Cyclohexane oxidation over AFI molecular sieves: effects of Cr, Co incorporation and crystal size

Abstract

Heteroatom substituted AFI molecular sieves with different sizes and morphologies were synthesized under hydrothermal conditions and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), N₂ adsorption/desorption isotherm measurement, inductively coupled plasma-optical emission spectrometry (ICP-OES) and thermogravimetric analysis (TG-DTG). Uniformly sized plate-like crystals as thin as 400 nm were prepared using a seed inducing method. Cr and Co ions were introduced into the molecular sieves and some of them were transformed to the high-valence state through calcination. In the reaction of cyclohexane oxidation, bimetal substituted AlPO₄-5 exhibited higher catalytic activities than single metal substituted analogues although the doping amount is lower. The improvement of diffusion efficiency resulting from the shortening of the channels along the c-axis increased the conversion of cyclohexane significantly.