Catalytic Isomerization of Estragole to Anethole over Hydrotalcites and HT-like Compounds,

Abstract

The liquid phase isomerization of estragole to anethole, an industrially important reaction, was carried out over catalysts belonging to the family of hydrotalcites (HTs) of general formula M(II)M(III)-xHT, where M(II) = Mg, Ni, Co, Zn and Cu, and M(III) = Al, Fe and Cr with varying M(II)/M(III) atomic compositions (here represented as ‘x’). Among various binary hydrotalcites investigated Mg and Ni offered maximum activity wherein MgAl-4HT showed 99% conversion and NiAl–4HT showed 87% conversion with a cis:trans ratio close to 15:85 at 200 °C at a substrate:catalyst mass ratio of 5:1. The other binary systems, ZnAl-4HT, CoAl-4HT and CuAl-4HT showed no conversion under similar reaction conditions. Among the trivalent metal ions studied, Al offered maximum activity followed by Fe and Cr and the activity trend was in line with the crystallinity of the samples. End members of the series, namely Mg(OH)2 and Al(OH)3, yielded no conversion, while materials crystallized as hydrotalcite (containing Mg and Al) showed pronounced activity, however varied with Mg/Al atomic ratio. Among the synthesis methodologies studied for MgAl-4HT, co-precipitation under low supersaturation offered maximum activity and solvent variation studies indicated that more polar solvents favored the reaction. The catalyst, MgAl-4HT, was reusable without significant loss in the activity up to three cycles. Variation of co-bivalent metal ions in MgM(II)′Al-yz (where yz is Mg:M(II)′ atomic ratio and M(II)′ = Ni, Co, Zn, Cu, Mn and Fe) ternary hydrotalcites showed that the inclusion of Ni and Co in the HT framework resulted in synergism wherein MgNiAl-31 showed 98% conversion and MgCoAl-31 showed 88% conversion with cis:trans ratio close to 15:85 at 200 °C with a substrate:catalyst mass ratio of 10:1. In contrast, a sharp drop in the activity (<10%) was observed with other co-bivalent metal ions. A reaction mechanism involving hydroxyl groups of HT-like lattice is proposed for this reaction.