edited by: MICHAEL R. SLABAUGH Weber State College Ogden. Utah 84408
Chemicals from Wood and Other Biomass Part II: The Chemistry of Conversion David W. Goheen Pioneering Research Group. Central Research Division. Crown Zellerbach Corporation. Camas. WA 98607 In Part I (the dune 1981 issue ofTHlS ,JOURNAL), we traced the development of industrial organic chemistry from extremelv modest beginnines to its present day prominence as one of the worldh i&lustrial mainbtays. Projections for future erowth were offered with predictionsofincreased utilization ;;l renewable biomass fee&tocks with eventual almost total conversiun to the renewahle resource. In Part 11, we will consider the chemistry of biomass conversion. Supply and Technology As a first consideration, we must ask ourselves the question as to whether there is sufficient biomass availahle toserve as the major feedstock source for the chemical industry. The answer is ves. There is far more hiomass available in iust presently under-utilized waste materials in the United ~ t & s than the total oroduction of all plastics, adhesives, solvents. and other organic chemicals pk,duced by the chemical industry ( I ). A similar statement for the rest of the world is undoubtedly also valid. As a second consideration, we must ask if technology is available. The answer is again yes; ways of producing many, if not must, ofthe large-scale industrial organic chemicals from wood and other birnnass sources are already known (1.2). It has not been lack of technology which has retarded the development of the production of chemicals from biomass, but rather it has been a question of economics. As pointed out already. during the decades of incredibly cheap petroleum. processes based on feedstocks from all other sources were simply not economically competitive except for a very few isolated cases. When one sets out to evaluate the biomass as a chemical feedstock, one is struck by one very significant point-the advantage inherent in wood as compared to other forms of biomass. Woody plants not only synthesize the two abundant organic materials, cellulose and lignin on a massive scale, hut also they do it in such a way that the two materials are concentrated in the boles of trees. Almost all the other products of photosynthesis whether they be as algae, bacteria, grass, straw, or vegetable crops are very much more disperse and costly to collect. Furthermore, these forms of biomass are bulky, often seasonal and difficult to store to insure continuous and year round operation for any process based on their utilization. Wood, on the other hand, is relatively easy to collect and can he stored for lone periods. Wood, therefore. .. . like petroleum and coal, represents stored incident solar enerev and has the priceless advantage . over the fossil materials ofbeing renewable. The use of wood to make up for shortfalls in petroleum is not a new concept. More than forty years ago hoth Germany and Sweden, faced with cutoffs of petroleum importation during World War 11, proposed and in many cases developed programs designed to produce a variety of products hased on wMd as the raw material. These exoeriences weresummarized in 1949 by E. Glesinger in a hnok, he Coming Age of WocKt" (3).In addition, Glesinger offered many other proposals both ~
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for utilizing wood and for managing the world's forests to nrclduce a huee and never endine wuod supplv. . . . He believed ;hat with pm;;er mnnsyemrnt thr world's ~ ~ W Q S I S C O prlb~ I ~ ~ dure tin 111 1.1 X 10" tons i d u,,wd on a permanent hasis. Fgw all uses, the world's cut of timber is presently in the range of i-8 x IOU tons per year and the demand is expected to rise to ahout 1.8 X 109 tuns per year by the year 2000 ('1). The annual renewahle cut proposed hy Glesinger is many times greater than the estimated demand by the year 2000. Although the world prohahly will never achieve the g o w t h that he proposed, it is certainly reasonable to anticipate good management resulting in a doubling or even tripling of the erowth now takine..nlace. Thus...nlentv. of wood can reasonahlv he expected to he available to satisfy hoth conventional reouirements and large-scale utilization as a chemical feedstnck. The orot)osals described bv Glesinner came out a t an unfortunate iime. 1949. hat was right i t the beginning of the enormous exnansion of wtrochemical plants. With seemingly limitless supplies of inexpensive petroleum, little consideration was given to Glesinger's vision of a world run by conversion uf wood. Today, his proposals should be carefully considered. T o implement these proposals, management practices to increase~forestgrowth, and thus the allowable cut, would have to he pursued vigorously. As mentioned above, there is a lead time before exhaustion of oetroleum and coal hecomes critical, but, if we are to have a smooth transition to wood as an e n e r w source. this lead time must not be allowed to pass by without reversal of forestry mismanagement practices that still take place in the world, especially in the tropical forest regions. In many areas, forests are often
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This leafwe pesenU relevant appllcatims of chemistry to wayday II1e. The infamation presented might be vsed directly in class. posted on bulletin boards a otherwise used to stimulate student involvement in activities related lo chemistry. G3mribufions should be sent to fealUB -~~~ BdilM. ~~
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~ l k eSlabam received hls BS degree in Chemistry at Purdue University in 1965 and his dactaate in aganic chemistry at Iowa Slate Universily in 1970.His interest in biochemistry and natural wcducts rerearch (slkaioids)led lo a year of postdoctwal study in biochemistry at Texas A8M University in 1971. Dr. Siabaugh is now Profesxxof Chemistry at Weber Slate College, where in 1979 he was mcogn~zedas lhe "Prolerxx of Year." His If i i* I profersionai interests and goals are directed toward chemical education and mmmmity involvement !n soence actlvilies. He has been panicularly active in directing regional science fairs.
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"mined" for individually valuable timher species and the bulk of the forest is injured or even destroyed hy the harvesting practices. Chemicals from Non-Woody Plants Although wood has many good attributes and should be able to provide much of the required raw material for future chemical plants, the first large-scale biomass utilization for chemical and liquid fuel production will he (and already is to a limited extent) conversion of sugars and starches from agricultural crops to ethanol in large fermentation plants. The experience in Brazil and other warm climate countries with sugar from sugar cane gives evidence that ethanol can significantly extend gasoline supplies for automotive transport. Large-scale production of alcohol from the starch of corn and wheat has begun in the United States and this effort will he greatly expanded. Before too long, ethanol should begin to he used to supply raw material for petrochemical plants. At one time, ethanol was made by fermentation and was converted to ethylene hy dehydration. For at least 30 years, ethanol was made hy the reverse process, the hydration of ethylene produced from inexpensive ethylene made by cracking cheap natural gas. At the present time, the costs are about astand-off and in the future, ethylene should be produced again hy dehydration of ethanol. Thus, many petrochemical operations could continue with a renewable hiomass feedstock and with essentially no capital conversion costs. Other chemicals, e.g., furfural, butanedial, sorbitol, and numbers of others also can he produced from agricultural hiomass. There is a large moral aspect to the conversion of a significant part of the world's food chain to anything except food. Thus, future large-scale production of fermentation alcohol should he from glucose made from presently under-utilized cellulose. Acid-catalyzed hydrolysis of the cellulose of sugar cane bagasse and corn stover is already possible and enzyme technology for such conversion has reached pilot plant stages.
r
PYROLYSIS-+
The enzymatic process cives promise of higher vields and is less energy demanding than the conventional acid-catalyzed process. Of the utmost importance would he development of an organism that would ccmtain enzymes that would ferment cellulose directly trr ethanol. Work is underway through genetic manipulati~rnthat may result in development of such an organism. Other non-woody plants, such as algae and kelps grown on vast areas of the ocean over continental shelfs, are being studied with the eoal of convertine them to methane bv anaemhic fermentacon. Methane produced from this (and k h e r wastes such as animal manure) is competitive with natural gas
Chemicals from Woody Plants The most widely used biomass in the future promises to he wood. The technology for producing most of the basic commodities of modern industrial chemistrv is alreadv known. Wood is a matrix of about 70-80% polymeric carbohydrates and 20-30% lienin. This matrix can be changed and deeraded in a variety of ways as shown in the Figure 1. As can he seen, with these hasic materials, most of the products of a modern chemical factory can he made. It should also he remembered that both cellulose and lignin in their high molecular weight polymeric form can be modified and utilized for many uses that have been developed for plastics and polymers~producedfrom petroleum. ~ a i o and n acetate fibers made from cellulose have long been staple commodities and improvements in their processing and properties suggest that they can recapture a great deal of the market from petroleum-based, manmade fibers. Cellulose esters and ethers can substitute for many petroleum-based plastics. Again, as with non-woody plants, the first major chemical production from wood in the U S . probably will he ethanol
Gas (carhon monoxide, carbon dioxide, hydrogen, hydrocarbons) Liquids (methanol, acetic acid, acetone, phenol derivatives) Charcoal Gas (hydrocarbons)
phenols and Cyclohexane derivatives Fermentation
Alcoh018 (ethyl-, butyl-, isopropyl-) Polyols (glycerol, ethylene, propylene glycol) Ketones (acetone) Acids (acetic-, lactic-, butyric-) .Yeast
Dehydration Hydrolysis
Hydroxymethylfurfural, Levulinic acid
Hydrogenation
Polyols
Crystallization
Glucose
rFermentation
Yeast
Dehydration
Furfural
Hydrogenation
Polyols (xylitol)
-Pentoses L~rystallization
Xylose
rHydrogenation
Phenol derivatives, Hydrocarbons Phenol derivatives, Catechols
L~xidation
Vanillin
Figure 1. Wood-based chemicals from various transformations.
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made by fermentation of glucose ohtained hy hydrolysis of cellulose. For manv years, such hydrolysis was considered uneconomical in Western countries although it has heen widelv oracticed in the Eastern-bloc countries, parlicularly the U.S.S.R. The economics has now changed, and it is no longer a question of whether hydrolysis pl&ts will he built in the US., hut onlv when. The economics, even with relatively low yield acid-catalyzed plants, appears to he reasonable. The hest return from a plant designed to utilize low grade hardwoods of the southeastern U.S. was determined to he direct burning for heat to save expensive petroleum. Hmvever, conversion of the wood to chemicals was also considered feasible and profitable. A plant designed to make ethanol, furfural, and phenol could nroduce from $8-$11 ner $1 of wood used. There was estimated to be sufficient wood supply in one 50-mile radius circle in North Carolina t i r supplv. indefinitely, three plants utilizing 1500 tons per day of wid ( 5 ) .
Utilization of lignin is a very intriguing part of wrmd utilization, and it is an important aspect r,fnearly every proposal fur chemically converting w a d Ever since ahout 1840 when A. Payen determined that, wood is a matrix of cellulose fibers txmnd with a material, which he called rnnlifire i n c r u s t o n l ~ ~ and which we now call lignin, the chemistry US this natural high polymer has been studied intensively. It was found during the latter part af the 19th century that lignin could he rhemically modified so that it would dissolve in water and free the cellulose fibers. Commercial applications of such chemical modifications have resulted in the pulp and paper industry. This industry is a hiomass utilizer of great size and rm a worldwide basis produces more than 10Qons of cellulose to satisfy the world requirement for pulp and paper products. This greatly exceeds the total production of all synthetic polymers. Along with the cellulose is pnduced about 6 X 10' tons of lignin. Most of this complex material is burned during pulping
r"
Mrod;-" I
HOCH.
I
HCOH HCOH
HC-
H-4
Figure 2. A structural model for softwood lignin. Sakakibara IS),
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I I H ('OH
chemical recovery operations and provides a considerahle amount of the energy required fin the pulping operation. When m e considers this lignin plus the lignin that should become available when large hydrolysis plants are huilt, it is apparent that it represents a renewahle organic resource of considerable magnitude. Determination of the chemical structure of lignin has proven to he an extraordinarily complex prohlem and has required the attention of many chemists for over 140 years. It was suggested at a relatively early stage in the studies that lignin has an aromatic nature, hut it was not until the work of K. Freudenherg and his co-workers at Heidelherg that a plausible structure of lignin was estahlished ( 6 ) . It is now recognized by most lignin investigators that lignin is produced in lignified plants hy a unique, almost nonenzymatic-controlled process. As shown in the equation, only the very first step in the synthesis is enzymatically induced. This is dehydrogenation of coniferyl alcohol (conifer trees, I) or sinapyl alcohol (deciduous trees, Ia) to give the mesomorphic structures 11, 111, IV, V. CHOH
CH.OH
OH
0
I
I
make it preferable to many heavy stocks that are derived from coal, shale oil, and other fossil carbun sources. For example. lignin can he nhtained with little or no ash and nu nitrr~gen. In should find greatly increased use in the future as a feedstock for chemical plants. As with cellulose, lignin can he utilized in its pdymeric form and many schemes for producing dispersants, adhesives, fillers, and resin ingredients have heen proposed. In the future, these uses can he expanded to take the place of products now made from non-renewable petroleum derivatives. Summary
Svnthetic organic chemistrv plays a maior role in our modern industrial civilization.-~iariingfrom modest heginnings with feedstocks derived from coal tar and agricultural crops, the industry has burgeoned to the point where its products affect the lives of almost everyone on earth. At the present time, the industry is virtually completely dependent on petroleum and natural gas for its raw materials. Although it has long been recognized that petroleum and other fbssil carhon reserves were finite and would eventually disappear, their exhaustion was believed to he many generations in the future. The incredihle appetite that has devehped for energy derived from the same fossil reserves has caused a reassessment of the time when supplies ofthe non-renewable resources will hecome seriouslv deoleted. It is now recognized
I
I.R=H Ia. R = OCH,
as producers begin i o visualize the depletion of their re-
I1
CHnH
CH.OH
I HC.
I HC
H('
HC
I
I11
CH.OH
I
HC
II
II
IV
v
These radicals combine in many possihle ways in an essentially random manner str that the final lignin is a complex polymeric material with no single repeating units commonly encountered in other naturally occuring polymers, ex., pmteins, hemicellulose, and cellulose. Various structural schemes incor~oratingall possible variations that could be concluded from resultsof degiadative studies have been offered from time to time by lianin rehearchers in the past 15 years. The first plausihle structure was that proposed by Freudenherg (6). This has been modified and updated hy Adler ( 7 ) and Sakakibara ( 8 ) .The structure proposed hy the latter author is shown in Figure 2 and incorporates all the known linkages that have heen shown to he present by degradative and spectral studies of various lignin samples. It must he concluded, however, that the determination of the "exact" structure of lignin, if indeed there really is one, may never he completely accomplished. This does not orevent or limit the mesent or future utilization uf this important renewahle resource. An inspection structures shows that lignin of anv of the modern .proposed . ran be an important source of aromatic chemicals. Pyrolysis and hydrogendysis of lignin can lead to good yields of phenolic materials ( 9 ) .Such hehavior suggests that lignin could serve as a feedstock in hydrocracking and hydrorefining plants developed to process petroleum. Lignin has attributes that
comparable to what has heen developed so far. For the near-term future, which is probably going to he a very conhsed and turhulent period in mankind's development, many procedures will require investigation. "Syn" fuels will be .oreoared . from stored carhon reserves in coal. shale. and tar sands. Alcohols will he made from agricultural crops. For the long-term future, if mankind can survive the turbulent years of petroleum depletion, there is considerable cause for optimism. Inexhaustible supplies of solar and geothermal energy may he developed. Conversion of cellulose and lignin from the earth's vast production of biomass s h d d free us from dependence on limited non-renewable petroleum feedstocks fbr chemical plants. Increased knowledge of low energy requiring natural processes should he gained and eventually photosynthesis itself may he accomplished in huge chemical factories, hut until that time, utilization of the natural hiomass should suffice. Interestingly, it appears that chemical feedstocks in the 300-year period following the industrial revolution may undergo a complete cycle starting with coal petroleum coal hiomass! biomass
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Literature Cited
191.771-778 11978). 11.W.. Paper PrpnntedfotheHth W18rld ForeitryCnneross.Oct.
(11 Sarkanm. K. V.. Sc8mue.
(2, Duheen.
16-28.1978,
171 Adler. ti.. Wood 90 T r i h n d . 11. 169-218 i1977l. 181 Snkakibara. A.. Wsod SCI ?erhnf>f. ld.H!lLllXl lL'J8III. 191 t i h e e n . 11. W.,"Liyninr: ocrurrenco. Fsrrnnwln.and Structure."lEditurs Ssrkanen. K. V. end Ludwip. C. H.lInlrrrrlt.nrr. New Ynrk. 1971. yp. 797-811.
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