Synthesis of β-MoO3 through evaporation of HNO3-added molybdic acid solution and its catalytic performance in partial oxidation of methanol
Two types of molybdenum trioxide were synthesized by evaporating the molybdic acid solution prepared via cation-exchange of an aqueous solution of Na2MoO4·2H2O. When the molybdic acid solution was evaporated to dryness at 323 K under reduced pressure and then calcined at 573 K in a stream of oxygen, α-MoO3 was exclusively produced. However, addition of HNO3 to the solution before the evaporation resulted in formation of a bright yellow powder which was identified as β-MoO3. The phase ratio of β/α was dependent on the quantity of HNO3; pure β-MoO3 was successfully synthesized when the molar ratio of HNO3/Mo in the solution was in the range of 1–2. It was also found that the HNO3 addition made no structural change of the isopolymolybdates in the solution and the dried precursor, but did affect the dehydration of the dried precursor to induce the crystallization of the β-phase. In partial oxidation of methanol, β-MoO3 exhibited much higher methanol conversion than the α-form. IR spectra of chemisorbed pyridine elucidated that the higher catalytic activity was caused by the relatively large number and high acidity of Lewis acid sites on the surface of β-MoO3.
Graphical abstractPure β-MoO3 was successfully synthesized through evaporation of a molybdic acid solution containing HNO3, whereas α-MoO3 was selectively produced from the HNO3-free solution. Compared to the α-form, β-MoO3 had the larger amount and higher acidity of Lewis acid sites on the surface, and thereby exhibited much higher catalytic activity in partial oxidation of methanol. Figure optionsDownload full-size imageDownload as PowerPoint slide
Journal: Applied Catalysis A: General - Volume 326, Issue 1, 30 June 2007, Pages 106–112