Analysis of Acidic Compound Classes in Crude Oil by Negative Ion Electrospray Ionization High Resolution FT-ICR Mass Spectrometry (ABSTRACT)

摘要

Characterization of acidic constituents of crude oil is important due to the problems associated with those functionalities during production and refining. Acidic constituents are associated with the formation of stable emulsions during production, making efficient oil recovery and subsequent processing very difficult. Acid molecules in crude oil are primarily implicated for corrosion in refineries and pipelines. Corrosion by naphthenic acids and acidic sulfur compounds at high fluid velocity and high temperature during distillation causes breakdown of transfer lines, furnace tubes, valves and pump fittings. Naphthenic acids are also significant because of their surface activity and marginal water solubility, so that they may leach to wastewaters and cause adverse environmental effects. However, acidic components of oil are not limited to carboxylic acids but also molecules that contain sulfur and nitrogen. Tomczyk et al. report that one-half of the acidic species in a crude oil contain nitrogen and at least one-fourth sulfur (Energy Fuels, 15(6): 1498-1504, (2001)). The most pragmatic approach to understand the multitude of problems associated with acidic oil components and design better solutions is an improved understanding of the chemistry and physics of petroleum at the molecular level. We have previously reported applications of negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) for speciation of acidic oil compound classes. Ammonium hydroxide is used in ~negative-ion ESI FT-ICR MS to deprotonate acidic species. It is a weak base because it gives a low concentration of hydroxide ions in solution. Molecules other than carboxylic acids may be only weakly or moderately acidic with high pKa values ranging from 9 to 34. Deprotonation efficiency of neutral or weakly acidic compounds is further diminished in the presence of carboxylic acids (matrix effect). Hence it is not easy to relate the observed ion relative abundances to those of neutral precursors in the original sample. We examine here the efficiency of quaternary tetramethylammonium hydroxide in speciation of acidic species in petroleum relative to ammonium hydroxide. The preliminary data is very promising in that this reagent allows us to ionize and detect species present at much lower concentration in the sample matrix. We demonstrate that a slight modification in the basic strength of the solvent system for negative-ion ESI-FTICR MS is extremely effective in generating a comprehensive compositional profile of crude oil acids over a wider DBE (double bond equivalents = number of rings plus double bonds to carbon) and carbon number range.