Forced composition cycling experiments were carried out at 0.1MPa total pressure and 250℃. A maximum in the time-averaged selectivity-period curve was found at the period of about 1 min

being higher by 5% than its optimal steady state value. A cycling operation model including all kinetic parameters of elementary steps was used to simulate the dynamic process of forced composition cycling. The results of simulation showed that this model can predict a maximum in the time-averaged selectivity-period curve but the period at which the maximum was found was much less than 1 min. It suggested that cycling operation could probably cause a slow change in the bulk phase or/and on the surface of the catalyst.