Vapour phase isopropylation of phenol over zinc- and iron-containing Al-MCM-41 molecular sieves

Abstract

Al-MCM-41 molecular sieves containing zinc and iron are hydrothermally synthesised. The low angle X-ray powder diffraction (XRD) analysis shows that Zn and Fe incorporated Al-MCM-41 retains the hexagonal structure. The higher d-spacing values of Zn- and Fe-Al-MCM-41 catalysts than that of Al-MCM-41 indicate the incorporation of Zn and Fe into the framework. The mesoporous nature of the materials was confirmed by nitrogen adsorption isotherms. Electron paramagnetic resonance (EPR) and diffuse reflectance spectrum (DRS) techniques confirm the tetrahedral co-ordination of iron into the framework. The total acidity of the synthesised catalysts was analysed by both TPD of ammonia and of adsorbed pyridine FT-IR spectroscopy. Acidity measurements indicate that Zn and Fe incorporation increases both Lewis and Brønsted acidity of the catalysts, due to strengthening of Brønsted acid sites by appropriately positioned Zn 2+ and Fe 3+ Lewis acid metals. Vapour phase isopropylation of phenol with a new alkylating agent isopropyl acetate (IPA) was carried out over the H-forms of the above catalysts. On comparison, the Zn and Fe incorporated Al-MCM-41 catalysts show significantly higher phenol conversion and selectivity towards the important product 4-isopropyl phenol (4-IPP). Further, the undesired and dialkylated products selectivity are found to be lower over Zn and Fe incorporated Al-MCM-41 than pure Al-MCM-41 catalysts.