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Journal ofMaterials Chemistry A
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aLaboratory for Sustainable Technology, Un
E-mail: [email protected]; tambDepartment of Energy Process Engi
Fuchsmuehlenweg 9, D-09596 Freiberg, GercInstitute of Thermal Engineering, Technica
Strasse 7, D-09596 Freiberg, Germany
† Electronic supplementary informacharacterisation data, determination osorbents supported on calcium aluminaCaO/meso-CaxAlyOz(20). See DOI: 10.1039
Cite this: J. Mater. Chem. A, 2014, 2,4332
Received 29th November 2013Accepted 4th February 2014
DOI: 10.1039/c3ta14953f
www.rsc.org/MaterialsA
4332 | J. Mater. Chem. A, 2014, 2, 433
Design and synthesis of stable supported-CaOsorbents for CO2 capture†
Nikki J. Amos,a Meilina Widyawati,a Sven Kureti,b Dimosthenis Trimis,c
Andrew I. Minett,a Andrew T. Harris*a and Tamara L. Church*a
CaO sorbents for CO2 capture are much more stable when supported on a ceramic; however, the rational
design of such supported sorbents has only recently been considered. We designed and prepared new CO2
sorbents by supporting CaO on meso- and macro/mesoporous supports made of materials that are stable
under conditions relevant to CO2 capture in pre-combustion CO2 sorption. The resulting sorbents were
used to capture and release carbon dioxide (CO2), and one demonstrated excellent stability over 30
cycles, taking up nearly twice as much CO2 over these thirty cycles as a related material that lacked a
tailored structure.
Introduction
Carbon capture is a well-known strategy to slow the rise inatmospheric CO2 concentration. CaO has been applied tocapture CO2 in both post-combustion capture, in which CO2 isremoved from ue gas following the combustion of coal orbiomass, and in pre-combustion capture, in which CaO is usedas an in situ sorbent to enhance the gasication (usually bysteam) of biomass or coal.1 CaO can theoretically absorb 785 gCO2 per kg, producing CaCO3 that can be calcined to re-formCaO and thus undergo repeated carbonation and calcinationreactions. Although these reactions must occur at hightemperature, the energy penalty for CaO-based CO2 capture canbe minimised by properly integrating the heat produced in thecarbonation with the steam cycles of a power plant.2
However, the operability of CaO in calcination–carbonationcycles is limited by several factors, including sintering.1 Thisfusion of CaO particles at high temperature, but below theirmelting point, decreases their surface area and pore volume,and thus their reactivity toward CO2.1 Attempts to combat thisproblem have recently been reviewed,3 and include screeningCaO precursors to obtain a high-surface-area material, incor-porating CaO into a support via co-precipitation, and graingCaO onto a support. CaO has been supported on oxides like
iversity of Sydney, NSW 2006, Australia.
neering and Chemical Engineering,
many
l University of Freiberg, Gustav-Zeuner-
tion (ESI) available: Additionalf maximum carbonation levels forte, synthesis and characterisation of/c3ta14953f
2–4339
CaTiO3,4 TiO2,5 Ca12Al14O33,6 mixed calcium aluminates,7
Al2O3,6d,8 MgO,9 SiO2,10 CexZryOz,10a La2O3,11 LaAlxMgyO310a and
cement7 to form sorbents that proved more stable than CaOalone.
Aiming to improve upon current supported-CaO-based CO2
sorbents, we considered the qualities an ideal supportedsorbent would possess. Grasa et al. found that the carbonationof CaO slowed following the formation of a �40 nm-thick layerof CaCO3 on its surface,12 so themost rapid adsorption would beobtained from a very thin layer of CaO. Moreover, the thin CaOlayer must be able to expand upon carbonation, as the molarvolume of CaCO3 is approximately twice that of CaO. Theseconsiderations have already led a few groups to produce sup-ported or functionalized sorbents with tailored nanostructures.Wu and Zhu coated the surfaces of nanosized (70 nm) CaCO3
particles with TiO2, then calcined to obtain CaTiO3/CaO parti-cles that were more stable to calcination–carbonation than asorbent formed by co-precipitating CaCO3 and TiO2 particlesand then calcining the product.13 Liu et al., on the other hand,synthesized hollow CaO/Ca12Al14O33 nanospheres that retained>90% of their initial CO2 capacity aer 30 carbonation–calci-nation cycles.14 We pursued ordered mesoporous and hierar-chically (macro/meso) porous materials in order to maximisethe amount of active, stable CaO in the sorbent. Two groupshave already supported CaO on SiO2 with ordered mesopor-es.10b,c However, SiO2 is not hydrothermally stable, meaning thatit may not be useful for pre-combustion applications (i.e. toincrease the yield of renewable H2 from biomass gasication),when water vapour is present at high temperature.15
Based upon these criteria, we pursued sorbents supportedon mesoporous SiC and CaxAlyOz, as well as on hierarchicallyporous CaxAlyOz. SiC has high thermal conductivity andmechanical strength, a low coefficient of thermal expansion,and low specic weight.16 It also displays high hydrothermal
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stability and resistance to oxidation due to the formation of aSiO2 lm on its surface.17 It is less reactive toward CaO thanAl2O3 and SiO2 are,17a,b which could help avoid the formation ofunreactive mixed oxides over the course of many carbonation–calcination cycles. Mesoporous SiC has been prepared frompolycarbosilane (PCS) using mesoporous SBA-15 silica as atemplate, and has a large surface area and highly orderedmesoporosity18 that may enable it to support large amounts ofCaO. Mesoporous calcium aluminate was chosen because non-mesoporous calcium aluminum oxides have proven very stablesupports for CaO sorbents,6–8 as have the hollow nanoshellsreported by Liu et al.14 Further, Chang et al. recently used adouble-layer hydroxidemethod to deposit CaCl2 onmesoporousalumina and calcined the material to produce CaO on a mixtureof calcium aluminates,19 and we reasoned that it would beinteresting to study the impact of synthesis method on acalcium aluminate system, as well as to study a hierarchicallyporous analogue. We therefore pursued CaO supported onmesoporous SiC and CaxAlyOz, as well as on hierarchicallyporous CaxAlyOz.
ExperimentalSynthesis of CaO supported on porous calcium aluminates
Mesoporous calcium aluminate (meso-CaxAlyOz, Fig. 1a) wassynthesised using a sol–gel procedure that employed evapora-tion-induced self-assembly, and that was based upon publishedprocedures for the syntheses of mesoporous Al2O3
20 and mes-oporous alumina-supported metal oxides.21 Thus, calcium
Fig. 1 Planned syntheses of tailored CaO sorbents supported on (a)mesoporous calcium aluminate, (b) hierarchically porous calciumaluminate, and (c) mesoporous silicon carbide. P123 ¼ poly(ethyleneoxide-b-propylene oxide-b-ethylene oxide); PS ¼ polystyrene.
This journal is © The Royal Society of Chemistry 2014
aluminate with a 1 : 1 mole ratio of Al3+ and Ca2+ ions wasprepared by combining the structure-directing agent PEO20P-PO70PEO20 (Pluronic P123, Sigma-Aldrich, Mn ¼ 5800 g mol�1,2.0 g) and ethanol (absolute, 20.0 mL) and stirring at 40 �C untilthe surfactant dissolved. Then, Ca(NO3)2$4H2O (Fluka, 2.36 g)was added to the solution and stirred vigorously until it dis-solved. In a separate ask, aluminium isopropoxide (Sigma-Aldrich, 2.04 g) was combined with nitric acid (67 wt%, 3.2 mL)and ethanol (absolute, 10.0 mL) and stirred at 40 �C until dis-solved. The solutions were combined and stirred at roomtemperature for 5 h, then aged at 60 �C for 48 h under N2 ow.The dried calcium aluminate sol was then heated in a GSL-1300X tube furnace (MTI) in air at 1.5 �C min�1 to 700 �C andheld at that temperature for 4 h.
The support synthesis was repeated in the presence of anarray of polystyrene beads (d¼ 220 nm), formed via evaporationand sedimentation, to produce meso/macroporous calciumaluminate (Fig. 1b), hier-CaxAlyOz, according to a combinationof methods developed for the synthesis of hierarchically porousamorphous and g-Al2O3.22 Specically, the meso/macroporouscalcium aluminate was prepared as per the procedure for itsmesoporous analogue; however, prior to the aging of the sol,dried polystyrene beads (an organic template for macropores)were immersed in the sol and allowed to stand for 10–15 min.The polystyrene array was then removed from the sol, andexcess sol was gently removed before the resulting inorganic–organic hybrid was aged and calcined as above.
To incorporate CaO onto the calcium aluminate supports,calcium acetate (Ca(OAc)2$H2O, 5.94 g) was dissolved indeionised (DI) water (28 mL). The solution was mixed with1.0 g calcium aluminate and stirred at 70 �C for 24 h. Thewater was evaporated in the fume hood overnight, and theresulting solid was dried in an oven at 100 �C for 24 h andheated in a GSL-1300X tube furnace (MTI) in air (230 mLmin�1) at 5 �C min�1 to 700 �C, allowed to dwell for 10 min,then heated at 1.5 �C min�1 to 900 �C and held at thistemperature for 1 h.
Synthesis of CaO supported on mesoporous silicon carbide
The mesoporous SiC support was produced by negative tem-plating (Fig 1c); that is, mesoporous SBA-15 silica was synthe-sised18a and used as a template for meso-SiC.18b In the rst step,SBA-15 silica was synthesised according to a modied literatureprocedure.18a PEO20PPO70PEO20 (P123, Sigma-Aldrich, Mn ¼5800 g mol�1, 6.45 g) was dissolved in a mixture of DI water(198.56 mL) and hydrochloric acid (HCl, 37%, 28.37 mL), andstirred at 38 �C overnight. Tetraethyl orthosilicate (TEOS,148.81 mL) was then added and the mixture was stirred vigor-ously at 38 �C for 24 h. The white suspension was hydrother-mally treated in a reux system for three days using theminimum heating power required to sustain gentle reux. Thecooled sample was ltered, washed with warmDI water (�60 �C)until the pH of the ltrate was neutral, and dried in a fume hoodfor �8 h, then in an oven at 100 �C for 24 h. The as-synthesisedSBA-15 was then heated in an AF-3 kiln (Woodrow) under air at1.5 �C min�1 to 550 �C and held at that temperature for 5 h.
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SiC was synthesised using the SBA-15 as a template.18 Poly-carbomethylsilane (PCMS, Sigma-Aldrich 1.223 g, Mw ¼ 800 gmol�1) was dissolved into a mixture of heptane (30.4 mL) and 1-butanol (0.2 mL), and the resulting solution ([PCMS] � 0.05 M)was poured onto SBA-15 (0.5705 g) until the silica wasimmersed. The white suspension was stirred in a fume hoodovernight to evaporate the solvent, and the resulting solid wasdried in an oven at 100 �C for 48 h. The sample was thenpyrolysed at 1300 �C under Ar (265 mLmin�1) using a GSL-1600-60X tube furnace (MTI) and the reported heating program.18a
The silica in the SiC/SiO2 composite was etched ve times inNaOH(aq) (2 M, approximately 120 mL NaOH(aq) per gcomposite) at 80 �C, each time using fresh NaOH solution. Theresulting SiC sample was washed several times with DI wateruntil the pH of the ltrate was neutral. The solid was driedunder ambient conditions in a fume hood overnight, then in anoven at 100 �C for 24 h.
To incorporate CaO onto the SiC support, calcium acetate(Ca(OAc)2$H2O, 0.0842 g) was dissolved in DI water (6 mL). Thesolution was poured onto SiC (0.05 g) until the solid wascompletely immersed, then stirred at 70 �C for 24 h. The waterwas evaporated in the fume hood overnight, and the resultingsolid was dried in an oven at 100 �C for 24 h and calcined underAr (265 mLmin�1) by heating at 1.5 �Cmin�1 to 800 �C in a GSL-1600-60X tube furnace (MTI), then holding at this temperaturefor 3 h.
Characterisation of materials
X-ray diffraction (XRD) patterns were measured on a SiemensD5000 or PANalytical X'Pert PRO MPD X-ray diffractometerequipped with a PIXcel detector, each using CuKa radiation (l¼1.5419 A), or on a D8 Discover from Bruker AXS (Cobalt tube,Gobel mirror, Vantec 1 Detector). Scanning electron micro-graphs (SEM) were recorded on a Zeiss Ultra+ instrument usinga 3 kV electron beam, and transmission electron micrographs(TEM) were recorded on a Philips Biolter or JEOL 1400 trans-mission electron microscope. Samples were prepared by soni-cating the material in ethanol and dispersing the resultingultrane sample on a copper grid, and imaged using a 120 kVelectron beam. N2 physisorption was measured at �196 �C onan Autosorb-1 or Autosorb iQ (Quantachrome) or ASAP2020(Micromeritics) instrument. Surface areas (SBET) were calculatedusing the BET method23 over P/Po ¼ 0.05–0.25. The pore sizesand pore size distributions were determined using the BJHmethod,24 and the total pore volume was calculated from theadsorbed volume at P/Po $ 0.97.
Cycles of carbonation (CO2 sorption) and calcination (CO2
release)
Carbonation and calcination were tested in a thermogravi-metric analyser (TGA, TA Instruments TGA-Q500). A sorbentsample (initial mass approximately 2–5 mg) was distributedevenly in a platinum pan and heated according to the programsbelow while the sample mass was monitored.
CaO/meso-CaxAlyOz and CaO/hier-CaxAlyOz were heated inN2 at 20 �C min�1 to 850 �C, then held isothermally for 10 min.
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The temperature was then lowered to 700 �C at 20 �C min�1,with N2 still owing. Once the temperature reached 700 �C, thegas supply was switched to 15% CO2/N2 (to approximate [CO2]in ue gas), and the sample was held isothermally for 30 min,similar to conditions tested for other CaO/calcium aluminatesorbents (see Table 2). This cycle was performed 30 times.
CaO/meso-SiC was heated in N2 at 10 �C min�1 to 890 �C,then held isothermally for 20 min. The temperature was thenlowered to 690 �C at 10 �Cmin�1, with N2 still owing. Once thetarget temperature was reached, the gas supply was switched to15% CO2/N2 and the sample was held isothermally for 30 minfor carbonation. The sample was heated again in N2 at 10 �Cmin�1 to 850 �C and held isothermally for 10 min. The cycle wasperformed 30 times.
Results and discussionCharacterisation of supports and sorbents
N2 physisorption measurements revealed Type II isothermswith H3 hysteresis for both meso- and hier-CaxAlyOz (Fig. S1aand S4a,† respectively),25 indicating the presence of large, slit-shaped pores. The hierarchically porous support had a highersurface area, as expected, though the difference was small (17vs. 15 m2 g�1; see Table 1). Neither support had a surface areathat approached that of mesoporous amorphous or g-Al2O3, oraluminates, made using the same or a similar method,19–21
suggesting that the incorporation of Ca2+ ions reduced thesurface area. TEM images of both supports (Fig. S2 and S5†)clearly showed their porous structures, though the mesoporeswere mostly disordered, with occasional regions of hexagonallyordered mesopores. This was supported by the pore sizedistributions measured for the supports, which were complex(Fig. S1a and S4a,† respectively). XRD analysis (Fig. S10†) indi-cated that meso-CaxAlyOz and hier-CaxAlyOz were amorphous.This was expected based upon the calcination temperature used(700 �C), as alumina and alumina-supported metal oxides syn-thesised from aluminium alkoxide precursors using similarsol–gel methods typically crystallise in the g phase between 750and 900 �C, and are converted to the a phase above 1000�C.21,22,26
Following impregnation with CaO, the pore structures of thecalcium aluminate sorbents were less evident in the TEMimages, though pores remained visible (Fig. 2; additionalimages in Fig. S3 and S6†). Somewhat counterintuitively, hier-CaxAlyOz appeared to suffer more pore blockage than meso-CaxAlyOz, losing �37% of its surface area and �30% of its porevolume upon CaO loading. meso-CaxAlyOz, on the other hand,lost less than 5% of its pore volume and surface area (Table 1,Fig. S1b and S4b†). The XRD patterns of the sorbents (Fig. 3), incontrast to those of the supports, showed crystalline materials,as expected based upon the high calcination temperature usedfor the sorbents. CaO/meso-CaxAlyOz and CaO/hier-CaxAlyOz
had very similar XRD patterns that showed primarily CaO (2q ¼32.4, 37.6 and 54.2� for the (111), (200), and (220) reections,respectively). Aluminium was present mostly as Ca3Al2O6
(JCPDS 32-0150), but also as Ca5(Al3O7)2 (JCPDS 11-0357);Ca12Al14O33 (JCPDS 09-0413) was not observed.
This journal is © The Royal Society of Chemistry 2014
Table 1 Textural properties of the porous supports and supported sorbents, as well as of the SBA-15 template for the SiC supporta
Entry Sample SBETb (m2 g�1) DA
c (nm) DDd (nm) VP
e (mL g�1)
1 meso-CaxAlyOz 15 3.5, 13 3.9, 10 0.102 CaO/meso-CaxAlyOz 15 Multimodal Multimodal 0.0973 hier-CaxAlyOz 17 Multimodal 15.3 0.164 CaO/hier-CaxAlyOz 11 3.3 Multimodal 0.105 SBA-15 1180 9.8 6.6 1.96 meso-SiC 470 1.4 1.4, 3.4 0.397 CaO/meso-SiC 129 3.1 3.1 0.24
a Measured by N2 adsorption–desorption at �196 �C. b Surface area calculated using the BET method23 over P/Po ¼ 0.05–0.25. c Maxima in the BJHpore-size distribution24 as calculated from the adsorption and desorption branches, respectively. d Maxima in the BJH pore-size distribution24 ascalculated from the adsorption and desorption branches, respectively. e Total pore volume at P/Po $ 0.97.
Fig. 2 Transmission electron microscope images of (a) CaO/meso-CaxAlyOz; (b and c) CaO/hier-CaxAlyOz; and (d) CaO/meso-SiC.
Fig. 3 X-ray diffraction patterns of sorbents made up of CaO onmesoporous and hierarchically porous supports. XRD patterns of thesupports themselves are shown in Fig. S10 of the ESI.†
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The SBA-15 used as a template for meso-SiC had anordered pore structure (ESI, Fig. S8†), and a pore size of 9.8nm (Table 1). The meso-SiC itself had a far lower surface areathan its SBA-15 parent (Table 1), contrary to literaturereports;18 this may have been due to the etching of thetemplate in hot NaOH(aq) rather than HF(aq).27 This treat-ment may also have caused the disordered mesoporosity ofthe SiC support (Fig. S9†). The N2 adsorption–desorptionisotherm of the porous SiC was also of Type II with H3hysteresis, revealing the presence of micro- and mesopores(ESI, Fig S7a†).28 When CaO was loaded on the support, theN2 adsorption–desorption isotherm displayed little hysteresis,consistent with some pore blocking by the CaO particles.Thus loading CaO onto any of the supports produced amaterial with lower surface area, as was observed by Wangand co-workers in their CaO/SBA-15 system;10c however, thiseffect was remarkably minor in the case of CaO/meso-Cax-AlyOz. The wide-angle XRD pattern of CaO/meso-SiC (Fig. 3)showed the same intense peaks for CaO as the CaO/calciumaluminates (vide supra). No other Ca-containing compoundswere observed, indicating that all of the Ca present wastheoretically available for carbonation. The b-SiC support gaveweak, broad reections observed at 36, 60 and 72�.18a
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CO2 sorption
As Ca2+ was added the to CaO/CaxAlyOz sorbents in two separatesynthesis steps, and because they could not readily be digestedfor elemental analysis, their maximum CO2 capacities wereestimated using extended carbonations (700 �C, 48 h, 100%CO2; see Fig. S11†). Each was able to take up approximately halfits weight in CO2, i.e. �500 g (11.3 mol CO2) per kg sorbent; thisis �64% of the per-kilogram capacity of pure CaO. The amountof Ca(OAc)2$H2O used in the synthesis of CaO/meso-SiC waschosen so that, upon complete carbonation, the CaCO3 formedwould occupy 90% of the pore volume of the support. Basedupon this amount, the theoretical uptake of CO2 on CaO/meso-SiC was calculated to be �274 g CO2 per g sorbent; thus, itcontained less Ca per unit mass than the sorbents supported onCaxAlyOz. To determine their practical reactivity toward CO2,each of the tailored sorbents were tested under carbonation and
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calcination conditions commonly used in the literature. Thus,carbonation was performed at 690 or 700 �C in 15% CO2 in N2
for 30 min and calcination at 850 �C in N2 for 10 min. Fig. 4shows the behaviour of the sorbents over 30 cycles, both interms of weight gain upon absorption and of CaO conversion toCaCO3 (based upon the theoretical uptake values above). InTable 2, the carbonation results obtained with these sorbentsare compared to those obtained for CaO supported on micro-porous calcium aluminate,6b as well as to several reportednanostructured supported sorbents.10c,14,19
Consistent with its lower CaO content, CaO/meso-SiC tookup signicantly less CO2 per unit weight than the sorbentssupported on calcium aluminate (Fig. 4a); however, all threesorbents attained good conversion in the rst carbonation cycle(Fig. 4b). The CaO/meso-SiC sorbent also had the greatest lossof CO2 capacity among the new sorbents tested, losing nearlyhalf of its capacity over 30 cycles (Table 2, entry 3). Thus furtherstudies will be required to improve this sorbent.
In the second and third cycles, the sorption capacities of thecalcium aluminate sorbents actually increased, indicating someself-activation. This phenomenon has also been observed forCaO on microporous Ca12Al14O33
6b or on Ca12Al14O33 with largemesopores,29 as well as for limestone30 and modied lime-stone.31 The increase in CO2 uptake over the rst few cyclescould be better understood by examining the uptake kinetics(Fig. 5). In the rst cycle (Fig. 5a), CO2 uptake in the CaO/calcium aluminate sorbents did not occur in distinct fast andslow stages, as is usually observed,30 but rather occurred grad-ually over the entire 30 min period. By the tenth cycle (Fig. 5b),the more familiar CO2-uptake pattern of CaO, in which rapidCO2 uptake occurs in the rst minutes but is then replaced by aslower diffusion-controlled phase, was observed. Manovic and
Fig. 4 Comparison of supported sorbents in 30 calcination–carbonationmeso-SiC), 15% CO2 in N2, 30 min; calcination: 850 �C, N2, 10 min). (a) Coof available CaO converted.
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Anthony suggested that this pattern of carbonation behaviourarises when ion diffusion through the fresh sorbent is difficultbut becomes easier as a more reactive outer layer of sorbentforms during the early carbonation cycles.30 Wheelock and co-workers, on the other hand, proposed that early carbonationcycles could causemore cracks and pores to appear in a sorbent,thus increasing the availability of CaO for carbonation.31 We arenot able to state whether either or both of these phenomenaproduced the self-reactivation of CaO/meso-CaxAlyOz and CaO/hier-CaxAlyOz.
In subsequent cycles, the CO2 capacity of the CaO/calciumaluminium sorbents decreased somewhat, but remained high.Aer 30 cycles, CaO/hier-CaxAlyOz retained 73% of its theoret-ical CO2 capacity (82% of its initial capacity; see Table 2, entry2), and CaO/meso-CaxAlyOz still took up a remarkable 87% of itstheoretical capacity, or 430 g CO2 per kg (Table 2, entry 1). CO2
uptake still occurred in a fast and then a slow stage in the 30th
cycle (Fig. 5c), though now less CO2 was absorbed before rapiduptake ceased. The relative reactivities of the two CaO/porouscalcium aluminate materials was initially surprising, as we hadhoped that the hierarchical pore structure of CaO/hier-CaxAlyOz
would improve the rate of gas transport through the solid, andthus the conversion of CaO to CaCO3. However, in terms ofconversion, CaO/meso-CaxAlyOz outperformed its hierarchicallyporous analogue in every cycle. This can be rationalised basedupon the surface area and porosity data in Table 1. Though thehier-CaxAlyOz support had a greater surface area and micro/mesopore volume than the mesoporous analogue, it lostsignicant surface area and pore volume upon calcium depo-sition, so that CaO/hier-CaxAlyOz had 25% less surface area thanCaO/meso-CaxAlyOz. Further, CaO/hier-CaxAlyOz lost 18% of itsCO2 capacity over 30 cycles, whereas CaO/meso-CaxAlyOz lost
cycles (carbonation: 700 �C (CaO/CaxAlyOz sorbents) or 690 �C (CaO/mparison in terms of mass gained. (b) Comparison in terms of fraction
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Table 2 Effectiveness and stability of CaO sorbents over 30 cycles of carbonation and calcination
Entry Sorbent
Carbonation CalcinationaWeight gain incycle #
Conversionin cycle #
Lossb [%]T [�C] t [min] [CO2] [% in N2] T [�C] t [min] 1 [%] 30 [%] 1 30
1 CaO/meso-CaxAlyOzc 700 30 15 850 10 46 43 0.92 0.87 6
2 CaO/hier-CaxAlyOzc 700 30 15 850 10 46 38 0.89 0.73 18
3 CaO/meso-SiC 690 30 15 850 10 24 13 0.86 0.49 4346b CaO/Ca12Al14O33
d,e 690 30 15 850 10 24 24 —f —f 056b CaO/Ca12Al14O33
d,g 690 30 15 850 10 37 32 —f —f 14629 CaO/Ca12Al14O33
d,h 600 25 25 900 15 8 16 —f —f �200i
719 CaO/meso-CaxAlyOzd,j 700 10 100 700 8 40 32 —f —f 20
810 CaO/SBA-15d,g 700 60 100 910 30 —f —f 0.99 0.82 17914 CaO/Ca12Al14O33 nanospheres
d 650 30 15 900 10 64 61 0.96 0.91 510 CaO/meso-CaxAlyOz(20)
c 700 30 15 850 10 40 28 0.92 0.64 30
a Calcination atmosphere was 100% N2.b Change in CO2 uptake capacity from the 1st to 30th cycle, as a percentage of the capacity in the rst cycle.
c Maximum conversion was calculated from an extended carbonation, see Fig. S11. d GraphClick soware32 was used to estimate valuesfrom published graphs; see relevant references. e The CaO precursor was Ca(OH)2.
f Not given. g The CaO precursor was Ca(OAc)2$H2O.h Sample was prepared from CaO and hydrated before use. i The CO2 capacity of the sorbent increased initially, then declined slowly; thus,uptake capacity increased in the rst 30 cycles. j The CaO precursor, CaCl2, was deposited in the presence of urea.
Fig. 5 Comparison of supported sorbents during the (a) first, (b) tenth, and (c) 30th carbonations. Carbonation conditions: 700 �C (CaO/CaxAlyOz
sorbents) or 690 �C (CaO/meso-SiC), 15% CO2 in N2, 30 min.
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only 6% of its capacity. These observations, taken together,suggest that the pore structure of the hier-CaxAlyOz supportactually enabled better transport of the calcium precursor,leading to the deposition of a CaO layer that was too thick.Scanning electron micrographs of the sorbents taken before(Fig. 6a and c) and aer (Fig. 6b and d) the 30 carbonation–calcination cycles conrmed that the surfaces of both CaO/meso-CaxAlyOz and CaO/hier-CaxAlyOz sintered during use.Though the sorbents were tested here over 30 cycles of calci-nation and carbonation in order to enable comparison toreported supported sorbents (vide infra), future work will testCaO/meso-CaxAlyOz over a very large number of cycles andcompare its residual CO2 capacity to that of limestone (�6–8%aer 1000–1200 cycles33).
Notably, both CaO/meso-CaxAlyOz and CaO/hier-CaxAlyOz
outperformed CaO sorbents supported on mesoporous calciumaluminate that was prepared via an AlOOH-supported layeredhydroxide (Table 2, entry 7),19 as well as CaO/SBA-15 (entry 8).10c
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Therefore, the lack of an ordered mesopore structure did nothinder CaO/meso-CaxAlyOz from acting as a very stable CO2
sorbent. This suggests that other calcium aluminate sampleswith non-ordered mesopores could prove interesting supportsfor CaO in the future; for example, a room-temperature route tohierarchically porous (though not ordered) calcium aluminateshas been reported.34 Though it showed superior stability amongour sorbents, CaO/meso-CaxAlyOz still lost a slightly greaterfraction of its CO2 capacity than the microporous sorbent CaO/Ca12Al14O33, prepared by Martavaltzi and Lemonidou from aCa(OH)2 precursor (Table 2, entry 4).6b However, that sorbentabsorbed only �24 wt% CO2, and thus over 30 cycles took upjust over half of the CO2 absorbed by CaO/meso-CaxAlyOz
(�164 vs. 312 mol CO2 per kg sorbent). A similar microporousCaO/Ca12Al14O33 sorbent prepared from Ca(OAc)2$H2O had agreater initial capacity than its Ca(OH)2-derived analogue(Table 2, entry 5); however, this value was still lower than theCO2 uptake capacity of CaO/meso-CaxAlyOz, and this sorbent
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Fig. 6 Scanning electron microscope images of (a and b) CaO/meso-CaxAlyOz before and after, respectively, 30 carbonation–calcinationcycles; and (c and d) CaO/hier-CaxAlyOz before and after, respectively,30 carbonation–calcination cycles. Carbonation: 700 �C, 15% CO2 inN2, 30 min; calcination: 850 �C, N2, 10 min.
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was also less stable. Even a CaO/Ca12Al14O33 sorbent preparedfrom CaO and then hydrated before use, which showed a netgain in uptake capacity over 30 cycles of carbonation andcalcination (Table 2, entry 6),28 took up much less CO2 per gramsorbent than CaO/meso-CaxAlyOz. Therefore, meso-CaxAlyOz
was able to support more useable CaO than a related non-mesoporous material with a similar surface area, and as aconsequence absorb more CO2 over 30 carbonation–calcinationcycles under near-identical conditions.
Although CaO/meso-CaxAlyOz outperformed both micropo-rous CaO/Ca12Al14O33 and an ordered mesoporous CaO/calciumaluminate sorbent in 30 carbonation cycles, it still took up lessCO2, and had a slightly less stable capture capacity, than thebest hollow CaO/Ca12Al14O33 nanospheres recently described byLiu et al. (Table 2, entry 9),14 and we were interested in under-standing the reason for this difference. Liu et al. noted animportant dependence of stability on the weight fraction ofCa12Al14O33 in the sample, with a minimum of 15% Ca12Al14O33
being necessary to ensure stability. Moreover, their samples hadhigher surface area (95 m2 g�1) than CaO/meso-CaxAlyOz. Wewere unable to measure the weight fraction of CaO or of calciumaluminates in our samples; however, we did produce a samplecontaining less Ca for comparison. Thus, the synthesis of meso-CaxAlyOz was repeated, but with less Ca(NO3)2$4H2O added tothe reaction mixture. As this sample was prepared with a molarratio of 4 Al3+ ions : 1 Ca 2+ ion, it was labeled meso-Cax-AlyOz(20). CaO was loaded onto this support according to thesame procedure used for meso-CaxAlyOz, giving CaO/meso-CaxAlyOz(20). Not only did this sample contain less Ca thanCaO/meso-CaxAlyOz, it also had a signicantly higher surfacearea (SBET ¼ 58 m2 g�1; see isotherm Fig. S12a†) and a clearmaximum in the pore size distribution (Fig. S12,† inset). TheXRD pattern of this sorbent (Fig. S13†) showed it to be primarilyCa(OH)2, indicating that it readily adsorbed H2O from the air.The primary Al-containing phase was Ca5(Al3O7)2, which made
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up �8.3% of the sample, with minor amounts (<5% each) ofCa3Al2O6 and CaAl2O4 (JCPDS 23-1036) being detected uponRietveld renement. Though the high surface area and readyuptake of atmospheric moisture were auspicious for CaO/meso-CaxAlyOz(20) as a sorbent, it showed disappointing stability over30 cycles of carbonation and calcination (Fig. S14†). Althoughthe sample gained 40% of its initial weight on the rstcarbonation (92% conversion), it took up only 27 wt% CO2 (64%conversion) in the 30th cycle (Table 2, entry 10). Thus, neither adecrease in Ca content, nor an increase in surface area,improved the CO2 uptake capacity of CaO/meso-CaxAlyOz. Wecannot rule out the different calcium aluminates present in ourCaO/meso-CaxAlyOz and the hollow nanospheres of Liu et al. ascontributors to the slightly higher stability of the latter sorbent;however, the Ca12Al14O33 phase is no guarantor of stability, as itwas the primary aluminium-containing phase in the CaO/mes-oporous calcium aluminate produced by Chang et al.,19 whichwas signicantly less stable than CaO/meso-CaxAlyOz.
Conclusions
Aer considering the best design for a supported CaO-basedCO2 sorbent, we synthesised three new materials—CaO onmesoporous silicon carbide, and on meso- and hierarchicallyporous calcium aluminate. Though syntheses were directedtowards ordered mesopore structures, the supports had meso-pores that were poorly ordered; nevertheless, the most stablesorbent, CaO on mesoporous calcium aluminate, took up moreCO2 over 30 cycles of carbonation and calcination than relatedmicro- and mesoporous sorbents tested under similar condi-tions. The superiority of this sorbent over its hierarchicallyporous analogue CaO/hier-CaxAlyOz, along with textural data,suggested that the macro/mesoporous structure permitted toothick a layer of CaO to be deposited on hier-CaxAlyOz.
Acknowledgements
We thank Drs Sebastian Perrier and Brian Hawkett of the KeyCentre for Polymers and Colloids at the University of Sydney forthe generous gi of polystyrene beads, Mr Peter L. H. Newmanfor recording SEM images, and Ms Emily Reynolds for assis-tance with XRDmeasurements. We are grateful for the nancialsupport of E.ON AG, ANLEC R&D, the University of Sydney, andthe German Academic Exchange Service.
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