No abstract

We present evidence for the dehydrogenation of zeolite Brønsted acid sites at temperatures above 500 °C forming hydrogen gas and a [AlO4]0 site that could participate in one-electron oxidations or react further to form extraframework aluminum.

A combination of experimental and computational methods has been used to investigate the effects of vanadium doping in ETS-10. Near edge X-ray absorption fine structure (NEXAFS) spectra reveal octahedrally coordinated VIV and VV species within V-doped ETS-10 materials, confirming substitution for TiIV sites only. Computational models, using hybrid density functional theory/molecular mechanics (DFT/MM) methods, have been developed that contain varying concentrations of VIV and VV within the O−M−O (M = Ti, V) chain. Geometry optimizations indicate that VV substitution leads to larger changes in the local chain geometry than VIV substitution. Substitution energetics for VIV and VV in different sites have been calculated to determine preferred locations of the two species, suggesting that long chains of VV are not stable and demonstrating the need for both VV and VIV within V-substituted materials. Wavefunctions for systems with an electron added or removed are used to identify electron and hole trapping sites associated with the VV and VIV doping centers respectively. An increase in photocatalytic activity is predicted at low [V] due to improved charge separation. However photocatalytic activity is expected to decrease at high [V] due to increased carrier recombination. These results are consistent with recent experimental data.

Various amounts of vanadium have been isomorphously substituted for titanium in ETS-10, creating samples with V/(V+Ti) ratios of 0.13, 0.33, 0.43, and 1.00 and characterized experimentally using Raman, near-edge X-ray absorption fine structure (NEXAFS), X-ray powder diffraction, N 2 adsorption, scanning electron microscopy (SEM), UV/vis spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Raman spectra reveal a disordered chain structure that contains different V-O bonds along with the presence of a V-O-Ti peak. The UV/vis spectra of the vanadium samples have three new absorption features in the visible region at 450, 594, and 850 nm, suggesting both V 4+ and V 5+ are present in the samples. NEXAFS results confirm the presence of both V 5+ and V 4+ in the vanadium samples, with a fraction of V 4+ within the range of 0.2-0.4. The addition of vanadium lowers the band gap energy of ETS-10 from 4.32 eV to a minimum of 3.58 eV for the 0.43ETVS-10 sample. Studies of the photocatalytic polymerization of ethylene show that the 594 nm transition has no photocatalytic activity. The visible transition around 450 nm in the vanadium-incorporated samples is photocatalytically active, and the lower-concentration vanadium samples have higher photocatalytic activity than that of ETS-10 and AM-6, the all-vanadium analogue of ETS-10.

Microporous vanadium-substituted titanosilicate ETS-10 solids are promising photocatalysts for decomposition of organic molecules. The dopant vanadium metal modulates the electronic environment of the titanosilicate matrix and plays a major role in the enhancement of the photocatalytic activity. However, the local electronic and geometric structure of the vanadium sites in these materials is a subject of controversy. Using vanadium electron paramagnetic resonance (EPR) and 51 V nuclear magnetic resonance (NMR) spectroscopy, we have characterized the local environments of the vanadium sites in vanadium-substituted ETS-10 samples with different vanadium loadings. The measurements reveal clearly the presence of V(IV) and V(V) oxidation states. The EPR results suggest that V(IV) is in octahedral sites and, therefore, must substitute for Ti in the framework. 51 V NMR studies indicate that the V(V) species are adjacent to the V(IV) species in most cases on the basis of significant electron-nuclear dipolar interaction between the V(V) nuclei and the unpaired electron on V(IV). The NMR chemical shift and electric field gradient parameters estimated from the NMR spectra are used in conjunction with density functional theory calculations to propose a model where the V(V) species preferentially occupy sites at the ends of the octahedral chains.