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Effects of porous carbon additives on the CO2 absorption performance of lithium orthosilicate
Accepted Manuscript Title: Effects of porous carbon additives on the CO2 absorption performance of lithium orthosilicate Author: Sungeun Jeoung Jae Hwa Lee Ho Young Kim Hoi Ri Moon PII: DOI: Reference:
S0040-6031(16)30131-9 http://dx.doi.org/doi:10.1016/j.tca.2016.05.010 TCA 77516
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
30-3-2016 18-5-2016 20-5-2016
Please cite this article as: Sungeun Jeoung, Jae Hwa Lee, Ho Young Kim, Hoi Ri Moon, Effects of porous carbon additives on the CO2 absorption performance of lithium orthosilicate, Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2016.05.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of porous carbon additives on the CO2 absorption performance of lithium orthosilicate Sungeun Jeoung,a Jae Hwa Lee,a Ho Young Kim,b and Hoi Ri Moon*a
a
Department of Chemistry, School of Natural Science, Ulsan National Institute of Science and
Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea. Fax: +82-52-217-2019; Tel: +82-52-217-2928; E-mail: [email protected]; b
Department of Chemical Engineering, School of Energy and Chemical Engineering, Ulsan
National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea.
1
Graphical Abstract
Highlights
Composites of Li4SiO4 and porous carbon materials were prepared for CO2 absorben ts.
The kinetic parameters of the composites were examined.
The pores of CMK-3 in Li4SiO4 aid the diffusion of CO2.
ABSTRACT
Lithium orthosilicate (Li 4SiO4) is an attractive high-temperature CO2 sorbent (> 650 C) because of its large theoretical absorption capacity of up to 36.7 wt%. However, slow kinetics and partial reactions with CO2 hinder its proper operation as a sorbent under
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practical conditions. To allow the use of this sorbent at lower operation temperatures, the present studies explored the way to improve the CO2 absorption kinetics and increase the degree of reaction of Li 4SiO4. Porous carbon materials such as CMK-3 were introduced into the sorbent to provide internal gas pathway. Upon calcination conditions, the carbon amount was controlled in the composites (Li4SiO4@CMK-X%, where X represents the amounts of CMK-3). In [email protected]%, CMK-3 is distributed over the whole solid; in contrast, the additive in Li [email protected]% is mainly observed near the surface of the solid. CO 2 gas sorption study of the composites showed that pores of CMK-3 in Li4SiO4 aid the diffusion of CO2. In addition, we found that the incorporation of porous carbon provides more active sites for interactions with CO2 through the formation of cavities between Li4SiO4 and CMK-3. [email protected]% had an increased CO2 absorption capacity (35.4 wt%) and rate (15.2 wt% for the first 5 min) at 600 C, compared to the CO2 absorption capacity (16.3 wt%) and rate (5.1 wt% for the first 5 min) of pristine Li 4SiO4 (p-Li4SiO4). To confirm the influence of porous carbon on the CO2 absorption properties, multi-walled carbon nanotube (MWCNT) was also examined as an additive instead of CMK-3. Li4SiO4@CNT showed similar trends with Li4SiO4@CMK sorbents.
Keywords: Lithium orthosilicate, Porous carbon, CO2 absorption, Gas pathway
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1. INTRODUCTION
Increasing atmospheric CO2 concentration, which is mostly due to the burning of fossil fuels, has been identified as a major contributor to global warming [1]. Because the direct capture of CO2 from power plants is economically viable, zeolites [2], amine-based materials [3], magnesium and calcium oxides [4-6], and lithium-based oxide composites [7-11] have been tested as high temperature CO2 sorbents. In particular, among lithium-based oxide sorbents, lithium orthosilicate (Li4SiO4) has been recognized as an attractive sorbent, since it has theoretical CO2 absorption capacity of up to 36.7 wt% of its original weight (Li4SiO4 + CO2 ↔ Li2CO3 + Li2SiO3) [12]. It is also known to have reasonable material costs. However, most reported Li4SiO4 sorbents have displayed slow kinetics and the partial reaction with CO2, which lead low absorption capacities [13,14]. Because the introduction of a high concentration of active sites on Li4SiO4 sorbents can solve this problem, various efforts have been made to synthesize the Li4SiO4 with a large surface area and small particle size. For example, diatomite has been used as a silica precursor to yield Li4SiO4 having a higher surface area. This high surface area arises from the uniform pore structure of diatomite (pore size, ~500 nm), which can produce macropores in the Li4SiO4 sorbent unlike solids synthesized using analytically pure silica [15,16]. As a result, this Li4SiO4 with a high surface area had superior CO2 absorption properties in terms of kinetics and capacity. Recently Choi et al. reported that decreasing the synthesis temperature prevented sintering, allowing the formation of a macroporous structure, and enhancing both the CO2 absorption capacity and rate in comparison to nonporous Li4SiO4 [17]. Ball-milling is a facile method to reduce particle size and increase the surface area of the nanomaterials. In the synthesis of Li4SiO4, ball-milling resulted in the formation of sorbent particles that were 30times smaller and had 12-times the surface area of bulk Li4SiO4 [18]. The sol-gel method was
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also used to generate nano-sized particles, which result from the better mixing of reactants and the higher reaction rate, which showed the distinguishable properties like CO2 absorption capacity and kinetics with Li4SiO4 synthesized by solid-state methods [19,20].
In spite of efforts to form advantageous structures for CO2 absorption, these morphologies are often not maintained during high-temperature CO2 absorption/desorption cycles, leading to capacity losses. Because Li2CO3, which is formed from the reaction between Li4SiO4 and CO2, exists as a molten phase in the range of the absorption temperature, the initial textural properties are lost by severe agglomeration [21]. Previously, as a trial to maintain the original morphology of some oxide materials, various additives have been introduced in the synthetic process. The Colon group used carbon-based material as an additive to prepare TiO2 nanoparticles, which play a role in preventing the formation of an irregular morphology; furthermore, this additive leads to the formation of a high surface area [22]. Mahinpey et al. reported that the composite of CaO sorbents with Al2O3 additives showed the improved cyclability by retaining its original capacity. The authors claimed that this result was attributed to the more active CaO sites induced by the co-existing alumina. [23]. On this wise, the introduction of additives to Li4SiO4 sorbents can be a feasible solution to improve the textural properties and physical stability.
Our strategy is the introduction of porous materials to provide the CO2 diffusion pathways and high surface area into Li4SiO4 sorbents. The additives chosen for use in the synthesis of Li4SiO4 must be non-reactive with the lithium precursors to avoid undesirable production of unnecessary species for CO2 absorption. In this sense, we selected highly porous carbon, CMK-3, which has a surface area of 1520 m2 g-1 and a total pore volume of 1.3 cm3 g-1,
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as an additive due to its low reactivity with lithium [24,25]. The embedded porous carbon in Li4SiO4 produced through the facile wet-mixing method affords more CO2 diffusion pathways in the resultant Li4SiO4 sorbents; this is expected to affect their CO2 absorption behavior. In this paper, we describe our investigation into the roles of the porous carbon materials on enhancing the CO2 absorption properties of Li4SiO4 by synthesizing Li4SiO4 and carbon composites with different kinds and amounts of porous carbon.
2. EXPERIMENTAL SECTION
2.1. Materials
Analytical grade chemicals lithium hydroxide monohydrate (LiOH.H2O, >98.0%), silica, fumed (SiO2, 0.007 µm powder), and carbon nanotube, multi-walled, carboxylic acid functionalized (MWCNT, >8% carboxylic acid functionalized) were purchased from Aldrich. All of these chemicals have been used without additional purification. CMK-3 was synthesized according to a previous report [25].
2.2. Preparation of the sorbents
Pristine Li4SiO4 (p-Li4SiO4): p-Li4SiO4 was synthesized according to a previous report [18]. LiOH.H2O (0.29 g, 6.8 mmol) and SiO2 (0.10 g, 1.7 mmol) were dissolved in water with stirring at 70 °C. Then, the mixture solution was heated at 105 °C to evaporate water, and the resulting powder was collected and calcined at 700 °C for 4 h in a tube furnace under nitrogen flow (500 mL min-1).
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Li4SiO4@CMK: LiOH.H2O (0.29 g, 6.8 mmol) and SiO2 (0.10 g, 1.7 mmol) were dissolved in water (10 mL) at 70 °C, and CMK-3 (0.02 g) was added to the mixture solution with vigorous stirring. Then, water was evaporated from the suspension by heating at 105 °C, and the obtained powder was calcined at 700 C for 4 h in a tube furnace under nitrogen flow (500 mL min-1). The resultant greyish powder was calcined again at 700 °C in air in muffle furnace. Upon retention time at 700 C, the carbon contents in the final product can be varied; no retention time yielded 1.8 wt% of carbon with respect to Li4SiO4, designated as [email protected]%; and retention for 4 h produced 0.5 wt%-carbon containing Li4SiO4, designated as [email protected]%.
Li4SiO4@CNT: The same experiment was performed to synthesize Li4SiO4@CNT with MWCNT (0.02 g) instead of CMK-3. After the first calcination under N2 flow, the powder was calcined at 700 °C in air without retention.
2.3. Material characterization
Elemental analyses (for C, H, N, S, and O) were performed by using a Thermo Scientific Flash 2000 series CHNS/O analyser. Thermogravimetric analyses (TGA) was performed under carbon dioxide atmosphere at a scan rate of 10 °C min-1, using Q50 from TA instruments. X-ray powder diffraction (XRPD) patterns were recorded on a Bruker D2 phaser diffractometer at 30 kV and 10 mA using Cu-Kα radiation (λ = 1.54059 Å) with a step size of 0.02° in 2θ at room temperature. Scanning electron microscope (SEM) images were taken using a Quanta 200 microscope (FEI) operating at 18 kV. Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS) were obtained with a JEOL JEM-2100F microscope.
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2.4. CO2 uptake performance tests
CO2 chemisorption experiments of dynamics, kinetics, and cyclability were performed using Q50 from TA instruments. To scan the absorption-desorption behavior of the prepared Li4SiO4 sorbents, dynamic thermograms were measured at a heating rate of 10 °C min-1 and a gas flow of 15% (v/v) CO2 in N2, which mimics flue gas, as well as 100% CO2 atmosphere from room temperature to 800 °C. The isothermal curves of the sorbents were obtained for 200 min at 500, 550, and 600 °C under 15% (v/v) CO2 in N2, and also at 550, 600, and 625 °C under 100% CO2 atmosphere. The stability of the Li4SiO4 sorbents was tested by cyclic absorption/desorption experiments as follows: absorption curves were obtained under a 15% (v/v) CO2 flow in N2 at 550 °C, and the desorption process was performed after switching the gas flow to N2 at 650 °C. The gas flow rate for all of the thermogravimetric analyses was 60 mL/min, and the sample amount used in each CO2 absorption experiment was ca. 10 mg.
3. RESULTS AND DISCUSSION
3.1. Synthesis and characterizations of p-Li4SiO4 and porous carbon-containing Li4SiO4 (Li4SiO4@CMK)
CMK-3 was added and well-dispersed in the water solution of the Li4SiO4 precursors (LiOH.H2O and SiO2) to intermingle it with Li4SiO4 homogeneously, and then water was eliminated by heating. Because, at high absorption temperatures, porous carbon species are not expected to absorb CO2, the excess quantity of CMK-3 can cause a gravimetric disadvantage. Thus, to control the effective amount of CMK-3, additional calcination was conducted in air at 700 °C (Scheme 1). For the second calcination, the reaction was carried out at 700 °C with no retention
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time, resulting in 1.8 wt% CMK-3 contained in the Li4SiO4, designated as [email protected]%. After calcination for 4 h, 0.51 wt% of carbon remained in the Li4SiO4, designated as [email protected]% (Table S1). The CO2 absorption properties of pristine p-Li4SiO4 were compared with those of Li4SiO4@CMK.
As shown in Fig. 1, the XRPD patterns for p-Li4SiO4, [email protected]%, and [email protected]% are well-matched with that of single-phase monoclinic Li4SiO4 (JCPDS no. 37-1472). Small amounts of amorphous carbon in [email protected]% and [email protected]% give no differences with p-Li4SiO4 in the XRPD patterns. The distribution and the shape of CMK-3 in both composites were investigated by TEM and EDS mapping (Fig. 2). For comparison, TEM images of p-Li4SiO4 are also shown in Fig. S1. CMK-3 in [email protected]% is well-distributed over the whole solid (Figs. 2a and b); in contrast, the additive in [email protected]% is mainly observed near the surface of the solid (Figs. 2d and e). EDS mapping images of carbon for both samples (Figs. 2c and f) also support this observation. While the porous carbon material, CMK-3, originally has a hexagonal shape with ordered mesoporosity [25], during synthesis of the composites the morphology of CMK-3 became tubular, and the material was embedded in the Li4SiO4 solid.
In the present work, since the introduction of porous carbon to Li4SiO4 aims to secure CO2 gas diffusion pathways, the surface area and the porosity ascribed to CMK-3 are essential in the composites. Thus, to confirm the porosity of these composites, nitrogen adsorption-
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desorption measurements were conducted. From the N2 adsorption-desorption isotherms (Fig. 3a), the Brunauer-Emmett-Teller (BET) surface areas and total pore volumes of three sorbents, p-Li4SiO4, [email protected]%, and [email protected]% are 1.06, 1.09, and 14.0 m2 g-1, and 0.005, 0.009, and 0.022 cm3 g-1, respectively. Increasing the amount of CMK-3 in the composites resulted in larger surface areas and higher pore volumes. To clearly see the effect of these changes on CO2 accessibility in the Li4SiO4@CMK composites, CO2 gas sorption experiments were conducted at 195 K (Fig. 3b). While p-Li4SiO4 has an adsorption capacity of 1.21 mL g-1, [email protected]% and [email protected]% had CO2 uptake capacities of 6.96 and 17.5 mL g-1, respectively. The enhanced CO2 adsorption at 195 K might be attributed to the cavities between Li4SiO4 and the porous carbon additive as well as the channels in CMK-3 itself, which possibly provide a pathway for CO2 molecules during the absorption at high temperature (Scheme 1).
3.2. CO2 absorption studies
To verify the advantages of incorporation of porous carbon in Li4SiO4 with regard to CO2 capture, CO2-absorption experiments were conducted using a TGA apparatus. Figs. 4a and b show the dynamic thermograms of the Li4SiO4 sorbents measured at a heating rate of 10 °C min1
under 15% CO2 and 100% CO2 conditions. Even though the CO2 absorption and desorption
temperatures of the three samples are similar, the maximum absorption capacities of three samples are different. Under a flow of 15% (v/v) CO2 in N2 (Fig. 4a), which mimics flue gas, the CO2-chemisorption process of Li4SiO4 sorbents began at 450 °C and reached their maxima at around 610 °C. This temperature showing the maximum uptake corresponds to equilibrium of
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absorption and desorption (613 °C for p-Li4SiO4, 610 °C for [email protected]%, and 603 °C for [email protected]%). The maximum absorption capacity at the apex increased as the amount of the porous carbon additives increased, and the uptakes were found to be 5.6, 9.0, and 10.7 wt% for p-Li4SiO4, [email protected]%, and [email protected]%, respectively. When measured in 100% CO2 atmosphere (Fig. 4b), the CO2-chemisorption process of Li4SiO4 sorbents began at 500 °C and reached those maxima around 680 °C, at which absorption and desorption are in equilibrium (681 °C for p-Li4SiO4, 684 °C for [email protected]%, and 678 °C for [email protected]%). Finally, at higher temperatures, the sorbents underwent desorption process to release the CO2 molecules. Although the equilibrium temperature for absorption and desorption shifted to higher temperatures due to the increased CO2 concentration [26], the order of CO2 absorption capacity is same with 15% CO2 condition. The maximum absorption capacities were 13.5, 20.2, and 26.0 wt% for p-Li4SiO4, [email protected]%, and [email protected]%, respectively.
To examine the temperature dependence of CO2-absorption, the behavior of three Li4SiO4-based sorbents was monitored isothermally under flowing mimicked flue gas at 500, 550, and 600 °C for 200 min (Figs. 4c and S2). At 550 °C, the absorption capacity and rate both increased as the amount of porous carbon additive increased (Fig. 4c). CO2 isotherms of those three samples display the same trends at 500 °C but the tendency is different at 600 °C particularly for the [email protected]% sorbent (Fig. S2). Despite its increased capacity, the initial absorption kinetics is drastically decreased, since its equilibrium temperature (603 °C) for adsorption and desorption of [email protected]% is very close to the measurement temperature (600 °C). For the isotherms at 625 °C that desorption process proceeds, there is no CO2 absorption for [email protected]%. On the contrary, [email protected]% and p-Li4SiO4
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sorbents indicate the small amounts of CO2 absorption with higher equilibrium temperatures (610 and 613 °C, respectively). The values about CO2 isotherm data are summarized in Table S2. On the other hand, Figs. 4d, S3, and Table S3 show the CO2 isotherm graphs of Li4SiO4 sorbents at 550, 600, and 625 °C under 100% CO2. The different temperature ranges are attributed to the shift of the equilibrium temperature of absorption and desorption, as previously described in Figs. 4a and b. The values of absorption capacity and rate of three sorbents exhibit the same trends as isotherm data under 15% CO2 condition. It is worth to note that the value of CO2 absorption capacity of [email protected]% sorbent is 35.8 wt%, which is 98% of the theoretical absorption capacity. To sum up, the porous carbon additives generate the broader reactive surface at the interface between Li4SiO4 and CMK-3. In addition, embedded CMK-3 provides CO2 diffusion pathways, which enables the inner part of the sorbents to effectively react with CO2 molecules. These factors improved both the CO2 absorption capacity and rate.
The CO2 absorption kinetics of those three Li4SiO4 sorbents is further analyzed by fitting the CO2 isotherm data with the double exponential model:
y = A exp –k1t + B exp –k2t + C where y represents the weight percentage of CO2 absorbed, t is the time, A, B, and C are the preexponential factors, and k1 and k2 are the exponential constants indicating the CO2 chemisorption directly produced over the Li4SiO4 particles and the diffusion rate kinetically controlled by lithium ions, respectively [10, 14, 26]. The exponential constant values obtained at each temperatures are presented in Table 1. The k1 values of the samples are generally one order of magnitude larger than the k2 values, which means that the limiting step of the total process is the lithium diffusion.
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Furthermore, the activation energies for each CO2 absorption processes can be estimated with these kinetic parameters (k1 and k2) by using the Arrhenius-type equation:
K = K0 exp –Ea/RT where Ko is the reaction rate constant, Ea is the activation energy of the chemisorption or the diffusion process, R is the gas constant, and T is absolute temperature. Fig. 5 shows the plots of ln K versus 1/T for the chemisorption and diffusion about [email protected]%, [email protected]%, and p-Li4SiO4. For both processes the linear trends are clearly shown, which consequently provide the gradients of the best-fit lines for estimation. Hence, the activation energies for the CO2 chemisorption on Li4SiO4 can be determined to be 64.7, 76.7, and 118.5 kJ/mol for [email protected]%, [email protected]%, and p-Li4SiO4, respectively. The calculated activation energies for the lithium diffusion throughout Li2CO3 also show the same trend as 19.7, 27.4, and 60.6 kJ/mol for [email protected]%, [email protected]%, and pLi4SiO4, respectively. This clearly enhanced sorption behavior could be attributed to the porous carbon additives providing CO2 diffusion pathways and accordingly favoring the chemisorption and diffusion processes, which decreases the energy necessary for the CO2 absorption reaction.
To investigate the stability and regenerability of [email protected]% sorbent, its CO2 absorption/desorption cycle test was carried out under 15% CO2 gas for absorption and 100% N2 gas for desorption every 80 min for 15 cycles (Fig. 6). The absorption amount of the sorbent at the first cycle was 21.1 wt%, but it gradually decreased to 10.0 wt% with cyclic loading. To clarify this behavior, we took TEM images (Fig. S4) and elemental analyses data (Table S4) of
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the [email protected]% sorbent after the CO2 cycling test. The additive dissipated and the percentage of carbon present in the sorbent decreased from 1.77% to 0.37%. We have previously prepared an unusual, coral-like Li4SiO4 through the thermolysis of a new lithium and siliconbased precursor [21]. Even though this porous sorbent efficiently chemisorbed CO2, its absorption capacity decreased and the morphology disappeared as the number of loading cycles increased. Likewise, in the present work, the capacity decrease upon cycling was caused by gradual loss of carbon additives at high temperature, leading agglomeration and deformation of the sorbents. In this sense, to maintain the initial textural properties of the sorbent, additives inert under harsh conditions should be secured.
3.3. Li4SiO4@CNT
To confirm the influence of porous carbon on the CO2 absorption properties of Li4SiO4 sorbents, the multi-walled carbon nanotube (MWCNT) was introduced as an additive instead of CMK-3. For evenly mixing in water, carboxylic acid functionalized MWCNT was chosen with the precursors, LiOH.H2O and fumed SiO2.
The XRPD patterns indicated that the Li4SiO4@CNT display diffraction peaks of singlephase monoclinic Li4SiO4, as with the Li4SiO4@CMK samples, which is coincident with JCPDS no. 37-1472 (Fig. S5). As shown in Figs. 7a and b, the additive is well-distributed over the whole solid in Li4SiO4@CNT. In nitrogen adsorption-desorption isotherm measurements (Fig. S6a), Li4SiO4@CNT displayed higher surface area and total pore volume than those of p-Li4SiO4 (4.52 m2 g-1 and 0.019 cm3 g-1 versus 1.06 m2 g-1 and 0.005 cm3 g-1). CO2 gas sorption isotherms of
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sorbents were also measured at 195 K (Fig. S6b) and Li4SiO4@CNT showed a characteristic isotherm of CO2 diffusion. From these results, we expected that Li4SiO4@CNT has a similar role to that of Li4SiO4@CMK sorbents.
To confirm the advantages of incorporation of porous carbon in Li4SiO4 with regard to CO2 capture, Fig. 7c displays the dynamic thermograms of Li4SiO4@CNT with p-Li4SiO4 under mimicked flue gas at a scan rate of 10 °C min-1. CO2-chemisorption process of Li4SiO4@CNT sorbent began at 450 °C and reach a maximum at 615 °C, at which absorption and desorption are in equilibrium. The maximum absorption capacity for the Li4SiO4@CNT sorbent was equal to 9.8 wt%, which is larger value than that of p-Li4SiO4. On the basis of dynamic thermogram of Li4SiO4@CNT, it was measured isothermally under flowing mimicked flue gas at 500, 550, and 600 °C (Fig. 7d). The absorption capacity and rate of Li4SiO4@CNT exhibited higher values than those of p-Li4SiO4.
To investigate the stability and regenerability of Li4SiO4@CNT sorbent, its CO2 absorption/desorption cycle test was carried out under 15% CO2 gas for absorption and 100% N2 gas for desorption every 80 min for 15 cycles (Fig. S7). The absorption amount of the sorbent at the first cycle was 14.8 wt%, but it gradually decreased to 10.0 wt% with cyclic loading. In common with [email protected]% sorbent, the capacity decrease upon cycling was caused by gradual loss of carbon additives at high temperature, leading agglomeration and deformation of the sorbents. Results from the CO2 absorption performances of Li4SiO4@CMK and Li4SiO4@CNT sorbents indicate that addition of porous carbon into Li4SiO4 gives CO2 diffusion pathways. In particular, considering that MWCNT is less porous than CMK-3, and
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Li4SiO4@CNT, which includes 1 wt% of carbon additive, contained less additive than [email protected]% (containing 1.8 wt% of additive), and Li4SiO4@CNT and [email protected]% show similar CO2 absorption capacity, the role of porous carbon additives mainly the voids between Li4SiO4 and porous carbon material.
4. CONCLUSIONS
We synthesized Li4SiO4 sorbents that included porous carbon, CMK-3, as an additive. The formed sorbents had higher surface areas and pore volumes than p-Li4SiO4, and showed characteristics of CO2 diffusion. These characteristics may be due to the addition of porous carbon additives into Li4SiO4 that gives CO2 diffusion pathways; in addition, this may be attributed to not only the porosity of the additive itself but also the voids between Li4SiO4 and porous carbon material. The Li4SiO4@CMK samples had better CO2 absorption properties compared with those of p-Li4SiO4. For control experiment investigating the effect of porous carbon on CO2 absorption properties, we introduced MWCNT as an additive. As a result, the Li4SiO4@CNT also had better CO2 absorption properties than the p-Li4SiO4.
In this paper, carbon materials, which are vulnerable to high temperature, were selected as additives; unfortunately, this proved problematic because, on cycling, the CO2 absorption ability of the sorbents decreased. Moreover, the industrial emissions in core fired power plants typically include the gases like O2, H2O, SO2 or NOx apart from CO2 and N2 [28, 29], which can induce the combustion of carbon at high temperature. For that reason, we expect that the use of an additive that is stable for any atmospheres at high temperature will have a significant effect on the CO2 absorption/desorption cycle. Therefore, we are currently investigating the use of alpha phase alumina, which is stable at high temperature [27], as an additive instead of porous carbon.
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Acknowledgments
This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea (NRF-2014M1A8A1049255; 2013K1A3A1A04076417). J.H.L. acknowledges the Global PhD Fellowship (NRF-2013H1A2A1033501). We are grateful to Prof. Sang Hoon Joo for helpful discussion.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/xxx.
AUTHOR INFORMATION Corresponding Author at: Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 689-798, Republic of Korea. Fax: +82-52-217-2019; Tel: +82-52-217-2928. E-mail: [email protected]
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[16] S. Shan, S. Li, Q. Jia, L. Jiang, Y. Wang, J. Peng, Impregnation precipitation preparation and kinetic analysis of Li4SiO4-based sorbents with fast CO2 adsorption rate, Ind. Eng. Chem. Res. 52 (2013) 6941-6945.
[17] H. Kim, H. D. Jang, M. Choi, Facile synthesis of macroporous Li4SiO4 with remarkably enhanced CO2 adsorption kinetics, Chem. Eng. J. 280 (2015) 132-137.
[18] I.C. Romero-Ibarra, J. Ortiz-Landeros, H. Pfeiffer, Microstructural and CO2 chemisorption analyses of Li4SiO4: effect of surface modification by the ball milling process, Thermochim. Acta 567 (2013) 118-124.
[19] K. Wang, X. Wang, P. Zhao, X. Guo, High-temperature capture of CO2 on lithium-based sorbents prepared by a water-based sol-gel technique, Chem. Eng. Technol. 37 (2014) 15521558.
[20] P.V. Subha, B.N. Nair, P. Hareesh, A.P. Mohamed, T. Yamaguchi, K.G.K. Warrier, U.S. Hareesh, Enhanced CO2 absorption kinetics in lithium silicate platelets synthesized by a solgel approach, J. Mater. Chem. A 2 (2014) 12792-12798.
[21] J.H. Lee, B. Moon, T.K. Kim, S. Jeoung, H.R. Moon, Thermal conversion of a tailored metal-organic framework into lithium silicate with an unusual morphology for efficient CO2 capture, Dalton Trans. 44 (2015) 15130-15134.
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[22] G. Colón, M.C. Hidalgo, J.A. Navío, A novel preparation of high surface area TiO2 nanoparticles from alkoxide precursor and using active carbon as additive, Catal. Today 76 (2002) 91-101.
[23] D. Karami, N. Mahinpey, Study of Al2O3 addition to synthetic Ca-based sorbents for CO2 sorption capacity and stability in cyclic operations, Can. J. Chem. Eng. 93 (2015) 102-110.
[24] H. Li, H. Zhou, Enhancing the performances of Li-ion batteries by carbon-coating: present and future, Chem. Commun. 48 (2012) 1201-1217.
[25] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, Synthesis of new, nanoporous carbon with hexagonally ordered mesostructured, J. Am. Chem. Soc. 122 (2000) 10712-10713.
[26] R. Rodríguez-Mosqueda, H. Pfeiffer, Thermokinetic analysis of the CO2 chemisorption on Li4SiO4 by using different gas flow rates and particle sizes, J. Phys. Chem. A 114 (2010) 4535-4541.
[27] C. Pecharromán, I. Sobrados, J.E. Iglesias, T. González-Carreño, J. Sanz, Thermal evolution of transitional aluminas followed by NMR and IR spectroscopies, J. Phys. Chem. B 103 (1999) 6160-6170.
[28] J. Koornneef, T.v. Keulen, A. Faaij, W. Turkenburg, Life cycle assessment of a pulverized coal power plant with post-combustion capture, transport and storage of CO2, Int. J. Greenhouse Gas Control 2 (2008) 448-467.
21
[29] R. Pacciani, J. Torres, P. Solsona, C. Coe, R. Quinn, J. Hufton, T. Golden, L.F. Vega, Influence of the concentration of CO2 and SO2 on the absorption of CO2 by a lithium orthosilicate-based absorbent, Environ. Sci. Technol. 45 (2011) 7083-7088.
22
Intensity (a.u.)
[email protected]%
[email protected]%
p-Li4SiO4
Li4SiO4 #37-1472
10
20
30
40
50
60
70
2 (Degrees) Fig. 1. XRPD patterns of three kinds of Li4SiO4 sorbents and the JCPDS file of Li4SiO4 as a reference.
23
(a)
[email protected]%
50 nm
(b)
5 nm
(f) (c)
500 nm
(d)
[email protected]%
50 nm
(e)
5 nm
(c) (f)
250 nm
Fig. 2. TEM images and EDS mapping of carbon (green dots). (a)-(c) [email protected]%, and (d)-(f) [email protected]%.
24
Quantity Adsorbed (mL g-1 STP)
(a)
16 14 [email protected]%
12 10 8 6
[email protected]%
4 2
p-Li4SiO4
0 0.0
0.2
0.4
0.6
0.8
1.0
-1
(b)
CO2 Adsorbed (mL g STP)
Relative Pressure (P/P0) 20 [email protected]%
18 16 14 12 10
[email protected]%
8 6 4
p-Li4SiO4
2 0 0.0
0.2
0.4
0.6
0.8
1.0
Pressure (bar) Fig. 3. (a) Nitrogen adsorption-desorption isotherms at 77 K, and (b) CO2 adsorption-desorption isotherms at 195 K of Li4SiO4 sorbents.
25
(a)
(b)
15% CO2 [email protected]%
100% CO2 [email protected]%
[email protected]% [email protected]%
p-Li4SiO4 p-Li4SiO4
(c)
(d)
15% CO2, 550 oC [email protected]%
100% CO2, 625 oC [email protected]% [email protected]%
[email protected]%
p-Li4SiO4
p-Li4SiO4
Fig. 4. Dynamic thermograms of the Li4SiO4 samples under (a) 15% CO2 in N2, and (b) 100% CO2 atmosphere. CO2 chemisorption isothermal analyses of the Li4SiO4 sorbents (c) at 550 °C under 15% CO2 in N2, and (d) at 625 °C under 100% CO2 atmosphere.
26
(a)
[email protected]%
(b)
y = -7.7872x + 6.70862 R = -0.99311
Chemisorption
(c)
y = -2.37383x – 2.22507 R = -0.95696
Diffusion
[email protected]%
(d)
y = -9.22528x + 7.90174 R = -0.97597
Chemisorption
(e)
[email protected]% y = -3.29464x - 1.2695 R = -0.94296
Diffusion
p-Li4SiO4
(f)
y = -14.25502x + 14.22331 R = -0.99999
Chemisorption
[email protected]%
p-Li4SiO4 y = -7.28582x + 3.75224 R = -0.90919
Diffusion
Fig. 5. Plots of ln k versus 1/T, for the two different processes, chemisorption (k1) and diffusion (k2), observed on (a), (b) [email protected]%, (c), (d) [email protected]%, (e), (f) p-Li4SiO4 sorbents.
27
Fig. 6. Cyclic performance at 550 oC of [email protected]%.
28
(a)
(b)
200 nm
20 nm
(c) 15% CO 2
(d) 15% CO 2 Li4SiO4@CNT
600 oC isotherm
550 oC isotherm
p-Li4SiO4
500 oC isotherm
Fig. 7. (a) TEM and (b) SEM images of Li4SiO4@CNT sorbents, (c) Dynamic thermograms of the Li4SiO4@CNT and added p-Li4SiO4 data as a reference, and (d) CO2 chemisorption isothermal analyses of the Li4SiO4@CNT at 500 oC, 550 oC, and 600 oC under 15% CO2 in N2.
29
N2
Air
700 oC
700 oC
Li4SiO4@CMK or Li4SiO4@CNT
Scheme 1. Scheme of preparation of porous carbon containing Li4SiO4 sorbents.
30
Table 1. Kinetic parameters of the Li4SiO4 sorbents derived from the CO2 isotherm fittings using a double exponential model.
Samples
[email protected]%
[email protected]%
p-Li4SiO4
Absorption temperature (oC)
k1 (s-1)
k2 (s-1)
R2
550
0.065
0.0061
0.9999
600
0.104
0.0068
0.9999
625
0.146
0.0079
0.9998
550
0.035
0.0052
0.9998
600
0.078
0.0060
0.9999
625
0.086
0.0075
0.9999
550
0.045
0.0065
0.9999
600
0.121
0.0083
0.9999
625
0.192
0.0146
0.9998
31
S0040-6031(16)30131-9 http://dx.doi.org/doi:10.1016/j.tca.2016.05.010 TCA 77516
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
30-3-2016 18-5-2016 20-5-2016
Please cite this article as: Sungeun Jeoung, Jae Hwa Lee, Ho Young Kim, Hoi Ri Moon, Effects of porous carbon additives on the CO2 absorption performance of lithium orthosilicate, Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2016.05.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effects of porous carbon additives on the CO2 absorption performance of lithium orthosilicate Sungeun Jeoung,a Jae Hwa Lee,a Ho Young Kim,b and Hoi Ri Moon*a
a
Department of Chemistry, School of Natural Science, Ulsan National Institute of Science and
Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea. Fax: +82-52-217-2019; Tel: +82-52-217-2928; E-mail: [email protected]; b
Department of Chemical Engineering, School of Energy and Chemical Engineering, Ulsan
National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea.
1
Graphical Abstract
Highlights
Composites of Li4SiO4 and porous carbon materials were prepared for CO2 absorben ts.
The kinetic parameters of the composites were examined.
The pores of CMK-3 in Li4SiO4 aid the diffusion of CO2.
ABSTRACT
Lithium orthosilicate (Li 4SiO4) is an attractive high-temperature CO2 sorbent (> 650 C) because of its large theoretical absorption capacity of up to 36.7 wt%. However, slow kinetics and partial reactions with CO2 hinder its proper operation as a sorbent under
2
practical conditions. To allow the use of this sorbent at lower operation temperatures, the present studies explored the way to improve the CO2 absorption kinetics and increase the degree of reaction of Li 4SiO4. Porous carbon materials such as CMK-3 were introduced into the sorbent to provide internal gas pathway. Upon calcination conditions, the carbon amount was controlled in the composites (Li4SiO4@CMK-X%, where X represents the amounts of CMK-3). In [email protected]%, CMK-3 is distributed over the whole solid; in contrast, the additive in Li [email protected]% is mainly observed near the surface of the solid. CO 2 gas sorption study of the composites showed that pores of CMK-3 in Li4SiO4 aid the diffusion of CO2. In addition, we found that the incorporation of porous carbon provides more active sites for interactions with CO2 through the formation of cavities between Li4SiO4 and CMK-3. [email protected]% had an increased CO2 absorption capacity (35.4 wt%) and rate (15.2 wt% for the first 5 min) at 600 C, compared to the CO2 absorption capacity (16.3 wt%) and rate (5.1 wt% for the first 5 min) of pristine Li 4SiO4 (p-Li4SiO4). To confirm the influence of porous carbon on the CO2 absorption properties, multi-walled carbon nanotube (MWCNT) was also examined as an additive instead of CMK-3. Li4SiO4@CNT showed similar trends with Li4SiO4@CMK sorbents.
Keywords: Lithium orthosilicate, Porous carbon, CO2 absorption, Gas pathway
3
1. INTRODUCTION
Increasing atmospheric CO2 concentration, which is mostly due to the burning of fossil fuels, has been identified as a major contributor to global warming [1]. Because the direct capture of CO2 from power plants is economically viable, zeolites [2], amine-based materials [3], magnesium and calcium oxides [4-6], and lithium-based oxide composites [7-11] have been tested as high temperature CO2 sorbents. In particular, among lithium-based oxide sorbents, lithium orthosilicate (Li4SiO4) has been recognized as an attractive sorbent, since it has theoretical CO2 absorption capacity of up to 36.7 wt% of its original weight (Li4SiO4 + CO2 ↔ Li2CO3 + Li2SiO3) [12]. It is also known to have reasonable material costs. However, most reported Li4SiO4 sorbents have displayed slow kinetics and the partial reaction with CO2, which lead low absorption capacities [13,14]. Because the introduction of a high concentration of active sites on Li4SiO4 sorbents can solve this problem, various efforts have been made to synthesize the Li4SiO4 with a large surface area and small particle size. For example, diatomite has been used as a silica precursor to yield Li4SiO4 having a higher surface area. This high surface area arises from the uniform pore structure of diatomite (pore size, ~500 nm), which can produce macropores in the Li4SiO4 sorbent unlike solids synthesized using analytically pure silica [15,16]. As a result, this Li4SiO4 with a high surface area had superior CO2 absorption properties in terms of kinetics and capacity. Recently Choi et al. reported that decreasing the synthesis temperature prevented sintering, allowing the formation of a macroporous structure, and enhancing both the CO2 absorption capacity and rate in comparison to nonporous Li4SiO4 [17]. Ball-milling is a facile method to reduce particle size and increase the surface area of the nanomaterials. In the synthesis of Li4SiO4, ball-milling resulted in the formation of sorbent particles that were 30times smaller and had 12-times the surface area of bulk Li4SiO4 [18]. The sol-gel method was
4
also used to generate nano-sized particles, which result from the better mixing of reactants and the higher reaction rate, which showed the distinguishable properties like CO2 absorption capacity and kinetics with Li4SiO4 synthesized by solid-state methods [19,20].
In spite of efforts to form advantageous structures for CO2 absorption, these morphologies are often not maintained during high-temperature CO2 absorption/desorption cycles, leading to capacity losses. Because Li2CO3, which is formed from the reaction between Li4SiO4 and CO2, exists as a molten phase in the range of the absorption temperature, the initial textural properties are lost by severe agglomeration [21]. Previously, as a trial to maintain the original morphology of some oxide materials, various additives have been introduced in the synthetic process. The Colon group used carbon-based material as an additive to prepare TiO2 nanoparticles, which play a role in preventing the formation of an irregular morphology; furthermore, this additive leads to the formation of a high surface area [22]. Mahinpey et al. reported that the composite of CaO sorbents with Al2O3 additives showed the improved cyclability by retaining its original capacity. The authors claimed that this result was attributed to the more active CaO sites induced by the co-existing alumina. [23]. On this wise, the introduction of additives to Li4SiO4 sorbents can be a feasible solution to improve the textural properties and physical stability.
Our strategy is the introduction of porous materials to provide the CO2 diffusion pathways and high surface area into Li4SiO4 sorbents. The additives chosen for use in the synthesis of Li4SiO4 must be non-reactive with the lithium precursors to avoid undesirable production of unnecessary species for CO2 absorption. In this sense, we selected highly porous carbon, CMK-3, which has a surface area of 1520 m2 g-1 and a total pore volume of 1.3 cm3 g-1,
5
as an additive due to its low reactivity with lithium [24,25]. The embedded porous carbon in Li4SiO4 produced through the facile wet-mixing method affords more CO2 diffusion pathways in the resultant Li4SiO4 sorbents; this is expected to affect their CO2 absorption behavior. In this paper, we describe our investigation into the roles of the porous carbon materials on enhancing the CO2 absorption properties of Li4SiO4 by synthesizing Li4SiO4 and carbon composites with different kinds and amounts of porous carbon.
2. EXPERIMENTAL SECTION
2.1. Materials
Analytical grade chemicals lithium hydroxide monohydrate (LiOH.H2O, >98.0%), silica, fumed (SiO2, 0.007 µm powder), and carbon nanotube, multi-walled, carboxylic acid functionalized (MWCNT, >8% carboxylic acid functionalized) were purchased from Aldrich. All of these chemicals have been used without additional purification. CMK-3 was synthesized according to a previous report [25].
2.2. Preparation of the sorbents
Pristine Li4SiO4 (p-Li4SiO4): p-Li4SiO4 was synthesized according to a previous report [18]. LiOH.H2O (0.29 g, 6.8 mmol) and SiO2 (0.10 g, 1.7 mmol) were dissolved in water with stirring at 70 °C. Then, the mixture solution was heated at 105 °C to evaporate water, and the resulting powder was collected and calcined at 700 °C for 4 h in a tube furnace under nitrogen flow (500 mL min-1).
6
Li4SiO4@CMK: LiOH.H2O (0.29 g, 6.8 mmol) and SiO2 (0.10 g, 1.7 mmol) were dissolved in water (10 mL) at 70 °C, and CMK-3 (0.02 g) was added to the mixture solution with vigorous stirring. Then, water was evaporated from the suspension by heating at 105 °C, and the obtained powder was calcined at 700 C for 4 h in a tube furnace under nitrogen flow (500 mL min-1). The resultant greyish powder was calcined again at 700 °C in air in muffle furnace. Upon retention time at 700 C, the carbon contents in the final product can be varied; no retention time yielded 1.8 wt% of carbon with respect to Li4SiO4, designated as [email protected]%; and retention for 4 h produced 0.5 wt%-carbon containing Li4SiO4, designated as [email protected]%.
Li4SiO4@CNT: The same experiment was performed to synthesize Li4SiO4@CNT with MWCNT (0.02 g) instead of CMK-3. After the first calcination under N2 flow, the powder was calcined at 700 °C in air without retention.
2.3. Material characterization
Elemental analyses (for C, H, N, S, and O) were performed by using a Thermo Scientific Flash 2000 series CHNS/O analyser. Thermogravimetric analyses (TGA) was performed under carbon dioxide atmosphere at a scan rate of 10 °C min-1, using Q50 from TA instruments. X-ray powder diffraction (XRPD) patterns were recorded on a Bruker D2 phaser diffractometer at 30 kV and 10 mA using Cu-Kα radiation (λ = 1.54059 Å) with a step size of 0.02° in 2θ at room temperature. Scanning electron microscope (SEM) images were taken using a Quanta 200 microscope (FEI) operating at 18 kV. Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDS) were obtained with a JEOL JEM-2100F microscope.
7
2.4. CO2 uptake performance tests
CO2 chemisorption experiments of dynamics, kinetics, and cyclability were performed using Q50 from TA instruments. To scan the absorption-desorption behavior of the prepared Li4SiO4 sorbents, dynamic thermograms were measured at a heating rate of 10 °C min-1 and a gas flow of 15% (v/v) CO2 in N2, which mimics flue gas, as well as 100% CO2 atmosphere from room temperature to 800 °C. The isothermal curves of the sorbents were obtained for 200 min at 500, 550, and 600 °C under 15% (v/v) CO2 in N2, and also at 550, 600, and 625 °C under 100% CO2 atmosphere. The stability of the Li4SiO4 sorbents was tested by cyclic absorption/desorption experiments as follows: absorption curves were obtained under a 15% (v/v) CO2 flow in N2 at 550 °C, and the desorption process was performed after switching the gas flow to N2 at 650 °C. The gas flow rate for all of the thermogravimetric analyses was 60 mL/min, and the sample amount used in each CO2 absorption experiment was ca. 10 mg.
3. RESULTS AND DISCUSSION
3.1. Synthesis and characterizations of p-Li4SiO4 and porous carbon-containing Li4SiO4 (Li4SiO4@CMK)
CMK-3 was added and well-dispersed in the water solution of the Li4SiO4 precursors (LiOH.H2O and SiO2) to intermingle it with Li4SiO4 homogeneously, and then water was eliminated by heating. Because, at high absorption temperatures, porous carbon species are not expected to absorb CO2, the excess quantity of CMK-3 can cause a gravimetric disadvantage. Thus, to control the effective amount of CMK-3, additional calcination was conducted in air at 700 °C (Scheme 1). For the second calcination, the reaction was carried out at 700 °C with no retention
8
time, resulting in 1.8 wt% CMK-3 contained in the Li4SiO4, designated as [email protected]%. After calcination for 4 h, 0.51 wt% of carbon remained in the Li4SiO4, designated as [email protected]% (Table S1). The CO2 absorption properties of pristine p-Li4SiO4 were compared with those of Li4SiO4@CMK.
As shown in Fig. 1, the XRPD patterns for p-Li4SiO4, [email protected]%, and [email protected]% are well-matched with that of single-phase monoclinic Li4SiO4 (JCPDS no. 37-1472). Small amounts of amorphous carbon in [email protected]% and [email protected]% give no differences with p-Li4SiO4 in the XRPD patterns. The distribution and the shape of CMK-3 in both composites were investigated by TEM and EDS mapping (Fig. 2). For comparison, TEM images of p-Li4SiO4 are also shown in Fig. S1. CMK-3 in [email protected]% is well-distributed over the whole solid (Figs. 2a and b); in contrast, the additive in [email protected]% is mainly observed near the surface of the solid (Figs. 2d and e). EDS mapping images of carbon for both samples (Figs. 2c and f) also support this observation. While the porous carbon material, CMK-3, originally has a hexagonal shape with ordered mesoporosity [25], during synthesis of the composites the morphology of CMK-3 became tubular, and the material was embedded in the Li4SiO4 solid.
In the present work, since the introduction of porous carbon to Li4SiO4 aims to secure CO2 gas diffusion pathways, the surface area and the porosity ascribed to CMK-3 are essential in the composites. Thus, to confirm the porosity of these composites, nitrogen adsorption-
9
desorption measurements were conducted. From the N2 adsorption-desorption isotherms (Fig. 3a), the Brunauer-Emmett-Teller (BET) surface areas and total pore volumes of three sorbents, p-Li4SiO4, [email protected]%, and [email protected]% are 1.06, 1.09, and 14.0 m2 g-1, and 0.005, 0.009, and 0.022 cm3 g-1, respectively. Increasing the amount of CMK-3 in the composites resulted in larger surface areas and higher pore volumes. To clearly see the effect of these changes on CO2 accessibility in the Li4SiO4@CMK composites, CO2 gas sorption experiments were conducted at 195 K (Fig. 3b). While p-Li4SiO4 has an adsorption capacity of 1.21 mL g-1, [email protected]% and [email protected]% had CO2 uptake capacities of 6.96 and 17.5 mL g-1, respectively. The enhanced CO2 adsorption at 195 K might be attributed to the cavities between Li4SiO4 and the porous carbon additive as well as the channels in CMK-3 itself, which possibly provide a pathway for CO2 molecules during the absorption at high temperature (Scheme 1).
3.2. CO2 absorption studies
To verify the advantages of incorporation of porous carbon in Li4SiO4 with regard to CO2 capture, CO2-absorption experiments were conducted using a TGA apparatus. Figs. 4a and b show the dynamic thermograms of the Li4SiO4 sorbents measured at a heating rate of 10 °C min1
under 15% CO2 and 100% CO2 conditions. Even though the CO2 absorption and desorption
temperatures of the three samples are similar, the maximum absorption capacities of three samples are different. Under a flow of 15% (v/v) CO2 in N2 (Fig. 4a), which mimics flue gas, the CO2-chemisorption process of Li4SiO4 sorbents began at 450 °C and reached their maxima at around 610 °C. This temperature showing the maximum uptake corresponds to equilibrium of
10
absorption and desorption (613 °C for p-Li4SiO4, 610 °C for [email protected]%, and 603 °C for [email protected]%). The maximum absorption capacity at the apex increased as the amount of the porous carbon additives increased, and the uptakes were found to be 5.6, 9.0, and 10.7 wt% for p-Li4SiO4, [email protected]%, and [email protected]%, respectively. When measured in 100% CO2 atmosphere (Fig. 4b), the CO2-chemisorption process of Li4SiO4 sorbents began at 500 °C and reached those maxima around 680 °C, at which absorption and desorption are in equilibrium (681 °C for p-Li4SiO4, 684 °C for [email protected]%, and 678 °C for [email protected]%). Finally, at higher temperatures, the sorbents underwent desorption process to release the CO2 molecules. Although the equilibrium temperature for absorption and desorption shifted to higher temperatures due to the increased CO2 concentration [26], the order of CO2 absorption capacity is same with 15% CO2 condition. The maximum absorption capacities were 13.5, 20.2, and 26.0 wt% for p-Li4SiO4, [email protected]%, and [email protected]%, respectively.
To examine the temperature dependence of CO2-absorption, the behavior of three Li4SiO4-based sorbents was monitored isothermally under flowing mimicked flue gas at 500, 550, and 600 °C for 200 min (Figs. 4c and S2). At 550 °C, the absorption capacity and rate both increased as the amount of porous carbon additive increased (Fig. 4c). CO2 isotherms of those three samples display the same trends at 500 °C but the tendency is different at 600 °C particularly for the [email protected]% sorbent (Fig. S2). Despite its increased capacity, the initial absorption kinetics is drastically decreased, since its equilibrium temperature (603 °C) for adsorption and desorption of [email protected]% is very close to the measurement temperature (600 °C). For the isotherms at 625 °C that desorption process proceeds, there is no CO2 absorption for [email protected]%. On the contrary, [email protected]% and p-Li4SiO4
11
sorbents indicate the small amounts of CO2 absorption with higher equilibrium temperatures (610 and 613 °C, respectively). The values about CO2 isotherm data are summarized in Table S2. On the other hand, Figs. 4d, S3, and Table S3 show the CO2 isotherm graphs of Li4SiO4 sorbents at 550, 600, and 625 °C under 100% CO2. The different temperature ranges are attributed to the shift of the equilibrium temperature of absorption and desorption, as previously described in Figs. 4a and b. The values of absorption capacity and rate of three sorbents exhibit the same trends as isotherm data under 15% CO2 condition. It is worth to note that the value of CO2 absorption capacity of [email protected]% sorbent is 35.8 wt%, which is 98% of the theoretical absorption capacity. To sum up, the porous carbon additives generate the broader reactive surface at the interface between Li4SiO4 and CMK-3. In addition, embedded CMK-3 provides CO2 diffusion pathways, which enables the inner part of the sorbents to effectively react with CO2 molecules. These factors improved both the CO2 absorption capacity and rate.
The CO2 absorption kinetics of those three Li4SiO4 sorbents is further analyzed by fitting the CO2 isotherm data with the double exponential model:
y = A exp –k1t + B exp –k2t + C where y represents the weight percentage of CO2 absorbed, t is the time, A, B, and C are the preexponential factors, and k1 and k2 are the exponential constants indicating the CO2 chemisorption directly produced over the Li4SiO4 particles and the diffusion rate kinetically controlled by lithium ions, respectively [10, 14, 26]. The exponential constant values obtained at each temperatures are presented in Table 1. The k1 values of the samples are generally one order of magnitude larger than the k2 values, which means that the limiting step of the total process is the lithium diffusion.
12
Furthermore, the activation energies for each CO2 absorption processes can be estimated with these kinetic parameters (k1 and k2) by using the Arrhenius-type equation:
K = K0 exp –Ea/RT where Ko is the reaction rate constant, Ea is the activation energy of the chemisorption or the diffusion process, R is the gas constant, and T is absolute temperature. Fig. 5 shows the plots of ln K versus 1/T for the chemisorption and diffusion about [email protected]%, [email protected]%, and p-Li4SiO4. For both processes the linear trends are clearly shown, which consequently provide the gradients of the best-fit lines for estimation. Hence, the activation energies for the CO2 chemisorption on Li4SiO4 can be determined to be 64.7, 76.7, and 118.5 kJ/mol for [email protected]%, [email protected]%, and p-Li4SiO4, respectively. The calculated activation energies for the lithium diffusion throughout Li2CO3 also show the same trend as 19.7, 27.4, and 60.6 kJ/mol for [email protected]%, [email protected]%, and pLi4SiO4, respectively. This clearly enhanced sorption behavior could be attributed to the porous carbon additives providing CO2 diffusion pathways and accordingly favoring the chemisorption and diffusion processes, which decreases the energy necessary for the CO2 absorption reaction.
To investigate the stability and regenerability of [email protected]% sorbent, its CO2 absorption/desorption cycle test was carried out under 15% CO2 gas for absorption and 100% N2 gas for desorption every 80 min for 15 cycles (Fig. 6). The absorption amount of the sorbent at the first cycle was 21.1 wt%, but it gradually decreased to 10.0 wt% with cyclic loading. To clarify this behavior, we took TEM images (Fig. S4) and elemental analyses data (Table S4) of
13
the [email protected]% sorbent after the CO2 cycling test. The additive dissipated and the percentage of carbon present in the sorbent decreased from 1.77% to 0.37%. We have previously prepared an unusual, coral-like Li4SiO4 through the thermolysis of a new lithium and siliconbased precursor [21]. Even though this porous sorbent efficiently chemisorbed CO2, its absorption capacity decreased and the morphology disappeared as the number of loading cycles increased. Likewise, in the present work, the capacity decrease upon cycling was caused by gradual loss of carbon additives at high temperature, leading agglomeration and deformation of the sorbents. In this sense, to maintain the initial textural properties of the sorbent, additives inert under harsh conditions should be secured.
3.3. Li4SiO4@CNT
To confirm the influence of porous carbon on the CO2 absorption properties of Li4SiO4 sorbents, the multi-walled carbon nanotube (MWCNT) was introduced as an additive instead of CMK-3. For evenly mixing in water, carboxylic acid functionalized MWCNT was chosen with the precursors, LiOH.H2O and fumed SiO2.
The XRPD patterns indicated that the Li4SiO4@CNT display diffraction peaks of singlephase monoclinic Li4SiO4, as with the Li4SiO4@CMK samples, which is coincident with JCPDS no. 37-1472 (Fig. S5). As shown in Figs. 7a and b, the additive is well-distributed over the whole solid in Li4SiO4@CNT. In nitrogen adsorption-desorption isotherm measurements (Fig. S6a), Li4SiO4@CNT displayed higher surface area and total pore volume than those of p-Li4SiO4 (4.52 m2 g-1 and 0.019 cm3 g-1 versus 1.06 m2 g-1 and 0.005 cm3 g-1). CO2 gas sorption isotherms of
14
sorbents were also measured at 195 K (Fig. S6b) and Li4SiO4@CNT showed a characteristic isotherm of CO2 diffusion. From these results, we expected that Li4SiO4@CNT has a similar role to that of Li4SiO4@CMK sorbents.
To confirm the advantages of incorporation of porous carbon in Li4SiO4 with regard to CO2 capture, Fig. 7c displays the dynamic thermograms of Li4SiO4@CNT with p-Li4SiO4 under mimicked flue gas at a scan rate of 10 °C min-1. CO2-chemisorption process of Li4SiO4@CNT sorbent began at 450 °C and reach a maximum at 615 °C, at which absorption and desorption are in equilibrium. The maximum absorption capacity for the Li4SiO4@CNT sorbent was equal to 9.8 wt%, which is larger value than that of p-Li4SiO4. On the basis of dynamic thermogram of Li4SiO4@CNT, it was measured isothermally under flowing mimicked flue gas at 500, 550, and 600 °C (Fig. 7d). The absorption capacity and rate of Li4SiO4@CNT exhibited higher values than those of p-Li4SiO4.
To investigate the stability and regenerability of Li4SiO4@CNT sorbent, its CO2 absorption/desorption cycle test was carried out under 15% CO2 gas for absorption and 100% N2 gas for desorption every 80 min for 15 cycles (Fig. S7). The absorption amount of the sorbent at the first cycle was 14.8 wt%, but it gradually decreased to 10.0 wt% with cyclic loading. In common with [email protected]% sorbent, the capacity decrease upon cycling was caused by gradual loss of carbon additives at high temperature, leading agglomeration and deformation of the sorbents. Results from the CO2 absorption performances of Li4SiO4@CMK and Li4SiO4@CNT sorbents indicate that addition of porous carbon into Li4SiO4 gives CO2 diffusion pathways. In particular, considering that MWCNT is less porous than CMK-3, and
15
Li4SiO4@CNT, which includes 1 wt% of carbon additive, contained less additive than [email protected]% (containing 1.8 wt% of additive), and Li4SiO4@CNT and [email protected]% show similar CO2 absorption capacity, the role of porous carbon additives mainly the voids between Li4SiO4 and porous carbon material.
4. CONCLUSIONS
We synthesized Li4SiO4 sorbents that included porous carbon, CMK-3, as an additive. The formed sorbents had higher surface areas and pore volumes than p-Li4SiO4, and showed characteristics of CO2 diffusion. These characteristics may be due to the addition of porous carbon additives into Li4SiO4 that gives CO2 diffusion pathways; in addition, this may be attributed to not only the porosity of the additive itself but also the voids between Li4SiO4 and porous carbon material. The Li4SiO4@CMK samples had better CO2 absorption properties compared with those of p-Li4SiO4. For control experiment investigating the effect of porous carbon on CO2 absorption properties, we introduced MWCNT as an additive. As a result, the Li4SiO4@CNT also had better CO2 absorption properties than the p-Li4SiO4.
In this paper, carbon materials, which are vulnerable to high temperature, were selected as additives; unfortunately, this proved problematic because, on cycling, the CO2 absorption ability of the sorbents decreased. Moreover, the industrial emissions in core fired power plants typically include the gases like O2, H2O, SO2 or NOx apart from CO2 and N2 [28, 29], which can induce the combustion of carbon at high temperature. For that reason, we expect that the use of an additive that is stable for any atmospheres at high temperature will have a significant effect on the CO2 absorption/desorption cycle. Therefore, we are currently investigating the use of alpha phase alumina, which is stable at high temperature [27], as an additive instead of porous carbon.
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Acknowledgments
This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea (NRF-2014M1A8A1049255; 2013K1A3A1A04076417). J.H.L. acknowledges the Global PhD Fellowship (NRF-2013H1A2A1033501). We are grateful to Prof. Sang Hoon Joo for helpful discussion.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/xxx.
AUTHOR INFORMATION Corresponding Author at: Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 689-798, Republic of Korea. Fax: +82-52-217-2019; Tel: +82-52-217-2928. E-mail: [email protected]
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[13] M. Puccini, M. Seggiani, S. Vitolo, CO2 capture at high temperature and low concentration on Li4SiO4 based sorbents, Chem. Eng. Trans. 23 (2013) 1279-1284.
[14] V.L. Mejía-Trejo, E. Fregoso-Israel, H. Pfeiffer, Textural, structural, and CO2 chemisorption effects produced on the lithium orthosilicate by its doping with sodium (Li 4xNaxSiO4),
Chem. Mater. 20 (2008) 7171-7176.
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Intensity (a.u.)
[email protected]%
[email protected]%
p-Li4SiO4
Li4SiO4 #37-1472
10
20
30
40
50
60
70
2 (Degrees) Fig. 1. XRPD patterns of three kinds of Li4SiO4 sorbents and the JCPDS file of Li4SiO4 as a reference.
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(a)
[email protected]%
50 nm
(b)
5 nm
(f) (c)
500 nm
(d)
[email protected]%
50 nm
(e)
5 nm
(c) (f)
250 nm
Fig. 2. TEM images and EDS mapping of carbon (green dots). (a)-(c) [email protected]%, and (d)-(f) [email protected]%.
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Quantity Adsorbed (mL g-1 STP)
(a)
16 14 [email protected]%
12 10 8 6
[email protected]%
4 2
p-Li4SiO4
0 0.0
0.2
0.4
0.6
0.8
1.0
-1
(b)
CO2 Adsorbed (mL g STP)
Relative Pressure (P/P0) 20 [email protected]%
18 16 14 12 10
[email protected]%
8 6 4
p-Li4SiO4
2 0 0.0
0.2
0.4
0.6
0.8
1.0
Pressure (bar) Fig. 3. (a) Nitrogen adsorption-desorption isotherms at 77 K, and (b) CO2 adsorption-desorption isotherms at 195 K of Li4SiO4 sorbents.
25
(a)
(b)
15% CO2 [email protected]%
100% CO2 [email protected]%
[email protected]% [email protected]%
p-Li4SiO4 p-Li4SiO4
(c)
(d)
15% CO2, 550 oC [email protected]%
100% CO2, 625 oC [email protected]% [email protected]%
[email protected]%
p-Li4SiO4
p-Li4SiO4
Fig. 4. Dynamic thermograms of the Li4SiO4 samples under (a) 15% CO2 in N2, and (b) 100% CO2 atmosphere. CO2 chemisorption isothermal analyses of the Li4SiO4 sorbents (c) at 550 °C under 15% CO2 in N2, and (d) at 625 °C under 100% CO2 atmosphere.
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(a)
[email protected]%
(b)
y = -7.7872x + 6.70862 R = -0.99311
Chemisorption
(c)
y = -2.37383x – 2.22507 R = -0.95696
Diffusion
[email protected]%
(d)
y = -9.22528x + 7.90174 R = -0.97597
Chemisorption
(e)
[email protected]% y = -3.29464x - 1.2695 R = -0.94296
Diffusion
p-Li4SiO4
(f)
y = -14.25502x + 14.22331 R = -0.99999
Chemisorption
[email protected]%
p-Li4SiO4 y = -7.28582x + 3.75224 R = -0.90919
Diffusion
Fig. 5. Plots of ln k versus 1/T, for the two different processes, chemisorption (k1) and diffusion (k2), observed on (a), (b) [email protected]%, (c), (d) [email protected]%, (e), (f) p-Li4SiO4 sorbents.
27
Fig. 6. Cyclic performance at 550 oC of [email protected]%.
28
(a)
(b)
200 nm
20 nm
(c) 15% CO 2
(d) 15% CO 2 Li4SiO4@CNT
600 oC isotherm
550 oC isotherm
p-Li4SiO4
500 oC isotherm
Fig. 7. (a) TEM and (b) SEM images of Li4SiO4@CNT sorbents, (c) Dynamic thermograms of the Li4SiO4@CNT and added p-Li4SiO4 data as a reference, and (d) CO2 chemisorption isothermal analyses of the Li4SiO4@CNT at 500 oC, 550 oC, and 600 oC under 15% CO2 in N2.
29
N2
Air
700 oC
700 oC
Li4SiO4@CMK or Li4SiO4@CNT
Scheme 1. Scheme of preparation of porous carbon containing Li4SiO4 sorbents.
30
Table 1. Kinetic parameters of the Li4SiO4 sorbents derived from the CO2 isotherm fittings using a double exponential model.
Samples
[email protected]%
[email protected]%
p-Li4SiO4
Absorption temperature (oC)
k1 (s-1)
k2 (s-1)
R2
550
0.065
0.0061
0.9999
600
0.104
0.0068
0.9999
625
0.146
0.0079
0.9998
550
0.035
0.0052
0.9998
600
0.078
0.0060
0.9999
625
0.086
0.0075
0.9999
550
0.045
0.0065
0.9999
600
0.121
0.0083
0.9999
625
0.192
0.0146
0.9998
31