Shape Selectivity Revisited: Higher Catalytic Rates in Smaller Zeolite Channels

2011 Spring Symposium

 
Aditya Bhan
Department of Chemical Engineering and Materials Science
University of Minnesota
Twin Cities


Abstract – Zeolites are crystalline inorganic framework oxides with channel and pocket dimensions typically smaller than 1 nanometer. Their constrained environments are well known to select for chemical reactions via steric mechanisms, typically, by exclusion of molecules or transition states based on size. The strong effects of pore size and shape as they become commensurate with those of reactant species and the concomitant effects on the enthalpy and entropy of adsorption have also been broadly and convincingly noted. We inquire instead, what are the effects of confinement in small channels? In this talk, I will present three examples where reactivity in small 8-membered ring pockets of H-MOR differs from that in larger 12-membered ring channels of MOR.

(i) We show that the apparent effects of proton density and of hydroxyl group environment on DME carbonylation turnover rates reflect instead the remarkable specificity of eight-membered ring zeolite channels in accelerating kinetically relevant *CH3-CO reaction steps.

(ii) In zeolite pores large enough to accommodate ethanol dimers, ethanol preferentially dehydrates via a bimolecular pathway to generate diethyl ether since the formation of ethanol dimeric species is energetically more favorable than the formation of ethanol monomers. In zeolite channels too small to accommodate ethanol dimers, ethanol is selectively dehydrated via a unimolecular reaction pathway to generate ethylene.

(iii) For isomerization reactions of n-hexane, 8-MR channels of H-MOR minimize the free energy of required carbocationic transition states, possibly via partial confinement effects that increase the entropy of the transition state at the expense of the reaction enthalpy. These findings show that confinement in zeolite channels influences rate and selectivity of hydrocarbon reactions more fundamentally than simple considerations of size and shape.

Speaker’s Biography – Aditya Bhan received his Bachelor of Technology (B. Tech.) in Chemical Engineering from IIT Kanpur in 2000. Subsequently, he moved to West Lafayette, Indiana and joined the group of Nick Delgass at Purdue, where he developed microkinetic models to describe propane aromatization on proton- and gallium- form ZSM-5 materials for his PhD. In 2005, he moved to the University of California at Berkeley to pursue post-doctoral studies in Professor Enrique Iglesia’s group to study the kinetics, mechanism, and site requirements of dimethyl ether carbonylation. In September 2007, Dr. Bhan took up his present position as an Assistant Professor in the Department of Chemical Engineering and Materials Science at the University of Minnesota. Dr. Bhan leads a research group that focuses on the structural and mechanistic characterization of inorganic molecular sieve catalysts useful in energy conversion and petrochemical synthesis. His research at Minnesota has been recognized with the McKnight Land Grant Professor and 3M Non-tenured Faculty awards.