Highly selective adsorption separation of light

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Jun 26, 2018 - porphyrinic zirconium metal-organic framework PCN-224. Renfeng .... an ideal Zr-MOF adsorbent possessing high adsorption uptake and se-.
Separation and Purification Technology 207 (2018) 262–268

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Highly selective adsorption separation of light hydrocarbons with a porphyrinic zirconium metal-organic framework PCN-224 ⁎

T



Renfeng Shia, Daofei Lva, Yongwei Chena, Houxiao Wua, Baoyu Liub, , Qibin Xiaa, , Zhong Lia a b

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal-organic framework Light hydrocarbons Adsorption selectivity Separation

Light hydrocarbons are important raw materials in the petrochemical industry, mainly achieved by a costintensive and energy-consuming cryogenic distillation. In this work, we reported a porphyrinic zirconium metalorganic framework material, PCN-224, for the adsorption separation of C2 (ethane, C2H6) and C3 (propane, C3H8) hydrocarbons over C1 (methane, CH4). The Brunauer-Emmett-Teller (BET) surface area of the synthesized PCN-224 was 2704 m3·g−1 and the pores were mainly centered at 12 and 16 Å. Gas adsorption experiments revealed that PCN-224 exhibited much higher adsorption capacities of C2H6 and C3H8 (2.94 and 8.25 mmol·g−1) than that of CH4 (0.47 mmol·g−1) at 298 K and 100 kPa. The corresponding adsorption selectivity of C2H6/CH4 predicted by the ideal adsorbed solution theory (IAST) was 12. Particularly, the C3H8/CH4 selectivity reached up to 609 at the same conditions. Furthermore, the breakthrough experiments results showed favorable dynamic performance of PCN-224 for efficient separation of C2/C1 and C3/C1 mixtures. Additionally, PCN-224 exhibited excellent water stability after retaining its framework in water for 48 h. Therefore, PCN-224 is a promising material for the separation of C1-C3 hydrocarbons.

1. Introduction Energy and environmental issues are currently becoming the focus of attention, and they are closely related to human survival. Natural gas, primarily consisting of CH4, has long been recognized as a clean and eco-friendly energy source alternative to traditional fossil fuels (e.g. coal and oil) because of its high ratio of hydrogen to carbon, thus resulting in lower CO2 emissions, nitrogen oxide, sulfur oxide and particles [1–3]. The worldwide demand of natural gas increases year by year, which is expected to surpass coal before 2030, and it will reach 25% coverage of total energy demand by 2035 [4]. However, the mined natural gas often contains small amounts of heavier hydrocarbons, such as C2H6 and C3H8, which can not only reduce the conversion rate and energy content, but also affect the steady state of safe storage and transportation in pipeline [5,6]. On the other hand, C2H6 and C3H8 hydrocarbons are important chemical feedstock. For example, C2H6 can be used as the cracking material for manufacturing ethylene. Therefore, to meet the high quality and purity demands of practical application, effective separation of CH4 from C2H6 and C3H8 is of great significant [7]. So far, traditional cryogenic distillation as the dominant technology to separate light hydrocarbons are cost-intensive and energyconsuming due to the operation conditions of high pressure and low



Corresponding authors. E-mail addresses: [email protected] (B. Liu), [email protected] (Q. Xia).

https://doi.org/10.1016/j.seppur.2018.06.064 Received 8 May 2018; Received in revised form 21 June 2018; Accepted 25 June 2018 Available online 26 June 2018 1383-5866/ © 2018 Published by Elsevier B.V.

temperature [8]. Hence, it is highly desirable to develop a cost-effective and energy-efficient separation technology operated under ambient conditions. Among the current alternative technologies, adsorption separation is recognized as one of the most promising approaches to achieve low cost and high efficiency due to its simple operation and low equipment investment [9]. Choosing suitable adsorbents with high adsorption capacity and selectivity is a key step during the adsorption separation process [10]. Compared to conventional solid adsorbents, such as zeolite and activated carbon materials [11,12], an emerging class of porous materials, metal-organic frameworks (MOFs), consisting of metal ions or clusters and organic linkers, have been widely studied for gas storage and separation, owing to their high surface areas, flexible structure, unique designable pore dimensions and functionality [13–17]. With respect to light hydrocarbons separation, Long and coworkers [18–20] have intensively investigated the adsorption separation performance of C1-C3 hydrocarbons of various isostructural MOF-74. Particularly, the adsorption capacities of Fe-MOF-74 [19] for C2H6 and C3H8 achieved 5.00 and 5.67 mmol·g−1 at 318 K and 1 bar, respectively. Besides, Bao et al. [10] synthesized a Mg-based MOF-74, exhibiting high C2H6 and C3H8 uptakes of 6.6 and 7.2 mmol·g−1 at 298 K and 1 bar, respectively. Similarly, Chen’s group reported a series of porous MOF materials, such

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every 8 h. Finally, the acetone was decanted, and the activated sample of PCN-224 was obtained by filtration and vacuum drying at 150 °C for 8 h.

as UTSA-30 [21], USTA-34 [22] and USTA-35a [23], for the application of hydrocarbons separation. Among these materials, USTA-35a exhibited 2.97 mol·g−1 C3H8 uptake and the highest C3H8/CH4 selectivity of 80 at 296 K and 1 bar. Besides, some new MOFs with high C2/C1 or C3/C1 adsorption selectivity were recently reported [24–27]. For instance, Li et al. [27] reported two finite binuclear MOFs (JLU-Liu37 and JLU-Liu38), where JLU-Liu37 exhibited remarkable C2H6 and C3H8 adsorption capacities of 4.42 and 7.95 mmol·g−1, and the C2H6/CH4 and C3H8/CH4 selectivities of JLU-Liu37 was high up to 11 and 206, separately, at 298 K and 1 bar. However, there is still a challenge that most MOFs based on lowvalence metals (e.g. Mg2+, Cu2+) are usually with poor resistance to water or acid/base in practical conditions [28,29]. From the perspective practical applications, highly robust and chemically stable zirconium-based MOFs hold great promising. Wang et al. [30] selectively investigated pore sizes of five Zr-MOFs for the application of C3/C1 hydrocarbons separation, and reported that the larger pore sizes and surface areas Zr-MOFs possess, the higher adsorption capacity C3 adsorbed; the smaller pore sizes and surface areas Zr-MOFs have, the higher adsorption selectivities C3/C1 achieved. Later, Zhang et al. [31] investigated adsorption separation performance of a Zr-based UiO-67 toward C1-C3 hydrocarbons. The CH4, C2H6 and C3H8 adsorption capacities of UiO-67 were 0.5, 3.0 and 8.2 mol·g−1; the corresponding IAST selectivities of C2H6/CH4 and C3H8/CH4 (v/v = 10:85) were at about 8 and 73, respectively, at 298 K and 100 kPa. However, the studies for separation of C2 and C3 from C1 based on Zr-MOFs are limited, an ideal Zr-MOF adsorbent possessing high adsorption uptake and selectivity still need to be developed. Recently, Zhou and coworkers [32] have synthesized a class of porphyrinic Zr (IV)-based MOFs, PCN-224, which are greatly attractive to researchers due to their ultrahigh surface areas, robust structure, exceptional thermal and chemical stability. However, to our best of knowledge, none of studies about light hydrocarbons separation on such porphyrinic Zr (IV)-based MOFs has been reported yet. This work is aimed to systematically demonstrate the adsorption separation performance of C1-C3 of porphyrinic Zr (IV)-based PCN-224. Besides, single-component isotherms of CH4, C2H6 and C3H8 and breakthrough experiments of C2H6/CH4 and C3H8/CH4 mixtures were performed to investigate its preferential separation performance of C2 and C3 over C1. Additionally, the water stable properties of PCN-224 were also demonstrated here.

2.3. Characterization Powder X-ray diffraction (PXRD) patterns of PCN-224 were collected on a Bruker D8 Advance X-ray diffractometer equipped with CuKα radiation (λ = 1.54 Å) operated at 40 kV and 40 mA in the 2θ range of 2–30°. Scanning electron microscopy (SEM) measurement was conducted using a Hitachi SU8220 instrument to confirm the morphology and size of the particle. To enhance the conductivity, a slim Au layer was sputter-coated on PCN-224 surface before measurement. Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209F1 apparatus, and the heating rate was of 10 °C·min−1 ranging from room temperature to 700 °C under nitrogen atmosphere. N2 adsorptiondesorption measurement at 77 K was carried out using a Micromeritics ASAP 2460 analyzer instrument. According to the N2 adsorption-desorption isotherms, the specific surface area and pore size were calculated using the BET equation and density functional theory (DFT) method, respectively. 2.4. Water stability test of PCN-224 240 mg of PCN-224 sample was separated into three vials containing 15 mL of deionized water. After placed at room temperature for 12 h, 24, and 48 h, the resulting PCN-224 products were collected by filtration and washed with acetone two times, followed by drying at 150 °C. The PXRD and N2 adsorption measurements were performed to analyze the samples, and the results were compared with the PCN-224 sample before water treatment. 2.5. Gas adsorption experiments To investigate the adsorption and separation properties of PCN-224, single-component gas adsorption experiments at different temperatures (278, 288 and 298 K) were conducted on a 3Flex Surface Characterization Analyzer (Micromeritics, USA). During this process, a circulating water bath was used to control the temperatures of gas adsorption experiments. Prior to each measurement, 80–100 mg of the sample was degassed at 150 °C for 6 h. The gas adsorption measurements were performed at the pressure ranging from 0 to 100 kPa. CH4, C2H6 and C3H8 were of ultrahigh purity grade (99.99%).

2. Experimental 2.1. Materials

2.6. Breakthrough experiments The organic linker meso-tetra(4-carboxyphenyl)porphyrin (TCPP) was purchased from J&K Scientific Co. Ltd. (China). Zirconium (IV) chloride (ZrCl4, 99.95%) was provided by Strem Chemicals, Inc. Benzoic acid (BA, 99.5%) and N,N-dimethylformamide (DMF, 99.5%) were obtained from Tianjin Chemical Plant (China). All the chemical reagents were directly used as obtained from commercial sources without further purification.

The breakthrough experiments of mixed gas C2H6/CH4 and C3H8/ CH4 (1:1, v/v) for PCN-224 were performed by using a homemade separation apparatus with a fixed-bed adsorber at 298 K, shown in Fig. S1. In this work, 200 mg PCN-224 was filled into the stainless steel column with a length of 28 cm and inner diameter of 4 mm, and the packed sample length was 5.5 cm. Before packing, the sample was activated at 150 °C under vacuum for 2 h, and then packed into the column with silica wool filled into its two ends. Afterwards, the column was linked to the six-way valve connected by gas chromatography analyzer. Prior to breakthrough test, a 60 min N2 flow (20 mL·min−1) was performed to remove the residuary gas in the pipeline system. After that, the equimolar of C2H6/CH4 or C3H8/CH4 gas mixture was introduced into the adsorption column at a flow rate of 4 mL·min−1 controlled by the mass flow controller. N2 was used as carrier gas with a flow rate of 42.4 mL·min−1. The effluent composition at the column outlet was monitored by a GC-9560 gas chromatography (Shanghai Wuhao, China) equipped with a flame ionization detector (FID). Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.seppur.2018.06.064.

2.2. Synthesis of PCN-224 The synthesis of PCN-224 sample was according to the previous procedure reported by Feng et al. [32] with some modifications. Typically, a mixture of 50 mg TCPP, 78 mg ZrCl4 and 2700 mg BA were homogeneously dissolved with 8 mL DMF under sonication in a 20 mL Pyrex vial, which was then tightly sealed and heated at 120 °C for 48 h in an oven. After the Pyrex vial was naturally cooled to ambient temperature, cubic dark purple crystals were collected by filtration and washed with fresh DMF and acetone. The activated PCN-224 was prepared by soaking the as-synthesized PCN-224 in 60 mL acetone for 24 h, in which process solvent-exchange with fresh acetone was carried out 263

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Fig. 1. The PXRD patterns of as-synthesized and activated PCN-224. Fig. 3. Powder X-ray diffraction patterns of PCN-224 upon water treatment for 0 h, 12 h, 24 h, 48 h.

3. Results and discussion

To investigate the permanent porosity of PCN-224 material, N2 adsorption-desorption measurement was carried out at 77 K. Fig. S3 shows the N2 adsorption isotherm of PCN-224 exhibiting a reversible type-I adsorption behavior, indicating the characteristic of microporous property. The N2 uptake of PCN-224 was up to 687 cm3·g−1. The BET surface area calculated from the N2 isotherm was 2704 m2·g−1 (Fig. S4). The corresponding total pore volume and micropore volume were 1.06 and 0.84 cm3·g−1, respectively. Furthermore, the pore size was found to center at around 12 and 16 Å, shown in the inset graph (Fig. S3), further confirming microporous structure of PCN-224. The establishment of intrinsic high porosity of PCN-224 inspired us to further investigate its potential applications for C1-C3 adsorption and separation.

3.1. Characterization The PXRD was employed to confirm the purity and crystallinity of PCN-224. As shown in Fig. 1, The strong characteristic peaks of 2θ at 4.6, 6.4, 7.9, 9.1, 11.2 and 13.7° were clearly observed both in the assynthesized and activated PCN-224, which matched well with that of the reported PCN-224 [32], reflecting that high-crystalline PCN-224 has been successfully prepared. Meanwhile, the PXRD pattern of the assynthesized PCN-224 was almost identical with that of the activated PCN-224, indicating that the activated PCN-224 still maintained structural integrity after the solvent-exchange treatment. Fig. 2 presents the SEM images of the activated PCN-224 sample. It is obviously observed that the PCN-224 crystals exhibited a cubic shape with sharp edges and an average crystallite size of around 3.5 µm. In addition, the PCN-224 crystals were found to have uniform and regular geometry, which was ascribed to the forming of well-crystallized PCN224 during the synthesis of PCN-224, well in consistency with the PXRD results above. TGA was performed to estimate the thermal stability of PCN-224. As depicted in Fig. S2, PCN-224 can be thermally stable up to 400 °C, revealing an excellent thermal stability of the framework. From the TGA curve, two distinct weight loss steps can be observed. The first weight loss step occurred in the temperature range of 30–200 °C, which showed a weight loss of 4.02%, mainly owing to the evaporation of solvent molecules adsorbed on the PCN-224 surface. The second weight loss step was observed at the temperature range of 400–700 °C, resulting in 32.28% weight loss accompanying with the structural decomposition of the framework.

3.2. Water stability Fig. 3 presents the PXRD patterns of PCN-224 before and after water treatment. It was noted that the diffraction peaks of PCN-224 samples soaked in water were in good agreement with that of the original PCN224, suggesting that no structure collapse or phase transition occurred upon the water treatment. To further confirm the water stability, N2 isotherms of the water-soaked PCN-224 were also measured to investigate the surface areas and porosity. As depicted in Fig. S5, only slight decrease of N2 uptake was found after immersing in water for 48 h compared to the freshly prepared PCN-224, which further proved that the porosity of PCN-224 was well retained, demonstrating its extraordinary water stability. Similar chemical or water stability has found in some reported trivalent and tetravalent based MOFs [28,33,34]. The excellent water stability of PCN-224 can be explained

Fig. 2. SEM images of PCN-224 crystals. 264

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Fig. 4. C1-C3 adsorption isotherms on PCN-224 at (a) 278 K; (b) 288 K; (c) 298 K, and (d) IAST selectivities of C2/C1 and C3/C1 for PCN-224 at 298 K.

Liu22 (4.15 mmol·g−1) [37] under the same conditions (Table S2). Another striking feature is the distinct adsorption performance difference of C1, C2 and C3 on PCN-224, suggesting that PCN-224 is a promising candidate to separate C1, C2 and C3 hydrocarbons.

by soft and hard and base theory, where Zr4+ belongs to hard Lewis acidic species and COO− from TCPP belongs to hard basic carboxylate, leading to that high-charge Zr4+ can offer strong electrostatic interaction with carboxylic oxygens of TCPP ligands and potentially be resistive to attack of water.

3.4. The adsorption selectivity of light hydrocarbons on PCN-224 3.3. Gas adsorption performance of light hydrocarbons on PCN-224 IAST model has been confirmed to be a reliable method for providing adsorption predictions, and it has been adopted by many research groups to predict adsorption selectivity of binary gas mixture in recent years [1,22,38]. Here, we employed the IAST to calculate the adsorption selectivity of PCN-224 for the following two binary mixtures: C2H6/CH4 and C3H8/CH4 (v/v, 50:50), which are related to the natural gas upgrading. Previously, the single-component isotherms of C3H8, C2H6 and CH4 at 298 K were fitted by utilizing a dual-site Langmuir-Freundlich (DSLF) model, and the DSLF equation is expressed as following:

The adsorption and separation performance of light hydrocarbons on PCN-224 was evaluated by pure component isotherms, and the isotherms of CH4, C2H6 and C3H8 were shown in Fig. 4(a)–(c). It is obviously found that the adsorption capacities of C1, C2 and C3 on PCN-224 decreased with the increasing temperature, indicating a physical adsorption behavior during the adsorption process. At any given temperature, the adsorption capacities of light hydrocarbons on PCN-224 followed the similar trend of C3 > C2 > C1, that is to say, the CH4, C2H6 and C3H8 uptakes of PCN-224 increased with the increasing carbon number. In detail, the C3H8 and C2H6 adsorbed amounts on PCN-224 could reach 10.16 and 4.69 mmol·g−1, respectively, at 278 K and 100 kPa. By contrast, the CH4 uptake of PCN-224 was only 0.70 mmol·g−1 at the same conditions. With the increase temperature to 298 K, the corresponding values of C3H8, C2H6 and CH4 adsorption were 8.25, 2.93 and 0.48 mmol·g−1, respectively. It is particularly stressed that the C3H8 adsorption capacity of PCN-224 at 298 K and 100 kPa (Fig. 4(c)) is comparable to that of UiO-67 (8.2 mmol·g−1) [31], JLU-Liu47 (8.12 mmol·g−1) [25], JLU-Liu37 (7.95 mmol·g−1) and JLU-Liu38 (8.39 mmol·g−1) [27], remarkably exceeded most the wellknown reported MOFs, such as Mg-MOF-74 (7.20 mmol·g−1) [10], UTSA-35a (2.97 mmol·g−1) [23], InOF-1 (4.25 mmol·g−1) [28], JUC100 (6.07 mmol·g−1) [35], JLU-Liu18 (5.18 mmol·g−1) [36] and JLU-

q = qm,1

bpn2 b1 pn1 + qm,2 n 1 1 + b1 p 1 + bpn2

where q (mmol·g−1) represents the equilibrium uptake for an adsorbent; qm,i (mmol·g−1) is the saturation capacity of site i; bi refers to the affinity coefficient of site i; ni is the ideal homogeneous surface deviation of site i. The adsorption selectivity for binary system is defined as:

S12 =

(x1/ x2) (y1 / y2 )

where xi and yi are the mole fraction of component i in the adsorbed phases and bulk phases, separately. 265

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Table S2 lists the fitting parameters of the DSLF model. It is obviously found that the corresponding correlation coefficients (R2) are all up to 0.9999, indicating that DSLF model could be well to fit the experimental isotherms. Fig. 4(d) shows the IAST-predicted selectivities of C2H6/CH4 and C3H8/CH4 (v/v, 50:50) mixtures separation on PCN-224 at 298 K. As displayed in Fig. 4(d), the selectivities for both binary mixtures increased with the increase of pressure, and analogous adsorption behaviors have been reported in other adsorbents and gases [25,27,31,38], which is possibly due to the stronger adsorbate-adsorbent interaction for C2 or C3 and PCN-224 [31]. In addition, PCN-224 showed remarkably higher selectivity of C3H8/CH4 than that of C2H6/CH4 under the same condition, indicating the favorable interaction with C3 over C2 for PCN-224, which is accordance with the pure component isotherms. When the pressure increased to 100 kPa, the adsorption selectivity of C3H8/CH4 was up to 609, while the C2H6/CH4 reached only 12. Besides, a comparison with some well-known MOFs with respect to light hydrocarbons separation performance has been listed in Table S2. From Table S2, it is obviously found that the C3H8/CH4 selectivity of PCN-224 at 100 kPa is significantly higher than other reported MOFs, such as JLU-Liu series (e.g. JLU-Liu22, 271.5 and JLU-Liu15, 461.5) reported by Liu’s group [37,39], JUC series (e.g. JUC-100, 80; JUC-103, 55 and JUC-106, 75) [35], UTSA series (e.g. UTSA-35a, 80) reported by Chen’s group [23], Zr-MOFs (e.g. Zr-FUM, 292) [5], which demonstrated that PCN-224 owns exceptional C3H8/CH4 selectivity among the best reported MOFs to the best of our knowledge. These consequences strongly supported the fact that CH4 can be effectively selective separated from C2 and C3 hydrocarbons by PCN-224 under ambient condition.

Fig. 5. Isosteric heats of adsorption (Qst) of C1-C3 for PCN-224.

adsorption of C3H8 and C2H6 over CH4. The consequence demonstrated the results of single-component isotherms and selectivities above. The differences of adsorption behaviors among C1, C2 and C3 can be attributed to as follows: (i) In general, for a given porous material with fixed aperture and pore volume, the larger size the adsorbate molecule have, the stronger attraction force with adsorbate molecule the surface force field of the adsorbent’s surrounding walls have [1,43]. Considering the kinetic size of C1-C3 hydrocarbons (CH4, 3.8 Å; C2H6, 4.4 Å and C3H8, 4.3–5.1 Å) [44], the adsorption capacity and Qst of light hydrocarbons on PCN-224 should followed the order of CH4 < C2H6 < C3H8. (ii) On the other hand, van der Waals attractive interaction between adsorbate and adsorbent based on the polarizability of adsorbate molecules is also a crucial parameter to described the adsorbate-adsorbent interaction potential [1,45]. The molecular polarizability increases with the molecular weight (CH4, 25.93 × 10−25 cm3; C2H6, 44.7 × 10−25 cm3 and C3H8, −25 3 62.9–73.7 × 10 cm ) [44], resulting the strongest affinity of PCN224 with C3H8 while the weakest affinity of PCN-224 with CH4.

3.5. Isosteric heat of adsorption The isosteric heat of adsorption (Qst) is an important thermodynamic parameter to assess the interaction strength between adsorbate and adsorbent, which can also provide credible energetic heterogeneity information of an adsorbent surface [1,40,41]. Herein, Qst was calculated by fitting singe-component adsorption isotherm at three temperatures (278, 288 and 298 K) using the Clausius–Clapeyron equation shown as follow:

3.6. Breakthrough experiments for equimolar C2/C1 and C3/C1 mixtures

∂ ln P ⎞ Qst = RT 2 ⎛ ⎝ ∂T ⎠q

To evaluate the applicability for dynamic separation of C2/C1 and C3/C1 mixtures with PCN-224, breakthrough experiments of equimolar two-component mixtures were performed on a fixed packed bed of PCN-224 with a total gas mixture flow rate of 4 mL·min−1 at 298 K. The breakthrough curves of CH4/C2H6 and CH4/C3H8 (v/v, 1:1) mixtures were displayed in Fig. 6(a) and (b). The breakthrough time of CH4 was shorter than that of C2H6 or C3H8, which was attributed to the stronger affinity of C2H6 or C3H8 with PCN-224, indicating the ability for PCN224 to effectively separate CH4 from C2H6 and C3H8. Furthermore, longer breakthrough times for the CH4/C3H8 mixture relative to CH4/ C2H6 mixture were observed. In the typical breakthrough process of CH4/C3H8 mixture, CH4 initially eluted through the column at 240 s and gradually reached saturation, while C3H8 was firstly detected till 792 s. By contrast, the breakthrough times of CH4 and C2H6 were 102 s min and 228 s in the separation process of CH4/C2H6 mixture, respectively. Meanwhile, a small roll-up was found in the breakthrough curve of CH4 desorption because the adsorption sites occupied by initially adsorbed CH4 were replaced with the incoming C2 or C3 until the latter one breakthrough, and similar phenomenon have reported in some previous MOFs [31,46]. The replaced of adsorbed CH4 was ascribed to the weak affinity between C1 and PCN-224. Considering the results of IAST-predicted selectivity and breakthrough experiments, PCN-224 is of great potential for application in separation of C1-C3 hydrocarbons.

−1

where Qst (kJ·mol ) represents the isosteric heat of adsorption at a given adsorbent surface loading; R (kJ·mol−1·K−1) refers to the universal gas constant; T (K) is the temperature; P (kPa) represents the pressure; q (mmol·g−1) is the adsorbed amount. Fig. 5 displays the calculated isosteric heats of C3H8, C2H6 and CH4 adsorption on PCN-224. As shown in Fig. 5, the variation of Qst is the function of gas loading, and the value of Qst reflects the affinity strength between adsorbate and PCN-224. The Qst of C1-C3 hydrocarbons of PCN-224 followed the order of CH4 < C2H6 < C3H8. In detail, the Qst of CH4, C2H6 and C3H8 are found to be in the range of 14–15 kJ·mol−1, 21–26 kJ·mol−1 and 28–34 kJ·mol−1, respectively. Obviously, none of them did maintain at a constant level when the adsorbed amounts increased, indicating the energetic heterogeneity of PCN-224 surface [28]. Besides, the isosteric heat of C2H6 adsorption decreased with the increasing loading amounts, possibly owing to the confinement of C2H6 molecules within the PCN-224 pore cavity [42]. In the case of C3H8, the adsorption heat first reduced with the loading up to 3.8 mmol·g−1 and subsequently increased slowly at high loading qualities, which can explained by van der Waals interaction between C3H8 molecules [10,20,42], and the isosteric heat of CH4 showed similar to C3H8. More importantly, the Qst of C3H8 and C2H6 were remarkably higher than CH4, which suggested that the interactions of PCN-224 with C3H8 or C2H6 were stronger than that with CH4, resulting in the preferential 266

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Fig. 6. Breakthrough curves of C2/C1 and C3/C1 (1:1, v/v) mixtures separation on pack bed of PCN-224.

4. Conclusions [9]

In summary, a porphyrinic zirconium MOF named PCN-224 has been studied for separation of C2/C1 and C3/C1 mixtures. According to the investigation of CH4, C2H6 and C3H8 adsorption and separation on PCN-224, C2H6/CH4 and C3H8/CH4 mixtures could be effectively separated under 298 K and 100 kPa. Both the adsorption capacities and the isosteric heat of C1, C2 and C3 on PCN-224 followed the trend: C3 > C2 > C1, suggesting a weaker interaction between C1 and the framework than that of C2 and C3. As a result, PCN-224 exhibited performance of preferential adsorption for C2 and C3 over C1. Furthermore, selectivity calculation and breakthrough experiments roundly demonstrated that PCN-224 features high selectivities for C2 and C3 relative to CH4. The IAST-predicted selectivities of C2H6/CH4 and C3H8/CH4 were up to 12 and 609, respectively. In addition, the fixed-bed adsorption measurements also displayed great potential in kinetic separation of C2/C1 and C3/C1 mixtures using PCN-224. More importantly, PCN-224 possessed exceptional water stability. From our study, PCN-224 was demonstrated to be a promising adsorbent to the purification of natural gas.

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Acknowledgments [19]

This work was supported by the National Natural Science Foundation of China (Nos. 21576092, 21436005 and U1662136), Guangdong Province Science and Technology Project (No. 2016A020221006).

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