Mechanocatalytic depolymerization of lignin

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Brittain, Alex David
Sievers, Carsten
Bommarius, Andreas S.
Meredith, J. Carson
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In this work, mechanochemical reactions were performed to depolymerize organosolv lignin with sodium hydroxide in a mixer ball mill at ambient conditions. GPC analysis revealed rapid depolymerization into small oligomers occurred within minutes of milling time, followed by a slower reduction in average relative molecular weight over the next eight hours of milling. Additionally, monomeric products were identified and quantified by GC-MS. The extent of depolymerization appeared to be limited by condensation reactions that form bonds between products. Suppression of these condensation reactions could be achieved through the addition of methanol as a scavenger or adjustment of the moisture content of the feedstock. These modifications resulted in lower average relative molecular weights and higher monomeric yields. The second portion of this work focuses on the mechanocatalytic depolymerization of lignin under ambient conditions using a variety of solid acid and base catalysts. The use of solid catalysts provides an easier separation from reaction products than sodium hydroxide, which will result in cost savings in industrial applications. Of the catalysts tested, only Mg(OH)2 proved to catalyze significant depolymerization, as determined by a decrease in relative average molecular weight measured by GPC, and creation of substantial yields of monomers as measured by GC-MS. Depolymerization with Mg(OH)2 was not as effective as sodium hydroxide catalyzed depolymerization, with smaller reductions in molecular weight and lower production of monomers for similar milling times. However, Mg(OH)2 was able to be easily completely recovered after the reaction through filtration, which was not possible with sodium hydroxide. The final section of this work involved the construction of a modified milling reactor to facilitate mechanocatalytic hydrogenation reactions at ambient conditions. Reactions of lignin model compound diphenyl ether using a Pt/Al2O3 catalyst revealed conversion to dicyclohexyl ether, cyclohexanol, and cyclohexanone. GC-MS quantification of products showed that reactivity was low for the first hour of milling, followed by a large increase in conversion at two hours of milling. This was followed by a slower increase in conversion between two and eight hours of milling. The volatile products of this reaction were carried out of the reactor by the effluent hydrogen stream and collected in a condenser downstream. The reactor designed for this work can be envisioned to provide an environment in which to simultaneously perform lignin depolymerization and hydrogenation reactions, creating a one-pot system for the creation and separation of fuel precursors.
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