Metal-Organic Framework-Based Catalysts with Single Metal Sites

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Bimetallic metal-organic frameworks and their derivatives

Bimetallic metal-organic frameworks (MOFs) have two different metal ions in the inorganic nodes. According to the metal distribution, the architecture of bimetallic MOFs can be classified into two main categories namely solid solution and core-shell structures. Various strategies have been developed to prepare bimetallic MOFs with controlled compositions and structures. Bimetallic MOFs show a synergistic effect and enhanced properties compared to their monometallic counterparts and have found many applications in the fields of gas adsorption, catalysis, energy storage and conversion, and luminescence sensing. Moreover, bimetallic MOFs can serve as excellent precursors/templates for the synthesis of functional nanomaterials with controlled sizes, compositions, and structures. Bimetallic MOF derivatives show exposed active sites, good stability and conductivity, enabling them to extend their applications to the catalysis of more challenging reactions and electrochemical energy storage and conversion. This review provides an overview of the significant advances in the development of bimetallic MOFs and their derivatives with special emphases on their preparation and applications.

Charting the Metal-Dependent High-Pressure Stability of Bimetallic UiO-66 Materials

In theory, bimetallic UiO-66(Zr:Ce) and UiO-66(Zr:Hf) metal-organic frameworks (MOFs) are extremely versatile and attractive nanoporous materials as they combine the high catalytic activity of UiO-66(Ce) or UiO-66(Hf) with the outstanding stability of UiO-66(Zr). Using in situ high-pressure powder X-ray diffraction, however, we observe that this expected mechanical stability is not achieved when incorporating cerium or hafnium in UiO-66(Zr). This observation is akin to the earlier observed reduced thermal stability of UiO-66(Zr:Ce) compounds. To elucidate the atomic origin of this phenomenon, we chart the loss-of-crystallinity pressures of 22 monometallic and bimetallic UiO-66 materials and systematically isolate their intrinsic mechanical stability from their defect-induced weakening. This complementary experimental/computational approach reveals that the intrinsic mechanical stability of these bimetallic MOFs decreases nonlinearly upon cerium incorporation but remains unaffected by the zirconium: hafnium ratio. Additionally, all experimental samples suffer from defect-induced weakening, a synthesis-controlled effect that is observed to be independent of their intrinsic stability.

Adsorption Properties of Ce5(BDC)7.5(DMF)4 MOF

In this article we report on the spectroscopic and adsorptive studies done on Ce(III)-based MOF possessing, upon desolvation, open metal sites, and a discrete surface area. The Ce-based MOF was synthesized from terephthalic acid linker (H2BDC) and Ce3+ cations by the classical solvothermal method. Preliminary powder X-ray diffraction analysis showed that the obtained materials corresponded to the ones reported by other authors. Spectroscopic techniques, such as XAS and in situ FTIR with probe molecules were used. In situ FTIR spectroscopy confirmed the successful removal of DMF molecules within the pore system at temperatures above 250 °C. Moreover, the use of CO as a probe molecule evidenced the presence of a Ce3+ open metal sites. Detailed volumetric and calorimetric CO2 adsorption studies are also reported.

The chemistry of Ce-based metal-organic frameworks

Metal-organic frameworks (MOFs) have gained widespread attention due to their modular construction that allows the tuning of their properties. Within this vast class of compounds, metal carboxylates containing tri- and tetravalent metal ions have been in the focus of many studies due to their often high thermal and chemical stabilities. Cerium has a rich chemistry, which depends strongly on its oxidation state. Ce(iii) exhibits properties typically observed for rare earth elements, while Ce(iv) is mostly known for its oxidation behaviour. In MOF chemistry this is reflected in their unique optical and catalytic properties. The synthetic parameters for Ce(iii)- and Ce(iv)-MOFs also differ substantially and conditions must be chosen to prevent reduction of Ce(iv) for the formation of the latter. Ce(iii)-MOFs are usually reported in comprehensive studies together with those constructed with other RE elements and normally they are isostructural. They exhibit a greater structural diversity, which is reflected in the larger variety of inorganic building units. In contrast, the synthesis conditions of Ce(iv)-MOFs were only recently (2015) established. These lead selectively to hexanuclear Ce-O clusters that are well-known for Zr-MOFs and therefore very similar structural and isoreticluar chemistry is found. Hence Ce(iv)-MOFs exhibit often high porosity, while only a few porous Ce(iii)-MOFs have been described. Some of these show structural flexibility which makes them interesting for separation processes. For Ce(iv)-MOFs the redox properties are most relevant. Thus, they are intensively discussed for catalytic, photocatalytic and sensing applications. In this perspective, the synthesis, structural chemistry and properties of Ce-MOFs are summarized.

Bimetallic hexanuclear clusters in Ce/Zr-UiO-66 MOFs: in situ FTIR spectroscopy and modelling insights

In situ FTIR and DFT modelling were used to study the stoichiometry of the hexanuclear clusters in Ce/Zr-UiO-66 compounds.

A spectroscopic and computational study of a tough MOF with a fragile linker: Ce-UiO-66-ADC

A spectroscopic and computational insight in the defective nature of the acetylene dicarboxylic acid based Ce-MOF, having UiO-66 topology and denoted as Ce-UiO-66-ADC MOF.

Selective catalytic reduction of NO by cerium-based metal–organic frameworks

NO has been catalytically converted by Ce-UiO-66 and bimetallic Ce/Zr-CAU-24.

Metal-Organic-Framework-Supported and -Isolated Ceria Clusters with Mixed Oxidation States

The formation of oxygen vacancies via reversible transitions between Ce(IV) and Ce(III) plays a crucial role in the propensity of cerium oxide to act as a supporting promoter in oxidative heterogeneous catalysis. An open challenge is, however, preparation of high-porosity, supported arrays of isolated ceria(IV, III) clusters with high porosity. Herein, we report two examples of oxy-Ce(IV, III) clusters supported and spatially isolated on an oxy-zirconium MOF, NU-1000. The clusters are introduced using either of two Ce complexes (precursors): Ce^IV(tmhd)₄ (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate) or Ce^III(iPrCp)₃ (iPrCp = tris(isopropyl-cyclopenta-dienyl), via SIM (solvothermal installation in MOFs). The prepared materials are named Ce-l-SIM-NU-1000 and Ce-n-SIM-NU-1000, respectively. X-ray photoelectron spectroscopy characterization shows that the ratio of Ce(III) to Ce(IV) oxidation states can be modulated. Difference envelope density analyses of X-ray scattering show that Ce_xO_yH_z clusters in Ce-n-SIM-NU-1000 are located between pairs of Zr₆ nodes, whereas in Ce-l-SIM-NU-1000, they are sited on MOF linkers throughout the micropores of NU-1000. Cluster size differences were further evaluated by pair function distribution (PDF) analyses of total X-ray scattering reveal that the node sited clusters contain of only a few cerium ions, whereas the linker-sited clusters each contain ∼90 cerium ions. The observed size appears to be defined by the size of NU-1000s triangular pores, that is, cluster formation appears to be pore templated. The Ce-SIM functionalized materials are catalytically active for hydrolysis of DMNP (dimethyl 4-nitrophenyl phosphate), a nerve-agent simulant. Conversion of a small fraction of the Ce(IV) ions in which the presence of small fractions of the cerium(IV) ions in Ce-l-SIM-NU-1000 to cerium(III) significantly enhances catalytic activity-perhaps by labilizing aqua ligands and facilitating simulant binding to the clusters Lewis-basic metal ions. While not explored here, the larger clusters, when partially reduced, are, we believe, candidate catalysts for O₂ activation and subsequent selective oxidation of organic substrates.

High-performance electrochemical enzyme sensor for organophosphate pesticide detection using modified metal-organic framework sensing platforms

A practical electrochemical biosensor with high sensitivity was developed for detecting organophosphorus (OP). Initially, Ce metal was introduced into an UiO-66-template to form Ce/UiO-66. Later, graphene oxide (GO), carbon black (CB) and multi-walled carbon nanotubes (MWCNTs) were separately added to Ce/UiO-66 to compare the effect of different carbon-based material types on the performance of the biosensor. Exclusively, Ce/UiO-66/MWCNTs with a Ce (7%) and MWCNT (30%) matrix was found to not only load more acetylcholinesterase (AChE) onto vacant sites but also increase electron transfer and decrease the number of diffusion pathways between the thiocholine and electrode surface. Moreover, the appropriate oxophilicity of Ce coupled with the high surface area and good conductivity of MWCNTs in the UiO-66 structure revealed a high affinity to acetylthiocholine chloride (ATCl) and possible catalysis of the hydrolysis of ATCl with a Michaelis-Menten constant of 0.258 mM. This biosensor, under optimal conditions, demonstrated a rapid and sensitive detection of paraoxon over a wide linear range of 0.01-150 nM, with a low detection limit of 0.004 nM. As a result, the AChE/Ce/UiO-66/MWCNTs/GCE biosensor can be employed in laboratory and field experiments to determine paraoxon levels.

Mixed-Metal MOFs: Unique Opportunities in Metal-Organic Framework (MOF) Functionality and Design

Mixed-metal metal-organic frameworks (MM-MOFs) can be considered to be those MOFs having two different metals anywhere in the structure. Herein we summarize the various strategies for the preparation of MM-MOFs and some of their applications in adsorption, gas separation, and catalysis. It is shown that compared to homometallic MOFs, MM-MOFs bring about the opportunity to take advantage of the complexity and the synergism derived from the presence of different metal ions in the structure of MOFs. This is reflected in a superior performance and even stability of MM-MOFs respect to related single-metal MOFs. Emphasis is made on the use of MM-MOFs as catalysts for tandem reactions.

Metal ion exchange in Prussian blue analogues: Cu(ii)-exchanged Zn–Co PBAs as highly selective catalysts for A3 coupling

The occurrence of metal ion exchange in Zn₃[Co(CN)₆]₂ and Cu₃[Co(CN)₆]₂ Prussian blue analogues (Zn–Co and Cu–Co PBAs) was demonstrated for the first time.

Mixed-metal metal–organic frameworks

Mixed-metal MOFs contain at least 2 different metal ions presenting promising potential in heterogeneous catalysis, gas sorption/separation, luminescence and sensing.

Three-dimensional hollow porous raspberry-like hierarchical Co/Ni@carbon microspheres for magnetic solid-phase extraction of pyrethroids

A three-dimensional magnetic hollow porous raspberry-like hierarchical Co/Ni@carbon microspheres (3D Co/Ni@carbon) were synthesized by using a bimetal-organic framework (Co/Ni-MOF) as a precursor and subsequent calcination under nitrogen. The 3D Co/Ni@carbon is a novel solid phase extractant that displays outstanding extraction capability and separation efficiency for the pyrethroid pesticides ethofenprox and bifenthrin. This is ascribed to the beneficial effects of facile analyte transport (due to the presence of free pores), the abundant number of adsorption sites (which warrant efficient extraction), and the excellent structural stability of the material. The 3D Co/Ni@carbon was applied to dispersive magnetic solid-phase extraction (d-MSPE), and the two pyrethroids were quantified by HPLC (UV detection wavelength: 220 nm). The method has a high preconcentration factor (937-1012) and give recoveries that range between 85.6-106.9%, with RSDs (for n = 5) of <6% in case of real samples. Graphical abstract The hierarchical porous Co/Ni@carbon microsphere as adsorbent was fabricated, and it showed high extraction efficiency for two pyrethroids.

A precursor method for the synthesis of new Ce(iv) MOFs with reactive tetracarboxylate linkers

A precursor method has been developed to synthesize Ce(iv) MOFs that could not be prepared directly from Ce(iv) salts. Starting from Ce₆ clusters, two Ce-UiO-66 analogues and four tetracarboxylate-based Ce(iv) MOFs could be synthesized. The applied method facilitates framework formation by evading reactive individual Ce(iv)-ions thereby paving the way for further development of Ce-MOFs.

Adsorptive removal of organic dyes from aqueous solution by a Zr-based metal–organic framework: effects of Ce(iii) doping

Ce(iii)-doped UiO-66 nanocrystals exhibited better dye adsorption performance than pristine UiO-66, showing the advantages of high stability and excellent porosity.

A multi-purpose reaction cell for the investigation of reactions under solvothermal conditions

A new versatile and easy-to-use remote-controlled reactor setup aimed at the analysis of chemical reactions under solvothermal conditions has been constructed. The reactor includes a heating system that can precisely control the temperature inside the reaction vessels in a range between ambient temperature and 180 °C. As reaction vessels, two sizes of commercially available borosilicate vessels (V_max = 5 and 11 ml) can be used. The setup furthermore includes the option of stirring and injecting of up to two liquid additives or one solid during the reaction to initiate very fast reactions, quench reactions, or alter chemical parameters. In addition to a detailed description of the general setup and its functionality, three examples of studies conducted using this setup are presented.

The duality of UiO-67-Pt MOFs: connecting treatment conditions and encapsulated Pt species by operando XAS

XAS study of Pt-functionalized UiO-67 MOFs shows that 2 types of catalytically active sites can be formed in MOF cavities isolated Pt-complexes and Pt nanoparticles.