Sodium cations, catalysis

S.Y. Kim and J.G. Goodwin, Jr., Effect of Back-Exchange of Sodium Cations on PdY Catalysis, unpublished results. 

A DFT Study of the acid catalysis of the mutarotation of erythrose and threose has looked at reaction in the gas phase, and in a continuum water model.Sodium cation can act as an inhibitor, whereas borane acts as a Lewis acid catalyst. Brpnsted acids H+ and HjO" " are particularly effective, with the activation energy being further lowered using H30" with one bridging H2O. 

In this section the influence of micelles of cetyltrimethylammonium bromide (CTAB), sodium dodecylsulfate (SDS) and dodecyl heptaoxyethylene ether (C12E7) on the Diels-Alder reaction of 5.1a-g with 5.2 in the absence of Lewis-add catalysts is described (see Scheme 5.1). Note that the dienophiles can be divided into nonionic (5.1a-e), anionic (5.If) and cationic (5.1g) species. A comparison of the effect of nonionic (C12E7), anionic (SDS) and cationic (CTAB) micelles on the rates of their reaction with 5.2 will assess of the importance of electrostatic interactions in micellar catalysis or inhibition. 

Micellar catalysis of azo coupling reactions was first studied by Poindexter and McKay (1972). They investigated the reaction of a 4-nitrobenzenediazonium salt with 2-naphthol-6-sulfonic and 2-naphthol-3,6-disulfonic acid in the presence of sodium dodecylsulfate or hexadecyltrimethylammonium bromide. With both the anionic and cationic additives an inhibition (up to 15-fold) was observed. This result was to be expected on the basis of the principles of micellar catalysis, since the charges of the two reacting species are opposite. This is due to the fact that either of the reagents will, for electrostatic reasons, be excluded from the micelle. 

Experience in PTC with cationic catalysts showed that, in general, the most suitable compounds have symmetrical structures, are lipophilic, and have the active cationic charge centrally located or sterically shielded by substituents. For anionic catalysis sodium tetraphenylborate fulfills these conditions, but it is not stable under acidic conditions. However, certain derivatives of this compound, namely sodium tetra-kis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB, 12.162) and sodium tetrakis[3,5-bis-(l,l,l,3,3,3-hexafluoro-2-methoxy-2-propyl)phenyl]borate (HFPB) are sufficiently stable to be used as PTC catalysts for azo coupling reactions (Iwamoto et al., 1983b 1984 Nishida et al., 1984). These fluorinated tetraphenylborates were found to catalyze strongly azo coupling reactions, some of which were carried out with the corresponding diazotization in situ. 

With aliphatic amines, the decomposition catalysis is moderate with heterocyclic aromatic amines (pyridine, quinoline), 0.1 % of amine is sufficient to cause maleic anhydride to decompose. An accident has also been mentioned with NaOH. This decomposition also takes place in the presence of sodium, lithium, ammonium, potassium, calcium, barium, magnesium and beryllium cations. 

Other reactions in which cations other than protons are catalyti-cally effective are esterification and acetal formation, catalyzed by calcium salts,277 and the bromination of ethyl cyclopentanone-2-carboxylate, catalyzed by magnesium, calcium, cupric, and nickel, but not by sodium or potassium ions.278 One interpretative difficulty, of course, is the separation of catalysis from the less specific salt effects. The boundary line between salt effects (medium effects) and salt effects (catalysis) is not sharp either in concept or experimentally. 

The synthesis of ionic liquids with BF4 and PF6 as cations has been the subject of much research since they are the most widely used in catalysis. However, it is difficult to make these ionic liquids in a pure form. The original route used to prepare ionic liquids with these anions consists of a metathesis (anion-cation exchange) reaction in which the imidazolium chloride is reacted with the sodium salt of the anion in a suitable solvent [8], The reaction is illustrated in Scheme 4.2 for the tetrafluoroborate salt. 

Very striking results on the interactions of molecules with a catalyst have been recently reported in zeolite catalysis because of the well ordered structure of these materials it is worth mentioning the subjects of zeolite design [10] and of acidic properties of metallosilicates [11]. In other areas where polycrystallinic or even amorphous materials arc applied, highly interesting results are now numerously emerging (such as hydrocarbon oxidation on vanadium-based catalysts [12] location of transition metal cations on Si(100) [13] CO molecules on MgO surfaces [14] CH4 and O2 interaction with sodium- and zinc-doped CaO surfaces [15] CO and NO on heavy metal surfaces [16]). An illustration of the computerized visualization of molecular dynamics of Pd clusters on MgO(lOO) and on a three-dimensional trajectory of Ar in Na mordenitc, is the recent publication of Miura et al. [17]. 

An early example of a comparison between the microemulsion approach and the process of phase transfer catalysis was a study by Menger et al. on the hydrolysis of trichlorotoluene to form sodium benzoate see Fig. 6 [47]. As can be seen from Table 1, hydrolysis in the presence of the cationic surfactant... 

Restructuring of the metal surface by adsorbed sulfur appears to be a general phenomenon (58,63,65, 70, 71, 74,81, 92,93). One observation is of particular interest here. It was observed that the Pt(l 11) surface reorients to the (100) plane in the presence of H2S (81). McCarroll (94) reported, on the other hand, that when Ca+ or Na+ ions were added to a clean Pt(100) surface, it caused a reorientation of surface Pt atoms to a (111) plane. He suggested that the switching between (100) and (111) planes may be the result of the cationic/anionic role played by Na+ or Ca+/S, respectively, and that this might possibly explain the role of promoters in catalysis. The reorientation of the metal surface is not unique to sulfur, sodium, or calcium only. It has been observed, for example, that the Ni(l 11) surface reorients to the (100) surface in the presence of ethylene or benzene (55). Somorjai (95) has... 

Micellar effects were found to be variable and dependent on the type of micelle employed. Cationic micelles, such as cetyltrimethylammonium bromide, inhibited hydration. Anionic micelles formed from sodium lauryl sulfate (NaLS) produced a small amount of catalysis at low concentration, exp actually passing through a maximum at [NaLS] < the critical micelle concentration (cmc). On the other hand, micelles formed from monopotassium -dodecyl phosphate in unbuffered water give impressive catalysis relative to water itself. Detailed discussion of these effects is given in Reference 80. 

The surfactant-aided Lewis acid catalysis was first demonstrated in the model reaction shown in Table 13.1 [22]. While the reaction proceeded sluggishly in the presence of 10 mol% scandimn triflate (ScfOTOs) in water, a remarkable enhancement of the reactivity was observed when the reaction was carried out in the presence of 10 mol% Sc(OTf)3 in an aqueous solution of sodium dodecyl sulfate (SDS, 20 mol%, 35 mM), and the corresponding aldol adduct was obtained in high yield. It was found that the type of surfactant influenced the yield, and that Triton X-100, a non-ionic surfactant, was also effective in the aldol reaction (but required longer reaction time), while only a trace amount of the adduct was detected when using a representative cationic surfactant, cetyltrimethylammonium bromide (CTAB). The effectiveness of the anionic surfactant is attributed to high local concentration of scandium cation on the surfaces of dispersed organic phases, which are surroimded by the surfactant molecules. 

Hexacyanoferrate(II). K4[Fe(CN)6] has been used to photosensitize Ti02. The acidity constants forH4[Fe(CN)6] are pKi = 2.54, p 2 = T08, pA 3 = 2.65, p 4 = 4.19. Association constants have been pubhshed for alkali metal cations ion pairing with hexacyanoferrate(II). Substitution at hexacyanoferrates(II) is very difficult, though it can be catalyzed by metal ions such as Hg +. Such catalysis can be augmented by surfactants such as sodium dodecyl sulfate (SDS), and indeed SDS-catalysis of Hg +-catalyzed replacement of cyanides in [Fe(CN)6]" ... 

Selection of the cations was based on several criteria. First, the raw coal contained alkaline-earth, mainly Ca and Mg, and some alkali metals on its carboxyl groups. Also, McKee (13) and Walker et al. (12) have shown that sodium, potassium and calcium are excellent catalysts for the C-O2 reaction. Thus, these cations may have a significant effect on the char burnout rate. In addition. Mg was back exchanged on the coal since it was contained on the raw coal and, as shown by Walker et al. (12), it is a poor catalyst for the C-O2 reaction. The purpose of using this alkaline-earth metal was to determine if catalysis of the heterogeneous C-O2 reaction affected the char burnout rate. This would help to elucidate whether the char burnout step was chemically or physically rate controlled. 

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