Chemical structural models for coalified wood (vitrinite) in low rank coal

Examination of a series of coalified gymnospermous woods ranging in rank from brown coal to subbituminous coal by solid-state 13C NMR and analytical pyrolysis has provided sufficient information to construct structural models depicting the changes that occur to lignin, the primary precursor of vitrinite, during coalification. Progressive changes in the chemistry of coalified wood suggest the following series of reactions: (1) demethylation to form catechol-like structures that are dominant components of brown coal and lignite A; (2) cleavage of aryl ether linkages to form phenols and reactive carbocations that alkylate the catechol rings; (3) dehydration of the catechol rings; (3) dehydration of the catechol-like structures to form the structures of subbituminous coal dominated by alkylphenols; and (4) reduction of the 3-carbon alkyl side chain derived from lignin to form propyl substituents. The models developed for each stage of coalification are derived from chemical modifications of the structure of lignin.