Isomerization of citronellal to isopulegol. Activity and diastereoselectivity on sulfated and phosphated zirconia

The influence of the acid properties of phosphated (ZP) and sulfated (ZS) zirconia on the activity and diastereoselectivity in the isomerization of (+/-)-citronellal to (+/-)-isopulegol was studied. The catalysts were prepared from Zr(OH)(4) obtained by the precipitation method. The synthesis of ZP and ZS was achieved through the immersion of Zr(OH)(4 )in NH4H(2)PO(4 )and (NH4)(2)SO(4 )solutions, respectively. The synthesized materials were characterized by several techniques, including N(2 )physisorption, X-ray diffraction (XRD), Raman spectroscopy, KBr pellets by FTIR, pyridine FTIR (Py-FTIR), and NH3temperature-programmed desorption (NH3-TPD). The results demonstrated that the density and strength of Lewis and Bronsted acid sites, as well as the Bronsted/Lewis acid sites density ratio (B/L), increased with the concentration of sulfate or phosphate species. Reaction tests revealed that ZS catalysts exhibited greater activity than ZP catalysts. The activity increased with the density of total acid sites, their strength, and the Bronsted and Lewis acid sites density ratio. The diastereoselectivity for (+/-)-isopulegol exhibited an inverse trend, with the most diastereoselective catalysts being those with only Lewis acid sites