One-pot tandem conversion of furfural to γ-valerolactone over a series of modified zirconium-based metal–organic frameworks with variational Lewis and Brønsted acid sites

The one-pot tandem conversion of furfural (FUR) to γ-valerolactone (GVL) is desirable due to the high value of GVL and the efficiency of the synthesis process. While MOF-808 presented a good catalytic potential among Zr-MOFs, it lacked the necessary Brønsted acidity for the tandem conversion. In this study, Zr-based metal–organic framework (MOF-808) was modified with acids of different strengths (triflic acid, sulfuric acid, sulfamic acid, and trifluoroacetic acid) using a two-step post-synthesis impregnation method. EPR results revealed the appearance of coordinatively unsaturated sites of Zr (Zr-CUS) in the first step of HCl-treated Zr-808-HCl and their disappearance in the subsequent acid-modified catalysts, clarifying the impregnation process. These modified frameworks (Zr-808-TF, Zr-808-S, Zr-808-SN, and Zr-808-CF) were employed as catalysts for the tandem conversion of FUR to GVL. Zr 3d XPS and solid-state 31 P MAS NMR spectra of TMPO adsorbed on the catalysts proved that Zr-808-TF outperformed other catalysts in terms of both Brønsted/Lewis acid strength and ratio. With such catalytic active sites, Zr-808-TF exhibited superior catalytic activity, achieving 83% GVL yield over an 8-hour reaction period. The four acid-modified catalysts not only differed in Lewis/Brønsted acid sites, but also exhibited different catalytic performances in the tandem conversion. The in-depth insight into the relationship between the modified functional groups and the acid-base sites provides a new avenue to optimize the acid-base sites and expands the application of MOFs in cascade catalysis.