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

Abstract

Confinement of molecules in nanoporous crystalline materials results in the unique and diverse characteristics of intracrystalline diffusion and adsorption, which can significantly affect the efficiency of gas separation and/or catalysis. However, understanding the interplay between confinement and intracrystalline diffusion and adsorption remains elusive at the quantitative level. In this work, it is found that the intracrystalline diffusion could be related to the hopping rate, which might be further connected to the translational and rotational motion of molecules and quantified by corresponding partition functions. Based on this analysis, the correlations capable of predicting the intracrystalline diffusivity and the adsorption entropy are developed. It is shown that the correlations can well capture the experimental and simulation results of more than 20 frameworks, including zeolites and MOFs, for a wide range of guest molecules. This approach can potentially serve as rapid screening tool for nanoporous crystalline materials in gas separation and catalysis.

Experimental and Theoretical Evidence for the Promotional Effect of Acid Sites on the Diffusion of Alkenes through Small‐Pore Zeolites

“…As Ghysels et al noted, diffusion is an activated process,w hich can be described as ah opping event between adjacent cages. [32,41,42] Thea ctual crossing time for ah ydrocarbon species is negligible compared to the residence time inside as ingle cage.T os ample these rare events and extract activation barriers,t he umbrella sampling (US) technique is applied (cf.S ection S1.1 of the SI). This technique has previously proven its validity for studying catalysis and diffusion in zeolites.…”

Directed transforming of coke to active intermediates in methanol-to-olefins catalyst to boost light olefins selectivity

“…As an archetypical co-catalysis reaction, catalytic performance in MTO is manipulated by the organic intermediates confined in zeolite channels or cavities through the sophisticated hydrocarbon pool mechanism 3 , 7 – 9 . These hydrocarbon pool species (HCPs), typically including the methyled-benzene carbocations 10 , 11 and cyclopentadienyl species 12 , 13 , are decisive for light olefins selectivity, owning to the altering of acidity 14 , reaction paths 15 , kinetics 8 , 9 , molecular transport 16 , and among others. However, the HCPs are also coke precursors that can readily evolve to polycyclic aromatic hydrocarbons (PAHs), the typical coke species, through cyclization 17 and cross-linked mechanism 18 , 19 , accelerating catalyst deactivation 20 .…”

Quantitative principle of shape‐selective catalysis for a rational screening of zeolites for methanol‐to‐hydrocarbons

“…In Equation ( 6), we defined the ratio D C ¼ y =D C ¼ 2 to quantify competitive diffusion between binary olefins within zeolite framework. As shown in Figure S22, using Equations (S42)-(S46) 33 to predict the ratio…”

“…For zeolite framework decorated with various type of heteroatom, Studt et al 25,26 summarized the linear scaling relations between the transition-state enthalpies of methylation reaction and adsorption enthalpy of ammonia, which illustrates that heteroatom type can regulate microenvironments within zeolite (e.g., acid site strength) and control the reaction kinetics. Through advanced microscopy imaging 27,28 and the-state-of-the-art simulation techniques, [29][30][31][32][33][34][35] tremendous efforts have been made to construct and understand the relation between molecular diffusivity and zeolite framework structure (e.g., the maximum included sphere diameter (MISD) and maximum free sphere diameter (MFSD) 36 ) under non-reaction condition. Molecular diffusion-limitation imposed by zeolites plays a crucial role in determining characteristics of shape selectivity.…”

“…Intracrystalline diffusion is mainly controlled by the topology of zeolite framework, type of guest molecules, temperature and loading of molecules. In an early effort, we developed a correlation between the intracrystalline diffusivity of guest molecules and geometry of zeolite framework 33 (Equations S42–S46). However, surface barriers, which can be dominated by various factors, would be affected not only by the zeolite framework topology 41 and type of guests, 40 but also the external surface properties and environment of exposure 42 .…”