Novel biochemical and catalytic microwave-assisted methods for the synthesis of biobutanol

Abstract

Biofuels such as bio-ethanol, bio-butanol and biodiesel are attractive fuel substitutes to negate the negative effects of fossil fuel use. Bio-butanol, in particular, is of interest because of its properties, such as; low volatility, less ignition problems and high intersolubility. These properties resemble that of petroleum fuel, making it a drop-in fuel that is more acceptable to internal combustion engines than bio-ethanol. Bio-based butanol is mostly produced through fermentation processes with relatively low yields and high cost. If bio-based butanol is to be used as a bulk fuel to replace petroleum, methods need to be developed to lower production and fuel costs. Therefore, in this study, alternative synthesis methods for lowering the cost of bio-based butanol through biological and chemical pathways were investigated and compared.

An extensive literature review reported on the butanol production protocols covering both traditional Acetone-Butanol-Ethanol (ABE) fermentation and catalytic upgrading of bioethanol to n-butanol. The literature review was published as a review article (B. Ndaba, I. Chiyanzu, S. Marx. 2015. n-Butanol derived from biochemical and chemical routes: A review, Biotechnology Reports, 8: 1-9, https://doi.orgl10.1016lj.btre.2015.08.001 ). Given the shortcomings of ABE fermentation, such as long fermentation times and difficulty in obtaining high butanol yield due to co-production of acetone and ethanol, the review highlighted the chemical process route as the most prudent method for producing bulk bio-based butanol. An ABE fermentation process of sweet sorghum juice was investigated as a comparative baseline study using different inoculum concentrations (3, 5, 10 %vlv). A single culture of C. acetobutylicum at culture loading of 10 %vlv resulted in a high bio-based butanol concentration of 6.49 gll (0.06 gig of sweet sorghum juice) after 96 hrs of fermentation, while fermentation with C. tetanomorphum fermentation produced a bio-based butanol concentration of 2.66 gll (0.02 gig of sweet sorghum juice) at a 5 %vlv inoculation. The baseline study showed that C. acetobutylicum could enhance bio-based yield compared to C. tetanomorphum, but, the butanol yield was however still low. The findings from the baseline study were published as a research article (B. Ndaba, Chiyanzu, S. Marx. 2015. Direct fermentation of sweet sorghum juice by Clostridium acetobutylicum and Clostridium tetanomorphum to produce bio-butanol and organic acids, Biofuel Research Journal, 6: 248-252, https://www.biofueljournal.com/article 9690.html, DOI: 10.18331/BRJ2015.2.2.7). It was concluded from these results that high enough butanol yields cannot be achieved with current biological pathways to meet fuel demands at an affordable price. Therefore, chemicals pathways for the production of bio-based butanol from bioethanol was investigated.

Bio-based butanol production through microwave-assisted catalytic conversion of biobased ethanol was investigated using three catalysts, i.e. magnesium oxide (MgO), platinum on alumina (Pt/Al2O3), and nickel on alumina (Ni/Al2O3). All catalysts were characterised in terms of physical properties using transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), Powder X-ray diffraction (P-XRD) and thermogravimetric analysis (TGA). The reaction was conducted in an industrial microwave reactor according to a 2-level factorial design with reaction temperatures (200°C and 250°C), catalyst loading (1 and 5 wt.%) and reaction times (30 and 60 min) as manipulated variables. Catalyst performance was compared by measuring and comparing the bio-based butanol selectivity and bio-based ethanol conversion. The Ni/Al2Q3 catalyst was shown to have the best performance in terms of bio-based butanol selectivity (58%) and bio-based ethanol conversion (20.Bwt.%) compared to the Pt/Al2Q3 catalyst which achieved only a 9.2wt.% ethanol conversion in 30 min with an 11.5% selectivity towards n-butanol produced. The MgO catalyst produced mostly acetaldehyde at a concentration of 140 g/L (0 .13 g/g of bio-ethanol) as a main product at an ethanol conversion of 10.7 wt.%. Therefore, the Ni/Al2Q3 catalyst was selected for further investigations. The effect of residence time on bio-based butanol yield using Ni/Al2O3 catalyst was investigated at residence times between 5 and 60 min. Product yield and distribution was determined using high performance liquid chromatography (HPLC). The highest n-butanol concentration of 161 g/L (0.22 gig of bio-ethanol) was achieved at an n-butanol selectivity of 60% and ethanol conversion of up to 76.6 wt.% at a residence time of 60 min. Conversion was shown to be higher in a shorter time (60 min) compared to the ABE fermentation process (96 hrs).

This study introduced a new microwave-assisted method for the rapid conversion of biobased ethanol to bio-based butanol. Overall, a microwave-assisted system for n-butanol production offers advantages of shorter reaction time and lower reaction temperatures compared to conventional reaction methods and ABE fermentation. Furthermore, biobased ethanol can be produced from a wide range of feedstock, including biomass which does not compete with food or feed. In addition, the use of agricultural residues to produce ethanol for bio-based butanol production by the method presented in this study, ensures that no land use change is involved, resulting in a much more environmentally friendly fuel production chain. This study presents a new and innovative method for bridging the gap towards environmentally responsible and economically sustainable biofuels production