Novel M (Mg/Ni/Cu)-Al-CO₃ layered double hydroxides synthesized by aqueous miscible organic solvent treatment (AMOST) method for CO₂ capture

Highlights

• Ni-Al-CO3 and Cu-Al-CO3 LDHs were synthesized by aqueous miscible organic solvent treatment method for the first time.

• Ni-Al-CO3 displayed a uniform flower-like morphology and greatly increased surface area (249.45 m2/g).

• Three un-calcined M (Mg, Ni, Cu)-Al-CO3 LDHs were used as CO2 capture adsorbents.

• The highest CO2 adsorption capacity of 0.87 mmol/g and CO2/N2 selectivity of 166 were achieved for Ni-Al-CO3 at 50 °C under 1200 mbar.

• The un-calcined Ni-Al-CO3 shows superior CO2 capture performance than the well-reported Mg LDHs.

Abstract

Layered double hydroxides (LDHs) have been intensively studied in recent years owing to their great potential in CO₂ capture. However, the severe aggregation between platelets and low surface area restricted it from exhibiting very high CO₂ adsorption capacity and CO₂/N₂ selectivity. In this research, we for the first time synthesized Ni-Al-CO₃ and Cu-Al-CO₃ LDHs using aqueous miscible organic solvent treatment (AMOST) method. The as-synthesized materials were evaluated for CO₂ adsorption at three different temperatures (50, 80, 120 °C) applicable to post-combustion CO₂ capture. Characterized with XRD, N₂ adsorption-desorption, TEM, EDX, and TGA, we found the newly synthesized Ni-Al-CO₃ LDH showed a nano-flower-like morphology comprising randomly oriented 2D nanoplatelets with both high surface area (249.45 m²/g) and pore volume (0.59 cc/g). Experimental results demonstrated that un-calcined Ni-Al-CO₃ LDH is superior in terms of CO₂ capture among the three LDHs, with a maximum CO₂ adsorption capacity of 0.87 mmol/g and the ideal CO₂/N₂ selectivity of 166 at 50 °C under 1200 mbar for typical flue gas CO₂/N₂ composition (CO₂:N₂ = 15:85, v/v). This is the first report of a delaminated Ni-Al-CO₃ LDH showing better CO₂ capture performance than the well-reported optimal Mg layered double hydroxide.

Introduction

With the acceleration of industrialization, the rapid development of modern society has been accompanied with the huge consumption of fossil fuels, leading to the emission of large amount of greenhouse gases, especially, carbon dioxide (CO₂) [1]. The atmospheric concentration of CO₂ increased from its pre-industrial level of 280 ppm to 380 ppm in 2005, and is expected to reach 550 ppm by 2050 if CO₂ emission continues at the same rate in the next three decades [2,3]. This exponentially increasing anthropogenic emission of CO₂ has already resulted in the rise of average global temperature, a phenomenon referred to as global warming. The temperature due to global warming is predicted to increase by 1.1˜6.4 °C during the 21^st century [3], threatening the survivals of both humans and animals on the earth. Therefore, there is an urgent need to take measures to reduce the carbon emission. CO₂ capture is viewed as one of the potential strategies for immediate action towards climate change mitigation. Technologies attempted for CO₂ capture are mainly based on solvent absorption [4], adsorption on solid adsorbents [5] and membrane separation [6].Among these technologies, the use of solid adsorbents for CO₂ capture is deemed to be more economical and simpler than the technologically mature solvent absorption process. The choice of adsorbents plays a central role in deciding the overall efficiency of adsorption technology for CO₂ selective separation and capture. Many types of solid adsorbents, such as activated carbon [7,8], zeolites [[9], [10], [11], [12], [13], [14], [15], [16]], and metal-organic frameworks [[17], [18], [19], [20], [21]] etc., have been exploited for CO₂ capture. However, these adsorbents usually show low adsorption efficiency at a relatively high temperature for flue gas, which is usually above 100 °C. Therefore, it is important to develop high performance adsorbents capable of working at such high temperatures for CO₂ capture.

Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds (HTs) or anionic clays, are a kind of ionic lamellar compounds consisting of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules [22]. A generic formula of most studied LDHs can be written as [M²⁺_1−xM³⁺_x(OH)₂] [Aⁿ⁻]_x/n·zH₂O, where M²⁺ and M³⁺ are divalent (e.g., Mg²⁺, Zn²⁺, Ni²⁺) and trivalent cations (e.g., Al³⁺, Ga³⁺, Fe³⁺, Mn³⁺), respectively. Aⁿ⁻ is a non-framework charge compensating anion (e.g., CO₃²⁻, Cl⁻, SO₄²⁻), and x is usually in the range of 0.2˜0.4 [[22], [23], [24]]. LDHs have found a wide range of applications, such as being used as CO₂ adsorbents [[25], [26], [27]], heavy metal ions adsorbents [28,29], catalysts [[30], [31], [32]], and drug delivery hosts [33]. Among various LDHs, the most widely studied one for CO₂ capture is Mg-Al-CO₃ LDH as well as its derived materials because LDHs would transform into mixed metal oxides (MMOs) upon high temperature calcination, which could provide active adsorption sites Mg-O for CO₂ [3,27], thus increasing CO₂ adsorption capacity. It is generally acknowledged that the fresh LDH has no CO₂ adsorption ability [3].

Due to the great potential of LDHs as CO₂ adsorbents, researchers developed several synthesis methods for LDHs with the aim of optimizing these materials. The commonly used one is co-precipitation method. This method involves simultaneous addition of divalent and trivalent metal salts solution into a base and interlayer anion solution (e.g., NaOH, Na₂CO₃) at a constant pH (e.g.,10) to allow both metal salts to co-precipitate under the same pH conditions. LDHs synthesized through this process typically show severe aggregation with ab-face stacking, resulting in large-sized crystal particles and low surface areas, which leads to underutilization of adsorption sites of the adsorbent. In order to solve this problem, O’Hare and co-workers [34] managed to optimize synthetic procedures using aqueous miscible organic solvent treatment (AMOST) method to successfully obtain Zn₂Al-borate and Mg₃Al-borate LDHs containing delaminated nanosheets with a uniform particle size of ca. 5 μm and high specific surface areas of 458.6 and 263 m² g⁻¹, respectively. Regarding the AMOST method, the LDHs are initially formed using a conventional coprecipitation approach but before final isolation the solid is re-dispersed in an aqueous miscible organic solvent, such as methanol or acetone, which can change the structure of LDHs during redispersion process. Buffet et al. [35] subsequently reported methylaluminoxane modified AMOST Mg₆Al₂(OH)₁₆CO₃·4H₂O LDH and used it as a catalyst support for the slurry phase polymerisation of ethylene. Chen et al. [36] also adopted AMOST method to tune the surface area and particle morphology of Mg/Al−CO₃ LDH by adjusting the organic solvent amount, re-dispersion time, and acetone washing steps.

The above-mentioned studies show that the modifications during the synthesis of LDH with AMOST are effective in improving the morphology, dispersity, surface area, and pore volume. In this context, we hypothesized that the optimized LDH could show large CO₂ adsorption capacity and high CO₂/N₂ selectivity. Herein, we synthesized three types of M (Mg/ Ni/ Cu)-Al-CO₃ layered double hydroxides using AMOST method and investigated their CO₂ adsorption performance at three different temperatures (50, 80, 120 °C) applicable to post-combustion CO₂ capture. We found that the novel Ni-Al-CO₃ LDH displayed the best CO₂ capture ability at all three temperatures among the three LDHs. This is the first report of an un-calcined Ni-Al-CO₃ LDH possessing the superior CO₂ adsorption ability to the nominal best LDH-based CO₂ adsorbent, i.e., Mg-Al-CO₃ LDH. This newly synthesized Ni-Al-CO₃ LDH using AMOST method may afford a good candidate as CO₂ adsorbent and also provide a route for tuning other LDH materials in a wide range of applications.