Metal-Organic Frameworks have a complex, regulary repeating crystal structure. Shutterstock | fractal-an
Due to their use in the creation of ‘designer’ catalysts, metal-organic frameworks (MOFs) are becoming an increasingly important tool in synthetic chemistry.
These catalysts can be designed and developed according to specification, meaning that catalytic sites can be tweaked to favor specific catalytic reactions and substrates. It is essential to understand the structure of MOFs in order to gain a better insight into this working mechanism.
MOFs are complexes that contain metal ions or clusters synchronized to organic ligands to develop multi-dimensional crystal structures. MOFs are often described as the synthetic equivalent of zeolites. Zeolites are naturally occurring microporous aluminosilicate crystals, employed in catalysis. Both zeolites and MOFs are highly porous, and the porosities are usually over 50% of the total volume of crystals.
Unlike zeolites, the structure of MOFs can be designed. A building block technique can be used to connect organic ligands (which operate as scaffolding) and central metal ions (which operate as joints), to achieve synthesis of MOFs. Given the easily customized quality of MOFs, more than 20,000 different MOFs have been synthesized and analyzed.
Given their high customizability and porosity, MOFs have become useable in various applications such as the capture of carbon dioxide, the storage of methane and hydrogen for use as fuel, and catalysis.
Microporous Materials as Catalysts
Catalysts must have highly definite structures and must have reactive sites of a specific geometry and size for them to be effective. Molecules to be catalyzed bind specifically to the reactive site where they can experience reactions at lower energy levels than would otherwise be possible.
MOFs are exceptionally good catalysts for numerous reasons including;
- A wide variety of ligands and metals are available to create MOFs, each with their own chemistries. This allows the MOFs active site to be designed to specification with tweaked properties.
- Reaction rates are further increased due to the high porosity and surface area of MOFs
- MOFs can be exposed to high temperatures, acidic environments, and other harsh conditions because of their high stability
- MOFs participate in heterogenous catalysis. They can be easily extracted for recycling following reactions as they are solid components.
MOFs can be employed as catalysts in conventionally hard reactions where selectivity is a problem, because of their ‘tunable’ nature. For instance, MOF catalysts can efficiently process asymmetric ligands for chiral syntheses due to the specific geometry of the active site.
Catalytic selectivity on the basis of substrate size is possible because of control over the pore size, i.e. while some reactants will not react as they are too big to fit in the pores, smaller substrates can undergo catalysis as they can enter the pores.
The size of an MOF's pore can determine if a substrate will undergo catalysis. Shutterstock | Igor Petrushenko
Designing Novel MOFs for Catalysis
The organic chemist has dreamed of designing very specific catalysts for a long time. Earlier, the development of catalysts was based mostly on serendipity and informed trial and error research.
MOFs belong to a new era of catalyst design where specific catalysts can be designed to specification, in a similar way to designer enzymes.
A solvothermal process is generally used to synthesize MOFs in order to create a crystal with a characteristic scaffold structure. Specific ligands are then used to functionalize the metal binding sites of the scaffold.
In order to activate binding sites or to keep them open to allow their binding to substrates, the process employs coordinating solvents such as diethylformamide (DEF) or dimethylformamide (DMF).
An example of MOF synthesis. A polyoxometalate catalyst holds the reactants in place as the ligands form a scaffolding around the zinc cations. Credit: Nature Comms | Qiuxia Han
Case Studies using FTIR
The structure-activity relationship between the substrate and the catalytic site is highly emphasized during the design process of MOF catalysts. The structure-activity relationship explains how the structure of the active site is directly related to its catalytic power towards a particular substrate.
In order to determine this relationship, the presence of catalyst-substrate bonds must be observed and evaluated. FT-IR spectroscopy is one of the best techniques to achieve this.
A Brønsted acid-derived MOF was created as a heterogeneous catalyst for a cycloaddition, in a recent study. Aromatic sulphonyl groups were incorporated with anhydridic reagents using post-synthetic modification.
FTIR was used to characterize the MIL-101- NH-RSO3H MOF catalyst. The catalyst can be used in a cycloaddition of substituted 2-vinyl-substituted phenol, which was also observed using FTIR.
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)
Specialized techniques such as Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) are highly beneficial in the FT-IR experiments of powdered solids such as MOFs.
The method DRIFTS uses powdered sample with little to no preparation. The IR spectrum of a sample is gathered from the bulk powder through diffuse reflection from the rough surfaces of the samples. Before processing and detection are performed, the infrared radiation is reflected in all directions and is collected using an ellipsoid mirror.
DRIFTS was employed to observe the reduction of copper species on the surface of the catalyst from copper (I) oxide to copper (II) oxide, in a study of copper nanoparticles employed for the catalysis of CO2 hydrogenation. The research also displayed the formation of methane at extended reaction periods and formate carbonyl bonds at temperatures as low as 70°C.
Infrared Accessories for MOF Analysis
Given the little to no sample preparation and monitoring of bond formation in ‘real-time’, FT-IR is an ideal technique for examining MOF catalysts.
There a several methods to perform the sampling: DRIFTS for crystalline and pure powder samples, attenuated total reflection (ATR) for heterogeneous liquid samples, and a pressed KBr sample pellet for high-resolution measurements.
ATR and DRIFTS are the preferred techniques, as measurements can be performed quickly, given that zero sample preparation is required.
Specac's Minidiff Plus DRIFTS accessory and the Golden Gate ATR
Specac, a specialist in infrared, provide a Golden Gate ATR accessory range which employ a single reflection monolithic diamond. Using the Golden Gate ATR, sampling can be carried in a reaction chamber with heating for in situ catalysis experiments or at ambient temperature.
A complete range of sampling accessories for DRIFTS experiments are provided by Specac, including a reaction cell for in situ experiments that enables the monitoring of a catalysts surface during a reaction and probing of the structure-activity relationships of catalysts.
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This information has been sourced, reviewed and adapted from materials provided by Specac.
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