MAS NMR Spectroscopy Research Articles

Investigation on the structure and viscosity of BaO-bearing slag melt through molecular dynamics simulation, Raman and 27Al MAS NMR spectra

Studying the structure and viscosity of BaO-bearing slag was helpful for explaining the micro mechanism of its physicochemical property variations, and further guiding the utilization of bearing-barium ores for hot metal production. The structure of CaO-SiO2-10 wt% MgO-12 wt% Al2O3-BaO slag with BaO content varying from 0 to 5 wt% was studied based on the combined computational-experimental method, including the molecular dynamic (MD) simulation and experimental measurements of Raman spectroscopy and 27Al magic angles pinning nuclear magnetic resonance (27Al MAS-NMR) spectroscopy. The viscosity of the slag was measured, and the relationship between the viscosity and polymerization degree of the slag was established. MD simulation results indicated that the coordination numbers (CNs) of Si-O kept unchanged. The concentrations of four-coordinated Al units and bridged oxygen increased with an increase in BaO. The distribution of Qi units of Si and Al indicated that the structural units tended to be more complicated with the increasing BaO content. The result from Raman spectroscopy indicated that the amount of Q2 and Q3 units increased with the addition of BaO, while that of Q0 and Q1 units decreased. The results from 27Al MAS-NMR spectroscopy showed that the relative concentration of [AlO4] structural units increased from about 48 to 53 wt%, and an opposite trend was observed in [AlO5] and [AlO6] units, indicating that higher BaO content enhanced the polymerization degree of the slag. In addition, the viscosity of the slag increased with the rising BaO content, which was caused by the dominant effect of Ba2+.

Structure of lead borate glasses by Raman, 11B MAS, and 207Pb NMR spectroscopies

A multi-spectroscopic investigation of glasses xPbO-(100-x)B2O3, where 30 ≤ x ≤ 80, was undertaken employing Raman, 207Pb static NMR, and 11B MAS NMR spectroscopies. The Raman results were quantified and combined with quantitative NMR results to produce a coherent structural model. The fraction of four-coordinated borons (N4) has a maximum near x = 50, in contrast with alkali modified borates where the maximum is near x = 35–40. In the range x = 30–50, both NMR and Raman spectroscopies indicate that lead ion behaves more ionically. From x = 50–65, the spectroscopic results indicate a change in the lead-oxygen bonding character from ionic to covalent. At similar compositions, the borate network becomes more ionic while remaining under-modified compared to alkali borates; consistent with lead-oxygen bonding being important to the structural evolution. When x > 65, the lead-oxygen bonding becomes significantly covalent.

Silsesquioxane-based porous polymer derived from organic chromophore with AIE characteristics for selective detection of 2,4-dinitrophenol and Ru3+

A novel silsesquioxane-based polymers (PCS-DPB) was prepared by the Friedel-Crafts reaction of octavinylsilsesquioxane (OVS) with AIE-active 4-[1-cyano-2-[4-(diphenylamino)phenyl]ethenyl]-α-[[4-(diphenylamino)phenyl]methylene]benzeneea-cetonitrile (CN-DPB). FTIR, solid state 29Si MAS and 13C CP MAS NMR spectroscopy analyses are performed to confirm its chemical structure and the presence of silsesquioxane units in the cross-linked network. XRD and TEM results prove its amorphous feature of the PCS-DPB material. TGA shows a high thermal stability of PCS-DPB with 5% mass loss temperature about 438 °C due to the highly cross-linking density and the presence of the inorganic silsesquioxane cage. The CN-DPB unit endows PCS-DPB with an excellent luminescence and a large red shift of 120 nm, which makes it to act as a probe to detect 2,4-dinitrophenol (DNP) and Ru3+ with high selectivity and sensitivity.

CURD: a Single-Shot Strategy to Obtain Assignments and Distance Restraints for Proteins Using Solid-State MAS NMR Spectroscopy.

We present a strategy dubbed CURD (correlations using recycle delays) to acquire chemical-shift assignments and distance restraints for proteins in a single experimental block under slow-moderate magic-angle spinning conditions. This is done by concatenating the 3D-CCC and 3D-NNC experiments, both of which individually require long experimental times for sufficient resolution and sensitivity to be realized. Unlike previous approaches, the CURD strategy does not increase the amount of radio-frequency deposition on the sample and does not require lengthy procedures to optimize any of the pulse sequence elements. Instead, time savings is obtained by using the hitherto unused recycle delay of one of the experiments (2D-CC/3D-CCC) to establish inter-residue correlations for the second experiment (2D-NN/3D-NNC). Experiments are demonstrated on a model protein at the MAS frequency of 12.5 kHz and are shown to result in time savings of the order of days for most of the routine cases.

Belite-calcium sulphoaluminate cement prepared by EMR and BS: Hydration characteristics and microstructure evolution behavior

Belite-calcium sulphoaluminate (B-CSA) cement is regarded as one of the most promising alternatives to ordinary Portland cement (OPC). The production of B-CSA cement from industrial solid waste is a critical technology for maximising mineral resource utilisation. The morphology and microstructure of hydration products produced during the hydration process of B-CSA cement have a significant impact on the mechanical properties of the cement. In this paper, B-CSA cement prepared by electrolytic manganese residue (EMR) and barium slag (BS) was taken as the research object, and the changes of PH, turbidity and electrical conductivity in the early hydration process were monitored. Furthermore, the phase composition and microstructure of hydration products of cement paste were analysed using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric analysis (TG), solid state MAS-NMR spectroscopy, nitrogen adsorption test, scanning electron microscope (SEM) and electron microprobe analyzer (EPMA). The results show that with the sharp change of pH value, turbidity and electrical conductivity in the early hydration process of B-CSA cement, a large amount of ettringite is formed during the rapid hydration of ye’elimite, the metastable AFt part is transformed into AFm and the hydration reaction of belite was inhibited. The hydration products are mainly ettringite, monosulfate and AH3, which are responsible for the strength development of B-CSA cement. The hydration rate of belite in the early stage is slow, and the C-S-H gel phase can be formed in an aluminum-rich environment. The microstructure evolution of cement paste was revealed by nitrogen adsorption tests and SEM. The crystal state of ettringite changes from columnar to fibrous to construct the hardened framework of disorderly distributed cement paste, and the gel phase fills in the void formed by the ettringite skeleton to form a dense structure and reduce the porosity. The interface between the unreacted belite phase and the AFt skeleton is clear, and the gel phase plays a role in connecting them. Mn2+ was enriched on the C-S-H gel phase to form the Ca-Si-Mn compound and solidified. Ba2+ preferentially forms BaSO4 with SO42− which precipitates at the interface between the ettringite phase and C-S-H gel phase. Based on the analysis of the test results, the hydration characteristics and microstructure evolution of B-CSA cement were determined.

High Na+ conducting Na3Zr2Si2PO12/Na2Si2O5 composites as solid electrolytes for Na+ batteries

AbstractSodium superionic conductor Na3Zr2Si2PO12 (NZSP) is a promising material as a solid electrolyte for sodium‐ion batteries. The highest conductivity of ∼1.0 mS/cm at room temperature (RT) was reported for the compound with a Na content of approximately 3.3 per formula unit (f. u.) and when the material is synthesized with a final sintering temperature ≥1220°C. Herein, we propose a new synthesis method to enhance the conductivity of the NZSP by liquid‐phase sintering with the optimum amount of additive of amorphous‐Na2Si2O5. In this regard, a series of composite materials were prepared by mixing Na3Zr2Si2PO12 with amorphous‐Na2Si2O5 (NZSP/NS‐x wt.%; with x = 0.0, 2.5, 5.0, 7.5, 10.0) and sintering at a lower temperature of 1150°C. Enhanced conductivity of 1.7 mS/cm at RT has been achieved for the Na3Zr2Si2PO12/Na2Si2O5‐5.0 wt.% (NZSP/NS‐5.0) composite. The effects of additives on the NZSP phase formation, microstructure, and ion conductivity have been investigated by XRD, MAS NMR, SEM, and impedance spectroscopy. Our study demonstrates that the higher conductivity of the NZSP/NS‐5.0 composite is caused by the combined effect of increased Na content in the NZSP phase (by diffusion of Na+ ions from the liquid phase of NS to bare NZSP phase), higher density, and microstructures with lesser pores.

Quantification of Biologically and Chemically Bound Phosphorus in Activated Sludge from Full-Scale Plants with Biological P-Removal.

Phosphorus (P) is present in activated sludge from wastewater treatment plants in the form of metal salt precipitates, extracellular polymeric substances, or bound into the biomass, for example, as intracellular polyphosphate (poly-P). Several methods for a reliable quantification of the different P-fractions have recently been developed, and this study combines them to obtain a comprehensive P mass-balance of activated sludge from four enhanced biological phosphate removal (EBPR) plants. Chemical characterization by ICP-OES and sequential P fractionation showed that chemically bound P constituted 38–69% of total P, most likely in the form of Fe, Mg, or Al minerals. Raman microspectroscopy, solution state 31P NMR, and 31P MAS NMR spectroscopy applied before and after anaerobic P-release experiments, were used to quantify poly-P, which constituted 22–54% of total P and was found in approximately 25% of all bacterial cells. Raman microspectroscopy in combination with fluorescence in situ hybridization was used to quantify poly-P in known polyphosphate-accumulating organisms (PAO) (Tetrasphaera, Candidatus Accumulibacter, and Dechloromonas) and other microorganisms known to possess high level of poly-P, such as the filamentous Ca. Microthrix. Interestingly, only 1–13% of total P was stored by unidentified PAO, highlighting that most PAOs in the full-scale EBPR plants investigated are known.

Evaluation of Zeolite Acidity by 31P MAS NMR Spectroscopy of Adsorbed Phosphine Oxides: Quantitative or Not?

31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy of adsorbed alkyl-substituted phosphine oxides has witnessed tremendous progress during the last years and has become one of the most informative and sensitive methods of zeolite acidity investigation. However, quantitative evaluation of the number of sites is still a challenge. This study clarifies the main origin of errors occurring during NMR experiments, introduces the appropriate standards (both internal and external), and determines the relaxation parameters and the conditions for the acquisition and integration of spectra. As a result, a methodology for the quantitative measurement of the content of Brønsted and Lewis sites and the amount of internal and external silanol groups is established. The application of probe molecules of different sizes (namely, trimethylphosphine oxide (TMPO), tri-n-butylphosphine oxide (TBPO), and tri-n-octylphosphine oxide (TOPO)) is shown to be a good tool for distinguishing between the active sites inside the zeolite pores, mesopores, and on the outer crystal surface. The methodology proposed is verified on BEA zeolites different in composition, texture, and morphology.

The way to increase the monovalent ion selectivity of FujiFilm® anion-exchange membranes by cerium phosphate modification for electrodialysis desalination

Recently, the selectivity of ion-exchange membranes and its control have been the subject of numerous fundamental and applied studies. This paper reports an enhanced monovalent anion selectivity of hybrid membrane based on the FujiFilm® Type I anion-exchange membrane and in situ synthesized cerium phosphate (CeP) with a gradient distribution over the membrane thickness. The Cl/SO4-selectivity coefficient (P(Cl/SO4)) of hybrid membranes reached 6.2. The effect of modification on the Cl/SO4 selectivity passed through a minimum as the total concentration of the external solution increased from 0.04 M to 1 M. For low concentration electrolytes, Cl/SO4-selectivity improving was attributed to partial neutralization of positive fixed charges by negative fixed charges of cerium phosphate, which causes an effective decrease of ion-exchange capacity and reduction of electrostatic selectivity factor favouring sulfate transport; for high concentration electrolytes, to the formation of narrow transport channel between the surface of the inorganic nanoparticle and the pore wall, which causes a decrease of sulfate transport due to the size sieving effect. The hybrid membranes and cerium phosphate were characterized using ATR FTIR and 31P MAS NMR spectroscopy, XRD analysis and scanning electron microscopy with EDX microanalysis.

Impact of sulphate source on the hydration of ternary pastes of Portland cement, calcium aluminate cement and calcium sulphate

The present work investigates the hydration and evolution of solid phase assemblage as a function of sulphate source in ternary Portland cement (PC), calcium aluminate cement (CAC) and calcium sulphate (CS¯HX) cement pastes. Two binders are compared, a PC‒rich and a CAC ‒ CS¯Hx ‒rich paste, containing gypsum and anhydrite as sulphate carrier, using a multi-method approach including calorimetry, XRD, TGA, MAS NMR spectroscopy, microscopy and thermodynamic modelling. The overall phase assemblage is very similar for the two-sulphate source in both PC-rich and CAC ‒ CS¯Hx−rich systems. However, the quantitative X-ray analysis, TGA and the 27Al, 29Si NMR show that the amounts of the crystalline hydrates and X-ray amorphous phases are influenced by the type of sulphate. In the long-term hydration, the anhydrite-bearing formulations exhibit the highest amount of ettringite, whereas the gypsum-containing samples develop a higher fraction of AFm phases and X-ray amorphous hydrates. This difference may relate to faster dissolution kinetics of gypsum compared to anhydrite in the studied ternary blends. For the CAC ‒ CS¯Hx −rich pastes, gehlenite from CAC (C2AS) shows hydraulic activity, which primarily results in the precipitation of strätlingite. Higher amounts of strätlingite are identified in the gypsum bearing samples, suggesting that the type of sulphate source impacts the hydration of silicate-bearing phases. Finally, the phase assemblages from thermodynamic modelling (using the GEMS software) are found to be in good agreement with those observed experimentally, although some differences occur as a result of kinetic effects.

Characterization of Monochromate and Hemichromate AFm Phases and Chromate-Containing Ettringite by 1H, 27Al, and 53Cr MAS NMR Spectroscopy

The calcium aluminate hydrate AFm and AFt phases formed upon hydration of Portland cement have an important role in the stabilization and solidification of hazardous chromate ions in hardened cement. AFm monochromate (Ca4[Al(OH)6]2(CrO4)·12H2O), AFm hemichromate (Ca4[Al(OH)6]2(CrO4)0.5(OH)·12H2O) and the chromate-containing AFt phase, Ca6[Al(OH)6]2-(CrO4)3·24H2O, were synthesized and investigated by 1H, 27Al, and 53Cr MAS NMR spectroscopy. 27Al quadrupolar coupling parameters (CQ, ηQ) and isotropic chemical shifts (δiso) were determined for the three phases, including two distinct Al sites in chromate-AFt, as observed by 27Al MAS and MQMAS NMR. Two dominant peaks are apparent in the 1H MAS NMR spectra of each of the phases. For the AFm phases, these resonances are assigned to framework hydroxyl groups (1.7–2.0 ppm) and water molecules/hydroxyls (5.0–5.5 ppm) in the interlayer. For chromate-AFt, the peaks are ascribed to framework hydroxyl groups in the [Ca6Al2(OH)12]6+ columns (~1.4 ppm) and water molecules (~4.8 ppm) associated with the Ca ions. 53Cr MAS NMR spectra acquired at 22.3 T for the samples show a narrow resonance for both chromate AFm phases, whereas indications of three distinct Cr resonances are apparent for the chromate AFt. The absence of any second-order quadrupolar effects in the 53Cr NMR spectra strongly suggests that the chromate ions are highly mobile in the anionic sites of the AFm and AFt structures. The NMR data reported in this work are in agreement with the reported crystal structures for the chromate AFm and AFt phases and may be useful for identification and characterization of chromate fixation in cementitious systems, complementing information gained from conventional powder X-ray diffraction studies.

Design of plate-like H[Ga]MFI zeolite catalysts for high-performance methanol-to-propylene reaction

Constructing plate-like zeolites with the controllable acidity represents an efficient strategy to promote their catalytic performance in methanol-to-olefins conversion. Herein, the plate-like H[Al/Ga]MFI zeolites with the similar crystal morphology but controllable acidity have been prepared via a fluoride-assisted low-temperature crystallization approach. The acidic properties of the plate-like zeolites are determined by NH 3 -TPD and probe-assisted 1 H and 31 P MAS NMR spectroscopy. The impact of acidity including the strength and density of the plate-like H[Al/Ga]MFI on the dual-cycle mechanism and the corresponding catalytic performance in the methanol-to-propylene conversion is subsequently confirmed. Reducing the acid strength and density can efficiently suppress the aromatic-based cycle, and thus hinder the accumulation of the polycyclic aromatics. The plate-like H[Ga]MFI zeolites with weak acid strength and most suitable acid density can well balance the dual-cycle mechanism, thereby achieving high propylene selectivity and long catalyst lifetime simultaneously. • Plate-like H[Al/Ga]MFI zeolites with the controllable acidity successfully constructed. • Impact of acidic strength and density of plate-like HMFI zeolites on the dual-cycle mechanism well elucidated. • Remarkable MTP performance achieved on plate-like H[Ga]MFI zeolites with weak acid strength and suitable acid density.

Planar Si5 and Ge5 Pentagons beside Isolated Phosphide Anions in Lithium Phosphide Tetrelides Li10+xSi5P and Li10+xGe5P

AbstractBinary lithium silicides and germanides as well as ternary lithium phosphidotetrelates have been intensively investigated with regard to their suitability as anode materials for all‐solid‐state‐batteries. In this context we studied the ternary phase diagram Li/Tt/P on the phosphorus‐poor side. The phosphide tetrelides Li10.68(8)Si5P and Li10.1(2)Ge5P are ternary compounds in the respective phase systems so far uniquely featuring discrete building blocks, which were previously found exclusively in binary alkali metal tetrelides and phosphides. Li10.68(8)Si5P and Li10.1(2)Ge5P contain aromatic Tt5 pentagons with an environment of mostly ordered Li atoms and isolated P3− anions with a coordination environment of predominantly disordered Li atoms. The five‐membered ring subunits are stacked under formation of [Li6Tt5] chains (Tt=Si, Ge). chains. The compounds crystallize in the space group Pnma (63) (Z=4) with cell parameters of a=10.2434(3) Å, b=4.2788(1) Å, c=23.9767(3) Å for Li10.68(8)Si5P and a=10.3599(11) Å, b=4.3072(2) Å, c=24.267(2) Å for Li10.1(2)Ge5P. The single crystal structure determination is supplemented by X‐ray powder diffraction experiments and 7Li, 29Si and 31P solid state MAS NMR spectroscopy. With respect to their electron count the compounds are placed between compounds comprising aromatic five‐membered rings such as the electron‐precise Zintl phase Li8MgSi6 and the non‐classical Zintl phase Li12Si7 having two extra electrons.

Exploring Hydrothermal Synthesis of SAPO-18 under High Hydrostatic Pressure.

The effect of external hydrostatic pressure on the hydrothermal synthesis of the microporous silicoaluminophosphate SAPO-18 has been explored. The crystallization of the SAPO-18 phase is inhibited at 150 °C under high pressures (200 MPa) when using relatively diluted synthesis mixtures. On the contrary, the use of concentrated synthesis mixtures allowed SAPO-18 to be obtained in all the studied conditions. The obtained solids were characterized with XRD, SEM, ICP-AES, TG and 27Al and 31P MAS NMR spectroscopy. The results highlight the importance of the external pressure effect on the hydrothermal synthesis of molecular sieves and its influence on the interaction between the organic molecule and the silicoaluminophosphate network.

Site-specific protein methyl deuterium quadrupolar patterns by proton-detected 3D 2H-13C-1H MAS NMR spectroscopy.

Determination of protein structure and dynamics is key to understand the mechanism of protein action. Perdeuterated proteins have been used to obtain high resolution/sensitivty NMR experiments via proton-detection. These methods utilizes 1H, 13C and 15N nuclei for chemical shift dispersion or relaxation probes, despite the existing abundant deuterons. However, a high-sensitivity NMR method to utilize deuterons and e.g. determine site-specific deuterium quadrupolar pattern information has been lacking due to technical difficulties associated with deuterium's large quadrupolar couplings. Here, we present a novel deuterium-excited and proton-detected three-dimensional 2H-13C-1H MAS NMR experiment to utilize deuterons and to obtain site-specific methyl 2H quadrupolar patterns on detuterated proteins for the first time. A high-resolution fingerprint 1H-15N HSQC-spectrum is correlated with the anisotropic deuterium quadrupolar tensor in the third dimension. Results from a model perdeuterated protein has been shown.

The crystal structure of mineral magadiite, Na2Si14O28(OH)2∙8H2O

Abstract Magadiite from Lake Magadi was structurally analyzed based on X-ray powder diffraction data. The idealized chemical composition of magadiite is Na16[Si112O224(OH)16]∙64H2O per unit cell. The XRD powder diffraction pattern was indexed in orthorhombic symmetry with lattice parameters a0 = 10.5035(9) Å, b0 = 10.0262(9) Å, and c0 = 61.9608(46) Å. The crystal structure was solved from a synthetic magadiite sample in a complex process using 3D electron diffraction combined with model building as presented in an additional paper. A Rietveld refinement of this structure model performed on a magadiite mineral sample in space group F2dd (No. 43) converged to residual values of RBragg = 0.031 and RF = 0.026 confirming the structure model. Physico-chemical characterization using solid-state NMR spectroscopy, SEM, TG-DTA, and DRIFT spectroscopy further confirmed the structure. The structure of magadiite contains two enantiomorphic silicate layers of, so far, unknown topology. The dense layers exhibit no porosity or micro-channels and have a thickness of 11.5 Å (disregarding the van der Waals radii of the terminal O atoms) and possess a silicon Q4 to Q3 ratio of 2.5. 16 out of 32 terminal silanol groups are protonated, and the remaining groups compensate for the charge of the hydrated sodium cations. Bands of edge-sharing [Na(H2O)6/1.5] octahedra are intercalated between the silicate layers extending along (110) and (110). The water molecules are hydrogen bonded to terminal silanol groups with O···O distances of 2.54–2.91 Å. The structure of magadiite is slightly disordered, typical for hydrous layer silicates (HLS), which possess only weak interactions between neighboring layers. In this respect, the result of the structure refinement represents a somewhat idealized structure. Nevertheless, the natural magadiite possesses a higher degree of structural order than any synthetic magadiite sample. The structure analysis also revealed the presence of strong intra-layer hydrogen bonds between the terminal O atoms (silanol/siloxy groups), confirmed by 1H MAS NMR and DRIFT spectroscopy. The surface zone of the silicate layers, as well as the interlayer region containing the [Na(H2O)6/1.5] octahedra, are closely related to the structure of Na-RUB-18.

New types of hyperbranched 1,2,3-triazole-alkoxysiloxane functional polymers for metal embedded nanocomposite surface coatings.

In this study, using the "click-chemistry" azide-alkyne cycloaddition reaction, two new types of polymers, namely, water-soluble amine-functional and water-insoluble pyridine-functional hyperbranched 1,2,3-triazoleorganoethoxysiloxane polymers, capable of stabilizing ultra-small silver nanoparticles and efficient for chemical surface modification were synthesized and characterized by 1H-NMR, 13C-NMR, 29Si-NMR and FT-IR spectroscopy, MALDI and GPC. Coordinatively active hetero-organic polymers with a flexible branched polyethoxysiloxane backbone bearing chelating 1,2,3-triazolyl-dimethylamine and -pyridine conjugated functional groups were exploited towards coordination with Ag+ metal ions, and formation and stabilization of narrowly dispersed silver nanoparticles (Ag-NPs) obtained in the process of radiation induced metal ion reduction. The influence of the chemically assisted radiation process on the Ag-NP size and size distribution was investigated. Hyperbranched polyorganoethoxysiloxanes loaded with Ag-NPs were covalently cross-linked on a Stöber silica surface, demonstrating the efficiency of the developed hetero-organic functional polymers in the preparation of functional nanocomposite coatings for various applications (heterogeneous catalytic systems, antibacterial materials, nanoparticle-based optical sensing devices, etc.). The nanocomposites were investigated and characterized by TEM-EDS, DLS, UV-Vis spectroscopy and 29Si MAS NMR spectroscopy.

Hydronium ion and water complexes vs. methanol on solid catalyst surfaces: how confinement influences stability and reactivity

Nature and stability of adsorbed water species on typical solid catalysts are assigned and their stability against desorption is compared with methanol by using quantitative 1H MAS NMR spectroscopy.

Urea-based covalent organic crown polymers and KI electrostatic synergy in CO2 fixation reaction: A combined experimental and theoretical study

Multifunctional organic polymer materials are supposed to be the most promising catalysts in the utilization of carbon dioxide (CO2) to five-membered cyclic carbonates (5CCs). Herein, functional one dimensional (1D) organic polymer materials (1D-UCP and 1D-UP) were successfully synthesized and their structural features were thoroughly characterized by FT-IR, 13C CP MAS NMR, SEM, TGA and XPS spectroscopy. These materials showed excellent reaction performance in the CO2 cycloaddition reaction. Notably, the 1D-UCP decorated by crown ether group and urea unit simultaneously showed excellent yields of 5CCs under solvent-free conditions and low pressure of CO2. The outstanding performance was attributable to the synergistic effect of activated KI by coordinating with crown ether fragment and urea unit as hydrogen bonding donor facilitating implementation of speed-determined step of this reaction. In combination with density functional theory (DFT) calculations including intermediates structure optimization and transition states free energy profile, a possible catalytic mechanism was proposed that the urea group accelerated the reaction by vicinal dual hydrogen bonding and increasing nucleophilicity of I anion.

Porous SiC and SiC/Cf Ceramic Microspheres Derived from Polyhydromethylsiloxane by Carbothermal Reduction.

A simple and inexpensive method for the preparation of porous SiC microspheres is presented. Polysiloxane microspheres derived from polyhydromethylsiloxane (PHMS) cross-linked with divinylbenzene (DVB) were ceramized under conditions leading to the removal of oxygen from the material. The content of free carbon (Cf) in highly crystalline silicon carbide (SiC) particles can be controlled by using various proportions of DVB in the synthesis of the pre-ceramic material. The chemical structure of the ceramic microspheres was studied by elemental analysis for carbon and oxygen, 29Si MAS NMR, 13C MAS NMR, SEM/EDS, XRD and Raman spectroscopies, and their morphology by SEM, nitrogen adsorption and mercury intrusion porosimetries. The gaseous products of the thermal reduction processes formed during ceramization created a porous structure of the microspheres. In the SiC/Cf microspheres, meso/micro pores were formed, while in carbon-free SiC, microspheres macroporosity dominated.