TECHNOLOGY
Electrochemical Theories Open Up Chemistry of Wood Digestion Anthraquinone pulping is studied in depth by two research groups who propose dianions, radical anions speed lignin degradation Joseph Haggin, C&EN Chicago
The chemical mechanisms involved in wood pulping are becoming bet ter understood as the result of new research. One conclusion already reached is that single electron trans fer (SET) reactions are important, and may be the key, to deciphering the chemistry of pulping. This conclusion has been reached jointly by two groups working in concert: Donald Dimmel and Lois Perry at the Institute of Paper Chem istry (IPC), Appleton, Wis., and Hel ena Chum and Peter Palasz at the Solar Energy Research Institute (SERI), Golden, Colo. They are us ing classic and electrochemical tech niques to study in depth the chem istry of anthraquinone pulping. Many chemists and chemical en gineers justifiably consider the pulp digester to be an example of indus trial sledgehammer chemistry. It de veloped over a long period of time in productive service by trial and error. In North America, the kraft process is the most widely used pulping process. It was patented a hundred years ago, before either the chemistry of pulping or the struc ture of wood was even partially understood. According to Dimmel, they are still not well understood and are the foci of renewed research interest. Alkaline pulping processes are aimed at removing lignin from the wood while retaining the carbohy drate materials, specifically the 20
October 15, 1984 C&EN
cellulose. Delignification frees the cellulose fibers, which are then used in slurry to make paper and board products. In kraft pulping, both hy droxide and hydrosulfide ions frag ment the lignin resulting in its dissolution. However, the hydrox ide ions also dissolve some of the carbohydrate polymers, with a con sequent lowering of the pulp yield. The research at IPC is aimed at im proving pulp yields by increasing delignification rates and minimiz ing carbohydrate degradation. This requires a detailed understanding of pulping chemistry. Much of the research in pulping chemistry is conducted with the aid of model compounds because of the complexity of wood chemistry and physics. Wood, like many natural products, is hard to define struc turally. Lignin, for example, is a very irregular, randomly crosslinked polymer of phenylpropane units joined by many different
linkages. Lignin model compounds are generally dimers of the basic building blocks and contain the most abundant linkage in lignin— namely the β-aryl ether bond. The model compounds are capable of forming quinonemethide interme diates upon heating in aqueous al kali solutions. Reactive quinone methide species appear to be cen tral to lignin reactions. Pulping with anthraquinone ad ditives was introduced in 1977. In a departure from previous experience, this resulted from research instead of trial and error on the production line. The addition of about 0.1% of anthraquinone to an alkaline pulp ing system sharply increases the pulping rate and pulping selectivity. The apparent catalytic effect and se lectivity improvement are major fac tors in the renewed interest in pulping chemistry. From the beginning of the IPC/ SERI studies it was apparent that
Redox cycle explains catalysis
Carbohydrates
Solubilized lignin
Stabilized carbohydrates
Lignin
Yield gain
Rate gain
Proposed redox cycle explains the catalytic effect of anthraquinone in a pulp di gester. Addition of 0.1% anthraquinone sharply increases the pulping rates and the selective removal of lignin
anthrahydroquinone also played an important role in digester chemistry. Anthrahydroquinone dianions and radical anions are produced when anthraquinone is reduced by some of the functional groups in the car bohydrates and lignin. Dimmel notes that model compound studies indicate that anthrahydroquinone increases the rate of delignification by promoting lignin fragmentation and retarding lignin condensation. During the course of these reactions anthrahydroquinone is oxidized to anthraquinone, thereby completing a redox cycle. Repetition of this cy cle accounts for the catalytic activi ty of anthraquinone. Two theories have been advanced to explain how anthrahydroquinone causes lignin fragmentation. The adduct theory proposes that anthrahy droquinone dianions add to lignin quinonemethide species and the resulting product fragments off an thraquinone and a monomer from one end of the polymer. If the cor
A N G U S
rect functional groups are present, the new polymer end unit can re peat the process, thereby gradually d e g r a d i n g t h e l i g n i n to watersoluble materials. Adducts contain ing anthrahydroquinone bonded to the α-carbon of a quinonemethide and also having a /3-aryl ether link age have been synthesized. When warmed with alkali, the adducts fragment, yielding anthraquinone and phenolate ions. Below 60 °C, anthrahydroquinone couples with simple quinonemeth ide species with high yields of adducts. At 60 °C, the reaction is reversible. At 100 °C, the adducts disproportionate to anthraquinone and an anthrone product. Dimmel says that an attractive feature of the adduct theory is that its mechanism resembles that proposed for hydrosulfide-promoted delignification wherein SH~ adds to the α-carbon of a lignin quinonemethide inter mediate and assists in cleaving the carbon//3-aryl ether bond through
C H E M I C A L
d i s p l a c e m e n t of a n e i g h b o r i n g group. The second theory is the SET theory, which proposes that anthra hydroquinone dianions and radical anions function as soluble electrontransfer catalysts, which mediate the transfer of electrons from the insol uble carbohydrate polymers to the insoluble lignin polymers. By such a mechanism, no bonding or ad ducts are involved. The electron shift helps to stabilize the carbohy drates by an oxidation reaction and helps to fragment the lignin by reduction. The species that accept the electrons from the anthrahydro quinone ions are presumed to be lignin quinonemethides. These spe cies appear to be good substrates for the SET reactions because exten sively resonance-stabilized benzylic phenolate radicals would result. Also, anthrahydroquinone radical anion should be an active partner in electron transfer because of ex tensive resonance stabilization and
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21
Technology
Lignin fragmentation: adduct theory OCri,
QCH,
Adduct theory is illustrated by synthesis of adduct (3) and subsequent fragmentation (II) to anthraquinone (4) and phenolate ions (5) and (6)
the good stability of both the oxidized and reduced forms. The research groups at IPC and SERI have electrochemically generated anthrahydroquinone radical anions and dianions and have shown that both transfer electrons model quinonemethide species, causing the latter to fragment at their /3-aryl-ether linkages. In the absence of anthraquinone, direct electrolysis of quinonemethide intermediates at more negative potentials than the initial anthraquinone reduction causes further quinonemethide fragmentation. Under the appropriate electrolysis conditions, electron transfer reactions appear to occur in preference to adduct formation and to produce the same products postulated by the adduct theory. These electrochemical experiments demonstrate a unique kind of chemistry — namely, electron transfer reactions between anthrahydroquinone and various quinonemethide species. Such reactions can
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Lignin fragmentation: SET theory Considerable support for the single electron transfer theory comes from the cyclic voltametry of mixtures of anthraquinone and such quinonemethides as
H3C—CH—Ο
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explain the high efficiencies of an thraquinone pulping systems and, possibly, the chemistry of other pulping systems as well. The appli cability of these reactions under ac tual pulping conditions is under fur ther investigation at IPC. The group at SERI is continuing experiments to determine the rates of electron transfer from anthrahydroquinone to quinonemethide intermediates, the rates of the fragmentation, and other related subjects. Dimmel believes that establish ing that quinonemethide fragmen tation follows from electron trans fer reactions will substantially change the way chemists attack the problems of wood pulping technol ogy. The possible use of electro chemical methods to monitor and/or promote pulping presents another potential means for improvement. D
ATTENTION — GC/MS Laboratories!!! The U. S. Environmental Protection Agency will hold a PREBID CONFERENCE iron) 8:30-5:00 p.m. on November 14 and 15, 1984 at the Clarion Four Seasons Hotel, 2500 Carlisle, NE, Albuquerque, New Mexico. This conference is in anticipation of the expansion of the organic GC/MS analyses requirements for the Superfund Pro gram. The conference will address the Government's technical and administrative requirements for the analysis of hazardous waste samples, in order to facilitate realistic bidding by new contractors. All interested laboratories must request the 10/84 Organics Analysis Invitation For Bid (IFB) in writing from Marian Bernd, Con tracting Officer, USEPA Office of Administration, Procurement & Contracts Mgmt. Div. (PM-214), 401 M St. SW, Washington, DC 20460. A $10 registration fee per participant is required to cover conference expenses. Conference participants should contact the Clarion Four Seasons Hotel at (800) 545-8400 by October 30, 1984, to reserve hotel rooms. When calling, individuals should identify themselves with the EPA Prebid Conference. In addition, each participant should complete the registration form below, and return it along with a check by October 30th. Questions concerning the conference should be directed to Mr. Steven Manzo, Sample Man agement Office, at (703) 557-2490.
REGISTRATION FORM - U. S. ENVIRONMENTAL PROTECTION AGENCY PREBID CONFERENCE I will be attending the EPA Prebid Conference on November 1 4 - 1 5 , 1984, in Albuquerque, New Mexico. Enclosed is my registration f e e check for $ 1 0 , payable to VIAR A N D COMPANY, INC.
(Name and Title — Please Print) (Name of Organization) (Address) (Area Code and Telephone Number) RETURN TO: MR. STEVEN MANZO. USEPA SAMPLE MANAGEMENT OFFICE, P.O. BOX 818, ALEXANDRIA. VA 22313
October 15, 1984 C&EN 23
