Metals in Zeolites for Oxidation Catalysis

Abstract

Zeolites have long been used as solid acid catalysts. The fluid catalytic cracking of heavy fractions of petroleum by the use of Y-type zeolites included in catalyst matrices is the world's largest catalytic process. Acidic zeolites have also widely replaced mineral and Lewis acids in large-scale chemical manufacturing such as alkylation of aromatics and the Beckmann rearrangement. Zeolites are also endowed with the redox property by the incorporation of a variety of metals, and this chapter deals with incorporation of metals and the resultant oxidation catalysis.The ways to introduce heteroatoms into zeolites are classified into two categories: heteroatoms can be introduced into the framework as well as into voids as extraframework species. Most zeolites have an intrinsic ability to exchange cations [1]. This ability to exchange is a result of isomorphous substitution of a cation of trivalent (mostly Al) or lower charges for Si as a tetravalent framework cation. As a consequence of this substitution, a net negative charge develops on the framework of the zeolite, which has to be neutralized by cations present within the channels or cages that constitute the microporous part of the crystalline zeolite. These cations may be any of the metals, metal complexes, or alkylammonium cations. If these cations are transition metals with redox properties, they can act as active sites for oxidation reactions. As a pioneering work, Wacker-type reactions were catalyzed by Y zeolite into which Pd 2+ and Cu 2+ were incorporated by ion exchange [2].Research on coordination chemistry in zeolites started in 1970s and early work was summarized by Lunsford [3]. A metal complex of the appropriate dimensions can be encapsulated in a zeolite, being viewed as a bridge between homogeneous and heterogeneous systems. Complexes that are smaller than the free diameters of the channels and windows have access to the cavities. On the other hand, complexes that are larger than the diameters of the windows must be synthesized in situ, namely, by the adsorption of the ligands into the zeolites containing transition metal ions or by the synthesis of the ligands in those zeolites [4][5][6]. Herron et al. first referred to such zeolite guest molecules as ship-in-a-bottle complexes [7]. Cationic complexes can be tethered to zeolites through the electrostatic interaction. However,

Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts

“…This interaction allows various ions to be exchanged into the framework cages and channels in order to tailor a zeolite material for a desired chemistry. An alternative way to modify zeolites with metal cations is the substitution of framework Al with other metals. ,, An important advantage of framework substitution is a decrease in metal leaching rates when compared to simple ion-exchanged cations …”

CTAB-templated mesoporous TS-1 zeolites as active catalysts in a desulfurization process: the decreased hydrophobicity is more favourable in thiophene oxidation

“…11,[17][18][19] Among them, titanium silicalite-1 (TS-1) zeolites, because of its superior hydrophobicity and intrinsically higher catalytic activity than Ti-Beta, is on the top of a few highly active catalysts in thiophene oxidation. [20][21][22][23][24][25] With the incorporation of Ti into a MFI framework, TS-1 zeolites are crystalline with an extensive microporosity of about 0.55 nm in pore diameter. On the other hand, Ti and Si own the same formal valence which is different from the situation of the substitution of trivalent metals for Si as happens in ZSM-5 zeolites.…”

“…26,27 Generally, it is believed that these attributes are critical to the superior catalytic performances for TS-1 zeolites in the oxidative reactions, thus making TS-1 an important milestone in zeolite preparation and fine chemical synthesis. 23,26,28,29 However, the small micropores impede the free diffusion of molecules as demonstrated by the poor performance in BT…”