ABSTRACT
A straightforward synthesis of a series of hybrid organic-inorganic materials (HOIMs) containing imidazolium moieties was achieved. The preparation of the imidazolium acetate precursor was performed in a single-step procedure using the Debus-Radziszewski reaction. The as-synthesized alkoxysilane was employed in combination with tetraethyl orthosilicate to generate an HOIM presenting a high specific surface area. Two different structure-directing agents (SDAs), an anionic (sodium dodecyl sulfate (SDS)) or a cationic (cetyltrimethylammonium bromide) surfactant, were used to investigate the role played by the SDA on the distribution of the imidazolium-based active sites within the silica structure. After the synthesis, the acetate ion was replaced with Cl- and Br- via a simple acid treatment. This procedure favors also the removal of the surfactant, thus releasing the porosity of the solids. The HOIMs synthesized were fully characterized via low-angle X-ray diffraction, N2 physisorption, transmission electron microscopy, 13C and 29Si MAS NMR, combustion chemical analysis, X-ray photoelectron spectroscopy, and CO2 physisorption to assess their physicochemical and structural features, as well as the successful incorporation of imidazolium salts. Their catalytic activity in the conversion of CO2 was tested over different epoxides to produce the corresponding cyclic carbonates. The key role of the SDS (anionic surfactant) as a templating agent was proved. The best material was stable under the selected reaction conditions, reusable over multiple cycles, and active on a series of different epoxides, thus proving its versatility.
ABSTRACT
Three different carbon nanoforms (CNFs), single-walled and multi-walled carbon nanotubes (SWCNTs, MWCNTs) and carbon nanohorns (CNHs), have been used as supports for the direct polymerization of variable amounts of a bis-vinylimidazolium salt. Transmission electron microscopy confirmed that all CNFs act as templates on the growth of the polymeric network, which perfectly covers the nanocarbons forming a cylindrical (SWCNTs, MWCNTs) or spherical (CNHs) coating. The stability of these hybrid materials was investigated in the conversion of CO2 into cyclic carbonate under high temperature and CO2 pressure. Compared with the homopolymerized monomer, nanotube-based materials display an improved catalytic activity. Beside the low catalytic loading (0.05-0.09 mol%) and the absence of Lewis acid co-catalysts, all the materials showed high TON values (up to 1154 for epichlorohydrin with SW-1:2). Interestingly, despite the loss of part of the polymeric coating for crumbling or peeling, the activity increases upon recycling of the materials, and this behaviour was ascribed to their change in morphology, which led to materials with higher surface areas and with more accessible catalytic sites. Transmission electron microscopy analysis, along with different experiments, have been carried out in order to elucidate these findings.
