Environmental Benign Synthesis of Lithium Silicates and Mg-Al Layered Double Hydroxide from Vermiculite Mineral for CO 2 Capture

This research introduces a completely new environmental benign synthesis route for obtaining two kinds of inter-mediate and high temperature CO2 sorbents, Mg-Al layered double hydroxide (LDH) and Li4SiO4, from vermiculite. The mineral vermiculite was leached with acid, from which the obtained SiO2 was used for the synthesis of Li4SiO4 and the leaching waste water was used for the synthesis of Mg-Al LDH. Therefore, no waste was produced during the whole process. Both Li4SiO4 and Mg-Al LDH sorbents were carefully characterized using XRD, SEM, and BET analyses. The CO2 capturing performance of these two sorbents was comprehensively evaluated. The influence of the Li/Si ratio, calcination temperature, calcination time, and sorption temperature on the CO2 sorption capacity of Li4SiO4, and the sorption temperature on the CO2 sorption capacity of LDH, were investigated. The optimal leaching acid concentration for vermiculite and the CO2 sorption/desorption cycling performance of both the Li4SiO4 and Mg-Al LDH sorbents were determined. In sum, this demonstrated a unique and environment-friendly scheme for obtaining two CO2 sorbents from cheap raw materials, and this idea is applicable to the efficient utilization of other minerals.


Introduction
Carbon dioxide (CO 2 ) has been regarded as one of the most important greenhouse gases that contributes to the effects of global warming and climate change [1].Fossil fuel power plants are responsible for >85% of the worldwide electrical energy demand [2].Fossil fuel combustion is a major CO 2 source, which accounts for >40% of the total manmade CO 2 among all of the different types of CO 2 emissions [3].In recent years, pre-combustion CO 2 capture from sorption enhanced hydrogen production (SEHP) processes has attracted considerable attention for capturing CO 2 from flue gas [4][5][6][7][8][9][10][11].For the SEHP technique, one of the key challenges is the selection of a suitable solid CO 2 sorbent [12,13].To date, several types of high-temperature CO 2 sorbents have been investigated, for instance, layered double hydroxide-(LDH) derived mixed oxides [14,15], and MgO [16] for the sorption enhanced water gas shift (SEWGS) process, and CaO [17,18], alkali zirconates [19], alkali dititanates [20], and alkali silicates [21,22] for the sorption enhanced steam reforming (SESR) process.
Compared with many other high-temperature CO 2 sorbents, Li 4 SiO 4 is considered as one of the most promising sorbents due to its high sorption capacity, good thermal stability and reproducibility [1].The Li 4 SiO 4 sorbent mainly works in the temperature range of 500-750 • C for the SESR process.The CO 2 capture mechanism on Li 4 SiO 4 can be explained by the reversible reaction between the Li 4 SiO 4 crystal structure and CO 2 , as described in reaction (1).The maximum CO 2 sorption capacity can thus be theoretically calculated as 36.7 wt %.However, the CO 2 capture capacities of synthesized Li 4 SiO 4 reported in the literature are often much lower than the theoretical value, due to the mass transport limitation and severe sintering issues.In order to achieve an appreciable CO 2 capture performance of Li 4 SiO 4 sorbents, several attempts have been made, including microstructural modification, alkali promotion, transition metal doping, and Li substitution by Na, etc. [12].Li 4 SiO 4 + CO 2 ↔ Li 2 SiO 3 + Li 2 CO 3 (1) LDH-derived mixed oxides have been intensively studied as intermediate temperature CO 2 for the SEWGS process.According to mechanistic studies, the active Mg-O basic sites are believed to be responsible for CO 2 capture and the possible reaction can be described using Equation (2) [15,23].Previously, great effort was devoted to the improvement of the CO 2 capture capacity and the long-term stability of LDH-based sorbents.For instance, the research activities mainly focused on the influence of cations (both divalent and trivalent), influence of anions (both organic and inorganic), influence of LDH morphology and particle size, the promoting effect of alkali carbonates, and the preparation of LDH-based inorganic hybrids [12,24,25].
In addition to the work on improving the CO 2 capture capacity and stability, another research effort has been made to reduce the cost of sorbents by using cheap raw materials, for instance, industrial solid wastes or minerals.So far, there are a few reports on the synthesis of Li 4 SiO 4 from waste silicon-containing materials including fly ash [26], rice husk ashes [27], and halloysite [28].The Li 4 SiO 4 which was synthesized with rice husk ashes achieved a 30.5 wt % maximum sorption at 680 • C and the Li 4 SiO 4 which was synthesized with halloysite reached a maximum adsorption capacity of 34.75 wt % at 700 • C.However, there are almost no reports on the synthesis of LDH-derived CO 2 sorbents from either industrial solid wastes or minerals.
Vermiculite, a lamellar hydrated aluminum iron magnesium silicate, is a clay mineral with a 2:1 crystalline structure, which contains silicon dioxide (SiO 2 ), magnesium oxide (MgO), aluminum oxide (Al 2 O 3 ), and some other metal oxides [29][30][31].Vermiculite has an unusual property of expanding due to the interlamellar generation of steam.Due to the large content of the silicon element, vermiculite has been a cheap precursor for the synthesis of SiO 2 [24,[32][33][34].However, in order to remove the impure elements, a leaching pretreatment in strong acids is normally required.During this pretreatment process, a large amount of acidic wastewater with metal ions including Mg, Al, and Fe, is produced.The discharge of such wastewater causes severe environmental problems [27,[35][36][37].Thus, an environmentally benign scheme for the utilization of the vermiculite mineral is highly desired for a more sustainable society.
In this contribution, we developed a novel environmental benign synthesis route for obtaining two types of CO 2 capturing materials (lithium silicates and LDHs) from vermiculite.Throughout the whole synthesis process, no waste is emitted.The performance of the obtained lithium silicates and LDH-based CO 2 sorbents were evaluated under each of their suitable conditions.The long-term stability of the sorbents was investigated during the CO 2 sorption and desorption cycles.

Environmentally Benign Synthesis and Characterization of Li 4 SiO 4 and LDHs
The environmentally benign synthesis scheme for obtaining Li 4 SiO 4 and Mg-Al-CO 3 LDH from the vermiculite mineral is illustrated in Figure 1.The raw vermiculite mineral was first thermally pretreated at 900 • C to produce expanded vermiculite (EXVMT) with a higher specific surface area.The chemical composition of the samples was determined by X-ray fluorescence (XRF) spectroscopy (Panalytical Axios-mAX), which included SiO 2 (40.8 wt %), MgO (21.7 wt %), Al 2 O 3 (20.8wt %), K 2 O (6.5 wt %), Fe 2 O 3 (5.0wt %), CaO (2.9 wt %), Na 2 O (1.4 wt %), and TiO 2 (0.9 wt %).Then, the obtained EXVMT was leached in acidic solution at 80 • C, by which SiO 2 powder and an acidic wastewater containing Mg 2+ , Al 3+ , and other metals, were formed (Figure 1a).Following this, Li 4 SiO 4 could be synthesized using a solid-state method by reacting the above obtained SiO 2 powder with lithium nitrate (Figure 1b), and Mg-Al-CO 3 LDH could be prepared using a co-precipitation method from the above obtained acidic wastewater (Figure 1c).During the whole synthesis procedure, both the obtained SiO 2 and the acidic wastewater were utilized, representing a promising and environmentally benign route for converting vermiculite minerals into high-value added products.The chemical composition of the samples was determined by X-ray fluorescence (XRF) spectroscopy (Panalytical Axios-mAX), which included SiO2 (40.8 wt %), MgO (21.7 wt %), Al2O3 (20.8 wt %), K2O (6.5 wt %), Fe2O3 (5.0 wt %), CaO (2.9 wt %), Na2O (1.4 wt %), and TiO2 (0.9 wt %).Then, the obtained EXVMT was leached in acidic solution at 80 °C, by which SiO2 powder and an acidic wastewater containing Mg 2+ , Al 3+ , and other metals, were formed (Figure 1a).Following this, Li4SiO4 could be synthesized using a solid-state method by reacting the above obtained SiO2 powder with lithium nitrate (Figure 1b), and Mg-Al-CO3 LDH could be prepared using a co-precipitation method from the above obtained acidic wastewater (Figure 1c).During the whole synthesis procedure, both the obtained SiO2 and the acidic wastewater were utilized, representing a promising and environmentally benign route for converting vermiculite minerals into high-value added products.In this synthesis process, one of the key parameters that affects the yield and quality of the SiO2 and LDH products is the concentration used during leaching.In this work, four different concentrations of acid, including 1, 3, and 5 M HNO3, and 6 M aqua regia, were used, and the obtained amounts of SiO2 and LDH are summarized in Table 1.With the increase in the concentration of acid, the weight of the obtained SiO2 decreased from 5.83 g with 1 M HNO3, to 3.13 g with 6 M aqua regia.The XRD analyses of these SiO2 samples suggest that a mild acid leaching treatment induces a low purity SiO2.In contrast, a severe acidic treatment led to the leaching out of almost all of the elements, thus producing a high-purity SiO2.This is the reason why a smaller SiO2 sample was obtained when the concentration of the acid was high.In contrast, the weight of the obtained LDHs increased with an increase in the concentration of the acid.This is because more elements were leached out into the acidic wastewater, which was consequently precipitated into LDHs by reacting with Na2CO3 and NaOH.This result clearly indicates that the concentration of leaching acid greatly influences the final In this synthesis process, one of the key parameters that affects the yield and quality of the SiO 2 and LDH products is the concentration used during leaching.In this work, four different concentrations of acid, including 1, 3, and 5 M HNO 3 , and 6 M aqua regia, were used, and the obtained amounts of SiO 2 and LDH are summarized in Table 1.With the increase in the concentration of acid, the weight of the obtained SiO 2 decreased from 5.83 g with 1 M HNO 3 , to 3.13 g with 6 M aqua regia.The XRD analyses of these SiO 2 samples suggest that a mild acid leaching treatment induces a low purity SiO 2 .In contrast, a severe acidic treatment led to the leaching out of almost all of the elements, thus producing a high-purity SiO 2 .This is the reason why a smaller SiO 2 sample was obtained when the concentration of the acid was high.In contrast, the weight of the obtained LDHs increased with an increase in the concentration of the acid.This is because more elements were leached out into the acidic wastewater, which was consequently precipitated into LDHs by reacting with Na 2 CO 3 and NaOH.This result clearly indicates that the concentration of leaching acid greatly influences the final products, which may eventually influence the CO 2 capture performance of the final Li 4 SiO 4 and LDH-derived sorbents.The synthesis process was monitored using various material characterization tools.Figure 2a shows the XRD patterns of VMT and EX-VMT.VMT is a clay mineral consisting of negatively charged silicate layers, with a thickness of about 1 nm.The negative charge arises from the substitution of Al 3+ by Si 4+ in tetrahedral sites.Before thermal treatment, the raw VMT showed an interlayer spacing of 1.10 nm, and the characteristic d 002 reflection peak was observed at 8.02 • .This interlayer spacing is slightly narrower than the reported value of 1.44 nm.This phenomenon might be due to the drying in an oven at 100 • C prior to XRD analysis, which caused the partial dehydration of the sample.After thermal treatment at 900 • C, the interlayer distance of the EX-VMT was further narrowed.The characteristic d 002 peak shifted to a higher 2θ degree of 8.78 • , corresponding to an interlayer spacing of 1.01 nm.This change can be attributed to the loss of the hydration water of the interlayer cations [38,39].Figure 2b shows the SEM image of raw VMT, from which the layered structure can be clearly observed, in which all of the layers are tightly packed.However, an abrupt heating of vermiculite at 900 • C induced the exfoliation of VMT, as shown in Figure 2c.In addition, the deformation of the surface of EX-VMT can also be seen, which was due to the spontaneous release of interlayer water molecules [40].The XRD pattern of residual solids confirmed that its majority phase was amorphous SiO 2 (Figure 2d).
Then, the obtained Li 4 SiO 4 and LDH were also characterized by XRD, BET, and SEM techniques, as shown in Figure 3.The XRD patterns in Figure 3a) clearly indicate that both Li 4 SiO 4 and LDH were successfully synthesized.For Li 4 SiO 4 , characteristic peaks at 22.18 • , 33.79 • , and 38.19 • , were observed, which can be attributed to the reflections of −110, 200, and −121 planes.For LDH, the characteristic peaks at 11.68 • , 23.53 • , 35.61 • , 39.62 • , and 47.19 • were observed, being attributed to the reflections of 003, 006, 009, 015, and 018 planes.The interlayered distance d 003 is 0.74 nm, which is similar to the literature reports [41].Figure 3b shows the N 2 adsorption/desorption isothermal of synthesized Li 4 SiO 4 , which suggests that the sample is non porous and that its specific surface area was quite low (1.3 m 2 •g −1 ).The morphologies of the Li 4 SiO 4 and LDH samples were studied using SEM analysis, as shown in Figure 3c,d.Li 4 SiO 4 exhibited relatively dense polyhedral particles, which is common for samples synthesized by the solid-state method.The SEM analysis is consistent with the BET result.The Mg-Al-CO 3 LDH showed a petaloid structure consisting of a nanosheet [15,[42][43][44].The formation of this petaloid-like morphology can be attributed to the synthesis pH of 10.When the pH value equals the isoelectric point of Mg-Al-CO 3 LDH, its surface will be close to neutral and the LDH particles will preferably grow into a petaloid structure [45].All of the aforementioned analyses indicate that both Li 4 SiO 4 and Mg-Al-CO 3 LDH can be successfully synthesized from the vermiculite mineral.In the following section, their CO 2 capturing performance will be comprehensively evaluated under various conditions.characteristic d002 peak shifted to a higher 2θ degree of 8.78°, corresponding to an interlayer spacing of 1.01 nm.This change can be attributed to the loss of the hydration water of the interlayer cations [38,39].Figure 2b shows the SEM image of raw VMT, from which the layered structure can be clearly observed, in which all of the layers are tightly packed.However, an abrupt heating of vermiculite at 900 °C induced the exfoliation of VMT, as shown in Figure 2c.In addition, the deformation of the surface of EX-VMT can also be seen, which was due to the spontaneous release of interlayer water molecules [40].The XRD pattern of residual solids confirmed that its majority phase was amorphous SiO2 (Figure 2d).

CO 2 Capture Performance of Synthesized Li 4 SiO 4 and LDH-Based Sorbents
In order to investigate the effect of the Li/Si molar ratio on the CO 2 sorption performance, Li 4 SiO 4 -based sorbents were synthesized with different Li/Si mole ratios, ranging from three to 10.
The SiO 2 was obtained from EX-VMT, which was treated with 6 M aqua regia.All Li 4 SiO 4 -(EX-VMT)-x (x = 3, 4, 5, 6, 7, 8, 9, and 10) were synthesized by a thermal treatment of 750 • C for 6 h.The CO 2 sorption capacities of all samples were evaluated at 650 • C for 2 h, under pure CO 2 .Figure 4a and Table 2 show that when the Li/Si molar ratio was three, the sorption capacity was relatively low (ca.25.21 wt %).When the Li/Si molar ratio was increased to four, the CO 2 sorption capacity rapidly improved to 30.47 wt %, reaching 93.7% of the theoretical sorption capacity of Li 4 SiO 4 (32.52 wt %).However, when the Li/Si molar ratio was in the range of five to nine, the CO 2 sorption capacities did not show any significant increase, and were all around 25.97-36.21wt %.Some of the values were even higher than the theoretical sorption capacity of Li 4 SiO 4 (32.52 wt %), which is probably due to the formation of Li 8 SiO 6 .The following XRD analysis of all of the samples clearly confirms this hypothesis.In particular, when the Li/Si molar ratio was 10, the CO 2 sorption capacity sharply improved to 57.93 wt %.However, although the formation of Li 8 SiO 6 can significantly improve the CO 2 sorption capacity, its sorption/desorption cycling performance was normally very poor.In other words, due to the even worse reversible CO 2 sorption capacity, the formation of Li 8 SiO 6 is normally prevented and Li 4 SiO 4 is preferred.

CO2 Capture Performance of Synthesized Li4SiO4 and LDH-Based Sorbents
In order to investigate the effect of the Li/Si molar ratio on the CO2 sorption performance, Li4SiO4-based sorbents were synthesized with different Li/Si mole ratios, ranging from three to 10.The SiO2 was obtained from EX-VMT, which was treated with 6 M aqua regia.All Li4SiO4-(EX-VMT)x (x = 3, 4, 5, 6, 7, 8, 9, and 10) were synthesized by a thermal treatment of 750 °C for 6 h.The CO2 sorption capacities of all samples were evaluated at 650 °C for 2 h, under pure CO2. Figure 4a and Table 2 show that when the Li/Si molar ratio was three, the sorption capacity was relatively low (ca.25.21 wt %).When the Li/Si molar ratio was increased to four, the CO2 sorption capacity rapidly improved to 30.47 wt %, reaching 93.7% of the theoretical sorption capacity of Li4SiO4 (32.52 wt %).However, when the Li/Si molar ratio was in the range of five to nine, the CO2 sorption capacities did not show any significant increase, and were all around 25.97-36.21wt %.Some of the values were even higher than the theoretical sorption capacity of Li4SiO4 (32.52 wt %), which is probably due to the formation of Li8SiO6.The following XRD analysis of all of the samples clearly confirms this hypothesis.In particular, when the Li/Si molar ratio was 10, the CO2 sorption capacity sharply improved to 57.93 wt %.However, although the formation of Li8SiO6 can significantly improve the CO2 sorption capacity, its sorption/desorption cycling performance was normally very poor.In other words, due to the even worse reversible CO2 sorption capacity, the formation of Li8SiO6 is normally prevented and Li4SiO4 is preferred.In order to explain the influence of the Li/Si molar ratio on the CO 2 sorption capacity of synthesized Li 4 SiO 4 -based sorbents, an XRD analysis of all samples was performed, as shown in Figure 4b.For all samples, the characteristic peaks of Li 4 SiO 4 were detected, suggesting the existence of the Li 4 SiO 4 phase.When the Li/Si molar ratio was three, a small portion of Li 2 SiO 3 was detected.Li 2 SiO 3 has no CO 2 sorption ability at high temperatures, which explains the relatively low CO 2 sorption capacity of Li 4 SiO 4 -(EX-VMT)-3 [46].When the Li/Si molar ratio reached four and five, relatively pure Li 4 SiO 4 was synthesized.Moreover, when the Li/Si molar ratio increased from six to nine, the phases of both LiOH and Li 8 SiO 6 appeared.In particular, at Li/Si = 10, the characteristic peaks of Li 2 O and Li 8 SiO 6 became more obvious, thus explaining the high CO 2 sorption capacity of Li 4 SiO 4 -(EX-VMT)-10.According to the literature, the regeneration performance of Li 2 O and Li 8 SiO 6 is very poor, which means that the sample is not promising for practical applications [46].In order to obtain the Li 4 SiO 4 -based CO 2 sorbent that has a high CO 2 sorption capacity and good cycling performance, the Li 4 SiO 4 -(EX-VMT)-4 sample was selected as the optimal sample for further study.
Then, the synthesis temperature for the Li 4 SiO 4 -based sorbents was optimized to further improve the CO 2 sorption capacity.Different Li 4 SiO 4 -(EX-VMT)-4 samples were synthesized at temperatures of 650, 700, 750, and 800 • C. Their CO 2 sorption capacities were evaluated using TGA analysis at 650 • C, as shown in Figure 5a.The CO 2 sorption capacity initially increased slightly from 34.12 wt % to 34.39 wt %, with an increase in the synthesis temperature from 650 to 700 • C.However, the CO 2 sorption capacity started to decrease with a further increase in the synthesis temperature.At 750 and 800 • C, the CO 2 sorption capacity diminished to 29.46 wt % and 18.74 wt %, respectively.These results confirm that the best synthesis temperature of the Li 4 SiO 4 -(EX-VMT)-4 sorbent is 700 • C. Figure 5b shows the XRD patterns of the sorbents synthesized at different temperatures.For all samples, the Li 4 SiO 4 phase is the main phase.With an increase in the synthesis temperature, the peak intensity became slightly sharper, suggesting that the decrease in the CO 2 sorption capacity might be due to the sintering of sorbents at high temperatures.
Besides, the influence of the calcination time on the CO 2 sorption capacity of Li 4 SiO 4 -(EX-VMT)-4 sorbents was also studied, as shown in Figure 5c.When the samples were synthesized for 5, 6, and 7 h, the CO 2 sorption capacity of all samples was quite similar, exhibiting values of 33.63 wt %, 34.39 wt %, and 33.95 wt %, respectively.The XRD patterns of these three samples are presented in Figure 5d.The results confirmed that an optimized synthesis temperature of 700 • C led to the Li 4 SiO 4 main phase being barely affected by the calcination time.
For a practical application purpose, the CO 2 sorption capacity of the Li 4 SiO 4 -(EX-VMT)-4 sorbent under different operating conditions was investigated.Figure 6a shows the influence of sorption temperature on the CO 2 sorption capacity of the Li 4 SiO 4 -(EX-VMT)-4 sorbent.The sorbent showed the best sorption performance and the fastest sorption kinetics at 650 • C, with a CO 2 sorption capacity of 34.39 wt %.Compared with other Li 4 SiO 4 -based sorbents that were also synthesized from cheap SiO 2 precursors, the Li 4 SiO 4 -(EX-VMT)-4 sample obtained in this research exhibited a higher CO 2 capacity [27,28].When the sorption temperature dropped to 600 • C, the CO 2 sorption capacity of Li 4 SiO 4 -(EX-VMT)-4 was only 22.11 wt %, which may be due to the kinetic limitation at a low sorption temperature.However, when the sorption temperature reached 700 • C, the Li 4 SiO 4 -(EX-VMT)-4 sorbent displayed almost no sorption capacity (2.51 wt %).This may be due to the activation of CO 2 desorption from Li 4 SiO 4 at 700 • C. Considering these findings, it can be concluded that the best sorption temperature for the Li 4 SiO 4 -(EX-VMT)-4 sorbent is 650 • C. 750 and 800 °C, the CO2 sorption capacity diminished to 29.46 wt % and 18.74 wt %, respectively.These results confirm that the best synthesis temperature of the Li4SiO4-(EX-VMT)-4 sorbent is 700 °C. Figure 5b shows the XRD patterns of the sorbents synthesized at different temperatures.For all samples, the Li4SiO4 phase is the main phase.With an increase in the synthesis temperature, the peak intensity became slightly sharper, suggesting that the decrease in the CO2 sorption capacity might be due to the sintering of sorbents at high temperatures.Besides, the influence of the calcination time on the CO2 sorption capacity of Li4SiO4-(EX-VMT)-4 sorbents was also studied, as shown in Figure 5c.When the samples were synthesized for 5, 6, and 7 h, the CO2 sorption capacity of all samples was quite similar, exhibiting values of 33.63 wt %, 34.39 wt %, and 33.95 wt %, respectively.The XRD patterns of these three samples are presented in Figure 5d.The results confirmed that an optimized synthesis temperature of 700 °C led to the Li4SiO4 main phase being barely affected by the calcination time.
For a practical application purpose, the CO2 sorption capacity of the Li4SiO4-(EX-VMT)-4 sorbent under different operating conditions was investigated.Figure 6a shows the influence of sorption temperature on the CO2 sorption capacity of the Li4SiO4-(EX-VMT)-4 sorbent.The sorbent showed the best sorption performance and the fastest sorption kinetics at 650 °C, with a CO2 sorption capacity of 34.39 wt %.Compared with other Li4SiO4-based sorbents that were also synthesized from cheap SiO2 precursors, the Li4SiO4-(EX-VMT)-4 sample obtained in this research exhibited a higher CO2 capacity [27,28].When the sorption temperature dropped to 600 °C, the CO2 sorption capacity of Li4SiO4-(EX-VMT)-4 was only 22.11 wt %, which may be due to the kinetic limitation at a low sorption temperature.However, when the sorption temperature reached 700 °C, the Li4SiO4-(EX-VMT)-4 sorbent displayed almost no sorption capacity (2.51 wt %).This may be due to the activation of CO2 desorption from Li4SiO4 at 700 °C.Considering these findings, it can be concluded that the best sorption temperature for the Li4SiO4-(EX-VMT)-4 sorbent is 650 °C.
In addition to the Li4SiO4-based sorbent, the CO2 capture performance of the Mg-Al-CO3 LDHderived sorbents was also studied in this work, as shown in Figure 6b.All of the samples were calcined at 400 °C before the CO2 sorption tests were performed, and the performances at different temperatures of 200, 300, and 400 °C were evaluated.The results indicated that the Mg-Al-CO3 LDHderived CO2 sorbent led to the highest CO2 sorption capacity at 200 °C, being 2.08 wt %.This value is similar to the LDH-derived CO2 sorbent reported in the literature [45].However, the CO2 sorption capacity became much lower when the sorption temperature was higher, with values of 1.21 wt % and 1.23 wt % at 300 and 400 °C, respectively.This result suggests that the LDHs synthesized from the vermiculite leaching wastewater can deliver a similar CO2 capture performance to those of pure LDH samples.In addition to the Li 4 SiO 4 -based sorbent, the CO 2 capture performance of the Mg-Al-CO 3 LDH-derived sorbents was also studied in this work, as shown in Figure 6b.All of the samples were calcined at 400 • C before the CO 2 sorption tests were performed, and the performances at different temperatures of 200, 300, and 400 • C were evaluated.The results indicated that the Mg-Al-CO 3 LDH-derived CO 2 sorbent led to the highest CO 2 sorption capacity at 200 • C, being 2.08 wt %.This value is similar to the LDH-derived CO 2 sorbent reported in the literature [45].However, the CO 2 sorption capacity became much lower when the sorption temperature was higher, with values of 1.21 wt % and 1.23 wt % at 300 and 400 • C, respectively.This result suggests that the LDHs synthesized from the vermiculite leaching wastewater can deliver a similar CO 2 capture performance to those of pure LDH samples.

Optimization of the Synthesis Strategy
As we have shown in the above section that the concentration of leaching acid has an important influence on the chemical composition of obtained SiO 2 and LDHs samples, we found it worthy to investigate the influence of the leaching acid on the CO 2 capture performance of synthesized Li 4 SiO 4 and LDH-based sorbents.From an economical and environmental point of view, using a lower concentration of leaching acid, while maintaining a relatively good CO 2 capture performance, is warranted.Therefore, in this section, the synthesis strategy for obtaining two types of CO 2 sorbents, Li 4 SiO 4 and LDH, from a vermiculite mineral was further optimized by using different concentrations of leaching acids, such as 1, 3, and 5 M HNO 3 and 6 M aqua regia.The obtained samples were denoted as Li 4 SiO 4 -(EX-VMT)-4 (xM) (x = 1, 3, 5, and 6) and Mg-Al-CO 3 LDH-xM (x = 1, 3, 5, and 6), respectively.
Figure 7a shows the CO 2 capture performance of Li 4 SiO 4 -(EX-VMT)-4 (xM) (x = 1, 3, 5, and 6) samples in pure CO 2 at 650 • C. When comparing all of the samples, the CO 2 sorption capacity of Li 4 SiO 4 -(EX-VMT)-4 (3M) has the lowest sorption capacity (31.87 wt %).With an increase in the concentration of acid, the CO 2 sorption capacity of Li 4 SiO 4 -(EX-VMT)-4 (5M) and Li 4 SiO 4 -(EX-VMT)-4 (6M) also increased, reaching 34.21 wt % and 34.39 wt %, respectively.It is surprising that the highest CO 2 sorption capacity was achieved with the 1 M acid.This is due to the synthesis scheme we used for the synthesis of the Li 4 SiO 4 -(EX-VMT)-4 (xM) samples.When calculating the Li/Si ratio of four, we assumed that all of the obtained solid product was SiO 2 .However, the SiO 2 sample obtained with 1 M acid still contained impurities, which led to a Li/Si ratio higher than four.The high CO 2 sorption capacity of the Li 4 SiO 4 -(EX-VMT)-4 (1M) sample may be due to the existence of the Li 2 O phase.
In order to gain a deeper understanding of the influence of the concentration of acid on the CO 2 sorption performance, the XRD patterns of Li 4 SiO 4 -(EX-VMT)-4 (xM) samples were examined, as shown in Figure 7b.When the concentration of acid was 6 M, the sample only consisted of Li 4 SiO 4 .When decreasing the concentration of acid from 5 to 1 M, the MgO phase appeared, which proved that a low acidic concentration will produce impurities in SiO 2 .Hence, this consequently influences the CO 2 sorption capacity of the samples.However, when the concentration of acid was too low, the content of SiO 2 became lower, which means that the Li/Si molar ratio was increased.Excess lithium led to the formation of the Li 2 O phase in the Li 4 SiO 4 -based sorbent.Previous studies have confirmed that Li 2 O has a higher CO 2 sorption capacity, but worse regeneration property, than Li 4 SiO 4 .
Figure 8a shows the CO 2 sorption capacities of Mg-Al-CO 3 LDH-xM (x = 1, 3, 5, and 6) samples.It appears that the CO 2 sorption capacity gradually increased with an increase in the concentration of acid from 1 to 6 M. For example, with a concentration of acid of 1, 3, 5, and 6 M, the CO 2 sorption capacity was 1.3wt %, 1.57 wt %, 1.63 wt %, and 2.08 wt %, respectively.Mg-Al-CO 3 LDH-6 M showed the highest CO 2 sorption capacity of 2.08 wt % at 200 • C. Figure 8b shows the XRD patterns of Mg-Al-CO 3 LDH-xM (x = 1, 3, 5, and 6).All of the synthesized samples formed a typical layered double hydroxide structure.This data suggests that there is no significant difference in their phase structure.The difference in their CO 2 capture capacity might be due to the Mg content.As is shown in Figure 7b, when the concentration of acid was low, the Mg element could not be completely leached out from the SiO 2 .Thus, the relatively lower Mg content in Mg-Al-CO 3 LDH-xM results in a lower CO 2 sorption capacity.In order to gain a deeper understanding of the influence of the concentration of acid on the CO2 sorption performance, the XRD patterns of Li4SiO4-(EX-VMT)-4 (xM) samples were examined, as shown in Figure 7b.When the concentration of acid was 6 M, the sample only consisted of Li4SiO4.When decreasing the concentration of acid from 5 to 1 M, the MgO phase appeared, which proved that a low acidic concentration will produce impurities in SiO2.Hence, this consequently influences the CO2 sorption capacity of the samples.However, when the concentration of acid was too low, the content of SiO2 became lower, which means that the Li/Si molar ratio was increased.Excess lithium led to the formation of the Li2O phase in the Li4SiO4-based sorbent.Previous studies have confirmed that Li2O has a higher CO2 sorption capacity, but worse regeneration property, than Li4SiO4.
Figure 8a shows the CO2 sorption capacities of Mg-Al-CO3 LDH-xM (x = 1, 3, 5, and 6) samples.It appears that the CO2 sorption capacity gradually increased with an increase in the concentration of acid from 1 to 6 M. For example, with a concentration of acid of 1, 3, 5, and 6 M, the CO2 sorption capacity was 1.3wt %, 1.57 wt %, 1.63 wt %, and 2.08 wt %, respectively.Mg-Al-CO3 LDH-6 M showed the highest CO2 sorption capacity of 2.08 wt % at 200 °C.Figure 8b shows the XRD patterns of Mg-Al-CO3 LDH-xM (x = 1, 3, 5, and 6).All of the synthesized samples formed a typical layered double hydroxide structure.This data suggests that there is no significant difference in their phase structure.The difference in their CO2 capture capacity might be due to the Mg content.As is shown in Figure 7b, when the concentration of acid was low, the Mg element could not be completely leached out from the SiO2.Thus, the relatively lower Mg content in Mg-Al-CO3 LDH-xM results in a lower CO2 sorption capacity.

Cycling Stability of Obtained CO2 Sorbents
For the practical application of CO2 sorbents, particularly at high temperatures, the CO2 sorption/desorption cycling stability is another important aspect.Thus, the cycling stability of the Li4SiO4-(EX-VMT)-4 (xM) (x = 1, 3, and 6) samples was investigated.Both sorption and desorption were performed at 650 °C for 30 min.Figure 9a shows that when the concentration of acid was 6 M, the cycle stability of the obtained Li4SiO4-(EX-VMT)-4 (6M) was poor.The CO2 sorption capacity started to decline quickly after its seventh cycle, which might be due to the sintering of the sorbent.After 10 cycles, the capacity decreased from its initial 29.37 wt %, to 25.86 wt %.However, with the decrease in leaching acid concentration, the cycling stability of the sorbents was slightly improved.For instance, the decline rate of Li4SiO4-(EX-VMT)-4 (3M) was slower than that of Li4SiO4-(EX-VMT)-4 (6M), and its CO2 sorption capacity started to decline during its tenth cycle.This improved stability might be attributed to the separating and supporting effect from the impurity phases within this sample.This speculation was further confirmed by the result from Li4SiO4-(EX-VMT)-4 (1M).When the leaching acid concentration was as low as 1 M, more impurities remained in the sorbent, thus leading to a decreased CO2 sorption capacity (1.17 wt %).However, it is very clear that the overall stability was improved, without almost any change during the 15 cycles.Thus, these data suggested a promising way to design highly stable Li4SiO4-based CO2 sorbents, by introducing certain inert separating and supporting materials.

Cycling Stability of Obtained CO 2 Sorbents
For the practical application of CO 2 sorbents, particularly at high temperatures, the CO 2 sorption/desorption cycling stability is another important aspect.Thus, the cycling stability of the Li 4 SiO 4 -(EX-VMT)-4 (xM) (x = 1, 3, and 6) samples was investigated.Both sorption and desorption were performed at 650 • C for 30 min.Figure 9a shows that when the concentration of acid was 6 M, the cycle stability of the obtained Li 4 SiO 4 -(EX-VMT)-4 (6M) was poor.The CO 2 sorption capacity started to decline quickly after its seventh cycle, which might be due to the sintering of the sorbent.After 10 cycles, the capacity decreased from its initial 29.37 wt %, to 25.86 wt %.However, with the decrease in leaching acid concentration, the cycling stability of the sorbents was slightly improved.For instance, the decline rate of Li 4 SiO 4 -(EX-VMT)-4 (3M) was slower than that of Li 4 SiO 4 -(EX-VMT)-4 (6M), and its CO 2 sorption capacity started to decline during its tenth cycle.This improved stability might be attributed to the separating and supporting effect from the impurity phases within this sample.This speculation was further confirmed by the result from Li 4 SiO 4 -(EX-VMT)-4 (1M).When the leaching acid concentration was as low as 1 M, more impurities remained in the sorbent, thus leading to a decreased CO 2 sorption capacity (1.17 wt %).However, it is very clear that the overall stability was improved, without almost any change during the 15 cycles.Thus, these data suggested a promising way to design highly stable Li 4 SiO 4 -based CO 2 sorbents, by introducing certain inert separating and supporting materials.
In addition, the cycling stability of the synthesized LDH-derived CO 2 sorbent was also studied, and Mg-Al-CO 3 LDH-6M was taken as the representative.The sorption was performed at 200 • C in pure CO 2 flow, and the desorption was performed at 400 • C in pure N 2 , both of which were performed for 30 min.Figure 9b shows that the Mg-Al-CO 3 LDH-6M-derived sorbent has an excellent cycling stability during the fifteenth cycle.Almost no change was observed and the CO 2 sorption capacity remained at 1.26 wt %.To summarize, we can conclude that via the newly developed environmentally benign synthesis strategy, two types of high-temperature CO 2 sorbents, Li 4 SiO 4 and LDH, can be obtained from the cheap raw material vermiculite, and their performance can be tuned by optimizing the synthesis procedure and parameters.In addition, the cycling stability of the synthesized LDH-derived CO2 sorbent was also studied, and Mg-Al-CO3 LDH-6M was taken as the representative.The sorption was performed at 200 °C in pure CO2 flow, and the desorption was performed at 400 °C in pure N2, both of which were performed for 30 min.Figure 9b shows that the Mg-Al-CO3 LDH-6M-derived sorbent has an excellent cycling stability during the fifteenth cycle.Almost no change was observed and the CO2 sorption capacity remained at 1.26 wt %.To summarize, we can conclude that via the newly developed environmentally benign synthesis strategy, two types of high-temperature CO2 sorbents, Li4SiO4 and LDH, can be obtained from the cheap raw material vermiculite, and their performance can be tuned by optimizing the synthesis procedure and parameters.

Synthesis of Li4SiO4 and LDH from Vermiculite
Vermiculite (VMT) was taken from the Xinjiang Province, China.First, the vermiculite was calcined at 900 °C for 2 h to form expanded vermiculite (EXVMT), which has a higher specific surface area compared to vermiculite.The chemical composition of EXVMT was determined by X-ray fluorescence (XRF) spectroscopy (Panalytical Axios-mAX).Then, the obtained EXVMT (10 g) was leaching pretreated in 200 mL of acidic solution at 80 °C for 24 h.The acidic solution used for this pretreatment of vermiculite was selected from 6 M aqua regia solution, 5 M HNO3, 3 M HNO3, or 1 M HNO3.When the leaching pretreatment process was ended, the suspension was filtered to separate

Synthesis of Li 4 SiO 4 and LDH from Vermiculite
Vermiculite (VMT) was taken from the Xinjiang Province, China.First, the vermiculite was calcined at 900 • C for 2 h to form expanded vermiculite (EXVMT), which has a higher specific surface area compared to vermiculite.The chemical composition of EXVMT was determined by X-ray fluorescence (XRF) spectroscopy (Panalytical Axios-mAX).Then, the obtained EXVMT (10 g) was leaching pretreated in 200 mL of acidic solution at 80 • C for 24 h.The acidic solution used for this pretreatment of vermiculite was selected from 6 M aqua regia solution, 5 M HNO 3 , 3 M HNO 3 , or 1 M HNO 3 .When the leaching pretreatment process was ended, the suspension was filtered to separate the solid sample and the acidic washing wastewater.The obtained solid powder, which mainly contained SiO 2 , was washed with distilled water until the filter liquor was neutral, and dried in an oven.
The Mg-Al-CO 3 LDHs were synthesized using a co-precipitation method from the above obtained acidic washing wastewater.First, the collected acidic washing wastewater was neutralized using 4 M NaOH until pH = 10.Then, 200 mL of this neutralized solution containing mainly Mg 2+ and Al 3+ was added drop-wise to 100 mL of 0.4 M Na 2 CO 3 solution.The pH value was kept at a constant value of 10 during the whole process, by adding 4 M NaOH.The suspension was stirred for 24 h at room temperature for aging.The samples were washed with deionized water until the filter liquor was neutral.The sample was further dispersed in acetone, stirred for 2 h, and then thoroughly washed with acetone.Finally, the obtained LDH was dried at 100 • C in an oven.The corresponding LDO can be obtained by calcining at 400 • C for 5 h.

Characterization of Samples
The phase structure of the samples was analyzed by powder X-ray diffraction (XRD) measurements performed on a Shimadzu XRD-7000 instrument in reflection mode with Cu K radiation.The accelerating voltage was set at 40 kV with a 30 mA current (λ = 1.542Å) from 1 to 70 • , with a scan rate of 2 • min −1 and a step size of 0.02 • .The morphology of the samples was studied using scanning electron microscopy (SEM, Hitachi S-3400N II).Before observation, the dried samples were sputtered and coated with gold for ~30 s, under an argon atmosphere.The specific surface area and pore information of the samples were determined by N 2 sorption/desorption measurements (BET SSA-7000).

Evaluation of CO 2 Capture Performance
The CO 2 sorption capacity and the long-term cycling stability of the synthesized sorbents were evaluated using TGA apparatus (Q50 TA Instrument).For the Li 4 SiO 4 -based sorbent, after placing the sample in the sample pan of TAG, it was heat-treated from room temperature to 650 • C, and kept at this temperature for 30 min N 2 .The CO 2 capture capacity of Li 4 SiO 4 was then determined isothermally under pure CO 2 (40 mL/min) at 650 • C, for 120 min.For the LDH-derived sorbent, the sample was first calcined at 400 • C for 5 h to obtain LDO.The LDO sample was further calcined in situ at 400 • C for 1 h under a flow of high purity N 2 (40 mL min −1 ), before performing the CO 2 sorption test.The temperature was then lowered to 200 • C, and the gas was switched from N 2 to CO 2 (40 mL min −1 ), and the CO 2 sorption was measured for 2 h.
The cycling performance of Li 4 SiO 4 -based CO 2 sorbents was measured using a pressure swing adsorption process.CO 2 sorption was performed at 650 • C in CO 2 flow (40 mL/min) for 30 min and desorption was performed at the same temperature in N 2 (29 mL/min) for 30 min.For the LDH-derived sorbents, a temperature swing adsorption process was applied, with a sorption temperature of 200 • C and a desorption temperature of 400 • C. In a typical run, the sorbent was heated to 200 • C at a rate of 10 • C min −1 under N 2 and maintained for 60 min.When the temperature was reached and stabilized, the N 2 was switched to CO 2 .After sorption for 30 min, the gas was switched to N 2 , and the TGA analyzer was heated to 400 • C at a rate of 10 • C min −1 and kept at 400 • C for 30 min.After the completion of desorption, the temperature was decreased to 200 • C for the next cycle of CO 2 sorption.

Conclusions
In this contribution, we demonstrated that two kinds of high-temperature CO 2 sorbents (Li 4 SiO 4 -based and Mg-Al-CO 3 LDH-derived) can be economically and environmentally synthesized from a cheap mineral vermiculite.The Li/Si molar ratio, calcination temperature and time, and sorption temperature, have shown an important impact on the CO 2 sorption capacity of Li 4 SiO 4 -based and Mg-Al-CO 3 LDH-derived sorbents.When the Li/Si molar ratio reached four and five, relatively pure Li 4 SiO 4 was synthesized.However, when the Li/Si molar ratio was high (six-nine), LiOH and Li 8 SiO 6 phases appeared and their cycling performance became worse due to their poor regeneration property.The best calcination temperature was determined to be 750 • C. The optimized Li 4 SiO 4 and Mg-Al-CO 3 LDH achieved a good CO 2 sorption capacity of 34.39 wt % at 650 • C and 2.08 wt % at 200 • C, respectively.We also proved that the concentration of leaching acid greatly influenced the chemical composition of these two sorbents, which consequently affected their CO 2 sorption/desorption cycling performance.For the Li 4 SiO 4 -based sorbent, it exhibited a good initial CO 2 sorption capacity but poor cycling performance with a high leaching acid concentration and poor initial CO 2 sorption capacity, but a good cycling performance with a low leaching acid concentration.The improved cycling performance can be attributed to the remaining impurity phases in the Li 4 SiO 4 -based sorbent that work as inert separating and supporting materials.The cycling performance of the LDH-derived sorbent was very stable during (at least) 15 cycles.

Figure 1 .
Figure 1.Schematic illustration of the synthesis of Li4SiO4 and LDH from vermiculite, (a) the vermiculite is treated with acid to obtain a silica and an acidic solution, (b) the synthesis of Li4SiO4 from the SiO2 obtained in step (a), and (c) the synthesis of LDH from the acidic solution obtained in step (a).

Figure 1 .
Figure 1.Schematic illustration of the synthesis of Li 4 SiO 4 and LDH from vermiculite, (a) the vermiculite is treated with acid to obtain a silica and an acidic solution, (b) the synthesis of Li 4 SiO 4 from the SiO 2 obtained in step (a), and (c) the synthesis of LDH from the acidic solution obtained in step (a).

Figure 2 .
Figure 2. (a) The XRD patterns of VMT and EX-VMT, (b) SEM image of VMT, (c) SEM image of EX-VMT, and (d) XRD pattern of SiO2 sample obtained with 6 M aqua regia.

Figure 2 .
Figure 2. (a) The XRD patterns of VMT and EX-VMT, (b) SEM image of VMT, (c) SEM image of EX-VMT, and (d) XRD pattern of SiO 2 sample obtained with 6 M aqua regia.

Figure 3 .
Figure 3. XRD, BET, and SEM analysis of synthesized Li 4 SiO 4 and Mg-Al-CO 3 LDH when 6 M aqua regia was used as the leaching acid, (a) XRD patterns of synthesized Li 4 SiO 4 and Mg-Al-CO 3 LDH, (b) BET analysis of synthesized Li 4 SiO 4 , (c) SEM image of the Li 4 SiO 4 sample, and (d) SEM image of the LDH sample.

Figure 6 .
Figure 6.(a) CO 2 sorption performance of Li 4 SiO 4 -(EX-VMT)-4 sorbent evaluated at different sorption temperatures of 600, 650, and 700 • C, and (b) the influence of sorption temperature on the CO 2 capture performance of Mg-Al-CO 3 LDH synthesized from the vermiculite leaching wastewater.

Table 1 .
The quantities of the obtained SiO 2 and Mg-Al-CO 3 LDH when the vermiculite was treated with different concentrations of leaching acids.