.0 0.0 0.0 0.0 3321.2 5.6 0.0 0.0 0.0 122.1 20.5 0.0 0.0 0.0 0.0 148.2 3.75 0.00 0.00 0.00 82.40 13.86 0.00 0.00 0.00 0.00 100.00

0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.00 100.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 100.00

0.0 0.0 334.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 334.0 0.0 0.0 7.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 7.6 0.00 0.00 100.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 100.00

50.6 16.3 0.0 1.1 2447.2 123.6 349.1 0.0 0.0 0.0 2987.8 0.3 0.6 0.0 0.0 136.0 20.5 7.6 0.0 0.0 0.0 165.0 0.17 0.39 0.00 0.01 82.39 12.44 4.60 0.00 0.00 0.00 100.00

50.6 16.3 0.0 0.3 2038.5 123.6 8.6 0.0 0.0 0.0 2237.8 0.3 0.6 0.0 0.0 113.2 20.5 0.2 0.0 0.0 0.0 134.9 0.21 0.48 0.00 0.00 83.95 15.22 0.14 0.00 0.00 0.00 100.00

0.0 0.0 0.0 0.7 408.7 0.0 340.6 0.0 0.0 0.0 750.0 0.0 0.0 0.0 0.0 22.7 0.0 7.4 0.0 0.0 0.0 30.1 0.00 0.00 0.00 0.03 75.39 0.00 24.58 0.00 0.00 0.00 100.00

0.0 0.0 132.3 0.0 179.8 0.0 0.0 91.3 346.5 0.0 750.0 0.0 0.0 3.0 0.0 10.0 0.0 0.0 45.7 12.4 0.0 71.0 0.00 0.00 4.23 0.00 14.07 0.00 0.00 64.28 17.42 0.00 100.00

0.0 0.0 0.0 0.0 179.8 0.0 0.0 0.0 0.0 0.0 179.8 0.0 0.0 0.0 0.0 10.0 0.0 0.0 0.0 0.0 0.0 10.0 0.00 0.00 0.00 0.00 100.00 0.00 0.00 0.00 0.00 0.00 100.00

0.0 0.0 132.3 0.0 0.0 0.0 0.0 91.3 346.5 0.0 570.2 0.0 0.0 3.0 0.0 0.0 0.0 0.0 45.7 12.4 0.0 61.0 0.00 0.00 4.93 0.00 0.00 0.00 0.00 74.80 20.28 0.00 100.00

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 198.0 198.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 6.2 6.2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 100.00 100.00

0.0 0.0 676.9 0.0 0.0 0.0 0.0 91.3 0.0 0.0 768.2 0.0 0.0 15.4 0.0 0.0 0.0 0.0 45.7 0.0 0.0 61.0 0.00 0.00 25.20 0.00 0.00 0.00 0.00 74.80 0.00 0.00 100.00

Mass (g) Glucose Yeast CO2 Aliphatics Water Salts Ethanol H2 CO O2 Total mol Glucose Yeast CO2 Aliphatics Water Salts Ethanol H2 CO O2 Total mol% Glucose Yeast CO2 Aliphatics Water Salts Ethanol H2 CO O2 Total

can deactivate easily, and so the use of this yeast is essential for improving the SR process. As shown in streams 3 and 7 in Fig. 1, CO2 is released during both fermentation and CO-PROX purification. The total CO2 produced in fermentation was 0.96 kg CO2/kg ethanol, which is close to the values reported from life cycle assessments on ethanol production from sugarcane in Nepal (0.51 kg CO2/kg ethanol), India (0.55 kg CO2/kg ethanol), and Brazil (0.37 kg CO2/kg ethanol) [51]. Recently, studies into fermentationbased biohydrogen from CH4 reforming reported values of approximately 5 kg CO2-equivalents/kg H2, which is lower than the 12 kg CO2-equivalents/kg H2 reported for natural gas [52]. However, these calculations include both CO2 production during transport and distribution, and CO2 consumption in farming, which were not considered in this study. The total CO2 production for the process shown in Fig. 1 is 3.14 kg CO2/

kg ethanol or 7.41 kg CO2/kg H2. During fermentation, 30% CO2 is produced, while 70% is released during the CO-PROX process. Although there is a mitigation potential in biomass growth, this CO2 should be captured to control greenhouse gas emissions into the atmosphere.

Energy balance Table 4 shows the possible energy produced when employing either ethanol or H2 as fuel, based on 1 kg of fermented glucose. In this comparison, ethanol could be used in an internal combustion engine, as opposed to H2, which would be used in a fuel cell. Based on the lower heating value of ethanol, it possesses 9377.6 kJ of available energy ðDHAV Þ for release during combustion. The distillation of bioethanol (15 vol%) to anhydrous ethanol (99.9 vol%) requires at least 5000 kJ/kg ethanol [53], with 1745.7 kJ being required for the separation process ðDHS Þ. The

Table 4 e Energy efficiencies of ethanol and hydrogen for use as fuels in combustion engines and fuel cells, respectively. 1 kg of fermented glucose as calculus base. Fuel Ethanol H2

(LHV) (kJ/kg)

mFuel (kg)

DHAV (kJ)

DHS (kJ)

DHR (kJ)

DHC (kJ)

DHCOP (kJ)

DHT (kJ)

h (%)

DHEW (kJ)

26,870 120,210

0.349 0.091

9381 10,976

1746 882

NA 1641

NA 1791

NA 61

7636 10,183

20 50

1527 5092

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net energy value ðDHT Þ delivered by ethanol as a fuel must therefore be applied to the internal combustion engine, with average efficiency (h) of 20% [38], giving a useful energy work output ðDHEW Þ of 1526.5 kJ/kg glucose. Furthermore, the possible energy produced by H2 in a fuel cell when 1 kg of glucose is fermented is shown in Table 4. H2 possesses 12,386 kJ of available energy ðDHAV Þ. Moreover, during the distillation of raw bioethanol (15 vol%) to distilled bioethanol (D-B, S/E ¼ 3:1, Table 2), the energy consumption is 295 kJ/kg (obtained from Aspen Plus® V7.2 simulation). Thus, 1368 kJ is required during the separation process ðDHS Þ, while 1641 kJ is required ðDHR Þ during the SR at 700
C. In addition, water condensation prior to CO-PROX delivers 1791 kJ ðDHC Þ, while the CO-PROX step ðDHCOP Þ requires 61 kJ. Thus, the total net energy would be DHT ¼ 10,183 kJ. When multiplying this net energy ðDHT Þ by an average PEMFC efficiency of 50% [2], H2 delivers a useful energy work output of DHEW ¼ 5092 kJ/kg glucose, which is 3.3  higher than that delivered by ethanol in internal combustion engines. We have therefore demonstrated that it is feasible to produce H2 by combining biological and catalytic processes to give good yields, catalyst stability, and energy efficiency. This process can be considered feasible due to the following characteristics: (1) Higher quantities of H2 are produced by the process described in Fig. 1 compared to direct glucose steam reforming; (2) The catalyst is active during 20 h TOS at 700
C with no carbon deposition detected over the catalyst surface during this time; and (3) H2 delivers higher energy for use in fuel cells than ethanol delivers in internal combustion engines.

Conclusions We report a technical analysis of hydrogen (H2) production by coupling fermentation, distillation, and reforming. The production of H2 from glucose involving biological, separation, and catalytic processes is a suitable technology, producing up to 8.2 mol H2/mol glucose. In this process, bioethanol can be obtained from glucose fermentation (S. cerevisiae, 200 rpm, 30
C, pH 5.50), and purified in a flash distillation (rotary evaporator, 70
C, 0.2 bar, 100 rpm, 30 min). The distilled bioethanol can subsequently be reformed over a Rh0.4Pt0.4/CeO2 catalyst, giving complete ethanol conversion, a stable H2 yield of 2.6 mol H2/mol inlet, and no carbonaceous depositions over 20 h time-on-stream (TOS) at 700
C. The produced H2 is suitable for use in a fuel cell, and delivers a useful energy work output of 5092 kJ/kg glucose, which is approximately three times more than ethanol delivers in internal combustion engines. However, this process can produce up to 7.41 kg CO2/kg H2, which must be captured to avoid release into the atmosphere. Future work will be focused on testing the catalyst for the H2 production from sugarcane industry wastes and integrating the process in a pilot plant.

Acknowledgments The authors are grateful to the Universidad de La Sabana for the financial support of this work through the project ING-135.

Abbreviations nF, CO2 r1 YP/S nCO-PROXCO2 D-B 1B XCO CFU DTG DNS h DHAV DHC DHR DHCOP DHTCOP DHTR XEt NSR Et;0 EA FID FC GHSV Cpi xCOP i;0 NSR 0 NCOP 0 mSG;0 xSR i;0 x0i LHVFuel mFuel mW 3M1B 2M1P DHT NRTL NCOP F xCOP i;F xSR i;F NSR F mSG;F xFi OSC PFR PDA 1P PEM PEMFC R-B r3 r2 DHS ST

CO2 from fermentation Fermentation reaction progress (mol) Yield of ethanol from glucose mol CO2 released in the CO-PROX reaction Bioethanol after distillation 1-Butanol CO conversion Colony forming units Derivative thermogravimetric analysis 3,5-Dinitrosalicylic Efficiency Energy available Enthalpy of condensation Enthalpy of reaction (SR) Enthalpy of reaction (COP) Enthalpy of reaction (CO-PROX) at 120
C Enthalpy of reaction (SR) at 700
C Ethanol conversion Ethanol inlet molar flow to SR Ethyl acetate Flame ionization detector Fresh catalyst Gas hourly space velocity Heat capacity of component i Inlet mol fraction of component i for the COPROX Inlet molar flow (SR) Inlet molar flow rate for the CO-PROX process Inlet syn-gas mass Inlet mol fraction of component i for the SR Inlet syn-gas mol fraction of component i Lower heating value Mass of fuel produced Mass of water condensed 3-Methyl-1-butanol 2-Methyl-1-propanol Net energy of fuel Non-random two-liquid Outer molar flow rate for the CO-PROX process Outlet mol fraction of component i for the COPROX Outlet mol fraction of component i for the SR Outlet molar flow (SR) Outlet syn-gas mass Outlet syn-gas mol fraction of component i Oxygen storage capacity Plug flow reactor Potato dextrose agar 1-Propanol Proton exchange membrane Proton exchange membrane fuel cell Raw bioethanol after glucose fermentation Reaction progress (CO-PROX) Reaction progress (SR) Separation energy Stability test

Please cite this article in press as: Sanchez N, et al., Hydrogen from glucose: A combined study of glucose fermentation, bioethanol purification, and catalytic steam reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.155

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DHCOP
DHR SR sCOP i sSR i SGM TCD TGA TOS DHEW VHG YPD WGS WGSR

Standard enthalpy reaction of the CO-PROX Standard enthalpy of reaction for the SR Steam reforming Stoichiometric coefficient of component i for the CO-PROX Stoichiometric coefficient of component i for the SR Synthetic glucose medium Thermal conductivity detector Thermogravimetric analysis Time-on-stream Useful energy work output Very high gravity Yeast, peptone, glucose Water gas shift Water gas shift reaction

[13]

[14]

[15]

[16]

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Please cite this article in press as: Sanchez N, et al., Hydrogen from glucose: A combined study of glucose fermentation, bioethanol purification, and catalytic steam reforming, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.01.155