Polymer-monolith pre-oxidation to improve shape retention during subsequent heat treatment, and monolith produced

Abstract

High purity carbon sorbent monoliths, particularly effective for the removal of trace-contaminants such as ammonia, formaldehyde, and methyl mercaptan from a gas flow, are fabricated by 3D-printing polymer monoliths, pre-oxidizing them in a flow of air at a temperature below the melting point of the polymer precursor, carbonizing them, and subsequently activating them to a weight loss of about 20 percent. The pre-oxidation step effectively prevents polymer-monolith swelling and melting during carbonization.

Claims

1. A method for the production of a carbon sorbent monolith for removing at least one gas from a gaseous environment in which the at least one gas is contained, comprising the steps: (a) producing a shaped polymer monolith using 3-D printing of a polymer precursor consisting essentially of polyether ether ketone (PEEK); (b) pre-oxidizing said shaped polymer monolith in a flow of an oxidizing gas at a temperature below the melting point of said polymer precursor for a time sufficient to preserve the shape of said polymer monolith during a subsequent carbonizing step; (c) carbonizing said pre-oxidized shaped polymer monolith of step (b) so as to produce a carbon sorbent monolith by exposure to elevated temperatures in a non-oxidative atmosphere; and (d) optionally, activating said carbon sorbent monolith of step (c) by exposure to an oxidizing environment under conditions sufficient to cause a carbon weight loss in said carbon sorbent monolith of at least about 1 weight percent and thereby producing a microporous sorbent monolith with a surface area of at least about 100 m.sup.2/g, with the majority of pores being smaller than about 2 nm.

2. The method of claim 1, wherein said pre-oxidizing step (b) is carried out at a temperature at least 3 C. below said melting point of said polymer precursor.

3. The method of claim 1, wherein said pre-oxidizing step (b) is carried out at a temperature at least 5 C. below said melting point of said polymer precursor.

4. The method of claim 1 wherein said sorbent monolith has a honeycomb cell structure.

5. The method of claim 4, wherein the walls that define the cells of said honeycomb structure are about 0.10 mm to 1.0 mm thick.

6. The method of claim 1, wherein said polymer precursor includes reinforcement in the form of fibers, at a fiber content of about 1-20 weight percent, for improved shape retention during said carbonizing step (c) and for enhanced strength.

7. The method of claim 6, wherein said fibers are carbon fibers.

8. The method of claim 1, wherein said pre-oxidizing step (b) is carried out in a flow of air.

9. The method of claim 1, wherein said pre-oxidizing step (b) is carried out at a temperature at least 1 C. below said melting point of said polymer precursor.

10. The method of claim 1, wherein said time in step (b) sufficient to preserve the shape of said polymer monolith during subsequent carbonizing step (c) is at least about 3 hours.

11. The method of claim 1, wherein said oxidizing environment in step (d) is selected from the group consisting of air, oxygen, carbon dioxide, steam, ozone, hydrogen peroxide, nitric acid, and mixtures thereof.

12. The method of claim 1, wherein said oxidizing environment in step (d) comprises air at a temperature in the range of 150 C. to 400 C.

13. The method of claim 12, wherein said temperature range in step (d) is 250 C. to 325 C.

14. The method of claim 1, wherein said carbon weight loss of at least about 1 weight percent in said step (d) is at least about 5 weight percent.

15. The method of claim 1, wherein said carbon weight loss of at least about 1 weight percent in said step (d) is at least about 20 weight percent.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 comprises three views of a short, cylindrical monolith embodying the present invention, view A being a plan view, view B being a side elevational view, and view C being an isometric view.

(2) FIG. 2 comprises two plots, the plot of part (a) showing ammonia-sorption performance data obtained during a rapid adsorption-desorption cycling procedure, and the plot of part (b) showing a portion of the large data set constituting part (a), drawn to an enlarged scale.

DETAILED DESCRIPTION OF THE INVENTION

(3) Turning initially in detail to FIG. 1 of the drawings, therein illustrated is a short, cylindrical sorbent monolith, generally designated by the numeral 10, embodying the present invention. The monolith consists of a circumferential surrounding wall 12 and a grid of mutually perpendicular internal walls 14 defining a multiplicity of open-ended cells (channels) 16 extending axially through the monolithic body.

(4) The fabrication method provided by the present invention comprises the following steps: (1) preparation of the precursor material (polymer) in a desired shape using 3D printing or any other suitable manufacturing method, such as injection molding or extrusion; (2) polymer-monolith pre-oxidation by exposure to air at a temperature below the polymer melting point for at least about 30 minutes, and preferably for three or more hours; (3) pre-oxidized polymer-monolith carbonization (pyrolysis) so as to provide a carbon monolith by exposure to elevated temperatures in a non-oxidative atmosphere; and ultimately (4) carbon-monolith activation to obtain the desired pore-structure characteristics, the desired surface chemistry (functional groups), and good TC-sorption performance (high sorption capacity and rapid vacuum-regeneration).

(5) In the pre-oxidation step (2), the polymer monolith is exposed to an activating environment in a flow of air at a temperature that is at least 1 C., preferably at least 3 C., and most desirably at least 5 C. below the melting point, and for a period of at least 30 minutes, preferably at least one hour, and most desirably at least three hours. The pre-oxidation treatment sets the polymer structure, and at least substantially prevents monolith swelling and meltdown during subsequent thermal processing. The mechanism of this step is not presently understood, but it is quite possible that the presence of oxygen facilitates polymer cross-linking, which solidifies the monolith structure and makes it resistant to the loss of shape.

(6) The activation step (4) can be implemented by means of contacting carbon with an oxidizing agent, such as air, oxygen, steam, or carbon dioxide. The activation process is usually associated with a carbon weight loss of normally at least 1.0 weight percent, preferably at least 5.0 weight percent, and most desirably 20 weight percent or more. Carbon activation creates additional internal surface area and porosity which, together with the oxygen species present on the carbon surface, are instrumental in the effective sorption of trace contaminants. The specific surface area of such carbon monoliths is normally higher than 100 m.sup.2/g, preferably at least 300 m.sup.2/g, and most desirably at least 500 m.sup.2/g. In preferred embodiments, the majority of pores in the sorbent monolith will not exceed 2 nanometers in size.

EXAMPLE

(7) Exemplary of the efficacy of the present invention is the following example. It employs, as the polymer precursor for monolith fabrication, polyether ether ketone reinforced with 10 weight percent carbon fiber 3D printer filaments (i.e., PEEK/CF). The material was obtained from 3DXTECH Additive Manufacturing of Grand Rapids, Michigan.

(8) 3D Monolith Printing

(9) An Intamsys Funmat HT 3D printer was used. The printer is designed specifically for higher melting temperature polymer materials, such as PEEK and PEEK reinforced with carbon fiber (PEEK/CF). The monolith produced and employed herein was 3D printed using the PEEK/CF filament. With reference to FIG. 1 of the drawings, the monolith had 0.5-mm square channels (open-ended cells), a diameter of 18 mm and a height (axial length) of 6 mm, the channel-defining walls being 0.25 mm thick and the circumferential wall being 0.5 mm thick.

(10) Pre-Oxidation

(11) Oxidative treatment of the monolith was effected at temperatures below the PEEK melting point (343 C.). This treatment was performed, prior to carbonization, in a flow of air at 340 C. for three to 20 hours. This step was found to successfully preserve the original PEEK/CF monolith shape during carbonization, without the need for supporting structures and thus greatly simplifying the fabrication process.

(12) Various pre-oxidation conditions (temperature and time) were employed to determine the effects thereof on polymer-monolith shape retention. Pre-oxidation was carried out in a flow of air at atmospheric pressure. It was found that polymer monoliths pre-oxidized at 355 C. for 1 hour, i.e., above the PEEK melting point of 343 C., exhibited a severe loss of the original shape, with the honeycomb monolith channels being completely fused together and the monolith itself turning into a lump of melted polymer, which solidified upon cooldown. Subsequent carbonization in a flow of high-purity nitrogen at 800 C. produced even more severe loss of shape, due to material softening, melting, and swelling. The foregoing result is hardly unexpected since thermoplastic polymers are well known to go through a fluid stage and melting at temperatures close to and above their melting points. Naturally, whenever carbonization was attempted without the preceding pre-oxidation step monolith meltdown (i.e. the complete loss of shape) was always observed. Subjecting PEEK/CF honeycomb monoliths to pre-oxidation at temperatures lower than the PEEK melting point, e.g. at 340 C., for times longer than about 30 minutes, resulted in very good shape retention during pre-oxidation, and also during subsequent thermal processing, i.e. carbonization and activation. The resultant monoliths showed perfectly preserved channels as well as overall monolith shape, even though some shrinkage of the channel walls and the overall monoliths dimensions was observed. Shrinkage was on the order of 10 to 20 percent, was somewhat dependent on the thermal processing (heat-treatment) conditions, notably the time-temperature profile, and did not at all adversely affect the functional usefulness of the resultant monolithic TC sorbent.

(13) Carbonization and Activation

(14) A standard laboratory tube furnace was used for carbonization, and several heating profiles were utilized, some of them with a single heating rate of 5 K/min and others with hold times at about 500 C. The final carbonization temperature used was 800 to 900 C., and nitrogen was employed as a carrier gas. Carbon activation was carried out in a flow of air at 325 C. to a weight loss (burn-off) of 20 weight percent using a tube furnace. It should be noted that, although generally advantageous, the activation step is optional, as carbons produced by polymer-precursor carbonization may already possess fair amounts of internal surface area and porosity.

(15) Pore-Structure Characterization Methodology

(16) An automated gas-sorption system ASiQwin (manufactured by Quantachrome Instruments of Ashland, Virginia) was used for collecting and processing nitrogen-isotherm data for carbon sorbents. Prior to making adsorption-isotherm measurements, each sample was outgassed under vacuum at 300 C. for at least three hours. Nitrogen-adsorption isotherms were determined at 77 K, and the resulting data were used to perform the following analyses: (a) Brunauer, Emmett, and Teller (BET) surface area; (b) pore volume; (c) Dubinin-Radushkevich (D-R) micropore surface area and micropore volume; and (d) pore-size distribution of micropores using the Density Functional Theory (DFT).

(17) The BET surface area for sorbent monoliths derived from PEEK/CF was found to be about 585 m.sup.2/g, and most of the pore volume (70 percent) was found to consist of microporosity (pores smaller than 2 nm).

(18) Sorbent Testing System and Procedure

(19) Sorbent-monolith performance for ammonia, formaldehyde, and methyl mercaptan sorption and regeneration was tested, as described in a recent paper (Wjtowicz, M. A., Cosgrove, J. E., Serio, M. A., Carlson, A. E., Hostetler, J. M, Espinosa, N. and Chullen, C., Pressure-Swing Adsorption of Trace Contaminants Using Carbon Sorbent Monoliths, paper ICES-2021-385, 50th International Conference on Environmental Systems, 2021). The tests performed demonstrated excellent monolith shape retention during sorbent-monolith fabrication, and also very good trace-contaminant removal performance. More than 2,000 adsorption-desorption cycles were carried out, with a half-cycle period of 5 minutes, for about 14 days of continuous pressure-swing adsorption operation at trace-contaminant levels close to their seven-day Spacecraft Maximum Allowable Concentrations (SMACs) Thus, TC-removal efficiencies were shown to be adequate for keeping ammonia and formaldehyde below SMAC levels at all times.

(20) Data illustrating the removal of ammonia, the most important trace contaminant of interest within the space-suit ventilation loop, are shown in FIG. 2. As previously indicated, part (a) of FIG. 2 is a plot of ammonia-sorption performance during a rapid adsorption-desorption cycling test with a half-cycle of 5 minutes. The sorbent employed in the test was derived from a 3D-printed PEEK/CF cylindrical monolith carbonized at 900 C. and activated in a flow of air at 325 C. to a burn-off of 20 weight percent; it had 0.5-mm square channels with defining walls of 0.25 mm in thickness, a diameter of 17 mm, an axial length of 3.5 mm, and a weight of 0.466 g. The gas flow rate through the sorbent was 0.52 L/min, and the ammonia inlet concentration was 5.5 ppm. The sorbent was regenerated by exposure to vacuum created by a rotary-vane pump (0.3 Torr). As indicated previously, FIG. 2 (b) reproduces a portion of the large data set shown in FIG. 2 (a).

(21) Ammonia slip is defined as ammonia concentration at the sorbent outlet, expressed in terms of the percentage of the ammonia inlet concentration. Thus, the ammonia-removal efficiency is equal to 100 percent minus ammonia slip. The data in FIG. 2 show that the sorbent monolith performs well over about 330 hours (14 days), i.e. 2,000 rapid adsorption-desorption cycles, at a residence time of 40 ms, which is relevant to full-scale operation of the Exploration Portable Life Support System (xPLSS) within the NASA space suit, using modest-quality vacuum for sorbent regeneration (0.3 Torr). Data in FIG. 2 also show that ammonia-removal efficiency (initially close to 100 percent) decreases gradually with time, to reach about 70 percent at the end of this long test. Sorbent-design calculations similar to those described by Paul et al. (Paul, H. L., Jennings, M. A., and Waguespack, G. M., Requirements and sizing investigation for Constellation space suit Portable Life Support System Trace Contaminant Control, Proc. 40th Int. Conf. on Environmental Systems, AIAA, Barcelona, Spain, Paper No. AIAA 2010-6065, 2010) were carried out, and showed that an ammonia-removal efficiency of at least 50 percent would be sufficient to make sure the astronaut breathes oxygen with ammonia under 7-day SMAC (2 mg/m.sup.3) (Wjtowicz et al. 2021). Therefore, the 70 percent removal efficiency shown in FIG. 2 is more than sufficient to meet NASA requirements.

(22) Prophetic Examples

(23) Monoliths having the honeycomb structure illustrated in FIG. 1 and described above are fabricated using each of the polymer precursors: PEEK, PEI, PEI/CF, PC, and PC/CF, as defined herein. Each of the monoliths is oxidized under temperature and time conditions suitable for the particular polymer of fabrication, as will be evident to or readily determined by those of ordinary skill in the art, based upon the operative example hereinabove set forth and other teachings of the specification. The thus pre-oxidized monoliths are thereafter carbonized and activated, again under suitable temperature and time conditions which will be evident to those of ordinary skill in the art.

(24) It is expected that shape-retention and other properties comparable to those of the PEEK/CF sorbent monoliths produced in accordance with the foregoing operative example would be exhibited by sorbent monoliths produced in accordance with the present prophetic examples.

(25) Precursor Selection

(26) The following criteria may be applied beneficially for polymer-precursor selection: compatibility with 3D printing (in the preferred embodiments); good shape retention upon carbonization and activation (a high carbon yield during carbonization, i.e. a low volatile-matter content, which is favorable for shape retention; furthermore, high carbon yields improve process economics); good TC sorption and sorbent regeneration demonstrated by the final product (sorbent monolith).

(27) As will be appreciated by those skilled in the art, despite specific descriptions and disclosures provided herein many variations in the form, size, and shape of the sorbent monolith, and in the steps and conditions employed for producing it, may be made without departing from the effective scope of the appended claims.

(28) The invention provides a novel method for improving shape-retention of polymer monoliths resulting from 3D printing or other suitable manufacturing techniques, during subsequent thermal processing steps of carbonization and activation. It also provides a novel carbon sorbent monolith produced by the method. Thus, the stated objects of the invention are attained by the instant method and product, as described in the foregoing specification and defined by the appended claims.