Thursday, February 05, 2009

J Phys Chem C, vol 113, Issue 5 and 6

Structure and Dynamics of Hydrous Surface Species on Alumina−Boria Catalysts and Their Precursors from 1H, 2H, 11B, and 27Al MAS NMR Spectroscopy

Michael Ryan Hansen, Hans J. Jakobsen and Jørgen Skibsted*
Instrument Centre for Solid-State NMR Spectroscopy and Interdisciplinary Nanoscience Center (iNANO), Department of Chemistry, University of Aarhus, DK-8000 Aarhus C, Denmark
J. Phys. Chem. C, 2009, 113 (6), pp 2475–2486
DOI: 10.1021/jp807595m
Abstract: The hydrous surface of a pseudoboehmite precursor impregnated with 1.3 wt % boron, its dehydration upon gentle heat treatment, and the corresponding alumina boria catalyst obtained by calcination at 750 °C have been studied by 1H and 2H MAS NMR as well as double-resonance 1H{11B}, 1H{27Al} TRAPDOR, and 11B{1H}, 27Al{1H} CP/MAS NMR experiments. Resonances from bulk and surface AlOH groups, surface BOH sites, and water molecules adsorbed on the surface-aluminate and -borate species have been identified and characterized. The dynamics of the water molecules and their removal from the surface by heat treatment have been studied by the double-resonance experiments. Optimizations of the 1H −11B/27Al cross-polarization conditions for the individual 11B and 27Al sites demonstrate that the BO4 as well as the BO3 sites on the alumina surface exhibit hydroxyl groups in their near vicinity. The same holds for the AlO4, AlO5, and AlO6 sites on the γ-Al2O3 surface of the alumina−boria catalyst.

2J Si−O−Si Scalar Spin−Spin Coupling in the Solid State: Crystalline and Glassy Wollastonite CaSiO3
Pierre Florian
*, Franck Fayon and Dominique Massiot
CNRS, UPR3079 CEMHTI, 1D Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France, and Université d’Orléans, Avenue du Parc Floral, BP 6749, 45067 Orléans Cedex 2, France
J. Phys. Chem. C, 2009, 113 (6), pp 2562–2572
DOI: 10.1021/jp8078309

Abstract: We have investigated the 29Si−O−29Si 2J scalar spin−spin coupling constant in two crystalline polymorphs and in a glass of 29Si isotopically enriched wollastonite CaSiO3 composition. In the crystalline samples, line widths of less than 1 Hz are observed in the indirect dimension of a simple J-resolved NMR experiment leading to 2JSiOSi values of 1.5, 3.6, and 8.0 Hz determined with a high accuracy of ±0.1 Hz. The very high resolution obtained with those two compositions enabled us to evidence strong coupling effects in the J-resolved two-dimensional spectra. In particular, additional lines in J-spectra are seen here for the first time in solid-state NMR experiments of inorganic samples. Cluster ab initio calculations led us to propose a stereochemical analysis of 2J, leading to a close-to-linear relationship between 2JSiOSi and the Si−O−Si bonding angle Ω ≃ 3.41J + 127 which takes into account the predominant influence of the Ca atoms. In the glass, this relationship allowed the analysis of the distribution of angle within each Q(n) species. The classical relationship between the 29Si isotropic chemical shift and the average Si−O−Si bond angle is experimentally confirmed and quantified. The most probable set of Ω’s are 128.0° for Q(1), (131.8°,135.4°) for Q(2), and (132.5°,137.1°,144.3°) for Q(3), and the presence of three-membered rings is evidenced. An unexpectedly strong correlation between the bonding at each corner of a given Q(n) unit and the bonding at adjacent corners is found.

Synthesis, Spectroscopic Properties, and Stabilities of Ternary Europium Complex in SBA-15 and Periodic Mesoporous Organosilica: A Comparative Study
Xianmin Guo
, Huadong Guo, Lianshe Fu§, Ruiping Deng, Wan Chen, Jing Feng, Song Dang and Hongjie Zhang*
State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, P. R. China, Graduate School of the Chinese Academy of Sciences, P. R. China, and Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
J. Phys. Chem. C, 2009, 113 (6), pp 2603–2610

Abstract: Ternary europium complex Eu(tta)3phen was covalently bonded with the general mesoporous material SBA-15 and SBA-15-type of periodic mesoporous organosilica (PMO) material via impregnation of Eu(tta)3·2H2O into phen-S15 and phen-PMO, respectively, through a ligand exchange reaction. The parent materials of phen-S15 and phen-PMO were synthesized by co-condensation of tetraethylorthosilicate (TEOS) or 1,2-bis(triethoxysilyl)ethane (BTESE) and the functionalized chelate ligand 5-(N,N-bis(3-triethoxysilyl)propyl)ureyl-1,10-phenanthroline (phen-Si) in the presence of Pluronic P123 surfactant as template, which were confirmed by SEM, XRD, FTIR, 29Si CP-MAS NMR, and N2 adsorption measurements. The photophysical properties of the hybrids, such as the photoluminescence (PL) spectra, PL intensities, symmetry properties, luminescence decay times, and Judd−Ofelt parameters, were investigated in detail. Compared to the sample of Eu(tta)3phen-PMO, the mesoporous hybrid material Eu(tta)3phen-S15 exhibited longer luminescent decay time and higher luminescence intensity, emission quantum efficiency (q), and absolute quantum yield (Φ), which were quantitatively stressed by the emission spectra and the calculated values of q of Eu3+ ion. Meanwhile, the result of thermal treatment demonstrated that the europium complex in Eu(tta)3phen-S15 material possessed a better thermal stability than that in Eu(tta)3phen-PMO. The above photoluminescence and thermal stability features indicated that SBA-15 is a better host material for lanthanide complex than SBA-15-type PMO material.

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