**3. Conclusion**

374 Biodiesel – Feedstocks and Processing Technologies

Consequently, if this part of the reaction pathway is similar to homogeneous systems, the others steps commonly involved in esterification reactions (e.g. protonation carbonyl group FA, attack of the alcohol molecule on protonated FA, water elimination, etc) may then

O

**.**

<sup>H</sup> <sup>H</sup> +

R` OH

**C17H33 C**

**step 6**

**. .**

**..**

**.. ..**

Fig. 17. Mechanism of formation ester catalyzed by free H+ ion in solution

**CH3CH2O**

**H<sup>+</sup>**

**C17H33**

**<sup>+</sup> CH3CH2O**

**H2O**

**OH2**

**+**

**step 5**

**C**

**+OH**

**C**

**OCH2CH3 C17H33**

**OH**

**C17H33**

the solution

**C**

**OH**

**step 4 step 3**

Fig. 18. Proposal of an alternative mechanism of formation ester catalyzed by free H+ ion in

**OH**

**H**

**..**

R OH

**O**

OR'

**.. ..**

**..**

**..**

**..**

Conversely, is also possible that other FA molecules can be activated via protonation on surface of supported-catalyst. Thus, an alternative proposal is displayed in Figure 18.

**OCH2CH3**

**H<sup>+</sup>**


**..**

**C17H33 C**

OH

**..**

**.**

**.. ..**

**CH3CH2OH**

**Bronsted site of the HPW/niobium catalyst**

R OR´

**O**

**step1**

**OH**

**step 2**

**C C17H33 OH**

**+OH**

**C C17H33 OH**

**+**

**OH**

<sup>+</sup> -H<sup>+</sup>

**. .**

**..**

O

**.**

**..**

R OR´

**..**

proceed as described in Figure 16.

**. .**

<sup>H</sup><sup>+</sup> OH

R OH +

**..**

**..**

O

**.**

**..**

R OH

**..**

The efficiency of tungstophosphoric acid (H3PW12O40) immobilized by impregnation method on silicon, zirconium and niobium oxides was assessed in the esterication of oleic acid with ethanol, at 60 C temperature. As a general tendency, it was observed that the catalytic activity decreases in the series HPW/Nb2O5 > HPW/ZrO2> HPW/SiO2 with all catalyst being treated on temperature range 100 to 300 C. Moreover, good yielding of recovery of HPW 50% w/w/Nb2O5 catalyst (*ca*. 75 %) and high conversions of acid oleic were obtained in recycle experiments. From leaching tests and of the rates of recovery may be concluded that the HPW/Nb2O5 catalysts are stable under reaction conditions used; however the recovery procedure employed it should be enhanced. Thus, it can be concluded that although yet non-finished, present methodology offers several advantages such as high yields, simple procedure for recovery and reuse of catalyst and mild reaction conditions. The authors hope that with this work a significant advance on the field of recoverable catalysts can has been proved.
