New insights in the formation of silanol defects in silicalite-1 by water intrusion under high pressure

Abstract

The "water-silicalite-1" system is known to act as a molecular spring. The successive intrusion-extrusion cycles of liquid water in small crystallites (6 × 3 × 0.5 μm(3)) of hydrophobic silicalite-1 were studied by volumetric and calorimetric techniques. The experiments displayed a decrease of the intrusion pressure between the first intrusion-extrusion cycle and the consecutive ones, whereas the extrusion pressures remained unchanged. However, neither XRD studies nor SEM observations revealed any structural and morphological modifications of silicalite-1 at the long-range order. Such a shift in the value of the intrusion pressure after the first water intrusion-extrusion cycle is attributed to the creation of silanol groups during the first water intrusion. Detailed FTIR and solid-state NMR spectroscopic characterizations provided a molecular evidence of chemical modification of zeolite framework with the formation of local silanol defects created by the breaking of siloxane bonds.

Synergetic effect of temperature and pressure on energetic and structural characteristics of {ZIF-8 + water} molecular spring

“…The thermal effects of intrusion and extrusion are presented on Figure 3. For HLSs [16,17,26] the intrusion is typically followed by an endothermal effect, while the extrusion is exothermal (Figure3 insert). Which means that during the intrusion/extrusion (compression/decompression) HLS stores/restores not only mechanical energy, but also thermal energy.…”

“…To our knowledge there are no studies devoted to the concomitant effects of pressure and temperature on structure of ZIFs. High pressure compression-decompression cycles over wide temperature range are typical operational conditions for porous Heterogeneous Lyophobic Systems (HLS) used for energy storage, transformation or harvesting (depending on the type of HLS) [14][15][16][17] . Recently the class of porous materials for HLS was extended to hydrophobic ZIF-8 [18][19][20][21] and ZIF-71 [22] metal organic frameworks.…”

“…In order to explore combined effects of pressure and temperature on the structure of ZIF-8 (and hence on the stability of {ZIF-8 + water system}) additional X-Ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR) spectroscopy and Scanning Electron Microscopy (SEM) characterizations have been performed before and after compression-decompression cycles of the {ZIF-8 + water} system. The operational principle of the {ZIF-8 + water} system is similar to the one for other HLSs formed by a lyophobic porous matrix and a non-wetting liquid [14][15][16][17] : the non-wetting condition eliminates the spontaneous penetration of the liquid into the pores of the matrix. By increasing the pressure in the system to some critical value (intrusion pressure ) the liquid can be forced into the pores (similarly like for the mercury porosimetry technique); the mechanical energy necessary to break the intermolecular bonds of the liquid during intrusion is supplied to the system during the intrusion process.…”

“…On the PVdiagram (on the macroscopic scale) this process is followed by a significant increase of compressibility of the system (Figure1): the corresponding plateau stands until the pores of the matrix are completely filled. As a consequence that the breaking of intermolecular bonds of the liquid (during intrusion) is an endothermic process [16,17] , the system stores not only mechanical, but also thermal energy during the intrusion. Since lyophobic pores constitute an energetically unfavorable environment for molecules of the non-wetting liquid, the decrease of the pressure in the system down to some critical value (extrusion pressure ) leads to the extrusion of the liquid from the pores of the matrix, which is followed by the release of mechanical (large expansion of the system, see Figure1) and thermal energy (renewal of intermolecular bonds of the liquid with an exothermic effect).…”

Low-temperature plasma-modified zeolite (LTPMZ) vs. natural Bulgarian zeolite (NBZ) – Comparative physicochemical, UV/VIS and FTIR studies

“…11). The FTIR analyses of the fresh NBZ and LTPMZ proved the breakdown of siloxane bonds, which induces the formation of local silanol effects [14]. The nature of these effects, however, could be different.…”