Through-space 11 B-27 Al correlation: Influence of the recoupling channel.

Through-space heteronuclear correlation (D-HETCOR) experiments based on heteronuclear multiple-quantum correlation (D-HMQC) and refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) sequences have been proven to be useful approaches for the detection of the spatial proximity between half-integer quadrupolar nuclei in solids under magic-angle spinning (MAS) conditions. The corresponding pulse sequences employ coherence transfers mediated by heteronuclear dipolar interactions, which are reintroduced under MAS by radiofrequency irradiation of only one of the two correlated nuclei. We investigate herein using numerical simulations of spin dynamics and solid-state NMR experiments on magnesium aluminoborate glass how the choice of the channel to which the heteronuclear dipolar recoupling is applied affects the transfer efficiency of D-HMQC and D-RINEPT sequences between 11 B and 27 Al nuclei. Experimental results show that maximum transfer efficiency is achieved when the recoupling scheme is applied to the channel, for which the spin magnetization is parallel to the B0 axis in average.

Significant promotion effect of the rutile phase on V2O5/TiO2 catalysts for NH3-SCR.

A large amount of polymeric vanadyl species owing to higher interaction energy between vanadia and anatase than rutile and the synergistic effect of vanadium oxides, anatase and rutile TiO2 contributes to an excellent NH3-SCR activity of the vanadia-based catalysts with high rutile content and low specific surface area.

Resolving the puzzle of single-atom silver dispersion on nanosized γ-Al2O3 surface for high catalytic performance.

Ag/γ-Al2O3 is widely used for catalyzing various reactions, and its performance depends on the valence state, morphology and dispersion of Ag species. However, detailed anchoring mechanism of Ag species on γ-Al2O3 remains largely unknown. Herein, we reveal that the terminal hydroxyls on γ-Al2O3 are responsible for anchoring Ag species. The abundant terminal hydroxyls existed on nanosized γ-Al2O3 can lead to single-atom silver dispersion, thereby resulting in markedly enhanced performance than the Ag cluster on microsized γ-Al2O3. Density-functional-theory calculations confirm that Ag atom is mainly anchored by the terminal hydroxyls on (100) surface, forming a staple-like local structure with each Ag atom bonded with two or three terminal hydroxyls. Our finding resolves the puzzle on why the single-atom silver dispersion can be spontaneously achieved only on nanosized γ-Al2O3, but not on microsized γ-Al2O3. The obtained insight into the Ag species dispersion will benefit future design of more efficient supported Ag catalysts.

The acidic nature of "NMR-invisible" tri-coordinated framework aluminum species in zeolites.

The unambiguous characterization of different acid sites in zeolites is of great importance for understanding their catalytic performance and the rational design of highly efficient zeolite catalysts. In addition to various well-characterized extra-framework Al species, a tri-coordinated framework aluminum species can also serve as a Lewis acid site in zeolites, which is "NMR-invisible" owing to its extremely distorted local environment. Here we provide a feasible and reliable approach to elucidate the acidic nature of the tri-coordinated framework Al in dehydrated H-ZSM-5 zeolites via sensitivity-enhanced two-dimensional (2D) multiple nuclear correlation NMR experiments coupled with trimethylphosphine oxide (TMPO) probe molecules. Two types of tri-coordinated framework Al sites have been unambiguously identified, which amount to 11.6% of the total Brønsted and Lewis acid sites. Furthermore, it was found that synergistic effects arising from the close spatial proximity between the tri-coordinated framework Al site and the Brønsted acid site lead to the generation of superacidity (with an acid strength stronger than 100% H2SO4) in the zeolite.

Uniform signal enhancement in MAS NMR of half-integer quadrupolar nuclei using quadruple-frequency sweeps.

We introduce two MAS schemes that allow manipulating the satellite-transition (ST) populations of half-integer quadrupolar nuclei, and which both exhibit improved robustness to the quadrupolar coupling constant (CQ). These schemes, called quadruple frequency sweep (QFS) or quadruple WURST (QWURST) are the sums of two DFS or four WURST to efficiently invert the ST populations of nuclei subject to large or small quadrupole interactions, simultaneously. These quadruple sweeps methods only require 6% more rf-power than the double sweeps ones. We demonstrate, both numerically and experimentally, that the QFS and QWURST schemes benefit from robustness to CQ and rf amplitude and offset and hence achieve uniform enhancement of the CT signal for 27Al nuclei subject to different quadrupole interactions. Although the version of QFS with repetitive accumulation can achieve higher enhancement in the S/N of the 27Al MAS spectrum, the final sensitivity gains mainly depend on the longitudinal relaxation time of different 27Al sites. We also confirm that these schemes provide an improved acceleration of the 31P-{27Al} coherence transfer in PT-J-HMQC experiments.

Heteronuclear correlation experiments of 23Na-27Al in rotating solids.

We demonstrated that the heteronuclear correlation experiments between two quadrupolar nuclei, 23Na and 27Al, with close Larmor frequencies can be achieved via D-HMQC and D-RINEPT approaches by using a diplexer connected to a conventional probe in magic-angle-spinning solid-state NMR. Low-power heteronuclear dipolar recoupling schemes can be applied on 23Na or 27Al to establish polarization transfers between the central transitions of 23Na and 27Al for a model compound, NaAlO2. Further, we showed a practical implementation of the two dimensional 23Na-27Al dipolar-based heteronuclear correlation experiment on a heterogeneous catalyst, Na2CO3/γ-Al2O3. This allows to determine spatial proximities between different 23Na and 27Al sites, thus the surface Na species adjacent to octahedral-coordination Al can be clearly discriminated.

Insights of the Crystallization Process of Molecular Sieve AlPO4-5 Prepared by Solvent-Free Synthesis.

Crystallization of AlPO4-5 with AFI structure under solvent-free conditions has been investigated. Attention was mainly focused on the characterization of the intermediate phases formed at the early stages during the crystallization. The development in the long-range ordering of the solid phases as a function of crystallization time was monitored by XRD, SEM, IR, UV-Raman, and MAS NMR techniques. Particularly, the UV-Raman spectroscopy was employed to obtain the information on the formation process of the framework. J-HMQC (27)Al/(31)P double-resonance NMR experiments were used to identify the P-O-Al bonded species in the intermediate phases. For the first time the P-O-Al bonded species in the intermediate phases can be correctly described through using this advanced NMR technique. The crystallization under solvent-free conditions appears to follow the pathway: The initial amorphous raw material is converted to an intermediate phase which has four-/six-membered ring species, then gradually transformed into crystalline AlPO4-5. This observation is not consistent with the common idea that the intermediate phase is the semicrystalline intermediates with a three-dimensional structure.