Rhodium-based catalyst system and method for preparing adipic acid by catalyzing tetrahydrofuran double carbonylation through rhodium-based catalyst system

Abstract

The invention discloses a method for preparing adipic acid by catalyzing tetrahydrofuran double carbonylation through a rhodium-based catalyst and a special catalyst system. The method comprises the following steps: placing tetrahydrofuran and a catalyst composed of a rhodium source, an iodine simple substance and a triphenylphosphine ligand in a mixed solvent composed of acetic acid and water, carrying out a carbonylation reaction at a certain temperature and pressure in a CO / H2 mixed atmosphere, and carrying out post-treatment to obtain adipic acid. The molar ratio of the active component of the catalyst to the accelerant to the ligand is 1: (4-6): (1.5-2.5). By optimizing the catalyst composition and reaction conditions, efficient and high-selectivity conversion of tetrahydrofuran is achieved, the conversion rate of tetrahydrofuran can reach 98.5% or above, and the selectivity of adipic acid can reach 87% or above. The process has the advantages that the reaction conditions are relatively mild, the catalyst can be repeatedly used, the product is easy to separate, few byproducts are generated and the like, and has a good industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of organic chemical synthesis and catalyst technology, specifically relating to a rhodium-based catalyst system for preparing adipic acid from tetrahydrofuran via a dicarbonylation reaction, its preparation method, and a method for synthesizing adipic acid using the catalyst. Background Technology

[0002] Adipic acid is an important chemical raw material, mainly used in the production of nylon 66, polyurethane, plasticizers, and food additives. The traditional industrial production method for adipic acid is the nitric acid oxidation of cyclohexane (or cyclohexanol / cyclohexanone). This method suffers from problems such as the use of highly corrosive nitric acid, the generation of large amounts of the greenhouse gas N2O, high energy consumption, and safety hazards, resulting in significant environmental pressure.

[0003] To overcome the shortcomings of traditional processes, researchers have developed a one-step route for synthesizing adipic acid from cyclic ethers (such as tetrahydrofuran) via carbonylation. This route boasts high atom economy, theoretically consuming only CO and H₂O with few byproducts, making it a green and promising synthetic pathway. The core of this route lies in an efficient and stable catalyst system. However, existing catalyst systems often suffer from low activity, poor selectivity and stability, low cost, and difficulty in recovery.

[0004] Therefore, developing a rhodium-based catalytic system that is highly active, selective, stable, and easy to recycle, and matching it with an optimized reaction process, is of great significance for promoting the industrialization of adipic acid preparation via tetrahydrofuran carbonylation.

[0005] Based on this, this application was developed. Summary of the Invention

[0006] The purpose of this invention is to address the problems in the existing technology of preparing adipic acid by tetrahydrofuran carbonylation, such as the difficulty in balancing catalyst activity and selectivity, high cost and poor recyclability of precious metal catalysts, and harsh reaction conditions, and to provide a novel rhodium-based catalyst system.

[0007] The present invention also provides a method for preparing the above-mentioned rhodium-based catalyst system and its application in the catalytic dicarbonylation of tetrahydrofuran to prepare adipic acid.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a rhodium-based catalyst system for the preparation of adipic acid by the dicarbonylation of tetrahydrofuran, which mainly consists of an active component, a promoter, and a ligand; the active component is a rhodium halide or a carbonyl halide, etc.; the promoter is elemental iodine; and the ligand is triphenylphosphine; wherein, the preferred molar ratio of rhodium, promoter, and ligand in the active component is 1:(4-6):(1.5-2.5). This preferred molar ratio ensures the efficient formation and stability of the rhodium active center, and elemental iodine serves both as a ring-opening promoter and as a component of the catalyst system.

[0009] Specifically, the active component is preferably selected from at least one of rhodium trichloride, dichlorotetracarbonyl dirhodium, etc.

[0010] Secondly, the present invention provides a method for preparing the above-mentioned rhodium-based catalyst system, which is simple and easy to implement: the active component, promoter and ligand are simply dissolved in a mixed solvent of acetic acid and water as the reaction medium in proportion and stirred evenly to form a highly active catalytic system in situ, without the need for pre-synthesizing complex rhodium complexes.

[0011] Specifically, in the mixed solvent, the volume ratio of acetic acid to water is (1-4):1.

[0012] Thirdly, the present invention provides an application of the above-mentioned rhodium-based catalyst system in the catalytic dicarbonylation of tetrahydrofuran to prepare adipic acid.

[0013] Fourthly, the present invention also provides a method for preparing adipic acid by catalytic dicarbonylation of tetrahydrofuran using the above-mentioned rhodium-based catalyst system, comprising the following steps: S1. Preparation of reaction raw materials: Tetrahydrofuran and rhodium-based catalyst (including active components, promoters and ligands) are added to a mixed solvent composed of acetic acid and water to form a homogeneous or suspension reaction solution; S2. Carbonylation reaction: The reaction solution is placed in a high-pressure reactor, and the air inside the reactor is replaced with an inert gas (such as nitrogen, argon, etc.). Then, a mixture of CO and H2 is introduced until the initial pressure is 2.5-3.5 MPa, wherein the volume ratio of CO to H2 is 1:1-2. The reaction system is then heated to 160-180°C and stirred at this temperature and pressure for 8-12 hours. S3. Product separation: After the reaction is completed, the reaction system is cooled and the pressure is slowly released to separate the catalyst. The reaction solution is then post-treated to obtain adipic acid.

[0014] Specifically, in step S1, the mass-to-volume ratio of the tetrahydrofuran to the mixed solvent composed of acetic acid and water is 1 g : (3-5) mL; the mass of the rhodium element in the rhodium-based catalyst accounts for 0.5%-1.5% of the mass of the tetrahydrofuran.

[0015] More preferably, in step S2, the reaction temperature is 165-175℃ and the initial pressure is 2.8-3.2 MPa.

[0016] Furthermore, in step S2, the stirring speed is preferably 400-600 r / min.

[0017] Specifically, in step S3, the separation catalyst includes a solid catalyst recovered through filtration or centrifugation (which can be recycled after washing and drying); the post-treatment includes: sequentially subjecting the filtrate after catalyst separation to vacuum distillation to remove the solvent to obtain a crude product, and then recrystallizing and purifying the crude product to obtain a high-purity adipic acid product; the solvent used for recrystallization is water.

[0018] The reaction mechanism of this invention is briefly described (non-limiting) as follows: In the catalytic system provided by this invention, the reaction may follow the following pathway: First, under acidic conditions and with the aid of an iodine promoter, tetrahydrofuran undergoes ring-opening to form a 1,4-diiodobutane intermediate; subsequently, this intermediate undergoes two consecutive carbonylation-hydrolysis reactions with CO / H2 under the action of an active catalytic species composed of rhodium-phosphine-iodine, ultimately generating adipic acid. Optimizing the amount of ligand is crucial for stabilizing the active rhodium species and suppressing excessive carbonylation or hydrogenolysis side reactions.

[0019] Compared with the prior art, the present invention has the following significant advantages and beneficial effects: 1) High catalyst activity and strong selectivity: By optimizing the molar ratio of rhodium, iodine and phosphorus, a highly efficient catalytic active center was formed. Under relatively mild reaction conditions (≤3.5 MPa, ≤180℃), near-complete conversion of tetrahydrofuran was achieved (conversion rate ≥98.5%), and the selectivity for the target product adipic acid was as high as 87% or more, which is significantly higher than most existing reports. 2) High cost-effectiveness of the catalyst: Using readily available triphenylphosphine as a ligand results in relatively low cost. More importantly, the catalytic system exhibits excellent stability, can be recovered through simple filtration, and shows less than 10% catalytic activity decay after five reuses, significantly reducing the consumption of the precious metal rhodium and process costs. 3) The process is green and simple to operate: the reaction uses acetic acid-water as a solvent, avoiding highly corrosive media; the main byproducts are small amounts of glutaric acid and succinic acid, which are few in number and easy to separate from the main product. The entire process, from catalyst preparation to product separation, is simple in steps, requires relatively conventional equipment, and is easy to scale up for industrial production. 4) High product purity: Adipic acid products with a purity of ≥99.8% can be easily obtained through post-processing methods such as vacuum distillation combined with water recrystallization, meeting the requirements of high-quality applications. Detailed Implementation

[0020] The technical solution of the present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0021] Unless otherwise specified, all reagents used in the examples are commercially available analytical grade or chemically pure products. Tetrahydrofuran was dried before use. Example 1

[0022] (1) Catalytic reaction: 200 mL of acetic acid, 133 mL of deionized water (volume ratio approximately 3:2), 0.01 mol (calculated as rhodium) RhCl3·x H2O, 0.05 mol of elemental iodine, 0.02 mol of triphenylphosphine, and 100 g of tetrahydrofuran were added sequentially to a 500 mL Hastelloy autoclave equipped with a stirrer and temperature control device. The autoclave was sealed, and the air inside was replaced with nitrogen three times. Subsequently, a 1:1 mixture of CO and H2 was introduced until the pressure reached 3.0 MPa. The stirrer was started and the rotation speed was set to 500 r / min. The temperature was raised to 170 °C and the timer was started. The reaction was carried out under these conditions for 10 hours. During the reaction, the pressure was maintained at a relatively constant level by adding gas.

[0023] (2) Product processing: After the reaction was completed, the reactor was cooled to room temperature, and the residual gas inside the reactor was slowly released. The reactor was opened, the entire reaction mixture was poured out, filtered, and the solid residue was washed twice with a small amount of acetic acid. The filtrate and washing liquid were combined. The filtrate was distilled under reduced pressure at 60℃ and -0.09 MPa using a rotary evaporator to distill off most of the acetic acid and water. 500 mL of hot water (about 80℃) was added to the remaining viscous substance to dissolve it, and the insoluble matter was removed by hot filtration. The filtrate was slowly cooled to 0-5℃ to crystallize, filtered, and the crystals were washed with a small amount of ice water and dried in a vacuum drying oven at 60℃ for 8 hours to obtain white flaky adipic acid crystals.

[0024] (3) Analysis and detection: The reaction filtrate was analyzed by gas chromatography (GC), and the conversion rate of tetrahydrofuran and the product distribution were calculated by the internal standard method. The purity of the obtained solid product was determined by melting point and high performance liquid chromatography (HPLC).

[0025] Results: The conversion rate of tetrahydrofuran was 99.2%, the selectivity of adipic acid was 88.5%, the selectivity of glutaric acid was 5.2%, the selectivity of succinic acid was 2.1%, and the total of other byproducts was 4.2%. The purity of the obtained adipic acid crystal product was 99.8%, and the melting point was 151-152℃. Example 2

[0026] The active component RhCl3 in Example 1 was replaced with an equal molar amount of dichlorotetracarbonyl dirhodium (Rh2(CO)4Cl2), and the remaining reaction conditions, material ratios and operating steps were the same as in Example 1.

[0027] Reaction results: Tetrahydrofuran conversion rate was 98.8%, and adipic acid selectivity was 87.2%. Example 3

[0028] Change the reaction conditions: the reaction temperature is 165℃, the initial pressure is 3.2 MPa, and the other conditions are the same as in Example 1.

[0029] Reaction results: Tetrahydrofuran conversion rate was 98.7%, and adipic acid selectivity was 88.1%. Example 4

[0030] The amount of ligand was changed: the amount of triphenylphosphine was adjusted to 0.015 mol (Rh:P = 1:1.5), and the other conditions were the same as in Example 1.

[0031] Reaction results: Tetrahydrofuran conversion rate was 99.0%, and adipic acid selectivity was 87.4%. Example 5

[0032] The amount of ligand was changed: the amount of triphenylphosphine was adjusted to 0.025 mol (Rh:P = 1:2.5), and the other conditions were the same as in Example 1.

[0033] Reaction results: Tetrahydrofuran conversion rate was 98.5%, and adipic acid selectivity was 87.9%. Example 6

[0034] Catalyst reuse experiment: The reaction was carried out under the conditions of Example 1. After the reaction was completed, the distillation operation of step (2) was not performed. Instead, the entire reaction mixture was filtered, and the separated solid catalyst residue (containing rhodium species, iodides, ligands, and possibly adsorbed organic matter) was washed twice with 20 mL of acetic acid and then used directly in the next reaction (the small amount of lost iodine and triphenylphosphine were added back to the initial molar amount, and tetrahydrofuran and solvent were added back). This cycle was repeated. The results are shown in Table 1 below.

[0035] Table 1. Experimental Cyclic Performance of Catalysts

[0036] The results in Table 1 above show that after the catalyst was reused 5 times, the conversion rate of tetrahydrofuran was still above 97.6% and the selectivity of adipic acid was still above 86.1%, indicating that both the activity and selectivity remained good.

[0037] Comparative Example 1

[0038] Iodine was not used as an accelerator, and all other conditions were the same as in Example 1.

[0039] Reaction results: The conversion rate of tetrahydrofuran was less than 5%, adipic acid was not detected in gas chromatography, and the main products were unreacted THF and other unknown substances. This demonstrates that elemental iodine is crucial to this catalytic system.

[0040] Comparative Example 2

[0041] Triphenylphosphine ligand was not used; all other conditions were the same as in Example 1.

[0042] Reaction results: Tetrahydrofuran conversion was 95.0%, but adipic acid selectivity was only 35.1%, with a significant increase in byproducts, mainly glutaric acid and butyric acid. This demonstrates that the ligand plays a crucial role in improving reaction selectivity.

[0043] Comparative Example 3 It reacts with sodium iodide at a higher temperature (190°C) and pressure (5 MPa).

[0044] Reaction results: Within the same reaction time (10 h), the conversion rate of tetrahydrofuran was 85.3%, and the selectivity of adipic acid was 72.5%. The comparison shows that the rhodium-based system of this invention achieved superior catalytic performance under milder conditions.

[0045] The above embodiments and comparative examples fully illustrate the excellent effects of the catalyst system and its application method provided by the present invention. The present invention is not limited to the specific embodiments described above, and any changes and substitutions made by those skilled in the art within the scope of the inventive concept and spirit should be included within the protection scope of the present invention.

Claims

1. A rhodium-based catalyst system, characterized in that, It is mainly composed of an active component, a promoter, and a ligand; the active component is a rhodium halide or a carbonyl halide; the promoter is elemental iodine; the ligand is triphenylphosphine; wherein, the molar ratio of rhodium, promoter, and ligand in the active component is 1: (4-6): (1.5-2.5).

2. The rhodium-based catalyst system as described in claim 1, characterized in that, The active component is selected from at least one of rhodium trichloride and dichlorotetracarbonyl dirhodium.

3. The method for preparing the rhodium-based catalyst system according to claim 1 or 2, characterized in that, The active component, promoter, and ligand are directly dissolved in a mixed solvent of acetic acid and water, and stirred until homogeneous to obtain the final product.

4. The preparation method of the rhodium-based catalyst system as described in claim 3, characterized in that, In the mixed solvent, the volume ratio of acetic acid to water is (1-4):

1.

5. The application of the rhodium-based catalyst system according to claim 1 or 2 in the catalytic dicarbonylation of tetrahydrofuran to prepare adipic acid.

6. A method for preparing adipic acid by catalytic dicarbonylation of tetrahydrofuran using the rhodium-based catalyst system according to claim 1 or 2, characterized in that, Includes the following steps: S1. Preparation of reaction raw materials: Tetrahydrofuran and rhodium-based catalyst are added to a mixed solvent composed of acetic acid and water to form a reaction solution; S2. Carbonylation reaction: The reaction solution is placed in a reaction vessel, the air is replaced with an inert gas, and a mixture of CO and H2 is introduced to an initial pressure of 2.5-3.5 MPa, wherein the volume ratio of CO to H2 is 1:1-2; then the reaction system is heated to 160-180°C and stirred at this temperature and pressure for 8-12 hours; S3. Product separation: After the reaction is completed, the reaction system is cooled and depressurized, the catalyst is separated, and the reaction solution is post-treated to obtain adipic acid.

7. The method as described in claim 6, characterized in that, In step S1, the mass-to-volume ratio of the tetrahydrofuran to the mixed solvent is 1 g : (3-5) mL; the mass of the rhodium element in the rhodium-based catalyst accounts for 0.5%-1.5% of the mass of the tetrahydrofuran.

8. The method as described in claim 6, characterized in that, In step S2, the reaction temperature is 165-175℃ and the initial pressure is 2.8-3.2 MPa.

9. The method as described in claim 6, characterized in that, In step S2, the stirring speed is 400-600 r / min.

10. The method as described in claim 6, characterized in that, In step S3, the catalyst separation is achieved by filtration or centrifugation; the post-treatment includes: sequentially subjecting the filtrate after catalyst separation to vacuum distillation, recrystallization purification, and drying; the solvent used for recrystallization is water.

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