Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 4, 1979 329
The Effect of Composition and Pretreatment on the Activity of Nickel-Molybdenum Based Hydrodesulfurization Catalysts Jorge Laine, Kerry C. Pratt," and David L. Trimm Deparfment of Chemical Engineering, Imperial College, London, S W7 2BY, England
Studies of the effect of pretreatment and composition of nickel-molybdenum based catalysts for the hydrodesulfurization of thiophene have been carried out. Optimal pretreatment has been found to involve heating the catalysts at 400 O C in an atmosphere of pure hydrogen sulfide. For this pretreatment procedure, the optimal composition was found to be 3% Ni0:35% Moo3 deposited on y-alumina. The activity patterns observed were found to be consistent with a reaction mechanism in which the catalytic activity was proportional to the positive hole carrier density in the solid.
Introduction The catalytic hydrodesulfurization (HDS) of organic molecules containing sulfur has long been a very important process in the petroleum and petrochemicals industry (Weisser and Landa, 1973; de Beer and Schuit, 1976; Amberg, 1974). In recent years this importance has been extended to include hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), and hydrocracking (HDC), as a result of renewed interest in coal as a chemical feedstock (Schuit and Gates, 1973; Spitz, 1977; Badilla-Ohlbaum et al., 1979a). The oxides and sulfides of molybdenum (or tungsten), promoted to a greater or lesser extent by cobalt, nickel, zinc, or manganese have been found to be effective catalysts (Frank and Schmid, 1973), with cobalt-molybdenum combinations being favored for HDS and nickel-molybdenum combinations being preferred where HDS, HDN, HDO, and HDC are all required. The present studies are intended to focus attention on some aspects of the preparation and pretreatment of nickel molybdenum catalysts, as part of an investigation into their activity for the hydrotreating of coal derived liquors. Using the HDS of thiophene as a test reaction, the influence of the Ni:Mo ratio, of prereduction, and of presulfiding have been examined. Although nickel molybdates have been less well st,udied than cobalt molybdates (Weisser and Landa, 1973; de Beer and Schuit, 1976), the mode of action of both catalysts is supposed to be very similar. There is still some measure of uncertainty as to the details of their operation, particularly with respect to the structure of the active catalyst. Three models have been proposed. These include the monolayer model, which supposes that an epitaxial monolayer of Moo3 is formed on y-Al,O, (de Beer and Schuit, 1976; Lo Jacono et al., 19731, the pseudo-intercalation model, which supposes that promoter ions may be located between layers of disulfides (de Beer and Schuit, 1976; Voorhoeve and Stuiver, 19711, and the synergetic model, which is based on the observation that mixtures of Co& and MoS2 show considerable synergism (Grange and Delmon, 1974; Hagenbach et al., 1973). These models have been interrelated by the work of de Beer and his associates (de Beer et al., 1972, 1974), who suggest that precursor cobalt-molybdenum oxide structures are best described by the monolayer model but that, during presulfiding or re-
* Address correspondence to this author at C.S.I.R.O., Division of Materials Science, Catalysis and Surface Science Laboratory, University of Melbourne, Parkville, Vic. 3052, Australia. 0019-7890/79/1218-0329$01.00/0
action, the catalyst is best described in terms of the intercalation or the synergetic models. Wentrcek and Wise (1978) prefer to relate catalytic activity with semiconductivity rather than structure. Since most sulfur or nitrogen containing substances of interest are electron donating, they suggest that catalytic activity could be related to p-type semiconductivity enhanced by the addition of the promoter. This was shown to be true for cobalt molybdate HDS catalysts. At least part of the difficulty in studying HDS catalysts rests in the uncertainties involved in preparation. A conventional technique involves impregnation of alumina by dissolved metal salts, followed by drying and calcination to give supported metal oxides. These may be reduced and presulfided before use as a catalyst. Inspection of the literature shows no clear optimization of different variables in the preparation of nickel molybdate based catalysts. Thus, for example, although Hagenbach et al. (1973) have shown that cobalt molybdate catalysts are most active when Co/(Co + Mo) equals 0.34.4, no such optimalization has been completed for nickel molybdates. The phase composition of Ni-Mo-A1 oxides has been studied (Pilipenko et al., 1973; Plyasova et al., 1974; Talipov et al., 1972), but no correlations with HDS activity have been attempted. This uncertainty extends throughout the preparation. Thus, for example, Sultanov et al. (1972) recommended that Mo should be impregnated on alumina before Ni, as this increases the HDS activity. However, Badilla-Ohlbaum et al. (197913) find that maximal HDS, HDO, HDN, and HDC activity is obtained if Ni is impregnated on alumina before Mo. As a result of this situation, a systematic study of the preparation of nickel-molybdenum catalysts has been undertaken. The optimal starting metal salts and the optimal calcining conditions have been previously reported (Laine et al., 1979), and the present studies have been concerned to establish the optimal Ni:Mo ratio and the effect of prereduction and presulfiding. The HDS of thiophene was used as a test reaction. Experimental Section All catalysts were prepared by impregnating B.D.H. 7-A1203powder, having a surface area of 116 m2 g-l, a pore volume of 0.25 cm3 g-' (mean pore diameter 90 A) and a particle size of 100-300 mesh. The alumina was co-impregnated with aqueous solutions of nickel nitrate and ammonium molybdate (Laine et al., 1979) to a composition reported below as x % Mo03:y% NiO. The solid was dried
0 1979 American Chemical Society
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Ind. Eng. Chern. Prod. Res. Dev., Vol. 18, No. 4, 1979
H2 Prereductlon
Ornt
350°C
43
I/ L ~ 10/10 0 1
0
10
5
15
Time lhourrl
Figure 3. The activity of catalysts as a function of Ni loading for 10% MOO,. Activation: "minimal" reduction (see text). Reaction: 3 g catalyst; 400 O C ; 740 mL min-' H,; molar ratio H,/thiophene = 45. I 0.:
0
-
5
10
15
,
20
,
25
% Moo3
omld 0
,
1
1
,
2
3
,
4
,
5
I
Figure 4. The activity of catalysts as a function of Mo loading for 3% NiO. Activation: "minimal" reduction (see text). Reaction: as in Figure 1; 0 , maximum activity; 0,steady-state activity.
J
6
1
Time ( H o u r s )
Figure 2. The activity of catalysts in the absence of presulfiding for 3% Ni0:10% Moo3. Activation: reduced at 300 O C . Reaction: 3 g of catalyst; 330 mL min-' Hz; molar ratio H,/thiophene = 23.
(120 "C; 16 h) and calcined (500 "C; 5 h) before use. The activity of the catalysts was measured at atmospheric pressure using a flow reactor fitted with an on-line gas chromatograph (Laine et al., 1979). The reactor consisted of a 22-mm diameter Pyrex tube, externally heated. The catalyst charge (about 3 g) was supported on a sintered glass disk. Mass transfer limitations were shown to be absent under the test conditions. Because of uncertainties as to the composition of the catalysts after different pretreatments, all compositions are reported as NiO and MOO, as deposited on the support. Results Preliminary experiments showed that a catalyst containing 5% NiO and 10% MooBhad a high activity for thiophene HDS, and this catalyst was used to study the effect of prereduction. Using the conversion of thiophene as a measure of activity, the effect of reduction in hydrogen for 16 h a t various temperatures was studied (Figure 1). I t is seen that highest activity was obtained a t the lowest prereduction temperature (300 "C) and a high activity was still obtained if no prereduction was carried out. However, the steady state catalytic activity was also dependent upon the condition under which the HDS reaction was carried out (Figure 2). For a catalyst prereduced a t 300 "C, the
Table I. Sulfur and Carbon Contents of Used Reduced Catalysts catalyst, %NiO
cornpn, %MOO,
%sulfur
%carbon
0 1 3 10 5
10 10 10 10 25
3.0 2.6 4.9 6.6 5.6
0.21 0.17 0.12 0.13
0.20
steady-state activity increased with the HDS reaction temperature and the approach to the steady-state activity also changed. The effect of changing reaction temperature once the steady-state activity had been reached is also shown in Figure 2. The effect of catalyst composition was then examined using a pretreatment procedure involving the minimum of reduction. Catalysts were heated to reaction temperature under hydrogen as fast as possible (
