Adsorptive Separation of 1-Butanol from Aqueous Solutions Using MFI

of Chemical Engineering and Materials Science, ‡Chemical Theory Center, and ... Zeolite MFI adsorbent for recovery of butanol from ABE fermentat...
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Adsorptive Separation of 1‑Butanol from Aqueous Solutions Using MFI- and FER-Type Zeolite Frameworks: A Monte Carlo Study Robert F. DeJaco,†,‡ Peng Bai,†,‡ Michael Tsapatsis,† and J. Ilja Siepmann*,†,‡,§ †

Department of Chemical Engineering and Materials Science, ‡Chemical Theory Center, and §Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States S Supporting Information *

ABSTRACT: Anaerobic fermentation can transform carbohydrates to yield a multicomponent mixture comprising mainly of acetone, 1-butanol, and ethanol (ABE) in a typical weight ratio of 3:6:1. Compared to ethanol, 1-butanol, the main product of ABE fermentation, offers significant advantages as a biofuel or a fuel additive. However, the toxicity of 1-butanol for cell cultures requires broth concentrations to be low in 1-butanol (≈1−2 wt %). An energy-efficient recovery method that performs well even at low 1-butanol concentrations is therefore necessary to ensure economic feasibility of the ABE fermentation process. In this work, configurational-bias Monte Carlo simulations in the Gibbs ensemble are performed to probe the adsorption of 1-butanol/ water solutions onto all-siliceous zeolites with the framework types MFI and FER. At low solution concentration, the selectivity and capacity for 1-butanol in MFI are larger than those in FER, while the opposite is true for concentrations at or above those of ABE broths. Structural analysis at various loadings sheds light on the different sorbate−sorbate and sorbate−sorbent interactions that govern trends in adsorption in each zeolite.

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INTRODUCTION As the world’s energy consumption increases, reliance on diminishing petroleum resources escalates the need for more and more alternative sources of energy. Environmental and practical considerations initiating from this dependence on limited fossil fuels require that transportation fuels be sustainable. Among several alternatives,1 biofuels, fuels produced mainly from biomass, are currently the only sustainable transportation fuels2 that can be produced on a large scale as either additives or complete replacements of gasoline. For these reasons, the annual production of 36 billion gallons of biofuels by 2022 was called for by the US Congress in the Energy Independence and Security Act (EISA 2007). However, partially due to the limited availability of nonethanol renewable fuels, the renewable fuel volume requirements set by this Act for 2014, 2015, and 2016 were not reached.3 Ethanol has received widespread attention industrially as a biofuel; this was in part due to the biotechnological readiness of the process.2 Ethanol’s disadvantages as a biofuel are well documented.4 Its high miscibility with water and hygroscopicity enable water at the bottom of storage tanks, pipes, or air to enter gasoline−ethanol blends.5 This results in driveability problems due to a higher water content in these blends. If phase separation occurs into hydrocarbon and aqueous phases, which can happen with decreasing temperature, less ethanol will remain in the fuel, and gasoline additives can be extracted into the aqueous phase, enabling the gasoline to be off-spec.5 Consequently, ethanol can only be blended with gasoline at 15 vol % for use in internal combustion engines, and pipelines cannot pump gasoline−ethanol blends, resulting in additional costs in the use of tanker trucks for transportation of blends to © XXXX American Chemical Society

commercial stations. While gasoline has a lower heating value (LHV) of 32.3 MJ/L, the LHV for ethanol is only 21.3 MJ/L, only about two-thirds the energy density of gasoline.2 1-Butanol (hereafter referred to as butanol), on the other hand, overcomes many of these disadvantages. It is miscible with gasoline in any proportion for use in internal combustion engines.2 It has a LHV of 27.8 MJ/L, corresponding to roughly 85% of the energy density of gasoline, can be transported more safely due to its lower vapor pressure, and is less hygroscopic, enabling it to be transported via pipelines.4,6 Despite these advantageous physical properties, butanol is yet to become a dominant biofuel since the ABE fermentation process is currently unable to compete economically with petrochemical production of butanol. This follows mainly from the inhibition and toxicity of butanol which severely restricts ABE broths to dilute concentrations ranging from 1 to 2 wt %, resulting in high costs for product separation. Although wellvalidated and conventional, distillation requires about 13 tons of steam for each ton of ABE solvent produced at these outlet concentrations7 and has been reported by some to use more energy to recover butanol than butanol provides.4,8 An energyefficient in situ recovery method is therefore needed to decrease recovery costs and water usage and increase volumetric productivity.9 By probing the energetic efficiency of different recovery techniques,10,11 adsorption onto zeolites, zeolitic imidazolate frameworks, and other metal−organic frameworks has emerged Received: December 8, 2015 Revised: January 17, 2016

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investigation of solution concentrations outside of the Henry’s law region), and both butanol and water are permitted to adsorb in the zeolite. The current study focuses on adsorption in zeolites MFI and FER because the former was used in previous experimental and computational studies for the butanol/water mixture and the latter was found to perform extremely well for the extraction of ethanol from water at low solution-phase concentrations.18

as a leading method, with important characteristics of adsorbents being rate of adsorption, capacity, selectivity for butanol, desorption capability, and cost.8,12,13 Hydrophobic allsilica zeolites are desired9 for their high capacity, selectivity for butanol over water, and simplicity in regeneration by heating. Previous studies of butanol/water adsorption on high-silica zeolites have demonstrated this high adsorption capacity of butanol at equilibrium on commercial MFI powders (0.118 g/ g14 and 0.12 g/g15,16). By showing that the single-component saturation loading of butanol is reached with butanol/water mixtures at solution concentrations of above ≈1 g/L, Oudshoorn et al.14 argued that competition by water does not play an important role in adsorption of butanol from solution by zeolite CBV28014, an aluminosilicate of the MFI framework type with Si/Al ratio of 140 and ammonium cations. Multicomponent adsorption of acetone, butanol, and ethanol from solution demonstrated that low amounts of acetone and ethanol were adsorbed compared to butanol in MFI, with the amount of acetone adsorbed being in general higher than that of ethanol.14,15 However, these studies were unable to measure competitive adsorption by water, and the loading of butanol was calculated assuming the total volume of solution did not change with adsorption. The suitability of a column process for adsorption and desorption with CBV28014 was evaluated by Saravanan et al.16 Multicomponent adsorption in a column process showed that acetone and ethanol initially appeared to adsorb completely, but both were displaced by butanol at later times in the unsteady process. At 100% breakthrough, the total acetone and butanol loadings were 0.01 and 0.11 g/g, respectively, and the loading of ethanol was negligible. These authors then showed that a 1.28 wt % binary aqueous solution of butanol could be concentrated to 84.3 wt % in a single step, achieving a concentration factor of 65, which is higher than other recovery techniques. Temperature-driven desorption was found to be incomplete, however, due to strong binding sites and the conversion of 1-butanol to 1-butene at high temperature. An optimal temperature of 150 °C was found where only 80% of the adsorbed butanol could be desorbed. Three consecutive adsorption/desorption cycles exhibited the same adsorption capacities, showing that the setup could be used repeatedly. In order to probe the separation characteristics of all-silica zeolites, which can be synthesized in many different crystalline forms, molecular simulation can be applied to provide molecular-level insight into equilibrium adsorption properties which vary widely with crystalline structure. Specifically, molecular simulation of butanol adsorption onto MFI is needed to characterize the highly favorable adsorption sites that prevent complete desorption in a column process and to find other framework types without this disadvantage. In addition, it is a relatively fast and inexpensive method to determine usually unavailable data10 describing adsorbate loading with respect to aqueous butanol concentration at different temperatures. In previous work, Xiong et al.17 used grand canonical Monte Carlo simulations to investigate the adsorption isotherms for 1-butanol and other alcohols on MFI, but for their simulations, the butanol fugacity was estimated from the excess chemical potential at infinite dilution and the system was treated as pseudo-unary, i.e., water coadsorption was not considered. Herein, Gibbs ensemble Monte Carlo (GEMC) simulation for adsorption equilibria of butanol from aqueous solution phases has been carried out where the solution phase is treated explicitly (i.e., allowing the

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METHODOLOGY Molecular Models. The zeolites used in this study are the all-silica forms of MFI (silicalite-1) and of ferrierite. Silicalite-1 has sinusoidal ten-ring channels (rings with 10 Si atoms) in the x-direction and straight ten-ring channels in the y-direction which intersect and form a 3-dimensional network of channels. The ortho form of silicate-1 with Pnma symmetry19 is adopted. For siliceous ferrierite (FER), the structure resolved by Morris et al.20 with Pnnm symmetry is employed. This structure has accessible oval-shaped, ten-ring channels oriented in the xdirection as well as straight eight-ring channels in the ydirection which intersect to form a 2-dimensional network of channels. The unit cell of each zeolite is replicated to obtain a supercell with atom positions fixed at the structures referenced above with periodic replication in all three directions about the supercell. Lennard-Jones (LJ) 12−6 and Coulomb potentials with parameters obtained from the TIP4P model21 for water and the transferable potentials for phase equilibria (TraPPE) force field for alcohols22 and zeolites23 are used to model sorbate−sorbate and sorbent−sorbate intermolecular interactions. While the water model is rigid, the alcohol model is semiflexible with harmonic angle bending and cosine series torsional potentials but rigid bond lengths. The parameters for oxygen and silicon in the zeolite frameworks were fit to a training set of sorption isotherms for alkanes, carbon dioxide, and ethanol,23 and previous work has validated them for the adsorption of water and ethanol/water mixtures.18,24 LJ parameters for all unlike interactions are obtained using Lorentz−Berthelot combining rules.25 The LJ interactions for sorbate−sorbate interactions and those in the solution phase are spherically truncated at a distance of 14 Å with analytical tail corrections to estimate interactions beyond this distance in both the zeolite and solution phases.26 Coulomb interactions are described using the Ewald summation method26 with a screening parameter of κ = 3.2/rcut and an upper bound of the reciprocal space summation at Kmax = int(κLbox) + 1. All host−guest interactions within zeolites are pretabulated and interpolated during simulation18,27 using here a grid spacing of 0.1 Å. Simulation Details. The simulations are carried out in the isobaric−isothermal version of the Gibbs ensemble28,29 with three simulation boxes to represent the zeolite phase, the solution phase, and a vapor-phase transfer medium. A system size of 2000 total molecules of butanol and water is used for simulations at a temperature of 323 K with an external pressure of 1 atm, and the mass of each zeolite supercell is the same. For each zeolite, seven starting compositions with the number of butanol molecules evenly spaced from NBuOH = 20 to 140 are utilized, with additional feeds for MFI at NBuOH = 110 and for FER at NBuOH = 5 and 10. Two additional ethanol molecules are used as impurity molecules to aid in sampling efficiency, as explained below. Equilibration and production periods consist of 250 000− 400 000 and 300 000 Monte Carlo cycles (MCCs), respectively B

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not reach its adsorption capacity even at the solubility limit. The loading on FER becomes higher than that on MFI at CBuOH ≈ 10 g/L, which is less than the butanol titers (the concentration of a fermentation broth at the end of each cycle) of typical clostridia strains.36 Compared to previous simulation data by Xiong et al.17 for 1-butanol adsorption in MFI, the present simulations yield an initial-uptake concentration that is shifted to lower CBuOH by about a factor of 2. In addition to difference in the molecular models, it is also likely that coadsorption of water (that was not considered by Xiong et al.) shifts the CBuOH of initial uptake to lower concentration because a fraction of the butanol molecules can interact favorably with water molecules. The present simulations also yield slightly higher QBuOH values for the flat part of the isotherm (CBuOH > 0.2 g/L) than those found by Xiong et al. At CBuOH = 10 g/L, the experimental measurements37 give a QBuOH that is about 4% and 11% higher than the present simulations and those of Xiong et al., respectively, and the slope of the experimental isotherm is much steeper at this point. The combination of TIP4P and TraPPE-UA force fields used here was previously found to underestimate the solubility limit for 1-butanol in water by a factor of 1.7 ± 0.2 at T = 298 K.39 Assuming that a similar deviation also holds at the slightly higer temperature used for this adsorption study, then the predicted solubility limit would be 39 ± 5 g/L, and this places the highest C for adsorption in MFI into the metastable region. To probe the thermodynamics of the different isotherms, Gibbs free energies of solvation (from vapor to solution), ΔGsolv = −RT ln(ρliq/ρvap), and of adsorption (from solution to zeolite), ΔGads = −RT ln(ρzeo/ρliq), are calculated from ratios of number densities40,41 as a function of CBuOH. As can be seen in Figure 2, there is no variation in ΔGsolv of water across the

(except in cases of very dilute solution concentrations for which production periods consisted of up to 600 000 MCCs to accumulate sufficient statistics). A MCC consists of 2002 randomly selected trial moves. Sixteen independent simulations are run at each feed concentration on each zeolite. The statistical uncertainties reported in the tables and shown in some of the figures reflect the standard errors of the mean computed from these independent simulations. Three types of coupled−decoupled configurational-bias Monte Carlo (CD-CBMC) moves are employed to improve system sampling: moves that regrow molecules without transferring boxes,30−33 swap moves that transfer particles to between simulation boxes,31,32 and switch moves34 that convert an ethanol molecule to a butanol molecule in one box and vice versa in the another box. In addition to CD-CBMC moves, volume moves for the solution-phase box, rigid-body center-ofmass translations, and rotations around the center of mass are carried out. As opposed to directly transferring molecules from the solution phase to the zeolite phase, a constant-volume vapor box is utilized as an intermediate transfer medium.35 The identity switch moves are performed to transfer butanol and ethanol only between solution and vapor boxes. An external biasing potential on ethanol in the vapor phase is used to yield on average about one ethanol molecule in both solution and vapor phases.35

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RESULTS AND DISCUSSION Binary Adsorption. The adsorption isotherms for butanol/ water solutions on MFI and FER as a function of solution concentration, CBuOH, are presented in Figure 1. (The

Figure 1. Binary adsorption isotherms at T = 323 K for butanol/water on MFI and FER as a function of alcohol concentration in the solution phase. The open red squares and green crosses depict the simulation data of Xiong et al.17 and the experimental data of Milestone and Bibby.37 The orange dashed line represents the experimental solubility limit.38

Figure 2. Gibbs free energies of adsorption (from solution to zeolite) and of solvation (from vapor to solution). The orange dashed line represents the experimental solubility limit.38

corresponding isotherms as a function of 1-butanol vaporphase number density are shown in Figure S1 of the Supporting Information.) The CBuOH of initial uptake differs between the two zeolites by almost a factor of 100 with MFI first taking up butanol at lower CBuOH, and >95% of MFI’s capacity is reached at the low concentration of ≈1 g/L. On the contrary, FER does

highly dilute solution concentrations investigated here. The wavering ΔGsolv for butanol at low CBuOH reflects the statistical uncertainties arising from a very low number of butanol molecules in such dilute solutions that corresponds to NBuOH ≈ 0.5 CBuOH/[g/L] for the system size used here; i.e., only ≈0.01 butanol molecules on average are found in the solution-phase C

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Langmuir box for CBuOH ≈ 0.02 g/L. (The numerical values of the number of butanol molecules in the solution-phase box are provided in Table S1 of the Supporting Information.) On the other hand, ΔGads varies widely across the CBuOH range investigated here. For butanol, there is a plateau in ΔGads corresponding to CBuOH at the initial onset of adsorption in each adsorption isotherm. At CBuOH corresponding to loadings of butanol above 75 mg/g zeolite, ΔGads begins to increase (i.e., becomes less favorable). This is likely the result of a larger entropic penalty of adsorption on a zeolite with less free volume becoming more significant than the enthalpic decrease associated with sorbate−sorbate interactions. For water in MFI, there is an initial decrease in ΔGads, with a minimum of 13.19 ± 0.04 kJ/mol, followed by a relatively large increase in ΔGads to 17.87 ± 0.07 kJ/mol. This increase of ≈5 kJ/mol, opposite in direction to the trends observed in a previous study of adsorption of ethanol/water and methanol/water solutions on MFI,24 is due to the longer chain length alcohol which provides fewer enthalpically favorable hydrogen bonding sites relative to the amount of occupied volume that increases the entropic penalty. A similar trend of ΔGads of water onto FER with increasing CBuOH is observed. In both zeolites, ΔGads values of butanol increasing at a higher rate than those of water with increasing CBuOH corresponds to a decrease in selectivity. The selectivity for butanol adsorption on each zeolite is displayed in Figure 3. While the selectivity of MFI for butanol adsorption varies by a factor of 400 from 1.2 × 106 at the lowest CBuOH to 2.9 × 103 at the highest CBuOH (corresponding to a change of nearly 4 orders of magnitude in CBuOH), that of FER has a smaller dependence on CBuOH, ranging from 4.4 × 104 to 7.8 × 103 for a factor of 150 change in CBuOH. At low CBuOH, the

selectivity for butanol and mole fraction of butanol in FER are less than those in MFI, while the opposite is true at CBuOH > 10 g/L, a lower bound on butanol titers from ABE broths. Given the low water loading (see Figure 1) and adsorbed mole fraction (see Figure 3), experimental measurement of the selectivity using mass balance for the solution concentrations before adsorption and in equilibrium is very challenging. Recently, Zhou et al.42 used infrared spectroscopy to estimate the 1-butanol and water loadings from integration of the adsorption bands for the C−H and O−H regions, respectively, and found a butanol/water selectivity of 165 at T = 308 K for adsorption from a 20 g/L solution on MFI single crystals made by the fluoride route and having a platelike habit with a length of less than 170 nm, an aspect ratio (length/width) of about 1.2, and a thickness of less than 40 nm. One would expect that the selectivity would be higher for larger and less platelike crystals (due to fewer surface silanols) and for crystals with even fewer interior defects. Clearly, the predicted selectivity of 7000 obtained here at a comparable state point is the upper bound and would be achievable only for very large single crystals with zero silanol defects. Structural Analysis. Analysis of molecular details of adsorption of butanol/water solutions on MFI and FER can provide deep insight into their differing adsorption profiles. The hydrogen bonding criteria used here consist of solely distance constraints of rOO ≤ 3.3 Å and rOH ≤ 2.5 Å. This loose distance criteria yields slightly more hydrogen bonds than more strict criteria43 which include an additional angular bound. It is chosen here to provide an upper bound on the number of hydrogen bonds for relative comparison of the dominating hydrogen bonding interactions in frameworks and sorbates. In Figure 4, strong correlations between the number of hydrogen bonds per molecule of both butanol and water with loading of butanol are observed. This explains that, at loadings above the initial onset of adsorption, hydrogen bonding plays a significant role in the loading of butanol. Unlike the aforementioned trends for loading, selectivity, and xBuOH, the number of hydrogen bonds per sorbate in FER does not exceed that in MFI at high loadings. For butanol and water in MFI, NHB per molecule ranges from 0.922 ± 0.003 and 1.396 ± 0.009, in that order, at the lowest CBuOH, to 1.260 ± 0.004 and 2.36 ± 0.02, respectively, at the highest CBuOH. These values are nearly the same as those observed by Bai et al.24 for ethanol and water in MFI, indicating that NHB per molecule for alcohol/water mixtures in MFI is independent of chain length for primary alcohols with four or fewer carbon atoms. This is not contradictory to the previously discussed differing trends of ΔGads between butanol/water in this study and ethanol/water in the work of Bai et al., as NHB is on a per molecule basis. For butanol and water in FER, NHB per molecule ranges from 0.869 ± 0.004 and 1.11 ± 0.03, respectively, at CBuOH = 0.75 g/L to 1.165 ± 0.002 and 2.11 ± 0.05 at the highest CBuOH = 37.0 ± 0.6 g/L. At the loadings of butanol investigated, there are around 0.25 less hydrogen bonds per water molecule in FER than MFI. Providing an environment for increased hydrogen bonding of water in its framework enables water to be adsorbed more favorably in MFI than FER, leading to a lower selectivity of butanol in MFI than in FER at the higher CBuOH investigated. The numbers of hydrogen bonds per molecule in solution for low, intermediate, and high concentrations are listed in Table 1. As expected from the slight variation in Gibbs free energies of solvation, there is only a small decrease in NHB for each molecule in solution with an order of magnitude increase in

Figure 3. Comparison of butanol adsorbed-phase mole fraction (top) and adsorption selectivites (bottom) on MFI and FER as a function of solution-phase concentration. The orange dashed line represents the experimental solubility limit.38 D

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1.89 ± 0.02 at high loading, indicating that at high loading water is essentially “sandwiched” between two butanol molecules, as has been seen for ethanol/water systems in MFI.24,44 Decomposing the hydrogen bonding for sorbates in FER by the types of molecules involved in each hydrogen bond demonstrates that it supports a completely different hydrogen bonding environment than MFI. At the low and intermediate loadings, Hother for butanol in FER is about one-third of the corresponding value in MFI. Additionally, the loading of water in FER at these low and intermediate loadings is also about one-third of the loading in MFI, indicating that the reason FER has a higher xBuOH at each loading is due to water coadsorbing more with butanol in MFI through hydrogen bonding. Unlike the observations of decreasing Hzeo of butanol in MFI, Hzeo for butanol in FER increases nearly 2-fold with increasing loading. At the same time, Hsame for butanol in FER stays constant relative to that in MFI as the loading of butanol increases 6fold. This suggests that, as loading on FER increases, the butanol molecules adsorb in sites that are more friendly toward the formation of (weak) hydrogen bonds, whereas the opposite holds for butanol hydrogen bonding in MFI. Also interestingly, Hsame does not become negligible for water in FER as it does in MFI at the highest loading investigated where a very small amount of water is adsorbed. This indicates that water aggregates in local regions in FER at high loading. From the tabulated potential for each zeolite, an energy grid is constructed for the oxygen atom in TIP4P water at each grid point with the framework. A gradient descent is performed at each grid point to determine energy basins with local minima. Subsequently, the watershed algorithm as implemented in scikit-image45 with periodic boundary conditions is employed to merge basins with an energy tolerance of 2.5 kJ/mol. This yields well-defined coordinate regions for the zigzag and straight channels in MFI, as shown in Figure 5. For FER, the regions output from the energy basin analysis are the straight channels of 8 Si atom rings (8Si, not including the intersection) and a region consisting of the straight channels of 10 Si atom rings and the intersection of the two accessible straight channels (10SINT), as shown in Figure 5. In Figure 6, channel-specific isotherms are presented for each framework type. For MFI at low loading, there is a similar amount of butanol molecules adsorbed in each channel. This continues with increasing solution concentration until the zigzag channels reach their capacity for butanol at CBuOH ≈ 0.2 g/L. Subsequent increases in loading are attributed to additional adsorption of butanol in the straight channels, which have about one more molecule than the zigzag channels per unit cell at the highest loading investigated. As the loading increases rapidly with solution concentration at the inflection point in the isotherm, water absorbs much more in the straight channels than the zigzag channels. This preference of about twice the number of water molecules in the straight channels continues as solution concentration increases, although the total number of water molecules adsorbed decreases considerably. For FER at low loading, however, butanol shows a strong preference for the 10SINT region, and water slightly favors to adsorb in the 8Si region. As the loading of butanol increases with increasing solution concentration, this preference subsides and the number of butanol molecules adsorbed in each region becomes nearly equivalent at the highest loading investigated. Similarly, the loading of water in the 8Si region remains

Figure 4. Number of hydrogen bonds per 1-butanol molecule on MFI and FER (top, denoted by red squares and blue diamonds, respectively) and number of hydrogen bonds per water molecule (bottom, denoted by magenta circles and upward cyan triangles, respectively).

Table 1. Hydrogen Bonds per Molecule in Solutiona CBuOH [g/L]

NHB,water

NHB,BuOH

0.020(2) 0.054(4) 4.3(1) 14.3(4) 37.0(6) 57.0(3)

3.7475(3) 3.7474(4) 3.7448(3) 3.7383(5) 3.7238(7) 3.7124(5)

2.6(1) 2.45(4) 2.419(9) 2.415(7) 2.404(4) 2.365(5)

a

The numbers in parentheses provide the statistical uncertainty in the last digit, as determined by the standard error of the mean. For the solution phase, the number of water molecules far exceeds that of butanol molecule; thus, the magnitudes of the errors are different.

solution concentration from 4.3 ± 0.1 to 57.0 ± 0.3 g/L. The slight decrease in number of hydrogen bonds per molecule at the higher concentrations is presumably due to an increasing number of hydrophobic alkane chains in solution as the CBuOH increases. NHB per molecule in each zeolite is decomposed into the identities of the sorbates involved in each hydrogen bond in Table 2 for low, intermediate, and high loadings. The number of hydrogen bonds by type for butanol in MFI follows from expectations as the loading of butanol increases. While Hsame increases 3-fold with increasing loading as a result of more adsorbed butanol and a higher xBuOH, Hother decreases for the same reason and becomes almost negligible at high loading. Similarly, Hsame for water becomes negligible at high butanol loading where only a small amount of water is adsorbed. Hother for water in MFI increases from 0.575 ± 0.005 at low loading to E

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Langmuir Table 2. Hydrogen Bonds with Same Sorbate Type, with Other Sorbate Type, and with Zeolite per Moleculea zeolite

CBuOH [g/L]

sorbate

Q [mg/g]

Hsame

Hother

Hzeo

MFI

0.009(1)

1-butanol water 1-butanol water 1-butanol water 1-butanol water 1-butanol water 1-butanol water 1-butanol water

21.4112(5) 2.64(4) 85.635(2) 3.83(5) 117.17(7) 0.65(2) 20.96(2) 0.93(3) 83.1(1) 1.25(4) 120.2(2) 0.77(3) 128.7(2) 0.45(3)

0.362(2) 0.52(1) 0.730(4) 0.583(8) 1.025(5) 0.083(7) 0.455(2) 0.23(2) 0.438(3) 0.242(9) 0.511(2) 0.28(2) 0.560(3) 0.26(3)

0.279(3) 0.575(5) 0.191(3) 1.064(5) 0.043(1) 1.89(2) 0.085(6) 0.48(2) 0.068(4) 1.08(2) 0.035(3) 1.30(4) 0.021(2) 1.42(5)

0.281(1) 0.305(3) 0.2171(5) 0.360(2) 0.1920(7) 0.384(6) 0.328(2) 0.410(8) 0.5112(9) 0.406(6) 0.5767(7) 0.410(7) 0.5837(5) 0.432(9)

0.054(4) 57.0(3) FER

0.75(4) 4.4(2) 14.3(4) 37.0(6)

a

The numbers in parentheses provide the statistical uncertainty in the last digit, as determined by the standard error of the mean.

Figure 5. Merged energy basins for MFI (left) and FER (right). For MFI, the yellow, magenta, blue, and red basins correspond to the zigzag (ZZ) channels, while the cyan and bright-green regions correspond to the straight (S) channels. For FER, the magenta and light-green regions correspond to the eight-ring straight channels (8Si), while the red and yellow regions correspond to the ten-ring channels and intersection of the two channels (10SINT).

relatively constant and then decreases as the loading of water in the 10SINT region reaches a maximum. This leads to a strong preference for water adsorption in the 10SINT region at the highest loading investigated. Three-dimensional probability density distributions for the water and butanol center-of-mass positions in MFI are displayed in Figure 7. At the low loading seen from the NBuOH/Nwater feed of 20/1980 (CBuOH ≈ 0.009 g/L), water seems to adsorb equally favorably in both the zigzag and straight channels. For butanol in MFI at this low loading, the most favorable sites are seen in the zigzag channels, but there is a higher quantity of less favorable sites in the straight channels. This leads to the about even adsorbed amount for both channels (see Figure 6). As the overall loading increases at intermediate CBuOH, the relative propensity for butanol to adsorb in the zigzag channels increases and water gets displaced to the straight channels. At CBuOH = 57.0 g/L, the fraction of high-density locations for butanol in the straight channels increases compared to CBuOH ≈ 0.054 g/L. This reflects the saturation of the zigzag channels by butanol (see Figure 6). The filling of both zigzag and straight channels with butanol molecules leads to an overall decrease in the water loading and its locations of highest density shift to the intersections. The corresponding three-dimensional probability density distributions for adsorption in FER are displayed in Figure 8. At CBuOH ≈ 0.75 g/L, the butanol molecules adsorb mostly in the 10SINT region. At the same time, water molecules adsorb relatively evenly throughout the unit cell, having a maximum

Figure 6. Channel-specific isotherms for MFI (top) and FER (bottom). Since supercells with the same number of SiO2 units are used, a direct comparison of loading can be made on a per molecule basis. The orange dashed line represents the experimental solubility limit.38

probability density of only 0.025, with more probable locations at the center of each ring. As the loading of butanol increases at CBuOH ≈ 4.4 g/L and then 37.0 g/L, the most probable location for butanol adsorption switches from the 10SINT regions to the smaller 8Si pores in the z-direction. As this happens with increasing loading of butanol, the water molecules are forced out of the 8-membered ring pores to the larger 10SINT regions. Despite that the water loading at CBuOH = 37.0 g/L is a factor of 2 lower than at 0.75 g/L, its more localized adsorption in the 10SINT regions at the higher concentration leads to a doubling of the maximum probability density. This localization explains the previous observation for Hsame of water being relatively high in FER at high CBuOH. F

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Figure 7. Three-dimensional probability density distributions of water (top row) and 1-butanol (bottom row) center-of-mass positions in one unit cell of MFI. Volume rendering is performed with splatting and transfer functions for the opacity are shown in black in the gray bar for each legend. The view of the image is a projection onto the xy-plane with the zigzag channels aligned along the horizontal direction. Si atoms of the unit cell are colored in light brown, while O atoms are red. The columns correspond to CBuOH ≈ 0.009, 0.054, and 57.0 g/L from left to right column, respectively.

Figure 8. Three-dimensional probability density distributions of water (top row) and 1-butanol (bottom row) center-of-mass positions in one unit cell of FER. Color coding and shading for probability density and zeolite atoms as in Figure 7. The larger ten-ring channels parallel to the z-direction are formed along each edge with their minimum images. Not easily seen are the eight-ring channels parallel to the y-direction. The columns correspond to CBuOH ≈ 0.75, 4.4, and 37.0 g/L from left to right column, respectively.

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CONCLUSIONS Equilibrium adsorption properties onto zeolites MFI and FER relevant to ABE fermentation are obtained from Gibbs ensemble Monte Carlo simulations. This computational study indicates that all-silica zeolites are highly advantageous for separation of butanol from ABE fermentation broth. The solution concentration of initial uptake for MFI is found to be 2 orders of magnitude smaller than that for FER. Although the zeolite loading, mole fraction of butanol, and selectivity for butanol are higher in MFI than in FER below solution concentrations of 10 g/L, all of these values become higher for FER than MFI above this concentration. Both zeolites are very selective for butanol over water with selectivities exceeding

30 000 for CBuOH < 1 g/L in both zeolites and still exceeding 2000 in MFI and 7000 in FER near the solubility limit. An analysis of the hydrogen bonding patterns analysis and of the location for sorbates provides more insight into the differing adsorption isotherms and selectivities. More hydrogen bonding occurs for each sorbate in MFI than in FER. With increasing loading, water molecules form more hydrogen bonds with butanol and are often bridging via hydrogen bonds between two butanol molecules. In FER specifically, water adsorbs in local regions as loading is increased and the number of water−water and water−framework hydrogen bonds does not change appreciably, whereas those with butanol increase at higher CBuOH. Butanol molecules form more weak hydrogen G

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bonds with zeolite oxygen atoms in FER than in MFI. In the latter framework, the number of butanol−butanol hydrogen bonds is about 5 times larger than the sum of butanol−water and butanol−framework hydrogen bonds. Probability histograms at different loadings show that the location of sorbates is strongly dependent on loading. With increasing loading in MFI, water molecules are displaced from zigzag channels by butanol molecules. As the loading of butanol in FER is increased, butanol molecules change from adsorbing mainly in the 10SINT regions to mostly in the 8Si regions. At the same time, water molecules become less evenly distributed with increasing loading, adsorbing locally in the 10SINT regions at high loading.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b04483. Table listing the average number of butanol molecules in the solution phase, figure depicting the loading as a function of butanol number density in the gas phase, and numerical data in tabular form for Figures 1, 2, 3, 4, and 6 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +1 (612) 624-1844; Fax +1 (612) 626-7541 (J.I.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0001004. The authors thank the Minnesota Supercomputing Institute for part of the computer resources used in this work.

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DOI: 10.1021/acs.langmuir.5b04483 Langmuir XXXX, XXX, XXX−XXX