Conversion of ethyllactate into lactide over acid SnO2/SiO2 catalyst

Abstract

Polylactide is a biodegradable polymer used as an environmentally friendly packaging material. The monomer for its production is lactide obtained in industrial settings by the two-stage process. At first stage, lactic acid, in the presence of Sn-octanoate as a catalyst, is condensed into oligomer, which then is depolymerized to lactide. A significant amount of by-products is formed in this liquid-phase process. A vapor-phase method of ethyl lactate condensation into lactide can be considered as an alternative to the industrial liquid-phase process. The advantage of this single-stage method is the use of solid catalysts. This paper studies the vapor-phase transformation of ethyl lactate over supported SnO2/SiO2 catalyst at 250 °C in continuous flow mode. Ethyl lactate solution in toluene and pure ethyl lactate were used as feed stocks. It has been found that increase of concentration of ethyl lactate in toluene from 20 to 50 % leads to reduction of ethyl lactate conversion and lactide yield from 44 to 27 % and from 22 to 13 %, respectively, while lactide selectivity remains practically unchanged and makes up 47–51 %. At that, the space-time yield of lactide increases from 0.15 to 0.26 mmol/gcat/h. It has been shown that in the case of using pure ethyl lactate and carrier gas (N2) with similar catalyst load, the ethyl lactate conversion increases from 27 to 30 %, lactide selectivity – from 47 to 68 %, lactate yield – from 13 to 20 %, and catalyst productivity on lactide – from 0.26 to 0.36 mmol/gcat/h. It has been found that ethyl lactate conversion and lactide productivity on regenerated SnO2/SiO2 catalyst declines by 3–5 % compared with the initial catalyst, which is associated with change of structural parameters of the regenerated catalyst.