The adsorption of methanol on hydrophilic, hydrophobic and mixed silicas

Abstract

The adsorption isotherms for methanol on hydrophilic and hydrophobic Aerosil, and on a solid mixture of both, are used to show that the effect of the hydrophobicity on the well known interaction between alcohols and the surface of silica manifests itself not on the energetic, but on the geometric characteristics of adsorption.There have been many publications dealing with the adsorption of alcohols on hydrophilic silicas, particularly using methanol as the ads~rbate,l-~ On the other hand, little has been done on the adsorption on mixed solidse4 This paper deals with a series of experiments on the adsorption of methanol, in an attempt to clarify the effect of mixing silicas having varying degrees of hydrophobicity. This work is related to research on the phenomenon of surface heterogeneity that is being followed in this laboratory. During the last 30 years this phenomenon has been interpreted by a large number of models,5 most of them based on the assumption of patchwise heterogeneity. As far as the authors are aware, however, the a priori construction of such surfaces has not been attempted, even though this would certainly clarify some of the assumptions of these kinds of models.

Surface Properties of Silica-Based Biomaterials: Ca Species at the Surface of Amorphous Silica As Model Sites

“…At low pressure, the ads II [ q int mol ] p values for A200/Ca x turn out to be still as high as in ads I (∼60 kJ/mol), indicating that a fraction of the strong component of the ads I process was not completely quenched within the first run. In addition, for the parent A200 system, the low-pressure [ q int mol ] p value is still relatively large (∼50 kJ/mol), probably as a consequence of the presence of multiple H-bonding interactions between CH 3 OH and surface Si−OH groups. , Conversely, the high-pressure [ q int mol ] p ads II values are typical of low-energy processes for both plain and Ca-modified silica (∼45 kJ/mol).…”

“…In addition, for the parent A200 system, the low-pressure [q int mol ] p value is still relatively large (∼50 kJ/mol), probably as a consequence of the presence of multiple H-bonding interactions between CH 3 OH and surface Si-OH groups. 47,50 Conversely, the high-pressure [q int mol ] p ads II values are typical of low-energy processes for both plain and Ca-modified silica (∼45 kJ/mol). To describe in molecular detail the energetic of CH 3 OH interaction with plain and Ca-modified silica, it is convenient to consider adsorption differential heats, as shown in the plots of Figure 4 [ads I in section a, ads II in section b].…”

“…The complex CH 3 OH/SiO 2 system has been long investigated in most of its experimental and theoretical aspects, as witnessed by a huge amount of work done over the years and present in the literature. ,, The initial heat of adsorption of CH 3 OH on silica (78 kJ/mol) reported by Natal-Santiago and Dumesic, is consistent with the formation of H-bonded adducts that can be removed by evacuation at relatively low temperature. In the case of microporous silica-based materials, it has been observed that methanol molecules tend to form stable, ringlike H-bonded structures …”

Dehydration of methanol to dimethyl ether over Nb2O5 and NbOPO4 catalysts: Microcalorimetric and FT-IR studies

“…However, the methanol coverage in terms of the amount per unit surface area on NbOPO 4 was lower than on Nb 2 O 5 , in agreement with what was observed for NH 3 adsorption (see Table 2). Both Nb 2 O 5 and NbOPO 4 strongly interact with methanol as indicated by the differential heats, which are higher than those on silica (Q init = 60 to 90 kJ mol −1 ) [26,28,[32][33][34] and H-ZSM-5 (Q init = 115 kJ mol −1 ) [32] but lower than on ␥-Al 2 O 3 (Q init > 200 kJ mol −1 ) [35].…”

“…Polar molecules with different gas phase affinities such as alcohol and water are usually used to measure the hydrophilic and hydrophobic properties of the surface of materials [26][27][28][29][30]. The differential heat and saturation coverage reflect the proton affinity and the strength of H-bonds or van der Waals forces on those samples.…”