Available online at www.sciencedirect.com Journal of Molecular Catalysis A: Chemical 278 (2007) 135–144 Solvent free aldol condensation of propanal to 2-methylpentenal using solid base catalysts Sumeet K. Sharma a,b , Parimal A. Parikh b , Raksh V. Jasra a,∗ a Discipline of Inorganic Materials & Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), G.B. Marg, Bhavnagar 364 002, Gujarat, India b Department of Chemical Engineering, SV National Institute of Technology, Surat 395 007, Gujarat, India Received 26 June 2007; received in revised form 28 August 2007; accepted 1 September 2007 Available online 7 September 2007 Abstract Catalytic activities of alkali ion-exchanged zeolites, alumina, alkali treated alumina and hydrotalcite having varied Mg/Al molar ratio were evaluated for aldol condensation of propanal in liquid phase under solvent free conditions. The maximum conversion (97%) of propanal with 99% selectivity of 2-methylpentenal was obtained at 100 ◦ C and 10 h using activated hydrotalcite of Mg/Al molar ratio of 3.5 as a catalyst. The basicity of the activated hydrotalcite of Mg/Al ratio 1.5, 2.5 and 3.5 was evaluated by isomerization of -isophorone to -isophorone as a model test reaction reported for basicity measurement. The effect of amount of catalyst and temperature on the conversion of propanal, selectivity of 2-methylpentenal and initial rate of reaction were studied in detail. The activation energy for propanal condensation under solvent free condition was found to be 58 kJ using activated hydrotalcite of Mg/Al molar ratio 3.5 as a catalyst. The catalyst was recycled six times without any significant loss in conversion and selectivity. © 2007 Elsevier B.V. All rights reserved. Keywords: Aldol condensation; Propanal; Cation exchanged zeolites; Hydrotalcite; Kinetics; 2-Methylpentenal 1. Introduction 2-Methylpentenal is a commercially important chemical that finds applications in pharmaceuticals, fragrances, flavors, cos- metics and as an intermediate for the synthesis of various pharmacologically active compounds [1]. Commercially, 2- methylpentenal is synthesized by the aldol condensation of propanal in the presence of liquid base like KOH or NaOH in sto- ichiometric amount [1–3]. Under optimum reaction conditions, 99% conversion of propanal is achieved with 86% selectivity of 2-methylpentenal using liquid base [1]. The existing process for the synthesis of 2-methylpentenal from propanal is not eco- friendly and have other drawbacks like the use of KOH or NaOH in higher than stoichiometric amount, lower selectivity, separa- tion of spent KOH/NaOH from post-synthesis reaction mixture, effluent treatment and disposal of spent liquid base. Besides, the alkali metal hydroxides require large amount of water for the ∗ Corresponding author. Tel.: +91 278 2471793; fax: +91 278 2567562. E-mail address: rvjasra@csmcri.org (R.V. Jasra). neutralization and washing after the completion of reaction. It is desirable to find solid base catalysts which could substitute liq- uid bases and would still show the advantages of heterogeneous catalysis, i.e. ease of separation of the products, decreased cor- rosion of the reactor, and possible regeneration of the catalyst. Therefore, research efforts are directed to develop a catalytic process, which can produce 2-methylpentenal from propanal with high selectivity employing eco-friendly reusable solid base catalyst (Scheme 1). The synthesis of C 6 aldehydes from propanal via aldol condensation reaction in ionic liquid media with NaOH as a catalyst was studied by the Mehnert et al. The authors have reported 100% conversion of propanal with 83% selectivity for C 6 aldehydes at 80 ◦ C in 3 h reaction time using [bdmin]PF 6 ionic liquid as solvent medium with sodium hydrox- ide as a catalyst [4]. Matsui et al. studied the effect of CO 2 pres- sure on the selectivity of condensation products in the presence of MgO with small amount of water. In the supercritical region, 94% selectivity of 2-methylpentenal was obtained near the crit- ical pressure of 12 MPa [5]. The vapor phase aldol condensation of the propanal was studied by Scheidt, who reported higher than 95% selectivity of 2-methylpentenal by passing the vapors 1381-1169/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2007.09.002