Communication pubs.acs.org/EF
On Exclusivity of Vanadium and Nickel Porphyrins in Crude Oil Edward Furimsky*
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IMAF Group, 184 Marlborough Avenue, Ottawa, Ontario K1N 8G4, Canada to 1240 ppm, respectively. Table 18,10 showed that there is no correlation between the natural abundance of metals and their content in crude oil. Thus, on the basis of the former, one would expect Fe, Ca, Mg, and Mn rather than V and Ni to be dominant metals in crude oil. This suggests that other factors must be responsible for the dominant occurrence of V and Ni in petroleum. In this regard, the attention should be focused on potential reactions leading to the formation of porphyrins as the main structures of V and Ni metals in crude oil.
INTRODUCTION Mutual effects of mineral matter, i.e., clays (silica−alumina), clay minerals (silicates−aluminates of alkali and alkali earth metals), metal carbonates, pyrite, oxides and sulfides of trace metals, etc., and biomass are part of the diagenesis process transforming biomass into hydrocarbon-rich organic matter. In other words, both biomass and mineral matter are mutually influencing each other while being simultaneously transformed. Early stages of biomass conversion are accompanied by the release of H2O and CO2. The formation of carbonates on the account of the latter is an important reaction. Consequently, alumina, silica, and silica−alumina are liberated from clay minerals. Catalytic participation of these oxides during digestion of biomass and maturation of crude oil can be anticipated. For example, alumina can catalyze dehydration of alcohols,1 while acidic silica−alumina can catalyze decarboxylation of fatty acids2 as well as cracking and isomerization of hydrocarbons.3 Similarity in the structure of the organic skeleton of porphyrins found in crude oil as well as that of chlorophyll and hemoglobin (Figure 1) is quite evident.4,5 During the digestion of biomass, the metals are removed from these structures, e.g., either via hydrolysis or reaction with CO2, leaving behind an organic skeleton comprising four pyrrole rings. A modified structure of the skeleton may survive until the advanced stage of crude oil maturation.6 In an extreme case, a structure approaching that of porphyne (Figure 1) is formed. Thus, esteric, carboxylic, and hydroxyl groups attached to a tetrapyrrole skeleton may not survive. Under hydrostatic pressures exceeding 100 MPa, they can be replaced by other substituents. Indeed, porphyrins with a great variety of substituents have been identified in crude oil.7 Because they are found exclusively in crude oil, Ni- and VO-containing porphyrins must have been formed during the diagenesis of biomass via an intimate contact of porphyne (Figure 1) with mineral matter. The content of vanadium (V) relative to nickel (Ni) in crude oils varying widely in composition has been well-documented.6,7 As a result of a much lower content, other metals have been receiving less attention. For example, the analyses of some 30 samples of crude oil obtained from different wells in Venezuela Alturitas field confirmed the presence of Cd, Cu, Mo, Fe, P, Zn, Ca, and Mg in addition to V and Ni.8 However, as the results in Table 1 show, V and Ni metals were far predominant, accounting for more than 80% of all metals, in agreement with the extensive database on the content of metals in conventional crude oil.6 No significant correlation between the content of V and Ni as well as the content of asphaltenes and organic carbon in crude oils could be established.9 This was concluded after extensive evaluations of a series of petroleum samples, with the content of V and Ni varying from 0.2 to 4760 ppm and from 7 © 2016 American Chemical Society
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REDUCIBILITY OF METAL OXIDES
An exclusive occurrence of Ni and VO porphyrins and the absence of other metal porphyrins in crude oil can be attributed to a high reducibility of the oxidic forms of V and Ni present in mineral matter. Thus, a reduced form of metal may be a precursor to coordination with the tetrapyrrole macroligands (Figure 1). Table 2 shows that, among transition metal oxides, the largest driving forces (most negative ΔG) for the reduction are exhibited by NiO and CoO. The reduction of V2O5 to V is not favorable, while that to VO is favorable. The ΔG values were estimated using the extensive database on thermochemical properties of inorganic substances published by Barin and Knacke.11 Then, the equilibrium constants for the reduction reactions can be estimated (e.g., log K = −ΔG/4.6T) using ΔG values in Table 2. These data would show that the reduction of V2O5 to VO is more than 2 orders of magnitude more favorable than that of NiO to Ni. The driving force for the latter is about 3 times greater than that for the reduction of CoO to Co. Therefore, not only a smaller natural abundance of Co compared to that of Ni and V (Table 2) but also a much lower reducibility may be responsible for the absence of Co porphyrins in crude oil. Despite the large abundance in the earth crust (Table 1), Fe and Mn metals are not available for the reaction with the porphyrin skeleton because of a low reducibility of their oxides at temperatures encountered during diagenesis (Table 2). The most abundant oxides, such as SiO2 and Al2O3, as well as the oxides of alkali and alkali earth metals are non-reducible under diagenesis conditions. Therefore, no porphyrins containing such metals should be formed during diagenesis. The reduction of metal oxides is facilitated by availability of readily transferable hydrogen and hydrogen produced in dehydrogenation reactions involving some components of lipids, shown in Figure 2. In microalgae biomass, the content of lipids may vary from 8 to 80%.12 Besides microalgae, large quantities of triglycerides and free fatty acids are present in vegetable oil biomass. The characterization of 39 crude oils of Received: September 19, 2016 Revised: October 29, 2016 Published: October 31, 2016 9978
DOI: 10.1021/acs.energyfuels.6b02385 Energy Fuels 2016, 30, 9978−9980
Communication
Energy & Fuels
Figure 1. Structures of chlorophyll, hemoglobin, porphyn, and porphyrin.
Table 1. Natural Abundance of Metals (A) and Their Content in Venezuelan Crude (B) iron manganese vanadium nickel zinc copper cobalt molybdenum cadmium calcium magnesium
A
B
50000 1000 160 80 75 50 20 1.5 0.1 40000 23000
4−75 na 440−617 70−94 1−12 2−17 na 3−22 1−5 11−57 1−10
Table 2. Driving Forces (ΔG) for Reduction of Transition Metal Oxides (MeO + H2 = Me + H2O) temperature (K) MeO
300
400
500
NiO CoO V2O5a V2O5b FeO CuO MnO
−4.0 −3.4 −8.3 +33.4 +5.4 +24.2 +31.6
−5.2 −4.1 −9.0 +30.0 +4.8 +25.3 +31.2
−6.5 −5.2 −12.0 +28.1 +4.4 +26.3 +30.6
a
From reaction 0.5V2O5 + 1.5H2 = VO + 1.5H2O. bFrom reaction 0.5V2O5 + 2.5H2 = V + 2.5H2O.
NiO + −CH 2CH 2− → Ni + −CHCH−+H 2O
different ages from all over the world conducted by Nytoft et al.13 revealed the presence of hopanes (Figure 2) and benzohopanes, confirming that such structures present in biomass can survive until the advanced stages of crude oil maturation. Moreover, the structures, such as isoprenoid alkanes, steranes, and hopanoids, were clearly identified in kerogen obtained from the Western Canada Sedimentary Basin.14 The origin of such structures can be traced to the terpenes, sterols, and hopanes present in biomass. Such hydrogen-rich structures can reduce transition metal oxides (e.g., NiO and V2O5) according to the following tentative reactions:
V2O5 + 3−CH 2CH 2− → 2VO + 3−CHCH− + 3H 2O
The mineral-matter-aided dihydrocyclization of long alkane chains is another source of hydrogen.15,16 This suggests that the rates of transformation of the same type of biomass should be different if it is in intimate contact with mineral matter of different composition. The occurrence of both oxidic and sulfidic forms of transition metals in the earth crust has been well-documented.17 The comparison of driving forces in Table 2 to those in Table 3 9979
DOI: 10.1021/acs.energyfuels.6b02385 Energy Fuels 2016, 30, 9978−9980
Communication
Energy & Fuels Notes
The author declares no competing financial interest.
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Figure 2. Some hydrogen donors present in biomass.
Table 3. Driving Forces (ΔG) for Reduction of Transition Metal Sulfides (MeS + H2 = Me + H2S) FeS2 → FeS FeS FeS2 NiS CoSa Cu2S MnS a
300
400
500
+8.4 +15.5 +23.9 +13.7 +13.3 +15.0 +44.3
+6.0 +14.7 +20.7 +12.4 +12.2 +15.5 +43.6
+3.7 +14.2 +17.9 +11.4 +11.4 +16.6 +43.0
Calculation was for CoS0.89.
shows that metal sulfides are much more resistant to reduction than corresponding metal oxides. For example, if the ΔG values of NiO and CoO (Table 2) are used for the estimate of the equilibrium constant, the value of the latter would be larger by more than an order of magnitude compared to those for the reduction of NiS and CoS (Table 3). In fact, during the commercial production of Ni and Co metals from sulfide ores, the latter have to be converted to oxides before pure metals can be obtained by subsequent reduction. Then, the natural abundance of the oxidic forms relative to sulfidic forms of V, Ni, and Co appears to be an important parameter for estimating relative amounts of porphyrins in crude oil. Unfortunately, such information is lacking. Therefore, the absence of Co porphyrins in crude oils may suggest either a predominance of nonreductive sulfided forms of cobalt ores or much lower abundance of Co relative to Ni and V.17 In conclusion, the correlations between the occurrence of metal porphyrins in crude oil and composition of surrounding mineral matter cannot be established because of an incomplete geochemical database. A high reducibility of V and Ni oxides to VO and Ni, respectively, compared to other metal oxides provides a plausible explanation for the exclusive occurrence of VO and Ni porphyrins in crude oil.
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REFERENCES
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DOI: 10.1021/acs.energyfuels.6b02385 Energy Fuels 2016, 30, 9978−9980
