Thursday, February 19, 2009

J Phys Chem C, Vol 113, Issue 7

Ordered and Hydrothermally Stable Cubic Periodic Mesoporous Organosilicas with SBA-1 Mesostructures: Synthesis, Characterization, Solid-State NMR Spectroscopy, and DFT Calculations
Yu-Chi Pan
, Hao-Yiang Wu, Guang-Liang Jheng, Hui-Hsu Gavin Tsai* and Hsien-Ming Kao*

J. Phys. Chem. C, 2009, 113 (7), pp 2690–2698
Abstract: Periodic mesoporous organosilicas (PMOs) based on the cubic SBA-1 mesostructure (Pm3n mesophase) were synthesized by co-condensation of tetraethoxysilane (TEOS) and 1,2-bis(triethoxysilyl)ethane (BTEE) under acidic conditions using cetyltriethylammonium bromide (CTEABr) as a structure-directing agent. The ethane-bridged PMO materials thus obtained were characterized by powder X-ray diffraction (XRD), solid-state 13C and 29Si NMR spectroscopy, thermogravimetric analysis (TGA), and nitrogen sorption measurements. The maximum BTEE contents that can be incorporated into the pore wall without degrading the Pm3n mesostructure were up to 60% (based on silica). The resulting materials were hydrothermally stable up to 120 h in boiling water with only a slight decrease in their structural properties, whereas the structure of the pure silica counterpart SBA-1 material was completely collapsed after such treatment. The presence of the ethane groups in the mesoporous wall led to a more hydrophobic environment and thus enhancement of hydrothermal stability, as revealed by water adsorption. The combined results of 2D 29Si{1H} heteronuclear correlation (HETCOR) NMR and density functional theory calculations suggested that the T3−T2−Q4−Q3 motif could be the favorable framework building unit in PMOs.

Studies of the Active Sites for Methane Dehydroaromatization Using Ultrahigh-Field Solid-State 95Mo NMR Spectroscopy
Jian Zhi Hu
*, Ja Hun Kwak, Yong Wang, Charles H. F. Peden*, Heng Zheng§, Ding Ma and Xinhe Bao

J. Phys. Chem. C, 2009, 113 (7), pp 2936–2942
Abstract: In this contribution, we show that the spin−lattice relaxation time, T1, corresponding to zeolite exchanged molybdenum species in Mo/HZSM-5 catalysts is about 2 orders of magnitude shorter than the corresponding T1 for small MoO3 crystallites. Such a difference is utilized to differentiate the exchanged Mo species from MoO3 agglomerates in Mo/HZSM-5 catalysts and to readily estimate their relative fractions present in catalysts with varying Mo loading. A good linear correlation between the amount of zeolite exchanged species and the aromatics formation rate during catalytic methane dehydroaromatization is obtained. This result significantly strengthens our prior conclusion that the exchanged Mo species are the active centers for this reaction on Mo/HZSM-5 catalysts (J. Am. Chem. Soc. 2008, 130, 3722−3723). Of more general interest for Mo-exchanged zeolites, the results may provide useful data for analyzing the binding of exchanged Mo species in zeolite cages. In particular, the NMR data suggest a possible saturation loading for the exchanged Mo species at a Mo/Al ratio of approximately 0.5 for the ZSM-5 zeolite used in this study (Si/Al = 25). Furthermore, for polycrystalline MoO3 powder samples, the parameters related to the electric field gradient (EFG) tensor, the chemical shift anisotropy (CSA), and the three Euler angles required to align the CSA principal axis system with the quadrupolar principal axis system are determined by analyzing both the magic angle spinning (MAS) and static 95Mo spectra. The new results obtained from this study on MoO3 powders should help to clarify some of the contradictions in prior literature reports of studies of Mo-containing solids by 95Mo NMR.

1 comment:

Anonymous said...

Aaron, who's covering Nature ?