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and properties as the traditional ones (e.g. polyethylene [PE] and polyethylene terephthalate [PET]). However, the process chemistry limits the efficiency to sugars. On the other hand, other bio‐based plastics with new properties have been developed, e.g. polyethylene furanoate (PEF) or poly‐lactate (PLA). The former is a durable plastic based on furan and the latter a compostable plastic. Developing bio‐based plastics that are also biodegradable – a fundamental challenge in biomass valorization – can ensure a higher sustainability at the waste management stage, as their waste is less dangerous to animals and humans (microplastics, trapped in fishing nets). However, differentiation in the lifetimes of plastics will also require the development of durable bioplastics.

      Accumulating plastic waste is just one of the many concerns that is helping to drive sustainable practices forward. Other concerns from the fossil‐driven industrial revolution include the following:

       Irreversible depletion of fossil fuels (i.e. oil and gas) and their detrimental environmental issues [7,8].

       Higher average temperatures and aggravation of weather conditions worldwide (e.g. heavier rains) from an increase of greenhouse gases and record levels of CO2 in the atmosphere [9].

       Global population growth (>9 billion projected by 2050) leading to higher energy, food, and chemical demands [10].

      These concerns require a sustainable chemical industry that embraces the concepts of green chemistry [11], circular [12] and low‐carbon economies [13], and high resource efficiency [14]. As such, biomass valorization and conversion of renewable feedstocks through green processes are advancing to fully shift toward a safer and sustainable chemical industry.

1.2 Biomass Valorization

The sustainable production of chemicals and products can be achieved from conversion of biomass, an inherently renewable source. Biomass covers a wide range of bio‐based resources from plants or animals. These resources include plant‐based materials, biowastes, and aquatic organisms. Valorizing renewable biomass feedstocks can offer environmental benefits that include reduced emissions, safer feedstocks, better geographic distribution of resources, and achievement of a circular economy [12,15–18].

      In a circular economy, resources – such as carbon, nitrogen, and phosphorous compounds – are used with a circular “take–make–reuse/recycle” approach, as opposed to a linear “take–make–dispose” approach [12]. A closed cycle can be achieved with biomass valorization processes by recycling the generated CO₂ through natural photosynthetic processes [19,20]. This process happens particularly with biodegradable plastics. Further, the existence of nonedible and rapidly growing plants parallel to the development of high‐throughput agricultural technologies can lead to a carbon‐neutral cycle in short periods of time, readjusting the increased levels of CO₂ emission given by the fossil industries [21].

      In the context of biofuels, biomass has been subdivided in three categories given as follows along with the major evidenced drawbacks:

      1 First‐generation biomass: This includes all edible biomasses (e.g. sugarcane, corn, whey, barley, and sugar beet) that are composed of sucrose or starchy carbohydrates, hence relatively simple macromolecules with low recalcitrance. Biological fermentation of said sugar polymers yields bioethanol, one of the most studied drop‐in biofuels with current industrial production [22]. Food‐derived vegetable oils are also considered as first‐generation biomass and they yield biodiesel through transesterification [23]. The main issue of this type of biomass is the clear competition with food resources (which will be continuously more precious, given the increase of world population) as well as the intensive use of water and land for the growth of said crops [24].

      2 Second‐generation biomass: Nonfood raw materials, including by‐products and waste materials. Generally, second‐generation biofuels are produced from lignocelluloses (e.g. grasses, soft or hard wood, and forestry residues) or various wastes/by‐products (e.g. agricultural: stover, wheat straw, corn cob, rice husk, and sugarcane bagasse; industrial: glycerol, grains from distilleries, and paper sludge; or urban: household and municipal solid wastes). Given the structural composition of these feedstocks (mixtures of cellulose, hemicellulose, and lignin), pretreatment is usually required for fermentation to biofuels and biochemicals, and the process economics are hindered by the use of multiple steps, leading to lower overall conversions [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

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