Shantanu Kelkar

Structural characterization of alkaline hydrogen peroxide pretreated grasses exhibiting diverse lignin phenotypes

BackgroundFor cellulosic biofuels processes, suitable characterization of the lignin remaining within the cell wall and correlation of quantified properties of lignin to cell wall polysaccharide enzymatic deconstruction is underrepresented in the literature. This is particularly true for grasses which represent a number of promising bioenergy feedstocks where quantification of grass lignins is particularly problematic due to the high fraction of p- hydroxycinnamates. The main focus of this work is to use grasses with a diverse range of lignin properties, and applying multiple lignin characterization platforms, attempt to correlate the differences in these lignin properties to the susceptibility to alkaline hydrogen peroxide (AHP) pretreatment and subsequent enzymatic deconstruction.ResultsWe were able to determine that the enzymatic hydrolysis of cellulose to to glucose (i.e. digestibility) of four grasses with relatively diverse lignin phenotypes could be correlated to total lignin content and the content of p-hydroxycinnamates, while S/G ratios did not appear to contribute to the enzymatic digestibility or delignification. The lignins of the brown midrib corn stovers tested were significantly more condensed than a typical commercial corn stover and a significant finding was that pretreatment with alkaline hydrogen peroxide increases the fraction of lignins involved in condensed linkages from 88–95% to ~99% for all the corn stovers tested, which is much more than has been reported in the literature for other pretreatments. This indicates significant scission of β-O-4 bonds by pretreatment and/or induction of lignin condensation reactions. The S/G ratios in grasses determined by analytical pyrolysis are significantly lower than values obtained using either thioacidolysis or 2DHSQC NMR due to presumed interference by ferulates.ConclusionsIt was found that grass cell wall polysaccharide hydrolysis by cellulolytic enzymes for grasses exhibiting a diversity of lignin structures and compositions could be linked to quantifiable changes in the composition of the cell wall and properties of the lignin including apparent content of the p-hydroxycinnamates while the limitations of S/G estimation in grasses is highlighted.

A mild approach for bio-oil stabilization and upgrading: electrocatalytic hydrogenation using ruthenium supported on activated carbon cloth

Electrocatalytic hydrogenation (ECH) offers a new approach for bio-oil stabilization (a.k.a. partial upgrading). Water-soluble bio-oil, obtained by aqueous extraction of the liquid product of biomass pyrolysis, was hydrogenated using ECH at room conditions. A new electrocatalyst, ruthenium supported on activated carbon cloth, was used as the catalytic cathode. After electrocatalytic hydrogenation, aldehydes and ketones were reduced to the corresponding alcohols or diols, forms less prone to condensation chemistry. Carbon recovery into the liquid product, important when making liquid fuels from biomass, was more than 80%, while less than 0.1 wt% of the water-soluble bio-oil formed solid precipitate. The stability of the ECH-treated water-soluble bio-oil was checked via an accelerated aging test followed by size exclusion chromatography analysis and viscometry. Besides stabilization of bio-oil for subsequent fuel production, hydrogen and valuable diols were produced during ECH. Strategies to optimize the energy efficiency of this approach by altering the cell design, modifying the catalyst and adjusting the reaction conditions were also explored.