Dehydroxylation Density

Though silica supports are amorphous, the surface may exhibit some local order, such as that of the mineral /3-crystoballite (Fig. 5.23). The surfaces of silica support contain OH groups at densities of between 4 and 5.5 OH per nm that of cristobal-lite is 4.55 OH per nm. Silica surfaces contain only terminal OH groups, i.e. bound to a single Si atom. Heating leads to dehydroxylation, and at high temperatures only the isolated OH groups remain. 

Table 9.1 OH group density according to the dehydroxylation temperature. (Adapted from Reference [4].)...

Two different approaches have been used to graft molybdenum on alumina, namely, either a two-step process involving gas-phase impregnation and further decomposition at high temperature (GPID) or the direct contact of [Mo(CO),5] vapor with the alumina support placed in a hot zone so as to achieve its decomposition. All of the relevant studies point to the existence of a close relationship between the OH group density on the support and the amount of deposited molybdenum as well as the chemical nature of such deposits. Hence, we successively deal with three types of alumina highly, partially and fully dehydroxylated surfaces. 

The catalytic activity of aluminas are mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as well as to the high ionicity of the surface Al-O bond [67,92]. The number of such very strong Lewis sites present on aluminum oxide surfaces depends on the dehydroxylation degree and on the particular phase and preparation. Depending on the activation temperature, the density of the strongest Lewis acid sites tends to decrease as the calcination temperature of the alumina increases (i.e., upon the sequence y — 5 —> 9, which is also a sequence of decreasing surface area and increasing catalyst stability). 

It has to be pointed out that Lewis acid and base sites produced during the regular dehydroxylation process can hardly be all involved in catalytic reactions as active sites. It has already been indicated [44] that only defect sites can be considered as active sites because of their low site density. The configuration of such defect sites can hardly be predicted from idealizing model considerations. 

The relationship between acid site density and effective acidity may account for the interesting observation of Hopkins that maximum cracking activity of n-hexane was obtained over a partially dehydroxylated hydrogen zeolite Y (45). While the normal hydrogen form would contain a greater overall concentration of acid sites, the partially dehydroxylated form may have a greater overall acid activity because of the increased effective acidity of the remaining sites. 

There are three traditional structures usually adopted as probable BASs in amorphous aluminosilicates a water molecule coordinated by an electron-acceptor center (I), a bridged OH group (II), and a surface H30 + ion (III) (125,126). The catalytic activity of these sites is obviously determined by their properties and surface concentrations. Pelmenshchikov et al. (127) have attempted to compare these characteristics for the above types of BAS in aluminosilicates in terms of the cluster approach. For this purpose they considered a sequence of states of the model fragment of a dehydroxylated surface plus two water molecules (Fig. 15). State S0 corresponds to a dehydroxylated surface, states S, Sn, and Sm represent the sites of the I, II, and III types and states Sla and SIla correspond to centers I and II at a higher coverage. The relative energies of these structures obtained using the CTP scheme and the CNDO/BW technique are presented in Fig. 15. The relative surface density of the sites, og(nJnf), was estimated as the relative probability of their occurrence ... 

For T = 573 K, log(n,/nn) log(nIa/n,Ia) ss 0, that is, the surface densities of BASs of the I and II types are approximately equal and independent of coverage. Also, log(nIa/nin) = log(nna/nin) = 12, that is, the surface density of BASs of the III type is too low to be of any practical interest in catalysis. Consideration of the extended fragment of a dehydroxylated surface plus five water molecules, where a surface H30+ ion receives additional possibilities for solvation, did not qualitatively change the conclusion. 

The low-coverage energy data for the adsorption of n-hexane and benzene on various non-porous solids in Table 1.4 illustrate the importance of the surface structure of the adsorbent and the nature of the adsorptive. Since n-hexane is a non-polar molecule, Em > Esp, and therefore the value of E0 is dependent on the overall dispersion forces and hence on the density of the force centres in the outer part of the adsorbent (i.e. its surface structure). Dehydroxylation of a silica surface involves very little change in surface structure and therefore no significant difference in the value of E0 for n-hexane. However, replacement of the surface hydroxyls by alkylsilyl groups... 

A1r Separation Properties. Self-bound LSX adsorbents have an enhanced ability to selectively adsorb nitrogen from air. For thermodynamically driven adsorption processes, the quantity of a gas adsorbed by a zeolite at a given pressure and temperature Is a function of Its the affinity for the cationic adsorption sites as well as the quantity of sites available for Interaction. Electronic charge balance dictates that the LSX will have the maximum number of cationic sites available for direct Interaction with weakly Interacting adsorbates. The electric field within the zeolite cavity 1s dependent on both structure and the charge density of the extra-framework cation. Small polyvalent cations 1n the dehydrated/dehydroxylated state, especially calcium, show high selectivity for N2 from a1r.(l2)... 

The extremely high temperature in a plasma jet leads, even during the very short residence time (hundreds of microseconds to few milliseconds, depending on particle density and size) of the hydroxyapatite particles, to dehydroxylation and finally thermal decomposition by incongruent melting. This thermal decomposition of hydroxyapatite in the hot plasma jet occurs in four consecutive steps as shown in Table 6.7. 

The present acid Y-zeolite dehydroxylated at 650°C (in vacuo), more than 100°C higher than the NH4-HY system (4), probably because of the difference in site density between the 2 Y samples. A similar difference in thermal stability (toward dehydroxylation) was reported (28) between 50 and 90% ammonium-exchanged NaY. 

For strongly dehydroxylated silica the surface concentration of siloxane groups is high (Figure 3.19). Because of the shift in the electronic density in going from O atom to Si atom, the formation of a hydrogen bond between the oxygen on the surface of the sample and water molecules is not favored. Thus, the siloxane surface is hydrophobic. 

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