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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.


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Kinetic scheme for hydrothermal reactions of straw biomass.
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Figure 3: Kinetic scheme for hydrothermal reactions of straw biomass.

Mentions: For this study a scheme of linked first order reactions was proposed to account for the production and degradation of furanic species, outlined schematically in FigureĀ 3, where the individual first order rate constants are shown for each step in the pathway. Various assumptions are implicit in the scheme, which have been introduced to improve the manageability of analysis, hopefully without sacrificing chemical reality. Firstly, it was assumed that there is a very low statistical chance of sugar monomers being directly released by endwise loss from polymer chains held in the cell wall. The apparent mono-exponential kinetics of initial hemicellulose solubilisation from the cell wall (rate constant ks) also suggested that it was only necessary to consider one initial reacting species and hence a single initial reaction step. Also, it was considered that only sugars converted to monomer form (rate constant k2) could undergo subsequent conversion to furanic species (k3), via dehydration mechanisms [29]. Initially it was assumed that all hemicellulose followed the furan pathway, although in the later discussion it is postulated that some might react to give formic acid. Degradation of furans via all possible pathways was approximated by a single rate constant (k4).


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)

Kinetic scheme for hydrothermal reactions of straw biomass.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 3: Kinetic scheme for hydrothermal reactions of straw biomass.
Mentions: For this study a scheme of linked first order reactions was proposed to account for the production and degradation of furanic species, outlined schematically in FigureĀ 3, where the individual first order rate constants are shown for each step in the pathway. Various assumptions are implicit in the scheme, which have been introduced to improve the manageability of analysis, hopefully without sacrificing chemical reality. Firstly, it was assumed that there is a very low statistical chance of sugar monomers being directly released by endwise loss from polymer chains held in the cell wall. The apparent mono-exponential kinetics of initial hemicellulose solubilisation from the cell wall (rate constant ks) also suggested that it was only necessary to consider one initial reacting species and hence a single initial reaction step. Also, it was considered that only sugars converted to monomer form (rate constant k2) could undergo subsequent conversion to furanic species (k3), via dehydration mechanisms [29]. Initially it was assumed that all hemicellulose followed the furan pathway, although in the later discussion it is postulated that some might react to give formic acid. Degradation of furans via all possible pathways was approximated by a single rate constant (k4).

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