Isomer sieving and the selective formation of terminal methyl isomers in reactions of linear alkanes on one-dimensional zeolites

Gina Noh, Stacey I. Zones, Enrique Iglesia

Research output: Contribution to journalArticlepeer-review

27 Scopus citations

Abstract

Diffusional hurdles enhance secondary interconversions of monomethyl isomers formed as primary products in bifunctional metal-acid isomerization of n-alkanes. One-dimensional medium-pore zeolites preferentially place methyl branches near the end of the isoalkane chains in what has been described as “pore mouth catalysis”; they also lead to low β-scission selectivities compared with three-dimensional frameworks similar in void size. n-Heptane and n-hexane reactions on acid forms of aluminosilicates of different channel size and connectivity (Al-MCM-41, FAU, SFH, BEA, MFI, MEL, SVR, TON, MTT; as mixtures with a Pt function) and crystallite size and proton density show that intracrystalline alkene concentration gradients are more consequential for larger and more branched molecules, as a result of their higher reactivity and lower diffusivity, and that the products form using all intracrystalline protons instead of only those near pore mouths. For both C6 and C7 reactants, 2-methyl to 3-methyl isoalkane ratios are those expected from equilibrium on large-pore zeolites (FAU, SFH, BEA). In contrast, these isomer ratios reflect the preferential sieving of the faster diffusing 2-methyl isomers from isoalkene products present as equilibrated mixtures within crystallites on medium-pore (10-MR) zeolites. These sieving effects are most evident for one-dimensional 10-MR frameworks because their void structures lead to stronger concentration gradients and more selective sieving of the faster diffusing isoalkenes. These effects are weaker on three-dimensional 10-MR structures because of less severe diffusional constraints, but the intersecting channels in these frameworks create local cage-like structures that favor β-scission through the preferential retention of dimethylpentenes, which act as the sole precursors to scission products. One-dimensional structures lack such channel undulations and cage-like intersections; as a result, one-dimensional medium-pore TON and MTT frameworks lead to low β-scission selectivities. This work demonstrates the essential, yet often overlooked, coupling between reactive and diffusive properties of zeolite materials in determining reactivity and selectivity in practice.

Original languageEnglish (US)
Pages (from-to)255-270
Number of pages16
JournalJournal of Catalysis
Volume377
DOIs
StatePublished - Sep 2019

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Physical and Theoretical Chemistry

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