Bronsted activity

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidie sites of such solid acids must be eon-sidered. Nafion-H contains acidic -SO3H groups in clustered pockets. In the acidic zeolite H-ZSM-5 the active Bronsted and Tewis acid sites are in close proximity (—2.5 A). 

A number of curing agents and catalysts used in epoxies are complex metal salts that are added to cure at room temperature or with heat. Curing agents or catalysts such as cationic dinonato (acetylacetone, etc.) complexes of Si, B, Ge, and P behave as hydrolytic activated Bronsted acid precursors, e.g. ... 

Figure 2 shows for HYUS-5 that there are several types of differently coordinated aluminium. Indeed, there are bands at 60, 30 and 1.4 ppm, which are related to tetrahedral, pentahedral (5) or tetrahedrally distorted aluminium (6), and different types of octahedrally coordinated aluminium, respectively. The tetrahedr aluminium appearing at 60 ppm has always being attributed to FAL (7). However, the intensity of the 60 ppm lime (34%) becomes difficult to reconcile with the value of 140 for the zeolite framework Si/Al ratio obtained from the unit cell size (24.24 A). Indeed, from the X-ray unit cell contraction one should expect 4% of the total aluminium to be in framework tetrahedral position. Then we conclude that in steam dealuminated zeolites there is some aluminium which is not in framework positions but it is also tetrahedrally coordinated (EFAL ). The same is observe in the HYD samples. We have recently claimed (8) that the EFAL could be present as a special type of non-crystalline silica-alumina formed during dealuminatij. If this is so, there is not doubt that at least some of the EFAL could be associated with BronsJ d acid sites, which would not be taken into account if only the FAL is considered to be related to active Bronsted sites. 

The importance of the desorption step in the reaction network may originate from the specific interaction of the 5-ring hydrocarbon molecules with the erionite pore system. The shape-selectivity of the erionite component strongly influences the equilibrium reactions n-heptane o 5-iing naphthenes methyl-cydohexane toluene because only n-heptane and the cyclopentane derivatives are able to reach the active Bronsted sites. In addition, the mobility of these hydrocarbons in the erionite should be quite different. 

A further finding was that only the moderately active Bronsted acid centers are responsible for dehydration, and that Lewis acid centers such as Al " are not involved. Evidence for this is that the addition of small amoimts of bases such as NH3 or pyridine does not inhibit the reaction. The formation of ether on AI2O3 is explained by a Langmuir-Hinshelwood mechanism, in which two adjacently adsorbed... 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

We found a way to overcome charge-charge repulsion when activating the nitronium ion when Tewis acids were used instead of strong Bronsted acids. The Friedel-Crafts nitration of deactivated aromatics and some aliphatic hydrocarbons was efficiently carried out with the NO2CI/3AICI3 system. In this case, the nitronium ion is coordinated to AICI3. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

Linear free energy relationships, see Bronsted equation, Dual substituent parameter (equations), Hammett equation(s), Quantitative structure-activity relationships, Ritchie nucleophilicity equation... 

Thomas and Long488 also measured the rate coefficients for detritiation of [l-3H]-cycl[3,2,2]azine in acetic acid and in water and since the rates relative to detritiation of azulene were similar in each case, a Bronsted correlation must similarly hold. The activation energy for the reaction with hydronium ion (dilute aqueous hydrochloric acid, = 0.1) was determined as 16.5 with AS = —11.3 (from second-order rate coefficients (102At2) of 0.66, 1.81, 4.80, and 11.8 at 5.02, 14.98, 24.97, and 34.76 °C, respectively). This is very close to the values of 16.0 and —10.1 obtained for detritiation of azulene under the same condition499 (below) and suggests the same reaction mechanism, general acid catalysis, for each. 

Bronsted-acid-catalyzed Diels-Alder reactions are not frequent because of the proton sensitivity of many dienes and cycloadducts, especially when long reaction times and high temperatures are required. Examples in aqueous medium involving imines activated by protonation as dienophiles and a proton-promoted Diels-Alder reaction of glyoxylic acid with cyclopentadiene are considered in Section 6.1. 

Bronsted-Evans-Polanyi (BEP) linear activation energy-reaction energy relationships ... 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

Linear relations between the activation energies and heats of adsorption or heats of reaction have long been assumed to be valid. Such relations are called Bronsted-Evans-Polanyi relations [N. Bronsted, Chem. Rev. 5 (1928) 231 M.G. Evans and M. Polanyi, Trans. Faraday Soc. 34 (1938) 11]. In catalysis such relations have recently been found to hold for the dissociation reactions summarized in Pig. 6.42, and also for a number of reactions involving small hydrocarbon fragments such as the hydro-... 

This is an example of a reversible reaction the standard electrode potential of the 2PS/PSSP + 2c couple is zero at pH 7. The oxidation kinetics are simple second-order and the presence of a radical intermediate (presumably PS-) was detected. Reaction occurs in the pH range 5 to 13 with a maximum rate at pH 6.2, and the activation energy above 22 °C is zero. The ionic strength dependence of 2 afforded a value for z Zg of 9 from the Bronsted relation... 

Bronsted acidity is the principal source of activity with the relative concentration of protonated and non-protonated reactants being dependent upon the nature of the exchangeable cation. Using FeCls - graphite intercalates - formed using a photochemical procedure and subsequently reduced using K/naphthalide - an efficient catalyst for the production of acetylene from syngas has been produced. 

---