Glucose-to-Fructose Isomerization: Optimisation and Mechanistic Insights Using Cheap Polyaluminium Chloride Catalyst

The catalytic isomerization of glucose to fructose is a pivotal step in the valorisation of lignocellulosic biomass. In this study, polyaluminium chloride (PAC), a low-cost, industrially available material characterised by polynuclear aluminium-oxo species, is investigated for the first time as an alternative catalyst for this transformation. The catalytic performance of PAC was systematically investigated by varying the temperature (70-130 °C), solvent (pure water, H₂O:MeOH 1:1 or 4:1) and reaction time (30-120 min). A fructose yield of up to 55% with a selectivity of 85% was obtained by using PAC at 120 °C in H₂O:MeOH 1:1 for 120 min, confirming its effectiveness in promoting glucose isomerization to fructose. Mechanistic insights and quantitative monitoring were achieved using benchtop NMR spectroscopy. ²⁷Al-NMR of PAC in aqueous solution exhibits two main signals at 1.12 ppm (attributed to the hexacoordinated Al complex) and at 63.3 ppm (associated with a tetrahedral Al centre typical of the Al₁₃ Keggin-type cluster). With the increase in temperature, as well as by changing the reaction media from pure aqueous to a mixed aquo-alcoholic system, new Al species were generated that are more reactive than the starting AlCl₃∙6H₂O and PAC. Overall, this work demonstrates that PAC represents a viable, scalable, and more sustainable alternative to conventional aluminium-based catalysts, offering a promising route toward more efficient biomass conversion processes.