Characterization of Petroporphyrins Using Ultraviolet− Visible

Jan 20, 2005 - Sackville Street Building, Sackville Street, Manchester M60 1QD, United Kingdom. Received February 13, 2004. Revised Manuscript Receive...
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Energy & Fuels 2005, 19, 517-524

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Characterization of Petroporphyrins Using Ultraviolet-Visible Spectroscopy and Laser Desorption Ionization Time-of-Flight Mass Spectrometry Hai Xu, Guohe Que, and Daoyong Yu State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, University of Petroleum, Dongying 257061, China

Jian R. Lu* Biological Physics Group, School of Physics and Astronomy, The University of Manchester, Sackville Street Building, Sackville Street, Manchester M60 1QD, United Kingdom Received February 13, 2004. Revised Manuscript Received November 8, 2004

Petroporphyrins were extracted from two typical Chinese heavy crude oils, Tahe and Du84, and then purified by silica gel chromatography, followed by demetallization by methyl sulfonic acid. The extraction and purification were monitored using ultraviolet-visible spectroscopy, and the final petroporphyrins were analyzed using laser desorption ionization time-of-flight mass spectrometry. The soft ionization mass spectrometric technique proved to be effective for the characterization of petroporphyrins. The results show that, in Tahe crude oil, vanadium is more abundant than nickel and 75% of the vanadyl porphyrins are of the etioporphyrin (ETIO) type, with remaining fractions attributed to deoxophylloerythroetioporphyrin (DPEP) and benzo types. The ∑DPEP/∑ETIO ratio was found to be 0.18. In contrast, the Du84 heavy crude oil contains more abundant nickel than vanadium, with its nickel porphyrins comprising mainly DPEP and ETIO types, with each occupying 45%, and the tetrahydrobenzo-DPEP and benzo types attributed to the remaining 10%. The ∑DPEP/∑ETIO ratio is ∼1.1. These results suggest that the Tahe crude oil has higher thermal maturity than the Du84 crude oil, and the former is in its mature stage, whereas the latter is in its evolution stage.

Introduction Despite being present in petroleum with low concentration, metals often cause detrimental consequences in upgrading processes because they poison catalysts used for sulfur and nitrogen removal and those used for cracking and molecular reconstitution. Hence, it is highly desirable to remove these metals from petroleum before catalytic hydrogenation and cracking. In recent years, with the increasing production of heavy petroleum that has a higher metal concentration in general, much effort has been made to develop the demetallization technology. It has been widely perceived that the better understanding of the chemical environment of the metals in petroleum, their amount and distribution, and their physicochemical properties should aid in the development of more efficient demetallization and catalytic processes. Vanadium and nickel are the two most abundant trace metals in petroleum, both of which are generally thought to occur in petroleum in two forms, metalloporphyrins (petroporphyrins) and metallononporphyrins. Because of significant geochemical importance, petroporphyrins have been extensively studied since their first * To whom correspondence should be addressed. Phone: 44-1612003926. E-mail: [email protected].

discovery in fossil fuel in 1934.1 It has been found that petroporphyrins usually exist as mixtures of several types. The two major types are etioporphyrin (ETIO, Figure 1a) and cycloalkanoporphyrins (CAPs, also referred to as deoxophylloerythroetioporphyrin (DPEP), Figure 1b-e), and among the CAP family, the most common one is the meso-β-cycloethanoporphyrin containing a five-membered exocyclic ring (DPEP-5, Figure 1b). Some other types with diverse carbon skeletons, such as tetrahydrobenzo-DPEP (Figure 1f), benzo (Figure 1g), and benzo-DPEP (Figure 1h), have also been identified in petroleum.2-9 The ∑DPEP/∑ETIO porphyrin ratio is (1) Treibs, A. Justus Liebigs Ann. Chem. 1934, 510, 42. (2) Filby, R. H.; Van Berkel, G. J. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 2-39. (3) Chicareli, M. I.; Kaur, S.; Maxwell, J. R. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 40-67. (4) Ocampo, R.; Callot, H. J.; Albrecht, P. In Metal Complexes in Fossil Fuels; Filby, R. H., Branthaver, J. F., Eds.; ACS Symposium Series 344; American Chemical Society: Washington, DC, 1987; pp 68-73. (5) 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 308-331. (6) Callot, H. J.; Ocampo, R.; Albrecht, P. Energy Fuels 1990, 4, 635. (7) Kaur, S.; Chicareli, M. I.; Maxwell, J. R. J. Am. Chem. Soc. 1986, 108, 1347. (8) van Berkel, G. J.; Glish, G. L.; McLuchey, S. A. Energy Fuels 1990, 4, 720.

10.1021/ef0499574 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/20/2005

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Figure 1. Different types of petroporphyrins (M ) VO2+ or Ni2+): (a) ETIO, (b-e) DPEP, (f) tetrahydrobenzo-DPEP, (g) benzo, and (h) benzo-DPEP.

an important geochemical parameter, and many studies have indicated a close correlation between this ratio and the geochemical state of petroleum: with increasing maturity of petroleum, this ratio tends to decrease.10-17 Owing to the low concentration of petroporphyrins, it is necessary to separate them from petroleum before they can be analyzed. To this end, numerous separation approaches have been reported in the literature, and most of them usually involve these steps: solvent extraction, chromatography, and acid demetallization. Quirke has given a good summary of these separation methods as well as analytical approaches utilized.5 Ultraviolet-visible (UV-vis) spectroscopy and mass spectrometry (MS) are indispensable and invaluable for (9) Czernuszewicz, R. S.; Rankin, J. G.; Lash, T. D. Inorg. Chem. 1996, 35, 199. (10) Didyk B.; Alturky, Y. I. A.; Pillinger, C. T.; Eglinton, G. Nature 1975, 256, 563. (11) Yen, T. F. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1975; pp 1-32. (12) Baker, E. W., Palmer, S. E. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 1, pp 485-551. (13) Barwise, A. J. D.; Robert, I. Org. Geochem. 1984, 6, 167. (14) Sundararaman, P.; Biggs, W. R.; Reynolds, J. G.; Fetzer, J. C. Geochim. Cosmochim. Acta 1988, 52, 2337. (15) Chen, J. H.; Philp, R. P. Chem. Geol. 1991, 91, 139. (16) Gallango, O.; Gassani, F. Org. Geochem. 1992, 18, 215. (17) Sundararaman, P.; Dahl, J. E. Org. Geochem. 1993, 208, 333.

the characterization of petroporphyrins. In addition to a strong Soret peak at ∼400 nm, petroporphyrins generally exhibit two visible peaks, called R and β peaks, between 500 and 600 nm. Petroporphyrin or porphyrinic metal concentrations can be determined by measuring the absorption of the Soret or the R peak. After removal of the central Ni2+ or VO2+, the corresponding free-base porphyrins have four visible peaks, which are labeled as I, II, III, and IV peaks from long wavelength to short wavelength, respectively, and the relative intensities of the peaks are useful in assigning porphyrin types.12,18,19 In general, MS has been utilized to obtain the molecular weight distributions of petroporphyrin mixtures. On the basis of mass spectra, the porphyrin types, the carbon number range and the relative percentage of each type, and the ∑DPEP/∑ETIO ratio can be further determined. Electron impact mass spectrometry (EI-MS) has been the most widely used technique in this field. To reduce the β cleavage of the peripheral constituents and obtain the molecular ions only, the ionizing voltage is often reduced from 70 to 10-20 eV. But this inevitably reduces its sensitivity.5,12 On the other hand, although (18) Baker, E. W. J. Am. Chem. Soc. 1966, 88, 2311. (19) Baker, E. W.; Yen, T. F.; Dickie, J. P.; Rhodes, R. E.; Clark, L. F. J. Am. Chem. Soc. 1967, 89, 3631.

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Table 1. Properties of Tahe and Du84 Heavy Crude Oils property

Tahe heavy crude oil

Du84 heavy crude oil

density(20 °C) viscosity(50 °C) carbon residue concentration sulfur concentration nitrogen concentration resin concentration heptane asphaltene concentration vanadium concentration nickel concentration

0.9666 g/cm3 903 mm2/s 15.76 wt % 2.38 wt % 0.46 wt % 17.2 wt % 14.2 wt % 252.0 µg/g 35.4 µg/g

1.0056 g/cm3 863 mm2/s 14.60 wt % 0.50 wt % 0.97 wt % 33.2 wt % 3.6 wt % 2.5 µg/g 132.9 µg/g

prior separation and purification procedures are performed on petroporphyrins, some petroleum matrix is usually well expected to be present in the purified fraction, which will undermine the mass spectrometric characterization of the petroporphyrins. Therefore, MS techniques that can optimize the production of molecular ions, accompanied by high sensitivity, good tolerance toward contaminants, and the ability to analyze complex mixture, will be promising in this field. The development of laser desorption ionization time-of-flight mass spectrometry (LDI-TOF-MS) offers such an opportunity. Despite its wide application in the analysis of both synthetic polymers and biopolymers (usually in high purity),20-24 such a technique has not yet been used to characterize petroporphyrins. In this paper, we present its first application to characterize petroporphyrins isolated from two Chinese crude oils, both of which have recently been exploited. The majority of Chinese crude oils have higher nickel contents than vanadium. The Tahe heavy crude oil from Tarim Basin, northwestern part of China, however, has much higher vanadium concentration than nickel. Furthermore, as shown in Table 1, it is also characterized by high carbon residue, sulfur, and n-heptane asphaltene contents. Thus, this crude is the focus of current geochemical and process research. However, little is known about the chemical nature of the vanadium compounds in this oil, especially their physicochemical properties, though it is widely recognized that the structure and composition of vanadyl petroporphyrins may have different implications to the catalysts for catalytic cracking. In contrast, the Du84 crude oil from the Liaohe Oilfield contains high nickel content, but again its nickel compounds have not been well characterized. In this paper, we report the isolation and purification of the vanadyl porphyrins from the Tahe heavy crude oil and of the nickel porphyrins from the Du84 heavy crude oil, supported by characterization from a combined approach of UV-vis and LDI-TOF-MS. Experimental Section The Tahe heavy crude oil used in this study was provided by the Tarim Petrochemical Factory, and the Du84 heavy crude oil was obtained from the Liaohe Petrochemical Factory. (20) Chen, Y. Z.; Tu, Y. P. Principle and Application of Organic Mass Spectrometry; Science Press: Beijing, 2001. (21) Hoffmann, E.; Stroobant, V. Mass Spectrometry: Principles and Applications, 2nd ed.; Wiley: Chichester, U.K., 2001. (22) Bahr, U.; Karas, M.; Hillenkamp, F. Fresenius J. Anal. Chem. 1994, 348, 783. (23) Strupat, K.; Karas, M.; Hillenkamp, F.; Eckerskorn, C.; Lottspeich, F. Anal. Chem. 1994, 66, 464. (24) Overberg, A.; Hassenbu¨rger, A.; Hillenkamp, F. In Mass Spectrometry in the Biological Sciences: A Tutorial; Gross, M. L., Ed.; Kluwer: Dordrecht, The Netherlands, 1992; pp 181-198.

Scheme 1. Procedures for Separation and Purification of Vanadyl Porphyrins in Tahe Heavy Crude Oila

a CH ) cyclohexane, DCM ) dichloromethane, CF ) chloroform, and MSA ) methyl sulfonic acid.

Scheme 2. Procedures for Separation and Purification of Nickel Porphyrins in Du84 Heavy Crude Oila

a CH ) cyclohexane, DCM ) dichloromethane, CF ) chloroform, and MSA ) methyl sulfonic acid.

Table 1 lists their main properties. Sulfur and nitrogen concentrations were measured by using a WK-3 sulfur tester (Jiangsu Electroanalysis Instrument) and a Ren-1000A nitrogen tester (Jiangsu Jianghuan Instrument), respectively. Vanadium and nickel contents were determined by atomic absorption spectrophotometry (Hitachi Z8000). The contents of resin and asphaltene were measured using the SARA technique as described by Liang.25 Methanol, acetonitrile, cyclohexane, dichloromethane, chloroform, toluene, n-heptane, tetrahydrofuran, and 2-propanol used in our work were analytical grade purchased from Beijing Chemical Co. Methyl sulfonic acid (MSA) used for the demetallization of petroporphyrins was chemical grade supplied by Shanghai Chemical Co. Vanadyl 2,3,7,8,12,13,17,18-octaethylporphyrin (VOOEP) and nickel 2,3,7,8,12,13,17,18-octaethylporphyrin (NiOEP) were obtained from Sigma-Aldrich Chemicals. All the chemicals were used as received. The chart describing the separation of vanadyl porphyrins from the Tahe heavy crude oil is illustrated in Scheme 1, and that describing the separation of nickel porphyrins from the Du84 heavy crude oil is shown in Scheme 2. In both schemes, solvent extraction was first applied: methanol was used to extract vanadyl porphyrins from the Tahe crude, and acetonitrile was used to extract nickel porphyrins from the Du84 crude. To extract petroporphyrins effectively, the oil samples were dispersed on diatomite and extracted with a Soxhlet extractor; more details relating to the extraction procedure have previously been reported.26 It is a common practice in (25) Liang, W. J. Petroleum Chemistry; University of Petroleum Press: Dongying, China, 1995; pp 135-136.

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petroleum research to utilize a combination of different solvents to achieve the optimal efficiency of extraction and separation of different petroporphyrins from crude oils. The two extracts were further separated by adsorption chromatography on a column (1 × 40 cm) packed with silica gel (100-200 mesh). The methanol extract was eluted using cyclohexane/dichloromethane (9:1, 7:3, 1:1, and 1:4, v/v), dichloromethane, and chloroform in sequence to give six fractions. The acetonitrile extract was eluted sequentially with cyclohexane, cyclohexane/dichloromethane (4:1, 1:1, and 1:4, v/v), dichloromethane, and chloroform to give six fractions as well. UV-vis spectrophotometry was employed to monitor each fraction to determine nickel and vanadyl porphyrins in these fractions. For the Tahe heavy crude oil, the fractions containing vanadyl porphyrins were combined and solvents were then removed. The sample obtained was dissolved in a minimum amount of dichloromethane and charged to an activated silica gel plate (GF254) by a capillary tube. After evaporation of dichloromethane, the plate was developed with n-heptane/ tetrahydrofuran (5:1, v/v). The bands observed were scraped off, extracted with dichloromethane, and analyzed by UVvis spectrophotometry. Ali et al.27 have tried to purify nickel porphyrins using thinlayer chromatography (TLC), but they found that the nickel porphyrins decomposed on the silica gel plate. Chakraborty et al.28 have also observed this phenomenon, and they attributed it to the lability of nickel porphyrins as compared with vanadyl porphyrins. For nickel porphyrins isolated from Du84 heavy crude oil, therefore, we carried out MSA demetallization straight from column chromatography and avoided the TLC purification step. Both vanandyl and nickel porphyrins were demetallized with MSA prior to further analysis. It has been confirmed that the porphyrin structures are unchanged during MSA demetallization.19 Compared with the previous MSA demetallization procedures,29 harsher conditions were used in the present MSA demetallization experiments: the vanadyl porphyrins were treated with MSA at 130 °C for 4 h, and the nickel porphyrins were treated at the same temperature for 3 h. Finally, the resultant free-base porphyrins were subjected to UV-vis spectrophotometry and mass spectrometry analyses. The treatments led to a complete removal of metal ions with no indication of any disturbance to the porphyrin structure. All UV-vis spectrophotometric analyses were performed on a Cary 50 spectrophotometer (Varian), using a 1 cm cuvette and dichloromethane as solvent. Samples were scanned from 700 to 350 nm. The LDI-TOF mass characterization was carried out on a BIFLEX III mass spectrometer (Bruker Daltonics). A 1 µL sample of free-base porphyrin solution in chloroform was spotted on the LDI-TOF-MS sample holder and the solvent allowed to evaporate under ambient conditions. The porphyrins were ionized by nitrogen laser pulses (337 nm) and accelarated under 19.5 kV, with time-delayed extraction. The analyzer was operated in reflection mode, and the reflector voltage was 20 kV.

Results and Discussion Methanol is often used to extract vanadyl porphyrins from crude oils.27,30 In contrast, acetonitrile is more effective in extracting nickel porphyrins.26 The solvent extraction procedure, however, is just a preliminary step because there still exists a lot of petroleum matrix in (26) Xu, H.; Yu, D. Y.; Sui, X. H.; Que, G. H. Chin. J. Appl. Chem. 2001, 18, 419. (27) Ali, M. F.; Perzanowski, H.; Bukhari, A.; Al-Haji, A. A. Energy Fuels 1993, 7, 179. (28) Chakraborty, S.; Bhatia, V. K. Indian J. Technol. 1981, 19, 92. (29) Erdman, J. G. U.S. Patent 3,190,829, 1965. (30) El-Sabagh, S. M. Fuel Process. Technol. 1998, 57, 65.

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Figure 2. UV-vis spectra of vanadyl porphyrins in Tahe heavy crude oil: (a) methanol extract, (b) after column chromatography, and (c) after TLC.

Figure 3. UV-vis spectra of an acetonitrile extract of Du84 heavy crude oil: (a) acetonitrile extract and (b) after column chromatography.

the extracts, which may interfere with the analysis of vanadyl and nickel porphyrins. It can be obviously seen from Figures 2a and 3a that the Soret peaks are all masked by strong matrix absorption when the two extracts are characterized by UV-vis spectroscopy although the R peaks are all evident. The silica column chromatography of the solvent extracts not only led to the enrichment of petroporphyrins but also resulted in the separation of nickel porphyrins from vanadyl porphyrins. For the methanol extract, the UV-vis spectra of the cyclohexane/dichloromethane (9:1, v/v), cyclohexane/dichloromethane (1:1, v/v), and chloroform fractions showed no character-

Characterization of Petroporphyrins

istic absorption peaks, indicating the absence of petroporphyrins in these fractions. The UV-vis spectra from the cyclohexane/dichloromethane (1:4, v/v) fraction and dichloromethane fraction are shown in Figure 2b. There are three noticeable peaks at 407, 533, and 572 nm, which are attributed to the typical Soret, β, and R peaks of vanadyl porphyrins, respectively. Besides these peaks, a shoulder peak at 592 nm is also visible, which was previously observed and regarded as characteristic of the benzo type (benzo and benzo-DPEP) of vanadyl porphyrins.19,30,31 The cyclohexane/dichloromethane (7:3, v/v) fraction exhibited a very weak peak at 552 nm, the typical R peak of nickel porphyrins, corresponding to the presence of a minor quantity of nickel porphyrins in the fraction. For the acetonitrile extract of Du84 crude oil, a UV-vis scan of the cyclohexane, dichloromethane, and chloroform fractions showed no characteristic absorption peaks, indicating the absence of petroporphyrins in these fractions. UV-vis absorption peaks at 390, 511, and 552 nm were detected in the 4:1 (v/v) and 1:1 (v/v) cyclohexane/dichloromethane fractions (Figure 3b), and the three peaks are Soret, β, and R peaks of nickel porphyrins, respectively, thus confirming the presence of nickel porphyrins in these two fractions. The 1:4 (v/v) cyclohexane/dichloromethane fraction exhibited a weak visible peak at 572 nm, corresponding to the presence of a small amount of vanadyl porphyrin in this fraction. Vanadyl porphyrins are likely to have higher polarity than nickel porphyrins because of the presence of the VO group. As a result, the former are usually eluted following the latter when separated on the silica gel column. On the other hand, it has been suggested that the difficulty in the enrichment of nickel porphyrins might be related to their lower polarity.30 The higher purity of vanadyl porphyrins in this work is easily noted when Figures 2b and 3b are compared. When the vanadyl porphyrins from the Tahe heavy crude oil were subjected to TLC, four bands were observed from the bottom to the top of the silica gel plate, which had Rf values of 0.00-0.14, 0.14-0.36, 0.36-0.50, and 0.50-0.86, respectively. The third TLC band (Rf ) 0.36-0.50) showed a red color, characteristic of vanadyl porphyrins, and the others were all yellow. Each was scraped off, extracted with dichloromethane, and analyzed by UV-vis. The third TLC band had UVvis absorption peaks at 407, 533, and 572 nm and a shoulder at 592 nm (Figure 2c). The other TLC bands did not exhibit any porphyrinic absorption. It can be clearly seen from Figure 2c that, after the column chromatography and TLC purification, the background absorption of the vanadyl porphyrins decreases substantially. Furthermore, it can be calculated from Figure 2c that the peak intensity ratio of R to β was about 1.9. For ETIO vanadyl porphyrins, this value is 2. It is about 1.3 for DPEP vanadyl porphyrins.12 Therefore, ETIO is the prevailing type of vanadyl porphyrins in the Tahe heavy crude oil. The harsher demetallization conditions were used in our work to convert thoroughly metalloporphyrins into free-base porphyrins, and the UV-vis analysis of the demetallized residues showed no porphyrinic absorption (31) Pena, M. E.; Manjarrez, A.; Campero, A. Fuel Process. Technol. 1996, 46, 171.

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Figure 4. UV-vis spectra of metal-free porphyrins from (a) vanadyl porphyrins of Tahe heavy crude oil and (b) nickel porphyrins of Du84 heavy crude oil.

peaks, indicating that the treatment achieved an almost complete demetallization. Figure 4a shows the UV-vis spectrum of free-base porphyrins from vanadyl porphyrins in the Tahe heavy crude oil. Compared with Figure 2c, the Soret peak is blue shifted from 407 to 399 nm, and four visible peaks, I-IV, are clearly observed at 619, 566, 535, and 498 nm, respectively. This spectrum is similar to that previously reported by other researchers as being typical of petroporphyrins.18,19,32,33 Thus, these visible peaks from the free-base porphyrins are useful in assigning the porphyrin type. The ETIO-type visible spectrum of free-base porphyrins is characterized by peak intensities in the order IV > III > II > I. The order becomes IV > I > II >III for the DPEP type, while for the benzo type (benzo series and benzo-DPEP series) the intensity order is III > IV > II > I.12 In Figure 4a, the intensity order is apparently IV > III > II > I, and this again confirms that ETIO is the dominant type of vanadyl porphyrins in Tahe heavy crude oil. For the nickel porphyrins in Du84 heavy crude oil, the UV-vis spectrum of the free-base version is shown in Figure 4b. The Soret peak is red shifted from 390 to 398 nm as compared with Figure 3b. The four visible peaks I-IV are seen at 617, 564, 533, and 498 nm, respectively, and the intensity order is IV > II > III > I. The spectrum is intermediate between those of ETIO and DPEP, indicating that nickel porphyrins in Du84 heavy crude oil are likely to consist of a mixture of ETIO and DPEP. In addition, it is interesting that the UVvis peaks (except IV) in Figure 4a are at slightly longer wavelengths than their corresponding peaks in Figure 4b. It has been known that the positions of the UV-vis peaks of ETIO are rather close to those of DPEP, but because of the presence of the fused benzene ring in the benzo type with the conjugation effect, its peaks are red shifted compared with those of ETIO and DPEP. Thus, the differences of the peak positions in Figure 4 imply (32) Dunning, H. N.; Moore, J. W.; Myers, A. T. Ind. Eng. Chem. 1954, 46, 2000. (33) Chen, P. R.; Xing, Z.; Wang, Z. J.; Liao, Z. Q.; Huang, D. F. Chin. J. Spectrosc. Lab. 2001, 15, 1.

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Figure 6. LDI-TOF mass spectra of metal-free porphyrins from vanadyl porphyrins in Tahe heavy crude oil.

Figure 5. LDI-TOF mass spectra of (a) VOOEP and (b) NiOEP.

that the concentration of the benzo type in the vanadyl porphyrins of Tahe heavy crude oil is possibly higher than that in the nickel porphyrins of Du84 heavy crude oil, and we will unambiguously confirm it in the subsequent MS analysis. Unlike traditional mass spectrometry, soft ionization mass spectrometry is mostly utilized for obtaining information about molecular weight. Soft ionization mass spectrometric techniques, e.g., electrospray ionization mass spectrometry and laser desorption ionization mass spectrometry, have been extensively developed to characterize large protein molecules in recent years.20,21 Despite their potential, their application in the analysis of petroporphyrins has been little reported. A few recent studies have however utilized plasma desorption mass spectrometry and electrospray ionization mass spectrometry.34-37 In our work, LDI-TOF-MS was used to characterize petroporphyrins for the first time. To test the feasibility of this technique, two model compounds, VOOEP and NiOEP, were first subject to the LDI-TOFMS analysis. Their mass spectra are shown in Figure 5. Besides the pronounced singly charged molecular ion peaks incorporating the isotopic contributions, no fragmental ion peak was observed. The isotopic relative abundance distributions of the molecular mass obtained from the experiment matched the theoretical values well within experiment error. In addition, the signal-to-noise ratio was very high as can be seen from Figure 5, and (34) Wood, K. V.; Bonham, C. C.; Chou, M. I. M. Energy Fuels 1990, 4, 748. (35) Wood, K. V.; Bonham, C. C. Energy Fuels 1993, 7, 97. (36) Vanberkel, G. J.; Quinones, M. A.; Quirke, J. M. F. Energy Fuels 1993, 7, 411. (37) Rodgers, R. P.; Hendrickson, C. L.; Emmett, M. R.; Marshall, A. G.; Greaney, M.; Qian, K. Can. J. Chem. 2001, 79, 546.

the noise level was almost negligible. These results thus show that LDI-TOF-MS is reliable and effective for characterizing porphyrins. The experiment also served as a useful standardization of the technique before petroporphyrins were analyzed. It is worthwhile to note that for the LDI-TOF mass spectrum of NiOEP (Figure 5b), the M + 2 isotopic peak is elevated due to the distinctive isotopic distribution of Ni (68.3% 58Ni, 26.1% 60Ni). As the M + 2 isotopic peaks of the DPEP series are commensurate with those of ETIO,12 it is therefore difficult to determine accurately the ∑DPEP/∑ETIO ratio on the basis of the mass spectrum of nickel porphyrins. Once the metal ion is removed, however, the intensity of the M + 2 isotopic peak will decrease, and its interference on the determination of the ∑DPEP/∑ETIO ratio will also be reduced accordingly. The C31 free-base DPEP, for example, has a mass of 462, and its M + 2 isotopic mass is 464. However, because the abundance of the M + 2 isotope is only 6%, its influence is clearly very small. In the case of the predominance of ETIO as shown in Figure 6 for petroporphyrins, the M + 2 isotopic peaks of DPEP have almost no effect on the ∑DPEP/∑ETIO ratio either. It has been well established that the molecular weights of petroporphyrins are related to their types.12 Free-base porphyrins of the ETIO series, which are now defined as the M + 2 series for subsequent comparison, have molecular weights of 310 + 14n, where n is an integer. Likewise, free-base porphyrins of the DPEP series (M series) have molecular weights of 336 + 14n, whereas the tetrahydrobenzo-DPEP series (M - 2 series) corresponds to a formula of 376 + 14n. The benzo series (M - 4 series) of free-base porphyrins has molecular weights of 360 + 14n, and the benzo-DPEP series (M - 6 series) corresponds to 386 + 14n. The LDI-TOF MS spectra of free-base porphyrins from the vanadyl porphyrins of Tahe heavy crude oil and the nickel porphyrins of Du84 heavy crude oil are shown in Figures 6 and 7, respectively. The two figures show a number of characteristic features. The first is that no fragmental ion peak is formed. In Figure 6, only parent molecular ion peaks are observed. In Figure 7, apart from the pronounced parent molecular ion peaks, some petroleum matrix peaks occur in the low m/e region, which are obviously induced by the lower purity of this sample as explained previously.

Characterization of Petroporphyrins

Energy & Fuels, Vol. 19, No. 2, 2005 523 Table 2. Characteristic Parameters of Vanadyl Porphyrins in Tahe Heavy Crude Oil relative porphyrin carbon number percentage, type range max % ETIO DPEP benzo

Figure 7. LDI-TOF mass spectra of metal-free porphyrins from nickel porphyrins of Du84 heavy crude oil.

The second interesting feature is the striking difference in porphyrin types and their distributions as shown in the two figures. In Figure 6, it can be clearly seen that the ETIO series, with a carbon number range of C26-C40, constitutes the major portion (more than 70%) of the total porphyrins, and in addition to the ETIO series, minor amounts of DPEP with a carbon number range of C29-C40 and the benzo series with a carbon number range of C31-C40 are also present. In contrast, the petroporphyrins in Figure 7 consist of the ETIO series with carbon numbers ranging from C25 to C35 and DPEP series with carbon numbers ranging from C28 to C38, and the two types are comparable in quantity. Minor amounts of tetrahydrobenzo-DPEP (C26-C38) and benzo (C29-C36) also coexist with the two main series. These differences are likely to be caused by the difference in the thermal maturity of the oil, which will be discussed below. On the other hand, there is a good agreement between the LDI-TOF-MS analyses and the foregoing UV-vis analyses, indicating again that the LDI-TOF-MS technique is suited for the characterization of petroporphyrins. The third feature is the shape of the mass distribution of each porphyrin series. It can be easily observed that every series exhibits a Gaussian-like mass distribution, which is often regarded as typical of porphyrins originating from petroleum. In Figure 6, the most abundant molecular ion peak is shown at m/e 450 corresponding to C30 ETIO porphyrin, and for the DPEP and benzo series, the maximum molecular ion peaks are at m/e 462 and 528, corresponding to C31 DPEP porphyrin and C36 benzo porphyrin, respectively. In Figure 7, the most abundant peak for C31 DPEP is at m/e 462, and for the ETIO series, the maximum molecular ion peaks are at m/e 436 and 450, corresponding to C29 and C30 ETIOs. Compared with ETIO and DPEP, it is difficult to determine the maximum molecular ion peaks for the tetrahydrobenzo-DPEP series and the benzo series due to their low concentrations and the interference of petroleum matrix peaks. Since we have already explained that the M + 2 isotopic peaks of the DPEP (M) series as shown in Figure 6 have almost no effect on the intensity of the molecular ion peaks of the ETIO (M + 2) series due to the predominance of the ETIO type, the relative abundance of the porphyrin peaks in this LDI-TOF-MS

C26-C40 C29-C40 C31-C40

C30 C31 C36

76.3 13.7 10.0

∑DPEP/∑ETIO 0.18

spectrum can be calculated following the approach by Baker et al.12 The relative percentage of each porphyrin series in the order of ETIO, DPEP, and benzo was found to be 76.3%, 13.7%, and 10.0%, respectively. However, when the ETIO (M + 2) series is comparable to the DPEP (M) series as in the case of the data shown in Figure 7, the contribution of the M + 2 isotopic peaks of the DPEP series to the relative abundance of the ETIO series may not be ignored. When isotopic effects were not taken into account, the relative percentage of each series was calculated to be 45.0%, 44.0%, 9.0%, and 2.0% for DPEP, ETIO, tetrahydrobenzo-DPEP, and benzo, respectively. Comparatively, when isotopic effects were considered, the relative percentage of each series was found to be 46.3%, 42.4%, 9.3%, and 2.0%, respectively. The largest deviation is under 2%, and this range of errors is well acceptable. We now extend the above assessment further by incorporating the VO2+ and Ni2+ into the data so that the characteristic parameters of vanadyl porphyrins in the Tahe heavy crude oil (Table 2) and those of nickel porphyrins in Du84 heavy crude oil (Table 3) can be obtained. Among these parameters, the most widely cited one is the ∑DPEP/∑ETIO ratio. It is still unclear what mechanism is involved in the alteration of this parameter. For example, Didyk et al.10 thought that, with increasing thermal maturity, ETIO arises from the thermal cleavage of DPEP at the isocyclic ring. But Barwise et al.13 disagreed with this mechanism, and they proposed that the reduction in the ratio was due to the higher thermal stability of ETIO compared with DPEP. Despite this ambiguousness, it is generally accepted that the ∑DPEP/∑ETIO ratio decreases with increasing maturity of petroleum. Thus, for the Tahe heavy crude oil, in which the concentration of vanadium is far higher than that of nickel due to its marine origin, the ∑DPEP/∑ETIO ratio of vanadyl porphyrins is 0.18, suggesting that the oil has higher thermal maturity and is in its mature stage. For Du84 heavy crude oil, which has far more nickel than vanadium because of its freshwater origin, the ∑DPEP/∑ETIO ratio is about 1.1, indicating that this oil is less thermally mature than Tahe heavy crude oil and is most likely to be in its evolution stage. It is also worthwhile to note that the above MS data have convincingly revealed that the concentration of the benzo series (10.0%) in Tahe heavy crude oil is higher than that in Du84 heavy crude oil (∼2%). This result explains the slightly different red shifts as observed from UV-vis spectra from the free-base porphyrins (see Figure 4). This work has demonstrated that LDI-TOF mass spectrometry is well suited for the identification and characterization of porphyrin mixtures isolated from petroleum. The LDI-TOF mass spectra of petroporphy-

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Table 3. Characteristic Parameters of Nickel Porphyrins in Du84 Heavy Crude Oil porphyrin type

carbon number range

ETIO DPEP tetrahydrobenzo-DPEP benzo

C25-C35 C28-C38 C26-C38 C29-C36

rins only include the monocharged molecular species, and no fragments are recorded. From these spectra, useful information concerning the porphyrin type, the carbon number range and the relative percentage of each type, and the ∑DPEP/∑ETIO ratio can be determined. The large difference in these parameters obtained from the two crude oils studied in this work is well correlated to their entirely different thermal maturities and depositional environments. Because of the inability to fragment the porphyrin macrocycle, however, this technique cannot provide more detailed structural information about the location of substituent groups on the porphyrin macrocycle, and the size of the exocyclic ring. Such information can be obtained by using other MS techniques such as chemical ionization MS, where MS is often utilized in the form of MS coupled with another MS technique or is combined with HPLC,8,38,39 NMR,3 and resonance Raman spectroscopy.9,40,41 It is relevant to state at this point that characterization of petroporphyrin structures in crude oils has direct implication to petroleum processing. Catalytic hydrodemetallization (HDM) is a potential technology for removing metals from heavy crude oils, and it has been demonstrated that the HDM reaction behavior of metalloporphyrins is sensitive to their types. Compard to nickel porphyrins, vanadyl porphyrins have higher catalytic HDM reactivity due to their greater polarity.42,43 Furthermore, if the central metals are the same, the peripherical groups of the porphyrin macrocycle may also affect their HDM reactivity. The HDM mechanism of metalloporphyrins generally consists of two sequential steps, the first being the reversible hydrogenation of peripheral double bonds, leading to the formation of metallochlorins, and the second being the irreversible hydrogenolysis on the methine position, resulting in the fragmentation of the big ring and the final removal of the central metal.42-46 Thus, if there is no substituent on the methine position, metalloporphyrin can be more easily attacked and the hydrogenolysis reactivity will be higher. After the first reversible hydrogenation step, metalloporphyrin of the ETIO type will become more (38) Van Berkel, G. J.; Glish, G. L.; Mcluckey, S. A.; Tuinman, A. A. Anal. Chem. 1990, 62, 786. (39) RosellMele, A.; Maxwell, J. R. Rapid Commun. Mass Spectrom. 1996, 10, 209. (40) Rankin, J. G.; Cantu, R.; Czernuszewicz, R. S.; Lash, T. D. Org. Geochem. 1999, 30, 201. (41) Boggess, J. M.; Czernuszewicz, R. S.; Lash, T. D. Org. Geochem. 2002, 33, 111. (42) Hung, C. W.; Wei, J. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 250. (43) Hung, C. W.; Wei, J. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 257. (44) Ware, R.A.; Wei, J. J. Catal. 1985, 93, 100. (45) Mitchell, P. C. H. Catal. Today 1990, 7, 439. (46) Furimsky, E.; Massoth, F. E. Catal. Today 1999, 52, 381.

max C29,C30 C31

relative percentage, % 46.3 42.4 9.3 2.9

∑DPEP/∑ETIO 1.1

easily hydrogenolysized than the DPEP type. Comparing vanadyl porphyrins in Tahe heavy crude oil and nickel porphyrins in Du84 heavy crude oil under the same HDM conditions, it is obvious that the former, in which the ETIO series dominates, are more easily demetallized than the latter in which the ETIO series and DPEP are comparable. Conclusions This study has shown that LDI-TOF-MS is very effective for the analysis of petroporphyrins. The porphyrin type, the carbon number range and the most abundant molecules of each porphyrin series, and the ∑DPEP/∑ETIO ratio in the two Chinese heavy crude oils were determined. The LDI-TOF-MS data strongly support the information derived from the more conventional UV-vis spectrometry. The results show that the Tahe heavy crude oil, newly exploited from Tarim Basin, is of marine origin in China and has much higher vanadium concentration than nickel. The vanadyl porphyrins in this oil were found to consist mainly of the ETIO series (C26-C40), with the most abundant carbon number at C30, a minor amount of the DPEP series (C29-C40), with the most abundant carbon number at C31, and the benzo series (C31-C40), with the most abundant carbon number at C36. The ETIO, DPEP, and benzo series were found to account for 76.3%, 13.7%, and 10.0% of the total porphyrins, respectively, and the ∑DPEP/∑ETIO ratio is 0.18, indicating that the oil is in its mature stage. In comparison, the Du84 heavy crude oil from the Liaohe Oilfield is of freshwater origin and contains far more nickel than vanadium. The nickel porphyrins in this oil were extracted and purified using a carefully selected set of procedures, followed by a combined analysis of UV-vis and LDI-TOF-MS. The nickel porphyrins in Du84 crude oil were found to consist of a mixture of DPEP (C25-C35), with the most abundant carbon number at C31, and ETIO (C28-C38), with the most abundant carbon numbers at C29 and C30. In addition, minor amounts of the tetrahydrobenzo-DPEP series (C26-C38) and benzo series (C29-C36) were found to coexist with the two main series. The DPEP, ETIO, tetrahydrobenzo-DPEP, and benzo series were found to account for 46.3%, 42.4%, 9.3%, and 2.0%, respectively, and the ∑DPEP/∑ETIO ratio is about 1.1, suggesting that the oil is in its evolution stage. Characterization of petroporphyrins separated from these oils provides useful information toward the design of oil refinery and related processes, especially upgrading treatments such as catalytic hydrodemetallization. EF0499574