tio

ng

ng-g

Available online 7 January 2010

Keywords:CO2 adsorptionPolyhedral oligomeric silsesquioxanes

opoicat

BET and 29Si MAS-NMR. CO2 adsorption/desorption proles of OAPS grafted chlorofunctionalized SBA-

being

to be the mid-term solution for the use of fossil fuel energytechnologies against environmental calamities. Initially amine-based chemical adsorbents are preferred, as CO2 in the gas phaseeasily dissolves into the aqueous solution of amine compounds[4]. Recently, fabrication of amines on porous supports such assilica and carbon, through grafting and impregnation, has provedto be a feasible and benecial route [59], as it overcomes thedisadvantages of using amines compounds directly in aqueous

present study, we planned to use rice husk ash (RHA) obtainedon heat-treating rice husk from rice milling as the silica sourcefor the direct synthesize of chlorofunctionalized SBA-15,subsequently loaded with organicinorganic nanocomposite,octa(aminophenyl)silsesquioxane (OAPS) by grafting, and thensubjected to CO2 adsorption.

OAPS is one of the derivatives of organicinorganic nanocompositepolyhedral oligomeric silsesquioxanes (POSS) which has attractedresearchers attention in the eld of nanomaterial science [1620].POSS compounds have (SiO1.5)n core cage structures (n = 8, 10, 12)with one substituent attached to every edge silicon and its size

* Corresponding author. Tel.: +82 41 660 1423; fax: +82 41 688 1343.

Microporous and Mesoporous Materials 131 (2010) 265273

Contents lists availab

e

lseE-mail address: htjang@hanseo.ac.kr (H.T. Jang).sources, which are renewable or have less environmental impact.The EIA predicts a 57% increase in energy demand by 2030 [1]and no doubt, fossil fuel energy will play a signicant role. Whenfossil-fueled power plants and coal-red plants are constructed,the major outlet will be CO2 emission [2], one of the main greenhouse gases. The challenge towards the use of fossil and coal fuel,targets on CO2 sequestration, as Intergovernmental Panel onClimate change (IPCC) states that by the year 2100, the CO2concentration will reach 570 ppm, which results in ultimate globalwarming around 1.9 [3]. Therefore, CO2 sequestration is expected

against the economic and environmental costs of large-scalemanufacture of mesoporous materials such as MCM-41, MCM-48and SBA-15 is due to the use of both hazardous templates andexpensive silica sources including sodium silicate, fumed silica,silicon tetraethoxide, etc. Recently, the use of the waste productslike rice husk and y ash from rice milling and coal combustion,respectively, have been proved to be potential silica sources forthe synthesis of mesoporous materials. The use of coal combustionwaste, y ash, for the synthesis of SBA-15, MCM-41 and MCM-48has already been reported [1115]. Based on such reports, in theOAPSMesoporous silicaRice husk ash

1. Introduction

Man-kind, in the 21st century is1387-1811/$ - see front matter 2010 Elsevier Inc. Adoi:10.1016/j.micromeso.2010.01.00115 (Cl-SBA-15/ OAPS) at 25, 50 and 75 C were obtained by TGA under atmospheric pressure. A maximumof 8 wt.% CO2 adsorption capacity (80 mg/g of adsorbent) was achieved over Cl-SBA-15/50% OAPS and itwas also shown that the absorbent was recyclable, selective and thermally stable. Neat OAPS adsorbsalmost no CO2 due to internally hydrogen-bonded amine. Cl-SBA-15/OAPS withstands high temperature(180 C) signicantly higher than the sorbent polyethyleneimine (PEI), which decomposes beyond 115 C.Therefore, the use of rice husk ash (RHA) as silica source and high thermal stability of Cl-SBA-15/OAPS areadvantageous for CO2 adsorption. Thus, grafting of octa(3-aminophenyl)silsesquioxanes on mesoporousSBA-15 broadens the applications of polyhedral oligomeric silsesquioxanes (POSS) compounds andveried to be a new candidate for CO2 capture.

2010 Elsevier Inc. All rights reserved.

forced to seek energy

solution [10]. The amine-grafted and/or impregnated porous solidsorbents become the prototype owing to their good selectivity,high surface area and higher tolerance to water. However, threatReceived in revised form 24 December 2009Accepted 3 January 2010

silane in presence of Pluronic 123 (surface directing agent). Octa(3-aminophenyl)octasilsesquioxane(OAPS) was grafted on chlorofunctionalized mesoporous silica SBA-15 and characterized by XRD, FTIR,Octa(aminophenyl)silsesquioxane fabricaSBA-15 for CO2 adsorption

Margandan Bhagiyalakshmi a, Ramani Anuradha a, SaaDepartment of Chemical Engineering, Hanseo University, Seosan 360-706, South KoreabCDRS (Carbon Dioxide Reduction & Sequestration R&D Center), 71-2 Jang-dong Yuseo

a r t i c l e i n f o

Article history:Received 5 June 2009

a b s t r a c t

Chlorofunctionalized mescondensation of sodium sil

Microporous and M

journal homepage: www.ell rights reserved.n on chlorofunctionalized mesoporous

Do Park b, Hyun Tae Jang a,*

u, Daejeon 305-343, South Korea

rous SBA-15 was synthesized by a direct method, through one-stepe solution obtained from rice husk ash (RHA) and chloropropyltrimethoxy-

le at ScienceDirect

soporous Materials

vier .com/locate /micromeso

ranges from 1 to 2.5 nm in diameter. Over the past decade, POSSmolecules have attracted considerable interest, as in these compos-ites, the cubic silica cores are completely dened as hard particles

CPTMS was added and the nal solution was aged under stirringfor 24 h at 35 C. The solution was subsequently heated for another24 h at 100 C. The crystallized product was ltered, washed with

266 M. Bhagiyalakshmi et al. /Microporous and Mesoporous Materials 131 (2010) 265273with 0.53 nm diameter and spherical radius of 12 nm includingperipheral organic units. Since they contain mono-dispersed,nanometer-scaled cubic silsesquioxanes, these composites aresuitable for a variety of interesting applications such as space-survivable coatings [21], low-k dielectric materials [22], astemplates for the preparation of nanostructured materials such asliquid crystalline polymers [23], catalysts [24], dendrimers [25],andmulti-armed star polymers [26]. Furthermore, POSS compositeswere reported to have wide applications for the encapsulation ofmetal nanoparticles and their catalytic activity is appreciable dueto formation of isolated metal-POSS material [27]. This led us toconsider POSS for CO2 adsorption.

Organicinorganic hybrid composites based on several types ofocta-functional POSS have been prepared and studied by Lainesgroup [28,29]. Until now, POSS nanocomposites have never beenconsidered for CO2 adsorption; nor are there reports available forgrafting of POSS on mesoporous silicas. Hence, in this study weselected octa(3-aminophenyl)octasilsesquioxane (OAPS), whichpossesses eight free amine group, for CO2 adsorption. A graftingmethod has been adopted for the loading of OAPS on 3-chloropro-pyltrimethoxysiliane (CPTMS) functionalized mesoporous SBA-15symbolized as Cl-SBA-15. Cl-SBA-15 was synthesized by directmethod, one-step condensation of sodium silicate solutionextracted from rice husk ash (RHA) and CPTMS using P123 astemplate. The one-step condensation is an added advantage of thisstudy as there are typically multi-steps involved in graftingprocess. The maximum CO2 adsorption of 8% (80 mg/g ofadsorbent) was obtained with 50% loading of OAPS compound onCl-SBA-15. To our knowledge there are no reports on POSSnanocomposites fabricated on mesoporous silicas for CO2adsorption.

2. Experimental

2.1. Materials

Octa(3-aminophenyl)octasilsesquioxane was purchased fromHybrid Plastics Inc., Hattiesburg, MS, EO20PO70EO20 (PluronicP123, BASF, 99.99%) and 3-chloropropyltrimethoxysilane (CPTMS,Aldrich, 95%) were procured and all these chemicals were usedwithout further purication. Rice husks obtained from a local farmwere milled and heated to 700 C for 24 h to obtain RHA. Sodiumsilicate solution was prepared by reuxing 4 g of RHA with100 ml of 2 M NaOH (Merck, 99%) in H2O at 70 C for 24 h[15,30]. Table 1 shows the ICP-AES analysis of rice husk ash andSiO2 concentration after extraction with sodium hydroxidesolution.

2.2. Synthesis of Cl-SBA-15 and grafting of OAPS on Cl-SBA-15

CPTMS functionalized mesoporous SBA-15 materials wassynthesized by one-step co-condensation of sodium silicatesolution from rice husk ash, and CPTMS in the presence of PluronicP123 (MW5800) as structure directing agents, using HCl as acidcatalyst. In a typical synthesis, 10 g of P123 was added to 380 mLof 1.6 M HCl (37%, Aldrich), followed by the addition of 200 mLof sodium silicate extracted from RHA. To this solution, 1 g of

Table 1Chemical composition of the rice husk ash (wt.%).SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 MnO P2O

93.2 0.13 0.07 1.23 0.25 0.78 0.08 0.006 0.33 0.15warm distilled water, and dried at 80 C. Then surfactant wasselectively removed by soxhlet extraction with ethanol for 24 h[31]. The obtained materials were denoted as Cl-SBA-15.

The OAPS grafting on Cl-SBA-15 was achieved by dissolving0.3 g of OAPS in 50 mL of dry THF, and then adding 0.7 g ofCl-SBA-15 into the solution, to obtain 30% OAPS loading. Themixture was reuxed at 70 C, for 24 h with continuous stirring,then ltered, washed and dried at 80 C for 8 h [32]. Variouspercentage loadings of OAPS (10%, 30% and 50%) were preparedfor comparison. The products were denoted as Cl-SBA-15/x OAPS(where x = 10%, 30% and 50%). The OAPS grafting on Cl-SBA-15 isshown in Scheme 1.

2.3. Characterization

Powder X-ray diffraction pattern were recorded using a RigakuMiniex diffractometer with Cu Ka radiation (k = 0.154 nm). Thediffraction data were recorded in the 2h range 0.510 at 0.02 stepsize and 1 s step time. The nitrogen adsorptiondesorptionisotherms were measured at 196 C on a Micromeritics ASAP2010 volumetric adsorption analyzer. Prior to each adsorptionmeasurement the samples were evacuated at 100 C under vacuum(p < 105 mbar) in the degas port. The specic surface area, SBETwas determined from the linear part of the BET equation, the porevolume was calculated using BET plot from the amount of nitrogengas adsorbed at last adsorption point (P/P0 = 0.95) and the pore sizedistribution using the BarrettJoynerHalenda (BJH) method.Fourier Transform Infrared (FTIR) spectra of the samples wererecorded at room temperature on a Nicolet 6700 spectrometerequipped with an ATR (attenuated total reection) cell. Eachsample was scanned 20 times at 4 cm1 resolution over the range4000400 cm1. Proton-decoupled 29Si MAS-NMR spectra wererecorded on a JEOL (JNM-LA400WB) 400 MHz spectrometer at79.4 MHz with a sample spinning frequency of 5 kHz. The aminecontent was examined by elemental analysis using a FlashEA1112, Thermonnigan instrument. Samples were dried beforeexamination without any further pretreatments. The amounts ofOAPS grafted to Cl-SBA-15 were measured by a thermogravimetricanalysis (TGA, SCINCO thermal gravimeter S-1000); 10 mg of OAPSloaded Cl-SBA-15 was heated at 10 C/min to 700 C under airow(50 ml/min).

2.4. CO2 adsorption

CO2 adsorptiondesorption measurements for Cl-SBA-15/OAPSsamples were performed using Thermogravimetric Analyser(TGA). A sample weight of ca. 10 mg was loaded into an aluminasample pan in a TG unit (SCINCO thermal gravimeter S-1000)and tested for CO2 adsorptiondesorption performances. The initialactivation of the samples was carried out at 105 C for 1 h innitrogen atmosphere. Then adsorption run was carried out usinghigh purity CO2 (99.999%) gas, while the desorption run wasconducted in N2 ow. The adsorption runs were conducted at 25,50 and 75 C under atmospheric condition and desorption at105 C. Both the gases, CO2 and N2 were passed through automaticvalve, assisted with timer for continuous adsorption and desorptionprole.5 Loss on ignition SiO2 concentration after extracting with NaOH solution

3.66 2.00

HH

d MeCl Cl Cl Cl

Cl-SBA-15 + NH2

Cl Cl Cl Cl

Cl-SBA-15

Cl Cl Cl Cl

Cl-SBA-15 + NH2

M. Bhagiyalakshmi et al. /Microporous an3. Results and discussion

3.1. Characterization

From our previous report, the XRD patterns of the rice husk,heat-treated at 500, 600, 700 and 800 C for 12 h [15] showsthe presence of crystalline, cristobalite form of silica at 800 Cand retain amorphous form at 700 C. As solubilization of RHAin crystalline form is difcult, rice husk was heat-treated onlyup to to 650 C for 24 h in order to retain its amorphous phasefor the effective extraction of silica in sodium hydroxide solution.The XRD patterns in the 2h range 0.55 of Cl-SBA-15 and Cl-

H2

NH2

NHO

O

O

OO

O

O

O

OSi

SiSiO

Si

Si

OSi

SiO

Si

NH2

NH2

NH

NH2

NH

NH2

N

H

H

NH

N

H

H

NH

N

H

H

N N

O

O

O

OO

O

O

O

OSi

SiSiO

Si

Si

OSi

SiO

Si

H

H

H

H

HCl H2

NH2

NHO

O

O

OO

O

O

O

OSi

SiSiO

Si

Si

OSi

SiO

Si

NH2

NH2

NH

NH2

NH

NH2

N

H

H

NH

N

H

H

NH

N

H

H

N N

O

O

O

OO

O

O

O

OSi

SiSiO

Si

Si

OSi

SiO

Si

H

H

H

H

HCl

Scheme 1. OAPS grafting and CO2 adsorpNH2

NH2

O

O

O

OO

O

O

O

OSi

SiSiO

Si

Si

OSi

SiO

Si

NH2

NH2

N2

OAPS

NH2

NH2

O

O

O

OO

O

O

O

OSi

SiSiO

Si

Si

OSi

SiO

Si

NH2

NH2

N2

OAPSOAPS

soporous Materials 131 (2010) 265273 267SBA-15/(10%, 30%, 50%) OAPS are shown in Fig. 1. The presenceof well-resolved diffraction peaks at 2h = 0.8 and two weakpeaks at 1.6 and 1.7 corresponds to (1 0 0), (1 1 0) and (2 0 0)Bragg reections, respectively, indicating the characteristic hex-agonal mesoporous structure of Cl-SBA-15. The Cl-SBA-15/OAPSpattern shows no signicant change or shift in XRD peaks posi-tion and only a decrease in the intensity of peaks is observed.The decrease in intensity of XRD peaks which is reported to bethe function of scattering contrast between the silica wall andthe pore channel due to pore lling and/or structural construc-tion by OAPS grafting, can be correlated with pervious reports[15,33].

NH2N

2

OAPS

OAPS OAPSOAPS

3.77 nm

N

H

H

CO2(ad)

OAPSCO

2(ad)

OAPS

CO2

(ad)OAPSCO

2(ad)

OAPS

NH2N

2

OAPS

OAPS OAPSOAPS

3.77 nm

N

H

H

CO2(ad)

OAPSCO

2(ad)

OAPS

CO2

(ad)OAPSCO

2(ad)

OAPS

tion on mesoporous Cl-SBA-15/OAPS.

.52

1

d Me0

500

1000

1500

2000

2500

3000

3500

0.5 1 1.5 2 2

Inte

nsit

y (a.

u.)

0

200

400

600

800

1000

1200

1400

1600

1.5In

tens

ity (a

.u.)

268 M. Bhagiyalakshmi et al. /Microporous anThe N2 adsorptiondesorption isotherms (Fig. 2) for Cl-SBA-15and Cl-SBA-15/(10%, 30%, 50%) OAPS gives information aboutporosity of the materials. The textural properties of the materialsare summarized in Table 2. The nitrogen adsorption/desorptionisotherm of Cl-SBA-15 sample shows type IV hysteresis loop(Fig. 2) in the IUPAC classication with a sharp increase in nitrogenuptake between partial pressures P/P0 = 0.680.75 indicatescapillary condensation in mesopores. The specic surface areaswere found to decrease signicantly upon OAPS grafting. Such asignicant decrease in these textural properties for mesoporousmaterials due to complete pore lling of Cl-SBA-15 at higher OAPSloading, was inline with a previous reports [5]. Since the moleculardiameter of OAPS is around 11.5 nm [34], the mesoporosity ofCl-SBA-15 materials remains undisturbed, even at high percentageloadings of OAPS.

The FTIR spectra of Cl-SBA-15 and Cl-SBA-15/(10%, 30%, 50%)OAPS are shown in Fig. 3. The presence of POSS is revealed byseveral bands, in particular, by the strong band at about1110 cm1, due to asymmetric stretching vibration of SiOSibonds [35,36]. The peaks at 3307 and 3378 cm1 are due to NHvibrations of free amino groups attached to phenyl ring in OAPSnanocomposite, whose intensity increases on increasing the OAPSloading, whereas in Cl-SBA-15, peaks around 35003700 cm1 aredue to OH formed during hydrolysis of methoxy groups in CPTMS.Peaks at 697 and 746 cm1 are related to out-of-plane deformationof phenyl groups, signals at 1137 cm1 (partially overlapped withSiO band) and 1432 cm1 are due to Siphenyl group bondsdeformation; the stretching vibration of C@C bonds is shown bypeak at 1595 cm1 [37], while bands around 3000 cm1 areattributed to CH stretching modes.

Fig. 4 shows 29Si MAS-NMR spectra of Cl-SBA-15 and Cl-SBA-15/50% OAPS. Cl-SBA-15 shows distinct peaks at 101.3 and109.3 ppm due to Q3 and Q4 species corresponds to SBA-15 and

Fig. 1. XRD patterns of Cl-SBA-15; Cl-SBA-15/10% OAPS; Cl-SBA-15/30% OAPS; a3 3.5 4 4.5 5(degrees)

Cl-SBA-15

Cl-SBA-15/50% OAPS

Cl-SBA-15/30% OAPS

Cl-SBA-15/10% OAPS

.6 1.7 1.8 1.9 22 (degrees)

Cl-SBA-15

Cl-SBA-15/50% OAPS

Cl-SBA-15/30% OAPS

Cl-SBA-15/10% OAPS

soporous Materials 131 (2010) 265273T1 and T2 silicones at 47.1 and 56.0 ppm which evidenceone-step condensation of sodium silicate solution and CPTMS[32]. Cl-SBA-15/50% OAPS exhibit major peaks at 47.1, 56.0,63.9, 67.6, 70.8, 77.4, 101.3 and 109.3 ppm. These signalsrepresents three types of silicone species, Q3 and Q4 speciescorresponding to SBA-15 at 101.3 and 109.3 ppm, T1 and T2 ofCPTMS at 47.1, 56.0, and cubic silicon cages of OAPS inCl-SBA-15/50% OAPS at 63.9, 70.8 and 77.4 [36]. The peaksdue to cubic silsesquioxane cages in Cl-SBA-15/50% OAPS,evidences the grafting of OAPS inside the pores of mesoporousCl-SBA-15.

The surface coverage of OAPS was calculated based on theamount of nitrogen content present in Cl-SBA-15/OAPS sampleswere quantied by elemental analysis and the TGA results(Fig. 5) and the results are shown in Table 2. The results indicatedthat 0.65 mmol/g, 1.97 mmol/g and 3.28 mmol/g of OAPS groupsare attached on Cl-SBA-15/10% OAPS, Cl-SBA-15/30% OAPS andCl-SBA-15/50% OAPS, respectively. The thermal behavior ofCl-SBA-15/30% OAPS (Fig. 5) shows two step weight loss in therange of 350450 C and 450650 C corresponding to massloss of aminophenyl analogue and cubic cage silsesquioxane,respectively [38]. The total weight loss in the range of350650 C is around 7, 23 and 44 for Cl-SBA-15/10% OAPS,Cl-SBA-15/30% OAPS and Cl-SBA-15/50% OAPS, respectively, whichshows the amount of OAPS loaded into the pores of Cl-SBA-15.

3.2. CO2 adsorption

CO2 adsorption/desorption measurement for Cl-SBA-15/30%OAPS at 25, 50 and 75 C were carried out using TGA (Fig. 6) underatmospheric pressure. The weight gain in TGA plot on feeding CO2gas was calculated as CO2 adsorption capacity. Initial weight of thesample was considered as 100% and after preliminary activation at

nd Cl-SBA-15/50% OAPS. Inset is magnied XRD patterns from 2h = 1.5 to 2.

d MeM. Bhagiyalakshmi et al. /Microporous an105 C in N2 atmosphere, the observed weight loss around 9 wt.% isdue to moisture content. Thereafter, the sample was cooled to25 C, CO2 gas is passed, slow and continual CO2 uptake up tomaximum of 6.2 wt.% was attained in rst 25 min. On prolongedexposure to CO2 feed gas for another 60 min, no further changein weight gain was observed illustrating complete saturation ofCO2 adsorption over Cl-SBA-15/30% OAPS. Thus, consideringweight gain in rst 25 min, CO2 adsorption capacity of Cl-SBA-15/30% OAPS is 6.2 wt.% (62 mg/g of adsorbent) at 25 C. The CO2adsorbed Cl-SBA-15/30% OAPS was subsequently regenerated at105 C by purging N2 gas for 60 min to achieve completedesorption. Desorption prole once again conrms 6.2% weightloss which is equivalent to the CO2 adsorption capacity. Thisimplies that the interaction between acidic CO2 and amine groupsof OAPS might be weak chemisorption reaction which iscompletely reversible. The interaction of CO2 with isolated amine,on the eight corners of cubic OAPS is illustrated in Scheme 1.Herein, the presence of amine groups at eight corners of the OAPSprovides a selective afnity for CO2 via the formation ofammonium carbamate species under atmospheric conditions. Therealistic mechanism behind the carbamate formation is the weakbonding attributed to the nucleophilic attack of NH2 group onCO2 [39]. The primary NH2 group in OAPS is weakly alkaline,and the lone pair of electrons on N atom can easily attack the C

Fig. 2. The N2 adsorptiondesorption isotherm of Cl-SBA-15 and OAPS grafted Cl-SBA-15400, 600 and 800 cm3/g for Cl-SBA-15/10% OAPS, Cl-SBA-15/30% OAPS and Cl-SBA-15/5

Table 2Characteristics properties of Cl-SBA-15 and Cl-SBA-15/OAPS.

Catalyst Surface area(SBET) (m2 g1)

Mean pore volume(Vp) (cm3 g1)

Pore diameter (Dp)

Cl-SBA-15 752.8 0.72 3.77Cl-SBA-15/10% OAPS 560.8 0.47 3.33Cl-SBA-15/30% OAPS 251.3 0.25 2.25Cl-SBA-15/50% OAPS 86.2 0.13 2.02soporous Materials 131 (2010) 265273 269atom in acid gas CO2. Subsequently, there exist bond between Hatom on NH2 and O atom on CO2 to form carbamate. Thereafter,the active H atom in carboxyl may then form a hydrogen bond withthe nearby amine group, and thus stabilize the chemisorption ofCO2.

The effect of temperature on CO2 adsorption, for Cl-SBA-15/30%OAPS at 50 and 75 C was carried out continuous cooling/heatingand switching gas between CO2 and N2 through automatic valveassisted with timer (Fig. 6). It is observed that the increase intemperature decreases the CO2 uptake around 2% (20 mg/g ofadsorbent). Since at higher temperature the weak bonding is notfavored which evidences that CO2 adsorption involves weakbonding transformations via carbamate formation. In addition,decrease in accessibility of CO2 to the active amine sites causedby the increased kinetic energy of CO2 molecules at slightly hightemperatures might also inuence CO2 adsorption capacity.

The amine efciency of the OAPS groups was calculated fromelemental analysis using the equation,

g sorbed CO2 mmol=gnitrogen content mmol=g

which was already reported [40]. The amine efciencies are 0.69,0.73 and 0.57 for Cl-SBA-15/10% OAPS, Cl-SBA-15/30% OAPS and

. To improve the clarity of the main plot the amount of N2 adsorption are shifted by0% OAPS, respectively.

(nm) TGA weight loss in therange of 200600 C (%)

C (%) H (%) N (%) NH2 (mmol/g)

7 4.94 0.43 0.92 0.65

23 14.82 1.29 2.76 1.9744 24.70 2.15 4.60 3.28

Cl-SBA-15

-101.3 (Q 3 )

-109.3 (Q 4 )

- 47.1 (T 1 )-56.0(T 2 )

-63.9-70.8

-77.6

Cubic silicon cage in OAPS

Cl-SBA-15/ 50% OAPSInte

nsity

(a.u.

)

Cl-SBA-15

-101.3 (Q 3 )

-109.3 (Q 4 )

- 47.1 (T 1 )-56.0(T 2 )

-63.9-70.8

-77.6

Cubic silicon cage in OAPS

Cl-SBA-15/ 50% OAPS

Cl-SBA-15

-101.3 (Q 3 )

-109.3 (Q 4 )

- 47.1 (T 1 )-56.0(T 2 )

-63.9-70.8

-77.6

Cubic silicon cage in OAPS

Cl-SBA-15/ 50% OAPS

-200 -150 -100 -50 0

-200 -150 -100 -50 0

Cl-SBA-15

-101.3 (Q 3 )

-109.3 (Q 4 )

- 47.1 (T 1 )-56.0(T 2 )

-63.9-70.8

-77.6

Cubic silicon cage in OAPS

Cl-SBA-15/ 50% OAPS

Chemical shift (ppm)Fig. 4. 29Si MAS-NMR spectra of Cl-SBA-15 and Cl-SBA-15/50% OAPS.

Fig. 3. FTIR spectrum of Cl-SBA-15; Cl-SBA-15/10% OAPS; Cl-SBA-15/30% OAPS; and Cl-SBA-15/50% OAPS.

270 M. Bhagiyalakshmi et al. /Microporous and Mesoporous Materials 131 (2010) 265273

d Me90

100

M. Bhagiyalakshmi et al. /Microporous anCl-SBA-15/50% OAPS, respectively. Therefore, it could be concludedthat Cl-SBA-15/30% OAPS is optimum as efciency increases onincreasing the OAPS loading, but beyond 50% OAPS efciencydecreases.

CO2 adsorption capacities of various OAPS loading on Cl-SBA-15are given in Table 3. The CO2 adsorption capacity of Cl-SBA-15/10%

100 200 3000

10

20

30

40

50

60

70

80

Wei

ght (

%)

TemperatureFig. 5. TG/DTGA prole of

Fig. 6. Continuous CO2 adsorption at 25, 50 and 75 C with (99%) CO2 ga1.6

soporous Materials 131 (2010) 265273 271OAPS is 2 wt.% (20 mg/g of adsorbent), and further increasing theOAPS loading, the adsorption capacity increases. Specically, 6.2wt.% (60 mg/g-adsorbent) over Cl-SBA-15/30% OAPS and amaximum of 8.3 wt.% (80 mg/g adsorbent) over Cl-SBA-15/50%OAPS are observed, while pure OAPS shows almost no CO2adsorption. In the neat phase, the isolated amine in OAPS remain

400 500 600 700-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Der

ivativ

e W

eigh

t (%

min)

( oC)Cl-SBA-15/30% OAPS.

s/desorption at 105 C by purging N2 gas over Cl-SBA-15/30% OAPS.

internally hydrogen-bonded similar to PAMAM dendrimer [41]and, therefore, unavailable to interact with CO2. The CO2adsorption capacity of Cl-SBA-15/50% OAPS is comparable withthe already reported CO2 sorbents like zeolite 13X (90.2 mg/g-adsorbent at 25 C) and PEI (80 mg/g of sorbent) [42,43].Furthermore, the direct synthesis Cl-SBA-15/OAPS overcomes thedifculty in adopting multi-step procedure, involved successivegrafting of chloropropyltrimethoxysilane followed by OAPS overmesoporous SBA-15.

(SBA-15/30% OAPS), tested by three consecutive CO2 adsorption (at25 C) and desorption (at 105 C) runs, shown in Fig. 7, representsno signicant change in adsorption capacity. This conrms thereproducibility and stability of Cl-SBA-15/30% OAPS compositematerials. The above performances test results are in agreementwith well-established CO2 sorbents like PEI supported mesoporoussilicas [43,44].

CO2 adsorbed in Cl-SBA-15/50% OAPS at 25 C was regeneratedat various temperatures (115, 150 and 180 C) and the results are

Table 3The effect of adsorption temperature, desorption temperature, OAPS loading and CO2 concentration on CO2 adsorption in OAPS grafted Cl-SBA-15.

Samples CO2 adsorption temperature (C) Temperature of N2 purging (C) Adsorption capacity (mg/g) CO2 gas concentration (%)

Cl-SBA-15-10% OAPS 25 105 20 9950 105 10 9975 105 8 99

Cl-SBA-15-30% OAPS 25 105 62 9950 105 40 9975 105 18 9925 105 63 15.21

Cl-SBA-15-50% OAPS 25 105 83 9950 105 61 9975 105 29 9925 115 80 9925 150 82 9925 180 83 99

272 M. Bhagiyalakshmi et al. /Microporous and Mesoporous Materials 131 (2010) 265273In order to study the selectivity of Cl-SBA-15/30% OAPS, the CO2sorption/desorption were carried out with diluted (15.21%) andpure carbon dioxide (99%) feed gas and results are summarizedin Table 3. Adsorption step was carried out with 15.21% CO2 inair and desorbed by purging with N2 gas. The observed CO2 uptakeis similar with that of pure CO2 feed gas. This indicates that nosignicant co-adsorption of N2 occurred, and concludes the highCO2 selectivity of Cl-SBA-15/30% OAPS. The stability of the materialFig. 7. Recycle runs of CO2 adsorption at 25 C andillustrated in Table 3. The CO2 adsorption capacity remainsunaltered regardless of higher regeneration temperature (180 C),which shows thermal stability of Cl-SBA-15/50% OAPS. Whilepreviously reported amine compounds such as PEI, MEA andAPTMS. decomposes on regenerating at higher temperature(115 C) [45]. Therefore, Cl-SBA-15/50% OAPS is recognized asCO2 adsorbents with high thermal stability; however, it has lowadsorption capacity compared to PEI loaded mesoporous silicas.desorption at 105 C on Cl-SBA-15/50% OAPS.

4. Conclusion

Novel OAPS grafted SBA-15 mesoporous silicas have beensuccessfully prepared and observed to have CO2 adsorptioncapacity of 80 mg/g of adsorbent over Cl-SBA-15/50% OAPS. Thisattempt emerges to be a breakthrough both in the chemistry ofmesoporous materials and POSS nanocomposites. This Cl-SBA-15with pore diameter ca. 3.77 nm can accommodate larger amountof OAPS to capture CO2. Furthermore, the cost and ease ofpreparing the Cl-SBA-15 material in comparison with multi-steppost-synthetic grafting procedure adopted for amine-graftedmesoporous silica materials, is also a benet. Therefore, it isconsidered that both textural properties of mesoporous silicamaterials and the size of POSS nanocomposites can inuence theadsorption performances of CO2. The 29Si MAS-NMR result showsthe presence of OAPS composites grafted on Cl-SBA-15 by theirspecic peak in the resolved position. This Cl-SBA-15/OAPS alsopasses the general performance tests recommended for CO2

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M. Bhagiyalakshmi et al. /Microporous and Mesoporous Materials 131 (2010) 265273 273sorbent such as selectivity towards CO2, reproducibility, andmaterials strength. In addition the materials also posses highthermal stability, as their CO2 adsorption capacity is not affectedat higher regeneration temperature (180 C) and completelyregenerable at low temperature (105 C). This Cl-SBA-15/OAPSmaterial also adsorbs CO2 through the traditional carbamatemechanism. The last and foremost importance of this investigationis the use of rice husk silica source for the synthesis of mesoporoussilica, which is an efcient alternate for hazardous silica sources.

Acknowledgements

This study was supported by a grant (Code CD3-201) fromCarbon Dioxide Reduction & Sequestration Research Center, oneof the 21st Century Frontier funded by the Ministry of Scienceand Technology of Korean Government.

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Octa(aminophenyl)silsesquioxane fabrication on chlorofunctionalized mesoporous SBA-15 for CO2 adsorptionIntroductionExperimentalMaterialsSynthesis of Cl-SBA-15 and grafting of OAPS on Cl-SBA-15CharacterizationCO2 adsorption

Results and discussionCharacterizationCO2 adsorption

ConclusionAcknowledgementsReferences