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Winner of the IUPAC Prize
for Young Chemists - 2000

 

Chandra Saravanan wins one of the first 4 IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Physical chemistry of organic molecules in nanoporous materials"

Current address (at the time of application)

c/o Head-Gordon Group
Department of Chemistry
University of California
Berkely, CA 94720, USA

Tel.: +1 510 642 9340
Fax: +1 510 643 1255
E-mail: saru@bastille.cchem.berkeley.edu

Academic degrees

  • Ph.D., University of Massachusetts, Amherst, MA, USA
  • M. S., Georgetown Universtity, Washington, DC, USA
  • B. Tech., Central Electrochemical Research Institute, India

Ph.D. Thesis

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 Tsapatsis

Essay

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.

Motivation:
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.

Significance:
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 KMC simulations.

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.

Results:
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 (< 20) 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.

Conclusions:
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.


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