Keep ′em separated: Bioalcohols are alternatives to petroleum‐based chemicals. Particularly biobutanol is an interesting compound, with properties superior to bioethanol. It is difficult, however, to separate biobutanol from the mixtures produced by the fermentation of biomass. This Communication describes a zeolitic imidazolate framework suitable for the separation of biobutanol from fermentation broths by adsorption.

This work discusses the adsorption of polar and apolar molecules on the copper–benzene-1,3,5-tricarboxylate (Cu–BTC) metal–organic framework. Vapor phase adsorption isotherms of various polar adsorbates such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 1-hexanol, water, acetone, acetonitrile, tetrahydrofuran, and N,N-dimethylformamide, as well as some apolar adsorbates such as n-hexane, n-heptane, m-xylene, and cyclohexane, on the Cu–BTC framework are presented. We show that water exposure of the Cu–BTC framework has an adverse effect on the uptake capacity. However, with minimized water exposure, we find high adsorption capacities, exceeding 0.65 cm3/g for all adsorbates with the exception of water, and we show that small polar adsorbates exhibit a two-step adsorption behavior. This behavior is further studied using molecular simulation and proposed to occur due to the presence of the various Cu–BTC cages. The cages containing the exposed coordinatively unsaturated copper sites have a more polar character, while the other cages behave in a more apolar way, causing a two-step adsorption behavior depending on the character of the adsorbate.

The low coverage adsorption properties of alkanes, alkenes, and aromatics of the linear, branched, and cyclic type (ca. 70 molecules) were studied using inverse pulse gas chromatography at zero coverage on the zirconium metal− organic framework UiO-66 and its functionalized analogues UiO-66-Me, UiO-66-NO 2 , UiO-66-Me 2 in the temperature range 433−573 K. In our study, we determined and analyzed the adsorption enthalpy, Henry constants, and entropic factors. Preferential adsorption of bulky molecules is observed with specific adsorbate and cage size effects, yielding very specific, preferential adsorption. Remarkably high adsorption selectivity factors (up to 14) for cyclo-compared to nalkanes were found. The presence of additional groups (methyl, nitro) on the linkers in the framework influences adsorption properties significantly, mainly by reducing the effective pore size. Whereas increased selectivity is observed for UiO-66-Me, this effect decreases again upon addition of a second methyl group, UiO-66-Me 2 . The latter allows for tuning confinement factors inside the pores, thus adsorption properties of the metal−organic framework. The selective adsorption results from the interaction in the smallest octahedral cage. The extreme confinement in the tetrahedral cage allows for stereoselective separation of disubstituted cycloalkanes and cis/trans alkenes. Monte Carlo simulations were performed for the unfunctionalized UiO-66 framework. First, a comparative study between the force fields Dreiding and UFF is performed with n-alkanes to obtain accurate and reproducible values. The simulations show adsorbate molecular size−adsorbent cage size effects similar to window/cage effects reported for zeolites (e.g., silicalite). Second, adsorption properties were simulated for selected cases, including stereoisomers. Careful analysis of the adsorbate's molecular positioning in the framework confirms the experimental data. The framework's selectivity results from adsorption in the tetrahedral cage at zero coverage. Furthermore, simulations show important contributions of entropic factors to the observed adsorption selectivity.

Computational high-throughput screening was carried out to assess a large number of experimentally reported metal−organic frameworks (MOFs) and zeolites for their utility in hexane isomer separation. Through the work, we identified many MOFs and zeolites with high selectivity (S L+M > 10) for the group of n-hexane, 2-methylpentane, and 3methylpentane (linear and monobranched isomers) versus 2,2dimethylbutane and 2,3-dimethylbutane (dibranched isomers). This group of selective sorbents includes VICDOC (Fe 2 (BDP) 3 ), a MOF with triangular pores that is known to exhibit high isomer selectivity and capacity. For three of these structures, the adsorption isotherms for a 10-component mixture of hexane and heptane isomers were calculated. Subsequent simulations of column breakthrough curves showed that the DEYVUA MOF exhibits a longer process cycle time than VICDOC MOF or MRE zeolite, which are previously reported, high-performing materials, illustrating the importance of capacity in designing MOFs for practical applications. Among the identified candidates, we synthesized and characterized a MOF in a new copper form with high predicted adsorbent capacity (q L+M > 1.2 mol/L) and moderately high selectivity (S L+M ≈ 10). Finally, we examined the role of pore shape in hexane isomer separations, especially of triangular-shaped pores. We show through the potential energy surface and three-dimensional siting analyses that linear alkanes do not populate the corners of narrow triangular channels and that structures with nontriangular pores can efficiently separate hexane isomers. Detailed thermodynamic analysis illustrates how differences in the free energy of adsorption contribute to shapeselective separation in nanoporous materials.

A vapor-phase adsorptive recovery process is proposed as an alternative way to isolate biobutanol from acetone-butanol-ethanol (ABE) fermentation media, offering several advantages compared to liquid phase separation. The effect of water, which is still present in large quantities in the vapor phase, on the adsorption of the organics could be minimized by using hydrophobic zeolites. Shape-selective all-silica zeolites CHA and LTA were prepared and evaluated with single-component isotherms and breakthrough experiments. These zeolites show opposite selectivities; adsorption of ethanol is favorable on all-silica CHA, whereas the LTA topology has a clear preference for butanol. The molecular sieving properties of both zeolites allow easy elimination of acetone from the mixture. The molecular interaction mechanisms are studied by density functional theory (DFT) simulations. The effects of mixture composition, humidity and total pressure of the vapor stream on the selectivity and separation behavior are investigated. Desorption profiles are studied to maximize butanol purity and recovery. The combination of LTA with CHA-type zeolites (Si-CHA or SAPO-34) in sequential adsorption columns with alternating adsorption and desorption steps allows butanol to be recovered in unpreceded purity and yield. A butanol purity of 99.7 mol % could be obtained at nearly complete butanol recovery, demonstrating the effectiveness of this technique for biobutanol separation processes.