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Biomass Pretreatment with Water and High-pressure Oxygen. The Wet-Oxidation Process Gary D. McGlnnls,'t Wllbur W. Wllson,r and CIM E. Mullen" Forest Products Laboratory and the Department of Chemistry, Mississippi State U n h w i s ~Mississlppl , State, Mississippi 39762
A biomass pretreatment process called wet oxidation that utilized water, oxygen (240-480 psi), and temperatures above 120 O C was applied to loblolly pine, black oak, and a mixture of low-grade hardwoods. The process was found to be effective for fractionatlng the hemicellulose, Ilgnln, and cellulose components of wood. Acid hydrolysis studies showed that the wet oxidation also enhanced the rate at which cellulose was hydrolyzed by acids to glucose.
Introduction Since about 1970, the need for alternative sources of energy and chemicals now obtained from petroleum has led to extensive research activities to develop processes for converting agricultural, forestry, and municipal wastes into fuels and chemicals. This research has involved studies on both old and new processes for biomass conversion. Some of the older processes, such as acid hydrolysis and fermentation to ethanol, are being reinvestigated by use of newer technology to determine if these processes can economically compete with those based on petrochemicals for production of certain organic chemicals. At the same time, studies are also proceeding on new processes for biomass conversion. One example is the development of enzymatic procedures which have become possible because of the availability of new strains of bacteria that produce enzymes which react with polysaccharides at faster rates, are more temperature tolerant, and can convert a wider variety of sugars into ethanol or other chemicals than those enzymes previously used. Biomass generally consists of three major components: cellulose, hemicellulose, and lignin. Cellulose is normally the major component. Ita chemical structure is similar in all plants and consists of repeating 1-4 @-D-glUCOpyR"Se units. The structures of the hemicellulosesand lignin vary, depending on the plant material. The relative abundance of these components in various types of biomass is shown in Table I (Emert and Katzen, 1979; Cowling and Kirk, 1976). Biomass residue could be used to partially replace petroleum as a source of fuel and chemicals. The amount of residual biomass in the United States has been estimated (Brink, 1976; Emert and Katzen, 1979) and is summarized in Table 11. This amount could replace 5-10% of the petroleum used in the United States (Goldstein, 1981a), but only if efficient and economical methods for converting the major component(s) into chemicals and liquid fuels can be found. Proposed processes for making ethyl alcohol from cellulosic materials (Worthy, 1981; Hajny, 1981) involve three major steps: a pretreatment step to partially break down the biomass, a hydrolysis step to convert the polysaccharides into monosaccharides, and a yeast fermentation step to convert the monosaccharides into ethyl alcohol. The primary purpose of the pretreatment step is to break down the lignin and overcome the resistance of cellulose to hydrolytic cleavage due to ita crystalline structure. In other words, the pretreatment should disrupt the physical t Forest Producta Laboratory. *Departmentof Chemistry.
Table I. Composition of Plant Materials hemi-
cellulose, cellulose, source
%
%
municipal solid wastea wood (softwood)
61 45-50 40-55 25-40
22 25-35 24-40 25-50
wood (hardwood) stems of monocotyledons
lignin, %
9 25-35 18-25 10-30
(grasses, bamboo, wheat, rice, sugarcane) The cellulosic materials remaining after removal of the plastic and metals from the solid waste. Table 11. Biomass Generated in the United States amount generated, source tonslyear 1. municipal solid waste 140 000 000 2. agricultural residue 300 000 000 3. forestry residue a. forest residues b. wood processing residues c. pulp and paper mill waste
168 000 000 145 000 000 20 000 000 3 000 000
structure of the cellulose material so that the rate and extent of hydrolysis are increased. However, a pretreatment should do more than loosen the plant cell structure if it is to be used as the first step in a commercial process. Ideally, the pretreatment would (a) use inexpensive chemicals and require simple equipment and procedures, (b) solubilize the lignin and hemicelluloses, (c) reduce the crystallinity and thereby enhance the hydrolysis of the cellulose to D-glucose, and (d) be suitable for pretreating a wide variety of plant species. The reason for the use of inexpensive chemicals or no chemicals and simple equipment is self-evident. The only way to attract large-scale investments from the private sector is to be able to produce ethyl alcohol from cellulosic material at a cost which is comparable to the market cost of gasoline. Consequently, each step in the process of converting biomass to alcohol must be relatively inexpensive. Another desirable characteristic of the pretreatment process is the ability to fractionate lignin and hemicellulose components from the biomass. There are several reasons why this is very important. The yield of alcohol is dependent on the cellulosic content of the biomass, since most hemicelluloses consist of five-carbon sugars which are not fermentable to ethyl alcohol using commercial strains of yeast. For example, in the same size production unit, a pretreated sample of wood that has 80% cellulosic con-
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tent would give twice the yield of alcohol as a nontreated wood sample, which normally has a cellulosic content of approximately 40%. Another advantage in removing the lignin and hemicellulose is that the hydrolysis step is generally enhanced when these components are removed (Lipinsky, 1979; Goldstein, 1981b). This is particularly true if enzymatic hydrolysis is used, since steric factors caused by the lignin tend to reduce the rate of hydrolysis. The importance of decreasing the crystallinity and molecular size of the cellulose material is due to the difficulty of hydrolysis of cellulose. Even cellulose which contains no lignin or hemicellulose is difficult to hydrolyze without pretreatment (Emert and Katzen, 1979; Goldstein, 1981a; Lipinsky, 1979). For acid hydrolysis, the ratio of the rates of conversion of cellulose to glucose and degradation products can be improved by disrupting the crystalline structure of the cellulose. For enzymatic hydrolysis, the hydrolysis rate can be enhanced by increasing the surface area on which the celluloses can work. One of the more important characteristics of a pretreatment is that it should be effective with a wide variety of biomass material. Many pretreatments are very dependent on the type of plant material used. Variations are found not only between broad groups of plant materials (e.g., softwoods and hardwoods) but also among individual species. Pretreatments with sodium hydroxide (Lora and Wayman, 1978; Millet and Baker, 1970),ammonia (Tarkow and Feist, 1969; Bender et al., 1970; Millet et al., 1970; Baker et al., 1975; Goldstein, 1981b),sulfur dioxide (Baker et al., 1975), steaming (Bender et al., 1970; Lora and Wayman, 1978), steam explosion (Jurasek, 1978), organosolv pretreatment (Sarkanen, 1980), and electron irradiation (Pritchard et al., 1962; Emert and Katzen, 1979) all indicate a strong species bias. Hardwoods are generally more susceptible to hydrolysis than softwoods. The two major types of forestry biomass residue available for chemical utilization are softwood residue at mill sites and large amounts of mixed low-grade hardwoods that are not suitable for lumber or pulp manufacturing. If biomass processes are to be developed for the utilization of such forestry residue, a pretreatment is required that will be equally effective with a wide variety of woods.
Wet-Oxidation Process Wet oxidation is the process of treating material with water and air or oxygen at temperatures above 120 "C. The process has been used commercially for the production of vanillin from pulped lignin (Salvesen et al., 1952), in pulping recovery systems (Zimmerman and Diddams, 1960), and for increasing the bulk of municipal solid waste (Guccione, 1964). Most of the initial work on the wetoxidation process was performed at the University of California (Schaleger and Brink, 1977,1978). It was found that the initial reaction during wet oxidation is the formation of acids. The acids are formed by the solubilization of the acidic hemicellulose components (xylans), by deesterification of the acetate groups on the hemicellulose and by oxidation. As the acid concentration increases and the pH drops, hydrolytic reactions become favorable. More and more of the hemicelluloses are broken down into lower-molecular-weight fragments which dissolve in the water. This reaction affects not only the hemicelluloses but also the cellulose and lignin fractions. One important effect on cellulose is that its rate of acid hydrolysis is substantially increased after wet oxidation (Schaleger and Brink, 1978); i.e., the accessibility of the cellulose is increased. Most of the earlier work was done at relatively low oxygen partial pressures (50 psi of air pressure). Recent
Table 111. Typical Wet-Oxidation Reaction Conditions temperature oxygen pressure range reactor volume sample size time
120-238 "C 120-480 psi 2000 mL 50 g of wood and 500 mL of water 30 min
work, which we have performed at the Mississippi Forest Products Laboratory, indicates that higher oxygen pressures and the addition of metal catalysts lead to a more specific reaction, both at low and high oxidation temperatures.
Materials and Methods The wet-oxidation pretreatment involves treating wood with oxygen and water at temperatures above 120 "C. Typical reaction conditions for the wet oxidation are given in Table 111. Three types of woody biomass were studied loblolly pine (Pinus taeda L.),a commercial softwood; black oak (Quercus uelutina Lam.), a low-grade hardwood; and a sample of mixed hardwood materials obtained by whole tree chipping. The latter material is currently the feed stock for a 1200 ton/day hardboard manufacturing plant and probably represents the best short-term source of woody biomass for alcohol production. After the bark was removed, the dried wood was ground to pass a 2-mm screen in a Wiley mill. Wood and water were mixed in a weight ratio of 1:lO and placed in a 2000-mL reactor (Model 4521) from Parr Instrument Co. The reactor was constructed from type 316 stainless steel and had a maximum working pressure and temperature of 1400 psi and 350 "C, respectively. At the conclusion of a reaction, the reactor was cooled to room temperature, the reactor pressure was released, and the solids and liquids were separated by filtration. For the catalyzed runs, exactly the same procedure was used except that 0.33 g of ferric sulfate was added to 50 g of wood. After the reaction was complete, the solids were air-dried until the moisture content was around lo%, weighed, and stored at 20 "C in air-tight containers for further analysis. Two samples of the air-dried wood were weighed out accurately and dried in an oven for 24 h at 115 "C to determine the exact moisture content. This value was used to calculate the yield of solid materials from the reactor runs. The yield of lignin was determined by the Klason lignin procedure while the total carbohydrates were determined by the hydrolysis procedure of Laver (Laver et al., 1967) and analyzed by a gas chromatography procedure (Chen and McGinnis, 1981). Kinetic studies were performed by suspending 3.0 g of the cellulosic material in 200 mL of 20.0% H2S04solution. The reaction mixture was placed in a bath maintained at 100 "C. Samples were taken at specific time intervals and analyzed for glucose content by a variety of analytical techniques (polarimeter, gas chromatograph, high-performance liquid chromatograph, and an enzymatic glucose analyzer). Since the samples had different amounts of cellulose present in the biomass material, the results are based on the glucose originally present in the material.
Results and Discussion Two distinct types of wet-oxidation reactions occur with the woody biomass: a low-temperature reaction, which is mainly hydrolytic, and a high-temperature oxidative reaction. At the lower temperatures (120-172 "C), the major reaction occurring during wet oxidation leads to solubilization of the hemicellulose and partial solubilization of the
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I ,,,,/I
SOLUBILIZED WOOD (%)
40
140
120
163
Po
180
m
143
240
Figure 1. Solubilization of wood vs. wet-oxidation temperature: loblolly pine (*); black oak (m); mixed hardwoods (e). Table IV. Stability of Monosaccharides to Wet Oxidationa at 154 "C recovered, 7% D -xylose D-mannose
88 91 100 91
g galactose
D-glucose
a Wet-oxidation conditions: 240 psi, 30 min; monosaccharide concentration = 10% by weight.
lignin component. This leaves behind a solid residue which contains a high percentage of cellulose. 1%-172 O C
wood
240 psi O,,30 min'
I63
180
ma
220
240
W T OXIDATION TEMPERATURE(TI
TEMPERATURE P C I
+
+
liquid fraction (hemicellulose lignin) solid fraction (cellulose + lignin) The wet-oxidation curves (amount of wood solubilized vs. the wet-oxidation temperature) are shown in Figure 1. The total amount of material solubilized is very similar for all three materials. The major reaction at the low temperatures is initiated by the production of small amounts of organic acids. The acids cause the partial hydrolysis of the glycosidic linkages between the hemicellulose units, which leads to a decreased molecular weight and water solubility. Under these conditions, the basic structure of the carbohydrates remains intact. Studies with monosaccharides as model compounds indicate that 88-100% of the monosaccharides can be recovered after wet oxidation at 154 "C (Table IV). The oligosaccharides obtained from the partial hydrolysis of the wood are probably more stable to oxidation than the monosaccharides because the glycosidic linkage in the oligosaccharides is more stable to oxidation than the al-
Figure 2. The effect of metal salts on the wet-oxidation reaction of loblolly pine: uncatalyzed (m); catalyzed with 0.67% CuS04 ( 0 ) ; catalyzed with 0.67% Fe2(S04),(*).
dehyde group of the monosaccharides. The relationship between wet-oxidation temperature and the yield of dissolved hemicellulose fraction from loblolly pine is given in Table V. Analysis of the total carbohydrates was done after complete hydrolysis to the monosaccharides. Loblolly pine contains approximately 24 % hemicellulose, consisting of a glucomannan, a galactoglucomannan, and a xylan (Koch, 1972). The maximum yield of the solubilized hemicellulose after wet oxidation was obtained at 154-160 "C and represents approximately 75% of the total hemicellulose in the starting wood. At temperatures above 160 "C,the yield of all solubilized sugars, except D-glucose, decreased even though the hemicelluloses were still being extracted in the liquid fraction. The reason for the reduction in total sugars was that the hemicelluloses were also oxidized and further broken down at a rapid rate. The reason that D-glucose increased was that at the higher temperatures the sugar was formed from the hydrolysis of the cellulose fraction. With hardwoods, the maximum yield of hemicellulose was slightly less, averaging between 60 and 70% at 154 "C. The wet-oxidation reaction occurs considerably faster than the reaction in air (Table VI). At all temperatures studied, the extent of wood solubilization was greater with oxygen than with air, especially at temperatures above 140 "C, where the amount of wood solubilized is two to three times greater with oxygen. This is not a pressure effect but is due to the higher concentration of oxygen in the liquid phase, since essentially the same results are obtained if runs are made with oxygen at 120 psi or air at 600 psi. Preliminary evidence indicates that the higher oxygen pressures are more effective in wet-oxidation reaction because they increase the rate of acid formation and the rate of lignin breakdown. In this study, the normal resi-
Table V. Analysis of the Carbohydrates Formed in the Liquid Fraction from Loblolly Pine after Wet Oxidation wet-oxidn temp, "C 149 154
carbohydrates in solution, %" D-glUCOSe
D mannose
D -galactose
D -xylose
L-arabinose
total
6.4 2.2 3.3 1.4 15.4 6.2 2.4 3.2 1.0 15.2 2.4 160 2.8 8.0 2.1 3.8 1.1 17.8 165 3.9 4.6 1.4 3.0 0.6 13.5 171 5.5 3.0 0.9 2.1 0.4 11.9 7.8 1.4 0.2 0.8 0.3 10.5 182 193 9.8 trace 9.8 9.8 trace 9.8 2 04 Based o n the oven-dried weight of starting material. This analysis was done by taking a known volume of the liquid after oxidation, hydrolyzing it completely with an inorganic acid, and by analyzing by gas chromatography after derivative formation. 2.1
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
355
Table VI. Comparative Amounts of Wood Solubilized in Air and Oxygena solubilized wood, %
temn. "C
air
0,
120 140 160 180 200
10 9 17 32 38
13
17 41 57 83
This study was done with loblolly pine wood using a 30-min residence time in the reactor. Wet-oxidation reactions in air were done at atmospheric pressure while the oxygen runs were done by pressurizing the reactors with 240 psi of oxygen.
I20
140
160 180 TEMERATURE ( ' C )
220
200
Figure 4. Cellulose content of black oak as a function of the wet-
LIGNIN x
=[ 20
oxidation temperature: based on starting weight (@); based on nonsolubilized material after wet oxidation (W.
7
lool
I \L
10
I
120
140
, \ ,
160
I
180
200
220
TEMPEWTURE (.C)
GLUCOSE (XI 40
-
20
-
Figure 3. Lignin content of the nonsolubilized wood (based on starting weight) as a function of the wet-oxidation temperature: loblolly pine (*); black oak ( 0 ) .
dence time in the reactor was 30 min. Slightly higher yields can be obtained by using longer residence times. For example, at 171 "C,the yields of solubilized wood for a 30-min and a 1-h reaction time are 51% and 61%, respectively; at 182" C, the yields are 61% and 70%. Another factor that increases the amount of solubilized wood is the addition of metal salts. From a large number of metal salts, which have been evaluated as potential catalysts for the wet-oxidation reaction, the two that have been found to be most effective are ferric sulfate and cupric sulfate. The effectiveness of these two catalysts is shown in Figure 2. At all temperatures studied, the presence of ferric sulfate increased the total amount of solubilized wood as compared to the uncatalyzed reaction. The metal salts were particularly effective at the lower temperatures. At the present time, it is not known how ferric sulfate catalyzes the reaction; however, the metal salts lead to the formation of much higher concentrations of acids, even at very low wet-oxidation temperatures, which could account for the more rapid wood solubilization. Ferric sulfate has also been found to be an effective catalyst in other delig nification reactions of carbohydrates including organosolv delignification and sodium hydroxide-oxygen pulping (Ericsson et al., 1971; Lowendahl and Samuelson, 1974). The effects of wet oxidation on the lignin depend on whether the starting materials are softwood or hardwood (Figure 3). In this study, two types of wood were used: black oak, which has a lignin content of 28%, and loblolly pine, which has a lignin content of 30%. With black oak, the lignin content of the solid material decreases quite rapidly, even at relatively low temperatures, and at 140 "C,well over 50% of the lignin is broken down and solubilized. In the case of the loblolly pine, the lignin is degraded at a much slower rate with the major solubilization
L
1
I
140
120
160
I
I
1
200
220
240
I
180
TEMPERATURE (T)
Figure 5. Cellulose content of loblolly pine as a function of the wet-oxidation temperature: based on starting weight ( 0 ) ;based on nonsolubilized material after wet oxidation (W.
occurring between 170 and 180 "C. In general, hardwood lignins are more susceptible to oxidation and to reactions with water than softwood lignins (Lora and Wayman, 1978; Glasser, 1980). The cellulose content, which was measured by hydrolysis and gas-chromatography analysis, was strongly dependent on the wet-oxidation temperature (Figures 4 and 5). With black oak, the cellulosic content of the solid reached a maximum of 80-85% at wet-oxidation temperatures of 160-180 "C,whereas, the cellulose content of the pine reached approximately 70%. The higher cellulosic content obtained from black oak was due to the increased solubilization of the lignin from this material. These results indicate that the wet-oxidation reaction fractionates the hemicellulose and lignin from woody biomass, leaving behind a solid fraction which contains a much higher percentage of cellulose. Another desirable feature of a pretreatment is that the process renders the cellulose more easily hydrolyzable. T h e generalized reaction scheme for production of glucose from raw (nonpretreated) cellulose by acid hydrolysis is cellulose (A)
ki
H ' , H20
k2
D-glucose (B) degradation products
where k l and k2 are first-order rate constants for the for-
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
138
I , , , , , 8 C
t
-
TIME [ hr)
Figure 7. Production of D-glUCOSe by the acid-catalyzed hydrolysis of wet-oxidized loblolly pine.
mation and degradation of glucose, respectively. For wet-oxidized wood, we have wet-oxidized (A')
k,'
H+,HZO
204
( OC)
Figure 6. Decomposition of D-glucose with 20% by volume sulfuric acid at 100 OC.
30
182
W E T - O X I D A T I O N TEMPERATURE
TIME ( hri
40
160
D-glucose (B)
k2
degradation products The ratio (kl'/kl) will be a measure of the effectiveness of the wet-oxidation pretreatment in enhancing cellulosic hydrolyzability. The overall reaction is consecutive first order so that
The rate constant, k,, for the degradation of glucose can be determined independently for the hydrolysis reaction conditions employed. Figure 6 shows that the degradation of glucose in 20% H2S04 at 100 "C follows first-order kinetics with k2 = 0.110 h-l. A typical hydrolysis curve for the production of glucose from pretreated cellulose under the same reaction conditions is shown in Figure 7. Using eq 3, a value for k l was determined for the raw cellulose and kl' was obtained for the cellulose which resulted from each wet-oxidationreaction condition. The relative change in k{ as a function of wet-oxidation temperature is shown in Figure 8. A t low wet-oxidation temperatures, the hydrolysis of the cellulose proceeded at a rate similar to that for the nonpretreated material. The crystalline structure of the cellulose had not been altered significantly by the pretreatment process. Increasing the wet-oxidation temperature led to a gradual increase in the reaction rate constant, kl', but a significant increase was not observed until the pretreatment temperature reached approximately
Figure 8. Relative change in the ratio k , ' / k , as a function of wetoxidation temperature; loblolly pine ( 0 ) ;mixed hardwoods (0).
190 "C. At this temperature, for both the pine and especially the mixed hardwoods, a marked increase in the reaction rate constant was observed, presumably due to the destruction of the cellulose crystalline structure. Relative changes in kl' were most pronounced for the mixed hardwoods as compared to the softwood. This is consistent with the general observation that cellulose from hardwoods is more easily hydrolyzed than that from softwoods (Emert and Katzen, 1979). The decrease in kl' at about 204 "C was observed for both the softwoods and hardwoods and may be due to the formation of an oxidized form of cellulose which is less susceptible to hydrolysis. Conclusions The wet-oxidation reaction appears to be a very promising pretreatment for biomass. The major advantages of this pretreatment process are that: (1)it uses inexpensive chemicals-only oxygen and water; (2) it fractionates the biomass into a liquid fraction, which consists of hemicellulose and part of the lignin, and a solid fraction, which is rich in cellulose; (3) it appears to be suitable for a wide range of woody biomass; and (4)it increases the accessibility of the cellulose to acid hydrolysis. Registry No. CuSO,, 7758-98-7; Fe2(S04)3,10028-22-5; hemicellulose, 9034-32-6; lignin, 9005-53-2; cellulose, 9004-34-6; Dglucose, 50-99-7; D-XylOSe, 58-86-6; D-mannOSe, 3458-28-4; Dgalactose, 59-23-4; L-arabinose, 5328-37-0.
Literature Cited Baker, A. J.; Miilet, M. A.; Satter, L. C. Cellulose Technology Research, ACS Symposium Series 10; Amerlcan Chemical Society: Washingon, DC, 1975. Bender, F.; Heaney, D. P.; Bowden, A. Forest Prod. J. 1970, 20, 36. Brink, D. L. "Pyrolysls-Gasification-Combustion: A Process for Utilization of Plant Materials"; Applied Polymer Symposium No. 28;Wiiey: New York, 1976. Chen, C. C.; McGinnis, G. D. Carbohydr. Res. 1981, 9 0 , 127. Conrad, E. C.; Palmer, J. D. Food Techno/. 1978, 84. Cowling, E. B.; Kirk, T. K. "Properties of Cellulose and Lignocellulosic Materiais as Substrates for Enzymatic Conversion Processes", Biotechnol. and Bioeng. Symp. No. 6; Wiiey: New York, 1976. Emert, G. H.; Katzen, R. "Chemicals from Biomass by Improved Enzyme Technology, Biomass as a Non-Fossil Fuel Source"; American Chemical Society: Washington, DC, 1979. Ericsson, B.; Lindgren, B. 0.; Theander, 0. Svensk Papperstidn. 1971, 22, 757. Glasser, W. G. "Pulp and Paper", 3rd ed.; Casey, J. P.. Ed.; Wiley-Interscience: New York, 1980 Voi. I , Chapter 2. Goldstein, 1. S. "Biomass Availability and Utility for Chemicals, Organic Chemicals from Biomass"; CRC Press: Boca Raton, FL, 1981a. Goldstein, 1. S. "Chemicals from Ceiiuiose. Organic Chemicals from Biomass"; CRC Press: Boca Raton, FL, 1981b Guccione, E. Chem. Eng. 1964, 7 1 , 118. Hajny, G. J. "Biological Utilization of Wood for Production of Chemicals and Foodstuffs"; U.S.D.A.: Washington, DC, Research Paper FPS-385, 1981. Jurasek, L. Dev. Ind. Mlcroblol. 1978, 2 0 , 177. Koch, P. "Chemical Constituents, Utilization of the Southern Pine"; U.S.D.A. Forest Service: Washington, DC, 1972; Vol. I . Laver, M. L.; Root, D. F.; Shaflzadeh, F.; Lowe, J. C . TAPPI 1967, 50(12), 618.
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Llpinsky, E. S. "Hydrolysis of Cellulose: Mechanisms of Enzymatic and AcM Catalysis"; Advances in Chemlstry Series No. 181; American Chemlcal Soclety: Washington. DC, 1979. Lora. J. H.; Wayman, M. TAPPI 1978, 61, 08. Lowendahl, L.; Samwlson, 0. Svensk PapperstMn. 1974, 16, 593. Mlllet, M. A.; Baker, A. J.: Felst, W. C.; Mellenberger, R. W.; Sattler, L. D. J . Anim. Sci. 1970. 31, 701. Prltchard, 0. I.; PMgen, W. J.; Minson, D. J. Can. J . Anim. Sci. 1982, 42, 215. Salvesen, J. R.; Brink, D. L.; Diddams, D. G.; Owzarskl, P. Vanlllln, U S . Patent 2434626, 1952. Sarkanen, K. V. "Progress in Biomass Conversion"; Academic Press: New York, 1980.
Schaieger, L. L.; Brink, D. L. OxMative Hydrolysis of Lignocellulose; Tappi Conference, Forest BioiogyIWood Chemlstry, Madison, WI, 1977. Schaleger, L. L.; Brlnk, D. L. TAPPI 1978, 61, 85. Tarkow, H.; Felst, W. C. "Mechanism for Improvingthe Dlgestibility of Lignocellulosic Materlals with Dilute Alkali and LlquM Ammonia"; Advances in Chemistry Serles No. 95; American Chemical Society: Washlngton, DC, 1969. Worthy, W. Chem. Eng. News 1981, 59(49), 35. Zimmermann, F. J.; DMdams, D. 0.TAPPI 1980, 43, 710.
Received for review July 26, 1982 Accepted December 14, 1982
Esters from Branched-Chain Acids and Neopentylpolyols and Phenols as Base Fluids for Synthetic Lubricants Tal S. Chao,' Manley Kjonaas, and James DeJovlne ARC0 Petroleum products Company, Division of Atlantic Richfieid Company, Harvey, Illinois 60426
Esters providing greater oxidation resistance than neopentylpolyol esters of straight-chain acids were prepared and evaluated as base fluids for synthetic lubricants. Acids employed included 2,2-, 3,3-, and 4,4dimethylpentanoic acids, 4,4dimethylhexanoic acid, and acids having single branchings. Polyols and phenols employed included trimethylolpropane, pentaerythrltol, resorcinol, and dihydroxybenzophenones. As shown by oxygen absorption test, oxidation resistance of these esters was improved when both hydrogen atoms in a methylene group were replaced by methyl groups. This improvement is substantlated by Erdco bearing head tests showing lower viscosity increase, lower acid number, and reduced deposits. Further improvement in oxidation resistance was seen when the neopentylpolyols were replaced by polyhydric phenols. Esters of 2,2- and 3,3dimethylpentanoic acids with dihydric phenols have induction periods ten times those of Type 2 base fluids. However, most of them are deficient in low-temperature fluidii. Data showing variations of crltical physical properties and oxidation resistance with structure are presented.
Introduction Esters of straight-chain carboxylic acids with neopentylpolyols such as pentaerythritol (PE), dipentaerythritol (diPE), and trimethylolpropane (TMP) are widely used as base fluids for Type 11/2and Type 2 lubricants for aircraft gas turbine engines (Yaffee, 1965; Dukek, 1964; Robson, 1971). Lubricants formulated from these base fluids are more thermally and oxidatively stable than Type 1lubricants based on dibasic acid esters (Dukek, 1964; Barnes and Fainman, 1957). Lubricants of even greater stability than Type 2 are required for advanced aircraft engines such as those equipped for supersonic transports. Aerodynamic heating raises the skin temperature of the aircraft and denies air cooling to the lubricant. The possible extent of cooling with fuel is limited (Dukek, 1964). Considerable research and development efforts have been carried out, both in the United States (Yaffee, 1965) and abroad (Dukek, 1964; Byford and Edginton, 1971; Robson, 1971; Byford, 1971) to search for lubricants which have better high-temperature characteristics than Type 2 lubricants but are less expensive than Type 3 lubricants such as polyphenyl ethers, fluorine compounds, pyrazines, etc. This paper describes some of the efforts made in our laboratory toward the development of what can be called Type 21/2 lubricants. Such lubricants are described by one jet engine manufacturer as having a temperature capability 50 O F higher than that of a Type 2 lubricant. Our efforts in developing these Type 21/2 lubricants involved both base oil and additive studies. For the base oil our effort was concentrated on the development of branched-chain acid esters of neopentylpolyols and phe0196-432118311222-0357$01.50/0
nolic compounds. The branched-chain acids studied included neo-acids, e.g., 2,2-dimethylpentanoic acid, gemdimethyl acids, e.g., 3,3-dimethylpentanoic acid, and acids containing single branching such as 2-ethylhexanoic acid. The phenols included resorcinol, dihydroxybiphenyls, dihydroxybenzophenones,etc. The esters and many of the branched-chain acids were prepared in our laboratories. The esters were evaluated as base fluids based on their physical characteristics and oxidative resistance as determined by oxygen absorption testing. The choice of esters of branched-chain acids is based upon their potential for improved oxidative stability. Earlier work by Crouse and Reynolds (1963) and by Metro (1964) indicated that oxidative and thermal stability can be improved by replacing straight-chain acids with pivalic, 2,2-dimethylpentanoic, and 2,2-dimethylhexanoic acids. These esters, however, are often solids and therefore unsuitable as lubricant base fluids. Our work was devoted to the optimization of the physical properties of these esters through selection of structural forms, while at the same time maintaining a suitable high level of oxidation stability. This was accomplished through the use of acids containing double branching remote from the 2- or CYposition and through the incorporation of n-valeric acid. Of the C, through C9 acids previously investigated in our laboratories, n-valeric was found to provide PE esters with the highest oxidation resistance.
Experimental Section Preparation of Branched Chain Acids. Many of the acids used in this work were prepared in our laboratories by established methods. 2,2-Dimethylbutyric acid was 0 1983 American Chemical Society
