Design of solid acid catalysts for aqueous-phase dehydration of carbohydrates: The role of Lewis and Brønsted acid sites

► Furfural selectivity increases with the Brønsted to Lewis acid site ratio. ► Lewis sites catalyze a side reaction between xylose and furfural to form humins. ► Catalyst pore confinement has an adverse effect on furfural selectivity. ► Gas-phase characterization can predict catalytic activity in the aqueous phase.