DNP enhanced solid state NMR of green and sustainable materials

Lead Research Organisation: University of Nottingham
Department Name: Sch of Chemistry

Abstract

Project background (identification of the problem and its importance and relevance to sustainability)
For the development of green and sustainable materials, greater understanding of structure-property relationships will help to streamline their design and synthesis allowing for more efficient processes. To gain this deep understanding, advanced analytical techniques are required.

Solid-state NMR is a powerful technique for studying the molecular-level detail of complex and heterogeneous materials. However, even with the high magnetic fields available today, solid-state NMR suffers from low sensitivity, because of the small nuclear spin polarizations involved, so that long acquisitions or large samples are required. Fortunately, weak NMR signals can be enhanced at low temperatures (~100 K) by dynamic nuclear polarization (DNP) in which the large electron spin polarization from an implanted radical is transferred to nearby nuclei. Progress with high-power microwave sources (gyrotrons) has made DNP possible at the high fields found in modern NMR spectrometers. Large signal enhancements (up to 300-fold) have been achieved for frozen biomolecules, corresponding to a reduction by a factor of 100,000 in experiment time.
More recently, DNP-enhanced NMR signals have been recorded for the functionalizing groups at the pore surfaces in meso-structured hybrid silica materials using radicals implanted from a solution impregnated into the pores. Despite the modest (20-fold) enhancement, structural details, such as the conformation of the functionalizing groups and their distribution were established from 13C and 29Si NMR spectra. This study triggered a revolution in materials NMR, making many new applications feasible for the first time.

DNP has the potential to transform solid-state NMR into the technique of choice for the characterization of green and sustainable materials. Currently, most DNP studies of this type involve impregnating the sample with a solution of a specially designed biradical with polarization transfer to the nuclei of interest, followed by polarization transport through the solvent matrix. This process is a critical factor, with changes in solvent, radical concentration, sample morphology and surface area etc. requiring empirical optimization to maximize the enhancement. The sample preparation step represents a problem for DNP-enhanced solid-state NMR, since the resulting enhancement is not reproducible, and the impregnating solution may not be compatible with the material of interest.

To improve the applicability of DNP-enhanced solid-state NMR this project will develop new sample preparation techniques. These new techniques will be used to analyse a variety of green and sustainable materials, chosen from research ongoing in the CDT, including polymers, metal organic framework materials and catalysts.

The new sample preparation techniques proposed are:
1) Supercritical carbon dioxide as an impregnation solvent.
2) Mesocellular foams as DNP matrices
(details about each technique available on request)

Areas: Analytical Science, Catalysis, Energy Storage, Functional Ceramics and Inorganics, Materials for Energy Applications, Polymer Materials and Surface Science

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/S515528/1 01/10/2018 30/09/2022
2115050 Studentship EP/S515528/1 01/10/2018 30/09/2022 Alison Daly McLellan