Urea Clathrate Research Articles

Separation of Propane and Propylene by Urea Clathrate Formation

Separation of Propane and Propylene by Urea Clathrate Formation

Piezoelectric electrospun polyacrylonitrile with various tacticities

ABSTRACTElectrospinning of urea clathrate polymerized polyacrylonitrile (PAN) with isotacticity 25% and 52% was achieved in N,N‐dimethylformamide (DMF) at room temperature. Although the molecular weights of the 25% and 52% were found to be comparable by size exclusion chromatography, creation of uniform nanofibers with comparable diameters (average of ~450 nm) required concentrations of 5 % w/v and 3.5% w/v, respectively. X‐ray diffraction (XRD) analysis demonstrated that the polymer retained semicrystalline structure and suggested that crystallinity was correlated with increasing isotacticity. Fourier transform infrared spectroscopy (FTIR) also confirmed increased crystallinity as compared to commercially purchased free‐radical polymerized PAN due to a shift in the ~1250 cm−1 methine peak. Periodic semistatic normal load piezoelectric testing of the electrospun isotactic PAN samples also exhibited an average of ~30% of the piezoelectric response of electrospun (65:35) poly(vinylidene fluoride‐trifluoroethylene) (PVDF‐TrFE), a current gold standard for piezoelectric polymers, whereas commercially purchased free‐radical polymerized PAN exhibited no observable piezoelectric response. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47530.

Enclathration of Ethane, Propane, and Propylene into Urea Clathrates and Roles of Methanol on Urea Clathrate Formation.

As a guest molecule of urea clathrate, a long-chain normal alkane and its derivative with low substituents in methanol solutions have been reported. To investigate the role of methanol in the urea clathrate formation, in the present study, we used propane (C3H8), propylene (C3H6), ethane (C2H6), and methane (CH4) as guest molecules. Raman spectra and powder X-ray diffraction profiles revealed that, regardless of the existence of methanol, the C3H8, C3H6, and C2H6 molecules are enclathrated into urea clathrates with a hexagonal structure, whereas there is no urea clathrate formation enclathrating CH4. The pressurization of the urea clathrates including C2H6 and C3H8 reveals that no pressure-induced structural phase transition occurs at pressures up to 200 MPa. In spite of the guest molecule much shorter than the lattice constant of the c-axis of the hexagonal channel structure, the urea clathrates have a fairly rigid structure against the compression. Methanol as an auxiliary solution is not always necessary for the urea clathrate formation. Methanol plays a role in decreasing the activation energy of the urea clathrate formation, although it makes urea clathrate thermodynamically unstable due to the high solubility of urea in methanol.

Determination of polycyclic aromatic hydrocarbons in infant formula using solid state urea clathrate formation with gas chromatography – tandem mass spectrometry

An analytical method has been developed for the quantitative determination of the 16 EU priority polycyclic aromatic hydrocarbons (PAHs) in infant formula. Prior to analysis infant formula is subjected to sample preparation including the extraction of PAHs from the sample, transesterification of the triglyceride content of the extract to fatty acid methyl esters (FAMEs), and solid state urea clathrate formation to remove the FAMEs. Measurements were carried out with gas chromatography – tandem mass spectrometry. The developed method has been evaluated in terms of trueness, precision (repeatability), limit of detection (LOD), limit of quantification (LOQ), selectivity, sensitivity and linearity. Measurement range of 200–1500ngkg−1 was selected in accordance with the limit values set by the European Commission (835/2011/EC). LOD values were 75ngkg−1 for dibenzo(a,e)pyrene, dibenzo(a,i)pyrene and dibenzo(a,h)pyrene while 50ngkg−1 for the rest of the PAHs (benzo(c)fluorene, benzo(a)anthracene, cyclopenta(cd)pyrene, chrysene, 5-methylchrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(j)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, dibenzo(a,h)anthracene, benzo(g,h,i)perylene, dibenzo(a,l)pyrene), LOQ was 200ngkg−1 for all 16 PAHs. Trueness ranged between –29.7 and 29.8% while precision values were between 0.5 and 24.7% (RSD, n=3).

Combined cluster and discriminant analysis: An efficient chemometric approach in diesel fuel characterization

Combined cluster and discriminant analysis (CCDA) as a chemometric tool in compound specific isotope analysis of diesel fuels was studied. The stable carbon isotope ratios (δ13C) of n-alkanes in diesel fuel can be used to characterize or differentiate diesels originating from different sources. We investigated 25 diesel fuel samples representing 20 different brands. The samples were collected from 25 different service stations in 11 European countries over a 2 year period. The n-alkane fraction of diesel fuels was separated using solid-state urea clathrate formation combined with silica gel fractionation. The stable carbon isotope ratios of C10–C24 n-alkanes were measured with gas chromatography–isotope ratio mass spectrometry (GC–IRMS) using perdeuterated n-alkanes as internal standards. Beside the 25 samples one additional diesel fuel was prepared and measured three times to get totally homogenous samples in order to test the performance of our analytical and statistical routine.Stable isotope ratio data were evaluated with hierarchical cluster analysis (HCA), principal component analysis (PCA) and CCDA. CCDA combines two multivariate data analysis methods hierarchical cluster analysis with linear discriminant analysis (LDA). The main idea behind CCDA is to compare the goodness of preconceived (based on the sample origins) and random groupings. In CCDA all the samples were compared pairwise.The results for the parallel sample preparations showed that the analytical procedure does not have any significant effect on the δ13C values of n-alkanes. The three parallels proved to be totally homogenous with CCDA.HCA and PCA can be useful tools when the examining of the relationship among several samples is in question. However, these two techniques cannot be always decisive on the origin of similar samples. The initial hypothesis that all diesel fuel samples are considered chemically unique was verified by CCDA. The main advantage of CCDA is that it gives an objective index number about the level of similarity among the investigated samples. Thus the application of CCDA supplemented by the traditionally used multivariate methods greatly improves the efficiency of statistical analysis in the CSIA of diesel fuel samples.

Optimisation of solid-state urea clathrate formation as a chemical separation method coupled to compound-specific stable carbon isotope analysis

ABSTRACTSolid-state urea clathrate formation (SSUCF) as a chemical separation method prior to stable carbon isotope fingerprinting of diesel fuel contaminations was studied. The stable carbon isotope ratios (δ13C) of n-alkanes in diesel fuel can be used to trace the origin of a contamination. The accurate measurement of the stable isotopic composition of individual compounds requires baseline separation from any other co-eluting compounds. For this purpose silica gel column chromatography (SGCC) and SSUCF were applied. Detailed optimisation of SSUCF was performed: different activators, clathrate formation temperatures, activator volumes, clathrate formation times and sample capacity were investigated. The main benefits of the developed method are reduced clathrate formation time and increased recoveries for lower molecular weight n-alkanes. The recoveries of the developed SSUCF method ranged between 63 and 100% for C10–C24 n-alkanes with relative standard deviation no more than 7%. The precision of the gas chromatography-isotope ratio mass spectrometry measurement was acceptable with a standard deviation of the δ13C values ranging between 0.08 and 0.15‰. The absence of isotopic fractionation was also investigated.The robustness of the method was tested within a model experiment. Nine different water samples including distilled water, tap water, river water, industrial wastewaters and groundwater samples were spiked with the same diesel fuel. The water samples were extracted with n-hexane and after purification with both SGCC and SSUCF n-alkanes were measured. The δ13C values of n-alkanes were found to be similar for all samples. The importance of sample purification prior to compound-specific isotope analysis (CSIA) was also demonstrated within this model experiment by analysing samples from different stages of the sample preparation.Our results show that the proposed method can remarkably improve the precision of compound-specific stable carbon isotope analysis of n-alkanes originating from diesel contamination of the aquatic environment.

ChemInform Abstract: Molecular Dynamics of Paraffins in the n-Alkane/Urea Clathrate.

Simulations of the molecular dynamics of n‐CNH2N+2 in the n‐alkane/urea clathrate have been performed for N=13,23,33, for a period of 1 ns. Parallel simulations were performed starting from conformations in which all CH2–CH2 bonds were in the trans conformation, and starting from a conformation in which one of the terminal CH2–CH2 bonds was in a gauche conformation. These simulations suggest less than 3% of the terminal CH2–CH2 bonds would be in a gauche state at equilibrium. The gauche content is much smaller at the CH2–CH2 bonds further removed from the ends of the chain. No significant differences are seen in the simulations performed with n‐alkanes with N=13,23,33. The mobility of the C–H bond vectors about the axis defined by the channel is greater at the ends than in the middle of the chain. The angular distribution is nearly randomized in 1 ns for the C–H vector in a methyl group, but a longer time scale would be required for randomization of this distribution for the C–H vector bonded to the centr...

Why is Stereoregular Polyacrylonitrile Obtained by Polymerization in Urea Canals Isotactic?

AbstractBecause stereoregular i‐PAN is obtained from the constrained polymerization of acrylonitrile monomer in expandable urea clathrates, we studied the inclusion compounds formed between guest AN and PANs and host cyclodextrins whose rigid channel diameters are in the range of the canals in urea clathrates. ICs of AN were successfully formed with α‐ and γ‐CDs but did not yield PANs, and only ICs between γ‐CD, and a‐ and i‐PANs were formed in solution. Modeling of AN and s‐ and i‐PAN‐CD‐ICs were performed using PM3 parameters to estimate their stabilities. We conclude that the polymerization of AN in urea clathrate channels produces predominantly i‐PAN as a consequence of its improved fit compared with s‐PAN.magnified image

Crystal structure of 1,10-dibromodecane and its infrared intensity in a urea clathrate and in the crystal

The crystal structure of 1,10-dibromodecane belongs to the monoclinic system and the space group is P2 1/c with lattice dimensions of a=5.4574(3) Å, b=5.2814(4) Å, c=21.088(1) Å and β=92.897(2)° and z=2. Infrared spectra of 1,10-dibromodecane in a urea clathrate and in the crystal were observed to investigate the effect of molecular interaction on infrared intensity. The infrared intensity of the CH 2 waggings in the crystalline state is 1.5–1.9 times stronger on the relative basis than that in a urea clathrate, whereas those of CH 2 stretching, CH 2 rocking and CH 2 bending are almost the same in both states. The former enhancement is explained in terms of increase in the bond moment of the C αH 2 group on the basis of crystal structure and the electrostatic model. The relative intensity of two CH 2 asymmetric stretching changes between the two states. This is also analyzed by the use of the electrostatic model.

Enantioselective Synthesis of DIANANE, a Novel C2-Symmetric Chiral Diamine for Asymmetric Catalysis

DIANANE (endo,endo-2,5-diaminonorbornane) is a novel chiral C(2)-symmetric diamine, based on the rigid bicyclo[2.2.1]heptane scaffold. Schiff-base ligands derived from DIANANE have already found use in asymmetric catalysis, e.g., in the highly enantioselective Nozaki-Hiyama-Kishi reaction. We herein describe a practical synthesis of enantiomerically pure DIANANE, starting from norbornadiene in four steps: (i) Pd-MOP catalyzed Hayashi-hydrosilylation/Tamao-Fleming oxidation, (ii) oxidation to norbornane-2,5-dione, (iii) endo-selective reductive amination with benzylamine, and (iv) hydrogenolytic debenzylation. None of the steps involves chromatographic purification. For the Tamao-Fleming oxidation, the use of hydrogen peroxide in the form of its urea clathrate instead of aqueous solution proved beneficial. By the above sequence, enantiomerically pure (ee >or=99%) DIANANE was obtained from norbornadiene in 40-50% overall yield. The relative and absolute configuration of DIANANE was confirmed by X-ray crystallography of the DIANANE bis-tosylamide, and of its bis-camphorsulfonamide. Furthermore, the synthesis and X-ray crystal structure of the Schiff-base ligand derived from DIANANE and 3,5-di-tert-butyl salicylic aldehyde are reported.

Infrared intensity in dielectric media by an electrostatic model

A change in an infrared intensity in dielectric media is treated by an electrostatic model. The basic model is originally formalized for a dipolar liquid. The model is satisfactorily applied to the infrared intensity of the CH stretching of chloroform, which changes 22 times large in the liquid state at −43 °C as in the gaseous state. A change in the infrared intensity in lithium ammonium tartrate, where a ferroelectric phase transition takes place, is analyzed on the basis of a local polarization above T c or a spontaneous polarization below T c, deducing important quantities on a phase transition. A difference in the infrared intensity of the CBr stretching of 1,10-dibromodecane between the urea clathrate and the crystalline state is analyzed by evaluating electric fields due to bond moments and oscillating dipoles. These analyses confirm the mechanism of the change in the absolute infrared intensity, which originates from an electrostatic interaction with an electric field applied to a molecule or a functional group closely related to a normal mode.

Investigation of the mobility of paraffin molecules in channel urea clathrates by nuclear magnetic resonance relaxation spectroscopy

The temperature dependence of the proton spin-lattice relaxation time (T1) is investigated for channel urea clathrates with paraffin molecules. The results obtained are interpreted within the reorientational model of paraffin molecules and their fragments in clathrate channels. The specific features of the dynamics of normal paraffins in urea clathrates are associated with incommensurate regions in the structure of these compounds.

An Anomalous Tacticity−Crystallinity Relationship: A WAXD Study of Stereoregular Isotactic (83−25) Poly(Acrylonitrile) Powder Prepared by Urea Clathrate Polymerization

A detailed wide-angle X-ray diffraction (WAXD) study was carried out for a series of stereoregular poly(acrylonitrile)s (PANs) having different isotacticities (83−25%). The degree of crystallinity ...

Conformation Effect and FT-IR Diffuse Reflection Spectroscopy of Stereoregular Isotactic Poly(acrylonitrile) Prepared by Urea Clathrate Polymerization

The conformation effect on the infrared intensity of the characteristic bands (1230/1250 cm-1) of stereoregular isotactic poly(acrylonitrile) (PAN) was studied by the diffuse reflection (DR) and KB...

Solid-state characterization and the thermal properties of stereoregular poly(vinyl chloride) prepared by urea clathrate polymerization

The solid-state characterization of highly stereoregular poly(vinyl chloride) (PVC) prepared by urea clathrate polymerization was carried out by using various instrumental analyses. The structural differences of PVC appeared most remarkably in solubility to organic solvents, IR, WAXD, and solid-state 13 C-NMR spectra. The value of the glass transition temperature (T g ) was about 90°C, not as high as expected, although its detection was quite difficult. The thermal stability was poor, as evidenced by the easy discoloration of this polymer by heat treatment, which was related to the absence of a termination reaction. Dynamic ESR spectra in the solid state clearly indicate that the radical formation occurs at such a low temperature as 160°C in the initial degradation stage. The degradation characteristics of urea clathrate PVC were critically discussed.

Solid-state characterization and the thermal properties of stereoregular poly(vinyl chloride) prepared by urea clathrate polymerization

Solid-state characterization and the thermal properties of stereoregular poly(vinyl chloride) prepared by urea clathrate polymerization

Molecular dynamics of guest radicals in urea inclusion compounds

Molecular dynamics simulations of several radical guests included in urea clathrates have been performed for a period of 1 ns. The starting conformations of the radicals have been obtained by AM1 quantum mechanical computations. The host matrix has been modelled as a single channel formed by 47 or 72 urea molecules, and different constraints imposed on the urea molecule positions have been tested to optimize the effect of the host confinement on the guest radicals. Two kinds of motion have been considered, the internal dynamics of the methylene group b to the radical centre and the rotation of the guest inside the host channel. The simulation results have been compared with experimental data obtained by electron paramagnetic resonance spectroscopy. Good agreement is found for the internal motion when a finite harmonic constraint is imposed on the urea carbonyl. The correlation time for the molecular rotation is estimated to be longer than the simulation period. The slow reorientation process should be controlled by the concurrent rearrangement of the host matrix.

13C n.m.r. and g.p.c.—low-angle laser light scattering measurements on polyacrylonitrile prepared by urea clathrate polymerization in the solid state for the optimization of tacticity

The molecular weight and molecular weight distribution of isotactic polyacrylonitrile (PAN) prepared by urea clathrate polymerization in the solid state were studied by the g.p.c.—low-angle laser light scattering (l.a.l.l.s.) method. The experimental details of this method for PAN are described. Measurements were carried out on three different kinds of urea clathrate PAN, and the results were compared with those of free-radical PAN (i.e. prepared using the aqueous redox slurry system). The characteristics of the urea clathrate PAN were (1) an asymmetrical shape in the g.p.c.—L.a.l.l.s. curve and (2) a remarkable contribution of the lower-molecular-weight component to the high stereoregularity (>80%) of the sample. An experimental prediction for the solid-state synthesis of a perfectly stereoregular PAN sample (isotacticity of 100%) has been derived from a combination of the g.p.c.—l.a.l.l.s, and 13 C n.m.r. results.

Radiolysis mechanisms in clathrates systems: EPR spectroscopy investigation of the long chain carboxylic esters urea adducts

The mechanism of the gamma radiolysis of long chain aliphatic carboxylic esters from decanoic and hexadecanoic acids, in the state of urea clathrates has been investigated by EPR spectroscopy. The major species identified are the carboxylate anion-radicals, trapped electrons, and the neutral primary alkyl radicals of type RCH · 2 arising from the OC and CCOOR bond scissions. Also present among the species trapped at 77 K are the secondary alkyl radicals CH 2CHCH 2 formed by CH bond rupture in the aliphatic chains. On warming the radical anions are converted into RCH · 2 species and intermediate acyl type radicals RCO have been observed. At room temperature all the primary and secondary radicals gradually disappear, being partially converted into the α-carboxylate radicals RCH 2CHCOOR. This process is thought to involve the intrachain migration of the free valences by hydrogen abstraction. The comparison with the neat compounds has shown that the clathrate structure has a profound influence on the radiolysis mechanism the major effects observed being the increase of the OC scission products, the increase of the radical anions yield and the decrease of the CH bond ruptures in the aliphatic chains.

13C n.m.r. observation of poly(l-lactide) in the narrow channels of its inclusion compound with urea

Abstract The high resolution CP/MAS/DD 13C n.m.r. spectra of bulk, semicrystalline poly( l -lactide) (PLLA) and PLLA confined to the narrow channels of its urea clathrate (PLLA-U) are compared and contrasted to the recently reported CP/MAS/DD spectrum of the crystalline 1:1 stereocomplex (PLLA∗PDLA) formed by mixing dilute solutions of PLLA and poly( d -lactide) (PDLA) followed by their cocrystallization. Single resonances are observed for all three carbon types in PLLA-U, while the carbonyl carbons in PLLA∗PDLA and all three carbon types in bulk PLLA evidence multiple resonances. Differences in the conformations and packing of PLLA chains in these distinct, ordered solid-state environments are the likely sources of their very different high resolution, solid-state 13C n.m.r. spectra.