Dependence of zeolite topology on alkane diffusion inside diverse channels

Abstract

Zeolites have been widely used for the processes of adsorption, separation, and catalysis, which are strongly correlated with molecular diffusion. However, the correlation between pore dimension and diffusion properties has not been systematically investigated so far. In this work, the diffusion properties of alkanes in six zeolites with similar pore sizes but different pore dimension have been examined. It is found that the diffusion coefficients of alkanes in zeolites are 2–5 orders of magnitude smaller than that in gas phase. Moreover, the diffusion of alkanes inside zeolites is sensitive to the pore dimension, and can be differentiated by 1‐D straight, 1‐D tortuous, and 3‐D intersecting channels, based on the derived quantitative correlation between the diffusion behavior and pore dimension. Our work may not only provide deep insights into the effects of pore dimension on diffusion, but also benefits for the future design and practical applications of zeolite catalysts.

An approach for predicting intracrystalline diffusivities and adsorption entropies in nanoporous crystalline materials

“…is 0.758 for PON and 0.229 for BOF. Approximately, the diffusivity of methane at infinite dilution in PON is five times large than that in BOF 54 . Basically, the increase of MFSD prompts the intracrystalline diffusion of methane in the framework.…”

“…35 As shown in Figure 5b, the predicted results agree well with the MD results with R 2 of 0.9689. In Figure 5b, 54 Basically, the increase of MFSD prompts the intracrystalline diffusion of methane in the framework. For MFSD less than 0.6 nm where the confinement of the framework is severe, the contribution of rotational partition function to the diffusivity is prominent as shown in Figure S8.…”

“…When molecules migrate to the window by translation, the change of the dimension of the confined spaces (i.e., change from cavity with large space to narrow window or vice versa) or pore constrictions 32,33 can enhance the frequency of collisions between molecules and wall 54 and lead to the decrease of diffusivity 39 . Marbach et al 39 showed that the diffusivity can be related to the fluctuations of nanopores by including the effect of structural constrictions: where d cage is the size of cavity, and d pore is the window diameter (nm).…”

“…The interatomic interactions were described by the condensed‐phase‐optimized molecular potentials for atomistic simulation studies (COMPASS) force field. The initial adsorption structures of molecules in zeolitic structures with fixed loading were obtained via Monte Carlo (MC) simulations in the NVT ensemble, where the particles number (N), volume (V), and temperature (T) are constant, based on the Metropolis method 54 . To ensure that the system is sampled under an equilibrated state, a minimum of 1 × 10 5 MC steps in the NVT ensemble are implemented.…”

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

“…38 Elucidating the diffusion of guest molecules in the channel or cage of zeolites remains a great challenge. [39][40][41][42][43][44][45][46][47][48] Several different techniques, e.g., pulsed field gradient (PFG) NMR characterization, 46,47 time-resolved in situ FT-IR spectroscopy, 49 MD simulations 37,[50][51][52][53][54][55][56] and transition state theory (TST), 57 were previously employed to address this critical issue. By combining PFG NMR and MD simulation, Liu et al disclosed the cavity-controlled diffusion of methane, ethane and propane in three cage-structured 8-ring zeolites (LEV, CHA and RHO) with close window sizes, and the diffusion rate trend of RHO > CHA > LEV was revealed.…”