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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

Kinetics of generation of inhibitor compounds in hydrolysate liquors following hydrothermal processing of straw. a) Furans; b) acetic acid; and c) formic acid. ♦, 180°C; ■, 200°C; ▲, 220°C.
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Figure 2: Kinetics of generation of inhibitor compounds in hydrolysate liquors following hydrothermal processing of straw. a) Furans; b) acetic acid; and c) formic acid. ♦, 180°C; ■, 200°C; ▲, 220°C.

Mentions: The hemicellulose oligomers liberated into solution undergo continuing hydrolysis to form a variety of pentose monomers, which then undergo dehydration reactions to give furfural as a major product, which then further degrades to a variety of molecules, including various organic acids and condensation products [29]. The UV analysis method provided a fast, reliable measure of the evolution of total furans with time, as shown for wheat straw at the three reaction temperatures in Figure 2a. A minor amount of cellulose may be solubilised under hydrothermal conditions, although other soluble C6 sugars may be derived from minor glucan or galactan constituents of hemicellulose. These C6 sugars will undergo dehydration to form 5-(hydroxymethyl)furfural (HMF), which will be detected by UV within the total furan response. For subsequent kinetic analyses all furanic species are assumed to be derived from hemicellulose. Separate analyses by HPLC confirmed that concentrations of HMF were present at a constant 7% proportion of furfural in the hydrolysates from this study, and it was therefore assumed that both C6 and C5 dehydration followed the same rate behaviour.


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)

Kinetics of generation of inhibitor compounds in hydrolysate liquors following hydrothermal processing of straw. a) Furans; b) acetic acid; and c) formic acid. ♦, 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 2: Kinetics of generation of inhibitor compounds in hydrolysate liquors following hydrothermal processing of straw. a) Furans; b) acetic acid; and c) formic acid. ♦, 180°C; ■, 200°C; ▲, 220°C.
Mentions: The hemicellulose oligomers liberated into solution undergo continuing hydrolysis to form a variety of pentose monomers, which then undergo dehydration reactions to give furfural as a major product, which then further degrades to a variety of molecules, including various organic acids and condensation products [29]. The UV analysis method provided a fast, reliable measure of the evolution of total furans with time, as shown for wheat straw at the three reaction temperatures in Figure 2a. A minor amount of cellulose may be solubilised under hydrothermal conditions, although other soluble C6 sugars may be derived from minor glucan or galactan constituents of hemicellulose. These C6 sugars will undergo dehydration to form 5-(hydroxymethyl)furfural (HMF), which will be detected by UV within the total furan response. For subsequent kinetic analyses all furanic species are assumed to be derived from hemicellulose. Separate analyses by HPLC confirmed that concentrations of HMF were present at a constant 7% proportion of furfural in the hydrolysates from this study, and it was therefore assumed that both C6 and C5 dehydration followed the same rate behaviour.

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