Written by: Wienke Reynolds – Co-Founder and CTO at Lignopure
Lignin is a major constituent of wood, grasses, cereals and other woodifying biomasses. Consequently, lignin is, after cellulose, the second most abundant biopolymer and the world’s largest phenolic resource on earth. In the plant cell wall of wood or herbaceous plants, lignin is connected to cellulose, hemicellulose and minor constituents, forming a complex structure similar to steel-enforced concrete, named lignocellulose.
The complexity of lignin isolation
Inside plants, lignin not only provides plants with their rigidity, strength and resilience but also works as a protective barrier against external factors, whereas cellulose fibres give the plant its tear strength and flexibility. Consequently, separating cellulose, lignin and hemicellulose requires quite harsh treatments, and in the case of conventional pulping involves strong chemicals.
To make it even more complex, lignin composition and structure vary with biomass type, even with growth conditions or harvesting season. Therefore, lignocellulose processing technologies have specialised in the treatment of different biomasses and targeted products and so far, their main and almost exclusive target has been cellulose. This has led to lignin ending up as a side stream, which mainly is incinerated to cover the process energy costs and/or to recover pulping chemicals in a closed loop.
Enabling the use of lignin as a drop-in raw material for chemical or material applications requires additional downstream processing steps (and investment!), which as well have a strong impact on the final lignin quality – which can be in terms of composition or chemical, thermal and physical properties. Even properties like the lignin colour and odour, which might seem profane, are strongly influenced by the lignin isolation process. This makes lignin an incredibly versatile and fascinating, but also challenging resource to work with.
The principles for lignin isolation
To understand lignin isolation possibilities, we need to have a look at the degradation and dissolution behaviour of lignin and polysaccharides. Lignin depolymerizes and dissolves in alkaline pH at elevated temperatures, but also in certain organic solvents or specific strong acids. On the other hand, Hemicellulose would also not withstand these conditions, whereas cellulose in most cases would stay intact. Cellulose and hemicellulose on the contrary also can be hydrolysed to sugar oligomers and monomers in acidic pH at elevated temperatures, whereas lignin stays intact unless you aim for really high temperatures. To complement the toolbox, polysaccharide and lignin-degrading enzymes can be used. Many of these processes also show a good synergistic effect with mechanical treatments.
Around these before-mentioned basic working principles, an impressive variety of lignocellulose processing approaches have been developed, which of course cannot all be put in certain “drawers”. Nevertheless, grouping the processes helps us understand why and how the lignin obtained from these processes behaves in a certain way.
Categorization of lignin isolation processes
Basically, lignin production processes can be categorized based on both biomass source and the isolation process: At one end, biomass sources can be hardwood, softwood or herbaceous plants (mainly poaceae). On the other hand, processes can roughly be grouped into pulping processes (solubilizing the lignin, yielding polymeric cellulose), biorefinery processes (dissolving the sugars, yielding polymeric lignin), and a third group which we like to call hybrid processes, as they combine working principles or process steps of pulping and biorefining approaches. In this context, e.g. organosolv and acetosolv processing come to mind. Some approaches even combine biorefinery pre-treatments with a subsequent pulping step, not to forget about novel technologies such as ionic liquid, deep eutectic solvent or even supercritical fluid processes.
Kraft pulping process
For decades, cellulose processing technologies have been optimized by the pulp and paper industry. One of these processes and the by far most used process is the so-called Kraft pulping, which converts softwoods with high resin content e.g. eucalyptus into large units. This process uses strong alkali and sulphur-based pulping chemicals, in addition, it is an incredibly integrated process which internally recovers pulping chemicals and energy using lignin as an energy source. To take into consideration is that precipitating lignin from the liquid effluent of the Kraft process (black liquor) always means extra processing expenses, but nevertheless, some pulping companies are already going the extra mile and successfully marketed their purified Kraft lignin e.g. for resin applications. Unfortunately, the skin-irritating properties and the strong, repulsive odour of Kraft lignin limit its application for consumer goods. A few deodorization approaches exist but have not made it to scale so far.
Soda pulping process
In line with the Kraft process, we should name the soda pulping, which can be considered the sulphur-free brother of the Kraft process. Soda pulping is often used for grasses and cereal straws and its working principle as well as lignin isolation processes are quite similar to the Kraft process, so much so that sometimes both processes are operated synergistically. Recently, soda pulping and non-wood pulp have gained higher popularity by making and having available good quality sulfur-free lignins, which have the potential to go into higher-value products.
Acidic sulphite pulping process
Another group of lignins, which already has an impressive list of commercial applications and is even listed in some personal care products, are lignosulfonates from the acidic sulphite pulping. This process mostly works with hardwoods, which are low in resins and silicates. Compared to most other lignins, lignosulfonates are water soluble in the full pH range, which makes them very interesting for many applications. However, similarly to the Kraft lignin, the strong odour hinders applications with direct customer contact.
Second generation biorefineries process
Next are the so-called second generation (2G) biorefineries process which approach the lignocellulose degradation the reverse way compared to pulping processes. There, the cellulose is hydrolysed to fermentable sugars, yielding depolymerized hemicellulose and macromolecular lignin as side products. Typically, these processes combine mechanical disintegration steps with thermal treatment (e.g. mild acid, steam extraction/explosion, hot water extraction) and a subsequent enzymatic saccharification of the cellulose. Classical glucose-derived products from this process are bioethanol or other chemicals such as glycols.
Larger scale 2G biorefineries started to be implemented in Europe in the 2010s and are still in strong development across the world. Interestingly, the hydrolytic biorefining approach is actually quite old: The former USSR, for example, operated a large number of chemical hydrolysis plants for lignocellulose, producing ethanol, furfural or fodder yeast – landfilling the lignin which was of quite poor quality compared to what can be achieved today.
Lastly, hydrolytic biorefinery lignins are really close to the native structure of lignin in plants, show low cytotoxicity and odour and have great potential for applications in the life sciences. However, biorefinery lignins exhibit a very limited solubility in multiple solvents, which is a hurdle for specific applications. For this reason, good particle engineering plays a crucial role in their valorisation.
With this said, the growing portfolio of lignin isolation and transformation technologies, and the increasing versatility and quality of lignin, we believe that disruptive lignin applications in Life Science sectors will finally take off, including lignin for pharmaceutical, nutraceutical and cosmetic applications.
We, at Lignopure, have devoted ourselves to making lignin utilization in our daily lives a reality, so much so that some of our team members can identify and differentiate lignin types solely by colour, odour and powder appearance – like real lignin sommeliers. To achieve the full utilization of lignin a big part consists in understanding the origin of the different lignins, their biomass of origin and the resulting properties depending on the isolation process used on them to therefore know how to add value to multiple products in the Life Science sector by transferring lignin’s unique natural functionalities.