22 research outputs found
Ring Opening of Biomass-Derived Cyclic Ethers to Dienes over Silica/Alumina
We
show that cyclic ethers, such 2-methyltetrahydrofuran (2-MTHF),
can undergo dehydration to produce pentadienes over SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub>. The catalyst exhibited reversible deactivation
due to coke deposition, with the yield to pentadienes decreasing from
68% to 52% at 623 K over 58 h time on stream. A reaction network for
2-MTHF dehydration was proposed on the basis of the results of space
time studies. Pentadienes can be produced directly by a concerted
hydride shift and dehydration of carbenium intermediates or indirectly
through dehydration of pentanal and pentenol. Reaction kinetics studies
were performed at temperatures ranging from 573 to 653 K and 2-MTHF
partial pressures from 0.21 to 2.51 kPa. The apparent activation energy
barrier for 2-MTHF conversion to pentadienes and the reaction rate
order for ring opening were determined to be 74 kJ mol<sup>ā1</sup> and 0.24, respectively, indicating strong interaction between 2-MTHF
and the SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> surface. Other
solid acids such as Ī³-Al<sub>2</sub>O<sub>3</sub>, H-ZSM-5,
and Al-Sn-Beta were found to be active for 2-MTHF dehydration to pentadienes.
The rate of ring opening decreased in the order 2,5-dimethyltetrahydrofuran
> 2-MTHF > tetrahydropyran > tetrahydrofuran. Over SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub>, the dehydration of 2,5-dimethyltetrahydrofuran
resulted in 75% yield to hexadiene isomers
Mechanocatalytic Depolymerization of Dry (Ligno)cellulose As an Entry Process for High-Yield Production of Furfurals
Driven by mechanical forces, the
acid-catalyzed depolymerization of solid biomass completely overcomes
the problems posed by the recalcitrance of lignocellulose. The solid-state
reaction leads to water-soluble oligosaccharides, which display higher
reactivity than cellulose and hemicellulose. Here, we show that water-soluble
oligosaccharides are useful feedstock for the high-yield production
of 5-hydroxymethylfurfural (HMF) and furfural in biphasic reactors.
This is because they readily undergo hydrolysis upon microwave heating,
selectively forming monosaccharides as intermediates in the aqueous
phase. Short reaction times are possible with the use of microwave
heating and limit the extent of degradation reactions. This work provides
an ionic-liquid-free approach to process lignocellulosic substrates
into HMF and furfural with high yields. In fact, starting this novel
approach with Ī±-cellulose, yields of HMF of 79% and furfural
of 80% at 443 K for 9 min were obtained. The processing of real lignocellulose
(e.g., beechwood and sugar cane bagasse) also achieved high yields
of HMF and furfural. Thereby, the current results indicate that the
process limitation lies no longer in the recalcitrance of lignocellulose,
but in the extraction of highly reactive HMF and furfural from the
aqueous phase in the biphasic reactor
Selective Hydrogenation of Unsaturated CarbonāCarbon Bonds in Aromatic-Containing Platform Molecules
The
combination of chemical and biological catalysis enables the
production from biomass of coumarin and dihydrocoumarin (DHC), opening
new routes to the formation of fine chemicals and pharmaceutical building
blocks. Each of these products requires the hydrogenation of 4-hydroxycoumarin
(4HC) to 4-hydroxydihydrocoumarin (4HDHC), which, in turn, requires
the reduction of an unsaturated CāC bond in the presence of
an aromatic ring. Using <i>in situ</i> attenuated total
reflection Fourier transform infrared (ATR-FTIR) spectroscopy, we
show that reaction at 348 K over monometallic Pd catalysts leads to
the partial reduction of the aromatic ring in 4HC, obtaining 93% selectivity
for Cī»C bond hydrogenation at 82% 4HC conversion and with a
low turnover frequency (TOF). Decreasing the Pd dispersion from 70%
to 6% not only leads to an increase in the rate of 4HC hydrogenation,
but it also leads to an increase in the rate of overhydrogenation.
However, the formation of bimetallic PdAu nanoparticles inhibits the
overhydrogenation reaction while also doubling the TOF to a value
of 6 ks<sup>ā1</sup> for 4HDHC production. A bimetallic PdAu
catalyst supported on SiO<sub>2</sub> leads to 97% selectivity for
Cī»C bond hydrogenation at 86% 4HC conversion, while an acidic
support such as amorphous silicaāalumina can be used to produce
DHC directly from 4HC
Mechanistic Insights into Ring-Opening and Decarboxylation of 2āPyrones in Liquid Water and Tetrahydrofuran
2-Pyrones,
such as triacetic acid lactone, are a promising class
of biorenewable platform chemicals that provide access to an array
of chemical products and intermediates. We illustrate through the
combination of results from experimental studies and first-principle
density functional theory calculations that key structural features
dictate the mechanisms underlying ring-opening and decarboxylation
of 2-pyrones, including the degree of ring saturation, the presence
of Cī»C bonds at the C<sub>4</sub>ī»C<sub>5</sub> or C<sub>5</sub>ī»C<sub>6</sub> positions within the ring, as well as
the presence of a Ī²-keto group at the C<sub>4</sub> position.
Our results demonstrate that 2-pyrones undergo a range of reactions
unique to their structure, such as retro-DielsāAlder reactions
and nucleophilic addition of water. In addition, the reactivity of
2-pyrones and the final products formed is shown to depend on the
solvent used and the acidity of the reaction environment. The mechanistic
insights obtained here provide guidance for the selective conversion
of 2-pyrones to targeted chemicals
Production of 5-Hydroxymethylfurfural from Glucose Using a Combination of Lewis and BrĆønsted Acid Catalysts in Water in a Biphasic Reactor with an Alkylphenol Solvent
We report the catalytic conversion of glucose in high
yields (62%)
to 5-hydroxymethylfurfural (HMF), a versatile platform chemical. The
reaction system consists of a Lewis acid metal chloride (e.g., AlCl<sub>3</sub>) and a BrĆønsted acid (HCl) in a biphasic reactor consisting
of water and an alkylphenol compound (2-<i>sec</i>-butylphenol)
as the organic phase. The conversion of glucose in the presence of
Lewis and BrĆønsted acidity proceeds through a tandem pathway
involving isomerization of glucose to fructose, followed by dehydration
of fructose to HMF. The organic phase extracts 97% of the HMF produced,
while both acid catalysts remain in the aqueous phase
Sustainable Solvent Systems for Use in Tandem Carbohydrate Dehydration Hydrogenation
Monophasic separation-friendly solvent
systems were investigated
for the sustainable acid-catalyzed dehydration of fructose to 5-hydroxymethylfurfural
(HMF). The HMF selectivity depends on both fructose conversion, temperature,
and the amount of cosolvent present in the aqueous solvent mixture.
Use of HMF-derived 2,5-(dihydroxymethyl)Ātetrahydrofuran (DHMTHF) or
low-boiling tetrahydrofuran (THF) as co-solvents results in increased
selectivity (>70%) to HMF at fructose conversions of ca. 80%. Analysis
of the fructose tautomer distribution in each solvent system by <sup>13</sup>C NMR revealed higher furanose fractions in the presence
of these and other protic (tetrahydrofurfuryl alcohol) and polar aprotic
co-solvents (DMSO) relative to water alone. Formation of fructosides
and/or difructose anhydrides in the presence of the co-solvents causes
lower selectivity at early reaction times, but reversion to fructose
and dehydration to HMF at longer reaction times results in increasing
HMF selectivity with fructose conversion. In 9:1 DHMTHF:water, a 7.5-fold
increase in the initial rate of HMF production was observed relative
to water alone. This mixed solvent system is proposed for use in a
tandem catalytic approach to continuous DHMTHF production from fructose,
namely, acid-catalyzed dehydration of fructose to HMF, followed by
its catalytic hydrogenation to DHMTHF
Tuning AcidāBase Properties Using MgāAl Oxide Atomic Layer Deposition
Atomic layer deposition (ALD) was
used to coat Ī³-Al<sub>2</sub>O<sub>3</sub> particles with oxide
films of varying Mg/Al atomic ratios, which resulted in systematic
variation of the acid and base site areal densities. Variation of
Mg/Al also affected morphological features such as crystalline phase,
pore size distribution, and base site proximity. Areal base site density
increased with increasing Mg content, while acid site density went
through a maximum with a similar number of Mg and Al atoms in the
coating. This behavior leads to nonlinearity in the relationship between
Mg/Al and acid/base site ratio. The physical and chemical properties
were elucidated using scanning electron microscopy (SEM), energy-dispersive
X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), N<sub>2</sub> physisorption, and CO<sub>2</sub> and NH<sub>3</sub> temperature-programmed desorption (TPD). Fluorescence
emission spectroscopy of samples grafted with 1-pyrenebutyric acid
(PBA) was used for analysis of base site proximity. The degree of
base site clustering was correlated to acid site density. Catalytic
activity in the self-condensation of acetone was dependent on sample
base site density and independent of acid site density
Conversion of Furfural to 1,5-Pentanediol: Process Synthesis and Analysis
A new process for
the production of 1,5-pentanediol (1,5-PDO) from
biomass-derived furfural is studied. In this process, furfural is
converted to 1,5-PDO in a high overall yield (80%) over inexpensive
catalysts via multiple steps involving hydrogenation, dehydration,
hydration, and hydrogenation subsequently. To effectively recycle
H<sub>2</sub> as well as recover 1,5-PDO, detailed separation subsystems
have been designed and integrated with reaction subsystems. Furthermore,
a pioneer plant analysis is performed to estimate the risk on the
cost growth and plant performance shortfalls. The integrated process
leads to a minimum selling price of $1973 ton<sup>ā1</sup> for
1,5-PDO, which suggests that it could be a promising approach for
converting biomass into oxygenated commodity chemicals, which are
difficult to produce from petroleum-derived feedstocks. The sensitivity
analysis also identifies that the most important economic parameters
for the process include the furfural feedstock price and plant size
Methane Conversion to Ethylene and Aromatics on PtSn Catalysts
Pt
and PtSn catalysts supported on SiO<sub>2</sub> and H-ZSM-5
were studied for methane conversion under nonoxidative conditions.
Addition of Sn to Pt/SiO<sub>2</sub> increased the turnover frequency
for production of ethylene by a factor of 3, and pretreatment of the
catalyst at 1123 K reduced the extent of coke formation. Pt and PtSn
catalysts supported on H-ZSM-5 zeolite were prepared to improve the
activity and selectivity to non-coke products. Ethylene formation
rates were 20 times faster over a PtSn(1:3)/H-ZSM-5 catalyst with
SiO<sub>2</sub>:Al<sub>2</sub>O<sub>3</sub> = 280 in comparison to
those over PtSn(3:1)/SiO<sub>2</sub>. H-ZSM-5-supported catalysts
were also active for the formation of aromatics, and the rates of
benzene and naphthalene formation were increased by using more acidic
H-ZSM-5 supports. These catalysts operate through a bifunctional mechanism,
in which ethylene is first produced on highly dispersed PtSn nanoparticles
and then is subsequently converted to benzene and naphthalene on BrĆønsted
acid sites within the zeolite support. The most active and stable
PtSn catalyst forms carbon products at a rate, 2.5 mmol of C/((mol
of Pt) s), which is comparable to that of state-of-the-art Mo/H-ZSM-5
catalysts with same metal loading operated under similar conditions
(1.8 mmol of C/((mol of Mo) s)). Scanning transmission electron microscopy
measurements suggest the presence of smaller Pt nanoparticles on H-ZSM-5-supported
catalysts, in comparison to SiO<sub>2</sub>-supported catalysts, as
a possible source of their high activity. A microkinetic model of
methane conversion on Pt and PtSn surfaces, built using results from
density functional theory calculations, predicts higher coupling rates
on bimetallic and stepped surfaces, supporting the experimental observations
that relate the high catalytic activity to small PtSn particles
Mechanistic Insights into the Hydrogenolysis of Levoglucosanol over Bifunctional Platinum SilicaāAlumina Catalysts
Herein,
we report on the hydrogenolysis of the biorenewable intermediate
levoglucosanol (Lgol) over bifunctional platinum catalysts supported
on silicaāalumina in tetrahydrofuran solvent. <sup>13</sup>C radiolabeling is used to confirm the ring rearrangement forming
tetrahydrofurandimethanol. The reaction rate and product selectivity
are comparable between 1.1 and 5.3 wt % Pt loadings, indicating that,
at these metal loadings, the rate-limiting step is acid catalyzed.
The measured zero-order dependence in hydrogen indicates that a non-rate-determining
hydrogenation step follows an acid-catalyzed irreversible rate-determining
step. The measured first-order dependence in Lgol indicates that the
acid sites are not highly covered by Lgol. A physical mixture of Pt/SiO<sub>2</sub> and SiAl catalysts displayed product selectivity similar
to that of the Pt/SiAl catalyst, indicating that nanoscale proximity
of metal and acid sites is not required to carry out Lgol hydrogenolysis
selectively. As the Pt loading in Pt/SiAl catalysts is decreased,
or when the bare SiAl support is separated from a Pt/SiO<sub>2</sub> catalyst in a dual-layer configuration, the selectivity toward identified
products decreases. These results suggest that degradation reactions
are avoided when the reactive intermediates formed over acid sites
are rapidly hydrogenated over metal sites. First-principles simulations
are performed to investigate the energetics of the proposed reaction
pathway. A detailed reaction mechanism for Lgol hydrogenolysis is
proposed on the basis of a combination of the experimental and computational
results. These findings provide a fundamental understanding of the
catalytic conversion of levoglucosanol over bifunctional metalāacid
catalysts, facilitating rationally designed processes to produce renewable
chemicals from biomass-derived levoglucosenone