Friday, May 08, 2009

J. Phys. Chem. C Vol. 113, Issues 16-19

Hyperpolarized 129Xe NMR Investigation of Ammonia Borane in Mesoporous Silica

Li-Qiong Wang
*, Abhi Karkamkar, Tom Autrey and Gregory J. Exarhos
Fundamental & Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354
J. Phys. Chem. C, 2009, 113 (16), pp 6485–6490
DOI: 10.1021/jp810994p

Abstract: Hyperpolarized (HP) 129Xe NMR was used to probe the porosity of mesoporous silica (MCM) infused with ammonia borane (AB). Variable-temperature HP 129Xe NMR measurements have been systematically carried out on a series of MCM-41 materials with AB loading ranging from 33 to 75 wt % (1:2 to 3:1 AB:MCM). Three distinct types of pore environments are clearly evident: pristine mesopores, pores coated with AB inside the meso-channels, and interparticle spacing formed from AB aggregates outside the meso-channels. We found similarly uniform coating of AB on mesoporous silica channels with 1:2 and 1:1 AB:MCM loading (ratio of weight percent). When the loading of AB to MCM is greater than 1:1, AB starts to aggregate outside the meso-channels. Further increases in loading (≥3:1) result in the formation of partially blocked meso-channels as a result of excessive AB. The detailed information obtained from this study on how supported AB resides in nanoporous channels and how it evolves with the increase of AB loading is helpful for the rational design of novel materials with optimal hydrogen storage and release properties



High-Resolution 89Y and 45Sc NMR Spectroscopic Study of Short-Range Structural Order in Nanocrystalline Y- and Sc-doped CeO2 and ZrO2

Pragati Jain, Hugo J. Avila-Paredes, Christine Gapuz, Sabyasachi Sen
* and Sangtae Kim

J. Phys. Chem. C, 2009, 113 (16), pp 6553–6560
Abstract: The effect of crystallite size on cation coordination environments and oxygen vacancy ordering has been investigated in micro- and nanocrystalline Y- and Sc-doped ZrO2 and CeO2 by using high-resolution 89Y and 45Sc magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Our results indicate that irrespective of crystallite size the vacancies are preferentially associated with the host cation (i.e., Zr) in Y-doped ZrO2 while they display a preference for the dopant cation (i.e., Sc) in Sc-doped ZrO2. On the other hand, vacancies prefer to be associated with the dopant cation in both Y- and Sc-doped CeO2. However, the reduction of crystallite size to a few nanometers shows an unexpected and remarkable effect of increasing randomness in the vacancy distribution in all materials. Such an effect is hypothesized to result from a higher degree of short-range structural disorder in the cation coordination environments in nanocrystals compared to that in their microcrystalline counterparts that controls the energetics of vacancy ordering via a complex balance between electrostatic and strain energy terms. Finally, a clear connection is established between vacancy ordering, oxygen ion transport, and electrical conductivity in microcrystalline Y-doped CeO2 and its possible implications on ionic transport in nanocrystalline materials are discussed.




Probing Porosity and Pore Interconnectivity in Crystalline Mesoporous TiO2 Using Hyperpolarized 129Xe NMR

Li-Qiong Wang
*, Donghai Wang, Jun Liu and Gregory J. Exarhos, Shane Pawsey and Igor Moudrakovski
J. Phys. Chem. C, 2009, 113 (16), pp 6577–6583
DOI: 10.1021/jp809740e

Abstract: Hyperpolarized (HP) 129Xe NMR was used to probe the porosity and interconnectivity of pores in crystalline mesoporous TiO2. We have demonstrated that HP 129Xe NMR can be used to differentiate between similar sized pores within different crystalline phases. Pores of 4 nm size resident in mixed anatase and rutile mesoporous TiO2 phases were identified. Complementary to other pore characterization techniques, HP 129Xe NMR is able to probe the interconnectivity between pores present in these different phases. The cross peaks in 2D exchange (EXSY) NMR spectra between the signals of xenon in two types of pores are visible on millisecond timescale, indicating substantial pore interconnectivity. The obtained information on porosity and interconnectivity is important for the understanding of ion transport mechanisms in mesoporous TiO2 anode materials.




Structure, Activity, and Stability of Triphenyl Phosphine-Modified Rh/SBA-15 Catalyst for Hydroformylation of Propene: A High-Resolution Solid-State NMR Study

Xijie Lan, Weiping Zhang
*, Li Yan, Yunjie Ding, Xiuwen Han, Liwu Lin and Xinhe Bao*
J. Phys. Chem. C, 2009, 113 (16), pp 6589–6595
DOI: 10.1021/jp810432p
Copyright © 2009 American Chemical Society

Abstract: A ligand (triphenyl phosphine, PPh3)-modified heterogeneous PPh3−Rh(CO)/SBA-15 catalyst and supported Wilkinson complex HRh(CO)(PPh3)3/SBA-15 catalyst were prepared and examined in the hydroformylation of propene. Heterogeneous PPh3−Rh(CO)/SBA-15 catalyst showed much higher activity and stability in this reaction. Multinuclear 1H, 29Si, 31P, and 17O MAS NMR and two-dimensional 17O MQ MAS NMR together with XRD and N2 adsorption were employed to study the local structures of these two catalysts. Quantitative 1H and 29Si MAS NMR and qualitative one- and two-dimensional 17O MAS and MQ MAS NMR indicate that in the presence of CO the silanols on the surface of SBA-15 can react with rhodium carbonyls to form the Si−O−Rh bonds at the interface between the catalyst and the support. 31P MAS NMR spectra demonstrate a similar Wilkinson complex structure is produced on the heterogeneous PPh3−Rh(CO)/SBA-15 catalyst. The formation of Si−O−Rh bonds at the interface may immobilize the Rh complex during the long reaction. These may be correlated to the higher performances of heterogeneous PPh3−Rh(CO)/SBA-15 catalyst in propene hydroformylation




Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations

Daniel Weber, Jonathan Mitchell
*, James McGregor and Lynn F. Gladden
J. Phys. Chem. C, 2009, 113 (16), pp 6610–6615
DOI: 10.1021/jp811246j
Copyright © 2009 American Chemical Society

Abstract
Two-dimensional nuclear magnetic resonance (NMR) relaxation time correlation measurements have been used to observe the behavior of liquids inside porous catalyst pellets; in particular, liquids of relevance to the hydrogenation of 2-butanone over a silica-supported ruthenium catalyst (Ru/SiO2). The behavior of 2-butanone is studied and compared to that of water and 2-propanol, which are used as solvents in this hydrogenation reaction. From the ratio of NMR relaxation times, T1/T2, for the liquids confined in the pores, it is possible to infer the relative strengths of the surface interaction for each liquid. Water is seen to have the strongest surface interaction, and 2-butanone has the weakest surface interaction. These results are supported by displacement experiments, in which one liquid replaces the other over time within the pore space of the catalyst. For comparison, the behavior of the same liquids in an alumina-supported palladium catalyst (Pd/Al2O3) was also studied. The variation in the strengths of surface interactions was more pronounced in the Pd/Al2O3 catalyst than in the Ru/SiO2 catalyst. This work demonstrates the applicability of NMR relaxation time correlation experiments to real catalytic systems containing metallic components. From these measurements, information on the access of reactants to surface adsorption sites can be inferred.





Characterization of HNbWO6 and HTaWO6 Metal Oxide Nanosheet Aggregates As Solid Acid Catalysts

Caio Tagusagawa
, Atsushi Takagaki, Shigenobu Hayashi§ and Kazunari Domen*
J. Phys. Chem. C, 2009, 113 (18), pp 7831–7837

Abstract: Nanosheet aggregates prepared from protonated layered tungstates HMWO6 (M = Nb, Ta) are examined as potential solid acid catalysts. The nanosheet aggregates are formed by soft chemical processing of the layered compound with tetra(n-butylammonium) hydroxide, and the catalytic activity and acid strength of the aggregates are compared with those for HTiNbO5, HNb3O8, and a range of conventional solid acids. The catalytic activity for the Friedel−Crafts alkylation of anisole in the presence of benzyl alcohol increases in the order HTiNbO5 < m =" Nb," m =" Ti,">




First-Principles Nuclear Magnetic Resonance Structural Analysis of Vitreous Silica

Thibault Charpentier
*, Peter Kroll and Francesco Mauri§
J. Phys. Chem. C, 2009, 113 (18), pp 7917–7929
DOI: 10.1021/jp900297r

Abstract:Gauge including projector augmented wave (GIPAW) NMR calculations combined with hybrid Monte Carlo/molecular dynamics simulations are carried out in order to investigate the relationships between the oxygen-17 and silicon-29 NMR spectra of vitreous silica and its local structure in terms of the Si−O−Si bond angle and Si−O distance distributions. Special attention is paid to the structure and NMR parameters of three- and four-membered rings, and the effect of their concentration on glass density is studied. It is shown that our simulations provide a new insight into the features of the 17O NMR parameters distribution. Accordingly, a new analytical model is presented and applied for the reconstruction of the Si−O−Si angle from the NMR spectrum. The reliability of the procedure is demonstrated conclusively through the excellent consistency of the analysis of the oxygen-17 and silicon-29 NMR experimental data of vitreous silica. Si−O−Si angle distribution mean values of 147.1° and 148.4°, respectively, and standard deviations of 11.2° and 10.8°, respectively, are obtained from the oxygen-17 and silicon-29 NMR experimental spectrum (Clark et al., ref
4) of the same sample.




SBE-Type Metal-Substituted Aluminophosphates: Detemplation and Coordination Chemistry

Daphne S. Bel n-Cordero
, Chul Kim§, Son-Jong Hwang and Arturo J. Hern ndez-Maldonado*
J. Phys. Chem. C, 2009, 113 (19), pp 8035–8049

Abstract: The detemplation process in Me-SBE (Me = Co2+, Mg2+, and Mn2+) aluminophosphates was studied to elucidate materials stability and framework characteristics. In addition, the hydrothermal synthesis conditions were optimized to obtain materials with minimal phase impurities. This was accomplished by means of decreasing reaction temperature and increasing aging periods. Scanning electron microscopy analysis of the Mg- and Mn-SBE as-synthesized samples revealed square plates with truncated corner morphologies grown in aggregated fashion and contrasting with the previously reported hexagonal platelike morphology of Co-SBE. Cautious detemplation in vacuum, using an evacuation rate of 10 mmHg/s and a temperature of 648 K, resulted in surface areas of about 700, 500, and 130 m2/g for Mg-, Co-, and Mn-SBE, respectively. Thermal gravimetric analysis and in situ high-temperature powder X-ray diffraction analyses indicate the frameworks for all of the SBE variants experienced collapse upon treatment with helium at temperatures above 700 K and subsequently formed an aluminophosphate trydimite dense phase. Detemplation in air at all times resulted in framework destruction during detemplation. In situ differential scanning calorimetry−powder X-ray diffraction data showed that the SBE frameworks experience breathing modes related to specific endothermic and exothermic scenarios during air treatment. Decomposition and elimination of the organic template during vacuum treatment was verified by Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy revealed that most of the Co atoms in vacuum-treated samples are in tetrahedral coordination, while the Mn atoms exhibit various coordination states. Ultraviolet-visible, electron paramagnetic resonance, and magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy corroborated the latter result in addition to providing evidence for the formation of Mn extra framework species. 27Al MAS NMR spectra for vacuum-detemplated Mg-SBE samples prior to and after dehydration confirmed the reversible formation of aluminum octahedral sites. This, however, did not affect the porous nature of detemplated Mg-SBE samples as these are capable of adsorbing 19 water molecules per super cage at 298 K.



Low Temperature 1H MAS NMR Spectroscopy Studies of Proton Motion in Zeolite HZSM-5

Hua Huo
, Luming Peng and Clare P. Grey*
J. Phys. Chem. C, 2009, 113 (19), pp 8211–8219
Abstract:
Low temperature 1H MAS NMR spectroscopy is used to study protonic motion in zeolite HZSM-5 in both samplethat have been dried using procedures that are standard in the literature and samples that have been more carefully dehydrated. A significant enhancement of proton mobility is seen for the “standard” dehydrated HZSM-5 sample in comparison to that seen for the much drier sample. This is ascribed to a vehicle-hopping mechanism involving the residual water that is present in these zeolites. A gradual change of the framework structure is observed on cooling to approximately 213 K, as monitored via the change in 1H chemical shift values of the Br√łnsted acid resonances and by X-ray diffraction. A more sudden change in structure is seen by differential scanning calorimetry and NMR at approximately 220−230 K, which is associated with changes in both the mobility and the modes of binding of the residual water to the Br√łnsted acid sites and the zeolite framework.



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