PLATINUM-PROMOTED AND UNPROMOTED SULFATED ZIRCONIA CATALYSTS PREPAREDBY A ONE-STEP AEROGEL PROCEDURE .2. CATALYTIC ACTIVITY IN THE ISOMERIZATION OF N-BUTANE

The catalytic activity in the isomerization of butane of a variety of zirconia sulfate catalysts, both unpromoted (AZS samples) and promoted with Pt (PZS samples), prepared by an aerogel technique, is reported. Catalysts have either different sulfate contents (AZS samples) or dif ferent Pt content and constant sulfate content (PZS samples). Catalyst s are studied in a continuous flow system at 150 and 250 degrees C, bo th in the absence and in the presence of hydrogen in the feed. AZS sam ples have identical specific activity at 150 degrees C, while differen ces are observed at 250 degrees C both in the absence and in the prese nce of H-2. PZS samples are more active and stable, particularly when activated in air only. No effect of Pt content on the catalytic activi ty is observed in the range explored (0.65-4.96%), while a negative ef fect of hydrogen can be evidenced under suitable conditions. An FTIR a nalysis performed on both AZS and PZS samples using a procedure that m imick the catalysis conditions suggest that contamination of the activ e sites on stream is mainly due to water and, in the absence of H-2 in the feed, to coke or oligomers deposition. A reaction mechanism is pr oposed involving an intermediate intermolecular butane dimerization wh ere Zr4+ Lewis acid centers promote the initial C4 carbenium ion forma tion, followed by addition to in situ formed butene. The roles of Fit in the catalysis are also discussed. (C) 1997 Academic Press.