11B Magic Angle Spinning NMR Research Articles

Boron Adsorption Kinetics of Microcrystalline Cellulose and Polymer Resin

Tailoring boron-polysaccharide interactions is an important strategy for developing functional soft materials such as hydrogels, fire retardants, and sorbents for environmental remediation, for example, using lignocellulosic biomass. For such applications to be realized, it is paramount to understand the adsorption kinetics of borate anions on cellulose and their local structures. Here, the kinetic aspects of boron adsorption by microcrystalline cellulose, lignin, and polymeric resin are investigated and compared. Borate anions interact with the vicinal diols in the glucopyranoside moieties of cellulose to yield chemisorbed boron chelate complexes. In contrast to cellulose, technical lignin contains fewer cis-vicinal diols, and it does not have a tendency to form such chelate complexes upon treatment with the aqueous boric acid solution. The formation kinetics and stability of these chelate complexes strongly depend on nanoscale structures, as well as reaction conditions such as pH and concentration of the sorbate and sorbent. Specifically, insights into the distinct boron adsorption sites were obtained by solid-state one-dimensional (1D) 11B magic-angle spinning NMR and the local structures and intermolecular interactions in the vicinities of boron chelate complexes are elucidated by analyzing two-dimensional (2D) 1H-13C and 11B-1H heteronuclear correlation NMR spectra. The total boron adsorption capacity of cellulose is estimated to be in the 1.3-3.0 mg range per gram of sorbent, which is lower than the boron adsorption capacity of a polystyrene-based resin, ∼17.2 mg of boron per gram of Amberlite IRA 743. Our study demonstrates that the local backbone and side chain flexibility as well as the structures of polyol groups play a significant role in determining the kinetic and thermodynamic stability of chelate complexes, yielding to different boron adsorption capabilities of lignocellulosic polymers.

Phase Diagram for the NaBH4–KBH4 System and the Stability of a Na1–xKxBH4 Solid Solution

The binary system NaBH4–KBH4 has been systematically investigated by several complementary techniques, including in situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermal analysis, and in situ 11B and 23Na magic-angle spinning (MAS) NMR. Full solubility in the system NaBH4–KBH4 and formation of a solid solution, Na1–xKxBH4, 0 110 °C, are observed. The Na1–xKxBH4 solid solution is most clearly observed by 11B MAS NMR, which reveals a binomial distribution of Na+ and K+ ions over the six metal sites in the first coordination sphere of the BH4– unit, i.e., seven almost equally distinct peaks in the range −36.4 to −42.6 ppm with a systematic change of 1.0 ± 0.1 ppm toward lower frequency for each replacement of K+ by Na+. The solid solution Na1–xKxBH4 is metastable at room temperature, and 75% of Na0.5K0.5BH4 has separated into NaBH4 and KBH4 after 24 h at ∼24 °C, as observed by in situ 23Na and 11B MAS NMR. The composition 0.682NaBH4–0.318KBH4 has the lowest m...

Synthesis of hyperbranched polymethylvinylborosiloxanes and modification of addition‐curable silicone with improved thermal stability

This paper reports a non‐catalyzed environmentally friendly method of synthesizing hyperbranched polymethylvinylborosiloxanes (PMVBSs) and their use to improve the thermal stability of normal addition‐curable silicones (ACSs). PMVBSs were synthesized by the direct polycondensation of dimethoxymethylvinylsilane with boric acid at 80–130°C in 1,4‐dioxane or diglyme. They were characterized by gel permeation chromatography; FT‐IR; 1H, 13C, 29Si and 11B NMR; and TGA. PMVBSs were composed of Si―O―Si and Si―O―B bridges with some unreacted B―OH groups remaining, and had a ceramic yield up to 65.97% at 900°C. PMVBS‐modified ACSs (PBS‐ACSs) were prepared by curing the PMVBSs with hydrogen‐containing silicone oil under Karstedt (platinum divinyltetramethyldisiloxane) catalysis. Thermal stability of PBS‐ACSs was characterized by TGA in N2 or air, and ceramic yields as high as 76.7% were obtained. Gas decomposition during the ceramization of PBS‐ACSs was examined by TG/mass spectroscopy. The SiBOC ceramics formed were characterized by FT‐IR, Raman, 29Si and 11B magic angle spinning NMR and elemental analysis. This method provides a valuable way to improve the thermal stability of ACSs. Copyright © 2013 John Wiley & Sons, Ltd.

Mixed Network Former Effect in Ion-Conducting Alkali Borophosphate Glasses: Structure/Property Correlations in the System [M2O]1/3[(B2O3)x(P2O5)1–x]2/3 (M = Li, K, Cs)

Glasses in the system [M2O]1/3[(B2O3)x(P2O5)1–x]2/3(M = Li, K, Cs) (0.0 ≤ x ≤ 1.0) were prepared by standard melt-quenching procedures, and their physical properties were characterized by thermal analysis, density measurements, and impedance spectroscopy. Their atomic level structures were comprehensively characterized by Raman spectroscopy, by X-ray photoelectron spectroscopy (XPS), and by 11B, 31P, and 7Li as well as 133Cs high resolution solid state magic-angle-spinning (MAS) NMR techniques. 31P MAS NMR peak assignments were aided by the presence or absence of homonuclear indirect 31P–31P spin–spin interactions, “J-coupling”, as detected via refocused INADEQUATE techniques. Consistent speciations of the phosphate and borate network former components in terms of the various PnmB and BnmP units, where n is the number of bridging oxygens (BOs) and m is the number of B or P units bonded to P or B, respectively, present in these glasses were derived from 11B MAS NMR, combined with both 31P MAS NMR and XPS l...

Structure and Ionic Conductivity in the Mixed-Network Former Chalcogenide Glass System [Na2S]2/3[(B2S3)x(P2S5)1–x]1/3

Glasses in the system [Na2S]2/3[(B2S3)x(P2S5)1–x]1/3 (0.0 ≤ x ≤ 1.0) were prepared by the melt quenching technique, and their properties were characterized by thermal analysis and impedance spectroscopy. Their atomic-level structures were comprehensively characterized by Raman spectroscopy and 11B, 31P, and 23Na high resolution solid state magic-angle spinning (MAS) NMR techniques. 31P MAS NMR peak assignments were made by the presence or absence of homonuclear indirect 31P–31P spin–spin interactions as detected using homonuclear J-resolved and refocused INADEQUATE techniques. The extent of B–S–P connectivity in the glassy network was quantified by 31P{11B} and 11B{31P} rotational echo double resonance spectroscopy. The results clearly illustrate that the network modifier alkali sulfide, Na2S, is not proportionally shared between the two network former components, B and P. Rather, the thiophosphate (P) component tends to attract a larger concentration of network modifier species than predicted by the bulk composition, and this results in the conversion of P2S74–, pyrothiophosphate, Na/P = 2:1, units into PS43–, orthothiophosphate, Na/P = 3:1, groups. Charge balance is maintained by increasing the net degree of polymerization of the thioborate (B) units through the formation of covalent bridging sulfur (BS) units, B–S–B. Detailed inspection of the 11B MAS NMR spectra reveals that multiple thioborate units are formed, ranging from neutral BS3/2 groups all the way to the fully depolymerized orthothioborate (BS33–) species. On the basis of these results, a comprehensive and quantitative structural model is developed for these glasses, on the basis of which the compositional trends in the glass transition temperatures (Tg) and ionic conductivities can be rationalized. Up to x = 0.4, the dominant process can be described in a simplified way by the net reaction equation P1 + B1 ⇆ P0 + B4, where the superscripts denote the number of BS atoms for the respective network former species. Above x = 0.4, all of the thiophosphate units are of the P0 type and both pyro- (B1) and orthothioborate (B0) species make increasing contributions to the network structure with increasing x. In sharp contrast to the situation in sodium borophosphate glasses, four-coordinated thioborate species are generally less abundant and heteroatomic B–S–P linkages appear to not exist. On the basis of this structural information, compositional trends in the ionic conductivities are discussed in relation to the nature of the charge-compensating anionic species and the spatial distribution of the charge carriers.

Formation Process of [B<SUB>12</SUB>H<SUB>12</SUB>]<SUP>2−</SUP> from [BH<SUB>4</SUB>]<SUP>−</SUP> during the Dehydrogenation Reaction of Mg(BH<SUB>4</SUB>)<SUB>2</SUB>

The existence of Mg(B12H12) as an intermediate compound during the dehydrogenation process of Mg(BH4)2 has been verified. To elucidate the formation process of Mg(B12H12), the dehydrogenation products of Mg(BH4)2 at different temperatures were characterized via X-ray diffraction and 11B magic angle spinning NMR measurements. The experimental results indicate that several new intermediate compounds such as Mg(B2H6) and Mg(B5H9) or Mg(B5H8)2 are expected to be formed prior to the formation of Mg(B12H12). Thus, it is suggested that the formation of [B12H12]2− from [BH4]− occurs via the gradual evolution of the B-H complex anion, i.e., [BH4]−→[B2H6]2−→[B5H9]2− or [B5H8]−→[B12H12]2−.

Complexation of boric acid with vitamin C

Mono-chelate (1 : 1) and bis-chelate (1 : 2) anionic complexes of boric acid with vitamin C (L-ascorbic acid, H2A) were isolated from aqueous solutions in salt form with Li+, Na+ and Ca2+ ions. The complexes were characterized by FTIR, 13C and 11B MAS (magic angle spinning) NMR techniques. The spectral data agreed with the calculated structures that H2A complexes with boric acid through its cis-enediol groups by forming five-membered chelate rings. Side-chain OH groups do not participate in complexation as corroborated by the results of the test experiments conducted with iso-propylideneascorbic acid (i-H2A) where the side chain is blocked. Empirical relations, that can be used as diagnostics for the 1 : 1 and 1 : 2 ascorbatoborate complexes, were derived from the observed chemical shift values. Bis-chelate complexes were found to have higher thermal and hydrolytic stabilities than their mono-chelate homologs. The effect of the cation on the stabilization of the complexes was also investigated. The observed relative stability of the 1 : 1 Na–ascorbatoborate complex with respect to the analogous Ca complex, correlated with the recent reports about the role of alkali metal ions in the stabilization of ribose in the prebiotic world.

A high-energy X-ray diffraction, 31P and 11B solid-state NMR study of the structure of aged sodium borophosphate glasses

The structure of aged melt-quenched sodium borophosphate glasses of composition (P 2O 5) 40(B 2O 3) x (Na 2O) 60− x (with x in the range 10–40) has been studied by high-energy X-ray diffraction (HEXRD), 31P and 11B magic angle spinning (MAS) NMR. Similar to the fresh samples, both P O P and P O B linkages are found to be present in these glasses. All three techniques show that the cross-linking between borate and phosphate units increases with boron oxide content. Distinctively upon aging, the glass is found to hydrolyze causing the network to degrade. At the same time, crystalline phases are now also observed. XRD and DTA show that the samples have a higher tendency towards crystallization with increasing boron oxide content upon exposed to moisture. 31P and 11B MAS NMR results are in agreement with these findings. TGA data show that samples with higher boron oxide content take up more moisture upon aging, suggesting that crystallization may be associated with glass hydrolysis. HEXRD results also suggest that sodium ions are preferentially associated with borate units with increasing boron oxide content.

Boron speciation and non-bridging oxygens in high-alkali borate glasses

High-resolution 11B MAS NMR has become a routine approach for quantifying three- and four-coordinate borons in borate glasses. However, its usefulness in probing finer structural details, such as differentiating amongst trigonal borons, has been limited by severe peak overlap. We apply 11B multiple-quantum (MQ) magic-angle spinning (MAS) NMR to high-alkali borate glasses, to separate signals from trigonal borons with varying numbers of non-bridging oxygens (NBOs). A comparison between lithium and cesium borate glasses at 41 and 47 mol% alkali oxide reveals significant differences in the types of trigonal borons present, as well as the degree of structural disorder. The spectral signature of NBO-bearing trigonal boron is clearly apparent in the cesium borate glasses, but is obscured in the corresponding lithium borates, both because of greater disorder and a lower population of such sites. Isotropic projections of the two-dimensional MQMAS data sets are consistent with progressive network depolymerization, as expected for increasing alkali loading. 11B MQMAS of a lithium borate invert glass (71 mol%) shows remarkable resolution of trigonal borons with two and three NBOs. These species are quantified and found to be in good agreement with the charge-balance requirements at this Li + concentration.

15N cross‐polarization using the inversion–recovery cross‐polarization technique and 11B magic angle spinning NMR studies of reference compounds containing B—N bonds

Borane–ammonia complexes, trialcynylborazine and N-trimethylcycloborazane were characterized by 15N and 11B solid-state magic angle spinning (MAS) NMR. They are representative of compounds containing B—N bonds and presenting various N and B environments, either tri- or tetracoordinated. Inversion–recovery cross-polarization (IRCP), a spectral editing technique first introduced for 13C, was extended to 15N. For all compounds, characterized by either weak or strong 15N–1H dipolar couplings, the polarization inversion behavior follows the theoretical equations describing the spin dynamics associated with the IRCP sequence. This technique thus proved to be an excellent tool for identifying 15N sites according to their proton environments. The isotropic chemical shift, quadrupolar coupling constant and asymmetry factor were extracted from the 11B MAS NMR spectra. However, the unusual shape of the center band was tentatively related to the presence of residual 11B–14N dipolar coupling, which could be possibly reduced at higher fields. © 1998 John Wiley & Sons, Ltd.

15N cross-polarization using the inversion-recovery cross-polarization technique and11B magic angle spinning NMR studies of reference compounds containing B—N bonds

Borane–ammonia complexes, trialcynylborazine and N-trimethylcycloborazane were characterized by 15N and 11B solid-state magic angle spinning (MAS) NMR. They are representative of compounds containing B—N bonds and presenting various N and B environments, either tri- or tetracoordinated. Inversion–recovery cross-polarization (IRCP), a spectral editing technique first introduced for 13C, was extended to 15N. For all compounds, characterized by either weak or strong 15N–1H dipolar couplings, the polarization inversion behavior follows the theoretical equations describing the spin dynamics associated with the IRCP sequence. This technique thus proved to be an excellent tool for identifying 15N sites according to their proton environments. The isotropic chemical shift, quadrupolar coupling constant and asymmetry factor were extracted from the 11B MAS NMR spectra. However, the unusual shape of the center band was tentatively related to the presence of residual 11B–14N dipolar coupling, which could be possibly reduced at higher fields. © 1998 John Wiley & Sons, Ltd.

Synthesis and Solid State 11B-{1H} Mas Nmr Investigations of some Group 11 arachno-2-Metallatetraboranes

The preparation and characterization (melting point, solution 11B-{1H} NMR, IR) of some new 2-arachno-argentatetraboranes and 2-arachno-cupratetraboranes, [2-{(R3P)2M}B3H8] {M = Ag, R = 2-MeC6H4, 4-CIC6H4, 4-MeOC6H4, 2,6-(MeO)2C6H3; M = Cu, R = 4-CIC6H4, 2,6-(MeO)2C6H3} are reported and their solid state 11B MAS (Magic Angle Spinning) NMR spectra are compared with those obtained from the previously known metallatetraboranes {M = Ag, R = Ph, 4-MeC6H4; M = Cu, R = Ph} and [NMe4][2-{M(CO)4}B3H8] (M = Cr, W). In contrast to the Cr and W derivatives, the Ag and Cu clusters exhibit only one signal in their solid state 11B spectra consistent with an intramolecular boron exchange process which occurs in the solid state. Solid state 11B MAS NMR spectra of Cs[B3H8] and [NMe4][B3H8] are also reported. Referee I: H. Beall Referee II: L. Barton

Structural state of boron in MFI-titanium boralites and their catalytic properties

A series of titanium boralites with the MFI structure has been synthesized. The catalysts were characterized using X-ray diffraction (XRD), IR and magic-angle spinning (MAS) NMR spectroscopic techniques and the incorporation of boron into the framework in the presence of titanium has been investigated in as-synthesized and in calcined samples. Tetrahedral framework boron was determined quantitatively by 11B MAS NMR. The activity of titanium boralites in the oxidation of hexane using hydrogen peroxide as oxidizing agent was evaluated. In the as-synthesized samples, boron and titanium were simultaneously incorporated into the siliceous MFI framework. At constant boron loading, a higher titanium loading leads to a higher fraction of boron removed from its framework position after calcination. The lower activity and H2O2 selectivity toward oxyfunctionalization products when boron is present are related to the mild acidic Brønsted sites of the boralite. Acid leaching selectively removes boron and allows a partial recovery of the activity of the titanium silicalite.

Quantitative interpretation of NMR spectra of disordered solids

Profile analysis of quadrupole-broadened NMR spectra of disordered solids is a complex and computationally intensive task that we have undertaken by developing general purpose optimization programs. After presenting the spectroscopic problem (first- or second-order quadrupolar effects) and the different types of optimizers (deterministic or stochastic), we discuss how efficiently the “solution” of a simple problem can be found, how a model of disorder can be introduced and progressively refined, which kind of information can be obtained. As example, 11B magic angle spinning NMR spectra in vitreous borates are quantitatively interpreted.

Solids containing tetrahalophosphonium cations: An investigation using 31P and 11B magic-angle spinning NMR and Raman spectroscopy

A variety of solid complexes containing PCl + 4 or PBr − 4 has been studied by magic-angle spinning (MAS) 31P and 11B NMR (the la