Full length articlePhase evolution during one-pot synthesis of amine modified mesoporous silica materials: Preparation, properties, carbon dioxide adsorption
Graphical abstract
Introduction
Self-organized aggregates of surfactants and block copolymers are widely used in chemistry as structure directing media in the synthesis of a variety of nanostructured materials [1]. These aggregates of surfactants are used as templates, whose size and structure determine the properties of the newly-formed material. In the last two decades, a great deal of scientific attention was paid to the synthesis of systems containing mixed inorganic and organic components, in which organic structure-directing agents interact with the inorganic component across a hydrophobic-hydrophilic interface to achieve desirable macroscopic properties in the final product material [2].
Soon after the first synthesis of the mesoporous silica MCM-41 by Beck, Kresge, and co-workers in 1992 [3,4], this approach led to a series of discoveries in materials chemistry. By adjustment of the synthesis conditions, ordered mesoporous materials with various hexagonal (MCM-41), cubic (MCM-48) and lamellar (MCM-50) structures were prepared. All three phases in the M41S family can be synthesised by slightly varying the reaction conditions, but the stability decreases in the order: lamellar phase, hexagonal MCM-41, cubic MCM-48 [5].
With the goal of extending the range of properties of self-assembled nanoporous materials, further scientific attention focused on producing hybrid inorganic-organic solids in which an organic moiety (e.g. amine ligand) is covalently linked to the silica backbone. The incorporation of the organic ligands is usually carried out in two ways: a.) by covalent binding of the ligands on the inorganic walls of the material by treatment of the calcined porous materials with organosiloxanes (i.e. post-synthesis treatment, called grafting) [6]; b.) by direct incorporation of the organic functions during the synthesis process, involving co-condensation of tetraalkoxysiloxanes, such as tetraethyl orthosilicate (TEOS), with organosiloxanes, such as 3-aminopropyltriethoxysilane (i.e. one-pot synthesis) [7]. The former synthesis pathway has several shortcomings. First, the attachment of a layer of functional groups on the pore surface results in a reduced pore size and pore volume, being often undesired as it can lead to pore blockage. Secondly, the loading level of the functional groups, which can be grafted on the surface, is limited because of the limited density of the reactive surface silanols [8]. Moreover, grafting is significantly hindered because of the restricted access to most of the surface active groups located in the microporous domains of the material [7,8].
Compared with the post-synthesis grafting, the co-condensation method presents several advantages, such as homogeneous distribution of the amine groups, controllable incorporation amount and avoidable pore blockage [[9], [10], [11], [12]]. Co-condensation offers a higher and more uniform surface coverage of functional groups and a better control over the surface properties of the resultant materials [[9], [10], [11], [12]]. This method has been widely employed to functionalize the mesoporous materials by various functional groups, such as aliphatic hydrocarbon, phenyl, thiol, amine, sulfonic groups, etc. [10,[13], [14], [15], [16], [17]]. However, sometimes, especially at higher concentrations of organosiloxanes, the co-condensation may lead to nonporous or microporous solids [7,18].
Our interest focuses on the use of ordered nanoporous silica materials for carbon dioxide adsorption. It is well documented that functionalization of porous silica with amine ligands leads to the formation of solid porous materials with high affinity to carbon dioxide. Different types of amines (mono-, di-, tri-, polyamines) were used for the silica functionalization [[19], [20], [21], [22], [23], [24]]. In such materials, the adsorption of carbon dioxide is related to amount of the surface amine centers, amine surface density [[25], [26], [27], [28], [29], [30], [31]], but the adsorption of carbon dioxide is influenced also by the basicity of amine ligands [32], the dimensionality of the silica matrix [33] as well as by entropy-driven effects [34].
In our previous work, dealing with the influence of amine basicity on carbon dioxide adsorption [32], we found that the carbon dioxide sorption capacities ranged between 0.68 mmol/g and 1.04 mmol/g, depending on the type of amine. In the light of these results, our interest turned to the effect of incorporating a larger amount of functional groups into the mesoporous silica materials, while using the same amine ligands. Since the co-condensation offers the possibility of homogeneous distribution of the amine groups and their controllable incorporation, in the present work we have focused on the one-pot synthesis of MCM-41-like materials modified with different amines. For the co-condensation with TEOS, we have used the three amino-organosiloxanes reported in Scheme I, namely 3-aminopropyltrimethoxysiloxane (APS), 3-(methylamino)propyltrimethoxysiloxane (MAPS) and 3-(phenylamino)propyltrimethoxysiloxane (PAPS), with the aim of producing materials with hexagonal symmetry and high amine loading. We have observed that the ordered porous structures were obtained only at small ratios of amino-organosiloxane precursors to TEOS and, in some cases, a phase change from the hexagonal to lamellar structure was observed. Even though the synthesis of amine modified silica materials prepared by co-condensation method has been described in the literature, to the best of our knowledge, a comparative study of the influence of concentration of three different amines (APS, MAPS, PAPS) on the phase evolution of the resulting material is missing. Not only does our work report on the synthesis and structural properties of MCM-41-like porous materials, but it also provides an additional insight from molecular simulation, which is key to understand some details that are not easily accessible by experiments. Moreover, to test the performance of our materials, we also analyse their ability of adsorbing carbon dioxide under a spectrum of conditions.
Section snippets
Experimental
All chemicals were purchased from Sigma-Aldrich® and used as received without further purification. TEOS (97%) was used as silica source and cetyltrimethylammonium bromide (CTAB) was used as the structure directing agent. APS, MAPS, PAPS were used as a sources of amines (see Scheme 1). These amino-organosiloxanes were mixed with TEOS in different wt% ratios during the synthesis.
Results and discussion
As mentioned above, there are two main methods to modify periodic nanoporous silica (PNS) by amine functionalities: grafting and one-pot synthesis. The method of grafting is based on the reaction of surface hydroxyls of PNS with alkoxysiloxanes bearing organic functional group (e.g. amine). Therefore, the amount of grafted ligands (amines), as active sites for carbon dioxide capture, strongly depends on the amount of the surface hydroxyls.
Another approach to load various amounts of amines on
Conclusions
In summary, APS, MAPS, and PAPS modified silica materials were prepared by direct co-condensation of TEOS and respective amino-organoalkoxysilanes. The well-ordered nanoporous materials were prepared in low concentrations of amino-organosiloxanes in the synthesis gel. The unit cell parameter of the hexagonally ordered amino-functionalized MCM41 samples is strongly dependent on the content of amino-organoalkoxysilanes, their type and size, suggesting that the interaction between the more
Acknowledgements
This work was supported by the Slovak Research and Development Agency under the contract APVV-15-0520 and by the Scientific Grant Agency of the Slovak Republic (VEGA) project no. 1/0745/17. The authors thank the Ministry of Education, Science, Research and Sport of the Slovak Republic and Accreditation Commission of the Slovak Republic for the financial support of the TRIANGEL team in the frame of the scheme Top Research Teams in Slovakia. AP and FRS acknowledge funding from the European
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