In the past few decades, almost all butanol were produced via petrochemical routes.
In recent years, increasing crude oil price and concerns about environmental prob- lems have renewed the interests in butanol production through biological fermentation
processes. Great efforts have been focused on the academic research on ABE fermen- tation with solventogenic clostridia. Transcriptional analyses and metabolic engineer- ing have been extensively performed onC. acetobutylicum(Green et al., 1996; Alsaker et al., 2004; Alsaker and Papoutsakis, 2005; Shao et al., 2007; Papoutsakis, 2008;
Alsaker et al., 2010). A bacterial group II intron-based genetic engineering system (ClosTron or TargeTron) has been recently developed for directly constructing muta- tion inClostridiumspecies (Heap et al., 2007), and various applications based on this system have been reported (Cooksley et al., 2012; Lehmann et al., 2012a,b). In addi- tion, conversion of low-value feedstocks like agricultural residues and optimization of fermentation process and downstream manipulations have also been studied (Ezeji et al., 2005, 2007a,b; Qureshi et al., 2008; Mariano et al., 2011). In industry, in recent years, a number of big companies started to market butanol and develop the biobu- tanol processes. Many venture companies have also been established focusing on the commercialization of biobutanol. Several major players include Cathay Industrial Biotech (Shanghai, China), Cobalt Technologies (Mountain view, CA, USA), Green Biologics (Abingdon, England), METabolic EXplorer (Clermont-Ferrand, France), etc. (Table 9.4) (Mascal, 2012). With the blending wall issue for bioethanol indus- try and the expiration of the blenders tax credit (NYTimes, 2012), many ethanol producers are considering to retrofit and transform their existing ethanol production infrastructures for butanol production (WSJ, 2009; Ethanolproducer.com, 2012). The prospects for biological butanol production look really bright.
However, there are still many challenges for the large-scale industrial produc- tion of butanol through clostridia fermentation processes. For example, high cost of the substrates, low yield and final titer, diversion of substrates to other unwanted bioproducts (like acetone and acids), high energy requirement for butanol recovery from dilute streams. Great efforts on overcoming these challenges should be made in order to significantly improve the butanol production process. First, the search and utilization of low-value and abundant substrates that do not compete with food and feed supplies would offer tremendous economic benefits. These include the cellulosic residues or by-products from agricultural and industrial processes, municipal wastes, and other energy biomasses. Effective pretreatment and biomass conversion tech- niques must be developed to satisfy the efficient fermentation requirement. Second, search or development of new strains and improvement of the existing strains should be performed in order to acquire better degradation capabilities on vast ranges of substrates, higher butanol yield, titer, and productivity. Recently, several new solven- togenic clostridia strains have been reported, and the genomes have been sequenced (Bao et al., 2011; Hu et al., 2011; Wu et al., 2012). For strain improvement, transcrip- tional analysis, metabolic engineering, and system biology tools can play key roles.
On one hand, butanol producers such as solventogenic clostridia can be mutated or engineered for desired properties. On the other, butanol production pathways can be introduced into desired industrial microorganisms for butanol production pur- pose. For example,E. coli has been attempted as the host and engineered for this objective (Ranganathan and Maranas, 2010; Shen et al., 2011). AdaptedP. putida strains that can grow in the presence of up to 6% (vol/vol) butanol has been reported, which could be potential hosts for biobutanol production (Ruhl et al., 2009). Third,
TABLE9.4MajorCompaniesCurrentlyMarketingn-Butanol CompanyTimefoundedLocationTechnologyDevelopmentstatus Butylfuel,LLC1991Gahanna,Ohio,USABasedonatwo-stagefermentation thatdecouplesbutyricacid productionfrombutanol formation
Apilotplantincorporatingthis technologyinafibrousbed bioreactorisunderdevelopment. GreenBiologicsmergedwith ButylfuelinJanuary2012(Green Biologicspressrelease,2012) CathayIndustrial Biotech1997Shanghai,ChinaContinuousclostridiaABE fermentation,withcornstarchas thefeedstock;optimizedprocess reducedwaterandenergyusage significantly
Currentlythelargestbiobutanol producerintheworld,witha productioncapacityof21million gallonsofbiobutanolperyear METabolicEXplorer1999Clermont-Ferrand, FranceUsingitscombinedexpertisein molecularbiology,metabolic engineering,andbioinformatics todesignhigh-performance microorganismsthatcan transformplant-derivedraw materialsintoanexistingbulk chemical,includingbutanol
Withabroadproductportfolio,the aredeveloping“anextremely flexibleandcompetitive fermentationprocessforbutanol production,”whichcanutilize sugar,starch,andhemicellulosic feedstocks GreenBiologics2003Abington,UKThetechnologyplatformincludesa largecollectionofnativeand geneticallymodifiedclostridia, fermentationprocessdesign, productionstraintofeedstock matching,andseparations MergedwithButylfuelinJanuary 2012;isaimingatretrofit existingethanolplantsand internationalmarketexpansion includingChina,Brazil,etc. (Ethanolproducer.com,2012) (continued
249
TABLE9.4(Continued) CompanyTimefoundedLocationTechnologyDevelopmentstatus CobaltTechnologies2005MountainView,CA, USAAnimmobilizedcelltechnology usingaproprietary, nongeneticallymodified clostridiastrainwithhigh productivityandabilityto convertbothC5andC6sugars intobutanolforoperationina continuousfermentationreactor Currentlyoperatinga5000GPY pilotfacilitysinceJune2009. Cobaltisnowintheprocess commissiona470,000GPY cellulosicbiobutanolfacility TetravitaeBioscience (Eastman Chemical)
2006Chicago,IL,USATheplatformisbasedonastable, nongeneticallymodified C.beijerinckiistrainwhichoffers highbutanolyields,reduced productinhibition,andtheability toutilizebothC5andC6sugars
Tryingtoadaptthetechnology processcellulosicfeedstocks; acquiredbyEastmanChemical CompanyinNovember2011 (Eastmanpressrelease,2011) Butalco2007CantonofZug, SwitzerlandBasedongeneticallyoptimized yeastsforproductionofadvanced biofuelslikeethanolandbutanol byusingpentosesugars Workingtogetherwithpartners developintegratedproduction processes(Butalco.com,2012)
250
development of novel fermentation processes and improvement of the downstream processing would be significant for improving the butanol production economics.
With the advancement of separation technology and fermentation engineering, more efficient and cost-effective downstream processes are expected in the future. With the rapid development in molecule biology and process engineering, significant advances have been and will be made on this old traditional process; it is expected that the fermentative production of biobutanol from renewable resources could become com- mercially viable in the near future.
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