The thermal behaviour of microporous materials is of essential importance both for their characterization and for their industrial applications and varies significantly from one material to another. In particular, boron substituted zeolites (borosilicates) display a lower acidic strength compared to their aluminosilicates parent structures, which are useful when mild conditions are required (e.g. toluene alkylation with ethanol, propylene oligomerization and conversion of methanol [1-3]). In this study, the thermal dehydration and template burning process of three borosilicates with different topology (MFI, sodalite and levyne, respectively) were studied by in situ synchrotron powder diffraction. The evolution of the structural features was followed through full profile Rietveld refinements, in the temperature range 25 to 900°C. TG and DTG analyses of the as-synthesized samples (heating rate of 5°C/min) were carried out from 25 to 900°C under a constant flux of air. The overall weight losses span between 15.4 and 27.2%, respectively. Decomposition temperature decreases from about 900°C for B-sodalite and B-levyne to about 750°C in the case of B-ZSM-5. Above this temperature, the large pore structure of MFI framework collapses with formation of cristobalite. An examination of the unit cell parameters in the temperature range investigated shows that remarkable change of the parameters occurs during the decomposition and expulsion of organic molecules used as templates. At the same time, a dramatic change in the slope of the TG analyses is reported. The mean thermal expansion coefficients over the experimental temperature range from Tamb to 900°C were calculated according to Fei [4]. Analyzing our data, we have established that the volume thermal expansion in the large pore MFI (αV= 2.97738E-05) is slightly larger than in the other phases (αV= -1.30091E-05 and αV=-1.77495E-05 for B-sodalite and B-levyne, respectively), thus suggesting a possible role of the framework density on the structural deformations. [1]. L.Z. Chen and Y.Q. Feng, Zeolites 12 (1992) 347. [2]. L. Occelli, J.T. Hsu and L.G. Galya, J. Mol. Catal. 32 (1985) 377. [3]. E. Unnenberg and S. Kolboe, Stud. Surf. Sci. Catal. 98 (1995) 144. [4]. Y Fei AGU Reference Shelf 2: Mineral Physics and Crystallography – A Handbook of Physical Constants. Ahrens TJ ed. : (1995) 29-44. Washington AGU.
Thermal stability of borosilicates: an in situ synchrotron powder diffraction study
LEARDINI, Lara;MARTUCCI, Annalisa;ALBERTI, Alberto;CRUCIANI, Giuseppe
2009
Abstract
The thermal behaviour of microporous materials is of essential importance both for their characterization and for their industrial applications and varies significantly from one material to another. In particular, boron substituted zeolites (borosilicates) display a lower acidic strength compared to their aluminosilicates parent structures, which are useful when mild conditions are required (e.g. toluene alkylation with ethanol, propylene oligomerization and conversion of methanol [1-3]). In this study, the thermal dehydration and template burning process of three borosilicates with different topology (MFI, sodalite and levyne, respectively) were studied by in situ synchrotron powder diffraction. The evolution of the structural features was followed through full profile Rietveld refinements, in the temperature range 25 to 900°C. TG and DTG analyses of the as-synthesized samples (heating rate of 5°C/min) were carried out from 25 to 900°C under a constant flux of air. The overall weight losses span between 15.4 and 27.2%, respectively. Decomposition temperature decreases from about 900°C for B-sodalite and B-levyne to about 750°C in the case of B-ZSM-5. Above this temperature, the large pore structure of MFI framework collapses with formation of cristobalite. An examination of the unit cell parameters in the temperature range investigated shows that remarkable change of the parameters occurs during the decomposition and expulsion of organic molecules used as templates. At the same time, a dramatic change in the slope of the TG analyses is reported. The mean thermal expansion coefficients over the experimental temperature range from Tamb to 900°C were calculated according to Fei [4]. Analyzing our data, we have established that the volume thermal expansion in the large pore MFI (αV= 2.97738E-05) is slightly larger than in the other phases (αV= -1.30091E-05 and αV=-1.77495E-05 for B-sodalite and B-levyne, respectively), thus suggesting a possible role of the framework density on the structural deformations. [1]. L.Z. Chen and Y.Q. Feng, Zeolites 12 (1992) 347. [2]. L. Occelli, J.T. Hsu and L.G. Galya, J. Mol. Catal. 32 (1985) 377. [3]. E. Unnenberg and S. Kolboe, Stud. Surf. Sci. Catal. 98 (1995) 144. [4]. Y Fei AGU Reference Shelf 2: Mineral Physics and Crystallography – A Handbook of Physical Constants. Ahrens TJ ed. : (1995) 29-44. Washington AGU.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
