We have investigated the effect of acid site density on the catalytic performance of high-silica (50 <= Si/Al <= 380) H-HPM-1 zeolite in the 1-butene skeletal isomerization. Changes (<= 0.5) in the number of framework Al atoms, and thus of Bronsted acid sites, per unit cell (60 tetrahedral atoms) led to striking variations in both 1-butene conversion and isobutene selectivity. Combining isobutene yield with catalyst durability, H-HPM-1 with a framework Si/Al ratio of 350 was found to perform better than any of the catalysts studied for this reaction thus far. (C) 2015 Elsevier Inc. All rights reserved.