Effect of surface chemistry of polyethyleneimine-grafted polypropylene fiber on its CO₂ adsorption

Abstract

A high amino density of grafted solid amine adsorption fiber (PP-AM-PEI) for CO₂ adsorption was prepared by grafting acrylamide (AM) onto the surface of a polypropylene (PP) fiber and subsequently modified with polyethylenimine (PEI). The grafting of AM on PP fibers was beneficial for introducing and assembling PEI onto the fibers. An examination by infrared and scanning electron microscopies confirmed that AM and PEI were successfully introduced onto the surface of PP fiber. PP-AM-PEI showed good thermo-stability, a high adsorption capacity for CO₂ (5.91 mmol g⁻¹). It also has good regeneration performance and could maintain almost the same adsorption capacity for CO₂ after 5 recycle numbers. In addition, two other polypropylene-based adsorbents, PP-GMA-PEI (using glycidyl methacrylate (GMA) as the active monomer) and PP-AM-EDA (using ethylenediamine (EDA) as the functional monomer) were prepared. Compared with PP-GMA-EDA, PP-AM-PEI was more suitable for CO₂ capture in the presence of moisture due to its hydrophilicity. The results also showed that the adsorption capacity of PP-AM-PEI was superior to PP-AM-EDA due to the high amino density of PEI.

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1. Introduction

Global warming is a huge challenge for mankind in the 21st century. CO₂, which mostly comes from flue gas, has been identified as a critical component of greenhouse gases. To reduce CO₂ emissions and its negative impact on the environment, a large number of effective materials^1–3 and technologies^4–7 have been researched. Among the different types of materials, liquid amine absorbents^8–10 are widely used, owing to their higher selectivity and absorption capacity for removing CO₂ from flue gas. However, there are some unavoidable issues such as viscosity, excessive corrosion equipment, high energy consumption and foaming that severely hindered its wide application. Thus, it is necessary to develop a promising alternative adsorbent, which has higher adsorption selectivity, larger adsorption capacity and better regeneration ability, but lower danger during transport and storage. Fortunately, solid adsorption materials¹¹ can meet these requirements.

Solid adsorption materials, particularly porous silica-supported amine materials,^12–16 which can adsorb CO₂ by physical and chemical adsorption,¹⁷ display outstanding adsorption properties. There are two main methods for preparing this array of silica-supported amine materials. One method is known as wet impregnation, which can introduce liquid amines like polyethylenimine (PEI)^12,18 and tetraethylenepentamine (TEPA)¹⁹ directly into the porous silica supports by physical impregnation. The other method uses chemical grafting to combine amines and supports.²⁰ Compared with the former, the latter can not only introduce more different types of amines onto the supports, but can also avoid amine leaking and agglomeration,²¹ which can endow the material with higher stability. In both methods, PEI was used on a large scale because it contains abundant primary, secondary and tertiary amines. Therefore, by employing PEI, the CO₂ adsorption capacity of the porous silica supports can be significantly improved. For porous silica supports, the pore volume²² and pore structure²³ are favorable to enhance the CO₂ adsorption capacity. However, large amount of amine loading on these supports, particularly microporous and mesoporous silica supports may cause serious pore blockage and CO₂ diffusion resistance, leading to the reduction of adsorption capacity.

Compared with porous silica supports, the adsorption capacity of fibrous adsorbent is unaffected by pore volume and pore diameter. For one thing, it can be prepared in various forms (including films, filters, nonwoven fabric, etc.), for another, it has large external surface area, short transit distance, low pressure drops, high chemical stability and desirable flexibility. Besides, grafting has little influence on the surface areas of the fibers, which can effectively improve their adsorption capacity for CO₂, even with high grafting rate.²⁴ Hence, solid amine fibrous adsorbents will have great promise in future applications.

In this study, polypropylene (PP) fiber was employed as a support. By combining graft polymerization with chemical modification on the surface of the PP fiber, a novel solid amine fiber (PP-AM-PEI) for CO₂ adsorption was prepared. The PP fiber was functionalized via covalent binding with PEI, which is rich in amines and exhibits an outstanding adsorption ability for CO₂. The effect of the grafting monomer, adsorption performance and regeneration ability of PP-AM-PEI were studied.

2. Experimental section

2.1 Materials and reagents

All reagents were purchased as analytical grade (AR) and used without further purification, except glycidyl methacrylate (GMA). The inhibitor in the glycidyl methacrylate monomer was removed with neutral aluminum oxide before use. Polypropylene (PP) fibers were provided by Xinshun Special Fiber Company (Zhongshan, China). Branched polyethylenimine (PEI, M_w = 600; 1800; 10 000; and 70 000) was purchased from Aladdin. The others, including acrylamide (AM) and ammonium ferrous sulfate [(NH₄)₂SO₄·FeSO₄·6H₂O], were purchased from Tianjin Fuchen Chemical Reagents Factory.

2.2 Preparation of PP-AM-PEI

The preparation process of PP-AM-PEI was illustrated in Scheme 1. First, raw PP was subjected to γ-ray irradiation at a dosage rate of 0.837 kGy h⁻¹ to obtain preirradiated PP fibers. Then, AM was grafted onto PP fibers according to the following procedure: 85 g of H₂O and 0.08 g (NH₄)₂SO₄·FeSO₄·6H₂O were put in a 100 mL three-necked flask, and the oxygen in the solution was removed by purging nitrogen for 30 min. Further, 1.0 g PP and 15 g AM were introduced into the flask, and the grafting reaction was carried out at 70 °C for 2 h. After the grafting step, the fiber was washed several times with boiling deionized water to completely remove the residual monomer and homopolymers. Afterward, the grafted product PP-AM was dried in a vacuum at 60 °C for 24 h. The grafting degree of PP-AM was calculated using the following eqn (1):where W₀ and W₁ are the weights (g) of the original and grafted PP fiber, respectively.

Scheme 1 Reaction scheme of PP-AM-PEI preparation.

PEI was introduced onto PP-AM fibers by reacting with PEI (10 wt% aqueous solution) for 6 h. The obtained fiber PP-AM-PEI was rinsed with deionized water and ethanol, followed by drying under vacuum at 60 °C for 24 h. The grafting degree of PEI on PP-AM-PEI was calculated according to the following equation:

Nitrogen content (N, mmol g⁻¹) of PP-AM-PEI (amide excluded) was calculated by the equation

where M_w is the molecular weight (g mol⁻¹) of the repeat unit of PEI, and W₁ and W₂ are the weights (g) of PP-AM and PP-AM-PEI, respectively.

2.3 Preparation of grafted solid amine fibers with different chemical structures

In order to investigate relationships between structure and CO₂ capture performance, a number of grafted solid amine fibers with different chemical structures were designed and prepared. The solid amine fibers (PP-x-y) were fabricated by grafting different monomers (x) and subsequently reacting with different types of amines (y) using a process similar to that of PP-AM-PEI. All materials and structures were listed in Table 1.

Table 1 Material and reagents used for the fabrication of solid amine fibers

Substrate material	Grafting monomer (x)	Amine (y)
Polypropylene (PP)	Acrylamide (AM)	Ethylenediamine (EDA)
	Glycidyl methacrylate (GMA)	Branched poly(ethyleneimine) (PEI)

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2.4 Physical and chemical characterization

Infrared spectra in the range of 700 to 4000 cm⁻¹ were obtained with an FT-IR analyzer (Tensor-27, Germany) equipped with a continuum microscope and an attenuated total reflection (ATR) objective.

A thermogravimetric (TG) analyzer (Netzch TG-209C) was employed to determine the thermal stability of PP, PP-AM and PP-AM-PEI. The tests were carried out under a nitrogen atmosphere by heating the fiber from ambient temperature to 600 °C, with a heating rate of 10 °C min⁻¹.

Elemental analyses (EA) were employed to determine the composition of the fibers. The nitrogen, carbon and hydrogen contents were determined by an Analysensysteme GmbH ElementarVario EL (Germany).

Fabric moisture transmission instrument model YG (B) 216-II (China) was employed to measure the hygroscopic rate of samples.

The morphology and diameter of the fiber samples were observed by an ultra-depth three-dimensional microscope (VHX-1000C, Japan).

2.5 CO₂ adsorption experiment

Breakthrough curves were used to characterize the CO₂ adsorption performances of all samples. Approximately 1.00 g fiber sample was tightly placed in an adsorption column (Φ = 1.3 cm), into which a dry nitrogen flow was introduced at a flow rate of 30 mL min⁻¹ for 0.5 h to remove the air and excess water in the tube. Then, the dry CO₂/N₂ mixture gas was introduced through the tube at a flow rate of 30 mL min⁻¹. The flow rate of the gas was controlled by electronic mass-control instruments. The inlet/outlet concentration of CO₂ was analyzed at 2 min intervals, using a Techcomp 7900 gas chromatograph with a thermal-conductivity detector (TCD). The effect of adsorption temperature on the adsorption was investigated in the range of 25 to 75 °C. The corresponding adsorption temperature was controlled by a water bath. After adsorption, pure nitrogen gas at a flow rate of 30 mL min⁻¹ was introduced through the tube at 85 °C to regenerate the spent fibers.

3. Results and discussion

3.1 Chemical structure and the thermal stability of PP-AM-PEI

ATR-FTIR was employed to evaluate a two-step grafting process. Fig. 1 shows FTIR spectra of PP, PP-AM and PP-AM-PEI, respectively. Compared with the PP fiber, the PP-AM fiber was characterized by the peaks at 1653 cm⁻¹ (ref. 25) and 1606 cm⁻¹ corresponding to the stretching vibrations of C=O, –NH and –C–N–, which represented the existence of amide bonds. Two broad adsorption peaks at 3000–3500 cm⁻¹, which could be attributed to primary amine, were also observed. After the grafting reaction of the PP-AM fiber with PEI, the N–H bond of primary amine, symmetric and asymmetric stretching vibrations of –CH₂– at 3000–3500 cm⁻¹, 2820 cm⁻¹ and 2928 cm⁻¹, respectively, all became much stronger, whereas the disappearance of two absorption peaks at 3000–3500 cm⁻¹ corresponding to the primary amine of amide and the emerging of the absorption peak at 1573 cm⁻¹ and to the bending of secondary amines (–N(R)H) in PEI^25,26 testified the substituting PEI for amino group in amide. Briefly, the presence of these new bands in the FTIR spectra confirms the success of the grafting process.

Fig. 1 FT-IR spectra of PP, PP-AM and PP-AM-PEI.

The thermogravimetric analysis results of PP, PP-AM and PP-AM-PEI were shown in Fig. 2. It was found that the PP fiber began to lose its weight when the temperature was raised to 380 °C, and it showed only one platform at a mass loss of almost 100%. In the case of PP-AM, there were two platforms whose starting decomposition temperatures were 250 and 380 °C, respectively. It was believed that the first platform resulted from the degradation of the grafted AM, and the degradation of its inner PP substrate led to another loss when the temperature reached 380 °C. For PP-AM-PEI, its weight loss below 100 °C and between 100 and 200 °C could be attributed to the desorption of physically adsorbed and chemically adsorbed water,^27,28 respectively. The weight losses in the temperature range from 200 °C to 380 °C could be attributed to grafted PEI and PAM; the weight losses in the temperature range from 380 °C to 500 °C could be attributed to PP. The TG results indicated that PP-AM-PEI fiber was stable when it was used at a temperature below 200 °C.

Fig. 2 Thermal stability of PP, PP-AM and PP-AM-PEI.

3.2 Adsorption properties of PP-AM-PEI for CO₂

3.2.1 Effect of PEI average molecular weight (M_w). To investigate the effect of PEI molecular weight on the CO₂ sorption performance of PP-AM-PEI at 25 °C, various PEI species with different molecular weights (M_w = 600; 1800; 10 000; and 70 000) were grafted onto the fibers (70 wt% PEI grafting), and their adsorption capacities were compared. The relevant results of these adsorbents are depicted in Fig. 3 and Table 2. As shown in Fig. 3, with the increase in PEI molecular weight, the CO₂ sorption capacity increases as well. It is clear that the adsorbent with a PEI M_w of 70 K showed the best sorption capacity among all the samples, which was approximately 33% higher than that of the lowest molecular PEI (M_w = 600, 4.12 mmol g⁻¹). In light of the elemental analysis data that the contents of N and H of the adsorbent with a PEI M_w of 70 K were the highest among the adsorbents, although their grafting degrees of PEI were the same, it is not difficult to understand why it had the highest adsorption capacity. Moreover, after the grafting of PEI, the content of N and H on PP-AM-PEI were higher than that on PP-AM, thereby providing further evidence of the successful functionalization with PEI. However, two of them on PP-AM-PEI with different molecular weights were slightly different. According to Scheme 1, the expected product generation was accompanied by the removal of ammonia. Accordingly, it can be inferred that the reaction process between the amide groups and a higher molecular weight of PEI was unlike that with the lower because the greater steric hindrance of the side chain of the higher molecular weight of PEI, the less the reactive chance.

Fig. 3 (a) Breakthrough curves of CO₂ over PP-AM-PEI grafting 70 wt% PEI with different average molecular weights and (b) their adsorption capacities (fiber mass: 1.0 g; adsorption temperature: 25 °C; N₂: 27 mL min⁻¹; CO₂: 3 mL min⁻¹).

Table 2 Elemental analyses of PP, PP-AM and PP-AM-PEI

Fiber	Elemental composition (wt%)
	C (%)	H (%)	N (%)
PP	84.30	13.66	0.00
PP-AM (G = 360%)	53.16	9.16	13.27
PP-AM-PEI (PEI M.W. 600)	45.47	9.72	15.25
PP-AM-PEI (PEI M.W. 1800)	49.49	10.56	15.00
PP-AM-PEI (PEI M.W. 10 k)	46.60	10.77	18.64
PP-AM-PEI (PEI M.W. 70 K)	48.31	11.01	18.32

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3.2.2 Effect of PEI grafting degree. The effect of PEI content on the CO₂ sorption capacity at room temperature was further examined, and the results are shown in Fig. 4. It is clearly shown that the grafting of PEI on the surface of PP-AM significantly increases the CO₂ sorption capacity. This is because PEI itself contains a great number of such adsorption sites as primary, secondary and tertiary amines. When the PEI grafting amount increased from 0 to 83 wt%, the CO₂ sorption capacity increased to a maximum of 5.91 mmol g⁻¹. However, the CO₂ sorption capacity decreased slightly with the further increase of the PEI grafting amount. It could be attributed to the external film becoming thicker with the increase of PEI, and under this condition, film-diffusion resistance became a key factor for the kinetics of CO₂ adsorption. Moreover, the number of the accessible sorption groups was reduced because they may be wrapped more easily by the long chains of PEI.

Fig. 4 CO₂ adsorption capacity and adsorption rate as a function of PEI (M_w = 70 K) content on PP-AM-PEI obtained with 10% CO₂ in N₂.

3.2.3 Effect of adsorption temperature. The CO₂ adsorption capacity of PP-AM-PEI with a series of temperatures was presented in Fig. 5 to evaluate the impact of temperature on CO₂ capture, as well as the thermo-stability of PP-AM-PEI. The breaking curves displayed in Fig. 5(a) showed that PP-AM-PEI was able to thoroughly adsorb CO₂ under different temperatures in the range of 25–75 °C at the initial phase, whereas for the effluent CO₂, concentration remained zero for a certain time before a breakthrough occurred. Afterward, it was observed that the lower the temperature, the longer it would take to breakthrough. As shown in Fig. 5(b), the maximum capacity reached 5.91 mmol g⁻¹ at 25 °C. With the increase of temperature, the adsorption capacity of PP-AM-PEI became lower, which would be ascribed to the fact that the adsorption of CO₂ onto PP-AM-PEI is an exothermic process. When the temperature was increased to 75 °C, the adsorption capacity significantly decreased to 3.3 mmol g⁻¹. In addition, CO₂ and water molecules are inclined to be in the form of gas with increasing temperature, and water has been proved to have important effects on adsorption. Moreover, these results are slightly different from our previous work;²⁹ the adsorption capacity decreased dramatically (3.98 mmol g⁻¹ to 1.44 mmol g⁻¹, decreased by 63.83%) as the temperature increased (30 °C to 70 °C). Hence, these results reveal that the thermostability of the PEI-grafting fiber is superior to that of the PEI-coating fiber. Furthermore, it should be noted that for many amine-grafted porous adsorbents,^15,30 CO₂ adsorption capacities did increase with the increase of temperature up to about 75 °C and then significantly decrease when the temperature was further increased. This was because when the temperature was below 75 °C, their adsorption process was diffusion controlled, and the elevation of the temperature could accelerate the diffusion of CO₂ into the interior of the thick layer of grafted amine, which was beneficial for the reaction between CO₂ and amine groups and the increase of the adsorption capacity, even though CO₂ adsorption is an exothermic process. Raising the temperature beyond 75 °C had a negative effect on the adsorption capacity; an increase in temperature was to accelerate the decomposition of the ammonium carbonate, which was unfavorable to the adsorption. With respect to PP-AM-PEI, the fiber-based adsorbents possessed superior properties of a large external surface area, thin grafting layer and thus, short transit distance. Therefore, for PP-AM-PEI, diffusion resistance did not play a predominant role. PEI-grafted fiber adsorbent exhibited easily accessible adsorption sites, more dispersed amino groups and weakened diffusion resistance of CO₂ adsorption, even at low temperatures. Furthermore, it can be seen that with the increase in the temperature, its adsorption capacity continuously decreased.

Fig. 5 (a) Breakthrough curves of CO₂ adsorption at different temperatures and (b) effect of adsorption temperature on adsorption capacity (Gy_(PEI) = 83 wt%; M_w(PEI) = 70 K; concentration of CO₂: 10%).

3.2.4 Effect of grafting monomer on adsorption capacity. In this paper, the grafting monomer served as an indispensable part of the solid amine adsorbent; in addition, a certain different type of grafting monomer can endow the grafted fiber with specific characteristics. In order to get a clear perspective of the effect of the grafting monomer on adsorption properties, acrylamide (AM) and glycidyl methacrylate (GMA) were used in the investigation. The measured results of PP-AM-PEI and PP-GMA-PEI were presented in Fig. 6 and Table 3. Under dry conditions, the dynamic adsorption capacities of both PP-AM-PEI and PP-GMA-PEI for CO₂ were very low, whereas the adsorption capacities were increased greatly with the presence of moisture. The phenomenon corresponded to the conclusion that water plays an important role in adsorption performance.^31,32

RNH₂ + H₂O ⇔ RNH₃⁺ + OH⁻ (4)

CO₂ + OH⁻ ⇔ HCO₃⁻ (5)

RNH₃⁺+HCO₃⁻ ⇔ RNH₃⁺HCO₃⁻ (6)

Fig. 6 Breakthrough curves of PP-AM-PEI and PP-GMA-PEI. (fiber mass: 1.0 g; adsorption temperature: 25 °C; gas flow rate: 30 mL min⁻¹).

Table 3 Hygroscopic rate (%) and adsorption properties of PP-AM-PEI and PP-GMA-PEI at 10% CO₂ concentration

	Temperature (°C)	Amino content (mmol g^−1)	Hygroscopic rate (%)	Adsorption capacity (mmol g^−1)	Amine utilization (%)
PP-AM-PEI (G = 63%)	25	8.99	55.08 ± 1.5	4.8	53.39
	40			3.56	39.6
	65			2.96	32.91
PP-GMA-PEI (G = 68%)	25	9.41	20.38 ± 0.11	2.51	26.9
	40			1.56	16.58
	65			1.26	13.39

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It is noteworthy that under moist conditions, the adsorption capacity and amine utilization (here, amine utilization is defined as the percentage of the actual number of amino for CO₂ adsorption in the total number of amino) of PP-AM-PEI were both far higher than those of PP-GMA-PEI by more than 50%, although the grafting amount of PEI was nearly equal. Hygroscopic rate tests indicated PP-AM-PEI (55.08%) was more hydrophilic than PP-GMA-PEI (20.38%) after grafting modification. Hydrophilism of the fiber was markedly enhanced by the hydrophilic nature of AM, whereas less obviously by the hydrophobic monomer GMA. Therefore, it was assumed that CO₂ adsorption with PP-AM-PEI, under moist conditions, was better facilitated by stronger moisture promoting effects.

Moreover, swelling experiments demonstrated that PP-AM-PEI was easier to swell than PP-GMA-PEI. As can be seen in Fig. 7, the swelling ability of the adsorbent was also distinctly promoted by the nature of AM in water, leading to more propitious opportunity for the diffusion of CO₂ on the surface of PP-AM-PEI, eventually. In contrast, with the increase in temperature, their adsorption capacities and amine utilization declined, but the decrease rate of PP-GMA-PEI was faster than that of PP-AM-PEI. This could be ascribed to that for PP-GMA-PEI, which was more hydrophobic, water molecules are more inclined to be in the form of gas at high temperatures. In summary, using AM, instead of GMA, as the grafting agent can significantly improve the CO₂ adsorption capacity of the PEI-grafting fiber under moist conditions.

Fig. 7 Micrograph images of (a) dry PP-AM-PEI and (b) PP-GMA-PEI; (c) swollen PP-AM-PEI and (d) PP-GMA-PEI adsorbents also included after water adsorption for 20 min.

3.2.5 Effect of molecular structure on adsorption capacity. In order to explore the influence of molecular structure on the CO₂ adsorption performance, ethanediamine (EDA) was also employed and immobilized onto PP-AM fiber. The adsorption capacities of PP-AM-PEI and PP-AM-EDA were compared. As shown in Fig. 8(b), the breaking curve of PP-AM revealed that amides on PP-AM could scarcely capture CO₂ because of the existence of the electron-withdrawing carboxyl group (C=O), which reduce the alkalinity of amine, leading to the significantly lower reactivity between amine and CO₂. Despite this, a small amount of CO₂ was still captured by PP-AM, which might be due to the affinity generated by physical adsorption such as electrostatic interaction, Van der Waals force, and hydrogen bonding, rather than chemisorption. It is assumed that both EDA and PEI consumed one active prime amine to react with one amide of PP-AM along with a new amide generated. Therefore, the amides should not be treated as effective amines for EDA, which means its amine utilization for CO₂ capture must be calculated by using half of the total amino groups. However, in the case of PEI, as it contains all primary, secondary and tertiary amino groups, and it is difficult to clarify the contents of different amino groups reacted onto fibers. Therefore, two limiting cases are assumed: one is that only primary amines exist on the PEI-grafted adsorbent, whose repeating unit is –CH₂–CH₂–NH₂ and its M_w is 44 (g mol⁻¹) and the other is that only tertiary amines present on the PEI-grafted adsorbent, whose repeating unit is and its M_w is 42 (g mol⁻¹). Using the two M_w limiting values to further calculate the N and amino group content, the range of 10.31–10.80 mmol g⁻¹ can be obtained, close to that calculated by using M_w of 43 of the –CH₂–CH₂–NH– (10.55 mmol g⁻¹). Therefore, it is reasonable to employ the intermediate value 43 as the M_w of the repeating unit to analyze the adsorption properties of PP-AM-PEI, which could significantly simplify the analysis process. Based on the above-mentioned analysis, the related results are illustrated in Fig. 8(a) and Table 4, which show that the saturated adsorption time decreased with the increase in temperature for both PP-AM-EDA and PP-AM-PEI. However, compared with PP-AM-EDA, it took longer for PP-AM-PEI to approach the saturated adsorption at 25 or 65 °C. Meanwhile, the amine utilization of PP-AM-EDA can reach up to 91.11%, but for PP-AM-PEI, it was just 56.01%. Moreover, under the same conditions, both the adsorption rate and the amine utilization of PP-AM-EDA were higher than those of PP-AM-PEI, because of the lower mass transfer resistance of CO₂ on the surface of the former than that of the latter. Compared to the results at room temperature, the adsorption rate and the amine utilization of PP-AM-EDA decreased dramatically at 65 °C. Inversely, PP-AM-PEI can still maintain a fair adsorption rate and amine utilization at higher temperatures because the increase in temperature could drive up a higher mobility of PEI and significantly increase the total number of accessible sorption amine sites. Therefore, from what has been discussed above, we may safely draw the conclusion that the high density of amino adsorbent is more suitable for adsorption at high temperatures and in the presence of water.

Fig. 8 (a) Adsorbed amount of PP-AM-PEI and PP-AM-EDA for CO₂ at different temperatures and (b) breakthrough curves of both types of samples (fiber mass: 1.0 g; adsorption temperature: 25 °C and 65 °C; N₂: 27 mL min⁻¹; CO₂: 3 mL min⁻¹).

Table 4 Adsorption properties of PP-AM-X solid amine fibers

	Temperature (°C)	Effective amino content (mmol g^−1)	Adsorption capacity (mmol g^−1)	Amine utilization (%)
PP-AM-PEI	25	10.55	5.91	56.01
	65		3.81	36.11
PP-AM-EDA	25	4.95	4.51	91.11
	65		2.77	55.95

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4. Regeneration performance

To satisfy the demands of practical use, the adsorbent should possess excellent chemical stability and regenerability for moderate desorption and long-term cyclic operation. In order to measure the stability of PP-AM-PEI, 5 cycles of adsorption-desorption were carried out, and the results are shown in Fig. 9(b). After 5 cycles, no significant change in CO₂ sorption capacity was observed: the adsorption capacity of adsorbent decreased by just 8%. Furthermore, the breakthrough curves of fresh and cycle 4 were displayed in Fig. 9(a). It was evident that the regenerated fibers exhibited nearly the same adsorption behaviour as the fresh ones. All these results confirmed that the solid amine adsorbent PP-AM-PEI could keep good stability after multiple regeneration cycles and maintain its adsorption capacity for CO₂.

Fig. 9 (a) Breakthrough curves of CO₂ adsorption on fresh and regenerated PP-AM-PEI and (b) CO₂ adsorption capacities at each regeneration cycle (fiber mass: 1.0 g; desorption temperature: 85 °C; N₂: 30 mL min⁻¹).

The FT-IR spectra of the fresh PP-AM-PEI, the PP-AM-PEI fiber after adsorption of CO₂ and the regenerated fiber are illustrated in Fig. 10. It was evident that compared with fresh fiber, the N–H adsorption bands of adsorbed PP-AM-PEI at 3000–3500 cm⁻¹ became significantly weaker, which could be attributed to the adsorption of CO₂ onto amine. The N–H and C–H bands of fresh and regenerated PP-AM-PEI showed nearly identical IR intensity. The weight of the sample after regeneration was close to that before adsorption (with a deviation of less than 0.5%), indicating that CO₂ was completely released from the PP-AM-PEI fiber after the desorption process, and the grafting PEI stayed intact during the CO₂ adsorption-desorption cycles.

Fig. 10 FTIR-IR spectra of the fresh, adsorbed and regenerated PP-AM-PEI.

5. Conclusion

This study has shown that PEI-grafted polypropylene fiber could effectively remove CO₂ from a simulated flue gas containing 10% CO₂ at ambient conditions. The amount of PEI grafting on PP-AM has a significant influence on the adsorption performance of the adsorbents. A grafting of 83 wt% PEI shows the largest adsorption capacity of 5.91 mmol g⁻¹ at 25 °C. It is found that a decrease in temperature favors the CO₂ adsorption of the grafted PEI fibers. The correlation between the CO₂ sorption capacity and the grafting monomer indicates that AM, a hydrophilic monomer, can endow the adsorbent with excellent swelling ability, which can improve the adsorption capacity during the adsorption process in the presence of water. As for the effect exerted by the amine structure, results of the amine utilization suggested that PEI was more effective and efficient to adsorb CO₂ molecules due to the stretching of wrapped chain at high temperatures. Furthermore, the adsorbent can be easily and completely regenerated under mild conditions and is stable in the cyclic operations for 5 cycles. Besides, the IR studies have suggested that the PP-AM-PEI can maintain good regenerability and stability.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (Grant no. 51173211), Science and Technology Project of Guangdong Province (2011B090400030), and the Science and Technology Project of Zhuhai (2010B050102024).

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