36
Energy & Fuels 1987,1, 36-44
On the other hand, in HzO, the reaction profiles for both catalysts were different: two stages with nickel and a single stage with iron. This may be explained by the difference in the chemical form of the catalyst during the reaction. The nickel catalyst existed in a form of metallic nickel, whereas the chemical form of the iron catalyst was magnetite in H20. The minimum H2/H20ratios required to convert the lowest metallic oxide to the metal at 923 K are approximately 3 for Fe and 0.005 for Ni. Thus, only nickel can be in the metallic state in the presence of small amounts of reducing gases such as volatile matter and/or gasification products. The first-stage gasification in H2 terminated at a coal conversion around 75 w t % with the iron catalyst and 85 wt% with the nickel catalyst, and the rate became very small thereafter (Figure 1). As the reaction proceeded, the particle size of iron metal increased from 29 nm on devolatilization to 54 nm at the ultimate conversion of 76 w t % (Table 111). The size of the nickel particles also increased from 8 nm at the initial stage to 32 nm at the termination point. This sintering is due to the consumption of char around the metal particles. The termination of the
first-stage gasification may be related to the deactivation of the catalyst owing to sintering. The disappearance of the first stage at a high temperature of 1273 K (Figure 2) may also be due to the sintering of iron catalysts. Similar results were obtained for the nickel-catalyzed hydrogasification. At 1273 K nickel particles of about 0.1 pm in size were observed even at the initial stage.26 It is emphasized again that the presence of fine metallic particles is necessary for the occurrence of the two-stage gasification. Acknowledgment. Y. Ohuchi and E. Sat0 are thanked for their assistance in carrying out experiments. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan, Special Project for Energy (No. 59040033). Registry No. Fe(N03)3, 10421-48-4;(NH.J3Fe(C20J3, 14221-47-7;FeC13, 7705-08-0;Fe2(S04)3,10028-22-5;KNOB, 7757-79-1; Ca(N03)2, 10124-37-5; Fe, 7439-89-6; Fe304,1309-38-2; Ni, 7440-02-0. (26)Higashiyama, K.;Tomita, A.; Tamai,Y.Fuel 1985,64,1525-1530.
Characterization of the Binding Sites of Vanadium Compounds in Heavy Crude Petroleum Extracts by Electron Paramagnetic Resonance Spectroscopy John G. Reynolds*+ and Emilio J. Gallegos Chevron Research Company, Richmond, California 94802
Richard H. Fish* and John J. Komlenic Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Received March 27, 1986. Revised Manuscript Received October 16, 1986 The high-performance liquid chromatographic (HPLC) separated fractions of pyridine/water extracts of selected crude petroleums were analyzed by electron paramagnetic resonance (EPR) spectroscopy to determine the first coordination sphere around the vanadium. The extracts were separated by polarity on an octadecylsilane column (ODS) into three fractions-low, moderate, and high polarity. As determined by EPR, the Boscan moderate-polar and Prudhoe Bay moderate- and low-polar fractions exhibited NzS2coordination. Cerro Negro and Wilmington moderate-polar fractions showed S4 coordination, and the Boscan low-polar fraction showed N4 coordination. The coordination of the Cerro Negro low-polar fraction had distinctly different parameters, showing a NOS2coordination. These results are important in the identification of non-porphyrin metal-containing compounds in heavy crude petroleums and residua. Introduction We recently reported on the molecular size and polarity of the vanadium and nickel compounds found in selected heavy crude petroleums, Boscan, Cerro Negro, Wilmington, and Prudhoe Bay, and their We examined the metal-containing compounds by size-exclusion chromatography in conjunction with graphite furnace atomic absorption selective metal detection (SEC-HPLC-GFAA), and found the metal distribution to be unique for each petroleum, while the histogrammic metal profiles may have utility for fingerprinting the heavy crude petroleums. For t Present address: Lawrence Livermore National Laboratory, University of California, Livermore, CA 94550.
0887-0624/87/2501-0036$01.50/0
vanadium, the majority of the metal-containing compounds fall into the 2000-9000-Da molecular mass (MW) range, while 3-7% was found in the MW range of >go00 Da. The low molecular mass range was calibrated by vanadyl and nickel model compounds, with 23 to 30% having MW in the 2000 Da. The extract consists predominantly of low molecular mass compounds with an average MW of approximately 350 Da. By comparison with nickel and vanadyl model compounds of similar molecular mass, we also feel this further substantiates metallo-non-porphyrin coordination spheres. These pyridinelwater extracts were separated by reversed-phase chromatography (RP-HPLC) into three fractions based on polarity, and the fractions were monitored by element specific detection (GFAA) and UV-Vis spectroscopy (Figure 1).2 Most of the nickel-containing compounds eluted in the most polar fraction. Comparison with model compound retention times indicates these compounds are non-porphyrin bound. Most of the vanadium-containing compounds were found in the moderate-polar fractions. Rapid scan (RS) UV-vis data show a prominent Soret band in this fraction for Boscan and Cerro Negro crude petroleum, indicating at least some of the vanadium is bound as petroporphyrin. This was not the case for Wilmington and Prudhoe Bay moderate-polar fractions, where the RS-UV-vis exhibited little or no Soret data. At this time, we have no adequate model compounds that coelute in this region. The least polar fraction also contains vanadium (up to 30% of total in crude). Model compound retention times show the porphyrins elute in this fraction, but the absence of a prominent Soret band in this fraction for most of the crudes suggests the presence of metallo-non-porphyrin compounds. Some researchers disagree on the existence of the nonporphyrin-bound metals. Recent X-ray absorption spectroscopy (EXAFS/XANES) results from the examination of asphaltenes from Boscan, Cabimas, and Aramco crude oils indicate that only vanadyl porphyrin model compounds fit the fine structure data.5t6 Selected nonporphyrin model compounds did not fit well. Photoelectron spectroscopy (LAMMA) of vanadium-containing compounds in the same asphaltenes show energy levels similar to those of N4-coordinatedmodel compounds.7B It was noted, though, when model compounds with coordination spheres other than N4 were placed in Aramco asphaltenes, the energy level for the V(2 3/2) assumed a value very similar to those of the N,-coor$inated model compounds and that of the vanadium petroporphyrins. Solid-state 34-GHz EPR measurements of a variety of asphaltenes from selected crudes and bitumens show calculated isotropic EPR parameters suggesting various coordination spheresQ The interpretation of these results, based on a statistical model, indicates the vanadium bound only as petroporphyrin. In order to further elucidate the structure of the vanadium-containing molecules in the moderate and low polarity RP-HPLC fractions, we have applied EPR to de( 5 ) Goulon, J.; Esselin, C.; Friant, P.; Berthe, C.; Muller, J.-F.; Poncet, J.-L.; Guilard, R.; Escalier, J.-C.; Neff, B. Collect. Collog. Semin. (Inst.
Fr. Pet.) 1984, 40, 158.
(6) Goulon, J.; Retournard, A.; Friant, P.; Goulon-Ginet, C.; Berthe, C.; Muller, J.-F.; Poncet, J.-L.; Guilard, R.; Escalier, J.-C.; Neff, B. J . Chem. SOC.,Dalton Trans. 1984, 1092. (7) Berthe, C.; Muller, J.-F.;Cagniant, D.; Grimblot, J.; Bonnelle, J.-P. Collect. Collog. Semin. (Inst. Fr. Pet.) 1984, 40, 164. (8) Novelli, J.-M.; Medina, J.-C.; Lena, L.; Maire, Y.; Ruiz, J.-M.; Vincent, E.-J. Collect. Collog. Semin.(Inst.Fr. Pet.) 1984, 40, 169. (9) Malhotra, M.; Buckmaster, H. A. Fuel 1986, 64, 335.
Energy & Fuels, Vol. l , No. l , 1987 37
Nickel
I Il Wilmington Crude E x t r a c l
(9 I conc 1
Nickel
Prudhoe Bay Crude Extract
( 2 7 . 1 conc 1
VIS
Vanadium
0
17.5
35
min
Figure 1. RP-HPLC-GFAA profiles for (a) standards, (b) Wilmington petroleum extract, and (c) Prudhoe Bay petroleum extract: AA(vanadium) a t 318.4 nm and AA(nicke1) a t 232.0 nm. Porphyrin standards were monitored a t 408 nm, and nonporphyrin standards were monitored a t 320 nm.
termine the first coordination sphere around the vanadyl ion. Model compound studies show the isotropic g and a values can reflect the chemical environment about the vanadyl ion when the nephelauxetic series S > N > 0 is considered. From the isotropic parameters of the RPHPLC fraction, the average first coordination sphere around the vanadyl ion can be determined by using the model compound correlation diagrams. Previous studies employing EPR have shown that the existence of coordination spheres other than porphyrin N4 are possible in metallo-non-porphyrin components of crudes and residua. These studies have shown metallonon-porphyrin coordination spheres of NOS2, N3S, S202, N30, and N4.10
gg
38 Energy & Fuels, Vol. 1, No. 1, 1987 Table I: Isotropic EPR Parameters of Selected Model VOL Comoounds Used in Correlation Diaarms no. ah, G gi, 1 etioporphyrin 1.9788 95.20 2 1.9798 95.08 octaethylporphyrin 3 benzotetraaza[ 14lannulene 1.9813 90.17 4 bis(salicyla1dimine) 1.9752 98.20 5 1.9750 100.10 bis(benzoy1acetone)ethylenediimine 6 bis(acety1acetonate) 1.9701 106.73 bis(benzylsalicyla1dimine) 7 1.9746 99.50 8 1.9801 89.00 bis(diethy1dithiocarbamate) 1.9801 89.49 bis(dipheny1dithiocarbamate) 9
A brief discussion of the EPR correlation techniques used to determine the first coordination sphere in this study is presented in the Appendix. These techniques have been known in the literature for several years. The reader is encouraged to review the arguments presented there to understand and appreciate the applicability of these techniques and their limitations. The purpose of the discussion is not to prove or disprove these EPR techniques but to show that they provide useful information, with some caveats. In this paper, we present and discuss the results of applying the EPR correlation techniques to the RP-HPLC fractions of Boscan, Cerro Negro, Wilmington, and Prudhoe Bay crude petroleums, and combine these results with UV-vis and mass spectral results to help determine the structure of the vanadium compounds. Experimental Section The reversed-phase separations were performed by methods and on equipment described previously.'v2 The ODS separations were performed with a solvent gradient consisting of an initial linear ramp from 100% methanol/water (3:l v/v) to 30% THF for 1-3 min and this mixture was held constant for 3-22 min. A second linear ramp was applied from 30% to 70% T H F for 22-24 min, and this mixture was held constant for 1min. A final linear ramp to 100% THF was applied for 25-27 min. The flow rate was constant a t 1.5 mL/min. T h e RP-HPLC fractions were collected as the following: high polar, 0-8 min; moderate polar, 8-24 min; low polar, 24-35 min (Figure 1). The model compounds were synthesized as described in the l i t e r a t ~ r e . ' * ~ ~ ~ ~ - ' ~ The E P R spectra were recorded and analyzed on equipment described e l ~ e w h e r e . ' ~ J T ~ h e spectra were recorded a t room temperature on dilute solutions of the fractions in chloroform or toluene. Because vanadyl (Z = 7/2) has significant second-order Zeeman electronic effects, the E P R parameters had to be calculated from a nonlinear least-squares fitting of the second-order spin Hamiltonian, where
E = (g)PH + (a)mi + ( ( ~ ) ~ / 2 ( g ) P H ) [ z+( z1) + mi21 (g) = isotropic g value; ( a ) = isotropic a value; other symbols have standard meaning. The E P R techniques employed here utilize vanadyl squarepyramidal model compounds to elucidate the first coordination sphere of the vanadium compounds in the reversed-phase fractions. Model compound studies show the isotropic g and a values can reflect the equatorial heteroatom binding around the vanadyl The coordination sphere ion when S, N, and 0 are ~onsidered.'~'~
(IO) Reynolds, J. G.; Biggs, W. R.; Fetzer, J. C. Liq. Fuels Technol.
1985. ., 423. - - -, 3 . .-.
(11) Bo&, R. A.; McCormick, B. J. Znorg. Chem. 1970, 9, 1541. (12) McCormick, B. J. Znorg. Chem. 1968, 7,1965. (13) Boucher, L. J.; Tynan,E. C.; Yen, T. F. Znorg. Chem. 1968,7,731. (14) Reynolds, J. G. Liq. Fuels Technol. 1985, 3, 73. (15) Dickson, F. E.; Kunesh, C. J.; McGinnis, E. L.; Petrakis, L. Anal. Chem. 1972,44,978. (16) Dickson, F. E.; Petrakis, L. Anal. Chem. 1974, 44, 1129. (17) Yen, T. F.; Boucher, L. J.; Dickie, J. P.; Tynan, E. C.; Vaughan, G. B. J.Znst. Pet. 1969,55,87. (18)Yen, T. F. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.; Ann Arbor Science: Ann Arbor, MI, 1975; Chapter 1, p 1. (19) McCormick, B. J.; Bellott, E. M. Znorg. Chem. 1970, 9, 1779.
\' '
0
Reynolds et al.
-
4
2
5
6
10
14
I6
I5
Figure 2. Selected model vanadyl compounds used for E P R parameter correlation studies. Table 11. EPR Parameters and Derived Coordination Spheres for Reversed-Phase Moderate- and Low-Polar Fractions sample ,g ais,,,G coordination Boscan Prudhoe Bay Cerro Negro Wilmington
Moderate-Polar Fraction" 1.9801 95.6 1.9801 94.1 1.9819 94.2 1.9813 94.6
Boscan Prudhoe Bay Cerro Negro
Low-Polar Fractionb 1.9789 95.3 1.9800 93.7 1.9776 94.2
NzS2 NzSz s 4
SP,SBN N4 N4, N 8 NOS2
a 8-24-min fraction from RP-HPLC separations. fraction from RP-HPLC separations.
24-35-min
geometry is assumed to be essentially square pyramidal with the 0 occupying the apex of the pyramid with the vanadium near the center of the basal plane. Correlations are made from parameters obtained from the RP-HPLC fractions to those of the model compounds, indicating certain combinations of heteroatoms in the metal coordination sphere. Several model compounds were used in developing the technique^,'"'^ and selected compounds are shown in Figure 2. These compounds and their isotropic EPR parameters are listed in Table I. In general, the isotropic values of these model compounds are in agreement with literature values with some exceptions and fall in the ranges shown on the correlation charts. The mechanics of applying the techniques are described in the Appendix. Electron-impact mass spectra (EIMS) were obtained on equipment described previously.20 The vanadyl etio- (Etio) and deoxophylloerythroetiopetroporphyrins (DPEP) were observed, when present, around m / e 500. In addition, a plethora of peaks is also evident. This overwhelming continuum was characteristic of all fractions studied and prevented us from using EIMS for any metal characterization other than vanadium petroporphyrin detection. The RS-UV-vis spectra were reported elsewhere.'**
Results and Discussion EPR Parameters of the RP-HPLC-GFAAFractions. Table I1 lista the isotropic EPR parameters calculated from spectra of moderate- and low-polarity fractions from the RP-HPLC separations. The measured parameter values deviate from porphyrin model compound values (Figure 2, Table I; compounds 1 and 2),21t22 to suggest the existence (20) Sundararaman,P.; Gallegos, E. J.; Baker, E. W.; Slayback, J. R. B.; Johnston, M. R. Anal. Chem. 1984,56, 2552. (21) Subramanian, J. In Porphyrin and Metalloporphyrins;Smith, K., Ed.; Elsevier Scientific: Amsterdam, 1976; p 584.
Energy & Fuels, Vol. 1, No. 1, 1987 39
Binding Sites of Vanadium Compounds 1.9663 1.9683
5
'&
1.9703 1.9723
Low -BOP Polar
100.0-
4:
n
g
Cerro NegroLOW Polar VON4
j 90.0 -
Moderalr Boacan Polar Wllmlnglon /Moderate Polar /em Negro \Moderole Polar Prudhoe Bay
:
1.9743
Low Polar
Prudhoe LOW Polar B a y ~ ~ V O ~ ~ d e r a ~ e
Borcan
1.9783
Low Polar Low Polar
1.9803 80.01 1.362
Borcan Moderate Polar
I
I 1.970
1.980
isolroplc g
Figure 3. Isotropic EPR parameter correlation diagram for vanadyl model compounds and RP-HPLC-GFAA fractions (taken after ref 17 and 18). of non-porphyrin vanadium compounds. The high-polarity fractions had very little vanadium and were not examined by EPR.2 Wilmington low-polar fraction was not obtained in large enough quantities to examine. The g values fall into two groups, those above g = 1.9800 and those below. This grouping also corresponds to the polarity of the fraction and shows the more polar the fraction, the greater the possibility of sulfur being incorporated into the coordination sphere. This is consistent with the polarity behavior of the model compounds. We found the S4 model compounds elute in the most polar fraction, while the N4 porphyrins elute in the least polar fractions. However, we cannot discount modifying influences on the polarity of the exogenous ligand structure. First Coordination Sphere Correlations. Figures 3 and 4 show the correlation diagrams used to determine the first coordination sphere of the RP-HPLC separated fractions. Both diagrams are shown for comparison. Figure 3 is of the type most commonly used in the literature and utilizes both isotropic g and a values. Figure 4 uses isotropic g values of known model compounds and average electronegativities to predict g values of coordination spheres that are not yet known synthetically. This diagram conveys information different from that of Figure 3, but assumes that the electronegativity of the first coordination sphere is solely dependent on the electronegativities of the binding atoms and is not affected by exogenous ligand atoms. This assumption has been discussed in the Appendix. Table I1 also lists the first coordination spheres derived from Figure 4. In the cases where two coordination spheres are shown, an unambiguous assignment could not be made from the parameters. Most of the coordination spheres derived in Table I1 contain sulfur. This is not a surprising result because all the crude petroleums are high in sulfur (Boscan has over 5 wt %). Boscan and Prudhoe Bay moderate-polar fractions have parameters indicating N2S2coordination. Because the EPR parameters are marginally close to the values for N4 coordination, vanadium petroporphyrins cannot be excluded. RS-UV-vis data indicate vanadium petroporphyrins are present in the Boscan fraction; however, in minor amounts. Previous determinations have shown that only 25-30% of the vanadium is bound as petroporphyrin in the crude p e t r ~ l e u m . ~ ~RS-UV-vis -~~ data for the (22) More, K. M.; Eaton, S. S.; Eaton, G. R. J . Am. Chem. SOC.1981, 103,1087.
Cerro Negro
\;oderate 2.5
Moderate Polar
Polar
Moderate Polar I
I
3.0
3.5
Electronegativity
Figure 4. Isotropic EPR parameter correlation diagram for vanadyl model compounds and RP-HPLC-GFAA fractions (taken after ref 15 and 16). Prudhoe Bay fraction clearly indicates no vanadium petroporphyrin complexes present.2 The N2S2coordination sphere must be considered with reservation. The correlation for this coordination sphere came from an extrapolation based on elemental electronegativities.16 It is our view that to say these coordination spheres are definitely N2S2would be overinterpretation of the data. We are saying the g values and a values deviate enough to speculate coordination spheres other than N4 porphyrins with the probable inclusion of S. At this time, the EPR techniques employed here are not accurate enough to be interpreted that closely, and therefore other characterization techniques must be coordinated with these studies, for example, UV-vis and EIMS. Cerro Negro and Wilmington moderate-polar fractions have a dominance of sulfur in the coordination spheres. The S3N coordination sphere is not known synthetically and should be considered with the reservations cited above. The S4coordination sphere is modeled by the vanadyl dithiocarbamates (Figure 2; compounds 8 and 9).15 Even though the first coordination sphere is valid for the EPR correlation, the outer ligand environment is probably not representative of sulfur structures in crude oil. Sulfur is more likely to be bound as thiophenic s u l f ~ r . ~The ' outer ligand structure may have little effect on the EPR parameters, but it may affect comparisons based on model compound reversed-phase behavior. We tested vanadyl dialkyl- and diaryldithiocarbamates and found they eluted in the high-polar fraction. This does n6t make them the best chromatographic models but does not invalidate them as EPR models. Recently, bis(ethy1ene) dithiolato complexes of vanadyl and thiovanadyl have been synthesized.% These vanadyl complexes show isotropic EPR parameters in the ranges expected for square-pyramidal S4coordinat i ~ nsupporting ,~~ the validity of the dithiocarbamates as EPR models. (23) Biggs, W. R.; Brown, R. J.; Fetzer, J. C.;Reynolds, J. G. Liq. Fuels Technol. 1985, 3, 397. (24) Spencer, W. A., Galobardes, M. A., Curtis, M. A.; Rogers, L. B. Sep. Sci. Technol. 1982, 17, 797. (25) Sugihara, J. M.; Bean, R. M. J . Chem. Eng. Data 1962, 7, 269. (26) Dean, R. A.; Whitehead, E. V. World Pet. 1963, 34(6), 261. (27) McKay, J. F.; Harnsberger, P. M.; Erickson, R. B.; Cogswell, T. E.; Latham, D. R. Fuel 1981,60, 17. (28) Money, J. K.; Huffman, J. C.; Christou, G. Inorg. Chem. 1985,24, 3297. (29) Christou, G., personal communication; results submitted for publication.
Reynolds et al.
40 Energy &Fuels, Vol. 1, No. 1, 1987
DG1a
12.51I
6.2
5
I 6kie
7ie
mle
50.8
488
461
491
25.0
Figure 5. Electron-impactmass spectra of Cerro Negro moderate-polar fraction (top) and Prudhoe Bay low-polar fraction (bottom).
The RS-UV-vis and EIMS data for the Cerro Negro moderate-polar fraction show some vanadium petroporphyrins. The EIMS data for Cerro Negro moderate-polar fraction is shown in Figure 5. The homologous series
attributed to VO(Etio) and VO(DPEP) are clearly evident. The same is observed for the Wilmington moderate-polar fraction, except the porphyrin spectral features were not as intense. We do not have model compounds with a Soret
Binding Sites of Vanadium Compounds
Energy &Fuels, Vol. 1 , No. 1, 1987 41
band at 408 nm that coelute in the moderate-polar fraction. Table 111. Average First Coordination Spheres Around the There are metalloporphyrins that may have this polarity Vanadium in Selected Crudes and Residua behavior.. Carboxylic acid derivatives of tetraphenyl sample coordination ref porphyrins have been synthesized and exhibit typical EPR 14 Arabian Heavy resins N4, N3S parameters similar to those of alkylpetroporphyrins.22 In Arabian Heavy asphaltenes NOSz 14 addition, several nickel petroporphyrin acids have been Arabian Heavy non-porphyrins N4, NOSz 10 isolated from Messel oil shale.30 These porphyrins have 14 Beta resins N4, NOS2 14 the spectroscopic characteristics of regular petroporphyBeta asphaltenes N4 10 NOS2 Beta non-porphyrins rins, but have polarity properties that are distinctively 14 Kern River resins N4 different from those of the alkylpetroporphyrins, which 14 Kern River asphaltenes NOS2 could make them possibly elute in the moderate-polar 10 Boscan non-porphyrins N4 fraction. Dimers of metallopetroporphyrins have been 14 Maya resins NOS2 suggested from mass spectra and gel permeation chro14 Maya asphaltenes NOS2 10 SzOz, N30, NOS2 Maya non-porphyrins matography of oil shalese31B2 These exhibit Soret ab15 Kuwait resins s 2 0 2 , NZOS sorptions similar to monomeric petroporphyrins, but have 15 Kuwait asphaltenes N4, NOS2 different solubility proper tie^,^^ which might allow them 15 Kuwait aromatics s 4 to elute in the moderate-polar fraction. From the SEC10 Morichal non-porphyrins N4, NOS2 HPLC-GFAA profiles of these extracts, there could be a Mara non-porphyrins NzOS, Sz02, N30, NOSz 16 few compounds of this size (approximately 1100 Da).2 39 N4 Khafji Vacuum residuum 39 Boscan and Prudhoe Bay low-polar fractions have EPR Athabasca bitumen N4 parameters indicating N4coordination spheres. RS-W-vis non-porphyrin metal-containingcompounds in heavy crude data show some vanadium petroporphyrins in the Boscan petroleums and residua. Porphyrins are the first and only fraction, and the porphyrin model compounds elute in this nickel and vanadium compounds so far isolated and fraction. RS-UV-vis data on the Prudhoe Bay fraction identified. Their physicochemical properties, for example, show little or no Soret indicating non-porphyrin N and/or UV-vis spectroscopy and MS, are very distinctive, and are S in the coordination sphere. Figure 5 shows the EIMS usually easily detected. The metallo-non-porphyrin comdata for the Prudhoe Bay low-polar fraction and indicates pounds have been elusive so far. The EIMS and UV-vis that no discernible petroporphyrins were evident. spectra of these RP-HPLC fractions show no discernible Various studies of Boscan crude petroleum revealed the non-porphyrin vanadium species, and as a result, the majority of the vanadium is non-porphyrin b o ~ n d . ~ ~ - ~ ~ compounds remain undifferentiated in the organic matrix. EPR studies on the residual oil after the vanadium petKnowing the implied first coordination sphere gives us roporphyrins were extracted still show N4 coordination.1° direction in the possible identification of metallo-nonThese compounds could have a variety of ligand structures, porphyrin compounds. The predominance of sulfur in with ch10rins~~ and c o r r i n ~(reduced ~~ porphyrins) being these coordination spheres predicts the polarity of the obvious choices (laboratory demetalation experiments show compounds. In our separations, the N4 model complexes these compounds may be formed in the reductive sequence are much less polar than our S4 model complexes. before demetalation is c ~ m p l e t e ) . ~In. ~addition, ~ highly Knowing the implied first coordination sphere also assists conjugated petroporphyrins, such as benzo derivatives or in determining what type of species to look for in the mass RHODO petroporphyrins, also have been suggested.Is spectral identification of the metallo-non-porphyrin comThese highly condensed structures may have physical and pounds. spectroscopic properties not typical of petroporphyrins. Sulfur is present in all these coordination spheres, and Different ring structures such as aza[1 4 ] a n n u l e n e also ~~~ with the exception of NOS2, we have not seen it combined have the non-porphyrin N4 coordination, but with a smaller in this way before. Of the heavy crude petroleums sepacoordination radius. rated by RP-HPLC, Boscan is the only crude examined The Cerro Negro low-polar fraction has EPR paranieters previously by EPR, and the results did not show sulfur. that suggest the NOS2coordination, which is unique to the The sulfur-ligated species may be more labile due to size fractions studied here. RS-UV-vis data show a low-inor solubility and are preferentially extracted. Alternately, tensity 408-nm absorption, but EIMS data indicate no the extraction removes small metal-containing compounds vanadium petroporphyrins. from all molecular size ranges. These small molecules may Comparisons with Other Vanadium-Containing have different polarities when they are bound in the polar Fractions. The results shown in Table I1 add to the list matrix of the crude. of non-porphyrin coordination spheres determined by EPR It is unlikely the vanadium complexes extracted are for metals in heavy crude petroleums, residua, and sepaartifacts of low molecular mass polar compounds that rerated fractions. Table I11 lists some of the correlations combine to chelate the vanadyl ion after extraction. We previously reported in the literature. The results of these have done TLC analyses and have seen similar groups of correlations are important in determining and isolating the vanadium compounds in both the heavy crude petroleums and their RP-HPLC fractions. (30)Ocampo, R.; Callot, H. J.; Albrecht, P. J . Chem. Soc., Chem. Commun. 1985,198. (31)Blumer, M.;Synder, W. D. Chem. Geol. 1967,2,35. (32)Blumer, M.;Rudrum, R. J . Inst. Pet. 1970,56, 99. (33)White, W. I. The Porphyrins; Academic: New York, 1978; Vol. 5,Chapter 7, p 303. (34)Baker, E.W.; Louda, H. W. Adu. Org. Geochem.,Proc. Int. Meet., 10th 1981. (35)Babior, B. M.,Ed. Cobalamin; Wiley: New York, 1975. (36)Agrawal, R.;Wei, J. Ind. Eng. Chem. Process Des. Dew. 1984,23, 505. (37) Kameyama, H.;Yamada, M.; Amano, A. J.Japan Pet. Inst. 1981, 24,317. (38)Burchill, H.;Honeybourne, C. L. Inorg. Synth. 1978,28,44.
Conclusions and Comments We have separated pyridinelwater extracts of selected crude petroleums by RP-HPLC-GFAA chromatography and examined the vanadium compounds in the moderateand low-polar fractions by EPR spectroscopy. By applying model compound EPR correlation techniques, we have found strong evidence for non-porphyrin ligation for the vanadium in heavy crude petroleums. The results found here suggest the metallo-nonporphyrin compounds have a wide variety of structures
42 Energy &Fuels, Vol. 1, No. 1, 1987
Figure 6. Square-pyramidalvanadyl complex.
that are not readily discernible by spectroscopic techniques. As in the case of the porphyrins and biomarkers in general, the metallo-non-porphyrin compounds may be very obvious once fully characterized. (The MS of VO(Etio) is now easily detected by EIMS, although once undifferentiated in the oil matrix.) The above results suggest at least some structural features for the metallonon-porphyrin compounds. (1)The average coordination sphere contains heteroatoms other than nitrogen. This has been proposed in the literature for several years18 and has been also observed in previous EPR and MS studies.10J4-16s39(2) The average coordination sphere appears to be crude dependent. This is also seen for other crude petroleums and fractions (see Tables I1 and III).'0J4-16139 (3) At least some of the metallo-non-porphyrin compounds are small molecules that are influenced by a tertiary matrix. The pyridine/water extraction shows small molecular mass and size components are removed from all molecular size ranges in the crude.lP2 In addition, the polar-separated fractions from these extractions show spectral characteristics that are different from those in the source (for example, the EPR parameters). We are continuing our molecular characterization studies to determine the structures of the highly polar nickel compounds and further our attempts to identify the vanadyl non-porphyrin compounds.
Acknowledgment. We thank Dr. Norman M. Edelstein of Lawrence Berkeley Laboratory for the use of the EPR spectrometer, Mary S. Grady of Chevron Research Co. for useful discussions, and Professor George Christou of Indiana University for the EPR results before publication. The Lawrence Berkeley Laboratory studies were supported by the Assistant Secretary of Fossil Energy, Division of Oil, Gas and Shale Technology, and the Bartlesville Project Office of the U.S. Department of Energy under Contract DE-AC03-76SF00098. Appendix In efforts to identify the elusive non-porphyrin metalcontaining species, several researchers have developed techniques using the isotropic EPR parameters. These techniques utilize vanadyl square-pyramidal model compounds to elucidate the average first coordination sphere of the vanadium-containing compounds in the sample studied. Model compound studies show that the isotropic g and a values may reflect the equatorial heteroatom binding around the vanadyl ion when S, N, and 0 are c~nsidered.'"'~ Figure 3 shows the correlation diagram from the literature utilizing both the g and a values from model comp o u n d ~ . 'The ~ ~ circles ~ represent the range for which both isotropic values vary for a set of model compounds with given coordination spheres. To obtain the coordination (39)Asaoka, S.;Nakata, S.; Shiroto, Y.; Takeuchi, C. Ind. Eng. Chem. Process Des. Deu. 1983, 22, 242.
Reynolds et al. sphere of the vanadium in the actual sample studied, the isotropic EPR parameters are calculated from the raw data, and this point is located on Figure 3. The circle in which the point is located reveals the average coordination of the vanadium-containing compounds in that sample. Other correlation diagrams have been d e ~ e l o p e d . ' ~ ' ~ Figure 4 shows a correlation diagram from the literature that utilizes isotropic g values and the electronegativity of the first coordination sphere.16 The net electronegativity of the first coordination sphere was calculated from the average of the electronegativities of the atoms directly bound to the metal. This allows for the extrapolation of g values from known model compounds (the solid lines), to give g values for coordination spheres that were not synthetically known at the time (the triangles and circles). This diagram is then used in the same manner as Figure 3. Assumptions of the Correlation Techniques. As in most average parameter techniques, some assumptions had to be made during the d e ~ e l o p m e n t . Because ~~ of these assumptions, controversy exists about the interpretation of the results. We present a discussion of some of the assumptions so the reader will be aware of the limitations of the techniques. Assumption 1: The coordination geometry around the vanadium is square pyramidal with the 0 occupying the apex of the pyramid with the vanadium near the center of the basal plane. This coordination is seen in Figure 6. Depending upon the substitutions in the corners of the equatorial plane, the complex can have as high as idealized CZuor C4?symmetry. This can be important in the interpretation of the spectroscopic properties of the compounds. Coordination is also possible in the vacant axial position, which also can affect spectroscopic properties (see Assumption 2). Several single-crystal X-ray structures show the predominant coordination geometry of the vanadyl complexes is square pyramidal with idealized CZVor C4usymmetry in the first coordination sphere. This is explained by the Ballhausen-Gray energy level schemea4' S4,4204, 43-45 N202,46and SZO2l1model compounds generally fit this electronic scheme well. The correlation techniques were developed by using these compounds. There are some structural exceptions to these symmetries observed in the solid-state geometry that also exhibit deviations in the electronic and EPR parameters from those expected from the idealized symmetries. The crystal and molecular structure of bis(2-methyl-8quinolinolato)oxovanadium(IV),47 structure 10, exhibits a trigonal-bipyramidal geometry. The isotropic g and a values in toluene solution are 1.985 and 89.8 G respectively, which are substantially different from those predicted by the correlation diagrams used in this study (1.975 and 90 G). A phenomemological crystal field model has been d e v e l ~ p e dwhich , ~ ~ explains the structural symmetry as distorted C2",and the Ballhausen-Gray model is adequate Shenkin, P. S. Liq. Fuels Technol. 1984,2, 233. Ballhausen, C. J.; Gray, H. B. Inorg. Chem. 1962,1, 111. Henrick, K.; Raston, C. L.; White, A. H. J. Chem. Soc., Dalton Trans. 1976, 26. (43) Cooper, S. R.; Koh, Y. B.; Raymond, K. N. J. Am. Chem. SOC. 1982, 104, 5092. (44)Dodge. R. P.: Temuleton, D. H.: Zalkin, A. J. Chem. Phys. 1961, 35,55. (45)Hon, P.-K.; Belford, R. L.; Pfluger, C. E. J. Chem. Phys. 1965,43, 1323. (46) Boucher, L. J.; Bruins, D. E.; Yen, T. F.; Weaver, D. L. J. Chem. Soc., Chem. Commun. 1969, 363. (47) Shiro,M.; Fernando, Q.J.Chem. Soc., Chem. Commun.1971,63. (48)Stoklosa, H. J.; Wasson, J. R.; McCormick, B. J. Inorg. Chem. 1974, 13, 592.
Binding Sites of Vanadium Compounds except for splitting of the d,, and dyzlevels. This deviation from idealized symmetry causes enough distortion to account partially for the anomalous isotropic g and a values of the complex.4g These complexes should show an orthorhombic instead of an axial frozen-solution EPR spectrum, if the distortion is large enough. However, this is not always observed. In addition, there are some tridentate Schiff base ligands that exhibit pseudo-square-pyramidal coordination, but because of the tris-chelating nature of the ligand, these are thought to be coordinated only in three sites on the equatorial EPR parameters have not been reported on these complexes, but this ligand conformation could easily distort the idealized square-pyramidal symmetry required for the correlation diagrams and also give anomalous g and a values. This type of tris chelation is conceivable in asphaltic type compounds found in the polar portion of heavy crude petroleums.ls In addition to the above structural examples, it should be noted that there is controversy whether all the vanadium in crude petroleums is found as V=02+. Patent claims53 and early e ~ p e r i m e n t s ~utilized ~ p ~ ~ EPR as a technique for determining total vanadium content in crude oils. This technique necessitated the vanadium to be all V=02+. Several crude oils high in vanadium have been examined and the concentrations of vanadium determined by EPR are in good agreement with the values determined by other methods. The technique has probably not been utilized more because of the advent of several easy and cost-effective metal analytical techniques. Some researchers, though, have observed that in the analysis of certain asphaltenes, not all the vanadium is accounted for by various spectroscopic techniques which detect specifically V(IV).5v6924This has been rationalized either by (1) the vanadium not being in the IV state or by (2) microinhomogeneity. It is important to note; however, matrix effects are important in EPR correlation times and therefore in the quantitation of vanadium in V=02+ c o m p l e x e ~ . ~These ~ - ~ ~effects have been used to explain EPR-silent V=02+ in extracts of Antrim oil shale.69 Assumption 2: Each vanadium is square pyramidal without strong axial coordination. This coordination primarily affects the isotropic a values but can, in some cases, affect the g values.60 Some vanadyl compounds complex with strong bases;61for example, (diethyldithiocarbamato)oxovanadium(IV),structure 8, changes from an isotropic g value of 1.980 in benzene to 1.975 in Me2S0.15 This is important to consider for model compounds, because it is often difficult to find EPR data measured in common solvents in the literature. To realistically correlate the coordination spheres in the crude oil, the solvent system for both the crude and the model compounds should be similar and noncoordinating. Values of model (49) Boucher, L. J.; Yen, T. F. Inorg. Chem. 1968, 7, 2665. (50) Kulkarni, V. H.; Patil, B. R.; Prabhakar, B. K. Curr. Sci. 1981, 50, 585. (51) Syamal, S.; Kale, K. S. Inorg. Chem. 1979, 18, 992. (52) Seangprasertkij, R.; Riechel, T. L. Inorg. Chem. 1985,24,1115. (53) Saraceno, A. J. U.S. Patent 3087888,1963 (reissued 1967 as U S . Patent 0026312). (54) Saraceno, A. J.; Fanale, D. T.; Coggeshall, N. D. Anal. Chem. 1961, 33, 500. (55) Roberta, E. M.; Rutlidge, R. L.; Wehner, A. P. Anal. Chem. 1961, 33, 1879. (56) Wilson, R.; Kivelson, D. J. Chem. Phys. 1966, 44, 154. (57) Bruno, G.V.; Harrington, J. K.; Eastman, M. P. J.Phys. Chem. 1977.81. 1111. (58) Campbell, R. F.; Freed, J. H. J . Phys. Chem. 1980, 84, 2668. Faraday Trans. (59) Garrett, B. B.; Gulick, W. M., Jr. J.Chem. SOC., I 1983. 79.1733. (60)Boucher, L. J.; Yen, T. F. Inorg. Chem. 1969,8, 689. (61) Vigee, G.S.; Watkins, C. L. J. Inorg. Nucl. Chem. 1972,34,3936.
Energy &Fuels, Vol. 1, No. 1, 1987 43 compounds measured in water solutions will be difficult to utilize. Conceivably, the environment of the vanadium in carbonaceous materials will also have potential axial donor molecules. Coordinating bases such as quinolines and carbazoles have been found in the nondistillable portion of several crude oils.62 Depending upon the strength of interaction, this type of coordination could effect the interpretation of the g and a values for the oil fraction. Assumption 3 T h e ligand structure of the atoms directly bonded to the metal has no effect on the isotropic EPR parameters. This holds true for several vanadyl coordination spheres. For example, vanadyl dithiocarbamates,12and vanadyl dithiolates,%.%structures 8,9, and 11, have ligand structures that are vastly different and carbon-sulfur bonding that is electronically different. However, the isotropic EPR parameters are the same when measured in similar solvents. The same is true for compounds such as the vanadyl acetylacetonatesmand vanadyl
cat echo late^.^^^^^ Vanadyl porphyrins, structures 1 and 2, and vanadylTADA complexes, structures 3,12, and 13, are an exception to this. Vanadyl-TADA complexes have a [14]annulene system as opposed to the [16]annulene system of the porphyrins. The EPR parameter values are a matter of debate, but appear not to be the same as those for the porphyrins. Structures 3, 12, and 13 all have TADA ligands, but do not have isotropic g values near those of the porphyrins (3, 1.989; 12, 1.989; 13, 1.973).65966We measured the isotropic g value for 3 to be 1.9813 (see above). None of these values would be interpreted to be four-nitrogen coordination by the correlation diagrams employed by the techniques discussed. This disagreement could be due to atoms outside the first coordination sphere. Although not reported in detail, t,he structural data for 13 shows a localized ClU but the balance of the ligand does not have the planar characteristics in the meso position that the porphyrins have. In addition to the TADA complexes, there are other examples where atoms in the outer ligand structure also affect the isotropic EPR parameters. Phosphorus hyperfine coupling is seen in dithiophosphato complexes of V=02+.68 This is probably not pertinent in these studies because phosphorous is seldom found in crude oils in any concentration. Assumption 4: T h e parameters for intermediate coordination spheres are a n average of parameters of the pure coordination spheres. (This assumption applies to Figure 4, which utilizes the electronegativity and isotropic g values of the model comp~unds.'~J~) Several model complexes follow this behavior. Schiff base compounds, with N2O2coordination geometry, have measured g values of 1.973-1.97513y4gthat match the parameters derived from statistical averages of N4 (1.979-1.980) and O4 (1.969-1.970) coordination spheres. The same behavior holds true for the monothio-/3-diketone S202parameters also." There are some compounds that appear to violate this assumption. These compounds, however, are not charac(62) McKay, J. F.; Harnsberger, P. M.; Erickson, R. B.; Cogswell, T. E.: Latham. D. R. Fuel 1981. 60. 17. '(63) Walker, F. A.; Carlin,' R.'L.;Rieger, P. H. J. Chem. Phys. 1966, 45, 4181. (64) Cass, M. E.; Greene, D. L.; Buchanan, R. M.; Pierpont, C. G.J. Am. Chem. SOC. 1983,105, 2680. (65) Goedken, V. L.; Ladd, J. A. J.Chem. SOC., Chem. Commun. 1981, 910. (66) Sakata, K.; Hashimoto, M.; Tagami, N.; Marakami, Y. Bull. Chem. SOC.Jpn. 1980,53, 2262. (67) Ladd, J. A. PhD Thesis, Florida State University, 1982. (68) Wasson, J. R. Inorg. Chem. 1971, 10, 1531.
44 Energy &Fuels, Vol. 1, No. 1, 1987
Reynolds et al.
terized by X-ray diffraction studies, so their true structural the charge distribution should highly favor the mercapto geometry is not known. Monocoordinated catechol comsulfur. Such distortions are probably similar to those plexes, with bipyridine as a coordinating base giving N202 found in the hydroxyquinoline complex,47structure 10, coordination spheres, have been recently s y n t h e ~ i z e d . ~ ~ therefore suggesting similar deviations in the g and a Although no single-crystal X-ray structures have been values. We are currently examining this in more detail by done, the complexes probably have asymmetric equatorial X-ray ~rystallography,~~ bonding due to ligand design, as in structure 14. These Assumption 5 Each vanadium is bound as V=O. No complexes show isotropic EPR g and a values of 1.994 and vanadium is bound as V=S. The softer sulfur ligand 78 G , respectively, far from the values of the model comshifts the EPR parameters away from any isotropic values pounds sited above for N202coordination. ever observed in crude oils and residua, to values similar A bis(cysteine methyl ester)oxovanadium(IV) complex ~~~~~~~~ to those for O4 and N202vanadyl c o m p l e ~ e s .Alhas been synthesized, and the elemental and spectrophothough this may not affect correlations with crude oils, it tometric characterization indicates a four-coordinate vacould be significant in other carbonaceous materials like nadyl complex with the equatorial ligand coordination to humic acids, where the first coordination sphere around be S2Nz. EPR parameters do not match those suggested the vanadium is found to be 04.73 by the correlation diagram~,'"'~ where the measured isoIn addition to the assumptions listed above, it must be tropic g and a values are 1.990 and 79.2 G , respe~tively.~~ mentioned that the EPR techniques are average parameter Because no magnetic measurements were made to check techniques. An informative review of the limitations of for vanadium-vanadium interactions and no single-crystal average parameter techniques has been p ~ b l i s h e d . ~ In~ structure is available, the true coordination geometry is terpretation of the results, as with any average parameter not known. The values were also measured in an untechnique, must be taken with reservation. Oil fractions specified solvent, probably buffered water. are probably mixtures of more than one type of metalIn our efforts to synthesize model compounds with an containing species. Unless the isotropic parameters are S2N2coordination sphere, we have found parameters insufficiently different (for example, O4and S4),complete consistent with the correlation diagram^.^' We have resolution of a mixture of coordination spheres is impossynthesized a vanadyl derivative of 2-mercaptopyridine, sible under the applied experimental conditions (for exproposed structures 15 or 16. The ligand was selected ample, N4 and S4). This ambiguity stresses the need for applying more than one technique for characterization, for because the nitrogen is in an environment that may be more representative of crude oils. The ligand field should example, the use of UV-vis and mass spectroscopic techbe distorted from the square-pyramidal geometry, because niques. (69) Galeffi, B.; Poetel, M. Nouu. J. Chim. 1984,8, 481. (70) Sakurai, H.: Hamada,. Y.:. Shimomura. S.: Yamahita. S. Inorg. Chim. Acta 1980,46, L119. (71) Gallegos, E. J.; Reynolds, J. G.; Fish, R. H., manuscript in preparation.
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(72) Poncet, J.-L.; Guilard, R. Polyhedron 1983, 2, 417. (73) Templeton, G. D., III; Chasteen, N. D. Geochim. Cosmochim. Acta 1980, 44, 741. (74) Boduszynski, M. M. Li9. Fuels Technol. 1984,2, 211.
