Patent ID: 6858754

Claim:
A process for the preparation of acrylic acid comprising: passing a reaction gas starting mixture containing propane, molecular oxygen and at least one diluent gas comprising steam over a multimetal oxide material at elevated temperature, wherein the multimetal oxide material has the stoichiometry I Mo 1 V b M 1 c M 2 d O n (I), where M 1 is Te, Sb or both Te and Sb, M 2 is at least one element selected from the group consisting of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Ga, Fe, Ru, Co, Rh, Ni, Pd, Pt, La, Bi, B, Ce, Sn, Zn, Si and In, b is 0.01 to 1, c is >0 to 1, d is >0 to 1 and n is a number which is determined by the valency and frequency of the elements other than oxygen in (I), wherein the propane is at least partially oxidized to acrylic acid, said process further comprising changing the composition of said reaction gas starting mixture at least once while the process is being carried out, the change to the composition of said reaction gas starting mixture being such that the proportion of the steam contained in the reaction gas starting mixture, before the change and relative to the molar amount of propane contained in the reaction gas starting mixture, is lower after the change.