Polymer scientists at Stellenbosch University have developed a novel method to break down lignin – a by-product in the paper manufacturing process – into valuable chemicals that can be used as raw materials in the chemical and polymer manufacturing industry. 

Lignin is one of the main components of trees and plants, but chemically it is extremely difficult to break down into simple molecules. Worldwide, the paper industry produces approximately 50 million tonnes of lignin per year as a by-product. Even though it could be exploited to provide us with an alternative to fossil fuel-based polymers, most of it (98%) is simply burned to fuel the paper mills.

Dr Ndumiso Sibanda, who developed the method as part of his doctoral thesis, says he is specifically interested in finding innovative and tailored scientific solutions to some of the more challenging questions facing the polymer, pharmaceutical and plastic industry. He will receive his PhD during Stellenbosch University’s autumn graduation this week. 

“My interest in biorefinery development was stimulated when I became part of Prof Harald Pasch’s research group in 2017 as an MSc student. Biorefinery methods are very valuable in our current society, where waste accumulation and resource depletion have negative social and environmental impacts,” he explains. 

Prof Pasch’s research group in SU’s Department of Chemistry and Polymer Science is regarded as one of the leading groups in this field. Another expert in the group is Dr Helen Pfukwa, who has done extensive research on lignin valorization. On top of revalorising an agricultural waste product, this type of research opens the way for lessening our dependence on fossil fuels, by replacing fossil fuel-derived polymers with environmentally friendly bio-based polymers.

Dr Sibanda says lignin has a very complex but versatile molecular structure, which makes it possible to synthesise new functional polymers that can be used as drug delivery vehicles, biomaterial synthesis and smart polymer synthesis.

“Generally, polymers that have been synthesized from lignin-derived monomers and their derivatives have high glass transition temperatures (called Tg’s) and are thermally stable with good mechanical properties. This makes them appealing replacements for fossil-fuel derived styrene in the synthesis of high temperature-resistant plastics, advanced composites and resins.”

The method has since been patented, and they are now working in collaboration with SU’s Department of Process Engineering to upscale the method: “We are currently at technological readiness level 5, which means we are working on an integrated approach to provide a bridge from laboratory testing and validation of the protocol to pilot testing in the factory.”

How does the technology work?

Lignin is a very complex macromolecule with a unique structure. The structure also differs depending on the origin of the wood and how it was pre-treated.

Chemists use oxygen to break down these complex macromolecules (called polymers) into smaller molecules (called component monomers) in a process called depolymerisation. In the case of lignin, however, most mild oxidative techniques do not lead to the complete conversion of lignin. Furthermore, if the process is not controlled effectively, over-oxidation may lead to structures that are even more complex than the starting lignin itself, Dr Sibanda explains.

His method, which rests on the work done by Dr Pfukwa, addresses some of these challenges. While it still does not lead to complete conversion, it improves the process significantly by preventing recombination reactions.

In the case of lignin, he succeeded in converting the macromolecules to various useful chemicals, such as vanillin (the compound that gives vanilla its distinctive aroma) and syringaldehyde (an organic compound found in wood that can serve as a building block for functional polymeric materials. Syringaldehyde is also responsible for the spicy, smoky, hot and smoldering wood aromas in whisky). These chemicals have an aromatic functionality, which means they have what chemists call a benzene ring – the simplest, organic and aromatic hydrocarbon and parent compound of numerous important aromatic compounds.

Since the 1950s, benzene was obtained solely from petroleum and is still an important building block in the manufacturing of polystyrene.

Their research takes place in the context of a wider effort in academic chemistry to promote sustainable use of resources. By converting what would have been regarded as a waste product into valuable chemicals, means that the value chain of lignin is extended, while at the same time reducing our dependence on crude oil.

Dr Sibanda says a green future for the chemical industry is possible: “We need policies that encourage the chemical and material manufacturing industries to include sustainable development in their long-terms plans. This will encourage industries to provide the same and sometimes even better quality products in a way that is environmentally sustainable. We also need more opportunities for collaborative research between industry and academia,” he concludes. 

Caption: Stellenbosch University polymer scientist Dr Ndumiso Sibanda developed a novel method to break down waste lignin as part of his doctoral research. He will receive his doctoral degree in Chemistry and Polymer Science during SU’s April 2022 graduation ceremonies. Photo: Stefan Els

US polimeerwetenskaplikes ontwikkel nuwe metode om lignien af te breek 

Polimeerwetenskaplikes by die Universiteit Stellenbosch (US) het ’n nuwe metode ontwikkel om lignien – ’n neweproduk in die papiervervaardigingsproses – af te breek tot waardevolle chemikalieë wat gebruik kan word as rou materiale in die chemikalieë- en polimeervervaardigingsbedryf.  

Lignien is een van die hoof komponente van bome en plante, maar om dit in eenvoudige molekules af te breek, is geen eenvoudige proses nie. Wêreldwyd genereer die papierindustrie ongeveer 50 miljoen ton lignien per jaar as neweproduk. Alhoewel dit ontgin kan word as plaasvervanger vir fossielbrandstofgebaseerde polimere, word die meeste daarvan (98%) slegs verbrand as brandstof vir die papiermeule. 

Dr Ndumiso Sibanda, wat die metode ontwikkel het as deel van sy doktorale proefskrif, sê hy stel spesifiek daarin belang om innoverende en doelgemaakte wetenskaplike oplossings te vind vir van die meer uitdagende vrae wat die polimeer-, farmaseutiese en plastiekindustrie in die gesig staar. Hy ontvang sy PhD hierdie week gedurende die US se herfsgradeplegtigheid. 

“My belangstelling in bioraffinadery-ontwikkeling is geprikkel toe ek in 2017 as MSc-student deel geword het van Prof Harald Pasch se navorsingsgroep. Bioraffinaderymetodes is baie waardevol in ons huidige samelewing waar die ophoop van afval en hulpbronuitputting ’n negatiewe impak op sosiale en omgewingsvlak het,” verduidelik hy. 

Prof Pasch se navorsingsgroep in die US se Departement Chemie en Polimeerwetenskap word beskou as een van die voorste navorsingsgroepe op hierdie gebied. Nog ’n kundige in hierdie groep is Dr Helen Pfukwa, wat uitgebreide navorsing gedoen het oor waardetoevoeging tot lignien as ’n landbouafvalproduk. Boonop baan hierdie tipe navorsing ook die weg vir ons om ons afhanklikheid van fossielbrandstowwe te verminder, deurdat polimere afkomstig van fossielbrandstowwe, vervang word met omgewingsvriendelike biogebaseerde polimere. 

Dr Sibanda sê lignien het ’n baie komplekse, dog veelsydige molekulêre struktuur. Daarom is dit moontlik om nuwe funksionele polimere soos biomateriale, slim polimere en selfs polimere wat as afleweraars van geneesmiddels kan optree, te sintetiseer.

“Oor die algemeen het polimere wat gesintetiseer is uit lignien-afgeleide monomere en hulle afgeleides, hoë glasoorgangstemperature (Tg’s genoem) en is hulle termies stabiel met goeie meganiese eienskappe. Dit maak hulle aantreklike plaasvervangers vir fossielbrandstofafgeleide stirene in die sintese van plastiek wat weerstandig is teen hoë temperature, gevorderde samestellings en resene.”

Die metode is sedertdien gepatenteer en hulle werk nou saam met die US se Departement Prosesingenieurswese om die metode  op ’n groter skaal te ontwikkel: “Ons is tans op tegniese gereedheidsvlak 5, wat beteken dat ons aan ’n geïntegreerde benadering en waardetoevoeging van die protokol vir ’n loodsstudie in die fabriek werk.”

Hoe werk die tegnologie?

Lignien is ’n baie komplekse makromolekule met ’n unieke struktuur. Die struktuur kan verskil, afhangend van die oorsprong van die hout en hoe dit vooraf behandel is. 

“Chemici gebruik suurstof om hierdie komplekse makromolekules (polimere genoem) in kleiner molekules (bekend as komponentmonomere) af te breek deur middel van ’n proses wat bekend staan as depolimerisasie. In die geval van lignien lei die ligste oksidatiewe tegnieke egter nie tot die volledige omskakeling van lignien nie. Boonop kan oor-oksidasie lei tot strukture wat selfs méér kompleks is as die lignien waarmee die proses afgeskop het as die proses nie effektief gekontroleer word nie,” verduidelik Dr Sibanda.

Sy metode, wat berus op die werk wat deur Dr Pfukwa gedoen is, hanteer sommige van hierdie uitdagings. Alhoewel dit steeds nie lei tot volledige omskakeling nie, verbeter dit die proses aansienlik deurdat dit rekombinasie-reaksies verhoed.

In die geval van lignien het hy daarin geslaag om die makromolekules tot verskeie nuttige chemikalieë af te breek: vanillien (die verbinding wat vanielje sy kenmerkende aroma gee) en siring-aldehiede (’n organiese verbinding wat in hout voorkom en as bousteen kan dien van funksionele polimeriese materiale. Siring-aldehiede is ook verantwoordelik vir die rokerige, warm en smeulende houtaromas in whiskey). Hierdie chemikalieë het ’n aromatiese funksionaliteit, wat beteken dat hulle ’n benseenring het − die eenvoudigste organiese en aromatiese koolwaterstof- en moederverbinding van talle belangrike aromatiese verbindings.

Sedert die 1950s is benseen uitsluitlik uit petroleum verkry en dit is steeds ’n belangrike bousteen in die vervaardiging van polistireen. 

Hulle navorsing vind plaas in die konteks van ’n wyer poging in akademiese chemie om die volhoubare gebruik van hulpbronne te bevorder. Deur dit wat as ’n afvalproduk beskou word in waardevolle chemikalieë om te skakel, word die waardeketting van lignien uitgebrei en  ons afhanklikheid van ruolie terselfdertyd verminder. 

Dr Sibanda sê ’n groener toekoms vir die chemiese industrie is moontlik: “Ons het beleide nodig wat die chemiese en materiaalvervaardigingsbedryf aanmoedig om volhoubare ontwikkeling by hulle langtermynplanne in te sluit. Dit sal nywerhede aanspoor om dieselfde en soms selfs beter kwaliteit produkte te verskaf op ’n wyse wat omgewingsvolhoubaar is. Ons het ook meer geleenthede nodig vir samewerkende navorsing tussen die industrie en akademia,” sluit hy af.  

Opskrif: Dr Ndumiso Sibanda, ’n polimeerwetenskaplike van die Universiteit Stellenbosch het as deel van sy doktorale navorsing ’n nuwe metode ontwikkel om afvallignien af te breek. Hy ontvang sy doktorsgraad in Polimeerwetenskap gedurende die US se herfs 2022-gradeplegtigheid. Photo: Stefan Els