Silica-based monoliths for CO2 capture

https://doi.org/10.1590/1980-5373-MR-2019-0285

“Fig. 12 shows the CO₂ adsorption curves at 30 ºC for the PEHA-functionalized samples. According to the cycles presented in the cited figures, it can be seen that the shapes of the adsorption and desorption curves for the functionalized samples are similar and that the impregnation of amines in the materials had a positive impact on the adsorption. As can be seen, adsorption and desorption curves are more pronounced in the materials incorporated with heteroatoms and functionalized with amines compared to the functionalized and non-functionalized material M1. The CO₂ adsorption kinetics of the functionalized samples prepared in this work is similar to that reported for some adsorbents present in the literature ^34,63.”

“Figure 12 CO₂ adsorption cycles of samples M1, M1-P, Al/M1-P, Ti/M1-P and Zr/M1-P.”

https://doi.org/10.1590/1980-5373-MR-2019-0285

“Table 1 shows the values for the adsorption capacity of PEHA-functionalized materials. It is found that the CO₂ adsorption capacity of the sample M1 had a significant increase after addition of heteroatoms and, subsequently, with amine functionalization. The results showed that Ti/M1-P exhibited the highest adsorption capacity (56.9 mg/g) among the other samples, with an increase of approximately 675% in relation to Ti/M1. For a better visualization of the performance of all the synthesized materials, a bar chart was constructed, which can be observed in Fig. 13. In this chart, it is possible to observe the significant contributions of the heteroatom and amine functionalizations to the adsorption capacity of the resulting materials, both when used separately and combined.”

“Figure 13 Adsorption capacities of all materials synthesized in this work.”

“In view of the obtained results, it could be noticed that the incorporation of heteroatoms modified the surfaces of the materials. One hypothesis to explain this alteration of the surface of the samples is the creation of Lewis and Brønsted acid sites ⁶⁴. The substitution of silicon by the Al³⁺ ion in the silica-based samples, for instance, leads to the formation of stronger Brønsted acid sites on the surface, since the difference between Si and Al atoms is balanced by protonation of an O atom, after each substitution ^34,64. According to Perez-Beltran et al. ⁶⁴, changes caused by aluminum do not imply major changes in the acid properties of the silanol groups initially present on the surface of the samples. On the other hand, the surface acidity properties are improved by the isomorphic substitution of Ti and Zr ⁶⁴. This is because the replacement of silicon with tetravalent ions, Zr⁴⁺ and Ti⁴⁺, results in the formation of acidic Lewis sites which, in turn, are more efficient at retaining amines than the Brønsted acid sites ³⁴. In the Zr insertion mainly Lewis acids sites are formed, but there are also traces of weak Brønsted acid sites. In the incorporation of Ti, only weak Lewis acid sites are produced. The acid-base interaction between the amines and the Lewis acid sites can stabilize or modify the structure of the amines, increasing the amine access to the available CO₂ and, therefore, a greater amount of CO₂ can be captured. In contrast, an inefficient interaction may be a consequence of the contact of the amines with the Brønsted acid sites, which in turn, will not corroborate with the capture of a large number of CO₂ molecules ³⁴.”