<p>Catalysis
has a responsibility to provide solutions to the growing grand challenge of
sustainability in the fuels and chemical industry to help combat climate
change. These changes; however, cannot be realized without a more fundamental
understanding of the active sites that catalyze chemical reactions, and how
they can be tuned to control rates and selectivities. Four specific examples of
active site modification will be considered in this work: the speciation of
isolated metals in zeolite frameworks, solvent thermodynamics and structure at
defects in zeolite frameworks, the electronic modification of platinum through
alloying in well-defined intermetallic nanoparticles, and the mobility and
shape of gold nanoparticles in zeolite channels. Each will highlight how
quantum chemistry calculations can provide a fundamental understanding of how
these active site modifications influence the kinetics of chemical reactions,
and how they can be controlled to pursue solutions to the reduction of carbon
through sustainable utilization of shale gas as well as renewable chemicals
production through biomass upgrading.</p>
<p>Zeolites
exchanged with metal heteroatoms can behave as solid Lewis or Br<a>ø</a>nsted acids depending on heteroatom identity.
Lewis acid heteroatoms can adsorb water and hydrolyze to speciate into “open
sites” which have been shown to differ in their ability to catalyze reactions
such as glucose isomerization as compared to “closed sites” which are fully
coordinated to the zeolite framework. The structure and catalytic properties of
these sites are interrogated by a gas phase reaction, ethanol dehydration, in
Sn-Beta by a combined Density Functional Theory (DFT) and experimental study.
DFT is used to map the possible reaction mechanisms for ethanol dehydration,
including the speciation of Sn sites into hydrolyzed configurations from water
or ethanol. A microkinetic model for ethanol dehydration including unselective
and inhibitory intermediates is constructed. This microkinetic model predicts
the population of reactants and products on the catalyst surface as well as the
sensitivity of individual elementary steps to the total rates. Powerful
anharmonic entropy methods using <i>ab-initio </i>molecular dynamics (AIMD) is
used to capture the entropy of confined reactive intermediates, which is shown
to be necessary to compare with experiment. Results on closed and hydrolyzed
open zeolite sites can then be compared with ethanol dehydration on “defect
open” sites which were shown experimentally to occur at material stacking
faults. A grain boundary model is constructed of zeolite Beta, where unique
sites have similar ligand identity as hydrolyzed open sites. These defect open
sites are found to not contribute to the observed reaction rate as they cannot
stabilize the same transition state structures that were observed in internal
Beta sites. </p>
<p>Intuition
about the ethanol dehydration reaction in Sn-Beta was then used to map a more
expansive and diverse chemical network, the synthesis of butadiene from
acetaldehyde and ethanol. For elementary reactions in this mechanism, which
included aldol condensation, MPV reduction, and crotyl alcohol dehydration in
addition to ethanol dehydration, the hydrolyzed open sites were found to be
crucial reactive intermediates. Hydrolyzed sites were necessary to stabilize
favorable transition states, which requires reconstruction of the local
framework environment. Methods to preferentially stabilize hydrolyzed sites
were then explored, using a screening algorithm developed to consider all
possible sites in each zeolite framework. It was found that the stability of
these hydrolyzed sites could be correlated to the local strain exerted by the
surrounding silica matrix. This provides a new descriptor that stabilizes
intermediates relevant to the synthesis of butadiene and ethanol dehydration.</p>
<p>Next,
the structure and thermodynamic stability of water networks around Sn-Beta
defects and heteroatom active sites was considered using AIMD. As many biomass
reactions occur in the presence of water, the interactions of water with
hydrophobic and hydrophilic functionalized defects dictate how the stability of
reactive intermediates and transition states is affected by a solvating
environment. Locally stable and strongly nucleated clusters of water were
observed to form at Sn defects, with less densely packed water structures
stable at hydrophilic defects. This is in comparison to defect-free siliceous
Beta, where significantly less water uptake is observed. These local clusters
are in equilibrium with the less dense liquid-like phase that extends between defects.
These results motivate localized cluster models around active sites in Lewis
acids, as well as advance the fundamental understanding of
hydrophobic/hydrophilic interactions in microporous materials. The local
cluster models are then applied to the ethanol dehydration reaction in
protonated aluminum Beta zeolites where experimentally observed non-unity
coefficient ratios are rationalized by quantifying a different degree of
solvation for the ethanol reactant state as opposed to the transition state, validated
by a thermodynamic phase diagram.</p>
<p>Changes
in the electronic energy levels of <i>d</i> electrons upon alloying was studied
in conjunction with a new spectroscopic technique being performed at Argonne
National Laboratory to develop new descriptors to predict the degree of coking
for different alloys. Resonant Inelastic X-ray Scattering (RIXS) simultaneously
probes the occupied and unoccupied valence states of platinum in nanoparticles
at ambient conditions. The specific excitation process of this spectroscopy is
particularly amendable to DFT modeling, which was used to provide richer
chemical insight into how changes in observed RIXS signature related to the
electronic structure changes of platinum upon alloying. From a suite of
multiple 3d alloy promoter catalysts synthesized, a quantitative comparison
with DFT modeled spectroscopy was developed. The stability of DFT calculated
coke precursors, relevant to dehydrogenation catalysts to convert light alkanes
into olefins, was then correlated to DFT modeled RIXS spectra, which is a
better descriptor for adsorption of unsaturated chemical intermediates that
used previously, as well as being a descriptor accessible to direct
experimental usage.</p>
<p>Finally,
the diffusion of gold nanoparticles in the TS-1 catalyst was studied using AIMD
to help understand what structural motifs of gold are present under reaction
conditions and how the shape and binding sites of gold is strongly influenced
by deformation by the zeolite framework. This is used to help predict new zeolites
for use in direct propylene epoxidation using molecular oxygen and hydrogen.
The optimization of this catalyst is environmentally relevant to reduce the
usage of inorganics and reduce the cost associated with production of hydrogen
peroxide. Following these discussions, the role of computation in the prediction
of active site structures and kinetics in conjunction with experiment was
included. The broader impact of these findings will also be considered, which
span beyond these specific reactions and materials.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8792588 |
| Date | 13 August 2019 |
| Creators | Brandon C Bukowski (6919304) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Density_Functional_Theory_Investigations_of_Zeolite_and_Intermetallic_Alloy_Active_Site_Structures_for_Kinetics_of_Heterogeneous_Catalysis/8792588 |
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