Environ. Sci. Technol. 2009, 43, 6275–6282
Ozonolysis of Lignin Models in Aqueous Solution: Anisole, 1,2-Dimethoxybenzene, 1,4-Dimethoxybenzene, and 1,3,5-Trimethoxybenzene E I N O M V U L A , †,‡ S E R G E J N A U M O V , † A N D C L E M E N S V O N S O N N T A G * ,‡,§ Leibniz-Institut fu ¨ r Oberfla¨chenmodifizierung (IOM), Permoserstrasse 15, D-04318 Leipzig, Germany, Max-Planck-Institut fu ¨ r Bioanorganische Chemie, Stiftstrasse 34-36, P.O. Box 101365, D-45470 Mu ¨ lheim an der Ruhr, Germany, and Universita¨t Dortmund, Fachbereich Bio- und Chemieingenieurwesen, Lehrstuhl Umwelttechnik, Emil-Figge-Strasse 70, 44227 Dortmund, Germany
Received March 18, 2009. Revised manuscript received June 25, 2009. Accepted July 1, 2009.
The lignin models anisole, 1,2-dimethoxybenzene, 1,4dimethoxybenzene, and 1,3,5-trimethoxybenzene were reacted with ozone in aqueous solution, and major products were identified and quantified with respect to ozone consumption when reference material was available. Hydroxylation products in yields equivalent to those of singlet oxygen and muconic products (in analogy to the Criegee mechanism) dominate. The formation of quinones points to the release of methanol. Hydroxyl radicals (•OH, quantified, main precursor: O3•-), singlet oxygen (quantified), O2•- radicals (quantified), and as counterparts of the •OH radicals radical cations of these methoxybenzenes must each play an important role as intermediates. In the case of 1,4-dimethoxybenzene, for example, the following products were identified (yields in parentheses when quantified): methyl(2Z,4E)-4-methoxy-6-oxo-hexa-2,4dienoate 5 (52%), hydroquinone 6 (2%), 1,4-benzoquinone 7 (8%), 2,5-dimethoxyhydroquinone 8, 2,5-dimethoxy-1,4-benzoquinone 9, singlet oxygen (6%), hydrogen peroxide (56%), •OH (∼17%), O2•- (e9%). Gibbs energies for the various potential reaction pathways were calculated with the help of the Jaguar 7.5 program.
Introduction Ozonation is widely used in drinking-water processing for disinfection as well as for the removal of unwanted contaminants (1-3). There is an increasing concern about potential unwanted byproducts. The omnipresent natural organic matter (NOM) may contain aromatic lignin-derived residues containing methoxy groups (4-6). In the pulp and paper industry, ozone has been used since the early 1990s as a bleaching agent for the production of chlorine-free paper. In this process, the elimination of coloring components due to lignin contamination is intended, but the integrity of the cellulose fibers should not be affected. Yet under the conditions of producing a pulp of full * Corresponding author e-mail:
[email protected]. † Leibniz-Institut fu ¨ r Oberfla¨chenmodifizierung (IOM). ‡ Max-Planck-Institut fu ¨ r Bioanorganische Chemie. § Universita¨t Dortmund. 10.1021/es900803p CCC: $40.75
Published on Web 07/17/2009
2009 American Chemical Society
brightness, some degradation of the cellulose fibers also occurs. This was attributed to very reactive, i.e., nonselective, free radicals, notably •OH (5). With a number of lignin models, the formation of •OH upon ozonation has, in fact, been observed by scavenging •OH with 2-propanol and measuring the resulting acetone (5). In their reactions with carbohydratebased polymers, •OH are most effective in inducing chain scission (7), and this must cause a decrease in fiber strength. In aqueous solution, however, the reaction •OH with the cellulose fibers is a heterogeneous reaction, and the cross section for their reaction with the fibers must be low compared to that of other omnipresent water impurities. Studies on the degradation of cellulose using ionizing radiation (8) may be interpreted on this basis. Beyond bleaching, there is another interest in the reaction of ozone with lignin. The effluents of the paper industry still contain a high load of organic matter (DOC). Most of this is biodegradable, but there is a component which is refractory to biodegradation. It is possible to enhance biodegradation by a partial oxidation of the biorefractory effluent with ozone (9). In the reaction of ozone with lignin, the main target must be the aromatic rings, and the aliphatic chains linking the aromatic residues will be barely attacked. This can be deduced from the generally low reactivity of ozone toward C-H bonds, even when activated by adjacent OH groups, k e 2 M-1 s-1 (10). The reactivity of the parent of aromatic compounds, benzene, is also very low, but upon substitution with electrondonating substituents, the rate of reaction increases dramatically (see Supporting Information). Thus, the aromatic lignin moieties substituted to a higher degree with electron-donating functional groups (e.g., alkoxyl) will be attacked preferentially. For obtaining some mechanistic insight into potential degradation pathways, we have chosen as model systems anisole, 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, and 1,3,5-trimethoxybenzene.
Not all the structural elements given by these four methoxylated benzenes may be found in lignin, but from the pathways displayed by these three types (ether functions in ortho, meta, and para positions) major aspects of lignin ozonolysis may be deduced. Moreover, the ozone chemistry of aromatic compounds is still poorly understood. The reason for this is the usually low ozone rate constant (104 M-1 s-1) that prevent a quantification of primary products even at low ozone to starting material ratios. 1,4-Dimethoxybenzene and 1,3,5-trimethoxybenzene have ozone rate constants >105 M-1 s-1 (see Supporting Information), and this problem largely falls away. In the pioneering work on the reactions of ozone with lignin models (5, 6), the formation of •OH radicals has been shown, but there is no information as to the final product. In the present paper, final primary products (and intermediates such as singlet oxygen, •OH and O2•-) have been identified and a large number of them have been quantified. This will shed some light on the reactions that are induced by ozone in its reactions with aromatic VOL. 43, NO. 16, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Structures of the identified products. compounds. While mechanistic considerations will be based on intermediates and products, the energetics of the various proposed pathways will be addressed by quantum-chemical calculations.
Experimental Section Anisole, 1,2-dimethoxybenzene, 1,4-dimethoxybenzene, and 1,3,5-trimethoxybenzene (all Aldrich) were used as received. For the quantification of the products, cis,cis-muconic acid and 1,4-benzoquinone (resublimed) were available from Merck as reference material. Solutions were made up in MilliQ-filtered (Millipore) water. An oxygen-fed ozonator (BMT 802X, BMT Messtechnik, Berlin) was used to produce the ozone stock solutions. Their ozone content was determined spectrophotometrically using ε(260 nm) ) 3300 M-1 cm-1 (11, 12). Ozonations were carried out in unbuffered aqueous solutions by mixing varying amounts of methoxybenzenescontaining solutions and ozone stock solution. For the quantification of the products, ozone concentrations remained
