Sulphide catalysts on silica as a support

Applied Catalysis, 18 (1985) 33--46

33

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

SULPHIDE CATALYSTS ON SILICA AS A SUPPORT VI. ON THE STRUCTURESOF THE ACTIVE PHASE AND ACTIVE CENTERS OF HYDRODESULPHURIZATION CATALYSTS Yu. I. YERMAKOV, A.N. STARTSEV, V.A. BURMISTROV, O.N. SHUMILO and N.N. BULGAKOV I n s t i t u t e of Catalysis, Novosibirsk 630090, U.S.S.R. (Received 5 May 1983, accepted 16 A p r i l 1985) ABSTRACT Based on the experimental data of the study of h i g h l y dispersed sulphided (Ni-W)/ SiO2 and (Ni-Mo)/Si02 catalysts prepared from organometallic precursors, the structures of the active phase and a c t i v e centers for C-S bond hydrogenolysis are discussed. The active phase of hydrodesulphurization catalysts represents layered sulphide structures with the same M-S and M-M (M=W or Mo) distances as in bulk disulphides. A single MS2 sheet is a s u f f i c i e n t s t r u c t u r a l element for the format i o n of active centers. I t was suggested that the active centers include Ni atoms, on which thiophene adsorption occurs, and W(Mo) atoms of the side planes of the MS2 sheet. Sulphur atoms bound with one Ni atom are involved in the c a t a l y t i c cycle. INTRODUCTION In previous works of t h i s series the c a t a l y t i c properties [ I ] ,

dispersion and

morphology [ 2 ] , e l e c t r o n i c state of elements [ 3 ] , and s t r u c t u r a l c h a r a c t e r i s t i c s [4,5] of the active phase of (Ni-W)/SiO 2 and (Ni-Mo)/SiO 2 sulphided catalysts were studied. The c a t a l y s t s ' preparation procedure included a step i n v o l v i n g the synthesis of Ni, Mo and W surface complexes using a l l y l compounds of these metals [ I ] . Catalysts prepared from organometallic precursors were more a c t i v e , more dispersed and had a more uniform surface p a r t i c l e size d i s t r i b u t i o n as compared to the samples prepared by a conventional impregnation method [ 1 , 2 ] . However, i t is possible to claim that the nature of the active species in "organometallic" and impregnation c a t a l y s t s is the same, as i t follows from the comparison of the data on the k i n e t i c s of thiophene hydrogenolysis, as well as on morphology [2] and structure [ 4 , 5 ] of surface p a r t i c l e s of the c a t a l y s t s of both series. In t h i s work we consider the structure of the active phase and possible models of active centers in b i m e t a l l i c hydrodesulphurization catalysts ( i . e . catalysts containing sulphided compounds of two metals) by taking i n t o account a l l our previous r e s u l t s of the study of sulphided catalysts prepared using organometallic precursors [ I - 1 1 ] . The main experimental results which must be taken i n t o account at t h i s consideration are the f o l l o w i n g : I. Addition of Ni to WS2/SiO2 or MoS2/SiO2 samples results in a marked synergetic e f f e c t ( a c t i v i t y per u n i t mass of supported metals increases up to 80 times), 0166-9834/85/$03.30

© 1985 Elsevier Science Publishers B.V.

34

the maximum a c t i v i t y being observed at the atomic r a t i o ~ = Ni/(Ni÷M) ~ 0.3 [ I ] ~here M:Mo or W). 2. Highly dispersed sulphided p a r t i c l e s are present on the support surface of cata l y s t s prepared from organometallic precursors. These p a r t i c l e s are p r i m a r i l y o

monolayered " t h r e a d - l i k e " structures about 25 A long [ 2 ] . 3. Mixed (containing two d i f f e r e n t metals) sulphided p a r t i c l e s are formed in b i m e t a l l i c c a t a l y s t s . A synchronous change of the electron state of Ni and W(Mo) in b i m e t a l l i c catalysts compared to monometallic (containing sulphided compound of one metal) catalysts was observed by XPS [3]. A s i m i l a r change in the bindin energies of Ni 2P3/2 level was observed in the catalysts prepared by supporting Ni(C3H5) 2 on the bulk tungsten disulphide [10]. 4. The main interactomic distances, M-S and M-M typical for the structures of bulk disulphides are retained in supported sulphided catalysts both of the "organom e t a l l i c " o r i g i n and prepared by impregnation technique (radial electron dist r i b u t i o n ~E~ and EXAFS [5,11] data). 5. For supported sulphided catalysts new interactomic distances, which are not c h a r a c t e r i s t i c of the structure of bulk WS2 (MoS2), are observed. RED curves show a maximum, corresponding to the distance of about 3.8 A [ 4 ] . EXAFS data indicate that part of metal atoms has W-O [5] and Mo-O [11] bonds. 6. According to EXAFS data, both h i g h l y dispersed "organometallic" and coarsely dispersed impregnation monometallic catalysts have lower coordination numbers of W(Mo) r e l a t i v e to S and W(Mo) than bulk WS2 (MoS2). In b i m e t a l l i c catalysts the i n t r o d u c t i o n of Ni results in a considerable increase in the coordination number of W(Mo). This increase may be caused by the l o c a l i z a t i o n of Ni in the o

WS2 ~MoS2) s t r u c t u r e at a Ni-M distance of about 2.4 and 3.4 A. 7. In the f i r s t

coordination sphere of Ni atoms, S atoms are at a distance of

o

2.22 A, which is considerably shorter than the Ni-S distance in nickel sulphide [5]. None of the physical methods used (XPS, RED, EXAFS) indicated the formatio of nickel sulphide in b i m e t a l l i c catalysts at an optimal value of ~ [ 2 - 5 ] . Note, that the conclusions about the r e t a i n i n g of the structures of bulk d i sulphides in the active phase of b i m e t a l l i c sulphide systems and about the formati of mixed compounds were f i r s t

reported by Tops~e and co-workers who studied mainly

alumina-supported sulphided Co-Mo catalysts [12-15]. DISCUSSION On the structure of the active phase of sulphided hydrodesulphurization catalysts By the active phase (or active component) we mean surface structures containing active centers of c a t a l y t i c reaction. Our experimental results indicate that in both highly dispersed "organometallic and impregnation sulphided catalysts the layered structure of bulk W and Mo d i sulphides is e s s e n t i a l l y retained. Transmission electron microscopy (TEM) data [2] suggest that a s i n g l e WS2 (MoS2) sheet is a minimal s t r u c t u r a l element s u f f i c i e n t

35 f o r the formation of active centers. A conclusion that the active centers are localized on a single Mo sheet was made e a r l i e r by Ratnasamy and Sivansanker [16] and Tops~e with co-workers [12,13]. As was discussed e a r l i e r [2,5] in supported catalysts the disulphide sheets may be e i t h e r " p a r a l l e l "

( i . e . attached to the support surface with a basal plane) or

"perpendicular" (attached to the support with a side plane) (see Figure 10 in [ 2 ] ) . As was discussed in [ 2 , 5 ] the second s i t u a t i o n is more probable. The predominant "perpendicular" o r i e n t a t i o n of the sheets r e l a t i v e to the support surface seems to be due to a high energy of the i n t e r a c t i o n between the support and the side plane because of the presence of c o o r d i n a t i v e l y unsaturated W(Mo) ions on t h i s plane. To determine possible l o c a l i z a t i o n of active centers of hydrodesulphurization catalysts, i t is necessary to consider in more detail the possible structure of the side planes of layered sulphides. The f o l l o w i n g cases (see Figure I) may be considered: Case A. Double-bonded atoms ( i . e . S atoms with a coordination number (CN) r e l a t i v e to metal atoms equal to 2) are exposed to the surface (side plane (10#0), see Figure IA). Case B. Coordinatively unsaturated W(Mo) atoms (CN r e l a t i v e to S atoms is 4) are present on the surface. T r i p l e bonded sulphur atoms (CN=3) are adjacent to the surface (side plane (3300),

see Figure IB).

Case C. Double-bonded S atoms and c o o r d i n a t i v e l y unsaturated metal atoms are present on the surface (side planes (0330) and (#010), see Figure IC). The structures shown in Figure I (A,B,C) are idealized. In the case A, for the conservation of the e l e c t r o n i c a l l y neutral state of a microcrystal as a whole and f o r the conservation of the local e l e c t r o n e u t r a l i t y on the surface an excess negative charge can be compensated by a proton, thus producing the surface -S-H groups. Another p o s s i b i l i t y , as indicated by Farragher [17,18], is the transfer of S atoms i n t o i r r e g u l a r position a f t e r the removal of excessive S atoms from the side plane according to the scheme:

where ~

is the removable sulphur atom, 0

and

0

are, respectively, S and

W(Mo) atoms (see also Figure IA). In the case B, to provide the e l e c t r o n - n e u t r a l i t y i t is necessary that additional sulphur atoms be chemisorbed on the side plane (see Figure I B ' ) . As a r e s u l t , single-bonded sulphur atoms, SC, appear on the side plane. The plane shown in Figure IC is e l e c t r o n e u t r a l . I t can be regarded as superposition of planes IA and IB. Rearrangements s i m i l a r to those discussed above may

16

o a ~ -~~

4-

//I

~

v

•~

-

m

~

"0

"El

E

~

tO

4J

-

O .E

q--

,,~

~ .R 8 ~_:g m

o

O

o~.~

0

$..

(

E:) v

(. q-.O

~

O

~ ~

O

.~ I_

tn v

E

E

E

~

b ~ ¢,./

C1.

U

~

E

I

I

I

I

4J

~

4J

O

I

oooli I

o

N k&°

37

,take place on the corresponding sites of t h i s plane (Figure I C ' ) . As shown by EXAFS [5,11], in supported catalysts part of metal atoms can have oxygen atoms in the f i r s t

coordination sphere. The formation of the metal-support

surface oxygen bonds occurs at the step of formation of surface complexes during the i n t e r a c t i o n of organometallic compounds with the surface OH-groups of the support [19]. Upon f u r t h e r sulphurization during the formation of disulphide sheets part of metal atoms can conserve the bond with the surface oxygen of the support. This seems to be a reason f o r a high dispersion of surface sulphides in catalysts of the "organometallic" o r i g i n . The distance between W and Si atoms in the W-O-Si fragment is ca. 3.8 A. Such a distance was observed by RED technique [ 4 ] . The presence of more electronegative oxygen ions of the support (as compared to S2-) in the coordination sphere of W(Mo) is the most probable reason f o r a noticeable decrease in the electron density on M and S atoms in comparison with the bulk d i sulphides [ 6 , 8 ] . The conservation of the sheet structure of bulk disulphides in microcrystals can be f a c i l i t a t e d by the close values of interatomic distances, 0-0, on SiO2 surface and S-S distances in the WS2(MoS2) [ 4 , 5 ] . Localization of nickel ions in the active phase of b i m e t a l l i c sulphided catalysts Elucidation of the Ni (or Co) l o c a l i z a t i o n in the active phase of sulphided hydrodesulphurization catalysts is one of the key problems in the study of possible structures of active centers. By the active center we mean the surface structure containing a minimum number of surface atoms necessary f o r the c a t a l y t i c cycle to proceed. The idea of the l o c a l i z a t i o n of Co atoms on the side planes of W(Mo) crystals was put forward by Farragher and Cossee [20]. The authors have suggested the socalled "pseudointercalation" model, according to which Co is localized between two disulphide sheets. Localization of Co on the side plane of a s i n g l e MoS2 sheet has been considered by Ratnasamy and Sivansanker [16]. Tops~e has also analyzed the possible l o c a l i z a t i o n sites of Co [12]. His conclusion is that the most probable l o c a l i z a t i o n of Co is the side plane of a single disulphide sheet [14], on which Co produces the so-called mixed "Co-Ho-S" phase active in the hydrogenolysis of the C-S bond. We have also obtained an experimental support f o r that Ni atoms are localized on the side plane of WS2(MoS2).;The evidence f o r Ni l o c a l i z a t i o n on the side WS2 are given in ref. [ 5 ] , where the phenomenon of increasing the coordination number of W atoms r e l a t i v e to the metal atoms upon addition of Ni to WS2 is analyzed. Now proceed to possible structures of surface sites that may form at l o c a l i z a t i o n of Ni atoms on the side plane of the disulphide sheet. The r e s u l t s of t h i s analysis are given f o r WS2 microcrystals. However, a s i m i l a r s i t u a t i o n is also expected f o r MoS2 (and f o r l o c a l i z a t i o n of Co atoms). Fragments of the WS2 l a t t i c e with Ni atoms localized on the side plane are i l l u s t r a t e d in Figure 2. Interatomic distances in various sites of Ni l o c a l i z a t i o n which were estimated usino the s t r u c t u r a l parameters of WS2, a=3.18 A and c=12.5 A are l i s t e d in Table I .

38

o£

S atom FIGURE 2

@

W alorn

II Ni atom

Possible l o c a l i z a t i o n of the second metal (Co or Ni) on the side plane of

MS2 crystals. A v e r t i c a l cross-section by plane (0002) is shown. Atoms in the cross section are hatched. Sulphur atoms which are removed or added during the c a t a l y t i c cycle, are indicated by dotted c i r c l e s . Site ~.

At t h i s s i t e the Ni atom occupies a regular position of the disulphide

l a t t i c e thus s u b s t i t u t i n g the W atom. Sulphur atoms, which must be removed (or adde during the c a t a l y t i c act ( f o r brevity we shall f u r t h e r call them "active" atoms), are double-bonded atoms, Sb. Site ~ is met on the planes shown in Figure IA, I A ' , IC, IC'. In Figure 2 the s i t e ~ is l o c a l i z e d on {7010) plane (see also Figure IA). Site B.

"Active" S atoms of this s i t e are single-bonded (Sc). Depending upon the

l o c a l i z a t i o n of " a c t i v e " sulphur, two types of 6 sites are possible: 61 - Ni atom occupies a regular position in the disulphide l a t t i c e ; "active" S atom, SC, is attached to Ni atom; 62 - Ni atom occupies the same position in the WS2 l a t t i c e as in ~I' but " a c t i v e " sulphu r is bound with W atom located nearby Ni atom. The active s i t e 6 is met on planes shown in Figures I B , I B ' , I C , l C ' ; this s i t e on (2300) plane is shown in Figure IB. Site y.

This s i t e comprises a Ni atom "buried" from the regular p o s i t i o n in the

disulphide l a t t i c e i n t o the center of a square consisting of four S atoms. The "active" S atom is single-bonded. The s i t e y may be typical f o r the planes shown in Figures IB, I B ' , IC, IC'; Figure 2 i l l u s t r a t e s l o c a l i z a t i o n of this s i t e on (0330) plane. Site ~.

This s i t e is formed as a r e s u l t of the s u b s t i t u t i o n of W atoms f o r Ni

atoms on the side planes with a rearranged structure (see Figures I A ' , I B ' , IC'). Figure 2 shows the s i t e a on the plane i l l u s t r a t e d in Figure IA. The "active" sulphur of this s i t e is single-bonded SC. I f one assumes that Ni is added to the sur-

39 TABLE I Interatomic distances in active centers of b i m e t a l l i c sulphided catalysts a

Active center

61 , 62

o b Interatomic distances/A Ni-S

Ni-W

2.48 (x 6)

3.19 (x 4)

2.48 (x 4)

3.19 (x 4)

2.23 (x 4)

2.46 (x 2)

2.20 (x 2)

2.40 (x 1)

2.40 (x 1)

3.30 (x 2)

3.36 (x 2)

E

2.48 (x 4)

1.84 (x I) 3.67 (x 2)

astructure of the active centeres is shown in Figure 2. bln brackets the number of distances around one active center is given. face together with an additional S atom, the environment of Ni occurs and the squareplanar structure of the s i t e ~ becomes s i m i l a r to that of the s i t e y. Site ~.

This s i t e is formed at the l o c a l i z a t i o n of Ni atoms on surface S atoms on

the planes shown in Figures IA, I A ' , IC and IC'. Note here t h a t , f or structural reasons, the formation of such sites can hardly be expected, because i t requires o

too short Ni-W distances ( f o r a Ni-S distance of 2.48 A, that o f Ni-W must be o

1.84 A, f o r a Ni-S distance of 2.23 A the Ni-W distance must decrease to ca. 0.9 A). Figure 2 shows s i t e ~ on the plane IA'. From the comparison of the data in Table I with structural data on studied b i m e t a l l i c catalysts i t follows that the most probable structure of active centers is that of s i t e ¥ (or ~). In this case the interatomic Ni-W and Ni-S distances are close to those estimated from EXAFS data. Thus, the l o c a l i z a t i o n of Ni on the side planes that have c o o r d i n a t i v e l y unsaturated tungsten ions should r e s u l t in the formation of the centers, in which the Ni-S and Ni-W distances correspond to those obtained from EXAFS data. The r e l a t i v e number of l o c a l i z a t i o n sites of Ni atoms w i l l depend upon the size and form of the WS2 microcrystal. We may reasonably expect a maximum a c t i v i t y when a l l potential l o c a l i z a t i o n sites w i l l be occupied by Ni atoms. In t h i s case the content of Ni w i l l determine a th e o r e t i c a l optimal value of ~ ( X t h e o r . o p t . ) . At a smaller content of Ni, the number of active centers w i l l be lower. At a larger content of Ni, part of the active centers w i l l be blocked by the fragments of nickel sulphide. The value Ofo~theor.opt. w i l l depend upon the size of a disulphide sheet. For a sheet of 25 A in size ~theor.opt. is w i t h i n 0.2 - 0.4 (a precise value of ~theor.opt.

40

depends on the plane in which the l o c a l i z a t i o n of Ni is considered). This value is close to the value of ~ - 0.3 at which the maximum a c t i v i t y was observed [ I ] .

Upon

increasing the size of a sheet ~theor.ont. decreases and becomes nearly ~ 0.1 for the sheets of a regular form and 50 ~ size whose basal planes are adjacent to the suDport (the sheets of the type shown in Figure 4 in reference [ 5 ] ) . However, i f a sheet is attached to the surface with i t s side plane (see Figure 4b [5]) ~theor.opt w i l l s l i g h t l y depend on the size of the sheet. Note that experiment shows that an optimal value of ~ does not depend on dispersion ( t h i s ~opt. is nearly the same for dispersed "organometallic" and less dispersed "standard" c a t a l y s t s ) , which provides an additional evidence for that sulphided sheets are attached to the support surfac with t h e i r side planes. On the role of Ni (or Co) in b i m e t a l l i c sulphided catalysts based on WS2 (MoS2~ Sulphides of metals, such as Co, Ni, Mo and W, i . e . elements that usually are in the composition of hydrotreating c a t a l y s t s , have nearly the same a c t i v i t y in hydrogenolysis of the C-S bond (per mol of metal) [22]. Upon mixing Mo(W) and Co(Ni sulphides the a c t i v i t y

sharply increases. According to the widely adopted i n t e r -

pretation of t h i s synergetic e f f e c t , a sulphide of VI Group metal, MVI, is regardec as the main c a t a l y s t component responsible f o r the a c t i v a t i o n of S-containing compound, and a sulphide of V I I I Group metal, MVlII, is regarded as promotor. The d i f f e r e n t reasons f o r the promoting action have been proposed (see, e.g. [ 2 3 ] ) , hov ever, they f a l l I.

i n t o two main groups:

The role of the promotor consists in the s t r u c t u r a l modification (increase in

dispersion and s t a b i l i t y of MvIS2). 2. The role of the promotor consists in the increasing of the number of active sites in the MvIS2 phase (anion vacancies, MVI reduced ions) due to hydrogen a c t i v a t i o n by the promotor. For the model of the "Co-Mo-S" phase Tops~e

[12] has suggested that Co is

d i r e c t l y involved in the active centers. I t is assumed that Co weakens the Mo-S bond, which f a c i l i t a t e s

sulphur removal under the reaction conditions and, thus,

leads to the formation of c o o r d i n a t i v e l y unsaturated Mo ions on which thiophene adsorption occurs. As found f o r the catalysts obtained by supporting Ni(C3H5) 2 on the bulk WS2 [9,10], the a c t i v i t y per Ni is almost constant independently of Ni concentration, surface and morphology of WS2. The same turn-over number is t y p i c a l f o r the suppori catalysts at ~ smaller than an optimal value. This ellows one to propose that when introduced into the c a t a l y s t containing W sulphide, Ni enters the composition of active centers. To analyze the role of Ni(Co) atoms in the~hydrogenolysis of thiophene, the r e a c t i v i t i e s of various sites of mono- and b i m e t a l l i c catalysts were estimated using the i n t e r a c t i n g bonds method (IBM). This method is described in d e t a i l in [24]. I t s a p p l i c a t i o n to the c a l c u l a t i o n of adsorption heats of oxygen on t r a n s i t i (

41 metal oxides [ 2 5 ] , of nitrogen [26], of oxygen and of hydrogen [27] on metals, as well as of the a c t i v a t i o n energies of the i n t e r a c t i o n of hydrogen with oxygen chemisorbed on copper-magnesium c a t a l y s t s [28], has indicated good agreement between theoretical and experimental data. We assume that the c a t a l y t i c cycle in thiophene hydrogenolysis reaction consists of two main steps: I) Removal of sulphur from the active center during i t s i n t e r a c t i o n with hydrogen:

S

H2

+

~

[]

~

H2 S

,

/H s

+

I/2 H 2

where a.c. is an active center and

[]

~-

[]

-F

H2S

,

(la)

is a vacant coordination s i t e .

2) Adsorption of thiophene with i t s subsequent hydrogenolysis:

[]

4_

C4H4 S

+ 2H 2

).

S

+

C4H8

(2)

/ []

+

C4H4 S

+

5/2I_12

~

s

+

c4H8

For the r e a c t i o n s that follow s i m i l a r two-step mechanisms, the rate of the c a t a l y t i c process is maximum i f the e n t h a l p i e s of both steps are close to each other [29]. So, for an optimal a c t i v e c e n t e r , i t is necessary t h a t the enthalpy of r e a c t ion (1) be close to that of reaction (2). To discriminate between the possible compositions of active centers, enthalpies (z~H) of reaction ( I ) and (2) on the above centers have been estimated. The values of &H were calculated as a difference of the sums of atomization enthalpies of the products and i n i t i a l

reagents of reactions (I) and (2) w i t h i n the IBM technique.

Using t h i s technique atomization enthalpy is calculated as follows

aHat = ~ i ( 2 - ui)Ei - ikSz UiUkAik

(3)

where Ei and Aik are semiempirical parameters t h a t , r e s p e c t i v e l y , characterize the i - t h bond strength and the i n t e r a c t i o n between i - t h and k-th bonds which belong to the same atom. Parameter c o e f f i c i e n t s of the bonding are determined from the condition of the maximum Hat and change from 0 to I. Parameters E and A were calculated using Equation (3) at analysis in terms of the IBM technique of the compounds (containing bonds of the corresponding type), f o r which experimental data on the structures and formation enthalpies were known. The calculated parameters E and A used in t h i s work are: EW_S = 123.8, ENi_S = 86.3, EMo_S = 109.6, ECo_S = 85.0,

42

TABLE 2 Enthalpies (AH) of the steps of thiophene hydrogenolysis on monometallic c a t a l y s t s Active

"Active"

Metal

sulphur

of

center a

H(1)

H(2)

/kcal mol -I

/kcal mol -I

W

95.1

-130.2

Mo

82.3

-117.4

disulphide t r i p l e - b o n d e d Sa

C~

I

double-bonded Sb

single-bonded Sc

I

single-bonded Sc

C

W

21.9

- 57.0

Mo

21.5

- 56.6

W

2.8

- 32.3

Mo

5.2

- 29.9

W

8.8

- 26.3

-10.2

- 24.9

Mo astructures of the a c t i v e centers ( ~ ' ,

8 ' , ~ ' ) of monometallic c a t a l y s t s were tak~

s i m i l a r to those of b i m e t a l l i c c a t a l y s t s (~, B, ¥) (see Figure 2) assuming the s u b s t i t u t i o n of a W(Mo) atom f o r a Ni(Co) atom.

EMo_Mo = 93.1, EW_W = 120.8, ENi_W = 90.7, ECo_Mo = 76.8; AW = 28.1, AMo = 21.6, ANi _ 14.1, -I AC° = 14.1, AS = 45.0. Here and below a l l energy values are given in kcal mol

. An e s s e n t i a l f e a t u r e of the formalism of the IBM technique is t h a t i t

provides the p o s s i b i l i t y

to consider any fragments o f a s o l i d w i t h o u t s i m p l i f y i n g

them i n t o c l u s t e r s . Thus, a l l below c a l c u l a t i o n s were made f o r surface structures t h a t are components of an i n f i n i t e

crystal

(real p a r t i c l e dimensions in c a t a l y s t s

are too high to expect a n o t i c e a b l e s i z e e f f e c t f o r them). Calculations o f the enthalpies of r e a c t i o n s ( I ) and (2) f o r mono- and b i m e t a l l i c a t a l y s t s are l i s t e d

in Tables 2 and 3, r e s p e c t i v e l y .

Consider now the obtained r e s u l t s . Monometallic systems.

For MvIS2 sulphides the removal of a t r i p l e - b o n d e d (S a)

and double-bonded (S b) S atoms is thermodynamically unfavorable (see AH ( I ) in Table 2). Note t h a t t h i s has also been i n d i c a t e d by Farragher and Cossee [ 2 0 ] . Th~ d i f f e r e n c e between AH ( I ) and AH (2) is high at removal Sa and Sb, t h e r e f o r e p a r t i c i p a t i o n of the centers containing such S atoms in c a t a l y s i s is hardly probable. Only centers of the types 8 ' , ~' c o n t a i n i n g single-bonded S atoms seem to take pan in c a t a l y s i s . However, in t h i s case the d i f f e r e n c e in the e n t h a l p i e s of steps ( I ) and (2) f o r WS2 and MoS2 is high. This may be a reason f o r a low a c t i v i t y

of the

above systems. As f o r nickel s u l p h i d e , thermodynamic e s t i m a t i o n , i n d i c a t e s t h a t in r e a c t i o n c o n d i t i o n s (T = 673 K, PH2s/PH2 = 4.5 x 10-5 - 1.3 x 1 0 - ~ b u l k phases of nickel

43

TABLE 3 Enthalpies (AH) of the steps of thiophene hydrogenolysis on b i m e t a l l i c sulphided catalysts Active center a

"Active" sulphur double-bonded Sb

Metal

H(1)

H(2)

/kcal mol -I

/kcal mol

W, Ni

14.7

-49.8

Mo, Co

12.2

-47.3

sulphide

BI

single-bonded Sc

W, Ni Mo, Co

-12.8 -13.7

-22.3 -21.4

B2

single-bonded Sc

W, Ni Mo, Co

2.8 - 5.2

-32.3 -29.9

y

single-bonded Sc

W, Ni

-22.5

-12.6

Mo, Co

-23.0

-12.1

single-bonded Sc

W, Ni Mo, Co

-13.3 -14.3

-21.8 -20.8

single-bonded Sc

W, Ni Mo, Co

-16.7 -17.5

-18.4 -17.6

6

-I

aFor the s t r u c t u r e of the active centers see Figure 2. sulphides (NiS, Ni3S 2) are unstable and w i l l be transformed to metallic nickel. The data on thermally programmed reduction (TPR) [7] show t h a t in conditions of hydrogenolysis of thiophene the supported nickel sulphide is completely reduced to the metal at 700-800 K. B i m e t a l l i c systems.

Estimation of AH ( I ) f o r the removal of double-bonded

sulphur from the b i m e t a l l i c active center containing two d i f f e r e n t metals ( ~ - s i t e ) indicates that t h i s process is unfavorable. The difference between AH ( I ) and AH (2) is large in the case of the centers containing double-bonded S atoms. Therefore, the p a r t i c i p a t i o n of ~ sites in c a t a l y s i s is hardly probable. Thus, in b i m e t a l l i c c a t a l y s t s the active centers also have to contain single-bonded S atoms. These centers may be of two types. (i)

Centers, in which the MVIII atom located on the side plane of MvIS2 disulphide

is not bound with a metal producing t h i s disulphide (centers BI and B2). In t h i s case the layered MvIS2 disulphide provides the f u n c t i o n of a matrix f o r l o c a l i z a t i o n of the active metal, MVIII. ( i i ) Centers, in which MVI atoms are located at a distance from MVlII atoms s u f f i cient f o r the formation of the metal-metal bond (centers y, ~, ~). In t h i s case the role of the MVI (in addition to that considered in ( i )

is to p a r t i c i p a t e in the

44 formation of the MVI - MVlllbOnd,which f a c i l i t a t e s removal of sulphur from the active center (compare AH for centers y and 61, Table 3). Estimation of AH ( I ) and AH (2) for the active center with single-bonded sulphur on the W(Mo) atom has indicated that these values do not d i f f e r from those for the corresponding center B' in monometallic systems (see Tables 2 and 3). Thus, the a c t i v i t y of the center of a b i m e t a l l i c c a t a l y s t containing "active" (single-bonded to a W(Mo) atom) sulphur d i f f e r s s l i g h t l y from the a c t i v i t y of the center in a monometallic c a t a l y s t . These centers may provide a small contribution to the a c t i v i t y of

bimetallic systems. So, i t is possible to suppose that high a c t i v i t y of bimetalliq

catalysts is due to the presence of active centers containing d i r e c t l y interacting Ni(Co) and W(Mo) atoms and single-bonded to N i ( C o ) " a c t i v e " sulphur. The AH ( I ) and ~H (2) values for t h i s active center are nearly independent of the type of the side plane (see Figure IA, IB or IC) on which i t may be located. Nor does i t depend upon the type of sulphur atoms (Sa or Sb) in the f i r s t

coordination sphere of the metal

bonded to "active" sulphur. For example, for s i t e y of the Ni-W c a t a l y s t with Ni in the square composed of 4 S atoms: 2Sa, 2Sb; plane IC; AH ( I ) = - 2 2 . 5 , AH (2) =-12.6 2Sa, 2Sb; plane IC; AH ( I ) = - 2 1 . 4 , AH (2) =-13.7 4Sa;

plane IB; AH ( I ) =-21.0, AH (2) =-14.1

4Sb; plane IB; AH ( I ) = - 2 2 . 3 , AH (2) =-12.8 These data indicate that for the hydrogenolysis of thiophene no special requirements are imposed on the faceting of the side plane of the disulphide sheet; but the structure of the active center is important. Note that substitution of one of sulphur atoms, in the coordination sphere of the metal atom bonded to the "active" sulphur, by an -S-H group enhances AH ( I ) by 1.4 kcal mol -I both f o r b i - and monometallic systems. Consideration of structural characteristics of active centers (Table I) and thermodynamics of two steps of thiophene hydrogenolysis (Tables 2 and 3) suggests that the most probable structure of the active center in bimetallic catalysts is that of center y. In t h i s center the interatomic distances are close to experimental values, and the enthalpy of reaction ( I ) is close to that of reaction (2). Structural and energetical characteristics of center ~ also s a t i s f y the above requirements, However, i t should be anticipated that t h i s center is unstable and may be transformed to y - s i t e upon binding an additional sulphur atom. CONCLUSIONS We now formulate the main conclusions of t h i s series of work on the study of supported sulphided catalysts for thiophene hydrogenolysis. Active phase of sulphided catalysts is microcrystal of layered sulphide (MoS2 or WS2). This conclusion was made already by Tops~e et a l . The high dispersion of sulphided sheets in catalysts prepared via anchoring of metal complexes on SiO2 is caused by the bonding of part of W(Mo) atoms to the oxygen atoms of the support surface.

45

Active centers of bimetallic catalysts are located on the side planes of MvIS2 sheets and contain MVIII atoms (Ni or Co) and MVI atoms (W or Mo). The atoms of MVIII d i r e c t l y p a r t i c i p a t e in the c a t a l y t i c cycle (proceeding through the steps of removal/addition of single-bonded S atoms). The possible role of MVI atoms is as follows: f i r s t , MvlS2 sulphide serves as matrix for the s t a b i l i z a t i o n of MVIII atoms in the sulphide environment; second, MVl atoms seem to a f f e c t the r e a c t i v i t y of sulphur attached to MVIII atoms (e.g. t h i s "ligand e f f e c t " manifests i t s e l f in the f a c i l i t a t i o n of the removal of "active" sulphur via the formation of the MVI - MVIII bond). In this series of work we have not obtained data on hydrogen activation centers. I t is possible to suppose hydrogen a c t i v a t i o n occurs on W(Mo) and Ni(Co) ions. REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Yu. I. Yermakov, A.N. Startsev and V.A. Burmistrov, Appl. Catal., 11 (1984) I . V.I. Zaikovskii, L.M. Plyasova, V.A. Burmistrov, A.N. Startsev and Yu. I. Yermakov, Appl. Catal., 11 (1984) 15. A.P. Shepelin, P.A. Zhdan, V,A. Burmistrov, A.N. Startsev and Yu. I . Yermakov, Appl. Catal., 11 (1984) 29. E.M. Moroz, S.V. Bogdanov, S.V. Tsybulya, V.A. Burmistrov, A.N. Startsev and Yu. I. Yermakov, Appl. Catal., 11 (1984) 173. D.I. Kochubei, M.A. Kozlov, K.I. Zamaraev, V.A. Burmistrov, A.N. Startsev and Yu. I. Yermakov, Appl. Catal., 1984, in press. Yu. I. Yermakov, B.N. Kuznetsov, A.N. Startsev, P.A. Zhdan, A.P. Shepelin, V.I. Zaikovskii, L.M. Plyasova and V.A. Burmistrov, J. Molec. Catal., 11 (1981) 205. V.A. Burmistrov, A.N. Startsev, Yu. I. Yermakov, React. Kinet. Catal. L e t t . , 22 (1983) 107. Yu. I. Yermakov, B.N. Kuznetsov, A.N. Startsev, V.A. Burmistrov, P.A. Zhdan, A.P. Shepelin and V.I. Zaikovskii, Kinetika i k a t a l i z , 24 (1983) 688. V.A. Burmistrov, A.N. Startsev and Yu. I . Yermakov, React. Kinet. Catal. L e t t . , 24 (1984) 365. V.I. Zaikovskii, A.P. Shepelin, V.A. Burmistrov, A.N. Startsev and Yu. I. Yermakov, React. Kinet. Catal. L e t t . , 25 (1984) 17. D.I. Kochubei, M.A. Kozlov, K.I. Zamaraev, V.A. Burmistrov, A.N. Startsev and Yu. I. Yermakov, Kinetika i K a t a l i z , in press. H. Tops~e, in: Surface properties and catalysis by non-metals, ed. J.P. Bonnelle et al. D. Reidel Publishing Company, 1983, p.329. B.S. Clausen, H. Tops~e and R. Candia, et a l . J. Phys.Chem., 85 (1981) 3868. H. Tops~e, B.S. Clausen, R. Candia, C. Wivel and S. M~rup, Bull. Soc. Chim. Belg., 90 (1981) 1189. H. Tops~e, B.S. Clausen, R. Candia, C. Wivel and S. M~rup, J. Catal., 68 (1980) 433, 453. P. Ratnasamy and S. Sivasanker, Catal. Rev.-Sci. Eng., 22 (1980) 401. A.L. Farragher, Adv. Colloid Interface Sci., 11 (1979) 3. A.L. Farragher, in:"The role of solid state chemistry in c a t a l y s i s " , A. Chem. Soc. Div. Petr. Chem., Washington, 1977. Yu. I. Yermakov, B.N. Kuznetsov and V.A. Zakharov, Catalysis by supported complexes, Elsevier, Amsterdam, 1981. A.L. Farragher and P. Cossee, In: Proceedings, 5th International Congress on Catalysis, Palm Beach, 1972 (J.W. Hightower, Ed.) p.1301, North-Holland, Amsterdam, 1973. B.F. Ormont, Struktura neorganicheskikh vestchestv. M.-L. (1950) 968. T.A. Pecoraro and R.R. Chianelli, J. Catal., 67 (1981) 430. K.C. Pratt and J.V. Sanders, J. Catal., 66 (1980) 82. N.N. Bulgakov, Yu. A. Borisov and V.V. Popovskii, Kinetika i Kataliz, 14 (1973) 468.

46

25 26 27 28 29

N.N. Bulgakov, V. Yu. Aleksandrov and V.V. Popovskii, React. Kinet. Catal. L e t t . , 4 (1976) 473; 8 (1978) 59. A.I. Boldarev, V . l . Avdeev, N.N. Bulgakov and I . I . Zakharov, Kinetika i Kataliz, 17 (1976) 706. V.A. Sobyanin, N. N. Bulgakov and V.V. Gorodetskii, React. Kinet. Catal. L e t t . , 6 (1977) 125. K,P. Pavysova, N.N. Bulgakov, V. Yu. Aleksandrov and V.V. Popovskii, Kinetika i Kataliz, 21 (1980) 988. M.I. Temkin, Kinetika i Kataliz, 25 (1984) 299.