Genesis of a bi-functional acid-base site on a Cr-supported layered double hydroxide catalyst surface for one-pot synthesis of furfurals from xylose with a solid acid catalyst

A Cr-supported Mg-Al layered double hydroxide (Cr/Mg-Al LDH) comprised bi-functional Lewis acid-Bronsted base active sites on the catalyst surface at close boundaries between Cr3+ oxide and Mg-Al LDH. These enabled efficient aldose-ketose isomerization. The combined use of solid Bronsted acid Amberlyst-15 and bi-functional acid-base Cr/Mg-Al LDH exhibits higher activity for one-pot transformation of xylose into furfural via aldose-ketose isomerization and successive dehydration under mild conditions than those exhibited by basic bare Mg-Al LDH, substituted Mg-Cr LDH, and Lewis acidic Cr3+ supported catalysts with Amberlyst-15. The correlation between the activity and the Cr loading amount for Mg-Al LDH was evaluated using several characterization techniques and MPV reduction as a model reaction for Lewis acidity investigation. The results show the following: (1) the monomer of Lewis acidic Cr3+ oxide is supported on the Mg-Al LDH surface below 1 wt%. (2) Thereafter, Cr3+ species form small Lewis acidic Cr3+ oxide dimers and/or trimers up to 5 wt%. Then, they cover the Mg-Al LDH surface to generate highly active bifunctional Lewis acid-Bronsted base sites. (3) Above 5 wt%, the excess Cr3+ species generate inert Mg-Cr and/or Mg-Al-Cr LDH-like composites. They are deposited on some active sites, leading to decreased activity. The results show that furfural yields vary significantly in accordance with Cr loadings and that the 5 wt% Cr/Mg-Al LDH achieved the highest value (59% yield, 18 h) because the most effective interaction between the Lewis acidic Cr3+ oxide and the Mg-Al LDH basic site is observed at 5 wt% Cr loading.