EXAFS STUDIES OF PILLARED CLAY CATALYSTS

Cr K-edge X-ray absorption spectroscopy shows that partially hydrolysed Cr(III) nitrate solutions contain zig-zag chains of cis-linked octahedra. These are retained once intercalated into smectite clays, in which the chains run parallel with the alumino-silicate layers. Partial oxidation to tetrahedral Cr(VI) sites occurs on calcination in air at 523 K, and to a lesser extent, in vacuo. For the three clays studied the fraction of oxidised sites followed the order laponite > bentonite > beidellite. After hydrogenation the Cr(VI) sites are lost and the octahedral Cr(III) units form a more condensed, but disordered structure. The sample so-formed from beidellite is the most active catalyst for methanol dehydration. Most products are alkanes of > C4. These undergo isotopic exchange, presumably with the Bronsted acid sites in this clay.