Dynamic Nuclear Polarization Surface Enhanced NMR spectroscopy (DNP SENS): Principles, protocols, and practice

A Formulation Protocol with Pyridine to Enable DNP-SENS on Reactive Surface Sites: Case Study with Olefin Polymerization and Metathesis Catalysts

“…It is thus not surprising that this approach has gained importance to characterize the surfaces of materials and has been used to detect reaction intermediates in heterogeneous catalysts. [5,[8][9][10][11] The main principle of DNP-SENS lies in exploiting the large polarization of electrons and to transfer this polarization to targeted nuclei (with a nuclear spin) at the surface of a material [9].…”

“…An efficient DNP formulation requires that upon freezing at 100 K, a homogenous distribution of the polarizing source (the radical) into a glassy matrix is obtained. [5,9] It also requires that the polarizing agents (the radicals) do not react or even interact with the substrate as the latter would lead to (paramagnetic) signal quenching. Such experiments can be done in aqueous or non-aqueous environment depending on the material(s) of interest.…”

“…Such experiments can be done in aqueous or non-aqueous environment depending on the material(s) of interest. [9,12] Enhancements up to 250 have been obtained with TEKPol radical in TCE (1,1,2,2tetrachloroethane) solution. [9,12,13] While a broad range of polarizing agents are now available, these exogeneous radicals and the polarization solution may not be innocent (interaction with the surface) and can even be reactive towards the surface sites, such as organometallic fragments, [14, 15] radicals and/or acid sites present on the surfaces, eventually quenching the NMR signals as evidenced by the lack of DNP enhancement of the frozen solution [10].…”

“…[9,12] Enhancements up to 250 have been obtained with TEKPol radical in TCE (1,1,2,2tetrachloroethane) solution. [9,12,13] While a broad range of polarizing agents are now available, these exogeneous radicals and the polarization solution may not be innocent (interaction with the surface) and can even be reactive towards the surface sites, such as organometallic fragments, [14, 15] radicals and/or acid sites present on the surfaces, eventually quenching the NMR signals as evidenced by the lack of DNP enhancement of the frozen solution [10]. Thus, although DNP-SENS has already helped to characterize a broad range of catalytic materials, the choice of the DNP formulation is critical, and the exploration of DNP formulation compatible with reactive surfaces remains an important goal [9,[16][17][18][19][20].…”

“…[9,12,13] While a broad range of polarizing agents are now available, these exogeneous radicals and the polarization solution may not be innocent (interaction with the surface) and can even be reactive towards the surface sites, such as organometallic fragments, [14, 15] radicals and/or acid sites present on the surfaces, eventually quenching the NMR signals as evidenced by the lack of DNP enhancement of the frozen solution [10]. Thus, although DNP-SENS has already helped to characterize a broad range of catalytic materials, the choice of the DNP formulation is critical, and the exploration of DNP formulation compatible with reactive surfaces remains an important goal [9,[16][17][18][19][20]. Several strategies have already been explored, comprising the use of mesoporous supports in combination with bulky radicals to protect the active sites present within the mesopores [21,22] or the use of dendritic radicals that cannot interact with the reactive sites.…”