Co-Mo-Al2o3 hydrodesulphurisation catalysts: Correlation of activities with properties of the catalysts

JournaE of the Lea-common Metats, (1977) 333 - 342 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

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CO-MO-A&& HYDRODESULPHURISATION TION OF ACTIVITIES WITH PROPERTIES

CATALYSTS: CORRELAOF THE CATALYSTS*

NELSON P. MARTINEZ, CI-IXPLUNKER

and (in part) PRAGNYA

Department

PHILIP C. H. MITCHELL

of Chemistry, The University, Whiteknights, Reading RG6 2AD jGt. Britain)

Summary The UV-visible reflectance spectra, magnetic susceptibilities and adsorption-desorption behaviour of cobalt and molybdate ions at surfaces in contact with an aqueous phase have been measured for a series of Co-MO-A120, hydrodesulphurisation (HDS) catalysts. The object was to discover correlations between properties of the catalysts and their activities in thiophene desulphurisation. Two ways in which cobalt acts as a promoter are identified: (a) in the presence of cobalt, the fraction of the surface of the alumina carrier covered by molybdate increases and with it the catalytic activity and (b) the initial rate of butane formation correlates with the ~oncen~atio~ of cobalt at the cataIyst surface; cobalt promotes hydrogenation reactions during HDS. The significance of the results in terms of the formation of Co-promoted Moss crystallites, which are the catalytic species, is discussed.

Introduction In this paper we describe our recent work on Co-MO-AlsOa hydrodesulphurisation (HDS) catalysts. Our object was to discover if there were correlations between activities in thiophene HDS and the UV-visible reflectance spectra and magnetic su~eptib~ities of the catalysts and also the adsorptiondesorption behaviour of cobalt and molybda~ ions at catalyst surfaces in contact with an aqueous phase. The purpose was to get information about the structures of the cobalt and molybdenum species in the oxide forms of the catalysts and to see how they relate, if at all, to catalytic activities and also to see if the techniques are of any value in discriminating between active and less active catalysts. There is considerable technical interest in developing physicochemical methods of screening catalysts prior to activity testing and so reducing the need for activity testing. For example, a recent patent [ 11 claims a method of assessing the activities of Ni-Mo-Al,03 and Ni-W-AlsO catalysts by measuring their UV-visible reflectance spectra. *Presented at the Conference on “The Chemistry and Uses of Molybdenum”, University of Oxford, England, 31 August - 3 September, 1976, sponsored by Climax Molybdenum Co. Ltd. and the Chemical Society (Dalton Division).

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Although the active species (formed in situ in the reactor) is considered to be MoS, promoted by Co(I1) ions, we have investigated the oxide form (which may be regarded as the catalyst precursor) for the following reasons: the catalysts are always prepared, supplied, handled and tested as the oxide forms; their activities and other properties are influenced by the compositions and properties of the oxide forms; preparative variables which influence activities are applied at the oxide stage. We consider that an important aspect of understanding Co-MO-Al,O, catalysts is to know how variation in the structure and composition of the oxide form influences the development of the catalytic sites in the sulphided catalyst. Experimental Catalysts Co-MO-AlsOs catalysts were supplied by J. R. H. Ross of the University of Bradford [2]. They had been prepared by sequential impregnation of freshly precipitated pseudoboehmite with aqueous solutions of cobalt(I1) formate and ammoniacal molybdate(VI), followed by filtration, drying (383 K, 48 h) and calcination (873 K, 17 h). Surface areas were 270 - 330 m2 g-l. Activities Details are given by Hargreaves and Ross [2]. Activities for thiophene hydrodesulphurisation were measured with PC, n 4 s = 420 N m- 2, PH, = 2950 N me2 and T = 521 K or 545 K and were expressed as the initial rate of thiophene hydrodesulphurisation. The initial rate of butane formation during hydrodesulphurisation was also measured. Spectroscopic measurements Spectra were measured by the diffuse reflectance technique on a Unicam SP 700 spectrophotometer with reference to silica. We used a special vacuum cell, which could be evacuated (lop3 Torr) and in which catalysts could be dosed with reactants. The catalysts were ground to mesh size 35 - 60. Standardisation of the mesh size of the particles is essential if reproducible spectra are to be obtained. We expressed light absorbance in terms of the KubelkaMunk function (f(R)) calculated from per cent reflectance at various wavenumbers [ 31. log f(R) = const. + log C where C is the concentration of the absorbing species. We expect a linear relationship provided that the chemical nature or structure of the absorbing species does not change as C changes. Magnetic measurements Magnetic susceptibilities were measured at about 293 K by the Gouy method. The balance was calibrated with tris(ethylenediamine)nickel(II)

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thiosulphate. Measured susceptibilities were corrected for diamagnetic contributions and apparent magnetic moments (II) were calculated [ 41 using p = (3klN, r-l,) (xmolar T)“‘. Desorption of cobalt and molybdenum species from the catalyst surface Catalyst samples (0.25 g) ground to mesh size 35 - 60 were shaken for up to 12 h with 31.0 ml water at about 293 K. Portions of the aqueous phase were withdrawn at intervals (usually 1 h) and filtered; their UV-visible spectra were then recorded (Unicam SP 1800). We determined the time taken for equilibration (about 3 h) and, from Beer’s Law calibrations, the concentrations of Co(I1) (h 505, 515 nm) and MoOi- (X 208,230 nm) in the aqueous phase. Adsorption of cobalt and molybdenum(VI) on y-alumina [5] y-Alumina (Ketjen, surface area 300 m2 g-l) (1.0 g) was shaken mechanically with aqueous solutions (100 ml) containing molybdenum(V1) (Analar ammonium heptamolybdate, (NH4)sMo,024*4H20) in conical flasks in a water thermostat (25 + 1 “C). Adsorption was followed to equilibrium by analysing portions of the aqueous phase. Molybdenum was determined by atomic absorption or by light absorbance at 400 nm (Unicam SP 500 spectrophotometer); concentrations were read from a calibration graph which was linear [0 - 4 X 10m3 (mol MO) cme3]. Adsorption of molybdenum(V1) on samples of y-alumina impregnated with cobalt(I1) was determined similarly.

Results and discussion UV visible reflectance spectra of the calcined catalysts A typical spectrum is shown in Fig. 1. UV absorption is assigned to (0 -+Mo) charge transfer transitions. Compounds containing molybdate tetrahedra, e.g. Na2Mo04, show only one peak. Compounds containing molybdate octahedra, e.g. Moo3 and CoMo04, show two peaks [6]. The UV spectrum of the catalysts showed only one peak. Hence the molybdate is predominantly tetrahedral. Visible spectra are assigned to crystal field transitions of Co(I1) [6] . The peaks of tetrahedral Co(I1) (e.g. in CoAl,O,) compared with octahedral Co(I1) are at lower wavenumbers, are more intense and show more structure. The spectrum of the catalysts corresponded to tetrahedral Co(I1). (But note that the presence of octahedral Co(I1) is also shown by our magnetic measurements. The spectrum of octahedral Co(I1) is weaker than that of tetrahedral Co(I1) and cannot be observed when the latter is present.) Intensity of the cobalt(II) absorption The intensity of the peak at 17 000 cm-’ is plotted as a function of the total cobalt concentration for Series 3 catalysts (MO constant, Co increasing)

336

Or

80 40

30

20 ~(10~ cm-l)

10 10 log (

Fig. 1. UV-visible calcining (B).

reflectance

spectra

of catalyst

[Co1 (g Co) (1 oog catalyst)

3-5 after calcining

-1

)

(A) and before

Fig. 2. Relation between the total concentr$ion of cobalt in the Series 3 catalysts and (A) , (B) the activity for thiophene HDS (initial the Kub$ka-Munk function at 17 000 cm -’ s-l )), (C) the activity for butane formation rate (10 (mol thiophene) (ptalyst) during HDS (initial rate (lo(mol butane) (g catalyst)-’ s-l)).

in Fig, 2. We expect a linear relationship if the chemical nature and structure of the absorbing species does not change with concentration and this is what is observed (with possibly some deviation for catalyst 3-5). We conclude that, as far as the cobalt spectrum is concerned, the structure of the cobalt species does not change with concentration. The activities of the catalysts are also plotted in Fig. 2. The hydrodesulphurisation activity (but not the activity for butane formation) correlates with increasing absorption intensity. But note that this is also a correlation with the total cobalt content of the catalysts; only if there had been a significant deviation from linearity could we have associated a change in the activity of the catalysts with a change of structure of the cobalt species. In Fig. 3 we show the cobalt(I1) absorption intensity for Series 4 catalysts (Co constant, MO increasing) and the activities plotted against the total molybdenum concentration. The intensity hardly changes. Thus the structure of the cobalt species is apparently not affected by change of molybdate concentration. The activity for butane formation correlates with the cobalt absorption intensity (i.e. hardly changes over the whole range of catalysts) but the hydrodesulphurisation activity does not. (The change of activity at catalyst 3-3 is associated with a change in the molybdate species, see Fig. 5.)

8

-2 ,

o

0

*r

A - 22

2

G z B 0

418 I I

-I

I

I

1 6

7

(g

*

[Mel

MO)

(1009

catalv~t)~’

9

-1

2

3

4

[Co1 (g CO) (1009

catalyst)

-1

Fig. 3. Relation between the total concentration of molybdenum in the Series 4 catalysts and (A) the activity for thiophene HDS (initial rate [ 10mg (mol thiophene) (g catalyst)-’ s-l I), (B) the activity for butane formation during HDS (initial rate [ lo-l1 (mol butane) (g catalyst)-l s-l I), (C) the Kubelka-Munk function at 17 000 cm-l. Fig. 4. Relation Kubelka-Munk

between functions

the total cobalt concentration in Series 3 catalysts at (A) 30 000 cm-l, (B) 40 000 cm-l.

and the

Intensity of the molybdate absorption Figure 4 shows the change in intensity of the molybdate absorption at 40 000 and 30 000 cm-l as the cobalt concentration increases (molybdate is constant). There is a general increase of absorption intensity as the cobalt increases although the shape of the spectrum does not change. We attribute this to increased “spreading” of the molybdate on the catalyst surface, i.e. an increase in the number of particles of molybdate. This interpretation is supported by our measurement of the isotherms of adsorption of molybdate from aqueous solution on Co-Al,Os samples (see below). The activities of the catalysts (Fig. 2) increase as the molybdate absorption increases, but more rapidly. Thus the increase of activity is apparently associated with the increased coverage of the surface by molybdenum(V1). But this is not the only factor; there is a specific effect of cobalt. In Fig. 5 we show the molybdate absorption intensities and activities of Series 4 catalysts (Co constant, MO increasing). The absorption intensities for catalysts 4-3,4-4 and 4-5 lie on straight lines but there is a change of gradient between 4-2 and 4-3. This suggests that there are two absorbing species. Our determinations of the adsorption isotherms for molybdate on Co-Al,Os samples (see below) showed that a double layer or multilayer of molybdate begins to build up at molybdate concentrations greater than those of catalyst 4-2. So we consider that the species in 4-3, 4-4 and 4-5 is a polymolybdate possibly with some octahedral molybdate and that in 4-l and probably 4-2 we have essentially a monolayer of tetrahedral molybdate. The hydrodesulphurisation activity does not correlate with the absorption intensities but the activity for butane formation correlates remarkably well (except for catalyst 4-5).

(g MO) (1OOg catalyst)-’

Fig. 5. Relation between the total concentration of molybdenum in Series 4 catalysts and the Ku~lka-Monk functions at (A) 40 000 cm -l, (B) 30 000 cm-l, (C) the activity for thiophene HDS (initial rate (10B9 (mol thiophene) (g catalyst)-l s-l)), (D) the activity for butane formation during HDS (initial rate (1O-11 (mol butane) (g catafyst)-’ s-l)).

Magnetic moments of the calcined catalysts Values of p are given in Table 1. Variations are hardly outside the limits of experimental error. The values correspond to about 40 - 50% octahedral cobalt(I1). TABLE 1 Magnetic moments of the calcined catalystsa Catalyst

Co(%w/w) /G.M.)

3-l

3-2

3-3

3-4

3-5

4-l

4-2

4-3

4-4

4-5

1.24 4.66

2.30 4.72

2.80 4.69

3.22 4.64

3.88 4.60

2.94 4.50

2.94 4.61

2.96 4.61

2.97 4.63

2.81 4.63

aEffective magnetic moments p(B.M.) at about 293 K.

Adsorption and desorption of cobalt and molybdenum species Adsorption iso therms Previously we have shown that 1 - 2% Co w/w constitutes a monolayer on y-alumina f6]. (By monolayer concentration we mean the ~on~en~a~on of a species in the catalyst at the horizontal portion of the isotherm, i.e. when

339

6

z

k

”

1 0

0

1

2

3

4

5

6

, 7

3

0

1

(mol (a)

Mo)(cm3

1 3

2

lOq[Mo]

4

5

I 6

[Co1 solution)-’

(g CO)

(1009

catalyst)-’

(b)

Fig. 6. (a) Adsorption isotherms for molybdenum(V1) in aqueous solutions at 25 “C on (A) r-alumina and on Co-alumina containing (B) Co l%, (C) Co 3%, (D) Co 6%. (b) Relation between amount of molybdenum adsorbed at the horizontal parts of the isotherms and the amount of cobalt in Co-Al203 samples. (A) Co--Al203 samples dried at 120 “C. (B) Co-Al203 samples calcined at 500 “C.

adsorption sites on the surface are saturated. We do not imply that the surface is completely covered by a particular species.) Typical isotherms for adsorption of molybdate on r-alumina and CoAlso3 are shown in Fig. 6(a). Almost identical isotherms were obtained for the adsorption of molybdate on boehmite (the hydrated precursor of yalumina) [5]. At the horizontal portion of the isotherm the amount of molybdate adsorbed was (4.1 + 0.1) X 10m4 (mol MO) (g A1203)-l (equivalent to MO 4.1%). The fraction of the alumina surface covered by the molybdate monolayer is only about 20 - 25% (in agreement with Fransen et al.) [7] . In Fig. 6(b) we show the molybdenum adsorbed at the horizontal portion of each isotherm (i.e. the monolayer coverage) as a function of the cobalt concentration in the Co-Also3 samples. We see that the effect of cobalt is to increase the fraction of the surface covered by molybdate in agreement with our spectroscopic results (Fig. 4). It is striking that the fraction covered is greatest at a composition close to that of the most active technical hydrodesulphurisation catalysts (Co 3%, MO 7%). Desorp tion The equilibrium amounts of cobalt desorbed into water for Series 3 catalysts are shown in Fig. 7. The curve becomes horizontal at higher cobalt concentrations (catalysts 3-3,3-4, 3-5), i.e. the amount of cobalt desorbing becomes constant and independent of the total cobalt content of the catalysts (which increases from 3-l to 3-5). This means that the surface is saturated

340

i

o

2

4

6

8 IOWO]

(mol Co){0

259 catalyst)-’

Fig. 7. Desorption of cobalt from Series 3 catalysts into water (A) and activities in thiophene HDS (initial rate (1O-g (mol thiophene) (g catalyst)-l s-l)) (B) and butane mation (initial rate (lo-l1 (mol butane) (g catalyst)-l s-l)) (C).

for-

Fig. 8. Desorption of molybdate from Series 4 catalysts into water (A) and activities in thiophene HDS (initial rate (lop9 (mol thiophene) (g catalyst)-l s-l)) (B) and butane formation (initial rate (10-l’ (mol butane) (g catalystfP1 s-l)) (C).

with cobalt, i.e. a cobalt monolayer has been established. The cobalt content of the catalysts corresponding to the monolayer was 1.8% w/w. The activities of the catalysts are also plotted in Fig. 7. There is no obvious correlation with hy~odesulphu~sation activity, but there is a striking correlation with activity for butane formation in that values for catalysts 3-2, 3-3 and 3-4 are on the horizontal portion of each curve. This suggests that the initial rate of butane formation correlates with the surface concentration of cobalt(I1). (We are unable to explain the increased activity of catalyst 3-5; possibly we are seeing here an influence on the catalyst structure of lattice cobalt.) For catalyst 3-l the cobalt content of the catalyst was less than that required for a cobalt monolayer. This is the least active catalyst of both series. Evidently one cause of low activity is insufficient cobalt at the surface. Desorption of molybdate is shown in Fig. 8. The values fall on two lines with different gradients. There is an inflexion between catalyst 4-l (MO 6.1%) and 4-2 (MO 7.2%). This is in the region where, according to our adsorption experiments, the molybdenum concentration corresponds to a monolayer {cf. Fig. 6). We conclude, in agreement with the reflectance spectra of the catalysts, that at the higher concentrations the nature of the molybdate species changes, possibly through the formation of a second layer of molybdate (cf. Fig. 6(a)).

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Conclusions Co and No species at the catalyst surface An obvious point, which has been somewhat neglected in previous discussions of activity-structure correlations for HDS catalysts, is that the catalytically significant species are those which are able to come into contact with the reactants, i.e. species at the catalyst surface. We conclude from our work that most, if not all, of the molybdate is at the surface and so activity correlations with total molybdenum content of the catalysts may be meaningful (increase of activity at least up to the point at which the molybdate monolayer is complete, about MO 7%) 181. However, only a fraction of the cobalt is at the surface (
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under steady state conditions hydrogenation is rate-limiting. working (i.e. sulphided) Co-MO-AlaOs catalysts essentially MoS, hydrogenation catalysts.

Activity-structure

Thus we regard as Co-promoted

correlations

Our results shed further light on the relation between activity and structure in HDS catalysts. In use the catalysts inevitably become sulphided. Activity is considered to reside in small crystallites of MO&, the active sites being edge-exposed Mo3 + ions. The number of active Mo3+ sites is increased by cobalt ions which intercalate at the layer edges of MO& crystallites [lo] . We consider that the maximum number of small MO& crystallites will be formed from oxide catalysts which have the maximum number of molybdate ions in a monolayer; our results suggest that this is indeed the situation in the most active catalysts. With less molybdenum than the optimum we get fewer MO& crystallites and hence fewer active sites; with more molybdenum than required for a monolayer we get interacting molybdate ions, polymolybdates and hence larger Moss crystallites. With less cobalt than the optimum, there is less dispersion of molybdate ions over the carrier, less intercalation and hence less activity. Our results focus attention on surface cobalt which is the cobalt most likely to intercalate in MO& crystallites. Acknowledgments We thank Climax Molybdenum Co. Ltd. (P.C. and P.C.H.M.) and the Government of Venezuela (N.P.M.) for financial support and Dr. J. R. H. Ross of Bradford University for catalyst samples. References

4 5 6 7 8 9 10

U. S. Patent 3,900,267 (1975) to M. F. L. Johnson. A. E. Hargreaves and J. R. H. Ross, 6th Int. Congr. on Catalysis, London, 1976, Paper B33. W. W. Wendlandt and H. G. Hecht, Reflectance Spectroscopy, Interscience, London, 1966. G. Kortum, Reflectance Spectroscopy, Springer, Berlin, 1969. R. B. Heslop, Numerical Aspects of Inorganic Chemistry, Elsevier, London, 1970. P. Chiplunker, Ph.D. Thesis, University of Reading, 1975. J. H. Ashley and P. C. H. Mitchell, J. Chem. Sot. A, (1968) 2821; (1969) 2730. P. C. H. Mitchell and F. Trifiro, J. Chem. Sot. A, (1970) 3183. T. Fransen, P. C. van Berge and P. Mars, in B. Delmon, P. A. Jacobs and G. Poncelet (eds.), Preparation of Catalysts, Elsevier, Amsterdam, 1976, pp. 405, 417 - 8. V. H. J. de Beer and G. C. A. Schuit, in B. Delmon, P. A. Jacobs and G. Poncelet teds), Preparation of Catalysts, Elsevier, Amsterdam, 1976, p. 343. P. C. H. Mitchell, The Chemistry of Some Hydrodesulphurisation Catalysts Containing Molybdenum, Climax Molybdenum Co. Ltd., London, 1967, p. 6. R. J. H. Voorhoeve and J. C. M. Stuiver, J. Catal., 23 (1971) 228, 243. R. J. H. Voorhoeve, J. Catal., 23 (1971) 236. A. L. Farragher and P. Cossee, in J. W. Hightower (ed.), Proc. 5th Int. Congr. on Catalysis, North-Holland, Amsterdam, 1973, p. 1301.