C. S. Gong, Ningjun Cao, and G. T. Tsao

Laboratory of Renewable Resources Engineering, Purdue University,
West Lafayette, IN 47907

A simple and effective method of treatment of lignocellulosic material was used for the preparation of poplar wood chip and com cob for the production of 2,3-butanediol by Klebsiella oxytoca ATCC 8724 in a simultaneous saccharification and fermentation (SSF) process. During the treatments, lignin and hemicellulose fractions of lignocellulosic materials were sequentially removed by aqueous ammonia (10-20%) steeping at 24°C for 24 h followed by, dilute hydrochloric acid (1%, w/v) hydrolysis at 100-108°C for 1 h. The cellulose fractions (80 g/L) were then converted to butanediol by K. oxytoca in the presence of a fungal cellulase (8.5 g IFPU per g cellulose). The butanediol concentrations of 24 and 25.5 g/L, and ethanol concentrations of 6 and 7 g/L were produced by K. oxytoca from wood chip and com cob, respectively. The average butanediol volumetric productivity was 0.26 g/L/h from wood chip and 0.35 g/L/h from com cob.

2,3- Butanediol (2,3-butylene glycol) is a metabolic product of simple carbohydrates produced by many species of enterobacteria through a fermentative metabolic pathway (/). It is a colorless and odorless liquid that has a high boiling point of 180­184° C and a low freezing point of -60°C. The heating value of butanediol (27,198 J/g) is very similar to that of ethanol (29,055 J/g) and methanol (22,081 J/g) which makes it a potentially valuable liquid fuel and fuel additive (2). Butanediol can be dehydrated to methyl ethyl ketone (MEK) and used as an octane booster for gasoline or as high-grade aviation fuel (2). MEK can also be further dehydrated to 1,3- butadiene and dimerized to styrene (5). Therefore, butanediol has a diverse industrial usage, particularly as a polymeric feedstock, in addition to its use for manufacturing butadiene or antifreeze. Currently, butanediol is enjoying an annual growth rate of 4 to 7 percent, buoyed by the increased demand for polybutylene terephthalate resins, y — butyrolactone, Spandex, and its precursors (4).

© 1997 American Chemical Society

2,3- Butanediol is the only isomer, among many, that can be produced by microorganisms. Bacterial species, particularly those belonging to Klebsielleae, are known to metabolize carbohydrates to produce neutral compounds such as 2,3- butanediol, acetoin, and ethanol as metabolic products. Other groups of enterobacteria, such as Erwinia, produced

channeled into butanediol production. The enzymes of the butanediol pathway can constitute as much as 2.5% of the total protein in E. aerogenes (18).

Zeng et al. (19) investigated the effect of pH on growth and product formation of glucose by E. aerogenes in a continuous culture operation and found the optimal pH range of 5.5-6.5 for butanediol and acetoin production. Similar pH optimum was also observed in K. oxytoca (7). In general, the biomass concentration increases steadily with increased pH. At high pH, butanediol concentration decreases with the increase of acetic acid production. Acetic acid has a dual role in the regulation of butanediol formation. It serves as the activator for butanediol accumulation at low concentrations, and at a concentration of 10 g/L or higher, it inhibits butanediol production (19-21). The strength of acetic acid inhibition depends on the concentration of its undissociated form, HAc. The concentration of HAc was in turn determined by the pHs.

The production of butanediol from lignocellulosic materials has been considered as an alternative approach in the conversion of biomass substrates to liquid fuels and chemical feedstocks (3,22). Over the years, there have been many studies conducted utilizing agricultural residues and wastes for butanediol production. The materials studied include: citrus waste (23), water hyacinth (24), wheat and barley straws (25), com stover (25), and hard wood hemicellulose fraction (25-30).

For the biological production of 2,3-butanediol to be economically competitive with petrochemical-based processes, the substrate source must be inexpensive, while reactor yields and productivity should be high. Lignocellulosic materials from forestry and agriculture residues, such as wood chips and com crop residues, are inexpensive and abundant and can be used in many bioprocesses for the production of products of high economical value. An effective utilization of xylose, arabinose, and other minor sugars in addition to glucose is important in the process economics. K. oxytoca ATCC 8724 is capable of producing butanediol from both hexoses and pentoses with good yield (7). In this study, we used K. oxytoca to produce butanediol from pretreated poplar wood chip and ground com cob cellulose fraction in the presence of a fungal cellulase.