Metathesis of ethylene and 2-butylene to propylene over WO₃/SiO₂ catalysts

The WO₃/SiO₂ catalysts were prepared by impregnation (NH₄)₂WO₄ onto SiO₂ supports and their physicochemical properties were characterized by means of UV-Vis DRS and NH₃-TPD. Effects of reaction conditions on the catalytic performance of WO₃/SiO₂ for metathesis of ethylene and 2-butylene to produce propylene were investigated. The optimal amount of WO₃ loading was ~8%. Under the conditions of 473K~573K, 3.0MPa and 1.6h^-1, the initial 2-butene conversion was higher than 80% with an exceeding 89.2% selectivity to  propylene, using 8%WO₃/SiO₂ as the catalyst. It was shown that the 2-butene conversion and propylene selectivity decreased slightly during 140h continuous operation, and the metathesis activity of catalyst can be recovered through increasing the reaction temperature from 473K to 513K. The characterization revealed that three types of W⁺⁶ species (tetrahedral tungsten oxide species, octahedral polytungstate species and WO₃ crystallites) were present in fresh WO₃/SiO₂ catalysts and the reduced form of tungsten oxide species (W⁺⁴, W⁺⁵ and W^+(6-y) (0<y<1)) may be the suitable state of W species acting as metathesis active centers. NH₃-TPD results demonstrated that new acid sites were introduced into the catalysts and the total amount of acidity was increased when SiO₂ supports were loaded with WO₃.