Elsevier

Thermochimica Acta

Volume 637, 10 August 2016, Pages 31-37
Thermochimica Acta

Effects of porous carbon additives on the CO2 absorption performance of lithium orthosilicate

https://doi.org/10.1016/j.tca.2016.05.010Get rights and content

Highlights

  • Composites of Li4SiO4 and porous carbon materials were prepared for CO2 absorbents.

  • The kinetic parameters of the composites were examined.

  • The pores of CMK-3 in Li4SiO4 aid the diffusion of CO2.

Abstract

Lithium orthosilicate (Li4SiO4) 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 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 Li4SiO4. Porous carbon materials such as CMK-3 were introduced into the sorbent to provide an 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 Li4SiO4@CMK-1.8%, CMK-3 is distributed over the whole solid; in contrast, the additive in Li4SiO4@CMK-0.5% is mainly observed near the surface of the solid. CO2 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. Li4SiO4@CMK-1.8% 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 Li4SiO4 (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.

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], [5], [6], and lithium-based oxide composites [7], [8], [9], [10], [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 30-times smaller and had 12-times the surface area of bulk Li4SiO4 [18]. The sol-gel method was 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, 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.

Section snippets

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].

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)

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

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

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.

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