For this, several different biomass valorization routes can be envisioned with a wide range of obtainable products.
1.4 Key Biomolecules
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 |
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