A metal–organic framework based propylene nano-trap with dual functionalities for highly efficient propylene/propane separation

Abstract

Propylene/propane (C₃H₆/C₃H₈) separation represents one of the most challenging and energy-intensive processes in the petrochemical industry due to their very similar sizes and physical properties. Most of the reported physisorbents still face the challenge of achieving simultaneously high C₃H₆ uptake and selectivity with moderate adsorption enthalpy. Herein, we realize an efficient propylene nano-trap in a microporous MOF (ZJUT-2, Ni(pyz-SH)₂SiF₆) for highly efficient C₃H₆/C₃H₈ separation. This MOF-based propylene nano-trap features a suitable pore cavity decorated with dual functionalities (–SH and SiF₆²⁻) to optimally interact with the C₃H₆ molecule, affording both large C₃H₆ capture capacity (123.5 cm³ cm⁻³ at 296 K and 0.5 bar) and high C₃H₆/C₃H₈ selectivity of 17.2 achieved with moderate C₃H₆ adsorption enthalpy (45 kJ mol⁻¹). Theoretical calculations revealed that the appropriate pore cavity and dual functionalities synergistically construct an efficient nano-trap to match better with the C₃H₆ molecule and thus provide stronger multipoint interactions with C₃H₆ over C₃H₈. Actual breakthrough experiments demonstrated that this material can efficiently capture C₃H₆ from C₃H₆/C₃H₈ mixtures (50/50 and 10/90, v/v) under ambient conditions, affording both top-tier C₃H₆ capture amount (2.6 mmol g⁻¹) and dynamic selectivity of 10.

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Introduction

Propylene (C₃H₆) is one of the most critical chemical intermediates and an essential raw material used in the production of polypropylene, acrylonitrile, isopropanol, and propylene oxide.¹ In 2018, the global production of polypropylene, as the second most important synthetic plastic (second to polyethylene), was estimated at 56 Mt and will continuously increase to 88 Mt by 2026.² For most end uses, the propylene must have a purity of at least 99.5% (polymer-grade).³ In industry, propylene is typically obtained by steam cracking of naphtha or during fluid catalytic cracking of gas oils in refineries, which involves propane (C₃H₈) as a coproduct.⁴ The production of polymer-grade C₃H₆ involves the separation of C₃H₆ from a C₃H₆/C₃H₈ mixture. A conventional method for this separation mainly relies on cryogenic distillation, executed at about 243 K and 0.3 MPa in a column containing over 100 trays.⁵ Evidently, such a heat-driven separation process is highly energy-intensive. It is highly demanded to develop alternative and energy-efficient separation technologies to potentially supersede traditional methods.⁶

Non-thermally driven processes, such as adsorptive separation by porous materials, have been considered to dramatically reduce the cost and energy required to purify olefins. In this regard, microporous metal–organic frameworks (MOFs) have been demonstrated to be promising adsorbents for gas separation and purification owing to their tunable pore size/shape and surface functionality.^7–11 Amongst various gas separations, C₃H₆/C₃H₈ separation represents one of the highest separation difficulties due to the subtle molecular size difference between the two components (<0.4 Å). A number of MOFs have been developed in recent years to show high C₃H₆/C₃H₈ separation performance based on equilibrium-based, kinetic-based, molecular sieving or gating-opening mechanisms.^12–30 Nevertheless, there commonly exists a trade-off challenge between uptake capacity and separation selectivity for most of the materials. For instance, several size-selective adsorbents with well-matched pore sizes (e.g., KAUST-7 and Co-gallate) enable complete size-exclusion of C₃H₈ from C₃H₆ to show record C₃H₆/C₃H₈ selectivities but are commonly impaired by their relatively low gas uptakes.^16,18 In contrast, those large-pore MOFs (e.g., HKUST-1 and FeMIL-100) show high C₃H₆ uptakes over 120 cm³ g⁻¹; however, large pores cannot efficiently discriminate the two similar molecules, resulting in low selectivity below 5.²⁹ To overcome this trade-off dilemma, some specific MOFs with gating-opening or thermodynamic–kinetic effects have been realized for benchmark separation properties, while such kinds of materials are difficult to universally design in most cases.^12–19 Another more popular strategy is to immobilize strong open metal sites (OMSs) into MOFs for boosting preferential binding of C₃H₆ over C₃H₈.^25–29 For example, the incorporation of high-density OMSs in MOF-74 or Ag(I) centers in MIL-101-SO₃H can improve C₃H₆/C₃H₈ selectivity up to ca. 40 while maintaining a high C₃H₆ adsorption amount.^25–28 However, such high selectivities arise from ultra-strong metal−olefin interactions that are commonly greater than 60 kJ mol⁻¹, leading to high regeneration energy. Evidently, there is a high demand to immobilize suitable functional sites with moderate binding affinity to boost C₃H₆/C₃H₈ selectivity while maintaining high adsorption amounts.

Recent studies have shown that SIFSIX materials (SIFSIX = hexafluorosilicate (SiF₆²⁻)) are very promising adsorbents for hydrocarbon separations because their pore size can be finely tuned and SiF₆²⁻ anions have moderately strong interactions with hydrocarbon molecules.^23,31 For example, two SIFSIX materials (GeFSIX-2-Cu-i and SIFSIX-2-Cu-i) with pore sizes of 4.5–4.7 Å exhibit the selective separation of C₃H₆ over C₃H₈, while the relatively large pore sizes lead to the insufficient selectivity of below 5.^23b Optimizing the pore size to 3.5 Å in NbOFFIVE-1-Ni can afford full molecular sieving toward C₃H₆/C₃H₈ separation; however, the extremely small pore spaces severely delimit its C₃H₆ uptake.¹⁶ Therefore, simple control of pore sizes with single functionality in SIFSIX materials cannot fully address the trade-off dilemma to target both high C₃H₆ adsorption and selectivity. Herein, we realized the immobilization of dual functionalities in a SIFSIX material (ZJUT-2, Ni(pyz-SH)₂SiF₆, pyz-SH = 2-mercaptopyrazine),³⁴ to construct an efficient C₃H₆ nano-trap for highly efficient C₃H₆/C₃H₈ separation. This MOF-based nano-trap features not only a small pore cavity with a suitable size of 3.9 × 3.9 × 7.5 ų that matches better with a kinetic diameter of C₃H₆ (4.0 Å) than C₃H₈ (4.3 Å),^12,18 but also is decorated with dual functionalities (–SH and SiF₆²⁻) to optimally interact with the C₃H₆ molecule. This material thus exhibits both top-tier C₃H₆ capture capacity (123.5 cm³ cm⁻³ at 0.5 bar and 296 K) and C₃H₆/C₃H₈ selectivity of 17.2 under ambient conditions, achieved by a moderate C₃H₆ heat of adsorption (45 kJ mol⁻¹). The C₃H₆ uptake and selectivity of ZJUT-2a are obviously higher than those of the pristine SIFSIX-3-Ni (76.6 cm³ cm⁻³ and 6.6) and most of the top-performing materials reported. Highly efficient separation of C₃H₆ from both 50/50 and 10/90 C₃H₆/C₃H₈ mixtures was confirmed by experimental breakthrough tests, providing both large C₃H₆ uptake (2.6 mmol g⁻¹) and high dynamic selectivity (10). Both values outperform or are comparable to those of some promising MOFs, such as Y-abtc (1.26 mmol g⁻¹ and 8.3),¹⁷ KAUST-7 (1.16 mmol g⁻¹ and 12),¹⁶ and Ni-NP (2.3 mmol g⁻¹ and 9.6).^22a

Results and discussion

The powder sample of ZJUT-2 was prepared by the reaction of NiSiF₆ and pyz-SH in methanol solution at 85 °C according to the previously reported literature.³⁴ The phase purity and crystallinity of bulk ZJUT-2 were confirmed by powder X-ray diffraction (PXRD), which matched well with that of the simulated patterns (Fig. S1, ESI†). As shown in Fig. 1a, detailed structure analysis revealed that this material consists of two-dimensional (2D) nets based on pyz-SH linkers and metal nodes, which are further pillared by SiF₆²⁻ anions to form the resulting 3D network. Each pore channel is separated by four SiF₆²⁻ anions to form cylindrical nanocages with a size of 3.9 × 3.9 × 7.5 ų. From a kinetics point of view, the nanocage aperture of 3.9 Å matches better with the kinetic diameter of C₃H₆ (4.0 Å) than C₃H₈ (4.3 Å), making it an ideal single-molecule trap for the capture of single C₃H₆ molecules (Fig. 1b). Most importantly, the incorporated dual functionalities of –SH and SiF₆²⁻ groups are located around the pore channels, which can create a multi-binding nano-trap to optimize the adsorption and recognition of C₃H₆ molecules. Thus, the optimized nano-trap with a suitable size and dual functionalities may provide a single-molecule trap for highly selective capture of C₃H₆ over C₃H₈.

Fig. 1 Structure description of ZJUT-2. (a) Illustration of the square-shaped arrangement in the Ni–pyrazine (4,4′) square grid that is further pillared by anion SiF₆²⁻ blocks to generate a 3D framework. (b) The pore channel structures of ZJUT-2, viewed along the a/c-axes and the b-axis, respectively. (c) The nanocages of ZJUT-2 that are separated by four SiF₆²⁻ anions along the b-axis. (d) View of the nano-trap with a size of 3.9 × 3.9 × 7.5 ų, decorated with dual functionalities (–SH and SiF₆²⁻). Color code: F, red; Si, cyan; C, gray; H, white, N, blue; Ni, green; S, yellow.

The permanent porosity of ZJUT-2a was first confirmed using the CO₂ adsorption isotherms at 196 K (Fig. S2, ESI†), affording a Brunauer–Emmett–Teller (BET) surface area of 387.8 m² g⁻¹. Single component gas adsorption isotherms of C₃H₆ and C₃H₈ for ZJUT-2a were collected at 296 K and 273 K up to 1 bar (Fig. 2a, S3 and S4, ESI†) and compared with those of pristine SIFSIX-3-Ni (Fig. S6, ESI†). As illustrated in Fig. 2a, ZJUT-2a exhibits a steep and high C₃H₆ uptake at 296 K, which is larger than that of C₃H₈ in the whole pressure region. Even at a low pressure of 0.1 bar, ZJUT-2a shows a very high C₃H₆ uptake of 72.3 cm³ cm⁻³, which is the highest among the reported MOFs relevant for C₃H₆/C₃H₈ separation except for the MOF-74 series (Fig. 2b).^25–27 In comparison, the C₃H₈ uptake at 0.1 bar (11.9 cm³ cm⁻³) is very low, affording a notably high C₃H₆/C₃H₈ uptake ratio of 6.1 at 0.1 bar. Such high low-pressure C₃H₆ uptake indicates that this MOF-based nano-trap can provide a stronger binding affinity with C₃H₆ over C₃H₈, probably attributed to the suitable cage size and dual functionalities that match better with the size and shape of the C₃H₆ molecule. When the pressure increases to 0.5 bar, the C₃H₆ uptake amount can be improved to 123.5 cm³ cm⁻³, which is higher than that of most relevant MOFs except for the MOF-74 series.^25–27 It is worth noting that this uptake is superior to that of SIFSIX-3-Ni (76.6 cm³ cm⁻³) and most of the current best-performing materials reported (Fig. 2c), such as HIAM-301 (87.0 cm³ cm⁻³),¹³ UTSA-400 (85.5 cm³ cm⁻³),¹⁴ MFM-520 (74.6 cm³ cm⁻³),²⁰ JNU-3a (69.3 cm³ cm⁻³),¹² and KAUST-7 (46.0 cm³ cm⁻³).¹⁶ At 1 bar and 296 K, the C₃H₆ uptake of ZJUT-2a can further increase to 138.9 cm³ cm⁻³. These adsorption behaviors can be supported by the experimental isosteric heat of adsorption (Q_st), wherein the initial Q_st value of C₃H₆ for ZJU-2a (45 kJ mol⁻¹) is higher than that of C₃H₈ (Fig. S9, ESI†). Further, as shown in Fig. S11 (ESI†), the initial Q_st value of ZJUT-2 for C₃H₆ is much higher than that of SIFSIX-3-Ni (38.7 kJ mol⁻¹), indicating that the immobilization of –SH groups can strengthen the C₃H₆ binding affinity. As shown in Fig. 2d, due to the lack of strong OMSs, the Q_st value of ZJUT-2a is much less than that of the MOF-74 series with high-density OMSs (55–70 kJ mol⁻¹).²⁵ Therefore, ZJUT-2a exhibits a remarkably top-tier C₃H₆ uptake achieved by a moderate Q_st value, compared with all the indicated MOFs as evidenced in Fig. 2d and S12 (ESI†).

Fig. 2 (a) Adsorption isotherms of C₃H₆ (red) and C₃H₈ (black) for ZJUT-2a at 296 K. (b) Comparison of C₃H₆ uptake capacity for ZJUT-2a and other best-performing materials at 0.1 bar and room temperature. (c) The absorption of C₃H₆ at 0.5 bar and room temperature for ZJUT-2a compared to the indicated best-performing materials. (d) Comparison of heats of adsorption (Q_st) of C₃H₆ and C₃H₆ uptake at 0.5 bar and room temperature for ZJUT-2a and other reported materials. (e) The IAST selectivity of 50/50 and 10/90 C₃H₆/C₃H₈ mixtures at 296 K. (f) Comparison of the C₃H₆ uptake capacity at 0.5 bar and C₃H₆/C₃H₈ selectivity for ZJUT-2a and other top-performing adsorbents reported.

The adsorption selectivity of ZJUT-2a for 50/50 and 10/90 C₃H₆/C₃H₈ mixtures was calculated by the ideal adsorbed solution theory (IAST) method. As indicated in Fig. 2e, ZJUT-2a shows a high selectivity of up to 17.2 and 20.9 for 50/50 and 10/90 C₃H₆/C₃H₈ mixtures at 1 bar and 296 K, respectively, which are much higher than that of the pristine SIFSIX-3-Ni (6.6) and other SIFSIX materials such as SIFSIX-2-Cu-i (4.5),^23b GeFSIX-2-Cu-i (4),^23b and ZU-36-Ni (10.8).^23a These values also outperform those of some promising MOFs including Ni–Np (10.5),^22b MAF-23-O (8.8)¹⁹ and MFM-520 (17),²⁰ but lower than those of molecular-sieving materials. It should be pointed out that this selectivity value is only for the qualitative comparison purpose. Besides gas selectivity, C₃H₆ uptake capacity at partial pressure is also an important criterion to determine the final separation performance. As shown in Fig. 3f, we comprehensively compared C₃H₆ uptake and selectivity of ZJUT-2a with some promising materials. Although those molecular-sieving MOFs exhibit the record-high selectivity due to the small pore sizes, their C₃H₆ uptakes are relatively low. If we set the C₃H₆ uptake and selectivity as concurrent objectives, most of the reported materials suffer from either unsatisfactory selectivity or inadequate uptake capacity. Evidently, ZJUT-2a, Co₂(m-dobdc),²⁵ UTSA-400 (ref. 14) and HIAM-301 (ref. 13) exhibit more balance between adsorption uptake and gas selectivity for C₃H₆/C₃H₈ separation. Thus, both top-tier C₃H₆ uptake capacity (123.5 cm³ cm⁻³ at 0.5 bar) and selectivity (17.2) along with moderate adsorption heat make this material among the best-performing materials reported for this separation.

Fig. 3 Illustration of (a) and (b) C₃H₆ adsorption sites in the nano-trap of ZJUT-2a, revealed by theoretical calculations. (c) Dense packing of the adsorbed C₃H₆ molecules within the pore channel of ZJUT-2a, viewed along the c axis. Color code: F (red), Si (cyan), C (grey in ZJUT-2, orange in C₃H₆), H (white), N (blue), Ni (green), S (yellow).

To gain better insight into both high C₃H₆ uptake and selectivity of ZJUT-2, grand canonical Monte Carlo (GCMC) simulations were performed to study the sorbate–sorbent interactions between the framework and gas molecules. The optimal adsorption sites for C₃H₆ and C₃H₈ in the pores of ZJUT-2a are approximated as shown in Fig. 3. Since the cavity size in ZJUT-2a matches well with the C₃H₆ molecule, each nano-trap can only capture one C₃H₆ molecule through multiple hydrogen bonding and van der Waals (vdW) interactions between dual functionalities (–SH and SiF₆²⁻) and the C₃H₆ molecule. As shown in Fig. 3a and b, each C₃H₆ molecule interacts with four SiF₆²⁻ anions through eleven C–H⋯F hydrogen bonds with the distances of 2.33–3.09 Å and also binds with four –SH groups through six C–H⋯S interactions (2.73–3.27 Å). Evidently, the immobilized SiF₆²⁻ and –SH groups synergistically contribute to enforcing the interactions with the C₃H₆ molecule. Further, the dense distribution of these nano-traps within the framework enables the C₃H₆ molecules to be in close proximity to each other with a contact distance of 4.16 Å along the b axis (Fig. 3c). Such observed contact distances in ZJUT-2a are notably shorter than those found in JNU-3a (4.53 Å), HIAM-301 (6.03 Å) and also comparable to the C₃H₆⋯C₃H₆ average distance in the crystalline C₃H₆ (4.47 Å) collected at 65 K.^15,32 This reveals that ZJUT-2a shows the dense packing of C₃H₆ molecules within the pores, thus resulting in its high C₃H₆ uptake capacity. In comparison, the overall H-bonding interactions between the pore surface and C₃H₈ molecule are much less than that of C₃H₆ (Fig. S14 and S15, ESI†). In addition, the cavity sizes of this nano-trap were found to be smaller than the kinetic diameter of C₃H₈. The weaker binding affinity and poor size match with the nano-trap may lead to partially populating the binding sites for C₃H₈. The above reasons thus afford both high C₃H₆ uptake capacity and selectivity.

To evaluate the actual separation performance of ZJUT-2a, dynamic breakthrough experiments on binary C₃H₆/C₃H₈ gas mixtures were carried out under ambient conditions. As presented in Fig. 4a, ZJUT-2a exhibited a clear separation for the 50/50 C₃H₆/C₃H₈ mixture, wherein pure C₃H₈ first eluted through the adsorption bed at 33 min, while C₃H₆ was retained for a longer time of 65 min. During this breakthrough interval, the C₃H₆ dynamic uptake was calculated to be 2.6 mmol g⁻¹, which is 79% of the saturated uptake (3.3 mmol g⁻¹) obtained from single-component adsorption isotherms at 296 K and 1 bar. This value is not only much higher than that of other reported SIFSIX materials (Fig. 4b), including SIFSIX-3-Ni (1.25 mmol g⁻¹),^23a GeFSIX-2-Cu-i (2.2 mmol g⁻¹),^23b and SIFSIX-2-Cu-i (2.0 mmol g⁻¹),^23b but also outperforms some top-performing MOFs such as JNU-3a (2.45 mmol g⁻¹),¹² HIAM-301 (2.07 mmol g⁻¹)¹³ and KAUST-7 (1.16 mmol g⁻¹).¹⁶ Further, the dynamic C₃H₆/C₃H₈ selectivity was estimated to be up to 10, which exceeds that of SIFSIX-3-Ni (2.3)^23a and most top-performing materials, such as Y-abtc (8.3),¹⁷ Co-MOF-74 (6.5)²⁵ and Ni-NP (9.6).^22a As shown in Fig. 4c, ZJUT-2a thus shows a rare combination of simultaneously high C₃H₆ dynamic uptake and selectivity, placing it among the best-performing materials reported so far for C₃H₆/C₃H₈ separation. It should be noted that the feed gases in some production processes might contain only a small amount of C₃H₆, which requires adsorbents to efficiently capture C₃H₆ at low partial pressures. Therefore, we performed the breakthrough experiments for 10/90 C₃H₆/C₃H₈ mixtures (Fig. 4d). More difference in the breakthrough times of C₃H₆ and C₃H₈ was observed, with a high dynamic C₃H₆ uptake of 1.5 mmol g⁻¹, indicating the superior separation ability of ZJUT-2a for some gas mixtures with low C₃H₆ content. Three continuous cycles on 50/50 and 10/90 mixtures showed the full retention of the separation performance and easy recyclability of ZJUT-2a (Fig. 4e and S20, ESI†). Given that water vapor is a ubiquitous component in industrial gas mixtures,³³ we conducted the breakthrough experiments for a wet C₃H₆/C₃H₈ mixture at 50% relative humidity. As shown in Fig. 4f, the almost unchanged breakthrough times of both C₃H₆ and C₃H₈ demonstrated the excellent moisture tolerance properties of ZJUT-2a, which can avoid the deleterious effect of water vapor on C₃H₆/C₃H₈ separation performance. As inferred from the PXRD performed on associated samples (Fig. S22, ESI†), the framework of ZJUT-2a remains stable after multiple breakthrough experiments.

Fig. 4 (a) Experimental breakthrough curves for a 50/50 C₃H₆/C₃H₈ mixture with a flow rate of 2 mL min⁻¹ under ambient conditions. (b) Comparison of C₃H₆ dynamic uptake for ZJUT-2a and other benchmark materials. (c) Comparison of C₃H₆ dynamic uptake and selectivity for ZJUT-2a and other benchmark materials. (d) Experimental breakthrough curves for a 10/90 C₃H₆/C₃H₈ mixture with a flow rate of 2 mL min⁻¹ under ambient conditions. (e) Cycling column breakthrough curves for 50/50 C₃H₆/C₃H₈ separation under ambient conditions. (f) Breakthrough curves of ZJUT-2a for 50/50 C₃H₆/C₃H₈ separation at 50% humidity.

Conclusions

In summary, we have realized the immobilization of dual functionalities into a suitable MOF to construct a single-molecule nano-trap for highly efficient C₃H₆/C₃H₈ separation. The incorporated bifunctional groups (–SH and SiF₆²⁻) combined with the appropriate cavity size/shape in ZJUT-2a can synergistically create multiple binding environments to densely and selectively trap the C₃H₆ molecule, as revealed by theoretical calculations. This MOF-based nano-trap thus exhibited both top-tier C₃H₆ capture capacity (123.5 cm³ cm⁻³ at 0.5 bar) and C₃H₆/C₃H₈ selectivity of 17.2 under ambient conditions, achieved by a moderate C₃H₆ adsorption enthalpy (45 kJ mol⁻¹). Breakthrough experiment data revealed both remarkably large C₃H₆ dynamic uptake of 2.6 mmol g⁻¹ and high dynamic selectivity of 10 for the separation of actual C₃H₆/C₃H₈ mixtures, surpassing most of the top-performing materials reported to date. This work provides some guidance to design porous materials with multiple soft functionalities to highly boost C₃H₆/C₃H₈ separation performance through moderate gas adsorption enthalpy.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (92163110, U1909214 and 92061126) and the Key Research and Development Program of Zhejiang Province (2021C01182).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Stability tests, PXRD patterns, adsorption isotherms and breakthrough curves. See DOI: https://doi.org/10.1039/d3ta02737f

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