Prospective and development of butanol as an advanced biofuel

An investigation into the adsorption mechanism of n-butanol by ZIF-8: a combined experimental and ab initio molecular dynamics approach

The zeolitic imidazolate framework, ZIF-8, has been shown by experimental methods to have a maximum saturation adsorption capacity of 0.36 g g^-1 for n-butanol from aqueous solution, equivalent to a loading of 14 butanol molecules per unit cell or 7 molecules per sodalite β-cage. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) shows the presence of hydrogen bonding between adsorbed butanol molecules within the cage; the presence of three different O-H stretching modes indicates the formation of butanol clusters of varying size. Ab initio molecular dynamics simulations show the formation of intermolecular hydrogen bonding between the butanol molecules, with an average hydrogen-bond coordination number of 0.9 after 15 ps simulation time. The simulations also uniquely demonstrate the presence of weaker interactions between the alcohol O-H group and the π-orbital of the imidazole ring on the internal surface of the cage during early stages of adsorption. The calculated adsorption energy per butanol molecule is -33.7 kJ mol^-1, confirming that the butanol is only weakly bound, driven primarily by the hydrogen bonding. Solid-state MAS NMR spectra suggest that the adsorbed butanol molecules possess a reasonable degree of mobility in their adsorbed state, rather than being rigidly held in specific sites. 2D ¹³C-¹H heteronuclear correlation (HETCOR) experiments show interactions between the butanol aliphatic chain and the ZIF-8 framework experimentally, suggesting that O-H interactions with the π-orbital are only short lived. The insight gained from these results will allow the design of more efficient ways of recovering and isolating n-butanol, an important biofuel, from low-concentration solutions.

Ethanol Upgrading to n-Butanol Using Transition-Metal-Incorporated Poly(triazine)imide Frameworks

The upgrading of ethanol to n-butanol was performed using a molecular catalyst integrated into a carbon nitride support, one of the first examples of a supported molecular catalyst performing the Guerbet process. Initial studies using crystalline poly(triazine)imide (PTI) with lithium or transition-metal cations imbedded in the support together with a base as the catalyst system did not produce any significant amounts of n-butanol. However, when using the catalyst material formed by treatment of PTI-LiCl with [(Cp*)IrCl₂]₂ (Cp* = pentamethylcyclopentadienyl) along with sodium hydroxide, a 59% selectivity for butanol (13% yield) was obtained at 145 °C. This PTI-(Cp*)Ir material exhibited distinct UV-vis absorption features and powder X-ray diffractions which differ from those of the parent PTI-LiCl and [(Cp*)IrCl₂]₂. The PTI-(Cp*)Ir material was found to have a metal loading of 27% iridium per empirical unit of the framework. Along with the formation of n-butanol from the Guerbet reaction, the presence of higher chain alcohols was also observed.

Implementation of graphitic carbon nitride nanomaterials and laser irradiation for increasing bioethanol production from potato processing wastes

Agricultural and agro-industrial wastes (e.g., potato peel waste) are causing severe environmental problems. The processes of pretreatment, saccharification, and fermentation are the major obstacles in bioethanol production from wastes and must be overcome by efficient novel techniques. The effect of exposing the fungi (yeast) Saccharomyces cerevisiae to laser source with the addition of graphitic carbon nitride nanosheets (g-C₃N₄) with different concentrations on bioethanol production was investigated through the implementation of a batch anaerobic system and using potato peel waste (PPW). Dichromate test was implemented as quantitative analysis for quantification of the bioethanol yield. The benefits of this test were the appearance of green color indicating the identification of ethanol (C₂H₅OH) by bare eye and the ease to calculate the bioethanol yield through UV-visible spectrophotometry. The control sample (0.0 ppm of g-C₃N₄) showed only a 4% yield of bioethanol; however, by adding 150 ppm to PPW medium, 22.61% of ethanol was produced. Besides, laser irradiations (blue and red) as influencing parameters were studied with and without the addition of g-C₃N₄ nanomaterials aiming to increase the bioethanol. It was determined that the laser irradiation can trigger the bioethanol production (in case of red: 13.13% and in case of blue: 16.14% yields, respectively) compared to the control sample (in absence of g-C₃N₄). However, by adding different concentrations of g-C₃N₄ nanomaterials from 5 to 150 ppm, the bioethanol yield was increased as follows: in case of red: 56.11% and, in case of blue: 56.77%, respectively. It was found that using fungi and exposing it to the blue laser diode source having a wavelength of 450 nm and a power of 250 mW for a duration of 30 min with the addition of 150 mg L^-1 of g-C₃N₄ nanomaterials delivered the highest bioethanol yield from PPW.

Upgrading bio-based acetone to diacetone alcohol by aldol reaction using Amberlyst A26-OH as catalyst

The aldol reaction of bio acetone in presence of a strongly basic ion exchange resin was carried out with and without the addition of water in a temperature range between − 30 °C and 45 °C. The conversion, selectivity and service time of the ion exchange resins were investigated in a stirred batch reactor and a continuous fixed bed reactor. For the batch experiments, both conversion and selectivity increased with decreasing temperature. Furthermore, the addition of water to the reaction medium has a positive effect on selectivity and catalyst service time of the resins. For the continuous flow experiments carried out in a fixed bed reactor, the selectivity towards diacetone alcohol is higher than in a batch reactor. This high selectivity is favored by a short contact time which inhibits as expected most of the consecutive reactions.

State of art of valorising of diverse potential feedstocks for the production of alcohols and ethers: Current changes and perspectives

Alcohols could be the biggest factor for the improvement of world biofuel economy in the present century due to their excellent properties compared to petroleum products. The primary concerns of sustainable alcohol production for meeting the growing energy demand owing to the selection of viable feedstock and this might enhance the opportunities for developing numerous advanced techniques. In this review, the valorization of alcohol production from several production routes has been exposed by covering the traditional routes to the present state of the art technologies. Even though the fossil fuel conversion could be dominant method for methanol production, many recent innovations like photo electrochemical synthesis and electrolysis methods might play vital role in production of renewable methanol in future. There have been several production routes for production of ethanol and among which the fermentation of lignocellulose biomass would be the ultimate choice for large scale shoot up. The greenhouse gas recovery in the form of alcohols through electrochemistry technique and hydrogenation method are the important methods for commercialization of alcohols in future. It is also observed that algae based renewable bio-alcohols is highly influenced by carbohydrate content and sustainable approaches in algae conversion to bio-alcohols would bring greater demand in future market. There is a lack of innovation in higher alcohols production in single process and this could be bounded by combining dehydrogenation and decarboxylation techniques. Finally, this review enlists the opportunities and challenges of existing alcohols production and recommended the possible routes for making significant enhancement in production.

Effects of orphan histidine kinases on clostridial sporulation progression and metabolism

Solventogenesis and sporulation of clostridia are the main responsive adaptations to the acidic environment during acetone-butanol-ethanol (ABE) fermentation. It was hypothesized that five orphan histidine kinases (HKs) including Cac3319, Cac0323, Cac0903, Cac2730, and Cac0437 determined the cell fates between sporulation and solventogenesis. In this study, the comparative genomic analysis revealed that a mutation in cac0437 appeared to contribute to the nonsporulating feature of ATCC 55025. Hence, the individual and interactive roles of five HKs in regulating cell growth, metabolism, and sporulation were investigated. The fermentation results of mutants with different HK expression levels suggested that cac3319 and cac0437 played critical roles in regulating sporulation and acids and butanol biosynthesis. Morphological analysis revealed that cac3319 knockout abolished sporulation (Stage 0) whereas cac3319 overexpression promoted spore development (Stage VII), and cac0437 knockout initiated but blocked sporulation before Stage II, indicating the progression of sporulation was altered through engineering HKs. By combinatorial HKs knockout, the interactive effects between two different HKs were investigated. This study elucidated the regulatory roles of HKs in clostridial differentiation and demonstrated that HK engineering can be effectively used to control sporulation and enhance butanol biosynthesis.

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.

Overview of Current Developments in Biobutanol Production Methods and Future Perspectives

Renewable biobutanol production is receiving more attention toward substituting fossil-based nonrenewable fuels. Biobutanol is recognized as the top most biofuel with extraordinary properties as compared with gasoline. The demand for biobutanol production is increasing enormously due to application in various industries as chemical substituent. Biobutanol production technology has attracted many researchers toward implementation of replacing cost-effective substrate and easy method to recover from the fermentation broth. Sugarcane bagasse, algal biomass, crude glycerol, and lignocellulosic biomass are potential cost-effective substrates which could replace consistent glucose-based substrates. The advantages and limitations of these substrates have been discussed in this chapter. Moreover, finding the integrated biobutanol recovery methods is an important factor parameter in production of biobutanol. This chapter also concentrated on possibilities and drawbacks of obtainable integrated biobutanol recovery methods. Thus, successful process involving cost-effective substrate and biobutanol recovery methods could help to implementation of biobutanol production industry. Overall, this chapter has endeavored to increase the viability of industrial production of biobutanol.

Response characteristics of the membrane integrity and physiological activities of the mutant strain Y217 under exogenous butanol stress

Butanol inhibits bacterial activity by destroying the cell membrane of Clostridium acetobutylicum strains and altering functionality. Butanol toxicity also results in destruction of the phosphoenolpyruvate-carbohydrate phosphotransferase system (PTS), thereby preventing glucose transport and phosphorylation and inhibiting transmembrane transport and assimilation of sugars, amino acids, and other nutrients. In this study, based on the addition of exogenous butanol, the tangible macro indicators of changes in the carbon ion beam irradiation-mutant Y217 morphology were observed using scanning electron microscopy (SEM). The mutant has lower microbial adhesion to hydrocarbon (MATH) value than C. acetobutylicum ATCC 824 strain. FDA fluorescence intensity and conductivity studies demonstrated the intrinsically low membrane permeability of the mutant membrane, with membrane potential remaining relatively stable. Monounsaturated FAs (MUFAs) accounted for 35.17% of the mutant membrane, and the saturated fatty acids (SFA)/unsaturated fatty acids (UFA) ratio in the mutant cell membrane was 1.65. In addition, we conducted DNA-level analysis of the mutant strain Y217. Expectedly, through screening, we found gene mutant sites encoding membrane-related functions in the mutant, including ATP-binding cassette (ABC) transporter-related genes, predicted membrane proteins, and the PTS transport system. It is noteworthy that an unreported predicted membrane protein (CAC 3309) may be related to changes in mutant cell membrane properties. KEY POINTS: • Mutant Y217 exhibited better membrane integrity and permeability. • Mutant Y217 was more resistant to butanol toxicity. • Some membrane-related genes of mutant Y217 were mutated.

Quantitative proteomic analysis to reveal expression differences for butanol production from glycerol and glucose by Clostridium sp. strain CT7

Clostridium sp. strain CT7 is a new emerging microbial cell factory with high butanol production ratio owing to its non-traditional butanol fermentation mode with uncoupled acetone and 1,3-propanediol formation. Significant changes of metabolic products profile were shown in glycerol- and glucose-fed strain CT7, especially higher butanol and lower volatile fatty acids (VFAs) production occurred from glycerol-fed one. However, the mechanism of this interesting phenomenon was still unclear. To better elaborate the bacterial response towards glycerol and glucose, the quantitative proteomic analysis through iTRAQ strategy was performed to reveal the regulated proteomic expression levels under different substrates. Proteomics data showed that proteomic expression levels related with carbon metabolism and solvent generation under glycerol media were highly increased. In addition, the up-regulation of hydrogenases, ferredoxins and electron-transferring proteins may attribute to the internal redox balance, while the earlier triggered sporulation response in glycerol-fed media may be associated with the higher butanol production. This study will pave the way for metabolic engineering of other industrial microorganisms to obtain efficient butanol production from glycerol.

How to outwit nature: Omics insight into butanol tolerance

The energy crisis, depletion of oil reserves, and global climate changes are pressing problems of developed societies. One possibility to counteract that is microbial production of butanol, a promising new fuel and alternative to many petrochemical reagents. However, the high butanol toxicity to all known microbial species is the main obstacle to its industrial implementation. The present state of the art review aims to expound the recent advances in modern omics approaches to resolving this insurmountable to date problem of low butanol tolerance. Genomics, transcriptomics, and proteomics show that butanol tolerance is a complex phenomenon affecting multiple genes and their expression. Efflux pumps, stress and multidrug response, membrane transport, and redox-related genes are indicated as being most important during butanol challenge, in addition to fine-tuning of global regulators of transcription (Spo0A, GntR), which may further improve tolerance. Lipidomics shows that the alterations in membrane composition (saturated lipids and plasmalogen increase) are very much species-specific and butanol-related. Glycomics discloses the pleiotropic effect of CcpA, the role of alternative sugar transport, and the production of exopolysaccharides as alternative routes to overcoming butanol stress. Unfortunately, the strain that simultaneously syntheses and tolerates butanol in concentrations that allow its commercialization has not yet been discovered or produced. Omics insight will allow the purposeful increase of butanol tolerance in natural and engineered producers and the effective heterologous expression of synthetic butanol pathways in strains hereditary butanol-resistant up to 3.2 - 4.9% (w/v). Future breakthrough can be achieved by a detailed study of the membrane proteome, of which 21% are proteins with unknown functions.

Milling byproducts are an economically viable substrate for butanol production using clostridial ABE fermentation

Butanol is a platform chemical that is utilized in a wide range of industrial products and is considered a suitable replacement or additive to liquid fuels. So far, it is mainly produced through petrochemical routes. Alternative production routes, for example through biorefinery, are under investigation but are currently not at a market competitive level. Possible alternatives, such as acetone-butanol-ethanol (ABE) fermentation by solventogenic clostridia are not market-ready to this day either, because of their low butanol titer and the high costs of feedstocks. Here, we analyzed wheat middlings and wheat red dog, two wheat milling byproducts available in large quantities, as substrates for clostridial ABE fermentation. We could identify ten strains that exhibited good butanol yields on wheat red dog. Two of the best ABE producing strains, Clostridium beijerinckii NCIMB 8052 and Clostridium diolis DSM 15410, were used to optimize a laboratory-scale fermentation process. In addition, enzymatic pretreatment of both milling byproducts significantly enhanced ABE production rates of the strains C. beijerinckii NCIMB 8052 and C. diolis DSM 15410. Finally, a profitability analysis was performed for small- to mid-scale ABE fermentation plants that utilize enzymatically pretreated wheat red dog as substrate. The estimations show that such a plant could be commercially successful.Key points• Wheat milling byproducts are suitable substrates for clostridial ABE fermentation.• Enzymatic pretreatment of wheat red dog and middlings increases ABE yield.• ABE fermentation plants using wheat red dog as substrate are economically viable. Graphical abstract.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

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.

Minimalistic Cellulosome of the Butanologenic Bacterium Clostridium saccharoperbutylacetonicum

Clostridium saccharoperbutylacetonicum is a mesophilic, anaerobic, butanol-producing bacterium, originally isolated from soil. It was recently reported that C. saccharoperbutylacetonicum possesses multiple cellulosomal elements and would potentially form the smallest cellulosome known in nature. Its genome contains only eight dockerin-bearing enzymes, and its unique scaffoldin bears two cohesins (Cohs), three X2 modules, and two carbohydrate-binding modules (CBMs). In this study, all of the cellulosome-related modules were cloned, expressed, and purified. The recombinant cohesins, dockerins, and CBMs were tested for binding activity using enzyme-linked immunosorbent assay (ELISA)-based techniques. All the enzymes were tested for their comparative enzymatic activity on seven different cellulosic and hemicellulosic substrates, thus revealing four cellulases, a xylanase, a mannanase, a xyloglucanase, and a lichenase. All dockerin-containing enzymes interacted similarly with the second cohesin (Coh2) module, whereas Coh1 was more restricted in its interaction pattern. In addition, the polysaccharide-binding properties of the CBMs within the scaffoldin were examined by two complementary assays, affinity electrophoresis and affinity pulldown. The scaffoldin of C. saccharoperbutylacetonicum exhibited high affinity for cellulosic and hemicellulosic substrates, specifically to microcrystalline cellulose and xyloglucan. Evidence that supports substrate-dependent in vivo secretion of cellulosomes is presented. The results of our analyses contribute to a better understanding of simple cellulosome systems by identifying the key players in this minimalistic system and the binding pattern of its cohesin-dockerin interaction. The knowledge gained by our study will assist further exploration of similar minimalistic cellulosomes and will contribute to the significance of specific sets of defined cellulosomal enzymes in the degradation of cellulosic biomass.IMPORTANCE Cellulosome-producing bacteria are considered among the most important bacteria in both mesophilic and thermophilic environments, owing to their capacity to deconstruct recalcitrant plant-derived polysaccharides (and notably cellulose) into soluble saccharides for subsequent processing. In many ecosystems, the cellulosome-producing bacteria are particularly effective "first responders." The massive amounts of sugars produced are potentially amenable in industrial settings to further fermentation by appropriate microbes to biofuels, notably ethanol and butanol. Among the solvent-producing bacteria, Clostridium saccharoperbutylacetonicum has the smallest cellulosome system known thus far. The importance of investigating the building blocks of such a small, multifunctional nanomachine is crucial to understanding the fundamental activities of this efficient enzymatic complex.

Metabolic Engineering of Histidine Kinases in Clostridium beijerinckii for Enhanced Butanol Production

Clostridium beijerinckii, a promising industrial microorganism for butanol production, suffers from low butanol titer and lack of high-efficiency genetical engineering toolkit. A few histidine kinases (HKs) responsible for Spo0A phosphorylation have been demonstrated as functionally important components in regulating butanol biosynthesis in solventogenic clostridia such as C. acetobutylicum, but no study about HKs has been conducted in C. beijerinckii. In this study, six annotated but uncharacterized candidate HK genes sharing partial homologies (no less than 30%) with those in C. acetobutylicum were selected based on sequence alignment. The encoding region of these HK genes were deleted with CRISPR-Cas9n-based genome editing technology. The deletion of cbei2073 and cbei4484 resulted in significant change in butanol biosynthesis, with butanol production increased by 40.8 and 17.3% (13.8 g/L and 11.5 g/L vs. 9.8 g/L), respectively, compared to the wild-type. Faster butanol production rates were observed, with butanol productivity greatly increased by 40.0 and 20.0%, respectively, indicating these two HKs are important in regulating cellular metabolism in C. beijerinckii. In addition, the sporulation frequencies of two HKs inactivated strains decreased by 96.9 and 77.4%, respectively. The other four HK-deletion (including cbei2087, cbei2435, cbei4925, and cbei1553) mutant strains showed few phenotypic changes compared with the wild-type. This study demonstrated the role of HKs on sporulation and solventogenesis in C. beijerinckii, and provided a novel engineering strategy of HKs for improving metabolite production. The hyper-butanol-producing strains generated in this study have great potentials in industrial biobutanol production.

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.

Pleiotropic regulation of a glucose-specific PTS in Clostridium acetobutylicum for high-efficient butanol production from corn stover without detoxification

BACKGROUND

Corn stover (CS) is evaluated as the most favorable candidate feedstock for butanol production via microbial acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum. By independent acid pretreatment and enzymatic hydrolysis, fermentable sugars (mainly glucose and xylose) were released, of which glucose was naturally utilized as the most preferred carbon source by C. acetobutylicum. However, the ABE fermentation using corn stover hydrolysate (CSH) without detoxification is typically limited to poor sugars utilization, butanol production and productivity. In the presence of pretreatment-derived inhibitors, the intracellular ATP and NADH, as important factors involved in cell growth, solventogenesis initiation and stress response, are exceedingly challenged owing to disrupted glucose phosphotransferase system (PTS). Therefore, there is a necessity to develop effective engineering approaches to overcome these limitations for high-efficient butanol production from CSH without detoxification.

RESULTS

PTS-engineered C. acetobutylicum strains were constructed via overexpression and knockout of gene glcG encoding glucose-specific PTS IICBA, which pleiotropically regulated glucose utilization, cell growth, solventogenesis and inhibitors tolerance. The PTS^GlcG-overexpressing strain exhibited high fermentation efficiency, wherein butanol production and productivity was 11.1 g/L and 0.31 g/L/h, compared to those of 11.0 g/L and 0.15 g/L/h with the PTS^GlcG-deficient strain. During CSH culture without detoxification, the PTS^GlcG-overexpressing strain exhibited desirable inhibitors tolerance and solventogenesis with butanol production of 10.0 g/L, increased by 300% and 400% compared to those of 2.5 and 2.0 g/L with the control and PTS^GlcG-deficient strains, respectively. As a result of extra glucose and 10 g/L CaCO₃ addition into CSH, butanol production and productivity were further maximized to 12.5 g/L and 0.39 g/L/h. These validated improvements on the PTS^GlcG-overexpressing strain were ascribed to not only efficient glucose transport but also its cascading effects on intracellular ATP and NADH generation, solventogenesis initiation and inhibitors tolerance at the exponential growth phase.

CONCLUSION

The PTS^GluG regulation could be an effective engineering approach for high-efficient ABE fermentation from lignocellulosic hydrolysates without detoxification or wastewater generation, providing fundamental information for economically sustainable butanol production with high productivity.

Effective continuous acetone-butanol-ethanol production with full utilization of cassava by immobilized symbiotic TSH06

BACKGROUND

Butanol production by fermentation has recently attracted increasingly more attention because of its mild reaction conditions and environmentally friendly properties. However, traditional feedstocks, such as corn, are food supplies for human beings and are expensive and not suitable for butanol production at a large scale. In this study, acetone, butanol, and ethanol (ABE) fermentation with non-pretreated cassava using a symbiotic TSH06 was investigated.

RESULTS

In batch fermentation, the butanol concentration of 11.6 g/L was obtained with a productivity of 0.16 g/L/h, which was similar to that obtained from glucose system. A full utilization system of cassava was constructed to improve the fermentation performance, cassava flour was used as the substrate and cassava peel residue was used as the immobilization carrier. ABE fermentation with immobilized cells resulted in total ABE and butanol concentrations of 20 g/L and 13.3 g/L, which were 13.6% and 14.7% higher, respectively, than those of free cells. To further improve the solvent productivity, continuous fermentation was conducted with immobilized cells. In single-stage continuous fermentation, the concentrations of total ABE and butanol reached 9.3 g/L and 6.3 g/L with ABE and butanol productivities of 1.86 g/L/h and 1.26 g/L/h, respectively. In addition, both of the high product concentration and high solvent productivity were achieved in a three-stage continuous fermentation. The ABE productivity and concentration was 1.12 g/L/h and 16.8 g/L, respectively.

CONCLUSIONS

The results indicate that TSH06 could produce solvents from cassava effectively. This study shows that ABE fermentation with cassava as a substrate could be an efficient and economical method of butanol production.

CRISPR/Cas9-mediated engineering of Escherichia coli for n-butanol production from xylose in defined medium

Butanol production from agricultural residues is the most promising alternative for fossil fuels. To reach the economic viability of biobutanol production, both glucose and xylose should be utilized and converted into butanol. Here, we engineered a dual-operon-based synthetic pathway in the genome of E. coli MG1655 to produce n-butanol using CRISPR/Cas9 technology. Further deletion of competing pathway followed by fed-batch cultivation of the engineered strain in a bioreactor with glucose-containing complex medium yielded 5.4 g/L n-butanol along with pyruvate as major co-product, indicating a redox imbalance. To ferment xylose into butanol in redox-balanced manner, we selected SSK42, an ethanologenic E. coli strain engineered and evolved in our laboratory to produce ethanol from xylose, for integrating synthetic butanol cassette in its genome via CRISPR/Cas9 after deleting the gene responsible for endogenous ethanol production. The engineered plasmid- and marker-free strain, ASA02, produced 4.32 g/L butanol in fed-batch fermentation in completely defined AM1-xylose medium.

Continuous Butanol Fermentation of Dilute Acid-Pretreated De-oiled Rice Bran by Clostridium acetobutylicum YM1

Continuous fermentation of dilute acid-pretreated de-oiled rice bran (DRB) to butanol by the Clostridium acetobutylicum YM1 strain was investigated. Pretreatment of DRB with dilute sulfuric acid (1%) resulted in the production of 42.12 g/L total sugars, including 25.57 g/L glucose, 15.1 g/L xylose and 1.46 g/L cellobiose. Pretreated-DRB (SADRB) was used as a fermentation medium at various dilution rates, and a dilution rate of 0.02 h^-1 was optimal for solvent production, in which 11.18 g/L of total solvent was produced (acetone 4.37 g/L, butanol 5.89 g/L and ethanol 0.92 g/L). Detoxification of SADRB with activated charcoal resulted in the high removal of fermentation inhibitory compounds. Fermentation of detoxified-SADRB in continuous fermentation with a dilution rate of 0.02 h^-1 achieved higher concentrations of solvent (12.42 g/L) and butanol (6.87 g/L), respectively, with a solvent productivity of 0.248 g/L.h. This study showed that the solvent concentration and productivity in continuous fermentation from SADRB was higher than that obtained from batch culture fermentation. This study also provides an economic assessment for butanol production in continuous fermentation process from DRB to validate the commercial viability of this process.

Butanol Synthesis Routes for Biofuel Production: Trends and Perspectives

Butanol has similar characteristics to gasoline, and could provide an alternative oxygenate to ethanol in blended fuels. Butanol can be produced either via the biotechnological route, using microorganisms such as clostridia, or by the chemical route, using petroleum. Recently, interest has grown in the possibility of catalytic coupling of bioethanol into butanol over various heterogenic systems. This reaction has great potential, and could be a step towards overcoming the disadvantages of bioethanol as a sustainable transportation fuel. This paper summarizes the latest research on butanol synthesis for the production of biofuels in different biotechnological and chemical ways; it also compares potentialities and limitations of these strategies.

Co-production of solvents and organic acids in butanol fermentation by Clostridium acetobutylicum in the presence of lignin-derived phenolics

Co-production of solvents (butanol, acetone, and ethanol) and organic acids (butyrate and acetate) by Clostridium acetobutylicum using lignocellulosic biomass as a substrate could further enlarge the application scope of butanol fermentation. This is mainly because solvents and organic acids could be used for production of fine chemicals such as butyl butyrate, butyl oleate, etc. However, many phenolic fermentation inhibitors are formed during the pretreatment process because of lignin degradation. The present study investigated the effects of five typical lignin-derived phenolics on the biosynthesis of solvents and organic acids in C. acetobutylicum ATCC 824. Results obtained in 100 mL anaerobic bottles indicated that butanol concentration was enhanced from 10.29 g L⁻¹ to 11.36 g L⁻¹ by the addition of 0.1 g L⁻¹ vanillin. Subsequently, a pH-control strategy was proposed in a 5 L anaerobic fermenter to alleviate the “acid crash” phenomenon and improve butanol fermentation performance, simultaneously. Notably, organic acid concentration was enhanced from 6.38 g L⁻¹ (control) to a high level of 9.21–12.57 g L⁻¹ with vanillin or/and vanillic acid addition (0.2 g L⁻¹) under the pH-control strategy. Furthermore, the butyrate/butanol ratio reached the highest level of 0.80 g g⁻¹ with vanillin/vanillic acid co-addition, and solvent concentration reached 13.85 g L⁻¹, a comparable level to the control (13.69 g L⁻¹). The effectiveness and robustness of the strategy for solvent and organic acid co-production was also verified under five typical phenolic environments. In conclusion, these results suggest that the proposed process strategy would potentially promote butanol fermentative products from renewable biomass.

Lignin-derived phenolics enhance solvent and organic acid biosynthesis in butanol fermentation by Clostridium acetobutylicum ATCC 824.

Effect of lignocellulose-derived weak acids on butanol production byClostridium acetobutylicumunder different pH adjustment conditions

The effects of formic acid, acetic acid and levulinic acid on acetone–butanol–ethanol (ABE) fermentation under different pH adjustment conditions were investigated using Clostridium acetobutylicum as the fermentation strain. CaCO₃ supplementation can alleviate the inhibitory effect of formic acid on ABE production. The ABE titers from the medium containing 0.5 g L⁻¹ formic acid with pH adjusted by CaCO₃ and KOH were 11.08 g L⁻¹ and 1.04 g L⁻¹, which reached 64.8% and 6.3% of the control group, respectively. Compared with CaCO₃ pH adjustment, fermentation results with higher ABE titers and yields were obtained from the medium containing acetic acid or levulinic acid, when the pH was adjusted by KOH. When formic acid, acetic acid, and levulinic acid co-existed in the medium, better fermentation result was achieved by adjusting the pH by CaCO₃. Moreover, 12.50 g L⁻¹ ABE was obtained from the medium containing 2.0 g L⁻¹ acetic acid, 0.4 g L⁻¹ formic acid, and 1.0 g L⁻¹ levulinic acid as compared to 3.98 g L⁻¹ ABE obtained from the same medium when the pH was adjusted by KOH. CaCO₃ supplementation is a more favorable pH adjustment method for ABE medium preparation from lignocellulosic hydrolysate.

The effects of formic acid, acetic acid and levulinic acid on acetone–butanol–ethanol (ABE) fermentation under different pH adjustment conditions were investigated using Clostridium acetobutylicum as the fermentation strain.

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

Butanol is acknowledged as a drop-in biofuel that can be used in the existing transportation infrastructure, addressing the needs for sustainable liquid fuel. However, before becoming a thoughtful alternative for fossil fuel, butanol should be produced efficiently from a widely-available, renewable, and cost-effective source. In this regard, lignocellulosic materials, the main component of organic wastes from agriculture, forestry, municipalities, and even industries seems to be the most promising source. The butanol-producing bacteria, i.e., Clostridia sp., can uptake a wide range of hexoses, pentoses, and oligomers obtained from hydrolysis of cellulose and hemicellulose content of lignocelluloses. The present work is dedicated to reviewing different processes containing pretreatment and hydrolysis of hemicellulose and cellulose developed for preparing fermentable hydrolysates for biobutanol production.

n-Butanol production in S. cerevisiae: co-ordinate use of endogenous and exogenous pathways

n-Butanol represents a key commodity chemical and holds significant potential as a biofuel. It can be produced naturally by Clostridia species via the ABE pathway. However, butanol production via such systems can be associated with significant drawbacks. Therefore, substantial efforts have been made toward engineering a suitable industrial host for butanol production. For instance, we previously generated a metabolically engineered Saccharomyces cerevisiae strain that produces ~300 mg/L butanol from combined endogenous and exogenous pathways. In this current study, the endogenous and exogenous pathways of butanol production were further characterised, and their relative contribution to the overall butanol titre was assessed. Deletion of any single component of the exogenous ABE pathway was sufficient to significantly reduce butanol production. Further evidence for a major contribution from the ABE pathway came with the discovery that specific yeast deletion mutants only affected butanol production from this pathway and had a significant impact on butanol levels. In previous studies, the threonine-based ketoacid (TBK) pathway has been proposed to explain endogenous butanol synthesis in ADH1 mutants. However, we find that key mutants in this pathway have little impact on endogenous butanol production; hence, this pathway does not explain endogenous butanol production in our strains. Instead, endogenous butanol production appears to rely on glycine metabolism via an α-ketovalerate intermediate. Indeed, yeast cells can utilise α-ketovalerate as a supplement to generate high butanol titres (> 2 g/L). The future characterisation and optimisation of the enzymatic activities required for this pathway provides an exciting area in the generation of robust butanol production strategies.

Novel distillation process for effective and stable separation of high-concentration acetone-butanol-ethanol mixture from fermentation-pervaporation integration process

BACKGROUND

One of the major obstacles of acetone-butanol-ethanol (ABE) fermentation from renewable biomass resources is the energy-intensive separation process. To decrease the energy demand of the ABE downstream separation processes, hybrid in situ separation system with conventional distillation is recognized as an effective method. However, in the distillation processes, the high reflux ratio of the ethanol column and the accumulation of ethanol on top of the water and butanol columns led to poor controllability and high operation cost of the distillations. In this study, vacuum distillation process which is based on a decanter-assisted ethanol-butanol-water recycle loop named E-TCD sequence was developed to improve the conventional separation sequence for ABE separation. The permeate of in situ pervaporation system was used as the feed.

RESULTS

The distillation processes were simulated and optimized by iterative strategies. ABE mixture with acetone, butanol and ethanol concentrations of 115.8 g/L, 191.4 g/L and 17.8 g/L (the other composition was water) that obtained from fermentation-pervaporation integration process was used as the feed. A plant scaled to 1025 kg/h of ABE mixture was performed, and the product purities were 100 wt% of butanol, 99.7 wt% of acetone and 95 wt% of ethanol, respectively. Results showed that only 5.3 MJ/kg (of butanol) was required for ABE separation, which was only 37.54% of the energy cost in conventional distillation processes.

CONCLUSIONS

Compared with the drawbacks of ethanol accumulation in butanol-water recycle loop and the extremely high recovery rate of ethanol in conventional distillation processes, simulation results obtained in the current work avoided the accumulation of ethanol based on the novel E-TCD sequence.

Biobutanol production from coffee silverskin

BACKGROUND

Coffee silverskin, a by-product from coffee roasting industries, was evaluated as a feedstock for biobutanol production by acetone-butanol-ethanol fermentation. This lignocellulosic biomass contained approximately 30% total carbohydrates and 30% lignin. Coffee silverskin was subjected to autohydrolysis at 170 °C during 20 min, with a biomass-to-solvent ratio of 20%, and a subsequent enzymatic hydrolysis with commercial enzymes in order to release simple sugars. The fermentability of the hydrolysate was assessed with four solventogenic strains from the genus Clostridium. In addition, fermentation conditions were optimised via response surface methodology to improve butanol concentration in the final broth.

RESULTS

The coffee silverskin hydrolysate contained 34.39 ± 2.61 g/L total sugars, which represents a sugar recovery of 34 ± 3%. It was verified that this hydrolysate was fermentable without the need of any detoxification method and that C. beijerinckii CECT 508 was the most efficient strain for butanol production, attaining final values of 4.14 ± 0.21 g/L acetone, 7.02 ± 0.27 g/L butanol and 0.25 ± 0.01 g/L ethanol, consuming 76.5 ± 0.8% sugars and reaching a butanol yield of 0.269 ± 0.008 gB/gS under optimal conditions.

CONCLUSIONS

Coffee silverskin could be an adequate feedstock for butanol production in biorefineries. When working with complex matrices like lignocellulosic biomass, it is essential to select an adequate bacterial strain and to optimize its fermentation conditions (such as pH, temperature or CaCO3 concentration).

Solvent production from xylose

Xylose is the second most abundant sugar derived from lignocellulose; it is considered less desirable than glucose for fermentation, and strategies that specifically increase xylose utilization in wild type or engineered cells are goals for biofuel production. Issues arise with xylose utilization because of carbohydrate catabolite repression, which is the preferential utilization of glucose relative to xylose in fermentations with both pure and mixed cultures. Taken together the low substrate utilization rates and solvent yields with xylose compared to glucose, many industrial fermentations ignore the xylolytic portion of the reaction in lieu of methods to maintain high glucose. This is shortsighted given the massive potential for xylose generation from a number of sustainable biomass feedstocks, based on utilization of the hemicellulose fraction(s) that enter pretreatment. A number of strategies have been developed in recent years to address xylose utilization and solvent production from xylose in systems with just xylose, or in systems with mixtures of glucose plus xylose, which are more typical of pretreated lignocellulose. The approaches vary in terms of complexity, stability, and ease of introduction to existing fermentation infrastructure (i.e., so-called drop-in fermentation strategies). Some approaches can be considered traditional engineering approaches (e.g., change the reaction conditions), while others are more subtle cellular approaches to eliminate the impacts of catabolite repression. Finally, genetic engineering has been used to increase xylose utilization, although this can be considered a relatively nascent approach compared to manipulations completed to date for glucose utilization.

Recent advances and strategies in process and strain engineering for the production of butyric acid by microbial fermentation

Butyric acid is an important platform chemical, which is widely used in the fields of food, pharmaceutical, energy, etc. Microbial fermentation as an alternative approach for butyric acid production is attracting great attention as it is an environmentally friendly bioprocessing. However, traditional fermentative butyric acid production is still not economically competitive compared to chemical synthesis route, due to the low titer, low productivity, and high production cost. Therefore, reduction of butyric acid production cost by utilization of alternative inexpensive feedstock, and improvement of butyric acid production and productivity has become an important target. Recently, several advanced strategies have been developed for enhanced butyric acid production, including bioprocess techniques and metabolic engineering methods. This review provides an overview of advances and strategies in process and strain engineering for butyric acid production by microbial fermentation. Additionally, future perspectives on improvement of butyric acid production are also proposed.

Rapid and stable production of 2,3-butanediol by an engineered Saccharomyces cerevisiae strain in a continuous airlift bioreactor

Utilization of renewable feedstocks for the production of bio-based bulk chemicals, such as 2,3-butanediol (2,3-BDO), by engineered strains of the non-pathogenic yeast, Saccharomyces cerevisiae, has recently become an attractive option. In this study, to realize rapid production of 2,3-BDO, a flocculent, 2,3-BDO-producing S. cerevisiae strain YPH499/dPdAdG/BDN6-10/FLO1 was constructed from a previously developed 2,3-BDO-producing strain. Continuous 2,3-BDO fermentation was carried out by the flocculent strain in an airlift bioreactor. The strain consumed more than 90 g/L of glucose, which corresponded to 90% of the input, and stably produced more than 30 g/L of 2,3-BDO over 380 h. The maximum 2,3-BDO productivity was 7.64 g/L/h at a dilution rate of 0.200/h, which was higher than the values achieved by continuous fermentation using pathogenic bacteria in the previous reports. These results demonstrate that continuous 2,3-BDO fermentation with flocculent 2,3-BDO-producing S. cerevisiae is a promising strategy for practical 2,3-BDO production.

Unique genetic cassettes in a Thermoanaerobacterium contribute to simultaneous conversion of cellulose and monosugars into butanol

The demand for cellulosic biofuels is on the rise because of the anticipation for sustainable energy and less greenhouse gas emissions in the future. However, production of cellulosic biofuels, especially cellulosic butanol, has been hampered by the lack of potent microbes that are capable of converting cellulosic biomass into biofuels. We report a wild-type Thermoanaerobacterium thermosaccharolyticum strain TG57, which is capable of using microcrystalline cellulose directly to produce butanol (1.93 g/liter) as the only final product (without any acetone or ethanol produced), comparable to that of engineered microbes thus far. Strain TG57 exhibits significant advances including unique genes responsible for a new butyrate synthesis pathway, no carbon catabolite repression, and the absence of genes responsible for acetone synthesis (which is observed as the main by-product in most Clostridium strains known today). Furthermore, the use of glucose analog 2-deoxyglucose posed a selection pressure to facilitate isolation of strain TG57 with deletion/silencing of carbon catabolite repressor genes-the ccr and xylR genes-and thus is able to simultaneously ferment glucose, xylose, and arabinose to produce butanol (7.33 g/liter) as the sole solvent. Combined analysis of genomic and transcriptomic data revealed unusual aspects of genome organization, numerous determinants for unique bioconversions, regulation of central metabolic pathways, and distinct transcriptomic profiles. This study provides a genome-level understanding of how cellulose is metabolized by T. thermosaccharolyticum and sheds light on the potential of competitive and sustainable biofuel production.

Catalytic Upgrading of Ethanol to n-Butanol: Progress in Catalyst Development

Because n-butanol as a fuel additive has more advantageous physicochemical properties than those of ethanol, ethanol valorization to n-butanol through homo- or heterogeneous catalysis has received much attention in recent decades in both scientific and industrial fields. Recent progress in catalyst development for upgrading ethanol to n-butanol, which involves homogeneous catalysts, such as iridium and ruthenium complexes, and heterogeneous catalysts, including metal oxides, hydroxyapatite (HAP), and, in particular, supported metal catalysts, is reviewed herein. The structure-activity relationships of catalysts and underlying reaction mechanisms are critically examined, and future research directions on the design and improvement of catalysts are also proposed.