Subscriber access provided by University of Kansas Libraries
Fossil Fuels
Investigation into the Effect of Heteroatom Content on Kerogen Structure Using Advanced 13C Solid-State Nuclear Magnetic Resonance Spectroscopy Wenying Chu, Xiaoyan Cao, Klaus Schmidt-Rohr, Justin E. Birdwell, and Jingdong Mao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01909 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on January 4, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Energy & Fuels
Investigation into the Effect of Heteroatom Content on Kerogen Structure Using Advanced 13C
2
Solid-State Nuclear Magnetic Resonance Spectroscopy
3 4
Wenying Chu1, Xiaoyan Cao2, Klaus Schmidt-Rohr2, Justin E. Birdwell3,*, Jingdong Mao1,*
5 6
1Department
7
23529, United States
8
2Department
9
3Central
10
of Chemistry and Biochemistry, Old Dominion University, Norfolk, Virginia,
of Chemistry, Brandeis University, Waltham, Massachusetts, 02453, United States
Energy Resources Science Center, U.S. Geological Survey, Denver, Colorado 80225,
United States
11 12
Submitted to Energy & Fuels
13 14 15
*Corresponding authors: J.E. Birdwell; Email:
[email protected]; Tel: (303) 236-1534 J. Mao; Email:
[email protected]; Tel: (757) 683-6874
1 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
16
ABSTRACT
17
To elucidate how different extreme heteroatom concentrations in oil shale kerogen may
18
present and contribute to various structural features, three shale samples, containing kerogen with
19
high oxygen content, low heteroatom content, and high sulfur content, were analyzed using
20
advanced 13C solid-state nuclear magnetic resonance (NMR) techniques, including multiple cross-
21
polarization/magic angle spinning (multiCP/MAS), dipolar dephasing (multiCP/DD), and two-
22
dimensional 1H-13C heteronuclear correlation (2D HETCOR). We found that oxygen in Estonian
23
kukersite was present mostly in aromatic C-O structures, and that non-protonated aromatic carbons
24
connecting with phenols and alkyl chains led to more diverse aromatic signal distributions and
25
structures in the kukersite organic matter than were observed in the other shales. The low-
26
heteroatom kerogen present in Australian Glen Davis torbanite had the simplest structural pattern
27
and the lowest aromaticity, despite having a lower atomic H/C ratio than the kerogens present in
28
the other shales. The organic sulfur-rich Ghareb marinite from Jordan contained the highest
29
aromaticity and most diverse alkyl structures among the three shales. 2D HETCOR with 1H spin
30
diffusion showed that structural heterogeneity of the Glen Davis kerogen was less than 1 nm,
31
indicating preservation of original structures present in precursor organic matter. This analysis of
32
organic matter in whole shale samples with unusual heteroatom contents and previous NMR
33
studies of shales and kerogens demonstrate that structural characteristics in organic matter are not
34
necessarily captured by kerogen typing based solely on elemental ratios (H/C, O/C) or
35
programmed pyrolysis parameters and that NMR provides deeper insights into kerogen structure.
2 ACS Paragon Plus Environment
Page 2 of 29
Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
36
Energy & Fuels
1. INTRODUCTION
37
Oil shales are fine-grained sedimentary rocks containing refractory organic matter
38
(kerogen) that generates synthetic crude oil upon pyrolysis at high temperatures (>350 °C) which
39
can then be refined into liquid hydrocarbon fuels
40
and organic solvents and represents the largest repository of organic matter on Earth
41
hydrogen, carbon, and oxygen contents of kerogen are used to determine kerogen types by plotting
42
the atomic H/C and O/C ratios in a van Krevelen diagram 3, 5. The different elemental ratios provide
43
information on the relative amount of liquid oil or natural gas that the kerogen may generate during
44
thermal decomposition 6. The nitrogen, oxygen, and sulfur (e.g., heteroatom) content of kerogens
45
with different origins vary based on the organic matter source materials and conditions in the
46
depositional environment. This is reflected in both elemental ratios (H/C, O/C, S/C, etc.) and the
47
distributions of functional groups present, which can indicate how the elements are connected
48
making it possible to infer structural properties
49
presence of O-containing functional groups such as phenols, esters, ethers, and carboxylic acids.
50
Organic sulfur can be present in the form of sulfoxide, sulfone, aliphatic sulfur, and aromatic sulfur
51
8.
52
reveal much more detailed and comprehensive structural information. Specifically, solid-state
53
nuclear magnetic resonance (NMR) spectroscopy provides information on the distribution of
54
carbon moieties and how they are connected, making it one of the best methods for non-destructive
55
characterization of oil shale and source rock kerogens.
56
1-2.
Kerogen is insoluble in common inorganic
5, 7.
3-4.
The
Abundant oxygen content indicates the
While elemental analysis may suggest overall chemical composition, spectroscopic methods can
The
13C
cross polarization/magic angle spinning (CP/MAS) method is the most widely
57
used solid-state NMR technique in source rock organic matter studies. For instance, it has been
58
applied to investigate structures of Green River oil shale kerogen 9, kerogen from Tertiary deposits
3 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
10,
Page 4 of 29
Aleksinac oil shale kerogen
11,
in Queensland, Australia
60
Australian Glen Davis shale 13. This technique was also applied in studying thermal alteration of
61
Cretaceous black shale kerogen 14, thermal evolution of a group of kerogen samples from China
62
15,
63
combined with Rock-Eval pyrolysis and vitrinite reflectance 17. It also has been combined with X-
64
ray photoelectron spectroscopy (XPS) to characterize organic nitrogen and sulfur functional
65
groups in oil shale and source rock organic matter
66
significantly advance our knowledge of the structure of kerogen 19-22. However, shortcomings of
67
simple
68
ambiguous assignments; quantitative information cannot be obtained due to those or other
69
limitations. Systematic advanced solid-state NMR techniques
70
applied to studies of natural organic matter in geologic samples in order to make quantitative
71
measurements 22, better functional group assignments 23, and aromatic cluster size estimation 24.
and thermal maturity of the New Albany Shale
13C
16
Estonian kukersite kerogen
12,
59
and
among others; the technique is usually
8, 18.
Solid-state
13C-NMR
has been used to
CP/MAS measurements include spinning sidebands, baseline distortion, and
19-21
have been developed and
72
While several studies have been conducted on the extractable organic fraction of
73
Ordovician Estonian kukersite, Permian Australian Glen Davis torbanite, and Cretaceous
74
Jordanian Ghareb marinite deposits using gas chromatography-mass spectrometry (GC-MS),
75
Fourier-transform infrared spectroscopy (FTIR), and X-ray absorption near-edge structure
76
(XANES) spectroscopy 25-30, few investigations have been carried out on isolated kerogen samples
77
or the whole shales 12-13, 31. In this study, we determined organic matter carbon moiety distributions
78
and connectivities in samples of these oil shales using advanced 13C solid-state NMR techniques
79
including
80
multiCP/MAS plus dipolar dephasing (multiCP/DD), two-dimensional 1H-13C heteronuclear
81
correlation (2D HETCOR), and 2D HETCOR with 1H spin diffusion. Whole shales were used in
13C
multiple
cross-polarization/magic
angle
4 ACS Paragon Plus Environment
spinning
(multiCP/MAS),
13C
Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
82
this work due to the high organic carbon content of the selected samples (20.9 to 56.2 wt.% organic
83
carbon) and in part to avoid kerogen alteration effects due to the use of strong acids in
84
demineralization procedures
85
information on the connectivity between these structures for organic matter present in these three
86
shales in order to compare structural features of immature kerogens with very different heteroatom
87
compositions to assess how they affect bulk kerogen structure.
22.
Our objectives were to obtain carbon moiety distributions and
88 89
2. MATERIALS AND METHODS
90
2.1. Shale samples
91
Ordovician kukersite shale of Estonia covers an area of more than 50,000 km2 and has been
92
extensively exploited and processed to fuel electric power plants and produce synthetic crude oil,
93
petrochemicals, and other products 32. The sample of Estonian kukersite used in this study contains
94
type I/II kerogen and was collected from the Narva-E mine. Upper Permian torbanite shale of the
95
Glen Davis Formation (New South Wales, Australia) is an organic-rich sedimentary unit
96
containing kerogen primarily originating from Botryococcus braunii algae
97
methanogenic bacteria 34. Jordanian marinite of the Upper Cretaceous Ghareb Formation contains
98
type IIS kerogen and is rich in a complex distribution of organic sulfur compounds, among which
99
the alkylthiophenes can be used as molecular biomarkers to indicate paleoenvironmental changes
26, 33
and some
100
35-36.
101
The samples were crushed, pulverized, and sieved (-60 mesh) to obtain homogenized powders for
102
NMR analysis.
All the shale samples were provided by the U.S. Geological Survey (collected by M. Lewan).
103
The mineralogy of shale samples was determined by X-ray diffraction (XRD; see Table 1).
104
Corundum was added as an internal standard (20 wt. %) to each sample prior to micronization and
5 ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 29
105
a PANalytical X’Pert Pro MPD X-ray diffractometer (Westborough, Massachusetts) was used to
106
collect diffractograms. Semiquantitative mineralogy was obtained by interpreting diffractograms
107
using the Jade Software package (Materials Data Inc., Livermore, California). Elemental
108
compositions of isolated kerogen samples were determined after demineralization following a
109
method described previously 37. Total organic carbon (TOC) content and programmed pyrolysis
110
parameters were determined using a LECO C744 Series analyzer and Wildcat Technologies
111
Hydrocarbon Analyzer With Kinetics (HAWK), respectively, following manufacturers’
112
instructions. The results from kerogen elemental analysis and XRD, TOC, and programmed
113
pyrolysis analyses on shale samples are shown in Table 1.
114
2.2. NMR spectroscopy
115
All 13C NMR analyses were performed on a Bruker Avance 400 spectrometer at 100 MHz
116
for 13C. Samples were packed in 4-mm-diameter zirconia rotors with Kel-F caps, and experiments
117
were run in a double-resonance probe head. The
118
tetramethylsilane (TMS), with 13COO- labeled glycine at 176.49 ppm as a secondary reference.
119
2.2.1. 13C multiple cross-polarization magic angle spinning (multiCP/MAS) NMR
13C
chemical shifts were referenced to
120
The 13C multiCP/MAS technique was employed to obtain quantitative solid-state 13C MAS
121
NMR spectra with good signal-to-noise ratios while measuring time was significantly reduced in
122
contrast to direct-polarization (DP/MAS) NMR 38. The spectra were collected at a spinning speed
123
of 14 kHz, with very small (
