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During the first attempts of biomass valorization, drop‐in energy solutions have been investigated as they could directly substitute the use of fossil resources for transportation vehicles. The most common examples are the use of bioethanol and biodiesel as additives to common automotive fuels. Bioethanol is mostly produced in industry using yeast fermentation of C6‐sugars. With an increase of 25 billion gallons (roughly 75 Mt) worldwide, bioethanol is one of the most mass‐produced bio‐based molecules. However, starchy feedstocks (i.e. first generation) are mostly used in the production of bioethanol, causing direct competition with the food market, widespread deforestation, and concerns on the presence of enough food sources for both humans and animals [42]. Also, bioethanol has limited competitiveness with petroleum options because of low product value and relatively high price, especially when considering food sustainability. To add perspective, the price of oil would have to be above $70–80 per barrel for bioethanol to be competitive from a cost standpoint, while today, oil is at <$40 per barrel [43].

Alternatively, another approach is to obtain different platform molecules from biomass that can be used for production of a wide variety of chemicals. With a shift on how we perceive platform molecules, new chemical (and biological) pathways can be envisioned. In order to induce this shift, several important bio‐products unique from petrol‐based ones were identified in a 2004 US Department of Energy report [44] later updated by Bozell and Petersen [45] and further reviewed by Gallezot [46]. Bio‐based platform chemical families and their respective processes, industrial applications and current technical challenges, are summarized in Table 1.1 [44–47]. For more information on the industrial challenges for biomass valorization, the reader is referred to Chapter 13 of this book.

Table 1.1 represents only a small fraction of all the molecules that could be identified as valuable platform chemicals, opening a significant number of possibilities for the synthesis of petrol‐like or new molecules. However, apart from established processes such as those of sorbitol and glycerol, all other biomolecules generally suffer from high production costs that might be caused by

1 high price of feedstocks (depending on the required sugar purity).

2 low resource efficiency (e.g. synthesis of by‐products that lower conversions and intensify purification/separation processes).

3 high investment and operational cost required for the reactor volumes or design, or need to maintain sterile conditions during production.

4 inefficient catalysts, which could be (a) biological (enzyme and bacteria), which require metabolic engineering for higher efficiency and durability; (b) homogeneous, which tend to be corrosive, toxic, or difficult to reuse and recycle; or (c) heterogeneous, which have lower conversions even if they can be recovered and reused, but are prone to irreversible adsorption of organic molecules, leading to coke and thus reactor fouling.

Particularly, when compared to petrol‐like compounds, the disadvantages of chemicals from biomass processing become increasingly apparent in terms of overall costs. Even when only considering feedstock transportation, the advantage goes to petroleum, as it is a fluid that can be pumped (or natural gas through pipelines). Biomass tends to occupy larger volumes, given its physical nature, and is much more difficult to transport as a result. Nevertheless, the most notable difference that gives petrol‐like compounds the slight edge is the absence of oxygen functionalities (aliphatics/aromatics/olefins), which reduces their reactivity but yields larger production volumes by the addition of heteroatoms. In fact, although fossil compounds are modified via oxidation, bio‐derived compounds often require oxygen removal. In this sense, larger initial volumes are required for biomass to reach the same final product volume, making it economically inefficient. Moreover, the reactivity of oxygen groups in biomass gives inefficient processes, especially if targeting petrol‐like compounds. In this regard, a better route is to build off these different functionalities and explore new platform chemicals that are specific for biomass products. Most of the advances have been achieved largely because of catalytic pathways that allow for lower energy requirements and higher resource efficiency.

Table 1.1 Key examples of the possible bio‐based products, state‐of‐the‐art processes, and challenges [44–47].

Bio‐product platform (example)	Process	Industrial application	Technological challenge
1,4‐Diacid (succinic acid)	Anaerobic fermentation	Pharmaceutical, food, polymers, solvents	Separation/purification of products
Furanics (HMF)	Acid‐catalyzed dehydration of C‐5 and C‐6 sugars/oxidation	Food/cosmetics, polymers, construction, textiles, fuels	Low resource efficiency
3‐Hydroxypropionic acid (acrylic acid)	Aerobic fermentation	Polymers, textiles	Low resource efficiency
Under metabolic engineering research
Amino acid (aspartic and glutamic acids)	Microbial process	Biodegradable polymers, pharmaceuticals	Need of sterile conditions
Complex separation
Under metabolic engineering research
Gluconic acid (methylglucoside)	Aerobic fermentation/catalytic oxidation	Food, pharmaceuticals	Low resource efficiency/catalyst deactivation
Itaconic acid (itaconic anhydride)	Aerobic fermentation	Specialty polymers (including biodegradable)	Low resource efficiency
Under metabolic engineering research
Glycerol (dihydroxyacetone)	By‐product of biodiesel/soap manufacture	Cosmetics, food, pharmaceuticals, lubricants, polymers, Li batteries	Low market price
Expensive purification
Catalyst separation/deactivation in upgrade
Levulinates (γ‐valerolactone)	Acid‐catalyzed dehydration of C‐6 sugars	Fragrances, food, fuels, solvents, pharmaceuticals, polymers	Low resource efficiency
Sorbitol (isosorbide)	Hydrogenation of C‐6 sugars	Food, pharmaceuticals	Established technology
Low market price
Lactones (3‐hydroxybutyrolactone)	Oxidative degradation of C‐5 and C‐6 sugars	Pharmaceuticals, chiral building block, polymers	Inefficient oxidation
Low resource efficiency unless starch is used
Inhibitory effect of biomass
Lactic acid (oxalic acid)	Anaerobic fermentation	Cosmetics, pharmaceutical, biodegradable polymers	High feedstock cost (high‐purity lignocellulosic sugars or food derived)
Separation/purification of products
Biohydrocarbons (isoprene)	Aerobic fermentation	Polymers	Investment cost (reactors)
High feedstock cost (high‐purity lignocellulosic sugars or food derived)
ABE (acetone, butanol, ethanol)	ABE fermentation	Fuels, solvents	High feedstock cost
Low resource efficiency
Lignin	Catalytic decomposition	Polymers, food, pharmaceuticals, fuels	Low resource efficiency

Sources: Werpy et al. [44], Bozell et al. [45], Gallezot [46], Isikgor et al. [47].

For instance, by taking the case study of plastic production from biomass, a variety of options can be imagined [47], giving strong environmental benefits. If traditional plastics (e.g. PE, polyamides, and PET) are produced starting from biomass, a possible reduction of ca. 310 Mt of CO₂‐equiv per year could be achieved with the substitution of less than 66% of the current fossil‐based plastics [48]. This footprint reduction is solely based on the process and not on the product as the degradation characteristics of these plastics (i.e. nonbiodegradable) are the same regardless of the feedstock type (e.g. biomass and petroleum). At the same time, new and innovative platform chemicals can be produced with fewer chemical steps (e.g. furanics as opposed to aromatics), opening new opportunities in the production of bio‐based plastics. For instance, a furan‐based plastic was synthesized via the polymerization of 2,5‐furandicarboxylic acid (FDCA), an oxidized product of 5‐hydroxymethylfurfural (HMF) (see Table 1.1), and ethylene glycol [49,50]. This plastic, known as polyethylene furanoate (PEF), can be conceived as the bio‐based parallelism of the common PET, where the C‐6 aromatic functionality is substituted by a furan ring. Further, the furan‐based polymer was found to perform better compared to PET in terms of physical properties such as gas barrier [49]. The production of PEF relies on the acid‐catalyzed dehydration of lignocellulosic biomass and/or sugars (e.g. glucose) into HMF with subsequent oxidation to FDCA (Figure 1.2) (see further Chapter 13).

However, this process is hindered by the coproduction of substances known as humins (at times, referred to as coke) in the dehydration step [51–53]. The lower resource efficiency of the first step increases the overall cost of PEF, limiting its competition with fossil‐based PET regardless of the improved properties. This low atom economy is common to many bio‐based processes.

Generally, when producing chemicals from whole lignocelluloses, the yields of conversion processes are lower compared to the same synthesis starting from the sugar (e.g. fructose, xylose, and glucose); hence, process costs tend to increase. Predominantly, the differences of lignin content and feedstock density depending on the considered biomass (e.g. grasses vs. softwood vs. hardwood) cause variation on the process yields as well as the amount of volumes to be processed (e.g. grasses require bigger volumes). In particular, the hydrogen bonding between the different components (i.e. lignin, hemicellulose, and cellulose) reduces the available surface to processing, increasing the structural complexity and the recalcitrant nature of lignocellulosic feedstocks. Furthermore, the inorganic metals (e.g. Mg and Ca) naturally present in plants may induce reactor fouling by induced precipitation of salts or heterogeneous catalyst (e.g. zeolites) deactivation by ion exchange [54,55]. Above all, the aforementioned large presence of heteroatoms (particularly oxygen) increases the moieties' reactivity, leading to low atom efficiency and occurrence of undesired side reactions, such as the synthesis of humins that act as a catalyst deactivator (in a similar way to coke) and promote reactor fouling.

Figure 1.2 Parallelism between the production of PET (left) and PEF (right).

In order to overcome these challenges, strategies include the use of unconventional solvents, milder conditions, and various pretreatment methods in order to separate the single components (e.g. decompose cellulose to glucose) and allow targeted valorization.