Amine-modified MCM-41 mesoporous silica for carbon dioxide capture
Graphical abstract
An easy way to obtain a material with CO2 sorption properties by anchoring 2.48 mmol g−1 of amine groups on commercially available MCM-41 is presented. A few carefully chosen spectroscopic techniques demonstrated that not only amine groups are covalently bounded to mesoporous silica but also the carbamate formation after CO2 uptake experiments. Microcalorimetry experiments performed at room temperature showed that this material has potential for CO2 recovery at low pressures/concentrations.
Highlights
► MCM-41 commercially available was covalently modified with 3-aminopropyl groups. ► Carbamate was formed as a product between the CO2 and amine groups. ► MCM-41-NH2 has CO2 sorption capacity 5.8 times higher than that for MCM-41.
Introduction
Human activities, such as the burning of fuels (oil, coal, petroleum and gas), deforestation and hydrogen production from hydrocarbons (steam conversion and partial oxidation) [1] have increased CO2 concentrations in the atmosphere. CO2 being one of the major greenhouse gases responsible for global warming, the emission control of CO2 has become a serious and challenging research topic [2], [3]. Thus the development of efficient processes for cost-effective CO2 capturing, separation and use is of great importance [2]. Currently CO2 capture involves chemisorption on alkylamine liquids, such as monoethanolamine (MEA), diethanolamine (DEA) and methyldiethanolamine, resulting in carbamate under anhydrous conditions, followed by conversion into bicarbonate or carbonate salts in water [4], [5], [6], [7], [8]. This method has several disadvantages, such as high-energy consumption to regenerate the liquids, equipment corrosion and high viscosity. In order to address these problems, the functionalization or impregnation of amine onto porous supports has attracted much attention [9], [10], [11], [12]. Ordered mesoporous silica MCM-41 [13], [14], MCM-48 [14], SBA-15 [14], [15], [16], [17], [18], [19], SBA-16 [14] and SBA-12 [16], [20] have been investigated for this purpose owing to their high pore volume, large surface area, and ease of functionalization [21], [22], [23].
Amine groups can be grafted onto porous materials, either through impregnation, or by postsynthesis methods. The latter methods have been widely employed for the synthesis of materials investigated for CO2 capture. CO2 uptake in postsynthetically modified mesoporous silica occurs by chemisorption. Infrared spectroscopy has shown that the chemisorbed species are carbamate−ammonium ion pairs, resulting from the quantitative transformation of primary amine groups during CO2 adsorption [11], [24].
The effect of both the mesoporous silica pore size and the nature of the amine groups on the adsorption of CO2 have been studied. Zelenak et al. [20] have investigated the electronic effects caused by 3-aminopropyl (AP), 3-(methylamino)propyl (MAP) and 3-(phenylamino)propyl (PAP) groups bound onto SBA-12 on CO2 capture. They showed that the higher the basicity, the better the CO2 adsorption. In the case of the PAP group, the presence of the aromatic ring led to a decrease in the amine basicity which rendered the adsorption of CO2 energetically much less favorable. MAP and AP presented almost the same CO2 adsorption capacity although the electron donor methyl group was expected to increase its basicity. Steric hindrance effects were probably responsible for the nitrogen of the secondary amine being less accessible to CO2. Adsorption of CO2 on aminopropyl-grafted mesoporous silica with different pore sizes, namely MCM-41 (33 Å), SBA-12 (38 Å) and SBA-15 (71 Å) was investigated and showed that the sorption capacity depended on both pore size and surface density of amine groups [16]. The total amount of CO2 adsorbed was 0.57, 1.04 and 1.54 mmol g−1 for MCM-41, SBA-12 and SBA-15, respectively.
In the current work we describe a simple methodology of anchoring of 3-aminopropyl groups in just one step using commercial MCM-41 and its surface characterization by a variety of analytical and spectroscopic techniques. The potential use of this material as CO2 adsorbent was investigated by isothermal microcalorimetry experiments.
Section snippets
Synthesis of the functionalized MCM-41
Mesostructured MCM-41 silica (Sigma–Aldrich) was previously activated under vacuum and heating at 180 °C for 10 h, before the reaction. 3-Aminopropyltriethoxysilane (Aldrich) was used without prior treatment. Toluene (Vetec, Brazil) was previously dried with sodium under argon.
Functionalization of the MCM-41 silica (Aldrich) with 3-aminopropyltriethoxysilane was carried out using a modified procedure described in the literature [25] in order to improve the amount of amino group bound onto silica
Characterization of MCM-41 and MCM-41-NH2
TEM micrographs (Fig. 2) confirmed that both materials have typical MCM-41 type, highly parallel channel-like porous structure packed in a hexagonal symmetry. MCM-41 functionalization (Fig. 2b) did not affect the hexagonal structure of MCM-41.
N2 adsorption/desorption isotherms of MCM-41 at −196 °C correspond to a typical type IV isotherm characteristic of a mesoporous material (Fig. 3a). Micropores adsorption and multilayer film formation on the pore walls is observed for the initial part of the
Conclusions
This work described an easy way to obtain a hybrid functional material for CO2 capture by anchoring amine groups onto commercially available MCM-41. The samples were characterized by several techniques that proved that amino groups are covalently attached to MCM-41. Nitrogen adsorption/desorption isotherms showed a remarkable surface modification after functionalization, whereas transmission electron microscopy showed that the hexagonal structure of the amine-modified MCM-41 remained unchanged.
Acknowledgments
Authors gratefully acknowledge FAPERJ (Primeiros Projetos, Jovens Emergentes, G.Q.S fellowship), CNPq (Jovens Pesquisadores em Nanotecnologia and Projeto Universal) and Alfa II Nanogastor (European Union) for financial support. C.M.R. is recipient of CNPq research fellowship. We thank Dr. A. Faro (IQ-UFRJ, Brazil) for the DRIFT spectra, Dr. Alvicler Magalhães for solid-state 13C and 29Si NMR experiments.
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