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Catalytic hydropyrolysis of lignin: Influence of reaction conditions on the formation and composition of liquid products
Bioresource Technology 40 (1992) 171-177
Catalytic Hydropyrolysis of Lignin: Influence of Reaction Conditions on the Formation and Composition of Liquid Products Dietrich Meier, Ronald Ante & Oskar Faix Bundesforschungsanstalt fiir Forst und Holzwirtschaft, Institut f/Jr Holzchemie und chemische Technologie des Holzes, Leuschnerstrasse 91, D-2050 Hamburg 80, FRG (Received 13 December 1990; revised version received 20 April 1991 ; accepted 26 April 1991 )
Abstract
Catalytic hydropyrolysis of different lignins has been studied under various reaction conditions. With a palladium catalyst on active charcoal, liquid products (oil) up to 80 wt% were obtained and the solid residue amounted to only 1 wt%. Nickelmolybdenum on aluminosilica catalysts led to oil yields of about 65 wt%, containing larger amounts of monomeric phenols. Therefore, nickel-molybdenum catalysts were selected to study the influence of the reaction conditions. Under initial hydrogen pressures ranging from 3 to 12 MPa the yield of oil increased from 18"4 wt% to 63"2 wt%. Reaction time had only a slight influence on the product yields, while increasing reaction temperatures promoted fragmentation to monomeric compounds. About one third of the oil was detectable by gas chromatography. The composition of this monomeric fraction did not change under the influence of changing process parameters. The main components of this fraction were phenol and cresols. Some alkylated phenols were found in minor amounts. Guaiacol was found only at a reaction temperature of 350°C. In none of the nickel-molybdenum experiments were catechols detected. Key words: Lignin, hydropyrolysis, liquefaction, catalysts, gas chromatography.
INTRODUCTION Since the aromatic character of lignin is known, its hydrogenolytic treatment has had two different
objectives: elucidation of the lignin structure; production of chemical feedstocks or fuels. In the light of the energy crisis in the early 1970s the production of fuels has attracted more interest, while in the meantime emphasis is laid on the degradation of lignin to commercially morevaluable low-molecular-weight chemicals. Many approaches have been made to find an economically reasonable utilization of the pulping byproduct lignin as a renewable source of organic carbon (Schweers, 1966; Goldstein, 1975; Meier, 1980). Hitherto, all attempts at hydroconverting lignin have been carried out either in water or in various solvents. One reason for this approach was that black liquors already contained the lignin in aqueous solution. The utilization of precipitated lignin as a dry powder has not given acceptable results. The first to succeed in the production of phenolic components from lignin in an aqueous alkaline solution were Lautsch and Freudenberg (1943). They established the effect of the different catalytic systems which are still used in lignin hydrotreatment. More than 10 years later the Swiss Inventa AG (1955)investigated the hydrogenolytic degradation of lignin in an aqueous solution. But high reaction-pressures and the restriction to water-soluble lignins required the use of high-boiling-point solvents. Therefore, in subsequent studies xylenol and a process-derived pasting oil were used as solvents. The latter seemed to be a promising solvent system, particularly on economic grounds. In a two-stage process 21% distillable phenols were obtained (Giesen et al., 1963). Workers at the Japanese Noguchi Institute studied the catalytic liquefaction of lignin with
171 Bioresource Technology 0960-8524/92/S05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain
172
D. Meier, R. Ante, O. Faix
phenolic solvents or process-derived pasting oil (Oshima et al., 1964). They reported high yields of monomeric phenols (up to 42%), and these could not be improved on in the experiments of the American Crown Zellerbach Co. (Goheen, 1966). Goheen found that the phenolic solvent which was alkylated during the process caused the high yield of monomeric phenols; but the yield of phenols, at 21%, was still remarkable. These results were only exceeded by the data of the Lignol process. Huibers et al. (1980) reported a yield of monomeric phenols up to 37.5% in a twostage ebullated-bed process. Up to now their results have not been confirmed. Connors et al. (1980) established the hydrogen-donating effect of tetralin upon the hydrocracking of kraft 'lignin. With the addition of cresol to the tetralin solvent Vuori et al. (1988) demonstrated the accelerating effect of phenolic compounds on the formation of ether-soluble phenolics and acids. Under mild thermolytic conditions, without catalyst, a maximum yield of ether-soluble phenols of 20.3 wt% was obtained. With the intention of developing a ligninderived pasting oil Johnson et al. (1990) investigated the behaviour of m-cresol as a stable solvent for lignin-hydrocracking. According to their results, hydrotreatment of cresol led not only to the formation of low-boiling products, but also to undesirable high-boiling compounds, mostly anthracene. The purpose of the present study was to exclude any influence of a solvent system on the catalytic hydrogenation of lignin. Therefore, we applied hydropyrolysis in a process where iignin reacted directly with hydrogen in a gas-solid reaction. Since solvents or pasting oils were avoided, there was no doubt about the origin of the degradation products. Hence, the question of whether hydropyrolysis in an autoclave equipped with an effective stirrer was suitable for producing monomeric phenols was investigated.
METHODS Feedstocks Three different technical lignins were tested. Their analytical data are given in Table 1. Elemental composition was determined on a CHOAnalyzer (Her~ius, Hanau, FRG). The organocell lignin was obtained from Organocell GmbH Munich, FRG. It was derived from the second stage of a technical two-stage pulping process of
Table 1. Analytical data of technical lignins in wt%. A s h and moisture content are on dry basis, elemental composition is based on maf feedstock
C H O" S OCH 3
Ash Moisture
Organocell
Kraft
A cetosolv
66.85 6.29 26.86 0.0 13"98 5"03 4"32
67.51 6.09 24.32 2.11 13"1 5"5 4"32
61.47 5.68 32.84 0-0 12"7 0"5 1"88
"Bydifference.
spruce wood (Picea sp.) and was precipitated in sulphuric acid. In the first step, a mixture of methanol/water was used as pulping liquor. In the second stage, caustic soda was added. A detailed characterization of the organocell lignin is given by Lindner and Wegener (1988). Kraft lignin (type K 27) was supplied by Holmens Bruk AB, Sweden. The acetosolv lignin was produced at Kunz, Gschwend, FRG, by treating spruce wood with acetic acid and 0"17% hydrochloric acid for 5 h at 110°C. Various commercially-available catalysts were used: their compositions are listed in Table 2. To obtain optimum distribution between lignin and catalysts the latter were ground to a fine powder in a ZM 1 mill (Retsch, Haan, FRG).
Hydropyrolysis experiments About 40 g air-dry lignin, equivalent to 36.5 g moisture- and ash-free (maf) material, were mixed intensively with 15 g of catalyst (c. 6 wt% metallic compounds based on maf lignin). This mixture was introduced into a 250 ml 'magne-drive' autoclave manufactured by Autoclave-Engineers Burton Corblin, France (Typ ABC-0025-SS-04DC, material 316 Ti). The system was pressurized with hydrogen after leak testing and purging twice with hydrogen to remove oxygen. The temperature was controlled at + 2°C and monitored on a plotter. During the reaction time, a few irregularities in the heating curve were observed due to endothermal or exothermal reactions. The stirrer speed was 1500 rpm. Experimental details are given in Table 3. After the reaction time, the heating was stopped and the system cooled down naturally. The hydrocracking products were treated as follows: gases were rejected; the brown, viscous, oil was taken out of the reactor, then filtered and the
Catalytic hydropyrolysis of lignin
173
Table 2. Information on the catalysts
Catalyst Pd/C
5
Loading
Support
l¥oducer
wt%Pd pre-reduced
Active charcoal
DEGUSSA, Hanau, FRG
Red mud (RM)
Fe_~O3
No
Siidchemie, Miinchen, FRG
Raney Ni
Raney nickel
No
DEGUSSA
M8-81
6 15
wt% NiO wt%MoO~
Aluminosilica
BASF, Ludwigshafen, FRG
M8-82
5-5 wt% NiO 18 wt%MoO 3
Aluminosilica
BASF
M8-86
5-5 wt% NiO 18 wt% MoO 3
Zeolite
BASF
Table 3. Reaction conditions for all hydropyrolysis experiments
Run No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Lignin
Organocell Organoceli Organocell Organocell Organocell Organocell Organocell Organocell Organocell Organocell Organocell Organoceli Organocell Organocell Organoceil Organocell Organocell Organocell Kraft Acetosoiv Kraft Kraft Acetosolv
Catalyst
Fe_,O a Raney Ni Pd/C None M8-81 M8-81 M8-81 M8-81 M8-81 M8-81 M8-81 M8-81 M8-81 M8-86 M8-82 M8-81 M8-81 M8-81 M8-81 M8-81 M8-82 M8-81 M8-82
Pressure (MPa) Initial
Maximum
10 10 10 10 3 12 10 8 6 10 10 10 10 10 10 10 10 10 10 10 12 10 10
30 22 18 30 15 26.5 22 19 17 22 22 21 23 30-5 25 21 22 22"5 19 21 23 20 21
autoclave washed with dichloromethane. This solution was used for the extraction of the solid residue for 24 h in a Soxhlet apparatus. The remaining solid residue was dried overnight at 103°C and extracted again for 4 h with acetone. Char was determined as weight of solid residue minus catalyst. The extract was called 'acetoneextract'. The fraction soluble in dichloromethane was concentrated to about 200 ml and dried for 12 h over Na2SO4, then filtered and washed. The solvent was evaporated under vacuum and the remaining liquid was designated as 'oil'.
Temperature (°C)
Heat time (minutes)
Reaction time (minutes)
380 380 380 380 420 420 420 420 420 400 350 400 450 420 420 420 420 420 400 420 420 420 420
52 43 39 48 62 55 65 63 81 47 35 45 73 64 63 62 60 61 48 63 58 55 54
15 15 15 15 15 15 15 15 15 15 15 0 15 15 15 60 0 120 15 15 15 15 15
Analysis of the oil Oil analysis was performed by means of a Chrompack 438 A gas chromatograph equipped with an FID and: fused silica capillary column, 50 m x0.32 mm; film SE 54 of 1/~m thickness (Macherey & Nagel, Dfiren, FRG); carrier gas, hydrogen; column inlet pressure 100 kPa; inj. temperature 250°C; det. temperature 280°C; oven temperature programmed 4 min constant at 45°C then 4°C/min to 280°C; internal standard: fluoranthen. The samples were dissolved in a mixture of acetone/dichloromethane (50 : 50) without prior derivatization. For quantitative estimations
174
D. Meier, R. Ante, O. Faix
the internal-standard method was used and the relative response factors were ascertained from known mixtures.
wt.% based
on lignin
100 4
Run
number
3
80-
60-
RESULTS AND .DISCUSSION
4020
The amount of products recovered varied in all experiments between 60 and 80 wt% based on maf lignin. The differences in the material balances must be ascribed to the formation of water, gases and volatile hydrocarbons. In prior experiments Meier et al. (1988) analysed the gas from biomass hydroliquefaction and found mainly carbon monoxide, carbon dioxide and methane.
Catalyst screening The yield of oil The results obtained with the different catalysts are depicted in Fig. 1. The highest oil yields (80.6 wt%) were obtained with the Pd/C catalyst. Without catalyst the oil amounted to only 15-1 wt%. A small increase up to 17 wt% was caused by the addition of Fe203. The negligible effect of Fe203 has been reported earlier (Schultz et al., 1982). Raney nickel as catalyst led to oil yields of 53.6 wt%, while nickel-molybdenum on aluminosilica (M8-81 ) caused a further increase of oil formation up to 66 wt%. In addition to M8-81, which contained 6% nickel oxide and 15% molybdenum oxide the catalysts M8-82 and M8-86 were tested. Both preparations had a lower NiO (5"5%) and a higher MoO 3 (18%) content than the catalyst M881. M8-82 yielded the maximum 53.6 wt% of oil, while M8-86, with a zeolitic support, caused a definite decrease in the oil yield to 17.3 wt%. Similar low yields were obtained in experiments without catalyst. Since the catalytic activity of M881 and M8-82 has been already demonstrated in our experiments, this low yield must be due to the unsuitable zeolitic channel system which prevents the access of lignin-cracking products to the active surface of the catalyst. Composition of the oils Owing to its low volatility, only a maximum of about 33 wt% of the M8-81 oil, corresponding to 22 wt% based on maf lignin, could be analysed by gas chromatography. This monomeric fraction was composed mainly of phenols, which amounted to 23% (based on oil or 15 wt% based on maf lignin). The amounts reported by Goheen
0 A n o cal.
M8-86
RM
Rane¥ NI M 8 - 8 2
M8-81
Pd/C
Catalyst
m
detectable
~
Oft
m
Char
Fig. l. Influence of different catalysts on liquid and solid hydropyrolysis products from organocell iignin.
(1966) were higher (max. 22% based on lignin) but they were obtained in the presence of a pasting oil or phenol. As discussed in the introduction, these solvents prevent an accurate determination of the product's origin and a quantification of single phenols. Huibers et al. (1980) reported still higher phenolic yields, but these data are doubtful because they were only estimated. The composition of the oils depended on the type of catalysts. The oil obtained with Pd/C contained large amounts of ethyl-, methyl-, and propyl-cyclohexanones and minor quantities of phenols, guaiacols and catechols. Compared with this, mainly monophenols were detected in the oils from the other catalysts. Therefore, NiMo catalysts were chosen to study the influence of the reaction conditions. The differences in the oil compositions can be seen in Table 4 where yields of monomeric components of two different runs, catalysed with palladium and nickel-molybdenum, are compiled. Besides phenol, the main components were cresols, dimethylphenols, ethyl- and propylphenol. The presence of guaiacol was proved only in one case when the reaction temperature was low (350°C). At higher reaction temperatures guaiacol was completely demethoxylated. Catechols were not detected in any of the experiments catalysed by NiMo. This finding confirms the high demethoxylating power of nickel-molybdenum. Mainly demethylation to catechols was observed in experiments without catalyst or with palladium catalyst.
Yields and composition of oils from different lignins When kraft lignin was used as feedstock, the oil amounted to 61.6 wt% and 1-3 wt% char was found. This can be ascribed to the sulphur in the
Catalytic hydropyrolysis of lignin 4. Identified compounds in the oils from runs no. 16 and no. 3 listed in the order of GC elution
Table
Substance
Ethylcyclohexane m/p-xylol Cyclohexanone Propylcyclohexanone 2-methylcyclohexanone 3-methylcyclohexanone 4-methylcyclohexanone Phenol m/p-methylanisol o-cresol 3-ethylcyclohexanone m/p-cresol 4-ethylcyclohexanone Guaiacol 2,6-dimethylphenol 2-ethylphenol 2,4-dimethylphenol 2,5-dimethylphenol 4-ethylphenol 3-ethyl-/ 3,5-dimethylphenol 2,3-dimethylphenol Naphthalene/ 3,4-dimethylphenol Methylguaiacol 2,4,6-trimethylphenol 2,3,6-trimethylphenol Methylethylphenol Methyltetralin 4-propylphenol 2,3,5-trimethylphenol 4-ethylguaiacol 2-methylnaphthalene 1-methylnaphthalene 4-propylguaiacol Ethylcatechol Propylcatechol Total Unknown GC-detectable
Run no. 16 (wt% based on lignin)
Run no. 3 (wt% based on lignin)
nd 0"08 nd nd nd nd nd 3-68 0-07 1-24 nd 3.69 nd nd 0-13 0.24 0.41 0-62 0.60 1-61
0-11 nd 0.43 0.09 0"14 0.14 0.31 0-14 nd 0.10 0.28 nd 0-76 0.61 nd nd 0.02 nd 0.82 0.18
0.35 0.57
0.06 0-06
nd 0-08 0-09 0-02 0"22 1.19 0.34 nd 0.38 0-28 nd nd nd
0.86 nd nd 0"37 0-06 0"37 nd 1-17 nd 0"06 0.49 0.61 0"42
16.26 5"57 21-84
8.78 2.66 11.44
nd, not detected.
kraft lignin which affected a sulphidation of the catalyst. It is well known that sulphided NiMo is more active than the oxidized form (Baker & Elliott, 1988). Hence, an increase in oil formation was observed compared to organocell lignin (Fig. 2). The lowest yields (37.9 wt%) were obtained with acetosolv lignin. This can be explained by the presence of acetyl groups introduced during the pulping process. The acetic structures might cause surface problems and hinder the contact of lignin molecules with the surface of the catalyst. But this phenomenon needs further investigation.
100 80-
175
wt.% based on lignin t4 ][ Run number J/T
6040-
20-
I
Char
~
oil
m
detectable
0 Acetosolv
Organocell
Kraft
Lignin Fig. 2. Yields of liquid and solid hydropyrolysis products obtained from different technical lignins.
Surprisingly, there were no differences in the composition of the GC-detectable part of the three lignin oils. This indicates that lignins from different technical processes, having different chemical structures, do not affect the quality but only the quantity of the oil. Although the oil yields were higher with sulphur-containing kraft lignin, sulphur-free organocell lignin was selected to study further the influence of the reaction conditions. Influence of the reaction conditions
In hydropyrolysis with NiMo catalysts, variation of reaction parameters produced no change in the composition of the monomeric fraction, indicating that the resulting compounds were rather stable under the conditions applied. Hence, the degradation mechanisms of all lignins must be the same. Corresponding observations were reported by Davoudzadeh et al. (1985) who did not find essential differences in hydrogenolysis products of lignin when the reaction conditions were varied.
Temperature The influence of the reaction temperature was studied in experiments at 350, 400, 420 and 450°C. The results are shown in Fig. 3. The highest yield of oil (66 wt% of maf lignin) was achieved at a temperature of 400°C, with an initial pressure of 10 MPa and a reaction time of 0 min (run no. 12). Under these conditions, char amounted to 5.7 wt% and the acetone extract was 4-4 wt%. The monophenols quantified by gas chromatography amounted to only 11 wt%, based on dry feedstock. It seems obvious that high oilyields do not automatically imply a large monomeric portion.
176
D. Meier, R. Ante, O. Faix
At 350°C the oil yield amounted to about 40 wt% while the acetone extract increased to 22 wt%. According to Meier et al. (1986) the acetone extract contains high-molecular-weight lignin fragments, showing that complete degradation does not take place at the relative low temperature of 350°C. Furthermore, only at that temperature was guaiacol detected in the oil. At temperatures above 400°C the oil yield decreased slightly, but both the GC-detectable fraction and the char formation were increased. The amount of detectable compounds increased from 17 wt% (6"8 wt% based on lignin) at 350°C to 44.7 wt% (22.7 wt% based on lignin) at 450°C, indicating that the cleavage of lignin bonds was governed by temperature. The formation of char increased with increasing temperature because not all radicals formed during the reaction could be saturated with hydrogen. These findings correspond with the results of Johnson et al. (1990) who performed hydrotreating experiments with lignin dissolved in cresol, and cresol alone. Reaction temperature was varied between 350°C and 425°C at a hydrogen pressure of about 7 MPa. They observed both an increased formation of highly-condensed aromatics (anthracenes) and an enhanced lignin depolymerization at higher temperatures. Hydrogen pressure
The influence of the partial hydrogen pressure was investigated by obtaining an initial hydrogen pressure (cold) of 3, 6, 8, 10 and 12 MPa in the reaction vessel. Due to the absence of any solvent the pressure at reaction temperature (see Table 3) was not influenced by other than lignin-derived components. At higher partial hydrogen pressures oil yields increased from 18.4 wt% to 63-2 wt% (see Fig. 4). Coincidently, the amount of char decreased from 41 wt% to only 3 wt%. No effect on the amount of the acetone extract, which reached about 0.5 wt%, was observed. The amount of the monomeric fraction increased with increasing pressure, but it reached a maximum at 10 MPa initial pressure. As pointed out before, the oil obtained at 3 MPa initial pressure contained the same components, in the same relative amounts, as the oil obtained at 12 MPa. The great influence of the hydrogen pressure on the oil yield demonstrates that sufficient available hydrogen can prevent recondensation reactions. As expected, hydrogen pressure is much more important when the hydrotreatment is carfled out without solvent. In a hydrogen-donating
wt.% b a s e d on l i g n i n '
I1
Run n u m b e r 10
a0
Acetone extract
40
I I Char •S.3 Oil
20-
delectable 0-
aso
400 420 450 T e m p e r a t u r e [o C]
Fig. 3. Influence of reaction temperature on liquid and solid hydropyrolysisproducts from organocell lignin.
wt.% b a s e d o n l i g n i n lOO-~
Run n u m b e r
S0 60' 40" Char
20.
~
Oll deteclable
3
6
8
l0
12
I n i t i a l pressure [MPa] Fig. 4. Influence of cold initial hydrogen pressure on liquid and solid hydropyrolysis p r o d u c t s from organocell lignin.
wt.% b a s e d on l i g n l n /I Run n u m b e r so/I 17 16 7
I00
-
18
6040 m
20
~
Char
Oil detectable
0
0
15
60
120
Reaction t i m e [mini Fig. 5. Influence of reaction time on liquid and solid hydropyrolysis products from organocell lignin.
solvent system, e.g. in tetralin, the influence of the hydrogen atmosphere becomes less important, as reported before (Davoudzadeh et al., 1985; Sugita et al., 1988). Time
To determine the influence of the reaction time, experiments were conducted at 0, 15, 60 and 120 min. The results are shown in Fig. 5. A reaction time of 0 rnin means that the heating was stopped
Catalytic hydropyrolysis of lignin
immediately after reaching the reaction temperature. In this case an oil yield of 58-6 wt% was obtained. A portion of 26.4 wt% (15.5 wt% based on lignin) was detectable by gas chromatography. After a reaction time of 60 min the oil yield remained more or less constant, but the detectable fraction increased up to 37.8 wt% (22 wt% based on lignin). A maximum of 6.4 wt% phenol (3.7 wt% based on lignin) was obtained in this experiment. After 120 min the oil yield and the monomeric fraction decreased slightly. Schultz et al. (1982) also reported decreasing amounts of solid residues with increasing reaction time, but the yield of soluble phenolics decreased coincidently, depending on the temperature. Connors et al. (1980) obtained a maximum of phenolic components (7 wt% of original lignin) after 15 min of reaction time. After 10 h the monomeric phenols amounted to only 1.1 wt%. ACKNOWLEDGEMENTS
This work was financially supported by the Commission of the European Communities, contract no. EN 3B-0120 D and the Federal Ministry of Food, Agriculture and Forestry, FRG, contract no. 89 NR 015. Thanks are due to Harald Voss who performed some of the experiments. REFERENCES Baker. E. G. & Elliott, D. C. (1988). Catalytic hydrotreating of biomass-derived oils. ACS Syrup. Ser., 376, 228-40. Connors, W. J., Johanson, L. N., Sarkanen, K. V. & Winslow, P. (1980). Thermal degradation of kraft lignin in tetralin. Holzforschung, 34, 29-37. Davoudzadeh, F., Smith, B., Avni, E. & Coughlin, R. W. (1985). Depolymerization of lignin at low pressure using Lewis acid catalysts and under high pressure using hydrogen donor solvents. Holzforschung, 39 (1985) 159-66. Giesen, J., Ploetz, T., Richtzenhain, H. & Deiters, W. (1963). Verfahren zur Herstellung von niedrigsiedenden Spalt-
177
produkten aus Lignin durch kontinuierliche Druckhydrierung in Gegenwart organischer Fliissigkeiten. Pat. No. 114 2853, FRG. Goheen, D. W. (1966). Hydrogenation of lignin by the Noguchi process. Adv. Chem. Ser., 59, 205-25. Goldstein, 1. S. (1975). Perspectives on production of phenols and phenolic acids from lignin and bark. Appl. Polym. Symp., 28,259-67. Huibers, D. T. A., Parkhurst, H. J. & Jones, M. W. (1980). Production of phenol from lignin. In Symp. Altern. Feedst., Vol. 25, Petrochemicals Division of Petroleum Chemistry, Inc., American Chemical Society, pp. 24-9. lnventa AG (1955). Verfahren zur Herstellung von Phenolen durch Spaitung des Lignins in Sulfitablauge. Pat. No. 305 712, Switzerland. Johnson, D. K., Chum, H. L., Anzick, R. & Baldwin, R. M. (1990). Preparation of a lignin-derived pasting oil. Appl. Biochem. BiotechnoL, 24-5, 31-40. Kanazawa, K., M6rck, R., Koyama, M. & Kringstad, K. P. (1987). Chemical structure of hydrocracked lignin: characterization of hydrocracked lignin oligomers. Holzforschung, 41,299-303. Lautsch, W. & Freudenberg, K. (1943). Verfahren zur Gewinnung von Phenolen, deren Abk6mmlingen und hydrierten Phenolen durch Druckhydrierung von Lignin oder ligninhaltigen Stoffen. Pat. No. 741686, FRG. Lindner, A. & Wegener, G. (1988). Characterization of lignins from organosolv pulping according to the organosolv process. J. Wood Chem. TechnoL, 8, 323-40. Meier, D. (1980). Verwertbarkeit von Ethanol/WasserLigninen durch Oxidation und Hydrogenolyse sowie ihre chemische und spektroskopische Charakterisierung. Dissertation, Universit~it Hamburg, FRG. Meier, D., Larimer, D. R. & Faix, O. (1986). Direct liquefaction of different lignocellulosics and their constituents. 1. Fractionation, elemental composition. Fuel, 65, 910-15. Meier, D., Jakobi, L. & Faix, O. (1988). Catalytic hydroliquefaction of spruce wood -- elemental balance and effect of catalyst. J. Wood Sci. Technol., 8, 523-42. Oshima, M., Maeda, Y. & Kashima, K. (1964). Method for liquefying lignin. Pat. No. 700 210, Canada. Schultz, T. P., Preto, R. J., Pittman, J. L. & Goldstein, 1. S. (1982). Hydrotreating of hydrochloric acid lignin in a hydrogen donor solvent. J. Wood Chem. Technol., 2, 17-31. Schweers, W. (1966). IJber die Hydrogenolyse von Lignin. 1. Mitt. Paperija Puu, 48, 161-74. Sugita, T., Mori, H. & ltoh, O. (1988). Hydrogenolysis of kraft lignin: acidic lignin is more effective. Chemistry Express, 3,667-70. Vuori, A. & Bredenberg, J. B.-son (1988). Liquefaction of kraft lignin. 1. Primary reactions under mild thermolysis conditions. Holzforschung, 42, 155-61.
Catalytic Hydropyrolysis of Lignin: Influence of Reaction Conditions on the Formation and Composition of Liquid Products Dietrich Meier, Ronald Ante & Oskar Faix Bundesforschungsanstalt fiir Forst und Holzwirtschaft, Institut f/Jr Holzchemie und chemische Technologie des Holzes, Leuschnerstrasse 91, D-2050 Hamburg 80, FRG (Received 13 December 1990; revised version received 20 April 1991 ; accepted 26 April 1991 )
Abstract
Catalytic hydropyrolysis of different lignins has been studied under various reaction conditions. With a palladium catalyst on active charcoal, liquid products (oil) up to 80 wt% were obtained and the solid residue amounted to only 1 wt%. Nickelmolybdenum on aluminosilica catalysts led to oil yields of about 65 wt%, containing larger amounts of monomeric phenols. Therefore, nickel-molybdenum catalysts were selected to study the influence of the reaction conditions. Under initial hydrogen pressures ranging from 3 to 12 MPa the yield of oil increased from 18"4 wt% to 63"2 wt%. Reaction time had only a slight influence on the product yields, while increasing reaction temperatures promoted fragmentation to monomeric compounds. About one third of the oil was detectable by gas chromatography. The composition of this monomeric fraction did not change under the influence of changing process parameters. The main components of this fraction were phenol and cresols. Some alkylated phenols were found in minor amounts. Guaiacol was found only at a reaction temperature of 350°C. In none of the nickel-molybdenum experiments were catechols detected. Key words: Lignin, hydropyrolysis, liquefaction, catalysts, gas chromatography.
INTRODUCTION Since the aromatic character of lignin is known, its hydrogenolytic treatment has had two different
objectives: elucidation of the lignin structure; production of chemical feedstocks or fuels. In the light of the energy crisis in the early 1970s the production of fuels has attracted more interest, while in the meantime emphasis is laid on the degradation of lignin to commercially morevaluable low-molecular-weight chemicals. Many approaches have been made to find an economically reasonable utilization of the pulping byproduct lignin as a renewable source of organic carbon (Schweers, 1966; Goldstein, 1975; Meier, 1980). Hitherto, all attempts at hydroconverting lignin have been carried out either in water or in various solvents. One reason for this approach was that black liquors already contained the lignin in aqueous solution. The utilization of precipitated lignin as a dry powder has not given acceptable results. The first to succeed in the production of phenolic components from lignin in an aqueous alkaline solution were Lautsch and Freudenberg (1943). They established the effect of the different catalytic systems which are still used in lignin hydrotreatment. More than 10 years later the Swiss Inventa AG (1955)investigated the hydrogenolytic degradation of lignin in an aqueous solution. But high reaction-pressures and the restriction to water-soluble lignins required the use of high-boiling-point solvents. Therefore, in subsequent studies xylenol and a process-derived pasting oil were used as solvents. The latter seemed to be a promising solvent system, particularly on economic grounds. In a two-stage process 21% distillable phenols were obtained (Giesen et al., 1963). Workers at the Japanese Noguchi Institute studied the catalytic liquefaction of lignin with
171 Bioresource Technology 0960-8524/92/S05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain
172
D. Meier, R. Ante, O. Faix
phenolic solvents or process-derived pasting oil (Oshima et al., 1964). They reported high yields of monomeric phenols (up to 42%), and these could not be improved on in the experiments of the American Crown Zellerbach Co. (Goheen, 1966). Goheen found that the phenolic solvent which was alkylated during the process caused the high yield of monomeric phenols; but the yield of phenols, at 21%, was still remarkable. These results were only exceeded by the data of the Lignol process. Huibers et al. (1980) reported a yield of monomeric phenols up to 37.5% in a twostage ebullated-bed process. Up to now their results have not been confirmed. Connors et al. (1980) established the hydrogen-donating effect of tetralin upon the hydrocracking of kraft 'lignin. With the addition of cresol to the tetralin solvent Vuori et al. (1988) demonstrated the accelerating effect of phenolic compounds on the formation of ether-soluble phenolics and acids. Under mild thermolytic conditions, without catalyst, a maximum yield of ether-soluble phenols of 20.3 wt% was obtained. With the intention of developing a ligninderived pasting oil Johnson et al. (1990) investigated the behaviour of m-cresol as a stable solvent for lignin-hydrocracking. According to their results, hydrotreatment of cresol led not only to the formation of low-boiling products, but also to undesirable high-boiling compounds, mostly anthracene. The purpose of the present study was to exclude any influence of a solvent system on the catalytic hydrogenation of lignin. Therefore, we applied hydropyrolysis in a process where iignin reacted directly with hydrogen in a gas-solid reaction. Since solvents or pasting oils were avoided, there was no doubt about the origin of the degradation products. Hence, the question of whether hydropyrolysis in an autoclave equipped with an effective stirrer was suitable for producing monomeric phenols was investigated.
METHODS Feedstocks Three different technical lignins were tested. Their analytical data are given in Table 1. Elemental composition was determined on a CHOAnalyzer (Her~ius, Hanau, FRG). The organocell lignin was obtained from Organocell GmbH Munich, FRG. It was derived from the second stage of a technical two-stage pulping process of
Table 1. Analytical data of technical lignins in wt%. A s h and moisture content are on dry basis, elemental composition is based on maf feedstock
C H O" S OCH 3
Ash Moisture
Organocell
Kraft
A cetosolv
66.85 6.29 26.86 0.0 13"98 5"03 4"32
67.51 6.09 24.32 2.11 13"1 5"5 4"32
61.47 5.68 32.84 0-0 12"7 0"5 1"88
"Bydifference.
spruce wood (Picea sp.) and was precipitated in sulphuric acid. In the first step, a mixture of methanol/water was used as pulping liquor. In the second stage, caustic soda was added. A detailed characterization of the organocell lignin is given by Lindner and Wegener (1988). Kraft lignin (type K 27) was supplied by Holmens Bruk AB, Sweden. The acetosolv lignin was produced at Kunz, Gschwend, FRG, by treating spruce wood with acetic acid and 0"17% hydrochloric acid for 5 h at 110°C. Various commercially-available catalysts were used: their compositions are listed in Table 2. To obtain optimum distribution between lignin and catalysts the latter were ground to a fine powder in a ZM 1 mill (Retsch, Haan, FRG).
Hydropyrolysis experiments About 40 g air-dry lignin, equivalent to 36.5 g moisture- and ash-free (maf) material, were mixed intensively with 15 g of catalyst (c. 6 wt% metallic compounds based on maf lignin). This mixture was introduced into a 250 ml 'magne-drive' autoclave manufactured by Autoclave-Engineers Burton Corblin, France (Typ ABC-0025-SS-04DC, material 316 Ti). The system was pressurized with hydrogen after leak testing and purging twice with hydrogen to remove oxygen. The temperature was controlled at + 2°C and monitored on a plotter. During the reaction time, a few irregularities in the heating curve were observed due to endothermal or exothermal reactions. The stirrer speed was 1500 rpm. Experimental details are given in Table 3. After the reaction time, the heating was stopped and the system cooled down naturally. The hydrocracking products were treated as follows: gases were rejected; the brown, viscous, oil was taken out of the reactor, then filtered and the
Catalytic hydropyrolysis of lignin
173
Table 2. Information on the catalysts
Catalyst Pd/C
5
Loading
Support
l¥oducer
wt%Pd pre-reduced
Active charcoal
DEGUSSA, Hanau, FRG
Red mud (RM)
Fe_~O3
No
Siidchemie, Miinchen, FRG
Raney Ni
Raney nickel
No
DEGUSSA
M8-81
6 15
wt% NiO wt%MoO~
Aluminosilica
BASF, Ludwigshafen, FRG
M8-82
5-5 wt% NiO 18 wt%MoO 3
Aluminosilica
BASF
M8-86
5-5 wt% NiO 18 wt% MoO 3
Zeolite
BASF
Table 3. Reaction conditions for all hydropyrolysis experiments
Run No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Lignin
Organocell Organoceli Organocell Organocell Organocell Organocell Organocell Organocell Organocell Organocell Organocell Organoceli Organocell Organocell Organoceil Organocell Organocell Organocell Kraft Acetosoiv Kraft Kraft Acetosolv
Catalyst
Fe_,O a Raney Ni Pd/C None M8-81 M8-81 M8-81 M8-81 M8-81 M8-81 M8-81 M8-81 M8-81 M8-86 M8-82 M8-81 M8-81 M8-81 M8-81 M8-81 M8-82 M8-81 M8-82
Pressure (MPa) Initial
Maximum
10 10 10 10 3 12 10 8 6 10 10 10 10 10 10 10 10 10 10 10 12 10 10
30 22 18 30 15 26.5 22 19 17 22 22 21 23 30-5 25 21 22 22"5 19 21 23 20 21
autoclave washed with dichloromethane. This solution was used for the extraction of the solid residue for 24 h in a Soxhlet apparatus. The remaining solid residue was dried overnight at 103°C and extracted again for 4 h with acetone. Char was determined as weight of solid residue minus catalyst. The extract was called 'acetoneextract'. The fraction soluble in dichloromethane was concentrated to about 200 ml and dried for 12 h over Na2SO4, then filtered and washed. The solvent was evaporated under vacuum and the remaining liquid was designated as 'oil'.
Temperature (°C)
Heat time (minutes)
Reaction time (minutes)
380 380 380 380 420 420 420 420 420 400 350 400 450 420 420 420 420 420 400 420 420 420 420
52 43 39 48 62 55 65 63 81 47 35 45 73 64 63 62 60 61 48 63 58 55 54
15 15 15 15 15 15 15 15 15 15 15 0 15 15 15 60 0 120 15 15 15 15 15
Analysis of the oil Oil analysis was performed by means of a Chrompack 438 A gas chromatograph equipped with an FID and: fused silica capillary column, 50 m x0.32 mm; film SE 54 of 1/~m thickness (Macherey & Nagel, Dfiren, FRG); carrier gas, hydrogen; column inlet pressure 100 kPa; inj. temperature 250°C; det. temperature 280°C; oven temperature programmed 4 min constant at 45°C then 4°C/min to 280°C; internal standard: fluoranthen. The samples were dissolved in a mixture of acetone/dichloromethane (50 : 50) without prior derivatization. For quantitative estimations
174
D. Meier, R. Ante, O. Faix
the internal-standard method was used and the relative response factors were ascertained from known mixtures.
wt.% based
on lignin
100 4
Run
number
3
80-
60-
RESULTS AND .DISCUSSION
4020
The amount of products recovered varied in all experiments between 60 and 80 wt% based on maf lignin. The differences in the material balances must be ascribed to the formation of water, gases and volatile hydrocarbons. In prior experiments Meier et al. (1988) analysed the gas from biomass hydroliquefaction and found mainly carbon monoxide, carbon dioxide and methane.
Catalyst screening The yield of oil The results obtained with the different catalysts are depicted in Fig. 1. The highest oil yields (80.6 wt%) were obtained with the Pd/C catalyst. Without catalyst the oil amounted to only 15-1 wt%. A small increase up to 17 wt% was caused by the addition of Fe203. The negligible effect of Fe203 has been reported earlier (Schultz et al., 1982). Raney nickel as catalyst led to oil yields of 53.6 wt%, while nickel-molybdenum on aluminosilica (M8-81 ) caused a further increase of oil formation up to 66 wt%. In addition to M8-81, which contained 6% nickel oxide and 15% molybdenum oxide the catalysts M8-82 and M8-86 were tested. Both preparations had a lower NiO (5"5%) and a higher MoO 3 (18%) content than the catalyst M881. M8-82 yielded the maximum 53.6 wt% of oil, while M8-86, with a zeolitic support, caused a definite decrease in the oil yield to 17.3 wt%. Similar low yields were obtained in experiments without catalyst. Since the catalytic activity of M881 and M8-82 has been already demonstrated in our experiments, this low yield must be due to the unsuitable zeolitic channel system which prevents the access of lignin-cracking products to the active surface of the catalyst. Composition of the oils Owing to its low volatility, only a maximum of about 33 wt% of the M8-81 oil, corresponding to 22 wt% based on maf lignin, could be analysed by gas chromatography. This monomeric fraction was composed mainly of phenols, which amounted to 23% (based on oil or 15 wt% based on maf lignin). The amounts reported by Goheen
0 A n o cal.
M8-86
RM
Rane¥ NI M 8 - 8 2
M8-81
Pd/C
Catalyst
m
detectable
~
Oft
m
Char
Fig. l. Influence of different catalysts on liquid and solid hydropyrolysis products from organocell iignin.
(1966) were higher (max. 22% based on lignin) but they were obtained in the presence of a pasting oil or phenol. As discussed in the introduction, these solvents prevent an accurate determination of the product's origin and a quantification of single phenols. Huibers et al. (1980) reported still higher phenolic yields, but these data are doubtful because they were only estimated. The composition of the oils depended on the type of catalysts. The oil obtained with Pd/C contained large amounts of ethyl-, methyl-, and propyl-cyclohexanones and minor quantities of phenols, guaiacols and catechols. Compared with this, mainly monophenols were detected in the oils from the other catalysts. Therefore, NiMo catalysts were chosen to study the influence of the reaction conditions. The differences in the oil compositions can be seen in Table 4 where yields of monomeric components of two different runs, catalysed with palladium and nickel-molybdenum, are compiled. Besides phenol, the main components were cresols, dimethylphenols, ethyl- and propylphenol. The presence of guaiacol was proved only in one case when the reaction temperature was low (350°C). At higher reaction temperatures guaiacol was completely demethoxylated. Catechols were not detected in any of the experiments catalysed by NiMo. This finding confirms the high demethoxylating power of nickel-molybdenum. Mainly demethylation to catechols was observed in experiments without catalyst or with palladium catalyst.
Yields and composition of oils from different lignins When kraft lignin was used as feedstock, the oil amounted to 61.6 wt% and 1-3 wt% char was found. This can be ascribed to the sulphur in the
Catalytic hydropyrolysis of lignin 4. Identified compounds in the oils from runs no. 16 and no. 3 listed in the order of GC elution
Table
Substance
Ethylcyclohexane m/p-xylol Cyclohexanone Propylcyclohexanone 2-methylcyclohexanone 3-methylcyclohexanone 4-methylcyclohexanone Phenol m/p-methylanisol o-cresol 3-ethylcyclohexanone m/p-cresol 4-ethylcyclohexanone Guaiacol 2,6-dimethylphenol 2-ethylphenol 2,4-dimethylphenol 2,5-dimethylphenol 4-ethylphenol 3-ethyl-/ 3,5-dimethylphenol 2,3-dimethylphenol Naphthalene/ 3,4-dimethylphenol Methylguaiacol 2,4,6-trimethylphenol 2,3,6-trimethylphenol Methylethylphenol Methyltetralin 4-propylphenol 2,3,5-trimethylphenol 4-ethylguaiacol 2-methylnaphthalene 1-methylnaphthalene 4-propylguaiacol Ethylcatechol Propylcatechol Total Unknown GC-detectable
Run no. 16 (wt% based on lignin)
Run no. 3 (wt% based on lignin)
nd 0"08 nd nd nd nd nd 3-68 0-07 1-24 nd 3.69 nd nd 0-13 0.24 0.41 0-62 0.60 1-61
0-11 nd 0.43 0.09 0"14 0.14 0.31 0-14 nd 0.10 0.28 nd 0-76 0.61 nd nd 0.02 nd 0.82 0.18
0.35 0.57
0.06 0-06
nd 0-08 0-09 0-02 0"22 1.19 0.34 nd 0.38 0-28 nd nd nd
0.86 nd nd 0"37 0-06 0"37 nd 1-17 nd 0"06 0.49 0.61 0"42
16.26 5"57 21-84
8.78 2.66 11.44
nd, not detected.
kraft lignin which affected a sulphidation of the catalyst. It is well known that sulphided NiMo is more active than the oxidized form (Baker & Elliott, 1988). Hence, an increase in oil formation was observed compared to organocell lignin (Fig. 2). The lowest yields (37.9 wt%) were obtained with acetosolv lignin. This can be explained by the presence of acetyl groups introduced during the pulping process. The acetic structures might cause surface problems and hinder the contact of lignin molecules with the surface of the catalyst. But this phenomenon needs further investigation.
100 80-
175
wt.% based on lignin t4 ][ Run number J/T
6040-
20-
I
Char
~
oil
m
detectable
0 Acetosolv
Organocell
Kraft
Lignin Fig. 2. Yields of liquid and solid hydropyrolysis products obtained from different technical lignins.
Surprisingly, there were no differences in the composition of the GC-detectable part of the three lignin oils. This indicates that lignins from different technical processes, having different chemical structures, do not affect the quality but only the quantity of the oil. Although the oil yields were higher with sulphur-containing kraft lignin, sulphur-free organocell lignin was selected to study further the influence of the reaction conditions. Influence of the reaction conditions
In hydropyrolysis with NiMo catalysts, variation of reaction parameters produced no change in the composition of the monomeric fraction, indicating that the resulting compounds were rather stable under the conditions applied. Hence, the degradation mechanisms of all lignins must be the same. Corresponding observations were reported by Davoudzadeh et al. (1985) who did not find essential differences in hydrogenolysis products of lignin when the reaction conditions were varied.
Temperature The influence of the reaction temperature was studied in experiments at 350, 400, 420 and 450°C. The results are shown in Fig. 3. The highest yield of oil (66 wt% of maf lignin) was achieved at a temperature of 400°C, with an initial pressure of 10 MPa and a reaction time of 0 min (run no. 12). Under these conditions, char amounted to 5.7 wt% and the acetone extract was 4-4 wt%. The monophenols quantified by gas chromatography amounted to only 11 wt%, based on dry feedstock. It seems obvious that high oilyields do not automatically imply a large monomeric portion.
176
D. Meier, R. Ante, O. Faix
At 350°C the oil yield amounted to about 40 wt% while the acetone extract increased to 22 wt%. According to Meier et al. (1986) the acetone extract contains high-molecular-weight lignin fragments, showing that complete degradation does not take place at the relative low temperature of 350°C. Furthermore, only at that temperature was guaiacol detected in the oil. At temperatures above 400°C the oil yield decreased slightly, but both the GC-detectable fraction and the char formation were increased. The amount of detectable compounds increased from 17 wt% (6"8 wt% based on lignin) at 350°C to 44.7 wt% (22.7 wt% based on lignin) at 450°C, indicating that the cleavage of lignin bonds was governed by temperature. The formation of char increased with increasing temperature because not all radicals formed during the reaction could be saturated with hydrogen. These findings correspond with the results of Johnson et al. (1990) who performed hydrotreating experiments with lignin dissolved in cresol, and cresol alone. Reaction temperature was varied between 350°C and 425°C at a hydrogen pressure of about 7 MPa. They observed both an increased formation of highly-condensed aromatics (anthracenes) and an enhanced lignin depolymerization at higher temperatures. Hydrogen pressure
The influence of the partial hydrogen pressure was investigated by obtaining an initial hydrogen pressure (cold) of 3, 6, 8, 10 and 12 MPa in the reaction vessel. Due to the absence of any solvent the pressure at reaction temperature (see Table 3) was not influenced by other than lignin-derived components. At higher partial hydrogen pressures oil yields increased from 18.4 wt% to 63-2 wt% (see Fig. 4). Coincidently, the amount of char decreased from 41 wt% to only 3 wt%. No effect on the amount of the acetone extract, which reached about 0.5 wt%, was observed. The amount of the monomeric fraction increased with increasing pressure, but it reached a maximum at 10 MPa initial pressure. As pointed out before, the oil obtained at 3 MPa initial pressure contained the same components, in the same relative amounts, as the oil obtained at 12 MPa. The great influence of the hydrogen pressure on the oil yield demonstrates that sufficient available hydrogen can prevent recondensation reactions. As expected, hydrogen pressure is much more important when the hydrotreatment is carfled out without solvent. In a hydrogen-donating
wt.% b a s e d on l i g n i n '
I1
Run n u m b e r 10
a0
Acetone extract
40
I I Char •S.3 Oil
20-
delectable 0-
aso
400 420 450 T e m p e r a t u r e [o C]
Fig. 3. Influence of reaction temperature on liquid and solid hydropyrolysisproducts from organocell lignin.
wt.% b a s e d o n l i g n i n lOO-~
Run n u m b e r
S0 60' 40" Char
20.
~
Oll deteclable
3
6
8
l0
12
I n i t i a l pressure [MPa] Fig. 4. Influence of cold initial hydrogen pressure on liquid and solid hydropyrolysis p r o d u c t s from organocell lignin.
wt.% b a s e d on l i g n l n /I Run n u m b e r so/I 17 16 7
I00
-
18
6040 m
20
~
Char
Oil detectable
0
0
15
60
120
Reaction t i m e [mini Fig. 5. Influence of reaction time on liquid and solid hydropyrolysis products from organocell lignin.
solvent system, e.g. in tetralin, the influence of the hydrogen atmosphere becomes less important, as reported before (Davoudzadeh et al., 1985; Sugita et al., 1988). Time
To determine the influence of the reaction time, experiments were conducted at 0, 15, 60 and 120 min. The results are shown in Fig. 5. A reaction time of 0 rnin means that the heating was stopped
Catalytic hydropyrolysis of lignin
immediately after reaching the reaction temperature. In this case an oil yield of 58-6 wt% was obtained. A portion of 26.4 wt% (15.5 wt% based on lignin) was detectable by gas chromatography. After a reaction time of 60 min the oil yield remained more or less constant, but the detectable fraction increased up to 37.8 wt% (22 wt% based on lignin). A maximum of 6.4 wt% phenol (3.7 wt% based on lignin) was obtained in this experiment. After 120 min the oil yield and the monomeric fraction decreased slightly. Schultz et al. (1982) also reported decreasing amounts of solid residues with increasing reaction time, but the yield of soluble phenolics decreased coincidently, depending on the temperature. Connors et al. (1980) obtained a maximum of phenolic components (7 wt% of original lignin) after 15 min of reaction time. After 10 h the monomeric phenols amounted to only 1.1 wt%. ACKNOWLEDGEMENTS
This work was financially supported by the Commission of the European Communities, contract no. EN 3B-0120 D and the Federal Ministry of Food, Agriculture and Forestry, FRG, contract no. 89 NR 015. Thanks are due to Harald Voss who performed some of the experiments. REFERENCES Baker. E. G. & Elliott, D. C. (1988). Catalytic hydrotreating of biomass-derived oils. ACS Syrup. Ser., 376, 228-40. Connors, W. J., Johanson, L. N., Sarkanen, K. V. & Winslow, P. (1980). Thermal degradation of kraft lignin in tetralin. Holzforschung, 34, 29-37. Davoudzadeh, F., Smith, B., Avni, E. & Coughlin, R. W. (1985). Depolymerization of lignin at low pressure using Lewis acid catalysts and under high pressure using hydrogen donor solvents. Holzforschung, 39 (1985) 159-66. Giesen, J., Ploetz, T., Richtzenhain, H. & Deiters, W. (1963). Verfahren zur Herstellung von niedrigsiedenden Spalt-
177
produkten aus Lignin durch kontinuierliche Druckhydrierung in Gegenwart organischer Fliissigkeiten. Pat. No. 114 2853, FRG. Goheen, D. W. (1966). Hydrogenation of lignin by the Noguchi process. Adv. Chem. Ser., 59, 205-25. Goldstein, 1. S. (1975). Perspectives on production of phenols and phenolic acids from lignin and bark. Appl. Polym. Symp., 28,259-67. Huibers, D. T. A., Parkhurst, H. J. & Jones, M. W. (1980). Production of phenol from lignin. In Symp. Altern. Feedst., Vol. 25, Petrochemicals Division of Petroleum Chemistry, Inc., American Chemical Society, pp. 24-9. lnventa AG (1955). Verfahren zur Herstellung von Phenolen durch Spaitung des Lignins in Sulfitablauge. Pat. No. 305 712, Switzerland. Johnson, D. K., Chum, H. L., Anzick, R. & Baldwin, R. M. (1990). Preparation of a lignin-derived pasting oil. Appl. Biochem. BiotechnoL, 24-5, 31-40. Kanazawa, K., M6rck, R., Koyama, M. & Kringstad, K. P. (1987). Chemical structure of hydrocracked lignin: characterization of hydrocracked lignin oligomers. Holzforschung, 41,299-303. Lautsch, W. & Freudenberg, K. (1943). Verfahren zur Gewinnung von Phenolen, deren Abk6mmlingen und hydrierten Phenolen durch Druckhydrierung von Lignin oder ligninhaltigen Stoffen. Pat. No. 741686, FRG. Lindner, A. & Wegener, G. (1988). Characterization of lignins from organosolv pulping according to the organosolv process. J. Wood Chem. TechnoL, 8, 323-40. Meier, D. (1980). Verwertbarkeit von Ethanol/WasserLigninen durch Oxidation und Hydrogenolyse sowie ihre chemische und spektroskopische Charakterisierung. Dissertation, Universit~it Hamburg, FRG. Meier, D., Larimer, D. R. & Faix, O. (1986). Direct liquefaction of different lignocellulosics and their constituents. 1. Fractionation, elemental composition. Fuel, 65, 910-15. Meier, D., Jakobi, L. & Faix, O. (1988). Catalytic hydroliquefaction of spruce wood -- elemental balance and effect of catalyst. J. Wood Sci. Technol., 8, 523-42. Oshima, M., Maeda, Y. & Kashima, K. (1964). Method for liquefying lignin. Pat. No. 700 210, Canada. Schultz, T. P., Preto, R. J., Pittman, J. L. & Goldstein, 1. S. (1982). Hydrotreating of hydrochloric acid lignin in a hydrogen donor solvent. J. Wood Chem. Technol., 2, 17-31. Schweers, W. (1966). IJber die Hydrogenolyse von Lignin. 1. Mitt. Paperija Puu, 48, 161-74. Sugita, T., Mori, H. & ltoh, O. (1988). Hydrogenolysis of kraft lignin: acidic lignin is more effective. Chemistry Express, 3,667-70. Vuori, A. & Bredenberg, J. B.-son (1988). Liquefaction of kraft lignin. 1. Primary reactions under mild thermolysis conditions. Holzforschung, 42, 155-61.