Hydrotalcite, also known as aluminum-magnesium layered double hydroxide (LDH) or anionic clay, is a synthetic compound that was broadly investigated in the past decade due to its many potential applications. In this work, we present an environmentally benign process for the transesterification (methanolysis) of neem oil to fatty acid methyl esters (FAME) using Zn-Mg-Al hydrotalcites as solid base catalysts in a heterogeneous manner. The catalysts were characterized by XRD, FT-IR, TPD-CO2, and the BET surface area analysis. It is well-known that the catalytic performance of hydrotalcite is dramatically increased through the incorporation of Zn into the surface of Mg-Al hydrotalcite material. The optimized parameters, 10？:？1 methanol/oil molar ratio with 7.5？g catalysts reacted under stirring speed 450？rpm at 65°C for 4？h reaction, gave a maximum ester conversion of 90.5% for the sample with Zn-Mg-Al ratio of 3？:？3？:？1. 1. Introduction Biodiesel is a promising nontoxic and biodegradable renewable fuel comprised of monoalkyl esters of long chain fatty acids, which are derived from vegetable oils or animal fats [1]. It has attracted attention during the past few years as a renewable and environmental friendly fuel. The transesterification reaction consists of transforming triglycerides into fatty acid alkyl esters, in the presence of an alcohol, such as methanol or ethanol, and a catalyst, such as an alkali or acid, with glycerol as a byproduct [2]. Nearly 60–80% of the total biodiesel production cost is attributed to biodiesel feedstock. Using cheaper feedstock, such as nonedible oil, animal fats, untreated crude edible oil, or waste cooking oil, has been suggested to lower the feedstock cost [3–5]. Neem oil was selected as a nonedible feedstock for biodiesel production. Mixed metal oxides (hydrotalcites) are ones of the base catalysts affording good catalytic activity for the methanolysis of vegetable oil. Because the actual active sites of the hydrotalcites participating in catalysis situated at the edges of crystals, the smaller crystals have a larger number of active sites [6]. The hydrotalcite is a kind of double-layered hydroxides (LDHs) composed of the brucite-like layers, and Mg2+ and Al3+ cations coordinate an octahedral structure. When Al3+ replaces Mg2+, a positive charge is generated in the layer, which is balanced by or OH？ located between the brucite-like cation layers [7, 8]. The interesting property of the oxides obtained by calcination of LDHs around 400°C is the formation of highly active homogenous mixed oxides which are potentially used
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