Chemistry, Physics and Technology of Surface

ISSN 2518-1238 (Online), ISSN 2079-1704 (Print)

The study is directed to the search of effective catalysts for ethyl lactate obtaining from fructose as renewable raw material. A series of SnO₂-containing oxides for transformation of 13 % fructose solution in 98 % ethanol was prepared by impregnation of different carriers. The reaction was carried out in autoclaves while rotating at a speed of 60 rpm for 3 h at 160 °C. The products of fructose conversion were analyzed using ¹³C NMR spectroscopy. The textural characteristics, strength and concentration of acid and base sites of the synthesized mixed oxides are given. All samples are characterized by a developed surface – 90–380 m²/g, a sufficiently large pore volume –     0.5–0.95 cm³/g, and a wide distribution of pores diameter – 10–20 nm. It has been shown that the acidic oxides SnO₂ and SnO₂/SiO₂ dehydrate fructose to 5-hydroxymethylfurfural, levulinic and formic acids and their esters. The basic SnO₂/MgO-ZrO₂ mixed oxide promotes the conversion of fructose towards the formation of 1-hydroxy-2-butanones, hydroxyacetone, ethyl propionate and ethyl formate. SnO₂/ZrO₂-TiO₂ samples catalyze the dehydration of fructose to 5-hydroxymethyl furfural with 77-90 % selectivity. The addition of SnO₂ to Al₂O₃ leads to a significant increase in the yield of the target product - ethyl lactate. Thus, the 20SnO₂/Al₂O₃ catalyst provides 97 % fructose conversion with a 49 % yield of ethyl lactate. It has been found that the addition of zinc ions to SnO₂/Al₂O₃ leads to the formation on the surface of a mixed oxide of a weak basic sites with H_0max =+7.2. Thus on 10SnO₂-5ZnO/Al₂O₃ catalyst the selectivity of ethyl lactate formation increases to 56 % at 100 % fructose conversion. The schemes for fructose conversion to ethyl lactate at acid L-centers of Sn ⁴⁺ are discussed. The obtained results show that namely acid ^IVSn⁴⁺ L-sites catalyze aldol decondensation of fructose as the first stage of the ethyl lactate formation as well as initiate the isomerization of hemiacetal into ethyl lactate.

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