Silica and alumina based functional materials: Substructures, adsorption and gas chromatographic properties
Graphical abstract
Introduction
Silica and alumina have found a well-defined, but rather limited use in molecular gas chromatography [1]. This is primarily due to their large specific surface area defining a limited number of substances, as a rule, light hydrocarbons, being analyzed [1], [2]. Besides, the existence of geometrical nonuniformity, often caused by the method of material preparation, leads to the elution of chromatographic peaks in the band tailing form. Thermochemical activation and hydrothermal treatment used sometimes don’t lead generally to significant improvement of the oxides texture A final materials have pores with uncontrollable size and often a broad size distribution [2], [3].
Modern technologies of synthesis of porous adsorbents and supports, using primarily templates as structure directing agents and varying the preparation conditions, allow obtaining controllable structural and adsorption textures of silica and alumina with predominant contribution of meso- [4], [5], [6] or macroporosity [7], [8], [9], [10]. Surfactants are used as templates for mesostructure producing and latex – for macroporosity obtaining. Regular texture in both cases is obtained by filling the space between template particles by the precursor solution, which subsequently forms a solid skeleton as a template replica. Among the materials with controllable adsorption texture mesoporous mesophase material should be mentioned. They represent a specific class of materials with a solid state, defined as the mesophase, representing an intermediate state between the amorphous and crystalline phases [4], [5], [6]. Their adsorption texture is typical for mesoporous materials. Many materials, including silica, alumina, zeolites, have been synthesized which have in addition to the typical texture an additional system of regular transport macropores [7], [8], [9], [10].
Interest to the study of the templated silica and alumina, in particular by molecular gas chromatography, is based on the assumption of availability of improved adsorption and chromatographic properties, required in practical gas chromatography, associated with their well-organized substructure compared with the unstructured counterparts. In addition, ideas existing in chromatography about a predominant influence of the alumina surface hydroxyl groups on to the interaction with adsorbates, in particular, unsaturated hydrocarbons [11], [12], turned out to be at least incomplete. Moreover the statement, that the alumina has less polarizing effect than silica has [11], is suspected to be also disputable.
In this regard the present work is devoted to the study of the following aspects: (a) an intermolecular interactions between adsorbate and adsorbent, where the silica and alumina were used as adsorbents in the conventional forms and modified by different ways including template method and carbonization; (b) the texture, morphology and phase properties of these functional materials using different techniques with the view of their influence on the chromatographic properties; and (c) the extended possibility of modified silica and alumina application in practical gas chromatography.
Section snippets
Experimental
Templated mesoporous mesophase silicate and macroporous silicas and alumina as well as their unstructured counterparts have been synthesized. Synthesis methods are given below. The spectrum of silica- and alumina-containing materials was broad, in particular silica materials had a wide range of specific surface area beginning from 28.3 m2 g−1 for macroporous silica (SiO2–M) up to 1086 m2 g−1 for aerogel (Table 1). The last one was synthesized by drying of the precursor gel in the supercritical
Textural and phase properties of silica and alumina materials according to SEM, TEM and XRD analyses
The use of templates as structure-directing agents leads not only to the formation of meso- or macroporous structures depending on the template size during the material synthesis but also to the ordering of their substructures. Electron micrographs obtained by SEM (Fig. 1a) and TEM (Fig. 1b) of the sample SBA-15, produced with the use of Pluronic P123, are an evidence of a highly ordered substructure, practically whole sample being regularly mesoporous. The material consists of particles having
Conclusions
X-ray diffraction and electron microscopy, adsorption techniques and gas chromatography were used to study silicas and aluminas, initial and modified by different methods, including templated and carbonized.
The chemical nature of the surface of all materials characterized by concentrated positive charge integrates them as a class of II-type adsorbents. In this case, the elution order, for example, of saturated and unsaturated hydrocarbons is determined not only by the presence of this charge
Acknowledgements
The authors thank N.A. Rudina and D.A. Zyuzin for their help in the adsorbent characterization, A.F. Danilyuk for aerogel synthesis, and V.N. Krivoruchko for assistance in gas chromatographic analyses. The work was performed in the framework of the joint Research and Educational Center for Energy-efficient Catalysis (Novosibirsk State University, Boreskov Institute of Catalysis). The work was partially financed by RFBR (Russian Foundation for Basic Research) Grant 12-03-93116_a.
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