Surface dehydroxylation

The effect of the surface area is far from being a simple one. It was shown for titania that when the surface area changes from 110 to 12 m2/g, the average time required for a complete mineralization of organic substrates increased from 40 to 75 and 50 to 75 min for salicylic acid and phenol, respectively [135], These results clearly show that textural properties, particularly the surface area, strongly affect the photoreactivity, although a high-temperature treatment improved their crystallinity [18], Therefore, this phenomenon may be explained only in connection with the catalyst surface dehydroxylation. 

Fig. 19. The termination rate, plotted here as relative melt index potential (RMIP), reflects the extent of surface dehydroxylation in two series of Cr/silica-titania catalysts, calcined in (Y) air or ( ) CO and then air to reoxidize the chromium, both at the temperatures shown. The third series ( ) shows the additional benefit of low-temperature attachment. It was calcined in CO at the temperatures shown, then air at a lower temperature (760°C).

The rate coefficients of reactions (15)-(27) were taken from the results of ab initio calculations. Reactions (28) and (29) describe the process of surface dehydroxylation/hydroxylation. We used a value of 1013 sec-1 as an estimation of the preexponential factor (this value corresponds to the characteristic frequency of internal vibrations of the reaction center) for the desorption reaction. To describe the experimental dependence of the growth rate vs. temperature adequately, we considered that the water adsorption energy is a linear function of the hydroxylation degree / ... 

However, they found that the more completely the surface was dehydroxylated, the longer the time required for rehydroxylation. A surface dehydroxylated at 1173 K for 10 h required several years in water at ambient temperature to become fully rehydroxylated. When the same silica sample was hydrothermally treated in boiling water, 60 h sufficed to obtain full rehydroxylation. Caution is necessary however in using hydrothermal treatment, since some silicas tend to undergo drastic changes in structure and reduction in surface area under these conditions. 

The early work of Kiselev (1957) revealed that the adsorption isotherms of n-pentane and n-hexane on non-porous quartz were intermediate in character between Types II and m. Values of C(BET) <10 were obtained and the differential enthalpies of adsorption decreased steeply at low surface coverage. More recently, the isotherms of isobutane (at 261 K) and neopentane (at 273 K) on TK800 have been found to be of a similar shape (Carrott et al., 1988 Carrott and Sing, 1989). Unlike those of benzene, these alkane isotherms do not undergo a pronounced change of shape as a result of surface dehydroxylation. This is consistent with the non-specific nature of their molecular interactions (see Chapter 1). 

The effect of surface dehydroxylation of a mesoporous silica on the Ar and N2 energetics of adsorption is illustrated in Figure 10.12. In the work of Rouquerol et al. (1979b) Tian-Calvet microcalorimetry was used to determine the variation of the differential enthalpy of adsorption as a function of surface coverage. Although strong... 

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 progressive deactivation of the catalyst in the absence of water could be due to some surface dehydroxylation and/or to the formation of stable intermediate species which can not evolve in the absence of water so that they remain strongly adsorbed on the catalyst surface. It is worth reporting that the... 

FIGURE 90 LCB levels in the polymers made with three series of Cr/silica catalysts, each prepared from the same silica but in a different way. Two independent and additive contributions are evident, one from structural sintering and another from surface dehydroxylation. A Cr/silica activated as shown, B Silica, calcined as shown, aqueous chromium, air 500 °C, C Silica, calcined as shown, anhydrous chromium, air 500 °C. 

The strong correlation between catalyst performance and surface dehydroxylation, both in the one-step and two-step activations, holds for some chemical methods of dehydroxylation too. Calcination of the support in CO further lowers silanol concentrations, probably through a water-gas-shift type of reaction such as that shown in Scheme 32 [33,145,208,382]. Calcination in H2 had no effect. Compounds containing carbon and sulfur, such as CS2, COS, or CH3SH, were especially effective in the promotion of dehydroxylation. Sulfur compounds containing no carbon, such as H2S or S02, had no effect. Exactly how these organic sulfur compounds aid in the removal of surface silanols is less clear. Presumably, the H in a surface OH group is removed as H2S and the O is removed as CO (or when COS is used, perhaps as C02). Sometimes, elemental sulfur vapor is also evolved from the reaction, but it is not left on the... 

A partly hydrophobic and organophilic silica can be made very simply by a process patented by Goebel (507b). Fine silica powder is dehydrated and the surface dehydroxylated to an optimum degree by heating it for 1.5 hr in air at 470-530 C. The BET area was 297 m g" but the resulting specific hydroxylated area was 184 m g . When this powder was cooled and refluxed at 118 C in butanol at atmospheric pressure or even when simply exposed to butanol vapor in a closed container for a week, esterification occurred and the surface became about half covered with attached butoxy groups. 

In a study of dissociative chemisorption of water, ammonia, alcohol, or amines on highly strained two-membered rings. Bunker and coworkers [30] concluded that two types of rings with different acid-base sites for adsorption exist on surfaces dehydroxylated above 650°C ... 

Based on the preceding discussions of water adsorption on dehydroxylated surfaces, the most likely mechanism of rehydration of silicate surfaces dehydroxylated above 450°C is adsorption on acidic silicon sites contained in strained two- and three-membered rings, followed by dissociative chemisorption. Since two-membered rings comprise a small fraction of the silica surface, the rehydration kinetics will initially reflect the rate of dissociative chemisorption of three-membered rings which cover approximately one quarter of the dehydroxylated surface. Subsequent water adsorption occurs preferentially on silanols formed by hydrolysis of three-membered rings. 

Schematic illustration of various stages of surface dehydroxylation. Stage 1 removai of hydrogen bonded siianols resulting in four-membered and larger rings. Stage 2 condensation reactions between isoiated siianols resulting in strained two- and three-membered rings. Stage 3 fully dehydroxylated surface.

As shown in Fig. 11, surface dehydroxylation progressively removes physisorbed water, hydrogen-bonded vicinal (or gemlnal) hydroxyls, and isolated hydroxyls. Because surface hydroxyls have been shown to be the principal sites for physisorption of water (see, e.g., Fig. 7), dehydroxylation makes the surface progressively more hydrophobic. The extent of physical adsorption of other molecular species capable of hydrogen bonding, such as ammonia and alcohol, is reduced as well. 

The rehydration of dehydroxylated surfaces has been investigated extensively. (See, e.g. [1,75,76,86,87].) The rehydration process involves water adsorption followed by dissociative chemisorption (Eq. 15). Either the adsorption step or the subsequent dissociative chemisorption step could be rate-determining [30]. Hair [22] states that rehydration is sluggish for surfaces dehydroxylated above about 450°C due to surface hydrophobicity (physisorption rate determining). Below this temperature there is sufficient hydroxyl coverage that the adsorption step is facile. 

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