Synergy effect between solid acid catalysts and concentrated carboxylic acids solutions for efficient furfural production from xylose

• Concentrated solutions of acetic acid combined with solid acid catalysts are efficient systems for furfural formation.
• Superficial Lewis acid sites operate in cooperation with the homogeneous weak Brønsted acidity of acetic acid solutions.
• Addition of NbOH to the carboxylic acid solution was shown to enhance the furfural stability.
• Among a panel of solid acid catalysts, NbOH is active, selective and stable.

An efficient furfural formation from xylose was demonstrated combining a concentrated aqueous solution of acetic acid and solid acid catalysts. Higher furfural yields and selectivities were obtained by comparison to the catalytic performances obtained in pure water. The evident synergy effect observed at 150 °C between the aqueous carboxylic acid solution and the solid acid catalysts is tentatively explained by the occurrence of two phenomena: 1) the contribution of Lewis acid sites which would operate in cooperation with the homogeneous weak Brønsted acidity brought by the aqueous acetic acid solution. According to the literature, the two steps mechanism involving the xylose-xylulose isomerization over Lewis acid sites and the successive Brønsted acid catalyzed cyclodehydration to furfural would be the prevailing reaction pathway in the heterogeneous–homogenous catalytic system at 150 °C. 2) an enhancement of the surface solid acid coverage by the carbohydrate and furfural owing to the presence of carboxylic acid in the aqueous solution as shown by comparative liquid phase adsorption experiments done in pure water and in aqueous acetic acid solutions. Among a series of solid acid catalysts, ZrW, Cs2HPW12O40, HY (Si/Al = 15), K10 and NbOH, the latter one, NbOH used non-calcinated was shown to be active, selective and stable in the aqueous acetic acid media. HY and K10 are as active and selective for furfural formation but suffer for a strong Al leaching which precludes their utilization as true solid acid catalyst in acetic acid media.

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