日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

The rates of solid-acid-catalyzed reactions are greatly enhanced by the presence of hydrogen in the system. Thus, the catalytic activities of prereduced Ag⁺-exchanged Y-zeolites for the disproportionation of ethylbenzene and isomerization of o-xylene are several times higher than that of H⁺-exchanged Y-zeolite, but only in the presence of gaseous hydrogen. The similar effect of hydrogen was also observed in other acid catalyzed reactions such as isomerization of 1-butene over not only Ag⁺-exchanged zeolites, but also silver dodecatungstophosphate (Ag₃PW₁₂O₄₀).

Heterolytic dissociation of H₂ proceeded to form acidic protons and silver-hydride species over Ag⁺-exchanged zeolites. For example, Ag₃-H and acidic protons are produced upon exposing Ag-A to H_2. The effects of the degree of Ag⁺ ion exchange and the temperature of hydrogen exposure on the formation of hydrogen chemisorbed species were examined with ¹H MAS NMR. Furthermore, the thermal stability of silver hydride species and the temperature dependence of the line shape of the peak due to acidic protons were studied with raising the temperature.

The heterolytic dissociation of methane over silver cationic clusters (Ag_n⁺) in Ag⁺-exchanged zeolites also proceeds. This result leads to the formation of silver hydride (Ag_n-H) and CH₃^δ+, which then reacts with ethene to form propene. Under these reaction conditions,H⁺-exchanged zeolites, such as H-ZSM-5 only catalyze ethene conversion to higher hydrocarbons, and no methane conversion occurs.

Conversion of ¹³C-labeled methane (¹³CH₄) in the presence of ethene (C₂H₄) over Ag⁺-exchanged zeolites (Ag-A, Ag-Y and Ag-ZSM-5) at 673 K affords a mixture of ¹³Clabeled propene (¹³CC₂H₆) and unlabeled propene (C₃H₆) at a ratio of approximately 80/20. The same reaction over Ag₃PW₁₂O₄₀ and proton-exchanged zeolites (H-ZSM-5 and H-Y) yields only unlabeled propene. These results show that silver cationic clusters (Ag_n⁺) in Ag⁺- exchanged zeolites are responsible for the activation of methane to form silver hydride species (Ag_n-H) and highly polarized CH₃^δ+, which reacts with ethene to form propene and acidic protons. Regeneration of Ag_n⁺ proceeds by reaction of acidic protons with Ag_n-H accompanied by the formation of H₂. Silver cations in Ag₃PW₁₂O₄₀, a silver salt, do not dissociate the C-H bond of CH₄ and thus cannot catalyze the reaction of methane and ethene.