Energy & Fuels 1993, 7, 179-184
179
Nickel and Vanadyl Porphyrins in Saudi Arabian Crude Oils Mohammad F. Ali,* H. Perzanowski, A. Bukhari, and Adnan A. Al-Haji Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received September 10, 1992. Revised Manuscript Received December 21, 1992
This study reports the isolation and characterization of nickel and vanadyl porphyrins in the residue (535 "C+) and asphaltenes from Arabian Heavy crude oil. The residue was obtained by distillation of crude oil, whereas the asphaltenes were precipitated by n-heptane from the crude oil. Metalloporphyrins were first extracted from the residue and the asphaltenes. The nickel porphyrins were separated from vanadyl porphyrins by means of adsorption chromatography on silica gel and alumina using solvents of increasing polarity. The chromatographic separation process was monitored by ultraviolet-visible spectrophotometry. Both nickel and vanadyl porphyrins were identified and characterized by ultraviolet-visible spectrophotometry and mass spectrometry.
Introduction Porphyrins and metalloporphyrins have been noted in various branches of science and industry. Their presence in crude oil is important not only from the scientific point of view but also from both economic and environmental standpoints. The identification of metalloporphyrins in crude oils and source rock is of fundamental interest in understanding the geochemical origins of petroleum sources, the diagenetic and the catagenetic pathways in the oil formation, and maturation, correlation, depositional, and environmental reconstruction studies. There are, however, several disadvantages resulting from the presence of these metalloporphyrins in petroleum. As is known, some heavy petroleum oils contain large quantities of metals, mainly vanadium and nickel, which are complexed in porphyrins. This causes considerable difficulty in hydrocatalytic upgrading of such heavy oils. Therefore, considerable attention has been focused on vanadyl and nickel porphyrins in crude oils to understand their nature and effects. Metalloporphyrins were the first compounds isolated from petroleum claimed to be of conclusive biological origins. Treibs discovered in 1934 that a wide variety of petroleum and bitumen contain porphyrin.' Since his discovery, researchers continued to search for these complex compounds in a variety of samples of different geological origins. In the early studies of porphyrins in the geosphere, the complexity of these pigments was not understood, but with the advent of mass spectrometry, electronic absorption spectrometry, infrared spectroscopy, nuclear magnetic resonance, and electron spin resonance spectroscopy, much of the complex heterogeneity of porphyrins has been resolved. Five main types of porphyrins along with their homologues have been reported in the literature to be present in petroleum and bitumen. These are etioporphyrins (Etio), deoxophylleoerythroetioporphyrin (DPEP), tetrahydrobenzo DPEP, and benzo (rhodo) Etio and benzo (rhodo) DPEP. Baker and Louda2 have elucidated the different types of the porphyrins and metalloporphyrins (1) Treibs, A. Ann. Chem. 1934, 500, 42-62.
0887-0624/93/2507-0179$04.00/0
found in the geosphere along with their structure and precursors. Chicareli et alS3have summarized the occurrence of sedimentary porphyrins whose structures have been fully or partially established. Types of these porphyrins and their structures are also listed. The porphyrins in petroleum are present predominantly as vanadyl and nickel chelated compounds. These compounds are very similar in many of their physical properties to the material with which they are associated. There is no single procedure for the isolation of porphyrin complexes from their host materials. Many isolation and purification methods are reported in the literature. Generally, the methods can be classified into three types: acid demetallation, liquid extraction, and chromatography. Quirke4 has reviewed those methods that have general application to the analysis of the geoporphyrin mixtures. Baker et aL5 have given a good summary of the methods of isolation, fractionation, and purification of the geoporphyrins. They also provided a comparison of these methods, their field of application, and efficiency. Rankel studied the decomposition of the Arabian Heavy crude The total petroporphyrins were extracted by demetallation with methanesulfonic acid from the Arabian Heavy crude oil before and after it was subjected to the processing conditions of heat, air, hydrogen, and hydrogen sulfide. The extracted petroporphyrins were measured spectrophotometrically. It was found that more than 90 ?4 of the petroporphyrins from Arabian Heavy crude oils decompose to polypyrrolics in 24 h at 240 "C, and more than 95% is degraded on Vz05 catalyst in l/2 h. It was concluded that petroporphyrins decompose if subjected ~
~~
(2) Baker, E.;Louda, J. W. In Biological Markers in the Sedimentary Record; John, R. B., Ed.; Elsevier: Amsterdam, 1986; pp 125-225. (3) Chicarelli, M. I.; Kaur, S.;Maxwell, J. R. In Metal Complexes in FossilFuels; Filby, R. H., Branthaver,J. F. Eds.;ACS Symposium Series 344; American Chemical Society: Washington DC, 1987; pp 40-67. (4) Quirke,J. M. E. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F. Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 74-83. (5) Baker, E. W.; Palmer, S. E. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 1, pp 485-550. (6)Rankel, L. A. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver,J. F., Eds.; ACS Symposium Series 344;American Chemical Society: Washington, DC, 1987; pp 257-264.
0 1993 American Chemical Society
Ali et al.
180 Energy & Fuels, Vol. 7, No. 2, 1993
to thermal treatment a n d / o r a mixture of reactive gases, viz., H2 and H2S. Hajlbrahim studied the HPLC fingerprinting of freebase petroporphyrins of different crudes7including Saudi Arabian Safaniya crude. He was able to detect up to 17 different peaks. He did not provide identification of the metalloporphyrins present in the Arabian Heavy Oil. This s t u d y reports the isolation and characterization of nickel and vanadyl porphyrins in the distillate residue (535 "C+) and asphaltenes from Arabian Heavy crude oil. The residue was obtained by distillation of crude oil, whereas the asphaltenes were precipitated b y n-heptane from the crude oil.
Experimental Section The crude oil was obtained from Saudi Arabian American Oil Co. The apparatus used for crude fractionation was Semi-Cal Series 3650 made by Podbielniak Inc. IL. The distillates boiling up to 370 "C were removed by atmospheric and vacuum distillation. The residue boiling above 535 "C was obtained by passing the 370 T+residue through a wiped-film, molecular still. The temperature during distillation was kept below 300 "C, and thermal exposure was minimized by a very short residence time in the heated zone. No decomposition was detected during the distillation. Asphaltenes from Arabian Heavy crude oil were precipitated by the addition of 20 volumes of n-heptane to the crude oil. The mixture wm stirred, allowed tostand for 16h, and thencentrifuged at 3500 rpm for 60 min. The supernatant liquid was decanted and fresh n-heptane was added to the residue. The mixture was stirred and again centrifuged. The asphaltenes were collected from the bottom of the centrifuging tube, washed with n-heptane, dried in a vacuum oven a t 60 "C, and weighed as impure asphaltenes. These asphaltenes were purified by Soxhlet extraction with n-heptane for 24 h, and then dried in a vacuum oven a t 60 " C . The dried asphaltenes were weighed. The elemental analysis of the carbon, hydrogen, nitrogen, and sulfur were done on a Carlo Erba elemental analyzer, Model 1106. The vanadium and nickel content of the samples were analyzed by X-ray fluorescence, XRF (Kevex 700). The samples were dissolved in toluene and then charged to the XRF auto sampler. The data were acquired and reduced by the computer software, Kevex Tool Box. All solvents used were HPLC grade solvents. Conventional silica gel column chromatography was performed using Fischer silica gel 80-200 mesh that was activated a t 250 "C for 24 h. Neutral alumina, 80-200 mesh, purchased from Bio-Rad was used for alumina column chromatography. The two-stage separation schemes for residue and asphaltenes are shown in Figures 2 and 3, respectively. In scheme 1 (Figure 2), the residue (6.0 g) was dissolved in toluene (10 mL) and extracted with anhydrous methanol by a previously reported procedure.* The methanol extracted residue was eluted from a silica gel column with cyclohexane, benzeneicyclohexane (1:4 and 3:7), benzene, and methanol. UV-visibleabsorption benzeneiethyl acetate (l:l), spectroscopy was used for the determination of nickel and vanadyl porphyrins in the fractions. The fractions containing vanadyl porphyrins were combined and further purified by chromatography on an a!umina column. In scheme 2 (Figure 3 ) , the asphaltene sample (10 g) was extracted first with benzeneiacetonitrile solvent and the combined extracts were eluted from a silica gel column with cyclohexane, benzene/cyclohexane (1:4,3:7, and L l ) , benzene, ethyl acetate1 benzene (l:l),ethyl acetate, and methanol. Nickel and vanadyl porphyrins in the fractions were monitored by UV-visible absorption spectroscopy. The fractions containing vanadyl (7) HajIbrahim, S.K. Int.J. Enuiron. Anal. Chem. 1982,12,123-130. (8) Chakarhorty: Bhatia, V. K. Indian J. Technol. 1981,92-99. ---I
Table I. General Properties of Crude Oils' method AM (ASTM no.) AEXL AL test gravity, OAPI D 287 38.7 34.0 30.5 20 D 482 44 58 ash, ppm -17.8 -15.0 -12.2 pour pt, " C D 97 trace trace trace sediment and water D 96 1.10 sulfur, wt % D 129 1.81 2.59 2.0 C residue, wt % D 189 3.58 5.87 3 15 28 vanadium, ppm 1 3 8 nickel, ppm 0.04 0.10 0.12 nitrogen, wt %
AH 28.1 110 -10 trace 3.35 7.53 69 21 0.16
"AEXL = Arabian Extra Light, AL = Arabian Light, AM = Arabian Medium, AH = Arabian Heavy. porphyrins were combined and further purified by chromatography on an alumina column. The high performance liquid chromatography (HPLC) instrument used in this study was Waters Associate, Milford, MA. The instrument is equipped with two pumps (Waters 501), programmatic variable multiwavelength detector (Waters 490), and a data station (Digital, Professional 380). Solvent systems that were used were isocratic or gradient. The columns were 5-pm (25 cm x 0.46 cm) CIS(Waters Assoc.). The samples were dissolved in methylene chloride and loaded to the injection loop by means of a 1O-lrL syringe. A 2-3-wL sample was injected. Detection (UV-vis) was monitored a t 405 nm. For thin layer chromatography, a multichannel silica gel analytical plate was used. The plate was activated a t 110 "Cfor half an hour in an oven. The sample of porphyrins was dissolved in a minimum amount of methylene chloride and charged to the plate by means of a capillary tube. The plate was allowed to dry and then developed in a jar containing 1:l decane/chloroform until the solvent front was 3 cm from the top end of the plate. The bands observed were scraped and extracted in methylene chloride and analyzed by UV-vis spectrophotometer. The ultraviolet-visible spectrophotometric analyses were performed on a double beam spectrophotometer, Cary (Varian, Model 2390) using a 1-cm cuvette and methylene chloride as a solvent. The samples were scanned from 700 to 350 nm. The mass spectrawere collected with a Jeolspectrometer (JMSDX 300) and a data system (JMA-DA 5000). A direct probe was used to charge the sample into the mass spectrometer. The samples were dissolved in methylene chloride and charged into glass cups with a microsyringe; the solvent was evaporated and then probed and scanned under the same conditions. A temperature program of 70-350 "C at 16 "C/min was used. A low voltage of 18 eV and a current of 4 pA were used. The mass spectra were measured over a range of scans to take into account the different volatility of the porphyrin complexes.
Results and Discussion Arabian Heavy (AH) crude oil used in this study was obtained from Saudi Aramco. It was crude oil from an offshore field, Safaniya, the world's largest offshore petroleum field, which is located about 125miles northwest of the exporting terminal Ras Tanura. Oil production f r o m Safaniya is from Cretaceous age of Wasia sandstone. Other than AH, S a u d i Aramco produces three other marketable qualities crude oils, namely: Arabian medium (AM), Arabian light (AL), and Arabian extra light (AEL). The four Arabian crude oils represent a wide range of crude oil types. The bulk properties of the four crude oils are tabulated in Table I. Specificgravity, viscosity, carbon residue, ash content, and sulfur, nitrogen, and metal content show a gradual increase f r o m Arabian extralight to Arabian heavy crude oils. Nickel and vanadium content are generally low for the four crude oils. Arabian Heavy
Energy & Fuels, Vol. 7,No. 2, 1993 181
Porphyrins in Saudi Arabian Crude Oils Table 11. Physical Properties and Elemental Analysis Arab Heavy crude vanadium, ppm nickel, ppm c,wt % H,wt % N, P P ~ s,wt % C residue, w t % API (60/60 O F ) viscosity, CS pour pt, O C
69 21 83.3 12.0 1600 3.35 7.53 28.1 21.5 (40 "C) -10
distillate 3 85.2 12.0 1000 3.38 12.36 18.6 29.5 (100 "C)
18
residue 189 62 85.3 10.2 4600 5.50 13.91 3.2
mphaltenes 710 195 81.9 7.4 9500 7.61
I
I
Dlrtlll8tm
R8SldU~(535°C+)
I bkthLal Extractlon Silk 0.1
Columl
(1:4)
70
Benzene/Ethyl8cetate
IAlumk
Column
Methylenechlorlde Y8thYl~n~ChlOrld~/n-H8~8~8
M8thyl8n8chlorld8/n-Hexana
Vanidyl
Figure 1. UV-vis spectrum of (a) Arab Heavy crude and (b) Arab Heavy residue.
has the highest content of nickel and vanadium and was selected for the present study. The physical and the chemical properties of the AH crude oil, 370-535 "C distillate, the 535 "C+ residue, and asphaltenes from AH crude oil are shown in Table 11. X-ray fluorescence analysis of the nickel and vanadium contents indicated that Arabian Heavy crude oil contained vanadium (69 ppm) and nickel (21 ppm), its distillate had an undetectable amount of nickel and only 3 ppm of vanadium, and the residue cut contained 189 ppm of vanadium and 62 ppm of nickel. The asphaltenes contained 710 ppm of vanadium and 195 ppm of nickel. On the basis of these results, the Arabian Heavy crude residue and asphaltenes were chosen for this study. Preliminary UV-vis spectrophotometric analysis of Arabian Heavy crude oil and its residue showed no characteristic absorption peaks for nickel or vansdyl porphyrins (Figure 1). This is due to the nature of the matrix of the sample that mask the absorption of the metalloporphyrins. Therefore, it was thought necessary to separate metalloporphyrins from the residue and asphaltenes matrix before they were analyzed. The chromatographic separation of vanadyl and nickel porphyrins from crude oil residue and asphaltene was attempted using a number of methods described in the literature. These methods of separation gave unsatisfactory results. Because of these unsatisfactory results and the failure to detect any of the nickel porphyrins in the sample, two new schemeswere developed for the separation of nickel and vanadyl porphyrins from the crude oil residue (scheme 1)andcrude asphaltenes (scheme 2). The solvent sequences for these schemes are shown in Figures 2 and 3. In scheme 1, the methanol-extracted 535 OC+ residues were eluted from a silica gel column with cyclohexane, benzene/cyclohexane(1:4and 3:7), benzene, benzenelethyl acetate (l:l),and methanol. The weight percentages of these fractions, calculated with respect to the residue sample, were 48, 2, 5, 8, and 3, respectively.
Porphyrins
Figure 2. Scheme no. 1: Separation of nickel and vanadyl porphyrins from Arab Heavy crude oil residue. Arabl8n IiMbY Crud8 011
Mdtener
AcetonltrlImIBenrene Extr8ctlon
I
Cyclohexane BenzendCyclohex8ne (1:4)
I
I
II
I
Benzene BenzenelCyclohexane (1:l)
I
I
Bszenm/Ethyl8cet8ta (1:1) Methlnol Ethylacetate
Vanidyl
Porphyrins
Figure 3. Scheme no. 2: Separation of nickel and vanadyl porphyrins from Arab Heavy crude oil asphaltenes.
UV-vis analysis of the above fractions showed no absorption peaks for the 1:4 benzenelcyclohexane fraction. A weak absorption band at 550 nm that is characteristic of nickel porphyrins was detected in the 3:7 benzene1 cyclohexane fraction. UV-vis absorption bands at 405, 530, 570 and 590 nm were detected for the last three fractions, benzene, 1:l benzenelethyl acetate, and methanol. This indicated that the third fraction contained only nickel porphyrins while the last three fractions contained vanadyl porphyrins. The fraction containing the nickel porphyrins was subjected to thin layer chromatography purification on silica gel plate developed in 1/1acetoneln-heptane. Three
Ali et al.
182 Energy &Fuels, Vol. 7,No. 2, 1993
bands were detected: top, middle, and bottom, had Rf values of 0.46-0.69,0.31-0.46, and 0.023-0.31. Each was scraped off and extracted in methylene chloride and then analyzed by UV-vis spectrophotometer. The three TLC fractions showed no absorption bands for nickel porphyrins, Chakraborty and Bhatias have noticed this phenomenon and attributed it to the lability of nickel porphyrins as compared to the vanadyl porphyrins. They explained that the nickel porphyrins decomposed on silica gel. The last three fractions (benzene, 1:l ethyl acetate/ benzene, and methanol) were combined and further purifed by chromatography on an alumina (80-200 mesh) column. The column was eluted with 1:19 cyclohexane/benzene, 1:lmethylene chlorideln-hexane, and methylene chloride. The calculated weight percentage yields on residue were 3 % , 0.5%, and 2%. The second fraction (1:l methylene chlorideln-hexane) was of a red color and gave wellresolved UV-vis absorption peaks at 405,530, and 570 nm and a shoulder at 590 nm, which were characteristic of vanadyl porphyrins. The XRF analysis showed 534 ppm vanadium and traces of nickel. The other two fractions showed no porphyrinic absorption bands. The isolated vanadyl porphyrins were subjected to a thin layer chromatographic study. The sample was applied as a methylene chloride solution to an analytical silica gel plate, which was activated at 110 "C for half an hour and developed in chloroform/decane (1:l). Five bands were observed. One of these had a red color, characteristic of vanadyl porphyrins, and had an Rfof 0.54-0.69. This band when removed and extracted with methylene chloride showed UV-vis absorption peaks at 405,530, and 570 nm and a sh'oulder at 590 nm. The other bands had Rf values of 0.00-0.64, 0.69-0.84, 0.84-0.89, and 0.89-0.94, respectively, and did not show any porphyrinic absorption. In scheme 2, asphaltenes (10 g) previously precipitated by n-heptane from Arabian Heavy crude oil was extracted in Soxhlet by solvents of successivelyincreasing proportion of benzene in acetontrile. Each extraction was continued until the extracting solvent remained colorless. The extraction process was monitored by UV-vis spectrophotometer, and it was discontinued when the UV/vis absorption band at 405 nm had diminished. All of the extracts (3:17, 6:17, 12:17, 1:1, and 2:l viv of benzene/ acetonitrile) showed characteristic porphyrins UV-vis absorptions at 405, 530, 570, and 590 nm. Since both nickel and vanadium were found in each extract and the concentration of vanadium was much higher than that of nickel as observed by XRF analysis, the fractions were combined. The combined asphaltene extracts were eluted from a silica gel column with cyclohexane, benzene/cyclohexane (1:4, 3:7, and l:l), benzene, ethyl acetate/benzene (l:l), ethyl acetate, and methanol to produce 3.21 %,0.70%,0.27% ,0.42 %,0.585% , 3.776, 0.3876, 0.1996, and 0.23% with respect to the asphaltenes sample (see scheme 2). No UV-vis absorption was observed for the first four fractions from the relatively low polar solvents. The last four eluted fractions obtained from relatively more polar solvents showed UV-vis absorption at 405,530,570, and 590 nm that was attributed to vanadyl porphyrins. For further purification, the last four fractions were combined,adsorbed on alumina (80-200 mesh) and packed on top of a glass column that was already prepacked with fresh alumina. The sample was then eluted with successive
~1
OA0.006 T'
4
0.002
0.001 0.000 I
I
0
10
20
30
40
50 MINUTES
Figure 4. HPLC of vanadyl porphyrins from AH residue, isocratic elution: MeOH, 2C18 column (25 cm X 0.46 mm).
solvents of increasing polarity. The fraction eluted from the column with 19:l benzene/cyclohexane, 0.070 % with respect to asphaltenes, did not show any porphyrins UVvis absorption. The fractions eluted with methylene chloridein-hexane (3:7) and withmethylene chloride (0.20 and 0.07 wt 5% of the asphaltenes sample, respectively) were red in color and had intense UV-vis absorption bands at 405,530, and 570 nm and a shoulder at 590 nm. These fractions, therefore, were combined for further TLC purification and identification studies. The vanadyl porphyrins sample was applied to a thin layer silica gel analytical plate and developed in decane/ chloroform (1:l). Four bands were observed. The Rfvalues of which were 0.00-0.48, 0.48-0.63, 0.63-0.85, and 0.850.91. The band whose Rf0.48-0.63 was red in color. This layer was removed and extracted in methylene chloride. The UV-vis analysis of which showed very resolved peaks at 405, 530, and 570 nm and a shoulder at 590 nm. The model compound vanadyl (IV) etioporphyrin 111 (STREM Chemicals, U.S.A.) was subjected to a thin layer chromatographic analysis under similar conditions. This compound had an Rf value of 0.59. It can be noted that the Rf value (0.59)of the model compound was within the Rf value of the red band observed for the vanadyl porphyrins separated from the Arabian Heavy asphaltenes (0.48-0.69) and the residue (0.54-0.69). It may be concluded that the vanadyl compounds present in Arabian Heavy samples are not a single type of compounds. The HPLC behavior of the vanadyl porphyrins isolated from the residue of Arabian Heavy crude can be seen in Figure 4. Two main peaks, a small peak, and a shoulder were detected at 10, 12, 17, and 9 min, respectively. On the basis of the retention volume of the standard compound measured under similar conditions, the residue vanadyl porphyrins may contain C32 Etio (111)vanadyl porphyrins as one of its components. The HPLC profile of the vanadyl porphyrins separated from the residue was different from the HPLC profile of the vanadyl porphyrins separated from asphaltenes (see Figures 4 and 5). This suggested that they were either different types of vanadyl porphyrins or the relative amount of each type was different in the residue and the asphaltenes. Mass spectral analysis, to be discussed later, showed that the residue and asphaltenes contained the same types of vanadyl porphyrins but the relative amount of each type was different. The DPEP type predominated in asphaltenes, whereas the Etio predominated in the residue. Mass Spectrometric Measurements. Baker et al. have recommended that mass spectra of fossil porphyrins
Porphyrins in Saudi Arabian Crude Oils
Energy & Fuels, Vol. 7, No. 2, 1993 183
-
0.005
-
0.004 0.003
0.002 0.001
-
-
O.Oo0 I 0
I
I
I
I
10
20
30
40
50 MINUTES
Figure 5. HPLC of vanadyl porphyrins (TIC band) from AH asphaltenes, isocratic elution: MeOH, 2 C l ~column (25 cm X
4#)
450
500
550
0.46 mm). m
-
1
80
-I
Figure 6. E l mass spectra of vanadyl porphyrins from Arab Heavy asphaltenes. be measured at aprobe temperature of 260 0C.9 However, we measured our separated vanadyl porphyrins at a programmed temperature from 70 to 350 OC at 16 "C/min. A very low voltage of 18 eV was used in order to minimize further fragmentation and to maximize the molecular ion peaks. The total ion chromatogram (TIC) profiles have indicated that the bulk of vanadyl porphyrins was thermally vaporized after 11-15 min in the probe of the mass spectrometer. This means that the vanadyl porphyrins obtained from both the residue and the asphaltenes of the Arabian Heavy crude oil can be volatilized at 240-310 "C. The differential volatilization of the vanadyl porphyrins can be solved by averaging the spectra over the range of the volatility of the sample according to Q ~ i r k e . ~ The molecular weight of a porphyrin nucleus that has no alkyl substituents, "the basic structure" of an etioporphyrin molecule incorporating vanadium (Vanadyl Etio), is 375. The alkyl-substituted series of this molecule has molecular weights corresponding to 375 + 14n, where n is an integer (M series). Likewise, the vanadyl deoxophylleoerythroetioporphyrin (vanadyl DPEP) series corresponds to 401 + 14n (M-2 series). The tetrahydrobenzo DPEP series corresponds to the 455 + 14n (M-4 series), whereas benzo Etio (M-6 series) and benzo DPEP (M-8 series) correspond to 425 + 14n and 451 + 14n, respectively. The mass spectra of Arabian Heavy residue and asphaltene vanadyl porphyrins were observed to exhibit a Gaussian-like mass distribution (Figures 6 and 7). This distribution is characteristic of the petroleum vanadyl porphyrin^.^
600
650
700 M/z
Figure 7. E l mass spectra of vanadyl porphyrins from Arab Heavy residue.
Mass spectral analysis of vanadyl porphyrins isolated from both the asphaltenes and residue of Arabian Heavy crude oil indicated that they contained mainly two homologous series of vanadyl DPEP and vanadyl Etio porphyrins (M-2 and M, respectively) as well as small amount of M-4, M-6, and M-8 series. Within each series, there are numerous compounds that differ in the total number of carbon atoms on the skeleton of the porphyrin ring. The mass spectra of these vanadyl porphyrins showed that they were very complex mixtures. The mass spectra of the vanadyl porphyrins isolated from the asphaltenes of the Arabian Heavy indicated that it contained more vanadyl DPEP than vanadyl Etio porphyrins. This finding agreed with the results of Didyk et a1.'0 It indicated that the asphaltenes preferentially concentrated the vanadyl DPEP due to incorporation of this type of porphyrins into the asphaltic host by forming p-p molecular complexes with the asphaltenes aromatics. In addition, the incorporation of the porphyrins in the asphaltic host was affected by their polarity and thus resulted in the preferential incorporation of the DPEP type porphyrins.1° Benzo vanadyl porphyrins, basically an alkylbenzo vanadyl porphyrin proposed by Baker et a1.,9 were found to coexist with the main two series, vanadyl DPEP and Etio porphyrins, which agrees with Didyk et a1.'0 Vanadyl porphyrins isolated from asphaltenes were observed to consist of M and M-2 series (Etio, DPEP) vanadyl porphyrins types with carbon numbers ranging from CZZto c42. The maximum peak molecular at mle 501 corresponds to C29 Etio and a molecular peak at mle 513 was found to correspond to C30 DPEP type. The carbon number of the benzo Etio vanadyl type has a range of C24-C42 with a maximum molecular peak at mle 523 and 537 corresponding to c31-c32 benzo Etio. The benzo DPEP was found to have a carbon range of C28-C42 with a maximum molecular peak at m/e 493 corresponding to (329. The maximum molecular peak at mle 539 was for C32 tetrahydrobenzo DPEP type that was found to have a carbon range of C28-C42. Table I11 lists different types of vanadyl porphyrins found in the asphaltenes, their carbon range, and the maximum peak for each type. On the basis (9) Baker, E. W.; Teh Fu Yen; Dickie, J. P.; Rhodes, R. E.; Clark L.
F.J. Am. Chem. SOC.1967,89, 2311-2315.
(10) Didyk, B.; Alturki, Y. A.; Pillinger, C. T.; Eglinton, G. Chem. Geol. 1975,15, 193-208.
Ali et al.
184 Energy & Fuels, Vol. 7,No. 2, 1993 Table 111. Mass Spectrometric Data for Vanadyl Porphyrins from Asphaltenes
porphyrin type" Etio DPEP BEtio BDPEP TBD
MW range
Cnumber
formula 375+ 14n 401 t 14n 425 14n 451 t 14n 4 5 5 t 14n
403-683 401-681 425-677 479-675 483-679
c22-c42 C22-C42 c24-c42 C28-C42 c28-c42
+
range
nb 2-22 0-20 0-18 4-16 2-16
max peak 501 513&527 523 & 537 493 539
BEtio and BDPEP = benzo Etio, & DPEP; TBD = tetrahydrobenzo DPEP. n = an integer. Table IV. Mass Spectrometric Data for Vanadyl Porphyrins from Residue
porphyrin type" Etio DPEP BEtio BDPEP TBD
formula 375 t 401 t 425 451 455 t
+ +
14n 14n 14n 14n 14n
MW range
nb
max peak
403-683 429-681 453-677 465-675 483-679
2-22 2-20 2-18 2-14 2-14
501 527 495 521 553
BEtio and BDPEP = benzo Etio and DPEP; TBD = tetrahydrobenzo DPEP. n = an integer. 0
Table V. Mass Spectrometric Data for Nickel Porphyrins from Residue
porphyrin type" Etio BDPEP a
MW range
Cnumber
formula 366+ 14n 442+ 14n
408-590 442-596
C23-c36 C26-C36
range
nb 3-16 0-10
max peak 450 498
BDPEP = benzo DPEP. n = an integer.
188
450 a7a I
Figure 8. E l mass spectra of nickel porphyrins from Arab Heavy Residue.
of the molecular ion peak's abundance, DPEP was more abundant than Etio in the asphaltenes. The vanadyl porphyrins isolated from the residue of the Arabian Heavy crude were also found to contain the same types of the porphyrinic series with the same range of carbon distribution up to c42. On the basis of the molecular peak relative abundance, porphyrins with Etio type were found to predominate. It was also found that the relative amounts vary as benzo Etio > tetra benzo DPEP > benzo DPEP vanadyl porphyrins. Table IV lists the vanadyl porphyrins (found in residue) types, their carbon number range, and their maximum molecular peaks. Etio had a maximum molecular peak at m/e 501 corresponding to C29 vanadyl Etio porphyrin. The DPEP
porphyrins have their maximum molecular peak at mle 527 that corresponded to C31 DPEP vanadyl porphyrin. The most abundant molecule in the homologous series of benzo Etio vanadyl porphyrin was found to be C29 whereas CS1was predominant in the homologous series of benzo DPEP series. Tetrahydrobenzo vanadyl porphyrins homologous series had its maximum molecular peak at mle 553 corresponding to C33. The mass spectra of the nickel porphyrins were similarly characterized with the presence of homologous series of porphyrins. The nickel porphyrins fraction separated from the residue cut with 3:7 chloroformln-hexane, which showed a visible absorption band at 550 nm, was further analyzed by alow-voltage (18eV) mass spectrometer. From the mass spectra only two types of nickel porphyrins could be identified: Etio and benzo DPEP (Figure 8). The homologous series of Etio with a carbon number range of c23-c36 was identified with a maximum molecular peak corresponding to c 2 6 , whereas benzo DPEP type has a homologous series ranging from c 2 6 to c36with a maximum molecular peak corresponding to c30 (see Table V for more details). The absence of nickel DPEP in the crude oil having vanadyl DPEP is difficult to explain. A model nickel compound was not available to us during the course of this research therefore it could not be ascertained whether this unusual behavior is due to the lability of nickel DPEP on silica gel. Further work on the isolation and characterization of nickel porphyrins from the asphaltenes was not done due to unavailability of model compounds and the lability of the nickel porphyrins. Conclusions The vanadyl porphyrins from Arabian Heavy Crude Oil asphaltene and residue (535 "C+) fraction are found to be a mixture of Etio and DPEP homologues (c22-c42) maximizing at and c30, respectively. Minor amounts of benzo porphyrins (benzo Etio, benzo DPEP, and tetrahydrobenzo DPEP) are also present. The fraction of vanadyl porphyrins isolated from the asphaltenes contains a higher proportion of DPEP homologues than do the vanadyl porphyrins isolated from the residue. The nickel porphyrins from Arabian Heavy Crude Oil residue (535 "C+) are found to be chiefly Etio homologues (c23-c36)maximizing at c26. The mass spectral analysis did not give an indication for the presence of nickel DPEP homologues in the porphyrins isolated from the residue. Minor amounts of benzo DPEP homologues (c26-c36) maximizing at C30 are present. The porphyrin composition of Arabian Heavy Crude Oil residue and asphaltenes reported in this work are quite consistent with results reported in the literature for other crude oils. The relative amount of DPEP and Etio varies with each petroleum. Acknowledgment. The authors acknowledge the support facilities provided by King Fahd University of Petroleum, and Minerals.
