CHEMICAL MODELING OF THE THERMAL AND CATALYTIC DEPOLYMERIZATION OF LIGNIN (PYROLYSIS, HYDRODEOXYGENATION, MODEL COMPOUNDS).

摘要

Lignin depolymerization was experimentally and mathematically modeled.;The catalytic liquefaction of lignin was probed through the reactions of the model compounds 4-methylguaiacol, 4-methylcatechol, eugenol, vanillin, o,o'-biphenol, o-hydroxydiphenylmethane and phenyl ether over a sulfided CoOMoO(,3)/(gamma)-Al(,2)O(,3) catalyst. Hydrodeoxygenation of substituted guaiacols and catechols occurred readily at temperatures where the predominant thermal reactions result in coke formation. The demethoxylation of methylguaiacol and the dehydroxylation of methylcatechol had Arrhenius parameters of (log A (l/g cat(.)s), E* (kcal/mol)) = (6.14, 28.3) and (6.83, 29.3), respectively. Cleavage of thermally stable diphenylmethane and phenyl ether interunit linkages was facile. Biphenol was converted to mostly 2-phenylphenol and dibenzofuran.;The foregoing model compound reaction pathways and kinetics were combined with a probabilistic description of lignin structure into a priori mathematical models of the kinetics of lignin depolymerization. Model predictions of individual molecular products allowed summation to the usual gas, light liquid, single-ring, multiring and residue product fractions. Kraft lignin pyrolysis was simulated for reactors where either only primary or also secondary reactions are important; results indicating that greater yields of reactive single-ring guaiacols would be obtained in the former instance. Catalytic liquefaction of both native and kraft lignin was modeled using an effectiveness factor function to describe the moderation of catalytic reaction rates due to the diffusion of large lignin fragments into the catalyst pores. The level of diffusional resistance was seen to have a marked effect on predicted product yields. These models can help define the proper protocols for future lignin experiments as well as explore novel, perhaps experimentally unattainable, lignin processing schemes.;Although the approach developed in this thesis was specific to the thermal and catalytic depolymerization reactions of lignin, it could have general applications to the processing of many complex substrates.;Kraft lignin pyrolysis was analyzed in terms of the reactions of its models 1,2-diphenylethane, cis- and trans-stilbene, diphenylmethane and 1,1,2-triphenylethylene. The major primary pathway for pyrolysis of diphenylethane or either stilbene isomer was cleavage to form toluene. These substrates also reacted to phenanthrene. Diphenylmethane pyrolysis yielded benzene, toluene and fluorene, and triphenylethylene pyrolyzed to these three and also diphenylmethane, diphenylethane, stilbene and phenanthrene.