Solid State Chemistry in Catalysis - American Chemical Society

13 The Role of Solid State Chemistry in Catalysis by Transition Metal Sulfides R. R. CHIANELLI

Downloaded by UNIV OF PITTSBURGH on September 14, 2013 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

Corporate Research Science Laboratories, Exxon Research & Engineering Company, Annandale, NJ 08801 The Transition Metal Sulfides are a group of solids which form the basis for an extremely useful class of industrial hydrotreating and hydroprocessing catalysts. Solid state chemistry plays an important role in understanding and controlling the catalytic properties of these sulfide catalysts. This report discusses the preparation of sulfide catalysts, the role of disorder and anisotropy in governing catalytic properties, and the role of structure in the promotion of molybdenum disulfide by cobalt. The Transition Metal Sulfides have been widely used i n petroleum upgrading processes for many years, and due to their c a t a l y t i c and structural s t a b i l i t y i n feedstocks containing large amounts of s u l f u r , the demand for better sulfide catalysts w i l l continue as we are forced to upgrade an increasingly heavier feedstock supply (Ο. Although i n d u s t r i a l l y Important for over sixty years, i t has been only recently that progress has been made in forming a basis for a fundamental understanding of how these solids catalyze impor tant reactions. An understanding of the solid state chemistry of these solids has played a key role in this recent progress. The c a t a l y t i c a l l y important s o l i d state chemistry of the sulfides includes not only "classical" s o l i d state areas such as the struc ture of supported and unsupported catalysts, but also areas which are at the forefront of solid state chemistry i t s e l f . These areas include novel low temperature methods for producing the s o l i d catalysts at low temperature, the study of disorder and i t s effect on the c a t a l y t i c properties of the solids and the importance of c r y s t a l l i n e anisotropy in determining and controlling the r e a c t i v i t y of the s o l i d , both to the c a t a l y t i c environment and to other metals which may cause poisoning or promotion. This report discusses these issues as the basis for understanding the relationship between the properties of the solids and their a b i l i t y to catalyze a reaction. 0097-6156/ 85/ 0279-0221 $06.00/ 0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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B i n a r y T r a n s i t i o n M e t a l S u l f i d e H y d r o d e s u l f u r i z a t l o n (HPS) C a t a l y s t s I n the p e r i o d between WWI and WWII, work p r i m a r i l y i n Germany focused on M0S2 and WS~ as b e i n g the best s u l f i d e s f o r h y d r o g é n a t i o n and heteroatom removal r e a c t i o n s i n the presence of hydrogen and s u l f u r (2). O r i g i n a l l y , these c a t a l y s t s were used i n an unsupported form and w i t h o u t a d d i t i o n a l t r a n s i t i o n metals which serve to promote catalytic activity. R e c e n t l y , the b i n a r y t r a n s i t i o n m e t a l s u l f i d e s ( b i n a r y r e f e r s to the simple t r a n s i t i o n metal s u l f i d e s c o n t a i n i n g one t r a n s i t i o n m e t a l and s u l f u r ) have been i n v e s t i g a t e d f o r t h e i r a c t i v i t y i n a model HDS r e a c t i o n ( 3 ) . The model HDS r e a c t i o n was the d e s u l f u r i z a t i o n o f d i b e n z o t h i o p h e n e (DBT) and a l l the group I V , V , V I , V I I and V I I I t r a n s i t i o n metal s u l f i d e s , w i t h the e x c e p t i o n of T c , were s t u d i e d . The a c t i v e s u l f i d e phases as determined by X - r a y d i f f r a c t i o n a f t e r r e a c t i v i t y measurements, along w i t h the a c t i v i t y of these phases i n the HDS r e a c t i o n are presented i n F i g u r e 1. Most of the phases which were i d e n t i f i e d a f t e r a p p r o x i m a t e l y e i g h t hours under c a t a l y t i c c o n d i t i o n s (A00°C and 1300 kpa) were p o o r l y c r y s t a l l i n e , as determined by broadened Bragg d i f f r a c t i o n peaks. However, i n some cases (Os and I r ) the d i f f r a c t i o n p a t t e r n obtained contained only diffuse scattering and no h i n t of any r e m a i n i n g Bragg p e a k s . The d i s o r d e r ( d i s c u s s e d f u r t h e r below) which appears i n these c a t a l y s t s c o n t r i b u t e s ( i n some c a s e s ) t o an u n c e r t a i n t y r e g a r d i n g the p r e c i s e phase which i s the s t a b l e s t a t e of the a c t i v e s u l f i d e under c a t a l y t i c c o n d i t i o n s . For example, i n the cases o f Os and I r amorphous phases o f the m e t a l s u l f i d e s were obtained. These phases have a p p r o x i m a t e l y a 1:1 metal-to-sulfur s t o i c h i o m e t r y , but c u r r e n t l y t h e i r s t r u c t u r e s are unknown. In the cases of V and Fe the p o o r l y c r y s t a l l i n e s t a t e of the c a t a l y s t s d i d not permit an exact c h o i c e of phase based s o l e l y on X - r a y d i f f r a c t i o n data from among s e v e r a l c l o s e l y r e l a t e d phases ( 4 ) . Nevertheless, i n most cases the c r y s t a l s t r u c t u r e , upon which the c a t a l y s t i s based, can be d i s c e r n e d . On the l e f t of the p e r i o d i c t a b l e i n group I V , V and V I we f i n d s t r u c t u r e s which are l a y e r e d types e i t h e r cadmium i o d i d e or m o l y b d e n i t e l i k e ( T i S , Z r S , NbS^, T a S , M o S , and WS ) or n i c k e l a r s e n i d e r e l a t e d ( V S , C r ^ ) . A l l c o n t a i n o n l y s i x c o o r d i n a t e m e t a l atoms, but as we move f u r t h e r to the r i g h t i n the p e r i o d i c t a b l e we f i n d t h a t the s t r u c t u r e s v a r y to a g r e a t e r e x t e n t . In the f i r s t row we have d i f f e r e n t s t r u c t u r e s s t a r t i n g w i t h MnS through to N i g S , which are dominated by t e t r a h e d r a l c o o r d i n a t i o n as w e l l as v a r i a b l e s t o i c h i o m e t r y . I n the second row we f i n d R u S w i t h a p y r i t e s t r u c t u r e (whereas i n the f i r s t row F e S i s not s t a b l e under c a t a l y t i c c o n d i t i o n s ) , R h S ^ w i t h a nickel arsenide related structure and PdS w i t h a unique structure. I n the t h i r d row we f i n d R e S w i t h a d i s t o r t e d l a y e r e d s t r u c t u r e , Os and I r w i t h undetermined amorphous s t r u c t u r e s , PtS w i t h a unique s t r u c t u r e and Au not forming a s t a b l e s u l f i d e under catalytic conditions. In o t h e r words, we f i n d t h a t as we move a c r o s s the p e r i o d i c t a b l e the s t a t e of the s u l f i d e c a t a l y s t is c o n s t a n t l y changing i n r e g a r d to c r y s t a l s t r u c t u r e , s t o i c h i o m e t r y and degree o f o r d e r . Yet the c a t a l y t i c a c t i v i t y f o r the model r e a c t i o n i s v a r y i n g i n a c o n t i n u o u s f a s h i o n as we move from element to element. F u r t h e r m o r e , the a c t i v i t y i s v a r y i n g i n a way f a m i l i a r 2

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Catalysis by Transition Metal Sulfides

PERIODIC POSITION

Figure 1. Periodic trend for the HDS of Dibenzothiophene by Transition Metal Sulfides.


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i n other areas of c a t a l y s i s : the " v o l c a n o " p l o t ( 5 ) . From F i g u r e 1 i t can be seen t h a t the f i r s t row t r a n s i t i o n m e t a l s u l f i d e s a r e i n a c t i v e r e l a t i v e to the second and t h i r d row t r a n s i t i o n m e t a l s u l f i d e s which e x h i b i t maxima i n the group V I I I m e t a l s , at R u S i n the second row and between Os and I r i n the t h i r d row. This b e h a v i o r p o i n t s to the secondary r o l e of c r y s t a l s t r u c t u r e and t o the dominance of the 4 and 5 d e l e c t r o n s i n d e t e r m i n i n g c a t a l y t i c a c t i v i t y i n the t r a n s i t i o n m e t a l s u l f i d e s . A c o n s i d e r a b l e amount o f work has gone i n t o u n d e r s t a n d i n g the o r i g i n of t h i s e f f e c t termed the " e l e c t r o n i c e f f e c t " i n s u l f i d e c a t a l y s i s , but a d e t a i l e d d i s c u s s i o n of t h i s work i s beyond the scope of t h i s paper ( 6 ) . Downloaded by UNIV OF PITTSBURGH on September 14, 2013 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch013

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Preparation of T r a n s i t i o n Metal S u l f i d e C a t a l y s t s The c a t a l y s t s d e s c r i b e d above were prepared v i a low temperature precipitation from non-aqueous solution (7). This technique i n v o l v e s the p r e c i p i t a t i o n of the t r a n s i t i o n m e t a l s u l f i d e from a non-aqueous solvent such as ethyl a c e t a t e by d i s s o l v i n g the a p p r o p r i a t e t r a n s i t i o n m e t a l h a l i d e i n the s o l v e n t and r e a c t i n g i t m e t a t h e t i c a l l y w i t h a s u l f i d i n g agent such as l i t h i u m s u l f i d e to p r e c i p i t a t e the i n s o l u b l e s u l f i d e f o r example: ethyl MoCl

4

+

2 Li S

>

2

MoS

2

Ψ +

4 UCl

(1)

acetate The b l a c k p r e c i p i t a t e i s s e p a r a t e d from the product L i CI by e x t e n s i v e washing w i t h e t h y l a c e t a t e . Because the product i s formed r a p i d l y at room t e m p e r a t u r e , i t i s c o m p l e t e l y amorphous t o X - r a y s . The amorphous MoS w h i c h has i n i t i a l l y low surface a r e a (~ 5m /gm) may then be c o n v e r t e d i n t o h i g h e r s u r f a c e area p o o r l y c r y s t a l l i n e MoS by heat t r e a t i n g i n a f l o w i n g gas of H /15% H S at e l e v a t e d temperature ( 8 ) . For example, i f amorphous M o S , prepared as d e s c r i b e d above, i s t r e a t e d at 400°C or 600°C i n a H /15% H S m i x t u r e , the r e s u l t i n g s u r f a c e a r e a s w i l l be 63 and 44 M /gm, r e s p e c tively. Thus, by c o n t r o l l i n g the temperature of the heat treatment a c o n t i n u o u s s e r i e s o f MoS c a t a l y s t s can be prepared which have v a r i a b l e surface areas. I n a s i m i l a r manner, a l l the t r a n s i t i o n m e t a l s u l f i d e s can be prepared i n an o x i d e free form w i t h s u f f i c i e n t s u r f a c e a r e a f o r c a t a l y t i c measurements. The lower temperature p r e c i p i t a t i o n method o f f e r s an a d d i t i o n a l advantage. In p r e p a r i n g a l l the t r a n s i t i o n m e t a l s u l f i d e s the c h e m i s t r y i s v a r i e d as l i t t l e as p o s s i b l e from one s u l f i d e to the n e x t . No o t h e r preparative method c u r r e n t l y a v a i l a b l e o f f e r s a l l these a d v a n t a g e s . Further more, by changing the s o l v e n t to propylene carbonate a homogeneous c o l l o i d a l d i s p e r s i o n o f MoS can be o b t a i n e d which i s s t a b l e over long periods of time. By s l u r r y i n g a support m a t e r i a l such as AUO^, Si0 o r MgO w i t h the c o l l o i d a l M o S , the c a t a l y s t can be s e l e c t i v e l y adsorbed on the support and thus the e f f e c t of s u p p o r t i n g the t r a n s i t i o n m e t a l s u l f i d e s can be s t u d i e d , a g a i n keeping the method o f p r e p a r a t i o n as c l o s e l y r e l a t e d as p o s s i b l e . Another method used i n the p r e p a r a t i o n of the t r a n s i t i o n m e t a l s u l f i d e s i s the thermal d e c o m p o s i t i o n o f a s u i t a b l e p r e c u r s o r i n a 2

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s u l f i d i n g environment. For example, MoS may be prepared v i a thermal d e c o m p o s i t i o n o f ammonium t h i o m o l y b d a t e w h i c h proceeds through the amorphous i n t e r m e d i a t e M0S3 ( 9 ) : 2

250 °C (NH ) MoS 4

2

>

4

MoS

+

3

H S+ 2

+

2NH^+

(2)

400 °C MoS

>

3

MoS

2

+

S°

(3)

T h i s method has the advantage of b e i n g s i m p l e r than the above method and of p r o v i d i n g c a t a l y s t s w i t h s u r f a c e areas which are g e n e r a l l y h i g h e r than those o b t a i n e d by o t h e r methods (>100 M / g m ) . A third method of p r e p a r a t i o n which i s c o n v e n i e n t i s the d i r e c t s u l f i d a t i o n of the a p p r o p r i a t e ammonium h e x a c h l o r i d e i n H / H S ( 1 0 ) :


2

2

2

350 °C (NH.).OsCl, 4 2 6

+

2H S 2

>

0

0sS

H /H S 2 2

+

o

2

2NH.C1+ 4

+

4HC1 + K

}m

T h i s method, which has been a p p l i e d to the noble m e t a l s u l f i d e s , has the advantage t h a t a l l b y - p r o d u c t s are e a s i l y removed i n the f l o w i n g gas phase. Both the thermal d e c o m p o s i t i o n method and the d i r e c t s u l f i d a t i o n method can be a p p l i e d to s p e c i f i c s u l f i d e s o n l y i n cases where the p r e c u r s o r m a t e r i a l can be e a s i l y s y n t h e s i z e d . The low temperature p r e c i p i t a t i o n t e c h n i q u e i s the o n l y method which can be a p p l i e d to a l l the t r a n s i t i o n m e t a l s u l f i d e s . A l l the above methods e a s i l y p r o v i d e reasonable q u a n t i t i e s of h i g h s u r f a c e a r e a c a t a l y s t s for further study. The E f f e c t Catalysts

of

Crystal

Structure

in

Transition

Metal

Sulfide

I n the p r e c e d i n g p a r t o f t h i s paper the predominance of the "periodic" effect on HDS by s u l f i d e c a t a l y s t s was d e s c r i b e d . Because p e r i o d i c i t y dominates, c r y s t a l s t r u c t u r e i s of secondary importance. However, i n t h i s s e c t i o n we b r i e f l y examine the e f f e c t of c r y s t a l s t r u c t u r e on the c a t a l y t i c p r o p e r t i e s of the t r a n s i t i o n metal s u l f i d e s . In the case of c a t a l y s t s such as MoS and W S , the most i n d u s t r i a l l y important c a t a l y s t s , the e f f e c t of c r y s t a l s t r u c t u r e i s q u i t e pronounced. An u n d e r s t a n d i n g of the e f f e c t of c r y s t a l s t r u c t u r e i n these c a t a l y s t s i s e s s e n t i a l to o p t i m i z i n g t h e i r c a t a l y t i c properties for a given a p p l i c a t i o n . The e f f e c t of c r y s t a l s t r u c t u r e may be i n v e s t i g a t e d by p r e p a r i n g c a t a l y s t s , as d e s c r i b e d above, at v a r i o u s temperatures which a s s u r e s a set of c a t a l y s t s h a v i n g v a r i a b l e surface a r e a s , pore s i z e d i s t r i b u t i o n s , and c r y s t a l l i n i t y . Measuring the c a t a l y t i c a c t i v i t y as a f u n c t i o n of these p h y s i c a l p r o p e r t i e s w i l l h e l p to d e f i n e the r o l e of c r y s t a l s t r u c t u r e f o r the p a r t i c u l a r t r a n s i t i o n m e t a l sulfide. I n g e n e r a l , the HDS i s p o o r l y c o r r e l a t e d to N BET s u r f a c e area. T h i s n o n - c o r r e l a t i o n can be most e a s i l y seen by p r e p a r i n g a 2

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s e r i e s of M0S2 c a t a l y s t s by a v a r i e t y of methods and measuring the HDS a c t i v i t y as a f u n c t i o n o f BET s u r f a c e a r e a . There i s v i r t u a l l y no c o r r e l a t i o n between h y d r o d e s u l f u r i z a t i o n and BET s u r f a c e a r e a . There i s , however, a r a t h e r good c o r r e l a t i o n to a s p e c i f i c c h e m i s o r p t i o n t e c h n i q u e , i n t h i s case 0 chemisorption (11). It is b e l i e v e d t h a t t h i s r e s u l t a r i s e s from the a n i s o t r o p y o f the l a y e r e d structure (Figure 2). I n t h i s s t r u c t u r e s i n g l e l a y e r s of t r a n s i t i o n m e t a l s are sandwiched between two l a y e r s of c l o s e - p a c k e d c h a l c o g e n atoms. W i t h i n these l a y e r s the t r a n s i t i o n metal atoms are bound to s i x s u l f u r atoms which are arranged t r i g o n a l p r i s m a t i c a l l y about the metal. Each s u l f u r atom b r i d g e s t h r e e t r a n s i t i o n m e t a l atoms w i t h i n the same l a y e r , forming the o n l y s t r o n g i n t r a l a y e r f o r c e s , and the l a y e r s can be viewed as t w o - d i m e n s i o n a l macromolecules which s t a c k , bound o n l y by van der Waals f o r c e s , to form three d i m e n s i o n a l c r y s t a l s (12). As a r e s u l t of t h i s , the b a s a l planes of M0S2 c a t a l y s t s are e x t r e m e l y i n e r t c o n t r i b u t i n g to the N s u r f a c e area measurements but not to the c a t a l y t i c a c t i v i t y . 0 , on the o t h e r hand, c h e m i sorbs on the MoS edge p l a n e s where the c a t a l y t i c a l l y a c t i v e s i t e s a r e presumed to be l o c a t e d . The i n e r t n e s s of the b a s a l p l a n e s has been demonstrated i n h i g h vacuum s t u d i e s on MoS s i n g l e c r y s t a l s ( 1 3 ) . A d s o r p t i o n and b i n d i n g s t u d i e s of t h i o p h e n e , H S and r e l a t e d m o l e c u l e s were c a r r i e d o u t . These s t u d i e s i n d i c a t e d t h a t o n l y p h y s i c a l a d s o r p t i o n o f t h e s e m o l e c u l e s occur on the b a s a l plane of M o S . The probe m o l e c u l e s desorbed without detectable d e c o m p o s i t i o n i n d i c a t i n g v e r y low c h e m i c a l a c t i v i t y of the b a s a l p l a n e s of M o S . I t was f u r t h e r shown t h a t the b a s a l plane o f MoS was i n e r t to 0 exposure at 520 Κ (14). Only s p u t t e r i n g w i t h He i o n s , which caused the d e s t r u c t i o n of the hexagonal feed p a t t e r n , would induce r e a c t i v i t y toward 0 . The basal plane could be annealed at 100°K and its inertness recovered. From t h i s study i t was concluded t h a t d e f e c t s may be i n t r o d u c e d i n t o the s u r f a c e by s p u t t e r i n g , and t h i s produced a d r a s t i c i n c r e a s e i n the r a t e o f o x i d a t i o n of the s u r f a c e and i n removal of s u l f u r . T h i s i n d i c a t e s t h a t oxygen c h e m i s o r p t i o n (and thus HDS) i s a s s o c i a t e d w i t h d e f e c t s i t e s i n MoS and not o r d e r e d basal planes. I n a w e l l c r y s t a l l i z e d c a t a l y s t these d e f e c t s l i e on the edge p l a n e s of the c r y s t a l l i t e s . Because of the a n i s o t r o p i c n a t u r e of M o S , i t tends to grow i n v e r y t h i n c r y s t a l s which do not permit easy study o f edge p l a n e s . However, Tanuka and Okuhara showed t h a t the edge p l a n e s of MoS were r e a c t i v e f o r c e r t a i n types o f r e a c t i o n s by c u t t i n g s i n g l e c r y s t a l s i n t o p i e c e s and comparing the r a t e s of cut and uncut c r y s t a l s ( 15). F u r t h e r e v i d e n c e f o r 0« i n t e r a c t i o n at the edge planes o f MoS comes from s t u d i e s on s i n g l e c r y s t a l s . When s i n g l e c r y s t a l s of MoS are p l a c e d i n an o x i d i z i n g environment at e l e v a t e d temperatures ( > 4 0 0 ° C ) , o x i d a t i o n of the MoS can be seen to proceed through the edge p l a n e s ( 1 6 ) . F u r t h e r m o r e , oxygen enrichment at edge planes o f MoS s i n g l e c r y s t a l s has been demonstrated by scanning Auger s t u d i e s on s i n g l e c r y s t a l s ( 1 7 ) . R u S , on the o t h e r hand, has a c o m p l e t e l y i s o t r o p i c c u b i c s t r u c t u r e i d e n t i c a l to p y r i t e ( F e S ) . The i s o t r o p i c n a t u r e o f R u S i n the HDS r e a c t i o n can be seen by the l i n e a r c o r r e l a t i o n s between HDS a c t i v i t y and 0 c h e m i s o r p t i o n and HDS a c t i v i t y and BET s u r f a c e


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Figure 2.

S t r u c t u r e of a s i n g l e l a y e r o f MoS^. Reproduced w i t h

p e r m i s s i o n from R e f . 8.

C o p y r i g h t 1982, T a y l o r & F r a n c i s L t d .


228


area (18). RuS^ and MoS e x h i b i t the two extremes which can occur i n HDS by s u l f i d e s . Presumably, the e f f e c t of c r y s t a l s t r u c t u r e i n a l l o t h e r t r a n s i t i o n m e t a l s u l f i d e s l i e s somewhere i n between. 2

The R o l e Catalysts

of

Edge

Planes

in

Transition

Metal

Sulfide

The importance of edge p l a n e s a l s o a r i s e s i n the i n d u s t r i a l l y i m p o r t a n t promoted t r a n s i t i o n m e t a l s u l f i d e c a t a l y s t systems. I t has been known f o r many years t h a t the presence of a second m e t a l such as Co o r N i to a MoS o r WS c a t a l y s t l e a d s to promotion (an i n c r e a s e i n a c t i v i t y f o r HDS or h y d r o g é n a t i o n i n excess of the a c t i v i t y of the i n d i v i d u a l components) ( 2 ) . Promotion e f f e c t s can e a s i l y be observed i n supported o r unsupported c a t a l y s t s . The supported c a t a l y s t s are c u r r e n t l y the most i m p o r t a n t i n d u s t r i a l c a t a l y s t s , but the unsupported c a t a l y s t s are e a s i e r to c h a r a c t e r i z e and s t u d y . Unsupported, promoted c a t a l y s t s have been prepared by many d i f f e r e n t methods, but one c o n v e n i e n t way of p r e p a r i n g these catalysts is by a p p l y i n g the nonaqueous p r e c i p i t a t i o n method d e s c r i b e d above. F o r example, f o r Co/Mo, a p p r o p r i a t e m i x t u r e s of C o C l and MoCl^ are r e a c t e d w i t h L i S i n e t h y l a c e t a t e : 2


Promoted

2

2

2

ethyl xCoCl

2

+ yMoCl

4

+ (x + 2 y ) L i S

>

2

a c e t a t e 25°C x"CoS"(amorphous) + yMoS + 2(x + 2 y ) L i C l

(5)

2

The amorphous p r o d u c t s are then heat t r e a t e d i n 15% H / H S at the d e s i r e d t e m p e r a t u r e , as d e s c r i b e d above f o r the b i n a r y systems. By u s i n g t h i s t e c h n i q u e , the r a t i o of Co/Mo i n the c a t a l y s t can e a s i l y be c o n t r o l l e d . I t has been w e l l e s t a b l i s h e d t h a t somewhere i n the r e g i o n of a Co/Mo r a t i o of O.2 to O.3 a maximum i n c a t a l y t i c a c t i v i t y w i l l occur ( 2 ) . The enhancement i n a c t i v i t y can be as much as an order of magnitude above the unpromoted systems. The p r e c i s e shape of the a c t i v i t y v s . promotion c u r v e s depends g r e a t l y on the method of p r e p a r a t i o n of the c a t a l y s t . The unsupported c a t a l y s t , a f t e r heat t r e a t m e n t , c o n t a i n s b o t h Co

Solid State Chemistry in Catalysis - American Chemical Society

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