(189a) Liquid-Phase Adsorption and Characterization of Carbon-Silica Composites

Porous carbon materials are widely used in separation and purification as they can be easily produced and tuned for a particular application. Their disordered structure is however a disadvantage in many applications where some shape selectivity is required for separation. Ordered porous silica materials have been widely used as templates for ordered carbon materials. When the dissolution of the template is omitted, a carbon-silica composite or CSC is obtained. Preparing larger particles of around 5 µm opens the road towards HPLC applications. In this work, such carbon-silica composites were prepared in different ways. First small MCM-41 particles of 0.4µm have been prepared by the procedure described by Glover and LeVan [1] resulting in a mesoporous silica with 2.1 nm pore diameter. These silica templates were subsequently loaded with carbon using different amounts of carbon precursor (ex. furfuryl alcohol). Their shape selectivity and tuneability have been demonstrated [3]. Larger perfectly spherical particles of 5.3µm as required for use in HPLC were produced by the synthesis method of Galarneau [2]. Nucleosil 100-5 uniform silica particles (5µm particles with 16 nm pore size) were converted to MCM-41 material by pseudomorphic synthesis. Columns of the material were slurry packed and tested in a standard HPLC setup. Liquid-phase experiments were carried out using both batch adsorption and HPLC measurements and indicate presence of polar groups in all the CSC carbon materials, suggesting that the silanol groups of the MCM-41 silica wall remain accessible or that heteroatoms are still present on the surface. The adsorption capacity decreases with carbon content, but exceeds that of zeolitic materials. A large number of screening experiments in HPLC have been performed and trends in retention behavior of several components have been identified. Mobile phases such as hexane + 0, 4, 6% methanol and acetonitrile + 5 to 65% water were used with molecules such as benzene, alkylbenzenes, chlorobenzene, bromobenzene, aniline, phenol, benzyl alcohol, nitrobenzene, xylenes, thiophene, naphthalene, methyl acrylate, alkylparabenes, anthraquinone, propyl acetate. HILIC separations were also performed with an acetonitrile/water mixture and showed an increased retention of all probe molecules at a higher water content of the mobile phase. Homologous series of aromatic components (benzene, toluene, ethylbenzene, propylbenzene) show increased retention in HILIC with increasing number of methylene groups, giving indications of a certain hydrophobic character of the stationary phase. More detailed characterizations in liquid phase chromatography are under way and will help identify if this family of materials could be useful in HPLC applications replacing existing supports and yielding improved properties for certain applications. The presentation will give an overview of separation properties using HPLC probing with a range of molecules in several phase systems and the influence of material synthesis variations (base material, carbon deposition and synthesis conditions).

[1] T. G. Glover, K. I. Dunne, R. J. Davis, M. D. Levan, ?Carbon-Silica composite adsorbent: Characterization and adsorption of light gases?, Micropor. Mesopor. Mater. 111, 1-11 (2008) [2] T. Martin, A. Galarneau, F. Di Renzo, D. Brunel, F. Fajula, ?Great Improvement of Chromatographic Performance Using MCM-41 Spheres as a Stationary Phase in HPLC?, Chem. Mater. 16,1725-1731 (2004) [3] De Clippel F, Harkiolakis A, Ke X, T. Vosch, G. Van Tendeloo, G.V. Baron, P.A. Jacobs, J.F.M. Denayer, B.F.Sels, ?Molecular Sieve Properties of Mesoporous Silica with Intraporous Nanocarbon?, Chem. Comm. 46, 928-930 (2010)