Tungstated zirconia-alumina

Also for this reaction, namely esterification plus transesterification, mixed oxides, this time acidic in nature, appear to be the most promising alternative. Tungstated zirconia-alumina (WZA), sulfated zirconia-alumina and sulfated tin oxide were shown to be active in the transesterification of soybean oil with methanol at 200-300 °C and in the esterification of n-octanoic acid with methanol at 175-200 °C. Although the order of activities is different for the two reactions, WZA gives high conversions in both readions and it is stable under the reaction conditions [31]. Titania on zirconia, alumina on zirconia and zirconia on alumina also showed good performances [32, 6]. 

Furuta et al. tested a series of strong solid acids (alumina promoted sulfated zirconia, alumina promoted tungstated zirconia and sulfated tin oxide) for the transesterification of soybean oil with methanol at 200-300°C. Reaction yields over 90% were obtained for the alumina promoted tungstated zirconia at reaction times of 20 h using a flow reactor T = 250°C). The activity of the same catalyst was maintained for up to 100 h. 

Tungstate-promoted zirconia (W0 /Zr02) has been established as an efficient and eco-friendly solid acid catalyst for the S5mthesis of 2-arylbenzimidazoles [50]. Both benzimidazoles and quinoxalines have been prepared using nanosulfated zirconia, nanostructured ZnO, nano-y-alumina, and nano-ZSM-5 zeolites, as the catalyst [51] (Scheme 30). Ethanol was found to be the most effective solvent in this case. 

---