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Peculiar ESR of Mn2+ in kerogen: the Zvonce graptolitic black shale (Serbia) Bratislav Z. Todorovi#, Pavle I. Premovic, Sreten B. Stojanovic, and Dragan T Stojiljkovi# Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00851 • Publication Date (Web): 28 Jul 2016 Downloaded from http://pubs.acs.org on August 1, 2016
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Peculiar ESR of Mn2+ in kerogen: the Zvonce graptolitic black shale (Serbia) Bratislav Ž. Todorović1, Pavle I. Premović1*, Sreten B. Stojanović2, Dragan T. Stojiljković2 1
Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of Niš, P.O. Box 224, 18000 Niš, Serbia,
2
Faculty of Technology, University of Niš, Bulevar Oslobodjenja 124, 16000 Leskovac, Serbia.
*Corresponding author. E-mail address:
[email protected] (P.I. Premovic)
Abstract. Mn2+ ions incorporated into kerogen isolated from the Zvonce black shale (Silur, Eastern Serbia) is detected by electron spin resonance (ESR). Similar species are found in the dark shale (Silur/Devon, central Morocco). No Mn2+ ions was previously detected by ESR in any geoorganic material. The Mn2+ ions in question have been investigated at X- and Q-band frequencies at temperatures of liquid helium (ca. 4.2 K) to 250 K. The spectra exhibits the usual six line pattern with the high g-factor of 2.154±0.005 and the isotropic hyperfine constant A of 7.56±0.1 mT. These six lines are identical but assymetrical at all temperatures and microwave (X- and Q-) bands. A computer simulation revealed that the line assymetry is probably due to the supperposition of the two very similar Lorentzians (or possible more) Mn2+ sites. The linewidth [∆Hp-p(T)] of the Mn2+ ESR lines shows a strong dependence on temperature with a T-2 variation of the relaxation time. It is suggested that the Raman relaxation process is mainly responsible for this behavior. 1. Introduction Mn2+ (3d5) has been extensively studied by ESR in a wide range of geo-inorganic materials such as clays, clay minerals, limestones, natural minerals and crystals.1 To our knowledge, no ESR active Mn2+ in geo-organic materials has been ever reported in the literature. We report the first case of the ESR active Mn2+ associated with the kerogen isolated from the Silurian black (graptolitic) shale from Zvonce of marine origin.2 The sedimentary geochemistry of Mn is dominated by the redox control of its speciation and its geochemical behavior is therefore quite different in oxic and anoxic environments, In oxygenated 1 ACS Paragon Plus Environment
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seawaters Mn probably exists in its higher oxidation states, Mn3+ (ESR inactive but magnetic) or Mn4+ (3d3), and its solubility is limited by equlibria with highly insoluble MnO23. The Zvonce graptolitic shale is believed to have been deposited under stagnant anoxic conditions in a sheltered area of a shallow epicontinental sea.2 In a such highly reducing sea environment, characterized by almost the absence of atmospheric O2 and the presence of H2S/pyrite (Fe2S3), Mn is readily reduced to very soluble Mn2+. Mn is present in anoxic solutions as Mn2+, MnHCO3 and in organic complexes.3 2. Material and metods 2.1. Kerogen sample preparation The kerogen isolation procedure is similar to that used by Premović et al.4, (and references therein). The shale sample was ground to a fine powder (200-400 mesh) with a ball mill and were treated with 20 % hydrochloric acid (HCl) to remove carbonates. After filtration and washing, the remaining minerals were acid leached by digestion for 72 h at room temperature using 1:1 (v/v) mixture of concentrated hydrofluoric acid (HF) (48 %) and HCl (20 %). The mixture was filtered and the residue washed repeatedly with boiling distilled water to pH 7. After drying, the organic concentrate was exhaustively extracted with benzene/methanol azeotrope until the solvent siphoning to the flask was clear. This extraction step removed the organic soluble fraction (bitumen) yielding kerogen. The kerogen was dried at 80 °C and stored in a desiccator. The ESR experiments with the kerogen isolated from the black shale from Zvonce (hereinafter Zvonce shale) have been given the most attention in this work and provide the main background for the ensuing discussion. Similar experiments were also performed with the kerogen of the dark shale from central Moroccan Basin.5 2.2. Electron spin resonance The ESR measurements were performed on finely ground powders of the kerogen sample, which were placed in pure silica tubes (Suprasil grade). Spectra were recorded on a Bruker ER-200 series ESR spectrometer with either a Bruker ER-044 X-band bridge or a Bruker ER-053 Q-band bridge, using standard 100 kHz field modulation. X-band measurements were made at 9.4 GHz utilizing a rectangular TE cavity, and those at Q-band at 35 GHz using a cylindrical TE cavity. Frequency was measured using a Hewlet-Packard frequency meter. Magnetic field calibration was performed with the DPPH standard g = 2.0037±0:0005.6 The ESR linewidth ∆Hp-p(T) temperature T is measured as the distance between two peaks of the absorption line derivative. 2 ACS Paragon Plus Environment
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ESR measurements at X- and Q-bands were carried out at room temperature and at different low temperatures (150 K, 175 K, 195 K, 215 K, 230 K, 245 K) using a nitrogen-flow device for cooling. Spectra at liquid nitrogen temperature of 77 K were obtained using a Bruker (liquid nitrogen) Dewar. No saturation of the paramagnetic Mn2+ signal was observed at these temperatures with increasing power up to 100 mW. All spectra were recorded using the same microwave power (100 mW), modulation amplitude (0.1 mT), conversion time (42 ms), time constant (82 ms) and sweep time (168 s). Liquid helium measurements at temperature of approximately 4.2 K were made on a Bruker ESP-380E spectrometer equipped with an Oxford Instruments helium-flow cryostat operating at X-band frequency with a 100 kHz modulation frequency. The spectra were recorded at this temperature in the following conditions: microwave power: 6 mW and modulation amplitude: 0.1 mT. The ESR line simulation was performed using a program package, EasySpin working on the MATLAB software.7 3. Results. The Mn2+ ion has the unpaired electron configuration 3d5, so that in general there are six spin levels (S = ±5/2, ±3/2, ±½) between which a maximum of five allowed transitions are possible according to the selection rule ∆S = ±1. In addition, however, the spin of the 55Mn nucleus (I = 5/2) results in a splitting of each fine-structure line into 2I+1, i. e. six hyperfine components. Mn2+ is usually detected by ESR in the approximate cubic or axial environment of the ligand atoms (in fact, the ligand donor atoms). In these cases the zero field splittings (ZFS) are small; frequently in a powder or solution spectrum the five ∆S transitions are unresolved due to their small anisotropy and only one group of six lines usually appears near g = 2.0. The ESR spectra of the Zvonce kerogen at approximately 4.2 K up to 245 K shows two principal resonances. The resonance with a sharp signal at g ~2.003 which can be attributed to a stable organic free radical of kerogen.8 The remaining resonance is composed of assymetric six identical lines with the isotropic g-value of 2.154 ± 0.005 and the isotropic hyperfine coupling constant A = 7.56 ±0.1 mT. For the sake of clarity, Fig. 1 shows only the Q-band spectrum of the Mn2+ resonance at 151 K. We assigned these six lines to Mn2+ in the Zvonce kerogen structure. Mn2+ was not detected by ESR in Zvonce kerogen from room temperature down to about 250 K. It is possible to speculate that the ESR spectra in question may be attributed to an ESR active Mn4+ ions (Mn3+ ion is ESR inactive) incorporated into the Zvonce kerogen but in this case the ESR spectra would be rather complex widely depending of axial D rhombic E zero-field splitting 3 ACS Paragon Plus Environment
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parameters, and microwave quantum value hν.9 In addition, as we pointed above, in marine anoxic environment Mn2+ is by far predominant. Therefore, the question also arises, how the oxidation of Mn2+ to Mn4+ was possible under very strong reducing conditions of the Zvonce black shale. The assymmetry of the six ESR lines of Mn2+ was observed in all spectra, at all temperature and at the X-and Q bands. Fig. 2 shows one of these assymetric lines recorded at approximately 4.2 K. It was found that this assymetric line could be well fitted to two Lorentzian lines of nearly the same intensities (with the intensity ratio 1:1.02) somewhat different linewidths [0.6(6) mT vs. 0.7(0) mT] centred around g = 2.154 (the magnetic field difference of their line centers is 0.2 mT). Hence, it is likely that this line assymetry is due to the supperposition of the two Lorentzians (or possible more) Mn2+ sites in the Zvonce kerogen with slightly different isotropic g values and linewidths. These sites are probably very similar but not identical. Of note, Gupta et al.10 recorded similar assymetric lines for the ESR spectra of the Mn3+-oxo complex with the g = 8.08 . It is intruguing that all Mn2+ of the Zvonce kerogen nicely agree with a metallic Lorentzian, frequently called also Dysonian. One may attribute this lineshape to the low skin-depth, δ. Indeed, the ESR lineshape has a strong dependence on the electrical conductivity, σ. This shape can vary from Lorentzian (low σ) to Dysonian (high σ and low δ). The average kerogen (like the Zvonce kerogen) is a very complex three-dimensional (non-conductive) polymer with a very low low σ ≤ 10-9 s cm-1. Thus, the lineshape of the Mn2+ resonance line studied is expected to be pure Lorentzian. We also approximately estimated the ESR line asymmetry using the parameter A/B which is defined as a ratio of the low field to the high field peak heights of the line. This estimation shows that A/B at liquid helium temperature (4.2 K) is about 4, at 115 K about 3 and at 175 K about 1. It is clear that the line asymmetry readily decreases with temperature. A close inspection reveals that there is no difference of the linewidths of the first and the sixth hyperfine components at 4.2 K [∆Hp-p (4.2 K) = 0.5±0.1 mT)] up to 245 K, indicating that the axial distortion is probably very small.11 In addition, there are no weaker doublets between these components. These doublets, referred to as "forbidden transitions" (∆m = ±l), are caused by axial distortion of the field of the ligands.12 Their absence in the ESR spectrum of the kerogen Mn2+ is
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a further support for our notion that there is no such axial distortion around Mn2+ in the Zvonce kerogen and this ion is surrounded by the cubic ligand arrangmement. Due to half-filled d shell (3d5) with spin S = 5/2, angular momentum L = 0 and nucleus spin I = 5/2, the ESR resonance of an Mn2+ ion located in the cubic site may be interpreted with the aid of the rather simple spin Hamiltonian H = βH·g·S + I·A·S The first term is the Zeeman energy and the second represents the hyperfine interaction between electron spin (S = 5/2) and nuclear spin (Mn2+; I = 5/2) with a hyperfine coupling constant A. Other symbols have their usual ESR meanings.13 The constant A is a qualitative measure of the covalency of Mn-bonds with the ligandssince the constant A is due to the core polarization through a magnetic coupling of inner s-electrons with unpaired d-electrons.13,14 Thus, A in more ionic ligand environment is larger than in more covalent ligand environment, i. e. the greater the covalent bonding, the smaller will be the hyperfine splitting. In general, the covalency (or ionicity) of the bond between Mn2+and ligands can be very roughly estimated using Matumura’s plot.12 Using value obtained from the ESR spectrum of the Mn2+ studied A = 7.56 mT (or 76×10−4 cm−1) we estimated a low covalency of about 15 %. Futhermore, this splitting would suggest that Mn2+ is probably bonded to oxygen or sulfur atoms of the ligands in sixfold coordination.14,15 One of the pecularities of Mn2+ in the Zvonce kerogen is its unusually high g value of 2.154. Most of the Mn2+ systems reported in literature show g values close to free spin value of 2.0023. Furthermore, the g value is also indicative of the nature of bonding. If the g value shows negative shift with respect to free electron g value of 2.0023, the bonding is ionic and conversely, if the shift is positive, the bonding is more covalent in nature.16 In the present work, a rather large positive (2.154 - 2.0023 = +0.152) shift in the g value would indicate that the Mn2+ in a highly covalent environment. This contradicts previous conclusion derived from the isotropic A that the Mn2+ in the ligand (oxygen or sulfur) environment of low covalency. Worth to note here, that it has been reported only two cases of Mn2+ having an isotropic g value higher than 2.11.17,18 Stepwise heating from about 4.2 K up to about 245 K led to an increase of the linewidth ∆Hp-p (T), but the spectral parameters A and g remained constant. The relaxation linewidth broadening is given by 5 ACS Paragon Plus Environment
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∆H (T) = ∆Hp-p (T) - ∆Hp-p (4.2 K) where ∆H(T) is the spin-phonon part of the ESR linewidth which was evaluated by subtracting from the total linewidth value ∆Hp-p(T) the value of temperature-independent linewidth obtained at the lowest temperature (liquid helium temperature of 4.2 K). The temperature variation of ∆H (T) is proportional to the temperature variation of the inverse of the relaxation time, 1/T1.1 Fig. 3 shows the dependence of ∆H (T) (i. e. 1/T1) vs. temperature in a log-log graph. This dependence indicates that the relaxation time is proportional to T-2 for temperature above 77 K. Such behavior is characteristic for a Raman relaxation process1. Acknowledgments. This paper was significantly improved through the helpful and detailed review of two anonymous referees. Funding for P. I. Premović’s ESR work at Université Pierre et Marie Curie (Paris) was obtained from le Ministere Francais de l’Éducation National de l’Enseignement Supérieur et de la Recherche. We thank to Drs. Jörg Sichelschmidt and Sergey Androneneko for their insightful comments and suggestions. We are also grateful to Drs. Nikolai A. Poklonski and Pavlo Aleshkevych who contributed to this report by providing essential bibliographic material.
References (1) Pilbrow, J. R. Transition Ion Electron Paramagnetic Resonance, Clarendon Press: Oxford, 1990. (2) Premović, P. I.; Jovanović, Lj. S.; Popović, G. B.; Pavlović, N. Z.; Pavlović M.S. J. Serb. Chem. Soc. 1988, 53, 427-431. (3) Anschutz, P.; Dedieu, K.; Desmazes, F.; Chaillou, G. Chem. Geol. 2005, 218, 265-279. (4) Premović, P. I.; Allard, T.; Nikolić, N. D.; Tonsa, I. R.; Pavlović, M. S. Fuel. 2000, 79, 813819. (5) Lüning, S. D. K.; Loydell, D. K .; Sutcliffe O.; Ait Salem A.; Zanella E.; Craig J.; Harper D. A. T. J. Petrol. Geol. 2000, 23, 293-311. (6) Swartz, H. M.; Bolton, J. R.; Borg, D. C. Biological Application of Electron Spin Resonance; New York: Wiley-Interscience; 1972. (7) Stoll, S.; Schweiger, A. J. Magn. Resonan. 2006, 178, 42-45.
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(8) Premović, P. I. Organic free radicals in Precambrian and Paleozoic rocks: Origin and significance.. In: Implications for Mineral and Energy Resources Early organic evolution; M. Schidlowski et al., eds.; Springer-Verlag, Heidelberg, 1992; pp 251-256. (9) Pedersen, E.; Toftlund. Inorg. Chem. 1974, 13, 1603-1612. (10) Gupta, R.; Taguchi, T.; Lassalle- Kaiser, B.; Bominaar, E. L.; Yano, J.; Hendrich, M. P.; Borovik A. S. Proc. Natl. Acad. Sci. USA. 2015, 112, 5319–5324. (11) Tikhomirova, N. N.; Dobryakov, S. N.; Nikolaeva, A. V. Phys. Star. Sol. 1972, 10, 593-603. (12) Wertz, J. E.; Bolton, J. R. Electron Spin Resonance, McGraw-Hill: New York, 1972. (13) Weil, J. A.; Bolton, J. R.; Wertz, J. E. Electron Paramagnetic Resonance: Elementary Theory and Applications, John Wiley and Sons: New York, 1994. (14) Matumura, O. J. Phys. Soc. Japan. 1959, 14, 108. (15) Šimanek, E.; Müller, K. A. J. Phys. Chem. Sol. 1970, 31, 1027-1040. (16) Van Wieringen, J. S. Disc. Faraday Soc. 1955, 19, 118–126. (17) Udomkan, N.; Memos, S.; Limsuwan, P.; Winotai, P.; Chaimanee, Y. Chinese Phys. Lett. 2005, 22, 1780-1783. (18) Kripal, R.; Shukla, A. K. Z. Naturforsch. 2006, 61a, 683-687.
Figure captions
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Figure 1. Narrow-range (Q-band) ESR spectrum of Mn2+ ions in the Zvonce kerogen at 151 K.
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Figure 2. Assymetric ESR line of Mn2+ ions associated with the Zvonce kerogen recorded at Xband and at ca. 4.2 K: experimental noisy (black) line; (b) the superimposed simulated (red dashed line), based on the ESR details given in the text.
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Figure 3. Spin-photon part of the ESR linewidth ∆Hp-p [mT] vs temperature T [K] (log-log scale), for Mn2+ ions associated with the Zvonce kerogen . Solid circles: experimental (X-band) data. Solid/dotted line: simulated temperature dependence.
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