Bifunctional Heterometallic Metal-Organic Frameworks for Solvent-Free Heterogeneous Cascade Catalysis

Abstract

A family of heterometallic metal-organic frameworks (MOFs) (CPM200s) harmoniously coexisting as Lewis acids and base (azo) sites were prepared. Seven CPM200s were employed as multifunctional heterogeneous cascade catalysts for the one-pot deacetalization-Knoevenagel reaction in a solvent-free system. Benefiting from the cooperation between Lewis acids from the open metal sites and base sites from the ligands, the CPM200s showed high activity and selectivity for the tandem reaction. The heterometallic 3D porous framework reported here not only offers a combination of two opposite active sites in the same framework of materials but also increases mass transfer of the substrate, thus maximizing the efficiency and substrate selectivity of the bifunctional catalysts. The CPM200s showed the highest turnover frequency (TOF), outperforming that of the reported MOFs in tandem with the deacetalization-Knoevenagel reaction. A strong correlation between the TOF and charge-to-radius ratio (z/r) of metal ions in the CPM200s was observed for the first time. The bifunctional CPM200s catalysts can be reused five times without significant loss of activity.Catalysts 2020, 10, 309 2 of 11 tailorable and accessible active sites, are actively used for heterogeneous catalysis [20][21][22]. With diverse functionalization capability on both organic ligands and inorganic nodes, MOFs could be a great candidate for bridging the gap between homogeneous and crystalline heterogeneous catalysts with high porosity and abundant active sites [23][24][25].By tailoring the pore system in terms of shape and size of the pore, MOFs could introduce shape and size selectivity to the substrates [23,24]. For example, MIL-101 functionalized both with amino and sulfo groups [26] and JUC-199 with both open metal sites (Zn 2+ ) and basic sites (-NH 2 ) [3] was used to catalyze the one-pot deacatalization-Knoevenagel condensation in the presence of organic solvent. Nevertheless, the solvent-free tandem reaction catalyzed by MOFs was rarely evaluated.MOFs with highly tunable inorganic building units especially with the easier integration of different metal cations into the same structure can be a great advantage for their applications [27][28][29] such as catalysis and gas adsorption and separation [30][31][32]. In addition to the flexible inorganic building units, organic ligands can also be controlled and tuned to further enhance the material properties. For example, MOFs containing both basic sites from the ligands part and Lewis acid sites from the metallic nodes possessing unsaturated open metal sites (OMSs) can potentially be integrated together. The synthesized MOFs could be used as bifunctional heterogeneous catalysts for one-pot tandem reactions [33][34][35]. Recently, a group of heterometallic MOFs [(M III M II 2 (OH)(ABTC) 1.5 (H 2 O) 3 ) (ABTC = full form of this ligand) with clear mention of the trimeric (M II 2 M III (µ 3 -OH)(CO 2 ) 6 ) clusters] with combinations of trivalent M III (Fe 3+ , V 3+ , Sc 3+ ) and divalent M II (Mg 2+ , Mn 2+ , Co 2+ ...

Robust Mg(Ca)Zr-Doped Acid–Base Bifunctional Mesoporous Silica and Their Applications in the Deacetalization-Knoevenagel Reaction

“… , Subsequently, the CO bond of benzaldehyde was polarized by the acid sites to allow a nucleophilic attack at the C atom . Meanwhile, the base sites activated α-H of CH 2 (CN) 2 to produce carbanion and further formed the target product 3 . − …”

Polyaromatic Carboxylate Ligands Based Zn(II) Coordination Polymers for Ultrasound-Assisted One-Pot Tandem Deacetalization–Knoevenagel Reactions

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