Process for the preparation of colorless transparent orlan

Abstract

The present application relates to the field of fine chemical product refining, and specifically discloses a preparation method of colorless and transparent olaflur (bis(hydroxyethyl)-aminopropyl-N-hydroxyethyl-octadecylamine dihydrofluoride). The method comprises sequentially performing hydrogenation decolorization, extraction separation, adsorption and salt formation on colored olaflur. The method has the advantages of coherent process operation and remarkable decolorization effect, and has realized mass production. The olaflur product treated by the method can be completely changed from a significant yellow color to colorless, the APHA color value can be stably reduced to below 10, the product is clear and transparent, the color is stable during long-term storage, and the quality requirements of high-end application fields are fully met.

Description

Technical Field

[0001] This invention relates to the field of fine chemical product refining, specifically to a method for preparing colorless and transparent olaflu. Background Technology

[0002] Olafur is an active ingredient widely used in oral care for preventing tooth decay. Its core structure is a long-chain fatty tertiary amine hydrofluoric acid salt with multiple hydroxyl groups (bis(hydroxyethyl)-aminopropyl-N-hydroxyethyl-octadecylamine dihydrofluoric acid). The tertiary amine group in this molecule is chemically reactive and is easily oxidized by trace amounts of oxygen during production and storage, generating brownish-yellow oxide impurities. Simultaneously, the nitrogen and oxygen atoms in its structure readily form stable, dark complexes with trace amounts of iron, copper, and other metal ions introduced from equipment or raw materials. These impurities collectively cause the final product to exhibit a stubborn yellowish-brown color similar to salad oil, severely affecting the product's appearance and purity.

[0003] Traditional decolorization methods (such as activated carbon adsorption, conventional reducing agent treatment, or simple acid-base washing) have limited effectiveness against molecules with this special structure, often resulting in incomplete decolorization, low product yield, easy emulsification, cumbersome operation, or inability to remove multiple types of impurities simultaneously. Therefore, developing a specialized process that can achieve targeted and efficient deep decolorization and purification of olaflue has become a pressing technical challenge to improve the quality of the final olaflue product. Summary of the Invention

[0004] This invention provides a method for preparing colorless and transparent olafon to solve the above-mentioned technical problems. This method can simultaneously and deeply remove coloring impurities such as oxides and metal complexes, obtaining a product with low color, high purity, and good stability. Furthermore, the process conditions are mild, the yield is high, and it is suitable for continuous production.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A method for preparing colorless and transparent olaflume, wherein the method involves sequentially subjecting colored olaflume to hydrogenation decolorization, extraction separation, adsorption, and salt formation.

[0006] In the above scheme, the colored olaflu refers to olaflu ranging from light yellow to dark brown.

[0007] In the above scheme, the hydrogenation decolorization step involves dissolving the colored olaflu in an organic solvent to obtain an olaflu solution, adding a catalyst, stirring the solution under a hydrogen atmosphere for decolorization, and filtering to remove the solvent after the process is completed.

[0008] In the above scheme, the organic solvent is one of methanol, ethanol, isopropanol or tetrahydrofuran, preferably ethanol.

[0009] In the above scheme, the catalyst is one of palladium on carbon, platinum on carbon, or ruthenium on carbon, the catalyst loading is 1% to 10%, the catalyst dosage is 0.1% to 5% of the mass of the olaflu solution, the catalyst is preferably 5% to 10% palladium on carbon (Pd / C), and the catalyst dosage is preferably 1% to 2%.

[0010] In the above scheme, the system pressure of the stirring decolorization reaction is 0.5~3.0 MPa; the reaction temperature is 50~120℃; and the reaction time is 0.5~12 hours; preferably, the system pressure is 1.0~2.0 MPa, the reaction temperature is 70~90℃, and the reaction time is 2~6 hours. These mild conditions ensure efficient and selective hydrogenolysis of the NO bond while avoiding the dehydroxylation and excessive reduction of hydroxyl groups in the molecule.

[0011] In the above scheme, the extraction and separation step involves adding an extractant and an alkaline aqueous solution to the hydrogenated and decolorized material and mixing them thoroughly, then separating and collecting the organic phase and drying the organic phase.

[0012] In the above scheme, the extractant is a mixture of two or more of ethyl acetate, toluene, n-hexane, cyclohexane, methyl tert-butyl ether, and n-butanol. Preferably, the extractant is a mixed solvent of toluene and n-hexane (volume ratio 1:1 to 3:1). This mixed solvent has extremely high extraction efficiency (>95%) for olaflurane, can significantly reduce emulsification, promote rapid and clear phase separation, and has a moderate boiling point for easy recovery.

[0013] In the above scheme, the alkaline aqueous solution is one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, and potassium bicarbonate aqueous solutions, and the pH value is adjusted to above 8.0. Preferably, the alkaline aqueous solution is a sodium hydroxide aqueous solution, and the pH is adjusted to 10.0~12.0.

[0014] In the above scheme, the extraction temperature is 40~60℃. Preferably, the extraction temperature is 40~50℃.

[0015] In the above scheme, the adsorption step involves desolvating the organic phase obtained after extraction and separation by adsorption treatment through a composite adsorbent bed.

[0016] In the above scheme, the composite adsorbent bed is composed of the following layers according to the material flow direction: a first layer is a weakly acidic cation exchange resin for adsorbing trace metal ions; and a second layer is activated carbon or a macroporous neutral adsorbent resin for removing organic pigments and macromolecular impurities. Preferably, the first layer of weakly acidic cation exchange resin is a D113 type weakly acidic acrylic cation exchange resin; and the second layer of porous adsorbent material is a CAD-40 type macroporous neutral resin.

[0017] In the above scheme, the salt formation step involves dissolving the adsorbed product in a solvent, adding hydrofluoric acid or introducing hydrogen fluoride gas, and then stirring to form a salt, thereby obtaining a colorless and transparent oraflu product.

[0018] In the above scheme, the solvent is one of 1,2-propanediol, 1,3-propanediol, or pure water.

[0019] In the above scheme, the concentration of hydrofluoric acid is 30~60 wt%.

[0020] In the above scheme, the APHA color of the obtained colorless and transparent olaflue product is less than 10, the sample is clear and transparent, the Fe content is less than 2 ppm, and the Cu content is less than 1 ppm.

[0021] The core of the three-step synergistic decolorization process adopted in this invention lies in the targeted design of each step and the sequential dependence between the steps. This interlocking synergistic mechanism is the key to achieving deep decolorization of oraflu products.

[0022] The first step, hydrogenation decolorization, is fundamental to the entire process. This step selectively hydrogenates the NO bonds in oxides using a catalyst, reducing them to colorless tertiary amine structures and eliminating the primary chromophore at its chemical origin. The crucial role of this step is that without prior reduction of the oxides via hydrogenation, subsequent extraction and adsorption steps cannot effectively remove these chemically bonded chromophore structures—extraction can only achieve physical phase separation, and adsorption can only capture free molecules, but is ineffective against covalently bound oxide structures. Therefore, the hydrogenation step creates the necessary preconditions for subsequent purification.

[0023] The second step, extraction and separation, is crucial, serving as a link between the preceding and following steps. This step utilizes an alkaline environment to allow olaflume to enter the organic phase of the extractant, achieving efficient physical separation from some of the polar pigments remaining in the alkaline aqueous phase. Simultaneously, ion exchange is used to extract catalytic metal ions into the aqueous phase, which are then discharged with the wastewater. The core value of this step lies in two aspects: First, it builds upon the results of the hydrogenation step, extracting the reduced product from a reaction system containing numerous impurities; second, it reduces the load on the third step, adsorption—by pre-removing most of the metal ions and pigments, the load on the subsequent adsorption column is significantly reduced, preventing the adsorbent from becoming saturated too quickly and extending its lifespan. If this step is omitted and adsorption proceeds directly, a large number of metal ions and pigments will rapidly penetrate or clog the adsorption bed, resulting in a significant reduction in adsorption efficiency.

[0024] The third step, composite adsorption, is the final quality assurance, and its sequential design—removing metals first and then pigments—is crucial. The material first flows through a weakly acidic cation exchange resin layer, where its carboxylic acid groups specifically chelate trace metal ions that were not completely removed in the first two steps. Even trace amounts of these metal ions can catalyze the re-oxidation of amines during storage, leading to discoloration of the product. After complete removal of metal ions, the material then flows through a polar macroporous adsorption resin layer, selectively adsorbing residual trace polar pigment molecules. This order cannot be reversed: if pigments are removed before metals, the pigments adsorbed on the resin may be catalytically degraded by subsequently flowing metal ions, producing new colored substances, or side reactions may occur in the resin bed; simultaneously, metal ions not preferentially captured will remain in the final product, affecting long-term stability.

[0025] The synergistic effect of the three-step process is manifested in the following ways: the hydrogenation step resolves chemically bonded pigments, the extraction step resolves physically mixed pigments and most metal ions, and the adsorption step resolves trace residual impurities. Each step creates conditions and reduces the burden on the next, while the next step compensates for any shortcomings of the previous step, ultimately achieving a complete decolorization chain from chemical structure transformation to physical separation and then to deep purification. This sequential dependence and functional complementarity enable the three-step process to achieve a decolorization effect that cannot be achieved by any single step or any combination of two steps, as an organic whole.

[0026] Compared with the prior art, the present invention has the following significant advantages: Excellent and thorough decolorization effect: Through the triple guarantee of chemical reduction, physical separation and targeted adsorption, it can deeply remove various types of coloring impurities. The APHA color of Auraflu products can be reduced from more than 500 to less than 10, and the storage stability is excellent with no color return.

[0027] High process selectivity and high yield: Each step is optimized for the molecular characteristics of olaflu, with few side reactions and an overall yield of over 90%.

[0028] Overcoming the inherent drawbacks of multi-step processes: This invention features a three-step synergy, a linearized process, and a step that reduces the burden on subsequent steps (such as hydrogenation reducing adsorption load and extraction pre-removing metal ions), resulting in smoother overall operation, longer adsorbent life, and lower overall cost, achieving a transformation from complex and inefficient to highly efficient and synergistic.

[0029] Suitable for industrial production: The equipment used is all conventional chemical equipment, the conditions are mild and controllable, and it is easy to achieve large-scale and continuous production. Attached Figure Description

[0030] Figure 1 This is a color comparison diagram of the olaflu products prepared in some embodiments and comparative examples of the present invention. Detailed Implementation

[0031] The present invention will be further described below with reference to embodiments, but the scope of protection of the present invention is not limited thereto.

[0032] Test methods The colorimetric determination described in this invention uses a Lovibond PFX-i series automatic colorimeter, and is performed according to the standard methods of ASTM D1209-05 (2019) and GB / T 605-2006. The specific operation is as follows: An appropriate amount of olafon sample is placed in a cuvette with a 10 mm optical path. The platinum-cobalt colorimetric value (APHA) is directly read under constant temperature conditions of 25℃. For samples with APHA greater than the upper limit of detection (500), the sample is diluted with pure water to one-tenth of its original value before testing. The instrument's detection accuracy is ±1 APHA unit.

[0033] The clarity determination method described in this invention employs visual turbidimetry. An appropriate amount of olaflurane sample (approximately 5-10 mL) is placed in a 25 mL colorless, transparent, stoppered colorimetric tube. The sample is observed axially (from the tube opening to the bottom) and laterally (from the side) against a white background under fluorescent light, and the clarity state is recorded. (Clear and transparent: The sample is completely transparent, without any visible suspended matter, precipitate, or opalescence, and has a transparency comparable to pure water or solvent; almost clear: The sample is transparent, but has a slight opalescence or a very small amount of visible particles; turbid: The sample has obvious opalescence, turbidity, or visible suspended matter / precipitate.) Metal ion content was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). 0.5 g of sample was accurately weighed, microwave-digested, and then measured on an Agilent 725-ES spectrometer. Instrument operating conditions: RF power 1550 W, plasma gas flow rate 15 L / min, helium collision cell mode to eliminate polyatomic ion interference, Sc and Ge as internal standards, and external standard method for quantitative calculation of residual iron, copper, and other metal elements. The method limit of quantitation was 0.1 ppm.

[0034] Example 1

[0035] Step 1: Take 100 g of the olaflue sample prepared according to CN119661380A and dissolve it in 200 g of ethanol. Add 1.0 g of Pd / C catalyst (Pd content 5%). After purging the air, introduce hydrogen gas to a pressure of 1.5 MPa, heat to 80℃ and stir for 4 hours. After the reaction is complete, cool, filter to recover the catalyst, and desolvate the filtrate.

[0036] Step 2: After solvent removal, add 200 g of a mixed solvent of toluene and n-hexane (toluene: n-hexane = 2:1) to the residue, then add 10 wt.% NaOH solution to adjust the pH to 11, mix and stir at 50°C for 30 minutes, let stand and separate the liquid, collect the organic phase and dry it with anhydrous sodium sulfate.

[0037] Step 3: Pass the above organic phase through an adsorption column (the upper layer of the column is 100 mL of D113 type weakly acidic acrylic cation exchange resin, and the lower layer is 100 mL of CAD-40 type macroporous neutral resin). The effluent is concentrated under reduced pressure to recover the solvent, and the residue is dried in a vacuum drying oven for 12 hours. After cooling, 29.1 g of white solid is obtained.

[0038] Step 4: Dissolve the solid in 58.8 g of 1,2-propanediol, add 6.3 g of hydrofluoric acid (40 wt.% concentration) dropwise, stir for 30 minutes, and obtain 94.2 g of colorless and transparent oraflu product, with a total yield of 94.2%. The product's APHA color was determined to be 8, the sample was clear and transparent, and the Fe content was <2 ppm and the Cu content was <1 ppm.

[0039] Example 2

[0040] This embodiment is largely the same as Example 1, except that the olaflu sample used for decolorization and purification is commercially available, as detailed below: Step 1: Take 20 g of commercially available olafluroline sample and perform hydrogenation decolorization as in Step 1 of Example 1.

[0041] Step 2: The extraction and separation steps are the same as Step 2 in Example 1.

[0042] Step 3: The adsorption purification step is the same as step 3 in Example 1.

[0043] Step 4: The preparation of the olaflu product is the same as step 4 in Example 1.

[0044] The final product yielded 18.1 g of colorless and transparent olafluroline, with a total yield of 90.5%. The product's APHA color was determined to be 6, the sample was clear and transparent, and the Fe content was <2 ppm and the Cu content was <1 ppm.

[0045] Example 3

[0046] This embodiment is largely the same as Example 1, except that the hydrogenation catalyst used is platinum-carbon (Pt / C), as detailed below: Step 1: Take 100 g of the olaflue sample prepared according to CN119661380A and dissolve it in 200 g of ethanol. Add 1.0 g of Pt / C catalyst (Pt content 5%). After purging the air, introduce hydrogen gas to a pressure of 1.5 MPa, heat to 80℃ and stir for 4 hours. After the reaction is complete, cool, filter to recover the catalyst, and desolvate the filtrate.

[0047] Step 2: The extraction and separation steps are the same as Step 2 in Example 1.

[0048] Step 3: The adsorption purification step is the same as step 3 in Example 1.

[0049] Step 4: The preparation of the olaflu product is the same as step 4 in Example 1.

[0050] Finally, 91.2 g of colorless and transparent olafluroline product was obtained, with a total yield of 91.2%. The product's APHA color was determined to be 9, the sample was clear and transparent, and the Fe content was <2 ppm and the Cu content was <1 ppm.

[0051] Comparative Example 1

[0052] The difference between this comparative example and Example 1 is that: commercially available olaflue products were used to directly measure color and metal ion content, as detailed below: Step 1: Commercially available olaflu products are distinctly dark yellow. The APHA color value was measured to be 743. The sample was almost clear, with an Fe content of 9.8 ppm and a Cu content of 8.7 ppm.

[0053] Comparative Example 2

[0054] The difference between this comparative example and Example 1 is that the olafron sample prepared according to CN119661380A is not subjected to decolorization and purification treatment; the color and metal ion content are directly measured, as follows: Step 1: The sample of olaflu prepared according to CN119661380A is obviously dark yellow. After measurement, the APHA color of the product is 596. The sample is almost clear, with Fe content of 17.1 ppm and Cu content of 10.7 ppm.

[0055] Comparative Example 3

[0056] The difference between this comparative example and Example 1 is that only activated carbon treatment is used, as detailed below: Step 1: Take 20 g of the same batch of olaflu sample, dissolve it in 40 mL of toluene, add 2 g of granular activated carbon, reflux for 2 hours, hot filter, concentrate and desolvent, to obtain 17.5 g of decolorized olaflu sample, with a yield of 87.5%.

[0057] The olaf product obtained after decolorization with activated carbon is distinctly yellow with an APHA color of 260. The sample is clear and transparent, with an Fe content of 16.8 ppm and a Cu content of 9.9 ppm.

[0058] Comparative Example 4

[0059] The difference between this comparative example and Example 1 is that only the hydrogenation step is performed, and the product is directly concentrated after filtration, as detailed below: Step 1: The hydrogenation decolorization step is the same as step 1 in Example 1.

[0060] 20 g of olafluroline sample was decolorized by hydrogenation to obtain 19.6 g of high-quality product, with a total yield of 98.0%. The obtained olafluroline product was pale yellow with an APHA color of 112. The sample was almost clear, with an Fe content of 15.9 ppm and a Cu content of 10.2 ppm.

[0061] Comparative Example 5

[0062] The difference between this comparative example and Example 1 is that the olaflu sample was directly extracted and separated, followed by direct desolvation, as detailed below: Step 1: The extraction and separation steps are the same as step 2 in Example 1.

[0063] Step 2: The preparation of the olaflu product is the same as step 4 in Example 1.

[0064] 20 g of olaflue sample was extracted and separated to obtain 19.1 g of high-quality product, with a total yield of 95.5%. The obtained olaflue product was distinctly yellow with an APHA color of 389. The sample was almost clear, with an Fe content of 7.2 ppm and a Cu content of 4.3 ppm.

[0065] Comparative Example 6

[0066] The difference between this comparative example and Example 1 is that only the olaflue sample was directly dissolved and then purified by composite adsorption, as detailed below: Step 1: The adsorption purification step is the same as step 3 in Example 1.

[0067] 20 g of olafluroline sample was purified by composite adsorption to obtain 19.7 g of high-quality product, with a total yield of 98.5%. The obtained olafluroline product was distinctly yellow with an APHA color of 366. The sample was clear and transparent, with an Fe content of 6.4 ppm and a Cu content of 4.1 ppm.

[0068] Comparative Example 7

[0069] The difference between this comparative example and Example 1 is that: after hydrogenation, extraction and separation are performed, adsorption is omitted, and direct desolvation is carried out, as detailed below: Step 1: The hydrogenation decolorization step is the same as step 1 in Example 1.

[0070] Step 2: The extraction and separation steps are the same as Step 2 in Example 1.

[0071] Step 3: The preparation of the olaflu product is the same as step 4 in Example 1.

[0072] 20 g of olafluroline sample was subjected to "hydrogenation decolorization + extraction separation" to obtain 18.6 g of high-quality product, with a total yield of 93.0%. The obtained olafluroline product was almost colorless, with an APHA color of 42, and the sample was almost clear. The Fe content was 5.2 ppm and the Cu content was 3.1 ppm.

[0073] Comparative Example 8

[0074] The difference between this comparative example and Example 1 is that: after hydrogenation reduction, composite adsorption purification is performed directly, omitting extraction and separation, as detailed below: Step 1: The hydrogenation decolorization step is the same as step 1 in Example 1.

[0075] Step 2: The adsorption purification step is the same as step 3 in Example 1.

[0076] 20 g of olafluroline sample was purified by hydrogenation decolorization and adsorption to obtain 19.1 g of high-quality product, with a total yield of 95.5%. The obtained olafluroline product was almost colorless, with an APHA color of 27. The sample was clear and transparent, with an Fe content of 8.6 ppm and a Cu content of 3.6 ppm.

[0077] Comparative Example 9

[0078] The difference between this comparative example and Example 1 is that the order of the composite adsorption bed is reversed: it passes through CAD-40 resin first, then D113 resin. The rest is the same as in Example 1, as detailed below: Step 1: The hydrogenation decolorization step is the same as step 1 in Example 1.

[0079] Step 2: The extraction and separation steps are the same as Step 2 in Example 1.

[0080] Step 3: Pass the extracted organic phase through an adsorption column (the upper layer of the column is 100 mL of CAD-40 macroporous neutral resin, and the lower layer is 100 mL of D113 weakly acidic acrylic cation exchange resin). The eluent is concentrated under reduced pressure to recover the solvent, and the residue is dried in a vacuum drying oven for 12 hours. After cooling, a white solid is obtained.

[0081] Step 4: The preparation of the olaflu product is the same as step 4 in Example 1.

[0082] 100 g of olaflurane sample was purified by hydrogenation decolorization, extraction separation, and adsorption purification to obtain 93.1 g of high-quality product, with a total yield of 93.1%. The product's APHA color was determined to be 19, the sample was clear and transparent, and the Fe content was 4.9 ppm and the Cu content was 2.4 ppm.

[0083] Comparative Example 10

[0084] This comparative example is largely the same as Example 1, except that toluene is used as the extractant in the extraction and separation step, while the rest is the same as in Example 1, as detailed below: Step 1: The hydrogenation decolorization step is the same as step 1 in Example 1.

[0085] Step 2: After solvent removal, add 200 g of toluene to the residue, then add 10 wt.% NaOH solution to adjust the pH to 11, mix and stir at 50°C for 30 minutes, let stand and separate (severe emulsification occurs, and phase separation takes more than 6 hours), separate and collect the organic phase and dry with anhydrous sodium sulfate.

[0086] Step 3: The composite adsorption purification step is the same as step 3 in Example 1.

[0087] Step 4: The preparation of the olaflu product is the same as step 4 in Example 1.

[0088] 72.3 g of high-quality product was obtained from 100 g of olafluroline sample, with a total yield of 72.3%. The product's APHA color was determined to be 12, the sample was clear and transparent, and the Fe content was <2 ppm and the Cu content was <1 ppm.

[0089] Table 1 shows detailed comparative data on the color and Fe and Cu metal ion content of the olaf products prepared in the embodiments and comparative examples of the present invention.

[0090] Table 1

[0091] The invention has been described in detail in the specification with specific examples. However, those skilled in the art can make various modifications and alterations to the invention within the scope and technical concept of the invention, and such modifications and alterations also fall within the scope of the claims of the invention.

Claims

1. A method for preparing colorless and transparent olaflu, characterized in that, The method involves sequentially hydrogenating and decolorizing colored olaflume, followed by extraction, separation, adsorption, and salt formation.

2. The method for preparing colorless and transparent olafluuron according to claim 1, characterized in that, The hydrogenation decolorization step involves dissolving the colored olaflu in an organic solvent to obtain an olaflu solution, adding a catalyst, stirring the solution under a hydrogen atmosphere for a decolorization reaction, and then filtering to remove the solvent after the process is complete.

3. The method for preparing colorless and transparent olafluuron according to claim 2, characterized in that, The catalyst is one of palladium on carbon, platinum on carbon, or ruthenium on carbon, the catalyst loading is 1% to 10%, and the amount of catalyst used is 0.1% to 5% of the mass of the olaflu solution.

4. The method for preparing colorless and transparent olaflume according to claim 2, characterized in that, The system pressure for the stirring decolorization reaction is 0.5~3.0 MPa; the reaction temperature is 50~120℃; and the reaction time is 0.5~12 hours.

5. The method for preparing colorless and transparent olafluuron according to claim 1, characterized in that, The extraction and separation step involves adding an extractant and an alkaline aqueous solution to the hydrogenated and decolorized material and mixing them thoroughly, then separating and collecting the organic phase and drying the organic phase.

6. The method for preparing colorless and transparent olafluuron according to claim 5, characterized in that, The extractant is a mixture of two or more of ethyl acetate, toluene, n-hexane, cyclohexane, methyl tert-butyl ether, or n-butanol.

7. The method for preparing colorless and transparent olafluuron according to claim 5, characterized in that, The alkaline aqueous solution is one of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, or potassium bicarbonate aqueous solution.

8. The method for preparing colorless and transparent olafluuron according to claim 5, characterized in that, The extraction temperature is 40~60℃.

9. The method for preparing colorless and transparent olafluuron according to claim 1, characterized in that, The adsorption step involves desolvating the organic phase obtained after extraction and separation by adsorption through a composite adsorbent bed.

10. The method for preparing colorless and transparent olafluuron according to claim 9, characterized in that, The composite adsorbent bed is composed of the following layers according to the material flow direction: the first layer is a weakly acidic cation exchange resin and the second layer is activated carbon or macroporous neutral adsorption resin.

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