Synthesis, characterization and catalytic oxidation of alcohols with chromium-substituted aluminophosphates

Abstract

Chromium-substituted aluminophosphate-5 and aluminophosphate-11 (CrAPO-5 and CrAPO-11) have been hydrothermally synthesized and characterized by XRD, scanning electron microscopy, n-butane and N₂ absorption, IR spectroscopy of [²H₃]acetonitrile adsorption and diffuse reflectance spectroscopy, showing that they are pure and possess high crystallinities and pore volumes. Calcined samples contain both strong Lewis-acid (Cr^VI) and weak Brønsted-acid (P—OH) sites, and the Cr^VI species is bonded to the internal AlPO₄-5 framework. CrAPO-5 and CrAPO-11 are selective and recyclable solid catalysts for the oxidation of benzylic and cyclic alcohols with O₂ or tert-butyl hydroperoxide (TBHP) as oxidant. It is proposed that the CrAPO-5-catalysed oxidation of alcohols with TBHP in the absence of O₂ proceeds via a heterolytic mechanism involving β-hydrogen elimination from an oxochromium(VI) alkoxide followed by reoxidation of the reduced Cr^IV by TBHP, giving high selectivities with respect both to substrate (>89%) and TBHP (>88%). The CrAPO-5-catalysed oxidation of alcohols with O₂ gives the corresponding ketones in selectivities of 72–97% at conversions of 26–78% when only 10 mol% of TBHP is added. The mechanism of the CrAPO-catalysed oxidations with O₂ and the changes in CrAPO-5 and CrAPO-11 structures after oxidation are discussed. Evidence is provided in support of the alcohol oxidation essentially taking place within the micropores of CrAPO-5.