Dehydroxylated silica gels

It should be noted in this connection that hydroxyl groups are not necessarily required to be present on the SiC>2 surface, since completely dehydroxylated silica gel appears still to be an excellent support for active CrC>3-based ethylene polymerisation catalysts [225]. 

Figure 1 illustrates differential distributions of the adsorption energy of n-hexane on partially dehydroxylated silica gel (1) and on carbon-silica adsorbents prepared through the pyrolysis of n-heptyl (2) and benzyl (3) alcohols and their different mixtures (4-6) on the solid surface. The energy distribution function X(E) of n-hexane was measured chromatographically from the pressure dependence of the retention volume [24,25]. 

Figure 1. Energy distribution X(E) of n-hexane adsorbed on partially dehydroxylated silica gel (1) and on the carbon-silica adsorbents (2-6) (see text).

When a silica gel is heated to progressively higher temperatures water is lost first from any mesopores and supermicropores then from any ultramicropores, and finally by the decomposition of surface hydroxyls. The dehydroxylation of the surface... 

As noted above, many investigations have been made of the adsorption of water on non-porous silicas. Less attention has been given to the dehydroxylation of porous silicas. An early study by Dzhigit et al. (1962) of the adsorption water vapour on a mesoporous silica gel involved both isotherm and calorimetric measurements. It was found that at very low surface coverage the adsorption enthalpy was not significantly affected by dehydroxylation, but a large difference became apparent as the surface coverage increased. A slow uptake of water vapour, which occurred after dehydroxylation, was attributed to chemisorption. 

Chlorine could be even used for efficient dehydroxylation of silica gels [79] at 800 °C due to the reaction ... 

Experiments with CPGs subjected to progressive thermal treatment leading to partial dehydroxylation of the surface and its enrichment in boron atoms showed specific adsorption properties of these materials differentiating them from silica gels [30,31,73]. It was found that depending on the type of adsorbate two trends in the adsorption behaviour of heated CPG can be observed [30,31]. The increase of thermal treatment time leads to the increase of CPG adsorption properties in relation to water, chloroform or hexane [30] (e.g. see Fig. 13). 

The adsorption of diethyl ether or tetrahydrofurane decreases for shortly heated glasses in comparison to non-heated ones and increases for longer heated CPG (e.g. see Fig. 14). This can be attributed to the described processes occuring on the CPG surface during its thermal treatment (dehydroxylation of Si-OH groups, creation of B-OH groups, increase of surface boron and sodium concentration, borate cluster formation). Each of these processes differently influences the adsorption properties of CPG. The adsorption properties of silica gel treated thermally in the same conditions (above 400°C) significantly decrease as the result of surface dehydroxylation. 

The rehydroxylation of a wide-pore silica gel sample calcined in air at 850 °C and held in water at 100 °C for periods covering 1 to 100 h was found to take 5-10 h for complete rehydroxylation. These and other results indicate that rehydroxylation of dehydroxylated silica (calcined at >400 °C) in the presence of water requires considerable energy to activate the process of dissociative adsorption, Ed. Chemisorption of water appears to take place, resulting in the formation of hydroxyl groups bound through... 

The first mention of chemical modification of silica in the former Soviet Union goes back to 1950, when A. Kiselev, Dzhigit, and co-workers (248) observed the irreversible adsorption of methanol vapors on silica gel. The mechanism of the chemisorption of methanol on dehydroxylated SiC>2, which involves the breaking of the siloxane bond, was described by Belyakova et al. (251). 

Eltekov, Pavlova, Shikalova, Bogacheva and co-workers (Moscow State University Institute of Physical Chemistry, the U.S.S.R. Academy of Sciences, Moscow) (17, 290, 297-307) investigated the effect of the chemical nature and the dimensions of the pores in silica adsorbents on the adsorption from solutions. The adsorption of benzene from n-hexane solutions on silica gels with a hydroxylated and a dehydroxylated surface was studied. Dehydroxylation sharply lowered the heat of adsorption of benzene. A comparison was made of the adsorption isotherms for a series of n-hexane solutions of aromatic hydrocarbons on hydroxylated silica gel. The intermolecular interaction of aromatic hydrocarbons with this adsorbent is stronger than the interaction of saturated hydrocarbons because aromatic hydrocarbons form hydrogen bonds with the silanol groups on the SiC>2 surface. 

The surface of pure silica gel is covered with silanol groups, at a maximum concentration of 8 Broensted acid OH groups per nm. They are mostly found as geminal or isolated pairs and are neither very acidic nor very basic (pKa 6). The hydroxylated surface is hydrophilic and easily adsorbs moisture from the air. This physically adsorbed water can be desorbed by raising the temperature to 100—200 °C. In the course of this heating a partial dehydroxylation of... 

Oxides.—The adsorbent properties of silica gel are known to be highly dependent on the degree of hydroxylation of the surface, and this dependency has been widely investigated in relation to gas adsorption.Khopina and Eltekov have continued earlier work of Kiselev and his collaborators on the influence of the surface chemistry of silica on adsorption from solution. In the present paper a comparison is made of the adsorption of a series of aromatic hydrocarbons (benzene, naphthalene, biphenyl, phenanthrene, o- and m-terphenyl) from n-heptane solution by hydroxylated silica, dehydroxylated silica, and graphitized carbon black, using data obtained in the present work together with earlier published data. The results are analysed in terms of the separation factor(partition coefficient)... 

Based on decomposition of the MAS Si NMR results (Fig. 44) [170] and estimates of the extent of dehydroxylation according to Eq. 14, the maximum percentage of silicons incorporated in three-membered rings in the two-step acid-base-catalyzed gels studied by Drinker and coworkers exceeds 20%. The NMR results indicate that after heating at intermediate temperatures (e.g., 600°C), the silica gel surface is composed primarily of OH terminated ... 

---