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Department of Chemistry

Lund University

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Nicola Rehnberg

Research interests

Summary

Forest and field are assumed to play an important role in the future supply of raw materials to the manufacturing of chemicals and polymers. A big challenge to a nascent biorefining industry is the defunctionalisation of biomass to an optimal degree with economical viability. Seemingly, “bio” is not enough to market new materials; performance and prise are parameters that can not be ignored. Targeting functional monomers or performance chemicals rather than products providing merely bulk may prove a successful approach of adding value to forestry, agriculture, and consumer products such as plastics.

Background

Forêt, forêt, forêt, et un lac” Forest, forest, forest, and a lake, that was what a French friend of mine said when we travelled north through Sweden. In his home area in southern France, he described the landscape as “maïs, maïs, maïs, et un forêt”, corn, corn, corn, and a forest. Even if Europe is a heavily populated continent, there are huge areas for farming and forestry. It is easily assumed that the amount of biomass available as substitute for fossil energy and material sources is enormous. That is however not the case. Most of the trees or the field crops that one sees when traveling thru the landscape have already an obvious use. Much of what is considered forest or agricultural by-products are often products with important uses. Straw is e.g., used as beddings in stables, beet pulp as feed for cows, saw dust to make fiberboards, lignin to generate heat and electricity etc. Furthermore, to stay fertile, the soil must be allowed to keep some organic material after harvest.

Much effort is put into research on methods of converting biomass into fuels for objects with engines that can not be supplied by an electric cord, e.g., cars and trucks. This is sad from at least two points of view as it to a large extent is in vain. The need for transportable energy concentrates like petrol and diesel exceeds what possibly is available from biomass. The ultimate sustainable solution to the transportation problem is electric reduction of carbon dioxide to form methane, methanol or the like. In large and practically unlimited scale, electricity is available from nuclear power plants or wind mills. The challenge is not the availability of energy but how to generate it at low cost. Furthermore, it is sad to defunctionalize biomass to hydrocarbons or other chemically trivial compounds. An example of this is production of ethene from sucrose. Sucrose is largely available from sugar cane or beet and can be fermented to ethanol that on dehydration gives ethene. Since so much functionality is lost during the transformation, 1 kg of sucrose delivers only 0,164 kg of ethylene, assuming 100 percent yields. The ethylene, can be polymerised to polyethylene, an important plastic material but the added value for the consumer of the production route is obscure.

The birth of modern polymer production took place around year 1900. On of the most noticeable achievements was the discovery of Bakelite by Leo Baekeland in 1909. It is a condensation product of phenol and formaldehyde with good resistance to heat and chemicals and high electrical resistance. Properties that were crucial to rapid electrification of the society. In the little south Swedish town of Perstorp, an Indian chemist Das Gupta, developed a similar polymer, named Indolack, from formaldehyde and cresol. The formaldehyde was prepared by oxidation of methanol formed by dry distillation of wood. The process also delivered a phenolic fraction containing, among other constituents, cresol. Thus, the polymer was 100% synthetic but also 100% non-fossil derived.

By time, and especially after WW II, biological raw materials, as source for polymers were superseded by fossil such as coal and oil. The interest in chemicals from renewable raw materials was there all thru the 20:th century but on a low level. In the last decades, the interest has risen substantially as “bio-materials” in the broadest sense has come to be a mean of reducing such diverse problems as resource depletion, energy crisis, air pollution, global warming, full landfills, and littering of the seas etc. This is of course not realistic. The limited availability of renewables, necessitates utilisation in applications where its full potential is taken advantage of. One such is as functional monomer (functiomer) in polymerisation reactions or other high value chemicals. A functiomer confer special property to a polymer besides bulk. Some classical examples are methacrylic acid and 2,2-bis(hydroxymethyl)propanoic acid that make poly(meth)acrylates and polyurethanes, respectively, water compatible. Allyl ethers like trimethylolpropane allyl ether that reduces oxygen inhibition in the curing of unsaturated polyesters, alcoholic monomers like 2-hydroxyethyl methacrylate and 4-hydroxybutyl acrylate that make poly(meth)acylates crosslinkable with melamines or isocyanates, and diacetone acrylamide that is used for the crosslinking of poly(meth)acrylates.

Most of these functiomers are considerably more expensive than monomers providing merely bulk. That is, their incorporation in a polymer increases the usefulness, i.e., the value of the polymer. A synonymous word to functiomer could be valuemer. One of the most important properties of a polymer is the ability of latent cross linking. During formation of an object, it is advantageous if the polymer is flexible and easily adopts the desired shape. Thereafter, it is desirable to have new bonds formed between the polymer molecules to lock them in the shaped positions. This process has been known for centuries in the case of oil paints. The oil, that is a kind of prepolymer, is applied to a surface and by the action of atmospheric oxygen the fatty acid residues polymerise to form a crosslinked network that greatly improves the protective properties of the coating. Oil paints have several shortcomings such as yellowing and chalking. During the 20:th century alkyds were developed that greatly reduced these problems. Alkyds have a high content (60-80%) of vegetable oil but a much higher molecular weight. In the last years, the inherent formation of volatile fragmentation products, during cure, such as aldehydes have become a health issue. Apparently, biogenic materials are not always all thru benign.

For decades, large efforts have been made to convert biomaterials to starting materials for organic synthesis. Not very many have been delivered. An unsurmountable request is a short synthetic path as a synthetic step means investment (capital cost), resource consumption (energy, water, etc.), and extremely few chemical reactions goes with 100% yield (loss of material and production of waist). As pointed out, biomaterials are often functionalised to the degree that they do not fit into the regular toolbox of the organic chemist, thus, defunctionalisation to an adequate degree is a necessity. Remaining, or formed functional groups should preferably be versatile allowing for a variety of further chemical transformations.

Some examples of the more promising biogenic platform chemicals are vanillin, furfural, 5-hydroxymethyl furfural, and 5-chloromethylfurfural.

 

Besides furfural, non of these are commercially available in an industrial meaning but they are all possible to synthesise in only one step from carbohydrates (furans) and lignin (vanillin). The yields are not always great but as the staring materials are inexpensive and the route short, it is realistic to hope for a successful commercialisation in an industrial scale. The above compounds are all aldehydes that can be acetalized, oxidised, or reduced to other important chemical functional groups. The hydroxyl group of 5-(hydroxymethyl)furfural is nucleophilic and the chloromethylgroup of the 5-(chloromethyl)furfural is electrophilic, thus they provide a complementary set of reactivity. The hydroxylgroup of vanillin is phenolic, which is interesting as such and to increase its versatility even more, it can be converted to an aliphatic alcohol in a simple reaction between vanillin and ethylene carbonate. Vanillin is also easily halogenated to give 5-halovanillin, thus available for interesting coupling reactions of Heck type.

 

Developing valuable and useful products from the above furan or vanillin derivatives should encourage interest and further efforts in their manufacturing. 

Nicola Rehnberg

Adjungerad Professor i Polymerteknologi

Docent i Organisk Kemi

Elektroniska adresser:

nicola.rehnberg@chem.lu.se

+46-73-6557337

Postadress:

Centrum för Analys och Syntes

Lunds Universitet

P. O. Box 124

SE-221 00 Lund

Besöksadress:

Kemicentrum

Naturvetarvägen 14

Lund