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

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.