Brensted acids

The pyridinium ion (acid 2) as the analyte can be titrated with quaternary ammonium hydroxide (base 3) as it concerns the determination of H+ of the Brensted acid pyridinium, a potentiometric measurement of the pH titration curve and its inflection point is most obvious. In the aprotic, but protophilic, solvent pyridine no stronger acid can exist (see reactions 4.37 and 4.38) than the pyridinium ion itself hence there is a levelling effect but in theory only on the acid side. 

K. Manabe, Y. Mori, S. Kobayashi, A Brensted Acid-Surfadant-Combined Catalyst for Mannich-Type Readions of Aldehydes, Amines, and Silyl Enolates in Water Synlett. 1999,1401-1402. 

It would surpass the scope of this article to discuss in detail all the papers written on the complex question of whether Lewis or Brensted acids are responsible for the acidity of silica-alumina. 

IV. Bridge and Endohydrogens and Relative Lowry-Brensted Acidity... 

Fig. 2. Relation (8b) between the activity of ZSM-5 and AAS in isobutyl alcohol dehydration to butene at 3971C and the number of (a) Brensted acid sites [B] (b) strong Lewis acid sites, [LL , and (c) weak Lewis acid sites, [L]weak. (A) NaHZSM-5 sample 2 (9) HZSM-5, sample I (O) HZSM-S, sample 3 ( ) HZSM-5, sample 4 ( ) amorphous aluminosilicate, AAS.

In their hydrated forms zeolites are used for ion exchange purposes, for example, water softening by replacement of Ca2-with Na+ or another ion. When dehydrated they have important catalytic applications, promoted by the Brensted acid sites, and by the large area of internal surface. They are used for the cracking of petroleum and for the isomerisation of hydrocarbons, where limited pore size exerts a shape selectivity, which allows one desirable product to be formed in high yield. 

Brensted acidity arises from the possibility transferring a proton to a base, which may be the same compound. Basicity is possible when non-bonding electron pairs are present. Basicity towards protons decreases towards the right and down each group in the periodic table, so that ammonia is the strongest base among simple hydrides. 

Table 3.3 pA, values for some Lowry-Brensted acids... 

Neutralization reactions between Lowry-Brensted acids and bases are frequently employed in chemical analysis. Methods based on them are sometimes termed acidimetric or alkalimetric. 

From the DSC experiment, it is evident that the iron cation is less reactive in epoxide polymerization then Brensted acids obtained from, e.g. sulfonium salts (see Fig. 11). Therefore, a heat treatment after the illumination step is necessary. The narrow enthalpy peak observed could refer to a very uniform pathway. 

Scheme 2. Reactions of the enamine complexes 2a, 2b with Brensted acids. Table 2. Si CP/MAS NMR data of compounds 3a - 3f.

As it could be expected, for D-NH4Y the increase of activation temperature caused a reduction of the concentration of Brensted acid sites with a simultaneous increase in the amount of "true" Lewis acid sites (Table 1). The disappearance of the h.f and l.f bands upon activation at 975 K showed that framework Bronsted acidity is practically eliminated at this temperature. However, the existence of a tiny band assigned to pyridinium ions (1540 cm l). 

For PtCu/NaY, however, most of the coke is presumably not in intimate contact with metal clusters, its oxidation therefore takes place at a higher temperature. Since splll -over of active oxygen from the oxidized metal over finite distance can be excluded, the oxidation of this coke will not be catalyzed by Pt or Cu. In the reduced form of the catalysts the Ft ensembles at the surface of the encaged bimetal particles are diluted with Cu and additional protons of hi Brensted acidity were formed in the reduction of each Cu ion to Cu°. The combination of both phenomena, small Pt ensembles and hl concentration of protons of strong Brensted acidity, results in an Increase of the RE/RO activity ratio as observed and reported(S). It is therefore reasonable to attribute the TPO peak at higher temperature to the coke deposited on the acid sites of the zeolite via carbenium ion formation and polymerization. The results are thus quite similar to those observed and... 

Figure 10 Correlation between Brensted acid site density as determined by MAS>NMR and nonneutrallzed lattice Al atoms for a series of dealuminated H>Y zeolites. ...

Figure 11 Measured Brensted acid site density as a function of the nominal concentration from ammonium ion exchange, (o) infrared spectroscopy ( ) 2-propanol dehydration ) TPD of ammonia. Line represents perfect agreement. ...

Water hydroxylates the surface producing more Brensted acid sites. Conclusively, water slightly inhibits the catalytic activity at temperatures lower than 663 K and significantly blocks the formation of N2O at the surface of the catalyst [82]. These results are comparable with those of Odenbrand et al. [83]. The presence of 1 vol.-% of water suppressed the formation of N2O below 775 K, whereas the maximum of the NO conversion shifted to higher temperatures. 

The concentration of acid sites was determined by temperature programmed desorption (t.p.d.) of NH3. The degree of ion exchange was calculated from the difference in concentration of the strong Brensted acid sites present before and after ion exchange. The structure of the zeolites after synthesis or postsynthetic modification was verified by XRD. 

Formation and Structure of Highly Dispersed PdO Interacted with Brensted Acid Sites 429... 

---