Structure-performance relationship of additive-incorporated tetraethylenepentamine-functionalized SiO₂ in direct air capture of CO₂

Abstract

Direct air capture (DAC) using amine-functionalized solid adsorbents holds promise for achieving negative carbon emissions. In this study, a series of additive-incorporated tetraethylenepentamine-functionalized SiO₂ adsorbents with varying tetraethylenepentamine and additive contents were prepared via a simple impregnation method, characterized by various techniques, and applied in the DAC process. The structure-performance relationship of these adsorbents in DAC was investigated, revealing that the quantity of active amine sites (or the tetraethylenepentamine content in the exposed layer), as determined by CO₂-TPD measurement, was an important factor affecting the adsorbent performance. This factor, which varied with the tetraethylenepentamine content, additive type, and additive content, showed a positive correlation with the CO₂ adsorption capacity of the adsorbents. The optimal adsorbent, 40TEPA-10PEG/SiO₂ containing 40 wt % tetraethylenepentamine and 10 wt % polyethylene glycol (Mn = 200), exhibited a stable CO₂ capacity of 2.1 mmol·g^–1 and amine efficiency of 0.22 over 20 adsorption–desorption cycles (adsorption at 400 ppm CO₂/N₂ and 30 °C for 60 min, and desorption at pure N₂ and 90 °C for 20 min). Moreover, even after deliberate accelerated oxidation treatment (pretreated in air at 100 °C for 10 h), the CO₂ capacity of 40TEPA-10PEG/SiO₂ remained at 2.0 mmol·g^–1. The superior thermal and oxidative stability of 40TEPA-10PEG/SiO₂ makes it a promising adsorbent for DAC applications.

Keywords

tetraethylenepentamine-functionalized adsorbent / direct air capture / active amine sites / CO₂ adsorption capacity / thermal and oxidative stability

1 Introduction

Carbon dioxide (CO₂), mainly originating from the combustion of fossil fuels, is a primary greenhouse gas responsible for global warming. Despite a decrease in demand for coal, oil, and natural gas in recent years, global energy-related CO₂ emissions still rose to 37.4 Gt in 2023, marking a 1.3% increase over 2022 [1]. Consequently, atmospheric CO₂ concentration reached 419.3 ppm in 2023, showing a 2.8 ppm increase over 2022 [2]. Direct air capture (DAC), initially proposed by Lackner et al. [3], is an important technique for negative carbon emissions. Unlike conventional CO₂ capture technologies, DAC can directly remove CO₂ from the atmosphere and offers advantages such as location flexibility and convenient CO₂ transport. In 2021, the International Energy Agency released “Net Zero by 2050: A Roadmap for the Global Energy Sector”, highlighting DAC as a new pathway to achieving net-zero emissions [4].

Nowadays, the expense of DAC utilizing solid adsorbents remains high due to the extremely low CO₂ concentration in the atmosphere, necessitating superior adsorption performance of adsorbents [5]. CO₂ adsorbents are typically classified into physical (e.g., carbon-based materials [6], zeolites [7], and metal-organic frameworks [8]) and chemical (primarily amine-functionalized adsorbents [9,10]) categories based on their adsorption mechanisms. Physical adsorbents often have low CO₂ adsorption capacity and slow kinetics [11], while chemical ones, like diethanolamine [12], triethylenetetramine [13], tetraethylenepentamine (TEPA) [14−16], polyethyleneimine [17], polypropyleneimine [18], and polyacrylamide [19], exhibit higher capacity and faster kinetics, especially suitable for DAC processes at room temperature. Among these amines, the short-chain TEPA features primary and secondary amine groups with a high nitrogen content, showing high CO₂ capacity. As a result, the TEPA-functionalized adsorbents have attracted considerable attention from researchers. However, the poor cyclic stability of TEPA-functionalized adsorbents restricts their practical applications.

The addition of additives to amine-functionalized adsorbents has been recognized as an effective method for improving their CO₂ capture performance [20]. Nevertheless, most investigations have focused on simulated flue gas or pure CO₂ conditions [21−23], with less attention given to DAC. Qi et al. [15] studied the cetyltrimethylammonium chloride-incorporated TEPA-functionalized micro-mesoporous silica, and claimed that cetyltrimethylammonium chloride facilitated the conversion of the adsorption product from carbamate to carbamic acid, thereby enhancing the DAC performance of adsorbent. However, they did not examine the adsorption–desorption cyclic stability. Goeppert et al. [14] synthesized propylene oxide-modified TEPA-based adsorbents, which showed enhanced adsorption kinetics and long-term stability. Unfortunately, the stability test was performed at 1000 ppm CO₂ rather than the approximately 400 ppm present in the atmosphere. Samaddoost et al. [24] investigated the sodium triacetoxyborohydride-modified TEPA-functionalized resin, which exhibited a 44% increase in CO₂ adsorption capacity compared to the sodium triacetoxyborohydride-free adsorbent. Nevertheless, the investigation into the structure-performance relationship of the adsorbent was limited. Moreover, the adsorbent was only assessed for 3 cycles, leaving its longer-term stability uncertain. He et al. [25] prepared TEPA-DEA/SBA-15 adsorbents, demonstrating improved adsorption and desorption kinetics attributed to the synergetic interaction between the hydroxyl groups of diethanolamine and the amine groups of TEPA, which alters the adsorption mechanism. Although 10 cycles were conducted (adsorption at 400 ppm CO₂ and desorption at pure N₂), the antioxidation property or oxidative stability, an important indicator for DAC adsorbents given the abundance of oxygen in the air, was not investigated. Very recently, Wang et al. [26] systematically studied additive-incorporated TEPA-functionalized adsorbents using seven types of additives. They proposed that the hydroxyl groups of additives weakened the hydrogen bonding between adjacent amine groups, consequently increasing the number of active amine sites. However, no relevant data were provided to quantitatively support this conclusion.

Literature survey indicates that only a few studies focused on the effects of additives on the DAC performance of TEPA-functionalized adsorbents. Furthermore, the structure-performance relationship of these DAC adsorbents has been studied qualitatively, lacking quantitative correlation. If a crucial property that quantitatively affects the CO₂ adsorption capacity of adsorbents could be revealed, it would significantly facilitate the development of high-performance solid adsorbents for DAC applications.

In this study, three typical additives, namely hexadecyltrimethylammonium bromide (CTAB), sorbitan monooleate (Span80) and polyethylene glycol (PEG, Mn = 200), were incorporated with TEPA and impregnated into commercial SiO₂. The effects of TEPA and additive contents on the structure and performance of adsorbents were carefully explored using various techniques. In particular, the quantity of active amine sites in the adsorbents was determined and found to correlate well with the CO₂ adsorption capacity. In addition, the thermal and oxidative stability of the optimal PEG-incorporated adsorbent were examined, demonstrating its superior DAC performance.

2 Experimental

2.1 Materials

Nano-silica (SiO₂, 99.8%) and Span80 (≥ 99.5%) were purchased from Shanghai Maclin Biochemical Technology Co., Ltd. TEPA (≥ 95%), CTAB (≥ 99.9%), PEG (Mn = 200, ≥ 99.7%), and CH₃OH (≥ 99.5%) were obtained from Shanghai Titan Technology Co., Ltd. The N₂ gas (≥ 99.999%) and the simulated ambient CO₂ (400 ppm CO₂ in N₂) were supplied by Air Liquide (Shanghai) Co., Ltd. All the reagents were used without further purification.

2.2 Adsorbent preparation

The TEPA-functionalized SiO₂-based adsorbents were synthesized using an impregnation method. In a typical procedure, 0.4 g of TEPA was dissolved in 80 mL of methanol at room temperature under vigorous stirring. Subsequently, 0.6 g of SiO₂ was added, and the mixture was continuously stirred for 10 h. The solvent was then removed using a rotary evaporator at 70 °C. Finally, the resulting solid powder was dried in a vacuum oven at 60 °C for 3 h. The as-prepared adsorbent contained 40 wt % TEPA. For adsorbents incorporated with additives (i.e., Span80, CTAB and PEG), the additive was introduced simultaneously with TEPA in the first step. The subsequent steps remained identical to those mentioned above. For simplicity, the adsorbents are designated as xTEPA-yA/SiO₂ (A = Span80, CTAB, or PEG), where x and y represent the mass percentages of TEPA and additive in the adsorbent, respectively. For instance, 40TEPA-10PEG/SiO₂ signifies an adsorbent containing 40 wt % TEPA and 10 wt % PEG.

2.3 Adsorbent characterization

The textural properties of the adsorbents, including specific surface area, pore volume and average pore diameter, were assessed using a Micromeritics ASAP 2460 instrument via N₂ physisorption measurements at –196 °C. Prior to measurement, the samples underwent degassing at 100 °C and 133 Pa for 8 h. The CO₂ adsorption isotherms at 0 and 30 °C were acquired on a Micromeritics 3Flex instrument. The pretreatment procedure for the samples was consistent with that employed for N₂ physisorption. The crystalline structures of the adsorbents were examined using X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation (λ  =  1.5406 Å) within the 2θ range of 10°–80°. Microstructural observations were conducted through field-emitted scanned electron microscopy (FESEM) using a Nova NanoSEM 450 instrument. Elemental analysis was performed on an energy dispersive spectrometer (EDS, Oxford system). Surface functional groups of the adsorbents after CO₂ adsorption were characterized using Fourier transform infrared spectroscopy (FTIR, Nicolet 6700). The TEPA distribution within the adsorbents was analyzed by temperature-programmed desorption of CO₂ (CO₂-TPD, Micromeritics AutoChem 2920). Initially, CO₂ adsorption on 50 mg of sample was carried out at 30 °C in 400 ppm CO₂/N₂ for 7 h; subsequently, the sample underwent heating to 120 °C in He at 1 °C·min^–1, with the CO₂ desorption continuously recorded. The actual content of TEPA (or sum of TEPA and additive) in the adsorbents was determined by a thermogravimetric analyzer (TGA, WRT-3P, Shanghai Precision & Scientific Instrument Co., Ltd.). The temperature program involved ramping from room temperature to 100 °C at 10 °C·min^–1 in N₂, followed by a 30-min hold at 100 °C and subsequent heating to 800 °C at 10 °C·min^–1.

2.4 Adsorbent test

The CO₂ adsorption capacity of each adsorbent was assessed using a fixed-bed adsorber (U-type quartz tube, 10 mm I.D.). In each test, around 0.1 g of adsorbent was loaded on a quartz wool plug in the adsorber. Prior to CO₂ adsorption, the adsorbent was pretreated at 90 °C in N₂ to eliminate adsorbed water and CO₂. This pretreatment process continued until the outlet concentration of CO₂ approached zero. Unless specified otherwise, the adsorption and desorption processes were conducted at 30 and 90 °C, respectively, with separate control by two thermostatic water baths. In addition, adsorption was performed in a stream of 400 ppm CO₂/N₂ (100 mL·min^–1) for 7 h, while desorption was carried out in pure N₂ (100 mL·min^–1) for 1.5 h. The outlet CO₂ concentration was real-time monitored by a CO₂ sensor (Vaisala GMP252).

The CO₂ adsorption capacity and amine efficiency are calculated by the following equations.

(1)$\mathrm{C}{\mathrm{O}}_2\mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}=\frac{1}{m}\underset{0}{\stackrel{t_{\mathrm{a}\mathrm{d}\mathrm{s}}}{\int }}F(c_{{\mathrm{C}\mathrm{O}}_2,\mathrm{b}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{k}}-c_{{\mathrm{C}\mathrm{O}}_2,\mathrm{a}\mathrm{d}\mathrm{s}})\mathrm{d}t,$

(2)$\mathrm{A}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=\frac{{\mathrm{C}\mathrm{O}}_2\mathrm{c}\mathrm{a}\mathrm{p}\mathrm{a}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}}{n_{\mathrm{N}}},$

where m is the mass of adsorbent, t_ads represents the duration of CO₂ adsorption, F denotes the molar flow rate of a gas mixture comprising 400 ppm CO₂ in N₂, $c_{{\mathrm{C}\mathrm{O}}_2,\mathrm{b}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{k}}$ and $c_{{\mathrm{C}\mathrm{O}}_2,\mathrm{a}\mathrm{d}\mathrm{s}}$ correspond to the molar fractions of CO₂ present in the outlet stream during blank and adsorption experiments, respectively, and n_N signifies the moles of amine groups contained in the adsorbent per unit mass. It is noteworthy that inert quartz sands were utilized in blank experiments, thereby precluding CO₂ adsorption.

3 Results and discussion

3.1 Effect of TEPA content

A series of TEPA-functionalized xTEPA/SiO₂ adsorbents with TEPA content ranging from 10 to 70 wt % were synthesized to assess the impact of TEPA content on their structural properties and DAC performance. Fig.1(a) shows the TGA curves of SiO₂ and xTEPA/SiO₂ (x = 10–70). In contrast to the absence of weight loss observed for SiO₂, two distinct weight loss stages are evident for all xTEPA/SiO₂. The first weight loss at ≤ 100 °C is primarily attributed to the desorption of physically adsorbed H₂O and CO₂ [27,28], while the second loss occurring at around 120–400 °C is ascribed to the evaporation and decomposition of TEPA [29,30]. Therefore, in order to mitigate TEPA loss during the desorption process, a desorption temperature of 90 °C is chosen in this study. Based on the weight loss during the second stage, the actual TEPA content in each adsorbent can be calculated, as presented in Tab.1. It is clear that the actual TEPA contents closely match the theoretical values, indicating the successful deposition of TEPA on SiO₂.

Fig.1 (a) TGA curves, (b) XRD patterns, (c) N2 adsorption-desorption isotherms, and (d–k) FESEM images of xTEPA/SiO2 adsorbents with different TEPA contents.

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Tab.1 Textural properties of different xTEPA-yA/SiO2 adsorbentsa)

Samples	SBET/(m2·g–1)	Vpb)/(cm3·g–1)	dpb)/nm	WT (or WTA)c)/wt %
SiO2	383.4	0.90	14.4	–
10TEPA/SiO2	214.5	0.80	16.7	11.3
20TEPA/SiO2	212.2	0.88	17.8	19.5
30TEPA/SiO2	184.1	1.11	24.2	30.8
40TEPA/SiO2	118.3	0.84	29.4	39.7
50TEPA/SiO2	47.9	0.47	39.2	49.7
60TEPA/SiO2	7.7	0.05	36.3	58.9
70TEPA/SiO2	2.2	0.01	22.9	69.0
40TEPA-10PEG/SiO2	40.7	0.49	41.9	50.6
40TEPA-10CTAB/SiO2	34.2	0.43	49.5	48.5
40TEPA-10Span80/SiO2	27.5	0.41	54.2	47.8
40TEPA-5PEG/SiO2	42.8	0.59	46.3	45.2
40TEPA-15PEG/SiO2	17.9	0.21	48.0	55.7
40TEPA-20PEG/SiO2	5.9	0.08	52.1	59.1

a) xTEPA-yA/SiO2 (A = PEG, CTAB, or Span80), where x and y signify the mass percentages of TEPA and additive, respectively; b) pore volume (Vp) and average pore diameter (dp) are determined from the adsorption branch using the BJH method; c) mass percentage of TEPA (WT) or the combined mass percentage of TEPA and additive (WTA) is measured by TGA.

Fig.1(b) presents the XRD patterns of SiO₂ and xTEPA/SiO₂. SiO₂ exhibits a broad diffraction peak at 2θ = 15°–30° (JCPDS No. 29-0085), indicative of its amorphous nature. In xTEPA/SiO₂, this peak maintains its shape and position, implying that the immobilization of TEPA on SiO₂ does not alter the amorphous structure of SiO₂. However, the peak intensity increases with the TEPA content, indicating the presence of amorphous TEPA. In a study on TEPA/MCNTs adsorbents, Irani et al. [31] reported a broad diffraction peak ranging from 10°–30° for TEPA.

Fig.1(c) displays the N₂ adsorption-desorption isotherms of these samples. SiO₂ shows a type IV isotherm with an H3 hysteresis loop [32], indicating the presence of mesopores (Fig. S1, cf. Electronic Supplementary Material, ESM). Upon the introduction of TEPA, the hysteresis loop widens and changes into type H2, progressively increasing with TEPA content up to 50 wt %, suggesting an enlargement of the mesopores. However, for TEPA content ≥ 60 wt %, the hysteresis loop becomes rather smaller, implying fewer mesopores. The BET surface area, pore volume and average pore diameter of these materials are summarized in Tab.1. Compared to SiO₂, the specific surface area of xTEPA/SiO₂ gradually decreases as expected with an increase in TEPA content, owing to pore occupation by TEPA. Nevertheless, the pore volume exhibits an interesting but unexpected trend: it initially increases with TEPA content from 10 to 30 wt %, and then decreases. Remarkably, the pore volume of 30TEPA/SiO₂ (1.11 cm³·g⁻¹) even surpasses that of SiO₂ (0.90 cm³·g⁻¹). This phenomenon differs from many results reported in the literature [15,33,34], where the pore volume of the adsorbent is typically smaller than that of the support and shows an inverse relationship with TEPA content. However, Zhao et al. [35] reported a similar result where 30TEPA/SiO₂ and 40TEPA/SiO₂ had larger pore volumes than SiO₂. We consider this phenomenon is likely associated with the distribution of TEPA in SiO₂: a portion of TEPA is located within the pores, leading to a reduction in pore volume, whereas another portion is situated outside, potentially generating additional pores. These opposing effects result in the largest pore volume for 30TEPA/SiO₂. As the TEPA content exceeds 40 wt %, the pores progressively become fully saturated, particularly at ≥ 60 wt % TEPA, where all pores are nearly filled, resulting in a significant decrease in pore volume. It is notable that even for a low TEPA content (e.g., 10 and 20 wt %), a fraction of TEPA is present on the outer surface of SiO₂. This result is supported by the larger average pore diameters of the adsorbents compared to SiO₂, because the former would otherwise exhibit smaller pore diameters than the latter if TEPA is only positioned inside the pores. Zhao et al. [35] and Zhao et al. [36] also found that the average pore diameter of TEPA-functionalized adsorbents was larger than that of the support materials and increased with the TEPA content. The reason is attributed to either the formation of additional 3D meso/macroporous frameworks on the surface of the support [35], or the filling of smaller pores while larger pores remain [36].

Fig.1(d–k) display the FESEM images of SiO₂ and xTEPA/SiO₂. It is apparent that with increase of the TEPA content, the grain size of the adsorbent also increases, further indicating the deposition of TEPA outside the pores. In particular, when the TEPA content exceeds 60 wt %, a smooth surface is observed for the adsorbents, with few small grains visible. This observation aligns with the textural property data.

The adsorption performance of xTEPA/SiO₂ in simulated ambient CO₂ was studied using a fixed-bed adsorber, with the results shown in Fig.2. It is evident that with an increase in TEPA content, the CO₂ adsorption capacity initially increases and then decreases. Specifically, at an amine content of 50 wt %, the adsorption capacity reaches a maximum value of 2.96 mmol·g^–1. Excessive amine loading does not necessarily result in higher CO₂ adsorption capacity, as overloading can lead to pore blockage in SiO₂ due to amine aggregation, thereby hindering CO₂ diffusion [37−39]. Regarding the amine efficiency, a similar trend is observed as that of CO₂ adsorption capacity, with a maximum of 0.25 for 30TEPA/SiO₂ and 40TEPA/SiO₂, and a minimum of 0.10 for 70TEPA/SiO₂. Similar results were also reported by other researchers [34,40,41]. Taking into account both CO₂ adsorption capacity and amine efficiency, a 40 wt % loading of TEPA is considered to be optimal.

Fig.2 Variation of CO2 adsorption capacity and amine efficiency with TEPA content in xTEPA/SiO2 (conditions: adsorption at 400 ppm CO2/N2 and 30 °C for 7 h in a fixed-bed adsorber).

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Fig.3 presents the CO₂-TPD profiles of xTEPA/SiO₂ following 7 h of CO₂ adsorption at 30 °C. These profiles can be deconvoluted into two peaks, except for 10TEPA/SiO₂ that shows a single peak. Based on the findings of Wang and Song [42], the lower temperature peak (65–70 °C) is attributed to the exposed TEPA layer, characterized by active amine sites that are extensively open and readily accessible to CO₂. In contrast, the higher temperature peak (75–90 °C) is associated with the bulky TEPA layer, where amine sites are densely packed and aggregated, resulting in a significantly higher barrier to CO₂ diffusion compared to the exposed layer. For xTEPA/SiO₂, where x increases incrementally by 10 wt % from 10 to 70 wt %, the TEPA content in the exposed layer (calculated by multiplying x by the percentage of the exposed layer peak area relative to the total peak area) is 10, 16, 20, 25, 32, 22, and 18 wt %, respectively. It is interesting that this sequence correlates well with the CO₂ adsorption capacity of xTEPA/SiO₂ (Fig.2), indicating the crucial role of the exposed TEPA layer in CO₂ adsorption. In addition, the TEPA content in the bulky layer is 0, 4, 10, 15, 18, 38, and 52 wt %, respectively. Increased TEPA content likely results in a denser bulky layer, leading to greater difficulty in CO₂ diffusion during both adsorption and desorption stages. Consequently, the peak temperature of the bulky layer increases noticeably with its TEPA content, as evidenced by Fig.3.

Fig.3 CO2-TPD profiles of xTEPA/SiO2 adsorbents with different TEPA contents.

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3.2 Effect of additive type

Considering the good performance of 40TEPA/SiO₂ in adsorbing 400 ppm CO₂, the adsorbent is subject to modification through the incorporation of three typical additives, i.e., PEG, CTAB, and Span80. The resulting adsorbents are 40TEPA-10PEG/SiO₂, 40TEPA-10CTAB/SiO₂, and 40TEPA-10Span80/SiO₂, each containing 40 wt % TEPA and 10 wt % additive.

Fig.4(a) displays the TGA curves of additive-incorporated 40TEPA-10A/SiO₂ adsorbents. Besides the two-stage weight loss noted for xTEPA/SiO₂ (Fig.1(a)), a third reduction at higher temperatures is observed for 40TEPA-10A/SiO₂, indicative of additive decomposition. Furthermore, the actual content of TEPA and additive for each adsorbent (Tab.1), derived from the second and third weight decrements, is close to the nominal value (i.e., 50 wt %). These results evidence the effective immobilization of both TEPA and additive on SiO₂. However, only the CTAB species (JCPDS No. 48-2454) is discernible on the XRD pattern of 40TEPA-10CTAB/SiO₂ (Fig.4(b)), with peaks at 2θ = 21.5° and 24.5° indexed to the (211) and (020) planes of CTAB, respectively. In contrast, PEG and Span80 are not detected, likely due to their amorphous nature [43].

Fig.4 (a) TGA curves, (b) XRD patterns, (c) pore size distribution curves, and (d–f) FESEM and EDS mapping images of 40TEPA-10PEG/SiO2, 40TEPA-10CTAB/SiO2, and 40TEPA-10Span80/SiO2.

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The incorporation of additives does not alter the type of hysteresis loops for the adsorbents. As depicted in Fig. S2 (cf. ESM), type H2 hysteresis loops are maintained for 40TEPA-10PEG/SiO₂, 40TEPA-10CTAB/SiO₂, and 40TEPA-10Span80/SiO₂. The pores of the adsorbents mainly fall within the 40–60 nm range, as shown in Fig.4(c). The morphology of 40TEPA-10A/SiO₂ (Fig.4(d–f)) resembles that of 40TEPA/SiO₂, exhibiting a rough surface and small grains. Moreover, EDS mapping analysis reveals a uniform distribution of TEPA and additive on SiO₂. However, compared to 40TEPA/SiO₂, the specific surface areas of 40TEPA-10A/SiO₂ are greatly reduced by 65%–77%, accompanied by a decrease in pore volume of around 42%–51% (Tab.1). The specific surface area and pore volume follow the sequence of 40TEPA-10PEG/SiO₂ > 40TEPA-10CTAB/SiO₂ > 40TEPA-10Span80/SiO₂, implying the dependency of adsorbent textural properties on the type of additives.

Fig.5(a) compares the CO₂ adsorption capacity and amine efficiency of additive-incorporated adsorbents. Both CO₂ capacity and amine efficiency follow the order 40TEPA-10PEG/SiO₂ (CO₂ capacity of 2.87 mmol·g^–1 and amine efficiency of 0.27) > 40TEPA-10CTAB/SiO₂ (2.68 mmol·g^–1 and 0.26) > 40TEPA/SiO₂ (2.64 mmol·g^–1 and 0.25) > 40TEPA-10Span80/SiO₂ (2.54 mmol·g^–1 and 0.24). Apparently, 40TEPA-10PEG/SiO₂ shows the highest CO₂ adsorption performance, while 40TEPA-10Span80/SiO₂ exhibits the lowest performance, even inferior to the additive-free 40TEPA/SiO₂. PEG is recognized for its abundance of hydroxyl groups, which are capable of interacting with amines. This interaction results in decreased amine viscosity [44], smaller amine agglomerates [45], and disruption of the hydrogen bonding network between amines [46]. Consequently, the diffusion barrier of CO₂ within amines is reduced and more active amine sites are provided. Although Span80 contains hydroxyl groups, its high viscosity (1212–2020 mm²·s^–1 [47]) creates a substantial diffusion barrier for CO₂.

Fig.5 (a) CO2 adsorption capacity and amine efficiency, and (b) CO2-TPD profiles of 40TEPA-10PEG/SiO2, 40TEPA-10CTAB/SiO2, and 40TEPA-10Span80/SiO2.

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Fig.5(b) presents the CO₂-TPD profiles of the three 40TEPA-10A/SiO₂. Similar to xTEPA/SiO₂ (x = 20–70), each profile of 40TEPA-10A/SiO₂ can be resolved into two peaks representing the exposed layer (peaking at 68 °C) and the bulky layer (84 °C). Analysis of CO₂-TPD profiles reveals that the TEPA content in the exposed layer is 30, 26, and 22 wt % for 40TEPA-10PEG/SiO₂, 40TEPA-10CTAB/SiO₂, and 40TEPA-10Span80/SiO₂, respectively. The presence of abundant hydroxyl groups in PEG results in more active amine sites, whereas the high viscosity of Span80 leads to fewer active amine sites. This sequence of TEPA content in the exposed layer aligns with their CO₂ capacity, consistent with the above observation for xTEPA/SiO₂.

Next, 40TEPA-yPEG/SiO₂ adsorbents with varying PEG contents (y = 0–20) are investigated. As the PEG content increases, both the specific surface area (118.3–5.9 m²·g^–1) and pore volume (0.84–0.08 cm³·g^–1) of the adsorbents exhibit a monotonous decrease (Tab.1). However, the type H2 hysteresis loops and mesoporous structures are retained across all compositions (Fig. S3, cf. ESM). Fig.6(a) presents the CO₂ adsorption capacity and amine efficiency of 40TEPA-yPEG/SiO₂. 40TEPA-5PEG/SiO₂ shows performance similar to that of 40TEPA/SiO₂, likely owing to the limited amount of PEG. Nevertheless, when the PEG content exceeds 15 wt %, both CO₂ capacity and amine efficiency decrease, particularly evident for 40TEPA-20PEG/SiO₂, which implies that excessive additive molecules impede CO₂ access to the amine sites [24]. This trend is probably attributed to differences in TEPA distribution within the adsorbents. Indeed, based on the CO₂-TPD profiles depicted in Fig.3, Fig.5(b), and Fig.6(b), the TEPA content in the exposed layer for 40TEPA-yPEG/SiO₂ (y = 0–20 with an increment of 5) is determined to be 25, 25, 30, 27, and 18 wt %, respectively, a sequence in good agreement with that of CO₂ capacity and amine efficiency.

Fig.6 (a) CO2 adsorption capacity and amine efficiency, and (b) CO2-TPD profiles of 40TEPA-yPEG/SiO2.

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Fig.7(a) and 7(b) show the CO₂ adsorption isotherms of 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂ at 0 and 30 °C. For both adsorbents, the CO₂ uptake increases with temperature. This phenomenon was also reported by other researchers, such as the TEPA-functionalized KIT-6-TEPA by Liu et al. [48], GMCM-41-TEPA50% by Wang et al. [49], KIL-2-TEPA by Ojeda et al. [50], TiO₂/NRs-T_27.4 by Kapica-Kozar et al. [51], and MIL-101(Cr)_TEPA(50) by Rim et al. [16]. Despite the exothermic nature typically associated with CO₂ adsorption, the observed temperature-dependent increase in CO₂ uptake suggests that the adsorption process within 0–30 °C is predominantly influenced by kinetics rather than thermodynamics. In addition, 40TEPA-10PEG/SiO₂ exhibits higher CO₂ uptake than 40TEPA/SiO₂ at both 0 and 30 °C, in accordance with the aforementioned results.

Fig.7 CO2 adsorption isotherms of (a) 40TEPA/SiO2 and (b) 40TEPA-10PEG/SiO2, as well as (c) isosteric heat of CO2 adsorption versus CO2 uptake.

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The CO₂ uptake data ($q_{{\mathrm{C}\mathrm{O}}_2}$) shown in Fig.7(a) and 7(b) can be described by the Toth isotherm model as follows [52,53],

(3)$q_{{\mathrm{C}\mathrm{O}}_2}=\frac{q_{\mathrm{m}}bp}{{[1+{\left(bp\right)}^n]}^{1/n}},$

where q_m is the maximum CO₂ adsorption capacity, p is the CO₂ partial pressure, and b and n are the equation constants. The isosteric heat of CO₂ adsorption (Q_st) is an important parameter reflecting the adsorbent-adsorbate interaction [27,54−56], which can be calculated using the Toth isotherms obtained at 0 and 30 °C through the van’t Hoff equation,

(4)$Q_{\mathrm{s}\mathrm{t}}=-R{\left[{\displaystyle \frac{\mathrm{\partial }\left(\mathrm{l}\mathrm{n}p\right)}{\mathrm{\partial }\left({\displaystyle \frac{1}{T}}\right)}}\right]}_q,$

where R denotes the gas constant and T is the adsorption temperature. The isosteric heat of adsorption is a function of CO₂ uptake, as presented in Fig.7(c). As the CO₂ uptake increases, the corresponding Q_st decreases for both adsorbents, dropping to levels even below 20 kJ·mol^–1. This trend implies the occurrence of CO₂ physisorption at high CO₂ uptake [57,58]. The isosteric heats of adsorption at zero coverage are around 109 and 91 kJ·mol^–1 for 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂, respectively, signifying strong CO₂ adsorption. However, the isosteric heat of adsorption for 40TEPA-10PEG/SiO₂ is lower than that for 40TEPA/SiO₂, indicative of a relatively weaker interaction. This discrepancy may facilitate the desorption process of CO₂-saturated 40TEPA-10PEG/SiO₂.

3.3 Analysis of cyclic stability

The cyclic stability of adsorption-desorption processes is another critical factor influencing the performance of adsorbents. Fig.8 illustrates the CO₂ adsorption capacity of 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂ over 10 consecutive cycles in the fixed-bed adsorber. For 40TEPA/SiO₂, the CO₂ capacity exhibits a consistent decrease from 2.64 mmol·g^–1 at the first cycle to 2.05 mmol·g^–1 by the 10th cycle, representing a reduction of 22.3%. In contrast, the CO₂ capacity of 40TEPA-10PEG/SiO₂ undergoes a modest decline from 2.87 to 2.62 mmol·g^–1 over seven cycles (8.7% reduction), thereafter remaining unchanged, indicating favorable cyclic stability.

Fig.8 CO2 adsorption capacity of 40TEPA/SiO2 and 40TEPA-10PEG/SiO2 over 10 consecutive adsorption-desorption cycles in a fixed-bed adsorber (conditions: adsorption at 400 ppm CO2/N2 and 30 °C for 7 h; desorption at pure N2 and 90 °C for 1.5 h).

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To acquire data on amine efficiency during cyclic operation, 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂ are further assessed using TGA. Following 10 h of CO₂ adsorption in the TGA, the capacities are measured at 2.61 and 2.80 mmol·g^–1 for 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂, respectively (Fig. S4, cf. ESM). These values are very close to those obtained from the fixed-bed adsorber (2.64 and 2.87 mmol·g^–1), providing evidence for the reliability of experimental data derived from both the fixed-bed adsorber and TGA. Fig.9 displays the weight change profiles of the two adsorbents over 20 cycles in the TGA, along with the corresponding CO₂ capacity and amine efficiency for each cycle. Note that the adsorption time for the cyclic evaluation is adjusted to 1 h instead of 10 h. This adaptation is justified by the observation that the CO₂ capacity attained after 1 h of adsorption accounts for a significant portion of the total capacity (around 75%). Moreover, this shorter adsorption time helps to save time and is conducive to performing more cycles.

Fig.9 (a, b) Weight change and (c, d) CO2 adsorption capacity and amine efficiency of 40TEPA/SiO2 and 40TEPA-10PEG/SiO2 over 20 consecutive adsorption-desorption cycles in a TGA. Conditions: adsorption at 400 ppm CO2/N2 and 30 °C for 60 min; desorption at pure N2 and 90 °C for 20 min.

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Both adsorbents experience weight loss during cycling due to the evaporation and decomposition of TEPA [14,59], albeit to varying degrees. For 40TEPA/SiO₂, its weight consistently decreases, showing a reduction of 14.5% after 20 cycles (Fig.9(a)). Conversely, for 40TEPA-10PEG/SiO₂, after undergoing a weight loss of around 9.5% following 15 cycles, its weight remains nearly constant (Fig.9(b)). It is noteworthy that the weight loss for 40TEPA-10PEG/SiO₂ is ascribed to TEPA as well, because a preliminary experiment (not shown here) has demonstrated the stability of PEG during cyclic operation between 30 and 90 °C in the TGA. Concerning the CO₂ adsorption capacity and amine efficiency, both parameters exhibit a decline over 20 cycles for 40TEPA/SiO₂ (Fig.9(c)). Specifically, the CO₂ capacity decreases from 2.0 to 1.5 mmol·g^–1, and the amine efficiency drops from 0.19 to 0.16. However, for 40TEPA-10PEG/SiO₂, the CO₂ capacity remains consistent during cycling, with an average value of around 2.1 mmol·g^–1 (Fig.9(d)). Moreover, a slight enhancement in amine efficiency is even observed (from 0.20 to 0.22) attributed to the reduced TEPA content.

The 20 cycle-used 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂ were characterized by FESEM-EDS mapping (Fig. S5, cf. ESM), N₂ physisorption (Fig. S6, cf. ESM), and CO₂-TPD (Fig. S7, cf. ESM). The micromorphology of the used adsorbents appears similar to that of the fresh counterparts (Fig.1(h) and Fig.4(d)), with no particle agglomeration observed. In addition, the uniform elemental distribution is preserved for both used adsorbents. However, the used adsorbents exhibited reduced textural properties due to the loss of TEPA. For 20 cycle-used 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂, the specific surface areas are 70.0 and 36.2 m²·g^–1, respectively, and the pore volumes are 0.29 and 0.33 cm³·g^–1, respectively. By comparing the textural data of fresh and used adsorbents, 40TEPA/SiO₂ exhibits a greater reduction than 40TEPA-10PEG/SiO₂. Furthermore, according to the CO₂-TPD analysis, the used 40TEPA-10PEG/SiO₂ possesses more active amine sites than the used 40TEPA/SiO₂. In summary, the above findings unveil the promoting effect of PEG on the thermal stability of TEPA-functionalized SiO₂.

Apart from the thermal stability, the oxidative stability is also a crucial consideration, given the presence of around 21% O₂ in the atmosphere. Fig.10(a) presents the CO₂ adsorption capacity of 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂, which were deliberately pretreated in air at 100 °C for 5 or 10 h to accelerate oxidation. These adsorption experiments were conducted in the fixed-bed adsorber. The CO₂ capacities of 40TEPA/SiO₂ pretreated for 5 and 10 h are 2.23 and 1.21 mmol·g^–1, respectively, corresponding to declines of 15.5% and 54.2% compared to the fresh sample. In contrast, 40TEPA-10PEG/SiO₂ samples pretreated for 5 and 10 h show CO₂ capacities of 2.45 and 2.02 mmol·g^–1, respectively, with declines of 14.6% and 29.6%. In addition, considering that the evaporation of TEPA possibly contributes to the decreased CO₂ capacity of air-pretreated adsorbents, 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂ were also pretreated in N₂ at 100 °C for 10 h, during which no oxidative degradation occurs. As shown in Fig. S8 (cf. ESM), the CO₂ capacities of N₂-pretreated 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂ are 1.75 and 2.13 mmol·g^–1, respectively. By comparing the CO₂ capacity of N₂- and air-pretreated samples (10 h treatment), it is apparent that significant oxidative degradation occurs in 40TEPA/SiO₂ (from 1.75 to 1.21 mmol·g^–1), while less degradation happens in 40TEPA-10PEG/SiO₂ (from 2.13 to 2.02 mmol·g^–1). Therefore, 40TEPA-10PEG/SiO₂ demonstrates increased oxidative stability.

Fig.10 (a) CO2 adsorption capacity and (b) FTIR spectra of 40TEPA/SiO2 and 40TEPA-10PEG/SiO2 in different states.

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Fig.10(b) displays the FTIR spectra of 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂ after CO₂ adsorption. For all samples in various states (fresh, 5-h oxidation, and 10-h oxidation), several peaks are observed: the peaks at 1570, 1405, and 1315 cm^–1 are ascribed to carbamate ions [60−62], while the peak at 1480 cm^–1 is associated with ammonium ions [60,63]. Both 40TEPA/SiO₂ and 40TEPA-10PEG/SiO₂ show similar IR spectra for the fresh and 5-h oxidized samples, suggesting that the functional groups of TEPA remain largely intact after 5 h of oxidation pretreatment, probably owing to the shorter duration of oxidation. However, the 10-h oxidized 40TEPA/SiO₂ shows a new peak at 1630 cm^–1, which is attributed to the stretching vibration of C=O and N–H groups from amide or imine species ultimately originating from TEPA degradation [64−66]. These amide and imine species generally cannot capture CO₂ due to their weak basicity. Conversely, this peak is absent in the 10-h oxidized 40TEPA-10PEG/SiO₂. This observation unfolds the enhanced oxidative stability of 40TEPA-10PEG/SiO₂, consequently leading to its higher CO₂ capacity compared to 40TEPA/SiO₂ after 10 h of oxidation. It is considered that the hydrogen bonding between the hydroxyl groups of PEG and primary and secondary amine groups of TEPA is able to inhibit the oxidative degradation of TEPA [46,64,67].

4 Conclusions

We synthesized a series of additive-incorporated TEPA-functionalized SiO₂ adsorbents for DAC applications. The successful immobilization of TEPA and additive species on SiO₂ was confirmed through TGA, XRD, N₂ physisorption and FESEM analyses, with their uniform distribution further validated by EDS mapping analysis. The effects of TEPA content (10–70 wt %), additive type (CTAB, PEG, and Span80), and additive content (0–20 wt %) on the CO₂ adsorption performance of the adsorbents in terms of CO₂ capacity and amine efficiency were systematically studied. The results showed that the PEG-modified adsorbent exhibited superior CO₂ adsorption performance compared to those modified with CTAB and Span80. Moreover, optimal values for TEPA and PEG contents in the adsorbents were 40 and 10 wt %, respectively. The investigation into the structure-performance relationship of the adsorbents indicated that the quantity of active amine sites positively influenced the CO₂ capacity. Both low and high loadings of TEPA and PEG resulted in a reduced quantity of active amine sites, consequently leading to lower CO₂ capacity and amine efficiency. The CO₂ adsorption isotherms of 40TEPA-10PEG/SiO₂ and 40TEPA/SiO₂ were well described by the Toth isothermal model, with a lower CO₂ adsorption heat of 91 kJ·mol⁻¹ for the former and 109 kJ·mol⁻¹ for the latter. Furthermore, 40TEPA-10PEG/SiO₂ displayed higher thermal and oxidative stability than 40TEPA/SiO₂ in DAC. The superior CO₂ adsorption performance of 40TEPA-10PEG/SiO₂ suggests its potential as a promising DAC adsorbent. Subsequent research will evaluate its performance under practical DAC conditions, including humid air, over extended cycles of adsorption and desorption.

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Competing interests

The authours declare that they have no competing interests.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant No. 22278143) is gratefully acknowledged.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11705-024-2512-3 and is accessible for authorized users.

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