Process intensification strategies for green solvent mediated biomass pretreatment
Graphical abstract
Introduction
The demand for energy and valuable materials for sustainable development has gone through the ceiling due to the massive growth of industrial sectors to meet the need for food, electronics, and products for the huge population in the world. Our society largely depends on limited fossil resources, many of which are not well distributed. Due to the excessive usage of fossil fuels, the emission of greenhouse gases has skyrocketed in the last few decades, which is considered one of the main culprits of global warming and leads to climate change. According to the Fourth National Climate Assessment report, approximately 85 % of total U.S. greenhouse gas emissions come from burning fossil fuels in various agricultural, industrial, and waste management processes (Reidmiller et al., 2019). Therefore, extensive research is going on to produce fuels and chemicals from green and renewable sources for sustainable development with the least economic and environmental impact. Lately, research on lignocellulosic biomass conversion into biofuels, chemicals, and bioproducts has received remarkable attention because it is a naturally available non-toxic, biodegradable, and low-cost feedstock material that is an excellent source of sugar (e.g., cellulose and hemicellulose) and aromatic polymers (e.g., lignin). These sugars and aromatic polymers can be converted to biofuels and valuable chemicals by biological and chemical processes (Chen and Mu, 2019, Sharma et al., 2022, Wang and Lee, 2021).
There are various sources of lignocellulosic biomass, such as agricultural & forestry residues, energy crops, animal and industrial food waste, municipal solid waste, etc. In the USA alone, a vast amount of lignocellulosic biomass feedstocks is available for immediate utilization. According to the US Department of Energy's 2016 Billion-Ton Report, a billion tons of biomass feedstocks from the above sources can be potentially produced, and if converted to biofuels and bioproducts, it could replace 30 % of 2005 US petroleum consumption without affecting the food or other agricultural products productions (Langholtz et al., 2016). If this target is achieved, it not only benefits the environment by reducing greenhouse gas emissions but also can provide sustainable economic growth and better energy security (Langholtz et al., 2016).
The chemical structure of lignocellulosic biomass is complex due to the existence of various physical and chemical linkages among the different chemical units present in it. The key chemical components of lignocellulosic biomass are cellulose (30–50 %), hemicellulose (20–30 %), and lignin (15–30 %), along with other minor components (Haldar and Purkait, 2021, Wang and Lee, 2021). The chemical structure of cellulose contains linear chains of β-d-glucopyranose that are linked by β-1,4-glycosidic bonds. The linear polysaccharide chains in the cellulose arrange themselves parallelly, which further link themselves by hydrogen bonds and van der Waal's forces to form an ordered structure. Due to this, the cellulose is crystalline, firm, and strong. Hemicellulose is a branched heteropolymer of different sugar units (e.g., mannose, arabinose, galactose, glucose, and xylose) with a degree of polymerization (DP) ranging from 50 to 300. Unlike cellulose, it is non-crystalline and connected with the cellulose microfibrils by non-covalent interactions. They are linked with lignin via covalent bonds. Due to the amorphous structure and lower DP of hemicellulose compared to cellulose, molecules can easily be solubilized, hydrolyzed, and degraded. Lignin is another main component of lignocellulosic biomass. It is the polymer of various aromatic phenols in which p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol are the main units connected by irregular and complex cross-linkings. Due to these irregular cross-linkings, lignins are amorphous and possess a highly resistant structure that is challenging to break down by using various enzymatic and biological degradation processes to convert them to small molecules (Ashokkumar et al., 2022, Sethupathy et al., 2022).
Effective conversion of lignocellulosic biomass to sugar (from cellulose and hemicellulose) and smaller aromatic compounds (from lignin) is hindered due to the complex structure of lignocellulose and the recalcitrance nature of the cell wall of the plant. Therefore, various physical (e.g., ball milling) and physiochemical (e.g., acids, alkali, ammonia fiber explosion, hot water, and solvet based, etc.), and biological (e.g., enzyme and microbial) pretreatment methods for have been developed to disintegrate the biomass into its components (cellulose, hemicellulose, and lignin) to facilitate conversion of the recovered biomass components into biofuels and valuable chemicals. Many of these pretreatment methods are not environmentally friendly, energy intensive, expensive due to costly equipment and chemicals, have low-solid loading, and need to operate long time to process biomass (Baruah et al., 2018, Duque et al., 2017, Wang and Lee, 2021).
In recent years, deep eutectic solvents (DESs) have gained interest for biomass pretreatment because of their biomass solubilizing ability, easy synthesis, low volatility, non-flammability, biodegradability, and low toxicity. Due to these remarkable properties, DESs are being extensively explored as an alternative to ionic liquids (ILs) (Chen and Mu, 2019, Sharma et al., 2022). DES-based biomass pretreatment is promising; however, several unresolved challenges, such as pretreatment efficiency, solvent loading, and recyclability, are needed to be addressed to enable large-scale deployment of this technology. Therefore, process intensification strategies have recently gained much attention for developing sustainable DES-based technologies that enhance not only the efficiency of the pretreatment processes but also lower the cost and minimize the other associated issues (Acciardo et al., 2022).
This review aims to critically evaluate the recent progress and provide perspectives for the future development of DES pretreatment technology. First, the design/selection of DESs for the pretreatment of a variety of biomass feedstocks, the recent progress in molecular simulations to understand the interactions between DESs and biomass components, and the guiding principles for designing new DESs suitable for applications in biomass deconstruction are discussed. Next, a brief overview is provided of the current reports on process intensification of DES-based biomass pretreatment, one-pot enzymatic hydrolysis and fermentation, recovery of reaction products, and regeneration and recycling of DES. After that, the techno-economic and life cycle analyses of DES-based pretreatment are discussed. Finally, the challenges and future research direction to overcome the difficulties associated with DES pretreatment are provided.
Section snippets
Basics of deep eutectic solvent (DES)
DES refers to a liquid eutectic mixture of two constituents, generally a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD), with a lower melting point than its individual constituents. The number of DES components could also be extended to three or more. The prevailing opinions hold that the intermolecular interactions within the DES system, such as hydrogen bonding, van der Waals interactions, and entropy of mixing, are responsible for decreased melting temperature and DES formation
Process intensification for biomass pretreatment using DES
Despite the extraordinary potential of DESs in a biorefinery concept, the key to an economically viable and scalable DES-based biomass pretreatment technology relies on 1) design and discovery of low-cost, biocompatible and effective DESs for biomass pretreatment applications; 2) producing high-value/specialty chemicals and materials as co-products; 3) development of efficient and cost-effective separation to recover co-products and to recycle the DES; 4) a better understanding of the scaleup
Recovery and recycling strategies in DES-based biomass pretreatment
DES loses efficacy to some extent due to the leftover chemicals as dissolved impurities (from cellulose and lignin) which are generated during the pretreatment or conversion of biomass. Therefore, recovery and purification steps are essential for recycling DESs back to the biomass pretreatment and conversion processes. However, few reports have been made on the recovery and recycling of DESs when they are used in biomass processing (Isci & Kaltschmitt, 2021). Various recovery and purification
Techno-economical assessment (TEA) of biomass pretreatment using DES
Compared to the other widely explored biomass pretreatment technologies, the TEA and LCA for DES-based biomass pretreatment are relatively scarce. This is probably because the technology is still in its nascent stage and evolving rapidly. Nevertheless, summarized here are a few examples of recent TEA work on DES-based technologies. In a recent study, Kumar et al. (2020) evaluated the economic feasibility of an integrated NADES-based biorefinery plant which can have a capacity of 1 ton per day
Challenges and future perspectives
DESs are promising green solvents that have shown potential to be used in biomass pretreatment because they are easy to prepare, cost-effective, less toxic, and, most importantly, biodegrade. In addition to the benefits, there are still several challenges and issues with DES-based pretreatment technology, such as DES recycling, pretreatment performance optimization, viscosity, etc., that are needed to be addressed before realizing application in industrial processes.
Conclusions
DES-based biomass pretreatment methods are promising for sustainable industrial applications. Recent advances in developing process intensification strategies have shown an improvement not only in the deconstruction of biomass but also in the separation, recovery, and conversion of biomass constituents to valuable products. However, several challenges that are discussed here still need to be addressed in order to take this green technology to the next level. Further development of DES-based
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jian Shi reports financial support was provided by National Institute of Food and Agriculture.
Acknowledgments
The authors acknowledge US Department of Agriculture, National Institute of Food and Agriculture (under accession number #1015068 and #1018315), and University of Kentucky Igniting Research Collaborations for financial support.
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