Pretreatment and hydrolysis of lignocellulosic wastes for butanol production: Challenges and perspectives

Application of fed-batch strategy to fully eliminate the negative effect of lignocellulose-derived inhibitors in ABE fermentation

BACKGROUND

Inhibitors that are released from lignocellulose biomass during its treatment represent one of the major bottlenecks hindering its massive utilization in the biotechnological production of chemicals. This study demonstrates that negative effect of inhibitors can be mitigated by proper feeding strategy. Both, crude undetoxified lignocellulose hydrolysate and complex medium supplemented with corresponding inhibitors were tested in acetone-butanol-ethanol (ABE) fermentation using Clostridium beijerinckii NRRL B-598 as the producer strain.

RESULTS

First, it was found that the sensitivity of C. beijerinckii to inhibitors varied with different growth stages, being the most significant during the early acidogenic phase and less pronounced during late acidogenesis and early solventogenesis. Thus, a fed-batch regime with three feeding schemes was tested for toxic hydrolysate (no growth in batch mode was observed). The best results were obtained when the feeding of an otherwise toxic hydrolysate was initiated close to the metabolic switch, resulting in stable and high ABE production. Complete utilization of glucose, and up to 88% of xylose, were obtained. The most abundant inhibitors present in the alkaline wheat straw hydrolysate were ferulic and coumaric acids; both phenolic acids were efficiently detoxified by the intrinsic metabolic activity of clostridia during the early stages of cultivation as well as during the feeding period, thus preventing their accumulation. Finally, the best feeding strategy was verified using a TYA culture medium supplemented with both inhibitors, resulting in 500% increase in butanol titer over control batch cultivation in which inhibitors were added prior to inoculation.

CONCLUSION

Properly timed sequential feeding effectively prevented acid-crash and enabled utilization of otherwise toxic substrate. This study unequivocally demonstrates that an appropriate biotechnological process control strategy can fully eliminate the negative effects of lignocellulose-derived inhibitors.

Extracellular expression of agarolytic enzymes in Clostridium sp. strain and its application for butanol production from Gelidium amansii

In this study, Clostridium sp. strain WK-AN1 carrying both genes of agarase (Aga0283) and neoagarobiose hydrolase (NH2780) were successfully constructed to convert agar polysaccharide directly into butanol, contributing to overcome the lack of algal hydrolases in solventogenic clostridia. Through the optimization by the Plackett-Burman design (PBD) and response surface methodology (RSM), a maximal butanol production of 6.42 g/L was achieved from 17.86 g/L agar. Further application of utilizing the butyric acid pretreated Gelidium amansii hydrolysate demonstrated the modified strain obtained the butanol production of 7.83 g/L by 1.63-fold improvement over the wild-type one. This work for the first time establishes a novel route to utilize red algal polysaccharides for butanol fermentation by constructing a solventogenic clostridia-specific secretory expression system for heterologous agarases, which will provide insights for future development of the sustainable third-generation biomass energy.

Strategies to enhance enzymatic hydrolysis of lignocellulosic biomass for biorefinery applications: A review

Global interest in lignocellulosic biorefineries has increased in the recent past due to technological advancements in sustainable and cost-effective production of numerous commodity and speciality chemicals and fuels from renewable lignocellulosic biomass (LCB). As a result, the market value of biorefinery products has also increased over the time, with an estimated worth of USD 867.7 billion by 2025. However, biorefinery operations, especially enzymatic hydrolysis, suffer from many challenges that limits the cost-effectiveness of conversion of LCB. Therefore, it is essential to understand and address these challenges in future biorefineries. The paper focuses on recent trends and challenges in enzymatic hydrolysis of LCB during lignocellulosic biorefinery operation for greener synthesis of energy, fuels, chemicals and other high-value products. Insights into the gaps in knowledge and technological challenges have also been addressed together with focus on future research needs and perspectives of enzymatic hydrolysis of LCB for biorefinery applications.

Production of butanol from lignocellulosic biomass: recent advances, challenges, and prospects

Due to energy and environmental concerns, biobutanol is gaining increasing attention as an alternative renewable fuel owing to its desirable fuel properties. Biobutanol production from lignocellulosic biomass through acetone-butanol-ethanol (ABE) fermentation has gained much interest globally due to its sustainable supply and non-competitiveness with food, but large-scale fermentative production suffers from low product titres and poor selectivity. This review presents recent developments in lignocellulosic butanol production, including pretreatment and hydrolysis of hemicellulose and cellulose during ABE fermentation. Challenges are discussed, including low concentrations of fermentation sugars, inhibitors, detoxification, and carbon catabolite repression. Some key process improvements are also summarised to guide further research and development towards more profitable and commercially viable butanol fermentation.

Post-hydrolysis of cellulose oligomers by cellulase immobilized on chitosan-grafted magnetic nanoparticles: A key stage of butanol production from waste textile

The recently developed technologies for immobilization of cellulase may address the challenges in costly hydrolysis of cellulose for cellulosic butanol production. In this study, a "hybrid" hydrolysis was developed based on chemical hydrolysis of cellulose to its oligomers followed by enzymatic post-hydrolysis of the resulting "soluble oligomers" by cellulase immobilized on chitosan-coated Fe₃O₄ nanoparticles. This hybrid hydrolysis stage was utilized in the process of biobutanol production from a waste textile, jeans waste, leading to selective formation of glucose and high yield of butanol production by Clostridium acetobutylicum. After validating the immobilization process, the optimum immobilization parameters including enzyme concentration and time were achieved on 8 h and 15.0 mg/mL, respectively. The reusability of immobilized enzyme showed that immobilized cellulase could retain 51.5% of its initial activity after three times reuses. Dilute acid hydrolysis of regenerated cellulose at 120-180 °C for 60 min 0.5-1.0% phosphoric acid led to less than 10 g/L glucose production, and enzymatic post-hydrolysis of the oligomers resulted in up to 51.5 g/L glucose. Fermentation of the hydrolysate was accompanied by 5.3 g/L acetone-butanol-ethanol (ABE) production. The simultaneous co-saccharification and fermentation (SCSF) of soluble and insoluble oligomers of cellulose led to 17.4 g/L ABE production.

Smart sustainable biorefineries for lignocellulosic biomass

Lignocellulosic biomass (LCB) is considered as a sustainable feedstock for a biorefinery to generate biofuels and other bio-chemicals. However, commercialization is one of the challenges that limits cost-effective operation of conventional LCB biorefinery. This article highlights some studies on the sustainability of LCB in terms of cost-competitiveness and environmental impact reduction. In addition, the development of computational intelligence methods such as Artificial Intelligence (AI) as a tool to aid the improvement of LCB biorefinery in terms of optimization, prediction, classification, and decision support systems. Lastly, this review examines the possible research gaps on the production and valorization in a smart sustainable biorefinery towards circular economy.

Transcriptomic studies of solventogenic clostridia, Clostridium acetobutylicum and Clostridium beijerinckii

Solventogenic clostridia are not a strictly defined group within the genus Clostridium but its representatives share some common features, i.e. they are anaerobic, non-pathogenic, non-toxinogenic and endospore forming bacteria. Their main metabolite is typically 1-butanol but depending on species and culture conditions, they can form other metabolites such as acetone, isopropanol, ethanol, butyric, lactic and acetic acids, and hydrogen. Although these organisms were previously used for the industrial production of solvents, they later fell into disuse, being replaced by more efficient chemical production. A return to a more biological production of solvents therefore requires a thorough understanding of clostridial metabolism. Transcriptome analysis, which reflects the involvement of individual genes in all cellular processes within a population, at any given (sampling) moment, is a valuable tool for gaining a deeper insight into clostridial life. In this review, we describe techniques to study transcription, summarize the evolution of these techniques and compare methods for data processing and visualization of solventogenic clostridia, particularly the species Clostridium acetobutylicum and Clostridium beijerinckii. Individual approaches for evaluating transcriptomic data are compared and their contributions to advancements in the field are assessed. Moreover, utilization of transcriptomic data for reconstruction of computational clostridial metabolic models is considered and particular models are described. Transcriptional changes in glucose transport, central carbon metabolism, the sporulation cycle, butanol and butyrate stress responses, the influence of lignocellulose-derived inhibitors on growth and solvent production, and other respective topics, are addressed and common trends are highlighted.

Thermostable Cellulases / Xylanases From Thermophilic and Hyperthermophilic Microorganisms: Current Perspective

The bioconversion of lignocellulose into monosaccharides is critical for ensuring the continual manufacturing of biofuels and value-added bioproducts. Enzymatic degradation, which has a high yield, low energy consumption, and enhanced selectivity, could be the most efficient and environmentally friendly technique for converting complex lignocellulose polymers to fermentable monosaccharides, and it is expected to make cellulases and xylanases the most demanded industrial enzymes. The widespread nature of thermophilic microorganisms allows them to proliferate on a variety of substrates and release substantial quantities of cellulases and xylanases, which makes them a great source of thermostable enzymes. The most significant breakthrough of lignocellulolytic enzymes lies in lignocellulose-deconstruction by enzymatic depolymerization of holocellulose into simple monosaccharides. However, commercially valuable thermostable cellulases and xylanases are challenging to produce in high enough quantities. Thus, the present review aims at giving an overview of the most recent thermostable cellulases and xylanases isolated from thermophilic and hyperthermophilic microbes. The emphasis is on recent advancements in manufacturing these enzymes in other mesophilic host and enhancement of catalytic activity as well as thermostability of thermophilic cellulases and xylanases, using genetic engineering as a promising and efficient technology for its economic production. Additionally, the biotechnological applications of thermostable cellulases and xylanases of thermophiles were also discussed.

Soybean hulls as carbohydrate feedstock for medium to high-value biomolecule production in biorefineries: A review

Soybean is one of the major world crops, with an annual production of 359 million tons. Each ton of processed soybean generates 50-80 kg of soybean hulls (SHs), representing 5-8% of the whole seed. Due to environmental concerns and great economic potential, the search of SHs re-use solutions are deeply discussed. The lignocellulosic composition of SHs has attracted the attention of the scientific and productive sector. Recently, some studies have reported the use of SHs in the production of medium to high value-added molecules, with potential applications in food and feed, agriculture, bioenergy, and other segments. This review presents biotechnological approaches and processes for the management and exploitation of SHs, including pre-treatment methods and fermentation techniques, for the production of different biomolecules. Great potentialities and innovations were found concerning SH exploration and valorisation of the soybean chain under a biorefinery and circular bioeconomy optic.

Butanol production from Saccharina japonica hydrolysate by engineered Clostridium tyrobutyricum: The effects of pretreatment method and heat shock protein overexpression

Macroalgal biomass is currently considered as a potential candidate for biofuel production. In this study, the effects of pretreatment method and heat shock protein overexpression were investigated for efficient butanol production from Saccharina japonica using engineered Clostridium tyrobutyricum. First, various pretreatment methods including acid hydrolysis, acid hydrolysis and enzymatic saccharification, and ultrasonic-assisted acid hydrolysis were employed to obtain the fermentable sugars, and the resulted hydrolysates were evaluated for butanol fermentation. The results showed that ultrasonic-assisted acid hydrolysate obtained the highest butanol yield (0.26 g/g) and productivity (0.19 g/L⋅h). Then, the effects of homologous or heterologous heat shock protein overexpression on butanol production and tolerance were examined. Among all the engineered strains, Ct-pMA12G exhibited improved butanol tolerance and enhanced butanol production (12.15 g/L butanol with a yield of 0.34 g/g and productivity of 0.15 g/L⋅h) from 1.8-fold concentrated S. japonica hydrolysate, which was the highest level ever reported for macroalgal biomass.

Current understanding of the inhibition factors and their mechanism of action for the lignocellulosic biomass hydrolysis

Biorefining of lignocellulosic biomass is a relatively new concept but it has strong potential to develop and partially replace the fossil derived fuels and myriad of value products to subsequently reduce the greenhouse gas emissions. However, the energy and cost intensive process of releasing the entrapped fermentable sugars is a major challenge for its commercialization. Various factors playing a detrimental role during enzymatic hydrolysis of biomass are inherent recalcitrance of lignocellulosic biomass, expensive enzymes, sub-optimal enzyme composition, lack of synergistic activity and enzyme inhibition caused by various inhibitors. The current study investigated the mechanism of enzyme inhibition during lignocellulosic biomass saccharification especially at high solid loadings. These inhibition factors are categorized into physio-chemical factors, water-soluble and -insoluble enzyme inhibitors, oligomers and enzyme-lignin binding. Furthermore, different approaches are proposed to alleviate the challenges and improve the enzymatic hydrolysis efficiency such as supplementation with surfactants, synergistic catalytic/non-catalytic proteins, and bioprocess modifications.

Valorization of pelagic sargassum biomass into sustainable applications: Current trends and challenges

Since long ago, pelagic Sargassum mats have been known to be abundant in the Sargasso Sea, where they provide habitat to diverse organisms. However, over the last few years, massive amounts of pelagic Sargassum have reached the coast of several countries in the Caribbean and West Africa, causing economic and environmental problems. Aiming for lessening the impacts of the blooms, governments and private companies remove the seaweeds from the shore, but this process results expensive. The valorization of this abundant biomass can render Sargassum tides into an economic opportunity and concurrently solve their associated environmental problems. Despite the diverse fields where algae have found applications and the relevance of this recurrent situation, Sargassum biomass remains without large scale applications. Therefore, this review aims to present the potential uses of these algae, identifying the limitations that must be assessed to effectively valorize this bioresource. Due to the constraints identified for each of the presented applications, it is concluded that a biorefinery approach should be developed to effectively valorize this abundant biomass. However, there is an urgent need for investigations focusing on holopelagic Sargassum to be able to truly valorize this seaweed.

Three-stage repeated-batch immobilized cell fermentation to produce butanol from non-detoxified sugarcane bagasse hemicellulose hydrolysates

To enable the production of butanol with undiluted, non-detoxified sugarcane bagasse hemicellulose hydrolysates, this study developed a three-staged repeated-batch immobilized cell fermentation in which the efficiency of a 3D-printed nylon carrier to passively immobilize Clostridium saccharoperbutylacetonicum DSM 14923 was compared with sugarcane bagasse. The first stage consisted of sugarcane molasses fermentation, and in the second stage, non-detoxified sugarcane bagasse hemicellulose hydrolysates (SBHH) was pulse-fed to sugarcane molasses fermentation. In the next four batches, immobilized cells were fed with undiluted SBHH supplemented with molasses, and SBHH-derived xylose accounted for approximately 50% of the sugars. Bagasse was a superior carrier, and the average xylose utilization (33%) was significantly higher than the treatment with the 3D-printed carrier (16%). Notably, bagasse allowed for 43% of the butanol to be SBHH-derived. Overall, cell immobilization on lignocellulosic materials can be an efficient strategy to produce butanol from repeated-batch fermentation of non-detoxified hemicellulose hydrolysates.

Deep Eutectic Solvents as Catalysts for Upgrading Biomass

Deep eutectic solvents (DESs) have emerged as promising green solvents, due to their versatility and properties such as high biodegradability, inexpensiveness, ease of preparation and negligible vapor pressure. Thus, DESs have been used as sustainable media and green catalysts in many chemical processes. On the other hand, lignocellulosic biomass as an abundant source of renewable carbon has received ample interest for the production of biobased chemicals. In this review, the state of the art of the catalytic use of DESs in upgrading the biomass-related substances towards biofuels and value-added chemicals is presented, and the gap in the knowledge is indicated to direct the future research.

Unveiling Modifications of Biomass Polysaccharides during Thermal Treatment in Cholinium Chloride : Lactic Acid Deep Eutectic Solvent

A deep analysis upon the chemical modifications of the cellulose and hemicelluloses fractions that take place during biomass delignification with deep eutectic solvents (DES) is lacking in literature, being this a critical issue given the continued research on DES for this purpose. This work intends to fill this gap by disclosing a comprehensive study on the chemical modifications of cellulose (microcrystalline cellulose and bleached kraft pulp) and hemicelluloses (xylans) during thermal treatment (130 °C) with cholinium chloride/lactic acid ([Ch]Cl/LA) at molar ratio 1 : 10, one of the best reported DES for biomass delignification. The obtained data revealed that [Ch]Cl/LA (1 : 10) has a negative impact on the polysaccharides fractions at prolonged treatments (>4 h), resulting on substantial modifications including the esterification of cellulose with lactic acid, shortening of fibers length, fibers agglomeration and side reactions of the hemicelluloses fraction (e. g., humin formation, lactic acid grafting). Wood delignification trials with [Ch]Cl/LA (1 : 10) at the same conditions also corroborate these findings. Moreover, the DES suffers degradation, including the formation of lactic acid derivatives and its polymerization. Therefore, short time delignification treatments are strongly recommended when using the [Ch]Cl/LA DES, so that a sustainable fractionation of biomass into high quality cellulose fibers, isolated lignin, and xylose/furfural co-production along with solvent recyclability could be achieved.

Comparison of dilute acid pretreatment of agave assisted by microwave versus ultrasound to enhance enzymatic hydrolysis

A comparison between microwave and ultrasound irradiations in the agave pretreatment using dilute sulfuric acid as catalyst was assessed for the first time. Pretreatments were performed using a Taguchi Orthogonal Array L9 (3⁴) to improve the hemicellulose removal and the agave digestibility. The results showed that under optimal conditions, the hemicellulose removal was superior in the pretreatment assisted with microwave (77.5%) compared to ultrasound (28.2%). Enzymatic hydrolysis yield of agave pretreated with microwave (MW_OC) was 2-fold higher than agave pretreated with ultrasound (US_OC). The relatively mild conditions of pretreatment with MW_OC allowed to obtain a hydrolyzed free of inhibitors with a high glucose concentration (47.7 g/L) at low solids loading (10% w/v). However, these conditions did not have a significant effect over the agave pretreated with ultrasound. The pretreatment assisted with MW_OC allowed to reduce time and temperature of the process compared to pretreatment with conventional heating.

Arundo donax Refining to Second Generation Bioethanol and Furfural

Biomass-derived sugars are platform molecules that can be converted into a variety of final products. Non-food, lignocellulosic feedstocks, such as agroforest residues and low inputs, high yield crops, are attractive bioresources for the production of second-generation sugars. Biorefining schemes based on the use of versatile technologies that operate at mild conditions contribute to the sustainability of the bio-based products. The present work describes the conversion of giant reed (Arundo donax), a non-food crop, to ethanol and furfural (FA). A sulphuric-acid-catalyzed steam explosion was used for the biomass pretreatment and fractionation. A hybrid process was optimized for the hydrolysis and fermentation (HSSF) of C6 sugars at high gravity conditions consisting of a biomass pre-liquefaction followed by simultaneous saccharification and fermentation with a step-wise temperature program and multiple inoculations. Hemicellulose derived xylose was dehydrated to furfural on the solid acid catalyst in biphasic media irradiated by microwave energy. The results indicate that the optimized HSSF process produced ethanol titers in the range 43–51 g/L depending on the enzymatic dosage, about 13–21 g/L higher than unoptimized conditions. An optimal liquefaction time before saccharification and fermentation tests (SSF) was 10 h by using 34 filter paper unit (FPU)/g glucan of Cellic® CTec3. C5 streams yielded 33.5% FA of the theoretical value after 10 min of microwave heating at 157 °C and a catalyst concentration of 14 meq per g of xylose.

Synergy of municipal solid waste co-processing with lignocellulosic waste for improved biobutanol production

Co-processing of lignocellulosic wastes, e.g., garden and paper wastes, and the organic matters fraction of municipal solid waste (OMSW) in an integrated bioprocess is a possible approach to realize the potential of wastes for biobutanol production. Dilute acid pretreatment is a multi-functional stage for breaking the recalcitrant lignocellulose's structure, hydrolyzing hemicellulose, and hydrolyzing/solubilizing starch, leading to a pretreated solid and a rich hydrolysate. In this study, dilute-acid pretreatment of the combination of wastepaper and OMSW, composite I, as well as garden waste and OMSW, composite II, at severe conditions resulted in "pretreatment hydrolysates" containing 33.7 and 19.4 g/L sugar along with 18.9 and 33.2 g/L soluble starch, respectively. In addition, the hydrolysis of solid remained after the pretreatment of composite I and II resulted in "enzymatic hydrolysates" comprising 19.4 and 33 g/L sugar, respectively. The fermentation of the pretreatment hydrolysates and enzymatic hydrolysates resulted in 3.5 and 6.4 g/L ABE from composite I and 15 and 5.2 g/L ABE from composite II, respectively. In this process, 148 and 173 g ABE (60 and 100 g gasoline equivalent/kg) was obtained from each kg composite I and composite II, respectively, where co-processing of OMSW with lignocellulosic wastes resulted in 10 and 49% higher ABE than that produced from the individual substrates.

Acetone, butanol, and ethanol production from puerariae slag hydrolysate through ultrasound-assisted dilute acid by Clostridium beijerinckii YBS3

In this study, puerariae slag (PS) was evaluated as a renewable raw material for acetone-butanol-ethanol (ABE) fermentation. To accelerate the hydrolysis of PS, the method of ultrasound-assisted dilute acid hydrolysis (UAAH) was used. With this effort, 0.69 g reducing sugar was obtained from 1 g raw material under the optimal pretreatment condition. Subsequently, the butanol and total solvent production of 8.79 ± 0.16 g/L and 12.32 ± 0.26 g/L were obtained from the non-detoxified diluted hydrolysate, and the yield and productivity of butanol were 0.19 g/g and 0.12 g/L/h, respectively. Additionally, the changes in the structure of PS after different pretreatment methods were observed using SEM and FT-IR. UAAH resulted in more severe and distinct damage to the dense structure of PS. This study suggests that the UAAH is an attainable but effective pretreatment method, thereby is a promising technique for lignocellulose hydrolysis and improve butanol production.

Reassessment of the role of CaCO3 in n-butanol production from pretreated lignocellulosic biomass by Clostridium acetobutylicum

In this study, the role of CaCO₃ in n-butanol production was further investigated using corn straw hydrolysate (CSH) media by Clostridium acetobutylicum CICC 8016. CaCO₃ addition stimulated sugars utilization and butanol production. Further study showed that calcium salts addition to CSH media led to the increase in Ca²⁺ concentration both intracellularly and extracellularly. Interestingly, without calcium salts addition, intracellular Ca²⁺ concentration in the synthetic P2 medium was much higher than that in the CSH medium despite the lower extracellular Ca²⁺ concentrations in the P2 medium. These results indicated that without additional calcium salts, Ca²⁺ uptake by C. acetobutylicum CICC 8016 in the CSH medium may be inhibited by non-sugar biomass degradation compounds, such as furans, phenolics and organic acids. Comparative proteomics analysis results showed that most enzymes involved in glycolysis, redox balance and amino acids metabolism were up-regulated with CaCO₃ addition. This study provides further insights into the role of CaCO₃ in n-butanol production using real biomass hydrolysate.

Metabolic and Process Engineering of Clostridium beijerinckii for Butyl Acetate Production in One Step

n-Butyl acetate is an important food additive commonly produced via concentrated sulfuric acid catalysis or immobilized lipase catalysis of butanol and acetic acid. Compared with chemical methods, an enzymatic approach is more environmentally friendly; however, it incurs a higher cost due to lipase production. In vivo biosynthesis via metabolic engineering offers an alternative to produce n-butyl acetate. This alternative combines substrate production (butanol and acetyl-coenzyme A (acetyl-CoA)), alcohol acyltransferase expression, and esterification reaction in one reactor. The alcohol acyltransferase gene ATF1 from Saccharomyces cerevisiae was introduced into Clostridium beijerinckii NCIMB 8052, enabling it to directly produce n-butyl acetate from glucose without lipase addition. Extractants were compared and adapted to realize glucose fermentation with in situ n-butyl acetate extraction. Finally, 5.57 g/L of butyl acetate was produced from 38.2 g/L of glucose within 48 h, which is 665-fold higher than that reported previously. This demonstrated the potential of such a metabolic approach to produce n-butyl acetate from biomass.

Recent advances in n-butanol and butyrate production using engineered Clostridium tyrobutyricum

Acidogenic clostridia naturally producing acetic and butyric acids has attracted high interest as a novel host for butyrate and n-butanol production. Among them, Clostridium tyrobutyricum is a hyper butyrate-producing bacterium, which re-assimilates acetate for butyrate biosynthesis by butyryl-CoA/acetate CoA transferase (CoAT), rather than the phosphotransbutyrylase-butyrate kinase (PTB-BK) pathway widely found in clostridia and other microbial species. To date, C. tyrobutyricum has been engineered to overexpress a heterologous alcohol/aldehyde dehydrogenase, which converts butyryl-CoA to n-butanol. Compared to conventional solventogenic clostridia, which produce acetone, ethanol, and butanol in a biphasic fermentation process, the engineered C. tyrobutyricum with a high metabolic flux toward butyryl-CoA produced n-butanol at a high yield of > 0.30 g/g and titer of > 20 g/L in glucose fermentation. With no acetone production and a high C4/C2 ratio, butanol was the only major fermentation product by the recombinant C. tyrobutyricum, allowing simplified downstream processing for product purification. In this review, novel metabolic engineering strategies to improve n-butanol and butyrate production by C. tyrobutyricum from various substrates, including glucose, xylose, galactose, sucrose, and cellulosic hydrolysates containing the mixture of glucose and xylose, are discussed. Compared to other recombinant hosts such as Clostridium acetobutylicum and Escherichia coli, the engineered C. tyrobutyricum strains with higher butyrate and butanol titers, yields and productivities are the most promising hosts for potential industrial applications.

Key process parameters for deep eutectic solvents pretreatment of lignocellulosic biomass materials: A review

Deep eutectic solvent (DES) has been considered as a novel green solvent for lignocellulosic biomass pretreatment. The efficiency of DES pretreatment is affected by the synergy of various process parameters. The study of effect of DES physicochemical properties and pretreatment reaction conditions on the mechanism of lignocellulose biomass fractionation was of great significance for the development of biomass conversion. Form the view of process control, this review summarized recent advances in DES pretreatment, analyzed the challenges, and prospected the future development of this research field.

Biobutanol production from municipal solid waste: Technical and economic analysis

Novel processes for the production of acetone-butanol-ethanol (ABE) from municipal solid waste (MSW) were developed and simulated using Aspen Plus®. In scenario 1, a conventional distillation system was used, while a gas stripping system was coupled with a fermenter in scenario 2. In scenario 3, pervaporation (PV) and gas stripping systems right after the fermentation reactor were applied. Gas stripping increased the total ABE produced while the addition of the PV module decreased the number of distillation columns from 6 to 2 as well as created 6.4% increments in the amount of butanol in comparison with scenario 1. Economical evaluation resulted in having payout periods of 15.9, 4.4, and 2.9 years for scenarios 1 to 3, respectively. These results show that using MSW as an inexpensive sugar-rich feedstock together with gas stripping PV system is a promising solution to overcome the major obstacles in the way of the ABE production.

Recent developments in the application of kraft pulping alkaline chemicals for lignocellulosic pretreatment: Potential beneficiation of green liquor dregs waste

Lignocellulosic waste has offered a cost-effective and food security-wise substrate for the generation of biofuels and value-added products. However, its recalcitrant properties necessitate pretreatment. Of the various pretreatment methods, alkaline techniques have gained prominence as efficient catalysts. The kraft pulping industry represents a major hub for the generation of white, black and green liquor alkaline solutions during the paper making process. Despite its well-known significance in the kraft pulping process, green liquor (GL) has been widely applied for lignocellulosic pretreatment. Recently, green liquor dregs (GLD), an alkaline waste generated from the kraft pulping industry has piqued interest. Therefore, this review outlines the general flow of the kraft pulping process and the alkaline chemicals derived. In addition, the extensively studied GL for lignocellulosic pretreatment is discussed. Subsequently, the potential beneficiation of GLD for lignocellulosic pretreatment is presented. Furthermore, the challenges and prospects of lignocellulosic pretreatments are highlighted.

A Two-Step Ferric Chloride and Dilute Alkaline Pretreatment for Enhancing Enzymatic Hydrolysis and Fermentable Sugar Recovery from Miscanthus sinensis

A two-step process was proposed to enhance enzymatic hydrolysis of Miscanthus sinensis based on a comparative study of acid/alkaline pretreatments. Ferric chloride pretreatment (FP) effectively removed hemicellulose and recovered soluble sugars, but the enzymatic hydrolysis was not efficient. Dilute alkaline pretreatment (ALP) resulted in much better delignification and stronger morphological changes of the sample, making it more accessible to enzymes. While ALP obtained the highest sugar yield during enzymatic hydrolysis, the soluble sugar recovery from the pretreatment stage was still limited. Furthermore, a two-step ferric chloride and dilute alkaline pretreatment (F-ALP) has been successfully developed by effectively recovering soluble sugars in the first FP step and further removing lignin of the FP sample in the second ALP step to improve its enzymatic hydrolysis. As a result, the two-step process yielded the highest total sugar recovery (418.8 mg/g raw stalk) through the whole process.

Pathway dissection, regulation, engineering and application: lessons learned from biobutanol production by solventogenic clostridia

The global energy crisis and limited supply of petroleum fuels have rekindled the interest in utilizing a sustainable biomass to produce biofuel. Butanol, an advanced biofuel, is a superior renewable resource as it has a high energy content and is less hygroscopic than other candidates. At present, the biobutanol route, employing acetone-butanol-ethanol (ABE) fermentation in Clostridium species, is not economically competitive due to the high cost of feedstocks, low butanol titer, and product inhibition. Based on an analysis of the physiological characteristics of solventogenic clostridia, current advances that enhance ABE fermentation from strain improvement to product separation were systematically reviewed, focusing on: (1) elucidating the metabolic pathway and regulation mechanism of butanol synthesis; (2) enhancing cellular performance and robustness through metabolic engineering, and (3) optimizing the process of ABE fermentation. Finally, perspectives on engineering and exploiting clostridia as cell factories to efficiently produce various chemicals and materials are also discussed.

Effect of detoxification methods on ABE production from corn stover hydrolysate by Clostridium acetobutylicum CICC 8016

In this study, effects of different single biomass derived inhibitors on acetone-butanol-ethanol (ABE) production by Clostridium acetobutylicum CICC 8016 were first investigated. The results showed that formic acid, coumaric acid, and furfural at 0.5 g/L (sodium formate equivalent) inhibited ABE production. Furthermore, corn stover hydrolysate media were prepared following dilute acid pretreatment, enzymatic hydrolysis, and detoxification with different methods. Among overliming, steam stripping, acetone-ethyl ether extraction, and ion exchange with five anion resins, adsorption with resin D301 showed the highest efficiency for inhibitor removal (99-100% of phenolics and 87-99% of sugar degradation products). Without detoxification, ABE production was lower than 1.0 g/L from 28.1 g/L sugars whereas ABE production with medium detoxified by D301 resin achieved higher ABE concentrations and yields than control with synthetic medium. Correlation analysis further revealed that formic acid, coumaric acid, and total phenolics were the major compounds inhibiting ABE production. The results also showed that the single detoxification method was sufficient to detoxify the hydrolysate for ABE production at the pretreatment conditions used in this study.

Clostridium sp. as Bio-Catalyst for Fuels and Chemicals Production in a Biorefinery Context

Clostridium sp. is a genus of anaerobic bacteria capable of metabolizing several substrates (monoglycerides, diglycerides, glycerol, carbon monoxide, cellulose, and more), into valuable products. Biofuels, such as ethanol and butanol, and several chemicals, such as acetone, 1,3-propanediol, and butyric acid, can be produced by these organisms through fermentation processes. Among the most well-known species, Clostridium carboxidivorans, C. ragsdalei, and C. ljungdahlii can be highlighted for their ability to use gaseous feedstocks (as syngas), obtained from the gasification or pyrolysis of waste material, to produce ethanol and butanol. C. beijerinckii is an important species for the production of isopropanol and butanol, with the advantage of using hydrolysate lignocellulosic material, which is produced in large amounts by first-generation ethanol industries. High yields of 1,3 propanediol by C. butyricum are reported with the use of another by-product from fuel industries, glycerol. In this context, several Clostridium wild species are good candidates to be used as biocatalysts in biochemical or hybrid processes. In this review, literature data showing the technical viability of these processes are presented, evidencing the opportunity to investigate them in a biorefinery context.

Deep Eutectic Solvents for Pretreatment, Extraction, and Catalysis of Biomass and Food Waste

Valorization of lignocellulosic biomass and food residues to obtain valuable chemicals is essential to the establishment of a sustainable and biobased economy in the modern world. The latest and greenest generation of ionic liquids (ILs) are deep eutectic solvents (DESs) and natural deep eutectic solvents (NADESs); these have shown great promise for various applications and have attracted considerable attention from researchers who seek versatile solvents with pretreatment, extraction, and catalysis capabilities in biomass- and biowaste-to-bioenergy conversion processes. The present work aimed to review the use of DESs and NADESs in the valorization of biomass and biowaste as pretreatment or extraction solvents or catalysis agents.

Production of n-butanol from cassava bagasse hydrolysate by engineered Clostridium tyrobutyricum overexpressing adhE2: Kinetics and cost analysis

The production of biofuels such as butanol is usually limited by the availability of inexpensive raw materials and high substrate cost. Using food crops as feedstock in the biorefinery industry has been criticized for its competition with food supply, causing food shortage and increased food prices. In this study, cassava bagasse as an abundant, renewable, and inexpensive byproduct from the cassava starch industry was used for n-butanol production. Cassava bagasse hydrolysate containing mainly glucose was obtained after treatments with dilute acid and enzymes (glucoamylases and cellulases) and then supplemented with corn steep liquor for use as substrate in repeated-batch fermentation with engineered Clostridium tyrobutyricum CtΔack-adhE2 in a fibrous-bed bioreactor. Stable butanol production with high titer (>15.0 g/L), yield (>0.30 g/g), and productivity (~0.3 g/L∙h) was achieved, demonstrating the feasibility of an economically competitive process for n-butanol production from cassava bagasse for industrial application.

n-Butanol production from lignocellulosic biomass hydrolysates without detoxification by Clostridium tyrobutyricum Δack-adhE2 in a fibrous-bed bioreactor

Acetone-butanol-ethanol fermentation suffers from high substrate cost and low butanol titer and yield. In this study, engineered Clostridium tyrobutyricum CtΔack-adhE2 immobilized in a fibrous-bed bioreactor was used for butanol production from glucose and xylose present in the hydrolysates of low-cost lignocellulosic biomass including corn fiber, cotton stalk, soybean hull, and sugarcane bagasse. The biomass hydrolysates obtained after acid pretreatment and enzymatic hydrolysis were supplemented with corn steep liquor and used in repeated-batch fermentations. Butanol production with high titer (∼15 g/L), yield (∼0.3 g/g), and productivity (∼0.3 g/L∙h) was obtained from cotton stalk, soybean hull, and sugarcane bagasse hydrolysates, while corn fiber hydrolysate with higher inhibitor contents gave somewhat inferior results. The fermentation process was stable for long-term operation without any noticeable degeneration, demonstrating its potential for industrial application. A techno-economic analysis showed that n-butanol could be produced from lignocellulosic biomass using this novel fermentation process at ∼$2.5/gal for biofuel application.

Direct microbial transformation of carbon dioxide to value-added chemicals: A comprehensive analysis and application potentials

Carbon dioxide storage in petroleum and other geological reservoirs is an economical option for long-term separation of this gas from the atmosphere. Other options include applications through conversion to valuable chemicals. Microalgae and plants perform direct fixation of carbon dioxide to biomass, which is then used as raw material for further microbial transformation (MT). The approach by microbial transformation can achieve reduction of carbon dioxide and production of biofuels. This review addresses the research and technological processes related to direct MT of carbon dioxide, factors affecting their efficiency in operation and the review of economic feasibility. Additionally, some commercial plants making utilization of CO₂ around the globe are also summarized along with different value-added chemicals (methane, acetate, fatty acids and alcohols) as reported in literature. Further information is also provided for a better understanding of direct CO₂ MT and its future prospects leading to a sustainable and clean environment.

Statistical and sequential (fill-and-draw) approach to enhance rhamnolipid production using industrial lignocellulosic hydrolysate C6 stream from Achromobacter sp. (PS1)

Statistical optimization using industrial rice-straw hydrolysate (C₆ stream) containing 5.0% total sugars was carried out for enhancing the rhamnolipid production from Achromobacter sp. (PS1) with subsequent adoption of a sequential fermentation approach with fill-and-draw operation for further increment. The interactive effects of six influential variables obtained from one-factor-at-a-time approach as sodium nitrate, yeast extract, ferrous sulphate, phosphate concentrations and agitation in presence of lignocellulosic hydrolyzed sugars as a basal medium using central composite design revealed the experimental rhamnolipid yield of 5.46 g/L at optimum conditions of total sugars 40 g/L (w/v), sodium nitrate 6.0 (g/L), yeast extract 2 (g/L), ferrous sulphate 0.2 (mg/L) and phosphate 1000 mM at 100 rpm at 30 °C in 8 days. The sequential approach further resulted in an overall yield of 19.35 g/L of rhamnolpid in five sequential-cycles with an increase of 258% over the batch process on account of nutrients replenishment and dilution of toxic by-products.

Adsorption of 2-nitrophenol using rice straw and rice husks hydrolyzed by subcritical water

The potential of rice husks and straw as adsorbents after being processed by subcritical water hydrolysis (SWH) was investigated. The influences of temperature (453, 493 and 533 K) and liquid/solid ratio (7.5 and 15 g water/g biomass) on the rice straw and rice husks characteristics and on the adsorption capacity of 2-nitrophenol were evaluated at pH 4 and 7. Adsorption kinetics, equilibrium and thermodynamic parameters were also studied. The adsorption capacity was favored at pH 7. Pseudo-first-order model was suitable to predict the kinetic curves for 2-nitrophenol concentrations of 25, 50, 75 and 100 mg/L and the isotherm data obeyed the Freundlich model. Overall, the thermodynamic results revealed a spontaneous and exothermic process. The maximum adsorption capacity (92.97 ± 1.31 mg/g) was obtained for rice straw that has undergone an SWH at 453 K and 7.5 g water/g straw. The integration of processes to valorize co-products can make the production of cellulosic bioethanol more feasible.

Hybrid dilute sulfuric acid and aqueous ammonia pretreatment for improving butanol production from corn stover with reduced wastewater generation

An efficient hybrid pretreatment method was developed for butanol production from corn stover using dilute sulfuric acid (DA) and aqueous ammonia (AA). With the optimized AA concentration, treatment temperature and time of 10% AA, 80 °C and 24 h, the hybrid pretreatment could effectively dissolve hemicellulose and lignin with solid recovery rate of 37.45% and lignin reduction rate of 86.77% compared to those of 57.75% and 45.84% from single DA pretreatment. By washing 1 time after each step treatment, sugar yield and butanol production were increased to 401.76 mg/g-CS and 10.89 g/L from 346.04 mg/g-CS and 9.33 g/L obtained without washing. Compared with conventional single DA and AA pretreatment methods, wastewater generation was reduced to 0.83 L/g-butanol from 2.11 and 3.46 L/g-butanol, indicating this hybrid pretreatment could be an effective approach for improving butanol production from lignocellulosic feedstocks.