H3PW12O40 encapsulation by nanoporous metal organic framework HKUST-1: Synthesis, characterization, activity and stability

Abstract

Hybrid composite material was obtained through encapsulation of H 3 PW 12 O 40 (PW) into HKUST-1 (Cu 3 (BTC) 2 , BTC = 1,3,5-benzenetricarboxylic acid), in molar composition of 5 Cu(NO 3 ) 2 · 3H 2 O /2.8 BTC/ 0.3 PW/ 0.6 CTAB by adding solutions of PW and copper salts to mixture of BTC and surfactant. The catalyst was characterized by various techniques including powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), laser particle size analyzer, Brunauer Emmett-Teller (BET). The acidity of the catalyst was measured by a potentiometric titration with n-butylamine and PW/HKUST-1 presented very strong acidic sites with Ei > 100 mV. This nano catalyst was successfully used for the synthesis of various β-keto enol ethers at 45 °C with 51-98% yield after 5-75 min. The catalyst was easily recycled and reused at least four times without significant loss of its activity (94% yield after forth run). The presence of the PW in PW/HKUST-1 and reused PW/HKUST-1 structure, eliminating any doubt about collapse of the HKUST-1 after catalytic reaction and can be followed by FT-IR, XRD and SEM techniques. Brönsted and Lewis acidity of the PW/HKUST-1 catalyst was distinguished by studying the FT-IR and determined by chemisorption of pyridine. The strength and dispersion of the protons on PW/HKUST-1 was considerably high and active surface protons became more available for reactant.

Strategies for Incorporating Catalytically Active Polyoxometalates in Metal–Organic Frameworks for Organic Transformations

“…Consisting of copper-paddlewheel nodes and benzene-1,3,5-tricarboxylate (BTC) linkers, HKUST-1 (also known as MOF-199 and Cu-BTC) and its isostructural Fe, Co, and Zr derivatives have been extensively used as POM supports. − Note that POM@HKUST-1 composites are also described in some literature as NENU- n . Due to the structure of HKUST-1 possessing only 9 and 3.5 Å cages, various Keggin POMs have been trapped within the larger cage (Figure ) to perform oxidative and acid-catalyzed transformations. − ,− In several examples, the [PW 12 O 40 ] 3– POM has been installed in HKUST-1 for esterification and hydrolysis of esters. ,, Traditional solvothermal synthesis of PW 12 @HKUST-1 yields relatively large particles, which are not ideal for catalysis. To form smaller particles, the reaction could be stopped by freezing during encapsulation and lyophilization to remove solvent .…”