Elsevier

Journal of CO2 Utilization

Volume 34, December 2019, Pages 688-699
Journal of CO2 Utilization

Research Article
CO2 sorption properties of Li4SiO4 with a Li2ZrO3 coating

https://doi.org/10.1016/j.jcou.2019.08.022Get rights and content

Highlights

  • Three different CO2 sorbents based on lithium orthosilicate were prepared and characterized.

  • Li4SiO4@Li2ZrO3 was the best performing material in terms of sorption capacity, stability and rate of carbonation-decarbonation.

  • Core-shell structure saturated with Li, allowing for easy restructuring during carbonation and fast regeneration.

Abstract

The application of a CO2 sorbent which releases CO2 at a lower temperature than calcium oxide is of interest in view of reducing the operating temperature or increasing the operating pressure of the super-dry reforming process. To this end, three different CO2 sorbents based on lithium orthosilicate (Li4SiO4) were prepared and characterized: (i) Li4SiO4 as such; (ii) zirconia coated Li4SiO4, denoted Li4SiO4@ZrO2; (iii) Li4SiO4, coated with lithium metazirconate and denoted as Li4SiO4@Li2ZrO3. While the carbonation properties of Li4SiO4 were found satisfactory, its decarbonation was incomplete. Li4SiO4@ZrO2 on the other hand, was found to contain Li2SiO3 and Li2ZrO3 rather than the anticipated Li4SiO4 and showed fast carbonation and decarbonation of Li2ZrO3. The best performing material in terms of CO2 sorption capacity, stability and rate of carbonation-decarbonation was Li4SiO4@Li2ZrO3. Its superior performance is ensured by the core-shell structure saturated with Li, allowing for easy restructuring during carbonation and fast regeneration upon decarbonation.

Introduction

Since the adoption of the Kyoto Protocol in 1997, solid carbon dioxide (CO2) sorbents have gained tremendous attention among researchers for their potential application in CO2 separation and utilization processes [[1], [2], [3], [4]]. A particular application of interest for high-temperature CO2 sorbents in view of indirect or direct carbon capture and utilization (CCU) is efficient conversion of methane via reforming processes, as in Sorption Enhanced Chemical Looping Reforming of Methane (SE-CL-RM), and Super-Dry Chemical Looping Reforming of Methane (SD-CL-RM) [5]. SE-CL-RM, first proposed by Lyon and Cole [6], minimizes the economic and environmental barriers of conventional reforming and offers high efficiency, inherent CO2 separation and pure H2 in one step [[7], [8], [9]]. This emerging concept (Scheme 1A) can run under near auto-thermal conditions as the heat released from the exothermic sorbent carbonation reaction can drive the endothermic reforming reaction. The endothermic sorbent regeneration is driven by the exothermic oxygen carrier oxidation, minimizing again heat requirements. Thermodynamics-based calculations on the combined SE-CL-RM process employing only two reactors showed that a decrease of up to 55% of energy demands is possible compared to conventional steam reforming [10]. The produced stream of pure CO2 allows for easy downstream utilization.

Another intensified reforming process for the valorization of methane (biogas) is ‘Super-dry’ reforming of CH4 (SDsingle bondCLsingle bondRM) [5,11]. Dry reforming of methane (DRM), the catalytic reaction of CH4 with CO2, recently gained much attention as it reduces greenhouse gases, applying CO2 as carbon source, and aims at utilizing biogas and natural gas with a significant amount of CO2 [[12], [13], [14]]. SDsingle bondCLsingle bondRM was developed for enhanced CO production from CH4 and CO2. The process uses a CO2 sorbent (CaO/Al2O3), an oxygen carrier (Fe2O3/MgAl2O4) and a CH4 reforming catalyst (Ni/MgAl2O4) as materials, and CH4 and CO2 in a 1:3 ratio (Scheme 1B). It combines an exothermic process, CaCO3 formation (from CaO and CO2), with two endothermic processes, CH4 reforming and Fe3O4 reduction, thereby looping the energy utilization. The isothermal coupling of these three different processes results in direct utilization of three times more CO2 than conventional dry reforming with a concomitant higher CO production.

Because of the important role of CO2 capture from and release to the gas phase in many processes, the temperature at which carbonation and decarbonation occur influences the operating temperature and pressure of the entire process. For example, super-dry reforming can operate in broad ranges of temperature (900–1100 K) and pressure (1–10 bar). However, decreasing the operation temperature gives less rise to material deactivation. Hence, the choice of the CO2 sorbent material is of importance for the overall process operating conditions. When using a calcium oxide based sorbent material, for example, the process can be run at 1023 K and 1 bar. Decreasing the operating temperature at a fixed pressure or increasing the operating pressure at a fixed temperature is only feasible when using a CO2 sorbent that releases CO2 at a lower temperature, i.e. a CO2 sorbent that has a lower affinity for CO2. To this end, lithium oxide based materials such as lithium orthosilicate (Li4SiO4) and lithium metazirconate (Li2ZrO3) are used to run the process at lower temperature. At the same time, the formation of hydroxides (such as Ca(OH)2 or LiOH), upon exposure to high partial pressures of steam and low temperatures, should be avoided (Fig. 1).

Minerals that contain alkali or earth-alkali metal oxides form a particular class of CO2 sorbents, which can typically store CO2 in the form of carbonates over a broad temperature range, and release this CO2 by decarbonation, either at an elevated temperature or in the presence of an inert sweep gas. Among the most popular high-temperature CO2 sorbents are calcium oxide [1,2], magnesium oxide [2,15] and lithium oxide [3,4] based materials, each of which have their own characteristic behavior in terms of CO2 sorption and release. The temperature at which the transition between carbonation and decarbonation occurs is governed by thermodynamics (Fig. 1), and is typically more than 150 K lower for lithium oxide based materials than for calcium oxide based materials [16].

While the first mentioning of Li2ZrO3 may be traced back to 1958 with the work of Hon and Bray [19], the structure of Li4SiO4, consisting of SiO4 tetrahedrons interlinked by Li ions, was first reported by Völlenkle and coworkers [20,21] in 1967. It was soon found to be a promising solid lithium ion conductor [22]. Even though the conductivity of pure ordered Li4SiO4 is quite low, it can be improved by a factor 103 or 104 by introducing vacancies [23]. The supercell structure of a pure ordered Li4SiO4 was resolved by Tranqui and coworkers as consisting of distorted SiO4 tetrahedra and LiOn (n = 4, 5, 6) polyhedral sites [23]. Out of 126 available Li sites (categorized in six distinctive types) in a unit cell of the Li4SiO4 superstructure, 56 are occupied and interlinked in a three-dimensional network [23].

Besides the widespread application of lithium in lithium-ion batteries, lithium oxide based materials such as lithium orthosilicate have also been studied for their application as tritium breeder materials in nuclear fusion reactor experiments [24]. The first use of Li2ZrO3 and Li4SiO4 as high-temperature CO2 sorbent dates back to work published in 1998 and 2001 by Nakagawa and coworkers [3,4]. Eq. (1) presents the carbonation reaction of Li2ZrO3 with formation of zirconia (ZrO2) and lithium carbonate (Li2CO3).Li2ZrO3s+CO2gZrO2s+Li2CO3sΔH°298K=160kJmol1

As for Li4SiO4, two subsequent carbonation reactions, given by Eqs. (2) and (3), may theoretically occur: Li4SiO4 is first carbonated with formation of Li2CO3 and lithium metasilicate (Li2SiO3), after which Li2SiO3 may be further carbonated with formation of more Li2CO3 and silica (SiO2). In practice, however, reaction 3 is not relevant for high-temperature carbonation since it is only thermodynamically favorable at low temperature [4] where kinetic limitations are likely to prevail.Li4SiO4s+CO2gLi2SiO3s+Li2CO3sΔH°298K=142kJmol1Li2SiO3s+CO2gSiO2s+Li2CO3gΔH°298K=84kJmol1

Since the first reports on lithium oxide based CO2 sorbents, researchers have also investigated the effect of different preparation methods and dopants on their reactivity and stability. Large efforts were dedicated to improve the pore structure of Li4SiO4, such as the application of organic precursors, which is beneficial to the formation of pores, and doping with eutectic salts, favorable for the decrease of CO2 diffusion resistance [25,26]. Among the most popular dopants are alkali metal carbonates such as potassium carbonate (K2CO3) [[27], [28], [29], [30], [31], [32], [33], [34], [35]] and sodium carbonate (Na2CO3) [30,33,[35], [36], [37], [38]]. Besides their pioneering work in CO2 sorption by lithium oxide based materials, Kato, Nakagawa and coworkers [16] were also the first to use a physical mixture of Li4SiO4 and Li2ZrO3 pellets as a way to inhibit grain growth. Indeed, although Li4SiO4 as such was found to significantly lose CO2 sorption capacity over subsequent cycles of carbonation and decarbonation, this effect was much less pronounced when mixed with the more thermally stable Li2ZrO3.

As a continuation of the above, in the present work, Li4SiO4 based materials, modified either by ZrO2 or Li2ZrO3, were synthesized and tested. In view of enhanced stability, here, the modifying phase was applied as a coating by means of a nanocoating strategy with a surfactant and precursor, yielding core-shell structured materials: Li4SiO4@ZrO2 and Li4SiO4@Li2ZrO3. Both materials were compared with non-modified Li4SiO4.

Section snippets

Material preparation

Four different Li4SiO4 based CO2 sorbent materials were tested: (i) Li4SiO4 as such, prepared using a wet physical mixing method with fumed silica, lithium hydroxide and citric acid as precursors; (ii) ZrO2 coated Li4SiO4, prepared using a coating method with P-123 as surfactant and zirconium propoxide as precursor; (iii) Li2ZrO3 coated Li4SiO4, prepared using a similar coating method as in (ii), followed by impregnation with LiOH; (iv) Commercially available Li4SiO4, obtained from Alfa Aesar®.

N2 adsorption

As a first characterization of the as prepared lithium orthosilicate (Li4SiO4) based sorbent materials, N2 adsorption measurements were performed. Table 1 presents the specific surface area, pore volume and average pore size of commercially available Li4SiO4, as prepared Li4SiO4 and its fumed silica precursor, Li4SiO4@ZrO2 and Li4SiO4@Li2ZrO3.

Introducing lithium in the silica structure to form Li4SiO4 is accompanied by a strong deterioration of morphological properties with a decrease in

Conclusions

In this work, Li4SiO4, Li4SiO4@ZrO2 and Li4SiO4@Li2ZrO3 CO2 sorbents were synthesized, characterized and compared in view of their applicability. By coating Li4SiO4 with ZrO2, the material’s CO2 sorption capacity was deteriorated through the formation of Li2ZrO3, a CO2 sorbent less reactive than Li4SiO4, and Li2SiO3. Based on CO2-TPR and (in situ) XRD characterization, the Li4SiO4@ZrO2 CO2 sorbent was considered inferior to both Li4SiO4 and Li4SiO4@Li2ZrO3. Applying a Li2ZrO3 coating, however,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Long Term Structural Methusalem Funding of the Flemish Government. L. C. Buelens acknowledges financial support from the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT Vlaanderen). The authors thank Geert Rampelberg, Davy Deduytsche and Ranjith K. Ramachandran for help with in situ XRD measurements (Department of Solid State Sciences, Ghent University).

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