Corn stover lignin is modified differently by acetic acid compared to sulfuric acid

In this study, two acid catalysts, acetic acid (HAc) and sulfuric acid (H2SO4), were compared in thermal pretreatments of corn stover, in particular to assess the less understood fate of lignin. HAc-insoluble lignin, analyzed by pyrolysis GC-MS, showed decreasing levels (%) of Cα-oxidized (from 3.7 ± 0.2 to 1.8 ± 0.1), propenyl (from 2.5 ± 0.1 to 1.0 ± 0.1), vinyl-G (from 34.5 ± 1.8 to 28.4 ± 0.9), vinyl-S (from 4.2 ± 0.2 to 3.7 ± 0.1) and methylated (from 4.9 ± 0.04 to 2.8 ± 0.1) lignin units at increasing HAc amounts. Concurrently, unsubstituted and vinyl-H units increased (from 7.5 ± 0.5 to 11.3 ± 0.2 and from 40.5 ± 1.9 to 49.9 ± 0.9, respectively). Similar trends were seen for residual lignin in H2SO4 catalyzed pretreatments, although the composition differed from that of residual HAc-lignin. In particular, H2SO4-lignin showed slightly lower values (%) for unsubstituted (9.9 ± 0.2) and vinyl-H (45.7 ± 4.1) units, while Cα-oxidized (3.4 ± 0.4), propenyl (1.9 ± 0.1), vinyl-G (28.5 ± 0.9), vinyl-S (4.4 ± 0.6) and methylated (4.6 ± 0.2) lignin units remained higher compared to HAc-catalysis at similar pH values. Xylan yields and corresponding enzymatic conversions of the solids were similar regardless the type of acid. Our findings show that HAc in pretreatments decreased lignin complexity, possibly due to cleavage reactions, although subsequent recondensation reactions increased solid lignin yields, more than H2SO4, while removal of xylan and enzymatic conversion of solids were equal.