Bronsted-type acidity

The catalyst acid sites are both Bronsted and Lewis type. The catalyst can have either strong or weak Bronsted sites or, strong i)i weak Lewis sites. A Bronsted-type acid is a substance capable of donating a proton. Hydrochloric and sulfuric acids are typical Bronsted acids. A Lewis-type acid is a substance that accepts a pair of electrons. Lewis acids may not have hydrogen in them but they are still acids. Aluminum chloride is the classic example of a Lewis acid. Dissolved in water, it will react with hydroxyl, causing a drop in solution pH. 

Concerning the nature of the acid sites on dehydrated alumina, the chemical evidence is more in favor of Lewis-type than of Bronsted-type acids. Trambouze and Perrin (343) estimated the content of Lewis acid sites by thermometric titration of a slurry in benzene with dioxane. The authors stated that Lew is acidity was not observed in boehmite and its dehydration products, only in the products obtained from hydrargillite. As mentioned earlier, Webb (339) found no indication of Bronsted acidity ammonia did not form ammonium salts. The quantity of ammonia chemisoi-bed per unit area in the range from 175° to 500° was not a function of the hydrogen content. After exten.sive dehydration, more ammonia was chemisorbed at 100 mm pressure, even at 500°, than corresponded to the hydrogen present (341b). 

Probably, protons may interact with oxygen attached to silicon and form Bronsted-type acid sites of the following type SiOH-H+, while the CeOH3+ cations present on the surface can act as Lewis-type acid sites. 

While there is no question that the stronger acidic and basic oxides can speed up the typically slow aldol step, it is also true that few Bronsted-type acidic or basic sites are necessary. For example, dealumination of Pd/H-FAU or Pd/SAPO-11 molecular sieve catalysts was shown to increase activity by a factor of 5 in the acetophenone to 1,3-diphenylbutan-l-one reaction, or by 20 /o in... 

Silicon-29 MAS NMR spectra of the expanded clays contain two separate resonances representing silicon atoms in a and environment. Furthermore, IR results have indicated that these expanded clays contain both Bronsted and Lewis acid sites, and that SCD enhances Bronsted-type acidity. Steaming greatly decreases acid site strength in these catalysts. 

Mironova, L. V, Shmidt, E K., Tkach, V. S., and Omistriev, V I., Propylene dimerization under action of nickel complexes activated with Lewis type and Bronsted type acids, Neftekhimia, 18, 205, 1978. 

C. The IR spectra in Figure 7D indicate that steaming (at 760 C/5h) increases the ACH-rectorite Bronsted type acidity. Some substitution of Si(IV) with Al(IV) must be present also in the montmorillonite-like layers and therefore new Si(IV)-0H-Al(IV) groupings are formed when ACH-rectorite is steam-aged. Steaming drastically reduced Lewis type acidity as well as acid site strength ... 

Using the n-buthylamine titration method, Scherzer and Humphries (18) have shown that USY-B zeolites have considerably less acidity than USY-A zeolites. This is due to the more advanced thermal dealumination of USY-B, which reduces both Bronsted and Lewis type acidity. 

This does not occur in the case of catalyst and reactants here described. With Bronsted-type catalysis, the reaction between the benzoyl cation, Ph-C" =0, and the hydroxy group in phenol is quicker than the electrophilic substitution in the ring. This hypothesis has been also confirmed by running the reaction between anisole and benzoic acid in this case the prevailing products were (4-methoxy)phenylmethanone (the product of para-C-benzoylation) and methylbenzoate (obtained by esterification between anisole and benzoic acid, with the co-production of phenol), with minor amounts of phenylbenzoate, phenol, 2-methylphenol and 4-methylphenol. Therefore, when the 0 atom is not available for the esterification due to the presence of the substituent, the direct C-acylation becomes the more favored reaction. 

The stronger Lewis-type acidity in the equilibrated catalyst agrees with the many literature evidences about the existence of these sites in well-crystallized VPP (14). Also, a discrete amount of ions may contribute to an enhancement of Lewis-type acidity, due to the presence of defects associated to the anionic vacancies (5). The Bronsted acidity in the oxidized catalyst (sample ox3sp) can be associated to... 

They exhibit strong acidity, which is usually of the Bronsted type, partly because of the influence of the strong internal electrostatic fields (ca. 106 Vcm 1) exerted on the interlamellar water (which generates protons by dissociation) or, because of the additional influence of certain hydrated interlamellar cations, notably Al3+. Cation hydrolysis, just as with strongly polarizing cations in zeolites, yields free protons, thus ... 

The spectrum of ammonia chemisorbed on a silica-alumina cracking catalyst was studied to determine whether the acidity of these catalysts is due to a Lewis (nonprotonic) or a Bronsted type of acid (28, 29). This work was based on the premise that ammonia chemisorbed on Lewis sites would retain a NH3 configuration while ammonia chemisorbed on a Bronsted site would form NHt. The NH3 configuration was expected to have bands near 3.0 and 6.1 p and the NHt near 3.2 and 7.0 p. 

To avoid confusion, when the term acid or base is used in this text, it refers to a proton acid or base—that is, a Bronsted-Lowry acid or base. The term Lewis acid or Lewis base will be used when the discussion specifically concerns this type of acid or base. 

A vast generalization beyond the Bronsted—Lowry acids and bases concepts is the concept of a Lewis29 base (an electron pair donor) and a Lewis acid (an electron pair acceptor). This concept has been used extensively in all branches of chemistry. In physical organic chemistry, quantities of the type pA = —logio[A] have used extensively to study reactivities—for example, in the Hammett equation. 

A second type of salt that produces an acidic solution is one that contains a highly charged metal ion. For example, when solid aluminum chloride (AICI3) is dissolved in water, the resulting solution is significantly acidic. Although the Al3+ ion is not itself a Bronsted-Lowry acid, the hydrated ion A1(H20)63+ formed in water is a weak acid ... 

Some important reaction types not shown here are redox, combustion, Bronsted-Lowry acid-base, and Lewis acid-base. We will cover these types later in this book. Reaction types are not mutually exclusive, so one reaction can fall into more than one type. 

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