The high thermal stability of the synthetic zeolite K–L: Dehydration mechanism by in situ SR-XRPD experiments

Abstract

Thermally induced structural modifications of a synthetic zeolite L [K8.46 (Al8.35 Si27.53) O72 *17.91H2O, framework type LTL, s.g. P6/mmm, a =18.3367(1) and c = 7.5176(1) Å] were studied by temperature-resolved synchrotron X-ray powder diffraction. In the investigated temperature range (RT-814°C), neither structure breakdown nor phase transitions occurred. The largest unit cell deformation was observed between 100º and 240°C, accompanied by an increase and decrease of the a and c cell parameters, respectively. After complete water release, an inversion of the a and c parameter behavior was observed, while the cell volume continued to increase, although following a more flattened curve. Overall, in the investigated T range, a small cell volume increase of 0.7% was observed. The release of the five water molecules present in zeolite L started with the most weakly bonded one and occurred between 80º and 240°C. During dehydration the framework underwent minor rearrangements, which facilitated water release: the apertures of the main 12-ring and the 8-ring channels became more circular and the 6-membered rings became more hexagonal. The thermal expansion of zeolite L, very unusual for a non-siliceous zeolite, was interpreted and compared with previous data reported in literature for this porous material, and with the behavior of the synthetic phases ITQ-4 and CIT-5. HighlightsIn-situ dehydration of zeolite L was studied between RT and 814 °C. > The largest unit cell deformation was observed in correspondence of water release. > In the investigated T range, a small cell volume increase of 0.7% was observed. > Water release is facilitated by minor framework deformations. >The thermal expansion of zeolite L is very unusual for a non-siliceous zeolite. were studied by temperature-resolved synchrotron X-ray powder diffraction. In the investigated temperature range (RT-814°C), neither structure breakdown nor phase transitions occurred. The largest unit cell deformation was observed between 100º and 240°C, accompanied by an increase and decrease of the a and c cell parameters, respectively. After complete water release, an inversion of the a and c parameter behavior 3 was observed, while the cell volume continued to increase, although following a more flattened curve. Overall, in the investigated T range, a small cell volume increase of 0.7% was observed. The release of the five water molecules present in zeolite L started with the most weakly bonded one and occurred between 80º and 240°C. During dehydration the framework underwent minor rearrangements, which facilitated water release: the apertures of the main 12-ring and the 8-ring channels became more circular and the 6-membered rings became more hexagonal. The thermal expansion of zeolite L, very unusual for a nonsiliceous zeolite, was interpreted and compared with previous data reported in literature for this porous material, and with the behavior of the synthetic phases ITQ-4 and CIT-5. *Highlights (for review)

Close-Packed Dye Molecules in Zeolite Channels Self-Assemble into Supramolecular Nanoladders

“…The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 39 Dehydrated ZL was mixed, in inert atmosphere (operating in a glow box fluxed with high purity dry N 2 ), with FL powder -in ratios corresponding to the desired loadings -and placed in a rotating oven. The mixtures were kept at 120 °C for 24 h in order to ensure the encapsulation of the dye and its homogenous distribution in the zeolite channels.…”

“…The framework and potassium atom coordinates reported in Ref. 39 for the room temperature refinement were used as a starting model. For all tetrahedral atoms, the Si scattering factor was used, neglecting the amount of Al atoms.…”

Hiding extra-framework cations in zeolites L and Y by internal ion exchange and its effect on CO2 adsorption

“…The temperature for internal ion exchange was established via an initial set of experiments in which a partly La-exchanged zeolite L (La0.9K7.1-L) was heated at temperatures from 523 K to 1073 K and the structure response monitored using PXRD. Zeolite K-L has previously been found to possess very high thermal stability and remains wellcrystalline up to 1087 K. 21 The most obvious changes in relative peak intensities and positions were observed at a temperature of 1073 K. This temperature was therefore chosen as the standard calcination temperature required to achieve internal cation exchange in La-exchanged zeolite L (SI, S5). This temperature is much higher than required for internal ion exchange processes in other zeolites (ca.…”

The Different Organization of Water in Zeolite L and Its MOF Mimic

“…The free diameter of zeolite L channels (corresponding to the size of the channel opening) is 7.2 Å, while the free diameter of the ZLMOF channels is limited to 7.55 Å by the ligands' thiomethyl groups 115 . Both zeolite L and ZLMOF feature hexagonal arrays of channels, running along the c-axis; moreover, for both as-prepared materials, the channels contain clusters of water molecules 115,116 .…”