Catalytic activity of SnO2- and SO4/SnO2-containing clinoptilolite in the esterification of levulinic acid

Catalysts based on natural zeolite – clinoptilolite loaded with either SnO2 (TOHCLI) or sulfated SnO2 (STOHCLI) were prepared and tested in the esterification of levulinic acid (LA) with octanol or ethanol. The Sn content in TOHCLI and STOHCLI varied from 4.5 to 12.3 wt.%. The catalysts were characterized by powder X-ray diffraction method, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, thermal analysis, X-ray photoelectron spectroscopy, N2 physisorption at −196 °C, 27Al and 29Si MAS NMR solid state spectroscopy and FTIR spectroscopy for analysis of acidic centers. A high conversion rate of LA into octyl- (OLA) or ethyl levulinate (ELA) was obtained for both TOHCLI and STOHCLI. TOHCLI showed a high activity in the conversion of LA into OLA (55%) and a moderate activity in the conversion to ELA (22%). STOHCLI led to a total conversion of LA to OLA and ELA due to the presence of a high amount of Brønsted and Lewis acid sites in the catalysts. The catalytic activity decreased to 86% for OLA and to 66% for ELA after next five cycles. Lower catalytic activity in the repeated cycles during ELA formation was explained by pore blockage due to coke formation.