A.P.M. Kentgens

A practical guide to solid-state NMR of half-integer quadrupolar nuclei with some applications to disordered systems

Abstract The practical aspects for obtaining useful NMR spectra of half-integer quadrupolar nuclei in diamagnetic solids are discussed in some detail. The general appearance of the Magic-Angle Spinning (MAS) line shape is described. The effects of a distribution in isotropic chemical shift and quadrupole interaction on the MAS line shape is shown. The necessity of fast MAS and short excitation pulses is demonstrated. The possibilities of 27Al MAS NMR at high fields is shown for some clay minerals. Techniques to obtain spectra free of quadrupolar broadening, such as DOuble Rotation (DOR), Dynamic Angle Spinning WAS) and Multiple-Quantum Magic-Angle Spinning (MQMAS), are discussed with some emphasis on the newly developed MQMAS method. The latter is used to describe structural details of phosphorus-impregnated alumina and aluminosilicate sol-gel materials. These examples give a good indication of the capabilities of this new technique.

Particle Morphology and Chemical Microstructure of Colloidal Silica Spheres Made from Alkoxysilanes

Abstract Monodisperse colloidal silica spheres with radii in the range 10–500 nm were prepared by hydrolysis and condensation of tetraethoxysilane (TES) in a mixture of water, ammonia and a lower alcohol at several temperatures. It was attempted to establish a relation between the morphology and the chemical microstructure of the particle. Particle morphologies were examined with transmission electron microscopy and static and dynamic light scattering. The microstructure of the spheres was studied with quantitative direct excitation 29Si nuclear magnetic resonance (NMR) spectroscopy and a combination of qualitative cross-polarization 13C NMR and elemental analysis. A comparison was made between these particles and particles prepared from TES in an ammonia/water in cyclohexane microemulsion and also with Ludox® silica particles. The siloxane microstructure was found to show almost no variation as a function of concentration of reagents, catalyst, co-solvent and temperature. Around 65% of the silicon nuclei was bonded through siloxane bonds with four other silicons, approximately 30% was bonded with three other silicons and a few percent with only two. It was shown that under most experimental conditions several percent of the ethoxy groups never leave the TES molecule and end up inside the silica. The Ludox particles were found to consist of a more condensed silicon structure as compared with particles synthesized in alcohol, ammonia, water mixtures, whereas the spheres prepared in the microemulsion were less condensed and contained more alkoxy groups. The differences in particle morphologies — ranging from irregularly shaped rough particles to perfect, smooth spheres — are not caused by differences in siloxane and ethoxy microstructure. It is proposed instead, that a smooth and spherical particle shape is the result of the growth by monomers or small oligomers, and that a rough, irregular shape is the result of growth by larger silicon structures.

Two-dimensional solid-state nutation NMR of half-integer quadrupolar nuclei.

The 2-dimensional solid-state nutation NMR expt. for the detn. of quadrupole parameters, as introduced by Samoson and Lippmaas (1983) is evaluated. A complete series of spectra (for spin I = 3/2, 5/2, 7/2, and 9/2) resulting from d.-matrix calcns. is presented, and some exptl. aspects of the method are discussed. Applications of the method to 27Al (I = 5/2) in spodumene and to 45Sc (I = 7/2) in Sc2(SO4)3 are shown.

Sodium Environments in Dry and Hydrous Albite Glasses: Improved 23Na Solid State NMR Data and Their Implications for Water Dissolution Mechanisms

The sodium environments in albite glasses with water concentrations ranging from 0 to 60 mol% were studied using 23Na off-resonance quadrupole nutation and magic angle spinning (MAS) NMR spectroscopy. Crystalline albite was used as a model compound to demonstrate that off-resonance nutation is a suitable method for determination of the quadrupole coupling constant (Cq) for 23Na. Off-resonance nutation experiments gave a mean Cq = 1.75 ± 0.2 MHz for all the albite glasses studied here. MAS NMR experiments were performed at three magnetic fields, 7.05 T, 9.4 T, and 14.1 T in order to deduce the mean isotropic chemical shift, δiso, and to provide an independent measurement of the values of Cq. The mean isotropic chemical shift is a strong function of dissolved water concentration, but the mean Cq is essentially constant at 2.1-2.2 ± 0.2 MHz over the water concentration range studied. The distributions of both chemical shift and quadrupolar interactions decreases markedly with increasing water concentration, consistent with earlier suggestions that the hydrous glasses have a much more ordered structure. These new data using off-resonance nutation and faster MAS combined with higher applied magnetic fields supersede the 23Na NMR data of Kohn et al. 1989a and should be used in preference in devising or testing models for water dissolution mechanisms in albite melts and glasses. Our revised data provide no evidence for a change in water dissolution mechanism at 30 mol% H2O, but the other conclusions of Kohn et al. 1989a and the principal features of the dissolution mechanism developed by Kohn et al. 1989a, Kohn et al. 1992, Kohn et al. 1994 are essentially unchanged.

Calcination and deboronation of [B]-MFI single crystals

Abstract Calcination and deboronation of [B]-MFI and silanol nest reoccupation of deboronated MFI were studied by variable temperature FT i.r. spectroscopy, X-ray diffraction, and 1 H, 11 B, and 29 Si MAS n.m.r. spectroscopy techniques. Boron loss from the framework upon calcining [B]-MFI was minimized by using an improved calcination procedure, during which boron is kept in a stable tetrahedral/saturated coordination and boron hydrolysis and extraction as well as T-atom reorganizations are avoided. Framework boron can be extracted from the calcined boron MFI material by refluxing in aqueous hydrochloric acid (pH 2 ) homogeneously distributed silanol nests, which are useful reactive sites for postsynthesis reoccupation.

Ultraslow molecular motions in crystalline polyoxymethylene. A complete elucidation using two‐dimensional solid state NMR

The slow magic angle spinning 2D exchange NMR experiment applied to semicrystalline polyoxymethylene shows the existence of ultraslow molecular rotations in crystalline regions of the material. By simulating the two‐dimensional spinning sideband spectra, which are characteristic for the molecular motion involved, it is found that a rotation of the polyoxymethylene helical chains over 200° is responsible for the observed spectra. The activation energy is 20 kcal/mol in good agreement with values obtained for the α relaxation in dynamic mechanical and dielectric studies.

A 29Si solid-state magic angle spinning nuclear magnetic resonance study of TEOS-based hybrid materials

Tetraethylorthosilicate (TEOS)-based materials were prepared via the sol—gel method by acid catalysed hydrolysis and condensation of mixtures of trifunctional alkoxysilanes (T) and tetraethylorthosilicate (TEOS, Q). The size of the organic tail and the mole percentage of T-substitution were varied. The effect of the length of the organic tail of the trifunctional silicon-alkoxide on the degree of condensation was studied with solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR). Cross-polarisation (CP) and single-pulse excitation (SPE) MAS-NMR measurements were compared. CP can not be used to obtain quantitatively reliable data for these systems due to the large distance between protons and part of the Q4 atoms. For quantitatively reliable data SPE MAS-NMR measurements with a repetition delay of 300 s have to be performed (at 7.05 T). The actual functionality (the realised number of network bonds) of the T-atoms decreases with increasing size of the organic tail due to steric hindrance. The functionality of the Q-atoms (AFQ), on the other hand, increases in the presence of T-atoms. An impressive increase in the AFQ parameter is observed for the samples containing 3-glycidoxy-propyltrimethoxysilane (Glymo). In all cases, except for low levels of Glymo substitution, the total number of network bonds decreases with increasing level of substitution. The evolution of the system starting from the sol phase indicates differences in the size of the primary particles formed from a pure TEOS sample on the one hand, and Glymo/TEOS or Phenyltrimethoxysilane (PhTMS)/TEOS samples on the other. In the latter two cases smaller primary particles are obtained. The different time dependences observed for the formation of the T-and Q-distributions for the sample containing Glymo implies phase segregation, in agreement with the results of CP experiments. The formations of the T- and Q-distributions for the PhTMS/TEOS sample appear to be synchronous. This effect, together with the results of the CP and 1H/29Si 2D heteronuclear correlation experiments, indicates the formation of a homogeneous system for PhTMS/TEOS systems.

Optimization of stripline-based microfluidic chips for high-resolution NMR

We here report on the optimization, fabrication and experimental characterization of a stripline-based microfluidic NMR probe, realized in a silicon substrate. The stripline geometry was modelled in respect of rf-homogeneity, sensitivity and spectral resolution. Using these models, optimal dimensional ratios were found, which hold for every sample size. Based on the optimized parameters, a simple integrated stripline-based microfluidic chip was realized. The fabrication of this chip is described in detail. We achieved a sensitivity of 0.47 nmol/square root(Hz) and a resolution of 0.7 Hz. The rf-homogeneity (A(810 degrees)/A(90 degrees)) was 76% and was proved to be suitable for 2D-NMR analysis of glucose.

Synthesis of molecular sieve [B]-BEA and modification of the boron site

The synthesis field and product composition of the (B,AI]-BEA molecular sieve were studied in order to prepare highly crystalline essentially aluminum-free (SiAl > 400) boron-containing Beta, using tetraethylammonium hydroxide as temptating agent. Fully crystalline [B.AI]-BEA was obtained with 4-6.8 trivalent ions per unit cell, i.e., Si(B + Al) = 8.4–15. BEA is proposed to be nucleated and grown by aluminosilicate species for solSiAl 21, both boro- and aluminosilicate entities provide charge compensation for the template ions and, hence, stabilization of the structure. Consequently, the boron incorporation efficiency is much higher for lower aluminum contents of the gel. Essentially aluminum-free [B]-BEA could be synthesized in the narrow compositional range, 5.6-6.6 B/unit cell, i.e., SiB = 8.7-10.4. Calcination and deboronation of [B]- and [B,AI]-BEA, silanol nest reoccupation of deboronated BEA, and direct Si⇋ substitution in calcined boron-containing BEA were studied by variable temperature FTi.r. spectroscopy. X-ray diffraction, and 11B, 27Al, and 29Si MAS n.m.r. spectroscopy techniques. By performing template removal in an ammonia atmosphere, boron hydrolysis and extraction as well as T-atom reorganizations could be avoided. Subsequently, mild extraction of lattice boron (> 90%) could be achieved by leaching with aqueous hydrochloric acid (pH < 5) at room temperature. The deboronated materials showed 20% XRD crystallinity loss, but no micropore volume or surface area reduction and could withstand thermal treatments up to 300°C in nitrogen. Preliminary investigations on silanol nest reoccupation and direct Si⇋ B replacement by exposure to silicon-introducing reagents showed only partial incorporation of silicon. Partial silylation of the silanol nests was achieved in the liquid phase using dichloromethylsilane (DCMS) or tetramethyl orthosilicate (TMOS) at 62 and 110°C, respectively. Direct substitution of framework boron by silicon was carried out by a gas phase reaction (300°C) using tetrachlorosilane (TCS).

27Al nutation NMR of zeolites

Abstract An NMR nutation experiment for quadrupolar nuclei developed by Samoson and Lippmaa has been applied to 27 Al spins in the zeolite catalyst H-ZSM-5. The 27 Al nutation spectra change with water content in the pores of this zeolite, from which it can be concluded that in the hydrated state many 27 Al nuclei are in highly symmetric surroundings. On dehydration the quadrupole interaction of these spins increases drastically, beyond detection. A second type of 27 Al spin with an intermediate quadrupole interaction that hardly changes on hydration/dehydration is detected.