Polyethyleneimine-Modified Amorphous Silica for the Selective Adsorption of CO2/N2 at High Temperatures.

Mechanochemistry is very attractive as an efficient, solvent-free, and simplified technique for the preparation of composite adsorbents. Here, a series of polyethyleneimine (PEI)-modified SiO2 adsorbents were prepared via mechanical ball milling for selective adsorption of CO2 at high temperatures. The structural properties of these adsorbents were characterized by XRD, SEM, TGA, FTIR, and N2 adsorption-desorption. This method can better disperse the PEI evenly in the SiO2 as well as maintain the porous structure of the adsorbents by comparing with the impregnated adsorbents. These adsorbents presented appreciable performance in separating CO2 at high temperatures, and the CO2 adsorption capacity of PEI(70%)/SiO2 is up to 2.47 mmol/g at 70 °C and 1.5 bar, which is significantly higher than that of the same type of CO2 adsorbent reported in the literature. Furthermore, the adsorbent of PEI(70%)/SiO2 provided an ideally infinite selectivity for CO2/N2 (15:85) at 70 °C. These results showed that mechanical grinding methods are a simple and effective approach to producing amine-modified silica composite adsorbents.

Introduction

Atmospheric CO2 is continuously increasing at a high rate, which is mainly due to the intensive use of fossil fuels.[1] Further, the environmental issue caused by the increase in CO2 has become a global problem that cannot be ignored.[2,3] Many options for reducing CO2 emissions had been investigated and liquid amine adsorption was generally considered to be one of the most effective methods of CO2 capture.[4,5] However, this method has some disadvantages such as high cost, high energy consumption, amine losses, and high corrosiveness. These issues limit the potential of liquid amines for CO2 capture applications.[6,7]

There has been a great concern for the development of efficient solid adsorbents for the capture of CO2. Particularly, it is of high significance for the capture of high-temperature flue gases in thermal power plants (CO2/N2 of about 0.15/0.85).[8] Solid amine adsorbents have the advantage of both physisorption and chemisorption, which makes them one of the most promising methods for capturing CO2.[8,9] Therefore, many porous materials are widely used as an amine support for the physical and chemical adsorption of CO2, including porous carbon,[10,11] mesoporous silica,[12−14] zeolites,[15,16] porous polymers,[17,18] and metal–organic frameworks (MOFs).[19,20] As mentioned above, mesoporous silica is widely used as a support to immobilize amines due to its good thermal stability, high pore volume, and abundance of surface silica hydroxyl groups.[9,21,22]

There are two main approaches to preparing solid amine adsorbents: (a) wet impregnation and (b) silane chemistry.[8,13,23−25] Wet impregnation is a simple and most common technique for the addition of organic amines to mesoporous silica. Nevertheless, these methods require large amounts of solution and long synthesis times. In addition, the removal of the solvent also requires energy, which has a significant impact on production costs. In the case of the industrial synthesis of solid amine adsorbents and protection of the environment, mechanochemistry could be used as a simple, efficient, and sustainable method.[26,27] The mechanochemical method has the obvious effect of changing the physicochemical properties of the materials, i.e., changing the size of the particles, creating structural defects, enhancing the chemical reactions during the grinding process, etc.[27−32] There have been reports of condensation reactions between hydroxyl groups on the surface of silica and siloxane groups by mechanochemical methods.[32,33] Amrute et al.(33) reported the mechanochemical functionalization of different supports (SBA-15, SiO2 gel, etc.) with various silicone compounds without the use of any solvent in only 5 min at room temperature, which provides a new idea for the modification of silica with organic amines.

Polyethyleneimine (PEI) has high CO2 absorption capacity, which is used as a modifier to modify silica for improving CO2 adsorption performance and selectivity.[8,34−37] The most commonly used method for constructing amine-silica adsorbents is the solvent impregnation method, in which amine can be uniformly dispersed on the silica. However, the silica with high amine loading is easy to agglomerate, and the use of organic solvents for dispersion also increases the production cost. Therefore, it is highly desirable to develop an effective method to prepare high amine loading silica adsorbents for effective separation of CO2. Herein, we demonstrated a straightforward method to prepare a silica adsorbent with high PEI loading by the mechanical grinding method without any solvents. The structural properties of adsorbents synthesized using different methods were systematically characterized by XRD, SEM, TGA, FTIR, and N2 adsorption–desorption. The CO2 adsorption performance of adsorbents was investigated, and the adsorption selectivity of CO2/N2 and CO2/CH4 binary mixtures was also calculated from the ideal adsorbed solution theory (IAST). In addition, the CO2 adsorption isotherms and the isosteric heats of CO2 adsorption were discussed by the dual-site Langmuir–Freundlich (DSLF) model and Clausius–Clapeyron equation.

Results and Discussion

Characterization of PEI-Modified SiO2 Adsorbents

The PEI-modified SiO2 adsorbents were prepared by impregnation and ball milling methods, and the obtained adsorbents were marked as IM-PEI(70%)/SiO2 and PEI(70%)/SiO2, respectively, where 70% represents the mass fraction of PEI. The structure of the PEI-modified SiO2 adsorbents was characterized by XRD. As shown in Figure a, a broadened peak from 2θ of 10° to 35° corresponds to the amorphous silica phase.[38] The crystalline structure of silica did not change after 70% PEI was loaded by either the impregnation method or ball milling method. In addition, changing the loading amount of PEI has no significant effect on the crystal structure of silica (Figure b). It is suggested that PEI was fixed on silica through physical interaction.

Figure 1

XRD patterns of (a) SiO2 before and after PEI modification and (b) different loading amounts of PEI-modified SiO2.

The microstructure of adsorbents was studied by SEM, and results are shown in Figure . SiO2 has a rich porous structure and is very favorable for supporting amine (Figure a,b). The surface texture of the amine-modified silica became denser after supporting 70% PEI. The pores of SiO2 also significantly reduced, indicating that PEI was uniformly loaded on the support due to the strong interaction of mechanical grinding (Figure c,d). Furthermore, elemental mapping images of Figure g show uniform dispersions of N throughout the support SiO2, thus indicating that the PEI units are uniformly dispersed in the adsorbent. In contrast, the agglomeration of IM-PEI(70%)/SiO2 prepared by the impregnation method is easier. Large pore channels were also obtained by removing the solvent from the adsorbent during the impregnation synthesis (Figure e,f).

Figure 2

SEM images of (a, b) silica, (c, d) PEI(70%)/SiO2, and (e, f) IM-PEI(70%)/SiO2. (g–j) Elemental (O, Si, and N) mapping images of PEI(70%)/SiO2.

The N2 adsorption/desorption isotherms and BJH pore size distribution curves of SiO2 and PEI-modified SiO2 adsorbents are drawn in Figure . After directly ball-milling SiO2, the specific surface area of ball-milled SiO2 decreases observably, indicating excessive collapse of the pore structure. This was due to the absence of protective agents (e.g., PEI and ethanol) during ball milling.[39,40] The specific surface area of SiO2 was further reduced by loading different masses of PEI, and all PEI-modified SiO2 adsorbents exhibit almost classic type IV isotherms.[41] Notably, PEI(70%)/SiO2 exhibits a prominent type H3 hysteresis, indicating the presence of mesopores. Even if the loading of PEI is augmented, there is still no significant change in hysteresis loops of PEI(70%)/SiO2 (Figure S1). In contrast, the hysteresis loops of IM-PEI(70%)/SiO2 shift from type H3 to type H1 with the increase in PEI loading.[42,43] Furthermore, the BJH pore size distribution curves indicate that modified adsorbents prepared by mechanical grinding have narrower aperture distribution. This narrower pore size is conducive to the adsorption and mass transfer of CO2. As shown in Table , the SBET of all the PEI-modified SiO2 adsorbents decreases with the increase in PEI loading, which is in line with expectations. According to the pore structure information of the PEI-modified SiO2 adsorbents prepared by the ball milling method and impregnation method, both methods are effective for loading amines onto SiO2. From the perspective of environmental-friendliness, the ball milling method has more advantages because it can achieve a uniform load of a large amount of PEI and maintain the porosity of the adsorbent to a certain extent.

Figure 3

N2 adsorption–desorption isotherms and BJH pore size distributions of (a, b) SiO2, (c, d) ball-milled SiO2, (e, f) PEI(70%)/SiO2, and (g, h) IM-PEI(70%)/SiO2.

Table 1

Textural Properties of the Synthesized Adsorbents

sample	SBET (m2/g)	Vp (cm3/g)a	dBJH (nm)b	CO2 uptakec
SiO2	393.28	1.59	14.51	0.59
ball-milled SiO2	223.71	0.55	8.91
PEI(30%)/SiO2	122.32	0.56	10.04	1.21
PEI(50%)/SiO2	100.14	0.53	11.91	1.54
PEI(70%)/SiO2	70.86	0.63	20.06	1.80
PEI(100%)/SiO2	55.84	0.51	19.33	1.67
IM-PEI(30%)/SiO2	118.91	1.46	26.95	1.29
IM-PEI(50%)/SiO2	81.46	1.10	27.08	1.66
IM-PEI(70%)/SiO2	72.37	0.63	19.89	1.70
IM-PEI(100%)/SiO2	19.85	0.24	22.83	1.06

Cumulative BJH desorption pore volume.

Average BJH desorption pore diameter.

Adsorption capacity of CO2 at 25 °C (1.0 bar).

The thermostability of PEI-modified SiO2 was analyzed by TGA analysis.[14,42,44] As shown in Figure a, the initial weight loss below 125 °C was attributed to desorption of water, CO2, and other volatile gases. As shown in Figure b, IM-PEI(70%)/SiO2 had a further mass loss (about 4%) in the temperature range of 150 to 230 °C, while no weight loss was observed in PEI (70%)/SiO2. It indicated that the adsorbent prepared by mechanical ball milling is more thermally stable. Furthermore, the dramatic weight loss after 230 °C shows the significant loss of PEI. This is mainly due to the decomposition of the amino group. In summary, the TGA results show the good thermal stability of ball-milled adsorbents.

Figure 4

(a) TGA curves of the PEI-modified SiO2 adsorbents prepared by the ball milling method with different PEI loads and (b) PEI(70%)/SiO2vs IM-PEI(70%)/SiO2.

The FTIR spectra of the adsorbents are shown in Figure . The corresponding functional groups of SiO2 are indicated in Figure a. The peaks at 3750–3400 cm–1 and 967 cm–1 are attributed to O–H vibrations of the silica surface. After PEI modification, peaks at 1643 and 1573 cm–1 correspond to the vibrations of secondary amine (R2NH) and primary amine (RNH2), respectively.[8] The chemical structure of PEI chains was also observed at 2957 and 2836 cm–1. The adsorbents synthesized by the impregnated method also show a similar characteristic infrared absorption peak (Figure S2). Therefore, these PEI-modified SiO2 adsorbents can chemically adsorb CO2.[45] Notably, it is worth noting that for the characterization of PEI(70%)/SiO2 after adsorbing a large amount of CO2, physically adsorbed CO2 could be clearly identified at 2340 cm–1 (Figure b).[22] The above results indicate that CO2 can be adsorbed through synergistic physical and chemical adsorption.

Figure 5

(a) FTIR spectra of the silica adsorbent prepared by ball milling with different PEI loads and (b) FTIR spectra of PEI(70%)/SiO2 before and after adsorption of CO2.

CO2 Adsorption

The influence of the PEI loads and different synthesis methods on the CO2 adsorption capacity at 25 °C was studied, and results are shown in Figure . For both the mechanical grinding and impregnation methods, the CO2 adsorption capacity of PEI-modified SiO2 increases with increasing amine content (Figure a,b). When the mass ratio of PEI to SiO2 was 0.7:1, the CO2 adsorption capacity of the adsorbents synthesized by both methods reached maximum values. The CO2 adsorption capacity of PEI(70%)/SiO2 is 1.88 mmol/g at 1.5 bar and 25 °C. However, the CO2 adsorption capacity of the impregnated adsorbents decreased sharply with further increasing PEI load. That is a result of blocking in the silica pores and the amine covering the SiO2 surface in caking form, which prevents CO2 molecules from entering the active center of adsorption and leads to an obvious reduction in CO2 adsorption capacity.[42,46] Furthermore, PEI(70%)/SiO2 has a faster CO2 adsorption rate (Figure S3), which may be due to the better retention of the pores of PEI(70%)/SiO2 than IM-PEI(70%)/SiO2. These all suggest that mechanical grinding is a promising way to prepare amine-modified adsorbents.

Figure 6

CO2 absorption isotherms of (a) ball-milled PEI-modified adsorbents and (b) PEI-impregnated adsorbents at 25 °C. (c) CO2 absorption isotherms of PEI(70%)/SiO2 at different temperatures. (d) CO2 absorption trends for PEI(70%)/SiO2 and IM-PEI(70%)/SiO2 at different temperatures.

The CO2 adsorption by PEI-modified SiO2 adsorbents was further investigated at temperatures ranging from 0 to 90 °C, as shown in Figure c. A high temperature is unfavorable for CO2 adsorption, but the viscosity of PEI and the diffusion resistance of CO2 in the inner layer are reduced.[8] Therefore, more amine sites are exposed, enabling enhanced CO2 adsorption with increasing temperature. The adsorption capacity reached a maximum of 2.47 mmol/g at 70 °C and 1.5 bar. Then, CO2 adsorption decreased with further temperature increase but still maintained a high capacity. More importantly, CO2 uptake is already up to 1.95 mmol/g at 70 °C and 0.15 bar, indicating the effective adsorption of low-pressure CO2 by PEI(70%)/SiO2. The adsorption performance of IM-PEI(70%)/SiO2 for CO2 was also tested under the same conditions (Figure S4). It is found that the PEI-modified SiO2 adsorbent prepared by the ball milling method has a higher adsorption capacity (Figure d) and speculated that the ball milling method has a higher amine utilization rate than the impregnation method.

N2 and CH4 Adsorption

We also investigated the N2 and CH4 adsorption of the material. As shown in Figure , the isotherms of CH4 and N2 were significantly different from the shape of the CO2 isotherm. CO2 adsorption is a combined adsorption through physical and chemical interactions. In contrast, the N2 and CH4 isotherms are almost linear, indicating that the adsorption is physisorption and adsorbents lack specific adsorption sites for N2 and CH4. As shown in Figure S5, even at 0 °C and 1.5 bar, the adsorption of N2 (0.037 mmol/g) and CH4 (0.086 mmol/g) was very low. These results showed that adsorbents have a strong affinity for CO2. Interestingly, the adsorbent showed no uptake of N2 at 70 °C, demonstrating that the adsorbent could completely separate CO2 and N2 at 70 °C.

Figure 7

CO2, N2, and CH4 absorption isotherms of PEI(70%)/SiO2 at (a) 25 °C and (b) 70 °C.

Selectivity and Isosteric Heat of Adsorption

In this work, we have calculated the gas selectivity of PEI(70%)/SiO2 by IAST. At first, the DSLF equation was employed to fit the adsorption isotherms for CO2, N2, and CH4, as expressed by:[19]where q is the equilibrium absorption (mmol/g) at gas pressure p (bar), q is the capacity of sites i (mmol/g) at saturation, b is the affinity coefficients of sites i, and n is the ideal homogeneous surface deviation. The fitting parameters for the DSLF model are listed in Table S1, and Figure a shows the experimentally CO2 adsorption isotherms and model curves. The results indicated that the model gives a good fit to the experimental isotherms of CO2, CH4, and N2. Then, the adsorption isotherms of the gas mixture were predicted by IAST and the selectivity of the binary gas mixture was calculated. The equation is shown as:[48]where x is the molar fraction in the adsorbed phase and y is the molar fraction in the gas phase. At 25 °C and 1 bar, the selectivity values of PEI(70%)/SiO2 for CO2/N2 (15:85) and CO2/CH4 (40:60) binary gas mixtures were 561.0 and 148.5, respectively. Moreover, PEI(70%)/SiO2 exhibits a high CO2 adsorption capacity as well as high CO2/CH4 and CO2/N2 selectivity at low CO2 pressure and 70 °C. As shown in Table , the selectivity of PEI(70%)/SiO2 is higher than that of many reported porous materials. It is a promising CO2 capture adsorbent especially for the separation of high-temperature flue gas.

Figure 8

(a) CO2 adsorption isotherms and dual-site Langmuir–Freundlich model fits for PEI(70%)/SiO2 at 0, 25, and 70 °C. (b) Isosteric heat of adsorption for CO2 on PEI(70%)/SiO2 according to the CO2 adsorption data at 0 and 25 °C.

Table 2

Comparison of Selectivity for CO2/N2 and CO2/CH4 Mixtures

adsorbent	T (°C)	CO2 uptaked	SCO2/N2	SCO2/CH4	ref
PEI(70%)/SiO2a	25	1.40	561.0	148.5	this work
PEI(70%)/SiO2a	70	2.04	infinitee	148.5
MCM-41b	25		11		(47)
TRI-PE-MCM-41b	25		308f		(47)
MOF-505@5GOa	25	1.25	37.2	8.6	(19)
C-COP-P-Mnc	25	0.13		6.3	(48)
PAN-NPc	0	0.80	101.1	16.5	(49)
nanosized zeolite La	25	1.96	198.6	75.3	(50)
cylindrical zeolitea	25	1.65	188.6	72.9	(50)
NPC-3-500a	0	1.39	160.4		(51)

The gas mixtures of CO2/N2 (15:85) and CO2/CH4 (40:60) at 1 bar.

The gas mixtures of CO2/N2 (20:80) at 1 bar.

The gas mixtures of CO2/N2 (15:85) and CO2/CH4 (50:50) at 1 bar.

CO2 uptake at 0.15 bar CO2 (mmol/g).

The adsorbent showed no uptake of N2 at 70 °C; thus, the adsorbents showed ideally infinite selectivity.

CO2/N2 molar selectivity ratio for materials at 1 bar.

The isotropic heat of adsorption (Qst) was calculated from the CO2 adsorption data at 0 and 25 °C using the Clausius–Clapton equation:[17]where p and T represent the pressure and corresponding temperature of isotherm i, respectively, and R is the constant (8.314 J/K·mol). The CO2 adsorption of PEI(70%)/SiO2 is a typical chemical adsorption. As shown in Figure b, the Qst of CO2 adsorption on the adsorbent decreased significantly as the amount of CO2 adsorption increased. Afterward, the Qst value was lower than the energy of forming chemical bindings (40 kJ/mol), indicating that it was a physical adsorption process.[22] Overall, this implies the heterogeneity of the adsorbent surface and obvious advantage for low-pressure CO2 uptake.

Cycle Performance

To test the regenerative performance of PEI(70%)/SiO2, CO2 adsorption–desorption experiments were carried out, as shown in Figure . It can be observed that PEI(70%)/SiO2 has excellent regeneration capacity, and the CO2 adsorption capacity has only little fluctuations after 10 subsequent cycles. There was no significant difference in the CO2 absorption, which means no loss of amine active centers. Therefore, PEI(70%)/SiO2 is very stable and has outstanding regenerability.

Figure 9

Cyclic adsorption performance of PEI(70%)/SiO2 for CO2 at 25 °C and 1.5 bar.

Adsorption Mechanism

Combined with the current experimental results and previous studies,[9,52−54] a plausible carbon dioxide adsorption mechanism is proposed and shown in Figure . After immobilization of PEI, SiO2 adsorbents have more active adsorption sites (e.g., −NH2). Thus, CO2 can be adsorbed chemically to form zwitterions with immobilized amines and, finally, carbamates with higher thermal stability. In addition, CO2 can also be fixed by hydrogen bonding between silicon hydroxyl and CO2. In the adsorption process, abundant alkaline −NH2 sites are the key reason for the highly selective separation of CO2 and N2.

Figure 10

Possible CO2 adsorption mechanism of the PEI-modified SiO2 adsorbent.

Conclusions

In conclusion, a series of amine-modified amorphous silica was prepared by a simple, green, and efficient mechanical grinding method. The influence of mechanical grinding on amine content, porous structure, and CO2 adsorption performance was investigated by characterization. The results showed that this method has significant advantages for amine dispersion. The absorbents also have a remarkably CO2 adsorption capacity at high temperatures. At 25 °C and 1 bar, its IAST selectivity values for CO2/N2 (15:85) and CO2/CH4 (40:60) were up to 561.0 and 148.0, respectively. The adsorption–desorption tests showed the stability and regeneration of the adsorbent. Therefore, it is proved to be a promising adsorbent for the capture and separation of CO2 from power-plant flue gas.

Experimental Section

Materials

Polyethyleneimine (PEI, Mw = 600 Da) and silica (SiO2, hydrophilic-380 type) were purchased from Energy Chemical Technology Co., Ltd., and Aladdin Biochemical Technology Co., Ltd. China, respectively. All chemicals were used directly without further purification. CO2 (>99.99%), N2 (>99.999%), and CH4 (>99.999%) were supplied by Guiyang Sanhe Special Gas Center, China.

Preparation of Adsorbents by the Ball Milling Method

The PEI-modified SiO2 adsorbents were prepared by mechanical ball milling. In a typical synthesis procedure, 0.7 g of PEI was dissolved in 1 mL of methanol and allowed to stir for 30 min. Then, the solution and 1 g of SiO2 were added to a 45 mL ZrO2 grinding bowl with 10 ZrO2 balls (diameter, 10 mm) and ball-milled at 500 rpm for 1 h (forward and reverse rotation for 30 min each) using a vertical planetary mill (YXQM-0.4L, MITR Instrument & Equipment Co., Ltd., China). Finally, the sample was allowed to dry at 80 °C for 12 h. The obtained sample was denoted as PEI(70%)/SiO2. Similarly, 30, 50, and 100% PEI-modified SiO2 adsorbents were synthesized.

Preparation of Adsorbents by the Impregnation Method

The PEI-modified SiO2 adsorbents were prepared by the impregnation method. In a typical synthesis procedure, a desired amount of PEI was dissolved in 30 mL of methanol. After complete dissolution, 1 g of SiO2 was added to the solution and stirred for 6 h at 25 °C. Then, the methanol in the mixture was evaporated at 90 °C under vacuum. The sample was dried at 80 °C under vacuum for 12 h. The obtained sample was denoted as IM-PEI(70%)/SiO2. Similarly, 30, 50, and 100% PEI-modified SiO2 adsorbents were synthesized.

Characterization of Adsorbents

FTIR spectra of adsorbents were recorded on a Nicolet iS50 FTIR spectrometer. Thermogravimetric analysis (TGA) was performed using an STA 449F5 simultaneous thermal analyzer over the temperature range of 30 to 500 °C with a heat rate of 10 °C·min–1 in a N2 atmosphere. The pore structure of the samples was recorded by a BSD-PS(M) surface area and porosity analyzer (Beishide Instrument-S&T). The surface morphology of adsorbents was determined by scanning electron microscopy (SEM, Hitachi S-3400 N). X-ray diffraction (XRD) was performed on a small-angle X-ray diffractometer (D8 Advance) in the range of 2θ from 5° to 90°.

Gas Adsorption Experiment

CO2, N2, and CH4 adsorption isotherms were determined using the BSD-PS(M) by varying the temperature from 0 °C to 90 °C, and the gas pressure was changed from 0 bar to 1.5 bar. Before the test, the samples were degassed in vacuum at 110 °C for 3 h. The cycle performance was tested by the above process repeating 10 times. Adsorption kinetics was evaluated by a typical procedure, and CO2 gas (99.99%) was bubbled at a flow rate of about 10 mL·min–1 through the absorbent. The CO2 uptake was measured with an electronic balance.