1. INTRODUCTION

The catalytic selective oxidation of light parafins to olefines or oxygenated compounds is an attractive way to utilise alkane feed. Propane oxidative dehydrogenation (ODH) to propene and propane selective oxidation to acrolein or acrylic acid have been extensively studied in the last twenty years both in academia and in industry. For ODH reaction basic catalyst and vanadium element appear to be two crucial parameters, as for VMgO.1 ^,2 For oxidation of propane to acrylic acid, MoNbTe(Sb)V mixed oxides appear to be the most promising catalysts. ^{3 5} Mo and V containing Keggin-type heteropolycompounds (H _3+xPMo _12-xV _xO ₄₀) HPCs have also been studied ^{6 8} but for propane oxidation the selectivity to propene was observed to be low and carbon oxides were the main products.9 In the latter case the stability of the compounds was the major problem and very often the compounds transformed to lacunary HPCs or even to a mixture of the constituting oxides. However it is known that alkaline salts of heteropolycompounds are thermally more stable.10 Moreover, the acidity of such compounds is known to be particularly strong,11 which could be detrimental to selective oxidation, although acidity is necessary to activate the poorly reactive alkane. As Mo-based heteropolyacids are better catalysts for oxidation reactions than their W counterparts6, which are known to present a stronger acidity,6 ^,11 i.e. a higher ability for alkane activation, our...