Silver-Based Alcohol-Functionalized Solvent for Efficient Separation of Olefins

Abstract

Olefins are important chemical raw materials, and separating alkanes/olefins in oil products is very important. However, alkanes and olefins with the same number of carbons have similar physicochemical properties and structures; therefore, separating them is difficult. This study designed and synthesized a series of silver-based alcohol-functionalized solvents for separating the cyclohexene/cyclohexane system, based on the salt effect and the π–π* coordination between Ag⁺ and the unsaturated double bonds of olefins. We screened BF₄ ^–, which has a large ionic radius and weak interactions, as the anion and 1,2-propanediol (1,2-PG), which has two hydroxyl groups (−OH), as the organic ligand. We relied on the interactions between –OH in the ligand to affect the electron cloud density of Ag⁺ after it coordinated with –OH. This allows the empty orbitals of Ag⁺ remaining after coordination with two 1,2-PGs to ligate with the carbon–carbon double bond (–CC), achieving efficient, highly selective cyclohexene separation. At the same time, at a low cyclohexene concentration, compound [Ag­(cyclohexene)₂(1,2-PG)₂]­BF₄ also exhibited a strong salt effect, which further improved the selectivity and extraction capacity of cyclohexene. Under optimal operating conditions, with an initial cyclohexene concentration of 20 mol %, the distribution coefficient of cyclohexene was as high as 5.456, and the selectivity was as high as 1362. The single-stage extraction rate of cyclohexene reached 94.24%. Furthermore, in the 1-hexene/n-hexane system, the distribution coefficient of 1-hexene was as high as 6.733, and the selectivity was 647.4. These results are significantly better than those reported for other extractants. The silver-based alcohol-functionalized solvents developed in this study exhibit high efficiency and robust separation performance and are environmentally benign.

1. Introduction

As society transitions toward a more low-carbon, more environmentally friendly lifestyle, the energy aspect of petroleum is gradually becoming less important, while its value as a resource is becoming increasingly prominent. The petrochemical industry is shifting toward the production of higher-value components. Olefin, for example, is an important raw material for a variety of high-end chemicals, such as polymers and halogenated hydrocarbons, due to the high activatability of carbon–carbon double bonds (–CC). The efficient separation of olefins and alkanes in petroleum products holds significant industrial importance. It also facilitates the promotion of the high-value transformation of resources, improves the overall utilization value of petroleum products, and mitigates environmental pollution and carbon emissions.

Cyclohexane and cyclohexene are essential chemical feedstocks that are widely used in the production of cyclohexanol and cyclohexanone, which are key intermediates in the synthesis of nylon. Primary industrial processes involving the separation of these compounds include the deep refining of naphthenic oils, the generation of cyclohexene as a byproduct during the hydrogenation of benzene to cyclohexane, and the oxidative dehydrogenation of cyclohexane to produce cyclohexene, among others. Therefore, achieving the efficient separation of cyclohexane and cyclohexene is critically important for the petrochemical industry. However, this task remains technically challenging due to the following factors: (i) similar physicochemical propertiesthe boiling points of cyclohexene and cyclohexane differ by only 2.25 °C (82.97 and 80.72 °C, respectively), and their relative volatility is low; (ii) similar molecular geometriesthe molecular dimensions of cyclohexane (7.315 × 7.122 × 5.068 ų) and cyclohexene (7.187 × 6.769 × 5.243 ų) are nearly identical; and (iii) highly similar structuresboth compounds possess six-membered rings and have a molecular weight difference of only two units (cyclohexane: 84.15; cyclohexene: 82.15). The main methods currently used to separate alkanes and olefins include adsorption, extractive distillation, membrane separation, and liquid–liquid extraction. Among these, liquid–liquid extraction is considered a promising technique due to its mild operating conditions, ease of scaling up, and low energy requirements. It has gained considerable attention for separating mixtures of alkanes and olefins with similar boiling points. The design and synthesis of efficient extractants play a crucial role in this process, as they directly determine its effectiveness.

Currently, most extractants used in industry are conventional organic solvents, such as cyclobutylsulfone, dimethyl sulfoxide, N,N-dimethylformamide, and N-formylmorpholine. Zhu et al. evaluated the separation performance of four organic solvents in the 1-hexene/hexane system at various temperatures and reported selectivity values ranging from 1 to 3. However, traditional organic solvents are typically volatile, increasing energy consumption and operational costs and also contributing to environmental pollution. In recent years, ionic liquids (ILs) and deep eutectic solvents (DESs) have been investigated for separating alkane/olefin mixtures. , Delgado-Mellado et al. examined five ILs for separating cyclohexene and cyclohexane, finding that the most effective, [4BMPY]­[TCM], achieved a selectivity of 2.4. Karpińska et al. synthesized eight [DCA]-based ILs, which exhibited cyclohexene/cyclohexane selectivities ranging from 1.97 to 3.25. However, their extraction capacity for cyclohexene was low. Blidi et al. evaluated the separation performance of 12 DESs for the cyclohexene/cyclohexane system at 298.15 K; among them, TBAB/triethylene glycol (TEG) (1:4) exhibited the highest selectivity, at 2.29. Domańska et al. investigated the separation performance of four ILs for both the cyclohexene/cyclohexane system and the 1-hexene/hexane system. Among these, [AMIM]­[DCA] demonstrated good selectivity in liquid–liquid extraction. Li et al. evaluated the separation performance of 17 ILs for the 1-hexene/hexane system at 313 K, with selectivity values ranging from 1.5 to 3.0. Numerous studies have demonstrated that using conventional extractants alone for alkane/olefin separation results in low selectivity and an inefficient separation performance.

In recent years, efforts to improve the efficiency of extraction separation have mainly focused on the following areas: computer-aided molecular design of extractants, selection of extractants capable of forming hydrogen bonds with target compounds, application of the salting-out effect, and incorporation of metal ions to create ligand-activated sites within extractant structures. Fu et al. investigated the effects of salting-out extraction on the separation of butyric acid. By selecting highly efficient inorganic salts and optimizing the extraction process, they achieved total recovery rates of 99.5% for butyric acid and 88.9% for acetic acid. It is important to note that the primary structural difference between alkanes and olefins with the same number of carbons is the presence of a CC bond in the olefin molecule. Therefore, olefins can be efficiently separated from alkanes by designing extractants that selectively recognize –CC. Encouragingly, studies have demonstrated that π–π* coordination interactions can form between metal ions and CC bonds, offering new insights for the development of high-performance extractants. Of the various metal ions, Cu⁺ and Ag⁺ are considered to be more effective olefinic coordination carriers due to their (n – 1)­d¹⁰s⁰ electronic configurations. However, Cu⁺ tends to undergo disproportionation, resulting in the formation of stable copper metal (Cu) and Cu²⁺, which limits its stability in practical applications. In contrast, silver salts have received more attention due to their superior stability. The π orbitals of–CC in cyclohexene donate electrons to the empty orbitals of Ag⁺, while, at the same time, electrons in the d orbitals of Ag⁺ back-donate into the π* antibonding orbitals of–CC in cyclohexene, forming a π–π* coordination interaction. Wu et al. synthesized hydrophobic Ag-based ILs by dissolving cyclohexene in an aqueous solution of AgNO₃ and LiNTf₂. These ILs exhibited selectivities of 33.4 and 51.9 for the cyclohexene/cyclohexane and cyclohexene/n-hexane systems, respectively, at 298 K. Li et al. synthesized a novel silver-based DES using AgCF₃SO₃ as the hydrogen-bond acceptor and DMBA as the hydrogen-bond donor. Selectivity for 1-octene over n-octane reached 16.11 at 298.15 K with a 10 wt % concentration of 1-octene. However, the limited solubility of silver salts and the complex synthesis of ILs and DES hinder their industrial application.

Silver salts are solid at room temperature, making them difficult to apply directly to liquid–liquid extraction. Consequently, researchers have prepared functionalized solvents by dissolving silver salts in organic solvents. In this system, Ag⁺ ions, produced by the dissociation of the salts, can be anchored through coordination with the solvent molecules, thereby forming a stable active site. The hydroxyl group (−OH) is an excellent candidate for Ag⁺ coordination in this system. Furthermore, alcohols offer advantages such as high polarity, low viscosity, and abundance, demonstrating good potential for application in the separation of alkanes and olefins. Ren et al. exploited the synergistic interaction among ILs, ethylene glycol (EG), and Ag⁺ to synthesize [Emim]­[BF₄]/AgBF₄/EG (2:1:0.1), achieving an ethylene uptake of 102.6 mg/g and a selectivity of 37.8. This represents a 11.6% increase in uptake compared to the system without EG. Wentink et al. dissolved silver nitrate (AgNO₃) in EG to investigate its effectiveness in separating 1-hexene from Fischer–Tropsch-synthesized C₆ fractions. The highest selectivity for 1-hexene over trans-2-hexene was found to be 5.87 at 300 K using a 3 mol/L AgNO₃/EG solution. Compared to traditional single solvents, functionalized solvents offer certain advantages in the separation of alkane/olefin systems. However, a lack of research into the separation mechanism hinders further improvements in the separation efficiency. To enhance the separation efficiency further, silver salts can be incorporated into the extractant to improve the selectivity and extraction capacity for olefins by utilizing the coordination between Ag⁺ and CC. Additionally, the salt effect can facilitate olefin separation further. Based on this concept, a series of silver-based alcohol-functionalized solvents were designed and synthesized to achieve efficient separation of alkane/olefin systems.

In summary, this study designed and synthesized a series of silver-based alcohol-functionalized solvents for the separation of the cyclohexene/cyclohexane system. First, based on the salt effect of silver salts and the π–π* coordination interaction between Ag⁺ and–CC of cyclohexene, a series of silver-based alcohol-functionalized solvents were designed, and AgBF₄/1,2-propanediol (1,2-PG) (1:2) was screened as the optimal extractant through extraction experiments. The structures of the functionalized solvents were characterized and analyzed by using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H NMR) spectroscopy, and high-resolution mass spectrometry (HRMS). Coordination interactions between Ag⁺ and 1,2-PG were also studied in depth using quantum chemical (QC) calculations to determine the molecular structure of the solvents. Differences in the interactions of the extractant molecules (consisting of Ag⁺, BF₄ ^–, and 1,2-PG) with cyclohexane and cyclohexene molecules were examined to provide a microscopic mechanistic analysis of the cyclohexene/cyclohexane extraction separation process. Finally, a series of cyclohexene/cyclohexane extraction experiments were conducted to systematically investigate the effects of extraction time, temperature, initial olefin concentration, and agent-to-oil ratio on the separation efficiency. The aim was to optimize extraction conditions and evaluate the extract’s recycling performance. Additionally, the performance of the prepared silver-based alcohol-functionalized solvents in separating other unsaturated/saturated hydrocarbon systems (e.g., alkanes/olefins and alkanes/aromatics) was also evaluated.

2. Materials and Methods

2.1. Materials

The analytical reagents utilized in this study, including their chemical names, CAS numbers, purity levels, and supplier details, are listed in Table S2. All reagents were used as is without further purification.

2.2. Synthesis and Characterization of Silver-Based Alcohol-Functionalized Solvents

A specified amount of silver salts and alcohols was weighed and added to a 100 mL brown round-bottom flask. The air inside the flask was removed using a vacuum pump to maintain a vacuum, and the flask was stirred at 300 rpm in a constant-temperature oil bath set to 60 °C for 3 h, until a homogeneous solution was obtained. The solution was then stored in a brown reagent bottle. Structural characterization was performed using FT-IR (Nicolet 8700, Thermo Scientific), ¹H NMR (Bruker AVANCE), and mass spectrometry (Thermo Scientific Q Exactive). Thermal stability was evaluated by using TGA (TGA 550) and DSC (DSC 200 F3, Netzsch).

2.3. Liquid–Liquid Extraction

The extractants and simulated oil were added into 10 mL glass sample bottles, followed by stirring at 30 °C for 30 min at a speed of 800 rpm. The mixture was then allowed to settle until the interface between the two phases became distinct. Subsequently, 200 μL of the extraction phase and 100 μL of the residual phase were extracted using a syringe, mixed with n-heptane as the internal standard, and diluted with anhydrous ethanol for gas chromatographic analysis. Experimental parameters that can be varied include extraction time, extraction temperature, initial olefin concentration, and agent-to-oil ratio.

2.4. Analysis Method

The samples were analyzed using a Shimadzu GC-2014 gas chromatograph equipped with an HP-5 column. The partition coefficient (D) and selectivity (S) describe the equilibrium distribution of components between the two phases, serving as key indicators of the extraction performance. The extraction rate (R) reflects the absolute change in the quantity of target compounds extracted from the oil phase. (See Supporting Information, Section S1, for details). To verify the reliability of the liquid–liquid equilibrium data, the Othmer–Tobias and Hand equations were used to analyze the experimental results (see Supporting Information, Section S2, for details).

2.5. QC Calculation

All density functional theory (DFT) calculations in this study were carried out using the Gaussian 16 software (see Supporting Information Section S3 for details). The intermolecular interaction energy of the resultant stable configuration was calculated by

$$\mathrm{\Delta }{E}_{\mathrm{A}-\mathrm{B}}=E_{\mathrm{A}+\mathrm{B}}-(E_{\mathrm{A}}+E_{\mathrm{B}})+E_{\mathrm{B}\mathrm{S}\mathrm{S}\mathrm{E}}$$ 1

where E _A, E _B, and E _A+B are the energy of single A, single B, and mixture A and B, respectively. Basis set superposition error (BSSE) refers to the basis set overlap error of molecules A and B. When calculating interaction energy, the basis functions from molecules A and B overlap in the complex system, which is equivalent to enlarging the basis set of the complex and lowering the energy EAB, and BSSE correction is used. The wave function analysis is completed by Multiwfn program.

3. Results and Discussion

3.1. Design and Synthesis of Silver-Based Alcohol-Functionalized Solvents

Seven structurally stable and cost-effective silver salts were selected as candidates for the design and synthesis of silver-based functionalized solvents. These were: silver chloride (AgCl); silver nitrate (AgNO₃); silver tetrafluoroborate (AgBF₄); silver trifluoroacetate (AgCF₃COO); silver trifluoromethanesulfonate (AgCF₃SO₃); silver bis­(trifluoromethanesulfonyl)­imide (AgNTf₂), and silver sulfadiazine (AgSD). The oxygen atom is a common ligand atom with moderate electronegativity that can provide lone-pair electrons and is widely found in organic compounds. The oxygen atom in –OH of alcohols can act as a coordination site with Ag⁺. Additionally, alcohol molecules can form stable hydrogen-bonding networks through hydrogen bonding, enhancing the structural stability of the functionalized solvents. On the other hand, taking environmental and economic factors into account, choosing a polyol that provides more –OH groups from a single molecule can reduce the use of organic solvents. Furthermore, interactions between –OH groups may alter the electron cloud density around Ag⁺, enabling it to retain vacant orbitals for coordination with cyclohexene. Taking all of the above into account, three diols (EG, 1,2-PG, and 1,3-PG) and one triol (glycerol and Gly) were selected as organic ligands for the silver-based functionalized solvents examined in this study. To explore the influence of ligand structure on extraction performance, long-chain TEG and tetraethylene glycol triacetonitrile (TTEG) were also selected for the study. The structural formulas of the selected compounds are shown in Figure a. The formation of low-viscosity, stable, and homogeneous solutions is essential for the successful synthesis of silver-based alcohol-functionalized solvents. Therefore, the solubility of seven silver salts in six alcohol substances was first explored. At a molar ratio of 1:8, AgNO₃, AgNTf₂, AgCF₃SO₃, and AgBF₄ dissolved completely, whereas AgCl, AgCF₃COO, and AgSD struggled to form stable solutions. This difference in solubility is attributed to the fact that larger anion radii correspond to lower lattice energies, weakening the electrostatic interactions and increasing the solubility of the silver salts.

1.

Design and synthesis of silver-based alcohol-functionalized solvents. Structural formulas of silver salts and alcohols (a); partition coefficients of cyclohexene when functionalized solvents were prepared as extractants with different silver salt-to-alcohol molar ratios of 1:8 (b); cyclohexene-to-cyclohexene selectivities when functionalized solvents were prepared as extractants with different silver salt-to-alcohol molar ratios of 1:8 (c); partition coefficients of cyclohexene in the synthesis of functionalized solvents as extractants with different molar ratios of AgBF₄ to EG (1,3-PG, 1,2-PG) (d); cyclohexene-to-cyclohexene selectivities in the synthesis of functionalized solvents as extractants with different molar ratios of AgBF₄ to EG (1,3-PG, 1,2-PG) (e).

Next, the separation performance of various silver-based alcohol-functionalized solvents in the cyclohexene/cyclohexane system was evaluated through extraction experiments, and the results are shown in Figure b,c. The anion of silver salt has a certain influence on the extraction separation effect. From the partition coefficients of cyclohexene, the effect of AgNTf₂ is observed to be the best, while the effect of AgNO₃ is observed to be the worst, and the effects of AgCF₃SO₃ and AgBF₄ are similar. This is because the smaller the radius of the anion corresponding to the silver salt, the stronger its interaction with Ag⁺, and the greater the interference with Ag⁺’s coordination ability, resulting in poorer separation performance of the extractant. From the selectivity of cyclohexene to cyclohexane, AgBF₄ demonstrates the highest performance, while AgNTf₂ shows the lowest. This is attributed to the strong solubility of AgNTf₂ for alkanes. The distribution coefficient of cyclohexene reflects the affinity and solubility of the extractant for cyclohexene; the larger the distribution coefficient, the higher the affinity. The selectivity of cyclohexene over cyclohexane reflects the ability of the extractant to separate the cyclohexene/cyclohexane system; the higher the selectivity, the better the separation performance. In AgBF₄-based extractants, the distribution coefficient of cyclohexene is slightly lower than that in AgNTf₂-based extractants. However, the selectivity of cyclohexene over cyclohexane is highest in AgBF₄-based extractants. To achieve an efficient separation of the cyclohexene/cyclohexane system, AgBF₄ was chosen to construct silver-based alcohol-functionalized solvents.

Silver-based alcohol-functionalized solvents prepared using different alcohols as organic ligands exhibited marked differences in the extraction and separation performance. Experimental results indicate that EG, 1,2-PG, and 1,3-PG perform excellently in terms of the distribution coefficient of cyclohexene and its selectivity to cyclohexane. In contrast, Gly, TEG, and TTEG performed significantly worse. To explore the microscopic mechanism of these differences, we optimized the structures of the complexes formed between AgBF₄, various alcohol solvents, and cyclohexene (or cyclohexane), labeling the distances between the key atoms in the process (see Figure S1 for details).

Next, three diols were selected for further investigation. A series of silver-based alcohol-functionalized solvents were prepared using various molar ratios of AgBF₄ to EG (or 1,2-PG and 1,3-PG). Extraction experiments were then conducted, and the results are shown in Figure d,e. It is important to note that the extractant is unstable when the molar ratio of EG (or 1,3-PG) to AgBF₄ is 1:2. Following liquid–liquid extraction, the extractant undergoes oxidation, resulting in a black precipitate. The distribution coefficient of cyclohexene and its selectivity over cyclohexane increased with the increasing concentrations of AgBF₄. This improvement is primarily due to the reduced alcohol content and elevated Ag⁺ content in the extractant, facilitating the formation of a coordination bond between Ag⁺ and cyclohexene and thereby enhancing the extraction efficiency. Additionally, [Ag­(cyclohexene)₂(1,2-PG)­2)­BF₄] is formed based on coordination interaction. After Ag⁺ reaches coordination saturation, at a low Ag⁺ content, because the electronegativity of cyclohexene is lower than that of cyclohexane, the ion-induced dipole interaction between Ag⁺ and cyclohexene molecules is stronger than that with cyclohexane, meaning that the salt effect of the silver salt further enhances its separation ability. The distribution coefficient of cyclohexene was highest at a molar ratio of 1:2 when the silver-based alcohol-functionalized solvent prepared with 1,2-PG as the solvent was used as the extractant. Taking into account both the extraction separation effect and the stability of the solvent, 1,2-PG was ultimately chosen as the organic ligand for the silver-based functionalized solvent.

3.2. Characterization of Silver-Based Alcohol-Functionalized Solvents

The thermal stability of silver-based functionalized solvents was evaluated via TGA and DSC (see Supporting Information, Section S4, for details). The structures of the synthesized silver-based alcohol-functionalized solvents were characterized using FT-IR spectroscopy. The FT-IR spectra of 1,2-PG, AgBF₄, and the AgBF₄:1,2-PG (1:2) solution are shown in Figure a. In the AgBF₄:1,2-PG (1:2) solution, the stretching vibration peak of BF₄ ^– shifted from 771.3 cm^–1 to 763.4 cm^–1 and that of –OH shifted from 3343.3 cm^–1 to 3513.7 cm^–1. This is due to hydrogen bonding between the fluorine atoms in BF₄ ^– and the hydroxyl hydrogen atoms in 1,2-PG. Additionally, the stretching vibration peak of the C–O single bond shifted from 1045.2 cm^–1 to 1033.7 cm^–1, and the vibrational amplitude was intensified. This was attributed to the coordination between the Ag⁺ ion and the hydroxyl oxygen atom of 1,2-PG. The FT-IR spectra of 1,2-PG, AgBF₄:1,2-PG (1:2), and AgBF₄:1,2-PG (1:3) are shown in Figure b. At a molar ratio of 1:2, the –OH stretching vibration peak was completely displaced, whereas at a molar ratio of 1:3, a weak peak appeared at 3343.3 cm^–1, and the peak at 3513.7 cm^–1 was weaker than that at a molar ratio of 1:2. This indicated that the free 1,2-PG decreased as the AgBF₄ concentration increases. At a molar ratio of 1:2, nearly all 1,2-PG molecules coordinate with Ag⁺. This explains why silver-based alcohol-functionalized solvents exhibit a better extractive separation effect at a molar ratio of 1:2 than that at a molar ratio of 1:3 (See Figure S2 for additional details).

2.

FT-IR spectra of 1,2-PG, AgBF₄, and the AgBF₄:1,2-PG (1:2) solution (a); FT-IR spectra of 1,2-PG, AgBF₄:1,2-PG (1:2), and AgBF₄:1,2-PG (1:3) (b); ¹H NMR spectra of 1,2-PG (c); ¹H NMR spectra of AgBF₄:1,2-PG (1:2) (d); HRMS spectra of AgBF₄:1,2-PG (1:8) (e).

¹H NMR analyses were performed on 1,2-PG and AgBF₄:1,2-PG (1:2), and the results are shown in Figure c,d, respectively. In AgBF₄:1,2-PG (1:2), the chemical shifts of the hydrogen atoms vary to different degrees, particularly that of the hydroxyl hydrogen, which moves to a higher field. This phenomenon is attributed to the empty orbital of Ag⁺ accepting electrons from the hydroxyl oxygen atom in 1,2-PG. This coordination reduces the electronegativity of the oxygen atom, thereby weakening its attraction to the hydroxyl hydrogen atom. Consequently, the electron cloud density around the hydroxyl hydrogen atom increases, leading to chemical shifts to higher fields. These results further confirm the coordination complexation between Ag⁺ and 1,2-PG. Additionally, HRMS analysis was conducted on AgBF₄:1,2-PG (1:8) silver-based alcohol-functionalized solvents. The results are shown in Figure e. In the mass spectra, peaks with mass-to-charge ratios (m/z) of 59.05, 108.9, 182.96, and 261 were attributed to C₃H₆O⁺ (a cation formed by the loss of an –OH group from 1,2-PG), Ag⁺, Ag⁺(C₃H₈O₂), and Ag⁺(C₃H₈O₂)₂, respectively. These findings further confirm the coordination complexation between Ag⁺ and 1,2-PG.

In summary, in the AgBF₄:1,2-PG (1:2)-functionalized solvent, the hydroxyl oxygen atom in 1,2-PG acts as an electron donor by contributing lone-pair electrons. Meanwhile, Ag⁺ acts as an electron acceptor, providing null orbitals that can form coordination interactions. Additionally, hydrogen bonding occurs between the BF₄ ^– anion and the hydroxyl hydrogen atoms in 1,2-PG, costabilizing this functionalized solvent.

3.3. ESP Analysis

This study examined interaction trends between different monomers at the molecular level by analyzing the electrostatic potential (ESP) of cyclohexane, cyclohexene, and silver-based alcohol-functionalized solvents using DFT. The results are shown in Figure . The ESP distribution within the cyclohexane molecule is relatively uniform, with the minimum value (−1.84 kcal/mol) distributed near the carbon atoms and the maximum value of 4.99 kcal/mol distributed near the hydrogen atoms. In contrast, the ESP distribution of the cyclohexene molecule differs significantly. A minimum value of −14.8 kcal/mol appears near the CC bond, while a maximum value of 7.38 kcal/mol is found near the hydrogen atoms attached to this bond. These results suggest that the CC bond in cyclohexene and its attached hydrogen atoms in cyclohexene are more likely to interact with other monomers than the corresponding atoms in cyclohexane. In the prepared silver-based alcohol-functionalized solvents, the positive surrounding ESP of the Ag⁺ cation is distributed around the four fluorine atoms of the BF₄ ^– anion. In the solvent molecule 1,2-PG, the minimum ESP value of −34.91 kcal/mol is located around the hydroxyl oxygen atom, while the maximum value (44.61 kcal/mol) is located around the hydroxyl hydrogen atom. It is worth noting that there are some differences in the ESP distribution around different –OH groups.

3.

ESP and extreme points of cyclohexane (a), cyclohexene (b), BF₄ ^– (c), 1,2-PG, (e)­Ag⁺-(1,2-PG) (d), Ag⁺-(1,2-PG)-BF₄ ^– (f), Ag⁺-2­(1,2-PG)-BF₄ ^– (g), and Ag⁺-3­(1,2-PG)-BF₄ ^– (h) (the unit of extreme value is kcal/mol).

To investigate the distribution of Ag⁺, BF₄ ^–, and 1,2-PG further, an ESP analysis was conducted on the optimized structure formed by Ag⁺ and 1,2-PG. The results revealed that the minimum value of ESP (−69.20 kcal/mol) was located around the hydroxyl oxygen atom, while the maximum value (136.54 kcal/mol) was distributed around the Ag⁺ ion. This suggests that both Ag⁺ and the hydroxyl hydrogen atoms of the hydroxyl group can interact with the fluorine atom in BF₄ ^–. Furthermore, an ESP analysis of the optimized structures formed by Ag⁺, BF₄ ^–, and one to three 1,2-PG molecules revealed consistent ESP distributions around Ag⁺, BF₄ ^–, hydroxyl hydrogen atoms, and hydroxyl oxygen atoms with those observed in their respective monomer forms. These results confirm the potential of the extractant molecule to achieve cyclohexene/cyclohexane separation.

3.4. IGMH and AIM Analysis

The coordination behavior of Ag⁺ in AgBF₄-based alcohol-functionalized solvents was investigated through QC calculations (see Supporting Information, Section S5, for details). Based on the 1:2 molar ratio of AgBF₄ to 1,2-PG in the silver-based alcohol-functionalized solvents prepared in this study, we conclude that the predominant distribution of monomers in the extractant molecules follows the pattern shown in Figure a. The optimized structures of Ag⁺-2­(1,2-PG)-BF₄ ^– complexes with cyclohexane (or cyclohexene) were further analyzed by using IGMH and AIM methods. The intermolecular interaction energies were annotated, and the results are shown in Figure a–d. As can be seen in the figure, Ag⁺ retains vacant orbitals after coordinating with 1,2-PG, enabling further coordination with cyclohexene. Additionally, the hydroxyl oxygen atom of 1,2-PG and the fluorine atom of BF₄ ^– form weak van der Waals and hydrogen bonds with cyclohexane and cyclohexene, respectively. Notably, the interaction energy between the Ag⁺-2­(1,2-PG)-BF₄ ^– complex and cyclohexene is significantly lower than that with cyclohexane. This difference indicates that salt effects in silver-based alcohol-functionalized solvents contribute to molecular recognition. The coordination between Ag⁺ and cyclohexene, combined with the salt effect of the silver salt, facilitates the efficient separation of the cyclohexene/cyclohexane system.

4.

IGMH isosurfaces, bond critical point, and bond path of (Ag⁺-2­(1,2-PG)-BF₄ ^–)-cyclohexene (a), (Ag⁺-2­(1,2-PG)-BF₄ ^–)-cyclohexane (b), (Ag⁺-2­(1,2-PG)-BF₄ ^–)-2 cyclohexene (c), and (Ag⁺-2­(1,2-PG)-BF₄ ^–)-2 cyclohexane (d).

3.5. Mechanism Analysis

This study involved designing and preparing a silver-based alcohol-functionalized solvent for the efficient separation of cyclohexene/cyclohexane. This was achieved by effectively anchoring the Ag⁺ site of action through coordination between the –OH group in the 1,2-PG molecule and the Ag⁺ ion. The extractant molecular monomers were distributed in the form of Ag⁺-2­(1,2-PG)-BF₄ ^–. The interactions between the –OH groups in the ligand adjust the electron cloud density around the Ag⁺ ion, enabling it to retain vacant orbitals for the characteristic π–π* coordination with the CC group in cyclohexene. This realizes the efficient recognition and selective binding of the cyclohexene molecule. Further investigations revealed that the interactions between Ag⁺-2­(1,2-PG)-BF₄ ^– and cyclohexene were consistently stronger than those with cyclohexane. This suggests that the salt effect positively contributes to the separation process following the incorporation of silver salts into the functionalized solvent. Consequently, the silver-based alcohol-functionalized solvent developed in this study successfully achieved the efficient separation of the cyclohexene/cyclohexane system by leveraging the π–π* coordination between Ag⁺ and CC, as well as the salt effect induced by the silver salt.

3.6. Optimal Conditions for Cyclohexene/Cyclohexane Extraction Separation

The effects of extraction time, extraction temperature, cyclohexene content, and solvent-to-oil ratio on the separation of the cyclohexene/cyclohexane system were investigated by extraction experiments (see Supporting Information, Section S6, for details). Under optimal operating conditions (an extraction temperature of 30 °C and an agent-to-oil ratio of 3:1), the distribution coefficient of cyclohexene reached 5.456 when the initial cyclohexene concentration was 20 mol %, with its selectivity to cyclohexane reaching 1362. The single-stage cyclohexene extraction rate was also the highest reported for cyclohexene/cyclohexane extractive separation at 94.24%.

3.7. Separation Performance of Silver-Based Alcohol-Functionalized Solvents in Other Systems

The extractant developed by our institute demonstrates excellent extraction and separation performance in other olefin/alkane and aromatic/alkane systems (see Supporting Information, Section S7, for details).

In the n-hexane/1-hexene system (see Figure ), the silver alcohol-functionalized solvent prepared in this study outperforms other reported extractants in terms of both the distribution coefficient of 1-hexene and the selectivity of 1-hexene over n-hexane. ,− The results indicate that the silver-based alcohol-functionalized solvents prepared in this study are effective.

5.

Comparison of the silver-based alcohol-functionalized solvents prepared in this study with other literature-reported extractants for extraction in n-hexane/1-hexene system.

3.8. Recycling Performance

Five recycling experiments were performed to assess the cyclic stability of the silver-based alcohol-functionalized solvents prepared in this study. The extraction time was set to 30 min, the extraction temperature to 30 °C, the initial olefin concentration to 20 mol %, and the agent-to-oil ratio to 1:1. After each extraction, the extractant underwent rotary evaporation at 50 °C for 2 h to recover the cyclohexene and regenerate the extractant. As shown in Figure a,b, the partition coefficients of cyclohexene and its selectivity for cyclohexane changed minimally after five cycles. The silver-based alcohol-functionalized solvents maintained a good extraction separation performance after five regenerations. To further evaluate the structural stability of these solvents, we compared the FT-IR spectra of AgBF₄:1,2-PG (1:2) before and after five cycles (see Figure c). The results showed that the characteristic infrared peaks of the functional groups in AgBF₄:1,2-PG (1:2) did not shift after five cycles, indicating that its structure remained stable throughout the regeneration processes. AgBF₄:1,2-PG (1:2) can efficiently separate the cyclohexene/cyclohexane system, effectively enriching cyclohexene in the extraction phase and achieving a high cyclohexene production capacity. At the same time, the extractant maintaining stable performance over multiple cycles means that the amount needed for replenishment is low, which can significantly reduce long-term operating costs. In addition, its regeneration process is simple and energy-efficient, further lowering overall costs.

6.

Cyclicity stability of silver-based alcohol-functionalized solvents. Effect of the number of cycles on the partition coefficients of cyclohexene and cyclohexane (a) and the selectivity of cyclohexene for cyclohexane (b); FT-IR spectra of AgBF₄:1,2-PG (1:2) before use and after five cycles (c).

4. Conclusion

This study investigated the extraction and separation properties of silver-based alcohol-functionalized solvents in the cyclohexene/cyclohexane system as well as their separation mechanisms. First, from seven silver salts and six alcohols, AgBF₄ and 1,2-PG were selected based on their ability to form a stable solution in alcohols, as well as on the extraction separation effect of the synthesized extractants. A series of characterizations were then conducted on the solvent to confirm its successful synthesis. These revealed that Ag⁺ interacts with the hydroxyl oxygen atoms of 1,2-PG, while BF₄ ^– associates with the hydrogen atoms of 1,2-PG. QC calculations revealed that the extractant molecule’s structural form is Ag⁺-2­(1,2-PG)-BF₄ ^–. Interaction energy calculations demonstrated that the interactions between the extractant molecules and different numbers of cyclohexene molecules were all stronger than the interactions between extractant molecules and cyclohexane molecules.

Finally, a series of extraction experiments showed that the silver-based alcohol-functionalized solvent showed a good extraction effect under various conditions. With an initial cyclohexene concentration of 20 mol %, the distribution coefficient of cyclohexene reached 5.456 under optimal operating conditions (30 °C extraction temperature and a 3:1 agent-to-oil ratio). The selectivity for cyclohexane was found to be 1362, and the single-stage cyclohexene extraction efficiency was 94.24%. Compared to the lower separation performance improvements reported for similar compounds in the literature, thanks to the exploration of the separation mechanism and the mechanism-based design of the extractant, the extraction and separation performance of the silver-based alcohol-functionalized solvents synthesized in this study show a significant improvement over that of traditional solvents. Additionally, cyclic experiments showed that the silver-based alcohol-functionalized solvent retained its structural stability after five cycles of use. Notably, the extractant also exhibits excellent extraction separation performance in α-olefins/alkanes, chain alkane/aromatic, cycloalkane/aromatic, and other systems, among others, and has a wide range of potential applications. The silver-based alcohol-functionalized solvent developed in this study offers a novel approach for the efficient, stable, and environmentally friendly separation of cyclohexene/cyclohexane systems, offering a promising solution for the industrial separation of alkanes/olefins.

All characterization and experimental data in this work are available and reproducible. All data were tested twice to ensure the usability of the data.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/cbe.5c00120.

• Extraction experimental analysis methods; analysis methods for liquid–liquid phase equilibrium data; QC calculations; thermal stability of AgBF₄:1,2-PG (1:2); coordination of Ag+ in functionalized solvents; optimization of conditions for extraction experiments; and scalability of AgBF₄:1,2-PG (1:2) (PDF)

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H.K., G.Y., and J.X. contributed equally to this work. Hongwei Kang: Methodology; investigation; validation; visualization; data curation; software, writingoriginal draft (equal). Jingyi Xu: Investigation; data curation; validation; methodology; writingoriginal draft (equal); formal analysis. Guojia Yu: Investigation; validation. Shuying Wang: Investigation; validation. Zhiyong Zhou: Investigation; validation; supervision. Chenchan Du: Supervision; investigation; methodology. Yuming Tu: Investigation; methodology; supervision; data curation. Junjun Shi: Investigation; methodology; supervision; data curation. Zhongqi Ren: Conceptualization; methodology; investigation; validation; supervision; funding acquisition; project administration; resources; writingreview and editing; formal analysis; software.

The authors declare no competing financial interest.