Homonuclear correlation in solid-state NMR : developing experiments for half-integer quadrupolar nuclei

The objective was to develop solid-state nuclear magnetic resonance (NMR) homonuclear correlation experiments for half-integer quadrupolar nuclei so as to study atomic proximities and connectivities in disordered materials. Nearby nuclear spins are coupled through space via their magnetic dipole moments. Dipolar broadening is removed by magic angle spinning (MAS) for isolated spin pairs. However, the noncommutation of the electric quadrupolar interaction with the dipolar interaction means that the latter will not be removed by MAS. This interplay between the dipolar and quadrupolar interactions, combined with the effects of multiple noncommutating homonuclear dipolar couplings, was investigated by observing spin-echo dephasing curves as well as magnetisation transfer in 2D spin diffusion experiments. Polycrystalline lithium diborate samples were synthesised to act as model compounds. The preparation of samples with differing 11B isotopic abundances enabled a comparison of samples with either predominantly isolated spin-pairs or multiple coupled nuclei. Spin diffusion experiments probed 11B–11B correlation at three magnetic field strengths, 80% and 25% 11B isotopic abundances, MAS rates from 4427 Hz to 7602 Hz and under DOR. Enhanced magnetisation transfer was observed for the higher 11B isotopic abundance and at slower spinning speeds. The latter dependence was reproduced by four-spin computer simulations. Secondorder quadrupolar broadened spin diffusion cross-peaks under MAS had a mixed positive and negative appearance for the 80% 11B sample. A similar effect was previously observed for four dipolar-coupled I = 1/2 nuclei. Spin-echo dephasing curves were recorded for 5%, 25% and 100% 11B isotopic abundances and MAS rates of 5 kHz to 20 kHz. Depletion of 11B isotopic abundance prolonged the coherence dephasing time because of a reduction of noncommuting homonuclear dipolar couplings. Faster dephasing was observed for the smaller CQ = 0.51MHz site; four-spin computer simulations showed this is consistent with the reintroduction of the dipolar coupling being most efficient when the MAS rate and first-order quadrupolar interaction are of the same magnitude. Speeding-up the MAS rate prolonged the dephasing time for the CQ = 2.56MHz site but not for the CQ = 0.51MHz site because of an interplay between the quadrupolar and multiple dipolar interactions. Through-bond Jcouplings between 11B nuclei were not detected, setting an upper bound of 2JBB <3 Hz in polycrystalline lithium diborate

Homonuclear correlation in solid-state NMR : developing experiments for half-integer quadrupolar nuclei

The objective was to develop solid-state nuclear magnetic resonance (NMR) homonuclear correlation experiments for half-integer quadrupolar nuclei so as to study atomic proximities and connectivities in disordered materials. Nearby nuclear spins are coupled through space via their magnetic dipole moments. Dipolar broadening is removed by magic angle spinning (MAS) for isolated spin pairs. However, the noncommutation of the electric quadrupolar interaction with the dipolar interaction means that the latter will not be removed by MAS. This interplay between the dipolar and quadrupolar interactions, combined with the effects of multiple noncommutating homonuclear dipolar couplings, was investigated by observing spin-echo dephasing curves as well as magnetisation transfer in 2D spin diffusion experiments. Polycrystalline lithium diborate samples were synthesised to act as model compounds. The preparation of samples with differing 11B isotopic abundances enabled a comparison of samples with either predominantly isolated spin-pairs or multiple coupled nuclei. Spin diffusion experiments probed 11B–11B correlation at three magnetic field strengths, 80% and 25% 11B isotopic abundances, MAS rates from 4427 Hz to 7602 Hz and under DOR. Enhanced magnetisation transfer was observed for the higher 11B isotopic abundance and at slower spinning speeds. The latter dependence was reproduced by four-spin computer simulations. Secondorder quadrupolar broadened spin diffusion cross-peaks under MAS had a mixed positive and negative appearance for the 80% 11B sample. A similar effect was previously observed for four dipolar-coupled I = 1/2 nuclei. Spin-echo dephasing curves were recorded for 5%, 25% and 100% 11B isotopic abundances and MAS rates of 5 kHz to 20 kHz. Depletion of 11B isotopic abundance prolonged the coherence dephasing time because of a reduction of noncommuting homonuclear dipolar couplings. Faster dephasing was observed for the smaller CQ = 0.51MHz site; four-spin computer simulations showed this is consistent with the reintroduction of the dipolar coupling being most efficient when the MAS rate and first-order quadrupolar interaction are of the same magnitude. Speeding-up the MAS rate prolonged the dephasing time for the CQ = 2.56MHz site but not for the CQ = 0.51MHz site because of an interplay between the quadrupolar and multiple dipolar interactions. Through-bond Jcouplings between 11B nuclei were not detected, setting an upper bound of 2JBB <3 Hz in polycrystalline lithium diborate.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Dynamic nuclear polarization enhanced solid-state NMR studies of surface modification of gamma-alumina

Dynamic nuclear polarization (DNP) gives large (>100-fold) signal enhancements in solid-state NMR spectra via the transfer of spin polarization from unpaired electrons from radicals implanted in the sample. This means that the detailed information about local molecular environment available for bulk samples from solid-state NMR spectroscopy can now be obtained for dilute species, such as sites on the surfaces of catalysts and catalyst supports. In this paper we describe a DNP-enhanced solid-state NMR study of the widely used catalyst gamma-alumina which is often modified at the surface by the incorporation of alkaline earth oxides in order to control the availability of catalytically active penta-coordinate surface Al sites. DNP-enhanced 27Al solid-state NMR allows surface sites in gamma-alumina to be observed and their 27Al NMR parameters measured. In addition changes in the availability of different surface sites can be detected after incorporation of BaO

Ion exchange and binding in selenium remediation materials using DNP-enhanced solid-state NMR spectroscopy

Selenate-loaded selenium water remediation materials based on polymer fibres have been investigated by dynamic nuclear polarization (DNP) enhanced solid-state NMR. For carbon-13 a significant reduction in experiment time is obtained with DNP even when compared with conventional carbon-13 NMR spectra recorded using larger samples. For the selenium remediation materials studied here this reduction allows efficient acquisition of {1H}-77Se heteronuclear correlation spectra which give information about the nature of the binding of the remediated selenate ions with the grafted side chains which provide the required ion exchange functionality

Silicon redistribution, acid site loss and the formation of a core-shell texture upon steaming SAPO-34 and their impact on catalytic performance in the methanol-to-olefins (MTO) reaction

IBM has received funding from the Engineering and Physical Sciences Research Council (EPSRC, Centre for Doctoral Training in Critical Resource Catalysis, EP/I017008/1) and Scotland's Chemistry departments (ScotCHEM). IBM also received a scholarship from the SCI and Santander. Johnson Matthey is thanked for in-kind contributions and hosting IBM in their R&D labs. ABN gratefully acknowledges support from the EPSRC (grants EP/L017008/1 and EP/R023751/1). The research data supporting this publication can be accessed at: https://doi.org/10.17630/09ddc03e-f121-4e79-9b55-674f64d9c8c4 [62].SAPO-34 is a commercially-implemented silicoaluminophosphate catalyst for selective high yield production of ethene and propene from methanol, but high temperature regeneration in the presence of steam leads to its deactivation. A comprehensive investigation of the effect of prolonged hydrothermal treatment on the structure and properties of SAPO 34 explains the changes in its catalytic methanol-to-olefins (MTO) performance. Microcrystalline powdered SAPO-34 (ca. 3 µm crystals, Al17.1P15.6Si3.3O72) and two batches of larger single crystals of SAPO-34 of different Si concentration (20-100 µm; Al17.3P14.7Si4.0O72 and Al17.7P12.3Si5.9O72 ) were steamed (pH2O = 0.95 atm) at 873–1023 K for up to 240 h. The acidity (NH3-TPD), crystallinity (PXRD), framework cation environment (solid-state 27Al, 29Si and 31P MAS NMR) and porosity were followed for all materials; larger crystals were amenable to single crystal X-ray diffraction, FIB-SEM and synchrotron IR microspectroscopy, including operando study during methanol and dimethyl ether conversions. Some level of steaming improved the lifetime of all SAPO-34 materials in MTO catalysis without affecting their olefin selectivity, although more severe conditions led to the formation of core-shell structures, microporosity loss and eventually at 1023 K, recrystallization to a dense phase. All these irreversible changes occurred faster in crystals with higher Si contents. The initial increase in catalytic lifetime results from an activated reduction in acid site density (Eact = 146(18) kJ mol⁻1), a result of redistribution of Si within the SAPO framework without porosity loss. Operando IR with online product analysis during methanol conversion suggests similar reaction pathways in calcined and steamed crystals, but with greatly reduced methoxy group densities in the latter. The gradual development of optically dark crystal cores upon progressive steaming was shown by FIB-SEM to be due to the formation of regions with meso- and macropores, and these were shown by IR mapping to possess low hydroxyl densities.PostprintPostprintPeer reviewe

Counting the Acid Sites in a Commercial ZSM-5 Zeolite Catalyst

This work was funded by Johnson Matthey plc. through the provision of industrial CASE studentships in partnership with the EPSRC (AZ (EP/N509176/1), APH (EP/P510506/1)). Experiments at the ISIS Neutron and Muon Source were made possible by beam time allocations from the Science and Technologies Facilities Council.45,46 Resources and support were provided by the UK Catalysis Hub via membership of the UK Catalysis Hub consortium and funded by EPSRC grants EP/R026815/1 and EP/R026939/1Peer reviewedPublisher PD

On the transition to Gasoline-to-Olefins chemistry in the cracking reactions of 1-octene over H-ZSM-5 catalysts

The cracking reactions of 1-octene over H-ZSM-5 zeolite are studied via micro-reactor and off-line spectroscopic techniques over a period of up to 72 hours on stream and a temperature range of 473 – 673 K. 1-Octene is found to react via a two-cycle hydrocarbon pool mechanism, with strong similarities to that reported for methanol-to-hydrocarbons chemistry. This dual-cycle mechanism requires temperatures of 673 K or higher to function with full efficiency, with lower temperatures deactivating portions of the cyclic mechanism, leading to premature deactivation of the catalyst through over-production of coke species. Inelastic neutron scattering is used to study the coke composition, identifying two distinct deactivation mechanisms depending on reaction temperature. The catalyst is also found to slowly progress from an aromatic-heavy to an olefin-heavy product regime even at full efficiency due to progressive blockage of active sites by amorphous carbon-rich coke. Artificial aging of the zeolite, through steam treatment, is found to shift the catalyst lifetime so that it commences at a later stage in this process, resulting in increased light olefin production. The reduced aromatic production also means that deactivation of the catalyst occurs more slowly in steamed catalysts than in fresh ones, after an equivalent time-on-stream. Collectively, these observations connect with the application of ZSM-5 catalysts to facilitate gasoline-to-olefins chemistry in fluidized catalytic cracking unit operations

The potential psychological benefits of Active Video Games in the rehabilitation of musculoskeletal injuries and deficiencies: A narrative review of the literature

Background: Recent literature suggests that Active Video Games (AVGs) may offer potential psychological benefits during the rehabilitation of musculoskeletal injuries and their corresponding deficiencies. Objectives: To review existing literature regarding the potential psychological benefits of AVGs within the context of rehabilitation from musculoskeletal injury or debilitation. Method: A narrative review of the literature that used the Population, Intervention, Comparison, and Outcomes PICO method was conducted. The literature review included studies that discussed and/or investigated potential psychological benefits of AVGs during musculoskeletal rehabilitation. Of the total 163 papers that were identified, 30 met the inclusion criteria. Results: The Nintendo® WiiTM (Nintendo Co., Ltd, Kyoto, Japan) was the most commonly-used games console that was employed in AVG interventions (15 out of 21), and these studies that investigated potential psychological benefits were typically conducted with elderly populations. These studies reported that using AVGs in musculoskeletal rehabilitation resulted in a number of positive psychological effects (e.g., enjoyment, effects on self). However, most studies lacked a clear theoretical framework, and varied greatly in their designs and methodologies. Conclusion: Despite encouraging findings of AVG use, insufficient evidence exists to reliably verify or refute the potential psychological benefits of AVGs in musculoskeletal rehabilitation. It is recommended that future studies in this area contain a theoretical framework to ensure greater consistency in the methodology used and the execution of the intervention. The potential findings of such investigations may result in the development of optimal, client-tailored rehabilitation programmes

Measurements of Deuteron Photodisintegration up to 4.0 GeV

The first measurements of the differential cross section for the d(gamma,p)n reaction up to 4.0 GeV were performed at Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab. We report the cross sections at the proton center-of-mass angles of 36, 52, 69 and 89 degrees. These results are in reasonable agreement with previous measurements at lower energy. The 89 and 69 degree data show constituent-counting-rule behavior up to 4.0 GeV photon energy. The 36 and 52 degree data disagree with the counting rule behavior. The quantum chromodynamics (QCD) model of nuclear reactions involving reduced amplitudes disagrees with the present data.Comment: 5 pages (REVTeX), 1 figure (postscript

A Study of the Quasi-elastic (e,e'p) Reaction on 12^{12}C, 56^{56}Fe and 97^{97}Au

We report the results from a systematic study of the quasi-elastic (e,e'p) reaction on $^{12}$C, $^{56}$Fe and $^{197}$Au performed at Jefferson Lab. We have measured nuclear transparency and extracted spectral functions (corrected for radiation) over a Q$^2$ range of 0.64 - 3.25 (GeV/c)$^2$ for all three nuclei. In addition we have extracted separated longitudinal and transverse spectral functions at Q$^2$ of 0.64 and 1.8 (GeV/c)$^2$ for these three nuclei (except for $^{197}$Au at the higher Q$^2$). The spectral functions are compared to a number of theoretical calculations. The measured spectral functions differ in detail but not in overall shape from most of the theoretical models. In all three targets the measured spectral functions show considerable excess transverse strength at Q$^2$ = 0.64 (GeV/c)$^2$, which is much reduced at 1.8 (GeV/c)$^2$.Comment: For JLab E91013 Collaboration, 19 pages, 20 figures, 3 table