2.1.2 LDH synthesized by conventional co-precipitation method

For the synthesis of traditional LDHs, LDHs was prepared by co-precipitation method. The salt solution with a fixed concentration of 0.03 mol L<sup>1</sup> Mg(NO3)2 and 0.01 mol L<sup>1</sup> Al(NO3)3–A (200 mL) with a certain rate of addition and 1 mol L<sup>1</sup> of NH3H2O were simultaneously added to the mixture at the second step, where the pH of the whole solution was always kept to be 10 0.2 and continuous stirring rate is 400 r min<sup>1</sup> . The mixture was then aged for further 4 h with stirring maintained. The final obtained materials were filtered and washed with distilled water until pH = 7, followed by drying at 100°C in an oven. The obtained material was denoted as CC-LDHs.

## 2.1.3 LDH synthesized by hybrid two-step method

MgAl layered double hydrotalcites (MgAl LDHs) were prepared using a hybrid two-step preparation approach. The hybrid two-step preparation consists of the mother solution prepared in 'T-mixer' accompanying with ultrasonic processing (first step) and the following step of co-precipitation (second step) process. The mother solution was synthesized according to the preparation method of the above section. About 50 mL of mother solution from the above solution was put into a beaker for continuous stirring with a rate of 400 r min<sup>1</sup> . Then, 150 mL of salt solution with a fixed concentration of 0.03 mol L<sup>1</sup> Mg(NO3)2 and 0.01 mol L<sup>1</sup> Al (NO3)3 and 1 mol L<sup>1</sup> of NH3H2O were simultaneously added to the mixture at the second step. The addition rate of salt solution was controlled by regulating the speed of peristaltic pump, where the pH of the whole solution was always kept to be 10 0.2. The whole preparation process of MgAl LDHs by two steps were illustrated in Figure 1. The final obtained materials were filtered and washed with distilled water until pH = 7, followed by drying at 100°C in an oven. The obtained material was denoted as TUC-LDHs.

2.3 Carbon dioxide adsorption measurements

DOI: http://dx.doi.org/10.5772/intechopen.86608

(80, 150 and 200°C) at the rate of 10°C min�<sup>1</sup>

sorbent (mg) and M is molar mass of CO2 (44 mol g�<sup>1</sup>

Normalized standard deviation : Δqð Þ¼ %

Correlation coefficient equation : <sup>R</sup><sup>2</sup> <sup>¼</sup> <sup>1</sup> � <sup>∑</sup><sup>N</sup>

capacity of the sample-qt (mmol g�<sup>1</sup>

adsorbent:

cycles.

127

2.4 Error analysis

which are given by

functions, i.e. chi-square (X<sup>2</sup>

The CO2 adsorption test and regenerability of the synthesized adsorbents were measured by Netzsch TG 209 F1 thermogravimetric analyser (TGA) at atmosphere pressure under dry conditions. Pure CO2 gas was used to carry out the whole adsorption/desorption measurement. Before adsorption test, all the samples were calcined at 500°C for 5 h in an Ar atmosphere. About 10 mg of sample loaded into an alumina crucible was heated from 25 to 105°C at 10°C min�<sup>1</sup>

Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO2 Adsorption

under N2 atmosphere for 60 min and then switched to desired temperature

gain of the sample and expressed as the mole of CO2 absorbed per gram of

qt <sup>¼</sup> mt � mo m � M

for 60 min and then switched to 80°C at a cooling rate of 10°C min�<sup>1</sup>

where mO and mt (mg) are the initial mass of adsorbent for CO2 adsorption and mass of adsorbent for CO2 adsorption at time t, respectively. m is the total mass of

In the absorption/desorption cycle, about 10 mg of the above calcined samples was heated from 25 to 105°C at 10°C min�<sup>1</sup> under N2 atmosphere (40 mL min�<sup>1</sup>

adsorption, the desorption process was carried out as the gas input was switched from CO2 to N2 at 105°C. The adsorption/desorption process was repeated in six

To determine the validity of isotherm and kinetics models, two different error

tion coefficient equation R2 were evaluated between experimental and calculated data,

n i¼1

s

0 @

where qt and qe are CO2 uptake determined by experiment and computed by model, respectively. qe is the mean experimental data, p is the number of model parameters and N is total number of experimental points. The most suitable model

to describe the CO2 adsorption process is the one with highest R2 value.

Chi � square : <sup>X</sup><sup>2</sup> <sup>¼</sup> <sup>∑</sup>

the gas input changed from N2 to CO2 and held for 90 min. The CO2 adsorption

. During the isothermal stage,

� 1000 (1)

)

(2)

� 100 (3)

N � 1 N � p � � (4)

. After

) was calculated according to the weight

).

) and normalized standard deviation Δqð Þ % , and correla-

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

n � 1

1 A �

<sup>i</sup>¼<sup>1</sup> qt � qe � �=qe � �<sup>2</sup>

<sup>j</sup>¼<sup>1</sup> qt � qe � �<sup>2</sup>

<sup>j</sup>¼<sup>1</sup> qt � qe � �<sup>2</sup>

qt � qe � �<sup>2</sup> qe

∑<sup>n</sup>

∑<sup>N</sup>

#### Figure 1.

Schematic illustrating the preparation process of MgAl LDHs by hybrid two steps.
