Preparation method of mesophase pitch regulated and controlled by plasma-assisted directional bromination and catalytic debromination

Abstract

The invention discloses a preparation method of mesophase pitch regulated and controlled by plasma-assisted directional bromination and catalytic debromination, and belongs to the technical field of high-value utilization of heavy oil and preparation of advanced carbon materials. The method comprises the following steps: (1) by taking aromatic hydrocarbon-rich heavy oil as a raw material, carrying out low-temperature plasma-assisted liquid-phase bromination reaction, and directionally introducing bromine atoms on an aromatic ring as activation and orientation sites, so as to construct bromination modified linear precursor asphalt; (2) carrying out supported metal hydride catalytic debromination-directional polycondensation reaction to synchronously realize total removal of bromine atoms, linear coupling between aromatic rings and improvement of molecular planarity, so as to prepare pre-mesophase pitch; and (3) carrying out heat treatment in a strong and uniform magnetic field, and enabling the liquid crystal state molecules to be highly and orderly arranged along the direction of the magnetic field by virtue of the orientation effect of the magnetic field on the pi electron cloud, so as to finally form the spinnable mesophase pitch with a wide-area streamline structure. The method realizes full-chain accurate regulation and control from molecular structure design to macroscopic orientation, and the prepared mesophase pitch has the characteristics of high anisotropic content (greater than 98%), moderate softening point (260-300 DEG C), highly ordered molecular arrangement and low quinoline insoluble content (less than 0.1%). The process is novel, and a brand new path is provided for preparing the high-performance mesophase pitch.

Description

Technical Field

[0001] This invention relates to a method for preparing mesophase pitch, and more particularly to a method for selectively and directionally activating aromatics with bromine using a low-temperature plasma field, combined with catalytic debromination and directional polycondensation, to directionally prepare a precursor with a high linear molecular structure, and then convert it into high-performance spinnable mesophase pitch. This method belongs to the field of high-value-added utilization of heavy oil and advanced carbon material preparation technology. Background Technology

[0002] Mesophase pitch is a nematic liquid crystal material formed during the liquid-phase carbonization of polycyclic aromatic hydrocarbon molecules. As a precursor for the preparation of key materials such as high-performance carbon fibers and carbon foam, its molecular structure directly determines the mechanical and thermal conductivity properties of the final carbon material. Ideally, it should be composed of highly planar, linearly linked aromatic macromolecules, exhibiting a highly ordered "wide-area streamlined" optical anisotropic structure in the molten state. This structure endows mesophase pitch with a suitable softening point, excellent melt flowability, high char residue, and easy graphitization, making it a high-value-added functional carbon material that can be used directly or after further processing in high-end thermal conductive materials, composite matrix, electrochemical electrodes, and other fields.

[0003] Currently, the industrial preparation of mesophase pitch mainly relies on thermal polycondensation. This method is usually carried out at high temperatures above 400 °C in an inert atmosphere, relying on the free radical thermal reaction of aromatic molecules to achieve condensation. Although the process is simple, its inherent defects are significant: (1) poor reaction selectivity: high temperature leads to violent reaction, and the condensation sites and directions of various aromatic molecules are completely random, which easily forms branched, distorted or cross-linked molecular structures, rather than ideal linear aromatic oligomers; (2) poor product uniformity: the difficult-to-control reaction process often leads to excessive polycondensation, generating a large amount of insoluble quinoline insolubles (QI), while the product has a wide molecular weight distribution and a high softening point; (3) difficulty in structural control: the obtained mesophase is mostly in the form of fine mosaic and small flow domain structures, which is difficult to spontaneously form large-size wide-area streamline structures, limiting the upper limit of its subsequent application performance.

[0004] To overcome the shortcomings of thermal polycondensation, researchers have developed catalytic polycondensation methods, such as using Lewis acid catalysts like AlCl3 and HF / BF3. These catalysts can activate aromatic rings at relatively low temperatures (300-380 °C), improving reaction efficiency and some selectivity. However, this method faces new bottlenecks: (1) Catalyst separation and contamination: the catalyst is difficult to completely remove from high-viscosity asphalt products, and the residue corrodes equipment, contaminates products, and affects subsequent processes; (2) Insufficient control over molecular configuration: although the reaction conditions are milder, there is still a lack of precise guidance on the connection modes between aromatic molecules (such as ortho, para, and the number of condensed rings), limiting its effectiveness in constructing highly linear molecular skeletons. In recent years, some technologies have attempted to introduce hydrogen-donating solvents (such as tetrahydronaphthalene) or specific extraction and separation steps to regulate the composition of raw materials or the reaction environment (such as CN110629326B and CN119552674A). However, these methods mainly adjust the reactant ratio or remove some components from a physical perspective, without fundamentally intervening in the reaction pathway and bonding direction of aromatic condensation.

[0005] Therefore, a common predicament of existing technologies lies in the lack of an effective means to actively guide aromatic molecules to undergo "directional condensation" at the molecular reaction mechanism level, thereby precisely constructing a linear, planar molecular framework. This makes it difficult to achieve breakthrough improvements in the molecular linearity, structural order, and overall performance of the prepared mesophase pitch. Developing a novel preparation process capable of molecular-level structural design has become a key technical problem urgently needing to be solved in this field. It is against this backdrop that this invention proposes an innovative solution. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a novel method for preparing mesophase pitch with directional control over molecular configuration. This method innovatively combines a low-temperature plasma-assisted bromine-mediated molecular directional activation mechanism with a supported metal hydride catalytic debromination-directional polycondensation technology. First, directional bromination modification of aromatic molecules is achieved under mild conditions. Then, linear coupling and deep polycondensation are simultaneously completed via catalytic debromination to construct a highly linear precursor pitch. Finally, a highly ordered spinnable mesophase pitch is obtained through magnetic field orientation.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for preparing mesophase pitch with plasma-assisted directional bromination and catalytic debromination control, using aromatic-rich heavy oil as raw material, is characterized by comprising the following steps:

[0009] (1) Plasma-assisted bromination and directional activation: Under the protection of an inert atmosphere, a bromine source is quantitatively added to the aromatic oil feedstock and placed in a low-temperature plasma reaction system equipped with parallel electrode plates. The plasma-assisted liquid-phase bromination reaction is carried out at 80~180 °C. The active particles generated by the plasma are used to enhance the directional introduction of bromine atoms on the aromatic ring, and activation and directional sites are precisely constructed on the aromatic molecules. The reaction time is 2~10 h, and bromination-modified linear precursor asphalt is obtained.

[0010] (2) Catalytic debromination and directional polycondensation: The brominated modified linear precursor pitch obtained in step (1) is mixed with a supported metal hydride catalyst (such as NaH / Al2O3 or CaH2 / SiO2) and subjected to catalytic debromination and directional polycondensation at 280~360 °C under an inert atmosphere for 5~20 h. This step aims to completely remove bromine atoms and simultaneously utilize the active sites formed during the debromination process to induce directional linear coupling and deep planar condensation between aromatic rings, forming a "pre-mesophase pitch" rich in linear biaromatic structures.

[0011] (3) Magnetic field-assisted liquid phase carbonization: The pre-mesophase pitch obtained in step (2) is placed in a strong uniform magnetic field environment (magnetic field strength ≥ 1 Tesla) and heat-treated at 380~420 ℃ for 2~10 h. Under the action of the magnetic field, the molecular magnetic moments of planar aromatic molecules with large π bonds will be oriented along the direction of the magnetic field, which greatly promotes the formation of a wide-area streamlined mesophase structure, and finally spinnable mesophase pitch is obtained.

[0012] The bromine source mentioned in step (1) of this invention is one of liquid bromine, N-bromosuccinimide (NBS) or copper bromide, and the amount added is 10% to 50% of the molar equivalent of the aromatic hydrocarbon in the raw material.

[0013] The low-temperature plasma conditions for the plasma-assisted bromination reaction described in step (1) of this invention are: power 50~500 W, frequency 10~40 kHz, plasma treatment and bromination reaction are carried out simultaneously, and the reaction time is 2~10 h.

[0014] The amount of the supported metal hydride catalyst added in step (2) of the present invention is 0.5% to 5% of the mass of the precursor pitch.

[0015] The direction of the magnetic field in step (3) of this invention can be set according to the needs of subsequent applications.

[0016] The softening point of the spinnable mesophase pitch obtained in step (3) of this invention is 260~300 ℃, the anisotropy content is >98%, and the H / C atomic ratio is 0.35~0.50.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] Molecular-level structure design: This pioneering approach applies the precise molecular synthesis strategy of "plasma-assisted directional bromination-catalytic debromination-guided linear coupling" to the preparation of asphalt materials. The plasma field enhances the directional introduction of bromine atoms at specific sites on the aromatic rings. These bromine atoms act as "temporary anchors" and activation points, simultaneously achieving directional linear coupling between aromatic rings during the subsequent catalytic debromination process. This synergistic process enables the controllable construction of a linearly ordered precursor from disordered heavy aromatic hydrocarbons—a feat impossible with traditional thermal methods.

[0019] Precise control of energy field: A strong magnetic field is introduced to macroscopically intervene in the orientation of the liquid crystal mesophase molecules, so that the molecular sheets achieve a highly consistent arrangement during the formation process, which significantly improves the optical anisotropy content and structural regularity of the mesophase pitch, and obtains a wide-area streamlined structure with a suitable softening point and good fluidity.

[0020] The process is green and efficient: the low-temperature plasma-assisted bromination process has low energy consumption and high bromination selectivity, significantly improving the utilization rate of bromine atoms; the catalytic debromination step can completely remove and effectively fix halogens, avoiding environmental pollution. The entire process is carried out at a relatively low temperature, effectively inhibiting excessive polycondensation and the formation of quinoline insolubles.

[0021] The product boasts superior performance: Through the above-mentioned multi-level fine control, the resulting mesophase pitch exhibits excellent molecular linearity and planarity, presenting an ultra-large-area streamlined structure. It possesses the comprehensive advantages of high anisotropy content, suitable softening point, and low quinoline insoluble content, making it a high-quality spinnable mesophase pitch. Attached Figure Description

[0022] Figure 1 is an orthogonally polarized light micrograph of the spinnable mesophase pitch prepared in Example 1 of the present invention.

[0023] Figure 2 shows an orthogonally polarized light micrograph of mesophase pitch prepared by the conventional thermal polycondensation method in the comparative example. Detailed Implementation

[0024] The present invention will be further described below with reference to the embodiments, but the present invention is not limited to these embodiments.

[0025] Implementation Example 1:

[0026] Catalytic cracking slurry was used as raw material. 100 g of slurry was taken and N-bromosuccinimide (NBS) with a molar equivalent of 30% was added. The mixture was placed in a plasma reactor equipped with parallel electrode plates. Under nitrogen protection at 120°C, a low-temperature plasma (200 W power, 20 kHz frequency) was turned on to carry out an auxiliary bromination reaction simultaneously. The reaction was stirred for 6 h to obtain light brown brominated modified linear precursor pitch A.

[0027] Asphalt A was mixed with 2 wt% NaH / Al2O3 catalyst and placed in a tube furnace. Under N2 protection, it was heat-treated at 330 °C for 12 h to complete the catalytic debromination and directional linear polycondensation, yielding pre-mesophase asphalt B.

[0028] Pitch B was placed in a graphite crucible and placed in a 1.5 T vertical uniform magnetic field generated by a superconducting magnet. It was heat-treated at 400 °C for 5 h to obtain spinnable mesophase pitch C. Its softening point was 285 °C. Microscopic observation under crossed polarized light (see...) Figure 1 The anisotropy content is >99%, it has a wide-area streamlined structure, H / C=0.42, and the quinoline insoluble content is 0.08%.

[0029] Implementation Example 2:

[0030] Coal tar pitch was used as raw material. Following the process described in Example 1, the bromine source was adjusted to liquid bromine (20% molar equivalent), and the plasma-assisted bromination conditions were 150 W power, 20 kHz frequency, simultaneous reaction for 8 h, catalytic debromination and polycondensation temperature of 300 °C, treatment for 8 h, and magnetic field strength of 2.0 T. The resulting mesophase pitch had a softening point of 295 °C, an anisotropy content of 98.5%, and a quinoline insoluble content of 0.05%.

[0031] Comparative example:

[0032] Mesophase pitch was prepared directly from the same batch of catalytic cracking slurry using the conventional thermal polycondensation method (430 ℃, atmospheric pressure, N2 protection, reaction for 20 h). The resulting product had a high softening point (310 ℃), and cross-polarized light microscopy (see Figure 2) showed an anisotropy content of approximately 90%, exhibiting a fine mosaic and fluidized mixed structure, with a quinoline insoluble content of 2.1%.

[0033] As can be seen from the comparison of the examples and comparative examples, the method of the present invention has significant advantages in terms of the regularity of the mesophase structure, the control of the softening point, and the reduction of the content of quinoline insoluble matter.

Claims

1. A method for preparing mesophase pitch with plasma-assisted directional bromination and catalytic debromination control, using aromatic-rich heavy oil as raw material, characterized in that, Includes the following steps: (1) Plasma-assisted bromination directional activation: Under an inert atmosphere, a bromine source is added to the raw material and placed in a low-temperature plasma reaction system to carry out a plasma-assisted liquid-phase bromination reaction, bromine atoms are directionally introduced as activation and orientation sites to obtain bromination-modified linear precursor pitch; (2) Catalytic debromination and directional polycondensation: Linear precursor pitch is mixed with a supported metal hydride catalyst and catalytic debromination and directional polycondensation are carried out under an inert atmosphere to simultaneously achieve complete removal of bromine atoms, linear coupling of aromatic rings and planarization improvement, and obtain pre-intermediate pitch; (3) Magnetic field-assisted liquid phase carbonization: The pre-mesophase pitch is placed in a strong uniform magnetic field environment for heat treatment to obtain spinnable mesophase pitch.

2. The method according to claim 1, characterized in that, The bromine source mentioned in step (1) is one of liquid bromine, N-bromosuccinimide or copper bromide, and the amount added is 10% to 50% of the molar equivalent of the aromatic raw material; the plasma-assisted bromination reaction is carried out in an inert atmosphere, the low-temperature plasma power is 50 to 500 W, the frequency is 10 to 40 kHz, the plasma treatment and bromination reaction are carried out simultaneously, and the reaction time is 2 to 10 h.

3. The method according to claim 1, characterized in that, The supported metal hydride catalyst in step (2) is NaH / Al2O3 or CaH2 / SiO2, and the amount added is 0.5%~5% of the mass of the linear precursor pitch; the reaction temperature is 280~360℃, and the reaction time is 5~20 h, so as to achieve the complete removal of bromine atoms and the directional linear condensation of aromatic molecules.

4. The method according to claim 1, characterized in that, The uniform magnetic field strength in step (3) is not less than 1 T, the heat treatment temperature is 380~420 ℃, and the heat treatment time is 2~10 h; the direction of the magnetic field is consistent with the flow direction of the asphalt during subsequent melt spinning.

5. The method according to claim 1, characterized in that, The softening point of the spinnable mesophase pitch obtained in step (3) is 260~300 ℃, the optical anisotropy content is greater than 98%, and the H / C atomic ratio is 0.35~0.50.

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