Adsorptive recovery of biobutanol in vapour phase with hydrophobic zeolites

1. Introduction
The depletion of oil reserves and the increasing costs for crude oil have attracted the attention of sustainable and more environmentally-friendly alternatives for petroleum based fuels. Biobutanol, produced by ABE (acetone, butanol and ethanol) fermentation of renewable feedstocks (biomass), is a promising candidate. A typical ABE fermentation generates a mixture of acetone, butanol and ethanol with a total concentration of 2 wt% (3:6:1 ratio, respectively) diluted in water.1 However, high separation costs due to the presence of other co-products and low final concentration prevents biobutanol to become a viable competitor to other biofuels.2,3 Adsorption has been identified as an energy-efficient technique. In previous work, we have already proposed ZIF-8 and SAPO-34 as shape selective materials for the recovery of butanol in liquid phase conditions.4,5
In this work, hydrophobic zeolites were synthesized and applied for the vapour phase separation and recovery of butanol in the headspace of the fermentation chamber. Operation in vapour phase offers several advantages; ABE concentrations relative to water vapour are higher, adsorbents suffer less from stability issues (aqueous conditions and low pH) and no clogging or fouling problems due to microbial cells and inorganic salts occurs.

2. Experimental Part
The adsorbents are fully characterized via high resolution Ar isotherms and vapour phase isotherms of the most important components (acetone, butanol, ethanol and water) present in the fermenter. From Ar porosimetry, micropore volume and pore size distribution were calculated. The adsorbents were pelletized and used as separation medium in vapour phase breakthrough experiments. This experimental study was supplemented with theoretical and modeling work (Density Functional Theory simulations) to have a better understanding of the adsorption properties.

3. Results and discussion
Adsorption isotherms and breakthrough experiments reveal shape selective properties for these zeolitic frameworks. Based on these adsorption isotherms, DFT simulations were performed to gain more insight in the interaction between each guest molecule and either host system for comparison. The most favorable adsorption site for each guest molecule is identified, as well as the energy profile for each adsorbate molecule to pass through the cage windows. Separation performance was tested in dynamic conditions by performing breakthrough experiments, where the effect of ABE composition, amount of water, contact time and total pressure on selectivity was experimentally investigated. Remarkable selectivity is obtained in both adsorbents, allowing recovery of the individual compounds of the mixture. Even in the presence of high concentrations of water, materials remained their selectivity. Furthermore, process simulations have been performed, proposing optimized separation schemes to recover these ABE compounds as pure as possible.

4. Conclusions
Two very promising zeolites were identified for the recovery of bioalcohols from an ABE model mixture in the presence of water vapour. Experimental findings were confirmed and further explained by simulations.

References
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