Double bond migration of eugenol to isoeugenol over as-synthesized hydrotalcites and their modified forms Original Research Article

Double bond migration of eugenol to isoeugenol was carried out over as-synthesized hydrotalcites and their modified forms. The catalysts of general formula M(II)M(III)-xHT with carbonate as interlayer anion were synthesized by a co-precipitation method where M(II)=Mg, Ni, Co, Zn, Cu and M(III)=Al, Cr, Fe, La, V with varying M(II)/M(III) atomic compositions (here represented as ‘x’). The synthesized catalysts were tested for the reaction. Among various binary hydrotalcites investigated, Mg and Ni offered maximum activity, wherein MgAl-4HT showed nearly 73% conversion and NiAl-4HT showed 75% conversion with 15:85 cis:trans ratio at 200 °C with a substrate:catalyst mass ratio of 2:1. The other binary systems showed poor activity (less than 5%) under similar reaction conditions. The preservation of HT-like lattice is presumed to be crucial for this reaction, as evidenced from “in situ” powder X-ray diffraction (PXRD) and thermogravimetric (TG) analysis measurements. Variation in the trivalent metal ions indicated a maximum activity for Al, followed by Fe and Cr, in accordance with the crystallinity. A co-operative phenomenon was noted when both Mg and Ni were present together in a ternary MgNiAl-HT, however the activity varied with Mg/Ni atomic composition. Solvent variation studies indicated that more polar solvents favored the reaction. Significant promotional influence in the activity was noted with alkali and ruthenium impregnation on MgAl-4HT, wherein maximum activity was showed by catalysts modified with Cs (among the alkali metal ions studied) and higher content of ruthenium. Comparison of the activity with conventional bases such as KOH and KOBut revealed a superior performance of HT-based catalysts, although conventional bases had been used under stoichiometrically excess conditions (around 9% conversion for KOH with 1:10 and 5% conversion for KOBut with 1:3 substrate:catalyst mole ratio). The good performances of these catalysts encouraged further studies. A reaction mechanism involving the hydroxyl group of HT-like lattice is proposed for this isomerization reaction.