An international research collaboration has developed a novel, porous, and hydrophobic metal–organic framework (MOF) based on copper ions and carborane–carboxylate ligands that can efficiently separate biobutanol from the broth of fermented biomass needed for the fuel’s production. A paper on the work is published in the Journal of the American Chemical Society.

The researchers are now looking to partner with industry to try to scale up the separation method using the new metal organic framework, says the study’s corresponding author, Kyriakos Stylianou of Oregon State University.

Biobutanol has recently emerged as an attractive option compared to bioethanol and biodiesel, but a significant challenge in its production lies in the separation stage. The current industrial process for the production of biobutanol includes the ABE (acetone–butanol–ethanol) fermentation process from biomass; the resulting fermentation broth has a butanol concentration of no more than 2 wt% (the rest is essentially water).

Therefore, the development of a cost-effective process for separation of butanol from dilute aqueous solutions is highly desirable. The use of porous materials for the adsorptive separation of ABE mixtures is considered a highly promising route, as these materials can potentially have high affinities for alcohols and low affinities for water. To date, zeolites have been tested toward this separation, but their hydrophilic nature makes them highly incompetent for this application.

The use of metal–organic frameworks (MOFs) is an apparent solution; however, their low hydrolytic stabilities hinder their implementation in this application. So far, a few nanoporous zeolitic imidazolate frameworks (ZIFs) have shown excellent potential for butanol separation due to their good hydrolytic and thermal stabilities.

—Gan et al.

The team’s new MOF, mCB-MOF-1, exhibits excellent stability when immersed in organic solvents, water at 90 °C for at least two months, and acidic and basic aqueous solutions.

The researchers found that, like ZIF-8, mCB-MOF-1 is non-porous to water (type II isotherm), but it has higher affinity for ethanol, butanol, and acetone compared to ZIF-8. This is reflected in their study by the separation of a realistic ABE mixture in which mCB-MOF-1 recovers butanol more efficiently compared to ZIF-8 at 333 K.

Our work demonstrates a step forward toward the discovery of novel water stable MOFs for biobutanol recovery from a mostly water-containing ABE mixture. To date, only ZIFs have been tested toward this application, and based on our findings, carborane-based MOFs can compete with ZIFs and can even outperform them in the separation of biobutanol. Future work includes scaling-up of mCB-MOF-1 synthesis, shape-engineering its powder form into a more industrially favored form, and testing its performance using different bed configurations as dictated by process modeling.

—Gan et al.

Resources

• Lei Gan, Arunraj Chidambaram, Pol G. Fonquernie, Mark E. Light, Duane Choquesillo-Lazarte, Hongliang Huang, Eduardo Solano, Julio Fraile, Clara Viñas, Francesc Teixidor, Jorge A. R. Navarro, Kyriakos C. Stylianou, and José G. Planas (2020) “A Highly Water-Stable meta-Carborane-Based Copper Metal–Organic Framework for Efficient High-Temperature Butanol Separation” Journal of the American Chemical Society doi: 10.1021/jacs.0c01008