IRMOF-74(n)-Mg: a novel catalyst series for hydrogen activation and hydrogenolysis of C-O bonds

Abstract

Metal-Organic Frameworks (MOFs) that catalyze hydrogenolysis reactions are rare and there is little understanding of how the MOF, hydrogen, and substrate molecules interact. In this regard, the isoreticular IRMOF-74 series, two of which are known catalysts for hydrogenolysis of aromatic C-O bonds, provides an unusual opportunity for systematic probing of these reactions. The diameter of the 1D open channels can be varied within a common topology 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-monotonically with pore size. These catalysts are recyclable and reusable, retaining their crystallinity and framework structure after the hydrogenolysis reaction. 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. Counterintuitively, however, the conversion also follows the trend IRMOF-74(III) > IRMOF-74(IV) > IRMOF-74(II) > IRMOF-74(I), with little variation in the corresponding selectivity. DFT calculations suggest the unexpected behavior is due to much stronger ether and phenol binding to the Mg(ii) open metal sites (OMS) 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 indicate that both H₂ and ether molecules interact with the Mg(ii) OMS and hydrogen-deuterium exchange reactions show that these MOFs activate dihydrogen bonds. The results suggest that both confinement and the presence of reactive metals are essential for achieving the high catalytic activity, but that subtle variations in pore structure can significantly affect the catalysis. Moreover, they challenge the notion that simply increasing MOF pore size within a constant topology will lead to higher conversions.

Scheme 1. Reactions catalyzed by IRMOF-74(I–IV)Mg, with computed gas-phase Gibbs free energies at 393 K (ref. 47).

Fig. 1. Conversion of aryl ethers by IRMOF-74 catalysts. Left: activated MOFs. Center: TiCl_x-infiltrated MOFs. Right: Ni-infiltrated MOFs.

Fig. 2. H–D isotope exchange experiment showing the formation of HD (m/z = 3) in the PPE/p-xylene/hydrogen reaction mixture in the presence of IRMOF-74(I)–Mg (p = 0.1 MPa). The data are qualitative as no calibration of the relative amounts for the different species were performed.

Fig. 3. Left: ²⁵Mg NMR spectra of IRMOF-74(I) catalyst with p-xylene and PPE under H_2(g) pressure, the catalyst with p-xylene and PPE with no H_2(g), the difference spectrum and the powder lineshape of the best fit of the difference (red) calculated with a C_q of 4.79 MHz and η_q of 1. Right: ²⁵Mg NMR spectra of IRMOF-74(I) catalyst with p-xylene and PPE under H_2(g) pressure, the catalyst with p-xylene and H_2(g), the difference spectrum and the powder lineshape of the best fit of the difference (red) calculated with a C_q of 3.14 MHz and η_q of 1.

Fig. 4. A close up of the preferred binding orientation of DPE in IRMOF-74(II), left, and IRMOF-74(III), right. The distance shown correspond to the Mg–O bond length.

Fig. 5. Powder XRD patterns of the activated and cycled IRMOF-74(I–IV) catalysts.