Vapour-phase conversion of methyl lactate into lactide over TiO2/SiO2 catalyst at the lowered pressure

Abstract

Now polylactide (PLA) widely use as biodestructive packing material. Usually monomeric lactide produce from lactic acid. Also, methyl (ethyl) lactate could be used for lactide obtaining via its vapour-phase condensation on TiO₂/SiO₂ catalyst at 260⁰C in N₂ carrier-gas flow. However, at that it is necessary to heat carrier-gas to the reaction temperature. In this communication the results on methyl lactate vapour condensation into lactide with the lowered pressure of 100-150 mbar, without carrier-gas, are presented. Supported TiO₂/SiO₂ catalyst with 5 wt.% titania content has been prepared by impregnation of silica with Ti(OC₄H₉)₄. After calcination at 500⁰C prepared catalyst has amorphous mesoporous structure with 335 m²/g surface area and 0.83 сm³/g pore volume. The catalytic experiments were performed in such way. Liquid methyl lactate was dosed into the evaporator (250⁰C) and further in the flowing reactor (260⁰C) under pressure of 100 mbar that produced by vacuum pump. Load on a catalyst was varied from 25 to 55 mmol ML/(g_cath). Cooled product was analyzed on Agilent 7820A chromatograph and NMR Bruker Avance-400 spectrometer. It was shown that TiO₂/SiO₂ catalyst provide 74% selectivity towards lactide at 50-53% methyl lactate conversion with lactide productivity of 7.8 mmol L/(g_cath) at 260⁰C/100 mbar. The catalyst stable work is more 80 h. Main impurities are methyl lactoillactate, 1-methoxyethanol and 1,1-dimethoxyethane.