A Ni₄O₄-cubane-squarate coordination framework for molecular recognition

Abstract

Molecular recognition is a fundamental function of natural systems that ensures biological activity. This is achieved through the sieving effect, host-guest interactions, or both in biological environments. Recent advancements in multifunctional proteins reveal a new dimension of functional organization that goes beyond single-function molecular recognition, emphasizing the need for artificial multifunctional materials in industrial applications. Herein, we have designed a porous Ni₄O₄-cubane squarate coordination polymer as an artificial molecular recognition host, drawing inspiration from the structural and functional features of natural enzymes. A comprehensive assessment of the material’s ability to distinguish target species under different operating conditions was carried out. The results confirm its sieving function through hexane isomers separation, host-guest interaction function via xenon/krypton separation, and dual presence of sieving and interaction through carbon dioxide/nitrogen separation. Additionally, the material demonstrates good stability and feasibility for large-scale production, indicating its practical potential. Our findings provide a bio-inspired multifunctional recognition material for chemical separations as proof-of-concept while offering solutions to advance artificial multifunctional materials adaptable to other applications beyond chemical separations.

Materials for multifunctional molecular recognition are essential in industry. Here, the authors present a cost-effective, stable and multifunctional Ni-cubane MOF that effectively separates hexane isomers, xenon/krypton, and carbon dioxide/nitrogen.

Introduction

Molecular recognition in biological systems is essential^1–3, playing a central role in understanding and advancing enzyme catalysis, substance transportation, and pharmacology^4–6. Due to the highly complex and flexible characters of natural structures, the molecular recognition process typically involves two routes for distinguishing target substances from mixtures: sieving and host-guest interaction^4,7. The sequential combination of these two functions is commonly observed for greater efficiency. While most natural recognition systems are single-functioned, allowing for high individual efficiency, they also create complex and fragile biological systems that require perfect coordination among all functions to avoid disorders and dysfunctions. However, recent discoveries on multi-functional proteins like cytochrome c have revealed another dimension of function organization in biological systems where different functions are exerted based on cellular localization and operating conditions^8,9. This highlights the critical significance and urgent necessity for the advancement of artificial molecular recognition systems that achieve multifunctionality through various physical interactions. This development is particularly vital for industrial chemical separations, which are among the most energy-intensive processes.

Traditional chemical separations, especially those involving gas or vapor mixtures with similar physicochemical properties, primarily rely on liquid media, making them highly energy-intensive. In contrast, solid-based technology using porous adsorbents or membranes offers an energy-efficient alternative^10,11. These solid separation media utilize structural porosities and/or active sites to distinguish and recognize target adsorbates, resembling biological systems. However, most of these materials are single-functioned, highlighting the need for exploring multifunctional molecular recognition solids in future separation processes. On the other hand, cytochrome c, a multifunctional protein mentioned earlier, can be described as a supramolecular assembly of metal ions and organic ligands through strong chemical bonds or weak interactions. Therefore, it is logical to learn from nature and synthesize artificial metal-organic coordination materials that incorporate naturally occurring inorganic building blocks and pore confinements to facilitate the discovery and exploration of multifunctional molecular recognition solids for industrial separation purposes. It is important to highlight that porous metal-organic coordination materials have not only demonstrated significant potential in crucial physical separations in recent years—such as hydrocarbons, CO₂/N₂, Xe/Kr, and isotopic compounds^12–16—but also possess the ability to integrate both physical and chemical interactions^17,18, which expands the scope of multifunctional molecular recognition applications. Consequently, this has established a valuable foundational basis for the future development of multifunctional separation materials.

Coordinative cubane moieties are commonly observed in flexible confinement and serve as active sites in various metalloenzymes, such as NiFe-CODH¹⁹, nitrogenases²⁰, and ferredoxin²¹. They exhibit good tolerance towards both terminal species and bonded ligand varieties of the cubane structure. Therefore, nickel and iron-based coordinative cubanes are bio-inspired ideal units for constructing molecular recognition materials with structural confinement and active sites when an organic linker with suitable flexibility is used for synthesis. While research on iron-cubane compounds mainly focuses on the catalytic activity of discrete complexes^22,23, nickel-cubane compounds have already been extended from discrete coordination complexes to infinite coordination frameworks, which have potential applications as separation materials, although only a few examples have been reported so far^24–26. However, due to limitations in their crystal structures and stabilities, the known nickel–cubane-based coordination frameworks are not yet efficient molecular recognition materials for chemical separations. Nevertheless, they demonstrate that it is possible and practical to achieve dual sieving capability and host-guest interactions with a made-to-order nickel–cubane-based network^24–26.

Here we present a metal-organic coordination framework assembled by Ni₄O₄-cubane unit and squaric acid, denoted as USTC-740 (USTC: University of Science and Technology of China). It possesses micropore window for potential sieving and host active sites on the cubane unit for interaction with guest molecules. We have selected three quintessential chemical separations in the industry to evaluate the multifaceted physical molecular recognition capabilities of USTC-740, each corresponding to distinct molecular recognition processes observed in biological systems: separation of hexane isomers in petrochemical industry, CO₂ capture and CO₂/N₂ separation in energy industry, as well as xenon/krypton (Xe/Kr) separation in nuclear energy applications. Both single-component and competitive sorption measurements demonstrated that USTC-740 efficiently recognizes target molecules in all three separations, displaying a great balance between high working capacity and high selectivity under near-working conditions. These experimental results were further supported by molecular simulation and computational calculation to understand their corresponding separation mechanisms. Furthermore, USTC-740 features a good stability and scalable synthesis with a high space-time yield under mild conditions using cost-efficient feedstocks. Therefore, our results highlight the promise of bio-inspired simple architecture enabled by rational synthesis in discovering efficient multifunctional molecular recognition materials, providing alternatives to classic separation material design and synthesis.

Results and discussion

Synthesis and structure

The function of a material is typically determined by its structural characteristics. Therefore, the multifunctionality of a material depends on its ability to flexibly adapt to external stimuli within its structure. Following this guide, we anticipate that incorporating the rigid Ni-cubane unit into a confined space with suitable flexibility will be advantageous for the selective accommodation of guest molecules, thereby achieving the desired separation performance. Consequently, significant efforts have been dedicated to identifying the appropriate organic linker for this purpose. Squaric acid, commonly used in the optical fiber industry, was chosen for the following reasons: (1) It possesses a coplanar structure with a robust square geometry and stretchable/rotatable carbon-oxygen bonds in coordination environments, which promotes local flexibility rather than framework flexibility in corresponding coordinative compounds²⁷; (2) It exhibits strong interactions with metal ions while having low steric hindrance during coordination reactions, resulting in the preferential formation of microporous materials characterized by uniform pore size distribution, well-defined inner environments and nearly absence of coordinatively unsaturated metal sites^16,28; (3) Metal–squarate compounds demonstrate significantly enhanced thermal and water stability compared to most metal-carboxylate solids^29,30. The history of crystalline Ni–squarate compounds can be traced back to the 1960s, as indicated by the CCDC database search^31,32. Various coordination and connection modes of squaric acid have been observed in reported Ni–squarate solids, wherein the Ni ions consistently exist as mononuclear centers regardless of the reaction conditions. This highlights the challenge of synthesizing a Ni–cubane inorganic unit under conventional reaction conditions. Through systematic evaluation and optimization of key parameters governing the reaction product, it was determined that high reactant concentration with trace water addition in N, N’-dimethylformamide (DMF) is suitable for refluxing synthesis of the powder form of the Ni–cubane–squarate coordination framework (USTC-740) under ambient pressure. Solvothermal reactions were employed to obtain single-crystal products suitable for structure determination (Supplementary Note 1).

The structure analysis reveals that USTC-740 crystallizes in cubic Fm−3m (225) space group with the unit-cell parameters of a = b = c = 18.8755(7) Å and V = 6725.0(7) ų, associated with the formula Ni₄(OH)₂(H₂O)₂(C₄O₄)₃. Each Ni²⁺ ion is octahedrally coordinated with three oxygen atoms from the squarates and three bridging oxygen atoms. The cubane unit is formed by four adjacent Ni²⁺ ions and four terminal oxygen atoms, where two diagonal-positioned Ni²⁺ ions connect to an oxygen pair from the squarate linker, resulting in a six-connection node (Fig. 1a). Each squarate linker molecule adopts a linear connection mode to bridge two nearby cubane units. From a crystallographic perspective, it is challenging to accurately determine whether the terminal oxygen atoms in the cubane unit correspond to hydroxyl groups or water molecules. However, based on charge compensation considerations, there are two hydroxyl groups and two water molecules present. The cubic structural unit of USTC-740 consists of eight Ni₄O₄-cubanes connected by twelve squarates. Due to the bonding between squarate and diagonal-positioned Ni²⁺ ions in the cubane, a dihedral angle of approximately 45° is formed between the squarate and vertical planes, resulting in two types of pores with different sizes. When all the squarates twist outwards from the cavity center, a large sphere-shaped cavity with a diameter of ~11.1 Å (Fig. 1b) is created. This arrangement generates six small cavities (~7.5 Å in diameter, Fig. 1c) along each facet of the large pore structural unit, and vice versa. Nevertheless, both small and large pores share a common pore window opening (~4 Å), which plays a crucial role in controlling guest molecule diffusion and maintaining uniformity within the crystal structure of USTC-740 (Supplementary Fig. 1). Therefore, USTC-740 exhibits a three-dimensional (3D) pore system formed by alternating packing arrangements of large and small cages with well-defined pore aperture (Fig. 1d). The accessible porosity was confirmed by CO₂ sorption measurement at 195 K, demonstrating a Brunauer-Emmett-Teller (BET) area of ~800 m² g⁻¹ and a free pore volume of ~0.30 cm³ g⁻¹ (Supplementary Fig. 2). It should be noted that this linkage mode minimizes the occurrence of coordinatively unsaturated metal sites in USTC-740, thereby reducing water molecule interference during molecular recognition processes while enhancing material stability and regeneration capabilities.

Fig. 1. Crystallographic structure of USTC-740.

a Neighboring cubane units are interlinked by squarate molecules. b The large pore (pink sphere) in the structure is formed when the linker square moiety twists out in the opposite direction of the pore center. c The formation of the small pore (green sphere) occurs when the linker square moiety twists inward towards the center of the pore. d The 3D porous framework of USTC-740, viewed along the a-axis, reveals each large pore being surrounded by six adjacent small pores on its front, back, left, right, up and down sides respectively. Consequently, each Ni₄O₄ cubane unit is positioned at the center of four large pores and four neighboring small pores. Ni, blue; C, gray; O, red.

Molecular recognition by USTC-740

The porous structure of USTC-740 possesses large cavities with small opening windows, making it an ideal candidate for chemical separation based on size- or shape-dependent sieving. Furthermore, the presence of both Ni²⁺ ions and oxygen species on the Ni₄O₄-cubane offers promising potential as active centers for interacting with guest molecules. Consequently, we have chosen to assess the multifunctional recognition capabilities of USTC-740 by evaluating its performance in three distinct chemical separations relevant to the energy industry, each associated with different mechanisms.

The initial scenario involves the separation of hexane isomers within the petrochemical industry, typically accomplished through size- and shape-exclusion in adsorptive processes³³, thereby demonstrating the molecular kinetic sieving effect. The second instance pertains to Xe/Kr separation within the nuclear energy sector, commonly achieved through differential thermodynamic host-guest interactions within porous environments³⁴, thus emphasizing the interaction effect of molecular recognition. Lastly, CO₂ capture involving CO₂/N₂ separation from energy industry integrates both kinetic sieving and thermodynamic host-guest interaction to successfully accomplish target completion in adsorptive recognition processes³⁵. The molecular properties of the guest associated with the selected system have been compiled in Supplementary Table 1.

Hexane isomers recognition by sieving

Hexanes, a class of six-carbon alkanes with five structural isomers, play an essential role as feedstocks and fuel additives in the petrochemical industry³⁶. This class includes the linear n-hexane (nHEX), the mono-branched 2-methylpentane (2MP) and 3-methylpentane (3MP), as well as the di-branched 2,3-dimethylbutane (23DMB) and 2,2-dimethylbutane (22DMB). Of particular importance are the di-branched isomers, which are highly sought after for their high research octane numbers (RONs) as ideal gasoline components³⁷. The most straightforward method for separating and enriching the di-branched isomers is through pore size/shape dependent kinetic sieving using porous materials. This approach capitalizes on the distinct differences in molecular sizes and shapes between di-branched isomers and linear/mono-branched ones³⁸.

We have chosen nHEX, 3MP, and 22DMB as the representative isomers with varying degrees of branching for assessing the adsorption and separation capabilities of USTC-740 in this particular application. Initially, single-component vapor phase adsorption isotherms of the three isomers were collected at 363 K and 393 K (Fig. 2a and Supplementary Fig. 3). The findings revealed that USTC-740 serves as a good molecular sieve, effectively recognizing and obstructing the passage of 22DMB through its structural cavity.

Fig. 2. Hexane isomers separation by USTC-740.

a Single-component adsorption isotherms of hexane isomers at 363 K. b The uptake ratios of the hexane isomers under different relative pressure at 363 K. c Uptake ratios of nHEX/22DMB and 3MP/22DMB for selected adsorbents that show size-exclusion for mono- and di-branched alkanes. d Column breakthrough curve of an equimolar ternary mixture of hexane isomers collected at 393 K.

In contrast, both nHEX and 3MP isomers can be efficiently adsorbed even within a low relative pressure range. The uptakes of both linear and mono-branched isomers at such elevated temperature are noteworthy compared to most reported sorbents, showing a great combination of high working capacity and selectivity. Correspondingly, the uptake ratios of nHEX/22DMB and 3MP/22DMB at 363 K reach their peak values at 39.6 and 30.7 respectively at a relative pressure of 0.05 P/P₀ before decreasing to 18.8 and 17.7 at 1 P/P₀ respectively (Fig. 2b). To our knowledge, USTC-740 displays the highest value of 3MP/22DMB ratio among all the reported materials for this application, and its performance is comparable to the record achieved by Zn-tcpt³⁹ (Fig. 2c and Supplementary Table 2). It is worth noting that the USTC-740 structure’s cavity with a large surface area provides a suitable pore window for effective adsorption of nHEX and 3MP at low pressure, as well as accurate recognition and blocking of 22DMB, which closely resembles the real relative pressures of hexanes in naphtha⁴⁰. Density functional theory (DFT) calculations (Supplementary Note 2) were used to determine the free penetrating energy of hexane molecules entering the pore of USTC-740 through the cavity window. The calculated energies are 0.2 eV for nHEX, 0.4 eV for 3MP, and 1.2 eV for 22DMB, supporting the molecular recognition and exclusion mechanism (Supplementary Fig. 4).

In order to further evaluate the recognition capability of the USTC-740 structure towards 22DMB in a ternary hexane isomers mixture, breakthrough experiments were carried out at 393 K using a fixed-bed column packed with USTC-740 solid and fed with an equimolar mixture of nHEX, 3MP, and 22DMB. The results indicated that the sorbent effectively separated hexane isomers into individual components (Fig. 2d). 22DMB with a high RON value was immediately identified by the sorbent, resulting in its elution from the column at the outset of the process. This supports the molecular recognition of 22DMB by USTC-740 with high sensitivity, accuracy, and efficiency. In contrast, the size exclusion effect that operates on 22DMB did not impact the adsorption of 3MP and nHEX isomers for approximately one hour; it took 61 min g⁻¹ for 3MP to penetrate throughout the column, while nHEX broke out at a substantially longer time of 157 min g⁻¹. These observations align with single-component adsorption results and demonstrate USTC-740’s sieving capability for sensitive and accurate recognition towards target molecules even in complex environments.

Xe/Kr recognition by host-guest interaction

Xenon and krypton are found in the exhaust gases of the nuclear energy industry at significantly higher concentrations (400 ppm for Xe and 40 ppm for Kr) than in the ambient air (0.087 ppm for Xe and 1.14 ppm for Kr). This not only underscores the critical importance of properly managing radioactive waste but also highlights the substantial value of extracting xenon from the waste gases of nuclear energy⁴¹. The utilization of porous solids for adsorptive separation of xenon through thermodynamic host-guest interaction has experienced notable advancements in recent years, demonstrating great potential as a viable alternative to traditional distillation methods⁴¹. Due to their close dynamic atomic diameters (4.1 Å for Xe and 3.7 Å for Kr) as well as a lack of dipole and quadrupole moments, physisorption-based separation of xenon and krypton primarily depends on the difference in their polarizabilities (40.4 × 10⁻²⁵ cm³ for Xe and 24.8 × 10⁻²⁵ cm³ for Kr), which corresponds to their host-guest interaction discrepancy with the adsorbent porosity. Hence, the ability to separate Xe/Kr mixtures is a suitable measure for evaluating a porous material’s molecular recognition capability in terms of host-guest interactions.

To that end, single-component adsorption isotherms of xenon and krypton were measured at three different temperatures as the initial step to check whether there is sieving effect of the USTC-740 structure. The results showed that both xenon and krypton atoms can effectively enter the pore with increasing relative pressure (Fig. 3a), displaying fully reversible adsorption with no notable diffusion restrictions. This indicates the adsorption is thermodynamically driven with an absence of molecular recognition based on size-sieving. The isotherms of xenon adsorption exhibit a type I shape with slight decreases in uptakes as the temperature increases, suggesting a strong interaction between the USTC-740 structural cavity and guest xenon atoms. In contrast, the isotherms of krypton are almost linear with notable changes in uptakes along different temperatures, indicating a much weaker interaction between the sorbent and krypton atoms. The corresponding isosteric heats of adsorption (Q_st) calculated from sorption isotherms are 29 kJ mol⁻¹ for xenon and 19 kJ mol⁻¹ for krypton, which was further confirmed by DFT binding energy calculation (Supplementary Fig. 5). Therefore, it can be concluded that the USTC-740 pore structure exhibits favored recognition of xenon through host-guest interaction, resulting in a balanced combination of xenon uptake and Xe/Kr IAST selectivity comparable to benchmark adsorbents in this application (Fig. 3b and Supplementary Table 3). A column breakthrough measurement was conducted on a gas mixture that simulates the exhaust gases of the nuclear energy industry, and the corresponding results are presented in Fig. 3c. The low concentration of Xe did not impact the recognition ability of USTC-740, which performed as effectively as in breakthrough results achieved with a binary Xe/Kr (20:80, v/v) mixture (Supplementary Fig. 6). This highlights the practical capability of the USTC-740 structure to selectively recognize specific molecules from complex mixtures through host-guest interactions with high sensitivity and selectivity.

Fig. 3. Xenon/Krypton separation by USTC-740.

a Single-component adsorption isotherms of xenon and krypton at 273, 283, and 298 K, respectively. b Comparison of the reported best performance in terms of IAST selectivity and xenon uptake at 0.2 bar for typical porous materials with rigid structures (Materials with Xe/Kr separation selectivity above 10 were included for comparison). c Single column breakthrough experiments using USTC-740 fixed-bed under low concentrations at 298 K and 1 bar. The column is initially purged with He and then injected with a gas-mixture with 400 ppm Xe and 40 ppm Kr balanced with dry air. The flow rate of He and gas-mixture is 2 mL min⁻¹.

CO₂/N₂ recognition by sieving and host-guest interaction

The capture of CO₂ is a critical task in achieving global carbon neutrality¹⁴. Specifically, the adsorptive separation of CO₂ from the flue gas produced by fossil fuel combustion (primarily a mixture of CO₂ and N₂) represents a complex molecular recognition process that combines the kinetic sieving effect and thermodynamic host-guest interaction³⁵. This process differs from common hexanes and Xe/Kr separations mentioned earlier, particularly in terms of the mechanism for molecular recognition. In general, a sorbent structure that exhibits an appropriate sieving effect at the pore entrance, followed by a suitable host-guest interaction between the inner cavity and CO₂ molecule, as well as ample pore space for CO₂ diffusion, will enhance high separation efficiency and selectivity. Therefore, parameters such as sieving effect, adsorptive capacity, and separation selectivity are valuable for evaluating a material’s ability to recognize CO₂ in applications involving CO₂/N₂ separation.

To assess the pore entrance sieving function of the USTC-740 structure for CO₂ and N₂ during the pore filling process, single-component gas sorption measurements are carried out at their boiling point temperatures. The material exhibited limited access to N₂ at 77 K across the entire pressure range, possibly due to the pore blocking effect. However, a reversible type I isotherm was observed for CO₂ sorption at 195 K. This demonstrated a substantial increase in uptake compared to that of N₂ at 77 K (Fig. 4a), indicating an effective molecular sieving effect for CO₂ over N₂. The adsorptive capacity of CO₂ and N₂ were analyzed by physisorption results collected in the temperature range from 273 K to 338 K (Fig. 4b). Constantly higher CO₂ uptakes than N₂ ones are observed for all the data collection temperatures, with both adsorbates showing continuous decreases in uptake as temperature increased. For example, the N₂ uptake at 1 bar decreases by 76.8% from 0.82 mmol g⁻¹ at 273 K to 0.19 mmol g⁻¹ at 338 K. In contrast, CO₂ uptake only shows a decrease of 37.7% under the same conditions (4.85 mmol g⁻¹ at 273 K to 3.02 mmol g⁻¹ at 338 K). This indicates a stronger interaction between the USTC-740 structure and CO₂ compared to N₂. The Q_st values calculated from isotherms (44 kJ mol⁻¹ for CO₂ vs 21 kJ mol⁻¹ for N₂) and DFT calculation of binding energy supported the interaction difference (Supplementary Fig. 7). Therefore, due to the combined effect of pore entrance sieving and inner active site binding interaction, USTC-740 demonstrates good CO₂ recognition ability, which can effectively balance CO₂ uptake and CO₂/N₂ selectivity, comparable to reported benchmarks (Fig. 4c and Supplementary Table 4). It is worth noting that the material exhibits performance similar to that of industrialized CALF-20¹⁴ as well as that of the benchmark ALF⁴² at a practical working temperature of 338 K. Furthermore, dynamic breakthrough measurements confirm the molecular recognition performance of USTC-740 towards CO₂ over N₂, even in wet environments (Fig. 4d and Supplementary Figs. 8 and 9). Thus, under certain conditions, USTC-740 has the capability to combine both kinetic sieving effect and thermodynamic host-guest interaction.

Fig. 4. Carbon dioxide/nitrogen separation by USTC-740.

a Single-component adsorption isotherms of CO₂ at 195 K and N₂ at 77 K. b CO₂ and N₂ sorption isotherms from 273 to 338 K. c Comparison of the reported best performance in terms of IAST selectivity and CO₂ uptake at 0.15 bar for representative porous materials (Materials with CO₂ uptakes above 2.5 mmol g⁻¹ at 0.15 bar were included for comparison). d Column breakthrough curves of USTC-740 at 318 K for the mixed gases of CO₂/N₂ (15/85 vol%).

As demonstrated above, USTC-740 has shown the ability to exhibit various molecular recognition functions that can adapt to different environments and conditions. Each function has been proven to be highly sensitive and accurate in detecting the target guest species, resembling biological recognition systems. Therefore, the lab-scale proof of concept for the multifunctionality of USTC-740 in energy-related separations supports the scientific validity and feasibility of artificial multifunctional recognition materials achieved through bio-inspired metal-organic coordination compounds.

Stability and production evaluation

Given the diverse applications of multi-functional molecular recognition materials, it is essential to evaluate their stability under various operational conditions to ensure the effectiveness of their recognition function. Specifically, assessing the thermal, chemical, and mechanical resistance of the material is crucial. Thermogravimetric analysis (TGA), temperature-dependent powder X-ray diffraction (PXRD) data, and gas sorption measurements on samples after thermal treatments demonstrate that the long-range order of the USTC-740 structure remains stable up to 250 °C (Supplementary Figs. 10–12), which is adequate for most adsorptive chemical separations. Furthermore, the material exhibits good chemical stability when subjected to different conditions such as common organic solvents, water, and acidic SO₂ gas (Supplementary Figs. 13–15). Additionally, USTC-740’s framework demonstrates strong resistance to pressure applied on its powder sample (Supplementary Fig. 16), indicating its potential for shaping and packing processes in scale-up applications.

The scalability of material production is a crucial factor to consider for advancing beyond the proof-of-concept stage in lab-scale. This is primarily determined by production cost, synthesis method, and space-time yield (STY). In order to compare the overall cost of producing one kilogram of each synthetic sorbent, we have obtained and calculated the lowest prices of major chemicals used for synthesis from commercial suppliers in China (Fig. 5, and Supplementary Note 3). The large-scale synthesis of USTC-740 can be achieved through refluxing reaction under ambient pressure (Fig. 5b). This process results in the production of a significant quantity of material from a single reaction (Fig. 5c). Consequently, when compared to other synthetic sorbents for hexane isomers separation, USTC-740 demonstrates pronounced advantages in terms of STY and unit cost, surpassing all reported coordination materials and being comparable to industrial zeolite benchmarks in this application (Fig. 5a). It is worth noting that the same conclusion can be drawn when comparing the STY of USTC-740 synthesis and the unit cost of the product with other sorbents reported for Xe/Kr and CO₂/N₂ separations (Supplementary Figs. 17 and 18, and Supplementary Tables 5–10). Therefore, USTC-740 holds great potential for large-scale multi-functional molecular recognition applications.

Fig. 5. Production evaluation of USTC-740.

a Space-time yield and the cost to synthesize one kilogram of product for USTC-740 and typical metal-organic coordination polymers for hexane isomers separation. Inset, detailed data comparison in a zoom-in region (Data were taken from literature, and were summarized in Supplementary Tables 5 and 6). b Reflux synthesis in scale using a large round-bottom flask (1 L) under ambient pressure. c USTC-740 product obtained from one large-scale reflux synthesis.

Drawing inspiration from natural recognition systems and building upon previous research on single-functioned metal-organic coordination materials for chemical separations of industrial importance, we have designed and synthesized USTC-740 as a proof-of-concept example of the next generation material for artificial molecular recognition. Through a comprehensive assessment that combines experimental data with computational results, we have demonstrated USTC-740’s ability to recognize target guest molecules under various operating conditions. This includes its sieving function in separating hexane isomers, host-guest interaction function in Xe/Kr separation, and dual functions combining sieving and interaction in CO₂/N₂ separation. In all three applications related to the energy industry, USTC-740 has shown high sensitivity and accuracy in distinguishing target components from adsorbate mixtures, confirming its practical multifunctional recognition potential when applied under different conditions. Furthermore, our evaluation of the stability and feasibility of large-scale production supports the notion that USTC-740 could be a promising candidate for practical applications once related industrial engineering processes are established. This work presents a viable solution for developing artificial multifunctional molecular recognition materials by emulating natural systems, which can be adapted to other applicable fields beyond chemical separations.

Methods

Synthesis of USTC-740 single-crystal product

Square acid (43 mg, 0.4 mmol), NiCl₂·6H₂O (120 mg, 0.5 mmol) and aqueous solution of HCl (3 M, 0.5 mL) were combined in N, N’-dimethylacetamide (DMA, 15 mL) in a 25 mL autoclave reactor. The mixture was stirred at room temperature (RT) for dispersion, followed by heating at 150 °C for 48 h in the sealed autoclave. After cooling down to RT, the resulting green single crystals suitable for structure analysis were filtered off and washed with acetone.

Synthesis of USTC-740 powder product

To a 1 L round bottom flask, squaric acid (38.7 g, 340 mmol), KOH (50.4 g, 900 mmol), DMF (600 mL) and H₂O (30 mL) were added and stirred at RT. NiCl₂·6H₂O (107.1 g, 450 mmol) was added while stirring. The reaction mixture was refluxed at 150 °C for 10 h. After cooling to RT, the crude powder product of USTC-740 was collected by filtration, washed with MeOH and air dry.

Sorption of Hexanes

Single-component adsorption equilibrium isotherms of hexane vapors were collected on a Belsorp Max II analyzer. USTC-740 sample was activated under dynamic vacuum at 393 K for 10 hours prior to data collection. Multicomponent column breakthrough measurement was performed with a lab-scale fix-bed reactor at 393 K. 0.75 g of USTC-740 sample was packed into a quartz column (4.0 mm I.D. × 300 mm) with silane treated glass wool filling the void space. A helium flow (15 mL min⁻¹) was used to purge the adsorbent. The sample was activated at 393 K overnight and the helium flow was then turned off while another dry helium flow at a rate of 1 mL min⁻¹ was bubbled through a mixture of hexane isomers according to the following volumes: 5.84 mL of nHEX, 4.12 mL of 3MP, and 2.57 mL of 22DMB, to achieve an equimolar nHEX/3MP/22DMB ternary vapor mixture. The effluent from the column was monitored using an GC equipped with HP-PONA column and FID.

Sorption of xenon and krypton

Single-component sorption isotherms of Xe and Kr were collected on a Micromeritics 3Flex analyzer. USTC-740 sample was activated under dynamic vacuum at 393 K for 8 h prior to data collection. The Xe/Kr breakthrough experiments were performed in a home-built dynamic gas breakthrough equipment. The activated USTC-740 sample (∼510 mg) was packed into a steel column (the steel column is about 12 cm long with a 4 mm inner and 6.4 mm outer diameter) and heated under vacuum at 393 K for 8 h before breakthrough tests. A stream of helium flow was injected into the column to further purge the materials and then the helium gas stream was stopped and the target gas (400 ppm Xe, 40 ppm Kr balanced with dry air) was introduced with a flow rate of 2 mL min⁻¹ at 298 K. The downstream was monitored by Hiden mass spectrometer (HPR 20). Then the breakthrough test of a binary gas mixture of Xe/Kr (20:80, v/v) was measured under a flow rate of 2 mL min⁻¹ at 298 K under identical conditions. The desorption test of USTC-740 was carried out using the same dynamic gas breakthrough equipment, the downstream was monitored by a Hiden mass spectrometer (HPR 20).

Sorption of CO₂ and N₂

Single-component sorption isotherms of CO₂ and N₂ from 273 K to 338 K were collected on a Micromeritics 3Flex analyzer. USTC-740 sample was activated under dynamic vacuum at 393 K for 8 hours prior to data collection. The CO₂/N₂ breakthrough test was performed on a 3 P MIXSORB S dynamic gas adsorption apparatus. 0.38 g of USTC-740 sample was packed into the column (I.D. 6 mm, length 100 mm). The column was first purged with a helium flow at a rate of 14 mL min⁻¹ at 393 K for 2 h. The fixed bed was then cooled to the analysis temperature (318 K) and the helium flow was switched to a CO₂/N₂ gas mixture (CO₂: 0.75 mL min⁻¹, N₂: 4.25 mL min⁻¹, total flow rate: 5 mL min⁻¹). The gas concentrations of N₂ and CO₂ at the outlet were measured with an on-line mass spectrometer (MKS). After the adsorption reached dynamic equilibrium, the helium flow was purged into the column with a flow rate of 10 mL min⁻¹ at 393 K for desorption. Breakthrough test under humid condition was performed analogously with a humidity generator setting the relative humidity as 50%.

Author contributions

Conceptualization, S.W., H.W. and S.X.; Investigation, Q.Y., S.A., L.Y., S.L., X.W., S.D., S.X., H.W., S.W. and J.D.; Writing-Original Draft, S.W.; Writing-Review & Editing, Q.Y., S.A., S.X., H.W., S.W. and J.D.; Supervision, H.W., S.W. and J.D.

Peer review

Peer review information

Nature Communications thanks Nobuhiko Hosono and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

All data involved in this work are included in this article and the corresponding supplementary materials. Crystal structure of USTC-740 has been deposited in the CCDC database under accession code CCDC-2343041. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-54348-1.