An experimental and modeling study on the catalytic effects of select metals on the fast pyrolysis of hardwood and softwood lignin

Abstract

Naturally occurring alkali and alkaline earth metals (AAEM) play an important catalytic role in the pyrolysis of lignin. Other metals also potentially play a role in the catalytic deconstruction of lignin but have only been qualitatively investigated. A combination of experiments and computational modeling were performed to explore the catalytic activity of ferrous iron in comparison to AAEM. Pyrolysis experiments with extracted lignin and density functional theory (DFT) calculations for model lignin dimers showed agreement between theory and experiment. Ferrous iron proved to be a stronger catalyst than either potassium or calcium. The activity order of the AAEM cations was less clear as model and experiments agreed for hardwood lignin but disagreed for softwood lignin. DFT predicted calcium to be a stronger catalyst than potassium for breaking β-O-4 ether bonds while experiments indicated potassium to be more catalytically active as a result of higher turnover frequency. Pyrolysis of softwood lignin had a lower apparent activation energy (9.2 kcal mol⁻¹) than for hardwood lignin (15.3 kcal mol⁻¹). Of the catalysts tested only ferrous iron prevented the melting of lignin during pyrolysis due to its low apparent activation energy of 3.6 kcal mol⁻¹ and 8.6 kcal mol⁻¹ for softwood and hardwood lignin, respectively.