Basicity of the catalyst

To ensure that proton transfer takes place from the protonated catalyst 64-H and not from the acidic reagent itself, apolar solvents favoring contact rather than solvent separated ion pairs as well as a slow addition of the acidic substrate RX-H are required. In addition, it was sometimes found beneficial to lower the basicity of the catalyst, thus rendering the protonated species [catalyst-H" ] more acidic for the stereo-determining protonation of the enolate. This was accomplished by formally replacing NR2 by Me (see 64e, Fig. 36). 

TPD that Cs loadings above 3% showed a significant decrease in the basicity of the catalyst, suggesting the growth of caesium oxide particles. 

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

An interesting spiro compound (59) that contains the thietane ring was obtained in minor amounts by dimerization of dimethylketene and subsequent treatment with P2S5. Pyrolysis of the spiro structure produced 60 and the thietanethione 61, which can also be prepared by base-catalyzed rearrangement of 62, a process that can be carried out as well with 63 to give the 2-thietanone 60. The solvent and the basicity of the catalyst are important parameters in this rearrangement. 

The Surface Properties of the Praseodymium Compound. Although the efficiency of catalysts in methane conversion has been ascribed to a variety of properties, a number of researchers have demonstrated the importance of basicity of the catalysts employed in this process (14-16). Thus estimates of basicity, such as may be obtained from the adsorption of carbon dioxide, are of some value in characterizing the catalysts. It is obvious that the surface state of a working catalyst at 750°C is different from that at room temperature. However, measurements of the adsorption of carbon dioxide at the latter temperature provide semiquantitative information on sites capable of donating electrons. 

When hard bases are the catalysts, the rate of elimination of a compound depends on the proton basicity of the catalyst as shown in Equation 7.34 (where kB is the rate constant for bimolecular elimination) 89... 

While THF or CH2CI2 are the most commonly used solvents, the solubility of the reagents or the catalyst may dictate the use of other solvents. Reactions are usually slow in DMF and in CH3CN when DABCO, DBU or DMAP were used as catalysts. An often-observed byproduct of the aza-MBH reaction is a bridged compound of type 97. This product is the result of a stepwise addition of 95 to 75 via the Mannich reaction, followed by an intramolecular conjugated addition (Michael addition) of the formed anion to the a,/ -unsaturated ketone, and thus due to the elevated basicity of the catalyst (Scheme 5.21) [92]. 

The interpretation given to the constant a is that it measures the sensitivity of the reaction (catalysis) to the acidity (or basicity) of the catalyst. In terms of the free energy changes we might say that it is a measure of the amount of the free energy change of ionization that occurs in the formation of the transition state. ... 

A comparison of the catalytic effects of compounds containing different groups is difficult when accounting only for the basicity. For example, Ph2NH and pyrazine (p/ a = 0.79 and 0.61, respectively) show different eatalytic effects. Besides the basicity of the catalysts other electronic and steric effects also play an important role. The hydrogenation is also catalyzed by DAD (Fig. 3). 

Besides the basicity of the catalyst, the acceptor strength of the disilane is important for the course of the hydrogenation. If the formation of a base adduct is the first step, in mixtures of several disilanes the disilane with the highest acceptor strength is hydrogenated first. 

Many different types of catalysts have been employed, including both metal oxides, such as PbO (5-7), which react in a redox cycle to Pb metal, and catalysts containing metals with fixed valence, such as Li promoted MgO (jg), which produce active Li 0" sites for methane dimerization. Such catalysts were discussed in four papers at the recent Ninth International Congress on Catalysis 8)- The most effective catalysts have several common traits. Low surface area has been found to be very important 2 9) in the conversion of methane. Also of importance is the basicity of the catalyst. (However, not every basic material causes C2 formation.)... 

Various sodium compounds were compared and shown in Table 2. Only Na2C03 showed an excellent conversion. But, other compounds including strong alkali compound represented a lower value. This result suggests that strong basicity of the catalyst is not required in gasification of rice straw. 

The W7 catalyst is prepared in the same way as the W4 but replacing the continuous wash by a brief decantation wash process. The resulting catalyst is quite alkaline and should be used only for those hydrogenations that are compatible with a strongly basic reaction medium. The basicity of the catalyst is probably responsible for its high activity. 

Other catalytic reactions may require an electron-rich metal to facilitate the ratedetermining (slowest) step of the catalytic cycle - Lewis basicity of the catalyst then plays a key role in substrate activation. For example, oxidative addition of is frequently rate determining in olefin hydrogenation reactions. The rate of alkene hydrogenation catalyzed by Rh(I)-diphosphinoferrocene complexes of type 2a (Figure 2) are known to increase the basicity of the phosphine moieties. In conventional approaches this can be achieved by... 

One of the key factors controlling the overall product spectrum is the "basicity" of the catalyst. This depends not only on the amount and type of alkali promoter present but also on its dispersion and how it has interacted with other promoters and impurities present (ref. 2). In the case of the fluidized-bed Synthol process the CH selectivity has been progressively lowered over the years from 15 to the current 1%. As the market for fuel gas is limited in South Africa, the excess CH must be catalytically reformed back to CO and (ref. 2). Not only is the production of CH wasteful in that it consumes more and CO than is needed for the formation of olefins but the reforming process itself is inefficient. Cutting back on the CH selectivity has therefore greatly benefited the overall economics of the Synthol FT process. 

In a similar manner, the sorption of weak acids, such as acetic acid, should give information on the basicity of the catalyst. 

ABSTRACT Over a copper chromite type catalyst, a DMEA (dimethylethylamine) yield of 70 % was obtained from monoethylamine (MEA) and methanol with diethylmethylamine (DEMA) as the main by-product. The formation of DMEA was increased to 85 % by just changing the basicity of the catalyst resulting from a change of the rate of the determining steps. The rate of the MEA condensation compared to that of the MEA methylation decreased. Moreover the mechanism of the second methylation step which could involve an intermediate amide ((MEFA) or aminoalkoxide) was different from that of the first methylation step. 

These data suggest that ionization of sodium metal on the disordered aluminate in B plays a key role in exhibiting the extremely strong basicity of the catalyst. 

The sum of results published in the literature on phenol alkylation using methanol are not clear and one cannot easily conclude to the relation between the acidity and the basicity of the catalyst and the selectivity in O or C alkylated products. However it seems that O-alkylation products can be obtained by the use of acidic catalysts (ref. 11). An increase of the acidity of oxide type catalysis (ref. 3) or mixed aluminium phosphate-alumina (ref. 12) gives rise to an increase of selectivity in O-alkylated products. However for strongly acidic catalysts C-alkylated products, which are the more thermodynamically stable, can be obtained either by isomerization or by reaction between phenol and methylarylether. 

The results on the basicity of the catalysts [measured by the STD of CO2 (chemisorbed at 50°C) from 50 to 980 C] are included in Table 1. The STD data on these catalysts are given elsewhere [18]. The results show that the basicity distribution for all the catalysts is hroad and it is strongly influenced by the promoters in the catalyst. 

In order to know the effect of the catalyst-support interaction on the surface basicity and base strength distribution, the chemisorption of CO2 at 100 C and TPD of CO2 from 50 to 900°C for the unsupported La-CaO cmd supported La-CaO ( with or without precoating the support by MgO or La203) catalysts have been measured. The data on the surface basicity of the catalysts is included in Table 3. 

The improved catalytic activity/selectivity of the MgO, CaO and La203 precoated supported La-CaO catalysts is attributed to the formation 0 f a protective layer of the precoated metal oxide between the reactive components of the supports and the deposited La-CaO. The improved catalytic performance is consistent with the increase in the strong basicity of the catalysts. 

MBA selectivity below 20 % were obtained. These results lead to the conclusion that the V2O5 phase still acts as an active phase whereas the formation of alkali metal-containing bronze-like species effects an increase in basicity of the catalyst surface and a separation of the active vanadyl sites and, hence, leads to the increase in aldehyde selectivity. 

Even though the relative basicities of the catalysts determined by benzoic acid were not quantitative, examination of Table VI reveals that, at least for the three catalysts examined, there is a correlation between catalyst basicity and activity. The failure of active alumina-based catalysts to respond significantly to the base additions indicates that these untreated catalysts approached their maximum basicity under the reaction conditions. This explanation agrees with the observations of Liu (22). 

In such a mechanism, what is the effect of increasing the nucleophilicity of the nucleophile and the basicity of the catalyst For a constant nucleophile changing the basicity of the catalyst would affect the rate as shown in the Eigen curve of Fig. 2a. When proton transfer from to B is thermodynamically favourable the rate of proton transfer is diffusion controlled and hence independent of the basicity of the catalyst B. When it is thermodynamically unfavourable the rate decreases proportionally to the decreased basicity of the catalyst, and is given by Kk K] /, where ATj and AT are the acid dissociation constants of T- and BH", respectively. 

Fig. 2. Hypothetical Br0nsted plot showing how the observed rate of the reaction outlined in Eqn. 7 of text varies with nucleophilicity of the attacking nucleophile and basicity of the catalyst (a) constant nucleophile, increasing basicity of catalyst when proton transfer from the intermediate is thermodynamically favourable increasing the nucleophilicity of the nucleophile increases the rate from (c) to (b).

The physicochemical properties of the catalysts are reported in Table II. A quantitative estimate of the acidity and the basicity of the catalysts, expressed in terms of the amounts of desorbed NH3 and COj per g of catalyst, is also given in this table. The NH, desorption data showed no clear correlation between acidity and metal loading for either of the two types of samples. This observation suggests that the HTC support possesses a high concentration of acid sites which appeared to be unaffected by the metal impregnation process. 

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