Dual-acidic Sn(IV)-based polyoxometalates for one-pot catalytic transfer hydrogenation-alcoholysis cascade reactions

A simple and highly efficient one-pot synthesis protocol for producing isopropyl levulinate (IPL) in cascade reactions of furfural (FAL) with 2-propanol was established using dual-acidic Keggin-type polyoxometalates (POMs) with adjustable acid properties. The dual-acidic POMs were synthesized via an ion-exchange strategy, where the H ions of Keggin-type heteropoly acid were partially replaced with metal chlorides. By varying the transition metal elements and Keggin units, the Lewis and Brønsted acid (LA and BA) properties of the prepared catalysts could be finely tuned, which can avoid the poor synergistic effect of traditional multi-component catalytic systems and the poor catalytic activity and selectivity caused by the lack of regulation of the LA and BA properties in the catalytic transfer hydrogenation (CTH)-alcoholysis cascade reactions. This work exhibited the highest FAL conversion of 95.4 % and IPL yield of 91.6 % over Sn_0.25H₂PW₁₂O₄₀ at 443 K for 120 min, with a low activation energy (E_a) of 32.74 kJ mol^-1, outperforming compared to traditional and commercial catalysts. The advantages of the strong LA acidity of Sn(IV) species and suitable ratio of LA-to-BA sites accelerated both the CTH and alcoholysis reaction processes. More importantly, the well synergistic effect of multiple active centers on clusters significantly played a crucial role in boosting catalytic performance. A plausible reaction mechanism was proposed based on experimental results, isotope tracing analysis, and density functional theory (DFT) calculations. The catalytic performance of Sn_0.25H₂PW₁₂O₄₀ remained unchanged after four cycles, which exhibited potential applications in one-pot cascade reactions. This study highlights the great potential of multifunctional Keggin-type POMs-based systems with multiple active centers for one-pot efficient conversion of biomass resources.