Hydrotalcite-SBA-15 composite material for efficient carbondioxide capture

Journal of CO2 Utilization 12 (2015) 109–115

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Hydrotalcite-SBA-15 composite material for efficient carbondioxide capture C.V. Pramoda , K. Upendara , V. Mohana , D. Srinivasa Sarmab , G. Murali Dharc , P.S. Sai Prasada , B. David Rajua , K.S.Rama Raoa,* a b c

Inorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India Geochemistry Division, CSIR- National Geophysical Research Institute, Hyderabad 500607, India Department of Chemical Engineering, Gayatri Vidya Parishad college of Engineering, Visakhapatnam 530041, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 July 2014 Received in revised form 25 May 2015 Accepted 28 May 2015

SBA-15 supported Mg–Al hydrotalcite (Mg/Al = 2) composite materials with varied hydrotalcite loadings were prepared by precipitation–deposition method. These materials showed promising activity towards CO2 adsorption capacity. The reproducibility studies showed that the material is efficient even after 5 cycles with >96% desorption ability. Analysis of kinetics for CO2 adsorption experiments showed that the regressed plots demonstrate excellent fits to the experimental data. The structural, textural and morphological characteristics of the materials have been determined by XRD, N2 adsorption, FTIR, SEM and TEM techniques. ã2015 Elsevier Ltd. All rights reserved.

Keywords: Adsorption CO2 Hydrotalcite SBA-15 Composite materials

1. Introduction Carbon dioxide (CO2) is one of the major greenhouse gases causing global warming. Although CO2 is naturally present in the Earth’s atmosphere, its concentration has been rising gradually and is expected to reach 610 ppm due to increase in the use of fossil fuels [1]. The cause of this explosive rise in concentration is largely due to the burning of the fossil fuels. The negative influence of these emissions on the global climate is becoming stronger, and has to be reduced in order to stabilize the CO2 concentration in the atmosphere. Hence, there is a serious urge to curb the concentration of CO2 level in the earth’s atmosphere and in space shuttles. Reduction in the concentration levels of CO2 can be realized by several measures: energy efficiency improvements, reduction in our energy demand, using alternative primary energy sources like solar, wind or biomass energies and CO2 capture and storage. Among these CO2 capture is a relatively new concept and the most effective one. At present, CO2 capture and its usage/sequestration has been receiving considerable attention [2]. The assessment from the Intergovernmental Panel on Climate Change (IPCC) states that CO2 emissions could be reduced by 80–90% for modern power plants those are equipped with suitable carbon dioxide capture and storage (CCS) technologies [3] Several adsorbents have been

* Corresponding author. Tel.: +91 40 27191712; fax: +91 40 27160921. E-mail address: [email protected] (K.S.R. Rao). http://dx.doi.org/10.1016/j.jcou.2015.05.002 2212-9820/ ã 2015 Elsevier Ltd. All rights reserved.

identified for the capture of CO2 like alkali metal titanate nanotubes [4], modified silicas [5] and activated carbons [6]. Good quality reviews are available in the literature on CO2 adsorption[7–9]. Materials with high surface areas like metal organic frame works are being tried as adsorbents for CO2 capture. Nevertheless, their preparation cost and thermal stability are of great concern, and thus restricting their applicability [10]. The ability to regenerate an adsorbent and the ease of this regeneration are also important considerations. The need for extreme conditions such as high temperatures or very low vacuum makes regeneration more complicated and expensive. Various basic and base functionalized materials like alkali metal carbonates and amine modified silicas have been reported for the CO2 adsorption [11,12]. Recently, NASA came out with Carbon dioxide And Moisture Removal Amine Swing bed (CAMRAS) that is associated with amine based technology. It has been reported that the HTs are suitable materials for CO2 adsorption [13–15]. Although various reports are available on hydrotalcite (HT) by exchanging the interlayer anions [16], effect of trivalent anions [17], promoted hydrotalcites [18], much work has not been done on the supported hydrotalcite materials although recently, meso-AlOOH supported hydrotalcites have been reported [19]. Generally, HT’s are considered as high temperature adsorbents, however they can be tried as low temperature adsorbents particularly in a restricted or finite compartment like space craft or coal mine.

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Hydrotalcite, with a general formula of [Mg+2(1
x)Al+3x(OH)2] (CO3)x/22
.nH2O is a class of double layered anionic clay having brucite-like Mg(OH)2 layers, where Magnesium cations are octahedrally coordinated with hydroxyl ions and share edges to form brucite-like layers. When a Magnesium cation is replaced by an Aluminium cation; a positive charge is generated in the layer, which is balanced by an anion, such as carbonate or hydroxyl, located between the layers. Water molecules can also be present in the interlayer space. Interesting property of hydrotalcites is their memory effect; after relatively gentle calcination, the double layered hydroxide structure can be regenerated by exposure to anions, and hence is used as regenerative CO2 sorbents [20]. The intention of the present work is the in-situ synthesis of Mg–Al hydrotalcite material on a high surface area SBA-15 support and to test composites for CO2 adsorption studies at low temperature.

Scanning electron microscope (SEM) images of the adsorbent materials were taken by using XL30 ESEM scanning electron microscope (M/s. Philips, Netherlands) using a beam voltage of 20 kV. Prior to SEM imaging, an ultra-thin layer of gold was coated using JEOL Fine coat Ion Sputter FC-1100 (operating at 1 KV and 10 mA for 5 min) in order to enhance the conductivity of the samples. Transmission electron microscope (TEM) analysis was made using a Technai G2 FEI F12 (M/s. Philips) at an accelerating voltage of 80–100 kV. FT-IR spectra of the materials were recorded on a Spectrum GX FT-IR spectrometer (M/s. PerkinElmer, Germany) at ambient conditions by KBr disc method with a nominal resolution of 4 cm
1 and averaging 5 spectra.

2. Experimental

CO2 adsorption studies were conducted in a dynamic fixed bed adsorption flow system. The adsorption studies were performed in a fixed bed reactor (SS, 410 mm length: 9 mm i.d.). 1 g of adsorbent mixed with 1 g of glass beads is suspended in the middle of the reactor between two quartz plugs and was activated in Helium gas at 473 K for 1 h. After cooling the reactor to the required temperature, a gas mixture of 10% CO2 balanced by He was passed through the bed at a required flow rate with the help of mass flow controllers. The outlet concentration was monitored online by using a 7820 A gas chromatograph (M/s. Agilent Technologies, USA) equipped with a thermal conductivity detector and a Porapak Q column. After collecting the adsorption data at 343 K, the sample was flushed with pure He gas for 30 min and proceeded with desorption up to 413 K with a ramping of 5 K/min.

2.1. Chemical and gases Tetra ethyl ortho silicate (TEOS), Pluronic P123 (EO20PO70EO20), Mav = 5800 were purchased from M/s. Sigma– Aldrich, USA. Aluminium nitrate, Magnesium Nitrate, HCl, NH3 solution, (NH4)2CO3 all of analytical grade were purchased from M/ s. S.D. Fine-Chem. Ltd., India. De-ionized water was used in the preparation of the materials. Helium, used as purge gas during the activation and the cyclic CO2 desorption, has a purity of 99.995%. 5% CO2 balanced He mixture gas was used for the adsorption studies with 99.99% purity.

2.5. CO2 adsorption measurement

2.2. Preparation of SBA-15 3. Results and discussion SBA-15 has been prepared in accordance with the literature procedure [21]. A solution of EO20PO70EO20:2 M HCl: TEOS: H2O = 2:60:4.25:15 (mass ratio) was prepared, stirred for 12 h at 313 K and then hydrothermally treated at 373 K under static condition for 12 h, subsequently filtered, dried at 373 K and calcined at 773 K for 8 h to get the parent SBA-15 support. 2.3. Synthesis of SBA-15 supported hydrotalcites (SHT) To the suspension that contains SBA-15 in water, solution A containing required amounts of nitrate precursors of Mg and Al (Mg/Al = 2) have been precipitated simultaneously by a solution B containing 5% NH4OH and 5% (NH4)2CO3, maintaining a constant pH of 9–12. The precipitate was then aged for 18 h at 353 K. The resultant mass was filtered, washed several times with deionized water, dried in oven at 393 K for 12 h. The composite materials were designated as 20SHT, 30SHT, 40SHT, 50SHT where the numerical indicates the percentage of hydrotalcite (HT) loading in the composites.

3.1. Structural and textural aspects Low and wide angle X-ray diffraction patterns (XRD) of the adsorbents are shown in Figs. 1 and 2 respectively. Low angle XRD patterns suggest that the mesoporous structure of SBA-15 is still preserved even after depositing HT. The structure is maintained as care has been taken during the synthesis of HT on SBA-15 by adopting precipitation-deposition route with a mixture of NH4OH and (NH4)2CO3 instead of alkali metal carbonate and hydroxide

2.4. Characterization of materials Low and wide angle X-ray diffraction patterns were recorded for all the composites using Ultima-IV, X-ray diffractometer (M/s. Rigaku Corporation, Japan) with Cu ka radiation. The specific surface area and pore size distribution of the samples were determined by N2 adsorption–desorption experiments using Autosorb-1 instrument (M/s. Quantachrome Instruments, USA) at liquid N2 temperature. The powders were first out-gassed at 473 K to ensure a clean surface prior to construction of adsorption isotherm. A cross-sectional area of 0.164 nm2 of the N2 molecule has been taken in the calculation of the surface area using the BET method.

Fig. 1. Low angle XRD patterns of the composites A) SBA-15 B) 20SHT C) 30SHT D) 40SHT E) 50SHT.

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Fig. 2. Wide angle XRD patterns of the composites A) Dried HT B) 20SHT C) 30SHT D) 40SHT E) 50SHT.

Fig. 3. N2 sorption isotherms of the composites A) HT B) 20SHT C) 30SHT D) 40SHT E) 50SHT F) SBA-15.

mixture as precipitant. It is expected that usage of the latter mixture damages the mesoporous structure of SBA-15 by forming Sodium Silicate. Further, the XRD signals in 50SHT (wide angle pattern) are identical to that of bare HT which confirms the formation of HT structure without disturbing the structure of SBA15. The absence of HT signals in lower loadings may be because of two reasons: HT structure might not have formed or it could be in amorphous form where the signal fell below the XRD detection limit. No substantial variation in the structural parameters, like d spacing, unit cell length for SBA-15 implies the retention of the parent SBA-15 mesoporous structure in all the composites. BET surface area and pore volume data are shown in Table 1. The surface area of pure SBA-15 is 642 m2 g
1 which decreased upon deposition of HT. Pore volume also followed the same trend suggesting that the HT is formed inside the pores of SBA-15. After complete filling, the excess HT material may be expelled out and deposited on the outer surface of SBA-15. This shows the limit of confinement of HT structure inside the pores. The sudden fall in the average pore volume of 40SHT (0.25 cc/g) by nearly 75% with respect to that of SBA-15 is a clear indication of pore saturation with HT. The nitrogen sorption isotherms of the materials are depicted in Fig. 3. All the isotherms are of type IV with a H1-type hysteresis loop, as defined by IUPAC [21]. This kind of hysteresis loop represents capillary condensation and desorption in openended cylindrical mesopores [22,23]. However, the unlikely shape of isotherm of 40SHT is not understood. Scanning electron microscope images of the 50SHT composite along with bare SBA-15 are depicted in Fig. 4. The morphology of

SBA-15 shown in Fig. 4(A) is in accordance with the reported literature. A flake like morphology is observed in 50SHT composite on deposition of HT which is shown in Fig. 4(B). 50SHT composite is also characterized by transmission electron microscopy, and the picture is shown in Fig. 5 which shows that the HT is well dispersed on SBA-15. The mesoporous structure of the support material SBA15 is still preserved even after the deposition of HT which is in accordance with XRD data. Fig. 6 shows the FTIR spectra of SBA-15, HT and SHT materials. The spectrum of SBA-15 shows bands at 461, 801 and 1080 cm
1. Bands at similar wave numbers in the spectra of crystalline and amorphous SiO2 have been assigned to the characteristic vibrations of Si–O–Si bridges cross linking the silica network. The band at 1633 cm
1 is ascribed to the adsorbed water, and a broad band at 3430 cm
1 in the spectrum of SBA-15 can be assigned to the hydrogen bonding silanol groups and adsorbed water which is also present in HT spectrum. The region between 3000–3750 cm
1 indicates “

OH” stretching frequency. In the SHT composites, clear observation shows that the intensity of peak at 1382 cm
1 which is a characteristic of interlayer carbonate species is increasing. This indicates the formation of layered structure of HT. Therefore, it can be affirmed that the stacking in HT increases with HT loading. This is clearly observed in X-ray diffraction studies that the signals corresponding to HT cannot be seen in lower loadings.

Table 1 Physico-chemical properties of the catalysts. Material

Vt (cc/g)a

SBET (m2/g)b

d100 (nm)c

a0 (nm)d

SBA-15 20SHT 30SHT 40SHT 50SHT HT

0.99 0.76 0.61 0.25 0.24 0.15

642 480 454 422 401 75

9.72 9.2 9.8 9.8 8.85 -

11.22 10.65 11.32 11.32 10.22 -

a b c d

Pore volume. BET surface area. Periodicity of SBA-15 derived from low angle XRD. p Unit cell length, (a0 = 2d100/ 3).

3.2. Carbon dioxide sorption capacity The break through curves for CO2 adsorption capacities of the adsorbent materials have been depicted in Fig. 7 and the total CO2 adsorption uptake values derived from breakthrough curves are shown in Table 2. Blank runs were conducted following the procedure reported by Shung Li et al. [24], however not presented in Fig. 7. Time versus outlet flow rate patterns of the gas mixture containing Ar and He were generated with and without the adsorbent in the adsorber. Under the same experimental conditions, the curve with the adsorbent and that without it followed the same path. Therefore, the resistance to mass transfer is considered negligible. Among them, 50SHT material shows a relatively higher adsorption capacity which shows the limiting ratio of SBA-15 to HT. Beyond 50% loading of hydrotalcite, HT attained bulk nature which observed a decrease in CO2 uptake

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Fig. 4. Scanning electron microscope images of the composites SBA-15 B) 50SHT.

Fig. 5. Transmission electron microscope image 50SHT composite.

Fig. 6. FTIR patterns of SHT composites A) 20SHT B) 30SHT C) 40SHT D) 50SHT E) SBA-15 F) HT.

(results not shown). 20SHT composite shows exceptional patterns of break through curves observed due to lower CO2 adsorption capacity than the others. This is because of the contribution of the parent SBA-15 micro and mesopores rather than the HT material for the magnitude of adsorption [25]. This is in accordance with the

literature information was confirmed by decrease in the BET surface area as well as pore volume of composites (Table 1). XRD and FTIR patterns show the structural confirmation and good crystalline nature of HT structure in 50SHT composite. These factors are responsible for higher adsorption capacity along with synergistic effect. Water tolerance ability studies on 50SHT composite showed good resistance towards water with a break through CO2 adsorption capacity of 2.404 mmol/g. Various hydrotalcite like materials have been reported for CO2 adsorption studies. Valuable information is available in the literature [13] regarding the anion size, effect of water and the hydrotalcite modification. Other report in the literature [26] claimed that the HT prepared with Fe (CN)64
as an interlayer anion and 37% Al substitution had higher adsorption capacity (0.02 mmol/g) which is far lower than that of the present value. Higher CO2 adsorption capacities have also been reported on Calcium based adsorbents [27,28]. However, they require higher desorption temperatures. A similar problem has been reported over Li based materials [29,30]. Encouraging results have been shown for zeolite materials despite their having poor thermal stability [31–33]. Molecular basket type adsorbents have been reported by several workers where polyethylenimine (PEI) has been impregnated over MCM-41 support [34–36]. These reports claim that the total adsorption capacity of 2.79 mmol/g is higher than that of parent MCM-41 and PEI materials. Another report [37] summarized various porous solids for CO2 capture studies. Hyper-branched aminosilica material (SBA-HA) [38] showed 3.1 mmol/g adsorption capacity while PEI impregnated over SBA-15 adsorbent [39] claimed 3.18 mmol/g capacity at 348 K and 1 atm. On the other hand, the present composite material (50SHT) shows a total CO2 uptake of 3.66 mmol/g under similar experimental conditions providing a breakthrough adsorption value at 2.5 mmol/g, with >96% desorption ability which turns out to be an interesting material for the CO2 adsorption studies. The present adsorption capacity value is higher than that of our previous report (break through value at 1.94 mmol/g) with a total CO2 uptake of 3.02 mmol/g on modified TiO2 nanotubes [4]. Tables 3 and 4 show the CO2 capture capacities and the kinetic results of various adsorbents available in the literature. The consolidated list in both the tables show that the present material is more advantages or at least on par with the other materials. Hence, this can be seen as a promising regenerative material for CO2 capture. Moreover, the combination of SBA-15 and HT improved the adsorption capacity in all those composite materials more importantly in 50SHT when compared to that of parent materials (SBA-15 0.78 mmol/g and HT 1.79 mmol/g) which suggests the existence of synergistic effect between them due to the increase in surface area and uniform distribution and confinement of HT within the pores of SBA-15. The reproducibility of the 50SHT composite material is shown in Fig. 8. The composite material found to show good adsorption

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Fig. 8. Recyclability of 50SHT composite. Fig. 7. Adsorption capacities (breakthrough curves) of adsorbents at 1 bar pressure and 343 K temperature SBA-15 B) HT C) 20SHT D) 30SHT E) 40SHT F) 50SHT. Table 2 Yoon-Nelson Kinetic parameters for CO2 adsorption on SHT composites. Material

q0a (mmol/ g)

KYN (min
1)

R2

Total CO2 adsorption (mmol/g)b

HT 20SHT 30SHT 40SHT 50SHT SBA-15

0.850 0.622 0.823 1.581 2.506 1.470

0.240 0.229 0.238 0.283 0.309 0.241

0.978 0.994 0.979 0.989 0.996 0.964

1.795 0.947 1.813 2.434 3.665 1.790

a b

Adsorption capacity at a temperature and pressure of 343 K and 1 atm. Calculated from Fig. 7.

capacity with >96% desorption ability even after 5 cycles which clearly shows that the typical memory effect of HT plays an important role in repeated adsorption and desorption processes confirming this composite material ideal for CO2 adsorption. 3.3. Kinetics of CO2 adsorption

Fig. 9. Yoon and Nelson kinetic plots for the adsorption of CO2 on composites.

used to obtain kinetic data of CO2 adsorption. The proposed linear form of Yoon–Nelson equation for the study of column adsorption kinetics is:

Kinetic studies for the CO2 adsorption have been made and Yoon–Nelson method reported in the literature [4,40,41] has been

Table 3 Comparison of the CO2 adsorption capacities of various materials. Material

Adsorption conditions Temperature (K), Pressure (bar)

Adsorption capacity (mmol/g)

Reference

Carbon Alkali metal carbonates Titania nanotubes Ca(OH)2 Hydrotalcites Na-X Present material

275, 1 333, 1 323, 1 923, 0.41 481, 1 298, 0.4 343, 1

3.5 9.4 4.4 10.7 0.9 3.9 3.6

[39] [40] [3] [26] [41] [30] –

Table 4 Comparison of the kinetic parameters for CO2 adsorption of various materials. Material

Temperature (k)

q0

KYN (min
1)

R2

Reference

Karanja cake Activated carbon HP20/PEI-50 Present

343 323 348 343

2.52 0.98 3.62 2.5

0.362 0.032 0.32 0.309

0.990 0.986 0.998 0.996

[39] [42] [43] –

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 ln



C C0
C

¼ kYN t
t  kYN

where kYN is the Yoon–Nelson rate constant in min
1; T is the time required for 50% adsorbate breakthrough (min) and t is the sampling time (min). The parameter kYN is determined from the plot of ln[C/(C0
C)] vs. t. C0, C are the initial and reported concentrations of CO2, respectively. A plot is drawn for ln(C/(C0
C)) vs. T for the data obtained in the laboratory. Analysis of the regression coefficients indicates that the regressed lines provide excellent fits to the experimental data with R2 values ranging from 0.964 to 0.996. The values of kYN and adsorption capacities of composite adsorbents are reported in Table 2. Data showed that the Yoon–Nelson model can be used to describe the behaviour of the adsorption of SHT composites in a fixed bed column. Yoon and Nelson kinetic plots for the adsorption of CO2 on adsorbents are shown Fig. 9. 50SHT composite CO2 break through adsorption capacity is higher than that of the parents and other composites; this higher adsorption capacity is established with higher value of Yoon–Nelson rate constant (kYN) as shown in Table 2. Kinetic parameters of CO2 adsorption for various adsorbents reported in the literature are summarized in Table 4 which shows that the present material is efficient or on par with the other materials. 4. Conclusions The synthesized hydrotalcite-SBA-15 composite materials are versatile, eco-friendly and repeatable CO2 capture materials. The magnitude of adsorption capacity increased with HT content (up to 50 wt%) over SBA-15 and decreased thereafter. This indicates the formation of HT structure has a great impact on the CO2 capture capacity and an existence of synergy between both the parent materials. Furthermore, Yoon–Nelson model is used to follow the fixed bed kinetics. The calculated breakthrough curves agreed well with the measured results. Hence, it is worthy to note that the high surface area mesoporous materials like SBA-15 can be modified using hydrotalcite like materials which can bring encouraging results in CO2 adsorption studies. Acknowledgements Authors thank CSIR and UGC, New Delhi for the award of fellowship and DST, New Delhi, India for financial grant. References [1] W.M. Budzianowski, Modelling of CO2 content in the atmosphere until 2300: influence of energy intensity of gross domestic product and carbon intensity of energy, Int. J. Global Warming 5 (2013) 1–17. [2] X. Liu, L. Zhou, X. Fu, Y. Sun, Su, Y. Zhou, Adsorption and regeneration study of the mesoporous adsorbent SBA-15 adapted to the capture/separation of CO2 and CH4, Chem. Eng. Sci. 62 (2007) 1101–1110. [3] IPCC, IPCC Special Report on Carbon Dioxide Capture and Storage, in: B. Metz, O.J.G.J. Davidson, H. de Coninck, M. Loos, L.A. Meyer (Eds.), Cambridge University Press, Cambridge, 2005. [4] K. Upendar, A. Sri Hari Kumar, N. Lingaiah, K.S. Rama Rao, P.S. Sai Prasad, Lowtemperature CO2 adsorption on alkali metal titanate nanotubes, Int. J. Greenhouse Gas Control 10 (2012) 191–198. [5] S. Hyun-Kon, C. Kil Won, K.H. Lee, Adsorption of carbon dioxide on the chemically modified silica adsorbents, J. Non-Cryst. Solids 242 (1998) 69–80. [6] B. Guo, C. Liping, X. Kechang, Adsorption of carbon dioxide on activated carbon, J. Nat. Gas Chem. 15 (2006) 223–229. [7] S. Choi, J.H. Drese, C.W. Jones, Adsorbent materials for carbon dioxide capture from large anthropogenic point sources, ChemSusChem 2 (2009) 796–854. [8] J.R. Li, M. Yuguang, M. Colin McCarthy, S. Julian, Y. Jiamei, H.K. Jeong, B. Perla Balbuena, Z. Hong-Cai, Carbon dioxide capture-related gas adsorption and separation in metal-organic frameworks, Coord. Chem. Rev. 255 (2011) 1791– 1823. [9] D.M. D’Alessandro, B. Smit, J.R. Long, Carbon dioxide capture: prospects for new materials, Angew. Chem. Int. Ed. 49 (2010) 6058–6082.

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