Influence of the Incorporation of Transition Metals on the Basicity of Mg,Al-Mixed Oxides and on Their Catalytic Properties for Transesterification of Vegetable Oils
The transesterification of vegetable oils produces fatty acid methyl esters (biodiesel). Biodiesel is a nonpolluting alternative fuel produced from renewable resources whose chemical and physical properties closely resemble those of the petroleum diesel fuel and does not contribute to the greenhouse effect. The use of solid catalysts makes biodiesel production processes environmentally friendly. In this work, Mg,Al-mixed oxides derived from Mg,Al-hydrotalcite-like compound with an M3+/( ) molar ratio of 0.25, in which Mg or Al was partially replaced by Co2+, Cu2+, Cr3+, or Fe3+, were prepared, characterized, and evaluated as catalysts for the transesterification of soybean oil with methanol. The results have indicated that the incorporation of transition metal influenced both textural and base properties. All the evaluated catalysts were active for the studied reaction. The catalytic activity followed the order Cr-MO < Co-MO < Cu-MO < Fe-MO < MO and could be explained by mesoporous volume. 1. Introduction Hydrotalcite-like compounds (HTLCs) are layered double hydroxides (LDHs) that have been investigated during the last two decades as adsorbents, anion exchangers, and catalysts or catalyst precursors for different types of chemical reactions. The general formula of the HTLCs is where the divalent ion can be , , , , , or , the trivalent ion can be , , or , the compensation anion can be , , , , or , and can take values between 0.17 and 0.33 [1]. Mg,Al-hydrotalcite is the most common LDH, and it can also be synthesized in a ternary form in which Mg or Al is partially replaced by a transition metal cation. Thermally treating HTLCs induces dehydration, dehydroxylation, and loss of compensation anions, forming mixed oxides/hydroxides with basic properties and a poorly crystallized structure [2]. These oxides present small particle size, large specific surface area, and basic properties attributed to hydroxyl groups, different - acid-base pairs, and anions [3–6]. The acid-base properties and, as a consequence, the catalytic activity and selectivity of these Mg,Me,Al-mixed oxides depend on chemical composition (type of Me cation and Mg/Me/Al molar ratio) and on the conditions of the thermal treatment used to decompose the hydrotalcite precursor. So, they can be tailored to the process under study aiming at increasing their activity and/or selectivity. Their main applications as basic catalyst include aldol, nitroaldol, Knoevenagel, and Claisen-Schmidt condensations, alkylation of ketones and phenols, Michael additions, glycerolysis of fats for the manufacture
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