RESUMO
We demonstrate a synthetic route toward the production of propene directly from poly(ß-hydroxybutyrate) (PHB), the most common of a wide range of high-molecular-mass microbial polyhydroxyalkanoates. Propene, a major commercial hydrocarbon, was obtained from the depolymerization of PHB and subsequent decarboxylation of the crotonic acid monomer in good yields (up to 75 mol %). The energetics of PHB depolymerization and the gas-phase decarboxylation of crotonic acid were also studied using density functional theory (DFT). The average activation energy for the cleavage of the R'C(O)O-R linkage is calculated to be 163.9 ± 7.0 kJ mol(-1). Intramolecular, autoacceleration effects regarding the depolymerization of PHB, as suggested in some literature accounts, arising from the formation of crotonyl and carboxyl functional groups in the products could not be confirmed by the results of DFT and microkinetic modeling. DFT results, however, suggest that intermolecular catalysis involving terminal carboxyl groups may accelerate PHB depolymerization. Activation energies for this process were estimated to be about 20 kJ mol(-1) lower than that for the noncatalyzed ester cleavage, 144.3 ± 6.4 kJ mol(-1). DFT calculations predict the decarboxylation of crotonic acid to follow second-order kinetics with an activation energy of 147.5 ± 6.3 kJ mol(-1), consistent with that measured experimentally, 146.9 kJ mol(-1). Microkinetic modeling of the PHB to propene overall reaction predicts decarboxylation of crotonic acid to be the rate-limiting step, consistent with experimental observations. The results also indicate that improvements made to enhance the isomerization of crotonic acid to vinylacetic acid will improve the direct conversion of PHB to propene.
RESUMO
BACKGROUND: During the pretreatment of biomass feedstocks and subsequent conditioning prior to saccharification, many toxic compounds are produced or introduced which inhibit microbial growth and in many cases, production of ethanol. An understanding of the toxic effects of compounds found in hydrolysate is critical to improving sugar utilization and ethanol yields in the fermentation process. In this study, we established a useful tool for surveying hydrolysate toxicity by measuring growth rates in the presence of toxic compounds, and examined the effects of selected model inhibitors of aldehydes, organic and inorganic acids (along with various cations), and alcohols on growth of Zymomonas mobilis 8b (a ZM4 derivative) using glucose or xylose as the carbon source. RESULTS: Toxicity strongly correlated to hydrophobicity in Z. mobilis, which has been observed in Escherichia coli and Saccharomyces cerevisiae for aldehydes and with some exceptions, organic acids. We observed Z. mobilis 8b to be more tolerant to organic acids than previously reported, although the carbon source and growth conditions play a role in tolerance. Growth in xylose was profoundly inhibited by monocarboxylic organic acids compared to growth in glucose, whereas dicarboxylic acids demonstrated little or no effects on growth rate in either substrate. Furthermore, cations can be ranked in order of their toxicity, Ca++ > > Na+ > NH4+ > K+. HMF (5-hydroxymethylfurfural), furfural and acetate, which were observed to contribute to inhibition of Z. mobilis growth in dilute acid pretreated corn stover hydrolysate, do not interact in a synergistic manner in combination. We provide further evidence that Z. mobilis 8b is capable of converting the aldehydes furfural, vanillin, 4-hydroxybenzaldehyde and to some extent syringaldehyde to their alcohol forms (furfuryl, vanillyl, 4-hydroxybenzyl and syringyl alcohol) during fermentation. CONCLUSIONS: Several key findings in this report provide a mechanism for predicting toxic contributions of inhibitory components of hydrolysate and provide guidance for potential process development, along with potential future strain improvement and tolerance strategies.
RESUMO
The reversion reactions of glucose in mildly acidic aqueous solutions have been studied, and the kinetics of conversion to disaccharides has been modeled. The experiments demonstrate that, at high sugar loadings, up to 12 wt % of the glucose can be converted into reversion products. The reversion products observed are primarily disaccharides; no larger oligosaccharides were observed. Only disaccharides linked to the C1 carbon of one of the glucose residues were observed. The formation of 1,6-linked disaccharides was favored, and alpha-linked disaccharides were formed at higher concentrations than beta-linked disaccharides. This observation can be rationalized on the basis of steric effects. At temperatures >140 degrees C, the disaccharides reach equilibrium with glucose and the reversion reaction competes with dehydration reactions to form 5-hydroxymethylfurfural. As a result, disaccharide formation reaches a maximum at reaction times <10 min and decreases with time. At temperatures <130 degrees C, disaccharide formation reaches a maximum at reaction times >30 min. As expected, disaccharide formation exhibits a second-order dependence upon glucose concentration. Levoglucosan formation is also observed; because it shows a first-order dependence upon glucose concentration, its formation is more significant at low concentrations (10 mg mL(-1)), whereas disaccharide formation dominates at high concentrations (200 mg mL(-1)). Experiments conducted using glucose and its disaccharides were calibrated with readily available standards. The kinetic parameters for hydrolysis of some glucodisaccharides could be compared to published literature values. From these experiments, the kinetics and activation energies for the reversion reactions have been calculated. The rate parameters can be used to model the formation of the disaccharides as a function of reaction time and temperature. A new and detailed picture of the molecular mechanism of these industrially important reversion reactions has been developed.