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Lis Nimani
2011 Wisconsin Bioenergy Summit
October 6th, 2011

University of Wisconsin - Madison
Biological Systems Engineering

 Introduction

 Lignin Isolation

 Depolymerization and Conversion of Lignin
◦ Pyrolysis (thermolysis)
◦ Gasification
◦ Hydrogenolysis
◦ Chemical Oxidation
◦ Catalysis
◦ Hydrolysis under Supercritical Conditions

 Renewable Energy sources are needed to replace
Fossil Fuels
◦ Depletion of Traditional Fossil Fuels
◦ Global warming due to GHG’s
◦ National Security
 Biomass as an Energy source
◦ Corn/Sugarcane (1st Generation)
◦ Lignocellulosic Biomass (2nd Generation)
◦ Algae (3rd Generation)
 Lignocellulosic Biomass
◦ Promising Biomass Source
◦ Cellulose, Hemicellulose and Lignin

 Biorefineries convert biomass into useful energy
and value added products.
◦ Cellulose and Hemicelluloses are generally converted to
ethanol.
◦ Lignin is normally combusted for Heat.
 Low value
 Underutilized
 Conversion of Lignin into Value Added Products.
◦ 15-30 % of biomass is Lignin
◦ Higher Energy Content than Cellulose
 9000-11000 Btu/lb vs. 7300-7500 Btu/lb
◦ Challenge: Lignin is a very complex molecule. It is
difficult to decompose and generates very high amounts
of solid residue.

 Lignin is a copolymer of three different
phenylpropane monomer units:
◦ para-courmaryl alcohol, coniferyl alcohol and
sinapyl alcohol

 Peroxidase and laccase enzymes in the plant cell
walls cause the dehydrogenation of phenolic OH
groups and generate free radicals from these
lignin precursors.
 Polymerization of the lignin monomers starts
with the coupling of two radicals forming a dimer
◦ Β-O-4 aryl ether bonds are the most frequent coupling
linkage formed
 Polymerization progresses with further coupling
of monomeric radicals with dimers, trimers and
oligomers resulting in complex branched
polymer.

Exact structure of Lignin in its Native form in plants is still unclear.

 Composition and amount of Lignin varies from species to
species as well as woods from different parts of the same tree.
◦ Softwoods (mostly G)>Hardwoods ( G and S)>Grasses
 Functional groups that influence the reactivity of lignin consist
of: methoxyl, phenolic, and aliphatic hydroxyl, benzyl
alcohol, noncyclic benzyl ether and carbonyl groups

 Mechanical vs. Chemical Processes
◦ Lignin can be isolated in a variety of methods
 Methods can be grouped into two Major Pathways
◦ Cellulose and Hemicelluloses are removed by solubilization,
leaving lignin as insoluble residue.
 Ex. Lignin available as by-product from a lignocellulosic ethanol
fuel biorefinery
◦ Methods involving dissolution and removal of lignin, leaving
cellulose and hemicelluloses as insoluble residues, followed
by the recovery of lignin from the solution.
 Ex. Kraft and Sulfite Lignin
 Other Types of Extracted Lignin:
◦ Organosolv (more pure and unaltered lignin due to milder
conditions), Milled Wood Lignin (Bjorkman Process), Steam
Explosion Lignin, Cellulolytic Enzyme Lignin, Klason Lignin,
Acid Hydrolysis Lignin, Soda Lime Lignin, etc.
 Isolation Method has an influential role in determining
the nature and structure of lignin.

 Degradation and conversion of lignin occurs through
thermochemical treatments.
◦ Includes thermal treatment of lignin in the presence or absence
of solvents, chemical additives and catalysts.
 Pyrolysis: Thermal treatment of biomass in the absence of
oxygen, with or without catalyst.
 Gasification: converts lignin to gasses. Major products are
H2,CO,CO2 and CH4.
 Hydrogenolysis: involves thermal treatment in the presence of
hydrogen so that the cleavage of bongs is assisted by the
addition of hydrogen. Used at lower temperatures favoring
higher yields of liquid.
 Oxidation: Thermal treatment in the presence of oxygen. Lignin
to aldehydes.
 Comubustion: Burning the Lignin to create heat and or
electricity.
 Note: Yields and composition of degradation products vary based on the process and the
conditions applied. Also, the nature of the lignin and its various composition and functional
groups have significant effects on the lignin conversion and product yields.

Lignin Depolymerization and Conversion

 Pyrolysis is the most studied method for the
conversion of biomass to lower-molecular weight
liquid or gaseous products.

 Def: Heating an organic substance in the absence of
air so that the molecular structure is broken down
into smaller units while the limited oxygen available
for the reaction ensures that there is no further
combustion to Carbon Dioxide.

 Pyrolysis of lignin is complex and is affected by
several factors: feedstock type, heating rate, reaction
temperature, additives, etc.

 Products: Hydrocarbons (gaseous), CO, CO2, volatile liquids
(methanol, acetone, acetaldehyde), monolignols,
monophenols, char
 Major monomeric aromatics obtained from pyrolysis:

 Lignin degradation through pyrolysis begins with
cleavage of weaker bonds at lower temperatures
and proceeds through breaking of stronger
bonds at higher temperatures.
◦ 120-300 °C: water, formaldehyde, formic acid.CO, CO2
◦ >500°C: aromatic ring cracking and condensation occur
releasing Hydrogen

 Gasification of Lignin at HIGHER temperatures
yields
◦ Hydrogen (cracking aromatic rings)
◦ CO2 (by reformation of Carbon-Oxygen double bond and COOH)
◦ CO (cracking of C-O-C and Carbon-Oxygen double bond)
◦ CH4 (cracking of methoxy groups

 Kinetics: A single step first order reaction
model is assumed.
◦ Arrhenius equation is used to determine rate
parameters
◦ Parameters vary for the lignin type or the isolation
method, as well as methods used to compute the
parameters.

 Pyrolysis performed in the presence of Hydrogen.
 Application of suitable solvents and catalysts can
speed up the reaction and increase the product yield.
 Very promising method for producing phenols from
lignin.
◦ Leads to higher net conversion, higher yields of
monophenols, and less char formation.
 Yields can be increased even greater if pretreatment techniques
such as microwave and/or ultrasound irradiation is performed
before hydrogenolysis.
 Reaction temperature range is: 300-600 °C.
 Performed by two different methods
◦ Treating lignin to gaseous Hydrogen
◦ Treating lignin to hydrogen donating solvent.

 A method using gaseous hydrogen to depolymerize
lignin is called Base Catalyzed Depolymerization
(BCD). In general this method is used before
hydrogenation to convert lignin to gasoline.
 Shabtai et al. proposed a multi step process for
converting lignin into reformulated gasoline that
includes BCD followed by hydrogenation and
hydrocracking.
 The BCD reaction uses a catalyst-solvent system of
an alkali hydroxide (KOH) and a supercritical alcohol.
◦ Reaction Conditions: Temperatures around 270 °C and
pressure of 140 bar for 1-5 minutes
◦ Reactions cause about a 50% reduction in Oxygen
compared to native lignin.

 After BCD the following stages occur to
produce reformulated gasoline:
◦ 2nd stage: Depolymerized lignin is subjected to
hydrodeoxygenation (HDO) in the presence of
catalyst. (ex. CoMo/Al2O3)
◦ 3rd stage: Involves partial ring hydrogenation and
mild hydrocracking in a hydrogen-rich catalytic
environment.
◦ Final product compares to reformulated gasoline.
◦ If Chemicals are desired then the step can be
skipped to produce phenolics.

 Process of BCD-HT can be improved by separating
the monomers (chemicals) and oligomers
(intermediates for fuel).
 Also lignin concentration in base is inversely
proportional to monomer yield.
◦ Lower concentration results in higher monomer yield.
◦ 10 % yield was observed to be optimal.
 Economics: (2004 Prices)
◦ Based on 100 kg lignin feedstock
◦ A yield of 7 wt-% of monomers provides revenue of $14 -
21 in chemicals
◦ Based on 45 wt-% of lignin, dimers, trimers and oligomers
can account for about $13-21.6.
◦ Total worth = $27-42.6.
◦ Lignin for combustion is worth $10

 Gasoline additives/blending agents can also be made
in a similar fashion.
 Johnson et al. had a goal to convert lignin into a
hydrocarbon product compatible with blending in
gasoline that has an octane number greater than 110.
 BCD is first performed on lignin. Then it is isolated by
acidification and then by solvent extraction (Diethyl
Ether). Then catalytic hydroprocessing (HPR) is
performed to convert the BCD lignin into aromatic
hydrocarbons that can be blended with gasoline.
◦ Production cost of BCD-HPR is $0.60-0.75/gallons
◦ Value of lignin-derived gasoline blending component is
$0.97-1.14/gallons

 The target characteristics for the hydrocarbon
product are:

 BCD reactions cause the lignin to be partially depolymerized.
Also Demethoxylation occurs during the BCD reaction.

 Lignin than is hydroprocessed to produce mixtures of aromatic and
napthenic hydrocarbons that can be blended with gasoline.
◦ It is preferred that napthenes are minimized (limiting hydrogenation).
 Analyses of the BCD-HPR products indicate that complete
deoxygenation with minimal hydrogenation is not easily attained

 Studies have also performed BCD-HT on
different types of Isolated Lignin
◦ Kraft Lignin, Organosolv Lignin, Ethanol Lignin
 Results show similar product spectrum and
small differences in chemical reactivity
 Analysis of products from model compound
reactions revealed that phenyl ether linkages
were effectively broken while Carbon-Carbon
linkages were less affected.

 Using Tetralin as an Hydrogen Donating Solvent
◦ Davoudxadeh et al. used tetralin with phenol for lignin
hydrogenolysis to give greater liquid yields than neat
pyrolysis.
 A yield of 20 wt-% was observed at 345 °C
 Upon dehydrogenation, tetralin releases 4
Hydrogen atoms at hydrocracking severities and
is converted to napthalene.
◦ Therefore hydrocracking severities stops reaction.
◦ Presence of tetralin for long residence times causes the
reduction in the amount of guaiacol yield and increases
the yield of phenol.

 Using Formic acid as an Hydrogen Donating Solvent
 One step alternative for the conversion of lignin into a low-oxygen
fuel and monomeric phenols using formic acid has been published.
◦ This method uses formic acid as a hydrogen donor and alcohol as
the solvent.
 When heated, formic acid decomposes completely to CO2 and active
Hydrogen, which combines with oxygen from the methoxy groups of
lignin to form water. The following reaction occurs:

 Lignin is good for oxidation or oxidative cracking due
to the presence of hydroxyl groups.
 The oxidative cracking involves the cleavage of the
lignin rings, aryl ether bonds, or other linkages within
the lignin.
 Products: Range from aromatic aldehydes to
carboxylic acids based on the severity of the reaction
conditions.
◦ Softwood: vanillin (industrially produced), vanillic acid
◦ Hardwood: syringaldehyde, syringic acid
 Nitrobenzene, metal oxides, and hydrogen peroxide
(not good for aldehyde production) are the most
popular oxidants for lignin, while catalytic oxidation
with oxygen is also possible.

 Solvents in supercritical conditions behave
differently than in subcritical conditions.
◦ Many advantages. Ex. Have ability to dissolve nonpolar
organic molecules and inorganic solvents.
 Decomposition of Lignin in supercritical water
occurs first by hydrolysis and then by
dealkylation yielding low-molecular-weight
fragments.
 Conversion of Lignin in Supercritical water has
been studied
◦ Conditions: T=374.15 °C, P= 22.1 MPa
◦ Monomeric yields have been low due to repolymerization
into char
 Can increase yields by introducing phenols.

 Catalysts used in lignin depolymerization should promote
high conversion and suppress char formation and
condensation.
◦ Conditions are not severe
◦ Also should assist in selective bond cleavage, leading to high
selectivity values for particular compounds.
 Various catalysts are used for different processes and
substrates.
◦ Zeolites and amorphouse silica-alumina catalysts
 Disrupting the lignin polymer by cracking and upgrading pyrolysis oils.
 Zeolites produced more aromatic hydrocarbons
 Silica-alumina favors aliphatic hydrocarbons
◦ Base catalysts (KOH and NaOH)
 effective in lignin hydrolysis (BCD)
◦ Hydrogenation Catalysts (Co, W, Pd, Pt, Ni, Ru…)
 Increases the yield and promotes hydrodeoxygenation

 Selective Degradation of wood lignin over Noble Metal Catalysts.
◦ Yan et al. have demonstrated that Lignin can be selectively
cleaved at the ether units to produce monomers and dimers.

 The reaction took place in a Parr bomb pressurized to 4 MPa and the
temperature set at 200 °C for 4 hours.
◦ Different catalysts were used to identify selectivity for specific monomers.

 They also have demonstrated that after lignin depolymerization these species
can be further hydrogenated to destroy the aromaticity and destroy the C-O
bonds to transform lignin to C9 and C14-C18 alkanes

 Summary of Literature on Catalytic Lignin Conversion:

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Lignin Depolymerization and Conversion

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