Catalysts, general basic types

Acid Halides (Lewis Acids). AH metal haUde-type Lewis catalysts, generally known as Friedel-Crafts catalysts, have an electron-deficient central metal atom capable of electron acceptance from the basic reagents. The most frequendy used are aluminum chloride and bromide, followed by... 

The alkylation of aniline by methanol is carried out at 623-723K over LiY and NaY non-protonic and non-basic zeolites to study the effect of reaction temperature on the catalytic activity and the selectivity. The reaction temperature influences strongly the aniline conversion, the selectivity and the catalyst life. Two types of products are observed, C- or N- alkylates whether the alkylation takes place on the ring or on the N atom. A general trend is that as the reaction temperature increases the catalytic activity and selectivity of N-alkylation increase while the selectivity of C-alkylation decreases. The IR results of benzene adsorption have been used to try to explain the catalytic properties of these two catalysts. 

Resol-Type Foam Chemistry. Resol is obtained as a result of the reaction between phenols (P) and formaldehyde (F) in the presence of basic catalyst. Generally, the reaction is made at a temperature below... 

A further application of the heterobimetallic lanthanoid catalysts of the LLB type to the field of catalytic asymmetric Diels-Alder reactions [47,48] was also achieved by Shibasaki et al. [49]. In general, LLB type complexes are multifunctional asymmetric catalysts, showing both Bronsted basicity and Lewis acidity. Nevertheless, in this study the use of LLB type catalysts acting as asymmetric Lewis acids alone was examined and led to the development of an LLB (type) catalyzed asymmetric Diels-Alder reaction [49]. Representative results for the catalytic asymmetric Diels-Alder reactions using 48 and cyclopentadiene in toluene as a solvent are shown in Scheme 16. 

X-ray photoelectron spectroscopy ( XPS ) reveals that impregnation of Raney catalysts with basic-pH salts, e.g. ammine complex types containing Pd or Pt, rather than previously reported acid salts, modifies the placement of PM within the Ni catalyst s sponge stmcture. The PM placement is expressed as a surfaceAiulk ( S/B ) ratio for the normalized dopant concentration. The resulting basic salt catalysts have lower S/B ratios and also generally perform better in activity and life tests than the acidic salt types. 

LASC-catalyzed Michael reaction in water has also been reported (Scheme 12.65) [165]. Michael reactions are generally performed under basic conditions that sometimes cause serious side reactions. Using LASC, these problems could be solved. Sc(DS)3 was shown to be the most active catalyst among several types of Lewis acids or LASCs [166]. Similarly, Friedel-Crafts-type reactions of aromatic compounds, the Reactions of indoles with electron-deficient olefins, proceeded smoothly in the presence of a catalytic amount of Sc(DS)3 (Scheme 12.66) [167],... 

Generally catalysts used for the anionic polymerization can be divided into two basic types ... 

The deviations that are observed from either the free-growth or Avrami relations can be attributed in part to the general problem encountered in homopolymer crystallization, i.e. the role of chain entanglements. In addition, there is a major contribution to the deviations due to the decreasing availability of eligible sequences as the transformation proceeds. This is due to the decrease in the undercooling at constant temperature. As a consequence, in contrast to homopolymer crystallization, copolymer isotherms are not superposable. Deviations from theory are observed at much lower levels of crystallinity, although the same basic type of nucleation is involved with both homopolymers and copolymers. Nucleation catalysts influence copolymer crystallization in a similar manner to that of homopolymers.(33b)... 

Michael-Type Additions. Michael additions are generally used to prepare methyl 3-mercaptopropionate (eq. 10) and mercaptopropionitrile (eq. 11) by the reaction of methyl acrylate or acrylonitrile and hydrogen sulfide using a basic catalyst. This reaction proceeds as shown ... 

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

Majetich and Hicks <96SL649> have reported on the epoxidation of isolated olefins (e.g., 61) using a combination of 30% aqueous hydrogen peroxide, a carbodiimide (e.g., DCC), and a mildly acidic or basic catalyst. This method works best in hydroxylic solvents and not at all in polar aprotic media. Type and ratios of reagents are substrate dependent, and steric demand about the alkene generally results in decreased yields. 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

The remarkable variety of redox systems which can already be derived from the Weitz type underline the wide scope of the general structure A and C as a basic principle for two step redox systems. The empirical material as well as general rules regarding structural influences on potentials and Ksem have been developed to such an extent, that redox systems can be taylored to meet special purposes. Catalysts for electron transfer, light positive systems and compounds of high electrical conductivity are some fields in which these redox systems could occupy key positions. Some applications have already been discussed in a previous review of wider scope h)... 

Tertiary amines catalyze the homopolymerization of epoxy resins in the presence of hydroxyl groups, a condition which generally exists since most commercial resins contain varying amounts of hydroxyl functionality (B-68MI11501). The efficiency of the catalyst depends on its basicity and steric requirements (B-67MI11501) in the way already discussed for amine-catalyzed isocyanate reactions. A number of heterocyclic amines have been used as catalytic curatives pyridine, pyrazine, iV,A-dimethylpiperazine, (V-methylmorpholine and DABCO. Mild heat is usually required to achieve optimum performance which, however, is limited due to the low molecular weight polymers obtained by this type of cure. 

Polymerization of THF was first observed by Meerwein and his coworkers (8) and was studied extensively by this group in the 1930 s and 1940 s. However, this work did not become generally known until after World War II and even then it was available only in the form of microfilmed reports (9). It was not until 1960, with the publication of Meerwein s review (3), that the scope of this excellent work became generally available. The catalysts used by this group are basically of the trialkyl oxonium ion type, either preformed or generated in situ. Meerwein classified combinations which generate oxonium ions into three broad groups ... 

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