Entrapment of Heteropolyacids in Metallic Silver Matrices: Unique Heterogenized Acid Catalysts

The general method of doping metals with organic, bio-organic, and polymeric dopants is extended to inorganic dopants. Specifically, the heteropolyacids H₃[P(M₃O₁₀)₄] (PMA; M=Mo, W) were successfully entrapped within a metallic silver matrix. The obtained PMA@Ag composites were fully characterized by PXRD, surface area, SEM, TEM and EDX measurements, showing a homogenous distribution of the catalyst in the support. The new composite materials are successfully applied in the catalytic alkylation of arenes, as demonstrated by the successful adamantylation of toluene or anisole with 1-bromoadamantane. Furthermore, this reaction is applied with the less reactive 1-chloroadamantane in both the supported and unsupported case. PMoA, which easily decomposes under the applied reaction conditions, is protected by entrapment and shows increased activity when supported in the silver matrix. In the same reaction, the entrapped PWA shows a drastically increased reaction rate compared to the free acid, which further confirms the positive synergistic effects of the entrapment process. Both heterogenized catalysts are successfully recycled and reused, but the reaction time steadily increases in successive cycles due to agglomeration of the catalyst, which results in a lower accessibility of the dopant. Moreover, the alkylation can be extended to other alkyl chloride substrates, as demonstrated for tert-butyl chloride.