SOME HYDROGENATION, OXIDATION, AND ISOMERIZATION REACTIONS OVER ZIRCONIUM-DIOXIDE CATALYST
HALCOM, DONALD BRUCE
Doctor of Philosophy
The mechanisms and selectivities of several hydrogenation, isomerization, and partial oxidation reactions that occur over a zirconia catalyst were investigated. The thermal stability of the catalyst in air and in vacuum were determined. The catalyst was predominately cubic in crystal structure and had a BET surface area of 60 m('2)/mg. During the addition of a deuterium to 1,3-butadiene, the molecular identity of D(,2) is conserved, and all three n-butenes (C(,4)H(,6)D(,2)) are produced directly as primary products. The relative amounts of products formed at 75(DEGREES)C or less are trans>>1-butene>cis. The D(,2) addition is zero order in butadiene and first order in deuterium with an activation energy of 14.1 Kcal/mole. In contrast to this preference for 1,4 addition, the reduction of 2,3-dimethyl-1,3-butadiene produced almost singularly 2,3-dimethyl-1-butene, the 1,2 addition product. The isomerization of n-butenes and of 2,3-dimethyl-1-butene at 75(DEGREES)C or less are first order reactions. During the isomerization of n-butenes, there is no direct cis trans conversion. The presence of butadiene suppresses the isomerization rates. The isomerization occurs with an intramolecular hydrogen transfer and a kinetic isotope effect of about three. The isomerization reactions occur on different sites than those used for butadiene reduction The oxidation of carbon monoxide to carbon dioxide at 300(DEGREES)- 400(DEGREES)C has an activation energy of 17.1 Kcal/mole. This oxidation rate can be described by a Langmuir-Hinshelwood kinetic model with CO(,2) desorbing in a second order manner. Temperature programmed desorption of CO(,2) is also second order with an activation energy of 20.5 Kcal/mole. The CO oxidation is first order in CO and zero order in oxygen. Propylene can be oxidized to CO, CO(,2), and H(,2)O over zirconia. This reaction is zero order in propylene and first order in oxygen with an activation energy of 19.2 Kcal/mole. A constant CO/CO(,2) ratio of 1.07 is obtained in the product. Butane is oxidatively dehydrogenated to 1-butene at 375(DEGREES)- 460(DEGREES)C with an activation energy and 55.9 Kcal/mole. The OXD is first order in butane and zero order in oxygen. A side reaction to produce a constant CO/CO(,2) ratio of 1.41 occurs on separate sites. The deep oxidation is first order in oxygen and zero order in butane. The OXD reaction rate is suppressed by water, whereas the deep oxidation is not affected by the presence of gaseous water. The deep oxidation has an activation energy of 45.0 Kcal/mole. Isotopic oxygen exchange at 400(DEGREES)C indicated that at least 86% of the oxygen in the zirconia can be statistically scrambled with the oxygen in the carbon dioxide. This suggests that CO(,2) is dissociatively chemisorbed. A permanent carbonate equivalent to 2.7 x 10('13) molecules of CO(,2)/cm('2) is retained on the surface. Thermogravimetric experiments showed that a carbonaceous residue is formed on the zirconia by the cracking of butane. The residue can be removed by oxidation with the apparent adsorption of oxygen in two separate steps. This suggests that oxygen reacts directly with the carbonaceous residue. A frontier analysis is used to describe the hydrogenation and isomerization reaction mechanisms. Proposed mechanisms for oxidation and oxidative dehydrogenation are given.