Probe molecules nitrogen-containing compounds

Prins reaction, heteropolyacid catalysis, 41 156 Probe molecules, 42 119 acidic dissociation constant, 38 210 NMR solid acidity studies, 42 139-140 acylium ions, 42 139, 160 aldehydes, 42 162-163 alkyl carbenium ions, 42 154-157 allyl cation, 42 143-144 ammonia, 42 172-174 arenium ions, 42 150-154 carbonium ions, 42 157-160 chalcogenenonium ions, 42 161-162 cyclopentenyl cations, 42 140-143 indanyl cations, 42 144-147 ketones, 42 162,163-165 nitrogen-containing compounds, 42 165-170... 

As examples of probe molecules directly introduced onto solid acids, we consider ketones and aldehydes, amines and other nitrogen-containing compounds, phosphines, and molecules that form multiple hydrogen bonds. 

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

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