Nowadays, the greenhouse effect mainly caused by CO₂ is a worldwide problem to be solved [1-3]. High-temperature flue gas (600–700 ℃) from fossil fuel based thermal power plants and cement plants is long-term stable sources of CO₂ emission [4, 5]. Using the adsorbents, especially Li₄SiO₄-based adsorbents to adsorb CO₂ from flue gas has been a potential method to avoid the greenhouse effect in recent years. Additionally, the Li₄SiO₄-based adsorbents can be further utilized in reforming reactions. By adsorbing CO₂, the lowering of CO and CO₂ concentration in the product leads to the increased H₂ yield [6, 7]. Li₄SiO₄ captures CO₂ at high temperatures through the reversible reaction (1) [8, 9]:

(1)

The optimum adsorption temperature of Li₄SiO₄ in reaction (1) is ~700 ℃ [9, 10], it limits the application of Li₄SiO₄ in many aspects [4-7]. Thus, it is necessary to broaden the range of Li₄SiO₄ adsorption temperature to increase its application fields.

In recent years, using cheap and clean silica sources for the preparation of Li₄SiO₄-based adsorbents has become a trend, Shan et al. use diatomite as silicon source and prepared Li₄SiO₄-based adsorbents [11, 12]. The Inner Mongolia coal series kaolinite is widely used in various processes. The main components of kaolin clay in coal deposit from Inner Mongolia are approximately 37.5% Al₂O₃ and 45.8% SiO₂ [13]. Nowadays, aluminum adhesive can be extracted from the coal based kaolin, leaving SiO₂ as the main residue that cannot be utilized in other processes, causing the waste of resources [14, 15]. To reduce the waste of SiO₂ derived from coal based kaolin, we reuse it as a silica source to synthesize Li₄SiO₄-based adsorbents in the present work.

Zhang et al. [16] demonstrated that K doping can remarkably improve the CO₂ chemisorption process at high temperatures (e.g., 600–750 ℃). K₂CO₃ can co-melt with Li₂CO₃ at high temperatures to form a eutectic "liquid shell", in which facilitates CO₂ diffusion throughout the product layer [17, 18]. In addition, J. Ortizlanderos et al. [19] reported that doping Al can also improve the adsorption performance of the Li₄SiO₄-based adsorbent, especially at medium temperatures (e.g., 400–600 ℃). It is because forming solid solution can accelerate the diffusion of Li⁺ in the adsorbents, which promotes the adsorption of CO₂ [19, 20]. However, the coeffect of K and Al on the adsorption performance of the Li₄SiO₄-based adsorbent is still unclear and needs to be further investigated.

The previous research of our group [21] has compared the performance of the Li₄SiO₄ adsorbent using kaolin or commercial silica as the silica source. The results show that the CO₂ adsorption capacity of kaolin-Li₄SiO₄ is larger than that of commercial silicaLi₄SiO₄, when the temperature is below 700 ℃. Therefore, we selected kaolin as a silica source to synthesize the Li₄SiO₄-based adsorbents in this work.

In the present study, we synthesized a series of Li₄SiO₄-based adsorbents using kaolin as the silica source, and the effects of codopedK and Al on the CO₂ adsorption properties were systematically investigated. We expect this work can provide guidance for the rational design and fabrication of Li₄SiO₄-based adsorbents with wide effective adsorption temperature for various processes.

In a typical procedure, kaolin clay was firstly pretreated with acid and alkali, and then washed and calcined to obtain the kaolinSiO₂. Thereafter, the Li₄SiO₄-based adsorbents from kaolin-SiO₂ were prepared. Briefly, LiNO₃ and kaolin-SiO₂ were mixed in ethanol with a Li:Si molar ratio of 4:1. After continuous stirring for 4 h at 40 ℃, the ammonia was added slowly into the above solution. With another 0.5 h stirring and 3 h aging, the mixture was dried and calcination at 750 ℃ for 4 h to obtain Li₄SiO₄.

KNO₃ was used as the K source, and halloysite nanotube (HTNs) was used as the Al source. The K-Li₄SiO₄ and Al-Li₄SiO₄ were prepared in accordance with the above method. Typically, among them, the mass fractions of K were 0.4 wt%, 0.8 wt%, 1.2 wt%, and named as 4K-Li₄SiO₄, 8K-Li₄SiO₄, 12K-Li₄SiO₄, respectively. The mass fractions of Al were 1.0 wt%, 1.5 wt%, 2.0 wt%, and named as 10Al-Li₄SiO₄, 15 Al-Li₄SiO₄, 20Al-Li₄SiO₄, respectively. Then, K and Al were simultaneously doped into Li₄SiO₄ adsorbent by addition of 0.8 wt% K and 1.5 wt% Al, respectively. The obtained sample was named as 8K15Al-Li₄SiO₄.

X-ray diffraction (XRD) characterization of the samples was performed using a Philips PW 1050/25 X-ray diffractometer with Cu-Ka radiation (λ = 0.1542 nm). The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. The scanning electron microscopy (SEM) images of the samples were taken on a Hitachi S-4800 scanning electron microscope (5 kV). Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance Ⅱ spectrometer with a magnetic field strength of 7.05 T, corresponding to a ²⁷Al Larmor frequency of 78.3 MHz. ²⁹Si MAS NMR spectra were obtained by operating the spectrometer at a resonance frequency of 59.59 MHz.

The CO₂ adsorption performances of the adsorbents were performed on a thermo-gravimetric analyzer (TG-DTA Instrument, PerkinElmer). Prior to the detection, to remove the adsorbed water and CO₂, 10 mg of adsorbent was heated to 760 ℃ at a heating rate of 10 ℃/min, and then cooled to 100 ℃ in a N₂ atmosphere (100 mL/min). Thereafter, the adsorbent was heated in a CO₂ atmosphere (60 mL/min) at a heating rate of 10 ℃/min. Additionally, to examine the stability of the selected adsorbents 10 absorption/desorption cycles were carried out: CO₂ was adsorbed at 600 ℃ for 20 min, and subsequently desorbed in N₂ atmosphere for 1 h.

The kinetics experiment was also performed on TG-DTA. The adsorbents were firstly heated to 760 ℃ in N₂ atmosphere, then cooled down to 550, 575, 600 and 650 ℃, respectively. Finally, we switched to the CO₂ atmosphere to get the isothermal adsorption results.

Figs. S1 and S2 (Supporting information) show the XRD patterns of the Li₄SiO₄-based adsorbents with K and Al doping. All the adsorbents were successfully prepared. Figs. S1C and S1D show the diffraction peaks of the samples doped with K or Al shift to lower angles, which indicates that K and Al were successfully doped into the Li₄SiO₄ lattice.

Fig. S3A (Supporting information) shows the ²⁹Si MAS NMR spectra of the adsorbents. Notably, all of the adsorbents exhibit a resonance peak at δ -67. The peak intensities of 8K-Li₄SiO₄ have little change compared with the pristine Li₄SiO₄, indicating that K cannot modulate the neighborhood of [SiO4]^4- tetrahedral units [22, 23]. However, the peak intensity of ²⁹Si becomes weaker after doping with Al, which suggests the modified angle of Si-O in the [SiO₄]^4- tetrahedral. Fig. S3B displays the corresponding ²⁷Al MAS NMR spectra of 15Al-Li₄SiO₄ and 8K15Al-Li₄SiO₄. The peaks reveal two types of aluminum 4-fold coordinated in the lattice [19]. The main peak is located at δ 78, and the other convoluted peak is located at δ 62. After doped with K and Al, the peak located at δ 62 vanishes, indicating the increase of lattice order [23, 24]. It also proves that K and Al are co-doped into Li₄SiO₄.

Fig. 1 shows the SEM images of the CO₂ adsorbents. Doping K and Al in Li₄SiO₄ directly affects the morphology of the samples. Compared with the pristine Li₄SiO₄, the surface of the doped adsorbents becomes rough, which is helpful to the contaction of CO₂ and the adsorbents.

Fig. 2A shows the CO₂ adsorption capacity of Li₄SiO₄-based adsorbents. The maximum CO₂ adsorption of the doped adsorbents locates at around 700 ℃, and the adsorption is much higher than that of the pristine Li₄SiO₄ below 700 ℃. Fig. S4 (Supporting information) shows the influence of K or Al on the CO₂ adsorption performance of the Li₄SiO₄-based adsorbents. The CO₂ adsorption capacity is significantly enhanced at medium or high temperatures, respectively. And the temperature of the maximum CO₂ adsorption capacity is lower than that of Li₄SiO₄ about 30 ℃. The adsorbent 8K15Al-Li₄SiO₄ in Fig. 2A shows that it takes advantages of high and medium temperature adsorption performance, and the adsorption capacity is much higher than that of the pristine Li₄SiO₄ among 350–700 ℃. The temperature range of 8K15Al-Li₄SiO₄ for carbon dioxide adsorption is greatly broadened. The results illustrate that the addition of K and Al alters the morphology and accelerates the CO₂ adsorption rate and enhances the CO₂ capture capacity of the Li₄SiO₄-based adsorbents at the medium and high temperatures.

In Fig. 2B, the adsorption/desorption test was carried out at a fixed temperature of 600 ℃ to investigate the cyclic adsorption performance of 8K15Al-Li₄SiO₄. As shown in Fig. 2B, the CO₂ adsorption capacity reaches 24.6 wt% in the first cycle. With the cycle times increased, the CO₂ adsorption amount gradually decreases, and then maintains at a relatively high adsorption capacity (about 22.0 wt%) after 3 cycles, better than reported [25]. Thus the 8K15Al-Li₄SiO₄ exhibits not only an excellent adsorption performance, but also good cycle adsorption stability at lower temperatures.

The saturated adsorption capacity of the adsorbents is summarized in Table 1. Notably, the CO₂ adsorption capacity of 8K15Al-Li₄SiO₄ achieves about 120.0% and 24.5% higher than that of the pristine Li₄SiO₄ at 500 and 600 ℃, respectively. Additionally, the saturation adsorption capacity of 8K15Al-Li₄SiO₄ at 600 ℃ is very close to that at 700 ℃. It can also be inferred that the 8K15AlLi₄SiO₄ has a great preponderance and application prospect for the removal of CO₂ in a wide temperature region.

The kinetic information about the CO₂ adsorption and additional isothermal analysis over the Li₄SiO₄-based adsorbents are displayed in Fig. S5 (Supporting information). The CO₂ adsorption capacity of these adsorbents was improved with the increase of temperatures. Both the adsorption capacity and the adsorption rate of the doped adsorbents are higher than those of the pristine Li₄SiO₄. To analyze the reasons for the increasement of CO₂ adsorption amount after doping K and Al into Li₄SiO₄ we use the double shell model, which is most widely accepted in many literatures [12, 26, 27].The double exponential model was successfully established to simulate the CO₂ adsorption on different adsorbents such as Li₄SiO₄ [28], Li₂ZrO₃ [29], and Na₂ZrO₃ [30]. The model can be represented as Eq. (2):

(2)

Where y represents the percentage of adsorbents weight change by CO₂ adsorption; t is the time; k₁ and k₂ are the rate constants, and k₁ is the rate constant of surface chemical reaction, while k₂ is the rate constants of lithium migration process; A, B, and C are the preexponential factors. The exponential constant values obtained at each temperature are listed in Table S1 (Supporting information). The k₁ values are much higher than k₂ for all the adsorbents, indicating that the chemisorption step is faster than lithium diffusion step. Lithium diffusion is the rate-determining step in the total process.

To further investigate the kinetic results, the incremental quantities of k₁ and k₂ values were calculated at corresponding temperatures. The data are listed in Table 2. After doping K and Al, the k₁ and k₂ values were increased, and the range of k₂ values increased more. It indicates that lithium migration process have been much more affected by doping method. The addition of K mainly reflected lithium migration at above 600 ℃, and k₂ values increased by more than 65%. However, when doping with Al, the lithium migration rate has been greatly improved at below 600 ℃, and k₂ values increased by almost 60%. The lithium migration rate of 8K15Al-Li₄SiO₄ is ~2 times higher than that of the pristine Li₄SiO₄ at 575–650 ℃ compared with the pristine Li₄SiO₄.

Lithium diffusion is the rate-determining step in the total process. As a result, the reaction rate enhances with the increase of the lithium migration process rate, which promotes the reaction at lower temperatures. Therefore, the adsorption capacity of the 8K15Al-Li₄SiO₄ is 22% at 500 ℃, and 32% at 650 ℃, which is close to that of 700 ℃. It means that 8K15Al-Li₄SiO₄ has a high adsorption capacity at low temperature. The CO₂ adsorption range of 8K15Al-Li₄SiO₄ is greatly widened.

In summary, we use kaolin as the silica source to synthesize a series of Li₄SiO₄-based adsorbents. The doped K or Al in Li₄SiO₄ promoted both surface chemical reaction and lithium migration process, especially for the later one. The lithium migration rate of the adsorbent 8K15Al-Li₄SiO₄ is ~2 times higher than that of the pristine Li₄SiO₄. Additionally, the 8K15Al-Li₄SiO₄ has good reproducibility and can be used repeatedly. The co-doping of K and Al improves the adsorption performance of Li₄SiO₄-based adsorbents, especially at the range of 500–650 ℃. The co-doped K and Al not only broaden the effective adsorption temperature range, but also greatly improve the adsorption performance of Li₄SiO₄, which can provide guidance for the rational design and synthesis of Li₄SiO₄-based adsorbents with wide effective adsorption temperature for various processes.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21476160, 21476159] and the Natural Science Foundation of Tianjin (Nos.15JCYBJC23000, 15JCZDJC37400).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2017.07.031.