INDEX acid strength

Rates of model reactions are more commonly used to determine relative rather than absolute surface acidities and a variety of acid-catalyzed reactions have been used for this purpose (1-3). Xylene isomerization is a particularly well-substantiated model reaction, thanks to work by Ward and Hansford (43). They demonstrated that the conversion of o-xylene to p- and /n-xylenes over a series of synthetic silica-alumina catalysts increases as the alumina content is increased from 1 to 7%. The number of strong Brdnsted acids in each member of the catalyst series was measured by means of infrared spectroscopy. Since conversion of o-xylene was found to be a straight-line function of the number of Br0nsted acids (see Fig. 9), rate of xylene isomerization appears to be a valid index of the amount of surface acidity for this catalyst series. This correlation also indicates that the acid strengths of these silica-alumina preparations are roughly equivalent. 

These reactions were considered as indicator reactions. The corresponding pK values are included in Table 9.2.3. Increase in acid strength in iodide melt causes that the pK shifts coincide for both reactions. This magnitude may be treated as the oxobasicity index of molten sodium iodide, pl aj equal to -0.2. The negative sing means that the iodide... 

The minute quantity of adsorbate remaining on the column after weakly bound probe has desorbed is chemisorbed to strongly acidic or basic sites on the substrate. The desorption profile obtained by ramping the column temperature is an index of the range of effective bond strength between the solid and adsorbed vapor. The flame ionization detector also registers desorption of adventitious organic contaminants polytherms with no probe on the column must be obtained separately so that sorbate and contaminant desorption can be deconvolved. 

The lack of correlation between isosteric heats of adsorption and catalytic activity illustrates the point we have been stressing the strength of attachment of a base to a catalyst surface is not a valid index of catalytic activity when a substantial portion of the base is strongly bonded to inactive portions of a catalyst surface. Putting it more explicitly, isosteric heats of adsorption are useful indexes for the prediction of acid-catalyzed... 

Modification of ZSM-5 zeolite can result in improved liquid yields and a doubling of the isoparaffin index of the liqnid fuels which indicates higher liquid quality compared with the parent ZSM-5 zeolite. The high catalytic activity of modified ZSM-5 was explained by its nniqne acidic properties with a sharp increase of the number and strength of weak acid sites and a decrease of strong acid sites [13]. 

---