Technological method for co-production of caprolactone and 2, 4-dimethylbenzoic acid

Abstract

The invention discloses a process method for co-production of caprolactone and 2, 4-dimethyl benzoic acid. According to the method, cyclohexanone is taken as a raw material, oxygen is introduced as an oxidant, 2, 4-dimethyl benzaldehyde is added as a pro-oxidant, and caprolactone and 2, 4-dimethyl benzoic acid are generated through oxidation reaction. According to the technological method, oxygen and 2, 4-dimethyl benzaldehyde are used as a co-oxidation system, in the presence of the carbon framework material loaded Ti-based catalyst, cyclohexanone can be subjected to oxidation reaction under mild conditions to generate caprolactone, the reaction is safe and efficient, the selectivity of the product caprolactone and the co-product 2, 4-dimethyl benzoic acid is high, the raw material conversion rate is high, and the method is suitable for industrial production. The product yield can be obviously improved, and the economic applicability is good.

Description

Technical Field

[0001] This invention relates to a co-production process, and more particularly to a process for the co-production of 2,4-dimethylbenzoic acid from caprolactone. Background Technology

[0002] Polycaprolactone (PCL) is a biodegradable aliphatic synthetic polyester with excellent biodegradability and compatibility. It is widely used in the production and processing of drug carriers, plasticizers, biodegradable plastics, nanofiber spinning, and molding materials. Caprolactone is a key intermediate in PCL synthesis. Currently, the main industrial method for synthesizing ε-caprolactone is the Baeyer-Villiger oxidation method, proposed in 1899 by German Nobel laureate Adolf von Baeyer and his student Victor Villiger. This method uses peroxyacid (currently peracetic acid / propionic acid is commonly used industrially) as the oxidant to oxidize cyclohexanone, resulting in a six-membered ring oxygen insertion reaction in the ketone to obtain caprolactone.

[0003] Currently, the main oxidant used in industrial Baeyer-Villiger oxidation is peroxyacid, which requires high safety standards during preparation, storage, and reaction. Historically, major industrial producers of caprolactone, such as Pastor in Sweden and Daicel in Japan, have experienced serious explosions, highlighting significant safety concerns. Furthermore, peroxyacid is typically prepared using hydrogen peroxide, resulting in high storage, transportation, and usage costs, further increasing production costs. While inexpensive and clean oxygen is the ideal oxidant, its direct application in the Baeyer-Villiger oxidation of ketones exhibits low reactivity and poor reaction results. Alternatively, additional sacrificial agents are needed to generate an active oxidant in situ capable of oxidizing ketones to lactones. Using benzaldehyde as a co-oxidant can achieve higher yields (Yuta Nabae, ACS Catal. 2013, 3, 230-236), but the byproduct benzoic acid has limited applications, restricting the commercial application of this route and generally limiting its use to academic research. Furthermore, patents CN111100105A and CN108558819A, which use acrolein as a co-oxidant, show results of 19.1% cyclohexanone conversion, 86.2% caprolactone selectivity, and 54.3% yield, respectively. This indicates that the acrolein co-oxidation system still suffers from low yield and high cost, and is far from industrial application. The key to the co-oxidation process for caprolactone is developing suitable co-oxidants and co-products; currently, no system with industrial-scale capability has been published. Additionally, and importantly, a highly efficient catalyst is crucial for the co-oxidation process of caprolactone. Researchers have developed various catalysts, such as various metal salts or oxides, but these are generally only suitable for the cyclohexanone / benzaldehyde system, with poor results in other systems. Researchers have also developed some novel catalyst systems, such as patents CN111100105A and CN108558819A, which use fluorine-modified carbon nanotubes as catalysts. However, these still suffer from low catalyst activity and low yield, failing to meet the requirements for industrial applications.

[0004] In summary, the existing caprolactone production process using peracetic acid and perpropionic acid as oxidants still poses significant safety risks. The process using O2+ as a co-oxidant is greener and safer, but research on it is still insufficient, and there are no reports of industrial-scale plants being put into operation. Summary of the Invention

[0005] To address the above technical problems, this invention proposes a process for the co-production of 2,4-dimethylbenzoic acid from caprolactone. This process avoids the safety and economic challenges associated with the use of peroxyacid / hydrogen peroxide. Furthermore, the co-product, 2,4-dimethylbenzoic acid, has broader applications and market potential, significantly reducing equipment investment and depreciation costs, and possesses the prospect of large-scale industrial application.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A process for the co-production of caprolactone and 2,4-dimethylbenzoic acid is disclosed, using cyclohexanone as raw material, introducing oxygen as an oxidant, and adding 2,4-dimethylbenzaldehyde as a co-oxidant. The oxidation reaction produces caprolactone and co-produces 2,4-dimethylbenzoic acid.

[0008] The preparation of caprolactone and the corresponding carboxylic acid by the cyclohexanone / aldehyde co-oxidation process is well-known in the industry. Benzaldehyde is generally used as the co-oxidizing aldehyde to ensure high product selectivity. However, the industrialization of this project requires not only consideration of reaction selectivity but also the wide applicability and economic viability of the co-products. Therefore, the selection of the co-oxidizing aldehyde is of paramount importance in developing new synthetic routes with industrial value.

[0009] While studying the synthesis of caprolactone from cyclohexanone via the Baeyer-Villiger oxidation reaction using oxygen as an oxidant, the researchers of this invention made a surprising discovery: using 2,4-dimethylbenzaldehyde as a co-oxidant, and in the presence of a preferred catalyst, a product selectivity similar to that of the benzaldehyde co-oxidation system can be obtained, with a higher raw material conversion rate. Moreover, the co-product 2,4-dimethylbenzoic acid is a raw material for the industrial preparation of trimellitic anhydride (global market size of 200,000 tons / year), making it more industrially applicable, with a wider range of uses, and potentially high-value market demand.

[0010] Furthermore, the reaction system of this invention has mild process conditions, high product yield, easy separation, and low production cost, which is conducive to large-scale industrial application.

[0011] In some preferred embodiments of the present invention, the oxidation reaction occurs in the presence of a catalyst; besides a suitable co-oxidant, an efficient catalyst is also one of the key factors in the Baeyer-Villiger oxidation reaction. The oxidation system of oxygen + 2,4-dimethylbenzaldehyde for cyclohexanone oxidation is generally not significantly superior to the oxidation system of oxygen + benzaldehyde. However, the inventors have surprisingly discovered that this oxidation system, when combined with a specific catalyst, can achieve satisfactory reaction results, exhibiting not only high reaction selectivity but also higher product yields, demonstrating strong industrial potential.

[0012] The catalyst is a Ti-based catalyst supported on a carbon skeleton material. This type of catalyst is a heterogeneous catalyst, which, compared with traditional homogeneous catalysts, does not have problems such as equipment corrosion, difficulty in separation, and excessive waste, and exhibits significant technological advantages. More importantly, the study found that after the carbon skeleton material is modified by supporting metallic Ti, it can significantly improve the reaction effect of the aforementioned co-oxidation system, exert active catalytic activity in this reaction system, has high product selectivity, and significantly improves the raw material conversion rate.

[0013] Preferably, the carbon framework material is selected from one or more of activated carbon, carbon nanotubes, graphene, graphene oxide, and their modified materials;

[0014] Preferably, the catalyst is TiO2 supported by carbon nanotubes and / or modified carbon nanotubes and / or modified TiO2, more preferably nano-TiO2 supported by nitrogen-doped modified carbon nanotubes, further preferably the nano-TiO2 loading is 0.5-5 wt%, preferably 1-2 wt%, and even more preferably the nitrogen doping amount in the nitrogen-doped modified carbon nanotubes is 0.1-10 wt%, preferably 0.5-5 wt%.

[0015] This invention does not impose any restrictions on the specific source of the nitrogen-doped modified carbon nanotubes. They can be purchased directly from commercially available finished products, or they can be custom-synthesized using any known technology. Feasible custom-synthesizing methods include, for example, 1) direct in-situ synthesis using conventional carbon nanotube production methods in the presence of nitrogen precursor compounds, and 2) doping modification after blending nitrogen precursor compounds and carbon nanotubes.

[0016] For the above method 1), further methods such as electric arc method, laser ablation method, catalytic method, etc., based on economic reasons, the catalytic method is preferred. For example, Maldonado et al. (Carbon 2006, 44(8), 1429-1437) disclosed a typical implementation of the prior art "floating catalyst" method, the main feature of which is that the catalytic component (ferrocene) is decomposed in situ in the presence of NH3 and xylene or pyridine.

[0017] For the above method 2), further examples include the CVD method in patent CN111100105A, the grinding method under nitrogen atmosphere in patent CN102414123A, and the calcination modification method of modified carbon nanotubes in patents CN117004216A and CN113856720B.

[0018] The nitrogen precursor compound can be a known small-molecule nitrogen-containing compound such as nitrogen, NH3, acetonitrile, pyridine, or pyrrole, or an organic oligomer or polymer such as dicyandiamide or melamine, without any restrictions.

[0019] Based on the consideration of having the widest possible sources of raw materials and the simplest possible preparation methods, this invention preferentially selects the following methods to prepare nitrogen-doped modified carbon nanotubes:

[0020] It is prepared by reacting and calcining components including nitrogen precursor compounds and carbon nanotubes. The reaction temperature can be 350-500℃, and the reaction time can be 2-6h. Favorable calcination conditions are, for example, calcination at 500-800℃ for 2-12h.

[0021] The nitrogen precursor compound may be selected from at least one of pyridine, pyrrole, dicyandiamide, and melamine;

[0022] Preferably, the ratio of carbon nanotubes to nitrogen precursor compounds can be (1-100):1, more preferably (10-80):1, and even more preferably (20-50):1, depending on the amount of nitrogen doping designed in the nitrogen-doped modified carbon nanotubes.

[0023] Furthermore, the preparation method of the nitrogen-doped modified carbon nanotube supported nano-TiO2 can refer to the general preparation methods of supported catalysts, such as impregnation method, precipitation method, vapor deposition method, sol-gel method, etc. The above preparation methods are very conventional and easy to select and adjust for those skilled in the art, and will not be described in detail here.

[0024] In some preferred embodiments of the present invention, the amount of catalyst used is 0.2-5 times the mass of cyclohexanone, preferably 0.5-2 times.

[0025] In some preferred embodiments of the present invention, the amount of oxygen used makes the reaction pressure reach 0.11 MPaA to 3 MPaA, preferably 1 MPaA to 2 MPaA;

[0026] Preferably, the amount of 2,4-dimethylbenzaldehyde used is 1-4 times the molar amount of cyclohexanone, more preferably 1-2 times.

[0027] In some preferred embodiments of the present invention, the reaction temperature of the oxidation reaction is 20-100°C, preferably 40-80°C;

[0028] Preferably, the oxidation reaction takes 0.5-5 hours, more preferably 2-4 hours.

[0029] In some preferred embodiments of the present invention, the oxidation reaction may or may not use a solvent, preferably without a solvent, which can simplify the refining and purification process and reduce equipment investment and energy consumption, etc.

[0030] Preferably, the solvent is selected from one or more of hexafluoroisopropanol, dichloromethane, dichloroethane, and acetonitrile.

[0031] The process in this invention uses oxygen + 2,4-dimethylbenzaldehyde as a co-oxidation system. In the presence of a Ti-based catalyst supported on a carbon skeleton material, cyclohexanone can be oxidized to caprolactone under mild conditions. The reaction is safe and efficient, with high selectivity for the product caprolactone and the coproduct 2,4-dimethylbenzoic acid. The raw material conversion rate is high, which is beneficial to significantly improve the product yield and has good economic applicability. Detailed Implementation

[0032] The present invention will be further illustrated below with specific embodiments. These embodiments are merely illustrative and do not limit the scope of the invention.

[0033] Unless otherwise specified, all raw materials used in the following embodiments of the present invention can be obtained through commercially available channels.

[0034] The catalyst AD was prepared according to the following method:

[0035] (1) Preparation of nitrogen-doped modified carbon nanotubes: Pyridine and carbon nanotubes (Beijing Deco Island Gold Technology Co., Ltd., CNT-104) were mixed and reacted at 80℃ for 2 hours. The reaction product was washed, dried, and calcined at 500℃ for 6 hours to obtain nitrogen-doped modified carbon nanotubes. The amounts of pyridine and carbon nanotubes were adjusted to obtain modified carbon nanotubes with different nitrogen contents. The nitrogen content was tested using a total nitrogen analyzer.

[0036] (2) Preparation of nitrogen-doped carbon nanotube-supported nano-TiO2: A titanium-based gel precursor was prepared by mixing tetrabutyl titanate and triethanolamine in an ethanol solvent. The nitrogen-doped carbon nanotubes prepared earlier were then added, and the solvent was gradually evaporated to fully impregnate the titanium-based gel. The reaction product was washed, dried, and calcined at 800℃ for 6 h to obtain nitrogen-doped carbon nanotube-supported nano-TiO2. The amounts of tetrabutyl titanate and nitrogen-doped carbon nanotubes were adjusted to obtain nitrogen-doped carbon nanotubes with different TiO2 loadings. The TiO2 loading was tested using ICP.

[0037] Catalyst A: Nitrogen-modified carbon nanotubes supported on TiO2, with TiO2 loading of 2 wt% and nitrogen doping in the carbon nanotubes of 0.5 wt%.

[0038] Catalyst B: Nitrogen-modified carbon nanotubes supported on TiO2, with TiO2 loading of 1 wt% and nitrogen doping in the carbon nanotubes of 0.5 wt%.

[0039] Catalyst C: Nitrogen-modified carbon nanotubes supported on TiO2, with TiO2 loading of 1 wt% and nitrogen doping in the carbon nanotubes of 1 wt%.

[0040] Catalyst D: Nitrogen-modified carbon nanotubes supported on TiO2, with TiO2 loading of 2 wt% and nitrogen doping in the carbon nanotubes of 5 wt%.

[0041] Catalyst E: Nitrogen-modified carbon nanotubes, with a nitrogen doping content of 0.5 wt%.

[0042] Catalyst F: TiO2 supported on carbon nanotubes, with a TiO2 loading of 2wt%, was synthesized according to step (2) of the preparation method of catalyst A, except that nitrogen-doped modified carbon nanotubes were replaced with carbon nanotubes.

[0043] Catalyst G: TiO2 supported on fluorine-modified carbon nanotubes, with a TiO2 loading of 2 wt% and a fluorine doping content of 0.5 wt% in the carbon nanotubes. It was synthesized according to the preparation method of catalyst A, except that pyridine was replaced with polytetrafluoroethylene.

[0044] The following examples and comparative examples all use a high-pressure stirred reactor as the reactor.

[0045]

Example 1

[0046] Weigh 10.45g of cyclohexanone, 24.59g of 2,4-dimethylbenzaldehyde, and 2.1g of catalyst A, add them to the reactor, heat to 80℃, introduce oxygen, maintain the reaction pressure at 1MPaA, react for 2 hours, cool to room temperature, filter, and take liquid phase samples to analyze the product composition. The raw material conversion rate and product selectivity are shown in Table 1.

[0047]

Example 2

[0048] Weigh 10.45g of cyclohexanone, 12.8g of 2,4-dimethylbenzaldehyde, and 2.1g of catalyst B, add them to the reactor, heat to 60℃, introduce oxygen, maintain the reaction pressure at 2MPaA, react for 2 hours, cool to room temperature, filter, and take liquid phase samples to analyze the product composition. The raw material conversion rate and product selectivity are shown in Table 1.

[0049]

Example 3

[0050] Weigh 10.45g of cyclohexanone, 24.59g of 2,4-dimethylbenzaldehyde, and 5g of catalyst C, add them to the reactor, heat to 40℃, introduce oxygen, maintain the reaction pressure at 1MPaA, react for 4 hours, cool to room temperature, filter, and take liquid phase samples to analyze the product composition. The raw material conversion rate and product selectivity are shown in Table 1.

[0051]

Example 4

[0052] 10.45g of cyclohexanone, 24.59g of 2,4-dimethylbenzaldehyde, 2.1g of catalyst D, and 100g of dichloromethane were weighed and added to the reactor. The temperature was raised to 40℃, oxygen was introduced, and the reaction pressure was maintained at 1MPaA. After reacting for 2 hours, the mixture was cooled to room temperature. After filtration, liquid phase samples were taken to analyze the product composition. The raw material conversion rate and product selectivity are shown in Table 1.

[0053]

Example 5

[0054] 10.45g of cyclohexanone, 24.59g of 2,4-dimethylbenzaldehyde, 15g of catalyst D, and 100g of dichloromethane were weighed and added to the reactor. The temperature was raised to 50℃, oxygen was introduced, and the reaction pressure was maintained at 2MPaA. After reacting for 2 hours, the mixture was cooled to room temperature. After filtration, liquid phase samples were taken to analyze the product composition. The raw material conversion rate and product selectivity are shown in Table 1.

[0055] Comparative Example 1

[0056] Caprolactone was prepared using essentially the same process as in Example 1, except that catalyst A was replaced with catalyst E of the same mass.

[0057] Comparative Example 2

[0058] Caprolactone was prepared using essentially the same process as in Example 1, except that 2,4-dimethylbenzaldehyde was replaced with the same molar amount of benzaldehyde, and catalyst A was replaced with catalyst E of the same mass.

[0059] Comparative Example 3

[0060] Caprolactone was prepared using essentially the same process as in Example 1, except that catalyst A was replaced with catalyst F of the same mass.

[0061] Comparative Example 4

[0062] Caprolactone was prepared using essentially the same process as in Example 1, except that catalyst A was replaced with catalyst G of the same mass.

[0063] Table 1. Reaction Results

[0064]

[0065]

[0066] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention.

Claims

1. A process for the co-production of 2,4-dimethylbenzoic acid from caprolactone, characterized in that, Using cyclohexanone as a raw material, oxygen is introduced as an oxidant, and 2,4-dimethylbenzaldehyde is added as a co-oxidant. The oxidation reaction produces caprolactone and co-produces 2,4-dimethylbenzoic acid.

2. The process for co-producing 2,4-dimethylbenzoic acid from caprolactone according to claim 1, characterized in that, The oxidation reaction occurs in the presence of a catalyst; the catalyst is a Ti-based catalyst supported on a carbon framework material. Preferably, the carbon framework material is selected from one or more of activated carbon, carbon nanotubes, graphene, graphene oxide, and their modified materials; Preferably, the catalyst is TiO2 supported by carbon nanotubes and / or modified carbon nanotubes and / or modified TiO2, more preferably nano-TiO2 supported by nitrogen-doped modified carbon nanotubes, further preferably the nano-TiO2 loading is 0.5-5 wt%, preferably 1-2 wt%, and even more preferably the nitrogen doping amount in the nitrogen-doped modified carbon nanotubes is 0.1-10 wt%, preferably 0.5-5 wt%.

3. The process for co-producing 2,4-dimethylbenzoic acid from caprolactone according to claim 2, characterized in that, The amount of catalyst used is 0.2-5 times the mass of cyclohexanone, preferably 0.5-2 times.

4. The process for co-producing 2,4-dimethylbenzoic acid from caprolactone according to claim 1, characterized in that, The amount of oxygen used makes the reaction pressure reach 0.11 MPaA to 3 MPaA, preferably 1 MPaA to 2 MPaA; Preferably, the amount of 2,4-dimethylbenzaldehyde used is 1-4 times the molar amount of cyclohexanone, more preferably 1-2 times.

5. The process for co-producing 2,4-dimethylbenzoic acid from caprolactone according to any one of claims 1-4, characterized in that, The reaction temperature of the oxidation reaction is 20-100℃, preferably 40-80℃; Preferably, the oxidation reaction takes 0.5-5 hours, more preferably 2-4 hours.

6. The process for co-producing 2,4-dimethylbenzoic acid from caprolactone according to any one of claims 1-5, characterized in that, The oxidation reaction may or may not use a solvent, preferably without a solvent; Preferably, the solvent is selected from one or more of hexafluoroisopropanol, dichloromethane, dichloroethane, and acetonitrile.

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