Catalyst made of NIMo sulfide supported on hydroxyapatite: Influence of Al addition on support properties and on the catalytic conversion of thiophene

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Catalyst made of NiMo sulfide supported on hydroxyapatite: Influence of Al addition on support properties and on the...

Conference Paper in Journal de Physique IV (Proceedings) · March 2005

DOI: 10.1051/jp4:2005123035

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J. Phys. IV France123(2005) 203–206

C EDP Sciences, Les Ulis DOI: 10.1051/jp4:2005123035

Catalyst made of NiMo sulfide supported on hydroxyapatite:

Influence of Al addition on support properties and on the catalytic conversion of thiophene

N. El Azarifi

, A. El Ouassouli

, M. Lakhdar

, A. Ezzamarty

, C. Moreau

, A. Travert

and J. Leglise

1Laboratoire de Catalyse Hétérogène, Faculté des SciencesAïnChock, BP. 5366, Maarif, Casablanca, Maroc

e-mail: ezzamarty@caramail.com

2Laboratoire Catalyse et Spectrochimie, UMR CNRS 6506, ISMRA, 6 Bd. du Maréchal Juin, 14050 Caen Cedex, France

3Laboratoire de Matériaux Catalytiques et Catalyse en Chimie Organique, ENSCM, Montpellier, France

Abstract.A hydroxyapatite and two apatites enriched with alumina have been synthesized to support NiMo sulfide. Hydroxyapatites are efficient supports of NiMo sulfide catalyst for desulfurization of thiophene.

Addition of Al ions during the preparation results in an intimate mixture of crystalline apatite and amorphous AlPO4. This leads to an important increase in surface area and porous volume that is beneficial to the catalytic activity. The highest activity is obtained with calcium-deficient apatite because the presence of apatitic HPO42−groups or related surface defects is required to improve the dispersion of the Mo ions in the oxidic precursor. The apatitic catalysts are superior to commercial NiMoP/Al2O3catalysts, which makes NiMo/apatite a good candidate for hydrotreating.

1. INTRODUCTION

The anti-pollution legislations have been more and more restrictive as to the contents of sulfide. These restraints necessitate the elaboration of new catalysts more efficient than alumina-based catalysts. Since phosphorus is found in the formulation of all the industrial sulfur catalysts, we are examining in this study the catalytic properties of the NiMo/apatite systems in hydrotreatment, with the aim of finding a noble application to phosphate materials.

We showed previously that synthesized hydroxyapatite was a good support for the sulfided NiMo phases for thiophene hydrodesulfurization [1]. The synergy between Ni and Mo was the same as that on alumina. However, the apatite has a surface 80 m²/g inferior to that of alumina, which makes apatitic catalysts less active than the alumina based industrial catalysts. The specific surface of the hydroxyapatite support should therefore be increased so as to make it more competitive.

In this study, we are investigating hydroxyapatite-based catalysts weakly enriched with aluminium.

The NiMo/phosphate catalysts have been prepared and confronted to a commercial NiMoP/Al₂O₃catalyst for thiophene hydrodesulfurization.

2. EXPERIMENTAL

The phosphate supports were prepared by precipitation as previously reported [2]. The calcium phosphate is denoted later as CP, the Al-enriched solids as ACPx, with x the Al weight content (Table 1), and the aluminum phosphate as AP.

*Present address:DRRT, 2 rue Grenet Tellier, 51038 Châlons en Champagne, France.

204 JOURNAL DE PHYSIQUE IV

NiMo/apatite catalysts have been prepared by impregnation with a solution of ammonium heptamolybdate (pH = 7), then with a solution of nickel nitrate (pH = 5). The Mo content was fixed at 3.5 atoms per nm², and the Ni/(Ni+Mo) atomic ratio was fixed at 0.38 because the catalytic activity was found to be maximum for this composition [2]. The catalysts were sulfided by an H2-15%H2S mixture at 400^◦C then tested for thiophene reaction (8 kPa) under H2 flow. Solids have been characterized by XRD, IR-TF and N2adsorptiometry. The NiMo catalysts were confronted to a commercial NiMoP/Al2O3, HR348 from Procatalyse.

3. RESULTS AND DISCUSSION

X-rays diagrams of the three solids CP, ACP1.5 and ACP3.4 calcined at 500^◦C resemble that of the hydroxyapatite [3]. Parameters, a and c, of the hexagonal unit-cell of samples ACP1.5 and ACP3.4 compared to those of the bare apatite support (table I). The cell parameters did not change because of Al addition. This indicates that the aluminium ions do not substitute for calcium in the apatite latrice. In fact, the aluminium atomic radius is smaller than that of calcium, which does not favor the substitution. Thus, this suggests the existence of an amorphous phase in the ACP1.5 and ACP3.4 solids. After calcination at 900^◦C (figure 1. a), the X-ray diagram of ACP3.4 was not affected, while the ACP1.5 apatite lines disappeared and were replaced by lines characteristic to-Ca3(PO4)2(-TCP). This result is in accordance with the work done by H. Tanaka andal.[4], which indicates that (-TCP) is produced by dehydroxylation of HPO42−ions. The stoichiometric apatite was found to be structurally stable up to 1000^◦C.

The AP solid was found to be amorphous after calcination at 500^◦C, and, after calcination at 900^◦C, broad lines appear in the X-ray diagram, characteristics to an AlPO₄aluminium phosphate [5].

Table 1. Elemental analysis, textural properties, and unit-cell parameters of the phosphate supports.

Chemical analysis Adsorptiometry^a X-rays diffraction^b

Support Ca Al P SBET Vp Rp a c

wt.% wt.% wt.% m²g⁻¹ cm³g⁻¹ nm nm nm

CP 38.3 0 18.8 83 0.32 7.7 0.942 0.689

ACP1.5 36.0 1.5 19.4 140 0.54 7.7 0.942 0.689

ACP3.4 3.8 3.4 18.0 194 0.60 6.2 0.943 0.688

AP 0 25.1 24.5 155 0.91 11.7 - -

asolids treated under vacuum at 400^◦C –^bcalcined at 500^◦C.

The results agree with those obtained by IR spectroscopy reported in a previous study, where it was noticed that the spectrum of sample ACP1.5 displayed a band at 875 cm⁻¹ characteristic to HPO42−

groups of Ca-deficient apatite, non observed on the infrared spectra of sample ACP3.4. As we have noticed in the infrared spectra of samples ACP1.5 and ACP3.4, there exist two bands at 3679 cm⁻¹ and 3791 cm⁻¹, which intensities increase with Al content. These bands have also been noticed on the infrared spectrum of sample AP. Other authors also noticed them with pure AlPO4[5].

The acidity of the supports was measured by infrared spectra adsorbing 2,6-lutidine (pKa = 6.7). The infrared spectrum of sample CP (figure 1. b) shows bands at 1581 cm⁻¹and 1608 cm⁻¹that indicate the presence of acidic Lewis sites. Infrared spectra of ACP1.5 and ACP3.4 show, in addition to the acidic Lewis sites, two bands at 1630 cm⁻¹and 1650 cm⁻¹ indicating the presence of Brønsted sites. These latters are probably related to the presence of an aluminium phosphate phase, since the spectrum of sample AP, with an Al/P atomic ratio close to 1, show only Brønsted sites bands.

Therefore, assuming that all Al ions in the Al-enriched solids belong to an AlPO₄ phase, the Ca/P ratio of the apatitic component in sample ACP1.5 was calculated at 1.56; it is thus similar to that of the calcium-deficient apatite CP. On the other hand, the Ca/P atomic ratio in sample ACP3.4 was equal to 1.66; the apatitic component is then stoichiometric. In summary, we deduce that the ACP1.5 and ACP3.4

REMCES IX 205

1680 1620 1560 Wavenumbers (cm^-1)

AP ACP3.4 ACP1.5 CP

1630 1582

1650 1608

15 20 25 30 35 40 45 50 55 60

AP

ACP3.4

ACP1.5

2 θ/ °

15 20 25 30 35 40 45 50 55 60

(a) (b)

Figure 1. a) X-rays diagrams of the ACP1.5, ACP3.4 and AP supports calcined at 900^◦C. b) Infrared spectra of adsorbed 2,6-lutidine over of the supports calcined at 500^◦C, after evacuation under vacuum at 150^◦C.

supports are made up of crystalline apatite and amorphous aluminium phosphate. The apatite phase of the ACP1.5 solid is deficient and that of ACP3.4 is stoichiometric. The sulfided Ni-Mo catalysts supported on every apatitic material convert thiophene to C4and H2S, whereas the bare supports were inactive. The Mo/ACPx catalysts are very low active compared to NiMo. The expected promoting effect of Ni on the activity of the Mo-containing catalyst was found, showing that both Ni and Mo metal ions interact in the sulfided state. The NiMo/ACP1.5 catalyst was found to be more active than its homologous NiMo/CP (table 2), because it accommodates more NiMo due to its higher surface area (table 1).

Table 2. Rate constants of thiophene HDS determined at 400^◦C over various NiMo catalysts^a and degree of sulfidation of NiMo.

Catalysts Rate constant S/(Ni+Mo)

l/hg at./at.

NiMo/CP 40.0 1.36

NiMo/ACP1.5 66.8 1.5

NiMo/ACP3.4 38.9 0.63

NiMo/AP 35.3 0.52

NiMoP/Al2O3 50.0 -

a3.5 Mo atoms nm⁻², the Ni/(Ni+Mo) atomic ratio was fixed at 0.38.

The addition of low aluminium quantities at the synthesis stage results in an increase of the specific surface area, which leads to catalysts more active in thiophene hydrodesulfurization. The low activity of the NiMo/ACP3.4 is linked to the presence of the stoichiometric apatitic phase, since the specific surface area of the ACP3.4 support is high (194 m²/g).

In conclusion, HPO₄²⁻ groups and surface defects are then necessary to obtain a good dispersion of the Ni-Mo oxidic precursors. Those groups and defects are grafting sites of oxides. The addition of aluminium ions at the synthesis stage has a beneficial effect on the texture, which in turn results in improving dispersion of the Ni-Mo sulfide. The X-rays diagrams of the NiMo/ACP1.5, Mo/ACP3.4 and NiMo/ACP3.4 catalysts (figure 2) are in agreement with these suggestions since the Mo/ACP3.4 and NiMo/ACP3.4 catalysts X-rays diagrams exhibited, in addition to the apatite lines, a line at 2 = 29^◦ (d=3.1) characteristic to bulk MoO3[5]. This line persisted after catalyst sulfidation, that proves that the sulfidation of the NiMo supported on ACP3.4 is incomplete.

206 JOURNAL DE PHYSIQUE IV

20 30 40 50 60

MoO₃

2θ/°

NiMo/ACP1.5 Mo/ACP3.4 NiMo/ACP3.4

Figure 2. X-rays diffraction patterns of some Mo and NiMo catalysts calcined at 500^◦C.

The NiMo/ACP1.5 catalyst is the best sulfided (table 2) and its X-rays diffraction patterns do not show MoO3lines which confirms that the NiMo sulfides are better dispersed on this catalyst.

Acknowledgement

The authors thank the French-Moroccan committee CMIFM for financial support in the frame of the Action Int´egr´ee program through grant Ma/02/36.

References

[1] El Ouassouli A., Th`ese d’Etat, Universit´e de Casablanca (2001).

[2] Elazarifi N., El Ouassouli A., Lakhdar M., Ezzamarty A., van Gestel J., Leglise J., Phosphorus Res.

Bull.,10(1999) 430-435.

[3] Elliot J.C., in “Structure and Chemistry of the Apatites and Other Calcium Orthophosphates”, Elsevier, Amsterdam (1994).

[4] Tanaka H., Watanabe T., Chikazawa M., J. Chem. Soc., Faraday Trans.,93(1997) 4377-4381.

[5] Peri J.B., Discuss. Farad. Soc., (1971) 52-55.

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