CO₂ absorption properties of Ti- and Na-doped porous Li₄SiO₄ prepared by a sol–gel process

Abstract

To improve the carbon dioxide (CO₂) absorption performance of lithium orthosilicate (Li₄SiO₄), tablet-like Li₄Si_1−x Ti _x O₄ and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ sorbents with loose and porous texture were prepared by a sol–gel process. The relationship between the Ti doping and volume expansion was studied for the first time. The results indicated that the Ti presence into the Li₄SiO₄ structure inhibited the growth of grains and abated the volume expansion. The X-ray diffraction and scanning electron microscopy results showed that the loose and porous solid solutions with similar phase crystallite but different grain sizes could obtain by heat treatment of precursor at 700 °C. The optimum Ti content of Li₄Si_1−x Ti _x O₄ seems to be 0.04 of Li₄Si_0.96Ti_0.04O₄. And the CO₂ capture behaviors of Li_3.9Na_0.1Si_0.96Ti_0.04O₄ were better than Li₄Si_0.96Ti_0.04O₄.

Introduction

It is no secret that the world is getting warmer thanks to carbon dioxide (CO₂), a key anthropogenic greenhouse gas [1]. Hence, CO₂ capture technologies have aroused extensive researches. Hitherto, a number of ways have been developed to decrease CO₂ emissions, for example, wet absorption, dry adsorption, membrane separation, and cryogenic separation. However, these technologies are costly and ineffective since they operate at low temperature [2], i.e., the high-temperature exhaust gas needs a cooling process before removing CO₂, which not only adds input but decreases efficiency. Therefore, high-temperature CO₂ absorbents such as hydrotalcites, calcium oxides, and lithium-containing ceramics, have been considered as alternative adsorbents. Among the various solid sorbents, lithium-containing materials, e.g., Li₂ZrO₃, Li₄SiO₄, Li₂CuO₂, Li₅AlO₄, Li₂SiO₃, Li₆Zr₂O₇, and Li₈SiO₆ are suggested to be potential materials [3–6]. In addition, Li₂ZrO₃ and Li₄SiO₄ can theoretically adsorb CO₂ in amounts up to 28.7 and 36.7 wt% at high temperature, respectively. Furthermore, the CO₂ adsorption by Li₄SiO₄ is much faster (about 30 times) than Li₂ZrO₃. Besides, Li₄SiO₄ are lighter and cheaper than Li₂ZrO₃ [7–10]. Consequently, Li₄SiO₄ is suggested to be the most promising solid sorbent at high temperature.

Several researches on the kinetic and sorption mechanism of Li₄SiO₄ have been reported [11–13]. A double-shell mechanism and Avrami–Erofeev kinetic model were proposed to describe the sorption process [3]. Figure 1 shows the double-shell structure for the CO₂ sorption on Li₄SiO₄. According to the model and mechanism, the sorption process is not only controlled by the chemisorption but Li⁺ and O²⁻ diffusion. Hence, the whole process can be split into two stages: the rapid reaction and diffusion control. The rapid stage is the reaction of CO₂ molecules and Li₄SiO₄ to form solid Li₂CO₃ and Li₂SiO₃ nuclei on the surface (Fig. 1b). The diffusion control stage is the diffusion of Li⁺ and O²⁻ through the Li₂SiO₃ shell covering the unreacted Li₄SiO₄. Therefore, in order to improve the sorption properties, the uptake of rapid stage should be enlarged. Meanwhile, the diffusion resistance should be reduced. Carbonates (e.g., Na₂CO₃, K₂CO₃) have been studied as eutectic melts with the outside Li₂CO₃ shell, which can facilitate CO₂ diffusion throughout the first shell [5, 8, 14]. Furthermore, doping hetero elements can enhance ion mobility and reduce diffusion resistance by forming point defects or secondary phases [8]. For example, Li_4−x Na _x SiO₄ [5], Li_3.7Al_0.1SiO₄, and Li_3.7Fe_0.1SiO₄ [8], Li_2−x Na _x ZrO₃ [15, 16], Li_2−x K _x ZrO₃ [17], Na₂(Zr_1−x Al _x )O₃ [18], and Li_4+x (Si_1−x Al _x )O₄ [19] showed that even small quantity of a doping component can improve the sorption property [9]. And lithium silicate platelets synthesized by a sol–gel approach had been studied to enhance CO₂ absorption kinetics [20]. Currently most researches focus on introducing Na or K to replace Li element for improving surface chemisorption sorption properties. And the Al, Fe, and V were studied to occupy Si atom [8, 21]. However, there are barely relevant investigations about the relationship of doping and volume expansion, which is extremely important for potential industry application.

Fig. 1

Illustration of double-shell structure for the CO₂ sorption process on Li₄SiO₄

As the Ti–O bond is stronger than the Si–O bond [22], the Li–O bond interaction is decreased in Li₄SiO₄ structure when the Si atom is replaced by Ti atom, and then the Li ion conductivity should be improved. Meanwhile, Mejía-Trejo reported that Na-doped Li₄SiO₄ increased the kinetic reaction of CO₂ absorption [5]. Therefore, in this paper, Ti and Na elements were co-doped into Li₄SiO₄ to enhance the sorption properties. The relationship of doping and volume expansion of Li₄SiO₄ was studied. Additionally, the crystal structure, microstructure, sorption, and cyclability of the doped Li₄SiO₄ were investigated.

Materials and methods

Experiment

Li₄Si_1−x Ti _x O₄ (x = 0, 0.005, 0.01, 0.02, 0.04, 0.06) and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ solid solutions were synthesized by sol–gel process. A flow chart of the process is shown in Fig. 2. Raw materials were lithium nitrate (LiNO₃), tetrabutyl titanate (Ti(OC₄H₉)₄), tetraethyl orthosilicate (Si(OC₂H₅)₄), sodium hydroxide (NaOH), ethanol (CH₃CH₂OH), and citric acid (C₆H₈O₇). All the chemicals were analytical reagent grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Firstly, solution of Ti(OC₄H₉)₄ and CH₃CH₂OH was added dropwise into a stirring aqueous solution of LiNO₃, C₆H₈O₇, and CH₃CH₂OH. After the mixed solution became transparent, a solution of Si(OC₂H₅)₄ and CH₃CH₂OH was mixed with the transparent solution, and then the solution was heated at 50 °C with continuously stirring for several hours till it became a yellowish dry gel. After that, the dry gel was ground and pressed into about 20 mm diametric tablet and calcined at 700 °C for 4 h.

Fig. 2

Flow chart of synthesis of tablet-like Li₄Si_1−x Ti _x O₄ (0 ≤ x ≤ 0.06) and Li_3.9Na_0.1Si_0.96Ti_0.04O₄

Characterization techniques

Thermal behaviors of Li₄SiO₄ dry gel were examined by thermogravimetry–differential thermal analysis (TG–DTA) instrument (STA409C NETZSCH, Germany) from room temperature to 800 °C with 10 °C/min under air for determining the sintering temperature. All samples were characterized by powder X-ray diffraction (XRD, D/Max-RB, RB Rigaku, Japan), and the cell parameters were refined by utilizing MDI Jade 5.0 software. The nitrogen adsorption measurements were carried out using Quadrasorb-18, Quantachrome, USA. The morphologies and particle size of the solid solutions were observed by scanning electron microscopy (SEM, JSM-6480LVJEOL, Japan). CO₂ absorption properties of Li₄Si_1−x Ti _x O₄ (0 ≤ x ≤ 0.06) and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ were carried out by TG–DTA instrument (STA409C NETZSCH, Germany) under CO₂ flux. Additionally, the cyclability of Li_3.9Na_0.1Si_0.96Ti_0.04O₄, Li₄Si_0.96Ti_0.04O₄, and Li₄SiO₄ were measured at 650 °C under CO₂ and N₂ flux (for desorption).

Results and discussions

Characterization

To determine the sintering temperature, TG–DTA curves (Fig. 3) of the Li₄SiO₄ precursor were carried out in air. The curves indicated that the small endothermic peak at about 270 °C might be assigned to the removal of combined water, and two sharp exothermic peaks at about 380 °C and 650 °C could be due to the organics combustion and the Li₄SiO₄ phase formation, respectively. No additional endothermic or exothermic peaks and weight changes were detected above 700 °C, which suggested that the synthesis process had completely finished. Therefore, the Li₄Si_1−x Ti _x O₄ (0 ≤ x ≤ 0.06) and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ dry gel precursor were heat treated at 700 °C for 4 h for further investigation.

Fig. 3

TG and TDA curves of Li₄SiO₄ dry gel precursor in argon

After the series samples were synthesized, the samples were characterized by XRD (Fig. 4a). The x = 0 (Li₄SiO₄) diffraction pattern fitted very well to the JCPDS file 37–1472 (cell parameters: a = 5.297 Å, b = 6.101 Å, c = 5.150 Å, β = 90.251°), which manifested that the sol–gel precursor calcined at 700 °C for 4 h could obtain pure Li₄SiO₄ phase. In this experiment, once the Ti addition started, the intensity of diffraction peak weakened with the increase of Ti content. Meanwhile, the diffraction peaks shifted slightly toward lower angles. These effects indicated that titanium doping inhibited the crystallization of Li₄SiO₄. It might be attributed to the difference in ion radius of Si⁴⁺ (0.42 Å) and Ti⁴⁺ (0.68 Å) and the closed packed structure of the silicate [23, 24]. In Li₄SiO₄ structure, Si locates in tetrahedral site of oxygen. Hence, if the Ti replaced the Si, the cell parameters of doped Li₄SiO₄ would increase, while diffraction angle would decrease by the Bragg’s law (λ = 2dsinθ). Figure 4c showed that the cell parameters (a, b, c, and volume (v)) increased with x value, and the increasing rate of c was bigger than a and b. It implied that Ti might be incorporated into the crystal lattice. However, when the x value was 0.06, tiny amounts of Li₂SiO₃ peaks were detected. Although Li₂SiO₃ absorbs CO₂ as well, the kinetic behavior of CO₂ absorption on Li₂SiO₃ is much slower than that on Li₄SiO₄ [5, 22]. Li₂SiO₃ hardly adsorbed CO₂ below 900 °C [25]. In this case, the doping limitation of Ti into Li₄SiO₄ should be 0.04, Li₄Si_0.96Ti_0.04O₄. Meanwhile, Mejía-Trejo reported that the solubility limit of sodium into Li₄SiO₄ is 0.1, Li_3.9Na_0.1SiO₄ [5]. Hence, NaOH was doped into Li₄SiO₄. In this experiment, Li₄SiO₄ (main phase) and tiny Li₃NaSiO₄ phase were detected (Fig. 4a). We found that the diffraction peaks shifted toward higher angles, and cell parameters a and c decreased about 0.15 and 0.11 %, while the cell parameter b increased about 0.053 %. It indicated that the Na might be incorporated into the crystal lattice, and the difference in atomic radius of Li (2.05 Å) and Na (2.23 Å) might account for the result. In order to understand the sorption process, the samples after absorbing CO₂ at 650 °C for 1 h were examined by XRD (Fig. 4b). Li₂SiO₃ and Li₂CO₃ peaks were found in all absorbed samples and their intensities were stronger with the increase of Ti content. It was ascribed to the fact that the samples absorbed CO₂ to form Li₂SiO₃ and Li₂CO₃, and the reaction may be described as follows.

Fig. 4

XRD patterns of Li₄Si_1−x Ti _x O₄ (0 ≤ x ≤ 0.06) and Li_3.9Na_0.1Si_0.96Ti_0.04O₄; a before absorption; b after absorption; c the relationship of cell parameters and x value

$$ {\text{Li}}_{ 4} {\text{SiO}}_{ 4} {\text{ + CO}}_{ 2} \leftrightarrow {\text{Li}}_{ 2} {\text{SiO}}_{ 3} {\text{ + Li}}_{ 2} {\text{CO}}_{3} $$

(1)

$$ \begin{aligned} {\text{Li}}_{ 4} {\text{Si}}_{1 - x} {\text{Ti}}_{x} {\text{O}}_{4} + {\text{CO}}_{2} \leftrightarrow \left( {1 - x} \right){\text{Li}}_{2} {\text{SiO}}_{3} + {\text{Li}}_{2} {\text{CO}}_{3} + x{\text{Li}}_{2} {\text{TiO}}_{3} \hfill \\ \hfill \\ \end{aligned} $$

(2)

$$ {\text{Li}}_{3.9} {\text{Na}}_{0.1} {\text{Si}}_{0.96} {\text{Ti}}_{0.04} {\text{O}}_{4} + {\text{CO}}_{2} \leftrightarrow 0. 9 5 {\text{Li}}_{2} {\text{CO}}_{3} + 0.05{\text{Na}}_{2} {\text{CO}}_{3} + 0. 9 6 {\text{Li}}_{2} {\text{SiO}}_{3} + 0. 0 4 {\text{Li}}_{2} {\text{TiO}}_{3} $$

(3)

However, no Na₂CO₃ and Li₂TiO₃ peaks were identified in the XRD patterns as its abundance is beyond the XRD resolution.

The morphologies of the unabsorbed and absorbed samples are shown in Fig. 5. As observed in Fig. 5a, all the unabsorbed tablet samples had a loose and porous microstructure. Table 1 shows the surface area, pore size, and pore volume of Li₄Si_1−x Ti _x O₄. The surface area and total pore volume increased with x value, and the pore size decreased in some degree, indicating that the dopant would hinder the growth of grains. During sintering process, the combustion products (CO₂ and H₂O steam) of the dry gel precursor moving from the interior to the exterior could be responsible for the loose and porous texture. Knitter reported that Ti doping could enhance the strength of Li₄SiO₄ [22]. Hence, these ceramic pores might offer firm tunnel for CO₂ carriage, which might be better than the tiny and unstable pores of powder adsorbent as the latter may be blocked up after few absorption cycles [18]. In addition, we found that the grain size decreased with Ti content increasing (Fig. 5a). The average size of grains abated from 3 μm (Li₄SiO₄) to 0.23 μm (Li₄Si_0.96Ti_0.04O₄). It seemed that Ti inhibited the growth of the particles, which is consistent with the results of Table 1. Some papers have reported that small particles have high surface area and better chemisorb efficiencies [10, 26, 27]. However, secondary phase was found in Li₄Si_0.96Ti_0.06O₄, and the EDS images (Fig. 5a(c)) show that the O/Si ratio of the polyhedral shape gray phase was higher (4.43) than the bright dendritic phase (2.93). It indicated that the gray phase and the secondary phase should be Li₄SiO₄ and Li₂SiO₃, respectively, which was also proved by XRD pattern (Fig. 4a). Figure 5b shows the microstructure of the absorbed samples. In all samples, white agglomerate particles covered the light dark phase, and it increased with the x value.

Fig. 5

The microstructure of Li₄Si_1−x Ti _x O₄ (0 ≤ x ≤ 0.06); a before absorption; b after absorption; c EDS patterns of x = 0.06

Table 1 Parameters of Li₄Si_1−x Ti _x O₄ (0 ≤ x ≤ 0.06)

The CO₂ absorbent expansivity α, α = (M − M ₀)/M ₀, where the M and M ₀ are the radius (R, R ₀), high (H, H ₀) or volume (V, V ₀) of absorbed and unabsorbed samples, respectively, is extremely important for industry application. In this experiment, we found that the absorption expansion ratio of Li₄SiO₄ was so big that a quartz crucible was cracked by its expansion force. Therefore, the relationship between doping and absorbent expansivity α was investigated for the first time. All samples performed at 650 °C under 160 ml/min CO₂ and 200 ml/min N₂ (desorption gas) for 10 cycles. As can be seen in Fig. 6, the α _R, α _H, and α _V declined sharply as the Ti content increased, and when the x ≥ 0.04, the α almost reached its limitation. The α _V of Li₄SiO₄ was about 22 %, while the Li₄Si_0.96Ti_0.04O₄ was only about 5.6 %, which almost decreased four times. The stable Ti–O bond can be responsible for these results. Therefore, Ti-doped Li₄SiO₄ could decrease the expansivity.

Fig. 6

The relationship between CO₂ absorbent expansivity and Ti doping

Investigation of general sorption behavior

The enthalpy changes (ΔH), entropy changes (ΔS), and free energy changes (ΔG) of Li₄SiO₄ absorption reaction were calculated using HSC chemistry 5.11 software (Table 2). Under 720 °C, all ΔG values are <0, revealing that Li₄SiO₄ can absorb CO₂ from room temperature to 720 °C. Figure 7 illustrates the CO₂ sorption behaviors of synthesized samples. All the samples appeared to display similar absorption behaviors. The weight increased slowly from around 200 °C and sharply at about 500 °C. And then it reached a peak at about 650 °C, while over 730 °C the absorption process almost finished. However, around 400 °C, a lightly weight loss was detected. Meanwhile, the weight increased with the x value except x = 0.06. To further understand the absorption process, all the samples were tested at 200, 300, 400, 500, 550, and 650 °C under 160 ml/min CO₂ for 3 h (Fig. 8).

Table 2 Calculated ΔH, ΔS, and ΔG for the Li₄SiO₄ absorption reaction

Fig. 7

CO₂ sorption behaviors of Li₄Si_1−x Ti _x O₄ (x = 0, 0.005, 0.01, 0.02, 0.04, and 0.06) and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ under 160 ml/min CO₂ from room temperature to 880 °C with a heating rate of 10 °C/min

Fig. 8

CO₂ sorption behaviors of Li₄Si_1−x Ti _x O₄ at 200, 300, 400, 500, 550, and 650 °C under 160 ml/min CO₂

As can be seen in Fig. 8, all the weight increased with the x value and temperature except at 400 °C. At relative low temperature (200–400 °C), the increase in tendency showed a linear trend, however, over 500 °C, it increased sharply in a short time to a plateau. These results indicated that the absorption process had two different principles, i.e., initially, at relative low temperature, the CO₂ chemisorption occurred over the absorbent surface forming an external shell, which resisted the Li⁺, O²⁻, and CO₂ diffusion and hindered the further reaction [12]. Nevertheless, at high temperature, the chemisorption can be reactivated as the Li⁺, O²⁻, and CO₂ obtained sufficient driving force throughout the bulk of the external shell [5]. However, at 400 °C, the weight was more minor than at 300 °C. Similar behaviors have been reported on Li_4−x (Si_1−x Al _x )O₄ and Na₂(Zr_1−x Al _x )O₃ at 400 and 300 °C, respectively [18, 21], which further provided evidence for a superficial chemisorption–desorption equilibrium. Therefore, the superficial desorption process does not seem to be modified by the Ti doping. However, it improved the absorption performance. At 300 °C, the weight increased from 7.3 wt% (x = 0) to 8.6 wt% (x = 0.06). Continuous decreasing grain size caused by Ti doping (Fig. 5a) may account for the changes.

Figure 9 shows Li₄Si_1−x Ti _x O₄ and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ absorption performance at 650 °C. As can be seen, Li₄Si_0.96Ti_0.04O₄ and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ showed a significant improvement on the absorption. It was up to 20.1, 30.4, and 31.6 wt% for Li₄SiO₄, Li₄Si_0.96Ti_0.04O₄, and Li_3.9Na_0.1Si_0.96Ti_0.04O₄, respectively. For Li_4−x (Si_1−x Al _x )O₄ series, a slight decline was found when x = 0.06. The decrement should be attributed to the presence of secondary phase Li₂SiO₃ (Fig. 4a) which is not as good as Li₄SiO₄ in terms of CO₂ absorption kinetic behavior [13, 26]. And the augment of Li_4−x (Si_1−x Ti _x )O₄ series may be due to the enhancement of Li⁺ diffusion and the decrease of grain size by doping Ti. For Li_3.9Na_0.1Si_0.96Ti_0.04O₄, the Ti and Na incorporated into the crystal lattice should be responsible for the augment as they could enhance Li⁺ and O²⁻ diffusion by forming lattice distortion (Fig. 4c), dislocations, interstitial atoms, and vacancies. Meanwhile, we found that the chemisorption time at 650 °C, exponential curves reached their plateaus, decreased with x value increasing (15 min, Li₄Si_0.96Ti_0.04O₄) since the grain size diminished (Fig. 5a).

Fig. 9

CO₂ sorption behaviors of Li₄Si_1-x Ti _x O₄ and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ at 650 °C under 160 ml/min CO₂

In order to evaluate the regeneration properties and the thermal stability, Li₄SiO₄, Li₄Si_0.96Ti_0.04O₄, and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ were exposed to CO₂ (160 mL/min) for 1 h and then to N₂ (200 mL/min) for 40 min at 650 °C (Fig. 10). The results indicated that the chemisorption capacity increased with the increasing of cycle numbers. For Li_3.9Na_0.1Si_0.96Ti_0.04O₄, the chemisorption capacity reached 32.5 wt% for the first cycle, and after 10 cycles it stabilized at 33.1 wt%. In contrast, for Li₄SiO₄ and Li_3.9Na_0.1Si_0.96Ti_0.04O₄, the chemisorption, after 10 cycles, stabilized to 29.2 and 32.6 wt%. Therefore, Li_3.9Na_0.1Si_0.96Ti_0.04O₄ had better regeneration properties due to Na and Ti co-doping. However, in the first two and three cycles, a slightly decrement was detected. A particle sintering effect may account for these changes as it decreased the surface area [18]. Meanwhile, we found that the saturate time slightly decreased with cycle numbers. These behaviors may be related to the decreased grains as the chemisorption produced tiny agglomerate lithium-containing oxide particles (Fig. 5b).

Fig. 10

Multi-cycle performance of CO₂ chemisorption/desorption on Li₄SiO₄, Li₄Si_0.96Ti_0.04O₄, and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ at 650 °C

It is worth mentioning that all the samples for absorption performance testes were in the form of pills. Hence, the loose and porous Li_3.9Na_0.1Si_0.96Ti_0.04O₄ pills prepared by sol–gel process could be used as a high-temperature CO₂ absorbent.

Conclusions

Tablet-like Li₄Si_1−x Ti _x O₄ and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ sorbents were prepared by a sol–gel process. The analysis performed by XRD suggests that Ti and Na are incorporated to the Li₄SiO₄ structure, and the solubility limit of Ti was 0.04, Li₄Si_0.96Ti_0.04O₄. The SEM analysis indicated that the Ti doping not just decreased the grain sizes but abated the volume expansion of pure Li₄SiO₄. Li₄Si_1−x Ti _x O₄ and Li_3.9Na_0.1Si_0.96Ti_0.04O₄ were able to chemisorb CO₂ from 200 to 700 °C, exhibiting higher CO₂ absorption performance than pure Li₄SiO₄. The isothermal analyses suggested that the absorption process had two principles: (1) the chemisorption occurred over the absorbent surface forming an external shell at low temperature phase (200–400 °C), (2) the chemisorption controlled by Li⁺, O²⁻, and CO₂ diffusion processes. The absorption capacities of Li₄SiO₄ at 650 °C, 30.4 and 31.6 wt% for Li₄Si_0.96Ti_0.04O₄ and Li_3.9Na_0.1Si_0.96Ti_0.04O₄, respectively, were improved by Ti doping and Na and Ti co-doping. The augments could be attributed to that Ti and Na incorporated into the crystal lattice and enhanced Li⁺ and O²⁻ diffusion. The cyclic experiments indicated that Li_3.9Na_0.1Si_0.96Ti_0.04O₄ exhibited better CO₂ capture behaviors than Li₄Si_0.96Ti_0.04O₄.