1. Introduction

The removal of CO2 emission from fossil fuel combustion, especially from coalfired power stations, has attracted much attention in recent years because of its potential negative impact on global warming and on human beings. It was reported that CO2 concentration in atmosphere now is close to 400 ppm, which is significantly higher than the reported industrial level of 300 ppm [1]. CO2 capture and storage (CCS) technology is considered to be an effective means to cope with the global demand of CO2 reduction in the long run. Among many technologies existing for CO2 capture, the use of amine-based solutions is still the main practical

technology on a large scale capture operation which could produce a series of bad effects such as toxicity, degradability, high regeneration energy requirements and corrosivity [2]. Hence, developing efficient and environmental friendly CO2 adsorbents will be crucial to CO2 capture. Layered double hydroxides (LDHs) are considered as good candidates for CO2 adsorption because of their fast sorption/ desorption kinetics and simple regenerability [3, 4]. LDHs are a class of ionic lamellar compounds made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules. The typical structure of LDHs consists of positively charged brucite-like layers, containing anions and water molecules in the interlayer spaces. Metal cations occupy the centre of octahedral structures and hydroxides occupy the vertices. The general formula of LDHs can be expressed as [M2+1xm3+x(OH)2][An]x/n zH2O, where M2+are divalent cations, such as Mg2+, Zn2+, Ni2+, etc., and M3+ are trivalent cations, such as Al3+, Ga3+, Fe3+, Mn3+, etc. An is a non-framework charge compensating anion, such as CO3 <sup>2</sup>, Cl, SO4 <sup>2</sup>, etc., and the value of x is between 0.10 and 0.33 [5].

characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and BET analysis. Their performance of CO2 adsorption was evaluated using thermogravimetric analysis (TGA). The effect of the rate of addition of Mg(NO3)2 and Al(NO3)3 on improvement of the CO2 capture capacity of MgAl LDHs and initial NH3H2O is also investigated.

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

Salt solution A (100 mL) containing a mixture of 0.03 mol L<sup>1</sup> Mg(NO3)2 and 0.01 mol L<sup>1</sup> Al(NO3)3 and solution B-NH3H2O (100 mL) with certain concentration was simultaneously transported into a 'T-type' impinging-stream reactor by means of metering pumps at the rate of 100 r min<sup>1</sup> to produce MgAl LDHs. Ultrasonic processing was also applied during this process and the frequency of ultrasonic processing kept to be 20 kHz. The pH of the whole solution was always kept to be 10 0.2 though regulating the concentration of NH3H2O. The resulting mixture washed with water for several times until pH = 7 and then dried at 100°C in

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

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

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

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

. The mixture was then aged for further 4 h with stirring maintained.

. Then, 150 mL of salt

2. Experimental

is 400 r min<sup>1</sup>

as CC-LDHs.

125

2.1 Materials preparation

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

2.1.1 LDH synthesized in T-Mixer with ultrasonic process

an oven. The resulting material was denoted as TU-LDHs.

2.1.2 LDH synthesized by conventional co-precipitation method

2.1.3 LDH synthesized by hybrid two-step method

material was denoted as TUC-LDHs.

beaker for continuous stirring with a rate of 400 r min<sup>1</sup>

The performance of LDHs and derived CO2 adsorbents have been investigated for several years, and most of the studies are focused on the effects of divalent cations [6, 7], trivalent cations [8], charge compensating anions [9, 10], Mg-Al ratio [6, 11, 12], alkaline metal cations (e.g. K<sup>+</sup> , Cs+ , Na+ , etc.) [13–16], synthetic method [17], the presence of SO2 and H2O [18, 19], particle size [3, 20] and calcination temperature or adsorption temperature [21, 22]. Yong and Rodrigues compared several commercial hydrotalcite-like compounds that can have the average CO2 adsorption capacity (0.2–0.5 mmol g<sup>1</sup> ) at 300°C and 1 bar of CO2 [6]. Wang et al. found that similar CO2 capture capacities (0.41–0.46 mmol g<sup>1</sup> ) can be obtained when using Mg3Al1, Mg3Ga1 and Mg3Fe1 at different calcination temperatures [8]. Except for changing the composition of LDHs, controlling its particle size is also believed to be an effective way for improvement of the CO2 capture capacity. Significant amount of efforts have been made on developing new methods to control its particle size. Hanif et al. investigated the effect of synthetic routes (co-precipitation, ultrasonication and microwave irradiation) on improving the CO2 adsorption capacity of hydrotalcite-based sorbents in the temperature range 300–400°C [17]. They have reported that the CO2 adsorption capacity of LDHs prepared by ultrasound-assisted route and microwaving are better than that of co-precipitation method.

Adoption of confined impinging T-jet mixer (CITJ) is a simple component that contains two inlet tubes and let two streams flow out from the tube. The local micromixing effect could be intensified during the CITJ reactor, which is beneficial for a fast homogenization of reactors. The mass transfer rate and chemical reaction rate can also be enhanced during the preparation process. This has been confirmed by the studies on the preparation of FePO4 nanoprecursor particles of LiFePO4 cathode material where the high specific areas can be obtained [23, 24]. Coprecipitation method is the conventional procedure used for synthesis of hydrotalcites. Ultrasonication and microwave irradiation of the synthesis gel during hydrotalcite precipitation leads to disruption in the layer stacking which in turn increases surface area [17]. To the best of our knowledge, the effect of the synthesis for the preparation of LDHs by applying jointly co-precipitation and ultrasonication in a T-jet mixer on the CO2 absorption capacity of LDHs has not yet been reported in the literature.

In the present study, we will present a hybrid two-step method approach for preparation of MgAl layered double hydroxide (MgAl LDHs). The novel two-step preparation route utilising the MgAl LDHs synthesized from confined impinging T-jet mixer (CITJ) as seeds for future preparation. The synthesized samples were

characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and BET analysis. Their performance of CO2 adsorption was evaluated using thermogravimetric analysis (TGA). The effect of the rate of addition of Mg(NO3)2 and Al(NO3)3 on improvement of the CO2 capture capacity of MgAl LDHs and initial NH3H2O is also investigated.
