CATALYTIC ISOMERIZATION OVER ACID, METAL AND HYBRID SITES
Professor Wolfgang W. H. Sachtler
Center for Catalysis and Surface Science
Wednesday, February 12, 1997, 4:00 p.m.
Room 102, Mines & Minerals Resources Bldg., UK
(Please note that this seminar is on campus, not at the CAER.)
In the 1930's Herman Pines and Vladimir Ipatieff discovered that paraffins can be isomerized and alkylated at low temperatures in strong liquid acids. In the 1950's it became clear that the reaction mechanism involved carbenium ion intermediates. Whereas primary carbenium ions have a prohibitively high energy, protonated cyclopropane derivatives are crucial in many alkane isomerizations and in ring enlargements or contractions of cycloalkanes.
Carbenium ions have never been detected on solid acid catalysts; alkoxy groups are formed instead. Still, the terminology used for isomerization in liquid acids is also useful in describing the mechanism of alkane isomerization and cracking over solid catalysts. High stability is achieved with bifunctional catalysts containing platinum metals in addition to acid sites.
Transition metal/zeolite catalysts show that these catalytic functions are not simply additive, but that chemical interaction between metal clusters and protons creates "collapsed bifunctional sites." Protons also act as "chemical anchors" for metal clusters, which results in extremely high metal dispersions. The geometry of the zeolite cavities affects the stereoselectivity and transport rates of the catalyzed reactions.
Revisiting solid acid catalysis with modern spectroscopic and isotopic labeling techniques shows that the isomerization of butane avoids primary carbenium ions by using a bimolecular route involving formation of butene and C8 intermediates. In contrast, isomerization of pentane follows a largely monomolecular route.
The seminar is sponsored jointly by the Center for Applied Energy Research and the Tri-State Catalysis Society.