Effect of Al-SBA-15 support on catalytic functionalities of hydrotreating catalysts: I. Effect of variation of Si/Al ratio on catalytic functionalities

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Abstract

SBA-15 and Al-SBA-15 of varying Si/Al ratios (10–40) were synthesized by well-known procedures. Characterization using XRD, pore size distribution, 27Al NMR confirmed that hexagonal mesoporous structure is obtained and majority of Al is in tetrahedral positions. Mo, CoMo and NiMo catalysts prepared using these supports were examined by XRD, oxygen uptakes in sulfided state and temperature programmed reduction (TPR). The catalysts were tested for hydrodesulfurization (HDS) of thiophene and hydrogenation of cyclohexene. Systematic variation of oxygen chemisorption and catalytic activities were observed as a function of Si/Al ratio. The systematic variation of activities, oxygen chemisorption and TPR hydrogen consumption of Mo indicated that the molybdenum dispersion and anion vacancies, and catalytic activities are significantly influenced by support Al content. Compared to SBA-15, Al-SBA-15 supported Mo, CoMo and NiMo catalysts show outstanding activities for hydrogenation.

Introduction

Environmental restrictions on petroleum products to limit the sulfur level in fuels to 50 ppm or lower necessitated new generation hydrodesulfurization catalysts. In addition, preparing hydrocarbon fuel feeds to the fuel cell set up requires sulfur reduction to 0.1 ppm. Such a demanding task requires catalysts that are seven times more active than the present catalysts used to achieve 500 ppm sulfur [1]. It is not only the high activity but they should also have different activity profiles with respect to different functionalities. In order to modify the activity to achieve the above said objectives several approaches have been pursued among which variation of support is an important one [2].

γ-Al2O3 is the only support used in commercial HDS catalysts. Many other supports like SiO2 [3], carbon [4], oxides [5], [6], [7] and mixed oxides [8] have been studied. In recent times, the attention is shifted to zeolites like Y, USY and mesoporous materials like MCM-41 [9], [10], HMS [11], [12], SBA-15 [13] and mesoporous Al2O3 [14] systems. MCM-41 supported CoMo catalysts are reported to exhibit higher activities for conversion of benzothiophene and petroleum residues. Song and Reddy [15] demonstrated that CoMo/MCM-41 is substantially more active than CoMo/γ-Al2O3 catalysts at high molybdenum loadings. Chiranjeevi et al. [16] reported that Al-HMS materials based CoMo, NiW catalysts showed superior activities compared to γ-Al2O3 supported catalysts. Highly active MoS2 catalysts supported on mesoporous Al2O3 were reported by Zdrazil and co-workers [14]. Vradman et al. [17] reported higher activites for HDS and hydrogenation using Ni-W-S/SBA-15 catalysts. Results reported by Klimova et al. [18] on Al-SBA-16 supported NiMo catalysts for HDS of 4,6-DMDBT hydrodesulfurization indicated that these materials exhibited high activities in conversion of the above said refractory sulfur compound. Zepeda et al. [19] reported on Ti-HMS of various Si/Ti ratios for HDS of benzothiophene suggested that Ti containing HMS supported catalysts displayed higher activities than the γ-Al2O3 supported catalysts. Chiranjeevi et al. reported systematic relationships for HDS and HYD activites as a function of Si/Al ratio of Al-HMS supported Mo catalysts [20]. In this investigation we are reporting a detailed systematic study on Mo, CoMo and NiMo supported on Al-SBA-15 of various Si/Al ratios for HDS and HYD functionalities.

Section snippets

Synthesis

The SBA-15 and Al-SBA-15 materials were synthesized by following published procedure [21], [22]. The incorporation of Al into SBA-15 mesostructure was carried out by direct synthesis method using Al-isopropoxide as Al source following published procedure [22]. In a typical synthesis, 9 ml tetra ethyl ortho silicate (TEOS) and a calculated amount of aluminum isopropoxide in order to obtain a Si/Al ratio equal to 10, 20, 30 and 40 are added to 10 ml of aqueous HCl solution at pH 1.5. This solution

Synthesis and characterization of the support

Al-SBA-15 materials with varying Si/Al ratios were characterized by XRD, BET surface area, pore size and 27Al NMR methods. The results indicated the hexagonal mesoporous structure is obtained as supported by low angle XRD peak (Fig. 1) and narrow pore size distribution centered around 78 Å. 27Al NMR results indicated that majority of Al is in tetrahedral surroundings [25], [26]. It was inferred from the results that the high surface area hexagonal mesoporous Al-SBA-15 is obtained at all Si/Al

Conclusions

The Mo, CoMo and NiMo catalysts were prepared by using high surface area Al-SBA-15 supports of varying Si/Al ratios. The surface areas of promoted catalysts are comparable to that of Mo catalysts. XRD examination of Mo, CoMo and NiMo catalysts indicated very high dispersion of Mo in unpromoted and promoted catalysts. The TPR results indicated that the promoters decreased the reduction temperature of molybdenum. The oxygen chemisorption on sulfided catalysts varied linearly with Si/Al ratio of

Acknowledgements

The authors are grateful to Director, Indian Institute of Petroleum, Dehradun, for his encouragement and Mr. G. Muthu Kumaran thanks CSIR, India, for a Senior Research Fellowship.

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