Generation of Strong Basicity in Metal–Organic Frameworks: How Do Coordination Solvents Matter?

Abstract

Solid strong bases with an ordered pore structure (OPS-SSBs) have attracted much attention because of their high catalytic activity and shape selectivity as heterogeneous catalysts in various reactions. Nevertheless, high temperatures are required to fabricate OPS-SSBs by using traditional methods. Herein, we report for the first time that the coordination solvents affect basicity generation in metal−organic frameworks (MOFs) greatly and that strong basicity can be formed at comparatively low temperatures. A typical MOF, MIL-53, was employed, and three different solvents, namely, water, methanol, and N,N-dimethylformamide (DMF), were coordinated, respectively, by means of solvent exchange. Thermogravimetry-mass spectrometer analysis shows that the conversion temperature of base precursor KNO 3 is quite different on MIL-53 coordinated with different solvents. The conversion of KNO 3 to basic sites takes place at 350, 300, and 250 °C on MIL-53 coordinated with water, methanol, and DMF, respectively. It is fascinating to observe the generation temperature of strongly basic sites at 250 °C, which is noticeably lower than that on various supports, such as mesoporous silica SBA-15 (600 °C), zeolite Y (700 °C), and metal oxide ZrO 2 (730 °C). This is due to the redox interaction between coordination solvents and KNO 3 , leading to a significant decrease in the temperature for KNO 3 conversion. Consequently, OPS-SSBs were prepared successfully with an ordered pore structure and strong basicity. The obtained OPS-SSBs show good shape selectivity in Knoevenagel condensation of aromatic aldehydes with different active methylene compounds. Moreover, these solid bases are highly active in the synthesis of dimethyl carbonate through transesterification reaction. This work might open up a new avenue for the fabrication of various functional materials at low temperatures through redox interactions.

Catalytically Stable Potassium Single‐Atom Solid Superbases

“…The precursor KNO 3 /G gives an IR band at 1384 cm À 1 ascribed to NÀ O vibration of nitrate (Supporting Information, Figure S1). [9] After treated at 400 °C, the IR band of nitrate disappears and the spectrum is similar to the support graphene, indicating the successful conversion of KNO 3 . The N1s X-ray photoelectron spectrum (XPS) exhibit a peak derived from nitrate at the binding energy (BE) of 407 eV for KNO 3 /G, which is absent in K 2 O/G and K 1 /G (Figure 1c).…”

“…The conversion of KNO 3 is also verified by infrared (IR) spectra. The precursor KNO 3 /G gives an IR band at 1384 cm −1 ascribed to N−O vibration of nitrate (Supporting Information, Figure S1) [9] . After treated at 400 °C, the IR band of nitrate disappears and the spectrum is similar to the support graphene, indicating the successful conversion of KNO 3 .…”