Hydrogen reduced transition metal zeolites

Another characteristic feature of the hydrogen-reduced transition metal zeolites is their acidic properties, as demonstrated by their catalytic behavior (7). Naccache and Ben Taarit (8) gave IR evidence of the subsequent formation of protons on hydrogen-reduced Cu(II)-Y zeolite. Furthermore, transition metal ions have various oxidation states. Owing to the shielding effect caused by the zeolite network and the electric fields, the transition metal ions may be stabilized in unusual oxidation states—i.e. Ni(I) (9). 

In addition to performing acid/base catalysis, zeolite structures can serve as hosts for small metal particles. Transition metal ions, e.g., platinum, rhodium, can be ion exchanged into zeolites and then reduced to their zero valent state to yield zeolite encapsulated metal particles. Inside the zeolite structure, these particles can perform shape selective catalysis. Joh et al. (16) reported the shape selective hydrogenation of olefins by rhodium encapsulated in zeolite Y (specifically, cyclohexene and cyclododecene). Although both molecules can be hydrogenated by rhodium supported on nonmicroporous carbon, only cyclohexene can be hydrogenated by rhodium encapsulated in zeolite Y since cyclododecene is too large to adsorb into the pores of zeolite Y. 

Not only metals but some oxide catalysts are active in diene hydrogenation ZnO modified by Sn(CH3)4 afforded 1-butene in hydrogenation of butadiene at room temperature. Reduced and sulfided moUbdena on alumina catalyst hydrogenated butadiene and cyclohexadiene selectively5 . When the transition metal complex Mo(CO)6 was encapsulated in NaY zeolite cages, it converted //wnv-l, 3-pcnladicnc to cA-2-pentene and 1,4-pentadiene to c -l,3-pentadiene at 150°C °. Cr(CO)3 encaged in LiX or NaX zeolite was efficient and selective in butadiene hydrogenation to cw-2-butene . ... 

The reduction of cations such as Pd + and other transition metals proceed analogously to the C-H activation events discussed above for Zn +. Hydrogen dissociatively adsorbs to form [PdH] + and a zeolite proton. This is followed by the subsequent activation of a second H2 molecule to form PdH2 and another zeolitic proton. Hydrogen readily desorbs from Pd, leaving a reduced metal atom next to two zeolitic protons . 

The metal oxide catalysts used for hydrogenation reactions are reduced to an active form of the metal before use. Apart from metallic platinum and silver, which are used to oxidize ammonia and methanol, respectively, oxidation catalysts are usually transition metal oxides. Acidic oxides, such as alumina, sihca alumina, and zeolites are used in cracking, isomerization, and dehydrogenation reactions. These are only a few examples of the catalysts now being widely used. A more detailed list is given in Table 1.4. 

Auto-reduction processes in the absence of hydrogen can occur during thermal treatments in vacuum or inert atmospheres. In the case of first-row transition cations,reduction stops at low-valent states. Thus, Fe " [106] or Cu +Y [107] zeolites heated under vacuum were reduced to the Fe + and Cu+ forms, respectively. Platinum-group cations can be reduced to metals by heating in the presence of extra-framework species. Thus, Pd + ions in Y zeolite were reduced to metal particles by heating at 773 K in the presence of water molecules [108] since oxygen was formed, the reaction probably proceeded according to the overall reaction ... 

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