Pentane Conversion on Dealuminated H-Y and HZSM-5

Abstract

The steady-state rate ofn-pentane conversion has been related to the Brønsted acidity of dealuminated (D) acid (H) Y zeolites (DHY) and ZSM-5 (DHZ). Low-temperature CO adsorption FTIR detects two kinds of Brønsted sites in DHZ and a distribution of Brønsted sites in DHY. In DHZ no Lewis sites were detected by this technique. In conjunction with results obtained by high-resolution29Si MAS REDOR spectroscopy and reported elsewhere, a distinction is made between the Brønsted acidity of an OH group bridging a silicon with only one aluminum within its first coordination shell and at least one aluminum in its third shell (Q1), from an OH group on a silicon withnoaluminum in its third coordination shell (FAl1). The latter OH group is more acid than the former. In DHY it is shown that the apparent activation energy of the overalln-pentane transformation increases to about 110 kJ mol−1as FAl1/Q2decreases below 1/3. Above 1/3 the apparent activation energy is about 60 kJ mol−1. The activity of DHY and DHZ scales with FAl1. The selectivity to isomerization vs. cracking depends mainly on the zeolite structure and the partial clogging of the pores by nonframework aluminum.