Towards efficient synthesis of sugar alcohols from mono- and poly-saccharides: role of metals, supports & promoters

Abstract

Biomass derived sugar alcohols (xylitol, arabitol) find numerous uses in the food, oral hygiene and pharmaceutical industries. Their direct synthesis from poly-saccharides, however, still remains an immense challenge. In this study, we demonstrate in detail the effects of metals, supports and promoters in enhancing the yields of sugar alcohols from mono- and poly-saccharides. We undertook synthesis of bimetallic catalysts, M–M′/S (M, metal = Pt, Ru; M′, promoter = Sn, Ga, Fe; S, support = γ-Al₂O₃ (AL), SiO₂–Al₂O₃ (SA), carbon (C)) with varying metal loadings (Pt/Ru = 2, 3.5 wt%; Sn = 0.22, 0.43, 0.87, 1.5, 3.5 wt%; Ga/Fe = 0.25 wt%) by a co-impregnation method. The catalytic activities of these catalysts were evaluated in the synthesis of sugar alcohols from xylose (mono-saccharide) and hemicellulose (xylan, poly-saccharide) at 130–190 °C. Among all of the bimetallic catalysts, the Pt(3.5)Sn(0.43)/AL catalyst (50%) showed 2.8 times improvement in the yield of sugar alcohols compared to a monometallic Pt(3.5)/AL catalyst (18%). Similarly, in the xylose reaction a 2.4 times enhancement in the yield of sugar alcohols over Pt(3.5)Sn(0.43)/AL (79%) was observed compared to the 33% yield obtained with Pt(3.5)/AL. By conducting several experiments it is confirmed that the residual Cl⁻, which remained on the catalyst even after calcinations and reductions carried out at 400 °C, does not play any role in catalysis. The stability of the Pt(3.5)Sn(0.43)/AL catalyst confirmed by XRD and ICP analysis was responsible for achieving reproducible activity in at least 5 consecutive runs. Formation of electron deficient Sn confirmed by XPS analysis helped to polarize the carbonyl group, which in turn enhanced the sugar alcohols’ yields. Formation of PtSn and Pt₃Sn species was observed when Sn loading was more than 0.87%.