Transport and reactivity of hydrocarbon molecules in a shape-selective zeolite

Abstract

For thl: catalytic cracking of Cs to C9 hydrocarbons on ZSM-5, we demonstrate quantitatively the contributions of each of two mechanisms for molecular shape selectivity. Using crystallites of different sizes and activities, and classical methods for evaluating diffusion inhibition of the reaction rate, we separate the effects of mass-transport-induced selectivity from that created by steric inhibition by the size of a reaction complex. The selective cracking of n-paraffins compared to monomethyl paraffins (from Cs to C,) is due to a higher intrinsic rate constant of the n-paraffin, with diffusional mass transport playing no appreciable role. In contrast, dimethyl paraffin cracking is strongly diffusion-inhibited. The methyl paraffin/n-paraffin discrimination is a result of steric constraint on the sizeable methyl paraffin/carbonium ion reaction complex. This structural selectivity is shown to be absent for the corresponding olefins where such complexes do not arise. The diffusivities at reaction conditions have been determined. For the linear hydrocarbon, diffusivity notably exceeds that expected from the Knudsen model. This reminds us to review assumptions of conventional concepts of mass transport. The availability of zeolites now allows us to probe many basic phenomena in catalysis, molecular configuration and dynamics, including mass transport.

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…495 Whereas these early experiments involved reactants that are categorically excluded from the LTA-type zeolite pores, 495 later experiments focused at differences in mass transfer rates as a dominant cause for reactant shape selectivity. 432 …”

“…For example, van der Waals interactions between a transition state and the zeolite framework can decelerate the reaction by increasing the free energy of formation of transition states that are incommensurate with the particular zeolite topology. [427][428][429][430]432 Alternatively, ionic interactions between a transition state and the zeolite framework can accelerate the reaction by decreasing the free energy of formation of transition states that are commensurate with the particular zeolite topology. 32 To quantity these effects, one has to perform a detailed quantum chemical calculation in the pores of the zeolite and to determine the transition state in the pores.…”

“…In our view, this did not happen until an elegant study by Haag, Lago, and Weisz from Mobil. 432 Interestingly, a recent quantum chemical study by Sauer and co-workers et al 44 indicates that the conclusions of the seminal study into transition state shape selectivity 426 were probably incorrect and that mass transfer effects are the dominant cause for the shape selectivity observed. This aside, by the early 1970s an appealingly simple, "classic 433 " concept of shape selectivity had emerged: It was either due to mass transport or due to transition state control of the reactions 428,429 seven though it was also well established that the phenomenon was considerably more complex.…”

Supercritical catalysts of light hydrocarbon conversion. DOE PETC eighth quartery report, July 1, 1995--September 30, 1995

“…As another example, alkane cracking catalyzed by HZSM-5 is af5ected by restricted transition state shape selectivity [38,39]. In this case, a branched alkane (which is more reactive than a linear alkane in an unhindered environment) is subject to more steric repulsion during the bimolecular cracking step than the linear alkane.…”

One-pot synthesis of nano-crystalline MCM-22

“…The MWW pore topology endows promising catalytic properties to MCM-22 zeolites. For instance, MCM-22 exhibits shape selectivity properties in the trans-alkylation of toluene to p-xylene [2]. Its potential for the hydroisomerization of n-alkanes into branched alkanes [3,4] and the alkylation of benzene to cumene and ethylbenzene [5,6,7] has also been demonstrated.…”