TEPA impregnation of electrospun carbon nanofibers for enhanced low-level CO₂ adsorption

Abstract

The CO₂ adsorption selectivity of plain activated carbon nanofibers (ANF) is generally low. For enhancement, nitrogen functionalities favorable for CO₂ adsorption are usually tethered to the ANF. In the current study, we adopted chemical impregnation using 0.5 wt% tetraethylenepentamine (TEPA) solution as an impregnant. To enhance the impregnation of TEPA further, preliminary oxidation of the nanofibers with 70% HNO₃ was conducted. The effects of HNO₃ and TEPA treatments on the modified ANFs were investigated for physical (using N₂ monosorb, thermogravimetric analyzer, scanning electron microscopy) and chemical (X-ray photoelectron spectrometer) changes. From the results, we found that although TEPA impregnation reduced the specific surface area and pore volume of the ANFs (from 673.7 and 15.61 to 278.8 m²/g and 0.284 cm³/g, respectively), whereas the HNO₃ pre-oxidation increased the number of carboxylic groups on the ANF. Upon TEPA loading, pyridinic nitrogen was tethered and further enhanced by pre-oxidation. The surface treatment cumulatively increased the amine content from 5.81% to 13.31%. Consequently, the final adsorption capacity for low (0.3%) and pure CO₂ levels were enhanced from 0.20 and 1.89 to 0.33 and 2.96 mmol/g, respectively. Hence, the two-step pre-oxidation and TEPA treatments were efficient for improved CO₂ affinity.

1 Introduction

Besides the highest contributor to anthropogenic global warming, CO₂ could also be harmful at relatively low concentrations, especially in confined indoor spaces, depending on whether they are, stationary (such as offices, homes, and subway stations) or mobile environments (such as cars, airplane, and submarines) [1, 2]. Therefore, the control of indoor CO₂ at 1,000 ppm or higher concentrations [the limit set by the Environmental Protection Agency (EPA)] require efficient technologies for safe indoor activities with a long duration [3].

In the last two decades, indoor air quality (IAQ) researchers have intensified efforts to improve the adsorption of elevated levels of indoor CO₂. The use of amine-based or amine-functionalized adsorbents is popular among such research approaches [4]. Usually, these adsorbents are fabricated using grafting or the impregnation method [5]. Some researchers have confirmed that both impregnation and grafting are effective for increasing the CO₂ adsorption capacity of carbon-based sorbents [6,7,8]. Furthermore, Na Rao et al. [9] compared the two methods and their results showed that impregnation is superior to grafting with respect to amine-loading efficiency and eventual CO₂ adsorption capacity. Impregnation of amines is usually a wet process during which amines are physically adhered onto a support via non-covalent attachment to improve the affinity of the support for a target adsorptive [10]. For example, the effects of three types of organic amines (i.e., diethylenetriamine, triethylenetetramine, and tetraethylenepentamine (TEPA)) on CO₂ adsorption capacity has been reported [11]. The results showed that if TEPA was used as a chemical impregnant for a solid support, it could improve CO₂ adsorption of the support.

TEPA is a linear molecule bearing five amine groups per molecule: two primary amines (RNH₂) and three secondary amine (R₂NH) groups. The mechanism by which primary amine and secondary amine react with CO₂ are expressed in Eqs. (1) and (2), respectively [5]:

$${\text{C}}{{\text{O}}_{\text{2}}} + {\text{ 2RN}}{{\text{H}}_{\text{2}}} \leftrightarrow {\text{ RN}}{{\text{H}}_{\text{3}}}^ + + {\text{ RNHCO}}{{\text{O}}^ - }$$

(1)

$${\text{C}}{{\text{O}}_{\text{2}}} + {\text{ 2}}{{\text{R}}_{\text{2}}}{\text{NH }} \leftrightarrow {\text{ }}{{\text{R}}_{\text{2}}}{\text{N}}{{\text{H}}_{\text{2}}}^ + + {\text{ }}{{\text{R}}_{\text{2}}}{\text{NCO}}{{\text{O}}^ - }$$

(2)

However, to achieve optimum wet impregnation, adequate knowledge of certain properties of the support such as the specific surface area, pore size, pore structure, and surface pH is essential. In general, a porous support with a relatively high specific surface area and large pore volume has a good potential for organic amine immobilization [11].

In this study, various activated nanofibers (ANFs) were prepared by either physical or chemical activation. Further, all the ANFs were impregnated with TEPA solution. To improve the TEPA loading, some ANFs were pre-oxidized with nitric acid solution prior to impregnation.

2 Experimentals

2.1 Consumable materials

Polyacrylonitrile (PAN, M.W. = 150,000 mol/g), 70% nitric acid, and tetraethylene pentamine (TEPA, Reagent grade) were purchased from SIGMA-ALDRICH, Co. in Korea. N,N-dimethylformamide (DMF), potassium hydroxide, and ethanol solution were procured from DAEJUNG CHEMICALS & METALS CO., Seoul, Korea. All of the reagents were used as-received.

2.2 Preparation of PAN-based carbon nanofibers

First, two types of PAN solutions were prepared in weight ratios: (i) PAN/DMF solution (PAN:DMF = 1:9) for physical activation and (ii) KOH/PAN/DMF solution (PAN:DMF:KOH = 1:9:0.01–0.05) for chemical activation. Each polymer solution was electrospun at an inject velocity of 1.5 mL/h under 18 kV. The electrospun nanofiber was stabilized in an oven at 200 °C for 4 h before cooling to room temperature. Then, cooling it to room temperature, it was transferred into a tubular quartz reactor and heated at a ramping rate of 5 °C/min to 800 °C under N₂ flow that was fed at 200 cm. Samples prepared with weight ratios (i) were activated for 15, 30, 60 and 90 min, and denoted as 15-ANF, 30-ANF, 60-ANF, and 90-ANF, respectively, whereas those prepared with weight ratios (ii) were activated for 15 min, and similarly denoted as 0.01-ANF, 0.03-ANF, and 0.05-ANF. Furthermore, 0.5 wt% TEPA solution in absolute ethanol was prepared as an impregnating solution. Then, 100 mg of each sample was added to the solution, and stirred for 6 h at room temperature. Afterwards, the samples were evaporated to dryness in an oven. However, some of the ANFs were pre-oxidized with 70% HNO₃ solution at 80 °C for 1 h before TEPA impregnation. The HNO₃ pre-treatment was preferably carried out below 100 °C for no more than 2 h [12]. Aside from ensuring a complete surface treatment of the fibers, the depth of the ravines increased and the fiber matrix remained uncorroded under those conditions.

2.3 Characterization of nanofiber adsorbent

The specific surface area (S_BET) and porosity of all the ANFs were measured by N₂ adsorption at 77 K using a surface analyzer (Belsorp mini II, BEL, Tokyo, Japan) at relative pressures (p/p₀) of 0–1. To pre-clean the samples prior to measurement, outgassing under N₂ flow for 3 h at 100 °C was carried out. The equation based on BET theory (Brunauer–Emmett–Teller) was used to calculate the S_BET, whereas the mesopore volume (V_meso) and micropore volume (V_micro) were obtained using Barrett–Joyner–Halenda (BJH) and MP-plot [5]. The total pore volume (V_total) was analyzed at a relative pressure of 0.99.

The thermal stability of the adsorbents was investigated with the aid of a thermogravimetric analyzer (TGA N-1500, Scinco, USA). Each sample was heated from room temperature to 400 °C at 10 °C/min heating rate under N₂ atmosphere. The surface morphology and feature of the samples were observed by field emission scanning electron microscopy (FE-SEM, LEO SUPRA 55, Germany) with the magnification of 50,000×. It was coupled with an energy-dispersive X-ray spectroscopy (EDS) for simultaneous elemental distribution analysis. In details, the changes in the surface chemical speciation, especially of the nitrogen functional groups, were investigated using X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, USA).

2.4 Measurement of CO₂ adsorption capacity

The adsorption capacity of the adsorbents for pure CO₂ was obtained using a monosorb instrument. The measurement was carried out under pressures ranging from vacuum to ambient (i.e., p/p₀ = 0–1) at 273 K. Furthermore, the selective adsorption capacity for 0.3% CO₂ (99.999%, in binary mixture with 99.99% N₂) was examined using a lab scale set-up as shown in Fig. 1. The 0.3% (3000 ppm) CO₂ represents the mean concentration of the gas commonly found in indoor spaces [13]. A CO₂ detector (SenseAir, Sweden), equipped with a non-dispersive infrared (NDIR) sensor was used to monitor the CO₂ concentration in the exiting gas stream. The same set-up was used for adsorption regeneration exercise carried out at 100 °C.

Fig. 1

A schematic diagram of the low-level CO₂ adsorption test set-up

3 Results and discussion

3.1 Physical properties of the ANFs

Table 1 shows the textural characteristics of all of the ANF samples. We observed that the S_BET and V_total of ANFs were enhanced by either activation time or the quantity of the activation reagent. However, after impregnation with TEPA, the S_BET and V_total were reduced proportionately, an observation that is consistent with those of other studies [11, 14]. In particular, micropores of the chemically activated ANFs undetectable because the percolation of the TEPA solution blocked the tiny pores. On the contrary, the microporosity of the ANFs derived by physical activation increased after impregnation, probably due to the thermal shrinking of boundary mesopores. Since the kinetic diameter of CO₂ is 0.33 nm, a factor that is significant to its physisorption, the proportion of microporosity is essential to the adsorption of the gas. Therefore, we inferred that ANFs prepared by physical activation are more suitable for surface modification by alkaline solution impregnation, which would improve the chemisorption of the CO₂ molecules.

Table 1 Surface textural and porous properties of all of the ANF samples

The N₂ adsorption isotherms of a part of physically activated ANFs are compared in Fig. 2. All the isotherms followed a typical behaviour of Type I between 0 and 0.8 of the relative pressure, indicating single-layer reversible adsorption. Type I adsorption frequently occurs on microporous solids. The initial adsorption capacity is relatively large with the change of relative pressure, and then the adsorption capacity will not change greatly with the increase of relative pressure [15]. However, as P/Po approached 1, the adsorption capacity increased infinitely. This was caused by multiple molecular layers (physisorption) of the sorptive formed at various thicknesses on the solid surface, leading to the formation of Type II adsorption isotherm. Therefore, it can be seen that the ANFs prepared has both inner micropores and outer mesopores well developed.

Fig. 2

N₂/77 K isotherm of physically activated ANFs

To further enhance TEPA loading on the ANFs at the same impregnation concentration, the ANFs were oxidized with 70% HNO₃ solution for 1 h prior to TEPA impregnation. The HNO₃ oxidation formed crevasses and pits by eroding the graphene layer primarily from the basal planes of carbonaceous materials [16]. After HNO₃ oxidation, the surface became irregular and there were more oxygen-based active groups such as carboxyl, carbonyl and ether groups tethered onto the ANFs [17,18,19]. The presence of these active groups were advantageous for the attachment of TEPA onto the adsorbent surface via a displacement reaction [20, 21].

The optimization of HNO₃ pre-oxidation was conducted with 60-ANF because the textural quality (S_BET and V_micro/V_total) of the 60-ANF-TEPA sample was found best in the TEPA-impregnation step. After pre-oxidation, the V_total of 60-ANF decreased significantly from 15.61 to 0.463 cm³/g. This was a result of mesopore shrinkage, resulting in a large increase in the proportion of micropore volume (V_micro/V_total) from 1.63 to 49.4%. The V_micro/V_total has been reported to be the driving force for selective adsorption of CO₂ [22]. Despite the significant improvement in this property, a slight decrease in the S_BET of the 60-ANF-HNO₃ sample was observed, which was attributed to the collapse of some mesopores. From the TEPA impregnation results of the adsorbent (before and after oxidation), the pre-treated 60-ANF-HNO₃-TEPA sample exhibited superior specific surface area and pore volume. However, the impregnation of TEPA depreciated the microporosity of the adsorbent (from 60-ANF-HNO₃ to 60-ANF-HNO₃-TEPA), i.e., a much superior microporosity was developed compared to that of 60-ANF. Finally, the pre-oxidized TEPA-doped ANFs showed more superior properties than those that were chemically activated.

The surface morphology of fibers is shown in the FE-SEM micrographs of ANFs (Fig. 3), produced at 50,000× magnification with 10 kV accelerating voltage. It can be seen that various surface modified ANFs exhibited uniform size distribution of fiber thickness. However, there were obvious etching on the surface of the TEPA-modified ANFs (Fig. 3b, d), while those of 60-ANF-TEPA sample (Fig. 3b) was deformed. Generally, the structural effects of HNO₃ pre-oxidation on the ANFs correspond with the differences in the S_BET and V_total of 60-ANF-HNO₃, which were higher than those of 60-ANF-TEPA and 60-ANF-HNO₃-TEPA (Table 1).

Fig. 3

FE-SEM images of a 60-ANF; b 60-ANF-TEPA; c 60-ANF-HNO₃; d 60-ANF-HNO₃-TEPA

To understand the thermal stability of modified ANFs, we performed a thermogravimetric analysis on the samples. The thermograms obtained are shown in Fig. 4. All of the samples showed different rates of weight loss as the volatile organics and moisture content were eluted below 100 °C. TEPA-treated samples evinced a less steep slope than those that were untreated, indicating more refractiveness of the contents introduced by TEPA treatment. Between 100 and 400 °C, the 60-ANF sample showed the most impressive thermal stability, as a steady plateau was observed. Such steadiness could be attributed to the homogeneity of the fibers, devoid of foreign dopants in the matrix as can be seen in Fig. 3a. However, the thermogram of pre-oxidized fibers exhibited steeper depreciation because the oxidized fiber surfaces were more easily decomposed under an increasing temperature [20]. The strong acidic functionalities such as carboxylic, anhydrides and lactones tend to be decomposed at low temperatures, while the weak acidic functionalities such as carbonyl, phenol and quinone are dissipated at high temperature [23, 24]. The steady weight loss of TEPA impregnated samples (60-ANF-TEPA and 60-ANF-HNO₃-TEPA) began at about 180 °C. From the difference in their thermograms, we inferred that chemical modification was most pronounced on the surface of the pre-oxidized 60-ANF-HNO₃-TEPA sample.

Fig. 4

Thermograms showing slopes that are indicative of the thermal stability of ANFs

3.2 Chemical properties of the ANFs

Through high resolution XPS analysis of the samples, the major peaks of the scan spectra determined to be due to the C_1s, O_1s, and N_1s photoelectrons. Table 2 lists the surface elemental compositions and atomic ratios of optimized 60-ANF samples. As expected, the HNO₃ treated sample (60-ANF-HNO₃) showed higher O_1s/C_1s ratios (14.75%) and N_1s/C_1s ratios (7.95%) than the unoxidized precursor (60-ANF). With HNO₃ treatment, the amount of O_1s and N_1s increased significantly, while the proportion of C_1s decreased due to surface eroding of the graphene layers, attributed to wet oxidation [25]. With the same impregnant concentration, the oxidized 60-ANF-HNO₃-TEPA sample showed a higher N_1s/C_1s ratios (17.19%) than that of the 60-ANF-TEPA (12.81%) sample, indicating that the HNO₃ oxidation treatment improved the loading of TEPA.

Table 2 The XPS-derived elemental compositions and atomic ratios of 60-ANF-based samples

To better understand the chemical speciation of C and N on the selected samples, deconvolution of the XPS spectra was performed in the C_1s and N_1s regions. Figure 5 shows the best curve fit for the high resolution XPS C_1s spectra of all of the samples. Table 3 provides the percentage composition of the functional groups in the C_1s regime as (i.e. C in polyaromatic structures (C (sp²), BE = 284.6 eV) or aliphatic structures (C (sp³), BE = 285.4 eV): in phenolic, alcohol, ether or C=N groups (BE = 286.0 eV): in carbonyl or quinone groups (BE = 287.6 eV): and in carboxyl, lactone, or ester groups (BE = 288.8 eV) [26]. With either HNO₃ or TEPA treatment, the appearance of –OH and C=O groups on the fiber surface resulted in a decrease in the relative proportion of C (sp²) and C (sp³). Wet oxidation often incorporates high amount of oxygen, as carboxylic and phenolic functionalities on the carbon surface [20]. As expected, upon HNO₃ oxidation (60-ANF-HNO₃), the amount of –COOH groups increased significantly. However, the content of –COOH groups in the TEPA impregnated 60-ANF-TEPA and 60-ANF-HNO₃-TEPA samples decreased by 0.4% and 2.87% respectively, compared to that of 60-ANF and 60-ANF-HNO₃. Of the two possible types of acidic reactive groups present on the surface (phenolic –OH and carboxylic –COOH), only the carboxyl groups could undergo a reaction with TEPA because both phenolic and aliphatic hydroxyl groups are essentially inert toward primary and secondary amino groups [27]. This result indicated the successful substitution reaction of the acidic oxygen functionalities by the N-bearing groups from TEPA.

Fig. 5

The high resolution of the deconvoluted XPS C_1s spectra of the ANF samples: a 60-ANF; b 60-ANF-TEPA; c 60-ANF-HNO₃; and d 60-ANF-HNO₃-TEPA

Table 3 Quantitative results of the fits of the XPS C_1s region, given in % of total intensity

The nature and amount of the N species tethered by the amination were investigated. Figure 6 shows the best curve fit for the high resolution XPS N_1s spectra of all samples, while Table 4 lists the percentage of nitrogen-containing functional groups. The primary surface nitrogen groups found on the ANFs were pyridine-type (BE = 398.1 eV), pyridone and pyrrole (BE = 400.3 eV), quaternary-N (BE = 401.5 eV), oxidized-N (BE = 402.8 eV) [28], and chemisorption of NO₂ at 405 eV [14]. Pyridinic and pyridine/pyrrole N were the two dominant N-groups found on the 60-ANF. Both accounted for about 74% of the total N, while the other groups were present in minor to trace quantities. However, upon the impregnation of TEPA (60-ANF-TEPA), there was a significant increase in the pyridinic and quaternary N-groups, possibly derived from the total conversion of shake-up satellites and adsorbed NO₂. Such observation which implied that the linear TEPA resulted in the increase of pyridine-like structures or six-member or five-member rings [14]. As expected, HNO₃ oxidation evinced lowering of basic N-groups (pyridines), while the least basic groups (shake-up satellites and adsorbed NO₂) were re-introduced to the ANFs. Furthermore, final TEPA amination of the pre-oxidized samples (60-ANF-HNO₃-TEPA) ensured that the quantity of the desired pyridinic N was optimized, while the unwanted shake-up satellites and adsorbed NO₂ groups were significantly lowered. We concluded that although TEPA is a viable amination agent, it also incorporates refractory quaternary-N while expunging unwanted N groups. In addition, HNO₃ was confirmed as an excellent pre-oxidant for TEPA amination of ANFs, as it improved the tethering of preferred pyridine groups which are favorable for selective adsorption of CO₂ over N₂ [29].

Fig. 6

The high resolution of deconvoluted XPS N_1s spectra of the ANF samples: a 60-ANF; b 60-ANF-TEPA; c 60-ANF-HNO₃; and d 60-ANF-HNO₃-TEPA

Table 4 Quantitative results of the deconvoluted XPS N_1s spectra values given in % of total intensity

To complement the elemental distribution analysis carried out with the XPS, an EDS analysis was performed on the optimized sample (60-ANF-HNO₃-TEPA). As shown in the Fig. 7, the EDS spectrum identified only a trace amount of N on the sample surface. Also, finite but uniform distribution of carbon and oxygen was observed. This was because XPS analysis is more sensitive than EDS, with a minimum detection concentration of 0.1%, although at the analysis depth of 10 –20 nm only, while EDS analysis occurs bulk measurement that reaches micron level [30, 31]. Since the impregnated N-groups were populated on the surface of the adsorbent, the EDS analysis could only detect a lesser percentage compared to those of the XPS. Still, both measurements give comparatively similar results when compared in proportion.

Fig. 7

The EDS spectrum of the sample 60-ANF-HNO3-TEPA (inset: EDS layered image)

4 Adsorption and regeneration of nanofiber adsorbents

4.1 CO₂ adsorption capacities of the ANFs

Table 5 shows the adsorption capacities for 0.3% and 100% CO₂ of all the test samples. We found that with either an increase in the activation time or the quantity of the activation reagent, the CO₂ adsorption capacity increased for both levels. This result is fundamentally true because gas adsorption is positively correlated with both the specific surface area and microporosity [32]. Although the S_BET of chemically activated ANFs was smaller than that of physically activated ANFs, the adsorption capacities of the former for pure CO₂ were still significantly improved. In regards to the 0.3% CO₂ test feed (Table 5), physically activated ANFs exhibited significantly higher adsorption capacities than their counterparts. We ascribed this result to the larger specific surface area induced by physical activation, which, in turn, favored TEPA impregnation and eventually CO₂ capture. In addition, the pre-oxidized 60-ANF-HNO₃-TEPA sample had higher amino loading (Table 2 and Fig. 4), resulting in the adsorption of 0.3% CO₂, which was about quadruple that of not pre-oxidized 60-ANF-TEPA.

Table 5 CO₂ adsorption capacities (q) of all ANF samples prepared in this work

Table 6 shows a comparison list of the CO₂ adsorption capacities of various adsorbents from open literatures. For all the sorbents, the higher S_BET and V_micro/V_total were, the higher the adsorption capacity of pure CO₂ was. In our study, an increase in amino group content on the surface of the modified samples also led to a relatively high CO₂ adsorption capacity. However, as most published works have focused on pure and flue gas CO₂ levels, our work is unique in that we attempted to improve the CO₂ adsorption selectivity from indoor low levels.

Table 6 Comparison of the CO₂ adsorption capacities (q) of the current work with those from literature

4.2 Regeneration capacity of the ANFs

The regeneration of our representative samples for each treatment were examined by a simple temperature programmed desorption (TPD) using the TGA. The desorption temperature was set at 100 °C under N₂ atmosphere. It has been confirmed that the desorption of CO₂ form CO₂–TEPA interaction is likely occurred in two stages: (i) slow transport via the decomposition of primary ammonium–carbamate species, followed by (ii) rapid desorption of primary and secondary ammonium–carbamate species at 100 °C. At 100 °C, the ammonium–carbamate species was eliminated, which liberated most of the adsorbed CO₂ [38]. Figure 8 shows the 5-cycle regeneration adsorptions of CO₂ (3000 ppm). Quantitatively, the adsorption capacity of sample 60-ANF, 60-ANF-TEPA and 60-ANF-HNO₃-TEPA decreased mildly by 2.62%, 11.49% and 3.58% respectively, after 5 cycles. The regeneration performance of the TEPA impregnated samples was relatively weak, which may be caused by the thermal-induced elution of some TEPA molecules from the surface.

Fig. 8

Adsorption capacity of carbon dioxide in five cycles

5 Conclusions

Nanofibers were prepared by electrospinning before physical or chemical activation to obtain activated nanofibers (ANFs) with varying structural and chemical characteristics. All of the ANFs were impregnated with a TEPA solution of varying amounts. By examination, we found that the ANFs that were physically activated were more suitable for TEPA impregnation because of the enhanced specific surface area and porosity. After TEPA impregnation, the specific surface area and total pore volume of the adsorbents decreased significantly. After the HNO₃ pre-oxidation treatment, the amine loading improved the incorporation of useful, basic pyridinic nitrogen, whereas the least basic N functionalities were effectively removed or converted. Therefore, the adsorption capacity of the optimized 60-ANF-HNO₃-TEPA for 0.3% and 100% CO₂ increased from 0.12 and 1.89 mmol/g (by 60-ANF-TEPA) to 0.33 and 2.96 mmol/g, respectively. A 5-cycle regeneration test showed a relatively stable adsorption of thermally refreshed adsorbents for proficient reusability. Consequently, HNO₃ pre-oxidation of TEPA-doped ANF is efficient for enhancing low-level CO₂ adsorption from indoor spaces.