Abstract
BackgroundThe use of a microwave synthesis reactor has allowed kinetic data for the hydrothermal reactions of straw biomass to be established from short times, avoiding corrections required for slow heating in conventional reactors, or two-step heating. Access to realistic kinetic data is important for predictions of optimal reaction conditions for the pretreatment of biomass for bioethanol processes, which is required to minimise production of inhibitory compounds and to maximise sugar and ethanol yields.ResultsThe gravimetric loss through solubilisation of straw provided a global measure of the extent of hydrothermal deconstruction. The kinetic profiles of furan and lignin-derived inhibitors were determined in the hydrothermal hydrolysates by UV analysis, with concentrations of formic and acetic acid determined by HPLC. Kinetic analyses were either carried out by direct fitting to simple first order equations or by numerical integration of sequential reactions.ConclusionsA classical Arrhenius activation energy of 148 kJmol−1 has been determined for primary solubilisation, which is higher than the activation energy associated with historical measures of reaction severity. The gravimetric loss is primarily due to depolymerisation of the hemicellulose component of straw, but a minor proportion of lignin is solubilised at the same rate and hence may be associated with the more hydrophilic lignin-hemicellulose interface. Acetic acid is liberated primarily from hydrolysis of pendant acetate groups on hemicellulose, although this occurs at a rate that is too slow to provide catalytic enhancement to the primary solubilisation reactions. However, the increase in protons may enhance secondary reactions leading to the production of furans and formic acid. The work has suggested that formic acid may be formed under these hydrothermal conditions via direct reaction of sugar end groups rather than furan breakdown. However, furan degradation is found to be significant, which may limit ultimate quantities generated in hydrolysate liquors.
Highlights
Physicochemical pretreatments are required to increase the efficiency of enzyme hydrolysis of the polysaccharide fraction of lignocellulosic biomass, in order to liberate fermentable sugars for production of ethanol
We have demonstrated how the hydrothermal deconstruction of wheat straw can be interpreted using standard or modified kinetic models, to gain understanding of the interrelationships between kinetic parameters and chemical pathways between different solubilised species
The combined mass loss from the depolymerisation and solubilisation of all species provides an unambiguous measure of the extent of deconstruction of wheat straw, so the derived kinetic parameters can be used to describe a global reaction ordinate, as discussed later
Summary
Physicochemical pretreatments are required to increase the efficiency of enzyme hydrolysis of the polysaccharide fraction of lignocellulosic biomass, in order to liberate fermentable sugars for production of ethanol. The lignocellulosic cell wall consists of an assembly of cellulose fibrils, sheaved in a layer of hemicellulose, which acts as an interface with a surrounding network of lignin [1]. This highly recalcitrant structure must be subjected to controlled deconstruction in order to increase the accessibility and reactivity of the cellulose fibrils, which is required to maximise the rate and yield of saccharification to liberate glucose. Access to realistic kinetic data is important for predictions of optimal reaction conditions for the pretreatment of biomass for bioethanol processes, which is required to minimise production of inhibitory compounds and to maximise sugar and ethanol yields
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