Influence of intracrystalline diffusion in shape selective catalytic test reactions

Abstract

Molecular dynamics simulations are used to measure the self diffusion constant D of linear decane and n-methylnonanes (n = 2, 3, 4, and 5) at a catalytically relevant temperature in seven 10 member ring zeolites. Two general behaviors are observed in D as the branch position is moved towards the center of the alkane chain. For three of the zeolites (MEL, MFI, and MTT), D decreases monotonically as expected based on a consideration of the bulkiness of the different isomers. For the other four, alkane diffusion is considered anamolous as D is not a monotonic function of branch position. For n-methylnonanes in three zeolites D shows a minimum at either n = 2 (FER), 3 (EUO), 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 provide a unique hindrance to molecular motion along the main diffusion channel. The ability of the zeolite to hinder motion depends on the molecular structure of the isoparaffin, resulting in the anamolous behavior observed. The 10 member ring zeolites selected for this study represent the entire group for which known structures exist and values of the modified constraint index have been published. The diffusion data presented indicates that product shape selectivity may play some part in determining the modified constraint index.