MgAlLi Mixed Oxides Derived from Hydrotalcite for Catalytic Transesterification
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MgAlLi Mixed Oxides Derived from Hydrotalcite for Catalytic Transesterification Cı´nthia S. Castro • Dilson Cardoso • Pedro A. P. Nascente • Jose´ Mansur Assaf
Received: 27 April 2011 / Accepted: 13 June 2011 / Published online: 29 June 2011 Ó Springer Science+Business Media, LLC 2011
Abstract MgAl hydrotalcite was synthesized and used as support for Li impregnation. MgAlLi oxides were obtained from heat treatment of the Li/MgAl hydrotalcite. These materials were characterized and evaluated as catalysts for model transesterification reactions. MgAl showed negligible activity under mild reaction conditions (50 °C and 0.5 h) whereas Li incorporation greatly increased the activity. The activities were correlated to their basicity determined by TPD of CO2. Reuse tests showed catalyst deactivation after the third cycle, probably due to lithium leaching. However, the contribution to a homogeneous reaction has been dismissed. MgAlLi revealed to be a promising catalysts for transesterification reaction and thus for biodiesel production. Keywords Mixed oxide Hydrotalcite Transesterification reaction Biodiesel
1 Introduction Hydrotalcite compounds are layered double hydroxides with x? 3? the general formula [M2? 1-xMx (OH)2] [Ax/n] mH2O, where M2? is a divalent cation, M3? is a trivalent cation, x (molar ratio): M3?/(M2? ? M3?) and A is a compensation
C. S. Castro (&) D. Cardoso J. M. Assaf Chemical Engineering Department, Federal University of Sa˜o Carlos, Rodovia Washington Luı´s, Km 235, Sa˜o Carlos, SP CEP 13565-905, Brazil e-mail: [email protected] P. A. P. Nascente Materials Engineering Department, Federal University of Sa˜o Carlos, Rodovia Washington Luı´s, Km 235, Sa˜o Carlos, SP CEP 13565-905, Brazil
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anion with charge n. The structure of these compounds is constituted by brucite-type octahedral layers, in which a part of M2? cations (e.g. Mg, Cu, Co, Ni, Mn, Zn, Fe) are substituted by those of M3? (e.g. Al, Cr, Fe, Co, Mn, V, Ga). The resulting excess of positive charge is compensated by hydrated anions in the interlayer space, the CO32- ions being the most common in the hydrotalcite-like clays [1]. Hydrotalcite calcination at high temperature promotes collapse of the crystalline structure leading to a basic solid with interesting properties for application in catalysis composed of homogeneous mixtures of oxides. These oxides are characterized as having very small crystal size (*10 nm), surface area of 100–300 m2 g-1, stable against thermal treatments and surface basic properties, presumably Lewis base sites [1]. Recently, these compounds have been applied to transesterification reactions in heterogeneous phase revealing to be promising heterogeneous catalysts for biodiesel production [2–6]. Moreover, lithium incorporation onto solid catalysts is reported in literature to be capable of increasing the catalysts basicity and therefore the activity in basic catalyzed reactions such as transesterification [7–10]. Compared to other alkali (Na, K and Cs) or alkaline earth (Ca, Sr and Ba) metal ion
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