Recent Approaches to

Lignin Valorization

Caitlin McMahon

Group Meeting 01/17/2014

Biomass

• Attractive feedstock for renewable fuels and chemicals

• Potential for 1 billion dry tons of biomass per year in the

U.S. alone

• U.S. Departments of Agriculture and Energy goals for 2030:

• 20% of transportation fuels from biomass

• 25% of U.S. chemical commodities from biomass

Weckhuysen, B. M. Chem. Rev. 2010, 110, 3552–3599.

Lignin • Lignocellulose (non-edible biomass) is the largest material by

volume produced in the world every year • Provides strength to trees and plants

• Composed of 40-50 wt% cellulose, 10-25 wt% hemicellulose, and 25-40 wt% lignin

• Lignin is an amorphous polymer of hydroxy-

and methoxy-substituted phenylpropane units

• Only renewable source of aromatic compounds

in large volumes

• Historically received little attention due to chemical recalcitrance

• “Waste” product of pulp and paper industry

Weckhuysen, B. M. Chem. Rev. 2010, 110, 3552–3599.

Lignin Structure • Lignin building blocks:

• Common linkages:

• β-O-4 linkages most common and weakest bonds

Weckhuysen, B. M. Chem. Rev. 2010, 110, 3552–3599.

Lignin Degradation • Enzymatic (in nature):

• White-rot fungi

• Lignin peroxidases, manganese peroxidases, laccases

• Strategies:

• Gasify syn gas

• Petroleum feedstock-like pyrolysis small molecules

• Defunctionalization simple aromatic compounds bulk and fine

chemicals

• Highly selective catalysts/break specific linkages valuable

functionalized

chemicals

Thielemans, W. Coord. Chem. Rev. 2010, 254, 1854–1870.

Weckhuysen, B. M. Chem. Rev. 2010, 110, 3552–3599.

Lignin Degradation • Cracking or Hydrolysis

• Commonly used in petroleum refineries – converts higher-boiling

hydrocarbons to smaller, more valuable chemicals

• Heterogeneous catalysts, high temperatures and H2 pressures

• Not specific

• Catalytic Reduction Reactions

• Mainly focused on production of bio-oils and fuels

• Production of bulk aromatic compounds (B,T,X, phenols)

• Remove functionality

• Catalytic Oxidation Reactions

• Focused on forming more complex aromatic compounds

• Platform chemicals or target fine chemicals Weckhuysen, B. M. Chem. Rev. 2010, 110, 3552–3599.

Recent Advances - Reduction • Generation of simple arenes – breaking C-O aryl bonds

• Dehydroxylation of phenols – Rinaldi

• Lignin model compounds and actual bio-oil & organosolv lignin

Rinaldi, R. Angew. Chem. Int. Ed. 2013, 52, 11499–11503.

Recent Advances - Reduction

• One-pot tandem reaction pathway:

Rinaldi, R. Angew. Chem. Int. Ed. 2013, 52, 11499–11503.

Recent Advances - Reduction • C-O bond cleavage and hydrodeoxygenation – Omar

• Lignin model compounds and synthetic polymer:

• Selectivity and activity of Zn/Pd/C maintained for at least 3 cycles

Abu-Omar, M. M. Chem. Sci. 2013, 4, 806–813..

Recent Advances - Reduction • Hydrogenolysis of Aryl Ethers – Hartwig

• Examples with lignin model compounds:

Hartwig, J. F. Science 2011, 332, 439-443.

Hartwig, J. F. JACS. 2012, 134, 20226–20229.

Recent Advances - Reduction

• Vanadium-catalyzed C-O bond cleavage – Toste

• Trimeric lignin model:

• Degradation experiments on lignin derived from M. giganteus

• Lowering of overall molecular weight based on GPC analysis

• Detection of various linkages determined by 2D NMR (β-O-4’ disappearance)

• GC/MS provided qualitative and quantitative analysis of volatile monophenolic

compounds produced

• Pretreatment and isolation of lignins affects degradation

Toste, F. D. Angew. Chem. Int. Ed. 2010, 49, 3791-3794.

Toste, F. D. ACS Catal. 2013, 3, 1369-1377.

Recent Advances - Reduction

• Compounds detected from Toste’s Vanadium-catalyzed

lignin degradation

Toste, F. D. ACS Catal. 2013, 3, 1369-1377.

Recent Advances - Oxidation

• Vanadium catalyzed C-C bond cleavage – Hanson

• Benzoquinone and acrolein products

• Different selectivity from Toste catalyst (C-C vs. C-O)

• Phenoxy group essential for C-C bond cleavage – potential phenoxy radical intermediate

Hanson, S. K. Angew. Chem. Int. Ed. 2012, 51, 3410–3413.

Recent Advances - Oxidation

• Cobalt-catalyzed oxidation of lignin models to benzoquinones

• Intermediate Co-superoxo complex generates

phenoxy radical

• Preliminary experiments on tulip poplar lignin

• Low absolute yields of monoaromatic compounds

due to isolation conditions (lower amounts of free

phenolic OH’s

Bozell, J.J. Org . Lett. 2013, 15, 2730-2733.

Recent Advances - Oxidation • Metal-free aerobic alcohol oxidation in lignin – Stahl

• 1° and 2° alcohols are oxidized, but selective for 2°

• Oxidized isolated Aspen lignin

Stahl, S. S. J. Am. Chem. Soc. 2013, 135, 6415–6418.

Recent Advances - Oxidation

• Redox-neutral C-O bond cleavage – Ellman & Bergman

• Proposed organometallic C-O activation to form Ru-enolate

• Quantitative depolymerization of poly(4’-hydroxy-1-phenethanol) to

4’-hydroxyacetophenone

Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2010, 132, 12554–12555.

Recent Advances - Oxidation

• Photochemical lignin degradation – Stephenson

• Catalytic oxidation of 2° alcohol C-O bond cleavage

• C-O bond significantly weakened after oxidation

• No intermediate purification necessary

• Generation of products not generated in high yields from other methods

• Batch-to-flow set-up gives high yields with lower catalyst loadings

Stephenson, C. R. J. J. Am. Chem. Soc. 2013.

.

• Lignin is a currently underutilized

source of fuel and chemicals

(especially aromatics) due to

recalcitrance, but has high potential

• Reductive C-O and C-C bond cleavage

to form simple aromatic compounds

• Oxidative cleavage to form highly

functionalized aromatic compounds

• Need for processes that can be applied to actual lignin

Conclusions