29Si MAS Research Articles

Reaction mechanisms, kinetics, and nanostructural evolution of magnesium silicate hydrate (M-S-H) gels

M-S-H gels were synthesised via reaction of Mg(OH)2 with silica fume, cured at 35 °C for up to 112 days, and their chemical and nanostructural evolution was examined. M-S-H gels with structural similarity to the thermodynamically stable serpentine-group mineral lizardite were formed. Quantification of 25Mg and 29Si MAS and 1H29Si CPMAS NMR, electron microscopy, and thermogravimetric data showed dissolution of brucite and silica fume, and M-S-H formation, all occurred linearly with time up to 56 days. Data showed strong correlation with the Avrami-Erofeyey nucleation kinetic model, indicating M-S-H formation was governed by nucleation reactions. After 112 days, two distinct M-S-H gels were formed: a Si-rich M-S-H gel with molar Mg/Si = 0.55(±0.2), and a Mg-rich M-S-H gel with molar Mg/Si = 0.80(±0.5). Nanostructural rearrangement of M-S-H continues up to 112 days, with increased crosslinking and polymerisation. This new insight is important for application of M-S-H binders in both construction and radioactive/toxic waste immobilisation.

Alteration in molecular structure of alkali activated slag with various water to binder ratios under accelerated carbonation

Carbonation of alkali activated materials is one of the main deteriorations affecting their durability. However, current understanding of the structural alteration of these materials exposed to an environment inducing carbonation at the nano/micro scale remains limited. This study examined the evolution of phase assemblages of alkali activated slag mortars subjected to accelerated carbonation (1% CO2, 60% relative humidity, up to 28 day carbonation) using XRD, FTIR and 29Si, 27Al, and 23Na MAS NMR. Samples with three water to binder (w/b) ratios (0.35, 0.45, and 0.55) were investigated. The results show that the phase assemblages mainly consisted of C-A-S-H, a disordered remnant aluminosilicate binder, and a minor hydrotalcite as a secondary product. Upon carbonation, calcium carbonate is mainly formed as the vaterite polymorph, while no sodium carbonate is found after carbonation as commonly reported. Sodium acts primarily as a charge balancing ion without producing sodium carbonate as a final carbonation product in the 28-day carbonated materials. The C-A-S-H structure becomes more cross-linked due to the decalcification of this phase as evidenced by the appearance of Q4 groups, which replace the Q1 and Q2 groups as observed in the 29Si MAS NMR spectra, and the dominance of Al(IV) in 27Al MAS NMR. Especially, unlike cementitious materials, the influence of w/b ratio on the crystalline phase formation and structure of C-A-S-H in the alkali activated mortars before and after carbonation is limited.

Utilization of 29Si MAS-NMR to Understand Solid State Diffusion in Energy Storage Materials

The properties of many solid-state materials arise from critical interfaces tied to the structure, morphology, and composition of the materials under study. For many materials, identifying components that may be invisible to diffraction techniques or other bulk sensitive techniques (i.e. inductively coupled plasma (ICP)), may cause important information to be overlooked. These can include grain boundary phases, nanoscale coatings, amorphous layers, or second phases that influence the materials environment. In this short review, the use of 29Si MAS NMR as a local probe to detect silicon-containing phases in complex energy storage systems is explored with a focus is on silicon-containing materials and silicon electrodes. Examples highlighting the utility of 29Si MAS NMR include 1) examining copper diffusion into silicon as a method to create 3 dimensional electrodes, 2) using Mg(II) electrolyte additives to create in-situ nanoscale silicide coatings to inhibit low voltage parasitic side reactions and extend calendar life, and 3) studying the lithiation reactions of passivated silicon on different time scales.

NMR Spectral Characteristics of Ultrahigh Pressure High Temperature Impact Glasses of the Giant Kara Crater (Pay-Khoy, Russia)

In this study, we carried out the analysis of the impact melt vein glasses from the Kara impact crater (Russia) in comparison to low-pressure impact melt glasses (tektites) of the Zhamanshin crater (Kazakhstan). 27Al, 23Na, and 29Si MAS NMR spectra of the samples of these glasses were analyzed. The samples of the natural glass contained inclusions of crystalline phases, paramagnetic elements that greatly complicate and distort the NMR signals from the glass phase itself. Taking into account the Mossbauer distribution of Fe in these glasses, the analysis of the spectra of MAS NMR of glass network-former (Si, Al) and potential network-modifiers (Na) of nuclei leads to the conclusion that the Kara impact melt vein glasses are characterized by complete polymerization of (Si,Al)O4 tetrahedral structural units. The NMR features of the glasses are consistent with the vein hypothesis of their formation under conditions of high pressures and temperatures resulting in their fluidity, relatively slow solidification with partial melt differentiation, polymerization, and precipitation of mineral phases as the impact melt cools. The 70 Ma stability of the Kara impact vein glass can be explained by the stabilization of the glass network with primary fine-dispersed pyroxene and coesite precipitates and by the high polymerization level of the impact glass.

Translating Race on the French Stage

Translating Race on the French Stage

Cross-Border Ethnic Relations at Vietnam-China Border Markets since 1990

At the Vietnamese-Chinese border, markets serve as places not only for exchanging and purchasing goods but also for socio-cultural exchanges among ethnic groups on both sides of the border. The Vietnamese economic renovation in the late 1980s and the process of normalizing Vietnam-China relations in1990 have contributed to the development of the market system at the borderline between these two countries, meeting the commodity trade needs of involved ethnic groups in the region and the surrounding areas, promoting cultural exchange, and maintaining and expanding kinship relations and friendship between the ethnic groups on both sides of the border. This article explores the cross-border ethnic relationships shown in trading, exchanging goods, and developing social networks of ethnic minorities in two typical border markets - Loc Binh market in Lang Son’s Loc Binh district and Can Cau commune market in Lao Cai’s Si Ma Cai district - thereby clarifying the dynamics of ethnic relations in the Vietnam-China border area since 1990.

Direct Observation of Carbonate Formation in Partly Hydrated Tricalcium Silicate by Dynamic Nuclear Polarization Enhanced NMR Spectroscopy

Solid-state NMR combined with dynamic nuclear polarization (DNP NMR) is used to study hydration processes in tricalcium silicate (Ca3SiO5, abbreviated as C3S) samples. The studied C3S samples have experienced early stage hydration (1–30 h) and slow aging (9 years) processes. The appearance of Q3 and Q4 lines in the 29Si MAS and 1H → 29Si CP MAS NMR spectra obtained for partly hydrated C3S samples indicated the formation of amorphous silica which corresponds to their carbonation, which was corroborated by complementary FTIR data. Significant DNP signal enhancements obtained for the studied samples allowed to further investigate the C3S carbonation process in detail employing the 1H → 29Si CP MAS FSLG HETCOR technique. Finally, DNP enhanced 1H → 13C CP MAS and 1H → 13C CP MAS FSLG HETCOR techniques enabled to directly observe the formation of carbonate moieties in partly hydrated C3S samples.

New heptacoordinate tungsten(II) complexes with α-diimine ligands in the catalytic oxidation of multifunctional olefins

New tungsten(II) and molybdenum(II) heptacoordinate complexes [MX2(CO)3(LY)] (MXLy: M = W, Mo; X = Br, I; LY = C5H4NCY = N(CH2)2CH3 with Y = H (L1), Me (L2), Ph (L3)) were synthesized and characterized by spectroscopic techniques and elemental analysis. The two tungsten complexes WXL1 (X = Br, I) were also structurally characterized by single crystal X-ray diffraction. The metal coordination environment is in both a distorted capped octahedron. The complexes with L1 and L2 ligands were grafted in MCM-41, after functionalization of the ligands with a Si(OEt)3 group. The new materials were characterized by elemental analysis, N2 adsorption isotherms, 29Si MAS and 13C MAS NMR. The tungsten(II) complexes and materials were the first examples of this type reported. All complexes and materials were tested as homogeneous and heterogeneous catalysts in the oxidation of multifunctional olefins (cis-hex-3-en-1-ol, trans-hex-3-en-1-ol, geraniol, S-limonene, and 1-octene), with tert-butyl hydroperoxide (TBHP) as oxidant. The molybdenum(II) catalyst precursors are in general very active, reaching 99% conversion and 100% selectivity in the epoxidation of trans-hex-3-en-1-ol. Their performance is comparable with that of the [Mo(η3-C3H5)X(CO)2(LY)] complexes, but it increases with immobilization. On the other hand, most of the W(II) complexes display an activity similar or inferior to that of the Mo(II) analogues and it decreases after they are supported in MCM-41. DFT calculations show that tungsten complexes and iodide ligands are more easily oxidized from M(II) to M(VI) than molybdenum ones, while the energies of relevant species in the catalytic cycle are very similar for all complexes, making the theoretical rationalization of experimental catalytic data difficult.

Platinum catalysts supported on ZSM5 zeolites with a hierarchical pore structure: characterization and performance in n-hexadecane hydroconversion

HZSM5 zeolites with a hierarchical pore structure were prepared from a microporous one using the top-down methodology by treatment with a sodium hydroxide solution in optimal conditions of time, alkali concentration and temperature. These materials were used as supports for platinum to produce bifunctional hydroconversion catalysts. The activity in n-hexadecane hydroconversion was one order of magnitude larger on Pt supported on the mesoporous than on the untreated zeolite and one order of magnitude larger than that of the supports. All Pt catalysts and supports were more selective to hydrocracking than to hydroisomerization, but the initial selectivity for hydroisomerization of the Pt catalysts supported on the mesoporous supports was considerably smaller than that of all other materials, including the supports. This was attributed to a smaller mass transfer limitation with the Pt catalysts supported on the mesoporous zeolites. All supports and catalysts were extensively characterized by techniques such as X-ray diffraction, hydrogen chemisorption, TPD of ammonia, IR spectroscopy of adsorbed pyridine, MAS 29Si and 27Al solid state NMR and transmission electron microscopy.

Temperature transformation of blended magnesium potassium phosphate cement binders

In this study, a multi-technique approach was utilised to determine the high temperature performance of magnesium potassium phosphate cement (MKPC) blended with fly ash (FA) or ground granulated blast furnace slag (GBFS) with respect to nuclear waste immobilisation applications. Conceptual fire conditions were employed (up to 1200 °C, 30 min) to simulate scenarios that could occur during interim storage, transportation or within a final geological disposal facility. After exposure up to 400 °C, the main crystalline phase, struvite-K (MgKPO4·6H2O), was dehydrated to poorly crystalline MgKPO4 (with corresponding volumetric and mass changes), with MgKPO4 recrystallisation achieved by 800 °C. XRD and SEM/EDX analysis revealed reaction occurred between the MgKPO4 and FA/GBFS components after exposure to 1000–1200 °C, with the formation of potassium aluminosilicate phases, leucite and kalsilite (KAlSi2O6 and KAlSiO4), commensurate with a reduced relative intensity (or complete elimination) of the dehydrated struvite-K phase, MgKPO4. This was further supported by solid-state NMR (27Al and 29Si MAS), where only residual features associated with the raw FA/GBFS components were observable at 1200 °C. The high temperature phase transformation of blended MKPC binders resulted in the development of a glass/ceramic matrix with all existing porosity infilled via sintering and the formation of a vitreous phase, whilst the physical integrity was retained (no cracking or spalling). This study demonstrates that, based on small-scaled specimens, blended MKPC binders should perform satisfactorily under fire performance parameters relevant to the operation of a geological disposal facility, up to at least 1200 °C.

Accessibility and strength of H-acceptor hydroxyls of ordered mesoporous silicas probed by pyridine donor

Ordered mesoporous silica (OMS) is an important and useful material for a variety of applications, including catalysis, adsorption, sensing and controlled drug delivery. The surface chemistry and the silanol groups on OMS pores are key properties for the potential modification and application of this material. This research aimed to synthesize (using standard protocols) and differentiate the accessibility and strength of the H-acceptor Si–OH from FDU-12, SBA-16, MCM-41 and SBA-15 by pyridine (Py) donor, where the first two have cubic pore structures and the last two have hexagonal pore structures. Donor–acceptor properties were assessed by calculation of the surface Si–OH densities by thermogravimetry (TG), H2O-TPD/MS, and 29Si MAS and CP/MAS NMR. The nature of the Si–OH groups on these materials was determined to be hydrogen-bonding sites using FT-IR spectroscopy of Py adsorption. The reactivity of these silanol groups was probed by Py-TG and slurry microcalorimetry of Py adsorption in cyclohexane. Differences in accessibility and reactivity were discussed considering the total potential sites on the surface (nOH) versus the actual sites that can react with the Py molecule (nPy). By using microcalorimetry, it was possible to quantitatively distinguish the strength of the sites: The acidity order was approximately the same as the relative amount of silanol groups (Si–OH) and Py on the surface of the OMS materials (αPy): FDU-12 > MCM-41 ≥ SBA-16 > SBA-15.

Enhanced Catalytic Performance of Cr/MOR for Ethane Dehydrogenation Through Dealumination

Dehydrogenation of ethane to ethylene assisted by CO2 was investigated over CrOx catalysts supported on MOR zeolites. The catalysts were characterized by XRD, N2 adsorption, SEM, 27Al and 29Si MAS NMR, XPS, DRIFTS, H2-TPR and laser Raman spectroscopy. The catalysts exhibit enhanced activity and stability after MOR dealumination, with an optimal ethane conversion of ~ 38% and ethylene yield of ~ 33%. Characterization results indicate that the improved catalytic performance derives from the increased silanol nests of MOR support created by acidic dealumination, which are in favor of dispersion and stabilization of CrOx species, resulting in more reducible Cr(VI) considered as the active site for dehydrogenation.

Mutual Aid and Survival as Resistance in Puerto Rico

I grew up hearing a common saying in Puerto Rico: “Mandeme mas si mas me merezco”—send me more if I deserve more. I always heard this used to signal suffering. It was a way to indicate that a highe...

Study on lithium storage in silicon species of Si-O-C materials

Lithium exchange mechanism, local structure, and its possible evolutions on lithium ion extraction and reinsertion are studied in Si-O-C materials. As the silicon distributions and microstructure change with the synthetic method, the relationship between the capacity and silicon structures is evaluated. By comparing derivative capacity curves of the ordinary anode materials composed of Si, C, or O, silicon species in terms of SiOnC4-n (1 ≤ n ≤ 4) are deduced to be the main source of lithium storage in Si-O-C materials. Of all the silicon species, SiOnC4-n (2 ≤ n ≤ 4) are confirmed to be reversible with lithium, but SiOC3 species be irreversible, by tracing the silicon structural changes during lithium insertion/distraction utilizing 29Si MAS NMR. Galvanostatic tests show the reversible capacity increases with the increase of the sum of SiOnC4-n (2 ≤ n ≤ 4) and decreases with the increase of SiOC3 content. Furthermore, a model is established to express the relationship between the capacity and silicon species. According to the model, 1 mol of SiO4, SiO3C, SiO2C2, or SiOC3 species can reversible deliver 3.0, 2.5, 0.5, or 0 mol of lithium ions, respectively. Applied to the reported data, the predictive capacities following the model agree well with the experimental values.

Dissolution kinetics of calcined kaolinite and montmorillonite in alkaline conditions: Evidence for reactive Al(V) sites

AbstractCalcined clays are attracting significant research attention because of their potential to partially replace CO2‐intensive portland cement. This potential depends largely on their reactivity, especially dissolution under alkaline conditions. Identification and characterization of reactive sites in these amorphous calcined clays has so far not been reported. Here, we investigate kaolinite (1:1 clay) and montmorillonite (2:1 clay) calcined at different temperatures under alkaline conditions (0.1 mol/L NaOH). Solution compositions are determined in batch dissolution experiments, whereas 27Al and 29Si MAS and CP/MAS NMR are used to investigate the structure of the undissolved residues to identify sites that are reactive and undergo preferential dissolution. The highest Si and Al dissolution rates for kaolinite are observed for calcination at 700°C, corresponding to the temperature of optimum reactivity, whereas the rates decrease and become increasingly incongruent at higher temperatures. The Si and Al dissolution rates for optimally calcined kaolinite are 4 and 12 times larger than the corresponding rates for optimally calcined montmorillonite, in accord with the much higher reactivity of calcined kaolinite compared to calcined 2:1 clays. This superior performance of kaolinite is explained by novel 27Al NMR results, which show strong evidence for preferential dissolution of highly reactive pentahedral aluminum sites.

Getting that sugar, one way or another.

Getting that sugar, one way or another.

Adsorption and Desorption Characteristics of Alkali Ions in Hydrated C3S-nano SiO2 Pastes

C3S pastes containing 0%, 5%, 10%, and 15% nano-SiO2 mixed with de-ionized water and alkali solutions were prepared. When C3S was completely hydrated, the pastes were ground into powders with a particle size less than 80 μm. Adsorption and desorption characteristics of alkali ions adsorbed by C3S-nano SiO2 pastes mixed with de-ionized water immersed in alkali solutions and those in C3S-nano SiO2 pastes mixed with alkali solutions, were investigated. Meawhile, the adsorption mechanisms of alkali ions were discussed. Results showed that the contents of alkali ions adsorbed by C3S-nano SiO2 pastes mixed with de-ionized water increased with increasing substitution levels of nano-SiO2 and/or the initial alkali concentrations. In C3S-nano SiO2 pastes mixed with de-ionized water, each paste was characterized by having a fixed alkali-adsorption capacity that was essentially independent of alkali concentration. No obvious difference between the adsorption capacity of a given paste for K+ and Na+ was observed. Adsorption of alkali ions in the pastes is considered to be caused by surface force which is related to the BET specific surface area of the paste, and charge compensation of C-S-H gel, mainly by electrostatic interactions. In C3S-nano SiO2 pastes mixed with alkali solutions, alkali ions may enter the structure of C-S-H gel to replace a part of Ca2+ in the interlayer. This assumption is supported by the structural characterization of C-S-H gel using 29Si MAS NMR.

A new synthetic route to molecularly imprinted ORMOSIL: Characterization and evaluation

ABSTRACTA new strategy for synthesis of molecularly imprinted organically modified silica (MIORMOSIL) has been described. The MIORMOSIL was prepared by reacting vinyl acetate and 3-(trimethoxysilyl) propyl methacrylate, followed by condensation and hydrolysis with tetraethyl orthosilicate in the presence of caffeine. The ORMOSIL was characterized by SEM, FTIR, 29Si MAS NMR, 13C MAS NMR, TGA, nitrogen adsorption/desorption and tested to extract analyte from coffee samples. The percentage recovery of caffeine 88 ± 5% (n = 9), limits of detection and quantification, 5.14 × 10–4 and 1.71 × 10−3 mmolL−1 respectively, were achieved. Cross reactivity studies for theophylline and theobromine showed high recovery (77% and 82% respectively).Abbreviations: HMIP: Molecularly Imprinted ORMOSIL; HNIP: non imprinted ORMOSIL; TMSPM: 3-(Trimethoxysilyl)propylmethacrylate

Crystal structures of two oxygen-deficient perovskite phases in the CaSiO3–CaAlO2.5 join

The crystal structures of Ca(Al0.5Si0.5)O2.75 and Ca(Al0.4Si0.6)O2.8 ordered oxygen-deficient perovskite phases are synthesized at 7 and 11 GPa, respectively, and 1500 °C, and were studied using NMR and synchrotron powder X-ray diffraction. 29Si MAS NMR, 27Al MAS and 3Q MAS NMR measurements revealed a single tetrahedral Si and single octahedral Al peak for the Ca(Al0.5Si0.5)O2.75 phase, and a tetrahedral and an octahedral Si peak and a single octahedral Al peak for the Ca(Al0.4Si0.6)O2.8 phase. Using this structural information as constraints, the crystal structures were solved from synchrotron X-ray diffraction data by the structure determination from powder diffraction (SDPD) technique. To double-check the structures, first-principles calculations of NMR parameters (chemical shifts and electric field gradients) were also conducted after relaxing the obtained structures. The calculated NMR parameters of both phases are consistent with the observed NMR spectra. The crystal structures of both phases consist of a perovskite-like layer of (Al,Si)O6 octahedra and a double-layer of SiO4 tetrahedra that are stacked alternatively in the [111] direction of ideal cubic perovskite. The perovskite-like layer is made of a double-layer of Al octahedra for the Ca(Al0.5Si0.5)O2.75 phase, and a triple-layer with a Si octahedral layer sandwiched between two Al octahedral layers for the Ca(Al0.4Si0.6)O2.8 phase. A unique feature common to both structures is that each SiO4 tetrahedron has one terminal oxygen that is not shared by other Si or Al. Homologous relation among these phases and merwinite (Ca3MgSi2O8) in terms of different numbers (1–3) of octahedral layers within the perovskite-like layer is noted.

Macroporous microspheres and microspheroidal particles from polyhydromethylsiloxane

Polysiloxane macroporous microspheres and macroporous spheroidal particles were generated from a linear siloxane polymer using a simple emulsion procedure. Osmotic pressure produced by nano-dispersed ionic salt solution in polysiloxane droplets formed in aqueous emulsion is proposed as a mechanism of macropore formation. The competition between cross-linking of the polymer and osmotic swelling of the microspheres governs the shape and porosity of the particles which were characterized by SEM, 29Si MAS NMR, mercury intrusion porosimetry, and X-ray computed tomography.