Encapsulation of polymers in CTA-MCM-41 via microemulsion
Graphical abstract
Introduction
Since the discovery by Mobil of the type MCM-41 mesoporous silicas [1], the synthesis of which employs surfactants to guide the structure, efforts have been made to modify the structure of the channels, for example causing expansion [2] or producing composites [3]. Increasing interest in the synthesis of materials with hybrid organic and inorganic composition has opened up a wide new field of research [4], [5]. Of particular interest is the insertion of monomers into the channels of the calcined inorganic matrix, followed by polymerization [6]. The objective is to improve the mechanical and thermal stability of the final material by means of the presence of the inserted polymeric phase [7].
The as-synthesized MCM-41 (designated here as CTA-MCM-41) contains the surfactant in the interior of the channels, and exhibits basic catalytic properties. This is due to the presence of siloxy anions (SiO−) associated with cetyltrimethylammonium (CTA) cations, as shown in Fig. 1 [8]. The reactions of industrial interest in which these solids can be used are the Knoevenagel condensation [9], [10] and the transesterification of vegetable oils [11]. In the latter case, a conversion rate of 65% has been achieved using the silica in the transesterification reaction of canola oil with ethanol. Loss of activity of the catalyst after successive usages has been associated with leaching of the CTA cations from within the silica channels [11], [12].
The motivation for the present work therefore arose from the need to stabilize the activity of this catalyst. It was proposed to modify the synthesis by employing a new procedure involving the encapsulation of polymers in the interior of the CTA-MCM-41channels. Since the polymers would be formed in the interior of the channels, together with the micelles, it was anticipated that they would interact with the surfactant, impeding the exit of the latter when the molecular sieve was in contact with a liquid medium. Use of polymerization in microemulsion for encapsulation of polymers in the interior of the channels of CTA-MCM-41 has not been previously reported in the literature as a method for modification of the synthesis.
Microemulsions are transparent and thermodynamically stable, and are formed spontaneously by mixing water, emulsifier, and monomer, without any need for vigorous agitation [13], [14], [15], [16]. The polymerization proceeds with the aid of an initiator (soluble in water or organic solvent) whose radicals are formed by application of heat or ultraviolet radiation [17]. When an initiator soluble in organic solvent is used, the polymerization only occurs within the micelles [18], where the three fundamental stages of a radical-based polymerization occur: initiation, propagation, and termination [19]. Polymerization in microemulsion is characterized by the formation of a turbid dispersion of emulsified polymers [20]. It has been reported that the polymerization occurs in the liquid phase within the micelles, which are maintained stable by the presence of the surfactant [21].
The objective of this work was to synthesize the mesoporous silica CTA-MCM-41, with the channels filled with polymers. To this end, polymerization in microemulsion was employed, with the step of formation of an emulsified polymeric phase being followed by the steps used in the conventional synthesis of MCM-41, as described in the literature, using the addition of a base for hydrolysis of the source of the silica and ageing. Details of the synthesis are provided in the Experimental section.
Section snippets
Material preparation
The synthesis of the mesoporous silicas was based on two the synthesis of the mesoporous silicas was based on two previous studies: Schumacher et al. [22] and Grün et al. [23]. The procedure was followed as described, however alcohol was not added in the synthesis. This was because the monomers employed are soluble in ethanol, which would have resulted in their removal from the interior of the micelles. After adaptation of the procedure, the new compositions of the reaction mixtures were SiO2:
Results and discussion
Table 2 presents the solubility of (a) the monomers in the aqueous solution of CTABr (%), and (b) the photoinitiator in the monomer (% by mass). The aqueous solutions prepared with these monomers presented turbidity after submission to UV radiation.
Fig. 5 presents the SAXS curves for the aqueous solutions with different butyl methacrylate/CTABr molar ratios. According to Aswal et al. [24], the most intense band, near q = 0.6 nm−1, is related to X-ray scattering in the nucleus of the micelle, and
Conclusions
This work presents a new methodology for incorporating polymers in the interior of mesoporous CTA-MCM-41 silica, employing microemulsion. Small-angle X-ray scattering analysis of aqueous solutions of the CTABr surfactants confirmed that the monomers used were emulsified by the surfactant, and remained in the hydrophobic interior of the micelles. Part of the polymers continued emulsified after exposure to ultraviolet radiation. Synthesis of MCM-41 in these emulsions resulted in an expansion of
Acknowledgments
The authors thank CNPq, CAPES, and PRH-ANP/MCT for grants, and the Brazilian National Synchrotron Light Laboratory (LNLS) for the small-angle X-ray scattering (SAXS) analyses.
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