Isomerization of n-butane over ultrastable H-Y zeolites with different Si/Al atomic ratio

Ultrastable H-Y zeolites with different Si/Al atomic ratios (3 < Si/Al < 11) are active and selective catalysts for the n-butane isomerization. The initial activity of these catalysts is lower than that measured on tungsta supported on zirconia catalysts (WOx/ZrO2) and acidic mordenite catalysts; however, the Brønsted acid sites of the ultrastable H-Y zeolites are stable and selective towards isobutane. No deactivation of the catalysts was observed after 5 h of time on stream. In contrast, WOx/ZrO2 and acidic mordenite catalysts under the same experimental conditions are largely deactivated in less than 1 h of time on stream. The stability of the ultrastable H-Y zeolite in comparison to H-mordenite catalysts may be due to the three-dimensional structure of H-Y made of large supercages interconnected by apertures of 12 oxygen atoms. This structure may favour the diffusion of reactant and product decreasing the residence time and the ensuing degradation to coke. Acidic molecular sieves with monodimensional structure may favour the formation of the precursors of the coke responsible of the catalyst deactivation.