Selective Oxidation of n-Butane over Highly Crystalline Vanadyl Pyrophosphate Catalyst Synthesized by Intercalation-exfoliation-reduction of Layered Vanadyl Phosphate Dihydrate

摘要

A novel synthetic method to prepare a highly crystalline (VO)2P2O7 catalyst by the intercalation-exfoliation-reduction of VOPO4·2H2O in alcohol is presented. Stepwise thermal treatment of layered VOPO4·2H2O in alcohols, such as 2-butanol, caused intercalation, followed by exfoliation of VOPO4·21120. Further thermal treatment of the exfoliated VOPO4 sheets in the alcohol at the reflux temperature afforded crystallites of the precursor VOHPO4·0.5H2O, which were 1 μm in length and 150 nm in thickness and had leaf-like shapes. On the other hand, VOHPO4·0.5H2O crystallites prepared by the direct reduction of VOPO4·21120 with 2-butanol were large, thick, well-developed platelets with a length of 5 μm and a thickness of 250 nm. The precursor synthesized by the intercalation-exfoliation-reduction process could be transformed into a highly crystalline (VO)2P2O7 phase under the reaction conditions. The resulting catalyst showed high activity and selectivity in the oxidation of n-butane due to the high crystallinity of (VO)2P2O7.The intercalation-exfoliation-reduction of VOPO4·2H2O in a mixture of 2-butanol and ethanol gave much smaller and thinner crystallites of VOHPO4·0.5H2O with uniform sizes (ca. 300 nm long and 35 nm thick). The catalyst derived from this precursor exhibited remarkably high selectivity for maleic anhydride with a maximum selectivity of ～ 84% in the oxidation of n-butane. These results show that the intercalation-exfoliation-reduction of VOP04·2H2O is an effective method to synthesize a highly active and selective catalyst. In addition, we showed that the size of the crystallites of VOHPO4·0.5H2O is a key factor in controlling the catalytic performance, especially selectivity, in the oxidation of n-butane.