Indian Journal of Chemical Technology Vol. 11, May 2004, pp 326-330

Preparation and characterization of molybdenum hydrotreating catalyst supported on MgO/Al2O3 mixed oxide

Folorunsho Aberuagbaa*, Manoj Kumarb, Gudimella Muralidharb & Lakshmi Datt Sharmab aChemical Engineering Department, Federal University of Technology, PMB 65, Minna, Nigeria

bCatalyst Laboratory, Indian Institute of Petroleum, Dehradun 248005, India

Received 1 May 2003; revised received 3 December 2003; accepted 4 March 2004

A series of molybdenum catalysts supported on MgO/Al2O3 (1:1) mixed oxide were prepared and characterized by BET surface area, X-ray diffraction, temperature programmed reduction and oxygen chemisorption. The catalytic activities for hydrodesulpurization(HDS), hydrogenation(HYD) and hydrocracking(HYC) were determined using thiophene, cyclohexene and cumene as model compounds respectively. Results indicate that at 2- 8% Mo loading, the catalyst is more dispersed and crystallite growth occurred beyond this range. The optimum metal loading for maximum oxygen uptake and catalytic activities for HDS, HYD and HYC was concluded to be 8%. The order of catalytic activity for HDS, HYD and HYC was: HYD>HYC>HDS. However, the Co promoted catalyst showed remarkable increase in both the HDS and HYD activity with no significant change in HYC activity, with the following order: HDS>HYD>HYC, the optimun Co loading being 3%.

IPC Code: B01J 23/28

Keywords: Hydrotreating, Mo catalyst, hydrosulpurization, hydrogenation, hydrocracking, Co catalyst

Hydrotreating is undertaken through a catalytic hydrogenation process, which saturates unsaturated hydrocarbons and removes S, N, O and other metals from different petroleum streams in a refinery. These processes represent some of the most important catalytic processes and the annual sales of hydrotreating catalysts represent close to 10% of the total world market of catalyst1. Hydrotreating catalysts are used extensively both for the conversion of heavy feedstock and for improving the quality of final products. It also plays an essential role in pretreating streams for other refinery processes such as catalytic reforming, fluid catalytic cracking (FCC) and hydrocracking reactions.

Sulphided Co and Ni promoted Mo on gamma-alumina catalysts are active for hydrodesulphuri-zation1,2. However the ever-changing environmental regulations on the quality of petroleum products has made the search for a more active and selective hydrodesulpurization catalyst a continuous process.

In recent years, promising results have been obtained with Ni or Co promoted Mo on mixed oxide

support for hydrotreating compared to alumina supported catalyst. But of all the mixed oxides supported catalysts, not much has been studied on catalyst supported on MgO/Al2O3 mixed oxides3-5. In a recent work on the preparation and characterization of MgO/Al2O3 mixed oxides support for hydrotreating catalysts6, it was found that the mixed oxide containing 50% magnesium and 50% alumina had the highest surface area and had bimodal pores. In this communication the results of the characterization of the Mo loaded MgO/Al2O3 (1:1) catalyst are presented.

Experimental Procedure Materials

The materials used are magnesium nitrate [Mg(NO3)2.6H2O] of analar grade, (S.D. Fine Chemicals Ltd., India), extra pure aluminum nitrate [Al(NO3)3.9H2O] (S.D. Fine Chemicals Ltd., India), extra pure urea [(NH2)2CO] (E. Merck ( India) Ltd) and analar grade ammonium heptahydrate [(NH4)6Mo7O24.4H2O] (E. Merck (India) Ltd). Support and catalyst preparation

A MgO/Al2O3 (1:1) mixed oxide was prepared by mixing equal amounts of 1M solution of aluminium

__________ * For corresspondence

ABERUAGBA et al.: PREPARATION OF MOLYBDENUM HYDROTREATING CATALYST

327

nitrate [Al(NO3)3.9H2O] and 1M solution of magnesium nitrate [Mg(NO3)2.6H2O] and solution of 60 wt% urea [(NH2)2CO] as the hydrolyzing agent in a 20 litre flask. The mixtures were heated to 90oC under total reflux for 4 h. During heating the pH of the mixture was maintained at 10.5. The precipitate formed was washed repeatedly with distilled water. After washing, the precipitate was oven dried at 110oC for 48 h followed by calcination at 550oC for 6 h. Water equivalent to the pore volume of the prepared support was used to dissolve the quantity of ammonium heptamolybdate [(NH4)6Mo7O24.4H2O] required to dope the support with molybdenum in percentages ranging from zero to 14%. The solution of appropriate concentration was then used to wet the support by incipient wetness. The catalyst prepared was first dried at room temperature for 24 h followed by drying at 110oC for 24 h and finally calcined at 550oC for 4 h in a furnace. Incipient wetness method was similarly used to prepare the Co promoted catalyst using appropriate solution of cobalt nitrate [Co(NO3).6H2O] to dope the catalyst with Co in the range of 1-5%.

Catalyst characterization

The five point BET surface area for each sample was determined by nitrogen (99.99% pure) physorption at 77K using micromeritics (USA) ASAP 2010 unit. The catalyst was dried under vacuum at 523 K in situ prior to analysis for surface area.

The XRD patterns for powder catalyst samples were obtained using a Rigaku model D-max 111 TB diffractometer. Temperature programmed reduction (TPR) was carried out using a TPR/TPD 2900 equipment produced by micromeritics, USA at a temperature ranging from ambient to 1000 °C.

Oxygen chemisorption was measured at –77oC in a high vacuum glass unit on catalyst sulphided at 400oC for 2 h using CS2/H2 mixture. Ultra pure oxygen from a storage bulb, connected to the high vacuum manifold was allowed to enter the catalyst chamber with a known dead space. Leveling off within 10 min followed an initial quick fall in pressure and the equilibrium pressure was noted. This process was repeated at different equilibrium pressures and the first isotherm, representing both the chemisorbed and physisorbed oxygen was generated. After the catalyst was evacuated at –77oC for 1 h at 10-6 torr to remove the physisorbed oxygen a second isotherm representing only the physisorbed oxygen

was generated in an identical manner. From these two linear and parallel isotherms the amount of chemisorbed oxygen was determined.

The catalytic activities were determined from an all glass differential flow microreactor operating under normal atmospheric pressure interfaced to a gas chromatograph by a six way sampling valve for product analysis. In a typical experimental run, 0.2g of a catalyst sample of particle size 18-40 mesh mixed with glass beads of the same size range in the ratio 1:1 to ensure uniform heating were secured between two plugs of quartz wool inside the glass reactor (pyrex glass tube, 0.8 cm, i.d.) and sulphided at 400oC for 2 h. Sulphiding was carried out using a mixture of CS2 and H2 at a flow of 40 mL/min. After sulphidation, the catalyst was maintained at a temperature of 400oC and flushed in H2 flow until no CS2 could be detected in the effluent gas. Model reactant viz. thiophene for hydrodesulpurization (HDS) or cyclohexene for hydrogenation (HYD) or cumene for hydrocracking (HYC) was then introduced. The reaction rates were calculated directly from the equation

r = X/(W/F)

assuming differential reactor behaviour at pseudosteady state, where r is the rate in mol/h.g.catal., X is the fractional conversion, W is the weight of the catalyst in grams and F is the total flow rate of gas in mol/h.

Results and Discussion BET surface areas

The BET surface areas (BETSA) per gram of the sulphided catalyst and per gram of the support is shown in Table 1.

The results show that the BET surface area per gram of the catalyst reduces with Mo loading probably due to blockage of pore mouth with the addition of molybdenum oxide during preparation and because of decreasing amount of support. However the BET surface areas per gram of the support did not vary much between 2-10%, indicating that Mo is well dispersed in that range. The use of surface area per gram of the support as a means of obtaining an insight into metal dispersion had been previously reported7. X-Ray diffraction

X-ray diffractograms of the support and the catalyst show that the incorporation of molybdenum

INDIAN J CHEM TECHNOL, VOL 11, MAY 2004

328

to the support induces a loss in the crystallinity of the magnesia/alumina peaks which appear less intense as the Mo loading is increased from 2-14%. This indicates a form of interaction between Mo and the support. A similar loss in crystallinity of magnesia when incorporated with Mo had been reported by Klimova et al.5 and they attributed the loss to an interaction between them. Temperature programmed reduction

The temperature programmed reduction (TPR) profiles of the catalyst show two characteristic peaks: the low temperature peak around 559-614oC due to the reduction of Mo6+ Mo4+ and a high temperature peak around 927-975oC due to formation of Mo metal. The total H2 consumption increases with increasing Mo content and the reduction of MoO3 MoO2 is facilitated with increasing Mo content in the catalyst. It is well known that at low Mo loading molybdenum oxide is present mainly as tetrahedral species, which is difficult to reduce due to strong interaction with the support8. At moderate loading both tetrahedral and octahedral species are present, and compared to tetrahedral species, the octahedral species are easily reducible8. The temperature at which the characteristic peaks occur as well as the total H2 consumed in arbitrary units is shown in Table 2. Oxygen chemisorption

The oxygen uptake values as a function %Mo loading is shown in Fig. 1. The figure shows that oxygen uptake increases linearly with %Mo loading up to 8% Mo, after which, there was a slight decline

with a further increase. The 8% Mo loading at which maximum oxygen is chemisorbed is an evidence of the loading at which maximum dispersion occurred. A similar loading value had been reported9 for MgO.

The equivalent molybdenum surface area (EMSA) calculated from 0.566616*LTOC (i.e., low temperature oxygen chemisorbed) as reported by Fierro et al.16 and the dispersion (O/Mo) obtained as the ratio of atoms of oxygen uptake to atoms of Mo, the surface coverage calculated by dividing EMSA by the BET surface area (BETSA) and the crystallite size (calculated using EMSA values) are tabulated in Table 3.

The results in Table 3 show that the equivalent molybdenum surface area (EMSA), and the surface coverage, rose with Mo up to 8%, after which,no significant changes were observed. In general, low surface coverage was obtained, suggesting that Mo occupies selective sites on the surface. The dispersion

Table 1 ⎯ BET surface area (BETSA)

% Mo BETSA m2/gcat BETSA m2/g-support

0

2

4

6

8

10

12

14

200

167

161

156

155

144

119

119

200

170

168

168

168

169

135

138

Table 2 ⎯ Temperature of characteristic TPR peaks

%Mo Low temperature peak (oC)

High temperature peak (oC)

Total H2 consumed

(arbitrary units)

2

4

6

8

10

12

14

614

572

559

559

571

565

577

955

927

927

948

958

958

975

2.14 × 108

3.7 × 108

4.16 × 108

7.43 × 108

8.06 × 108

9.13 × 108

11.2 × 108

Fig. 1 ⎯ Oxygen uptake versus % Mo loading

ABERUAGBA et al.: PREPARATION OF MOLYBDENUM HYDROTREATING CATALYST

329

was essentially constant between 2-8%, after which a decline was observed. The crystallite sizes did not change appreciably with 2-8% Mo loading, however a further increase in Mo loading resulted in crystallite growth.

The foregoing results show that at up to 8% wt MO loading, molybdenum oxide on the mixed oxide support is more dispersed and beyond this point molybdenum crystallites are formed. Catalytic activities

All the experiments were conducted in the absence of interpellet and intrapellet diffusional resistances. The catalytic activities for hydro-desulpurization (HDS), hydrogenation (HYD) and hydrocracking (HYC) as a function of molybdenum loading is illustrated in Fig. 2. From the figure it can be seen that HDS, HYD and HYC rose with molybdenum loading up to 8% Mo, after which a further increase resulted in a decline. This result also demonstrates that 8% Mo is the optimum metal loading for maximum catalytic activities for HDS, HYD and HYC. The figure further reveals that the order of catalytic activity for HDS, HYD and HYC on the metal surface is: HYD>HYC>HDS. This suggests that this catalyst will be most suitable for hydro-treating reactions requiring strong hydrogenation activity. However, it will be seen latter that, the cobalt promoted catalyst showed remarkable increase in both HDS and HYD activities without any significant increase in HYC activity with the order, HDS > HYD > HYC.

The quasi turnover frequencies (QTOF) were calculated as rate/Mo loading for all the three reactions and were plotted against Mo loading in Fig. 3. The QTOFs are virtually constant with Mo loading up to 8% wt and then start to decrease at higher loading. The QTOF is a measure of the ratio of moles of reactant consumed per hour per gram molybdenum. QTOF is proportional to the dispersion because the latter is a measure of the ratio of surface molybdenum atoms to the total molybdenum atoms.

The correlation of oxygen chemisorbed to catalytic activities for HDS, HYD, and HYC is illustrated in Fig. 4. The straight lines obtained through the origin for each of the catalytic functionalities show that there is a good correlation between the oxygen chemisorbed and each of the catalytic functionalities. This suggests that there is proportionality between oxygen chemisorption and all the three functionalities. Similar correlation have also been observed by other workers 8,11-13. Since there is a strong evidence for different sites for HDS and

Table 3 ⎯ EMSA, dispersion, surface coverage and crystalline size values

%Mo EMSA m2/g

%O/Mo (dispersion)

Surface coverage

(%)

Crystalline size (oA)

2

4

6

8

10

12

14

7.93

15.31

23.48

30.93

24.58

20.89

20.87

13.4

13.0

13.3

13.1

8.3

5.9

5.0

4.3

9.49

15.04

20.0

17.0

17.6

17.5

26.27

27.22

26.62

26.94

42.38

59.84

69.88

Fig. 2 ⎯ Activity versus % Mo loading

Fig. 3 ⎯ Variation of QTOF with % Mo loading

INDIAN J CHEM TECHNOL, VOL 11, MAY 2004

330

HYD14, oxygen chemisorption cannot distinguish them and, therefore, it is not specific to any one of these functionalities and it may be measuring some parameters related to dispersion. The promotional effect of Co on the catalyst impregnated with 8% Mo for HDS, HYD and HYC functionalities was also investigated and the results obtained are tabulated in Table 4.

The results clearly show that the incorporation of 3% Co on the catalyst enhanced the catalytic activities more than both the 1 and 5% Co incorporation. Furthermore, the enhancement in catalytic activities for HDS and HYC is quite remarkable whereas the effect on HYC activity was insignificant. From Massbauer emission spectroscopy (MES) Co in sulphided Mo/Al2O3 catalyst has been reported to be present in several forms15, the ones easily identifiable being Co-Mo-S, Co9S8 and Co-Al2O3. These various forms however, depend on catalyst preparation method and sulphidation. Of all the forms of Co structure, Co-Mo-S structure was reported to be responsible for the promotional effect on catalyst activities for HDS, and the activity was reported to be proportional to the amount of Co present as Co-Mo-S. Co-Mo-S structures are not restricted to CoMo-Al2O3 catalysts alone. Such structures have been observed in

carbon supported Co.Mo catalyst16. A similar structure could be attributed to the promotional effect observed here.

Conclusion The results of this studies show that at 8%

loading, the Mo is more dispersed on the catalyst surface and is the optimum metal loading for maximum activities for HDS, HYD, and HYC. Futhermore, the Co promoted catalyst showed a remarkable increase in both HDS and HYD activity without any significant effect on HYC activity with the following order of catalytic functionalities HDS > HYD > HYC.

References 1 Henric Topsoe, Bjerne S Clausen & Franklin E Masoth,

Hydrotreating Catalysis in Catalysis (Science and Technology) edited by Anderson J R & Boudart Michel (Springer-Verlag, New York), 1996.

2 Gates B C, Katzer J R & Schuit G C A, Chemistry of Catalytic Processes (McGraw-Hill Book Company, New York), 1979.

3 Hillerova H, Vit Z & Zdrazil M, App Catal, 118 (1994) 111. 4 Shimada H, Sato T, Yoshimura Y, Hirasishi J & Nishijima

A, J Catal, 110 (1988) 275. 5 Klimova T, Casados D S & Ranirez J, Catal Today, 43

(1998) 135. 6 Aberuagba F, Kumar M, Gupta J K, Muralidhar G & Sharma

L A, React Kinet Catal Lett, 75 (2002) 245. 7 Segawa K, Soeya T, Kim D S, Sekiyu Gakkaishi, 33 (1990)

347. 8 Maity S K, Rana M S, Srinivas B N, Beji S K, Muralidhar G

& Prasada Rao T S R, J Mol Catal: A Chemical, 153 (2000) 121.

9 Ramakrishna H, Chary K V R, Rama Rao K S & Muralidhar G, Recent Developments in Catalysis edited by Viswanathan B & Pillai C N (Norasa Publishing House, New Delhi), 1990.

10 Fierro J L G, Gonzalez T L, Lopez A L & Weller S W. J Catal, 89 (1984) 111.

11 Srinivas B N, Maity S K, Prasad V V D N, Rana M S, Kumar M, Muralidhar G & Prasada Rao T S R, Stud Surf Sci Catal, 113 (1998) 147.

12 Maity S K, Srinivas B N, Prasad V V D N, Singh A, Muralidhar G & Prasada Rao T S R, Stud Surf Sci Catal, 113 (1998) 579.

13 Bacheller J, Dudhet J C & Cornet D, J Phy Chem, 84 (1980) 1925.

14 Muralidhar G, Ramakrishna H & Prasada Rao T S R, Catal Lett, 22 (1993) 351.

15 Henrick Topsoe & Bierne S Clausen, Catal Rev Sci Eng, 26(3&4) (1984) 395.

16 Brysse M, Bennett B A, Chadwick D & Vrinat M, Bull Sci Chiro Bel, 90 (1982) 1271.

Fig. 4 ⎯ Correlation between activity and oxygen uptake

Table 4 ⎯ Promotional effect of Co on catalytic functionalities

%Co HDS mol/h.g.cat

HYD mol/h.gcat

HYC mol/h.gcat

0 11.78 × 10-3 30.5 × 10-3 36.25 × 10-3

1 42.3 × 10-3 35.9 × 10-3 36.35 × 10-3

3 58.5 × 10-3 47.3 × 10-3 37.6 × 10-3

5 48.8 × 10-3 37.5 × 10-3 32.87 × 10-3