Hybrid silica with bimodal mesopore system: Synthesis and catalytic evaluation

https://doi.org/10.1016/j.molcata.2015.12.016Get rights and content

Highlights

  • Hybrid silicas were synthesized via emulsion polymerization.

  • SAXS of dispersions containing surfactant and monomers showed swollen micelles.

  • Micellar expansion depends on the length of the monomer alkyl chain.

  • The polymers inside the CTA–MCM-41 channels improved the activity stability.

Abstract

As-synthesized CTA–MCM-41 silica possesses basic siloxy (triple bondSiO-) catalytic sites, enabling it to be used in reactions such as transesterification and Knoevenagel condensation. Its application in basic catalysis has shown promising results, but attempts to reuse it in more than one cycle has revealed rapid loss of activity due to the removal of cetyltrimethylammonium (CTA) cations from the interior of the channels. The present work describes the encapsulation of nanoparticulate methacrylic polymers during the conventional synthesis of MCM-41, resulting in hybrid silicas containing polymers in the interior of the inorganic and organic matrix. Aqueous dispersions containing surfactant and monomers were characterized by small-angle X-ray scattering (SAXS), which confirmed the expansion of the micelles due to the presence of the encapsulated monomers. Transmission electron microscopy (TEM) was used to observe the formation of polymer nanoparticles stabilized by the surfactant (cetyltrimethy lammonium bromide). Infrared absorption spectroscopy and chemical analysis showed that these particles were present in the material after the synthesis. Analysis of the silicas using SAXS and nitrogen physisorption showed the expansion of the channels of the CTA–MCM-41 and the formation of two mesopore systems in the silica. Catalytic evaluation demonstrated that the silicas synthesized in the presence of the emulsified polymers possessed better catalytic stability when low monomer contents and monomers with larger alkyl chains were used. The encapsulation of nanoparticles of methacrylic polymer in the interior of the CTA–MCM-41 channels helped to avoid leaching of the CTA cations during the reaction.

Introduction

The growing demand for energy in the industrialized world, together with the problems of pollution caused by the use of fossil fuels [1], [2], [3], has given impetus to the production and use of fuels considered clean, especially biofuels such as bioethanol and biodiesel. The advantages of these fuels include decreased emissions of carbon dioxide, sulfur dioxide, unburned hydrocarbons, and particulates [4], [5], [6]. The production of biodiesel usually involves the use of transesterification [7], [8], [9]. The catalysts employed in the reaction include: (a) acidic catalysts, which are associated with low reaction rates and require high methanol/oil ratios, (b) basic catalysts, which provide shorter reaction times and require mild conditions of temperature and pressure, and (c) enzymatic catalysts, which can be inactivated in the presence of large quantities of alcohol, produce glycerol as the main product, and can only be used at low temperatures [10], [11], [12], [13]. Biodiesel is produced commercially by homogeneous catalysis, with disadvantages including corrosion of equipment and high costs associated with the neutralization of residues and separation of the product from the catalyst [9], [14], [15]. For these reasons, heterogeneous catalysts offer a promising option for the transesterification reaction, because they can be easily separated from the reaction mixture without the need for solvents, and are easily regenerated. They are also less corrosive by nature, safer, and less harmful to the environment [16], [17], [18].

A variety of solids have been used in transesterification processes, notably as-synthesized MCM-41 silica (here denoted CTA–MCM-41), whose basic properties are due to the presence of siloxy (triple bondSiO-) anions in the mouths of the pores. This characteristic was first identified by Kubota et al. [19]. and Martins et al. [20], who evaluated this catalyst for use in the Knoevenagel condensation reaction. Fabiano et al. [21]. investigated the use of as-synthesized MCM-41, MCM-48, and MCM-50 silicas in the catalytic transesterification reaction of canola oil, and obtained methyl ester conversions exceeding 60%. However, when they were reused in successive batches, these silicas suffered from progressive loss of activity. This deactivation could be explained by weak bonding between the CTA cations and the siloxy (triple bondSiO-) groups present in the silica matrix [20].

The main objective of this work was to synthesize hybrid silicas containing nanoparticles of polymer encapsulated together with the CTA cations, in order to improve the catalytic stability of the silicas. The nanoparticles were prepared by the emulsion polymerization method [22], in which cetyltrimethylammonium bromide (CTABr) is used to stabilize the polymer particles. The encapsulation was achieved by successive hydrolysis and condensation reactions of the silica source (tetraethyl orthosilicate) on micelles swelled by the polymer. Catalytic activity was evaluated using the transesterification of ethyl acetate with methanol as a model reaction.

Section snippets

Synthesis of CTA–MCM-41

The synthesis of the CTA–MCM-41 silica was based on the work of Schumacher et al. [23], with modifications. The composition of the synthesis mixture employed was SiO2:12.5NH3:0.4CTABr:174H2O:4EtOH. The addition of alcohol used in the original synthesis was eliminated in order to avoid solubilization of the monomers evaluated (the four molecules of alcohol in the mixture used resulted from hydrolysis of the silica source). The following sequence of steps was employed: (1) the surfactant

Modification of CTA–MCM-41 (encapsulation of the polymer)

Modification of the synthesis of CTA–MCM-41 involved addition of the monomer and the photoinitiator (benzoin, Aldrich) to CTABr in an aqueous mixture, until a clear dispersion was obtained. Ultraviolet irradiation (λ  200 nm) was then applied for 12 h. This resulted in the formation of a white-colored turbid dispersion, indicating the formation of emulsified polymer particles. This characteristic is typical of aqueous dispersions containing emulsified polymer produced by the emulsion

Results and discussion

Fig. 1 presents the SAXS curves obtained for the aqueous dispersions of surfactant with butyl, octyl, and dodecyl methacrylate. According to Aswal et al. [30], the most intense peak (q   0.5 nm−1) is due to X-ray scattering caused by the micelles, while the least intense peak (q  1.0 nm−1) results from scattering by the Br anions arranged around the micelles. The curves revealed that the intensity of the first peak (∼0.6 nm−1) increased in line with the monomer/surfactant ratio (R), indicating the

Conclusions

Modification of the synthesis of CTA–MCM-41 resulted in silicas with channels that were expanded due to the presence of polymer nanoparticles in their interior. The emulsion polymerization method produced polymer particles with a broad pore diameter distribution, which then generated silicas with a bimodal pore diameter distribution. These modified silicas presented improved catalytic stability, with leaching of the CTA cations being hindered by the presence of the polymeric phase in the

Acknowledgments

The authors are grateful for support provided by CNPq. We also thank the Brazilian National Synchrotron Light Laboratory (LNLS) for the small-angle X-ray scattering (SAXS) analyses.

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