23 research outputs found
Enhanced characteristics of genetically modified switchgrass (Panicum virgatum L.) for high biofuel production
Background
Lignocellulosic biomass is one of the most promising renewable and clean energy resources to reduce greenhouse gas emissions and dependence on fossil fuels. However, the resistance to accessibility of sugars embedded in plant cell walls (so-called recalcitrance) is a major barrier to economically viable cellulosic ethanol production. A recent report from the US National Academy of Sciences indicated that, “absent technological breakthroughs”, it was unlikely that the US would meet the congressionally mandated renewable fuel standard of 35 billion gallons of ethanol-equivalent biofuels plus 1 billion gallons of biodiesel by 2022. We here describe the properties of switchgrass (Panicum virgatum) biomass that has been genetically engineered to increase the cellulosic ethanol yield by more than 2-fold. Results
We have increased the cellulosic ethanol yield from switchgrass by 2.6-fold through overexpression of the transcription factor PvMYB4. This strategy reduces carbon deposition into lignin and phenolic fermentation inhibitors while maintaining the availability of potentially fermentable soluble sugars and pectic polysaccharides. Detailed biomass characterization analyses revealed that the levels and nature of phenolic acids embedded in the cell-wall, the lignin content and polymer size, lignin internal linkage levels, linkages between lignin and xylans/pectins, and levels of wall-bound fucose are all altered in PvMYB4-OX lines. Genetically engineered PvMYB4-OX switchgrass therefore provides a novel system for further understanding cell wall recalcitrance. Conclusions
Our results have demonstrated that overexpression of PvMYB4, a general transcriptional repressor of the phenylpropanoid/lignin biosynthesis pathway, can lead to very high yield ethanol production through dramatic reduction of recalcitrance. MYB4-OX switchgrass is an excellent model system for understanding recalcitrance, and provides new germplasm for developing switchgrass cultivars as biomass feedstocks for biofuel production. Keywords:
Switchgrass; Bioenergy; Biofuel; Feedstock; Cellulosic ethanol; PvMYB4; Transcription factor; Cell wall; Recalcitrance; Lignin; Hemicellulose; Pecti
Hydrocarbon Renewable and Synthetic Diesel Fuel Blendstocks: Composition and Properties
We
examined the chemical composition and properties of several
diesel fuels and blendstocks derived from Fischer–Tropsch (FT)
synthesis, hydroisomerization of lipids, and fermentation of sugar
via the terpenoid metabolic pathway. Comprehensive two-dimensional
gas chromatographic analysis with nonpolar and polar columns, <sup>13</sup>C NMR, GC-MS, and elemental analysis were used to assess
fuel chemistry. Performance properties included density, heat of combustion,
cetane number, and cloud point, as well as other properties. The fuels
consisted almost entirely of normal and iso-paraffins. Three samples
contained residual oxygen below 0.1 mass %. All of the renewable and
synthetic diesel fuels have significantly lower density than is typical
for a petroleum-derived diesel fuel. As a result, they have slightly
higher net heat of combustion on a mass basis (2%–3% higher),
but lower heat of combustion on a volume basis (3%–7% lower).
Two critical diesel performance properties, cetane number and cloud
point, were correlated with iso-paraffin content and chain length.
The results confirm that properties of hydroisomerized fats and oils,
as well as FT diesel, can be tuned by increasing the degree of isomerization
to lower cloud point which also lowers the cetane number. In spite
of this trade-off between cloud point, and cetane number, the cetane
numbers were still over 70 for fuels with cloud points as low as −27
°C. The terpenoid biofuel exhibited a cloud point below −70
°C and a cetane number of 58
Unraveling the <sup>13</sup>C NMR Chemical Shifts in Single-Walled Carbon Nanotubes: Dependence on Diameter and Electronic Structure
The atomic specificity afforded by nuclear magnetic resonance
(NMR)
spectroscopy could enable detailed mechanistic information about single-walled
carbon nanotube (SWCNT) functionalization as well as the noncovalent
molecular interactions that dictate ground-state charge transfer and
separation by electronic structure and diameter. However, to date,
the polydispersity present in as-synthesized SWCNT populations has
obscured the dependence of the SWCNT <sup>13</sup>C chemical shift
on intrinsic parameters such as diameter and electronic structure,
meaning that no information is gleaned for specific SWCNTs with unique
chiral indices. In this article, we utilize a combination of <sup>13</sup>C labeling and density gradient ultracentrifugation (DGU)
to produce an array of <sup>13</sup>C-labeled SWCNT populations with
varying diameter, electronic structure, and chiral angle. We find
that the SWCNT isotropic <sup>13</sup>C chemical shift decreases systematically
with increasing diameter for semiconducting SWCNTs, in agreement with
recent theoretical predictions that have heretofore gone unaddressed.
Furthermore, we find that the <sup>13</sup>C chemical shifts for small
diameter metallic and semiconducting SWCNTs differ significantly,
and that the full-width of the isotropic peak for metallic SWCNTs
is much larger than that of semiconducting nanotubes, irrespective
of diameter
Application of a Pyroprobe–Deuterium NMR System: Deuterium Tracing and Mechanistic Study of Upgrading Process for Lignin Model Compounds
In this study, a pyroprobe–deuterium
(<sup>2</sup>H) NMR
system has been used to identify isotopomer products formed during
the deuteration and ring opening of lignin model compounds. Several
common model compounds for lignin and its upgraded products, including
guaiacol, syringol, toluene, <i>p</i>-xylene, phenol, catechol,
cyclohexane, methylcyclohexane, and methylcyclopentane, have been
examined for selective ring opening. Similar pathways for upgrading
of toluene and <i>p</i>-xylene has been found, which will
undergo hydrogenation, methyl group elimination, and ring opening
process, and benzene, cyclohexane, and methylcyclohexane have been
found as major intermediates before ring opening. Very interestingly,
the <sup>2</sup>H NMR analysis for the deuterium-traced ring opening
of catechol on Ir/γ-Al<sub>2</sub>O<sub>3</sub> is almost identical
to the ring opening process for phenol. The ring opening processes
for guaiacol and syringol appeared to be very complicated, as expected.
Benzene, phenol, toluene, cyclohexane, and methylcyclohexane have
been determined to be the major products