Surface hydroxyl groups interaction with probe molecules

The formation of H-bonds between OH groups and various probe molecules is widely used to assess the acidity of surface hydroxyls. A potential problem is that a given probe may be protonated by one kind of surface hydroxyls while it forms only an H-bond with another (less acidic) kind of OH. Another important phenomenon is that a probe molecule may form an H-bond with a hydroxyl group at low surface coverage but may accept a proton at high surface coverage when the OH group interacts simultaneously with two probe molecules. In this section, we will exclusively consider H-bond formation. However, whenever data for protonation exist, it wiU be explicitly mentioned. 

Kinetic studies indicate that the product composition, and therefore the overall success of the process, depends on a series of competitive elementary steps that involve a range of adsorption sites [54]. Adsorption of HCI is postulated to occur at two types of site, only one of which leads to CH3CI formation. The unwanted formation of (CH3)20 is postulated to involve two adsorbed CH3OH molecules, this process competing with HCI adsorption. The interaction between CH3CI and a y-alumina surface, as probed by vibrational spectroscopy, leads to the formation of CH3O bound to Al centers as the dominant surface species. These arise from the loss of HCI and the consumption of surface hydroxyl groups [55],... 

B. Brunner, Solid state NMR-a powerful tool for the investigation of surface hydroxyl groups in zeolites and their interactions with adsorbed probe molecules. J. Mol. Struct. 355, 61-85 (1995). doi 10.1016/0022-2860(95)08867-U... 

Various types of neutron scattering can be utilized to extract data on structure and dynamics for novel catalytic materials. By selectively deuterating an SSZ-13 zeolite, Cheetham and others" used ND performed on the Dual Beam Neutron Spectrometer (DUALSPEC) diffractometer at the Chalk River Laboratories and found that two acid sites are present in the unit cell of the zeohte. INS can be used to probe the mechanism of the catalytic reaction by looking at the change in the vibrational modes of the adsorbed molecules on the surface. Lennon et alP found that the interaction of HCl with a ]-alumina catalyst results in the dissociative adsorption of HCl, in which the hydroxyl groups terminally bound to A1 are replaced by chlorine. INS spectra reveal an in-plane deformation mode, 5 (OH), that can be resolved into two bands located at 990 and 1050 cm. ... 

To assess the surface acid/base site modification due to and Li incorporation into the lattice, FTIR spectroscopy of adsorbed methanol as a probe molecule was used. Methanol is known to interact with the acid-base pair O ") of the oxide surface [19] and to form methoxy and hydroxyl groups by dissociation of its O-H bond. The Vco (C-O stretching) and VcH3 (C-H stretching of the methyl group) bands of the methoxy species are measured since they are known to be sensitive to Lewis acid site nature [20]. Fig. 3 shows spectra of methanol adsorbed on y-Al203 and Mg -modified Y-AI2O3... 

The interaction of molecules with OH groups is investigated for two reasons (i) to characterize the surface hydroxyls and (ii) to obtain information on the activation (and conversion) of particular molecules. In the first case, the molecule has to act as a probe. To be a good probe, a molecule should fulfill a number of requirements, as summarized in several review articles... 

In principle, when a probe molecule forms an H-bond with only the hydrogen atom of an OH group, the interaction yields information on the acidity of the hydroxyl. On the contrary, if the molecule (hypothetically) interacts only with the oxygen atom, the information concerns the basicity. When the interaction is with both, the oxygen atom and the hydrogen atom, the situation is more comphcated and needs a careful analysis. A complicated situation also arises when the probe molecule interacts not only with an OH group, but with one or more other surface sites. 

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