Fast room temperature lability of aluminosilicate zeolites

Abstract

Aluminosilicate zeolites are traditionally used in high-temperature applications at low water vapour pressures where the zeolite framework is generally considered to be stable and static. Increasingly, zeolites are being considered for applications under milder aqueous conditions. However, it has not yet been established how neutral liquid water at mild conditions affects the stability of the zeolite framework. Here, we show that covalent bonds in the zeolite chabazite (CHA) are labile when in contact with neutral liquid water, which leads to partial but fully reversible hydrolysis without framework degradation. We present ab initio calculations that predict novel, energetically viable reaction mechanisms by which Al-O and Si-O bonds rapidly and reversibly break at 300 K. By means of solid-state NMR, we confirm this prediction, demonstrating that isotopic substitution of 17O in the zeolitic framework occurs at room temperature in less than one hour of contact with enriched water.

Water‐Induced Structural Dynamic Process in Molecular Sieves under Mild Hydrothermal Conditions: Ship‐in‐a‐Bottle Strategy for Acidity Identification and Catalyst Modification

“…To further clarify the specific functions of water under mild hydrothermal conditions on molecular-sieve framework, exploratory usage of H 2 17 Ot oi nteract with SAPO-34 molecular sieve was attempted at the temperature range of 100-300 8 8C( see Experimental Section in Supporting Information) and the achieved steam-treated samples were measured by 17 OM AS NMR spectroscopy.S urprisingly,t he 17 OM AS NMR spectra exhibited in Figure 1a clearly demonstrate two broad resonance signals at 21 and 48 ppm, attributing to the framework 17 Oatoms in the Si-O-Al and P-O-Al species as the linking oxygen confirmed by 2D 17 O MQMAS spectrum ( Figure 1b). [29,42,43] These unexpected results imply that 17 Oa toms from H 2 17 Oa re incorporated into molecular-sieve framework, suggesting that part of the tetrahedrally coordinated structure of SAPO-34 may undergo ad ynamic T-O-T bonds breaking by hydrolysis when interacting with water vapor,b ut in af ollowing step, dehydration reaction completely recovers the framework immediately.A fter this dynamic procedure,n ot erminal hydroxy groups are newly generated and the crystallinity remained unchanged, comparing with the untreated sample ( Figure 1c,F igure S3 and S4), while the incorporated and retained 17 Oa toms on the steamed SAPO-34 reveal the reversible hydration-dehydration process (Figure 1d). Considering the observation failure of T-O-T bonds changes by in situ solid-state NMR spectroscopy,itcan be inferred that this dynamic and reversible process involving of hydrolysisdehydration step is av ery fast process beyond the time resolution of in situ NMR technique.G enerally,t he framework of molecular sieves was considered to be less affected by water and the hydrothermal stability can be well-kept at moderate-temperature based on the weak adsorption of water and the unchanged crystallinity after dehydration.…”

Direct 17O-Isotopic Labeling of Oxides Using Mechanochemistry

“…1 Indeed, thanks to the very wide range of variation of 17 O NMR parameters, detailed insight into the bulk and surface structure of a variety of materials can be obtained. For example, recent studies have used 17 O NMR to elucidate the structure of porous materials like zeolites and metal organic frameworks, [2][3][4][5] of mixed metal oxides used as electrode materials, [6][7] or of nanoparticle surfaces. [8][9][10][11] Natural abundance 17 O NMR unfortunately suffers from a very poor absolute signal sensitivity, due to the very low abundance of the NMR-active isotope (only 0.04%).…”

Steam‐Induced Coarsening of Single‐Unit‐Cell MFI Zeolite Nanosheets and Its Effect on External Surface Brønsted Acid Catalysis

“…During their use, zeolites are typically exposed to hydrothermal treatments which may alter their structure at the atomic to the nanometer scale with desirable or undesirable effects on performance . Understanding and controlling the water vapor‐induced structural rearrangements at the nanometer (single‐unit‐cell) level is of particular significance for two‐dimensional (2D) zeolites and thicker nanosheets that constitute an emerging class of catalysts, adsorbents and membranes . Herein, we demonstrate that an all‐silica single‐unit‐cell meso /microporous MFI‐type zeolite (SPP: self‐pillared pentasil) retains its crystallinity and micro‐ and mesoporosity under steaming at 350 °C, while small but detectable changes take place in the content of silanol groups and the enthalpy of transition (Δ H Transition ) relative to α‐quartz (the most stable polymorph of silica under ambient conditions).…”