Production of Methyl Lactate with Sn-USY and Sn-β: Insights into Real Hemicellulose Valorization

Abstract

Potassium exchanged Sn-β and Sn-USY zeolites have been tested for the transformation of various aldoses (hexoses and pentoses), exhibiting outstanding catalytic activity and selectivity toward methyl lactate. Insights into the transformation pathways using reaction intermediates-dihydroxyacetone and glycolaldehyde-as substrates revealed a very high catalytic proficiency of both zeolites in aldol and retro-aldol reactions, showcasing their ability to convert small sugars into large sugars, and vice versa. This feature makes the studied Sn-zeolites outstanding catalysts for the transformation of a wide variety of sugars into a limited range of commercially valuable alkyl lactates and derivatives. [K]Sn-β proved to be superior to [K]Sn-USY in terms of shape selectivity, exerting tight control on the distribution of produced α-hydroxy methyl esters. This shape selectivity was evident in the transformation of several complex sugar mixtures emulating different hemicelluloses-sugar cane bagasse, Scots pine, and white birch-that, despite showing very different sugar compositions, were almost exclusively converted into methyl lactate and methyl vinyl glycolate in very similar proportions. Moreover, the conversion of a real hemicellulose hydrolysate obtained from Scots pine through a simple GVL-based organosolv process confirmed the high activity and selectivity of [K]Sn-β in the studied transformation, opening new pathways for the chemical valorization of this plentiful, but underutilized, sugar feedstock.

Figure 1

Product distribution obtained with [K]Sn-USY and [K]Sn-β catalyst in the transformation of hemicellulose monosaccharides (A, glucose; B, mannose; C, xylose, and D, arabinose) in methanolic media. Reaction conditions: monosaccharide concentration = 48 g·L^–1; catalyst loading = 0.75 g; reaction volume = 75 mL; 150 °C; 13 bar (autogenous pressure).

Figure 2

Product distribution obtained with the [K]Sn-USY and [K]Sn-β catalyst in the transformation of hemicellulose monosaccharides (A, glucose; B, mannose; C, xylose and D, arabinose) in methanol/water (96:4 wt %) media. Reaction conditions: monosaccharide concentration = 48 g·L^–1; catalyst loading = 0.75 g; reaction volume = 75 mL; 150 °C; 13 bar (autogenous pressure).

Figure 3

Product distributions obtained with [K]Sn-USY and [K]Sn-β catalyst (fresh catalyst, left; first reuse, right) in the transformation of reaction intermediates (A, dihydroxyacetone (C3); B, glycolaldehyde (C2), and C, dihydroxyacetone and glycolaldehyde equimolar mixture) in methanol/water (96:4 wt %) media. Reaction conditions: dihydroxyacetone concentration = 28.8 g·L^–1; glycolaldehyde concentration = 19.2 g·L^–1; catalyst loading = 0.75 g; reaction volume = 75 mL; 150 °C; 13 bar (autogenous pressure).

Figure 4

Starting composition and product distribution obtained in the transformation of several sugar mixtures representing hemicelluloses from Scots pine, white birch, and sugar cane bagasse in the presence of [K]Sn-β. Reaction conditions: initial total sugar loading: 48 g·L^–1; catalyst loading = 0.75 g; reaction volume = 75 mL; 150 °C.

Figure 5

Product distribution obtained from the transformation of methanolic solutions of GVL-organosolv Scots pine hemicellulose in the presence of [K]Sn-β. Reaction conditions: initial total sugar loading: 48 g·L^–1; catalyst loading = 0.75 g; reaction volume = 75 mL; 150 °C. The composition of the starting sugar mixture is listed in Table S2 (Supporting Information).