13C CP/MAS NMR study of isobutyl alcohol dehydration on H-ZSM-5 zeolite. Evidence for the formation of stable isobutyl silyl ether intermediate

Dehydration of isobutyl alcohol selectively labelled with a¹³C nucleus in the CH₂ group (i-BuOH[1–¹³C]) has been studied on H-ZSM-5 zeolite within the temperature range 296–448 K using¹³C CP/MAS NMR. The formation of the isobutyl silyl ether intermediate (IBSE) has been detected. It is stable below 398 K. Within the temperature range 398–423 K IBSE decomposes gradually to produce first a butene dimer, probably 2,5-dimethyl-l-hexene and then other butene dimers and oligomers. AtT > 423 K scrambling of the selectively labelled carbon of the initial dimeric product over various positions in the carbon skeleton of the final dimers (oligomers) is observed. This is explained in terms of the formation of carbenium ion as the reaction intermediate.     

Carbenium ion properties of octene-1 adsorbed on zeolite H-ZSM-5

It is shown that octene-1 adsorbed on zeolite H-ZSM-5 at ambient temperature exhibits carbenium ion properties. Namely: (1) According to²H NMR, the proton of the acidic Al-OH-Si group of the zeolite is transferred into the CH₂= group of the octene-1 molecule. (2) According to¹³C NMR the¹³C label inserted into the terminal CH₂= group of the octene-1 molecule is scrambled over its hydrocarbon skeleton. Thermodynamic and kinetic parameters for carbon scrambling are measured within the temperature range 290–343 K. The zeolite framework is shown to favour the formation of the linear rather than branched carbeniumion.     

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.     

C NMR study on structure, composition and curing behavior of phenol-urea-formaldehyde resole resins

Phenol-urea-formaldehyde (PUF) resole resins were synthesized and analyzed by both liquid and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. The liquid ¹³C NMR analysis indicated that the co-condensation reactions between the phenolic ring and the urea unit occurred during the synthesis of the resins. The addition of the urea component effectively reduced the free formaldehyde content in the resin systems. Methylene ether bridges in the resins were found to be mainly associated with the urea units. pH had significant influences on the structure and composition of the resins. Solid-state ¹³C NMR measurements of the cured resins suggested that the pH probably affected the curing mechanism. A longer time and a higher temperature can generally accelerate the curing process and increase the rigidity of the cured network.     

Molecular model of Estonian kukersite kerogen evaluated by C MAS NMR spectra

The chemical structure of Estonian kukersite kerogen is evaluated using a simulation of ¹³C MAS NMR spectrum. A reasonable fit to the experimental NMR spectrum is obtained by assuming a model of the geomacromolecule with empirical formula C₄₂₁H₆₃₈O₄₄S₄NCl and a set of structural elements comprising mainly alkylated phenolic structures particularly alkyl-1,3-benzenediols and condensed alicyclic rings. From the presented model new views are coming up on the carbon skeleton of kerogen and constraints on the phenol formation pathways in the retorting process, i.e. up to 80% of methylene groups in kerogen are located in aliphatic chains and the complicated mixture of phenols in the retort oil seems to result mainly from the thermal conversion of alkyl-1,3-benzenediol units originally present in kerogen.     

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.     

Heterogenization of 12-tungstophosphoric acid on stabilized zeolite Y

Topics in Catalysis - 12-tungstophosphoric acid (PW12) has been supported on dealuminated zeolite Y, containing a secondary pore system with the predominant pore radii of 15 Å. The interaction...     

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 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.     

Solid-state C NMR investigation of the structure and hydrogen bonding for stereoregular poly(vinyl alcohol) films in the hydrated state

The structure and hydrogen bonding of the hydrated stereoregular poly(vinyl alcohol) (PVA) films have been investigated by high-resolution solid-state ¹³C NMR spectroscopy. It is found by the ¹³C spin-lattice relaxation analysis that there exist three components with different T_1C values assigned to the crystalline, less mobile and mobile components for the hydrated syndiotactic PVA (S-PVA) and highly isotactic PVA (HI-PVA) films. The line shape analysis indicates that the probability of intramolecular hydrogen bonding is appreciably increased in the crystalline region for the S-PVA films by the hydration but a slightly helical structure, which is probably allowed by the formation of the successive intramolecular hydrogen bondings along the chains in the crystalline region, seems not to undergo any significant change by the hydration for HI-PVA. This fact indicates that intramolecular hydrogen bonding is more stable in the hydrated state in the crystalline region. As for the less mobile component, the line shape of the CH resonance line for the hydrated S-PVA or HI-PVA films is found to be very similar to that of the corresponding crystalline component, probably being due to the successive formation of intermolecular or intramolecular hydrogen bonding in the interfacial region, which mainly contributes to the less mobile component, for the S-PVA or HI-PVA films even in the hydrated state. The mole fractions of the mm, mr and rr sequences are also estimated for the mobile component that is produced in each stereoregular PVA sample by swelling with water and it is concluded that no prominent preferential partitioning of the mm, mr and rr sequences occurs in the crystalline and noncrystalline regions for the PVA films with different tacticities.     

A 13C and 15N Solid State NMR Study of the Reactions of Acetone Oxime Adsorbed on FeZSM-5

The reactions of acetone oxime, a proposed reaction intermediate for the SCR (Selective Catalytic Reduction) of NO with propane on FeZSM-5, have been studied with ¹³C and ¹⁵N solid state MAS NMR (magic angle spinning nuclear magnetic resonance). FeZSM-5 with three different loading levels was prepared by the sublimation method. The thermal reactions of acetone [2-¹³C] oxime adsorbed on FeZSM-5 samples with different iron loadings were monitored by ¹³C MAS NMR by heating to the desired temperature and then cooling to room temperature for data acquisition. For the sample with the lowest iron loading (Fe/Al = 0.11), acetic acid and N-methyl-2-propanamine were formed by the decomposition of acetone oxime. For the samples with the higher iron loadings (Fe/Al = 0.69 and 0.91), acetone, N,N-methyl-2,2-propanediamine, and N-methyl-2-propanimine were formed by the decomposition of acetone oxime. ¹⁵N MAS NMR was used to investigate reactions of ¹⁵NO and acetone oxime on the FeZSM-5 samples. The formation of gas phase N₂ and N₂O was observed.     

Olefin as an Intermediate in n-Butane Isomerization on Sulfated Zirconia. An In Situ 13C MAS NMR Study of n-Octene-1 Conversion

By using in situ ¹³C MAS NMR and ex situ GC-MS, the analysis of hydrocarbon products formed from n-octene-1 adsorbed on sulfated zirconia catalyst (SZ) has been performed. It is shown that a mixture of alkanes and stable alkyl substituted cyclopentenyl cations (CPC) is formed as the basic reaction products. Formation of both alkanes and CPC from n-octene-1, a precursor of C₈ ⁺ cation, the key intermediate in n-butane isomerization via a bimolecular pathway, implies that formation of the isomerized alkane occurs by a complex process of conjunct polymerization, rather than isomerization itself. CPC deposited on the SZ surface can be in charge of the catalyst deactivation.     

Chemical characterization and agronomic effectiveness of phosphorus applied as a polyphosphate-chitosan complex

This work was undertaken to characterize a chitosan-polyphosphate complex (CH-PP) and to evaluate its agronomic effectiveness as a source of phosphorus for ryegrass (Lolium perenne) grown on loamy and clayey soils. High resolution solid state³¹P and¹³C nuclear magnetic resonance were used to characterize this complex and to monitor the structural changes occurring to it during an 8-week incubation period in a loamy soil. A pot experiment was conducted on the two soils after labelling the available P with³²PO₄ ions. This experiment allowed for the determination of the agronomic effectiveness of the chitosan-polyphosphate complex compared to polyphosphate and to monocalcium phosphate. Results showed that chitosan immobilized up to 147 mg P kg^–1 as pyrophosphate and hexametaphosphate. This reaction did not involve major structural changes in the pyrophosphate or hexametaphosphate groups nor in the chitosan. The chitosan-polyphosphate complex was as efficient as the polyphosphate alone to sustain the P nutrition of ryegrass. The relative agronomic effectiveness of these P sources was slightly lower compared to that of monocalcium phosphate. The high P fertilizing value of the chitosan-polyphosphate complex was attributed to its gradual hydrolysis in the soil. The potential interest of chitosan to remove polyphosphates from waste waters while preserving the high P fertilizing value of polyphosphates was addressed.     

On the Quantification of Lignin Hydroxyl Groups With 31P and 13C NMR Spectroscopy

Factors affecting the accuracy of the analysis of lignin hydroxyl and carboxyl groups with ³¹P NMR have been further elucidated. Two modifications of ³¹P NMR analysis of lignin, namely the protocols using 1,3,2-dioxaphospholane (PR-I) and 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (PR-II) as phosphorylation reagents with different internal standards, were studied. The previous ³¹P NMR standard protocol with PR-II underestimated OH groups by about 30%, whereas the ³¹P NMR standard protocol with PR-I tended to produce overestimated data. It has been shown that cholesterol is not an appropriate internal standard, resulting in underestimated values for OH groups due to incomplete baseline resolution. The best internal standard has been found to be endo-N-hydroxy-5-norbornene-2,3-dicarboximide. Strong care should be taken related to the stability of the internal standards to avoid inflated results due to IS degradation. Under modified optimized conditions, both methods show a good correlation with the ¹³C NMR protocol in the quantification of hydroxyl groups as average, with the variability between the methods in the range of 5–15%. However, the ³¹P NMR protocols report COOH content that is twice as low as that of ¹³C NMR data. Finally, the best approach for the use of the ³¹P and ¹³C NMR methods in lignin analysis is discussed.     

Effects of Solvation and Ionization on 13C NMR Spectra of Lignin Model Compounds,Spruce Milled Wood Lignin and Kraft Lignin

The effects of alkali and polar aprotic solvent on the aromatic carbons signals in ¹³C NMR (Carbon-13 nuclear magnetic resonance) spectra of lignin model compounds and spruce milled wood lignin (MWL) were studied. It was found that in 1 M aqueous NaOH signal shifts of C-1 and C-4 carbon atoms in the aromatic ring were the most noticeable in lignin models with free phenolic hydroxyl groups, which are ionized under the conditions. A similar effect in the spectra of the studied model compounds was observed in 0.5 M aqueous NaOD-deuterated dimethyl sulfoxide (DMSO) mixture (DMSO: water ratio 3:7 v/v). The model data help explaining changes in the ¹³C NMR spectra of MWL and lignin in situ dissolved in spruce kraft black liquor caused by ionization. In the ¹³C NMR spectra of spruce black liquor the signals of phenolic and non-phenolic lignin units are clearly separated and do not overlap with the signals of the carbon atoms of carbohydrates and other aliphatic products of wood degradation. The data obtained are useful in understanding the important role of solvation and ionization processes leading to lignin solubilization.     

Zeolite ZnY catalysts prepared by solid-state ion exchange

Zeolites ZnY with various overall zinc contents were prepared from mixtures of zeolite NH₄Y and crystalline zinc chloride by solid-state ion exchange. The obtained materials were investigated with carbon monoxide, xenon, and nitrogen adsorption as well as with ¹²⁹Xe NMR and XRF spectroscopy. From the results of these measurements, the zinc cation distributions between the different types of cages of the faujasite framework as well as between the crystallographic positions SIII and SII within the large voids (supercages) were quantitatively determined. The concentrations of zinc cations in the supercages of the presently prepared zeolites are considerably higher than in materials obtained from NaY by conventional wet ion exchange using aqueous zinc salt solutions. Experimental evidence is provided for salt inclusion under certain conditions of preparation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Analysis of sequence distribution in methyl methacrylate-methyl acrylate copolymers by C NMR spectroscopy

Methyl methacrylate (MMA)-methyl acrylate (MA) copolymers, prepared in bulk at 50 °C using AIBN as initiator, were characterized by 150 MHz ¹³C NMR spectroscopy, including use of the Distortionless Enhancement by Polarization Transfer (DEPT) experiment to obtain methylene and methine carbon resonances as subspectra. Dyad, triad, tetrad and partial pentad distributions were measured from the α-methyl, methine and methylene carbon resonances. These were in good agreement with distributions calculated for the copolymers based on monomer feed compositions, conversions and reactivity ratios of 2.60 and 0.27 for methyl methacrylate and methyl acrylate, respectively.     

Fully quantitative carbon-13 NMR characterization of resol phenol-formaldehyde prepolymer resins

Different resol phenol-formaldehyde prepolymer resins have been synthesized with different Formaldehyde/Phenol (F/P) ratios or different catalysts and characterized by ¹³C NMR spectroscopy in solution. A fast quantitative measuring protocol is proposed based on the use of chromium(III)acetylacetonate as a relaxation agent. APT (attached proton test) and DEPT (distortionless enhancement by polarisation transfer) spectra were acquired to enable proper resonance assignments, especially in the regions with severe signal overlap. Equations are presented in which the methylene bridges (MB), the methylol groups (MG) and the dimethylene ether bridges (DMEB) of resol resins are quantitatively taken into account. Important structural factors determined quantitatively for resol prepolymer resins are the F/P ratio after reaction, the degree of polymerization (n), the number average molecular weight (M_n) and the content of free ortho and para positions.     

C NMR Molecular dynamic investigation of tropical wood Angelin Pedra (Hymenolobium paetrum)

Time has shown wood as a very important material in several areas related to civil construction. Because of its organic origin, wood presents different physical and mechanical properties for different species. In consequence of these variations, there is the necessity to study chemical composition and molecular dynamic to better understand its property that will promote the use in civil construction. The focus of this work is to evaluate the Angelin Pedra wood in relation to the main chemical components, the domain type and the chemical components that constitute these domains. Solid state nuclear magnetic resonance (NMR) will be used for that, since the main advantage of NMR comparing to other techniques specially for the sample in question, is the potential of this spectroscopy to provide analysis of all functional group without pre-treatment of the sample. Some solid state NMR techniques were used and it was observed that the wood in investigation presents different packing, cells arrangements and chains ordination along the fibers in the different parts of the wood, because of the different distribution of the main wood components along the fibers.     

Semiquantitative analysis of thiophenic compounds in light cycle oil (LCO) using C NMR spectroscopy

S-methyltetrafluoroborate salts of the thiophenic compounds (CH₃-S⁺:BF₄⁻) present in LCO petroleum fractions were obtained and analyzed by ¹H and ¹³C NMR spectroscopy. The methylation of the samples was carried out using 99.5% ¹³C enriched methyl iodine, to improve the sensitivity of the technique. The amount of the methylated derivatives was determined by the internal standard method; using dioxane as a reference, 37 sulphur compounds were detected. Among them, benzo[b]thiophene, dibenzo[b,d]thiophene, and several isomers of methyl, dimethyl and trimethyl[b]benzothiophenes were the most abundant. With this research, it was demonstrated that NMR spectroscopy can be used to analyze thiophenic compounds from petroleum medium fractions.