(525g) Single Event Microkinetic Modeling of n-Hexane Hydroisomerization On Pt/H-ZSM-5

The hydroisomerization of C5 -C7 compounds is of practical interest in the refining industry in the wake of stringent emission norms restricting the aromatics and oxygenate content in the gasoline pool. Though the product distribution is generally governed by the equilibrium composition on a bi-functional catalyst, the use of a medium-pore zeolite such as ZSM-5 gives a shape-selective product distribution deviating from that obtained on a large-pore zeolite. The n-hexane hydroisomerization on a Pt/H-ZSM-5 catalyst is explored in the present work. It primarily leads to mono-branched isomers with negligible formation of di-branched isomers or cracked product (Figure 1). A Single Event MicroKinetic (SEMK) model has been developed and accounts for the shape selectivity in terms of the differences between reacting and product species with respect to the physisoprtion and diffusion in the zeolite pores. n-hexane has been selected as model component in order to avoid transition state shape selectivity effect. One of the main features in the present model is the rigorous multi-component diffusion model. The diffusion coefficients are calculated as a function of the species concentration, the pore connectivity and the acid properties on the lattice scale considering the mean field approximation [1]. The molecular interactions in a multi-component environment are modeled by considering the inter species correlation through the Stefan-Maxwell formulation [2]. The effect of simultaneous diffusion and reaction are modeled at the crystallite scale. The strategic advantage of the calculation of diffusion coefficients by such a rigorous model is shown through the model regression to the experimental data. The n-hexane conversion can be adequately reproduced by estimating only the protonation enthalpy for formation of secondary alkylcarbenium ion while the physisorption parameters and the diffusion coefficients are fixed at the literature values. The protonation enthalpy is a measure of the average acid strength and a value of -65.4 ?b 2.3 kJ mol-1 is obtained which is comparable to a reference US-Y catalyst. The description of the mono-branched isomer distribution by the model can be further improved by also estimating the diffusion coefficients of the latter, cf. figure 1 but this result in more pronounced differences in their values than experimentally observed. The same fitting can be obtained by slight adjustment in other model parameters which has higher contribution in the product yield such as considering a difference in the physisorption enthalpies of 2-methyl- and 3-methyl-pentane within 1.0 kJ mol-1 which results in realistic values of the diffusion coefficients. Therefore a common strategy for parameter estimation for hydrosiomerization on ZSM-5 catalyst may involve two steps. First, the estimation of the physisorption and/or protonation enthalpies which results in a first-order response in the product distribution followed by the estimation of diffusion coefficients for a fine-tuning of second-order. The developed SEMK model can be further used to explore the structure-activity relationship for ZSM-5 catalyst and related materials. References: [1] Coppens M-O, Bell A.T, Chakraborty A.K, Chem. Eng. Sc, 54, 3455-3463, 1999. [2] Keil F.J, Krishna R, Coppens M-O, Rev. Chem. Eng, 16, 71-197, 2000.