Mastering simultaneous reaction and diffusion through Single Event Microkinetic modeling of n-alkane hydroconversion on Pt/H-ZSM-5

The shape selective catalysis based on ZSM-5 zeolite has found widespread application (1,2) in refining and petrochemical industry since long time. The product distribution associated with hydrocracking and hydroisomerization processes on ZSM-5 are predominantly controlled by the pore geometry and component diffusion. The various aspects of diffusion inside ZSM-5 pore structure have been widely covered in the literature (1,3) in the absence of reaction. On the other hand, detailed kinetic models generally make use of a simple diffusion model. With the augmentation of computational power, molecular modeling tools like Monte Carlo simulations are popularly performed nowadays (3,4 and the references therein) considering both diffusion and reactions in a zeolite matrix though these methods are yet to be established in relevant time scales. To understand the simultaneous shape selectivity effects on diffusion and reaction in ZSM-5 in a fundamental way, the detailed modeling of both the aspects is required. In the present work, a kinetic model has been developed for n-alkane hydroconversion on Pt/H-ZSM-5 using the single event microkinetic (SEMK) methodology along with a detailed multi-component diffusion model. It accounts explicitly for the pore structure by distinguishing between physisorption of branched and non-branched alkanes. The pore connectivity enters into the model via its effect on the occupancy of the active sites in determining the values of the diffusion coefficients. The model is validated with n-hexane hydroconversion data on Pt/H-ZSM-5 catalyst and allows to describe the differences of the of the product distribution when compared to Pt/HUSY.