Nature of Alumina Catalysts

In spite of the fact that alumina is an excellent and widely used catalyst for the dehydration of alcohols, there is no agreement in the literature with regard to the mechanism of this reaction or the nature of the olefinic products. For example, 1-alkenes have been obtained from primary alcohols such as 1-butanol (19-22), 1-pentanol (23), 1-hexanol (24-26), 1-heptano (27), and 1-octanol (25) but mixtures of olefins differing in the position of the double bond (13, 26, 28) or even in the carbon skeleton (29) have been reported from other primary alcohols. 

It was further reported that olefins such as unbranched hexenes (24, 30) undergo only double bond shift without skeletal rearrangement over alumina. On the other hand, rearrangement of the carbon skeleton has been observed in the interconversion of cyclohexene to methylcyclopentenes (14, 15). 

While double bond migration in olefins might arise from base (31) as well as acid catalysis (32), the occurrence of skeletal isomerization under these conditions can be ascribed to acid catalysts. This presumption would attribute acidic properties to the alumina. 

Abundant evidence has been gathered to show that pure alumina, prepared either from aluminum isopropoxide or aluminum nitrate and ammonia and calcined at 600-800°, has intrinsic acidic sites. Several physical methods have been used to study the acidity of alumina. Titration with butylamine (33), dioxane (34), and aqueous potassium hydroxide (35) as well as chemisorption of gaseous ammonia (35), trimethylamine (36), or pyridine (37) gave apparent acidity values which approximated those of silica-alumina. On the other hand, the indicator method for testing the acidity of solids as developed by Walling (3S) showed no indication of even weak acids (39, 40). 

The activity of alumina for dehydration and isomerization is markedly decreased by adsorbed sodium or potassium ions (36, 37, 41, 42). The approximately parallel decrease in conversion with increasing sodium content indicates that the catalytic centers for dehydration are the same as those for isomerization (36). 

Many of the earlier investigations are difficult to interpret as the importance of the chemical nature of the alumina was not appreciated and analytical techniques lacked the accuracy of present available methods. The numerous reports of the nature of alumina catalysts have been summarised by Winfield and Pines and Manassen . Ample evidence is presented to show that pure alumina, prepared from aluminium isopropoxide or aluminium nitrate and ammonia and calcined at 600-800°C, contains intrinsic acidic sites. Acidity has been estimated by titration against basic solvents , chemisorption of 

Aluminas prepared from sodium aluminate retain about 0.1% of sodium ions. 

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The actual selectivity depends on the nature of the catalyst. For example, the following data were reported for n-hexane transformed over platinum and palladium supported on the same alumina 44) (pulse system, hydrogen carrier gas, T = 520°C) ... 

The work with l-bromo-2-chloroethane allowed the influence of the nature of the halogen on its reactivity to be observed as either vinyl bromide or vinyl chloride are formed. The ratio of the chloride to the bromide in the products changed with the nature of the catalyst, being around 0.1 for sulphates of Ni, Co, Mn, Cu, Zn and for silica—alumina, 0.6 for alumina and 5 for KOH—Si02 [179]. 

In order to achieve the goal of reducing sulfur levels in fuels, there is a clear need for understanding the mechanism of the reaction (Chapter 4) in conjunction with the nature of the catalyst and support. Most of the work has been carried out with the traditional cobalt-molybdenum catalyst supported on alumina. This system is a time-tested and effective. 

Catalysts based on 7r-allylic derivatives of transition metals supported on alumina, silica or silica-alumina gels exhibit generally enhanced activity by comparison with their unsupported counterparts, while the stereospecificity depends on the nature of the catalyst carrier. For instance, Cr(All)3, which predominantly produces 1,2-polybutadiene [137], becomes a stereospecific catalyst for the formation of trans- 1,4-polybutadiene when supported on silica or silica-alumina gel and for the formation of cis- 1,4-polybutadiene when supported on alumina [148]. However, an increase in the content of cis-1,4 monomeric units in polybutadiene with increasing silica concentration in n-allylnickel-alumina-silica catalysts has been observed [149]. 

In the case of multiply unsaturated carbonyl compounds, regioselectivity is also sensitive to the nature of the catalyst, to reaction conditions, and to the structure and degree of substitution of the hydrogenated double bonds. For example, hydrogenation of 3,5-heptadien-2-one over nickel on alumina or nickel on zinc oxide occurs mainly at the y,8-double bond. But if the catalyst is modified by the addition of lead or cadmium, reduction occurs mainly at the a,p-double bond (Scheme 24). 

Catalytic amination of phenol (7) at 425 °C and around 200 atmospheres has been developed by Mitsui Petrochemical Industries of Tokyo. The nature of the catalyst is unspecified, though various metallic oxides and cocatalysts have been described. One Mitsui process employs a low alkali, weakly acidic alumina catalyst. Mild conditions, high yield and selectivity are claimed. Mitsui operates both the four-step phenol (starting from benzene) and two-step nitrobenzene processes12. 

Metathesis catalysts may be either homo- or heterogeneous. Although complexes of Ru, Mo, and W seem to show the most activity, metathesis may be catalyzed in some instances by Ti and Ta species. Heterogeneous substances, such as WOj/silica or Mo03 and Mo(CO)6 supported on alumina, catalyze the Triolefin Process and others performed on an industrial scale. Mechanistic studies of the type described in the previous section were essentially impossible to do with heterogeneous catalysts because the nature of these catalysts was so ill defined. Further discussion of the mechanism of metathesis under heterogeneous catalysis is beyond the scope of this textbook. 

Seco-5a-androstane-8,9,ll-trione derivatives (287) were cyclized with alumina or silica in acetonitrile to furnish the tetracyclic compounds (288) and (289) and the corresponding ene-diones. Introduction of various substituents and at positions 3 and 17 caused systematic changes in the ratios of the cyclization products. The cyclization process was also affected by the nature of the catalyst and of the solvent. 

On reviewing these data it is apparent that the most probable structure of calcined silica-alumina catalysts is a mixture of silica and alumina particles with the silicon and aluminum ions sharing oxygen ions at the points of contact. If this structure is actually present, the chemical properties of alumina in its various crystal forms will be the main controlling factor of the behavior of the mixed oxide system. The crystal habits of silica can be expected to be of merely secondary importance in determining the nature of the catalyst. 

Thiols add to propiolic acid esters under the influence of acid or base catalysts [150], The ratio of Z E isomers produced depends on both the solvent and the nature of the catalyst used. When neutral alumina is employed as catalyst the reaction gives high yields of the olefinic product and high Z.E ratios (e.g. equation 4,39) in comparison to the results with homogeneous, basic catalysts [151]. 

Prepared by heating ammonium mucate, or from butyne-l,4-diol and ammonia in the presence of an alumina catalyst. The pyrrole molecule is aromatic in character. It is not basic and the imino-hydrogen atom can be replaced by potassium. Many pyrrole derivatives occur naturally, e.g. proline, indican, haem and chlorophyll. 

Many solids have foreign atoms or molecular groupings on their surfaces that are so tightly held that they do not really enter into adsorption-desorption equilibrium and so can be regarded as part of the surface structure. The partial surface oxidation of carbon blacks has been mentioned as having an important influence on their adsorptive behavior (Section X-3A) depending on conditions, the oxidized surface may be acidic or basic (see Ref. 61), and the surface pattern of the carbon rings may be affected [62]. As one other example, the chemical nature of the acidic sites of silica-alumina catalysts has been a subject of much discussion. The main question has been whether the sites represented Brpnsted (proton donor) or Lewis (electron-acceptor) acids. Hall... 

Because the synthesis reactions are exothermic with a net decrease in molar volume, equiUbrium conversions of the carbon oxides to methanol by reactions 1 and 2 are favored by high pressure and low temperature, as shown for the indicated reformed natural gas composition in Figure 1. The mechanism of methanol synthesis on the copper—zinc—alumina catalyst was elucidated as recentiy as 1990 (7). For a pure H2—CO mixture, carbon monoxide is adsorbed on the copper surface where it is hydrogenated to methanol. When CO2 is added to the reacting mixture, the copper surface becomes partially covered by adsorbed oxygen by the reaction C02 CO + O (ads). This results in a change in mechanism where CO reacts with the adsorbed oxygen to form CO2, which becomes the primary source of carbon for methanol. 

Dehydration of ethanol has been effected over a variety of catalysts, among them synthetic and naturally occurring aluminas, siUca-aluminas, and activated alumina (315—322), hafnium and 2irconium oxides (321), and phosphoric acid on coke (323). Operating space velocity is chosen to ensure that the two consecutive reactions. 

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