Surface acidity nonaqueous indicator

Nonaqueous methods for the determination of surface acidity represent a considerable improvement over aqueous methods because the solvents used (e.g., benzene, iso-octane) do not react with catalyst surfaces as previously described in the case of water. Of the available types of nonaqueous methods (1-3), the simplest is that employing adsorbed indicators. It can be used to determine acid strengths and amounts of surface acids as described in the following section. 

If the objective is to fingerprint surface acidity (so that one acidic solid can be distinguished more clearly from another), the nonaqueous titration method (26) would be appropriate. The original method could be made more selective for titration of Br0nsted acids by choosing recently recommended indicators (21) and using a sterically hindered amine (54) as the basic reagent. However, even an improved indicator method may have its drawbacks a sterically hindered amine is not perfectly selective,... 

Before we discuss characteristic features of a metal sulfate catalyst, it is to be noted that the model reaction should be one which has as straightforward a mechanism as possible, preferably in a homogeneously catalyzed reaction. This is the only way we can critically evaluate the efficiency of the present solid catalyst system. The depolymerization of paraldehyde was most extensively studied in view of the foregoing criterion. For the homogeneous acid catalysis of the depolymerization of paraldehyde, there are ample data given by Bell and his associates in nonaqueous solvent (by proton acid as well as Lewis acid) and also in aqueous solution (55,56). Since most Hammett indicators change their color when adsorbed on the surface acids of both Bronsted and Lewis type, it is fortunate that this depolymerization proceeds easily by acids of both types. Evidently the dotted line in Fig. 2 shows excellent... 

Studebaker (63) studied the potentiometric titration of surface oxides in nonaqueous medium. Using sodium aminoethoxide in ethylene diamine, he found indications for the appearance of two breaks in the titration curves. The first break was attributed to carboxyl groups or groups of similar acidity, the second one to phenols. Only two-thirds of the acidity that was determined by NaOH neutralization could be titrated in nonaqueous medium. 

One of earliest methods of obtaining information about the acid-base properties of the catalyst surface, in particular of the silica-alumina, was to adsorb from nonaqueous solvents on to them a series of known organic indicators and to notice the color change.J... 

Reactions at the Silicon Nitride - Solution Interface - A study was conducted to determine the extent of aqueous reactions at the silicon nitride-water interface. It was demonstrated that up to 27 days are required to stabilize reactions at the interface as indicated by Ph and particle electrophoresis measurements. A semiautomatic titrator was also purchased and set up to utilize acid-base titrations to study the silicon nitride-solvent interface. A particular emphasis of this work will be on the nonaqueous potentiometric and conductometric titration to determine the strength of acid and base sites on the silicon nitride surface. 

Reactions at the silicon nitride - solution interface - The use of potentiometric titrations in nonaqueous solvents indicates that there are possibly several different kinds of acid and base sites on the silicon nitride depending on the prior environmental history of the powd. The results are generally consistent with potentiometric titrations us d to evaluate the silicon dioxide surface. Future work will focus on further potentiometric as well as conductometric titrations on silicon nitride as well as model systems such as silicon dioxide. 

Chemical Characterization. Titration methods in aqueous medium are not very informative, because H2O tends to strongly modify surface properties by molecular or dissociative chemisorption (70). Therefore, nonaqueous methods have been proposed, where the solvent (eg, benzene or isooctane) does not (or not strongly) interact with the catalyst surface. Hammett indicators were used to determine the acid strength in terms of the Hammett-Deyrup function Hq which is defined as... 

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