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Scott M. Auerbach

Materials Research Society: Boston 1998

The research in my group has focused on developing novel theory and simulation methods for modeling molecules in zeolites and other shape-selective, nanoporous catalysts. The technological importance of zeolites cannot be overstated, considering that the value of zeolite catalysis to petroleum cracking is well in excess of 100 billion dollars. The wide-ranging applicability of these materials results from strong host-guest interactions, which can severely retard guest mobility, making theoretical modeling nearly intractable. To address this issue, we have applied chemical dynamics and kinetics to diffusion in zeolites, yielding efficient and illustrative theoretical approaches, and important predictions of material properties:

• Ising Model of Diffusion. Developing a predictive analytical theory for diffusion in zeolites has remained challenging, due to the coupling between infrequent event dynamics and strong adsorbate-adsorbate interactions. To address this issue, we have developed a new theory for self-diffusion in zeolites based on mean field dynamics of cage-to-cage motion, yielding excellent quantitative agreement with KMC.[1] Our theory provides deep understanding of the microscopic physics essential to these transport phenomena, and has elucidated a long-standing discrepancy since our results are in excellent agreement with pulsed field gradient NMR, and in disagreement with tracer zero-length column data.
• Phase Transitions in Zeolites. There are very few reports of phase transitions in nanopores, presumably because confinement into such small cavities (< 20 Å) reduces the vapor-liquid critical temperature to extremely low values. We have used Grand Canonical lattice simulation techniques and Gibbs-Duhem thermodynamic integration to demonstrate that cooperative interactions can lead to phase transitions for benzene in faujasite, for temperatures as high as 370 K.[2] We expect that careful adsorption experiments will reveal this sort of phase transition for a wide array of other systems.
• Zeolite Membranes. Modeling permeation through zeolite membranes is of increasing importance because of recent progress in synthetic techniques. We have developed a new Grand Canonical KMC algorithm for modeling activated permeation, which predicts that for ultra-thin membranes, discrepancies among different permeation measurements should arise, but can easily be explained in terms of sorption-limited transport.[3]

These exciting advances make us hopeful for even further breakthroughs in our understanding of hydrocarbon mobility and reactivity in nanopores.

• References