Reaction Behaviors and Kinetics during Induction Period of Methanol Conversion on HZSM-5 Zeolite

The reaction behavior in the induction period of the methanol to hydrocarbon (MTH) reaction over HZSM-5 (Si/Al = 19) zeolite has been investigated in a fixed-bed reactor. It is found that the induction period could be more than 2 h when the reaction was performed at a temperature of 255 degrees C and below. Meanwhile, three reaction stages can be clearly distinguished in the induction period: i.e., the initial C-C bond formation stage, the hydrocarbon pool (HCP) species formation stage, and the autocatalysis reaction stage. For each reaction stage, the kinetic parameters as well as the apparent activation energies have been evaluated. The HCP species formation stage is shown to be the rate-determining step. Addition of a ppm amount (molar) of benzene, toluene, or p-xylene to the methanol feed leads to a shortened induction period due to a lower energy barrier for both the HCP species formation and the autocatalysis reaction. A critical value of HCP species, [HCP](c), that is required for starting the autocatalysis reaction (the third stage) has been proposed. This critical value was measured to be 1 toluene molecule per 276 unit cells for HZSM-5 zeolite when toluene is cofed with methanol.