PNNL

Abstract

The MOFs of the isoreticular IRMOF-74 series are efficient catalysts for hydrogenolysis of aromatic C-O bonds, but a detailed mechanistic understanding of the reaction has not been established. An important advantage of these MOFs is that diameter of their 1D open channels can be varied systematically, owing to the common secondary building unit (SBU) and controllable length of the hydroxy-carboxylate struts. We show that the first four members of the IRMOF-74(Mg) series are inherently catalytic for aromatic C-O bond hydrogenolysis and that the conversion varies non-monotically with pore size. The hydrogenolysis conversion of phenylethylphenyl ether (PPE), benzylphenyl ether (BPE), and diphenyl ether (DPE) varies as PPE > BPE > DPE, consistent with the strength of the C-O bond, whereas the conversion follows the trend IRMOF-74(III) > IRMOF-74(IV) > IRMOF-74(II) > IRMOF-74(I) with little change in selectivity. DFT calculations suggest the unexpected behavior is due to much stronger ether binding to the Mg(II) open metal sites of IRMOF-74(III) resulting from a structural distortion that moves the Mg2+ ions toward the interior of the pore. Solid-state 25Mg NMR data reveal low-symmetry local Mg environments in IRMOF-74(I)-Mg and indicate binding of hydrogen and substrates to the Mg(II) site. The MOF catalysts are recyclable and reusable and retain their crystallinity and framework structure after the hydrogenolysis reaction. The results suggest that both confinement and the presence of reactive metals are essential for achieving the high catalytic activity. Moreover, they challenge the notion that simply increasing MOF pore size will lead to higher conversions.