Time-resolved Energy Dispersive X-ray Diffraction Research Articles

On the advantages of the use of the three-element detector system for measuring EDXRD patterns to follow the crystallisation of open-framework structures

Time-resolved energy-dispersive X-ray diffraction (EDXRD) studies, employing a new detector technology, of a range of crystallisations of open framework materials are described. We consider four distinct categories of phenomena where new information has been gained specifically from the use of the multi-element (as opposed to the single element) detector system. The systems investigated are: (a) the competitive formation of small-pore and large-pore aluminophosphates (AlPO's), and the effect of concentration of CoII in the mother liquor (precursor gel) in directing the relative amounts of AlPO-18 (AEI) and chabazite (CHA) structures that are formed; (b) the influence of both template (structure directing) molecules and synthesis time on the stabilities of the AlPO-5 (AFI) structures; (c) a study of both the rate of formation of the open framework titanosilicate (ETS-10) structure and the dissociation rate of crystalline TiO2 used in the preparation of ETS-10; and (d) tracking of the intermediate formed during the synthesis of the gallophosphate structure known as ULM-3. The advantages of using a three-element detector configuration are illustrated.

Open-framework fluorinated gallium and aluminium phosphates: an in situ study of the hydrothermal synthesis by X-ray diffraction using synchrotron radiation

The formation mechanisms of microporous materials synthesized under mild hydrothermal conditions are still not elucidated and it is necessary to utilize in situ characterizations for gaining information and new insights concerning the chemical processes which take place within the cell. The hydrothermal crystallizations of the open-framework gallium and aluminium fluorinated phosphates with the ULM-3 and ULM-4 structural types have been investigated by time-resolved energy-dispersive X-ray diffraction. It has been shown that the nature of the phosphorus source (H3PO4, polyphosphoric acid or P2O5) could drastically affect the crystallization behaviour. For example, in the gallium system, an intermediate appears at short reaction times before the formation of the ULM-3 structure when P2O5 is used instead of H3PO4, whereas no other phase is observed during the crystallization of ULM-4. However, with aluminium, an intermediate phase is systematically formed before the crystallization of the ULM-3 solid. Kinetics data for each system have been modelled using the Avrami–Erofe’ev equation and these calculations indicate the reactions to be diffusion controlled.

Time-Resolved in Situ X-ray Powder Diffraction Study of the Formation of Mesoporous Silicates

In situ, time-resolved energy-dispersive X-ray diffraction has been used to investigate the formation of the mesoporous silicates FSM-16 and MCM-41. The data suggest that the silica−surfactant mesophases formed are highly dependent on the reactant medium, the effect of the silica source being one of the main determining factors. Kanemite, a layered polysilicate, proves to be an excellent silicate source, giving rise to relatively ordered mesophases and subsequent highly ordered mesoporous silicate products. The time-resolved in situ X-ray diffraction data of the kanemite−alkytrimethylammonium system indicated that the silica−surfactant mesophase precursor to FSM-16 forms from a medium containing a number of intercalated silicate phases, while in contrast, the hexagonal mesophase precursor to MCM-41 forms from a medium containing no other ordered silicate−surfactant phases detectable by in situ X-ray diffraction.

Kinetics of the Solid-State Phase Transformation of Form β to γ of Sulfanilamide Using Time-Resolved Energy-Dispersive X-ray Diffraction

The kinetics of the solid-state phase transformation of form beta to gamma of sulfanilamide in powdered samples have been investigated using energy-dispersive X-ray diffraction (EDXRD) combined with synchrotron radiation. The beta to gamma transformation which is relatively fast has been followed in real time, courtesy of the high time resolution of the EDXRD method. The data obtained yield alpha-time curves of high accuracy and precision. The observed kinetics are atypical in that the transformation does not always proceed to completion but plateaus off, the rate and extent being higher with increasing temperature. This phenomenon suggests a distribution of activation energies in the powdered samples. Despite this complication the data have been analyzed by considering only the fraction transformed. Of the various kinetic models considered, the Avrami-Erofeyev (n = 3.5) and the Cardew model were found to best describe the data. The data fitting with both of these models, however, was not totally satisfactory. The Avrami-Erofeyev model was found to depart increasingly from the observed data at high alpha values. The Cardew model, being specific for powdered or polycrystalline samples, was significantly better, but only up to alpha values of about 0.85. Above this point the Cardew model deviates markedly from the observed data. Direct visual observation using hot-stage microscopy has revealed that the transformation always proceeds from a single nucleation event in each crystallite and that coalescence of growing surfaces and ingestion of potential nuclei are unimportant, which is consistent with the Cardew model. Also, extinction studies using polarized light have shown that the transformation in the crystallites is generally of the type single crystal to single crystal but does not exhibit any orientational relationship. The overall activation energy and the individual nucleation and growth activation energies for the beta to gamma transformation based on the Cardew model were determined to be 101 +/- 7, 142 +/- 14, and 70 +/- 4 kJ/mol, respectively. The activation energy based on the Avrami-Erofeyev model was 89 +/- 8 kJ/mol. These magnitudes are within the expected range for molecular crystals.