Studio “In-Situ” del comportamento termico di borosilicati mediante diffrazione da polveri con luce di sincrotrone

The isomorphous substitution of Al and/or Si by other tri- and tetravalent metal ions is generally considered to be 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 verified, leading to the formation of microporous materials with different catalytic properties from those of their parent aluminosilicates. When boron ions are incorporated into the tetrahedral framework sites of zeolites, selective heterogeneous catalysts are produced (referred to as “boralites” or “borosilicates”). Compared to zeolites, boralites display lower acidity. Accordingly, they have proven to be efficient catalysts in reactions requiring low acid strength (e.g. the double-bond isomerisation of linear olefins, cracking MTBE, N-alkylation of aniline, etc…). Catalysts need to have the appropriate thermal stability to withstand the extreme conditions which are frequently involved in their use and regeneration. So, information regarding their response to heating is essential not only for their characterization but also for their industrial applications. The aim of this PhD thesis is to study the thermal behaviour of boron-substituted zeolites with different framework topologies, using Rietveld structure analysis of temperature-resolved powder diffraction data collected using synchrotron radiation. Such experimental conditions are ideal for the rapid collection of the diffraction data necessary to monitor each step of the dehydration process in detail. For this purpose, the following materials (which were all synthesized by Eni S.p.A.) were selected: -B-sodalite (SOD) (s.g I -43m): Na0.15[B0.07Si11.93O24] · 1.8TR -B-ZSM-5 (MFI) (s.g. Pnma): Na4.6 [B9.3Si86.7O192] · 11.5EN · 6.5H2O -B-levyne (LEV) (s.g. R -3m): Na0.27 [B3.00Si51.00O108] · 4.5Q · 29H2O (where TR=1,3,5-trioxane=C3H6O3, EN=ethylenediamine=C2H8N2, Q=quinuclidine=C7H13N). Time resolved powder diffraction patterns were collected at the GILDA beamline at ESRF (Grenoble) in the 25-900°C temperature range (heating rate of 5°C/min). The evolution of the structural features was followed by full profile Rietveld refinements. Thermal analyses (TG, DTA and DTG) of the as-synthesized samples were carried out in the same temperature range under a constant flux of air (heating rate 5°/min). The overall weight losses span between 15.4 and 27.2%, respectively. XRD diffraction patterns indicated that B-sodalite and B-levyne maintain their crystallinity up to about 800°C, whereas B-ZSM-5 collapses above 730°C and at 800°C is converted into �-cristobalite. B-ZSM-5 unit-cell parameters increase with increasing temperatures up to the breakdown temperature. This increase is more evident in the 230-500°C temperature range, where expulsion of the template occurs. This result is at complete odds with the case of boron-free MFI materials, both in their as-synthesized and calcined forms. Hence, B-ZSM-5 is the first example of positive thermal expansion in MFI topology. The cell volume of B-sodalite does not change remarkably until template expulsion, which occurs at above 380°C, as inferred by TG analysis weight loss and by the structure refinement of trioxane molecule occupancy. When trioxane molecule decomposition begins, volume expansion becomes negative. Organic molecules expulsion at over 380°C apparently justifies the negative thermal expansion over this temperature. In the case of B-levyne, a dramatic change in the unit-cell parameters is observed at about 500°C, when the decomposition and expulsion of organic molecules occurs. Unit-cell volume firstly increases up to 600°C and then decreases up to the highest investigated temperature. To explain this anomalous behaviour, the distribution of quinuclidine molecules inside the levyne cage should be taken into account. Quinuclidine assumes two different positions within the cage (labelled Q1 and Q1b, respectively) which are differently occupied. One of these two molecules (Q1b) is rapidly expelled at 600°C, corresponding to a sharp positive volume expansion. The second molecule (Q1) is lost at higher temperatures, with negative thermal expansion in the volume. Template degradation and expulsion strongly affect the B-levyne structure inducing tetrahedral tilting and pore system deformation. Moreover, the evolution of the O-T-O angles and T-O distances suggest the formation of trigonal boron according to NMR and IR spectroscopy. This is the first example of tetrahedral to trigonal boron conversion detected by XRD analysis.