3. Results and discussion

#### 3.1 Synthesis and characterization of LDHs prepared by different ways

by the type of metal cation (Mg2+ and Al3+). Although LDHs are prepared by three different ways, the metal cation of Mg2+ and Al3+ still keep the same without any change. Similar observations can be found in the research of Wang et al. for the formation of LDHs by co-precipitation and IEP method [20]. Concerning the lattice parameter c, it was affected by three main factors: the average charge of the metal cations, the nature of the interlayer anion and the water content. TU-LDHs have lowest lattice parameter c compared to other two LDHs. All the samples use the same anion and metal cations so that the slight difference between samples may be

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

The crystalline size of particles was calculated using the Debye-Scherrer's formula

<sup>β</sup> cos<sup>θ</sup> (5)

<sup>D</sup> <sup>¼</sup> <sup>0</sup>:89<sup>λ</sup>

where D is the grain size (nm), λ is the wavelength of the X-ray radiation, β is the full width at half maximum and λ, θ is the angle of diffraction. The crystallite size along the c direction was calculated according to (003) and (006) diffraction peaks. The values given in Table 1 showed that the crystallite size in the c direction by TU-LDHs and TUC-LDHs were a slightly higher than the crystallite size of CC-LDHs. There is no obvious reduction of crystallite size in the c direction of

N2 adsorption-desorption isotherm of MgAl LDHs is given in Figure 3. All the adsorbents show a Type IV isotherm according to the IUPAC classification, which is associated with mesoporous materials [26]. TUC-LDHs and CC-LDHs show a H3 type hysteresis loop, suggesting that the pores are produced by 'slit-shaped' of plate-like particles [27]. This type of isotherm is commonly observed in the mesoporous stacking structure of sheet-like 2D crystallites [28]. In the case of TU-LDHs, it shows a H2 type hysteresis loop corresponding to a complex and interconnected pore structure, indicating that the pores are produced by rapid nucleation process. This difference could be related to the different synthetic route applied in this work. The surface area (SBET) of the samples were determined using BET (Brunauer, Emmett and Teller) model. The total volumes (VTotal) were calculated according to the amount of nitrogen (N2) absorbed at a relative pressure (P/PO) of 0.99. The pore volumes were calculated from the desorption branch of the isotherms using the Barrett-Joyner-Halenda (BJH) method, for the pores between 1.7 and 300.0 nm. Table 2 lists the textural parameters of MgAl LDHs obtained from N2 adsorption-desorption isotherms. It can be seen clearly that the SBET of TUC-LDHs is the highest of 235.3 m<sup>2</sup> g�<sup>1</sup> compared to 198.7 m<sup>2</sup> g�<sup>1</sup> for TU-LDHs and 148.1 m2 g�<sup>1</sup> for CC-LDHs as well as the pore sizes (90.83 Å) and highest pore

Property TU-LDHs CC-LDHs TUC-LDHs d<sup>003</sup> (nm) 0.7977 0.8058 0.8011 d<sup>110</sup> (nm) 0.1518 0.1516 0.1519 a<sup>a</sup> (nm) 0.3036 0.3032 0.3038

(nm) 2.3931 2.4174 2.4033 Crystallite size in c direction (nm)<sup>c</sup> 8.65 7.35 8.63

Value calculated from (003) and (006), according to Debye-Scherrer's formula.

Lattice parameters of LDHs prepared by different methods.

related to a slow water content in the layer.

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

materials prepared by three different methods.

cb

Table 1.

129

a a=2d<sup>110</sup> <sup>b</sup> c=3d<sup>003</sup> <sup>c</sup>

Co-precipitation method is a common method used in preparation of MgAl LDHs materials in the previous studies. The main requirement for improving the CO2 adsorption capacity of MgAl LDHs is to develop it with high surface area and pore volume. It is well known that the formation process of MgAl LDHs includes both nucleation and growth process. Controlling the process of nucleation properly may have an effect on the whole size of MgAl LDHs, thus increasing its surface area. Here, we reported the first synthesize of MgAl LDHs using a hybrid two-step method. At first step, the salt solution and initial concentration of NH3H2O solution were transported into 'T-type' impinging-stream reactor. Two solutions interact on each other on the 'T-type' impinging-stream reactor and ultrasonic wave were also applied in the interaction process. At second step, the conventional co-precipitation method was used. This means that a certain rate of salt solution and NH3H2O were simultaneously added to the mixture to produce the MgAl LDHs. The chemical and physical effects of ultrasonic irradiation originated from acoustic cavitation lead to rapid reaction rate and change the selectivity performance of the reaction [25], thus improving the nucleation of MgAl LDHs.

The XRD patterns of MgAl LDHs prepared using three different methods are shown in Figure 2. For all the samples, the characteristic reflections based on the structure of (PDF #35-0965) were clearly observed, which can be indexed to (i) (00l) peaks of (003) and (006) with corresponding to the basal spacing and higher order reflections; (ii) (0kl) peaks of (012), (015) and (018) with broad reflections; (iii) sharp (hk0) and (hkl) peaks of (110) and (113). It can be seen that LDHs samples synthesized under different conditions have similar structures. The lattice parameter a and c are calculated according to the parameters of (110) and (003) plane assuming a 3R stacking of layer structure, where the value a (a = 2d110) represents average distance between two metal ions and c is three times of the brucite-like layer and interlayer distance (d003). The lattice parameter a is almost independent of the synthesis method, which can be explained that it was affected

Figure 2. XRD patterns for MgAl LDHs prepared using (a) TU method, (b) CC method and (c) TUC method.

#### Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO2 Adsorption DOI: http://dx.doi.org/10.5772/intechopen.86608

by the type of metal cation (Mg2+ and Al3+). Although LDHs are prepared by three different ways, the metal cation of Mg2+ and Al3+ still keep the same without any change. Similar observations can be found in the research of Wang et al. for the formation of LDHs by co-precipitation and IEP method [20]. Concerning the lattice parameter c, it was affected by three main factors: the average charge of the metal cations, the nature of the interlayer anion and the water content. TU-LDHs have lowest lattice parameter c compared to other two LDHs. All the samples use the same anion and metal cations so that the slight difference between samples may be related to a slow water content in the layer.

The crystalline size of particles was calculated using the Debye-Scherrer's formula

$$\mathbf{D} = \frac{\mathbf{0}.89\lambda}{\beta \cos \theta} \tag{5}$$

where D is the grain size (nm), λ is the wavelength of the X-ray radiation, β is the full width at half maximum and λ, θ is the angle of diffraction. The crystallite size along the c direction was calculated according to (003) and (006) diffraction peaks. The values given in Table 1 showed that the crystallite size in the c direction by TU-LDHs and TUC-LDHs were a slightly higher than the crystallite size of CC-LDHs. There is no obvious reduction of crystallite size in the c direction of materials prepared by three different methods.

N2 adsorption-desorption isotherm of MgAl LDHs is given in Figure 3. All the adsorbents show a Type IV isotherm according to the IUPAC classification, which is associated with mesoporous materials [26]. TUC-LDHs and CC-LDHs show a H3 type hysteresis loop, suggesting that the pores are produced by 'slit-shaped' of plate-like particles [27]. This type of isotherm is commonly observed in the mesoporous stacking structure of sheet-like 2D crystallites [28]. In the case of TU-LDHs, it shows a H2 type hysteresis loop corresponding to a complex and interconnected pore structure, indicating that the pores are produced by rapid nucleation process. This difference could be related to the different synthetic route applied in this work. The surface area (SBET) of the samples were determined using BET (Brunauer, Emmett and Teller) model. The total volumes (VTotal) were calculated according to the amount of nitrogen (N2) absorbed at a relative pressure (P/PO) of 0.99. The pore volumes were calculated from the desorption branch of the isotherms using the Barrett-Joyner-Halenda (BJH) method, for the pores between 1.7 and 300.0 nm. Table 2 lists the textural parameters of MgAl LDHs obtained from N2 adsorption-desorption isotherms. It can be seen clearly that the SBET of TUC-LDHs is the highest of 235.3 m<sup>2</sup> g�<sup>1</sup> compared to 198.7 m<sup>2</sup> g�<sup>1</sup> for TU-LDHs and 148.1 m2 g�<sup>1</sup> for CC-LDHs as well as the pore sizes (90.83 Å) and highest pore


a a=2d<sup>110</sup> <sup>b</sup>

3. Results and discussion

Sorption in 2020s

improving the nucleation of MgAl LDHs.

Figure 2.

128

3.1 Synthesis and characterization of LDHs prepared by different ways

Co-precipitation method is a common method used in preparation of MgAl LDHs materials in the previous studies. The main requirement for improving the CO2 adsorption capacity of MgAl LDHs is to develop it with high surface area and pore volume. It is well known that the formation process of MgAl LDHs includes both nucleation and growth process. Controlling the process of nucleation properly may have an effect on the whole size of MgAl LDHs, thus increasing its surface area. Here, we reported the first synthesize of MgAl LDHs using a hybrid two-step method. At first step, the salt solution and initial concentration of NH3H2O solution were transported into 'T-type' impinging-stream reactor. Two solutions interact on each other on the 'T-type' impinging-stream reactor and ultrasonic wave were also applied in the interaction process. At second step, the conventional co-precipitation method was used. This means that a certain rate of salt solution and NH3H2O were simultaneously added to the mixture to produce the MgAl LDHs. The chemical and physical effects of ultrasonic irradiation originated from acoustic cavitation lead to rapid reaction rate and change the selectivity performance of the reaction [25], thus

The XRD patterns of MgAl LDHs prepared using three different methods are shown in Figure 2. For all the samples, the characteristic reflections based on the structure of (PDF #35-0965) were clearly observed, which can be indexed to (i) (00l) peaks of (003) and (006) with corresponding to the basal spacing and higher order reflections; (ii) (0kl) peaks of (012), (015) and (018) with broad reflections; (iii) sharp (hk0) and (hkl) peaks of (110) and (113). It can be seen that LDHs samples synthesized under different conditions have similar structures. The lattice parameter a and c are calculated according to the parameters of (110) and (003) plane assuming a 3R stacking of layer structure, where the value a (a = 2d110) represents average distance between two metal ions and c is three times of the brucite-like layer and interlayer distance (d003). The lattice parameter a is almost independent of the synthesis method, which can be explained that it was affected

XRD patterns for MgAl LDHs prepared using (a) TU method, (b) CC method and (c) TUC method.

c=3d<sup>003</sup> <sup>c</sup>

Value calculated from (003) and (006), according to Debye-Scherrer's formula.

#### Table 1.

Lattice parameters of LDHs prepared by different methods.

Figure 3. Nitrogen adsorption-desorption isotherm for TU-LDHs, CC-LDHs and TUC-LDHs.


#### Table 2.

Textural parameters of MgAl LDHs obtained from N2 adsorption-desorption isotherms.

volume (0.48 cm<sup>3</sup> g<sup>1</sup> ). Both of TUC-LDHs and CC-LDHs facilitate the macrostructure of pores. Although TU-LDHs has the second largest SBET, its pore sizes (24.4 Å) and pore volume (0.08 cm3 g<sup>1</sup> ) are the lowest in comparison with other two materials. This indicates that the TU method contributes to the formation of mesoporous structure. The increase in surface area of TU-LDHs is very likely caused by the enhanced micromixing in the 'T-mixer', where the use of ultrasonication can intensify the turbulent eddies and those microbubble bursting that erode the surface area of hydrotalcites of layered structure through removal of the interlayer anions [17]. This effect may be more evident as TUC-LDHs compared with CC-LDHs possess smaller size so that the erosion can be taking place for TUC-LDHs. This explanation seems to be supported by SEM and TEM observation of TUC-LDHs.

thickness of 10–20 nm. However, the average size and thickness of CC-LDHs (shown in Figure 4(e)) are significantly larger than in the case of TUC-LDHs. In addition, EDX analysis of TUC-LDHs (shown in Figure 4(d)) is given as a rough molar ratio of MMg/Al ≈ 3, which is consistent with chemical composition of raw

SEM images of MgAl LDHs for (a) TU-LDHs, (b) CC-LDHs and (c) TUC-LDHs; EDS of (d) TUC-LDHs;

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

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

From the above XRD, BET, SEM and TEM results, further analyses are as follows. Mixing is very slow, relying on the molecular diffusion process due to the absence of turbulence in a low Reynold number. However, the mixing can be enhanced by changing the geometry of mixing channel from conventional agitated vessel to 'T-mixer'. Ultrasonic wave, an external energy force, can generate additional force to interfere with the flow field in the 'T-mixer' channel so that a high level of supersaturation is generated and micromixing process of chemical reaction can be enhanced in extremely short time (<sup>10</sup><sup>7</sup> s) [23, 24]. Thus, the high level of

supersaturation contributes to form large amounts of nuclei in a short time, resulting in a small average particle size accumulating together, which is consistent with the BET results in Table 2. Finally, the layered structure of TU-LDHs seems to stack together tightly. For TUC-LDHs, the presence of TU-LDHs during the growth of TUC-LDH may introduce defects into LDH structure, through modified nucleation conditions or induced curvature [29]. The final particles exhibit fluffy shapes

materials.

131

Figure 4.

TEM of (e) CC-LDHs and (f) TUC-LDHs.

The size, morphologies and structure of MgAl LDHs prepared by different methods are characterized using FESEM. It can be clearly seen from Figure 4(a) that the morphology and structure of TU-LDHs looks like a house-of-cards-type stacking structure, which is similar to that from 'exfoliation-self-assembly' method. However, no significant differences can be observed between CC-LDHs and TUC-LDHs, as shown in Figure 4(b) and (c). Both of them show rose-like morphology, while morphology of TUC-LDHs looks quite loose compared with CC-LDHs. In order to further explore the morphology and structure of the prepared LDHs, CC-LDHs and TUC-LDHs were also characterized using TEM analysis. The TEM image in Figure 4(f) shows that the TUC-LDHs has an average size of 100 nm and

Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO2 Adsorption DOI: http://dx.doi.org/10.5772/intechopen.86608

#### Figure 4.

volume (0.48 cm<sup>3</sup> g<sup>1</sup>

Figure 3.

Sorption in 2020s

Table 2.

of TUC-LDHs.

130

(24.4 Å) and pore volume (0.08 cm3 g<sup>1</sup>

Sample name BET surface area (m2 g<sup>1</sup>

). Both of TUC-LDHs and CC-LDHs facilitate the macro-

) are the lowest in comparison with other

) Average pore diameter (Å) Pore volume (cm<sup>3</sup> g<sup>1</sup>

)

structure of pores. Although TU-LDHs has the second largest SBET, its pore sizes

TU-LDHs 198.7 24.4 0.08 CC-LDHs 148.1 85.61 0.28 TUC-LDHs 235.3 90.83 0.48

Nitrogen adsorption-desorption isotherm for TU-LDHs, CC-LDHs and TUC-LDHs.

Textural parameters of MgAl LDHs obtained from N2 adsorption-desorption isotherms.

two materials. This indicates that the TU method contributes to the formation of mesoporous structure. The increase in surface area of TU-LDHs is very likely caused by the enhanced micromixing in the 'T-mixer', where the use of

ultrasonication can intensify the turbulent eddies and those microbubble bursting that erode the surface area of hydrotalcites of layered structure through removal of the interlayer anions [17]. This effect may be more evident as TUC-LDHs compared with CC-LDHs possess smaller size so that the erosion can be taking place for TUC-LDHs. This explanation seems to be supported by SEM and TEM observation

The size, morphologies and structure of MgAl LDHs prepared by different methods are characterized using FESEM. It can be clearly seen from Figure 4(a) that the morphology and structure of TU-LDHs looks like a house-of-cards-type stacking structure, which is similar to that from 'exfoliation-self-assembly' method. However, no significant differences can be observed between CC-LDHs and TUC-LDHs, as shown in Figure 4(b) and (c). Both of them show rose-like morphology, while morphology of TUC-LDHs looks quite loose compared with CC-LDHs. In order to further explore the morphology and structure of the prepared LDHs, CC-LDHs and TUC-LDHs were also characterized using TEM analysis. The TEM image in Figure 4(f) shows that the TUC-LDHs has an average size of 100 nm and

SEM images of MgAl LDHs for (a) TU-LDHs, (b) CC-LDHs and (c) TUC-LDHs; EDS of (d) TUC-LDHs; TEM of (e) CC-LDHs and (f) TUC-LDHs.

thickness of 10–20 nm. However, the average size and thickness of CC-LDHs (shown in Figure 4(e)) are significantly larger than in the case of TUC-LDHs. In addition, EDX analysis of TUC-LDHs (shown in Figure 4(d)) is given as a rough molar ratio of MMg/Al ≈ 3, which is consistent with chemical composition of raw materials.

From the above XRD, BET, SEM and TEM results, further analyses are as follows. Mixing is very slow, relying on the molecular diffusion process due to the absence of turbulence in a low Reynold number. However, the mixing can be enhanced by changing the geometry of mixing channel from conventional agitated vessel to 'T-mixer'. Ultrasonic wave, an external energy force, can generate additional force to interfere with the flow field in the 'T-mixer' channel so that a high level of supersaturation is generated and micromixing process of chemical reaction can be enhanced in extremely short time (<sup>10</sup><sup>7</sup> s) [23, 24]. Thus, the high level of supersaturation contributes to form large amounts of nuclei in a short time, resulting in a small average particle size accumulating together, which is consistent with the BET results in Table 2. Finally, the layered structure of TU-LDHs seems to stack together tightly. For TUC-LDHs, the presence of TU-LDHs during the growth of TUC-LDH may introduce defects into LDH structure, through modified nucleation conditions or induced curvature [29]. The final particles exhibit fluffy shapes

and the flake-like structures are formed. Another important reason may be related to the isoelectric point (IEP) of MgAl LDHs. The IEP of MgAl LDHs is around 10, so it can be positively (pH > IEP), neutrally (pH = IEP), or negatively (pH < IEP) charged depending on the relationship between the IEP and the pH. Because it is electronic neutrally on its surface, its growth is inhibited due to the repulsive force between negatively charged TUC-LDH primary particles and Al(OH)4 , CO3 2, OH anions. Hence, the formation rate of LDHs is so fast that the growth in all directions under such basic conditions. It is well known that the formation process of TUC-LDH includes both nucleation and growth process. In our experiment, the growing of TUC-LDH is based on the mixture solution, which suggests that it supplies a nucleation environment for TUC-LDH growth, then the directions of LDHs growth could be determined. As reported in literature, the deposition of a colloidal suspension of TUC-LDH on substrates, such as glass or silicon, generally leads to the TUC-LDH nanoplatelets having preferred orientation with their c-axis perpendicular to the substrate. The fact that the MgAl-CO3 LDH nanoplatelets are perpendicularly attached to the surface via their edges suggests they are grown onto the substrate via a strong chemical interaction. Wang et al. reported the first synthesize of nanosized spherical MgAl LDHs using IEP method [20].

(14%) was found to occur between 50 and 170°C. In the temperature range of 50 and 214°C, TUC-LDHs observed a weight loss of 14%. There are no much differ-

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

The CO2 capture capacity of the above-mentioned three types of LDHs, including TU-LDHs, TUC-LDHs and CC-LDHs, were evaluated using isothermal CO2 adsorption tests. In the present work, we are particularly interested in whether the modified methods would influence the final CO2 adsorption capacity or not. The CO2 adsorption capacities of the different types of LDHs are given in Figure 6. All the samples were first calcined at 500°C before each CO2 adsorption test. Then, the thermogravimetric adsorptions of CO2 on the samples were measured at 80, 150 and 200°C using a TGA analyser. It can be found that the CO2 adsorption capacities of TU-LDHs, CC-LDHs and TUC-LDHs at 80°C are 0.30, 0.22 and 0.28 mmol g<sup>1</sup>

respectively. While for the sample at 200°C, the CO2 adsorption capacities of TU-

The CO2 adsorption capacity of LDHs at 200°C was higher than that of 80 and 150°C. The main reason maybe that a surface phenomenon and chemical interactions are restricted at 80°C due to the higher activation energy [19]. We can see that TUC-LDHs had better adsorption capacity than other two TU-LDHs and CC-LDHs independent of adsorption temperature. The removal of water during calcination process leads to the formation of channels and pores. As the BET surface area of TUC-LDHs is higher than TU-LDHs and CC-LDHs, it could increase more basic sites for CO2

In order to get a good adsorbent, fast adsorption kinetics is considered as one of the most important parameters to evaluate the adsorbent in a dynamic process. Hence, the ability of withstanding large adsorbate flows are connected with the rate of adsorption. Here, we mainly study the PFO model and Avrami model on the function of CO2 adsorption on our LDHs [30]. The general sorption rate equations

,

, respectively.

ences during the second weight loss process among three samples.

LDHs, CC-LDHs and TUC-LDHs are 0.33, 0.25 and 0.36 mmol g<sup>1</sup>

3.4 Kinetic studies of CO2 capture performance

CO2 adsorption capacities of the different types of LDHs at different temperatures.

3.3 CO2 adsorption capacity

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

adsorption [7].

are expressed as:

Figure 6.

133

#### 3.2 TGA analysis

Calcination is a very important step for activating the MgAl LDHs, because the fresh LDHs do not contain much basic sites so that the CO2 uptake capacity is quite low. Figure 5 shows the TGA spectrum of the three different types of synthesized LDHs. The curves for the LDHs prepared by three methods are all fairly similar in shape. The TGA spectra of TU-LDHs show a weight loss of 13% between 50 and 207°C due to the loss of the physisorbed water. In the second weight loss, 33.2% occurred between 207 and 600°C, which is mainly caused by dehydroxylates and decarbonates of LDHs to a large extent, finally leading to the formation of a mixed oxide with a three-dimensional network [8]. For CC-LDHs, the first weight loss

Figure 5. TGA spectrum of the three different types of synthesized LDHs.

Hybrid Two-step Preparation of Nanosized MgAl Layered Double Hydroxides for CO2 Adsorption DOI: http://dx.doi.org/10.5772/intechopen.86608

(14%) was found to occur between 50 and 170°C. In the temperature range of 50 and 214°C, TUC-LDHs observed a weight loss of 14%. There are no much differences during the second weight loss process among three samples.
