Knock on Wood

With the universal growth in environmental awareness, society is making increasingly loud demands for the use of alternative plastics. “Alternative” means the combination of material-specific properties with sustainability factors: plastics are supposed to fulfil their functional purpose, be manufactured from natural, renewable raw materials that are not required for food production, be disposed of without residue and pollutants at the end of their life cycle or be transformed into new products without any loss of quality. Industry is trying to achieve these objectives with scientific support. It remains to be seen whether they can be reached in their entirety. Steps are, however, being taken in the right direction. Initial success is fuelling hopes that it will be possible to replace conventional plastics obtained primarily from oil to an increasing extent in future by equivalent, high-performance plastics based on renewably produced biomass. The crucial question in this context is: what are the monomers supposed to be made from that are the basic components of all plastics (polymers)? What biomass satisfies the requirements of being the appropriate kind and being available in sufficient quantities? And can plastics with the desired material properties really be produced from it? The numerous different approaches for replacing fossil carbon sources, about which k-online specifically has already reported in many different ways – particularly in the sections “Topic of the Month” and “Apropos K” – are now being supplemented by another promising idea contributed by two scientists from Germany and the USA. Their research is leading the plastics industry to bark up what may well turn out to be – quite literally – the right tree.

There are supposed to be people for whom Christmas is the best time of the year. Some Christmas junkies demonstrate their enthusiasm not least of all by setting their Christmas tree – the most typical decoration of the season – up in the living room as early as possible and often waiting until as late as the end of January or early February to throw it out of the house again – provided that they use a real tree rather than a plastic one that they store in the basement or attic. And since we are on the subject: can you imagine that the original, i.e. the forest-grown tree, and its plastic counterpart are related? The work done by Professor Dr Till Opatz from the Institute of Organic Chemistry at Johannes Gutenberg University (JGU) in Mainz / Germany, and Professor em. Dr Anthony J. Arduengo III. from the Department of Chemistry and Biochemistry at the University of Alabama / USA, suggests that this might at any rate be the case in the foreseeable future.

According to the statistical authorities, about 30 million Christmas trees are sold in Germany alone every year. They fulfil their purpose and then end up by the side of the road, where municipal waste management companies pick them up and dispose of them: the final destination for the trees is either a composting plant or – as fuel – in a combined heat and power station or refuse incineration plant. It is not immediately obvious that anything about this standard form of Christmas tree recycling might change in future, i.e. with respect to the manufacturing of plastics and other chemical products, but this could happen if the ideas proposed by Till Opatz and Anthony J. Arduengo III. about a more sustainable approach to chemistry are implemented. These research scientists think that wood has good potential for this and that what is known as xylochemistry (in Ancient Greek, “xylon” means “wood”) provides the solutions for the synthesis processes needed in this context.

A number of years ago, Till Opatz and Anthony J. Arduengo III. got talking about sustainable chemistry at a conference. At the time, Opatz and his task force were focussing on the synthesis of natural and active medical substances. As a former industrial chemist, Arduengo was interested in chemical components, smaller molecules and their reactivity. Both the scientists were quick to realise that these fields would complement each other excellently and permit effective modernisation by using renewable raw materials – with specific implementation of the decision taken by the international community in Rio in 1992 to use biomass to an increasing extent both as a source of energy and as a raw material for chemical production. Back then, Opatz and Arduengo already shared similar views about the potential of wood biomass too. Both of them felt that wood was a suitable candidate for replacing oil as the most important source up to now of carbon compounds, which are essential for the production of active medical substances, dyes, adhesives, fuels and, not least of all, plastics. Since the two scientists also found that the chemistry between them both was very good, Opatz and Arduengo decided to join forces in carrying out research in this sector.

A look at the scientific facts confirms that the two scientists are right in their assessment of wood as a carbon source, not least of all because oil is formed from such biomass under the influence of high temperature and pressure levels over geological periods. However, in the course of this process hydrocarbons without their original molecular properties were produced from the many different organic compounds like carbohydrates, proteins and lipids via successive defunctionalisation (kerogenesis). “When oil is used as the basic material for industrial chemistry, the functionalities that have been lost are recovered with what is in many cases considerable expense and effort”, says Professor Opatz. As he points out, although the direct use of biomass as a raw material for chemical production or as an alternative source of energy has the potential to make the indirect route via kerogenesis superfluous, this has rarely been done in practice up to now.

In other words: before oil became what it is today, it was originally biomass – living organic tissue that has been modified chemically and altered at the molecular level in the course of time in the prevailing environmental conditions. All that has remained of the molecular diversity created by the attachment of functional groups to carbon structures are hydrocarbon compounds, which are processed effectively by petrochemistry but need to have functional groups added to them again by complicated means for further use in different industrial fields.

Opatz and Arduengo think that this roundabout route is not necessary, if a switch is made from petrochemistry to xylochemistry (wood chemistry). Professor Opatz: “50 per cent of the dry matter of wood consists of carbon (C), while oxygen (O) accounts for 43 per cent and hydrogen (H) contributes another 6 per cent. Nitrogen and potassium (ash) as well as sulphur and phosphorus make up about one per cent; wood would leave absolutely no residue on combustion otherwise.” Half of wood matter is cellulose, which is particularly important in paper manufacturing, but could be available more and more as a resource for different applications in the course of the digital revolution. By means of hydrolysis, for example, cellulose can be converted into glucose, which in turn is basically an oligomeric form of formaldehyde, one of the most important basic substances and raw materials in the chemical industry.

The other half of the dry matter of wood consists to roughly equal extents of lignin and hemicellulose. Hemicellulose is a collective term for polysaccharides (“multiple sugars”) that occur in plants and can be processed into major raw materials for the production of solvents and nylon when treated by appropriate biochemical and technical processes. Lignins, in turn, are strong biopolymers that are deposited in the walls of plant cells and lead to the typical “woodification” phenomenon; lignins account for 20 to 30 per cent of the dry matter of lignified plants. Alongside cellulose and chitin, they are among the most frequent organic compounds on our planet. The total amount of lignin produced by Mother Nature is estimated to be about 20 billion tonnes per year. An unbelievably large amount, particularly in view of the fact that lignins are highly suitable materials for the production of biopolymers. (The second part of our series “Knock on wood” is about lignin.)

In the way scientists like Opatz and Arduengo understand the term, xylochemistry has already proved on many occasions to be able to provide raw materials for the production of active medical substances, dyes or plastics from wood, which – in contrast to oil – is renewable, sustainable and climate-neutral. Opatz, Arduengo and members of their staff write in the trade magazine “Angewandte Chemie”: it is an attractive alternative to our current chemical infrastructure, which continues to be based essentially on oil and natural gas – resources that developed over millions of years under conditions that it is practically impossible to reproduce on a technical scale. The scientists also say that oil and natural gas are obtained from underground deposits and consumption of them leads to an imbalance in the carbon dioxide cycle of our ecosystem. It is therefore necessary “to develop an alternative, sustainable chemical infrastructure that is not based on resources that are finite (available in limited amounts), avoids environmental imbalances and is nevertheless economic”. To their way of thinking, the use of biomass as both a source of energy and a raw material for chemical production is becoming increasingly important in this context. Opatz, Arduengo et al. write that implementing the sustainability principles as specified by the international community in Rio in 1992 “requires a number of different assignments to be carried out”. Firstly, renewable raw materials need to be exploited as a source of chemical components and reagents. It is important, secondly, to maintain and take advantage of functionalities that occur naturally as far as is possible – at the chemical level, i.e. with respect to the functional groups, chirality, heteroatoms etc. The third assignment is to come up with catalysts and reagents that exploit all the structural chemistry potential of biomass. And, as Opatz et al. put it in a nutshell: “Fourthly and lastly, chemical transformation, solvents and syntheses should be chosen and configured in such a way that they can be carried out and produced preferably in a continuous (flow) process.” The scientists are aware that the four assignments they have formulated are tremendous challenges on the road towards a sustainable chemical infrastructure. In view of the limited oil and natural gas reserves that are still available, anthropogenic climate change and the consequences of this for mankind, they consider it to be time to adopt sustainable new approaches. According to Opatz and Arduengo, xylochemistry is a good option here and a good place to start.

To be continued