Ind. Eng. Chem. Res. 1997, 36, 2087-2095
2087
Thermal Decomposition of Wood and Cellulose in the Presence of Solvent Vapors Emma Jakab,*,† Kui Liu, and Henk L. C. Meuzelaar‡ Center for Microanalysis and Reaction Chemistry, University of Utah, Salt Lake City, Utah 84112
The thermal decomposition of white birch wood and filter pulp was studied in water and methanol vapor at 2 MPa pressure in a flow-through reactor. The abundance of the volatile products was monitored by on-line GC/MS using repetitive sampling in combination with fast separation on a short capillary column. The reactor was heated to 400 °C at 20 °C/min and the intensity profile of the product ions within the 30-200 amu range recorded. The system was capable of separating the profiles of typical hemicellulose products evolved at lower temperature from the characteristic cellulose and lignin products detected from wood. Char yields in methanol were similar to those in an inert gas atmosphere; however, the presence of water markedly increased the amount of char produced. The product distribution of cellulose was strongly affected by the solvents. In methanol, pyran derivatives dominate besides levoglucosan and glycolaldehyde, whereas the relative abundance of 2-furaldehyde and 5-(hydroxymethyl)-2-furaldehyde increased in the presence of water. Water catalysis was also indicated by lowering the decomposition temperatures of cellulose. High-pressure (6.5 MPa) thermogravimetric experiments in helium or hydrogen atmospheres were also found to lower the reaction temperature of wood. This observation can be explained by the catalytic effect of reaction water released during the thermal decomposition of wood. Introduction Lignocellulosic materials can be converted into liquid fuels and chemical feedstocks by thermochemical processes (Chornet and Overend, 1985; Diebold et al., 1994). The reactions of biomass liquefaction processes in solvent environments can be divided into two groups: substrate-solvent interactions and thermal decomposition reactions. The solvent stabilizes the reaction intermediates and reduces the formation of secondary char or coke. For process economic and/or environmental reasons, solvents of industrial interest tend to be limited to those derivable from the biomass itself (e.g., alcohols, phenols) or to water. A few aqueous procedures have been proposed such as steam explosion (Overend and Chornet, 1987), steam liquefaction (Boocock et al., 1988), and aqueous thermomechanical (Bouchard et al., 1991) decomposition. In these processes water hydrolyzes most of the hemicellulose and appreciable amounts of lignin degrade during water treatment. It is well-known (Mok et al., 1992a) that dilute acid hydrolysis of cellulose at moderate temperature (about 200 °C) leads to the formation of glucose with 55-70% yield. Although low 5-(hydroxymethyl)-2furaldehyde yields are even observed under pyrolysis conditions (Shafizadeh and Fu, 1973; Helleur, 1987), substantial conversion of hexoses into 5-(hydroxymethyl)-2-furaldehyde and related products requires concentrated acid and higher temperatures (Sjo¨stro¨m, 1981). The thermal decomposition of biomass and its components has been studied for a long time (Shafizadeh, 1985; Antal, 1985). The product distribution is determined by various factors, including substrate type, heating conditions, reactor atmosphere, and catalysis (Windig et al., 1984; Chum et al., 1990; Milosavljevic * Corresponding author. † Visiting scientist, currently at Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budao¨rsi 45, H-1112 Budapest, Hungary. Telephone: (36 1) 319-3119. Fax: (36 1) 319-2537. E-mail:
[email protected]. ‡ E-mail:
[email protected]. S0888-5885(96)00335-1 CCC: $14.00
et al., 1996; Drummond and Drummond, 1996). Cellulose comprises up to 50% of biomass; therefore, its decomposition has been investigated most thoroughly. Recently, Radlein et al. (1991) gave a review of the current knowledge of cellulose pyrolysis mechanisms. Two primary decomposition pathways have been identified: (1) transglycosidation, leading to the formation of levoglucosan; and (2) reverse aldolization (Richards, 1987), explaining hydroxyacetaldehyde evolution. A third, tentative Ei-elimination was proposed by Lomax et al. (1991) in order to explain the formation of dehydrated products (e.g., oligomers). Extensive lists of products, together with their chromatographic retention data and mass spectral patterns, have been published for cellulose pyrolysis (Pouwels et al., 1989) and for wood decomposition (Faix et al., 1991a,b; Pouwels et al., 1987). These lists reveal that a variety of pyran and furan derivatives, linear carbonyl compounds, and even aromatics, e.g., phenols, can be obtained from cellulose. The product distribution is strongly influenced by the experimental conditions. It is well established (van der Kaaden et al., 1983; Pan and Richards, 1989; Va´rhegyi et al., 1988) that small amounts of inorganic substances can cause profound changes in the decomposition mechanism of cellulose. It is less well-known that lignin decomposition is also strongly affected by the presence of cations (Jakab et al., 1993). Piskorz et al. (1986) called attention to the fact that hydroxyacetaldehyde could be obtained in fair yields from the rapid pyrolysis of celluloses containing near trace quantities of alkali cations, and Richards (1987) achieved good yields of hydroxyacetaldehyde by vacuum pyrolysis of pure cellulose at 350 °C. Shafizadeh et al. (1979) observed that 5% phosphoric acid increases the yield of levoglucosenone up to 11% at high heating rates. The choice of pathways is also a function of the degree of polymerization (Julien et al., 1991). Piskorz et al. (1989) have reported that pretreatment of wood by acid hydrolysis increases the liquid yields up to 80% from wood. When biomass is pyrolyzed under industrial conditions, large sample sizes are used, often in closed (batch © 1997 American Chemical Society
2088 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 1. On-line GC/MS monitoring of the volatile products from the flow-through microreactor.
type) reactors. Higher reaction rates were detected (Jo¨nsson, 1985) in the center of a cellulose cylinder than on the surface. This was explained by the autocatalytic effects of the products. Antal and co-workers (Mok et al., 1992b; Va´rhegyi et al., 1993) found that the use of steam or closed sample holders decreased the decomposition temperature of cellulose and increased the char yield. They explained this observation as an autocatalytic effect of water on the decomposition and proposed a new kinetic model for cellulose pyrolysis (Va´rhegyi et al., 1994; Antal and Va´rhegyi, 1995). The objective of this study is to investigate the volatile decomposition products of wood and cellulose in a tubular reactor by on-line GC/MS. We used solvent vapor media (methanol and water) to model some thermal liquefaction processes. The second purpose of these experiments is to study the effect of water on the reaction mechanism, because under industrial conditions water vapor formed during the decomposition process tends to remain in the pores. Finally, a third objective is to demonstrate the feasibility of on-line GC/ MS analysis under high-pressure, high-temperature conditions. In our laboratory a novel process monitoring technique has been developed to interface thermobalances and high-pressure microreactors to GC/MS instruments (McClennen et al., 1993; Nie et al., 1993). Short-column GC/MS was applied using a patented, fluidics switched vapor sampling inlet (McClennen et al., 1990; Arnold et al., 1991). Experimental Section A Hewlett-Packard gas chromatograph-mass selective detector (HP 5971) system was coupled to a vertical flow-through microreactor, as shown in Figure 1. The reactor consists of a 300 mm long, 3.2 mm i.d. Hastelloy
tube heated by means of a temperature-programmed furnace. About 140 mg of white birch (Betula papyrifera) wood chips or ash-free analytical filter pulp (Millipore) was placed in the middle section of the reactor tube and heated up to 400 °C at a 20 °C/min heating rate. The fluid medium (methanol, water, or methanol + 5% water) was continuously pumped from a glass reservoir into the reactor at 0.2 mL/min by means of an HPLC pump (Waters Associates Inc.). The pressure was set to 2 MPa by using a suitable backpressure valve (Grove Valve & Regulator Co., Model 91W). A small aliquot of the product vapor flow was introduced into the vapor sampling device through a 2 m × 0.1 mm i.d. fused-silica capillary that reduced the pressure to near ambient. Since analysis samples were taken directly from the high-pressure region, the delay between GC injection and product formation was only about 30 s. The transfer line and the sampling unit were kept at 250 °C in order to minimize condensation losses. The automated vapor sampling inlet provided pulsed sampling at 1.3 min intervals. A 2 m × 0.18 mm i.d. DB-1 fused-silica capillary column performed the fast separation of the products at 80 and 120 °C isothermal temperatures. The GC column was directly coupled to the MSD ion source, operating in electron impact mode at 70 eV electron energy. The decomposition products were identified on the basis of the mass spectra and retention times by comparison with literature data (Pouwels et al., 1987, 1989; Faix et al., 1991a,b). Thermogravimetric (TG) experiments were carried out in a Cahn Model TG-151 high-pressure thermobalance at 6.5 MPa and ambient pressures. Wood chips (30 mg) were placed in a quartz sample holder and heated at 10 °C/min while flushed with helium or hydrogen at 500 mL/min flow rate (at ambient pressure
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2089
Figure 2. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of white birch under various conditions.
and temperature). A more detailed description of the instrument can be found elsewhere (Liu et al., 1994). Results and Discussion Wood Decomposition in High-Pressure Thermogravimetry. A few experiments were performed in the high-pressure thermobalance to study the effect of pressure and gaseous atmosphere on the weight loss profile of white birch wood. As seen in Figure 2a, no measurable difference is observed during the course of the decomposition in hydrogen and helium atmospheres up to 400 °C. Above this temperature hydrogen promotes devolatilization during the coking process, especially at high pressure. Two peaks can be distinguished on each DTG curve (Figure 2b); the first one can be attributed to hemicellulose decomposition, and the second peak represents cellulose and lignin products, as confirmed by GC/MS analysis of the evolved products (not shown here). The separation of the two decomposition steps becomes more pronounced at high pressure. The most significant influence of pressure is that it lowers the decomposition temperature by about 35 °C at 6.5 MPa. A similar phenomenon was observed by DSC experiments in closed crucibles (Mok et al., 1992b) and was explained by the autocatalytic effect of water. Although there is a continuous flow of the flushing gas in the thermobalance, the residence time of the volatile products is increased at high pressure, so it seems plausible that the self-generated atmosphere catalyzes the decomposition.
Figure 3. Wood conversion in water vapor at 20 °C/min heating rate. (a) Total ion chromatogram (TIC) with multiple sampling. (b) TIC with an expanded time scale. (c) Mass spectum averaged from 12 to 27 min run time.
Wood Decomposition in Solvent Vapors. In order to clarify the effect of water further, white birch wood was subjected to thermal decomposition at 2 MPa pressure in a tubular reactor flushed by a continuous flow of water vapor, and the volatile products were analyzed by on-line GC/MS. Since the TG experiments showed that the main decomposition stage ends around 400 °C, the samples were heated up at 20 °C/min and kept at 400 °C for 20 min. Small aliquots of the volatile reaction products were injected automatically onto the GC column at 1.3 min intervals. Figure 3a shows the total ion chromatogram taken between 10 and 30 min revealing that the main evolution of volatiles occurs at around 300-400 °C. Two sampling intervals of the chromatogram are magnified (expanded time base) in Figure 3b. GC analyses were performed at an isothermal column temperature of 120 °C. This enabled elution of the monomeric lignin products (guaiacol and syringol derivatives). However, it should be pointed out that separation of the low molecular mass products is not complete. The first broad peak contains several compounds. Major components include carbon dioxide (m/z 44) as well as acetic acid and hydroxyacetaldehyde (m/z 60). Figure 3c shows the mass spectrum averaged over the entire interval. The characteristic products of hardwood decomposition can be identified. White birch has relatively low lignin content (19% on extractive-free wood). Two series of lignin products are marked in Figure 3c; in agreement with the original lignin structure of the birch wood (guaiacol:syringol ≈ 2:3), the guaiacol de-
2090 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 4. Wood conversion in water vapor at 20 °C/min heating rate. (a) Total ion chromatogram with the integrated peak area (I) values summed up within each sampling interval. (b-d) Selected ion chromatograms.
Figure 5. Wood conversion in methanol vapor at 20 °C/min heating rate. (a) Total ion chromatogram. (b-c) Selected ion chromatograms. (d) Mass spectum averaged from 12 to 27 min run time.
rivatives have smaller intensities than the syringol derivatives. Molecular ion peaks dominate the mass spectra of the phenolic compounds, whereas polysaccharide products strongly fragment at 70 eV electron energy. Nevertheless, characteristic hexose (m/z 126) and pentose markers (m/z 114), as well as more degraded cellulose and hemicellulose products (e.g., m/z 60, acetic acid + hydroxyacetaldehyde; m/z 96, furaldehyde), can be identified besides the numerous fragment ions. The mass spectrum reveals the formation of wellknown products of wood decomposition in the water atmosphere. The repetitive sampling approach, however, enables monitoring of the time profiles of various products. Figure 4 shows the evolution of selected hemicellulose, cellulose, and lignin products together with the total ion current signal (TIC). The envelope of the TIC profile only represents the evolution profiles of the products with the highest signal intensities (viz. carbon dioxide and acetic acid). The TIC envelope shows a maximum at 15.80 min, whereas the integrated peak area (summed up within each analysis interval) reaches a maximum between 18.34 and 19.64 min (Figure 4a) when the most intense cellulose and lignin products evolve. Note that white birch contains as much as 35 % O-acetyl-4-O-methylglucuronoxylan and
only 3% other hemicelluloses. Figure 4b shows the evolution of a xylan marker, 4-hydroxy-5,6-dihydro-2Hpyran-2-one. This pentose product is released from the hemicellulose at substantially lower temperatures than the cellulose and lignin components decompose. The other compound contributing to the signal intensities at m/z 114 (3-methyltetrahydrofuran-2,4-dione) evolves at higher temperature, indicating its cellulose origin. The peak signal at m/z 126 represents several cellulose products. The profiles of two major compounds (maltol and 5-(hydroxymethyl)-2-furaldehyde) are marked in Figure 4c. The presence of water vapor promotes the formation of 5-(hydroxymethyl)-2-furaldehyde, whereas the yield of maltol (3-hydroxy-2-methyl-4H-pyran-4-one) is higher in a methanol atmosphere. The evolution of aromatic products such as 4-ethylsyringol and syringaldehyde, originating from lignin (Figure 3d), indicates that lignin and cellulose decomposition occur at similar temperatures. This figure demonstrates the beneficial combination of short-column GC separation and MS detection. The GC column at 120 °C isothermal temperature provides a useful degree of separation for furan, pyran, and phenol derivatives. Although the separation is not complete during the 1.3 min analysis interval, the information content can be enhanced by using selected ion profiles.
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2091
Figure 6. Total ion chromatograms of cellulose decomposition at 20 °C/min heating rate in various solvent vapors.
Figure 5 shows the thermal decomposition of white birch wood in methanol vapor. The first m/z 60 peak (Figure 5b) may represent two products, acetic acid and hydroxyacetaldehyde. Acetic acid is released from the side groups of O-acetyl-4-O-methylglucurono-β-D-xylan (hemicellulose), whereas hydroxyacetaldehyde can be attributed to cellulose decomposition. The evolution of the main m/z 60 components takes place at lower temperature than the formation of methyl syringol (m/z 168). This observation indicates that the contribution of the hemicellulose product acetic acid is more significant to this peak than that of hydroxyacetaldehyde. This is confirmed by the fact that the profile of fragment ions at m/z 45 (primarily the acetic acid fragment) reaches its maximum at the same time. In the experiments conducted in water vapor the key fragment ion of hydroxyacetaldehyde (m/z 31) was also detected, and it was found that the m/z 45 acetic acid fragment ion evolves earlier and at higher intensity than m/z 31 (hydroxyacetaldehyde). Figure 5b shows the formation of a key cellulose product, levoglucosan. Its main fragment ion is known to be m/z 60 (Evans et al., 1984). Under the chromatographic conditions applied levoglucosan elutes together with 4-methylsyringol. Although its yield could not be measured reliably due to adsorption problems, the intensity of the levoglucosan signal is comparable to that of methylsyringol. The profiles of cellulose and lignin products are quite similar. Notwithstanding the fact that the time resolution is not high in these experiments, it can be seen that the separation of hemicellulose decomposition was somewhat greater in water vapor (Figure 4) than in methanol. It is known (Chornet
Figure 7. Average mass spectra of the volatile products of cellulose decomposition in various solvent vapors.
and Overend, 1985) that hemicellulose can be hydrolyzed in water at low temperature during longer time periods. The solvation effect of water may enhance the decomposition of hemicellulose in water vapor. At 2 MPa pressure the char yield of white birch was 22% and 45% in methanol and in water atmospheres, respectively. However, the decomposition is likely not complete since the temperature was only raised to 400 °C for 20 min. In the thermogravimetric experiments (Figure 2a), the amount of solid residue was 25-30% around 400 °C in a gaseous atmosphere at 0.1 and 6.5 MPa. Consequently, the char yield in methanol is similar to that in an inert atmosphere. However, the presence of water markedly increased the char yield. Comparison of the product distribution in water and methanol shows (Figures 3c and 5d) that the quality of the lignin products is not measurably influenced by the choice of solvent. The levoglucosan yield was about the same as that of 4-methylsyringol in both water and methanol atmospheres. Nevertheless, the wood experiments indicated that the intensity of several cellulose products (e.g., furan derivatives) was substantially affected by the presence of solvents. These findings prompted us to examine the decomposition of cellulose in different solvents. Effect of Solvent Vapors on Cellulose Decomposition. Millipore ash-free filter pulp, which has almost identical TG curves as Whatman cellulose, was used as a cellulose sample. Figure 6 shows the total ion chromatograms of the volatile products of filter pulp as a function of time and temperature. Chromatographic analyses were performed at 80 °C so as to get good separation of the lower molecular weight products.
2092 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 1. Assignment of Cellulose Products Numbered in Figure 8 no.
structure
assignment
1
2-furaldehyde
2
2-acetylfuran
2,3-dihydro-5-methylfuran-2-one 3
5-methyl-2-furaldehyde
4
4-hydroxy-5,6-dihydro-2H-pyran-2-one
5
2-hydroxy-3-methyl-2-cyclopenten-1-one
6
3-methyltetrahydrofuran-2,4-dione
7
3-hydroxy-6-methyl-3,4-dihydro2H-pyran-2-one
8
levoglucosenone
9
2-methyl-3-hydroxy-4H-pyran-4-one
Figure 8. One sampling interval of the total ion chromatograms of cellulose decomposition in various solvent vapors. Peak numbers refer to the compounds listed in Table 1.
10
3,5-dihydroxy-2-methyl-5,6-dihydro-4Hpyran-4-one
Compounds up to MW ) 144 could be eluted and 3-hydroxy-6-(hydroxymethyl)-5,6-dihydro-4H-pyran-4one was detected. However, levoglucosan did not elute under the conditions applied. In methanol solvent the evolution profile of cellulose products is quite sharp and, similar to the decomposition in an inert atmosphere (Va´rhegyi et al., 1988), the degradation rate reaches a maximum at about 360 °C. All products have similar profiles; there is no sign of the presence of contaminants. Although 4-hydroxy-5,6-dihydro-2H-pyran-2-one is a characteristic product of xylan decomposition, it is also detected among the cellulose pyrolysis products too (Pouwels et al., 1989). The time profile of this compound (not shown here) indicates that it is formed from cellulose and not from xylan residues. As seen in Figure 6, the temperature range of cellulose decomposition becomes wider and shifts to lower temperatures with increasing water content of the solvent. This observation is in agreement with the DSC experiments of Mok et al. (1992b) and supports the view (Va´rhegyi et al., 1993) that water has a catalytic effect on the decomposition. The average mass spectra in Figure 7 reveal that the spectrum becomes simplified in pure water vapor; 2-furaldehyde and 5-(hydroxymethyl)-2-furaldehyde dominate, and their intensities are comparable to those of the low-MW fragment ions (m/z 31, 39, 43). In the presence of methanol, a more complex mixture of products appears to be formed. However, in comparison with the main ion signal at m/z 44, their abundance is lower. The average mass spectrum is dominated by m/z 44 (CO2) and the fragment ions of m/z 43 (CH3CO+)
11
2-formyl-5-(hydroxymethyl)tetrahydrofuran-3-one
12
3,5-dihydroxy-2-methyl-4H-pyran-4-one
13
1,4:3,6-dianhydro-R-D-glucopyranose
14
5-(hydroxymethyl)-2-furaldehyde
15
3-hydroxy-6-(hydroxymethyl)-5,6dihydro-4H-pyran-4-one
and m/z 57 (C2H5CO+). These fragments generally appear in the spectra of carbonyl compounds, which are formed by ring cleavage mechanisms during cellulose pyrolysis. The yield of these small molecules is very low during decomposition in water vapor (Figure 7c). The type of furan derivatives also changes in different solvents: in water, 5-(hydroxymethyl)-2-furaldehyde is abundant and furan is practically absent, while in methanol, loss of substituents takes place and marked amounts of 5-methyl-2-furaldehyde and furan are formed. Some ion signals represent more than one compound. Furthermore, several molecules (e.g., pyran derivatives) are strongly fragmented during ionization. Therefore, careful comparisons of the chromatograms are necessary
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2093
Figure 9. Proposed pathway for the formation of furan derivatives during decomposition of cellulose in the presence of water.
to examine the product distribution in detail. Figure 8 compares one typical sampling interval from each cellulose experiment, indicating a clear change in the chromatographic pattern from one solvent to another. Obviously, the largest variety of products can be observed in the presence of mixed solvents (methanol + 5% water). Peak numbers in Figure 8 refer to the compounds listed in Table 1. Peak identification was based on comparison of the retention sequences and mass spectra with literature data (Pouwels et al., 1987, 1989; Faix et al., 1991a,b). In the medium-MW range, 2-furaldehyde (1) and 5-(hydroxymethyl)-2-furaldehyde (14) are the dominating products in the presence of water vapor, whereas 5-methyl-2-furaldehyde (3), 2-furaldehyde (1), 2-hydroxy-3-methyl-2-cyclopenten-1-one (5), and levoglucosenone (8) are the major products in a methanol matrix. A ring-opening mechanism can explain the formation of 5, which is known as a product formed from carbohydrates under alkaline conditions (van der Kaaden et al., 1983) but was also detected during flash pyrolysis of cellulose (Pouwels et al., 1989). Levoglucosenone (8) and 1,4:3,6-dianhydro-R-D-glucopyranose (13) are known minor pyrolysis products of cellulose (Shafizadeh et al., 1979; Schulten, 1984) which can be formed by dehydration of levoglucosan. The yields of compounds 5, 8, and 13 are only significant in the presence of methanol. The type of solvent affects the quality of pyran derivatives too. In water, more saturated (dihydro)pyran compounds are formed (7, 10, and 15), whereas in methanol mostly unsaturated pyran derivatives are released (9, and 12). The substitution pattern and relative abundance of furan compounds are also influenced by the solvents, as discussed above. In water, saturated forms of furan derivatives are also observed, such as 2-formyl-5-(hydroxymethyl)tetrahydrofuran-3-one (11). The foregoing results clearly show that the formation of furan derivatives is enhanced by the presence of water vapor under neutral conditions. It has been established (Sjo¨stro¨m, 1981) that hydrolysis of glucose in strongly acidic media leads to the production of 5-(hydroxymethyl)-2-furaldehyde. However, the mech-
anism proposed for the formation of furan derivatives in acidic media does not describe the reactions under neutral conditions. Kilzer and Broido (1965) suggested a mechanism for the low-temperature cross-linking and dehydration of cellulose which involves the formation of furanose rings. An analogous mechanism can explain the water catalysis during thermal decomposition. We propose a pathway for the formation of furan derivatives, in which water initializes the opening of the pyranose ring followed by scission of the cellulose chain, as seen in Figure 9. Furthermore, 2-formyl-5-(hydroxymethyl)tetrahydrofuran-3-one (11) appears to be an intermediate product in the mechanism of 5-(hydroxymethyl)-2-furaldehyde (14) formation. The intensity of compound 11 is highest in the presence of small amounts of water (5% water in methanol vapor). Since 5-(hydroxymethyl)-2-furaldehyde readily undergoes condensation reactions, the proposed mechanism can also explain the increased char yield in the presence of water. The formation of furan derivatives gives an explanation for the findings of Mok et al. (1992a) that hydrolysis of cellulose in neutral media produces compounds that cannot be further hydrolyzed to sugars. Conclusions Fast, repetitive, on-line GC/MS detection methods can provide detailed information about the decomposition of wood and cellulose in various atmospheres and at elevated pressures. The intensity vs time profiles of the volatile products at various pressures and solvent vapor atmospheres show that the hemicellulose decomposition step can be separated from the cellulose and lignin decomposition stage by this method. Decomposition of wood and cellulose occurred at similar temperatures and produced similar amounts of char in methanol vapor as in gaseous atmospheres. However, in water the char yield increased significantly and the decomposition shifted to lower temperatures. A similar shift in the decomposition behavior was observed by high-pressure TG experiments, where at 6.5 MPa pressure wood decomposed at a 35 °C lower
2094 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
temperature than at ambient pressure. This shift can also be explained by the catalytic effect of water (formed in the course of degradation) as suggested by Mok et al. (1992b) and Va´rhegyi et al. (1993). GC/MS analysis of the volatile products revealed that the product distribution is strongly affected by the type of solvent. High yields of furan derivatives were observed in the presence of water. In each solvent marked amounts of products are evolved, the formation of which can be explained by opening of the pyranose rings. In methanol, 2-hydroxy-3-methyl-2-cyclopenten1-one and carbonyl compounds of low MW are characteristic ring-opening products, whereas furan derivatives dominate in a water medium. The substitution and saturation are also influenced by the solvents. Water tends to prevent the cleavage of functional groups from the furan rings. The degree of saturation is higher in the decomposition products in water (e.g., tetrahydrofuran and dihydropyran derivatives) than in methanol (e.g., pyran derivatives, levoglucosenone). The fact that hydroxy- and formyl-substituted furan compounds are prone to condensation reactions could explain the higher char yield in the water atmosphere. The furan-forming mechanism may only play a secondary role during the decomposition of cellulose in vacuum, when products tend to be released fast. However, in industrial-scale processes the water produced cannot be removed rapidly from the large sample mass and/or closed reactor environments. This may be expected to have a major effect on the decomposition pathways. Acknowledgment The expert assistance and advice of W. H. McClennen, L. Mihamou, and P. Cole with the experiments are gratefully acknowledged. Financial support for the work reported here was provided by the U.S. Department of Energy through the Consortium for Fossil Fuel Liquefaction Science (Grant No. UKRF-4-43576-90-10), the Advanced Combustion Engineering Research Center (funded by the National Science Foundation, the State of Utah, and 23 industrial participants), and Rocketdyne Division of Rockwell Corp. Literature Cited Antal, M. J. Biomass Pyrolysis: A Review of the Literature. Part II. Lignocellulose Pyrolysis. In Advances in Solar Energy; Boer, K. W., Duffie, J. A., Eds.; American Solar Energy Society: New York, 1985; pp 175-253. Antal, M. J.; Va´rhegyi, G. Cellulose Pyrolysis Kinetics: The Current State of Knowledge. Ind. Eng. Chem. Res. 1995, 34, 703-717. Arnold, N. S.; McClennen, W. H.; Meuzelaar, H. L. C. Vapor Sampling Device for Direct Short Column Gas Chromatography/ Mass Spectrometry Analyses of Atmospheric Vapors. Anal. Chem. 1991, 63, 299-304. Boocock, D. G. B.; Allen, S. G.; Chowdhury, A.; Fruchtl, R. Producing, Evaluating and Upgrading Oils from Steam Liquefaction of Poplar Chips. In Pyrolysis Oils from Biomass; Soltes, J., Milne, T. A., Eds.; American Chemical Society: Washington, DC, 1988; pp 92-103. Bouchard, J.; Nguyen, T. S.; Chornet, E.; Overend, R. P. Analytical Methodology for Biomass Pretreatment. Part 2: Characterization of the Filtrates and Cumulative Product Distribution as a Function of Treatment Severity. Bioresour. Technol. 1991, 36, 121-131. Chornet, E.; Overend, R. P. Biomass Liquefaction: An Overview. In Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier: London and New York, 1985; pp 967-1002.
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Received for review June 12, 1996 Revised manuscript received February 3, 1997 Accepted February 10, 1997X IE960335I
X Abstract published in Advance ACS Abstracts, April 1, 1997.
