Urea-assisted synthesis of hydrothermally stable Zr-SBA-15 and catalytic properties over their sulfated samples

Ordered hexagonal mesoporous Zr-SBA-15(x)s (where x stands for Si/Zr) with a large amount of zirconium heteroatoms (Si/Zr at 5.0–9.0) have been successfully synthesized under the assistance of urea by using tri-block copolymer as a template. The results obtained from XRD and N2 isotherm techniques indicate that these Zr-SBA-15s have more excellent hydrothermal stabilities in both boiling water and 100% steaming than SBA-15 does. UV–vis spectroscopy suggests that zirconium species are mainly amorphous zirconia in Zr-SBA-15(4) and Zr-SBA-15(5), while they are highly isolated Zr4+ ion in Zr-SBA-15(7) and Zr-SBA-15(9). After sulfation, Zr-SBA-15(7) and Zr-SBA-15(9) basically maintain their ordered hexagonal mesostructure and isolated Zr4+ ion. Catalytic tests in esterification and transesterification show that sulfated Zr-SBA-15(7) and Zr-SBA-15(9) with isolated Zr4+ exhibit much higher catalytic activities than sulfated Zr-SBA-15(4) and Zr-SBA-15(5) with amorphous zirconia, indicating that the isolated Zr4+ species are more favorable for the formation of acidic sites than amorphous zirconia. The sulfation of isolated Zr4+ species in porous silica materials with large surface area may offer a novel route for preparation of good acidic catalysts in the future.