Synthesis of phosphotungstic acid-supported versatile metal–organic framework PTA@MIL-101(Fe)–NH2–Cl

Abstract

The catalyst PTA@MIL-101(Fe)–NH₂–Cl was synthesized in various solvents to investigate the effect of solvents on the catalyst crystal structure. PTA@MIL-101(Fe)–NH₂–Cl can only be obtained in DMF because of its excellent ability to deprotonate the organic ligands and promotion to yield MIL-101 crystals by forming a Cl–H–DMF complex. The water-containing solvents were unfavorable to yield MIL-101 crystals since MIL-101 crystals reorganized and other crystals were formed in this kind of solvents. The metal cation concentration kinetically controlled the crystal structure. MIL-101 crystals were fabricated only at low metal cation concentration. In the DMF synthesis system, the supported phosphotungstic acid (PTA) retained its Keggin structure. The acidity dramatically decreased compared to PTA caused by the double electrostatic interaction of amino groups and positively charged exterior surface of carrier MIL-101(Fe)–NH₂–Cl. The roles of both amino groups as PTA immobilization sites and chlorine groups as substrate absorption sites were confirmed in starch hydrolysis catalyzed by PTA@MIL-101(Fe)–NH₂–Cl.