A vapor-phase adsorptive recovery process is proposed as an alternative way to isolate biobutanol from acetone-butanol-ethanol (ABE) fermentation media, offering several advantages compared to liquid phase separation. The effect of water, which is still present in large quantities in the vapor phase, on the adsorption of the organics could be minimized by using hydrophobic zeolites. Shape-selective all-silica zeolites CHA and LTA were prepared and evaluated with single-component isotherms and breakthrough experiments. These zeolites show opposite selectivities; adsorption of ethanol is favorable on all-silica CHA, whereas the LTA topology has a clear preference for butanol. The molecular sieving properties of both zeolites allow easy elimination of acetone from the mixture. The molecular interaction mechanisms are studied by density functional theory (DFT) simulations. The effects of mixture composition, humidity and total pressure of the vapor stream on the selectivity and separation behavior are investigated. Desorption profiles are studied to maximize butanol purity and recovery. The combination of LTA with CHA-type zeolites (Si-CHA or SAPO-34) in sequential adsorption columns with alternating adsorption and desorption steps allows butanol to be recovered in unpreceded purity and yield. A butanol purity of 99.7 mol % could be obtained at nearly complete butanol recovery, demonstrating the effectiveness of this technique for biobutanol separation processes.

We report the use of a 3D-printing fiber deposition method to synthesize ZIF-8 monoliths. A binder recipe was developed containing 16.7 wt % of methylcellulose and 16.7 wt % of bentonite binder. The effect of the used binders on the adsorption of n-butanol and Ar was investigated, showing a decrease in equilibrium loading of 33 wt % for n-butanol due to the presence of both binders. To maximally recover the adsorption capacity, a thermal activation procedure was developed to remove the organic binder after 3D-printing without destroying the ZIF-8 structure. The use of different activation temperatures for the removal of the organic binder was investigated, showing activation at 450 °C to be optimal. Adsorbent monoliths using a nozzle with two different diameters (250 and 600 μm) were subsequently printed and the efficacy of the monoliths for the recovery of biobutanol was tested via breakthrough experiments. The developed structures showed to retain the selectivity of ZIF-8 for biobutanol, adsorbing 0.2 g/g n-butanol in dynamic conditions, opening perspectives for the further design and optimization of 3D-printed ZIF-8 structures.

We report the use of 3D-printed ZIF-8 adsorbent monoliths for the recovery of biobutanol from model fermentation mixtures. Two ZIF-8 monoliths, obtained by layer-by-layer printing, with different fiber thickness (250 and 600 μm), front channel size (350 μm × 350 μm and 760 μm × 760 μm), and side channel size (<60 and 260 μm) were studied. Equilibrium isotherms of acetone−butanol−ethanol (ABE) fermentation products showed that both monoliths retained a high saturation capacity (0.2 g/g) for n-butanol and a low saturation capacity for water (0.04 g/g). Mixture breakthrough experiments demonstrate that a high amount of butanol is adsorbed (0.2 g/g) in dynamic conditions, close to the pure component capacity. When increasing the carrier gas flow rate, broadening of the n-butanol breakthrough profile was observed for the 250 μm fiber monolith, while no broadening of the profile was observed for the 600 μm fiber monolith. Computational fluid dynamics (CFD) simulations show that the small side channels of the 250 μm monolith (<60 μm) leads to maldistribution of the flow at the inlet. Finally, the thermal regeneration of the monoliths was investigated, showing the capacity of the ZIF-8 monoliths could be fully restored for different adsorption−desorption cycles.

The vapor phase adsorption of butanol from ABE fermentation at the head space of the fermenter is an interesting route for the efficient recovery of biobutanol. The presence of gases such as carbon dioxide that are produced during the fermentation process causes a stripping of valuable compounds from the aqueous into the vapor phase. This work studies the effect of the presence of carbon dioxide on the adsorption of butanol at a molecular level. With this aim in mind Monte Carlo simulations were employed to study the adsorption of mixtures containing carbon dioxide, butanol and ethanol. Molecular models for butanol and ethanol that reproduce experimental properties of the molecules such as polarity, vapor-liquid coexistence or liquid density have been developed. Pure component isotherms and heats of adsorption have been computed and compared to experimental data to check the accuracy of the interacting parameters. Adsorption of butanol/ethanol mixtures has been studied in absence and presence of CO2 on two representative materials, a pure silica LTA zeolite and a hydrophobic metal-organic framework ZIF-8. To get a better understanding of the molecular mechanism that governs the adsorption of the targeted mixture in the selected materials, the distribution of the molecules inside the structures was analyzed. The combination of these features allows obtaining a deeper understanding of the process and to identify the role of carbon dioxide in the butanol purification process.

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