In this study the high-pressure (HP) behavior of a potassium alluminosilicate zeolite L (ZL)  has been investigated by in situ synchrotron X-ray powder diffraction, using the “penetrating” pressure transmitting medium methanol:ethanol:water=16:3:1 (m.e.w.) and the “non penetrating” one silicone oil (s.o.). ZL is an interesting and well know porous material used to produce organic-inorganic hybrid compounds [2,3], currently employed in strategic areas from sustainable energy technologies to biomedical sciences. This investigation is a preliminary study to understand the effect of pressure on hybrid ZL composite materials, in order to improve their optical properties. The HP experiments were performed from ambient pressure (Pamb) to 6.2 GPa. and upon decompression to Pamb. The evolution of the unit cell parameters of the ZL, followed both in m.e.w. and s.o. up to 6.2 GPa, did not show neither amorphization nor phase transition. ZL undergoes to an anisotropic compression both in m.e.w. and s.o. This effect is higher in s.o. than in m.e.w., accounting for a total volume decrease ΔV= -6.2% and -9.8% in m.e.w. and s.o., respectively. The severe compression observed in silicone oil induces almost reversible structural changes in the framework: a)the main 12MR channel, becomes more circular up to 3.1GPa, b) the double 6 ring thickness, connecting the cancrinite cages along the c axis- decrease linearly with the pressure up to 3.1 GPa, c) at 1.8 GPa the extraframework potassium site in the main 12MR channel, moves in a new potassium site. When compressed in m.e.w. between Pamb and 2.5 GPa, ZL undergoes a reversible over-hydration (from 18 to 25 water molecules p.u.c.), as already observed for K or Rb-galliosilicate ZL, [4,5]. The water molecules hosted in the 12MR channel are strongly hydrogen-bonded each other giving rise to a water nanotubes above 0.5 GPa. This over-hydration is probably responsible for the lower compression observed with respect to s.o. experiment. These results - in agreement with those present in literature for other ZL samples with different framework and extraframework composition - indicate that the capacity of this system to incorporate nanostructures inside its channels is mainly driven by its peculiar one-dimensional spatial constrains. The study was financed by the ZAPPING project, financed by Italian MIUR.
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