Investigation of solid catalysts for glycolysis of polyethylene terephthalate

Three series of solid catalysts including SO42−/ZnO, SO42−/TiO2 and SO42−/ZnO-TiO2 were prepared by precipitation or co-precipitation method with different calcination temperature. The textural properties of the catalysts were studied by means of XRD, Raman, SEM, N2 adsorption–desorption and energy dispersive X-ray analysis. Acidity properties of the catalysts were tested by NH3–TPD and pyridine–FTIR methods. Glycolysis of Polyethylene terephthalate (PET) into monomer of bis(2-hydroxyethyl terephthalate) (BHET) over solid acid catalysts has been studied. Relationships between the textural properties, the surface acidity and the catalytic activity have been investigated. Experimental results showed that the surface area and the amount of middle strength acid sites on the sulfated binary oxides of SO42−/ZnO-TiO2-200–300 °C were higher than those on unitary oxide of zinc or titanium. The FTIR study showed that the Lewis acid sites predominated over the SO42−/ZnO-TiO2-300 °C catalyst, while the peak corresponding to Brönsted acid sites was weak or absent. The SO42−/ZnO-TiO2-200–300 °C exhibited a catalytic activity with 100% conversion of PET and 72% selectivity of BHET after 3 h at 180 °C under atmospheric pressure. The catalyst has been reused for four times and the catalytic activity of recycled catalyst maintained the same as that of the fresh catalyst.