Anhydrous diluents for the propylene oxidation reaction to acrolein and acrolein oxidation to acrylic acid

In a process for producing acrolein by the catalytic oxidation of propylene and a process for producing acrolein and acrylic acid by a two-step catalytic oxidation of propylene, wherein the first-stage reaction produces primarily acrolein and the second stage reaction produces primarily acrylic acid by the oxidation of acrolein, said processes optionally using one or more recycle streams to either or both stages, both stages operating on feed streams containing oxygen and inert diluent gas, the improvement comprising utilizing one or more essentially inert essentially anhydrous diluent gases free of any intentionally added water in a mole ratio of about 2.0 to about 32.0 moles of diluent per mole of propylene as the essentially inert essentially anhydrous diluent feed added to the first stage, said added essentially inert essentially anhydrous gas feed having a composite heat capacity of at least about 7.95 calories/gram-mole (.degree.C.) and an oxygen-containing stream containing from about 1.1 to about 2.1 moles of molecular oxygen per mole of propylene and utilizing one or more essentially inert essentially anhydrous diluent gases as the inert gas feed which is free of any intentionally added water to the second stage, said added essentially inert essentially anhydrous diet comprising one or more inert gases having a composite heat capacity of at least about 7.95 calories/gram-mole (.degree.C.)