High-resolution Solid-state 29Si Research Articles

Structure and Chemical Durability of Lead Crystal Glass.

Silicate glasses containing lead, also called lead crystal glasses, are commonly used as food product containers, in particular for alcoholic beverages. Lead's health hazards require major attention, which can first be investigated through the understanding of Pb release mechanisms in solution. The behavior of a commercial crystal glass containing 10.6 mol % of PbO (28.3 wt %) was studied in a reference solution of 4% acetic acid at 22, 40, and 70 °C at early and advanced stages of reaction. High-resolution solid-state 17O and 29Si NMR was used to probe the local structure of the pristine and, for the first time, of the altered lead crystal glass. Inserted into the vitreous structure between the network formers as Si-O-Pb bonds, Pb does not form Pb-O-Pb clusters which are expected to be more easily leached. A part of K is located near Pb, forming mixed Si-O-(Pb,K) near the nonbridging oxygens. Pb is always released into the solution following a diffusion-controlled dissolution over various periods of time, at a rate between 1 and 2 orders of magnitude lower than the alkalis (K and Na). The preferential release of alkalis is followed by an in situ repolymerization of the silicate network. Pb is only depleted in the outermost part of the alteration layer. In the remaining part, it stays mainly surrounded by Si in a stable structural configuration similar to that of the pristine glass. A simple model is proposed to estimate the Pb concentration as a function of glass surface, solution volume, temperature, and contact time.

On the origin of high ionic conductivity in Na-doped SrSiO3.

Understanding the local structure and ion dynamics is at the heart of ion conductor research. This paper reports on high-resolution solid-state 29Si, 23Na, and 17O NMR investigation of the structure, chemical composition, and ion dynamics of a newly discovered fast ion conductor, Na-doped SrSiO3, which exhibited a much higher ionic conductivity than most of current oxide ion conductors. Quantitative analyses reveal that with a small dose (<10 mol%) of Na, the doped Na integrates into the SrSiO3 structure to form Na x Sr1-x SiO3-0.5x , and with >10 mol% Na doping, phase separation occurs, leading to the formation of an amorphous phase β-Na2Si2O5 and a crystalline Sr-rich phase. Variable-temperature 23Na and 17O magic-angle-spinning NMR up to 618 °C have shown significant changes in Na ion dynamics at high temperatures but little oxide ion motion, suggesting that Na ions are responsible for the observed high ionic conductivity. In addition, β-Na2Si2O5 starts to crystallize at temperatures higher than 480 °C with prolonged heating, resulting in reduction in Na+ motion, and thus degradation of ionic conductivity. This study has contributed critical evidence to the understanding of ionic conduction in Na-doped SrSiO3 and demonstrated that multinuclear high-resolution and high-temperature solid-state NMR is a uniquely useful tool for investigating ion conductors at their operating conditions.

Preparation and characterization of sol–gel-modified pineapple leaf fiber/polylactic acid composites

In this study, the fibers generated from agricultural waste-pineapple leaf are obtained through exposure, drying, crushing and sifting. A novel sol–gel method is utilized to modify the pineapple leaf fiber (PALF), which is subsequently treated by a coupling agent. The aim is to improve the compatibility between PALF and the polymer matrix and to enhance the heat resistance and mechanical properties of the composite material. Furthermore, a series of modified PALF/polylactic acid (PLA) composites are prepared. FTIR, high resolution solid-state 13C and 29Si NMR experiments show that the PALF was successfully modified by the silane coupling agent and sol–gel method. Polarizing optical microscopy analysis reveals PLA crystal growth of a sufficiently high density along the polymer-fiber interface. Moreover, the storage and loss moduli of PLA are increased by adding the modified PALF. Apart from the enhancement of the mechanical properties, the incorporation of modified PALF reduces the amount of agricultural waste and extends the application of PLA.

Reversibly meltable layered alkylsiloxanes with melting points controllable by alkyl chain lengths

Meltable layered alkylsiloxanes (CnLSiloxanes) were synthesized from tetraethoxysilane and alkyltrialkoxysilanes with carbon numbers (n) of 12, 16, and 18 via hydrolysis and polycondensation at 100 and 150 °C under basic conditions. Differential scanning calorimetric (DSC) measurements revealed that CnLSiloxanes melted reversibly at −0.8 to 51.3 °C, the melting points being dependent on n. Scanning electron microscope (SEM) images showed that thin plates were stacked. X-ray diffraction (XRD) peaks were observed at angles corresponding to distances (d) of 2.1, 2.5, and 2.8 nm for C12-, C16-, and C18LSiloxanes, respectively. High-resolution solid-state 13C nuclear magnetic resonance (NMR) measurements showed that the organic moieties were alkylsilyl groups with long alkyl chains and that an all-trans conformation was dominant. This was supported by the XRD peak corresponding to a d value of 0.41 nm. High-resolution solid-state 29Si NMR measurements demonstrated the presence of SiO4 and CSiO3 units. A structural model has been proposed for CnLSiloxanes, where siloxane sheets consisting of the SiO4 and CSiO3 units are stacked with the ordered interdigitated monolayer of the alkyl chains in between and are bonded covalently with the alkyl chains.

The role of the defect groups on the Silicalite-1 zeolite catalytic behavior

Silicalite-1 samples were synthesized from gels of composition: 0.08 Na2O – 0.08 TPABr – 1 SiO2 – 20 H2O (TPABr=tetrapropylammonium bromide) at 170°C under autogeneous pressure during 24h. The gels used without aging yielded large particles of ca. 15μm, while the aged gel led to small particles of ca. 2μm. The acidity of the samples was checked by FTIR of the silanol groups, by the IR spectra of adsorbed pyridine and by temperature programmed desorption of ammonia. The silanol groups were also identified by High Resolution Solid State 29Si NMR spectroscopy. The samples not containing H-bonded silanol groups were not active either in the dehydration of propan-2-ol or the cumene cracking. The samples calcined and NH4+ exchanged by NH4Cl either at pH=5.5 or pH=10.5 showed activity in the propan-2-ol dehydration, but not in cumene cracking. These samples contained acid sites of medium acid strength characterized by the H-bonded silanol groups. A second calcination of the samples was necessary in order to show some activity in cumene cracking. A ZSM-5 sample of Si/Al=25 was synthesized and studied for the sake of comparison. A Silicalite-1 sample composed of small particles first calcined at 550°C, then NH4+ exchanged with NH4Cl at pH=5.5 and finally calcined at 550°C showed the highest activity (10–11%) in cumene cracking. A hypothesis could be advanced on the production of Lewis acid centers, to explain the catalytic activity of this sample.

Elemental and structural analysis of silicon forms in herbal drugs using silicon-29 MAS NMR and WD-XRF spectroscopic methods

The objective of this work was to study concentration of silicon and its structural forms present in herbal drugs. Equisetum arvense and Urtica dioica L. from teapot bags, dietary supplements (tablets and capsules) containing those herbs, dry extract obtained from a teapot bag of E. arvense, and samples of the latter herb harvested in wild habitat over four months were studied using wavelength dispersive X-ray spectroscopy (WD-XRF) and high-resolution solid-state 29Si NMR. The highest concentration of Si, ca. 27 mg/g, was found in the herbal material from the teapot bags containing E. arvense. The Si content in natural E. arvense (whole plants) increased from May to August by ca. 7 mg/g, reaching value 26 mg/g. Three different silicon forms were detected in the studied herbal samples: Si(OSi )4 (Q 4), Si(OH)(OSi )3 (Q 3) and Si(OH)2(OSi )2 (Q 2). Those sites were populated in E. arvense in the following order: Q 4 ≫ Q 3 > Q 2. A dramatic, ca. 50-fold decrease of the Si concentration during the infusion process was observed. The infusion process and the subsequent drying procedure augmented population of the Q 4 sites at the cost of the Q 2 sites. The WD-XRF and 29Si NMR methods occurred useful and complementary in the study of herbal materials.

Characterization of pozzolanic reaction and its effect on the C-S-H Gel in fly Ash-cement paste

High resolution solid-state 29Si MAS NMR, combined with XRD, SEM and FTIR were used to characterize the pozzolanic activity of FA, type of main pozzolanic reaction products, and the effect of pozzolanic reaction on the C-S-H microstructure in fly ash-cement (FC) paste. The experimental results indicate that in the hydrated FC paste with 30% dosage of FA at 3 d, FA partially participated in the pozzolanic reaction, while, at 120 d, FA largely reacts. During the hydration of FC paste at laboratory temperature, the pozzolanic reaction products are C-S-H gel rather than zeolitic gel. Moreover, after the covalent bonds of Si-O-Si, Si-O-Al and Al-O-Al in the structure of FA are broken, monosilicates Si-OH and Al-OH groups form, these chemical species can connect C-S-H dimers, thus producing more Al-free C-S-H and aluminous C-S-H than in the plain cement paste. The increased content of Al for Si substitution in the bridging tetrahedra of C-S-H may decrease the stability of C-S-H, which results in a rather obvious loss in the mechanical strength of hardened FC paste.

An efficient peak assignment algorithm for two-dimensional NMR correlation spectra of framework structures

An algorithm is described for efficiently assigning the resonances in NMR spectra to the inequivalent atoms in the structure under study based on the information in two-dimensional NMR correlation experiments and the ‘connectivities’ known from the structure. The algorithm, which is based on basic graph theory concepts, finds all possible assignments sets which are consistent with the experimentally observed correlations and known connectivities in a very efficient manner. It is designed to deal with less than ideal experimental data in which there may be overlapping peaks and uncertainty about the presence or absence of correlation peaks. The algorithm was primarily developed for assigning the peaks in the high-resolution solid-state 29Si MAS NMR spectra of highly siliceous zeolites based on two-dimensional 29Si INADEQUATE spectra and is described using the zeolites ZSM-12 and ZSM-5 as working examples. Peak assignment for zeolite frameworks is particularly challenging since there is often little or no information to distinguish peaks from one another such as characteristic chemical shifts, relative intensities, or different relaxation times. The algorithm may be a useful tool for easily, reliably, and efficiently working out peak assignments from other types of correlation experiments on other types of systems and further examples are provided in the Supplementary material.

29Si and 27Al MAS/3Q-MAS NMR Studies of High Silica USY Zeolites

High-resolution solid state 29Si, 27Al magic angle spinning (MAS) and 27Al triple quantum magic angle spinning (3Q-MAS) NMR spectroscopic techniques were used to characterize highly crystalline ult...

Chain dynamics and stability of the poly(3-methacryloxypropyltrimethoxysilane-co-vinylimidazole)

The chain dynamics and the apparent activation energy of dehydration and decomposition of the ester bond for poly(3-methacryloxypropyl-trimethoxysilane-co-vinylimidazole) copolymer (PMPS–PVI), were examined by high-resolution solid-state 29Si and 13C nuclear magnetic resonance spectroscopy and thermogravimetric analysis (TGA). The 29Si–1H cross-polarization process revealed that the 3-methacryloxypropyltrimethoxysilane (MPS) components species had similar CP efficiency. The spin–lattice relaxation time in rotating frame (T1ρH) of the copolymers indicated that the molecular motion of the imidazole ring was independent of the MPS content. The apparent activation energy of dehydration under nitrogen was dependent on the MPS content, while that of decomposition ester bond was independent of the MPS content.

Organic-Inorganic Hybrid Materials 4: NMR Study of the Poly(imide-silica) Hybrids

The microstructure of the poly(imide-silica) hybrid materials containing various proportions of silica was examined by high-resolution solid-state 29Si and 13C nuclear magnetic resonance spectroscopy. The 29Si—1H cross-polarization (CP) process revealed that the Q 2, Q 3 and Q 4 species had different CP efficiency. Moreover, the time constant for the energy exchange between 1H and 29Si spin systems (T SiH) for the siloxane units was less restricted by silica than by imide. The spin-lattice relaxation time in rotating frame (T H 1p ) of hybrids indicated that the 1H spin diffusion between imide and siloxane units were on a nanometer scale.

ORGANIC-INORGANIC HYBRID MATERIALS 6: NMR STUDY OF THE POLYVINYLIMIDAZOLE-SILICA HYBRIDS

The microstructure of the polyvinylimidazole-silica hybrids (PVI-SiO2) containing various proportions of silica was examined by high resolution solid-state 29Si and 13C nuclear magnetic resonance spectroscopy. The 29Si-1H cross-polarization (CP) process revealed that the Q3 and Q4 species had different CP efficiency. Moreover, the time constant for the energy exchange between 1H and 29Si spin systems (T SiH) for the siloxane units was less dependent on the silica content, and that was in the range of 2.0 to 4.0 ms. The spin-lattice relaxation time in rotating frame (T H 1p) of hybrids suggested that the 1H spin-diffusion between imidazole and siloxane units were on a nanometer scale estimated by spin-diffusion path length.

Characterization of offretite during hydrothermal treatment by high-resolution solid-state29Si magic angle spinning NMR and27Al triple-quantum magic angle spinning NMR spectroscopy

Characterization of offretite during hydrothermal treatment by high-resolution solid-state29Si magic angle spinning NMR and27Al triple-quantum magic angle spinning NMR spectroscopy

Characterisation of alkali ions in the Y-type zeolites by multi MAS NMR studies

The NaY zeolites exchanged with other alkali cations (Li +, K +, Rb + and Cs +) are characterised by high resolution solid state 29Si-, 27A1-, 23Na- and 133Cs-NMR. The chemical shifts of the various Si( nAl) n=0, 1, 2, 3 and 4 configurations show particular changes as a function of the alkali counter-cations. The NMR line stemming from the Si(1Al) shows the greatest linewidth suggesting that the distribution of the chemical shift is the highest. The quantitative 23Na-NMR data allowed one to follow the Na + exchange by the other alkali cations. The 23Na-NMR data show unambiguously that both the cation exchange and an eventual chemical reaction occur through the Na + ions occupying the cation positions in the sodalite and super cages. The 133Cs-NMR of a dehydrated sample confirm the literature data showing that Cs + cations may occupy any of the available cationic sites.

The structural analysis of Cardo silicone polyimides by high resolution solid-state 13C, 15N and 29Si NMR

It is well-known that polymers containing a fluorene moiety (so-called ‘Cardo-type polymers’) have high thermal stability, good mechanical properties and excellent solubility in common organic solvents. For the benefit of the full use of these polyimides in industrial applications, cross-linked Cardo silicone polyimides, which have both high thermal stability and high resistivity to organic solvents, were designed. The cross-link density was determined by solid-state 13C and 29Si NMR because no other direct method for the determination of the cross-link density is available. Furthermore, natural abundance CPMAS 13C and 15N NMR measurements were made for three different forms of Cardo silicone polyimides which exhibit a high oxygen permselective property. An explanation for the gas separation mechanism is given.

Step-by-step observation of the hydration of C3S by magic-angle spinning 29Si nuclear magnetic resonance: the masking effect of D2O

High-resolution solid-state 29Si nuclear magnetic resonance (NMR) characterization of hydrated C3S provides a means of selectively observing the hydrogen-containing phases. In particular, by applying the cross-polarization (CP) technique, we demonstrated that a contrast can be obtained, not only for hydrated against anhydrous phases, but also for hydrogen-hydrated against deuterium-hydrated phases. Selectively deuterated samples were thus prepared by a H2O/D2O alternated hydration procedure. The progress of the polycondensation of orthosilicates as derived by H2O was followed, demonstrating that a separate microphase of anhydrous C3S prevailingly reacted at any step. An induction period, showing a low reactivity and mainly formation of Qo hydrated was observed during the first hydration steps of selectively deuterated samples. The efficiency of the masking effect of D2O was also indirectly proven by comparison with calorimetric measurements.

Instrumental investigations of hydration of Class J oilwell cement

The hydration of cement clinker frequently results in the intimate formation of several hydrates, often of variable crystallinity, and is further complicated by the incorporation of impurities into the hydrate structures. The hydrothermal hydration products of a Class J cement have been characterized by X-ray diffraction techniques, infrared (IR) spectroscopy, scanning electron microscopy (SEM) and high-resolution solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy. IR and NMR spectroscopy show promise for identifying constituent phases of the silicate matrix in hydrothermal cement pastes.

High-resolution solid-state 13C and 29Si NMR investigations of the dynamic properties of tetrakis(trimethylsilyl)silane

High-resolution solid-state 13C and 29Si NMR spectroscopy have been used to investigate the dynamic properties of tetrakis(trimethylsilyl)silane (TTMSS) as a function of temperature; from the temperature dependence of the spectra, it is clear that TTMSS is not useful as a chemical shift reference material below ca. 238K, but the spectral changes associated with the phase transition at this temperature nevertheless provide a precise method for controlling temperature calibration in low-temperature solid-state 13C NMR studies.

Influence of synthesis conditions and postsynthesis treatments on the nature and quantity of structural defects in highly siliceous MFI zeolites: A high-resolution solid-state 29Si n.m.r. study

The formation and annealing of various kinds of defects in the framework of highly siliceous MFI-type zeolites, as evidenced by high-resolution solid-state 29 Si n.m.r., can be explained by considering the systematic influence of synthesis parameters and postsynthesis treatments. High pH values and high concentrations of alkali metal and fluoride ions prevent a complete polycondensation of the silicate species and generate materials that contain significant levels of nonbonding point defects. By contrast, a high crystallization temperature results in the ready hydrolysis of the terminal Si-O − groups, leading to structures containing fewer defects. A large number of T vacancies (hydroxyl nests) are created in zeolites crystallized at low temperatures in the presence of high Pr 4 N + concentations and in the absence of F − or Na + ions. Such conditions favor the formation of double-5-ring silicate anions. The hypothesis, that such species may further condense, conducting to frameworks with an important number of empty T-sites, is proposed. On the basis of these findings, ideal synthesis conditions can be proposed to produce (Si) MFI zeolites that contain a minimum number of defects. Various postsynthesis treatments, namely, different calcination conditions followed, or preceded, by selected ionic (akali cation) exchanges, result in a partial or a nearly total healing of these defects in the final materials.

Silicon-29 NMR Study of Alkali-Exchanged NaY Zeolites

High-resolution solid-state 29Si NMR has been applied to the study of partially exchanged Li, K, and Cs NaY zeolites. The order of the 29Si chemical shifts of dehydrated samples is Li, Na-Y < Na-Y < K, Na-Y. The correlation between the 29Si chemical shift and the Li or K loading on Li, Na-Y or K, Na-Y was rationalized in terms of the interaction between the framework and the cations inside the small cages. Because of the restrictive migration of large Cs+ ions from the supercages to the small cages, the 29Si chemical shift of Cs, Na-Y was found to be similar to that of Na-Y.