Catalytic Materials - American Chemical Society

6 Magnetic Resonance Studies of Metal Deposition on Hydrotreating Catalysts and Removal with Heteropolyacids 1

Downloaded by CORNELL UNIV on May 30, 2017 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch006

B. G. SILBERNAGEL, R. R. MOHAN, and G. H. SINGHAL Corporate Research, Science Laboratories, Exxon Research and Engineering Company, Linden, NJ 07036 Nuclear magnetic resonance and electron spin resonance techniques have been used to trace the deposition of vanadium on Co-Mo/alumina catalysts employed to hydrotreat heavy, highly metalized pe troleum feedstocks. The vanadium is deposited on the catalyst in a sequence of chemical forms which vary with the level of metals loading: initially a para magnetic VO form dominates, followed by a diamagnetic vanadium form believed to be associated with the alumina surface. At levels in excess of several percent, the dominant form is a vanadium sulfide of approximate composition V S . Molybdophosphoric acid exposure facilely removes the sulfides, with the surface and paramagnetic forms being much more refrac tory. A scheme for enhancing metals removal by com plete conversion to the sulfide is discussed. 2+

2

3

The present paper w i l l describe the a p p l i c a t i o n of magnetic resonance spectroscopy, both nuclear magnetic resonance (NMR) and e l e c t r o n spin resonance (ESR), for analyzing the process of metals deposition on c a t a l y s t s during the treatment of heavily metalized petroleum feedstocks and for studying the removal of these metals during e x t r a c t i o n with h e t e r o p o l y a c i d s . We begin with a b r i e f d e s c r i p t i o n of the hydrotreating process and the systematics of metals d e p o s i t i o n , as traced by elemental a n a l y s i s and microprobe t e c h n i q u e s . We then compare NMR and ESR studies of model systems synthesized in the laboratory with analyses of discharged c a t a l y s t s , i l l u s t r a t i n g the different vanadium forms present on the c a t a l y s t . F i n a l l y , we describe the heteropolyacid e x t r a c t i o n process and demonstrate the role that magnetic resonance plays in i t s a n a l y s i s . As the world's known crude o i l reserves d i m i n i s h , we are confronted with the prospect of t r e a t i n g p r o g r e s s i v e l y less d e s i r a b l e crude o i l s . These materials contain high l e v e l s (typ i c a l l y several percent) of organic s u l f u r and n i t r o g e n , as well as o r g a n i c a l l y complexed vanadium and nickel at the level of 1

Current address: Baytown Research and Development Laboratories, Exxon Research and Engineering Company, Baytown, TX 77520. 0097-6156/84/0248-0091$06.00/0 © 1984 American Chemical Society

Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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CATALYTIC MATERIALS

hundreds of parts per m i l l i o n . These heavier feedstocks also contain a s i g n i f i c a n t high-molecular-weight f r a c t i o n which i s d i f f i c u l t to r e f i n e . A preliminary step in r e f i n i n g such mat e r i a l s i s c a l l e d h y d r o t r e a t i n g , a process intended to demetali z e the feed, s i g n i f i c a n t l y reduce the l e v e l s of s u l f u r and n i t r o g e n , and to reduce the molecular weight of the petroleum. Such a process must contend with the high l e v e l s of s u l f u r generated as a by-product. Treating 25,000 barrels/day of a feed c o n t a i n i n g 5 wt% s u l f u r produces 200 tons/ day of s u l f u r ! The c a t a l y s t s must thus be s u l f u r - t o l e r a n t , and i t has been the p r a c t i c e to use s u l f i d e c a t a l y s t s . ( 1 ) A high-surface-area a l u mina support is impregnated with metals such as cobalt and molybdenum (or p o s s i b l y nickel and tungsten), c a l c i n e d , and then exposed to an environment which "sulfides" the c a t a l y s t . The p r e c i s e form of the c a t a l y t i c metals a f t e r such a treatment i s a subject of present research i n t e r e s t . ( 2 , 3 ) We w i l l focus the present d i s c u s s i o n on the t o p i c of meta l s , mostly vanadium and n i c k e l , which are deposited from the feed onto the c a t a l y s t during the hydrotreating p r o c e s s . This i s not an i n s i g i f i c a n t amount of m a t e r i a l , since 25,000 b a r r e l s of a feed containing 100 ppm. metals w i l l deposit one-half ton of vanadium and nickel on the c a t a l y s t being employed to t r e a t it. This metals chemistry has been the subject of considerable recent r e s e a r c h . ( 4 , 5 ) P r o f i l e s of the metals d i s t r i b u t i o n in i n d i v i d u a l c a t a l y s t p e l l e t s using energy-dispersive x-ray t e c h niques show s u b s t a n t i a l metals accumulation near the pellet s u r f a c e , the form of the p r o f i l e depending on the s i z e d i s t r i b u t i o n of the c a t a l y s t pores and the character of f e e d s t o c k . ( 5 , 6 ) As we w i l l demonstrate, i t i s very useful to know the amount and chemical form of metals deposited throughout the reactor, changes in deposit chemistry as a function of time, and the i n f l u e n c e of deposited metals on c a t a l y s t a c t i v i t y and s e l e c t i v ity. Vanadium and nickel in the s t a r t i n g petroleum f r a c t i o n s occur l a r g e l y as o r g a n i c a l l y complexed s p e c i e s . Figure 1 shows a t y p i c a l ESR absorption from a crude o i l , revealing the w e l l a r t i c u l c ^ e d , sixteen-component spectrum associated with a vanadyl (V0 ) species. ESR g-values and hyperfine coupling cons t a n t s , deduced from the magnetic f i e l d p o s i t i o n s and s p l i t t i n g s of these components, i n d i c a t e that t h i s i s a vanadyl porphyrin species.(J) The integrated i n t e n s i t y of the spectrum i n d i c a t e s that more than 80% of the vanadium in the sample i s in the porphyrin form. Figure 2 shows the ESR spectrum of a discharged hydrotreating c a t a l y s t . Two other paramagnetic species are a l s o prominent in t h i s spectrum; a very i n t e n s e , narrow signal a s s o c i a t e d with carbon r a d i c a l s in the c a t a l y s t coke and an absorption associated with an oxygen-coordinated form of M o . However, the dominant ESR features are again those of a vanadyl species. The spectral components are s i g n i f i c a n t l y broader, and a detailed analysis of the g-value and hyperfine coupling c o n 5 +


93

Metal Deposition on Hydrotreating Catalysts

SILBERNAGEL ET AL.


6.

Figure

2:

ESR spectrum of Carbon R a d i c a l s , Mo° Species Vanadyl Ions on a Discharged Catalyst (u = GHz, H = 3240G, f i e l d scan = 2 KG). Q

0


and 9.1

94

CATALYTIC MATERIALS

stants reveals that t h i s V(r i s not a vanadyl p o r p h y r i n . The signature observed here appears associated with a V 0 ^ ion a s s o c i a t e d with defect s i t e s on the alumina support and has no r e l a t i o n s h i p to the s t a r t i n g organic s p e c i e s . This was demon s t r a t e d by exposing pure alumina support material and alumina pre-impregnated with cobalt and molybdenum to aqueous s o l u t i o n s of V O S O 4 . Upon reduction in an H 2 / H 2 S environment at 350°C 45 minutes, a paramagnetic signal was observed which was i d e n t i cal to that shown in Figure 2, independent of the presence of the a c t i v e m e t a l s . ( 7 j The number of paramagnetic vanadyl spe c i e s as determined by i n t e g r a t i n g the ESR spectrum is found to be proportional to the surface area of the alumina support em ployed, suggesting that s p e c i f i c surface s i t e s on the alumina serve as the residence for these V 0 ^ i o n s . The i n t e n s i t y data a l s o suggest that the maximum number of vanadium atoms that can be accommodated in t h i s form i s on the order of a f r a c t i o n of a percent (by weight) of the c a t a l y s t . The balance of the vanadium must occur in other forms, which we +


+

Figure

3:

NMR Spectrum of Vanadium and Aluminum Forms on a Discharged Catalyst (υ = 15 MHz, H = 13401G, f i e l d scan = 400G). Q


for

6.

95

have traced by NMR t e c h n i q u e s . Figure 3 shows an NMR absorption spectrum f o r a discharged c a t a l y s t . This spectrum e x h i b i t s an NMR absorption from the alumina support, a second component from the aluminum metal of the NMR bridge and two absorptions a s s o c i ated with vanadium s p e c i e s . The w e l l - d e f i n e d signal at lower f i e l d i s i d e n t i f i e d on the basis of i t s shape and resonance position as a vanadium sulfide of approximate composition 2$3.(Ζ) poorly defined signal at somewhat higher f i e l d values i s related to a diamagnetic vanadium s p e c i e s . The lack of d e f i n i t i o n of the signal suggests an amorphous vanadium form; the i n t e r a c t i o n of the vanadium quadrupole moment with a disordered environment causes the kind of "smearing" observed here. The amount of material associated with t h i s diamagnetic vanadium phase (several weight percent) would be consistent with a t h i n surface layer on the c a t a l y s t . These data taken together suggest that vanadium i s depos i t e d on the c a t a l y s t in three successive forms. The i n i t i a l vanadium which appears on the c a t a l y s t i s p r i m a r i l y an i s o l a t e d V0 s p e c i e s , presumably associated with alumina defect s i t e s . This i s followed by the diamagnetic vanadium surface phase and f i n a l l y by the vanadium s u l f i d e s . This progression i s i l l u s t r a t e d by the a n a l y s i s of c a t a l y s t samples taken from d i f f e r e n t p o s i t i o n s in a reactor which had been employed in a p i l o t - p l a n t treatment of a petroleum residuum (Figure 4 ) . Note that a l l of ν



SILBERNAGEL ET A L .

T

h

e

m

o

r

e

2 +

ΙΟΟι

VANADIUM LOADING, RELATIVE UNITS

OIAMAGNETIC V VP?* 0.5

1.0

FRACTIONAL BED LENGTH Figure

4:

Vanadium D i s t r i b u t i o n and Chemical Forms at D i f ferent P o s i t i o n s in a Reactor Used to Treat a Heavy Feed.



96

CATALYTIC MATERIALS

the samples show the VCr ESR absorption, while four samples show the diamagnetic vanadium s p e c i e s . This i n d i c a t e s that these vanadium s p e c i e s , once deposited, do not convert as the c a t a l y s t ages. A recent conjecture that the d i f f e r e n t forms of vanadium encountered on the c a t a l y s t r e s u l t from d i f f e r e n t organic host molecu1es(8) i s not consistent with these systematics. There are several i n c e n t i v e s to regenerate such discharged c a t a l y s t s by removing the deposited metals and carbon, which w i l l become i n c r e a s i n g l y greater when l a r g e r amounts of heavy, metals-bearing petroleum must be r e f i n e d . The high metals l e v e l s of heavy feeds t y p i c a l l y contemplated for RESIDFINING w i l l r e s u l t in a high metals deposition r a t e . The amount of deposited metals w i l l equal the s t a r t i n g weight of the c a t a l y s t for t r e a t ment of feeds in the 300-500-ppm range a f t e r a run of several months' d u r a t i o n . This discharged c a t a l y s t must be e i t h e r r e generated or r e p l a c e d . The increased expense of the c a t a l y s t , coupled with the s u b s t a n t i a l e c o l o g i c a l problems associated with the disposal of large volumes of used c a t a l y s t s , suggests that regeneration merits careful c o n s i d e r a t i o n . Furthermore, these c a t a l y s t s could prove to be valuable sources of vanadium and n i c k e l metal, since discharged c a t a l y s t s may have a higher content of these metals than many native o r e s . For regeneration to be t e c h n i c a l l y v i a b l e , i t must be able to remove deposited vanadium and nickel q u a n t i t a t i v e l y as well as the carbonaceous coke which was c o - d e p o s i t e d . The c a t a l y t i c a l l y a c t i v e metals should remain unaffected in amount, chemist r y , and state of d i s p e r s i o n . The alumina support should remain i n t a c t , with the surface area, p o r e - s i z e d i s t r i b u t i o n and crush strength a f t e r treatment comparable to that of the o r i g i n a l . To be economical1y v i a b l e , the process should be accomplished in a minimum of steps at nearly ambient temperatures and preferably in aqueous s o l u t i o n . The ultimate proof of any such scheme i s f o r the c a t a l y t i c a c t i v i t y of the regenerated c a t a l y s t to be equal to that of a fresh one. The simultaneous deposition of metals and coke during hyd r o t r e a t i n g provides a p a r t i c u l a r c o m p l i c a t i o n . In the absence of the deposited metals, these c a t a l y s t s can be regenerated by burning the coke o f f the c a t a l y s t surface.(9J However, the presence of vanadium and nickel on the c a t a l y s t surface leads to s u b s t a n t i a l s i n t e r i n g of the c a t a l y s t support. Similarly, acid e x t r a c t i o n of metals from such c a t a l y s t s has been proposed, but such techniques n o n s e l e c t i v e l y remove metals from the c a t a l y s t surface and often do severe damage to the support.(10) One c l a s s of acids in which these problems are minimized i s the heterpolyacids,(ll) one example being molybdophosphoric acid ( M P A ) : H3PMO 2Û4Q.XH20. discussed below, these acids are easily synthesized in aqueous s o l u t i o n and s t a b l e under the reaction c o n d i t i o n s . They can be made to provide a s u f f i c i e n t l y low pH in water s o l u t i o n to s o l u b i l i z e vanadium and nickel s u l 1

A

s


fides. The reaction a s s o c i a t e d with t h e i r ly described by: α M 0 O

3

+ H 0 ——>

4

2

formation

H PMo 0.xH 0 3

Y

2

is

general

(

1

)

The phase behavior of these molybophosphoric acid complexes has been studied as a function of pH and a v a r i e t y of complexes are observed.(10) For pH values below 4, the dominant complex i s 3 1 2 ° 4 0 . 2 . experiments described here have been perrormed with p H - 2 . Batch e x t r a c t i o n studies were employed with 0 . 4 wt% MPA s o l u t i o n s , at temperatures of ~ 4 5 ° C . The e x t r a c t i o n process was continuously monitored by examining the metals in the e x t r a c t . The e x t r a c t i o n process was e s s e n t i a l l y complete in a time s c a l e of several days, although the c a t a l y s t type and pore geometry i n f l u e n c e d the e x t r a c t i o n r a t e . The amounts of metals extracted a f t e r such a treatment are shown in Table I, for a r e l a t i v e l y small-pore (~50Â diameter) c a t a l y s t and a l a r g e r - p o r e (~150Â) one. The nickel i s nearly q u a n t i t a t i v e l y e x t r a c t e d , while no changes in e i t h e r molybdenum or aluminum level were observed. Conversely, only t h r e e - q u a r t e r s of the deposited vanadium was removed. A conspicuous negative feature of t h i s extraction procedure i s that nearly h a l f of the cobalt was l o s t during the treatment. NMR and ESR provide some reasons f o r the lack of complete vanadium removal during the treatment. NMR shows that the vanadium s u l f i d e species has been completely removed to within the p r e c i s i o n of the observation, while l o s s of the diamagnetic surface phase i s considerably l e s s pronounced. ESR observations reveal l i t t l e i f any change in the number of paramagnetic vanadyl species on the c a t a l y s t . Thus i t appears that the s u l f i d e forms are p a r t i c u l a r l y s u s c e p t i b l e to e x t r a c t i o n by MPA, while the other forms are n o t . NMR studies of the extract solution suggest that the vanadium removed from the c a t a l y s t i s i n c o r porated into the MPA c l u s t e r , g i v i n g i t a composition of H 3 P V Η


+ H P0

3

97


6. SILBERNAGEL ET AL.

Μ

Ρ

Ο

Μ

Ο

Χ

Η

0

M

o

s

t

o

f

t

h

e

1 1 ° 4 ρ . X H 2 O .

The cobalt loss i s a very great concern f o r the p r a c t i c e of t h i s technology. One means of remedying the d i f f i c u l t y i s to r e s t o r e the cobalt by a separate impregnation a f t e r the MPA treatment. This has been done in two ways: by impregnating the c o b a l t in a s i n g l e step and then c a l c i n i n g , and by doing two successive impregnations with h a l f the t o t a l amount of c o b a l t added in each c a s e . As Table II i n d i c a t e s , t h i s process leads to a nearly complete recovery of surface area and pore volume of the support. The successive increase of -50% in both q u a n t i t i e s following the MPA treatment and again a f t e r the c a l c i n a t i o n stages produces s u r f a c e - a r e a and pore-volume values comparable to the o r i g i n a l c a t a l y s t . Perhaps as s i g n i f i c a n t i s the f a c t that the crush strength of the treated p e l l e t s i s comparable to the s t a r t i n g c a t a l y s t support. Hydrodesulfurization t e s t s on


CATALYTIC MATERIALS

98

Table

I:

Metals Extraction

SPENT CATALYSTS

PERCENT EXTRACTED NICKEL COBALT MOLYBDENUM

SMALL PORE (d-50A)

70-80

98

-45

LARGE PORE (d~150A)

70-80

93

-40

Table Downloaded by CORNELL UNIV on May 30, 2017 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch006

VANADIUM

Effects

II:

Surface Area and Pore Volume

TREATMENT

Recovery

SURFACE AREA

PORE VOLUME

(m /g)

(cm /g)

2

3

DISCHARGED CATALYST

136

0.24

AFTER MPA TREATMENT

200

0.34

SINGLE CO IMPREGNATION

225

0.43

DOUBLE CO IMPREGNATION

275

0.55

-275

-0.50

FRESH CATALYST

ALUMINUM

these regenerated c a t a l y s t s y i e l d a c t i v i t i e s of 75% of fresh c a t a l y s t for the s i n g l e c a l c i n a t i o n sequence and -100% for the dual c a l c i n a t i o n . In reviewing these r e s u l t s , we would l i k e to emphasize the information they provide about metals deposition chemistry as well as the p o t e n t i a l u t i l i t y of the proposed regeneration process. The metals e x t r a c t i o n studies confirm the fact that three d i s t i n c t phases of vanadium occur on these c a t a l y s t s and that they are of varying r e a c t i v i t y . By c o n t r a s t , the nickel removal i s e s s e n t i a l l y complete under s i m i l a r c o n d i t i o n s . We suggest that the chemical s t a b i l i t y of the nickel s u l f i d e phase causes i t to form under a broader range of temperatures and s u l f u r p a r t i a l pressures than the vanadium s u l f i d e s , and that the n i c k el s u l f i d e i s r e a d i l y soluble in MPA. A l l vanadium species can be converted to the s u l f i d e by treatments in H S at temperatures 2


6.

SILBERNAGEL ET AL.


99

of 5 0 0 ° C - 6 0 0 ° C for durations of several hours.(10,13) Samples r e c e i v i n g that pretreatment can be q u a n t i t a t i v e l y demetalized by MPA e x t r a c t i o n . ( 1 3 ) The economic p r a c t i c e of an e x t r a c t i o n technology based on MPA hinges on several f a c t o r s . The e x t r a c t i o n rate can be en hanced d r a m a t i c a l l y by the a d d i t i o n of Η 0 to the MPA s o l u tion.(^4) The removal of cobalt during e x t r a c t i o n i s a cause f o r concern because recovering the cobalt from the extract and reimpregnation of the c a t a l y s t adds steps to the treatment pro cess. The d e t a i l s of such a regeneration process in actual p r a c t i c e would be defined by the s p e c i f i c nature of the c a t a l y s t to be t r e a t e d .


2

2

Literature Cited 1.

2. 3. 4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14.

For an introduction, see Weisser, O.; Landa, S. "Sulfide Catalysts, Their Properties and Applications"; Pergamon: New York, 1973. Silbernagel, B. G.; Pecoraro, T. A. Chianelli, R. R., J. Catalysis 1982, 78, 380-88. Clausen, B. S.; Topsøe, H.; Candia, R; Lengeler, B. This volume. Sie, S. in "Catalyst Deactivation, vol. 6", Delmon, B.; Froment, G., Eds.; Elsevier: Amsterdam, 1980, p. 545-69. Silbernagel, B. G.; Riley, K. L. in "Catalyst Deactivation, vol. 6," Delmon, B.; Froment, G., Eds.; Elsevier: Amster dam, 1980, pp. 313-21. Tamm, P. W.; Harnsberger, H. F.; Bridge, A. G. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 262-73. Silbernagel, B. G. J. Catalysis, 1979, 56, 315-20. Mitchell, P.C. H.; Valero, B., 1983, Inorganica Chimica Acta 71, 179-84. See, e.g. Satterfield, C. N. "Heterogeneous Catalysis in Practice", McGraw-Hill: New York, 1980, pp. 272-7. Gamble, F. R.; Levy, R. B., U.S. Patent 4,014,815, 1977. See, e.g. Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shum, W. Inorg. Chem, 1979, 18, 93-103. Mohan, R. R.; Singhal, G. H., U.S. Patent, 4,272,401, 1981. Silbernagel, B. G.; Mohan, R. R.; Singhal, G. H. U.S. Pat ent 4,272,400, 1981. Mohan, R. R.; Silbernagel, B. G.; Singhal, G. H., U.S. Patent 4,268,415, 1981.

R E C E I V E D December 5, 1983


Catalytic Materials - American Chemical Society

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