Introduction

Global climate change caused by increased concentrations of greenhouse gases in the atmosphere has become a worldwide urgent environmental issue [1, 2]. It is well known that CO2 is the major anthropogenic greenhouse gas (GHG) in the atmosphere mainly due to the extensive utilization of fossil fuels as energy source. The atmospheric concentration of CO2 has increased to 384 ppm in 2007 from its pre-industrial level of ca. 280 ppm, and is expected to reach 550 ppm by 2050 even if CO2 emission is stable for the next four decades [3]. Therefore, a great deal of effort has been devoted to stabilize the CO2 concentration in the atmosphere by controlling the emission of CO2 from various sources. Carbon capture and storage (CCS) is believed to be one of the keys to reducing greenhouse gas emissions.

Adsorption is potentially a cost-effective technique to capture CO2 from flue gases of fixed sources or directly from air. As a consequence of this, various kinds of materials have been investigated as possible CO2 adsorbents, such as activated carbon, carbon molecular sieves, zeolites, mesoporous materials, metal oxides, anionic clays or hydrotalcite-like materials, metal–organic frameworks (MOFs), amine functionalized solid sorbents and alkali-metal carbonate-based sorbents [4,5,6].

Modified clay materials have been studied for different gas adsorptions, such as N2, O2, CH4, CO, CO2, and C2H2 etc. [7,8,9]. Venaruzzo et al. [20] reported the adsorptions of CO, CO2 and SO2 gases by two bentonitic clay minerals. The clay minerals were tested for gas adsorption at 25 °C and 100 Kpa. The adsorption values of the CO2 were in the range of 0.218–0.516 mmol/g. Obviously, the CO2 adsorption capacities of the above modified clay materials are still very low compared to other sorbents.

Pillared clays are inorganic oxide-clay nanocomposites, which are an interesting class of 2-dimensional microporous materials. Due to their high surface area and permanent porosity, they are widely used as adsorbents and catalyst supports [10, 11]. However, there is scarce information about the CO2 gas adsorption properties of pillared clays.

In this paper, Al2O3, ZrO2 and TiO2-SiO2 pillared clays were prepared using montmorillonite clay and characterized by XRD and N2 physisorption technique. In addition, the CO2 adsorption isotherms of these samples were measured and compared for determining their potential use in CO2 separation.

Experimental

Materials

Preparation of Al2O3-pillared montmorillonite (MMT)

Al polymer pillaring solution was prepared as follows: The pillaring precursor solution was prepared by titrating 500 ml of 0.4 molar NaOH (≥ 96.0%) into 250 ml of 0.4 molar Al(NO3)3·9H2O (≥ 99.0%) at 2.5 ml per min under vigorous stirring (Na:Al = 2:1). After the titration and 1 h stirring, the solution was heated at 65 °C for 4 h in a sealed container and then stored at room temperature in a container before use.

Pillaring process with Al2O3

A 0.2 g of Na-montmorillonite, Wyoming or a sodium montmorillonite supplied by Kunimine Industrial Company, Japan (designated as Kunipia from hereafter) was added slowly to 20 ml of deionized water. After 30 min stirring 20 ml of the pillaring solution was added into the suspension. The suspension was stirred for 8 h after the precursor addition and then centrifuged to collect the solid. To make sure that the intercalation of Al pillars was complete, the above solid was again treated with 20 ml of deionized (DI) water and 20 ml of the pillaring solution by stirring for 1 day. The resultant pillared clays were washed with DI water several times and finally with ethanol. The final product was dried in air. The above preparation of Al2O3-pillared montmorillonite was based on a previously described procedure by Malla and Komarneni [12]. The Al2O3-pillared montmorillonites (MMT) are hereinafter referred to as Al-pillared kunipia and Al-pillared Na-MMT.

Preparation of ZrO2-pillared montmorillonite

Hydroxy zirconium solution was first prepared as follows: A solution of 0.2 M ZrOCl2 (≥ 99.0%) was heated at 60 °C for 48 h to synthesize hydroxyl Zr polymers. For obtaining a ZrO2-pillared montmorillonite, a 1% suspension of montmorillonite, Wyoming was mixed with excess amount of pillaring (> 25 times the CEC of clay) solution and reacted for 2 h at 25 °C. The above preparation of ZrO2-pillared montmorillonite was based on a previously described procedure by Malla et al. [13].

Preparation of TiO2–SiO2 pillared montmorillonite

Titania Sol was first prepared as follows: Ti isopropoxide (≥ 98.0%) was added to 1 M HCI solution to hydrolyze and obtain Ti polymeric sol and the resulting slurry was peptized to a clear solution by stirring for 3 h at room temperature. The molar ratio of HCI to the alkoxide was about four. Silica Sol was then prepared as follows: Silica sol solution was prepared by mixing tetraethoxysilane, 2 M HCI and ethanol in a ratio of 41.6 g/l0 ml/12 ml. Silica sol is negatively charged and therefore, it cannot be directly used as a pillaring agent but can be used in the form of mixed sols with titanium (IV) hydroxide deposited on the silica sols. Therefore, titania-silica mixed sol was prepared as follows: The silica and the titania sol solutions prepared above were mixed in a molar ratio of TiO2/SiO2 = 1/10 and stirred for 1 h at room temperature. For obtaining TiO2–SiO2 pillared montmorillonite (Scheme 1), about 1% suspension of montmorillonite, Wyoming in water was reacted with the mixed sol solution. The ratio of the mixed sol of TiO2–SiO2 to the cation-exchange capacities (CEC) equivalent of the clay was 30. The mixture was allowed to react for 3 h under stirring at 50 °C and then the products were separated by centrifugation, washed with water several times, and then dried at room temperature. The above preparation of TiO2–SiO2 pillared montmorillonite was based on a previously described procedure by Yamanaka et al. [14].

Scheme 1
scheme 1

Schematic illustrating the preparation of TiO2–SiO2 pillared montmorillonite

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All the above pillared clays were heated/calcined at 450 °C for 5 h to convert polymeric hydroxy cations to ceramic oxides. The as-prepared pillared clays and calcined pillared clays were characterized by different techniques as described below.

Characterization

X-ray diffraction patterns for the samples were recorded on a Panalytical Xpert PRO X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Ǻ, 45 kV, 40 mA). Nitrogen adsorption and desorption isotherms were measured on an Autosorb-1 sorption analyzer by Quantachrome instrument at liquid nitrogen temperature. The samples were degassed at 150 °C for 6 h prior to analysis. The total pore volume (Vt) was determined as the volume of liquid nitrogen adsorbed at a relative pressure of 0.99. The pore size distribution was calculated from the desorption isotherm using the BJH method.

CO2 adsorption

The CO2 adsorption isotherms at 273 K were measured by using a Quantachrome Autosorb-1 sorption analyzer with pure CO2 (99.999%). Prior to each adsorption experiment, the sample was degassed for 2 h at 373 K under vacuum.

Results and discussion

Figure 1a, b show the XRD patterns of Al-pillared kunipia, Al-pillared Na-MMT, ZrO2-pillared MMT and TiO2 + SiO2 pillared MMT. It can be seen that both Al-pillared Kunipia and Al-pillared MMT show the characteristic peaks of the montmorillonite crystalline layered structure in the position (2 theta) of ~ 7.5°, as well as peaks at 20° and 26.5°, corresponding to the quartz commonly found in clays. The ordered pillaring of layered materials will result in the shifting of the (001) to lower 2θ region [15]. The d001 value of Al-pillared Kunipia is 1.54 nm, which is slightly larger than that of Al-pillared MMT (1.40 nm). These d001 values of calcined pillared clays decreased compared to the as-prepared Al pillared clays, which showed d001 values of around 1.8 nm. For ZrO2 and TiO2 + SiO2 pillared MMT samples, the (001) peaks obtained are almost absent in the lower 2θ region, which is because of the lack of a sufficiently ordered and oriented silicate layer structure. The poor long range ordering may be due to the disordered distribution of different sizes of pillaring oxides in the clay layers.

Fig. 1
figure 1

XRD patterns of pillared clays calcined at 450 °C for 5 h (a) Al-pillared kunipia, and Al-pillared Na-MMT and (b) ZrO2-pillared MMT and TiO2 + SiO2 pillared MMT

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Figure 2 shows the N2 adsorption–desorption isotherms of the four different kinds of pillared clays, and Table 1 summarizes the textural properties of the samples prepared in this work. The isotherms of all the four different pillared clay samples are of type IV and show steep hysteresis of type H4 at high relative pressures corresponding to the mesoporous nature of the pillared clay materials (Fig. 2a). It can be seen that TiO2 + SiO2 Pillared clay showed the largest surface area of 437 m2/g and total pore volume of 0.28 cm3/g, whereas Al-pillared Na-MMT showed the lowest surface area and pore volume of 169 and 0.19 cm3/g, respectively (Table 1). All the four pillared clay samples showed a very close pore size distribution with a peak centered at 3.9 nm (Fig. 2b). Note that, TiO2 pillared clay were reported by many researchers [16,17,18,19], the specific surface areas of the samples were around 250 m2/g. However, with the adding of SiO2, the surface area of TiO2 + SiO2 pillared clay increased to more than 400 m2/g, because the composite pillar particles were probably tightly packed in the interlayers forming small pores between the particles and silicate layers. The presence of positively charged TiO2 particles coated on the negatively charged SiO2 particles enabled the pillaring with SiO2 producing more micropores and hence the surface area increased significantly [12, 14].

Fig. 2
figure 2

Nitrogen adsorption–desorption isotherms (a) and pore size distributions (b) of pillared clays. The isotherms for samples ZrO2 pillared clay, Al-pillared Na-MMT, Al-pillared Kunipia and TiO2 + SiO2 pillared clay are shifted by 0, 30, 60 and 90 cm3/g, STP

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Table 1 Textural properties and CO2 uptake of pillared clays
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The adsorption–desorption isotherms of CO2 gas measured at 273 K on various pillared clay adsorbents are depicted in Fig. 3. All the isotherms show a modest increase in the amount of CO2 adsorbed with an increase of pressure and the gas adsorption data are fully reversible. All the samples exhibited a slight hysteresis, which indicated their good affinity for CO2. Furthermore, it is noticed that there are no plateaus in adsorption isotherms in the pressure range investigated. The equilibrium adsorption capacities of these adsorbents at 1 atm are compiled in Table 1. It can be seen that the TiO2 + SiO2 Pillared clay sample showed the highest adsorption capacity (1.18 mmol/g at 273 K and 1 atm) among these pillared clay samples correlating with its surface area, which is the highest among the four pillared clays studied here. All the pillared clays showed much higher sorption capacities than unmodified bentonite (montmorillonite) which only showed a CO2 adsorption capacity of 0.218 mmol/g (25 °C and 1 atm) [20]. The CO2 adsorption capacity increased with the increase of pore volume (Table 1) of these four different pillared clays and correlated well with the surface areas of these samples except with that of the ZrO2 pillared clay. This appears to suggest that ZrO2 has less affinity for CO2 than Al2O3 in the interlayers of clay. As has been found previously with many adsorbents, physical adsorption of CO2 generally correlates with surface areas of the adsorbents [4], which is what was found in this study with various pillared clays. Further tailoring of the pillared clays could lead to increased CO2 adsorption.

Fig. 3
figure 3

Adsorption–desorption isotherms of CO2 on various pillared clay adsorbents. (Filled and open symbols represent adsorption and desorption, respectively)

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Conclusions

Montmorillonite clay minerals pillared with metal oxides were found to be effective in improving clay’s adsorption for CO2. The CO2 adsorption capacities of four different pillared clays were found to increase with the increase of their pore volume and correlated with surface areas in general. Among the four pillared clays studied here, the N2 BET surface areas of the TiO2 + SiO2 pillared montmorillonite greatly increased to more than 400 m2/g as compared to about 27 m2/g for the Na-montmorillonite and therefore, the TiO2 + SiO2 pillared montmorillonite exhibited the best CO2 adsorption capacity of 1.18 mmol/g at 273 K and 1 atm.