Metal–organic frameworks for purification of methanol-to-olefin (MTO) products

Abstract

Ethylene and propylene are fundamental feedstocks in the chemical industry, and their production currently relies predominantly on petroleum cracking. In recent years, the maturation of methanol-to-olefin (MTO) technology has opened new avenues for the production of low-carbon olefins. Traditionally, separation of MTO products has relied on energy-intensive distillation. The development of physical separation technologies could reduce energy consumption by one-third or more. As an emerging class of crystalline porous materials, metal–organic frameworks (MOFs) exhibit great promise for gas adsorption and separation, owing to their highly tunable pore environments. However, the similar physicochemical properties of MTO products present significant challenges in designing MOF adsorbents with high selectivity for their separation. To date, studies on MOF-based separation of MTO products remain limited. In this review, we provide a concise overview of recent advances in MOF-based adsorbents for the capture and separation of MTO products. We summarize the underlying separation mechanisms and strategies for enhancing MOF performance and finally discuss the remaining challenges and future prospects in this field.

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1. Introduction

Ethylene (C₂H₄) and propylene (C₃H₆) are essential building blocks in the modern chemical industry, widely used in the synthesis of polymers, solvents, and a variety of fine chemicals.^1–3 Traditionally, these light olefins are primarily produced via steam cracking of petroleum.^4,5 However, with the growing depletion of global petroleum resources and the push toward carbon neutrality, the development of sustainable, non-petroleum-based olefin production technologies has become a major research focus.⁶ In recent years, methanol-to-olefin (MTO) technology has emerged as a promising alternative to petroleum cracking for light olefin production, owing to its diverse feedstock options—such as coal, biomass, and CO₂-derived methanol—and excellent product selectivity.^7–9

The MTO process typically yields approximately 51 wt% of C₂H₄ and 21 wt% of C₃H₆, along with smaller amounts of ethane (C₂H₆), propane (C₃H₈), C₄, and C₅+ hydrocarbons.^10–12 To meet the high-purity requirements of downstream polymerization and fine chemical processes, efficient separation of target olefins from MTO products is essential. However, conventional separation methods rely heavily on cryogenic distillation under high pressure, which is energy-intensive, costly, and demands stringent operational and equipment conditions.¹³ As a result, the development of more energy-efficient and cost-effective alternative technologies is key to advancing the industrialization of MTO.

Porous adsorbents, which function through adsorption/desorption cycles, have attracted considerable attention due to their simple operation and low energy consumption, particularly in gas-phase hydrocarbon separations.^14–20 Metal–organic frameworks (MOFs), as a rapidly advancing class of porous materials, offer exceptional advantages in olefin/paraffin separations, owing to their ultra-high surface areas, structural diversity, and tunable pore sizes and surface functionalities.^21–25 In the context of MTO product separation, finely tuned MOFs have demonstrated great potential for highly selective separation of complex multi-component mixtures such as C₃H₆/C₂H₄, C₃H₆/C₂H₆/C₂H₄, and C₃H₆/C₃H₈/C₂H₄.

Nonetheless, the molecular similarity in size and polarity between C₂–C₃ olefins and their corresponding paraffins poses substantial challenges for efficient separation.²⁶ These challenges necessitate precise control over key structural parameters of MOFs, including pore size matching, pore volume tuning, open metal sites, and N/O/F-containing functional groups.^27–31

To date, comprehensive reviews on the application of MOFs for MTO product separation remain scarce. This review aims to summarize recent progress in the use of MOFs for separating MTO products—including C₃H₆/C₂H₄, C₃H₆/C₂H₆/C₂H₄, and C₃H₆/C₃H₈/C₂H₄—with a focus on representative MOF structures and their separation performance in practical applications. Furthermore, the key challenges are discussed and potential future directions are proposed to stimulate deeper research and discussion in this emerging field.

2. Discussion

2.1. C₃H₆/C₂H₄ separation

As the two principal products of the MTO process, the separation of C₃H₆ and C₂H₄ is both fundamental and critical for product purification, offering substantial economic benefits. Although C₃H₆ and C₂H₄ exhibit similar physical properties (Table 1), which poses challenges for their separation, there has been a surge of interest in using MOFs for the separation of C₃H₆/C₂H₄ in recent years. As summarized in Table 2, many MOF materials have demonstrated excellent performance in C₃H₆/C₂H₄ separation, exhibiting significant adsorption capacity and high selectivity.^32–51 MFM-202a reported by Martin Schröder et al., NEM-7-Cu by Wenyue Guo et al., and LIFM-38 by Jijun Jiang et al. are among the earliest MOFs developed for C₃H₆/C₂H₄ separation.^46,51,52 With continued advances in MOF research, new materials with superior separation performance have been successively reported. At present, most MOF-based strategies for C₃H₆/C₂H₄ separation are centered on several key design principles, including the use of functionalized organic linkers, the introduction of open metal sites (OMSs), the synergistic integration of OMSs with functionalized linkers, and the construction of single-molecule nanoscopic traps.

Table 1 The physical properties of C₂H₄, C₂H₆, C₃H₆ and C₃H₈

Olefin/paraffin	Molecular dimensions (Å)	Kinetic diameter (Å)	Polarizability × 10^25 cm^−3	Boiling point (K)
C2H4	3.25 × 4.18 × 4.84	4.16	42.5	169.4
C2H6	3.81 × 4.08 × 4.44	4.44	44.3–44.7	184.6
C3H6	4.20 × 5.30 × 6.40	4.68	62.6	225.5
C3H8	4.20 × 4.60 × 6.80	4.30–5.11	62.9–63.7	231.0

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Table 2 Summary of representative MOFs for C₃H₆/C₂H₄ separation

MOFs	S BET (m^2 g^−1)	C3H6 uptake (cm^3 g^−1)	C2H4 uptake (cm^3 g^−1)	Q st (C3H6) (kJ mol^−1)	Q st (C2H4) (kJ mol^−1)	C3H6/C2H4Sads	Main interactions	Ref.
a 296 K. b 10 kPa.
Co2(OATA)(DPA)	823	121.3	94.5	28.5	21.7	23.1	C–H⋯π; C–H⋯O/N	32
FJI-W9	1026	83	66	38.0	20.9	20.5	C–H⋯F/O	33
JLU-MOF132	450	52.4	15.8	29.2	25.4	9.2	C–H⋯F/O	34
Fe-BQDC-BTC-TPBTC	834	69.3	46.2	42.5	28.5	9.6	OMSs	35
FJI-H8-Me	2054	211.0	173.1	44.3	34.3	9.9	C–H⋯π; C–H⋯O	36
Cu3(OH)2(Me2BPZ)2	1533	138	52	30.27	20.68	7.4	C–H⋯π; C–H⋯O	37
Zn-BPZ-SA	925	68.3	63.9	33.65	23.13	4.8	C–H⋯π; C–H⋯O/N/C	38
Mn-dtzip	881	216.4	76.7	32.3	24.2	8.6	OMSs; C–H⋯O/N	39
1	1958	146.3	47.2	37.6	28.0	9.0	C–H⋯O/N	40
2	1421	127.3	41.7	34.0	27.3	5.4	C–H⋯O/N	40
Spe-MOF	3292	236.9	48.9	29.4	22.5	7.7	OMSs; C–H⋯O/N	41
CoFA	400	92.7	88.0	56.05	33.76	10.43	C=C⋯H; C=O⋯H	42
ZJU-74a-Pd^a	590	108.5	105.7	55.8	30.5	23.4	OMSs; C–H⋯π; C–H⋯N≡C/C	43
Fe2Mn-L	3105	291.1	87.5	39.9	38.9	7.8	OMSs	44
MIL-101(Cr)	2692	196.6	62.1	34.3	35.8	6.6	OMSs	44
ZIF-8	1511	80.6	26.4	29.0	13.8	6.7	OMSs	44
Activated carbon	1086	128.9	98.8	38.5	32.5	11.6	van der Waals	44
HKUST-1	1514	167.0	136.5	48.5	45.1	16.3	OMSs	44
Iso-MOF-1	3211	209.0	51.0	—	—	5.08	OMSs; C–H⋯π; C–H⋯O/N	45
Iso-MOF-2	3154	254.1	71.4	—	—	6.60	OMSs; C–H⋯π; C–H⋯O/N/F	45
Iso-MOF-3	3067	234.7	66.0	—	—	7.04	OMSs; C–H⋯π; C–H⋯O/N	45
Iso-MOF-4	2925	254.5	73.1	30.9	25.4	7.74	OMSs; C–H⋯π; C–H⋯O/N/F	45
NEM-7-Cu	786	75.5	29	36.9	22.5	8.6	C–H⋯O/N	46
CoV-bco-tpt	1042	110.0	102.8	—	—	7.83	van der Waals	47
FDM-201	1965	180.5	68.2	27.1	19.1	8.6	C–H⋯π; C–H⋯O/N; π⋯π	48
SIFSIX-Cu-TPA	—	128.9	26.2^b	—	—	—	C–H⋯π; C–H⋯F; π⋯π	49
UPC-33	934	94.3	31.1	48.93	10.31	5.7	NH2 groups	50
LIFM-38	803	58	20	27.3	28.1	6.4	C–H⋯O/N/F	51
MAF-68	1724	151.9	84.7	31.6	19.9	8.6	C–H⋯π; π⋯π	53
JLU-MOF125	1638	148.3	104.8	35.8	26.4	9.7	CH3 groups	54
JLU-MOF126	1587	130.1	76.6	38.2	21.8	11.0	NH2 groups	54
UiO-68-2CH3OCH3	3133	238.7	58.5	26.2	17.1	6.6	C–H⋯π	55

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2.1.1. Linker functional sites. Co₂(OATA)(DPA), reported by Zhonghua Zhu et al., achieves preferential capture of C₃H₆ through a synergistic combination of optimal pore size and both –NHCO– and –NH– functional groups. At 298 K and 1 bar, it exhibits a C₃H₆ uptake of 121.3 cm³ g⁻¹—significantly higher than the 94.5 cm³ g⁻¹ uptake for C₂H₄—with a high C₃H₆/C₂H₄ selectivity of 23.1.³² Notably, Co₂(OATA)(DPA) maintains an exceptionally high C₃H₆ uptake of 97.8 cm³ g⁻¹ even at a low pressure of 0.05 bar. In breakthrough experiments conducted at 298 K, Co₂(OATA)(DPA) was able to produce 97.9 L kg⁻¹ of C₂H₄ (99.9% purity) and 75.0 L kg⁻¹ of C₃H₆ (99.9% purity) in a single run. GCMC simulations indicate that the primary adsorption mechanism involves C–H⋯π interactions between C₃H₆ or C₂H₄ and the pyridine moieties of the ligand, as well as C–H⋯O and C–H⋯N hydrogen-bonding interactions with the framework. These interactions are generally stronger for C₃H₆ than for C₂H₄. Moreover, it retained high productivity and purity even at 353 K, underscoring its strong potential for application in MTO product separation (Fig. 1).

Fig. 1 (a) Three-dimensional framework of Co₂(OATA)(DPA) viewed along the c-axis; (b) adsorption isotherms of C₂H₄ and C₃H₆ at 298 K; (c) breakthrough curves for an equimolar C₂H₄/C₃H₆ mixture (1 : 1) at 298 K under varying pressures in Co₂(OATA)(DPA); and (d) desorption profiles of a 1 : 1 C₂H₄/C₃H₆ mixture at 2.0 bar.³² Reproduced from ref. 32 with permission from Springer Nature, copyright 2024.

In 2024, Mingyan Wu et al. reported the synthesis of a layered Y-MOF (FJI-W9) assembled from bent diisophthalate ligands and 2-fluorobenzoic acids with Y–O clusters.³³ Benefiting from the exposed –F groups within the pores, FJI-W9 exhibited notable adsorption capacities of 83 cm³ g⁻¹ for C₃H₆ and 66 cm³ g⁻¹ for C₂H₄ (298 K, 100 kPa), with a C₃H₆/C₂H₄ IAST selectivity reaching 20.3. GCMC simulations were employed to investigate the adsorption mechanism of FJI-W9, revealing that the preferred adsorption sites for C₃H₆ and C₂H₄ are located around the –F groups and oxygen atoms, where multiple C–H⋯F/O interactions occur, accompanied by significant differences in static binding energies. Breakthrough experiments confirmed that FJI-W9 can effectively separate C₃H₆/C₂H₄ mixtures and produce polymer-grade C₂H₄, maintaining its separation performance over multiple cycles. These results indicate that FJI-W9 is a promising MOF material for MTO separation.

Yunling Liu et al. synthesized a mixed multinuclear cluster-based MOF, JLU-MOF132, using the small V-shaped ligand 2,5-thiophene dicarboxylate.³⁴ Due to the high connectivity of the multinuclear yttrium clusters, JLU-MOF132 exhibited excellent thermal and solvent stability. After Li⁺ ion exchange, the adsorption capacities of C₃H₆ and C₂H₄ at 298 K and 1 bar for JLU-MOF132 were 52.4 cm³ g⁻¹ and 15.8 cm³ g⁻¹, respectively, with an IAST selectivity of 9.2 (50/50). In further dynamic breakthrough experiments, JLU-MOF132 demonstrated good separation performance across a wide range of ratios. Under 50/50 (v/v, 298 K) conditions, the C₂H₄ productivity reached 26.1 L kg⁻¹. Under conditions closer to industrial production ratios (20/50, v/v), the C₂H₄ productivity increased to 73.1 L kg⁻¹, surpassing most reported MOFs. In repeated dynamic breakthrough cycles, JLU-MOF132 maintained stable separation performance, and due to its lower Q_st, the regeneration conditions for JLU-MOF132 were relatively mild.

2.1.2. Open metal sites. Open metal sites (OMSs) represent a well-established strategy for the selective binding of unsaturated hydrocarbons, as they can form strong π-complexation interactions with unsaturated C=C bonds, thereby imparting MOF materials with exceptional adsorption capabilities.^56,57 In 2024, Bin Li et al. synthesized a high-density open-metal-site MOF, ZJU-74a-Pd, featuring sandwich-like nanoscopic traps that are well-matched to C₃H₆ molecules and exhibiting unprecedented binding affinity toward C₃H₆.⁴³ At 0.1 bar, ZJU-74a-Pd demonstrated an exceptionally high C₃H₆ uptake of 103.8 cm³ g⁻¹. GCMC simulations revealed that the adsorbed C₃H₆ molecules strongly interact with two adjacent Pd(II) sites, underpinning the high affinity observed. Breakthrough experiments confirmed the excellent performance of ZJU-74a-Pd in separating C₃H₆/C₂H₄ mixtures across various compositions, enabling the production of high-purity C₂H₄ (>99.95%) with a yield of 3.4 mol kg⁻¹ in a single-step process. Moreover, the material retained its separation performance over five consecutive cycles. In summary, ZJU-74a-Pd exhibits outstanding performance for C₃H₆/C₂H₄ separation, establishing it as a highly promising MOF candidate for purification of MTO products.

2.1.3. Integration of OMSs and functionalized linkers. In 2019, Daofeng Sun et al. synthesized a series of iso-MOFs by employing ligand modification strategies to finely tune the pore environment within the frameworks.⁴⁵ Among them, iso-MOF-4 exhibited a maximum C₃H₆ uptake of 254.5 cm³ g⁻¹ and a C₂H₄ uptake of 73.1 cm³ g⁻¹ at 298 K and 1 bar, with an IAST selectivity of 7.74. Further GCMC simulations revealed that the exceptionally high C₃H₆ uptake of iso-MOF-4 is attributed to its abundance of phenyl and pyrazine rings, appropriately sized pore spaces, and accessible open metal sites. Further breakthrough experiments confirmed the excellent dynamic separation performance of iso-MOF-4 for C₃H₆/C₂H₄ (298 K, v/v = 50/50), achieving a C₂H₄ productivity of 14.30 mol kg⁻¹. In addition, iso-MOF-4 demonstrated outstanding cycling stability, maintaining its C₂H₄ productivity over multiple adsorption–desorption cycles without significant degradation (Fig. 2).

Fig. 2 (a) Structural modulation of iso-MOFs; (b) C₃H₆ and C₂H₄ adsorption isotherms of iso-MOFs at 298 K; and (c) breakthrough curves for a C₃H₆/C₂H₄ gas mixture obtained using iso-MOF-4 at 298 K.⁴⁵ Reproduced from ref. 45 with permission from American Chemical Society, copyright 2019.

2.1.4. Single-molecule nanoscopic traps. In 2025, Yuanbin Zhang et al. reported a low-cost, easily scalable cobalt formate MOF material, CoFA, for C₃H₆/C₂H₄ separation (Fig. 3).⁴² Due to the presence of single-molecule C₃H₆ nano-traps in the CoFA structure, it exhibits preferential adsorption of C₃H₆. Under conditions of 298 K and 1 bar, CoFA synthesized at a 10 g scale had a C₃H₆ adsorption capacity of 94.9 cm³ g⁻¹, while the C₂H₄ adsorption capacity was 89.3 cm³ g⁻¹. At 298 K, the IAST selectivity of CoFA for C₃H₆/C₂H₄ was 11.84, which is higher than that of most reported MOF materials used for C₃H₆/C₂H₄ separation. Five dynamic breakthrough experiments were conducted on CoFA under the conditions of C₃H₆/C₂H₄/Ar (10/10/80, v/v/v, 298 K), with an average C₂H₄ productivity of 26.1 ± 2.6 cm³ g⁻¹, indicating excellent separation performance and good cycling stability. The C₃H₆ and C₂H₄ adsorption capacities of CoFA@5%PES granules, made by using PES as a binder, did not show significant decreases compared to the CoFA powder, indicating their potential for industrial application (Fig. 3). This work provides a low-cost, high-performance MOF material for MTO separation processes.

Fig. 3 (a) Structure of CoFA; (b) photographs of CoFA granules after granulation; and (c) adsorption isotherms of C₂H₆ and C₃H₆ on CoFA@5%PES at 298 K and 1 bar.⁴² Reproduced from ref. 42 with permission from Elsevier, copyright 2025.

2.2. C₃H₆/C₂H₆/C₂H₄ separation

2.2.1. Nonpolar channels functionalized with N/O/F sites. In actual MTO processes, small amounts of C₂H₆ are present in the product, and the separation of C₂H₆/C₂H₄ is already a highly challenging separation process in MOFs.^58–60 Further separation of C₃H₆/C₂H₆/C₂H₄ is undoubtedly even more difficult. Currently, the construction of MOFs capable of separating C₃H₆, C₂H₆, and C₂H₄ primarily relies on avoiding the presence of open metal sites, while incorporating accessible O, N, or F binding sites within the pore environment. This design enables strong binding affinities toward C₃H₆ and C₂H₆ over C₂H₄ through a combination of electrostatic interactions and multiple types of supramolecular binding forces. Table 3 summarizes some representative MOFs for C₃H₆/C₂H₆/C₂H₄ separation.^{50,52,61–72} In the separation of MTO products, the goal is not only the purification of C₂H₄ but also the recovery of high-purity C₃H₆.

Table 3 Summary of representative MOFs for C₃H₆/C₂H₆/C₂H₄ separation

MOFs	S BET (m^2 g^−1)	C3H6 uptake (cm^3 g^−1)	C2H6 uptake (cm^3 g^−1)	C2H4 uptake (cm^3 g^−1)	C3H6/C2H4Sads	C2H6/C2H4Sads	Main interactions	Ref.
a C3H6/C2H4 (2/5). b C2H6/C2H4 (10/90). c 293 K.
FJI-Y9	1420	131	105	88	9.6^a	1.79^b	OMSs; C–H⋯π; C–H⋯O	61
Zn2(oba)2(dmimpym)	354	76.0	56.4	48.3	15.6	1.8	C–H⋯π; C–H⋯O/N/C	62
Zn-BPZ-TATB	1124	114.0	105.1	91.8	7.4	1.7	C–H⋯π; C–H⋯O/N/C	63
MAC-4	1218	127	124	83	8.4	2.0	C–H⋯π; C–H⋯O/N	64
Al-TCPP	1548	162.4	118.5	101.2	10.1	2.0	C–H⋯π; C–H⋯O/N; C⋯O–H	65
JNU-74a	1006	94.6	82.1	71.5	13.0	2.3	C–H⋯π; C–H⋯O/N/F	66
ZJNU-401	1224	101.5	76.8	65.9	15.45	1.75	C–H⋯π; C–H⋯O/N	67
Zn(bdc)(dabco)0.5	1710	—	—	—	—	—	C–H⋯π; C–H⋯O; π⋯π	68
Zn(bdc-Cl)(dabco)0.5	1723	—	—	—	—	—	C–H⋯π; C–H⋯O/Cl; π⋯π	68
Zn(bdc-Cl2)(dabco)0.5	1446	—	—	—	3.0	1.7	C–H⋯π; C–H⋯O/Cl; π⋯π	68
Zn(bdc-ndc)(dabco)0.5	1485	—	—	—	—	—	C–H⋯π; C–H⋯O; π⋯π	68
DMOF-1-Cl2	1222	115.3	110.0	96.8	12.1	1.9	C–H⋯π; C–H⋯O/Cl	69
DMOF-1-Br2	979	85.3	83.4	71.4	11.4	—	C–H⋯π; C–H⋯O/Br	69
NKU-0210	1290	180.41	106.10	—	16.4	1.60	C–H⋯π; C–H⋯O/N	70
NTUniv-75	889	102	98	97	14.3^a	1.86^b	C–H⋯π; C–H⋯O/N	71
PL-Co-MOF	1344	135.52	120.05	103.8	8.61	1.49	C–H⋯π; C–H⋯O/N	72
MFM-202a	2220	160.8^c	94.3^c	65.0^c	8.4	1.4	C–H⋯π	52

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In 2023, Zhonghua Zhu et al. designed Zn-BPZ-TATB, featuring active O/N sites and nonpolar pores, to achieve one-step purification of C₂H₄ (99.9%) and recovery of C₃H₆ (99.5%) from C₃H₆/C₂H₆/C₂H₄ mixtures (Fig. 4).⁶³ At 298 K and 1 bar, the adsorption capacities of Zn-BPZ-TATB for C₂H₄, C₂H₆, and C₃H₆ are 4.10 mmol g⁻¹, 4.69 mmol g⁻¹, and 5.09 mmol g⁻¹, respectively, indicating that Zn-BPZ-TATB is a selective adsorbent for C₂H₆ and C₃H₆. GCMC simulations were conducted to gain deeper insight into the selective adsorption behavior of Zn-BPZ-TATB. The results reveal that the preferred adsorption sites are primarily located near methyl and carboxyl groups. C₂H₄ forms two C–H⋯N hydrogen bonds and one C–H⋯π interaction with the framework, whereas C₂H₆ engages in multiple C–H⋯C/N/O interactions. In contrast, C₃H₆ exhibits a combination of C–H⋯π and C–H⋯C/N/O interactions with the framework. These interactions collectively result in stronger binding affinities for C₂H₆ and C₃H₆ compared to C₂H₄. Further dynamic breakthrough experiments show that Zn-BPZ-TATB can achieve 99.9% C₂H₄ purity from mixtures of C₃H₆ and C₂H₄ at various temperatures (273 K and 298 K) and ratios (10/10, 20/20, and 30/30, v/v). Dynamic breakthrough experiments for a ternary mixture of C₃H₆/C₂H₆/C₂H₄, which more closely resembles actual production conditions, show that Zn-BPZ-TATB can produce 99.9% C₂H₄ and obtain 99.5% C₃H₆ after inert gas-assisted thermal desorption, with yields of 38.2 L kg⁻¹ and 12.7 L kg⁻¹, respectively.

Fig. 4 (a) Structure of Zn-BPZ-TATB; (b) adsorption isotherms for C₂H₄, C₂H₆, and C₃H₆ at 298 K; (c) dynamic breakthrough curves of Zn-BPZ-TATB for C₂H₆/C₂H₄ mixtures with different volume ratios; and (d) dynamic breakthrough and desorption curves of Zn-BPZ-TATB for a C₂H₆/C₃H₆/C₂H₄ (5/5/5, v/v/v) mixture.⁶³ Reproduced from ref. 63 with permission from John Wiley and Sons, copyright 2023.

Another typical example of enhancing the separation of C₃H₆/C₂H₄ and C₂H₆/C₃H₆/C₂H₄ through the design of rich O/N sites and nonpolar channels is MAC-4, reported by Lei Hou et al. in 2024 (Fig. 5).⁶⁴ At 298 K and 1 bar, the adsorption capacities of C₃H₆, C₂H₆, and C₂H₄ on MAC-4 were 127 cm³ g⁻¹, 107 cm³ g⁻¹, and 83 cm³ g⁻¹, respectively, showing preferential adsorption of C₃H₆ and C₂H₆ over C₂H₄. GCMC simulations confirmed that the primary adsorption sites are located within the cavity formed by two [Zn₂(COO)₄] SBUs and two triazolate-trinuclear [Zn₃(OH)(dmtrz)₃] SBUs. Multiple C–H⋯O/N hydrogen bonds are observed between the ligands and C₂H₄, C₂H₆, and C₃H₆ molecules. However, the interactions between the framework and C₂H₆/C₃H₆ are both more numerous and stronger than those with C₂H₄, consistent with the adsorption behavior of MAC-4. To evaluate its performance in practical separation processes, the dynamic breakthrough curves of MAC-4 for C₂H₆/C₂H₄ and C₂H₆/C₃H₆/C₂H₄ mixtures at various temperatures and ratios were further tested (Fig. 5). The experimental results show that MAC-4 exhibited good separation performance. For a C₂H₆/C₃H₆/C₂H₄ mixture (2/10/25, v/v/v) at 298 K, MAC-4 produced 27.4 L kg⁻¹ of C₂H₄, and after thermal desorption, 36.2 L kg⁻¹ of C₃H₆, surpassing most of the reported MOFs. Notably, MAC-4 can be synthesized on a gram scale using inexpensive commercial ligands under heated stirring conditions, making it a potential candidate for industrial adsorption applications.

Fig. 5 (a) Structural diagram of MAC-4; (b) large-scale synthesis of MAC-4; (c) adsorption isotherms of C₂H₄, C₂H₆, and C₃H₆ on MAC-4 at 298 K; (d) dynamic breakthrough curves of MAC-4 for a C₂H₆/C₂H₄ mixture (1/1, v/v, 298 K); and (e) dynamic breakthrough and desorption curves of MAC-4 for a C₂H₆/C₃H₆/C₂H₄ mixture (5/10/25, v/v/v).⁶⁴ Reproduced from ref. 64 with permission from John Wiley and Sons, copyright 2024.

Dan Li et al. enhanced the surface polarity of non-polar pore surfaces by introducing highly electronegative atoms, thereby strengthening binding affinity for C₃H₆ and C₂H₆ over C₂H₄ through a combination of electrostatic interactions and multiple supramolecular binding forces. Based on this strategy, they synthesized a MOF material, JNU-74a, which is rich in O/N/F binding sites.⁶⁶ Benefiting from a high density of supramolecular binding sites in its structure, JNU-74a exhibits excellent purification performance for both C₃H₆/C₂H₄ and C₃H₆/C₂H₆/C₂H₄ mixtures. Under various dynamic breakthrough conditions, JNU-74a enables the separation of MTO products to yield high-purity C₂H₄ (99.95%), followed by the desorption of high-purity C₃H₆ (99.5%). Notably, in the ternary system, a single adsorption–desorption cycle with JNU-74a yields 12.7 L kg⁻¹ of C₂H₄ and 38.2 L kg⁻¹ of C₃H₆. More importantly, JNU-74a can be synthesized on a 10-gram scale using relatively inexpensive reagents, making it a promising candidate for the industrial separation of MTO products.

2.3. C₃H₆/C₃H₈/C₂H₄ separation

Reports on C₃H₈ separation in the MTO process are currently scarce, mainly due to the limitations in the practical separation performance of MOF-based materials for C₃H₆/C₃H₈ separation, despite the development of various separation mechanisms, including thermodynamic,^74,75 kinetic,^76,77 kinetic-thermodynamic synergy,^78,79 gate effect,^80–85 and molecular sieving.³⁰ The addition of another product, C₂H₄, further complicates the separation process.

In 2025, Lei Hou et al. reported the first MOF material, Cd-dtzip-H₂O, capable of achieving C₃H₆/C₃H₈/C₂H₄ separation.⁷³ This material features a honeycomb-like channel structure, which facilitates the preferential adsorption of C₃H₆ through a vortex effect induced by H₂O in the pores, demonstrating excellent C₃H₆ adsorption performance. At 298 K and 1 bar, the adsorption amount of C₃H₆ reached 112.0 cm³ g⁻¹, significantly surpassing those of C₃H₈ (66.5 cm³ g⁻¹) and C₂H₄ (56.7 cm³ g⁻¹). GCMC simulations revealed that the optimal adsorption sites are located near vortex-like regions formed by water molecules within the framework. The oxygen atoms of the water molecules and the nitrogen atoms in the ligands form multiple C–H⋯O/N hydrogen bonds with the gas molecules. Among these, C₃H₆ forms a significantly greater number of hydrogen bonds than C₃H₈ and C₂H₄, corroborating the experimental gas adsorption results that indicate a pronounced affinity for C₃H₆. In dynamic experiments, Cd-dtzip-H₂O was able to achieve 5.2 L kg⁻¹ of C₃H₆ (99.9%) and 21.7 L kg⁻¹ of C₂H₄ (99.5%) in a C₃H₆/C₃H₈/C₂H₄ mixture (Fig. 6). Table 4 presents key data on the adsorption behavior of Cd-dtzip-H₂O.⁷³ Cd-dtzip-H₂O represents a significant step forward in the use of MOFs for the separation of MTO process products, while also presenting greater challenges.

Fig. 6 (a) Pore accumulation diagram of Cd-dtzip-H₂O; (b) gas adsorption isotherms of Cd-dtzip-H₂O at 298 K; (c) breakthrough curves at different ratios of C₂H₄/C₃H₈/C₃H₆ mixtures; and (d) desorption curves of C₂H₄/C₃H₈/C₃H₆ (15/5/5) mixtures.⁷³ Reproduced from ref. 73 with permission from John Wiley and Sons, copyright 2025.

Table 4 Summary of representative MOFs for C₃H₆/C₃H₈/C₂H₄ separation

MOFs	S BET (m^2 g^−1)	C3H6 uptake (cm^3 g^−1)	C3H8 uptake (cm^3 g^−1)	C2H4 uptake (cm^3 g^−1)	C3H6/C2H4Sads	C3H6/C3H8Sads	Main interactions	Ref.
Cd-dtzip-H2O	804	112.0	66.5	56.7	9.0	1.7	C–H⋯π; C–H⋯O/N/C	73

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3. Conclusion and outlook

As mentioned above, significant progress has been made in the use of MOF materials for MTO product separation, with some MOFs demonstrating excellent performance, highlighting their tremendous potential to replace traditional separation technologies. However, several key challenges remain in further developing efficient and practical MOF materials for industrial MTO separation processes, necessitating in-depth research:

(1) Trade-off between adsorption capacity and selectivity: MOF materials often face a “selectivity-capacity” trade-off during structural design. Some highly selective materials may sacrifice total adsorption capacity, while excessive interaction strength can increase the energy consumption of the regeneration process. These conflicts limit their processing capacity and practical application efficiency. Therefore, achieving high adsorption capacity while maintaining excellent selectivity remains one of the key challenges in the design of MOFs for MTO product separation. Future studies may further explore flexible MOFs, which enable selective adsorption of target molecules through their structural responsiveness, offering the potential for enhanced separation efficiency. In addition, the design and synthesis of kinetically selective MOFs provide a promising strategy for achieving efficient separation of MTO products.

(2) Structural stability: some MOFs, although exhibiting good separation performance under laboratory conditions, may experience rapid performance degradation in practical industrial applications due to structural collapse, deactivation of active sites, or adsorbate competition. Therefore, the development of MOFs with long-term structural stability and good regenerability is essential for advancing their industrial application in MTO product separation. The development of highly stable MOFs with high connectivity, such as Zr-MOFs, represents a promising direction for achieving robust and durable separation of MTO products.

(3) Synthesis cost and scalability: many high-performance MOF materials involve complex precursors and low-yield synthesis routes or are highly sensitive to reaction conditions, severely restricting their large-scale production and engineering applications. Future research should focus on developing green, cost-effective, and scalable synthetic strategies to accelerate the transition of MOF materials for MTO product separation from laboratory studies to practical industrial applications.

(4) Separation behavior in multi-component gas systems: most current research focuses on the separation performance of binary and ternary systems, while MTO products are essentially a complex mixture containing C₂H₄, C₃H₆, C₂H₆, C₃H₈, C₄+, and C₅+. Therefore, the competitive adsorption behavior of MOFs under multi-component conditions, their selectivity retention capabilities, and interactions with impurities all require further systematic investigation.

(5) MTO separation via MOF-based membranes: although MOF-based membrane separation technologies have made notable progress in C₂/C₃ separations, research on their application to more complex MTO product mixtures remains relatively limited. In the future, continued advances in the understanding of separation mechanisms and design strategies for MOF membranes are expected to enable efficient separation of multicomponent MTO gas mixtures. This progress will not only contribute to energy savings and emission reductions in low-carbon olefin production, but also lay the groundwork for the practical application of membrane separation technologies in the MTO industry. Further exploration of membrane materials specifically tailored for the complex composition of MTO products represents a critical direction for future research.

Author contributions

Conceptualization: B. T., X. Y. and Y. L.; writing – original draft: B. T. and X. Y.; writing – review & editing: Y. L.; supervision: Y. L.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 22171100 and U23A20360) and the ‘111 Center’ (B17020).

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