13 research outputs found
Bimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels
The
bimolecular thermal reactions of carboxylic acids were studied
using quantum mechanical molecular modeling. Previous work investigated the unimolecular decomposition of
a variety of organic acids, including saturated, α,β-unsaturated,
and β,γ-unsaturated acids, and showed that the type and
position of the unsaturation resulted in unique branching ratios between
dehydration and decarboxylation, [H<sub>2</sub>O]/[CO<sub>2</sub>].
In this work, the effect of bimolecular chemistry (water–acid
and acid–acid) is considered with a representative of each
acid class. In both cases, the strained 4-centered, unimolecular transition
state, typical of most organic acids, is opened up to 6- or 8-centered
bimolecular geometries. These larger structures lead to a reduction
in the barrier heights (20–45%) of the thermal decomposition
pathways for organic acids and an increase in the decomposition kinetics.
In some cases, they even cause a shift in the branching ratio of the
corresponding product slates
Comparison of Unimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels
Quantum
mechanical molecular modeling is used [M06-2X/6-311++GÂ(2df,p)] to
compare activation energies and rate constants for unimolecular decomposition
pathways of saturated and unsaturated carboxylic acids that are important
in the production of biofuels and that are models for plant and algae-derived
intermediates. Dehydration and decarboxylation reactions are considered.
The barrier heights to decarboxylation and dehydration are similar
in magnitude for saturated acids (∼71 kcal mol<sup>–1</sup>), with an approximate 1:1 [H<sub>2</sub>O]/[CO<sub>2</sub>] branching
ratio over the temperature range studied (500–2000 K). α,β-Unsaturation
lowers the barrier to decarboxylation between 2.2 and 12.2 kcal mol<sup>–1</sup> while increasing the barriers to dehydration by ∼3
kcal mol<sup>–1</sup>. The branching ratio, as a result, is
an order of magnitude smaller, [H<sub>2</sub>O]/[CO<sub>2</sub>] =
0.07. For some α,β-unsaturated acids, six-center transition
states are available for dehydration, with barrier heights of ∼35.0
kcal mol<sup>–1</sup>. The branching ratio for these acids
can be as high as 370:1. β,γ-Unsaturation results in a
small lowering in the barrier height to decarboxylation (∼70.0
kcal mol<sup>–1</sup>). β,γ-Unsaturation also leads
to a lowering in the dehydration pathway from 1.7 to 5.1 kcal mol<sup>–1</sup>. These results are discussed with respect to predicted
kinetic values for acids of importance in biofuels production
Diffusion of Biomass Pyrolysis Products in H‑ZSM‑5 by Molecular Dynamics Simulations
Diffusion of biomass
pyrolysis vapors and their upgraded products
is an essential catalytic property of zeolites during catalytic fast
pyrolysis and likely plays a critical role in the selectivity of these
catalysts. Characterizing the diffusivities of representative biofuel
molecules is critical to understand shape selectivity and interpret
product distribution. Yet, experimental measurements on the diffusivities
of oxygenated biofuel molecules at pyrolysis temperatures are very
limited in the literature. As an alternative approach, we conducted
MD simulations to measure the diffusion coefficients of several selected
molecules that are representative of biomass pyrolysis vapors, namely
water, methanol, glycolaldehyde, and toluene in H-ZSM-5 zeolite. The
results show the diffusion coefficients calculated via MD simulations
are consistent with available NMR measurements at room temperature. The effect of molecular weight and molecular
critical diameter on the diffusivity among the chosen model compounds
is also examined. Furthermore, we have characterized the diffusivities
of representative biofuel molecules, namely xylene isomers, in H-ZSM-5.
Our calculations determined that the ratio of the diffusion coefficients
for xylene isomers is <i>p-</i>xylene:<i>o-</i>xylene:<i>m-</i>xylene ≈ 83:3:1 at 700 K. Additionally,
our results also demonstrate the different diffusivity between <i>p</i>-xylene and toluene is due to the molecular orientations
when the molecules diffuse along the channels in H-ZSM-5 and provide
deep insight into the effect of molecular orientation on its diffusivity
Group Additivity Determination for Oxygenates, Oxonium Ions, and Oxygen-Containing Carbenium Ions
Bio-oil produced from biomass fast
pyrolysis often requires catalytic
upgrading to remove oxygen and acidic species over zeolite catalysts.
The elementary reactions in the mechanism for this process involve
carbenium and oxonium ions. In order to develop a detailed kinetic
model for the catalytic upgrading of biomass, rate constants are required
for these elementary reactions. The parameters in the Arrhenius equation
can be related to thermodynamic properties through structure–reactivity
relationships, such as the Evans–Polanyi relationship. For
this relationship, enthalpies of formation of each species are required,
which can be reasonably estimated using group additivity. However,
the literature previously lacked group additivity values for oxygenates,
oxonium ions, and oxygen-containing carbenium ions. In this work,
71 group additivity values for these types of groups were regressed,
65 of which had not been reported previously and six of which were
newly estimated based on regression in the context of the 65 new groups.
Heats of formation based on atomization enthalpy calculations for
a set of reference molecules and isodesmic reactions for a small set
of larger species for which experimental data was available were used
to demonstrate the accuracy of the Gaussian-4 quantum mechanical method
in estimating enthalpies of formation for species involving the moieties
of interest. Isodesmic reactions for a total of 195 species were constructed
from the reference molecules to calculate enthalpies of formation
that were used to regress the group additivity values. The results
showed an average deviation of 1.95 kcal/mol between the values calculated
from Gaussian-4 and isodesmic reactions versus those calculated from
the group additivity values that were newly regressed. Importantly,
the new groups enhance the database for group additivity values, especially
those involving oxonium ions
Predictive Model for Particle Residence Time Distributions in Riser Reactors. Part 1: Model Development and Validation
In
this computational study, we model the mixing of biomass pyrolysis
vapor with a solid catalyst in circulating riser reactors with a focus
on the determination of solid catalyst residence time distributions
(RTDs). A comprehensive set of 2D and 3D simulations were conducted
for a pilot-scale riser using the Eulerian–Eulerian two-fluid
modeling framework with and without subgrid-scale models for the gas–solid
interaction. A validation test case was also simulated and compared
to experiments, showing agreement in the pressure gradient and RTD
mean and spread. For simulation cases, it was found that for accurate
RTD prediction, the Johnson and Jackson partial slip solid boundary
condition was required for all models, and a subgrid model is useful
so that ultra high resolutions grids, which are very computationally
intensive, are not required. We discovered a 2/3 scaling relation
for the RTD mean and spread when comparing resolved 2D simulations
to validated unresolved 3D subgrid-scale model simulations
Furan Production from Glycoaldehyde over HZSM‑5
Catalytic fast pyrolysis of biomass
over zeolite catalysts results
primarily in aromatic (e.g., benzene, toluene, xylene) and olefin
products. However, furans are a higher value intermediate for their
ability to be readily transformed into gasoline, diesel, and chemicals.
Here we investigate possible mechanisms for the coupling of glycoaldehyde,
a common product of cellulose pyrolysis, over HZSM-5 for the formation
of furans. Experimental measurements of neat glycoaldehyde over a
fixed bed of HZSM-5 confirm furans (e.g., furanone) are products of
this reaction at temperatures below 300 °C with several aldol
condensation products as coproducts (e.g., benzoquinone). However,
under typical catalytic fast pyrolysis conditions (>400 °C),
further reactions occur that lead to the usual aromatic product slate.
ONIOM calculations were utilized to identify the pathway for glycoaldehyde
coupling toward furanone and hydroxyfuranone products with dehydration
reactions serving as the rate-determining steps with typical intrinsic
reaction barriers of 40 kcal mol<sup>–1</sup>. The reaction
mechanisms for glycoaldehyde will likely be similar to that of other
small oxygenates such as acetaldehyde, lactaldehyde, and hydroxyacetone. This study provides a generalizable mechanism of oxygenate coupling
and furan formation over zeolite catalysts
Understanding Trends in Autoignition of Biofuels: Homologous Series of Oxygenated C5 Molecules
Oxygenated
biofuels provide a renewable, domestic source of energy that can enable
adoption of advanced, high-efficiency internal combustion engines,
such as those based on homogeneously charged compression ignition
(HCCI). Of key importance to such engines is the cetane number (CN)
of the fuel, which is determined by the autoignition of the fuel under
compression at relatively low temperatures (550–800 K). For
the plethora of oxygenated biofuels possible, it is desirable to know
the ignition delay times and the CN of these fuels to help guide conversion
strategies so as to focus efforts on the most desirable fuels. For
alkanes, the chemical pathways leading to radical chain-branching
reactions giving rise to low-temperature autoignition are well-known
and are highly coincident with the buildup of reactive radicals such
as OH. Key in the mechanisms leading to chain branching are the addition
of molecular oxygen to alkyl radicals and the rearrangement and dissociation
of the resulting peroxy radials. Prediction of the temperature and
pressure dependence of reactions that lead to the buildup of reactive
radicals requires a detailed understanding of the potential energy
surfaces (PESs) of these reactions. In this study, we used quantum
mechanical modeling to systematically compare the effects of oxygen
functionalities on these PESs and associated kinetics so as to understand
how they affect experimental trends in autoignition and CN. The molecules
studied here include pentane, pentanol, pentanal, 2-heptanone, methylpentyl
ether, methyl hexanoate, and pentyl acetate. All have a saturated
five-carbon alkyl chain with an oxygen functional group attached to
the terminal carbon atom. The results of our systematic comparison
may be summarized as follows: (1) Oxygen functionalities activate
C–H bonds by lowering the bond dissociation energy (BDE) relative
to alkanes. (2) The R–OO bonds in peroxy radicals adjacent
to carbonyl groups are weaker than corresponding alkyl systems, leading
to dissociation of ROO<sup>•</sup> radicals and reducing reactivity
and hence CN. (3) Hydrogen atom transfer in peroxy radicals is important
in autoignition, and low barriers for ethers and aldehydes lead to
high CN. (4) Peroxy radicals formed from alcohols have low barriers
to form aldehydes, which reduce the reactivity of the alkyl radical.
These findings for the formation and reaction of alkyl radicals with
molecular oxygen explain the trend in CN for these common biofuel
functional groups
Ab Initio Surface Phase Diagrams for Coadsorption of Aromatics and Hydrogen on the Pt(111) Surface
Supported
metal catalysts are commonly used for the hydrogenation
and deoxygenation of biomass-derived aromatic compounds in catalytic
fast pyrolysis. To date, the substrate–adsorbate interactions
under reaction conditions crucial to these processes remain poorly
understood, yet understanding this is critical to constructing detailed
mechanistic models of the reactions important to catalytic fast pyrolysis.
Density functional theory (DFT) has been used in identifying mechanistic
details, but many of these works assume surface models that are not
representative of realistic conditions, for example, under which the
surface is covered with some concentration of hydrogen and aromatic
compounds. In this study, we investigate hydrogen-guaiacol coadsorption
on Pt(111) using van der Waals-corrected DFT and ab initio thermodynamics
over a range of temperatures and pressures relevant to bio-oil upgrading.
We find that relative coverage of hydrogen and guaiacol is strongly
dependent on the temperature and pressure of the system. Under conditions
relevant to ex situ catalytic fast pyrolysis (CFP; 620–730
K, 1–10 bar), guaiacol and hydrogen chemisorb to the surface
with a submonolayer hydrogen (∼0.44 ML H), while under conditions
relevant to hydrotreating (470–580 K, 10–200 bar), the
surface exhibits a full-monolayer hydrogen coverage with guaiacol
physisorbed to the surface. These results correlate with experimentally
observed selectivities, which show ring saturation to methoxycyclohexanol
at hydrotreating conditions and deoxygenation to phenol at CFP-relevant
conditions. Additionally, the vibrational energy of the adsorbates
on the surface significantly contributes to surface energy at higher
coverage. Ignoring this contribution results in not only quantitatively,
but also qualitatively incorrect interpretation of coadsorption, shifting
the phase boundaries by more than 200 K and ∼10–20 bar
and predicting no guaiacol adsorption under CFP and hydrotreating
conditions. The implications of this work are discussed in the context
of modeling hydrogenation and deoxygenation reactions on Pt(111),
and we find that only the models representative of equilibrium surface
coverage can capture the hydrogenation kinetics correctly. Last, as
a major outcome of this work, we introduce a freely available web-based
tool, dubbed the Surface Phase Explorer (SPE), which allows researchers
to conveniently determine surface composition for any one- or two-component
system at thermodynamic equilibrium over a wide range of temperatures
and pressures on any crystalline surface using standard DFT output
Catalytic Upgrading of Biomass-Derived Compounds via C–C Coupling Reactions: Computational and Experimental Studies of Acetaldehyde and Furan Reactions in HZSM‑5
Catalytic
C–C coupling and deoxygenation reactions are essential for
upgrading of biomass-derived oxygenates to fuel-range hydrocarbons.
Detailed understanding of mechanistic and energetic aspects of these
reactions is crucial to enabling and improving the catalytic upgrading
of small oxygenates to useful chemicals and fuels. Using periodic
density functional theory (DFT) calculations, we have investigated
the reactions of furan and acetaldehyde in an HZSM-5 zeolite catalyst,
a representative system associated with the catalytic upgrading of
pyrolysis vapors. Comprehensive energy profiles were computed for
self-reactions (i.e., acetaldehyde coupling and furan coupling) and
cross-reactions (i.e., acetaldehyde + furan) of this representative
mixture. Major products proposed from the computations are further
confirmed using temperature controlled mass spectra measurements.
The computational results show that furan interacts with acetaldehyde
in HZSM-5 via an alkylation mechanism, which is more favorable than
the self-reactions, indicating that mixing furans with aldehydes could
be a promising approach to maximize effective C–C coupling
and dehydration while reducing the catalyst deactivation (e.g., coke
formation) from aldehyde condensation
Polarized Matrix Infrared Spectra of Cyclopentadienone: Observations, Calculations, and Assignment for an Important Intermediate in Combustion and Biomass Pyrolysis
A detailed
vibrational analysis of the infrared spectra of cyclopentadienone
(C<sub>5</sub>H<sub>4</sub>î—»O) in rare gas matrices has been
carried out. <i>Ab initio</i> coupled-cluster anharmonic
force field calculations were used to guide the assignments. Flash
pyrolysis of <i>o</i>-phenylene sulfite (C<sub>6</sub>H<sub>4</sub>O<sub>2</sub>SO) was used to provide a molecular beam of C<sub>5</sub>H<sub>4</sub>î—»O entrained in a rare gas carrier. The
beam was interrogated with time-of-flight photoionization mass spectrometry
(PIMS), confirming the clean, intense production of C<sub>5</sub>H<sub>4</sub>î—»O. Matrix isolation infrared spectroscopy coupled
with 355 nm polarized UV for photoorientation and linear dichroism
experiments was used to determine the symmetries of the vibrations.
Cyclopentadienone has 24 fundamental vibrational modes, Γ<sub>vib</sub> = 9a<sub>1</sub> ⊕ 3a<sub>2</sub> ⊕ 4b<sub>1</sub> ⊕ 8b<sub>2</sub>. Using vibrational perturbation theory
and a deperturbation–diagonalization method, we report assignments
of the following fundamental modes (cm<sup>–1</sup>) in a 4
K neon matrix: the a<sub>1</sub> modes of X̃ <sup>1</sup>A<sub>1</sub> C<sub>5</sub>H<sub>4</sub>O are found to be ν<sub>1</sub> = 3107, ν<sub>2</sub> = (3100, 3099), ν<sub>3</sub> = 1735, ν<sub>5</sub> = 1333, ν<sub>7</sub> = 952, ν<sub>8</sub> = 843, and ν<sub>9</sub> = 651; the inferred a<sub>2</sub> modes are ν<sub>10</sub> = 933, and ν<sub>11</sub> = 722; the b<sub>1</sub> modes are ν<sub>13</sub> = 932, ν<sub>14</sub> = 822, and ν<sub>15</sub> = 629; the b<sub>2</sub> fundamentals are ν<sub>17</sub> = 3143, ν<sub>18</sub> = (3078, 3076) ν<sub>19</sub> = (1601 or 1595), ν<sub>20</sub> = 1283, ν<sub>21</sub> = 1138, ν<sub>22</sub> = 1066, ν<sub>23</sub> = 738, and ν<sub>24</sub> = 458.
The modes ν<sub>4</sub> and ν<sub>6</sub> were too weak
to be detected, ν<sub>12</sub> is dipole-forbidden and its position
cannot be inferred from combination and overtone bands, and ν<sub>16</sub> is below our detection range (<400 cm<sup>–1</sup>). Additional features were observed and compared to anharmonic calculations,
assigned as two quantum transitions, and used to assign some of the
weak and infrared inactive fundamental vibrations