Lignin strong oxidants

The carboxyl contents of different lignins are shown in Table 7.5.1. In the case of lignins isolated or modified without recourse to strong oxidation procedures, the carboxyl content is generally less than 10%. However, when lignin is subjected to the action of strong oxidants, as is the case, for example, in the bleaching of chemical pulps, the carboxyl content may substantially exceed this value. 

Chum HL, Ratcliff M, Schroeder HA, Sopher DW (1984)Electrochemistry of biomass-derived materials. I. Characterization, fractionation, and reductive electrolysis of ethanol-extracted explosively depressurized aspen lignin. J Wood Chem Technol 4 505-532 Crozier TE, Johnson DC, Thompson NS (1979) Changes in a southern pine dioxane lignin on oxidation with oxygen in sodium carbonate media. Tappi 62 107-111 Ekman K, Enkvist T (1955) Some determinations of weak and strong acids in various lignin preparations and pulps. Pap Puu 37 369-382... 

Ce(IV) sulfate is widely used as a catalyst for various reactions, for instance grafting acrylamide on alkali lignin (Meister et al., 1984) and the Belousov-Zhabotinskii oscillating reaction (Winfree, 1984). It can be also used as a strong oxidant for several classes of organic compounds (Rao et al., 1983). 

The relatively basic (hydroxyalkyl)phosphines act toward LMCs as reductants and, compatible with this, also as strong nucleophiles. We have studied such reactions in aqueous and D2O solutions by P-, H-, and C-NMR spectroscopies (including 2D correlation methods), product isolation and, when possible, X-ray analysis of isolated compounds or their derivatives. Thus, aromatic aldehyde moieties present in lignin (e.g., 3) are reduced to the corresponding alcohols (see 4) with co-production of the phosphine oxide in D2O, -CH(D)OD is formed selectively (36). The mechanism proceeds via a phosphonium species formed by initial nucleophilic attack of the P-atom at the carbonyl C-atom, i.e., via ArCH(OH)P%, where Ar is the aromatic residue and R is the hydroxyalkyl substituent (36). When the aldehyde contains a 4-OH substituent, the alcohol product... 

The C.O.P. of a compound depends strongly on the electronic effects of substituents on o- and -positions and is linearly related to its normal redox potential. Using a modified procedure we determined the C.O.P. of several lignin models and lignins and found that a relationship exists between C.O.P. and initial oxidation rate. The oxidizability of monohydric... 

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