DE69420079T2 - Activated carbon impregnated with organic amines - Google Patents

Description

Organic amine impregnated activated carbon The invention relates to a breathing gas filter composition comprising activated carbon impregnated with an organic amine compound to improve the performance of the activated carbon against toxic perfluorocarbons, particularly against trifluoronitrosomethane (TFNM), as well as methods for impregnating activated carbon with organic amine compounds.

Activated carbon has been used in both commercial and military applications in respiratory gas filter cartridges of gas masks to remove toxic gases. Activated carbons typically contain certain metals (copper, chromium and silver), usually in the form of an oxide crystallite. Typically, these adsorbents are known in the industry as ASC Whetlerite carbon or ASC/TEDA when the carbon also contains triethylenediamine, TEDA. The function of these metals or metal compounds is primarily to break down HCN or CNCl by chemical reaction(s) into harmless gaseous products and/or products that are readily physically or chemically adsorbed onto the activated carbon.

Trifluoronitrosomethane (TFNM) is a blue-colored perfluorocarbon with a boiling point of -84ºC. Under sunlight (UV radiation), this gas dimerizes, losing its intense blue color and becoming pale yellow. TFNM is sufficiently toxic to pose a high risk, even to people protected by standard carbon adsorption filters. The amine-impregnated activated carbon described here has been found to be particularly effective in the chemical adsorption or chemisorption of this type of compound.

From a methodological point of view, there are several ways to apply an organic amine to the surface of the activated carbon. A proposal for application A method for applying TEDA to the surface of activated carbon is found in U.S. Patent No. 4,531,953, issued to JE Groose et al. on July 30, 1985. The Groose patent proposes direct sublimation of TEDA onto the activated carbon surface at atmospheric pressure. Such a method eliminates the subsequent drying process since no solvent is involved. Another method for applying TEDA to the surface of activated carbon under reduced pressure is described in detail in U.S. Patent No. 5,145,820, issued to SH Liang et al. on September 8, 1992.

An object of the present invention is to provide an organic amine impregnated activated carbon that can effectively remove toxic perfluorocarbons, particularly TFNM, from the breathing air stream and that has a longer shelf life and useful life.

In one aspect, the invention consists in a respiratory gas filter composition providing protection against toxic perfluorocarbons comprising substantially dry activated carbon and an organic amine impregnating agent in an amount of from 1.5 to 1.8% by weight based on the weight of the carbon, characterized in that the composition additionally comprises from 0.1 to 10% by weight based on the weight of the carbon of an alkali hydroxide.

According to a second aspect, the invention consists in a process for impregnating activated carbon by contacting an organic amine having a low vapor pressure with substantially dry activated carbon, characterized by the following steps: applying an overpressure of 1-5 Pa to the carbon and the amine in an oxygen-free inert gas atmosphere and then heating the carbon and the amine for a prolonged period while maintaining the overpressure of 1-5 Pa in an oxygen-free inert gas atmosphere and pretreating the activated carbon with a compound selected from the group consisting of an alkali hydroxide in a proportion of 0.1 to 10% by weight based on the weight of the carbon, a suitable halogen and a suitable halide salt. The above-mentioned heating step can typically be carried out at 40°C or 60°C for a period of 1 to 72 hours. depending on the amine impregnant, and the oxygen-free inert gas can be nitrogen, helium, argon, etc.

In another aspect, the invention consists in a method for impregnating activated carbon with a solid amine compound, comprising the steps of dissolving the amine in a solvent selected from water and organic solvents, and adding the amine solution to the activated carbon, characterized by the step of pretreating the activated carbon with a compound selected from the group consisting of an alkali hydroxide in an amount of 0.1 to 10% by weight based on the weight of the carbon, a suitable halogen and a suitable halide salt.

In some known amine-impregnated activated carbons, the amines exhibit a tendency to dissociate due to the presence of acidic surface sites. In the present invention, the activated carbon is pretreated to prevent the decomposition of the amine compounds on the carbon surface. The pretreatment process may include neutralization to a substantially neutral pH (by an alkali solution such as KOH or NaOH) or alkalization to a basic pH in the same manner, or reaction of the acidic sites (which are typically oxygen-containing components) by halogenation (such as by Cl2 and Br2). These treatments effectively deactivate the acidic surface groups on the carbon and thus improve the stability of the amines adsorbed on the carbon surface. U.S. Patent No. 4,072,479, issued to Sinha et al. on February 7, 1978, has detailed the use of NaOH in treating the activated carbon to remove malodorous compounds, particularly sulfur-containing compounds, from the air stream. However, in the present invention, treatment with NaOH is neither essential nor necessary in protecting against perfluorocarbons. Our pretreatment of the activated carbon with NaOH or KOH is solely to improve the storage time and useful life of the amine-impregnated carbon, particularly in the presence of moist air.

More specifically, the adsorption of amines on activated carbon is largely physical in nature and occurs via undissociated molecules. However, some of the amine molecules form hydrogen bonds with the acidic, oxygen-containing components (e.g. a CO2 complex) on the carbon surface. The acidic groups at which the amine is split on the carbon surface can be easily removed or deactivated by treatment with halogens or alkaline solutions (such as KOH or NaOH). An excess of alkalinity on the carbon surface (in the form of OH groups) helps to stabilize the amine on the surface and prevent its dissociation (i.e. shift the equilibrium towards the undissociated molecule). A similar advantage is achieved by pretreating the carbon surface by halogenation. The halogen reacts with the acidic oxygen-containing components on the carbon surface and thereby "deactivates" them so that the subsequent physical adsorption or physisorption of the active amine impregnant is not accompanied by reactions with the acidic sites on the surface which would deplete the proportion of amine available in the desired form. Like the alkali pretreatment, halogenation serves a dual purpose since the resulting halide salts, e.g. hydrogen halide salts, on the surface also stabilize the amine molecules on the carbon surface by reducing the degree of dissociation. Thus, alkali or halogen pretreatments of the carbon surface eliminate the undesirable effect that would result from reaction with acidic surface oxides and at the same time provide a means of stabilizing the active amine impregnant in the undissociated form. Experiments have shown that this step extends storage time and useful life.

The application of the method according to the second aspect of the invention facilitates the evaporation of the amine, the penetration of the amine molecules into the micropores of the activated carbon surface and then the adsorption of the amine molecules on the activated carbon surface. In addition, the inert environment hinders the oxidation of the amine impregnating agents on the carbon surface, as discussed by Hershman et al. in U.S. Patent No. 4,264,776, issued April 28, 1981. The lengthy heating step, preferably carried out with constant mixing in a closed vessel, allows "surface distillation" of the amine (by adsorption-desorption processes) and thus promotes uniform distribution of the amine molecules on the activated carbon surface.

The use of essentially dry activated carbon was deemed necessary after testing the impregnation process on two types of activated carbon. The first type was freshly prepared impregnated ASC Whetherlite activated carbon. Since ASC activated carbon needs to be dried after whetlerizing (pre-impregnation with copper, chromium and silver), the drying process can be included in the impregnation process. The other type of activated carbon was aged activated carbon with various moisture contents. For amine impregnation in the presence of moisture/humid air, the amine molecules must displace water from the activated carbon to penetrate the large surface area available in the meso- and micropores. When the micropores on the activated carbon surface are fully or partially filled with water, the adsorption of the amine is limited by the rate at which adsorbed water can be displaced, which is a kinetic problem. In the presence of water, reactions with some of the disclosed amines may also occur. Therefore, the activated carbon should be dried before introducing the amine.

The impregnation step involves the following series of processes:

(i) sublimation or evaporation of amine;

(ii) external diffusion of amine molecules to the activated carbon surface;

(iii) internal diffusion of amine molecules within the activated carbon pores; and

(iv) Adsorption of amine molecules.

After impregnating the activated carbon with amine, an equilibration period is provided to allow the amine adsorption process to reach a state of equilibrium. Carrying out the process at slightly elevated temperatures of 40ºC to 60ºC allows repeated desorption and redeposition of the amine to occur. In this way, a more even distribution of the amine molecules on the activated carbon surface is achieved. In an oxygen-free inert gas environment, competition between amine and inert gas for adsorption sites on the carbon surface is minimal because inert gas does not adsorb to any significant extent on a carbon surface. This environment also enhances the adsorption of amine molecules on the carbon surface. In addition, by avoiding an oxidizing environment, reactions such as the decomposition of tertiary amines in the presence of oxygen and carbon are inhibited.

Short description of the drawings:

Fig. 1 shows a diagram of the breakthrough profile of TFNM on several fresh (i.e. dry) amine-impregnated activated carbons.

Fig. 2 shows a diagram of the breakthrough profile of perfluoroisobutene at a content of 1000 mg/m³ on an activated carbon impregnated with triethylamine.

Fig. 3 shows a diagram of the breakthrough profile of hexafluorocyclobutene at a content of 1000 mg/m³ on an activated carbon impregnated with triethylamine.

Experimental - Chemicals

All amines were purchased from commercial manufacturers and used without further purification. A wide range of activated carbons have been shown to be suitable for impregnation. Examples given here are ASC-Whetlerite (i.e. carbons previously impregnated with copper, chromium and silver), ASC/T (ASC-Whetlerite with 2% triethylenediamine) or BPL (original carbon without chemical impregnating agents).

Pretreatment of coal Halogenation

This reaction can be carried out either in the gas phase or in the aqueous phase. Typically, the carbon is stirred in an aqueous Br2 or Cl2 solution for several hours (typically 2 to 4 hours). The solution is then removed by suction filtration and the carbon is washed with copious amounts of water. The carbon is then dried at 105 to 150ºC and under reduced pressure (typically < 100 Pa). Following this step, the carbon can be further activated at temperatures between 200 and 400ºC for up to 4 hours under reduced pressure (typically < 100 Pa).

Neutralization

A solution containing activated carbon is neutralized to an alkaline pH with an alkali solution containing an alkali hydroxide, such as KOH or NaOH. The carbon containing 0.1 to 10 wt.% alkali hydroxide is then washed until a substantially neutral pH is achieved, dried, and activated in the same way as in the halogenation process.

Alkalization

A sufficient amount of solution containing 0.1 to 12 wt% alkali hydroxide (NaOH or KOH) is added to the activated carbon until visible saturation. The slurry is then dried for at least 30 minutes. Heat may be applied to speed up the process. The solution is then removed by suction filtration and the carbon is dried at 105 to 150ºC and reduced pressure (typically < 100 Pa). Further activation at 200 to 400ºC for up to four hours may be beneficial for some carbons.

Impregnation with amines

Three methods were used to impregnate the ASC compositions with amine compounds. The first method (described above) uses the internal (albeit quite low) vapor pressure of the amines (in solid or liquid form) to drive the adsorption process. This method works extremely well for all liquid amines and some solid amines, e.g. triethylenediamine and hexamethyleneamine.

A second method has been developed for solid amides, such as ortho- and para-toluenesulfonamide, which do not have sufficient vapor pressure for the first method to work. First, a 250 mL solution is prepared with the desired loading concentrations (usually 1.5 to 18 wt.%) of the amide dissolved in 95% ethanol. The solution is then added to 250 g of ASC charcoal. Once the addition of the impregnating solution is complete, the ethanol is removed from the "wet" charcoal under vacuum. This typically takes 1/2 to 3/4 hour. By then, the charcoal is sufficiently dry and completely free-flowing. The charcoal is then spread evenly (approximately 0.5 cm high) on two watch glasses and placed in a vacuum oven. The coal is dried at less than 100 Pa and 50-60ºC for up to 16 hours. Mass balance calculations show that after this treatment essentially all the ethanol is removed and that all the amide impregnant remains within the pore structure of the coal.

A third method takes advantage of the water solubility of the desired impregnant, such as urea. The preparation of the solution is very similar to the method proposed by Joshua C. Whetzell and EW Fuller, which gave rise to the term "Whetlerite". An ammoniacal solution is prepared containing about 7% copper, 2% chromium, and the desired amide impregnant. BPL coal is typically used as the starting material in the preparation of these amide impregnated carbons. The first preparation step is to prepare an ammonia solution by adding 250 mL of NH4OH to 400 mL of deionized distilled water. Then 37 g of CrO3 is added, followed by 164 g of ammonium carbonate, 31 g of urea, and 117 g of CuCO3. Cu(OH)2 is added (in the above order) and dissolved in the solution. This solution is then added dropwise to 440 g of coal until an excess of liquid is clearly visible. The ratio of impregnation solution to coal is approximately 1.4 ml/g. The "wet" impregnated coal is then removed and allowed to drain for 30 to 40 minutes with occasional shaking. The coal is then spread out to a uniform thickness on a flat tray and dried gradually in an induced draught oven over a period of 220 minutes as follows: 80ºC (40 minutes), 100ºC (30 minutes), and finally 120ºC (150 minutes). The resulting impregnated coal typically has the following composition (in wt%): 3% Cr, 10% Cu, 3% urea and certain amounts of CO₂ and NH₃. The development of this impregnation process was an attempt to make the process more cost effective since all of the impregnating agents could be added to the coal in one step. A disadvantage of this process, however, is that the drying step in the impregnation process must be carried out at lower temperatures (say 120-150ºC) to avoid spontaneous combustion of the coal. This can produce coal with a moisture content of more than 2%.

Assessment of amine-impregnated coals

The amine impregnated carbons prepared above were evaluated for protection against toxic perfluorocarbons, such as trifluoronitrosomethane. The impregnated carbons were placed in a container with an internal diameter of 10.5 cm and to a bed height of 1.25 or 2 cm. A steady stream of the perfluorocarbon test gas, diluted to 1000 mg/m3 in an air stream, was introduced into the carbon bed at a flow rate of 30 l/min at 30ºC and 80% relative humidity, and the effluent from the carbon bed was monitored by a calibrated gas chromatograph for this perfluorocarbon gas. The breakthrough time is set at the time when the concentration of the effluent perfluorocarbon gas reaches 100 mg/m3, 1/10 of the original influent concentration.

Some selected amine-impregnated carbons were also tested and found to be effective against other perfluorocarbons, such as perfluoroisobutene and hexafluorocyclobutene.

First pre-selection of amine impregnating agents

In order to systematically evaluate all candidate amine impregnant agents, the amine compounds used in this work were divided into three main groups: aliphatic, aromatic and heterocyclic groups, which were further divided into three classes: primary, secondary and tertiary, as shown in Table 1 below. In addition, an additional group was added under "functional substituents" to indicate amine compounds containing additional substituents, such as a nitro group (as in 4-(4-nitrobenzyl)pyridine). In this way, a simple 3 x 4 experimental matrix was formed so that for each trend or anomaly, a correlation could be made between activity (protection against trifluoronitrosomethane) and position in the matrix.

The amine impregnated carbons were evaluated as follows: 125 or 170 ml layers of impregnated carbon were placed in a 10.5 cm diameter container and tested for trifluoronitrosomethane at 1000 mg/m³ and an air flow rate of 30 l/min at 25ºC and 38% relative humidity. The breakthrough time results are summarized in Table 2 below. The effluent from the filter cartridge was monitored for TFNM until a concentration of 100 mg/m³ was reached and the time from time zero to this concentration was arbitrarily designated as the "breakthrough time." A typical TFNM loading profile for passage through some of these amine impregnated carbons is shown in Figure 1. Specifically, it is TFNM at 1000 mg/m³ on activated carbon (125 ml, 10.5 cm diameter layer) impregnated with 7 wt% triethylamine, which had been preconditioned at 80% relative humidity until a constant weight was reached. Table

Note that in Table 2 below, some secondary amines such as diisopropylamine, pyrrole and piperidine and some tertiary amines such as triethylamine appeared to provide the best protection against TFNM. Carbons impregnated with these amines all gave longer breakthrough times than carbons impregnated with another amine. In addition, it was determined from stoichiometry that 2 moles of TFNM were consumed per mole of secondary and tertiary amines. Therefore, aliphatic secondary and tertiary amines are expected to provide better protection against trifluoronitrosomethane than the other candidate amines. Diamines did not appear to provide better protection against TFNM than other amines. Table 2 First pre-selection of selected amine impregnating agents as carbon impregnating agents for increased protection against perfluorocarbons primary/aliphatic primary/aromatic secondary/aliphatic Table 2 First pre-selection of selected amine impregnating agents as carbon impregnating agents for increased protection against perfluorocarbons* (continued) secondary/aromatic secondary/heterocyclic tertiary/aliphatic tertiary/aromatic Table 2 First pre-selection of selected amine impregnating agents as carbon impregnating agents for increased protection against perfluorocarbons (continued) tertiary/heterocyclic Functional substituents

*Note: All tests were carried out using 170 ml of the amine impregnated carbon contained in a filter cartridge body. All carbons were conditioned at 30ºC and 80% relative humidity until an equilibrium water weight was achieved prior to testing. Each loading test consisted of a continuous flow of 1000 mg/m³ CF₂NO in an air stream of 30 1/min at 80% relative humidity and 30ºC. The breakthrough time is defined as the time required for the effluent test gas to reach 100 mg/m³.

Second pre-selection of suitable amines as carbon impregnating agents

In this section, the "better amine impregnants" were applied to the coal at various loading levels and tested on trifluoronitrosomethane as shown in Table 3 below. The purpose is to determine the optimum loading level of the amines on the coal.

In general, the higher the amine loading level, the better the protection against TFNM was. All candidate amines shown in Table 3 performed very well. In addition, dipropylamine, piperidine and triethylamine appeared to give the longest breakthrough times against TFNM at a loading level of 7%, and pyrrole was even better at a loading level of 5%. Diisopropylamine appeared to be the best candidate amine, as no penetration of TFNM occurred at any loading level. In all cases, the level of protection against TFNM improved with increasing amine loading level. Table 3 Second pre-selection: Protection against TFNM provided by selected amine-impregnated dry carbons

Note: For test conditions, see footnote to Table 2.

Third preselection of amine-impregnated carbons

Further tests were carried out on other toxic perfluorocarbons, such as perfluoroisobutene (PFIB) and hexafluorocyclobutene (HFCB), using carbon impregnated with triethylamine at a loading level of 7 wt%.

As can be seen in Figures 2 and 3, the results were very promising. At a continuous exposure of 1000 mg/m³ PFIB, the breakthrough time is 155 minutes (i.e. it takes 155 minutes for the PFIB effluent from the filter cartridge to reach 100 mg/m³). The breakthrough time for tris(trifluoromethyl)methane (TRIS), a non-toxic breakthrough product from the reaction between the activated carbon and PFIB, is 163.2 minutes. For HFCB, the breakthrough time under similar conditions is 28.8 minutes. More specifically, in both Figures 2 and 3, it is an activated carbon (125 ml, 10.5 cm diameter bed) impregnated with 7 wt% triethylamine, which was preconditioned at 80% relative humidity until a constant weight was reached (note in Figure 2 that perfluoroisobutene (denoted PFIB) converts to tris(trifluoromethyl)methane (denoted TRIS)).

To verify the generality of this amine impregnation procedure, other activated carbons, namely BPL carbon (carbon with no impregnation history) and ASC/TEDA carbon were tested. The uptake of amine impregnants on these carbons and the protection against trifluoronitrosomethane and other perfluorocarbons were evaluated. The procedures for loading the carbons with these amine impregnants were similar to those described above for ASC-Whetlerite. The results are summarized in Table 4. As shown, the choice of starting carbons has very little effect on the performance of the impregnated carbon against TFNM. Table 4 Protection against trifluoronitrosomethane by using different starting carbons

Notes: (1) All amine impregnating agents were loaded at 5 wt%.

(2) All stress tests were carried out using the same conditions as in Table 2.

Similar experiments using ASC/TEDA carbons (i.e., without the amine impregnant mentioned in the present disclosure) result in much shorter breakthrough times than those indicated in Figures 1-3, depending on the perfluorocarbon.

Claims (14)

1. A respiratory gas filter composition providing protection against toxic perfluorocarbons comprising essentially dry activated carbon and an organic amine impregnating agent in an amount of 1.5 to 18% by weight based on the weight of the carbon, characterized in that the composition additionally comprises 0.1 to 10% by weight based on the weight of the carbon of an alkali hydroxide. 2. Composition according to claim 1, wherein the organic amine is contained in a proportion of 4 to 7 wt.%, based on the weight of the coal. 3. A composition according to claim 1 or claim 2, wherein the organic amine is selected from the group consisting of substituted amines, amides and sulfonamides. 4. A composition according to claim 1 or claim 2, wherein the organic amine is selected from the group consisting of aliphatic secondary and tertiary amines. 5. The composition of claim 1 or claim 2, wherein the organic amine is selected from the group consisting of ethylenediamine, dipropylamine, isopropylamine, aniline, N,N- diethyl-1,4-phenylenediamine, 4-phenylazoaniline, diisopropylamine, diethylamine, diethylenetriamine, diphenylamine, pyrrole, piperidine, triethylamine, triisobutylamine, trioctylamine, N,N- diethylaniline, 4-(4-nitrobenzyl)pyridine, hexamethyleneamine, monoethanolamine, urea, N,N-dimethylformamide, o-toluenesulfonamide and p-toluenesulfonamide. 6. A process for impregnating activated carbon by contacting an organic amine having a low vapor pressure with substantially dry activated carbon, characterized by the following steps: applying an overpressure of 1-5 Pa to the carbon and the amine in an oxygen-free inert gas atmosphere and then heating the carbon and the amine for a prolonged period while maintaining the overpressure of 1-5 Pa in an oxygen-free inert gas atmosphere and pretreating the activated carbon with a compound selected from the group consisting of an alkali hydroxide in a proportion of 0.1 to 10 wt.%, based on the weight of the carbon, a suitable halogen and a suitable halide salt. 7. The method of claim 6, comprising the additional step of storing the amine-impregnated carbon in an oxidant-free casing. 8. A process according to claim 6 or claim 7, wherein the heating step is carried out at 40° to 60°C for 1 to 72 hours. 9. The method of any one of claims 6 to 8, wherein the oxygen-free inert gas is selected from the group consisting of nitrogen, argon and helium. 10. The process of any one of claims 6 to 9, wherein the amine is selected from the group consisting of aliphatic secondary and tertiary amines. 11. Process according to one of claims 6 to 11, wherein the amine is present in a proportion of 4 to 7% by weight, based on the weight of the coal. 12. The method of any one of claims 6 to 11, wherein the activated carbon is selected from the group consisting of copper, silver and chromium impregnated carbon, non-impregnated base carbon and copper, silver, chromium and triethylenediamine impregnated carbon. 13. A process according to any one of claims 6 to 12, wherein the amine is selected from dipropylamine and triethylamine. 14. A process for impregnating activated carbon with a solid amine compound, comprising the steps of dissolving the amine in a solvent selected from water and organic solvents and adding the amine solution to the activated carbon, characterized by the step of pretreating the activated carbon with a compound selected from the group consisting of an alkali hydroxide in an amount of 0.1 to 10% by weight based on the weight of the carbon, a suitable halogen and a suitable halide salt.