The calculated self-diffusion coefficients of the aliphatic ester promoters are shown in fig. . The diffusion coefficients decrease as the alkyl chain length increases. The activation enthalpies of diffusion increase with chain length (as do enthalpies of adsorption ) due to increased dispersion interactions with the zeolite pore walls. Smaller promoters exhibit a preference for diffusion through the straight pore, consistent with greater self-diffusion coefficients in the straight pores (b direction) than the sinusoidal pores (a direction) (fig. ). This preference decreases as the alkyl chain length increases, and completely disappears for methyl n- hexanoate for which the diffusion coefficients through each type of pore are comparable.