Butanol synthesis fermentation

As acetone is the least attractive product in the ABE fermentation, many efforts have been made to engineer the bacterial cells to produce butanol with minimal or no acetone. This goal has been achieved both in C. acetobutylicum natively and in many foreign hosts. However, all these successful cases rely on the direct n-butanol synthesis from acetyl-CoA, a process that will be discussed later in this chapter, and not via the native acid reassimilation. 

So far, the only known natural butanol produced are numerous clostridial species in a process called ABE (acetone-butanol-ethanol) fermentation. Butanol pathway in ABE fermentation consists of condensing two acetyl-CoA molecules (catalysed by a thiolase) and then reducing the product to butanol (requiring four reductases and one dehydratase). Except butanol, acetone and ethanol, Clostridia can also synthesise different chiral substances whose classical chemical synthesis would be challenging (Rogers et al. 1986), and they can degrade several toxic compounds (Francis et al. 1994 Spain 1995). 

O2 is required for butanol synthesis, but in industrial practice anaerobic fermentation is preferred due to lower costs (e.g. for reactor cooling) and lower risk of explosion (no air/isobutene mixtures). 

Process engineering is important for process optimisation, to complement the properties of the microorganism. In principle, a continuous fermentation mode is preferred in industry, due to higher productivity. In case Clostridia are used, fermentation should be designed as a multistage process to correspond to the biphasic character of butanol synthesis. 

A fermentation route to 1-butanol based on carbon monoxide employing the anaerobic bacterium, Butyribacterium methjlotrophicum has been reported (14,15). In contrast to other commercial catalytic processes for converting synthesis gas to alcohols, the new process is insensitive to sulfur contaminants. Current productivities to butanol are 1 g/L, about 10% of that required for commercial viabiUty. Researchers hope to learn enough about the bacteria s control mechanisms to be able to use recombinant DNA to make the cells produce more butanol. 

Fermentation. Fermentation is defined (Ref 3) as the production of chemicals by a series of enzyme catalyzed reactions with bacteria, yeasts, or molds under aerobic or anaerobic conditions. At present, fermentation is used to produce complex molecules not easily synthesized such as penicillin and other antibiotics, vitamin BI2, and enzymes. Formerly, glycerine (See Fetmentol), acetone, butanol, and citric lactic acids were some of the chemicals produced by fermentation process. Synthesis is now a more economical route to these materials (See also Refs 1 2) Refs 1) P.A. Wells G.E. Ward, IEC 31, 172-77(1939) 2) H.E. Silcox S.B. Lee,... 

Since the twenties and thirties butanol together with acetone was produced by fermentation of carbohydrates (corn). In the sixties the process was replaced by the hydroformylation of propene. In the OXO process alkenes react with synthesis gas in the presence of a homogeneous catalyst to give a mixture of branched and linear aldehydes ... 

This continual modification of the process allowed it to compete with the petrochemical synthesis of butanol and acetone (see sections 9.8.1 and 9.9.7 of Vol. I) (Figure 6.6), until molasses found a competing use as an animal feed and its price began to rise. British production stopped in 1957, but the process survives in South Africa and Taiwan, where oil supplies are limited. A process which made use of waste cellulose might still be economic. However, its main legacy to biotechnology lies in that early phase of its development which laid the foundations of the modern fermentation industry. 

It is possible, for example, to set down pathways from glucose to the l-isomers of glutamate, aspartate and lysine (Figures 6.2, 6.9 and 6.10) and to portray their synthesis in the same manner as that of citrate or butanol. However, the use of micro-organisms to produce amino acids is dependent on a knowledge of the interactions which exist within the metabolic pathways which catalyse their synthesis. This fact sets the development of these fermentations apart from that of the acids and the organic solvents. 

Worden RM, Grethlein AJ, Jain MK, Datta R (1991) Production of butanol and ethanol from synthesis gas via fermentation. Fuel 70 615-619... 

In the United States, a subsidiary of the DuPont Company, Lazote, Inc., made synthetic methanol at Belle, West Virginia. The Belle operation was part of the ammonia plant at the site. The methanol production was actually a step in the ammonia process for removing carbon monoxide, which was an impurity in the ammonia synthesis gas. Commercial Solvents was the first to employ the high-pressure synthesis process, developed by BASF, in the United States. The plant, located in Peoria, lUinois, began operation a few months after the Lazote plant at Belle. The Commercial Solvents plant used an off-gas from a fermentation operation. The off-gas contained carbon dioxide and hydrogen from the production of butanol from corn. This first of a kind plant in the United States was rated at about 4000 t per year. 

As described above, the clostridial ABE fermentative path leads to synthesis of butanol, together with smaller amounts of acetone, ethanol and acetic and butyric acids, together with carhon dioxide and hydrogen (Branduardi et al. 2014). Normally, the solvent ratio of acetone, butanol and ethanol, respectively, is 3 6 1, and the total solvent concentration is around 20 g/L (Connor and Liao 2009). Many natural clostridial strains have the upper butanol tolerance limit at about 11-12 g/L. However, some mutants and engineered strains can tolerate up to 19 g/L of butanol (Jang et al. 2012a). 

The organic chemicals that fall into this category and can be produced by fermentation include ethanol, butanol, acetone, 2,3-butane-diol, and glycerol. 2.3-Butanediol and glycerol fermentations have been developed at laboratory and pilot-plant scales, but have not been commercialized. Ethanol, butanol, and acetone have been produced industrially by fermentation, but chemical synthesis is the manufacturing practice of choice for economic reasons. However, as price and availability of ethylene and propylene as feedstocks for the synthetic processes become subjects of concern, there is renewed interest in examining the fermentation pocesses as means of producing all or a portion of the future needs of ethanol, butanol, and acetone. 

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