(314c) Mastering Simultaneous Reaction and Diffusion through Single Event Microkinetic Modeling of N-Alkane Hydroconversion on Pt/H-Zsm-5

Catalyst activity and selectivity in hydroconversion on medium pore zeolites such as ZSM-5 are predominantly controlled by the pore geometry and component diffusion. To understand the simultaneous shape selectivity effects on diffusion and reaction in a fundamental way, a kinetic model has been developed for n-alkane hydroconversion on Pt/H-ZSM-5 using the single event microkinetic methodology. The reactant shape selectivity in terms of different physisorption parameters for linear and branched alkanes is considered through multicomponent Langmuir equation, while product shape selectivity is accounted for through diffusion inside the ZSM-5 micropores. The latter is modeled based on activated jumps of physisorbed molecules between channel and intersection sites. The rates of the activated jumps depend on the sorbate concentration as well as on the strength of the acid sites and the distribution between weak and strong sites. The multicomponent environment is dealt with through inter species diffusivity as described by the Stefan-Maxwell formulation. The transport diffusivity at the crystallite scale is calculated from self-diffusivity phenomenon at unit cell level.

These types of reactant and product shape selectivity are sufficient to model the observed phenomena in n-C₆hydroconversion. For heavier components transition state shape selectivity effects will also be accounted for. A typical calculated concentration profile alculated inside the crystallite (Figure 1) allows assessing the effects of the simultaneous occurrence of diffusion and reaction as well as of the interaction between different species. Various scenarios such as the dependency of diffusion coefficients on the sorbate concentration, the magnitude of inter-species interactions, the distribution between weak and strong acid sites,? can be tested using this model. Though the dependency of diffusion coefficients on sorbate concentration is substantial, it mainly affects the global catalyst activity, rather than the product distribution, vide Figure 2, whereas the inter species diffusivity has minimal effect towards catalyst activity and selectivity.

Model parameter values is currently being validated against experimental n-C₆hydroconversion data in a CSTR set-up through regression. Slightly more negative protonation enthalpies, i.e., about 5 kJ mol^-1, are obtained compared to a reference USY-zeolite. Activation energies for isomerisation and cracking reactions were not further adjusted to the experimental data.. Having validated the model, it will be a useful tool for understanding diffusion and reaction induced shape selectivity in hydroconversion on ZSM-5 zeolites and other medium pore zeolites.

Figure 1 Concentration profile for n-hexane and its isomers along diffusion path in zeolite crystallite  

Figure 2  Plot for conversion with a change in W/F for simulation of n-hexane hydroconversion on Pt/H-ZSM-5 in CSTR set-up;  ♦ diffusion coefficients are concentration independent, ▲ strong dependence on concentration; other operating conditions : Pressure: 30 bar, H2/HC : ~ 55 mol/mol, Temperature: 250 °C