Isovalent Substitutions in Composite Catalysts Na/W/Mn/SiO2

Abstract

Isovalent analogues of the composite catalyst Na/W/Mn/SiO₂ prepared by solid-phase reactions, namely, Li/Na(Rb)/W/Mn/SiO₂, Na(Li,Rb,Cs)/Mo/Mn/SiO₂, Na/W/Re/SiO₂, Na/W/Mn/GeO₂, and a nonisovalent analogue Mg(Ca,Sr,Ba)/W/Mn/SiO₂ were studied by X-ray powder diffraction. The composites Li/Na/W/Mn/SiO₂ and Li/Rb/W/Mn/SiO₂ having an equimolar contents of alkali elements with nonadditive changes in SiO₂ matrix polymorphism and the formation of mixed tungstates show equally high product yields (23–24%) in oxidative coupling of methane (OCM), these yields being comparable to those observed for Na/W/Mn/SiO₂. Substitutions of W by Mo, Mn by Re, and Si by Ge in the composite Na/W/Mn/SiO₂, as well as substitutions of Na by Mg, Ca, Sr, and Ba, change the phase constitution of the composite and reduce its catalytic activity.