Relationship between hydrodesulfurization activity and morphological and structural changes in NiW hydrotreating catalysts supported on Al2O3–TiO2 mixed oxides
Introduction
In the search for better hydrotreating catalysts, it has been found that the nature of the support plays an important role to improve the HDS activity. For example, it has been found that the titania supported CoMo catalysts have 1.9 times the HDS activity of their alumina supported counterparts [1]. However, since the titania polymorph, anatase, shows low-thermal stability and -surface area, some other alternatives must be used. Among these, the use of titania–alumina mixed oxides has been proposed as an interesting alternative [2]. Also, in the case of W-based catalysts, the use of alumina as support leads to a strong interaction between the tungsten oxidic phases and the alumina support, which diminishes the degree of sulfidation of the former ones 3, 4. To overcome this problem, in a previous work, we studied the effects that the change in the nature of the support produced in the state of the W oxidic supported phases as the nature of the support changed from alumina to titania [5]. The results from this study showed that W catalysts supported on titania-rich Al2O3–TiO2 mixed oxide supports have a greater HDS activity than those supported on alumina and that it is possible to obtain catalysts with higher surface areas than TiO2 even for Al2O3–TiO2 mixed oxides with 95% mol content of TiO2. Also, good dispersion and reducibility of the W oxidic phases was obtained by the incorporation of titania to the alumina support.
It is well known that the presence of the Ni promoter also improves the reducibility and dispersion of the W phases. Therefore, it seems interesting to study the behavior of Ni-promoted W based catalysts supported on Al–Ti(x) mixed oxides to benefit from both, the Ni promotion and the presence of TiO2 in the support, which leads to higher HDS activities. To this end, a series of W and NiW/Al2O3–TiO2 catalysts with different support compositions were prepared and characterized in their oxided and sulfided forms. The results from these characterizations will be used to estimate the changes in the degree of reducibility (TPR), dispersion (XPS, FTIR of adsorbed NO) and catalyst sulfidation (HREM) with the titanium content and to build a geometrical model of the sulfided catalysts to explain the catalytic activity changes for the thiophene hydrodesulfurization reaction, in terms of the architectural changes of the WS2 crystallites.
Section snippets
Supports and catalysts preparation
A series of nickel–tungsten catalysts supported on alumina–titania mixed oxides with a molar ratio x=TiO2/(Al2O3+TiO2)=0, 0.1, 0.5, 0.7, 0.95 and 1.0 have been prepared. The supports were prepared by the co-precipitation of the required amounts of aluminum (AIP) and titanium (TIP) isopropoxides following the same method described previously [5]. In short, after the dissolution of AIP in n-propanol, under vigorous stirring the required amount of TIP was added. The hydrolysis of the alkoxides was
Catalytic activity
The results from the catalytic activity of the NiW/Al–Ti(x) catalysts are presented in Fig. 1. To allow a good comparison of the catalysts, since they have widely different surface areas, the rates of hydrodesulfurization have been normalized by the total amount of W atoms and are therefore expressed as molecules of thiophene converted per second and per atom of W impregnated on the catalyst. It is clear from the results in Fig. 1 that high titania contents in the catalyst (x≥0.7) lead to a
Discussion
According to our TPR results, the increase of titanium in the catalyst support leads to tungsten species with less interaction with the support which therefore reduce more easily than those supported on alumina. This improvement in reducibility may be due to the decrease of the interaction between the tungsten oxidic species and the support as the titanium loading is increased. In addition, effects due to the reducible behavior of TiO2 and its semiconductor character may play a role considering
Conclusions
From the above results the following conclusions can be drawn:
- 1.
The incorporation of titania to the support in W and NiW/Al–Ti (x) catalysts increases the reducibility and sulfidability of the W species. The incorporation of the Ni promoter also enhances the reducibility of the tungsten species.
- 2.
During the sulfidation process, a thin layer of the TiO2 present in the support is also sulfided.
- 3.
The XPS results indicate that for the titania-rich catalysts (x≥0.7), which show tungsten concentrations
Acknowledgements
We kindly acknowledge financial support from PEMEX-Refinación, IMP (FIES program), and DGAPA-UNAM.
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