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Chemical acceleration - catalysis in chemistry and biochemistry
David Bradley
Without catalysts chemistry would be nothing more than an interminable chore and life would not exist. Compounds would sit indolently in the bottom of reaction flasks, while life processes would take an age. But, the presence of a chemical catalyst, an enzyme in nature, will arouse the most sluggish of reactions so reactants quickly become products.
From the research laboratory and industrial reactor to the insides of every living cell, catalysts are the chemical accelerators that speed up reactions. Catalysts have allowed us to make everything from fuels and plastics to targeted agrochemicals and pharmaceuticals on a day-to-day timescale and underpin life itself.
By definition a catalyst is a material added to a reaction that changes the rate of the reaction, but on completion remains unchanged itself, although catalytic poisoning and side-products blur that text-book picture.
Scientists have gathered substantial evidence, from experimental and theoretical studies, that define catalytic reactions, emphasising the intermediates and transition states that are formed en route from reactant to product and detailing the reaction profiles that provide the chemist and the biochemist with the best picture of catalytic chemistry. The latest understanding of catalysis is leading to new advances in our understanding of how reactions can be pushed preferentially down one of two possible reaction paths and how nature has achieved this desirable ability through millions of years of evolution.
How catalysts and enzymes do their job was the subject of the Royal Society's meeting "Catalysis in chemistry and biochemistry" held 14-15th June. Understanding their secrets could help chemists make the next generation of catalysts, reduce waste and lead to "greener" ways of making the myriad products we need.
Catalytic prospects
Research carried out by Professor RJP Williams FRS of the inorganic laboratory at Oxford University and Sir John Meurig Thomas FRS of the Royal Institution was described by Sir John in the first talk of the meeting. The researchers have carried out an assessment of the similarity between the design and mode of action of homogeneous, that is solution and gas phase catalysts, and enzymatic and solid (heterogeneous) catalysts. A catalyst usually functions by lowering the activation energy but sometimes it works by changing the path along which the uncatalysed reaction travels.
In such a catalytic reaction, the catalyst undergoes structural and electronic change.
The researchers have used free-energy/reaction-coordinate diagrams to examine the nature of such changes. It was pointed out that the basic physical characteristics governing catalytic selectivity are the energy barriers that block the entry of reactants, substrates in the case of enzymes, to the
catalyst, or enzyme, or block the release of products from their active sites. The diagrams plot the energy of the process as time passes and show the energies of the reactants, the catalyst, the product, and any chemical species that exist in between. It was emphasised that the aim of such studies is to find novel ways to improve catalytic performance by engineering the catalysts so that the energetics of the process are optimised.
The atomic environment and electronic state of atoms in and around a catalyst's active site play a crucial role in the catalyst's activity. The extended framework within which the active site sits also plays a critical role and it cited the fact that scientists generally find the active site and its surroundings to be as important in nanoporous solid catalysts as in metal-organic complexes and metalloenzymes. But, it is the atoms of the active site, above all, that chemists must rationalise in order to understand catalysts and to design new ones.
An improved understanding of catalysis is already leading to new technology. For instance, biological methods have allowed researchers to convert the natural enzyme cytochrome C into an efficient catalyst for the epoxidation of alkenes, an industrially important conversion not generally seen in nature, that is used in making epoxy resins. Also, solid, heterogeneous, catalysts are now being designed that retain the recyclability of their predecessors but have the high efficiency of their solution-based, homogeneous, counterparts.
Crystallising thoughts on enzymes
A group of liver enzymes known as the cytochrome P450 enzymes catalyse hydroxylation reactions of a vast array of organic compounds, explained Professor Thomas Poulos of the University of California, Irvine. This reaction is key to the metabolism of the majority of pharmaceuticals we use as well as countless other toxic compounds processed by the liver. Most P450 reactions require insertion of an oxygen molecule, O2, into a bond between a carbon and, a hydrogen atom (a C-H bond); a reaction needing a potent oxidising agent. Scientists, however, are yet to directly observe this crucial oxidant forming but reason that it involves an iron-oxygen radical, Fe(IV)=O.
The Fe(IV)=O radical has been experimentally observed in several enzymes, such as peroxidases and catalases. Poulos pointed out that he and his colleagues have obtained detailed crystal structures of a peroxidase Fe(IV)=O intermediate that is helping them understand why the Fe(IV)=O intermediate is relatively unreactive in peroxidases but is very fast reacting in P450 enzymes. They have coupled this structural information with chemical structures of oxygen complexes formed between natural and engineered cytochrome P450 enzymes. This shows how an oxygen molecule, O2 first becomes attached to an iron atom, and then splits to produce the active Fe(IV)=O oxidant.
The structures and analysis of the enzymatic reactions using rapid reaction kinetic results and cryogenic trapping methods by other researchers, reveal that a rigid network of water molecules delivers protons (positive hydrogen ions) to the iron-linked oxygen atom. This provides the chemical energy needed to split the O-O bond, explained Poulous. There is now evidence that other enzymes such as monooxygenases exploit water networks similarly, so a clearer picture of how many enzymes function is emerging from the research.
Oxygen levels
Living things must utilise oxygen but also protect themselves from its harmful effects, explained Professor Christopher Schofield of Oxford University. Knowledge about the interplay between enzymes and oxygen will not only improve our understanding of a wide range of biological processes, perhaps even the origin of aerobic life as we know it, but could have implications in treating cancer and ischaemic heart disease, two states in which oxygen levels are critical.
One family of oxygenase and oxidase enzymes differs from others in that it does not rely on a reactive iron atom attached to a haem group to work. Instead, 'non-haem' oxygenases and oxidases use an iron bound directly to the protein. Several of the reactions catalysed by the oxygenase and oxidase enzymes have no synthetic precedent, so how they work intrigues chemists looking for novel laboratory catalysts.
Non-haem oxygenases and oxidases are involved in a range of biological processes including antibiotic production and the biosynthesis of plant signalling molecules including the ripening hormone ethylene. Researchers recently discovered a new role for such iron-dependent oxygenases in regulating DNA transcription. Schofield explained that the activity of the signalling compound HIF (hypoxia inducible factor) is inhibited by a process catalysed by iron(II)-dependent oxygenases and appears to occur in organisms ranging from worms to people.
HIF, it turns out, regulates the transcription of an array of genes. Among this array is the gene for a chemical involved in blood vessel formation, vascular endothelial growth factor. Schofield explained how tumour cells replicate rapidly and so run out of oxygen-providing blood vessels. The resultant lack of oxygen limits the activity of the oxygenases which inhibit the activity of HIF. In turn, this allows HIF to escape modification and to enable production of the growth factor causing new blood vessels. These blood vessels give tumours their characteristic claw-like blood vessels and so the name of the disease -- cancer, the crab.
Schofield discussed the latest structural and mechanistic studies on the 2-oxoglutarate dependent oxygenases to show how the various enzyme family members are involved in signalling and the regulation of blood vessel formation and red blood cell production, suggesting an additional medicinal application for such studies.
On the surface
Professor Wyn Roberts of Cardiff University discussed the activation of oxygen (O2) on catalytic metal surfaces. The exposed metal atoms at the surface interact with other atoms and molecules that touch the surface. Once these atoms or molecules have been adsorbed on to a surface there is great scope for change and for those chemicals to interact with other chemical species that pass over the surface.
Studies of the way oxygen sticks to a metal surface, its chemisorption, have established the
importance of transient, short-lived oxygen states in controlling the reactivity of the surface. By co-adsorbing oxygen with various "probe" molecules, such as ammonia, on to a metal surface, Roberts and his colleagues have identified a role for the specific oxygen states in catalysis. They have further explained the activity using non-classical models based on spectroscopic studies which, coupled with scanning tunnelling microscopy, confirm the changes taking place on the metal surface at the atomic level.
Roberts focused on the development of oxygen states at two metal surfaces of interest copper, Cu(110), and magnesium, Mg(0001). He and his team have found evidence for so-called disordered states having an active role in oxy-dehydrogenation reactions that take place on these metal surfaces at temperatures low enough that the movements of the chemical species involved can be tracked readily. The results show how this disorder is actually lost at high temperatures leading to a reconstruction of the surface and effectively loss of catalytic activity.
Understanding precisely what occurs when a molecule such as oxygen is adsorbed on to a metal surface could lead to new ways of using such catalysts in the chemical industry, optimising old catalysts or leading to new ones that work efficiently at low temperatures.
Theory pumps out enzyme answers
Coupling experiment and theory to investigate in detail the catalytic chemistry of oxidase enzymes is key to understanding the intermediates and transition states formed between reactant and product, said Professor Per Siegbahn of Stockholm University in Sweden. For the past 6-8 years, his research group has attempted to understand how such metal-containing enzymes work.
Siegbahn explained that many interesting enzymes contain a transition metal in their active site. For instance, photosystem II, the catalytic system that allows green plants to use sunlight to make sugars, contains manganese atoms at its active site. Cytochrome c oxidase releases chemical energy from sugars for use by the cells of almost all living things also has an iron heart. Other enzymes, including methane monooxygenase, ribonucleotide reductase, methylcoenzyme M reductase, nitrogenase, and haem and non-haem oxygenases, are empowered by the presence of a transition metal.
According to Siegbahn, studies of these various enzymes have been assisted by a computational approach to modelling large biomolecules known as density functional theory (DFT). DFT can produce an accurate representation on the computer screen of the active sites of enzymes containing some 80 to 100 atoms. That threshold is continually being pushed higher as more computing power becomes available to scientists and the approach is further refined. Siegbahn added that one advantage of using such a computational approach is that short-lived intermediates and transition states can be studied with equal ease as though they were stable intermediates.
The accuracy of the energies revealed by DFT is very high and, added Siegbahn, in most cases this is adequate to distinguish between different enzyme mechanisms. He revealed details of one such mechanism laid bare in recent major study of the proton-pumping mechanism of cytochrome oxidase.
Bond options
Bonds between carbon and hydrogen atoms are ubiquitous - in living things and at the laboratory bench, very few organic chemicals have no C-H bonds. As such, their reactivity plays a central role in countless reactions: activate the C-H bond and almost any chemical group can replace the hydrogen atom in the C-H to make an entirely new molecule. Such a molecule will have different physical and chemical properties. Nature has a wide range of enzymes at its disposal to activate carbon-hydrogen bonds, explained Professor Stephen Lippard of the Massachusetts Institute of Technology. A clearer picture of how they work will not only give us new insights into nature's chemistry but could also yield new approaches to activating C-H bonds in the laboratory.
One particular group of enzymes acts on the C-H bonds in hydrocarbons, such as methane. The soluble methane monooxygenase (MMO) enzyme selectively swaps an H for the hydroxyl (OH) group to convert methane into methanol. At the heart of the active site of this and related enzymes are two linked iron atoms. Researchers have discovered that the reaction of the reduced, di-iron(II), form of this enzyme with oxygen molecules sequentially generates two intermediate chemical species. The first is a peroxo-diiron(III) and the second a di(oxo)diiron(IV) species. It is the latter that reacts with methane to produce methanol.
Lippard's team has used experimentally calibrated DFT to model the transformations taking place. In the laboratory, they "fed" molecules of the form CH3X (where one of methane's H atoms is substituted with another group, the X: methyl, nitro, cyano or hydroxyl) to the enzyme. They could see how different X-factors affect the rate of reaction, which provided them with clues about the nature of the enzyme's active site. They found that there are two types of reactivity depending on whether binding to the enzyme or C-H bond activation is the step that limits the overall reaction rate.
Lippard also reported how X-ray crystallography reveals how access to the active site in the hydroxylase (MMOH) component of MMO occurs through a series of water-repellent pockets in the enzyme. His team was also able to correlate the X-ray structure for this enzyme with that of a related enzyme, toluene monooxygenase hydroxylase (ToMOH). In the latter, they observed a wide channel in the enzyme along which molecules can pass. With this information, the team then built synthetic models of the enzymes, characterized them structurally and mechanistically, and derived information that helps them see how the di-iron centres react with substrates. These experiments could lead to new ways to accelerate the hydroxylation of specific bonds in organic molecules using oxygen as the O-atom source.
Catalysing hydrocarbons
Alkanes and alkenes are at the bottom of the chemical industry food chain acting as the feedstock for countless processes in the manufacture of everything from plastics to pharmaceuticals. Professor Gabor Somorjai of the University of California, Berkeley, described how the active site in a platinum catalyst affects the activation of C-H bonds in important feedstock molecules such as ethylene, propylene, isobutene, cyclohexene, and hexene) and methane, ethane, hexane, 2-methylpentane,
and 3-methylpentane.
By studying how changing temperature while keeping the pressure of the hydrocarbon constant, he and his colleagues track the effect on catalytic power. They have spectroscopy to characterize the structures of the molecules adsorbed on to the platinum surface and high-pressure scanning tunnelling microscopy to monitor how the adsorbed molecules move on the platinum surface at different temperatures. This way they can track activity in important reactions such as hydrogenation, dehydrogenation, and carbon monoxide (CO) poisoning of the catalyst.
Somorjai explained how C-H bond dissociation occurs at low temperatures, about 250 K, for all of the molecules studied, although it happens only at high pressures for the alkanes because they easily lose their grip on the surface of the catalyst unless there is a high pressure behind them. He added that because bond dissociation is sensitive to the structure of the catalytic surface, his team has found that these reactions cause a restructuring of the metal surface.
The movement of the hydrocarbons on the catalyst's surface is essential to catalytic activity, added Somorjai. His team has demonstrated this using CO molecules on the platinum surface. CO molecules block the movement of other molecules so "poison" the catalyst. Somorjai's team has also found that ethylene hydrogenation is insensitive to surface structure although both hydrogenation and dehydrogenation of cyclohexene are. Additionally, they have found that hexane and other alkanes six-carbon alkanes form either upright or flat lying molecules on the platinum surface and react to produce branched isomers or benzene, respectively.
These discoveries all provide new insights into how some of the most fundamental organic chemical reactions occur on platinum and provide a starting point for the optimisation of reactions critical to the chemical industry and perhaps the design of new catalysts.
Catalysis on the other hand
One of the most important ways catalysts can assist chemists is to push a reaction down one of two possible paths selecting one product over another. Professor Ryoji Noyori of Nagoya University in Japan explained how he and his colleagues are developing metal-containing catalysts that have this ability. They have made handed, or chiral, diphosphine/1,2-diamine-ruthenium(II) complexes that catalyse the rapid and productive hydrogenation of small organic molecules known as ketones to make just one of two possible forms - left or right handed. The alcohol products of the reaction are building blocks for pharmaceuticals and other chemicals, and making just one handed form can lead to a safer or more effective drug.
Noyori explained how he and his colleagues have found that this selective hydrogenation occurs at the carbon-oxygen double bond (C=O) in the ketone through what he described as a non-classical mechanism. The C=O group is "reduced" by the outer part of the catalyst, rather than by the usual metal atom interaction. The ruthenium atom then donates a negative hydrogen ion, a hydride, while the amino (NH2) part of the catalyst provides a positive hydrogen ion, a proton with a ring of atoms forming the intermediate. The ketone is thus converted directly into the alcohol but with only one face of the chiral catalyst acting as the reaction centre and so producing only one handed form of
the alcohol molecule.
Noyori pointed out that this hydrogenation reaction uses inexpensive, clean hydrogen gas and only a very small amount of the chiral catalyst to make the handed product in large quantities and with no chemical waste.
Function and form
The fundamental objective of chemical synthesis, announced Professor Barry Sharpless of The Scripps Research Institute in California, is not the production of compounds for their own sake, but the production of exploitable properties. Structures are important to our understanding but it is the functionality, whether that is producing comfort in polymeric upholstery, fuelling an aircraft or fighting a disease in the form of a pharmaceutical. The key to creating functionality, Sharpless added, is catalysis.
Sharpless pointed out how nature's solution to the problem of functional form, is not simple but in a sense the chemistry just "clicks". Nature's approach appears impossibly complex but this is where click chemistry is most helpful, he explained.
Rather than fighting to create new reactions to build useful molecules, Sharpless and his colleagues have developed an approach that allows them to create diverse chemical function using a handful of good reactions. This technique they call "click chemistry". These relatively simple chemical reactions can piece together building blocks to generate entirely new compounds.
There was little initial enthusiasm for Sharpless' approach, he conceded, now it is paying off. In one demonstration, his team exploited the ability of natural enzymes found in our bodies to hold together reactants long enough for them to click together into a useful new molecule. They chose the enzyme acetylcholine esterase (AChE), which is closely involved in the transmission of nerve impulses in the brain. Molecules that block this enzyme reversibly can be used to reduce Alzheimer's symptoms. The click chemistry product obtained is the most potent and specific inhibitor of the enzyme ever found, which bodes well for making new drugs for other enzymes and proteins using click chemistry in this way.
Modelling catalysis
Professor Richard Catlow of The Royal Institution of Great Britain has taken a computational approach to understanding how carbon-hydrogen bonds are activated by catalysts. Computer simulation techniques are now well-established tools, he explained, for developing models of catalytic processes at the molecular level. In particular, he showed how applications of modelling methods can provide accurate and detailed models of active site structures, of molecular conformations at the active site, and of the mechanisms of catalysed reactions.
The powerful modelling methods are, explained Catlow, complementary to experimental methods such as X-ray and neutron studies, which can plot the precise positions of atoms in the catalytic
system. The combined approaches will help chemists develop an understanding of catalysis at the molecular level.
Catlow and his colleagues have previously used molecular modelling to study catalysts but now their focus has shifted to revealing active site structures and reaction mechanisms in important transition metal substituted microporous catalysts. Included in this group are silica, aluminosilicate, and aluminophosphate catalysts, which are used to partially oxidise organic compounds. They are also investigating the hydrogenation mechanisms of oxide and metal supported oxides that are used in making the activated feedstock chemical methanol from simpler petrochemicals. Finally, Catlow's team is also looking at the mechanisms of hydrogen abstraction from C-H bonds in oxide catalysts.
All of these catalytic systems are amenable to studies using quantum mechanics rather than simple molecular modelling, Catlow explained. As such he and his colleagues have been able to learn much from "embedded cluster" techniques that follow the catalytic process and reveal the activation energies involved and tie in with experimental data. The research has begun to resolve many of the unanswered questions regarding major industrial catalysts such as "copper on zinc oxide" and could ultimately lead to new insights into optimising these reactions as well as leading to new catalysts.
Microbes ride the Hydrogen Cycle
Micro-organisms use hydrogenase enzymes containing iron or iron and nickel together to oxidise hydrogen (H2) or to reduce hydrogen ions (H ) in the biological Hydrogen Cycle. A clearer understanding of microbial activity with hydrogen, including the influences of oxygen and carbon monoxide, has important implications for optimising future energy technologies based on hydrogen as a fuel.
Simon Albracht's group at the University of Amsterdam discovered that all hydrogenases contain carbon monoxide (CO) and/or cyanide (CN-) groups at their
active site. They have looked particularly at the reactions of nickel-iron hydrogenases which contain a (CysS)2Ni3 (μ-'O')(μ-SCys)2Fe2 (CN)2(CO) group. Albracht explained how two temporarily inactive forms are produced by reaction with oxygen, which leads to coordination of an oxygen species 'O' that bridges the nickel and iron atoms. The 'O' species is either a hydroxide, (OH-) in what is called the "Ready" state, or a peroxide (OOH-), in the "Unready" state. The forms differ in their rates of activation. Activation takes place when the 'O' group is replaced by a hydride (H-), which takes less than a minute for the "Ready" state but several hours for the "Unready".
Recently the Amsterdam group has collaborated with the group of Fraser Armstrong at Oxford University, who have developed special electrochemical techniques for studying the catalytic properties of enzymes. The researchers investigated the hydrogenase from the microbe Allochromatium vinosum by attaching the enzyme directly to an electrode. This configuration makes possible new mechanistic experiments in which the electrode is used to impose strict potential control over the interconversions between different states of the enzyme. It was discovered that the enzyme oxidises molecular hydrogen at a rate comparable with an industrial platinum catalyst, but with much better resistance to carbon monoxide. This, explained Armstrong, suggests the possibilities of using hydrogenases,, or biomimetic compounds inspired by these catalysts, instead of
platinum in fuel-cell technologies.
On the surface
One of the most fundamental of processes in catalysis is the interaction of a diatomic molecule, such as carbon monoxide (CO) or nitrogen (N2), explained Professor Gerhard Ertl of the Max Planck Gesellschaft, in Berlin, with the surface of a well-defined single crystal of catalytic material. Researchers can study this process at the atomic scale and on the femtosecon timescale.
If a diatomic molecule approaches a surface it makes a bond with an atom on the surface and then it dissociates. Ertl explained how cooling a catalytic surface can allow the scientist to trap such a molecule on the surface. Warming the system allows researchers to study what happens as the energy rises and chemistry starts to occur. The interaction of H2 with an active site on the surface of ruthenium dioxide is an archetypal example of this process in action.
Ertl and his colleagues are also using rapid laser pulses to trigger particular events on such catalytic surfaces and spectroscopy to reveal what happens as the thermal equilibrium at the surface breaks down. Their studies are revealing how "steps", "terraces" and other types of defect, on the surface of the catalyst act as active sites for the activation of hydrogen molecules as well as for the adsorption of other diatomic molecules, such as nitrogen oxide (NO) and nitrogen (N2), which then split because of this activation. The latter process is decisive in ammonia synthesis and can now be understood on the atomic scale. Related studies are revealing more details about the mechanism and the rates of the catalytic oxidation of carbon monoxide and hydrogen, respectively.
Reducing nitrogen
Professor Richard Schrock of the Massachusetts Institute of Technology is interested in the reduction, of nitrogen (N2) to the important industrial and agrochemical ammonia. Importantly, Schrock and his colleagues have found a way to catalyse this reaction so that it occurs at room temperature and pressure rather than with the high temperatures and pressures necessary for the current manufacturing process so cutting the energy requirements. This is a major economic goal given that world production of ammonia is more than 100 million tonnes annually.
Schrock and his colleagues have focused on the catalytic reduction of dinitrogen at well-defined sites on single metals and have demonstrated that it is possible to reduce dinitrogen to ammonia catalytically at a single molybdenum centre. This catalytic site, he explained, is protected in terms of the surrounding structure, against decomposition reactions that occur when two metal atoms interact with the nitrogen and then the resulting ammonia molecules.
The catalysts developed by Schrock's team contain chemical groups known as meta-terphenyl-substituted triamidoamine ligands. They found that when they added a proton source and a chemical reducing agent they could obtain 65% efficiency at room temperature and pressure.
Their studies have revealed fourteen possible intermediates in this catalytic reaction, several of which have been isolated and shown to be active catalysts. This confirms the idea that the N2 is reduced at
a protected, single molybdenum metal centre and provides new insights into the FeMo nitrogenase centre. Catalysts like Schrock's might even one day be used in industrial ammonia manufacture.
Natural nitrogen fixation
Organometallic chemistry is taking great strides towards converting nitrogen from the air into ammonia, as MIT's Richard Schrock explained in an earlier talk. However, nature has always had the edge when it comes to chemistry and the biological "fixation" of N2 mediated by nitrogenase enzymes is far more efficient at reducing nitrogen. As Professor Douglas Rees of the California Institute Technology explained, the energy biomolecule ATP (adenosine triphosphate) drives the enzymatic highly efficient reduction of atmospheric dinitrogen to ammonia in the nodules of legumes for instance, and in other plant systems.
Understanding biofixation could ultimately lead to a novel approach to ammonia production, but a clearer understanding of the enzymes involved is necessary before that is possible. Nitrogenase is composed of two component metalloproteins, the MoFe-protein with the FeMo-cofactor which provides the active site for the reduction, and the Fe-protein that couples the release of energy from ATP to the transfer of electrons to reduce N2.
Rees' team has developed a perspective on the nitrogenase system that reveals details of the structures formed between the component proteins involved in the energy release and the electron transfer. He also explained how a high-resolution crystallographic study of the MoFe-protein established a previously unrecognized non-metal chemical group at the centre of the FeMo-cofactor. Although the mechanism of dinitrogen reduction by nitrogenase remains to be established, Rees conceded, it has been possible to make inferences about the mechanism from these studies, models of the chemistry and computational studies, which has taken chemists one step closer to mimicking nitrogen biofixation.
Catalytic Membrane Reactors
Membrane reactors are of increasing interest in catalytic conversions, suggested Professor Thomas Maschmeyer of the University of Sydney, Australia. This, he continued, is because of their inherent advantages over fixed-bed installations. Currently, the majority of catalytic (particularly bulk) chemical reactions are performed in fixed- or fluidised-bed reactors coupled to a variety of distinct separation operations. In membrane reactors these operations are integrated, i.e. the reactants are converted into product with the product separated simultaneously by the membrane from the reaction-zone. For certain types of applications this can result in higher conversions with fewer side products.
However, membranes allow much lower fluxes of product than a fixed-bed reactor, they can be unstable and currently are often too costly to make them economically viable for widespread industrial applications.
Maschmeyer and his colleagues hope to change that. They have worked on small molecule activation (O2, H2, and C-H bonds) in areas such as alkane hydro-isomerisation, reversible hydrogen storage,
oxidative coupling, direct oxidation of benzene to phenol and catalytic partial oxidation using membrane reactors. They have found that by tailoring the operating conditions for each reaction, they can reap the benefits of membranes and overcome the disadvantages.
Many different membrane materials have been studied, such as condensed oxides, zeolites, porous carbon, as well as mesoporous and macroporous oxides. Maschmeyer's team has developed a set of general "design-rules" to help potential users optimise the set-up of a membrane reactor. Their work also revealed the underlying concepts necessary to build continuous, well-behaved membranes.
The team's preliminary results in alkane hydro-isomerisation hint at industrially viable membrane reactors. Furthermore, combining catalytic partial oxidation of methane with solid oxygen conducting membranes might offer an alternative to the flaring of gas from oil fields, producing useful chemical products from a valuable resource that is otherwise wasted at a level equivalent to 1 million barrels of oil per day.
Membrane reactors might also find use in converting hydrogen into a form that can be stored for fuel cells. Indeed, catalytic membrane reactors could solve many problems and may even hold a key to a sustainable future.
Solid research into oxidation
Using catalysts to convert simple hydrocarbons obtained from oil, into oxygen-containing compounds is an important step on the road to a wide variety of chemicals from agrochemicals and pigments to drugs and polymers. The two main processes for converting such hydrocarbons into intermediates such as phenols, cresols, xylenols, aldehydes and carboxylic acids are oxidation and hydroxylation.
Dr Paul Ratnasamy of the National Chemical Laboratory in Pune, India, provided several examples of the processes in action. For instance, phenol is manufactured from benzene by alkylation with cumene followed by homogeneous oxidation of the latter to cumene hydroperoxide and cleavage to phenol and acetone. Terephthalic acid, a major commodity chemical, is manufactured by the oxidation of para-xylene in acetic acid with cobalt and manganese salts as catalysts and bromide promoters.
Such oxidations are currently carried out using catalysts that are themselves dissolved in the liquid phase of the reaction mixture. Such homogeneous catalysis is highly efficient as the reactants and the catalyst are in continuous contact. The disadvantage though is that separating the catalyst from the product once the reaction is complete is difficult. Moreover, these kinds of hydrocarbon oxidations generate toxic effluents which require extensive clean-up and catalyst separation.
He explained that finding solid catalysts for hydroxylation and side-chain oxidation of ring-shaped hydrocarbons using oxygen gas as the oxidant, presents a major challenge to chemists today. A solid, or heterogeneous catalyst, while putatively less efficient, can easily be separated from a liquid phase containing the product by simple filtration.
Ratnasamy and his colleagues have investigated two classes of solid catalysts for the hydroxylation of aromatic rings and oxidation of their side chains to carboxylic acids using O2 (and also hydrogen peroxide, H2O2) as the oxidant. The first category, he explained, consists of microporous solids in which are trapped polymeric transition metal complexes. The second category is based on porous titanosilicate containing transition metals which use O2 as the oxidant. He and his team have used various spectroscopic methods to demonstrated the efficacy of both classes of heterogeneous catalysts and elucidate the active sites in these catalysts. Ratnasamy suggested that such catalysts are close to rivalling homogeneous systems for many industrially important hydrocarbon oxidation reactions.
Auto conversion
Catalysts are not just about making useful chemical products, they also play an important role in converting noxious vehicle emissions into less harmful substances. Dr Martyn Twigg of Johnson Matthey in Royston, England, described the science underlying automotive exhaust emission control and the development of new, improved catalysts.
The main exhaust pollutants are hydrocarbons and their partially oxidised products, especially carbon monoxide, and nitrogen oxides (NOx), explained Twigg. Although harmful in themselves, these species can undergo photochemical reactions in sunlight to form other materials that are of greater environmental and health concern.
To remove these small molecules from exhaust gases requires activation with a platinum group metal. The first catalytic converters used in America were platinum-based oxidation catalysts to reduce exhaust hydrocarbon and carbon monoxide levels. NOx was reduced to nitrogen in a separate catalytic converter. Later developments in the technology led to the adoption of "three-way catalysts" which convert all three pollutants simultaneously using a single catalyst. Today, added Twigg, these catalyst systems are so well engineered they can last the lifetime of the car despite high operating temperatures, vibration and bad treatment, something industrial catalysts are not subjected to.
With that level of catalytic success, attention, he explained, has turned to the development of more fuel-efficient lower carbon dioxide emitting vehicles, that bring a series of new emissions control technology challenges. "Lean-burn" engines provide better fuel economy and here the oxidation of hydrocarbons and carbon monoxide in the exhaust gas is straightforward. However, direct reduction of NOx under lean conditions is practically impossible, he said. Researchers have turned to two novel approaches to cope with NOx in a lean-burn engine: NOx-trapping and Selective Catalytic Reaction (SCR) with ammonia or hydrocarbon. These have not yet been commercialised.
The most familiar lean-burn engine is the increasingly popular Diesel engine, added Twigg. Diesel engines are more efficient than petrol engines but produce more particulates, or soot; another cause for concern. The catalytic removal particulate matter from diesel exhaust is now being introduced. First it is filtered from the gas then periodically the collected particulates are combusted at a relatively low temperature with nitrogen dioxide formed over a catalyst from already present nitric oxide in the exhaust gas, or by catalytically oxidising hydrocarbon and carbon monoxide over a catalyst upstream
of the filter. The latter approach requires quite high temperatures but this can now be done very reliably.
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