Effect of Metals on Supercritical Water Gasification of Cellulose and

Fernando L. P. Resende and Phillip E. Savage*. Department of Chemical Engineering, University of Michigan, 2300 Hayward Street, Ann Arbor, Michigan ...
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Ind. Eng. Chem. Res. 2010, 49, 2694–2700

Effect of Metals on Supercritical Water Gasification of Cellulose and Lignin Fernando L. P. Resende and Phillip E. Savage* Department of Chemical Engineering, UniVersity of Michigan, 2300 Hayward Street, Ann Arbor, Michigan 48109-2136

We gasified cellulose and lignin in supercritical water, using quartz reactors, and quantified the catalytic effect of metals by adding them to these reactors in different forms. We used nickel, iron, copper, zinc, and zirconium wires, ruthenium powder, and Raney nickel slurry. The presence of metals was more likely to increase gas yields to a measurable extent when the catalyst surface area/biomass weight ratio was at least 15 mm2/mg (5.0 wt % biomass loading in our experiments). Nickel and copper typically provided higher gas yields at 5.0 wt % loading, and nickel provided the highest H2 yields at 1.0 wt % loading. SCWG at 500 °C with nickel at 240 mm2/mg generated 16 mmol/g of H2 from cellulose. CH4 yields were not strongly influenced by the presence of metals. With nickel wires in the reactor, gas with about 40-50% of the energy content in the original biomass was produced from cellulose and lignin. All of the metals tested except copper produced H2 from water when exposed to SCWG conditions with no biomass. It is important that this background H2 formation be accounted for when interpreting results from SCWG experiments in the presence of metals. Exposure of nickel wires to supercritical water did not reduce their activity for H2 production. Introduction Concerns about depletion and prices of fossil fuels have increased the interest in renewable sources of energy. Among them, lignocellulosic biomass appears as an interesting alternative because of its large availability. Also, using such biomass as a fuel can be part of a strategy to slow the increase of CO2 in the atmosphere. Moreover, because many potential biomass feedstocks are wastes, using them to obtain energy also helps to avoid waste accumulation. Gasification is a technology employed to break the organic structure of biomass into light gases, such as H2, CH4, CO2, and CO.1-3 Several applications are possible for this gaseous product. It can be burned to generate heat and/or power. H2 can be separated from the gaseous products for use in fuel cells. It is also possible to use the products as syngas for producing chemicals or liquid fuels. Some drawbacks prevent gasification from being used more widely in large-scale operations. Biomass feedstocks with more than 50 wt % moisture are common. With these wet materials, a large amount of energy is lost drying the feedstock, significantly decreasing the overall thermal efficiency4,5 of a gasifier. In addition, part of the carbon in the biomass is converted to char and tar3 during gasification, decreasing gas yields. Tar and char are also difficult to separate from the main gas stream.5,6 Supercritical water gasification (SCWG) has the potential to overcome these barriers. In this process, water at supercritical conditions (above 374 °C and 22 MPa) is the solvent and reaction medium. Because water is the solvent, the thermal efficiency of gasification is independent of the feedstock moisture content,7 and drying is not required.8,9 At supercritical conditions, water can have properties similar to those of organic solvents.10,11 This feature makes possible the dissolution of many organic compounds in supercritical water,6,12-14 including cellulose and lignin, the main components of biomass. The homogeneous reaction medium favors decomposition of organic compounds into gases, decreasing formation of tar and char.12,15,16 The use of catalysts in SCWG is a common strategy to achieve high gasification yields without substantially increasing * To whom correspondence should be addressed. E-mail: [email protected].

the severity of conditions (temperature, pressure). In general, catalysts increase the conversion of biomass to gases by decomposing reactive intermediates that are rapidly formed from the feed molecules by hydrolysis, pyrolysis, and dehydration. The gasification of the intermediates competes with the formation of polymeric materials and char.17 Once gas species are formed from the intermediates, a catalyst may also help by achieving fast equilibration of the water-gas shift reaction and the hydrogenation of CO and CO2 to CH4 and H2O.17,18 Catalysts are used not only to increase the rate of a desired chemical reaction (activity), but also to steer the reaction toward the desired product(s) (selectivity). Therefore, a catalyst may still be useful in the case of unfavorable thermodynamics, if reaching chemical equilibrium is not the goal.17 Metals are typical catalysts for SCWG, because they promote a high level of carbon conversion to gas at relatively low temperature.8 Elliott published a comprehensive review of the research efforts on catalytic SCWG.4 Table 1 summarizes the more active metals in SCWG according to previous studies. Nickel has been widely studied4,8,15,19-21 (also in the form of Raney nickel22). Only a few researchers mention the use of inexpensive metals such as iron and copper.23,24 Deactivation of nickel has been reported.4,17 Ruthenium6,25-28 and rhodium29 appear as good options for metal catalysts that can keep catalytic activity over longer periods of time. Other metals, such as chromium, tungsten, platinum, and palladium, have also been tested, but very low activity is reported.4 In addition to being potential SCWG catalysts, metals are also typically components of the reactor walls used in SCWG experiments where their presence can interfere with reaction rates even when no catalyst is added to the system. Lack of proper understanding of catalytic effects from reactor walls may lead to problems in the scale-up of the reactors.17 The catalytic effect of reactor walls on the products of SCWG has been the topic of previous studies.3,30-32 All of the studies in Table 1 were performed in metal reactors. The first studies of SCWG in the absence of metals were performed by Potic33 and DiLeo,34,35 using capillary quartz reactors. Potic gasified glucose, and DiLeo gasified methanol, glycine, and phenol. In previous papers,36-38 we published

10.1021/ie901928f  2010 American Chemical Society Published on Web 02/12/2010

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Table 1. Summary of Catalysts Used in SCWG catalyst nickel Raney nickel ruthenium rhodium

reactions promoted

comments 4,8,15,19-21

tar cracking, water-gas shift, methanation, hydrogenation same as Ni22 actively breaks C-C bonds6,25-28 effective to decompose benzene rings29

results for cellulose and lignin SCWG performed in quartz reactors in the absence of metals. While contamination from dissolution of quartz in SCW is still possible, at 600 °C, only about 0.04% of the reactor material would be leached out at equilibrium.39 Even though it is possible for species released to interfere with the reactions, this interference is likely to be very small. The primary contribution of the present Article is to evaluate the catalytic effect of different added metals by comparing these present results with those from the earlier metal-free experiments. This approach allows one to quantify the real effect of each metal in SCWG. We also performed a systematic study of the effect of experimental conditions (temperature, biomass loading, and water density) on catalytic activity. Experimental Methods We purchased microcrystalline cellulose powder (0.6 g/cm3, particle size 0.1 wt % +60 mesh, 70 wt % +325 mesh) and organosolv lignin (0.5 g/cm3, particle size 5.5% +40 mesh, 92.4% +100 mesh) from Sigma-Aldrich and used the materials as received. Quartz capillary tubes (2 mm i.d., 6 mm o.d., 18.0 cm length) served as mini-batch reactors. We loaded water into the reactor before the biomass, so its expansion during heating would favor mixing. We also typically inserted a metal wire (16 cm long, 0.25 mm diameter) into the reactor. The total surface area of a single catalyst wire was 40 mm2 (or 125 mm2/g). Using a catalyst in wire form is appropriate for the geometry of the reactor, because the maximum distance between any particle reacting and the catalyst is the internal diameter. Metals tested in wire form were copper, iron, nickel, zinc, and zirconium. The wires were purchased from Alfa-Aesar and Sigma-Aldrich, wiped with 320 grit sand paper (3 M Imperial Wetordry 413Q sandpaper 9 × 11 02004), and loaded into the reactors. Some catalysts of interest were not available in wire form. We purchased ruthenium as powder (∼325 mesh, Alfa Aesar) and Raney nickel as a slurry (Raney 2800 nickel, 50 wt % slurry in water, Sigma Aldrich). The amount of ruthenium needed to provide 40 mm2 was 3.6 mg. Loading only 40 mm2 of active nickel was not possible for Raney nickel because of its high surface area. Instead, we based the catalyst loading on the amount of water needed (43 µL) for the system to achieve the desired supercritical density. Therefore, 86 µL of the Raney nickel slurry was loaded to each reactor. This loading corresponds to 148 mg of catalyst, with a total surface area 1.34 × 107 mm2. SCWG was performed by placing the sealed capillary reactor either in a preheated, isothermal fluidized sand bath (Techne model SBL-2) or in a tube furnace (Type F21100 tube furnace from Barnstead Thermolyne). To collect the gases formed during SCWG, the cooled quartz reactor was first inserted into a 1/2 in. × 20 cm long metal tube, which is sealed with a 1/2 in. cap at one end and is connected to a valve at the other. Details of this experimental technique are presented elsewhere.38 N2 (present from the residual air in the reactor and metal tube) served as a standard during the gas analysis. Knowing the amount of N2 present and then determining its mole fraction

increases gas yields substantially4,8,15,19-21 provides colorless aqueous phase22 maintains activity for long time6,25-28 high activity for decomposition29

from the gas chromatographic analysis allowed us to calculate the number of moles of all other gases present. Every experiment was repeated at least once. If standard deviations were high, additional replicates were performed. In all cases, mean values are reported herein. The uncertainties shown in the graphs are standard deviations. Results The first part of this section provides results from control experiments examining the behavior of different metals in SCW, in the absence of cellulose/lignin. The second section shows the effect of temperature, water density, and biomass loading on SCWG done in the presence of inexpensive metal wires such as copper, iron, and nickel. This part of the study was aimed toward finding the conditions (within the range examined) that maximize catalytic activity. In the third section, we attempt to increase gas yields from catalytic SCWG by increasing the catalyst surface area/biomass ratio in the reactor and by testing ruthenium as a catalyst. The final section reports on the likelihood of catalyst deactivation by hydrothermal oxidation of the metal. Background Experiments. As noted in the Experimental Methods, we used nickel, iron, copper, zinc, and zirconium wires, ruthenium powder, and Raney nickel slurry as potential SCWG catalysts. To the best of our knowledge, no previous work has been reported with zirconium as a catalyst in SCWG. Elliott reports the use of zinc,4 but no useful activity is reported. Raney nickel is a porous nickel framework covered by hydrated alumina particles. With Raney nickel as a catalyst, high CH4 yields were achieved in the gasification of wood from 370 to 420 °C. Also, the water resulting was nearly tar-free.22 Metals can undergo oxidation in SCW and form metal oxides.4 A metal oxide layer on the surface of a metal wire could alter its catalytic activity. Also, oxidation of the metal can produce H2 from the water. Zinc, for example, can be used in a thermochemical cycle for producing H2 from water. Because H2 is one of the gases of interest in this study, it is important to know how much can be formed from oxidation of the metal by water alone under the experimental conditions tested for SCWG. Therefore, this section reports results for metals oxidation in the absence of cellulose and lignin. To determine the extent of hydrogen production from metal oxidation, we performed sets of experiments for 10 min at 500 and 600 °C with only water (0.08 g/cm3) and metals added to the reactors. In these tests, the zinc and zirconium wires transformed into a solid white powder inside the reactor (which matches the descriptions of ZnO and ZrO2). For zinc, the 16 cm wire inserted into the reactor corresponds to 860 µmol. If the zinc completely reacts with water, it would form 860 µmol of H2. We observed 700 µmol of H2 at 500 °C, suggesting that a large percentage of the mass of the wire was oxidized in SCW. High H2 yields were also obtained from the reactors with added zirconium and Raney nickel. At 600 °C, the H2 yield was 280 µmol from zirconium and 201 µmol from Raney nickel (the reactor had 148 mg of Raney nickel catalyst). The amount of H2 generated from SCW by these metals is much greater than the amounts that would be formed from cellulose or lignin by

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Ind. Eng. Chem. Res., Vol. 49, No. 6, 2010 Table 2. Effect of Metal, Temperature, Loading, and Water Density on Enhanced Yields for Cellulose SCWG at 10 min (Except Gw ) 0.05, 0.18 g/cm3, Where t ) 7.5 min) enhanced yield (mmol/g) T (°C)

wt %

Fw (g/cm3)

gas

Ni

Cu

500

9.0

0.08

9.0

0.08

500

9.0

0.05

1.9 -0.5 1.1 1.2 -0.6 0.1 4.5 2.0

0.3 -0.6

600

500

9.0

0.18

500

5.0

0.08

500

33.3

0.08

H2 CO CO2 H2 CO CH4 CO2 H2 CO CO2 H2 CO CH4 CO2 H2 CO CH4 CO2 H2 CO CH4 CO2

Figure 1. H2 yields from SCW and metal (10 min, 0.08 g/mL).

SCWG in our experiments. Therefore, these metals were considered no further. Figure 1 shows the H2 amounts obtained when Ni and Fe wires and Ru powder were exposed to SCW in the absence of biomass. All three materials produced about 0.5-1.5 µmol of H2 at 500 °C and even higher amounts at 600 °C. We also tested copper wires, and only these did not react with water to produce H2 at the supercritical conditions used. In some experiments with the metals and SCW, CH4 and CO2 were detected among the product gases as well. At 500 °C, 2 µmol of CO was produced from a reactor with water and ruthenium. Raney nickel produced 4 µmol of CH4 at 500 °C. At 600 °C, 12 µmol of CH4 and 6 µmol CO2 were produced. While no source of carbon was intentionally added to the reactors, it is possible that some of the metals may have carried along carbon impurities as surface contaminants. This possibility seems to be the likely reason for the appearance of these gases, because similar experiments carried out with copper and SCW at 600 °C did not show any formation of C-containing gases. Because Ni, Fe, and Ru can produce H2 from SCW in the absence of biomass, we subtracted the H2 amounts in Figure 1 from the amounts measured in SCWG experiments with biomass to determine the amount of H2 formed from biomass. It is these adjusted yields that we report herein. Of course, this adjustment to the H2 yields is based on the assumption that H2 formation from oxidation of the metal and H2 formation from gasification of the biomass are chemically independent. Catalytic SCWG. We considered temperatures of 400, 500, 600, and 725 °C, biomass loadings of 5.0, 9.0, and 33.3 wt %, and water densities of 0.05, 0.08, and 0.18 g/cm3. On the basis of the results of our background experiments, we report results from copper, nickel, and iron wires as potential catalysts for SCWG of lignin and cellulose. The enhanced yields (adjusted yield from biomass gasified in the presence of metal - yield in the absence of metal, at otherwise identical conditions) generated by the presence of a single copper, iron, or nickel wire are shown in Table 2 for cellulose and in Table 3 for lignin. Tables 2 and 3 report enhanced yields only when these values exceeded their standard deviations. This criterion was met for about one-half of the experimental conditions studied. Note too that the “enhanced” yield is negative in some cases (especially for CO). Here, the metal accelerated the disappearance of the compound, perhaps by accelerating the water gas shift reaction. For cellulose SCWG, nickel is the metal that most often led to enhanced yields exceeding the uncertainty in the data. It also seems to be the metal with the largest influence on the enhanced yields for H2. These yields were around 2 mmol/g at four different reaction conditions. For lignin SCWG, iron is the metal that most often led to enhanced yields exceeding the uncertainty in the data. The effect of temperature on the enhanced yields was small. Likewise, the effect of water density on the enhanced yields is

2.3 2.0 1.7 0.3 4.6 2.2 0.3 0.5 5.6 0.4 0.3

-0.9

Fe

1.2 -0.3 0.6 0.8

0.7 -1.0

2.0 2.2 1.1 10.0

1.2

3.8 0.9 1.6 0.2 2.2 0.6 0.5 0.4 2.8

Table 3. Effect of Metal, Temperature, Loading, and Water Density on Enhanced Yields for Lignin SCWG at 15 min enhanced yield (mmol/g) T (°C)

wt %

Fw (g/cm3)

gas

Ni

600

9.0

0.05

H2 CH4 CO2 CO CH4 CO H2 CO CH4 CO2 H2 CO CH4 CO2 H2 CO CH4 CO2 CO2 CO CH4

1.0 1.3 2.2

600

9.0

0.18

600

5.0

0.08

600

725

725 600

9.0

9.0

9.0 33.3

0.08

0.08

0.18 0.08

Cu

Fe

0.9

2.7

-0.2

2.2 -1.1

2.7

1.4 1.7

0.9 -0.5 1.0 1.8

1.9 4.3 0.9 1.6 1.5 2.9 -0.7 -0.4

1.0 -0.7 0.9 1.3 0.4 1.3 -1.3 -1.0 -0.6

small, and there are few clear trends. We had expected catalysts that promote the water-gas shift reaction, such as nickel, to become more effective as the water density increased, but the data did not bear out this expectation. For lignin SCWG, the enhanced H2 yield with added Fe changes with the water density, but it increases as the water density decreases. The biomass loading is the process variable with the largest influence on the enhanced yields. The enhanced yields of H2 and CH4 at the highest cellulose/lignin loading (33.3 wt %) are small (never more than 0.6 mmol/g) regardless of the feedstock and regardless of the metal used in the experiment. At the lowest biomass loading (5.0 wt %), however, the enhanced yields of H2 and CH4 reach their highest values. In the absence of metals, the effect of biomass loading on yields is small as compared to the effects of temperature and water density.38 In the presence of metals, however, with only one exception, the 5.0 wt % loading is the condition that gave the largest absolute increase

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Figure 2. Effect of metals on gas yields from cellulose SCWG (500 °C, 5.0 wt %, 10 min, 0.08 g/cm3).

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Figure 4. Effect of metals on yields from cellulose SCWG (500 °C, 5.0 wt %, 10 min, 0.08 g/cm3).

Figure 3. Effect of metals on gas yields from lignin SCWG (600 °C, 5.0 wt %, 15 min, 0.08 g/cm3).

in H2 and CH4 yields for all three metals. Because the 5.0 wt % loading is the situation in which metals had the greatest influence, we will focus on this situation in more detail in the next section. SCWG at 5.0 wt % Loading. Figure 2 compares the data from noncatalytic SCWG38 with the present results from SCWG with added metals at 5.0 wt % loading for cellulose. Without added metals, the H2 yield is 0.5 mmol/g. In the presence of metals, it is 2.7 mmol/g with nickel, 1.4 mmol/g with iron, and 2.5 mmol/g with copper. The CO yield is 3.1 mmol/g without metals, and it increases to about 4-5 mmol/g in the presence of any of the metals. The CH4 yield is largely unaffected by the presence of nickel or iron, but it increases to 2 mmol/g in the presence of copper. The CO2 yield is 4.9 mmol/g without metals, and it increases to 10.5 mmol/g with nickel, 7.1 mmol/g with iron, and 14.9 mmol/g with copper. Overall the effects of nickel and copper are comparable and typically larger than the effect of iron. For lignin (Figure 3), the presence of any of the metals increases the H2 yield from 1.3 to about 3-4 mmol/g. Nickel and copper also increase the CO2 yield from 4.4 to 6.2 and 8.7 mmol/g, respectively. The CH4 yield increases from 4.1 mmol/g with no metal to 5.2 with nickel and iron, and to 6.0 with copper. The effect of the metals on CO yields is small. Figures 4 and 5 show the effect of the metals on the carbon and hydrogen atom yields and the energy content of the product gas. For cellulose, 24% of the carbon atoms in the feedstock appear in the gases from noncatalytic SCWG. The C atom yield increases to 45% with nickel and 60% with copper. The hydrogen atom yield from noncatalytic SCWG is 8%. It increases to 20% with nickel and 21% with copper. The energetic content of the gas doubled when Ni or Cu was added to the reactor. For lignin, the carbon atom yield and the energy content from SCWG increased only slightly upon the addition of metals. The hydrogen atom yield, however, increased from 34% from noncatalytic SCWG to 45-55% in the presence of any of the metals.

Figure 5. Effect of metals on yields from lignin SCWG (600 °C, 5.0 wt %, 15 min, 0.08 g/cm3).

In general, it seems that nickel and copper are better catalysts than iron at the conditions of the cellulose SCWG experiments. At 600 °C, however, where lignin SCWG was examined, the three metals seem to be similar in terms of catalytic effect, and this effect is small. At the 5.0 wt % loading, each reactor contains 2.6 mg of biomass exposed to 40 mm2 of catalyst surface area (a ratio of 15.4 mm2/mg biomass). Our experimental results suggest that this ratio might be close to the minimum needed to provide observable catalytic activity for metals in SCWG, at least in the quartz reactors we used. The previous SCWG studies reported in the literature with stainless steel reactors (but no added catalyst) had a surface area/biomass ratio of about 18-19 mm2/mg. This value being close to that which demonstrated measurable catalytic activity in the present experiments suggests the possibility of catalytic effects from the reactor walls in those studies. Increasing Surface Area to Biomass Ratio. In an attempt to increase the contribution from the metal catalysts in this study, we conducted additional experiments with more catalyst surface area per unit biomass. This ratio can be increased either by increasing the total surface area of the catalyst or by decreasing the biomass loading. We examine both approaches. SCWG at 1.0 wt % Loading. We first present results from experiments with a lower biomass loading (1.0 wt %). This change increases the surface area/biomass ratio in the quartz capillary reactors to 80 mm2/mg. The uncertainties in the data at 1.0 wt % were much higher than at the previous conditions because of the difficulty in weighing precisely the small amount of biomass (0.5 mg) and because of the correspondingly small GC peaks generated for the gaseous products. In general, decreasing the biomass loading did not provide improved clarity regarding the effectiveness of the different metals as SCWG catalysts for lignin or for cellulose. The data for lignin had sufficiently large uncertainties that the differences between the gas yields from catalyzed and uncatalyzed SCWG

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Figure 6. Effect of metals on gas yields from cellulose SCWG (500 °C, 1.0 wt %, 10 min, 0.08 g/cm3). Figure 8. Effect of multiple nickel wires on cellulose SCWG at 500 °C (10 min, 1.0 wt %, 0.08 g/cm3).

Figure 7. Effect of metals on yields from cellulose SCWG (500 °C, 1.0 wt %, 10 min, 0.08 g/cm3).

were nearly always smaller than the sum of the uncertainties. The data from cellulose SCWG (Figures 6 and 7) likewise suffered from the uncertainty inherent in these experiments, but in some instances the influence of the added metal exceeded the uncertainties. Figure 6 shows that the H2 yield from noncatalytic SCWG of cellulose at 500 °C is 0.7 mmol/g. Copper increases it to about 2.5 mmol/g, ruthenium increases it to 2.9 mmol/g, and nickel increases it to 5.8 mmol/g. Only the result for nickel has an enhanced yield that exceeds the sum of the uncertainties, however. Nickel also appears to be the only metal that increased the CH4 yield (from 1.7 to 6.3 mmol/g) reproducibly enough to generate an enhanced yield that exceeded its uncertainty. The CH4 yields with added nickel or iron are much higher than the metal-catalyzed yields obtained at 5.0 wt % loading. For CH4 at least, it appears that increasing the metal area to biomass loading ratio increased the gas yield. The superior performance of nickel for SCWG of 1.0 wt % cellulose at 500 °C is consistent with its efficacy noted previously with 5.0 wt % loading. Ruthenium was not as effective in enhancing gas yields in our experiments as it has been reported to be in the literature.25,40 We believe the main reason for this discrepancy arises from our use of ruthenium powder (rather than wire). The biomass particles and fragments therefrom may not have had adequate opportunity to encounter the catalyst particles if the latter resided at one end of the quartz capillary reactor. We have observed low activity from powder but higher activities from wires in previous work with Ni-catalyzed SCWG of methanol.34 Figure 7 shows the effect of the metals at 1.0 wt % cellulose loading on the carbon and hydrogen atom yields and the energy content of the gas. The C yield from noncatalytic SCWG yield was 30%, and it increased to about 50% with added nickel and iron. This value is comparable to that observed at the 5.0 wt % loadings. The hydrogen yield, 13% in the absence of metal, increased to 55-60% with added nickel and iron. The energy content of the gas increases from 13% without metal to about 45% with nickel or iron. These improvements in the energy content of the gas with nickel and iron exceed the experimental uncertainties. Additionally, these yields are about 2-3 times

higher than those observed with added metals at 5.0 wt % loadings. Interestingly, nickel and iron appear to be the best catalysts at 1.0 wt % loading, whereas nickel and copper were the more active metals at 5.0 wt % loading. Even though there is uncertainty in the data, it does appear that nickel emerged as the best catalytic metal of those tested. SCWG with Larger Metal Surface. A second way to increase the surface area/biomass ratio is to increase the total surface area of the catalyst. We accomplished this increase by adding multiple wires to the quartz reactor. We chose nickel as the metal, because it was an effective catalyst at both 1.0 and 5.0 wt % loadings for cellulose SCWG. We performed new experiments with two wires and three wires added at 1.0 wt % loading. The surface area/biomass ratio is 80 mm2/mg for one wire, 160 mm2/mg for two wires, and 240 mm2/mg for three wires. Figure 8 shows results for cellulose. The H2 yield from noncatalytic SCWG was 0.7 mmol/g, and one nickel wire increased it to 5.8 mmol/g. With two Ni wires present, the H2 yield increased to 10.5 mmol/g, and with three wires it increased further to 16.0 mmol/g. This last value is the highest H2 yield obtained from any of the conditions studied in this work. The CO yield remained at about 3 mmol/g for most cases, except with three wires, where CO nearly vanishes. The CO2 yield increased from 6.8 mmol/g for noncatalytic SCWG to about 15 mmol/g with two or three Ni wires. The uncertainties in the CO2 yields are large, however. The CH4 yield stays around 2.5 mmol/g with two or three wires. In summary, at 240 mm2/mg (three wires), there was a great increase in the H2 yield relative to results with one nickel wire, little change in the CH4 yield, and a nearly CO-free gas was produced from cellulose SCWG. Because the energy content of the gas is strongly related to its CH4 content, adding more than one nickel wire to the reactor did not have much effect on the energy content. That is, the energy content in the gas increased tremendously upon adding the first nickel wire, but there was no additional increase with additional wires. For lignin SCWG there were no changes in the gas yields that exceeded the uncertainties, even when three nickel wires were used. Deactivation of Nickel Wires by Supercritical Water. Previous studies in the literature have reported deactivation of nickel catalysts in SCWG systems. Elliott4 mentioned that only nickel in its reduced form possesses catalytic activity, and our results indicate modest oxidation when nickel is exposed to SCW. To test for the possibility of deactivation by exposure to SCW at 0.08 g/mL, we pretreated nickel wires in quartz reactors for 2 h in SCW at the same temperatures we used for the gasification experiments (500 and 600 °C). We then repeated

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4,18,22

Figure 9. Effect of nickel exposure to SCW for 2 h on cellulose SCWG (500 °C, 10 min, 1.0 wt %, 0.08 g/mL).

SCWG, but these studies were performed at conditions milder than those used in the present study. We did not test whether Raney nickel could produce H2 from metal oxidation at these milder conditions. Nevertheless, the present results showing H2 production from metals and SCW alone provide a cautionary note for others doing research in this field. Exposure of nickel wires to SCW prior to their use in SCWG experiments does not diminish their activity for H2 formation. Previous work in the literature reports ruthenium as a catalyst that can actively break C-C bonds and increase gas yields.25,40 In the present work, though, ruthenium had little catalytic activity, especially for lignin SCWG, where it has been previously shown to increase gas yields by preventing recombination of intermediates. We believe this difference is a result of the physical form of the ruthenium metal (powder) in the present experiments. In quartz capillary reactors, metal wires are a better form for introducing metal surface because they run the entire length of the capillary and increase the likelihood of interactions between the catalyst and the biomass. Acknowledgment

Figure 10. Effect of nickel exposure to SCW for 2 h on lignin SCWG (600 °C, 15 min, 1.0 wt %, 0.08 g/mL).

SCWG experiments at 1.0 wt % loading with these pretreated nickel wires. Figures 9 and 10 show the results. For SCWG of both cellulose and lignin with the pretreated wires, the CH4 yield is lower than it was with fresh Ni wires. It seems that exposure to SCW may modestly reduce the already modest methanation activity of the nickel wires. The mean yields of H2, however, a compound whose formation is strongly promoted by the addition of nickel wires, actually increased when pretreatment is performed at 500 °C. This increase is evident for both cellulose and lignin, but it is also within the bounds of the experimental uncertainty, so it may not be statistically meaningful. In any event, it is clear that the exposure to SCW at 500 or 600 °C prior to a gasification experiment does not reduce the activity of the metal wires for the production of H2. Conclusions This work is the first to isolate the influence of potential metal catalysts on gas yields from SCWG. The effect of a single nickel, copper, or iron wire within the quartz reactor was low, apparently because the surface area was small. Adding multiple wires led to higher H2 yields from cellulose with nickel. A nickel surface area/biomass weight ratio of 240 mm2/mg (three wires, 1.0 wt % biomass loading), the highest ratio used in this study, produced the highest H2 yields (16.0 mmol/g). At these conditions, the hydrogen atom content of the product gas was 70% of that originally present in the biomass, and over 60% of the carbon atoms were gasified. All of the metals we tested, save copper, produced H2 from water under SCWG conditions but with no added biomass. Strong oxidation takes place for zinc, zirconium, and Raney nickel, but even nickel and iron underwent enough oxidation to produce detectable amounts of H2. Previous studies in the literature showed that Raney nickel is an effective catalyst for

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ReceiVed for reView December 6, 2009 ReVised manuscript receiVed January 19, 2010 Accepted January 28, 2010 IE901928F