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Impregnation Precipitation Preparation and Kinetic Analysis of Li4SiO4‑Based Sorbents with Fast CO2 Adsorption Rate ShaoYun Shan,† SanMei Li,† QingMing Jia,†,* LiHong Jiang,† YaMing Wang,† and JinHui Peng‡ †

School of Chemical Engineering, Kunming University of Science and Technology, Kunming 650093, China School of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China

‡

ABSTRACT: Li4SiO4-based sorbents from diatomite with fast CO2 adsorption rate (achieving the adsorption equilibrium at about 15 min) were developed by the impregnation precipitation method at lower temperature (600 °C). Influence of sintering temperatures on phase composition was investigated. Phase composition and morphologies were analyzed by XRD and SEM, and CO2 adsorption properties were investigated by TG. The as-prepared Li4SiO4-based sorbents reached a higher adsorption capacity of 34.02 wt % (7.73 mmol CO2/g Li4SiO4, corresponding to 92.70% efficiency) in a mixture gas (50 mL/min N2 and 50 mL/min CO2). The activation energies for chemisorption (28.868 kJ/mol) and diffusion (16.563 kJ/mol) were obtained by isothermal study. Compared with our previous research results by the solid reaction method, the activation energies for these two processes decreased measurably. In addition, the as-prepared Li4SiO4-based sorbents also exhibited faster adsorption rate and better adsorption−desorption performance.

1. INTRODUCTION CO2 emissions, mostly resulting from fossil fuel combustion, are considered as a major contributor to climate change. Sorption-enhanced steam methane reforming (SESMR) is a promising process for H2 production of high purity (95%) in one step, where CO2 captors are installed with the reforming catalyst.1,2 Lithium-based and sodium-based sorbents have been considered as promising CO2 captors.3−9 Compared with other sorbents, such as Li2ZrO3, Li4SiO4 sorbents show higher adsorption capacity and better durability.10,11 In general, Li4SiO4-based sorbents are synthesized by solid-state reactions at high temperatures (900−1000 °C), using analytical pure SiO2 and lithium salts as precursors.12−15 The resultant sorbents often present relatively large particles and poor purity, resulting in lower adsorption efficiency and adsorption rate. The adsorption rate, as well as the adsorption efficiency, are important examination factors for the sorbents performance. The previous studies showed that Li4SiO4 sorbents usually have higher adsorption efficiency and better cyclic adsorption durability, but the adsorption rate is still not high. The adsorption rate may be increased by doping or decreasing the particle size and diffusion distance. Xu et al.16 investigated effects of the particle size of quartz powder on CO2 adsorption properties of Li4SiO4, and the results showed the sorbents from smaller particle-size quartz powder showed a more rapid adsorption−desorption process and a higher adsorption efficiency. In our previous studies,13,14 we prepared Li4SiO4based sorbents from diatomite with higher adsorption efficiency and better cyclic stability by a solid-phase method, but the adsorption rate is lower (around 80 min is needed to achieve the maximum adsorption capacity). Marin et al.17 reported novel lithium-based sorbents from fly ashes can reach the plateau of maximum at about 15 min in pure CO2 atmosphere, but the sorbent had lower adsorption efficiency (29.16%). Li4SiO4-based sorbents prepared by Seggiani et al.15 showed lower CO2 adsorption efficiency and adsorption rate under low © XXXX American Chemical Society

CO2 concentration. Therefore, Li4SiO4-based sorbents with higher adsorption efficiency, faster adsorption rate, and better cyclic adsorption stability are desirable. In this paper, we developed Li4SiO4-based sorbents from diatomite with faster CO2 adsorption rate, higher adsorption efficiency, and better cyclic adsorption durability at lower temperature (600 °C) by an impregnation precipitation method. The effect of sintering temperatures on phase composition was investigated. The kinetic parameters for the chemisorption and diffusion processes were obtained by the isothermal study. Additionally, the adsorption−desorption properties were investigated by TG.

2. EXPERIMENTAL SECTION 2.1. Preparation of Li4SiO4-Based Sorbents. Diatomite (75% SiO2, C.R., Shanghai Fengxian Reagent Co. Ltd., China), LiNO3 (97%, A.R., Tianjin Fengchuan Chemical Reagent Co. Ltd., China), and NH3·H2O (25−28%, C.R., Shanghai Fengxian Reagent Co. Ltd., China) were used as reactants. The properties for diatomite are shown in Table 1. All reactions were performed with a Li:Si molar ratio of 5.2:1.14 For impregnation precipitation process, the required amounts of LiNO3 was first dissolved in ethanol. Diatomite was added to the solution, stirred, and impregnated for 4 h, then NH3·H2O was slowly added, drop by drop (taking 6 s between drops), to the solution. The final solution was placed and heated at 80 °C Table 1. Diatomite Composition Analysis composition

SiO2

Al2O3

Fe2O3

CaO

K2O

loss

others

content (wt%)

75.26

14.33

2.31

1.02

1.56

4.35

1.17

Received: March 8, 2013 Revised: May 2, 2013 Accepted: May 6, 2013

A

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33-0786) resulted from the reaction between the Al2O3 phase coming from diatomite and Li2CO3. Ortiz-Landeros et al.18 reported the aluminum presence into the Li4SiO4 structure highly improves the CO2 chemisorption. Additionally, sintering temperatures (600−700 °C) had no obvious influence on phase composition, which showed that the reactions were complete at lower temperature (600 °C). Compared with the previous reported Li4SiO4-based sorbents,12−15,19 the preparation temperature lowered measurably. Figure 2 shows the morphologies of diatomite and the sorbent IDS. Seen from Figure 2a, diatomite had rich uniform

until it dried, and the obtained powders were calcined at 600− 700 °C for 4 h in a muffle furnace. The resultant Li4SiO4-based sorbents were designated as IDS. The reactions were as follows: LiNO3 + NH3·H 2O → LiOH + NH4NO3

(1)

4LiOH + SiO2 → Li4SiO4 + 2H 2O

(2)

The sorbents SDS (using diatomite as silicon source) and SAS (using analytical SiO2 as silicon source) were prepared by a solid reaction method.13 2.2. Characterization Techniques. The phase compositions of the as-synthesized sorbent IDS were identified by XRD (D8ADVANCE, German)) at room temperature using a Cu Kα radiation. Specific surface areas were measured by using an ASAP2020-Physisorption analyzer (Micromeritics Instrument Corporation, USA). The morphologies were characterized by SEM (JSM-35C, JEOL Ltd., Japan). As Li4SiO4 is not a conductor material, the sorbents were coated with a layer of gold film to avoid a lack of electrical conductivity. Two different kinds of thermal analysis were performed in TG (STA 449 F3, Netzsch Co. Ltd., German). First, a series of sorbents (SAS, SDS, and IDS) with about 15 mg were placed in Al2O3 crucible and heat-treated dynamically, with heating rates of 10 °C/min from room temperature to 800 °C in the gas mixture (50 mL/min N2 and 50 mL/min CO2). Then a Li4SiO4-based sorbent (IDS) was tested isothermally under a gas mixture (50 mL/min N2 and 50 mL/min CO2) at different temperatures (500, 550, 600, and 620 °C) for 200 min. The CO2 adsorption reaction is as follows: Li4SiO4 + CO2 = Li 2SiO3 + Li 2CO3

Figure 2. SEM images of diatomite (a) and the sorbent IDS (b) at 600 °C for 4 h.

pore structure with the pore size of about 500 nm. Figure 2b shows that the sorbent IDS was mainly composed of homogeneous spherical particles with a particle size of about 1−2 μm. There is a marked difference with the morphologies of diatomite. The specific surface area for the sorbent IDS is 8.53 m2/g, higher than those of the sorbents SDS (1.75 m2/g) and SAS (1.15 m2/g).13 3.2. Effects of Different Preparation Methods and Silicon Sources on Adsorption Properties. Figure 3a shows the dynamic thermograms of different Li4SiO4-based sorbents (SAS, SDS, and IDS). All the thermograms seem to present similar behavior. At temperatures lower than 400 °C, three thermograms were very stable, which showed no reactions between Li4SiO4 and CO2. The sorbents began to

(3)

According to reaction 3, the theoretical adsorption capacity is 36.7 wt % (being 8.34 mmol CO2/g Li4SiO4)

3. RESULTS AND DISCUSSION 3.1. Phase Composition and Morphologies of Li4SiO4Based Sorbents. Figure 1 shows XRD patterns of the sorbent IDS with different sintering temperatures. As shown in Figure 1, Li4SiO4 phase (JCPDS: 24-0650) was the main phase for the sorbents with three different sintering temperatures. The occurrence of a small amount of LiAlSi2O6 phase (JCPDS:

Figure 1. XRD patterns of the sorbent IDS with different sintering temperatures.

Figure 3. Thermogravimetric and isothermic analyses of the Li4SiO4based sorbents (SAS, SDS, and IDS) calcined at 700 °C for 4 h. B

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absorb CO2 at about 450 °C, finishing this process at around 680 °C. Later, at temperatures higher than 680 °C, the sorbents presented a desorption process. Several aspects must be pointed out from Figure 3a. Different silicon sources and preparation methods had almost no impact on the maximum CO2 adsorption temperature (about 680 °C). It is reported that CO2 flows have a larger impact on the maximum CO2 adsorption temperature.22 Besides, the CO2 adsorption rate and the absorption capacity observed on three different sorbents changed as a function of silicon sources and preparation methods. The maximum adsorption capacity for the sorbents SAS, SDS, and IDS were equal to 11.34 wt %, 24.23 wt %, and 29.89 wt %, respectively. In comparison to the sorbents SAS and SDS, the sorbent IDS had the faster adsorption rate and the larger adsorption capacity, which is probably resulted from the particular morphologies of the sorbent IDS. To further understand the CO2 adsorption on Li4SiO4 sorbents, we performed some extra experiments. Figure 3b shows the isothermal graphs of the sorbents SAS, SDS, and IDS at 620 °C. Compared with our previous Li4SiO4-based sorbents (SAS and SDS), the as-prepared sorbent IDS showed higher adsorption capacity and faster adsorption rate. The sorbent IDS had much faster adsorption rate before 10 min, and the adsorption reaction reached the maximum adsorption capacity at about 15 min. The adsorption capacities of the sorbent IDS at 2, 5, 10, and 15 min were 78%, 88%, 93%, and 96% of the maximum adsorption capacity, respectively. While the adsorption capacities of the sorbents SAS and SDS at 15 min were only 75% and 45% of the maximum adsorption capacity, respectively. In addition, the adsorption reaction reached equilibrium at about 15 min for the sorbent IDS, while the sorbent SDS reached equilibrium at about 80 min, and the sorbent SAS did not still reach equilibrium until 200 min. The higher adsorption capacity and faster adsorption rate was probably resulted from larger specific surface area and smaller uniform particle size for the sorbent IDS. Besides this, the results were also explained by the following kinetic analysis. 3.3. Kinetic Analysis for the Sorbent IDS. Figure 4 shows the isotherms of the sorbent IDS at four different temperatures. The adsorption capacity and the adsorption rate

increased as a function of the temperature between 500 and 620 °C. Compared with the previous as-prepared sorbents (SDS and SAS),13 the sorbent IDS has much higher adsorption efficiency and faster adsorption rate at the same adsorption temperature. To obtain kinetic information about the CO2 adsorption rates, some isothermal studies were investigated at four different temperatures in the mixture gas (50 mL/min N2 and 50 mL/min CO2) (see Figure 4). Four adsorption curves at different adsorption temperatures showed similar trend, and the data were fitted to a double exponential model:12,15,17,20−23 y = A exp(−k1x) + B exp(−k2x) + C, where y represents adsorption capacity, x is adsorption time, k1 and k2 are the exponential constants, and A, B, and C are pre-exponential factors. The kinetic parameters obtained at four different temperatures are shown in Table 2. As can be seen, both k1 (chemisorption) and Table 2. Kinetic Parameters Obtained from the Experimental Data Fitted to a Double Exponential Model for the Sorbent IDS T (°C)

k1 (1/s)

k2 (1/s)

R2

500 550 600 620

0.112 0.137 0.184 0.202

0.018 0.021 0.025 0.025

0.99981 0.99925 0.99955 0.99935

k2 (diffusion) increased from 500 to 620 °C. The same behavior was observed for lithium-based and sodium-based sorbents.17,20 The k1 values are about seven times higher than those of k2. Compared with our previous studies,13 the ratio value of k1/k2 decreased, probably resulting from smaller particle size of the sorbent IDS. The fact that k1 > k2 is in agreement with literature,12,20,24 indicates that the limiting step of the reaction is the diffusion process. To quantitatively analyze these results and the temperature dependence of the different processes, some researchers reported that the k1 and k2 constant values were usually fitted according to the Arrhenius equation12 or the Eyring’s model.20,22,23,25 In the paper, we obtained the activation energies by the Arrhenius equation, so that ⎛ E ⎞ ⎟⎟ k = k 0 exp⎜⎜ − ⎝ R gT ⎠

where k is the reaction rate constant, k0 is the pre-exponential factor, E is the activation energy, Rg is the gas constant, and T is absolute temperature. The activation energies for the chemisorption and diffusion processes were estimated to be 28.868 and 16.563 kJ/mol, respectively. As can be seen, the activation energy of the chemisorption process is slightly higher than that of the diffusion process, which shows that the chemisorption process is more dependent on the temperature. In comparison with our previous results,13 the activation energies of the chemisorption and diffusion processes for the sorbent IDS were lower, indicating that the sorbent IDS had higher reactivity due to the formation of smaller particle size grains and structural differences produced by the aluminum and other elements present on the diatomite.10,18,26 The k1/k2 ratio of the sorbent IDS was lower than those of the sorbents (SAS and SDS), which showed the diffusion rate was relatively faster for the sorbent IDS. Seen from the above kinetic analysis, the

Figure 4. Isotherms of the sorbent IDS. C

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sorbent IDS for CO2 adsorption followed a double adsorption mechanism.27 3.4. Adsorption−Desorption Performance Measurement for the Sorbent IDS. For a continuous process to be feasible the material will need to operate over numerous adsorption−desorption cycles. To investigate cyclic adsorption properties of the sorbent IDS, the adsorption−desorption test was carried out at a fixed temperature (700 °C) in a Netzsch thermogravimetric analyzer. During the test, a mixture gas (50 mL/min N2 and 50 mL/min CO2 for adsorption and 100 mL/ min N2 for desorption) were introduced into the system every 20 min. Figure 5 showed the adsorption−desorption cycle

(2) According to the double exponential model and the Arrhenius equation, some kinetic information for CO2 capture was measured. The lower activation energies for the sorbent IDS (28.868 kJ/mol for chemisorption process and 16.563 kJ/mol for diffusion process) resulted from particular morphologies. (3) The sorbent IDS had excellent adsorption−desorption performance. The adsorption capacity only decreased 1.05 wt % from the first cycle (34.14 wt %, being 7.73 mmol CO2/g Li4SiO4) to the 15th cycle (33.09 wt %, being 7.52 mmol CO2/g Li4SiO4)).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-0871-65920353. Fax: +86-0871-3801114. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been sponsored by National Natural Science Foundations of China (Grants 51104075, 31160146, and 21266012).

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REFERENCES

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Figure 5. Adsorption−desorption performance of the sorbent IDS.

number of the sorbent IDS. As shown in Figure 5, with increasing cycle number, the adsorption capacity decreased 1.05 wt % from the first cycle (34.14 wt %, being 7.73 mmol CO2/g Li4SiO4) to the 15th cycle (33.09 wt %, being 7.52 mmol CO2/ g Li4SiO4). Compared with our previous as-prepared Li4SiO4based sorbents (SDS and SAS), the sorbent IDS showed faster adsorption rate and better cyclic adsorption stability. This is probably because the sorbent IDS have particular morphologies. Compared with the lithium-based sorbents prepared by Marin et al.,17 the as-prepared Li4SiO4-based sorbents showed higher CO2 adsorption efficiency in the premise of remaining faster adsorption rate. Therefore, the sorbent IDS should have a potential application prospect for high temperature CO2 capture.

4. CONCLUSIONS Li4SiO4-based sorbents from diatomite with superior adsorption properties were prepared at lower temperature (600 °C) by impregnation precipitation method. The effect of different sintering temperatures on phase composition was investigated. The kinetic parameters for the chemisorption and diffusion processes were obtained by the isothermal study. The following conclusions were obtained: (1) The sorbent IDS had higher adsorption efficiency and faster adsorption rate (achieving equilibrium at about 15 min) than the SDS and SAS. The maximum adsorption capacity of the sorbent IDS reached 34.02 wt % (7.73 mmol CO2/g Li4SiO4) in a gas mixture (50 mL/min N2 and 50 mL/min CO2), having 92.70% efficiency. D

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