Changing places

Catalysts represent one of the black arts of process engineering. Substances which allow reactions to proceed at low temperatures, and which promote the production of valuable products rather than useless or harmful by-products, they combine advanced chemistry with engineering.

The shape of the catalyst particles and their position within the reactor governs how the reacting substances come into contact with them and is as important as the processes taking place on the surface. Getting both tasks right is highly complex, and ABB's catalyst researchers are among many scientists and researchers involved with solving the problems.

ABB's catalyst team, based within its process technology division at a number of locations in the US and Europe, is mainly concerned with technologies connected with the 'heavier' end of the process spectrum. Catalyst systems here tend to be based around fixed-bed arrangements. Generally, catalysts are extruded as pellets and packed into the reactor. However, because these pellets are inflexible and relatively large, they can cause flow problems. If they are packed too tightly, they restrict flow and cause pressure drop. Also, the reactants can only contact the material on the outside of the catalyst pellets; moreover, the catalyst may not be in the best position inside the vessel to catch the reactants at their optimum conditions. The catalyst therefore performs below its optimum efficiency, which can lead to poor yields, waste of energy and production of unwanted by-products.

Micro-engineered catalysts (MECs) solve one of the problems of conventional fixed-bed catalysts. Developed by ABB researchers over a four-year period, MECs are designed to increase the reactants' access to the catalyst. The system consists of flexible ceramic microfibres, thinner than a human hair, coated with catalyst particles smaller than 15 microns. The fibres can be shaped inside the reactor to conform with the fluid dynamics of the vessel, and because the system is so porous, flow is impeded much less than in conventional fixed-bed reactors. This makes catalysis more efficient, as the reactants can contact far more active catalyst particles. Because of this, reactors can be made smaller.

The company is targeting two particular applications for MECs, both of which it expects to commercialise this year. The first is in removal of nitrogen oxides from process and power station waste gases. NOx removal is becoming an increasingly pressing problem as regulations over greenhouse gas emissions tighten, and the technologies currently available are not efficient. This means that catalyst market for deNOx is large - several hundred millions of dollars per year, according to ABB - and growing. The company is constructing an MEC-based deNOx plant for a petrochemical refining plant in the US, which it hopes to bring on stream later this year.

Another candidate for MECs is the selective hydrogenation of petrochemicals, a standard reaction carried out in refineries to convert unsaturated and aromatic molecules into clean-burning petroleum. There are a wide variety of processes and catalysts used for this, but ABB claims that MECs are as active, but significantly more selective, than current catalysts. ABB is operating a pilot scheme for this technology, involving the hydrogenation of impurities in the process stream of a propylene plant in Asia.

Nanotechnology is also at the root of another catalyst innovation, nano-sized catalysts. Working with researchers at the Massachsetts Institute of Technology, ABB is developing these systems for a variety of applications, ranging from the reformation of petroleum to systems which can burn methane in a stable flame.

The nanocatalysts are based on barium and aluminium alkoxide, combined so that they form a material with pores only a 25th of the size of those found in normal zeolites. The process to manufacture them is complex - the precursors for the materials are mixed with an emulsion in which nanometre-scale droplets of water are mixed with oil in what is known as a reverse microemulsion. The precursors move from the oil into the water, and each droplet acts as a tiny reactor, forming barium hexaaluminate crystals just 30nm in diameter.

In one research strand, MIT's Professor Jackie Ying has taken this technology one stage further. Ying is working on catalysts to stabilise the burning of methane in gas-fired power stations. This is a hot topic, as the high temperatures of methane flames - in excess of 1400°C - is normally too hot for catalysts to function, but encourages the formation of NOx. Combustion is a chemical reaction between methane and oxygen, and reactions can be catalysed - and a catalyst that will allow the reaction to proceed at a lower temperature would result in a cleaner, more stable burn.

Ying's goal was to reduce the temperature at which combustion starts - known as light-off - to around 400°C, but still allow temperatures to go as high as 1300°C without damaging catalyst activity. Nanocatalysts were an option, as barium hexaaluminate crystals are temperature-stable and reduce light-off temperature. The reverse microemulsion technique created crystals which reduced light-off to 700°C, but there was another, more tantalising target.

Ceria - CeO2 - supports combustion at temperatures below 600°C. However, above this temperature, ceria crystals stick together, which destroys the material's activity. How could Ying's team combine the materials' properties?

The solution came from the original emulsion. When the researchers added ceria to the reverse microemulsion, it too migrated from the oily phase into the water droplets and was incorporated into the surface of the BHA particles. Because they are firmly anchored, they cannot fuse together. The combined system allows the methane-oxygen reaction to light-off at 400°C, and is still stable at 1100°C. In a paper in Nature, Ying and her colleague, Andrey Zarur, said that the catalyst would have 'potential practical applications in the ultraclean catalytic combustion of methane', and also that it 'could be extended to other systems and reactions of interest.'

ABB goes a step further. The tiny size of the pores in nanocatalysts means that they have an even larger surface area than zeolites, making them potentially far more active and efficient. It hopes its research in nanocatalysts 'may result in a number of massive technological leaps in the chemical industry' - and hopes to be at the forefront of these.

Sidebar: Environment and products drive catalyst market

Demand for process catalysts is set to rise steeply, with the market reaching a value of $3.3billion by 2005, according to a report from industry analysts at the Freedonia Group. Chemical companies see catalysts as the best method to both increase the performance of their plants and to meet environmental targets, the report says, and the increasing demand is driving research.

Petroleum refining is the largest catalyst market and will remain so, the report says, because of government regulation over sulphur levels in fuel. 'To reduce sulphur content in gasoline and diesel fuel, refiners are expected to use improved catalysts and/or higher catalyst loadings in order to minimise the need for new capital equipment,' the report says.

However, the fastest growing sector will be polymerisation catalysts. Demand for metallocene catalysts, which produce polymers that combine properties such as toughness and transparency with good processability, will more than double, it says, and metallocenes will overtake Ziegler-Natta catalysts and reaction initiators as the dominant catalyst type by 2010.

Demand for syngas and oxidation catalysts has been very low for the past two years, because the high energy price saw production of high-volume, low-value basic chemicals like ammonia and nitric acid fall. Cheaper energy and a gradual improvement in world markets will see demand increase, Freedonia says, although much of the new capacity for these chemicals is likely to be in Eastern Europe and Asia. The fine chemical sector, however, will continue to be a strong market and a driver for R&D, as chiral and enzyme-based catalysts become more widely used in drug production.

The US market for catalysts is fragmented, the report reveals, with seven suppliers - Engelhard, Grace Davison, Akzo Nobel, Shell, UOP, Atofina and Sud-Chemie - accounting for around 45 per cent of the market. The emphasis on R&D is forcing costs up, and this is likely to lead to consolidation in the industry, it says.

Moreover, the larger end-users are likely to put pressure on the manufacturers to cut prices.