Regeneration of molecular filters

Abstract

A non-thermal method for regenerating an alkali metal permanganate impregnated molecular filter comprising: washing a spent molecular filter comprising oxidized products selected from manganate (MnO₄²⁻), manganese dioxide (MnO₂), and sulfate and nitrate salts using distilled water; contacting said washed and wet molecular filter with a solution comprising: (i) an alkali metal hydroxide, and (ii) an alkali metal persulfate, to promote in-situ oxidation of Mn(IV) or Mn(VI) species to Mn(VII), thereby obtaining a treated molecular filter; and drying the treated molecular filter to yield a regenerated molecular filter characterized by the reformation of permanganate ions.

Description

[0001]REGENERATION OF MOLECULAR FILTERS FIELD OF THE INVENTION: The present invention relates to molecular filters. Particularly, the present invention relates to regeneration of molecular filters. More particularly, the present invention relates to regeneration of alkali metal oxidizer impregnated molecular filters. BACKGROUND OF THE INVENTION: It is known that various detrimental airborne compounds such as oxides of nitrogen, sulfur compounds, ammonia, formaldehyde, carbon monoxide, oxides of sulfur, mercaptans, amines, and ethylene are present in the environment and causes pollution, toxicity, and disagreeable odors. Several molecular filters rely on oxidizing or reducing agents that are impregnated in them for abating pollutants in air or water, especially air. For example, permanganate media works by oxidizing Sox / NOx. NOx refers to total concentration of the most important oxides of nitrogen that are emitted by combustion sources: nitric oxide (NO) and nitrogen dioxide (NO2). Principal sources of NOx in a combustion process are fuel NOx and thermal NOx. SOx refer to oxides of Sulphur which include Sulphur dioxide (SO2) and Sulphur trioxide (SO3). SO2results from oxidation of Sulphur in fuels such as coal and oil. NOx and SOx are typically oxidized into salts such as sulphates and nitrates and, in turn, reduces to manganate, manganese dioxide, and states with lower oxidation number. For instance, US6004522 discloses solid media comprises a porous substrate impregnated with at least approximately 7% potassium permanganate by weight of the composition; at least approximately 10% water by weight of the composition; sodium bicarbonate, and activated alumina. US20220032232 discloses a filter medium for separating nitrogen oxides from a gas mixture such as ambient air, wherein the filter medium comprises three layers that contain different adsorbent materials. A layer (A) comprising non-impregnated active carbon, a layer (B) comprising a solid carrier material that is impregnated with a permanganate salt; a layer (C) comprising alkaline impregnated active carbon. Considering their high efficiency, molecular filters / media are used in various application areas, such as air dryers, scrubber tanks, air / exhaust systems, waste water treatment, odour control, refinery plants, chemical plants, and the like; in order to eliminate corrosive gas, airborne pollutants, and the like. Its regeneration is must in order to make repetitive use of such filters and in order to save costs. Currently, these filters are non-regenerable and generally sent into landfills after end of their life where the oxidizers have been used up. Accordingly, the present invention envisaged to provide a simple and cost-effective process for regeneration of the molecular filter which is more efficient. OBJECTS OF THE INVENTION: It is an object of the present invention to provide a process for regeneration of the molecular filter which is non-thermal and efficient It is an object of the present invention to provide a process for regeneration of the molecular filter which is a simple and cost effective and carried out at room temperature. SUMMARY OF THE INVENTION: According to this invention, there is provided systems and methods for regeneration of molecular filters. The present invention provides systems and methods for regenerating alkali metal permanganate-impregnated molecular filters through a controlled redox process that operates at ambient or mildly elevated temperatures (50°C–110°C). The regeneration involves: • Removing water-soluble byproducts (e.g., nitrates and sulfates) via aqueous washing. • Chemically reoxidizing reduced manganese species (Mn²⁺, MnO₂, MnO₄²⁻) into the active permanganate form (MnO₄⁻) using a stoichiometrically optimized composition comprising sodium hydroxide (NaOH) and sodium persulfate (Na₂S₂O₈). • Optionally, pretreating the filter with sodium metabisulfite to reduce residual permanganate before oxidation, improving yield and selectivity. • Monitoring the regeneration via colorimetric transition and confirmatory chemical tests. Notably, this regeneration mechanism is grounded in the thermodynamic feasibility of manganese oxidation states and leverages persulfate’s high oxidation potential under alkaline conditions to promote the conversion of Mn(IV) and Mn(VI) species back to Mn(VII), restoring the filter's functionality. According to this invention, there is provided a non-thermal method for regenerating an alkali metal permanganate impregnated molecular filter comprising a porous media selected from alumina, carbon, zeolites, or molecular sieves, the method comprising: a. washing a spent molecular filter comprising oxidized products selected from manganate (MnO₄²⁻), manganese dioxide (MnO₂), and sulfate and nitrate salts using distilled water to selectively remove water-soluble salts, thereby obtaining a washed and wet molecular filter; b. contacting said washed and wet molecular filter with a solution comprising: (i) an alkali metal hydroxide selected from sodium hydroxide (NaOH) or potassium hydroxide (KOH), and (ii) an alkali metal persulfate selected from sodium persulfate (Na₂S₂O₈) or potassium persulfate (K₂S₂O₈), in a weight ratio of hydroxide to persulfate ranging from 1:4 to 1:7 and maintaining the pH of the resulting mixture between 9.5 and 11.0 to promote in-situ oxidation of Mn(IV) or Mn(VI) species to Mn(VII), thereby obtaining a treated molecular filter; and c. drying the treated molecular filter at a temperature between 50°C and 110°C to yield a regenerated molecular filter characterized by the reformation of permanganate ions. In at least an embodiment, the regeneration process further comprises a pretreatment step involving contacting the washed molecular filter with a metal bisulfite solution selected from sodium metabisulfite (Na₂S₂O₅) in a concentration ranging from 10% to 50% wt. / wt., under mildly acidic conditions to ensure complete reduction of residual permanganate prior to reoxidation. In at least an embodiment, the regeneration process further comprises a pretreatment step involving contacting the washed molecular filter with a metal bisulfite solution selected from sodium metabisulfite (Na₂S₂O₅) in a concentration ranging from 10% to 50% wt. / wt., under mildly acidic conditions to ensure complete reduction of residual permanganate prior to reoxidation, said pretreatment step being conducted at ambient temperature for a duration of 5 to 30 minutes prior to regeneration. In at least an embodiment, the weight ratio of the molecular filter to alkali metal hydroxide to alkali metal persulfate is maintained within the range of 25:1:6. In at least an embodiment, the regeneration occurs at ambient pressure without the application of external thermal energy or catalysts. In at least an embodiment, the drying step is performed under forced air convection at 50 ± 5°C for 2–5 hours. DETAILED DESCRIPTION OF THE INVENTION: While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is intended to cover all modifications, equivalents, and alternatives falling within the scope of the disclosure. Further, the phraseology and terminology employed in the description is for the purpose of description only and not for the purpose of limitation. The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusion, such that a device, apparatus, system, assembly, method that comprises a list of components or a series of steps that does not include only those components or steps but may include other components or steps not expressly listed or inherent to such apparatus, or assembly, or device. In other words, one or more elements or steps in a system or device or process proceeded by “comprises… a” or “comprising …. of” does not, without more constraints, preclude the existence of other elements or additional elements or additional steps in the system or device or process as the case may be. According to this invention, there is provided systems and methods for regeneration of molecular filters. In accordance with the present invention there is provided a process for regeneration of an alkali metal permanganate impregnated molecular filter; said process comprises the following steps: a. washing the molecular filter using distilled water to remove one or more water-soluble salts and obtain a washed and wet molecular filter; b. adding alkali metal hydroxide and alkali metal persulfate to said washed and wet molecular filter to obtain a treated molecular filter; and c. drying said molecular filter at a temperature of about 500C to 1100C to obtain the regenerated molecular filter. In one embodiment of the present invention, the alkali metal permanganate is sodium permanganate or potassium permanganate. In one embodiment of the present invention, the alkali metal permanganate impregnated molecular filter is a used or spent filter. In one embodiment, the molecular filter comprises spherical alumina, carbons, zeolites or molecular sieve media impregnated with potassium permanganate. Generally, the percentage of the permanganate varies from 8% to 18%. In one embodiment of the present invention, the water-soluble salts are salt of nitrates and sulphates. In one embodiment of the present invention, the alkali metal hydroxide is sodium hydroxide or potassium hydroxide. In one embodiment of the present invention, the alkali metal persulfate is sodium persulfate or potassium persulfate. In one embodiment of the present invention, the metal permanganate is sodium permanganate or potassium permanganate. Typically, the percentage of sodium hydroxide for regenerating the spent molecular filter ranges from 2% to 10% wt. / wt. Typically, the amount of sodium persulfate in the composition ranges from 10% to 30% wt. / wt. In one embodiment of the present invention, the process comprises a step of treating the washed molecular filter with metal bisulfite. Typically, the amount of metal bisulfite in the composition is ranging from 10% to 50% wt. / wt. In one embodiment of the present invention, metal bisulfite is sodium meta bisulfite. In one embodiment, the weight proportion of metal hydroxide (NaOH) to metal persulfate is 1:4 to 1:7, preferably 1:6. In one preferred embodiment, the weight proportion of the molecular filter to sodium hydroxide (NaOH) to sodium persulfate is 20-30:1:4-7, preferably 25:1:6. In one embodiment of the present invention the process is carried out at a pH ranging from 8 to 11, preferably at 10 to 11. In one embodiment, in the step (c), the formation of alkali metal permanganate is confirmed by change of color of molecular filter from brown / green to pink. The formation of alkali metal permanganate is confirmed by a confirmatory test. Said confirmatory test comprises adding a mixture of sodium hydroxide and sugar to the alkali metal permanganate solution obtained by adding water to the said regenerated filters. The colour changes from pink to green to yellow indicates formation of alkali metal permanganate. Excess sodium persulfate in an alkaline medium result in the formation of sodium permanganate. Sodium or potassium manganate (Na₂MnO₄) is oxidized to form sodium permanganate (NaMnO₄) in an alkaline medium, using an oxidizing agent such as sodium persulfate (Na₂S₂O₈). Sodium manganate (Na₂MnO₄) is typically dark green in color. The green color is due to the presence of the manganate ion (MnO₄²⁻), which has a distinct green hue. This is in contrast to sodium permanganate (NaMnO₄), which is purple due to the permanganate ion (MnO₄⁻). The present invention is now illustrated with the help of reactions that takes place during the regeneration of the molecular filter. Reduction of KMnO4using Sodium metabisulphite Sodium metabisulphite (NaHSO3) reacts with potassium permanganate (KMnO4), and produces sodium sulfate (Na2SO4), manganese dioxide (MnO2), and potassium sulfate (K2SO4). In this reaction, sodium metabisulphite acts as a reducing agent, while KMnO4acts as an oxidizing agent. 2KMnO4+5NaHSO3+3H2SO4→K2SO4+2MnSO4+5NaHSO4+3H2O Potassium permanganate (KMnO₄) is reduced to manganese(II) sulfate (MnSO₄). Sodium bisulfite (NaHSO₃) is oxidized to sodium bisulfate (NaHSO₄). The reaction is typically carried out in an acidic medium (H₂SO₄) to facilitate the redox process. When potassium permanganate (KMnO₄) reacts with sodium metabisulfite (Na₂S₂O₅), the color change observed is due to the reduction of the permanganate ion (MnO₄⁻). Potassium permanganate is purple in color. When it reacts with sodium metabisulfite, it gets reduced to manganese(II) ion (Mn²⁺), which is typically pale pink or nearly colorless in dilute solution. Initially, the solution will be purple due to the KMnO₄. As the reaction proceeds, the purple color fades, indicating the reduction of MnO₄⁻ ions to Mn²⁺ ions. The final solution will be nearly colorless or very pale pink, depending on the concentration of the Mn²⁺ ions. The molecular filter can be fully reduced with sodium metabisulfite and then regenerated using NaOH and sodium persulfate. The synthesis of sodium permanganate (NaMnO₄) from sodium persulfate (Na₂S₂O₈) and manganese dioxide (MnO₂) involves the use of sodium hydroxide (NaOH) primarily to adjust the pH and facilitate the reaction conditions. The present invention relies on a carefully controlled chemical oxidation mechanism that regenerates manganese in lower oxidation states (Mn(IV), Mn(VI)) back into the permanganate oxidation state (Mn(VII)). The key reactive components include: • an alkali hydroxide (e.g., NaOH) providing an alkaline medium and stabilizing high-valent manganese species. • an alkali persulfate (e.g., Na₂S₂O₈), which is a strong oxidizing agent that decomposes to generate sulfate radicals (SO₄•⁻) in situ under basic aqueous conditions. The following stoichiometric reactions are proposed to occur during the regeneration step: Oxidation of Manganese Dioxide [Oxidation of Manganese Dioxide (MnO₂) – Mn(IV) to Mn(VII)]: Manganese dioxide (MnO₂) acts as a precursor and is oxidized by sodium persulfate in an alkaline medium. 2 MnO2+ 3 Na2S2O8+8 NaOH → 2 NaMnO4+ 6 Na2SO4+ 4 H2O In this reaction, sodium persulfate (Na₂S₂O₈) oxidizes manganese dioxide (MnO₂) to form sodium permanganate (NaMnO₄), sodium sulfate (Na₂SO₄), and water (H₂O). Under alkaline conditions, persulfate oxidizes MnO₂ to MnO₄⁻. This reaction assumes complete conversion with excess base. The intermediate radicals from persulfate, such as SO₄•⁻, are drive the oxidation step. Similarly, potassium persulfate can be used in reaction. MnO2+ 4 NaOH + K2S2O8→ K2MnO4+ 2 K2SO4+ 2 H2O → KMnO4 This reaction shows the transformation of manganese dioxide into potassium manganate in an alkaline medium using potassium persulfate as the oxidizing agent. Importance of NaOH: pH Adjustment: NaOH is essential to maintain an alkaline pH during the reaction. The presence of an alkaline environment (pH around 10- 11) is crucial, as it stabilizes the permanganate ion (MnO₄⁻) formed during the reaction. This pH range promotes the efficient oxidation of MnO₂ by Na₂S₂O₈. Reaction Medium: NaOH also serves as a medium in which the reaction between MnO₂ and Na₂S₂O₈ can occur effectively. It ensures that the components are in a suitable chemical environment for the oxidation reaction to proceed. Oxidation of Manganate Ion (MnO₄²⁻) – Mn(VI) to Mn(VII) Manganate can be oxidized further to permanganate: This reaction is especially pH-sensitive and occurs optimally between pH 9.5 and 11. Outside this range, disproportionation or precipitation of MnO₂ may dominate. Formation of sodium permanganate: After the reaction, sodium permanganate (NaMnO₄) is formed along with sodium sulfate and water. The sodium permanganate remains in the filter and does the job of oxidation of the SOx NOx etc gases. After the filter is spent the excess sodium sulphate is removed in the washing steps. Apart from sodium persulfate (Na₂S₂O₈), there are other reagents that can be used in the regeneration of sodium permanganate (NaMnO₄) from reduced species such as manganese dioxide (MnO₂). Potassium permanganate (KMnO₄): Potassium permanganate itself can be used as a starting material to produce sodium permanganate through metathesis reactions. KmnO4+NaOH→NaMnO4+KOH This reaction converts potassium permanganate into sodium permanganate and potassium hydroxide. Decomposition of Sodium Persulfate in Alkaline Medium Although often thermally initiated, under strongly basic and aqueous conditions, persulfate can undergo slow decomposition to generate sulfate radicals, which oxidize Mn species. These radicals drive the oxidation from Mn(IV) / Mn(VI) to Mn(VII), in situ. Preferred Ratios: • Filter: sodium hydroxide (NaOH) : sodium persulfate (Na₂S₂O₈) = 25:1:6 • sodium hydroxide (NaOH) : sodium persulfate (Na₂S₂O₈) = 1:6 • pH maintained at ~10.5 for optimal oxidation. The regeneration strategy is based on the redox cycling of manganese species within the filter substrate. Spent filters typically contain manganese dioxide (MnO₂) and manganate (MnO₄²⁻); lower oxidation state residues formed after permanganate (MnO₄⁻) oxidizes airborne pollutants. These lower-valent species are chemically inert under neutral conditions and pose a challenge for reactivation. However, under alkaline and oxidative environments, it is thermodynamically and kinetically favorable to drive these reduced species back into their Mn(VII) oxidation state: MnO₂ + Na₂S₂O₈ + NaOH → NaMnO₄ + Na₂SO₄ + H₂O This reaction benefits from: • Persulfate’s high oxidation potential; • strong nucleophilicity and solubilization of Mn species in high pH; • in situ formation of permanganate confirmed by distinct purple coloration and redox tests. Moreover, sodium hydroxide serves dual roles: • Establishes the required alkaline environment (pH 10–11); • Solubilizes manganate precursors and enhances mass transfer across the filter matrix. The regeneration reaction is critically dependent on maintaining a pH between 9.5 and 11.0, a range that: • prevents MnO₂ precipitation • avoids self-decomposition of MnO₄⁻ • enables radical-assisted oxidation without requiring elevated temperatures Unlike prior art that relies on thermal regeneration, acidic oxidizers, or mechanical substitution, the current process regenerates Mn(VII) chemically under mild, non-thermal, alkaline conditions, thereby preserving the filter integrity. Process Steps: 1. Aqueous Pre-Wash: o Removes nitrate, sulfate, and chloride salts which may impede redox kinetics or participate in side reactions. 2. Selective Reduction (Optional): o Addition of sodium metabisulfite reduces residual Mn(VII) into Mn(II) or Mn(IV), facilitating more uniform regeneration upon subsequent oxidation. 3. Reoxidation Treatment: o A composition comprising: ^ Sodium hydroxide (2–10% w / w); ^ Sodium persulfate (10–30% w / w); o Applied to the washed filter media under mild agitation or diffusion. 4. Drying: o Performed at 50°C–110°C to preserve substrate structure and allow moisture desorption. In the comparative experiments, various other alternative reducing agent were attempted which include but are not limited to hydrogen peroxide, oxalic acid, iron (II) sulfate, hydrochloric acid, sulfurous acid etc. The oxidizing agents tested include but are not limited to Sodium chlorate, ozone, nitric acid, chlorine, bromine, potassium dichromate etc. After several experiments, it was found that the alkali metal persulfate such as sodium persulfate or potassium persulfate are most efficient oxidizing agent in terms of regeneration efficiency. Similarly, the metal bisulfite such as sodium bisulfite is found to provide efficient reduction which in turn also supports efficient regeneration of the molecular filter. In one aspect there is provided a composition for regeneration of molecular filter; said composition comprises sodium hydroxide (NaOH) and sodium persulfate, wherein the weight proportion of sodium hydroxide (NaOH) to sodium persulfate is important. Preferably, the weight proportion of metal hydroxide (NaOH) to sodium persulfate is 1:6. Typically, the amount of sodium hydroxide in the composition is ranging from 1 to 10 g, preferably 4 g (4% w / w). The amount is calculated based on total weight of the molecular filer / media (100 g). Typically, the amount of sodium persulfate in the composition is ranging from 20 to 30 g, preferably, 24 g (24% w / w). In accordance with another aspect of the present invention there is provided a kit for regeneration of molecular filter; said kit comprising: a) a storing means containing sodium hydroxide (NaOH) and sodium persulfate; and b) an instruction leaflet covering the information about how to use the kit. Unlike prior systems that utilize: • High-temperature thermal regeneration (which destroys media), • Acid-base leaching (which deteriorates structural integrity), • Non-recyclable media configurations (which mandate disposal), The present invention uniquely achieves: • Redox regeneration of permanganate at ambient or mild temperatures; • Using commercially available, safe reagents (NaOH and Na₂S₂O₈); • With repeatable and reversible restoration of oxidation capacity; • In a manner that is selective, scalable, and applicable in situ. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher / lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary. Further, the patent also covers the in-situ regeneration of the molecular filters. A solution of persulphate and NaoH with composition as above is prepared in water and used as an in situ regenerant. The spent molecular filters are washed with water from an overhead shower in the tank and then optionally dried with a jet of air. The regenerant is then showered from the top and infuses into the molecular filters regenerating them and getting them back into the appropriate oxidation stage. The excess regenerant is collected in the tank and may be recirculated. EXAMPLE 1: Effect of Hydroxide-to-Persulfate Ratio Hydroxide:Persulfate pH of Mn(VII) Filter Efficiency Restored (%) (%) d 1^:^6 EXAMPLE 2: Effect of pH Range During Oxidation Step pH Value Permanganate Filter Observed Oxidation ) 8.5 (outside Not stable in ran e – CO S / CE: Only pH 10–11 fosters the formation and stabilization of permanganate; acidic or excessively basic conditions destabilize Mn(VII), undermining regeneration. EXAMPLE 3: Effect of Claimed Reactant Ratios Filter: NaOH : Na₂S₂O₈ Color Intensity of Regeneration Efficiency COMMENTS / INFERENCE: The 25:1:6 ratio yields efficiency outcomes. In sum, These tables support that each claimed range and process parameter directly contributes to regeneration efficiency, filter stability, and Mn(VII) formation. Deviation from these ranges causes: • suboptimal oxidation or incomplete MnO₄⁻ reformation, • structural or functional degradation, • poor repeatability and industrial infeasibility. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

Claims

CLAIMS, 1. A non-thermal method for regenerating an alkali metal permanganate impregnated molecular filter comprising a porous media selected from alumina, carbon, zeolites, or molecular sieves, the method comprising: a. washing a spent molecular filter comprising oxidized products selected from manganate (MnO₄²⁻), manganese dioxide (MnO₂), and sulfate and nitrate salts using distilled water to selectively remove water-soluble salts, thereby obtaining a washed and wet molecular filter; b. contacting said washed and wet molecular filter with a solution comprising: (i) an alkali metal hydroxide selected from sodium hydroxide (NaOH) or potassium hydroxide (KOH), and (ii) an alkali metal persulfate selected from sodium persulfate (Na₂S₂O₈) or potassium persulfate (K₂S₂O₈), in a weight ratio of hydroxide to persulfate ranging from 1:4 to 1:7 and maintaining the pH of the resulting mixture between 9.5 and 11.0 to promote in-situ oxidation of Mn(IV) or Mn(VI) species to Mn(VII), thereby obtaining a treated molecular filter; andc. drying the treated molecular filter at a temperature between 50°C and 110°C to yield a regenerated molecular filter characterized by the reformation of permanganate ions.

2. The method as claimed in claim 1, wherein the regeneration process further comprises a pretreatment step involving contacting the washed molecular filter with a metal bisulfite solution selected from sodium metabisulfite (Na₂S₂O₅) in a concentration ranging from 10% to 50% wt. / wt., under mildly acidic conditions to ensure complete reduction of residual permanganate prior to reoxidation.

3. The method as claimed in claim 1, wherein the regeneration process further comprises a pretreatment step involving contacting the washed molecular filter with a metal bisulfite solution selected from sodium metabisulfite (Na₂S₂O₅) in a concentration ranging from 10% to 50% wt. / wt., under mildly acidic conditions to ensure complete reduction of residual permanganate prior to reoxidation, said pretreatment step being conducted at ambient temperature for a duration of 5 to 30 minutes prior to regeneration.

4. The method as claimed in claim 1, wherein the weight ratio of the molecular filter to alkali metal hydroxide to alkali metal persulfate is maintained within the range of 25:1:6.

5. The method as claimed in claim 1, wherein the regeneration occurs at ambient pressure without the application of external thermal energy or catalysts.

6. The method as claimed in claim 1, wherein the drying step is performed under forced air convection at 50 ± 5°C for 2–5 hours.

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