Crystal Structure Directed Catalysis by Aluminum Metal-Organic Framework: Mechanistic Insight into the Role of Coordination of Al Sites and Entrance Size of Catalytic Pocket

The use of metal-organic frameworks (MOFs) in the field of catalysis is growing exponentially due to their high surface area and distinctive active sites. In this work, we report a novel understanding of the active sites responsible for the catalytic activity of aluminum trimesate MOFs and their crystal/framework structure dependency. Here, we have studied the nature of the active sites of Al-MOFs with two different framework structures (MIL-100 and MIL-96). We found that the MOFs with MIL-100 framework structures were highly catalytically active, while the same Al-MOFs with MIL-96 framework structures exhibited poor catalytic activity. This behavior is explained based on the effect of coordinated water molecules on their Brønsted acidity as well as the effect of the coordination of Al sites, specifically hexacoordinated Al 3+ 6c sites and pentacoordinated Al 3+ 5c sites, on their Lewis acidity. The different entrance sizes of the catalytic pocket of MIL-96 and MIL-100 also played critical roles in their catalytic performance.