1. Introduction
Acetylene (C2H2) is a critical basic chemical raw material, extensively utilized in the synthesis of industrial products such as polyester plastics, synthetic rubber, and fine chemicals [1]-[3]. Its purity directly determines the quality and production efficiency of downstream products [4]. Industrially, C2H2 is mainly produced by partial oxidation of natural gas or thermal cracking of hydrocarbons, and impurity gases such as CO2 and CH4 are inevitably mixed in the products [5]-[7]. C2H2 and CO2 exhibit high similarity in both molecular size (C2H2, 3.32 × 3.34 × 5.7 Å3; CO2, 3.18 × 3.33 × 5.36 Å3) and boiling point (C2H2, 189.3 K; CO2, 194.7 K) [8] [9]. The traditional cryogenic distillation method for their separation suffers from high energy consumption, low efficiency and harsh equipment requirements, making it difficult to meet the green and efficient production needs of modern industry [10]. Therefore, the development of low-energy-consumption and high-selectivity C2H2/CO2 separation technologies has become a research focus in the field of chemical separation.
Crystalline porous materials have become ideal alternatives to traditional separation methods due to their designable pore structures, controllable surface properties, and efficient physical adsorption characteristics. Among them, Metal-Organic Cages (MOCs), a novel class of crystalline porous materials, have shown great application potential in the field of gas adsorption and separation due to their modifiable pores, adjustable structures, and easy post-synthetic modification [11]-[15]. ZrT-1, a V4E6 type material constructed with zirconium as the metal center, is an important branch of MOCs. Its moderate pore size, good structural stability, and modifiable framework make it a research focus in the field of gas separation [16]-[19]. However, the intrinsic pore microenvironment of ZrT-1 has a relatively weak interaction with the C2H2 molecules, and its separation selectivity for C2H2/CO2 still needs to be improved. Introducing specific functional groups through ligand functionalization to regulate its pore size and surface properties is a key strategy to optimize its adsorption and separation performance [20].
Allyl (-CH=CHCH3), as a typical unsaturated alkyl functional group containing carbon-carbon double bonds, combines the electronic properties of unsaturated double bonds and the spatial structure advantages of alkyl chains, making it an ideal choice for ligand functionalization of ZrT-1. The carbon-carbon double bond in the allyl group, as the core structure of the unsaturated bond, its π-electron cloud can produce strong π-π stacking interactions with the π-electron system of C2H2 molecules [21]. At the same time, the uneven electron distribution of the double bond forms a weak polar characteristic, which can form weak interactions with the terminal hydrogen of C2H2, enhancing the adsorption capacity for C2H2 molecules; the alkyl carbon chain of the allyl group itself forms moderate steric hindrance, which can finely adjust the pore size without destroying the main cage structure of ZrT-1, further improving the sieving effect for C2H2/CO2.
Based on these considerations, this study designed and synthesized an allyl-functionalized terephthalic acid ligand using the parent ZrT-1 as the basis and adopting the ligand functionalization strategy, and constructed the ZrT-1-allyl through coordination assembly. The crystal structure and pore characteristics of the material were systematically characterized, and its adsorption performance and separation mechanism for C2H2 and CO2 were thoroughly investigated.
2. Synthesis
2.1. Ligand Synthesis
The synthesis of 2-allylterephthalic acid ligand was performed with reference to reported methods [22] (Scheme 1). 2-Bromoterephthalic acid (6.2 g, 25.3 mmol) and 5 mL of H2SO4 were added to 100 mL of CH3OH solution. The mixture was heated under reflux for 24 hours. After the reaction, NaHCO3 solution (50ml) was added to the mixture, and white precipitate of dimethyl 2-bromoterephthalate was obtained by filtration. And then dimethyl 2-bromoterephthalate, Pd (pph3)4, and allyltributyltin were dissolved in 200 mL of toluene, and reflux for 4 days under N2 atmosphere. After the reaction, the product was purified by silica gel column chromatography and distilled under reduced pressure to obtain a colorless oil. The intermediate product was dissolved in 10 mL of THF, and NaOH solution (1 mmol/L, 65 mL) was added. The mixture was stirred at room temperature for 12 hours. After the reaction, the organic solvent was removed under vacuum, the residue was acidified with 1 M HCl solution, and white precipitate was obtained by filtration (yield 81%). Figure 1 shows the 1H NMR spectrum of 2-allylterephthalic acid (BDC-allyl). 1H NMR (400 MHz, DMSO-d6, ppm): 13.24 (s, 2H), 7.86 (d, 3H), 5.97 (ddt, 1H), 5.03 (m, 2H), 3.75 (s, 2H)
Scheme 1. Synthetic procedures of the BDC-allyl ligand.
2.2. Synthesis of ZrT-1-allyl
BDC-allyl (5 mg) and zirconocene dichloride (15 mg) were dissolved in 1 mL of N,N'-dimethylacetamide (DMA), and four drops of deionized water were added. After full dissolution by ultrasound, the mixture was transferred to an electric thermostatic blast drying oven and reacted at 65˚C for 10 hours to obtain colorless and transparent cubic crystals (ZrT-1-allyl) with a yield of 73%.
3. Results and Discussion
3.1. Structural Analysis
To characterize the structure of ZrT-1-allyl, X-ray diffraction (PXRD) and high-resolution electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) were performed. The PXRD results show that the diffraction peak positions of ZrT-1-allyl are consistent with those of the parent structure, indicating that ZrT-1-allyl is isostructural with ZrT-1 (Figure 2(b) and Table 1). The ESI-TOF-MS results confirm the existence of cages in ZrT-1-allyl (Figure 2(a)). The +2, +3, and +4 ion peaks in the ZrT-1-allyl spectrum are 1682.8120, 1122.1914, and 841.8985, corresponding to [M-4Cl-2H]2+, [M-4Cl-H]3+, and [M-4Cl]4+ ions, respectively.
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Table 1. ESI-TOF-MS analyses of MOCs.
The average measured molecular weight after m/z deconvolution is 3509 Da, which is 240 Da higher than 3269 Da of ZrT-1. This difference is exactly equal to the molecular weight difference of functional groups between terephthalic acid (H₂BDC) and BDC-allyl in six ligands ([M(C3H5)-M(H)]), further confirming that ZrT-1-allyl has the same structure as ZrT-1, which is a tetrahedral cage structure. Meanwhile, due to the weak diffraction of ZrT-1-allyl, no single crystal data was obtained, so we simulated the structure of ZrT-1-allyl (Figure 2(c)). ZrT-1-allyl has a tetrahedral cage structure, arranged symmetrically in the tetragonal space group I41/a. Each cage is composed of BDC-allyl occupying the six edges, and four Cp3Zr3O(OH3) units at the four vertices. Cages were connected by H-Cl bonds, forming a tetrahedral structure inside the cavity and wavy pores between the cages.
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Figure 2. (a) ESI-TOF-MS analyses of ZrT-1-allyl, (b) PXRD analyses of ZrT-1-allyl, (c) The simulated structure of ZrT-1-allyl, (d) N2 sorption isotherms at 77 K for ZrT-1-allyl, (e-f) Single-component adsorption (solid) and desorption (open) isotherms of C2H2 and CO2 for ZrT-1-allyl at 298 K and 273 K.
3.2. Gas Adsorption and Separation Performance Tests
To further analyze the porosity and separation performance, N₂ adsorption isotherms were obtained at 77 K. The isotherms are consistent with Type I isotherms, and the N₂ adsorption capacity is 173.295 cm3/g (Figure 2(d)). The Brunauer-Emmet-Teller (BET) specific surface area was calculated to be 508.7899 m2/g. Subsequently, the adsorption curves of ZrT-1-allyl for C2H2 and CO2 were measured at 273 K and 298 K (Figure 2(e) & Figure 2(f)) At 298 K and 1 bar, the adsorption capacity of ZrT-1-allyl for C2H2 (42.54 cm3/g) is significantly higher than that for CO2 (24.89 cm3/g), with an adsorption difference of 17.65 cm3/g, indicating that ZrT-1-allyl has certain potential for C2H2/CO2 separation. To further evaluate the affinity of the material for C2H2 and CO2, the IAST selectivity and adsorption enthalpy (Qst) were calculated according to the Ideal Adsorbed Solution Theory (IAST) and Clausius-Clapeyron equation, respectively (Figure 3(a) & Figure 3(b)). The Qst value of C2H2 (20.79 kJ/mol) is higher than that of CO2 (14.62 kJ/mol), indicating that the material has higher affinity for C2H2 than CO2. At 298 K and 100 kPa, the selectivity of ZrT-1-allyl for C2H2/CO2 (50/50) is 2.8, which shows that the introduction of allyl functional groups effectively enhances the selectivity of the material for C2H2/CO2. Collectively, ZrT-1-allyl has high C2H2 adsorption capacity, moderate separation selectivity, and low adsorption enthalpy, indicating its potential for separating C2H2/CO2 mixed gases.
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Figure 3. (a) Isosteric heats of C2H2 and CO2 adsorption at low coverage for ZrT-1-allyl, (b) IAST calculations of ZrT-1-allyl for C2H2/CO2 (50/50) mixtures at 298 K, (c) Breakthrough curves of equimolar C2H2/CO2 mixtures for ZrT-1-allyl at 298 K and 1 bar (Ci/C0, outlet concentration/feed concentration), (d) Separation cycling test of ZrT-1-allyl for C2H2/CO2 (50/50).
To validate the practical separation capability of the material, breakthrough experiments were performed on C2H2/CO2 (Figure 3(c) & Figure 3(d)). The results demonstrate that ZrT-1-allyl can effectively separate C2H2/CO2. At 298 K and 1 atm, the C2H2/CO2 (50/50, v/v) mixed gas was passed through the sample tube at a flow rate of 1 mL/min, and the outlet gas was monitored by a mass spectrometer. Due to the weak affinity of ZrT-1-allyl for CO2, it was desorbed first from the sample tube, after a period of time C2H2 was desorbed after reaching adsorption saturation, with a separation time of 24 min/g (Figure 3(d)). The breakthrough experiment proves that ZrT-1-allyl possesses practical C2H2/CO2 separation capability and can serve as an ideal adsorbent separation material. Additionally, after three rounds of cycle tests, the separation performance of the material did not degradation significantly, indicating that the ZrT-1-allyl has excellent cycling stability.
3.3. Theoretical Simulation
The interaction between the framework and gases was further clarified and the adsorption behavior of gas molecules on the framework was determined via theoretical simulation. The H in C2H2 can form weak interactions with allyl groups through C-H・・・π (3.13 - 3.33 Å), and form C-H・・・O hydrogen bonds (3.03 - 3.2 Å) with carboxyl oxygen. Furthermore, multiple Cδ-・・・Hδ+ dipole-dipole interactions (3.05 - 3.23 Å) exist between adjacent C2H2 molecules adsorbed within the pores, which exert a synergistic effect to enhance the stability of C2H2 adsorption. This indicates that strong interaction between the cage and C2H2 (Figure 4(a) &
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Figure 4. Theoretical Simulation on Adsorption Sites (a-b) and Adsorption Density (c-d) of C2H2 and CO2 in ZrT-1-OMe.
Figure 4(b)). GCMC simulation results show that gas molecules are preferentially adsorbed in the ZrT-1-allyl cages, and the adsorption density of C2H2 is stronger than that of CO2 (Figure 4(c) & Figure 4(d)). Theoretical simulations and gas penetration experiment results mutually corroborate each other, proving that ZrT-1-allyl can effectively separate C2H2/CO2 mixed gases and has potential for practical application.
4. Conclusion
To optimize the separation performance of ZrT-1 for C2H2/CO2, this study adopted ligand functionalization as the core strategy to regulate pore structure. An allyl-functionalized terephthalic acid ligand was synthesized and used to construct ZrT-1-allyl through coordination assembly. Structural characterization confirms that the material retains the tetrahedral cage framework of ZrT-1 and only achieves fine pore regulation through allyl groups. At 298 K and 1 bar, its adsorption capacity for C2H2 is much higher than that for CO2; the separation selectivity for C2H2/CO2 (50/50, v/v) reaches 2.8, the dynamic separation duration in breakthrough experiments is 24 min/g, and the separation performance does not decay significantly after three rounds of cycle tests, showing good cycle stability. This study confirms that allyl functionalization is an effective method to regulate the pore structure and adsorption performance of ZrT-1. The prepared ZrT-1-allyl has excellent C2H2 adsorption affinity, good separation selectivity and cycle stability, providing new experimental basis and design ideas for the design of high-performance MOCs materials for C2H2/CO2 separation.
Acknowledgements
This work was supported by Oil & Gas Major Project (2025ZD1406703), the Key Basic Research Projects of Natu-ral Science Foundation of Shandong province (ZR2023ZD40), the National Natural Science Foundation of China (NSFC, Grant No. 22275210, 22201305, 22171288), the Key Re-search and Development Projects of Shandong Province (2023CXGC010315), the Taishan Scholar Foundation (tsqnz20221123), the Fundamental Research Funds for the Central Universities (25CX07001A), and the CNPC Innova-tion Found (2024DQ02-0202).