Isolated OH groups

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. 

Figure 8.9 Diffuse reflectance infrared spectrum of a silica support, showing silica vibrations at frequencies below 1300 cm1, overtones and combination bands between 1700 and 2050 cm-1, and various hydroxyl groups at frequencies above 3000 cm 1. The sharp peak at 3740 cm"1 is due to isolated OH groups, the band around 3550 cm 1 to paired, H-bonded OH groups, and the band around 3660 cm 1 to hydroxyls inside the silica (courtesy of R.M. van Hardeveld, Eindhoven).

Both spectra for the surfactant and the MoS-L- 16C material exhibit a broad absorption band at around 3450 cm 1 due to small amount of water molecules adsorbed on the external surface of the powder solids. There is no absorptions corresponding to isolated OH groups in... 

Heat treatment temperature (K) Total no. of OH groups/nm2 Isolated OH groups/nm2 H-bonded OH groups/nm2... 

The structures of hydrated borates and polyborates (Chapt. 1) are principially different from those of anhydrous species (Chapt. 2), although there are transitions between both the groups. These latter arise if the borates are not fully hydrated, or, for example, if the BO3 group is associated with isolated OH groups (Chapt. 2.2.2), but also for boracites containing OH groups (Chapt. 2.8). 

The shift is determined by the appearance of calcium ions in the structure in the vicinity of vanadium ions with vanadyl bond, as in the case of LiV30g (see above). In the spectra of activated mixtures one cannot observe the vibration band of isolated OH-groups, present in the spectrum of initial Ca(OH)2. Instead of it, the wide band appears at 27(X)-... 

The CO2H group will have a pK of about 4-5 and the OH group a pffa of about 10. This gives a comfortable margin between the two ionizations. At pH 7 the acid will be ionized and at pH 12 both itoups will be ionized. In concentrated mineral acid both will have their protons and the carboxylic acid will start to be protonated. It is more readily protonated than the isolated OH group as it gives = delocalized cation. 

The relative contribution from the P- and Al-associated peaks in the bimodal MW distribution varies with the aluminophosphate calcination temperature. As the temperature is raised, the low-MW (P-associated) peak begins to grow. This observation suggests that this peak results from reaction with isolated OH groups. 

The hydration-dehydration behavior of the hydroxyl groups on alumina (28) and titania (33) is similar in many respects to that on silica. However, there are some important differences. Peri and Hannan (28) found three absorption bands instead of one for isolated OH groups after a drastic calcination treatment. 

Methanol is oxidized to formate ions, acetone and acetaldehyde to acetate species. An enolic form of adsorbed acetone is seen, which accounts for rapid isotopic exchange of -OH. On O2 adsorption two bands at 1155 and 1020 cm are formed,which are however not assigned. Mixtures of CO plus O2 lead to the obvious formation of carbonates and bicarbonates. CO interaction with presorbed NH3 forms carbamate species. Oxidizing properties of the surface are shown by the reaction of trichloroacetone to trichloroacetate species. Nitriles are, however, hydrolysed to acetimidate species R-CNH . Similar reactions with surface -OH are shown by ethyl and phenyl isocyanate, which form urethanes by reaction with isolated -OH groups, and 1,3-diethylurea with -OH pairs. Reaction with surface -OH is also shown by trimethylchlorosilane, which, however, harshly attacks the surface and forms silicone species. ... 

Soapstone exhibits a lamellar nanostructure, which makes it able to promote intercalation reactions. Furthermore, they exhibit isolated OH groups, as indicated by FTIR data, and the major number of silicon atoms present in... 

FIGURE 28.2 A plot of the concentration of various types of hydroxyl groups on the temperature of preliminary treatment of the silica samples in vacuum [46,47] (curves 1-3) 1, total concentration of all the types of OH-groups 2, concentration of alone isolated OH-groups 3, concentration of vicinal OH-groups. The curve 4 gives a dependence of the content of silicon atoms in siloxane bridges on the temperature. 

Temperature, (°C) Total concentration of OH-groups, (OH/hm ) Occupation degree, oh Concentration of isolated OH-groups, (OH/hm ) Concentration of vicinal OH-groups, (OH/hm ) Concentration of geminal OH-groups, (OH/hm ) [16,17]... 

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