Effects of Zeolite Channel Size and Connectivity On the Rate and Selectivity of n-Hexane Hydroisomerization

Tuesday, November 9, 2010: 9:33 AM
254 C Room (Salt Palace Convention Center)
Hsu Chiang, Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN and Aditya Bhan, Chemical Engineering, University of Minnesota, Minneapolis, MN

n-Hexane hydroisomerization reactions on bifunctional metal-acid catalysts (Pt/Al2O3 and acid zeolites) were used to probe the effects of zeolite pore size and connectivity on 2-methylpentane (2MP) and 3-methylpentane (3MP) formation. The measured rate of n-hexane isomerization was linearly proportional to the molar ratio of H2 to n-C6H14 over four zeolites (BEA, MOR, MFI, and BEA), consistent with a bifunctional mechanism involving the facile dehydrogenation of n-hexane on the metal catalyst to n-hexene and a kinetically-relevant step involving isomerization of n-hexene to 2MP= and 3MP= on zeolitic acidic sites. The rate per acid site increased in the order BEA (12x12MR) > MFI (10x10MR) > MOR (12x8MR) > FER (10x8MR), suggesting that molecules obtain more freedom in large pore zeolites. We surmise that the higher rates of isomerization on BEA and MFI than on MOR and FER imply that isomerization reactions mainly occur at the channel intersections of BEA and MFI. The measured selectivity defined as the ratio of the rate of 2MP formation to the rate of 3MP formation under conditions where catalytic rates were proportional to the molar ratio of H2 to n-C6H14 was measured to be FER (2.1) > MFI (1.35) ≈ MOR (1.38) ≈ BEA (1.35). The measured selectivity to 2MP is consistent with the hypothesis that isomerization reactions mainly occur in the channel intersections of MFI and BEA and hence MFI behaves like a large pore zeolite. The specific effects of zeolite structure on enthalpic and entropic driving forces for hydroisomerization pathways will also be discussed in context of the temperature dependence of measured reaction rates and selectivity.

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