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 Bronsted 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 favor
the diffusion of reactant and product decreasing the residence time and
the ensuing degradation to coke. Acidic molecular sieves with
monodimensional structure may favor the formation of the precursors of the
coke responsible of the catalyst deactivation.