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Improving Gas Separation Performance of ZIF-8 by [BMIM][BF] Incorporation: Interactions and Their Consequences on Performance Burak Koyuturk, Cigdem Altintas, F. Pelin Kinik, Seda Keskin, and Alper Uzun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00848 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 24, 2017

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The Journal of Physical Chemistry

Improving Gas Separation Performance of ZIF-8 by [BMIM][BF4] Incorporation: Interactions and Their Consequences on Performance Burak Koyuturk, Cigdem Altintas, F. Pelin Kinik, Seda Keskin,* and Alper Uzun*

Department of Chemical and Biological Engineering, Koç University, Rumelifeneri Yolu, 34450 Sariyer, Istanbul, Turkey Koç University TÜPRAŞ Energy Center (KUTEM), Koç University, Rumelifeneri Yolu, 34450 Sariyer, Istanbul, Turkey *Corresponding authors: [email protected], [email protected]

ABSTRACT Gas separation performance of zeolitic imidazolate framework (ZIF-8) was improved by incorporating an ionic liquid (IL), 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]). Detailed characterization based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed that the morphology of ZIF-8 remains intact upon IL incorporation up to 28 wt%. Thermogravimetric analysis indicated the presence of direct interactions between the IL and metal organic framework (MOF). FTIR spectroscopy illustrated that the anion of the IL was shared between the imidazolate framework and [BMIM]+ cation. Adsorption isotherms of CO2, CH4, and N2 measured for pristine ZIF-8 and IL-loaded ZIF-8 samples, complemented by Grand Canonical Monte Carlo (GCMC) simulations, showed that these interactions influence the gas uptake performance of ZIF-8. CH4 and N2 uptakes decreased in the whole pressure range, while CO2 uptake first increased by approximately 9% at 0.1 bar in 20 wt% IL-loaded sample, and then, decreased as in the case of other gases. As a result of these changes in gas uptakes occurring at different extend, the corresponding CO2/CH4, CO2/N2, and CH4/N2 selectivities enhanced especially at low

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pressure regime upon IL incorporation. Results showed that CO2/CH4 selectivity increased from 2.2 to 4, while CO2/N2 selectivity more than doubled from 6.5 to 13.3, and CH4/N2 selectivity improved from 3 to 3.4 at 0.1 bar at an IL loading of 28 wt%. The heat of adsorption values (Qst) measured and simulated for each gas on each sample indicated that interactions between the IL and ZIF-8 strongly influence the gas adsorption behaviors. The change in Qst of CO2 upon IL-incorporation was more significant than that of other gases, leading to an almost doubling of CO2 selectivity over CH4 and N2, specifically at low pressures. On the other hand, the selectivity improvement was lost at high pressures because of a strong decrease in the available pore space due to the presence of IL in ZIF-8. These results suggest that such IL/MOF combinations with tunable structures have huge potential towards high performance gas separation applications.


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1. INTRODUCTION Adsorption-based gas separation processes by nanoporous materials are widely used because of their low energy demands and environmentally friendly natures. Traditional nanoporous materials, such as zeolites, activated carbons, and carbon nanotubes have been widely studied for these processes, especially for the separation of CO2 from CH4 and N2.1 Identifying new materials that can achieve CO2/CH4 and CO2/N2 separations with high selectivities is desired as natural gas purification and flue gas separation play important roles to mitigate the CO2 emissions. Among various nanoporous materials, metal organic frameworks (MOFs), which are composed of metals and organic linkers, have recently been considered as promising materials for adsorption-based CO2 separations due to their high surface areas, large porosities, tunable pore sizes and shapes.2 The growing interest on MOFs mainly arises from the infinite number of combinations of metal ions and organic linkers with tunable pore structures, which leads to pre-determined design of materials to achieve a desired gas separation.3 A recent technology to fine-tune the gas separation performance of a MOF is to utilize ionic liquids (ILs). ILs are organic salts with low melting points (2 cm-1) resulting from the strengthening of the C2-N bond. The corresponding shifts were -4, -6, and -6 cm-1, for IL4/ZIF-8, IL20/ZIF-8 and IL28/ZIF-8, respectively. Another peak at 1467 cm-1 assigned to ring δ(CH3) wagging shows red shifts of -8, -9, and -9 cm-1 for IL4/ZIF-8, IL20/ZIF-8, and IL28/ZIF-8, respectively. The remaining bands in the low frequency region were 1034 and 1046 cm-1 attributed to ν(BF4)asym. For each IL loading considered, the former one shifts to 1037 cm-1 (we note that this shift amount might be within our error range, considering the spectral resolution was 2 cm-1). The latter one, on the other hand, illustrates considerably high blue shifts of 14, 10, and 11 cm-1 on IL4/ZIF-8, IL20/ZIF-8, and IL28/ZIF-8, respectively. Similar to the changes in the band positions in lower region, bands in high wavenumber region also present shifts in their positions. Bands at 2939 and



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3113 cm-1 assigned to ring ν(CH3)sym and ring ν(CH3)asym are two of these bands. The ring ν(CH3)sym exhibits red shifts of -9, -7, and -9 cm-1 on IL4/ZIF-8, IL20/ZIF-8, and IL28/ZIF-8, respectively, and the ring ν(CH3)asym shows a red shift of -16 cm-1 on each IL/ZIF-8 samples. These red shifts indicate a weakening in the interaction between ring and methyl group in cation. The latter one in high frequency region (3121 cm-1) assigned to ν(C2H)sym on the ring shows the most significant shift on each sample; a blue shift of 26, 27, and 26 cm-1 for IL4/ZIF-8, IL20/ZIF-8, and IL28/ZIF-8, respectively. These changes in the band positions can be interpreted in different ways. However, based on the findings of a previous study,30 we interpret the interactions of ZIF-8 with the IL as described in the following: Due to the presence of imidazolium ring in ZIF-8, anion of IL interacts with MOF and receives electrons. This interaction lead to an increase in inner B−F bond strength, indicated by blue shifts in ν(BF4)asym. As anion prefers to interact more with the MOF, its interionic interactions weakens consistently with an increase in C2−H stretching in imidazolium ring.43-44 This change also leads to an increase in the electron density of cation and corresponding strengthening of the inner bonds in imidazolium ring. Stronger inner bonds can also be originated by the presence of hydrogen bonding between framework and cation. We also note that assignment of the IR features in bulk IL spectrum was done based on a DFT-based study reporting the most stable conformer of [BMIM][BF4],30 however ILs may have more than one stable conformer and depending on the conformers preferred inside the MOF cage, there may be different shifts observed in IR features. We suggest that detailed DFT-based studies can provide valuable information in the way of revealing these interactions between the ILs and MOFs in such IL-incorporated MOF materials in deep detail.23


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Figure 4. IR spectra of IL-incorporated MOFs, bulk [BMIM][BF4] and pristine ZIF-8: (a) 900-1600 cm-1, (b) 2800-3200 cm-1. 19 ACS Paragon Plus Environment


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Table 4. Shifts in IR bands of [BMIM][BF4] in the lower region.

Table 5. Shifts in IR bands of [BMIM][BF4] in the higher region.

As discussed above, detailed characterization of the samples prepared by the incorporation of [BMIM][BF4] into ZIF-8 provides direct evidence on the presence of interactions between anion of the IL and ZIF-8. These interactions can have a significant effect on the gas adsorption and separation performance of pure ZIF-8. Aiming at revealing the consequences of these interactions, volumetric gas adsorption experiments were carried out for CH4, N2, and CO2 in a pressure range of 0.1-10 bar at 298 K. Single-component gas adsorption isotherms of CH4 and N2 were fitted to the dual site Langmuir model, whereas CO2 uptakes were fitted to the Freundlich model. Experimental measurements on pristine ZIF-8 provide consistent data with the literature.16 Parameters of these models are provided in Table S5. GCMC simulations were also performed for each gas in the same pressure range and at the same temperature to complement the experimental measurements. Gas uptake results of experiments and simulations per gram of MOF measured up to 10 bar for the pristine MOF and IL/MOF samples with different IL loadings are given in Figure 5.


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Figure 5. Gas uptakes of ZIF-8 and IL-incorporated ZIF-8 obtained from experiments and simulations: (a) ZIF-8, (b) IL4/ZIF-8, (c) IL20/ZIF-8, (d) IL28/ZIF-8. Filled symbols: experimental uptake values, empty symbols: simulated uptake values multiplied with the factors given in Table S4. 22 ACS Paragon Plus Environment

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We note that IL molecules were loaded into ZIF-8 cages with fixed loading amounts as determined by the ICP-MS measurements to maintain consistency for the GCMC calculations. However, because of the limitations originated from the force field (UFF) that was used for IL loading, a maximum IL amount of 25 wt% could be reached for CO2. As illustrated above, there are interactions between MOF and IL molecules resulting in changes in electron densities on both MOF and IL. Thus, uptake values calculated for IL-incorporated ZIF-8 samples reported in Figure 5 were multiplied by a pressure dependent factor (f) as defined in Equation 3 in “Computational Methodology” section to fit the simulation results to the results of experimental gas uptake measurements. The corresponding m and n parameters for each IL-incorporated sample are given in Table S4 in SI. For comparison, unmodified simulation results are also given in Figure S7. Considering the values reported in Table S4, the strongest modification of the simulated results was needed for the CO2 uptake of IL/ZIF-8 samples. This can be explained with the strong interactions between MOF and IL, which do not behave as bulk species. As stated in the interpretation of the IR spectra, electronic interactions between the IL and MOF lead to a change in the partial charges of atoms when IL was incorporated into the MOF. This result might imply that modification of the sites for gas adsorption by the incorporation of IL was more pronounced for the CO2 adsorption, as we discussed further below. In Figure 6, gas uptakes of IL-incorporated MOF samples were compared with their corresponding values of pristine ZIF-8. Uptake values for CH4 and N2 become lower throughout the entire pressure range upon the IL incorporation. However, the case is different with CO2 on IL20/ZIF-8, for which the uptake value becomes slightly higher than its counterpart on pristine ZIF-8 up to 0.2 bar. For instance, at 0.1 bar it reaches a maximum of approximately 108.6% of its value on pristine ZIF-8. As the IL loading increased, a higher degree of decrease in uptakes is expected at high pressures because of a decrease in the



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available pore space of MOF as shown in Table 1. However, changes in the uptake behavior of IL/MOF samples for different gases specifically at low pressures cannot be simply described by a decrease in the available space of the framework. Interactions between the IL and MOF can either create new sites for gas adsorption or strongly modify the existing ones. This influence is more significant at low pressures, where the interactions between the adsorbates and adsorption sites is the dominant factor (not the available space) controlling the gas adsorption behavior.23


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Figure 6. Uptake change (%) of CO2, CH4, and N2 with respect to IL loading (wt%): (a) 0.1 bar, (b) 0.2 bar, (c) 1 bar, (d) 5 bar. Dotted lines are provided to guide the eye.

Results in Figure 6 illustrate different degrees of changes on the uptakes of different gases in IL/MOF samples prepared with different IL amounts. These results suggest that the gas separation performance of ZIF-8 can be tuned by incorporating ILs. To reveal the consequences of the changes in gas uptakes on the separation performance of the MOF, ideal



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gas selectivities of samples were calculated by dividing the higher gas uptake to lower one. Figure 7 shows that the ideal gas selectivities of IL-loaded ZIF-8 differ considerably from those observed on pristine ZIF-8. Relative selectivities calculated by normalizing the individual selectivities of IL-loaded samples to the corresponding selectivities of ZIF-8 for CO2/CH4, CO2/N2, and CH4/N2 are given in Figure 7a, 7b, and 7c, respectively. CO2/CH4, CO2/N2, and CH4/N2 selectivities of ZIF-8 and IL-loaded ZIF-8 samples are also shown in Figure S8. Data show that significant improvements were obtained especially for IL28/ZIF-8. According to Figure 7(a), IL28/ZIF-8 exhibits approximately 1.8 times higher CO2/CH4 selectivity than that of pristine ZIF-8 at 0.1 bar. Data also indicate that as the IL loading decreases, CO2/CH4 selectivity of IL-loaded sample approaches to the selectivity of ZIF-8. Higher separation performances observed at low pressures can be attributed to the higher electrostatic interactions between CO2 and IL-incorporated MOF samples. The best performances of IL-incorporated MOF samples were observed for CO2/N2 selectivities of IL20/ZIF-8 and IL28/ZIF-8. At 0.1 bar, it reached to 2 and 1.3 times of the corresponding values of pristine ZIF-8, respectively. Among all samples considered with different IL loadings, only IL20/ZIF-8 underperformed CH4/N2 selectivity of ZIF-8. CH4/N2 selectivities of IL28/ZIF-8 and IL4/ZIF-8 samples slightly surpass the selectivity of pure ZIF-8, since the difference between the amount of decrease in CH4 and N2 uptakes is lower than the other pairs.


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Figure 7. Normalized selectivities of samples (the ratio of IL/ZIF-8 samples to the selectivity of pure ZIF-8) for (a) CO2/CH4, (b) CO2/N2, (c) CH4/N2 separations.

The normalized selectivities of IL28/ZIF-8 were compared with the normalized selectivities of [BMIM][BF4]/Cu-BTC22 and [BMIM][PF6]/ZIF-823 that we previously studied and results given in Table 6. Before comparing the separation performances of ILincorporated MOFs, we first compared the ideal selectivities of the pristine ZIF-8 and CuBTC. It is seen that Cu-BTC has a higher separation performance for all gas pairs than ZIF-8, which can be attributed to the open metal sites available in Cu-BTC. Since these open metal sites are favorable for CO2 molecules,3 it is expected to see this kind of separation behavior. When the performance of two different MOFs, ZIF-8 and Cu-BTC, are compared in the presence of the same IL, [BMIM][BF4], our data summarized in Table 6 illustrates that ZIF-8 in general provides better performance. In another words, incorporation of [BMIM][BF4] improved the separation performance of ZIF-8 for CO2/CH4 and CO2/N2 but had a negative 28 ACS Paragon Plus Environment

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effect on the CO2/CH4 and CO2/N2 separation performance of Cu-BTC except at 0.1 bar for CO2/N2. This is probably because of the occupation of the open metal sites of Cu-BTC by IL molecules, however, there is no such case in ZIF-8. Therefore, the effect of IL is clearer on the gas separation performance of ZIF-8. These results indicate that the type of MOF plays a major role in determining the separation performance of the IL/MOF sample even though the same IL is considered. The same case is also valid for the type of IL. When two different samples were prepared with the same MOF using different ILs (such as [BMIM][PF6]- and [BMIM][BF4]-incorporated ZIF-8 samples), the separation performance varies significantly by the change in IL type. In [BMIM][BF4]/ZIF-8 and [BMIM][PF6]/ZIF-8 composites, different ILs might be affecting the performances because of the differences in their bulkphase physicochemical properties. The latter one showed better selectivity for all gas pairs except for CH4/N2. The only difference between these two systems is the anion of the IL. The bulk IL with [PF6]- anion is more favorable for CO2 because of the fact that it is more apolar than [BF4]- anion, therefore its affinity for CO2 is higher.46 Consequently, [BMIM][PF6] incorporated ZIF-8 showed better selectivity for CO2. Besides these effects, the presence of interactions between IL and MOF at different levels as illustrated by TGA and IR results might also play a major role in determining the selectivities of these two composites with the same MOF. According to TGA results, decomposition temperature of [BMIM][PF6]/ZIF-8 decreased to 236 °C,23 however the corresponding value for [BMIM][BF4]/ZIF-8, with the same IL loading, was 274 °C. These different T′onset were attributed to the interactions which are indicated by different degrees of shifts in different features of ILs observed in the IR spectra. For example, the shift in C2H band of [BMIM][BF4] (26 cm-1) is much larger than that of [BMIM][PF6] (5 cm-1). Also, after incorporation of [BMIM][BF4], remarkable red shifts were observed in ring ν(CH3)sym and ring ν(CH3)asym vibrations whereas no shifts were observed in the case of [BMIM][PF6]. As a result of these different behaviors, different



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performances were observed. These results and our ongoing studies illustrate that different combinations of IL-MOF pairs lead to different separation performances as they do not interact in the same way. Thus, we infer that each IL-MOF pair requires a separate investigation focusing on the elucidation of interactions and their consequences on the performance. Investigations on the effects of systematic changes in the structures of IL and MOF would add a significant value for the rational design of such novel materials with exceptional separation performance. Table 6. Comparison of normalized selectivities of [BMIM][BF4]/ZIF-8 with the normalized selectivities of other IL/MOF samples studied in our previous works.*22-23 IL loading in MOFs are 30, 26 and 28 wt% for [BMIM][BF4]/Cu-BTC, [BMIM][PF6]/ZIF-8, and [BMIM][BF4]/ZIF-8, respectively. [BMIM][BF4]/Cu-BTC22 [BMIM][PF6]/ZIF-823 [BMIM][BF4]/ZIF-8 Normalized pressure (bar) pressure (bar) pressure (bar) selectivities 0.1 0.5 1 0.1 0.5 1 0.1 0.5 1 0.69 0.64 0.64 4.08 2.54 2.02 1.82 1.52 1.42 CO2/CH4 1.09 0.84 0.73 3.70 2.27 1.79 2.04 1.69 1.57 CO2/N2 1.56 1.30 1.15 0.91 0.90 0.88 1.12 1.11 1.11 CH4/N2 *Ideal selectivities of Cu-BTC (ZIF-8) are 5.51, 5.70 and 5.57 (2.18, 2.43 and 2.50) for CO2/CH4; 18.27, 17.52 and 15.81 (6.54, 7.23 and 7.39) for CO2/N2; 3.35, 3.09 and 2.88 (2.99 2.97 and 2.95) for CH4/N2 at 0.1, 0.5 and 1 bar, respectively.

Aiming at revealing the reasons for improvements in selectivities as shown in Figure 7, isosteric heat of adsorption (Qst) values for each gas on pristine ZIF-8, IL4/ZIF-8, IL20/ZIF-8, and IL28/ZIF-8 were measured. Qst values of CO2, CH4, and N2 adsorbed on ZIF-8, shown in Figure 8, are consistent with the literature.33 Data show that the Qst of CO2 significantly increased from approximately 18 to 29 kJ/mol from ZIF-8 to IL28/ZIF-8 with an increase in the IL loading. Similar improvements in Qst were also observed for CH4 with increased IL loading. Qst of CH4 increased from 13 kJ/mol in pristine ZIF-8 to 20 kJ/mol in IL28/ZIF-8. The case is similar with N2, for which it increased from approximately 8 to 13 kJ/mol upon the incorporation of IL into ZIF-8 with a loading of 28 wt%. 30 ACS Paragon Plus Environment

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Figure 8. Isosteric heat of adsorption of: (a) ZIF-8, (b) IL4/ZIF-8, (c) IL20/ZIF-8, and (d) IL28/ZIF-8 calculated by Equation 2 using the adsorption isotherms obtained at 10 and 25 °C.


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Consequences of the changes in the heat of adsorption values on the gas selectivities were investigated by calculating the differences in Qst values (∆Qst) of CO2, CH4, and N2 for the corresponding selectivities on each sample as shown in Figure 9. Simulation results agreed well with the experimental results according to Figure 9. An increase in ∆Qst of the gas pair leads to an increase in corresponding selectivities especially for CO2/CH4 and CO2/N2 at low pressures such as 0.1 bar. However, at high pressures, for example at 5 bar, the change in ∆Qst does not significantly affect the gas selectivities. Thus, we infer that the interaction between MOF and IL is the dominant factor especially at low pressures influencing strongly the Qst values of respective gases as we previously discussed.23 This is why the effect of IL on the gas selectivity becomes dominant at low pressures. However, we note the increase in ∆Qst does not lead to a significant increase in the CH4/N2 selectivity. This behavior might suggest the presence of other factors; for instance, changes in pore volume (available space) might be a dominant factor for this gas pair.

Figure 9. Change in the isosteric heat of adsorption of corresponding gases in ZIF-8 and IL28/ZIF-8 with respect to the gas selectivities. Closed (open) symbols show selectivity of ZIF-8 (IL28/ZIF-8) sample. Each symbol indicates different pressure: rectangle (■): 0.1 bar, triangle (▲): 1 bar, circle (●): 5 bar. 33 ACS Paragon Plus Environment


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4. CONCLUSIONS One of the most commonly used ILs, [BMIM][BF4], was incorporated into a robust MOF, ZIF-8, at different loadings to investigate the consequences of IL incorporation on the gas uptake and separation performance of the MOF. Detailed characterization confirmed that both IL and ZIF-8 maintain their structural integrity upon IL incorporation. TGA results suggested the presence of direct interactions between IL and MOF, significantly modifying the thermal decomposition mechanisms. IR results provided additional evidence on the presence of these interactions and indicated that anion of the IL was shared between the imidazolate linkers of ZIF-8 and [BMIM]+. The consequences of these interactions on gas uptake and separation performance of ZIF-8 were investigated using volumetric uptake measurements complemented by the GCMC simulations. Data suggest that IL incorporation strongly modifies the individual uptake values of each gas at a different extent. At low pressures, CO2 uptake increases to the values higher than their corresponding counterparts in pristine ZIF-8, whereas CH4 uptake becomes lower than the one in parent material at similar conditions. As a result of this difference, the gas separation performance of the IL-loaded ZIF-8 was significantly modified. Results indicated that CO2/CH4 selectivity becomes almost twice of its value on pristine ZIF-8 at low pressures. Further analysis of the results indicated that the presence of IL modifies the gas adsorption behavior of MOF by significantly improving the heat of adsorption values. For instance Qst of CO2 increased by almost twice as much of that of N2 does. Because this increase occurs at different extents for each gas, increase in the CO2 being the largest, the MOF becomes more selective towards CO2 upon the incorporation of [BMIM][BF4]. Results presented here illustrate a huge potential towards tuning the gas storage and separation performance of MOFs by IL incorporation. For the rational design of such materials the interactions between ILs and MOFs, and their consequences on the performance should be considered. 34 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information Fit parameters for CO2, CH4 and N2 measured at different temperatures for ZIF-8, LJ parameters of ZIF-8 and [BMIM][BF4] used in simulations, LJ parameters and charges of CO2, CH4 and N2 used in simulations, coefficients used in the factor to match the simulated CO2, CH4, and N2 adsorption isotherms with the experiments, parameters of dual site Langmuir and Freundlich fits, EDX results on IL28/ZIF-8, linear correlations for IL loadingsurface area and IL loading-pore volume, N2 isotherms of ZIF-8 and [BMIM][BF4]/ZIF-8 samples at 77 K, derivative weight change of ZIF-8 and IL/ZIF-8 samples, deconvoluted IR peaks between 900-1600 cm-1 for IL4/ZIF-8, IL20/ZIF-8, IL28/ZIF-8 and ZIF-8, deconvoluted IR peaks between 2800-3200 cm-1 for IL4/ZIF-8, IL20/ZIF-8, IL28/ZIF-8 and ZIF-8, gas uptakes of ZIF-8 and IL-incorporated ZIF-8 obtained from experiments and simulations for IL4/ZIF-8, IL20/ZIF-8, IL28/ZIF-8 shown together with unmodified computational results, ideal adsorption selectivities of ZIF-8 and [BMIM][BF4]/ZIF-8 samples calculated from fitted isotherms. ACKNOWLEDGMENTS This work is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) under 1001-Scientific and Technological Research Projects Funding Program (project number: 114R093) and by Koç University Seed Fund Program. Support provided by the KUTEM (Koç University TUPRAS Energy Center) is gratefully acknowledged. A.U. acknowledges the BAGEP Award of Science Academy of Turkey and TUBA-GEBIP Award. Authors thank to Koç University Surface Science and Technology Center (KUYTAM) for providing help with the sample characterization.



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