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The kinetics of inhibitor production resulting from hydrothermal deconstruction of wheat straw studied using a pressurised microwave reactor.

Ibbett R, Gaddipati S, Greetham D, Hill S, Tucker G - Biotechnol Biofuels (2014)

Bottom Line: A 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.However, furan degradation is found to be significant, which may limit ultimate quantities generated in hydrolysate liquors.

View Article: PubMed Central - HTML - PubMed

Affiliation: BBSRC Sustainable Bioenergy Research Centre, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, UK. roger.ibbett@nottingham.ac.uk.

ABSTRACT

Background: The 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.

Results: The 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.

Conclusions: A 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.

No MeSH data available.


Related in: MedlinePlus

Simulation of kinetics of generation of furans, with inclusion of positive rate constant for formic acid generation from soluble hemicellulose. ♦, 180°C; ■, 200°C; ▲, 220°C.
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Figure 5: Simulation of kinetics of generation of furans, with inclusion of positive rate constant for formic acid generation from soluble hemicellulose. ♦, 180°C; ■, 200°C; ▲, 220°C.

Mentions: The pseudo-first order kinetic behaviour shown by formic acid may therefore conceal a more complicated reaction scheme, which may involve competition with the main pathway for generation of furfural. If it is assumed that formic acid can be formed by end group reactions of solubilised hemicellulose then this will result in a split in the k2 pathway from this point, which may be introduced into the scheme by inclusion of an additional parallel first order reaction with a rate constant kfa, as shown in Figure 3. A corresponding additional term is included in the differential equation (5). Attempts at calculation of formic acid kinetic profiles with this revised scheme were quite successful, with fitted data shown as continuous lines in Figure 2c, with parameters summarised in Table 3. The pseudo-first order behaviour then comes from the rapid solubilisation of hemicellulose, giving an apparent fixed reagent concentration. Subsequent adjustment of the other rate constants in the analysis were required to maintain good fits of furfural profiles, which led to a reduction in k2 for monosaccharide formation and an increase in k3 for furfural formation, with the rate constant k4 for furfural degradation unchanged. The alternative rate parameters including kfc are summarised in Table 3. The alternative fits to the furan experimental profiles are shown in Figure 5, which are acceptable for data at 180 and 200°C, but slightly less so at 220°C. However, overall the scheme is still reasonable as an explanation of formic acid generation from polysaccharide containing biomass under these hydrothermal conditions.


The kinetics of inhibitor production resulting from hydrothermal deconstruction of wheat straw studied using a pressurised microwave reactor.

Ibbett R, Gaddipati S, Greetham D, Hill S, Tucker G - Biotechnol Biofuels (2014)

Simulation of kinetics of generation of furans, with inclusion of positive rate constant for formic acid generation from soluble hemicellulose. ♦, 180°C; ■, 200°C; ▲, 220°C.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4230421&req=5

Figure 5: Simulation of kinetics of generation of furans, with inclusion of positive rate constant for formic acid generation from soluble hemicellulose. ♦, 180°C; ■, 200°C; ▲, 220°C.
Mentions: The pseudo-first order kinetic behaviour shown by formic acid may therefore conceal a more complicated reaction scheme, which may involve competition with the main pathway for generation of furfural. If it is assumed that formic acid can be formed by end group reactions of solubilised hemicellulose then this will result in a split in the k2 pathway from this point, which may be introduced into the scheme by inclusion of an additional parallel first order reaction with a rate constant kfa, as shown in Figure 3. A corresponding additional term is included in the differential equation (5). Attempts at calculation of formic acid kinetic profiles with this revised scheme were quite successful, with fitted data shown as continuous lines in Figure 2c, with parameters summarised in Table 3. The pseudo-first order behaviour then comes from the rapid solubilisation of hemicellulose, giving an apparent fixed reagent concentration. Subsequent adjustment of the other rate constants in the analysis were required to maintain good fits of furfural profiles, which led to a reduction in k2 for monosaccharide formation and an increase in k3 for furfural formation, with the rate constant k4 for furfural degradation unchanged. The alternative rate parameters including kfc are summarised in Table 3. The alternative fits to the furan experimental profiles are shown in Figure 5, which are acceptable for data at 180 and 200°C, but slightly less so at 220°C. However, overall the scheme is still reasonable as an explanation of formic acid generation from polysaccharide containing biomass under these hydrothermal conditions.

Bottom Line: A 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.However, furan degradation is found to be significant, which may limit ultimate quantities generated in hydrolysate liquors.

View Article: PubMed Central - HTML - PubMed

Affiliation: BBSRC Sustainable Bioenergy Research Centre, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, UK. roger.ibbett@nottingham.ac.uk.

ABSTRACT

Background: The 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.

Results: The 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.

Conclusions: A 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.

No MeSH data available.


Related in: MedlinePlus