Introduction

In recent years, the enormous amount of CO2 emitted from fossil fuel has become a problem that receives increasing attention and urgently needs to be solved since it has resulted in greenhouse effect as well as another series of environmental problems [1,2,3,4,5]. To reduce the emission of CO2, carbon capture and storage (CCS) is considered as a satisfactory approach, where the solid amine adsorption material that is developed with the adsorption method as the core is one of the most effective adsorption materials [6,7,8,9,10]. Currently, the solid amine adsorbent is commonly synthesized by employing porous materials as matrix and following by introducing amine reagents via physical or chemical method [11, 12]. However, the existing porous polymer materials are rather rigid and unable to carry out post-processing and secondary molding treatment, which greatly limits its application in practice [13, 14]. In addition, most of the porous matrices were synthesized in the form of powder and need to use a lot of organic solvents in the prepared process. When running CO2 capture, it is easy to cause the blockage of adsorption column, leading to poor dynamic performance [15,16,17,18].

In general, the adsorption capability of solid amine adsorbent largely depends on specific surface area of the matrix and its porosity, pore volume, constituent elements and so on [19,20,21]. According to the previous report, high nitrogen content in a way facilitates the adsorption performance of porous materials, especially in the selective adsorption [22,23,24,25,26]. And there is a new concept called N2 phobicity. Wang et al. [27] prepared porous polymers with surface area and nitrogen content of 744 m2/g and 10.21% using 1, 4-phthalaldehyde and benzene as monomers and DMSO as solvent. After carbonization, its nitrogen content reached 5.58–8.74% and showed good CO2 adsorption and selective adsorption performance. Under the conditions of 1 bar and room temperature, its adsorption capacity of CO2 was 7.41 μmol, which was the best value of porous carbon material for CO2 adsorption at that time. Lee et al. [28] used melamine and phenol as monomer and F127 as template to synthesize a highly ordered mesoporous polymer, whose nitrogen content was as high as 18%. Since it was rich in nitrogen and with great pore size distribution (2.5–2.9 nm), it showed favorable CO2 selectivity that increased with temperature and reached its maximum of 117 (Henry method) at 323 K.

According to the above, it can be seen that the nitrogen-rich porous material has good adsorption and selective adsorption to CO2 [28,29,30,31,32]. Among the many nitrogen-rich monomers, melamine has the advantages of weak basicity (pKa = 5.5), diverse reactivity, high nitrogen content, inexpensive and extensive sources, which promotes the relevant research and application as well as attracts widespread attention of scientific research workers [19, 20, 33]. It also has been reported that the absorbent, prepared by melamine immobilized on the surface of porous silicon or melamine combined into the covalent organic framework (COF) structure, showed better adsorption performance for CO2 [34]. In this paper, a series of MFM with different pore diameters were synthesized in water solvent by using melamine, formaldehyde and m-phenylenediamine as monomers and SiO2 microspheres with different particle diameters as templates. These hollow microspheres were used as the matrix to load PEI and prepared solid amine adsorbents with different pore sizes, whose influence on CO2 adsorption performance was studied.

Experimental

Reagents

Melamine (AR), poly (ethylenimine) (PEI, Mw = 600) and metaphenylene diamine were from Aladdin Chemistry Co., Ltd., China. Formaldehyde aqueous solution (CH2OH, 37 wt%) and ethyl alcohol were purchased from Guangzhou Chemical Reagent Factory, China. All reagents were used without further purification.

Synthesis of nitrogen-rich hollow microspheres (MFM)

In a typical preparation process, melamine (1.15 g, 9.13 mmol) and formaldehyde aqueous solution (2.23 g, 27.5 mmol) were dissolved in 20 mL water, and then, the mixture was stirred at 90 °C until it became transparent to prepare their prepolymer. The SiO2 microspheres with different particle sizes (12 nm, 20 nm, 30 nm, 50 nm) and 20 ml metaphenylene diamine aqueous solution were then added into the above prepolymer solution of melamine and formaldehyde. The mixture solution was then carefully transferred to a Teflon container and heated at 120 °C for 5 h to complete the polycondensation of prepolymer (melamine and formaldehyde aqueous solution) and metaphenylene diamine. The polycondensation product, a yellow solid, was ground, sieved and then successively washed with ethyl alcohol. The obtained granules were dried under vacuum at 80 °C and noted as MFM-SiO2-Xnm, where X presents the particle size of SiO2 microspheres. The formation mechanism is shown in Scheme 1.

Scheme 1
scheme 1

Formation mechanism of MFM

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Preparation of PEI impregnation sorbents (MFM-PEI)

The nitrogen-rich hollow microspheres (MFM) was modified by impregnating PEI. In a typical process, 0.5 g PEI was dissolved in 20 mL ethanol, and then, 0.5 g MFM was added into the solution. The mixture was continuously stirred at 70 °C for 5 h. After being centrifuged with 10000 r/min for 20 min, the adsorbent was obtained and was dried under vacuum at 80 °C. The resulting adsorbents were denoted as MFM-PEI.

Characterization

Nitrogen adsorption–desorption isotherms were characterized at 77 K on an automatic gas adsorption instrument (ASAP2020, Micromeritics Corp., USA) at the range of relative pressure from 10−6 to 1. Before each test, the sample was dried under vacuum at 100 °C overnight. Vtotal was calculated based on the nitrogen amount adsorbed at P/P0 = 0.95. Specific surface area and pore parameters were calculated by using the Brunauer–Emmett–Teller (BET) method and density functional theory (DFT) method, respectively. Scanning electron microscope (SEM, S4800, Hitachi, Japan) was used to observe the morphology and microstructure of the samples, and transmission electron microscope (TEM, JEM-2010HR, JEOL, Japan) was applied to observe the porous structure. 13C solid-state nuclear magnetic resonance (13C NMR) was performed on Mercury-Plus 300 spectrometer (Varian Co., Ltd., American) operating at 400 MHz, and the chemical shifts are quoted relative to the metaphenylene diamine and the polymer.

The CO2 adsorption measurement

The CO2 adsorption capacity was measured in a fixed bed flow system. Before measurement, the sample was dried under vacuum at 80 °C for 12 h. Then, 0.5 g dried adsorbent was loaded into a glass tube (Φ = 1.3 cm). The dry nitrogen with a flow rate of 30 mL/min was introduced into the column at 90 °C for 20 min to remove the air and residue water. Then, the column was cooled down to room temperature. A mixed gas that includes 10% CO2 and 90% N2 with a flow rate of 30 mL/min was introduced to the tube for the adsorption test. The CO2 concentrations at the inlet and outlet were determined by an Agilent 6820 gas chromatography that is equipped with a thermal conductivity detector. After completing CO2 adsorption, the adsorbent was regenerated by purging with N2 with a flow rate of 30 ml/min at 90 °C.

The adsorption capacity was calculated by the following equation:

$$ Q = \mathop \int \limits_{0}^{t} \left( {C_{\text{in}} - C_{\text{eff}} } \right) \cdot V \cdot {\text{d}}t/22.4W $$
(1)

where Q is the adsorption capacity of the adsorbent (mmol CO2/g); t is the adsorption time (min); Cin and Ceff are the influent and effluent concentrations of CO2 (vol%), respectively; V is the total flow rate, 30 mL/min; and W and 22.4 are the weight of the adsorbent (g) and molar volume of gas (mL/mmol), respectively.

Results and discussion

Chemical characterization and pore structure of MFM

MFM with different pore sizes have been successfully synthesized via precipitation polymerization of melamine, formaldehyde solution and metaphenylene diamine in aqueous solution by using different particle size SiO2 microspheres as template, followed by NaOH extraction. To confirm the chemical structure, solid-state 13C NMR was measured and is shown in Fig. 1. The signals at 165 ppm are indicative of triazine rings. The signal at 68 ppm is associated with the methylene groups in ether linkages, whereas the 53 ppm resonance is attributed to methylene linkages. Moreover, the benzene ring adsorption peak appears in 124 ppm and 150 ppm. The 13C NMR results confirm that the metaphenylene has reacted with methylated melamine and formed methylene groups by dewatering. In addition, according to the analysis of element of the material (Table 1), the nitrogen content of MFM reaches up to 38.52%.

Figure 1
figure 1

Solid-state 13C NMR spectra of MFM

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Table 1 Element analysis of MFM
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The N2 adsorption–desorption isotherms of MFMs which were prepared by using different particle size SiO2 templates are shown in Fig. 2. As illustrated in Fig. 2, the N2 adsorption–desorption isotherms of MFMs are consistent with type IV isotherm and both of them have hysteresis loops, which indicates that MFMs are mesoporous. The pore structure parameters of MFM hollow microspheres prepared by using different particle size SiO2 microspheres as templates are presented in Table 2. From Table 2, it can be seen that the specific surface area, pore volume and average pore size of MFM decreased with the increase in SiO2 particle size. When the particle size of SiO2 was 12 nm, the MFM hollow microsphere presents the maximum specific surface area, pore volume and average pore size, and their values are 183.67 m2/g, 0.91 cm3/g and 19.8 nm, respectively. In the case of the addition of same volume of SiO2 emulsion, the smaller the particle size of the SiO2 template, the larger the specific surface area of the MFM hollow microspheres. When the particle size of the template increased, it was difficult for the prepolymer to wrap the template, which resulted in the decrease in the specific surface area, pore volume and average pore size of MFM hollow microspheres.

Figure 2
figure 2

N2 adsorption–desorption isotherms of MFM-SiO2 prepared by using different particle sizes of SiO2 microspheres as template

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Table 2 Pore structure parameter of MFM-SiO2 prepared by using different particle sizes of SiO2 microspheres as template
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SEM and TEM images of MFM hollow microspheres prepared by using different particle size SiO2 microspheres as template are shown in Fig. 3. As shown in Fig. 3a, the microspheres show regular spherical morphology, good dispersion and uniform particle size. It can be observed from the TEM image that the MFM microspheres have no pore before the removal of the SiO2 template (Fig. 3b). The MFM microsphere showed hollow structure after the removal of the SiO2 template by NaOH dissolving (Fig. 3c–f). The pore sizes of MFM were getting larger as the size of SiO2 microspheres increased, which is different with the tendency of overall average pore size, implying that the pores in MFM originated not only from the SiO2 template, but also from the stacking of the polymer particles.

Figure 3
figure 3

SEM (a) and TEM (b) images of MFM before the removal of SiO2 template. The TEM (c–f) images of MFM after the removal of different particle size SiO2 templates (12, 20, 30 and 50 nm)

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The CO2 adsorption performance of MFM impregnating PEI (MFM-PEI)

The MFM prepared by using different particle sizes of SiO2 microspheres as template was used as support to load PEI for CO2 adsorption. With the increase in particle size of SiO2 template, the CO2 adsorption capacity of MFM-PEI decreases gradually (Fig. 4), which was positively correlated with the specific surface area of MFM, indicating that high specific surface area can increase the contact probability between PEI and CO2, thus increasing the CO2 adsorption.

Figure 4
figure 4

Breakthrough curves (a) and capacity (b) of CO2 adsorption on MFM-PEI prepared by using different particle size SiO2 templates

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The effect of template dosage on the CO2 adsorption performance is shown in Fig. 5. It is evident that the dosage of 12 nm SiO2 microspheres could affect the CO2 adsorption capacity of MFM-PEI. The adsorption capacity of MFM-PEI increased first and then decreased with the increased dosage of 12 nm SiO2 microspheres. When the dosage of SiO2 microspheres was 0.5 mL, the adsorption capacity of MFM-PEI was the largest. Because the pore formation of SiO2 microspheres was weak when the dosage of SiO2 microspheres was less, MFM had lower pore volume; accordingly, the loading amount of PEI was lower, leading to lower CO2 adsorption capacity. With the increase in the dosage of 12 nm SiO2 microspheres, the specific surface area of MFM increased, but the material became tightness (Fig. 6) and was not beneficial to the diffusion of CO2, which would also lead to the decrease in CO2 adsorption capacity.

Figure 5
figure 5

Breakthrough curves (a), capacity of CO2 adsorption on MFM-PEI and specific surface area of MFM (b) prepared by using different dosages of 12 nm SiO2 microspheres

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Figure 6
figure 6

SEM images of MFM prepared by using different dosages 12 nm SiO2 microspheres

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On the other hand, the CO2 adsorption capacity of MFM-PEI is largely dependent on the adsorption temperature as indicated in Fig. 7. When the adsorption temperature increased from 20 to 70 °C, the CO2 adsorption capacity of MFM-SiO2-12 nm-PEI increased first and then decreased, and the maximum adsorption capacity was 2.68 mmol/g with the corresponding utilization efficiency of amino as high as 40.66% at 60 °C. There are two aspects of the influence of temperature on the adsorption of CO2 [35, 36]. On the one hand, the adsorption process was controlled by diffusion at low temperatures. The increase in temperature would accelerate the diffusion of CO2 in thick layer of PEI, and meanwhile, it was beneficial to the complete extension of PEI chains. It exposed the entrapped amino and thus provided more adsorption sites, making it fully contact with CO2 and improving the CO2 adsorption capacity with the increase in temperature. On the other hand, the amino group on CO2 adsorption was exothermic process. The higher temperature made the reaction tend to proceed in the reverse direction, and desorption became the main process under high temperature. Taking these two aspects into account, the adsorption capacity of the material increased first and then decreased with the increase in temperature and had the maximum adsorption capacity at 60 °C.

Figure 7
figure 7

Breakthrough curve (a), capacity and amine utilization efficiency (b) of CO2 adsorption on MFM-SiO2-12 nm-PEI at different temperatures

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The regeneration performance of the solid amino sorbents is another significant parameter, as the adsorbent should remain stable CO2 adsorption capacity for long-term adsorption–desorption cycling in practical application. In Fig. 8, the regenerability of the MFM adsorbents is demonstrated in 20 cycles. After 20 cycles of adsorption (at 30 °C)–desorption (at 90 °C), the MFM-SiO2-12 nm-PEI adsorbent has no significant decrease in CO2 adsorption capacity and nearly maintain the same capacity as the fresh one. This result implied that the MFM-SiO2-12 nm-PEI adsorbents could keep stable for long-term adsorption–desorption cycling.

Figure 8
figure 8

Adsorption capacity of CO2 on fresh and regenerated MFM-SiO2-12 nm-PEI

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Primary exploration of adsorption mechanism

In order to better understand the adsorption behavior and adsorption kinetics, the fitting curves of pseudo-first-order models, pseudo-second-order model and Avrami model were drawn to analyze the kinetic mechanism. The fitting parameters with coefficient of determination (R2) are presented in Table 2. The equations can be arranged as:

$$ {\text{Lagergren pseudo-first-order}}{:}\,q_{t} = q_{e} \left( {1 - e^{{k_{f} t}} } \right) $$
(2)
$$ {\text{Pseudo-second-order}}{:}\,q_{t} = q_{e} \left( {q_{e} k_{s} /\left( {1 + q_{e} k_{s} t} \right)} \right) $$
(3)
$$ {\text{Avrami:}}\,q_{t} = q_{e} \left[ {1 - \exp \left( { - \left( {k_{a} t} \right)^{n} } \right)} \right] $$
(4)

where t is the time elapsed from the beginning of the adsorption process, qt is the amount adsorbed at a given point in time and qe represents the amount adsorbed at equilibrium. kf (min−1), ks (g mmol−1 min−1) and ka (min−1) are rate constants, respectively.

Figure 9 shows the CO2 adsorption amount of microspheres under different temperatures. The fitting results attained with coefficient of determination (R2) are listed in Table 3. It can be observed from Fig. 9 that the plots of pseudo-first-order models and pseudo-second-order model failed to match the experimental data as they overrated the CO2 adsorption at the initial stage and underrated it afterwards. In contrast, the Avrami model fits them well, which was confirmed by the significantly high R2 (> 0.99), and the forecasts of equilibrium adsorption (qe) were more approximate to experimental value. Thus, the Avrami model was selected in further analysis of CO2 adsorption behaviors.

Figure 9
figure 9

Experimental CO2 capacity on MFM-SiO2-12 nm-PEI under different temperatures and corresponding fit to kinetic models

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Table 3 Kinetic parameters for CO2 adsorption on MFM-SiO2-12 nm-PEI under different temperatures
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As shown in Fig. 9, the plots can be divided into two stages: At the first stage, the CO2 was adsorbed completely, leading to a rapidly rising of adsorption amount through time, while at the latter stage, the CO2 partly breakthrough the adsorbent and the adsorption amount increased slowly before equilibrium. As listed in Table 3, the kinetic orders (na) varied from 1.4 to 1.7, which means both chemisorption and physisorption play roles in the adsorption of CO2. And the rate constant (ka) decreased with temperature at low temperatures but then increased as the temperature exceeding 60 °C, which is consistent with the analysis of Fig. 9.

Whereas the pseudo-first-order models, pseudo-second-order model and Avrami model are only concerned with the CO2 adsorption process, the Boyd’s film-diffusion model, interparticle diffusion model and intraparticle diffusion model were utilized to analyze the experimental kinetic to further insight the rate-controlling step and diffusion rate.

The Boyd’s film-diffusion model is expressed as:

$$ B_{t} = \left( {\sqrt \pi - \sqrt {\pi - \frac{{\pi^{2} F}}{3}} } \right)^{2} (F < 0.85) $$
(5)
$$ B_{t} = - 0.4977 - \ln \left( {1 - F} \right)(F > 0.85) $$
(6)
$$ F = \frac{{q_{t} }}{{q_{e} }} $$
(7)

The interparticle diffusion model is expressed as:

$$ q_{t} = q_{e} - q_{e} \frac{6}{{\pi^{2} }}e^{{\frac{{ - \pi^{2} k_{f} t}}{15}}} $$
(8)

The intraparticle diffusion model is expressed as:

$$ q_{t} = k_{id} t^{1/2} + C $$
(9)

where qe (mmol g−1) and qt (mmol g−1) refer to the amount of CO2 adsorbed at equilibrium and at a given point of time t (min), respectively; C represents the boundary layer thickness; kf (mmol g−1) and kid (mmol g−1) refer to the interparticle diffusion and intraparticle diffusion rate constant, respectively; F is the fractional attainment of equilibrium at a given point of time; and Bt is a mathematical function of F.

Figure 10 displays the plots of Boyd’s film diffusion for Bt against time. According to the Boyd’s film-diffusion theory, the nonlinear plots indicated that the CO2 adsorption behavior of microspheres involves not only film diffusion but also the pore diffusion and chemical reactions. Figure 11 shows the CO2 adsorption amount of microspheres at the temperature ranging from 20 to 70 °C with plots of interparticle diffusion models drawn on it, and the results attained are listed in Table 4. As shown in Fig. 11, the plots shifted from experimental data with time. And the corresponding R2 was relatively low, while the predictive qe was overrated.

Figure 10
figure 10

Boyd’s film-diffusion simulation curves of MFM-SiO2-12 nm-PEI under different temperatures

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Figure 11
figure 11

Interparticle diffusion simulation curves of MFM-SiO2-12 nm-PEI under different temperatures

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Table 4 Parameters of interparticle diffusion simulation of MFM-SiO2-12 nm-PEI under different temperatures
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To better analyze the diffusion behavior of CO2 adsorption of microspheres, the intraparticle diffusion model [35, 37] was used. Figure 12 shows the Weber–Morris plots (the plots of intraparticle diffusion model), which can be divided into three parts that are corresponding to three stages of diffusion. At the first stage, which was the initial stage of adsorption, the CO2 was outside the particles where the film diffusion controlled the adsorption rate. At the second stage, the medium stage of adsorption, the CO2 penetrated through the interface layer into the particles, where pore diffusion controlled the adsorption. At the last stage, the CO2 was inside the pores of particles; thus, the surface reaction dominates the adsorption rate. Figure 12 illustrates that at the 40 °C, the slopes of three parts of plots were 0.27, 1.157 and 2.286. The first stage had the lowest slope, demonstrating the film diffusion controlled the adsorption rate. However, as for the other temperature, the last stages owned the lowest slope, indicating the surface reaction controlled the adsorption rate. Taking 30 °C for an example, the slopes were 0.27, 1.09 and 0.024. And for all of the temperatures considered, the slopes of second stages were always the highest, suggesting that pore diffusion of CO2 was rather efficient.

Figure 12
figure 12

Intraparticle diffusion simulation curves of MFM-SiO2-12 nm-PEI under different temperatures

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The mass transfer rate in MFM-PEI microspheres prepared in this study was compared with that of similar mesoporous amino resin powder adsorbents (MF powder). The time taken to adsorb 1 mmol of CO2 per unit volume of powder adsorbent and microsphere adsorbent was calculated. For this calculation, 0.5 g mesoporous amino resin powder (4.6 ml in volume) and 0.5 g microspheres (7 ml in volume) were separately packed in two glass tubes with the same diameter. The equilibrium adsorption time teq, the amount of CO2 adsorbed at equilibrium Q, and the time that taken to adsorb 1 mmol of CO2 per unit volume of powder or microspheres t were measured.

It could be seen from Table 5 that the time required for MFM to adsorb 1 mmol CO2 was obviously shorter than that required for the powder mesoporous amino resin, which indicated that the mass transfer rate of gas was obviously increased by the MFM.

Table 5 Adsorption time used for 1 mmol CO2 of mesoporous amino resin powder (MF powder) and MFM
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Conclusion

In summary, we have successfully prepared the MFM with different pore sizes via precipitation polymerization of melamine, formaldehyde solution and metaphenylene diamine in aqueous solution by using different particle size SiO2 microspheres as template. When 12 nm SiO2 microspheres was used as template, the prepared MFM had relatively high specific surface area, pore volume and pore size, which were 183.67 m2/g, 0.91 cm3/g and 19.8 nm, respectively. After loading with PEI, the CO2 adsorption capacity of MFM-PEI reached 2.68 mmol/g at 60 °C, with the corresponding utilization efficiency of amino as high as 40.66%. Avrami model could better describe the adsorption process of CO2, indicating both physical adsorption and chemical adsorption in the whole process. After 5 times, 10 times, 15 times and even after 20 cycles of adsorption (30 °C)–desorption (90 °C), the adsorption capacity of regenerated MFM-PEI at 30 °C was nearly the same as the that of fresh one, which showed that the adsorbent regeneration performance was stable, and showed higher quick mass transfer rate, with great advantages in practical production.