Macroscopic and Microscopic View of Competitive and Cooperative

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Macroscopic and microscopic view of competitive and cooperative adsorption of alcohol mixtures on ZIF-8 Benjamin Claessens, Ana Martin-Calvo, Juan Jose GutierrezSevillano, Nicolas Dubois, Joeri Denayer, and Julien Cousin-Saint-Remi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03946 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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Macroscopic and microscopic view of competitive and cooperative adsorption of alcohol mixtures on ZIF-8 Benjamin Claessens†, Ana Martin-Calvo†, Juan José Gutiérrez-Sevillano‡, Nicolas Dubois†, Joeri F.M. Denayer†, Julien Cousin-Saint-Remi†* *corresponding author: [email protected] †Department

of Chemical Engineering

Vrije Universiteit Brussel Pleinlaan 2 1050 Elsene Belgium ‡Departamento

de Sistemas Fisicos Quimicos y Naturales

Universidad Pablo de Olavide Ctra. Utrera, km 1 41013 Sevilla Spain

ACS Paragon Plus Environment

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ABSTRACT While in most adsorptive separations different mixture components tend to compete for different adsorption sites, we report the existence of cooperative effects in the adsorption of alcohols (ethanol & 1-butanol) from the vapor phase on ZIF-8. The presence of these molecules in binary mixtures leads to an increase in their equilibrium capacities, compared to the pure component isotherms. These effects were first observed when predicting the mixture equilibrium capacities using the Ideal Adsorbed Solution Theory (IAST) and were also observed via Grand Canonical Monte Carlo (GC MC) simulations. GC MC simulations showed that the interaction between adsorbate molecules leads to the cooperative effect predicted by IAST. The predicted cooperative adsorption could be confirmed via breakthrough experiments. In these experiments, a ‘roll-up’ of 1-butanol was observed during the regeneration of a ZIF-8 packed column. A dynamic breakthrough model employing IAST was developed and used to explain the effect of the adsorption equilibrium on the dynamic breakthrough profiles.


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Introduction Within the class of Metal Organic Frameworks (MOFs), the Zeolitic Imidazolate Frameworks (ZIFs) have gained a lot of attention due to their remarkable chemical and thermal stability compared to other MOFs.1 ZIFs possess zeolite-like structures consisting of metal ions coordinated with imidazolate linkers.1,2 For instance, the wellknown ZIF-8 is built of Zn2+-ions and methyl imidazolate linkers, joining into the sodalite topology with cages of 12.5 Å interconnected by 3.3 Å windows.3 Due to its hydrophobic nature and stability under humid conditions, ZIF-8 has been extensively studied for the adsorptive recovery of alcohols produced via fermentation, such as biobutanol.4–13 Single component adsorption (equilibrium) isotherms of n-alcohols on ZIF-8 show distinctive stepped or sigmoidal shapes with inflection points.4,5,11 Such macroscopic observations are typically observed with polar compounds adsorbing in hydrophobic pores or with flexible materials.14–17 Experimental studies on ZIF-8 have shown that two different structures exist: one which is observed at ambient pressures and one which can be observed at pressures above 1.47 GPa.18,19 This phase change of ZIF-8 is marked by a 30° rotation of the 2-methyl-imidazolate linkers, increasing the pore entrance by 0.5 Å, thus showing that the ZIF-8 framework possesses flexibility.19 However, XRD results showed that the structure of ZIF-8 when loaded with 1-butanol is very similar to its empty, ambient pressure structure.4 On the other hand, molecular simulations have been used to gain microscopic insights into this adsorption phenomenon on ZIF-8.19–29


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Monte Carlo simulations, using the ambient pressure structure of ZIF-8, were able to accurately describe the experimental adsorption isotherms.24–26,30,31 They, moreover, indicated that the flexibility of the framework has little influence on the adsorption equilibrium of alcohols on ZIF-8.

24–26,30,31

Instead, the adsorption mechanism seems to

follow that of a polar component adsorbing into hydrophobic pores.

21,27,30,32

Although

the pure component equilibrium of linear alcohols on ZIF-8 has been well studied,21,27,30,32 studies on the mixture adsorption equilibrium of alcohols is limited to binary alcohol/water mixtures.30,32 The (macroscopic) performance of an adsorbent under dynamic conditions with fixedbeds is known to be dramatically influenced by the shape of the adsorption isotherm.29,33 Depending on the shape of the adsorption isotherm, various elution profiles are typically observed with fixed beds. For instance, a concave isotherm, such as the Langmuir isotherm, results into the development of a sharp favorable profile (shock wave) during the adsorption step, while a broad unfavorable profile (dispersive wave) is formed during the desorption step.29,33 In contrary, convex type isotherms, like the antiLangmuir isotherm, lead to the inverse behavior.29,33 The S-shaped isotherms, as observed with alcohols on ZIF-8, generate more complex elution profiles, combining shocks and dispersive parts.4,6,7 The efficient development of adsorptive separation processes, relies on the proper understanding of the dynamic expression of multicomponent adsorption equilibria, rather than that of single compound systems.34


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In that perspective, an accurate multicomponent adsorption isotherm model is required for process simulations and design. However, obtaining information on the multicomponent adsorption equilibrium is experimentally tedious and not trivial. Although molecular simulations offer the opportunity to calculate multicomponent adsorption equilibria, they remain slow and unpractical for dynamic calculations. In general, macroscopic models are used to predict mixture behavior, which often solely make use of single component data. As a matter of simplicity, explicit (semi-)empirical models, such as the extended Langmuir model, are the preferred approach to describe adsorption of mixtures. On the other hand, a more rigorous approach can be followed via the Ideal Adsorbed Solution Theory (IAST), which directly extends from solution thermodynamics.35 The implementation of the IAST equations in a dynamic model remains challenging due to the many and mathematically stringent iterations necessary for adsorption equilibrium calculations. Different approaches have been developed in the past to accelerate IAST calculations as well as to combine it with dynamic simulations.33–45 Different authors simultaneously solved the dynamic equations and the IAST equilibrium equations numerically.33,34,38,39 Santos et al. used a spline interpolation method to speed up IAST calculations.40 The methods developed by Levan & Vermeulen and Frey & Rodrigues are based on Taylor or Padé approximations.41,42 FastIAS, developed by Myers & O’Brien allows a more efficient numerical solution of the governing equations, when


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an analytical expression for the spreading pressure is available.43,46 For some pure component isotherm models, including the Langmuir, BET and quadratic models, IAST equations can be solved explicitly for binary adsorbate systems exhibiting equal saturation capacities.35,36,44,45,47 More recently, Landa et al. developed a method based on solving the IAST equations as ordinary differential equations to combine IAST calculations with a breakthrough model based on equilibrium theory.37 In this work, the adsorption and separation of alcohol mixtures on ZIF-8 is studied at the microscopic and macroscopic level. First, the pure component ethanol and 1butanol isotherms are discussed and compared with Monte Carlo (MC) results. Secondly, multicomponent adsorption equilibria are calculated via the Ideal Adsorbed Solution Theory (IAST) and compared to Monte Carlo (MC) simulations to gain insight in the molecular mechanisms governing mixture adsorption on ZIF-8. The MC and IAST predictions are validated with experimental data obtained from single component vapor phase adsorption isotherm measurements and breakthrough experiments for different binary ethanol/butanol mixtures. Finally, a dynamic model based on IAST is developed to further explore the dynamic expression of the multicomponent adsorption equilibria during the separation of alcohol mixtures with ZIF-8 packed beds. Experimental Section Materials


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ZIF-8 powder was purchased from Sigma-Aldrich (Basolite® Z1200, BASF). The material properties were discussed elsewhere.4,6 Pellets were made by a compaction method.6 ZIF-8 powder was compressed to a cake at a pressure between 5 – 8 kPa. The obtained cake was crushed and sieved at a size range of 250-500 µm. The effect of compaction was observed to be negligible and discussed elsewhere.6 1-butanol (99.9% purity) was purchased from VWR and ethanol (99.9 % purity) from Merck. Methods Single component isotherms Vapor phase adsorption isotherms of 1-butanol and ethanol on ZIF-8 powder were measured with the gravimetric method (VTI corporation, SGA-100H). Nitrogen gas (Air Liquide) was bubbled through a vessel containing pure liquid alcohol (i.e. evaporator): enriched vapor stream was then sent to the sample chamber. The uptake of the vapor by the ZIF-8 sample was recorded until equilibrium. Prior being contacted with the vapor stream, the ZIF-8 powder was activated at 150 °C for 2 hours in the sample chamber under pure nitrogen gas. Adsorption isotherms were obtained by repeating the measurements for different alcohol partial pressures. IAST calculations Multicomponent adsorption isotherm equilibrium capacities were calculated via the Ideal Adsorbed Solution Theory (IAST). A detailed discussion of the theoretical foundations of IAST and can be found in literature.34,36 An overview of the governing


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equations was added in Supplementary Information. In this work, to calculate IAST mixture equilibria, the pure component isotherm data was fitted with a combined Langmuir-Sips model (eq. (1)) and the adsorption isotherm parameters can be found in Table S1: 𝑞0𝑖

= 𝑞𝑠𝑎𝑡,𝑙 ∙ 𝐾𝑙 ∙

𝑝0𝑖 1 + 𝐾𝑙 ∙ 𝑝0𝑖

+ 𝑞𝑠𝑎𝑡,𝑠 ∙

(𝐾𝑠 ∙ 𝑝0𝑖 )𝑛 𝑛

1 + (𝐾𝑠 ∙ 𝑝0𝑖 )

(1)

with 𝑞𝑠𝑎𝑡,𝑙 and 𝑞𝑠𝑎𝑡,𝑠 the saturation capacities of the Langmuir and Sips portions of equation (1), respectively (in mol/g), 𝐾𝑙 and 𝐾𝑠 the Langmuir and Sips constants (in Pa-1), 𝑛 the exponent of the Sips model and 𝑝0𝑖 the pressure of (pure) component 𝑖 in the gas phase (in Pa). The equilibrium adsorption capacities were calculated via IAST for the different ethanol and 1-butanol mixture compositions. Using equation (1), the integral of the reduced spreading pressure (eq. (S2)) can be evaluated analytically (eq. (S6)). Equations (S1), (S3), (S4) and (S6) were combined, leading to one algebraic equation with the adsorbed phase mole fraction of ethanol as unknown. This algebraic equation was solved numerically using a trust-region-reflective algorithm in Matlab® R2017b (lsqnonlin solver). Knowing the mole fraction of ethanol, the 1-butanol mole fraction can be easily determined via equation (S4). Equations (1) and (S5) can then be used to calculate the adsorption capacity of the different components. Molecular simulations


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Molecular simulations were carried out using the software RASPA.37,38 Adsorption isotherms of pure 1-butanol, ethanol and ethanol/1-butanol mixtures were calculated via Monte Carlo simulations in the Grand Canonical ensemble. During the simulations, random movements are applied to the adsorbate molecules, until equilibrium is reached. The imposed movements include rotation, translation, reinsertion and identity change of the molecules. Host-guest and guest-guest interactions were defined by Lennard-Jones (L-J) and Coulombic potentials. Lorentz-Berthelot mixing rules are applied to obtain cross-term L-J interactions. The Ewald summation method was used for Coulombic interactions, with a relative precision of 10-6. Lennard-Jones and Coulombic potentials were cut and shifted at a cutoff distance of 12 Å. Periodic boundary conditions were applied. The molecular models for 1-butanol and ethanol were validated in previous work.24 Both molecules were defined as flexible models. Each CHx group from the aliphatic chain was considered as a single interaction center. The oxygen and hydrogen atoms in the hydroxyl group were defined independently. Chemical bonds were kept fixed. Harmonic bend and TraPPE dihedral potentials were used for angles and torsions, respectively.39 The molecular parameters from TraPPE were taken for the Lennard-Jones parameters and partial charges of oxygen, hydrogen atoms and the connecting CH2 group. Lennard-Jones parameters of the other CHx groups were obtained from the previous work of Dubbeldam et al.40 In addition, ZIF-8 was considered as a rigid framework, using the crystallographic positions of the individual


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atoms (cubic unit cell of a=b=c 16.991Å3).1 Lennard-Jones parameters and partial charges of the atoms of the ZIF-8 structure were taken from the previous work of Gutierrez-Sevillano et al.41. Molecular simulations were carried out for a fixed total mixture pressure and different relative amounts of ethanol and 1-butanol. Breakthrough experiments Breakthrough experiments were carried out with ZIF-8 pellets packed columns (10 cm in length and 0.21 cm internal diameter) through which vapor streams were sent at a flow rate of 40 ml/min and at a temperature of 313 K. Vapor mixture streams of ethanol or 1-butanol were produced by bubbling helium (Air Liquide) through two evaporators containing either pure liquid 1-butanol or pure liquid ethanol. The partial pressures of ethanol and 1-butanol were controlled via the temperature inside the evaporator, as well as with an additional dilution stream of helium. Prior being contacted with the vapor enriched streams, the ZIF-8 packed bed was activated overnight at 150 °C under a He flow. The vapor mixture was analyzed at the outlet of the column via a gas chromatograph. So-called breakthrough curves were obtained by plotting the outlet concentrations in function of time. Similarly, after equilibration with the vapor enriched stream, desorption curves were measured by analyzing the outlet stream by purging the columns with pure helium at a flow rate of 40 ml/min and 313 K. From these breakthrough curves, mixture equilibrium capacities can be determined from a mass


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balance over the adsorption column.42 More details about these calculations can be found in the Supporting Information. Fixed-bed modelling A plug flow model containing axial dispersion was used to predict the concentration profiles of pure ethanol and 1-butanol vapor, and mixtures thereof, eluting from ZIF-8 pellets packed columns. The system was assumed to behave isothermally. Furthermore, the gas velocity was assumed to be constant over the column, considering the very diluted nature of the mixtures as well as the limited pressure drop (less than 0.5 bar). A linear driving force model was used to describe the adsorption kinetics and the equilibrium capacities were calculated using IAST. The governing equations and employed solution method were added in the Supporting Information. Axial dispersion and mass transfer coefficients were varied to describe the broadness of the experimental breakthrough profiles. The same parameters were used for all mixtures. The parameters’ values are provided in the Supporting Information. Results and Discussion Single component adsorption equilibria The single component adsorption isotherms of ethanol and 1-butanol on ZIF-8 are displayed in Figure 1. For ethanol (Figure 1, right), the adsorption (equilibrium) capacity increases gradually, until an adsorbate pressure of around 1000 Pa is reached. For higher ethanol partial pressures, the adsorption capacity shows a steep increase, but after the inflection point, at about 1500 Pa, it is seen to level off to reach saturation. A


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similar stepped adsorption isotherm shape is observed for 1-butanol (Figure 1, left), however, the steep increase and saturation appears at much lower pressures. The saturation of the ZIF-8 pores is reached at about 3.9 mmol/g 1-butanol, compared to 5.8 mmol/g for ethanol. The difference in saturation capacities between ethanol and 1butanol is simply an expression of the molecular weight.4 The values, shown in Figure 1, are also consistent with those previously reported in the literature.4,8 The stepped adsorption isotherm shape is typically observed with flexible materials14,43,44 or polar compounds concentrating in hydrophobic environments.34 In order to identify the governing (microscopic) adsorption mechanism, Grand Canonical Monte Carlo (GCMC) simulations were performed with the ZIF-8 framework considered being rigid. First, the GCMC equilibrium capacities are compared to the experimental results in Figure 1. For both adsorbates, the shape of the adsorption isotherm obtained by GCMC is comparable with that of the experimental data, although the capacity obtained in the simulations is larger than the experimental capacity. Since the GCMC simulations assumed ZIF-8 to be rigid, these results (Figure 1) already suggests that the well-known ZIF-8 flexibility, with its linker freely rotating,4,18–20 has little influence on the adsorption capacities of n-alcohols. This observation is also in line with previous studies.4,12 Next, while the GCMC adsorption capacities follow the same trend as the experimental data, still some (significant) differences can be observed. For instance, at saturation, the GCMC simulations predict larger loadings than those


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obtained by experiments, while at low pressures the adsorption capacity appears underestimated (Figure 1). These differences may be due to the GCMC simulation force fields,25 but also from differences between various real ZIF-8 powder samples.11 Although real samples typically contain defects, in MC simulations the crystal lattice is assumed to be perfect.

Figure 1. Comparison between the experimental (open symbols) pure component isotherms of ethanol (right) and 1-butanol (left) and those predicted by Monte Carlo simulations (closed symbols) on ZIF-8 at 313K. The fitted isotherm model (eq. 1) is shown as a solid line for pure ethanol and 1-butanol.

For our simulations, the size of adsorbate-framework and adsorbate-adsorbate interaction energies of ethanol and 1-butanol at different points in the isotherm are shown in Figure 2. It can be observed that, at very low loading, for both components, the interaction between the adsorbate and the framework is dominating. With


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increasing loading, the strength of the adsorbate-adsorbate interactions increases, until a capacity of around 0.5 mmol/g 1-butanol or 1.0 mmol/g ethanol is reached. Subsequently, for both ethanol and 1-butanol, the host-adsorbate and adsorbateadsorbate interaction energies remains constant, showing that the available space in the ZIF-8 cages is energetically homogenous. In the case of 1-butanol, the host-adsorbate interaction energy is larger than the adsorbate-adsorbate interaction energy. However, in the case of ethanol, the opposite trend is visible: the adsorbate-adsorbate interaction energies are larger compared to the framework-adsorbate interaction energies. Thus, especially for ethanol, adsorbate-adsorbate interactions play an important role in the adsorption mechanism.


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Figure 2. Adsorbate-adsorbate (circles – red) and host-adsorbate (blue – squares) interaction energies as a function of loading for 1-butanol (a) and ethanol (b). These results are in line with the mechanism reported by Zhang et al.

25,32

They

reported that the adsorption mechanism of methanol, ethanol and 1-butanol follows that of polar components on a rigid hydrophobic material. At low vapor pressures, adsorption occurs near the double bond of the imidazolate linkers. Due to adsorbateadsorbate interactions, the adsorption capacity increases subsequently at higher pressures, leading to a steep increase in capacity. Finally, after the inflection point, complete pore-filling starts to occur.25,32 A similar mechanism was also observed by Nalaparuju et al. when studying the adsorption of water, ethanol and methanol in the hydrophobic ZIF-71 framework.28 Mixture adsorption equilibria Next, the mixture adsorption equilibrium was investigated for various binary alcohol mixtures on ZIF-8 via IAST and GCMC predictions, as well as being compared with experimental data (see Methods). First, to calculate the equilibrium capacities of ethanol and 1-butanol using IAST, the (single-component) adsorption isotherms were fitted using a dual Langmuir-Sips isotherm model, consisting of the sum of a Langmuir and Sips model (eq. (1), Figure S1).6 The reason for choosing this model is twofold. First, the shape of the experimental data was well described. The inflection points of the 1butanol and ethanol isotherms were well approached by the Sips part of equation (1),


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while the Langmuir part describes the initial increase of the isotherm in a better way (Figure 1). Secondly, a Henry-law relationship is expected between the equilibrium capacity and the corresponding vapor pressure in the lower pressure region.35 Although the Langmuir model has a clear Henry-law limit, the derivative of the Sips isotherm model equals 0 in the low pressure region.34,45,46 The addition of a Langmuir term to the isotherm equation thus assures thermodynamic consistency, since the combined model will have a non-zero derivative in the low pressure region, leading to a non-zero Henry constant. The parameter values obtained from the fitting of equation (6) on the experimental data (Figure 1) are provided in the Supplementary Information (Table S1). Figure 3 shows the IAST equilibrium adsorption capacities of ethanol (Figure 3a,c,d) and 1-butanol (Figure 3b) on ZIF-8 from binary (vapor) mixtures. The edges of the surfaces in Figs. 2a-c correspond to the pure component ethanol and 1-butanol adsorption isotherms. Furthermore, also in line with the single component adsorption equilibria (Figure 1), much higher partial pressures are required to fill up the ZIF-8 pores with ethanol molecules compared to 1-butanol. In the higher partial pressure region for both components, the equilibrium capacity for both ethanol and 1-butanol is observed to be lower than the pure component capacity. However, it is intriguing to note that IAST predicts a higher ethanol adsorption capacity in the presence of 1-butanol in the low-pressure region than as a pure vapor (Figures. 3a,c). For a constant ethanol partial pressure, a steep increase can be observed at a 1-butanol partial pressures of about 20


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Pa, followed by a maximum in the ethanol loading between 30 Pa and 60 Pa 1-butanol partial pressure. For instance, the pure component ethanol loading at 360 Pa is predicted to be 0.15 mmol/g. At the same ethanol partial pressure, but in the presence of 40 Pa 1-butanol vapor, the ethanol adsorption capacity jumps to 0.55 mmol/g. At higher 1-butanol partial pressures, the ethanol equilibrium capacity starts to decrease, but remains significantly higher than in the pure state.

Figure 3. IAST (binary) adsorption equilibria capacities of ethanol and 1-butanol at 313K on ZIF-8. a) ethanol adsorption capacities, b) 1-butanol adsorption capacities, c) zoomin on Figure 3a and d) different cross-sections of Figure 3a at constant 1-butanol


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pressure. The pure ethanol isotherm model (eq. 1) and experimental isotherm (circles) is also shown.

In Figure 3d different cross-sections of Figure 3a are shown at various (constant) 1butanol partial pressures. The ethanol pure component experimental data and adsorption isotherm model are shown together with binary adsorption isotherms in the presence of 1-butanol. Two different regions can be observed comparing the single component and binary isotherms. First, for partial pressures below about 1500 Pa, in line with previous observations (see Figs. 2a,c), the ethanol adsorption capacity is clearly seen to be higher in the presence of 1-butanol than in the pure state. This suggests that a cooperative adsorption mechanism is induced by 1-butanol, with its magnitude being dependent on the 1-butanol partial pressure. For instance, for a binary mixture containing 10 Pa 1-butanol, the cooperative effect appears to be large between ethanol partial pressure of 500 Pa and 1500 Pa (Figure 3d). At 1000 Pa of ethanol partial pressure and 10 Pa 1-butanol, the capacity of ethanol increases from 0.6 mmol/g to 1.5 mmol/g. At a higher 1-butanol pressure of 80 Pa, the range of ethanol partial pressures where a cooperative effect is observed lies between 0 Pa and 1200 Pa. Second, at higher ethanol partial pressures, a different relationship with 1-butanol is observed. Above 1500 Pa ethanol partial pressure, the presence of 1-butanol leads to a decrease in ethanol adsorption capacity for all binary mixture compositions. For instance, for a


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binary ethanol/1-butanol mixture containing 2000 Pa ethanol and 10 Pa 1-butanol, the ethanol capacity is predicted to be 3.7 mmol/g. The predicted ethanol capacity for pure ethanol at 2000 Pa is 4.4 mmol/g. Thus, while at low pressure 1-butanol appears to induce a cooperative (adsorption) effect, at higher pressures a competitive effect is observed. In Figure 4, the (mixture) adsorption (equilibrium) capacities of ethanol and 1-butanol on ZIF-8 obtained by GCMC simulations are compared with those calculated by IAST and experimental data. The data predicted by IAST shows the same trend as discussed above (Figure 3): at 1-butanol partial pressures of 40 Pa, 80 Pa and 125 Pa an increase ethanol capacity is observed compared to the pure component ethanol isotherm. The increase shown is the highest for a binary mixture containing 40 Pa 1-butanol, while it is lower for a mixture containing 80 Pa and 125 Pa 1-butanol. The MC data shows a similar trend: for mixtures containing 40 Pa 1-butanol, a large increase in ethanol equilibrium capacity is observed (Figure 4). The increase is still present for mixtures containing a higher amount of 1-butanol, although its magnitude decreases with increasing 1butanol partial pressure. Since the IAST does not take molecular information into account, the fact that it predicts the same trends as the MC simulations is quite remarkable. In previous studies, excellent agreement between MC predictions and IAST has been observed by different authors for MOFs and zeolites used in CO2/N2 separation47–50, methane storage51 or xylene separation.52 However, IAST often fails to


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accurately reproduce MC predictions, when preferential adsorption of one component takes place inside a pore or pore window.53–55 As also shown in Figure 4, the experimental ethanol equilibrium capacities follow a similar trend as the MC and IAST simulations. With increasing partial pressure of 1butanol, the ethanol equilibrium loading increases, thus confirming the cooperative adsorption of ethanol in the presences of 1-butanol. Small differences between the equilibrium capacities obtained via the different techniques can be observed. Each of the used techniques has its own sensitivities. For instance, IAST is unable to account for specific adsorbate-adsorbate interactions between ethanol and 1-butanol. As mentioned previously, the predictions of MC simulations can be sensitive to the used force field models.25 Experimentally, other effects, such as the column pressure drop or heat effects might influence the calculated mixture capacities.56 In general, the lower the amount of adsorbate, compared to the amount in the adsorbed phase, the more accurate the calculation of the equilibrium capacity via breakthrough experiments. In our case, the system is very diluted. For 1-butanol, the most adsorbing component, a pure component breakthrough experiment at 80 Pa (Figure S6 & Figure 8) led to a calculated equilibrium loading (3.2 mmol/g) very similar to the gravimetric pure component isotherm (3.3 mmol/g). Overall, comparing the three used techniques, the largest difference was observed at a 1-butanol partial pressure of 40 Pa. As can be observed in Figure 1, this corresponds to the region just before the inflection point in


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the pure component isotherm of 1-butanol. Since in this region, the 1-butanol isotherm is the steepest, the largest difference can be expected.

Figure 4. Comparison between experimental equilibrium capacities of (a) ethanol and (b) 1-butanol (solid blue bars) and predictions by IAST (orange, dashed bars) and MC simulations (green, dotted bars) for binary mixtures containing 360 Pa ethanol and a variable partial pressure of 1-butanol. Average density profiles of a binary ethanol/1-butanol mixture at 360 Pa ethanol and 40 Pa 1-butanol are shown in Figure 5b, together with the corresponding pure component density profiles in Figure 5a. The plots of Figure 5 show a projection over the XY plane of the position of the centers of mass of the ethanol or 1-butanol molecules in the simulation box, averaged over all the simulation runs. In this region of the adsorption isotherm (i.e. at 360 Pa), a very low amount of ethanol is adsorbed


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(Figure 1). This is reflected in the pure ethanol average density profile (Figure 5a), with a very low amount of ethanol present in the ZIF-8 pores. Contrarily, for 1-butanol at 40 Pa, a significant amount is adsorbed, leading to a higher occupation density in Figure 5a. Comparing the average density profiles of pure ethanol (Figure 5a, right) with that of the mixture (Figure 5b, right), a higher occupational density of ethanol can be observed in Figure 5b. This result lies in line with the higher loading of ethanol predicted for a binary mixture as shown in Figure 4. For pure 1-butanol at 40 Pa (Figure 5a) and 1-butanol in the mixture (Figure 5b), a higher occupational density is observed than for ethanol. Comparing the two sides of Figure 5b, the ethanol in the binary mixture seems to adsorb in the vicinity of the 1-butanol molecules near the pore walls. As mentioned previously, in the case of pure ethanol, the size of the adsorbate-adsorbate interaction energy is higher than that of framework adsorbate interaction energy (Table 1). Thus, this result, together with the occupational density profiles, suggests that the interaction of ethanol with 1-butanol causes the increased equilibrium loading of ethanol.


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Figure 5. Average density profiles of 1-butanol (left) and ethanol (right) in ZIF-8 cavities in equilibrium with (a) pure 1-butanol (at 40 Pa) and pure ethanol (at 360 Pa) and (b) a mixture thereof at similar partial pressures (40 Pa 1-butanol and 360 Pa ethanol). The images show the projection over the XY plane of the center of mass of each ethanol and 1-butanol, averaged over all the simulation runs. The color gradation (from red to white) indicates most and less populated regions in the simulation box. Fixed-bed dynamics Figure 6 shows concentration profiles of binary alcohol (vapor) mixtures eluting from ZIF-8 packed beds during adsorption and desorption. These curves were recorded at partial pressures where the cooperative effect on the (mixture) adsorption equilibria was observed with IAST and GCMC predictions. In all cases, ethanol is the first component observed at the column outlet during adsorption (Figure 6, left). This result is consistent with the predictions of IAST & the MC simulations, with the equilibrium capacity of 1-


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butanol being higher than ethanol (Figure 3&3), as well as with previous work on the recovery of biobutanol with ZIF-8 in liquid phase.4,5,27 For all studied mixtures, the breakthrough of ethanol occurred very fast (< 1 min.). The adsorption breakthrough profiles of 1-butanol show a different shape dependent on its partial pressure. At a lower partial pressure of 40 Pa, a single broad breakthrough profile is observed (Figure 6a). At higher partial pressures of 80 Pa and 125 Pa, a step can be seen in the butanol breakthrough profiles: first the profile is very broad, followed by a sharp, fast breakthrough (Figure 6a&b). Although in general, such breakthrough shapes are not common, they have been observed before in liquid phase when studying ZIF-8 in the context of bio alcohol recovery.5–7 The shape of the observed 1-butanol breakthrough profiles can be linked to the shape of the pure component isotherm.6,33 As explained above, the 1-butanol adsorption isotherm consists of an anti-Langmuir and a Langmuir part. According to the wave theory of chromatography, the shape of a breakthrough profile is dependent on the derivative of the adsorption isotherm.33,34 An isotherm with a decreasing derivate as a function of pressure (Langmuir-type isotherm) leads to a very sharp, ‘shock’, concentration profile during adsorption, whilst an isotherm with an increasing derivative as a function of pressure (anti-Langmuir type) leads to a broad, ‘dispersive’, concentration profile during adsorption.33,34 At a lower partial pressure of 40 Pa, only a dispersive wave is formed, while at higher partial pressure, both a shock and a dispersive wave are observed. For a more detailed discussion of the influence of


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adsorption isotherm shape on breakthrough profiles, an excellent review by Helfferich & Carr is available in literature

33

and, more recently, the influence of the S-shape of the

ethanol isotherm on liquid phase breakthrough profiles on ZIF-8 was discussed by Cousin-Saint-Remi & Denayer.6 The breakthrough model employing IAST is able to describe the fast breakthrough of ethanol during adsorption as well as the two-step breakthrough profile of 1-butanol (Figure 6). Furthermore, for both the experimental and modelled profiles, the size of the shock part increases with increasing 1-butanol pressure. However, small differences exist between the predicted and experimental profiles. The predicted double-step 1-butanol breakthrough profiles show a faster transition towards a shock profile than the experimental profiles (Figure 6b&c, left). At the lowest 1-butanol partial pressure of 40 Pa, the transition towards a two-stepped breakthrough profile can already be observed in the modelled data, whilst only a dispersive profile is visible in the experimental data (Figure 6a, left). These small differences could be expected, considering that no exact match between the adsorption capacities predicted by IAST and the experimentally capacities was obtained (Figure 4). Overall, the breakthrough model employing IAST to calculate the binary equilibrium is able to describe the general experimental trends.


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Figure 6. Adsorption (left) and desorption (right) breakthrough profiles for different binary ethanol (red - squares) and butanol mixtures (green - circles).

Breakthrough

experiments were performed with a constant partial pressure of ethanol (360 Pa) and 40 Pa (a), 80 Pa (b) and 125 Pa 1-butanol (c). All experiments were performed with a total flow rate of 40 ml/min and at 313 K. IAST breakthrough predictions are shown as a solid line (ethanol – red, 1-butanol – green).


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While in all cases, during adsorption very fast breakthrough of ethanol is observed (< 1 min.), the breakthrough times of ethanol during desorption are very different for the different binary mixtures (Figure 6, right). For clarity, a zoom-in on the different ethanol concentration profiles during desorption was added in the Supplementary Information (Figure S2). To help explain the fast breakthrough of ethanol during adsorption, Figure 7 shows a developing concentration profile after 50 min in a packed ZIF-8 column as predicted by the breakthrough model. The mass transfer zone of 1-butanol can be observed, whilst the concentration of ethanol in the vapor phase is already almost equal to the feed. The ethanol equilibrium capacity is low in the zone were only ethanol is present, however, in the part of the column were 1-butanol is present, the equilibrium capacity increases. Thus, when the mass transfer zones of 1-butanol and ethanol separate during adsorption, the ethanol present in the zone before 1-butanol is almost unretained, leading to a very fast breakthrough of ethanol. However, only when the 1butanol concentration profile has moved through the column, complete equilibrium is achieved for both components. This fast initial breakthrough of ethanol during adsorption experiments makes it difficult to observe the increase in ethanol equilibrium capacity. During desorption, the full packed bed is saturated with the complete mixture, thus the extra ethanol which is adsorbed in presence of 1-butanol leads to the observed increase in desorption breakthrough times. Therefore, the desorption breakthrough


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times were used to calculate the equilibrium ethanol capacities (Figure 4). A comparison between the capacities calculated during adsorption and desorption was added in Supporting Information (Figure S3), showing only negligible differences for 1-butanol. On the other hand, for ethanol larger differences are observed, which are attributed to the difficulty to detect the mass transfer zone in adsorption mode compared to in desorption mode (vide supra).

Figure 7. Developing adsorption breakthrough profile inside the ZIF-8 column for a 360 Pa ethanol / 40 Pa 1-butanol mixture at 313 K and a flow rate of 40 ml/min. Profiles are shown after a time of 50 min.

While the ethanol desorption profiles are all S-shaped, the desorption profiles of 1butanol show an overshoot or ‘roll-up’ for the binary mixtures containing 40 Pa and 80 Pa 1-butanol (Figure 6a&b). For the higher 1-butanol partial pressure of 125 Pa, the overshoot is not present. Furthermore, the height of the overshoot appears to be


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dependent on the 1-butanol partial pressure, with a higher overshoot for the mixture containing a lower partial pressure of 1-butanol. A similar trend is predicted by the breakthrough model, with mixtures containing a lower 1-butanol partial pressure showing a bigger overshoot (Figure S4). Figure 8 shows a comparison between the regeneration of a ZIF-8 packed column after saturation with a binary mixture of 360 Pa ethanol and 80 Pa 1-butanol and the regeneration profile after saturation with pure 1butanol at 80 Pa. While the overshoot is present for the binary mixture, it is not visible when regenerating the column after saturation with pure 1-butanol. Furthermore, it can be observed that the overshoot occurs when the ethanol concentration at the outlet starts to decrease. Thus, the overshoot of 1-butanol appears to be linked to the presence or absence of ethanol.

Figure 8: Comparison of isothermal desorption breakthrough profiles on ZIF-8 after saturation with pure 1-butanol at 80 Pa (green-diamonds) and a binary mixture of 360 Pa ethanol (red – squares) and 1-butanol (green – diamonds). Adsorption and desorption were carried out at 313 K, using total flow rate of 40 ml/min.


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To investigate the mechanism behind the ‘roll-up’ of 1-butanol which was observed in this work (Figure 8), a contour plot of the binary adsorption equilibrium of 1-butanol as predicted by IAST is shown in Figure 9a. The contour plot which is shown in Figure 9a thus corresponds to a horizontal projection of Figure 3b in the XY plane. The solid black lines correspond to a fixed 1-butanol adsorption capacity. As can be seen from Figure 9a, the 1-butanol adsorption capacity increases when starting at the x-axis at a fixed 1butanol partial pressure and moving along the y-axis with increasing ethanol partial pressure. For example, the 1-butanol equilibrium capacity at 40 Pa is predicted to be 1.7 mmol/g, while for a mixture of 360 Pa ethanol and 40 Pa 1-butanol, IAST predicts a 1butanol capacity of 2.1 mmol/g. Similar to Figure 3c&d, a zoom-in on the 1-butanol binary isotherm and a cross section of the binary equilibrium surface was added in Supplementary Information (Figure S5). The cooperative effect of ethanol on 1-butanol is also clearly visible in this figure. IAST thus not only predicts an increased ethanol capacity due to 1-butanol adsorption, but also predicts an increase of 1-butanol loading due to the presence of ethanol. Also visualized in Figure 9a are the partial pressures of ethanol and 1-butanol observed at the column outlet during an adsorption-desorption cycle. In total, during adsorption and desorption, four different steps are observed, leading to four different lines in the equilibrium plane of Figure 9a. The corresponding parts of the breakthrough


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profiles (pressure vs. time) during adsorption and desorption are shown in Figures. 9b&c. In a first step ethanol is the first component to be observed at the column outlet during adsorption and no 1-butanol is visible (blue line in Figures 9a&b). Therefore, this line coincides with the y-axis in Figure 9a.

In a second step, ZIF-8 becomes fully

saturated with 1-butanol, leading to breakthrough of 1-butanol (orange line in Figure 9a&b). Due to the increasing 1-butanol partial pressure, the corresponding 1-butanol equilibrium capacity increases. The adsorption column is completely saturated when the point corresponding to 360 Pa ethanol and 40 Pa 1-butanol is reached in the equilibrium plane. Due the lower affinity of the adsorbent for ethanol, this component is the first to be completely desorbed, which is visualized by the yellow line in Figure 9a&c. As previously mentioned, the presence of ethanol also leads to a cooperative increase in 1-butanol loading. Since ethanol is being removed from the adsorption column, the equilibrium partial pressure of 1-butanol increases, and the yellow line runs parallel to the contour lines. The corresponding 1-butanol partial pressure increases from 40 Pa to 45 Pa. The removal of ethanol thus causes a shift in the equilibrium partial pressure of 1butanol. This shift in 1-butanol equilibrium pressure leads to the observed roll-up at the column outlet. Finally, the adsorbed 1-butanol is completely removed from the column, as visualized by the purple line in Figure 9a&c, which follows the x-axis as ethanol has already been completely desorbed.


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Figure 9: Isothermal adsorption – desorption cycle as predicted by IAST for a binary mixture of 360 Pa ethanol and 40 Pa 1-butanol at 313 K. (a) Shows a contour plot of the 1-butanol adsorption equilibrium as predicted by IAST. Black, solid contour lines show the points of constant 1-butanol adsorption capacity in mmol/g. Colored solid lines show the ethanol partial pressure as a function of 1-butanol partial pressure at the column outlet. The different steps in the adsorption and desorption breakthrough profiles are shown in different colors: breakthrough of ethanol during adsorption (blue), breakthrough of 1-butanol during adsorption (orange), breakthrough of ethanol during desorption (yellow) and breakthrough of 1-butanol during desorption (purple). (b) And (c) show the adsorption and desorption breakthrough profiles of ethanol (dotted lines)


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and 1-butanol (solid lines). The different colors correspond to the different zones shown in (A).

Although not common, similar ‘roll-up’ effects during desorption have been observed before in other adsorbate-adsorbent systems.57–62 The mechanism behind the overshoot, however, can be very different depending on the system. Davankov & coworkers observed an overshoot during desorption when studying the separation of HCl and CaCl2 on a microporous, hyper crosslinked polystyrene resin.59 According to these authors, size exclusion between H+ and Ca2+ caused the observed overshoot during loading and regeneration: removal of one of the components leads to an increase in the concentration of the other mixture components, when the concentration fronts move through the column. Heinonen et al. reported the cooperative adsorption of glucose, xylose, acetic acid and furfural in the presence of H2SO4.61 The authors mention a salting-out effect of H2SO4 on the other mixture components, leading to an increased equilibrium capacity on the resin. Since H2SO4 is the first component to elute during desorption, the equilibrium of the other components shifts towards higher concentrations as soon as it is removed from the column. Nowak et al. observed cooperative adsorption of fructose, glucose and surcrose on an ion-exchange resin, with all of these pure components showing and anti-Langmuir adsorption isotherm shape.62 Due to the cooperative effect of glucose on fructose adsorption, the fructose


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concentration at the column outlet increased when glucose eluted as first component during regeneration of the resin. The mechanism behind the overshoot observed in our work, thus appears to be similar to the one reported by Heinonen et al.61 and Nowak et al.62, with the least adsorbing component (ethanol) increasing the capacity of the strongest adsorbing component (1-butanol). However, for liquid phase mixtures, this cooperative effect can also be caused by different adsorbates decreasing each other’s solubility, such as in the case of Nowak et al.62 In this case, the cooperative adsorption of alcohols could be linked to adsorbate-adsorbate interactions inside the ZIF-8 cages. Summary and Conclusions In this work, the occurrence of cooperative effects in the adsorption of alcohol mixtures on ZIF-8 is demonstrated. IAST predicts an increase in adsorption equilibrium capacity for both components compared to their pure component isotherm. Molecular simulations helped gain more insight in the cooperative effect of ethanol on 1-butanol. Average density profiles and adsorbate-adsorbate interaction energies show that the interaction between ethanol and 1-butanol leads to an increased ethanol loading in the presence of 1-butanol. The cooperative adsorption of ethanol due to 1-butanol could also be observed experimentally via breakthrough experiments. Breakthrough experiments showed ‘roll-up’ of 1-butanol during the regeneration of a saturated ZIF-8 column. Using the developed breakthrough model, this effect could be linked to an increase in 1-butanol capacity due to the presence of ethanol.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Modelling parameters & methods, zoom-in on breakthrough profiles of Figure 6, zoom-in on the 1-butanol equilibrium of Figure 3, comparison between adsorption/desorption capacities (PDF) AUTHOR INFORMATION Corresponding Author * [email protected] Department of Chemical Engineering Vrije Universiteit Brussel Pleinlaan 2 1050 Elsene Belgium Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources


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For this research, J. Cousin-Saint-Remi was funded by the Research Foundation Flanders (FWO) with a Post-Doctoral Fellowship (Grant Number 12P2217N). ACKNOWLEDGMENT The authors are grateful to the Research Foundation Flanders (FWO) for financial support. REFERENCES (1)

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