A single-event kinetic model for
1-pentene cracking on ZSM-5 is
presented. In the kinetic model, the influence of the catalyst on
the reactivity is comprehended via steric constraints in the zeolite
pores. Compared to gas-phase chemistry, reaction pathways with sterically
demanding olefins are excluded from the reaction network because of
these constraints. The number of estimated kinetic parameters is reduced
by choosing operating conditions for which double-bond isomerization
and skeletal isomerization are in thermodynamic equilibrium. Thus,
only cracking and dimerization remain as rate-determining steps. The
data set comprises 23 different experimental conditions being varied
at different residence times. From this data set, significant kinetic
parameters with small confidence intervals are determined. A good
parity between model and experiment was obtained, which supports the
applicability of the single-event methodology to the complex reactivity
of olefins cracking on ZSM-5 and gives mechanistic insight into the
reaction pathways leading to the product distribution. Furthermore,
the possibility to use these kinetic parameters for extrapolation
purposes beyond the experimentally covered range of reaction conditions
is shown.
The Brønsted acid sites of H-ZSM-5 and ferrierite reversibly adsborb linear pentenes via hydrogen bonding, rapidly isomerizing the double bond. On H-ZSM-5, dimerization of adsorbed pentenes occurs at a slower rate and leads to pentyl ester covalently bound to the surface. Pentene adsorbed on zeolites with narrower pores, such as ferrierite, remained stable in a hydrogen-bonded state even up to 423 K. Comparing the differential heat of adsorption of 2-pentene on silicalite and ferrierite allowed for the determination of the enthalpy difference between physically adsorbed pentene in ZSM-5 and the localized hydrogen-bonded π-complex at Brønsted acid sites, -36 kJ/mol. The activation energy (35 kJ/mol) for dimerization is almost identical to this enthalpy difference, suggesting that the rate-determining step is associated either with the mobilization of π-bonded 2-pentene or with the equally large activation barrier to form an alkoxy group via a carbenium-ion transition state. In a closed system, the dimerization rate is first order in the concentration of the π-complex that is both in equilibrium with the mobile pentene phase and in production of the carbenium ion that reacts with the mobile pentene. Overall, the alkoxy group is -41 ± 7 kJ/mol more stable than physisorbed pentene, establishing a series of energetically well-separated groups of reactive surface species.
La 3+ cations exchanged into ultrastable zeolite Y and zeolite X promote catalytic isomerization, cracking, and alkylation of alkanes. La 3+ cations stabilize the zeolite lattices and, more importantly, polarize alkane C−H bonds to enhance the rates of all three reactions. This unique activity leads to stable cracking and isomerization of reactive alkanes, with polarizable C−H bonds with adjacent tertiary or quaternary carbon atoms below 370 K. The presence of La 3+ cations also enhances the zeolite catalyzed hydride transfer rate for isobutane alkylation with 2-butene leading to high catalyst stability. Solid state MAS NMR shows that the strongest positive effects are associated with nonhydroxylated La 3+ cations accessible to the reacting molecules in supercages of the zeolite. The high activity is the result of a cooperative polarization of C− H bonds of alkanes by La 3+ cations and the presence of stable and strong Brønsted acid sites.
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