(739f) Butanol Separation from Multicomponent Vapor Mixtures Using Zeolitic Imidazolate Frameworks

Biobutanol is a promising renewable alternative to conventional petroleum-derived liquid fuels, and can be used as either as an additive or as a direct gasoline replacement. It also has many industrial applications, e.g., in the production of paints and polymers. A major challenge in fermentative biobutanol production is to find a cost-effective butanol separation process from the extremely dilute aqueous butanol solution (which is usually in the form of an acetone-butanol-ethanol mixture). Vapor-phase adsorption after gas stripping of the butanol solution can be an effective option to separate butanol from water due to the favorable nature of butanol-water vapor liquid equilibrium. Zeolitic Imidazolate Frameworks (ZIFs), a set of nanoporous metal-organic frameworks (MOFs) with tunable porosity and functionality are possible candidates for this purpose. In previous work by some of the present authors on ZIF-8, ZIF-90 and ZIF-71 adsorbents, it was found from single-component adsorption isotherms and IAST predictions that ZIF-8 and ZIF-71 may be useful for this separation.¹ Previous work from our group has also demonstrated that the effective pore size, hydrophobicity, and organophilicity of ZIFs can be tuned using mixed-linker ZIFs.²

However, very little is known about separation of butanol from multicomponent vapor mixtures. In particular, an important issue is the presence of humidified acid gas (CO₂) in differing amounts in the vapor mixture, which may cause degradation of ZIF materials by attacking the Zn-N coordination bonds. To understand these effects and their impact on butanol separation, here we evaluate the separation performance of ZIF-8, ZIF-90, ZIF-71, hybrid ZIF-8_x-90_100-x and ZIF-8_x71_100-x adsorbents with a 1 mole % butanol-in-water system through vapor phase breakthrough experiments at 35 °C and 1 bar. For simplicity, acetone and ethanol were not included in the mixture. We conducted long-term stability studies of the adsorbents in humid air and CO₂ in addition to vapor breakthrough studies. The separation properties are presented in detail and qualitatively explained based upon the textural properties, hydrophobicity, and organophilicity of the ZIF frameworks. ZIF-8 and ZIF-71 are found to be best materials (in terms of butanol selectivities ~15 and capacities ~4.3 mmol/g) for this separation in the absence of humid CO₂ in the feed stream. However, in the presence of humid CO₂, ZIF-8, ZIF-90 and ZIF-8_x-90_100-x hybrids are found to be unstable upon long-term operation. The mechanistic aspects of this degradation are studied by several techniques including FTIR spectroscopy and explained. From a practical viewpoint, ZIF-71 appears to be an excellent candidate for butanol separation owing to its acid gas stability as well as good butanol adsorption capacity and selectivity.

References

1. Zhang, K.; Lively, R. P.; Dose, M. E.; Brown, A. J.; Zhang, C.; Chung, J.; Nair, S.; Koros, W. J.; Chance, R. R., Alcohol and Water Adsorption in Zeolitic Imidazolate Frameworks. Chem. Commun. 2013, 49, 3245-3247.

2. Eum, K., et al., Highly Tunable Molecular Sieving and Adsorption Properties of Mixed-Linker Zeolitic Imidazolate Frameworks. J. Am. Chem. Soc. 2015, 137, 4191-4197.