Br0nsted and Lewis Acids

Other materials that have been investigated include sulfated zirconia, Br0nsted and Lewis acids promoted on various supports, heteropolyacids, and organic resins, both supported and unsupported. On the whole, these materials also deactivate rapidly, and some of them also exhibit environmental and health hazards. 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

For zirconia, the species in Scheme 11.4 have been proposed [67], accounting for both Br0nsted and Lewis acidity. These Br0nsted acid sites are proposed to be present only if sulfates, and possibly poly(sulfates), are also present on the top terminations of Zr02 crystallites [66]. 

Catalytic activity in olefin polymerization is related to the presence of cationic metal-hydrocarbyl species [90], which can be obtained by (i) using oxide supports that have high Br0nsted and Lewis acidity, (ii) the addition of a co-catalyst to a neutral supported species or (iii) modification of the surface with Lewis acid cocatalysts prior to grafting of the metal-hydrocarbyl species (Scheme 11.8a-c) [91-97]. 

Rare-earth exchanged [Ce ", La ", Sm"" and RE (RE = La/Ce/Pr/Nd)] Na-Y zeolites, K-10 montmorillonite clay and amorphous silica-alumina have also been employed as solid acid catalysts for the vapour-phase Beckmann rearrangement of salicylaldoxime 245 to benzoxazole 248 (equation 74) and of cinnamaldoxime to isoquinoline . Under appropriate reaction conditions on zeolites, salicyl aldoxime 245 undergoes E-Z isomerization followed by Beckmann rearrangement and leads to the formation of benzoxazole 248 as the major product. Fragmentation product 247 and primary amide 246 are formed as minor compounds. When catalysts with both Br0nsted and Lewis acidity were used, a correlation between the amount of Br0nsted acid sites and benzoxazole 248 yields was observed. 

The infrared study of chemisorbed ammonia by Mapes and Eischens (55) was the first to demonstrate the power and utility of the infrared spectroscopic method for determination of surface acidity. These investigators demonstrated that IR spectra of ammonia chemisorbed on cracking catalyst contained H—N—H-bending bands that arose from NH4+ and coordinated NH3 (Fig. 6), a finding that constituted direct evidence for the existence of Br0nsted and Lewis acids on the surface of silica-alumina catalyst. Parry (23) subsequently suggested the use of pyri-... 

Lefrancois and Malbois (227) determined the types of acidity present on H-mordenite and various cationic forms by obtaining infrared spectra of pyridine adsorbed on the zeolite. H-mordenite activated at 400° contained both Br0nsted and Lewis acid sites. Upon addition of water, the band due to Lewis-bound pyridine disappeared and the Br0nsted site concentration increased. Removal of the added water by evacuation restored some of the Lewis acid sites. 

Jensen, W. B. (1980) The Lewis Acid-Base Concepts, Wiley-Interscience, New York. Bell, R. P. (1969) The Proton in Chemistry, Cornell University Press, Ithaca, NY. Shriver, D. F., Atkins, P. W., and Langford, C. H. (1990) Inorganic Chemistry, Oxford University Press, Oxford. Chapters 5 and 6 give a good introduction to Br0nsted and Lewis acids and bases. 

Zeolites Crystalline alumosilicates, three-dimensional network of silica and alumina with adjusted silica to alumina ratio Hydrophobic or hydrophilic depending on the silica/alumina ratio, cation exchanger, Br0nsted and Lewis acidity... 

Porous titania Anatas Br0nsted and Lewis acid and basic sites, cationic and anionic exchange groups, point of zero charge at pH 5, pJCa of Bronsted add sites 0.2-0.5... 

Selli and Forni [702] performed a detailed comparison of the determination of surface acidity by FTIR of pre-adsorbed pyridine on the one hand and temperature-programmed desorption of pyridine on the other, where the adsorbents were of Y-, Beta- or MOR-type. An important result of this study appears to be that the pyridine distribution between Br0nsted and Lewis acid sites imder equilibrium conditions (in FTIR) is not much different from that determined under kinetic control (in TPD). 

Br0nsted and Lewis acid-catalyzed cyclizations of enones have become important methods for the synthesis of fused-flirans and benzofurans. Mukai and co-workers recently applied this strategy to the synthesis of the tetracylic ring system of (+)-nakadomarin A, a compound with cyctotoxic, inhibitory and antimicrobial activity. Dihydroxylation of the isolated double bond of the a-pentenyl-a,P-cyclopentenone framework, followed by acid-catalyzed ring closure, gave the desired tetracyclic product in 80% yield. A similar acid-catalyzed strategy was employed by Dixon and co-workers in their synthesis of the furan moiety of (-)-nakadomarin A. ... 

In catalytic biomass cmiversion, the dehydration of polyol moieties is a key reaction, forming either an olefin bond, an ether, or a carbonyl group (after tautomerization). This type of reactimi usually requires the aid of acid catalysis. Both Br0nsted and Lewis acids are known to catalyze dehydration. The most famous dehydration in the context of biomass conversion is the formation of HMF from six-carbon sugars. As can be seen in Fig. 12, HMF from fructose (or glucose) preserves the F.C value of 1.17, while the FI index falls from 2 to 1. Similarly, pentoses lead to furfural in aqueous media under acid catalysis. The catalytic formation of HMF and furfural from hexoses and pentoses, respectively, and HMF production directly from cellulose, have been reported frequently over... 

Tables from 5 to 14 show, that the substitution of Al + and/or P + by divalent metal ions or tetravalent silicon in aluminophosphate structures creates both the Br0nsted and Lewis acid sites. These acid sites differ mutually in their donor-acceptor ability. The first one can transfer protons from the catalyst to the adsorbed molecules, whereas the latter can accept an electron pair from the adsorbed molecules. The strength and concentration of both types of acid sites determine the activity, selectivity, and lifetime of catalysts in acid-catalyzed reactions. The acid strength varies among aluminophosphates, and it is mainly dependent on the type of metal substituted in the framework. Also the catalytic performance is affected by structural characteristics of the framework such as the pore size, pore shape, or geometry.

These include oxidation sites with different oxidation states, as well as Br0nsted and Lewis acid and base sites. In addition, the sites responsible for O2 activation may be different from those for oxygen insertion or dehydrogenation. In order to stabilize particular reaction centers or catalyst phases, the formation of compounds with non-reactive cations is sometimes desirable. 

A great variety of homogeneous catalysts are known, including metal complexes and ions, Br0nsted and Lewis acids, and enzymes. Homogeneous transition metals are used in ... 

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