Water Mixtures

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Separations

Non-Ideality in The Adsorption of Ethanol/Ethyl Acetate/ Water Mixtures On ZIF-8 Metal Organic Framework Thomas Virdis, Valery Danilov, Gino Baron, and Joeri F. Denayer Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00719 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Industrial & Engineering Chemistry Research

Non-Ideality in The Adsorption of Ethanol/Ethyl Acetate/Water Mixtures On ZIF-8 Metal Organic Framework 1

Thomas Virdis*, 1Valery Danilov, 1Gino V. Baron, 1Joeri F. M. Denayer Department of Chemical Engineering, Pleinlaan 2, B-1050 Brussel – Belgium

1

*[email protected]

Abstract Fisher esterification of acetic acid (or acetic anhydride) with ethanol (EtOH) is the main industrial process for the synthesis of ethyl acetate (EA). Nonetheless, the separation of the produced ester from ethanol is challenging since these Volatile Organic Compounds (VOCs) form an azeotropic mixture. In this work, the adsorption and separation EtOH/EA/water mixtures on the ZIF-8 Metal-Organic Framework is studied. The present study aims to characterize the adsorptive behavior of ZIF-8 with non-ideal EtOH/EA/water mixtures. Single and multi-component adsorption isotherms, obtained by gravimetry and breakthrough experiments, show high adsorption capacity and selectivity towards ethyl acetate. Pulse chromatography experiments confirm the higher interaction strength between EA and ZIF-8 in the low-pressure range at low degree of pore filling (Henry’s region). The breakthrough profiles show development of intermediate plateaus in specific concentration ranges, with complex breakthrough evolution that has been modeled using a combined IAST/RAST model to accounts for deviations from ideality.

Keywords: Metal-Organic-Frameworks, Real Adsorption Solution Theory, Zeolitic Imidazolate Frameworks 8, azeotrope separation, breakthrough adsorption, Volatile Organic Compounds

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1. Introduction Ethyl acetate is a Volatile Organic Compound (VOC) of interest for a multitude of industrial and daily-life applications. This organic is commonly synthesized through esterification of ethyl alcohol with acetic acid (or acetic anhydride) following the Fisher esterification procedure or by direct ethanol dehydrogenation,1 albeit the latter being less attractive due to higher energy requirements. Therefore, the separation of ethyl acetate (EA), acetic acid and ethanol (EtOH) mixtures represents a crucial step for proper product recovery. However, while acetic acid can be easily removed (e.g. salification, ion exchange chromatography), the real issue comes from separating ethanol and ethyl acetate, since these two compounds have very similar boiling point (respectively 78.5 °C and 77.06 °C) and form an azeotropic mixture. Therefore, EtOH/EA mixtures are challenging to separate by conventional distillation. Despite the large number of known purification techniques, such as azeotropic distillation,2 extractive distillation,3 and membrane separation,4 none of those bears significant advantages as the procedure is often complex and highly energy demanding. Selective physisorption is generally referred to as a valid alternative5–7 to the aforementioned methods. Adsorption has captivated the attention of many, given its efficiency toward VOCs sequestration at low concentration, which implies the ability of removing pollutants from the environment. Metal-organic frameworks (MOFs) are a promising class of adsorbents that stand out from the other porous solids in the context of several applications.8–12 Nevertheless, VOCs adsorption on MOFs has not been fully explored yet. These crystalline nanomaterials have a wide variety of geometrical and specific physico-chemical properties, which can be selected by tuning their synthetics route. MOFs are also characterized by exceptionally large surface areas,13 leading to advantages in terms of 2 ACS Paragon Plus Environment

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adsorption capacity. Due to the organic nature of the framework, some of these adsorbents are afflicted by weak hydro- and thermal stability, while others exhibit strong structural cohesion.14–16 In few occasions, some MOFs with better overall stability have been developed by mimicking pre-existing structures; such is the case for the Zeolitic Imidazolate Frameworks (ZIFs). This sub-class of MOFs shares topological isomorphism with zeolites, and generally exhibits large tolerance to moisture, chemicals and high temperature.17 ZIF-8 is an archetypal member of this class of adsorbents, assembled from 2methylimidazole linkers coordinated with Zn2+ ions. The structure of this adsorbent is cubic-shaped with 8 sodalite (SOD) cages held at the corners. This geometrical rearrangement resembles that of Linde Type A (LTA) zeolites and other LTA-type materials (e.g. zeolite 5A). Nonetheless, ZIF-8 offers larger total pores volume (Vp ∼ 0.66 cm3 g−1) despite having same cavity size (11.6 Å) as the aforementioned adsorbents.18 The adsorption properties of ZIF-8 in relation to certain organic compounds have been disclosed in previous studies,18–22 and demonstrate that ZIF-8 may be an eligible material for the development of gas phase VOCs sequestration systems. In the present paper, we investigated the vapor phase adsorption of ethyl acetate and ethanol onto ZIF-8 (BASF Basolite Z1200), as single-components and in binary mixture. ZIF-8 properties are investigated to understand whether this material can be proposed as an efficient system for the recovery of ethyl acetate from reaction medium. Single component

(EtOH,

EA,

water)

and

multicomponent

adsorption

(EtOH/EA;

EtOH/EA/water) capacities are measured in isotherm conditions (30 °C), by means of gravimetric analysis and breakthrough experiments in a packed bed of ZIF-8. Ideal and Real Adsorption Solution Theory (IAST/RAST) are applied to determine multi3 ACS Paragon Plus Environment


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component selectivity from adsorption isotherm data, and to predict capacities for EtOH/EA mixtures with different composition. In addition, the thermodynamic adsorption constants at low coverage (Henry adsorption constant K’ and adsorption enthalpy ∆H0) are investigated using Pulse Gas Chromatography (Pulse GC) in the high temperature range (100 – 200 °C).

2. Methodology 2.1 Materials All organic solvents are HPLC grade (purity > 99.8%). Ethyl acetate was obtained from Sigma Aldrich, while ethanol is supplied by LiChromSolv®. Deionized Millipore water was used during this experimental study. The commercial reference ZIF-8 (Basolite® Z1200) is produced by BASF and purchased from Sigma Aldrich. The structure of ZIF-8 is depicted in Figure 1. The kinetic diameter of ZIF-8 micropores is nominally 3.4 Å.23 ZIF-8 pellets were prepared from bulk powder samples, applying pressure through a hydraulic press and sieving the solid fractions to isolate particles with size between 200 and 400 µm; the integrity of the porous network was assessed by Ar porosimetry measurements at 87.45 K using an AUTOSORB-1 device. The sample was regenerated by degassing at 200 °C under vacuum for 4 hours.

2.2 Pulse Gas Chromatography Pulse Gas Chromatography (Pulse GC) experiments were performed by using an Agilent gas chromatography unit (4890D), equipped with a PAL-GC auto-sampler system. The 4 ACS Paragon Plus Environment

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liquid sorbates were injected (0.1 µl) through an injector inlet heated at 220 °C. The sorbate vapors are then carried into a stainless-steel column with Swagelock connectors (5 cm length, 0.3175 cm OD, 0.1585 cm ID) packed with 60 mg of ZIF-8 pellet (200 – 400 µm), by using He as inert carrier gas (20 Nml/min). The outlet of the column was connected to a flame ionizing detector (FID). The experiments were performed at temperatures between 120 °C and 160 °C, with intervals of 10 °C. The first order moment (µi) of the eluted peaks was used to calculate the Henry’s constant (K’) as explained in other studies.24 Mass transfer limitations are evaluated by performing multiple injections in isothermal conditions with different flow rate, to assess the linear dependency between the first order moment of the peak and the inverse superficial gas velocity. Linear regression results in R2 > 0.998 for all the tested compounds, in the flow rate range between 5 Nml/min and 50 Nml/min, indicating absence of mass transfer limitations. Further experiments were performed to evaluate the relationship between µi and the injected volume, as in Henry’s region (low coverage regime) the first order moment is not influenced by the concentration of the sorbates. The results of this preliminary investigation show that deviation from low coverage behavior are observed above an injection volume of 0.2 µl. The results of this preliminary investigation are available in supporting information (Figure S1 and S2). An injection volume of 0.1 µl has been used throughout the Pulse GC experimental study. The adsorption enthalpy at zero coverage (∆H0) and the Van’t Hoff pre-exponential factor (K’0) were calculated using the Van’t Hoff equation:

𝐾 ′ = 𝐾0′ 𝑒

−∆𝐻0 𝑅𝑇

Eq. (1)



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2.3 Vapor phase isotherm adsorption Single component adsorption isotherms of ethyl acetate, ethanol and water in ZIF-8 were measured at 303 K via dynamic gravimetric method using a SGA-100H microbalance system (VTI Corporation, USA). The activation was performed prior to each experiment, by heating the sample at 200 °C under N2 flow for 4 h with a thermal gradient of 4 °C/min. The equilibrium criterion was set for all the experiments at a mass change 10-1. According to the gas porosimetry results, ZIF-8 pelletization does not lead to significant porous structure disruption.

3.2 Single component adsorption The single-component vapor adsorption isotherms measured for ethanol, ethyl acetate and water at 30 °C on ZIF-8 pellets are reported in Figure 3. Despite the molecular size of ethyl acetate and ethanol being larger than the nominal pores mouth size (5.2 Å and 4.5 Å respectively), both organics are adsorbed. The rotation of the imidazole-linkers in the windows of the ZIF-8 pores allows the uptake of molecules which kinetic diameter is larger in respect to the nominal pores width.19,23,27 Adsorption equilibrium is achieved for both organics in relatively short time (30 – 120 minutes); further information about the uptake dependency on time are given in the supporting information (Figure S7). Moreover, crystal size has been proved to be strongly related to diffusivity, with the ZIF8 ACS Paragon Plus Environment

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8 crystals size being easily tunable (e.g. different synthesis procedure) in order to achieve faster diffusion of the sorbates.29 Generally, the affinity of ZIF-8 for the studied molecules decreases in the order ethyl acetate > ethanol >> water. At high relative pressure, ZIF-8 provides large adsorption capacity corresponding to 27.7 wt% for EtOH (P/P0 = 0.8) and 34.8 wt% for EA (P/P0 = 0.98). As reported before in literature,28 the adsorbed amount of water is negligible over the entire relative humidity range (5 – 90% RH). The water isotherm follows Type III isotherm shape,29 indicating unfavorable adsorption in full agreement with the hydrophobic nature of ZIF-8. This further upholds the conclusions of previous studies about polar compounds sorption onto this MOF.22,30 At saturation pressure, the SOD cages of ZIF-8 hold about 16.5 molecules of ethanol and approximately 21 molecules of ethyl acetate, as calculated on the bases of ZIF-8 unit cell formula (Zn12N48C96H120). The alcohol adsorption isotherm (Figure 3) expresses a sigmoidal shape, as an effect of the low sorbate-adsorbent affinity in the low-pressure region. Short alcohols such as methanol and ethanol give rise to similar adsorption isotherms on analogous ZIFs (e.g. ZIF-68 and ZIF-77);31,32 the S-shaped profiles can be related to molecular clusters formation and subsequent cage loading mechanisms. Evidence of micropores filling is observed at P/P0 exceeding 0.06, where the uptake of the alcohols rapidly approaches saturation capacity (23.2 wt% at P/P0 = 0.25). Micropores loading with EA occurs at tenfold lower relative pressure, and the saturation capacity (30.3 wt%) is already achieved at 0.115 P/P0. The complete overlap between adsorption and desorption branches for both molecules (not shown) implies the reversibility of the uptake process, which is of great interest for ethyl acetate/ethanol mixtures separation and recovery. 9 ACS Paragon Plus Environment


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A Langmuir-type (Type I) adsorption isotherm shape is observed for ethyl acetate, implying stronger interactions between ethyl acetate and ZIF-8 compared to ZIF-8/EtOH and ZIF-8/water pairs. To further investigate the low-pressure adsorption properties, the strength of the guest-host interactions at zero coverage has been evaluated through pulse gas chromatography, to extract Henry’s adsorption coefficients (K’) via the first order moment method. An overview of the obtained K’ values is given in Table 1; ethyl acetate adsorption onto ZIF-8 leads to higher values of K’ compared to EtOH at any experimental temperature. The EA adsorption enthalpy at zero-coverage equals 38.6 kJ/mol, while less energy is released (23.4 kJ/mol) upon ethanol sorption. This remarkable difference may be explained by considering the direct correlation between the number of non-polar CHx units composing the two different molecules. Ethyl acetate provides a larger number of carbon units (4) in respect to ethanol (2), resulting in larger guest-host affinity associated to non-polar interactions. The π-electrons on the ester carbonyl group may further enhance adsorption affinity as these chemical groups have larger polarizability than hydroxyl groups.33 When considering adsorption onto ZIF-8, a similar effect has been already disclosed in the context of alkenes adsorption, where the greater dipole moment increases selectivity over saturated hydrocarbons.34 The selectivity at low concentration (Henry’s region) is assessed accordingly to the proportion of the K’ values, EA/EtOH selectivity corresponds to 6.0 at the lowest experimental temperature (120 °C). At increasing temperature, EA/ETOH selectivity decreases linearly; K’ values at 30 °C were extrapolated to calculate selectivity at low temperature, which equals to 12.8. ZIF-8 demonstrates attractive separation potential for ethyl acetate over ethanol, especially at very low concentrations.


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3.3 Adsorption and separation of mixtures Mixture breakthrough experiments were carried out at 30 °C on ZIF-8 pellets. The separation of EA/EtOH mixture has been measured in dry conditions and in presence of different levels of relative humidity (20%, 50% and 90%). 3.3.1 EtOH/EA binary mixture adsorption In Figure 4, selected breakthrough profiles obtained at different sorbates partial pressure in absence of water are shown. EtOH/EA mixture adsorption give rise to non-classical breakthrough profiles. At an early stage, EA and EtOH are both adsorbed into ZIF-8 and retained in the adsorption column; subsequently, ethanol breakthrough is observed (at relatively short time), pointing at the weaker adsorption of EtOH as compared to EA. Its breakthrough profile progresses following sigmoidal shape (intermediate step), showing a continuous increase in concentration accompanied by a steep step and overshoot as soon as EA starts eluting. The EA breakthrough profile generates a first small step in outlet concentration, followed by a curve with continuously increasing gradient till the feed concentration is reached. As expected, the displacement of EtOH by EA results in a rollup (overshoot) in the EtOH breakthrough profile. It is well-known that the combination of favorable, Langmuir type (type I) isotherms with unfavorable (anti-Langmuir, sigmoidal) isotherms gives rise to complex breakthrough profiles.35,36 A further analysis of the shape of the breakthrough profiles goes beyond the scope of the present study. The corresponding binary adsorption isotherms (30 °C) are presented in Figure 5. Differently from the pure component adsorption isotherm, ethyl acetate exhibits a doublestep adsorption branch. This points at a complex adsorption mechanism, in which mutual 11 ACS Paragon Plus Environment


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interactions between EtOH and EA might result in enhanced adsorption under specific conditions. Alternatively, the flexibility of the ZIF-8 framework23,27 could be invoked to explain the step in EA adsorption; the mechanical properties of this MOF, explored in previous studies,23 allows structural transformation as the transition between low and high loading occurs. The spatial rearrangement of the 2-methyl-imidazolate linkers leads to the formation of structures with larger pores opening (similar results have been observed for vapor phase bio-butanol separation on ZIF-8).20 However, no experimental data are available to support such an hypothesis. The ethanol adsorption isotherm almost shape follows a linear trend, indicating limited guest-host affinity. The binary mixture adsorption isotherms (Figure 5) of EtOH/EA mixtures display an initial EA adsorption plateau (20.5 – 21.8 wt%, corresponding to about 8.5 molecules of EA per cage) at relatively low partial pressure (0.2 kPa). EA saturation capacity corresponds to 25 wt% at a vapor pressure of 5.4 kPa. The calculated number of EA molecules inside ZIF-8 crystal cages at 12.8 kPa is lower (16) in comparison with single component adsorption (21). Multi-component ethanol uptake is whereas significantly lower (11.0 wt% at 12.8 kPa) in respect to pure component capacity, and each unit cell only hosts 6.5 molecules at the highest experimental pressure. These results confirm the selectivity towards EA in adsorption of EA/EtOH mixtures on ZIF-8 in dry conditions, especially in low-pressure regime as expected according to the Henry’s adsorption constants for single components. 3.3.2 EtOH/EA/water ternary mixture adsorption The effect of humidity on organics adsorption has been appraised by studying ternary mixture adsorption of EtOH/EA/water at various level of relative humidity. Water molecules are released throughout ethyl acetate synthesis, due to intermolecular condensation reactions;1 therefore, the moisture concentration increase during the process 12 ACS Paragon Plus Environment

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must be contemplated when developing an efficient separation strategy. The EtOH/EA/water adsorption isotherms were measured while moisturizing the adsorption column at 20%, 50% and 90% relative humidity (respectively 0.85 kPa, 2.1 kPa and 3.8 kPa of H2O at 30 °C). Ternary mixture breakthrough profiles (90% RH) are reported in Figure 6. Water breakthrough occurs immediately, in accordance with the hydrophobic nature of ZIF-8. Nevertheless, the loading of ZIF-8 pores with EA/EtOH seems to enhance water uptake since the molar flowrate of water decreases below feed level as soon as EtOH starts to elute. This is most likely because of the enhanced hydrophilicity within the pores as the concentration of EtOH/EA in the adsorbed phase increases (blue square in Figure 6). At later time, adsorbed water is displaced by the more slowly propagating EA front, resulting in a second roll up in the breakthrough profile of water when EA also starts to elute (red square in Figure 6). The resulting ternary adsorption isotherms are reported in Figure 7. The presence of water decreases the adsorption of EA and EtOH, already from the lowest water vapor pressure (20% RH). At higher relative humidity (50%, 90%), the adsorbed amount of EA further drops, even though no additional loss of adsorption capacity is observed when the relative humidity is increased from 50% to 90%. Above a relative humidity of 20%, the step in the EA isotherm is no longer visible. The number of EA molecules adsorbed per ZIF-8 cages is reduced from 21 in dry, pure component conditions to 9.0 (20% RH), 4.2 (50% RH) and 4.0 (90% RH). In absence of water in vapor phase but in presence of EtOH around 16.5 ester molecules are adsorbed per cage.



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3.4 IAST/RAST modeling of binary/ternary mixture adsorption In first instance, the Ideal Adsorption Solution Theory (IAST/RAST) was used to predict the binary adsorption isotherms of the EtOH/EA mixtures studied in this work. Figure 8 shows the IAST predictions together with the experimental data. Although IAST gives a reasonable prediction of the amounts adsorbed at low pressure (especially for EA), its account overestimates EtOH adsorption. At higher pressure, IAST predicts incorrect tendencies, i.e. a decrease in EA adsorption instead of a plateau value and a steep increase in the amount of adsorbed EtOH. The highly non-ideal character of the EtOH/EA/ZIF-8 system is incompatible with the assumption of ideal behavior in IAST.

Therefore, Real Adsorption Solution Theory (RAST) was applied to estimate the deviation from ideality.37 Activity coefficients were fitted to obtain a match between experimental isotherm data and RAST predictions (Figure 8). The variation of adsorbed phase activity coefficients with EtOH partial pressure is shown in Figure 9. The activity coefficients of both EtOH and EA are a complex function of partial pressure, showing a decrease with pressure, followed by an intermediate plateau and a continuous increase. For EA, activity coefficients vary between 1 and 2, while for EtOH, values between 0.35 and 1 are obtained in the experimental pressure range. Although there is no simple physical explanation for such a tendency, this analysis clearly demonstrates the complexity of the current adsorbate/adsorbent system. On the other hand, it should also be stressed that this RAST approach has no predicative character outside the range of experimental conditions of this study, since the activity coefficients were extracted by fitting of experimental mixture isotherms. RAST method allows to calculate adsorption selectivity of for EA/EtOH pairs on ZIF-8 in dependence of the EtOH concentration in 14 ACS Paragon Plus Environment

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the adsorbed phase (Figure 10). As shown in Figure 10, selectivity is a complex function of composition. IAST underestimates the selectivity for EA, while RAST allows to match the experimental selectivity, ranging between 4.6 and 14.0 in the experimental range of conditions.

4. Conclusions Zeolitic Imidazolate Framework 8 (ZIF-8) demonstrates interesting vapor phase adsorption properties towards ethyl acetate, both as single-component or in mixture with ethanol and water. Considering the emerging importance of adsorption as a greener alternative to conventional separation processes, these results open possibilities to promising scenarios in the context of industrial ethyl acetate separation. The results shown within this manuscript demonstrates that ZIF-8 has large adsorption capacity for both ethyl acetate and ethanol. The hydrophobic behavior of this adsorbent prevents excessive water uptake despite the high relative humidity (> 90% RH). The complete reversibility of the sorption process, and relatively mild conditions needed to regenerate the ZIF-8 samples, are important assets. In line with the mentioned results, we disclosed that binary mixture adsorption capacity of EA into ZIF-8 is few times larger than EtOH uptake; EA adsorption occurs already at very low relative pressure (P/P0 = 0.007). Breakthrough experiments demonstrated highly non-ideal behavior during adsorption and separation of EtOH/EA mixtures on ZIF-8, perceptible via the presence of intermediate plateaus and steps in the breakthrough curves and through the strongly compositiondependent activity coefficients needed to adequately describe the adsorption equilibrium of the mixture. IAST and RAST method were successfully used to determine EA/ETOH 15 ACS Paragon Plus Environment


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pair selectivity, accounting for deviation from ideality as a function of the liquid sorbates activity coefficients. The results of modeling reported in this study clearly show significant selectivity towards EA, especially when low partial pressure is considered. Despite the hydrophobic nature of ZIF-8, water does affect organics sorption as the capacity of both ethanol and ethyl acetate significantly. The synergic effect of EtOH/EA/H2O adsorption leads to complex breakthrough showing enhanced water adsorption, most likely associated to increased hydrophilicity of ZIF-8 adsorption cages; however, the mechanisms behind this adsorptive behavior must be further investigated to provide a full understanding of non-ideal VCs mixture adsorption on ZIF-8. Nonetheless, even though the saturation capacity of the organic compounds is effectively reduced, the multi-component selectivity in presence of water vapors is comparable to that observed in dry conditions.

Acknowledgements Thomas Virdis, Valerii Danilov, Gino Baron and Joeri Denayer are grateful to FWO Vlaanderen, for financial support in frame of the SBO CheckPack project.


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NOTATION µi

first order moment, s

EA

ethyl acetate

EtOH

ethanol

FID

Flame Ionizing Detector

GC

Gas Chromatography

IAST

Ideal Adsorption Solution Theory

ID

Inner Diameter

K’

Henry’s constant, mol Kg-1 Pa-1

K’0

Van’t Hoff pre-exponential factor

LTA

Linde Type A

MOF

Metal Organic Framework

OD

Outer diameter

P0

saturation pressure, Pa

q

adsorbed amount, mg g-1

RAST

Real Adsorption Solution Theory

RH

relative humidity

S

selectivity

SOD

Sodalite cage

VCs

Volatile Compounds

VOCs

Volatile Organic Compounds

Vp

total pores volume, cm3 g-1

VPB

Vapor Phase Breakthrough

X

fraction in vapor phase

Y

fraction in adsorbed phase 17 ACS Paragon Plus Environment


ZIF

Zeolitic Imidazolate Framework

Zm

reference value

ΔH0

differential adsorption enthalpy, kJ mol-1

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Greek letters γ

activity coefficient

Subscripts/superscripts A

compound A, ethanol

B

compound B, ethyl acetate

EA

ethyl acetate

EtOH

ethanol

i

generic compound


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Table 1: summary of K’ values obtained with first order moment method from raw Pulse GC data. *Data is obtained through extrapolation from empirical measurements.

Compound

Temperature (°C)

K’ (mol Kg-1 Selectivity Pa-1) (α) 2.48 · 10-4

Ethanol 120 Ethyl acetate

1.47 · 10-3

6.0

2.15 · 10-4


1.10 · 10-3

5.1

1.75 · 10-4


8.47 · 10-4

4.8

1.51 · 10-4


6.40 · 10-4

4.2

3.89 · 10-3

Ethanol* 30 Ethyl acetate*

4.97 · 10-2

12.8

Figure 1: Full-atom structure of ZIF-8, showing the SOD cavities at the corners (yellow spheres).


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Figure 2: Adsorption isotherm of Ar measured at 77.3 K on ZIF-8 samples; Full and empty green triangles respectively indicate Ar adsorption and desorption on ZIF-8 reference powder (crystal size 0.5 µm); Full and empty black circles represent Ar adsorption and desorption on ZIF-8 pellets (200 – 400 µm).

Figure 3: single-component adsorption isotherms for EtOH, EA and water measured at 30 °C on ZIF-8

pellets (200 – 400 µm) via gravimetric analysis.



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Figure 4: breakthrough profiles for ethanol/ethyl acetate mixture at 30 °C. Round and triangular markers respectively represent EtOH and EA; Colors indicate sorbates partial pressure: cyan) EtOH: 2.2 kPa; EA: 1.8 kPA; blue) EtOH: 1.1 kPa; EA: 1.0 kPA; purple) EtOH: 0.77 kPa; EA: 0.75 kPA.

Figure 5: multicomponent adsorption isotherms of ethyl acetate/ethanol binary mixture (50:50 v/v, X EtOH = 0.63; XEA = 0.37), measured on ZIF-8 pellets (200 – 400 µm) in dry conditions (0% RH).


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Figure 6: breakthrough profiles for ethanol/ethyl acetate/water vapor mixtures (90% RH) at 30°C and different pressures (partial pressure is 2.4 kPa for EtOH, 2.0 kPa for EA). Round (grey) and triangular (red) markers respectively represent EtOH and EA, square markers (blue) show water concentration profile.

Figure 7: EtOH (left) and EA (right) binary mixture adsorption isotherms measured at 30 ̊C, in dry conditions (0% RH) and at different levels of humidity (20%, 50% and 90%).



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Figure 8: EtOH/EA binary mixture adsorption isotherm (30 °C) together with IAST/RAST predictions. Points correspond to the experimental mixture isotherm data obtained on ZIF-8 pellets.

Figure 9: Estimated adsorbed phase activity coefficients for ethyl acetate/ethanol vapor mixture, extracted from experimental binary mixture adsorption isotherm measured on ZIF-8 pellets (30°C).


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Figure 10: Selectivity predictions from IAST/RAST model, calculated for EA/EtOH pairs in function of the increasing concentration of EtOH in gas phase.

Abstract graphic (4.0 cm x 8.4)



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ASSOCIATED CONTENT Supporting Information Available: additional information on the experimental equipment and the mathematical model used throughout this study: 1) preliminary tests on pulse gas chromatography to assess mass transfer limitation and optimal injection volume; 2) technical information about the gravimetric apparatus used to measure single component adsorption capacity; 3) technical information regarding the vapor phase breakthrough apparatus used to measure multi-component adsorption breakthrough profiles; 4) mathematical method for the calculation of sorbates partial pressure; 5) insight on the IAST/RAST method proposed to model experimental data obtained during this study; 6) Information about single component adsorption kinetics exploring the dependency of equilibrium time on the sorbates partial pressure.


Water Mixtures

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