Sn–Al-USY for the valorization of glucose to methyl lactate: switching from hydrolytic to retro-aldol activity by alkaline ion exchange

Abstract

Lactic acid and ester derivatives, which are important precursors of a wide variety of products, can be prepared via retro-aldol sugar condensation in the presence of Sn-containing zeolites. However, the preparation of these materials is often complex and requires zeolitic structures that are difficult to obtain on a commercial scale, hindering the access to this technology at the industrial level. In the present study, a comparative investigation was conducted on the preparation of a faujasite zeolite functionalized with Sn and Al species from a commercial USY material using simple post-synthetic methods. The Sn–Al-USY zeolite displayed poor selectivity towards methyl lactate when the methanolic solutions of glucose were treated with it; however, alkaline ion-exchange switched the catalytic activity of this zeolite, enhancing its performance in retro-aldol condensation and inhibiting the dehydration pathway towards methyl levulinate. The catalytic activities of these materials have been correlated to their acid–base properties and the interaction of the supported tin species with alkali metals. Na and K-exchanged Sn–Al-USY demonstrated high catalytic activity in the transformation of glucose into methyl lactate, providing yields above 40% from glucose as well as good stability and reusability.