Unraveling Diffusion and Other Shape Selectivity Effects in ZSM5 Usingn-Hexane Hydroconversion Single-Event Microkinetics

Abstract

Potentially dominant factors governing the shape selectivity in n-hexane hydroconversion over a Pt/H-ZSM5 catalyst were evaluated by means of single-event microkinetic (SEMK) model regression against experimental data. The observed product distribution could be adequately modeled, and a corresponding physically meaningful interpretation could be made only when accounting for intracrystalline diffusion limitations for each hexane isomer involved in the reaction network, rather than considering physisorption effects or transition-state shape selectivity. Simultaneous diffusion and reaction inside the catalyst crystallites were expressed via Fick's second law, while the alkane Fick diffusion coefficients were assessed by explicitly accounting for mixture nonideality effects. A 3-fold lower diffusion coefficient was found to be required for 3-methylpentane compared with 2-methylpentane to explain the typically high selectivity toward the latter alkane. Once formed inside the catalyst crystallite, dimethylbutane isomers remained nearly immobile as was evident from their significantly lower diffusion coefficients. Reaction at the crystallite external surface was primarily responsible for the marginal conversion toward the former species, as observed experimentally.

Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts forn-Heptane Hydroisomerization

“…These observations are therefore indicative of the large diffusional differences between skeletal isomers of heptane in ZSM-5 that determine the isomer product distribution. 14 , 23 , 24 The results also show that the product distribution is not affected by the location of Pt nanoparticles in composite supports. 82 …”

“…The results of the ZSM-5 catalysts show clear evidence of shape selectivity in n -heptane hydroisomerization, resulting in a very low selectivity to di- and tribranched isomers and a 2-methylhexane/3-methylhexane ratio that is deviating from the equilibrium value. 23 Previous research has pointed out the that the isomer product distribution of ZSM-5 is mainly governed by the free energy barriers for diffusion of different heptane isomers within micropores, that is, product shape selectivity. 24 The isomers formed within the zeolite with a high free energy barrier for diffusion are more likely to undergo cracking and leave the zeolite as fast diffusing propane and i -butane.…”

“… 23 Bulky dibranched hexanes and heptanes are formed to a limited extent over these catalysts because of steric constraints and fast cracking of these molecules in ZSM-5 micropores. 23 , 24 Large-pore (12-membered ring) zeolites such as zeolite Beta and zeolite Y impose much less steric constraints on branched isomers of hexane and heptane and are therefore often considered to be “nonshape-selective” zeolites for these processes, with an isomer product distribution that is close to thermodynamic equilibrium. 24 , 25 …”

Information-Driven Catalyst Design Based on High-Throughput Intrinsic Kinetics

“…By implementation of these models in an adequate reactor model accounting for transport phenomena [28,29], specific reactor configurations such as a riser reactor [30] or a slurry-bubble column [31] and catalyst deactivation [32], reliable, industrially relevant simulations can be made with these models. This also comprises the extension from model compound behavior, as typically measured at the laboratory scale, to realistic feeds [25,33].…”

Kinetic Modeling of Catalytic Olefin Cracking and Methanol-to-Olefins (MTO) over Zeolites: A Review

“…However, there are also conditions where this ideal scenario is not realized. In the literature, several microkinetic studies for hydrocracking using the single-event methodology are available [51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68]. Other approaches are possible and useful especially for complex feeds such as a Fischer-Tropsch product mixture or vacuum gas oil [69][70][71][72][73][74].…”