EFFECT OF THE ALKALI CATION ON HEPTANE AROMATIZATION IN L-ZEOLITE

Platinum was deposited on a series of BaL zeolites containing different alkali cations. The composition, Pt dispersion, infrared spectrum of adsorbed CO, and the turnover frequency for heptane reforming on each catalyst was determined. The infrared spectrum of a saturation coverage of CO on Pt/BaL at 25-degrees-C consists of a broad infrared band which shifts from 2085 to 2025 cm-1 as the alkali element in the L is changed from Li to Cs. Heating the samples to 225-degrees-C causes new bands to appear at 2030, 1975, and 1940 cm-1. These bands are assigned to CO adsorbed on Pt in which the oxygen end of the molecule weakly bonds to cations in the L lattice. This and other evidence indicates that the Pt particles are primarily located inside the zeolite cavities. The specific activity of Pt/BaL for heptane reforming at 440-degrees-C, 0.16 atm heptane and 0.95 atm H-2 is sensitive to the distribution of alkali cations. As the lighter elements, Li and Na, are exchanged for heavier ones, Rb and Cs, the turnover frequencies for formation of C1-C6 hydrocarbons, ethylcyclopentane, benzene and toluene increase on average by 3.4, 3.3, 8.8, and 3.8 times, respectively. These data indicate that adsorption and catalysis on Pt/BaL is strongly influenced by metal-support interactions.