7 Oct 2016

Developing novel MOF catalysts using infrared spectroscopy

Metal-organic frameworks (MOFs) are becoming increasingly important in synthetic chemistry as they can be used to create ‘designer’ catalysts. They can be designed and created to specification meaning catalytic sites can be fine-tuned to favour certain substrates and catalytic reactions. To understand how this works, an understanding of the structure of MOFs is necessary.  

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Metal-organic frameworks (MOFs) are complexes consisting of metal ions or clusters coordinated to organic ligands to produce multi-dimensional crystal structures. They are frequently described as the synthetic equivalent of zeolites, which are naturally occurring microporous aluminosilicate crystals which are also used in catalysis. Both MOFs and zeolites are highly porous, with porosities that tend to be greater than 50% of the total crystals volume.

However, unlike zeolites, the structure of MOFs can be designed. This is possible as the synthesis of MOFs can be achieved using a building block methodology to link organic ligands (which act as scaffolding) with central metal ions (which act as joints). The easily customised nature of MOFs means over 20,000 different MOFs have been synthesised and studied.

Due to their extensive porosity and customisability MOFs have become ideal for many applications including the storage of hydrogen and methane for use as fuel, the capture of carbon dioxide, and catalysis. [1]

Microporous materials as catalysts

In order to be effective catalysts must have highly specific structures with reactive sites of a specific size and geometry. Molecules to be catalysed bind specifically to the reactive site where they can then undergo reactions at a lower energy than would be possible otherwise.

MOFs are excellent catalysts for several reasons [2] including;

  • a wide range of metals and ligands available to construct MOFs, each with their own chemistries. This means the MOFs active site can be designed to specification with fine-tuned properties
  • the high porosity, and surface area, of MOFs means reaction rates are further increased
  • MOFs are highly stable meaning they can be exposed to harsh conditions such as acidic environments and high temperatures
  • MOFs participate in heterogenous catalysis. As the MOF is solid component it is easily removed for recycling following reaction

The ‘tunable’ nature of MOFs means they can act as catalysts in traditionally difficult reactions where selectivity can be an issue. For example, the specific geometry of the active site allows MOF catalysts to efficiently process asymmetric ligands for chiral syntheses. Control over the pore size also means catalytic selectivity on the basis of substrate size is possible, i.e. some reactants will be

Designing Novel MOFs for catalysis

The design of very specific catalysts has long been a dream of the organic chemist. In the past the development of catalysts has mostly on serendipity and educated trial and error experiments.

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MOFs represent a new era of catalyst design where, in a similar fashion to designer enzymes, specific catalysts can be developed according to specification. Active sites to be specifically designed to specification.

MOFs are typically synthesised using a solvothermal process to produce a crystal with a characteristic scaffold structure. The metal binding sites of the scaffold are then functionalised using specific ligands. This process tends to use coordinating solvents such as dimethylformamide (DMF) or diethylformamide (DEF) to activate binding sites (or to keep them open) [3,4].

Case studies and FTIR

When designing MOF catalysts, a lot of emphasis must be placed upon studying the structure-activity relationship between the catalytic site and substrate. It is this relationship that is the key towards creating even more efficient and effective catalysts. Determination of this relationship requires the presence of catalyst-substrate bonds to be determined, with one of the best methods of achieving this being FT-IR spectroscopy.

A recent study [5] showed a Brønsted acid-derived MOF developed as a heterogeneous catalyst for a [4+2] cycloaddition. Aromatic sulphonyl groups were added using anhydridic reagents by post-synthetic modifications. The MIL-101-NH-RSO3H MOF catalyst was characterized by using FTIR, and showed utility in a [4+2] cycloaddition of substituted 2-vinyl-substituted phenol, which was also monitored by using FTIR.

Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

FT-IR experiments on powdered solids, such as MOFs, benefit greatly by using a specialised technique such as DRIFTs (Diffuse Reflectance Infrared Fourier Transform Spectroscopy)

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Image: analyticalspectroscopy.net

DRIFTS is a technique that uses powder samples with little (or no) preparation required. The IR spectrum of a sample is collected from the bulk powder by diffuse reflection from the samples rough surfaces. The infrared radiation is reflected in all directions and collected by an ellipsoid mirror prior to processing and detection.

A study of copper nanoparticles used for the catalysis of CO2 hydrogenation [6] used DRIFTS and to monitor the reduction of copper species on the surface of the catalyst from copper (I) oxide to copper (II) oxide. The study also showed the formation of formate carbonyl bonds at temperatures as low as 70°C as well as methane formation at extended reaction times.

Infrared Accessories for MOF Analysis

FT-IR is an excellent technique to examine MOF catalysts as there is very little sample preparation and bond formation can be monitored in ‘real-time’.

Sampling can be carried out in several ways. These include using a pressed KBr sample pellet for high resolution measurements, DRIFTS for pure powder and crystalline samples and Attenuated Total Reflection (ATR) for heterogeneous liquid samples.

DRIFTS and ATR are the favoured method because no sample preparation is required, allowing measurements to be carried out rapidly.

Specac, experts in infrared, produce the Golden Gate ATR spectrometer accessory, which uses a single reflection monolithic diamond which allows for sampling at ambient temperature, or in a reaction cell with heating for in situ catalysis experiments.

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The Golden Gate ATR

Specac also provide a full range of sampling accessories for DRIFTS experiments including a reaction chamber for in situ experiments.

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The Golden Gate ATR with Reaction Cell attachment

DRIFTS is ideal for observing the catalytic activity of MOF surfaces and to an environmental chamber. It is also useful for monitoring catalytic surfaces and to determine the structure-activity relationship of a catalyst’s active site. 

Check out #SpectroscopySolutions for more.

References

  1. Hiroyasu Furukawa, Kyle E. Cordova, Michael O’Keeffe, Omar M. Yaghi, The Chemistry and Applications of Metal-Organic Frameworks, Science  30 Aug, 2013: Vol. 341, Issue 6149
  2. Adeel H. Chughtai,  Nazir Ahmad, Hussein A. Younus,  A. Laypkovc and  Francis Verpoort, Metal–organic frameworks: versatile heterogeneous catalysts for efficient catalytic organic transformations, Chem. Soc. Rev., 2015, 44, 6804-6849
  3. Mitch Jacoby, Materials Chemistry: Metal-Organic Frameworks Go Commercial, Chemical and Engineering News, Volume 91, Issue 51, pp. 34-35, December 23, 2013
  4. Carl K. Brozek, Vladimir K. Michaelis, Ta-Chung Ong, Luca Bellarosa, Nuria Lo ́ pez, ́ Robert G. Griffin, and Mircea Dinca , Dynamic DMF Binding in MOF‑5 Enables the Formation of Metastable Cobalt-Substituted MOF‑5 Analogues, ACS Cent. Sci. 2015, 1, 252−260
  5. Chao Qi, Daniele Ramella, Allison M. Wensley, Yi Luan, A Metal-Organic Framework Brønsted Acid Catalyst: Synthesis, Characterization and Application to the Generation of Quinone Methides for [4+2] Cycloadditions, Volume 358, Issue 16, August 18, 2016, Pages 2604–2611
  6. Marco Bersani and Kalyani Gupta, et al., Combined EXAFS, XRD, DRIFTS, and DFT Study of Nano Copper-Based Catalysts for CO2 Hydrogenation, ACS Catal., 2016, 6 (9), pp 5823–5833