A continuous process for the preparation of low odor dimethylcyclohexylamine

Abstract

The application belongs to the field of fine chemical organic synthesis, and provides a continuous preparation method of low odor dimethylcyclohexylamine, which comprises the following steps: cyclohexanone and dimethylamine are respectively pumped into a preheater by a metering pump, hydrogen is filled in, and after being mixed and preheated, they are fed into a trickle bed reactor filled with diatomite supported nickel-chromium catalyst; after reaction, the material is flowed into a condenser through a pipeline for condensation, and is separated through a gas-liquid separator; the condensate is separated from water in the product through a water separator, and then is fed into a rectifying tower for separation and purification to obtain product N,N-dimethylcyclohexylamine; a trace amount of hydrogen bond production agent is added to the product after rectification. Different from traditional batch reaction kettles and hydrogenation catalysts of noble metals such as palladium and platinum, the application uses commercially available diatomite supported nickel-chromium as a catalyst, and uses cyclohexanone and secondary amine as raw materials; the process is simple, the raw material conversion rate is high, the active component chromium in the catalyst can inhibit the conversion of cyclohexanone into cyclohexanol, the product selectivity is high, and the raw material cost is low.

Description

Technical Field

[0001] This invention belongs to the field of fine chemical organic synthesis, specifically relating to a continuous preparation method of low-odor dimethylcyclohexylamine. Background Technology

[0002] N,N-Dimethylcyclohexylamine, with its unique alicyclic tertiary amine structure, is primarily used as a catalyst in the production of polyurethane foam. Currently, there are two main methods for preparing N,N-dimethylcyclohexylamine. First, in industrial production, cyclohexanone and dimethylamine are used as raw materials in a batch reactor, where N,N-dimethylcyclohexylamine is obtained through reductive amination under the action of a hydrogenation catalyst. For example, patent CN 115814820 discloses a supported hydrogenation catalyst with selenium-palladium composite as the active component, used to catalyze the reaction of cyclohexanone and dimethylamine to prepare N,N-dimethylcyclohexylamine, achieving a selectivity of up to 99.7%. The advantage of this method is the availability and low cost of the raw materials; the disadvantage is that cyclohexanone easily hydrogenates to cyclohexanol under high pressure during the reaction, which has a similar boiling point to the product N,N-dimethylcyclohexylamine, making separation difficult and resulting in a product purity that is difficult to achieve. Second, N,N-dimethylcyclohexylamine is prepared by N-methylation reaction using cyclohexylamine and formaldehyde (or methanol) as raw materials under the action of a catalyst. Patent CN 111333520 discloses a method for reacting formaldehyde with cyclohexylamine using a supported palladium catalyst to obtain N,N-dimethylcyclohexylamine with a purity of up to 99.88%. While this invention avoids the hydrogenation of cyclohexanone to cyclohexanol, the use of an aqueous formaldehyde solution as the methylating agent inevitably generates N-methylcyclohexylamine, whose boiling point is close to that of N,N-dimethylcyclohexylamine, increasing the difficulty of subsequent separation and purification. Furthermore, cyclohexylamine and formaldehyde are significantly more expensive than cyclohexanone and dimethylamine, and the use of the precious metal palladium as the active component of the catalyst significantly increases raw material costs, resulting in a lack of market competitiveness.

[0003] With the increasing application of polyurethane in the home and automotive industries, customers are demanding higher quality polyurethane catalysts. The demand for low-odor N,N-dimethylcyclohexylamine with a purity of over 99.9% is also increasing dramatically, along with stricter requirements regarding odor. Therefore, it is necessary to develop a new method for preparing low-odor N,N-dimethylcyclohexylamine to meet market demands. Summary of the Invention

[0004] To address the problems existing in current production processes, this invention provides a method for preparing low-odor N,N-dimethylcyclohexylamine. This method differs from traditional batch reactors by employing a continuous reductive amination process in a trickle-bed reactor to prepare N,N-dimethylcyclohexylamine. The reaction conditions are mild, no precious metal catalysts such as palladium or platinum are required, and the product selectivity is high, reducing the difficulty of separation and purification. This method can produce high-purity, low-odor products that meet customer requirements.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a low-odor N,N-dimethylcyclohexylamine, characterized by comprising the following steps: Step 1: Using cyclohexanone and dimethylamine as raw materials, in a continuous production process, metering pumps preheat the high-temperature gaseous products generated in the trickle bed reactor in their respective heat exchangers. The preheated raw materials are then mixed with hydrogen gas and fed into a trickle bed reactor filled with diatomaceous earth-supported nickel-chromium catalyst. The reaction pressure is controlled at 0.1–3.0 MPa, the reaction temperature at 80–160 °C, and the liquid hourly space velocity (LHSV) of cyclohexanone is 0.1–1.0 h⁻¹. -1 The molar ratio of cyclohexanone to dimethylamine was controlled at 1:1.0~1.5, and the catalyst dosage in the trickle bed reactor was 0.01~3.0 mg / L. 3 Ketones / (h·m 3 (Catalyst), with hydrogen accounting for 10-30% of the molar ratio of the mixed gas.

[0006] The catalyst, calculated by weight percentage of the total supported catalyst, contains 25-50% nickel, 25-50% nickel oxide, and 10-25% chromium trioxide, with diatomaceous earth as the carrier, and the content of diatomaceous earth is 25-50%.

[0007] Step 2: After the reaction, the material is cooled by a heat exchanger, condensed by a condenser, and separated into gas and liquid phases. The gas phase is then pumped into a preheater by a compressor or circulating pump for recycling. The liquid phase is separated and purified in a distillation column after water separation to obtain the product N,N-dimethylcyclohexylamine. The dimethylamine and cyclohexanone separated by distillation are returned to the intermediate storage tank, and then preheated with supplemented hydrogen, dimethylamine, and cyclohexanone in a preheater before entering the trickle bed reactor for reaction, thereby improving the utilization rate of raw materials.

[0008] Step 3: Add a trace amount of hydrogen bonding agent to the product after distillation. By forming hydrogen bonds with dimethylcyclohexylamine, it prevents N,N-dimethylcyclohexylamine from escaping from the liquid phase to the gas phase, thereby reducing the product odor.

[0009] Preferably, in step 1, the reaction pressure is 0.1-1.0 MPa, the reaction temperature is 80-100℃, and the liquid hourly space velocity (LHSV) of cyclohexanone is 0.2-0.8 h⁻¹. -1 The molar ratio of cyclohexanone to dimethylamine is 1:1.01-1.20, and the catalyst dosage is 0.5~1.0 mg / L. 3 Ketones / (h·m 3 (Catalyst), the molar ratio of hydrogen in the mixed gas is 10-20%.

[0010] Preferably, hydrogen bonding agents such as crown ethers, azacrown ethers, phenanthroline, urea, and thiourea are added to the product after distillation, with an addition amount of 10-100 ppm.

[0011] This invention addresses the purity and odor issues of N,N-dimethylcyclohexylamine currently available on the market by providing a novel method for preparing low-odor N,N-dimethylcyclohexylamine. Using cyclohexanone and dimethylamine as raw materials, and commercially available diatomaceous earth-supported nickel-chromium as a catalyst, a trickle bed reactor is employed for a continuous gas-liquid-solid three-phase reaction. Cyclohexanone and dimethylamine are separately metered into a preheater, mixed with hydrogen gas, and then fed into the trickle bed reactor. The resulting mixture is condensed and separated into gas and liquid phases. The gas phase is recirculated back into the trickle bed via a compressor for reuse, while the liquid phase product undergoes distillation for purification. N,N-dimethylcyclohexylamine is collected as the final product. Unreacted cyclohexanone and dimethylamine are returned to the feed inlet, replenished with additional cyclohexanone and dimethylamine at a specific molar ratio, and then fed back into the trickle bed reactor with hydrogen gas to continue the reaction.

[0012] The beneficial effects of this invention are that, unlike traditional batch reactors and precious metal hydrogenation catalysts such as palladium and platinum, it uses commercially available diatomaceous earth-supported nickel-chromium as a catalyst and cyclohexanone and secondary amines as raw materials to continuously synthesize N,N-dimethylcyclohexylamine through reductive amination. The process is simple, the raw material conversion rate is high, the active component chromium in the catalyst can inhibit the conversion of cyclohexanone to cyclohexanol, the product selectivity is high, the reaction temperature and pressure are low, the energy consumption is low, the raw material cost is low, and it is suitable for industrial production. Attached Figure Description

[0013] Figure 1 This is a reaction flow diagram for this application.

[0014] In the diagram: 1. Cyclohexanone storage tank; 2. Dimethylamine storage tank; 3. Metering pump; 4. Heat exchanger; 5. Trickling bed reactor; 6. Preheater; 7. Hydrogen storage tank; 8. Condenser; 9. Gas-liquid separator; 10. Compressor. Detailed Implementation

[0015] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments. Example 1:

[0016] This invention uses a commercially available diatomaceous earth-supported nickel-chromium catalyst. Before the reaction, ceramic packing material with a diameter of 4 mm and a length of 10 mm is first loaded into a 316L stainless steel tubular reactor with an inner diameter of 35 mm and a length of 740 mm. The packing height is 50 mm. Then, a cylindrical diatomaceous earth-supported nickel-chromium catalyst with a diameter of 3 mm and a height of 3 mm is loaded into the tubular reactor. The catalyst loading height is 650 mm, and the catalyst loading amount is 420 g.

[0017] Before the reaction, the catalyst needs to be activated. First, nitrogen gas is introduced three times to purge the air from the entire reactor system, followed by three times with hydrogen gas. The system is then pressure tested and purged. Once the system is leak-free, a programmed temperature increase begins, while hydrogen is continuously introduced to activate the catalyst. The temperature is initially increased to 120°C at a rate of 20°C / h, while hydrogen is introduced at a rate of 400-500 ml / min through a hydrogen flow meter. Once the temperature stabilizes at 120°C, it is held for 1 hour. Then, the temperature is increased to 230°C at the same rate and held for another hour. After activation, the temperature is lowered to the reaction temperature, and the reaction pressure is adjusted by regulating the hydrogen flow rate.

[0018] After catalyst activation, feeding begins. Cyclohexanone in cyclohexanone storage tank 1 and dimethylamine in dimethylamine storage tank 2 are fed into the corresponding heat exchangers 4 at a molar ratio of 1:1.05 via metering pump 3 for initial preheating. The heat exchange medium channels of the two heat exchangers 4 are connected in series, and the heat exchange medium is the high-temperature finished product generated in the trickle bed reactor 5. After initial heat exchange, the raw materials enter the preheater 6 for preheating. At the same time, hydrogen is introduced into the preheater 6 from the hydrogen storage tank 7. After the hydrogen is mixed evenly with the raw materials in the preheater 6, it enters the catalyst bed in the trickle bed reactor 5 for reaction. The reaction temperature is controlled at 95℃. After the reaction, the mixture first undergoes heat exchange in two heat exchangers 4, and then is cooled by a condenser 8. The condensate enters a gas-liquid separator 9. The non-condensable gas phase, mainly hydrogen, is drawn out by a compressor 10 and sent to a preheater to participate in the mixing reaction. The liquid product is first separated from the water in a water separator before entering a distillation process for further separation, yielding the final product N,N-dimethylcyclohexylamine. Cyclohexanone and dimethylamine separated during distillation are collected in an intermediate storage tank and subsequently sent to the preheater to replenish hydrogen before continuing the reaction. A sample of the liquid product from the gas-liquid separator is taken for gas chromatography monitoring, confirming that the main component is N,N-dimethylcyclohexylamine with a content of 95.6%, while the contents of the raw materials cyclohexanol and dimethylamine are 3.1% and 1.3%, respectively. The purity of N,N-dimethylcyclohexylamine after distillation is 99.9%. Examples 2-8:

[0019] By changing the following reaction conditions in Example 1: space velocity of cyclohexanone, reaction temperature, reaction pressure, and molar ratio of cyclohexanol to dimethylamine, Examples 2-8 were obtained. Specific data are shown in Table 1.

[0020]

[0021] As shown in Table 1, the single-pass conversion rate of cyclohexanone is no less than 80% under the action of diatomaceous earth-supported nickel-chromium catalyst, especially at 95℃ and 0.8MPa, where the molar ratio of cyclohexanone to dimethylamine is 1:1.1 and the mass hourly space velocity of cyclohexanone is 0.2h⁻¹.-1 At this stage, the conversion rate of cyclohexanone can reach 95.4%, the selectivity of N,N-dimethylcyclohexylamine is higher than 99%, and the purity of the product after distillation can reach 99.9%. Examples 9-15:

[0022] To further reduce the product odor, we took 99.9% N,N-dimethylcyclohexylamine obtained in Example 5 and added 10-100 ppm of various hydrogen bonding agents, including crown ether, azacrown ether, phenanthroline, urea, and thiourea, to it for sensory evaluation to verify the odor change of N,N-dimethylcyclohexylamine after adding different hydrogen bonding agents. The specific test was as follows: First, we took 50ml capped glass bottles, numbered 1-8, and added 40ml of 99.9% N,N-dimethylcyclohexylamine. Then, we added 30 ppm of crown ether, azacrown ether, phenanthroline, urea, thiourea, 50 ppm of azacrown ether, and 100 ppm of azacrown ether, respectively. After mixing thoroughly, the bottles were capped and left to stand for 30 minutes for testing. The odor level was assessed using a six-level scoring system, from level 1 (no odor) to level 6 (strong, pungent odor). The odor intensity of the samples was determined by sensory evaluation by five professional odor evaluators, and the test results are shown in Table 2.

[0023]

[0024] Table 2 shows that adding a certain amount of hydrogen bonding agent can improve the odor of N,N-dimethylcyclohexylamine to some extent. The improvement is particularly significant when azirconium crown ether is added; after adding 50 ppm of azirconium crown ether, four out of five professional odor evaluators gave it an odor rating of Level 1, indicating a significantly lower odor than N,N-dimethylcyclohexylamine without added hydrogen bonding agent.

[0025] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A continuous preparation method for low-odor dimethylcyclohexylamine, using cyclohexanone and dimethylamine as raw materials, characterized in that, Includes the following steps: Step 1: Cyclohexanone and dimethylamine are separately metered into their respective heat exchangers for heating, then mixed in a preheater. Hydrogen gas is simultaneously introduced into the preheater. The preheated and mixed feedstock then enters a trickle bed reactor filled with diatomaceous earth-supported nickel-chromium catalyst. The reaction pressure is controlled at 0.1–3.0 MPa, the reaction temperature at 80–160 °C, and the liquid hourly space velocity (LHSV) of cyclohexanone is 0.1–1.0 h⁻¹. -1 The molar ratio of cyclohexanone to dimethylamine was controlled at 1:1.0~1.5, and the catalyst dosage in the trickle bed reactor was 0.01~3.0 mg / L. 3 Ketones / (h·m 3 (Catalyst), with hydrogen comprising 10-30% of the molar ratio of the mixed gas; Step 2: After the reaction, the material flows into the heat exchanger through the pipeline to exchange heat with cyclohexanone and dimethylamine in step 1, and is condensed by the condenser. The condensate is separated by the gas-liquid separator. The water at the bottom of the mixture in the gas-liquid separator is separated and sent to the distillation column for further purification to obtain the product N,N-dimethylcyclohexylamine. Step 3: Add a trace amount of hydrogen bonding agent to the product after distillation.

2. The continuous preparation method of low-odor dimethylcyclohexylamine according to claim 1, characterized in that: The catalyst, calculated by weight percentage of the total supported catalyst, contains 25-50% nickel, 25-50% nickel oxide, and 10-25% chromium trioxide, with diatomaceous earth as the carrier, and the content of diatomaceous earth is 25-50%.

3. The continuous preparation method of low-odor dimethylcyclohexylamine according to claim 1, characterized in that: The preferred reaction pressure is 0.1-1.0 MPa, the preferred reaction temperature is 80-100℃, and the preferred liquid hourly space velocity (LHSV) of cyclohexanone is 0.2-0.8 h⁻¹. -1 The preferred molar ratio of cyclohexanone to dimethylamine is 1:1.01-1.20, and the preferred catalyst dosage is 0.5-1.0 mg / L. 3 Ketones / (h·m 3 (Catalyst), the molar ratio of hydrogen in the mixed gas is preferably 10-20%.

4. The continuous preparation method of low-odor dimethylcyclohexylamine according to claim 1, characterized in that: The gas separated by the gas-liquid separator in step 2 is drawn out by the compressor and sent to the preheater for recirculation reaction.

5. The continuous preparation method of low-odor dimethylcyclohexylamine according to claim 1, characterized in that: The dimethylamine and cyclohexanone separated in step 2 enter the intermediate storage tank. After sampling and testing, hydrogen is added to the intermediate storage tank, and the mixture is preheated in the preheater before entering the trickle bed reactor to continue the reaction.

6. The continuous preparation method of low-odor dimethylcyclohexylamine according to claim 1, characterized in that: Hydrogen bonding agents that can be added to the distilled product include crown ethers, azacrown ethers, phenanthroline, urea, thiourea, etc., with an addition amount of 10-100 ppm.

7. The continuous preparation method of low-odor dimethylcyclohexylamine according to claim 1, characterized in that: The cyclohexanone and dimethylamine each correspond to a heat exchanger, and the heat exchange medium channels of the two heat exchangers are connected in series. The high-temperature gas generated by the trickle bed reactor enters the condenser for condensation after passing through the heat exchange channels of the two heat exchangers in sequence.

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