Sulfide catalysts made of cobalt and hydroxyapatite | [Catalyseurs sulfures a base de cobalt et d'hydroxyapatite]

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J. Phys. IV France 123 (2005) 319–322

C

EDP Sciences, Les Ulis DOI: 10.1051 / jp4:2005123057

Sulfided catalysts based on apatite modified by adding zirconium ions: Characterization and catalytical behaviour

M. Aït Chaoui

¹

, A. El Ouassouli

¹

, A. Ezzamarty

¹

, M. Lakhdar

¹

, C. Moreau

³

, J. Leglise

²

and A. Travert

²

1

Laboratoire de Catalyse Hétérogène, Université Hassan II, Faculté des Sciences Aïn Chock, BP. 366, MAARIF, Casablanca, Maroc

e-mail: [email protected]

2

Laboratoire Catalyse et Spectrochimie, UMR CNRS 6506, ISMRA, 6 Bd. du Maréchal Juin, 14050 Caen Cedex, France

3

Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, UMR CNRS 5618, ENSCM, 8 rue de l´École Normale, 34296 Montpellier Cedex 5, France

Abstract.

Supports based on apatite and apatite weakly enriched with zirconium phosphate were synthesized.

On these supports, NiMo sulﬁdes were transplanted. The NiMo/Apatites catalysts convert selectively the dimethyldisulﬁde (DMDS) to methanthiol contrary to their counterpart NiMo/Al

2

O

3

. Adding low amounts of zirconium ions improves the catalytical activity. The activity improvement is due to the increase of surface area, as well as to the good dispersion of the NiMo sulﬁde phase.

1. INTRODUCTION

Phosphorus is often found in the formulation of commercial hydrotreating catalyst such as NiMo/Al

2

O

3

. The presence of phosphate groups improves the dispersion of the sulfided NiMo phase on the alumina support, resulting in higher activity for hydrogenation and C-heteroatom breakage [1]. Recently, we have shown that hydroxyapatite can be used to support NiMo sulfide catalyst [2], the activity of which widely largely improved for a calcium deficient apatite.

The present work consists in preparing apatitic calcium phosphate in presence of Zr ions during the synthesis step with the aim of improving the textural properties. The phosphates solids were used as support of NiMo sulfide catalyst and tested for the conversion of dimethyldisulfide. The sulfided NiMo/apatite catalysts will be compared to NiMo/Al

₂

O

₃

series.

2. EXPERIMENTAL 2.1 Supports and catalysts

Two phosphated supports were prepared using coprecipitation method in aqueous medium [3]. A volume of 250 ml of calcium nitrate (0.3 M) or solution containing Ca (0.28 M) - Zr (0.015 M) mixture was added dropwise onto a 1000 ml diammonium hydrogenophosphate boiling solution (0.09 M). The pH was kept in the range between 9-10 during the overall precipitation process using ammonia solution (25%). The solids were ﬁltered, washed and dried under vacuum during 12 h at 80

^◦

C. Calcium phosphate is noted Ao, zirconium enriched apatite as AZr1.7 (with 1.7 the zirconium weight content) and the zirconium phosphate is noted ZrP. The structure of the obtained supports is characterized using X-Ray diffraction, Infrared spectroscopy as well as chemical analysis. The textural properties were determined by nitrogen adsorptiometry at -196

^◦

C. The Mo and NiMo oxidic precursors were prepared by impregnation using a solution excess (1.5 of porous volume) of apatite supports and an alumina ketjen 000.1.5E(220 m

²

/g, 0.6 cm

³

/g). NiMo catalysts were prepared by successive impregnation with an ammonium heptamolybdate

Article published by EDP Sciences and available at http://www.edpsciences.org/jp4or http://dx.doi.org/10.1051/jp4:2005123057

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320 JOURNAL DE PHYSIQUE IV

solution at pH 7, after intermediate calcinations at 400

^◦

C, and with a nickel nitrate solution at the same pH 5. Before impregnation, the supports were treated at 400

^◦

C and 500

^◦

C for 4 h. The Mo content is ranged from 0 to 13 wt%, the nickel content is up to 3.5 wt % and the Ni/Ni+Mo atomic ratio is ﬁxed at 0.4. For NiMo/Al

2

O

3

the Ni/Ni+Mo ratio is about 0.3.

2.2 Catalytic measurement

The Mo and NiMo catalysts are again calcined at 500

^◦

C. They were sulﬁded in a reactor at 360

^◦

C for 2h.

The sulﬁdation was carried out with H

2

S (3.0810

⁻³

mol/h) and H

2

(0.05 mol/h). The DMDS conversion was studied at 150 or 200

^◦

C. The catalyst activity was evaluated at conversion lower than 10 %. In these conditions DMDS was selectively transformed to CH

₃

SH, identiﬁed by GC (OV101, FID detector). The catalytic activity is given by the ﬁrst-order rate constant: K

_d

= - F

^◦

Ln (1-X) /WC in l/hg. F

^◦

is the DMDS ﬂow rate (1.8 to 5 ml/h), C the concentration (0.83 mmol/l) in the reactor at 150

^◦

C, and W the weight of catalyst (0.02-0.2 g). The selectivity was measured at 200

^◦

C at conversion higher than 50%.

3. RESULTS AND DISCUSSION 3.1 Supports characterization

The X-rays diffraction patterns of solids Ao and AZr1.7 (ﬁg 1.a) calcined at 500

^◦

C showed one structure only with space group P6

_3/m

[3].

Figure 1.

a) X-Ray diffraction patterns of Ao, AZr1.7 and ZrP solids calcined at 500

^◦

C. b) IR spectra of Ao and AZr1.7 (self supported disk 20 mg).

The infrared spectroscopy supported the data obtained by XRD. The IR spectra (Fig 2.b) are constituted of the characteristic bands of calcium hydroxyapatite [3]. They include the band at 875 cm

⁻¹

due to HPO

42−

groups substituting for PO

³₄⁻

ions in the lattice of the calcium-deﬁcient apatite [3].

The Ca/P atomic ratio of the calcium phosphate Ao is equal to 1.58 that agrees with a calcium deﬁcient apatite [3]. The Ca/P atomic ratio of AZr1.7 solid is equal to 1.42, this value is unaccustomed for apatites prepared in basic medium for which the value of the Ca/P ratio is lying between 1.5 and 1.67 [3]. The cell parameters did not change because of Zr addition (table 1). This indicates that the zirconium ions do not substitute for calcium in the apatite lattice. Presumably Zr ions are precipitated within another phosphated phase.

ZrP compound was prepared using the same experimental procedure but without Ca ions. Its X-ray diffraction patterns showed an amorphous phase (ﬁg 1.a). The compound crystallises at 1000

^◦

C. Its diffraction pattern (ﬁg 2.a) is similar to the zirconium diphosphate ZrP

2

O

7

[4]. The infrared spectrum is

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REMCES IX 321

Figure 2.

a) X-Ray diffraction pattern of ZrP calcined at 1000

^◦

C. b) Inﬂuence of Mo coverage on activity of suﬁded NiMo catalysts. Supports are calcined at 500

^◦

C.

Table 1.

Elemental analysis, textural properties, and unit-cell parameters of the phosphate supports calcined at 500

^◦

C. Chemical analysis Adsorptiometry XRD

Solides Ca Zr P Ca/P A

BET

Vp Rp a c

% g % g % g at/at m

²

g

⁻¹

cm

³

g

⁻¹

nm nm nm

Ao 38.3 0 18.8 1.57 83 0.32 7.7 0.943 0.688

AZr1.7 35.3 1.7 19.2 1.42 126.6 0.66 10.4 0.942 0.688

also similar to that obtained for zirconium diphosphate. It shows that ZrHPO

₄

decomposes at 1000

^◦

C yielding to ZrP

₂

O

₇

[5]. As the Zr/P ratio is of about 0.5, we supposed that zirconium within AZr1.7 is involved in a zirconium phosphate phase.

Assuming that the Zr ions belong to zirconium diphosphate, the Ca/P ratio of the apatite component in AZr1.7 is evaluated to 1.51, which exhibits the existence of a Ca-deﬁcient apatite.

The surface area of the zirconium containing compound is higher than that of calcium phosphate (Ao). The compound has an open texture because of its large pores radius and its higher porous volume.

So zirconium ions improve the textural performances.

3.2 Catalytic activity

DMDS is totally converted into H

₂

S and CH

₄

at 360

^◦

C. Therefore, the sulﬁded NiMo/apatite catalyst exhibits hydrodesulﬁdation properties as the classical NiMo/Al

2

O

3

hydrotreating catalyst. At 150

^◦

C, DMDS is selectively reduced to CH

3

SH. With both catalysts, the activity increases proportionally to the NiMo content up to a limiting coverage, then remains constant beyond (ﬁg 2.b) strikingly. A similar limit, about 3 Mo atoms per nm

²

, is found with calcium-deﬁcient apatitic supports Ao or AZr1.7 and with alumina.

The NiMo/AZr1.7 catalysts are more active than their counterpart NiMo/Ao (ﬁg 2.b) because they accommodate more NiMo due to their higher surface area. NiMo phase is thus better sulﬁded, which shows again the importance of adding Zr during the precipitation.

The activity of phosphated catalysts with support calcined at 400

^◦

C before the deposit of the active phase (Mo and Ni) is distinctly higher than that of the catalyst whose support is calcined at 500

^◦

C (table 2).

This gain of activity would be related to the presence of HPO

₄²⁻

groups which are decomposed at 500

^◦

C [5]. We concluded that the presence of apatitic HPO

₄²⁻

groups is required to improve the dispersion of Mo ions in the oxidic precursor, resulting in a better sulﬁde dispersion and improved sulﬁdation.

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322 JOURNAL DE PHYSIQUE IV

Table 2.

Catalytical activity determined at 150

^◦

C on NiMo/apatite sulﬁded catalysts compared to coventional NiMo/Al

2

O

3

. The supports are calcined at 400

^◦

C and at 500

^◦

C before impregnation. NiMo catalysts sulﬁdation degree.

Support calcined at 500

^◦

C Support calcined at 400

^◦

C Catalyst K

d1

(l/hg) S/Ni+Mo (at/at) K

d2

(l/hg) S/Ni+Mo (at/at)

NiMo/Ao 10.2 0.99 25.4 1.36

NiMo/AZr1.7 13.53 1.2 31.2 1.56

NiMo/Al

₂

O

₃

15.54 1.5 14.5 -

The performances of our catalysts were confronted to those of the catalyst NiMo/Al

₂

O

₃

. The activity of phosphate catalysts whose support is calcined at 400

^◦

C is higher than the alumina-based catalyst. The sulﬁded phase was well dispersed on apatitic catalyst.

The selective DMDS reduction to CH

3

SH on NiMoS/Al

2

O

3

catalyst yields not only CH

3

SH but also DMS [6]. This latter is formed on catalysts possessing acido-basic sites, strong Lewis acidic and medium basic sites. Compared to alumina, apatite based catalysts are selective for CH

3

SH (table 3). Indeed, apatites, according to the study on the acid sites nature carried out by 2,6-dimethylpyridine adsorption, possess weaker Lewis acid sites. This is an attractive feature for NiMo/apatite catalysts because CH

3

SH is an important intermediate in organic synthesis [6].

Table 3.

Selectivity for CH

3

SH at different conversion rates determined at 200

^◦

C. Catalysts NiMoS/Ao NiMoS/AZr1.7 NiMoS/Al

2

O

3

DMDS Conversion (%) 65.8 94.2 72.3 100 60.4 89.3

CH

3

SH (% mol) 99.5 99.0 99.3 97.3 89.9 25.4

4. CONCLUSION

The hydroxyapatites can be used as supports of NiMo sulﬁded catalysts for selective conversion of DMDS to CH

₃

SH. The addition of zirconium ions during the preparation yields a mixture of hydroxyapatite and an amorphous zirconium diphosphate and leads to an increase of the surface area as well as of the porous volume which is thus beniﬁcial for the catalytical activity. The activity is maximal when the support is calcined at 400

^◦

C and higher than that of NiMo/Al

2

O

3