Method for preparing 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compound, reaction system and obtained product

Abstract

The invention discloses a method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound and a reaction system and the obtained product, the method comprising: adding 2,2,6,6-tetramethylpiperidine to a reactor, adding or not adding an organic solvent and stirring and dissolving; adding a catalyst containing a modified titanium silicon molecular sieve, adding a hydrogen peroxide solution, 2,2,6,6-tetramethylpiperidine and hydrogen peroxide are subjected to an oxidation reaction; the catalyst is removed from the product obtained, and the liquid-phase product is desolvated to obtain 2,2,6,6-tetramethylpiperidine nitroxide free radical compound. The method provided by the present invention prepares 2,2,6,6-tetramethylpiperidine nitroxide free radical compound in a one-step reaction, has a high yield, is easy to purify the product, and has a high purity of the piperidine nitroxide free radical compound product, and the catalyst can be reused.

Description

Technical Field

[0001] The present invention relates to the technical field of organic synthesis, and in particular to a method for preparing a piperidine nitroxide free radical compound. Background Art

[0002] Piperidine nitroxides are stable free radicals with strong resistance to both light and heat. Therefore, they are often used as light stabilizers in polymerization, inhibitors in olefin production, thermal degradation stabilizers for polymers, and catalysts for the oxidation of fatty alcohols to produce aldehydes and ketones. Their polymerization inhibition is superior to most common inhibitors, and they are widely used to prevent scaling in the production of low-carbon olefins, acrylates, and methacrylates.

[0003] Currently, the synthesis method of piperidine nitroxide free radical compounds mainly involves oxidation of piperidine compounds. CN101691352A discloses a method for preparing 4-hydroxy-2,2,6,6-tetramethylpiperidine nitroxide free radical. 4-hydroxy-2,2,6,6-tetramethylpiperidine and hydrogen peroxide are used as raw materials. An oxidation reaction is carried out in the presence of a mixture of sodium tungstate, an oxidation promoter, and disodium ethylenediaminetetraacetic acid as a catalyst. The product is extracted with petroleum ether and then the solvent is removed by vacuum distillation to prepare 4-hydroxy-2,2,6,6-tetramethylpiperidine nitroxide free radical. CN102001993A discloses a method for preparing 4-allyl-2,2,6,6-tetramethylpiperidine nitroxide free radical. The method comprises the following steps: using 4-allyl-2,2,6,6-tetramethylpiperidine and hydrogen peroxide as raw materials, carrying out an oxidation reaction in the presence of a composite catalyst of sodium tungstate, an oxidation promoter, and a quaternary ammonium salt; allowing the reaction to stand and separate the phases; performing reduced pressure distillation on the organic phase to remove the solvent; and then washing the organic phase with hydrochloric acid and drying the organic phase to obtain the 4-allyl-2,2,6,6-tetramethylpiperidine nitroxide free radical. CN200910025094.7 discloses a method for preparing 4-substituted-2,2,6,6-tetramethylpiperidine nitroxide free radicals, comprising using 4-substituted-2,2,6,6-tetramethylpiperidine and hydrogen peroxide as raw materials, in the presence of a magnesium salt catalyst, to obtain a product, filtering the product to remove the catalyst, and then performing reduced pressure distillation to remove the solvent to obtain the 4-substituted-2,2,6,6-tetramethylpiperidine nitroxide free radical. CN102464609A discloses a method for preparing 4-carbonyl-2,2,6,6-tetramethylpiperidine nitroxide free radicals, comprising using 4-carbonyl-2,2,6,6-tetramethylpiperidine and hydrogen peroxide as raw materials, in the presence of a magnesium hydroxide catalyst, to obtain a product, which is then filtered and distilled under reduced pressure to obtain the 4-carbonyl-2,2,6,6-tetramethylpiperidine nitroxide free radical. CN108689916A discloses a method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical, which uses 2,2,6,6-tetramethylpiperidine and hydrogen peroxide as raw materials, obtains the product under the action of a composite catalyst of magnesium hydroxide and diatomaceous earth, and then obtains 2,2,6,6-tetramethylpiperidine nitroxide free radical by filtration, extraction with ethyl acetate, and reduced pressure distillation of the organic phase.

[0004] The raw materials used in the above methods are all piperidine compounds and hydrogen peroxide. The main difference lies in the catalyst. Currently, there are two main types of catalysts: sodium tungstate and magnesium salts. The sodium tungstate system has a low yield of nitroxide free radicals, while the magnesium hydroxide system may gradually deactivate due to absorption of acidic gases such as CO2 in the air. Summary of the Invention

[0005] One of the technical problems to be solved by the present invention is to provide a method and a continuous reaction system for preparing 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compounds based on the existing technology.

[0006] The second technical problem to be solved by the present invention is to provide a 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compound prepared by the above method.

[0007] A method for preparing a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound comprises the following steps:

[0008] S1. Add 2,2,6,6-tetramethylpiperidine into a reaction kettle, add or not add an organic solvent and stir to dissolve;

[0009] S2. Adding a catalyst containing a modified titanium silicalite into a reactor, adding a hydrogen peroxide solution, 2,2,6,6-tetramethylpiperidine and hydrogen peroxide to carry out an oxidation reaction;

[0010] S3. removing the catalyst from the obtained product and removing the solvent from the liquid product to obtain a 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compound.

[0011] The 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compound product is prepared by the above method.

[0012] A reaction system for preparing a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound comprises a reaction kettle, a buffer tank, an extraction tower, a vacuum distillation tower and a solvent recovery tower, which are connected in sequence. The bottom of the buffer tank is connected to the feed inlet of the extraction tower via a material pump. The reaction materials are 2,2,6,6-tetramethylpiperidine, hydrogen peroxide and a catalyst containing a modified titanium silicon molecular sieve, with or without an organic solvent.

[0013] The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compounds and the product provided by the present invention have the following beneficial effects:

[0014] The present invention uses modified titanium silicalite to catalyze the reaction of 2,2,6,6-tetramethylpiperidine and hydrogen peroxide, and only requires one step of reaction to prepare a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound. The method has a high reaction yield and is easy to purify the product. The obtained piperidine nitroxide free radical compound product has high purity, and the catalyst containing the modified titanium silicalite can be reused, thereby improving economic benefits.

[0015] The reaction system for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compounds provided by the present invention is suitable for the method provided by the present invention and can be continuously operated to obtain 2,2,6,6-tetramethylpiperidine nitroxide free radical compound products. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 The present invention provides a schematic flow diagram of a continuous reaction system for preparing 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compounds.

[0017] in:

[0018] 1-reactor, 2-extraction tower, 3-vacuum distillation tower, 4-solvent recovery tower, 5-buffer tank, 6-raw material, 7-product, 8-extraction solvent, 9-organic phase, 10-aqueous phase, 11-product, 12-mixed solvent, 13-first recovery solvent, 14-second recovery solvent. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is described in detail and clearly below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0020] In a first aspect, the present invention provides a method for preparing a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound, comprising the following steps:

[0021] S1. Add 2,2,6,6-tetramethylpiperidine into a reaction kettle, add or not add an organic solvent and stir to dissolve;

[0022] S2. Adding a catalyst containing a modified titanium silicalite into a reactor, adding a hydrogen peroxide solution, 2,2,6,6-tetramethylpiperidine and hydrogen peroxide to carry out an oxidation reaction;

[0023] S3. removing the catalyst from the obtained product and removing the solvent from the liquid product to obtain a 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compound.

[0024] Wherein: the chemical structural formula of the 2,2,6,6-tetramethylpiperidine compound is:

[0025]

[0026] In the method provided by the present invention, in S1, the organic solvent is an alcohol or aromatic hydrocarbon organic solvent.

[0027] Preferably, the organic solvent is an aliphatic alcohol or cycloalkanol containing 1 to 6 carbon atoms, and an alkane or aromatic hydrocarbon containing 5 to 9 carbon atoms.

[0028] More preferably, the organic solvent is selected from one or more of methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, tert-amyl alcohol, cyclohexanol, cyclohexane, n-hexane, benzene and toluene.

[0029] Optionally, in S1, the mass ratio of the organic solvent to the 2,2,6,6-tetramethylpiperidine is 0 to 20:1. Preferably, the organic solvent is added, and the mass ratio of the organic solvent to the 2,2,6,6-tetramethylpiperidine is 0.2 to 4:1.

[0030] In the method provided by the present invention, the catalyst containing the modified titanium silicate molecular sieve is composed of 20-100 wt% of the modified titanium silicate molecular sieve and 0-80 wt% of a heat-resistant inorganic oxide, based on the total weight of the catalyst, wherein the heat-resistant inorganic oxide is aluminum oxide and / or silicon oxide;

[0031] Preferably, based on the total weight of the titanium silicate molecular sieve, the modified titanium silicate molecular sieve contains 93-99.9 wt% of the titanium silicate molecular sieve and 0.1-7 wt% of the transition metal oxide;

[0032] Preferably, the modified titanium silicate molecular sieve contains 98-99.8 wt% of titanium silicate molecular sieve and 0.2-2 wt% of transition metal oxide;

[0033] Preferably, the transition metal oxide is selected from one or more of Ag, Zn and Cu metal oxides;

[0034] Preferably, the titanium silicate molecular sieve is a TS-1 titanium silicate molecular sieve having an MFI topological structure.

[0035] In the method provided by the present invention, the preparation method of the modified titanium silicate molecular sieve is:

[0036] C1. impregnating the titanium silicon molecular sieve with a transition metal salt solution to obtain an impregnated titanium silicon molecular sieve;

[0037] C2, drying the impregnated titanium silicon molecular sieve obtained in step C1 to obtain a dried impregnated titanium silicon molecular sieve;

[0038] C3. calcining the dried and impregnated titanium silicate obtained in step C2 to obtain a modified titanium silicate.

[0039] Among them, in step C1, the transition metal salt solution is preferably one or more of AgNO3, Cu(NO3)2 and Zn(NO3)2 aqueous solutions, the mass concentration of the transition metal salt solution is 0.1~15wt%, the mass ratio of the transition metal salt solution to the titanium silicon molecular sieve is 0.5~3:1, the immersion temperature is 10~80℃, and the immersion time is 10~30 hours.

[0040] In step C2, the drying temperature is 100 to 160° C., and the drying time is 1 to 8 hours.

[0041] In step C3, the calcination temperature is 400-650° C., and the calcination time is 1-20 hours.

[0042] The catalyst containing the modified titanium silicate can be prepared using conventional methods for preparing catalysts in the art, and the present invention is not limited thereto. For example, the modified titanium silicate and a heat-resistant inorganic oxide precursor are mixed and formed, and then calcined to obtain the catalyst containing the modified titanium silicate.

[0043] In the method provided by the present invention, in S2, a catalyst containing modified titanium silicate is added to a reactor and evenly mixed with the solution obtained in S1. A hydrogen peroxide solution is slowly added dropwise at 20°C-100°C and stirred thoroughly until the reaction is complete.

[0044] In S2, the reaction temperature is preferably 25°C-85°C.

[0045] In S2, the hydrogen peroxide solution is a 1 wt% to 70 wt% hydrogen peroxide solution, preferably a 5 wt% to 40 wt% hydrogen peroxide solution;

[0046] The molar ratio of the hydrogen peroxide to 2,2,6,6-tetramethylpiperidine is 0.5-10:1, preferably 1-5:1.

[0047] In the method provided by the present invention, in S3, the obtained product is filtered to remove the catalyst, the liquid phase product is allowed to stand and phase-separated, and then the organic phase is subjected to reduced pressure distillation to remove the solvent; or the liquid phase product is first extracted with an organic solvent, then allowed to stand and phase-separated, and the organic phase is subjected to reduced pressure distillation to remove the organic solvent; to obtain a 2,2,6,6-tetramethylpiperidine nitroxide free radical product, and the separated catalyst is reused. Preferably, the separated catalyst is dried and then reused.

[0048] In the method provided by the present invention, the liquid phase product is composed of an organic solvent, water, a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound, and a small amount of unreacted 2,2,6,6-tetramethylpiperidine. After phase separation, the organic phase is composed of the 2,2,6,6-tetramethylpiperidine nitroxide free radical compound, a small amount of 2,2,6,6-tetramethylpiperidine, and the organic solvent. The aqueous phase is composed of water and a trace amount of the 2,2,6,6-tetramethylpiperidine nitroxide free radical compound. After the organic phase is subjected to reduced pressure distillation to remove the organic solvent, it is composed of 2,2,6,6-tetramethylpiperidine nitroxide free radical with a purity of not less than 99% by weight and a small amount of 2,2,6,6-tetramethylpiperidine.

[0049] The second separation and purification method is adopted, wherein the extraction phase is composed of an extraction solvent, an organic solvent, a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound and a small amount of 2,2,6,6-tetramethylpiperidine, and the raffinate phase is water and a small amount of organic solvent. After standing for phase separation, the organic phase is composed of the extraction solvent, the 2,2,6,6-tetramethylpiperidine nitroxide free radical compound and a small amount of 2,2,6,6-tetramethylpiperidine. After the organic solvent is removed by reduced pressure distillation, the composition is 2,2,6,6-tetramethylpiperidine nitroxide free radical with a purity greater than 99wt% and a small amount of 2,2,6,6-tetramethylpiperidine.

[0050] In S3, the organic solvent used in the extraction step is one or more of petroleum ether, dichloromethane, dichloroethane, ethyl acetate, alkanes containing 5 to 9 carbon atoms, cycloalkanes and aromatic hydrocarbons.

[0051] The present invention has no limitation on the extraction operation conditions, which are generally as follows: the temperature is room temperature 15° C. to 40° C., and the pressure is 0.1 MPa.

[0052] In the method provided by the present invention, the organic solvent is removed by vacuum distillation to obtain a 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compound product. The operating conditions of the vacuum distillation are: a pressure of 0.001 to 0.05 MPa and a distillation tower bottom temperature of 50 to 110°C.

[0053] In the method provided by the present invention, preferably, the reactor used is an isothermal reactor with stirring, and is equipped with a catalyst filtering device, wherein the filter core is a ceramic membrane or a hollow carbon fiber membrane.

[0054] In a second aspect, the present invention provides a continuous reaction system for preparing 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compounds, comprising a reactor, a buffer tank, an extraction tower, a vacuum distillation tower and a solvent recovery tower connected in sequence, wherein the bottom of the buffer tank is connected to the feed inlet of the extraction tower via a material pump; the reaction materials are 2,2,6,6-tetramethylpiperidine, hydrogen peroxide and a catalyst containing a modified titanium silicon molecular sieve, with or without an organic solvent.

[0055] Preferably, the reactor is an isothermal reactor equipped with a stirring system and a filtration system, wherein the filtration medium of the filtration system is a ceramic membrane or a hollow carbon fiber membrane.

[0056] The continuous reaction system and reaction process of the 2,2,6,6-tetramethylpiperidine nitroxide free radical compound provided by the present invention are described in detail below with reference to the accompanying drawings. However, the present invention is not limited thereto.

[0057] Attachment Figure 1 The schematic flow diagram of the continuous reaction system for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compounds provided by the present invention is shown in the attached figure. Figure 1 As shown, the continuous reaction system for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compounds provided by the present invention comprises a reactor 1, a buffer tank 5, an extraction tower 2, a vacuum distillation tower 3, and a solvent recovery tower 4, which are connected in sequence. The bottom of the buffer tank 5 is connected to the feed inlet of the extraction tower via a material pump. The liquid phase product of the oxidation reaction product in the reactor enters the buffer tank after liquid-solid separation and then enters the extraction tower via a material pump. After countercurrent contact and extraction with an extraction solvent, the liquid phase is separated to produce an upper aqueous phase 10 and a lower organic phase 9. The organic phase enters the vacuum distillation tower 3 and is subjected to vacuum distillation to obtain a product 11 containing at least 99 wt% of the 2,2,6,6-tetramethylpiperidine nitroxide free radical compound. The mixed solvent 12 enters the solvent recovery tower 4 and is further separated into a first recovered solvent 13 and a second recovered solvent 14, which are the organic solvent added in the reaction step and the extraction solvent added in the extraction step, respectively, based on their boiling points.

[0058] In a third aspect, the present invention provides a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound product prepared by any of the above methods for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compounds.

[0059] The product is a piperidine nitroxide free radical compound with a purity of more than 99%. The remainder is unreacted 2,2,6,6-tetramethylpiperidine.

[0060] The invention uses 2,2,6,6-tetramethylpiperidine and hydrogen peroxide as raw materials and a modified titanium silicon molecular sieve as a catalyst. Only one-step reaction is required to prepare the 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compound. Subsequent separation is easy to obtain a high-purity product, and the catalyst can be recovered and reused.

[0061] The method of the present invention is further illustrated below by way of examples, but the present invention is not limited thereto.

[0062] In the Examples and Comparative Examples:

[0063] Titanium silicate molecular sieve TS-1 was prepared by the method disclosed in CN1421389A.

[0064] 2,2,6,6-Tetramethylpiperidine is commercially available with a purity of 99%.

[0065] The yield is the actual mass of the product obtained / theoretical mass of the product that can be obtained × 100%;

[0066] The mass of 2,2,6,6-tetramethylpiperidinyl nitroxide free radical in the product was obtained by gas chromatography analysis. The purity of the product was calculated as follows: mass of 2,2,6,6-tetramethylpiperidinyl nitroxide free radical in the actual product / total mass of the product × 100%.

[0067] Preparation Example 1 of Modified Titanium Silica Molecular Sieve Catalyst

[0068] 0.103 g of silver nitrate was dissolved in 15.0 g of deionized water to obtain a silver nitrate aqueous solution. 15.0 g of titanium silicate molecular sieve TS-1 was then added and impregnated at room temperature (25-30°C) for 24 hours. The solution was then dried in air at 120°C for 6 hours and finally calcined at 550°C for 8 hours in air. The resulting silver oxide-supported titanium silicate molecular sieve had an impregnation content of 0.94 wt%, designated Catalyst G1.

[0069] Preparation Example 2 of Modified Titanium Silica Molecular Sieve Catalyst

[0070] 0.342 g of zinc nitrate hexahydrate was dissolved in 20.0 g of deionized water to obtain a zinc nitrate aqueous solution. 20.0 g of titanium silicate molecular sieve TS-1 was then added and impregnated at room temperature (25-30°C) for 12 hours. The solution was then dried in air at 105°C for 12 hours and finally calcined at 600°C in air for 6 hours. The resulting zinc oxide-supported titanium silicate was impregnated at a concentration of 0.47 wt%, designated Catalyst G2.

[0071] Preparation Example 3 of Modified Titanium Silica Molecular Sieve Catalyst

[0072] 0.281 g of copper nitrate trihydrate was dissolved in 16.0 g of deionized water to obtain a copper nitrate aqueous solution. 16.0 g of titanium silicate TS-1 was then added and impregnated at room temperature (25-30°C) for 20 hours. The solution was then dried in air at 130°C for 5 hours and finally calcined at 600°C in air for 5 hours. The result was a copper oxide-supported titanium silicate impregnated with 0.58 wt% copper oxide, designated Catalyst G3.

[0073] Preparation Example 4 of Modified Titanium Silica Molecular Sieve Catalyst

[0074] 0.835 g of zinc nitrate hexahydrate was dissolved in 20.0 g of deionized water to obtain a zinc nitrate aqueous solution. 20.0 g of titanium silicate molecular sieve TS-1 was then added and impregnated at room temperature (25-30°C) for 12 hours. The solution was then dried in air at 105°C for 12 hours and finally calcined at 600°C in air for 6 hours. The resulting zinc oxide-supported titanium silicate catalyst, designated G4, had an impregnation concentration of 1.14 wt%.

[0075] Example 1

[0076] Add 14.1 g (0.1 mol) of 2,2,6,6-tetramethylpiperidine, 5.0 g of methanol, and 0.34 g of G1 catalyst to the reactor. Mix thoroughly using magnetic stirring. Control the temperature at 75°C. Add 34.1 g (0.3 mol) of 30 wt% hydrogen peroxide dropwise over 2 hours. Continue the reaction for 6 hours after the addition is complete.

[0077] After the reaction is completed, the catalyst is filtered off and 20 g of ethyl acetate is added for extraction. After thorough mixing and shaking, the mixture is allowed to stand for 5 hours to separate the phases. The organic phase mainly contains 2,2,6,6-tetramethylpiperidine nitroxide free radical compound, and the aqueous phase contains only a very small amount of organic solvent and product. The organic phase is obtained by fractionating with a separatory funnel and distilling the organic phase under reduced pressure to obtain 15.40 g of 2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.7% and a purity of 99.6%.

[0078] Example 2

[0079] The reaction conditions were the same as those in Example 1, except that catalyst G2 was used to obtain 15.41 g of 2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.8% and a purity of 99.3%.

[0080] Example 3

[0081] The reaction conditions were the same as those in Example 1, except that catalyst G3 was used to obtain 15.23 g of 2,2,6,6-tetramethylpiperidine nitrogen oxide free radical compound with a yield of 97.6% and a purity of 99.1%.

[0082] Example 4

[0083] The reaction conditions were the same as those in Example 1, except that catalyst G4 was used to obtain 15.38 g of 2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.6% and a purity of 99.5%.

[0084] Example 5

[0085] 14.1 g (0.1 mol) of 2,2,6,6-tetramethylpiperidine, 10 g of toluene, and 0.53 g of G2 catalyst were added to a reactor. The mixture was thoroughly mixed using magnetic stirring and the temperature was controlled at 60°C. 54.4 g (0.4 mol) of 25 wt% hydrogen peroxide was added dropwise over a period of 2 hours. After the addition was complete, the reaction was continued for 8 hours. After the reaction was complete, the catalyst was filtered off and 30 g of dichloromethane was added. After thorough mixing and oscillation, the mixture was allowed to stand for 5 hours to allow for phase separation. The organic phase primarily contained dichloromethane, toluene, and 2,2,6,6-tetramethylpiperidine nitroxide free radical compound, while the aqueous phase contained only trace amounts of organic solvent and product. The organic phase was separated using a separatory funnel and the solvent was removed by vacuum distillation to yield 15.41 g of 2,2,6,6-tetramethylpiperidine nitroxide free radical compound, with a yield of 98.8% and a purity of 99.6%.

[0086] Example 6

[0087] 14.1 g (0.1 mol) of 2,2,6,6-tetramethylpiperidine and 10 g of toluene were added to the reactor, along with 0.53 g of the filtered and dried catalyst from Example 2. The mixture was thoroughly mixed using magnetic stirring, and the temperature was controlled at 60°C. 54.4 g (0.4 mol) of 25 wt% hydrogen peroxide was added dropwise over a period of 2 hours. After the addition was complete, the reaction was continued for 8 hours. After the reaction was complete, the catalyst was filtered out, the mixture was allowed to stand, and the phases were separated. The organic phase was subjected to reduced pressure distillation to remove water, yielding 15.38 g of a 4-hydroxy-2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.6% and a purity of 99.5%.

[0088] Comparative Example 1

[0089] 14.1 g of 2,2,6,6-tetramethylpiperidine and 10 g of toluene were added to a reactor without a catalyst. Thoroughly mixed using magnetic stirring, the temperature was controlled at 60°C, and 54.4 g of 25 wt% hydrogen peroxide was added dropwise over 2 hours. After the addition was complete, the reaction was continued for 8 hours. After the reaction was complete, the mixture was allowed to stand and phase separated. The organic phase was subjected to reduced pressure distillation to remove the solvent, yielding 14.07 g of the 2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 90.2% and a purity of 32.8%.

[0090] Comparative Example 2

[0091] 14.1 g of 2,2,6,6-tetramethylpiperidine, 10 g of toluene, and 0.53 g of magnesium hydroxide catalyst were added to a reactor. The mixture was thoroughly mixed using magnetic stirring and the temperature was controlled at 60°C. 54.4 g of 25 wt% hydrogen peroxide was added dropwise over a period of 2 hours. After the addition was complete, the reaction was continued for 8 hours. After completion of the reaction, the catalyst was filtered off, the mixture was allowed to stand, and the phases were separated. The organic phase was subjected to reduced pressure distillation to remove the solvent, yielding 15.05 g of a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 96.5% and a purity of 99.2%.

[0092] Comparative Example 3

[0093] 14.1 g of 2,2,6,6-tetramethylpiperidine, 10 g of toluene, and 0.53 g of unmodified titanium silicalite TS-1 catalyst were added to a reactor. Thoroughly mixed using magnetic stirring, the temperature was controlled at 60°C, and 54.4 g of 25 wt% hydrogen peroxide was added dropwise over 2 hours. After the addition was complete, the reaction was continued for 8 hours. After the reaction was complete, the catalyst was filtered off, the mixture was allowed to stand, and the phases were separated. The organic phase was subjected to reduced pressure distillation to remove the solvent, yielding 15.38 g of 2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.6% and a purity of 90.1%.

Claims

1. A method for preparing a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound, characterized in that: The following steps are involved: S1. Add 2,2,6,6-tetramethylpiperidine into a reaction kettle, add or not add an organic solvent and stir to dissolve; S2, adding the modified titanium silicate catalyst into the reactor, adding hydrogen peroxide solution, 2,2,6,6-tetramethylpiperidine and hydrogen peroxide to carry out oxidation reaction; S3, removing the catalyst from the obtained product, and removing the solvent from the liquid product to obtain a 2,2,6,6-tetramethylpiperidine nitroxide free radical compound; removing the catalyst from the obtained product by filtering, and removing the solvent from the organic phase after the liquid product is allowed to stand and phase separated; Based on the total weight of the titanium silicate molecular sieve, the modified titanium silicate molecular sieve consists of 93-99.9 wt% of the titanium silicate molecular sieve and 0.1-7 wt% of a transition metal oxide; the transition metal oxide is selected from Ag and / or Cu metal oxides.

2. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound according to claim 1, wherein The modified titanium silicate molecular sieve consists of 98-99.8 wt% of titanium silicate molecular sieve and 0.2-2 wt% of transition metal oxide.

3. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound according to claim 1, characterized in that: The organic solvent described in S1 is an aliphatic alcohol containing 1 to 6 carbon atoms or a cycloalkanol containing 1 to 6 carbon atoms, and an alkane containing 5 to 9 carbon atoms or an aromatic hydrocarbon containing 6 to 9 carbon atoms.

4. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound according to claim 3, characterized in that: The organic solvent is selected from one or more of methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, tert-amyl alcohol, cyclohexanol, cyclohexane, n-hexane, benzene and toluene.

5. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compounds according to any one of claims 1 to 4, characterized in that: In S1, the mass ratio of the organic solvent to the 2,2,6,6-tetramethylpiperidine is 0 to 20:

1.

6. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compounds according to claim 5, characterized in that: The mass ratio of the organic solvent to the 2,2,6,6-tetramethylpiperidine is 0.2-4:

1.

7. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound according to any one of claims 1 to 4, characterized in that: In S2, the catalyst containing modified titanium silicalite is added to the reactor and evenly mixed with the solution obtained in S1. At 20°C-100°C, the hydrogen peroxide solution is slowly added dropwise and stirred thoroughly until the reaction is complete.

8. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound according to claim 7, characterized in that: The reaction temperature is 25°C to 85°C.

9. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound according to any one of claims 1 to 4, characterized in that: In S2, the hydrogen peroxide solution is a 1% to 70 wt% hydrogen peroxide solution, and the molar ratio of the hydrogen peroxide to 2,2,6,6-tetramethylpiperidine is 0.5 to 10:

1.

10. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound according to any one of claims 1 to 4, characterized in that: The hydrogen peroxide solution is a 5-40 wt% hydrogen peroxide solution; the molar ratio of the hydrogen peroxide to 2,2,6,6-tetramethylpiperidine is 1-5:

1.

11. The method for preparing 2,2,6,6-tetramethylpiperidine nitroxide free radical compound according to any one of claims 1 to 4, characterized in that: In S3, the obtained product is filtered to remove the catalyst, and the separated catalyst is reused.

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