Molecular Characterization Of Sulfur-containing Compounds In Crude Oils:Methodology And Application

A comprehensive understanding of the molecular composition of petroleum sulfur compounds can facilitate the development of hydrodesulfurization catalysts and optimize desulfurization refining processes.It can also explain the formation and the thermal evolution and biodegradation mechanism of sulfur compounds in petroleum,thus improve the theoretical system of organic geochemistry.In this study,based on Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS),analytical methods of sulfur compounds in crude oils,especially polar sulfur compounds,were further improved by solving the isolation/separation challenge of different types of sulfur-containing compounds from complex crude oil matrix,and the selective ionization of different types of sulfur-containing compounds in ionization source.(1)The Spray-DART ionization method was developed by combining direct analysis in real time(DART)ionization with an electrospray ionization source,achieving simultaneous ionization and selective detection of sulfiedes and thiophenes.Crude oil samples dissolved in toluene were directly infused into a spray needle,which produced a continuous and long-time stable ion current for FT-ICR MS analysis to obtain mass spectra with a broad dynamic range and high signal-to-noise ratio.In positive-ion DART mode,sulfides and thiophenes were simultaneously ionized but different ion forms were produced,realizing directly discrimination.The thiophenes were mainly ionized to molecular ion peak[M]^·+,while the sulfides were oxidized to sulfoxides in an open,high-temperature ionized environment to form[M+O+H]⁺ions.Therefore,the Spray-DART ionization method has potentially application in the rapid analysis of petroleum sulfur compounds.(2)The molecular compositions of the sulfoxides were characterized by selective separatioin and mass spectrometry.When using positive-ion ESI FT-ICR MS to analyze the molecular composition of petroleum samples,the O₁S₁ class species,protonated or sodiated,ubiquitously existed and generally present high relative abundance.The O₁S₁class species were successfully separated from the non-polar sulfur compounds and concentrated in the polar fraction.The chemical structure of the O₁S₁ class species was confirmed to be sulfoxide by Li Al H₄ reaction.The molecular composition of sulfoxides was revealed using+ESI FT-ICR MS.The molecular composition of sulfoxides is related to the geological environment and maturity of crude oils,and the molecular composition of sulfoxides in different crude oils was largely different,but mainly existed in the form of aliphatic sulfoxides with one to five rings(DBE<6).The quantitative result of sulfur element showed that polar sulfur species amount to a large proportion(generally more than one-third)of sulfur compounds in crude oils from various regions of the world,however,polar sulfur compounds in the form of sulfoxides account for only a small fraction(0.8%-5.7%)of total sulfur.(3)The selective ionization of the sulfoxides in+ESI FT-ICR MS was achieved by adding NH₄OH as ionization promoter into the sample solution.Sulfoxides are prone to form Na₁O₂S₂ dimers with high relative abundance in positive–ion ESI,which would seriously affect the the composition analysis of sulfoxides in crude oils.The association property and ESI ionization mechanism of Na₁O₂S₂ dimer were studied by collision-isolation dissociation(CID)experiments.The Na₁O₂S₂ dimers can be significantly eliminate when a small amount of NH₄OH is added into the sample solution,achieving selective ionization of the sulfoxides in positive–ion ESI.(4)The molecular composition of sulfur compounds in a Venezuelan crude oil was comprehensively characterized by using extrography and methylation/demethylation separations,elemental analysis and high resolution mass spectrometry,as well as hydrodesulfurization processe.Sulfur compounds in the crude oil were separated in five fractions,including sulfidic fraction,thiophenic fraction,sulfoxide fraction,carboxylic acidic sulfur compounds fraction,and a residue.The results showed that sulfoxides have similar distribution in carbon number and double bond equivalent(DBE)to sulfides in the crude oil,indicating that sulfoxides may be derived from the oxidation of sulfides.The molecular composition of carboxylic acid sulfur compounds combines the distribution characteristics of sulfides and thiophenes,but differs largely with the composition of carboxylic acid compounds.It is speculated that such compounds may be the biodegradation products of sulfides and thiophenes.The quantitative result of sulfur element in each fraction showed that sulfur in the form of sulfide and thiophene accounted for?60%of total sulfur,while polar sulfur compounds amount to a considerable proportion(?30%)of total sulfur for the Venezuelan crude oil and all other investigated crude oils,but sulfoxides and carboxylic acidic sulfur compounds occupied just a small proportion(?8.0%of total sulfur for Venezuelan crude oil)of the polar sulfur compounds.Other polar sulfur compounds(?20%of total sulfur)were hardly analyzed by mass spectrometry and found resistant to hydrodesulfurization and should be concerned in the following studies.