Porphyrin-Metal Complexes in Petroleum Stocks H. N. DUNNING AND hTANCYA. RABON P e t r o l e u m Experiment S t a t i o n , Bureau of Mines, U . S. D e p a r t m e n t of t h e Interior, Bartlesuille, Okla.
T
HE presence of porpyhrin-metal complexes in petroleum
and several heavy metals caused permanent inactivation of cracking catalysts. Because of their metal and nitrogen contents, porphyrin-metal complexes may poison petroleum refining catalysts and adversely affect the stability of fuels. A further complication is that such complexes are remarkably stable and, unlike inorganic salts or basic nitrogen compounds, cannot be removed from petroleum stocks by usual procedures, such as acid-treating. The porphyrins are readily volatilized, a property that further complicates their removal from petroleum stocks. Observation of porphyrin-metal complexes in the propanedeasphalted raffinate of a mid-continent crude oil prompted a n intensive investigation of their identities and characteristics. Chromatographic and solvent-extraction methods were combined to isolate the complexes. The vanadium and nickel-porphyrin complexes were identified by spectrophotometric analysis of the extracts. The identification was corroborated by deconiposiiig the complexes and identifying their constituent parts-metals and porphyrin aggregates.
and bituminous material was established by the work of Treibs in 1934 (11). Recently an increased interest in these remnants of plant and/or animal life has developed, as the implications of their occurrence on petroleum exploration and production have been indicated by several investigations ( 1 , 3-6,8). Porphyrin-metal complexes are commonly associated with the asphaltic residues of crude oils in which they occur (3, 4, 8). However, investigations in this and another laboratory (8) show that small amounts of these complexes often remain in partly refined petroleum products such as thermally cracked distillates and deasphalted stocks. Thompson and others (IO) showed that nitrogenous materials caused copious sludge formation in furnace oils. Mills ( 6 ) reported that minute amounts of nickel, vanadium, iron, or copper, accumulating from charge stocks, poisoned clays and synthetic cracking catalysts. Mills and Shabaker (7') reported that compounds containing basic nitrogen caused temporary inactivation
PROPANE DEASPHALTING
CHROMAJOGRAM ALUMINA
rL27
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CHROMATOGRAM NO / SILICA GEL
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Cycky&:$E- BENZENE
BENZENE. ACETONE
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Figure 3.
NO. 2
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Separation process for nickel-porphyrin complex
INSOLUBLE ZONE, I
-8
CHROMA.'OGRAM NO ALUMINA
I CHROMATOGR4M
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NO-3
ALUyiNA
HEXANE
~ENZENE
EXTRUDED " I
EXTRUDED
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*2
Figure 2.
3 0 % CHLOQOFORM
&+lj
6 0 % CHLOROFORM
ZONE E - A VANADIUM COMPLEX
ZONE I - C VANADIUM COMPLEX
Separation process for first vanadiumporphyrin complex
Figure 4. 95 1
Separation process for second vanadiumporphyrin complex
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
952
BlATERIALS
The petroleum sample used in this investigation was obtained from Tatunis field, Carter County, Okla. It was produced from sand of Pennsylvanian age a t a depth of 2260 feet. The sample had a specific gravity of 0.896. G0"/60" F. (26.4' h.P.1.) and a Saybolt Universal viscosity oi 165 seconds a t 100" F.; 115 seconds a t 130' F. The llmisbottoni carbon residue of the sample n-as 7.3%. The specifications of the solvents have been listed (3),except for acetone, propane, and hexane. The acetone was Fisher's certified reagent grade. The propane and hexane were obtained fioni the Phillips Petroleum Co. and were technical grade containing a minimum 95 mole yo of the respective solvent. Davison silica gel (28- t o 200-mesh) and Alcoa H-41 alumina (80- to 200-mesh) were used as adsorbents. EXPERIMENTAL METHODS AND RESULTS
The petroleum samples were deasphalted a t 60" C. in a pressure reactor equipped with a rocking mechanism. A propaneoil ratio of 8 to 1 was used in the original precipitation and in two subsequent washings of the asphalt. The propane-soluble portions 17-ere combined and the propane was distilled, leaving the raffinate. The petroleum, asphalt, and raffinate contained 0.37, 1.35, and 0.17 weight yo of nitrogen, respectively. Preliminary separation procedures are shown schematically in Figure 1 and the final procedures in Figures 2, 3 , and 4. Raffinate samples of about 20 grams were used in the early separation procedures. The largest sample of raffinate used weighed 115 grams. The raffinate was diluted with an equal volume of hexane and introduced int,o a silica gel column 2.5 em. in diameter and 125 em. in length, which had been prewet with hexa,ne. The column was eluted successively with hexane, cyclohexane, benzene, a solution of 10% acetone in benzene, and pyridine. -A distinct zone was eluted by cach of the solvents except cyclohexane; in all, 29 portions of eluate were collected. In this and following chromat,ograms the spectrum of each fraction was determined and the fractions of each zone richest in porphyrin complexes were combined for further study. For example, eight fractions of the hexane eluate were collected. Fractions 1, 6, 7 , and 8 contained no porphyrin complex. Therefore, they were discarded, and fractions 2, 3, 4, and 5 were combined and designated as zone I. The porphyrin-containing fractions of the hexane, benzene, and benzene and acetone eluates were designated zones I, 11, and 111, respectively. The pyridine
eluate (zone IV) contained no detectable amount of porphyrin complex. The spectra of zones I, 11, and 111 are shown in Figuie 5 The main absorption peak a t about 570 m p and the weaker band a t about 535 mp indicate the presence of the vanadium-porphyrin complex in zones I and 111; the peak a t 550 mp and the band a t about 510 mp indicate the presence of the nickel-porphyrin complex in zone 11. The spectrum of the raffinate is shown in Figure 6 for comparison. The spectrum of the raffinate obtained by deasphalting the crude oil Tvith pentane, at a pentane-oil ratio of 10 to 1, also is shown in Figure 6. Inspection of the spectra (Figure 5 ) shows that, in addition t o the metal complexes, zone I contains a relatively large amount of extraneous material, while zones I1 and I11 are progressively less contaminated. Accordingly, zones I and I1 were rechromatographed on alumina columns 1.1 cm. in diameter and 70 cm. in length. The columns were eluted successively with the solvents used previously, except that acetone was substituted for pyridine. In both chromatograms, the zones containing the porphyrinmetal complexes were diffuse and moved slowly (about 0.2 the rate of solvent movement) during benzene elution and rapidly (about 0.9 the rate of solvent movement) with the benzeneacetone miyture. I n each chlomatogram the last fraction of the hen7ene eluate and the first fiaction of the benzene-acetone
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400
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Figure 6.
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Absorption spectra of propane- and pentane-deasphalted raffinates 83.0 m g . per m l . of benzene 8.3 m g . per m l . of benzene
lUl
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, 500 WAVE LENGTH, inp
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Vol. 48, No. 5
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550
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Figure 5 ,
1
Absorption spectra of chromatographic zones of raffinate 8.3 mg. per ml. of benzene
ZONE I
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953
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1956
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Figure 8. Absorption spectra of alcohol-soluble and -insoluble portions of zone I-A Soluble. 8.3 m g . per ml. of benzene Insoluble. 0.83 mg. per m l . of benzene
eluate were rich in the metal-porphyrin complex. Therefore, these were combined to form zones I-A and 11-A containing the vanadium- and nickel-porphyrin complex, respectively. The spectra of these zones are shown in Figure 7. Alcohol extraction often is successful for enrichment of porphyrin-metal complexes in petroleum extracts (3, 8). Accordingly, zones I-A4and 11-A were extracted with absolute ethyl alcohol. Little separation was accomplished with zone 11-A, because over 99yo of this zone was soluble in alcohol. The separation of zone I-A, by alcohol eutraction, was more successful. About 80% of this zone was soluble in refluxing alcohol. The separation accomplished is shown in Figure 8, which presents the spectra of the alcohol-soluble and alcohol-insoluble portions of zone I-A. Zones I-B and 11-A were concentrated further by chromatographic separations on alumina columns 1 cm. in diameter and 50 cm. in length. The columns were developed with hexane, followed by benzene. Elution was stopped when the zones had been separated, the column was sectioned, and the several portions were extracted from the adsorbent with chloroform. Zone 111, diluted with hexane, was put on an alumina column and eluted with hexane, solutions of 30 and 607, chloroform in heuane, and acetone (Figure 4). The spectra of zones I-C and 111-A indicated the presence of a single metal-porphyrin complex in each. However, the spectrum of zone 11-B indicated the presence of two complexes. .Accordingly, zone 11-B was subjected to another chromatographic separation on a silica gel column 1 cm. in diameter and 50 em. in length. The eluents used were hexane, cyclohexane, a 50Yo solution of cyclohexane and benzene, benzene, and a 10% solution of acetone in benzene. The benzene-cyclohexane eluate was rich in a single metal porphyrin complex and was designated zone 11-C. The spectra of zones I-C, 11-C, and 111-A, shown in Figure 9, indicate that these zones are relatively very rich in porphyrin complexes. The peaks, in decreasing order of intensity, are listed in Table I.
Table I. Absorbance Peaks of Porphyrin-Metal Complexes Zone
I-c
11-C 111-A
Complex Vanadium Nickel Vanadium
Wave Lengths of Absorption Maxima, M*
410 395
408
574 564 573
535 516 534
O'
460
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500
550
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LENGTH,mu
Figure 9. Absorption spectra of final chromatographic fractions containing porphyrin complexes 3.0
mg. per m l . of henzene
The porphyrin components of the vanadium- and nickelporphyrin complexes were determined by the method of Groennings ( 5 ) with modifications of the colorimetric procedure ( 4 ) . The spectra of the porphyrin aggregates from the nickel and vanadium complexes did not differ appreciably from that of the porphyrin aggregate from the crude oil. The spectrum of the free porphyrins from the crude oil appears in Figure 10. The absorption peaks were located a t 401, 501, 534, 566, and 619 emp. The average wave lengths of the visible peaks determined with a Hartridge Reversion spectroscope were 501.1, 534.1, 565.0. and 617.8 mp. Vanadium was determined with an x-ray spectrograph equipped with a helium path. Qualitative identification was made by observation of K , emission a t 2.505 A. and K p emission a t 2.285 A. Semiquantitative estimations were made by comparing emission intensities of the complexes with intensities of samples that had been analyzed spectrographically. Kickel was determined with a Beckman DU flame spectrophotometer equipped with a direct-recording attachment. Qualitative identification was made by observation of the nickel peaks a t 352.5 and 346.1 mp. Semiquantitative estimates were based on comparison of emission intensities of the complex with emission intensities of prepared samples determined under identical operating conditions. The porphyrin contents of the crude oil and raffinate were 200 and 30 p.p.m., by weight, respectively. A small part of the porphyrin aggregate from the crude oil and a larger portion of the aggregate (about 30Yo)from the raffinate could not be extracted from ether by 10 weight % hydrochloric acid. These portions were extracted from ether by 20 weight 70 acid. The molar extinction coefficients of the vanadium- and nickelporphyrin complexes, a t wave lengths of maximum absorption
954
INDUSTRIAL AND ENGINEERING CHEMISTRY
in the visible region, are about 2.6 and 3 . 5 X lo4, respectively (9). With extracts of relatively high optical purity, calculations based on such extinction coefficients and absorbance values of the extracts give fair estimates of the amounts of porphyrinmetal complexes present. According to such calculations, the vanadium-porphyrin content of the raffinate isolated in zone I-C was between 8 and 10 p.p m. and in zone 111-A betn-een 3 and 4 p.p.m., by weight. The nickel-porphyrin content of the raffinate isolated in zone 11-C mas between 3 and 4 p.p.m., by ITeight. These values do not include all of the porphyrin content of the raffinate because of the extensive separations and selection of only the richest portions of the chromatographic zones
Vol. 48, No. 5
Figure 11. The fractions n w e diluted to the same absorbance value a t 574 mp to facilitate comparison. The absorption spectrum of the 260" C. fraction is compared with that obtained by chromatographic separation (zone I-C) in Figure 12. Vacuum distillation of mixtures of the nickel- and vanadium-porphyriii complexes indicated that the nickel complex was a t least PLC readily distilled as the vanadium complex, DISCUS SPBN
The occurrence of niet,al-porphyrin complexes in the propane deasphalted raffinates of crude oils apparently is caused by soliibilization rather than by entrainment or cont,amination. .\1though pure metal-porphyrin complexes are generally insoluble in hydrocarbons, their solubilization by naturally occurring SUI)stances which often accompany them has been observed repeatedly (3). Observations that t,he complexes viere increasingly difficult to elute from the adsorbents, as they viere separated from accompanying impurities on a single chromatogram and in successive chromatograms, corroborate this conclusion. Zone I contained considerable light-absorbing material, other than the porphyrin complex, as well as a. largc amount of wax. This zone contained 60 n-eight 7, of the raffinate. Zones 11 and I11 contained 15 and 1 weight' yGof the raffinate, respectively. The remainder of the raffinate was distributed throughout the portions of the eluates not used for further separation and in zone IV.
WAVE LENGTH, m g
Figure 10. -4bsorption spectrum of porphyrin aggregate from crude oil from Tatums field 160 mg. (original oil) per ml. of ohloroform 0 w
Although the various zones contained too little poiphyrin (an aveiage of about 0.1 micromole) for quantitative extraction, the amounts of porphyrin aggregates isolated from the zones were of the same order as the amounts calculated from the absorption spectra of the zones. The amounts of nickel and vanadium calculated in this manner agreed M ith the cor1esponding values determined analytically within i Z O 7 , . The interfacial activities and film-forming tendencies of the porphyrin aggregates from the crude oil and raffinate were determined with a pendent-drop apparatus in 0.12 weight yo benzene solutions a t the a-ater interface (3). The interfacial activities (decrease in interfacial tensions) were 15.5 and 8.2 dynes per em. for the porphyrin aggregates from the crude oil and 1affinate, respectively. Thp corresponding values of film formations were 90 and 607,. The volatiliiation of the first vanadium complex was investigated by high-vacuum distillation of portions of zone I-A. A 17-mg. portion of zone I-A was distilled a t a pressure of 10 microns in the separation on the largest scale. The condensing and heating areas of the appaiatus were flat suifaces 0.3 em. apait. The distillations mere accomplished by heating the distillation area slowly from room temperature to 360" C. and holding the temperature constant for about 15 minutes at 20" intervals from 200' to 360' C. The first fraction of distillate, a t about 130" C., was a clear wax. Then;no distillate was obtained until the temperature reached 240" C. The 20" fractions mere examined spectroscopically and combined to form major fractions which represented the distillates rollected from 240" to 280°, 280" to 320°, and 320' to 360' C. These three fractions, and similar fractions collected in earlier runs, contained an average of about 55, 25, and 10% of the complex in the charge material, respectively. The absorption spectra of two fractions of the distillate are compared with the spectra of the charge material and residue in
.25
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Figure 11, Absorption spectra of fractions during molecular distillation of first vanadium-porphyrin complex Zone I-A
Alumina was more effect'ive for separating the porphyrin complexes from nonporphyrin material than was silica gel. However, repeated chromatographic procedures with alumina coluinnt: did not succeed in separating the small amounts of vanadiuin complex (indicat'ed by the weak absorption band a t 570 1n.u in Figure 7) from the nickel complex in zone II-.1. A single silica gel chromat'ogram was sufficient to isolate the nickel-porphyrin complex in studies with a second portion of zone 11. Alternate silica gel and alumina chromatograms (Figures 2 and 3) weye effective for the separation of t'he met'al complexes from pitch other and from accompanying light-absorbing impurities. The difference in adsorption of the vanadium-porphyrin coniplex in zone I from that in zone I11 indicates a considerable difference in the polarity of the two vanadium complexes. The distinctness of the zones and the occurrence of the nickel-porphyrin complex zone between them strengthen the assumption that there is a valid difference between the two vanadium complexes. Unfortunately, comparisons of the rate of movement of
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
M a y 1956
such complexes on chromatographic columns are seriously hindered by the indeterminate effects of accompanying impurities, as discussed above. The most distinct differences in the polarities of porphyrins are caused by the presence or absence of carboxylic acid side chains (4). The low adsorption of zone I on silica gel indicates that the porphyrins in it are of the decarboxylated type.
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DILUTED .I \,
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450
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Figure 12. Absorption spectra of final chromatographic fraction and molecular distillation cut containing first vanadium-porphyrin complex Chromatographic fraction. 1.5 m g . per ml. Distilled fraction. 1.1 mg. per ml.
The interfacial activities and film-forming tendencies of the porphyrin aggregates from the crude oil and raffinate were low compared with the corresponding properties of the porphyrin aggregate obtained from an extracted California crude oil and indicated t o be of the decarboxylated type by studies of its infrared spectrum. The porphyrin aggregate from the raffinate also was less interfacially active than that from the crude oil. This aggregate was not soluble to a detectable extent in sodium hydroxide solution and required strong mineral acid for extraction from ether. These observations indicate that the porphyrin aggregates of zones I1 and 111, as well as zone I, probably are decarboxylated types. The spectra of the porphyrin aggregates from the two vanadium complexes and the nickel complex are not appreciably different. These spectra also are similar to that of the porphyrin aggregate from the crude oil (Figure 12). However, considerable alterations may be made in the side chains of porphyrins without affecting their absorption spectra. It appears logical to assume that the difference in polarity of the vanadium complexes in zones I and I11 is caused by small differences in the numerous side chains of their porphyrin constituents. The distribution of metal-porphyrin complexes among the various zones of chromatogram I and their difficult separation from nonporphyrin materials indicate that the removal of such complexes from stocks in which they occur will be difficult on a commercial scale. The most obvious method would involve freeing the metals from the complex, after which the inorganic metal and basic porphyrin aggregate could be removed readily. However, the nickel- and vanadium-porphyrin complexes are exceedingly stable. The nickel complex may be decomposed by treatment with sulfuric acid (2). However, the vanadium complex resists even this drastic action (8, 1 1 ) but may be decomposed by prolonged digestion a t elevated temperatures with glacial acetic acid saturated with hydrogen bromide. Nickel may be removed from its porphyrin complexes by less drastic digestion with the hydrobromic-acetic acid mixture. Digestion under atmospheric pressure with hydrogen bromide bubbling
955
through the flask, at 50' C. for an hour, gave results comparable to those obtained by the usual sealed-ampoule method. The low backgrounds of the spectra of the 260' and 300' C. distillates (Figure 11) indicate that a substantial separation has been accomplished. The residue has a very low content of the vanadium-porphyrin complex. Approximate calculations based on absorbance values indicate that an average of about 90y0 of the porphyrin complex was recovered in the distillate fractions. The spectra shown in Figure 12 indicate that the 260' C. distillate is of a purity comparable to that obtained (zone I-C) by alcohol extraction and extensive chromatographic separation of zone 1-8.Furthermore, approximately the same proportion of the complex originally present in zone I-A is contained in these two fractions. Therefore, molecular distillation, or sublimation, is a promising method for concentrating the stable metal-porphyrin complexes from petroleum extracts. The ready volatilization of metal-porphyrin complexes may be of considerable practical importance. The metal-porphyrin complexes and some of their products may be distilled in refining processes such as thermal cracking and, therefore, appear in the distillates. This would result in the presence of trace metals in such stocks, where they adversely affect storage and utilization. Much of the difficulty observed in attempting to remove trace metals, such as nickel and vanadium, from petroleum stocks may be explained by the identification of stable porphyrin complexes of these metals in such stocks. CONCLUSIONS
Vanadium- and nickel-porphyrin complexes have been isolated from the propane-deasphalted raffinate of an Oklahoma crude oil. Two portions of the vanadium-porphyrin complex were obtained. These were identical in optical properties but differed in their adsorption on silica gel. The porphyrin-metal complexes were identified by spectrophotometric analyses of the extracts. This identification was corroborated by decomposing the complexes and identifying their constituent parts-metals and porphyrin aggregates. The distillation (sublimation) of the metal-porphyrin complexes isolated in this investigation has been studied. Correlations of the properties of the porphyrin-metal complexes with the properties of their porphyrin aggregates indicate that the porphyrins involved are decarboxylated types. The occurrence of trace metals, such as vanadium and nickel, as stable porphyrin-metal complexes in petroleum stocks prevents their removal by usual methods of treatment. ACKNOWLEDGMENT
The authors are indebted to J. W. Moore, B. H. Eccleston, M. L. Whisman, and R. T. Johansen, of this station, for their aid in obtaining samples and metal analyses. LITERATURE CITED
(1) Blumer, Max, Helv. Chim. Acta 33, 1627-37 (1950). (2) Caughey, W. S., Corwin, A. H., J. Ant. Chem. SOC.77, 1509-13 (1955). (3) Dunning, H. N., iLIoore, J. W., Denekas, 13. O., IND.ENG. CHEM.45, 1759-65 (1953). (4) Dunning, H. N . , LToore, J. W., Myers, A. T., Ibid., 46, 2000-7 (1954). (5) Grocnnings, S.,ANAL.CHEM.25, 938-41 (1953). ( 6 ) Mills, G. A., IND. ENG.CHEM.42, 182-7 (1950). (7) Mills, G . -4., Shabaker, H. A., Petroleum Refiner 30, 97-102 (September 1951). (8) Skinner, D. A., IND. ENG.CHEM.44, 1159-65 (1952). (9) Stern, A , , Dezelic, M., 2. physiic. Chem. A-180, 131-8 (1937). (10) Thompson, R. B., Chenicek, J. A,, Druge, L. W., Symon, T., IND. ENG.CHEX.43, 935-9 (1951). (11) Treibs, A.,Ann. 509, 103-14 (1934); 510, 42-62 (1934); 517, 172-96 (1935). RECEIVED for review October 4, 1955.
ACCEPTEDJanuary 12, 1956. Division of Industrial and Engineering Chemistry, 128th Meeting, ACS, Minneapolis, Minn., September 1955.
