4-methylpentan-2-ol dehydration over zirconia catalysts prepared by sol-gel

Zirconia samples have been prepared from xerogels and aerogels obtained usi ng zirconium n-propoxide as precursor. Structure and texture have been inve stigated by X-ray diffraction, thermal analysis, transmission electron micr oscopy, nitrogen adsorption/desorption. Surface acidity and basicity have b een assessed by adsorption microcalorimetry, using ammonia and carbon dioxi de as probe molecules. 4-Methylpentan-2-ol dehydration has been tested at a tmospheric pressure in a fixed-bed flow microreactor. The xerogel gives tet ragonal zirconia upon calcination, during which a mesoporous system is form ed. The crystal phase depends on the presence of oxygen during the cooling step in the case of the aerogel, whose texture is partially retained upon c alcination. Both kinds of catalysts have well-balanced concentrations of ac id and base sites, but the acid sites are weaker in comparison with the bas ic ones. At 603 K the initial conversion of 4-methylpentan-2-ol over the ca lcined xerogel and aerogel is 45 and 63%, respectively; the selectivity to 4-methylpent-1-ene is 77% for both. The occurrence of an E2-like mechanism with the activated complex having a marked carbanionic character seems prob able. The aerogel catalyst is quite stable during operation, whereas change s in activity and selectivity are observed for the xerogel catalyst.