WO2015109385A1

WO2015109385A1 - Carbon monolith, carbon monolith with metal impregnant and method of producing same

Info

Publication number: WO2015109385A1
Application number: PCT/CA2014/050565
Authority: WO
Inventors: Yuxing CUI; Eva Gudgin DICKSON
Original assignee: Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of National Defence
Priority date: 2014-01-21
Filing date: 2014-06-17
Publication date: 2015-07-30
Also published as: WO2015109381A1

Abstract

A process for preparing a self-supporting monolithic porous carbonaceous adsorbent structure involves drying a mixture of non-gelling polymeric carbon precursor particles and an organic latex binder at a temperature of 100°C or less to form a solid monolith of polymeric carbon precursor particles bound by an organic polymer matrix in a pre-determined shape. The solid monolith in the pre-determined shape is carbonized at a temperature of 800°C or less to form a self-supporting monolithic porous carbonaceous adsorbent structure. A metal impregnant is introduced before or after carbonization of the solid monolith.

Description

CARBON MONOLITH, CARBON MONOLITH WITH METAL IMPREGNANT AND METHOD OF PRODUCING SAME

Cross-reference to Related Applications

This application is a continuation-in-part of International Patent Application PCT/CA2014/000041 filed January 21 , 2014, the entire contents of which is herein incorporated by reference.

Field

This application relates to air purification, especially in respirators, more particularly to monolithic carbonaceous adsorbents, especially metal impregnated monolithic carbonaceous adsorbents, and to a process for preparing the adsorbents.

Background

Exposure of civilians or military personnel and first responders to toxic gases and vapors without proper respiratory protection can be harmful, incapacitating and/or potentially lethal depending on the type and amount of substance(s) and duration of exposure. Efficient respiratory protection against such hazards is a problem which requires equipment such as air-purifying respirators designed to reduce or eliminate the inhalation exposure to harmful agents. Important factors in the performance of such air- purifying respirators include efficiency of removal of toxic agents, capacity to retain agents after removal, effective performance against a broad range of toxic agents, or design specific to particular agents, breathing resistance, size, weight and shape. All the preceding factors can influence the effectiveness of respiratory protection against toxic agents, and the ability of the wearer to perform useful or necessary functions while wearing the equipment.

Adsorbent materials housed in a filtering device attached to a close-fitting face mask provide protection from a variety of gases and vapors for a limited time. The adsorbents presently used in respiratory protection primarily consist of granular activated carbon packed at maximum packing density into a container forming a bed of adsorbent. The adsorbent bed must be immobilized in its final form by mechanical means, such as fixing a top and bottom plate in place under compression, with side walls providing the support for the top and bottom plates.

The granular adsorbents are characterized by a large internal surface and small pores. Manufacture involves the carbonization of an organic raw material, which in the case of respirator adsorbents is commonly derived from coal or natural cellulosic materials such as coconut shell. Following carbonization, the material is crushed and sized to select a distribution of granule sizes. The carbonized material is then activated, which is a process of modifying the material by application of heat, steam, and other chemicals or gases to increase the internal surface area.

Some adsorbent materials are modified by post-activation addition of inorganic materials (e.g. metal ions) and organic materials called impregnants, intended to augment the adsorptive capacity of the carbon. This is done to improve the adsorption of toxic substances or initiate the chemical decomposition of some highly toxic gases to less toxic products. Physical adsorption of these products and other harmful gases is accomplished by the adsorbent material. Important factors in the performance of the adsorbent material include specific surface area, loading of impregnants and accessibility of the active adsorbing sites (all to increase the ability to effectively remove the toxic gases and vapors from the airstream), and pressure drop of the air flow (to minimize breathing resistance). The method of impregnant introduction to prepare the impregnated adsorbent material can affect all the factors that influence air purifying capability.

The current activated carbons used in respiratory protection are produced by carbonization of raw materials such as wood, coal and coconut shells at temperatures of at least 800°C in the absence of oxygen, followed by chemical or physical activation of the carbonized product. These materials show good adsorbent properties attributed mostly to their high specific surface area of 800-1500 m²/g which is contained predominantly within micropores. However, they are characterized by some major disadvantages. The adsorbent bed must be made in a housing under compression to maintain the packing density and shape, and the granules in the housing must remain immobile after compression. In other words, forces applied to the housing such as rough handling, or a temperature increase resulting in expansion, would reduce the performances of the adsorbent bed due to air penetration through mobile granules resulting in less adsorbing interaction with micropores. The possible friction between granules would also give fine powdery carbon causing malfunction of the respirator. It is difficult to economically design an adsorbent structure which could be integrated into a respiratory mask instead of protruding in one direction. It is also of concern that the consistency of the end product depends on the natural source of the raw materials. The adsorbing capacity would vary due to the difference of raw materials such as coal and coconut shell in terms of texture or ingredients. Carbon structures have been prepared differently in prior art. Generally, the methods fall into two kinds. The first kind is to bind a carbonized porous material using binding materials such as polymer. A drawback is that the binder could block the pores which are essential to all applications. If the binding is followed by a second carbonization, the whole process is multistep, which is usually costly. As well, some adsorbing sites are not able to be accessed due to packing and molding. The second method involves mechanical forces such as pressure or extrusion to make a carbon structure. The voids between beads or granules are reduced due to compression. This is required for gas storage or other applications that need compact packing to increase the adsorbing capacity per unit volume. For respiratory protective application, a low breathing resistance or pressure drop is critical while maintaining the adsorbing capability by having a reasonably high specific surface area and bulk density.

A common method to prepare impregnated carbon structures is by soaking or imbibing the carbonized carbon structure in a solution that contains the desired impregnant. Once the carbon structure is carbonized, the desired impregnants are loaded into the carbon structure. In the case of metal ion impregnants, heat treatment is usually needed to convert the metal ions to a specific or stable state. There may be drawbacks of the current method of preparing impregnated carbon structures by soaking or imbibing the carbonized structure in a solution. Firstly, when preparing the carbon structure, pyrolysis or carbonization temperature can be up to 1000°C depending on the source materials. This carbonization process is energy consuming which increases the manufacturing cost. Secondly, due to crystallization of impregnants after soaking, nonuniform distribution of the impregnants (e.g. metal ions) on the active adsorbing sites of carbon may cause a decrease in specific surface area and blockage of the accessibility of the air flow to pores. Some impregnants may occlude the pores thus reducing the effective surface area. Therefore the loading level of impregnants must be limited to reduce the blockage. Consequently, solution impregnation methods may compromise the performance of carbonized structures in some cases.

A new generation of respirator design, which relies on the properties and structure of the adsorbent material, is needed to improve the general performance of both the respirator and users. Additionally, a cost effective and "green" preparation method is needed. Summary

There is provided a process for preparing a self-supporting monolithic porous carbonaceous adsorbent structure, the process comprising: drying a mixture of non- gelling polymeric carbon precursor particles and an organic latex binder at a temperature of 100°C or less to form a solid monolith of polymeric carbon precursor particles bound by an organic polymer matrix in a pre-determined shape; and, carbonizing the solid monolith in the pre-determined shape at a temperature of 800°C or less to form a self-supporting monolithic porous carbonaceous adsorbent structure.

There is further provided a process for preparing a self-supporting monolithic porous carbonaceous adsorbent structure, the process comprising: drying a mixture of non-gelling polymeric carbon precursor particles and an organic latex binder at a temperature of 100°C or less to form a solid monolith of polymeric carbon precursor particles bound by an organic polymer matrix in a pre-determined shape; carbonizing the solid monolith in the pre-determined shape at a temperature of 800°C or less to form a self-supporting monolithic porous carbonaceous adsorbent structure; and introducing a metal impregnant before or after carbonization of the solid monolith.

There is further provided a process for preparing a carbonaceous adsorbent structure, the process comprising: drying a mixture of poly(vinylidene chloride) particles in a solution of a metal salt to form a metal salt impregnated bed of poly(vinylidene chloride) in a pre-determined shape, the metal salt capable of catalyzing the decomposition of the poly(vinylidene chloride); and, carbonizing the bed in the pre-determined shape at a temperature of 650°C or less to form a metal impregnated carbonaceous adsorbent structure.

There is further provided a carbonaceous adsorbent structure produced by a process of the present invention.

There is further provided a self-supporting monolithic porous carbonaceous adsorbent structure comprising a free-standing monolith of carbon particles having a specific surface area of 800 m²/g or more.

The process involving organic latex binder allows binding of the precursor particles at low temperature, thus the process is cost effective because little or no heat is required at the binding stage, and the process is environmentally friendly because no decomposition occurs during binding which means that there is little or no generation of harmful and corrosive chemicals during binding. In addition, the organic latex binder, which is polymeric, is also carbonized during carbonization/pyrolysis to produce porous carbon. Further, organic latex binder forms a thin layer of continuous film binding the precursor particles into a single piece, thus the mechanical strength of the adsorbent structure is enhanced both before and after carbonization/pyrolysis. Because no compression is required to bind the precursor particles together, the resulting adsorbent structure retains voids between precursor particles, thus maintaining good air permeability resulting in low breathing resistance and/or pressure drop in the adsorbent structure. Further, gas flow channels throughout the adsorbent structure are formed naturally by gases formed during carbonization. The gas flow channels start from active sites in micropores within the adsorbent structure and lead to a surface of the adsorbent structure. Voids between carbon particles are integrated into the channels. Therefore, the gas flow channels ensure that air has access to active adsorbing sites within the monolithic porous carbonaceous adsorbent structure, especially when used for respiratory protection. The process comprises a single carbonization step producing a self-supporting adsorbent structure with desired shape. The adsorbent structure is formed before carbonization, so both the precursor particles and the latex binder when present are carbonized during the same pyrolysis process. The adsorbent structure forming process is simplified. There is little or no binder blocking pores or active adsorption sites. As a self-supporting structure, the adsorbent structure can be loaded into an air purification device without compression avoiding crushing of carbon particles during fabrication of the air purification device (e.g. a respirator). Ergonomic shapes or designs of adsorbent structures can be practically attained. The binding step can be performed without the addition of any heat if speed is not essential. One or more metal impregnants may be introduced into the monolithic porous carbonaceous adsorbent structure to improve adsorptive capacity of the adsorbent structure. The metal impregnant may be introduced before or after carbonization. The metal impregnant may be introduced in any suitable manner, for example, by contacting the monolithic porous carbonaceous adsorbent structure with a solution of one or more metal salts, by contacting the solid monolith of polymeric carbon precursor particles with a solution of one or more metal salts prior to carbonization or by contacting the polymeric carbon precursor particles with a solution of one or more metal salts prior to forming the solid monolith of predetermined shape. When the metal impregnant is introduced by contacting the monolithic porous carbonaceous adsorbent structure in a solution of one or more metal salts, the metal impregnated adsorbent structure is preferably heat treated after impregnation to activate the metal impregnant. It is particularly advantageous to introduce the metal impregnant prior to carbonization. Introducing the one or more metal salts prior to carbonization results in simultaneous carbonization of the polymeric carbon precursor and activation of the metal impregnant. Contacting the solid monolith of polymeric carbon precursor particles in a solution of one or more metal salts prior to carbonization is preferred.

The one or more metal salts are preferably capable of catalyzing the decomposition of the polymeric carbon precursor. Catalyzing the decomposition of the polymeric carbon precursor permits carbonizing the polymeric carbon precursor at a lower temperature, thereby representing considerable energy savings. Further, the process involving contacting the solid monolith of polymeric carbon precursor particles in a solution of one or more metal salts prior to carbonization advantageously permits increasing loading levels of the metal without unduly impacting adsorptive performance of the monolithic porous carbonaceous adsorbent structure. During the subsequent carbonization, metal impregnants from the decomposition of the one or more metal salts may migrate and embed into the monolithic porous carbonaceous adsorbent structure through pores that are created when the carbon precursor decomposes. Since both the metal salts and the carbon precursor are decomposing simultaneously at high temperature, the metal impregnants may distribute uniformly without filling pores. Some metal salts melt at the carbonizing temperature, which may promote homogeneous distribution of the metal impregnants. As a result, the introduced metal impregnants may neither decrease the specific surface area, nor block accessibility to the adsorbing sites.

The present process produces a self-supporting monolithic porous carbonaceous adsorbent structure of any desired shape useful for air purification, especially for respiratory protective applications. The adsorbent structure may be one integral piece that does not need compression to maintain the structure's integrity. The process is energy efficient and environment friendly. The process involves a pyrolysis/carbonizing step that creates efficient tortuous gas flow path in the whole monolith, low breathing resistance (pressure drop), and strong mechanical strength of the adsorbent structure. Further features will be described or will become apparent in the course of the following detailed description. It should be understood that each feature described herein may be utilized in any combination with any one or more of the other described features, and that each feature does not necessarily rely on the presence of another feature except where evident to one of skill in the art. Brief Description of the Drawings

For clearer understanding, preferred embodiments will now be described in detail by way of example, with reference to the accompanying drawings, in which:

Fig. 1A depicts a flow diagram illustrating steps in one embodiment of a process for producing a self-supporting monolithic porous carbonaceous adsorbent bed in accordance with the present invention;

Fig. 1 B depicts a flow diagram illustrating process steps for producing a metal- impregnated self-supporting monolithic porous carbonaceous adsorbent bed in accordance with the present invention; Fig. 2 depicts a a solid monolith of polymeric carbon precursor particles bound by an organic polymer matrix, where the organic polymer matrix is best seen in exploded area A;

Fig. 3 depicts a self-supporting monolithic porous carbonaceous adsorbent bed comprising gas flow channels therein, the carbonaceous adsorbent bed having maintained the same shape after carbonization as the monolith of Fig. 2 had before carbonization;

Fig. 4 depicts air flow in the adsorbent bed of Fig. 3 where air flows between and through carbon particles formed from carbonization of the polymeric precursor particles;

Fig. 5 depicts a graph of equivalent flow rate (SLPM) vs. pressure drop (mmH₂0/cm) for a self-supporting monolithic porous carbonaceous adsorbent bed of the present invention compared to a porous carbonaceous adsorbent bed having carbon particles not bound by an organic latex binder; and,

Fig. 6 depicts a graph of pH vs. time (sec) comparing ammonia breakthrough times for zinc-impregnated monolithic porous carbonaceous adsorbent structures over a range of zinc content and a monolithic porous carbonaceous adsorbent structure that was not impregnated with zinc.

Detailed Description

One embodiment of a process for producing a self-supporting monolithic porous carbonaceous adsorbent bed is depicted in Fig. 1A. In a first step, type and size distribution of the polymeric carbon precursor particles are selected. The size distribution of the precursor particles is particularly important for providing a good balance between adsorption capacity of the carbon and pressure drop across the adsorbent bed. This balance is a result of a balance between accessible particle surface area and void size between particles. Larger voids provides for lower pressure drop and faster air flow through the bed at the expense of adsorption capacity. Smaller particles provide for larger accessible surface area and better adsorption capacity at the expense of faster air flow and lower pressure drop. Low breathing resistance is enhanced by a properly chosen particle size distribution and resulting packing of the precursor particles. Pressure drop through a monolith of the same bed depth with properly chosen particle sizes can be significantly lower than the National Institute for Occupational Safety and Health (NIOSH) standard at a flow rate of 85 liter per minute (SLPM). The particle size distribution is preferably in a range of about 10-30 mesh US sieve size, which is equivalent to average particle diameters in a range of about 0.595-2.00 mm. A mixture of particles having different particle sizes within the range is preferred. The precursor particles are preferably beads or granules or a mixture thereof. Preferably the precursor particles have low aspect ratios, where aspect ratio is a ratio of a longest dimension (e.g. length) to a shortest dimension (e.g. width) of the particle. Aspect ratios of about 1 :2 or less, or about 1 : 1.5 or less, or about 1 :1.25 or less are preferred. An aspect ratio of about 1 :1 is most preferred. Spherical or nearly spherical particles are most preferred. Packing density of the precursor particles and tortuosity of the path through the adsorbent bed is determined by a combination of the particle size distribution and the aspect ratio and can be chosen to give the best combination of low breathing resistance and high tortuosity, leading to a longer residence time through the bed and better gas removal capacity.

The carbon precursor particles comprise an organic polymer that forms porous carbon when carbonized. The carbon precursor particles preferably yield when carbonized micropores having diameters of about 2 nm or less and mesopores having diameters of about 2-50 nm. The relative yield of these pores is dependent on the carbonization conditions and the presence of any promoting agents. The porous carbon derived from these precursor particles has a specific surface area greater than about 800 m²/g, which arises mainly from the micropores. The high specific surface area is desirable because higher surface area leads to greater efficiency of air purification as toxic gases and other pollutants have more surface area on which to adhere. The carbon precursor particles may be synthetic or from a natural source. Some examples of organic polymers include poly(vinylidene chloride) (PVDC), PVDC copolymer, polystyrene (PS), PS copolymer, phenolic resin, cellulosic polymer (e.g. cellulose) or any mixture thereof. Poly(vinylidene chloride) (PVDC) or copolymers thereof (e.g. poly(vinylidene chloride)-co- methyl acrylate (PVDC-co-MA), poly(vinylidene chloride)-co-methyl methacrylate (PVDC- co-MMA)) are particularly preferred. The use of a non-gelling polymeric carbon precursor is of particular note as gelling can occlude channels in the adsorbent bed thereby reducing effective surface area and reducing air permeability resulting in high breathing resistance and/or pressure drop in the adsorbent bed.

A latex is a stable dispersion (e.g. emulsion) of polymer microparticles in an aqueous medium. In a second step, a latex binder is selected and used to coat the precursor particles by mixing the latex with the precursor particles. The polymer in the latex binds the precursor particles together in a polymer matrix once the coated precursor particles are dried. The polymer in the latex preferably comprises a synthetic organic polymer. The latex polymer is also chosen to form porous carbon upon carbonization. Latex polymers may comprise, for example, poly(vinylidene chloride) (PVDC) latex which is an emulsion of PVDC and/or PVDC copolymer in aqueous medium, polystyrene (PS) latex which is an emulsion of PS and/or PS copolymer in aqueous medium, or mixtures thereof. Latexes of poly(vinylidene chloride) or copolymers thereof (e.g. poly(vinylidene chloride)-co-methyl acrylate (PVDC-co-MA), poly(vinylidene chloride)-co-methyl methacrylate (PVDC-co-MMA)) are particularly preferred. Solid content of the latex should be properly selected to provide sufficient polymer binder to effectively coat the precursor particles so that a monolith of sufficient structural integrity is formed upon drying, and so that the monolithic porous carbonaceous adsorbent bed formed after carbonization is self-supporting, while minimizing the amount of water that would need to be evaporated during drying. The solid content of the latex is preferably in a range of about 15-65% by weight of the latex, more preferably about 40- 55% by weight. Relative amount of precursor particles and latex also contributes to forming monoliths of sufficient structural integrity. The precursor particles are preferably coated with latex in a latex to precursor ratio in a range of about 1 :4 to 1 :20 by weight, more preferably about 1 :4 to 1 :6 by weight, where the weight of the latex includes the weight of the polymer binder and the water in the latex.

In a third step, the mixture of latex-coated precursor particles is molded into a desired shape, preferably in a mold. The presence of water in the latex provides a relatively fluid mixture so that that the latex-coated precursor particles may be poured and shaped in the mold. In a fourth step, the molded latex-coated precursor particles are dried, preferably in the mold. Drying is performed at a temperature of about 100°C or less, no higher than the boiling point of water. Drying may be performed at a temperature of 90°C or less, 80°C or less, 70°C or less, 60°C or less, 50°C or less, 40°C or less, or 30°C or less. Drying may be performed at ambient temperature or higher, for example 20°C or higher, or 25°C or higher. Drying temperatures at the lower end require less energy input but result in longer drying times. The latex polymer binds the precursor particles together at a low temperature in a range of from about ambient temperature to about 100°C. The latex polymer has a melting point greater than the drying temperature, for example greater than about 100°C, so that the latex polymer remains substantially in the solid state to reduce the chance of melted polymer flowing away from the precursor particles leaving the precursor particles uncoated with binder. Further, within this temperature range there is little or no decomposition of either the precursor particles or the latex polymer. In the case of PVDC-based precursor particles and/or PVDC-containing latex, if decomposition were to occur during binding of the precursor particles, hydrogen chloride (HCI) gas would be produced, which would cause a number of problems. Further, since the latex is an aqueous dispersion, the binding step comprises evaporation of water rather than softening of dry polymer. Therefore, no pressure is needed during molding and drying because the precursor particles are bound by a generally continuous film formed by latex polymer on the precursor particles' surfaces. Not having to apply pressure during drying is useful for maintaining spaces between precursor particles thereby maintaining good air permeability in the resulting adsorbent bed. With reference to Fig. 2, after the mixture is dried, an adsorbent bed 1 comprising a solid monolith of polymeric carbon precursor particles 5 (two labeled) bound by an organic polymer matrix 7 is demolded. In a fifth step, the demolded solid monolith 1 of desired shape is pyrolyzed to carbonize the now dry precursor particles and latex polymer. Both the precursor particles and the latex polymer are carbonized to form porous carbon. The porous carbon created by carbonization of the latex polymer preferably has a specific surface area of no less than about 800 m²/g. Carbonizing the solid monolith may be performed at a decomposition temperature of the carbon precursor and organic polymer matrix (e.g. 800°C or less) to form a self-supporting monolithic porous carbonaceous adsorbent bed.

The carbonization is preferably performed under a controlled stepwise temperature profile. Carbonization is preferably performed slowly, with a number of temperature plateaus and a slow ramp rate between temperature plateaus. The specific profile used depends to some extent on the size and/or weight of the monolith being carbonized, and the decomposition temperature of the precursor particles. Time and ramp rate for each step are generally different for samples with different sizes and/or weights, but sample size and/or weight is not necessarily proportional to time and ramp rate. In one embodiment, temperature plateaus may be at about 180°C, 200°C, 350°C and 700°C with ramp rates in a range of about 0.2-3°C/minute. Temperature plateaus at about 200°C and about 700°C are particularly useful for obtaining good results, especially for PVDC. Soaking times at a given temperature plateau may vary, for example soaking times may be in a range of from about 0.5 hours to about 4 hours.

In a specific embodiment, a monolith 20 mm in diameter and 40 mm in length) may be carbonized using the following temperature profile:

25°C to 180°C, ramp rate of 2°C/min;

180°C, soaking for 0.5 h;

180°C to 200°C, ramp rate of 0.5°C/min;

200°C, soaking for 3 h;

200°C to 350°C, ramp rate of 1°C/min;

350°C, soaking for 2 h;

350°C to 700°C, ramp rate of 1°C/min

700°C, soaking for 3 h; and,

700°C down to 25°C, ramp rate of -3°C/min.

Carbonization is preferably performed in an inert atmosphere, e.g. under Ar or N₂ or in vacuo, to reduce oxidation of carbon, for example to reduce oxidation of carbon to carbon dioxide and/or carbon monoxide. Because the demolded solid monolith already possesses the desired shape and already possesses structural integrity, the solid monolith may be carbonized without the need for a container thereby permitting process gases to flow through and around the monolith during carbonization resulting in a more homogeneous product. If PVDC-based precursor particles and/or PVDC-containing latex are used, hydrogen chloride (HCI) gas is produced on decomposition during carbonization, which must be scrubbed from the product gas stream during carbonization.

With reference to Fig. 3, a self-supporting monolithic porous carbonaceous adsorbent bed 10 produced after carbonization is a whole piece and is ready to be assembled into an air purification device, such as a respirator. Because the adsorbent bed is self-supporting, respirator design is simplified and more rugged. Gas evolution during carbonization encourages formation of tortuous gas flow paths 15 (one labeled) from one end of the adsorbent bed 10 to the other end. The tortuous gas flow paths 15 become air flow paths through which air may flow after the monolith is carbonized. The air flow paths are the paths through which air flows in the adsorbent bed during an air filtering application such as in a respirator. The created gas flow paths 15 ensure efficient adsorbing interaction by directing the air to microporous adsorbing sites in or on carbon particles 20 (two labeled). The carbon particles 20 are formed from carbonization of the polymeric precursor particles. Adsorbing sites arise from the micropores formed during carbonization of the precursor particles or in the carbonized latex polymer on the surfaces of the carbon particles. As seen in Fig. 4, air (depicted by arrows) may flow between the carbon particles 20 (only one labeled) formed from carbonization of the polymeric precursor particles or through internal channels 21 (only one labeled) in the carbon particles 20. The adsorbent structure preferably comprises carbon particles having a specific surface area of 800 m²/g or more. Performance of a self-supporting monolithic porous carbonaceous adsorbent bed of the present invention was compared to performance of a prior art carbonaceous adsorbent bed that was made using a conventional compressed bed method. The precursor particles were PVDC and the organic latex binder was PVDC-co-MMA. The prior art bed was formed with carbon particles of similar particle size distribution as those in the bed of the present invention, but the carbon particles were not bound together with an organic latex binder. The size distribution of the carbon particles is given in Table 1.

Table 1

Performance was determined by measuring pressure drop across the adsorbent bed at different equivalent flow rates. Fig. 5 depicts a graph of equivalent flow rate (SLPM) vs. pressure drop (mmH₂0/cm) illustrating the results. The results clearly show that the pressure drop is significantly reduced in an adsorbent bed of the present invention in comparison to the prior art bed, especially at a high flow rate. The pressure drop may be reduced by up to 75%. One or more metal impregnants may be introduced into the monolithic porous carbonaceous adsorbent structure to improve adsorptive capacity of the adsorbent structure. The metal impregnant may be, for example, one or more metal ions, e.g. one or more of zinc, copper, cobalt, silver or molybdenum ions. The metal impregnant may be introduced in any suitable manner, for example, by contacting (e.g. soaking or imbibing) the monolithic porous carbonaceous adsorbent structure with a solution of one or more metal salts, by contacting (e.g. soaking or imbibing) the solid monolith of polymeric carbon precursor particles in a solution of one or more metal salts prior to carbonization or by contacting (e.g. soaking or imbibing) the polymeric carbon precursor particles with a solution of one or more metal salts prior to forming the solid monolith of predetermined shape. It is particularly advantageous to introduce the one or more metal salts prior to carbonization. Contacting the solid monolith of polymeric carbon precursor particles with a solution of one or more metal salts prior to carbonization is preferred.

Examples of metal salts include zinc salts, copper salts, cobalt salts, silver salts, and molybdates. The counter anion to the metal cation may be for example, acetate, nitrate, halide (e.g. fluoride, chloride, bromide or iodide, especially chloride), carbonate or oxalate, among others. In the case of molybdates where a molybdenum cation is bound to oxygen atoms to form an anionic species, the counter cation may be, for example, ammonium. The one or more metal salts are preferably capable of catalyzing the decomposition of the polymeric carbon precursor. Catalyzing the decomposition of the polymeric carbon precursor permits carbonizing the polymeric carbon precursor at a lower temperature, thereby representing considerable energy savings. In the case of poly(vinylidene chloride), the carbonization temperature may be advantageously lowered to about 650°C or less, preferably to a temperature in a range of about 550-650°C, for example 600°C. The one or more metal salts preferably comprise one or more of zinc chloride, zinc acetate, copper chloride, copper acetate, silver nitrate or ammonium molybdate. The one or more metal salts more preferably comprise zinc acetate and/or zinc chloride.

The one or more metal salts are preferably used in an amount sufficient to provide a loading of metal in the monolithic porous carbonaceous adsorbent structure of about 0.01-30 wt%, based on total weight of the monolithic porous carbonaceous adsorbent structure. The loading is preferably in a range of about 0.05-20 wt%, more preferably in a range of about 1-10 wt%. The process involving contacting the solid monolith of polymeric carbon precursor particles in a solution of one or more metal salts prior to carbonization may have one or more of the following advantages. Increased loading levels of the metal impregnant may be permitted without unduly impacting adsorptive performance of the monolithic porous carbonaceous adsorbent structure. The metal impregnant may be more uniformly distributed in the monolithic porous carbonaceous adsorbent structure. There may be less or no blockage of and improved air accessibility to adsorbing sites. The process combines both carbonization and impregnation into one step, thereby simplifying the manufacturing process.

Fig. 1 B depicts a flow diagram illustrating an embodiment of process steps for producing a metal-impregnated self-supporting monolithic porous carbonaceous adsorbent bed in accordance with the present invention. The process is similar to the one illustrated in Fig. 1A, except that metal may be introduced at one or more of the steps. As illustrated in Fig. 1 B, Option 1 provides for mixing a solution of metal salt with the carbon precursor particles and then drying the metal salt-coated particles before further processing. Option 2 provides for soaking or imbibing the dried, demolded monolithic bed in a solution of metal salt prior to carbonization. Option 3 provides for soaking or imbibing the monolithic porous carbonaceous adsorbent bed after carbonization in a solution of metal. Any one of the three options may be employed, or a combination of any two or more of the options may be used. Other options for introducing the metal salt into the self- supporting monolithic porous carbonaceous adsorbent bed would be apparent to one skilled in the art. To further illustrate embodiments of metal impregnation of monolithic porous carbonaceous adsorbent structures produced from non-gelling polymeric carbon precursor particles and an organic latex binder, the following experiments were conducted.

E1 : PVDC beads treated with zinc acetate before monoliths were made using PVDC-based latex and before carbonization. Thus, 10.0 grams of PVDC bead mixtures with the sizes of 10, 12, 14, 16, 18, and 20 mesh were wetted by 3.5 grams of a zinc acetate aqueous solution having a concentration of 28.5% by weight. The mixtures were dried in an oven at 60°C overnight and then the dried mixtures were used to prepare green (uncarbonized) monoliths using PVDC-co-MMA latex according to the method described above in connection with Fig. 1. The green monoliths were carbonized at 600°C in a tubular furnace under controlled ramp rate and continuous nitrogen flow. The final zinc content in the monolithic porous carbonaceous adsorbent structures was 2.35% by weight. E2: Green monoliths prepared from PVDC beads and PVDC-based latex were treated with zinc acetate before carbonization. Thus, 10.0 grams of PVDC beads mixtures of 10, 12, 14, 16, 18, and 20 mesh were used to make green monoliths according to the method described above in connection with Fig. 1. Then, the green monoliths were treated with a zinc acetate solution and dried. The amount and concentration of the zinc acetate solution were set to bring the zinc content of the final carbonized monolith to 2.35% by weight. The zinc treated dry green monoliths were carbonized at 600°C as described in E1. The final zinc content in the monolithic porous carbonaceous adsorbent structures was 2.35% by weight. E3: Monoliths were treated with zinc acetate after carbonization. Thus, 10.0 grams of PVDC beads mixtures with size of 10, 12, 14, 16, 18, and 20 mesh were wetted by PVDC-co-MMA latex to make green monoliths as described above in connection with Fig. 1. The green monoliths were then carbonized at 700°C in a tubular furnace under controlled ramp rate and continuous nitrogen flow. The monolithic porous carbonaceous adsorbent structures were impregnated with zinc acetate solution and heat treated after imbibing in a conventional manner. The final zinc content in the monolithic porous carbonaceous adsorbent structures was 2.35% by weight.

E4: PVDC beads treated with zinc acetate before carbonaceous adsorbent bed prepared by carbonization and conventional compression bed technique. Thus, 10.0 grams of PVDC bead mixtures with the sizes of 10, 12, 14, 16, 18, and 20 mesh were wetted by 3.5 grams of a zinc acetate aqueous solution having a concentration of 28.5% by weight. The zinc acetate pre-treated PVDC beads were carbonized at 700°C in a tubular furnace under controlled ramp rate and continuous nitrogen flow, and then a carbonaceous adsorbent bed was made by compressing the carbonized beads in a cylindrical holder. The final zinc content in the carbonaceous adsorbent bed was 2.35% by weight.

E5 Carbonized PVDC beads impregnated with zinc acetate and then compressed to make carbonaceous adsorbent beds. Thus, 10.0 grams of PVDC bead mixtures with the sizes of 10, 12, 14, 16, 18, and 20 mesh were carbonized at 700°C in a tubular furnace under controlled ramp rate and continuous nitrogen flow. The carbonized beads were treated with a zinc acetate solution and dried. The amount and concentration of the zinc acetate solution were set to bring the zinc content to 2.35% by weight. A carbonaceous adsorbent bed was made by compressing the carbonized beads in a cylindrical holder. E6: Blank sample monolith without metal impregnation. A blank sample monolith was prepared as in E1 latex but without zinc treatment. Carbonization was performed at 700°C in a tubular furnace under controlled ramp rate and continuous nitrogen flow. The final zinc content in the monolithic porous carbonaceous adsorbent structures was 0% by weight.

Ammonia breakthrough time was measured for each of the monolithic porous carbonaceous adsorbent structures of E1 -E6. The results are shown in Table 2. The test conditions were: concentration of NH₃ in air was 2500 ppm; flow rate was equivalent to 64 SLPM for a diameter of 10.1 cm structure; relative humidity was <10%.

Table 2

As seen from Table 2, monolithic porous carbonaceous adsorbent structures without zinc impregnation show a short breakthrough time of 33 seconds per gram compared to zinc impregnated structures. This illustrates the value of metal impregnation in this example, particularly impregnation with a zinc salt to remove ammonia from a gas stream.

Comparing E1-E3 with E4-E5, it is evident that the latex method of producing monolithic porous carbonaceous adsorbent structures is far superior to the conventional compressed bed method of producing carbonaceous adsorbent beds, even when comparing E3 to E4, where E3 is the latex method together with a traditional zinc impregnation method and E4 is a conventional compressed bed method together with a zinc pre-treatment impregnation method. This clearly illustrates that the latex method of producing monolithic porous carbonaceous adsorbent structures provides better results over conventional compressed beds. Furthermore, comparing E2 to E3 illustrates that the latex processing method together with the zinc pre-treatment method provides superior results to using just the latex processing alone followed by a traditional zinc impregnation method after carbonization. While E1 -E3 and E6 in Table 2 involved the use of PVDC-co- MMA latex, experiments with PVDC-co-MA latex resulted in no observable differences in either binding or carbonization.

Comparing E1 to E3, bead packing also appears to be an issue for ammonia adsorption. The PVDC beads in E1 were pre-treated with zinc acetate first and then bound by latex, resulting in an ammonia breakthrough time that is 56 seconds per gram shorter than that of E2 in which the PVDC-based beads were first bound by latex and then treated with zinc acetate. It was observed that the packing of latex wetted zinc acetate pre-treated beads of E1 was not as compact as for latex wetted non-zinc acetate pre-treated beads of E2. However, although there is difference in breakthrough time between latex bound monoliths E1-E3, they all provide monolithic porous carbonaceous adsorbent structures having longer breakthrough times than porous carbonaceous adsorbent beds produced from a conventional compressed bed method.

E4 provides an ammonia breakthrough time that was 24 seconds per gram longer than E5, demonstrating the superiority of E4 over E5. For a 50 g carbonaceous adsorbent bed, the additional 24 seconds per gram can provide an additional 20 minutes of protection. In E4 the PVDC beads were treated with zinc acetate before carbonization, as opposed to E5 where treatment with zinc acetate was done using the traditional method after carbonization of the PVDC beads. Therefore, even though the carbonaceous beds of both E4 and E5 were both prepared by conventional compressed bed techniques, it is evident that pre-treating the PVDC beads with a zinc salt provides better results than the traditional method of impregnation after carbonization. Pre-treating the PVDC beads with the zinc salt appears to provide a more homogenous zinc distribution in the adsorbent structure, resulting in an improvement in gas adsorption.

Comparison of E3 to E2 demonstrates a similar pattern where zinc acetate pre- treatment of the green monolith made using the latex method results in an improvement in ammonia breakthrough time of 24 seconds per gram over the breakthrough time of a monolithic porous carbonaceous adsorbent structure in which the carbonized monolith is treated with zinc acetate.

To further illustrate improvements realized by pre-treating PVDC precursor with a zinc salt prior to carbonization, experiments were conducted in which PVDC beads were carbonized and their BET specific surface area measured. BET specific surface area represents accessible surface for adsorption. In one set of experiments (E7-E9), the PVDC beads were pre-treated with zinc acetate and then carbonized. In another set of experiments (E10-E13), PVDC beads were carbonized and then treated with zinc acetate. E7-E13 were compared to PVDC beads that were carbonized without zinc acetate treatment (E14). The sample parameters, carbonization conditions and BET specific surface areas are provided in Table 3.

Table 3

Theoretically, carbonization of the PVDC precursor is considered to be complete when the weight of obtained carbon is about 24.7% that of the weight of the precursor, i.e. a weight loss of about 75.3% by losing hydrogen chloride. For Samples E7-E14, the degree of carbonization was compared by weight loss and a weight loss of 77.5% was considered to be the point of complete carbonization. Complete carbonization of the PVDC precursor was possible for the zinc acetate-treated precursor (E7-E9) at a temperature of about 600°C, while for untreated precursor (E10-E14) complete carbonization was only possible at about 700°C. These results were confirmed by thermogravimetric analysis. This illustrates the usefulness of zinc salts in reducing carbonization temperature of the PVDC beads. Table 3 also illustrates that zinc acetate pre-treated PVDC beads (E7-E9) provide superior BET specific surface area (more surface available for adsorption) and larger pore volumes (less pore blocking) after carbonization than PVDC beads that were first carbonized and then treated with zinc acetate (E10-E13). It is also evident in comparing E9 to E10 that much higher zinc loading is possible while retaining higher surface area and pore volume when the PVDC beads are pre-treated with zinc acetate. Furthermore, comparing E7, E8 and E9, surface area and micropore volume remain relatively constant as zinc loading is increased, which indicates that even higher zinc loading is possible without significant degradation in adsorbent performance. Fig. 6 depicts a graph comparing ammonia breakthrough times for zinc- impregnated monolithic porous carbonaceous adsorbent structures over a range of zinc content (E7-E9) to a monolithic porous carbonaceous adsorbent structure that was not impregnated with zinc (E14). From Fig. 6 it is evident that as the zinc content increases from 0% to 5% to 7.5% to 8% (all wt% based on total weight of the adsorbent structure), the ammonia breakthrough time increases dramatically. This illustrates that the adsorbent structures (E7-E9) contain adequate zinc to effectively react with ammonia, significantly decreasing the ammonia breakthrough compared to an unimpregnated structure.

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The novel features will become apparent to those of skill in the art upon examination of the description. It should be understood, however, that the scope of the claims should not be limited by the embodiments, but should be given the broadest interpretation consistent with the wording of the claims and the specification as a whole.

Claims

Claims:

1. A process for preparing a self-supporting monolithic porous carbonaceous adsorbent structure, the process comprising: drying a mixture of non-gelling polymeric carbon precursor particles and an organic latex binder at a temperature of 100°C or less to form a solid monolith of polymeric carbon precursor particles bound by an organic polymer matrix in a pre-determined shape; carbonizing the solid monolith in the predetermined shape at a temperature of 800°C or less to form a self-supporting monolithic porous carbonaceous adsorbent structure; and introducing a metal impregnant before or after carbonization of the solid monolith.

2. The process according to claim 1 , wherein the metal impregnant is one or more metal ions.

3. The process according to claim 1 , wherein the metal impregnant is one or more of zinc, copper, cobalt, silver or molybdenum ions.

4. The process according to claim 1 , wherein the metal impregnant is zinc ions.

5. The process according to any one of claims 1 to 4, wherein the metal impregnant catalyzes decomposition of the polymeric carbon precursor.

6. The process according to any one of claims 1 to 5, wherein the metal impregnant is introduced by contacting the monolithic porous carbonaceous adsorbent structure with a solution of one or more metal salts, by contacting the solid monolith of polymeric carbon precursor particles with a solution of one or more metal salts prior to carbonization or by contacting the polymeric carbon precursor particles with a solution of one or more metal salts prior to forming the solid monolith of predetermined shape.

7. The process according to any one of claims 1 to 5, wherein the metal impregnant is introduced by contacting the monolithic porous carbonaceous adsorbent structure with a solution of one or more metal salts and then heat treating to activate the metal impregnant.

8. The process according to any one of claims 1 to 5, wherein the metal impregnant is introduced prior to carbonization.

9. The process according to any one of claims 1 to 5, wherein the metal impregnant is introduced by contacting the solid monolith of polymeric carbon precursor particles with a solution of one or more metal salts prior to carbonization.

10. The process according to any one of claims 6 to 9, wherein the one or more metal salts are one or more of acetate, nitrate, halide, carbonate, oxalate or ammonium salts.

1 1. The process according to any one of claims 6 to 9, wherein the one or more metal salts are zinc acetate and/or zinc chloride.

12. The process according to any one of claims 1 to 1 1 , wherein the metal impregnant is introduced in an amount sufficient to provide a loading of metal in the monolithic porous carbonaceous adsorbent structure of 0.01-30 wt%, based on total weight of the monolithic porous carbonaceous adsorbent structure.

13. The process according to claim 12, wherein the loading is in a range of 0.05-20 wt%.

14. The process according to claim 1 , wherein the organic latex binder has a melting temperature greater than the temperature at which the mixture is dried.

15. The process according to any one of claims 1 to 14, wherein the precursor particles comprise poly(vinylidene chloride) (PVDC), PVDC copolymer, polystyrene (PS), PS copolymer, phenolic resin, cellulosic polymer or any mixture thereof.

16. The process according to any one of claims 1 to 14, wherein the precursor particles comprise poly(vinylidene chloride).

17. The process according to any one of claims 1 to 16, wherein the latex binder comprises a poly(vinylidene chloride) (PVDC) latex, a PVDC copolymer latex, a polystyrene (PS) latex, a PS copolymer latex or any mixture thereof.

18. The process according to any one of claims 1 to 16, wherein the latex binder comprises a poly(vinylidene chloride) (PVDC) latex, a poly(vinylidene chloride)-co-methyl acrylate (PVDC-co-MA) latex, a poly(vinylidene chloride)-co-methyl methacrylate (PVDC- co-MMA) latex or any mixture thereof.

19. The process according to any one of claims 1 to 18, wherein the latex binder has a solid content in a range of 15-65% by weight of the latex so that the solid monolith formed upon drying has sufficient structural integrity to maintain the pre-determined shape.

20. The process according to any one of claims 1 to 18, wherein the latex binder has a solid content in a range of 40-55% by weight of the latex so that the solid monolith formed upon drying has sufficient structural integrity to maintain the pre-determined shape.

21. The process according to any one of claims 1 to 20, wherein the precursor particles are coated with the latex binder in a latex binder to precursor ratio in a range of 1 :4 to 1 :20 by weight.

22. The process according to any one of claims 1 to 20, wherein the precursor particles are coated with the latex binder in a latex binder to precursor ratio in a range of 1 :4 to 1 :6 by weight.

23. The process according to any one of claims 1 to 22, wherein the drying is at a temperature of 50°C or less.

24. The process according to any one of claims 1 to 23, wherein the mixture is molded into the pre-determined shape before being fully dried and no pressure is applied during molding and drying.

25. The process according to any one of claims 1 to 24, wherein the polymer matrix comprises a continuous film formed by polymer of the latex binder on surfaces of the precursor particles.

26. The process according to any one of claims 1 to 25, wherein carbonizing is performed in an inert atmosphere.

27. The process according to any one of claims 1 to 26, wherein carbonizing is performed with a temperature profile comprising temperature plateaus at about 200°C and about 700°C with soaking times at each temperature plateau in a range of from about 0.5 hours to about 4 hours and ramp rates between the temperature plateaus in a range of about 0.2-3°C/minute.

28. The process according to any one of claims 1 to 27, wherein the latex binder forms porous carbon upon carbonization.

29. A process for preparing a carbonaceous adsorbent structure, the process comprising: drying a mixture of poly(vinylidene chloride) particles in a solution of a metal salt to form a metal salt impregnated bed of poly(vinylidene chloride) in a pre-determined shape, the metal salt capable of catalyzing the decomposition of the poly(vinylidene chloride); and, carbonizing the bed in the pre-determined shape at a temperature of 650°C or less to form a metal impregnated carbonaceous adsorbent structure.

30. The process according to claim 29, wherein the temperature is in a range of 550°C to 650°C.

31. The process according to any one of claims 29 to 30, wherein the metal salt comprises a zinc salt.

32. The process according to claim 31 , wherein the zinc salt comprises zinc acetate or zinc chloride.

33. The process according to any one of claims 1 to 32, wherein the particles have average particle diameters in a range of 0.595-2.00 mm.

34. The process according to any one of claims 1 to 33, wherein the particles have an aspect ratio of 1 :1.5 or less.

35. The process according to any one of claims 1 to 33, wherein the particles have an aspect ratio of about 1 :1.

36. The process according to any one of claims 1 to 35, wherein the adsorbent structure has a specific surface area of 800 m²/g or more.

37. A carbonaceous adsorbent structure produced by the process as defined in any one of claims 1 to 36.

38. A self-supporting monolithic porous carbonaceous adsorbent structure comprising a free-standing monolith of metal impregnated carbon particles having a specific surface area of 800 m²/g or more.

39. A respirator comprising a carbonaceous adsorbent structure as defined in any one of claims 37 to 38.