Performance of Li₄SiO₄ Material for CO₂ Capture: A Review

Abstract

Lithium silicate (Li₄SiO₄) material can be applied for CO₂ capture in energy production processes, such as hydrogen plants, based on sorption-enhanced reforming and fossil fuel-fired power plants, which has attracted research interests of many researchers. However, CO₂ absorption performance of Li₄SiO₄ material prepared by the traditional solid-state reaction method is unsatisfactory during the absorption/regeneration cycles. Improving CO₂ absorption capacity and cyclic stability of Li₄SiO₄ material is a research highlight during the energy production processes. The state-of-the-art kinetic and quantum mechanical studies on the preparation and CO₂ absorption process of Li₄SiO₄ material are summarized, and the recent studies on the effects of preparation methods, dopants, and operating conditions on CO₂ absorption performance of Li₄SiO₄ material are reviewed. Additionally, potential research thoughts and trends are also suggested.

1. Introduction

The emission of anthropogenic CO₂ into the environment has aggravated the trend of global warming [1], which has become one of the most threatening problems in recent decades, and the largest emission sources of CO₂ are fossil fuel-fired power plants [2]. Hence, various techniques have been reported to capture CO₂ from the flue gas released from fossil fuel-fired power plants [3], and CO₂ capture and storage (CCS) has been recognized as one of the most effective techniques to mitigate CO₂ emission [4,5]. In the process of CCS, CO₂ is captured from flue gas and stored for utilization and sequestration instead of being released to the environment directly. Recent studies have found that various lithium-based materials, such as LiFeO₂ [6], Li₂CuO₂ [7], Li₂ZrO₃ [7,8,9], Li₈SiO₆ [10,11], and Li₄SiO₄ [9], are capable of effective CO₂ capture. Among these materials, Li₄SiO₄, with a variety of applications [12,13], has better application potential, owing to its higher CO₂ sorption capacity, cyclic stability than LiFeO₂, Li₂CuO₂, and Li₈SiO₆, and lower cost than that of Li₂ZrO₃ [9]. Additionally, the regeneration temperature of Li₄SiO₄ material is much lower compared with the calcium-based CO₂ sorbents, indicating that lower energy consumption is required for the regeneration. Li₄SiO₄ material is usually obtained by the solid-state reaction method with Li₂CO₃ and SiO₂ at high temperature, which is shown in Equation (1) [14]:

$$2\mathrm{L}{\mathrm{i}}_2\mathrm{C}{\mathrm{O}}_3+\mathrm{S}\mathrm{i}{\mathrm{O}}_2\leftrightarrow \mathrm{L}{\mathrm{i}}_4\mathrm{S}\mathrm{i}{\mathrm{O}}_4+2\mathrm{C}{\mathrm{O}}_2$$

(1)

The basic reversible reaction for CO₂ sorption by Li₄SiO₄ material follows Equation (2), and the process is shown in Figure 1. In the absorption reactor, CO₂ in flue gas from fossil fuel-fired power plants or syngas from hydrogen plants based on sorption-enhanced reforming is absorbed by Li₄SiO₄ at 500 to 600 °C, thus the gas, almost free of CO₂, is exhausted from the reactor. The generated Li₂SiO₃ and Li₂CO₃ are transported to the regeneration reactor, where Li₄SiO₄ is regenerated at temperatures higher than 700 °C and sent to the absorption reactor for the next CO₂ absorption cycle, and CO₂-rich gas can be obtained in the regeneration reactor.

$$\mathrm{L}{\mathrm{i}}_4\mathrm{S}\mathrm{i}{\mathrm{O}}_4+\mathrm{C}{\mathrm{O}}_2\leftrightarrow \mathrm{L}{\mathrm{i}}_2\mathrm{S}\mathrm{i}{\mathrm{O}}_3+\mathrm{L}{\mathrm{i}}_2\mathrm{C}{\mathrm{O}}_3$$

(2)

It can be calculated according to Equation (2) that the theoretical CO₂ absorption capacity of Li₄SiO₄ is 367 mg CO₂/g Li₄SiO₄, which is much higher than that of Li₂ZrO₃ (125 mg/g) [9].

Additionally, CO₂ absorption by Li₄SiO₄ material can also contribute to sorption-enhanced hydrogen production, as shown in Figure 1, where methane and ethanol are usually selected [15]. In the process of methane or ethanol reforming, CO₂ is a necessary while undesired product. With in situ CO₂ absorption by Li₄SiO₄ material, the concentration of CO₂ in syngas can be reduced, and the reaction equilibrium of reforming can be shifted to hydrogen production simultaneously, thus the CO₂ absorption capacity of Li₄SiO₄ material is the key factor to determine the hydrogen production efficiency [16,17]. This section will be discussed in detail in Section 6.

However, the CO₂ absorption capacity and cyclic stability of pristine Li₄SiO₄ material prepared by the solid-state reaction method is low, which is mainly due to the smooth surface of pristine Li₄SiO₄ particles generated at high temperature, thus the surface area and pore volume of Li₄SiO₄ material are low, and the reaction between CO₂ and Li₄SiO₄ is limited [18]. Therefore, a large number of works have been conducted to improve the pore structure of Li₄SiO₄, such as the application of organic precursors, which is conducive to the formation of pores, and doping with eutectic salts, which is favorable for the decrease of CO₂ diffusion resistance [19,20].

Since CO₂ absorption by Li₄SiO₄ material was firstly reported, abundant studies have been revealed the reaction mechanism and improved the cyclic CO₂ absorption performance [21]. This work introduces the latest research progress on CO₂ absorption performance of Li₄SiO₄ material. In addition, thermodynamic and kinetic comprehension of the reaction between CO₂ and Li₄SiO₄ are illustrated, and strategies to enhance the cyclic CO₂ absorption performance of Li₄SiO₄ material are summarized. Additionally, applications of Li₄SiO₄ material in sorption-enhanced hydrogen production are reviewed, and studies on CO₂ absorption by Li₄SiO₄ material at the molecular scale are also reviewed briefly. Finally, the major drawback that hinders the large-scale application of Li₄SiO₄ material for CO₂ absorption is introduced.

2. Thermodynamics and Kinetics of CO₂ Absorption by Li₄SiO₄

2.1. Reaction Model for Synthesis of Li₄SiO₄

Li₄SiO₄ material is usually synthesized by the solid-state reaction method, and the preparation process is illustrated by Equation (1) [22,23], and a core-shell model was suggested for the solid-state reaction between Li₂CO₃ and SiO₂, which is shown in Figure 2. In the first step, Li₂CO₃ reacts with SiO₂ at their contact part, and a thin Li₂SiO₃ layer is formed. Li₂SiO₃ is the intermediate product, which continues to react with Li₂CO₃ to form Li₄SiO₄ eventually. Li₄SiO₄ and Li₂SiO₃ layers become thicker with the reaction, and internal SiO₂ is covered by the layers in the meantime. Thus, Li⁺ and O²⁻ must diffuse through the product layer before contacting with internal SiO₂, and it is the limited step that limiting the synthesis of Li₄SiO₄, because the diffusion process is much slower than the reaction. Consequently, alternative synthesis methods and precursors for the synthesis of Li₄SiO₄ have been reported, which will be discussed in the following section [24].

2.2. Kinetic Study for CO₂ Absorption by Li₄SiO₄

Figure 3 shows the CO₂ absorption performance of Li₄SiO₄ materials prepared by the solid-state reaction method and the sol-gel method in a thermogravimetric analyzer (TGA) [25], and the CO₂ absorption amount (mg CO₂/g sorbent) was used to evaluate the CO₂ absorption capacity of Li₄SiO₄ materials, which is calculated according to Equation (3):

$$C_N=\frac{m_2-m_1}{m_1}$$

(3)

where C_N is the amount of CO₂ absorbed by the Li₄SiO₄ material, mg/g; N represents the number of cycles; m₁ represents the initial mass of Li₄SiO₄ material, g; and m₂ represents the mass of the sample during CO₂ absorption, mg. The CO₂ absorption stage of Li₄SiO₄ occurs at temperatures lower than 400 °C, and the CO₂ absorption rate increases suddenly when the temperature reaches around 500–600 °C. Weight losses of two Li₄SiO₄ materials are observed when the temperature exceeds 720 °C, indicating the reaction converts to the regeneration of Li₄SiO₄ materials, and the regeneration reaction is much faster than the absorption process. As shown in Figure 3, the CO₂ absorption capacity of Li₄SiO₄ material prepared by the sol-gel method is higher than that prepared by the solid-state reaction method, which will be discussed in Section 3.3.3.

2.3. Thermodynamic Study for CO₂ Capture by Li₄SiO₄

Figure 4 shows the equilibrium partial pressure of CO₂ over Li₄SiO₄ material as a function of temperature [26], and the maximum temperature of CO₂ absorption by Li₄SiO₄ material is determined by the corresponding CO₂ partial pressure. When CO₂ partial pressure is 100% at 1 atm, it can be inferred from Figure 4 that the equilibrium temperature is around 715 °C, which agrees well with the results that discussed above. The corresponding temperature of CO₂ absorption by Li₄SiO₄ material can be determined by the CO₂ partial pressure, and CO₂ absorption by Li₄SiO₄ material occurs when the temperature is lower than the equilibrium temperature, otherwise Li₄SiO₄ material is regenerated. As a result, CO₂ absorption and regeneration regions of Li₄SiO₄ material are divided by the equilibrium line.

2.4. Reaction Mechanism and Reaction Model of CO₂ Capture by Li₄SiO₄

The double-shell mechanism is regarded as the most appropriate model for the reaction between CO₂ and Li₄SiO₄ [27], which is schematically illustrated in Figure 5. At the beginning of the reaction, CO₂ molecules react with Li₄SiO₄ particles to generate a double shell composed of Li₂CO₃ and Li₂SiO₃, which covers the internal Li₄SiO₄. Then the reactants diffuse through the double shell to continue the reaction, and the thickness of the double shell increases as the reaction proceeds. Thus, the second stage is much slower than the first stage, owing to the high diffusion resistance of the reactants. Therefore, decreasing the diffusion resistance is conducive to the reaction between CO₂ and Li₄SiO₄. The presence of steam and doping of molten salts are believed to reduce the diffusion resistance in the double shell, which will be discussed in the following section. Additionally, the shrinking core model and the unreacted core model were well-reported in many studies, which are also involved with the external product shell and internal unreacted core, and the models are similar to that of the double-shell model.

TGA curves of CO₂ absorption by Li₄SiO₄ at various temperatures are present in Figure 6, and weight gain of Li₄SiO₄ was used to evaluate its CO₂ absorption performance, which can be calculated according to Equation (4):

$$W_N=(C_N+1)\times 100\%$$

(4)

where W_N is the weight gain of Li₄SiO₄ material during the Nth cycles, wt.%; and C_N is the amount of CO₂ absorbed by Li₄SiO₄ material during the Nth cycles, mg/g. CO₂ absorption capacity of Li₄SiO₄ increases with the temperature rising from 460 to 560 °C. Li₄SiO₄ shows a fast CO₂ absorption stage in a short time and a slow CO₂ absorption stage in the following long time, which are controlled by the chemical reaction and diffusion, respectively [28]. Most of the TGA curves are fitted to the double exponential model, which is shown in Equation (5):

$$y=A{\exp }^{-k_{1}t}+B{\exp }^{-k_{2}t}+C$$

(5)

where y represents the weight gain of Li₄SiO₄ material after CO₂ absorption; k₁ and k₂ denote two exponential constants for the chemical reaction-controlled stage and the diffusion-controlled stage, respectively; and two pre-exponential factors A and B are the intervals that control the corresponding stages [28].

Table 1. Kinetic parameters of reaction between Li₄SiO₄ and CO₂ for double exponential model [28].

T (°C)	k1(sec−1)	k2(sec−1)	A	B	C
460	8.0 × 10−4	1.4 × 10−4	−3.248	−5.560	108.8
480	1.72 × 10−3	1.3 × 10−4	−3.467	−7.821	111.2
500	2.96 × 10−3	1.7 × 10−4	−3.716	−8.668	112.3
520	4.26 × 10−3	2.1 × 10−4	−4.231	−8.711	112.8
540	4.17 × 10−3	2.1 × 10−4	−4.983	−9.003	113.8
560	4.27 × 10−3	2.5 × 10−4	−9.017	−8.911	116.6

presents the kinetic parameters of the double exponential model fitted to the reaction between CO₂ and Li₄SiO₄ [28]. As presented in Table 1, the values of k₁ are usually one order of magnitude higher than those of k₂, and B are always larger than A, indicating that CO₂ absorption over the surface of Li₄SiO₄ controlled by chemical reaction is a rapid process, and CO₂ absorption controlled by diffusion occurs in a large interval of time. Thus, CO₂ absorption controlled by diffusion is the limiting step hindering the absorption of CO₂ by Li₄SiO₄ [29,30].

Although the double exponential model is widely used due to its simplicity, Ortiz et al. [26] thought that this model was short of the theoretical mechanism to support its fitting with the experimental data. Zhang et al. [27] reported that the Avrami–Erofeev model was relevant to the reaction mechanism of the formation and growth of product crystals, which are shown as Equations (6) and (7):

$$\mathrm{d}\alpha /\mathrm{d}t=Kn(1-\alpha ){[-\ln (1-\alpha )]}^{(n-1)/n}$$

(6)

$$\ln [-\ln (1-\alpha \left)\right]=\ln k+n\ln t$$

(7)

where α refers to the degree of conversion; K denotes the kinetic constant; k equals to Kⁿ; and n is the kinetic parameter; t represents the time. Equation (7) is an equation of a straight line with slope n in the coordinates ln (−ln (1 − α)) vs. ln t. If the value of n is higher than 1, the absorption reaction is controlled by the formation and growth of product crystals. When n equals to 0.5 approximately, the absorption reaction is controlled by the diffusion of ions [31].

As illustrated in Figure 7, the curves of Avrami–Erofeev model look similar to TGA curves obtained from 550 to 700 °C, and the rapid chemical reaction-controlled stage and the slow diffusion-controlled stage can be easily distinguished at every temperature. Additionally, Zhang et al. [27] reported that the Avrami–Erofeev model suited the regeneration process of Li₄SiO₄ material, and the entire regeneration process was controlled by the rate of the formation and growth of product crystals, which was also confirmed by Xiang et al. [32]. Thus, the Avrami–Erofeev model is more suitable for CO₂ absorption by Li₄SiO₄.

3. Synthesis of Li₄SiO₄ Materials with Superior Cyclic Absorption/Regeneration Performance

It is clear that Li₄SiO₄ material synthesized by the solid-state reaction method from SiO₂ and Li₂CO₃ achieves low CO₂ absorption capacity, due to the low porosity of Li₄SiO₄ generated at high temperatures during the preparation. Thus, the CO₂ absorption capacity of Li₄SiO₄ decreases rapidly with the number of cycles, which is the main disadvantage of Li₄SiO₄ material in industrial applications. The various methods have been reported to enhance the cyclic performance of Li₄SiO₄ material prepared by solid-state reaction method, such as hydration [33] and ball milling [34,35]. The strategies to enhance the cyclic performance of Li₄SiO₄ can be categorized as follows: (i) reducing the diffusion resistance by adding solid solutions or molten salts; (ii) using alternative precursors; and (iii) using a more appropriate synthesis method.

3.1. Modification for Li₄SiO₄ Prepared by Solid-State Reaction

As discussed above, high temperature during the preparation by the solid-state reaction method of Li₄SiO₄ leads to sintering, and the core of Li₄SiO₄ usually cannot react with CO₂. Thus, Yin et al. [33] proposed a hydration process to improve the pore structure of Li₄SiO₄ material. First, Li₄SiO₄ material was prepared by the solid-state reaction method, and then distilled water was added to the samples and stirred at 80 °C for 4 h. They reported that dense particles formed during the solid-state reaction preparation could be split into fine particles, so the porous structure and high cyclic CO₂ absorption capacity of Li₄SiO₄ was obtained by hydration process.

Romero-Ibarra et al. [34] used the ball milling process to modify the surface properties of Li₄SiO₄ material, and they found that the ball milling process decreased the particle size and improved the surface area of Li₄SiO₄ material. Kanki et al. [35] reported that the ball milling process could promote CO₂ absorption of Li₄SiO₄ material at lower temperatures, and longer ball milling duration led to higher CO₂ absorption capacity. Additionally, the doping of K₂CO₃ in Li₄SiO₄ material improved its CO₂ absorption capacity under short ball milling duration.

3.2. Doping of Solid Solutions or Molten Salts

3.2.1. Solid Solutions

The CO₂ absorption rate of Li₄SiO₄ material is mainly controlled by the diffusion of ions and CO₂. Zhao et al. [36] reported that solid solution usually formed with the doping of Al₂O₃ during solid-state preparation, thus increased oxygen vacancies could promote the diffusion in the product layer. Ortiz-Landeros et al. [37] reported that Al₂O₃ addition and ball milling could extend the range of CO₂ absorption temperature. In addition, Ortiz-Landeros et al. [38] compared Li_4+x(Si_1−xAl_x)O₄ with Li_4−x(Si_1−xV_x)O₄ as the solid solutions, and the results showed that diffusion resistance of CO₂ and ions in Li_4+x(Si_1−xAl_x)O₄ was diminished, while the presence of V was adverse to the diffusion through the product layer.

3.2.2. Molten Salts

The doping of alkali metals, such as Na and K, could produce a layer of molten salts with low eutectic temperature, which reduced diffusion resistance effectively, thus the limiting step of Li₄SiO₄ material for CO₂ absorption could be resolved. The CO₂ absorption performance of various alkali metal-doped Li₄SiO₄ materials is summarized in

Table 2. Summary of CO₂ absorption performance of Na and K doped Li₄SiO₄ materials.

Materials	Molar Ratio of Li/Alkali Metal	Preparation Method	Absorption Conditions	Regeneration Conditions	Cycle No.	Weight Increase (wt.%)	Refs.
SiO2, Li2CO3, K2CO3	10.5:1	Solid-state reaction	4% CO2/N2; 580 °C; 60 min	N2; 800 °C; 10 min	4	24	[39]
SiO2, Li2CO3, K2CO3	10.83:1	Solid-state reaction	4% CO2/N2; 580 °C; 60 min	N2; 700 °C; 15 min	25	16	[40]
SiO2, Li2CO3, K2CO3	17.6:1	Solid-state reaction	CO2; 575 °C; 25 min	N2; 700 °C; 50 min	10	28	[41]
SiO2, Li2CO3, K2CO3	17.6:1	Solid-state reaction	CO2; 650 °C; 15 min	90% H2O/N2; 650 °C; 15 min	22	21	[42]
SiO2, CH3COOLi, K2CO3	43.7:1	Impregnated suspension	15% CO2/N2; 550 °C; 30 min	N2; 750 °C; 10 min	40	315	[43]
SiO2, CH3COONa, Na2CO3	16:1	Impregnated suspension	15% CO2/N2; 550 °C; 30 min	N2; 750 °C; 10 min	40	28	[43]
SiO2, Li2CO3, Na2CO3	49:1	Solid state	CO2; 700 °C; 30 min	Air; 900 °C; 30 min	5	32	[44]
SiO2, LiNO3, NaF	41:1	Template	15% CO2/N2; 600 °C; 35 min	N2; 700 °C; 20 min	10	31	[45]
SiO2, LiNO3, NaCl	133:1	Solid state with hydration	15% CO2/N2; 575 °C; 40 min	N2; 700 °C; 10 min	10	32	[46]

[39,40,41,42,43,44,45,46].

As presented in Table 2, Na and K were the most commonly reported alkali metals to enhance the CO₂ absorption performance of Li₄SiO₄ material. In order to determine the most appropriate doping method for K₂CO₃, Seggiani et al. [39] compared eutectic doping and simple mechanical addition of K₂CO₃, and they found that Li₄SiO₄ particles obtained from mechanical addition were smaller, as shown in Figure 8, so the mechanical doping method may be more appropriate for the doping of K₂CO₃. Olivares-Marín [47] et al. synthesized K₂CO₃-doped Li₄SiO₄ material with fly ash as the silicon precursor, and they reported that the CO₂ absorption capacity of the prepared Li₄SiO₄ material increased with the increase of the dopant amount. It is also worth noting that Zhang et al. [42] reported that the K₂CO₃ doped Li₄SiO₄ material cooperated well with the Ni/γ-Al₂O₃ catalyst in the sorption-enhanced steam methane reforming (SE-SMR) system, and high-purity hydrogen (>95%) could be obtained at lower temperatures ranging from 500 to 550 °C, and the presence of steam in the regeneration atmosphere could improve the reaction rate obviously. Mejía-Trejo et al. [48] prepared Na-doped Li₄SiO₄ material by doping Na₂CO₃ into the starting materials of TEOS and Li₂CO₃ through the co-precipitation route, and they noted that the addition of Na₂CO₃ increased the activity and reduced the equilibrium temperature of Li₄SiO₄ material for CO₂ absorption, and Li_3.85Na_0.15SiO₄ had the highest CO₂ absorption capacity among various Na-doped Li₄SiO₄ materials. Seggiani et al. [40] noted that dopants like K₂CO₃ and Na₂CO₃ could form eutectic mixtures with Li₂CO₃, which melted at high temperatures (>500 °C), so the diffusion of ions and CO₂ was enhanced in the diffusion-controlled stage. Yang et al. [43] reported that orderly crystalline arrangement of Li₄SiO₄ was broken by doped K₂CO₃ and Na₂CO₃ for their different crystal sizes, thus more pores and larger specific surface area were generated.

3.2.3. Other Dopants

Wang et al. [49] prepared K-, Mg-, Cr-, and Ce-doped Li₄SiO₄ and found that Ce was the most difficultly doped into the Li₄SiO₄ crystal lattice among the four elements. However, Ce was the most effective to inhibit the aggregation of Li₄SiO₄ grains, so Ce-doped Li₄SiO₄ achieved the highest CO₂ absorption performance. Subha et al. [50] studied the CO₂ absorption by Li₄SiO₄ material doped with Y₂O₃, Gd₂O₃ or LaPO₄, and found that both Y₂O₃ and Gd₂O₃ improved the CO₂ absorption capacity of Li₄SiO₄, and Y₂O₃-doped Li₄SiO₄ retained the highest CO₂ absorption capacity due to the segregation of second phase created by the doped unreacted Y₂O₃. Chen et al. [51] reported that Ca-doped Li₄SiO₄ material achieved high CO₂ absorption capacity and they proposed a modified double-shell mechanism to describe the CO₂ absorption and regeneration mechanism of Ca-doped Li₄SiO₄ as shown in Figure 9. The transformation of Ca₂SiO₄ to Li₂CaSiO₄ during CO₂ absorption process was beneficial of transferring CO₂ from Li₄SiO₄ surface to the core, which reduced the diffusion resistance and improved CO₂ absorption, and regeneration was also correspondingly enhanced.

Additionally, doping of organic matter can also enhance the CO₂ absorption property of Li₄SiO₄ material due to the formation of the porous structure. Wang et al. [30,52] prepared carbon-coated Li₄SiO₄ material by the sol-gel method, and gluconic acid and citric acid were used as the complexing agents, respectively. During the carbonization stage, gluconic acid and citric acid decomposed, and a mesoporous carbon coating covered the surface of Li₄SiO₄ material, which suppressed the growth of Li₄SiO₄ grains. As a result, the cyclic CO₂ absorption capacities and rates of carbon-coated Li₄SiO₄ materials were higher than that of uncoated Li₄SiO₄ during multiple absorption/regeneration cycles. Furthermore, CMK-3, as a kind of porous carbon material [53], was also introduced into the Li₄SiO₄ material. Jeoung et al. [54] prepared CMK-modified Li₄SiO₄, while the cyclic absorption capacity of CMK-modified Li₄SiO₄ decreased obviously with the number of cycles.

It has been reviewed in this part that doping of metal elements, such as K, Na, Ca, Ce, Y, Al, or organic matters, can enhance the CO₂ absorption capacities of Li₄SiO₄ material. The limitation in the diffusion-controlled stage for Li₄SiO₄ is reduced greatly with the doping of solid solution or molten salts, and the porous structure of Li₄SiO₄ by doping of organic matters is obtained. The additive amounts are minor, but the CO₂ absorption performance of Li₄SiO₄ can be greatly enhanced.

3.3. Selection of Alternative Precursors for Preparation of Li₄SiO₄

Li₄SiO₄ material is usually prepared from Li₂CO₃ and SiO₂, which are not able to create a favorable surface characteristic for CO₂ absorption. Recent studies have shown that Li₄SiO₄ materials prepared from alternative precursors, especially organic precursors, rather than Li₂CO₃ and SiO₂ achieve high CO₂ absorption capacities and cyclic stability. In this section, the effects of precursors on CO₂ absorption capacity of Li₄SiO₄ material are summarized.

3.3.1. Lithium Precursors

Kim et al. [29] synthesized Li₄SiO₄ material from LiOH and fumed silicate by the solid-state reaction method. They reported that the synthesis temperature could be reduced to 600 °C due to the use of LiOH, and the obtained Li₄SiO₄ showed higher CO₂ absorption capacity compared with those synthesized at 700 °C and 800 °C, which achieved 298 mg/g after 10 cycles. Wang et al. [55] synthesized Li₄SiO₄ with LiOH by the sol-gel technique and they found that LiOH-synthesized Li₄SiO₄ particles were primarily composed of porous grains, and the average grain size of Li₄SiO₄ prepared by the sol-gel method was much smaller than that synthesized by the solid-state reaction method.

Weng et al. [56] synthesized Li₄SiO₄ from LiNO₃ as lithium precursor and tetraethyl orthosilicate (TEOS) as a silicon precursor by the sol-gel method. The CO₂ absorption capacity of the obtained Li₄SiO₄ material increased with increasing temperature from 400 to 500 °C in 2% CO₂. Bretado et al. [14] reported that the solid-state reaction method was more appropriate than the sol-gel method for the preparation of Li₄SiO₄ material when LiNO₃ was used as the lithium precursor. However, Subha et al. [25] reported that the sol-gel method was superior to the solid-state reaction method for Li₄SiO₄ material prepared from LiNO₃ and colloidal silica. This indicates that the most appropriate synthesis method depends on the lithium and silicon precursors simultaneously.

Compared with inorganic lithium precursors, organic lithium-containing materials seems more appropriate as the lithium precursor for the preparation of Li₄SiO₄ material. Yang et al. [19] used lithium acetate and lithium lactate to prepare novel Li₄SiO₄ materials by the impregnated suspension method. As shown in Figure 10, the two novel Li₄SiO₄ materials showed a bulgier morphology and more porous structure, compared with Li₄SiO₄ synthesized by the solid-state reaction method. Absorption capacities of Li₄SiO₄ material prepared from lithium acetate or lithium lactate as the lithium precursors were almost six times higher than that of a conventional Li₄SiO₄ material. Additionally, the CO₂ absorption capacities and conversions of Li₄SiO₄ material prepared from lithium acetate or lithium lactate showed an incremental tendency over 40 cycles, and the conversion of Li₄SiO₄ prepared from lithium acetate was approximately 70% even in the last cycle, which was calculated according to Equation (8):

$$X_N=\frac{C_N}{m_0}$$

(8)

where X_N is the conversion of Li₄SiO₄ during the Nth cycle, %; and m₀ is the theoretical CO₂ absorption capacity of Li₄SiO₄ material, which is 367 mg/g. Lee et al. [57] used Li and a Si-containing metal-organic framework (MOF) as the silicon precursor, and the prepared Li₄SiO₄ material was able to convert into Li₄SiO₄ thermally. The as-prepared material had a coral-like morphology, so the contact area between CO₂ and Li₄SiO₄ material was enhanced, and the Li₄SiO₄ material showed higher CO₂ absorption capacity than that prepared by the conventional solid-state reaction method.

In this section, CO₂ absorption performance of Li₄SiO₄ material synthesized from various lithium sources was reviewed. It is known to all that the price of Li-containing materials is quite high now, including Li₂CO₃, LiOH, LiNO₃ or organic lithium precursors mentioned above, so it is necessary to find other alternative Li-containing materials, especially wastes, with lower prices as the lithium precursor for the preparation of Li₄SiO₄ material.

3.3.2. Silicon Precursors

SiO₂ is an essential raw material for the synthesis of Li₄SiO₄. In addition to pure SiO₂, there are many SiO₂-rich industrial wastes which have attracted researchers’ interests, such as rice husk ash (RHA) and fly ash (FA). In this section, the effects of alternative silicon precursors on CO₂ absorption performance of Li₄SiO₄ material are critically reviewed.

Wang et al. [20] selected two kinds of RHA samples as the silicon precursors for the preparation of Li₄SiO₄ material, which contained the SiO₂ contents of 94.71 and 98.84 wt.%, respectively. HCl aqueous solution was used to pretreat the two RHA samples, then Li₄SiO₄ materials were synthesized by the solid-state reaction method with Li₂CO₃. The employment of RHA produced a smaller particle size, larger pore volume, and surface area compared with pure Li₄SiO₄ material. They reported a weight gain of nearly 135 % over 15 cycles, which was much higher than that of pure Li₄SiO₄ material. Furthermore, Wang et al. [58] pretreated rice husk samples at 600 and 1000 °C, respectively, and cyclic performances of the two RHA-synthesized Li₄SiO₄ materials pretreated at 1000 °C achieved better CO₂ absorption performance, which was similar to that of the RHA-derived Li₄SiO₄ material mentioned above. To study the effects of RHA as the silicon precursor on the CO₂ absorption properties of Li₄SiO₄ material, Wang et al. [59] selected RHA and two kinds of nanosilica (Aerosil and quartz) to prepare Li₄SiO₄ materials by solid-state reaction method, and SEM images and BET analysis indicated that RHA-synthesized Li₄SiO₄ material possessed higher surface area and larger pore volume. Furthermore, the weight gain of RHA-synthesized Li₄SiO₄ material was higher and faster than that of the two nanosilica-synthesized Li₄SiO₄ materials, and its cyclic CO₂ absorption capacity reached nearly 30 wt.% over 15 cycles. The authors ascribed this phenomenon to the almost unchanged surface morphology of Li₄SiO₄ material prepared from RHA over multiple absorption/regeneration cycles. Qiao et al. [60] also noted that RHA-derived Li₄SiO₄ material could enhance the yield of H₂ and reduce the energy consumption in the process of sorption-enhanced steam ethanol reforming.

Fly ash (FA) is a kind of hazardous mineral residue released from coal-fired power plants, and it accounts for approximately 88% in the total coal ash content, which contains a high silicon content, thus it has been used to fabricate useful materials [61,62]. Therefore, Li₄SiO₄ materials can also be prepared from FA as a silicon precursor. Olivares-Marín et al. [47] fabricated Li₄SiO₄ material from Li₂CO₃ and three kinds of FA, and the samples were doped with several amounts ranging from 5 to 40 mol% of K₂CO₃. The cyclic CO₂ absorption capacity of one of the doped FA-Li₄SiO₄ was approximately 100 mg/g over 10 cycles, which was far below the theoretical absorption capacity of Li₄SiO₄ material synthesized from pure SiO₂, but it was relatively stable over multiple cycles. Sanna et al. [63] synthesized Na/Li-FA Li₄SiO₄ material with different molar ratios of Li₂CO₃, FA, and Na₂CO₃, and the material was doped with K₂CO₃. They reported that the CO₂ absorption capacity of the obtained Li₄SiO₄ material was approximately 50 mg/g in low CO₂ concentration in the presence of water vapor, and water vapor had no effect on the cyclic CO₂ absorption capacity.

Shan et al. [64] selected diatomite as silicon precursor, containing the SiO₂ content of approximately 75% [65], and zeolite was also chosen as precursor for comparison. Li₄SiO₄ was synthesized by the solid-state reaction method. Li₄SiO₄ synthesized from diatomite showed higher CO₂ absorption capacity. Li₄SiO₄ material synthesized from diatomite achieved better CO₂ absorption performance than that synthesized from pure SiO₂ because of the higher specific surface area of the former [66]. In order to determine the optimum molar ratio of Li₂CO₃ to SiO₂, Shan et al. [65] prepared a series of Li₄SiO₄ containing the molar ratios of Li₂CO₃ to SiO₂ ranging from 2.0 to 2.8 and their CO₂ absorption capacities carbonated under 50 vol.% CO₂ at 620 °C for 30 min were shown in

Table 3. CO₂ absorption performances of Li₄SiO₄ materials with different molar ratios of Li₂CO₃ to SiO₂ [65].

Molar Ratio	2.0:1	2.1:1	2.2:1	2.3:1	2.4:1	2.6:1	2.8:1
Weight gain (%)	116	122	124	129	129	130	116

.

As presented in Table 3, when molar ratio of Li₂CO₃ to SiO₂ was 2.6:1, CO₂ absorption capacity reached 30.32 wt.% (82.62% of the theoretical value). The CO₂ absorption capacity of Li₄SiO₄ material with this molar ratio decreased from 34.14 to 27.70 wt.% over 16 cycles. However, Shan et al. [67] pointed out that high temperature (900 °C) during the solid-state reaction preparation process resulted in the sintering of Li₄SiO₄ easily, so they selected the impregnation precipitation method to prepare Li₄SiO₄ materials, which was operated at lower temperature. Diatomite, LiNO₃, and NH₃·H₂O were selected as the starting materials with the Li:Si molar ratio of 5.2:1, and the reactions involved are shown in Equations (9) and (10). When carbonated in 50 vol.% CO₂ and regenerated in pure N₂ at 700 °C, both for 30 min, cyclic CO₂ absorption capacity of Li₄SiO₄ synthesized by the impregnation precipitation method was quite stable, which decreased from 34.14 to 33.09 wt.% as the cycle number increases from 1 to 15.

$${\mathrm{LiNO}}_3+{\mathrm{NH}}_3\cdot {\mathrm{H}}_2\mathrm{O}\to \mathrm{LiOH}+{\mathrm{NH}}_4{\mathrm{NO}}_3$$

(9)

$$4\mathrm{LiOH}+{\mathrm{SiO}}_2\to {\mathrm{Li}}_4{\mathrm{SiO}}_4+2{\mathrm{H}}_2\mathrm{O}$$

(10)

Halloysite is also a SiO₂-containing material with a SiO₂ content of about 50 wt.% [68]. Niu et al. [69] synthesized Li₄SiO₄ from treated halloysite nanotubes (HNTs) with HCl aqueous solution and Li₂CO₃ by the solid-state reaction method at 800 °C. The content of Al₂O₃ of HNTs is 43.859%, and the presence of Al³⁺ was beneficial to the enlargement of Li₄SiO₄ crystalline structure, which is beneficial for its CO₂ absorption performance [37]. The CO₂ absorption capacity of halloysite-synthesized Li₄SiO₄ material was approximately 30 wt.% over 10 cycles, which was higher than that of SiO₂-synthesized Li₄SiO₄ material.

In this section, CO₂ absorption performances of Li₄SiO₄ materials synthesized from various silicon precursors were reviewed. Li₄SiO₄ synthesized from RHA, diatomite and halloysite exhibited high CO₂ absorption capacity, while fly ash was not a good lithium precursor. Some elements in these silicon precursors other than pure SiO₂ are possibly beneficial for the CO₂ absorption properties of Li₄SiO₄ materials, which will be discussed in the following sections. There are a large number of Si-containing materials, especially industrial wastes like steel slag, so the following research will focus on these materials. The studies on alternative silicon precursors for the preparation of Li₄SiO₄ materials have obtained great progress, while the major problem that limits the practical application of Li₄SiO₄ is the high price of Li-containing materials, and the cost of Li₄SiO₄ will not be reduced by much even if SiO₂ is free of charge, so the future research should focus on alternative lithium precursors.

3.3.3. Synthesis Methods

Most of the Li₄SiO₄ materials were synthesized by the traditional solid-state reaction method at a relatively high temperature (900 °C). The solid-state reaction method has been widely used because of its simplicity, while Bretado et al. [14] reported that high temperature during the solid-state reaction process resulted in contamination and volatilization. In addition, the microstructure and composition of Li₄SiO₄ materials were difficult to control and agglomeration and sintering of the materials also occurred in the preparation process [24,29]. Thus, Bretado et al. [14] selected the impregnated suspension method to prepare Li₄SiO₄ material and they found that the conversion of the obtained Li₄SiO₄ material (98.4%) was higher than that prepared by the solid-state reaction method (94.9%).

Subha et al. [25] reported that the platelet-shaped Li₄SiO₄ material synthesized from LiNO₃ and colloidal silica by a sol-gel method achieved an absorption capacity of 350 mg/g. Additionally, the platelet-shaped Li₄SiO₄ material was coated with a porous carbon mesh, and the cyclic absorption/regeneration performance of the platelet-shaped Li₄SiO₄ material retained approximately 120 mg/g over eight cycles. The CO₂ absorption rates of the coated Li₄SiO₄ materials were faster than those of the uncoated ones. Additionally, the sol-gel method was superior to the solid-state reaction method when LiOH was selected as the lithium precursor [29,55]. However, the impregnation precipitation method was superior to the solid-state reaction method when diatomite was selected as the silicon precursor [67]. Venagas et al. [70] reported that Li₄SiO₄ materials synthesized by the sol-gel method was not completely pure, probably because the use of a microwave oven resulted in the sublimation of Li₄SiO₄.

4. Effects of Particle Properties on CO₂ Absorption Performance of Li₄SiO₄ Material

The newly synthesized Li₄SiO₄ powder is too fine, and elutriation might occur in the reactor, especially in fluidized bed reactors, in industrial applications. In addition, powdery Li₄SiO₄ materials cannot create effective fluidization, while most of the studies on CO₂ absorption by Li₄SiO₄ material were conducted on fixed bed reactors or TGA. Thus, pelletization may be an effective method for the practical application of Li₄SiO₄ materials. The effects of the particle properties on CO₂ absorption performance of Li₄SiO₄ material were critically reviewed in this section.

Pacciani et al. [71] studied the CO₂ absorption by the pelletized Li₄SiO₄ materials, which were doped with less than 10 vol.% K₂CO₃ and Li₂TiO₃ as a binder. The CO₂ absorption capacity of the pelletized Li₄SiO₄ material was 23 wt.% carbonated in 10 vol.% CO₂. However, Kato et al. [72] reported that the pelletized Li₄SiO₄ materials were more prone to lose their cyclic stability due to the sintering which was caused by the short length of material particles. Essaki et al. [73] prepared cylinder-type K₂CO₃ doped Li₄SiO₄ materials with the diameter of 3 mm and length of 6 mm, while the CO₂ absorption capacity of a Li₄SiO₄ pellet was not so high as that of Li₄SiO₄ powder. Puccini et al. [74] synthesized K₂CO₃-doped Li₄SiO₄ by the solid-state reaction method, and they selected cellulose fiber as the binder. The Li₄SiO₄ material pellets with a diameter of 6 mm and lengths of 1.5, 2.5, and 3.5 mm were prepared, but the prepared Li₄SiO₄-based pellets did not show superior cyclic performance and the conversion of the Li₄SiO₄ pellets decreased to below 28% after 10 cycles. Furthermore, Puccini et al. [75] selected layered graphite and carbon nanotubes as the binders, and thermogravimetric analysis showed that layered graphite was a more suitable binder than carbon nanotubes. It is noteworthy that the cyclic performance of Li₄SiO₄ pellets with a binder of layered graphite was more superior than that of the pellets mentioned in [74], as shown in Figure 11.

Pelletization is an essential procedure for the practical application of Li₄SiO₄, but few researchers studied the CO₂ absorption performance of the pelletized Li₄SiO₄ materials in fluidized bed reactors. Additionally, mechanical intensity and wearing characteristics of pelletized Li₄SiO₄ materials have seldom been reported.

5. Effects of Reaction Conditions on CO₂ Absorption Performance of Li₄SiO₄ Material

Realistic reaction conditions for CO₂ absorption by Li₄SiO₄ material is very complicated, which involves the absorption atmosphere, absorption temperature, regeneration (desorption) temperature, and operating pressure, etc. Hence, the effects of reaction conditions on the CO₂ absorption performance of Li₄SiO₄ materials are reviewed in this section.

5.1. Reaction Atmosphere

5.1.1. CO₂ Concentration

The practical CO₂ concentration in the flue gas from fossil fuel-fired power plant is about 15 vol.% [76], but pure CO₂ is usually selected as the absorption atmosphere of Li₄SiO₄, and the CO₂ absorption performance under the practical lower CO₂ concentration has been overlooked. In fact, CO₂ concentration in sorption-enhanced hydrogen production process is also usually low. Therefore, it is necessary to investigate the CO₂ absorption performance of Li₄SiO₄ material in low CO₂ concentrations.

Pacciani et al. [71] reported that the CO₂ absorption rate of Li₄SiO₄ material rose apparently when CO₂ concentration in absorption atmosphere increased from 2.5 to 24.5 vol.%. Essaki et al. [77] prepared the pelletized Li₄SiO₄ materials with an average particle size of 5 mm and K₂CO₃ and Li₂ZrO₃ were doped into the materials to promote the absorption reaction and prevent reduction of absorption capacity, respectively. The absorption property of Li₄SiO₄ pellets was investigated in 5 vol.% CO₂ at first, and they found that 500 °C was the most appropriate temperature in the range of 400–600 °C for the CO₂ absorption by Li₄SiO₄. However, when the absorption tests were carried out in 10 or 15 vol.% CO₂, it was found that the CO₂ absorption capacity rose as the temperature increased from 400 to 600 °C. Essaki et al. [77] ascribed this phenomenon to the influence of reaction equilibrium, as shown in Figure 12. The equilibrium temperature of CO₂ absorption and regeneration showed an increasing trend with increasing CO₂ concentration, and the weight increase was used to evaluate the CO₂ absorption performance of Li₄SiO₄ material, which can be calculated according to Equation (11):

$$I_N=W_N-100\%$$

(11)

where I_N is the weight increase of Li₄SiO₄ material during the Nth cycle, wt.%; W_N is the weight gain, wt.%; N is the number of cycles. It was also noteworthy that the CO₂ absorption process of Li₄SiO₄ was limited in low CO₂ concentration (5 vol.%), while it was controlled by the diffusion of Li⁺ and O²⁻ in high CO₂ concentration (15 vol.%).

Researchers found that the limits of low CO₂ concentration could be counteracted by the addition of dopants. Puccini et al. [78] found Li₄SiO₄ material doped with 30 wt.% K₂CO₃ maintained a stable CO₂ absorption capacity (approximately 160 mg/g) after 25 cycles in 4 vol.% CO₂ at 580 °C. It is worth noting that Seggiani et al. [39] reported that CO₂ absorption capacity of K₂CO₃-doped Li₄SiO₄ material was superior than 20 wt.% over four cycles in 4 vol.% CO₂. Furthermore, Seggiani et al. [40] also reported that the CO₂ absorption capacity of Na₂CO₃-doped Li₄SiO₄ material in 4 vol.% CO₂ was 7 wt.%, and it was quite stable over 25 cycles. Adding some dopants can improve the CO₂ absorption capacity of Li₄SiO₄ material, but the improvement in lower CO₂ concentration is still relatively lower compared with that in higher CO₂ concentration. The CO₂ absorption performance of Li₄SiO₄ material in high CO₂ concentration has been well studied by researchers. Thus, Li₄SiO₄ materials with high absorption capacity, fast absorption rate, and good cyclic stability in low CO₂ concentrations should be investigated for industrial application.

5.1.2. Presence of Steam

Apart from CO₂, steam also exists in realistic CO₂ absorption conditions, and the content of steam during the typical sorption-enhance hydrogen production process is more than 30%. Ochoa-Fernández et al. [79] reported that steam could promote the mobility of alkaline ions, indicating that the limiting resistance of the CO₂ absorption reaction could be reduced. Thus, the presence of steam in the absorption atmosphere also has non-negligible effect on the CO₂ absorption capacity of Li₄SiO₄ material.

Ochoa-Fernández et al. [80] reported that the presence of 10 vol.% steam in the absorption atmosphere could raise the CO₂ capacity from 9.5 to 29 wt.%. Additionally, they also found that the presence of steam accelerated the regeneration reaction: the regeneration process became faster and more thorough with the presence of steam, and cyclic CO₂ absorption performance of Li₄SiO₄ material degraded slightly after eight cycles with the presence of steam, almost the same as the experimental data obtained in dry atmosphere. Quinn et al. [81] used pelletized Li₄SiO₄ materials for CO₂ absorption in 14.7% CO₂, 2.6% steam in N₂ at 550 °C, and they found that the CO₂ absorption capacity after 10 min was almost three times higher than that in dry atmosphere. Furthermore, Sanna et al. [63] synthesized Li₄SiO₄ material from FA as SiO₂ precursor, and the CO₂ absorption capacity was enhanced by steam. Puccini et al. [82] also noted that the CO₂ absorption rate was accelerated correspondingly with increasing steam concentration from 10 to 30 vol.%.

As mentioned above, the presence of steam contributes to the CO₂ absorption by Li₄SiO₄ material, because the addition of steam maybe enhances the mobility of Li⁺ and O²⁻ [27,83], thus the resistance of diffusion is reduced, so the CO₂ absorption capacity of Li₄SiO₄ is enhanced by steam.

5.1.3. Gas Contaminants

NO_x and SO₂ are common gas contaminants which have done great harm to the environment and people’s health. Thus, the effects of NO_x and SO₂ in the flue gas on the CO₂ absorption by Li₄SiO₄ material should be considered. The effects of NO_x and SO₂ on the CO₂ absorption performance of Li₄SiO₄ material could be great despite of their minor contents [82].

Puccini et al. [82] performed CO₂ absorption tests in an atmosphere of 4 vol.% CO₂ and various concentrations of NO, and the results showed that NO in the absorption atmosphere does not show a harmful effect on the CO₂ absorption capacity of Li₄SiO₄ material. Furthermore, when the concentration of SO₂ in the absorption atmosphere increased from 0 to 2000 ppm, the weight change of Li₄SiO₄ material increased with the increase of SO₂ concentration, but the regeneration performance of Li₄SiO₄ material in the presence of SO₂ was worse compared with that in the absence of SO₂. Additionally, the cyclic CO₂ absorption performance of Li₄SiO₄ material was negatively influenced in the presence of SO₂ under absorption and regeneration atmospheres [71]. The authors ascribed this phenomenon to the nonreversible reaction between SO₂ and Li₄SiO₄, as shown in Equations (12) and (13):

$${\mathrm{Li}}_4{\mathrm{SiO}}_4{+\mathrm{SO}}_2\to {\mathrm{Li}}_2{\mathrm{SO}}_3{+\mathrm{Li}}_2{\mathrm{SiO}}_3$$

(12)

$${\mathrm{Li}}_2{\mathrm{SiO}}_3{+\mathrm{SO}}_2\to {\mathrm{Li}}_2{\mathrm{SO}}_4+\mathrm{SiO}$$

(13)

The formation of Li₂SO₃ and Li₂SO₄ prevented the regeneration of the materials, indicating that the presence of SO₂ in the absorption atmosphere has an adverse effect on the absorption performance of Li₄SiO₄ material [82]. In general, NO had no negative impact on the CO₂ absorption property of Li₄SiO₄ materials, while SO₂ had an adverse effect due to the formation of the irreversible Li₂SO₃ and Li₂SO₄, so SO₂ must be scrubbed prior to the trapping of CO₂. However, the exact joint role and acting mechanism of NO and SO₂ in the process of CO₂ capture are still unknown, and the effects of other contaminants, like HCl or H₂S, on the CO₂ absorption by Li₄SiO₄ material are not clear, thus further research is necessary.

5.2. Reaction Temperature

As shown in Figure 4, the equilibrium temperatures of absorption and regeneration increase as the CO₂ partial pressure rises monotonously. In other words, each equilibrium temperature corresponds with a partial pressure of CO₂ in the absorption atmosphere. Essaki et al. [77] reported that when the absorption temperature of Li₄SiO₄ pellets provided by Toshiba varied from 400 to 600 °C in 5 vol.% CO₂, and results showed that weight increase at 500 °C was 20 wt.%, which was much higher than those at 400 °C and 600 °C. Additionally, Quinn et al. [81] reported that 625 °C was the most appropriate temperature for the absorption of Toshiba-provided Li₄SiO₄ pellets in a pure CO₂ atmosphere. This confirmed the conclusion that the equilibrium temperature of the reaction between Li₄SiO₄ and CO₂ rises with increasing CO₂ partial pressure.

Different kinds of Li₄SiO₄ materials accommodate diverse appropriate absorption temperatures. Qiao et al. [60] synthesized Li₄SiO₄ material from RHA and Li₂CO₃, and they found that the most suitable temperature for absorption was 650 °C in a pure CO₂, while Puccini et al. [78] reported that 580 °C was the optimum temperature for K-doped Li₄SiO₄ materials, and Wang et al. [30] pointed out that 575 °C was the most appropriate for the CO₂ absorption by Li₄SiO₄.

Temperature during the regeneration process also had a deep effect on CO₂ absorption performance on Li₄SiO₄ material. Ochoa-Fernández et al. [80] reported that the ratio and degree of regeneration increased when the regeneration temperature rose from 525 to 575 °C. This indicates that a higher regeneration temperature is possibly advantageous for the regeneration of Li₄SiO₄ material, while too high a regeneration temperature intensifies the sintering of the material, which is extremely harmful.

6. Application of Li₄SiO₄ Material in Sorption-Enhanced Hydrogen Production

Sorption-enhanced hydrogen production is one of the most important applications of Li₄SiO₄ material as a CO₂ acceptor, which mainly consists of sorption-enhanced steam methane reforming (SE-SMR) and sorption-enhance steam ethanol reforming (SE-SER). Overall reactions of SE-SMR and SE-SER are shown in Equations (14) and (15), respectively:

$${\mathrm{CH}}_4{+2\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2{+4\mathrm{H}}_2$$

(14)

$${\mathrm{C}}_2{\mathrm{H}}_5{\mathrm{OH}+3\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{CO}}_2{+6\mathrm{H}}_2$$

(15)

In the SE-SMR and SE-SER processes, in situ CO₂ removal of Li₄SiO₄ material as the CO₂ acceptor shifts the reaction equilibrium to hydrogen production, and exothermal absorption of CO₂ by the Li₄SiO₄ material provides heat for reforming, thus high hydrogen yield can be achieved.

Rusten et al. [84] conducted SE-SMR with CO₂ absorption by Li₄SiO₄ material in a fixed bed reactor at 848 K and 2 MPa, and syngas with the hydrogen concentration of 87% was obtained, which was higher than that obtained when Li₂ZrO₃ was used as a CO₂ acceptor. Essaki et al. [85] introduced commercial Li₄SiO₄ pellets into SE-SMR process, and the experiments were carried out on a vertical furnace. It was reported that methane conversion at 550 °C was 80%, and hydrogen concentration reached 93.6 vol.% in syngas. The performance of Li₄SiO₄ pellets in the SE-SER process was also tested, and the results showed that the concentrations of hydrogen and CO in syngas were higher than 99 vol.% and less than 0.12 vol.%, respectively, indicating that Li₄SiO₄ pellets were promising as the CO₂ acceptor for the SE-SER process [86]. Zhang et al. [42] reported K₂CO₃-doped Li₄SiO₄ material coupled well with the Ni/γ-Al₂O₃ catalyst, and hydrogen concentration in the syngas was higher than 95 vol.%. In addition, they found that homogeneous distribution of Li₄SiO₄ material and catalyst led to higher hydrogen concentration in the syngas.

It can be concluded from the studies above that hydrogen yield and concentration were mainly dependent on the performance of Li₄SiO₄ materials, thus Li₄SiO₄ materials with superior CO₂ absorption performance should be investigated. Additionally, Li₄SiO₄ materials may be applicable to various sorption-enhanced hydrogen production, and raw materials for gasification could be biomass, sludge, coal, etc.

7. Density Functional Theory Studies on Li₄SiO₄ Material

Duan et al. [87] studied CO₂ absorption performance on monoclinic and triclinic phases of Li₄SiO₄ using density functional theory, and they found that the thermodynamic properties of the two phases were similar to each other. The calculation results showed that reaction heat of the reaction between Li₄SiO₄ and CO₂ was consistent with the experimental data. Kong et al. [88] reported that the (0 1 0) plane was the most stable low-Miller index plane of Li₄SiO_4, and the adsorption and dissociation behaviors of molecular H₂O on the Li₄SiO₄ (0 1 0) plane were investigated. They found that molecular H₂O was more inclined to be absorbed on O atoms on the surface.

8. Conclusions

Research progress of Li₄SiO₄ materials for CO₂ capture in energy production processes, including hydrogen plants based on sorption-enhanced reforming and fossil fuel-fired power plants, were reviewed in this paper. Thermodynamic and kinetic studies on the preparation and CO₂ absorption of Li₄SiO₄ material were demonstrated, and the diffusion of CO₂ and ions through the product layer seemed to be the limiting step for CO₂ absorption by Li₄SiO₄ material. Since Li₄SiO₄ material prepared by the traditional solid-state reaction method only achieved low CO₂ absorption capacity, methods to enhance the CO₂ absorption performance of Li₄SiO₄ material were illustrated. Introducing a solid solution and molten salts could reduce the diffusion resistance in the product layer, and using hydration, ball milling, or organic precursors could increase the contact area of CO₂ and Li₄SiO₄, which is beneficial for CO₂ absorption by Li₄SiO₄ material. The sol-gel method seemed to be most appropriate for preparation of Li₄SiO₄ material, which is beneficial for the formation of porous structure. The effects of gas contaminants and reaction conditions on CO₂ absorption performance of Li₄SiO₄ material and the applications of Li₄SiO₄ material in the sorption-enhanced hydrogen production process were summarized. In view of the current studies reviewed in this work, potential research thoughts and trends are suggested as follows:

(i) Most of the laboratory experiments were carried out on TGA or fixed-bed reactors, while fluidized bed was the common equipment in practical application for the absorption of CO₂ under most energy production conditions. Additionally, powdery Li₄SiO₄ materials could not create effective fluidization, while studies on the performance of pelletized Li₄SiO₄ materials were insufficient. As a result, more focus should be attached to the CO₂ absorption performance of pelletized Li₄SiO₄ material in fluidized bed reactors.

(ii) Application of Li₄SiO₄ materials on sorption-enhanced hydrogen production is an important aspect, and hydrogen yield and concentration were considerable, while fewer studies involved this area. Additionally, CO₂ absorption performance of Li₄SiO₄ materials in realistic sorption-enhanced hydrogen production conditions (i.e., low CO₂ concentration in the presence of steam) deserves to be studied.

(iii) Preparation cost of Li₄SiO₄ materials is the main problem that limits its industrial application, thus many studies investigated the feasibility of silicon-containing solid wastes as a silicon precursor. However, the main factor that controls the cost of Li₄SiO₄ materials is the expensive lithium precursor. As a result, it is suggested that lithium-containing wastes can be tested for the possibility as a lithium precursor, and Li₄SiO₄ materials prepared from inexpensive lithium-containing wastes may be promising for large-scale CO₂ absorption.