Synthesis and Catalytic Properties of New Metalloporphyrin-Based Porous Organic Framework Materials with Single and Accessible Sites

It is a big challenge to homogenize heterogeneous catalysts with molecular catalytic performance. With this target in mind, herein, we describe a facile strategy for direct incorporation of single catalytic sites in 3D open porous aromatic frameworks (PAFs). The synthesis of the PAF (denoted as PAF-76) as well as its derivatives (PAF-76-M, M=Fe, Mn, Zn) was achieved by the use of tetrakis(4-bromophenyl)methane as tetrahedral nodes and tetrakis(4-bromophenyl)porphyrin as planar nodes. The connection of monomers into an extended network of PAF-76 was monitored by ¹³C NMR and FTIR spectroscopies. The prepared PAF-76s showed 3D porous structures with surface areas of 450–700 m² g⁻¹, pore volumes of 0.3–0.4 cm³ g⁻¹, and pore sizes around 1.2 nm. The direct incorporation of metalloporphyrin components into the PAF-76-M frameworks has allowed the uniform distribution of metal ionic sites throughout the PAF-76-M particles. The combined merits of isolated metal sites and suitable pore size make PAF-76 a good candidate for heterogeneous catalysis. The catalytic performances of the porphyrin/metalloporphyrin-based active sites in the PAF-76s were evaluated by aerobic oxidation reactions of styrene, which are usually carried out with homogeneous systems. Metal-functionalized PAF-76s (PAF-76-M) exhibit enhanced turnover frequencies for styrene conversion (16.9–50.9 mol mol(M)⁻¹ h⁻¹) compared with molecular catalysts (0–35.0 mol mol(M)⁻¹ h⁻¹), and improved selectivity toward phenylacetaldehyde (85.7–99 %) in contrast to their corresponding monomers (0–75.5 %). The robustness of PAF-76 in terms of high thermal stability, good recyclability, and excellent solvent resistance showed that these PAF-76 materials hold great promise for developing heterogeneous catalysts.