Brpnsted acidic strength

Other adsorbents have been used in an effort to measure the acid strength of the sites or eliminate diffusion limitations. Kubelkova et al. used low temperature adsorption of CO on H-ZSM-5, H-Y, NaH-Y, and various AlPO sieves to measure the shift in the acidic OH stretching frequency upon CO adsorption. The authors argue that this shift is related to the proton affinity of the zeolites and thus to the Brpnsted acid strength. Tvaruzkova et al. used d3-acetonitrile to characterize both the Brpnsted and Lewis acidity of a number of zeolites. Using the band intensities and the frequency of the C-N band they obtained relative concentrations and strengths of the various acid sites. 

Yi D, Zhang H, Deng Z (2010) H and N chemical shifts of adsorbed acetonitrile as measures to probe the Brpnsted acid strength of solid acids a DFT study. J Mol Catal A Chem 326 88-93... 

The presence of wolframate species on both titania and zirconia causes a little increase of the Lewis acid strength of the residual Lewis sites, an almost full disappearance of the surface anions acting as basic sites and the appearance of a very strong Brpnsted acidity [268-271]. Strong Brpnsted and Lewis acidity are also found on pure WO3 [272], Similar effects have been found for sulfated titanias [261]. The Brpnsted acid strength of these materials, measured by the olefin oligomerization method (Table 9.8) is superior to that of silica-alumina and comparable to that of protonic zeolites [268]. 

Heeribout, L., Semmer, V., Batamack, R, Doremiueux-Morin, C., and Fraissard, J. Brpnsted acid strength of zeoUtes studied by H NMR scaling, influence of defects. Micrqpor. Meospor. Mater. 1998, 21, 565-570. 

The affinity towards water of the all-silica counterpart was lower than that of the proton exchanged zeolite, as expected, but it was not negligible. The reported isotherms indicated that hydrophilic sites, responsible for weak and reversible water H-bonding adducts, are developed in zeolites even in the absence of framework A1 atoms. Stmctural defects generating polar species consisting of Si—OH nests (which are characterized by a weak Brpnsted acidic strength),[25, 61] are always present in Al-free zeolites, unless especially prepared in order to obtain hydrophobic, inert materials, as claimed by Flanigen et al.[36]. See also Ref. [24]. 

The Brpnsted coefficient a represents the sensitivity of the rate to the acid strength of the catalyst. It is a measure of the degree of proton transfer from catalyst to substrate in the transition state. For nearly all reactions where BH+ contains acidic N-H or O-H groups, a is in the range 0-1. 

It is known that the activation temperature can influence the acid strength distribution. For example, measurements of the differential heats of ammonia adsorbed at 150°C for a HY zeolite have led to the conclusion that stronger acid sites, in the 150-180 kJ/mol range, are formed upon increasing the activation temperature from 300 to 650°C. Dehydroxylation at high temperature resulted in the formation of strong Lewis acid sites and the disappearance of intermediate and weak Brpnsted sites [62]. 

In general acid catalysis, the rate is increased not only by an increase in [SH ] but also by an increase in the concentration of other acids (e.g., in water by phenols or carboxylic acids). These other acids increase the rate even when [SH ] is held constant. In this type of catalysis the strongest acids catalyze best, so that, in the example given, an increase in the phenol concentration catalyzes the reaction much less than a similar increase in (HjO ). This relationship between acid strength of the catalyst and its catalytic ability can be expressed by the Brpnsted catalysis equation ... 

---