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, 29Si and 27Al 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 m2 g−1 and this value showed marked decreases if compared with the dealuminated (310 m2 g−1), iron impregnated (302 m2 g−1) and sulphated mordenite (247 m2 g−1). It suggested the pore plugging or blocking by EFAL species, further confirmed by NMR experiments. The dealuminated mordenite showed Brønsted 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 × 10−9 to 2 × 10−7 mol s−1 g−1); a significant initial activity was observed at a temperature as low as 200 °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 × 10−7 mol s−1 g−1) in comparison with modified mordenites.

Iron or sulphates impregnated on dealuminated mordenite showed an enhanced conversion in the transformation of n-butane to isobutane compared to non-dealuminated or dealuminated modernite. Fe or SO42− species produce new active sites that promote the catalytic activity due to the highest strength of few sites and/or to the occurrence of an initial red-ox activation step over iron species. However, modified mordenites remain less active than conventional sulphated zirconia, used as reference catalyst.Figure optionsDownload as PowerPoint slide