Metal Complexes in Fossil Fuels - American Chemical Society

by examination of the nature of hydrodemetallized vanadium deposited on catalysts and the ... ESR spectra were obtained using a JEOL EF-1 spectrometer...
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Chapter

18

Characteristics of Vanadium Complexes in Petroleum Before and After Hydrotreating S. Asaoka, S. Nakata, Y. Shiroto, and C. Takeuchi

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Chiyoda Chemical Engineering and Construction Company, R&D Center, 3-13 Moriya-cho, Kanagawa-Ku, Yokohama, 221 Japan

Vanadium removal from asphaltenes plays an important role i n hydrotreating of heavy o i l s . Vanadium i s mainly concentrated i n asphaltenes as vanadyl porphyrins involved i n associations with other large hydrocarbon molecules to form asphaltene micelles, which cause d i f f i c u l t i e s i n hydrodemetallization. Dissociation of asphaltene micelles enhances metal removal. Vanadyl porphyrins i n the non-asphaltene portion of heavy o i l s are less associated and are r e a d i l y removed. Vanadium removed during hydrotreatment of heavy o i l s i s deposited on catalysts initially as four-sulfur coordinated vanadyl compounds, which react further to form vanadium sulfide. Remaining vanadium complexes i n treated o i l s are involved i n smaller associations than the asphaltene micelles of the feed. During the hydrotreating of heavy oils, hydrodemetallization depends significantly on catalyst pore structure, which changes with hours-on-stream and depends on the molecular sizes of the reacting materials. The preferable catalyst pore structure for hydrotreating reactions i s determined by the nature of metal deposition on catalysts. I t also has been shown that vanadium s u l f i d e deposited on a catalyst during hydrotreating not only causes catalyst pore structure changes, but has autocatalytic a c t i v i t y . A hydrodemetallization mechanism on catalysts i s proposed which considers this autocatalytic a c t i v i t y derived from the deposited vanadium s u l f i d e . As i s generally known, i t i s not easy to h y d r o c a t a l y t i c a l l y upgrade heavy petroleum residues. This i s because they contain large quantities of metals, mainly vanadium, which are complexed with porphyrins and possibly with other large molecules containing condensed polyaromatic r i n g s . Much fundamental research on the c h a r a c t e r i s t i c s of metal complexes i n heavy o i l s has been carried

0097-6156/87/0344-0275$06.00/0 © 1987 American Chemical Society

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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METAL COMPLEXES IN FOSSIL FUELS

out to study hydrotreating behavior. Petroleum residues are mixtures of asphaltene micelles containing r e s i n components c o l l o i d a l l y dispersed i n maltenes (non-asphaltenes) as the dispersing medium (1). Asphaltenes are r i c h i n heteroatom compounds, and vanadium compounds also are concentrated i n asphaltenes. Vanadium compounds play a s i g n i f i c a n t role i n hydrotreating (,2) and this role may be c l a r i f i e d by examination of the nature of hydrodemetallized vanadium deposited on catalysts and the changes observed i n non-demetallized vanadium complexes. In this paper, extensive studies done during the develop­ ment of hydrotreating catalysts and processes for heavy o i l upgrading are described.

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f

Experimental A main feedstock used for the hydrotreating tests i s Boscan crude produced i n Venezuela. Other feedstocks used are Bachaquero atmospheric residue, Khafji vacuum residue, Gach Saran vacuum residue, Basrah Heavy vacuum residue, Athabasca t a r sand bitumen, etc. The hydrotreating tests were carried out by a high-temperature, high-pressure, flow-type unit. The feed o i l and hydrogen passed by the once-through upflow mode through a fixed-bed reactor, and the catalyst bed was maintained isothermally. The reactions were carried out under the following conditions: pressure, ^90 to 180 kg/cm ; temperature, 360 to 430°C; LHSV, 0.2 to 1.5 h~ ; and hydrogen to l i q u i d r a t i o i n volume, 600 to 1000 NL/L. The thermal treatments were done without catalyst i n a similar way. The catalysts, especially prepared for the t e s t s , were a l l of the cobalt-molybdenum type, supported on a c a r r i e r of an oxide. The sizes and shapes of these catalysts were the same, cylinders 0.8mm i n diameter and about 3mm i n length. A n a l y t i c a l gel permeation chromatograms (GPC) were obtained by means of a Japan Anal. Ind. LC-08 chromatograph, equipped with four columns of 600mm length and 20mm diameter i n series, f i l l e d with Shodex A-802 χ 1, A-803 χ 2 and A-804 χ 1, respectively. Molecular weight d i s t r i b u t i o n s were calibrated by the use of polystyrene. After fractionation by GPC, each f r a c t i o n was subjected to elemental analysis by radiation method so that the d i s t r i b u t i o n s of heteroatom compounds were obtained. ESR spectra were obtained using a JEOL EF-1 spectrometer operat­ ing at X-band frequency (9.2 GHz) with 100 kHz f i e l d modulation. The microwave cavity for measurements at high temperature was equipped with a hot-air blower. The ESR spectra were obtained at temperatures between 20 and 270°C. The ESR techniques and a n a l y t i c a l procedures used i n t h i s study were according to Tynan et a l . (4) The a n a l y t i c a l methods used for characterization of deposited vanadium were common ones c h a r a c t e r i s t i c of each a n a l y t i c a l i n s t r u ­ ment: Spark Source Mass Spectrometer (JEOL, 01BM, ED-01), X-ray Diffractometer (Rigaku Denki), X-Ray Fluorescence Spectrometer (Philips, PW-1400), Fourier Transform Infrared Spectrometer (JEOL, JIR-100) , Transmission Electron Microscope (JEOL), Scanning Electron Microscope (Hitachi, S-650), Energy Dispersive X-Ray Spectrometer (Philips, EDAX), Electron Probe Micro Analyzer (Shimadzu, EMX-SM&7), X-Ray Photoelectron Spectrometer (Shimadzu, ESCA-750).

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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18.

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Characteristics of Vanadium Complexes in Petroleum

Molecular Weight D i s t r i b u t i o n of Vanadium Containing Compounds When a heavy o i l such as Boscan crude i s analyzed by gel permeation chromatography (GPC), vanadium i s concentrated i n the heavy fractions, as i l l u s t r a t e d i n Figure l a . Asphaltenes, which are demetallized with d i f f i c u l t y , are of greater molecular weight than most of the sulfur and some of the vanadium compounds i n the nonasphaltene portion of Boscan crude. In a previous report (3) i t has been indicated that cracking of asphaltenes i s very important i n the hydroprocessing of heavy o i l s . Both hydrodemetallization and hydrodesulfurization are closely related to asphaltene cracking. I t i s observed that the number-average molecular weights of asphaltenes and of s u l f u r - and vanadium-containing components change as a r e s u l t of hydrotreating. However, the changes of the number-average molecular weights of the sulfur and vanadium compounds, which decrease from 1200 to 1100 and 1700 to 1500 respectively, i s small compared to the change observed for the asphaltenes, which are reduced from 4600 to 2200 (Figure 1). In addition, the molecular weight d i s t r i b u t i o n s of the aphsltenes and the sulfur and vanadium complexes become narrower. These facts i l l u s t r a t e that under the hydrotreating conditions we employed, the larger molecules are more e a s i l y cracked as the molecular weights of vanadium compounds are reduced. ESR Spectra of Vanadyl Ion i n Asphaltenes and Maltenes Vanadium compounds i n asphaltenes mainly exist as vanadyl porphyrins, which interact with other molecules to form large associations. Electron Spin Resonance (ESR) has been used to study the environment of vanadium i n petroporphyrins and other complexes (4). The behavior of vanadyl compounds i n asphaltene cracking reactions and i n hydrodemetallization has been described (3) . The c h a r a c t e r i s t i c changes in vanadyl porphyrins during hydrotreating and thermal treating have been further studied using ESR. In general, vanadyl groups i n petroleum exhibit two types of ESR signals. One i s a 16-line anisotropic spectrum due to "bound" vanadyl groups. The other i s an 8-line i s o t r o p i c spectrum due to "free" vanadyl groups. Asphaltenes dissolved i n a solvent have anisotropic ESR spectra s i m i l a r to asphaltenes i n the s o l i d state (Figure 2). Therefore vanadium compounds i n asphaltenes are considered to be involved i n very large molecular associations. When asphaltenes obtained from hydrotreated Boscan crude are examined by ESR, i s o t r o p i c spectral l i n e s are observed i n addition to the anisotropic spectrum. The i s o t r o p i c peaks are quite small. Maltenes of the parent crude dissolved i n a solvent exhibit anisotropic and i s o t r o p i c spectra of equal magnitude. Maltenes of hydrotreated Boscan crude have ESR spectra i n which the i s o t r o p i c peaks are larger than anisotropic peaks. Therefore, the r e l a t i v e sizes of the molecular associations involving vanadium are: free asphaltenes > product asphaltenes >> feed maltenes > product maltenes The degree of association of vanadium i n the hydrotreated product i s not greatly changed from that of the parent crude, although the number-average molecular weight of the asphaltenes of the product i s greatly changed. There also i s l i t t l e change i n association of vanadium remaining i n maltenes after hydrotreating. This suggests

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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METAL COMPLEXES IN FOSSIL FUELS

Figure 1. Molecular Weight Distributions of S-, V-compound and Asphaltenes (a)before and (b)after Hydrotreating (Boscan Crude).

feed

100 gauss

Figure 2. Crude).

ESR

Spectra

of Asphaltenes

and Maltenes

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

(Boscan

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Characteristics of Vanadium Complexes in Petroleum

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that maltene and asphaltene associations containing vanadium are not cracked without demetallization. Indeed, i t should be noted that those molecules that are cracked are almost free of vanadium. A model of t h i s behavior i s shown i n Figure 3. Characteristic Changes of Vanadyls i n Asphaltenes During Hydrotreating As shown i n Figure 2, the vanadium i n asphaltenes from both the feed and product o i l s give the same anisotropic ESR spectra. This i s a 16-line spectrum and i s c h a r a c t e r i s t i c of vanadyl ion i n petroleum asphaltenes at low temperatures. At higher temperatures, an 8-line i s o t r o p i c spectrum i s observed. This i s o t r o p i c spectrum has been quantitatively studied over a range of temperatures i n order to understand the e f f e c t s of hydrotreating on vanadyl coordination. This analysis has been described i n d e t a i l previously (_3) · analysis centers on the r a t i o of the magnitude of the No. 5 l i n e i n the i s o t r o p i c spectrum to that of the No. 6 l i n e i n the anisotropic spectrum. This r a t i o i s temperature dependent, and d i s s o c i a t i o n energies of ligands a x i a l l y coordinated to vanadyl groups may be calculated from the r a t i o by means of Arrhenius p l o t s . The r e l a t i v e amounts of "bound" and "free" vanadyl species present at a given temperature can be calculated from the r a t i o . Dissociation energies of vanadyl complexes i n asphaltenes from various feeds are calculated to be about 15 to 18 kcal/mol using the above method, whereas the vanadyl complexes i n the asphaltenes of the hydrotreated crudes have d i s s o c i a t i o n energies of 8 to 10 kcal/mol (Figure 4). I f d i s s o c i a t i o n energies and the amount of vanadyl i n the "bound" state r e f l e c t strengths and quantities of binding respectively, i t may be inferred that vanadyls i n feed o i l asphaltenes are associated with heteroatom-containing species. In the product o i l asphaltenes, many heteroatoms have been removed, and vanadyl a x i a l coordination i f believed to be involved with aromatic sheets. This analysis i s supported by results reported by Tynan, et a l . (4). These investigators report the energy of dimerization of formic acid to be 14 kcal/mol, and the energy of association of ethanol tetramer to be 23 kcal/mol. The energy of association between aromatic layers i n asphaltenes i s about 1 kcal/mol, while the energy of hydrogen bonding between aromatic sheets i s 2 to 8 kcal/mol. I t may be inferred that some asphaltene micelles are broken up during hydrotreating and that some vanadium i n them i s removed. The remaining vanadium compounds form new asphaltene associations by interaction with stacked aromatic sheets. Asphaltenes are not readily demetallized thermally, but undergo hydrodemetallization i n catalyst pores as a r e s u l t of o v e r a l l reduction i n molecular weight of asphaltenes i n c a t a l y t i c hydrotreating. T

n

e

Thermal Treatment (Visbreaking and Hydrovisbreaking), Hydrotreating, and Their Combination Many processes are known to accomplish heavy o i l upgrading. Among these are visbreaking and hydrovisbreaking. Visbreaking i s a simple thermal treatment, whereas hydrovisbreaking i s a thermal treatment under hydrogen pressure. A combination of the thermal treatment and a hydrotreatment has been proposed (5^) . The c h a r a c t e r i s t i c changes i n vanadyl coordination were examined

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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METAL COMPLEXES IN FOSSIL FUELS

Figure 3 . Model of Hydrotreating Analysis for Boscan Crude.

ô ε

20 ο

(Asphaltene Cracking)

from ESR

Jeed— ο Q ο; i hydrotreating

£

10 product

5h

ΙΟ

Figure 4. Change Hydrotreating.

10 10 vanadium in asphaltenes ,wt-ppm

of

Characteristics

for

Vanadyls

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

during

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Characteristics of Vanadium Complexes in Petroleum

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for an o i l subjected to these processes. As shown i n Figure 5, there i s only a small change i n vanadyl coordination i n products from the two thermal processes compared with the feed, and l i t t l e vanadium i s l o s t . Hydrotreating causes loss of vanadium, and that vanadium which survives i s i n a more "bound" state and has a lower d i s s o c i a t i o n energy. When hydrotreating follows a thermal process, the surviving vanadium i s converted almost e n t i r e l y into a "bound" state with a d i s s o c i a t i o n energy intermediate between the vanadium i n the feed and when the feed i s hydrotreated without a p r i o r thermal treatment. Obviously, the e f f e c t of a combination of processes on vanadium i s not additive (5). Vanadium i n Catalyst Pores The bonding of vanadyl compounds to the inner surface of catalyst pores also was studied by ESR. Large- and small-pore catalysts with sharp pore d i s t r i b u t i o n s were prepared. These pore diameters were completely d i f f e r e n t from each other. Each catalyst was contacted with solutions of asphaltenes from a feedstock and asphaltenes from a hydrotreated feedstock at room temperature. Vanadyl compounds from these solutions diffused into the catalyst pores. ESR spectra of these vanadyl complexes were taken at 100°C. As shown i n Figure 6, the vanadyl complexes from the feed asphaltenes which d i f f u s e into the small-pore catalyst are "free" when compared with the vanadyl complexes that diffused into the large-pore c a t a l y s t . The pore size i n the large-pore catalyst i s believed to approximate the size of asphaltene micelles. These micelles are dissociated when heated to 100°C, and some smaller vanadyl complexes become strongly adsorbed on the catalyst surface. These adsorbed vanadyl complexes are not reintegrated into micelle structures on cooling. A second increase i n temperature results i n vanadyl complexes having the same degree of freedom f o r feed asphaltenes i n both the small- and large-pore c a t a l y s t . Asphaltenes from the hydrotreated product do not have micellar aggregations. The vanadyl complexes i n these asphaltenes are strongly bound to the surface of the catalyst and would be expected to be reactive i n any subsequent c a t a l y t i c demetallization. Therefore i n c a t a l y t i c hydrodemetallization, i t i s important to consider molecular sizes of feed vanadyl complexes. I t must be remembered i n the design of hydrotreating catalysts for heavy o i l upgrading that the molecules to be treated can e x i s t i n large associations. Dependence of C a t a l y t i c A c t i v i t i e s on Pore Structure and Changes Due to Metal Deposition I t i s well known that pore structure (volume, diameter, surface area, etc. ) i s one of the most important properties of hydrotreating catalysts for heavy o i l s (6). For example, r e a c t i v i t i e s of catalysts with similar pore volumes depend on pore diameters. As i l l u s t r a t e d i n Figure 7, hydrodemetallization (HDM) a c t i v i t y depends on pore diameter. The pore diameter giving maximum a c t i v i t y i s about 25 nm for a fresh catalyst. As metals deposit, r e a c t i v i t y becomes maximized at larger pore diameters. Hydrodeasphaltening (HDA) a c t i v i t y shows a similar tendency to HDM a c t i v i t y . On the other hand, hydrodesulfurization (HDS) shows a maximum a c t i v i t y at pore diameters of about 10 nm. This means that the HDS a c t i v i t y i s d i s t i n c t l y d i f f e r e n t from HDM or HDA a c t i v i t y . HDA i s also affected by metal deposition.

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

281

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METAL COMPLEXES IN FOSSIL FUELS

20

hydrovisbreaking Q i visbreaking Ο feed A hydrovisbreaking /hydrotreating

10

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hydrotreating

hydrovisbreaking '-fc visbreaking Ο feed 10

] [^hydrotreating hydrovisbreaking • /hydrotreating _J I I I I I I I 1 I L 10' ΚΓ vanadium in asphaltenes,wt-ppm Figure 5. Vanadyls i n Combination of Those.

hydrotreating,

catalyst pore size 30 r

Ο lOnm

Thermal-treating

• 20nm

Ε

^ 25 S

r

feed

2 15

a>

\•o

product .2 10 .2 ο 5 ~

2nd elevation of temperature

(Λ «Λ

*

ο ο 1 st elevation of temperature

0

10'

;

,-2 10"'

m-'

ι

to

10

2

ratio of No.5 iso/No.6xaniso (at 100°C) Figure

6.

Vanadyls i n C a t a l y s t Pores.

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

and

18.

Characteristics of Vanadium Complexes in Petroleum

ASAOKA ET AL.

283

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Changes i n pore structure due to metal deposition are determined by the amount of metal deposited and i t s d i s t r i b u t i o n i n the catalyst particle. Theoretical studies of metal deposition and d i s t r i b u t i o n have been reported (7) . The characterization of deposited vanadium i s important to the understanding of catalyst a c t i v i t y , especially HDM a c t i v i t y . Characterization of Vanadium Deposited on Catalysts Vanadium deposits onto catalysts with a s p e c i f i c d i s t r i b u t i o n . As shown i n Figure 8, t h i s d i s t r i b u t i o n occurs along the reactor a x i a l length and along the radius of individual catalyst p a r t i c l e s . These p a r t i c l e s were examined by X-ray microanalysis (XMA). Vanadium deposits were found to be concentrated near the reactor i n l e t and near the outer surfaces of catalyst p a r t i c l e s . Metal deposits decrease along the reactor axis and toward the center of catalyst particles. At the reactor inlet itself, vanadium i s less concentrated. There i s also less vanadium i n a very thin layer at the outside of catalyst p a r t i c l e s . This phenomenon indicates that the HDM process requires hydrogen s u l f i d e . As shown i n the broken lines in Figure 8, the phenomenon disappears i f HDM i s performed with added hydrogen s u l f i d e . Deposited metals exist as sulfides and show X-ray d i f f r a c t i o n (XRD) patterns attributable to a c r y s t a l l i n e V^S phase (8) , as shown in Figure 9. In t h i s V^S^ phase, vanadium may be substituted by other t r a n s i t i o n metals such as n i c k e l . 4 ^ non-stoichiometric compound i n which the atomic r a t i o of sulfur to vanadium can vary from 1.2 to 1.5. When vanadium s u l f i d e deposits over catalyst outer surfaces, the p o l y c r y s t a l l i n e state can be observed by scanning electron microscopy (SEM), as shown i n Figure 10a. In catalyst pores, vanadium s u l f i d e forms rod-shaped deposits several tens of nanometers thick and hundreds of nanometers i n length, as determined by transmission electron microscopy (TEM) on an u l t r a - t h i n sample (Figure 10b). Electron d i f f r a c t i o n (ED) studies also were performed (Figure 10c). Since the deposited vanadium has been i d e n t i f i e d as ^^4 information about the surface of the vanadium s u l f i d e has been obtained by X-ray photoelectron spectroscopy (XPS) and ESR. The XPS spectra are shown i n Figure 11. The vanadium i n the V S^ exists p a r t l y as V , and on the outer surface of catalyst p a r t i c l e s partly as V . C a t a l y t i c a c t i v i t i e s of the surface thus w i l l vary with the makeup of the V^S metal cluster compound. ESR spectra (Figure 12) show that some vanadium dispersed on catalyst surfaces exists as vanadyl (V=0) i o n . 4

V

S

s a

3

r

+4

4

The Change of Vanadyl Coordination As reported previously (3) , the nature of the ligands involved i n chelation with vanadium may be determined by ESR spectra. This i s done by examining the i s o t r o p i c parameters g and A (the hyperfine constant). Ligand systems of vanadyl ion remaining i n treated crudes are of the four-nitrogen donor type ( ) (Figure 13). Vanadyl ion deposited on catalysts i s complexed with sulfur (VOS^). V 0 N

4

C a t a l y t i c A c t i v i t y of Vanadium Sulfide I t has been shown that vanadium sulfide deposits grow i n a d i r e c t i o n a l l y selective manner as the V S phase builds up. This growing

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

284

METAL COMPLEXES IN FOSSIL FUELS

θ h

0.3 0.4 0.5 I

[

Ο

ι

ι

200

,

I

400

[

1

600

Average Pore Diameter Based on Fresh Catalyst ( Â )

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θ : Loss in pore volume due to metal sulfides deposition

Figure 7 . Diameter.

Hydrodemetallization

inlet

Activity

Reactor Axial Length

with

Average

outlet

outer surface center Catalyst Particle Radius

Figure

8.

D i s t r i b u t i o n of Deposited Vanadium.

V3S4

10

20

30

40 2Θ ( )

50

60

e

Figure

9.

X-ray D i f f r a c t i o n Pattern.

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Pore

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Figure 10. and TEM.

Characteristics of Vanadium Complexes in Petroleum

Observation

of Deposited

Vanadium Surfide with SEM

sputtering time

(min) U s e d Catalyst

Surface

1

?m 520

285

0 1

513

Binding Energy (eV)

Figure 11. Oxidation State of Vanadium.

In Metal Complexes in Fossil Fuels; Filby, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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METAL COMPLEXES IN FOSSIL FUELS

Figure 12.

ESR Spectra of Vanadiums.

160 products

-supported

140

(ΗΤ,ΤΤ)

V0S04

feeds

7"—

spent catalyst

V0(N4K^-

V0S4

N

Ζ 120

ο