PEROXIDASE-CATALYZED POLYMERIZATION AND DEPOLYMERIZATION OF COAL IN ORGANIC-SOLVENTS

Peroxidases from horseradish roots (HRP) and soybean hulls (SBP) catalyze the efficient polymerization of a 4-kDa dimethylformamide (DMF)-soluble fraction of Mequininza (Spanish) lignite in 50% (v/v) DMF with an aqueous component consisting of acetate buffer, pH 5.0. Under these conditions, HRP and SBP catalyze the oxidation of free phenolic moieties in the coal matrix, thereby leading to oxidative polymerization of the low-molecular-weight coal polymers. The high fraction of nonphenolic aromatic moieties in coal inspired us to examine conditions whereby such coal components could also become oxidized. Oxidation of nonphenolic aromatic compounds was attempted using veratryl alcohol as a model substrate. SBP catalyzed the facile oxidation of veratryl alcohol at pH <3. HRP, however, was unable to elicit veratryl alcohol oxidation. The potential for SBP to catalyze interunit bond cleavage on complex polymeric substrates was examined using 1-(3,4-dimethoxyphenyl)-2-(phenoxy)propan-1,3-diol (1) as a substrate. SBP catalyzed the C alpha-C beta and beta-ether bond cleavage of this compound, suggesting that similar reactions on coal, itself, could lead to depolymerization. Depolymerization of a >50 kDa coal fraction was achieved using SBP in 50% (v/v) DMF with an aqueous component adjusted to pH 2.2. Approximately 15% of the initial high-molecular-weight lignite fraction was depolymerized to polymers <4 kDa in size. Hence, SBP is capable of catalyzing the depolymerization of coal in organic solvents, and this may have important ramifications in the generation of liquid fuels from coals.