Abstract
Over H3PW12O40 and its acidic cesium salts at 250°C, alkane transformations occur through the mechanisms previously proposed for sulfated zirconia and mordenite catalysts: propane is mainly transformed into butanes through a trimerization–isomerization–cracking process, n-butane into isobutane, propane and pentanes through a dimerization–isomerization–cracking process, n-hexane into methylpentanes and 2,3-dimethylbutane through a monomolecular mechanism. With all the samples, n-butane transformation is initially much faster than propane transformation, the difference in rate increasing significantly with the Cs content: from 25 times with H3PW12O40 to 350 times with Cs2.4H0.6PW12O40. On the other hand, n-hexane transformation is 2.3 to 7 times faster than n-butane transformation. A decrease in acid strength and in acid site density with Cs introduction is proposed to explain the increase in the rate ratios. For all the reactions, sulfated zirconia pretreated at 600°C is 2–3 times more active than the heteropolycompounds. HMOR10 which is the most active catalyst for n-hexane transformation is the least active for n-butane and especially propane transformation. This very low activity of mordenite for these bimolecular processes can be related to particularities of its pore system: bimolecular reactions are strongly unfavoured in the narrow non-interconnected channels of this zeolite.
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