Biomass and Bioenergy 22 (2002) 405–410

Catalytic hydrogen generation from biomass(glucose and cellulose) with ZrO2 in supercritical water

Masaru Watanabe, Hiroshi Inomata ∗, Kunio AraiResearch Center of Supercritical Fluid Technology, Department of Chemical Engineering, Tohoku University 07, Aoba, Aramaki,

Aoba-ku, Sendai 980-8579, Japan

Received 17 April 2001; received in revised form 17 December 2001; accepted 17 December 2001

Abstract

We conducted the batch experiments for hydrogen production from biomass (glucose and cellulose) with ZrO2 catalyst insupercritical water (673–713 K and 30–35 MPa). For comparison, we also conducted the experiments with alkali and withoutcatalyst at the same conditions. The yield of hydrogen with zirconia was almost twice as much as that without catalyst forall the starting materials (glucose and cellulose). ? 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Glucose; Cellulose; Catalyst; Alkali; Zirconia; Hydrogen generation; Biomass

1. Introduction

Generation of hydrogen from biomass is an envi-ronmentally benign method to produce energy. In or-der to generate H2 from biomass e;ectively, water isused as a suitable solvent and reactant as follows:

C6H10O5 + 7H2O → 12H2 + 6CO2: (1)

For these reasons, many researchers has conductedhydrogen generation from biomass in sub- and su-percritical water [1–4]. Yu et al. [1] reported thatthe e;ective gasiAcation of biomass must be con-ducted at low concentration of biomass because athigh concentrations of biomass polymerization of thedecomposition products occurred. However, higherconcentration of biomass must be gasiAed to be aneconomical source [2]. For this reason, Xu et al. [2]

∗ Corresponding author. Tel.: 81-22-217-7282; fax: 81-22-217-7283.

E-mail address: inomata@scf.che.tohoku.ac.jp (H. Inomata).

examined the e;ect of active carbon on the gasiA-cation of high-concentration biomass in supercriticalwater for achieving high degree of gasiAcation ofhigh-concentration biomass solution. Although theyield of hydrogen was low, biomass was completelydecomposed into gas even at a high concentrationof biomass (1:2 M). Minowa and Ogi [3] conductedthe experiments of gasiAcation of cellulose in hotcompressed water using a batch reactor and reportedthat the rate of gasiAcation was enhanced with a re-duced Ni catalyst. However, the yield of CH4 wasalso enhanced and the yield of H2 was suppressed.Kruse et al. [4] studied the e;ect of alkali (KOH) onthe gasiAcation of biomass in supercritical water. Theyield of H2 was almost three times greater with alkali(5 wt%), than without alkali. They considered that theenhancement of H2 yield by adding alkali was due tothe enhancement of water gas shift reaction (Eq. (2)):

CO + H2O� CO2 + H2: (2)

After evaluating the previous work on the high-temperature water gasiAcation of biomass [1,2,4],

0961-9534/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved.PII: S 0961 -9534(02)00017 -X

406 M. Watanabe et al. / Biomass and Bioenergy 22 (2002) 405–410

we considered the most important factors to be: (1)The rate of biomass decomposition to intermediatesthat form hydrogen has to be much greater than therate of formation of polymeric unreactive substances;(2) the rate of the water gas shift reaction has tobe promoted, and (3) the methanation of either car-bon monoxide or carbon dioxide has to be inhibited.Alkali catalyst is one of the suitable catalysts for thegasiAcation of biomass. However, the recovery of ho-mogeneous alkali catalyst such as sodium hydroxideis diLcult, and thus, the process using homogeneouscatalyst would be expensive. Therefore, a recoverableheterogeneous catalyst such as metal oxide must beselected for the development of a low cost process.Recently, we found that zirconia, which is a solidbase catalyst [5] and stable in supercritical water[6], is an e;ective catalyst for the decomposition ofcarbonyl compound such as carboxylic acid in super-critical water [7]. Since the decomposition of biomasssuch as glucose and cellulose in supercritical waterproduces aldehydes and ketones [8,9] intermediates,the yield of H2 could be promoted using zirconia asa catalyst for gasiAcation of biomass in supercriticalwater.In this paper, the experiments of gasiAcation of glu-

cose and cellulose were conducted with and withoutcatalyst (NaOH and zirconia) using a batch reactor.The e;ect of catalyst on the gasiAcation of biomass insupercritical water was discussed.

Table 1Experimental conditions

Run Reactanta Temperature Reaction time Water density Catalyst(no.) (K) (min) (g=cm3)

1 Glucose 673 15 0.35 —2 Glucose 673 15 0.35 ZrO2

b

3 Glucose 673 15 0.35 NaOHc

4 Glucose 713 10 0.2 —5 Glucose 713 10 0.2 ZrO2

b

6 Glucose 713 10 0.2 NaOHc

7 Cellulose 673 15 0.35 —8 Cellulose 673 15 0.35 ZrO2

b

9 Cellulose 673 15 0.35 NaOHc

10 Cellulose 713 10 0.2 —11 Cellulose 713 10 0.2 ZrO2

b

12 Cellulose 713 10 0.2 NaOHc

aLoaded weight of reactants to a reactor is 0:1 g.bLoaded weight of zirconia to a reactor is 0:3 g.c1 M of NaOH aqueous solution was loaded to a reactor instead of water.

2. Experimental

Zirconia catalyst was prepared by calcination of zir-conium hydroxide (purchased from Nakarai TesqueInc.) at 673 K for 3 h. The catalyst was used in theform of Ane powder (67:69 m2=g of surface area). Glu-cose was obtained from Wako Chemical. Cellulose(de-ashed microcrystalline cellulose) was purchasedfrom Merck. Glucose and cellulose were used as re-ceived. Sodium hydroxide (NaOH, 1 M aqueous solu-tion) was purchased from Wako Chemicals and usedwithout further puriAcation. Pure water, which wasdistilled after deionization, was obtained by a waterdistillation apparatus (Yamato Co., model WG-220).The reaction was carried out in an SS 316 stain-

less steel tube bomb reactor with an inner volumeof 6 cm3 as described elsewhere [10]. The experi-mental procedure was also reported elsewhere [10].The loaded amounts of samples were of 0:1 g, andthe loaded amount of water was 1:2 g (0:2 g=cm3) at713 K or 2:1 g (0:35 g=cm3) at 673 K. For the exper-iments using alkali hydroxide (NaOH), 1:2 g 713 Kor 2:1 g (673 K) of 1 M NaOH solution was loadedinto a reactor instead of pure water. In the case of theexperiments with solid catalysts, 0:3 g of a catalystwas loaded. The reaction temperatures were 673 and713 K, the reaction times ranged from 10 min (713 K)to 15 min (673 K). Experimental conditions are sum-marized in Table 1.

M. Watanabe et al. / Biomass and Bioenergy 22 (2002) 405–410 407

The identiAcation and quantiAcation of the gas prod-uct was conducted by GC-TCD (Shimadzu, modelGC-7A, and Hitachi, model GC163). Water insol-ubles (including the metal oxide catalyst and coke)were divided by Altration with a membrane Alter. Theamount of carbon in the water solution was evaluatedusing total organic carbon (TOC) detector, Shimadzu,model TOC-5000A). Zirconia catalyst before and af-ter the reaction were analyzed by X-ray di;ractometer(Mac Science, model M18XHF-SRA) with Mo K�radiation.The product yield (mol%) of carbon compound was

evaluated from carbon base as shown below:

Product yield [mol%] =The amount of carbon atom in a product

The amount of carbon atom in a loaded starting material× 100: (3)

Hydrogen yield (H2 yield, mol%) was evaluated fromhydrogen base of a loaded amount of samples asfollows:

H2 yield [mol%] =The amount of hydrogen atom in a produced H2

The amount of hydrogen atom in a loaded starting material× 100: (4)

3. Results and discussion

In this study, the products, which were identiAedandquantiAed,wereH2; CO; CO2; CH4; C2–C4 hydro-carbons (C2H4; C2H6; C3H6; C3H8; C4H8; C4H10),and TOC (total organic carbon in the recoveredwater). The yields of C1–C4 hydrocarbons werenegligibly small for all the experimental con-ditions. The lack of mass balance closure, i.e.100 − (TOC + gaseous carbon (CO; CO2, and hy-drocarbons)) was presumed to be due to the waterinsoluble products which were not measured.Zirconia crystal systemwas a mixture of monoclinic

and tetragonal forms before the reaction, after reactionit was changed into almost pure monoclinic structure.As reported previously [7], the stable phase of zirconiawas assumed to be monoclinic.Figs. 1 and 2 show the gas and liquid carbon com-

pounds distribution (C distribution: CO; CO2, andTOC) and the yield of H2 from glucose decomposi-tion at 673 K for 15 min with and without catalyst,respectively. Without catalyst, the yield of CO plusCO2 was only 10% and that of H2 was about 2%. Onthe other hand, the gasiAcation eLciency of glucosewith zirconia was twice (CO + CO2: about 20%, H2:5%) as much as without catalyst in supercritical water.

In Figs. 3 and 4, the C distribution and the yield ofH2 from cellulose decomposition at 673 K for 15 minwith and without catalyst were shown, respectively.As same as the case of glucose, while the yield ofCO plus CO2 was only 15% and that of H2 was about2% without catalyst, the gasiAcation of glucose withzirconia was twice as much as without catalyst (CO+CO2: about 20%, H2: 5%) in supercritical water.The e;ect of temperature on the gasiAcation of

glucose and cellulose in supercritical water was also

examined. The reaction temperature, the water den-sity, and the reaction time were 713 K; 0:2 g=cm3,and 10 min, respectively. The results are shown in

Fig. 1. Gas and liquid carbon compound yield of glucose de-composition in supercritical water (673 K; 15 min; 0:35 g=cm3 ofwater density, and 0:1 g of glucose).

408 M. Watanabe et al. / Biomass and Bioenergy 22 (2002) 405–410

Fig. 2. H2 yield of glucose decomposition in supercriticalwater (673 K; 15 min; 0:35 g=cm3 of water density, and 0:1 gof glucose).

Fig. 3. Gas and liquid carbon compound yield of cellulosedecomposition in supercritical water (673 K; 15 min; 0:35 g=cm3

of water density, and 0:1 g of cellulose).

Figs. 5–8. Figs. 5, 6, 7 and 8 show the C distribution ofglucose gasiAcation, the yield of H2 of glucose gasiA-cation, the C distribution of cellulose gasiAcation, andthe yield of H2 of cellulose gasiAcation, respectively.

Fig. 4. H2 yield of cellulose decomposition in supercritical wa-ter (673 K; 15 min; 0:35 g=cm3 of water density, and 0:1 g ofcellulose).

Fig. 5. Gas and liquid carbon compound yield of glucosedecomposition in supercritical water (713 K; 10 min; 0:2 g=cm3

of water density, and 0:1 g of glucose).

Even at a shorter reaction time (10 min) at 713 K thanthat (15 min) at 673 K, the gasiAcation eLciency at713 K was almost the same as that at 673 K for allthe experiments of glucose and cellulose. This indi-cates that the rate of gasiAcation has enhanced with

M. Watanabe et al. / Biomass and Bioenergy 22 (2002) 405–410 409

Fig. 6. H2 yield of glucose decomposition in supercritical water(713 K; 10 min; 0:2 g=cm3 of water density, and 0:3 g of glucose).

Fig. 7. Gas and liquid carbon compound yield of cellulosedecomposition in supercritical water (713 K; 10 min; 0:2 g=cm3

of water density, and 0:3 g of cellulose).

temperature. At a higher temperature (713 K), thegasiAcation eLciency with zirconia was twice asmuch as that without catalyst for glucose and cellu-lose. Thus, the activity of zirconia was found to bestill kept even at 713 K.

Fig. 8. H2 yield of cellulose decomposition in supercritical wa-ter (713 K; 10 min; 0:2 g=cm3 of water density, and 0:3 g ofcellulose).

For comparison, we conducted the experiments us-ing alkali hydroxide (NaOH) for glucose and cellu-lose at 673 and 713 K. The results are also shownin Figs. 1–8. For all the experiments, the gasiAcationeLciency with NaOH was the highest. In the case ofNaOH, the yield of CO was negligibly small. Thus,it was assumed that CO rapidly reacted with waterto produce CO2 and H2 (water gas shift reaction: Eq.(2)) as suggested by Kruse et al. [4]. Although theyield of gaseous compounds (CO; CO2, and H2) withzirconia was lower than that with NaOH at all the ex-perimental conditions, we believe that we could showthe possibility of zirconia as a catalyst for the gasiA-cation of biomass in supercritical water.Zirconia is well known to be an acid–base catalyst

[5]. The acidity and=or basidity on the surface of zirco-nia was controlled with the structure [5,11]. Further-more, the structure and stabilization of zirconia wascontrolled by doping Yttrium (the so-called stabilizedzirconia), alkali metal [11] and so on, and thus, thecatalytic property of zirconia was also controlled. Inthe near future, we will try the same metal-doped zir-conia in the reaction, and we will make clear the re-lationship between the acid–base property of zirconiaand gasiAcation of biomass in supercritical water.

410 M. Watanabe et al. / Biomass and Bioenergy 22 (2002) 405–410

4. Conclusion

In this study, the gasiAcation of biomass (glucoseand cellulose) was conducted to examine the e;ectof zirconia by use of batch-type reactors in supercrit-ical water (0:35 g=cm3 of water density for 15 minat 673 K and 0:2 g=cm3 of water density for 10 minat 713 K). As a result, the gasiAcation eLciency ofbiomass in supercritical water with zirconia was twiceas much as that without catalyst at all the experimen-tal conditions.

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