Selective Production of Ethyl Lactate from Rice Straw in the Presence of Lewis and Brønsted Acids.

In this study, direct conversion of rice straw to ethyl lactate (EL) was evaluated in acidic conditions with ethanol as reaction medium. The devised Lewis acid catalyst system (SnCl₂/SnCl₄) and, Lewis/Brønsted acid catalyst system (SnCl₂/ZSM-5) played vital role in the selective production of EL. The yield of EL was optimized up to 21.8% with SnCl₂/SnCl₄ followed by 20.0% with SnCl₂/ZSM-5 at 220 °C in sub-critical ethanol (100%) for reaction time of 4 h. On contrarily, the SnCl₂/γ-Al₂O₃ catalyst system was less effective in improving the EL yield (17.1%). As Brønsted acid, the catalytic activity of heterogeneous catalyst ZSM-5 and homogeneous catalyst H₂SO₄ was compared by using these catalysts along with SnCl₂ loading. Due to catalytic stability, ZSM-5/SnCl₂ with 3:1 mass ratio has proved to be slightly more influential than H₂SO₄ in enhancing Lewis acidity of SnCl₂. Overall both Lewis and Brønsted acid systems are significantly active in selective production of EL. On the basis of catalytic activity, it can be incurred that the Brønsted acid sites mainly facilitated the degradation of holocellulose into monosaccharides and respective dehydration reactions. Strong Lewis acid sites, on the other hand, catalyzed C–C bond cleavage and rearrangement reactions during isomerization, retro-aldol and finally esterification reactions. [ABSTRACT FROM AUTHOR]