Biobased materials: Part II

Lignin

= aside from cellulose and hemicellulose the most important type of biomass in nature

• contains a lot of aromatic residues, while cellulose and hemi-cellulose don’t (they’re composed of sugars)

drawback of lignin as SM in chemical industry

has a strong structural inhomogeniety because the linkages between the aromatic rings can be quite different

not the case for cellulose or starch which only contian alpha or beta glucose

lignin in chemical industry

Breakdown of lignin generally leads to monolignols

• wich are benzenes substituted with oxygen containing moieties and differ by amount of substituents

How?

1. lignocellulose is fractionated to get rid of cellulose and hemicellulose

   1. forms pure lignin

2. Lignin is depolymerized to monolignols or related products

3. these can be used for other purposes

   1. starting materials for

      1. pharmaceutical

      2. polymers

   2. defuncitonalization to be used as fuels

      1. (not a great idea)

   3. functionalized to value added chemicals

      1. can be used as additives to fossile fuels to improve performance (petrol)

Extraction of lignin in the past

previously ligin was mostly a by product of the paper industyr

• paper is made from cellulose extracted from wood rendering lignin as a side prodcut

red arrows = yields strongly modified lignins rendering them useless (with incorporation of sulfur)

• cellulose used to be extracted by the Kraft process yielding pure cellulose using sulfides in base

  • but this funcitonalizes lignin with sulfur containing functional groups

  • now catalytic treatments for further processing and funcitonalization cannot be used as the sulfur containging groups can poison the catalyst needed for these processes

• Alternatives

  • H2SO4 —> Klason lignin

  • NaOH —> Alkali lignin

green arrow = yields only mild or minimial modification of ligin

• organic solvent extraction

  • attractive because not chemical modificaiton

• ILs= ionic liquids

  • also been attempted but higher in price

Treatments of biomass

Best way to is to treat lignocellulose biomass alltogether with attractive functionalization, 3 approaches

1. fractionation and separation = current pulping process

   1. separates modified lignin (preferably not to strongly modified

   2. also yields an untouched cellulose/hemicellulose fraction

2. RCF = reductive catalytic fractionation

   1. separates lignin and fully convert lignin into lignin monomers

   2. leaves the sugar moieties untouched

3. catalytic conversion = one-pot catalytic process

   1. full catalytic conversion to a mixture of lignin monomers and low molecular weight alcohols and alkanes

   2. mostly used for fuels and chemicals

      

Reductive catalytic fractionation (RCF)

Relies on catalytic hydrogenolysis and depending on the type of metal used, you obtain (but leaves sugars intact):

• alkylphenols = with aliphatic alcohols removed

  • metals with high oxophilicity

    • Pd not very oxophilic

    • Zn2+ is due to LA properties (not Zn metal)

    • so Pd/Zn2+/C removes aliphatic alchols

    • or: Ru/C

• monlignols = aliphatic alchols intact

  • Pd/C

  • Ni/C

lignine contains a lot of benzyilic carbon with an oxygen attached (benzylic alcohols or ethers) which are cleaved upon hydrogenolysis = what fragments the polymer

• aliphatic alcohols are not removed by hydrogenation and require a lewis acid (like Zn2+)

The first step on the left is not cleavage of a benzylic carbon with an alcohol attached but the neighbouring benzyilic alcohol activates it for cleavage (chatgpt)

Lignin in bioprocessing

After chemcial treatment of lignocellulose lignin monomers (monolignols), dimer, trimers are formed.

• this is type: ‘fractionation and separation’ although not entirely the same as before

• it still sepeartes the saccharides (cellulose and hemicellulose) from lignin but druing the process some bonds can be cleaved leading to lignin monomer, dimers and trimers apart form polymeric lignin

These can undergo futher biochemical transformations:

• like treatment with bacteria (pseudomonas)

• these do chemical modificaitons on the lignin derived chemcicals and get different results depending on whether they were genetically engineered or not:

  • not engineered: yields polyhydroxyacid

    • can be used for surgical yarns

    • much like PHB or PHBV obtained from starch through engineered biomass

  • engineered:

    • yields catechol which is converted to muconic acid in high yields

Lignin reductive processing

lignine derived chemcials through RCF can be further functionalized to fine chemicals and materials or defunctionalized to bulk fuels orchemicals

• Functionalization involves emerging processes

  • this means they are new with respect to the processes applied in the chemical industry nowadays

  • so they’re difficult to implement because the chemical industry does not like change

    • safety: processes done for a long time, know how to do them safely, new procedures would require safety assessment froms scratch

    • it’s a captial intensive industry: meaning that it tries to avoid new investments in new plants and procedures

• defunctionalization involves Drop-in

  • this is taking another feedstock and turning them into something that can be used by current chemcial plants

    • far easier to realise (less severs changes)

  • good solution on short term

    • but emerging processes are needed on long term to get rid of fossile fuels

Lignin derived building blocks for new materials (non-drop in = emerging)

Lignocellulose is either:

depolymerized

• via

  • fractionation and separation

    • can yield muconic acid after bacterial treatment

  • reductive catalytic fracitonation

    • monolignons with or without aliphatic alchohols

• then functionalized

• and finally polyerized again to new bio-based polymers

directly funcitonalized

• but challenging to control properties because they’re highly dependent on the lignin structure

lignin derived building blocks for new materials (mainly drop-in)

what can we do with muconic acid

1. partial hydrogneation (2 double bond)

   1. building block for further functionalization

2. hydrogenation + NH3 forms caprolactam

   1. for synthesis of Nylon-6

3. hydrogenation to diacid and reductive amination to di-amines

   1. combined these monomers form nylon-6,6

4. Dials alder

   1. forms terephtalic acid for the production of PET

5. polymerization

   1. yields unsaturated polyesters

      

plant oils as green resources

Smaller fraction of biomass next to cellulose, lignin hemicellulose some proteins and oils

• but chemically interesting

They can be easily extracted and have some structural diversity.

• can be obtained form reesterification of triglycerides yielding glycerol and the needed fatty acids

use:

• biobased polymers

• bio fuels

• starting point for commodity chemicals

ricinoleic acid is particularly interesting due becasue enantiomerically pure.

New plant oils

= oils found in plants that weren’t considered before as a source of plant oils

• will become increasingly available via screening of natural organisms, selective breeding and genetic engineering

• petroselenic acid

  • only 1 double bond and position relative to COOH allows for cyclisation to 6-ring

• calendic acid

  • trans double bonds can form diene for DA

• santalbic acid

  • alkyne

• vernolic acid

  • enantiomerically pure

  • contains epoxice

CNSL

= cashew nut shell oil

• inedible

• currently a large waste stream of 450,000 tonnes/year

• 

anacardic acid is the largest fraction

• but upon heating the COOH is cleaved of and it is fully converted to cardanol

• So in technical CNSL (means thermally treated) cardanol is the largest fraction

• 

plant oils for biobased polymers (3 resins)

Traditional application field = cross-linked systems

A resin is a solid or highly viscous liquid that can be converted into a polymer.

Epoxy resins form an important and versatile class of cross-linkable polymers made from monomers containing at least two strained-ring groups called oxiranes

Alkyd resins

• has been applied on large scale for a long time eg. for paints

• glycerides with unsaturated fatty acids are polyesterified with polyols and diacids

• fatty acid chains contain double bonds that are prone to H radical absraction by oxygen biradicals during drying leading to crosslinking causing the paint to dry

  • blending these polymerized oils with different dyes gvies different colored paints

Epoxy resins (new application of oils)

= material containing epoxide(s) that can react with crosslinkers to form polymers or highly crosslinked networks

Curing = epoxide ring opening and crosslinking that turns the resin into a solid material.

So here the epoxidized fatty acids are the epoxy resins that after curing or polymerization are turned to a crosslinked network for instance for coating.

• traditional approach

  • Bisphenol A attacks on epichlorohydrin forming a new epoxide

  • can be attacked by other O- of bisphenol A and so on forming prepolymers that need to be crosslinked

• However

  • bisphenol A is hormone dirsupting adn epichlorohydrin is carcinogenic so alternatives are needed

• Crosslinking = curing

  • Amine based crosslinking

    • reacts with epoxide terminal groups (only two per prepolymer)

    • 

    • so need at least a trifunctional crosslinker for crosslinking to occur (NH2 is difunctional)

  • anhydride based crosslinking (phtalic anhydride)

    • Although it phtalic anhydride is a bifunctional crosslinker

    • 

    • it reacts with both the epoxide end groups and the interal OH groups,

    • so the prepolymer acts as the multifuncional entity

• Alternatives to epichlorohydrin and bisphenol A: Epoxides form vegetable oil

• 

  • double bonds can be oxidized to epoxides using H2O2 which can be polymerized

    • H2O2 is a green oxidant

    • CHOOH only used in catalytic amounts

    • and only produces water as waste

    • can alos introduce more than 1 epoxid allready introducing some crosslinking

    • in these examples they show these vegetavl oil epoxides as crosslinkers because attached to glycerol

  • epoxidized fatty acids can then be crosslinked

    • degree of unsaturation and degree of epoxidation determine corss linking density and thus material properties like Tg and tensile strength

Polyurethane resisns (new application of oils)

• epoxides can also be turned into alcohols forming polyols if it contains more double bonds

  • higher crosslinking density is stronger material properties

  • higher I value = higher Tg and higher tensile strength

Linear polymers from vegetable oil (4)

Synthesis of diacids form vegetable oil

1. fatty acid can be oxidatively cleaved via ozonolysis

   1. traceless reaction: no additional reagents or waste

      1. good for sustainability

   2. still rather toxic (but can be generated in situ)

2. oxidative workup

3. generates diacids

   1. can be used for A2 + B2 polycondensation

Alternative

1. alcohol of ricinoleic acid reactis with succinic anhydride which ringopens to an ester and acid

   1. succinic anhydride can easily be prepared form bioprocess form succinic acid

2. double bond can be hydrogenated if not needed

3. product can be used again as diacid for polycondensation

   

Ring opening polymerization of epoxides

1. epoxidation with H2O2 as before

2. the epoxy resins can now be polymerized via anionic ROP

   1. to linear polymers instead of crosslinked networks

3. modifying X can also alter polymer properties (like hydrophilicity

Metathesis

= very attractive from sustainability point of view

methathesis of two different fatty acids with ethylene can form various starting points for further polymerizations

• equilibrium: shift by removing products you don’t need

• yields terminal double bond:

  • turn into epoxide for anionic ROP

  • copolymerize with ethylen to give bio-based modified PE

    • by playing with amount of comonomer you can alter properties like Tg and tensile strength

  • if two terminal ones: turn into diacid via ozonolysis for polycondensation

Acyclic diene metathesis (ADMET) polymerization

= metathtesis of monomer with two terminal alkenes

• this generates ethylene which can be easily removed under reduced pressure to shift the equilibrium

derivatization of cardanol

Cardanol is formed through decarboxylation of anacardic acid, the main component of cashew nut shell oil (CNSL)

• produced in large amounts and discarded as waste but it is useful

• it can be dervatizised in various ways

  • esterifications

  • hydrogenolysis

  • O sulfation

  • nitration,…

it has a safe substitution pattern:

• 4- alkyl phenols are toxic, because they can be oxidized at the benzyilic position to quinone interemediates which are very electrophilic (arom gets restored and michael acceptor) and thus prone to Nuc attack by cellular components

  • difficult to prepare better 3-substitued analogues due to +M effect of OH

• But can be derived from CNSL

Also useful for difuncitonal monomers:

• cardenol can be transformed to bisphenols that can serve as alternatives to bisphenol A to make epoxy resins

• 

  • as bisphenol A is hormone disrupting, this serves as a good biobased alternative

Oxidative enzymatic polymerization

not verry common

• horseradish peroxidase (HRP)

  • catallyses compounds using with the help of H2O2

  • consist of Iron surrounded by a porphyrin ring

  • H2O2 oxidizes Fe to compound 1 which accepts é from substrates generating radicals while going back to its original oxidation state

• laccases

  • Cu based complexes that use molecular oxygen for the oxidation of compounds genrating water as a side product

—> polymerization via radical coupling

oxidative enzymatic polymerization of phenols using HRP

HRP polymerziaton of pheonl leads to heterogenous polymers

• the radical can be delocalized over the entire ring, so couplin gcan occur through he carbons and oxygen

• so the structure is a little ill defined

Solution = regioselectivity via the template-like effect

• PEG contains a lot of H-bond acceptors (O from ethers)

• phenol can form H-bonds to these Oxygens forming a PEG chain decorated with phenol

• this makes coupling though O less likely because it’s sterically shielded

• but still not full homogeneity because coupling can still occur through several carbons

Good functional group tolerance

• polymerization allows various FG on phenol, even cationic ones

• this is good news because this helps increase solubility

  • good for aqeous environment of the enzymes needed

solubility

• if phenols are not well soluble - use cycldextrin,

  • this forms well soluble complexes with the phenols

  • it has an inside cavity that is hydrophobic while the outside is hydrophilic

• this helps perform the polymerzation in water

  • because the enzymes perform best in water or highly polar solvents

oxidative enzymatic polymerisation:

other applications

surface patterning:

1. take gold surface

   1. has high affinity for sulphar

2. using p-ATP (para-aminothiophenol) the entire surface can be covered forming a monolayer with the basic amine sticking out

3. Then a second layer of acid species is attatched

   1. using an AFM tip as a kind of pen dipped in a solution the acid to write something on nanoscale

4. after wrting detachment can be avoided by polymerizing the acid units via oxidative enzymatic polymerization

5. this gives polymer in certain patterns attached to a surface

   1. no real application

Other applications:

• polyaniline

• polythiophen

  • these are quick and dirty ways

  • no control over molar mass likke guy Koeckelbergs

  • but cheap and applicable at large scale = what industry likes