Influence of intracrystalline diffusion in shape selective catalytic test reactions

Molecular dynamics simulations are used to measure the self diffusion const ant D of linear decane and n-methylnonanes (n = 2, 3, 4, and 5) at a cataly tically relevant temperature in seven 10 member ring zeolites. Two general behaviors are observed in D as the branch position is moved towards the cen ter of the alkane chain. For three of the zeolites (MEL, MFI, and MTT), D d ecreases monotonically as expected based on a consideration of the bulkines s of the different isomers. For the other four, alkane diffusion is conside red anamolous as D is not a monotonic function of branch position. For n-me thylnonanes in three zeolites D shows a minimum at either n = 2 (FER), 3 (E UO), or 4 (TON). In AEL, D has a local maximum for n = 3. Alkane diffusion is anamolous in these zeolites because they have structural features that p rovide a unique hindrance to molecular motion along the main diffusion chan nel. The ability of the zeolite to hinder motion depends on the molecular s tructure of the isoparaffin, resulting in the anamolous behavior observed. The 10 member ring zeolites selected for this study represent the entire gr oup for which known structures exist and values of the modified constraint index have been published. The diffusion data presented indicates that prod uct shape selectivity may play some par? in determining the modified constr aint index.