Science: Chemistry:

Morphology Controlled MoO3/SiO2 Catalyst for Hydrodesulfurization

Seung Hyun Cho and Dr. Jin-Wook Ha+
Department of Chemical Engineering, Soonchunhyang University

ABSTRACT

Several series of morphologically controlled MoO3/SiO2 catalysts were prepared, characterized, and tested for hydrodesulfurization (HDS) of dibenzothiophene (DBT) activity. Molybdenum surface loaded with 4.0 atoms Mo/nm2 was prepared as sintered hexagonal and sintered orthorhombic, as well as a novel "well dispersed hexagonal" phase. Characterization by XRD, Raman, and O2 chemisorption results reveals that the dispersion of MoO3 over silica depends on the final MoO3 phase in the order of: sintered hexagonal < sintered orthorhombic < dispersed hexagonal phase. Temperature programmed reduction (TPR) results show that both bulk and dispersed microcrystalline of MoO3 reduce to MoO2 at 650C and to Mo metal at 1000C. HDS of DBT was performed in a differential reactor at 30 atm over the temperature range 350-500C. Activity of MoO3/SiO2 toward HDS of DBT is proportional to dispersion.

 

Key Words: Morphology of MoO3/SiO2, Hydrodesulfurization of Dibenzothiophene

 

1.      INTRODUCTION

 

Continual effort is put into characterization of the types of Mo oxide species which exist over various supports, their stability, and the optimal preparation techniques to obtain them. A recent series of comprehensive works performed with Raman and UV over a number of supports and supported oxides and with a wide array of preparation procedures have synthesized many of these concepts into a general theory [1-7]. First, it was postulated that the surface species of molybdenum present in the air exposed, hydrated catalyst is only a function of pH in the hydrated layer. Second, the amount of material deposited in well dispersed form depends on the number of reactive hydroxyl groups present. In all of the referenced work, MoO3 produced by a calcination at temperatures in excess of 450C, which produces the well known and characterized orthorhombic MoO3 phase. The present work with silica supported MoO3 has centered on investigating, and exploiting the behavior of the little studied hexagonal phase of MoO3. The morphology of silica supported catalysts was fully characterized by using XRD, Raman, BET, and TPR. Activity was tested by hydrodesulfurization (HDS) of dibenzothiophene (DBT) at 30 atm over the temperature range from 350 to 500C.

 

The current study seeks to further characterize the highly dispersed hexagonal phase and then to establish structure-function relationships for all types of highly loaded catalysts. It is desired to develop a comprehensive understanding of the highly loaded catalysts which will complement the current high degree of understanding of low loading catalyst.

 

2. EXPERIMENTAL

 

2.1 Catalyst Preparation

 

The support material used for the preparation of this study was Aerosil 380 (surface area is 380m2/gm), manufactured by Degussa. To densify the silica, it was wetted with deionized water, dried overnight at room temperature (24C), and then dried at 110C for 12h in a muffle furnace. Ammonium hepta-molybdate (IV) tetrahydrate (AHM), (NH4)6Mo7O24.4H2O, supplied by Aldrich Chemical Company Inc. was used as a precursor. Surface loading of MoO3 over silica was 4 atoms Mo/nm2. Loadings were prepared by physically mixing the desired amount of dry support and precursor, and then adding deionized water to reach incipient wetness. Acid or base impregnations were conducted with concentrated nitric acid (2.5N) or ammonium hydroxide (5N) in place of deionized water. After impregnation the samples were thoroughly stirred and dried in air overnight.

 

Table 1. Supported precursor and MoO3 patterns resulting from various preparation conditions for supported MoO3/SiO2

Precursor

 

Impregnation

(Dry T)

Phase of MoO3

formed

Calcination

Temp.(time)

Phase of MoO3

formed

AHM/SiO2

Acida

(25C)

Hexagonal

(NH3)0.15.MoO3.0.5

H2O/SiO2

300C (2h)

in air

Hexagonal

(sintered wrt precursor)

AHM/SiO2

Waterb

(25C)

AHM

(NH4)6Mo7O24.4H2O/

SiO2

500C (2h)

in air

Orthorhombic

(sintered wrt precursor)

AHM/SiO2

Basec

(25C)

Triclinic

(NH4)2Mo2O7/SiO2

300C (2h)

in air

Dispersed Hexagonal

a Nitric acid,HNO3(2.5N),b Deionized water,H2O(pH=5),c Ammonium hydroxide,NH4OH(5N).

 

The final distribution of material appeared homogeneous with the use of watchglasses. Calcinations were performed in air in a standard muffle furnace. A summary of sample treatments and final phases of supported MoO3 is given in Table 1. Three different MoO3 phase over silica will be referred as "SO" for sintered orthorhombic, "SH" for sintered hexagonal and "DH" for dispersed hexagonal phase of MoO3/SiO2.

 

2.2 Catalyst Characterization

 

A Siemens D 5000 X-ray powder diffractometer was used to identify the various crystalline phases. Measurements were done at 50 kV, 30 mA, and suitable two theta range (8-33), theta step (0.05) and count time (3 sec). Raman spectroscopy of catalysts (wet and calcined samples) under ambient conditions were obtained with an Ar+ laser (Spectra Physics Model 171) by utilizing about 10-40 mW of the 514.5 nm line for excitation. Temperature programmed reduction (TPR) was carried at the temperature range between 400 and 1000C with 5C increment. Reducing gas was 4 vol % of H2 in N2, flowing at 16.8 sccm. Dispersion of MoO3 over SiO2 was estimated by low temperature (25C) O2 chemisorption. Activity for hydrodesulfurization of dibenzothiophene (DBT) was performed in a differential reactor at 30 atm over the temperature range between 350 and 500C.

 

3. RESULTS AND DISCUSSION

 

The effect of the precursor pH (acidic, neutral and basic precursor) and calcination temperature was studied by XRD and results shown in Figure 1. Acidic precursor and 300C calcinations, sintered hexagonal phase of MoO3 was observed. Sintered Orthorhombic MoO3 was observed with neutral precursor and 500C calcinations. And for basic precursor and 300C calcination samples, both hexagonal and orthorhombic phase of MoO3 was observed.

 

The crystallite size of the fraction of crystallites observable by XRD was estimated from the Scherrer equation to be; 137 (SH), 88 (SO), and 59 (DH), using the full width at half maximum (FWHM) of the unsupported H(210) (hexagonal phase of MoO3 crystal) and R(110) (orthorhombic phase of MoO3 crystal) phases as standards for instrumental broadening. However, since the integrated intensities of the DH are well below those of the sintered phases a good portion of the material also exists as very small(less than 40 particles beyond the detection limit of XRD). These could be amorphous or microcrystalline. No material was lost through volatilization in this series; a high degree of crystallinity can be reestablished in the dispersed samples by subsequent high temperature treatment.

 

Figure 1. XRD patterns of calcined 4 atoms Mo/nm2 supported MoO3/SiO2: (a) basic precursor, 300C calcined in air (DH), (b) neutral precursor, 500C calcined in air (SO), (c) acid precursor, 300C calcined in air (SH).

 

The Raman spectra of supported MoO3/SiO2 catalysts under ambient conditions are shown in figure 2. Trends are almost same to XRD results, a much lesser degree of crystallinity for the well-dispersed hexagonal sample compared to the sintered hexagonal and orthorhombic.

 

The crystalline MoO3 peak is much more sensitive in Raman than the polymolybdate (Mo7O246-) peak; even though the intensity is high, the relative amount is low. Wachs' group has reported that the calcined samples exhibit the very weak Raman features due to the surface molybdenum oxide species, Mo7O246- and/or Mo8O264-. It means that dispersed hexagonal sample actually has less MoO3 crystalline and more surface molybdenum oxide species, Mo7O246-.


 

O2 uptake of sulfided MoO3/SiO2 catalysts is given in Table 2. Results also show that dispersion degree of DH is much larger than those of sintered catalysts (SH and SO).

 

Table 2. O2 uptake of 4 atoms Mo/nm2 supported sulfided MoO3/SiO2 catalysts

Catalysts

O2 / MoS2 (mmol / g)

O2 / Mo (mmol / mmol )

SH MoO3/SiO2

SO MoO3/SiO2

DH MoO3/SiO2

93.86

163.61

289.60

0.0150

0.0262

0.0464

 

Figure 2. Raman spectra of calcined 4 atoms Mo/nm2 supported MoO3/SiO2: (a) acid precursor, 300C calcined in air (SH), (b) neutral precursor, 500C calcined in air (SO), (c) basic precursor, 300C calcined in air (DH).

 

An initial series of experiments with physically mixed samples were performed to clarify the interpretation of TPR results. TPR profiles of silica supported MoO3/SiO2 and physical mixture of MoO3+SiO2 were shown in figure 3. TPR results show that MoO3 crystal can be reduced at two regions and the area of the first peak is always close to 1/2 that of the second one.

 

XRD data taken for the impregnated and physically mixed samples from TPR experiments stopped at 650C and 1000C were shown in figures 4 and 5, respectively. XRD results showed that MoO3 crystal reduced to MoO2 by low temperature reuction and displayed the pattern of metallic Mo after TPR to 1000C.

 

 

 

Figure 3. TPR profiles of samples: (a) bulk MoO3, (b) physically mixed 4 atoms Mo/nm2 MoO3+SiO2, calcined 4 atoms Mo/nm2 supported MoO3/SiO2: (c) basic precursor, 300C calcined in air (DH), (d) acid precursor, 300C calcined in air (SH), (e) neutral precursor, 500C calcined in air (SO).

 

These results implies that MoO3 crystal reduce by two step over silica as follows:

 

MoO3 (both dispersed and bulk) + H2 MoO2 (TPR up to 650C)

MoO2 (both dispersed and bulk) + 2H2 Mo (TPR up to 1000C)

Mo(VI) + 3H2 Mo(0) (overall reaction)

 

The percentage conversion of DBT and the selectivity to biphenyl (BP) were given in Tables 3 and 4. The selectivity to BP was about 80% at temperature ranges from 350 to 400C. However, selectivity to BP was almost 100% at 500C.

 

 

Figure 4. XRD patterns of samples after TPR upto 650C: (a) calcined 4 atoms Mo/nm2 supported MoO3/SiO2(SO), (b) physically mixed 4 atoms Mo/nm2 MoO3+SiO2.

 

 

Figure 5. XRD patterns of samples after TPR upto 1000C: (a) calcined 4 atoms Mo/nm2 supported MoO3/SiO2(SO), (b) physically mixed 4 atoms Mo/nm2 MoO3+SiO2.

Table 1. Supported precursor and MoO3 patterns resulting from various preparation conditions for supported MoO3/SiO2

 

Activity of catalysts toward hydrodesulfurization of DBT is proportional to dispersion in the order of: sintered hexagonal < sintered orthorhombic < dispersed hexagonal phase MoO3/ SiO2. TOFs of the crystalline phases appear higher than that of the dispersed phase. However, the predominant factor for activity per gram of MoO3 for hydrodesulfurization of DBT is dispersion; the higher activity of the crystallites does not compensate their lesser dispersion.

 

Table 3. The effect of temperature on DBT conversion and O2 uptake over MoO3/SiO2

Catalyst

Conversion (%)

O2 uptake

O2/Mo(mol/mol)

 

623K

673K

723K

773K

 

SH MoO3/SiO2

24

27

36

47

0.018

SO MoO3/SiO2

28

30

42

55

0.027

DH MoO3/SiO2

32

48

54

74

0.048

 

Table 4. The effect of temperature on selectivity of BP and CHB over MoO3/SiO2

Catalyst

Selectivity (%)

 

BP(biphenyl)

CHB (cyclohexyl benzene)

 

623K

673K

723K

773K

623K

673K

723K

773K

SH MoO3/SiO2

79

80

85

91

21

20

15

9

SO MoO3/SiO2

81

83

92

98

19

17

8

2

DH MoO3/SiO2

82

85

96

99

18

15

4

1

 

 

REFERENCES

 

1. K. Segawa and W. S. Millman, J. Catal., 77, 221 (1982).

2. L. Wang and W. K. Hall, J. Catal., 77, 232 (1982).

3. F. J. Gil-Llambias, A. M. Escudey-Castro, and A. Lopez Agudo, J. Catal. 90, 323 (1984).

4. J. Leyer, M. I. Zaki, Z. Shuxian, and H. Knozinger, Mater. Chem. Phys. 13, 301 (1985).

5. S. J. Moon and S. K. Ihm, Appl. Catal., 42, 307 (1988).

6. A. Datta, J.-W. Ha, and J. R. Regalbuto, J. Catal., 133, 55 (1992).

7.      J.-W. Ha, J. of Ind. & Eng. Chemistry, 2(2), 137 (1996).

 

 

[ back to "Publications & Special Reports" ]
[ BWW Society Home Page ]