|Title||Methanol-Based Chain Elongation with Acetate to n-Butyrate and Isobutyrate at Varying Selectivities Dependent on pH|
|Author(s)||Leeuw, Kasper D. de; Smit, Sanne M. de; Oossanen, Sabine van; Moerland, Marinus J.; Buisman, Cees J.N.; Strik, David P.B.T.B.|
|Source||ACS sustainable chemistry & engineering 8 (2020)22. - ISSN 2168-0485|
Biological Recovery & Re-use Technology
|Publication type||Refereed Article in a scientific journal|
|Keyword(s)||Biobased chemicals - Chain elongation - Isobutyrate - Methanol - n-Butyrate - Open-culture fermentation - Selection pressure|
Biomass fermentation technologies offer alternative methods to produce platform chemicals that currently originate from fossil sources. This research showed that an enriched microbiome was capable to produce isobutyrate (i-C4) from acetate via methanol-based chain elongation. A long-term continuous reactor experiment showed that the selectivity for i-C4 and/or n-butyrate (n-C4) could be reversibly adjusted by changing the reactor pH. A reactor pH of 6.75 led to formation of (carbon per total carbon of products) 0.78 n-C4 and 0.024 i-C4, whereas a reactor pH of 5.2 led to a selectivity of 0.24 n-C4 and 0.65 i-C4. This shift in product spectrum was also represented by a shift in microbial composition. The results suggest that a Eubacterium genus is responsible for the formation of n-C4, whereas a Clostridium luticellarii strain is responsible for the formation of a mixture of i-C4 and n-C4. The formation of n-C4 and i-C4 at a low pH was observed to be coupled according to the thermodynamics of isomerization. At a reactor pH of 5.5 and 5.2, the product ratio of i-C4:n-C4 approached 0.69 i-C4:0.31 n-C4, which is the theoretical ratio that would be achieved when determined by the equilibrium of isomerization. Various batch experiments at pH 5.5 and 5.2 confirmed that addition of either n-C4 or i-C4 at the start of the batch would directly lead to the formation of the other butyrate component. Moreover, batch experiments performed at pH 6.5 produced mainly n-C4 and led to the development of a completely different microbiome. The imposed pH is a strong selection pressure that can facilitate changes in product selectivities for n-C4 and i-C4 during methanol-based chain elongation of acetate.