Catalytic Furfural/5-Hydroxymethyl Furfural Oxidation to Furoic Acid/Furan-2,5-dicarboxylic Acid with H2 Production Using Alkaline Water as the Formal Oxidant.

Sayan Kar,Yehoshoa Ben-David,Quan-Quan Zhou,David Milstein

doi:10.1021/jacs.1c10908

Sayan Kar, Yehoshoa Ben-David + Show 2 more

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https://doi.org/10.1021/jacs.1c10908

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Abstract

Furfural and 5-hydroxymethyl furfural (HMF) are abundantly available biomass-derived renewable chemical feedstocks, and their oxidation to furoic acid and furan-2,5-dicarboxylic acid (FDCA), respectively, is a research area with huge prospective applications in food, cosmetics, optics, and renewable polymer industries. Water-based oxidation of furfural/HMF is a lucrative approach for simultaneous generation of H2 and furoic acid/FDCA. However, this process is currently limited to (photo)electrochemical methods that can be challenging to control, improve, and scale up. Herein, we report well-defined ruthenium pincer catalysts for direct homogeneous oxidation of furfural/HMF to furoic acid/FDCA, using alkaline water as the formal oxidant while producing pure H2 as the reaction byproduct. Mechanistic studies indicate that the ruthenium complex not only catalyzes the aqueous oxidation but also actively suppresses background decomposition by facilitating initial Tishchenko coupling of substrates, which is crucial for reaction selectivity. With further improvement, this process can be used in scaled-up facilities for a simultaneous renewable building block and fuel production.

Highlights

Owing to the negative consequences of fossil fuel use, intensive research is ongoing, focusing on transitioning toward a renewable framework for fuel and materials production.[1−5] Furfural and 5-hydroxymethyl furfural (HMF) are chemical feedstocks produced by hydrolysis of biomass waste.[6−8] Because of their renewable nature, the synthesis of commodity chemicals from furfural and HMF has garnered increasing attention.[9−12] Among many products obtainable from furfural and HMF, their oxidation products, furoic acid and furan dicarboxylic acid (FDCA), respectively, hold particular interest (Figure 1A)
Attempts toward aqueous oxidation of furfural at 135 °C by complex 1 (1 mol %) in 1,4-dioxane/ alkaline water resulted in complete decomposition of furfural with no generation of H2 or furoic acid
We report here molecular catalysts for the direct catalytic oxidation of furfural and HMF to furoic acid and FDCA, respectively, using alkaline water as the formal oxidant

Summary

■ INTRODUCTION

Owing to the negative consequences of fossil fuel use, intensive research is ongoing, focusing on transitioning toward a renewable framework for fuel and materials production.[1−5] Furfural and 5-hydroxymethyl furfural (HMF) are chemical feedstocks produced by hydrolysis of biomass waste.[6−8] Because of their renewable nature, the synthesis of commodity chemicals from furfural and HMF has garnered increasing attention.[9−12] Among many products obtainable from furfural and HMF, their oxidation products, furoic acid and furan dicarboxylic acid (FDCA), respectively, hold particular interest (Figure 1A). When furfural was heated at 135 °C in 1,4-dioxane in the presence of complex 9 (1 mol %) for 3 h, formation of furfuryl furoate in 65% yield was observed, signifying that complex 9 can catalyze the Tishchenko coupling of furfural (Figure 5B).[51,56] The resulting ester, under the reaction conditions of Table S1 is hydrolyzed to furoate salt and furfuryl alcohol (Figure 5C) This alternative disproportionation pathway via Tishchenko coupling followed by base-mediated hydrolysis leads to quick consumption of all furfural/HMF before the onset of decomposition and is crucial for the observed high oxidation selectivity with catalyst 6 or 9. After 68 h of reaction at 150 °C, 1.3 g of furoic acid (77% yield) was isolated from the reaction, along with 316 mL of H2 collected (87% yield), demonstrating the scalability of the process

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES