Elucidating the Collaborative Roles of Linker Functional Groups and Paired Bridging Methoxy Ligands on Al12 Ring Nodes of Metal-Organic Framework CAU-3 for Methanol Dehydration.

Metal-organic frameworks (MOFs) featuring aluminum oxo ring nodes are rare, including CAU-1 with Al8(OH)4(OCH3)8 and CAU-3 with Al12(OCH3)24 nodes. Previous studies on CAU-1 remain limited due to restricted linker options. Herein, we report the results of CAU-3 incorporating Al12 ring nodes coordinated by 1,4-benzenedicarboxylate (BDC) linkers and linkers bearing tunable active functional groups, including -NH2 and -OH. Surprisingly, only approximately half of the paired methoxy ligands (12 out of 24) on the Al12(OCH3)24 nodes are reactive upon water exposure during synthesis and posttreatment, converting to paired μ2-OH ligands. CAU-3 with unsubstituted BDC linkers exhibits very low catalytic activity for methanol dehydration to dimethyl ether; however, the activity can be dramatically enhanced by introducing linker function groups. Specifically, -NH2 ligands increase the activity by a factor of ∼3, while -OH groups lead to more than an order-of-magnitude increase, attributable to their stronger hydrogen-bond interactions with methanol. By combining catalytic results and density functional theory (DFT) calculations, we attribute the catalytic sites in CAU-3 for methanol dehydration to paired node sites bearing bridging methoxy ligands that collaborate with linker functional groups through hydrogen-bonded methanol molecules. These findings provide experimental and computational confirmation of mechanistic inferences previously proposed for CAU-1.