Elastic behavior of MFI-type zeolites: 1-Compressibility of Na-ZSM-5 in penetrating and non-penetrating media

We report the results of an in situ synchrotron X-ray powder diffraction study on the elastic behavior of Na-ZSM-5, performed using both silicone oil (s.o.) and (16:3:1) methanol:ethanol:water (m.e.w.), as ‘‘non-penetrating’’ and ‘‘penetrating’’ pressure transmitting media, respectively. In the range from ambient pressure (Pamb) to 6.2 GPa, the reductions of a, b, c, and V observed in s.o. are: 6.4, 6.3, 6.9 and 18.5%, respectively. From Pamb to 7.4 GPa, a unit-cell volume reduction of about 14.6% is observed for Na-ZSM-5 compressed in m.e.w., and the corresponding reductions of a, b, and c cell parameters are 6.3, 4.6, and 4.5%, respectively. In both cases no phase transitions are observed and the unit cell parameters of ambient conditions are recovered upon decompression. The complete structural refinements relative to the experiments performed in m.e.w. up to 1.6 GPa reveal a strong increase in the extra-framework content – with the penetration of additional water/alcohols molecules in the partially occupied extra-framework sites of as-synthesized Na-ZSM-5. This P-induced penetration, which does not induce any cell volume expansion, is only partially reversible, since a fraction of the extra-molecules remains in the channels upon decompression. Our results show that Na-ZSM-5 is the softest microporous material among those so far compressed in s.o. Moreover, its compressibility is higher in s.o. than in m.e.w. (K0 = 18.2(6) GPa, K0 = 4 (fixed) and 28.9(5) GPa, K0 = 4 (fixed), respectively). This can be ascribed to the penetration of the extra-water/alcohol molecules, which contribute to stiffen the structure and to contrast the channel deformations