(556a) Molecular Modeling of Pyrolysis

Modeling pyrolysis of lignocellulosic biomass with a network of true, elementary reactions is now a plausible goal, not a quixotic vision. The key for lignocellulosic biomass is to deconstruct kinetics as being dominated by cellulose, hemicellulose, and lignin, the three major cell-wall components. Biomass contains a great diversity of species and biopolymers, yet we can recognize chemical structures and features of kinetics that distinguish these three fractions:

Cellulose. Natural cellulose is a polysaccharide of β-D-glucose, linked by 1,4-glycosidic bonds. When aligned, the OHs in each polymer chain form a large number of hydrogen bonds that interlink the chains in an orderly fashion, allowing strong crystallinity and microfibril formation. Work in the past five years [1-5] has revealed that in pure cellulose, the material become amorphous by thermal disruption (a state empirically inferred as "active cellulose" previously), and then pericyclic reactions cleave the chain to a cello-n-san and a shortened cellulose oligomer, ultimately yielding levoglucosan. The pericyclic reactions may be either unimolecular steps or OH-catalyzed, bimolecular reactions. When ions are present, evidence suggests the chain components are chelated, aiding char formation [6].

Hemicellulose. Hemicellulose is a variety of polysaccharides including five- and six-membered ring monosaccharides and displaying branching that forces them to be amorphous. Characteristic hemicellulose biopolymers are xylan and galactoglucomannan. Xylan itself is a family of poly(xylose) polymers, representing about 10-35% and 10-15% of the hemicelluloses in hardwoods and softwoods, respectively, where the dominant xylans in those cases also differ (O-acetyl-4-O-methylglucuronoxylan and arabino-4-O-methylglucuronoxylans) [7]. The galactoglucomannans are more structurally diverse, being random copolymers of galactose, glucose, and mannose with some acetyl substitutions [8]. Work in my group pursues the hypothesis that pyrolysis transition states for hemicellulose structures are pericyclic and analogous to those found for cellulose.

Lignin. Lignin contains aromatics connected by aliphatic segments, some of which are covalently bonded with hemicellulose. Aromatics content of lignin is substantially phenolic, giving it a high energy content and lower, benzylic-type bond energies. Experiments and modeling at high temperature (e.g., [9,10,11]) show that pyrolysis is dominated by homolytic bond-breaking to form radicals, which then break down by β-scission and abstractions. The chemical steps are amenable to radical-chemistry modeling that has been developed for combustion and thermal cracking of polymers (e.g., [13]).

In this presentation, I will review these aspects, along with new data and modeling that move us toward elementary models of lignocellulosic pyrolysis.

References:

[1] R.S. Assary, L.A. Curtiss, CatChemComm 4:2 (2012) 200-205.

[2] H.B. Mayes, L.J. Broadbelt, J. Phys. Chem. A 116:26 (2012) 7098-7106.

[3] V. Agarwal, P.J. Dauenhauer, G.W. Huber, S.M. Auerbach, J. Am. Chem. Soc. 134 (2012) 14958−14972.

[4] V. Seshadri. P.R. Westmoreland, J. Phys. Chem. A 116:49 (2012) 11997-20013.

[5] X. Zhou, M.W. Nolte, H.B. Mayes, B.H. Shanks, L.J. Broadbelt, Ind. Eng. Chem. Res. 53:34 (2014) 13274–13289 and 13290–13301.

[6] J.M. Jones, M. Nawaz, M. Pourkashanian, A. Williams A, Bioenergy Review, 1:1 (2003) 37-41.

[7] H. Sixta (ed.), Handbook of Pulp 1. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 28–30 (2006).

[8] S. Willför, K. Sundberg, M. Tenkanen, B. Holmbom, Carbohydrate Polymers, 72:2 (2008) 197-210.

[9] M.T. Klein, P.S. Virk, Ind. Eng. Chem. Fundam. 22 (1983) 35-45.

[10] M.T. Klein, P.S. Virk, Energy & Fuels 22 (2008) 2175–2182.

[11] J. Cho, S. Chu, P.J. Dauenhauer, G.W. Huber, Green Chem. 14 (2012) 428-439.

[12] T. Faravelli, A. Frasoldati, G. Migliavacca, E. Ranzi, Biomass and Bioenergy 34 (2010) 290-301.