Acidic catalytic activity

Acidic Cation-Exchange Resins. Brmnsted acid catalytic activity is responsible for the successful use of acidic cation-exchange resins, which are also soHd acids. Cation-exchange catalysts are used in esterification, acetal synthesis, ester alcoholysis, acetal alcoholysis, alcohol dehydration, ester hydrolysis, and sucrose inversion. The soHd acid type permits simplified procedures when high boiling and viscous compounds are involved because the catalyst can be separated from the products by simple filtration. Unsaturated acids and alcohols that can polymerise in the presence of proton acids can thus be esterified directiy and without polymerisation. 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

Amorphous Sn-, Si-, and Al-containing mixed oxides with homogeneous elemental distribution, elemental domains, and well-characterized pore architecture, including micropores and mesopores, can be prepared under controlled conditions by use of two different sol-gel processes. Sn-Si mixed oxides with low Sn content are very active and selective mild acid catalysts which are useful for esterification and etherification reactions [121]. These materials have large surface areas, and their catalytic activity and selectivity are excellent. In the esterification reaction of pentaerythritol and stearic acid catalytic activity can be correlated with surface area and decreasing tin content. The trend of decreasing tin content points to the potential importance of isolated Sn centers as active sites. 

Eu, Gd, Dy, Er, Yb, and Th forms. Infrared spectral measurements were conducted in situ to characterize the OH groups and acidity. Catalytic activities were measured by cumene cracking. 

Most industrial shape selective catalytic processes today use medium-pore zeolites from the "pentasil" femily. (The name refers to the five-membered rings in their framework and to their high silicon content.) ZSM-5 is by far the most important member of this family. It has high acid catalytic activity and it is very stable The silica/alumina ratio in ZSM-S varies from the teens to the thousands. High silica/alumina ratios give hydrophobidty, high acid strength, and thermal, hydrothermal, and acid stability. 

When catalyst functions are regenerated by coke burning, the coke on the metal is burnt first, and metal activity is recovered. Then, coke on the acid function is eliminated and the acid catalytic activity is recovered. 

Table 1 shows the catalytic activity of bulk PMo and PMo-PSF-DMF film catalyst at 170°C. Acetaldehyde is formed by oxidation reaction while ethylene and diethylether are formed by acid-catalyzed reaction over PMo catalyst [24]. As shown in Table 1, PMo-PSF-DMF film catalyst shows the higher ethanol conversion than bulk PMo. The PMo-PSF-DMF shows remarkably enhanced yield and selectivity for acetaldehyde, but it shows obviously decreased yield and selectivity for ethylene and diethylether compared to the bulk PMo. The oxidation activity of the film catalyst was about 10 times higher than that of bulk PMo. It was believed that the enhanced oxidation activity of the film catalyst was due to the fine distribution of PMo through PSF matrix whereas the reduction of an acidic activity was due to DMF which was strongly adsorbed on the acid sites of PMo. The DMF effect was also confirmed by the catalytic activity of PMo-DMF in Table 1. PMo-DMF showed the suppressed acidic catalytic activity compared to the bulk PMo. Above results imply that the film...

Effect of silica binder on acidity, catalytic activity and deactivation due to coking in propane aromatization over H-gallosilicate (MFI)... 

In some instances, there still exist conflicting reports about the surface acidity-catalytic activity correlation. These differences may arise not only from the use of different reaction conditions and different approaches to preparing or modifying the catalysts but also from a poor characterization of the materials employed. Indeed, the detailed physicochemical characterization of the catalytic materials, as well as the study of their interaction with reagents and products, still represents well-recognized problems in the use of heterogeneous catalysis for organic syntheses. 

It is widely known that catalytic cracking of hydrocarbons takes place on the sites of the catalyst that possess the properties of Bronsted or Lewis acids catalytic activities for cracking and acidity run in parallel. 

The results show clearly that, in the presence of water, the acid catalytic activity of supported sulfonic acid groups is essentially the same on polymer and silica supports, except where the level of polymer sulfonation is high, when the sulfonic acid exhibits significantly enhanced activity. The trend in molar enthalpies of neutralisation with aqueous NaOH is similar. On silica supports, and polymer supports with low sulfonic acid concentrations, these enthalpies are very similar to those of strong mineral acid solutions. In contrast, resins with high levels of sulfonation show significantly higher molar enthalpies of neutralisation. 

G319A <4> (<4> isozyme PDK2, site-directed mutagenesis, mutation of conserved amino acid, catalytically active, but very poor binding of ATP [24]) [24]... 

A range of PILs was trialed in the Friedel—Craft alkylation of phenol with tert-butyl alcohol to produce tert-butyl alcohol as mentioned in section 6.1. These included a series of pyridinium PILs specifically designed for use as acid catalysts by Duan et al. and SO3H functionalized Bronsted acidic AILs. " It was shown successfully from the pyridinium PILs that a range of PILs could be easily produced that modified the acidities, catalytic activities, and, hence, selectivities and conversions for this reaction type, with the best performance by 2-methylpyridinium, with a conversion of 95%, and a selectivity toward 2,4-di- r butylphenol of 82%. In general, the SO3H functionalized AILs led to comparable selectivities and conversions for this reaction as the PILs. " Some nonstoichiometric salts containing pyridinium cations with HF anions have been used successfully as the catalyst and reaction media for the isobutane—olefin alkylation reactions. 

The tailored dinuclear aluminium Lewis acid displayed a high Lewis-acid catalytic activity due to a double electrophilic activation of a substrate s carbonyl group. This skilful ligand design turned a standard alkoxide into a real multitool for carbonyl chemistry. This type of catalyst was employed for instance in Mukaiyama aldol reactions, in MPV reductions and Oppenauer oxidations and related Tischchenko coupling reactions. 

The Si/Al ratio plays a significant role, since the aluminum atom is directly related to the acidic site and accounts for the formation of carbenium and/or carbonium ions or possibly cation radicals inside the zeolite. Dealumination processes can promote porous structure modifications, which may improve some interesting properties of zeohtes, like thermal and hydrothermal sta-bihty, acidity, catalytic activity, resistance to aging and low coking rate, and... 

In another study [138], authors presented evidence that heats of adsorption of ammonia on three different kinds of zeolite structure (Y, mordenite, ZSM-5) can be used to obtain the correlation plots that describe relationships between acidic and catalytic properties of these catalysts. Catalytic properties were tested in simple carbonium ion reactions. An approach to search acidity-catalytic activity correlations was based on the straightforward concept which implies that the heat of adsorption of the base is directly related to the energy needed for the protonation of hydrocarbon molecules leading to the carbocation formation. For example it was... 

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