14 research outputs found
Understanding Intermolecular Forces: DFT-SAPT studies on graphite-like acenes interacting with water
The interaction of water with graphene has been a quintessential example of hydrophobic interactions for many years. However, no reliable experimental or theoretical value exists for the water-graphene interaction energy. In the current document, the water-graphene interaction energy is explored using high-level ab initio methods. In addition, the water-graphene interaction energy is decomposed into its physical components in order to give further physical insight into the water-graphene interaction.Water is found in a variety of environments, ranging from small clusters to the bulk. Because of this, the development of accurate models capable of describing water in a wide range of environments has been an active area of research. In the second part of this document, the nature of the water--water interaction is explored and a new polarizable water model is presented
DFT Study of the Conversion of Furfuryl Alcohol to 2‑Methylfuran on RuO<sub>2</sub> (110)
Recently, our group proposed the
catalytic transfer hydrogenation for refining biomass-derived 5-hydroxymethylfurfural
and furfural to 2,5-dimethylfuran and 2-methylfuran, respectively.
With a metallic Ru/C catalyst, a selectivity of ∼30% was achieved.
The promotion of the Ru/C catalyst with the Lewis acid RuO<sub><i>x</i></sub> resulted in a selectivity of ∼80%. In the
current study, we employ density functional theory calculations on
the RuO<sub>2</sub> (110) surface in order to elucidate the role of
the Lewis acidity in the reduction of the biomass-derived furfuryl
alcohol to 2-methylfuran. We identify the rate-limiting step to be
the scission of the C–O bond of the side chain. In addition,
we find evidence for the activation of the furan ring via an insertion
of a hydrogen atom. However, the Lewis basicity of a neighboring bridging
oxygen results in the furan ring being deactivated. Finally, the formation
of water from the reduction process is facile. However, owing to the
strong binding energy between the RuO<sub>2</sub> surface and the
water molecule, poisoning of the catalytic surface by adsorbed water
is possible. Finally, we show that the RuO<sub>2</sub> (110) carries
out the reduction of furfural to furfuryl alcohol via the Meerwin–Ponndorf–Verley
(MPV) reaction fairly easily, consistent with published experimental
data
Erratum: A second generation distributed point polarizable water model (J. Chem. Phys. (2010) 132 (014309)
A second generation distributed point polarizable water model
A distributed point polarizable model (DPP2) for water, with explicit terms for charge penetration, induction, and charge transfer, is introduced. The DPP2 model accurately describes the interaction energies in small and large water clusters and also gives an average internal energy per molecule and radial distribution functions of liquid water in good agreement with experiment. A key to the success of the model is its accurate description of the individual terms in the n -body expansion of the interaction energies. © 2010 American Institute of Physics
Reaction Pathways and Intermediates in Selective Ring Opening of Biomass-Derived Heterocyclic Compounds by Iridium
While the catalytic hydrogenolysis
of biomass-derived aromatic
cyclic compounds to functionalized long chain alcohols and polyols
has been known for decades, the factors that control the selectivity
remain either unknown or controversial. Previous reports have hypothesized
full ring saturation of the aromatic ring is necessary prior to hydrogenolysis.
Contradictorily, recent studies have shown hydrogenolysis occurs prior
to the saturation of the conjugated bonds. Furthermore, it has been
assumed the functional groups present are fully reduced prior to hydrogenolysis;
however, this has not been shown a priori. In order to resolve these
controversies, we combine density functional theory and high-resolution
electron energy loss spectroscopy (HREELS) to probe the catalytic
hydrogenolysis of saturated and unsaturated heterocyclic molecules
(furan, furfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol)
on iridium. Our results reveal that full saturation of the aromatic
ring is not only unnecessary but leads to slower kinetics and differing
selectivities. In contrast to previous studies, we show selective
partial ring saturation can enhance the kinetics of the hydrogenolysis
process. Reduction/oxidation of the functional group leads to a change
in the electronegativity, resulting in a change in selectivity. These
results provide important mechanistic insights allowing for further
improvement of catalysts for the effective transformations of biomass-derived
oxygenates to value-added products
Reaction Pathways and Intermediates in Selective Ring Opening of Biomass-Derived Heterocyclic Compounds by Iridium
While the catalytic hydrogenolysis
of biomass-derived aromatic
cyclic compounds to functionalized long chain alcohols and polyols
has been known for decades, the factors that control the selectivity
remain either unknown or controversial. Previous reports have hypothesized
full ring saturation of the aromatic ring is necessary prior to hydrogenolysis.
Contradictorily, recent studies have shown hydrogenolysis occurs prior
to the saturation of the conjugated bonds. Furthermore, it has been
assumed the functional groups present are fully reduced prior to hydrogenolysis;
however, this has not been shown a priori. In order to resolve these
controversies, we combine density functional theory and high-resolution
electron energy loss spectroscopy (HREELS) to probe the catalytic
hydrogenolysis of saturated and unsaturated heterocyclic molecules
(furan, furfural, furfuryl alcohol, and tetrahydrofurfuryl alcohol)
on iridium. Our results reveal that full saturation of the aromatic
ring is not only unnecessary but leads to slower kinetics and differing
selectivities. In contrast to previous studies, we show selective
partial ring saturation can enhance the kinetics of the hydrogenolysis
process. Reduction/oxidation of the functional group leads to a change
in the electronegativity, resulting in a change in selectivity. These
results provide important mechanistic insights allowing for further
improvement of catalysts for the effective transformations of biomass-derived
oxygenates to value-added products
Site-Dependent Lewis Acidity of γ‑Al<sub>2</sub>O<sub>3</sub> and Its Impact on Ethanol Dehydration and Etherification
We
examine the heterogeneity of the Lewis acidity on the (100)
and (110) facets of γ-Al<sub>2</sub>O<sub>3</sub> by computing
the binding energies of various oxygenates, in addition to the reaction
barriers of dehydration and etherification reactions of ethanol. We
show that the ethanol dehydration barrier is moderately affected by
site heterogeneity (barriers between 1.2 and 1.6 eV); in contrast,
a nearly 3-fold change in the ethanol etherification barrier is found
among the various Al<sup>3+</sup> sites. In order to rationalize these
results, the <i>s</i>-conduction band mean of the Al<sup>3+</sup> sites is introduced as a descriptor to characterize the
ability to transfer electron charge from the adsorbate to the Lewis
acid site. It is shown for the first time that this descriptor quantitatively
correlates the oxygenate binding energies and the ethanol dehydration
reaction barriers. However, for the ethanol etherification reactions
the <i>s</i>-conduction band mean of the Al<sup>3+</sup> sites describes barriers only qualitatively due to the bimolecular
nature of this reaction, which results in a change in the nucleophilicity
of the ethoxy species by a nearby adsorbed ethanol. As a result, the
strength of the Lewis acid sites is not the only descriptor for etherification
chemistry. Hydration of the (110) facet indicates an increase in Lewis
acidity strength as described by the <i>s</i>-conduction
band mean that results in stronger binding. However, this increase
in Lewis acidity results in either a negligible change of the ethanol
dehydration reaction barriers on some sites or an increase due to
a reduction in the basicity of the adjacent oxygen by the dissociated
water. Similarly, ethanol etherification is slowed down by the presence
of water due primarily to the change in nucleophilicity of the ethoxy
species. Overall, our results clearly indicate that while the binding
energy is an excellent descriptor of Lewis acidity strength and dehydration
chemistry on the clean alumina surfaces, cooperative phenomena (i.e.,
modulation of the nucleophilicity of the ethoxy by the nearby oxygen
or water and the basicity of oxygen in the presence of water) are
key issues that lead to a breakdown in the correlation between Lewis
acid strength in terms of the binding energy or the <i>s</i>-conduction band mean and the reaction barriers
Mechanistic Insights into Metal Lewis Acid-Mediated Catalytic Transfer Hydrogenation of Furfural to 2‑Methylfuran
Biomass conversion to fuels and chemicals
provides sustainability,
but the highly oxygenated nature of a large fraction of biomass-derived
molecules requires removal of the excess oxygen and partial hydrogenation
in the upgrade, typically met by hydrodeoxygenation processes. Catalytic
transfer hydrogenation is a general approach in accomplishing this
with renewable organic hydrogen donors, but mechanistic understanding
is currently lacking. Here, we elucidate the molecular level reaction
pathway of converting hemicellulose-derived furfural to 2-methylfuran
on a bifunctional Ru/RuO<sub><i>x</i></sub>/C catalyst using
isopropyl alcohol as the hydrogen donor via a combination of isotopic
labeling and kinetic studies. Hydrogenation of the carbonyl group
of furfural to furfuryl alcohol proceeds through a Lewis acid-mediated
intermolecular hydride transfer and hydrogenolysis of furfuryl alcohol
occurs mainly via ring-activation involving both metal and Lewis acid
sites. Our results show that the bifunctional nature of the catalyst
is critical in the efficient hydrodeoxygenation of furanics and provides
insights toward the rational design of such catalysts
Solventless C–C Coupling of Low Carbon Furanics to High Carbon Fuel Precursors Using an Improved Graphene Oxide Carbocatalyst
Graphene
oxide, decorated with surface oxygen functionalities,
has emerged as an alternative to precious-metal catalysts for many
reactions. Herein, we report that graphene oxide becomes superactive
for C–C coupling upon incorporation of a highly oxidized surface
associated with Brønsted acidic oxygen functionality and defect
sites along the surface and edges. The resulting improved graphene
oxide (IGO) demonstrates significantly higher activity over commonly
used framework zeolites for the upgrade of low-carbon biomass furanics
to high-carbon fuel precursors. A maximum 95% yield of C<sub>15</sub> fuel precursor with high selectivity is obtained at low temperature
(60 °C) and neat conditions via hydroxyalkylation/alkylation
(HAA) of 2-methylfuran (2-MF) and furfural. Coupling of 2-MF with
carbonyl compounds ranging from C<sub>3</sub> to C<sub>6</sub> produces
precursors of carbon numbers 12 to 21 with a high yield. The catalyst
regains nearly full activity upon regeneration. Extensive microscopic
and spectroscopic characterization of the fresh and reused IGO carbocatalysts
indicates that defects and the enhanced oxygen content are strongly
correlated with the high activity of IGO. Density functional theory
calculations reveal defects at carbonyl sites as suitable Brønsted
acidic oxygen functional groups. A plausible reaction mechanism is
also hypothesized