Mesophase Formation Investigation in Pitches by NMR Relaxometry

Piches são utilizados como precursores de diversos materiais avançados de carbono. O objetivo deste trabalho foi combinar as metodologias de extração com solvente com a ressonância magnética nuclear de baixo campo, através da técnica de relaxação, caracterizando piches de petróleo tratados termicamente. O tempo de relaxação T 1 apresentou dois domínios: um na região aromática e o outro atribuído a mesofase. Os resultados mostraram que a técnica de relaxometria por RMN de H pode ser empregada como uma nova ferramenta para a caracterização desse tipo de sistema.


Introduction
Carbonaceous pitches are used as raw materials in advanced carbon products. 1The growth of the mesophase affects the physical properties of the pitch, softening point and viscosity, and also affects the final properties of the resultant carbon products.3][4][5][6][7] Polarized optical microscopy (POM) and solvent-insoluble fractions are conventional tools for the study and measurement of the amount of mesophase formation. 2,6,8Although POM is a standard identification tool, extensively used by liquid crystal researchers, Li et al. 9 have concluded that conventional POM observation could not be regarded as a good method to analyze the size and size distribution of the mesophase spheres in the isotropic matrix of heat-treated pitches, because of their random distribution.Even the statistical assumptions used in some works could not help to obtain the precise size, since the different apparent sizes could be caused by the random positions.In these random positions spheres were cut in the preparation of the samples and also caused by the size distribution of mesophase spheres in the pitches.
Another analytical method frequently used to follow the growth of the mesophase is solvent extraction.The literature reports a wide range of solvents, e.g.heptane, toluene, tetrahydrofuran, pyridine, quinoline and N-methyl pyrrolidinone.However, in different systems, each different extracted material and its extract behave differently.The mesophase spheres in the heat-treated coal tar or petroleum pitches are extracted at a very low yield.Besides, extraction and filtration are very tedious procedures. 2,3,5he aim of this work is to combine solvent extraction methodology by using quinoline, N-methyl pyrrolidinone and toluene, with low field nuclear magnetic resonance relaxometry technique in order to characterize heatedtreated samples of petroleum pitches.Longitudinal proton J. Braz.Chem.Soc.relaxation time data, which is conventionally characterized by relaxation time T 1 , was used to investigate the presence of domains in the samples studied.1][12][13] Torregrosa-Rodriguez et al. 14 and Evdokimov et al. 15 have also studied the formation of asphaltene dispersions in oil/toluene solutions by low field NMR relaxation, with measurement of the spin-spin relaxation times (T 2 ), identifying monomers below 10 mg L -1 .In this investigation, low field NMR relaxometry and insoluble fractions techniques were used to characterize different domains in the samples studied.

Experimental
The pitch precursor (sample A) comes from petroleum cracking residue submitted to heating treatment; it was heated at 430 o C per 4 hours in a N 2 atmosphere, and five different samples were obtained (samples B-E) as specified in Table 1.Two other pitch samples were obtained from the precursor by density difference with hot stage centrifugation, the upper (isotropic-sample F) and lower (anisotropic-sample G). 16 The low-field 1 H NMR relaxation measurements were done on a Resonance Instruments Maran Ultra 23 NMR analyzer, operating at 23.4 MHz (for protons) and equipped with an 18 mm variable temperature probe operating at 300 K. Proton spin-lattice relaxation times (T 1 H) were measured with the inversion-recovery pulse sequence (D 1 -πτπ/2 -acq.), using a recycle delay value greater than 5T 1 (e.g.D 1 of 10 s), and π/2 pulse of 4.5 µs calibrated automatically by the instrument software.The amplitude of the FID was sampled for twenty τ data points, ranging from 0.1 to 5000 ms, with 4 scans each point.The T 1 values and relative intensities were obtained with the aid of the program WINFIT by fitting the exponential data.Distributed exponential fittings as a plot of relaxation amplitude versus relaxation time were performed by using the software WINDXP.

Results and Discussion
The T 1 H relaxation time data obtained at 300 K for the samples studied are shown in Table 2.The mesophase formation can be followed across the distributed exponential fittings as a plot of relaxation amplitude versus relaxation time; this was performed using WINDXP software (Figure 1).In our studies of T 1 relaxation times, two distinct domains were observed: one referring to the aromatic region and the other was attributed to mesophase.Jurkiewicz et al. 10 have used spin-lattice characteristics of coal 1 H NMR signals and have suggested that two phases, one molecular and other macromolecular, could be distinguished in the coal structure.
In the present work, T 1 H longitudinal relaxation time and the insoluble fraction data were correlated to the presence of different domains in the samples studied (Figure 2).In Figure 2, the highest one belongs to the domain controling the relaxation process, which is a rigid one.The results from the insoluble fraction determinations were lower than those obtained with 1 H NMR relaxometry 10 and proved that the mesophase formation was understimated, probably due to the scale of the measurement.It was also observed a good correlation between lower insoluble fraction concentration and NMR relaxation data.

Conclusions
The NMR relaxation results showed that the system in investigation presented more than one domain, according to their molecular mobility, as a function of phase interaction and dispersion.These results also supported that NMR relaxometry could be used as a new tool for the characterization of this kind of system.

Figure 2 .
Figure 2. T 1 H longitudinal relaxation time data and insoluble fraction of the samples

Table 1 .
Characteristics of samples studied

Table 2 .
Proton spin-lattice relaxation times of the samples determined by low field NMR using WINFIT software * These values result from T 1 curve adjustment for three exponentials.