Analysis Of Molecular Structures For Heavy Oils Saturates By IR Laser Desorption/Tunable VUV Photoionization-high Resolution Mass Spectrometry

A new method using synchrotron radiation-highly resolved mass spectrometry has been surveyed for the analysis of molecular structures of heavy oils. The separation of desorption and ionization of heavy oils with the help of a IR laser and a synchrotron radiation beam respectively favors an ideal "soft" ionization condition for the analysis. The ion signals are differentiated by a highly-resolved time of flight mass spectrometry; and with the tunability of the light wavelengths, the photoionization efficiency (PIE) spectrum of each mass peaks is integrated. Measurement of the PIE spectra of some model molecules suggests that this method can measure the ionization energy of molecules accurately. The mass spectra and PIE spectra of saturates from Lungu residual oils are measured, and with the help of quantum chemical calculations, some molecular structures of the saturate fractions are assigned. The experiment has measured the photon ionization mass spectrometry of the Lungu residues newborn saturates and the secondary saturates at the different hydrogen-adding reactions conditions. The molecular weight of residues saturates mostly distributes between 210 ~ 400, and it is made up of a series of disciplinary ion peak clusters whose mass number difference is 14; each cluster contains some of the major ion peaks, the mass number difference of adjacent peaks is 2. The primary saturates molecules can be classified that: CnH2n+2,CnH2n,CnH2n-2,CnH2n-4,CnH2n-6,CnH2n-8 and CnH2n-10. When the synchrotron radiation photon energy increases to a stated extent, the parent ion range narrows, the intensity of great parent ion weakens, some of them disappear , the strongest peak leans to the direction of small mass, and that is due to the high-mass parent ion cracking more easily at the condition of high energy. And that a series of fragment ion peak clusters whose mass number difference is also 14 appear in the low-mass region, they are mainly the three species of CnH2n,CnH2n-2 and CnH2n-4,and the saturates mass spectrum also shows the impact of hydrogenation reaction conditions to the composition. The comparison of the mass spectrum under the different VUV photon energy indicates that the change of photon energy does not make great effect on measurement of average molecular weight. The effects of hydrogenation reactions and reaction condictons to saturates molecular structures have been analyzed, and found that the main products of residue hydrogenation are tetracyclic and pentacyclic alkanes. Hydrogenation of the LGAR resins can produce mono- to tetracyclic alkanes under relatively mild conditions (reaction temperature: 390°C); while the chain, pentacyclic, and especially hexacyclic alkanes are formed only under more rigorous conditions (reaction temperature: 420°C).By comparing the results of some small alkanes ionization energy calculational data at the theory level of B3LYP/6-31 + G (d, p) and the experimental results, and comparing the results of practical molecule of saturates sample and the experimental results, we confirm the theory calculational error is about -0.4 eV. The ionization data which is gained from the PIE spectra of each saturates mass number explains that: each mass number of the saturate contains many alkanes and cycloalkanes molecules, and does not contain the condensation of aromatic ring structure. The quantum chemical calculation results also explains that the saturate contains the above two molecules.