The numerous separation and environmental challenges associated with sulfuric acid have triggered the evaluation of sulfated zirconias and organosulphonic acids as transesterification catalysts. With unsupported systems, the SO4 2 may easily be lost in the reaction medium and thus the catalyst deactivation by acidity decay. Therefore the incorporation of porous silica or alumina as support material is necessary to solve this problem. Zirconia normally exists
as monoclinic, tetragonal and cubic phases. It has been established that, the tetragonal phase exerts much higher transesterification activity than the other phases, especially when doped with
appropriate quantity of amorphous WO3. In some cases, sulfated zirconia can be replaced with sulfated tin oxide or tungstatedzirconia
supported over alumina to obtain comparable activity
[49]. In addition to the phase properties, the loading of WO3 and
calcinations temperature influence the activity of ZrO2 (Table 3).
Calcination at 500 C ensures optimum ester yields, especially with
the lower loadings. Higher calcination temperatures sintered the
catalyst, with significant resistance associated with higher WO3
loadings [49].