Diffusion of ammonia in silicalite studied by QENS and PFG NMR

Ammonia diffusivities in silicalite have been studied by quasi-elastic neutron scattering (QENS) and pulsed-field gradient (PFG) NMR for temperatures between 200 and 500 K and loadings from 1.5 up to 4.3 molecules per unit cell. The diffusion coefficients obtained by both techniques increase with increasing ammonia concentration. The QENS diffusivities refer to only a certain fraction of molecules, because during the time scale of the measurement the other part of molecules is essentially immobile, in interaction with silanol groups. During the much larger time scale of the PFG NMR experiment, ammonia molecules assume both states of mobility, leading to an average diffusivity which is smaller than the diffusivity of the mobile molecules recorded by QENS. The difference between the diffusivities derived from both techniques decreases when the proportion of immobile molecules is taken into account. The residence time of ammonia in interaction with silanol groups is about two orders of magnitude longer than with oxygen atoms.     

PFG NMR and QENS diffusion study of n-alkane homologues in MFI-type zeolites

Recent methodological progress in pulsed field gradient (PFG) NMR and quasi-elastic neutron scattering (QENS) is exploited to enhance the range of diffusion measurement of the series of the n-alkane homologues in MFI-type zeolites (silicalite-1, ZSM-5) up to n-tetradecane. As observed already for the short-chain-length alkanes, the diffusivities are found to be intermediate between those for NaCaA and NaX, in agreement with the trend in the respective pore diameters. Similarly as in NaX, the diffusivities decrease monotonically with increasing chain length, in contrast to NaCaA, where recently for chain lengths above about six carbon atoms a remarkable deviation from this trend has been observed. Though agreeing in their trends, the diffusivities measured by PFG NMR are by up to one order of magnitude below the QENS data. This difference may be referred to internal diffusion barriers in the MFI-type zeolites: since PFG NMR monitors molecular displacements of typically micrometers the determined diffusivities are affected by these additional transport resistances, while these resistances are of no relevance for QENS, where much shorter displacements are recorded.     

Selective multi-component diffusion measurement in zeolites by pulsed field gradient NMR

The potential of pulsed field gradient (PFG) NMR for selective diffusion measurement in multi-component liquids is far from being fulfilled in multi-component diffusion studies with zeolites. We present two recent developments in PFG NMR instrumentation, which will significantly improve the measuring conditions for multi-component diffusion in zeolites and other nanoporous materials. They include options for an enhancement of the sensitivity with respect to smaller displacements by a novel principle of field gradient pulse matching and with respect to selectivity between different components by combining PFG NMR with magic angle spinning (MAS) NMR with a microimaging gradient system. The potentials and limitations of the two options are demonstrated by the first results of selective PFG NMR self-diffusion measurements with zeolitic adsorbate–adsorbent systems containing as much as four different species of guest molecules.     

Interaction of the perfluorinated polymer Nafion-H with water studied by proton broad-line NMR at 4 K and MAS NMR at room temperature

Broad-line¹H NMR at 4 K shows that, when Nafion-H interacts with water molecules at low concentrations, the only species formed is the hydronium ion. The chemical shift of the unsolvated hydronium ion in Nafion-H with 1 H₂O/SO ₃ ^– , determined by¹H MAS NMR at room temperature, is 10.4 ppm relative to external TMS.     

PFG NMR investigation of hydrocarbon diffusion in large NaX zeolite crystals: Effect of internal field gradients on diffusion data

Self-diffusion coefficients for intracrystalline diffusion of hydrocarbon molecules adsorbed in large crystals of NaX zeolite have been measured by the pulsed field gradient (PFG) NMR technique, at ambient temperature and at different diffusion times (from 6 to 12 ms). Two NMR pulse sequences, stimulated and 13-interval bipolar spin echo, were used to examine the influence of internal field gradients on diffusion data. For both sequences the effective self-diffusion coefficient of the guest molecules was found to decrease with increasing observation times. The extrapolated intracrystalline diffusion coefficient is independent of the NMR sequence. In contrast, the estimated extent of molecular diffusion depends strongly on the pulse program. For the small molecules (butane to hexane), the domain size, R, of restricted diffusion obtained with the stimulated spin-echo sequence is smaller than the crystal dimension whereas R is always comparable to it when the 13-interval pulse sequence is used. This shows the effect of internal field gradients on the diffusion data leading to wrong values of R if the stimulated pulse sequence is used. The light hydrocarbons diffuse freely inside the zeolite particles whereas the crystal boundaries act as reflecting surfaces, as previously observed. On the other hand, even with the 13-interval pulse sequence, the smaller values of R obtained for large molecules as n-heptane and octane shows that their displacement is hindered by restrictions in the NaX macro-crystals.     

NMR study of intrinsic diffusion and reaction in CsNaX type zeolites

In this contribution the catalytic behaviour of the zeolites CsNaX-60 and CsNaX-60·8CsOH (impregnated with eight CsOH per unit cell) was studied. As a model reaction, the conversion of isopropanol to acetone as the product of basic catalysis or to propene as the product of acid catalysis was chosen. The intracrystalline self-diffusivities of isopropanol, acetone and propene in these zeolites and in NaX were measured with the pulsed field gradient (PFG) NMR technique. Self-diffusivity measurements of propane and water were performed to characterize these three zeolites. ¹³C MAS NMR studies with proton decoupling were used to test the cesium containing zeolites for acid or basic catalytic activity.     

Diffusion of probe polystyrenes with different molecular weights in poly(methyl methacrylate) gels and inhomogeneity of the network structure as studied by time-dependent diffusion NMR spectroscopy

Poly(methyl methacrylate) (PMMA) gels have been prepared with radical polymerization by cross-linking methyl methacrylate monomer using ethylene glycol dimethacrylate as cross-linking monomer in toluene containing polystyrenes (PSs) with M_w from 4000 to 400,000. The diffusion coefficients of the PSs in the PMMA gels swollen in deuterated chloroform have been measured by pulsed field-gradient (PFG) ¹H NMR method with the diffusing time Δ varied. From the experimental results, it is found that the network structure of PMMA gels prepared in the presence of PSs with M_w=4000 and 400,000 are relatively homogeneous and inhomogeneous, respectively, within the Δ range from 40 to 500 ms.     

Studies using Al MAS NMR of AFm and AFt phases and the formation of Friedel's salt

This paper describes the application of the magic angle spinning (MAS) NMR spectroscopy to study the chemical environment of ²⁷Al-bearing phases in Portland cement-based concrete. A specific methodology is described that allows reliable spectra to be determined for combinations of different types of cements and fillers (in this case, Portland cement, fly ash, slag, silica fume, metakaolin and limestone filler). As well as the study of ‘molecular structure’ of cement matrix, the paper reviews the mechanism of Friedel's salt formation in cement systems. Mechanisms based on ion exchange of chloride for hydroxide in hydroxy-AF_m and on chloride absorption on formation are discussed. Finally, the nature of the chloride/hydrate binding phenomena are described to provide a reasonable robust and fundamental picture of the role different cements can play in the provision of overall concrete durability to chloride ingress from a chemical perspective.     

Solid-State 13C CP/MAS NMR for Alkyl-O-Aryl Bond Determination in Lignin Preparations

Several lignin preparations (Freudenberg lignin, Björkman lignin, and Pepper lignin), technical lignins (soda, soda-AQ, Kraft, Kraft-AQ, and hydrolysis), dimeric lignin model compounds, and different polysaccharides (galactoglucomannan, arabinogalactan, xylan, and arabinan) were analyzed by means of solid-state ¹³C CP/MAS NMR. Signals assignment in solid-state NMR lignin spectra was performed on the basis of the conducted studies and earlier published data. It was established that there exists strong linear correlation (r = 0.985) between Alkyl-O-Aryl inter-unit bond content in lignin and integral signals intensity in NMR spectra in the range of chemical shifts of 96–68 ppm. The integral signals intensity was measured in correlation with the reference integral signals in the range of chemical shifts of 162–102 ppm, typical for aromatic carbon atoms. To eliminate the effect, caused by carbohydrates contained in lignin, the correction factor of 0.67% of the area of integration per 1% of carbohydrates was determined. It was shown that the solid-state ¹³C CP/MAS NMR method allowed to determine Alkyl-O-Aryl bond content in both soluble and insoluble lignin preparations, and also to determine methoxyl groups content in soluble preparations.     

Elongation-induced phase separation of poly(vinyl alcohol)/poly(acrylic-acid) blends as studied by C CP/MAS NMR and wide-angle X-ray diffraction

The membrane samples of poly(vinyl alcohol)/poly(acrylic-acid) (PVA/PAA) blend with different draw ratios were studied by both ¹³C CP/MAS NMR and wide-angle X-ray diffraction (WAXD) measurements. Phase separation induced by elongation of the sample was observed and the change of the phase structure with draw ratio was found to be dependent on the composition of the blend samples.     

Surface complexes in zeolite-catalysed acylation reactions detected by13C MAS NMR spectroscopy

The state of the acylating agent acetyl chloride, adsorbed on a series of proton and metal ion exchanged zeolites X, Y (faujasite) and ZSM-5, was investigated by¹³C MAS NMR spectroscopy. The observed carbonyl signals were assigned to two species: chemisorbed acetyl chloride bound to lattice oxygen (signals near 182 ppm), and acetyl chloride complexed with counter cations in the lattice (signals near 172 ppm). In a few cases signals were observed which have been assigned to free acylium cation stabilized on the surface of the solid (signal 160–165 ppm). Experiments in which toluene was adsorbed on to ZnY pretreated with acetyl chloride showed the participation of various adsorbed species in the acylation reaction of toluene.     

Absorption and diffusion measurements of water in acrylic paint films by single-sided NMR

Artists’ acrylic paint surfaces can accumulate surface grime. Aqueous surface cleaning of these paints can be particularly problematic because they swell upon exposure to water and because of the presence of water extractable additives. Research to determine the parameters of water penetration and swelling as well as the role of the water soluble additives in these phenomena is important in order to understand the leaching mechanism and possibly describe the best conditions for removal of water soluble grime. In this work, the penetration and molecular dynamics of water with different salt concentrations in films of artists’ waterborne acrylic paint were studied for the first time in situ using noninvasive single-sided nuclear magnetic resonance (NMR). This novel approach demonstrated that the absorption of water in the acrylic film is independent of the salt concentration, whereas the absorption of water in water-washed acrylic paints, after the water soluble components have been removed, is not only 15% less than unwashed paint films but it also decreases with increasing salt concentration. In addition, the self-diffusion coefficients of water within the acrylic polymeric network were determined and analyzed. The diffusion coefficients of water depend only slightly on the salt concentration and are only marginally influenced by the amount of incorporated surfactants. Thus the diffusion rate of water within the acrylic polymer film appears to be almost independent of the salt concentration or the surfactant content.     

The 15N CP/MAS NMR Characterization of the Isocyanate Adhesive Bondline for Cellulosic Substrates

A 99% ¹⁵ 5N-labeled polymeric diphenylmethane diisocyanate (pMDI) resin with desirable adhesive properties was synthesized. This adhesive was used to make a series of cellulose/¹⁵N-labeled pMDI composites at various cellulose precure moisture contents. Cure chemistry and bondline morphology were monitored using ¹⁵N CP/MAS NMR. Biuret-type structures were identified as the predominant chemical moieties at low precure moisture conditions, while urea linkages were predominant at higher precure moisture. Significant amounts of urethane were not detected, however low amounts could be obscured by signal overlap. Relaxation studies using variable contact times were complicated by excessively long cross-polarization rates for nonprotonated nitrogens. Experiments using variable spin lock times prior to fixed contact periods suggest that the cured resin is homogeneous. The utility of ¹⁵N CP/MAS for elucidating fine structural and morphological information from complex isocyanate-cured composites is clearly demonstrated.     

CP/MAS C NMR analysis of the structure and hydrogen bonding of melt-crystallized poly(vinyl alcohol) films

The structure and hydrogen bonding of the melt-crystallized atactic poly(vinyl alcohol) (A-PVA) films, which were carefully prepared without significant thermal degradation, have been characterized by CP/MAS ¹³C NMR spectroscopy. The ¹³C spin-lattice relaxation analysis has revealed that there exist three components with different T_1C values, the crystalline, less mobile noncrystalline and mobile noncrystalline components, in good accord with the results for different PVA samples previously reported. It should be noted that the T_1C values of the crystalline and noncrystalline components are appreciably smaller for the melt-crystallized films than those for the un-annealed and annealed samples prepared by casting from the aqueous solution. The ¹³C NMR spectra of the crystalline and noncrystalline components are separately recorded by using the difference in T_1C and their CH lines are successfully resolved into three and seven constituent lines by the least-squares curve fitting, respectively. Moreover, the statistical analysis of the integrated intensities of the constituent lines thus obtained enables to determine the probability f_a for the formation of intramolecular hydrogen bonding in the successive two OH groups along each chain and another probability f_t of the trans conformation for the crystalline and noncrystalline components. It is found that the f_a value is relatively larger for the melt-crystallized films than those for the un-annealed and annealed samples. On the basis of these results, the features of the melt-crystallization and the resulting crystalline-noncrystalline structure are discussed by particularly considering effects of intra- and inter-molecular hydrogen bonding on the crystallization.     

Solid-state F MAS and H→F CP/MAS NMR study of the phase transition behavior of vinylidene fluoride-trifluoroethylene copolymers: 1. Uniaxially drawn films of VDF 75% copolymer

The changes in the phase structures and molecular mobility caused by the ferroelectric-paraelectric phase transition of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer, P(VDF₇₅/TrFE₂₅), were analyzed using variable temperature (VT) solid-state ¹⁹F MAS and ¹H→¹⁹F CP/MAS NMR spectroscopy. The CF₂ signal of the VDF chain sequence and the CHF signal at the head-to-head linkage of VDF-TrFE sequence showed higher frequency shift in the temperature range 43-92 °C, whereas no change was found for the CHF signal at the head-to-tail linkage of VDF-TrFE up to 92 °C. Hence, VT ¹⁹F MAS spectra revealed that the VDF-TrFE head-to-tail sequence is the most stable part in polymer chains against trans-gauche conformational exchange motions below the phase transition temperature (Curie temperature, T_c) on heating. However, all chain sequences including TrFE units undergo conformational exchange at around T_c. The phase transition behavior is clearly recognized in the ¹⁹F spectral shapes, in which the broad signals of the ferroelectric immobile phase disappeared between 115 and 119 °C. In addition, T_1ρ^F for all peaks decreased to a unique value (ca. 20 ms) at 119 °C, indicating that uniform molecular motion accompanied by a full chain rotation occurred at the temperature. The significantly longer T_1ρ^F for all peaks (ca. 20 ms) in the paraelectric phase (119 °C) than that in the amorphous domain (<4 ms) at ambient temperature supports the conclusion that there is restricted rotational motion of polymer chains around the chain axis in the paraelectric phase. On cooling from 119 to 85 °C, a gradual decrease in gauche conformers in the paraelectric phase was confirmed by the low-frequency displacement of CF₂ signals in VDF sequences accompanied by slight decreases in T₁^F and T_1ρ^F. The phase transition was observed between 85 and 77 °C on cooling, in which the characteristic signals of the paraelectric phase disappeared, the T_1ρ^F values of all peaks quickly increased, and the broad crystalline signals abruptly appeared at 77 °C.     

Structure and dynamics of a vinylidene fluoride oligomer and its cyclodextrin inclusion compounds as studied by solid-state F MAS and H → F CP/MAS NMR spectroscopy

The molecular structure and dynamics of a vinylidene fluoride oligomer telomerized by carbon tetrachloride (Cl-OVDF) and its inclusion compound (IC) with β-cyclodextrin (β-CD) have been investigated using solid-state ¹⁹F magic angle spinning (MAS) and ¹H → ¹⁹F cross-polarization (CP)/MAS NMR spectroscopy. The preferential IC formation of the lower-molecular-weight components with β-CD was used to refine as-received Cl-OVDF. The refined Cl-OVDF with larger molecular weight readily takes γ-form (tttg⁺tttg⁻) conformation, and it also forms ICs with β-CD (Cl-OVDF/β-CD IC) under a certain condition. ¹⁹F MAS NMR indicates that Cl-OVDF chains virtually isolated in the β-CD cavities take no specific conformations even at −40 °C. The temperature dependence of the magnetic relaxation times (T₁^F, T_1ρ^F) indicates that the Cl-OVDF chains in ICs undergo molecular motions similar to the amorphous phase in the bulk, although the intramolecular spin diffusion among ¹⁹F nuclei is more significant in the former because of the one-dimensional confinement.     

Diffusional behavior of amino acids in solid-phase reaction field as studied by H pulsed-field-gradient spin-echo NMR method

The diffusion coefficients (D) of tert-butyloxycarbonyl-l-phenylalanine (Boc-Phe) in Merrifield network polystyrene gels, used as a solid-phase reaction field have been determined as a function of the amino acid concentration over the temperature range from 30 to 50 °C by means of the ¹H pulsed-field-gradient spin-echo NMR method. From these experimental results, it was found that the D value of Boc-Phe in DMF-d₇ solution, in DVB 1 and 2% cross-linked network polystyrene gels depends on the amino acid concentration. The D value of Boc-Phe·Cs(tert-butyloxycarbonyl-l-phenylalanine cesium) salt in the solid-phase reaction field under chemical reaction was determined at 50 °C. Further, it was found that the D value depends on the NMR observation time, that is the applied two field-gradient pulse interval. Details of its analysis were discussed.     

Glycerin-Induced Conformational Changes in Bombyx mori Silk Fibroin Film Monitored by 13C CP/MAS NMR and 1H DQMAS NMR

In order to improve the stiff and brittle characteristics of pure Bombyx mori (B. mori) silk fibroin (SF) film in the dry state, glycerin (Glyc) has been used as a plasticizer. However, there have been very limited studies on the structural characterization of the Glyc-blended SF film. In this study, ¹³C Cross Polarization/Magic Angle Spinning nuclear magnetic resonance (CP/MAS NMR) was used to monitor the conformational changes in the films by changing the Glyc concentration. The presence of only 5 wt % Glyc in the film induced a significant conformational change in SF where Silk I* (repeated type II β-turn and no α-helix) newly appeared. Upon further increase in Glyc concentration, the percentage of Silk I* increased linearly up to 9 wt % Glyc and then tended to be almost constant (30%). This value (30%) was the same as the fraction of Ala residue within the Silk I* form out of all Ala residues of SF present in B. mori mature silkworm. The ¹H DQMAS NMR spectra of Glyc-blended SF films confirmed the appearance of Silk I* in the Glyc-blended SF film. A structural model of Glyc-SF complex including the Silk I* form was proposed with the guidance of the Molecular Dynamics (MD) simulation using ¹H–¹H distance constraints obtained from the ¹H Double-Quantum Magic Angle Spinning (DQMAS) NMR spectra.     

FTIR spectroscopic and1 H MAS NMR studies of the influence of lattice chemistry and structure on Brønsted acidity in zeolites

The influence of the Si/Al ratio, of the nature of the T-atom and of the pore size on the acidic strength of Brønsted sites in zeolites has been investigated using changes of the vibrational properties of Brønsted OH(OD) groups and a shift change of Brønsted protons in nuclear magnetic resonance upon adsorption of weak bases. Deuterated acetonitrile and trichloro-acetonitrile have been chosen to probe the acidic strengths of ZSM-5, FeZSM-5, mordenite and zeolite Y, which are often used as catalysts. From the results of the FTIR and 1 H MAS NMR studies it can be concluded that the chemical composition of the lattice dominates the acidic strength of the Brønsted sites in zeolites. Differences in structure or pore size play a much smaller role.     

n-Butane conversion on sulfated zirconia: in situ 13C MAS NMR monitoring of the kinetics of the 13C-label scrambling and isomerization

The kinetics of the conversion of ¹³C-labeled n-butane adsorbed on sulfated zirconia (SZ) were monitored by in situ ¹³C MAS NMR spectroscopy. Rate constants of n- to isobutane isomerization and of the ¹³C-isotope scrambling from the primary to the secondary carbon atoms in n-butane were determined. The monomolecular scrambling of the ¹³C-label in adsorbed n-butane has an activation energy of 17 ± 3 kcal mol^–1 and occurs faster than the bimolecular process of n-butane isomerization which has an activation energy of 15.1 ± 0.2 kcal mol^–1. The transfer of the selective ¹³C-label from the primary to the secondary carbon atom in the adsorbed n-butane seems to consist of two reaction steps: (i) a hydride abstraction by SZ leading to the formation of sec-butyl cations and (ii) a label scrambling in the sec-butyl cations. This two-step process with the formation of sec-butyl cations as intermediate increases the apparent activation energy for the ¹³C-label scrambling, which is almost twice as large compared with the activation energy for carbon scrambling of sec-butyl cations in a superacidic solution.