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Tuning gas separation performance of CuBTC by ionic liquid incorporation Kutay Berk Sezginel, Seda Keskin, and Alper Uzun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04123 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 14, 2016

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Tuning gas separation performance of CuBTC by ionic liquid incorporation Kutay B. Sezginel,a,b Seda Keskin,a,b,* Alper Uzuna,b,* a

Department of Chemical and Biological Engineering, Koc University, Rumelifeneri Yolu, Sariyer, 34450, Istanbul, Turkey;

b

Koç University TÜPRAŞ Energy Center (KUTEM), Koç University Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey; *Corresponding Authors: [email protected]; [email protected]

1 Environment ACS Paragon Plus

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ABSTRACT Efficient separation of gases has industrial, economic, and environmental importance. Here, gas separation performance of a metal organic framework (MOF) is enhanced by ionic liquid (IL) incorporation. One of the most commonly used ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), was incorporated into a commercially available MOF, CuBTC. Detailed characterization by combining spectroscopy with diffraction, electron microscopy, and thermal analysis confirmed that the structures were intact after incorporation. Adsorption isotherms of CH4, H2, N2, and CO2 in IL-incorporated CuBTC were experimentally measured and compared with those of pristine CuBTC. Consequently, ideal selectivities for CO2/CH4, CO2/N2, CO2/H2, CH4/N2, CH4/H2, and N2/H2 separations were calculated. Results showed that CH4 selectivity of CuBTC over CO2, H2, and N2 gases becomes at least 1.5 times higher than that of pristine CuBTC upon the incorporation of IL. For example, CH4/H2 selectivity of CuBTC increased from 26 to 56 at 0.2 bar when the IL loading was 30 wt%. These results show that incorporation of ILs into MOFs can lead to unprecedented improvements on gas separation performance of MOFs. The tunable physicochemical properties of ILs combined with a large number of possible MOF structures open up opportunities for rational design of novel materials for meeting future energy challenges.


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1. Introduction Adsorption-based gas separation has been a powerful alternative to the traditional separation methods, such as distillation, due to its low cost, energy efficiency, and smaller ecological footprint. Metal organic frameworks (MOFs) have been considered as promising alternatives to well-known nanoporous materials in adsorption-based gas separation applications because of their high porosities, large surface areas, low densities, and good mechanical and chemical stabilities. One of the most important advantages of MOFs is the possibility to generate multiple materials with varying pore sizes and chemical functionalities.[1] Several experimental and computational studies show that there are many MOFs that exhibit high adsorption selectivity in CO2/CH4, CH4/H2, CO2/N2, and CO2/H2 separations.[2] Remarkable structural properties and highly tunable nature of MOFs suggest that it is possible to find an ideal MOF for a specific gas separation. MOFs can be functionalized to create stronger adsorption sites for gas molecules and to increase gas selectivity of materials.[3] However, functionalization of MOFs is limited with the type of MOF and its available sites for the functionalization. One alternative to functionalization is incorporation of ionic liquids (ILs) into MOFs. ILs, salts with melting points below 100 °C, offer a wide range of physical and chemical properties as a result of almost unlimited number of possible combinations of different anion-cation pairs.[4] In theory, it might be possible to obtain ILs with high solubility towards gases, such as CH4 and CO2, and this solubility can be modified by tailoring the ion pairs. Majority of the studies that consider both MOFs and ILs have focused on utilization of ILs as reaction media in the synthesis of MOFs,[5] in addition to the studies on catalysis[6], phase behavior.[7] Aijaz et al.[8] fabricated high surface area, N- and BN- decorated nanoporous carbons by the carbonization of an IL-impregnated MIL-100(Al), (and thus, by


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losing IL and MOF structures), and reported remarkable CO2 and H2 adsorption capacities. Limited number of studies focused on adsorption-based separation performances of ILincorporated MOFs. These studies are all computational and employed molecular-level simulations. Chen et al.[9] examined the microscopic properties of [BMIM][PF6] in IL/IRMOF-1 composite and its capability for CO2 capture. They showed that even though CO2 uptake of ILincorporated IRMOF-1 decreases, its CO2/N2 selectivity increases compared to pristine IRMOF1. Gupta et al.[10] used molecular simulations to examine CO2 capture of IRMOF-1 supported IL membranes and concluded that these membranes outperform polymer membranes in CO2 permeability. Vicent-Luna et al.[11] performed molecular simulations to study the effect of different ILs with [EMIM]+ cation and various anions on CO2 adsorption and selectivity of CuBTC. They showed that the presence of ILs in the MOF’s pores enhances CO2 adsorption at low pressures, which leads to higher adsorption selectivity for CO2 over CH4 and N2. Lei et al.[12] investigated CO2 solubility in mixtures of ILs and ZIFs (zeolitic imidazolate frameworks, a subgroup of MOFs). They concluded that individual CO2 solubility of ILs and CO2 adsorption of ZIFs can be used to predict CO2 solubility of IL/ZIF systems. Li et al.[13] carried out molecular simulations to investigate the performance of IL-incorporated Cu-TDPAT for separation of H2S/CH4 mixture. [BMIM]+ cation with four types of anions was incorporated into Cu-TDPAT framework at different loadings. IL-incorporated Cu-TDPAT exhibited significantly higher H2S selectivity compared to pristine Cu-TDPAT. As evident from these computational studies, significant improvements can be obtained on the gas adsorption and gas separation performance of MOFs by the incorporation of ILs into their structures. Although recent experimental studies show that ILs can be used either as supports or fillers in MOF membranes to improve membrane-based gas or liquid separation


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performance,[14] to the best of our knowledge, there is no experimental study illustrating such performance improvement for adsorption-based gas separations including CO2/CH4, CO2/N2, CO2/H2, CH4/N2, CH4/H2, and N2/H2. In this work, we aim to perform an experimental proof-ofconcept study to examine the effects of IL incorporation on the adsorption-based gas separation performance of a MOF. We started with one of the most commonly examined MOFs, CuBTC, and incorporated one of the most commonly used ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), in it. CuBTC was chosen as it is reported to be a MOF with high CH4 storage capacity as well as being commercially available.[15] [BMIM][BF4] was selected since it is fairly cheap compared to other ILs so that its incorporation into the CuBTC does not significantly affect the material cost. Detailed characterization was performed by combining spectroscopy with diffraction, electron microscopy, and thermal analysis to check whether [BMIM][BF4] and CuBTC remain intact after incorporation. Adsorption isotherms of CO2, CH4, N2, and H2 gases in CuBTC and [BMIM][BF4]-incorporated CuBTC were experimentally measured using volumetric method at room temperature up to 100 bar. Using isotherm data, ideal selectivities of CuBTC and [BMIM][BF4]-incorporated CuBTC for CO2/CH4, CO2/N2, CO2/H2, CH4/N2, CH4/H2, and N2/H2 separations were compared to elucidate the consequences of IL incorporation on gas separation performance of MOFs. Data presented exceptionally promising results in the way of tuning the gas storage and separation performance of MOFs. 2. Methods 2.1.Materials and sample preparation

[BMIM][BF4], acetone (≥ 99.5%), ethanol (≥ 99.8%), CuBTC were purchased from Sigma-Aldrich. [BMIM][BF4] and CuBTC were stored in Ar glovebox (Labconco) and fresh 5 Environment ACS Paragon Plus

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materials were used for sample preparation. CuBTC was dehydrated at 105 °C overnight before sample preparation and no further purification was performed for other materials. For gas adsorption measurements CH4 (99.95 vol%), N2 (99.998 vol%), and CO2 (99.9 vol%) gases were purchased from Linde gas company and H2 (99.999 vol%) and He (99.999 vol%) gases were purchased from Messer gas company. IL-incorporated CuBTC samples were prepared in open atmosphere and the samples were dried after preparation and stored in sealed glass containers in desiccator to minimize water and impurity content. Two solvents were considered for sample preparation and acetone was selected as ethanol had destructive effects on the structure (as discussed in SI). As an example for the preparation of [BMIM][BF4]/CuBTC samples, 0.15 g of [BMIM][BF4] was dissolved in 15 ml of acetone in an empty glass container. 0.35 g of dehydrated CuBTC was added into the solution and the resulting mixture was continuously stirred at 30 °C for 6 hours. The solvent was evaporated and the resulting sample was dried in vacuum oven at 105 °C overnight. The resultant blue solid was labelled as [BMIM][BF4]/CuBTC (30 wt%) according its weight percent. Other samples with 5 and 20 wt% loadings of [BMIM][BF4] were prepared in a similar manner. The samples were kept sealed in desiccator to minimize their humidity and impurity contents. 2.2.Characterization and performance measurement methods 2.2.1. Scanning electron microscopy (SEM). A Zeiss FESEM Ultra Plus and a Zeiss Evo LS 15 were used for SEM imaging. In Zeiss FESEM Ultra Plus, samples were analyzed under vacuum with an accelerating voltage of 1 kV and working distance around 4 mm and the micrographs of the samples were obtained at different magnifications including: 20Kx, 10Kx, 2Kx, and 500x. Similarly, samples were analyzed under vacuum with an accelerating voltage of 1 kV and working distance around 10 mm in Zeiss Evo LS 15. In this equipment the SEM images were


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obtained up to 2Kx magnification due to limitations of the instrument. For both instruments secondary electron detectors were used. The sample preparation was performed in a similar manner for both equipment and for all samples investigated in this study. Approximately 5 mg of samples were placed on carbon tape and excess samples on the carbon tape surface were discarded. The samples were placed in the instrument and measurements were performed under high vacuum to remove impurities on the surface. 2.2.2. X-Ray diffraction. A Bruker D8 Advance with DAVINCI design was used with Cu Kα1 radiation source employing a wavelength of 1.5418 Å. The power rating of X-ray generator was adjusted to 40 kV and 40 mA and Vantec-1 detector was used with a slit size of 1 mm. The measurement was performed between 2Θ values of 2-90° with a step size of 0.01263°. For the sample preparation, approximately 10 mg of sample was placed on a double-sided tape attached on a glass slide. The glass slide was placed on a gum and levelled with the sample holder using a smooth glass piece. The sample was spread thoroughly to cover double-sided tape to prevent amorphous signals coming from the tape. In measurements where a slight background is observed due to tape, background subtraction was performed. 2.2.3. Thermal gravimetric analysis (TGA). For TGA experiments TA Instruments Q500 was used with a platinum pan. Initially, a clean pan was placed on the balance and set to tare. Approximately 15 mg of sample was weighed and the furnace was closed. Constant temperature ramp rate of 2 °C/min was employed up to 700 °C under N2 flow of 40 and 60 ml/min for balance and purge gases respectively. For isothermal treatment measurements, first a constant temperature ramp rate of 10 °C/min was employed up to 175 °C and then the temperature was kept constant for 15 h under same N2 flow conditions. After experiment was finished, platinum pan was cleaned carefully to get rid of any residuals. For the comparison of thermal


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decomposition temperatures, the onset (Tonset) and derivative onset (T′onset) temperatures were determined by extrapolation (within an error range of ± 3 °C) from TG and derivative TG curves. Tonset generally overestimates thermal decomposition temperature compared to T′onset values. Thus, in this study mainly T′onset values were considered. 2.2.4. Fourier transform infrared (FT-IR) spectroscopy. FT-IR measurements were performed on a Bruker Vertex 80v IR spectrometer equipped with an attenuated total reflection (ATR) cell and a room temperature DLaTGS detector. The data was collected between spectral range of 4000-500 cm-1 with 4 cm-1 resolution. For both background and sample measurements 128 scans were obtained with a scanner velocity of 10 kHz. Approximately 10 mg sample was placed on top of ATR crystal and measurements were performed at ambient conditions. The ATR crystal and the tip of the compression rod were cleaned gently with ethanol before each measurement. Extended ATR correction, atmospheric compensation, and baseline correction were performed for each IR spectrum. The deconvolution of the signals were performed using Fityk software. [16]34 2.2.5. Brunauer–Emmett–Teller (BET) surface area. For BET analysis, Micromeritics ASAP 2020 - Physisorption Analyser was used. For each measurement approximately 100 mg of sample was used and the samples were activated under vacuum in two steps at 90 °C and 150 °C for 1 h and 10 h, respectively. After activation, sample was cooled down to 77 K using liquid nitrogen and free space measurement was performed with helium gas. The sample was then held under vacuum for an additional 10 h to remove residual helium gas. Then the volumetric N2 adsorption was measured between 10-6 and 1 bar. 2.2.6. Elemental analysis. For elemental analyses in this study Thermo Scientific Flash 2000 CHNS/O analyser was used to determine C, H, and N contents of the materials. The sample was


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weighed in a tin capsule and then placed into an oxidation/reduction reactor kept at a temperature of 900–1000 °C. The exact amount of oxygen required for optimum combustion of the sample was delivered into the combustion reactor at a precise time. The reaction of oxygen with the tin capsule at elevated temperature generates an exothermic reaction which raises the temperature to 1800 °C. At this high temperature both organic and inorganic substances are converted into elemental gases which were separated in a chromatographic column and finally detected by a highly sensitive thermal conductivity detector. 2.2.7. High pressure volumetric adsorption analysis. Adsorption performance analyses were performed to measure gas storage and separation capabilities of the materials. For the measurements and activation (degassing) of the samples, High Pressure Volumetric Analyser HPVA II-200[17]35 Particulate Systems (Micromeritics) was used. Approximately 0.45 g of sample was weighed (designated as wet sample weight) and placed in the sample holder. The sample holder was assembled and connected to the degas port. The sample was activated at 150 °C for CuBTC and 125 °C for [BMIM][BF4]/CuBTC materials under vacuum for at least 12 h until the pressure reached 10-6 bar. After activation, the sample holder was cooled and connected to the analysis port inside a water bath at ambient temperature. Before running the experiment all gas lines were evacuated and flushed with helium three times to get rid of any residual gases from earlier measurements. The analysis gas was connected to either one of gas inputs and the regulator on the gas cylinder was set to maximum adsorption pressure which was 100 bar for CH4, N2, H2, and 35 bar for CO2. The ideal gas selectivities of the samples were calculated as the ratio of adsorbed amounts of gases at exactly same pressures. The single-component gas adsorption isotherms were fitted to dual-site Langmuir isotherms and selectivities at the predetermined pressures were obtained from these fits. All selectivities were calculated by


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dividing more selective component (higher adsorbed amount) to less selective component (lower adsorbed amount).

3. Results and Discussions The IL-incorporated MOF materials were prepared by mixing a solution of [BMIM][BF4] (Sigma-Aldrich) and acetone with freshly dehydrated CuBTC (Basolite C300, Sigma-Aldrich) for 6 h at 30 °C. Scanning electron microscopy (SEM) images and X-ray diffraction measurements of the acetone treated CuBTC confirmed that the solvent does not affect the morphology of the MOF (as given in Supporting Information, SI, Fig. S1 online). The solvent was then evaporated and the resultant blue solid ([BMIM][BF4]/CuBTC) was further dried overnight in a vacuum oven at 105 °C, then it was moved to the antechamber of an argon-filled glovebox (Labconco) and dried further under vacuum for 1 h before being transferred into the glovebox. The corresponding IL loading of the samples were 5, 20, and 30 wt%, as confirmed by elemental analysis within 10 % error (SI). Powder X-ray diffraction pattern of the IL-incorporated samples presented in Fig. 1 show that the framework of CuBTC does not change by the incorporation of [BMIM][BF4], consistent with the SEM images (provided as Fig. S2 in SI). As a result it can be inferred that structures did not show deformation and loss of crystallinity after impregnation. BET measurements indicate that the surface area of pristine CuBTC decreases from 1371 to 1229, 733, and 248 m2/g for an IL loading of 5, 20, and 30 wt% (as given in Table S1 and Fig. S3-S4 online), respectively. These results suggest that the IL was successfully incorporated into the pores of CuBTC. Although we do not have direct experimental evidence, we speculate that most of the IL is inside


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the MOF’s pores. The majority of the total surface area of the MOF is inside the pores and as the IL was loaded the surface area of the samples decreased as shown in Table S1. Moreover, after we incorporated 30 wt.% IL into the MOF, our samples still remained as dry powders further indicating that IL molecules are not at the external surface but inside the pores. We, however, note that at higher loadings (>30 wt.%), our samples become like mud showing that the pores are now filled and the external surface is also getting some IL as well. In fact, this is the reason of why we limit our IL loading at 30 wt.%.

CuBTC [BMIM][BF4]/CuBTC (5 wt%) [BMIM][BF4]/CuBTC (20 wt%) [BMIM][BF4]/CuBTC (30 wt%)

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Figure 1. X-ray diffraction pattern of CuBTC (black), [BMIM][BF4]/CuBTC (5 wt%) (red), [BMIM][BF4]/CuBTC (20 wt%) (blue), and [BMIM][BF4]/CuBTC (30 wt%) (purple).


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Thermogravimetric analysis (TGA) of the samples (can be found as Fig. S5-S6 in SI) showed that the IL-incorporated MOF samples have more weight loss than pristine CuBTC upon heating to 700 °C in flowing N2 suggesting the decomposition of the IL as well. Moreover, the derivative onset temperature of the samples, indicating the starting temperature of thermal decomposition shifts to lower temperatures with the incorporation of the IL into the MOF (as given in Table S2 in SI). Accordingly, the decomposition temperature of bulk [BMIM][BF4] is 376 °C and that of pristine CuBTC was approximately 312 °C. These limits decrease to 286, 276, and 275 °C when the IL is incorporated into the MOF with a loading of 5, 20, and 30 wt%, respectively. Such a decrease in the thermal stability of the samples upon IL incorporation can be originated from the interactions between [BMIM][BF4] and CuBTC as suggested by a similar trend on metal-oxide-supported imidazolium ILs.[18] Samples were also characterized by FT-IR spectroscopy. As seen in Fig. 2 (a) four peaks belonging to [BMIM][BF4] were also observed in [BMIM][BF4]/CuBTC samples with increasing absorbance with the IL loading. These peaks at 3161, 2960, 2936, and 2875 cm-1 were assigned to v(C4,5-H), vasym(CH2), vasym(CH2), and vsym(CH2) modes, respectively.[19] In 1700-500 cm-1 region, other peaks were observed with the incorporation of IL (Fig. 2 (b)). The new peaks resulting from [BMIM][BF4] incorporation at 1573, 1281, 1165, 1045, and 623 cm-1 are more noticeable compared to others. The peaks at 1573 and 1165 cm-1 are assigned to C=C bond stretching and in plane HCCH bending of the [BMIM]+ cation. Other peaks at 1281 and 1045 cm-1 are resulting from the stretching of [BF4]anion. There are two more peaks close to 1045 cm-1, which were also assigned to v(BF4), however, these peaks were united in [BMIM][BF4]/CuBTC samples. Two peaks at 623 and 653 cm-1 in the IL spectrum were both assigned to in-phase vibrations of (CH2(N), CH3(N)). It is difficult to observe the peak at 653 cm-1 in [BMIM][BF4]/CuBTC samples. For wavenumbers at


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approximately 650 cm-1, deconvolution of the IL/MOF spectra revealed two peaks at 653 and 662 cm-1 suggesting the existence of N atoms. The peak at 847 cm-1, assigned to γ(C4,5−H) is hardly visible in [BMIM][BF4]/CuBTC samples, however magnification of the region shows a slight peak especially for [BMIM][BF4]/CuBTC (30 wt%). Lastly, for wavenumbers approximately at 1450 cm-1, deconvolution of the IL/MOF spectra revealed three peaks at 1423, 1446, and 1470 cm-1. The peaks at 1470 and 1446 cm-1 were assigned to δsym(CH3) of [BMIM]+ cation and v(C−C) for CuBTC. As a result, [BMIM][BF4] incorporation can be observed from the IR spectrum in the [BMIM][BF4]/CuBTC samples. As the IL loading increases, peaks assigned to [BMIM][BF4] are intensified. Results presented in Fig. 2 show that both CuBTC and [BMIM][BF4] keep characteristic peaks in IL-incorporated samples suggesting both materials were intact after incorporation. However, as discussed above, data revealed some changes in either the position or the intensity of [BMIM][BF4] fingerprints in IL-incorporated samples. These changes confirm the presence of direct interactions between [BMIM][BF4] and CuBTC.[18] Such interactions can play a crucial role in determining the gas adsorption performance of ILincorporated MOFs. Thus, these CuBTC samples with different IL loadings were tested for gas storage performance.


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Figure 2. IR Spectra for CuBTC, [BMIM][BF4]-incorporated CuBTC samples, and [BMIM][BF4] between (a) 3300-2800 cm-1, and (b) 1700-500 cm-1. Volumetric CH4, CO2, N2, and H2 uptakes were measured at ambient temperature up to 100 bar except for CO2, which was measured up to 35 bar, using a Micromeritics High Pressure Volumetric Analyzer (HPVA II). Figure 3 shows the results of these measurements in comparison with those of pristine CuBTC. We note that elemental analyses of the used samples confirm that the IL amount does not change during the measurement, and the reproducibility measurements indicate that the storage performance is the same within 5 % error for repetitive measurements. As expected, storage capacity for all gases decreased compared to that of pristine CuBTC as the IL loading increased from 5 to 30 wt%. Because the surface area of the framework is reduced with IL-incorporation, the number of adsorption sites is also reduced causing a


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decrease in the storage capacity. The decrease was more pronounced for gas molecules that have stronger interactions with the framework of the structure, i.e., CO2 followed by CH4, N2, and H2. Additionally, the saturation pressure was decreased from 60 to 50 bar for CH4 and from 25 to 20 bar for CO2 in samples with 20 wt% and 30 wt% IL loadings, respectively.

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Figure 3. Single component excess adsorption isotherms of (a) CH4, (b) CO2, (c) N2, and (d) H2 measured at room temperature. Uptake amounts were reported cc (STP) gas per gram CuBTC. The decrease in gas uptakes at low pressures changes significantly as the identity of the gas changes (as shown in Fig. 4 for a representative pressure of 1 bar). This change suggests that even though the number of adsorption sites is decreased in total, new adsorption sites are created with the incorporation of [BMIM][BF4] into the framework. These new sites have different affinities for each gas, resulting in different degree of reduction in the adsorption capacities


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specifically at low pressures. As seen in Fig. 4, the adsorption capacity change compared to CuBTC at 1 bar is in the increasing order of CH4 < N2 < H2 < CO2 for all IL loadings tested. The uptake amounts at 1 bar decrease almost linearly with IL loading for N2, H2, and CO2. Among these decreasing trends that on CO2 is significant, it decreased to almost half of its original value on pristine MOF. This case is similar to that observed for H2 and N2, although not being that significant. CH4 uptake, however, exhibits a changing decrease rate in the uptake at 1 bar as a function of IL amount. The data show that incorporation of IL does not show any detectable effect on the CH4 uptake up to an IL loading of 20 wt% (within the error range of our measurements). However, it decreases to 80 % of its original value on pristine CuBTC when the IL loading was 30 wt%. 100 Adsorption Capacity Change (%)

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IL Loading (wt %) Figure 4. A representative data set for gas adsorption capacity change at different IL loadings at 1 bar upon incorporation of [BMIM][BF4] into CuBTC. Data are given based on per gram of MOF. Dotted lines are to guide the eye. 16 Environment ACS Paragon Plus

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Such distinct behavior of individual gases implies that IL-incorporated MOFs can be promising in gas separations by tuning the gas affinities of materials. Therefore, ideal selectivities of IL-incorporated MOFs given in Fig. 5 (focusing on a pressure range from 0 to 5 bar, complete data set for a larger pressure range are given in SI online) were calculated from the single-component gas adsorption isotherms as the ratio of more adsorbed component. It can be seen that selectivities differ significantly especially at low pressures up to 1 bar. For instance, the CO2/CH4 selectivities of IL-incorporated CuBTC (Fig. 5(a)) are less than the selectivity of ILfree CuBTC. This result is interesting since [BMIM][BF4] has a higher CO2 solubility compared to CH4 with a CO2/CH4 selectivity of approximately 17 at ambient temperature and pressure.[20] The decrease in CO2/CH4 selectivity of the bulk IL upon incorporation into the MOF structure can be attributed to the interactions between IL and CuBTC as suggested by the FT-IR data discussed above. Another interesting behavior was also observed for CO2/H2 selectivity as a function of pressure (Fig. 5 (b)). Data show that until 0.6 bar, CO2/H2 selectivity is higher for [BMIM][BF4]/CuBTC compared to pristine CuBTC. The CO2/H2 selectivity becomes maximum at 0.2 bar for [BMIM][BF4]/CuBTC with an IL loading of 30 wt%, reaching 208, almost 1.5 times that of pristine CuBTC. However, after 0.6 bar CO2/H2 selectivity of the IL-incorporated samples starts to decrease and after 1 bar these samples show lower CO2/H2 selectivity compared to pristine CuBTC. A similar behavior was also observed on CO2/N2 selectivity (Fig. 5 (c)). On the other hand, data on IL-incorporated CuBTC show that CH4 selectivity towards all gases increases at all pressures investigated, suggesting an increased CH4 interaction with IL/CuBTC compared to pristine CuBTC. At 1 bar the CH4/H2 selectivity (in Fig.5 (d)) was increased from 19 to 26 and became comparable with that of Zeolite 5A and activated carbon which have ideal CH4/H2 selectivities of 20 and 25 at 1 bar, respectively.[21] It is observed that


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higher loading of [BMIM][BF4] enhances selectivity towards CH4 at all pressures. For instance, the CH4 selectivity with respect to H2 increased from 26 to 56 at 0.2 bar when 30 wt% IL was incorporated. As a result it can be inferred that the IL is playing a significant role in determining the selectivity of the MOF material. Similarly, for CH4/N2 separation (Fig. 5 (e)), CuBTC with 30 wt% IL loading becomes almost 1.6 times more selective towards CH4 at low pressures than the pristine CuBTC, and then this value gradually decreases to the levels of IL-free MOF specifically above 1 bar. The CH4/N2 selectivity was only increased from 2.8 to 3.4 for [BMIM][BF4]/CuBTC (30 wt%) at 1 bar, however, this value is still higher than selectivities of MOF-5 (1.1),[22] Zeolite 5A (1.7),[21] and activated carbon (3)[21] at around 1 bar. At high IL loadings, data given in Fig. 5 (f) illustrate that the selectivity for N2 in N2/H2 mixture is always positively influenced at all pressures investigated. The N2/H2 selectivity reaches a maximum of 12 at 0.3 bar for [BMIM][BF4]/CuBTC (30 wt%), however, it decreases to 8 at 1 bar which is lower than 8.6 for activated carbon[21] and 12 for Zeolite 5A[21] at 1 bar. At lower loadings, the selectivity for N2 is negatively influenced after approximately 1 bar. Overall, the changes in gas selectivity values are intensified with an increase of IL amount in the structure. As seen in Fig. 5 the gas selectivity values for CuBTC and [BMIM][BF4]/CuBTC (5 wt%) are very similar; however, both [BMIM][BF4]/CuBTC (20 wt%) and [BMIM][BF4]/CuBTC (30 wt%) show significant deviations in gas selectivities compared to IL-free CuBTC. These deviations in gas selectivities result from distinct gas adsorption capacity changes because of different affinities of adsorption sites for each gas component (Fig. 4). In general, adsorption sites established or modified with the incorporation of [BMIM][BF4] have higher affinity towards CH4 compared to other gases. Thus, CH4 selectivity was increased with the incorporation of IL compared to IL-free CuBTC. We observed that 30 wt.% is the incipient


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wetness limit for this MOF. Therefore, any higher IL loading results in mud formation. This behavior also describes why better performance is obtained with 30 wt.% than the other cases with lower loadings. Obviously, 5 or 20 wt.% was not enough to fill the MOF’s pores. Thus, it does not fully reflect the maximum effect of IL incorporation on the gas separation performance of the MOF. One can fully observe the IL-incorporation effect only when the pores are fully filled with IL. We suggest that the gas affinities of IL/MOF pairs may change significantly based on the individual and combined properties of materials. Supporting our hypothesis, when this manuscript was under review, Yang et al.23 recently reported similar ideal selectivity improvements for [BMIM][Tf2N]-incorporated ZIF-8. Thus, theoretically unlimited number of possible MOF and IL combinations with tunable physicochemical properties offer tremendous potential in tuning gas storage and separation properties of MOFs towards the rational design of new materials for meeting future energy challenges.


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Figure 5. Ideal adsorption selectivities of [BMIM][BF4]/CuBTC samples for (a) CO2/CH4, (b) CO2/N2, (c) CO2/H2, (d) CH4/N2, (e) CH4/H2, and (f) N2/H2 at room temperature up to 5 bar. The first component is the more selective over the second component. The x-axes are given in log scale for clear representation of the data at low pressures.


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4. Conclusion In summary, data presented here illustrate that incorporation of [BMIM][BF4] into the structure of CuBTC at different loadings significantly influences the gas storage and separation performance of the MOF. There is a substantial amount of change in gas selectivity values specifically at low pressures with high IL loading, accompanied by a different degree of decrease in the uptake values of different gases. The IL/MOF couple that we studied in this work, [BMIM][BF4]/CuBTC, shows the highest affinity towards CH4 followed by N2, H2, and CO2 especially between 0-1 bar. CH4/H2 selectivity, for instance, increased from 26 to 56 at 0.2 bar upon the incorporation of [BMIM][BF4] with a loading of 30 wt%. The results from this proofof-concept study confirm the benefits of combining ILs with MOFs to achieve dramatic performance enhancements in gas storage and separation with MOFs. The results from this proof-of-concept study confirm the benefits of combining ILs with MOFs to achieve dramatic performance enhancements in gas storage and separation with MOFs. There is a huge potential for the rational design of new materials with unprecedented gas separation performances. Tunable physicochemical properties of both MOFs and ILs and their unlimited number of possible combinations offer opportunities to tailor the structures for the best gas storage and separation performance in any application.

Supporting Information Available: SEM images, elemental analysis, TGA, XRD, and IR characterization of samples and selectivity figures are provided. This material is available free of charge via the Internet at http://pubs.acs.org.


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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. Financial support provided by the KUTEM (Koç University TUPRAS Energy Center) is gratefully acknowledged. S.K. acknowledges TUBA-GEBIP and A.U. acknowledges the support by the Science Academy of Turkey under the BAGEP Award Program. Authors thank Koç University Surface Science and Technology Center (KUYTAM) for providing help with the sample characterization.

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Table of Contents/Abstract


Tuning the Gas Separation Performance of CuBTC ... - ACS Publications

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