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Role of Biomass in the Production of Chemicals

Layla Filiciotto, Evan Pfab, and Rafael Luque

Universidad de Córdoba, Campus de Rabanales, Departamento de Quimica Organica, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, Córdoba, Spain

1.1 Introduction

Chemistry is a fundamental part of everything around us. Nature is largely responsible for all of the chemistry that occurs and has been so from the dawn of time. However, societal advances and technological developments in recent years have allowed us to contribute far more chemistry than in the past. Quality-of-life improvements for major parts of the world with better food distribution, clothing, technological devices, and medical treatments have required the chemistry to progress further with detrimental unknown effects on the environment. Policies and scientists worldwide are now striving toward the development of a truly sustainable society, culminating into the implementation of the UN's 17 Sustainable Development Goals that tackle various issues including infrastructures, education, equality, peace, and environmental protection [1]. In the active search for solutions, biomass valorization has emerged as the most viable option for a more sustainable chemical industry.

The impact that sustainability could have on the chemical industry is best reflected in the magnitude of the chemical industry itself. Today, the chemical industry generates approximately $4 trillion in global sales with the production of more than 95% of all commodities [2]. One of the biggest turning points in the chemical industry, and what arguably led it to such heights, was the advent of catalytic cracking in the nineteenth century for the refining of fossil resources. Catalytic cracking allowed for most of the products we use daily to be easily sourced from petroleum [3]. Biomass valorization processes were also being explored around the same time. However, the complex nature of biomass and the wide availability of fossil resources gained all of society's attention on the use of the latter [4]. As such, petroleum processes have been the major focus of scientists and engineers for the past two centuries. Although significant developments have been achieved considering this with higher resource efficiency and cleaner technologies, the resulting environmental concerns driven by the emissions and spills have led much attention back to renewable processes such as biomass valorization.

Biomass valorization and more sustainable practices are important steps for overturning the "disposable society" mindset where resources are viewed as infinite, cheap, and harmless. One of the most straightforward examples of this can be seen with plastic. Advances in chemistry not only created plastics but also helped notice the alarming consequences.

Plastics were developed in conjunction with the advent of petroleum processes. Plastic products possess desirable characteristics (lightweight, durable, etc.) that allow for endless applications at a low manufacturing cost. Plastics found their way into daily use with things such as clothing and packaging. However, we were unequipped to properly handle this new technology. The characteristics that make plastic so appealing for a wide variety of applications (i.e. durable and heat resistant) are the same that make plastic so difficult to deal with. Its inherent non-degradability, along with extremely careless handling and littering, created a plastic waste crisis with the now widespread problem of microplastics in our oceans [5,6]. Biomass is a more attractive feedstock that can create bio-based and/or biodegradable plastics to help overturn the drastic impact from petroleum-based products. Much initial research has focused on using biomass for drop-in solutions, i.e. plastics with the same composition 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...