Synthesis of LiF-Containing Li4SiO4 as Highly Efficient CO2 Sorbents

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Synthesis of LiF-containing Li4SiO4 as highly efficient CO2 sorbents Ke Wang, Chunlei Wang, Zhongyun Zhou, Zhiwei Lin, and Pengfei Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01175 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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Industrial & Engineering Chemistry Research

Synthesis of LiF-containing Li4SiO4 as highly efficient CO2 sorbents Ke Wang, Chunlei Wang, Zhongyun Zhou, Zhiwei Lin, Pengfei Zhao* School of Electrical and Power Engineering, China University of Mining and Technology, Xuzhou 221116, China Corresponding author: Dr. Pengfei Zhao School of Electrical and Power Engineering China University of Mining and Technology Xuzhou 221116, China Fax: 86-516-83592000 E-mail: [email protected]

Abstract: Several types of lithium halide-containing Li4SiO4 sorbents were prepared via sacrificial carbon template technology to overcome the typical kinetic limitations under low CO2 partial pressures. The synthesized samples were characterized by XRD (X-ray diffraction), SEM (scanning electron microscopy), N2 adsorption, XPS (X-ray photoelectron spectroscopy) and thermogravimetric analysis. The results reveal that, among three lithium halide-containing Li4SiO4 sorbents, the highest uptakes were obtained by LiF addition. Different amounts of LiF addition also significantly affected the intrinsic properties and absorption characteristics of the Li4SiO4 sorbents. When the addition amount was 10 mol%, Li and F were incorporated into the Li4SiO4 structure to form a solid solution. Such features generated smaller crystallite sizes and 1

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particle sizes as well as a species of Li2O on Li4SiO4 surface, which was responsible for the maximum uptake capacity of 36.2 wt.% at 575 °C under 15 vol.% CO2. This value is the largest uptake reported in the literature to date. Moreover, a high capacity was maintained over 8 sorption/desorption cycles.

1. Introduction Excessive CO2 emissions, one of greatest possible contributors to global warming, have attracted widespread attention.1 Fossil fuel-fired power plants contribute to 33~40% of total CO2 emissions. CO2 capture and storage (CCS) in thermal power plants provides the most effective and direct solution to mitigate ongoing global warming issues.

2

In sorption-enhanced steam methane reforming

(SESMR), capturing the reaction product of CO2 can positively affect the reaction process, not only more efficiently producing H2 but also reducing CO2 emissions.3 However, during CCS in thermal power plants or SESMR processes, the operating costs of using amine-based absorbents are significantly high due because of the requirement of cooling the high-temperature flue gases, which is accompanied by a large energy loss.4 Conversely, high-temperature solid sorbents can directly trap CO2 at high temperatures, and thus, research on these materials is increasing. Several types of high-temperature candidate materials, including CaO-based sorbents

5-8

, layered

double hydroxides 9, 10 and ceramic materials 11-14, have been proposed. Among them, lithium salt compounds, especially Li4SiO4, have become one of the most anticipated absorbents because of their large absorption capacities, reasonable durability, better 2


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cyclic stability and reasonable materials costs. Based on the reversible reaction as follows 15, 16, a theoretical maximum adsorption of 36.7 wt.% was calculated. Li4SiO4+CO2 ↔ Li2CO3+Li2SiO3

(1)

Common Li4SiO4 prepared by a solid-state method has large particle sizes and an extremely low porosity, 17 therefore exhibiting low diffusion kinetics under typical hot flue gas conditions in power plants or SE-SMR conditions (4-15 vol.% CO2). It was reported that approximately only half of the sorbents participated in the reaction, even over a long reaction time (~120 min).18 Currently, there are two main routes to improve the CO2 adsorption kinetics. One feasible method is the use of synthetic routes to decrease the particle size and increase the specific surface area. The sorbents synthesized by superior methods, such as hydration19, sol–gel 20, 21, carbon templates22, 23

, impregnated precipitation24 and ball milling25, have better absorption

characteristics compared to Li4SiO4 fabricated by traditional solid-state methods. Since the formation of low-temperature eutectic compounds significantly reduces the diffusion resistance of CO2, doping several types of foreign elements alkali carbonates K2CO3-doped

26-32

, especially

33-35

, into the Li4SiO4 particles is an efficient technique.

Li4SiO4

was

prepared

by

a

solid-state

reaction

method.36

Co-precipitation has also been used to obtain Na2CO3-doped Li4SiO4.37 Several reports have revealed that the absorption capacity of doped sorbents can be greatly promoted by controlling the morphology of the Li4SiO4 precursor.38 For instance, relatively porous binary (Na−K)CO3-doped Li4SiO4 particles were facilitated by inducing an alternative silica precursor. Nanorod ternary (Li−Na−K)CO3-coated 3



Li4SiO4 was designed by a microwave sol–gel process.39 Recently, the new NaF or NaCl doping was developed to modify the intrinsic properties of Li4SiO4 through a sacrificial carbon template or hydration technology. 40, 41 Both NaF- and NaCl-doped Li4SiO4 had a CO2 uptake capacity as high as 32.0 wt.% under 15 vol.% CO2, demonstrating the important role of the additives. Despite those impressive results, it is still highly challenging to develop novel Li4SiO4 sorbents that exhibit both high activity and high stability. Therefore, we developed highly efficient LiF-containing Li4SiO4 through a sacrificial carbon template procedure. Our previous studies 22, 23 have shown that the sacrificial carbon template procedure facilitates increasing the specific surface area. Furthermore, rather than using common K2CO3 or Na2CO3 as dopants, lithium halides as new additives were used for the first time to tailor Li4SiO4 materials. In this study, we prepared various types of lithium halides-containing Li4SiO4 and examined the CO2 uptake to identify an appropriate CO2 absorbent. Notably, the impact of the different LiF concentrations on the intrinsic properties of Li4SiO4 sorbents was explored by determining their crystal phases, morphologies, surface compositions and CO2 absorption kinetics. 2. Experimental 2.1 Sorbents All chemicals (AR) were purchased from Aladdin Chemical Reagent Co., Ltd. All water used was deionized water. The lithium halide-containing Li4SiO4 samples were prepared by the sacrificial carbon template method 22. In the synthesis process, 4


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pure Li4SiO4 was first formed by a solid-state reaction according to the previous literature

19

. Then, the pure Li4SiO4 and a quantitative lithium halide reagent were

dissolved in the appropriate gluconic acid aqueous solution and stirred vigorously until the water was completely evaporated at 80 °C. In addition, the molar ratios of m(gluconate):m (Li4SiO4):m(additive) were 2:1:X. Next, the obtained aqueous slurries were dried in an oven at 105 °C overnight. The dried powders were pyrolyzed at 500 °C in a pure N2 flow and then further calcined at 700 °C in air for 4 h in a muffle furnace to obtain the sorbents. The resulting Li4SiO4 samples were named SCT-Y-X, where Y represents the additive material, for example, SCT-LiF-0.10 represents LiF-containing Li4SiO4 sorbents with the additive amount of 10 mol.%. SCT sorbents were synthesized by the same procedure but without addition. 2.2 Characterization Powder X-ray diffraction (XRD: Bruker D8 Advance powder diffractometer) was used to obtain the chemical composition and crystallographic structure of the materials. The size and morphology of Li4SiO4 were characterized by high-resolution scanning electron microscopy (SEM: Hitachi Model S-4800). This microscope was also equipped with an EDX-Mapping spectroscopy unit. The porous properties of samples

were

examined

by

a

Micromeritics

ASAP

2020

nitrogen

adsorption/desorption analyzer. The X-ray photoelectron spectroscopy (XPS: Perkin-Elmer, PHI 5600) spectra of the sorbents were examined to study the chemical compositions on the surface. The spectral data was further corrected by the adventitious C 1s peak with a fixed value of 284.6 eV and processed based on 5



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peak-differentiating analysis (XPSPEAK41 software). 2.3. CO2 sorption The CO2 uptake of the samples was measured by a thermal-gravimetric analyzer (TG: ZRY-1P, Techcomp Jingke Scientific Instrument). All TG experiments were carried out in a flow of 50 mL/min. The dynamic curves of the samples were performed from room temperature to 1000 °C with a heating rate of 10 °C per min in an atmosphere of 15 vol.% CO2 in N2. The isothermal absorption performance of the sorbents was initiated by switching the gas from 100% N2 to 15 vol.% CO2 in N2 and held at that value for 120 minutes at the desired temperature (475, 525 and 575 °C). The cycling tests were also examined in a flow of 15% CO2 at 575 °C for sorption and a flow of pure N2 at 700 °C for desorption. The performance was repeated for 8 cycles, and the sorption and desorption processes lasted for 35 min and 20 min, respectively. 3. Results and discussion 3.1 Characterization The dynamic curves for three lithium halide-containing sorbents performed in an atmosphere of 15 vol.% CO2 are shown in Figure 1. The reference SCT began to absorb CO2 at ~500 °C and release CO2 beyond 640 °C. In this case, the highest uptake attained was merely 10.2 wt.%. Upon lithium halide addition, the initial absorption

of

three

sorbents

shifted

to

a

lower

temperature.

Other

alkali-carbonate-doped Li4SiO4 materials were also reported to exhibit similar results 37

. It is also clear that all three mixed sorbents presented a markedly improved CO2

uptake. Among the three sorbents, the highest uptake (~30.4 wt.%) at ~640 °C was 6


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achieved by adding LiF. However, between 440 and 540 °C, LiCl-containing sample had better CO2 sorption than LiF-containing sample. It was probably due to the fact that LiCl had lower melting temperature than LiF, which may be easier to facilitate the formation of eutectic phases of LiX-Li2CO3 (X=Cl or F) during CO2 chemisorption, and thus reduce the diffusion resistance of CO2 more efficiently. Additionally, the reaction rate for CO2 uptake was also significantly increased when mixed with lithium halides. In particular, the fastest absorption rate was obtained for the LiF-containing sample because its slope (between 440 and 640 °C) reached 0.15 wt.% min-1. The superior performance of the LiF-containing sample between 540 and 640 °C demonstrates that LiF serves as the most efficient additive among the three types of lithium halides. To further determine the different amounts of LiF on the absorption properties of the LiF-containing sample, extra isothermal absorption experiments were performed at 575 °C under 15 vol.% CO2. As shown in Figure 2, the weight of SCT increased slowly. Its absorption was not saturated within 120 min with the final absorption amount of 23.8 wt.%. When mixed with LiF, all sorbents presented a noticeably enhanced CO2 uptake with a faster reaction rate and a higher maximal absorption capacity. When the proportion of LiF increased from 6 mol% to 10 mol%, the reaction rate and maximal absorption capacity were both increased. However, as the amount of LiF increased further to 15 mol%, the CO2 uptake slightly decreased, mainly due to its more sintered morphologies, which will be observed by the following SEM images. Apparently, the optimal additive ratio is 10 mol%. After 120 mins of absorption, 7



SCT-LiF-0.1 presented a maximum uptake capacity of 36.2 wt.% under 15 vol.% CO2, which is the largest value reported in the literature to date 33, 39. To explain the superior CO2 absorption performance of SCT-LiF-0.1, the impact of different concentrations of LiF on the intrinsic properties of Li4SiO4 sorbents was examined. Powder XRD was used to identify the crystal phases of the three mixed sorbents. The patterns of the reference sample (SCT) were also included. As shown in Figure 3a, SCT showed the typical diffraction patterns of Li4SiO4 (JCPDS No.37-1472). When the additive amount was low (6 mol%), in addition to Li4SiO4, no secondary phases were detected in all mixed samples, suggesting that LiF may have been incorporated into Li4SiO4 to form a solid solution. When the additive amount was increased to 10 mol% or 15 mol%, only Li4SiO4 was detected. The solubility limit of SCT-LiF was higher than 15 mol%. According to our previous study40, the solubility limit of the NaF-doped sorbent was lower than 10 mol% because of a minute amount of the Li3NaSiO4 phase identified in SCT-NaF-0.1. Compared with NaF, LiF is more easily introduced into the Li4SiO4 structure due to the smaller atomic radius of lithium. Moreover, the peak wide observed for mixed samples changed as a function of the additive amount. Further by applying Scherrer’s equation, the smallest crystallite size of Li4SiO4 was obtained for SCT-LiF-0.1. Similar to NaCO3-doped Li4SiO437, LiF addition also reduced the crystallite size of Li4SiO4. To further analyze shift in the peak positions of SCT-LiF-0.1, the peaks in 2θ range of 21.0-23.0 were provided in detail (Figure 3b). The addition of LiF to the SCT sample resulted in the peak positions shifted slightly to higher angle as compared to 8


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that of the Li4SiO4 sample. Such peak shift can presumably be attributed to the substitution of O by F in the crystal lattice of Li4SiO4 leading to a lattice shrinkage consistent with Vigards law. SEM was used to examine the morphologies of these mixed Li4SiO4 samples. The reference SCT sample showed the co-existence of large and small particles with developed macropores (Figure 4a). Clearly, LiF addition induced significant morphological changes. When mixed with 6 mol% LiF, due to the effects of the solid solution, the particles slightly aggregated to form a connected structure accompanied by the reduction of macropores (Fig. 4b). As the additive amount increased to 10 mol%, these macropores completely disappeared. Meanwhile, the particles became small with nub-like structures (Fig. 4c). It appears that mixed with LiF controlled the growth of the particles. However, as the additive amount was further increased to 15 mol%, the particles exhibited more sintering and had a large particle size (exceeding 20 µm), which presumably resulted from the presence of excessive LiF. Figure 5 shows the EDS mapping images of SCT-LiF-0.1. All elements (O, Si and F) filed all the mapping images without any clear border, revealing that SCT-LiF-0.1 obtained quite uniform distribution of LiF. The porous structures of SCT-LiF-0.1 were further characterized by isothermal N2 adsorption-desorption measurements. Based on the IUPAC classifications, the isotherms for SCT-LiF-0.1 presented type Ⅱ isotherms with narrow H3 and H4 hysteresis loops (Figure 6), confirming the non-porous structure. This structure agrees well with the characteristics of the sample induced by the effects of the solid solution. 9



Furthermore, compared with the case of SCT sample in our previous work22, a significant decline in the surface area and pore volume was obtained for SCT-LiF-0.1 (Table 1), highlighting the sintered morphologies observed by the SEM images via the LiF addition procedure. Based on the XRD, SEM and N2 adsorption characteristics, SCT-LiF-0.1 presented small crystallite sizes and particle sizes, thereby noticeably facilitating the chemisorption processes. However, as confirmed by the phase diagram of LiF–Li2CO3 from FT salt-FACT salt databases, the formation of low-temperature eutectic phases (the mixture of Li2CO3 and LiF) were involved in the absorption process, thus decreasing the diffusion resistance. Furthermore, superior CO2 absorption properties were attained for SCT-LiF-0.1. More importantly, considering that the LiF addition had an inert activity with CO2 (inset of Figure 1), SCT-LiF-0.1 obtained a maximum uptake capacity of 36.2 wt.%, which exceeds ~102% of carbonation conversion (Figure 2b). Therefore, some CO2 must be trapped by a different mechanism. To further determine this phenomenon, the surface composition of SCT-LiF-0.1 was examined. As observed in Figure 7a, the existence of fluorine in SCT-LiF-0.1 has been proved. Furthermore, the detected binding energy was 681.5 eV. This value is smaller than that of LiF (684.8 eV, insert figure), indicating that fluorine entered the lattice of Li4SiO4. The addition of fluorine induced the oxidation state of O as show in Figure 7b. The O 1s spectra of SCT contained a broad peak centered at 531.5 eV, matching the typical characteristic of Li4SiO4.42 Comparatively, an additional 10


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low-energy shoulder was observed for SCT-LiF-0.1, corresponding to the Li2O species.43 Changes in the oxidation state of O were also reported for several halogen-doped Li4SiO4 materials successfully

substituted

in

the

40, 41

and clearly proved that the F addition

oxygen

site.

Moreover,

based

on

the

peak-differentiating analysis, the amount of Li2O determined for SCT-NaF-0.1 was 11.52% (Figure 7c), indicating the F addition induced a species of Li2O on Li4SiO4 surface. Similar to the O 1s spectra, slight changes in the oxidation state of Li 1s were generated due to Li addition (Figure 7d). The peak at ~54.0 eV was observed for SCT, which is consistent with the formation of the LixSiOy phase.

44

Upon addition, the

peak for SCT-LiF-0.1 shifted to a higher binding energy, which was related to the formation of Li2O on the surface at higher binding energies (~55.5 eV)

43

. These

changes also confirmed that LiF addition induced variations on the Li surrounding of Li4SiO4, which is also in good agreement with the results from NaF-doped Li4SiO4. According to the previous study45, Li2O can react with CO2 through the following reaction: Li2O + CO2 ↔ Li2CO3

(2)

It was evident that the maximum uptake capacity for Li2O was 147.2 wt.%, which is significantly larger than that of Li4SiO4. Presumably, the presence of a small amount of Li2O contained in SCT-LiF-0.1 may trap CO2, which resulted in the experimental capacity exceeding the theoretical maximum uptake capacity. To confirm this hypothesis, the phase composition of the products after reacting with CO2 was further determined. Fresh SCT-LiF-0.1 was subjected to adsorption 11



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experiments (in 15% CO2 for 2 h) at 475, 525 and 575 °C. The XRD patterns of sorption products were provided in Figure 8. Increasing the temperature from 475 to 575°C, Li4SiO4 phase diminished and both Li2SiO3 and Li2CO3 were generated. No additional phase was included in the product, which confirmed that both Eq (1) and Eq (2) occurred. 3.2 Kinetic analysis and stability Different isothermal experiments (Figure 9) were conducted at 475 °C, 525 °C, and 575 °C under 15% CO2 in a pure N2 atmosphere to examine the absorption kinetics. As expected, the absorption capacity of CO2 increased with increasing temperature. The isothermal data were fitted to a double exponential model24: y=Aexp(-k1t) + Bexp(-k2t) + C

(3)

where y represents the CO2 absorption capacity; t represents the time; A, B and C are the pre-exponential factors; and k1 and k2 represent the exponential constants indicating the CO2 chemisorption directly over the Li4SiO4 particles and the CO2 chemisorption rate controlled by diffusion process, respectively. The parameters obtained from this model are shown in Table 2. For these two sorbents, the value of k1 is far greater than the value of k2, implying that the diffusion of lithium is the critical factor limiting the total adsorption. Similar results were also reported by previous researchers.33-35 In addition, the values of k1 and k2 are much larger than the values for SCT sample, demonstrating that the surface chemisorption and the diffusion were both improved due to mixed with LiF. The activation enthalpy was calculated using Eying’s model: 12


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ln (k/T) = (-∆H++/ R) 1/T + ln (kB/h) + ∆S++/ R

(4)

where k represents the reaction rate constant, T is the absolute temperature, ∆H++ represents the activation enthalpy, R is the gas constant, kB is the Boltzmann constant, h is Planck's constant and ∆S++ is the activation entropy. As shown in Figure 10, the correspondence between the reaction rate constant of Ln(k/T) and 1/T is revealed. The addition of LiF caused a slight decrease in the activation enthalpy for chemisorption (decreasing from 70.17 kJ/mol to 63.25 kJ/mol), which means that the LiF addition causes the chemisorption to depend less on temperature. For the diffusion process, the activation enthalpy presented a completely opposite change, showing that the diffusion process for the mixed sample had a greater dependence on temperature. Other alkali-carbonate-doped Li4SiO4 sorbents also presented similar results

37

. The addition of LiF induced small crystallite sizes

and particle sizes as well as a species of Li2O on Li4SiO4 surface, ensuring a great abundance of Li-O sites on the surface of SCT-LiF-0.1. At the same time, LiF were structurally incorporated into Li4SiO4, accelerating the transport of Li+ and O2-. Moreover, CO2 only needed to diffuse through a molten layer with low resistance. All these characteristics favor the chemisorption process, reducing the energy necessary for the reaction. However, the addition of LiF did not seem to help with the whole diffusion process because the ∆H‡ was increased. The regeneration under cyclic CO2 absorption/desorption is an important requirement in practical systems. The cycling performance of SCT-LiF-0.10 was examine for 8 cycles. As seen in Figure 11, the maximum absorption capacity was 13



attained at ~34.8 wt.% after the first cycle. After 8 cycles, its maximum uptake also exceeded 32.5 wt.%, which confirms the remarkable regeneration. Mixing with alkali metals 37, 46 can facilitate the desorption of the sorbent at lower temperatures, allowing the cycled sorbents to be less sintered and resulting in good cyclabilities. In addition, the reaction rate became slightly solower as a function of cycles. It may be attributed to the sublimation of Li2O during desorptions, which will be further examined. 4. Conclusions Through the sacrificial carbon template method, we obtained various types of lithium halide-containing Li4SiO4 materials. All mixed sorbents presented a markedly improved CO2 uptake compared with SCT. Among them, the highest uptake was achieved by LiF addition, suggesting that LiF acts as the most efficient additive. Different concentrations of LiF addition also have a large influence on the intrinsic properties and absorption characteristics of the Li4SiO4 sorbents. LiF was introduced in Li4SiO4 to form a solid solution. The solubility limit was even higher than 15 mol%. When the additive amount was 10 mol%, the smaller crystallite size and particle size as well as a species of Li2O on Li4SiO4 surface was obtained for SCT-LiF-0.10, accounting for a maximum uptake capacity of 36.2 wt.%, which was ~102% of the carbonation conversion. Moreover, a high capacity was preserved over 8 sorption/desorption cycles, demonstrating that LiF-containing Li4SiO4 may have great potential for high-temperature CO2 capture. Acknowledgement This work was supported by financial supports from the Fundamental Research 14


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Table 1. N2 adsorption results of SCT and SCT-LiF-0.10 Samples

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

SCT a

11.9

3.1×10-2

5.12

SCT-LiF-0.10

3.6

0.5×10-2

3.42

a: the results form reference 22

Table 2. Kinetic parameters prepared from the isotherms of samples. Samples

SCT

SCT-LiF-0.10

T(°C)

k1 (s-1)

k2 (s-1)

R

A

B

C

475

1.59×10-3

1.36×10-4

0.999

-3.802

-12.290

116.538

525

4.07×10-3

3.49×10-4

0.999

-3.609

-12.663

116.397

575

6.76×10-3

7.85×10-4

0.999

-7.237

-19.030

122.912

475

3.10×10-3

2.21×10-4

0.999

-8.330

-19.421

127.703

525

3.20×10-3

8.50×10-4

0.999

-15.658

-19.484

133.318

575

1.20×10-2

5.30×10-3

0.995

65.893

-96.059

136.017

18


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Figure captions: Figure 1. Dynamic CO2 absorption properties of the Li4SiO4 sorbents and LiF (inset figure) in a 15% CO2 flux in the range of 100-1000 °C. Figure 2. CO2 uptake characteristics of different LiF-doped samples and SCT at 575 °C. (a) Weight increase; (b) carbonation conversion. Figure 3. XRD patterns of the samples. (a) Raw sorbent; (b) SCT and SCT-LiF-0.10 samples in 2θ range from 22° to 23°. Figure 4. SEM images of different Li4SiO4 samples. (a) SCT; (b) SCT-LiF-0.06; (c) SCT-LiF-0.10 and (d) SCT-LiF-0.15. Figure 5. The EDX mapping of the particles of SCT-LiF-0.10. (a) SEM imagine; (b) O maping; (c) Si maping and (d) F maping. Figure 6. N2 adsorption-desorption isotherms for SCT-LiF-0.10. Figure 7. XPS spectra of the samples. (a) Wide XPS spectrum of SCT-LiF-0.10; (b) O1s spectra; (c) peak fitting of O 1s spectra for SCT-LiF-0.10; (d) Li 1s spectra. Figure 8 XRD patterns of the SCT-LiF-0.10 sorbent after sorption. Figure 9. Isothermal thermograms at different temperatures. Figure 10. Eyring's plots for the rate constants of surface chemisorption (k1) and bulk diffusion (k2). Figure 11. Cyclic performance of the SCT-LiF-0.10 sample during 8 cycles (a flow of 15% CO2 at 575 °C for sorption and a flow of pure N2 at 700 °C for desorption).

19



Figure 1.

20


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b

a

Figure 2.

21



a

b

Figure 3.

22


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a

b

c

d

Figure 4.

23



a

c

b

d

Figure 5.

24


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Figure 6.

25



b

a

c

d

Figure 7.

26


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Figure 8.

27



Figure 9.

28


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Figure 10.

29



Figure 11.

30


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TOC

31


Synthesis of LiF-Containing Li4SiO4 as Highly Efficient CO2 Sorbents

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