Diffusion of confined fluids in microporous zeolites and clay materials

Abstract

Fluids exhibit remarkable variation in their structural and dynamic properties when they are confined at the nanoscopic scale. Various factors, including geometric restriction, the size and shape of the guest molecules, the topology of the host, and guest–host interactions, are responsible for the alterations in these properties. Due to their porous structures, aluminosilicates provide a suitable host system for studying the diffusion of sorbates in confinement. Zeolites and clays are two classes of the aluminosilicate family, comprising very ordered porous or layered structures. Zeolitic materials are important due to their high catalytic activity and molecular sieving properties. Guest molecules adsorbed by zeolites display many interesting features including unidimensional diffusion, non-isotropic rotation, preferred orientation and levitation effects, depending on the guest and host characteristics. These are useful for the separation of hydrocarbons which commonly exist as mixtures in nature. Similarly, clay materials have found application in catalysis, desalination, enhanced oil recovery, and isolation barriers used in radioactive waste disposal. It has been shown that the bonding interactions, level of hydration, interlayer spacing, and number of charge-balancing cations are the important factors that determine the nature of diffusion of water molecules in clays. Here, we present a review of the current status of the diffusion mechanisms of various adsorbed species in different microporous zeolites and clays, as investigated using quasielastic neutron scattering and classical molecular dynamics simulation techniques. It is impossible to write an exhaustive review of the subject matter, as it has been explored over several decades and involves many research topics. However, an effort is made to cover the relevant issues specific to the dynamics of different molecules in microporous zeolites and clay materials and to highlight a variety of interesting features that are important for both practical applications and fundamental aspects.

Diffusive Skin Effect in Zeolites

“…Because of their excellent activity, hydrothermal stability, and high selectivity, zeolites have been widely used in petrochemical industries. − During the industrial catalytic process, the diffusion of reactants, intermediates, and products inside the confined zeolite channels has shown great importance to the activity of reaction and the selectivity of products. − To date, more than 200 kinds of zeolites with different topologies have been synthesized successfully, and they can be divided into channel-type, cage-type, and intersecting channel-type zeolites . However, it remains challenging to unify the transport properties under confinement as molecular diffusion behavior varies with the zeolite topologies. − Therefore, in-depth understanding of the diffusion mechanism and commonality of molecular motion was the key to elucidating the diffusion behaviors.…”

Neutron Scattering Studies of Heterogeneous Catalysis

“…Historically, most QENS studies of catalytically active materials have been concerned with the mobility of hydrocarbons in zeolites. Accordingly, several reviews in the existing literature focused on QENS application in catalytic science ,, with an emphasis on zeolites and related materials. Early QENS measurements of catalytic materials involved studies of diffusion of methane, ethane and propane, and isobutane in ZSM-5 zeolite.…”

Mesoporous Materials Make Hydrogels More Powerful in Biomedicine

“…According to the definition of the International Union of Pure and Applied Chemistry (IUPAC), porous materials can be simply classified based on their pore size. Materials with a pore size of less than 2 nm are microporous materials, including zeolites [ 1 ] and metal–organic frames (MOFs) [ 2 ], while those with a pore size larger than 50 nm are macroporous materials, with major representative materials such as aerogels [ 3 ] and porous ceramics [ 4 ]. When the pore size is between 2 nm and 50 nm, porous materials are named mesoporous materials.…”