Interaction of isobutene with the surface of different solid acids

The interaction of an olefin with relatively high reactivity toward acid-catalyzed polymerization such as isobutene, with four different solid acids was investigated by means of in situ diffuse reflectance infrared spectroscopy (DRIFTS), continuous flow studies using a packed-bed oscillating balance reactor (OBR), and temperature-programmed desorption/reaction (TPD/R) of the preadsorbed olefin. The uptake of isobutene and its oligomers produced by acid-catalyzed surface polymerization at 473 K is very slow over H-ZSM-5, presumably due to a diffusion-controlled reaction process. The narrow pore structure of the zeolite prevents coke formation (cyclization), and the isobutene oligomers desorb slowly upon heating under nitrogen. In light of TPD/R and DRIFTS results, tungstated zirconia appears to be unable to undergo irreversible retention of isobutene and oligomeric products, whereas the two forms of sulfated zirconia investigated in this study retain a relatively large amount of oligomeric species, and subsequently produce coke and lose the sulfate function. There is a redox reaction between the surface carbonaceous residues and the sulfate function in sulfated zirconias. Of the four catalysts studied, sulfated zirconia systems were the most vulnerable to chemical attack by isobutene.