Over the past two years I’ve been heavily involved in the chemistry of wood products. To an organic chemist such as myself, wood is an extremely attractive starting material. It’s cheap. It’s renewable, meaning that we can easily replace the stocks that we use. People may not realize this, but petroleum / petroleum chemicals are currently the largest source of starting materials for plastics, solvents, and pharmaceuticals. Oil supplies can be depleted and once they’re gone, there is no easy solution. The problem is that organic chemists have been lazy. Oil is almost too attractive of a source for the various molecules that make up the beginnings of our reactions. The various valuable fractions are there just waiting for us to isolate them, which is often a simple matter of distillation. It’s really not a surprise that we’ve become so dependent on oil supplies for organics, but the practice is one that has to be phased out, because we’re eventually going to run out of oil.
Wood is a much better choice of organic starting material. We’re in no danger of running out of tree stock. The reason that we haven’t made the jump sooner is that wood keeps it’s valuables locked away more tightly than oil. A typical hardwood contains three major structural components: cellulose, hemicellulose, and lignin, all of which are almost hopelessly intertwined and crosslinked. Of these, it’s typically only cellulose that is valuable for conversion into different types of biofuels (the liquids that we can easily burn in engines for energy). Cellulose is a long repeating sugar polymer and can be transformed into biofuels by a process known as pyrolysis. The wood fraction is heated in air in the absence of oxygen. This avoids simply “burning” the cellulose, transforming the organic material into hundreds of complex molecules. The process isn’t entirely understood. There are simply too many products and far too many reaction pathways for chemists to have a firm handle on the process. I’m well aware of this difficulty, as it’s something that I’ve tried to overcome for the past two years in my own work. That’s why I was very excited to read a recent article published in Nature: Chemistry which discussed using a model system to mimic the reactions taking place in cellulose.
This research (published by some chemists from Delaware) was particularly clever. A cellulose molecule is extremely long – hundreds and even thousands of molecules linked end to end. Its size makes the behavior in pyrolysis difficult to understand. The repeat unit, a simple sugar, can’t be used as the model because the free sugar groups in the non-repeating molecule throw off the analysis. To overcome this difficulty, the chemists turned to alpha-cyclodextrin, which is a cyclic molecule. Imagine taking a flexible straw and being asked to describe it’s “essence” without describing the ends of the straw. The solution in this paper was to bend the straw into a ring. That way, there are no ends, and the only repeating bonds are the ethers that react in the pyrolysis. It was definitely a smart technique. I’ve made macrocycles before, and there’s a certain beauty to their simplicity. The use of cyclodextrin as a model for cellulose allowed the researchers to lay out several new models for the pyrolysis process. It should allow chemists to design more effective and more productive means of converting wood into liquid fuels, because we finally understand some of the underlying processes.
The source of this article can be found at:
Pichon, A. “Cellulose conversion: a promising pyrolysis”. Nature: Chemistry 2012, 4, 68-69.