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