SELECTIVITY IN HETEROGENEOUS CATALYTIC PROCESSES

The selectivity of several catalytic systems was studied. Shape select ivity of Pt on carbon-fiber catalysts was demonstrated in the competit ive hydrogenation of 1-hexene and cyclohexene and in the parallel dehy drogenation of cyclohexanol to cyclohexanone and phenol. Both reaction s were carried out in a gas-phase fixed-bed reactor. Catalysts prepare d on carbon fibers, containing pores with small constrictions (5 Angst rom) yielded significantly higher rates of hydrogenation of 1-hexene c ompared to those of cyclohexene and selectively produced cyclohexanone from cyclohexanol. Other catalysts, supported on carbon fibers with l arge constrictions (7 Angstrom) or activated carbon, displayed compara ble rates of hydrogenation for both reactants and yielded cyclohexanon e as well as phenol from cyclohexanol. Nitration of o-xylene with nitr ogen dioxide was carried out in the gas phase over a series of solid a cid catalysts packed in a fixed bed. Several zeolites, supported sulfu ric acid, and sulfated zirconia were tested. Zeolite H-beta was found to be the most active and selective catalyst for the production of 4-n itro-o-xylene. A preliminary kinetic model indicated that the selectiv ity to 4-nitro-o-xylene increased with decreasing concentration of nit rogen dioxide. Alkylation of phenol with methanol was performed on zeo lites, supported sulfuric and phosphoric acids, and sulfated zirconia packed in a fixed-bed. The ratio of o- to c-alkylation, measured at 18 0 degrees C and methanol to phenol feed molar ratio of unity, ranged f rom 4 with the supported acids to 2 with zeolite H-beta. This ratio de creased with temperature. The ratio of o- to p-cresol changed from abo ut 2 in zeolites in supported sulfuric acid and to 0.5 in phosphoric a cid supported on carbon fibers.