Acid strength and catalytic activity

Knowledge of the properties of the texture and pore structure of solid materials is highly important for the development of catalytic materials and catalysts. It should, however, be mentioned that the assessment of the dynamic changes of the material under reaction conditions may be even more important than the assessment under pristine conditions, as the changes in material during use will determine its useful lifetime. Note also that physisorption markedly influences specific properties of porous solids, such as acid strength and catalytic activity which are crucial material parameters for sorption and catalysis. 

The relationship between acid strength and catalytic activity has been quantitatively investigated. The acid strength distribution (only Lewis sites were present on the aluminas) measured by thermal desorption of pyridine is shown in Table 3.16, and the rate of dehydration of 2-butanol was measured over the aluminas of which the acid... 

P.F Siril, A.D. Davison, J.K. Randhawa, D.R. Brown, Acid strengths and catalytic activities of sulfonic acid on polymeric and silica supports. J. Mol. Catal. A Chem. 267,72-78 (2007)... 

Side-Chain Alkylation. There is continued interest in the alkylation of toluene with methanol because of the potential of the process in practical application to produce styrene.430 Basic catalysts, specifically, alkali cation-exchanged zeolites, were tested in the transformation. The alkali cation acts as weak Lewis acid site, and the basic sites are the framework oxygen atoms. The base strength and catalytic activity of these materials can be significantly increased by incorporating alkali metal or alkali metal oxide clusters in the zeolite supercages. Results up to 1995 are summarized in a review.430... 

A. E. Hirschler (Sun Oil Co., Marcus Hook, Pa. 19061) I should like to make a comment relating to your statement that activity is not a simple function of acidity. This statement appears to contain the implicit assumption that all acid sites able to protonate pyridine would have the same catalytic activity. Actually, one might expect an intensity factor (acid strength) to be operative in acid catalysis as well as an extensive factor, and the apparent lack of correlation you observed may result from an influence of acid strength on catalytic activity. Pyridine adsorption data alone do not completely define Bronsted acidity. One may need also some measure of acid strength. Acidity measurement with Hr indicators, such as we and others have reported, may supply useful information relative to acid strength as well as number of acid sites. 

In relating acid strength to catalytic activity, it has been customary to use the thermodynamic dissociation constant in water at 25°, and this practice has been defended by Dippy (92). However, it has been shown that the order of acid strength is not independent of the medium or the temperature (Everett, 93 Harned and Embree, 94). As pointed out by Everett and Wynne-Jones (95), the order for AF298 is not always the same as for the other thermodynamic functions. The parallelism between... 

Strength and catalytic activity of such solid acids correspond to those of mineral acids. The major advantage of solid acids is their thermal stability, which allows them to be used at much higher temperatures. 

Catalytic resnlts are well correlated with the acid strength of the active species irrespective of their natnre (Lewis or Bronsted). On the other hand, there is no clear correlation between the density of the active sites and the catalytic performances. While the FS03H/Si02 catalyst is very active (yields 99.5 -100%, Table 48.2), AICI3/MCM shows only moderate yields (14.3-20.1%) to N-acylsulfonamide, even if both samples exhibit a similar density (25 x lO , Table 48.1). 

In conclusion, more efficient and clean solid (acid and superacid) catalysts will be used in the coming years to reduce not only the emission of environmentally harmful products but also the use of noxious catalysts. The optimal catalytic systems will be determined from the nature of acid strength of its active sites, the nature of the reaction, and the reaction conditions. 

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... 

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