Simple structure descriptors quantifying the diffusion of ethene in small-pore zeolites: insights from molecular dynamic simulations

Abstract

Small-pore zeolites with 8-rings are pivotal catalytic materials to produce light olefins from non-petroleum resources employing methanol-to-olefins or syngas-to-olefins processes. The constraints of cage openings on the diffusion of light olefins play a crucial role in tailoring catalytic performance. However, the relation between the elegant structures of zeolites and the underlying diffusion kinetics is not well understood yet. In this work, the diffusion of ethene in seven cage-structured 8-ring zeolites (LEV, CHA, AEI, ERI, AFX, SFW and RHO) was systematically investigated using molecular dynamic simulations at different ethene loadings and temperatures. The self-diffusion coefficient highly relates to the zeolite framework structures and increases with both ethene loading and temperature. Ethene diffuses the fastest in RHO and the slowest in ERI and LEV. The self-diffusion of ethene from one cage to the other follows the Arrhenius plot. The activation energy is irrelevant to the loading and decreases as ERI > LEV > SFW ∼ AFX > CHA > AEI > RHO. The pre-exponential factor linearly increases with the loading except in ERI and LEV, with smaller openings, and increases as ERI ∼ LEV < CHA ∼ AEI ∼ AFX ∼ SFW < RHO under similar loading with respect to the number of Si atoms. The activation energy and the pre-exponential factor of ethene diffusion are revealed to correlate well with two simple structure descriptors, i.e., the opening size of 8-rings and the accessible volume of each Si atom, respectively. This theoretical work thus unravels the intrinsic structure characteristics quantifying the individual diffusion kinetic parameters, and may offer some facile implications to tailor the diffusion behavior of light hydrocarbons in cage-structured 8-ring zeolites.