Isomerization of eugenol and alkenyl aromatics of perfumery interest over Ni-containing layered double hydroxides as solid base catalysts

Isomerization of eugenol was carried out over M(II)Al binary hydrotalcites where M(II) = Mg, Ni, Cu, Li or Co with M(II)/Al atomic ratio of 3.0. All samples were synthesized by coprecipitation under low supersaturation and characterized by powder X-ray diffraction (PXRD), FT-IR, thermogravimetry and BET N2 adsorption measurements. All samples showed PXRD profiles similar to HT-like phase except for copper, which showed additional reflections of impurity phases. Among the catalysts studied, NiAl3-HT offered maximum conversion of eugenol to isoeugenol of around 77% at 200 °C in 6 h using DMF as solvent. CuNiAl ternary hydrotalcites with (Cu + Ni)/Al atomic ratio of 3.0 with varying Cu:Ni atomic ratios were synthesized and tested for isomerization of eugenol. Addition of copper in NiAl HT-like lattice significantly reduced the isomerization activity wherein 25% substitution of Cu2+ for Ni2+ completely annihilated the activity. To study, the influence of synthesis methodology, NiAl3-HT was also synthesized through urea and hexamine hydrolysis wherein sample prepared by former method offered maximum crystallinity among all samples synthesized. However, no isomerization of eugenol was seen for these samples except for the sample obtained through coprecipitation. The observed activity trends could reasonably be correlated with the basicity of the catalysts obtained using phenol adsorption measurements. The catalyst could be reused for two times without much decrease in the activity while further reuse resulted in decrease in the activity. The versatility of the catalyst was checked through isomerization of various alkenyl aromatics wherein at 160 °C, allyl benzene showed maximum conversion to β-methyl styrene and eugenol showed minimum conversion.