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Acidic and Neutral Polar NSO Compounds in Severely Biodegraded Oils Characterized by Negative-Ion ESI FT-ICR MS Yinhua Pan, Yuhong Liao, Quan Shi, and Chang Samuel Hsu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef400191h • Publication Date (Web): 06 May 2013 Downloaded from http://pubs.acs.org on May 14, 2013
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Energy & Fuels
Acidic and Neutral Polar NSO Compounds in Heavily Biodegraded Oils Characterized by Negative-Ion ESI FT-ICR MS
Yinhua Pana, b, Yuhong Liaoa, *, Quan Shic, Chang Samuel Hsuc, d
a
The State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, P. R. China b
c
d
University of Chinese Academy of Sciences, Beijing 100049, China
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102200, China
(a) Petro Bio Oil Consulting, Tallahassee, FL 32312 (b) Department of Chemical and Biomedical Engineering, Florida State University Tallahassee, Florida 32310, United States of America
ABSTRACT: Five heavily biodegraded tar sand bitumens from an oil column were separated into maltene and asphaltene fractions for analysis by negative-ion electrospray (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). These bitumens have identical source, which have experienced a natural sequence of biodegradation. The polar NSO compounds in maltene fractions contain O1, S1O1, O2, S1O2, S2O2, O3, S1O3, O4, S1O4, N1, N1O1, N1O2, N1S1 and S1 classes; while the polar NSO compounds in asphaltene fractions contain O1, S1O1, S2O1, O2, S1O2, S2O2, O3, S1O3, S2O3, O4, S1O4, S2O4, O5, S1O5, S2O5, O6, N1, N1O1, N1O2, N1O3, N1S1 and N2 classes. Polar NSO compounds with stronger molecular polarity and larger molecular weight are readily fractionated into asphaltene fractions. The O2 class is prevalent in polar NSO compounds of both maltene and asphaltene fractions of all bitumen samples. The N1 class in maltene fractions is dominated by compounds with DBE values of 9, 10, 12 and 13, while N1 class in asphaltene fractions is dominated by compounds with DBE of 15. Most of these N1 compounds are likely pyrrolic compounds with various numbers of aromatic rings. The biodegradation pathways of nitrogen-containing compounds are also explored in this study. N1 species are likely converted to N1O1 and N1O2 species following biodegradation pathways as ring-opening reaction or carbazole dioxygenase (CARDO) catalytic oxidation reaction. S1O2-5 classes are identified as the dominant sulfur-containing compound classes under negative-ion ESI 1
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mode. These classes are considered to contain acid functionalities with higher polarity because the sulfur-containing compounds without oxygen are difficult to analyze by negative-ion ESI in which acids can be ionized by deprotonation. Both progressive oxidation and sulfuration may be involved in the anaerobic biodegradation of sulfur-containing acidic compounds.
1. INTRODUCTION All crude oils are hydrocarbon mixtures containing smaller amounts of sulfur, nitrogen, oxygen
and
trace
amounts
of
metals
(mostly
nickel
and
vanadium).1,2
Although
heteroatom-containing compounds only account for approximately 10% of normal oil, they have more than 20 000 distinct elemental compositions (CcHhNnOoSs).3-10 These polar NSO compounds often cause problems in oil production, refining, transportation and storage, resulting in corrosion, emulsion,
sedimentation,
plugging,
catalyst
poisoning
and
environmental
contamination/pollution.7,8,11 Biodegradation of crude oil in the reservoir is a principal alteration process of most heavy oils.3,12,13 Previous researches show that low molecular weight hydrocarbons in crude oils are preferentially destroyed while heteroatom-containing compounds are preferentially preserved during biodegradation,13-17 resulting in the increase of oil polarity, density, viscosity, sulfur/heteroatom content and acidity.18-20 The major oxygen-containing compounds are naphthenic acids and phenols in crude oils.21 Naphthenic acids exist as an organic mixture of compounds with broad polydispersity in molecular weight and structure.22-24 They are of concern in oil refining, storing and transportation because of corrosion problems.25,26 The molecular compositions of naphthenic acids have been a focus since the 1970’s. 22,23,27-31 The nitrogen-containing compounds in crude oils include neutral nitrogen compounds (such as pyrroles, carbazoles and indoles) and basic nitrogen compounds (such as pyridines and quinolines). Nitrogen-containing compounds are harmful in oil refining and storing because they can reduce activity of catalysts and increase product instability.32,33 Primary sulfur-containing compounds include sulfidic and thiophenic compounds, which are of environmental concerns.34-36 Biodegradation mechanisms of hydrocarbons in crude oils have been well studied;37-41 but biodegradation processes and mechanisms of polar NSO compounds are only poorly understood
2
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due to their thermal instability, low volatility and ineffective separations/detection by routine instrumental analysis. As an example, molecular compositions of polar compounds in biodegraded crude oils are so complex that gas chromatography (GC) often yields an “unresolved complex mixture (UCM)” hump on GC chromatogram.15 Thus, the routine analytical methods such as gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) are limited to analysis of polar NSO compounds with low molecular weight and simple molecular
structure
which
are
separable
by
chromatography,8
such
as
carbazoles,
benzocarbazoles,42,43 thiophenes and benzothiophenes.44,45 Comprehensive two-dimensional gas chromatography (GC×GC) has been particularly applied to characterize UCM in gas oil biodegraded oil
46,47
and
48
. GC×GC coupled to flame ionization detector (FID) or time-of-flight mass
spectrometry (TOFMS) is an ideal technique allowed for individual identification of most of nonpolar or less polar compounds such as saturated and unsaturated hydrocarbons,49,50 aromatic compounds 47,51,52 and heteroatomic compounds 53. Also, a critical column configuration is needed in GC×GC analysis.49,50 It seems a challenge to carry out the characterization of complex mixtures with stronger polarity such as naphthenic acids, dibenzocarbazoles and thiophenic acids together. Both metabolisms of aerobic 38,54 and anaerobic 39,55,56 biodegradation simulation of crude oil have been reported in detail in previous studies. Anaerobic mineralization transforms hydrocarbons into the products that can be oxidized via β-oxidation pathways.55,56 Watson et al.38 reported that hopanes can also be oxidized during aerobic biodegradation of crude oil with side chain oxidation. Since carbazole analogues are the most abundant nitrogen-containing compounds in many crude oils, carbazole has been generally chosen as a model compound to reveal biodegradation processes of nitrogen-containing compounds.57 Biodegradation simulation experiments by Sato et al.58,59 and Riddle et al.60 suggest that carbazole can be converted to 2´-aminobiphenyl-2,3-diol
via
carbazole
dioxygenase
(CARDO)
catalytic
oxidation.
Dibenzothiophene (DBT) was also used as a model sulfur-containing compound in biodegradation simulation experiments. The results suggested that some strains could selectively remove sulfur by a 4S-pathway, in which DBT is converted to 2-hydroxybiphenyl (2-HBP).35,36,61-63 In general, biodegradation process and mechanism of polar NSO compounds were infrequently reported or referred to in previous literatures, especially for those with larger molecular weight. Electrospray (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) 3
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has provided a powerful tool to characterize crude oils and crude oil distillates at molecular level.10,64,65 The key features of ultrahigh resolution FT-ICR mass analysis include high mass accuracy (
