Science: Chemistry:

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 450°C, 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 (24°C), and then dried at 110°C 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

^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 1000°C with 5°C 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 (25°C) 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 300°C calcinations, sintered hexagonal phase of MoO₃ was observed. Sintered Orthorhombic MoO₃ was observed with neutral precursor and 500°C calcinations. And for basic precursor and 300°C 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 Å (SH), 88 Å (SO), and 59 Å (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 Å 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 (Mo₇O₂₄^6-) 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, Mo₇O₂₄^6- and/or Mo₈O₂₆^4-. It means that dispersed hexagonal sample actually has less MoO₃ crystalline and more surface molybdenum oxide species, Mo₇O₂₄^6-.