VAPOR-PHASE TRIMERIZATION OF FORMALDEHYDE TO TRIOXANE CATALYZED BY 1-VANADO-11-MOLYBDOPHOSPHORIC ACID - A REACTION-RATE WITH AN UNUSUAL PRESSURE DEPENDENCY

The reaction rate of the trimerisation of anhydrous formaldehyde vapou r was examined in a Berty-type reactor at a temperature of 110 degrees C in the pressure range from 0.6 to 1.1 bar. 1-Vanado-11-molybdophosp horic acid on silicon carbide support was used as catalyst. The reacti on rate shows a step-wise increase with growing partial pressure of fo rmaldehyde. The reaction takes place in the pseudoliquid bulk phase of the catalyst. Intracrystalline diffusion within the heteropoly acid p articles obviously is the rate-determining step of the reaction. Catal yst crystals do incorporate trioxane molecules during the reaction yie lding H4PVMo11O40.nC(3)H(6)O(3) phases of well-defined compositions. T hese phases differ in their diffusion coefficients causing the excepti onal shape of trimerisation kinetics.