We introduce the various options of experimentally observing mass transfer in mesoporous materials. It shall be demonstrated that the exploration of the underlying mechanisms is excessively ...complicated by the complexity of the phenomena contributing to molecular transport in such systems and their mutual interdependence. Microscopic diffusion measurement by the pulsed field gradient (PFG) technique of NMR offers the unique option to measure both the relative amount of molecules adsorbed and the probability distribution of their displacements over space scales relevant to fundamental adsorption science just as for technological application. These advantages are shown to have cared for a recent breakthrough in our understanding. The examples presented include the measurement of diffusion in purely mesoporous materials and the rationalization of the complex concentration patterns revealed by such studies on the basis of suitably chosen micro-kinetic models. As an interesting feature, transition into the supercritical state is shown to become directly observable by monitoring a jump in the diffusivities during temperature enhancement, occurring at temperatures notably below the bulk critical temperature. PFG NMR studies with hierarchical materials are shown to permit selective diffusion measurement with each of the involved subspaces, in parallel with the measurement of the overall diffusivity as the key parameter for the technological exploitation of such materials. We refer to the occurrence of diffusion hysteresis as a novel phenomenon, found to accompany phase transitions quite in general. Though further complicating the measuring procedure and the correlation between experimental observation and the underlying mechanisms, diffusion hysteresis is doubtlessly among the new options provided by diffusion studies for gaining deeper insight into the structure and dynamics of complex porous systems.
Adding mesopore networks in microporous materials using the principles of hierarchical structure design is recognized as a promising route for eliminating their transport limitations and, therefore, ...for improving their value in technological applications. Depending on the routes of physico-chemical procedures or post-synthesis treatments used, very different geometries of the intentionally-added transport mesopores can be obtained. Understanding the structure-dynamics relationships in these complex materials with multiple porosities under different thermodynamical conditions remains a challenging task. In this review, we summarize the results obtained so far on experimental and theoretical studies of diffusion in micro-mesoporous materials. By considering four common classes of bi-porous materials, which are differing by the inter-connectivities of their sup-spaces as one of the most important parameter determining the transport rates, we discuss their generic transport properties and correlate the results delivered by the equilibrium and non-equilibrium techniques of diffusion measurements.
As an omnipresent phenomenon in nature, diffusion is among the rate-determining processes in many technological processes. This is in particular true for catalytic conversion in nanoporous materials. ...We provide a critical review of the possibilities of exploring diffusion phenomena over microscopic dimensions in such media by direct experimental observation. By monitoring the probability distribution of molecular displacements as a function of time, the pulsed field gradient technique of NMR (PFG NMR) records the rate of molecular re-distribution. By varying the observation time, PFG NMR is thus able to trace even hierarchies of transport resistances as occurring, e.g., in catalyst particles in the form of binder-compacted assemblages of zeolite crystallites. Alternatively, and complementary to this information, interference microscopy (IFM) and IR microscopy (IRM) are able to follow the evolution of intracrystalline concentration profiles during uptake and release. This allows, in particular, an accurate quantification of the transport resistances on the surface of the individual crystallites and of the probability that reactant molecules from the gas phase, upon colliding with the external surface, are able to penetrate through such "surface barriers" into the crystal bulk phase. Being able to distinguish between different molecular species, IRM is able to record the evolution of intracrystalline concentration profiles even during multi-component adsorption and catalytic reactions (169 references).
Advances in materials synthesis bring about many opportunities for technological applications, but are often accompanied by unprecedented complexity. This is clearly illustrated by the case of ...hierarchically organized zeolite catalysts, a class of crystalline microporous solids that has been revolutionized by the engineering of multilevel pore architectures, which combine unique chemical functionality with efficient molecular transport. Three key attributes, the crystal, the pore and the active site structure, can be expected to dominate the design process. This review examines the adequacy of the palette of techniques applied to characterize these distinguishing features and their catalytic impact.
The presence of one-dimensional MEL intergrowths in two-dimensional MDI zeolite nanosheets, inferred from experimental and theoretical analysis, allows for world-beating xylene separation performance.
Adsorption and desorption of hydrocarbons in a realistic model (2496 atoms) of ZSM-5 zeolite (MFI), including an external surface and a reservoir for molecules, have been studied using classical ...molecular dynamics, with special focus on surface barriers. Different degrees of surface blocking have been modeled according to experimental observations. Using previous molecular dynamics results, from the analysis of adsorption and desorption path lengths, we demonstrate that surface barriers are symmetric, i.e. with equal paths for desorption and adsorption, in agreement with the principle of microscopic reversibility and in contradiction with a model proposed recently. A new thermodynamic analysis confirms the symmetry of adsorption/desorption paths.
Molecular dynamics simulations have been carried out to determine the uptake and release rates for benzene in an idealized crystal of silicalite (the pure silica form of ZSM-5) with two external ...surfaces perpendicular to the straight channel 010. A realistic model has been developed to simulate a system in which the kinetics are controlled by the combined effects of surface resistance (pore blocking at the external surface) and intracrystalline diffusional resistance. The system has been treated using periodic boundary conditions and contains a finite reservoir in which (for uptake calculations) the benzene molecules are initially located. For the calculation of the release, the benzene molecules are all located initially within the crystal, and the periodicity along the length of the reservoir is removed so that the molecules are released at zero pressure. The effect of a surface barrier has been investigated by considering three systems with different degrees of channel entrance blocking (0, 50, and 87.5%). The resulting calculations of uptake and release make it possible to estimate the relative importance of surface resistance (pore blocking) and intracrystalline diffusion in determining the sorption rate. It is shown that the classical model based on the formal solution of the one-dimensional diffusion equation, taking account of the finite rate of permeation through the crystal surface (i.e., surface resistance) via the boundary condition, provides a good representation of the kinetic behavior. For comparison, self-diffusion and tracer exchange are also simulated for the same system under comparable conditions.
Nanoporous solids are attractive materials for energetically efficient and environmentally friendly catalytic and adsorption separation processes. Although the performance of such materials is ...largely dependent on their molecular transport properties, our fundamental understanding of these phenomena is far from complete. This is particularly true for the mechanisms that control the penetration rate through the outer surface of these materials (commonly referred to as surface barriers). Recent detailed sorption rate measurements with Zn(tbip) crystals have greatly enhanced our basic understanding of such processes. Surface resistance in this material has been shown to arise from the complete blockage of most of the pore entrances on the outer surface, while the transport resistance of the remaining open pores is negligibly small. More generally, the revealed correlation between intracrystalline diffusion and surface permeation provides a new view of the nature of transport resistances in nanoporous materials acting in addition to the diffusion resistance of the regular pore network, leading to a rational explanation of the discrepancy which is often observed between microscopic and macroscopic diffusion measurements.
Monitoring the recombination of OH groups in a ceramic sample after firing, also known as rehydroxylation (RHX), was proposed as a way to determine the time elapsed since the firing of a ceramic ...material, thus providing archeologists with the only up-to-date known method for determining the age of fired ceramics directly. A nuclear magnetic resonance (NMR) study was performed in order to understand the RHX dating of ceramic materials in archeology. We perform MAS NMR investigations on four pure clay minerals and one mixed ceramic. We point out a large discrepancy between NMR measurements and TG in the obtained total concentration of hydrogen. We are able to differentiate and investigate the dynamics (by monitoring H/D exchange) of the three types of hydrogen species present in the samples: T0 (physisorbed), T1 (interlayer), and T2 (chemisorbed) water. We use H/D tracer exchange to monitor the mobility of hydrogen species and obtain the exchange time constants of T2 water, which is in the order of a few to 100 days. Interestingly, we find that H/D exchange time constants do not significantly depend on temperature. The slow exchange times of T2 water, in the order of days, can be compared with the diffusion time scales of T1 water (in the order of 100 s) obtained with tracer desorption and with T0 water (order of 100 ms) obtained by PFG MAS NMR measurements.
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