Studio “In-Situ” \ndel comportamento termico \ndi borosilicati mediante \ndiffrazione da polveri \ncon luce di sincrotrone
Year: 2011
Type: dissertation
Abstract: The isomorphous substitution of Al and/or Si by other tri- and tetravalent metal ions is \ngenerally considered to be a tool for tailoring the catalytic properties of zeolites. A large number of \nelements have been incorporated into the framework, but only a few (B, Ga, Fe, V and Ti) have \nbeen verified, leading to the formation of microporous materials with different catalytic properties \nfrom those of their parent aluminosilicates. \nWhen boron ions are incorporated into the tetrahedral framework sites of zeolites, selective \nheterogeneous catalysts are produced (referred to as “boralites” or “borosilicates”). Compared to \nzeolites, boralites display lower acidity. Accordingly, they have proven to be efficient catalysts in \nreactions requiring low acid strength (e.g. the double-bond isomerisation of linear olefins, cracking \nMTBE, N-alkylation of aniline, etc...). \nCatalysts need to have the appropriate thermal stability to withstand the extreme conditions \nwhich are frequently involved in their use and regeneration. So, information regarding their \nresponse to heating is essential not only for their characterization but also for their industrial \napplications. \nThe aim of this PhD thesis is to study the thermal behaviour of boron-substituted zeolites \nwith different framework topologies, using Rietveld structure analysis of temperature-resolved \npowder diffraction data collected using synchrotron radiation. Such experimental conditions are \nideal for the rapid collection of the diffraction data necessary to monitor each step of the \ndehydration process in detail. \nFor this purpose, the following materials (which were all synthesized by Eni S.p.A.) were \nselected: \n-B-sodalite (SOD) (s.g I -43m): Na0.15[B0.07Si11.93O24] · 1.8TR \n-B-ZSM-5 (MFI) (s.g. Pnma): Na4.6 [B9.3Si86.7O192] · 11.5EN · 6.5H2O \n-B-levyne (LEV) (s.g. R -3m): Na0.27 [B3.00Si51.00O108] · 4.5Q · 29H2O \n(where TR=1,3,5-trioxane=C3H6O3, EN=ethylenediamine=C2H8N2, Q=quinuclidine=C7H13N). \nTime resolved powder diffraction patterns were collected at the GILDA beamline at ESRF \n(Grenoble) in the 25-900°C temperature range (heating rate of 5°C/min). The evolution of the \nstructural features was followed by full profile Rietveld refinements. Thermal analyses (TG, DTA \nand DTG) of the as-synthesized samples were carried out in the same temperature range under a \nconstant flux of air (heating rate 5°/min). The overall weight losses span between 15.4 and 27.2%, \nrespectively. \nXRD diffraction patterns indicated that B-sodalite and B-levyne maintain their crystallinity \nup to about 800°C, whereas B-ZSM-5 collapses above 730°C and at 800°C is converted into \n�-cristobalite. \nB-ZSM-5 unit-cell parameters increase with increasing temperatures up to the breakdown \ntemperature. This increase is more evident in the 230-500°C temperature range, where expulsion of \nthe template occurs. This result is at complete odds with the case of boron-free MFI materials, both \nin their as-synthesized and calcined forms. Hence, B-ZSM-5 is the first example of positive thermal \nexpansion in MFI topology. \nThe cell volume of B-sodalite does not change remarkably until template expulsion, which \noccurs at above 380°C, as inferred by TG analysis weight loss and by the structure refinement of \ntrioxane molecule occupancy. When trioxane molecule decomposition begins, volume expansion \nbecomes negative. Organic molecules expulsion at over 380°C apparently justifies the negative \nthermal expansion over this temperature. \nIn the case of B-levyne, a dramatic change in the unit-cell parameters is observed at about \n500°C, when the decomposition and expulsion of organic molecules occurs. Unit-cell volume firstly \nincreases up to 600°C and then decreases up to the highest investigated temperature. To explain this \nanomalous behaviour, the distribution of quinuclidine molecules inside the levyne cage should be \ntaken into account. Quinuclidine assumes two different positions within the cage (labelled Q1 and \nQ1b, respectively) which are differently occupied. One of these two molecules (Q1b) is rapidly \nexpelled at 600°C, corresponding to a sharp positive volume expansion. The second molecule (Q1) \nis lost at higher temperatures, with negative thermal expansion in the volume. Template degradation \nand expulsion strongly affect the B-levyne structure inducing tetrahedral tilting and pore system \ndeformation. Moreover, the evolution of the O-T-O angles and T-O distances suggest the formation \nof trigonal boron according to NMR and IR spectroscopy. This is the first example of tetrahedral to \ntrigonal boron conversion detected by XRD analysis.
Source: EprintsUnife
Author: Leardini, Lara
Language: English
Subfield: Inorganic Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7015369438