Diffusion of long n-alkanes in silicalite. A comparison between neutron scattering experiments and hierarchical simulation results.

New quasi-elastic neutron scattering (QENS) data are presented for the self-diffusion of normal alkanes up to hexadecane in the zeolite silicalite at low occupancy. Measured diffusivities at 300 K are higher than in Na-ZSM-5 and agree with the predictions of molecular dynamics and a hierarchical transition-state theory/Brownian dynamics conducted on the same systems. Activation energies for diffusion are around 5 kJ/mol for C1-C6 but rise with carbon number for C8-C16 to approximately 15 kJ/mol for C16. An explanation is suggested by the hierarchical simulation, which finds that C1-C6 tend to reside entirely within the interiors of single channel segments and diffuse through jumps across energetically less favorable channel intersections, while longer molecules straddle channel intersections and must undergo conformational rearrangements in order to diffuse. The agreement between QENS and simulations reflects that the two methods probe motion over the same small (nm) length scales. Self-diffusivities measured by pulsed field gradient nuclear magnetic resonance (PFG-NMR) in the same systems are appreciably lower. This is because PFG-NMR probes motion over significantly longer (microm) length scales and is therefore more sensitive to defects in the silicalite crystals.