EA (purity ≥ 99%), DEA (purity ≥ 99.5%), TEA (purity ≥ 99%), decahydronaphthalene (decalin) (reagent grade, 98%), dimethylformamide (DMF, ≥ 99.8%), GMA (purity ≥ 99%), s-PP (Mw = 174,000; Mn = 75,000) and tetrahydrofuran (THF) (anhydrous, ≥ 99.9%) were supplied by Sigma-Aldrich, Singapore. Acetone (Analytical grade, Fischer Scientific, USA) and methanol (Merck Millipore, USA) were used, as received, without any further purification. CO₂ and nitrogen (N₂) gas (>99.98% purity) were supplied by Linde AG, Malaysia. Deionized (DI) water was used for washing and preparation of solutions.

A fixed bed adsorption column fabricated by Solutions Sdn Bhd, Malaysia, was used to test the CO₂ adsorption capacity of the prepared samples under dynamic condition. A stainless-steel column with a height of 330 mm and an internal diameter of 250 mm was equipped with a porous plate located at a height of 30 mm, as a sample holder. Temperature control was carried out using a heating element coiled around the column and two K-type thermocouples, with an accuracy of ±1.5°C located at a height of 75 and 280 mm from the column base.

A gas mixture of CO₂/N₂ with a total flow rate of 50 ml/min with different CO₂ concentrations was used for adsorption experiments at atmospheric pressure. The flow of CO₂ and N₂ gasses was accurately controlled by two mass flow controllers (GFC series, Dwyer Instruments, USA) in the range of 1–50 and 1–100 ml/min, respectively. An online CO₂ analyzer (GfG Gesellschaft für Gerätebau IR 24) with a maximum detection limit of 50 vol% was used to monitor CO₂ concentration before and after adsorption. Nitrogen gas was humidified via a water bubbler at 60°C and then mixed with CO₂. The relative humidity of the gas stream was about 80% as determined by Fisher-Scientific lab hygrometer.

The calculation of the dynamic adsorption capacity (q) of the samples was carried out as in Equation (3) (Serna-Guerrero and Sayari, 2010):

where F is total flow rate (mmol/s), C₀ is the initial concentration of CO₂, W is the weight of the adsorbent (g), and t_q is the stoichiometric time (s), calculated using the breakthrough curve according to Equation (4) (Serna-Guerrero and Sayari, 2010).

where C and C₀ are CO₂ concentrations at the inlet and the outlet gas streams, respectively.

The ratio of the moles of CO₂ adsorbed on the material to the moles of nitrogen available on the sample is defined as amine efficiency. Amine type and presence or absence of water molecules determine the maximum amine efficiency of an adsorbent. Maximum amine efficiency of all types of amines (primary, secondary, and tertiary) in humid conditions used in these experiments is 1. Amine efficiency was calculated according to Equation (5):

where amine density is the moles of the amine functional groups per gram of the adsorbent and q is the adsorption capacity.

Furthermore, the influence of initial CO₂ concentration, temperature and degree of grafting/amination on the adsorption capacity of the samples was investigated. Initial CO₂ concentration, temperature and DG were changed in the range of 5–15%, 30–50°C, and 150–400%, respectively.

A scanning electron microscope (SEM, Hitachi S3400N, Japan) with an acceleration voltage of 10 kV and a magnification of 1000–5000 x was used to study the morphology of the samples after being coated with gold. To estimate the average fiber diameter of nanofibers, the diameters of 30 random fibers of each sample were measured and used. The changes in the chemical composition of the samples were investigated using a Nexus 670 Thermo Nicolet FTIR in the frequency range of 4,000–400 cm⁻¹ with a resolution of 4 cm⁻¹ and five-time scans.

The thermal properties such as melting temperature of the samples were investigated using a differential scanning calorimetry (DSC: Q200 TA Instrument, USA) under nitrogen atmosphere (flow rate: 50 mL min⁻¹) in the heating range of 30–300°C at a heating rate of 10°C min⁻¹. The thermal stability of the samples was monitored by thermogravimetric analysis (TGA) on a Mettler Toledo TGA/SDTA 851e unit in a temperature range of 30–600°C at a heating rate of 10°C min⁻¹.

The synthesis of s-PP nanofibrous sheets was carried out. Further, independent parameters of the electrospinning process including applied voltage, needle tip to collector distance (TCD) and flow rate were varied based on the combinations offered by Box-Behnken design (BBD) of RSM, as reported previously (Abbasi et al., 2018). Nanofibers with an average fiber diameter of 439 nm were obtained and used for grafting and amination due to their proper production rate and excellent fiber quality.

After being irradiated with an electron beam under different grafting parameters, the s-PP nanofibrous sheets were grafted with GMA. The effect of four main parameters, such as monomer concentration, absorbed dose, grafting time, and temperature on the DG was studied. Results showed a high dependency of the DG on reaction parameters, as reported elsewhere (Abbasi et al., 2018), and samples with DG in the range of 150–400% were obtained.

In this study, three different amines, including EA, DEA and TEA, were used for amination reactions. The effect of reaction parameters such as DG, amine concentration and reaction temperature on the degree of amination (DA) was studied.

The influence of reaction time on the degree of amination (DA) for samples with different DGs (150, 300, and 400%) using EA, DEA, and TEA is shown in Figures 1A–C where treatments with EA and DEA were performed under constant temperature and amine concentration of 60°C and 60 vol% in water, respectively. However, for TEA, a volume ratio of 50:50 (v/v) at 30°C was used. As for amination with EA (Figure 1A), the DA increased sharply, in the first 30 min, for all samples with various DGs, showing a fast reaction rate in the initial stage of the amination. This was followed by a very slight increase that continued up to 120 min beyond which it reached a plateau. The maximum DA achieved for the sample with the DG of 150% was 98% whereas samples with DGs of 300 and 400% reached 94 and 90%, respectively.

A similar trend was observed for amination using DEA. As can be seen in Figure 1B, the increase in reaction time led to an increase in DA for the three samples with various DGs. For the sample with 150% DG, DA sharply increased from 20% after 5 min to 98% after 60 min in an almost linear trend, indicating a high reaction rate at this stage. Subsequently, as time went by, DA reached a plateau. For samples with 300 and 400% DGs, a sharp linear increase in DA was observed as time increased from 5 to 30 min, reaching values of 65 and 58%, respectively. This was followed by a slight increase in DA up to 120 min beyond which a plateau was reached at 93% for DG of 300 and 86% for DG of 400%. The decrease in DA observed during the amination with EA and DEA at higher DGs was mostly caused by the decrease in sample porosity by the increase in grafting density, as revealed by SEM images. Thus, diffusion of the amination solution slowed down and hampered access to epoxy groups, leading to lower DA.

BET analysis showed that with increasing the degree of grafting from 150 to 300%, the total pore volume of the samples decreased from 0.009 to 0.006 cm³/g. However, it is worth noting that these very low values are related to intra-fiber pores, revealing an almost solid, non-porous structure of single fibers. Inter-fiber porosity which is the main type of porosity for these samples are formed through overlapping fibers (Abbasi et al., 2018). Therefore, with increasing the fiber size, and bead size and number, this type of porosity is decreased.

As for the amination reaction using TEA, which was carried out at 30°C to avoid a drastic change in its mechanical characteristics, reaction rate was much lower compared to that of EA and DEA. During the initial 10 h of the reaction, Figure 1C showed a significant increase in DA from about 8–70% for all grafted samples with various DGs. Then, a slight increase in DA followed up to 16 h beyond which it reached a plateau. At about 80%, DA remained constant. The trend was the same for all the samples with various DGs.

Figure 1 reveals that kinetics of the amination reaction using various amines was in the order of primary>secondary>tertiary. It could be attributed to the spatial hindrance of various amines having a different number of alkyl groups around the nitrogen atom. With increasing the number of alkyl groups covalently bound to the nitrogen atom in the molecular structure of the amines, the number of efficient collisions with epoxy rings decreases, leading to lower reaction kinetics.

Figure 2 displays the effect of temperature (Figure 2A) and amine concentration (Figure 2B) on the DA using EA and DEA. Regarding EA, the increase in temperature from 30 to 50°C led to an increase in DA from 76 to 95% beyond which it reached a plateau. Further increase in the temperature did not significantly affect the DA. Hence, optimal temperature for amination reaction was maintained at 50°C. A similar trend was observed for amination using DEA, showing an increase in DA from 15% at 25°C to 98% at 70°C. All reactions were carried out for 60 min using 50 vol% amine concentration in water. Increasing the reaction temperature not only enhanced the diffusion of the amine solution into the nanofibrous sheet but also provoked the ring opening reaction of the epoxy group of Poly-GMA grafted chains (Zhuang et al., 2013). Furthermore, the higher DA observed for amination using EA at lower temperatures was an indication of lower activation energy for EA amination reaction compared to that of DEA.

It was noted that increasing EA concentration from 20 to 100% in water resulted in a sharp rise in DA from 35 to 98%. This was attributed to improved frequency of efficient collisions between amine molecules and epoxy groups due to increased concentration of the loaded amine groups. On the other hand, a different trend was observed for amination with DEA. For instance, an increase in DEA concentration from 20 to 50% led to an increase in DA from 50 to 98%. However, a further increase in DEA concentration triggered a decrease in DA, reaching to 30% for 100% pure amine. This could be ascribed to the decrease in the diffusion of highly concentrated amine solutions into the nanofibrous sheets, leading to lower DA for higher concentrations. Thereafter, the reactions were carried out using 50% amine concentration. Thus, considering the high DA achieved in most of the samples at 70°C, it can be reasonable to assume that 100% amination had been achieved.

Figure 8 shows CO₂ adsorption breakthrough curves for s-PP nanofibers grafted by GMA and aminated using EA, DEA, and TEA. Temperature, degree of grafting of the samples and feed concentration was changed in the range of 30–50°C, 150–400, and 5–15% to study their effect on adsorption capacity. In Table 1, the results of CO₂ adsorption tests are summarized for all nanofibrous s-PP adsorbents grafted by poly(GMA) and aminated using three various amines.

As for the EA-aminated adsorbents, when adsorption temperature increased from 30 to 40 and 50°C, the breakthrough curve shifted to shorter times and the adsorption capacity decreased from 2.57 to 1.67, and 1.04 mmol CO₂/g of the adsorbent, respectively. A similar trend was observed for DEA-and TEA- aminated adsorbents. The adsorption capacity of the DEA-aminated sample decreased from 1.78 for 30°C to 1.49 and 1.18 mmol CO₂/g of the adsorbent for 40 and 50°C, respectively. Again, the TEA-aminated adsorbent showed a decrease in its adsorption capacity from 0.8 to 0.68 and 0.42 for 30 to 40 and 50°C, respectively. For all three types of the synthesized adsorbents, the entropy change and the heat of adsorption proved negative for CO₂ adsorption. Hence, it is evident that when the temperature increased, adsorption capacity decreased (Drese, 2010).

Three samples with various DGs of 150, 300, and 400% with maximum DA obtained for each sample was used to investigate the influence of DG/DA combination (determining amine density) on the CO₂ adsorption capacity of various synthesized adsorbents. The increase in DG led to a rise in the number of epoxy rings on the sample for the amination reaction. As a result, it was expected to have higher amine density for higher DG and therefore, higher adsorption capacity. On the other hand, when DG increased, the highest DA that was obtained for the sample decreased.

When DG increased for the EA-aminated adsorbents in the order of 150, 300 and 400%, the highest DA that was determined dropped from 98 to 94 and 90%, respectively. Thus, for the samples with DGs of 150, 300 and 400%, amine density rose in the order of 3.30, 3.81, and 3.87 mmol/g respectively. Besides, amine efficiency for the EA-aminated samples with DG of 150, 300, and 400% was found to be 68.5, 67.5, and 59%, respectively. Essentially, a combination of amine density and amine efficiency of an adsorbent determines its adsorption capacity. Consequently, the highest adsorption capacity (q) of 2.57 mmol CO₂/g of the adsorbent was obtained for EA-aminated samples having DG of 300%. For the samples with DG of 150 and 400%, the adsorption capacity proved to be 2.26 and 2.28 mmol CO₂/g of the adsorbent, respectively.

The EA-aminated sample with DG of 300% had about 15% higher amine density compared with the sample having DG of 150%. This led to 14% higher adsorption capacity which produced almost the same amine efficiency for the two samples. On the other hand, although the amine density of the sample with DG of 400% was a little bit higher than that of the sample with DG of 300%, both adsorption capacity and amine efficiency were seen to be lower. This demonstrated, therefore, high pore blockage of the adsorbent and less access of CO₂ molecules to amine groups.

As for DEA-aminated adsorbents, when DG increased from 150 to 300 and 400%, the highest DA decreased from 98 to 93 and 86%, respectively. As a result, the amine density of the sample with DG of 300% (3.61 mmol/g) was higher than that of the samples with DG of 150% (2.18 mmol/g) and 400% (3.57 mmol/g). On the other hand, amine efficiency for DEA-aminated samples was 50.6, 49.3, and 42.5% for 150, 300, and 400% DG, respectively. The combination of amine density and amine efficiency revealed the adsorption capacity for these samples to be 1.61, 1.78, and 1.52 mmol CO₂ per gram of adsorbent, respectively. As can be seen, the highest adsorption capacity was for the sample with 300% DG, followed by 150 and 400%. Even though the amine density of the sample with DG of 400% was higher than that of the sample with DG of 150%, because of pore blockage and less accessibility of amine functional groups to CO₂ molecules, its adsorption capacity was lower.

When DG increased from 150 to 300% for the TEA-aminated samples, the highest DA decreased only by 2% from 84 to 82%. In this regard, there was no difference between the samples with the DG of 300 and 400%. As a result, the amine density of the samples increased when DG increased, reaching to 2.61, 3.01, and 3.14 mmol/g for 150, 300, and 400%, respectively. However, TEA-aminated samples showed amine efficiencies of 28.9, 26.6, and 26.8% for 150, 300, and 400% DG, respectively. The combination of both amine density and efficiency resulted in adsorption capacities of 0.75, 0.80, and 0.84 mmol CO₂ per gram of adsorbent, respectively. Thus, it can be observed that DG did not have a strong influence on adsorption capacity of the TEA-aminated samples. Although the amine density of the samples with higher DGs were higher, the decrease in amine efficiency due to pore blockage and less accessibility of CO₂ molecules to amine functional groups compensated for the higher amine density; almost similar adsorption capacities were achieved for all three adsorbents.

Figure 8C shows the effect of feed concentration on the breakthrough curves of CO₂ adsorption. Consequently, when feed concentration increased, breakthrough point was reached in a shorter time and the curve shifted to the left. As for higher concentrations, shorter times were required to reach equilibrium point, and the adsorption kinetics was faster. Furthermore, adsorption capacity and amine efficiency also changed when feed concentration was modified.

As for EA-aminated adsorbents, when feed concentration increased from 5 to 15%, amine efficiency also increased from 49.8 to 75.3%. A similar rise in adsorption capacity from 1.90 to 2.87 mmol CO₂/g of the adsorbent was observed. As for the DEA-aminated samples, amine efficiency and adsorption capacity increased from 41.3 to 57.1% and from 1.49 to 2.06 mmol CO₂/g adsorbent, respectively, when feed concentration increased in the range of 5–15%. The TEA aminated samples also showed a comparable trend. When feed concentration increased from 5 to 15%, amine efficiency and adsorption capacity also increased from 20.4 to 31.1% and from 0.61 to 0.94 mmol CO₂/g adsorbent, respectively.

The highest adsorption capacities for the EA-, DEA- and TEA-aminated samples were 2.87, 2.06, and 0.94 mmol/g, obtained for the adsorbents with DG of 300%, CO₂ concentration of 15%, and DA of 94, 93, and 82%, respectively. Further, the highest amine efficiency for EA-, DEA-, and TEA- aminated samples were 75, 57, and 31% respectively. Therefore, high adsorption capacities are expected for these adsorbents. However, their capacities are considered to be moderate mostly due to the high weight percentage of s-PP base polymer which does not participate in the adsorption process.

Amine efficiency for EA-aminated sample was higher than the values previously reported for other amine-containing CO₂ adsorbents, namely Aziridine grafted SBA-15 (44%) (Hicks et al., 2008), PEI-modified glass microfibers (54%) (Li et al., 2008), amine-based Nano-fibrillated cellulose (28%) (Gebald et al., 2011), triethylenetetramine-bearing polypropylene microfibers (46%) (Zhuang et al., 2013), and (DAEAPTS)-grafted SBA-15 (23%) (Mittal et al., 2015).

When the CO₂ adsorption capacity of EA-, DEA-, and TEA-containing samples is compared, it was observed that capacity decreased in the order of primary>secondary> tertiary amines. This effect has already been reported in previous studies (Ko et al., 2011) showing the effect of amine chemical structure on its ability to attach to CO₂ molecules. A primary amine is attached to only one alkyl group and two hydrogen atoms. Since the molecule is smaller, the spatial hindrance is minimum for attaching to CO₂ molecules. Secondary and tertiary amines are attached to two and three alkyl groups, respectively. So, the size of various types of amines increases according to amine type and the number of alkyl groups attached to the nitrogen atom. This increase in size results in higher spatial hindrance and less chance of CO₂ molecules getting close to the nitrogen atom to form a covalent bonding. As a result, primary amines have higher affinity to CO₂ molecules as well as higher adsorption capacity. Secondary and tertiary amines are ranked second and third.

For real world applications, regeneration performance is one of the crucial characteristics of an adsorbent. In Figure 9, breakthrough curves (a) and CO₂ adsorption capacities (b) on fresh and regenerated EA-, DEA-, and TEA-aminated samples at each regeneration cycle can be seen. Hence, all types of adsorbents show high stability against regeneration process. Consequently, after four regeneration cycles, the adsorption capacity of the EA-aminated samples decreased from 2.57 to 2.35 mmol/g, showing an 8% decrease in capacity. Most of this decrease happened during the first and second regeneration cycles, after which the capacity remained almost constant.

For the adsorbents containing DEA, excellent stability was observed. After four cycles of regeneration, a 13% drop in adsorption capacity happened. Thus, capacity decreased from 1.78 to 1.55 mmol/g. The stability of the DEA containing adsorbent was less than that of the EA and TEA containing samples. The TEA aminated samples demonstrated excellent regeneration performance. Even after four cycles of regeneration, the adsorption capacity practically did not change and remained at around 0.80 mmol/g. This indicated very high stability of the samples containing TEA against thermal treatment during the regeneration process. FTIR spectroscopy on the amine-bearing adsorbents after 4 cycles of regeneration showed no considerable change in their chemical structure, confirming their stability during the regeneration process.

It is worth noting that the regeneration process was performed at 80°C for 15 min, which is considered as low temperature and short time compared to the regeneration process of other amine-bearing CO₂ adsorbents. For example, regeneration temperature/time of some of the adsorbents reported in the literature was: 105°C/40 min for PEI-containing polyamide-6 (Zainab et al., 2017), 130°C/180 min for Aziridine grafted SBA-15 (Hicks et al., 2008), 120°C/30 min for PEI-modified glass microfibers (Li et al., 2008) and benzyl amine-bearing ion exchange resin (Alesi and Kitchin, 2012), 90°C/60 min for amine-based Nano-fibrillated cellulose (Gebald et al., 2011), and 100°C/20 min for triethylenetetramine-bearing polypropylene microfibers (Zhuang et al., 2013). It decreases energy and time requirements for the regeneration process, making the new materials more viable for industrial applications.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.