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CHAPTER 4
SUMMARY AND CONCLUSION
A modern refinery is a highly integrated industrial enterprise, the main task of which is to efficiently produce a high yield of valuable products from a crude oil feed of variable composition. Transportation fuels, the major petroleum products are receiving the highest scrutiny because of the pollution from exhaust gas. Environmental restrictions regarding the quality of transportation fuels and the emissions from the refinery itself are currently the most important and most expensive issues. Hydrotreating or hydroprocessing refers to a variety of hydrogenation processes, which saturate unsaturated hydrocarbons and remove sulfur [by hydrodesulfurisation (HDS)], nitrogen [by hydrodenitrogenation (HDN)], oxygen [by hydrodeoxygenation (HDO)] and metals [by hydrodemetallization (HDM)] from different petroleum streams in a refinery. The main aim of hydrotreating is to diminish air pollution emissions, improve the fuel quality, to avoid poisoning of noble metals and acid catalysts used in catalytic reforming and cracking.
In order to improve the activity and to reduce the severity of experimental conditions, several approaches have been pursued. One such approach was to choose a proper support for the active components.
In this context, AlMCM-41 (Si/Al = 100) support was
synthesized hydrothermally by following a reported method employing cetyltrimethylammonium bromide as a template. The material was characterized by various physico-chemical methods such as XRD,
N2 adsorption-desorption studies, TGA and SEM. The mesoporous nature of the MCM-41 was confirmed by low angle XRD. BET measurement revealed high surface area of the molecular sieves. TG analysis indicates the removal of water and template at specific temperatures.
This material was used as a support for NiO-MoO3 catalysts.
H-AlMCM-41 was impregnated with Mo and Ni precursor salts by sequential impregnation method. For the preparation of nickel promoted catalysts,
H-AlMCM-41 was first impregnated with MoO3 then followed by NiO. The compositions of the catalysts were chosen as 12, 18, 24 and 30 wt.% of MoO3 with a constant amount of 7 wt.% NiO.
All the supported catalysts were characterized by XRD, N2 adsorption-desorption studies, ICP-AES elemental analysis, SEM, FT-Raman spectroscopy, UV-Vis DRS and FT-IR spectra of CO adsorption. XRD data of all the metal impregnated catalysts indicate that the crystallinity of the support was affected gradually with increase in MoO3 content. The formation of different phases like NiMoO4, MoO3 and NiO over the support’s surface was also revealed by the X-ray diffractograms. It was clearly observed from the textural analysis that on increasing the metal oxide loading, the surface area and pore volume of the catalysts were found to decrease significantly. The fine dispersion of NiO-MoO3 was clearly apparent from SEM photograph. FT-Raman results confirm the formation of NiMoO4, MoO3 and NiO phases over the support. UV-Vis DRS results reveal the coordination environment of Ni and Mo impregnated catalysts. FT-IR studies of CO adsorbed on 7% NiO-18% MoO3 and 7% NiO-24% MoO3 showed that the promoted Mo sites increases with increase in MoO3 loading, indicating the fine dispersion of NiO–MoO3 particles and more number of active Mo species on the surface of H-AlMCM-41. Among these catalysts, 7% NiO-24% MoO3 loaded catalyst shows better dispersion as well as NiMoO4 phase.
Using these supported catalysts, HDN of OT and CHA were carried out under optimized reaction conditions to optimize the wt.% loading of MoO3. The result shows that both the amines follow the same trend. Among these catalysts, hydrodenitrogenation activity followed the order 7% NiO-24% MoO3 > 7% NiO-18% MoO3 > 7% NiO-30% MoO3 > 7% NiO-12% MoO3. An optimum metal loading of 24% MoO3 has been chosen for further study.
Earlier literature clearly indicated that the catalytic performance was not only improved due to the high surface area of the support, but also the other changes that occur in their textural properties while modifying the preparation procedure. Considering this, attempts were made to improve the performance of 7% NiO-24% MoO3 loaded catalyst with different preparation procedures over H-AlMCM-41(100) support. The Ni-promoted catalysts were prepared by co-impregnation and by sequential impregnation method (normal order and reverse order).
These materials were characterized by TEM, XRD, N2 adsorption-desorption studies, XPS and FT-IR spectra of CO adsorption. TEM pictures showed the dispersion of metal oxide corresponding to the different preparation procedures. XRD data of these impregnated catalysts indicated the presence of different phases like NiMoO4, MoO3 and NiO over the surface of the support. It was clearly observed from the textural analysis that the method and the order of impregnation can significantly influence the surface area and pore volume of the catalysts. XPS results showed that the
co-impregnated and reverse order impregnated catalysts were completely reduced to their lower oxidation states, while the normal order impregnated catalysts were found to have some unreduced metal oxide over the support’s surface. The FT-IR studies of CO adsorption clearly showed that the reverse order impregnated catalysts possess more number of finely dispersed active metal species on the surface of H-AlMCM-41 than the other catalysts.
The catalytic activity of these catalysts was tested for HDN of OT and CHA on pre-reduced and pre-sulfided catalyst. HDN activity of pre-reduced catalysts was found to be better than pre-sulfided catalyst. Hydrodenitrogenation activity of pre-reduced catalysts shows that the reverse order impregnated catalyst is more active than the rest. The activities of the catalysts were in the following order: 24% MoO3-7% NiO > 7% NiO-24% MoO3 > (7% NiO.24% MoO3). This method of impregnation was chosen for further studies.
To study the effect of Si/Al ratio on MCM-41 supported NiO-MoO3 catalysts, SiMCM-41 and AlMCM-41 (Si/Al = 100, 60 and 30) were synthesized hydrothermally following a reported method in which cetyltrimethylammonium bromide was employed as a surfactant. The materials were characterized by XRD, ICP-AES elemental analysis, SEM, N2 adsorption-desorption studies, TPD of ammonia using adsorption techniques and MAS NMR. The mesoporous nature of the MCM-41 was confirmed by low angle XRD. BET measurement reveals high surface area of the molecular sieves. The strength of the acid sites was measured by TPD of ammonia revealing the substitution of aluminum in the framework. MAS NMR spectra evidenced the environment of Al and Si in the frame work.
The catalyst (24% MoO3-7% NiO) was loaded over all the support and characterized by different techniques like XRD, N2 adsorption-desorption studies, MAS NMR, UV-Vis DRS and FT-IR spectra of CO adsorption. The XRD results showed the formation of NiMoO4 phase along with NiO phase except for Si/Al = 100 support. The X-ray diffractograms of AlMCM-41 (100) supported catalyst shows the formation of NiMoO4 phase only. This result clearly indicates the dispersion of metal oxide over AlMCM-41 (100) supported catalysts was better than other supports. This result was further confirmed by FT-IR spectra of CO adsorption. DRS results revealed the different environments of the metal species. Compared to other catalysts, AlMCM-41 (100) supported catalysts showed large dispersion of metal oxides.
To study the effect of support, 7 wt.% NiO-24 wt.% MoO3 was impregnated over different supports like MCM-41, HZSM-5 and γ-Al2O3 at their optimum levels. H-AlMCM-41 supported catalyst showed higher activity due to its higher surface area.
In this present investigation, different supported metal catalysts were studied for HDN reaction. Among these catalysts, 24 wt.% MoO3-
7 wt.% NiO/H-AlMCM-41(100) catalysts (reverse order) was found to have higher HDN activity than the other catalysts.