Amine modified LDH for CO2 capture at different temperatures

Fig. 5 shows the CO2 adsorption capacity of amine modified LDHs at four different temperatures, where each sample had the same pre-treatment as described above. The amount of carbon dioxide adsorbed on MgAl N1 was about 1.0 mmol g−1 in the temperature range tested, which is within the range of values 0.57–2.05 mmol g−1 reported in the literature for CO2 adsorption on APTES modified mesoporous silica (Hiyoshi et al., 2005Huang et al., 2003Zelenak et al., 2008). The amine efficiency is defined as the ratio of the amount of CO2 adsorbed to the number of amine groups in the sample and thus provides a measure of how effective the grafted amines are in augmenting the adsorption capacity of the materials for CO2. The amine efficiencies for the samples tested in this study were in the range of 0.41–0.48 mol CO2 mol−1 N, which is close to the maximum efficiency of 0.5 based on carbamate formation mechanism (Satyapal et al., 2001). Under dry conditions, two moles of amine groups are required to remove every mole of CO2 to form carbamate.”


Fig. 5. CO2 adsorption capacity of amine modified layered double hydroxides at 25, 40, 60 and 80 °C.”

“For the samples MgAl N2 and MgAl N3, the grafted aminosilanes not only have primary amine end groups, but also have secondary amines in their linkers. MgAl N2 had a CO2 adsorption capacity of 1.1 mmol g−1 at 25 °C, which decreased to 0.8 mmol g−1 at 40 °C. At 60 and 80 °C, the CO2 adsorption capacity increased to 1.2 mmol g−1. Although the CO2 adsorption capacity of MgAl N2 was higher than that of MgAl N1, the amine efficiency of MgAl N2 was in the range of 0.15–0.24, which was less than that of MgAl N1. The CO2 adsorption capacity of MgAl N3 showed a steady increase from 0.74 mmol g−1 at 25 °C to 1.75 mmol g−1 at 80 °C. The trend is different from the previous results of hexagonal mesoporous silicas (HMSs) modified by DAEPTS, the same aminosilane used for preparation of MgAl N3, reported by Knowles et al. (2006). They reported that the CO2 adsorption capacity of the DAEPTS modified HMS decreased from 1.34 mmol g−1 at 25 °C to 0.45 mmol g−1 at 75 °C. This suggests that positively charged MgAl brucite-like layers might affect CO2 adsorption in a way which is different from silica based supports. The amine efficiency of MgAl N3 increased from 0.12 at 25 °C to 0.39 at 80 °C. Knowles et al. (2006) reported the AEAPTS and DAEAPTS modified HMSs had an amine efficiency of about 0.3. The authors suggested that the lower amine efficiencies than APTS modified HMS were due to reduced accessibility of CO2 to the surface amine groups, as longer hydrocarbon chains might lead to reduced mobility and relative proximity of amine pairs. Comparing adsorption capacities in the current study with those reported in the literature, Lwin and Abdullah (2009) reported adsorption capacity of 30 mg g−1 (0.68 mmol g−1) at 25 °C for CuAl-1.0 hydrotalcite, which decreased to 22 mg g−1 (0.5 mmol g−1) at 150 °C. The adsorption of 0.74 mmol g−1 recorded in this study at 25 °C was only slightly higher than that measured by Lwin and Abdullah (2009), but the marked increase in adsorption that occurred with increasing temperature up to 80 °C in this work was in contrast with their study. This suggests that the amine modified adsorbents reported here would be useful in higher temperature capture processes, such as post-combustion capture.”

Leave a Comment