Polarization Surface Enhanced NMR Spectroscopy Research Articles

INTERFACES. A program for determining the 3D structures of surfaces sites using NMR data

Dynamic nuclear polarization surface enhanced NMR spectroscopy has enabled the determination of high-resolution structures from surface-supported molecules, including single-site heterogeneous catalysts. Structure determinations have largely mimicked the approaches used in biomolecular NMR spectroscopy, namely, using distance measurements to constrain a conformational search. These early demonstrations made use of purpose-built software, which has limited the adoption of the technique. Herein, we describe the open-source program INTERFACES (Interpret NMR to Elucidate or Reconstruct the Full Atomistic Configurations of External Surfaces) which automates the analysis of RE(SP)DOR data as well as the structure determination for surface sites. Distances, angles, dihedral angles, complex orientation, and distance from the support can all be sampled to find all structures that agree with the experimental data. A χ2 metric is used to define the error ranges of the REDOR fits and produce structures with an arbitrary level of confidence. Structural solutions are then provided as both overlays and ORTEP-like probability ellipsoids.

Uncovering selective and active Ga surface sites in gallia-alumina mixed-oxide propane dehydrogenation catalysts by dynamic nuclear polarization surface enhanced NMR spectroscopy.

Gallia–alumina (Ga,Al)2O3(x : y) spinel-type solid solution nanoparticle catalysts for propane dehydrogenation (PDH) were prepared with four nominal Ga : Al atomic ratios (1 : 6, 1 : 3, 3 : 1, 1 : 0) using a colloidal synthesis approach. The structure, coordination environment and distribution of Ga and Al sites in these materials were investigated by X-ray diffraction, X-ray absorption spectroscopy (Ga K-edge) as well as 27Al and 71Ga solid state nuclear magnetic resonance. The surface acidity (Lewis or Brønsted) was probed using infrared spectroscopy with pyridine and 2,6-dimethylpyridine probe molecules, complemented by element-specific insights (Ga or Al) from dynamic nuclear polarization surface enhanced cross-polarization magic angle spinning 15N{27Al} and 15N{71Ga} J coupling mediated heteronuclear multiple quantum correlation NMR experiments using 15N-labelled pyridine as a probe molecule. The latter approach provides unique insights into the nature and relative strength of the surface acid sites as it allows to distinguish contributions from Al and Ga sites to the overall surface acidity of mixed (Ga,Al)2O3 oxides. Notably, we demonstrate that (Ga,Al)2O3 catalysts with a high Al content show a greater relative abundance of four-coordinated Ga sites and a greater relative fraction of weak/medium Ga-based surface Lewis acid sites, which correlates with superior propene selectivity, Ga-based activity, and stability in PDH (due to lower coking). In contrast, (Ga,Al)2O3 catalysts with a lower Al content feature a higher fraction of six-coordinated Ga sites, as well as more abundant Ga-based strong surface Lewis acid sites, which deactivate through coking. Overall, the results show that the relative abundance and strength of Ga-based surface Lewis acid sites can be tuned by optimizing the bulk Ga : Al atomic ratio, thus providing an effective measure for a rational control of the catalyst performance.

The Structure of Molecular and Surface Platinum Sites Determined by DNP-SENS and Fast MAS 195Pt Solid-State NMR Spectroscopy.

The molecular level characterization of heterogeneous catalysts is challenging due to the low concentration of surface sites and the lack of techniques that can selectively probe the surface of a heterogeneous material. Here, we report the joint application of room temperature proton-detected NMR spectroscopy under fast magic angle spinning (MAS) and dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS), to obtain the 195Pt solid-state NMR spectra of a prototypical example of highly dispersed Pt sites (single site or single atom), here prepared via surface organometallic chemistry, by grafting [(COD)Pt(OSi(OtBu)3)2] (1, COD = 1,5-cyclooctadiene) on partially dehydroxylated silica (1@SiO2). Compound 1@SiO2 has a Pt loading of 3.7 wt %, a surface area of 200 m2/g, and a surface Pt density of around 0.6 Pt site/nm2. Fast MAS 1H{195Pt} dipolar-HMQC and S-REDOR experiments were implemented on both the molecular precursor 1 and on the surface complex 1@SiO2, providing access to 195Pt isotropic shifts and Pt-H distances, respectively. For 1@SiO2, the measured isotropic shift and width of the shift distribution constrain fits of the static wide-line DNP-enhanced 195Pt spectrum, allowing the 195Pt chemical shift tensor parameters to be determined. Overall the NMR data provide evidence for a well-defined, single-site structure of the isolated Pt sites.

Imine Metathesis by Silica-Supported Catalysts Using the Methodology of Surface Organometallic Chemistry.

With this protocol, a well-defined singlesite silica-supported heterogeneous catalyst [(≡Si-O-)Hf(=NMe)(η1-NMe2)] is designed and prepared according to the methodology developed by surface organometallic chemistry (SOMC). In this framework, catalytic cycles can be determined by isolating crucial intermediates. All air-sensitive materials are handled under inert atmosphere (using gloveboxes or a Schlenk line) or high vacuum lines (HVLs, <10-5 mbar). The preparation of SiO2-700 (silica dehydroxylated at 700 °C) and subsequent applications (the grafting of complexes and catalytic runs) requires the use of HVLs and double-Schlenk techniques. Several well-known characterization methods are used, such as Fourier-transform infrared spectroscopy (FTIR), elemental microanalysis, solid-state nuclear magnetic resonance spectroscopy (SSNMR), and state-of-the-art dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS). FTIR and elemental microanalysis permit scientists to establish the grafting and its stoichiometry. 1H and 13C SSNMR allows the structural determination of the hydrocarbon ligands coordination sphere. DNP SENS is an emerging powerful technique in solid characterization for the detection of poorly sensitive nuclei (15N, in our case). SiO2-700 is treated with about one equivalent of the metal precursor compared to the amount of surface silanol (0.30 mmol·g-1) in pentane at room temperature. Then, volatiles are removed, and the powder samples are dried under dynamic high vacuum to afford the desired materials [(≡Si-O-)Hf(η2π-MeNCH2)(η1-NMe2)(η1-HNMe2)]. After a thermal treatment under high vacuum, the grafted complex is converted into metal imido silica complex [(≡Si-O-)Hf(=NMe)(η1-NMe2)]. [(≡Si-O-)Hf(=NMe)(η1-NMe2)] effectively promotes the metathesis of imines, using the combination of two imine substrates, N-(4-phenylbenzylidene)benzylamine, or N-(4-fluorobenzylidene)-4-fluoroaniline, with N-benzylidenetert-butylamine as substrates. A significantly lower conversion is observed with the blank runs; thus, the presence of the imido group in [(≡Si-O-)Hf(=NMe)(η1-NMe2)] is correlated to the catalytic performance.

Discerning γ-Alumina Surface Sites with Nitrogen-15 Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy of Adsorbed Pyridine

Low-temperature 15N dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) of adsorbed pyridine in combination with FTIR measurements and DFT calculations was applied to investigate the surface sites of γ-alumina, which can be divided into four groups: (1) groups 1 and 2, associated with less shielded 15N chemical shifts and the lowest ν(8a) frequencies of adsorbed pyridine, corresponding to weakly adsorbed H-bonding pyridine to hydroxyl group or chemisorbed water on alumina; (2) group 3, with intermediate 15N chemical shifts and ν(8a) frequencies, corresponding to pyridine coordinated to specific Lewis acid sites, namely five-coordinated (AlV) aluminum atoms of the (100) facet, as well as some H-bonded pyridine; (3) group 4, associated with the most shielded 15N chemical shift and the highest ν(8a) frequency, selectively assigned to Lewis acid Al sites located on the (110) facet and corresponding to both four- and five-coordinated aluminum atoms (AlIV and AlV). Noteworthy, a correlatio...

In Situ XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca3Al2O6-Stabilized CO2 Sorbents

CaO is an effective high temperature CO2 sorbent that, however, suffers from a loss of its CO2 absorption capacity upon cycling due to sintering. The cyclic CO2 uptake of CaO-based sorbents is improved by Ca3Al2O6 as a structural stabilizer. Nonetheless, the initially rather stable CO2 uptake of Ca3Al2O6-stabilized CaO yet starts to decay after around 10 cycles of CO2 capture and sorbent regeneration, albeit at a significantly reduced rate compared to the unmodified reference material. Here, we show by a combined use of in situ XRD together with textural and morphological characterization techniques (SEM, STEM, and N2 physisorption) and solid-state 27Al NMR (in particular dynamic nuclear polarization surface enhanced NMR spectroscopy, DNP SENS) how microscopic changes trigger the sudden onset of deactivation of Ca3Al2O6-stabilized CaO. After a certain number of CO2 capture and regeneration cycles (approximately 10), Ca3Al2O6 transformed into Ca12Al14O33, followed by Al2O3 segregation and enrichment at the...

Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity.

Identification of surfaces at the molecular level has benefited from progress in dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS). However, the technique is limited when using highly sensitive heterogeneous catalysts due to secondary reaction of surface organometallic fragments (SOMFs) with stable radical polarization agents. Here, we observe that in non-porous silica nanoparticles (NPs) (dparticle = 15 nm) some DNP enhanced NMR or SENS characterizations are possible, depending on the metal-loading of the SOMF and the type of SOMF substituents (methyl, isobutyl, neopentyl). This unexpected observation suggests that aggregation of the nanoparticles occurs in non-polar solvents (such as ortho-dichlorobenzene) leading to (partial) protection of the SOMF inside the interparticle space, thereby preventing reaction with bulky polarization agents. We discover that the DNP SENS efficiency is correlated with the hydrophilicity of the SOMF/support, which depends on the carbon and SOMF concentration. Nitrogen sorption measurements to determine the BET constant (CBET) were performed. This constant allows us to predict the aggregation of silica nanoparticles and consequently the efficiency of DNP SENS. Under optimal conditions, CBET > 60, we found signal enhancement factors of up to 30.

Correction to "Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy".

Correction to "Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy".

Three-Dimensional Structure Determination of Surface Sites.

The spatial arrangement of atoms is directly linked to chemical function. A fundamental challenge in surface chemistry and catalysis relates to the determination of three-dimensional structures with atomic-level precision. Here we determine the three-dimensional structure of an organometallic complex on an amorphous silica surface using solid-state NMR measurements, enabled through a dynamic nuclear polarization surface enhanced NMR spectroscopy approach that induces a 200-fold increase in the NMR sensitivity for the surface species. The result, in combination with EXAFS, is a detailed structure for the surface complex determined with a precision of 0.7 Å. We observe a single well-defined conformation that is folded toward the surface in such a way as to include an interaction between the platinum metal center and the surface oxygen atoms.

Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy.

Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) is an emerging technique that allows access to high-sensitivity NMR spectra from surfaces. However, DNP SENS usually requires the use of radicals as an exogenous source of polarization, which has so far limited applications for organometallic surface species to those that do not react with the radicals. Here we show that reactive surface species can be studied if they are immobilized inside porous materials with suitably small windows, and if bulky nitroxide bi-radicals (here TEKPol) are used as the polarization source and which cannot enter the pores. The method is demonstrated by obtaining significant DNP enhancements from highly reactive complelxes [([triple bond, length as m-dash]Si-O-)W(Me)5] supported on MCM-41, and effects of pore size (6.0, 3.0 and 2.5 nm) on the performance are discussed.

From single-site tantalum complexes to nanoparticles of Ta x N y and TaO x N y supported on silica: elucidation of synthesis chemistry by dynamic nuclear polarization surface enhanced NMR spectroscopy and X-ray absorption spectroscopy.

Air-stable catalysts consisting of tantalum nitride nanoparticles represented as a mixture of Ta x N y and TaO x N y with diameters in the range of 0.5 to 3 nm supported on highly dehydroxylated silica were synthesized from TaMe5 (Me = methyl) and dimeric Ta2(OMe)10 with guidance by the principles of surface organometallic chemistry (SOMC). Characterization of the supported precursors and the supported nanoparticles formed from them was carried out by IR, NMR, UV-Vis, extended X-ray absorption fine structure, and X-ray photoelectron spectroscopies complemented with XRD and high-resolution TEM, with dynamic nuclear polarization surface enhanced NMR spectroscopy being especially helpful by providing enhanced intensities of the signals of 1H, 13C, 29Si, and 15N at their natural abundances. The characterization data provide details of the synthesis chemistry, including evidence of (a) O2 insertion into Ta-CH3 species on the support and (b) a binuclear to mononuclear transformation of species formed from Ta2(OMe)10 on the support. A catalytic test reaction, cyclooctene epoxidation, was used to probe the supported nanoparticles, with 30% H2O2 serving as the oxidant. The catalysts gave selectivities up to 98% for the epoxide at conversions as high as 99% with a 3.4 wt% loading of Ta present as Ta x N y /TaO x N y .

Dendritic polarizing agents for DNP SENS.

Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) is an effective method to significantly improve solid-state NMR investigation of solid surfaces. The presence of unpaired electrons (polarizing agents) is crucial for DNP, but it has drawbacks such as leading to faster nuclear spin relaxation, or even reaction with the substrate under investigation. The latter can be a particular problem for heterogeneous catalysts. Here, we present a series of carbosilane-based dendritic polarizing agents, in which the bulky dendrimer can reduce the interaction between the solid surface and the free radical. We thereby preserve long nuclear T'2 of the surface species, and even successfully enhance a reactive heterogeneous metathesis catalyst.

Modern ssNMR for heterogeneous catalysis

This review is devoted to modern applications of solid state NMR (ssNMR) to the structural studies of heterogeneous catalysts. Recent developments of ssNMR combining high magnetic fields up to 23.5T, high rotation frequencies up to 120kHz, and the new software of spectra acquisition and processing coupled with quantum chemistry calculations allow unravel the 3D structure of active sites of the sophisticated structures, such as heterogeneous catalysts. Moreover, this opens a possibility of investigation of low sensibility nuclei that are generally assigned as inappropriate for NMR. Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP-SENS) is another powerful technique extending NMR application for surface feature investigation. Combination of modern ssNMR (including DNP) and DFT calculations synthesize the most powerful approach (so called NMR crystallography) of structural research of catalysts which is demonstrated on several remarkable application examples on challenging nuclei.

Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy.

Understanding the chemistry of colloidal quantum dots (QDs) is primarily hampered by the lack of analytical methods to selectively and discriminately probe the QD core, QD surface and capping ligands. Here, we present a general concept for studying a broad range of QDs such as CdSe, CdTe, InP, PbSe, PbTe, CsPbBr3, etc., capped with both organic and inorganic surface capping ligands, through dynamic nuclear polarization (DNP) surface enhanced NMR spectroscopy. DNP can enhance NMR signals by factors of 10-100, thereby reducing the measurement times by 2-4 orders of magnitude. 1D DNP enhanced spectra acquired in this way are shown to clearly distinguish QD surface atoms from those of the QD core, and environmental effects such as oxidation. Furthermore, 2D NMR correlation experiments, which were previously inconceivable for QD surfaces, are demonstrated to be readily performed with DNP and provide the bonding motifs between the QD surfaces and the capping ligands.

Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations.

Despite the widespread use of amorphous aluminosilicates (ASA) in various industrial catalysts, the nature of the interface between silica and alumina and the atomic structure of the catalytically active sites are still subject to debate. Here, by the use of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) and density functional theory (DFT) calculations, we show that on silica and alumina surfaces, molecular aluminum and silicon precursors are, respectively, preferentially grafted on sites that enable the formation of Al(IV) and Si(IV) interfacial sites. We also link the genesis of Brønsted acidity to the surface coverage of aluminum and silicon on silica and alumina, respectively.

Silica-surface reorganization during organotin grafting evidenced by ¹¹⁹Sn DNP SENS: a tandem reaction of gem-silanols and strained siloxane bridges.

Grafting reactive molecular complexes on dehydroxylated amorphous silica is a strategy to develop "single-site" heterogeneous catalysts. In general, only the reactivity of isolated silanols is invoked for silica dehydroxylated at 700 °C ([SiO(2-700)]), though ca. 10% of the surface silanols are in fact geminal Q2-silanols. Here we report the reaction of allyltributylstannane with [SiO(2-700)] and find that the geminal Q2-silanols react to form products that would formally arise from vicinal Q3-silanols that are not present on [SiO(2-700)], indicating that a surface rearrangement occurs. The reorganization of the silica surface is unique to silica dehydroxylated at 700 °C or above. The findings were identified using Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy (DNP SENS) combined with DFT calculations.