Selective Self-Condensation of Butanal over Zr-BEA Zeolites

Butanal conversion has been studied over Zr-BEA zeolites with different amount of partially hydrolyzed open Zr-sites and fully condensed closed sites. The kinetic study pointed that the main reaction pathway of butanal conversion involves its self-condensation into 2-ethyl-2-hexenal (2-EN), the selectivity to 2-EN being within 80–85 %. Besides, small amounts of 1-butanol and butanoic acid are formed via Cannizzaro disproportionation of butanal. Secondary reactions include butanoic acid ketonization to 4-heptanone and esterification to butyl butanoate and tandem hydrogenation/dehydration of 2-EN into octadienes. A number of techniques, including FTIR spectroscopy of adsorbed CO, pyridine, 2,6-di-tert-butylpyridine (DTBPy), TPD of isopropylamine and selective poisoning with DTBPy were applied for characterization of Zr-sites and elucidation of their activity. Open Zr-sites were found to be more active with respect to closed Zr-sites in butanal condensation: the TOF values estimated at 433 K being 1.4 s⁻¹ for open Zr-sites and 0.6 s⁻¹ for closed sites. On the contrary, selectivity into 2-EN was similar for open and closed sites. The results point that Zr-BEA catalysts with enhanced content of open sites are perspective catalysts for the selective synthesis of 2-EN.