Broensted acid

Organic molecules spontaneously form corresponding cation-radicals on inclusion within activated zeolites (Yoon and Kochi 1988, Yoon 1993, Pitchumani et al. 1997). Zeolites are crystalline alu-mosilicate minerals that are widely used as sorbents, ion exchangers, catalysts, and catalyst supports. As zeolites act as electron acceptors due to the presence of Lewis- or Broensted-acid sites, confined organic compounds occur to be electron donors. Frequently, the interaction of electron donor with electron acceptor centers spontaneously generates cation-radicals and traps the ejected electrons. 

Methylation - Carboxylic groups are determined by their methylether derivatives. Methylation may be carried out by diazomethane in the presence of a Broensted acid, such as HC1 or in the presence of a Lewis acid such as BFj, or in methanol. Phenolic-OH may also be estimated by this method if methylation in ether is followed by saponification to eliminate interference from -COOH. 

Considering the organometallic side of Scheme III (right), usually all the compounds located on this side are able to produce the neighboring systems on the left by Broensted acid-base-type reactions, e.g., alkyls might readily react with amines, cyclopentadienes and alcohols to yield amide, alkoxide and cyclo-pentadienyl complexes, respectively. The silylamide route has attracted enormous attention for the synthesis of pure alkoxides (A in Scheme III, Eq. 12)... 

The resurgence of organometallic lanthanide amide chemistry and in particular that of the silylamides is certainly connected with their use as key synthetic-precursors. The so-called silylamide route is standard procedure in synthetic lanthanide chemistry. Suitable substrates are generally more Broensted acidic compounds like alcohols, phenols, cyclopentadienyls, acetylenes, phosphanes, thiols as listed in Scheme 12 [133,140,254-263]. 

Van Santen RA, Kramer GJ (1995) Reactivity Theory of Zeolitic Broensted Acidic Sites, Chem Rev 95 637-660... 

Broensted acids proved less effective as catalysts and scale-up considerably larger (-60%) amounts of distillate than expected were noted, consisting... 

The activation energy of the reaction was 106 kjoules/mole. The product was moderately hydrogenated and predominantly straight chain, such that 82% of the C2 5 alkenes was 1-alkene. This catalyst deactivated at 1% per h. The hydrotalcite preparations were generally of lower activity than the chlorites, as illustrated in Table IV by the results from a preparation aged at 60 C. Remarkably, however, over a period of 24 h no fall in activity could be detected. For this catalyst, isomerization of the primary product was evident, in that while 78% of C2-.5 was alkene, only 43% was 1-alkene, thus demonstrating substantial Broensted acidity from the hydrotalcite residue support. 

Hydroxyl groups ( 3.3). H2O is almost ubiquitous and easily reacts with low-coordinated sites to form OH groups at the surface of MgO. These centers can act as nucleation centers in the growth of metal particles, induce asymmetries in the surface electric field, or exhibit a classical Broensted acid behavior. 

The substitution of aluminium for boron in zeolites leads to a material with decreased Broensted acidity. These properties have been successfully applied in industrial processes, such as the Assoreni (methyl tertiobuthylether into methanol and isobutene) and Amoco processes (xylene isomerization and ethylbenzene conversion) [1-3]. Recently, the methanol conversion, the alkylation of toluene with methanol and the xylene isomerization on borosilicalites were critically analyzed [4]. 

H-beta zeolite proved to be an active and selective catalyst for alkylation of benzene with propene. In situ spectroscopic methods were applied to follow the formation and the evolution of surface intermediates and products,. It was found that when benzene is taken alone on the zeolite surface, its adsorption is reversible up to 473 K. On the contrary propene undergoes to several transformations even at 295 K. Isopropylbenzene behaves as propene, giving the same intermediates and products by decomposition at higher temperatures. Isopropyl cations formed upon chemisorption of propene on Broensted acid sites are the key intermediates for the alkylation reaction and are responsible for the faster deactivation via unsaturated caibenium ions formation. 

Upon adsorption of increasing amount of cumene on H(3, the OH bands shifted to lower wavenumbers, and bands characteristic for adsorbed cumene appeared (Fig. 4). IR spectra clearly show that cumene first interacts with the bridging OH groups and then with the Broensted acid sites due to extraframework aluminium species. It is interesting to remark that even the terminal silanols interact with cumene. Cumene is not easily removed from the zeolite, the background spectrum of zeolite could not be restored. 

The presence of Lewis and Broensted acid sites in zeolites and their ratio influences the mechanism and product distribution for the catalytic redox reactions. In our previous works [7, 9, 10], we have shown that Lewis acid sites... 

Replacing the weaker Broensted acids by stronger carboxylic acids makes the substitution of the hydrogen atoms in compound 1 more rapid. In the absence of any catalyst hypervalent 1 reacts spontaneously with benzoylic acid under evolution of dihydrogen to produce the benzoyloxysilane 4 [6], Similar observations were made by Corriu et al. [6, 7], On heating 4 to 120°C elimination of benzaldehyde occurs and oligomeric 3 is formed. 

Tanaka, M., Fujiwara, M., Ando, H. Dual Reactivity of the Formyl Cation as an Electrophile and a Broensted Acid in Superacids. J. Org. Chem. 1995, 60, 3846-3850. 

One arm of the imidazolium scaffold contains the catalytic centre, a bridgehead nitrogen atom possessing the required nucleophilicity, the second arm contains a Broensted acidic primary alcohol capable to speed up the critical proton transfer step which leads to the P-ammonium enolate intermediate, direct precursor of the final Baylis-Hillmann product. The reaction of RiCHO and CH2=CH-R2 is carried out under solvent free conditions at room temperature, catalyst 10 can be readily recovered from the reaction mixture and reused for at least 6 times without significant loss of catalytic activity. A few results are reported in Table 3. 

Strategy C. The Broensted acid 16 smoothly catalyses the Mannich reaction shown in Figure 15, affording the corresponding P-amino carbonyl compounds in excellent yield and short reaction times. ... 

Mo and Xiao recently reported an upgraded reaction protocol (Figure 16) which is based on the use of Pd(OAc)2, dppp, and a Broensted acid. 

High intrinsic acidity which arises form the presence of a highly delocalized anion charge and of mobile protons in both aqueous and organic media the acidity is even higher than that of some mineral acids. The Broensted acidity can be controlled by partial neutralization of the protons. 

In the present investigation pure and reduced form of CuY zeolite were prepared, characterised and their catalytic activity towmds dehydration reaction of t-BuOH was compared with that of NaY and H-Y zeolite. The dehydration activity is correlated to the Broensted acid density of various catalysts. 

The percentage conversion of t-BuOH over various catalysts and the acidity of catalysts between 513-673° K is given in Table 4. The catalytic activity follows the order NaY < CuY < HY < CuY(R). The relatively low activity of NaY is because of its very low Broensted acidity due to the presence of strongly basic sodium ions. The acid zeolite HY is obtained by the decomposition of the corresponding ammonium zeolite [11]. 

The protons thus formed as above are present as surface hydroxyl groups and is a cause of Broensted acidity in zeolites. This accounts for the high acidity and catalytic activity of HY. CuY showed a better activity than NaY. This as probably due to Cu" ions which constitute Lewis acid sites generating Broensted acid sites during activation treatment of CuY carried out prior to the dehydration reaction. The precursor for the above protons and hence the new Broensted sites could be the molecular water that gets evolved during the activation treatment as seen from TG/DTA profile. 

The high catalytic activity of CuY and CuY(R) is due to conversion of Lewis sites to Broensted sites during activation of CuY and due to generation of protons during reduction of Cu by hydrogen respectively. The relatively low activity of NaY is due to very low Broensted acidity. High activity of HY is due to very large value of Broensted acidity. 

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