Alkali and alkaline-earth cation exchanged chabazite zeolites for adsorption based CO₂ capture

Abstract

In this study chabazite zeolites were prepared and exchanged with alkali cations – Li, Na, K and alkaline-earth cations – Mg, Ca, Ba and were studied to assess their potential for CO₂ capture from flue gas by vacuum swing adsorption for temperatures below 120 °C. Isotherm measurements (CO₂ and N₂) were made for all samples at 273 K, 303 K and 333 K using a volumetric apparatus and represented with the Dual-site Langmuir model for CO₂ and N₂. Henry’s constants and isosteric heats of adsorption were calculated and qualitative analyses performed for all samples. Adiabatic separation factor (ASF) and capture figure of merit (CFM) were proposed and used as indices for assessing adsorbent performance and compared with a commercial NaX-zeolite sample. It was found that NaCHA and CaCHA hold comparative advantages for high temperature CO₂ separation whilst NaX shows superior performance at relatively low temperatures.

Introduction

It is widely acknowledged that CO₂ emissions make the major contribution to global warming and their reduction is urgently needed [1]. The capture and storage of CO₂ emitted from major industries, such as the steel industry and power industry, have been proposed by many governments and major energy agencies. Various technologies for carbon capture and storage (CCS) from flue gas of power plants have been proposed, among which absorption, adsorption, membrane and cryogenic processes are the leading candidates. Because of its relatively low operating and capital costs, pressure/vacuum swing adsorption (PSA or VSA) has attracted much research effort. Research aspects of cyclic adsorption processes include cycle design, process optimization, new adsorbent development, and equipment innovation. Recent research in the field of adsorbents, which are the most important aspect of the adsorption process, has made rapid progress in the last decade and X, Y, A, ZSM-zeolites, chabazites, metal oxides, various carbons etc. have been widely studied [1], [2], [3], [4], [5].

Among those adsorbents, chabazite zeolite is a promising but often ignored option. Chabazite (CHA) is of great interest for its ion-exchange and gas separation possibilities. Naturally occurring chabazite is often found in cavities in basalt, andesite, and other igneous rocks, and as an alteration product of volcanic glass. Chabazite from the Bowie deposit has been used successfully at the Oak Ridge, Hanford, and Savannah River Nuclear Facilities to remove radioactive cesium-137 and strontium-90 from process waters [2], [3]. Chabazite is also used commercially as a desiccant. It is stable at a pH of 2.5 which makes it suitable for removing water from hydrogen chloride gas streams, and trace gas removal, such as nitrogen removal from argon, oxygen separation from argon [4], [5], [6].

The chabazite structure consists of double 6-ring (D6R) building units arranged in layers in the sequence ABCABC and linked by tilted 4-rings. This results in a 3-D framework which contains large ellipsoidal cavities each approximately 6.7 × 10 Å. Each cavity can be accessed by 6 openings each of which consists of 8-rings of somewhat variable dimension depending on extent and type of ion exchange and adsorption. These 8-ring windows have approximate dimension 3.7 × 4.2 Å (hydrated) or 3.1 × 4.4 Å (dehydrated). A cation-free CHA has a nominal 8-ring window dimension of 3.8 × 3.8 Å providing a maximum free dimension of 4.3 Å. This is considerably changed when cations are present. Kington’s systematic studies on the crystal structure of chabazites noted that the size of silicate rings and the cations were the influential factors for the adsorption energy of a given adsorbate [7]. Based on the results of structural study and further calorimetric experiments on natural and calcium chabazites, it was found that the heat of adsorption at low coverage is related to the electrostatic field within the intracrystalline voids; also it was noted that the major contribution to energetic heterogeneity of adsorption is the interaction of the adsorbate quadrupole moment with the electrostatic-field gradient in the adsorbent [8].

Regarding the specific research of cation effects on CO₂ adsorption in chabazite, there are very few studies. Barrer and Davies [9] conducted a study of adsorption of a range of gases with permanent quadrupole moment, including CO₂, on decationated (hydrogen) chabazite and found that the initial isosteric heats of adsorption of the gases correlated well with their polarizability. They concluded that the gases with permanent quadrupole moment were bound by considerable field-gradient quadrupole energy. A study conducted by Khvoshchev et al. indicated that Ca-chabazite had greater adsorption heat than Na-chabazite and partial cation exchange of Ca-Chabazite with Mg²⁺ lead to the sharp decrease of the heat of adsorption [10]. Inui, Okugawa and Yasuda studied the adsorption behaviour of different zeolites by the PSA process and claimed that chabazite and 13X were most appropriate for CO₂ separation among the zeolites studied, which included chabazite, clinoptilolite, mordenite, ferrierite, erionite, MS-5A, MS-4A, MS-13X, H-ZSM-5 [11]. Coe and Gaffney found that at around Si:Al = 2, chabazite has the largest adsorption capacity for N₂. They concluded that cation sitting, Si/Al ratio and activation temperature all played a role in N₂ capacity [4]. Although much work has been done in structural and functional analysis, synthetic cation selectivity and siting, and CO, O₂, N₂, CH₄ adsorption, little work has been done on the adsorption of CO₂ on chabazite exchanged with different cations in terms of CO₂ capture from flue gas. Data for the heat of adsorption are rarely reported and applications of chabazite in P/VSA processes for CO₂ capture from flue gas have not been reported.

In contrast, cation effects on CO₂ adsorption in zeolite X,Y and ZSM have been widely studied [12], [13], [14], [15]. Alkali metal cation effects on CO₂ adsorption in Y and X zeolites were studied and it was concluded that Li cation exchanged X/Y zeolites have the largest CO₂ capacities as a result of greatest ion–quadrupole interactions with CO₂, although no CO₂/N₂ selectivity was reported [14]. In an experimental adsorbent screening study conducted by Harlick et al., using isotherm equilibrium analysis, various zeolites (excluding chabazite) were examined and it was claimed that an adsorbent with a near linear isotherm and a low SiO₂/Al₂O₃ ratio is the most promising option for CO₂ capture [16]. However, this analysis used ideal pressure swing cycles and overlooked the important role of the heat of adsorption in altering the adsorbent temperature and hence its working capacity. It is essential to include this effect since operation of a PSA process with strongly adsorbed species such as CO₂ lead to large temperature swings. Indeed, temperature swings up to 20 K have been observed in our own pilot scale CO₂ PSA process [17]. A good adsorbent must have good adiabatic working capacity, and adiabatic working selectivity which can be determined either by experiments or simulation. These two parameters can be combined into appropriate adsorbent screening parameters: the adiabatic separation factor (ASF) and capture figure of merit (CFM) which are very good predictors of an adsorbents separation performance under actual process conditions.

The goal of our study is several-fold: First, we synthesize a range of alkali and alkaline-earth cation exchanged chabazite materials and measure the adsorption isotherms, zero-loading Henry’s constants and heats of adsorption of CO₂ and N₂ over a range in temperature and pressure and explain these effects based on energetics of the CO₂–cation interaction. We then determine the CO₂ working capacity, selectivity, adiabatic separation factor and capture figure of merit for the CO₂/N₂ separation on chabazite which allows us to assess the suitability of this adsorbent for CO₂ capture from flue gas streams.