Structural Specification Of Nitrogen Compounds In The Hydrotreatment Of Heavy Oil By High-resolution Mass Spectrometry

The existence of nitrogen compounds has negative effects,such as the cleanliness and stability of the heavy oil.Hydrotreatment is the main method for the production of clean fuel.The characterization at the molecular level provides suggestions for the processing technology improvement.High-resolution mass spectrometry is a powerful instrument for characterizing the molecular composition of heavy oil,but it cannot be used to specify the molecular structure alone.The molecular structure is necessary to be analyzed because it determines the physical and chemical properties.Nitrogen compounds in heavy oil before and after hydrotreatment were studied in this paper.The collision induced dissociation(CID)and hydrogen/deuterium exchange(HDX)are coupled with the positive-ion electrospray ionization(+ESI)high-resolution mass spectrometry to investigate the compositional variation of nitrogen-containing species with different functional groups before and after hydrotreatment,and to obtain its representative core structure.In this study,the narrow window CID is used to obtain the nitrogen compounds structure information of heavy oil and its hydrotreated products.First of all,the CID experiments of nitrogen model compounds provided the fragmentation rule of various functional groups.It was found that only amine compounds have hydrocarbon fragments that can be detected after CID.Setting the narrow isolation window to 0.4 Da can effectively reduce the mutual interference of multiple heteroatom compounds'parent ions,so the bond between the fragment ions and the parent ions is stronger.The presence of hydrocarbon fragments in hydrotreated heavy oil after CID represents the existence of amine compounds produced by hydrogenation.The narrow window CID experiments shows that the nitrogen compounds in the heavy oil mostly exist in the form of a combination of pyridine and long alkyl side chains,and containing naphthene rings and amine appear in the hydrogenation products.We coupled HDX with+ESI to investigate the specific distribution of nitrogen compounds with different functional groups before and after hydrotreatment of heavy oil using high-resolution mass spectrometry based on previous work.The solvent and additive were optimized for HDX+ESI through the systematic evaluations on model nitrogen-containing compounds.We found that adding deuterated water(D₂O)and formic acid(DCOOD)significantly increases HDX degree and thus facilitates the identification of nitrogen functional group.Through the HDX experiment of light oil,the appropriate addition of additives and the reaction temperature and time were selected.After application on the hydrotreated petroleum samples,the compositional variation of intermediate amine compounds compositional during the heavy petroleum hydrotreating process were clearly revealed.The saturation of pyridine or pyrrole compounds to cyclic amines,and the further reaction of high DBE cyclic amines and short side chain compounds were observed.