Hierarchically Structured HKUST-1 Nanocrystals for Enhanced SF6

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Hierarchically Structured HKUST‑1 Nanocrystals for Enhanced SF6 Capture and Recovery Chong Yang Chuah,† Kunli Goh,†,‡ and Tae-Hyun Bae*,†,‡ †

School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore Singapore Membrane Technology Centre, Nanyang Technological University, Singapore 637141, Singapore

‡

S Supporting Information *

ABSTRACT: HKUST-1, an inexpensive metal−organic framework possessing open metal sites, has a great potential for capture and recovery of SF6. In this work, the structural property of HKUST-1 was modified to yield a hierarchically structured HKUST-1 nanocrystal exhibiting a superior performance with higher SF6 uptake (4.98 mmol g−1 at 25 °C and 1 bar), better SF6/ N2 selectivity (∼70 at 25 °C), faster SF6 adsorpton kinetics, and lower energy penalty for regeneration compared to those of bulk HKUST-1 crystal as well as those of conventional zeolite and porous carbon adsorbents. Higher surface area and the presence of mesoporosity to facilitate the transport of SF6 to active sites residing in microporous spaces were found to be key factors contributing to such enhancement. The outstanding potential utility of our HKUST-1 crystal in industrial applications was also validated with an idealized vacuum swing adsorption model.

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INTRODUCTION Growing concern of global warming has generated extensive research efforts on the capture, storage, and utilization of CO2, the most common greenhouse gas. Sulfur hexafluoride (SF6), on the other hand, is a more potent greenhouse gas but has received relatively less research attention as compared to CO2. It was reported that the global warming potential (GWP) of SF6 is approximately 23 900 times greater than that of CO2 because of its strong capability of absorbing infrared radiation.1 Furthermore, the atmospheric lifetime of SF6 is a longer (800− 3200 years) than that of CO2 (30−95 years).2 Such behaviors can create a long-lasting negative effect on global warming, despite the emission of SF6 being far lower than that of CO2. By 2020, the projected SF6 emission will reach 4270 ± 1020 tonne/year based on an average growth rate of 7.4 ± 3.7%/ year.2 Thus, SF6 is categorized as one of the four regulated greenhouse gases (CO2, CH4, N2O, and SF6) under the Kyoto Protocol. SF6 is a widely used insulating gas in numerous applications such as in the metalworking industries and electronics sectors.3 In most circumstances, SF6 is used as a mixture with N2 for economic reason.4,5 In addition, SF6 feed often gets converted into SF6/N2 mixture during the blowing process.6 Therefore, effective separation and recovery of SF6 from the SF6/N2 mixture is of paramount importance not only for recycling the expensive SF6 but also for preventing release of the strong greenhouse gas to the atmosphere. The amount of SF6 in the mixture is generally too small to recover via an energy intensive liquefaction process. 7 Adsorptive separation using porous materials, on the other hand, can be offered as a lower energy consumption alternative © 2017 American Chemical Society

with high relevance for such applications. In this context, various porous materials including zeolites,8−12 activated carbon,13−15 carbon nanotubes,16 pillared clays,17 mesoporous alumina,7 metal−organic frameworks (MOFs),18−20 and porous organic cages21 have been studied for potential application in SF6/N2 separation. Among them, MOFs, which are a family of crystalline microporous network constructed by strong coordination bonding between metal ions (or metal clusters) and organic ligands, are known to be very effective in capturing SF6 owing to their large accessible surface areas and tunable physicochemical properties. Particularly, MOFs with coordinatively open metal sites such as MOF-74 and HKUST-1 have demonstrated good adsorption capacities for SF6. In this regard, HKUST-1 is chosen as a suitable adsorbent for SF6 capture and recovery for the following reasons. First, HKUST-1 contains an intersecting three-dimensional system of large square-shaped pores (9 × 9 Å) which is ideal for SF6 molecules with kinetic diameter of about 5.13 Å.8 Second, the synthesis of HKUST-1 is well-established and requires only inexpensive copper sources and trimesic acid to react under mild reaction conditions. Third and most importantly, HKUST-1 is already commercially available under the trade name Basolite. This suggests that HKUST-1 can be readily adopted by relevant industries. Comparatively, it is noteworthy that MOF-74, which is known to have an SF6 adsorption capacity higher than that of HKUST-1 crystals, has not yet been commercialized. Received: January 11, 2017 Revised: March 10, 2017 Published: March 13, 2017 6748

DOI: 10.1021/acs.jpcc.7b00291 J. Phys. Chem. C 2017, 121, 6748−6755

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The Journal of Physical Chemistry C

Figure 1. FESEM images and structural scheme of HKUST-1 crystals: (a) bulk crystal (HKUST-1a), (b) nanocrystal (HKUST-1b), and (c) nanocrystals with hierarchical structure (HKUST-1c).

Figure 2. (a) PXRD patterns for HKUST-1 crystals, (b) N2 sorption isotherm of HKUST-1 crystals, and (c) pore size distribution of HKUST-1 crystals.

tional activated carbon and zeolite 13X in an idealized vacuum swing adsorption (VSA) process. In essence, this study highlights a scalable synthetic method to potentially innovate hierarchically structured nanosized porous materials for enhanced SF6 capture and recovery.

As such, our strategy focuses on the design of hierarchically structured HKUST-1 nanocrystals to improve the performance as an adsorbent for SF6 capture. By synergistically downsizing HKUST-1 to offer an even larger surface area and generating hierarchical structures with mesoporosity in HKUST-1, the diffusion length in microporous spaces can be effectively shortened, resulting in enhanced SF6 adsorption−desorption. Various approaches, such as soft-templating method using surfactants22 and template-free methods via homogeneous ligand etching,23 CO2-directed assembling,24 and ball-mill synthesizing,25 can be employed to control the crystal size and create hierarchical structure in MOFs. However, as compared to these approaches, our synthesis method involves only refluxing and is straightforward for a more cost-effective scaled-up production. In addition, our hierarchically structured HKUST-1 nanocrystals also show promising results with enhanced SF 6 uptake of 4.98 mmol g−1 , better SF 6 /N 2 selectivity at ∼70, shorter SF6 adsorption kinetics, and the lowest energy penalty for regeneration. More importantly, the nanocrystals demonstrate better performances than conven-

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RESULTS AND DISCUSSION Synthesis and Characterization of Hierarchical HKUST-1 Nanocrystals. The hierarchical HKUST-1 nanocrystal design is realized by exploiting the synthesis reaction conditions. Conventionally, micrometer-sized bulk HKUST-1 crystals (HKUST-1a) were synthesized via a pressurized, hightemperature solvothermal reaction (Figure 1a).26 By alleviating the pressure during the synthesis of HKUST-1, downsizing of the HKUST-1a bulk crystals to nanosized HKUST-1 crystals (HKUST-1b and c) are made possible, as verified by the field emission-scanning electron microscopy (FE-SEM) images, which show HKUST-1a with a lateral dimension of ∼12 μm while HKUST-1b and c exhibit average crystal sizes of 300−500 and 70−120 nm, respectively (Figure 1a−c). Morphologically, 6749

DOI: 10.1021/acs.jpcc.7b00291 J. Phys. Chem. C 2017, 121, 6748−6755

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favorable interaction between SF 6 molecules and the coordinatively open metal sites residing in the Cu paddle wheels of HKUST-1. HKUST-1b shows SF6 uptake capacity that is better than that of the bulk crystals owing to its higher surface area and micropore volume. This is in agreement with a previous study which demonstrated a positive relationship between the surface area of HKUST-1 and the gas uptake capacity.29 Interestingly, hierarchically structured HKUST-1c demonstrates an even better performance than HKUST-1b despite the surface area and the micropore volume of both crystals being comparable (Table 1). The mesopores created in the HKUST-1c nanocrystal might facilitate the transport of SF6 molecules by reducing the diffusion length and allowing easy access to active sites in the HKUST-1c nanocrystals. Adsorption−desorption kinetics is as important as the adsorption capacity because it can determine the overall processing rate in industrial operation. As such, SF6 adsorption kinetics was measured at 25 °C and 1 bar of dosing pressure. Figure 3c shows fractional uptake, which is defined as the ratio of the gas uptake at given time to the equilibrium uptake, of three HKUST-1 crystals. Relatively rapid SF6 uptakes are observed for all three HKUST-1 samples. The mass transport model predicts that the rate of uptake significantly decreases as the size of the adsorbent increases. In this work, however, even bulk HKUST-1a crystals show a rapid SF6 uptake due to the presence of mesopores that can rapidly distribute SF6 molecules to the micropore domains. For HKUST-1b, the lack of mesoporosity is compensated by the downsized crystal size, which offers a larger surface area for improving the uptake rate (Table 1). Hence, the rate of SF6 uptake of HKUST-1b is almost comparable to that of HKUST-1a. Without a doubt, HKUST-1c nanocrystals give the best performance in the adsorption kinetics evaluation as a result of the synergistic effect brought about by downsizing of the crystals and the introduction of mesoporosity into the nanocrystals. To benchmark the uptake rate of HKUST-1c, we also measured adsorption kinetics of zeolite 13X and the activated carbon which do not possess any mesoporosity. As shown in Figure 3d, the two commercial adsorbents required an equilibration time that was much longer than that of the hierarchically structured HKUST-1c nanocrystals. Overall, results from the isotherm and adsorption kinetics studies suggest that nanoscale engineering of a porous material to create nanocrystals possessing both micropores and mesopores is an efficient way to enhance SF6 uptake and decrease the required time for SF6 uptake to achieve equilibrium. SF6/N2 Selectivity and Isosteric Heat of Adsorption. Besides the adsorption capacity, the selectivity or the separation efficiency is another important parameter to quantify so as to determine the purity of the recovered products. For this reason, N2 uptake properties of all adsorbents were measured and are displayed in Figure S4 alongside the SF6 adsorption isotherms. The SF6/N2 selectivities were calculated using the ideal adsorbed solution theory (IAST) as demonstrated in many previous studies with zeolites and MOFs.30−35 To do so, the isotherms of SF6 and N2 in Figure S4 were fitted with a dualsite and a single-site Langmuir−Freundlich isotherm model, respectively. Both SF6 and N2 uptake data were successfully described by the adsorption models with R2 values greater than 0.99. All the parameters used for fitting are summarized in Tables S1 and S2. The SF6:N2 ratio of the mixture commonly used by industries is 0.1:0.9 at ambient pressure. Therefore, the SF6/

the defined tetragonal bipyramidal shape of HKUST-1a is lost, giving irregularly shaped HKUST-1b and c nanocrystals (Figure 1d). Nevertheless, Fourier transform-infrared spectroscopy (FT-IR) confirms the successful coordination of trimesic acid into the paddle wheel Cu2(COO)4 unit for all the synthesized HKUST-1 samples (Figure S1).27 Thermogravimetric analysis (TGA) also reveals similar trends, with all the samples showing thermal stabilities up to 350 °C (Figure S2). Most importantly, the crystallinity of HKUST-1b and c remains intact, as illustrated by the similar powder X-ray diffraction (PXRD) patterns as compared to that of HKUST-1a (Figure 2a).28 These characterization results suggest that only the size and morphology of the HKUST-1 crystals are altered, but the physicochemical properties are not compromised during the downsizing synthesis. Next, the pore characteristics of the synthesized HKUST-1 were elucidated using N2 physisorption measured at 77 K. As shown in Figure 2b, all samples display high N2 uptakes at the low-pressure region, indicating the presence of large micropore volumes. Meanwhile, a hysteresis loop between adsorption− desorption branches for HKUST-1a and c proves the presence of mesoporosity within the samples (Figure 1d). The pore sizes estimated by Barrett−Joyner−Halenda (BJH) methods are about 4 nm for both HKUST-1a and c (Figure 2c). Essentially, the N2 physisorption study confirms that HKUST-1a and c possess a hierarchical structure in which both microporous and mesoporous domains coexisted, while HKUST-1b is purely microporous. The N2 physisorption isotherms are also used to calculate the surface areas and pore volumes, as summarized in Table 1. It is found that the Brunauer−Emmett−Teller (BET) Table 1. Surface Areas and Pore Volumes of HKUST-1 Crystals Calculated from N2 Physisorption at 77 K sample

SBETa (m2/g)

SLanga (m2/g)

Smicrob (m2/g)

Vmicrob (cc/g)

HKUST-1a HKUST-1b HKUST-1c

1090 1135 1328

1626 1727 1699

951 1093 1279

0.498 0.580 0.585

a

BET (SBET) and Langmuir (SLang) surface area are obtained at P/Po = 0.05−0.3. bMicropore area (Smicro) and volume (Vmicro) are obtained using t-plot method at the pressure range of P/Po = 0.4−0.6.

surface area of HKUST-1c is higher than that of HKUST-1a, indicating that the transition from bulk crystal to nanocrystal is indeed accompanied by an increase in the surface area. In addition, HKUST-1c also shows higher micropore surface area and volume. We infer that downsizing of HKUST-1a to nanocrystals can effectively shorten the diffusion length and create more accessible surface areas. The almost similar micropore volumes of HKUST-1b and c also suggest that the mesopores generated in HKUST-1c do not result in a loss in the microporosity and crystallinity of the HKUST-1c nanocrystals. SF6 Adsorption Capacities of HKUST-1 Crystals. Pure component SF6 adsorption isotherms measured at both 25 and 40 °C are displayed in Figure 3a,b. In addition to the three types of HKUST-1 crystals synthesized in this work, two commercial adsorbents, namely, zeolite 13X and an activated carbon, were also tested under the same conditions for benchmarking. Generally, all HKUST-1 crystals exhibit adsorption properties that are better than those of zeolite 13X and activated carbon. This is presumably attributed to the 6750

DOI: 10.1021/acs.jpcc.7b00291 J. Phys. Chem. C 2017, 121, 6748−6755

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Figure 3. Pure component SF6 adsorption isotherms measured at (a) 25 °C and (b) 40 °C; SF6 adsorption kinetics at 25 °C and 1 bar of dosing pressure for (c) HKUST-1 crystals and (d) zeolite 13X and activated carbon together with HKUST-1c.

and the activated carbon (Figure 4). This is attributed to the increased accessibility of SF6 molecules to the active sites in HKUST-1c as evidenced by the steeper rise of SF6 uptake in the low-pressure range (Figure 3a,b). Thus, results from the IAST study reveal that SF6/N2 selectivity can be enhanced by tuning the size and pore characteristics of the HKUST-1 crystals. Isosteric heat of adsorption (Qst), a measure for binding energy between adsorbent and adsorbate, was also calculated for SF6 after fitting adsorption isotherms with a virial equation. As shown in Figure S5, high-quality fittings were obtained using the parameters summarized in Table S4. Figure 5 shows the isosteric heat of adsorptions as a function of SF6 loading for all adsorbents tested. Surprisingly, HKUST-1c, which possesses the highest SF6 uptake, maintains the lowest Qst as the loading increases. Because of such low binding energy, the SF6 uptake of HKUST-1c shows only a marginal decrease of ∼5% when temperature increases from 25 to 40 °C. In contrast, raising the temperature causes a more significant decrease in SF6 uptake for both HKUST-1a and b (Figure 3a,b). Therefore, HKUST1c is highly advantageous for applications where feed gas temperature is relatively high. More importantly, the isosteric heat of adsorption or the binding energy, which is equivalent to the energy penalty for regeneration of medium, indicates that the hierarchically structured HKUST-1c can be operated at a lower energy consumption than the other two HKUST-1 crystals. In essence, this result may imply that the mesopores within HKUST-1c nanocrystals do not only shorten the diffusion length to enhance SF6 molecules transport to the active sites; they also increase the number of active sites that are readily accessible for SF6 molecules.

N2 selectivities were calculated based on this information and at two different temperatures, namely, 25 and 40 °C. The results are displayed in Figure 4. Other properties such as the amount

Figure 4. SF6/N2 selectivities calculated by IAST at 25 and 40 °C. The partial pressures of SF6 and N2 were 0.1 and 0.9 bar, respectively.

of uptake for mixture gas and the purity of the product are also calculated and summarized in Table S3. Among the three HKUST-1 samples, HKUST-1a shows the lowest SF6/N2 selectivity. When the bulk HKUST-1a crystals are downsized, an enhanced selectivity is exhibited for HKUST-1b, owing to an increase in the SF6 uptake with no noticeable change in the N2 adsorption (Figure S4). Even so, the best SF6/N2 selectivity is observed for HKUST-1c. Evidently, this shows that introducing mesoporosity into HKUST-1 nanocrystals can further improve the selectivity from 50 to 70 at 25 °C to provide a competitive advantage over commercial adsorbents, including zeolite 13X 6751

DOI: 10.1021/acs.jpcc.7b00291 J. Phys. Chem. C 2017, 121, 6748−6755

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HKUST-1c nanocrystals outperform zeolite 13X and the activated carbon, showing a high potential utility in realistic industrial applications.

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CONCLUSION In this work, we reported hierarchically structured HKUST-1 nanocrystals that showed improved SF6 uptake capacity and SF6/N2 selectivity, faster adsorption kinetics, and reduced energy penalty for regeneration as compared to conventional HKUST-1. Through the modification of the synthesis pressure and temperature, nanoscale engineering can be easily achieved to downsize and introduce mesoporosity in the HKUST-1 crystals. Nanosized crystals offer a large surface area, and the presence of mesopores facilitates SF6 molecular transport into and out of the active sites in the micropores of HKUST-1. As a result, the hierarchically structured HKUST-1 nanocrystals not only possess enhanced performances in SF6 capture and recovery but also show an excellent potential utility in a model VSA process. More importantly, our scalable synthetic method highlights the great promise of nanoscale engineering to synthesize porous materials as high-performing adsorbents for enhanced SF6 capture and recovery.

Figure 5. Isosteric heat of adsorption of all adsorbents as a function of SF6 loading.

Potential Utility in an Idealized Vacuum Swing Adsorption. In a realistic industrial operation, an adsorption−desorption cycle is repeated either by a temperature or pressure swing. Therefore, in this work, the potential utility of the synthesized HKUST-1 crystals was evaluated with an idealized VSA of which the operational principle is the same as a pressure swing adsorption (PSA). Conventional PSA where desorption is conducted at an ambient pressure may not be suitable for HKUST-1 as the SF6 uptake is almost saturated at approximately 1 bar. In this case, the feed gas must be compressed to a very high pressure for adsorption to secure positive working capacity, resulting in a high energy penalty for compression. This makes the overall process not economical. Rather, vacuum swing is more attractive for adsorbents showing early saturation at around ambient pressure. In our VSA model, it was assumed that a 1:9 SF6:N2 mixture gas at 1 bar (the partial pressure of SF6 is equal to 0.1 bar) is fed to the adsorber and desorption is carried out at 0.01 bar. We also assumed that, at desorption conditions, the adsorber is filled with SF6 gas which is released from the adsorbent so as not to overestimate the working capacity. As a result of this assumption, a 0.01 bar SF6 partial pressure is adopted at desorption. Using this VSA model, four criteria as defined in a previous work36 were calculated and are summarized in Table 2. As expected, the hierarchically structured HKUST-1c nanocrystals show the highest potential utility among all the adsorbents tested. In particular, the working capacity for HKUST-1c is 45% higher than that of bulk HKUST-1a crystal. The SF6/N2 selectivity also increased by more than 100% after downsizing from bulk crystal to hierarchically structured nanocrystals. Above all,

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EXPERIMENTAL SECTION Materials. Copper(II) nitrate trihydrate, copper(II) acetate monohydrate, trimesic acid, zeolite 13X, and an activated carbon (activated charcoal, Darco) were purchased from SigmaAldrich. Absolute ethanol was purchased from VWR. SF6 and N2 which were used as the testing gases were purchased from Air Liquide. All chemicals were used as received without further purification. Synthesis of HKUST-1. Three different HKUST-1, namely, bulk crystals (HKUST-1a), nanocrystals (HKUST-1b), nanocrystals with a hierarchical structure (HKUST-1c), were synthesized using the procedures shown below.22,25,29 HKUST-1a. A copper solution was prepared by dissolving 4.5 mmol (0.5465 g) of copper(II) nitrate trihydrate in 7.5 mL of distilled water in a 20 mL vial. In a separate 20 mL vial, 2.5 mmol (0.2625 g) of trimesic acid was dissolved in 7.5 mL of absolute ethanol. Next, the two solutions were mixed and placed in a 50 mL Teflon-lined autoclave reactor. Subsequently, the reaction was carried out at 120 °C for 12 h. After the reaction mixture cooled to room temperature, the resulting precipitate was filtered and washed with a copious amount of 1:1 ethanol:water mixture. HKUST-1b. A 1.2 g sample of copper(II) nitrate trihydrate and 0.6 g of trimesic acid were added sequentially into 20 mL of absolute ethanol. The resulting mixture in a 20 mL vial was vigorous stirred at room temperature for 24 h. Next, the formed precipitate was filtered and washed with a copious amount of 1:1 ethanol:water mixture. HKUST-1c. A 3 mmol (0.5988 g) sample of copper(II) acetate monohydrate and 2 mmol (0.840 g) of trimesic acid were added sequentially into a round-bottom flask which was filled with 40 mL of absolute ethanol. The resulting solution was refluxed at 75 °C for 20 h under argon flow. The formed precipitate was collected by a centrifugation and then dispersed in fresh 1:1 ethanol:water mixture. Such a centrifugation− redispersion cycle was repeated several times to remove residual impurities. Characterization. The adsorption properties of SF6 and N2 in HKUST-1 were measured by a volumetric gas sorption

Table 2. Four Parameters Evaluated in an Idealized VSA Model sample

a Nads (mmol/g) 1

ΔNb (mmol/g)

Rc (%)

d αads 12

HKUST-1a HKUST-1b HKUST-1c zeolite 13X activated carbon

0.981 1.196 1.372 0.923 1.006

0.868 1.067 1.256 0.861 0.858

88.5 89.2 91.5 93.3 85.3

38.2 48.3 80.6 51.2 30.3

b Nads 1 = SF6 uptake under adsorption conditions. ΔN = working = selectivity under adsorption capacity. cR = regenerability. dαads 12 conditions. a

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DOI: 10.1021/acs.jpcc.7b00291 J. Phys. Chem. C 2017, 121, 6748−6755

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In this expression, P is pressure in bar, T temperature in Kelvin, and N the total amount adsorbed in mmol/g; ai and bi are the virial coefficients (constants that are independent of temperature), and m and n are described as the total number of coefficients required to fit the isotherm accurately. In essence, determination of suitable values of m and n requires trial-anderror. Note that the value of a0 must be negative because adsorption is an exothermic process. After the isotherm is fit using eq 4, Qst can be calculated using the equation38,39

analyzer (Quantachrome, iSorb HP1). Prior to analysis, HKUST-1 crystals were activated at 180 °C under dynamic vacuum for 8 h, except for HKUST-1c which was activated at 150 °C. Similarly, Zeolite 13X and activated carbon were also separately activated at 180 °C for 8 h. The isotherms were measured in the range of 0−1 bar at both 25 and 40 °C, which were precisely controlled by a water circulator and an isothermal jacket, respectively. To quantify the surface areas and pore volumes, nitrogen physisorptions at 77 K were performed using Quantachrome Autosorb-6B. Prior to measurements, samples were activated at the same conditions mentioned above. The pore size distributions of adsorbents were also analyzed from the N2 desorption data using the BJH method. FT-IR spectra were measured by IR spectrometer (PerkinElmer, Spectrum One) with the resolution of 4 cm−1 in a range of 4000 to 500 cm−1. TGA was performed on a thermogravimetric/differential thermal analyzer (PerkinElmer, Diamond TG/DTA) at the heating rate of 10 °C/min in the temperature range of 30 to 800 °C. This analysis was carried out under a pure nitrogen purging. PXRD analysis was performed under ambient conditions at the step size of 0.02° in the range of 2θ from 5 to 35°. The morphology of HKUST-1 crystals was observed with FE-SEM (Joel, JSM6700). Evaluation of SF6 and N2 Uptake Performance. The following dual-site Langmuir−Freundlich model35 was used to describe SF6 adsorption: q=

qsat,1b1p1/ c 1 + b1p1/ c

+

m

Q st = −R ∑ aiN i i=0

Lastly, the potential utility of adsorbents in SF 6 /N 2 separation process was evaluated with an idealized VSA system. In this study, the adsorption pressure, Pads, and desorption pressure, Pdes, were set at 1 and 0.01 bar, respectively.40 It should be noted that the SF6/N2 mixture is supplied during the adsorption process, resulting in SF6 partial pressure less than 1 bar. However, in the desorption condition, SF6 molecules captured in the adsorbent are released and fill the void space of the adsorber column. Then, the partial pressure of SF6 at desorption conditions is close to the total pressure, which is 0.01 bar. Therefore, the working capacity or the useful capacity can be defined as the amount of uptake at the partial pressure of SF6 at adsorption conditions minus the amount of uptake at the partial pressure of SF6 at desorption conditions. Using this idealized model, the potential utility of the adsorbent was evaluated using the following measures:36 (a) SF6 uptake under adsorption condition, N1ads (mmol/g); (b) SF6 working capacity, ΔN1 (mmol/g); (c) regenerability, R; and (d) ads selectivity under adsorption condition, α12 . The sorbent selection parameter, S, was not calculated in this work because a negligible amount of nitrogen (y2) is presented in the adsorber column at desorption condition, leading to meaningless figures for αdes 12 . The criteria mentioned above can be mathematically defined as follows:

qsat,2b2p1/ f 1 + b2p1/ f

(1)

where q is the total quantity of SF6 absorbed and p is the partial pressure; qsat,1 and qsat,2 are the saturated loadings for sites 1 and 2, respectively; b1 and b2 are Langmuir parameters for sites 1 and 2, respectively; c and f are Freundlich parameters for sites 1 and 2, respectively. Meanwhile, N2 adsorption data were fit with a single-site Langmuir−Freundlich model: q=

ΔN1 = N1ads − N1des

qsatb1p1/ c 1 + b1p1/ c

(2)

R=

Determining saturation capacity of N2 by experiment at ambient temperature or above is often challenging because of the extremely low interaction between N2 and adsorbent. Thus, in this study, it was assumed that all adsorption sites in an adsorbent were equally accessible for both SF6 and N2. This assumption is often used to estimate the saturation capacities of weakly adsorbed components. We also confirmed that of SF6 and N2 in MOFs are fairly close to each other.19 In addition, the adsorption kinetics of SF6 was studied at the dosing pressure of 1 bar and 25 °C. Results were presented with the fractional uptake, which can defined as eq 3, versus time plot using the expression: Fractional uptake =

Amount of uptake at time t Equilibrium uptake

m

⎛1⎞ ⎜ ⎟ a Ni + ⎝T ⎠ ∑ i i=0

■

N1ads N2ads

×

(6)

(7)

y2 y1

⎛ (α ads)2 ⎞⎛ ΔN ⎞ 1 S = ⎜⎜ 12des ⎟⎟⎜ ⎟ ⎝ α12 ⎠⎝ ΔN2 ⎠

(8)

(9)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00291. Additional characterizations and fitting parameters for isotherm measurement and isosteric heat and adsorption (PDF)

(3)

■

n

∑ bjN j j=0

ΔN × 100 N1ads

α12ads =

The SF6/N2 selectivity under various conditions was calculated using IAST.37 The isosteric heat of adsorption (Qst) was computed from the virial plot.38 ln P = ln N +

(5)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

(4) 6753

DOI: 10.1021/acs.jpcc.7b00291 J. Phys. Chem. C 2017, 121, 6748−6755

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The Journal of Physical Chemistry C ORCID

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Tae-Hyun Bae: 0000-0003-0033-2526 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Academic Research Fund Tier-1 (Project Reference Number: RG10/13) from the Ministry of Education, Singapore.

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REFERENCES

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DOI: 10.1021/acs.jpcc.7b00291 J. Phys. Chem. C 2017, 121, 6748−6755

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DOI: 10.1021/acs.jpcc.7b00291 J. Phys. Chem. C 2017, 121, 6748−6755