Nonlinear Active Site Dynamic Behavior Over In Situ Generated Working Zeolites

Step response studies are combined with transient microkinetic models and the active site dynamic concept to untangle reaction and dynamics during the induction period of dimethyl ether (DME) conversion over zeolites. DME leads to propylene through a methoxymethyl mechanism. The reduction in induction period is due to a dynamic site-interconversion mechanism amongst three site ensembles (α, β, and γ). A dynamic turnover frequency (TOF) is introduced to rationalize site behavior over working catalysts. The dynamic TOF shows higher rates over the β and γ site-ensembles, reducing the induction period. An asymmetric cyclic orbit is observed on the phase portraits of working catalysts in comparison to freshly activated catalysts. This asymmetric cyclic orbit transforms in shape and magnitude as the initial flowrate of DME and water, acid site density distribution, and total fractional coverage change giving induction period control.