Elucidating the Collaborative Roles of Linker Functional Groups and Paired Bridging Methoxy Ligands on Al ₁₂ 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.