Heptane adsorption in silicalite-1: Neutron scattering investigation

Structural properties of confined deuterated n-heptane in silicalite-1 have been investigated by neutron scattering experiments during the adsorption process. At 300 K, the adsorption isotherm shows a sharp inflection at a loading near N-ads = 4 molecules per silicalite-1 unit cell. In addition, the diffusivities obtained from recent QENS data exhibit a loading dependence. Our motivation is to find structural signatures of the peculiar behavior of n-heptane in silicalite-1 and to check the numerous computer simulation findings of this behavior. Our detailed neutron diffraction investigation agrees with the MONO-ORTHO phase transition of the silicalite-1 above a N-ads value of 4 molecules per unit cell, while no transition is reported for the n-hexane/silicalite-1 system at 300 K. Besides, the adsorption Of C7D16 molecules in silicalite-1 is proved to be a two-step filling process. The location of the C7D16 molecules at increasing loading obtained from Rietveld refinement shows that the C7D16 molecules begin by first filling the straight channels, then near N-ads = 4 mol./u.c., C7D16 molecules fill both straight and sinusoidal channels. Our structural results happen to be in agreement with computer simulation calculations newly reported by Song et al. By contrast, the commensurate ''freezing'' of n-heptane in the sinusoidal channels of the silicalite-1 obtained from molecular simulation does not explain the two-step adsorption process. Rather, it corresponds to the final adsorption state similar to the molecular arrangement determined for the 8 n-hexane/silicalite-1 complex at 180 K. Recent thermodesorption studies on the both n-heptane/ZSM-5 and n-heptane/ZSM-11 systems showed also the defectiveness of the commensurate ''freezing'' interpretation.