Orthorhombic Mo3VOx oxide catalysts in various crystal sizes with different external surface areas and the same micropore volumes were synthesized hydrothermally by adding sodium dodecyl sulphonate (SDS, C12H25SO3Na) to preparation solution and by changing synthesis temperature. The synthesized catalysts were then tested for the selective oxidations of ethane, propane, and acrolein, in order to clarity catalysis field for the reactions. It was found that ethane converted to ethene in the heptagonal channel in the structure of the catalyst and propane was also oxidized to COx in the heptagonal channel. Acrolein, on the other hand, was converted to acrylic acid at the mouth of the heptagonal channel. It is concluded that the heptagonal channel is all responsible for the catalysis for ethane, propane, and acrolein. However, the catalysis field is different in a reflection of the molecular size of the reactants. (C) 2014 Elsevier B.V. All rights reserved.