Green chemistry - a synonym for innovation? | Feature

No comments

Products from renewable resources and synthetic procedures that use energy and raw materials economically with the help of novel catalysts are examples of the potential benefits of so-called 'green chemistry'. Rolf Froböse sets the scene.

’Green chemistry’ is a relatively new concept coined by the the American Paul Anastas, who publicised back in 1994 the ideal of synthetic methods that save natural resources and established what he called the Twelve Principles of Green Chemistry.

The following are examples of areas in which chemical industry is already using the principles of ’green chemistry’ in its processes, new process developments which are on the verge of large-scale application, and international research teams which are active in this area.

A: Sustainable resources on the up
Chemists are striving to develop innovative technologies to utilise the potential of renewable resources more efficiently than has been done so far. After all, Nature produces around 170 billion tons of plant biomass per annum globally, divided equally into wood, cereals and other crops. On average, however, only 3-4 per cent of this production is used by the economy.

Currently, the fraction of renewable resources among the raw materials used by industry in Germany and the US is around 10 per cent, although experts expect this number to rise significantly within the next few years. The US National Research Council concluded from a recent study that the market share of renewable resources will reach 25 per cent by 2020. ’In the long term, renewable resources are the only viable solution,’ argues J?rgen O. Metzger from Oldenburg University in Germany.

Practical examples of how Nature’s synthetic achievements can be used in ways that make ecological and economical sense are found at Cognis group, a company whose oleochemicals branch offers a complete range of chemical products made from natural materials to customers around the world. Starting materials include coconuts, palms, palm seeds and soy oils. Out of these, Cognis produces fatty acids, their methyl esters, glycerol and long-chain alcohols, which then serve as starting materials for the bodycare, pharmaceutical, and food industries. ’We practically invented this chemistry and developed it to perfection,’ emphasises Susanne Marell, Cognis’ vice president for corporate communications.

Plant oils can now compete with mineral oils even in the production of petrol and natural gas. One example is the new drilling fluid based on plant-derived esters of palm oil. ’The unobjectionable ecotoxicological properties of such rinsing agents have proved their worth especially in marine biotopes,’ explains Marell. The resulting rock powder can be returned to the seas without any concerns.

During their research, chemists sometimes come across old results that have simply fallen into oblivion. Thus, the chemists Donald Kiely and Hui-Xin Chen of the University of Montana at Missoula, US, found out from old documents that nylon used to be made from materials such as oat waste or corn cobs. They went on to develop a biotechnological process for the production of the nylon precursor adipic acid from glucose. The preliminary results look encouraging and experts regard this use of natural resources as an option for the future.

This view is shared by Birgit Kamm at the Research Institute for Bioactive Polymer Systems (Biopos) at Teltow, who is developing one of the first biorefineries in Germany. The initial target is to couple the refinery to an existing drying facility for green materials, which separates biomass into liquid extract and solid residue. While the residue is processed to make animal feed, composite fibre materials, and as a starting material for the production of levulinic acid, Biopos wants to produce proteins and fermentation media from the liquid phase.

The US company Cargill-Dow has already shown that biorefineries can make sense economically. In a biorefinery in Nebraska, corn waste is turned into 140 000 tons of eco-plastics annually. According to the company, these can be used for ordinary packaging like foil or plastic pots, and even for T-shirts. To this end, the starch is degraded to a glucose syrup and eventually converted to lactic acid, which is polymerised and spun to a thread.

B: From biocatalysts to ’white biochemistry’
In the course of evolution, Nature has developed synthetic processes whose selectivity and efficiency cannot be beaten by technology. Enzymes, the catalysts of biological transformations, have been used empirically since the beginnings of history. In 1981, Degussa developed the concept of an enzyme membrane reactor. This was an important milestone in the development of modern biocatalysis, because it combined the advantages of an enzyme reactor with those of homogeneous catalysis.

About three years ago, Degussa created a research centre called ’Projekthaus Katalyse’ in order to cover the entire spectrum of catalyst development, from the discovery of new catalyst recipes, via the testing, to the optimisation of parameters for process engineering. One of the goals of the centre is to investigate whether existing synthetic pathways can be carried out enzymatically.

One example of the high efficiency of such systems is the product L-tert-leucine (L-Tle). This amino acid does not occur in Nature but serves as a chiral auxiliary compound in asymmetric synthesis and as a building block for syntheses of drugs against cancer, inflammation and viral infection. The synthesis looks relatively simple on paper: the redox enzyme leucin dehydrogenase reduces a keto-carbonic acid precursor to form the L-Tle (see scheme opposite). The problem is that the natural cofactor NADH is required as a hydrogen donor in stoichiometric amounts, and the reactor does not include the NADH regeneration system that is in place in the cell. The high costs of such cofactors have so far blocked industrial application.

However, Maria-Regina Kula at the Institute for Enzyme Technology of the University of D?sseldorf has managed to find an enzyme that can regenerate the precious redox cofactors during the reaction. She isolated the enzyme formate dehydrogenase from the yeast Candida boidinii and discovered that it is suitable for a large number of redox systems requiring NADH as a cofactor. For this achievement, Germany’s president, Johannes Rau, awarded her (jointly with Martina Pohl) the Deutscher Zukunftspreis (German Future Award) in 2002.

’Another remarkable enzymatic method that competes with a chemical process is the production of L-Dopa,’ says Kula. The enzymatic production process of this drug used for the treatment of Parkinson’s disease starts with catechol, pyruvate, and ammonia, making use of the desaminating activity of the enzyme L-tyrosine phenol lyase. Dehydroserine is formed as an intermediate in the course of a reversible ?,?-elimination, and then converted to L-Dopa by stereoselective addition of catechol. The enzyme accepts catechol as a mimic of its natural substrate phenol. ’It has taken around 20 years to elucidate the reaction mechanism of this type of enzyme and optimise the conditions of the reaction accordingly,’ reports Kula. The resulting industrial process is now used by a Japanese company called Ajinomoto.

Kula thinks that the influence of biocatalysis in chemical industry will grow substantially because of its advantages in selectivity, sustainability, and environmental protection. The toolbox available today is waiting to be used for new and unusual production paths.

C: Green solvents - pressure for new syntheses
Supercritical media are regarded as promising candidates to replace organic solvents in a large number of processes. The removal of caffeine from coffee or tea was among the first applications of these solvents. Following the encouraging experience with this technology, extraction of other natural products soon followed. The range of applications pursued by Degussa alone spans from decaffeination via hop extraction to the fat reduction in cocoa and extraction of peanut oil.

The advantages of supercritical fluids benefit not only the chemistry of natural products. Another example is the production of well-defined polymer particles. Polyamides used in paints and varnishes, eg for the car industry, must be precisely defined in their characteristic properties such as particle diameter. If the grains are too fine, the spray painting will require disproportionate efforts to exclude dust. On the other hand, large grains are also undesirable, as they would appear as a defect in the finished lacquer. Traditionally, well-defined distributions of grain sizes are achieved by repeated sieving and sorting, which is both trouble-some and expensive. It would be desirable to have processes which can narrow down the distribution of particle sizes from the beginning.

’This is where the advantages of supercritical media such as CO₂ are apparent, as their solvent properties can be varied over a wide range,’ explains Stephan Pilz at Degussa. A narrow distribution of grain sizes, for example, can be achieved by rapidly changing the solubility. Sudden relaxation of the system can be used to end supercriticality, which leads to precipitation of the solvated particles.

DuPont has developed a process using supercritical CO₂ (scCO₂) instead of trichlorotrifluoroethane for the production of teflon.

In nanoelectronics, scCO₂ can replace ultrahigh purity water, as it does not influence the nanometric structures. Also, the controllable evaporation rates observed when scCO₂ is gradually moved out of supercriticality make it possible to deposit extremely thin homogeneous films of photo varnishes, using coating methods that easily surpass traditional methods.

D: Ionic liquids
A relatively recent development in green chemistry is the use of ionic liquids as ’green solvents’. They have the advantage of exhibiting very low vapour pressures. Researchers at the University of North Carolina at Chapel Hill, are experimenting with chloroaluminate compounds that are still liquid at room temperature and possibly useful for cationic polymerisations, electrophilic alkylations and acylations.

Ludwig Maase’s group at BASF (Ludwigshafen, Germany) has succeeded in developing the first commercial process based on an ionic liquid. It is now being used for the production of alkoxyphenylphosphine, a precursor for photoinitiators. The main attraction of the process is that the ionic liquid is formed in the course of the reaction, and that its formation absorbs hydrochloric acid, which is an unwanted byproduct of the process. In the conventional method, the acid had to be neutralised with triethylamine. The resulting salt, however, has a high melting point and had to be filtered out. Instead of the amine, BASF now uses N-methylimidazole (Mia), which forms a chloride that melts at 75 ?C. Thus, the costly separation becomes expendable. In spite of the higher cost compared with triethylamine, the ’Mia process’ is cost-effective, because the reaction achieves higher yields and Mia can easily be recycled and kept in the reaction cycle.

The path chosen by BASF looks like an approximation to the ideal described by the chemist Chao-Jun Li at the McGill University at Montreal, Canada: ’The best solvent is no solvent’. However, he admits that this ideal solvent is like the ideal gas state in that one can attempt to approach it as far as possible but cannot achieve the ideal state.

E: Green propellants
Biodiesel is well known as an environmentally friendly fuel, but away from the limelight the space industry has also been looking for alternative fuels, known as ’green propellants’ .

The goal is to replace engines using toxic fuel combinations with others that use environmentally benign fuels, to avoid future contamination of the Earth’s atmosphere and of space stations.

To design new engines, industry must understand the key parameters of selected green propellants. Open questions concerning the reactions in the combustion chamber and the heat transfer at the chamber walls need to be addressed.

So far, the main liquid fuels used for rockets are the hydrogen/oxygen combination, where both elements meet in the combustion chambers as liquids, and hydrazine. The latter is extremely harmful to the environment because of the high proportion of nitrous fumes generated during combustion. Another disadvantage of liquid gases is the energy required for their production and cooling.

Among the alternatives, the current favourite is hydrogen peroxide (H₂O₂), which decomposes without producing harmful substances and also qualifies as a green propellant in terms of the energy balance. So far there are two companies that have the expertise to produce the ultrapure H₂O₂ required for this application. ’Until now this fuel has been used only in small drives, for instance to correct the position of a satellite,’ states Egon Walzer, who is in charge of the application technology of hydrogen peroxide at Degussa. However, the possibility of using H₂O₂ in carrier systems is being discussed.

John Rusek of Swift Enterprises in West Lafayette, US, believes that this will be just a matter of time: ’I think that green rocket engines have a great future ahead of them.’ In his opinion, past experience and current discoveries lead to the conclusion that green propellants will gradually replace conventional fuels. In order to make sure that this development isn’t hampered by a lag in the knowledge transfer required, Rusek organises an annual conference in the US, where experts deal with the current developments in hydrogen peroxide technology.

Future outlook
These examples show that the principles of green chemistry can give valuable impetus for innovation in diverse branches of the chemical industry and can pave the way for a more sustainable kind of chemistry. Its potential is far from being realised, as many doors have only been opened slightly.