Synthesis and structure of the [AlPO4]12 Pr4NF molecular sieve with AFI structure

Qiu, Shilun; Pang, Wenqin; Kessler, H.; Guth, J.L.

doi:10.1016/0144-2449(89)90101-2

Cited by 172 publications

(123 citation statements)

References 2 publications

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“…Although we have not found any experimental adsorption and transport data of methane in SSZ-24, the siliceous AFI-type zeolite structure, 25 there are data for a similar structure with the same framework type: AlPO 4 -5, 26 the aluminum phosphate equivalent to SSZ-24. The experimental data [27][28][29][30][31] are displayed in Figure S5 along with our simulation predictions.…”

Section: Agreement Between Simulations and Measurementsmentioning

confidence: 95%

Transport into Nanosheets: Diffusion Equations Put to Test

2013

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Ultrathin porous materials, such as zeolite nanosheets, are prominent candidates for performing catalysis, drug supply, and separation processes in a highly efficient manner due to exceptionally short transport paths. Predictive design of such processes requires the application of diffusion equations that were derived for macroscopic, homogeneous surroundings to nanoscale, nanostructured host systems. Therefore, we tested different analytical solutions of Fick’s diffusion equations for their applicability to methane transport into two different zeolite nanosheets (AFI, LTA) under instationary conditions. Transient molecular dynamics simulations provided hereby concentration profiles and uptake curves to which the different solutions were fitted. Two central conclusions were deduced by comparing the fitted transport coefficients. First, the transport can be described correctly only if concentration profiles are used and the transport through the solid–gas interface is explicitly accounted for by the surface permeability. Second and most importantly, we have unraveled a size limitation to applying the diffusion equations to nanoscale objects. This is because transport-diffusion coefficients, D T, and surface permeabilities, α, of methane in AFI become dependent on nanosheet thickness. Deviations can amount to factors of 2.9 and 1.4 for D T and α, respectively, when, in the worst case, results from the thinnest AFI nanosheet are compared with data from the thickest sheet. We present a molecular explanation of the size limitation that is based on memory effects of entering molecules and therefore only observable for smooth pores such as AFI and carbon nanotubes. Hence, our work provides important tools to accurately predict and intuitively understand transport of guest molecules into porous host structures, a fact that will become the more valuable the more tiny nanotechnological objects get.

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Section: Agreement Between Simulations and Measurementsmentioning

confidence: 95%

Transport into Nanosheets: Diffusion Equations Put to Test

2013

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“…The example of methane desorbing from siliceous AFI-type zeolite membranes will be employed, the structure 27 of which exhibits one-dimensional channels (Figure 2b). The methodology introduced in ref 20 is extended, as described in detail in the Supporting Information (SI1).…”

Section: ■ Methodologymentioning

confidence: 99%

Predicting Local Transport Coefficients at Solid–Gas Interfaces

2012

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The regular nanoporous structure make zeolite membranes attractive candidates for separating molecules on the basis of differences in transport rates (diffusion). Since improvements in synthesis have led to membranes as thin as several hundred nanometers by now, the slow transport in the boundary layer separating bulk gas and core of the nanoporous membrane is becoming increasingly important. Therefore, we investigate the predictability of the coefficient quantifying this local process, the surface permeability α, by means of a two-scale simulation approach. Methane tracer-release from the one-dimensional nanopores of an AFI-type zeolite is employed. Besides a pitfall in determining α on the basis of tracer exchange, we, importantly, present an accurate prediction of the surface permeability using readily available information from molecular simulations. Moreover, we show that the prediction is strongly influenced by the degree of detail with which the boundary region is modeled. It turns out that not accounting for the fact that molecules aiming to escape the host structure must indeed overcome two boundary regions yields too large a permeability by a factor of 1.7−3.3, depending on the temperature. Finally, our results have far-reaching implications for the design of future membrane applications. ■ INTRODUCTIONMolecular exchange between a gas reservoir and a nanoporous crystalline solid (e.g., a zeolite membrane or crystal) represents a key design process in applications, such as adsorption, molecular-sieving, catalysis, and ion-exchange. Over the last few decades, a good understanding has been developed with regard to the role 1 and dependence 2−7 of guest diffusion in such regular host structures, that is, the transport mechanism of molecules inside the nanopores far away from the interface to the fluid phase. 8,9 For example, gas diffusion in zeolites is known to be an activated process 4−7 where molecules need to overcome a series of regularly distributed internal diffusion barriers which arise from nanopore shape in the unit cell alone. Many phenomena, including the loading-dependence of the self-diffusion coefficient, can be explained by the variation of such (free) energy barriers. In this context, molecular simulations have been proven to be invaluable, 4,5,10 owing to improved agreement with experiments.11−13 Despite these accomplishments, there are still unresolved problems, 14 many of which are related to the boundary layer separating the gasphase region from the core zeolite space.While cases exist in which the boundary layer may accelerate molecular exchange between the reservoir and the porous host, 15,16 it usually slows down the transport rate close to the surface, 6,7,17−20 leading to the name of this phenomenon: surface barriers. Exciting insights into their nature have been unraveled only recently. 6,7,20 Microscopy experiments 6 in conjunction with mesoscopic modeling 7 evidenced that exceptionally few accessible pore entrances together with a large number of lattice defects (i.e...

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“…A single unit cell consists of 96 oxygen and 48 silicon atoms, and its dimensions are 23.774, 13.726, and 8.484 Å in the x, y, and z direction, respectively. The original crystal structure, as taken from ref 18, was converted from monoclinic to orthorhombic for computational efficiency and thus accommodates 4 cages in total.…”

Section: Methodsmentioning

confidence: 99%

On the Effects of the External Surface on the Equilibrium Transport in Zeolite Crystals

2009

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With the aid of molecular simulation techniques (molecular dynamics, grand-canonical Monte Carlo, and reactive flux correlation function RFCF), the influence of the external surface on the equilibrium permeation of methane and ethane into and out of an AFI-type zeolite crystal has been studied. In particular, "extended dynamically corrected transition state theory", which has been proven to describe the transport of tracers in periodic crystals correctly, has been applied to surface problems. The results suggest that the molecules follow paths that are close to the pore wall in the interior and also at the crystal surface. Moreover, the recrossing rate at the surface turns out to be non-negligible, yet, in contrast to the intracrystalline recrossing rate, remains almost constant over loading which gives indication to diffusive barrier crossing at the crystal surface. As a consequence of very different adsorption and desorption barriers, the corresponding permeabilities are shown to be not equal for one and the same condition (T and p). The critical crystal length, beyond which surface effects can be certainly neglected, is computed on basis of flux densities. Entrance/exit effects, in the present cases, are practically important solely for ethane at low pressures. The influence of the type of external surface on the surface flux is, hereby, rather small, because the transport at the surface is controlled by the slow supply from the gas phase. This has been evidenced by a simplified thermodynamic model that has been derived within this work and which is based on rapidly assessable simulation data. Finally, we propose a procedure for estimating the importance of different factors that have an impact on surface effects.

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Synthesis and structure of the [AlPO4]12 Pr4NF molecular sieve with AFI structure

Cited by 172 publications

References 2 publications

Transport into Nanosheets: Diffusion Equations Put to Test

Transport into Nanosheets: Diffusion Equations Put to Test

Predicting Local Transport Coefficients at Solid–Gas Interfaces

On the Effects of the External Surface on the Equilibrium Transport in Zeolite Crystals

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