Mesophase evolution in heat-treated solid petroleum pitches

A combination of analytical techniques was used to follow the mesophase evolution of petroleum pitches in the solid phase. The samples were characterized by matrix assisted laser desorption and time of flight analysis by mass spectrometry (MALDI-TOF-MS), high field 13C nuclear magnetic resonance (13C NMR), low field ¹H NMR, electron spin resonance (EPR), polarized optical microscopy and thermogravimetric analysis (TGA). MALDI-TOF-MS revealed an oligomeric distribution of molecular weights in the pitch: monomeric (m/z 200 to 400), dimeric (m/z 400 to 650), trimeric (m/z 650 to 950) and tetrameric (m/z 950 to 1600), an essential factor for the production of carbon fibers, among other advanced composite of carbon. Relaxation time (T1) studies permitted to observe two large areas: the aromatics and the mesophases. These results also showed that the heating time increases the mesophasic area. Also through direct and non-destructive 13C NMR measurements, we obtained the content of aromatic and aliphatic carbons, as well as the degree of protonation of these carbons. Optical microscopy with polarized light showed the interfaces between the anisotropic and isotropic regions as well as the process of coalescence of mesophase. The results obtained by the other techniques corroborate the MALDI-TOF-MS and NMR results.Uma combinação de técnicas analíticas foi utilizada para acompanhar a evolução da mesofase em pixes de petróleo em fase sólida. As amostras foram caracterizadas por dessorção a laser assistida por matriz, seguida de espectrometria de massas por tempo de vôo (MALDI-TOF-MS), ressonância magnética nuclear de 13C (13C NMR) de alto campo, ¹H NMR de baixo campo, ressonância eletrônica de spin (EPR), microscopia ótica polarizada e análise termogravimétrica (TGA). A análise por MALDI-TOF-MS revelou uma distribuição oligomérica de pesos moleculares no pixe: monomérica (m/z 200 a 400), dimérica (m/z 400 a 650), trimérica (m/z 650 a 950) e tetramérica (m/z 950 a 1600), fator essential para a produção de fibras de carbono, bem como outros compósitos de carbono. Estudos de tempo de relaxação (T1) permitiram a observação de duas grandes áreas: a dos aromáticos e a de mesofase. Estes resultados também mostraram que o tempo de aquecimento aumenta a área de mesofase. Através de medidas não destrutivas de 13C NMR, foram obtidos os conteúdos de carbonos aromáticos e alifáticos, e também o grau de protonação desses carbonos. A microscopia ótica com luz polarizada mostrou as interfaces entre as regiões anisotrópicas e isotrópicas, bem como o processo de coalescência da mesofase. Os resultados obtidos pelas outras técnicas corroboram com os resultados de MALDI-TOF-MS e NMR.13551371Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

A Combined Experimental and Theoretical Study on the Formation of Crystalline Vanadium Nitride (VN) in Low Temperature through a Fully Solid-State Synthesis Route

An efficient method of synthesis of the vanadium nitride (VN) at low temperature is evaluated, and a mechanism for the crystallization process is proposed in this paper. From the mixture of ammonium <i>m</i>-vanadate with guanidinium carbonate an intermediate, guanidinium <i>m</i>-vanadate (GmV), is produced. GmV decomposed and underwent interesting structural transformations with increasing temperatures. This process is studied by theoretical (periodic DFT calculations) and experimental (⁵¹V MAS NMR, XRD, FTIR, and elemental analysis) methods. It is proposed that GmV is first decomposed into reactive species, then through solid-state transformations it is converted into vanadium oxynitride (VO_<i>x</i>N_1–<i>x</i>) with varying stoichiometry, and, last, GmV transforms itself into crystalline NaCl-type structure vanadium nitride. The DFT calculations show that this transformation is energetically favorable, and the formation of a VO_<i>x</i>N_1–<i>x</i> solid solution is feasible