Scalable synthesis of a low-cost Zn–MOF with a nonpolar pore surface for efficient separation of methanol-to-olefin products

Abstract

The pursuit of scalable metal–organic framework (MOF) adsorbents for efficient one-step purification of C₂H₄ and recovery of C₃H₆ holds significant importance for practical applications. It is desirable to incorporate high separation efficiency along with other key norms such as easy scalability, economic feasibility, and eco-friendliness into a single MOF structure. However, this presents a formidable challenge due to the high cost of specially designed ligands, rigorous synthetic conditions, and typically lengthy reaction times. Herein, we present a scalable MOF adsorbent denoted as Zn-hba decorated with a nonpolar pore surface, which successfully integrates all the aforementioned norms. Zn-hba exhibits selective uptake for C₂H₆ and C₃H₆ over C₂H₄ with exceptional selectivity and record-high C₃H₆ and exceptional C₂H₆ uptake at low pressure, allowing for one-step purification of C₂H₄ and recovery of C₃H₆ from a ternary mixture of C₂H₆/C₂H₄/C₃H₆. Importantly, Zn-hba is synthesized with a high yield of 42% using commercially available cheap reagents such as zinc acetate, 4-hydroxybenzoic acid, methanol and n-amyl alcohol in only 12 hours. The total cost for producing each gram of this adsorbent is as low as $0.14, comparable to that of commercial zeolites and nearly one-tenth the cost of the benchmark MOF MAC-4.

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Introduction

Ethylene (C₂H₄) and propylene (C₃H₆), primarily produced from hydrocarbon cracking and the methanol-to-olefin (MTO) reaction, are essential feedstocks for the production of downstream chemicals.^1,2 A crucial step in obtaining high-purity C₂H₄ and C₃H₆ is the removal of the main impurity C₂H₆ and recovery of C₃H₆. This is typically achieved through energy-intensive cryogenic cyclic distillation at high pressure, due to their similar physicochemical properties.^3,4 In the industrial field of polymerization, high-purity C₂H₄ and C₃H₆ are desirable, with a purity of over 99.9% and 99.5%, respectively. With the growing demand for C₂H₄ and C₃H₆, exceeding 300 million tons produced in 2023,^5,6 it is vital to replace the energy-intensive distillation methods with more energy-efficient, cost-effective, and environmentally friendly alternatives. Among these methods, adsorption and separation technology utilizing porous materials holds great promise.^4,7–14

Metal–organic frameworks (MOFs), as a new class of crystalline porous materials, have been developed for adsorptive separation due to their high modularity, exceptional porosity, and diverse functionality.^3,8,15–18 To achieve a simplified C₂H₄ and C₃H₆ purification procedure, the MOF structure must exhibit graded uptake for C₂H₄, C₂H₆ and C₃H₆, with high selectivity and capture capability. Most MOFs have been reported to preferentially adsorb C₂H₄ over C₂H₆ because the incorporation of C₂H₄ binding sites such as unsaturated metal sites into MOF structures is much more easily achieved.^19–21 However, these C₂H₄-selective MOFs are not ideal adsorbents in practical MTO separation as the pure C₂H₄ product must be obtained from the additional desorption process. Thus the development of C₂H₆-selective MOFs to produce the pure C₂H₄ product directly at the outlet is highly desired. For C₃H₆/C₂H₄ separation, previous reports demonstrated that the strategy of utilizing polar functional groups such as open metal sites (OMSs) in MOFs is feasible due to the higher acidity and more –CH groups in C₃H₆ in comparison to C₂H₄.^1,22 However, this strategy is paradoxical to the C₂H₆-selective MOFs because the OMSs in MOFs prefer to interact strongly with C₂H₄ instead of C₂H₆, leading to preferential adsorption of C₂H₄ over C₂H₆, while C₂H₄ purification and C₃H₆ recovery are both important in order to produce high-purity C₂H₄ and C₃H₆ products in the MTO process. Therefore, the development of MOF materials for the efficient separation of both C₂H₆/C₂H₄ and C₃H₆/C₂H₄ is a significant challenge.

Recent reports have indicated that enhancing the non-polarity of the pore surface in MOFs can facilitate the selective adsorption of C₂H₆ over C₂H₄.^23–27 This is because of the higher polarizability of C₂H₆ (44.7 × 10⁻²⁵ cm³) compared to that of C₂H₄ (42.52 × 10⁻²⁵ cm³).²³ At the same time, nonpolar pores generated by aromatic rings can offer more and stronger C–H⋯π interactions, resulting in enhanced paraffin affinity and selective C₂H₆ adsorption.^28–30 On the other hand, as aromatic moieties such as benzene rings possess delocalized π electrons, the pore surface decorated with benzene rings can provide stronger π⋯π interactions between the host and C₃H₆ with a conjugated π system in comparison to C₂H₄ molecules.³¹ For example, Chen et al. reported that a C₃H₆-selective MOF ZJNU-401 exhibited preferential C₃H₆ adsorption and good C₃H₆/C₂H₄ selectivity, which can be attributed to suitable pore size and benzene ring decorated pore walls.²⁷ Therefore, we speculate that the MOF structure with a nonpolar pore surface decorated by benzene rings and without OMSs could realize the efficient separation of both C₃H₆/C₂H₄ and C₂H₆/C₂H₄ mixtures.

Additionally, the practical application of high-performance MOFs faces several limitations, primarily high costs, challenges in scalability, and potential performance degradation. Ligand design is crucial for achieving desired structures and properties, but it often leads to significantly increased costs for adsorbents. The raw materials for synthesizing organic ligands can cost tens of dollars per gram, and the production of MOF adsorbents typically requires long reaction times, derailing scalable manufacturing. For example, the C₂H₆-selective benchmark MOF ZNU-10 is synthesized from the reaction of Cu(NO₃)₂·3H₂O and mixed linkers p-C₂B₁₀H₁₀–(COOH)₂ and DABCO in DMF/MeOH/H₂O solution, with the price not less than $7 per gram.³² Recently, Hou's group developed a high performance MOF adsorbent, MAC-4, which combines isophthalic acid and 3,5-dimethyl-1,2,4-triazole, and synthesis can be scaled-up with a cost as low as 1.35 $ per gram.²⁴ The scale-up synthesis of MAC-4 is not an environment-friendly protocol due to the use of DMF solvent, and a sustainable synthesis process can accelerate the commercialization of MOF materials, such as CALF-20,^33,34 MIL-100(Fe),^35,36 etc. To advance the industrial application of MOF adsorbents, it is essential to explore options that involve simple, green synthesis processes and very low costs, ensuring high scalability and practicality.

Taking above into consideration, herein we present an efficient C₃H₆- and C₂H₆-selective MOF adsorbent, Zn-hba, enabling the one-step purification of C₂H₄ and efficient recovery of C₃H₆ for the separation of MTO products. Zn-hba is synthesized using the common cheap ligand 4-hydroxybenzoic acid (HBA) and the late transition metal zinc (Zn) through a simple and scaled-up synthesis process, achieving a high yield of 42% (based on Zn) (see the Synthetic section for the detailed procedure). The simple structure avoids the existence of OMSs and features dense, periodic quadrate channels decorated with nonpolar benzene rings, facilitating efficient separation of C₂H₆/C₂H₄/C₃H₆ mixtures without performance degradation after cycling measurements. Zn-hba exhibits preferential adsorption of C₂H₆ and C₃H₆ over C₂H₄ with a record-high C₃H₆ packing density (257.54 g L⁻¹) at 1 kPa and extreme high C₂H₆ packing density (186.94 g L⁻¹) at 10 kPa. The Ideal Adsorption Solution Theory (IAST) selectivity for C₂H₆/C₂H₄ and C₃H₆/C₂H₄ is calculated to be 2.41 and 11.73, respectively. Theoretical calculations reveal that the stronger binding affinity for C₃H₆ and C₂H₆ over C₂H₄ between the nonpolar pore surface and guest molecules is responsible for the C₃H₆- and C₂H₆-selective adsorption. Breakthrough experiments confirm the good separation performance of Zn-hba for C₂H₆/C₂H₄ and C₃H₆/C₂H₄ mixtures even under humid conditions. Moreover, Zn-hba could separate the ternary C₂H₆/C₃H₆/C₂H₄ mixtures to produce high-purity C₂H₄ (>99.9%) and C₃H₆ (>99.5%) with a productivity of 3.32 and 19.42 L kg⁻¹, respectively. In addition, all reagents used for the synthesis of Zn-hba are commercially available and extremely low cost. Despite the fact that the synthesis is not very sustainable, Zn-hba exhibits a significant cost advantage. The cost per gram of Zn-hba is as low as $0.14, making it comparable to that of commercial zeolites and nearly one-tenth of the price of MAC-4.

Results and discussion

Synthesis and characterization

The evaporation of methanol in the n-amyl alcohol/MeOH solution containing Zn(OAc)₂·2H₂O and 4-hydroxy benzoic acid (hba) yielded colorless needle-like crystals of Zn-hba according to the literature.³⁷ The scale-up synthesis of Zn-hba also has been investigated via using an alternative simple protocol. As shown in Fig. S1, n-amyl alcohol was added into the MeOH solution containing Zn(OAc)₂·2H₂O and 4-hydroxy benzoic acid under heating conditions at 100 °C in a round flask to obtain the final microcrystalline Zn-hba sample, and the solvent methanol was recovered during the reaction. These mild synthetic conditions and simple synthetic procedures can produce around 5 g of Zn-hba with a high yield of 42% in 12 hours through one single reaction. Moreover, compared to most porous MOFs applied to gas adsorptive separation, the raw materials for synthesis of Zn-hba are commercially available and the price is very low. The cost of the Zn-hba material is calculated to be $0.14 per kg (Table S1), which makes it comparable to that of commercial zeolites and nearly one-tenth of the price of MAC-4. It is noted that Zn-hba made with this scalable synthesis method exhibited good crystallinity and high phase purity, which was confirmed by powder X-ray diffraction (PXRD) measurements (Fig. S2).

The network of Zn-hba is constructed with 1D tetrahedral zinc(II)-based helical chains as the second building unit (SBU) connected to μ₂-phenolate and carboxylate groups. The hba linkers extend from the metal chains in four directions and perpendicular to the 1D helical chains. Furthermore, each hba linker is linked to two adjacent helical chains, resulting in a 3D structure of 9.1 Å × 9.1 Å square channels with a formula of Zn(hba)·xH₂O (Fig. 1). Based on the Platon calculation, the framework of Zn-hba exhibits a porosity of 37.9%. Additionally, the 1D square channels in the 001 direction are decorated with aromatic moieties and the open metal sites are completely avoided in the structure, providing a nonpolar pore surface that facilitates selective C₃H₆ and C₂H₆ adsorption over C₂H₄ for Zn-hba. The PXRD pattern of as-synthesized samples is consistent with the corresponding simulated one from single crystal data, indicating that its bulk product was pure phase (Fig. S2). The thermogravimetric analysis (TGA) indicated that Zn-hba can be completely desolvated (Fig. S3), facilitating the examination of their permanent porosities. Nitrogen (N₂) adsorption isotherms at 77 K showed type-I curves of Zn-hba (Fig. S4), with a saturated loading of 177 cm³ g⁻¹, indicating a close Brunauer–Emmett–Teller (BET) surface area of 672 m² g⁻¹, and corresponding experimental pore volume of 0.259 cm³ g⁻¹. The pore size distribution of 8 Å agrees with the calculated results from the crystal structure. Moreover, the chemical stability of Zn-hba has been examined. The variable temperature VT-PXRD measurements show that Zn-hba can retain its structural rigidity up to 500 °C (Fig. S5). This ultrahigh thermal stability is rare among MOFs and surpasses that of all Zn–MOFs^{23,24,38–42} to the best of our knowledge. Besides, Zn-hba also exhibits high stability in humid air for one week (Fig. S6). Overall, these data demonstrate that Zn-hba is a robust porous MOF for gas adsorption.

Fig. 1 (a) Schematic illustration of a 1 D channel decorated with benzene rings and (b) the 3D network of Zn-hba.

Gas adsorption properties

The well-developed channels combined with the nonpolar pore surface and high stability encouraged us to investigate the gas adsorption performance of Zn-hba for light hydrocarbons. Single-component gas adsorption isotherms for C₂H₆, C₂H₄ and C₃H₆ were collected on Zn-hba small-scale samples at 273/298 K under 100 kPa. As illustrated in Fig. 2a and b, the maximum uptake of 75.9/67.3 cm³ g⁻¹ for C₂H₆ and 77.2/71.2 cm³ g⁻¹ for C₃H₆ at 273/298 K, were both much higher than that of 73.4/61.7 cm³ g⁻¹ for C₂H₄. Thus the adsorption capacity is in the order of C₃H₆ > C₂H₆ > C₂H₄, indicating a reversed adsorption behaviour in Zn-hba, which is quite rare among reported MOFs.^{23–25,27,43} Remarkably, Zn-hba exhibits abundant C₃H₆ and C₂H₆ adsorption at extremely low pressure at 273 or 298 K (Fig. 2c and d). The adsorption of C₃H₆ is record-high with an uptake of 38.91 cm³ g⁻¹ at 1 kPa and 298 K, being the highest among all MOFs for C₃H₆-selective adsorption (Table S2). The adsorption capacity of C₂H₆, with a value of 39.54 cm³ g⁻¹ at 10 kPa and 298 K, is also the highest among most MOFs for C₂H₆-selective adsorption, except slightly lower than that of NTU-70P (40.3 cm³ g⁻¹)⁴⁰ (Table S3). Afterwards, the packing density of C₃H₆ was calculated to be 258 g L⁻¹, which is the highest one at 1 kPa when compared with that of reported MOFs (Table S2). The C₂H₆ packing density was estimated to be 187 g L⁻¹, being the second highest value at 10 kPa in comparison to other MOFs (Table S3). It is worth noting that the C₂H₆ and C₃H₆ packing density outperforms that of some of the best-performing MOF materials such as Zn-BPZ-TATB,²³ MAC-4 (ref. 24) and ZJUN-401.²⁷ Meanwhile, the adsorption of C₂H₄, especially in the low-pressure regions, is quite low. Moreover, it is also clear that the slopes of C₂H₆ and C₃H₆ adsorption isotherms are much steeper than that of C₂H₄, indicating that the adsorption affinity in Zn-hba follows the order of C₃H₆ > C₂H₆ > C₂H₄, which suggests that Zn-hba has great potential for C₃H₆/C₂H₆/C₂H₄ gas mixture separation in the MTO process. Importantly, the scale-up synthesized Zn-hba exhibits similar adsorption isotherms to C₂H₄, C₂H₆ and C₃H₆ in comparison with the small-scale synthesized single crystal samples (Fig. S7). To further assess the durability of Zn-hba for gas adsorption performance, we conducted cyclic adsorption experiments. The continuous C₂H₆, C₂H₄ and C₃H₆ adsorption measurements without reactivation at 298 K showed the uptake of C₂H₆, C₂H₄ and C₃H₆ and no obvious decrease for atleast five adsorption cycles (Fig. S8), demonstrating excellent recyclability and regeneration capability of the material. The high crystallinity of Zn-hba post-experimentation was confirmed by PXRD analysis and N₂ adsorption results (Fig. S9). No obvious change in PXRD peaks and N₂ adsorption curves at 77 K (Fig. S9) was observed, indicating that Zn-hba possesses excellent durability and recyclability for this gas adsorption.

Fig. 2 Gas adsorption isotherms of C₂H₄, C₂H₆, and C₃H₆ on small-scale Zn-hba at (a) 273 K and (b) 298 K; logarithmic-scale plots within the range of (c) 0–100 kPa and (d) 0–5 kPa at 273/298 K.

The adsorption enthalpies (Q_st) were determined from sorption isotherms via the Clausius–Clapeyron equation at 298 K and 273 K to appraise the interaction strengths of the framework for C₂H₄, C₂H₆ and C₃H₆. The Q_st values at zero-coverage of C₃H₆, C₂H₆ and C₂H₄ were computed to be 31.0, 26.9 and 26.1 kJ mol⁻¹ (Fig. S10), respectively, corroborating that C₃H₆ and C₂H₆ have stronger affinity with the framework than C₂H₄. It is noteworthy that the Q_st values of all three gases are relatively low due to the nonpolar pore surface of Zn-hba, indicating that the regeneration treatment is simple and consumes a low amount of energy. In fact, a simple activation treatment at 373 K for 30 minutes prior to each cycle was sufficient to regenerate fully reproducible adsorption profiles for C₃H₆, C₂H₆ and C₂H₄.

As discussed above, Zn-hba exhibits a higher adsorption capacity of C₃H₆ and C₂H₆ than that of C₂H₄, making it a potential material for C₂H₄ purification in MTO application. Thus we have examined the separation of binary gas mixtures in the range of 0–100 kPa using ideal adsorption solution theory (IAST). As shown in Fig. 3a and S11, Zn-hba exhibits high selectivity values of 2.41/2.49/2.50, and 2.61/2.70/2.7 for 1/1, 1/9, and 1/15 C₂H₆/C₂H₄ mixtures at 298/273 K and 100 kPa, respectively.

Fig. 3 IAST selectivity curves of Zn-hba for (a) C₂H₆/C₂H₄ and (b) C₃H₆/C₂H₄ mixtures at 298 K; (c) comparisons of the selectivity in Zn-hba and some benchmark materials for C₃H₆/C₂H₄ (1/1); (d) comparison of C₃H₆ uptake at 1 kPa and C₃H₆/C₂H₄ IAST selectivity for Zn-hba and other benchmark materials.

The selectivity for the equimolar C₂H₆/C₂H₄ mixture at 298 K and 100 kPa is comparable to that of many top-performing C₂H₆-selective MOFs such as ZIF-8,⁴⁴ PCN-250,⁴⁵ and MAC-4 (ref. 24) (Fig. S12). For C₃H₆/C₂H₄ mixtures, the selectivity values were estimated to be 11.73/14.05/18.70, and 16.33/18.80/22.93 for 1/1, 2/5, and 1/9 C₃H₆/C₂H₄ mixtures at 298/273 K and 100 kPa, respectively (Fig. 3b and S13). Notably, this selectivity for the equimolar C₃H₆/C₂H₄ mixture at 298 K and 100 kPa is the highest among reported C₃H₆-selective MOFs,^22,23,25,46 but only lower than that of Zn₂(oba)₂(dmipym) (15.6)²⁶ (Fig. 3c and d).

To in-depth understand the adsorption mechanism of C₃H₆, C₂H₆ and C₂H₄ in Zn-hba, theoretical calculations based on first-principles dispersion-corrected density functional theory (DFT-D) were performed to reveal the host–guest interactions between the framework and adsorbates. As illustrated in Fig. 4 and S14, the preferential adsorption sites of C₃H₆, C₂H₆ and C₂H₄ are similar and located within the square channels. In Zn-hba, one C₂H₄ molecule interacts with two carboxylate groups in hba linkers via four C–H⋯O interactions (2.84, 3.13, 3.64 and 3.65 Å) and two aromatic moieties via three C–H⋯π interactions (3.41, 3.61 and 3.74 Å), providing a static binding energy of −38.73 kJ mol⁻¹ (Fig. 4a). For C₂H₆, there are five C–H⋯O interactions (2.84, 2.90, 3.27, 3.29 and 3.81 Å) between the gas molecule and carboxylates, and four C–H⋯π interactions (2.98, 3.41, 3.59 and 3.88 Å). The average binding energy for these C₂H₆ adsorption sites is calculated to be −44.94 kJ mol⁻¹, indicating a stronger binding affinity between the alkane adsorbate and nonpolar pore surface with aromatic rings (Fig. 4b). Due to more hydrogen atoms and the larger size of the C₃H₆ molecule, there are more contacts involving C–H⋯O interactions (2.78, 2.82, 2.83, 3.26, 3.65 and 3.79 Å) between C₃H₆ and carboxylate/hydroxyl groups in hba and C–H⋯π interactions (3.03, 3.57, 3.62, 3.67 and 3.95 Å) between C₃H₆ and benzene rings, with a much stronger binding energy of −50.87 kJ mol⁻¹, resulting in an enhanced host–guest interaction to facilitate the capture of propylene in the Zn-hba framework (Fig. 4c). These DFT calculation results provide a rational explanation for the experimental observations of preferential adsorption of C₂H₆ and C₃H₆ over C₂H₄. Apart from the thermodynamics of host–guest interactions, the kinetic adsorption effect is also very important for assessing porous materials' performance. Therefore, the adsorption kinetics were also studied using molecular dynamics (MD) simulations.

Fig. 4 The DFT-calculated configurations of (a) C₂H₄, (b) C₂H₆, and (c) C₃H₆ in Zn-hba. Green, Zn; red, O; gray, C; light green, H; orange, adsorbed gas molecules.

As shown in Fig. S15, the diffusion coefficients (D_s) of the three gas molecules were estimated to be 1.73 × 10⁻⁸ m² s⁻¹ for C₂H₄, 1.27 × 10⁻⁸ m² s⁻¹ for C₂H₆ and 1.05 × 10⁻⁸ m² s⁻¹ for C₃H₆, respectively, demonstrating faster diffusion behavior of C₂H₄ and C₂H₆ over C₃H₆. Additionally, kinetic mechanisms were investigated via adsorption experiments (Fig. S16). The adsorption rates were collected from the adsorption analyzer and the experimental kinetic diffusion coefficients (K_s) were determined to be 4.3 × 10⁻², 2.5 × 10⁻² and 1.1 × 10⁻² for C₂H₄, C₂H₆ and C₃H₆, respectively. The larger K_s of C₂H₆ and C₃H₆ than C₂H₄ indicates the faster adsorption kinetics of C₂H₆ and C₃H₆ compared to that of C₂H₄ in Zn-hba. The experimental data are consistent with the MD simulation results, demonstrating that smaller adsorbate molecular size causes faster guest diffusion. In brief, we thus reason that the preferential adsorption of C₂H₆ and C₃H₆ over C₂H₄ is determined by thermodynamic mechanisms instead of kinetic mechanisms.

Breakthrough experiments

To evaluate the practical C₂H₆/C₂H₄ separation performance of Zn-hba, dynamic breakthrough experiments were performed on Zn-hba large-scale samples for C₂H₆/C₂H₄ (v/v, 5/5, 1/9, and 1/15) mixtures using Ar as the carrier gas with a total flow rate of 10 mL min⁻¹ at 298 K. C₂H₄ eluted first from the column to produce the high-purity C₂H₄ (>99.9%) product (Fig. 5a and S17). The separation factor was estimated to be 2.36, 2.75 and 2.98 for 5/5, 1/9, and 1/15 C₂H₆/C₂H₄ mixtures, respectively, which are similar to the IAST selectivities. The separation performance of the Zn-hba material for C₃H₆/C₂H₄ (v/v: 5/5, 2/5, and 1/9) mixtures was also verified under similar conditions. As shown in Fig. 5b and S18, C₂H₄ passed through the column rapidly while C₃H₆ flowed out slowly at 92/163/172 min g⁻¹, respectively. Notably, these breakthrough time intervals for C₃H₆ in Zn-hba are the longest among most MOFs for C₃H₆/C₂H₄ separation, such as Zn-BPZ-TATB,²³ MAC-4 (ref. 24) and ZJNU-401,²⁷ suggesting the excellent separation ability of Zn-hba for C₃H₆/C₂H₄. Subsequently, the polymer-grade C₂H₄ (>99.95%) was estimated to be 56.49/85.17/120.00 L kg⁻¹, superior to those of [Zn₂(oba)₂(dmimpym)],²⁶ Zn-BPZ-SA⁴⁷ under similar conditions. As pure C₃H₆ is another very important product in the MTO process, high-purity C₃H₆ (>99.94%) can be obtained by the desorption experiments via Ar purging. As a result, the productivity of C₃H₆ for the C₃H₆/C₂H₄ mixture (v/v, 5/5) was calculated to be 25.81 L kg⁻¹, which is comparable to that of Cd-dtzip-H₂O (Fig. S19).⁴⁸ Furthermore, Zn-hba also exhibited good C₃H₆/C₂H₄ separation performance under humid conditions. The breakthrough time was almost the same for 3 cycles of breakthrough experiments under 50% and 100% humid conditions (Fig. S20 and S21), indicating the excellent moisture resistance of Zn-hba for C₃H₆/C₂H₄ separation. The structural stability of Zn-hba was confirmed by PXRD and N₂ adsorption results (Fig. S22 and S23), proving its excellent humidity-resistant recyclability and durability. Additionally, as illustrated in Fig. S24 and S25, Zn-hba shows good recycling performance for the separation of equimolar C₂H₆/C₂H₄ and C₃H₆/C₂H₄ mixtures at 298 K, making it a potential material for industrial C₂H₄ purification.

Fig. 5 (a) Breakthrough curves for C₂H₆/C₂H₄ (v/v, 1/15) mixtures at 298 K; (b) breakthrough curves for C₃H₆/C₂H₄ (v/v, 5/5) mixtures at 298 K; (c) breakthrough curves for the C₂H₆/C₃H₆/C₂H₄ mixture (v/v/v, 33.3/33.3/33.3) at 298 K; (d) comprehensive comparisons for Zn-hba and other porous materials.

During the real industrial process, there are some light hydrocarbons in the MTO products, which is a significant challenge for high-purity C₃H₆ recovery and C₂H₄ purification. Therefore, the dynamic breakthrough measurements for ternary C₂H₆/C₃H₆/C₂H₄ mixtures with various ratios were performed at 298 K to evaluate the separation performance of Zn-hba. As shown in Fig. 5c, for the 33.3/33.3/33.3 ternary mixtures, C₂H₄ passed through the column rapidly to produce high-purity C₂H₄ (>99.9%) with a productivity of 3.32 L kg⁻¹, following by C₂H₆ detected from the outlet, while C₃H₆ occurred until around 46 min. Then the desorption experiments were performed by purging with He gas with a flow rate of 5 mL min⁻¹ at 373 K. As a result, 19.42 L kg⁻¹ C₃H₆ (>99.5%) were recovered from the ternary mixtures in one single adsorption/desorption process. Moreover, when the ratios of ternary mixtures were set up to be 1/9/90 and 2/10/25, Zn-hba still exhibited efficient separation performance (Fig. S26 and S27), making it an excellent adsorbent for C₂H₄ purification and C₃H₆ recovery in MTO product separation. Overall, Zn-hba obtains good scores in uptake at low pressure, IAST selectivity, scalability, price and separation under humidity, which highlights its high promise for real industrial separation (Fig. 5d).

Conclusions

In summary, we developed a scalable and simple synthetic method to produce a low-cost and robust MOF, Zn-hba. Its production cost is only $0.14 per kg, significantly lower than that of most reported MOF materials. Moreover, the low polarity pore surface promotes that Zn-hba prefers adsorbing C₃H₆ and C₂H₆ over C₂H₄ and yields a record-high propylene packing density and exceptionally high ethane packing density at low pressure, which leads to significant C₃H₆/C₂H₄ and C₂H₆/C₂H₄ selectivities, exceeding that of most C₃H₆ and C₂H₆-selective MOFs. DFT calculations reveal that there are more multiple supramolecular interactions and a stronger binding affinity between the nonpolar pore surface of Zn-hba and C₃H₆ and C₂H₆ in comparison to C₂H₄. Breakthrough experiments confirm the good separation performance of Zn-hba for C₂H₆/C₂H₄ and C₃H₆/C₂H₄ mixtures with the longest breakthrough time intervals. More importantly, Zn-hba maintains its C₃H₆/C₂H₄ separation ability after multiple cycles of breakthrough experiments at high humidity. In addition, Zn-hba can produce polymer-grade C₂H₄ (>99.9%) and high-purity C₃H₆ (>99.5%) from ternary C₃H₆/C₂H₆/C₂H₄ mixtures via one adsorption–desorption process. In general, the synthetic feasibility, low-cost, and easy scalability in combination with good separation performance make Zn-hba the most promising material for industrial gas separation. This work highlights the significance of a nonpolar pore environment in MOF materials to facilitate the challenging separation and practical industrial application.

Author contributions

L. G. and H.-J. Y. conceived the idea. Y.-Y. Z. carried out the experiments and analyzed the data. L. G. and C.-Z. X. wrote the manuscript. All authors contributed to the preparation of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. Supplementary information is available: Synthetic procedure, experimental details, all characterisation data, additional adsorption and separation data. See DOI: https://doi.org/10.1039/d5sc03946k.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 22301138) and Jiangsu Specially Appointed Professorship, and the startup funding from Nanjing Normal University.

Notes and references

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