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: