Combining Homogeneous and Heterogeneous Catalysis
Recent work involving Prof. Lee Brammer and Dr Tony Haynes supported through the EPRSC UK Catalysis Hub and published in Angewandte Chemie International Edition highlights the opportunities and advantages of combining the best traits of homogeneous and heterogeneous catalysis. A summary written by Prof Brammer is included below.
Homogeneous and heterogeneous catalysts are both widely used in academic research and large-scale industrial processes. Each has its own advantages and disadvantages. Homogeneous catalysts are typically molecular compounds that dissolve in solution and carry out catalysis in the solution phase. Such catalysts are highly tuneable and the catalysts and catalytic reaction can be investigated in detail by spectroscopic and other experimental measurements. The main disadvantages of homogeneous catalysts are long-term instability and difficulty in separating the catalysts form the products of the catalytic reaction as all are in the same (solution) phase. Heterogeneous catalysts, by contrast, are often simple solids, which can easily be separated from reactants and products that are in solution or gas phase. Such catalysts, however, have the disadvantage of being difficult to study mechanistically and also provide limited opportunity for tuning of catalysis.
It has long been recognised that an optimum catalyst would include the advantages of both homogeneous and heterogeneous catalysts, and avoid the disadvantages of each. The UK Catalysis Hub project entitled “Catalysis in Confined Environments” focuses on the use of metal-organic frameworks (MOFs) as hosts for homogeneous catalysts. MOFs are periodic crystalline porous materials that can be subjected to detailed molecular-level characterisation by diffraction and spectroscopic methods, and their modular design lend them to use as host for molecular organometallic catalysts. The project team combines three research groups with expertise in MOFs and three with expertise in homogeneous catalysis across 4 universities (Liverpool, Sheffield, Oxford and Imperial College).
The recently published article reports the encapsulation of Crabtree’s catalyst, one of the most well-known and effective organometallic hydrogenation catalysts, into a large-pore MOF, known as MIL-101. The strategy involves confinement by electrostatics, enacted by using an anionic analogue of MIL-101 in which the framework ligands have anionic sulfonate groups to provide charge balance for the cationic molecular catalyst.
The encapsulated catalyst shows catalytic behaviour for hydrogenation of alkenes to alkanes in solution that is comparable with the homogeneous Crabtree’s catalyst. It is also able to hydrogenate small alkenes as gas-phase reactants. The catalyst also shows greater longevity due to protection from deactivation pathways in its encapsulated form and can as a solid it can easily be separated from the products in solution. Most encouraging is that the microenvironment of the MOF pores can be used to influence the catalytic reaction pathway. Functionalised alkenes, for example containing alcohol groups, can undergo either simple hydrogenation or, due to alcohol coordination to the iridium catalytic centre, can alternatively isomerise to give aldehydes. The isomerisation mechanism is almost completed prevented for the encapsulated (heterogeneous) catalyst. Such control brings to mind the catalytic behaviour of metalloenzymes, for which suitably-positioned amino acid residues close to the active site control reactivity and selectivity of the cataytic centre.