Efficient Propylene/Ethylene Separation in Highly Porous Metal–Organic Frameworks

Abstract

Light olefins are important raw materials in the petrochemical industry for the production of many chemical products. In the past few years, remarkable progress has been made in the synthesis of light olefins (C2–C4) from methanol or syngas. The separation of light olefins by porous materials is, therefore, an intriguing research topic. In this work, single-component ethylene (C₂H₄) and propylene (C₃H₆) gas adsorption and binary C₃H₆/C₂H₄ (1:9) gas breakthrough experiments have been performed for three highly porous isostructural metal–organic frameworks (MOFs) denoted as Fe₂M-L (M = Mn²⁺, Co²⁺, or Ni²⁺), three representative MOFs, namely ZIF-8 (also known as MAF-4), MIL-101(Cr), and HKUST-1, as well as an activated carbon (activated coconut charcoal, SUPELCO^©). Single-component gas adsorption studies reveal that Fe₂M-L, HKUST-1, and activated carbon show much higher C₃H₆ adsorption capacities than MIL-101(Cr) and ZIF-8, HKUST-1 and activated carbon have relatively high C₃H₆/C₂H₄ adsorption selectivity, and the C₂H₄ and C₃H₆ adsorption heats of Fe₂Mn-L, MIL-101(Cr), and ZIF-8 are relatively low. Binary gas breakthrough experiments indicate all the adsorbents selectively adsorb C₃H₆ from C₃H₆/C₂H₄ mixture to produce purified C₂H₄, and 842, 515, 504, 271, and 181 cm³ g⁻¹ C₂H₄ could be obtained for each breakthrough tests for HKUST-1, activated carbon, Fe₂Mn-L, MIL-101(Cr), and ZIF-8, respectively. It is worth noting that C₃H₆ and C₂H₄ desorption dynamics of Fe₂Mn-L are clearly faster than that of HKUST-1 or activated carbon, suggesting that Fe₂M-L are promising adsorbents for C₃H₆/C₂H₄ separation with low energy penalty in regeneration.

1. Introduction

Light olefins, particularly ethylene (C₂H₄) and propylene (C₃H₆), are important raw materials in the petrochemical industry for the manufacture of products such as plastics, solvents, cosmetics, paints, and drugs. Light olefins are traditionally produced from the thermal or catalytic cracking of crude oil. In the past decades, considerable attention has been paid to the production of light olefins (C₂⁼–C₄⁼) from other alternative feedstocks, such as coal, natural gas, and biomass, by the Fischer–Tropsch synthesis (FTS) and the methanol-to-olefins (MTO) reaction [1,2]. Some recent breakthrough results showed that coal- and biomass-derived syngas (a mixture of carbon monoxide and hydrogen) could be converted to light olefins in very high selectivities (up to 80%, relative to less valuable saturated hydrocarbons) by advanced catalysts [3,4]. These new innovations would lead to a high demand for efficient separation technology for light olefins in the future. Nowadays, the separation of C₂H₄ and C₃H₆ is commonly accomplished by cryogenic distillation, which is a mature but energy-intensive process. Many works have been reported to develop new materials and technologies to efficiently separate C₂H₄ and C₃H₆ [5,6,7,8,9]. Adsorptive separation of C₂H₄ and C₃H₆ with porous materials is regarded as a promising alternative [10].

Metal–organic frameworks (MOFs), a type of porous material composed of metal ions/clusters and organic ligands, have attracted intensive research attention in the past two decades [11,12,13,14]. The high porosity, tailorable structure, crystalline nature, and facile synthesis of MOFs endow this type of material with great potential in gas storage, separation, catalysis, and sensing, among others. Particularly, some recent works have shown that bimetallic MOFs could exhibit improved performance in gas adsorption and catalysis [15,16,17,18]. Separation is one of the most studied applications where the intrinsic porous structures of MOFs could be utilized. It has been demonstrated that MOFs are high potential in light hydrocarbon separations [19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37]. Relatively, the separation of C₂H₄ and C₃H₆ using MOFs is less explored, although C₂H₄ and C₃H₆ adsorption isotherms of some MOFs were reported [38,39,40].

In this work, the C₃H₆/C₂H₄ separation performances of three new isostructural MOFs, [Fe₂M(µ₃-O)(L)₂] (denoted as Fe₂M-L, M = Mn²⁺, Co²⁺, or Ni²⁺, H₃L = [1,1′:3′,1″-terphenyl]-4,4″,5′-tricarboxylic acid), three representative MOFs, ZIF-8 (a zinc(II) 2-methylimidazolate also known as MAF-4) [41,42], MIL-101(Cr) (a chromium(III) terephthalate) [43], and HKUST-1 (a copper(II) benzene-1,3,5-tricarboxylate) [44], and a commercial activated carbon (activated coconut charcoal, SUPELCO^©) have been evaluated by single-component gas adsorption isotherm measurements and binary gas mixture breakthrough experiments. Their C₃H₆ and C₂H₄ adsorption properties and C₃H₆/C₂H₄ separation performances have been compared and discussed.

2. Materials and Methods

2.1. Materials and Instrumentation

All the chemicals were purchased from chemical suppliers and utilized without purification. ZIF-8, MIL-101(Cr), and HKUST-1 were prepared by previously reported methods [45,46,47]. Activated carbon (activated coconut charcoal, SUPELCO^© (Bellefonte, PA, USA) was purchased from Sigma-Aldrich (Missouri, USA). Thermal gravimetric analyses (TGA) were carried out under zero air conditions with a SHIMADZU TGA-50 thermogravimetric analyzer (heating rate: 5 °C/min) (Kyoto, Japan). Fourier transform infrared (FT-IR) spectra were measured with a Shimadzu IRAffinity FT-IR spectrophotometer (Kyoto, Japan). The adsorption isotherms of N₂ and hydrocarbons were recorded with an ASAP2020 adsorption analyzer (Micromeritics, Norcross, GA, USA). PXRD measurements were performed with a Smartlab3 X-ray powder diffractometer (Rigaku, Tokyo, Japan). Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) measurements were performed with a PerkinElmer Optima 8300 spectrometer (Waltham, MA, USA).

2.2. Synthesis

Syntheses of [Fe₂M(µ₃-O)(CH₃COO)₆] (the precursors of Fe₂M-L, M = Mn²⁺, Co²⁺, or Ni²⁺): an aqueous solution (70 ml) of Na(CH₃COO) 3H₂O (42 g, 0.31 mol) was added to an aqueous solution (70 ml) of Fe(NO₃)₃·9H₂O (8 g, 0.02 mol) and M(NO₃) (0.1 mol). Dark-red precipitate was formed, which was collected after filtration, washing with solvents (water and ethanol), and drying in the air [17].

Syntheses of Fe₂M-L: to a 5 mL glass vial, [Fe₂M(µ₃-O)(CH₃COO)₆] (~0.015 g, 0.05 mmol) and the ligand H₃L (H₃L = 1,1′:3′,1″-terphenyl]-4,4″,5′-tricarboxylic acid) (0.012 g, 0.05 mmol) were added. Then, the solvent N,N-dimethylformamide (DMF, 2 mL) and the coordination modulator acetic acid (0.25 mL) were introduced to the vial. The solids were all dissolved after an ultrasonication treatment, and the vial was capped and placed in a preheated oven (120 °C) for 72 h. Block-shaped dark yellow or brown single crystals were formed and collected (yield: ca. 81% based on Fe).

2.3. Single-Crystal X-ray Diffraction

Single crystals of Fe₂M-L were picked for single-crystal diffraction experiments. The diffraction data were collected at 100 K with a Rigaku Supernova CCD diffractometer (Tokyo, Japan) equipped with a mirror-monochromatic enhanced Cu-Kα radiation (λ = 1.54184 Å). The dataset was corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The structure was solved by direct methods and refined by full-matrix least-squares on F² with anisotropic displacement using the SHELXTL software package (Göttingen, Germany) [48]. Non-hydrogen atoms on the frameworks were refined with anisotropic displacement parameters during the final cycles. The hydrogen atoms on the ligands were positioned geometrically and refined by using a riding model. The electron density of the disordered guest molecules in Fe₂M-L was flattened by using the SQUEEZE routine of PLATON (Utrecht, The Netherlands) [49]. The graphical representations of single-crystal structures of Fe₂M-L were performed by the Diamond software (Bonn, Germany) [50]. Some strong residual Q peaks were found near the water molecules coordinated with the metal ions in the final refinement cycles, indicating that partially coordinated solvent molecules may be DMF molecules rather than water molecules, which could not be fully modeled. The single-crystal structure data of Fe₂M-L have been deposited in the Cambridge Crystallographic Data Centre (CCDC deposition number: 2177907–2177909).

2.4. Estimation of C₃H₆ and C₂H₄ Adsorption Heat

The C₃H₆ and C₂H₄ adsorption heats were calculated from the adsorption data recorded at 298 and 273 K. The Toth equation [51] (Equation (1)) was first utilized for fittings the adsorption isotherms (Figures S10–S19), where N stands for the gas uptake, N_sat stands for the saturated uptake, P represents pressure, b and t represent two constants.

$$N=\frac{N_{sat}bP}{{(1+b{P}^t)}^{\frac{1}{t}}}$$

(1)

$$P=\frac{N}{{(b^t{N}_{sat}{}^t-b{N}^t)}^{\frac{1}{t}}}$$

(2)

The following Equation (2) could be obtained from the rearrangement of Equation (1).

The C₃H₆ and C₂H₄ adsorption heats (Q_st) were estimated by the Clausius–Clapeyron equation [52] (Equation (3)), where C represents a constant, R stands for the universal gas constant, and T stands for temperature. Assuming (lnP)_N is the function, and (1/T) is the variable, Q_st depending on the gas uptake (N), could be calculated from the slopes data points (Q_st/R).

$${(\ln P)}_N=-\frac{Q_{st}}{R}\frac{1}{T}+C$$

(3)

2.5. Prediction of IAST C₃H₆/C₂H₄ Selectivity

Ideal adsorbed solution theory [53] (IAST) is a well-accepted way to predict gas adsorption selectivity for gas mixtures by adsorption data of the individual gases. The IAST defines the following equations.

$$y_1+y_2=1$$

(4)

$$x_1+x_2=1$$

(5)

$$p_{mix}{y}_1=p_1^o{x}_1$$

(6)

$$p_{mix}{y}_2=p_2^o{x}_2$$

(7)

$${\pi }_1^o=\frac{RT}{A}{\displaystyle {\int }_0^{p_1^o}{n}_1}(p)d\ln p$$

(8)

$${\pi }_2^o=\frac{RT}{A}{\displaystyle {\int }_0^{p_2^o}{n}_2}(p)d\ln p$$

(9)

$$\pi ={\pi }_1^o={\pi }_2^o$$

(10)

In these equations, R stands for the universal gas constant, T stands for the temperature for adsorption experiment, A represents the surface area of adsorbent, x_i stands for the ratio of gas i in the gas mixture adsorbed by adsorbent, y_i stands for the ratio of gas i in the gas mixture before adsorption, p_mix represents the pressure of gas mixture before adsorption, p_i⁰ represents the pressure of gas i that corresponds to the spreading pressure π of the binary mixture, n_i(p) stands for the uptake of gas i at the pressure p.

From Equations (4)–(10), Equation (11) is obtained.

$${\displaystyle {\int }_0^{\frac{p_{mix}{y}_1}{x_1}}{n}_1}(p)d\ln p={\displaystyle {\int }_0^{\frac{p_{mix}{y}_2}{1-x_1}}{n}_2}(p)d\ln p$$

(11)

$$S_{12}=\frac{x_1{y}_2}{x_2{y}_1}$$

(12)

The adsorption selectivity of gas 1 over gas 2 (S₁₂) can be obtained by Equation (12).

For the adsorption of 1:9 C₃H₆/C₂H₄ gas mixture at 1 bar, C₃H₆ is gas 1, C₂H₄ is gas 2, p_mix is 1 bar, y₁ is 0.1, y₂ is 0.9, and n_i(p) can be calculated by fittings adsorption data of the individual gases (Figures S20–S24). x₁ can then be calculated by solving Equation (11). At last, x₂ can be obtained by applying the x₁ to Equation (5), and S₁₂ can be calculated from Equation (12). The C₃H₆/C₂H₄ selectivities for a 1:1 C₃H₆/C₂H₄ gas mixture or at other pressures could be calculated similarly.

2.6. Gas Mixture Breakthrough

Breakthrough tests for the gas mixture were performed with a 1:9 C₃H₆/C₂H₄ gas mixture. Quartz tubes (6 mm for outer diameter, 3 mm for inner diameter, and 100 mm in length) were packed with the adsorbents. The adsorbents were first activated at 100 °C overnight inside the tubes under a He flow with a flow rate of 10 SCCM. Mass flow controllers (Alicat Scientific, Tucson, AZ, USA) were used to control the gas flow rate. After the adsorbents were cooled down to room temperature, the breakthrough experiments started when the He flow was changed into a flow of the C₃H₆/C₂H₄ gas mixture (flow rate: 2 SCCM). The concentrations of C₃H₆ and C₂H₄ gases at outlet were determined by a mass spectrometer (Hiden HPR20). For monitoring the desorption dynamics of the adsorbed C₃H₆ and C₂H₄ gases in the adsorbents, the gas flow was changed from the binary C₃H₆/C₂H₄ gas to a He flow of 10 SCCM.

3. Results and Discussion

3.1. Crystal Structure and Porosity

Fe₂M-L were synthesized from solvothermal reactions of [Fe₂M(µ₃-O)(CH₃COO)₆] precursors and H₃L ligand in DMF at 120 °C. It was found that using the pre-synthesized [Fe₂M(µ₃-O)(CH₃COO)₆] precursors instead of mixtures of Fe/M metal salts as metal ion source is necessary for the formation of final crystalline products. Additionally, the introduction of an excess coordination modulator (acetic acid) is important for the production of Fe₂M-L crystals in high crystallinity and with a relatively large size (>0.1 mm). It is well-known that coordination modulators affect the nucleation and growth rate, morphology, and crystallinity of MOFs, and only intergrown aggregates with poor crystallinity or even amorphous phases could be obtained without modulators in some cases [17,54]. The presence of two types of metal ions with a Fe to M ratio of 2:1 in Fe₂M-L was confirmed by ICP-AES measurements for digested samples of the MOFs (Table S2).

Single-crystal X-ray diffraction (SCXRD) experiments and structure analyses revealed that Fe₂M-L are isostructural and in the R-3c space group (trigonal) (Table S1). There is one type of [Fe₂M(µ₃-O)(−COO)₆] (denoted as Fe₂M hereafter) clusters and one type of L³⁻ ligands in their structures. Each Fe₂M cluster is connected with six different but equivalent L³⁻ ligands, and each L³⁻ ligand bridges three Fe₂M clusters (Figure 1a). The interconnection of the Fe₂M clusters and L³⁻ ligands in such a way results in their 3D frameworks, which could be regarded as (3,6)-connected nets from a topological point of view (point symbol: {4·6²}₂{4²·6⁷ 8⁶}). The frameworks contain large volumes which are occupied by disordered guest molecules, as indicated by TGA results (Figure S1). The solvent-accessible volumes are ~74% of the whole structures, as estimated by Platon [49].

The highly open frameworks of Fe₂M-L can also be regarded as alternate packing of two sets of cages. One set is octahedral, each of which consists of 6 Fe₂M clusters and 8 L³⁻ ligands (Figure 1b). In the octahedral cage, two neighboring Fe₂M clusters are all bridged by one L³⁻ ligand with its two carboxylate groups. The other set of cages is in a tetrakaidecahedron shape, each of which is built from 12 Fe₂M clusters and 12 L³⁻ ligands, where each L³⁻ ligand links 3 neighboring Fe₂M clusters (Figure 1c). The cavities inside the octahedral and tetrakaidecahedral cages are ~6 and ~9 Å, respectively. Each small cage is surrounded by eight large cages, and each large cage is surrounded by eight small cages and six equivalent large cages by polyhedral face sharing (Figure 1d). The windows between two small cages or between one large cage and one small cage are in a diameter of ~3.8 Å, and the windows between two large cages are in a diameter of ~5.1 Å. Several isoreticular MOFs to Fe₂M-L were previously reported [55,56,57].

To confirm the permanent porosity of Fe₂M-L, N₂ adsorption isotherms were recorded at 77 K after the as-prepared crystals were activated by guest exchange (methanol as solvent) and then degassing at 80 °C. The three MOFs showed highly similar N₂ adsorption isotherms (Figure 2a), which is consistent with their isostructural structures and close unit cell parameters. The isotherms are type I isotherms typical for microporous materials, showing saturated N₂ adsorption capacities of 837, 854, and 847 cm³ g⁻¹ at ~1 P/P₀ for Fe₂Mn-L, Fe₂Co-L, and Fe₂Ni-L, respectively. The apparent BET/Langmuir surface areas of the three MOFs are estimated to be 3105/3600, 3168/3675, and 3169/3674 m² g⁻¹, respectively. The pore volumes are estimated to be 1.29, 1.32, and 1.31 cm³ g⁻¹, respectively, which are almost the same as the predicted pore volumes from their single crystal data (1.30, 1.32, and 1.32 cm³ g⁻¹). The results suggested that the MOF samples were in a pure phase, and their highly open frameworks remained unchanged after the evacuation of guests. The high purity of the batch MOF crystal samples was also confirmed by PXRD measurement results, which showed a good agreement between the PXRD patterns of the MOF samples and the single-crystal structure simulated ones (Figure S2).

3.2. Adsorption Study for C₂H₄ and C₃H₆

The adsorption isotherms of C₂H₄ and C₃H₆ were recorded for Fe₂M-L at 298 K as well as 273 K. As shown in Figure 2b, the three MOFs showed close gas adsorption capacities at all pressure ranges. The C₂H₄ uptakes were 87.5, 94.4, and 94.9 cm³ g⁻¹, and the C₃H₆ uptakes are 291.1, 302.3, and 304.2 cm³ g⁻¹ at 1 bar for Fe₂Mn-L, Fe₂Co-L, and Fe₂Ni-L, respectively. The slightly lower gas uptakes of Fe₂Mn-L with respect to those of the other two MOFs are consistent with the results of 77 K N₂ adsorption studies, which revealed that Fe₂Mn-L had a slightly lower porosity than Fe₂Co-L and Fe₂Ni-L. Overall, the results indicate the gas adsorption properties of Fe₂M-L are not very dependent on the nature of the M ions. Therefore, only Fe₂Mn-L was investigated for further experiments. For comparison, C₂H₄ and C₃H₆ adsorption measurements were also carried out for four benchmark adsorbents, namely, HKUST-1, MIL-101(Cr), ZIF-8, and a commercial activated carbon. ZIF-8, MIL-101(Cr), and HKUST-1 were all prepared by reported methods [45,46,47], and activated carbon was purchased from Sigma-Aldrich. Before the C₂H₄ and C₃H₆ adsorption measurements, PXRD measurements and/or N₂ adsorption experiments at 77 K were performed for those adsorbents (Figures S3–S9). According to the adsorption experiments, their porosities are evaluated and shown in

Table 1. The porosities of HKUST-1, MIL-101(Cr), ZIF-8, activated carbon, and Fe₂M-L estimated by their N₂ adsorption data.

Adsorbent	Pore Volume (cm3 g−1)	SBET (m2 g−1)	SLangmuir (m2 g−1)
HKUST-1	0.63	1513.6	1759.3
MIL-101(Cr)	1.47	2691.8	4702.1
ZIF-8	0.85	1510.8	2007.9
activated carbon	0.46	1086.4	1262.6
Fe2Mn-L	1.29	3105.2	3599.6
Fe2Co-L	1.32	3168.1	3674.5
Fe2Ni-L	1.31	3168.9	3674.3

.

As shown in Figure 2c, for HKUST-1 and activated carbon, the C₃H₆ uptakes increase abruptly at the low-pressure range (~140 and 90 cm³ g⁻¹ at 0.1 bar), and after a gradual increase at the high-pressure range, the uptakes reach 167.0 and 128.9 cm³ g⁻¹ at 1 bar, respectively. The C₂H₄ adsorption isotherms of the two adsorbents share a similar profile to the C₃H₆ adsorption isotherms with lower uptakes, being 136.5 and 98.8 cm³ g⁻¹ at 1 bar, respectively. In contrast, for MIL-101(Cr) and ZIF-8, the uptakes of C₃H₆ or C₂H₄ all gradually increase in the full pressure range. Notably, although MIL-101(Cr) has a larger pore volume (1.47 cm³ g⁻¹) than Fe₂Mn-L (1.29 cm³ g⁻¹), its C₃H₆ and C₂H₄ uptakes at 1 bar (196.6 and 62.1 cm³ g⁻¹) is obviously lower than those of Fe₂Mn-L (291.1 and 87.5 cm³ g⁻¹). Additionally, the gas uptakes of ZIF-8 are the lowest among all the tested adsorbents, being 80.6 and 26.4 cm³ g⁻¹ at 1 bar for C₃H₆ and C₂H₄, respectively, although its pore volume is clearly higher than those of HKUST-1 and activated carbon (Table 1). The C₃H₆ uptake of Fe₂Mn-L at 1 bar is higher than those of the other adsorbents, but its C₂H₄ uptake is lower than that of HKUST-1 and activated carbon. The C₃H₆/C₂H₄ uptake ratios at 1 bar are 3.3, 3.2, 3.1, 1.3, and 1.2 for Fe₂Mn-L, MIL-101(Cr), ZIF-8, activated carbon, and HKUST-1, respectively. The C₃H₆ and C₂H₄ adsorption isotherms of MIL-101(Cr), ZIF-8, and HKUST-1 were also previously reported [20,38,40,58,59,60,61], and those reported results are basically consistent with those presented in this work. Some slight differences may result from the subtle difference in sample preparation and/or activation.

To assess their capability of selectively adsorbing C₃H₆ from C₃H₆/C₂H₄ mixture, the IAST selectivities [53] were predicted for the five adsorbents to 1:1 and 1:9 binary C₃H₆/C₂H₄ gases, respectively. The results show that C₃H₆/C₂H₄ selectivities for HKUST-1 and activated carbon are higher than those for Fe₂Mn-L, MIL-101(Cr), and ZIF-8, especially at low-pressure range (Figure 3a,b). The selectivities of HKUST-1, activated carbon, Fe₂Mn-L, ZIF-8, and MIL-101(Cr) at 1 bar are 16.3, 11.6, 7.8, 6.7, and 6.6 for the 1:1 binary C₃H₆/C₂H₄ gas, and 18.0, 14.4, 7.6, 7.0, and 6.3 for the binary 1:9 C₃H₆/C₂H₄ gas, respectively. Noteworthily, the order of the adsorbents in their IAST selectivities at 1 bar (HKUST-1 > activated carbon > Fe₂Mn-L > ZIF-8 > MIL-101(Cr)) is dramatically different from the order of the adsorbents in their C₃H₆/C₂H₄ uptake ratios at 1 bar in adsorption experiments of pure gases (Fe₂Mn-L > MIL-101(Cr) > ZIF-8 > activated carbon > HKUST-1).

For a better understanding of the C₃H₆ and C₂H₄ adsorption behavior and the IAST predicted C₃H₆/C₂H₄ selectivities, the C₃H₆ and C₂H₄ adsorption heats (Q_st) were estimated for the adsorbents by Clausius–Clapeyron equation using Toth equation fitting parameters from adsorption results obtained at 273 and 298 K (Figures S10–S19) [51,52]. For C₂H₄ adsorption, the Q_st values for HKUST-1, activated carbon, Fe₂Mn-L, and MIL-101(Cr) decrease as the loadings increase, whereas the Q_st values for ZIF-8 increase at low loading and stabilize at higher loadings (Figure 3c). The Q_st values at low loading for HKUST-1, activated carbon, Fe₂Mn-L, MIL-101(Cr) (45.1, 32.5, 38.9, 35.8 kJ mol⁻¹) are obviously larger than the Q_st of ZIF-8 (13.8 kJ mol⁻¹), which is similar to the vaporization enthalpy of C₂H₄ (~14 kJ mol⁻¹). For C₃H₆ adsorption, the Q_st values for MIL-101(Cr) (from 34.3 to 26.7 kJ mol⁻¹) and activated carbon (from 38.5 to 32.5 kJ mol⁻¹) decrease as the loading increase, while the Q_st values for HKUST-1, ZIF-8 and Fe₂Mn-L first gradually decrease from low loading and then increase at high loading (Figure 3d), changing from 48.5, 29.0, and 39.9 kJ mol⁻¹, to 35.0, 26.8, and 28.4 kJ mol⁻¹, and eventually to 41.0, 35.6, and 34.1 kJ mol⁻¹, respectively. Q_st values of the adsorbents are obviously large than the vaporization enthalpy of C₃H₆ (~19 kJ mol⁻¹). The rise of Q_st at high loading probably results from the rising of guest-guest interactions.

The above results revealed HKUST-1 and activated carbon show high affinity to the C2 and C3 olefins, which resulted in their high loading of the olefins even at low-pressure ranges. Relatively, the interactions between the olefins and Fe₂Mn-L, MIL-101(Cr), or ZIF-8 are low, although, at low pressures, Fe₂Mn-L and MIL-101(Cr) also show quite high Q_st values. The interaction between Fe₂Mn-L and C₃H₆ is not strong, but it shows high C₃H₆ uptakes, which should be a result of its high porosity and moderate pore size. It is also suggested that the nature of metal ions in the adsorbents does not profoundly affect their C₃H₆ and C₂H₄ adsorption behaviors. For the adsorptive C₃H₆/C₂H₄ separation, three key parameters need to be considered, namely, C₃H₆/C₂H₄ adsorption selectivity, adsorption capacity, and regeneration energy. Based on the above-mentioned results, among the tested adsorbents, HKUST-1 may outperform the others in high adsorption selectivity, while Fe₂Mn-L may be advantageous in high adsorption capacity and facile regeneration for C₃H₆/C₂H₄ separation.

The C₃H₆/C₂H₄ separation performances of other types of adsorbents have also been reported. For example, Han et al. reported two covalent organic frameworks (COFs), CR-COF-1 and CR-COF-2, by one-pot Suzuki coupling and Schiff’s base reaction [5]. The C₃H₆ uptakes were 84 and 137 cm³ g⁻¹, and the C₂H₄ uptakes were 38 and 72 cm³ g⁻¹ at ~1 bar and 298 K for CR-COF-1 and CR-COF-2, respectively. The uptakes are obviously lower than those of Fe₂Mn-L (291.1 and 87.5 cm³ g⁻¹). Zhang et al. prepared a blend membrane by doping 15% (mass) polyethylene glycol (PEG600) into the poly(ether-block-amide) (Pebax® 2533) polymer matrix [6]. For a 1:1 binary C₃H₆/C₂H₄ gas, the Pebax® 2533/PEG600 membrane showed a C₃H₆ permeability of 273 barrer and a C₃H₆/C₂H₄ selectivity of 4.15 at 293 K and 2 bar, and a C₃H₆ permeability of 196 barrer and a C₃H₆/C₂H₄ selectivity of 8.90 at 238 K and 2 bar. The permselectivities are comparable to the C₃H₆/C₂H₄ IAST selectivity of Fe₂Mn-L at 298 K and 1 bar for a 1:1 binary C₃H₆/C₂H₄ gas (7.8).

3.3. Dynamic Breakthrough of Binary C₃H₆/C₂H₄ Gas

To confirm the separation capacities of the adsorbents for a real binary C₃H₆/C₂H₄ gas, breakthrough experiments were performed for the quartz tubes packed with the adsorbents by using a binary gas of C₃H₆/C₂H₄ (1:9) at room temperature and ambient pressure. The tested adsorbents all show capability to separate the C₃H₆/C₂H₄ gas mixture, which is indicated by the gaps between their C₂H₄ and C₃H₆ breakthrough curves (Figure 4a). Among the five adsorbents, HKUST-1 showed the longest breakthrough time for C₃H₆ (530 min g⁻¹), indicating a C₃H₆ adsorption capacity of 106 cm³ g⁻¹. The C₂H₄ breakthrough time for HKUST-1 was 62 min g⁻¹, corresponding to a C₂H₄ adsorption capacity of 112 cm³ g⁻¹. Accordingly, about 842 cm³ g⁻¹ purified C₂H₄ could be obtained in a breakthrough test of HKUST-1. The C₃H₆/C₂H₄ separation capacity is also high for activated carbon according to its breakthrough curves. It captured 69 cm³ g⁻¹ C₃H₆ and 106 cm³ g⁻¹ C₂H₄, and about 515 cm³ g⁻¹ purified C₂H₄ could be obtained for a breakthrough run. Fe₂Mn-L captured about 73 cm³ g⁻¹ C₃H₆ before C₃H₆ started to penetrate the MOF column. Meanwhile, 153 cm³ g⁻¹ C₂H₄ was adsorbed, and the productivity of purified C₂H₄ in each breakthrough run was 504 cm³ g⁻¹ for Fe₂Mn-L, slightly less than the productivity of activated carbon. The C₂H₄ and C₃H₆ adsorption capacities of ZIF-8 and MIL-101(Cr) were obviously lower, and only about 181 and 271 cm³ g⁻¹ purified C₂H₄ could be obtained in each of their breakthrough tests, respectively.

The binary gas breakthrough experiment results are in good accordance with the results of adsorption isotherm measurements for the individual gases, IAST selectivity, and adsorption heat calculations. Specifically, HKUST-1 exhibited high Q_st and uptakes for both C₃H₆ and C₂H₄, and its C₃H₆/C₂H₄ IAST selectivities were also high relative to those of the other tested adsorbents. A high C₃H₆/C₂H₄ separation performance was indeed observed in the binary gas breakthrough experiment for HKUST-1. Similar results were also observed for activated carbon, except that it exhibited lower adsorption capacities than HKUST-1. For Fe₂Mn-L, although its C₃H₆/C₂H₄ IAST selectivities were obviously lower than those of HKUST-1 and activated carbon, it still showed a high C₃H₆/C₂H₄ separation performance for a real binary gas. Compared with HKUST-1, Fe₂Mn-L shows lower productivity of purified C₂H₄ in a breakthrough run, and the C₂H₄ productivities of Fe₂Mn-L and activated carbon are close. However, it should be noted that the Q_st values of Fe₂Mn-L for C₃H₆ and C₂H₄ adsorption are much lower than those of HKUST-1 and activated carbon, which would lead to its advantage in regeneration with less energy consumption. This conjecture is further sustained by the comparison of C₃H₆ and C₂H₄ desorption dynamics of the adsorbents. After C₃H₆ and C₂H₄ were saturatedly adsorbed by the adsorbents during breakthrough experiments, the adsorbents were regenerated by purging a He flow. It was found that the evacuation of adsorbed gases in Fe₂Mn-L was clearly faster than that of HKUST-1 or activated carbon (Figure 4b).

4. Conclusions

In summary, three new isostructural MOFs, Fe₂M-L (M = Mn²⁺, Co²⁺, or Ni²⁺), have been obtained, which all show large surface areas (BET: ~3100 m² g⁻¹) and high pore volumes (~1.3 cm³ g⁻¹). Adsorption isotherms at room temperature suggest the MOFs uptake ~300 cm³ g⁻¹ C₃H₆ and ~90 cm³ g⁻¹ C₂H₄ at 1 bar. The potential of the MOFs in C₃H₆/C₂H₄ separation has been further evaluated by IAST selectivity prediction, adsorption heat calculations, and dynamic binary C₃H₆/C₂H₄ (1:9) gas breakthrough experiments. The predicted IAST C₃H₆/C₂H₄ adsorption selectivity for Fe₂Mn-L is ~8 at 298 K and 1 bar. The C₃H₆ and C₂H₄ adsorption heats for Fe₂Mn-L are estimated to be 28–40 kJ mol⁻¹ and 20–38 kJ mol⁻¹, respectively. A binary gas breakthrough experiment confirms the capability of Fe₂Mn-L to selectively adsorb C₃H₆ over C₂H₄, producing 469 cm³ g⁻¹ purified C₂H₄ in a breakthrough run. In addition, the C₃H₆/C₂H₄ separation performances of four other benchmark adsorbents, HKUST-1, MIL-101(Cr), ZIF-8, and activated carbon, were also studied for comparison. The results reveal that Fe₂M-L are promising adsorbents for C₃H₆/C₂H₄ separation with low energy penalty in regeneration, although HKUST-1 shows higher C₃H₆/C₂H₄ adsorption selectivity and productivity of purified C₂H₄ than Fe₂M-L and the other tested adsorbents.