Method for preparing piperidine nitroxide free radical compounds

Abstract

The present invention relates to the technical field of organic synthesis, and discloses a method for preparing piperidine nitroxide free radical compounds, the obtained products, and a continuous reaction system. The method comprises: adding a piperidine compound and a solvent to a reactor, stirring and dissolving; adding a catalyst containing a modified titanium silicalite molecular sieve to the reactor, adding a hydrogen peroxide solution, and performing an oxidation reaction between the piperidine compound and the hydrogen peroxide; removing the catalyst from the obtained product, and removing the solvent from the liquid product by vacuum distillation to obtain a piperidine nitroxide free radical compound. The method provided by the present invention prepares piperidine nitroxide free radical compounds by a one-step reaction, has a high yield, is easy to purify the product, and the obtained piperidine nitroxide free radical compound product has high purity. The catalyst containing the modified titanium silicalite molecular sieve can be reused, thereby improving economic benefits.

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-allyloxy-2,2,6,6-tetramethylpiperidine nitroxide free radicals, comprising: using 4-allyloxy-2,2,6,6-tetramethylpiperidine and hydrogen peroxide as raw materials, conducting 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, subjecting the organic phase to reduced pressure distillation to remove the solvent, and then washing with hydrochloric acid and drying to obtain the 4-allyloxy-2,2,6,6-tetramethylpiperidine nitroxide free radicals. CN101475525A 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 subjecting the product to reduced pressure distillation to remove the solvent to obtain the 4-substituted-2,2,6,6-tetramethylpiperidine nitroxide free radicals. CN102464609A discloses a method for preparing 4-carbonyl-2,2,6,6-tetramethylpiperidine nitroxide free radical, which uses 4-carbonyl-2,2,6,6-tetramethylpiperidine and hydrogen peroxide as raw materials, under the action of magnesium hydroxide catalyst to obtain a product, and then filters and distills under reduced pressure to obtain 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, under the action of magnesium hydroxide and diatomaceous earth composite catalyst to obtain a product, and then filters, extracts with ethyl acetate, and distills the organic phase under reduced pressure to obtain 2,2,6,6-tetramethylpiperidine nitroxide free radical.

[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 piperidine 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 piperidine nitrogen oxide free radical compounds prepared by the above method.

[0007] In a first aspect, the present invention provides a method for preparing a piperidine nitroxide free radical compound, comprising the following steps:

[0008] S1, adding a piperidine compound and a solvent into a reaction kettle, stirring and dissolving;

[0009] S2, adding a catalyst containing a modified titanium silicate molecular sieve into a reactor, adding a hydrogen peroxide solution, a piperidine compound 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 by vacuum distillation to obtain a piperidine nitroxide free radical compound;

[0011] The piperidine compound has the following structural formula:

[0012] Wherein R is a substituent.

[0013] In a second aspect, the present invention provides a piperidine nitrogen oxide free radical compound product prepared by the above method.

[0014] In a third aspect, the present invention provides a continuous reaction system for preparing piperidine nitroxide free radical compounds, comprising a reactor, a buffer tank, and a vacuum distillation column connected in sequence, wherein the buffer tank is connected to the feed inlet of the vacuum distillation column via a material pump; the reaction materials in the reactor are piperidine compounds, solvents, hydrogen peroxide, and a catalyst containing modified titanium silicon molecular sieves; the piperidine compounds have the following structural formula:

[0015] Wherein R is a substituent.

[0016] The beneficial effects of the method for preparing piperidine nitroxide free radical compounds provided by the present invention are:

[0017] The invention uses piperidine compounds and hydrogen peroxide as raw materials and modified titanium silicon molecular sieve as catalyst, and can prepare piperidine nitroxide free radical compounds in only one step. The method has 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 silicon molecular sieve can be reused, thereby improving economic benefits.

[0018] The continuous reaction system for preparing piperidine nitroxide free radical compounds provided by the present invention is suitable for the method provided by the present invention and can continuously operate to obtain piperidine nitroxide free radical compound products. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1The present invention provides a schematic flow diagram of a continuous reaction system for preparing piperidine nitrogen oxide free radical compounds.

[0020] in:

[0021] 1-reactor, 2-buffer tank, 3-vacuum distillation tower, 6-raw material, 4, 5, 7, 8-pipelines. DETAILED DESCRIPTION

[0022] 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.

[0023] In a first aspect, the present invention provides a method for preparing a piperidine nitroxide free radical compound, comprising the following steps:

[0024] S1, adding a piperidine compound and a solvent into a reaction kettle, stirring and dissolving;

[0025] S2, adding a catalyst containing a modified titanium silicate molecular sieve into a reactor, adding a hydrogen peroxide solution, a piperidine compound and hydrogen peroxide to carry out an oxidation reaction;

[0026] S3, removing the catalyst from the obtained product, and removing the solvent from the liquid product by vacuum distillation to obtain a piperidine nitroxide free radical compound;

[0027] The piperidine compound has the following structural formula:

[0028] Wherein R is a substituent.

[0029] In the piperidine compound, R is selected from carbonyl, hydroxymethylene, carboxymethylene, aminomethylene, C1-C4 alkylmethylene, C1-C4 alkoxymethylene or C1-C4 alkenylmethylene.

[0030] Preferably, R is selected from a carbonyl group, a hydroxymethylene group, a methoxymethylene group, or a carboxymethylene group. When R is a carbonyl group, the piperidine compound is 4-carbonyl-2,2,6,6-tetramethylpiperidine; when R is a hydroxymethylene group, the piperidine compound is 4-hydroxy-2,2,6,6-tetramethylpiperidine; when R is a methoxymethylene group, the piperidine compound is 4-methoxy-2,2,6,6-tetramethylpiperidine; and when R is a carboxymethylene group, the piperidine compound is 4-carboxy-2,2,6,6-tetramethylpiperidine.

[0031] In the method provided by the present invention, the solvent described in S1 is water and / or an organic solvent; the organic solvent is an aliphatic alcohol, cycloalkanol or ester compound containing 1 to 6 carbon atoms; preferably, the organic solvent is one or more selected from methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, tert-amyl alcohol, cyclohexanol, ethyl acetate, methyl propionate and ethyl propionate;

[0032] Preferably, the solvent is water.

[0033] In the method provided by the present invention, in S1, the mass ratio of the solvent to the piperidine compound is 0.1 to 20:1; preferably 0.2 to 4:1; more preferably 1 to 2:1.

[0034] In the method provided by the present invention, the catalyst containing modified titanium silicate molecular sieve is composed of 20wt%-100wt% of modified titanium silicate molecular sieve and 0-80wt% of 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.

[0035] Preferably, based on the total weight of the titanium silicalite, the modified titanium silicalite contains 90-99.9wt% of the titanium silicalite and 0.1-10wt% of the modified metal oxide, and the modified metal is an alkaline earth metal and / or a rare earth metal; more preferably, the modified titanium silicalite contains 98-99.8wt% of the titanium silicalite and 0.2-2wt% of the modified metal oxide.

[0036] Preferably, the titanium silicate molecular sieve is a TS-1 titanium silicate molecular sieve having an MFI topological structure;

[0037] Preferably, the modified metal oxide is selected from one or more of MgO, CeO2 and LaO2.

[0038] In the method provided by the present invention, the preparation method of the modified titanium silicate molecular sieve is:

[0039] C1. impregnating the titanium silicon molecular sieve with a modified metal salt solution to obtain an impregnated titanium silicon molecular sieve; the modified metal is an alkaline earth metal and / or a rare earth metal;

[0040] C2, drying the impregnated titanium silicon molecular sieve obtained in step C1 to obtain a dried impregnated titanium silicon molecular sieve;

[0041] C3. calcining the dried and impregnated titanium silicate obtained in step C2 to obtain a modified titanium silicate.

[0042] Among them, in step C1, the modified metal salt solution is preferably one or more of Mg(NO3)2, Ce(NO3)3, and La(NO3)3 aqueous solutions, the mass concentration of the modified metal salt solution is 0.1~15wt%, the mass ratio of the modified metal salt solution and titanium silicon molecular sieve is 0.5~3:1, the immersion temperature is 10~80℃, and the immersion time is 10~30 hours.

[0043] In step C2, the drying temperature is 100 to 160° C., and the drying time is 1 to 8 hours.

[0044] In step C3, the calcination temperature is 400-650° C., and the calcination time is 1-20 hours.

[0045] 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.

[0046] In the method provided by the present invention, in S2, preferably, a catalyst containing modified titanium silicalite is added to a reactor and evenly mixed with the solution obtained in S1. At 20°C-100°C, preferably 25°C-85°C, a hydrogen peroxide solution is slowly added dropwise and fully stirred until the reaction is complete.

[0047] In S2, the mass ratio of the catalyst to the piperidine compound is 0.005 to 0.1:1, preferably 0.01 to 0.05:1, and more preferably 0.01 to 0.04:1.

[0048] In S2, the hydrogen peroxide solution is a 1% to 70 wt% hydrogen peroxide solution, preferably, the hydrogen peroxide solution is a 5 to 40 wt% hydrogen peroxide solution; the molar ratio of the hydrogen peroxide to the piperidine compound is 0.5 to 50:1, preferably 1 to 10:1, and more preferably 3 to 6:1.

[0049] In the method provided by the present invention, in S3, the catalyst is removed from the obtained product by filtration, and the liquid product is directly subjected to reduced pressure distillation to remove the solvent to obtain a piperidine nitroxide free radical compound product; the separated catalyst is reused.

[0050] The vacuum distillation is operated under the following conditions: a pressure of 0.001 to 0.05 MPa, a distillation tower bottom temperature of 50 to 110°C, and a solvent discharged from the top of the tower. The solvent is water and / or an organic solvent, preferably water or a mixture of water and an organic solvent. The bottom discharge is a piperidine nitroxide free radical compound product.

[0051] The method provided by the present invention uses piperidine compounds and hydrogen peroxide as raw materials and modified titanium silicon molecular sieve as catalyst. Only one-step reaction is required to prepare piperidine nitrogen oxide free radical compounds, and subsequent separation is easy to obtain high-purity products, and the catalyst can be recovered and reused.

[0052] In a second aspect, the present invention provides a piperidine nitroxide free radical compound product prepared by any of the above methods for preparing piperidine nitroxide free radical compounds.

[0053] The product is a piperidine nitrogen oxide free radical compound with a purity of more than 99%, and the rest is unreacted piperidine compound.

[0054] In a third aspect, the present invention provides a continuous reaction system for preparing piperidine nitroxide free radical compounds, comprising a reactor, a buffer tank, and a vacuum distillation column connected in sequence, wherein the buffer tank is connected to the feed inlet of the vacuum distillation column via a material pump; the reaction materials in the reactor are piperidine compounds, solvents, hydrogen peroxide, and a catalyst containing modified titanium silicon molecular sieves; the piperidine compounds have the following structural formula:

[0055] Wherein R is a substituent.

[0056] 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.

[0057] The continuous reaction system of piperidine nitroxide free radical compounds provided by the present invention is described in detail below with reference to the accompanying drawings, but the present invention is not limited thereto.

[0058] Attachment Figure 1 The schematic flow diagram of the continuous reaction system for preparing piperidine 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 piperidine nitroxide free radical compounds provided by the present invention includes a reactor 1, a buffer tank 2 and a vacuum distillation tower 3 connected in sequence. The bottom of the buffer tank 2 is connected to the feed port of the vacuum distillation tower 2 via a material pump. The liquid phase product of the product after the oxidation reaction of the reaction material in the reactor 1 enters the buffer tank 2 after liquid-solid separation and enters the vacuum distillation tower 3 via the material pump. The top discharge 7 is the solvent, and the bottom discharge 8 of the vacuum distillation tower is used to obtain a piperidine nitroxide free radical compound product, which contains more than 99wt% of piperidine nitroxide free radical compounds.

[0059] The method of the present invention is further illustrated below by way of examples, but the present invention is not limited thereto.

[0060] In the Examples and Comparative Examples:

[0061] Titanium silicate molecular sieve TS-1 was prepared by the method disclosed in CN1421389A.

[0062] The piperidine compounds are all commercially available.

[0063] The yield is the actual mass of the product obtained / theoretical mass of the product that can be obtained × 100%;

[0064] The mass of the piperidine nitroxide free radical compound in the product is obtained by gas chromatography analysis. The purity of the product is the mass of the piperidine nitroxide free radical compound in the actual product / the total mass of the product×100%.

[0065] Preparation Example 1 of Modified Titanium Silica Molecular Sieve Catalyst

[0066] 0.352 g of magnesium nitrate hexahydrate was dissolved in 15.0 g of deionized water to obtain a magnesium 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 20 hours. The solution was then dried in air at 115°C for 5 hours and finally calcined at 550°C for 6 hours in an air atmosphere. The result was a magnesium oxide-supported titanium silicate molecular sieve impregnated with 0.369 wt%, designated catalyst GX-1.

[0067] Preparation Example 2 of Modified Titanium Silica Molecular Sieve Catalyst

[0068] 0.156 g of cerium nitrate hexahydrate was dissolved in 10.0 g of deionized water to obtain a cerium nitrate aqueous solution. 10.0 g of titanium silicate TS-1 was then added and impregnated at room temperature (25-30°C) for 30 hours. The solution was then dried in air at 130°C for 4 hours and finally calcined at 600°C for 4 hours in an air atmosphere. The resulting cerium oxide-supported titanium silicate, impregnated at 0.619 wt%, was designated catalyst GX-2.

[0069] Preparation Example 3 of Modified Titanium Silica Molecular Sieve Catalyst

[0070] 0.173 g of lanthanum nitrate hexahydrate was dissolved in 12.0 g of deionized water to obtain a lanthanum nitrate aqueous solution. 12.0 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 110°C for 6 hours and finally calcined at 600°C for 4 hours in an air atmosphere. The resulting lanthanum oxide-supported titanium silicate was impregnated at 0.543 wt%, designated catalyst GX-3.

[0071] Example 1

[0072] Add 15.7 g (0.1 mol) of 4-hydroxy-2,2,6,6-tetramethylpiperidine, 20.0 g of deionized water, and 0.34 g of GX-1 catalyst to a reactor. Thoroughly mix using magnetic stirring. Control the temperature at 65°C. Add 34.0 g (0.3 mol) of 30 wt% hydrogen peroxide dropwise over 2 hours. Continue the reaction for 6 hours after the addition is complete.

[0073] After the reaction was completed, the catalyst was filtered off, and the reaction product was distilled under reduced pressure to obtain 16.98 g of 4-hydroxy-2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.7% and a purity of 99.4%.

[0074] Example 2

[0075] 15.5 g (0.1 mol) of 4-carbonyl-2,2,6,6-tetramethylpiperidine, 18.0 g of methanol, and 0.34 g of GX-2 catalyst were added to a reactor. The mixture was thoroughly mixed using magnetic stirring and the temperature was controlled at 70°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 the reaction product was subjected to reduced pressure distillation to obtain 16.76 g of 4-carbonyl-2,2,6,6-tetramethylpiperidine nitroxide free radical compounds with a yield of 98.6% and a purity of 99.6%.

[0076] Example 3

[0077] 17.1 g (0.1 mol) of 4-methoxy-2,2,6,6-tetramethylpiperidine, 25.0 g of methanol, and 0.34 g of GX-3 catalyst were added to a reactor. The mixture was thoroughly mixed using magnetic stirring and the temperature was controlled at 60°C. 45.3 g (0.4 mol) of 30 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, and the reaction product was distilled under reduced pressure to obtain 18.32 g of 4-methoxy-2,2,6,6-tetramethylpiperidine nitroxide free radical compounds with a yield of 98.5% and a purity of 98.2%.

[0078] Example 4

[0079] The reaction conditions were the same as those in Example 1, except that GX-2 was used as the catalyst to obtain 16.87 g of 4-hydroxy-2,2,6,6-tetramethylpiperidine nitroxide free radical compounds with a yield of 98.1% and a purity of 99.2%.

[0080] Example 5

[0081] The reaction conditions were the same as those in Example 4, except that GX-3 was used as the catalyst to obtain 16.77 g of 4-hydroxy-2,2,6,6-tetramethylpiperidine nitroxide free radical compounds with a yield of 97.5% and a purity of 99.0%.

[0082] Example 6

[0083] The reaction conditions were the same as in Example 4, except that the catalyst used in Example 4 and filtered and dried was used as the catalyst to obtain 16.94 g of 4-hydroxy-2,2,6,6-tetramethylpiperidine nitroxide free radical compounds with a yield of 98.5% and a purity of 99.3%.

[0084] Example 7

[0085] 15.5 g (0.1 mol) of 4-carbonyl-2,2,6,6-tetramethylpiperidine, 25.0 g of tert-butanol, and 0.46 g of GX-1 catalyst were added to a reactor. The mixture was thoroughly mixed using magnetic stirring. The temperature was controlled at 80°C. 85.0 g (0.5 mol) of 20 wt% hydrogen peroxide was added dropwise over 2 hours. After the addition was complete, the reaction was continued for 4 hours.

[0086] After the reaction was completed, the catalyst was filtered off, and the reaction product was distilled under reduced pressure to obtain 16.69 g of 4-carbonyl-2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.2% and a purity of 99.1%.

[0087] Example 8

[0088] 17.1 g (0.1 mol) of 4-methoxy-2,2,6,6-tetramethylpiperidine, 30 g of ethyl acetate, and 0.52 g of GX-2 catalyst were added to the reactor. The mixture was thoroughly mixed using magnetic stirring. The temperature was controlled at 75°C. 43.7 g (0.45 mol) of 35 wt% hydrogen peroxide was added dropwise over 2 hours. After the addition was complete, the reaction was continued for 8 hours.

[0089] After the reaction was completed, the catalyst was filtered off, and the reaction product was distilled under reduced pressure to obtain 18.23 g of 4-methoxy-2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.0% and a purity of 99.1%.

[0090] Example 9

[0091] Add 18.5 g (0.1 mol) of 4-carboxy-2,2,6,6-tetramethylpiperidine, 35 g of deionized water, and 0.63 g of GX-3 catalyst to a reactor. Thoroughly mix using magnetic stirring. Control the temperature at 85°C. Add 68 g (0.6 mol) of 30 wt% hydrogen peroxide dropwise over 2 hours. Continue the reaction for 7 hours after the addition is complete.

[0092] After the reaction was completed, the catalyst was filtered off, and the reaction product was distilled under reduced pressure to obtain 19.58 g of 4-carboxyl-2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 97.9% and a purity of 99.3%.

[0093] Comparative Example 1

[0094] 15.7 g of 4-hydroxy-2,2,6,6-tetramethylpiperidine and 20 g of deionized water were added to a reactor without a catalyst. The mixture was thoroughly mixed using magnetic stirring and the temperature was controlled at 60°C. 34.0 g of 30 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 product was subjected to reduced pressure distillation to remove the solvent, yielding 15.05 g of a 4-hydroxy-2,2,6,6-tetramethylpiperidine nitroxide free radical compound with a yield of 98.5% and a purity of 40.1%.

[0095] Comparative Example 2

[0096] 15.7 g of 4-hydroxy-2,2,6,6-tetramethylpiperidine, 20 g of deionized water, and 0.34 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. 34.0 g of 30 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 completed, the product 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%.

Claims

1. A method for preparing a piperidine nitroxide free radical compound, characterized in that: The following steps are involved: S1, adding a piperidine compound and a solvent into a reaction kettle, stirring and dissolving; S2, adding the modified titanium silicate molecular sieve into a reactor, adding a hydrogen peroxide solution, a piperidine compound and hydrogen peroxide to carry out an oxidation reaction; S3, removing the catalyst from the obtained product, and removing the solvent from the liquid product by vacuum distillation to obtain a piperidine nitroxide free radical compound; The piperidine compound has the following structural formula: wherein R is a substituent, and R is selected from a carbonyl group, a hydroxymethylene group, a carboxymethylene group, a C1-C4 alkylmethylene group, a C1-C4 alkoxymethylene group, or a C1-C4 alkenylmethylene group; Based on the total weight of the titanium silicate molecular sieve, the modified titanium silicate molecular sieve is composed of 90-99.9wt% of the titanium silicate molecular sieve and 0.1-10wt% of the modified metal oxide, and the modified metal oxide is selected from one or more of MgO, CeO2 and LaO2.

2. The method for preparing piperidine nitroxide free radical compounds according to claim 1, wherein R is selected from carbonyl, hydroxymethylene, methoxymethylene or carboxymethylene.

3. The method for preparing piperidine nitroxide free radical compounds according to claim 1, wherein The solvent described in S1 is water and / or an organic solvent; the organic solvent is an aliphatic alcohol, cycloalkanol or ester compound containing 1 to 6 carbon atoms.

4. The method for preparing piperidine nitroxide free radical compounds according to claim 3, wherein The organic solvent is selected from one or more of methanol, ethanol, propanol, n-butanol, isobutanol, tert-butanol, tert-amyl alcohol, cyclohexanol, ethyl acetate, methyl propionate and ethyl propionate.

5. The method for preparing piperidine nitroxide free radical compounds according to claim 3 or 4, wherein: The solvent is water.

6. The method for preparing piperidine nitroxide free radical compounds according to claim 1, wherein In S1, the mass ratio of the solvent to the piperidine compound is 0.1 to 20:

1.

7. The method for preparing piperidine nitroxide free radical compounds according to claim 6, wherein The mass ratio of the solvent to the piperidine compound is 0.2-4:

1.

8. The method for preparing piperidine nitroxide free radical compounds according to claim 7, wherein: The mass ratio of the solvent to the piperidine compound is 1-2:

1.

9. The method for preparing a piperidine nitroxide free radical compound according to any one of claims 1-4 and 6-8, characterized in that: In S2, a catalyst containing modified titanium silicalite is added to a reactor and mixed evenly with the solution obtained in S1. The mass ratio of the catalyst to the piperidine compound is 0.01 to 0.05:

1. At 20°C to 85°C, a hydrogen peroxide solution is slowly added dropwise and stirred thoroughly until the reaction is complete.

10. The method for preparing piperidine nitroxide free radical compounds according to any one of claims 1-4 and 6-8, characterized in that: In S2, the hydrogen peroxide solution is a 5-40 wt% hydrogen peroxide solution, and the molar ratio of the hydrogen peroxide to the piperidine compound is 0.5-50:

1.

11. The method for preparing piperidine nitroxide free radical compounds according to claim 10, wherein: The molar ratio of the hydrogen peroxide to the piperidine compound is 1-10:

1.

12. The method for preparing piperidine nitroxide free radical compounds according to any one of claims 1-4 and 6-8, characterized in that: In S3, the obtained product is filtered to remove the catalyst, and the liquid product is directly subjected to reduced pressure distillation to remove the solvent to obtain a piperidine nitroxide free radical compound; the separated catalyst is reused.

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