Promoting effect of fluorine on cobalt—molybdenum/ titania hydrodesulfurization catalysts

Applied Catalysis, 71 (1991) 351-361

Elsevier Science Publishers B.V., Amsterdam

351

Promoting effect of fluorine on cobalt-molybdenum/ titania hydrodesulfurization catalysts

J. Ramirez, R. Cuevas and L. Gasque Departamento de Zngenieria Quimica, Facultad de Quimica, UNAM, Ciudad TJniuersitaria,

Mexico D.F. 04510 (Mexico), fax. (+52-5) 5483227

and

M. Vrinat* and M. Breysse

Znstitut de Recherches sur la Catalyse CNRS, 2 avenue Albert Einstein, 69626 Villeurbanne

Cddex (France), tel. (+ 33) 72445323, fax. (+ 33) 72445399

(Received 20 October 1990, revised manuscript received 15 December 1990)

Abstract

The effect of fluoride incorporation on the titania support on the surface structure of promoted cobalt-

molybdenum catalysts and their catalytic activity for thiophene hydrodesulfurization (HDS) has been

studied. The relative activity between fluoride-modified and unmodified catalysts was found to be max-

imum and close to 5 for a fluorine content of 0.8 wt.-%. From high-resolution electron microscopy

(HREM) examination of sulphided samples, the increase in HDS could be partly explained by smaller

molybdenum sulphide crystallites (increase in the dispersion of the active phase). Diffuse reflectance

spectroscopy (DRS) of the oxidic precursors shows that the effect of fluoride incorporation is also to

increase the amount of the MO”’ in tetrahedral surroundings and to enhance the part of cobalt involved

in the generation of the “CO-MO-S” active phase (probably cobalt in octahedral symmetry).

Keywords: cobalt-molybdenum/titania, fluorine, hydrodesulphurization, dispersion, catalyst charac-

terization (DRS, HREM).

INTRODUCTION

Most research on hydrodesulfurization (HDS) catalysts has been focused on cobalt molybdenum, nickel molybdenum or nickel tungsten supported on alumina. Nevertheless, recent studies have shown that the activity of molybdenum sulphide catalysts can be enhanced by using other carriers such as titania, zirconia, ceria and some binary oxides [l-5]. Explanations of the role of the support have been given in terms of an electronic effect or a geometrical effect, i.e. modification of the dispersion or the orientation of the molybdenum sulphide crystallites.

In a previous study [ 61, we reported that up to a molybdenum loading cor-

0166-9834/91/$03.50 0 1991 Elsevier Science Publishers B.V.

352

responding to 2.8 atoms/nm2 of molybdenum, Mo/TiO, catalysts are about five times more active per molybdenum atom than the corresponding MO/ AlzO, samples in the hydrogenation of biphenyl and the HDS of thiophene. It was proposed that the role of titania is to promote the formation of smaller molybdenum sulphide crystallites than on alumina. These molybdenum sulphide particles might be oriented edge-up on titania due to an interaction between the molybdenum sulphide edge planes and some planes of titania.

The promoting effect of cobalt on the catalytic activity of molybdenum supported on titania catalyst is lower than for the alumina-supported catalysts [3,7]. This lower promoting effect has been ascribed to (i) the formation of edge-up molybdenum sulphide particles more difficult to promote or (ii) to a greater interaction of cobalt with the support, leading to a lower percentage of the promoter involved in the formation of the active CO-MO-S phase. A modification of the support could change its interaction with the active phase and consequently the catalytic activity. The study of these changes may provide some information about the effect of the support on the synergy between cobalt and molybdenum. For alumina-supported CO-MO catalysts, in a recent study [8 ] we have found, by using X-ray photoelectron spectroscopy (XPS), that fluoride addition to the catalyst support increases the dispersion of molybdenum and promoter; this being linearly correlated to surface fluoride content. In the same study, the hydrogenation activity of the fluorinated catalysts was several times higher than the non-modified catalysts. Since titania-supported HDS catalysts have been found to be more active than those supported on alumina [ 7 1, and fluorine modified CO-MO and Ni-MO/alumina catalysts were also found more active than the non-modified alumina supported catalysts [ 8,9], we decided to investigate the changes in thiophene hydrodesulfurization (HDS) activity induced by the incorporation of fluoride into titania-supported CO-MO catalysts. A similar approach to the previous study of CO-MO and Ni- MO/alumina catalysts [ 91 was followed in the present work, which reports the thiophene HDS activity changes as fluoride was incorporated into the titania support. In addition, catalysts were characterized by high resolution electron microscopy (HREM) and UV-vis diffuse reflectance (DRS) measurements. Some catalysts without additive were also characterized for comparison.

EXPERIMENTAL

Catalyst preparation

All the catalysts were prepared by the incipient wetness impregnation method using Degussa P.25 titania (with a 78: 22 anatase-to-rutile ratio) which had a specific surface area of 50 m2 g-’ and a pore volume of 0.6. For the fluorinated catalysts, the support was first calcined at 773 K during four hours, then impregnated with the desired amount of fluoride using an aqueous solution of

TABLE 1

Catalyst composition and reference

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Reference Molybdenum loading (at nm-‘)

Weight (wt.-%)

Molybdenum Cobalt Fluorine

Co-Mo(1.4)/TiO,-F(0) 1.4 1.10 0.29 0 Co-Mo(2.8)/TiO,-F(0) 2.8 2.19 0.57 0 Co-Mo(5.6)/TiO,-F(0) 5.6 4.21 1.12 0 Co-Mo(2.8)/TiO,-F(0.2) 2.8 2.19 0.57 0.2 Co-Mo(2.8)/TiO,-F(0.8) 2.8 2.19 0.57 0.8 Co-Mo(2.8)/TiO,-F(2.0) 2.8 2.19 0.57 2.0

ammonium fluoride, dried at 393 K during twelve hours and calcined at 623 K during four hours.

The support surface area diminished slightly from 50 to 45 m2 gg’ with the incorporation of 2 wt.-% fluoride. However, all catalyst loadings were calculated on an unmodified surface area basis.

The impregnation of molybdenum was effected by using an heptamolybdate solution. The catalysts were then dried at 393 K for twelve hours and calcined at 623 K. Subsequently, the catalysts were impregnated with cobalt nitrate solutions, dried as previously described and finally calcined at 723 K. Sulfi- dation was carried out at 673 K during four hours in a stream of hydrogen- hydrogen sulphide (15% ). Table 1 gives the composition of the catalysts used. In the table, a CO-MO catalyst containing 2.8 atoms of molybdenum per square nanometer and supported on titania modified with 0.8 wt.-% fluorine was la- belled CO-MO (2.8) /TiO,-F (0.8).

Catalytic activities

The thiophene HDS activity of the catalysts was measured in a classical atmospheric microreactor apparatus. Hydrogen was bubbled through a satu- rator filled with thiophene and maintained at 273 K. The gas mixture was passed through a Pyrex reactor with temperature regulation which contained the catalyst. Product analysis was performed on-line by gas chromatography with a 5-m n-octane Porasil column held at 323 K. Catalytic activities were measured at low conversion, below 15%, in the temperature range 523-653 K.

Deactivation of the catalysts was observed during the first three hours on- stream, this deactivation being more pronounced for the fluoride-containing catalysts. Therefore all catalyst activities reported here are given after the catalysts were on-stream for three hours under reaction conditions.

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Catalyst characterization

DRS spectra of the catalysts in their oxidic form were obtained in a Perkin Elmer Lambda 9 spectrophotometer. HREM examinations of sulfided catalysts were performed in a Jeol 100 CX microscope.

RESULTS AND DISCUSSION

Catalytic activities

Fig. 1 summarizes the catalytic activities for the unmodified and fluoride- modified catalysts at different temperatures. It appears clearly that the addition of fluoride on the support increases the rate of hydrodesulfurization. A maximum increase of nearly five times the rate of the unmodified catalyst is obtained for the 0.8 wt.-% fluoride-modified catalyst which presents a specific activity two times higher than the conventional Co-Mo/Al,O, sample [ 71.

Electron microscopy examinations

In order to see if the rate enhancement was due to an increase in dispersion, HREM micrographs of the unmodified and 0.8 and 2.0 wt.-% fluorine-modified catalysts were analysed. The supported phase appears as small crystallites having the typical lamellar structure of molybdenum sulphide and constituted from one or several layers separated by 0.6 nm in good keeping with the molybdenum sulphide structure. These micrographs allow us to determine the number of layers, n, and the length, L, of the crystallites. Distribution of n and L can be obtained on the basis of a large sampling for each catalyst studied (several hundreds of crystallites for each sample ), and average values fi and L can be calculated for the different catalysts.

The results given in Fig. 2 show that both the mean length (c) and number of fringes (fi) of the molybdenum sulphide crystallites decrease with the incorporation of fluoride. These results indicate that the molybdenum sulphide- like phase is better dispersed on the surface of the support when this one is modified by fluoride.

For the CO-MO, Ni-Mo or Ni-W sulfided catalysts, it is now generally accepted that the promoter location is restricted to edge positions of the molybdenum (or tungsten ) disulfide as suggested by several authors [ 10-121. If it is further assumed that the catalytic activity is ascribed to the mixed sites, this activity would then be proportional to the number of molybdenum edge atoms.

Therefore, assuming that the molybdenum sulphide microcrystallites have an hexagonal shape with a diagonal corresponding to the L value, one can calculate the ratio molybdenum (edge-to-molybdenum total) between the

R

-2 10

-3 10

-4 10

1.5

l A

\ 0

1.7 ‘*’ (1 /T K)x103

Fig. 1. Arrhenius plots showing the activities of Co-Mo/TiO,-F (x) catalysts. n = wt.-% fluoride content in the TiOz support: (0) x=0.0, (A) x=0.2, (*) x=0.8, (m) x=2.0 (R= molecules thiophene MO atom-’ s-l).

number of molybdenum atoms on edge plane to the total number of atoms in these crystallites. The calculation gives a molybdenum edge-to-molybdenum total ratio of 0.33 for L=3.2 nm and 0.4 for L=2.6 nm. If each molybdenum edge atom accomodates one promoter atom this small increase of the ratio of molybdenum edge to molydenum total could not explain the high activity observed for fluorinated catalysts.

This calculation exercise considers that all the edge positions available have been promoted. However, it is possible that not all the cobalt present in the catalyst is involved in the formation of promoted sites. An explanation for the large increase of the activity of the modified catalysts could be found in a difference in the number of promoted sites with the addition of fluoride to the

356

100

2.0 4.0 6.0

length of crystallites (nrn’

. I .

. I

. I

. ,

. 1

. I

1 2 3 n

number of fringes

Fig. 2. Crystallite size distribution for the Co-Mo/TiOz-F (x) sulphided catalysts. (- - - ) CoMo/ TiO,-F(O), t~3.2, E= 1.5; (---) CoMo/TiO,-F(O.B),t=2.5, ti= 1.3; (*.a) CoMo/TiO,-F(B.O), L= 2.8, K= 1.2.

catalyst. This proposal in turn will be related to the preparation of the catalysts. To explore further this possibility, DRS studies have been performed on the oxidic precursors.

Characterization of the oxidic state of the catalysts

Since the DRS spectrum of titania overlaps to some extent with the molybdenum spectrum, titania was used as the reference. Moreover, as cobalt and molybdenum have been widely studied in the past by DRS, it was not consid- ered necessary to repeat the study of model compounds for the assignment of the characteristic bands of molybdenum and cobalt.

Molybdenum species

For alumina-supported samples, the bands of octahedral and tetrahedral oxidic forms of MO” (charge transfer transitions) appear in the region of 200- 300 nm, namely, both octahedral and tetrahedral at 225-240 nm, tetrahedral at 250-280 nm and octahedral at 290-330 nm [ 13 1. For titania-supported catalysts the bands are shifted toward the high wave-length side of the spectrum. In this case, due to the interference of titania bands (200-350 nm), only a part

357

of the octahedral molybdenum band which extends up to 450 nm can be observed.

Fig. 3 shows the DRS spectra of catalysts without modification by fluoride at different metal contents (1.4, 2.8 and 5.6 molybdenum atoms per square nanometer) and with (Co/ (Co + MO) ) = 0.3. In this figure it can be seen that the smallest molybdenum loading corresponds to the largest shift of the molybdenum band around 400 nm to the left. It has been shown in the past by Raman spectroscopy that at low molybdenum loadings, below the monolayer, a well-dispersed tetrahedral molybdenum species is obtained in oxidic MO/ TiO, catalysts [ 141. Therefore, the shift of the molybdenum band has also been interpreted here as a reduction in the octahedral MO”‘.

It has been previously reported [ 13,151 that the relative quantities of tetrahedral and octahedral MoV’ depend on catalyst preparation method, molybdenum content and calcination temperature. The higher the amount of molybdenum content and the calcination temperature, the higher the amount of octahedral MO”‘. In the present case, since the amount of molybdenum and calcination temperature were kept constant for the fluoride modified catalysts, any change in the molybdenum bands will be caused by the presence of fluoride on the support.

500 800 - .i(nm)-

Fig. 3. DRS spectra of the unmodified Co-Mo/TiO, catalysts with different molybdenum contents: (a) CO-MO (1.4)/Ti02; (b) CO-MO (2.8)/TiO,; (c) CO-MO (5.6)/TiO,

a

.*.......... . . . . . . . . . . r.

500 1000 1500

- -i(nm) -

Fig. 4. DRS spectra of the fluoride-modified Co-Mo/TiO, catalysts: (a) CO-MO (2.8)/TiO,-F (0); (b) CO-MO (2.8)/TiO,-F (0.8); (c) CO-MO (2.8)/TiO,-F (2.0). The spectrum of CO-MO (23)/TiO,-F(0.2) has been ommitted for clarity since it was too close to spectrum (a).

Fig. 4 shows that the incorporation of fluoride leads to a decrease in the high wavelength side of the molydenum band which has been interpreted as a reduction in the amount of MO”’ in octahedral symmetry. The maximum such effect is for the catalyst with 0.8 wt.-% fluorine. The same result can also be appreciated in the reduction of the intensity of the molybdenum band situated at 410 nm.

On the other hand, similar DRS spectra are obtained for the fluoride-modified catalysts with high molybdenum loadings and the unmodified catalysts containing low molybdenum concentrations. It is assumed therefore that at comparable molybdenum loadings, more MO”’ in tetrahedral surroundings is present in the modified catalysts. These well-dispersed MoV’ entities in tetrahedral symmetry have been proposed in the past to generate the most active species for thiophene HDS [ 3,14 1. This assumption agrees well with the HREM observations which indicate that incorporation of fluoride in the catalysts leads to smaller molybdenum sulphide like crystallites.

Cobalt species

For the cobalt species, a broad triple band situated between 500 and 700 nm, due to ligand field transitions can be assigned to tetrahedral Co”. In alumina- supported catalysts, this band has been assigned to tetrahedral Con in CoAl,O, [ 16-171. In our case, for samples containing only cobalt supported on titania, the triple band of Con in tetrahedral surroundings appeared in the same wavelength range although not so well defined, probably due to the presence of significant amounts of Co,O,.

Evidence for the presence of Co,O, is provided by the bands corresponding

359

to Co”’ in octahedral surroundings which consists of (i) an absorption band at 400 nm, (ii) a band between 650-900 nm with a maximum at 700 nm, and (iii) the bands appearing in the range 1000-1840 nm which can be assigned to both octahedral Co”’ and tetrahedral Con.

Fig. 3 shows the spectra of unmodified Co-Mo/TiO, catalysts at different molybdenum loadings in which the atomic ratio of molybdenum to molybdenum-plus-cobalt has been kept constant and equal to 0.3. In this figure, the Con triple band between 500-700 nm is best defined at low loading indicating that under these conditions there is a higher proportion of Con tetrahedral with respect to Co”’ in octahedral symmetry whereas at high metal loadings a badly defined spectrum is obtained probably due to increased amounts of Co304. Recently, a similar effect has been reported [ 181 from photoacoustic spectroscopy studies of CO-MO catalysts supported on alumina with various amounts of metal loading.

The shoulder at 550 nm present in our catalysts with no fluoride incorporated has been interpreted as a contribution of octahedral cobalt in Co304. In the same catalysts the band between 650-900 nm has also been assigned as a contribution of Co,O,.

An interesting feature of the spectra of the fluorine-modified catalysts, all of which have the same amounts of metals, is the difference in intensity of the cobalt bands (as could be seen in Fig. 4). The triple band of the tetrahedral cobalt at around 600 nm decreases with the incorporation of fluoride. This decreasing effect is maximum for the catalysts with the highest, activity containing 0.8 wt.-% fluorine. Also, in the region of high wavelengths between 1000 and 1840 nm, in which contributions of Co”’ octahedral and Con tetrahedral can be seen, the order of intensity of these bands from the smallest to the highest is: Co-Mo/TiO,-F (0.8) < Co-Mo/TiO,-F (2.0) < Co-Mo/TiO,- F (0.0). This diminishing effect can be interpreted as a reduction of the contribution of Co,O, due to fluoride incorporation.

The above results suggest that if the contribution of Con in tetrahedral symmetry and the contribution of Co30, which contains both tetrahedral and octahedral cobalt becomes less important, significant amounts of cobalt in other than tetrahedral symmetry must be present in the catalyst. Although from DRS spectra it is not possible to say that this non-tetrahedral cobalt may be octahedrally coordinated, recent studies [ 19-221 have postulated that octahedral cobalt (not in Co,O,) is present in the catalyst and is responsible for the promoting effect.

As it is generally accepted that the CosS, phase which originates from sulphidation of Co30, does not contribute to the “catalytic activity”, an inhibition in the formation of this phase will result in more cobalt available for the formation of the “CO-MO-S” active phase.

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CONCLUSIONS

The following general conclusions can be drawn from the above results: (i) The incorporation of fluoride into titania leads to CO-MO catalysts pre-

senting very high HDS activity, the maximum being at fluorine loadings around 0.8 wt.-%.

(ii) The addition of fluoride to the catalyst support results in higher dispersion of the molybdenum sulphide phase, i.e. smaller molybdenum sulphide crystallites with a lower average number of layers. The higher activity of the fluoride-modified catalysts can be related in part to this higher dispersion of the active phase.

(iii) The effect of incorporating fluoride into the catalysts is to reduce the amount of octahedral molybdenum, therefore increasing the MO”’ in tetrahedral surroundings; entities which have been proposed [3,14] to generate the most active species for thiophene HDS.

(iv) The fluoride added to the catalyst inhibits the formation of Co,O, and decreases the amount of Con in tetrahedral coordination symmetry in the oxidic precursor. These results suggest that the concentration of non-tetrahedral cobalt, possibly in octahedral symmetry, is enhanced by incorporation of fluoride leading during the catalyst sulphidation to more cobalt involved in the “CO-MO-S” active phase.

ACKNOWLEDGEMENTS

We thank UNAM-DGAPA, CONACYT and CNRS (CONACYT-CNRS 140-106G-202-021) for financial support for this project.

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Promoting effect of fluorine on cobalt—molybdenum/ titania hydrodesulfurization catalysts

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