The Chemical industry places its primary Focus on Fossil Resources for the Manufacturing of Carbon-Based Compounds: Pharmaceutical Analysis Thesis, QUB, Ireland

Biomass

1.1. What is Biomass?

The chemical industry places its primary focus on fossil resources for the manufacturing of carbon-based compounds. However, as the development of the economy and society progresses problems with these resources and the environment increases causing a decline in the supply of conventional fuels. Therefore, the search for alternative raw materials for chemical production has made biomass a more plausible resource that has considerable potential to produce chemicals and biofuels, paving the way for a sustainable future.

Biomass refers to any type of organic matter derived from plants and animals (fossil) to generate energy¹, it can be obtained from sugars, fats, cellulose, proteins and lignin and has been it’s been used as fuel since humans learnt how to make a fire to cook and keep warm until fossil fuels became more widely used in the 1900s. ² These sugars are carbohydrates and supply plants and the animals that eat this plant with energy, hence why food abundant in carbohydrates are good sources of energy for the human body³.

Fig 1. Sources of Biomass-based on feedstock origin⁴.

Biomass has been found to be prevalent in renewable green energy more recently, it had a prominent role before fossil fuels took over in the twentieth century and now its share in the energy mix is rising. The status of biomass as a primary source of renewable energy depends largely on geographical and socio-economical conditions for instance90% of the primary energy source in Nepal is derived from biomass but it constitutes less than 0.1% in most middle eastern countries⁵.

Biomass can be converted to substitute liquid fuels by a number of processes; biomass to Liquids (BtL) refers to a thermochemical process currently transitioning from pilot scale to demonstration scale that essentially converts a variety of biomass types to a variety of fuels and chemicals⁶, Presently, biomass to ethanol is produced on a large scale to provide a gasoline additive in the United States and Brazil, among other places.

The market for ethanol derived from biomass is influenced by government requirements and facilitated by generous tax subsidies. Research holds the promise of more economical ethanol production from cellulosic, or woody, biomass, but related processes are far from economic as reducing the cost of growing, harvesting, and converting biomass crops will be necessary to pull this off⁷.

1.2.        Benefits of biomass

1.2.1.   Greenhouse Gas Emissions Reduction

Using biomass energy over fossil fuels can significantly less greenhouse gas emissions even though burning biomass releases around the same amount of carbon dioxide as burning fossil fuels⁸.

Fossil fuels release carbon dioxide previously captured by photosynthesis from years ago this is considered “newer” greenhouse gas, whereas biomass releases carbon dioxide that is largely balanced by the carbon dioxide captured in its own growth depending on how much energy was used to grow, harvest, and process the fuel.

More precisely, the CO₂released when biofuel is burned simply returns to the atmospheric carbon dioxide and eventually is taken back into plants through the process of photosynthesis⁹.

However, studies have found that clearing forests to grow biomass results in a carbon penalty that takes decades to recuperate therefore in order to effectively allow the earth to continue the carbon cycle process biomass materials such as plants and forests would have to be sustainably farmed10.

1.2.2.   Waste reduction

According to a recent World Bank report, 1.3 billion tonnes per year of municipal solid waste (MSW) is currently generated worldwide and is expected to double by the year 2025¹. Energy generated from waste materials can cause a considerable change in waste management, which is now a major problem in most developed and developing countries.

The increasing availability of plant and animal waste due to the rise in consumption levels and figuring out how to reuse these supplies will be an important aspect of future goals for sustainability¹.Recently, there has been considerable interest in developing smaller-scale BtL and in using waste as a feedstock⁶.

Table 1.Energy recovery technologies that have contributed to increased biowaste valorisation¹.

1.2.3.   Ash yield and inorganic matter

The low ash yield of biomass which is more representative of wood, woody biomass and some agricultural biomass varieties is more advantageous and beneficial in terms of higher quality of fuel simply because thermochemical and biochemical conversions are made easier, increased heating value and there is also less fuel contamination by soil, dirt, rainfall, wind, fertilizers, pesticides, and additives during biomass growing, harvesting, transport and processing.

It is common knowledge that the biggest technological and environmental challenges that biofuel faces today are mostly related to the presence, quantity, source, and behaviour of inorganic matter in biomass. The inorganic elements in biomass are typically considerably smaller amounts than in solid fossil fuels disregarding aquatic biomass, animal biomass and some varieties of herbaceous and contaminated biomass11.

2.  Lignin

Exhaustion of traditional fuel reserves has prompted an investigation into renewable resources¹. Biomass being a wildly available renewable resource can be valorised to produce fuels, chemicals and so on. Lignin has been the most underutilised out of all the fractions of Biomass due to its complex structure12.

Lignin is a wood component that is known and proving to be an especially promising resource but is currently exclusively used for the purpose of generating energy. The function of lignin in the plant cell wall is to provide structural support, transport the water and nutrients, and deliver protection to prevent chemical or biological attacks. Though the chemical structure is very complicated, it is generally accepted that lignin is formed via irregular biosynthesis13.

The term lignin is derived from the Latin word “signum” meaning wood, and it accounts for approximately 15-30% of the total biomass content in plants12, It is very hydrophobic and insoluble in water and alcohol14. Lignin is a complex organic polymer formed of monolignol units, lignocellulose provides shape and stability to plants; its biopolymers strengthen the cell walls(makes plants resistant to pests and wind)and usually consists of three main components: the cellulose and hemicellulose with lignin acting as a connector of sorts and therefore solidifying the cell walls15.

Strands of cellulose make up the core of the cell wall of plants, this unique polymer is highly crystalline, insoluble in water and very much resistant to depolymerisation. The microfibril strands provide strength and flexibility and the crystalline core of cellulose are very resistant to chemical and biological degradation¹⁶.

Fig 2. Breakdown of lignocellulose showing the relationship between lignin, cellulose, pectin, and hemicellulose17.

Currently, lignocellulolytic enzymes such as cellulose, hemicellulose and pectinase all together make up for 20% of worldwide sales of commercially available enzymes; functional usage of hemicellulose and cellulase first started as a means to produce from it animal feed which then extended to the production of textiles and eventually paper in the 1980s but more recently these enzymes are used extensively in the food industries to flavour and texturize food products¹⁸.

Lignin is a major fraction of biomass and accounts for 40% of the total lignocellulosic biomass energy12and in comparison to fossil fuels lignocellulose obtained from wood, straw, or silver grass are renewable raw materials that can be grown in forests and are climate-neutral (amount of CO₂ released from burning them will not exceed the amount stored in them while they were growing)15.

Each year about 150 billion tonnes of lignin is produced by plants; besides cellulose and chitin lignin is the most abundant natural polymer in nature12. Presently, more than 80 million tons of lignin is being produced as a by-product from the lignocellulosic biorefinery and paper industry.

Though only a very small fraction of said lignin is used for fuel in the industry, therefore, researchers intend to synthesize various chemicals of high commercial value from lignin to make the biorefinery process more viable, this in itself is a challenge considering lignin’s poor solubility and complex phenolic ring structures with high bonding energy14.

2.1. Lignin structure

Lignin sources can be divided into three major groups: softwood lignin, hardwood lignin, and grass lignin, due to differences in the chemical structure of monomer units and the linkages between monomer units19. It is the only primary plant ingredient that has aromatic more specifically phenolic structural characteristics20 furthermore it has a branched three-dimensional chemical structure with various functional groups such as carbonyl(C=O), carboxyl (COOH) and methoxy group (OCH3), respectively14. It’s known that lignin macromolecules are comprised of complex covalent linkages with a variety of C-O and C-C bonds, the chemical and physical properties of which depends largely on inter and intramolecular interactions and also solution conformation21.

Fig 3. Section of lignin molecule, highlighted also are the structures of the phenylpropanoids of which lignin is composed20

Lignin varies in structure depending on its source, conditions it grew in, location of the tissue, age, and other natural conditions of living plants14. It’s clear from the structure above that lignin is indeed very complex but on a more detailed evaluation, it’s clear that there is a patternLignin is a polymeric compound composed of phenylpropanoid units derived from three cinnamyl alcohols (monolignols): p-coumarin(hydroxyphenyl), coniferyl (guaiacyl), and sinapyl (syringe) alcohols19.

Fig 4. Structures of the main monolignol groups along with their corresponding remain in lignin polymers21

• Guaiacyl lignin contains significantly high concentrations of coniferyl alcohol with the G:S: H ratio of 90:2:8. It is also named softwood lignin, mainly derived from coniferous trees12.
• Guaiacyl-syringyl lignin contains significant amounts of sinapyl alcohol in addition to coniferyl alcohol. It is also known as hardwood lignin, principally found in deciduous trees and shrubs12.
• Guaiacyl-syringyl p-hydroxybenzaldehyde lignin contains a significant proportion of p-hydroxybenzaldehyde, approximately 30% in combination with other phenyl propane subunits. Lignin derived from monocotyledons falls into this category12.

Table 2. The relative amount of the monolignols in different types of lignin Biomass14

Plants contain differing amounts of these monolignol units. There’s an abundance of G units in softwood, and hardwood is plentiful of both G and S units with a more complex structure. Non-woody species such as grasses contain a substantial amount of H units but a more significant amount of G units12. Studies have shown that the relative content of monolignols hugely affects the digestibility of biomass significantly. The ratio of S and G units in cell walls mirrors the performance of pre-treatments on biomass and is an issue that affects the degradability of lignin for example the high presence of S units aligns with the removal of Lignin. Although due to the complexities of biomass and the effects of other components, inconsistent results suggesting positive relation of S and G units to delignification were reported during pre-treatment²².

2.2.  Lignin model compounds

The diversity and complexity of lignin structure in different kinds of plants along with the combination of different amounts of monolignols with distinct distribution and substitution patterns impedes the understanding of the possible reactions that lignin can be subjected to and therefore limits its usage23; for this reason, model compounds which represent key linkages in the original structure of the compound have become a critical tool for the assessment of novel technologies and for delivering an insight into the mechanism of conversion24Common linkages found in heterogeneous, high molecular weight lignin are β-O-4, α-O-4, 5-5, β-5, 4-O-5, β-1, β–β.

Fig 5. Structures of the more common key linkages in lignin25.

Although the failure or success of a study largely depends on the selection of an appropriate model compound, translating and relating it to real lignin, however, has proven to be difficult considering the literature on the topic is scattered and little to no comprehensive comparison of synthetic methods to access the compound exists26. In the phenylpropanoid units that make up lignin as shown in Figure 3 above the carbon atoms in the rings are marked 1-6 starting at the carbon atom the propyl chain is attached to, and are then labelled with α, β, γ starting from the carbon atom next to the aromatic ring; for example, the β-O-4 would be mean that the β carbon atom of one propyl chain is connected to an oxygen atom at the 4^th position of another aromatic unit26.

2.2.1.    β -O-4

The β-O-4 linkage is the most abundant linking in native lignin, it has characteristic benzylic and primary aliphatic alcohol groups27 and makes up about 45-50% in softwood and approximately 60% in hardwood26. The β-O-4linkage is more frequently used and replicated in literature and studies. The simplest β-O-4 type a model is (2- phenoxy ethyl) benzene and it’s often used because it’s commercially available26. Several methods for the synthesis of dilignol model compounds with β-O-4 linkage have been reported over the years though most of these methods are either low-yielding, laborious or require special equipment.

The first known total synthesis of lignin model compounds surfaced in 1952, they studied the oxidative cleavage of the ether bond without side chains as models meaning the β-O-4 dimers were primarily used as they’re known as the non-condensed linkage in lignin28. Selective cleavage of these linkages, more specifically the β-O-4 linkage, is a vital step for the valorization of lignin to aromatic compounds29.

2.3.  Analytical methods for the determination of lignin characterisation

With the intention to improve the industrial use of lignin there must be a constant supply of lignin products which is to be of high quality relating to purity chemical plant compositions and functional properties. These requirements call for detailed characterization in these such cases chromatographic and spectroscopic methods are useful tools in the investigation of plant cell wall polymers like lignin30.

Although new methods such as Raman scattering microscopy, time of flight secondary ion mass spectroscopy are able to provide chemical and spectral imaging with high resolution and sensitivity. These techniques are not readily available in all labs and have not yet been explored widely by scientific groups13.

High-Performance Liquid Chromatography(HPLC) analysis has been critical for determining the structural and chemical complexity of the cell wall31, it is also an analytical technique used to determine the ratio of lignin monomers obtainedby the alkaline nitrobenzene oxidation method.

Lignin content has been commonly determined by wet chemical methods for example Klason lignin determination is a quick and accessible method32.A set of samples containing several hardwoods, softwood and wooden samples isolated by different processing technologies were studied by boeriu at al33.

This study mentioned that the Fourier Transform (FTIR) spectrasignifies avital tool for a quick qualitative and quantitative characterization of the chemical structure and functional properties of lignin. This analytical method combined with chemometrics can be used as a fast and reliable nondestructive technique in the characterization of lignin-based materials30.

¹³C NMR spectroscopy has proven to be a method with a sizeable potential in providing comprehensive structural information for lignin, it is essential in the quantitative determination of the amounts of different structural units in lignin and provides a sophisticated alternative method due to its larger chemical shift dispersion.

However, the use of¹³C NMR for the study of lignin is limited to mostly using the aromatic and methoxy signals as internal standards in expressing the various functional groups per methoxy unit although, works well with native lignin this could be because of some serious errors during analyses on lignin model compounds.

2.4. Applications of Lignin

The most critical concerns in the world these days are providing sustainable consumption for energy and natural resources. Lignin is very obviously an alternative due to its low cost, sufficient availability, and its positive impact on the environment due to its large structure and valuable properties it exhibits34.

Apart from some applications of lignosulfonate, low-value lignin generated from high-quality cellulose by the more established paper industry are burned as low-value fuel which is then applied to process heat generation. Nevertheless, more efficient resource utilization would be appropriate if looking from a sustainability and economic viewpoint.

That being said value extraction from the lignin fraction of lignocellulosic biomass has become a major focus area this includes the development of new fractionation methods as well as several new catalytic methodologies for depolymerization or modification of lignin to generate emerging lignin-derived chemical products. In order to generate value from the lignin biopolymer and its extremely intricate chemical structure needs to be understood and interpreted26.

2.4.1.   Lignin based wood adhesive.

In paper and wood industries polymeric resins have been widely produced since the beginning of the century and their average global production has reached approximately 6 million of tons per year.

Polymeric resins are used in broad applications such as adhesives for wood composites, fibre reinforced composites, electrical laminates, mineral wool/glass insulation, moulding materials, acid-resistant coating, and epoxy curing agent.

The substitution of phenol with lignin in wood adhesives has been explored and it has earned attention among the researchers due to the similar structure of lignin with phenol-formaldehyde resins34.

2.4.2.   Carbon materials derived from lignin.

Lignin’s aromatic structure makes it a potential forerunner for the preparation of high value-added carbonaceous materials by means of thermochemical conversion.g., pyrolysis, solvolysis, hydrothermal conversion. These methods are preferred over others due to their higher productivity and compatibility with existing infrastructure amenities³⁵.

These materials have attracted great interest in structural and functional applications for example, lignin-based carbon fibres (CFs) are widely studied as reinforcing components for designing advanced composite materials (e.g., automotive devices, sportive goods) with high mechanical property34.

2.4.3.   Usage in Thermoplastic and polymer blend

Due to its chemical structure, lignin is deemed to be a substituent for non-renewable compounds in polymer blends and thermoplastics.Particular methods have been tested to help combat lignin’s issue of poor miscibility in blends and solvents; some of these methods include the chemical modification of lignin, the addition of coupling agents e.g., Poly (ethylene-vinyl acetate). Studies have shown that the integration of lignin most especially modified lignin into thermoplastic and polymer blends offers promising ways to better the biodegradability of various materials³⁶.

2.4.4.   UV activity and sun-blocking properties

UV chromophoric groups are usually generated during polymerisation of monolignols this, therefore, confirms lignin’s absorption ability in the UV and visible region.³⁶A study conducted in 2015 by Qian et al. investigated the incorporation of lignin into various creams and shows the enhanced effects of sun-blocking properties on the addition of lignin to commercial sunscreen lotions. The blended lotion became much better than the original SPF15 after adding 10% wet ton lignin but drastically surpassed SPF50 more precisely in the UVA area³⁷.

2.5. Project objective

Fig.6 Proposed target compounds from photocatalysis by mass fragmentation

• The main objective of this project is the synthesis of proposed Lignin model compounds that are representative of lignin found in nature, the focus of the synthesis being on one of the most abundant linkages found in lignin; the β-O-4 linkage of the syringe dimer and its reduced derivative.
• Following this, analytical methods including NMR, HPLC, LCMS are employed so as to elucidate the structure of synthesised products.
• Design suitable experiments for the synthesis of each target compounds