Method for preparing 2,4-dichlorotoluene by using modified molecular sieve to catalyze 2,5-dichlorotoluene

Abstract

The present invention discloses a method for preparing 2,4-dichlorotoluene by catalytic isomerization of 2,5-dichlorotoluene using a modified molecular sieve. From the perspective of green chemistry, the relevant molecular sieve is modified to catalyze the preparation of 2,4-dichlorotoluene from 2,5-dichlorotoluene. This method has the advantages of environmental friendliness, high selectivity, good stability, low cost, easy availability, and easy recovery, and can form a green and sustainable industrial process. The method of the present invention comprises the following steps: loading the modified molecular sieve into the constant temperature zone of a fixed-bed reactor under an inert atmosphere, and feeding a mixture of 2,5-dichlorotoluene and a diluent to the preheater of the fixed-bed reactor through a constant flow pump. Under the action of high temperature in the preheater, the raw material mixture is rapidly vaporized, and its vapor is continuously carried into the catalyst bed in the constant temperature zone by a flowing inert carrier gas for isomerization reaction. After condensation by circulating water, the product is collected; wherein the modified molecular sieve is a modified HZSM-5 molecular sieve.

Description

Technical Field

[0001] The present invention relates to a method for preparing 2,4-dichlorotoluene from 2,5-dichlorotoluene, and more specifically to a method for preparing 2,4-dichlorotoluene by catalytic reaction of 2,5-dichlorotoluene with a modified molecular sieve. Background Art

[0002] 2,4-Dichlorotoluene is a very important fine chemical raw material for organic synthesis, which is used to prepare pharmaceuticals, pesticides, dyes and other chemical products. Through nitration, reduction, fluorination, chlorination, hydrolysis and acyl chlorination, 2,4-dichlorotoluene can form 2,4-dichloro-5-fluorobenzoyl chloride, which can be used as the raw material for the super antibacterial drug ciprofloxacin in medicine. After being oxidized by potassium permanganate and then acidified, 2,4-dichlorotoluene can be prepared into 2,4-dichlorobenzoic acid. As a raw material, it is further used in the production of the fungicide diniconazole in pesticides, and is also an intermediate for the herbicides pyrazoxyfen and pyrazolate. In medicine, it can be used to synthesize the antimalarial drug atabrine hydrochloride and the non-mercurial diuretic furosemide (frusemide). It can be seen that the wide application of 2,4-dichlorotoluene makes the preparation of 2,4-dichlorotoluene of great significance.

[0003] Existing preparation methods of 2,4-dichlorobenzyl chloride all have many deficiencies. For example: using p-chlorotoluene and chlorine as raw materials and Lewis acid as a catalyst to synthesize 2,4-dichlorotoluene, this method has a low selectivity for 2,4-dichlorotoluene in industrial production, a large amount of acidic wastewater, and many impurities in the generated chlorides. Taking 3-chloro-p-toluidine as a raw material, diazotization reaction is carried out with sodium nitrite to obtain a salt of 2-chloro-4-diazo-methyl, and the diazo group is converted into chlorine with cuprous chloride, and after separation and purification, 2,4-dichlorotoluene is obtained. This method requires diazotization and Sandmeyer reaction, with a complex process, low product yield, and the disadvantage of a large amount of wastewater, and is not suitable for industrialization. In the existing synthesis processes at home and abroad, the production process is complex, there is a large amount of acidic wastewater, and the catalyst consumption is large. In addition, in the production of 2,4-dichlorotoluene, it can generally be directly prepared by one-step chlorination method, using chlorine as the chlorine source. The process flow is not green and environmentally friendly, and 2,5-dichlorotoluene is produced as a by-product during the production process. 2,5-dichlorotoluene is only used as a solvent in industrial organic synthesis, its uses are extremely single, and the product added value is low. 2,5-dichlorotoluene is low in cost and easy to obtain. If it is used as a raw material for the isomerization method to prepare 2,4-dichlorotoluene, it has advantages that cannot be compared with the synthesis method and has broad application prospects. Using isomerization catalysis to prepare 2,4-dichlorotoluene from 2,5-dichlorotoluene in a fixed bed, on the one hand, using a fixed bed reactor for preparation, the operation is simple, the energy consumption is small, the generation of three wastes is less, and the reaction conditions are mild. On the other hand, isomerization can complete the preparation of 2,4-dichlorotoluene in the absence of chlorine, which not only ensures safety but also does not require the treatment of tail gas, improving the environmental protection performance.

[0004] At present, the production processes for preparing 2,4-dichlorotoluene by isomerization catalysis of 2,5-dichlorotoluene all adopt preparation methods using Lewis acids such as SbCl 3 , ZrCl 4 etc. as catalysts. For example, Patent CN106540738A discloses a method for catalyzing 2,5-dichlorotoluene. Using Hβ loaded with 10% Cu as a catalyst, 2,5-dichlorotoluene is reacted under the conditions of 350 °C, a raw material volume space velocity of 0.2h -1 , and normal pressure and hydrogenation. The conversion rate of 2,5-dichlorotoluene is 45.08%, and the selectivity of 2,6-dichlorotoluene is 14.25%. The raw materials of this reaction are easy to obtain, but the selectivity and yield of 2,6-dichlorotoluene in the reaction are both low. Patent CN105198696A discloses a method for preparing 2,6-dichlorotoluene by H-type molecular sieve catalyzing 2,5-dichlorotoluene. Hβ, HY, and HZSM-5 molecular sieves are selected for the isomerization reaction, and Hβ is selected as the optimal catalyst. At a reaction temperature of 350 °C, a volume space velocity of 0.6h -1 , and a carrier gas of N 2Under the optimal condition of 10 ml / min, the conversion rate of 2,5-dichlorotoluene is 30.9%, and the selectivity of 2,6-dichlorotoluene is 45.7%. The selectivity and yield of the target product 2,6-dichlorotoluene are both relatively low. In the patent JP2000191563A, a mixture of dichlorotoluenes is used as the raw material, and an acidic zeolite is used as the catalyst. Among them, mordenite or β-zeolite with a pore structure composed of 12 oxygen atoms is used as the preferred catalyst. 2,5-dichlorotoluene can be effectively converted into 2,4-dichlorotoluene and 2,6-dichlorotoluene; while under the condition of low content of 2,5-dichlorotoluene, 2,4-dichlorotoluene and 2,6-dichlorotoluene will be converted into 2,5-dichlorotoluene. For the methods described in the above prior art, the reaction conditions are harsh, the yield and product selectivity are not high, and it is difficult to reach the actual application level.

[0005] Solid acid catalysts mainly show the advantages of little corrosion to equipment, easy separation and recovery, good high-temperature stability, can be activated and regenerated, and reduce waste emissions, which is convenient for continuous chemical operation. It is an environmentally friendly catalyst. Therefore, how to develop a method with simple steps, high product yield, high selectivity, especially a highly efficient catalyst for isomerization catalysis of 2,5-dichlorotoluene to prepare 2,4-dichlorotoluene is the problem to be solved at present. Summary of the Invention

[0006] In view of the problems and deficiencies of the prior art, the present invention provides a method for preparing 2,4-dichlorotoluene by catalytic isomerization of 2,5-dichlorotoluene with a modified molecular sieve. From the perspective of green chemistry, this method modifies the relevant molecular sieve to catalyze the preparation of 2,4-dichlorotoluene from 2,5-dichlorotoluene, and has the advantages of environmental friendliness, high selectivity, good stability, low cost, easy availability and easy recovery, etc., and can form a green and sustainable industrial process.

[0007] The present invention is realized by the following technical solutions:

[0008] The method for preparing 2,4-dichlorotoluene by catalytic isomerization of 2,5-dichlorotoluene with a modified molecular sieve of the present invention includes the following steps:

[0009] The modified molecular sieve is loaded into the constant temperature area of the fixed bed reactor and under an inert atmosphere. The mixture of 2,5-dichlorotoluene and the diluent is transported to the preheater of the fixed bed reactor by a constant flow pump. Under the action of the high temperature of the preheater, the raw material mixture is quickly vaporized, and its vapor is continuously carried into the catalyst bed in the constant temperature area by a flowing inert carrier gas for isomerization reaction. After being condensed by circulating water, the product is collected; wherein the modified molecular sieve is a modified HZSM-5 molecular sieve.

[0010] The method for preparing 2,4-dichlorotoluene by catalytic reaction of the above-mentioned modified molecular sieve with 2,5-dichlorotoluene according to the present invention further comprises the technical solution that the modification method of the modified HZSM-5 molecular sieve is hydrothermal modification method or metal impregnation method. A further technical solution is that the steps of the hydrothermal modification method are as follows: loading the HZSM-5 molecular sieve into the middle constant temperature zone of a fixed-bed reactor, drying for more than 3 h under an inert atmosphere; raising the temperature of the fixed-bed reactor to 500-700 °C, and after the temperature is stabilized, controlling the water inflow with a constant flow pump, and setting the liquid hourly space velocity to 1-10 h -1 , treating under 100% water vapor for 1-20 h, and then cooling to room temperature under an inert gas flow to obtain the HZSM-5 molecular sieve modified by the hydrothermal modification method. A further technical solution is that the temperature of the middle constant temperature zone of the fixed-bed reactor is 300-500 °C; the liquid hourly space velocity is 2-4 h -1 ; the treatment time under water vapor is 4-8 h.

[0011] A further technical solution may also be that the steps of the metal impregnation method are as follows: dissolving a metal salt modifier in water to obtain a mixed solution, adding the HZSM-5 molecular sieve into the prepared mixed solution, fully stirring and mixing evenly, standing for impregnation for more than 9 h and then drying, and then calcining in a muffle furnace. After cooling to room temperature, taking out to obtain the HZSM-5 molecular sieve modified by the metal impregnation method. Among them, a further technical solution is that the metal salt modifier is one or a combination of ferric nitrate, zinc nitrate, nickel nitrate, cobalt nitrate, copper nitrate, lanthanum nitrate, cerium nitrate, neodymium nitrate and praseodymium nitrate; the impregnation duration is 9-48 h; the calcination temperature is 300 °C-900 °C; the metal loading in the HZSM-5 molecular sieve modified by the metal impregnation method is 0.1%-10%. A further technical solution is that the calcination temperature is 400 °C-600 °C; the metal loading in the HZSM-5 molecular sieve modified by the metal impregnation method is 1.5%-3.5%.

[0012] The method for preparing 2,4-dichlorotoluene by catalytic reaction of the above-mentioned modified molecular sieve with 2,5-dichlorotoluene according to the present invention further comprises the technical solution that the inert gas is one or more of Ar, Ne and N 2 .

[0013] The method for preparing 2,4-dichlorotoluene by catalytic reaction of the above-mentioned modified molecular sieve with 2,5-dichlorotoluene according to the present invention further comprises the technical solution that the temperature of the preheater is 100-500 °C; the isomerization reaction temperature of the catalyst bed is 200-600 °C, and the reaction pressure is atmospheric pressure - 4 MPa; the volume hourly space velocity of the liquid transported by the constant flow pump is 0.1-10 h -1; The diluent is one or a combination of benzene, chlorobenzene, and toluene, and the volume ratio of 2,5-dichlorotoluene to the diluent is 1:1 - 20:1. A further technical solution is that the temperature for the isomerization reaction in the catalyst bed layer is 300 - 500 °C; the volumetric space velocity of the liquid transported by the constant flow pump is 1 - 5 h -1 ; The volume ratio of the 2,5-dichlorotoluene to the diluent is 1:1 - 5:1.

[0014] The present invention has the following beneficial effects compared with the prior art:

[0015] (1) As a simple and effective modification method, hydrothermal treatment causes dealumination of the HZSM-5 molecular sieve framework, which can remove non-framework aluminum species in the pores, dredge the pores, and make the reactant molecules more easily contact the active sites in the pores. It modulates the properties such as pore volume, specific surface area, and acid amount of the HZSM-5 molecular sieve, changes the catalytic performance of the HZSM-5 catalyst, and achieves the purpose of adjusting the product components and improving the selectivity of the target product. At the same time, the non-noble metal modified HZSM-5 molecular sieve catalyst improves the activity of the catalyst while reducing costs and increasing economic benefits.

[0016] (2) In the traditional process scheme, the preparation of 2,4-dichlorotoluene requires the participation of chlorine gas, which is not only dangerous but also harmful to the environment. The isomerization preparation process, however, does not require the participation of chlorine gas, ensuring safety and eliminating the need for tail gas treatment, thus enhancing environmental protection. At the same time, compared with the pressurized isomerization conditions in industrial production, the process conditions of this reaction are less dangerous and more mild. The gas-solid reaction mode results in good conversion and high selectivity of the raw materials. The preparation is carried out in a fixed-bed reactor, with simple operation, low energy consumption, less generation of three wastes, and mild reaction conditions.

[0017] (3) The molecular sieve used in this method is easy to obtain, has a low cost, is non-toxic and harmless, has good catalytic activity, and can be regenerated by certain means for recycling, forming a green and sustainable industrial process.

[0018] (4) In industrial production, 2,5-dichlorotoluene is only used as a solvent, which is easy to obtain and has a low cost. By using it as a raw material for the isomerization reaction to prepare 2,4-dichlorotoluene with higher added value, its economic value is fully exploited, realizing the high-value utilization of organic raw materials. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of the initial adsorption configuration of reactants and products on the surface of HZSM-5

[0020] Figure 1In: a is 2,5 - dichlorotoluene, b is 2,4 - dichlorotoluene, c is 2,6 - dichlorotoluene, e is 2,3 - dichlorotoluene, f is 3,4 - dichlorotoluene

[0021] Figure 2 are the schematic diagrams of the adsorption configurations of the reactants and products after structure optimization

[0022] Figure 2 In: a is 2,5 - dichlorotoluene, b is 2,4 - dichlorotoluene, c is 2,6 - dichlorotoluene, e is 2,3 - dichlorotoluene, f is 3,4 - dichlorotoluene

[0023] Figure 3 is the thermodynamic step diagram of the reactants and products

[0024] Figure 4 is the process diagram of kinetic calculation Specific implementation mode

[0025] Example 1 (Modification of HZSM - 5 molecular sieve by hydrothermal modification)

[0026] Load the HZSM - 5 molecular sieve into a fixed - bed reactor, and dry it at 400 °C for 3 h in an N 2 atmosphere. Then, with a liquid hourly space velocity of 3 h -1 , a treatment time of 6 h, and steam temperatures of 500 °C, 600 °C, and 700 °C respectively for treatment, and cool down under N 2 to obtain HZSM - 5 molecular sieve catalysts of types HZ - 500, HZ - 600, and HZ - 700 after hydrothermal treatment respectively.

[0027] Example 2 (Preparation of 2,4 - dichlorotoluene by molecular sieve - catalyzed 2,5 - dichlorotoluene)

[0028] Taking the modified HZSM - 5 molecular sieve catalysts after different hydrothermal treatments in Example 1 as an example, load the modified molecular sieve into the constant - temperature region of a fixed - bed reactor and under an inert atmosphere. The mixture of 2,5 - dichlorotoluene and a diluent is transported to the pre - heater of the fixed - bed reactor through a constant - flow pump. Under the high - temperature action of the pre - heater, the raw material mixture vaporizes rapidly, and its vapor is continuously carried into the catalyst bed layer in the constant - temperature region by a flowing inert carrier gas for isomerization reaction. After condensation by circulating water, the product is collected. Among them, the pre - heater temperature is 200 °C, the reaction temperature is 400 °C, the volume ratio of 2,5 - dichlorotoluene to benzene is 5:1, and the liquid hourly space velocity is 2.5 h -1 under the conditions of gas - phase isomerization reaction of 2,5 - dichlorotoluene, and the reaction results are shown in Table 1.

[0029] Table 1 Raw material conversion rate and product selectivity of catalysts with different hydrothermal modification temperatures

[0030]

[0031] Note: 2,4-DCT is 2,4-dichlorotoluene, 3,4-DCT is 3,4-dichlorotoluene, BZ is benzene, TL is toluene, MX is xylene, BC is chlorotoluene, and CBZ is chloroethylbenzene.

[0032] When the hydrothermal modification temperature is 600 °C, the reaction temperature is 400 °C, and the volumetric space velocity of the raw material liquid is 2.5 h -1 , and the feed ratio is 5:1, the conversion rate of 2,5-dichlorotoluene is 22.31%; the selectivity of 2,4-dichlorotoluene is 79.32%; the selectivity of 3,4-dichlorotoluene is 11.66%, the selectivity of benzene is 1.66%, the selectivity of toluene is 0.70%, the selectivity of xylene is 0.26%, the selectivity of chlorotoluene is 4.68%, the selectivity of chloroethylbenzene is 0.50%, and others are 1.32%. The results show that the modified HZSM-5 molecular sieve has good selectivity for the target product in the isomerization reaction of 2,5-dichlorotoluene. The selectivity of 2,4-dichlorotoluene is increased by 13%, there are fewer impurity by-products, and the conversion rate is also higher.

[0033] Example 3 (Preparation of 2,4-dichlorotoluene by molecular sieve catalysis of 2,5-dichlorotoluene)

[0034] Taking the HZ-600 modified molecular sieve catalyst prepared in Example 1 as an example, the modified molecular sieve was loaded into the constant temperature zone of a fixed bed reactor and under an inert atmosphere. A mixture of 2,5-dichlorotoluene and a diluent was transported to the preheater of the fixed bed reactor by a constant flow pump. Under the action of the high temperature of the preheater, the raw material mixture was rapidly vaporized, and its vapor was continuously carried into the catalyst bed in the constant temperature zone by a flowing inert carrier gas for isomerization reaction. After condensation by circulating water, the product was collected. The temperature of the preheater was 200 °C. The gas-phase isomerization reaction of 2,5-dichlorotoluene was carried out at a reaction temperature of 400 °C and a volume ratio of 2,5-dichlorotoluene to benzene of 5:1. The reaction results are shown in Table 2.

[0035] Table 2 Raw material conversion rate and product selectivity at different volumetric space velocities of the raw material liquid

[0036]

[0037] It can be seen from Table 2 that when the space velocity is low, the reaction time becomes longer, and the raw material reacts fully with the catalyst, increasing the conversion rate of 2,5-dichlorotoluene. However, at low space velocity, 2,5-dichlorotoluene is more likely to undergo side reactions and disproportionation reactions, and there are more by-product impurities, and the product selectivity cannot be improved; appropriately increasing the space velocity can prevent the formation of other impurities and improve the selectivity of the main product. Therefore, 2.5 h -1 is an optimal condition for the liquid hourly space velocity.

[0038] Example 4 (Preparation of 2,4-dichlorotoluene from 2,5-dichlorotoluene catalyzed by molecular sieve)

[0039] Taking the HZ-600 modified molecular sieve catalyst prepared in Example 1 as an example, the modified molecular sieve was loaded into the constant temperature zone of a fixed bed reactor and under an inert atmosphere. The mixture of 2,5-dichlorotoluene and diluent was transported to the preheater of the fixed bed reactor by a constant flow pump. Under the high temperature of the preheater, the raw material mixture was rapidly vaporized, and its vapor was continuously carried into the catalyst bed in the constant temperature zone by the flowing inert carrier gas for isomerization reaction. After condensation by circulating water, the product was collected. Among them, the preheater temperature was 200 °C, the reaction temperature was 400 °C, the volume ratio of 2,5-dichlorotoluene to benzene was 5:1, and the space velocity of the reaction liquid was 2.5 h -1 Under the conditions of -1, the gas-phase isomerization reaction of 2,5-dichlorotoluene was carried out, and the reaction results are shown in Table 3.

[0040] Table 3 Raw material conversion rate and product selectivity of different diluents

[0041]

[0042] It can be seen from Table 3 that the conversion rate and selectivity of unmodified HZSM-5 after adding diluent are equivalent to those of HZ-600 without adding diluent after hydrothermal modification, indicating that hydrothermal modification can effectively improve the product selectivity by redistributing the pore structure and acidity of the catalyst; when the hydrothermally modified HZSM-600 catalyst is added with diluent, it is further verified that the use of diluent can effectively further improve the selectivity of the product and reduce the formation of other by-products. Therefore, the selectivity is the best when diluent is added and after hydrothermal modification.

[0043] Example 5 (Modification of HZSM-5 molecular sieve by metal impregnation method and preparation of 2,4-dichlorotoluene from 2,5-dichlorotoluene)

[0044] The HZSM-5 molecular sieve was modified by the metal impregnation method. Ferric nitrate was weighed and dissolved in distilled water. The HZSM-5 molecular sieve was added to the prepared mixed solution, stirred thoroughly and mixed evenly, left standing overnight for 9 h, dried in an oven at 160 °C, and finally calcined in a muffle furnace at 450 °C. After cooling to room temperature, it was taken out and stored to prepare Fe / HZ molecular sieve catalysts with loadings of 1.5%, 2.5%, and 3.5% respectively.

[0045] Taking the modified zeolite catalyst with different loadings of transition metal iron as an example, the modified zeolite catalyst was loaded into the constant-temperature zone of a fixed-bed reactor and placed under an inert atmosphere. A mixture of 2,5-dichlorotoluene and a diluent was delivered to the preheater of the fixed-bed reactor by a constant-flow pump. Under the high temperature of the preheater, the raw material mixture vaporized rapidly, and its vapor was continuously carried into the catalyst bed in the constant-temperature zone by a flowing inert carrier gas for isomerization reaction. After condensation by circulating water, the product was collected. The temperature of the preheater was 200 °C. Under the conditions of a reaction temperature of 400 °C, a volume ratio of 2,5-dichlorotoluene to benzene of 3:1, and a liquid hourly space velocity of 1.5 h -1 The gas-phase isomerization reaction of 2,5-dichlorotoluene was carried out, and the reaction results are shown in Table 4.

[0046] Table 4 Raw material conversion and product selectivity of modified catalysts with different Fe loadings

[0047]

[0048]

[0049] When the metal Fe loading was 3.5%, the reaction temperature was 400 °C, and the liquid hourly space velocity of the raw material was 1.5 h -1 , and the mixing ratio was 3:1, the conversion rate of 2,5-dichlorotoluene was 33.75%; the selectivity of 2,4-dichlorotoluene was 61.67%, the selectivity of 3,4-dichlorotoluene was 12.36%, the selectivity of benzene was 5.45%, the selectivity of toluene was 4.95%, the selectivity of xylene was 1.63%, the selectivity of chlorotoluene was 8.00%, the selectivity of chloroethylbenzene was 1.96%, and others were 3.98%.

[0050] The experimental results showed that when the Fe loading was 3.5%, the modified HZSM-5 zeolite had good selectivity for the target product and catalytic activity in the isomerization reaction of 2,5-dichlorotoluene, and the obtained yield was the largest.

[0051] Example 6 (Modification of HZSM-5 zeolite by metal impregnation method and catalysis for the preparation of 2,4-dichlorotoluene from 2,5-dichlorotoluene)

[0052] HZSM-5 zeolite was modified by the metal impregnation method. Cerium nitrate was weighed and dissolved in distilled water. The HZSM-5 zeolite was added to the prepared mixed solution, stirred well and mixed evenly, left to stand overnight for 9 h, dried in an oven at 160 °C, and finally calcined in a muffle furnace at 450 °C. After cooling to room temperature, it was taken out and stored. The Ce / HZ molecular sieve catalysts with loadings of 1.5%, 2.5%, and 3.5% were obtained.

[0053] Taking the modified molecular sieve catalyst with different cerium rare earth metal loadings as an example, the modified molecular sieve was loaded into the constant temperature zone of a fixed-bed reactor and placed under an inert atmosphere. A mixture of 2,5-dichlorotoluene and a diluent was transported to the preheater of the fixed-bed reactor by a constant flow pump. Under the high temperature of the preheater, the raw material mixture vaporized rapidly, and its vapor was continuously carried by a flowing inert carrier gas into the catalyst bed in the constant temperature zone for isomerization reaction. After condensation by circulating water, the product was collected. The temperature of the preheater was 200 °C. Under the conditions of a reaction temperature of 400 °C, a volume ratio of 2,5-dichlorotoluene to benzene of 3:1, and a liquid hourly space velocity of 1.5 h -1 The gas-phase isomerization reaction of 2,5-dichlorotoluene was carried out under the conditions of

[0054] Table 5 Conversion of raw materials and selectivity of products of modified catalysts with different Ce loadings

[0055]

[0056] When the metal Ce loading was 2.5%, the reaction temperature was 400 °C, and the liquid hourly space velocity of the raw material liquid was 1.5 h -1 , and the ingredient ratio was 3:1, the conversion rate of 2,5-dichlorotoluene was 35.94%; the selectivity of 2,4-dichlorotoluene was 56.93%, the selectivity of 3,4-dichlorotoluene was 10.10%, the selectivity of benzene was 7.74%, the selectivity of toluene was 6.04%, the selectivity of xylene was 2.17%, the selectivity of chlorotoluene was 7.12%, the selectivity of chloroethylbenzene was 1.39%, and other was 8.51%.

[0057] The experimental results showed that when the Ce loading was 2.5%, the modified HZSM-5 molecular sieve had good catalytic activity and product selectivity for the isomerization reaction of 2,5-dichlorotoluene, and the obtained yield was the largest.

[0058] Comparative Example 1

[0059] The unmodified HZSM-5 molecular sieve was loaded into a fixed-bed reactor and dried at 400 °C for 3 h in an N 2 atmosphere. Taking the unmodified HZSM-5 catalyst as an example, the gas-phase isomerization reaction of 2,5-dichlorotoluene was carried out under the conditions of a reaction temperature of 400 °C, a volume ratio of 2,5-dichlorotoluene to benzene of 5:1, and a liquid hourly space velocity of 2.5 h -1 .

[0060] The conversion rate of 2,5-dichlorotoluene was 18.17%; the selectivity of 2,4-dichlorotoluene was 66.36%, the selectivity of 3,4-dichlorotoluene was 12.65%, the selectivity of benzene was 5.60%, the selectivity of toluene was 4.00%, the selectivity of xylene was 3.22%, the selectivity of chlorotoluene was 4.18%, the selectivity of chloroethylbenzene was 1.98%, and impurities were 2.01%.

[0061] In addition, the adsorption structures of the reactants and products were optimized for their structures and single-point energies were calculated respectively, and the thermodynamic process of the isomerization of 2,5-dichlorotoluene to 2,4-dichlorotoluene, 2,6-dichlorotoluene, 2,3-dichlorotoluene and 3,4-dichlorotoluene on the HZSM-5 catalyst was studied. The initial adsorption configurations of the reactants and products on the HZSM-5 surface were set as Figure 1 shown, and the adsorption configurations of the reactants and products after structure optimization were as Figure 2 shown.

[0062] Compared with the initial placement positions, the centroid of dichlorotoluene is located between the Lewis acid and Bronsted acid and both move towards Al, playing the role of the adsorption active center in the isomerization reaction, which is consistent with the conclusion of the synergistic catalysis of Bronsted acid and Lewis acid in the experiment. Among the adsorption configurations of the four products, the average distances between 3,4-dichlorotoluene and 2,4-dichlorotoluene and the active sites of HZSM-5 are relatively close, indicating that the interaction between the two is the strongest on the surface. However, the distance between 2,6-dichlorotoluene and the active center is elongated, suggesting that the energy of this product may be higher and it is a thermodynamically unfavorable configuration.

[0063] By analyzing the reaction potential energy surface through density functional theory (DFT), according to the energy data on the reaction path, the reaction potential energy surface can be drawn to reveal the energy relationship between the reactants and products. To further compare the thermodynamic energies of the four products with respect to the reactants, the energies of the reactants and products before and after the reaction were calculated. According to the adsorption energy calculation formula:

[0064] E ads =E AB -E A -E B (1)

[0065] The energies of each structure were calculated by formula (1), and the adsorption energies of 2,5-dichlorotoluene, 2,4-dichlorotoluene, 2,6-dichlorotoluene, 2,3-dichlorotoluene and 3,4-dichlorotoluene were -0.038 eV, -0.580 eV, 0.399 eV, -0.039 eV and -0.462 eV respectively. Finally, the thermodynamic step diagram of the reactants and products was as Figure 3 shown.

[0066] Through the analysis of the thermodynamic steps Figure 3 it can be seen that among the products, 2,4-dichlorotoluene is the most thermodynamically favorable product, and 2,6-dichlorotoluene is thermodynamically less favorable.

[0067] The process of the catalyst catalyzing the reactants to be converted into products is controlled by both thermodynamics and kinetics. Thermodynamics determines whether the reaction can proceed theoretically, but the energy barrier of the reaction is controlled by the kinetic process. Therefore, it is necessary to calculate the kinetic process of the isomerization reaction to explore the isomerization mechanism and the role of acid sites in the whole process.

[0068] In the calculation of the kinetic process of the isomerization reaction, through the transition state search, it can be found that the reaction mechanism of HZSM-5 catalyzing the hydrogenation of 2,5-DCT is that the proton on the catalyst attacks the C-C bond connected to the benzene ring and the methyl group, causing the methyl group to fall off. After the charge rearrangement, it is concentrated on the C connected to the Cl at the ortho position, and the methyl group replaces the chlorine at the ortho position. After the ortho-chlorine undergoes transposition, H + returns to the HZSM-5 catalyst again. The whole process is a two-step reaction, involving two transition states, namely the transposition of the methyl group to the ortho position under proton attack (TS1) and the transposition of chlorine to the ortho position (TS2). The isomerization transformation is initiated by the proton in the bridging hydroxyl group in the Al-OH-Si structure attacking the methyl group, which is consistent with the experimental result of the synergistic catalysis of B acid and L acid.

[0069] Figure 4 (a) shows the kinetic calculation process of dichlorotoluene isomerization. The energy barrier of the whole reaction path is 1.974 eV, which appears in the process of the proton attacking the methyl group to form TS1. This step has a relatively high endothermic value and is the rate-determining step. Figure 4 (e) and 4(f) are the IRC path analysis diagrams. The results show that only one unique structure pointing to the product can be obtained when searching for the transition state configuration from both the forward and reverse directions, which proves the accuracy and uniqueness of the transition state search structure. Through the above simulation of the isomerization reaction of dichlorotoluene, the feasibility of this method is confirmed.

Claims

1. A method for preparing 2,4-dichlorotoluene by catalytic reaction of 2,5-dichlorotoluene with a modified molecular sieve, characterized in that, it comprises the following steps: The modified molecular sieve is placed in the constant temperature zone of a fixed-bed reactor and under an inert atmosphere. A mixture of 2,5-dichlorotoluene and a diluent is delivered to the preheater of the fixed-bed reactor through a constant flow pump. Under the high temperature of the preheater, the raw material mixture vaporizes rapidly, and its vapor is continuously carried by a flowing inert carrier gas into the catalyst bed in the constant temperature zone for isomerization reaction. After condensation by circulating water, the product is collected; wherein the modified molecular sieve is a modified HZSM-5 molecular sieve; the modification method of the modified HZSM-5 molecular sieve is a hydrothermal modification method, and the steps are as follows: The HZSM-5 molecular sieve is placed in the middle constant temperature zone of the fixed-bed reactor and dried for more than 3 h under an inert atmosphere; the temperature of the fixed-bed reactor is raised to 500 - 700 °C. After the temperature is stabilized, the water inflow is controlled by a constant flow pump, and the liquid hourly space velocity is set to 1 - 10 h -1 , and it is treated under 100% water vapor for 1 - 20 h, and then cooled to room temperature under an inert gas flow to obtain the HZSM-5 molecular sieve modified by the hydrothermal modification method.

2. The method for preparing 2,4-dichlorotoluene by catalytic reaction of 2,5-dichlorotoluene with a modified molecular sieve according to claim 1, characterized in that, The temperature of the middle constant temperature zone of the fixed bed reactor is 300 - 500 °C; the liquid hourly space velocity is 2 - 4 h -1 ; the treatment time under water vapor is 4 - 8 h.

3. The method for preparing 2,4-dichlorotoluene by catalytic reaction of 2,5-dichlorotoluene with a modified molecular sieve according to claim 1, characterized in that, The inert gas described is one or more of Ar, Ne, and N 2 in the above list.

4. The method for preparing 2,4-dichlorotoluene by catalytic reaction of 2,5-dichlorotoluene with a modified molecular sieve according to claim 1, characterized in that, The temperature of the preheater is 100 - 500 °C; the temperature for the isomerization reaction in the catalyst bed is 200 - 600 °C, and the reaction pressure is atmospheric pressure - 4 MPa; the volumetric space velocity during the liquid delivery by the constant flow pump is 0.1 - 10 h -1 ; the diluent is one or a combination of benzene, chlorobenzene, and toluene, and the volume ratio of 2,5 - dichlorotoluene to the diluent is 1:1 - 20:

1.

5. The method for preparing 2,4-dichlorotoluene by catalytic reaction of 2,5-dichlorotoluene with a modified molecular sieve according to claim 4, characterized in that, The isomerization reaction temperature of the catalyst bed described is 300 - 500 °C; the volumetric space velocity of the liquid delivered by the constant flow pump is 1 - 5 h -1 ; the volume ratio of 2,5 - dichlorotoluene to the diluent is 1:1 - 5:1.

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