Partial oxidation of propane over vanadium-containing zeolite catalysts

by Luo, Lin

Abstract (Summary)
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Over the past few decades, significant efforts have been devoted to the development of new catalysts and processes for the partial oxidation of cheap and abundant light alkanes directly into oxygenates and olefins. One of the main challenges in the partial oxidation of light alkanes is that they are usually less reactive than the desired products, and further oxidation to total oxidation products, [...], is thermodynamically favored. With a few exceptions, e.g., partial oxidation of n-butane to malefic anhydride by V-P-O and a oxidation of propane to acrylonitrile by V-Al-Sb, catalysts investigated for the partial oxidation of light alkanes consist of complicated elemental compositions of metal oxides that have less than desirable catalytic behavior. Compared to the traditional bulk metal oxides where the active sites selective for partial oxidation of hydrocarbons are usually lattice oxygens, small metal oxide clusters that do not have lattice energies are anticipated to offer oxygen with a lower energetic barrier. Thus, by using metal oxide clusters, the reaction temperature for oxidation of hydrocarbons may possibly be lowered and total oxidation reduced. Zeolites have been shown to be able to provide an excellent matrix for stabilizing metal oxide clusters. Here, a new approach is investigated for the partial oxidation of propane that combines the tunable advantages of zeolites with possibility of high reactivity of metal oxide clusters by using zeolite supported metal oxide clusters as catalysts. Various methods are employed to synthesize zeolite supported small metal oxide clusters, including ion-exchange-hydrolysis, liquid phase impregnation, vapor phase impregnation, and post-synthesis modification methods. Vanadium is used as the transition metal of interest and is combined with zeolite L, beta and SSZ-33. Vanadium oxide clusters are successfully incorporated inside zeolites and they have remarkably lower reduction temperatures than the bulk metal oxide. These zeolite based catalysts are studied for propane oxidation. The influence of the locations of the vanadium, the acidity of the zeolite matrix, the hydrophobicity of the zeolite framework and the addition of a second metal(Mo and Sb) are discussed. It is found that vanadium oxide cluster catalysts (VxOy/zeolite L, VxOy/zeolite beta, V-Sb/zeolite beta, V-Mo/zeolite beta) are not particularly selective for the partial oxidation of propane at the reaction conditions investigated (contact time 2 s, reaction temperature 350-450[degrees]C, feed gas molar ratio C3H8:O2:H2O:He=4:2:4:5), and most of the vanadium-containing zeolite beta catalysts are as active as V2O5 (as suggested by the turnover frequency). A considerable amount of acetic acid is produced with vanadyl ion-exchanged zeolite beta (VO-H-beta), with a selectivity to acetic acid of 21.1% at propane conversion 1.62% (350[degrees]C). It appears that more valuable oxygenates, e.g., acrylic acid, may have been produced in the reaction and overoxidized to COx, since feeding acrylic acid into this VO-H-beta reaction system at 350[degrees]C results in complete oxidation of the acid to COx. Motivated by these data, the reaction pathways for propane and propylene oxidation are investigated on this catalyst. For comparison, reaction pathways are also studied with a "Mitsubishi" type catalyst, [...], one of the best catalysts for propane partial oxidation to acrylic acid. With VO-H-beta, propylene is the primary product of propane oxidation and acetic acid is a sequential oxidation product of the formed propylene possibly through an acetone intermediate.[...] also gives propylene as the primary product of propane oxidation and the propylene thus formed further oxidizes to acrylic acid and acetone. Reactions of individual oxygenate compounds, e.g., propanal, acrolein, etc., suggest a need to further suppress the total oxidation and to improve allylic oxidation feature for the zeolite based catalysts.
Bibliographical Information:

Advisor:Mark E. Davis; George R. Gavalas; Richard C. Flagan

School:California Institute of Technology

School Location:USA - California

Source Type:Master's Thesis

Keywords:chemical engineering


Date of Publication:09/22/2000

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