Partial oxidation of hydrocarbons using titanium containing molecular sieves

Citation

Khouw, Charles B. (1995) Partial oxidation of hydrocarbons using titanium containing molecular sieves. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9dyq-m742. https://resolver.caltech.edu/CaltechETD:etd-10122007-103304

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The objective of this work is to investigate the reaction mechanism of alkane activation on titanium containing, aluminum-free ZSM-5 (TS-1) and to elucidate the relationships between the physicochemical properties of the catalyst and its reactivity. Samples of TS-1 have been synthesized using different methods. The activities of these catalysts for the oxidation of alkanes, alkenes and phenol at temperatures below 100 [...] using aqueous [...] as oxidant are reported and compared to those from other titanium containing materials, e.g., anatase and an amorphous [...] coprecipitate. Comparisons between the activities of TS-1 and [...] coprecipitate for alkane oxidation and alkene epoxidation using non-aqueous [...] indicate that the absence of water is crucial for the catalytic activity of silica-supported titanium oxide. Due to the hydrophobicity of TS-1, the concentration of water surrounding the titanium is maintained at low value so that aqueous [...] can be used as oxidant on this catalyst. Issues of mechanism of the alkane oxidation on TS-1 are investigated by analyzing the stereoselectivity pattern of cis- and trans-1,3-dimethylcyclopentane, the "radical clock" rearrangement of ethyl- and isopropylcyclopropane and the effect of oxidants on the catalytic activity of TS-1. The stereoselective reaction pattern of cis- and trans-1,3-dimethylcyclopentane indicates that radicals are formed during alkane oxyfunctionalization with TS-1. The presence of stereos crambling without any "radical clock" rearrangement during alkane oxidation on TS-1 reveals that the radicals formed may have a very short life-time or their movements are restricted such that no rearrangement can occur. A proposal for the mechanism of alkane oxidation on TS-1 is given and compared to a mechanism suggested for alkene epoxidation on TS-1 and the [...] coprecipitate. Alkyl hydroperoxides are active as oxidants for alkene epoxidation on the [...] coprecipitate but not for alkane oxidation reactions on both TS-1 and the [...] coprecipitate. A plausible explanation for the above results is provided. The presence of alkali metal ions in the synthesis mixture of TS-1 completely eliminates the catalytic activity of this material. However, the catalytic activity can be restored by washing the solid with acid solution prior to catalytic testing. The washing removes [...] ions from silanol groups adjacent to the framework titanium active centers. Thus, it is postulated that a silanol group in the neighborhood of the titanium atom is a necessary feature for catalytic activity. The acid treatment may be useful in overcoming the problems of synthesizing TS-1 from reagents that contain alkali metal ions, e.g., TPAOH solutions. More importantly, this treatment opens the possibility of synthesizing other titanium containing silicate structures that require the presence of alkali metal ions in the synthesis mixture for their formation.

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