Abstract
The synthesis of heterogeneous cooperative catalysts in which two or more catalytically active components are spatially separated within a single material has generated considerable research efforts. The multiple functionalities of catalysts can significantly improve the efficiency of existing organic chemical transformations. Herein, we introduce ruthenium (Ru) nanoparticles (NPs) on the surfaces of a metal–organic framework pre-encapsulated with polyoxometalate silicotungstic acid (SiW) UiO−66 (University of Oslo [UiO]) and prepared a 2.0% Ru/11.7% SiW@UiO−66 porous hybrid using the impregnation method. The close synergistic effect of metal Ru NPs, SiW, and UiO-66 endow 2.0% Ru/11.7% SiW@UiO-66 with increased activity and stability for complete methyl levulinate (ML) conversion and exclusive γ-valerolactone (GVL) selectivity at mild conditions of 80°C and at a H2 pressure of 0.5 MPa. Effectively, this serves as a model reaction for the upgrading of biomass and outperforms the performances of the constituent parts and that of the physical mixture (SiW + Ru/UiO−66). The highly dispersed Ru NPs act as active centers for hydrogenation, while the SiW molecules possess Brønsted acidic sites that cooperatively promote the subsequent lactonization of MHV to generate GVL, and the UiO−66 crystal accelerates the mass transportation facilitated by its own porous structure with a large surface area.
Highlights
Metal–organic frameworks (MOFs) are an emerging class of advanced functional materials that have generated tremendous research interest by virtue of their fascinating properties, such as the well-defined crystalline structures, large surface areas, tunable pore cavities, and abundant unsaturated metal sites in their scaffolds (Ma et al, 2009; Stock and Biswas, 2012; Furukawa et al, 2013; Chen et al, 2017; Jiao et al, 2017)
We reported the improved catalytic activity of Ru NPs supported on the acidified MOFs for the transformation of methyl levulinate (ML) to GVL (Lin et al, 2017, 2018)
ICP–AES analyses revealed that the Ru and SiW contents within Ru/SiW@UiO−66 were 2.0 and 11.7% by weight, respectively (Table 1). These were very close to the nominal amounts added during the catalyst preparation procedure, implying that the in-situ encapsulation of SiW and subsequent immobilization of Ru NPs within the MOF was a feasible technique for catalyst preparation
Summary
Metal–organic frameworks (MOFs) are an emerging class of advanced functional materials that have generated tremendous research interest by virtue of their fascinating properties, such as the well-defined crystalline structures, large surface areas, tunable pore cavities, and abundant unsaturated metal sites in their scaffolds (Ma et al, 2009; Stock and Biswas, 2012; Furukawa et al, 2013; Chen et al, 2017; Jiao et al, 2017). While the SiW molecules which possess Brønsted acidic sites, which are encapsulated within the cavities of UiO−66, promote the subsequent lactonization of MHV to generate GVL owing to their excellent dealcoholization properties.
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