Metal Complexes in Fossil Fuels - American Chemical Society

Chapter

18

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

Downloaded by UNIV OF PITTSBURGH on October 18, 2013 | http://pubs.acs.org Publication Date: July 6, 1987 | doi: 10.1021/bk-1987-0344.ch018

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.

276

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.


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).



18.

ASAOKA ET AL.

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


277


278


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


(Boscan

18.

ASAOKA ET AL.

279



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



280


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


during

18.

ASAOKA ET AL.



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.


281

282


20

hydrovisbreaking Q i visbreaking Ο feed A hydrovisbreaking /hydrotreating

10


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.


and

18.


ASAOKA ET AL.

283


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


284


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 )


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.


Pore


18.

ASAOKA ET AL.

Figure 10. and TEM.


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.



286


Figure 12.

ESR Spectra of Vanadiums.

160 products

-supported

140

(ΗΤ,ΤΤ)

V0S04

feeds

7"—

spent catalyst

V0(N4K^-

V0S4

N

Ζ 120

ο

Metal Complexes in Fossil Fuels - American Chemical Society

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