Diffusion of Aromatics in Silicalite-1: Experimental and Theoretical Evidence of Entropic Barriers

Abstract

A specific computational methodology based on transition state theory (Kolokathis, P. D. et al. Mol. Simul., 2014, 40, 80−100) is evolved and applied for calculation of the selfdiffusion coefficients of p-xylene and benzene in silicalite-1 at infinite dilution. In addition, we study the orientational distributions of phenyl rings and methyl stems of p-xylene and benzene sorbed in the zeolite and check for the existence of entropic barriers to translational motion. A new reduction method for the states appearing in the free energy profiles is presented and used for calculation of transition rate constants for elementary jumps. Quasi-elastic neutron scattering measurements are also conducted and compared with the simulation results. A major conclusion from both experiments and simulations is that p-xylene diffuses roughly 100 times faster than benzene when sorbed at low occupancy in silicalite. Benzene encounters strong entropic barriers to translational motion at channel intersections, where it can adopt a variety of orientations. The corresponding barriers for p-xylene are much lower, reflecting the inability of its major axis to reorient within channel intersections.

para‐Xylene Ultra‐selective Zeolite MFI Membranes Fabricated from Nanosheet Monolayers at the Air–Water Interface

“…Membranes prepared by gel‐free secondary growth of these nanosheets showed improved performance, compared to other membranes, with separation factors of p ‐xylene over o ‐xylene of 2000–8000. Simulations have predicted even higher separation factors . We hypothesize that these higher separation factors could be achieved by further reducing non‐selective paths in the membranes .…”

“…The xylene isomers used, p ‐ and o ‐xylene, have kinetic diameters of 0.58 and 0.68 nm, respectively. According to simulation, b ‐oriented defect‐free MFI membranes are capable of exhibiting separation factor over 10 000 . Permeation tests were performed for equimolar binary mixtures of p ‐/ o ‐xylene (ca.…”

para‐Xylene Ultra‐selective Zeolite MFI Membranes Fabricated from Nanosheet Monolayers at the Air–Water Interface

“…[28,29] We hypothesize that these higher separation factors could be achieved by further reducing non-selective paths in the membranes. [28,29] We hypothesize that these higher separation factors could be achieved by further reducing non-selective paths in the membranes.…”

Neutron spectroscopy as a tool in catalytic science

“…15,21 However the scarcity of instruments available means that very few studies in catalytic systems are available in the literature. [22][23][24] The velocities of polarized neutrons are compared before and after the scattering event, using the Larmor precession of the neutron spin. The spin vector acts as an internal timer linked to each neutron, connecting it to the velocity measurement, which allows the velocities before and after scattering of the neutron to be compared, and a direct measurement of the velocity difference can be performed.…”