Probing Interconnectivity in Hierarchical Microporous/Mesoporous Materials Using Adsorption and Nuclear Magnetic Resonance Diffusion

Abstract

Adsorption and transport in hierarchical materials are investigated by means of adsorption and nuclear magnetic resonance experiments. Using micro/mesoporous zeolites with well-defined mesoporosity, we show that adsorption at a given pressure can be described as a simple linear combination of the adsorbed amounts taken at the same pressure for the pure microporous (zeolite FAU-Y) and mesoporous (Al-MCM-41) solids. Such a quantitative decomposition allows us to demonstrate the ability of diffusion measurements by Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR) to probe interconnectivity in hierarchical solids. On the one hand, transport in the mechanical mixtures can be described as the superimposition of diffusion in pure microporous and mesoporous solids. On the other hand, PFG NMR for the hierarchical sample provides an effective diffusivity that is intermediate between those for the pure zeolite and mesoporous silica. Furthermore, this effective diffusivity is slower than the linear combination of the two diffusivities weighted by the number of molecules present in each phase (used in the independent domain and fast-exchange theories) clearly showing interconnectivities and transfer limitations between the microporous and mesoporous domains. We also discuss the ability of combining theories such as the fast exchange model and the effective medium theory to quantitatively predict diffusion in such microporous/mesoporous materials.

Revelation on the Complex Nature of Mesoporous Hierarchical FAU-Y Zeolites

“…22 Different experimental conditions have been reported in the literature for the synthesis of FAUmes starting from zeolite crystals with various Si/Al ratios, using several sources of alkali (NaOH, TMAOH, NH 4 OH), different temperatures (80, 115, 150 °C), different durations (12 to 24 h) and different types and amounts of alkyltrimethylammonium surfactants (CnTAX, n = 10 -22, X = Cl, Br). 4,[14][15][16][17][18][19][20][21][22][23][24] In the present study, a systematic investigation of the materials resulting from the transformation of FAU-Y (Si/Al = 15) into FAUmes has been performed. Materials have been prepared using C18TAB as surfactant and NaOH as the alkali from mixtures with molar ratios 1 SiO 2 / n NaOH / 0.1 C18TAB / H O (n = 0.025 -0.25) at 115 °C for h. Our aim was to analyze precisely their structural, textural and acidic properties as a function of NaOH/Si ratio.…”

Assessing Guest‐Molecule Diffusion in Heterogeneous Powder Samples of Metal–Organic Frameworks through Pulsed‐Field‐Gradient (PFG) NMR Spectroscopy

“…[1,2] Thus, much effort has been made to study such idealizedsystems. [8][9][10][11] It is true that recent progress in PFG NMR methodology [12] has facilitated diffusion studies in also more complex pore architectures [13][14][15][16] just as the investigationof highly heterogeneouss ystemsb eing characterized by ad istribution of diffusivities. [16,17] However,a pplication of such advanced techniques requires high accuracyi nt he spinecho attenuations, which cannotb ee xpectedt ob ea ttainable as ar ule.…”

PFG‐NMR as a Tool for Determining Self‐Diffusivities of Various Probe Molecules through H‐ZSM‐5 Zeolites

“…Although these values were attributed to intercrystalline diffusion of toluene molecules for the non-modified ZSM5-nano sample, the data indicate that it is not the case for the desilicated sample. This effect was shown by Coasne et al [53] when analyzing the effective diffusivity of nhexane in hierarchical FAU. However, for ZSM5-meso, the observed diffusion behavior remains at a higher value than the microporous samples which may imply that the molecules of toluene diffuse merely within the crystals.…”

“…As the structure possesses larger pores ( Figure S4), the diffusion becomes faster. This effect was shown by Coasne et al [53] when analyzing the effective diffusivity of nhexane in hierarchical FAU. These authors showed that a zeolite possessing both micro-and mesopores exhibits a diffusion value intermediate between the diffusion in purely microporous structure and purely mesoporous MCM-41.…”