29Si Magic Angle Spinning NMR Research Articles

Preparation and Surface Properties of Low-Density Gels Synthesized Using Prepolymerized Silica Precursors

Properties of silica xerogels and aerogels synthesized using a number of prepolymerized silica precursors were probed by 29Si magic-angle spinning (MAS) NMR spectroscopy, the small-angle X-ray scattering (SAXS) method, the nitrogen adsorption method, and transmission electron microscopy (TEM) to show that xerogels with attractive textural properties can easily be prepared using this type of precursors and the conventional one-step, base procedure. Pore sizes and overall pore volumes in these materials can be notably larger than those in the corresponding materials synthesized using tetraethoxysilane. This positive effect stems from the stronger structure of the polymeric network due to a higher degree of silica condensation on one side and a larger thickness of polymeric chains on the other. The thorough investigations of the fine silica structure demonstrate, however, that the relationship between the microstructure of the silica precursor and the micro- and macrostructures of dry gels is complex and the use of more condensed precursors favors, but does not necessarily ensure, more porous dry materials, under the same reaction conditions. Ethyl silicate 40 may be recommended as a low-cost precursor suitable for applications in this situation.

Small-pore framework zirconium and hafnium silicates with the structure of mineral tumchaite

Small-pore framework sodium silicates (AV-14) Na 2MSi 4O 11 · 2H 2O (M = Zr and Hf) possessing the structure of the rare mineral tumchaite have been prepared via hydrothermal synthesis. The AV-14 framework consists of corner-sharing MO 6 (M = Zr, Hf) octahedra and SiO 4 tetrahedra. The [Si 4O 11] silicate sheets may be described as a combination of the spiral chains formed from the corner-sharing SiO 4 tetrahedra. These sheets are connected by MO 6 octahedra, resulting in a three dimensional framework. AV-14 materials are the first examples of synthetic microporous zirconium and hafnium silicates whose structures are built up from silicate sheets. Hf- and Zr-AV-14 materials and the parakeldyshite-type phase, which forms at 1100 °C from the decomposition of Zr-AV-14, have been studied by bulk chemical analysis, powder X-ray diffraction, scanning electron microscopy, 23Na and 29Si magic-angle spinning NMR spectroscopy and thermogravimetry.

Effect of site isolation on the preparation and performance of silica-immobilized Ti CGC-inspired ethylene polymerization catalysts

Titanium constrained-geometry-inspired complexes (CGCs) are assembled using several different synthetic protocols on aminosilica scaffolds. In particular, a new synthetic method is reported that utilizes spatially patterned amines on the silica surface to create Ti–CGC sites that are substantially more active for ethylene polymerization than materials prepared using traditional methods. The materials are characterized using multiple techniques including thermogravimetric analysis (TGA), nitrogen physisorption, FT-Raman spectroscopy, 29Si and 13C cross-polarization magic-angle spinning (CPMAS) NMR spectroscopy, and diffuse–reflectance UV–visible (UV–vis) spectroscopy. While the new patterning protocol allows for quantitative addition of the cyclopentadienyl group to the surface amines and near quantitative metallation with the titanium source, these steps result in subquantitative additions on aminosilica surfaces prepared via traditional techniques (at high amine loadings) and excess titanium addition (at low amine loadings). Using methylaluminoxane (MAO) as a co-catalyst, the patterned catalyst is more productive than the control materials that were prepared using traditional techniques. However, use of MAO causes significant leaching of the metal complex from the solid support. Using a tris(pentafluorophenyl)borane/trialkylaluminum system as co-catalyst alleviates the leaching problem, allowing for the productivity of the immobilized species to be evaluated. In the polymerization of ethylene, the patterned catalyst is shown to be up to 10 times more productive than the solid control materials. The patterned catalyst is also more active than the homogeneous analogue.

Solubility and structure of calcium silicate hydrate

The poorly crystalline calcium silicate hydrate (C-S-H) phases that form near room temperature, which include the technically important C-S-H gel phase formed during the hydration of Portland cement, have a broad similarity to the crystalline minerals tobermorite and jennite, but are characterized by extensive atomic imperfections and structural variations at the nanometer scale. Relationships between the aqueous solubility and chemical structure are reported for specimens formed by different preparation methods and with a broad range of compositions. Both new and previously published data show that these phases generate a family of solubility curves in the CaO–SiO 2–H 2O system at room temperature. As demonstrated by 29Si magic-angle spinning (MAS) NMR data and by charge balance calculations, the observed solubility differences arise from systematic variations in Ca/Si ratio, silicate structure, and Ca–OH content. Based on this evidence, the solubility curves are interpreted as representing a spectrum of metastable phases whose structures range from purely tobermorite-like to largely jennite-like. These findings give an improved understanding of the structure of these phases and reconcile some of the discrepancies in the literature regarding the structure of C-S-H at high Ca/Si ratios.

Investigation of cation environment and framework changes in silicotitanate exchange materials using solid-state 23Na, 29Si, and 133Cs MAS NMR*1

Crystalline silicotitanate (CST), HNa3Ti4Si2O14·4H2O and the Nb-substituted CST (Nb-CST), HNa2Ti3NbSi2O14·4H2O, are highly selective Cs+ sorbents, which makes them attractive materials for the selective removal of radioactive species from nuclear waste solutions. The structural basis for the improved Cs+ selectivity in the niobium analogs was investigated through a series of solid-state magic angle spinning (MAS) NMR experiments. Changes in the local environment of the Na+ and Cs+ cations in both CST and Nb-CST materials as a function of weight percent cesium exchange were investigated using 23Na and 133Cs MAS NMR. Framework changes induced by Cs+ loading and hydration state were investigated with 29Si MAS NMR. Multiple Cs+ environments were observed in the CST and Nb-CST material. The relative population of these different Cs+ environments varies with the extent of Cs+ loading. Marked changes in the framework Si environment were noted with the initial incorporation of Cs+, however with increased Cs+ loading the impact to the Si environment becomes less pronounced. The Cs+ environment and Si framework structure were influenced by the Nb-substitution and were greatly affected by the amount of water present in the materials. The increased Cs+ selectivity of the Nb-CST materials arises from both the chemistry and geometry of the tunnels and pores.

Synthesis and Characterization of Bifunctionalized Ordered Mesoporous Materials

AbstractDirect synthesis (co‐condensation reaction) and post‐synthesis reaction (grafting) are combined for the first time to efficiently fabricate bifunctionalized ordered mesoporous materials (OMMs). Ethylenediamine‐containing OMMs (ED‐MCM‐41) were first synthesized via direct synthesis and then further modified by the phenyl (PH) group in a supercritical fluid (SCF) medium via grafting reaction, resulting in OMMs with ED and PH groups (PH‐ED‐MCM‐41). X‐ray diffraction (XRD) patterns, N2 sorption properties, transmission electron microscopy (TEM), 29Si and 13C magic angle spinning (MAS) NMR, chemical analysis, and hydrothermal treatment were used to characterize the bifunctionalized materials. Experiments show that bifunctionalized OMMs can be efficiently prepared by modifying the directly synthesized monofunctionalized OMMs via grafting reaction in a supercritical fluid medium. Both functional groups are distributed uniformly at the surfaces. The advantage of bifunctionalized OMMs over monofunctionalized OMMs was illustrated by introducing thiol groups into ED‐MCM‐41 materials and the subsequent formation of CdS nanocrystals inside thiol‐ and ED‐functionalized MCM‐41 (HS‐ED‐MC‐41). Because of the variety of the functional groups that can be introduced into OMMs by direct synthesis or post‐synthesis reaction, it is expected that the present strategy could provide a generally applicable approach to the design of OMMs with two functional groups.

State of Aluminum in Dealuminated, Nonhydrated Zeolites Y Investigated by Multinuclear Solid-State NMR Spectroscopy

A series of dealuminated zeolites Y with framework nSi/nAl ratios of 2.8−6.0 was prepared by steaming and characterized by atomic emission spectroscopy and 1H, 27Al, and 29Si NMR spectroscopy. The steaming of zeolite H-Y was performed under water vapor pressures of 3.4−81.5 kPa and at a temperature of 748 K. To exclude an additional modification of the dealuminated zeolites Y, the samples were investigated in the nonhydrated state, i.e., without hydration after the dealumination. By 29Si magic-angle spinning (MAS) NMR spectroscopy, a strong high-field shift of the signals of Si(3Al) and Si(2Al) sites in the spectra of nonhydrated zeolites Y in comparison with those of the hydrated samples was observed. This finding is explained by a change of the Si−O−T bond angles of Si(nAl) sites in the local structure of nonhydrated framework AlO4 tetrahedra. With increasing water vapor pressure during the dealumination, a systematic decrease of the total amounts of framework aluminum atoms in nonhydrated zeolites Y wa...

Study of Sialon S-phases M2AlxSi12-xN16-xO2+x, M = Ba and Ba0.9Eu0.1, by X-ray Single Crystal Diffraction, X-ray Powder Diffraction, and Solid-State Nuclear Magnetic Resonance

The crystal structure of the sialon Ba S-phase Ba2AlxSi12-xN16-xO2+x was refined, for x ≈ 2, using Mo Kα single-crystal data in Imm2 with a = 8.230(1) Å, b = 9.655(2) Å, c = 4.9093(8) Å, and V = 390.1 Å3, to a weighted R(Fobs2) = 9.5% for 520 unique reflections. The Si/Al tetrahedral framework is very similar to that for the iso-structural Sr S-phase. The Ba atom positions are shifted relative those of the Sr atoms by ∼0.34 Å along the c axis and the Ba atoms are coordinated by 10 N/O atoms at 2.85 to 3.35 Å, whereas the Sr atoms are coordinated by 8 N/O atoms at 2.66 to 3.22 Å. Powder samples of Ba S-phases (x ≈ 0 and 2) and an S-phase with 10% of the Ba atoms substituted by Eu atoms (x ≈ 2) were prepared by spark plasma sintering and postheat treatment at 1600 °C for 15 h. The S-phases were studied by 29Si and 27Al magic-angle spinning NMR and the results are discussed in relation to proposed distributions of Si/Al and O/N atoms in the structure.

Structural Study of Inorganic Oxides in a Hybrid Organic−Inorganic Solid Polymer Electrolyte

Polymer−inorganic composite electrolytes very often show superior properties as compared to simple polymer electrolytes, and this enhancement is often ascribed to the structure and interfacial properties of the composite. Here, the structure of aluminosilica (AlSi) domains formed within organic−inorganic solid polymer electrolytes was studied using solid state 29Si and 27Al magic angle spinning (MAS) NMR, transmission electron microscopy (TEM), and nitrogen sorption experiments following material calcinations, to determine how the composite affects the properties in this class of polymer electrolytes. The major feature of all the calcined AlSi's based on 600 MW poly(ethylene glycol) (PEG) is the presence of two types of morphologies: nanoparticles with sizes of about 20−60 nm, and larger platelike particles. Increasing the amount of AlSi in the organic−inorganic composite material (OICM) increases the fraction of platelike particles relative to nanoparticles. The nanoparticles are practically nonporous, ...

Investigation of cation environment and framework changes in silicotitanate exchange materials using solid-state 23Na, 29Si, and 133Cs MAS NMR

Crystalline silicotitanate (CST), HNa 3Ti 4Si 2O 14·4H 2O and the Nb-substituted CST (Nb-CST), HNa 2Ti 3NbSi 2O 14·4H 2O, are highly selective Cs + sorbents, which makes them attractive materials for the selective removal of radioactive species from nuclear waste solutions. The structural basis for the improved Cs + selectivity in the niobium analogs was investigated through a series of solid-state magic angle spinning (MAS) NMR experiments. Changes in the local environment of the Na + and Cs + cations in both CST and Nb-CST materials as a function of weight percent cesium exchange were investigated using 23Na and 133Cs MAS NMR. Framework changes induced by Cs + loading and hydration state were investigated with 29Si MAS NMR. Multiple Cs + environments were observed in the CST and Nb-CST material. The relative population of these different Cs + environments varies with the extent of Cs + loading. Marked changes in the framework Si environment were noted with the initial incorporation of Cs +, however with increased Cs + loading the impact to the Si environment becomes less pronounced. The Cs + environment and Si framework structure were influenced by the Nb-substitution and were greatly affected by the amount of water present in the materials. The increased Cs + selectivity of the Nb-CST materials arises from both the chemistry and geometry of the tunnels and pores.

Direct Method for Surface Silyl Functionalization of Mesoporous Silica

A direct method of surface silyl modification and simultaneous surfactant removal of mesoporous silica is investigated in its physicochemical details. Twelve different silanes of various functionalities are studied. The method employs an alcohol solution of silanes to allow the simultaneous surfactant/silyl exchange process, which results in a more uniform monolayer coverage of the surface and a higher amount of surface attachments of silane. We vary the solution concentration of silanes to study the effect on loadings. It is found that the variation of the surface loading of the silyl group follows a Langmuir adsorption model closely. The method gives one a well-controlled monolayer coverage of the surface. The loadings are determined by the exchange equilibrium. Fittings of the loading data to Langmuir adsorption isotherms give one the adsorption equilibrium constants and maximum surface loadings. We categorize the silanes into three different groups according to the values of the equilibrium constants and discuss them with respect to molecular structures. We also report on the extensive characterizations of the surface-functionalized mesoporous materials, such as nitrogen adsorptions, X-ray diffraction, 29Si magic-angle spinning NMR, 13C magic-angle spinning NMR, and IR spectroscopy. The method provides one with a convenient and highly controllable approach to the surface functionalization of mesoporous silica.

A Study of the Formation of Molecular Sieve SAPO-44

The formation of a silicoaluminophosphate-based molecular sieve SAPO-44 has been investigated in the present study. Particular attention was directed toward the characterization of the intermediate phases formed at different stages during the synthesis. The changes in the long-range ordering of the gel phases as a function of crystallization time was monitored by powder X-ray diffraction, and the development of the local structure of P, Al, and Si atoms was followed by 31P, 27Al, and 29Si magic-angle spinning NMR. 1H → 31P cross polarization (CP) was employed to differentiate the P sites in different phases coexisting in the sample. Several 27Al/31P and 27Al/29Si double-resonance NMR experiments such as CP and transferred echo double-resonance were also utilized to identify the nature of the intermediate phases, and to select the P−O−Al and Si−O−Al bonding connectivities. FT-Raman spectroscopy was used to gain the information regarding the evolution of the porosity in the framework and the immediate envir...

Variable temperature high resolution 29Si MAS NMR of siliceous zeolite ferrierite

Variable temperature high resolution magic angle spinning 29Si NMR spectroscopy is used to study the phase transition between the low and high temperature versions of pure silica ferrierite. Spectra collected at 133–400 K indicate that the low temperature phase is probably of lower symmetry than that previously reported using single crystal X-ray diffraction results. The NMR spectra around the phase transition are consistent with the description of the high temperature phase as a dynamic average of possible low temperature structures. A comparison of the NMR chemical shifts with the results of previous X-ray diffraction experiments confirms the validity of proposed correlations and indicates how they can be of predictive value in certain cases.

Characterization of white Portland cement hydration and the C-S-H structure in the presence of sodium aluminate by 27Al and 29Si MAS NMR spectroscopy

The effects of hydrating a white Portland cement (wPc) in 0.30 and 0.50 M solutions of sodium aluminate (NaAlO 2) at 5 and 20 °C are investigated by 27Al and 29Si magic-angle spinning (MAS) NMR spectroscopy. It is demonstrated that NaAlO 2 accelerates the hydration of alite and belite and results in calcium-silicate-hydrate (C-S-H) phases with longer average chain lengths of SiO 4/AlO 4 tetrahedra. The C-S-H phases are investigated in detail and it is shown that the Al/Si ratio for the chains of tetrahedra is quite constant during the time studied for the hydration (6 h to 2 years) but increases for higher concentration of the NaAlO 2 solution. The average chain lengths of “pure” silicate and SiO 4/AlO 4 tetrahedra demonstrate that Al acts as a linker for the silicate chains, thereby producing aluminosilicate chains with longer average chain lengths. Finally, it is shown that NaAlO 2 reduces the quantity of ettringite and results in larger quantities of monosulfate and a calcium aluminate hydrate phase.

Nuclear magnetic resonance studies of 0.139MO (or M ′2O) · 0.673SiO 2 · (0.188− x)Al 2O 3 · xB 2O 3 (M=Mg, Ca, Sr and Ba, M ′=Na and K) glasses

The 11B, 27Al and 29Si magic angle spinning (MAS) NMR spectra of 0.139MO (or M ′ 2O) · 0.673SiO 2 · (0.188− x)Al 2O 3 · xB 2O 3 · (M=Mg, Ca, Sr and Ba, M ′=Na and K) glasses were examined. When three network forming oxides such as SiO 2, Al 2O 3 and B 2O 3 coexist, Al 2O 3 reacts preferably with network modifier such as M ′ 2O or MO. The reactivity of residual network modifier with SiO 2 was high compared with B 2O 3 in the order of K 2O < Na 2O < BaO < SrO < CaO < MgO. The apparent equilibrium constants, K app, of the reactions, Si(Q 3) + B(Q 3)=Si(Q 4) + B(Q 4), were determined. Here, the Si(Q n ) or B(Q 3) and B(Q 4) represent the SiO 2 structural units or three- and four-coordinated boron atoms, respectively. The subscript are the number of bridging oxygen atoms. The K app values indicate the reactivity of MO or M ′ 2O with B 2O 3 and SiO 2 in the MO(M ′ 2O) · SiO 2 · Al 2O 3 · B 2O 3 glasses. The reaction proceeds to the left direction in the order of MgO > CaO > SrO > BaO > Na 2O > K 2O. The order of the reactivity is the same as that of the electronegativities among alkali and alkaline earth metals.

Synthesis and structural characterization of novel tin and titanium potassium silicates K 4M 2Si 6O 18

The synthesis of a new potassium titanosilicate, K 4Ti 2Si 6O 18 (Ti–AV-11), possessing the crystal structure of potassium stannosilicate AV-11, has been reported. The unit cell of this material is trigonal, space group R3 (no. 146), Z=3, a=10.012, c=14.8413 Å, γ=120°, V=1289 Å 3. The structure of AV-11 is built up of MO 6 ( M=Sn, Ti) octahedra and SiO 4 tetrahedra by sharing corners. The SiO 4 tetrahedra form helix chains, periodically repeating every six tetrahedra. These chains extend along the [001] direction and are linked by isolated MO 6 octahedra, thus producing a mixed octahedral–tetrahedral oxide framework. AV-11 materials have been further characterized by bulk chemical analysis, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), 29Si and 119Sn magic-angle spinning (MAS) NMR spectroscopy.

Aluminosilicate Species in the Hydrogel Phase Formed during the Aging Process for the Crystallization of FAU Zeolite

Faujasite zeolite is synthesized hydrothermally after an aging process under various conditions. Hydrogel is formed during the aging, and the amount increases with increasing the duration. Aluminosilicate species included in the hydrogel phase during the aging is investigated for the changes in Si/Al ratios and Q units during the aging and the crystallizing processes by means of 29Si magic-angle spinning NMR spectroscopy. Aging for more than 1 day is necessary to obtain faujasite with high crystallinity. The distribution in Q4 units in the hydrogel is gradually changed, and the aluminosilicate species having mainly a Q4(4Al) unit is obtained after 7 days of aging, while 2 days of aging results in remanent silicate species derived from the starting silica source. The former gives faujasite crystallites with higher crystallinity, sharper particle size distribution, and a lower Si/Al ratio. On the basis of these findings, the crystallization mechanism is discussed.

Polyoxoanion Occluded within Modified MCM‐41: Spectroscopy and Structure.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation of Scratch‐ and Abrasion‐Resistant Polymeric Nanocomposites by Monomer Grafting onto Nanoparticles, 4

AbstractTo obtain transparent, scratch‐ and abrasion‐resistant coating materials a high content of nanosized silica and alumina filler was embedded in radiation‐curable acrylate formulations by acid catalyzed silylation using trialkoxysilanes. 29Si magic angle spinning (MAS) NMR spectroscopy and matrix‐assisted laser desorption/ionisation time‐of‐flight mass spectrometry (MALDI‐TOF MS) were used to elucidate the structure of the surface‐grafted methacryloxypropyl‐, vinyl‐, and n‐propyltrimethoxysilane. 29Si MAS NMR measurements revealed a predominance of T2 and T3 structures of silicon atoms, i.e., silane oligomers have been formed by an extensive loss of water. For methacryloxypropyltrimethoxysilane, the proportion of the highly condensed T3 silicon atoms was estimated to be 75%. In accordance with these NMR findings, MALDI‐TOF MS showed highly condensed oligomeric siloxanes of more than 20 monomeric silane units. The degree of silane condensation is higher on alumina than on silica, irrespective to the addition of maleic acid as catalyst. Moreover, completely condensed silsesquioxanes, e.g., octamers, decamers, and dodecamers, were detected. Based on the observed MS pattern, a ladder‐like arrangement of two linked siloxane chains forming connected eight‐membered rings is proposed, which is similar for all of the condensed organosilanes under study. The grafted polysiloxanes lead to an organophilation of inorganic nanofillers and improve their dispersibility in acrylate suspensions. Proposed ladder‐like arrangement of the T3 structure of silicon atoms in polysiloxanes grafted on the silica surface.imageProposed ladder‐like arrangement of the T3 structure of silicon atoms in polysiloxanes grafted on the silica surface.

Interactions of bovine serum albumin and lysozyme with sodium silicate solutions

The interaction of lysozyme and bovine serum albumin (BSA) with diluted solutions of sodium silicate was studied at pH 4.7 and 7.2. At neutral pH, lysozyme leads to the formation of a silica precipitate incorporating the protein whereas a composite BSA–silica gel was obtained in acidic media. Using the colorimetric molybdosilicate method, the nature of the silicate species interacting with the proteins could be determined. The effect of pH on these interactions is discussed, taking into account the evolution of the charge of both organic and inorganic components. Characterisation of the solids by thermogravimetric analysis, 29Si magic angle spinning-NMR, electron microscopy and porosimetry confirms the proposed mechanisms of protein–silica interactions. Of particular interest is the fact that BSA–silicate interactions induce protein aggregation that controls silica particles size, mimicking biomineralization processes.