Access to Highly Fluorinated Silica by Direct F₂ Fluorination: Chemical Compositions and FTIR Investigations

Abstract

The direct F 2 gas fluorination of a mesoporous silica gel has been shown to be a unique method leading to very high levels of fluorination (up to 13 wt % F in the bulk). The final powders are homogeneous with a controlled amount of grafted fluorine. In this study, various conditions of fluorination were tested, such as duration, temperature, F 2 gas concentration of the fluorinating gas, or an annealing pretreatment. The content of grafted fluorine on silica was quantified by XPS and by the Seel method. Infrared spectroscopy measurements accounted for the consumption of different types of hydroxyl groups, that is, isolated (3740 cm -1 ), terminal (3715 cm -1 ), and bound (3520 cm -1 ), and also for the presence of unreacted internal hydroxyl groups inaccessible to D 2 O molecules and so to F 2 . Results showed that an F/OH substitution occurs during the fluorination process and that the grafted amount depends on the F 2 concentration of the fluorinating gas and on the concentration of surface hydroxyl groups and physisorbed water trapped on starting silica. Elemental analyses and FTIR data led to the bulk composition of fluorinated silicas: SiO 2-x-y (OH) 2x F 2y . Finally, on a quantitative basis, the elimination of silanol groups parallels the grafting of fluorine for low fluorine content. At higher fluorine contents, a reaction path takes place involving the Si-O-Si opening. Redox processes involving O 2 /OHand F 2 /Fcouples explain the wide range of reached F/OH substitution rates without formation of SiF 4 or SiF 6 2species which are observed in classical routes with aqueous fluorinating agents.

Template-assisted encapsulation of fluorinated silanes in silica films for sustained hydrophobic–oleophobic functionality

“…Surfactant templating has been used to fabricate mesoporous films that are popular for applications like catalysis and separations and so are generally engineered for high porosity and surface area. There are some reports on the fluorine functionalization of mesoporous silica [24][25][26][27][28]. Of these, only the works by Bae, Jung et al [26,27] involve synthesis of mesoporous films via co-condensation with functional moeties, which, unlike the post-synthesis grafting approach, is the only templating route that fully incorporates the functional groups in the bulk, in addition to being a single step process.…”

Fluorination of alumino-silicate minerals: The example of lepidolite

“…It is known that the fluoride route is able to modify the networks of various microporous compounds: silica [5,6,11], zeolites and metallophosphates [28,29], clays [30,31], rending these phases attractive for numerous purposes, including catalysis [32]. The use of various fluorination techniques including rfplasmas or direct F 2 -gas extends this type of surface modifications, which can functionalize in a very versatile way various silicates of the mica-type.…”

“…Direct F 2 -gas fluorination process was performed at various temperatures, between 100 and 200 8C in a ''fluorine line'' using handling procedures previously described, with adapted experimental set-up enabling to work in excellent safety conditions [5]. The samples were set in a Ni boat which had been previously passivated.…”

“…The properties of silica, silicates and alumino-silicates largely depend on the chemistry of their surface. Among the possible species that can help modify the surface properties of materials, the F À ions, generating ionic bonds, have been largely used, in particular for silica [1][2][3][4][5][6]. Recently, the properties of amorphous silica generated by the fluoride method have been investigated [7][8][9][10][11].…”

Structural architecture and physical properties of some inorganic fluoride series: a review

“…On the other hand, due to the straightforward reactivity of fluorine and fluorinated media, the properties of most materials could be easily modulated by surface treatments involving either fluorinated gases or plasmas [2]. For instance, direct F 2 -gas or radio-frequency (rf) plasma fluorination allows one to modify drastically at low temperatures the surfaces of various types of materials, including metals, powders, oxide ceramics [3], silica and silicates [4,5], carbons [6], elastomers and polymers [7], etc. This paper will not deal with this aspect of surface fluorine chemistry neither with the powerful possibility to change conduction properties by intercalation of fluorine/fluoride species in graphite and carbon materials, i.e.…”