Tripod Beta

Dorart, J.A. and van der Graaf, G.C. (1996). Tripod-beta incident investigation and analysis, hr Proceedings ofthe Third International Conference on Health, Safety and Environment New Orleans, LA Society of Petroleum Engineers. 

Tripod-Beta is a third variant based on the Tripod model of Section 5.2 (Reason, 1997). The associated PC software makes it possible for the analyst to document the findings of the investigation and to produce a graphic incident tree with the immediate circumstances of the accident (hazards and barrier failures) and the underlying active and latent failures and preconditions. 

The various spectroscopic techniques had revealed that Ti4+ ions in TS-1, Ti-beta and, Ti-MCM-41 are 4-coordinate in the dehydrated state. Tetrapodal Ti(OSi)4 and tripodal Ti(OH)(OSi)3 are the main Ti species. Upon exposure to H20, NH3, H202, or TBHP, they increase their coordination number to 5 or 6. On samples in which the Ti4+ has been grafted onto the silica (referred to as Ti f MCM-41), a dipodal Ti species (Ti(OH)2(OSi)2) may also be present. As a result of interaction with the oxidant ROOH (R = H, alkyl), the formation of 7)1- and p2-peroxo (Ti-O-O-), hydroperoxo (Ti-OOH), and superoxo (Ti02 ) species has been observed experimentally (Section III). A linear correlation between the concentration of the p2-hydroperoxo species and the catalytic activity for propene epoxidation has also been noted from vibration spectroscopy (133). 

The higher conversion in the presence of Ti-beta is probably a result of the higher temperature (343 v.v. 323 K). Diffusional constraints cannot account for the observed differences in selectivity. Ti-beta and TS-1 are distinctly more selective than the mesoporous material. Recalling that tetrapodal titanium sites are more predominant in the former two molecular sieves although tripodal titanium sites are the major surface species over the latter mesoporous material (Section II), we infer that the data indicate that high epoxidation selectivity is probably correlated with the presence of tetrapodal structures in these two molecular sieves. This correlation is discussed in Section VI. 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

Significant progress has been achieved in the preceding few years in the study of titanosilicate molecular sieves, especially TS-1, TS-2, Ti-beta, and Ti-MCM-41. In the dehydrated, pristine state most of the Ti4+ ions on the surfaces of these materials are tetrahedrally coordinated, being present in either one of two structures a tetrapodal (Ti(OSi)4) or a tripodal (Ti(OSi)3OH) structure. The former predominates in TS-1, TS-2, and Ti-beta, and the latter is prominent in Ti-MCM-41. The Ti ions are coordinatively unsaturated and act as Lewis acid sites that coordinatively bind molecules such as H20, NH3, CH3CN, and H202. Upon interaction with H202 or H2 + 02, the Ti ions form titanium oxo species. Spectroscopic techniques have been used to identify side-bound hydroperoxo species such as Ti(02H) and superoxo structures such as Ti(02 ) on these catalysts. 

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