Hydrogenation dehydroxylation

Allyl Complexes. Allyl complexes of thorium have been known since the 1960s and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a thorium haUde and an aHyl grignard. This synthetic method was utilized to obtain a rare example of a naked aHyl complex, Th(Tj -C2H )4 [144564-74-9] which decomposes at 0°C. This complex, when supported on dehydroxylated y-alumina, is an outstanding heterogeneous catalyst for arene hydrogenation and rivals the most active platinum metal catalysts in activity (17,18). 

In the case of oxide catalysts or alkali metal-doped oxide catalysts, basic surface sites can be generated by decarboxylation of a surface metal carbonate exchange of hydroxyl hydrogen ions by electropositive cations thermal dehydroxylation of the catalyst surface condensation of alkali metal particles on the surface and reaction of an alkali metal with an anion vacancy (AV) to give centers (e.g., Na + AV — Na + e ). 

Organothorium complexes such as [Th(r 3-allyI )4] supported on dehydroxylated y-alumina have been shown to exhibit activities rivaling those of the most active platinum metal catalysts.123 Thorium maintains its original +4 oxidation states at all times that is, the mechanism does not follow the usual oxidative addition-reductive elimination pathway. Partially hydrogenated products cannot be detected... 

Later, Cattanach, Wu, and Venuto did an elaborate thermogravi-metric study on the calcination of ammonium zeolite Y and the resulting products (19). They found that the hydrogen zeolite reacted with anhydrous ammonia to yield an ammonium zeolite identical in ammonia content with the initial ammonium zeolite. Further, these workers reported that after loss of chemical water ( dehydroxylation according to Uytter-hoeven, Christner, and Hall or decationization according to Rabo, Pickert, Stamires, and Boyle) the sample became amorphous when exposed to moisture. This observation conflicted with the statement of Rabo et al. (16) in which they emphasized the extreme stability of their decationized Y. The data of Cattanach, Wu, and Yenuto prove, beyond any doubt, that they obtained the expected normal hydrogen zeolite Y prior to the loss of chemical water above 450°. Rabo et al., however, did not prove that the material from which they removed chemical water, was in fact, the hydrogen zeolite. They probably prepared, unknown to them at the time, the ultrastable zeolite described below. 

The acid anhydride derived from the true hydrogen form, generally called dehydroxylated Y and assumed to be formed according to Reaction 3 of Uytterhoeven, Christner, and Hall (15). 

The chemistry and structure of the hydrogen form of zeolite Y have been thoroughly investigated 82) and are not considered further. The structure of the dehydroxylated zeolite proposed by Uytterhoeven, Christ-ner, and Hall 15) remains unchanged. Recently Ward, on the basis of infrared studies, suggested that this form may be amorphous 27). The extreme instability of dehydroxylated zeolite Y to moisture complicates detailed study 19). The elucidation of the detailed nature of this material lies in the future. At present, completely dehydroxylated Y is little understood and presents a challenging void in our knowledge of the nature of ammonium zeolite Y thermal decomposition products. 

Ambs and Flank correctly observed that variables can be introduced into the calcination of ammonium Y so that a variable series of products can be obtained 33). However, there is no doubt that the normal hydrogen zeolite can be obtained from the ammonium form by carefully controlled calcination. In addition, carefully controlled calcination of the acid yields the dehydroxylated form. The ultrastable form, which can be prepared by a number of procedures described below, differs drastically in stability and composition from the other two forms. That it may contain some sites similar to, or perhaps identical with, sites in the hydrogen and dehydroxylated forms cannot be refuted. Unquestionably, however, the ultrastable form differs significantly from the other two forms. 

The relationship between acid site density and effective acidity may account for the interesting observation of Hopkins that maximum cracking activity of n-hexane was obtained over a partially dehydroxylated hydrogen zeolite Y (45). While the normal hydrogen form would contain a greater overall concentration of acid sites, the partially dehydroxylated form may have a greater overall acid activity because of the increased effective acidity of the remaining sites. 

Nb-containing MCM-41 sieves represent Lewis acidity proven by FTIR study conducted after pyridine adsorption [3,4], Hydrogen forms of niobium-containing MCM-41 materials exhibit lower Bransted acidity than that in hydrogen aluminosilicate mesoporous molecular sieves (see the band at 1549 cm 1 in Figure 6 [3]). The dehydroxylation of H-NbMCM-41 samples causes the formation of the following lattice species ... 

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