4 research outputs found
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
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
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
Carbocation Stability in H‑ZSM5 at High Temperature
Zeolites
are common catalysts for multiple industrial applications,
including alcohol dehydration to produce olefins, and given their
commercial importance, reaction mechanisms in zeolites have long been
proposed and studied. Some proposed reaction mechanisms for alcohol
dehydration exhibit noncyclic carbocation intermediates or transition
states that resemble carbocations, and several previous studies suggest
that the <i>tert</i>-butyl cation is the only noncyclic
cation more stable than the corresponding chemisorbed species with
the hydrocarbon bound to the framework oxygen (i.e., an alkoxide).
To determine if carbocations can exist at high temperatures in zeolites,
where these catalysts are finding new applications for biomass vapor-phase
upgrading (∼500 °C), the stability of carbocations and
the corresponding alkoxides were calculated with two ONIOM embedding
methods (M06-2X/6-311GÂ(d,p):M06-2X/3-21G) and (PBE-D3/6-311GÂ(d,p):PBE-D3/3-21G)
and plane-wave density functional theory (DFT) using the PBE functional
corrected with entropic and Tkatchenko–Scheffler van der Waals
corrections. The embedding methods tested are unreliable at finding
minima for primary carbocations, and only secondary or higher carbocations
can be described with embedding methods consistent with the periodic
DFT results. The relative energy between the carbocations and alkoxides
differs significantly between the embedding and the periodic DFT methods.
The difference is between ∼0.23 and 14.30 kcal/mol depending
on the molecule, the model, and the functional chosen for the embedding
method. At high temperatures, the pw-DFT calculations predict that
the allyl, isopropyl, and <i>sec</i>-butyl cations exhibit
negligible populations while acetyl and <i>tert</i>-butyl
cations exhibit significant populations (>10%). Moreover, the periodic
DFT results indicate that mechanisms including secondary and tertiary
carbocations intermediates or carbocations stabilized by adjacent
oxygen or double bonds are possible at high temperatures relevant
to some industrial uses of zeolite catalysts, although as the minority
species in most cases