Applied Catalysis A: General 270 (2004) 227–235 Double bond migration of eugenol to isoeugenol over as-synthesized hydrotalcites and their modified forms Dasari Kishore, Srinivasan Kannan ∗ Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute (CSMCRI), GB Marg, Bhavnagar 364002, India Received in revised form 1 April 2004; accepted 6 May 2004 Abstract 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 KOBu t 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 KOBu t 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. © 2004 Elsevier B.V. All rights reserved. Keywords: Isomerization of eugenol; Crystallinity; “In situ” powder X-ray diffraction; HT-like lattice; Ru-incorporation; Alkali-doped hydrotalcite; Synergism 1. Introduction Eugenol is isomerized to its corresponding alkenyl alkoxy benzene, isoeugenol, wherein the double bond in the alkenyl group migrates to a position conjugated with the benzene ring (Scheme 1). The product finds applica- tion in pharmaceuticals and in a wide variety of blossom compositions in fragrances [1,2]. Isomerization of eugenol is generally carried out in a homogenous medium using KOH in alcoholic solutions (most often in higher alcohols) at high temperatures [3]. Loupy and Thach [4] have at- ∗ Corresponding author. Tel.: +91-278-2567760/+81-52-789-3192; fax: +91-278-2567562/+81-52-789-3193. E-mail address: kanhemad1@sancharnet.in (S. Kannan). tained quantitative yields when they used KOH or KOBu t (2.2 mol equiv.) as catalyst in the presence of a phase transfer agent, Aliquat 336, under solvent-free conditions. Subsequently, they have also tested the effect of several additives under similar reaction conditions and achieved 90% yield with pyridine as an additive at 150 ◦ C. Recently, Thach and Strauss [5] have carried out the isomerization of eugenol in a microwave batch reactor under aqueous conditions using 0.4 M NaOH; they obtained 73% conver- sion at 200 ◦ C in 15min. However, in all these cases, the amount of base employed was 10–20 times (w/w ratio) more than the substrate. In the literature, an alternative approach is described, wherein complexes of group VIII elements are employed for this reaction [6,7]. Kameda and Yoneda [8] have reported homogeneous isomerization of 0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.05.008