Bernard F. Mentzen

X-ray diffraction and solid-state n.m.r. studies of AEL molecular sieves: Effect of hydration

Abstract Interpretation of X-ray powder diffraction spectra by using profile refinement techniques allowed the determination of (i) the positions of P, Al and O atoms in AEL-type materials (AlPO 4 -11, SAPO-11) and (ii) the effect of adsorbed water molecules on the geometry of the previously evacuated AEL framework. The as-synthesized sample (hereafter labeled SYN form) and the 550°C calcined form (CAL form) crystallize in the acentric Ima2 space group with 3P + 3Al sites per asymmetric unit, whereas the rehydrated material (WAT form) adopts the acentric Pna 2 1 space group with 5P + 5Al sites per asymmetric unit. In MAS -n.m.r. spectroscopy, a broad 31 P peak observed for the SYN and the dehydrated CAL forms is centered near δ = −30.7 ppm and is interpreted as the evelope of three overlapping components with 2:1:2 intensity ratios (8P1:4P2:8P3). For the hydrated WAT form two peaks centered at δ = −23 and −29 ppm having an intensity ratio of 1:4 have been attributed to the 4P1 and the 4P2 + 4P3 + 4P4 + 4P5 sites, respectively. The first peak corresponds to the P1 species, which is attached to OH and/or water molecules, as shown by proton cross-polarization experiments. The second broad peak, which corresponds to the remaining P atoms, is interpreted as four overlapping peaks of equal intensity. Water molecules were shown by X-ray diffraction experiments to form dimeric clusters and to be located close to P1 in the elliptical channels. It is suggested that this atom could be substituted by Si atoms in the SAPO-11 samples, as supported by the observed 1:4 intensity ratio and by the fact that the intensity of the component corresponding to P1 decreases with increasing Si content.

Structural study of an MFI/naphthalene system by X-ray powder diffraction, 29Si MAS n.m.r, and energy-minimization calculations

Abstract Combining solid-state 29Si MAS n.m.r. studies, energy-minimization calculations using the atom—atom potential method (Buckingham and Lennard-Jones models), and X-ray profile-fitting structure refinements (Rietveld method with geometric constraints) shows that it is possible to locate the naphthalene (NPH) molecules sorbed at room temperature in a saturated silicalite-4 C10H8 complex. Interpretation of the n.m.r. spectrum and of the diffraction profile is consistent with orthorhombic Pnma framework symmetry. The entrapped sorbate molecules, which are localized at the channel intersections of the MFI framework, have their longer symmetry axis oriented parallel to the y-axis direction [010] and form infinite and loosely associated polymeric chains in the straight channels. The NPH does not enter the zigzag channels, where the presence of an additional extraframework water molecule could be seen. NPH adsorption provokes an elliptical deformation of the straight-channel sections (6.3 × 4.8 A). The zigzag channel sections remain almost circular. For the first time, an inversion in the a and b unit cell parameters is observed ( a b = 0.9984 ) and the NPH acronym is proposed for this deformation of the MFI framework. Time-resolved X-ray diffraction shows that at lower pore fillings (0 a b ratio is also observed for several other MFI/aryl systems.

Coordination chemistry in anhydrous monovalent metallic formates. I. The crystal structure of anhydrous potassium formate KHCOO. C2h-octahedral eight-coordination of the potassium atom

Abstract KHCOO crystallizes in the orthorhombic space group Cmcm, with a = 5.890(1), b = 6.793(1) and, cell. The structure was solved by conventional Patterson and Fourier methods and refined by full-matrix least-squares computations to R = 0.00522 (weighted R w = 0.0358), for the 360 unique data collected. The coordination of the potassium atom is discussed on the basis of a purely C 2h symmetry model. The formated groups are of C v symmetry and lie on the mirror planes at z = 1 4 and 3 4 .

Coordination chemistry in anhydrous monovalent metallic formates. II. The crystal structure of anhydrous thallium(I) formate TlHCOO

Abstract TlHCOO crystallizes in the orthorhombic space group Pna21 with a = 6.728(2), b = 5.937(2) and c = 7.808(2) A, with four formula-units per primitive unit-cell. The structure was solved by conventional Patterson and Fourier methods and refined by full-matrix least-squares computations to R = 0.023 for the 226 unique data collected. The results are compared to those reported for other monovalent metallic formates (including ammonium formate).

Structural study of the silicalite (MFI)/tetrachlorethylene (TCET) host–guest system by X-ray powder diffraction and 29Si NMR. Locations of the TCET molecules at low and high pore-fillings

Abstract Investigations on the silicalite (MFI)/tetrachlorethylene (TCET) system by X-ray powder diffraction show the existence of two sorbent/sorbate complexes, corresponding to four and eight molecules per unit-cell pore-fillings. Both phases present an orthorhombic symmetry and 29 Si NMR shows that their space groups are Pnma and P 2 1 2 1 2 1 respectively. Crystal structure refinements using the Rietveld method show that at lower pore-fillings only one adsorbed species occupies the channel–intersections of the MFI topology. At higher loadings, a second tetraethylene species resides in the zigzag channels.

Dehydration of ammonium trioxalatochromate(III) and trioxalatoferrate(III) in relation with the preparation of metallic mixed-oxides

Abstract The different steps appearing during the dehydration processes of ammonium trioxalatochromate (III) and ferrate(III) have been investigated by considering 1) - their crystallization diagrams, 2) - thermogravimetry (TG) under controlled water pressure and 3) - differential thermal analysis (DTA). In the case of ammonium trioxalatoferrate(III) trihydrate (NH 4 ) 3 [Fe (C 2 O 4 ) 3 ]. 3H 2 O the TG curves exhibit a plateau corresponding to the dihydrate intermediate. For the chromate(III) the dehydration equilibrium is divariant. These observations arc discussed in terms of the crystal structures of both investigated solids, especially on the basis of the specific role played by one of the three water molecules in the trihydrates.