CO2 adsorption and CO2/CH4 separation using fibrous amine-containing adsorbents: isothermal, kinetic, and thermodynamic behaviours

A series of fibrous aminated adsorbents for CO2 adsorption were prepared by covalent incorporation of poly (glycidyl methacrylate) (PGMA) by graft copolymerization of GMA onto electron beam (EB) irradiated polyethylenepolypropylene (PE/PP) fibrous sheets and subsequent amination with ethylenediamine (EDA), diethylenetriamine (DETA), or tetraethylenepentamine (TEPA). The physico-chemical properties of the adsorbents were evaluated using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric (TGA), X-ray diffraction (XRD), and Brunauer–Emmett–Teller (BET) analysis. All the adsorbents displayed typic primary and secondary amine features combined with a decrease in both of crystallinity and surface area of PE/PP, and such a decrease was higher in adsorbents with longer aliphatic chain of the amine. Of all adsorbents, TEPA-containing fibres showed the highest CO2 adsorption capacity and thus was further investigated for CO2 capture from CO2/CH4 mixtures of different gas ratios under various pressures and temperatures. The selectivity of CO2 over CH4 and equilibrium isotherms, kinetics, and thermodynamics of the adsorption on the fibrous aminated adsorbent were all investigated. The Sips model was found to best fit the isotherm of CO2 adsorption suggesting the presence of a combination of monolayer and multilayer adsorptions. The adsorption kinetic data was found to best fit Elovich model reflecting chemisorption. The ΔG°, ΔS°, and ΔH° showed positive values suggesting that the adsorption of CO2 on the present fibrous adsorbent was non-spontaneous with an increase in randomness implying that the process was endothermic. Overall, it can be suggested that PE/PP-g-PGMA/TEPA adsorbent has a strong potential for separation of CO2 from NG.


Introduction
Natural gas (NG) is the one of cleanest, safest, and most efficient fossil fuel, which emits roughly 26-41% less CO 2 than oil and coal (Adewole et al. 2013).Hence, the demands on NG keep growing with an expected 64% more consumption in the coming few decades.To meet the expanding global demand for NG, offshore NG fields offers an alternative reserves to onshore counterparts and the ocean provides about one-third of global NG (Chen et al. 2017).For offshore NG production, a complete production process is carried out on-site of the floating liquefied NG production storage and off-loading (LNG-FPSO) structures, including pre-treatment, liquefaction, storage, transportation, and Responsible Editor: Tito Roberto Cadaval Jr off-loading (Chen et al. 2020).The pre-treatment is the most crucial section of the offshore NG development to remove impurities from raw NG before commercializing it.Raw NG comes with high compositions of methane, while CO 2 and H 2 S are the major impurities (Adewole et al. 2013).Raw NG contains up to 70% of CO 2 depending on the geographical location, and the removal procedure is typically performed at a pressure range of 30-60 bar (Han and Ho 2021).Such high amounts of CO 2 in the NG stream reduce its energy content (calorific values) and create corrosion problems during transportation in pipelines and cylinders.Thus, CO 2 content should be minimized to 2-3% by volume (Lee et al. 2018).
To date, the leading technologies for CO 2 removal from NG are solvent scrubbing, adsorption, membranes separation, and cryogenic distillation.Among all, amine-scrubbing in a gas-liquid contactor is the most mature and commercially used method for CO 2 removal that offers higher capture efficiency (Mukhtar et al. 2020c).However, this process is high energy demanding, high operational cost, and operates in central plants making it not suitable for offshore NG treatment (Adewole et al. 2013;Chen et al. 2020).Adsorption technology offers alternative simplified operation, low energy demand, ease of control, and high efficiency in addition to wide range of adsorbent materials (Chen et al. 2020).Broadly, different types of solid adsorbents have been explored for CO 2 capture from NG, including carbon-based materials (Attia et al. 2020), zeolites (Wang et al. 2019), mesoporous silica (Ullah et al. 2015), microporous organic polymer (MOPs) (Xu et al. 2020), and metal-organic framework (MOFs) (Furukawa et al. 2010).All of such adsorbents are packed in fixed bed column for practical applications, and this is accompanied by pressure loss, channelling, and inhomogeneous gas flow when fitted for working in the adsorption columns processes such as pressure or temperature swing adsorption (Mallick et al. 2018).Moreover, MOFs bearing extraordinary CO 2 adsorption capacity have high cost, low hydrothermal stability, and are not practical for column system at a large scale of LNG-FPSO (Abid et al. 2012;Danaci et al. 2015).
Solid basic polymer adsorbents containing groups such as hydroxy, nitro, amine, imidazole, triazine, and imine provide alternative materials for CO 2 capture (Petrovic et al. 2021).Particularly, the development of new adsorbents with aminecontaining fibrous structures provides advantages in terms of rapid gas diffusion and enhanced gas-solid interaction while reducing pressure loss during gas treatment.Graft copolymerization is one of the most appealing methods to impart permanent functional groups to polymer substrates (non-woven fabrics, films, and porous particles) to prepare CO 2 polymer adsorbents with various morphologies (Zhao et al. 2020).Graft copolymerization can be initiated on polymer substrates using high energy radiation including gamma rays, electron beam, low energy radiation such as ultraviolet (UV), and plasma treatment in addition to conventional chemical initiators in the presence of vinyl monomers that can host different amine groups (Shoushtari et al. 2012).
Fibrous amine containing adsorbents is a class of solid polymer adsorbents that have potential to overcome the pressure drop and gas channelling problems when packed in adsorption columns, and thus, they are worthy further development.Moreover, most of these aminated fibrous adsorbents were tested for CO 2 capture from air and postcombustion effluent despite the presence of other major applications involving NG purification.Fibrous adsorbents were mainly prepared by radiation induced graft copolymerization (RIGC) of vinyl monomers onto polymer non-woven sheets made of polypropylene and polyethylenepolypropylene (PE/PP) (Nasef et al. 2014;Kavaklı et al. 2016;Rojek et al. 2017;Abbasi et al. 2018).Monomers such as glycidyl methacrylate (GMA) (Imanian et al. 2022), N-vinylformamide (Zubair et al. 2020), and acrylamide (Nasef and Güven 2012) were used to endow variety of amine groups after post-grafting treatments and demonstrated an appealing affinity to CO 2 (Nasef and Güven 2012).Applying RIGC for immobilization of functional groups offers advantages in terms of ability to control the level, location, and distribution of graft chains on the substrate by optimization of reaction parameters without leaving detrimental waste.This allows the adsorbent to be easily scaled and makes the preparation process rather environmentally friendly (Abou Taleb et al. 2008).
Several studies have reported the preparation of fibrous adsorbents by RIGC of GMA, which have an epoxy group allowing the amination of the grafted substrate with various amine groups (Abbasi et al. 2018(Abbasi et al. , 2019b;;Mohamad et al. 2021).However, the investigated fibrous CO 2 adsorbent have been mainly tested for adsorption of CO 2 from its mixtures with N 2 , whereas their application in CO 2 capture from its mixtures with CH 4 has not been reported in the literature.Moreover, the adsorption isotherms, kinetics, and thermodynamics of adsorption on such aminated fibrous adsorbents have not been established.The objective of the present study is to investigate the CO 2 adsorption behaviour from CO 2 / CH 4 mixtures on fibrous adsorbent immobilized with various amine groups hosted by poly (GMA) incorporated in PE/PP fibrous sheet by RIGC.The various properties of the adsorbents were evaluated.The performance was evaluated under various temperatures and pressures with different CO 2 /CH 4 mixtures resembling the conditions of industrial removal of CO 2 from NG.Moreover, the adsorption isothermal, kinetic, and thermodynamic behaviours were studied by fitting the data to common models.The obtained fundamental properties such as adsorption capacity, kinetics, thermodynamics, and selectivity are essential for laying the foundation for process design parameters for CO 2 capture from NG with such fibrous adsorbents.

Preparation of amine-functionalized fibrous adsorbent
The adsorbent was prepared in a two-step procedure started by RIGC of GMA onto pre-irradiated PE/PP fibrous sheet.The irradiation was performed by an electron beam (EB) accelerator (NHV-Nissin High Voltage, EPS3000) operated at an acceleration energy of 2 MeV and a beam current of 10 mA with a total dose of 50 kGy at 25 kGy pass −1 to create the free radicals.The irradiated PE/PP fibrous sheet was placed inside an ampoule containing an emulsion mixture comprising of 10% GMA in DI and containing 0.5% Tween 20 (surfactant) and bubbled with N 2 to remove O 2 .The reaction was carried out at 50 ℃ for 35 min and yielded samples denoted as PE/PP-g-PGMA with 200% degree of grafting (DOG) which is calculated according to Eq. 1 (Zubair et al. 2020).
where W i and W f are the weights of the samples before and after grafting reaction, respectively.More details on preparation procedure can be found elsewhere (Nasef et al. 2014).
The grafted samples (PE/PP-g-PGMA) were functionalized by treatment with pure EDA or DETA.The reaction was conducted in a round bottom flask at 90 °C under reflux and continuous stirring for 6 h.After reaction completion, the aminated sample was removed and repeatedly washed with DI water few times, then dried in a vacuum oven at 60 °C for 5 h.On the other hand, the amination with TEPA amine was (1) Degree of graf ting (DG%) = carried out by using 3:1 TEPA/isopropanol ratio and conducted at 83 °C under reflux and continuous stirring for 4 h as previously reported (Mohamad et al. 2021).The weight changes of PE/PP-g-PGMA before and after the amination reaction were determined and used to calculate the percent of amination (A%) as Eq. 2.
where W p , W g , and W a are the weights of pristine, grafted, and aminated PE/PP substrates, respectively.MW is the molecular weight of amine group, and 142.15 is the molecular weight of GMA.The final adsorbents are denoted as PE/PP-g-PGMA/EDA, PE/PP-g-PGMA/DETA, and PE/PPg-PGMA/TEPA.All experiments were repeated 3 times, and the reported data for the A% is an average of 3 readings.

Properties of adsorbent
The changes in the chemical structure, morphology, thermal stability, crystallinity, and textural of the aminated fibrous adsorbent were evaluated and compared with both the pristine PE/PP and the grafted fibrous substrates.The changes in chemical composition were analysed using a Nicolet iS50 FT-IR spectrometer.The spectra were obtained with 32 scans and a resolution of 4 cm −1 across a frequency range of 500-4000 cm −1 .The morphological changes of the samples with respect to fibre diameter were examined using a GEMINISEM 500 microscope.The thermal stability of the samples was tested using a Q50 thermogravimetric analyser (TA Instruments) at a heating rate of 10 °C min −1 in a temperature range of 30-700 °C.The crystalline structure of the samples was examined by X-ray diffraction (XRD) using a PANalytical Empyrean analyser at Bragg's angle in the range of 5-70°.BET test was performed with Quantachrome Instrument (Novatouch) to measure surface area and pore analysis.The samples were degassed at 80 °C for 4 h before analysis.

Gas adsorption measurements
The CO 2 adsorption capacity measurements were carried out using the magnetic suspension balance (MSB) known as isoSORP® gravimetric analyser manufactured by RUBOTHERM (Bochum, Germany).Details of MSB's basic principles, components, and operational procedure were described elsewhere (Fujii et al. 2010;Schell et al. 2012).One cycle of adsorption measurement consists of three steps comprising pre-treatment, buoyancy, and adsorption measurement.All the data were recorded by RUBOTHERM control system software (RSCS-2016).
The adsorbent sample was subjected to a pre-treatment (2) started by heating the samples at 80 °C for 4.5 h under vacuum until the recorded weight loss reached a constant to eliminate any retained moisture.Subsequently, the buoyancy measurement was carried out to precisely determine the weight and volume of adsorbent sample using purified helium gas at the required sorption temperature under varying pressure from vacuum condition to 30 bar.Finally, the adsorption measurements of pure gases (CO 2 and CH 4 ) and their gas mixture at different ratios (20-80% CO 2 /CH 4 ) were carried out at different operating conditions including different pressures up to 30 bar.The equilibrium sorption was achieved in about 50 min.for each pressure reading.

CO 2 /CH 4 selectivity
The ideal selectivity (S) of CO 2 over CH 4 was estimated using adsorption capacity data of pure gases using the following equation (Mukhtar et al. 2020a): where q is the adsorption capacity (mmol g −1 ) and P is the operating pressure (bar), respectively.

Kinetic adsorption on fibrous adsorbent
Few kinetic models were proposed to describe the kinetic behaviour of adsorption on solid adsorbents.The kinetic models that were tested in this work include the following (Rahafza et al. 2018): i. Pseudo-first-order kinetic model The non-linear form of this model is expressed in the following equation: where q t and q e are the amounts (mmol g −1 ) of the adsorbate adsorbed at time t (min), and at an equilibrium, respectively.k 1 is the pseudo-first-order rate constant (1 min −1 ).
ii. Pseudo-second-order kinetic model The non-linear form of pseudo second order equation is given by Eq. 5: where k 2 is the rate constant of pseudo second order model (mol g min.−1 ) iii.Elovich kinetic model The Elovich kinetic model is expressed as follows: where is the initial adsorption rate and is the desorption constant during each experiment.

Adsorption isotherms for CO 2 adsorption on fibrous adsorbent
Adsorption isotherms at equilibrium are used to describe the interaction between the adsorbent and targeted adsorbate.The equilibrium isotherm data provides significant explanations and information on the surface properties and mechanisms of the adsorption process.The fitting of adsorption isotherm data to an adsorption isotherm model is crucial in determining the model representing the experimental data for design purposes (Raganati et al. 2018).The adsorption isotherm models are described as follows: Langmuir isotherm: According to the Langmuir theory, only one adsorbate (CO 2 ) molecule can bind to each active site on the adsorbent surface, explaining the monolayer adsorption process.The non-linear form of the Langmuir isotherm model equation is as follow: where q e (mmol g −1 ) is the adsorbed amount of CO 2 , P is the pressure (bar), q m (mmol g −1 ) is the maximum mon- olayer adsorption capacity, and K L is the Langmuir equilib- rium constant related to adsorption capacity and adsorption intensity.
Freundlich isotherm: Freundlich isotherm model accounts for a heterogonous adsorption system.This isotherm can be used to define the multilayer uptake of CO 2 molecules adsorbed onto the adsorbent surface.The non-linear form of Freundlich isotherm model equation is expressed as follows: where K f is the Freundlich constant, while n is the factor of heterogeneity, which related to the adsorbate affinity to the adsorbent.
Sips isotherm: Sips isotherm model indicates that the adsorption process of CO 2 molecules onto the adsorbent surface follows a combination of Langmuir and Freundlich isotherm models.The non-linear equation of Sips isotherm model is expressed as follows: (6) q t = 1 ln( t + 1) where q s is the Sips isotherm maximum adsorption capacity, K s is the Sips isotherm constant, and s is the Sips exponent.
If s equals to 1, it represents a homogeneous system.

Thermodynamics of adsorption on fibrous adsorbent
Thermodynamic properties are crucial in the adsorption process to determine the spontaneity of the reaction between the adsorbent and adsorbate.Besides, the nature of adsorption including physisorption or chemisorption can be determine through the evaluation of the thermodynamic properties such as standard Gibbs free energy change (ΔG°), the enthalpy change (ΔH°), and the entropy change (ΔS°) by using Eq. 10 (Raganati et al. 2018).For instance, physisorption arises from relatively weak interactions such as van der Waals force, while chemisorption involves a stronger chemical interaction (covalent bonding).The ΔG° can be directly calculated from Eq. 11 once the thermodynamic constant is obtained.The thermodynamic constant can be derived from various isothermal model types, including Langmuir, Freundlich, and Sips (Tran et al. 2016).The enthalpy change (ΔH°), also known as heat of adsorption, and the entropy change ΔS° correspond to the slope and intercept from Van't Hoff plot correlating ln K S with 1T and calculated using Eq.12.
where T is the temperature (K), R is the universal gas constant (J mol K −1 ), and K S is the thermodynamic constant. (

Amine-containing adsorbents
Preparation of the adsorbents was carried out in a 2-step procedure (grafting of GMA and immobilization of amine).Both grafting and amination parameters were selected based on our previous investigation (Mohamad et al. 2021), and yielded samples with DG of 200% and amination percent (A%) in the range of 70-80% (A%) were obtained in all aminated samples.The preparation of the adsorbent by grafting of GMA and amination with TEPA is illustrated in the schematic diagram shown in Fig. 1.

Properties of adsorbent with different amines
Chemical composition properties Fig. 2 shows the FTIR spectra of pristine PE/PP sheet, PGMA grafted PE/PP sheet, and PE/PP-g-PGMA sheets aminated with EDA, DETA, and TEPA.It can be observed that the representative diagnostic peaks at 2917 and 2847 cm −1 corresponding to the antisymmetric and asymmetric stretching vibration of C-H coming from PE/ PP backbone, respectively.The peaks appeared at 1740 and 1255 cm −1 are attributed to group (-COC) originated from epoxide ring of PGMA formed after grafting, and remained in a weaken form after amination (Abbasi et al. 2019a).In addition, the incorporation of PGMA to the PE/PP structure was also proved by the emergence of two bands at 905 and 840 cm −1 , which are assigned for -CO of the epoxy group (Nasef et al. 2014).The evidence of -NH absorptions originating from the primary amine and secondary amine functionalization appeared in the peaks at around 1527 and 1658 cm −1 , respectively (Ullah et al. 2015).The peak at 1156 cm −1 represents the -CN stretching vibration in TEPA.Furthermore, the chemisorption mechanism of CO 2 onto primary or secondary amine groups involves the formation of carbamate (NCOO-) as the final product under dry conditions.Figure 2B shows the differential spectra of FTIR before and after CO 2 adsorption onto PE/PP-g-PGMA/TEPA.The adsorbent exhibited characteristic bands of NCOOskeletal vibration at 1320 and 1410 cm −1 .These FTIR results confirm the incorporation of PGMA side chain grafts and their subsequent immobilization with amines.Furthermore, CO 2 was proven to be adsorbed as a result of interaction with TEPA in the adsorbent through the formation of carbamate (Dao et al. 2020)

Morphological properties
The SEM images of pristine PE/PP, PE/PP-g-PGMA, PE/PP-g-PGMA/EDA, PE/PP-g-PGMA/DETA, and PE/ PP-g-PGMA/TEPA sheets are presented in Fig. 3.The images revealed that there are changes in the average fibre diameter (AFD) in the sheets after grafting and amine functionalization.For instance, the AFD of the pristine PE/PP was 15.3 ± 3.18 µm, which increased after grafting with PGMA to 20.0 ± 5.70 µm.Such an increase in AFD is coming from the covalent incorporation of PGMA around the fibres, and similar trend was reported for grafting of vinyl monomers on fibrous substrates (Othman et al. 2019).On the other hand, amination with EDA and DETA led to a reduction in the average fibre diameter by ~ 7.4% to reach values of 18.50 ± 5.6.This is likely due to minor degradation of tiny amount of unbound PGMA side chains caused by using of concentrated amine agents.However, the AFD was increased to 24.18 ± 6.20 µm after amination with TEPA diluted in

Crystalline structure properties
Figure 5 shows X-ray diffractograms of pristine PE/PP, grafted PE/PP-g-PGMA, and all three different types of amine-functionalized PE/PP-g-PGMA.The crystalline peaks in all diffractograms showed no changes in Bragg's angles regardless the of the type of immobilized amine suggesting that the backbone structure of PE/PP is well preserved and remain intact even after grafting and subsequent amination.It is confirmed that PGMA and all three types of amine have no contribution to the diffraction pattern (Nasef et al. 1998).However, the degree of crystallinity decreased after grafting and even more after the amination.For instance, the intensity of the crystalline peaks decreased in a sequence of pristine PE/PP > PE/ PP-g-PGMA > PE/PP-g-PGMA/EDA > PE/PP-g-PGMA/ TEPA > PE/PP-g-PGMA/DETA.This reduction in the peak intensity is due to the dilution of the inherent crystallinity by incorporating amorphous PGMA, and such effect became more profound after immobilization of amines with PE/PP-g-PGMA/DETA showing the lowest degree of crystallinity (Sharif et al. 2013).These results suggest that the aminated adsorbents retained reasonable physical integrity suitable for adsorption applications.

Surface area and pore analysis
The nitrogen adsorption-desorption isotherm was evaluated using BET analysis to determine the surface area, pore volume, and pore width of PGMA grafted and amine functionalized substrate which tabulated in Table 1.It can be seen that the surface area was obtained for PE/PPg-PGMA, PE/PP-g-PGMA/EDA, PE/PP-g-PGMA/DETA, and PE/PP-g-PGMA/TEPA fibrous sheets, where surface areas of 7.50, 5.46, 5.38, and 3.47 m 2 g −1 were observed, respectively.After functionalization with amine, the surface area decreased, and such a decrease was higher with longer aliphatic chain of the amine.The immobilization of amines to the grafted fibrous sheet not only makes the amine dominate the pore and reduces the surface area but also lowers the pore volume value.These results emphasize that surface area which is small in these adsorbents is not the crucial factor in the adsorption mechanism and it is the number of amine sites and their accessibility that will dominate the performance of these fibrous adsorbents.

Effect of amine types on the CO 2 adsorption
The adsorption capacity of pure CO 2 was measured at different pressures (up to 30 bar) for PE/PP-g-PGMAEDA, PE/ PP-g-PGMA/DETA, and PE/PP-g-PGMA/TEPA at room temperature, and the obtained adsorption data is presented in Fig. 6.The CO 2 adsorption capacity increased gradually with the increase in the pressure for all adsorbents without reaching saturation suggesting a trend close to type II adsorption isotherms (nonporous adsorbent) and the presence of a combination monolayer and multilayer adsorptions on the present adsorbent.Particularly, the CO 2 adsorption capacity increased gradually to reach 2.12 mmol g −1 for PE/PP-g-PGMA/TEPA at 30 bar compared to 1.49 and 1.21 mmol g −1 for to PE/PP-g-PGMA/DETA and PE/PPg-PGMA/EDA, respectively.The continuous increasing trend in CO 2 adsorption capacity can be attributed to the early interaction with energetic amine at the surface followed by the successive diffusion of CO 2 to the inner layers reaching deeper amine sites under the influence of the pressure increase (Ahmed et al. 2017).Since the secondary amine is more efficient than the primary amine at interacting with CO 2 molecules (Liu et al. 2017), the superior CO 2 adsorption of PE/PP-g-PGMA/TEPA can be attributable to the larger number secondary amine groups in TEPA compared to DETA and EDA.Thus, PE/PP-g-PGMA/TEPA is selected to further investigate the adsorption behaviour of CO 2 under various temperatures.

Effect of temperature on CO 2 adsorption on TEPA-containing adsorbent
Figure 7 shows the variation of pure CO 2 gas adsorption with the temperature in the range of 30-70 °C at 30 bar.It can be observed that the rise in the temperature leads to an increase in the adsorption capacity until 60 °C beyond which it declines.For instance, the CO 2 adsorption capacity reached a maximum value of 2.66 mmol g −1 at 60 °C and slipped to 2.62 mmol g −1 at 70 °C.The increased CO 2 adsorption capacity with the temperature rise is due not only to enhancement of CO 2 diffusion rate and the increase in the CO 2 molecules kinetic energy but also to the acceleration in CO 2 accessibility to the inner layers containing more TEPA sites (Zhao et al. 2022).Thus, the increase in adsorption capacity with temperature up to 60 °C indicates that the CO 2 capture process is controlled by kinetic rather than thermodynamic factors (Zhao et al. 2017).The observation of increasing adsorption with temperature suggests that the CO 2 adsorption on the present fibrous adsorbent does not follow the Clausius-Clapeyron equation (Yano et al. 2020).Similar deviation from adsorption thermodynamics was also report in literature for TEPA modified carbon nanotubes [48] and mesoporous carbon [49].It can be concluded that 60 °C is the best temperature for the maximum adsorption capacity which makes this adsorbent favourable for CO 2 capture from NG as the commercial technology of amine absorption typically operate at 50 to 60 °C.

Adsorption of pure CO 2 and CH 4 on PE/ PP-g-PGMATEPA adsorbent
Figure 8 a shows the adsorption behaviour of pure CO 2 and CH 4 gases on PE/PP-g-PGMA/TEPA adsorbent at different pressures and temperatures.It can be seen that the higher pressures and temperatures (30-60 °C) led to higher CO 2 adsorption performance unlike CH 4 which showed very tiny adsorption (0.02-0.04 mmol g −1 ) that seem to be insignificant at all temperatures and pressures.This observation suggests that PE/PP-g-PGMA/TEPA adsorbent has high CO 2 selectivity over CH 4 .The CO 2 /CH 4 ideal selectivity for PE/PP-g-PGMA/TEPA was evaluated and presented in correlation with pressures at different temperatures in Fig. 8b.As the pressure increases, the CO 2 /CH 4 selectivity decreased for all temperatures, which can be attributed to the pressure-driven mechanism through amine containing sites in different layers irrespective to those bound to the surface of the adsorbent (Mukhtar et al. 2020a, b).Overall, the CO 2 CH 4 selectivity for all pressure readings is considered high where all of them are above  value of 50, showing superior CO 2 adsorption compared to CH 4 , which is far better than those reported in the previous studies (Vosoughi et al. 2021) (Zohdi et al. 2019).Therefore, it can be suggested that TEPA-aminated adsorbent has a strong potential for separating CO 2 from it mixtures with CH 4 (Dao et al. 2020).This information is crucial for the development of NG treatment technologies since CO 2 capture from NG demands designing highly CO 2 selective adsorbent material (Mukhtar et al. 2020a).

Adsorption of CO 2 from CO 2 /CH 4 mixtures
The effect of variation of CO 2 /CH 4 gas mixture composition and pressure on the adsorption capacity of CO 2 on PE/ PP-g-PGMA/TEPA adsorbent is presented in Fig. 9. Pure CO 2 and CH 4 adsorption variations with the same pressure range were used as references.The CO 2 adsorption capacity increased with the rise in the pressure and CO 2 composition.The adsorption capacity was found to be in the following sequence at 30 bar: 100% CH 4 < 20% CO 2 < 40% CO 2 < 60% CO 2 < 80% CO 2 < 100% CO 2 with their adsorption capacity were 0.02, 1.50, 2.00, 2.30, 2.57, and 2.66 mmol g −1 , respectively.The decrease of the adsorption capacity with the CO 2 lessening in the feed mixture is most likely caused by accumulation of the CH 4 around TEPA aminated sites reducing their accessibility by CO 2 .This is going along with the fact that CH 4 has lighter molecular weight (16.04 g mol −1 ) than CO 2 (44.01 g mol −1 ), leading to higher rate of diffusion, which is inversely proportional to the square root of its molecular mass as per Graham's law of diffusion (Rani et al. 2018).Thus, the CO 2 access to TEPA groups in the adsorbent is hindered.This observation suggests that the present adsorbent works best at high CO 2 concentrations in the feed and is likely to be useful for purification of NG with high CO 2 concentration.

Adsorption/desorption cyclic stability
The effect of number of adsorption/desorption cycles on the adsorption capacity of pure CO 2 on PE/PP-g-PGMA/ TEPA adsorbent is evaluated in ten cycles, and the obtained data is presented in Fig. 10.The adsorption was conducted at a temperature of 30 °C with a pure CO 2 at a flowrate of 500 ml min −1 .The adsorbent was regenerated by placing it inside the sample container of MSB and heating to 80 °C for 4.5 h under vacuum conditions until the adsorbent showed a constant weight was obtained.The adsorbent showed a reasonable stability with an average adsorption capacity of 114 mg g −1 (~ 3% loss) after 10 cycles at a pressure of 30 bar.As can be clearly seen, the CO 2 adsorption capacity remained almost unvaried at the value of 2.66 mmol g −1 (the adsorption capacity of the first cycle is 2.65 mmol g −1 ).The absence of any significant adsorption capacity loss confirms the high stability of the present adsorbent and its suitability for prolonged cyclic operation.

Comparison of kinetic models
Three models including pseudo-first-order, pseudo-second order, and Elovich kinetic models were used to fit the experimental CO 2 adsorption results.The data are presented in Fig. 10 The effect of number of adsorption/desorption cycles on the adsorption capacity of pure CO 2 on PE/PP-g-PGMA/TEPA adsorbent of the adsorbent for capture is controlled by surface diffusion and chemical reactions at the gas-solid interface.The result best fitting of data to Elovich kinetic models suggests that the CO 2 adsorption process in the present system occurs as a group of reactions including surface diffusion, bulk phase diffusion, and active ionic surfaces.It also describes the chemisorption process in relation to the amount of surface coverage and decrease in the adsorption rate (Najafi et al. 2021).The reaction rate of CO 2 adsorption was enhanced as the β value was found to increase with the rise in the adsorption temperature from 30 to 60 °C (Rahafza et al. 2018).

Adsorption isotherms for CO 2 adsorption on PE/ PP-g-PGMA/TEPA
Figure 12 shows the adsorption isotherms and curve fitting for CO 2 adsorption on PE/PP-g-PGMA/TEPA at different temperatures using various isothermal models such as Langmuir, Freundlich, and Sips, whereas the relevant parametric data is presented in Table 3.To avoid inaccuracy from linearization, the magnitude of adsorption isotherm parameters in this study were obtained using the non-linear regression analysis.It can be seen that the highest value of R 2 with more than 0.99 and the lowest value of SE for Sips isotherm model among others for at all varied temperatures indicates that the adsorption process of CO 2 molecules on the PE/PP-g-PGMA/TEPA adsorbent surface follows Sips isotherm model.This suggests the presence of monolayer and multilayer adsorptions.Lastly, the value of Sips constant, K s , was increased with the temperature rise.This dem- onstrated that the binding affinity of CO 2 molecules with the fibrous surface of PE/PP-g-PGMA/TEPA becomes more vital with the rise of temperature up to 60 °C.
Figure 13 shows Van't Hoff plot for adsorption CO 2 on PE/PP-g-PGMA/TEPA adsorbent in which ln K S was plotted versus 1T.The K S in Eq. 12, which is the thermodynamic constant, was obtained from the Sips isothermal model fitting parameters reported in Table 3.To further investigate the effect of thermodynamics on the CO 2 adsorption on PE/ PP-g-PGMA/TEPA adsorbent and understand the mechanism of adsorption, the thermodynamic parameters for the CO 2 adsorption onto PE/PP-g-PGMA/TEPA adsorbent was studied and the obtained data are presented in Table 4.The intercept and slope from Van't Hoff plot were used in Eqs.10-12 to calculate ΔG°, ΔH°, and ΔS°, respectively.The value of ΔG° is positive at each temperature.This indicates that the adsorption process is not thermodynamically favourable, nonspontaneous, and needs energy to initiate the reaction between the adsorbent and adsorbate.Moreover, there is a decreasing value of ΔG° with temperature rise as depicted in Table 4 and this suggests that the adsorption process is decreasing in the non-spontaneity and it becomes more favourable at higher temperatures (up to 60 °C) as the energy needed to promote the adsorption reaction (Mukhtar et al. 2020a).A positive value of ΔH° was obtained in this study which shows an endothermic nature of the adsorption process, which reflects the increasing temperature that caused an increased adsorption capacity as presented in Fig. 7.The endothermic adsorption process (ΔH° > 0) suggests that adsorption of CO 2 is associated with energy adsorption in the form of heat from the surrounding.This endothermic phenomenon is mostly because the total energy absorbed in bond breaking is higher than that released in the bond making between CO 2 and adsorbent (Tran et al. 2016).Furthermore, ΔH° value also provides a clear understanding of the adsorption mechanism.Theoretically, ΔH° < 20 kJ mol −1 is for the physisorption process and ΔH° > 80 kJ mol −1 is for the chemisorption process (Raganati et al. 2018).Hence, 63.44 kJ mol −1 of ΔH° obtained in this study confirmed the presence of chemisorption interaction between the CO 2 molecules and PE/PP-g-PGMATEPA adsorbent.These results are in a complete agreement with   2021) who obtained a positive value of ΔH° for all samples that were functionalized with TEPA.Shi et al. (2021) and HaiyanYang and Liu (2022) reported similar adsorption behaviour and claimed the reaction between TEPA and CO 2 is an endothermic chemical reaction.In the meantime, the entropy (ΔS°) was used to determine the randomness of the adsorption process.A positive ΔS° value of 0.18 kJ mol −1 was obtained prevailing a typical chemisorption and suggesting not only an increase in the randomness at the solid-gas interface during the adsorption process but also the presence of a good affinity between the CO 2 molecules and PE/PP-g-PGMATEPA adsorbent surface (Ebelegi et al. 2020;Edet and Ifelebuegu 2020).

Conclusions
Amine-containing adsorbents with fibrous structures were prepared, characterized, and tested for adsorption of CO 2 and separation of mixtures of CO 2 CH 4 .The adsorbent was prepared by grafting of GMA onto EB irradiated PPPE fibrous sheets followed by immobilization of desired amine group under controlled conditions.The covalent grafting of aminated side chains was verified and the morphology as well as the structure and thermal stability were proven.
The PE/PP-g-PGMA/TEPA adsorbent with highest number of secondary amine groups demonstrated a maximum CO 2 adsorption capacity (2.12 mmol g −1 ) compared to those loaded with DETA and EDA.The increase in the pressure up to 30 bar led to an increase in the adsorption capacity, which was also increased with the rise in the temperature up 60 °C to a value of 2.66 mmol g −1 beyond which it declined.This suggested that the CO 2 capture process is controlled by kinetics rather than thermodynamic factors.The adsorbent demonstrated a high stability during ten adsorption-desorption cycles at relatively low regeneration temperature of 80 °C.The CO 2 adsorption isotherm was best represented by Sips model suggesting the presence of monolayer and multilayer adsorption in the TEPA containing adsorbent.The adsorption kinetics data was found to best fit Elovich model indicating that adsorption mainly proceeds by chemisorption mechanism involving steps such as surface diffusion, bulk phase diffusion, and active ionic interaction.The thermodynamic properties revealed that the adsorption process is non-spontaneous and endothermic in nature.Finally, the high CO 2 selectivity shown by the adsorbent towards CO 2 signifies its potential for separating CO 2 from its mixtures with CH 4 especially at high CO 2 concentrations in the feed.Thus, this adsorbent is likely to be promising for purification of NG with high CO 2 concentrations.

Fig. 1
Fig. 1 Schematic representation for preparation of fibrous adsorbent (PE/PP-g-PGMA/ TEPA) by RIGC of GMA on PE/PP sheet and subsequent immobilization of TEPA

Fig. 7 Fig. 8
Fig. 7 Variation of adsorption capacity of pure CO 2 on PE/PPg-PGMA/TEPA with temperature at 30 bar

Table 1
Surface area and pore properties of PE/PP-g-PGMA, PE/PP- Fig.6Variation of CO 2 adsorption capacity with pressure on adsorbents containing different amine types(EDA, DETA, and TEPA)

Table 2
Comparison of parameters of the kinetic models at 30, 40,

Table 3
Data of isothermal models for CO 2 adsorption on PE/PPg-PGMA/TEPA at different temperatures