In this study, a revolution in solid-state nuclear magnetic resonance (SSNMR) spectroscopy is taking place, attributable to the rapid development of high-field dynamic nuclear polarization (DNP), a ...technique yielding sensitivity improvements of 2–3 orders of magnitude. This higher sensitivity in SSNMR has already impacted materials research, and the implications of new methods on catalytic sciences are expected to be profound.
Our civilization relies on synthetic polymers for all aspects of modern life; yet, inefficient recycling and extremely slow environmental degradation of plastics are causing increasing concern about ...their widespread use. After a single use, many of these materials are currently treated as waste, underutilizing their inherent chemical and energy value. In this study, energy-rich polyethylene (PE) macromolecules are catalytically transformed into value-added products by hydrogenolysis using well-dispersed Pt nanoparticles (NPs) supported on SrTiO3 perovskite nanocuboids by atomic layer deposition. Pt/SrTiO3 completely converts PE (M n = 8000–158,000 Da) or a single-use plastic bag (M n = 31,000 Da) into high-quality liquid products, such as lubricants and waxes, characterized by a narrow distribution of oligomeric chains, at 170 psi H2 and 300 °C under solvent-free conditions for reaction durations up to 96 h. The binding of PE onto the catalyst surface contributes to the number averaged molecular weight (M n) and the narrow polydispersity (Đ) of the final liquid product. Solid-state nuclear magnetic resonance of 13C-enriched PE adsorption studies and density functional theory computations suggest that PE adsorption is more favorable on Pt sites than that on the SrTiO3 support. Smaller Pt NPs with higher concentrations of undercoordinated Pt sites over-hydrogenolyzed PE to undesired light hydrocarbons.
In spite of the widespread applications of amorphous silica-aluminas (ASAs) in many important industrial chemical processes, their high-resolution structures have remained largely elusive. ...Specifically, the lack of long-range ordering in ASA precludes the use of diffraction methods while NMR spectroscopy has been limited by low sensitivity. Here, we use conventional as well as DNP-enhanced
29
Si-
29
Si,
27
Al-
27
Al, and
29
Si-
27
Al solid-state NMR experiments to shed light on the ordering of atoms in ASAs prepared by flame-spray-pyrolysis. These experiments, in conjunction with a novel Monte Carlo-based approach to simulating RESPDOR dephasing curves, revealed that ASA materials obey Loewenstein's rule of aluminum avoidance. 3D
17
O{
1
H} and 2D
17
O{
1
H,
27
Al} experiments were developed to measure site-specific O-H and HO-Al distances, and show that the Brønsted acid sites originate predominantly from the pseudo-bridging silanol groups.
Advanced solid-state NMR methods, using dynamic nuclear polarization (DNP), are applied to probe the atomic-scale bulk structure of amorphous silica-alumina catalysts prepared by flame-spray pyrolysis, and the structure of their Brønsted acid sites.
A combination of solid‐state NMR techniques and theoretical calculations confirmed that unsubstituted metal‐free corroles form supramolecular systems with toluene through π–π interactions in the ...solid state. Ultrafast magic angle spinning (MAS) enabled the use of 1H–1H homonuclear correlation spectroscopy to explore those intermolecular interactions for which heteronuclear correlation methods were difficult to apply.
Two-dimensional through-bond
1H{
13C} solid-state NMR experiments utilizing fast magic angle spinning (MAS) and homonuclear multipulse
1H decoupling are presented. Remarkable efficiency of ...polarization transfer can be achieved at MAS rates exceeding 40
kHz, which is instrumental in these measurements. Schemes utilizing direct and indirect detection of heteronuclei are compared in terms of resolution and sensitivity. A simple procedure for optimization of
1H homonuclear decoupling sequences under these conditions is proposed. The capabilities of these techniques were confirmed on two naturally abundant solids, tripeptide
N-
formyl-
l-methionyl-
l-leucyl-
l-phenylalanine (f-MLF-OH) and brown coal.
The reaction of γ‐alumina with tetraethylorthosilicate (TEOS) vapor at low temperatures selectively yields monomeric SiOx species on the alumina surface. These isolated (‐AlO)3Si(OH) sites are ...characterized by PXRD, XPS, DRIFTS of adsorbed NH3, CO, and pyridine, and 29Si and 27Al DNP‐enhanced solid‐state NMR spectroscopy. The formation of isolated sites suggests that TEOS reacts preferentially at strong Lewis acid sites on the γ‐Al2O3 surface, functionalizing the surface with “mild” Brønsted acid sites. For liquid‐phase catalytic cyclohexanol dehydration, these SiOx sites exhibit up to 3.5‐fold higher specific activity than the parent alumina with identical selectivity.
Strong enough to get the job done: By selectively installing isolated (‐AlO)3SiOH species on a γ‐Al2O3 substrate, a unique “mild” Brønsted‐acid catalytic site is formed. These well‐defined sites evince highly uniform characteristics and are catalytically competent in the dehydration of cychohexanol, showing higher activity, no loss of selectivity, and no coking.
Single-site organolanthanum complexes supported on mesoporous silica nanoparticles, La{C(SiHMe2)3} n @MSNs, catalyze the ring-opening hydroboration reaction of aliphatic and styrenic epoxides with ...pinacolborane (HBpin). The surface-bound complexes, synthesized by reaction of the homoleptic tris(alkyl)lanthanum La{C(SiHMe2)3}3 and SBA-type MSN treated at 700 °C (MSN700), are mostly monopodal SiOLa{C(SiHMe2)3}2 and contain an average of one bridging La↼HSi per alkyl ligand. This structure was established through a combination of solid-state NMR (SSNMR) experiments, including J-resolved SiH coupling and quantitative 29Si measurements, diffuse reflectance IR, and elemental analysis. These rigorous analyses also established that grafting reactions in pentane provide a preponderance of SiOLa{C(SiHMe2)3}2 sites and are superior to those in benzene and THF, and that grafting onto MSN treated at 550 °C (MSN550) results in a mixture of surface species. The single-site supported catalysts are more selective and in most cases more active than the homogeneous analogue, allow easy purification of products from the catalyst, are strongly resistant to leaching into solution phase, and may be recycled for reuse at least five times. After reaction of La{C(SiHMe2)3} n @MSN and HBpin, species including SiOLa{C(SiHMe2)3}(H2Bpin) and SiOLa{C(SiHMe2)3}{κ2–pinB–O(CMe2)2OBH3} are identified by detailed 1D and 2D 11B SSNMR experiments.
Despite the growing number of successful applications of dynamic nuclear polarization (DNP)-enhanced magic-angle spinning (MAS) NMR in structural biology and materials science, the nuclear ...polarizations achieved by current MAS DNP instrumentation are still considerably lower than the theoretical maximum. The method could be significantly strengthened if experiments were performed at temperatures much lower than those currently widely used (∼100 K). Recently, the prospects of helium (He)-cooled MAS DNP have been increased with the instrumental developments in MAS technology that uses cold helium gas for sample cooling. Despite the additional gains in sensitivity that have been observed with He-cooled MAS DNP, the performance of the technique has not been evaluated in the case of surfaces and interfaces that benefit the most from DNP. Herein, we studied the efficiency of DNP at temperatures between ∼30 K and ∼100 K for organically functionalized silica material and a homogeneous solution of small organic molecules at a magnetic field
B
0
= 16.4 T. We recorded the changes in signal enhancement, paramagnet-induced quenching and depolarization effects, DNP build-up rate, and Boltzmann polarization. For these samples, the increases in MAS-induced depolarization and DNP build-up times at around 30 K were not as severe as anticipated. In the case of the surface species, we determined that MAS DNP at 30 K provided ∼10 times higher sensitivity than MAS DNP at 90 K, which corresponds to the acceleration of experiments by multiplicative factors of up to 100.
DNP enhancement, paramagnet-induced quenching/depolarization and build-up times are studied in a heterogeneous catalyst between 30 and 100 K. He-cooled MAS DNP at 30 K provides up to 100-fold better time performance than N
2
-cooled MAS DNP at 90 K.
The efficacy of dynamic nuclear polarization (DNP) surface-enhanced NMR spectroscopy (SENS) is reviewed for alumina, silica, and ordered mesoporous carbon (OMC) materials, with vastly different ...surface areas, as a function of the biradical concentration. Importantly, our studies show that the use of a “one-size-fits-all” biradical concentration should be avoided when performing DNP SENS experiments and instead an optimal concentration should be selected as appropriate for the type of material studied as well as its surface area. In general, materials with greater surface areas require higher radical concentrations for best possible DNP performance. This result is explained with the use of a thermodynamic model wherein radical-surface interactions are expected to lead to an increase in the local concentration of the polarizing agent at the surface. We also show, using plane-wave density functional theory calculations, that weak radical-surface interactions are the cause of the poor performance of DNP SENS for carbonaceous materials.
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•Optimal biradical concentrations for DNP SENS NMR are determined in a wide range of materials.•Silica, alumina and carbonaceous surfaces require tailored sample preparations.•Sensitivity improvements are explained using thermodynamic model and DFT calculations.
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