Selectivity in heterogeneous catalytic processes

The selectivity of several catalytic systems was studied. Shape selectivity of Pt on carbon-fiber catalysts was demonstrated in the competitive hydrogenation of 1-hexene and cyclohexene and in the parallel dehydrogenation of cyclohexanol to cyclohexanone and phenol. Both reactions were carried out in a gas-phase fixed-bed reactor. Catalysts prepared on carbon fibers, containing pores with small constrictions (5 Angstrom) yielded significantly higher rates of hydrogenation of 1-hexene compared to those of cyclohexene and selectively produced cyclohexanone from cyclohexanol. Other catalysts, supported on carbon fibers with large constrictions (7 Angstrom) or activated carbon, displayed comparable rates of hydrogenation for both reactants and yielded cyclohexanone as well as phenol from cyclohexanol. Nitration of o-xylene with nitrogen dioxide was carried out in the gas phase over a series of solid acid catalysts packed in a fixed bed. Several zeolites, supported sulfuric acid, and sulfated zirconia were tested. Zeolite H-beta was found to be the most active and selective catalyst for the production of 4-nitro-o-xylene. A preliminary kinetic model indicated that the selectivity to 4-nitro-o-xylene increased with decreasing concentration of nitrogen dioxide. Alkylation of phenol with methanol was performed on zeolites, supported sulfuric and phosphoric acids, and sulfated zirconia packed in a fixed-bed. The ratio of o- to c-alkylation, measured at 180 degrees C and methanol to phenol feed molar ratio of unity, ranged from 4 with the supported acids to 2 with zeolite H-beta. This ratio decreased with temperature. The ratio of o- to p-cresol changed from about 2 in zeolites in supported sulfuric acid and to 0.5 in phosphoric acid supported on carbon fibers.