Palladium(II) Immobilized on Metal–Organic Frameworks for Catalytic Conversion of Carbon Dioxide to Formate

Metalation of metal-organic frameworks: fundamentals and applications

Metalation of metal-organic frameworks (MOFs) has been developed as a prominent strategy for materials functionalization for pore chemistry modulation and property optimization. By introducing exotic metal ions/complexes/nanoparticles onto/into the parent framework, many metallized MOFs have exhibited significantly improved performance in a wide range of applications. In this review, we focus on the research progress in the metalation of metal-organic frameworks during the last five years, spanning the design principles, synthetic strategies, and potential applications. Based on the crystal engineering principles, a minor change in the MOF composition through metalation would lead to leveraged variation of properties. This review starts from the general strategies established for the incorporation of metal species within MOFs, followed by the design principles to graft the desired functionality while maintaining the porosity of frameworks. Facile metalation has contributed a great number of bespoke materials with excellent performance, and we summarize their applications in gas adsorption and separation, heterogeneous catalysis, detection and sensing, and energy storage and conversion. The underlying mechanisms are also investigated by state-of-the-art techniques and analyzed for gaining insight into the structure-property relationships, which would in turn facilitate the further development of design principles. Finally, the current challenges and opportunities in MOF metalation have been discussed, and the promising future directions for customizing the next-generation advanced materials have been outlined as well.

Hydrogenation of Carbon Dioxide to Formate by Noble Metal Catalysts Supported on a Chemically Stable Lanthanum Rod-Metal-Organic Framework

The conversion of carbon dioxide to formate is of great importance for hydrogen storage as well as being a step to access an array of olefins. Herein, we have prepared a JMS-5 metal-organic framework (MOF) using a bipyridyl dicarboxylate linker, with the molecular formula [La₂(bpdc)_3/2(dmf)₂(OAc)₃]·dmf. The MOF was functionalized by cyclometalation using Pd(II), Pt(II), Ru(II), Rh(III), and Ir(III) complexes. All metal catalysts supported on JMS-5 showed activity for CO₂ hydrogenation to formate, with Rh(III)@JMS-5a and Ir(III)@JMS-5a yielding 4319 and 5473 TON, respectively. X-ray photoelectron spectroscopy of the most active catalyst Ir(III)@JMS-5a revealed that the iridium binding energies shifted to lower values, consistent with formation of Ir-H active species during catalysis. The transmission electron microscopy images of the recovered catalysts of Ir(III)@JMS-5a and Rh(III)@JMS-5a did not show any nanoparticles. This suggests that the catalytic activity observed was due to Ir(III) and Rh(III). The high activity displayed by Ir(III)@JMS-5a and Rh(III)@JMS-5a compared to using the Ir(III) and Rh(III) complexes on their own is attributed to the stabilization of the Ir(III) and Rh(III) on the nitrogen and carbon atom of the MOF backbone.

Two novel metal-organic frameworks functionalised with pentamethylcyclopentadienyl iridium(III) chloride for catalytic conversion of carbon dioxide to formate

Hydrogenation of CO₂ to formate is a vital reaction, because formate is an excellent hydrogen carrier, which yields blue hydrogen. Blue hydrogen is comparatively cheaper and attractive as the world envisions the hydrogen economy. In this work, two isostructural lanthanide-based MOFs (JMS-6 and JMS-7 [Ln(bpdc)_3/2(dmf)₂(H₂O)₂]_n) were prepared and used as support materials for molecular catalysts. The bipyridyl MOF backbone were functionalised using pentamethylcyclopentadienyl iridium(III) chloride to give Ir(III)@JMS-6a and Ir(III)@JMS-7a. XPS of the functionalised MOFs show downfield shifts in the N 1s binding energy indicating successful grafting of the complex to the MOF. Hydrogenation experiments in the presence of an organic base showed that the functionalised MOFs were active towards converting CO₂ to formate. Ir(III)@JMS-6a and Ir(III)@JMS-7a exhibited the highest turnover numbers of 813 and 621 respectively. ICP-OES indicated insignificant leaching during catalysis. TEM images and XPS data of the recovered catalyst ruled out the presence of Ir(0), confirming that the activity observed was attributed to the molecular Iridium(III) centres.

Confinement Effects in Well-Defined Metal-Organic Frameworks (MOFs) for Selective CO2 Hydrogenation: A Review

Decarbonization has become an urgent affair to restrain global warming. CO₂ hydrogenation coupled with H₂ derived from water electrolysis is considered a promising route to mitigate the negative impact of carbon emission and also promote the application of hydrogen. It is of great significance to develop catalysts with excellent performance and large-scale implementation. In the past decades, metal-organic frameworks (MOFs) have been widely involved in the rational design of catalysts for CO₂ hydrogenation due to their high surface areas, tunable porosities, well-ordered pore structures, and diversities in metals and functional groups. Confinement effects in MOFs or MOF-derived materials have been reported to promote the stability of CO₂ hydrogenation catalysts, such as molecular complexes of immobilization effect, active sites in size effect, stabilization in the encapsulation effect, and electron transfer and interfacial catalysis in the synergistic effect. This review attempts to summarize the progress of MOF-based CO₂ hydrogenation catalysts up to now, and demonstrate the synthetic strategies, unique features, and enhancement mechanisms compared with traditionally supported catalysts. Great emphasis will be placed on various confinement effects in CO₂ hydrogenation. The challenges and opportunities in precise design, synthesis, and applications of MOF-confined catalysis for CO₂ hydrogenation are also summarized.

Construction of a MOF-Supported Palladium Catalyst via Metal Metathesis

A new MOF-supported heterogeneous palladium catalyst Pd/NBB-1 has been synthesized successfully through the effective metal metathesis between Pd(CF₃ COO)₂ and NBB-1. NBB-1 is a two-dimensional zinc metal-organic framework constructed from 2-aminoterephthalate (NH₂ -H₂ BDC) and 2,2'-bipyridine-5-carboxylate (HBPC) by solvothermal method. The replacement efficiency of Pd(II) to Zn(II) is up to 72% after only 24 hours, which is beneficial to the catalytic application. Pd/NBB-1 with a low loading of 2 mol% works efficiently in the 1,4-addition reaction of arylboronic acids with α,β-unsaturated ketones in air, and its catalytic activity keeps unchanged after 3 reaction cycles. This work provides a new strategy to effectively prepare supported noble metal/MOF catalysts, which would further increase the practical applications of metal-organic frameworks.

Hydrogenation of Carbon Dioxide to Formate Using a Cadmium-Based Metal–Organic Framework Impregnated with Nanoparticles

The burning of fossil fuels to meet energy demands has increased carbon dioxide (CO2) in the atmosphere, causing global warming and associated climate change. Therefore, new materials are being developed to capture CO2 effectively, limit its impact on the environment, and store and/or utilise it as an abundant C1 building block. In this study, we investigate a cadmium(II) metal–organic framework, [Cd(bdc)(DMF)]n (MOF1), synthesised by treating benzene-1,4-dicarboxylic acid with four equivalents of [Cd(NO3)2]. MOF1 was then used to support Pd, Ni, and Pt nanoparticles in forming MOF1/Pd MOF1/Ni and MOF1/Pt, respectively. These MOF-based materials were characterised using powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR), energy-dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED), and high-resolution transmission electron microscopy (HR-TEM). MOF1/Pd MOF1/Ni and MOF1/Pt proved highly active in the catalytic hydrogenation of CO2 to formate selectively; in contrast, MOF1 did not hydrogenate CO2 to formate. The MOF1/Pd, MOF1/Ni, and MOF1/Pt catalysts produced formate selectively, with the highest TON of 1500 (TOF of 69 h−1) achieved using MOF1/Pd as the catalyst at 170 °C within 2 h. A formate yield of 98% was obtained, which demonstrates that the combination of nanoparticles and MOFs greatly enhances the catalytic activity of the active sites.

Crystalline Cu(ii) metal–organic frameworks based on a carboxamide pincer ligand and an NCONCON–Pd(ii) pincer complex

Two copper MOFs based on carboxamide linkers are presented. This work shows how organometallic complexes with carboxylate functionalities can be repurposed to produce crystalline MOFs which can be used in carbon dioxide conversion.

Synthesis and Characterization of 2D Metal-Organic Frameworks for Adsorption of Carbon Dioxide and Hydrogen

The reaction of Cd(NO3)2·4H2O and Zn(NO3)2·6H2O with the bipyridyl dicarboxylate ligand H2bpydc (2,2'-bipyridine-4,4'-dicarboxylic acid) afforded two porous metal organic frameworks [Cd(bpydc)2(DMF)2·2DMF]n (JMS-3) and [Zn(bpydc)(DMF)·DMF]n (JMS-4). X-ray diffraction studies revealed that both JMS-3 and JMS-4 crystallize in the monoclinic crystal. The MOFs possess 2D interdigited networks with (sql) topology. Sorption studies showed that the activated phase of JMS-3 had CO2 volumetric uptakes of 26.50 and 30.89 cm3 (STP) g-1 (1.18 and 1.39 mmol g-1) whist JMS-4 gave 10.96 and 16.08 cm3 (STP) g-1 (0.49 and 0.71 mmol g-1) at 298 and 273 K respectively.