The isomorphous substitution of Al and/or Si by other tri- and tetravalent metal ions is generally considered a tool for tailoring the catalytic properties of zeolites. A large number of elements have been incorporated into the framework, but only a few (B, Ga, Fe, V and Ti) have been tested, leading to the formation of microporous materials with catalytic properties that are different from those of the parent aluminosilicates. In particular, boron substituted zeolites (borosilicates) display lower acidic strength, which is useful when mild conditions are required (e.g. toluene alkylation with ethanol, propylene oligomerization and the conversion of methanol [1-3]). In this work the influence of boron on the thermal behaviour of ZSM-5 was studied by in-situ time-resolved powder diffraction (GILDA, ESRF Grenoble). A B-ZSM-5 (B-MFI) sample (Na2[Si82 B14O192] ∙ 6EN∙ n H2O, s.g. Pnma) synthesized by Eni S.p.a. in the presence of ethylenediamine (EN) was selected for this study. The evolution of the B-ZSM-5 structural features was followed through full profile Rietveld refinements in the temperature range 25-900°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 using a STA 409 PC LUXX® - Netzch. XRD diffraction patterns indicated that B-MFI collapses at about 770°C and at 800°C is converted into β-cristobalite [4]. All the unit-cell parameters increase with increasing temperatures up to the breakdown temperature. This increase is more evident in the 300-500 °C temperature range where the expulsion of the template occurs. This result completely disagrees with the case of silicalites heated after calcination, where after a slight increase in volume, up to 100-150°C, negative thermal expansion is evident [5]. This fact is also present when a Si-fraction is substituted by Zr or Fe [5-6]. In the case of as-synthesized silicalites, the removal of the organic template leads to an overall contraction of the orthorhombic lattice [7]. References. [1] Chen, L.Z. & Feng, Y.Q. (1992): Zeolites, 12, 347-350; [2] Occelli, L., Hsu, J.T., Galya, L.G. (1985): J. Mol. Catal., 32, 377-390; [3] Unnenberg, E. & Kolboe, S. (1995): Stud. Surf. Sci. Catal., 98 , 144; [4] Howden, M. G. (1985): Zeolites, 5, 334-338; [5] Bhange, D. S. & Ramaswamy, V. (2006): Mater. Res. Bull., 41, 1392-1402; [6] Milanesio, M. , Artioli, G., Gualtieri, A., Palin, L., Lamberti, C. (2003): J. Am. Chem. Soc., 125, 14549-14558; [7] Geus, E. R. & Van Bekkum, H. (1995): Zeolites, 15, 333-341.
THERMAL BEHAVIOUR OF BORON-ZSM-5
LEARDINI, Lara;MARTUCCI, Annalisa;ALBERTI, Alberto;CRUCIANI, Giuseppe
2010
Abstract
The isomorphous substitution of Al and/or Si by other tri- and tetravalent metal ions is generally considered a tool for tailoring the catalytic properties of zeolites. A large number of elements have been incorporated into the framework, but only a few (B, Ga, Fe, V and Ti) have been tested, leading to the formation of microporous materials with catalytic properties that are different from those of the parent aluminosilicates. In particular, boron substituted zeolites (borosilicates) display lower acidic strength, which is useful when mild conditions are required (e.g. toluene alkylation with ethanol, propylene oligomerization and the conversion of methanol [1-3]). In this work the influence of boron on the thermal behaviour of ZSM-5 was studied by in-situ time-resolved powder diffraction (GILDA, ESRF Grenoble). A B-ZSM-5 (B-MFI) sample (Na2[Si82 B14O192] ∙ 6EN∙ n H2O, s.g. Pnma) synthesized by Eni S.p.a. in the presence of ethylenediamine (EN) was selected for this study. The evolution of the B-ZSM-5 structural features was followed through full profile Rietveld refinements in the temperature range 25-900°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 using a STA 409 PC LUXX® - Netzch. XRD diffraction patterns indicated that B-MFI collapses at about 770°C and at 800°C is converted into β-cristobalite [4]. All the unit-cell parameters increase with increasing temperatures up to the breakdown temperature. This increase is more evident in the 300-500 °C temperature range where the expulsion of the template occurs. This result completely disagrees with the case of silicalites heated after calcination, where after a slight increase in volume, up to 100-150°C, negative thermal expansion is evident [5]. This fact is also present when a Si-fraction is substituted by Zr or Fe [5-6]. In the case of as-synthesized silicalites, the removal of the organic template leads to an overall contraction of the orthorhombic lattice [7]. References. [1] Chen, L.Z. & Feng, Y.Q. (1992): Zeolites, 12, 347-350; [2] Occelli, L., Hsu, J.T., Galya, L.G. (1985): J. Mol. Catal., 32, 377-390; [3] Unnenberg, E. & Kolboe, S. (1995): Stud. Surf. Sci. Catal., 98 , 144; [4] Howden, M. G. (1985): Zeolites, 5, 334-338; [5] Bhange, D. S. & Ramaswamy, V. (2006): Mater. Res. Bull., 41, 1392-1402; [6] Milanesio, M. , Artioli, G., Gualtieri, A., Palin, L., Lamberti, C. (2003): J. Am. Chem. Soc., 125, 14549-14558; [7] Geus, E. R. & Van Bekkum, H. (1995): Zeolites, 15, 333-341.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
