Catalytic activity of alumina

Pines and Haag studied the correlation between trimethylamine adsorption and catalytic activities of aluminas for isomerization and dehydration (36). From the results obtained they reached the following general conclusions (a) there is a satisfactory correlation between catalytic activity and amine chemisorption values for aluminas obtained from the same methods of preparation—both measure acidity (6) there is no satisfactory correlation between catalytic activity and amine index of aluminas obtained from different sources. 

Most of the work with alumina was done, however, attempting to elucidate the nature of the catalytically active sites in dehydrated alumina. The catalytic activity of alumina is enhanced by treatment with hydrofluoric acid. Oblad et al. (319) measured a higher activity in the isomerization of 1- and 2-pentene. Webb (339) studied the effect of HF treatment on ammonia adsorption by alumina. There was no difference in the capacity. However, the ammonia was more easily desorbed at a given temperature from the untreated sample. Apparently, the adsorption sites grew more strongly acidic by the treatment. No NH4+ ions, only NHj molecules were detected by their infrared spectra, indicating that the ammonia was bound by Lewis acids rather than Bronsted acids. 

Freni [159] examined ESR reactions over Rh/Al203 catalyst. The results indicated that the catalytic activity of alumina (AI2O3) is not negligible ethylene and water are produced at 347 °C and their production increases and reaches equilibrium at 600 °C. 

The catalytic activity of aluminas are mostly related to the Lewis acidity of a small number of low coordination surface aluminum ions, as well as to the high ionicity of the surface Al-O bond [67,92]. The number of such very strong Lewis sites present on aluminum oxide surfaces depends on the dehydroxylation degree and on the particular phase and preparation. Depending on the activation temperature, the density of the strongest Lewis acid sites tends to decrease as the calcination temperature of the alumina increases (i.e., upon the sequence y — 5 —> 9, which is also a sequence of decreasing surface area and increasing catalyst stability). 

Aluminium oxide exists in many crystalline modifications, usually designated by Greek letters, some with hexagonal and some with cubic lattices (cf. refs. 11 and 24). The best known and mostly used forms are a- and 7-alumina but practical catalysts are seldom pure crystallographic specimens. This makes the surface chemistry of aluminas rather complicated. Moreover, the catalytic activity of alumina depends very much on impurities. Small amounts of sodium (0.08—0.65%) poison the active centres for isomerisation but do not affect dehydration of alcohols [10]. On the other hand, traces of sulphates and silica may increase the number of strong acidic sites and change the activity pattern. 

Effect of thermal treatment on the catalytic activity of alumina After Dunstan and Pincock [218]. 

It is interesting finding that the catalytic activity of alumina, which showed the highest activity for the disproportionation of E2, was very low for the alkylation. This may indicate the reaction can not be catalyzed by Lewis acid. HY hardly catalyzes the reaction, though its activity for the disproportionation was high. This may come from the shape selective nature of microporous zeolite. 

Although alumina is widely used as a catalyst support, very few data are available relevant to the catalytic activity of alumina alone for reactions other than dehydration. As Holm and Blue (1) have shown, alumina has appreciable hydrogenation-dehydrogenation activity, especially after drying at high temperatures. The activity was decreased by exposure of the dried catalyst to humid air at room temperature but not by exposure to dry air. 

Workers in the Ipatieff Laboratory were always aware of the differences in activity, particularly acid activity, of the silica and alumina they used. In the late fifties and early sixties, Pines and his students studied the intrinsic acidity and catalytic activity of aluminas as a function of their preparation method (24). They investigated the dehydration of alcohols (25-31) and many excellent papers were contributed in the alumina area and in the area of alumina supported dehydrogenation and aromatization catalysts (32, 33, 34). 

Reyes, P., Oportus, M., Pecchi, G., Frety, R. Moraweck, B. Influence of the nature of the platinum precursor on the surface properties and catalytic activity of alumina-supported catalysts. Catalysis Letters 37, 1993 (1996). 

Park, P.W. and Ledford, J.S. The influence of surface structure on the catalytic activity of alumina supported copper oxide catalysts — Oxidation of carbon monoxide and methane, Catal B Environ. 1998, 15, 221-231. 

In the petroleum (qv) industry hydrogen bromide can serve as an alkylation catalyst. It is claimed as a catalyst in the controlled oxidation of aHphatic and ahcycHc hydrocarbons to ketones, acids, and peroxides (7,8). AppHcations of HBr with NH Br (9) or with H2S and HCl (10) as promoters for the dehydrogenation of butene to butadiene have been described, and either HBr or HCl can be used in the vapor-phase ortho methylation of phenol with methanol over alumina (11). Various patents dealing with catalytic activity of HCl also cover the use of HBr. An important reaction of HBr in organic syntheses is the replacement of aHphatic chlorine by bromine in the presence of an aluminum catalyst (12). Small quantities of hydrobromic acid are employed in analytical chemistry. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

This paper describes the catalytic activity of nickel phosphide supported on silica, alumina, and carbon-coated alumina in the hydrodesulfurization of 4,6-dimethyldibenzothiophene. The catalysts are made by the reduction of phosphate precursors. On the silica support the phosphate is reduced easily to form nickel phosphide with hi catalytic activity, but on the alumina support interactions between the phosphate and the alumina hinder the reduction. The addition of a carbon overlayer on alumina decreases the interactions and leads to the formation of an active phosphide phase. 

The catalytic activity of Mg -modified alumina decreases with increasing Mg content in the samples as seen fi om Figure 3. 

For regeneration to be technically viable, it must be able to remove deposited vanadium and nickel quantitatively as well as the carbonaceous coke which was co-deposited. The catalyti-cally active metals should remain unaffected in amount, chemistry, and state of dispersion. The alumina support should remain intact, with the surface area, pore-size distribution and crush strength after treatment comparable to that of the original. To be economically viable, the process should be accomplished in a minimum of steps at nearly ambient temperatures and preferably in aqueous solution. The ultimate proof of any such scheme is for the catalytic activity of the regenerated catalyst to be equal to that of a fresh one. 

An exhaustive review of dehydration reactions has been written recently by Winfield (3) and most of the relevant literature can be found there. The purpose of this chapter is to review some recent developments and to point out the resemblance of alumina-catalyzed dehydration of alcohols to solvolytic reactions. It will be demonstrated that by careful selection of model compounds, such as olefins and alcohols, it is possible to throw light on the catalytic action of alumina and to reveal the presence of active catalytic sites. 

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). 

Fig. 20. Catalytic activity of low ignition manganese oxides on alumina as a function of manganese concentration [J. Mooi and P. W. Selwood, J. Avi. Chem. Soc. H, 1750 (1952)].

Erivanskaya and co-workers also studied the dehydrocyclization of 2-n-butylnaphthalene over supported palladium, rhodium, and iridium catalysts (56-55). Palladium-alumina showed the lowest C6-dehydrocyclization activity, but was the most active for the C5-dehydrocyclization of 2-n-butyl-naphthalene. A later study showed, however, that this enhanced activity was due to the high chlorine content of the palladium-alumina catalyst and not to some mysterious inherent catalytic activity of palladium (56). 

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