Zeolite adsorbents for the separation of n-butyric acid from fermentation broths via adsorption

Abstract

The production of n-butyric acid (n-BA) through microbial fermentation has attracted considerable attention owing to the potential to reduce the carbon footprint via the recycling of carbon biomass sources. However, the industrial use of n-BA produced via fermentation involves its separation and recovery, which require energy-intensive extraction processes. We explore the possibility of recovering n-BA with different types of zeolite adsorbents. Whereas Silicalite-1 (MFI), Zeolite 4A (LTA), and Zeolite Y (FAU) preferentially adsorb n-BA, [OH]_Silicalite-1 exhibits higher n-BA/acetic acid (AA) and n-BA/propionic acid (PA) selectivities (approximately 3.11 and 2.08, respectively). These results indicate that the selective adsorption of n-BA by zeolites is mainly driven by intermolecular interactions that depend on the molecular weight of the adsorbates, whereas their hydrophobicity hinders the adsorption of the more polar AA and PA. Lowering the pH values of fermentation broths suppresses the deprotonation of n-BA, thereby promoting n-BA adsorption by zeolites. Moreover, [F]_Silicalite-1 with fewer defective silanol groups and nests exhibits improved n-BA/AA and n-BA/PA selectivities (approximately 7.80 and 3.10, respectively); adsorbents made from [F]_Silicalite-1 demonstrate excellent recyclability and negligible adsorption of bulky glucose. According to the comparison of the extraction methods, [F]_Silicalite-1 has a distinctly higher selectivity for n-BA at low concentrations in mixtures. Thus, zeolite adsorbents can either be used as individual separation units for mixtures with low n-BA concentrations or retrofitted for already existing extraction processes.