29Si MAS NMR Spectra Research Articles

Structural and chemical investigation of the gel-derived bioactive materials from the SiO2–CaO and SiO2-CaO-P2O5 systems

Abstract A series of biomaterials from the binary SiO 2 –CaO and ternary SiO 2 -CaO-P 2 O 5 systems were obtained by the sol-gel route in the form of powders. The CaO/SiO 2 molar ratio, of the samples varied from 1:4 to 3:2. The effect of chemical composition on the structure was investigated with XRD diffraction and FTIR spectroscopy. The presence and relative contribution of silicon Q n (Si) and phosphorus Q n (P) structural units was determined by 29 Si and 31 P MAS NMR. For all of the materials theoretical and NMR network connectivity were calculated. Optical basicity and group basicity theories enabled determination of dependencies between particular types and amounts of Q n (Si) and Q n (P) units. The textural parameters of the powders were determined with the BET. Our studies revealed an increased tendency for crystallization with increasing amount of calcium, in both binary and ternary materials. 29 Si MAS NMR spectra revealed that silicon was present in Q 2 , Q 3 and Q 4 structural units. Moreover, 31 P MAS NMR indicated that phosphorus exist mainly as a monophosphate Q° complex. Results shown that calcium induced structure de-polymerization in binary materials, while in ternary system the formation of phosphorus-rich clusters binding calcium were formed, and re-polymerization of silica network took place. Assessment of the biomaterials dissolution and ion release is crucial in terms of further medical applications. Materials solubility expressed as a Ca and Si ion release was measured with the ICP-OES spectrometry. It showed that Ca ions release was more chemical composition dependent, while in contrary the Si release, depended more on the materials textural parameters and crystallinity.

Short range order characterization of the Na2S + SiS2 glass system using Raman, infrared and 29Si magic angle spinning nuclear magnetic resonance spectroscopies

Sodium thiosilicate glasses are of interest in electrochemical applications as fast ion conducting solid state electrolyte materials. While some structural studies have been performed on this glass series, their short range order (SRO) structure has not been fully realized. Specifically the SiS4 tetrahedra corner sharing behavior in these glasses appears to be less understood, particularly the existence and nature of the Si3, where the superscript describes the number of bridge bonds of the form SiSSi, structure in this glass system has not been fully examined. In this paper, glasses and polycrystals in the sodium thiosilicate system have been explored and characterized using a combination 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, infrared (IR) spectroscopy and for the first time Raman spectroscopy. Raman and IR spectroscopy have been used to determine the composition dependence of the structural units in this system. Specifically, the Raman spectra were used to differentiate corner shared Si3, Si2, and Si1 units and edge shared Si2 structure units.29Si MAS NMR spectra were then deconvoluted to determine the composition dependence of the populations of these different structural units.

Trivalent transition-metal cations and local structure in pyrope- and grossular-rich solid solutions investigated by 27Al and 29Si MAS NMR spectroscopy

Pyrope-rich garnet is an important “sink” for trivalent transition metals such as Cr 3+ , V 3+ , and Fe 3+ in Earth’s upper mantle. In order to obtain a better crystal-chemical understanding of transition metals in garnet and especially pyrope solid solutions, 27 Al and 29 Si MAS NMR spectra were collected on synthetic crystals of composition Mg 3 (Al 0.915, V 0.085 ) 2 Si 3 O 12 and Mg 3 (Al 0.98, Cr 0.02 ) 2 Si 3 O 12 , and compared with the 29 Si MAS NMR spectrum of a Fe 3+ -bearing grossular garnet [Ca 3 (Al 0.956 Fe 3+ 0.044 ) 2 Si 3 O 12 ]. The purpose of the study is to explore the nature of paramagnetic resonances, or their lack thereof, arising from interactions of Cr 3+ , V 3+ , and Fe 3+ with 27 Al and 29 Si nuclei. The 27 Al and 29 Si MAS NMR spectra of Cr 3+ -bearing pyrope show significant resonance broadening, similar to that of Fe 3+ -bearing grossular, which hinders any crystal-chemical interpretation. The 27 Al and 29 Si MAS NMR spectra of V 3+ -bearing pyrope, on the other hand, show several distinct and narrow paramagnetically shifted NMR resonances, even at relatively high concentrations of V 3+ . The various NMR resonances can be assigned to different local atomic configurations around AlO 6 or SiO 4 groups having zero, one, two or three next-nearest octahedrally coordinated V 3+ neighbors. The intensities of the experimental resonances were measured and compared to those calculated for the hypothetical case of statistically random mixing of V 3+ cations to investigate Al-V 3+ mixing behavior at the octahedral site. A simple analysis, based on the number of unpaired electrons for a given transition metal ( i.e ., Fe 2+,3+ , Cr 3+ , and V 3+ ) in pyrope, is made in an attempt to explain the marked differences in their behavior in 27 Al and 29 Si MAS NMR spectra.

An investigation of local Fe2 + order-disorder in a mantle grospydite garnet using paramagnetically shifted 27Al and 29Si MAS NMR resonances

An attempt is made for the first time to measure the local (short-range) distribution of the X-cations in a ternary almandine-pyrope-grossular garnet solid solution using paramagnetically shifted NMR resonances. Initial 27 Al MAS NMR measurements made on an almandine-rich garnet of composition Alm 45 Prp 42 Grs 13 proved to be uninterpretable because of the high Fe 2+ concentration. Thus, a grossular-rich and almandine-poorer garnet occurring in a mantle grospydite xenolith taken from the Roberts Victor kimberlite, South Africa, was chosen for detailed 27 Al and 29 Si MAS NMR spectroscopic study. The crystals are compositionally homogeneous based on electron microprobe analysis, showing no measurable zoning, and have the formula Alm 23 Prp 30 Grs 47 [(Fe 0.23 Mg 0.30 Ca 0.47 ) 3 Al 2 Si 3 O 12 ]. The garnet is cubic with the conventional space group Ia 3 d . The composition and, also importantly, the lack of measurable zoning or compositional heterogeneity, as well as the mode of occurrence, make this garnet a key sample for investigating the possible presence of short-range X cation order. The 27 Al MAS NMR spectrum shows a very broad asymmetric resonance located between about 100 and −50 ppm. It consists of a number of individual closely overlapping paramagnetically shifted resonances, which are difficult to analyze quantitatively. The 29 Si MAS NMR spectrum, showing better resolution, has two broad but observable resonances termed S0 and S4. S0 is located between about −60 ppm and −160 ppm and S4 is centered at roughly 95 ppm. Both S0 and S4 are composite resonances containing many overlapping individual peaks. S0 contains information on local X-cation configurations, where an isolated SiO 4 group in the garnet structure does not have any edge-shared Fe 2+ -containing dodecahedra. S4 involves various local configurations where there is one edge-shared dodecahedron containing Fe 2+ . The measured intensity of the resonances S0 and S4 are roughly similar to calculated intensities assuming random Fe 2+ -cation mixing. These first results do not indicate any overt short-range X-cation order in grospydite garnet.

Bioinspired Insights into Silicic Acid Stabilization Mechanisms: The Dominant Role of Polyethylene Glycol-Induced Hydrogen Bonding

Mono- and disilicic acids were stabilized by uncharged polyethylene glycols (PEGs) in silica-supersaturated solutions (the starting solution contained 500 ppm/8.3 mM sodium orthosilicate, Na2SiO3·5H2O, expressed as SiO2) at pH = 7, most likely by hydrogen bonding between the silanol groups and -CH2-CH2-O-ether moieties. The stabilization was monitored by measuring molybdate-reactive silica and also by a combination of liquid- and solid-state (29)Si NMR spectroscopy. It depends on PEG concentration (20-100 ppm) and molecular weight (1550-20,000 Da). Two narrow (29)Si NMR signals characteristic for monosilicic acid (Q(0)) and disilicic acid (Q(1)) can be observed in (29)Si NMR spectra of solutions containing PEG 10000 with intensities distinctly higher than the control, that is, in the absence of PEG. Silica-containing precipitates are observed in the presence of PEG, in contrast to the gel formed in the absence of PEG. These precipitates exhibit similar degrees of silica polycondensation as found in the gel as can be seen from the (29)Si MAS NMR spectra. However, the (2)D HETCOR spectra show different (1)H NMR signal shifts: The signal due to H-bonded SiOH/H2O, which is found at 6 ppm in the control, is shifted to ~7 ppm in the PEG-containing precipitate. This indicates the formation of slightly stronger H-bonds than in the control sample, most likely between PEG and the silica species. The presence of PEG in these precipitates is unequivocally proven by (13)C CP MAS NMR spectroscopy. The (13)C signal of PEG significantly shifts and is much narrower in the precipitates as compared to the pristine PEG, indicating that PEG is embedded into the silica or at least bound to its surface (or both), and not phase separated. FT-IR spectra corroborate the above arguments. The H-bonding between silanol and ethereal O perturbs the band positions attributed to vibrations involving the O atom. This work may invoke an alternative way to envision silica species stabilization (prior to biosilica formation) in diatoms by investigating possible scenarios of uncharged biomacromolecules playing a role in biosilica synthesis.

Sustainable Synthesis of Zeolites without Addition of Both Organotemplates and Solvents

The development of sustainable and environmentally friendly techniques for synthesizing zeolites has attracted much attention, as the use of organic templates and solvents in the hydrothermal synthesis of zeolites is a major obstacle for realizing green and sustainable synthesis ways. Recently, the introduction of the organotemplate-free synthesis method allowed avoiding the use of organic templates, but water as solvent was still required; solvent-free routes on the other hand beared the potential to significantly reduce the amount of polluted wastewater, but organic templates were still present. In this work, we have demonstrated a combined strategy of both organotemplate- and solvent-free conditions to synthesize aluminosilicate zeolites Beta and ZSM-5 (S-Beta and S-ZSM-5), two of the most important zeolites relevant for industry. The samples are thoroughly characterized by XRD patterns, SEM images, N2 sorption isotherms, UV-Raman spectra, and (29)Si and (27)Al MAS NMR spectra. The results demonstrate that S-Beta and S-ZSM-5 zeolites exhibit almost the same textural parameters (e.g., BET surface area and pore volume) and catalytic performance in cumene cracking and m-xylene isomerization as those of conventional Beta and ZSM-5 zeolites synthesized under hydrothermal conditions (C-Beta and C-ZSM-5). The organotemplate- and solvent-free syntheses of S-Beta and S-ZSM-5 take place at a low-pressure regime and are free of harmful gases as well as give high product yields together with highly efficient consumption of the starting raw materials. These advantages plus the very simple procedures opened the pathway to a highly sustainable zeolite synthesis protocol compared to conventional methods currently employed for C-Beta and C-ZSM-5. Very interestingly, this simple synthesis is a good model for understanding zeolite crystallization. The detail characterizations indicate that the S-Beta crystals are formed from the assembly of zeolite building units, mainly 4MRs, while the 5MRs in the framework are just formed in the crystallization of S-ZSM-5, rather than existence in the starting solid mixture. During the crystallization processes, small traces of water play an important role for the hydrolysis and condensation of silica and/or aluminosilicate species.

Factor analysis of 27Al MAS NMR spectra for identifying nanocrystalline phases in amorphous geopolymers

Nanostructured materials offer enhanced physicochemical properties because of the large interfacial area. Typically, geopolymers with specifically synthesized nanosized zeolites are a promising material for the sorption of pollutants. The structural characterization of these aluminosilicates, however, continues to be a challenge. To circumvent complications resulting from the amorphous character of the aluminosilicate matrix and from the low concentrations of nanosized crystallites, we have proposed a procedure based on factor analysis of (27)Al MAS NMR spectra. The capability of the proposed method was tested on geopolymers that exhibited various tendencies to crystallize (i) completely amorphous systems, (ii) X-ray amorphous systems with nanocrystalline phases, and (iii) highly crystalline systems. Although the recorded (27)Al MAS NMR spectra did not show visible differences between the amorphous systems (i) and the geopolymers with the nanocrystalline phase (ii), the applied factor analysis unambiguously distinguished these materials. The samples were separated into the well-defined clusters, and the systems with the evolving crystalline phase were identified even before any crystalline fraction was detected by X-ray powder diffraction. Reliability of the proposed procedure was verified by comparing it with (29)Si MAS NMR spectra. Factor analysis of (27)Al MAS NMR spectra thus has the ability to reveal spectroscopic features corresponding to the nanocrystalline phases. Because the measurement time of (27)Al MAS NMR spectra is significantly shorter than that of (29)Si MAS NMR data, the proposed procedure is particularly suitable for the analysis of large sets of specifically synthesized geopolymers in which the formation of the limited fractions of nanocrystalline phases is desired.

Probing the Relationship between Silicalite-1 Defects and Polyol Adsorption Properties

The relationship between polyol adsorption affinity and silanol defect density was investigated through the development of vapor and aqueous adsorption isotherms on silicalite-1 materials which vary in structural and surface properties. Silicalite-1 crystals prepared through alkaline synthesis, alkaline synthesis with steaming post-treatment, and fluoride synthesis routes were confirmed as crystalline mordenite framework inverted (MFI) by SEM and XRD and were shown to contain ~8.5-0 silanol defects per unit cell by (29)Si MAS, (1)H MAS, and (1)H-(29)Si CPMAS NMR. A hysteresis in the Ar 87 K adsorption isotherm at 10(-3)P/P0 evolved with a decrease in silanol defects, and, through features in the XRD and (29)Si MAS NMR spectra, it is postulated that the hysteresis is the result of an orthorhombic-monoclinic symmetry shift with decreasing silanol defect density. Gravimetric and aqueous solution measurements reveal that propylene glycol adsorption at 333 K is promoted by silanol defects, with a maximum 20-fold increase observed for aqueous adsorption at ~10(-3) g/mL with an increase from ~0 to 8.5 silanols per unit cell. A comparison of vapor and aqueous propylene glycol adsorption isotherms on defect-free silicalite-1 at 333 K, both of which exhibit the Type-V character, indicates that water enhances adsorption by a factor of ~2 in the Henry's Law regime. Henry's constants for aqueous C2-C4 polyol adsorption (concentrations below 0.004 g/mL) at 298 K are shown to have a linear dependence on the silanol defect density, demonstrating that these molecules preferentially adsorb at silanol defects at dilute concentrations. This systematic study of polyol adsorption on silicalite-1 materials highlights the critical role of defects on adsorption of hydrophilic molecules and clearly details the effects of coadsorption of water, which can guide the selection of zeolites for separation of biomass-derived oxygenates.

A neutron/X-Ray diffraction, IR, and1H/29Si NMR Spectroscopic investigation of armenite: behavior of extra framework Ca cations and H2O molecules in microporous silicates

The crystal chemistry of armenite, ideally BaCa 2 Al 6 Si 9 O 30 · 2 H 2 O, from Wasenalp, Valais, Switzerland was studied. Armenite typically forms in relatively low-temperature hydrothermal veins and fissures and has small pores containing Ca cations and H 2 O molecules as extra-framework species. Single-crystal neutron and X-ray diffraction measurements were made on armenite from the above locality for the first time. IR powder spectroscopic measurements were made from room temperature (RT) down to 10 K. 1 H and 29 Si NMR measurements were made at RT. Attention was given to investigating the behavior of the extra-framework species and hydrogen bonding. The diffraction results show new features not observed before in published diffraction studies on armenite crystals from other localities. The neutron results also give the first static description of the protons, allowing bond distances and angles relating to the H 2 O molecules and H-bonds to be determined. The diffraction results indicate Al/Si order in the framework. Four crystallographically independent Ca and H 2 O molecule sites were refined, whereby both sites appear to have partial occupancies such that locally a Ca atom can have only a single H 2 O molecule bonded to it through an ion-dipole interaction. The Ca cation is further bonded to six O atoms of the framework forming a quasi cluster around it. The IR spectrum of armenite is characterized in the OH-stretching region at RT by two broad bands at roughly 3470 and 3410 cm –1 and by a single H 2 O bending mode at 1654 cm –1 . At 10 K four intense OH bands are located at 3479, 3454, 3401 and 3384 cm –1 and two H 2 O bending modes at 1650 and 1606 cm –1 . The 29 Si MAS NMR spectra show four resonances at –81.9, –83.2, –94.9 and –101.8 ppm that are assigned to crystallographically different Si sites in an ordered structure, although their relative intensities deviate somewhat from those predicted for complete Al/Si order. The 1 H MAS spectra contain a single main resonance near 5.3 ppm and a smaller one near 2.7 ppm, which can be assigned to H 2 O molecules bonded to Ca and a second H 2 O type located in a partially occupied site, respectively. Bonding for the extra-framework “Ca-oxygen-anion-H 2 O-molecule quasi-clusters” and also the nature of H-bonding in the microporous zeolites scolecite, wairakite and epistilbite are analyzed. The average OH stretching wavenumbers shown by the IR spectra of armenite and scolecite are, for example, not far removed from that observed in liquid H 2 O, but greater than that of ice. What remains poorly understood in microporous silicates is how the ion-dipole interaction in quasi clusters affects H-bonding strength between the H 2 O molecules and the aluminosilicate framework.

MAS NMR measurements and ab initio calculations of the 29Si chemical shifts in dumortierite and holtite

Three samples from the dumortierite group of minerals were examined with magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR): a dumortierite [c. (Al,□)Al 6 (BO 3 )Si 3 O 13 (O,OH) 2 ] consisting of dark blue euhedral crystals from Madagascar (D34); a fine-grained pale blue dumortierite from Island Copper mine, British Columbia, Canada (D12); and a creamy white holtite [c. (Ta,Nb,□,Al)Al 6 (BO 3 )(Si,Sb,As) 3 O 12 (O,OH,□) 3 ] from Szklary, Lower Silesia, Poland (WPH). Restricted Hartree-Fock ab initio electronic structure calculations were performed on model clusters with the goal of matching local environments of Si atoms to peaks in the 29 Si MAS NMR spectra. The spectrum of D34 showed five resolved peaks at −95.2, −92.6, −91.3, −89.1, and −86.5 ppm with deconvoluted peak area contributions of 57, 19, 7, 10, and 7%. Electronic structure calculations, cross-polarization MAS NMR measurements and relative intensities support assigning the peaks at −95.2 and −92.6 ppm to Si2 and Si1 sites, respectively, adjacent to fully occupied Al1 sites (i.e., Q 4 Si sites), and assigning the three remaining peaks to Si sites adjacent to vacant Al1 sites (i.e., Q 3 Si sites). Due to the complexity of the dumortierite structure, clusters composed of at least the first four shells of nearest neighbor atoms to the target Si atom are necessary to model Q 4 sites. The spectrum of D12 showed two main peaks at −93 and −95 ppm, with minor peaks below −90 ppm and above −100 ppm. The spectrum of WPH showed one broad peak at −93 ppm, likely containing both Si1 and Si2 signals, and two minor peaks below −90 ppm. Single-crystal X-ray diffraction and structure refinement on D34 shows orthorhombic symmetry, Pnma , Z = 4, a = 4.6882(1), b = 11.7924(2), c = 20.1856(3) A, and V = 1115.97(4) A 3 with R 1 = 0.0124. Three distinct sub-sites of the face-sharing octahedral chain site Al1 were distinguished corresponding to sites with one vacancy above, with one vacancy below, and between two occupied sites; the vacancy-adjacent sites have the cation displaced to increase the Al 3+ -Al 3+ distance. Each sub-site is approximately ¼ occupied, suggesting that Al 3+ cations in individual face-sharing octahedral chains are ordered as □-Al-Al-Al, although cations from chain to chain are disordered, preserving Pnma symmetry. Powder X-ray diffraction measurements were performed on both D34 and D12. The unit cell of D12 was found to be a = 4.7001(7), b = 11.785(2), c = 20.277(3) A.

27Al and 29Si Solid-State NMR Characterization of Calcium-Aluminosilicate-Hydrate

Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties. Because of the environmental impact of cement industry, it is more and more common to replace a part of the clinker in cement by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is incorporated into the C-S-H. This may have consequences on the cohesion and durability of the material, and it is thus of importance to know the amount and the location of Al in C-S-H and what the parameters are that control these features. The present paper reports the (29)Si and (27)Al MAS NMR analyses of well-characterized C-A-S-H samples (C-S-H containing Al). These samples were synthesized using an original procedure that successfully leads to pure C-A-S-H of controlled compositions in equilibrium with well-characterized solutions. The (27)Al MAS NMR spectra were quantitatively interpreted assuming a tobermorite-like structure for C-A-S-H to determine the aluminum location in this structure. For this purpose, an in-house written software was used which allows decomposing several spectra simultaneously using the same constrained spectral parameters for each resonance but with variable intensities. The hypothesis on the aluminum location in the C-A-S-H structure determines the proportion of each silicon site. Therefore, from the (27)Al NMR quantitative results and the chemical composition of each sample, the intensity of each resonance line in the (29)Si spectra was set. The agreement between the experimental and calculated (29)Si MAS NMR spectra corroborates the assumed C-A-S-H structure and the proposed Al incorporation mechanism. The consistency between the results obtained for all compositions provides another means to assess the assumptions on the C-A-S-H structure. It is found that Al substitutes Si mainly in bridging positions and moderately in pairing positions in some conditions. Al in pairing site is observed only for Ca/(Si+Al) ratios greater than 0.95 (equivalent to 4 mmol.L(-1) of calcium hydroxide). Finally, the results suggest that penta and hexa-coordinated aluminum are adsorbed on the sides of the C-A-S-H particles.

Structural characterization of the lithium silicides Li15Si4, Li13Si4, and Li7Si3 using solid state NMR

Local environments and lithium ion dynamics in the binary lithium silicides Li(15)Si(4), Li(13)Si(4), and Li(7)Si(3) have been characterized by detailed variable temperature static and magic-angle spinning (MAS) NMR spectroscopic experiments. In the (6)Li MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200 K, whereas at higher temperatures partial or complete site averaging is observed on the ms timescale. The NMR spectra also serve to monitor the phase transitions occurring in Li(7)Si(3) and Li(13)Si(4) at 235 K and 146 K, respectively. The observed lithium isotropic shift ranges of up to approximately 50 ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl-Klemm-Busmann concept for the electronic structure of these materials. The (29)Si MAS-NMR spectra obtained on isotopically enriched samples, aided by double-quantum spectroscopy, are well suited for differentiating between the individual types of silicon sites within the silicon frameworks, and in Li(13)Si(4) their identification aids in the assignment of individual lithium sites via(29)Si{(7)Li} cross-polarization/heteronuclear correlation NMR. Variable temperature static (7)Li NMR spectra reveal motional narrowing effects, illustrating high lithium ionic mobilities in all of these compounds. Differences in the mobilities of individual lithium sites can be resolved by temperature dependent (6)Li MAS-NMR as well as (6)Li{(7)Li} rotational echo double resonance (REDOR) spectroscopy. For the compound Li(15)Si(4) the lithium mobility appears to be strongly geometrically restricted, which may result in a significant impediment for the use of Li-Si anodes for high-performance batteries. A comparison of all the (6)Li and (7)Li NMR spectroscopic data obtained for the three different lithium silicides and of Li(12)Si(7) previously studied suggests that lithium ions in the vicinity of silicon clusters or dimers have generally higher mobilities than those interacting with monomeric silicon atoms.

Calix[4]arene derivative functionalized lanthanide (Eu, Tb) SBA-15 mesoporous hybrids with covalent bonds: assembly, characterization and photoluminescence

Three kinds of novel macrocylic calix[4]arene derivatives functionalized SBA-15 type of mesoporous hybrids (Calix-S15, Calix-NO(2)-S15 and Calix-NH(2)-S15) are synthesized by co-condensation of tetraethoxysilane (TEOS) and modified organic ligand (Calix-Si, Calix-NO(2)-Si and Calix-NH(2)-Si) in the presence of Pluronic P123 surfactant as a template. The structural preservation of these three parent materials is confirmed by FTIR spectra, (29)Si MAS NMR spectra, XRD pattern, and N(2) adsorption-desorption measurements. The ternary mesoporous luminescent hybrids containing Ln(3+) (Eu(3+), Tb(3+)) complexes covalently attached to the functionalized ordered mesoporous SBA-15, which are designated as Ln(Calix-S15)phen, Ln(Calix-NO(2)-S15)phen and Ln(Calix-NH(2)-S15)phen, are obtained by introducing lanthanide ions and 1,10-phenanroline into the corresponding parent material via covalent bond assembling methods. XRD pattern, TEM and N(2) adsorption-desorption measurements are employed to characterize the mesostrcture of the resulting lanthanide mesoporous hybrids. The photoluminescent behavior (luminescence, lifetime, quantum efficiency, and energy transfer) for these chemically bonded mesoporous hybrids is studied in detail. Also, their quantum efficiencies are determined, which indicates that the different mesoporous hybrid material systems derived from different functionalized calix[4]arene derivative bridges present different luminescence behavior.

ChemInform Abstract: High-Resolution 27Al and 29Si MAS NMR Investigation of SiO2-Al2O3 Glasses

{sup 27}Al and {sup 29}Si MAS NMR spectra have been obtained for SiO{sub 2}-Al{sub 2}O{sub 3} glasses quenched from the melt at two rates: 10{sup 5}-10{sup 6} {degree}C s{sup {minus}1} (SQ) and 10{sup 2}-10{sup 3} {degree}C s{sup {minus}1} (NQ). TEM experiments detected phase separation in the S{sub 76}A{sub 24}SQ and S{sub 52}A{sub 48} SQ samples but no phase separation for the S{sub 41}A{sub 59} SQ composition. The NQ glasses were phase separated and, in the diffraction TEM experiments, showed diffuse spots indicating microcrystallinity. {sup 27}Al spectra for the SQ glasses show three peaks due to aluminum in 4-, 5-, and 6-coordination, whereas NQ glasses show only peaks associated with aluminum in 4- and 6-coordination. Quantitative results for the {sup 27}Al spectra are discussed. The effect of spin speed on the {sup 27}Al MAS NMR spectra for SQ and NQ samples is demonstrated. Magnitudes of mean quadrupolar coupling constants and mean isotropic chemical shifts have been estimated. {sup 29}Si spectra of SQ glasses reveal a distribution of 4-coordinate Si sites, whereas NQ glasses show two inequivalent 4-coordinate sites.

Incorporation of Phosphorus Guest Ions in the Calcium Silicate Phases of Portland Cement from 31P MAS NMR Spectroscopy

Portland cements may contain small quantities of phosphorus (typically below 0.5 wt % P(2)O(5)), originating from either the raw materials or alternative sources of fuel used to heat the cement kilns. This work reports the first (31)P MAS NMR study of anhydrous and hydrated Portland cements that focuses on the phase and site preferences of the (PO(4))(3-) guest ions in the main clinker phases and hydration products. The observed (31)P chemical shifts (10 to -2 ppm), the (31)P chemical shift anisotropy, and the resemblance of the lineshapes in the (31)P and (29)Si MAS NMR spectra strongly suggest that (PO(4))(3-) units are incorporated in the calcium silicate phases, alite (Ca(3)SiO(5)) and belite (Ca(2)SiO(4)), by substitution for (SiO(4))(4-) tetrahedra. This assignment is further supported by a determination of the spin-lattice relaxation times for (31)P in alite and belite, which exhibit the same ratio as observed for the corresponding (29)Si relaxation times. From simulations of the intensities, observed in inversion-recovery spectra for a white Portland cement, it is deduced that 1.3% and 2.1% of the Si sites in alite and belite, respectively, are replaced by phosphorus. Charge balance may potentially be achieved to some extent by a coupled substitution mechanism where Ca(2+) is replaced by Fe(3+) ions, which may account for the interaction of the (31)P spins with paramagnetic Fe(3+) ions as observed for the ordinary Portland cements. A minor fraction of phosphorus may also be present in the separate phase Ca(3)(PO(4))(2), as indicated by the observation of a narrow resonance at delta((31)P) = 3.0 ppm for two of the studied cements. (31)P{(1)H} CP/MAS NMR spectra following the hydration of a white Portland cement show that the resonances from the hydrous phosphate species fall in the same spectral range as observed for (PO(4))(3-) incorporated in alite. This similarity and the absence of a large (31)P chemical shift ansitropy indicate that the hydrous (PO(4))(3-) species are incorporated in the interlayers of the calcium-silicate-hydrate (C-S-H) phase, the principal phase formed upon hydration of alite and belite.

Porous properties of mesoporous silicas from two silica sources (acid-leached kaolinite and Si-alkoxide)

Two different mesoporous silicas (MesoPS) were synthesized by hydrothermal treatment in NaOH solution of two silica sources. One of these starting silicas was derived from selectively acid leached metakaolinite, and the other was from tetraethylorthosilicate (TEOS). The syntheses used a surfactant, cethyltrimethylammonium bromide (CTABr) and were carried out at different CTABr/ Si, NaOH/Si and H2O/Si ratios. In the MesoPS from kao- linite, the specific surface areas (SBET) of the products were (1500 m 2 /g when prepared with 0.2 B CTABr/Si B 0.4, 0.3 B NaOH/Si B 0.6 and H2O/Si = 150. These SBET values are higher than those obtained from TEOS (ca. 1300 m 2 /g). The XRD patterns of these products contain a hexagonal (10-) peak with a lattice parameter a0 = 4.2-5.2 nm in the MesoPS derived from kaolinite and a0 = 4.0-4.6 nm in the products from TEOS. The regu- larity of the hexagonal structure is higher in the MesoPS derived from TEOS than from kaolinite. The pore size distributions of the products show a sharp peak at 2.8 nm in the MesoPS from kaolinite and at 2.4-2.5 nm in those from TEOS. The meso-structure is found to be formed during the stirring step of the synthesis and becomes more regular after hydrothermal treatment. The differences in the porous properties of the two MesoPSs from kaolinite and TEOS are attributed to differences in the dissolution rates and silica concentrations in the synthesis solutions. The 29 Si MAS NMR spectra show a higher number of Q 3 (Si(OSi)3 OH) units in the MesoPS from kaolinite and this is sug- gested to be related to the difference in their porous properties.

Self‐Assembly of Alkyl‐Substituted Cubic Siloxane Cages into Ordered Hybrid Materials

Siloxane-organic hybrids with well-ordered mesostructures were synthesized through the self-assembly of novel amphiphilic molecules that consist of cubic siloxane heads and hydrophobic alkyl tails. The monoalkyl precursors functionalized with ethoxy groups (C(n)H(2n+1)Si(8)O(12)(OEt)(7), 1 Cn, n=16, 18, and 20) were hydrolyzed under acidic conditions with the retention of the siloxane cages, leading to the formation of two-dimensional hexagonal phases by evaporation-induced self-assembly processes. Analysis of the solid-state (29)Si MAS NMR spectra of these hybrid mesostructures confirmed that the cubic siloxane units were cross-linked to form siloxane networks. Calcination of these hybrids gave mesoporous silica, the pore diameter of which varied depending on the alkyl-chain length. We also found that the precursors that had two alkyl chains formed lamellar phases, thus confirming that the number of alkyl chains per cage had a strong influence on the mesostructures. These results expand the design possibility of novel nanohybrid and nanoporous materials through the self-assembly of well-defined oligosiloxane-based precursors.

Formation of Indium Nitride Nanorods within Mesoporous Silica SBA-15

We report the first formation of arrays of InN nanorods inside the nanoscale channels of mesoporous silica SBA-15. In(NO3)3 dissolved in methanol was incorporated into SBA-15 powder without prior pore surface functionalization. Formation of InN nanorod arrays was carried out by ammonolysis at 700 degrees C for 8 h. The final products have been characterized by FT-IR spectra, (29)Si MAS NMR spectra, Raman spectra, XRD patterns, TEM images, nitrogen adsorption-desorption isotherm measurements, and optical spectroscopy. The freestanding InN nanorods observed after silica framework removal with HF solution show diameters of 6-7.5 nm and lengths of 25-50 nm. Formation of a trace amount of In2O3 was also verified. The InN nanorods exhibit a broad band centered at around 550-600 nm, and a band gap energy of 1.5 eV was determined. No light absorption in the near-IR region was measured. The nanorods give a weak emission band centered at around 600 nm. These optical properties are believed to be related to the possible incorporation of oxygen during InN nanorod synthesis.

Cation order/disorder behavior and crystal chemistry of pyrope-grossular garnets: An 17O 3QMAS and 27Al MAS NMR spectroscopic study

The thermodynamic mixing properties of the pyrope-grossular solid solution show large deviations from ideality, which could be partly related to Ca-Mg order/disorder. In this study, synthetic pyrope-grossular garnets with X Mg = 1.00, 0.91, 0.75, 0.50, 0.24, 0.10, and 0.00 are observed using 17 O 3QMAS, 27 Al MAS, and 29 Si MAS NMR to examine Ca-Mg order/disorder behavior and crystal chemical variations. The 17 O 3QMAS NMR spectra show four distinct resonances, assigned to four different local oxygen coordination environments; two resemble end-member garnets (oxygen bonded to two Mg or two Ca) and two are intermediate (oxygen bonded to one Ca and one Mg), indicating that there are two distinct bond distances for the Mg-O and/or Ca-O bonds through the entire solid solution. Noticeable changes in the NMR peak position for two of the oxygen sites suggest that as X Mg increases, the longer Ca-O bond shortens. The relative areas for the different oxygen sites are close to those predicted using a model of random Ca/Mg mixing. The maximum allowed reduction in configurational entropy from first neighbor Ca-Mg ordering is insignificant relative to other configurational entropy reductions and excess vibrational entropy. These conclusions are not inconsistent with published theoretical calculations suggesting some Ca-Mg ordering that involves correlations beyond the first neighbor, as suggested by published theoretical calculations. Even at 18.8 Tesla, the 27 Al MAS NMR spectra do not resolve different local Al sites with varying combinations of X cation neighbors. The 29 Si MAS NMR spectra have resonance broadening, probably caused by the addition of 0.15 wt% Fe 2 O 3 in the synthetic samples, and are consistent with published results suggesting a small degree of Ca-Mg ordering that is not reflected in the 17 O NMR spectra.

Characterization and hydroisomerization performance of SAPO-11 molecular sieves synthesized in different media

Abstract Silicoaluminophosphates SAPO-11 molecular sieves were synthesized from a conventional aqueous medium, a two-liquid phase medium and a HF medium, respectively, and were characterized by various methods such as XRD, SEM, pyridine-FT-IR, NH 3 -TPD, 29 Si MAS-NMR. The morphologies of the SAPO-11 molecular sieves obtained from the conventional aqueous medium and the two liquid-phase media exhibit pseudo-spherical aggregates ranging from 7 to 10 μm assembled from cubic plate microcrystallites, whereas the SAPO-11 molecular sieves synthesized from the HF medium exhibit hexagonal prismatic aggregates of about 1–3 μm length. The synthesis media used highly influenced the number and the strength of SAPO-11 acid sites. Compared to SAPO-11 obtained from conventional aqueous medium, SAPO-11 prepared from two-liquid phase media possessed a similar number of BrOnsted acid sites but a much larger number of Lewis acid sites, whereas the SAPO-11 synthesized from HF medium had a similar number of Lewis acid sites but a much lower number of BrOnsted acid sites. The 29 Si MAS-NMR spectra showed that the HF could change the manner of Si incorporation into the SAPO-11 framework. The hydroisomerization of n -hexadecane on Pt/SAPO-11 samples showed that Pt/SAPO-11 obtained from the two-liquid phase media had the largest yield of isomers. The conversion activity of n- hexadecane was dependent on the amount of BrOnsted acid of SAPO-11 molecular sieves, while its hydrocracking activity was related to the strong acid sites of SAPO-11.