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 μm³) 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.