The purpose of this work is to study the synthesis, characterization, and catalytic performance of two types of solid heteropoly acid catalysts, namely, silicotungstic acid bulk (STAB) and STA-silica sol-gel (STA-SG) compared with sulfuric acid. From the XPS analyses, there was a significant formation of W-O-Si, W-O-W, and Si-O-Si bonding in STA-SG compared to that in STAB. The main spectra of O1s (90.74%, 531.5？eV) followed by other O1s peak (9.26%, 532.8？eV) were due to the presence of W-O-W and W-O-Si bonds, respectively. The STA-SG catalyst was found to be the more environmentally benign solid acid catalyst for the esterification reaction between oleic acid and glycerol due to its lower toxicity supported by silica via sol-gel technique. In addition, the ease of separation for STA-SG catalyst was attributed to its insoluble state in the product phase. The esterification products were then analysed by FTIR and HPLC. Both the H2SO4 and the STAB gave high conversion of 100% and 98% but at a lower selectivity of GME with 81.6% and 89.9%, respectively. On the contrary, the STA-SG enabled a conversion of 94% but with a significantly higher GME selectivity of 95%, rendering it the more efficient solid acid catalyst. 1. Introduction During the recent years, glycerol has been used as combustion materials around the world. The glycerol usage was expanded into many other high-quality products such as pharmaceutical, foods, and engine lubricant. To date, glycerol modification into glycerol monoester (GME) as lubrication materials that was based on the biosources was not really practised in the industry. The nature of the polar head group and the structure of the hydrocarbon tail of GME gave the strong impact as a friction reducer [1]. The GME is synthesized at present by acid catalyzed esterification of glycerol and fatty acids [2, 3]. Recently, list of studies involving alternative heterogeneous catalytic routes have been reported such as the glycerol esterification with lauric acid (LA) and oleic acid (OA) by using functionalized mesoporous materials [4], zeolitic molecular sieves [5, 6], and solid cationic resins [7, 8] as catalysts. In another work, the beta-zeolite catalyst gave the conversion of fatty acids above 20% at optimum condition of glycerol？:？LA molar ratio of 1？:？1 at 100°C for 24 hours [9]. In this work, the usage and activities of the silicotungstic acid bulk (STAB) and the silicotungstic acid-silica sol-gel (STA-SG) have been studied. The STAB consists of two molecular structures, namely, Keggin and Dawson structures with four protons. The
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