Comparative study of transformation of linear alkanes over modified mordenites and sulphated zirconia catalysts: Influence of the zeolite acidity on the performance of n-butane isomerization

Zeolite mordenite was dealuminated by steam treatment followed by acid leaching and further impregnated with 1 wt.% of iron or sulphated. The various solids were characterized by XRD, Si-29 and Al-27 NMR, IR of adsorbed pyridine as well as ammonia adsorption calorimetry and evaluated in n-butane isomerization. BET surface area of non-dealuminated mordenite was 462 m(2) g(-1) and this value showed marked decreases if compared with the dealuminated (310 m(2) g(-1)), iron impregnated (302 m(2) g(-1)) and sulphated mordenite (247 m(2) g(-1)). It suggested the pore plugging or blocking by EFAL species, further confirmed by NMR experiments. The dealuminated mordenite showed Bronsted and Lewis acid sites with medium strength. The sulphated modified mordenite showed highest acidity strength, however its isomerization activity remains modest. The activity in the transformation of n-butane to isobutane was markedly improved by dealumination and subsequently iron impregnation (from 4 x 10(-9) to 2 x 10(-7) mol s(-1) g(-1)); a significant initial activity was observed at a temperature as low as 200 degrees C. At higher temperatures, the conversion level did not change. The positive effect of iron impregnation could be related to the increase in acid strength and/or to the participation of the redox properties of iron to the initial activation step of the reaction. Nevertheless, sulphated zirconia. used as a reference catalyst, presented the highest performance (7 x 10(-7) mol s(-1) g(-1)) in comparison with modified mordenites. (C) 2008 Elsevier B.V. All rights reserved.