Natural eutectic solvent as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of chemical engineering separation, and provides a natural eutectic solvent and a preparation method and application thereof, the natural eutectic solvent comprises the following components: caprolactam, citric acid and water; wherein the molar ratio of caprolactam to citric acid is (1-4): (1-2); the mass of the water is 20-40% of the sum of the mass of the caprolactam and the mass of the citric acid. By introducing water as a third component, the viscosity of the system is effectively reduced, the mass transfer performance is improved, and the denitrification efficiency is improved. The eutecticevaporate solvent provided by the invention can provide more reaction sites, has relatively strong removal capacity and selectivity on nitrogenous organic matters, and can realize efficient denitrification under mild conditions. The natural deep-eutectic solvent prepared by the preparation method disclosed by the invention is applied to removal of basic nitrides of oil products, and has the advantages of short time consumption and low dosage, namely high removal efficiency.

Description

Technical Field

[0001] This invention relates to the field of chemical engineering separation technology, and in particular to a natural eutectic solvent, its preparation method, and its application. Background Technology

[0002] Fuel oil, as a key fuel in transportation, industrial production, and energy supply systems, directly impacts environmental emission control and the stable operation of downstream catalytic processes. Nitrogen-containing compounds are among the key hazardous components affecting oil quality and environmental safety, while also possessing significant utilization value. If their original structure can be preserved, these molecules can be used as raw materials for fine chemicals and as high-value monomers in the production of pharmaceuticals, dyes, and functional materials, achieving efficient resource utilization. Therefore, the efficient removal of nitrogen-containing heterocyclic compounds from oil products remains a crucial research topic in the field of oil purification and deep refining.

[0003] Currently, industrial denitrification mainly employs alkaline washing. While this method is mature, it suffers from drawbacks such as high solvent consumption, limited denitrification efficiency, and secondary pollution. Meanwhile, hydrodenitrification, as a highly efficient deep denitrification technology, can significantly reduce nitrogen-containing compound content, but it often comes with high hydrogen consumption, high cost of precious metal catalysts, and demanding operating conditions. In contrast, liquid-liquid extraction, due to its mild operating conditions, low energy consumption, and minimal damage to aromatic hydrocarbon structures, is considered an important supplementary method to hydrodenitrification.

[0004] While traditional organic solvents are abundant and their processes are simple, they generally suffer from high volatility, high toxicity, and environmental unsustainability. Ionic liquids, as representatives of green solvents, offer advantages such as high thermal stability and strong designability, but their high synthesis costs, purification difficulties, and potential environmental risks limit their large-scale application. Therefore, providing a novel oil denitrification process has become an urgent problem to be solved. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems in the prior art and provide a natural eutectic solvent, its preparation method, and its application.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a natural eutectic solvent comprising the following components: Caprolactam, citric acid, and water; The molar ratio of caprolactam to citric acid is 1~4:1~2; The mass of water is 20-40% of the combined mass of caprolactam and citric acid.

[0007] The present invention also provides a method for preparing the aforementioned natural eutectic solvent, comprising the following steps: The natural eutectic solvent is obtained by mixing caprolactam, citric acid and water.

[0008] Preferably, the mixing temperature is 40~70℃ and the time is 5~30min.

[0009] The present invention also provides the application of the aforementioned natural eutectic solvent in denitrification of oil products.

[0010] This invention provides a natural eutectic solvent comprising the following components: caprolactam, citric acid, and water; wherein the molar ratio of caprolactam to citric acid is 1-4:1-2; and the mass of water is 20-40% of the sum of the masses of caprolactam and citric acid. The natural eutectic solvent used in this invention is composed of natural or renewable components, with widely available and environmentally friendly raw materials.

[0011] This invention, by introducing water as a third component, effectively reduces the system viscosity, improves mass transfer performance, and increases denitrification efficiency. The eutectic solvent provided by this invention offers more reaction sites, exhibiting strong removal capacity and selectivity for nitrogen-containing organic compounds, achieving highly efficient denitrification under mild conditions. The natural eutectic solvent prepared by this invention achieves high removal efficiency with short processing time and small dosage in applications removing basic nitrogen compounds from oils.

[0012] The technical solution provided by this invention is low in cost, simple to operate, highly efficient in extraction, and environmentally compatible, and can efficiently remove nitrogen-containing heterocyclic compounds from oils under mild conditions. Attached Figure Description

[0013] Figure 1 This is a molecular surface charge distribution diagram of the natural eutectic solvent in Example 1; Figure 2 The infrared spectrum of the natural eutectic solvent in Example 1 is shown. Detailed Implementation

[0014] This invention provides a natural eutectic solvent comprising the following components: Caprolactam, citric acid, and water; The molar ratio of caprolactam to citric acid is 1~4:1~2; The mass of water is 20-40% of the combined mass of caprolactam and citric acid.

[0015] In this invention, the molar ratio of caprolactam to citric acid is preferably 1.5~3.5:1.1~1.9, more preferably 2~3:1.2~1.8, and even more preferably 2.4~2.6:1.4~1.6.

[0016] In this invention, the mass of water is preferably 25-35% of the combined mass of caprolactam and citric acid, more preferably 26-34%, and even more preferably 28-32%.

[0017] In this invention, caprolactam (hydrogen bond acceptor) and citric acid (hydrogen bond donor) are used as the main components, and water is introduced as the third component to obtain a natural eutectic solvent. The ability of caprolactam and citric acid to form a stable eutectic solvent system is mainly due to their high complementarity in molecular structure. The caprolactam molecule contains an amide group, whose carbonyl oxygen atom has a strong hydrogen bond acceptor ability, while the amide nitrogen can act as a weak hydrogen bond donor under certain conditions. Citric acid contains multiple carboxyl and hydroxyl groups, which can simultaneously provide abundant hydrogen bond donor and acceptor sites. When the two are mixed in an appropriate molar ratio, a dense and stable hydrogen bond network can be constructed through multi-point hydrogen bonding, thereby forming a eutectic solvent system with distinct polar characteristics.

[0018] The present invention also provides a method for preparing the aforementioned natural eutectic solvent, comprising the following steps: The natural eutectic solvent is obtained by mixing caprolactam, citric acid and water.

[0019] In this invention, the mixing temperature is preferably 40~70℃, more preferably 45~65℃, and even more preferably 50~60℃; the mixing time is preferably 5~30min, more preferably 10~25min, and even more preferably 15~20min.

[0020] The present invention also provides the application of the aforementioned natural eutectic solvent in denitrification of oil products.

[0021] In this invention, the oil product includes one or more of diesel oil, coal tar oil, gasoline, heavy oil, and residual oil.

[0022] In this invention, the nitrogen-containing compound that is denitrified includes one or more of quinoline and pyridine.

[0023] In this invention, the preferred mass ratio of the natural eutectic solvent to the oil is 1:7~10, more preferably 1:8~9, and even more preferably 1:8.4~8.6. The natural eutectic solvent and the oil are mixed and then extracted. After extraction, the mixture is allowed to stand and separate into layers. The upper layer is the denitrified oil, and the lower layer is the extract phase enriched with nitrogen compounds. Back-extraction with dilute alkaline water regenerates the extract phase and recovers the nitrogen compounds.

[0024] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0025] Example 1

[0026] Weigh caprolactam and citric acid in a molar ratio of 2:1. Then add 30 wt% water and place the mixture in a flask with a stirrer. React at 50°C for 30 minutes until a homogeneous and transparent liquid is formed, which is the final natural eutectic solvent.

[0027] The distribution of molecular surface charge of the natural eutectic solvent in this embodiment is as follows: Figure 1 As shown, from Figure 1 It can be seen that the σ-profile of quinoline shows a significant peak in the positive σ region, corresponding to the high electron density surface near the nitrogen atom of the heterocyclic ring. This distribution characteristic indicates that quinoline has a significant hydrogen bond acceptor ability and readily interacts specifically with hydrogen bond donors. In contrast, the distribution of quinoline in the negative σ region is weaker, indicating that it has almost no hydrogen bond donor ability, which also determines that its dissolution in nonpolar solvents mainly relies on weak dispersion. Citric acid exhibits a broad and strong distribution peak in the negative σ region, originating from the proton donor sites in its multiple carboxyl and hydroxyl groups. This high-density negative σ distribution indicates that citric acid is a typical strong hydrogen bond donor molecule, capable of multi-point synergistic interaction with strong hydrogen bond acceptors. This feature makes it a key component in natural eutectic solvent systems for forming stable hydrogen bonds with the nitrogen atom of the quinoline heterocyclic ring. Caprolactam shows a certain distribution in both the positive and negative σ regions in the σ-profile, reflecting the dual nature of its amide group as both a hydrogen bond acceptor (C=O) and a weak hydrogen bond donor (NH). This dual-purpose characteristic facilitates the construction of stable hydrogen bond networks in natural eutectic solvents, while simultaneously providing auxiliary synergistic sites for quinoline during extraction. Compared to single components, the σ-profile of the natural eutectic solvent system exhibits a wider distribution range and higher intensity in both positive and negative σ regions, indicating the formation of a complex and dense hydrogen bond network among multiple components. This network structure not only enhances the internal stability of the natural eutectic solvent but also significantly improves its ability to form multi-point hydrogen bonds with polar solutes. The σ-profile of n-heptane is almost entirely concentrated near σ=0, exhibiting neither hydrogen bond donor nor acceptor characteristics. This means that it cannot form effective hydrogen bond interactions with quinoline, and the dissolution of quinoline in it mainly depends on weak van der Waals interactions.

[0028] The above σ-profile analysis reveals that quinoline, as a strong hydrogen bond acceptor, exhibits significant complementarity in charge density distribution with the high-density hydrogen bond donor sites in the eutectic solvent. This high degree of σ-profile matching enables quinoline to form stable and synergistic hydrogen bond interactions in the natural eutectic solvent phase, thereby significantly reducing its dissolution free energy. Conversely, in the n-heptane phase, due to the lack of surface charge density regions for hydrogen bond formation, quinoline cannot achieve similar stabilizing effects. It is precisely this fundamental difference in hydrogen bond interaction capabilities that leads to a lower dissolution free energy for quinoline in the natural eutectic solvent phase, ultimately resulting in a negative transfer free energy and the efficient extraction of quinoline from the model oil in experiments using the natural eutectic solvent.

[0029] The infrared spectrum of the natural eutectic solvent in this embodiment is as follows: Figure 2 As shown, in the FT-IR spectrum of the eutectic solvent, the 3200-3600 cm⁻¹ region can be observed. -1 A significantly broadened and intensified absorption band appears in the region. Compared to the relatively narrow O–H stretching vibration peaks in pure citric acid and H₂O, this absorption band exhibits a redshift towards lower wavenumbers and displays a distinct asymmetry. This change is generally considered a typical spectral indicator of strong hydrogen bond interactions, suggesting the establishment of a highly cooperative hydrogen bond network structure in the natural eutectic solvent system. Notably, the significant broadening of the O–H absorption band in the eutectic solvent compared to the pure water system indicates that water molecules do not exist in a free state but are deeply embedded in the hydrogen bond network of the natural eutectic solvent, participating in the reconstruction of the overall hydrogen bond structure as structural regulating units. This characteristic suggests that water molecules play a role in structural regulation rather than simple dilution in this system. Within an appropriate water content range, water molecules can act as hydrogen bonds, forming new hydrogen bond interactions with the polar functional groups in caprolactam and citric acid, thereby reducing the viscosity of the system and increasing the degree of freedom of molecular motion without completely destroying the original hydrogen bond framework. This structural rearrangement facilitates the diffusion and migration of nitrogen-containing compound molecules between the oil phase and the eutectic solvent phase, thereby significantly improving the extraction rate and separation efficiency. However, when the water content increases further, the hydrogen bond network in the eutectic solvent system becomes excessively diluted, leading to a reduction in effective interaction sites and thus weakening the selective solubility of nitrogen-containing compounds. This indicates that water in this system has a dual effect of promoting mass transfer and weakening interactions, and its content needs to be controlled within an appropriate range. In the caprolactam-citric acid eutectic solvent system, water mainly functions as a structure modifier and mass transfer promoter, achieving synergistic optimization of extraction performance by balancing system viscosity and molecular interaction strength.

[0030] In the mid-infrared region, citric acid is located at 1700-1750 cm⁻¹. -1The C=O stretching vibration peak of the carboxyl group showed a significant shift and a decrease in intensity after the formation of the eutectic solvent, indicating that the carboxyl group participates in significant intermolecular interactions. Meanwhile, the peak value in caprolactam, located at 1650-1680 cm⁻¹, also showed a significant shift. -1 The C=O absorption peak of the amide group also exhibits a systematic shift and peak broadening in the spectrum of the natural eutectic solvent. These changes indicate that in the natural eutectic solvent system, the carboxyl and amide groups form a stable multi-center interaction structure through hydrogen bonding and acid-base synergistic effects, rather than a simple physical blending state. Furthermore, no new characteristic absorption peaks appear in the FT-IR spectrum of the eutectic solvent, indicating that the formation of the system does not involve the formation of new covalent bonds, and its stability mainly stems from reversible non-covalent interactions, such as hydrogen bonding and electrostatic interactions. This result is highly consistent with the basic physicochemical characteristics of eutectic solvents and further proves that the natural eutectic solvent system is a typical hydrogen-bonded dominant eutectic solvent.

[0031] In summary, the FT-IR characterization results clearly demonstrate that a dense and stable multi-center hydrogen bond network structure is constructed in the natural eutectic solvent system through the synergistic interaction between carboxyl groups, amide groups, and water molecules. This structural feature is not only a key basis for the stable existence of the eutectic solvent, but also provides important molecular-level support for its multi-point synergistic interaction with nitrogen-containing heterocyclic compounds during extraction.

[0032] Example 2

[0033] Weigh caprolactam and citric acid separately at a molar ratio of 1:1. Then add water with a mass fraction of 30 wt%. Place the mixture in a flask equipped with a stirrer and react at 70°C for 30 minutes until a homogeneous and transparent liquid is formed, which is the final natural eutectic solvent.

[0034] Example 3

[0035] Weigh caprolactam and citric acid in a molar ratio of 1:2, then add 30 wt% water and place the mixture in a flask with a stirrer. React at 70°C for 30 minutes until a homogeneous and transparent liquid is formed, which is the final natural eutectic solvent.

[0036] Performance testing

[0037] Simulated fuel oil was prepared by separately dissolving quinoline and pyridine in n-heptane simulated oil to achieve a nitrogen concentration of 200-1000 ppm. After denitrogenation, the concentrations of quinoline (313 nm) and pyridine (254 nm) in the n-heptane phase were determined using UV-Vis spectrophotometry. At least three parallel samples were taken from the n-heptane phase for quantitative determination of each target analyte, with the relative error controlled within 1% at equilibrium to ensure data accuracy. The extraction efficiency (E0%) of nitrogen compounds was calculated using the following formula:

[0038] In the formula, C0 represents the initial UV absorbance of quinoline and pyridine in the simulated oil, and C1 represents the UV absorbance of quinoline and pyridine after extraction from the model oil.

[0039] All application examples below are based on the natural eutectic solvent prepared in Example 1.

[0040] 1 g of natural eutectic solvent was weighed and placed in a 20 mL sample bottle. 7 g of simulated oil with a nitrogen concentration of 250 ppm was added. Extraction was carried out at 30 °C with magnetic stirring at 800 rpm for 20 min. After the reaction was completed, the mixture was allowed to stand for 10 minutes to allow the solution to separate into layers. The extraction efficiencies of quinoline and pyridine were measured to be 99.77% and 99.22%, respectively.

[0041] 1 g of natural eutectic solvent was weighed and placed in a 20 mL sample bottle. 10 g of simulated oil with a nitrogen concentration of 250 ppm was added. Extraction was carried out at 30 °C with magnetic stirring at 800 rpm for 20 min. After the reaction was completed, the mixture was allowed to stand for 10 minutes to allow the solution to separate into layers. The extraction efficiencies of quinoline and pyridine were measured to be 99.12% and 98.54%, respectively.

[0042] 1 g of natural eutectic solvent was weighed and placed in a 20 mL sample bottle. 7 g of simulated oil with a nitrogen concentration of 250 ppm was added. Extraction was carried out at 30 °C with magnetic stirring at 800 rpm for 5 min. After the reaction was completed, the mixture was allowed to stand for 10 minutes to allow the solution to separate into layers. The extraction efficiencies of quinoline and pyridine were measured to be 99.57% and 98.03%, respectively.

[0043] 1 g of natural eutectic solvent was weighed and placed in a 20 mL sample bottle. 7 g of simulated oil with a nitrogen concentration of 500 ppm was added. Extraction was carried out at 30 °C with magnetic stirring at 800 rpm for 20 min. After the reaction was completed, the mixture was allowed to stand for 10 minutes to allow the solution to separate into layers. The extraction efficiencies of quinoline and pyridine were measured to be 99.28% and 99.47%, respectively.

[0044] For the natural eutectic solvent after nitride extraction, the following regeneration process was used: 20 mL of deionized water was added, and dilute alkaline water was added as the back-extraction agent, with an equal volume of dilute alkaline water added to the aqueous phase. The mixture was back-extracted at 25°C for 30 min. After the system was allowed to stand until the two phases completely separated, the liquid was separated, and the lower aqueous phase containing the natural eutectic solvent was collected. The above back-extraction operation was repeated twice to fully remove nitrides from the natural eutectic solvent, yielding an aqueous solution of the regenerated natural eutectic solvent. After four regenerations of the natural eutectic solvent, the extraction efficiencies for quinoline and pyridine were 98.32% and 97.24%, respectively.

[0045] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A natural eutectic solvent, characterized in that, It contains the following components: Caprolactam, citric acid, and water; The molar ratio of caprolactam to citric acid is 1~4:1~2; The mass of water is 20-40% of the combined mass of caprolactam and citric acid.

2. The method for preparing the natural eutectic solvent according to claim 1, characterized in that, Includes the following steps: The natural eutectic solvent is obtained by mixing caprolactam, citric acid and water.

3. The method for preparing the natural eutectic solvent as described in claim 2, characterized in that, The mixing temperature is 40~70℃, and the time is 5~30min.

4. The application of the natural eutectic solvent of claim 1 in denitrification of oil products.

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