A synthesis method for preparing polymerization inhibitor 706 by using a sulfonated silicon dioxide catalyst

Abstract

The application provides a synthesis method of a polymerization inhibitor 706 by using a sulfonated silicon dioxide catalyst, and the synthesis method comprises the following steps: dissolving a polyoxypropylene polyoxyethylene copolymer solution in water, adding concentrated hydrochloric acid and heating to 40 DEG C, adding tetraethyl orthosilicate to the solution, adding 3-mercapto propyl trimethoxysilane and 30wt.% hydrogen peroxide after stirring, continuing to stir, and after the reaction is completed, filtering the solution to obtain a filter cake and a filtrate, mixing the filtrate with the filter cake after treating the filtrate with activated carbon, and obtaining the sulfonated silicon dioxide catalyst through aging, washing, acidification and drying; the sulfonated silicon dioxide catalyst synthesized by the method has the advantages of simple synthesis method, high catalytic reaction activity and high reaction selectivity in the synthesis of the 706 polymerization inhibitor.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation and chemical synthesis technology, specifically relating to a method for preparing polymerization inhibitor 706 using sulfonated silica catalyst. Background Technology

[0002] 706 polymerization inhibitor, chemically named bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate nitroxide radical, is a novel, highly efficient, and widely used free radical polymerization inhibitor with high inhibition efficiency, exhibiting excellent polymerization inhibition effects in both aerobic and anaerobic environments. This product is used to prevent the self-polymerization of polyolefin monomers during production, separation, purification, storage, or transportation, and to control and regulate the degree of polymerization of olefins and their derivatives in organic synthesis reactions. It exhibits good polymerization inhibition properties for acrylates, methacrylates, acrylic acid, acrylonitrile, styrene, butadiene, and other polymers.

[0003] Currently, the catalysts used in the synthesis of 706 polymerization inhibitors are mainly concentrated sulfuric acid or p-toluenesulfonic acid. These catalysts are used in large quantities, are costly, cause equipment corrosion, and are not recyclable, requiring large amounts of alkali for neutralization and subsequent discharge of neutralization wastewater. Sulfonated silica catalyst is a novel strong acid solid catalyst designed to replace concentrated sulfuric acid or p-toluenesulfonic acid in organic synthesis reactions. It is widely used in petrochemical and fine chemical industries. It solves the problems of high consumption, high cost, equipment corrosion, large discharge of neutralization wastewater, and catalyst recovery associated with the previous use of concentrated sulfuric acid or p-toluenesulfonic acid as catalysts. Summary of the Invention

[0004] To address the aforementioned problems, this invention discloses a method for synthesizing polymerization inhibitor 706 using a sulfonated silica catalyst.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A method for synthesizing polymerization inhibitor 706 using a sulfonated silica catalyst, the method comprising the following steps:

[0007] 2,2,6,6-Tetramethyl-4-hydroxypiperidine nitrogen oxide radical and dimethyl sebacate were sequentially added to a 100 mL four-necked flask equipped with a nitrogen protection and distillation apparatus. Then, sulfonated silica catalyst was added, and the reaction was carried out at 130–180 °C for 4–10 h to obtain the product 706 polymerization inhibitor.

[0008] The synthetic route for the reaction in this invention is as follows:

[0009]

[0010] Furthermore, the mass ratio of the sulfonated silica catalyst to the dimethyl sebacate is 1:50 to 120, and the molar ratio of the dimethyl sebacate to the 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide radical is 1:1.1 to 1:2.5.

[0011] The preparation method of the sulfonated silica catalyst includes the following steps:

[0012] Step (1): Take the polyoxypropylene-polyoxyethylene copolymer solution (P123) and stir it with distilled water until dissolved. Add concentrated hydrochloric acid and heat it to 40°C after mixing evenly to obtain mixture A.

[0013] Step (2): Add tetraethyl silicate dropwise to mixture A, stir for 2-8 hours, add 3-mercaptopropyltrimethoxysilane dropwise, stir, add 30 wt.% hydrogen peroxide, stir for 20 hours, and obtain mixture B;

[0014] Step (3): After filtering mixture B, filter cake and filtrate are obtained. The filtrate is treated with activated carbon and then mixed with the filter cake to obtain mixture C.

[0015] Step (4): Mixture C is placed in a forced-air drying oven and aged for 24 hours to obtain mixture D;

[0016] Step (5): Mixture D is filtered; ethanol is refluxed 3 times, each time for 4 hours; 400 ml of 2 mol / L hydrochloric acid is added for 4 hours; and the mixture is dried at 100℃ for 6 hours to obtain sulfonated silica catalyst.

[0017] Furthermore, the amount of concentrated hydrochloric acid added in step (1) is 10-30 ml.

[0018] Further, the molar ratio of tetraethyl silicate, 3-mercaptopropyltrimethoxysilane, and hydrogen peroxide in step (2) is 1:0.5 to 1.5:1.6.

[0019] Furthermore, the time interval between the addition of 3-mercaptopropyltrimethoxysilane and hydrogen peroxide in step (2) is 0 to 2 hours.

[0020] Furthermore, the aging temperature in step (4) is 90–120°C.

[0021] The beneficial effects of this invention are as follows:

[0022] (1) Sulfonated silica catalyst was synthesized for the first time using a one-pot method, and the synthesis method is simple;

[0023] (2) The sulfonated silica catalyst is composed of small nanoparticles with a spherical morphology (see [reference]). Figure 1 (As shown) provides more active sites and has a higher specific surface area (see...) Figure 2This is beneficial for improving reaction activity;

[0024] (3) Innovative use of sulfonated silica catalyst as the catalyst for the synthesis of 706 polymerization inhibitor. This solid catalyst has high activity, can be reused, has good repeatability, and maintains high activity and selectivity even after 25 repetitions. Attached Figure Description

[0025] Figure 1 This is a scanning electron microscope image of the sulfonated silica catalyst synthesized in Example 1 of the present invention;

[0026] Figure 2 The N2 adsorption-desorption isotherm and pore size distribution curve of the sulfonated silica catalyst synthesized in Example 1 of this invention are shown. Detailed Implementation

[0027] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0028] The manufacturers and models of the compounds used in the following examples are shown below; other chemical reagents used, unless otherwise specified, were obtained through conventional commercial channels.

[0029]

[0030]

[0031] Example 1

[0032] (1) Take 40g of polyoxypropylene-polyoxyethylene copolymer solution (P123) and stir with 980g of distilled water until dissolved. Add 20ml of concentrated hydrochloric acid. After mixing evenly, heat to 40℃ to obtain mixture A.

[0033] (2) Add 1 mol of tetraethyl silicate to mixture A, stir for 2 h, add 0.5 mol of 3-mercaptopropyltrimethoxysilane, and after the addition is complete, add 1.6 mol of 30 wt.% hydrogen peroxide, stir for 20 h, and obtain mixture B;

[0034] (3) After filtration, mixture B yields filter cake and filtrate. The filtrate is treated with activated carbon and then mixed with the filter cake to obtain mixture C.

[0035] (4) Mixture C was placed in a forced-air drying oven and aged at 90°C for 24 hours to obtain mixture D;

[0036] (5) Mixture D was filtered; ethanol was refluxed three times, each time for 4 hours; acidified with 400 ml of 2 mol / L hydrochloric acid for 4 hours; and dried at 100 °C for 6 hours to obtain the sulfonated silica catalyst, which was designated CAT-1. The scanning electron microscope image of the sulfonated silica catalyst is shown below. Figure 1 As shown;

[0037] (6) Dimethyl sebacate (14.0 g, 0.060 mol), 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide radical (22.7 g, 0.132 mol), and 0.36 g CAT-1 were added to a 100 mL four-necked flask equipped with a nitrogen-protected distillation apparatus. The mixture was heated to 160 °C, and methanol produced during the reaction was continuously distilled off. After 6 hours, the reaction was completed, and excess 1,2,2,6,6-pentamethylpiperidine alcohol was distilled off. The reaction system was washed with 90 °C hot water (50 mL × 2) to remove residual water from the product, yielding 28.62 g of an orange-red solid, with a yield of 93.4%. The product yield remained at 91.8% even after 25 cycles of catalyst CAT-1 recycling. The N2 adsorption-desorption isotherms and pore size distribution curves of the sulfonated silica catalyst are shown below. Figure 2 As shown.

[0038] Example 2

[0039] (1) Take 40g of polyoxypropylene-polyoxyethylene copolymer solution (P123) and 980g of distilled water and stir until dissolved. Add 10ml of concentrated hydrochloric acid and heat to 40℃ after mixing evenly to obtain mixture A.

[0040] (2) Add 1 mol of tetraethyl silicate to mixture A, stir for 2 h, add 0.5 mol of 3-mercaptopropyltrimethoxysilane, and after the addition is complete, add 1.6 mol of 30 wt.% hydrogen peroxide, stir for 20 h, and obtain mixture B;

[0041] (3) After filtration, mixture B yields filter cake and filtrate. The filtrate is treated with activated carbon and then mixed with the filter cake to obtain mixture C.

[0042] (4) Mixture C was placed in a forced-air drying oven and aged at 90°C for 24 hours to obtain mixture D;

[0043] (5) Mixture D was filtered; ethanol was refluxed 3 times, each time for 4 hours; acidified with 400 ml of 2 mol / L hydrochloric acid for 4 hours; dried at 100 °C for 6 hours to obtain sulfonated silica catalyst, which was numbered CAT-2.

[0044] (6) Dimethyl sebacate (14.0 g, 0.060 mol), 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide radical (24.8 g, 0.144 mol), and 0.42 g CAT-2 were added to a 100 mL four-necked flask equipped with a nitrogen-protected distillation apparatus. The mixture was heated to 180 °C, and methanol produced during the reaction was continuously distilled off. After 10 h, the reaction was completed, and excess 1,2,2,6,6-pentamethylpiperidine alcohol was distilled off. The reaction system was washed with 90 °C hot water (50 mL × 2) to remove residual water, yielding 27.73 g of an orange-red solid, with a yield of 90.1%. Even after 18 cycles of recycling using catalyst CAT-6, the product yield still reached 88.4%.

[0045] Example 3

[0046] (1) Take 40g of polyoxypropylene-polyoxyethylene copolymer solution (P123) and stir with 980g of distilled water until dissolved. Add 30ml of concentrated hydrochloric acid. After mixing evenly, heat to 40℃ to obtain mixture A.

[0047] (2) Add 1 mol of tetraethyl silicate to mixture A, stir for 2 h, add 0.5 mol of 3-mercaptopropyltrimethoxysilane, and after the addition is complete, add 1.6 mol of 30 wt.% hydrogen peroxide, stir for 20 h, and obtain mixture B;

[0048] (3) After filtration, mixture B yields filter cake and filtrate. The filtrate is treated with activated carbon and then mixed with the filter cake to obtain mixture C.

[0049] (4) Mixture C was placed in a forced-air drying oven and aged at 90°C for 24 hours to obtain mixture D;

[0050] (5) The mixture D was filtered; ethanol was refluxed 3 times, each time for 4 hours; acidified with 400 ml of 2 mol / L hydrochloric acid for 4 hours; dried at 100 °C for 6 hours to obtain sulfonated silica catalyst, which was numbered CAT-3.

[0051] (6) Dimethyl sebacate (14.0 g, 0.060 mol), 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide radical (24.8 g, 0.144 mol), and 0.36 g CAT-3 were added to a 100 mL four-necked flask equipped with a nitrogen-protected distillation apparatus. The mixture was heated to 150 °C, and methanol produced during the reaction was continuously distilled off. After 8 hours, the reaction was completed, and excess 1,2,2,6,6-pentamethylpiperidine alcohol was distilled off. The reaction system was washed with 90 °C hot water (50 mL × 2) to remove residual water, yielding 28.19 g of an orange-red solid, with a yield of 92.0%. Even after 25 cycles of catalyst CAT-3 recovery, the product yield still reached 90.7%.

[0052] Example 4

[0053] (1) Take 40g of polyoxypropylene-polyoxyethylene copolymer solution (P123) and stir with 980g of distilled water until dissolved. Add 20ml of concentrated hydrochloric acid. After mixing evenly, heat to 40℃ to obtain mixture A.

[0054] (2) Add 1 mol of tetraethyl silicate to mixture A, stir for 2 h, add 1 mol of 3-mercaptopropyltrimethoxysilane, and after the addition is complete, add 1.6 mol of 30 wt.% hydrogen peroxide, stir for 20 h, and obtain mixture B;

[0055] (3) After filtration, mixture B yields filter cake and filtrate. The filtrate is treated with activated carbon and then mixed with the filter cake to obtain mixture C.

[0056] (4) Mixture C was placed in a forced-air drying oven and aged at 90°C for 24 hours to obtain mixture D;

[0057] (5) The mixture D was filtered; ethanol was refluxed 3 times, each time for 4 hours; acidified with 400 ml of 2 mol / L hydrochloric acid for 4 hours; dried at 100 °C for 6 hours to obtain sulfonated silica catalyst, which was numbered CAT-4.

[0058] (6) Dimethyl sebacate (14.0 g, 0.060 mol), 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide radical (20.7 g, 0.12 mol), and 0.60 g CAT-4 were added to a 100 mL four-necked flask equipped with a nitrogen-protected distillation apparatus. The mixture was heated to 160 °C, and methanol produced during the reaction was continuously distilled off. The reaction was completed after 10 h, and excess 1,2,2,6,6-pentamethylpiperidine alcohol was distilled off. The reaction system was washed with 90 °C hot water (50 mL × 2) to remove residual water from the product, yielding 27.70 g of an orange-red solid, with a yield of 90.4%. The product yield was still 88.9% even after 22 cycles of recycling using the catalyst CAT-4.

[0059] Example 5

[0060] (1) Take 40g of polyoxypropylene-polyoxyethylene copolymer solution (P123) and stir with 980g of distilled water until dissolved. Add 20ml of concentrated hydrochloric acid. After mixing evenly, heat to 40℃ to obtain mixture A.

[0061] (2) Add 1.5 mol of tetraethyl silicate to mixture A, stir for 2 h, add 0.5 mol of 3-mercaptopropyltrimethoxysilane, and after the addition is complete, add 1.6 mol of 30 wt.% hydrogen peroxide, stir for 20 h, and obtain mixture B;

[0062] (3) After filtration, mixture B yields filter cake and filtrate. The filtrate is treated with activated carbon and then mixed with the filter cake to obtain mixture C.

[0063] (4) Mixture C was placed in a forced-air drying oven and aged at 90°C for 24 hours to obtain mixture D;

[0064] (5) The mixture D was filtered; ethanol was refluxed 3 times, each time for 4 hours; acidified with 400 ml of 2 mol / L hydrochloric acid for 4 hours; dried at 100 °C for 6 hours to obtain sulfonated silica catalyst, which was numbered CAT-5.

[0065] (6) Dimethyl sebacate (14.0 g, 0.060 mol), 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide radical (19.0 g, 0.11 mol), and 0.42 g CAT-5 were added to a 100 mL four-necked flask equipped with a nitrogen-protected distillation apparatus. The mixture was heated to 190 °C, and methanol produced during the reaction was continuously distilled off. The reaction ended after 8 hours, and excess 1,2,2,6,6-pentamethylpiperidine alcohol was distilled off. The reaction system was washed with 90 °C hot water (50 mL × 2) to remove residual water from the product, yielding 28.79 g of an orange-red solid, with a yield of 93.7%. The product yield remained at 92.2% even after 22 cycles of recycling using the catalyst CAT-5.

[0066] Example 6

[0067] (1) Take 40g of polyoxypropylene-polyoxyethylene copolymer solution (P123) and stir with 980g of distilled water until dissolved. Add 20ml of concentrated hydrochloric acid. After mixing evenly, heat to 40℃ to obtain mixture A.

[0068] (2) Add 1 mol of tetraethyl silicate to mixture A, stir for 4 h, add 0.5 mol of 3-mercaptopropyltrimethoxysilane, and after the addition is complete, add 1.6 mol of 30 wt.% hydrogen peroxide, stir for 20 h, and obtain mixture B;

[0069] (3) After filtration, mixture B yields filter cake and filtrate. The filtrate is treated with activated carbon and then mixed with the filter cake to obtain mixture C.

[0070] (4) Mixture C was placed in a forced-air drying oven and aged at 90°C for 24 hours to obtain mixture D;

[0071] (5) The mixture D was filtered; ethanol was refluxed 3 times, each time for 4 hours; acidified with 400 ml of 2 mol / L hydrochloric acid for 4 hours; dried at 100 °C for 6 hours to obtain sulfonated silica catalyst, which was numbered CAT-6.

[0072] (6) Dimethyl sebacate (14.0 g, 0.060 mol), 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide radical (20.7 g, 0.12 mol), and 0.42 g CAT-6 were added to a 100 mL four-necked flask equipped with a nitrogen-protected distillation apparatus. The mixture was heated to 150 °C, and methanol produced during the reaction was continuously distilled off. The reaction ended after 6 hours, and excess 1,2,2,6,6-pentamethylpiperidine alcohol was distilled off. The reaction system was washed with 90 °C hot water (50 mL × 2) to remove residual water, yielding 29.01 g of an orange-red solid, with a yield of 94.4%. After 25 cycles of recycling using the catalyst CAT-6, the product yield still reached 93.7%.

[0073] Comparative Example 1

[0074] The purpose is to compare with Example 1 to illustrate the effect of the activated carbon treatment step on catalyst activity.

[0075] (1) Take 40g of polyoxypropylene-polyoxyethylene copolymer solution (P123) and stir with 980g of distilled water until dissolved. Add 20ml of concentrated hydrochloric acid. After mixing evenly, heat to 40℃ to obtain mixture A.

[0076] (2) Add 1 mol of tetraethyl silicate to mixture A, stir for 2 h, add 0.5 mol of 3-mercaptopropyltrimethoxysilane, and after the addition is complete, add 1.6 mol of 30 wt.% hydrogen peroxide, stir for 20 h, and obtain mixture B;

[0077] (3) Mixture B was placed in a forced-air drying oven and aged at 90°C for 24 hours to obtain mixture C;

[0078] (4) The mixture C was filtered; ethanol was refluxed 3 times, each time for 4 hours; acidified with 400 ml of 2 mol / L hydrochloric acid for 4 hours; dried at 100 °C for 6 hours to obtain sulfonated silica catalyst, which was numbered CAT-7.

[0079] (4) Dimethyl sebacate (14.0 g, 0.060 mol), 2,2,6,6-tetramethyl-4-hydroxypiperidine nitric oxide radical (22.7 g, 0.132 mol), and 0.36 g CAT-7 were added to a 100 mL four-necked flask equipped with a nitrogen-protected distillation apparatus. The mixture was heated to 160 °C and the methanol produced during the reaction was continuously distilled off. The reaction ended after 6 h. The excess 1,2,2,6,6-pentamethylpiperidine alcohol in the reaction was distilled off. The reaction system was washed with 90 °C hot water (50 mL × 2) to remove residual water from the product, yielding 19.18 g of orange-red solid, with a yield of 62.6%.

[0080] Comparative Example 2

[0081] The purpose is to compare with Example 1 to illustrate the effect of the acidification step on catalyst activity.

[0082] (1) Take 40g of polyoxypropylene-polyoxyethylene copolymer solution (P123) and stir with 980g of distilled water until dissolved. Add 20ml of concentrated hydrochloric acid. After mixing evenly, heat to 40℃ to obtain mixture A.

[0083] (2) Add 1 mol of tetraethyl silicate to mixture A, stir for 2 h, add 0.5 mol of 3-mercaptopropyltrimethoxysilane, and after the addition is complete, add 1.6 mol of 30 wt.% hydrogen peroxide, stir for 20 h, and obtain mixture B;

[0084] (3) After filtration, mixture B yields filter cake and filtrate. The filtrate is treated with activated carbon and then mixed with the filter cake to obtain mixture C.

[0085] (4) Mixture C was placed in a forced-air drying oven and aged at 90°C for 24 hours to obtain mixture D;

[0086] (5) The mixture D was filtered; ethanol was refluxed 3 times, each time for 4 hours; and dried at 100°C for 6 hours to obtain sulfonated silica catalyst, which was numbered CAT-8.

[0087] (4) Dimethyl sebacate (14.0 g, 0.060 mol), 2,2,6,6-tetramethyl-4-hydroxypiperidine nitric oxide radical (22.7 g, 0.132 mol), and 0.42 g CAT-8 were added to a 100 mL four-necked flask equipped with a nitrogen-protected distillation apparatus. The mixture was heated to 160 °C and the methanol produced during the reaction was continuously distilled off. The reaction ended after 8 h. The excess 1,2,2,6,6-pentamethylpiperidine alcohol in the reaction was distilled off. The reaction system was washed with 90 °C hot water (50 mL × 2) to remove residual water from the product, yielding 24.94 g of orange-red solid, with a yield of 81.4%.

[0088] It should be noted that the above content merely illustrates the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. For those skilled in the art, various improvements and modifications can be made without departing from the principle of the present invention, and all such improvements and modifications fall within the scope of protection of the claims of the present invention.

Claims

1. A method for the synthesis of 706 polymerization inhibitor using a sulfated silica catalyst, characterized in that, The synthesis method comprises the following steps: 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide and dimethyl sebacate are sequentially added into a 100 mL four-necked flask provided with nitrogen protection and a distillation device, then a sulfonated silicon dioxide catalyst is added, and reaction is carried out at a temperature of 130-180 DEG C for 4-10 h to obtain the product 706 polymerization inhibitor, and the reaction route is shown as follows: ， The preparation method of the sulfonated silicon dioxide catalyst comprises the following steps: Step (1): polyoxypropylene-polyoxyethylene copolymer solution is stirred with distilled water until dissolved, concentrated hydrochloric acid is added, the mixture is heated to 40 DEG C after uniform mixing, and mixture A is obtained; Step (2): tetraethyl silicate is added dropwise to mixture A, stirring is carried out for 2-8 h, 3-mercaptopropyl trimethoxysilane is added dropwise, after stirring, 30 wt.% hydrogen peroxide is added, and stirring is carried out for 20 h to obtain mixture B; Step (3): mixture B is filtered to obtain filter cake and filtrate, the filtrate is treated with activated carbon, and then mixed with the filter cake to obtain mixture C; Step (4): mixture C is placed in a forced air drying oven and aged for 24 h to obtain mixture D; Step (5): mixture D is filtered, washed with ethanol reflux for 3 times, each time for 4 h, acidified with 400 ml 2 mol / L hydrochloric acid for 4 h, and dried at 100 DEG C for 6 h to obtain the sulfonated silicon dioxide catalyst.

2. The method of claim 1, wherein the method of preparing 706 polymerization inhibitor using a sulfated silica catalyst is characterized by, The mass ratio of the sulfonated silicon dioxide catalyst to the dimethyl sebacate is 1:50-120.

3. The method of claim 1, wherein the method of preparing 706 polymerization inhibitor using a sulfated silica catalyst is characterized by, The molar ratio of the dimethyl sebacate to the 2,2,6,6-tetramethyl-4-hydroxypiperidine nitroxide is 1:1.1-1:2.

5.

4. The process for the synthesis of 706 polymerization inhibitor using sulfonated silica catalyst as claimed in claim 1, wherein, The amount of the concentrated hydrochloric acid added in step (1) is 10-30 ml.

5. The method of claim 1, wherein the method of preparing 706 polymerization inhibitor using a sulfated silica catalyst is characterized by, The molar ratio of the tetraethyl silicate, 3-mercaptopropyl trimethoxysilane and hydrogen peroxide in step (2) is 1:0.5-1.5:1.

6.

6. The method of claim 1, wherein the method of preparing 706 polymerization inhibitor using a sulfated silica catalyst is characterized by, The interval between the addition of the 3-mercaptopropyl trimethoxysilane and the hydrogen peroxide in step (2) is 2 h.

7. The method of claim 1, wherein the method is characterized by, The aging temperature in step (4) is 90-120 DEG C.

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