**1. Introduction**

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Biodiesel is a source of energy derived from renewable sources, which can be a substitute for diesel fuel. It is biodegradable, and it has limited greenhouse gas emissions because of the closed CO2 cycle and a lower combustion emission profile (especially SOx). Biodiesel consists of fatty acid alkyl esters (usually methyl esters, FAME) derived from either the transesterifi‐ cation of triglycerides and/or the esterification of free fatty acids (FFAs) with low molecular weight alcohols [1-3].

Traditionally, the production of biodiesel is carried out in the presence of a homogeneous base catalyst; however, effluent disposal leads to environmental problems and economical incon‐ veniences. These problems can be overcome by the use of heterogeneous catalysts. Another problem with commercial production of biodiesel is the high cost of raw materials. In order to overcome this problem, waste oils and fats can be used as feedstocks [4-7]. However, waste cooking oils contain a high amount of free fatty acids, which is a problem for biodiesel production by the traditional process (homogeneous alkali-catalyzed transesterification) [8,9]. Different solid acids such as resins with sulfonic acid groups [10-14], zeolites [15-17], solid super acid catalysts [18] and carbonaceous catalysts [19] have been used as catalysts in the conversion of waste cooking oil containing a high amount of free fatty acids into biodiesel.

Heteropolyacids (HPAs) have several advantages as catalysts, which make them economically and environmentally attractive. On the one hand, HPAs have a very strong Bronsted acidity, approaching the superacid region; on the other, they are efficient oxidants, exhibiting fast reversible multielectron redox transformations under rather mild conditions. Their acid-base and redox properties can be varied over a wide range by changing the chemical composition.

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Solid HPAs possess a discrete ionic structure, comprising fairly mobile basic structural units, e.g., heteropolyanions and counter cations (H+ , H3O+ , H5O2 + , etc.) unlike the network structure of, e.g., zeolites and metal oxides [20-22]. HPAs have low specific surface areas (1–10 m2 /g). In order to increase the specific area of HPAs or, even better, to increase the number of accessible acid sites of the HPAs, a variety of supports like activated carbon [23-26], silica [27-32], zeolite [33-38] and polymeric matrix [39-43] have been used as support to immobilize HPAs.

HPAs and HPAs supported on different supports have been used as acid catalysts for biodiesel production [44-48].

Transesterification of waste cooking oil with high acid value and high water content was performed using heteropolyacid H3PW12O40.6H2O (PW) as homogeneous catalyst. PW was found to be the most promising catalyst, which exhibited the highest ester yield (87%) for transesterification of waste cooking oil and an ester yield of 97% for esterification of long-chain palmitic acid. The PW acid catalyst showed higher activity under the optimized reaction conditions compared with the conventional homogeneous catalyst sulfuric acid, and it can easily be separated from the products by distillation of the excess methanol and can be reused several times [45].

Biodiesel production in the presence of 20 wt% myristic acid from soybean oil was carried out over heteropolyacid immobilized on mesoporous Ta2O5 materials. Different catalysts were prepared. The network structures of the hybrid materials and the functions of the incorporated alkyl groups on the catalytic activity of the materials have been put forward [46].

Biodiesel synthesis from waste cooking oil was carried out over 12-tungstosilicic acid on SBA-15. The heteropolyacid was prepared using impregnation method. The effect of different reaction parameters like percentage loading, catalyst amount, mole ratio, time and tempera‐ ture were studied for the supreme conversion. The catalyst was recycled up to four times after simple workup without notable loss in the activity [48].

In this work, we report the transesterification of waste cooking oil with different alcohols over heteropolyacids immobilized on SBA-15. We also studied the effect of free fatty acid addition into waste cooking oil. A kinetic model is proposed.
