Size exclusion propyne/propylene separation in an ultramicroporous yet hydrophobic metal–organic framework

Abstract

Propyne/propylene separation is important in the petrochemical industry but challenging due to their similar physical properties and close molecular sizes. Herein, we present two isoreticular ultramicroporous Zn(II)-MOFs, Zn₂(ATZ)₂(TPDC) (BUT-305, H₂TPDC = [1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid, HATZ = 3-amino-1,2,4-triazole) and Zn₂(ATZ)₂(MeTPDC) (BUT-306, H₂MeTPDC = 5′-methyl-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid). The pore aperture of BUT-306 (∼1.6 Å) is smaller than that of BUT-305 due to the presence of extra gate-like methyl groups in the 1D channels of the former. With a narrow and hydrophobic pore aperture, BUT-306 exhibits high hydrophobicity and hydrolytic stability and adsorbs C₃H₄ but excludes C₃H₆ in a wide temperature range. The C₃H₆ and C₃H₄ adsorption capacities of BUT-306 at 298 K and ∼1 bar were 2.4 and 29.6 cm³ g⁻¹, respectively. Dynamic column breakthrough experiments confirmed the high capability of BUT-306 to remove C₃H₄ from the equimolar binary gas mixture of C₃H₄ and C₃H₆. The C₃H₄/C₃H₆ separation performance of BUT-306 was largely retained even when the binary C₃H₄/C₃H₆ gas for breakthrough experiments was pre-saturated with water vapor. In addition, the single-crystal structure of C₃H₄-loaded BUT-306 was determined, which revealed that the adsorbed C₃H₄ molecules were located in the center of channel cavities and interacted with the MOF by multiple weak C^δ−⋯C^δ+ dipole–dipole interactions and C–H⋯п interactions. This work demonstrates the high potential of an ultramicroporous, hydrophobic, and hydrolytically stable MOF in the removal of C₃H₄ from the C₃H₄/C₃H₆ gas mixture by size exclusion adsorption. The structure and gas adsorption studies shed light on the design and synthesis of new adsorbents for the separation of light hydrocarbons.

---

Introduction

Propylene (C₃H₆), as a raw material of many chemical products, plays a key role in the petrochemical industry.¹ C₃H₆ is mostly produced from the cracking of large hydrocarbon molecules or crude oil in the petrochemical industry commonly accompanied by propyne (C₃H₄) as an undesirable byproduct, which can poison the catalysts for propylene polymerization.² Efficient removal of C₃H₄ from the C₃H₄/C₃H₆ mixture is thus of high importance. Traditional C₃H₄/C₃H₆ separation methods, such as cryogenic distillation or catalytic partial hydrogenation, suffer from high cost and high energy penalties.^3,4 The physical adsorption method based on porous materials is attractive for gas separation owing to its environmentally friendly processes and energy efficiency.^5–11 Highly efficient C₃H₄/C₃H₆ separation using traditional porous materials (such as zeolites and activated carbons) remains challenging because of highly similar physical properties, molecular sizes (C₃H₆: 4.65 × 4.16 × 6.44 ų and C₃H₄ 4.01 × 4.16 × 6.51 ų) and kinetic diameters (C₃H₆: 4.68 Å and C₃H₄: 4.76 Å) of the two hydrocarbons.¹²

As a class of newly emerged porous materials, metal–organic frameworks (MOFs)^13–20 have been widely used for gas separation owing to their structural diversity,^21–26 adjustable pore size^27–30 and designable adsorption sites.^31–37 Most reported MOFs used for C₃H₄/C₃H₆ separation are based on their open metal sites or functional groups which can strongly interact with C₃H₄. For example, the MOF-74 series (Mg, Co, Ni, and Fe-MOF-74)³⁸ with open metal sites and the SIFSIX series (SIFSIX-3-Ni,³⁹ SIFSIX-1-Cu,⁴⁰ and SIFSIX-2-Cu-i¹⁹) with SiF₆²⁻ anion pillars can afford high adsorption capacity and strong interaction for C₃H₄.³⁴ However, these MOFs allow diffusion of both C₃H₄ and C₃H₆ into their pores,^2,34,41–46 resulting in a compromised separation efficiency. In addition, the regeneration processes are relatively energy-intense due to the high affinity of these MOFs to C₃H₄. Therefore, the development of adsorbents with high performance in selective adsorption of C₃H₄ over C₃H₆ primarily relying on the molecular sieving effect would be complementary. However, among the reported MOFs, only UTSA-200 showed molecular sieving C₃H₄/C₃H₆ separation performance with a high adsorption selectivity of over 20 000.³² In addition, stability and recyclability are also important for porous materials in gas separation applications,⁴⁷ especially when the gas mixtures to be purified contain reactive non-hydrocarbon impurities, e.g. water, which is ubiquitous.

Herein, we report two ultramicroporous MOFs, Zn₂(ATZ)₂(TPDC) (BUT-305, H₂TPDC = [1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid, HATZ = 3-amino-1,2,4-triazole) and Zn₂(ATZ)₂(MeTPDC) (BUT-306, H₂MeTPDC = 5′-methyl-[1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylic acid), which are 3D framework structures consisting of 2D [Zn(ATZ)] layers pillared by TPDC²⁻ and MeTPDC²⁻ ligands (Scheme 1), respectively. The difference between the two isoreticular MOFs is that there are extra gate-like methyl groups in the 1D channels of BUT-306. Owing to its narrow and hydrophobic pore aperture, BUT-306 exhibits high hydrolytic stability, hydrophobicity, and potential in the removal of C₃H₄ from the C₃H₄/C₃H₆ mixture. In addition, the location of C₃H₄ molecules inside the pore of BUT-306 and the host–guest interactions have been elucidated by single-crystal structural analyses.

Scheme 1 The construction and building blocks of BUT-305 and -306.

Results and discussion

Single-crystal structures and chemical stability

As-synthesized crystals of BUT-305 and -306 were obtained through the solvothermal reactions of zinc salt and organic ligands (HATZ and H₂MeTPDC or HATZ and H₂MeTPDC) in the mixture of DMF and a small amount of aqueous HBF₄ solution. The as-synthesized crystals were guest-exchanged in MeOH solvent for 3 days at 120 °C followed by heating at 120 °C for 18 hours to remove guest molecules, resulting in the formation of BUT-305 and -306 samples. The SCXRD measurement of BUT-306 revealed that it crystallizes in the tetragonal I4/m space group (Table S1†) and it is isoreticular to Zn₂(ATZ)₂(iPA) (H₂iPA = isophthalic acid) as reported by Chen and co-workers.⁴⁸ The 2D [Zn(ATZ)] layers are linked by the MeTPDC²⁻ pillars to form a 3D pillar-layered framework with 1D channels along the crystallographic c-axis (Fig. 1a). The cross-sectional diameters of the channel are estimated to be 1.6 to 5.0 Å by Materials Studio.⁴⁹ It is worthy of note that the channel is highly tortuous with three types of cavities (A, B, and C), and it is divided into parts by sets of gate-like methyl groups, each set of which are from 4 neighboring MeTPDC²⁻ ligands (Fig. 1b). The aperture defined by the 4 gate-like methyl groups is only about 1.6 Å in diameter. Regardless of the small aperture size, BUT-306 is still potentially porous as the total potential solvent area volume accounts for 23.6% of the unit cell volume of BUT-306 and the pore volume is estimated to be 0.17 cm³ g⁻¹ by Platon.⁵⁰

Fig. 1 The crystal structure of BUT-306: (a) the 3D framework view along the c axis and (b) side-view of the channel. Color codes: Zn, turquoise; O, red; N, blue; C, grey; H, white; and Connolly surface (probe radius 1.7 Å), green. The ligands coordinated with the metal ions on the walls of neighboring channels are shown in pink for clarity.

Due to the thin-plate-like shape of BUT-305 crystals, attempts to determine their structure by SCXRD failed. The PXRD pattern of BUT-305 resembled that of BUT-306, suggesting that BUT-305 is likely isoreticular to BUT-306 (Fig. 2a). Indexing and Pawley refinement results for the PXRD pattern of BUT-305 showed that it also crystallizes in the same space group (I4/m) with a slightly larger unit cell relative to that of BUT-306 (Fig. 2b and Table S2†). Due to the absence of methyl groups on the ligands of BUT-305, it is expected that the channels of BUT-305 are larger than those of BUT-306. The difference in the channels of the two MOFs results in their distinctive gas separation performance, which will be discussed in detail below. The PXRD patterns of BUT-305 and -306 samples well matched the simulated one from the single-crystal structure of BUT-306 (Fig. 2a), which confirmed that the prepared BUT-305 and -306 samples were pure phases. In addition, the PXRD measurements showed that the highly crystalline structures of BUT-305 and -306 samples remained essentially unchanged after they were treated with methanol at 120 °C for 3 days (Fig. S1†) or treated with water at room temperature for 24 hours (Fig. 2a), demonstrating their high hydrolytic stability.

Fig. 2 (a) The PXRD patterns of BUT-305 and BUT-306. (b) The results of Pawley refinement for the PXRD data of BUT-305. For comparison, the space group and cell parameters of BUT-306 are as follows: I4/m, a = 12.3420(2) Å, and c = 40.6826(9) Å.

Adsorption studies

To verify the accessibility of their pores, N₂ adsorption isotherms of BUT-305 and -306 were recorded at 77 K (Fig. S2†). Although the calculated pore volumes of the two MOFs are over 0.17 cm³ g⁻¹, type I N₂ adsorption isotherms were not obtained. The N₂ uptake at P/P₀ ≈ 1 was only 1.2 cm³ g⁻¹ for BUT-306, suggesting that N₂ molecules could not diffuse into its channels at 77 K. The N₂ uptake of BUT-305 gradually increased as the pressure is increased with an N₂ uptake of 77.1 cm³ g⁻¹ at P/P₀ ≈ 1, which might result from the comparable size of the N₂ molecule and the pore of BUT-305. The CO₂ adsorption isotherms of BUT-305 and -306 at 195 K were then recorded. As shown in Fig. S3,† the two MOFs showed an abrupt increase in CO₂ uptake (to ∼70 cm³ g⁻¹) at low pressures (P/P₀ < 0.1). At higher pressures, the CO₂ uptake of BUT-306 only slightly increased, while that of BUT-305 increased to 193 cm³ g⁻¹ at P/P₀ ≈ 1 by two additional stepwise increases, indicating the presence of some difference in the pore size, shape and/or flexibility of the two MOFs. According to the adsorption data at low pressures, the BET surface areas of BUT-305 and -306 were estimated to be 351 and 305 m² g⁻¹, respectively. Water adsorption isotherms were also recorded for the MOFs at 298 K. Type III water adsorption isotherms were obtained for both MOFs (Fig. 3a and S4†), and the water uptakes at P/P₀ ≈ 1 were 160 and 80 cm³ g⁻¹ for BUT-305 and BUT-306, respectively. These results indicated that the two MOFs were capable of accommodating small molecules and their pore surface was relatively hydrophobic.

Fig. 3 Adsorption and separation performance of BUT-306. (a) The H₂O adsorption isotherms of BUT-305 and -306 at 298 K. Single-component C₃H₄ and C₃H₆ adsorption isotherms of BUT-306 at (b) 298 K, (c) 273 K, (d) 283 K (e) 313 K and (f) 323 K, respectively.

Encouraged by the high stability, accessible porosity and small pore apertures of the two MOFs, we explored their performances in light hydrocarbon separations. Adsorption isotherms for C₂H₂, C₂H₄, C₂H₆, C₃H₄, C₃H₆, and C₃H₈ were recorded at 298 K, as shown in Fig. S5.† The adsorption capacities of BUT-305 for C₂H₂, C₂H₄ and C₂H₆ at ∼1 bar were similar (42.3–44.6 cm³ g⁻¹), and its adsorption capacities for C₃H₄ (44.4 cm³ g⁻¹) and C₃H₆ (43.9 cm³ g⁻¹) at ∼1 bar were also close, but that for C₃H₈ (33.8 cm³ g⁻¹) was lower. These results suggested that all these hydrocarbons could enter the pore of BUT-305, and the differences in adsorption capacities of BUT-305 for these hydrocarbons were not profound. In contrast, strong size exclusion effects were observed in the hydrocarbon adsorption isotherms of BUT-306. For C₂ hydrocarbons, the uptake of BUT-306 for C₂H₂ (36.6 cm³ g⁻¹) or C₂H₄ (30.3 cm³ g⁻¹) was obviously higher than that for C₂H₆ (11.0 cm³ g⁻¹) at ∼1 bar. Moreover, the uptakes of BUT-306 for C₃H₆ (2.4 cm³ g⁻¹) and C₃H₈ (0.33 cm³ g⁻¹) at ∼1 bar were nearly negligible (Fig. S5d†), but a type I-like C₃H₄ adsorption isotherm was obtained for BUT-306 with a relatively high uptake of 29.6 cm³ g⁻¹ at ∼1 bar (Fig. 3b and S6†), indicating its potential in the removal of C₃H₄ from the C₃H₄/C₃H₆ mixture. Although the C₃H₄ adsorption capacity of BUT-306 is lower than those of some reported MOFs under similar conditions, e.g., SIFSIX-2-Cu-i (84.4 cm³ g⁻¹), UTSA-200 (80.2 cm³ g⁻¹) and ELM-12 (62.1 cm³ g⁻¹), BUT-306 hardly permits the entrance of C₃H₆ molecules into its pores. The C₃H₆ adsorption capacity of BUT-306 was only 2.4 cm³ g⁻¹ at ∼1 bar and 298 K, significantly lower than those of the reported MOFs (Table S3†),^32,51,52 including SIFSIX-2-Cu-i (58.9 cm³ g⁻¹), UTSA-200 (26.9 cm³ g⁻¹), and ELM-12 (32.0 cm³ g⁻¹). The size exclusion effects of BUT-306 on the adsorption of C₃H₄ over C₃H₆ should be related to the small apertures defined by gate-like methyl groups in its 1D channels.

For a better comparison, the uptake ratios of BUT-306 for C₃H₄ over C₃H₆ at 0.99 bar or at 0.01/0.99 bars (0.01 bar for C₃H₄ and 0.99 for C₃H₆) and 298 K were calculated, being 10.75 and 3.67, respectively, which are much higher than those of the reported porous materials (1.05–3.07 at 0.99 bar; 0.03–2.49 at 0.01/0.99 bars) under similar conditions (Table S3†). Furthermore, ideal adsorbed solution theory (IAST) was used to predict the C₃H₄/C₃H₆ adsorption selectivity of BUT-306. Assuming the gas mixture with C₃H₄/C₃H₆ ratios (v/v) of 1 : 1, 1 : 9 and 1 : 99, the IAST C₃H₄/C₃H₆ adsorption selectivities of BUT-306 at ∼1 bar and 298 K were calculated to be 636, 717, and 989, respectively (Fig. S7†).

To explore the adsorption performances of BUT-306 for C₃H₄ and C₃H₆ in a broader temperature range, C₃H₄ and C₃H₆ adsorption isotherms were measured at 273, 283, 313 and 323 K, respectively. As shown in Fig. 3c–f, type I-like adsorption isotherms were obtained for C₃H₄ at all four temperatures, and the adsorption capacities at ∼1 bar were all over 25 cm³ g⁻¹, while the C₃H₆ uptakes were all relatively low (<5 cm³ g⁻¹). It is indicated that the size exclusion adsorption behavior of BUT-306 is not highly sensitive to the temperature. The adsorption capacity of BUT-306 for C₃H₄ or C₃H₆ gradually increased as the temperature increased from 273 to 323 K. For example, the C₃H₄ uptake at ∼1 bar increased from 24.2 cm³ g⁻¹ at 273 K to 35.0 cm³ g⁻¹ at 323 K. The adsorption data at these temperatures were further verified by repeated adsorption isotherm measurements (Fig. S8†). It is not common because physisorption is essentially exothermic, and the adsorption capacities at a certain equilibrium pressure should decrease when increasing the adsorption temperature for typical physical adsorption adsorbents. The abnormal adsorption behavior suggests the existence of structural flexibility in BUT-306.^53,54 To verify this, SCXRD measurements were carried out for the same one single crystal of BUT-306 at 273 K, 313 K and 298 K, respectively. As shown in Fig. S9 and Table S1,† the unit cell parameters were nearly unchanged and no obvious structural change was observed during varying the measurement temperatures, indicating that the structural flexibility was induced by the adsorption of hydrocarbons rather than the change of temperature. Otherwise, according to the static equilibrium crystal structure of BUT-306, the narrowest pore aperture is too small (diameter: 1.6 Å) to let the hydrocarbon molecules diffuse into the channels. The increase of measurement temperature should be beneficial for the diffusion of C₃H₄ molecules into BUT-306 as there is an energy barrier between the guest-free state and the guest-included state originating from the structural flexibility of BUT-306, which leads to the uncommon adsorption behavior of BUT-306 as mentioned above.

For a better understanding of the C₃H₄ adsorption process of BUT-306, the single-crystal structure of C₃H₄-loaded BUT-306 was determined. A single crystal of BUT-306 preheated at 50 °C under an N₂ atmosphere for 6 hours was exposed to the atmosphere of C₃H₄ at ambient pressure and temperature for 30 minutes to load the hydrocarbon molecules, which was then mounted on a single-crystal X-ray diffractometer under a low-temperature N₂ flow bath (200 K) for data collection. Structure determination and refinement results revealed that the framework of BUT-306 was essentially unchanged after C₃H₄ loading (Fig. S9 and Table S1†) and two crystallographically independent C₃H₄ molecules were found inside its channels. As the framework was retained, the narrowest channel apertures defined by the methyl groups of MeTPDC²⁻ ligands kept unchanged after C₃H₄ loading, indicating that there was a transient structural variation to open the gate-like apertures when C₃H₄ molecules diffused into the channels. The transient gate-opening might have resulted from the swing or distortion of the MeTPDC²⁻ ligands.

The two types of C₃H₄ molecules are located inside cavities A and B with a linear molecular shape along the 4-fold rotation axis, respectively (Fig. 4a). The C–C triple bond and single bond of the C₃H₄ molecule located at cavity A (C₃H₄-A) are 1.08(8) and 1.54(8) Å, respectively. The C₃H₄ molecule located at cavity B (C₃H₄-B) is normal to the reflection plane crossing its center C atom. Due to the symmetry-imposed disorder, the two C–C bonds of C₃H₄-B are same in length, being 1.24(8) Å, close to the average length of a C–C triple bond and a C–C single bond. Occupations of the two C₃H₄ molecules were set to be 20%, and no geometrical restraints were used in the structure refinements. The C₃H₄ molecules are confined inside the cavities by multiple weak host–guest interactions with the framework. Specifically, the distance between the terminal sp-hybridized C atom (with a negative charge) of C₃H₄-A and the 3-position C atoms (with a positive charge) of four neighboring ATZ⁻ ligands on the channel walls is 3.50 Å (Fig. 4b), slightly larger than the van der Waals radii of two C atoms (1.7 × 2 Å), indicating the presence of weak C^δ−⋯C^δ+ dipole–dipole interactions. In addition, there are fourfold weak C–H⋯п interactions between C₃H₄-B and the aromatic H atoms of four neighboring MeTPDC²⁻ ligands (C_C3H4⋯H distances: 3.76, 3.95 Å; <C–H⋯C angles: 85.9, 86.1°) (Fig. 4c). Though every single interaction is weak, the overall interactions between C₃H₄ and MOF should be moderate because each C₃H₄ molecule is interacting with four neighboring channel walls simultaneously. Assuming that the two adsorption sites are fully occupied, the calculated C₃H₄ uptake of BUT-306 is 26.8 cm³ g⁻¹, which is slightly lower than the experimentally observed uptake (29.6 cm³ g⁻¹) at ∼1 bar and 298 K. It is noteworthy that C₃H₄ uptake of BUT-306 at ∼1 bar gradually increased from 24.2 to 35.0 cm³ g⁻¹ as the temperature increased from 273 to 323 K. This fact suggests that the adsorbed C₃H₄ molecules could be also located inside the cavity C besides cavities A and B. When each of the cavities is occupied by one C₃H₄ molecule, the calculated C₃H₄ uptake of BUT-306 is 35.7 cm³ g⁻¹, close to the uptake (35.0 cm³ g⁻¹) observed at ∼1 bar and 323 K, further justifying the assignment of adsorption sites.

Fig. 4 (a) The two types of C₃H₄ adsorption sites in the cavities of BUT-306. The interaction between the BUT-306 framework and the C₃H₄ molecules located in cavity A (b) and in cavity B (c). Color codes: Zn, turquoise; O, red; N, blue; C, grey; and H, white. For clarity, the C atoms of C₃H₄-A and C₃H₄-B are in gold and pink colors, respectively.

Dynamic column breakthrough studies

Dynamic column breakthrough experiments of a binary gas of C₃H₄/C₃H₆ (1 : 1, v/v) were carried out at ambient pressure and temperature to further evaluate the separation performance of BUT-306 for a real gas mixture. As the binary gas flowed through the column packed with BUT-306, purified C₃H₆ was detected in the outlet of the column, while C₃H₄ was trapped in the column until the retention time reached ∼10 min g⁻¹ (Fig. 5a). After saturation of gas adsorption, the MOF was regenerated at 393 K under a He gas flow. Meanwhile, the gas eluted from the outlet was monitored. As shown in Fig. 5b, the peak area of desorbed C₃H₄ was significantly larger than that of desorbed C₃H₆, indicating the high adsorption selectivity of C₃H₄ over C₃H₆. The dynamic C₃H₄ productivity is estimated to be 0.87 mol kg⁻¹ h⁻¹ with a purity of 90.3% by analyzing the breakthrough and desorption curves. It is important to note that during the C₃H₄/C₃H₆ separation process, the fewer the C₃H₆ molecules the adsorbent adsorbs, the higher the C₃H₆ purification productivity and efficiency. The BUT-306 sample was regenerated and used for the C₃H₄/C₃H₆ breakthrough experiment for two more cycles, and its separation performance essentially unchanged (Fig. 5a), which indicated that BUT-306 could be fully regenerated for repeated use in the removal of C₃H₄ from the C₃H₄/C₃H₆ mixture. Breakthrough experiments were also carried out at 273, 313 and 323 K. As shown in Fig. 5c, the final retention times of C₃H₄ nearly kept unchanged after varying the measurement temperatures. Since BUT-306 shows high hydrolytic stability and hydrophobicity, the effect of water vapor on its C₃H₄/C₃H₆ separation performance was also investigated. The breakthrough experiments were carried out with the binary C₃H₄/C₃H₆ gas pre-saturated with water vapor (RH ≈ 100% at ambient temperature). It was found that the separation performance of BUT-306 was not lost under such a high humidity (Fig. 5d). The retention time of C₃H₄ was ∼6 min g⁻¹, about 60% of that under dry conditions. The breakthrough experiment under humid conditions was repeated after regeneration of the MOF, and the two breakthrough curves obtained from the two successive runs well overlapped, suggesting the high and stable separation performance of BUT-306 due to its ultramicroporous, stable and hydrophobic nature.

Fig. 5 Dynamic column breakthrough curves of BUT-306 for a binary C₃H₄/C₃H₆ (1 : 1, v/v) gas (a) at room temperature, (c) at 273, 313 and 323 K, and (d) under RH ≈ 100% at room temperature, respectively. (b) The MS signal peaks of C₃H₄ and C₃H₆ at the outlet when the MOF column was purged with a He gas flow at 393 K after a breakthrough test at room temperature.

Conclusions

Two new MOFs BUT-305 and -306 with isoreticular ultramicroporous structures have been synthesized. The pore aperture of BUT-306 (∼1.6 Å) is smaller than that of BUT-305 due to the presence of extra gate-like methyl groups in the channels of the former. This structural feature endows BUT-306 with a high adsorption selectivity of C₃H₄ over C₃H₆. The C₃H₆ and C₃H₄ adsorption capacities of BUT-306 at 298 K and ∼1 bar were 2.4 and 29.6 cm³ g⁻¹, respectively, indicating a size exclusion adsorption behavior. Based on the single-component gas adsorption isotherms, the IAST C₃H₄/C₃H₆ adsorption selectivity at 298 K and 1 bar was calculated to be 636 for an equimolar mixture of C₃H₄ and C₃H₆. Dynamic column breakthrough experiments of a binary gas confirmed the high capability of BUT-306 to remove C₃H₄ from a real C₃H₄/C₃H₆ mixture. Thanks to its high hydrolytic stability and hydrophobicity, the C₃H₄/C₃H₆ separation performance of BUT-306 was largely retained even when the binary C₃H₄/C₃H₆ gas for breakthrough experiments was pre-saturated with water vapor. In addition, the single-crystal structure of C₃H₄-loaded BUT-306 revealed that the adsorbed C₃H₄ molecules were located in the center of channel cavities and interacted with the MOF by multiple weak C^δ−⋯C^δ+ dipole–dipole interactions and C–H⋯п interactions. In short, this study reports an ultramicroporous, hydrophobic, and hydrolytically stable MOF with high performance in the removal of C₃H₄ from a C₃H₄/C₃H₆ gas mixture by size exclusion adsorption. Its structure and gas adsorption studies shed light on the design and synthesis of new adsorbents for light hydrocarbon separations.

Experimental section

Materials and characterization

All general reagents and solvents (AR grade) were purchased commercially and used directly. The single-crystal X-ray diffraction data (SCXRD) of BUT-306 (at 273, 298, and 313 K) and C₃H₄-loaded BUT-306 (at 200 K) were collected on a Rigaku Supernova CCD diffractometer equipped with a graphite-monochromatic enhanced Cu-Kα radiation source (λ = 1.54184 Å). These SCXRD data for BUT-306 have been deposited in the Cambridge Crystallographic Data Centre (CCDC accession numbers: 2168051–2168054†). The PXRD pattern (2θ range: 3–60°) of BUT-305 was measured with a Bruker D8 Advance X-ray powder diffractometer and Pawley refinement⁵⁵ of the PXRD data was performed in Materials Studio. The peak profiles, background, zero-shift, asymmetry, and unit cell parameters were together refined. The final Pawley refinement results are shown in Fig. 2a and Table S2.† The simulated PXRD pattern was obtained from single-crystal data of BUT-306via the Mercury program.⁵⁶ The other experimental PXRD data were recorded on a Rigaku SmartLab3 X-ray powder diffractometer that was equipped with a Cu sealed tube (λ = 1.54178 Å) with a scanning rate of 10° min⁻¹. The gas sorption measurements were performed using a Micromeritics ASAP 2020 surface area and pore analyzer.

Synthesis and activation

A mixture of HATZ (18 mg), H₂MeTPDC (30 mg) or H₂MeTPDC (32 mg) and Zn(CH₃COO)₂·(90 mg) was dissolved in 18 mL of DMF and placed in a 20 mL glass vial containing 720 μL of 48% aqueous HBF₄ solution. Then the mixture sealed in the vial was heated in an oven to 120 °C for 48 hours and the as-synthesized colorless crystals of BUT-305 or -306 were obtained. After the solution was cooled, the crystals were collected and washed several times with CH₃OH and dried under vacuum.

The as-synthesized samples of the two MOFs were first soaked in fresh DMF at 60 °C for 12 hours to remove the excess organic ligands and metal salts. Then, these samples were immersed in MeOH for 3 days at 120 °C. In order to ensure the activation effect, the extract was replaced with fresh methanol three times a day. Finally, the samples in MeOH were collected and dried at 120 °C under vacuum for 18 hours for gas adsorption measurements.

Gas adsorption measurements

The N₂ adsorption isotherms of BUT-305 and -306 were measured at 77 K, while their water adsorption isotherms were recorded at 298 K. Single-component gas (C₂H₂, C₂H₄, C₂H₆, C₃H₄, C₃H₆, and C₃H₈) adsorption isotherms of the two MOFs were measured at 298 K. In addition, the C₃H₄ and C₃H₆ adsorption isotherms of BUT-306 were also recorded at 273, 283, 313 and 323 K, respectively. The C₃H₄ and C₃H₆ adsorption isotherms of BUT-306 at 298 were fit by the Langmuir–Freundlich equation, respectively (Fig. S10†). Then, based on the single-component gas adsorption isotherms of BUT-306 for C₃H₄ and C₃H₆ at 298 K, ideal adsorbed solution theory (IAST) was used for the prediction of adsorption equilibria of the binary gas mixtures.⁵⁷ The selectivity of preferential adsorption of component 1 (C₃H₄) over 2 (C₃H₆) in a mixture containing 1 and 2 can be formally defined as an equation (eqn (1)), where q₁ and q₂ are the absolute loadings at the partial pressures p₁ and p₂, respectively. To calculate the C₃H₄/C₃H₆ adsorption selectivity of BUT-306 for gas separation, the gas mixture composition is assumed to be 1 : 99, 1 : 9 and 1 : 1, respectively.

Breakthrough experiments

The breakthrough experiments for the binary C₃H₄/C₃H₆ gas mixture (1 : 1, v/v) were carried out using a setup as shown in Fig. S11.† A powder sample of BUT-306 (330 mg) was packed in a quartz tube (o.d. 6 mm, i.d. 3 mm, and length 150 mm). The temperature of the MOF sample packed in the quartz tube was controlled by a heating jacket (298, 313, 323, and 393 K) or by an ice/water bath (273 K). The gas mixture flow rate was set to be 2 mL min⁻¹ by the mass flow controllers (MFC). The outlet gas compositions from the column were determined continuously using a mass spectrometer (MS, Hiden, HPR-20). For the breakthrough experiments with a humid gas mixture, the C₃H₄/C₃H₆ mixture was pre-saturated with water vapor (relative humidity (RH) ≈ 100% at room temperature) by bubbling it through deionized water before it passed through the MOF column. The sample was regenerated for repeated tests under a He flow for 4 hours at 393 K.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We would like to acknowledge the financial support from the National Natural Science Foundation of China (No. 22038001 and 51621003).

References

1. M. F. Friedrich, M. Lucas and P. Claus, Selective hydrogenation of propyne on a solid Pd/Al₂O₃ catalyst modified with ionic liquid layer (SCILL), Catal. Commun., 2017, 88, 73–76.
2. L. Li, R.-B. Lin, R. Krishna, X. Wang, B. Li, H. Wu, J. Li, W. Zhou and B. Chen, Flexible-Robust Metal-Organic Framework for Efficient Removal of Propyne from Propylene, J. Am. Chem. Soc., 2017, 139, 7733–7736.
3. B. Bridier, N. López and J. Pérez-Ramírez, Partial hydrogenation of propyne over copper-based catalysts and comparison with nickel-based analogues, J. Catal., 2010, 269, 80–92.
4. R. P. Lively and D. S. Sholl, Seven chemical separations to change the world, Nature, 2016, 532, 435–437.
5. J. Jiang, Z. Lu, M. Zhang, J. Duan, W. Zhang, Y. Pan and J. Bai, Higher Symmetry Multinuclear Clusters of MOFs for Highly Selective CO₂ Capture, J. Am. Chem. Soc., 2018, 140, 17825–17829.
6. K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Carbon Dioxide Capture in Metal-Organic Frameworks, Chem. Rev., 2012, 112, 724–781.
7. L.-H. Xie and M. P. Suh, High CO₂-Capture Ability of a Porous Organic Polymer Bifunctionalized with Carboxy and Triazole Groups, Chem. – Eur. J., 2013, 19, 11590–11597.
8. K.-J. Chen, H. S. Scott, D. G. Madden, T. Pham, A. Kumar, A. Bajpai, M. Lusi, K. A. Forrest, B. Space, J. J. Perry and M. J. Zaworotko, Benchmark C₂H₂/CO₂ and CO₂/C₂H₂ Separation by Two Closely Related Hybrid Ultramicroporous Materials, Chem, 2016, 1, 753–765.
9. H. G. Hao, Y. F. Zhao, D. M. Chen, J. M. Yu, K. Tan, S. Ma, Y. Chabal, Z. M. Zhang, J. M. Dou, Z. H. Xiao, G. Day, H. C. Zhou and T. B. Lu, Simultaneous Trapping of C2H2 and C2H6 from a Ternary Mixture of C2H2 /C2H4 /C2H6 in a Robust Metal-Organic Framework for the Purification of C2H4, Angew. Chem., Int. Ed., 2018, 57, 16067–16071.
10. K. Li, D. H. Olson, J. Seidel, T. J. Emge, H. Gong, H. Zeng and J. Li, Zeolitic imidazolate frameworks for kinetic separation of propane and propene, J. Am. Chem. Soc., 2009, 131, 10368–10369.
11. Z. Zhang, B. Tan, P. Wang, X. Cui and H. Xing, Highly efficient separation of linear and branched C4 isomers with a tailor-made metal–organic framework, AIChE J., 2020, 66, e16236.
12. P. Samaddar, Y.-S. Son, D. C. W. Tsang, K.-H. Kim and S. Kumar, Progress in graphene-based materials as superior media for sensing, sorption, and separation of gaseous pollutants, Coord. Chem. Rev., 2018, 368, 93–114.
13. H.-C. J. Zhou and S. Kitagawa, Metal-Organic Frameworks (MOFs), Chem. Soc. Rev., 2014, 43, 5415.
14. Y. Liu, H. Dong, F. Hu and Y. S. Zhao, Metal-organic framework microlasers, Sci. Bull., 2017, 62, 3–4.
15. W. G. Cui, T. L. Hu and X. H. Bu, Metal-Organic Framework Materials for the Separation and Purification of Light Hydrocarbons, Adv. Mater., 2020, 32, e1806445.
16. W. Fan, X. Zhang, Z. Kang, X. Liu and D. Sun, Isoreticular chemistry within metal–organic frameworks for gas storage and separation, Coord. Chem. Rev., 2021, 443, 213968–214024.
17. R.-B. Lin, S. Xiang, W. Zhou and B. Chen, Microporous Metal-Organic Framework Materials for Gas Separation, Chem, 2020, 6, 337–363.
18. M. R. Tchalala, P. M. Bhatt, K. N. Chappanda, S. R. Tavares, K. Adil, Y. Belmabkhout, A. Shkurenko, A. Cadiau, N. Heymans, G. De Weireld, G. Maurin, K. N. Salama and M. Eddaoudi, Fluorinated MOF platform for selective removal and sensing of SO₂ from flue gas and air, Nat. Commun., 2019, 10, 1328–1338.
19. P. Nugent, Y. Belmabkhout, S. D. Burd, A. J. Cairns, R. Luebke, K. Forrest, T. Pham, S. Ma, B. Space, L. Wojtas, M. Eddaoudi and M. J. Zaworotko, Porous materials with optimal adsorption thermodynamics and kinetics for CO₂ separation, Nature, 2013, 495, 80–84.
20. V. F. Martins, A. M. Ribeiro, M. G. Plaza, J. C. Santos, J. M. Loureiro, A. F. Ferreira and A. E. Rodrigues, Gas-phase simulated moving bed: Propane/propylene separation on 13X zeolite, J. Chromatogr. A, 2015, 1423, 136–148.
21. F. ZareKarizi, M. Joharian and A. Morsali, Pillar-layered MOFs: functionality, interpenetration, flexibility and applications, J. Mater. Chem. A, 2018, 6, 19288–19329.
22. C. Yu, Q. Ding, J. Hu, Q. Wang, X. Cui and H. Xing, Selective capture of carbon dioxide from humid gases over a wide temperature range using a robust metal–organic framework, Chem. Eng. J., 2021, 405, 126937–126944.
23. T. Liu, H. Cui, X. Zhang, Z. Y. Zhang, R. B. Lin, B. Liang, J. Zhang, D. Li and B. Chen, Doubly Interpenetrated Metal-Organic Framework of pcu Topology for Selective Separation of Propylene from Propane, ACS Appl. Mater. Interfaces, 2020, 12, 48712–48717.
24. X. Wang, R. Krishna, L. Li, B. Wang, T. He, Y.-Z. Zhang, J.-R. Li and J. Li, Guest-dependent pressure induced gate-opening effect enables effective separation of propene and propane in a flexible MOF, Chem. Eng. J., 2018, 346, 489–496.
25. L. Yang, X. Cui, Q. Ding, Q. Wang, A. Jin, L. Ge and H. Xing, Polycatenated Molecular Cage-Based Propane Trap for Propylene Purification with Recorded Selectivity, ACS Appl. Mater. Interfaces, 2020, 12, 2525–2530.
26. L. Yu, X. Han, H. Wang, S. Ullah, Q. Xia, W. Li, J. Li, I. da Silva, P. Manuel, S. Rudic, Y. Cheng, S. Yang, T. Thonhauser and J. Li, Pore Distortion in a Metal-Organic Framework for Regulated Separation of Propane and Propylene, J. Am. Chem. Soc., 2021, 143, 19300–19305.
27. Z. Zhang, Q. Ding, S. B. Peh, D. Zhao, J. Cui, X. Cui and H. Xing, Mechano-assisted synthesis of an ultramicroporous metal-organic framework for trace CO₂ capture, Chem. Commun., 2020, 56, 7726–7729.
28. B. Liang, X. Zhang, Y. Xie, R. B. Lin, R. Krishna, H. Cui, Z. Li, Y. Shi, H. Wu, W. Zhou and B. Chen, An Ultramicroporous Metal-Organic Framework for High Sieving Separation of Propylene from Propane, J. Am. Chem. Soc., 2020, 142, 17795–17801.
29. R. B. Lin, L. Li, H. Wu, H. Arman, B. Li, R. G. Lin, W. Zhou and B. Chen, Optimized Separation of Acetylene from Carbon Dioxide and Ethylene in a Microporous Material, J. Am. Chem. Soc., 2017, 139, 8022–8028.
30. H. Wang, X. Dong, V. Colombo, Q. Wang, Y. Liu, W. Liu, X. L. Wang, X. Y. Huang, D. M. Proserpio, A. Sironi, Y. Han and J. Li, Tailor-Made Microporous Metal-Organic Frameworks for the Full Separation of Propane from Propylene Through Selective Size Exclusion, Adv. Mater., 2018, 30, e1805088.
31. Z. Jiang, L. Fan, P. Zhou, T. Xu, S. Hu, J. Chen, D.-L. Chen and Y. He, An aromatic-rich cage-based MOF with inorganic chloride ions decorating the pore surface displaying the preferential adsorption of C₂H₂ and C₂H₆ over C₂H₄, Inorg. Chem. Front., 2021, 8, 1243–1252.
32. L. Li, H.-M. Wen, C. He, R.-B. Lin, R. Krishna, H. Wu, W. Zhou, J. Li, B. Li and B. Chen, A Metal-Organic Framework with Suitable Pore Size and Specific Functional Sites for the Removal of Trace Propyne from Propylene, Angew. Chem., Int. Ed., 2018, 57, 15183–15188.
33. X.-Q. Wu, J.-H. Liu, T. He, P.-D. Zhang, J. Yu and J.-R. Li, Understanding how pore surface fluorination influences light hydrocarbon separation in metal–organic frameworks, Chem. Eng. J., 2021, 407, 127183–127192.
34. L. Yang, X. Cui, Q. Yang, S. Qian, H. Wu, Z. Bao, Z. Zhang, Q. Ren, W. Zhou, B. Chen and H. Xing, Highly Efficient Removal of Propyne from Propylene with Anion-Pillared Ultramicroporous Materials, Adv. Mater., 2018, 30, 1705374–1705382.
35. Z. Zhang, Q. Ding, X. Cui, X. M. Jiang and H. Xing, Fine-Tuning and Selective-Binding within an Anion-Functionalized Ultramicroporous Metal-Organic Framework for Efficient Olefin/Paraffin Separation, ACS Appl. Mater. Interfaces, 2020, 12, 40229–40235.
36. B. Yu, S. Geng, H. Wang, W. Zhou, Z. Zhang, B. Chen and J. Jiang, A Solid Transformation into Carboxyl Dimers Based on a Robust Hydrogen–Bonded Organic Framework for Propyne/Propylene Separation, Angew. Chem., 2021, 133, 26146–26152.
37. R. Das, S. S. Dhankhar and C. M. Nagaraja, Construction of a bifunctional Zn(II)–organic framework containing a basic amine functionality for selective capture and room temperature fixation of CO₂, Inorg. Chem. Front., 2020, 7, 72–81.
38. S. R. Caskey, A. G. Wong-Foy and A. J. Matzger, Dramatic Tuning of Carbon Dioxide Uptake via Metal Substitution in a Coordination Polymer with Cylindrical Pores, J. Am. Chem. Soc., 2008, 130, 10870–10871.
39. A. Kumar, D. G. Madden, M. Lusi, K. J. Chen, E. A. Daniels, T. Curtin, J. J. T. Perry and M. J. Zaworotko, Direct Air Capture of CO₂ by Physisorbent Materials, Angew. Chem., Int. Ed., 2015, 54, 14372–14377.
40. S. D. Burd, S. Ma, J. A. Perman, B. J. Sikora, R. Q. Snurr, P. K. Thallapally, J. Tian, L. Wojtas and M. J. Zaworotko, Highly selective carbon dioxide uptake by [Cu(bpy-n)₂(SiF₆)] (bpy-1=4,4′-bipyridine; bpy-2=1,2-bis(4-pyridyl)ethene), J. Am. Chem. Soc., 2012, 134, 3663–3666.
41. L. Yang, X. Cui, Y. Zhang, Q. Yang and H. Xing, A highly sensitive flexible metal–organic framework sets a new benchmark for separating propyne from propylene, J. Mater. Chem. A, 2018, 6, 24452–24458.
42. L. Li, L. Guo, F. Zheng, Z. Zhang, Q. Yang, Y. Yang, Q. Ren and Z. Bao, Calcium-Based Metal-Organic Framework for Simultaneous Capture of Trace Propyne and Propadiene from Propylene, ACS Appl. Mater. Interfaces, 2020, 12, 17147–17154.
43. H.-M. Wen, L. Li, R.-B. Lin, B. Li, B. Hu, W. Zhou, J. Hu and B. Chen, Fine-tuning of nano-traps in a stable metal–organic framework for highly efficient removal of propyne from propylene, J. Mater. Chem. A, 2018, 6, 6931–6937.
44. M. Khraisheh, F. AlMomani and G. Walker, High Purity/Recovery Separation of Propylene from Propyne Using Anion Pillared Metal-Organic Framework: Application of Vacuum Swing Adsorption (VSA), Energies, 2021, 14, 609.
45. Z. T. Lin, Q. Y. Liu, L. Yang, C. T. He, L. Li and Y. L. Wang, Fluorinated Biphenyldicarboxylate-Based Metal-Organic Framework Exhibiting Efficient Propyne/Propylene Separation, Inorg. Chem., 2020, 59, 4030–4036.
46. Z. Zhang, Q. Ding, J. Cui, X. Cui and H. Xing, Fine-Tuning Pore Dimension in Hybrid Ultramicroporous Materials Boosting Simultaneous Trapping of Trace Alkynes from Alkenes, Small, 2020, 16, e2005360.
47. X. Suo, H. Pan, L. Chen, X. Cui and H. Xing, Control of Functionalized Pore Environment in Robust Ionic Ultramicroporous Polymers for Efficient Removal of Trace Propyne from Propylene, ACS Appl. Mater. Interfaces, 2021, 13, 42706–42714.
48. K.-J. Chen, R.-B. Lin, P.-Q. Liao, C.-T. He, J.-B. Lin, W. Xue, Y.-B. Zhang, J.-P. Zhang and X.-M. Chen, New Zn-Aminotriazolate-Dicarboxylate Frameworks: Synthesis, Structures, and Adsorption Properties, Cryst. Growth Des., 2013, 13, 2118–2123.
49. Materials Studio v5.5, Accelrys Inc., San Diego, CA, 2010.
50. A. L. Spek, Single-crystal structure validation with the program PLATON, J. Appl. Crystallogr., 2003, 36, 7–13.
51. Z. Li, L. Li, L. Guo, J. Wang, Q. Yang, Z. Zhang, Y. Yang, Z. Bao and Q. Ren, Gallate-Based Metal–Organic Frameworks for Highly Efficient Removal of Trace Propyne from Propylene, Ind. Eng. Chem. Res., 2020, 59, 13716–13723.
52. Y.-L. Peng, T. Wang, C. Jin, P. Li, S. Suepaul, G. Beemer, Y. Chen, R. Krishna, P. Cheng, T. Pham, B. Space, M. J. Zaworotko and Z. Zhang, A robust heterometallic ultramicroporous MOF with ultrahigh selectivity for propyne/propylene separation, J. Mater. Chem. A, 2021, 9, 2850–2856.
53. J.-P. Zhang and X.-M. Chen, Exceptional Framework Flexibility and Sorption Behavior of a Multifunctional Porous Cuprous Triazolate Framework, J. Am. Chem. Soc., 2008, 130, 6010–6017.
54. S. Ma, D. Sun, D. Yuan, X. Wang and H.-C. Zhou, Preparation and Gas Adsorption Studies of Three Mesh-Adjustable Molecular Sieves with a Common Structure, J. Am. Chem. Soc., 2009, 131, 6445–6451.
55. G. S. Pawley, Unit-Cell REFINEMENT FROM Powder Diffraction Scans, J. Appl. Crystallogr., 1981, 14, 357–361.
56. C. F. Macrae, I. Sovago, S. J. Cottrell, P. T. A. Galek, P. McCabe, E. Pidcock, M. Platings, G. P. Shields, J. S. Stevens, M. Towler and P. A. Wood, Mercury 4.0: from visualization to analysis, design and prediction, J. Appl. Crystallogr., 2020, 53, 226–235.
57. A. L. Myers and J. M. Prausnitz, Thermodynamics of mixed-gas adsorption, AIChE J., 1965, 11, 121–127.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 2168051–2168054. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi01152b

---