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Biobutanol: Beyond Traditional Fuel – Can Science Brew a Sustainable Future?

Butanol epitomizes a quintessential bulk chemical raw material, poised to transcend into the echelons of a nascent biofuel era. The utilization of renewable substrates in the microbial fermentation process, yielding butanol alongside coveted solvents such as acetone and ethanol, has garnered significant traction. The realm of ABE (Acetone-Butanol-Ethanol) fermentation, synonymous with ethanol bioprocessing, erstwhile ranked as the preeminent fermentative domain, second only to ethanol fermentation. However, the enduring slump in the price of petrochemical butanol has cast a pall over the economic viability of ABE fermentation, posing formidable challenges to its hegemony. Amidst such milieu, the augmentation of product valor, enhancement of substrate conversion efficacy, and harnessing cost-effective feedstocks emerge as cardinal imperatives in fortifying the fiscal fortitude of ABE fermentation endeavors.

ABE fermentation process schematic

1. The Saga of ABE through the Ages

The annals of biobutanol production unfurl a narrative steeped in historical reverie. Tracing its origins to the crucible of the First World War, wherein the anaerobic fermentation prowess of solvent-generating Clostridium catalyzed the synthesis of butanol, laying the cornerstone for the genesis of synthetic rubber. Concurrently, the vista of solvent fermentation, underpinned by carbohydrate substrates like maize flour, burgeoned apace, culminating in its zenith as the world’s second-largest fermentative enterprise, trailing only in the wake of alcohol fermentation. Yet, the specter of waning fortunes loomed with the onset of the 1950s, as the hegemony of ABE fermentation waned under the encroaching dominance of the oil conglomerates. The enclaves of Europe, North America, and Japan bore witness to the gradual ebbing of ABE fermentative endeavors, save for the enclave of China, ensconced within its unique economic and political crucible, where the flame of ABE fermentation endured. In 1955, the inaugural bastion of ABE fermentation, the Shanghai Solvent Factory, embarked upon its odyssey of corn fermentation to yield ABE. Over the ensuing decades, approximately thirty bastions of ABE fermentation arose, albeit the crescendo of petroleum synthesis and chemical industry heralded the twilight of ABE fermentation’s heyday, with domestic enterprises shuttering their operations since the nineties. The pantheon of industrial strains, emanating from the crucible of Clostridia, constitutes the lynchpin of solvent production, with Clostridium acetobutylicum reigning supreme as the vanguard species, endowed with an indomitable prowess in fermenting starchy substrates, thereby enshrining the quintessential ethos of ABE fermentation.

2. The Vicissitudes of Biobutanol Fabrication

The epoch of petrochemical ascendancy has cast a pall over the economic hinterlands of ABE fermentation, ensnaring it in a vortex of competitive exigency vis-à-vis petroleum-derived butanol. The crucible of economic exigency, wherein the chasm of butanol fermentation’s economic tenability yawns wide, finds its roots in multifarious quandaries: firstly, the exorbitant outlay entailed in sourcing carbon substrates for fermentation; secondly, the pall of diminished butanol concentration in the fermentative broth; and thirdly, the specter of diminished butanol selectivity in the fermentative crucible.

2.1. The Esoteric Quandary of Costly Substrates

The tapestry of butanol synthesis, woven through the loom of traditional fermentation, enshrines a pantheon of processes, encompassing solvent fermentation, distillation separation, and the labyrinthine straits of environmental stewardship. The annals of anaerobic fermentation, predicated upon the fecundity of starchy edibles like maize, cereals, and desiccated tubers, unfurl an insatiable appetite for resources, culminating in a cornucopia of demand, approximating 4.0 to 4.5 tons of maize, alongside copious streams of steam, water, and electricity, for every ton of solvent engendered. In a bid to uphold the imprimatur of food security and price stability, the nation stands resolute in curbing the rampant utilization of corn and its ilk for bioenergetic ends, thereby thrusting into the crucible the imperative of ameliorating the production outlay of butanol, via augments in conversion efficacy and the judicious adoption of non-edible substrates.

2.2. The Quixotic Pursuit of Enhanced Butanol Prowess

The epoch of traditional ABE fermentation bespeaks a tripartite alchemy, engendering acetone, butanol, and ethanol in equal measure. Yet, the clarion call of fiscal prudence resonates in the endeavor to burgeon the preeminence of butanol, eclipsing its compatriots in the fermentative pantheon, as the quintessence of fiscal alacrity. An analysis borne of sagacity proffers that elevating the butanol concentration from 12g/L to 19g/L heralds a halving of subsequent distillative outlays, yet the apogee of fermentative ambition finds itself fettered by the baleful specter of butanol toxicity, casting a pall over the viability of such pursuits.

2.3. The Melancholia of Bygone By-products

The hallowed precincts of traditional ABE fermentation, presided over by the aegis of Clostridium acetobutylicum, bear witness to the triumvirate of butanol, acetone, and ethanol, yet in the crucible of fiscal sagacity, butanol stands preeminent. Alas, the fecundity of fermentation, whilst bounteous, remains ensnared within the cauldron of inefficiency, with the bête noire of unrecoverable acetic and butyric acids, dimming the luminescence of raw material conversion efficacy, and auguring dire straits for production costs. Thus, the clarion call resounds for the augmentation of butanol selectivity, in a paean to fiscal acumen, and the judicious conservation of grain reserves, as the vanguards of ABE fermentation’s ascendancy.

3. Where lies the path?

Through the genetic manipulation of butanol-producing fermentation strains and the refinement of fermentation and product retrieval processes, the ensuing three objectives can be attained: 1) Augmenting the proportion of butanol in the yield; 2) Discovering economical alternative raw materials; 3) Amplifying the concentration of fermentation products; 4) Augmenting the value addition of fermentation products. This shall surmount the constraining bottleneck in the traditional fermentation method of manufacturing butanol, thereby enhancing the economic viability of biobutanol production.

3.1 Enhancing the butanol proportion

Elevating the butanol ratio is feasible through the reconstruction and optimization of the microbial butanol pathway. The archetypal butanol metabolic pathway emanates from solventogenic Clostridium. Apart from 60% to 70% butanol, solventogenic Clostridium also yields two low-value by-products: 20% to 30% acetone and 10% ethanol. By reconstructing and optimizing the butanol pathway, it is plausible to diminish the synthesis of acetone and ethanol, thereby further increasing the proportion of butanol in the overall solvents while upholding the original high conversion efficiency of the strain, consequently fortifying the economic competitiveness of butanol production.

3.2 Expanding the raw material repertoire in biobutanol preparation

3.2.1 Non-grain raw materials such as potatoes and Jerusalem artichokes

Potatoes are crops abundant in starch content, encompassing cassava, sweet potato, and potato, extensively cultivated worldwide and in my nation. The market valuation of potato raw materials is inferior to that of corn, wheat, and other grain raw materials, thus, it has been widely deployed in bioethanol production.

3.2.2 Woody fiber raw materials

Lignocellulose is generally deemed as a fermentation raw material with the utmost application potential. In recent years, there has been a plethora of foreign studies on the utilization of lignocellulose fermentation in the preparation of ABE solvents. The process entails pretreatment and hydrolysis of raw materials into monosaccharides; fermentation of sugar liquid to yield butanol; product distillation and recycling, etc. Despite the feasibility of the aforementioned process route, numerous technical hurdles necessitate surmounting. Concerning bacterial strains, the equitable utilization of five-carbon sugars and six-carbon sugars in fiber hydrolyzate poses a pressing challenge that demands resolution.

3.2.3 Syngas

Synthetic gas predominantly comprises a mixed gas blend of CO, H2, and CO2, derived from an extensive array of sources, including coal, oil shale, tar sand, heavy residues, subpar natural gas, and biomass. The production technology employing synthesis gas as raw material to synthesize chemical raw materials like ammonia, oxygenates, and hydrocarbons has been commercially operationalized.

3.3 Heighten the butanol concentration in ABE fermentation broth

Butanol exerts high toxicity to bacterial cells. Augmenting its butanol tolerance via genetic strain modification engenders conducive conditions for refining the selectivity and product concentration of butanol biosynthesis. To mitigate the inhibitory impact of fermentation product butanol on the production strains, diminishing butanol toxicity in the fermentation broth can be achieved through enhancements in fermentation and downstream processes, thus culminating in an elevation of butanol concentration at the culmination of fermentation.

3.4 Enhance the value addition of ABE fermentation broth

Researchers from the Institute of Microbiology, Chinese Academy of Sciences, were inspired by Clostridium beijerinckii’s proficiency in producing acetone, butanol, ethanol, and isopropanol from sugar substrates and devised an isopropanol biosynthesis module. The components encompass a secondary alcohol dehydrogenase gene derived from Clostridium beijerinckii and a potent promoter derived from Clostridium acetobutylicum. Introducing this module into Rh8, a laboratory-bred mutant strain exhibiting high butanol resistance and solvent yield (J. Proteome Res., 2010, 9:3046-3061), it was discerned that the acetone synthesized by the host cells can be efficiently and wholly converted into isopropanol, thereby establishing an isopropanol biosynthetic pathway and transforming the classic ABE fermentation into IBE (Isopropanol-Butanol-Ethanol) fermentation.

Isopropyl alcohol is a pivotal chemical product and chemical raw material. The global total demand for isopropyl alcohol currently approximates 2.3 million tons per annum. The realization of Clostridial IBE fermentation not only propels the biomanufacturing of isopropanol but also harbors the potential for utilization as biofuel by blending isopropanol, butanol, and ethanol.

Additionally, ABE fermentation broth typically exhibits low concentration, and its current utilization modality involves obtaining pure acetone, butanol, and ethanol products via purification processes such as concentration and separation, inevitably leading to issues like high energy consumption. Ergo, the optimal utilization modus of ABE fermentation broth is to directly convert it into fuels or chemicals sans separation.

The Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has devised a strategy capable of converting approximately 70% of the carbon in an aqueous ABE fermentation mixture (ABE: acetone-butanol-ethanol-water) into 4-heptanone (4-HPO). The reaction is catalyzed by tin-doped ceria (Sn-ceria) with an exceptionally high selectivity of 86%. Although Sn-ceria serves as a versatile catalyst for dehydrogenation, Guerbet alcohol reactions, condensation, and esterification reactions, all involving acid-base catalysis and redox reactions, it yields 4-HPO with remarkable selectivity. 4-HPO is a value-added intermediate suitable for producing jet fuel and fine chemicals.

4. Future outlook

As a pivotal chemical and a novel generation of biofuel, the biological preparation of butanol has gradually emerged as a global research focal point. The imperative is to further curtail the manufacturing cost of biobutanol to secure a competitive market edge over petrochemical synthesis routes. However, with the deepened exploration of lignocellulose hydrolysis and the diminishing cost of cellulase, the utilization of such renewable resources in butanol fermentation production is poised to become an inevitable developmental trajectory.

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