The free energy barrier of diffusion depends both on the enthalpy of the transition state and the probability that the molecules are in an appropriate location with an appropriate velocity towards the bottleneck (see above). Spatial probability distributions for the molecular centre of mass (COM) of benzaldehyde and 4-n-pentyl benzaldehyde can be seen in fig. , probability distributions on the other axes are in the ESI. † . In all cases the highest probability COM position is in the pore intersections. With increasing chain length in the the 4-n alkyl benzaldehydes, there is an increasing tendency to lie in the straight pore (b direction) and around the identified bottleneck. We suggest there is a smaller probability of benzaldehyde adopting the orientation need to pass through the bottleneck, in the straight pore, than for other aromatic aldehyde promoters, due to the greater rotational freedom it experiences in the intersection. The presence of the alkyl chains in the 4-n alkyl benzaldehydes limits the promoters' ability to rotate in the intersection, analogous to toluene and p-xylene in ZSM-5. There is no significant reduction in aromatic aldehyde promoter diffusion on addition of methanol. This trend aligns with the larger aliphatic ester promoters, where non-competitive diffusion is present. We observe that those promoters that have the highest catalytic efficiency exhibit non-competitive diffusion.