Cynthia J. Jameson

Professor

Physical Chemistry

Microporous solids, zeolites in particular, are widely used in heterogeneous catalytic processes, separations, oil recovery, and other industrial processes. An understanding of elementary processes at surfaces, such as adsorption and diffusion is an important fundamental problem and may assist in interpreting more complicated surface chemistry. In our laboratory we use NMR spectroscopy to obtain a direct measure of the equilibrium distribution of adsorbed molecules in a microporous solid, e.g., the fraction of cavities having exactly 3 molecules. We measure the individual rate constants for a Xe atom leaving from Xen in a cavity and going into the next cavity to form Xem . This is diffusion at the molecular level. Industrial processes involve adsorption from streams which have multiple components, and separations depend on competitive adsorption of mixtures in zeolites. In fundamental studies of these systems we measure directly the equilibrium distribution of molecules of type B in cages containing a known number of molecules of type A, a more complete description of competitive adsorption than ever possible. Our grand canonical Monte Carlo (GCMC) simulations have reproduced not only the distributions but also the NMR chemical shifts as a function of composition and temperature. The concept of the intermolecular chemical shift surfaces developed in our lab from quantum mechanical calculations and gas phase NMR have made possible such detailed interpretations in zeolites which are completely consistent with our work in NMR of gas mixtures. Our spin relaxation studies in gases provide cross sections associated with either the reorientation of a molecular frame or the changes in the rotational angular momentum vector of a molecule in collisions. Cross sections from quadrupolar and spin rotation relaxation are independent sensitive measures of the anisotropy of the intermolecular potential of the collision pair.