• Issue 1  |
  • August 2009

Lignin Thermal Decomposition Explained Through New Computational Studies

Ariana Beste, A. C. Buchanan III, and Robert J. Harrison

Lignin is the second most abundant naturally occurring biopolymer found in vascular plants, and is a byproduct of making paper. Currently, this byproduct gets burned, but with the proper understanding and development, it could possibly be used as a renewable source of fuel and chemicals. While there is already a significant understanding of the thermal breakdown processes in materials made of carbon and hydrogen, less is known about the thermochemical conversion of materials containing oxygen like lignin. Experimental studies have shown that the thermal decomposition of lignin is dependent on two hydrogen transfer reactions, one with phenoxyl radicals and one with benzyl radicals (see Figure 1).

Hydrogen transfer steps

Figure 1. Hydrogen transfer steps investigated.

When phenethyl phenyl ether (PPE), a lignin model compound, is decomposed, two sets of products are formed depending on which hydrogens are involved in the hydrogen transfer reaction. It is estimated that there is a 7 kcal energy difference between the product pair formed from the transfer of the hydrogen further from the oxygen (the a-hydrogen) and the product pair formed from the transfer of the hydrogen closer to the oxygen (the β-hydrogen). As shown in Oxygen Substituent Effects in the Pyrolysis of Phenethyl Phenyl Ethers, Britt et al. studied the compounds experimentally, and found that the product selectivity (the ratio of the α to β product pairs) was not as large as this energy difference suggested it would be. Intrigued, we used density functional theory – a method of computationally simulating the reaction – to study the intermediate stages of the reaction.

Imagine you are on a car trip. Say you have two possible destinations, and want to know which trip would give the best gas mileage. At first, all you know is that destination A is on the plain and destination B is on a hill. In that case, it’s seems that going to A would give better gas mileage. However, if you found a map to each destination and discovered that each trip required driving over a different mountain, it might change your conclusion. The size of the mountain would make more of an impact on the gas mileage than the height of the final destination. If the mountain between you and A is of a similar size to the mountain between you and B, traveling to A may still give better gas mileage. The difference, however, will depend largely on the comparative sizes of the mountains, not on the final locations.

Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US Department of Energy.