Biobutanol separation with the metal-organic framework ZIF-8.

Bioalcohols, such as bioethanol and biobutanol, are a promising alternative to petroleum-based chemicals. As a fuel, biobutanol has superior properties compared to bioethanol, including a higher energy density and a lower volatility. A major challenge in the economical production of biobutanol 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 energyand cost-intensive. Among alternative separation methods to recover butanol from fermentation broth, adsorption has been identified as the most energy-efficient technique. This requires adsorbents that, besides a high adsorption capacity and stability, have a high affinity towards alcohols (typically, the final butanol concentration is at most 20 g L ) and a low affinity for water. Typical adsorbents (i.e. , most zeolites, silica, and alumina) have a high preference to water and so are not suitable for this particular application. Oudshoorn et al. reported that among the commercially available hydrophobic zeolites, silicalitetype zeolites are the most selective for alcohols, but their adsorption capacity remains low. Although active carbon selectively adsorbs alcohols from water, the recovery of adsorbed alcohols is problematic. Metal–organic frameworks (MOFs) offer new opportunities in adsorption technology, with unprecedented capacities and chemical and structural tunability. Herein, it is demonstrated that the MOF ZIF-8, a member of the zeolitic imidazolate framework (ZIF) family, has promising features for the production of pure biobutanol from its fermentation medium. ZIFs contain tetrahedral Zn atoms linked by imidazolate ligands. A large variety of zeolite-like structures can be obtained by modification of the ligands. ZIFs offer high hydrothermal, chemical, and thermal stabilities. ZIF-8, discovered by Huang et al. , crystallizes into the zeolite sodalite topology, generating a resistant structure with cages of 12.5 connected via hexagonal windows of 3.3 . (Figure S2). Adsorption isotherms on ZIF-8 have been reported for Ar, CO2, CH4, N2, C2H6, C2H4, and H2, and also for longer alkanes, alkenes, and organic compounds. b, 7] Molecular simulations have been used to identify the adsorption sites of H2, N2, and CH4. [8] Selective ZIF-8-membranes have been designed, and their permeability for light gasses has been investigated. 9] ZIF-8 shows an only very low [a] J. Cousin Saint Remi, T. R my, V. Van Hunskerken, S. van de Perre, T. Duerinck, Prof. Dr. G. V. Baron, Prof. Dr. J. F. M. Denayer Department of Chemical Engineering Vrije Universiteit Brussel Pleinlaan 2, 1050 Brussel (Belgium) Fax: (+ 32) 2 629 17 98 E-mail : joeri.denayer@vub.ac.be [b] Dr. M. Maes, Prof. Dr. D. De Vos, Dr. E. Gobechiya, Prof. Dr. C. E. A. Kirschhock Centre for Surface Chemistry and Catalysis Katholieke Universiteit Leuven Kasteelpark Arenberg 23, 3001 Heverlee (Belgium) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201100261. Figure 1. Vapor-phase adsorption on ZIF-8. Full symbols: adsorption; open symbols: desorption. a) Adsorption isotherms at 50 8C. b) Adsorption capacity at 50 8C. c) Butanol isotherms at varying temperature. d) Isosteric heat of adsorption as a function of pore filling.