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Energy & Fuels 2006, 20, 446-454

Product Identification and Distribution from Hydrothermal Conversion of Walnut Shells Aiguo Liu,*,† YoonKook Park,‡,⊥ Zhiliang Huang,§ Baowu Wang,§ Ramble O. Ankumah,*,† and Prosanto K. Biswas† Department of Agricultural and EnVironmental Sciences, Chemical Engineering Department, and Department of Food and Nutritional Sciences, Tuskegee UniVersity, Alabama 36088 ReceiVed June 27, 2005. ReVised Manuscript ReceiVed December 7, 2005

Agricultural byproducts are a major source of biomass for biofuel/bioenergy conversion. The southeastern U.S. produces a great amount of nutshells from pecan, walnut, and peanut processing. In this study, walnut shells were selected as a representative agricultural byproduct, and a hydrothermal process catalyzed by both bases and acid was applied to convert the walnut shells into liquefied organic compounds. Conversion rate, major organic products, and their distribution were measured under different concentration of bases (0-1.0 M) and reaction temperature (200-300 °C, corresponding to a pressure range of 1.5-8.6 MPa). An increase in base concentration (KOH and Na2CO3 ) or reaction temperature generally resulted in higher conversation rates and was more favorable to the generation of organic compounds of lower molecular weights. HCl as a catalyst promoted the generation of levulinic acid, but the conversion rates were very low. Major compounds from hydrothermal process catalyzed by bases were phenol derivatives. Small amounts of cyclopenten derivatives and C12-18 fatty acids were detected. The effects of reaction conditions on the distribution of products were characterized by the relative abundance of each compound group categorized based on the GC retention time.

Introduction Biomass is one of the most important potential renewable energy sources. Agricultural byproducts are a major source of biomass for biofuel/bioenergy conversion. Currently, hydrothermal hydrolysis and high-temperature pyrolysis are two major areas of focus of extensive efforts in the biomass conversion research.1-3 The high-temperature pyrolysis process for biomass conversion produces charcoal, bio-oil, and syngas as major products.3 However, the high operation temperatures (550 to ∼950 °C) destroy the natural structure of phenolic compounds and may cause cross-linking between aromatic rings and hydrocarbon chains in these compounds to generate tars that require late stages of treatment. The hydrothermal hydrolysis process produces more liquefied compounds mainly consisting of oxygenated compounds, which are potential fuel additives and sources of valuable chemicals. The hydrothermal process provides considerable opportunities for the production of conventional and unconventional fuels, fuel additives, and other chemicals. Integrated production of fuel and chemicals is the most likely scenario for economic optimization of biomass utilization because the overall costs of production of commodity * Corresponding authors. E-mail: [email protected] (A.L.) or [email protected] (R.A.); fax: (334) 727-8552. † Department of Agricultural and Environmental Sciences. ‡ Chemical Engineering Department. § Department of Food and Nutritional Sciences. ⊥ Current address: Chemical System Engineering Department, Hongik University, Jochiwon, Korea. (1) Karago¨z, S.; Bhaskar, T.; Muto, A.; Sakata, Y. Effect of Rb and Cs Carbonates for Production of Phenols from Liquefaction of Wood Biomass, Fuel 2004, 83, 2293-2299. (2) Karago¨z, S.; Bhaskar, T.; Muto, A.; Sakata, Y.; Uddin, M. A. LowTemperature Hydrothermal Treatment of Biomass: Effect of Reaction Parameters on Products and Bioling Point Distributions, Energy Fuels 2004, 18, 234-241. (3) Bridgwater, A. V.; Meier, D.; Radlein, D. An Overview of Fast Pyrolysis of Biomass, Org. Geochem. 1999, 30, 1479-1493.

products are noncompetitive with large-scale petroleum industries. Developing new and high valued products is essential for future development of agricultural byproducts as renewal industrial sources.4 Catalytic hydrothermal hydrolysis has the advantage of a one-step generation of high proportions of organic compounds and the elimination of a second stage treatment such as syngas filtration for tar removal and water gas shift conversion.5 The southeastern U.S. produces a great amount of nutshells from pecan, walnut, and peanut processing. A major part of these shells is used to produce activated carbon, and a small portion of pecan or walnut shells is used as polishing powders and filling materials.6,7 After harvesting, these nuts are usually shipped to central facilities for further processing, including shell removal. This makes it more feasible to collect these byproducts for industrial scale centralized processing. The purpose of this preliminary research is to identify and characterize the major organic compounds from the hydrothermal hydrolysis conversion of these nutshells and to investigate the effects of treatment conditions including temperature and catalysts. The ultimate goal is to develop a process that is able to selectively generate desired products or at least narrow the distribution of compounds in terms of molecular weight or carbon numbers while maintaining a relatively high conversion rate. (4) Goldstein, I. S. Organic Chemicals from Biomass; CRC Press: Boca Raton, FL, 1981; pp 1-10. (5) Zhang, Y.; Draelants, D.; Engelen, K.; Baron, G. Development of nickel-activated catalytic filters for tar removal in H2S-containing biomass gasification gas, J. Chem. Technol. Biotechnol. 2003, 78, 265-268. (6) Ahmedna, M.; Marshall, W. E.; Rao, R. M. Production of Granular Activated Carbons from Select Agricultural By-Products and Evaluation of Their Physical, Chemical, and Adsorption Properties, Bioresour. Technol. 2000, 71, 113-123. (7) Wartelle, L. H.; Marshall, W. E.; Toles, C. A.; Johns, M. M. Comparison of Nutshell Granular Activated Carbons to Commercial Adsorbents for the Purge-and-Trap Gas Chromatographic Analysis of Volatile Organic Compounds, J. Chromatogr. A 2000, 879, 169-175.

10.1021/ef050192p CCC: $33.50 © 2006 American Chemical Society Published on Web 01/12/2006

Hydrothermal ConVersion of Walnut Shells

Energy & Fuels, Vol. 20, No. 2, 2006 447

Materials and Methods Materials. Raw walnuts were purchased from a local grocery store, and the shells were peeled off manually. The shells were further smashed with a hammer, milled, and sieved to obtain particle sizes in the range of 1-3 mm. Analytical grade or better chemicals, acetone, ethyl ether (EE), KOH, and Na2CO3, were purchased from Fisher Scientific. Water was purified by distillation and deionization through a Millipore deionizer. Experimental Process. A reactor system purchased from Parr Co. (Moline, IL) was used for all tests. A nitrogen cylinder was connected to the reactor to purge the solution to eliminate air and maintain an inert atmosphere. This was achieved by repeating for 3 times a cycle of pressurization (to 1.0 MPa, holding for 2 min while stirring) and release to atmosphere. A similar setup has been reported in the literature for hydrothermal hydrolysis of biomass.1,2,8-10 The reactor has an effective volume of 150 mL and a maximum operation temperature and pressure of 350 °C and 20 MPa, respectively. For each test, 2 g of walnut shell and 50 mL of water were used. Temperature was the principal parameter for reaction control. Catalysts were dissolved in the water and added into the reactor together with the walnut shells. Gaseous products were initially collected by using a gas-bag and qualitatively analyzed using GC/MS. Results showed a possible generation of a small amount of CO, but other possible gaseous products such as CH4, H2, and CO2 were minimal or insignificant. Therefore, gaseous products were released into a hood in later tests without further analysis. The three reaction temperatures, 200, 250, and 300 °C, were tested. The temperature was controlled by heating at a rate of 10 °C/min until the preset reaction temperature (200, 250, or 300 °C) was reached. The temperature was then maintained constant with less than (2 °C variation for 1 h. After 1 h, a fan was used to cool the reactor until the temperature was below 120 °C. The reactor was then put in a water bath to further cool to room temperature. Under these conditions, the average cooling rate was about 20 °C/ min. The overall procedure for product separation and analysis is illustrated in Figure 1. Briefly, after the reactor was cooled to room temperature, the solid and liquid products were transferred to 50 mL centrifuge tubes and separated at 10 000g for 10 min. The solid residue was extracted once by adding 5.0 mL of acetone and was shaken manually every 10 min 5 times. The mixture of solid residue and acetone was subjected to centrifugation at 10 000g for 5 min to separate solid and acetone liquid for further analysis. An aliquot of 5 mL of the liquid phase from the reaction mixture after centrifugation was acidified with 0.5 mL of concentrated HCl (37% w/w) before being mixed with an equal volume of ethyl ether (EE) in a 15 mL polyethylene centrifuge tube. The samples were shaken manually every 10 min 5 times. After the separation of the two phases, approximately 3.0 mL of the upper level organic phase from both acetone and EE extractions was removed with a 10 mL syringe and subsequently filtered with 0.2 µm nylon syringe filter (Millipore). The syringe filter was washed 1 time with pure acetone or EE before use. After discarding the first 1 mL of filtrate, approximately 1.5 mL of filtrate was collected in a glass vial for GC/MS analysis. Solid residue from acetone extraction was analyzed for elemental composition after drying at 105 °C. The aqueous phase from EE extraction was discarded without further analysis. Both catalytic and noncatalytic hydrothermal processes were carried out for the purpose of comparison. KOH as a catalyst was (8) Osada, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Lowtemperature Catalytic Gasification of Lignin and Cellulose with a Ruthenium Catalyst in Supercritical Water, Energy Fuels 2004, 18, 327-333. (9) Cortright, R. D.; Davda, R. R.; Dumesic, J. A. Hydrogen from Catalytic Reforming of Biomass-Derived Hydrocarbons in Liquid Water, Nature 2002, 418, 964-967. (10) Fang, Z.; Minowa, T.; Smith, R. L.; Ogi, T.; Kozinski J., Liquefaction and Gasification of Cellulose with Na2CO3 and Ni In Subcritical Water at 350 °C, Ind. Eng. Chem. Res. 2004, 43, 2454-2463.

Figure 1. Diagram of experimental procedures.

tested systematically at different temperatures and concentrations. Na2CO3 and HCl as catalysts were tested at 250 °C only. Analytical Procedures. The EE and acetone extracts were analyzed by GC/MS (Shimazhu QP2010) to identify the major products. A Restek XTI-5 column (5% bonded phenyl, 95% dimethyl polysiloxane, 30 × 0.25 mm i.d.) was utilized. The injection was set at 275 °C with a split ratio of 10 and injection volume of 1 µL. The column temperature was programmed at a starting temperature of 40 °C (holding for 1 min), to an ending temperature of 275 °C (holding for 5 min) with a heating rate of 5 °C/min.10 A Vario EL III elemental CHNS analyzer (Elementar Analyzensysteme, Germany) was used for total carbon analysis of the raw material, solid residues (after acetone extraction), and aqueous phase samples (before EE extraction).

Results and Discussion Conversion Rates. Elemental analysis showed that the raw walnut shell had about 48% C and trace amounts of N and S at 0.14 and 0.03 wt %, respectively. The contents of oxygen and hydrogen were not determined. A total of 17 tests was carried out under various combinations of catalyst concentration and reaction temperature (Table 1). The reaction time was set for 1 h for all tests. Two conversion rates, one based on the total solid weight and the other based on the total carbon, were calculated. The total solid conversion rate is defined as Cts ) (1 - W(s)/W)100, and the total carbon conversion is defined as Ctc ) Wc(ag)/Wc(100), where W(s) and W represent the weight of solid residue and the sample and Wc(ag) and Wc represent total carbon weight in aqueous phase and in the original sample of walnut shells, respectively. The weight of solid residues was measured after the acetone extraction and drying at 105 °C until constant weight was reached. The total carbon in the aqueous phase was measured before EE extraction. Results of the two conversion rates are also listed in Table 1. It can be seen from Table 1 that, as a general trend, a higher temperature or higher concentration of KOH resulted in higher

448 Energy & Fuels, Vol. 20, No. 2, 2006

Liu et al. Table 1. Test Summary

test no. and conditions

reaction T (°C)

total weight conversiona (%)

total C conversionb (%)

01-water 02-KOH-0.1 M 03-KOH-0.5M 04-KOH-1.0M 05-water 06-HCl-1% 07-HCl-5% 08-KOH-0.1M 09-KOH-0.5M 10-KOH-1.0M 11-Na2CO3-0.05M 12-Na2CO3 -0.1M 13-Na2CO3 -1.0M 14-water 15-KOH-0.1M 16-KOH-0.5M 17-KOH-1.0M

200 200 200 200 250 250 250 250 250 250 250 250 250 300 300 300 300

48.5 47.2 74.0 93.7 63.9 58.2 32.2 68.4 NSc NS 78.1 63.8 72.9 69.5 86.1 87.5 98.3

23.3 26.7 66.4 78.6 NAa 27.5 31.0 NA 100.0 100.0 NA NA NA 24.5 30.9 57.0 102.0

a Defined as C ) (1 - W /W)100. b Defined as C ) W ts (s) tc c(ag)/Wc(100); where W(s) and W represent the weight of solid residue and the sample and Wc(ag) and Wc represent total carbon weight in aqueous phase as water-soluble and in the original sample. c NS: no solid residue left after reaction; NA: no data available.

conversion rates. This is more obvious for reactions when KOH was used as catalyst (test nos. 01-04, 08-10, and 15-17). However, when the reaction temperature was 250 °C or higher, the effect of temperature on the conversion rates was not as obvious as that in the lower range (tests 08-10 vs 15-17). An increase in the concentration of Na2CO3 did not significantly affect the conversion rate based on total solid weight (test nos. 11-13). For reactions using Na2CO3 as a catalyst, the carbon concentration in aqueous phase was not measured because of a possible interference of dissolved CO2. An increase in HCl concentration resulted in a decrease in Cts and a minimal increase in Ctc. Comparing the two conversion rates, it was observed that for most tests, Cts is higher than Ctc. A possible reason for the difference is the deposition of water insoluble compounds on the solid residues. Because parts of these compounds were extracted by acetone before the solid residues were weighed, they were excluded from the calculations of both conversion rates. When there was no solid residue left after reaction, the two conversion rates were much closer (test nos. 09, 10, and 17). This also serves as proof to the previous explanation. The fact that moisture accounts for approximately 3% of total weight11 of raw walnut shell counts for a small part of the difference. The loss of gaseous products should be negligible based on the fact that GC/MS did not show any significant level of possible gaseous products such as CO, CO2, and CH4 and that the values of Ctc were very close to 100% (in the range of 5% relative error) when there was very little or no solid residue left (e.g., test nos. 09, 10, and 17). Also, biomass gasification research has shown that a much higher temperature (550 to ∼950 °C) and heating rate is required to achieve significant conversion.12,13 Major Products. Typical GC spectra of both EE and acetone extracts are shown in Figure 2a,b, respectively. The blank samples of EE and acetone were run under identical conditions (11) Gonzalez, J. F.; Ramiro, A. C.; Gonzalez-Garcia M.; Canan, J.; Encinar, J.; Sabio, E.; Rubiales, J. Pyrolysis of Almont Shells. Energy Applications of Fractions, Ind. Eng. Chem. Res. 2005, 44, 3003-3012. (12) Elliott, D. C.; Neunschwander, G. C.; Hart, T. T.; Butner, R. S.; Zacher, A. H.; Engelhard, M. H.; Young, J. S.; McCready, D. E. Chemical Processing in High-Pressure Agueous Environment. 7. Process Development for Catalytic Gasification of Wet Biomass Feedstocks, Ind. Eng. Chem. Res. 2004, 43, 1999-2004. (13) Kruse, A.; Meier, D.; Rimbrecht, P.; Schacht, M. Gasification of Pyrocatechol in Supercritical Water in the Presence of Potassium Hydroxide, Ind. Eng. Chem. Res. 2000, 39, 4842-4848.

to identify the solvent peak as well as the impurities in these solvents. The results showed that most components in EE and acetone had a retention time of less than 6.0 min, except one that was tentatively identified as 4-hydroxy-4-methyl-2-pentanone in acetone, which had a retention time of 6.320 min. Because of difficulties in distinguishing the impurities in the solvents from hydrolytic products of walnut shells, all the peaks that had a retention time of less than 6.0 min were excluded from the list of final products. As shown in Figure 2a, there are similarities in terms of major peaks between GC spectra of EE extracts from reactions in pure water (A), 1.0 M Na2CO3 (C), and 0.5 M KOH (D), although two major peaks with retention times of 10.25 and 18.75 did not exist in spectrum A. The 5% HCl catalyzed reaction (Figure 2a, spectrum B) showed fewer major peaks than the other spectra. A major peak that appeared at ∼13.3 min (Figure 2a, spectrum B) was identified as levulinic acid, which had about 12% of total peak area, which was unique for HCl catalyzed reactions. Except for reactions in 0.5 M KOH (Figure 2a, spectrum D), there were almost no visible peaks after 30 min. It can be concluded from GC spectra (Figure 2a, spectra C and D) that the major products in EE extracts were very similar for reactions catalyzed by Na2CO3 and KOH. The unique peaks with a retention time of longer than 30 min in a KOH catalyzed reaction were identified mainly as long-chain organic acids such as tridecanoic acid (at 34.8 min) and octadecanoic acid and its derivatives (with retention times between 38 and 40 min). GC spectra of products in acetone extracts are shown in Figure 2b. When HCl and Na2CO3 were used as catalysts (Figure 2b, spectra A and B), there were very few acetone extractable compounds left on solid residues. For samples of pure water and 0.5 M KOH (Figure 2b, spectra C and D), the major compounds in acetone extracts had retention time longer than 20 min. It was observed that an increase in the reaction temperature and/or in the KOH concentration usually resulted in an increase in the peak area of identical retention time, which indicates an increase in the quantities of the compounds produced in EE extracts (more GC spectra were provided in Supporting Information). This is more obvious for compounds with higher retention times such as octadecanoic acids (retention times of 38 to ∼40 min). A possible explanation is that walnut shells may contain trace amount of oils. At higher temperatures and KOH concentrations, walnut oil hydrolyzes into glycerol and fatty acids. The latter further reacts with KOH to become



Figure 2. Typical GC spectra at reaction temperature of 250 °C. (a) EE extracts from aqueous phases. A, water; B, 5% HCl; C, 1.0 M Na2CO3; D, 0.5 M KOH. (b) Acetone extracts from solid residues. A, 5% HCl; B, 1.0 M Na2CO3; C, water; D, 0.5 M KOH. (Note: spectra were arranged in a different order for clarity purposes).

water-soluble potassium salts at higher temperatures and higher KOH concentrations. The acidification of the aqueous phase would transform these salts back into the acidic form and were extracted by EE. However, at lower temperatures or KOH concentrations, the oils either did not hydrolyze or were not transformed to the potassium salts. These fatty acids are highly hydrophobic and would be attached to the surface of solid residues rather than being dissolved in water. The transformation between potassium salts and fatty acids might be the reason for the emulsification that was observed during the acidification of aqueous phase samples. The peaks of these fatty acids were not observed in EE or acetone extracts when HCl or Na2CO3 were used as catalysts. This probably indicates that these two catalysts were unable to hydrolyze the oils in walnut shells into acids. A list of representative compounds that were identified by using the NIST GC/MS library are summarized in Tables 2 and 3 for EE and acetone extracts, respectively. The two lists correspond to spectrum D in Figure 2a,b, which represents the EE and acetone extracts for the 0.5 M KOH catalyzed reaction. Compounds that were common in most tests and had larger peak areas are marked with an asterisk. The molecular structures for the most significant compounds were also shown in the tables. Compounds in the EE extracts (Table 2) can generally be divided into three groups. The first one includes low molecular weight acids and five-carbon ring compounds and usually had retention times of less than 13 min; the second group is the

phenol derivatives as represented by 2-methoxy-phenol, 1,2benzendiol, and the methoxy benzendiols. These phenolic compounds exhibited GC retention times in the range of 1325 min. The last group mainly consists of C12-18 organic acids and shows retention times of longer than 30 min. Some compounds in the acetone extract (Table 3), especially the long-chain acids, were also detected in the EE extracts. There were very few compounds with retention times less than 13 min in acetone extracts. Most of the compounds had retention times longer than 20 min and were mainly high molecular weight compounds including C12-18 organic acids. Interestingly, compounds containing N and S such as pyrimidine derivatives were also found in the acetone extracts. Using the hydrothermal treatment of sawdust under comparable conditions, Karago¨z et al.2 reported similar compounds with the same sequence of retention times in the EE extract as presented in this research. However, they did not report any compounds containing N or S in the acetone extracts. This probably indicates a major difference in composition between these hard shell materials and the woods. By comparing the peak areas and the relative abundance in terms of area percentage, it was found that three compounds, 2-mehoxy-phenol, 3,4-dimethoxy-phenol, and 1,2-benzenediol, were the most common and predominant in most tests. It is known from wood chemistry that these three compounds are the basic structure of lignin.14 In view of this, a close


Liu et al.

Table 2. Major Compounds in EE Extract of Aqueous Phasea

a

Asterisk: marked as major peaks in the spectrum; pound sign: group of compounds that were not completely separated.

examination of the effects of reaction conditions on the generation of these three representative compounds may reveal the possible mechanism of lignin degradation in hydrothermal hydrolysis reactions. Under identical GC/MS conditions, the duplicate measurements indicated that relative errors in the peak

areas of the same component were less than 5%. For the same compound, the peak area represents the relative concentration of the compound in different samples. Figure 3 summarizes the effects of reaction temperature and KOH concentration on the generation of these three compounds. For the purpose of



Table 3. Major Compounds in Acetone Extraction of Solid Residue

a

Asterisk: marked as major peaks in the spectrum.

comparison, the peak areas were normalized to the largest peak. It can be seen that moderate reaction conditions (∼0.5 M KOH and ∼250 °C) were more favorable to the formation of 2-methoxy-phenol and 3,4-dimethoxy-phenol (Figure 3a,b). A high temperature and high concentration of KOH were more favorable to the generation of 1,2-benzenediol (Figure 3c). It is known that phenyl propanoid forms the basic units of lignin, although the detailed structure varies for different materials. The three phenolic compounds, 2-methoxy-phenol, 3,4-dimethoxy-phenol, and 1,2-benzenediol, were most possibly generated from the cleavage of C-O ester bonds (R-o-4 and β-o-4) of lignin. The schematic diagram in Figure 4 better explains our results as presented in Figure 3. The cleavage of the C-O ester bonds catalyzed by strong bases in water forms methoxy benzene and phenolic compounds as the first step of decomposition.8,14,15 The methoxy benzene can further hydrolyze

to form methoxy phenolic compounds. The latter then hydrolyzes to form multihydroxyl benzene and methane. However, as shown in Table 2, most compounds are methoxy phenols, which indicates that the hydrolysis to form benzenediol and methane proceeds to a lesser degree under the reaction conditions used in this research. A lack of a significant amount of methane in the gaseous products as discussed earlier also confirms this assumption. A higher reaction temperature and higher base concentration may increase the hydrolysis of methoxy phenols (Figure 3c). (14) Minami, E.; Kawamoto, H.; Saka, S. Reaction Behavior of Lignin in Supercritical Methanol as Studied with Lignin Model Compounds, J. Wood Sci. 2003, 49, 158-165. (15) Wang, D.; Montane, D.; Chornet, E. Catalytic Steam Reforming of Biomass-Derived Oxygenates: Acetic Acid and Hydroxyacetaldehyde, Appl. Catal. A 1996, 143, 245-270.


Figure 3. Effects of KOH concentration on three representative products in EE extract. (a) 2-methoxy-phenol; (b) 3,4-dimethoxyphenol; and (c) 1,2-benzenediol.

Similar results were observed for reactions using Na2CO3 as a catalyst but not as obvious as that of KOH. It is worth noticing that these three compounds were also detected in EE extracts of HCl catalyzed reactions, but at a much lower abundance. An increase in HCl concentration resulted in a further decrease

Figure 4. Simplified scheme of lignin degradation.

Liu et al.

in the amount of these three compounds, with a concomitant increase in levulinic acid. An increase in the HCl concentration from 1 to 5% resulted in an almost 20% increase in the amount of levulinic acid as calculated by GC peak areas. This indicates that strong bases as catalysts mainly promote the hydrolysis of lignin, whereas strong acids enhance the conversion of cellulose. Effects of Temperature and KOH Concentration on the Distribution of Hydrothermal Products. The GC/MS results have shown that there were more than 100 compounds in the EE extracts from the hydrothermal reactions of walnut shells. It is almost impossible to quantitatively measure each compound. In an effort to evaluate the effects of catalyst and reaction temperature on the overall distribution of reaction products, as an approximation, the compounds as detected by GC/MS were arbitrarily divided into five groups according to their retention times. Table 4 lists the range of retention times and featured compounds for each group. The total peak area in each group was manually calculated by adding the integrated peak areas of all identifiable individual peaks. A total of 150 peaks with a half peak width of 0.2 s or longer was calculated for each spectrum. Relative abundance can be compared among the same group based on the total peak area of the group to estimate the effects of reaction conditions on the generation of compounds. Results for the EE extracts are shown in Figure 5a-c. In general, a low reaction temperature and low base concentration promote formation of group 1 compounds (Figure 5a). When pure water was used as reaction media, products were predominantly the group 1 compounds, in which the major peak was identified as 3-furaldehyde. When KOH was added as catalyst, the peak of 3-furaldehyde totally disappeared. An increase in the KOH concentration resulted in more group 5 compounds that were represented by the octadecadienoic acids (Figure 5a,b). When the reaction temperature was increased to 300 °C, differences in total peak areas for groups 2-4 became less obvious, especially when KOH was higher than 0.5 M (Figure 5c). It also shows that reactions carried out at a higher temperature and higher KOH concentration generated more compounds in



Figure 6. Distribution of products in acetone extracts as a function of temperature and KOH concentration.

Figure 5. Distribution of products as a function of KOH concentration. (a) 200 °C; (b) 250 °C; and (c) 300 °C.

Figure 7. Distribution of products for reactions catalyzed by Na2CO3 and HCl at 250 °C. (a) EE extracts from aqueous phase and (b) acetone extracts from solid residues.

each group. This indicates that more water-soluble compounds were generated. This observation is consistent with the results of the conversion rates as shown in Table 1. The product distributions in acetone extracts from solid residues were analyzed using the same arbitrary grouping method described in the preceding section, and the results at various KOH concentrations and reaction temperatures are presented in Figure 6. Results show that the total peak areas for groups 1-3 in acetone contracts were relatively low, which indicates that there were less compounds of lower molecular weight and shorter retention time left in solid residues. The

compounds in acetone extracts were mainly in groups 4 and 5, which typically include alkyl acids and branched phenolic compounds of higher carbon numbers. A general trend observed was that at higher KOH concentrations and higher reaction temperatures (e.g., 0.5 M KOH and 300 °C), there were less acetone extractable compounds left on solid residues. Using the same approximation method, product distributions in EE and acetone extracts from Na2CO3 and HCl catalyzed reactions were analyzed, and the results are shown in Figure 7a,b, respectively. Figure 7a shows that there was a very little amount of group 5 compounds in aqueous phase when Na2CO3


Liu et al.

Table 4. Retention Time for Grouping and Representative Compounds in Each Group group G1 G2 G3 G4 G5

Tr

(min)

6-10 10-15 15-20 20-30 >30

representative compounds and features DL-methyltartronic

acid; MW 98-134, median 102; relatively small and polar 2-methoxy-phenol; MW 110-174, median 124; mainly phenol and its derivatives; 3-methoxy-1,2-benzenediol, 1,2-benzenediol; MW 110-194, median 140; concentrated with aromatic branched ethers 3,4-dimethoxy-phenol; MW 138-208, mediam 154; similar structure to G3 9,12-octadecadienoic acid (Z,Z); MW 214-282, median 280; long-chain hydrocarbon acids

or HCl were used as catalysts. Although a higher concentration of Na2CO3 resulted in a relatively higher yield in each group, the effect of Na2CO3 concentration was not as obvious as compared to that of KOH (Figure 5b). For tests using HCl as a catalyst, a higher concentration of HCl resulted in the formation of compounds in groups 1 and 2 but to a lesser degree for other groups. A major peak in group 2 was identified as levulinic acid, which was the major compound that was detected only in HCl catalyzed reactions. Similarly, the products in acetone extracts for tests using Na2CO3 and HCl as catalysts were mainly compounds in groups 4 and 5 (Figure 7b). Conclusions Hydrothermal conversion of biomass is a process that can potentially be integrated with proper catalysts to simultaneously produce biofuels and possible value-added chemicals. This preliminary research has shown that the catalyst and temperature are main factors controlling the distribution of products and the conversion rates of walnut shells into liquid products. KOH is an effective catalyst in increasing the overall conversion rate of walnut shells. A moderate concentration of 0.5 M KOH and a reaction temperature of about 300 °C are more favorable to formations of the methoxy phenolic compounds. Na2CO3 and

HCl are much less effective as catalysts in terms of conversion rate and the distribution of products. It is more desirable to achieve more narrowly distributed products at high conversion rates. Further research is needed to test different catalysts and solvents to make the process more selective or controllable. Quantitative analyses are also needed for at least a few major compounds. Acknowledgment. Contribution No. 344 of the George Washington Carver Agricultural Experimental Station. Partial financial support by the U.S. Department of Energy (DOE cooperative agreement No. DE-FC04-90A.L.661581 HBCU/MI Environmental Technology Consortium) is gratefully acknowledged. The authors thank K. Kpomblekou and his graduate students for assistance in elemental analyses.

Note Added after ASAP Publication. After this article was published ASAP on January 12, 2006, the author affiliations had to be modified. The corrected version was published ASAP March 7, 2006. Supporting Information Available: GC/MS spectra for EE and Acetone extracts. EF050192P

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