Commercial Kevlar derived activated carbons for CO2 and C2H4 sorption


 The carbonaceous precursor was obtained via pyrolysis of commercial aramid polymer (Kevlar). Additionally the precursor was activated at 1000°C in CO2 atmosphere for different times. Obtained materials were characterised by BET; XPS; SEM and optical microscopy. The sorption capacities were determined by temperature swing adsorption performed in TGA apparatus for CO2 and C2H4 gases. The obtained materials exhibit high difference in sorption of these gases i.e. 1.5 and 2.8 mmol/g @30°C respectively and high SSA ~1600 m2/g what can be applied in separation applications. The highest uptakes were 1.8 and 3.1 mmol/g @30°C respectively. It was found that the presence of oxygen and nitrogen functional groups enhances C2H4/CO2 uptake ratio.


INTRODUCTION
Greenhouse gases accumulate in the atmosphere and create the heat-refl ective layer that keeps the Earth temperature friendly for life. It is widely acknowledged that climate change is caused by an excess of anthropogenic greenhouse gases. Some of the most common and worrisome greenhouse gases are CO 2 and CH 4 . Carbon dioxide is emitted whenever carbon-rich fossil fuels are burned. Although CO 2 is not the most potent anthropogenic greenhouse gas, it has the largest contribution to climate change due to its volume emitted mostly in the energy sector. On the other hand, anthropogenic methane is emitted in the process of decomposition of organic matter. It is also released from landfi lls, swamps, cattle farms, rice paddies, etc. CH 4 emissions are lower than CO 2 emissions but it is considered a major greenhouse gas because each methane molecule has 25 times the global warming potential of a carbon dioxide molecule.
To actively address these emerging challenges, capturing the greenhouse gases from the emission sources and utilizing them is considered a direct and highly effective strategy. Carbon dioxide is a very important multipurpose chemical and can be utilized as a freezing medium, foaming agent, green solvent and promising feedstock for the future production of key chemicals 1, 2, 3 . Before CO 2 utilization it has to be separated and recovered from the mixture of gasses, usually from the post-combustion mixture.
The liquid amine-based CO 2 separation commercialized for a very long time still suffers from high energy consumption in the recycling of amine, as well as the amines' toxic, corrosive, and volatile properties. The CO 2 can be also separated by the application of membranes, but the problem can be low durability of membranes at elevated temperatures of fl ue gases. However, the combination of photocatalysis and membrane separation in the application for CO 2 -derived products could alleviate the problem of CO 2 storage after separation 4 .
By contrast, solid sorbents that capture CO 2 via physical interactions are more appealing, since the whole process is energy-saving, eco-friendly, and easily reversible 5 . The solid adsorbents have been suggested as alternative candidates for CO 2 separation. Considerable efforts have been devoted to screening suitable CO 2 adsorbents for post-combustion capture application 6-23 .
Methane is the valuable chemical stock in chemical industries but the fi rst stage of methane transformation -syngas production is very energy consumed 24 . Despite being the most abundant and least expensive hydrocarbon feed stock available, methane fi nds limited use as a starting material within the chemical industry 25 . That's why the production of key chemicals and fuels considering the rapid advances in methane direct conversion technologies is very promising. Methane can be utilized as feedstock for methanol 26-28 , formaldehyde 29-31 , methyl bisulphate 25, 32-35 , carbon nanotubes combined with hydrogen 36-40 one-step production excluding the synthesis gas production stage. Methane can be also applied as alternative transportation in compressed natural gas and adsorbed natural gas technology 41 .
Although ethylene is not considered a potent global warming agent its removal via sorption is of importance due to its impact on fruit and vegetable ripening. Ethylene is the hormone for plants and its concentration control in the greenhouse atmosphere or during fruits transport can improve growth effi ciency and food preservation respectively 42  Among the variety of precursors from which carbonaceous materials may be derived the polymers are especially attractive. One of the advantages is the formation of the fi nal shape during polymerization stage. The other advantage is exact and the same characteristic for every batch of produced material. The polymer precursors usually exhibit very high purity. The disadvantage may be the higher price.
An interesting precursor is aramid polymer also known as a Kevlar ®. Kevlar is a kind of polyaramid fi bers which was fi rstly commercialized in 1972 by DuPont. This material has good properties like high-temperature decomposition, fl ame resistance, abrasion and stretching resistance, chemically stability, etc 60-62 . It has stab resistance and ballistic property which allows to produce fl exible body armours with light weight and high-temperature resistance capacity enables to produce personal protective clothing for the fi refi ghters such as gloves, suits and jackets, breeches. Moreover, plastrons are made for sportswear in motorcycling and car racing 63-66 . The good elongation capacity, abrasion resistance, strength, thermal stability and adaptable weariness resistant, make Kevlar good material to manufacture tires, braking lines and automobile brakes. Many useful carbonaceous materials are derived from Kevlar with great benefi ts of desired properties, such as Activated Carbon Fibers (ACFs) show fi brous morphology. It improves adsorption rate of the ACFs which is used for effective gas storage. High adsorption capacity is possible to achieve by robust structure, tunable porosity, lightweight, high thermal/chemical stability of Kevlar 67-68 . Aramid Nanofi ber (ANF) is made from Kevlar to make composite materials with good cation selectivity and high mechanical strength and are used in nanofl uidic osmotic power generators 69 . Polyaramid fi bers are used as precursors for microporous carbonaceous materials for gases (e.g. CO 2 , N 2 , CH 4 , O 2 ) and vapours (e.g. CH 2 Cl 2 , C 6 H 6 , C 6 H 12 ) adsorption 70-75 .
The aramid polymer contains oxygen and nitrogen in its structure. Therefore the Kevlar-derived carbonaceous materials are doped with these elements. The goals of this work were: checking how the time of physical activation infl uence C 2 H 4 and CO 2 uptakes; Determining how the elemental content is changing during activation; Determining the infl uence of nitrogen and oxygen on sorption properties.

EXPERIMENTAL
The commercial Kevlar fabric was used as a carbonaceous precursor. The Carburisation process was conducted under nitrogen atmosphere at 700°C for 2 hours (ramp 10°C/min). The material obtained in this way will be referred as Carb. The physical activation was conducted under CO 2 atmosphere at 1000°C (ramp 5°C/min). After reaching 1000°C the sample was kept for a given time.
The activated carbons will be referred as AC0; AC5; AC15 and AC30 for times of activation ranging from 0 to 30 minutes -e.g. AC15 denotes sample activated 15 minutes at 1000°C.
The textural characterization of the carbonaceous materials was carried out by N 2 adsorption-desorption isotherm measurements at 77 K (Quantachrome; Autosorb Instrument). The samples were degassed at 373 K under vacuum for 16 h prior to analysis. The specifi c surface areas (SSA) were calculated using the Brunauer-Emmett-Teller (BET) equation. The total pore volumes were evaluated based on the volume adsorbed at a relative pressure of about 0.99. Pore size distribution, as well as the volume of micro-(V micro ) and mesopores (V meso ), were calculated using QSDFT method.
The sub-micropores of diameters below 1.2 nm volumes were calculated by using the Density Functional Theory (DFT) to the CO 2 adsorption isotherms at 0°C. The software being the part of volumetric apparatus (QuadraWin; Quantachrome Instruments) was used to evaluate the data. The cumulative pore volumes V 0.7 , V 0.8 and V 1.0 of pores up to 0.7, 0.8 and 1.0 nm were determined respectively. Also, the specifi c surface area (S CO2 ) of pores up to 0.9 nm was determined.
The X-ray photoelectron spectroscopy measurements were performed in a commercial UHV surface analysis system (PREVAC), which operates at a base pressure in the low 10 -10 mbar range. The setup is equipped with nonmonochromatic X-ray photoelectron spectroscopy (XPS) and kinetic electron energy analyzer (SES 2002; Scienta). The calibration of the spectrometer was performed using Ag 3d 5/2 transition. Samples in the form of fi ne powder were thoroughly degassed prior to measurement so that during XPS measurements the vacuum was in the low 10 -9 mbar range. The X-ray photoelectron spectroscopy was performed using Al K α (hν = 1486.6 eV) radiation.
Scanning electron microscopy with cold emission (SU8020; Hitachi) was used to characterize the morphology of the samples. The accelerating voltage was 5 kV. Also, an optical microscope (Stemi 508; Zeiss ) was used for characterization in the visible light spectrum.

RESULTS AND DISCUSSION
In Fig 1. There are presented images obtained in optical microscope of Kevlar prior and after pyrolysis/ carburization. It can be noticed that net-like structure is preserved after thermal treatment. This feature is an advantage for gas adsorption applications.
In Fig 2. There are presented images obtained via scanning electron microscopy.
The activated material also preserves net-like structure but the individual threads shrink during the process. Closer inspection also reveals the formation of spongelike structure in the case of activated material. The size of most of the visible pores is about 50 nm. These pores are pathways for the diffusion of gases into much smaller pores below 1 nm size.
The porous morphology was determined from nitrogen adsorption/desorption isotherms presented in Fig. 3. The evaluation of these isotherms enabled the determination  Table 1. Moreover, the pore size distributions were determined and are presented in Fig. 4.
According to IUPAC, isotherms presented in Figure 3 indicate a combined type of I b and IVa. Type Ib is characteristic for materials with a broad range of micropores and narrow mesopores up to a diameter of 2.5 nm. Moreover, the microporous structure is confi rmed by steep uptake of nitrogen at very low p/p 0 range. On the other hand, type IVa is characteristic for materials with mesoporous structure, which is confi rmed by the hysteresis phenomenon occurring for pores wider than 4 nm in diameter.
The analysis of the data presented in Table 1 reveals activation process strongly develops SSA from 3 m 2 /g for not activated material (Carb) to about 1600 m 2 /g for all activated samples. The total pore volume is negligible for Carb and increases 84-96 times for other samples depending on activation time. Interestingly the Carb sample despite its negligible SSA and total pore volume exhibits signifi cant CO 2 and C 2 H 4 uptakes. One should note that for uptakes of these gases are responsible pores below 1.0 nm. The nitrogen sorption method enables pore volume determination in the range 1.2-100.0 nm. Therefore it is expected that Carb sample exhibit a considerable volume of pores below 1.2 nm.   The CO 2 sorptions at 0°C enabled the determination of pore volumes below 1.2 nm, the specifi c surface area of such pores and CO 2 uptake at this temperature ( Table 2).
The notation V 0.7 , V 0.8 and V 1.0 denotes cumulative pore volume of pores up to 0.7, 0.8 and 1.0 nm respectively. Pores 0.7 and 0.8 are crucial for sorption of CO 2 in temperatures 0°C and 30°C respectively. The pores 1.0 nm are crucial for sorption ethylene at 30°C.
The pore size distributions presented in Fig. 4. exhibit considerable amount of micropores in the range 1.2-2.0 nm and mesopores in the range 2.0-12.0 nm. The Carb material is not presented in the fi gure due to the negligible nitrogen sorption. The highly porous structure is a result of CO 2 activation process. Even sample AC0 underwent activation process by heating up to 1000°C followed by immediate cooling down in CO 2 atmosphere. The main difference one can observe in volume of mesopores which are irrelevant for the sorption of small molecules like CO 2 and C 2 H 4 but may be crucial for the sorption of larger molecules like benzene.
The sorption properties are also dependent on the surface chemistry of the sorbents. In Fig. 5 there are presented XPS spectra of all samples with denoted signals.
values for Kevlar fabric indicating that this element is embedded in polymer. Carbonisation of Kevlar at 700°C leads to the highest sodium surface concentration. Activation at 1000°C depletes sodium concentration. This tendency can be explained as follows: the sodium atoms embedded in the polymer diffuse to the surface in the carbonisation process. However, at higher temperatures volatile sodium compounds evaporate and thus lower concentrations are observed. The sodium presence in the polymer may be result of its production procedure. The polymer is produced in strong acid and fi nally, it requires neutralisation e.g. with sodium base.
It was also found that the nitrogen content in Kevlar polymer is lower than expected. The atomic ratio of Carbon/Oxygen/Nitrogen is expected to be 78/11/11. The XPS measured ratio is 81.2/15.5/3.3 respectively. The discrepancy may be result of surface oxidation of polymer and the presence of sodium and silicon oxides.
The analysis of Table 3 shows that oxygen and nitrogen contents decrease with activation time. This may indicate that heteroatoms may be considered as a defect over the carbon surface which facilitates the reaction of CO 2 with carbon.
In Fig. 6 there are presented CO 2 isobars obtained through heating and monitoring the mass of the sample in 250°C and cooling down to 30°C under CO 2 atmosphere. Due to that measurement procedure, the highest desorption occurs at 250°C and is considered as a reference point with 0.0 mmol/g uptake.
Such presentation of the results enables the determination of gas uptakes not only for a single temperature but for the whole 30-250°C range. The sample with the highest CO 2 uptake is AC0. Interestingly the sample without activation process i.e. Carb exhibits comparable CO 2 uptakes above 50°C.
In Fig. 7 there are presented C 2 H 4 isobars obtained in the same manner as in case of CO 2 .
One can observe that activation leads to signifi cant improvement of C 2 H 4 uptakes. The highest performance exhibit the sample AC30 with the longest time of activation.
In Fig. 8. there is a presented graph showing C 2 H 4 / CO 2 ratios for all samples.
One can notice that activation increases the preference for C 2 H 4 sorption. This feature can be utilised for the separation of these gases. The increase of the ratio   The Kevlar polymer consists of hydrogen, carbon, nitrogen and oxygen elements. Hydrogen is not detectable with XPS method. Surprisingly the presence of sodium and silicon was found in the samples. The quantitative analysis enabled the determination of elemental surface concentrations presented in Table 3.
The sodium and silicon may be result of commercialgrade material. The silicon element may be antifriction agent during manufacture of Kevlar threads. Therefore it is expected that silicon will be present mainly over the surface of threads. This confi rms the high concentration of this element for fabric of Kevlar while pulverised material after carbonisation exhibit lower content of this element. Conversely, sodium exhibits the lowest correlates well with the decrease of oxygen and nitrogen (c.f. Tab. 2).

SUMMARY AND CONCLUSIONS
Carbon materials derived from commercial Kevlar fabric retain their netlike structure after physical activation. This feature is benefi cial for the reduction of fl ow resistance in the sorption/desorption of liquids and gases. of oxygen and nitrogen in activated carbon the higher C 2 H 4 /CO 2 ratio. Therefore activated carbons for the separation of these gases should contain low content of heteroatoms.   These materials contain high nitrogen concentrations i.e. 4.8 and 2.6 at. % for not activated and activated material respectively. The commercial-grade Kevlar contains silicon and sodium additives. The highest obtained uptakes at 30°C were 3.1 and 1.8 mmol/g for C 2 H 4 and CO 2 respectively. It was observed that the lower content