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Cite this: Chem. Commun., 2014, 50, 4927

Chabazite and zeolite 13X for CO2 capture under high pressure and moderate temperature conditions†

Received 17th August 2013, Accepted 18th March 2014

Seung-Hwan Hong,a Min-Seok Jang,a Sung June Chob and Wha-Seung Ahn*a

DOI: 10.1039/c3cc46313c

Mesoporous chabazite ion-exchanged with Ca2+ was effective for CO2 capture at 20 bar and 473 K, whereas 13X as a support material enabled recyclable carbonation of ca. 8 wt% Mg(OH)2 approaching the theoretical maximum for CO2 capture with 10% H2O.

Reducing greenhouse gas emissions and increasing the efficiency in designing future power plants are pressing issues worldwide. The integrated gasification combined cycle (IGCC) is being considered for this purpose and pre-combustion is the favoured choice of the IGCC with regard to CO2 capture options due to the potential removal of a high concentration of CO2 in synthesis gas under high pressure and moderate temperature conditions.1 Targeting the pre-combustion technology, dry adsorbents such as zeolites (X, Y, A, SSZ-13 and chabazite),2 activated carbon,3 hydrotalcite,4 and layered double hydroxides/oxides5 have been considered. Zeolite 13X showed stable CO2 capture performance at temperatures ranging from 363–473 K,6 but had relatively low selectivity for CO2 and its hydrophilic nature might hinder CO2 capture under wet conditions.7,8 Chabazite (CHA) type zeolites are promising alternatives. CHA has a three-dimensional pore system with large ellipsoidal cages connected to an eight membered channel window of 3.8  3.8 Å and has a relatively high Si/Al ratio than 13X, which can lead to higher CO2 selectivity.9 Ca2+-exchanged CHA was reported to have higher CO2 capture capacity at 473 K under dry conditions.2c Pure silica CHA showed a higher CO2 capture capacity than 13X in the presence of water at 313 K.7 On the other hand, the CO2 capture capacities of all these microporous zeolite adsorbents tend to saturate at low pressures. Carbonation reaction-based processes are also feasible for CO2 removal from flue gases under moderate temperature conditions, a

Department of Chemistry and Chemical Engineering, Inha University, Incheon, 402-751, Republic of Korea. E-mail: [email protected]; Fax: +82 32 872 0959; Tel: +82 32 860 7466 b Department of Applied Chemical Engineering, Chonnam National University, Gwangju, 500-757, Republic of Korea † Electronic supplementary information (ESI) available: Experimental details, characterization, and CO2 adsorption data from volumetric, gravimetric, and breakthrough runs. See DOI: 10.1039/c3cc46313c

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and Mg(OH)2 is an excellent candidate sorbent for CO2 removal under IGCC conditions (473–623 K). The compound has a considerably higher CO2 capture capacity than that of the commercial Selexols process, and the energy required for the reverse carbonate decomposition to recover Mg(OH)2 is significantly lower than other processes.10 Butt et al.,11 however, reported that the carbonation of Mg(OH)2 disrupts the inward diffusion of CO2, causing limitations in Mg(OH)2 conversion; only ca. 17% of the full carbonation capacity per gram of Mg(OH)2 was possible. In this study, two different approaches were proposed to improve the CO2 capture performance of zeolite 13X and CHA under moderate temperature and high pressure conditions. Firstly, the CO2 capture capacities of CHA after ion-exchange were measured and an enhancement in the CO2 capture capacity under high pressure conditions was achieved by introducing mesopores in CHA. Secondly, zeolite 13X and CHA were utilized as a support material for Mg(OH)2 to increase its efficiency in CO2 capture via carbonation under pre-combustion conditions (20 bar and 473 K). Initially, the K+ form of CHA (KCHA) was synthesized hydrothermally, and Na+, Li+, Ca2+ and Mg2+ forms of CHA (NaCHA, LiCHA, CaCHA, and MgCHA) were prepared via ionexchange. All maintained high crystallinity (Fig. S1, ESI†) and exhibited type I N2-isotherms at 77 K (Fig. S2, ESI†), confirming their microporous structure. A sharp increase in the isotherms after P/P0 4 0.95 also indicated textural mesopores from the interparticle voids (see Fig. S3, ESI† for its morphology).12 Large K+ ions caused pore blockage in the CHA framework and produced a very low surface area, as estimated by N2 adsorption,9 but the ion-exchanged CHAs showed significantly higher BET surface areas (Table 1). KCHA, despite having a low surface area showed a high CO2 capture capacity, because CO2 can have sufficient access to the interior of the pore due to the smaller molecular size than N2. The CO2 adsorption capacity increased in the following order: LiCHA 4 CaCHA 4 NaCHA 4 MgCHA at 298 K (Fig. S4, ESI†). None, however, exceeded the adsorption capacity of 13X (260 mg g 1-adsorbent). For practical IGCC implementation, the CO2 adsorption capacity at 473 K is a more relevant parameter to

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Communication Table 1


CO2 capture performances of the ion-exchanged chabazites and 13X

CO2 adsorption capacity

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Textural properties Sample

SBET VPorea Vmesob [M+ or 2+]/ 298 K/ 473 K/ 298 K/ 473 K/ 298 K/ 473 K/ (m2 g 1) (cm3 g 1) (cm3 g 1) Al 1 barc 1 bard 1 bare 1 bare 20 bar f 20 bar f CO2 breakthrough


17 520 682 660 503 73 154 200

0.04 0.32 0.36 0.35 0.29 0.10 0.16 0.18

0.03 0.09 0.10 0.08 0.09 0.09 0.13 0.16

0.99 0.95 0.65 0.06 0.66 0.99 0.97 0.72

150 215 255 225 201 110 160 166

15 33 42 48 23 12 22 27

140 207 — 212 — 105 153 161

14 32 — 46 — 10 21 26

143 215 — 219 — 160 229 237

17 36 — 48 — 31 45 62

Dry conditions (CO2 : N2 = 3 : 7)

13X 13X–Mg(OH)2 CaCHA CaCHA–Mg(OH)2 CaCHA(M) CaCHA(M)–Mg(OH)2

804 750 660 610 200 173

0.34 0.46 0.35 0.49 0.18 0.30

0.05 0.19 0.08 0.16 0.16 0.27

— — 0.60 — 0.72 —

260 — 225 — 166 —

36 — 48 — 27 —

— — — — — —

21/21g 24/23 23/23 25/24 18/18 20/20

— — — — — —

31 93 26 50 50 77

Moisture conditions (CO2 : N2 : H2O = 3 : 6 : 1)

a VPore = total pore volume. b Vmeso = mesopore volume calculated by the BJH method (P/P0 r 0.95). c CO2 adsorption by the volumetric method (BELsorpII-mini, BEL, Japan). d CO2 adsorption by TGA. e CO2 adsorption by breakthrough runs. f CO2 adsorption by breakthrough runs. g CO2 adsorption by TGA using humid CO2.

be considered so that the CO2 adsorption capacities of CHAs at 473 K could be measured (Fig. S5, ESI†) using a TGA unit (Scheme S1, ESI†). As shown in Table 1, CaCHA showed the highest CO2 capture capacity (48 mg g 1-adsorbent), which exceeded the capacity shown by 13X (36 mg g 1-adsorbent). The possibility of an enhancement in the adsorption capacity accompanied by changes in textural properties was examined by synthesizing a microporous–mesoporous hierarchical chabazite (KCHA(M)) by introducing [3-(trimethoxysilyl)propyl] octadecyldimethylammonium chloride (TPOAC) as a mesopore generating agent (Si/TPOAC = 0.03–0.1). Both KCHA and KCHA(M) exhibited the same characteristic XRD peaks corresponding to the zeolite CHA structure, but the XRD peak intensities decreased with increasing TPOAC concentrations, and higher proportions of mesopores were formed in the product (Fig. S6, ESI†); KCHA with 10 mol% TPOAC showed low crystallinity, whereas KCHA with 3 mol% TPOAC had a low mesopore volume (Table S1, ESI†). Therefore, KCHA(M) with 5 mol% TPOAC was used as the reference material. The isotherms of KCHA(M) exhibited small but clear hysteresis at relative pressures higher than P/P0 = 0.4, and the volume of N2 adsorbed by KCHA(M) significantly surpassed that of KCHA (Fig. S7, ESI†) because the N2 blocked by K+ ions were reduced by the mesopores. TEM images of KCHA and KCHA(M) obtained using different amounts of TPOAC clearly revealed mesopore regions (Fig. S8, ESI†). KCHA(M) was also ion-exchanged with Na+ or Ca2+. The XRD patterns of KCHA(M), NaCHA(M), and CaCHA(M) showed no significant changes (Fig. S9, ESI†) and their mesoporous structures were maintained with a mean mesopore size of ca. 3.3 nm (Fig. S10–S12, ESI†). The CO2 capture capacities under dry conditions of 1 to 20 bar at 298 and 473 K were measured using a home-made breakthrough equipment (Scheme S2 and Fig. S13–S16, ESI†) employing a gas mixture (CO2 30% and N2 70%). Although the CO2 adsorption capacity over CHAs barely changed with pressure, the meso-CHAs surpassed the adsorption capacity of the

4928 | Chem. Commun., 2014, 50, 4927--4930

CHAs as the pressure was increased. As shown in Fig. 1, at 20 bar and 473 K, CaCHA(M) produced significantly improved performance over CaCHA. The introduction of mesopores in CHA was clearly effective in increasing the CO2 capture capacity under high pressure conditions. The selectivity of CO2 over N2 was calculated by integrating their respective breakthrough curves. CaCHA(M) had higher selectivity (19) than either CaCHA (17) or 13X (11) at 20 bar and 473 K, and the selectivity decreased with increasing adsorption temperature and pressure (Table S2, ESI†). The validity of the adsorption data obtained by the high pressure breakthrough run was confirmed by comparing the data obtained at 298 K with the high-pressure CO2 adsorption isotherms obtained by using a magnetic suspension balance (Rubotherm, Germany) (Fig. S17–S19, ESI†). 29 Si and 27Al magic angle spinning NMR spectroscopy of the microporous and mesoporous CHA zeolites (Fig. S20, ESI†) revealed the presence of defect sites in the latter that can act as selective adsorption sites, and the experimental high pressure CO2 adsorption isotherm obtained at 298 K on mesoporous CHA

Fig. 1

CO2 breakthrough curves at 20 bar and 473 K.

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zeolite was fitted very well with the dual Langmuir adsorption sites model (Fig. S21, ESI†); the corresponding estimated parameters are given in Table S3 (ESI†). Interestingly, mesoporous silica MCM-41 with a pore diameter of ca. 3.3 nm also exhibited a CO2 adsorption isotherm pattern linearly increasing with pressure as in the latter part of the isotherm for mesoporous CHA (Fig. S22, ESI†). Finally, the concept of applying the zeolites as a support material for Mg(OH)2 was examined, in which the chemical conversion of CO2 is governed by Mg(OH)2 + CO2 = MgCO3 + H2O, and regeneration is conducted by heating MgCO3 to MgO followed by hydration: MgO + H2O = Mg(OH)2. According to thermodynamic analysis, MgCO3 forms easily up to 700 K at 20 bar, whereas the decomposition of MgCO3 is initiated at approximately 600 K and 1 bar.10 The favourable products are MgO and CO2. MgO has low reactivity with CO2 and is easily hydrated to Mg(OH)2. Mg(OH)2 can be adsorbed preferentially via an acid-based reaction onto the bridging hydroxyl protons (SiOHAl) on the zeolite surface.13 As shown in Fig. 2(b), Mg(OH)2 impregnated on 13X (Si/Al = 1.4) clearly shows fine Mg(OH)2 nanopetals grown on the 13X surface (confirmed by SEM-EDX/TEM-EDS mapping and FT-IR spectroscopy: Fig. S23–S25, ESI†). Mg(OH)2 on the zeolite 13X surface was estimated to be ca. 8.0 wt% (Table S4, ESI†). The XRD peak of Mg(OH)2 was not visible, but the background with the phase indexing at 381 showed the presence of the broad peak corresponding to the Mg(OH)2 phase by the Rietveld refinement process (Fig. S26, ESI†). The CO2 capture capacity of Mg(OH)2 impregnated on the 13X sample was measured under high pressure conditions (10 and 20 bar) at 473 K employing a breakthrough instrument using a feed gas consisting of 30% CO2, 10% H2O and N2 as a balance to determine the effects of pressure. The CO2 capture capacities of Mg(OH)2, 13X and Mg(OH)2-impregnated 13X at 10 bar were 95, 26 and 88 mg g 1, respectively (Fig. S27, ESI†). These amounted to ca. 12.5% of Mg(OH)2 being used for the CO2 conversion, whereas ca. 99.1% Mg(OH)2 was used for the CO2 conversion in the case of 8 wt% Mg(OH)2 impregnated on 13X (excluding the contribution by 13X). As shown in Fig. 3, a CO2 capture capacity of 62.5 mg g 1 was obtained at 20 bar, which corresponds to ca. 99.9% of Mg(OH)2 on 13X used for the carbonation reaction. 13X impregnated with Mg(OH)2 after 4 recycle runs of CO2 capture-decomposition (Fig. S28, ESI†) was found to be collapsed in structure (Fig. S29–S31, ESI†). However, the fine distribution of the Mg(OH)2 phase was preserved (Fig. S32, ESI†) and the CO2 capture capacity at 20 bar and 473 K was also maintained.


Fig. 3 CO2 breakthrough curves at 20 bar and 473 K (CO2 : N2 : H2O = 3 : 6 : 1): (a) 13X, (b) Mg(OH)2 impregnated (8 wt%) on 13X, and (c) Mg(OH)2.

To further examine the case, breakthrough experiments were conducted using 13X alone under the same conditions (Fig. S33, ESI†). 13X was again found partially collapsed (Fig. S34, ESI†) and mesopores were formed accompanied by a decrease in surface area after 3 recycle runs (Fig. S35 and S36, ESI†). Nevertheless, the CO2 capture capacity at 20 bar was still maintained, probably because the collapsed 13X still retained the primary building units of aluminosilicates. No Mg(OH)2 nanopetals were formed over CaCHA or CaCHA(M). Instead, a continuous Mg(OH)2 phase was detected by SEM (Fig. S37 and S38, ESI†). Strong Mg(OH)2 XRD peaks were detected (Fig. S39 and S40, ESI†) and ca. 15 and 18 wt% Mg(OH)2 was found on CaCHA and CaCHA(M), respectively (Table S4, ESI†). The smaller aluminium content (Si/Al = 2.4) in CHA could be responsible for the failure of local creation of nucleation sites, as was also the case for mordenite or SiO2 as a support (Fig. S41 and S42, ESI†). At 20 bar and 473 K, Mg(OH)2 on CaCHA and CaCHA(M) offered only a 21.5 wt% and 20 wt% theoretical maximum CO2 conversion via carbonation, respectively (Fig. S43 and S44, ESI†). In this study, the CO2 capture performance of chabazite and zeolite 13X under high pressure and moderate temperature conditions was examined. The CO2 adsorption capacity increased in the following order: CaCHA 4 LiCHA 4 13X 4 NaCHA 4 MgCHA at 473 K. The introduction of mesopores in CHA was effective in increasing the CO2 capture capacity under high pressure conditions. Zeolite 13X was effective as a support material for Mg(OH)2 used in CO2 capture via carbonation under pre-combustion conditions (20 bar and 473 K), leading to the full utilization of Mg(OH)2 for CO2 capture approaching the theoretical maximum. This study was financially supported by the Samsung Advanced Institute of Technology in Korea (2012).

Notes and references

Fig. 2

SEM images of (a) Mg(OH)2 and (b) Mg(OH)2 impregnated on 13X.

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Chabazite and zeolite 13X for CO2 capture under high pressure and moderate temperature conditions.

Mesoporous chabazite ion-exchanged with Ca(2+) was effective for CO2 capture at 20 bar and 473 K, whereas 13X as a support material enabled recyclable...
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