(270b) Spectroscopic and Modeling Study of Aluminum Active Species in Glucose Isomerization

The need to reduce greenhouse gas emissions and our dependence on fossil fuels has prompted considerable research in the production of fuels and chemicals from lignocellulosic biomass.¹ Among the top biomass-derived chemicals, 5-hydroxymethyl furfural (HMF) has gained great interest as a platform chemical.² HMF synthesis includes the hydrolysis of cellulosic biomass to glucose, glucose isomerization to fructose, and fructose dehydration to HMF.¹ Currently, HMF production is not commercial because industrial glucose isomerization uses enzymes, which are costly and incompatible with the Brønsted acidity necessary for fructose dehydration.³ To overcome this challenge, homogeneous metal chlorides (e.g., CrCl₃, AlCl₃) have demonstrated effectiveness in direct glucose conversion to HMF in a single pot.⁴ This rather unexpected result was rationalized by tandem catalysis where the metal species catalyzes glucose isomerization via Lewis acidity and hydrolyzes to produce Brønsted acid sites (H₃O⁺) necessary for dehydration.⁴ Direct, in situ experimental evidence in support of multiple catalytic species has though been lacking. This in turn hinders our ability to develop molecular descriptors and ultimately to predict better homogeneous and heterogeneous catalysts. In this work, we investigate the effect of combining Lewis (AlCl₃) and Brønsted (HCl) acids in glucose isomerization. For the first time, we demonstrate the interplay between the acids and from a practical position, optimize the concentration of the Lewis and Brønsted acids to maximize the formation of fructose. Simultaneously, the aluminum speciation in water is explored using in situ ²⁷Al nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and inductively coupled plasma-mass spectrometry (ICP-MS), and the glucose-to-fructose isomerization is correlated with the various Al species. An AlCl₃ speciation model combined with kinetic results indicates that the hydrolyzed Al(III) complex [Al(H₂O)₄(OH)₂]¹⁺ is the active species in glucose isomerization and acts as a Lewis acid-Brønsted base bifunctional site.

References:

¹ van Putten, R. J., et al., Chem Rev 2013, 113(3), 1499-1597.

² Corma, A., et al. Chem Rev 2007, 107(6), 2411-2502.

³ Nguyen, H., Vlachos, D., et al., Acs Catal 2016, 6(3), 1497-1504.

⁴ Choudhary, V., Vlachos, D., et al., J Am Chem Soc 2013, 135 (10), 3997-4006.