[25–30]. The main technological challenge of these feedstocks is, in fact, the structural complexity that hinders the efficient use of the lignocelluloses as a whole, calling for pretreatments that in turn possess drawbacks depending on the method (vide infra).3 Third‐generation biomass: This includes nonedible feedstocks that do not require agricultural lands for their cultivation, namely, aquatic biomass, such as algae and other microorganisms (e.g. cyanobacteria). Depending on the strain, these feedstocks may contain mono/polyunsaturated hydrocarbons to produce gasoline‐like fuels via cracking or higher lipid content for biodiesel applications via transesterification. When considering algae, the main issue is correlated with the high water content that hinders transportation or requires significant energy inputs or long times to dry them, whereas microorganisms require specific operating conditions. Furthermore, the economic challenges of these feedstocks limit their industrial application, given the low cultivation volumes and resource efficiency in processing [31–33].
A fourth generation of biomass is also contemplated and exemplified as modified microorganisms considered in the third generation, finally used to harvest solar energy through photosynthetic processes [34,35]. However, these microbial species require improvements of genomics‐based breeding and carry the usual concerns of modified organisms, such as unexpected microbial resistance.
The available volumes of these types of biomass will play a major role in identifying the biggest driver for chemical sustainability. According to a 2018 report from the European Union (EU), the annual production of agricultural biomass (i.e. first generation) was estimated at 956 million tonnes (Mt) of dry matter of which 54% directly used for food consumption and 46% of residues (e.g. leaves and stems) partially used for animal bedding or bioenergy production. In fact, 80% of the agricultural biomass is used as food and feed, showing the limited potential of using first‐generation biomass for chemicals and energy production. As it concerns third‐generation biomass, in particular algae (including macro and micro), only 0.23 Mt of wet matter was estimated, corresponding to a mere 0.027 Mt of dry mass. On the other hand, the total woody biomass (above ground, second generation) was estimated at 18 600 Mt of dry weight [36]. Looking at the quantities of the different biomasses, the high availability of lignocelluloses in Europe makes them the most attractive. The >18 000 Mt of woody resources can make Europe competitive worldwide and support sustainable processes. Particularly, the efficient use of lignocelluloses and residues would improve the long‐term sustainability of the chemical industry, given the volumes and little impact on the food resources, although these feedstocks still rely on forest management constraints. Other waste materials (e.g. food and municipal) are increasing in volumes, given the concomitant increase of world population and improvement of their living conditions. For example, 61 Mt of food waste are produced yearly in the EU alone [37]. However, the major challenges of these products are the variable seasonal composition as well as the implementation of a proper supply chain of these anthropological side streams to biorefineries [38].
Conversion strategies of biomass, however, generally come with low resource efficiency, causing higher production costs and limited competitiveness with the well‐established petroleum market. Thus, for economic advantage, high volumes, ease of production, and limited competition with other markets (e.g. food) are required. In this sense, the use of lignocellulosic biomass may again offer a promising alternative to the fossil‐based industry. From an energetic perspective, lignocelluloses and other waste materials possess lower energy densities compared to nonrenewable resources such as coal, oil, and natural gas. However, biopower possesses negative emissions thanks to the photosynthetic process, whereas fossil fuels cause significant life cycle greenhouse gas emissions [39]. Also, conversion of biomass to key molecules (e.g. ethanol, 2‐methylfuran, and hydrogenated ethers and fatty acids) can offer biofuel diversification with various energy contents for different transport applications, including aviation; these processes rely on the separation of the different biomass components [21,40]. From a chemical point of view, the use of lignocelluloses can offer a wide variety of platform chemicals for the synthesis of not only traditional but also new products to satisfy different areas in the chemical industry (pharmaceuticals, textiles, and materials), which are discussed in the following paragraph. A separation of bio‐components will be required and explained therein.
1.3 Lignocellulosic Biomass
Of all types of biomass, lignocelluloses are the most available on the planet, ranging from wood and forestry waste to straw and agricultural waste. Lignocellulosic biomass is composed of cellulose (40–50%), hemicellulose (15–20%), lignin (25–35%), and other elements (Figure 1.1). Both cellulose and hemicellulose are carbohydrate‐based polymers, while lignin is an aromatic polymer. Cellulose is a linear, glucose‐based polymer, making it a good source of this C6‐sugar. Cellulose cross‐links with hemicellulose, a branched polymer composed of different C5‐carbohydrates, uronic acids, and C6‐sugars. Lignin, perhaps the most irregular component of lignocellulose, is a polyaromatic macromolecule composed of phenylpropane derivatives. Lignin is mostly responsible for structural rigidity within the lignocellulose. Further, lignocellulosic bio‐feedstocks include variable quantities of pigments; terpenes; inorganic elements such as Mn, K, P, Cl, Ca, Mg, and Na, as well as Al, C, Fe, N, S, Si, and Ti to a smaller extent; and various extractives, e.g. carbohydrates, proteins, lipids, waxes, chlorophyll, terpenes, tannins, and uronic acids.
Figure 1.1 Schematic representation of the components of lignocelluloses.
An extensive and systematic review on the composition of various types of biomass shows the significant changes in the composition of these elements depending on the type of biomass [41].
Overall, lignocelluloses are made of highly oxygenated C5‐ and C6‐derivatives. The oxygen functionalities make lignocelluloses a much different feedstock to petroleum sources that are mainly hydrocarbons. The oxygen functionalities in lignocelluloses are in some cases advantageous because they can minimize oxidation reactions, which usually have a negative environmental impact, and favor reduction reactions, which are typically milder processes and have less environmental impact. Further, the propensity to produce coke/humins and ash obliges the use of mild temperatures for these by‐products' minimization, as opposed to the traditional catalytic cracking/reforming of fossils. In fact, the presence of plenty of oxygen functionalities and low volatility tend to lead to the molecules' decomposition at high temperatures, generating carbonaceous residues.
Lignocelluloses have variable composition in their singular components depending on the plant origin. Water and inorganic residue contents also vary significantly from grass to wood. Although composition does vary significantly, biomass can source several useful compounds, including carbohydrates, aromatics, terpene, and fatty esters. These different components can be isolated and converted for use in many applications including pharmaceutical,
