Calcined Mg–Al, Mg–Cr and Zn–Al hydrotalcite catalysts for tert-butylation of phenol with iso-butanol—a comparative study

Abstract

Calcined hydrotalcites (CHTs) are studied for the tert-butylation of phenol using iso-butanol in the temperatures ranging from 350 to 500 °C. The major products of this reaction on calcined magnesium–aluminium hydrotalcites (CMA-HTs) are O-tert-butyl phenol (tert-butyl phenyl ether, OTBP) and 2-tert-butyl phenol (o-tert-butyl phenol, 2TBP) with O-butenyl phenol (butenyl phenyl ether, OBP) and 2-butenyl phenol (o-butenyl phenol, 2BP) as useful by-products. With a view to understand the reaction mechanism and the reaction path, tert-butylation of phenol is studied by changing both Mg[M(II)] and Al[M(III)] ions with Zn2+ and Cr3+, respectively. Thus, Mg–Al (MA), Mg–Cr (MC) and Zn–Al (ZA) hydrotalcites (with M2+:M3+ ratio=2) are prepared and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic absorption spectroscopy (AAS), differential thermal analysis/thermo gravimetric analysis (DTA/TG), BET-surface area (BET-SA) and acidity–basicity measurements (temperature-programmed desorption (TPD) of NH3 and CO2). Mixed oxides obtained from CHTs are found to be more active over their individual oxides for the tert-butylation of phenol. The butylation activities of the three different CHTs are in the order CMA-HT>CZA-HT>CMC-HT. A different product distribution is obtained over calcined Mg–Cr and Zn–Al hydrotalcites for the tert-butylation of phenol showing the influence of acid–base properties on the activity of the catalysts. A probable mechanism based on the experimental observations has been proposed in order to explain the formation of butenyl phenols.