Mechanistic tracer studies of a synthetic mica-montmorillonite: n-butene isomerizations
Milliron, Dennis Luke
Hightower, Joe W.
Master of Science
This thesis examines the catalytic characteristics of a synthetic mica-montmorillonite clay. The double bond isomerizations (including both double bond migration and rotation, but excluding structural rearrangements of the carbons) of the n-butenes were employed as the probe reactions. Product analyses were performed via gas chromatography and mass spectroscopy. The reaction products distribution was kinetically controlled, which suggests the possibility of a metastable intermediate. The only intermediate compatible with tracer data obtained herein is the secondary butyl carbonium ion. The isotopic data also indicate that both types of isomerization proceed via an intermolecular mechanism rather than intramolecularly. The isomerization reaction appears to follow first order kinetics. The existence of a significant isotope effect intimates that carbon-hydrogen bond cleavage occurs in the rate determining step of the reaction sequence. Bronsted type acid sites appear to be responsible for the catalytic activity of this synthetic clay for the isomerization of n-butenes. Bronsted type acid sites are associated with the hydroxyl groups which occur in this clay. Since up to 20% of the total catalyst hydrogen can occur on 12% of the surface (the circumferential surface area of the layer-lattice crystals), and up to 55% of the hydrogens which lie near the crystal surface can occur on this same 12% of the external surface area, it is hypothesized that the circumferential hydroxyls are sites of high catalytic activity.