Mixed oxide supported hydrodesulfurization catalysts—a review
Introduction
Increasing awareness of the impact of environmental pollution by automobiles has shifted the responsibility of pollution control to the refiner’s side. As a consequence stringent fuel specifications for gasoline and diesel fractions resulted. In the case of diesel fuels the sulfur level is expected to be lowered to 50–10 ppm range in large part of developed and developing countries by the end of this decade [1], [2]. In order to reach 50 ppm level from the existing 500 ppm sulfur specification, it was reported that the catalytic activity needs to be increased by four to five times that of the present ones [3]. In order to achieve this goal many approaches have been followed among which variation of support is an important one. The support effects are studied for other reasons such as understanding the role played by support in dispersing the active components and promoters and altering the catalytic functionalities through metal–support interaction [4], [5], [6], [7]. Many materials have been tried as supports to Mo and W active components. Some of which are SiO2 [8], MgO [9], ZrO2 [10], [11], TiO2 [12], [13], carbon [14], [15], [16], zeolites like Na–Y [17], USY [17], mesoporous materials like MCM-41 [18], [19], [20], HMS [21], [22], SBA-15 [23] clays, pillared clays [24] and mixed oxides derived from above-mentioned oxides. Some of the oxides like TiO2, ZrO2 showed outstanding activities [12], [13]. However, the low surface area, limited thermal stability and unsuitable mechanical properties prevented their commercial exploitation. With an aim to overcome these disadvantages mixed oxides of these materials with γ-Al2O3 have been used as supports to take advantage of favorable characteristics of both the systems [25], [26], [27], [28]. Some systems containing SiO2, MgO, ZrO2, B2O3 have also figured in the studies in order to understand the role of support in hydrotreating reactions [6].
Overall a number of mixed oxide supports containing M and W active components and Co or Ni promoters have been studied for hydrodesulfurization (HDS), hydrogenation (HYD), hydrodenitrogenation (HDN) and hydrodeoxygenation (HDO), etc. TiO2–Al2O3 system has been extensively studied because of its commercial prospects [27], [28]. Others such as ZrO2–TiO2, ZrO2–Al2O3 [29], ZrO2–Y2O3 [30], [31], [32], SiO2–Al2O3 [33], [34], [35], [36], [37], [38], [39], and Al2O3–B2O3 [39], [40], [41], [42], [43] also received considerable attention. Systems such as ZrO2–SiO2 [44], TiO2–SiO2 [45], MgO–Al2O3 [29], MgO–SiO2 [39], SiO2–CeO2 [46], and TiO2–MnO2 [47] also figured in some investigations related to exploratory studies. A review of available literature on this systems will be discussed with special emphasis on research carried out in our group. The available literature is grouped into Al2O3 containing mixed oxides, TiO2 containing mixed oxides, ZrO2 containing mixed oxides, and other mixed oxide systems, comprising of all others where limited information is only available, for the convenience of discussion.
Section snippets
SiO2–Al2O3 supported catalysts
Mo, CoMo, NiMo, NiW, W, supported SiO2–Al2O3 mixed oxides studied extensively by a number of authors on various aspects like structure, texture and catalytic activities [33], [34], [35], [36], [37], [38], [39], [48]. One of the variables studied is the composition of the support. The structure of MoO3 phase as a function of support composition was also studied. The dispersions were evaluated using ESCA and oxygen chemisorption, which indicated that the dispersion of molybdenum decreases with
TiO2–Al2O3 system
As mentioned earlier, TiO2 supported systems exhibited higher activities compared to Al2O3 supported Mo and CoMo systems. However, thermal instability, low surface area and poor mechanical properties came in their way of commercial exploitation. In order to overcome these disadvantages and also get more insight into the role played by the Ti, in Ti containing supports, a number authors followed several methods like grafting of TiO2 on γ-Al2O3, SiO2, etc. and forming mixed oxides of TiO2 with
SiO2–ZrO2 supported catalysts
We have studied SiO2–ZrO2 supported Mo, CoMo, NiMo catalysts using thiophene HDS, cyclohexene HYD and cumene cracking reactions [44], [75]. The variation of support composition and at a fixed support composition variation of Mo content, and at a fixed Mo content variation of Co or Ni were considered. Oxygen chemisorption was used to measure the anion vacancies or dispersion. The maximum in HDS activity was obtained at 0.15 wt.% Zr/Zr+Si ratio, whereas HYD activity showed maximum at 0.30 Zr/Zr+Si
Other mixed oxides
Some mixed oxides like CeO2–SiO2 [46], Al2O3–P2O5 [50], Al2O3–Ga2O3 [51] were reported as supports to CoMo active phase. Gulkova and Vit used SiO2–CeO2 as support to (P) CoMo system for HDS of thiophene and HDN of pyridine and reported that catalysts prepared SiO2–CeO2 has higher activities compared Al2O3 supported catalysts. Results on CoMo/Al2O3–B2O3 were reported by Flego et al. [51] indicated that this system exhibits lower activities for thiophene HDS than Al2O3 supported catalysts.
Mixed oxides as supports in deep desulfurization catalysis
Oxide and mixed oxides supported catalysts for deep desulfurization will be dealt with in detail in the same issue by Segawa et al. However, a brief account is given here for completeness of the information. Al2O3–B2O3 mixed oxides supported catalysts were found to exhibit higher activity compared γ-Al2O3 supported catalysts at an Al/B ratio of 3.5 [52]. Zhao et al. [56] reported TiO2–Al2O3 supported CoMo and NiMo catalysts showed superior activity for desulfurization of FCC gasoline compared
Concluding remarks
Past two decades there has been intense activity from several parts of the world on oxides and mixed oxides as supports to Mo, CoMo, NiMo, W and NiW. These supports have been prepared using several methods and effect of preparation on physico-chemical properties are dealt with in considerable detail. The support and catalysts were characterized in few cases, most of the times only activity data is available to say that mixed oxide supported catalysts perform on par or better than commercial
Acknowledgements
The authors would like to thank Director, Indian Institute of Petroleum for his encouragement, support and facilities. The authors also want to thank Mr. Muthu Kumaran and Vikram Singh Rawat for their help during preparation of manuscript.
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