Catalysis is an interdisciplinary science that encompasses breakthroughs from the smallest quantum scales to the largest engineered reactors. Homogeneous catalysts, heterogeneous catalysts and biocatalysts have sparked revolutionary advances in energy, agriculture, materials, medicine and environmental remediation. Advances in spectroscopy, computer-aided design and directed enzyme evolution are helping catalysts meet the challenges of the future.
The sunlight that floods our planet each day is the ultimate unlimited energy resource: it generates all of the carbon-based fuels and drives our food chain. Access to this power supply, however, critically depends upon catalysis, which is crucial to the photosynthetic conversion of light into organic matter1.
Catalysts work by modifying the energy barriers of chemical processes and accelerating reaction speeds without being consumed themselves. Normally, such processes are classified as one of the two types: homogenous if the catalyst is in the same phase (gas, liquid or solid) as the reactants, or heterogeneous if the phases are different. Many natural catalysts are homogenous; for example, the conversion of ozone into oxygen in the atmosphere is catalysed by chlorine radicals, all of which are in a gaseous phase. Heterogeneous systems commonly involve a solid metal or metal oxide catalyst interacting with gaseous reagents and can, for example, make possible the production of fertilizers or the reduction of pollution.
Catalysis research has entered a new age, marking the end of an era dominated by trial and error. Instead, sophisticated analytical tools and powerful computational methods are improving our understanding of catalysts under real conditions. Because the boundaries between homogenous catalysts, heterogeneous catalysts and biocatalysts are rapidly disappearing, breakthroughs will require contributions from multiple disciplines — making this a true scientific frontier.
Challenges for Catalysis
Despite much progress, molecular-level descriptions of real catalyst systems are still rare. In particular, reaction rates are not easy to predict or measure; the selectivity observed in catalytic transformations depends on minute differences in such reaction rates. Moreover, the role of solvent molecules during liquid-phase homogenous reactions and the intricate dynamic properties of the catalysts themselves are still poorly understood.
New computational chemistry techniques can be used to calculate the structure and properties of molecules using quantum-mechanical theories2. Sophisticated methods that simulate catalysts at multiple timescales and over different lengths can be coupled with advanced computational power to help address many of these outstanding problems and generate predictive insights into new catalytic substances.
Innovative experimental techniques will also play a vital role in improving catalysis. Ultra-fast lasers, which emit bursts of light quicker than atoms move, now allow real-time tracking of catalytic processes in action. By using laser and other spectroscopic methods to identify the elusive transition states between reactants, products and catalysts, scientists aim to improve catalyst design and efficiency significantly. Automation also has the potential to revolutionize catalyst discovery: using high-throughput combinatorial synthesis, robotic instruments can screen the activity of large collections of different catalysts and hence speed up the identification of promising catalysts.
Biology continues both to inspire and to challenge catalysis researchers. For example, many processes in nature are regulated by interconnected catalytic networks that carefully switch chemical pathways on or off3. Organizing catalysts into such controlled systems, however, remains difficult. Another way in which researchers emulate nature is through directed evolution of biocatalysts in test tubes (Fig. 1)4. By subjecting enzyme-coding DNA sequences to iterative rounds of mutagenesis, screening and selection, it is possible to generate biocatalysts that are able to perform specific chemical duties such as selective oxidation of hydrocarbon chains.
The emerging field of organocatalysis seeks to improve upon natural catalytic systems by replacing, for example, entire protein chains with small organic molecules that mimic the active sites and hydrogen-bonding properties of larger biological entities5. Whether such molecules can be endowed with other desirable attributes of biocatalysts, such as molecular switching or allosteric control, remains to be seen.
Because good catalysts must survive many process cycles, intrinsically stable noble metals, such as ruthenium and gold, play important roles in both homogenous and heterogeneous catalysis6. Nature, by contrast, has developed extraordinarily selective catalysts that are free of noble metals. Can researchers find cheap, robust and industrially viable alternatives that can take over the role of increasingly scarce noble metals7?
A primary objective of heterogeneous-catalysis research is unravelling the secrets behind the adsorption, activation and desorption processes occurring on metal and metal oxide surfaces. Scanning tunnelling microscopy represents one way in which to achieve this goal; for example, recent studies revealed that platinum covered with a thin iron oxide film acts as a better catalytic converter than clean platinum (Fig. 2)8,9. Such unexpected results demonstrate that improved analytical tools can produce more rational catalyst designs.
Catalysis is indispensable for materials science. Current techniques use petroleum-based starting materials, however, meeting the demand for high-performance materials such as advanced polymers will require a new generation of catalysts optimized to handle renewable chemical feed stocks, which, unlike simple petroleum hydrocarbons, are often over-functionalized with chemical groups. The future also promises downsized chemical reactors located close to these feed stocks. This presents another opportunity to exploit the inherent advantages of catalysis and will require the design of a new type of non-traditional reactor10.
Pharmaceutical production, which currently generates more waste per kilogram than the petrochemical industry, shows unique potential for catalyst innovation. Drug synthesis normally involves multiple, diverse synthetic steps; until every transformation can be done catalytically, there remains significant potential for innovation.
In the future, catalysts must be able to activate the least-reactive molecules that one can imagine: new methods of nitrogen fixation, the breaking down of carbon dioxide, the splitting of water and the selective activation of hydrocarbon bonds represent some of the grand challenges awaiting researchers, with potentially enormous ramifications for society.
Catalysis presents a way to save energy, reduce waste and meet the sustainability challenges facing our planet, making its study more important now than ever. As a new century demands increasing economic and environmental innovation, it is safe to forecast that the field of catalysis will remain forever young — and will only grow in importance.
How small can an enzyme get without losing its excellent properties as a biocatalyst? This and related questions fall into the realm of ‘organocatalysis’. This emerging field of research, which currently faces exponential growth and promises to complement the established fields of organometallic catalysis, heterogeneous catalysis and biocatalysis, is being shaped by one of the departments at the Max Planck Institute of Coal Research (List, B. et al. Science 313, 1584, 2006).