Speaker
Description
INTRODUCTION
Structural deformations in zeolites during adsorption processes have been previously reported for materials such as MFI [1], RHO [2], AlPO-LTA [3], or AlPO-CHA [4]. These changes are typically associated with an increase in adsorption capacity above a certain pressure, as evidenced by the presence of steep steps in the adsorption isotherm. Such behavior is often linked to second-order phase transitions, where atomic connectivity is preserved, but changes in symmetry may occur. These transitions depend on both the pressure and the nature of the adsorbate.
Here, we describe the structural response of pure silica zeolite ZSM-11, also known as silicalite-2 [5] (IZA code: MEL), during the adsorption of light hydrocarbons (propane and butane). This study combines in situ powder X-ray diffraction (PXRD) with adsorption isotherm measurements.
RESULTS AND DISCUSSION
Propane and butane adsorption isotherms at 298 K do not present abrupt steps (Figure 1, left), in contrast to Ar adsorption at 77-97 K, which exhibits a sharp step, as previously reported (Figure 1, right) [6]. This suggested the absence of phase transitions during hydrocarbon adsorption in silicalite-2 and a continuous deformation of the framework.
Figure 1: (Left) hydrocarbon adsorption isotherms at 298 K; red: propane, black: butane; (right) Ar adsorption isotherms at 77 K (black), 87 K (red), 92 K (green) and 97 K (blue) on silicalite-2
To further analyze these phenomena, in situ PXRD experiments were conducted under atmospheres with controlled partial pressures of the selected hydrocarbons diluted in helium at room temperature.
All diffraction patterns of silicalite-2 obtained at different partial pressures were indexed with the same tetragonal symmetry (I -4 m 2), indicating that there were no symmetry changes during the filling of the microporous space of the zeolite. However, a continuous increase in the unit cell parameters was observed when increasing the ratio of hydrocarbon in the gas flow (Figure 2). This expansion is not linear with respect to the hydrocarbon uptake, showing a small variation of the cell volume after the inclusion of the first molecules, which occupy the void spaces, that becomes larger once the pores are almost full (Figure 3). These results demonstrate that silicalite-2 exhibits a significant framework flexibility, accommodating hydrocarbon molecules through continuous expansion of the pore system while preserving its structural symmetry. Moreover, this process is fully reversible, returning to the original cell parameters when submitting the material to a flux of pure helium at room temperature.
Figure 2: Variation of the unit cell parameters of silicalite-2 with the hydrocarbon/helium ratio (XCn). Red: propane; black: butane
Figure 3: Expansion of the cell volume of silicalite-2 with the uptake of hydrocarbon molecules. Red: propane; black: butane
EXPERIMENTAL
Silicalite-2 was synthesized following a previously reported procedure [7]. Prior to adsorption measurements the calcined material was degassed at 400 ºC under high vacuum for 12 h. Ar adsorption isotherms were measured using an ASAP-2020 (Micromeritics) equipped with a Cryotune system (3P Analytics), and hydrocarbon adsorption isotherms were measured using a BELSORP Max-II instrument. PXRD experiments were performed in an Anton-Paar XRK-900 reaction chamber attached to an Empyrean X-ray diffractometer. Prior to the measurements, the calcined material was degassed at 400 ºC under a continuous flow of helium. Hydrocarbon/helium mixtures with different proportions were then fluxed through the sample. Diffraction patterns were collected after equilibrium was reached for each flux composition.
CONCLUSIONS
Although zeolite flexibility is often associated with phase transitions driven by cation movement or structural/symmetry rearrangements, the present study shows that silicalite-2, which possesses no cations on the channel system, responds differently to light hydrocarbon adsorption. No phase transitions were observed. Instead, the material undergoes a continuous elastic expansion of the unit cell. This behavior highlights the intrinsic flexibility of the MEL framework, which adapts to guest molecules through smooth structural deformation while preserving its symmetry. Further studies using synchrotron radiation are expected to improve the structural resolution and gain a deeper insight into the adsorption-induced flexibility and the atomic displacements in the framework.
REFERENCES
[1] E. García-Pérez, J.B. Parra, C.O. Ania, D. Dubbeldam, T.J.H. Vlugt, J.M. Castillo, P.J. Merkling and S. Calero (2008) J. Phys. Chem. C, 112, 9976–9979.
[2] M. Palomino, A. Corma, J.L. Jordá, F. Rey and S. Valencia (2012) Chem. Commun., 48, 215–217.
[3] A. Krajnc, J. Varlec, M. Mazaj, A. Ristic, N. Zabukovec Logar and G. Mali (2017) Adv. Energy Mater., 7, 1601815.
[4] J. Varlec, A. Krajnc, M. Mazaj, A. Ristic, K. Vanatalu, A. Oss, A. Samoson, V. Kaucic and G. Mali (2016) New J. Chem., 40, 4178–4186.
[5] D. Bibby, N. Milestone, L. Aldridge (1979) Nature 280, 664–665
[6] V. Sánchez-Gil, E. G. Noya, J. M. Guil, E. Lomba, S. Valencia, I. da Silva, L. Pusztai, L. Temleitner (2016, J. Phys. Chem. C, 120, 4, 2260–2270
[7] O. Terasaki, T. Ohsuna, H. Sakuma, D. Watanabe, Y. Nakagawa and R.C. Medrud (1996) Chem. Mater., 8, 463–468.
ACKNOWLEDGEMENTS
The authors thank the financial support of the Spanish Ministry of Science and Innovation (CEX2021–001230-S and PID2022-136934OB-100 grants funded by MCIN/AEI/10.13039/ 501100011033 funded by “ERDF A way of making Europe” and TED2021-130191B-C41 grant funded by the European UnionNext Generation EU/PRTR) and the Generalitat Valenciana (Prometeo 2021/077 and CIPROM/2024/050).