A method for preparing 6-aminocapronitrile from caprolactam

Abstract

The present invention discloses a method for preparing 6 - aminocapronitrile from caprolactam. This method adopts a two - stage amination reaction design, uses two catalysts with different performances respectively, and removes the water generated in the first amination reaction process through a combined process of heat exchange, gas - liquid separation and combined adsorption, breaking the reaction equilibrium. Through the secondary reaction in the second amination reactor, high conversion rate under high selectivity is finally achieved. In the method of the present invention, the conversion rate of caprolactam is greater than 98%, and the selectivity of 6 - aminocapronitrile is greater than 99%. It reduces the caprolactam rectification and recovery process necessary due to the low conversion rate of caprolactam in the current process, avoids the corresponding separation material loss and energy consumption, reduces the unit consumption and energy consumption of the whole preparation process, and is more suitable for large - scale industrial production.

Description

Technical Field

[0001] The present invention belongs to the field of organic chemical industry, relates to the technical field of hexamethylenediamine preparation, and particularly relates to a method for preparing 6-aminocapronitrile, a key intermediate of hexamethylenediamine, from caprolactam. Background Art

[0002] Nylon 66 is widely used due to its excellent properties. The key technologies of the industrial chain of its core raw material, hexamethylenediamine, have been monopolized by foreign countries. In order to break the monopoly and achieve domestic substitution, in recent years, many domestic enterprises and research institutes have developed a new route for preparing 6-aminocapronitrile by gas-phase catalytic amination dehydration of caprolactam and then synthesizing hexamethylenediamine by hydrogenation of 6-aminocapronitrile. The core of this route is the technology for preparing 6-aminocapronitrile by gas-phase catalytic amination dehydration of caprolactam.

[0003] The main reaction equation for preparing 6-aminocapronitrile is as follows:

[0004]

[0005] This reaction occurs on the catalyst surface. Since it is an equilibrium reaction, the single-pass conversion rate of caprolactam is generally below 90%. Increasing the reaction temperature can further increase the conversion rate, but due to the presence of water, caprolactam will self-polymerize and 6-aminocapronitrile will attack the carbonyl carbon of caprolactam to form ring-opening by-products, resulting in a rapid decrease in reaction selectivity. Existing catalyst technologies and synthesis processes have their own characteristics, but all have the problem of low single-pass conversion rate of caprolactam. To ensure selectivity, the single-pass conversion rate of some processes is even controlled below 60%. In order to reduce the unit consumption, the unreacted caprolactam must be rectified, recovered and reused. Thus, the separation and purification equipment is complex. Since caprolactam is unstable at high temperatures and is more likely to deteriorate under the catalysis of impurity amines, the material loss and energy consumption during the recovery and separation of caprolactam remain high. Summary of the Invention

[0006] Based on the problems existing in the above-mentioned prior art, the main object of the present invention is to provide an efficient method for preparing 6-aminocapronitrile, which can increase the conversion rate of caprolactam to more than 98% while ensuring that the selectivity is greater than 99%, thereby eliminating the subsequent rectification separation process for recovering caprolactam, avoiding the separation loss of caprolactam and the corresponding separation and recovery energy consumption, and thus reducing the unit consumption and energy consumption of the entire preparation process, and being more suitable for large-scale industrial production.

[0007] Based on this, the present invention provides a method for preparing 6-aminocapronitrile from caprolactam, and the method includes the following steps:

[0008] S1: A mixture of caprolactam and ammonia is preheated and then introduced into a first reactor for the first catalytic amination dehydration reaction to obtain a first reaction product, and the temperature of the first catalytic amination dehydration reaction is 400 - 500 °C;

[0009] S2: After the first reaction product is cooled, gas-liquid separation is carried out. The separated ammonia gas is recovered, and the separated liquid is dehydrated to obtain a dehydrated liquid mixture.

[0010] S3: After preheating the dehydrated liquid mixture described in step S2 and the separated and recovered ammonia gas, they enter the second reactor for a second catalytic amination dehydration reaction to obtain a second reaction product, namely 6-aminohexanenitrile; the temperature of the second catalytic amination dehydration reaction is 300-400 °C (excluding 400 °C).

[0011] In a specific embodiment, in step S1, the preheating temperature of the mixed gas is 400-500 °C, preferably 400-420 °C.

[0012] In a specific embodiment, in step S1, the catalyst loaded in the first reactor needs to have excellent catalytic performance at high temperature (400-500 °C), and it can be a silica catalyst modified by a combination of ytterbium nitrate and cesium nitrate.

[0013] In a specific embodiment, in the catalyst loaded in the first reactor, in terms of molar dosage, the molar ratio of ytterbium nitrate to cesium nitrate is 1:(5-20), preferably 1:(10-20), more preferably 1:(10-15), such as 1:10, 1:11, 1:12, 1:13, 1:14 or 1:15.

[0014] In a specific embodiment, in the catalyst loaded in the first reactor, the total mass of ytterbium nitrate and cesium nitrate accounts for 15%-20% of the mass of silica.

[0015] In a specific embodiment, the preparation method of the silica catalyst modified by a combination of ytterbium nitrate and cesium nitrate loaded in the first reactor is as follows:

[0016] 1) Prepare a mixed aqueous solution of ytterbium nitrate and cesium nitrate in a formulated amount for later use;

[0017] 2) Slowly add the formulated amount of fumed silica to the mixed aqueous solution described in step 1);

[0018] 3) Add the binder low-sodium pseudoboehmite; preferably, the dosage of the binder is 0.5-2% of the mass of the fumed silica, preferably 1-2%;

[0019] 4) Immerse at 60-100 °C for 6-10 h, preferably immerse at 80 °C for 8 h;

[0020] 5) After evaporation to dryness and drying, bake at 100-150 °C, preferably 120 °C for shaping, and then calcine at 1200 °C for 10-14 h, preferably 12 h, to obtain the catalyst.

[0021] In a specific embodiment, in step S1, the temperature of the first catalytic amination dehydration reaction is preferably 400-420 °C.

[0022] In a specific embodiment, in step S1, the molar ratio of caprolactam to ammonia is 1:10-40, and the preferred molar ratio is 1:17-30.

[0023] In a specific embodiment, in step S1, in the first reactor, the mass hourly space velocity of caprolactam is 0.5-10 h -1 , preferably 2.5-5 h -1 .

[0024] In a specific embodiment, in step S1, the reaction pressure is 0.1-1 MPa, preferably 0.3-0.8 MPa.

[0025] In a specific embodiment, in step S1, in the first catalytic amination dehydration reaction, the conversion rate of caprolactam is controlled above 85%, and the selectivity of 6-aminocapronitrile is greater than 99.5%.

[0026] In a specific embodiment, in step S2, the first reaction product is cooled to 50-150 °C, preferably cooled to 80-100 °C. Within this temperature range, sufficient separation of ammonia can be achieved while more water remains in the reaction solution for subsequent adsorption separation.

[0027] In a specific embodiment, in step S2, the first reaction product can be cooled through a heat exchanger. Preferably, the excess heat of the first reaction product can be transferred to the raw materials caprolactam and ammonia in step S1 through the heat exchanger to avoid energy waste.

[0028] In a specific embodiment, in step S2, the separated liquid can be dehydrated by an absorbent in a dehydrator to obtain a dehydrated liquid mixture.

[0029] In a specific embodiment, the absorbent can be molecular sieves of different sizes such as 3A, 4A, 5A, etc., or can also be absorbent reagents such as calcium oxide, anhydrous calcium chloride, anhydrous magnesium sulfate, anhydrous sodium sulfate, etc.

[0030] In a specific embodiment, in step S3, the preheating temperature is 300-350 °C, preferably 320-330 °C.

[0031] In a specific embodiment, in step S3, the temperature of the second catalytic amination dehydration reaction is preferably 320-330 °C.

[0032] In a specific embodiment, in step S3, the catalyst loaded in the second reactor needs to have excellent catalytic performance at a relatively low temperature (300-400 °C), and it can be a ZSM-5 molecular sieve catalyst with a specific silicon-aluminum ratio loaded with a specific proportion of ytterbium nitrate, calcium nitrate, and copper nitrate. For its preparation process, refer to CN202111086850.4.

[0033] Specifically, the preparation method of the ZSM-5 molecular sieve is as follows:

[0034] 1) Prepare a mixed aqueous solution of ternary active components of ytterbium nitrate, calcium nitrate, and copper nitrate for later use;

[0035] 2) Weigh the ZSM-5 silicon-aluminum molecular sieve and add the molecular sieve to the mixed aqueous solution of the ternary active components in step 1);

[0036] 3) Immerse at a temperature of 25-80 °C for 4-10 h;

[0037] 4) Evaporate to dryness, dry, form, and calcine to obtain the catalyst;

[0038] Among them, the mass ratio range of ytterbium nitrate:calcium nitrate:copper nitrate is 1:(1-20):(1-20), and the mass ratio of the total mass of ytterbium nitrate, calcium nitrate, and copper nitrate to the mass of the ZSM-5 silicon-aluminum molecular sieve is (0.01-0.4):1.

[0039] In a specific embodiment, in step S3, the loading amount of the catalyst loaded in the second reactor is 50%-300% of the loading amount of the first reactor, preferably 100%-200%.

[0040] In a specific embodiment, in step S3, in the second reactor, the mass hourly space velocity of caprolactam is 0.5-10 h -1 , preferably 2.5-5 h -1 .

[0041] In a specific embodiment, the reaction pressure in the second reactor is 0.1-1 MPa, preferably 0.3-0.8 MPa.

[0042] As described in the background technology, the main reaction of preparing 6-aminocapronitrile by gas phase catalytic amination dehydration of caprolactam is a reversible reaction, and the molar amount of water generated by the reaction is as much as the product 6-aminocapronitrile. When the conversion rate of caprolactam reaches 90%, if you want to further increase the conversion rate, the reaction process conditions will become more harsh, and the selectivity of 6-aminocapronitrile will show a trend of rapid decline. 6-aminocapronitrile self-polymerization, 6-aminocapronitrile attacking caprolactam carbonyl carbon to generate open-loop by-products, water promotes caprolactam ring-opening polymerization and other side reactions such as deamination and dehydrogenation are difficult to contain. For example, experiments show that when the content of 6-aminocapronitrile in the reaction system is large enough (the conversion rate of caprolactam is greater than 95%), 6-aminocapronitrile self-polymerization obviously occurs under the reaction conditions of 400° C., and only the dimerization product content reaches 1%-3%, which seriously affects the selectivity of the overall reaction.

[0043] The present invention finds that the key to solving this contradiction is: first, remove the byproduct water generated by the reaction from the system to break the original balance; second, react again at a lower temperature to maintain high selectivity while greatly improving the conversion rate. The technical solution of the present invention adopts three consecutive steps. In step S1, the first gas-phase catalytic amination dehydration reaction is carried out, the conversion rate of caprolactam is controlled between 85% and 90%, and the selectivity of 6-aminocapronitrile is greater than 99.5%. Step S1 adopts a modified silicon-based catalyst with excellent catalytic performance at high temperature, and adopts a suitable reaction temperature (400-500°C) and a high space velocity control method to maintain the high selectivity of 6-aminocapronitrile while increasing the conversion rate of caprolactam as much as possible. The first reaction product of step S1 can transfer excess heat to the reaction raw material of step S1 through heat exchange in step S2, avoiding energy waste, and at the same time, a large amount of ammonia generated by gas-liquid separation in step S2 can be collected and recycled for step S3. The liquid after cooling in step S2 enters a dehydrator, and removes the water generated in step S1 by adsorption dehydration, so as to prepare for entering the second reactor of step S3. The dehydrated material is passed into the second reactor of step S3, and the second reactor is loaded with a modified ZSM-5 molecular sieve catalyst with excellent catalytic performance at a relatively low temperature (300-400° C., excluding 400° C.). The material re-establishes a new equilibrium in the second reactor at a relatively low reaction temperature, and the content of caprolactam in the second reaction product is finally controlled to be below 2%, and the selectivity of 6-aminocapronitrile is maintained at more than 99%, thereby avoiding the old process of continuing to recover caprolactam after subsequent separation of 6-aminocapronitrile, and eliminating the energy consumption and material loss of recovering and purifying caprolactam. DETAILED DESCRIPTION

[0044] The present invention is specifically described below by way of examples, but the present invention is not limited by any of the examples.

[0045] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods.

[0046] For the materials, reagents, etc. used in the following examples, unless otherwise specified, they can all be obtained through commercial channels.

[0047] Reagents:

[0048] Caprolactam: CAS No.: 105 - 60 - 2, industrial qualified product, Cangzhou Xuyang Chemical Co., Ltd.

[0049] Liquid ammonia: CAS No.: 7664 - 41 - 7, 99%, Heze Huacheng Gas Factory.

[0050] Ytterbium nitrate pentahydrate: CAS No.: 35725 - 34 - 9, 99.9%, Aladdin Reagent Co., Ltd.

[0051] Cesium nitrate: CAS No.: 7789 - 18 - 6, analytical pure, Shanghai Chemical Reagent Co., Ltd.

[0052] Fumed silica: CAS No.: 112945 - 52 - 5, specific surface area greater than 400m 2 / g, Aladdin Reagent Co., Ltd.

[0053] Calcium nitrate tetrahydrate: CAS No.: 13477 - 34 - 4, 99%, Shanghai Chemical Reagent Co., Ltd.

[0054] Copper nitrate trihydrate: CAS No.: 10031 - 43 - 3, analytical pure, Shanghai Chemical Reagent Co., Ltd.

[0055] ZSM - 5 molecular sieve: sodium type, specific surface area greater than 350m 2 / g, Ost Catalysis Materials Dalian Co., Ltd.

[0056] Low - sodium pseudoboehmite: CAS No.: 63957 - 70 - 0, 68 - 72%, Hangzhou Zhihuajie Technology Co., Ltd.

[0057] Instruments:

[0058] Analytical balance: Mettler, one - ten - thousandth.

[0059] Oven: Vacuum drying oven, Guangdong Hongzhan Technology Co., Ltd.

[0060] Muffle furnace: Rated temperature 1400°C, Henan Sant Special Furnace Industry Technology Co., Ltd.

[0061] Extrusion machine: Changzhou Qianjiang Drying Equipment Engineering Co., Ltd.

[0062] Example 1

[0063] a. Preparation of silica catalyst modified by ytterbium nitrate and cesium nitrate combination:

[0064] It should be noted that in the translation, the "Aladdin Reagent Co., Ltd." in the translation of "Ytterbium nitrate pentahydrate" and "Fumed silica" is a common reagent company name for reference. If there is a more specific and accurate company name in the original text, it should be used accordingly. Also, for the "Ost Catalysis Materials Dalian Co., Ltd.", it is translated according to the name provided. If there is an error in the original name, it may need to be adjusted according to the correct information.Prepare a mixed aqueous solution: Add precisely weighed 2.5 g of ytterbium nitrate pentahydrate and 12 g of cesium nitrate to 400 ml of deionized water and dissolve. Weigh 90 g of fumed silica (i.e., silica prepared by the vapor phase method) and slowly add it to the above mixed aqueous solution. Weigh 1.3 g of binder low-sodium pseudo-boehmite and add it to the above aqueous solution. Keep it warm and shake at 80 °C for 8 h. Evaporate the excess water under reduced pressure with a water pump. The residual gel is formed into a four-leaf shape. The formed solid is baked at 120 °C for 4 h and calcined in a muffle furnace at 1200 °C for 12 h to obtain a modified silica catalyst.

[0065] b. Preparation of modified ZSM-5 molecular sieve:

[0066] For the detailed preparation method, refer to CN202111086850.4. The specific preparation method is as follows:

[0067] Prepare a ternary active component mixed aqueous solution: Add accurately weighed 1.0 g of ytterbium nitrate, 8.0 g of calcium nitrate, and 15.0 g of copper nitrate to 300 ml of deionized water. Weigh 100 g of ZSM-5 molecular sieve with a silica-alumina ratio of 100. Add the molecular sieve carrier to the ternary active component mixed aqueous solution. Keep it warm and shake at 65 °C for 6 h. Evaporate the excess water under reduced pressure with a water pump. The residual solid is baked at 110 °C for 4 h, dried and formed, and calcined in a muffle furnace at 600 °C for 5 h to obtain the finished modified ZSM-5 molecular sieve catalyst.

[0068] c. Preparation of 6-aminohexanenitrile:

[0069] S1: The mixed gas of caprolactam and ammonia (molar ratio 1:20) is preheated to 410 °C and then enters the first reactor filled with the silica catalyst modified by the above combination of ytterbium nitrate and cesium nitrate for the first gas-phase catalytic amination dehydration reaction to obtain the first reaction product. Among them, the mass space velocity of caprolactam is 5 h -1 , the reaction pressure is 0.5 MPa, and the bed temperature is 410 °C.

[0070] S2: The first reaction product enters the heat exchanger, exchanges heat with the raw materials caprolactam and ammonia in step S1, and cools down to 100 °C. It is separated into gas and liquid in the gas-liquid separator. The gas is recovered and recycled and compressed; the liquid enters the dehydrator and is dehydrated by adsorbing with the water absorbent anhydrous calcium chloride to obtain a dehydrated liquid mixture;

[0071] S3: The dehydrated liquid mixture and the separated and recovered ammonia are preheated to 320 °C and then enter the second reactor filled with the above modified ZSM-5 molecular sieve for the second gas-phase catalytic amination dehydration reaction to obtain the 6-aminohexanenitrile product. Among them, the mass space velocity of caprolactam is 5 h -1 , the reaction pressure is 0.5 MPa, the reaction temperature is 320 °C, and the loading amount of ZSM-5 molecular sieve is 1.5 times that of the catalyst loading amount in the first reactor.

[0072] Example 2

[0073] Except that the reaction temperature in the first reactor is 420 °C, other implementation manners are the same as those in Example 1.

[0074] Example 3

[0075] Except that the mass hourly space velocity of caprolactam in the first reactor is 2.5 h -1 , and the reaction temperature is 400 °C, other implementation manners are the same as those in Example 1.

[0076] Example 4

[0077] Except that the mass hourly space velocity of caprolactam in the first reactor is 2.5 h -1 , and the reaction temperature is 430 °C, other implementation manners are the same as those in Example 1.

[0078] Example 5

[0079] Except that the molar ratio of caprolactam to ammonia is 1:10, other implementation manners are the same as those in Example 1.

[0080] Example 6

[0081] Except that the molar ratio of caprolactam to ammonia is 1:17, other implementation manners are the same as those in Example 1.

[0082] Example 7

[0083] Except that the molar ratio of caprolactam to ammonia is 1:30, other implementation manners are the same as those in Example 1.

[0084] Example 8

[0085] Except that the reaction temperature in the second reactor is 330 °C, other implementation manners are the same as those in Example 1.

[0086] Example 9

[0087] Except that the reaction temperature in the second reactor is 310 °C, other implementation manners are the same as those in Example 1.

[0088] Comparative Example 1

[0089] Compared with Example 1, the reaction temperature in the first reactor is 390 °C, and other implementation manners are the same as those in Example 1.

[0090] Comparative Example 2

[0091] Compared with Example 1, the dehydrator is cancelled, that is, the dehydration link is cancelled, and other implementation manners are the same as those in Example 1.

[0092] Comparative Example 3

[0093] Compared with Example 1, the catalyst loaded in the second reactor is the same as that in the first reactor, both being modified silica. The reaction temperature of the second reactor is 410°C, and other implementation manners are the same as those in Example 1.

[0094] Comparative Example 4

[0095] Compared with Example 1, the catalyst loaded in the second reactor is the same as that in the first reactor, both being modified silica. The reaction temperature of the second reactor is 360°C, and other implementation manners are the same as those in Example 1.

[0096] Comparative Example 5

[0097] Compared with Example 1, the catalyst loaded in the second reactor is the same as that in the first reactor, both being modified silica. The reaction temperature of the second reactor is 430°C, and other implementation manners are the same as those in Example 1.

[0098] Test Example

[0099] The caprolactam conversion rate, 6 - aminocapronitrile selectivity of the first reaction product, and the total caprolactam conversion rate and 6 - aminocapronitrile selectivity of the second reaction product in the above - mentioned examples and comparative examples were detected and calculated. The calculation formulas are as follows:

[0100] Caprolactam conversion rate of the first reaction product = (1 - mass of caprolactam in the first reaction product / feed amount of caprolactam) * 100%;

[0101] 6 - Aminocapronitrile selectivity of the first reaction product = (mass of 6 - aminocapronitrile in the first reaction product / 112) / (caprolactam conversion rate of the first reaction product * feed amount of caprolactam / 113) * 100%;

[0102] Total caprolactam conversion rate of the second reaction product = (1 - mass of caprolactam in the second reaction product / feed amount of caprolactam) * 100%;

[0103] 6 - Aminocapronitrile selectivity of the second reaction product = (mass of 6 - aminocapronitrile in the second reaction product / 112) / (total caprolactam conversion rate of the second reaction product * feed amount of caprolactam / 113) * 100%;

[0104] The results are shown in Table 1.

[0105] Table 1.

[0106]

[0107]

[0108] In the above-described embodiment, after the by-product water generated by the first reaction is shifted out of the system, the second catalytic amination is carried out, and it is ensured that the second catalytic amination reacts again at a lower temperature, to significantly improve the conversion rate while maintaining high selectivity. From the result of Comparative Example 2, if there is no dehydration after the first reaction, in the second step reaction, the conversion rate and selectivity are all far below the expected value, proving that water has a huge impact on the reaction balance, and significantly causes the generation of side reactions. From the result of the above-mentioned Comparative Example 3, dehydration after the first reaction, and the second reaction maintains the same temperature as the first reaction, can obtain higher caprolactam conversion, but 6-aminocapronitrile poor selectivity. Further improving the second reaction temperature in Comparative Example 5 will cause a significant reduction in selectivity, and the higher the temperature, the worse the selectivity.

[0109] The first high-temperature reaction and the second low-temperature reaction need to match the catalyst, and the catalytic reaction temperature of the actual reaction also needs to be suitable for the catalyst. For example, in the first reaction of Comparative Example 1, the modified silica catalyst with good high-temperature adaptability as in Examples 1-3 is used, but the catalytic temperature does not reach the optimal catalytic temperature of the catalyst. The conversion rate of the first reaction is low, only 80%. Although the selectivity of more than 99% can be achieved in the second reactor, the total conversion rate is only 96%; in Example 4, the temperature of the first reactor is higher, and the conversion rate of the first reaction exceeds 90%, reaching more than 92%, but the selectivity of the first reaction is slightly lower, at 97.8%. After the second reaction, the selectivity is reduced to 96.1%. At the same time, combined with the results of Comparative Example 4, if a catalyst with excellent performance at high temperature is used in the second reaction, the requirements of conversion rate and selectivity cannot be met regardless of high temperature or lower temperature. The second reaction can only use a catalyst with excellent catalytic performance at a lower temperature.

[0110] Examples 5-7 further investigated the effect of the ratio of ammonia to caprolactam on the selectivity of the reaction. When the molar ratio of ammonia to caprolactam was as low as 10, the reaction selectivity was low, indicating that the ratio of ammonia to caprolactam needed to be controlled during the reaction.

[0111] From the above analysis, in order to ensure high conversion rate, water must be removed before the second reactor, and the temperature of the second reactor cannot be too high. The second reactor needs to be loaded with a catalyst with excellent catalytic performance at a relatively low temperature to ensure that the residual caprolactam is fully converted while curbing the side reaction at a low temperature to ensure selectivity, otherwise the deterioration side reactions such as the self-polymerization of 6-aminocapronitrile cannot be suppressed. At the same time, the first reactor is loaded with a catalyst with excellent catalytic performance and strong water resistance at high temperature to improve the conversion rate as much as possible, to ensure that the caprolactam conversion rate at the outlet of the first reactor is higher (generally required to be greater than 85%), and more than 85% is reached in Examples 1-3, and finally the high conversion rate and high selectivity of the second reaction solution are ensured.

[0112] The above is only used to illustrate the present invention. For those skilled in the art, the present invention may have various combinations and equivalent substitutions. Any modification and improvement within the spirit and principle of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for preparing 6 - aminocapronitrile from caprolactam, the method comprises the following steps: S1: The mixed gas of caprolactam and ammonia is preheated and then introduced into the first reactor for the first catalytic amination dehydration reaction to obtain a first reaction product. The temperature of the first catalytic amination dehydration reaction is 400 - 420 °C; S2: After the first reaction product is cooled, gas - liquid separation is carried out. The separated ammonia is recycled, and the separated liquid is dehydrated to obtain a dehydrated liquid mixture; S3: The dehydrated liquid mixture in step S2 and the recycled ammonia are preheated and then enter the second reactor for the second catalytic amination dehydration reaction to obtain a second reaction product, namely 6 - aminocapronitrile. The temperature of the second catalytic amination dehydration reaction is 320 - 330 °C, wherein, in step S1, the preheating temperature of the mixed gas is 400 - 420 °C; the catalyst loaded in the first reactor is a silica catalyst modified by a combination of ytterbium nitrate and cesium nitrate; in the catalyst modified by a combination of ytterbium nitrate and cesium nitrate, the molar ratio of ytterbium nitrate to cesium nitrate is 1:(5 - 20); in the catalyst modified by a combination of ytterbium nitrate and cesium nitrate, the total mass of ytterbium nitrate and cesium nitrate accounts for 15% - 20% of the mass of silica; in step S3, the preheating temperature is 320 - 330 °C; in step S3, the catalyst loaded in the second reactor is a ZSM - 5 molecular sieve catalyst loaded with ytterbium nitrate, calcium nitrate, and copper nitrate; in step S3, the loading amount of the catalyst loaded in the second reactor is 50% - 300% of the loading amount of the first reactor; In step S3, in the second reactor, the mass space velocity of caprolactam is 0.5 - 10 h -1 ; and / or, the reaction pressure in the second reactor is 0.1 - 1 MPa.

2. The method according to claim 1, wherein, in the catalyst modified by a combination of ytterbium nitrate and cesium nitrate, the molar ratio of ytterbium nitrate to cesium nitrate is 1:(10 - 20).

3. The method according to claim 1, wherein, in the catalyst modified by a combination of ytterbium nitrate and cesium nitrate, the molar ratio of ytterbium nitrate to cesium nitrate is 1:(10 - 15).

4. The method according to claim 1, wherein, the preparation method of the silica catalyst modified by a combination of ytterbium nitrate and cesium nitrate is as follows: 1) Prepare a mixed aqueous solution of ytterbium nitrate and cesium nitrate in a formulated amount for later use; 2) Slowly add the formulated amount of fumed silica into the mixed aqueous solution in step 1); 3) Add the binder low - sodium pseudoboehmite; the amount of the binder is 0.5% - 2% of the mass of the fumed silica; 4) Immerse at 60 - 100 °C for 6 - 10 h; 5) After evaporation to dryness and drying, bake at 100 - 150 °C to form a shape, and then calcine at 1200 °C for 10 - 14 h to obtain the catalyst.

5. The method according to claim 4, wherein, the preparation method of the silica catalyst modified by a combination of ytterbium nitrate and cesium nitrate is as follows: 1) Prepare a mixed aqueous solution of ytterbium nitrate and cesium nitrate in a formulated amount for later use; 2) Slowly add the formulated amount of fumed silica into the mixed aqueous solution in step 1); 3) Add binder low-sodium pseudo-boehmite; the dosage of the binder is 1-2% of the mass of the fumed silica; 4) Impregnate at 80 °C for 8 h; 5) After evaporation to dryness and drying, bake at 120 °C to form a shape, and then bake at 1200 °C for 12 h to obtain the catalyst.

6. According to the method described in claim 1, it is characterized in that, in step S1, in step S1, the molar ratio of caprolactam to ammonia is 1:17-30; And / or, in step S1, in the first reactor, the mass hourly space velocity of caprolactam is 0.5 - 10 h -1 ; and / or, in step S1, the reaction pressure is 0.1-1 MPa; and / or, in step S1, in the first catalytic amination dehydration reaction, the conversion rate of caprolactam is controlled above 85%, and the selectivity of 6-aminocapronitrile is greater than 99.5%.

7. According to the method described in claim 6, it is characterized in that, in step S1, In the first reactor, the mass hourly space velocity of caprolactam is 2.5 - 5 h -1 ; and / or, in step S1, the reaction pressure is 0.3-0.8 MPa.

8. According to the method described in claim 1, it is characterized in that, in step S2, the first reaction product is cooled to 50-150 °C; in step S2, the first reaction product is cooled by a heat exchanger; in step S2, the separated liquid is adsorbed and dehydrated by a water absorbent in a dehydrator to obtain a dehydrated liquid mixture.

9. According to the method described in claim 8, it is characterized in that, in step S2, the first reaction product is cooled to 80-100 °C; the first reaction product transfers the excess heat of the first reaction product to the raw materials caprolactam and ammonia in step S1 through a heat exchanger; in step S2, the water absorbent is 3A, 4A, 5A molecular sieve, or calcium oxide, anhydrous calcium chloride, anhydrous magnesium sulfate, anhydrous sodium sulfate.

10. According to the method described in claim 1, it is characterized in that, the preparation method of the catalyst filled in the second reactor is as follows: 1) Prepare a mixed aqueous solution of ytterbium nitrate, calcium nitrate, and copper nitrate ternary active components for use; 2) Weigh ZSM-5 silica-alumina molecular sieve, and add the molecular sieve to the mixed aqueous solution of the ternary active components described in step 1); 3) Impregnate at a temperature of 25-80 °C for 4-10 h; 4) Evaporate to dryness, dry, form, and bake to obtain the catalyst; wherein, the mass ratio range of ytterbium nitrate:calcium nitrate:copper nitrate is 1:(1-20):(1-20), and the total mass of ytterbium nitrate, calcium nitrate, and copper nitrate and the mass ratio of the ZSM-5 silica-alumina molecular sieve is (0.01-0.4):

1.

11. According to the method described in claim 1, it is characterized in that, the filling amount of the catalyst filled in the second reactor in step S3 is 100%-200% of the filling amount of the first reactor.

12. According to the method described in claim 1, it is characterized in that, In step S3, in the second reactor, the mass hourly space velocity of caprolactam is 2.5 - 5 h -1 ; and / or, the reaction pressure in the second reactor is 0.3-0.8 MPa.

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