Local structure and catalytic properties of molybdenum oxide supported on nanostructured SiO2 (SBA-15) were studied under reaction conditions by combined X-ray absorption spectroscopy (XAFS) and mass spectrometry. MoO3 supported on SBA-15 exhibits stability and catalytic properties different from those of binary bulk oxides. The interaction between support and molybdenum oxide stabilizes particular hexagonal MoO3 structure that is highly active and selective. The hex-MoO3-SBA-15 is stable under reducing (propene) and oxidizing reaction conditions in the temperature range from 20 to 500 degrees C. In contrast to bulk hex-MoO3, the onset of activity at about 250 degrees C is not accompanied by a transformation to alpha-MoO3. Moreover, in contrast to alpha-MoO3, hex-MoO3-SBA-15 is capable of directly oxidizing propene to acrylic acid without additional metal sites. (C) 2007 Elsevier Inc. All rights reserved.