TECHNOLOGY
Argonne Scientists Make Artificial Coal Process using clay catalysts to transform lignin provides new insights into coal structure and how to alter it Study of the basic chemical structure of coal has led to new ideas about the way coal was formed during geologic time. Chemists at Argonne National Laboratory have succeeded in making a type of artificial coal from naturally occurring materials. The process is much less severe than formerly thought to be necessary and provides some new insights into coal structure and how to alter it. The most widely held view of natural coalification—the sum of the geological processes that produced coal—is that organic plant material was transformed microbially to humic materials. These, in turn, were transformed by abiotic thermal processes to lignite, bituminous coal, and anthracite. Randall E. Winans, who leads the group from Argonne's chemical division investigating coal structure, says that as appealing as this view may be, it is no more firmly based than other views. In fact, he maintains, there is no incontrovertible evidence to support any particular theory of coalification. How coal was formed is still under debate. In this context it appears significant that Winans' group recently has demonstrated that artificial coalification reactions catalyzed by clay minerals easily transform lignin into an altered lignin that is much like that found in natural lignite. So it appears that the group has made some artificial lignite. 42
November 21, 1983 C&EN
Winans believes that the early results in the project support the idea that, in the natural coalification processes, much of the lignin survives the early diagenetic stage with little or no alteration of its chemical structure. Subsequently, it is transformed into aromatic-rich coal macromolecules (vitrinite) during the catalytic phase of coalification. Most of the plant materials that were converted to coal were probably woody varieties, and the chief interest is with these materials. Coalification of woody tissue appears to be primarily a loss of oxygenated compounds, mostly carbohydrates, followed by the direct chemical alteration of lignin to the macromolecules found in coal. The present study is examining the possibility that most, if not all, of the vitrinite macerals in lignite were formed directly from slightly altered lignin. The general approach that Winans' group is using includes three stages. The first is to determine the chemical identity of lignin, kerogens found in shale, and various coals by use of selective oxidative degradation, pyrolysis and other suitable means. This is followed by the transformation of lignin into artificial coal products by low-temperature catalytic reactions. Clays form the family of catalysts. The final stage is comparison of the natural and artificial coal macerals. The presumption is that close correspondence will help deduce the processes of coalification and, more important from a scientific viewpoint, the detailed structure of coal. Clay minerals have been long known to exert catalytic effects on lignin. In the Argonne experiments a typical example would involve heating of clay and lignin mixtures to temperatures from 150 to 200 °C.
The yield of insoluble products ranges from 54 to 67% by weight, with longer reaction times favoring more vitrinitic materials. There is an increase in aromaticity with time that generally parallels that observed for natural coals of increasing rank. Likewise, the general indications are that the processes at work in the artificial coalification are similar to those occurring in natural evolution. A typical experiment would run for more than two months. The oxidation results are also supported by a 13C nuclear magnetic resonance study of softwood lignin using cross polarization and magic angle spinning. Demethyloxylation occurs very rapidly and
H/C, O/C ratios provide coalification road map H/C 2.0 1.8 1.6 1.4 1-2
Lignin
Liptinite
1.0
2^
0.8 0.6
Vitrinite
#*^^ Inertinite
0.4 0.2 0 0.01
.
.
. . .^_L_
. .....
0.1
O/C The progress of coalification can be measured in terms of the hydrogen-to-carbon and oxygen-to-carbon ratios. There are three general paths. Argonne's artificial coalification experiments, generally, follow the line shown, from upper right to lower left. For example, lignin was^ subjected to artificial coalification for two months to yield the modified form at point 1. Longer times yielded the subsequent points 2, 3, and 4
1.0
there is at least an implication that ral coalification path for vitrinite pal precursors for alginite. There is, some of the aliphatic entities are and the artificial path for hydro- as yet, no evidence for the formaconverted to aromatics during coal- thermal carbonification of cellulose. tion of kerogens from diatoms. ification. A further implication is Heating cellulose with clay at 150 The present studies on the mechathat the loss of methylene carbons °C yields products in which the nisms of coalification at Argonne sometimes observed may typically celluloselike structures still pre- are still in the early stages, but the occur either by fragmentation reac- dominate. Winans notes that it is early evidence suggests that major tions or by conversion to aromatics. generally accepted that cellulose is progenitors such as lignin or lipids The increase in aromaticity of artifi- easily removed in the diagenetic could survive the diagenetic stages cial products is believed to be simi- stage of coalification, leaving lig- of coalification. They could then be lar to that observed in artificial coals nin to be preferentially concentrat- transformed directly to highly aroof higher rank. Reduction of the ed with the phenol derivatives and matic (vitrinite) and highly aliphataliphatic character in the artificial lipids. There is also independent ic (alginate) macromolecules by therproducts with greater reaction times evidence that the cellulose is prefer- mal catalysis. further reinforces this belief. As entially lost while the lignin conThe current research into direct coalification proceeds, the data sug- centrates during coal formation. At coal formation implies, among othgest that structures in the artificial present the artificial coalification of er things, that the fundamental orproducts become more and more cellulose does not produce solid ma- ganic structures in coal derived dicrosslinked through the aromatic terials chemically similar to vitrinite. rectly from lignin should be much groups. This is in agreement with However, the possibility of a minor less diverse than those from ran13 C NMR data acquired. contribution from cellulose and/or domly altered materials. In addition, Clay minerals are important to its derivatives in coalification can- the artificial coalifications provide excellent models for the complex this artificial coalification process. not be excluded totally. Acid treatment of the clays accelerOne of the more elusive mecha- macromolecular structure of coal and ates the reactions a bit but even the nisms is probably that for the for- yet are less chemically complex beuntreated clays are "convincingly mation of aliphatic macromolecules cause of the pure source materials. catalytic," according to Winans. in oil shale kerogens or coal al- The next step in the research is to Transformation of lignin certainly ginates. Such materials have been make use of isotopic labels to furdoes not occur in the absence of the synthesized from algal fatty acids. ther probe structural transformations clays. The clays of choice are mont- These acids are important lipid con- during coalification. Ultimately, the morillonite varieties, but kaolinite stituents of algae and are very object is to define coal structures and illite clays are also effective, if abundant. Nonmarine algae gener- and how they were formed. to a lesser degree. Combinations of ally are considered to be the princiJoseph Haggin, Chicago the various clays don't seem to have an effect different from that of the individual kinds. The presence of AlBr3 also appears to catalyze the artificial coal- A new gas-treating agent developed ture adjacent to the nitrogen atom ification process. However, it is not by Exxon Research & Engineering to form a hindered amine. Control certain that the structures of prod- Co., Florham Park, N.J., selectively of the amine molecular structure ucts obtained with AlBr 3 are similar removes h y d r o g e n sulfide from makes it possible to tailor the agents to those of the corresponding coal gases. The company says it can in- for specific treating functions. vitrinites. crease s c r u b b i n g capacity more The company has completed labHigh-temperature pyrolysis of lig- than 40%, compared to other gas- oratory, pilot plant, engineering, nin from 350 to 400 °C does not treating agents and processes. and commercial application studies effect the transformation into coalSelective removal of hydrogen on the new agent. The studies, it like materials c o r r e s p o n d i n g to sulfide is economically preferred in says, show that the agent is an atvitrinite and/or lignite. Even in the refinery and production applica- tractive replacement for other selecpresence of clay, such high-temper- tions, including Claus sulfur plant tive hydrogen sulfide removal proature reactions did not produce tail gas cleanup, when there are no cesses now in commercial use, inproducts consistent with the natu- process needs to remove other gases. cluding both methyldiethanol amine ral evolutionary path to vitrinite. The new agent, called Flexsorb SE, and direct conversion processes. Loss of catalytic effects by the clays has been designed specifically to Exxon R&E expects formally to was observed when the reactions react preferentially with hydrogen offer Flexsorb SE for licensing soon. were carried out in the absence of sulfide rather than carbon dioxide. It says it also plans to offer hindered air. It is apparent that the lignin The agent is one of a family of amine technology that can make imfunctional groups, which are suscep- gas-treating agents being developed provements in removal of both cartible to oxidation, are very easily by Exxon R&E based on a new amine bon dioxide and hydrogen sulfide oxidized. chemistry. Company researchers using amines in an organic solvent, Another matter that has piqued have found that the effectiveness and in removal of carbon dioxide the interest of the Argonne group of treating agents can be improved with an amine-promoted hot potasis the similarity between the natu- by placing a bulky molecular struc- sium carbonate solution. D
Exxon agent selectively removes H2S from gases
November 21,1983 C&EN
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