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Novel hydrotreating catalysts prepared from heteropolyanion complexes impregnated on alumina
Applied Catalysis, 48 (1989) 187-197 Elsevier Science Publishers B.V., Amsterdam -
187 Printed in The Netherlands
Novel Hydrotreating Catalysts Prepared from Heteropolyanion Complexes Impregnated on Alumina A.M. MAITRA and N.W. CANT* School of Chemistry, Macquarie University, NSW2109 (Australia) and D.L. TRIMM School of Chemical Engineering and Industrial Chemistry, University of New South Wales, P. 0. Box 1, Kensington, NS W 2033 (Australia) (Received 27 May 1988, revised manuscript received 20 September 1988)
ABSTRACT The preparation of hydrotreating catalysts by the impregnation of alumina with solutions of heteropolyanions has been studied. These anions have the general structure [H,A&O,]“-, where A may be Co or Ni and B may be MO or W. The anions have advantage over conventional impregnating agents in that both catalytically active metals are in the same complex and should be deposited in the same position in the pellet. In addition, the solubility of the anion is higher than conventional impregnating agents. This allows higher concentrations of catalytic metal (and particularly of tungsten) to be deposited on or in the support. Conditions for the preparation of “shell” or “uniform” impregnated catalysts have been established. Comparisons have been made of the efficiency of such catalysts and of conventional catalysts for the hydrotreating of a model feedstock. Heteropolyanion-based catalysts are very effective for hydrodesulphurization and hydrogenation but their relatively low content of promoter metals restricts hydrodenitrogenation activity.
INTRODUCTION
The use of hydrotreating catalysts based on molybdenum or tungsten salts, supported on alumina and promoted by cobalt or nickel, is well established [l31. The chemical composition and preparation of such catalysts have a significant effect on performance [ 4-61, as does the texture of the solids. Mass transfer in pores, particularly under conditions where foulants can be deposited on the catalyst [ 81, is a major factor in determining catalyst efficiency. Control of catalyst porosity is best achieved by controlling the pore size in the support and impregnating with appropriate metal salts [8]. Conditions may be adjusted to achieve impregnation only on the outside of the pellet (shell catalyst) or to obtain an even distribution throughout the pellet [ 91. However,
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in the latter instance, impregnation times may be long owing to the slow diffusion of large species [lo] and/or the limited solubility of given metals salts. Impregnation by tungsten salts poses particular problems in that highly polymeric species may be produced which lead to pore blockage and slow impregnation [lo]. A further difficulty with impregnation arises in that anionic base elements require acidic conditions to promote adsorption on alumina, while impregnation of cobalt or nickel salts is favoured in basic solution [lo]. As a result, two impregnation operations may be required and, under these conditions, uniformity of deposition is hard to achieve. In the light of these difficulties, attention has now been focused on impregnation of alumina supports using heteropolyanions which contain both base and promoting elements in a single complex [ 111. The general formula of these anions is (H,A,B,O,)+, where A may be Co or Ni and B may be MO or W. Impregnation with such a single salt thus allows introduction of both catalytically active metals at the same time. The preparation of Co-W-, CO-MO-, Ni-W- and Ni-MO-based catalysts from heteropolyanions has been studied. The efficiency of such catalysts for hydrotreating has been compared with that of catalysts prepared by conventional impregnation using a model feedstock. EXPERIMENTAL
A variety of heteropolyanion compounds were prepared by standard literature methods and their formulae checked by atomic absorption analysis against standard solutions of known concentration. With the exception of the cobalttungsten combination, the general formula of the anions was (H6XY6024)Xwith X=Co or Ni and Y=Mo or W. (NH4)4(Ni”MosOadHG), mixed complex and the (NH,), ( NinW60Z4Hs) *5Hz0 ( NH4)4 ( NinMo,W30Z4HB) were each prepared by the method of Matijevic et al. [ 121. The first compound was light blue with a room-temperature solubility limit of about 0.016 A4 (9 g/l MO, pH ~4.9). The tungsten analogue was also light blue with solubility 0.016 M (18 g/l W pH~6.5). The synthesis of ( NH4 j3 ( CO~~~MO~O~~H~) - 7H20 was similar to that described by Friedheim and Keller [ 131. In the final step the salt was recrystallized from water to remove a minor olive-green solid with a cobalt-to-molybdenum ratio of 2:lO. The bluish green CoMo, salt has a saturation limit of ca. 0.055 M (32 g/l MO, pH M4). The corresponding free acid was obtained by passage through a cation-exchange column (Amberlite IR-120, Rohm and Haas) in the H+ form [ 141. Solutions with pH l-l.5 and of concentrations up to 0.055 M were obtained with retention of the cobalt-to-molybdenum ratio of 1:6. Cobalt-tungsten heteropolyanion compounds were also prepared in both ammonium salt and free acid forms, but the metal-to-metal ratios in the struc-
189
tural units differed from each other and from the above. The cobalt-to-tungsten ratio was 2:ll in the ammonium salt (NH4)s(Con2W,1040H2) .13H,O prepared as described by Baker and co-workers [ 15,161. The material prepared was emerald green with solubility 0.021 M (45 g/l W, pH = 6.5) and a cobaltto-tungsten ratio of 1:4.9 by analysis. When passed through Amberlite IR-120 (acid form ) an intense blue solution (pH z 1) with a cobalt-to-tungsten ratio analysed as 1:11.6 was eluted. This corresponds to the conjugate base described by Baker and McCutcheon [ 171 as (Co”W,,O,,)“-. Nearly saturated solutions (0.016 M, 35 g/l) could be used for some time but, over a period of months, small amounts of yellow WO, were deposited. Solutions of the complexes were prepared and used to impregnate alumina cylinders of diameter 1.6 mm and length 3-5 mm (Norton SA6173, surface area 220 m’/g, pore volume 0.55-0.70 cm3/g, mean pore diameter 8 nm). The solids were washed in distilled water (ca. five times the volume of the pellets) and dried in air at room temperature and then in air at 140” C (24 h). The metal loading and the distribution were determined by atomic absorption spectrometry (AAS) (Varian Techtron Model 1100) and by electron microprobe analyses (ARL Model EMX) . The latter involved measurements with a spot size of 5 pm and polished half pellets. Calibration was based on non-porous standards. Metal contents as determined by the microprobe were up to 25% below the results of bulk analyses by AAS and the metal ratios showed similar variations. The difference reflects the porous nature of the support and possibly some non-uniformity in the deposited layers. Scatter was especially pronounced for the molybdenum systems. Before use as catalysts, the solids were calcined in air (465 ’ C for 16 h). The catalysts were presulphided in hydrogen sulphide-hydrogen (1:9, v/v) at 673 K for 1 h. The reactor was cooled to 573 K under an inert atmosphere and the activity of each catalyst was determined at 573,603 and 633 K. The test series included comparative measurements with two commercial catalysts of the same geometric form. One was based on nickel and tungsten (Harshaw 4303) and the other on nickel and molybdenum (Shell S324). The standard operating conditions were pressure 7 MPa, LHSV 2.2 g (g-cat.) -’ h-l and hydrogen flow-rate 500 ml min-l. Liquid product analysis was carried out by off-line gas chromatography on a Gow-Mac chromatograph (2.5-m column packed with OV-101, column temperature programmed from 333 to 523 K at 6 K min-‘, flame ionization detection). The model feed mixture contained 5% (w/w) of thiophene, 20% (w/w) of quinoline, 7% (w/w) of dibenzofuran and 8% (w/w) of phenanthrene in decalin (55%, w/w). 5% (w/w) of pseudo-cumene was added to the mixture as an internal standard.
190 RESULTS AND DISCUSSION
Catalyst preparation Cobalt-tungsten-based system Initial studies were focused on cobalt-tungsten containing heteropolyanions, as they were highly soluble and their colour gave a good visual indication of impregnation. The essential features of the impregnation may be discussed with the aid of the results summarized in Figs. 1 and 2. In all figures, the dominant (base) metal is shown on the left of the diagram and the minor (promoter) metal on the right. This description is adopted for convenience only. In fact the metal distributions represent measurements at identical points for each element taken along the same radius of the 1.6-mm alumina cylinders. The bars represent the range observed for repeated measurements made at short distances on either side of an initial determination. Impregnation with the acid complex was found to be shell progressive (Fig. 1). A nearly uniform distribution was achieved to a depth that varied with time. The measured tungsten-to-cobalt weight ratio in the impregnated band was ca. 30, reflecting the 12:l molar ratio of these elements in the starting (CoW,,O,,)“ion. Under the conditions pertinent to Fig. 1, about 16 h were needed for complete impregnation, when about 20% (w/w) tungsten was deposited. This corresponds to about one monolayer of ion on the alumina, a value similar to that observed by impregnating with polytungstates [lo]. The time required for impregnation was much longer than that predicted on the Tungsten;Cobalt
Tungsten
2
-2
E .o, 1
-1
i Cobalt
z
s -I
edge
centre
edge
Fig. 1. Profiles for impregnation using0.015 MH,(CO”W~~O,,,) 4 h; (c) 7 h; (d) 24 h.
edge
(35 g/l W) at pH 2: (a) 2 h; (b)
Fig. 2. Profiles for impregnation using 0.021 M (NH,),(Co”,W,,O,,H,) (45 g/l W) at pH 5.5-6: (a) 10 h; (b) 22.5 h; (c) 7 days; (d) 0.021 M (46 g/l) at pH 3.5 for 10 h.
191
basis of the simple model proposed by Weisz and co-workers [ 18-201, indicating that mass transfer limitations in the pore are important. The sharp impregnation front is not unexpected, in that the negatively charged anion should be strongly bound to positively charged alumina below the isoelectric point at pH 6 [21]. Weaker adsorption would be expected for impregnation by nearly neutral solutions, and this is reflected in the results summarised in Fig. 2. Adsorption of the ammonium salt (NH,),( Co2W11040H2) is seen to give a dish-shaped profile through the pellet, with gradual filling of the “dish” with time. The concentration of the impregnant slowly builds up until tungsten-to-cobalt weight ratio of ca. 12 (reflecting the molar ratio of 5.5 in the salt) is achieved throughout the pellet. At intermediate pH, where both species are expected to be present, impregnation profiles reflect both complexes and an average tungsten-to-cobalt weight. ratio of ca. 9.0 was achieved in the final pellet (curve d in Fig. 2). Comparison of Figs. 1 and 2 shows impregnation to be slower with the neutral salt, mainly reflecting the larger size of the (NH,‘), (CO~W~~O~~H~)*species. However, the concentration of the impregnating solution also affects the rate of impregnation, as seen in Fig. 3. With the acid salt (curves b and c) similar impregnation profiles to that found after 7 h with 0.015 M solution (Fig. lc) required approximately 50 h with 0.0025 M solution (Fig. 3b) and 120 h with 0.0008 M solution (Fig. 3~). With the neutral salt, the dish developed with 0.0011 M solution after 120 h (curve a) is much below that developed with 0.021 M solution after 10 h (curve d) . The amounts of cobalt and tungsten taken up at the point of uniform impregnation are presented in Table 1. The amount of complex used initially was
edge
centre
edge
Fig.3. Effect of concentration and pH on profiles when impregnating with cobalt-tungsten heteropolyanions: (a) 0.0011 A4 (2.25 g/l W) at pH 6.5 for 120 h; (b) 0.0008 M (1.75 g/l W) at pH 3 for 120 h; (c) 0.0025 M (5.83 g/l W) at pH 3 for 49 h; (d) 0.021 M (45 g/l W) at pH 6.5 for 10 h.
192
TABLE I Analytical results and impregnation times for visually uniform cobalt-tungsten Tungsten concentration (g/l)
catalysts
Impregnating species (CO”W,,O,,)~Time (h)
45 (335jj*
-20
17 10 5.5 1.75
(-10) -30 -40 -80 - 200
at pH 2
(COI~~W~~O~~H~)~at pH 6
W (%, w/w)
CO (%, w/w)
20.0 (23.1) 20.2 19.9 19.7 20.0
0.62 (0.65) 0.62 0.56 0.53 0.54
Time (h) -32 -10 (-24) -200 > 200 > 200 -
W (%, w/w)
co (70, w/w)
19.8 18.8 (21.5) 17.8 16.0 14.5 -
1.42 1.21 (1.46) 1.18 1.10 1.00 -
*200% excess solution above uptake.
about 30-50% in excess of that taken up, except for one preparation, when a 200% excess was used. Impregnation under acidic conditions is seen to give final uptakes that are independent of the starting concentration, although a small excess was observed for the more concentrated impregnating solution owing to deposition from residual solution in the pores during drying. Impregnation under nearly neutral conditions gave uptakes that varied significantly with the concentration of the starting solution, reflecting variable adsorption by the more weakly bound species. Tungsten-to-cobalt atomic ratios were 11.2 ? 0.8 for acidic conditions and 4.7 + 0.5 for neutral conditions, in agreement with those in the starting materials. Cobalt-molybdenum-based system Impregnation by the cobalt-molybdenum heteropolyanion showed many common features with the cobalt-tungsten system but also some significant differences. Impregnation by the acid complex was shell progressive with development of a flat profile corresponding to monolayer coverage in 2 h (Fig. 4). Neutral conditions gave a dished profile with lower concentration near the pellet rim. This presumably reflects removal of the weakly adsorbed complex by the minimal washing involved in the preparation. The metal contents of a visually uniform catalyst prepared from solution containing a 200% excess of material taken up are summarized in Table 2. The results are analogous to those for cobalt-tungsten catalysts with the lower uptake of molybdenum reflecting the relative atomic weights of molybdenum and tungsten. Impregnation is seen to be much faster with cobalt-molybdenum ions than with cobalt-tungsten ions. The cobalt-tungsten anions are known to be roughly
193
edge
centre
edge
Fig. 4. Effect of pH on profile development when impregnating with ( CO”‘MO,O,,H,)~-. M (15.8 g/l MO) at pH for 2 h; (b) 0.028 M (16.2 g/l MO) at pH 2 for 2 h.
(a) 0.027
TABLE 2 Analytical results and impregnation times for visually uniform catalysts with (Co”‘Mo,O,,H, as impregnating ion Molybdenum concentration (g/l) 31.5 15.8 9.0 5.3
Acid form at pH 2-3
j3-
Ammonium salt at pH 5-6
Time (h)
MO (%, w/w)
co (%, w/w)
Time (h)
MO (%, w/w)
co (70, w/w)
t2 -2 -4 -7
10.2 9.9 9.6 9.5
0.78 0.71 0.70 0.68
-2 -8 -16 -50
10.3 9.4 9.1 9.2
0.75 0.68 0.68 -
spherical with a diameter of ca, 1.2 nm [ 111, whereas the cobalt-molybdenum ions (and nickel-molybdenum ions) are thought to be disc-shaped with a diameter of ca. 1.2 nm but a thickness of ca. 0.57 nm [22]. Presumably the latter ions are adsorbed flat on the pore wall, thus producing less pore blocking and faster diffusion of the impregnating species. The occurrence of restricted diffusion of ions of 1.2-nm diameter in an alumina with a mean pore diameter of 8 nm (as used here) presumably reflects the presence of constrictions in the pore system. The effect should be less in a support with more uniform pores and/or pores of larger mean diameter. It
would be of interest to test this prediction experimentally. Nickel-tungsten-and nickel-molybdenum-based systems Although similarities existed between these systems and their cobalt-containing analogues, some interesting differences were noted. Impregnation with
194
( NiW,0,,H,)4produced shell progressive impregnation at low pH and dishshaped profiles under nearly neutral conditions (Fig. 5 ) . However, the ratio between the metals did not equal the ratio in the original complex at all points. For example, the tungsten-to-nickel ratio observed for catalyst, 5a was closer to 9 than the expected value of 6. Again, in Fig. 5b, tungsten is seen to penetrate further into the pellet than nickel producing a tungsten-to-nickel ratio that varies across the diameter. The absence of salts containing tungsten alone in the starting material was confirmed, and these results indicate that some disproportionation of the het-
1 15-
Tungsten
I
Nickel
- 1.5
centre
cage
edge
Fig. 5. Profile development when impregnating with (Ni”W,0,,H,)4-: (a) 0.01 M (4 g/l W) at pH 3.5 for 29 h; (b) 0.011 M (3.9 g/l W) atpH 6 for 96 h; (c) 0.032 M (12 g/l W) at pH 6 for 96 h. Tungsten I
edge
Fig. 6. Profile “Mo,W,O,,H,)~-
Molybdenum Nickel
centre
-
---
I
edge
development during impregnation with a saturated solution at pH 5.5: (a) 9 h; (b) 25 h; (c) 147 h.
of
(Ni-
195
eropolyanion in the catalyst pores must be occurring. The resulting tungstencontaining species would be expected to be larger than the nickel-containing fragment, and more penetration of the former species would only be expected if the nickel-containing fragment was adsorbed on the pore wall. Separation was even more pronounced on impregnation with (NiMo~W~O~~H~)~- (Fig. 6) with molybdenum migrating into the pellet far faster than tungsten or nickel. In contrast to all other samples, the molybdenum profile was an inverted dish, with the highest concentration being at the centre of the pellet. Tungsten was found to migrate faster than nickel, as would have been expected from the results in Fig. 5. In contrast, separation was not observed during preparations carried out with the corresponding complex containing nickel and molybdenum alone, (NiMo6024H6)4-. The metal ratios for the two elements were identical across entire pellets. Catalyst testing Determination of catalytic activity required amounts of catalysts larger than the 2-g lots prepared in the development work described above. Larger batches (15-50 g) of five types were made using similar procedures. Their profile type and bulk metal analyses are given in Table 3 together with the results of activity testing at 603 and 663 K. Results for the two commercial catalysts, one nickel-tungsten and the other nickel-molybdenum, are included. It can be seen that the five samples prepared by the heteropolyanion method have good hydrodesulphurization (HDS ) and hydrogenation (HY) activity. All surpass that of the commercial nickel-tungsten sample despite a lower content of metals, especially promoter metal. Within the heteropolyanion series, the activity increases with increasing metal content for both cobalt-tungsten (samples 1-3) and nickel-tungsten (samples 4 and 5). However, the increased activity is seemingly not directly proportional to the metal content when non-uniform catalysts are compared (e.g. HY activity for sample 1 versus 3). This probably reflects some mass transfer limitations on the reaction of large molecules and ineffective use of active ingredients at pellet centres as a result. The activity of sample 2 is significantly less than that of sample 3 despite a similar tungsten content and distribution. The difference is attributable to the lower cobalt content of the former (prepared with a lower pH so that some COW,, complexes were present in addition to CoZWll complexes). Compared with the commercial nickel-molybdenum (sample 7)) the best catalysts prepared by the heteropolyanion method (samples 3 and 5) exhibit similar HDS and HY activity but much lower HDN activity. This is probably not due to an intrinsic inferiority of tungsten-based as compared with molybdenum-based catalysts. Tischer et al. [ 231 showed that the two systems have comparable activities for the hydrotreatment of coal liquids when promotion is optimal in both instances. We obtained similar results for the model com-
196
TABLE 3 Hydrotreatment performance of cobalt-tungsten and nickel-tungsten catalysts prepared by the heteropolyanion method in comparison with commercial nickel-tungsten and nickel-molybdenum catalysts Reaction conditions : total pressure, 7 MPa; LHSV, 2.2 g (g-cat.))’ h-l; hydrogen flow-rate, 0.5 1 (STP) min’. Sample No.
1
Metal combination
type
Profile
co-w
Fig. 2a
Metal (%, w/w)* Co(Ni) 0.8
W(Mo) 7.2
2
co-w
Fig. 2d
1.2
16.1
3
co-w
Fig. 2c
1.8
18.2
4
Ni-W
Fig. 5a
(0.6)
9.3
5
Ni-W
Fig. 5c
(0.9)
11.8
6
Ni-W (commercial) Ni-Mo (commercial)
Uniform
4
19
Uniform
3.4
(20)
7
Test temperature (K)
Conversion (%) HDS
HDN
HY
603 633 603 633 603 633 603 633 603 633 603 633 603 633
66 80 51 85 80 94 67 80 86 a7 47 79 88 90
-
12 22 7 31 18 34 8 20 14 20 5 20 16 30
1 1 1 2 2 3 2 5 2 9 13 25
*Analysis by AAS; balance alumina.
pound mixture used here [ 241. The optimal atomic ratio of promoter element to base metal for the best HDN activity is in the range 0.5-l [23,24]. The relatively low HDN activity of the catalysts prepared by the heteropolyanion method can be traced to their low cobalt (nickel)-to-tungsten ratio. This is limited to at most 1:5.5 by the stoichiometry of the starting complexes themselves. In conclusion, this work has shown that heteropolyanions containing promoter and base metals in a single complex provide a convenient method for preparing catalysts with controlled profiles by impregnation of support pellets. Catalysts so prepared show promising hydrotreating activity but the promoterto-base metal ratio is too low for good hydrodenitrogenation activity. Work in progress indicates that their activity for this reaction can be improved by inclusion of additional promoter metal, as one would expect. ACKNOWLEDGEMENTS
The authors acknowledge, with gratitude, financial support from the Australian Research Council and the National Energy Research, Development and Demonstration Program.
197 REFERENCES 1 2 3 4 5 6
8 9 10
11 12 13 14 15 16 17 18 19 20 21 22 23 24
0. Weisser and S. Landa, Sulphide Catalysts and their Properties and Applications, Pergamon Press, London, 1973. P. Grange, Catal. Rev. Sci. Eng., 21 (1980) 135. B.C. Gates, J.R. Katzer and C.A. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, New York, 1979, Ch. V. D.S. Thakur, P. Grange and B. Delmon, J. Catal., 91 (1985) 318. H. Topsoe, NATO AS1 Ser. C, No. 105 (1983) 329. M. Houalla, C.L. Kibby, L. Petrakis and D.M. Hercules, J. Catal., 83 (1983) 50. D.L. Trimm and A. Stanislaus, Appl. Catal., 21 (1986) 215. J.M. Thomas and W.J. Thomas, Introduction to the Principles of Heterogeneous Catalysis, Academic Press, New York, 1967. S. Lee and R. Aris, Catal. Rev. Sci. Eng., 27 (1985) 207. A.M. Maitra, N.W. Cant and D.L. Trimm, Appl. Catal., 27 (1986) 9. M. Pope, Heteropoly and Isopoly Oxometallates, Springer, Berlin, 1983. E. Matijevic, M. Kerker, H. Beyer and F. Theubert, Inorg. Chem., 2 (1963) 581. C. Friedheim and F. Keller, Chem. Ber., 39 (1969) 4301. L.C.W. Baker, B. Loev and T.P. McCutcheon, J. Am. Chem. Sot., 72 (1950) 2374. L.C.W. Baker, V.S. Baker, S.H. Wasfi, G.A. Candela and A.H. Kahn, J. Am. Chem. Sot., 94 (1972) 5499. L.C.W. Baker, V.S. Baker, K. Eriks, M.T. Pope, M. Shibata, D.W. Rollins, J.H. Fang and L.L. Koh, J. Am. Chem. Sot., 88 (1966) 2329. L.C.W. Baker and T.P. McCutcheon, J. Am. Chem. Sot., 78 (1956) 4503. P.B. Weisz, Trans. Faraday Sot., 63 (1967) 1801. P.B. Weisz and J.C. Hicks, Trans. Faraday Sot., 63 (1967) 1808. P.B. Weisz and H. Zollinger, Trans. Faraday Sot., 63 (1967) 1815. G.A. Parks, Chem. Rev., 65 (1979) 177. M.T. Pope and L.C.W. Baker, J. Phys. Chem., 63 (1959) 2083. R.E. Tischer, N.K. Narain, G.J. Steigeland, D.L. Cillo, Am. Chem. Sot. Div. Petr. Chem. Prepr., 30 (1985) 459. M.E. Kural, PhD Thesis, University of New South Wales, 1986.
187 Printed in The Netherlands
Novel Hydrotreating Catalysts Prepared from Heteropolyanion Complexes Impregnated on Alumina A.M. MAITRA and N.W. CANT* School of Chemistry, Macquarie University, NSW2109 (Australia) and D.L. TRIMM School of Chemical Engineering and Industrial Chemistry, University of New South Wales, P. 0. Box 1, Kensington, NS W 2033 (Australia) (Received 27 May 1988, revised manuscript received 20 September 1988)
ABSTRACT The preparation of hydrotreating catalysts by the impregnation of alumina with solutions of heteropolyanions has been studied. These anions have the general structure [H,A&O,]“-, where A may be Co or Ni and B may be MO or W. The anions have advantage over conventional impregnating agents in that both catalytically active metals are in the same complex and should be deposited in the same position in the pellet. In addition, the solubility of the anion is higher than conventional impregnating agents. This allows higher concentrations of catalytic metal (and particularly of tungsten) to be deposited on or in the support. Conditions for the preparation of “shell” or “uniform” impregnated catalysts have been established. Comparisons have been made of the efficiency of such catalysts and of conventional catalysts for the hydrotreating of a model feedstock. Heteropolyanion-based catalysts are very effective for hydrodesulphurization and hydrogenation but their relatively low content of promoter metals restricts hydrodenitrogenation activity.
INTRODUCTION
The use of hydrotreating catalysts based on molybdenum or tungsten salts, supported on alumina and promoted by cobalt or nickel, is well established [l31. The chemical composition and preparation of such catalysts have a significant effect on performance [ 4-61, as does the texture of the solids. Mass transfer in pores, particularly under conditions where foulants can be deposited on the catalyst [ 81, is a major factor in determining catalyst efficiency. Control of catalyst porosity is best achieved by controlling the pore size in the support and impregnating with appropriate metal salts [8]. Conditions may be adjusted to achieve impregnation only on the outside of the pellet (shell catalyst) or to obtain an even distribution throughout the pellet [ 91. However,
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
188
in the latter instance, impregnation times may be long owing to the slow diffusion of large species [lo] and/or the limited solubility of given metals salts. Impregnation by tungsten salts poses particular problems in that highly polymeric species may be produced which lead to pore blockage and slow impregnation [lo]. A further difficulty with impregnation arises in that anionic base elements require acidic conditions to promote adsorption on alumina, while impregnation of cobalt or nickel salts is favoured in basic solution [lo]. As a result, two impregnation operations may be required and, under these conditions, uniformity of deposition is hard to achieve. In the light of these difficulties, attention has now been focused on impregnation of alumina supports using heteropolyanions which contain both base and promoting elements in a single complex [ 111. The general formula of these anions is (H,A,B,O,)+, where A may be Co or Ni and B may be MO or W. Impregnation with such a single salt thus allows introduction of both catalytically active metals at the same time. The preparation of Co-W-, CO-MO-, Ni-W- and Ni-MO-based catalysts from heteropolyanions has been studied. The efficiency of such catalysts for hydrotreating has been compared with that of catalysts prepared by conventional impregnation using a model feedstock. EXPERIMENTAL
A variety of heteropolyanion compounds were prepared by standard literature methods and their formulae checked by atomic absorption analysis against standard solutions of known concentration. With the exception of the cobalttungsten combination, the general formula of the anions was (H6XY6024)Xwith X=Co or Ni and Y=Mo or W. (NH4)4(Ni”MosOadHG), mixed complex and the (NH,), ( NinW60Z4Hs) *5Hz0 ( NH4)4 ( NinMo,W30Z4HB) were each prepared by the method of Matijevic et al. [ 121. The first compound was light blue with a room-temperature solubility limit of about 0.016 A4 (9 g/l MO, pH ~4.9). The tungsten analogue was also light blue with solubility 0.016 M (18 g/l W pH~6.5). The synthesis of ( NH4 j3 ( CO~~~MO~O~~H~) - 7H20 was similar to that described by Friedheim and Keller [ 131. In the final step the salt was recrystallized from water to remove a minor olive-green solid with a cobalt-to-molybdenum ratio of 2:lO. The bluish green CoMo, salt has a saturation limit of ca. 0.055 M (32 g/l MO, pH M4). The corresponding free acid was obtained by passage through a cation-exchange column (Amberlite IR-120, Rohm and Haas) in the H+ form [ 141. Solutions with pH l-l.5 and of concentrations up to 0.055 M were obtained with retention of the cobalt-to-molybdenum ratio of 1:6. Cobalt-tungsten heteropolyanion compounds were also prepared in both ammonium salt and free acid forms, but the metal-to-metal ratios in the struc-
189
tural units differed from each other and from the above. The cobalt-to-tungsten ratio was 2:ll in the ammonium salt (NH4)s(Con2W,1040H2) .13H,O prepared as described by Baker and co-workers [ 15,161. The material prepared was emerald green with solubility 0.021 M (45 g/l W, pH = 6.5) and a cobaltto-tungsten ratio of 1:4.9 by analysis. When passed through Amberlite IR-120 (acid form ) an intense blue solution (pH z 1) with a cobalt-to-tungsten ratio analysed as 1:11.6 was eluted. This corresponds to the conjugate base described by Baker and McCutcheon [ 171 as (Co”W,,O,,)“-. Nearly saturated solutions (0.016 M, 35 g/l) could be used for some time but, over a period of months, small amounts of yellow WO, were deposited. Solutions of the complexes were prepared and used to impregnate alumina cylinders of diameter 1.6 mm and length 3-5 mm (Norton SA6173, surface area 220 m’/g, pore volume 0.55-0.70 cm3/g, mean pore diameter 8 nm). The solids were washed in distilled water (ca. five times the volume of the pellets) and dried in air at room temperature and then in air at 140” C (24 h). The metal loading and the distribution were determined by atomic absorption spectrometry (AAS) (Varian Techtron Model 1100) and by electron microprobe analyses (ARL Model EMX) . The latter involved measurements with a spot size of 5 pm and polished half pellets. Calibration was based on non-porous standards. Metal contents as determined by the microprobe were up to 25% below the results of bulk analyses by AAS and the metal ratios showed similar variations. The difference reflects the porous nature of the support and possibly some non-uniformity in the deposited layers. Scatter was especially pronounced for the molybdenum systems. Before use as catalysts, the solids were calcined in air (465 ’ C for 16 h). The catalysts were presulphided in hydrogen sulphide-hydrogen (1:9, v/v) at 673 K for 1 h. The reactor was cooled to 573 K under an inert atmosphere and the activity of each catalyst was determined at 573,603 and 633 K. The test series included comparative measurements with two commercial catalysts of the same geometric form. One was based on nickel and tungsten (Harshaw 4303) and the other on nickel and molybdenum (Shell S324). The standard operating conditions were pressure 7 MPa, LHSV 2.2 g (g-cat.) -’ h-l and hydrogen flow-rate 500 ml min-l. Liquid product analysis was carried out by off-line gas chromatography on a Gow-Mac chromatograph (2.5-m column packed with OV-101, column temperature programmed from 333 to 523 K at 6 K min-‘, flame ionization detection). The model feed mixture contained 5% (w/w) of thiophene, 20% (w/w) of quinoline, 7% (w/w) of dibenzofuran and 8% (w/w) of phenanthrene in decalin (55%, w/w). 5% (w/w) of pseudo-cumene was added to the mixture as an internal standard.
190 RESULTS AND DISCUSSION
Catalyst preparation Cobalt-tungsten-based system Initial studies were focused on cobalt-tungsten containing heteropolyanions, as they were highly soluble and their colour gave a good visual indication of impregnation. The essential features of the impregnation may be discussed with the aid of the results summarized in Figs. 1 and 2. In all figures, the dominant (base) metal is shown on the left of the diagram and the minor (promoter) metal on the right. This description is adopted for convenience only. In fact the metal distributions represent measurements at identical points for each element taken along the same radius of the 1.6-mm alumina cylinders. The bars represent the range observed for repeated measurements made at short distances on either side of an initial determination. Impregnation with the acid complex was found to be shell progressive (Fig. 1). A nearly uniform distribution was achieved to a depth that varied with time. The measured tungsten-to-cobalt weight ratio in the impregnated band was ca. 30, reflecting the 12:l molar ratio of these elements in the starting (CoW,,O,,)“ion. Under the conditions pertinent to Fig. 1, about 16 h were needed for complete impregnation, when about 20% (w/w) tungsten was deposited. This corresponds to about one monolayer of ion on the alumina, a value similar to that observed by impregnating with polytungstates [lo]. The time required for impregnation was much longer than that predicted on the Tungsten;Cobalt
Tungsten
2
-2
E .o, 1
-1
i Cobalt
z
s -I
edge
centre
edge
Fig. 1. Profiles for impregnation using0.015 MH,(CO”W~~O,,,) 4 h; (c) 7 h; (d) 24 h.
edge
(35 g/l W) at pH 2: (a) 2 h; (b)
Fig. 2. Profiles for impregnation using 0.021 M (NH,),(Co”,W,,O,,H,) (45 g/l W) at pH 5.5-6: (a) 10 h; (b) 22.5 h; (c) 7 days; (d) 0.021 M (46 g/l) at pH 3.5 for 10 h.
191
basis of the simple model proposed by Weisz and co-workers [ 18-201, indicating that mass transfer limitations in the pore are important. The sharp impregnation front is not unexpected, in that the negatively charged anion should be strongly bound to positively charged alumina below the isoelectric point at pH 6 [21]. Weaker adsorption would be expected for impregnation by nearly neutral solutions, and this is reflected in the results summarised in Fig. 2. Adsorption of the ammonium salt (NH,),( Co2W11040H2) is seen to give a dish-shaped profile through the pellet, with gradual filling of the “dish” with time. The concentration of the impregnant slowly builds up until tungsten-to-cobalt weight ratio of ca. 12 (reflecting the molar ratio of 5.5 in the salt) is achieved throughout the pellet. At intermediate pH, where both species are expected to be present, impregnation profiles reflect both complexes and an average tungsten-to-cobalt weight. ratio of ca. 9.0 was achieved in the final pellet (curve d in Fig. 2). Comparison of Figs. 1 and 2 shows impregnation to be slower with the neutral salt, mainly reflecting the larger size of the (NH,‘), (CO~W~~O~~H~)*species. However, the concentration of the impregnating solution also affects the rate of impregnation, as seen in Fig. 3. With the acid salt (curves b and c) similar impregnation profiles to that found after 7 h with 0.015 M solution (Fig. lc) required approximately 50 h with 0.0025 M solution (Fig. 3b) and 120 h with 0.0008 M solution (Fig. 3~). With the neutral salt, the dish developed with 0.0011 M solution after 120 h (curve a) is much below that developed with 0.021 M solution after 10 h (curve d) . The amounts of cobalt and tungsten taken up at the point of uniform impregnation are presented in Table 1. The amount of complex used initially was
edge
centre
edge
Fig.3. Effect of concentration and pH on profiles when impregnating with cobalt-tungsten heteropolyanions: (a) 0.0011 A4 (2.25 g/l W) at pH 6.5 for 120 h; (b) 0.0008 M (1.75 g/l W) at pH 3 for 120 h; (c) 0.0025 M (5.83 g/l W) at pH 3 for 49 h; (d) 0.021 M (45 g/l W) at pH 6.5 for 10 h.
192
TABLE I Analytical results and impregnation times for visually uniform cobalt-tungsten Tungsten concentration (g/l)
catalysts
Impregnating species (CO”W,,O,,)~Time (h)
45 (335jj*
-20
17 10 5.5 1.75
(-10) -30 -40 -80 - 200
at pH 2
(COI~~W~~O~~H~)~at pH 6
W (%, w/w)
CO (%, w/w)
20.0 (23.1) 20.2 19.9 19.7 20.0
0.62 (0.65) 0.62 0.56 0.53 0.54
Time (h) -32 -10 (-24) -200 > 200 > 200 -
W (%, w/w)
co (70, w/w)
19.8 18.8 (21.5) 17.8 16.0 14.5 -
1.42 1.21 (1.46) 1.18 1.10 1.00 -
*200% excess solution above uptake.
about 30-50% in excess of that taken up, except for one preparation, when a 200% excess was used. Impregnation under acidic conditions is seen to give final uptakes that are independent of the starting concentration, although a small excess was observed for the more concentrated impregnating solution owing to deposition from residual solution in the pores during drying. Impregnation under nearly neutral conditions gave uptakes that varied significantly with the concentration of the starting solution, reflecting variable adsorption by the more weakly bound species. Tungsten-to-cobalt atomic ratios were 11.2 ? 0.8 for acidic conditions and 4.7 + 0.5 for neutral conditions, in agreement with those in the starting materials. Cobalt-molybdenum-based system Impregnation by the cobalt-molybdenum heteropolyanion showed many common features with the cobalt-tungsten system but also some significant differences. Impregnation by the acid complex was shell progressive with development of a flat profile corresponding to monolayer coverage in 2 h (Fig. 4). Neutral conditions gave a dished profile with lower concentration near the pellet rim. This presumably reflects removal of the weakly adsorbed complex by the minimal washing involved in the preparation. The metal contents of a visually uniform catalyst prepared from solution containing a 200% excess of material taken up are summarized in Table 2. The results are analogous to those for cobalt-tungsten catalysts with the lower uptake of molybdenum reflecting the relative atomic weights of molybdenum and tungsten. Impregnation is seen to be much faster with cobalt-molybdenum ions than with cobalt-tungsten ions. The cobalt-tungsten anions are known to be roughly
193
edge
centre
edge
Fig. 4. Effect of pH on profile development when impregnating with ( CO”‘MO,O,,H,)~-. M (15.8 g/l MO) at pH for 2 h; (b) 0.028 M (16.2 g/l MO) at pH 2 for 2 h.
(a) 0.027
TABLE 2 Analytical results and impregnation times for visually uniform catalysts with (Co”‘Mo,O,,H, as impregnating ion Molybdenum concentration (g/l) 31.5 15.8 9.0 5.3
Acid form at pH 2-3
j3-
Ammonium salt at pH 5-6
Time (h)
MO (%, w/w)
co (%, w/w)
Time (h)
MO (%, w/w)
co (70, w/w)
t2 -2 -4 -7
10.2 9.9 9.6 9.5
0.78 0.71 0.70 0.68
-2 -8 -16 -50
10.3 9.4 9.1 9.2
0.75 0.68 0.68 -
spherical with a diameter of ca, 1.2 nm [ 111, whereas the cobalt-molybdenum ions (and nickel-molybdenum ions) are thought to be disc-shaped with a diameter of ca. 1.2 nm but a thickness of ca. 0.57 nm [22]. Presumably the latter ions are adsorbed flat on the pore wall, thus producing less pore blocking and faster diffusion of the impregnating species. The occurrence of restricted diffusion of ions of 1.2-nm diameter in an alumina with a mean pore diameter of 8 nm (as used here) presumably reflects the presence of constrictions in the pore system. The effect should be less in a support with more uniform pores and/or pores of larger mean diameter. It
would be of interest to test this prediction experimentally. Nickel-tungsten-and nickel-molybdenum-based systems Although similarities existed between these systems and their cobalt-containing analogues, some interesting differences were noted. Impregnation with
194
( NiW,0,,H,)4produced shell progressive impregnation at low pH and dishshaped profiles under nearly neutral conditions (Fig. 5 ) . However, the ratio between the metals did not equal the ratio in the original complex at all points. For example, the tungsten-to-nickel ratio observed for catalyst, 5a was closer to 9 than the expected value of 6. Again, in Fig. 5b, tungsten is seen to penetrate further into the pellet than nickel producing a tungsten-to-nickel ratio that varies across the diameter. The absence of salts containing tungsten alone in the starting material was confirmed, and these results indicate that some disproportionation of the het-
1 15-
Tungsten
I
Nickel
- 1.5
centre
cage
edge
Fig. 5. Profile development when impregnating with (Ni”W,0,,H,)4-: (a) 0.01 M (4 g/l W) at pH 3.5 for 29 h; (b) 0.011 M (3.9 g/l W) atpH 6 for 96 h; (c) 0.032 M (12 g/l W) at pH 6 for 96 h. Tungsten I
edge
Fig. 6. Profile “Mo,W,O,,H,)~-
Molybdenum Nickel
centre
-
---
I
edge
development during impregnation with a saturated solution at pH 5.5: (a) 9 h; (b) 25 h; (c) 147 h.
of
(Ni-
195
eropolyanion in the catalyst pores must be occurring. The resulting tungstencontaining species would be expected to be larger than the nickel-containing fragment, and more penetration of the former species would only be expected if the nickel-containing fragment was adsorbed on the pore wall. Separation was even more pronounced on impregnation with (NiMo~W~O~~H~)~- (Fig. 6) with molybdenum migrating into the pellet far faster than tungsten or nickel. In contrast to all other samples, the molybdenum profile was an inverted dish, with the highest concentration being at the centre of the pellet. Tungsten was found to migrate faster than nickel, as would have been expected from the results in Fig. 5. In contrast, separation was not observed during preparations carried out with the corresponding complex containing nickel and molybdenum alone, (NiMo6024H6)4-. The metal ratios for the two elements were identical across entire pellets. Catalyst testing Determination of catalytic activity required amounts of catalysts larger than the 2-g lots prepared in the development work described above. Larger batches (15-50 g) of five types were made using similar procedures. Their profile type and bulk metal analyses are given in Table 3 together with the results of activity testing at 603 and 663 K. Results for the two commercial catalysts, one nickel-tungsten and the other nickel-molybdenum, are included. It can be seen that the five samples prepared by the heteropolyanion method have good hydrodesulphurization (HDS ) and hydrogenation (HY) activity. All surpass that of the commercial nickel-tungsten sample despite a lower content of metals, especially promoter metal. Within the heteropolyanion series, the activity increases with increasing metal content for both cobalt-tungsten (samples 1-3) and nickel-tungsten (samples 4 and 5). However, the increased activity is seemingly not directly proportional to the metal content when non-uniform catalysts are compared (e.g. HY activity for sample 1 versus 3). This probably reflects some mass transfer limitations on the reaction of large molecules and ineffective use of active ingredients at pellet centres as a result. The activity of sample 2 is significantly less than that of sample 3 despite a similar tungsten content and distribution. The difference is attributable to the lower cobalt content of the former (prepared with a lower pH so that some COW,, complexes were present in addition to CoZWll complexes). Compared with the commercial nickel-molybdenum (sample 7)) the best catalysts prepared by the heteropolyanion method (samples 3 and 5) exhibit similar HDS and HY activity but much lower HDN activity. This is probably not due to an intrinsic inferiority of tungsten-based as compared with molybdenum-based catalysts. Tischer et al. [ 231 showed that the two systems have comparable activities for the hydrotreatment of coal liquids when promotion is optimal in both instances. We obtained similar results for the model com-
196
TABLE 3 Hydrotreatment performance of cobalt-tungsten and nickel-tungsten catalysts prepared by the heteropolyanion method in comparison with commercial nickel-tungsten and nickel-molybdenum catalysts Reaction conditions : total pressure, 7 MPa; LHSV, 2.2 g (g-cat.))’ h-l; hydrogen flow-rate, 0.5 1 (STP) min’. Sample No.
1
Metal combination
type
Profile
co-w
Fig. 2a
Metal (%, w/w)* Co(Ni) 0.8
W(Mo) 7.2
2
co-w
Fig. 2d
1.2
16.1
3
co-w
Fig. 2c
1.8
18.2
4
Ni-W
Fig. 5a
(0.6)
9.3
5
Ni-W
Fig. 5c
(0.9)
11.8
6
Ni-W (commercial) Ni-Mo (commercial)
Uniform
4
19
Uniform
3.4
(20)
7
Test temperature (K)
Conversion (%) HDS
HDN
HY
603 633 603 633 603 633 603 633 603 633 603 633 603 633
66 80 51 85 80 94 67 80 86 a7 47 79 88 90
-
12 22 7 31 18 34 8 20 14 20 5 20 16 30
1 1 1 2 2 3 2 5 2 9 13 25
*Analysis by AAS; balance alumina.
pound mixture used here [ 241. The optimal atomic ratio of promoter element to base metal for the best HDN activity is in the range 0.5-l [23,24]. The relatively low HDN activity of the catalysts prepared by the heteropolyanion method can be traced to their low cobalt (nickel)-to-tungsten ratio. This is limited to at most 1:5.5 by the stoichiometry of the starting complexes themselves. In conclusion, this work has shown that heteropolyanions containing promoter and base metals in a single complex provide a convenient method for preparing catalysts with controlled profiles by impregnation of support pellets. Catalysts so prepared show promising hydrotreating activity but the promoterto-base metal ratio is too low for good hydrodenitrogenation activity. Work in progress indicates that their activity for this reaction can be improved by inclusion of additional promoter metal, as one would expect. ACKNOWLEDGEMENTS
The authors acknowledge, with gratitude, financial support from the Australian Research Council and the National Energy Research, Development and Demonstration Program.
197 REFERENCES 1 2 3 4 5 6
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11 12 13 14 15 16 17 18 19 20 21 22 23 24
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