Title Physical chemistry of organic molecules
in nanoporous materials
Adviser Prof.. Scott M. Auerbach
Thesis Committee Prof.. Scott M. Auerbach, Prof. Bret Jackson,
Prof. Pattricia Bianconi, Prof. Murugappan Muthukumar, Prof. Michael
My dissertation studies transport and phase equilibria
of organic molecules in zeolites. Zeolites are fascinating nanoporous
materials with a wide range of industrial applications such as separations
of organic molecules, catalysis, and ion-exchange. A fundamental understanding
of the physical chemistry of molecules in zeolites provides insight
into developing new nanoporous materials, thus making the study of
such materials exciting.
Both thermodynamics and transport of molecules confined in nanopores
play an important role in the fundamental understanding of reactivity
and separations in zeolites. Although there has been significant development
in these two areas, they have evolved as separate fields. In order
to establish a connection between these fields, we explore the effect
of a vapor-liquid phase transition, a thermodynamic phenomenon, on
the transport properties of molecules in zeolites. Next, we address
the issue of resolving some long-standing discrepancies among various
experiments that measure self-diffusion of benzene in faujasite (FAU)
type zeolites. We also invest a considerable effort in developing
a new theory for explaining observed concentration dependencies of
self-diffusion for various sorbates in FAU type zeolites.
Academic: Being the first to report high-temperature
phase transitions in FAU type zeolites, we have opened up new avenues
for discovering similar thermodynamic phenomena in other zeolites.
This work is also one of the first fundamental theoretical studies
that attempts to bridge the gap between research in transport and
thermodynamics in zeolites. A new theory that we developed to calculate
the mobility of molecules in FAU zeolites also indicates that our
approach is applicable for a wide array of host-guest systems.
Industrial: Our results indicate that the transport
properties of strongly associating molecules are extremely sensitive
to pressure changes that may occur in industrial reactors. Our fundamental
understanding of self-diffusion also provides valuable information
such as optimum loading and temperature for designing reactors for
separations and catalysis. The present study also provides insights
for separation of natural gas from aromatics.
Model and Methodology:
Molecular dynamics (MD) simulations have been successfully used to
model dynamics of molecules in siliceous zeolites. However, the "deep"
potential energy wells (i.e. adsorption sites) present in cation-containing
zeolites result in prohibitively large time scales for MD simulations,
thus restricting their application to cation-free siliceous zeolites.
In order to overcome the limitations of MD simulations, we performed
kinetic Monte Carlo (KMC) simulations on a realistic coarse-grained
lattice model for benzene in cation-containing FAU type zeolites.
In this model, the dynamics of a molecule is determined by its adhesive
interactions with the zeolite, its cohesive interactions with other
adsorbate molecules, and its jump rates between lattice sites. Using
this model, diffusion isotherms (i.e. diffusion coefficients at various
loadings of molecules at specific temperatures) were calculated using
The vapor-liquid equilibria of molecules in nanopores
was also thoroughly investigated by performing grand canonical Monte
Carlo (GCMC) simulations on the lattice model. This study was used
to understand the effect of phase transitions on the mobility of fluids.
We have shown that strongly confined benzene molecules exhibit subcritical
properties in Na-X, a FAU type zeolite. Our GCMC simulations for benzene
in Na-X show low to high density phase transitions at temperatures
as high as 300 K. Although confinement of molecules in small cavities
(< 20Å) can quench phase transitions, we have shown that cooperative
interactions between molecules in adjacent cages in FAU type zeolites
can lead to vapor-liquid transitions.
Impact: The existence of a phase transition for confined benzene
through this unique mechanism also opens up the possibility of observing
phase equilibria for other strongly associating molecules in nanopores.
We therefore expect careful adsorption experiments to reveal this
vapor-liquid phase equilibrium for benzene in Na-X, and possibly for
a wide array of other systems.
Our knowledge of the thermodynamics of the system from
the phase-equilibria work was used to understand transport of benzene
in FAU. We find that the mobility of molecules in the phase-separated
regime is dominated by the dynamics of "evaporation" between the liquid
and vapor phases within the zeolite. This evaporation process also
plays a key role in determining the shape of the diffusion isotherms.
Impact: This mechanism of diffusion reported for the first
time in zeolite science, stresses the fact that a clear understanding
of the thermodynamics of confined fluids is crucial for elucidating
the transport properties of molecules in zeolites. Our study emphasizes
the need for a careful analysis to check for phase-transition effects
before interpreting experimental diffusion isotherms for strongly
associating molecules. We also show that the transport properties
of molecules in some zeolites can be drastically altered for small
changes in pressure. This information is extremely vital for designing
reactors for molecular sieving and catalysis applications.
We also find that our theoretical approach provides
general qualitative explanations for diffusion-isotherm types observed
for a whole range of host-guest systems. Finally, on the issue of
resolving discrepancies for benzene diffusion through FAU, our sensitivity
analysis on the system parameters clearly indicates excellent qualitative
agreement with pulsed field gradient NMR diffusivities, and strong
disagreement with tracer zero length column (TZLC) data. We have proposed
experimental schemes for testing the validity of TZLC data.
My thesis presents a coarse-grained lattice-gas model for self-diffusion
in nanopores. We have shown that organic molecules in zeolites can
exhibit fascinating phenomena such as phase transitions and subcritical
diffusion. Our model has been useful in resolving experimental discrepancies
and has also helped in providing a qualitative understanding of self-diffusion
for a whole range of host-guest systems. We have also illustrated
that the present work has a significant impact on design of industrial
reactors. Finally, we have shown that this study provides a preliminary
step to bridge the gap between two separate fields: thermodynamics
and transport in zeolites.