Enhancing the central-transition NMR signal of quadrupolar nuclei by spin population transfer using SW-FAM pulse trains with a tangent-shaped sweep profile

Advanced solid-state NMR spectroscopy and its applications in zeolite chemistry

Solid-state NMR spectroscopy (ssNMR) can provide details about the structure, host-guest/guest-guest interactions and dynamic behavior of materials at atomic length scales. A crucial use of ssNMR is for the characterization of zeolite catalysts that are extensively employed in industrial catalytic processes. This review aims to spotlight the recent advancements in ssNMR spectroscopy and its application to zeolite chemistry. We first review the current ssNMR methods and techniques that are relevant to characterize zeolite catalysts, including advanced multinuclear and multidimensional experiments, in situ NMR techniques and hyperpolarization methods. Of these, the methodology development on half-integer quadrupolar nuclei is emphasized, which represent about two-thirds of stable NMR-active nuclei and are widely present in catalytic materials. Subsequently, we introduce the recent progress in understanding zeolite chemistry with the aid of these ssNMR methods and techniques, with a specific focus on the investigation of zeolite framework structures, zeolite crystallization mechanisms, surface active/acidic sites, host-guest/guest-guest interactions, and catalytic reaction mechanisms.

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 (C_Q). 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 C_Q 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.

Signal enhancement in solid-state NMR of quadrupolar nuclei

Recent progress in the development and application of signal enhancement methods for NMR of quadrupolar nuclei in solids is presented. First, various pulse schemes for manipulating the populations of the satellite transitions in order to increase the signal of the central transition (CT) in stationary and rotating solids are evaluated (e.g., double-frequency sweeps, hyperbolic secant pulses). Second, the utility of the quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) and WURST-QCPMG pulse sequences for the rapid and efficient acquisition of particularly broad CT powder patterns is discussed. Third, less frequently used experiments involving polarization transfer from abundant nuclear spins (cross-polarization) or from unpaired electrons (dynamic nuclear polarization) are assessed in the context of recent examples. Advantages and disadvantages of particular enhancement schemes are highlighted and an outlook on possible future directions for the signal enhancement of quadrupolar nuclei in solids is offered.