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 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.