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Additive introduction based on porous amine-silica hybrid materials is one of the key technologies to enhance the ability of CO2capture. Hence, we first developed a one-step strategy of directly introducing brush-like cetyltrimethylammonium chloride (CTAC) and bis-(γ-triethoxysilylpropyl)-tetrasulfide (BTES) additives into the silica support to prepare micro-mesoporous organosilica nanoparticles with double brush-like additives modified (MMON-D). Then, the amine-functionalized CO2adsorbent (MMON-D-50) was obtained via tetraethylenepentamine impregnation. The adsorbent with double additives exhibited a higher CO2adsorption performance (4.04 mmol/gadsorbentand 0.33 mmol CO2/mmol N under pure CO2and 25 °C conditions) than that of the adsorbents with none or a single additive. Furthermore, spectra analysis, kinetics, and adsorption heats were performed to clarify the adsorption mechanism of the synergistic action of CTAC and BTES. The results revealed that CTAC as a quaternary ammonium formed on the adsorbent surface can facilitate the formation of carbamic acid. Meanwhile, BTES as a swelling agent endows silica support with a large pore size and inhibits the formation of silylpropyl carbamate to enhance CO2adsorption performance. This work supplies a novel surface-modified strategy for developing adsorbents with excellent CO2capture performance, which is of prime importance for both academic interests and practical applications.

Graphical Abstract:

Solid adsorbents modified with N-containing functional groups have been extensively studied due to their excellent CO2adsorption performance. In this work, an adsorbent was obtained to capture CO2by impregnating polyethylenimine (PEI) into MCM-41 with a reserved template that had different alkyl chain lengths [myristyltrimethylammonium bromide (C14), cetyltrimethylammonium bromide (C16), and stearyltrimethylammonium bromide (C18)]. The thermogravimetric analysis, small-angle X-ray diffraction, N2adsorption–desorption, Fourier transform infrared (FTIR) spectroscopy analysis, and in situ FTIR spectra were employed to study the properties of adsorbents. The results showed that the template had been successfully introduced into the supports in the synthetic process, and the CO2adsorption performance was quite sensitive to the alkyl chain length of the template. Besides, the adsorbent with the template (C16) exhibited the highest CO2adsorption capacity (2.49 mmol/g) and amine utilization rate (0.28 mmol CO2/mmol N) under 25 °C and pure CO2. It is believed that the appropriate alkyl chain length of the template could act as an important role in the enhancement of PEI distribution and decrease the CO2transfer resistance, thereby improving the CO2adsorption capacity.

Amine adsorbents have been widely investigated for direct CO2capture from air. However, the low amine efficiency of the widely-studied amine sorbents is the critical factor to hinder the practical application under dry conditions. Herein, we developed the brush-like cetyltrimethylammonium chloride (CTAC) templating strategy to prepare micro-mesoporous silica nanoparticles (MMSN-X) and TEPA-functionalized adsorbents (MMSN-X/TEPA-Y). The character of samples was investigated by transmission electron microscopy, N2adsorption-desorption, Fourier transform infrared spectroscopy, thermal gravimetric analyzer and X-ray photoelectron spectroscopy. The adsorbent (MMSN-30/TEPA-50) with appropriate amount of brush-like CTAC exhibited higher CO2adsorption capacities (3.68 mmol/gadsorbentunder simulated dry air conditions) than that of the adsorbents with none or less additives. The kinetic and thermodynamic model were applied to investigate the adsorption rate control factors and adsorption behavior of CO2adsorbents with brush-like CTAC. Moreover, with the effect of brush-like CTAC, the significant variation of adsorption product (conversion from carbamate (CO2/N = 0.5) to carbamic acid (CO2/N = 1)) was detected by in situ Fourier transform infrared spectroscopy. These results demonstrated that the combination of CO2and amine was altered in the presence of brush-like CTAC and thereby the amine efficiency was enhanced, which could be expected to be an efficient adsorbent for CO2from air in practical applications.