Methanol based chain elongation with acetate to n-butyrate and isobutyrate at varying selectivities dependent on pH

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 iso-butyric acid via methanol based chain elongation of acetate. A long term continuous reactor experiment showed that the selectivity for isobutyrate (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) of 0.78 n-C4 and 0.024 i-C4, whereas a reactor pH of 5.2 led to a selectivity of 0.25 n-C4 and 0.63 i-C4 . This shift in product spectrum was also represented by a shift in microbial composition. The results suggest that an 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 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 the ratio of 0.684 i-C4 : 0.316 n-C4, which is the theoretical ratio that would be achieved when i-C4 and n-C4 would be formed at amounts 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 immediately lead to the formation of the other butyrate component. Moreover, batch experiments performed at pH 6.5 immediately produced mainly n-C4 and led to the development of a completely different microbiome. The imposed pH as a strong selective pressure can immediately facilitate changes in product selectivities for n-C4 and i-C4 during methanol based chain elongation of acetate.