(633c) Biobutanol Separation with the ZIF-8 Metal Organic Framework

Bio-alcohols like bio-ethanol and bio-butanol form a promising alternative to petroleum-based chemicals.A major challenge in the economical production of bio-butanol as chemical or fuel is its separation from the aqueous medium in which it is produced by the fermentation of biomass. Given the low concentration of the alcohols in the fermentation broth, separation of the butanol fraction via distillation would be energy- and cost-intensive.

Among the alternative separation methods, adsorption was identified as the most energy-efficient technique to recover butanol from fermentation broth.^[1] This requires adsorbents with, besides high adsorption capacity and stability, high affinity towards alcohols since the final butanol concentration is typically at most 20 g/l, together with low affinity for water. Typical adsorbents, i.e. most zeolites, silica or alumina, show a high preference to water, making them unsuitable for this particular application. Oudshoorn reported that among the commercially available hydrophobic zeolites, silicalite type zeolites are the most selective for alcohols, but their adsorption capacity remains low.^[2] Although active carbon selectively adsorbs alcohols from water, recovery of the adsorbed alcohols is problematic.^[3]

In this work, it is demonstrated that the ZIF-8 MOF, a member of the Zeolite Imidazolate Frameworks (ZIF) family, shows promising features for the production of pure biobutanol from its fermentation medium.

Since ethanol and acetone are major side products in the fermentative production of biobutanol, we have studied adsorption and separation of C1-C5 1-alcohols, acetone and water on ZIF-8 by determining gas and liquid phase pure component and mixture isotherms, measuring separation by pulse chromatography and breakthrough experiments and studying desorption.

Vapor phase adsorption isotherms of methanol-pentanol show an S-shaped profile. Very large adsorption capacities for butanol are observed (30wt%), in spite of the relatively large size of the 1-alcohols compared to the ZIF-8 window. The absence of hysteresis in adsorption and desorption together with the steep isotherm allows for easy desorption of adsorbed alcohols. The adsorption mechanism was elucidated via XRD and Rietveld refinement study of empty and butanol loaded ZIF-8 samples.

While water is not adsorbed at all by ZIF-8, ethanol and acetone are adsorbed to a much lower extend as compared to butanol. The exclusion of water, ethanol and acetone makes ZIF-8 a highly attractive adsorbent for biobutanol purification. A mixture of these components at concentrations comparable to those in fermentation broth is efficiently separated on a column packed with ZIF-8 pellets. Interestingly, concentrated butanol could be obtained by regeneration of the adsorption column with methanol and mild heating. TPD experiments furthermore demonstrate that pure butanol is desorbed from ZIF-8 powder which was first contacted with water/butanol mixtures.

Given the large capacity of ZIF-8, its high stability and its ease of regeneration and desorption, this offers perspectives for the economical and low energy intensive production of biobutanol.

References

[1] N. Qureshi, S. Hughes, Bioprocess Biosyst Eng 2005, 27, 215–222.

[2] Oudshoorn, A. Luuk, Biochem Eng J 2009, 48, 99–103.

[3] N. Qureshi, S. Hughes, Bioprocess Biosyst Eng 2005, 27, 215–222.