VAPOR-PHASE TRIMERIZATION OF FORMALDEHYDE TO TRIOXANE CATALYZED BY 1-VANADO-11-MOLYBDOPHOSPHORIC ACID

The kinetics of the trimerization of anhydrous formaldehyde vapour wer e examined in a tube reactor in the temperature range from 102.5 degre es C to 110 degrees C and in the pressure range from 0.7 to 1.15 bar, 1-Vanado-11-molybdophosphoric acid on a silicon carbide support was us ed as catalyst. Evaluation of the data obtained shows a steady increas e of the reaction rate with growing partial pressure of formaldehyde. A description of the rate by a power-law leads to a formal reaction or der of six. This high formal order could not be explained satisfactori ly. Investigations in a Berty-type recycle-reactor showed that the rea ction rate increased stepwise with growing partial pressure of formald ehyde. Intracrystalline diffusion within the pseudo-liquid phase of th e heteropolyacid particles obviously is the rate-determining step of t he reaction. The temperature dependence of the reaction rate was found to be inverse and extremely strong. Differences in the shapes of tube reactor and recycle reactor kinetic curves can be explained by the le velling effect of temperature gradients in the catalyst bed of the tub e reactor.