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Stability and strucural changes upon heating of ZSM-5 zeolites with different aluminium ratios: an in situ time-resolved powder diffraction study

Brognara A.;Beltrami G.;Ardit M.;Gigli L.;Cruciani G.;Martucci A.
2018
2018
The zeolite behavior upon heating is of particular importance since catalytic properties, molecular sieve effects are remarkably enhanced in the dehydrated/calcined state and at relatively high temperatures. The structural modifications occurring in the same zeolite framework are affected by several intrinsic chemical and structural factors (i.e. framework topology, Si/Al ratio, charge-compensating cations, coordination of bare cations, crystal size, heating rate, etc.) [1-3]. The interplay of the factors listed above renders difficult the thermal stability prediction of any given zeolite. Ideally, the effect on the thermal stability of varying the framework Si/Al ratio is best evaluated in a series of isostructural zeolites having different Si/Al ratio. In this work, the step by step thermal dehydration process of four ZSM-5 zeolites with different Si/Al ratio (SAR), has been studied by in situ synchrotron radiation powder diffraction. ZSM-5 samples were prepared starting from the following synthesis gel: TPABr (98%, Fluka), NaOH (97%, Carlo Erba Reagenti), Al(OH)3 (98%, Fluka), precipitated silica gel (100%, Merck) and distilled water [4]. In order to achieve different Si/Al ratio in the sample, the molar composition of the gel was changed by varying the amount of Al2O3 in the starting mixture: 0.1Na2O−0.08TPABr−xAl2O3−1SiO2–20H2O, with the alumina amount (0.005 < x < 0.033) varied according to the expected Si/Al molar ratio: 15, 25, 50 and 100 [4]. The time-resolved experiment was performed at the MCX beamline at Elettra. ZSM-5 powder samples were placed in a quartz-glass capillary using a gas blower to control the sample temperature. The evolution of the structural features was monitored through 36 structure refinements in the temperature range from 25 to 800 °C by full profile Rietveld analysis. A picture of both kinetics and dynamic behavior of these materials including Temperature-dependent variation of cell parameters and crystal-structure modifications upon heating is presented. High temperature experiments confirm that all ZSM-5 maintain their crystallinity within the investigated T-range. No significant differences in the thermal stability are highlighted. On the contrary, the beta angle variation was found to be affected by the Si/Al ratio. Specifically, the higher the Silicium content the greater the beta angle. Lattice parameters vary continuously on increasing temperature, with a strong change in their evolution close to the temperature at which the monoclinic to orthorhombic phase transition takes place (T100°C). Finally, a negative volume expansion associated with the temperature increase was observed in all investigated ZSM-5 samples. References [1] Alberti, A. & Martucci, A. (2005) In Studies in Surface Science and Catalysis, 155, 19-43, Elsevier. [2] Alberti, A. & Martucci, A. (2011) Microporous and Mesoporous Materials, 141, 192-198. [3] Cruciani, G. (2006) Journal of Physics and Chemistry of Solids, 67, 1973-1994. [4] Frusteri, F., Bonura, G., Cannilla C., Drago Ferrante, G., Aloise, A. Catizzone, E., Migliori, M. & Giordano, G. (2016) Applied Catalysis B: Environmental 176–177, 522–531
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2399706
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