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ideally remove lignin without affecting the desired carbohydrates, hence being energy effective while having a simple reactor design and low production of waste compounds (including solvents) [86]. Nevertheless, improvement of the current pretreatment technologies is still required to obtain economical solutions. Various pretreatment strategies and their advantages and disadvantages are illustrated in Table 1.2.

Overall, different pretreatment methods will be preferred for different applications. For example, applications requiring low toxicity would be better suited with microbial conversions. Alternatively, applications requiring high sugar yields would probably utilize chemical conversions. The main challenge for selective biomass processes is achieving a reasonable balance between cost considerations and efficient separation of each component. In addition, pretreatment requirements with current technologies further complicate the processes [87]. Innovative solutions that address these challenges will help push biomass processes closer to practical implementation.

Table 1.2 Advantages and disadvantages of pretreatment methods for lignocelluloses.

Method	Pretreatment	Advantages	Disadvantages
Biological	Fungi	Energy effective	Low hydrolysis rate
		Degrades lignin/hemicellulose network
Physical	Milling	Reduces cellulose crystallinity	Energy intensive
Chemical	Ozonolysis	Lignin reduction	Cost ineffective (ozone)
		Low microbial inhibitors
	Organosolv	Lignin and hemicellulose hydrolysis	Big solvent volumes
			Requires solvent recycle
	Alkali	Lignin removal	Inefficient for softwoods
			Large amounts of water
		Reduces cellulose crystallinity	Long pretreatment times
		Limited hemicellulose degradation	Base recycle
	Concentrated acid	High glucose yield	Large amounts of acids
		Energy effective	Requires acid recycle
			Reactor corrosion
	Diluted acid	Low microbial inhibitors	Low sugar yields
		Lower corrosion issues	Degradation products
	Ionic liquids	Reduces cellulose crystallinity	Cost ineffective (ionic liquids)
		Higher accessible surface area	Difficult recovery/separation of desired products
		Lignin removal	Potential toxicity and thermal instability of ionic liquids
		Degrades lignin/hemicellulose network
Physicochemical	Steam explosion	Lignin removal	High microbial inhibitors
		Hemicellulose solubilization
		Fair sugar yields	Partial hemicellulose degradation
		Economical
	Ammonia fiber expansion (AFEX)	Higher accessible surface area	Inefficient with lignin‐rich biomass
		Low microbial inhibitors	Big ammonia volumes (cost)
	CO2 explosion	Higher accessible surface area	No effect on lignin/hemicelluloses network
		Low microbial inhibitors	High pressure (cost, reactor)
		Economical
	Wet oxidation	Lignin removal	Cost ineffective (oxygen and alkaline catalyst)
		Low microbial inhibitors
		Energy effective

1.7 Conclusions and Perspectives

      The search of a true sustainable chemical industry is driven by the development of processes that rely on not only renewable feedstocks associated with low environmental impact techniques but also economic viability to compete with the well‐established oil and gas markets. To recede the dependency on polluting resources, creative solutions following a green design in the most restringing way are required. The following chapters in this book discuss various methods of biomass valorization, along with their respective challenges and innovative solutions, as means to progress toward chemical sustainability.

References

      1 1. United Nations Sustainable Development Knowledge Platform. (2019). Sustainable development goals. https://sustainabledevelopment.un.org/sdgs (accessed 20 November 2019).

      2 2. International Council of Chemical Associations. (2019). Sustainable development. https://www.icca-chem.org/sustainable-development/ (accessed 10 September 2019).

      3 3. Speight, J.G. (1999). The Chemistry and Technology of Petroleum. New York: Marcel Dekker.

      4 4. Filiciotto, L. and Luque, R. (2018). Nanocatalysis for green chemistry. In:

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