Solid base catalysed 5-HMF oxidation to 2,5-FDCA over Au/hydrotalcites: fact or fiction?

Abstract

Nanoparticulate gold has emerged as a promising catalyst for diverse mild and efficient selective aerobic oxidations. However, the mechanism of such atom-economical transformations, and synergy with functional supports, remains poorly understood. Alkali-free Mg-Al hydrotalcites are excellent solid base catalysts for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furan dicarboxylic acid (FDCA), but only in concert with high concentrations of metallic gold nanoparticles. In the absence of soluble base, competitive adsorption between strongly-bound HMF and reactively-formed oxidation intermediates site-blocks gold. Aqueous NaOH dramatically promotes solution phase HMF activation, liberating free gold sites able to activate the alcohol function within the metastable 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) reactive intermediate. Synergistic effects between moderate strength base sites within alkali-free hydrotalcites and high gold surface concentrations can afford highly selective and entirely heterogeneous catalysts for aqueous phase aldehyde and alcohol cascade oxidations pertinent to biomass transformation.

Fig. 1. In situ Au L_III XANES during thermal processing of the HAuCl₄/Mg–Al HT precursor: (a) thermal evolution of normalised XANES spectra; (b) representative least squares fitted XANES spectra to reference gold species; (c) quantitative thermal evolution of fitted Au species.

Fig. 2. pH dependence of HMF oxidation over a 2 wt% Au/HT catalyst after 7 h reaction, and possible mechanism for surface-initiated HMFCA at pH 9, and solution phase activation at higher pH.

Scheme 1. Impact of NaOH on kinetics of HMF oxidation over 2 wt% Au/HT.

Fig. 3. Operando Au L_III-edge XAS of a 2 wt% Au/HT catalyst during aqueous phase selective aerobic oxidation of HMF; catalytically active, metallic gold nanoparticles are unaffected by hot water or NaOH addition.

Fig. 4. (main) Impact of Au loading on the sensitivity of individual oxidation steps towards soluble base addition over Au/HT catalysts; (inset) Au loading dependent product selectivity in HMF oxidation in the absence of soluble base.

Fig. 5. (main) Disparate evolution of FDCA yield as a function of either HMF or HMFCA conversion over 25 mg () 0.5 wt%, () 1 wt%, () 2 wt%, () 5 wt%, and () 10 wt% Au/HT catalysts, and 50 mg 2 wt% Au/HT; (inset) normalised FDCA productivity per Au atom as a function of HMF : surface Au molar ratio for 2 wt% Au/HT highlighting self-poisoning by high HMF concentrations.