Zn- and Ti-Modified Hydrotalcites for Transesterification of Dimethyl Terephthalate with Ethylene Glycol: Effect of the Metal Oxide and Catalyst Synthesis Method

Abstract

The activity and selectivity of hydrotalcites (HTs) can be suitably enhanced by the addition of different metal oxides. Zinc and titanium are prospective candidates for such a modification. Transesterification of dimethyl terephthalate (DMT) with ethylene glycol (EG) using basic catalysts is an industrially important process for the production of bis(2-hydroxyethyl)terephthalate (BHET). BHET is a precursor for polyethylene terephthalate (PET) which is used in production of films, fibers, and molding materials. As against use of polluting liquid bases, solid bases could be employed. In the current work, transesterification of DMT with EG was studied over modified HT base catalysts wherein the HT was activated with the addition of zinc and titanium. These catalysts were prepared by the combustion synthesis using different fuels. The modified HT using Zn and Ti were well characterized by scanning electron microscopy, energy-dispersive X-ray spectrometry, Brunauer–Emmett–Teller surface area analyzer, temperature-programmed desorption, and X-ray diffraction. Effects of several parameters on the rate of reaction and conversion of the limiting reagent were investigated. Zinc-modified HT using glycine as fuel (Zn–HT–glycine) was found to be the most selective, active, and reusable catalyst. The Langmuir–Hinshelwood–Hougen–Watson model was used to establish the reaction mechanism and kinetics. All species were weakly adsorbed leading to a second-order kinetics. Using a mole ratio of 1:2 of DMT to EG and 0.05 g/cm3 Zn–HT–glycine loading resulted in to 64.1% conversion of DMT and 96.1% selectivity to BHET in 4 h at 180 °C. The apparent activation energy was 9.64 kcal/mol. The catalyst was robust and reusable.