(373c) Preparation of Amine Modified Bimodal Mesoporous Silica Particles for CO2 Separation

Mesoporous silica has been extensively studied for many applications such as CO₂ adsorbents, drug delivery, catalysis, and so on. Among them, bimodal mesoporous silica (BMS), which had two different sizes of mesopores, have drawn much attention recently because it exhibited unique diffusion properties in the pores. For this reason, when BMS is modified to have affinity with CO₂, it is expected that high CO₂ adsorption capacity is obtained by small pores and fast adsorption rate is achieved by large pores.

Therefore, we prepared various kinds of amine modified BMS, and the effects of pore structure of BMS samples and the type of introduced amine on CO₂ adsorption/desorption properties were examined in this study.

In the experiments, BMS were prepared by co-condensation of two different silica precursors. Tetraethylorthosilicate (TEOS) and triblock copolymer Plulonic 123 (P123) were used as silica precursors and templating agent, respectively. Polymethylhydrosiloxane (PMHS) was not only used as silica precursors but also as template for large pores. First, various BMS samples were prepared with different PMHS concentration, and the pore structures of the resulting samples were investigated.

The results showed that three types of mesoporous silica with different pore structures (monomodal and bimodal) were successfully prepared and it was found that the pore diameter and the pore volume could be controlled by changing PMHS concentrations. To introduce amine groups to the BMS samples, 3-Aminopropyltriethoxysilane (APTES), 3-(2-Aminoethoxyl amino)propyltriethoxysilane (AEAPTES), 3-[2-(2-Aminoethylamino)ethylamino]propyltrimethoxysilane (AEAEAPTES) were used.

It is known that when conventional monomodal mesoporous silica is modified with AEAEAPTES, although introduced amount of amine is large, amine utilization efficiency is low because long chain length of the moiety plugs the pores and diffusion of CO₂ in the pores is inhibited. On the other hand, our experimental results showed that although the samples were modified with AEAEAPTES which had longest amino group, and large amount of amino groups were introduced, the amino group utilization efficiencies for CO₂ capture within short period adsorption (30 min.) were still high. This is probably due to better pore structure of our samples compared to conventional mesoporous sample.

In conclusion, in this study, preparation procedure of bimodal mesoporous silica (BMS) with different pore structure was established. Then, the effect of pore structure on CO₂ adsorption characteristics was examined using the resulting BMS samples after modifying with various amino groups. The results showed that by applying BMS for CO₂ adsorption, it was found that high CO₂ adsorption capacity was achieved by small pores and fast adsorption rate was achieved by large pores.