Niobium oxide (Nb2O5) as support for CoMo and NiW catalysts in the hydrodesulfurization reaction of 3-methylthiophene

The efficiency of niobium oxide as catalytic support of hydrodesulfurization (HDS) catalysts (CoMo and NiW) has been investigated in the HDS of a model molecule representative of sulfur compounds present in FCC gasoline (3-methylthiophene: 3MT). The NiW catalyst presented higher catalytic activity than CoMo calcined and non-calcined catalyst, however a better ratio pentane/pentene has been achieved by CoMo catalysts, which implies a lower formation of hydrogenated products. Indeed, the activity order for the catalysts evaluated is: NiW/Nb 2 O 5 > CoMo/Nb 2 O 5 calcined support > CoMo/Nb 2 O 5 non-calcined support, despite the ratio pentane/pentene which has the inverse order. Furthermore, textural and chemical characterization techniques have been performed. From NH 3 -TPD analysis it was observed an acidity profile with a predominance of weak/strong and weak/moderate acid for CoMo and NiW catalysts, respectively. Meanwhile, the BET analysis has shown a low specific surface area for the catalysts supported by niobium oxide. Concerning the structure characteristic, the XRD analysis has suggested an amorphous phase in all catalysts analyzed.


Introduction
The reduction of pollutants in fossil fuels is a challenge in the refineries taking into account more strict regulations and the development of environmentally friendly energy.In order to achieve the requirement for environmental regulations, different alternatives need to be evaluated aiming at reducing contaminants present in the motor vehicle fuels (Lu et al., 2020;Santos et al., 2019).Hydrotreatment process is one of the alternatives and consists in removing impurities (S, N, O) from crudes under high partial pressure of hydrogen.
Regarding the hydrodesulfurization process of FCC gasoline, the challenge is to selectively hydrodesulfurize the sulfur species (HDS) and to avoid the hydrogenation of olefins (HYD) in order to preserve the octane number of the fuel (Silva & Secchi, 2018).
Due to the presence of sulfur compounds, the hydrotreatment process needs specific catalysts that combine high thioresistance and catalytic activity.In this regard, hydrotreating catalysts are usually in form of transition metal sulfides (TMS) such as molybdenum and tungsten (VIB group) promoted by elements of VIIIB group such as nickel or cobalt and supported on alumina, silica or aluminosilicates.Thus, the usual industrial catalyst for the hydrodesulfurization processes is the molybdenum sulfide catalysts promoted by nickel or cobalt, depending on the feed to be hydrotreated, with a Co(Ni)/[Co(Ni)+Mo] molar ratio between 0.3-0.6 (Santos et al., 2019).Furthermore, the catalytic support is responsible for loading the active components and disperse the active phase, improving the reaction effectiveness.It is the catalytic support that influences the diffusion, adsorption and determines the accessibility of reactants towards the active sites (Gutierrez et al., 2017;Huirache-Acuña et al., 2012).This fact presents the relevant importance of the support and the interaction metal-support in the preparation step of a hydrotreatment catalyst.
Currently, Brazil has more than 90% of the world's natural resource of niobium (Nb) and it is considered the largest explorer country in the world for this metal.In the hydrotreatment field, the literature has shown an increase in the number of publications about activity and selectivity enhancement as well as the chemical stability of traditional catalysts when a small amount of niobium is added (Aray et al., 2014;Méndez et al., 2017).Gaborit et al. (2000) used niobium as a dopant for hydrotreating NiMo catalyst to investigate the hydrodesulfurization of dibenzothiophene (DBT) and hydrogenation (HYD) of tetraline at P = 33 bar and T = 300ºC.The use of niobium as a dopant has increased the catalytic activity in both, HDS and HYD, model reactions where the highest activities were obtained with an optimum niobium content of 5%.In the case of DBT hydrodesulfurization, the selectivity for cracked products increased and isoalkylbenzenes appeared when Nb was added, which demonstrated that niobium sulfide enhanced the catalytic acidic properties.Since then, a crescent interest in the application of these materials has been noticed, especially with studies about niobium oxides as active phase or catalytic support.The Nb2O5 has already been used as support to Mo and Co (Ni) Mo as well as a precursor of active phase such as niobium sulfide (Faro et al., 2006;Kaluza & Zdražil, 2018;Méndez et al., 2017).For instance, Faro et al. (2006) have investigated the HDS of thiophene at 2.8 MPa and 523-573 K and concluded that the degree of niobium sulfidation increases in the following order: Nb2O5 < Ni/Nb2O5 < Mo/ Nb2O5 < NiMo/Nb2O5.Moreover, it was concluded that niobium sulfide has a strong influence on the activity of the niobium-supported catalysts in the cumene hydrocracking reaction.
Furthermore, the selective hydrodesulfurization reaction has also been studied over supported tungsten catalyst promoted by nickel under specific operating conditions, due mainly to the promising HDS activity over the WS2 slabs.Indeed, León et al. (2017) have studied the HDS of 3-methylthiophene at T = 280ºC and atmospheric pressure over a supported NiW catalyst under different supports.In this study, the catalytic performance of the NiW was attributed to an efficient metal-support interaction, particularly with the samples that used the proposed mixed supports.
In order to further increase the catalytic activity through enhancing the interaction of metallic ions, numerous studies have been published in the literature by using a chelating agent in the hydrotreatment catalysts (CoMo and NiMo, NiW) preparation (Haandel et al., 2017;Pereyma et al., 2018;Valencia et al., 2014;Valencia & Klimova, 2012).The chelating agent are molecules composed by two or more atoms donors of electrons that works as a ligand of a metallic ion behaving as acid/base of Lewis.This reaction occurs with a great contribution of entropy, due to the water molecules surrounding of metallic ion replaced by a ligand chelating (Kaluza et al., 2012).In the literature, the citric acid and ethylene diamino tetracetic acid (EDTA) have been studied as a chelating agent in the in the preparation step of catalysts for hydrotreatment of sulfur compounds such as thiophene and benzothiophene with NiMo, CoMo and NiW catalysts (Castillo-villalón et al., 2014;Kaluza & Zdražil, 2018;Lélias et al., 2010;Pereyma et al., 2018).Indeed, Kaluza & Zdražil (2018) have applied the nitrilotriacetic acid (NTA) in the preparation of CoMo catalyst supported in niobium in order to improve its activity in the HDS of thiophene at 1.0 MPa and 400ºC.A pronounced improvement (5.7 times) has been noticed when compared the specific activity normalized per total BET surface area with the commercial catalyst, which was attributed to a positive effect of NTA in the preparation of supported niobium catalyst as well as the high activity per m 2 of CoMo/Nb2O5 catalyst.The citric acid as a chelating agent was also used in the preparation of a supported NiW catalyst in the investigation of the effect of catalytic thermal treatment in the HDS of dibenzothiophene (T = 280ºC, P = 35 bar, H2/feed = 500 m 3 /m 3 ) (Pereyma et al., 2018).In this study, a direct correlation between the thermal treatment temperature and the catalytic activity of DBT HDS has been found and attributed to an increasing in the stacking degree of WS2 while maintaining a small length of sulfide slabs.This paper focuses on the efficiency of niobium oxide as a support for two types of catalysts, CoMo and NiW, in the hydrodesulfurization of a model sulfur molecule (3methylthiophene) representative of sulfur species present in the FCC gasoline.The role of the support as well as the active phase will be investigated through a catalytic activity test and explained by the proposed characterizations aiming at achieving the selective hydrodesulfurization of model molecule by avoiding the hydrogenation of olefins.

Catalyst and chemicals
The CoMo catalyst precursors were prepared using Ammonium Heptamolybdate The support for the catalysts (Nb2O5) has been calcined in a muffle for 4 hours at 400°C.Afterward, the sample was used for metal wet impregnation with a molar ratio CA/precursor = 1.The mass of precursors as well as the molar ratio Co:Mo and Ni:W for the catalysts prepared are presented in Table 1.
Table 1.Theoretical mass of precursors and the molar ratio of each metal used in the catalyst preparation.

Compound
CoMo/Al2O3  In all samples, the wet impregnation resulted in a solid catalyst that was dried in a rotary vacuum evaporator for 1h and in the oven at 110°C for 2h.The catalyst was crushed and sieved to a size range between 250 and 315 m and then sulfided in situ under H2S/H2 flow (10 mol% H2S) for 10 h at 400°C and atmospheric pressure.3-methylthiophene (98% purity) and n-heptane (>99% purity) were purchased from Sigma-Aldrich which was used without further purification.Hydrogen sulfide (1 vol% in mixture with H2) was purchased from Air Liquide.

Characterization techniques
The specific surface area of the catalysts was measured on a Micromeritics ASAP 2000 analyzer at -196ºC.Before N2 adsorption, the solid samples containing oxide precursors were degassed overnight under a secondary vacuum at 120°C.The specific surface area (SBET in m 2 /g) was calculated from the adsorption isotherm (P/P0 between 0.05 and 0.20) using the Brunauer-Emmett-Teller (BET) method.The total pore volume was calculated from the adsorbed volume of nitrogen at P/P0 equal to 0.99.
The acidity of the catalyst was measured by the NH3-TPD technique.The samples (150 mg) were pretreated with He (30 mL/min) at 200 °C for 30 min and cooled down to 100 ºC.The ammonia adsorption was then carried out (5% NH3 in He: 30 mL/min, for 30 min).
The physisorbed ammonia was purged with He (30 mL/min) for 1 h.The desorption of NH3 was measured from 100°C to 400°C with a rate of 5 ºC.min -1 under the inert gas helium.The amount of desorbed NH3 was analyzed using a TCD detector.
The decomposition temperature of the precursor as well as the stability of the support was evaluated by thermogravimetric analysis, using a thermobalance from Shimadzu model DTG-60 by heating (5mg) the sample between 25-900 °C (rate of 10 °C/min) under a nitrogen flow rate of 100 mL/min.
The crystallinity of the various samples was determined by X-ray diffraction (XRD).
using Shimadzu model XRD 6100 diffractometer with a Cu-Kα tube at 40 kV and 30 mA, with a sample drawer specific for powder-like material.The diffractograms were obtained in the 2θ range between 5 and 80°, with a scanning speed of 2° min -1 and 0.02° step at every

Reaction conditions
Catalytic activity measurements were carried out in a fixed bed reactor at 250°C under a total pressure of 2 MPa with a ratio H2/feed of 360 NL/L.The sulfur model feed (0.3wt% of 3-methylthiophene), containing 1000 ppmS in n-heptane was injected in the reactor by an HPLC Gilson pump (307 series, pump's head volume: 5 cm3).The mass of catalyst used was between 200 and 300 mg.The reaction products were injected on-line using an automatic sampling valve into a Varian gas chromatograph equipped with a PONA capillary column and a flame ionization detector as in previous works (Daudin et al., 2007;Lamic et al., 2008;Naboulsi et al., 2018;Pelardy et al., 2016Pelardy et al., , 2017;;Santos et al., 2019).Desulfurized products, resulting from the transformation of 3-methylthiophene are designated as HDS products corresponding also under these operating conditions to the conversion.The contact time is defined as the ratio between the total amount of feed and the mass of catalyst in the oxide form.The catalytic activity was calculated as the number of moles of HDS products formed per hour and per gram of catalyst.This parameter was calculated at a conversion lower than 30% in a differential regime.

Scientific Methodology
The scientific methodology of this work was focused on the comparative method since different catalysts have been evaluated and compared in different aspects from catalytic activity until the product distribution in a scope of laboratory research.According to Pereira et al. (2018) the nature of this study is focused on the quantitative methodology since the data have been collected from experimental results, treated and analyzed comparatively.

Catalyst Characterization
The physical-chemical properties of the different catalysts CoMo/Nb2O5 and NiW/Nb2O5 were characterized by their mass loss, specific surface area, structure and acidity measured by TGA, BET, XRD and NH3-TPD, respectively.
The thermogravimetric analysis has shown endothermic and exothermic phenomena for the three catalyst samples with and/or without a mass loss (Figure 1).The phenomena observed from room temperature up to 150ºC is attributed to mass loss due to the removal of water physically adsorbed in the oxide structure (Silva et al., 2000).The second phenomenon Research, Society and Development, v. 9, n.11, e74391110307, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.10307 9 observed at 550ºC and 660ºC, without significant mass loss, could be correlated to the formation of crystal phase Nb2O5 with a pseudohexagonal structure, in accordance to the results obtained by Falk et al. (2014).Regarding the non-calcined CoMo/Nb2O5 catalyst it is possible to notice a peak, of exothermic nature, related to mass losses at 150ºC and 220ºC (Figure 1a) attributed to thermal decomposition of precursor salts of Co and Mo present in the metal impregnation process.Meanwhile, for the CoMo/Nb2O5 calcined support catalyst (Figure 1b), it was observed a thermal decomposition in different steps.Indeed, the decomposition peaks occur at 400ºC and 425ºC, with exothermic nature, corresponding to a multiple-step of decomposition for the formation of different Co and Mo oxides (Gonzalez-Cortes et al., 2014;Salazar et al., 2017).Finally, for the NiW/Nb2O5 catalyst (Figure 1c), an exothermic event at 350ºC, with mass loss is observed, which could be attributed to the formation of different Ni and W oxides on the niobium surface.c) The specific surface areas are very low excepted for the CoMo/Nb2O5 non-calcined support catalyst (29 m 2 /g) (Table 2).Indeed, after the calcination step at 400°C, the specific surface area of CoMo/Nb2O5 and NiW/Nb2O5 calcined catalysts are 6 and 2 m 2 /g, respectively corresponding possibly to the decomposition of precursors as shown by TGA-DTA technique.
Furthermore, the loss of area could also be correspondent to the partial blocked of the pores by the presence of oxides.XRD results of the non-calcined and calcined support catalysts presented an amorphous structure.In all catalyst samples, it was observed two wide peaks at 2Ø = 25º and 52º which is characteristic of a niobium amorphous phase.The intensity and shape of the peaks observed in Figure 2 suggest that there is not a defined crystallinity structure or the particles are well dispersed that they could not be noticed in the X-Ray Diffraction.Indeed, the results of these three samples are in accordance with the Nb2O5 phase observed by Santos et al. (2017).Furthermore, the patterns observed for the calcined and non-calcined CoMo/Nb2O5 have suggested that even with the support calcination step, the Nb2O5 structure has been maintained.Fonte: Authors.
Finally, the total acidity of different samples has been investigated through the NH3-TPD.As reported in Figure 3, representing the curve of NH3 desorption as a function of time and temperature, it can be seen that, whatsoever the catalysts, various peaks were observed corresponding to different acidity strengths.Indeed, it is noticed that for the CoMo/Nb2O5 non-calcined support catalyst (Figure 3a) weak and strong acid sites are predominant whereas for the CoMo/Nb2O5 calcined catalyst (Figure 3b) there are mainly moderate and still more relevant strong acid sites.Indeed, the literature suggests that the enhancement of temperature promotes a decrease of acidity, creating new superficial sites with different acid strengths and modifying the material properties as an ion exchanger (Kitano et al., 2012).Regarding the NiW/Nb2O5 catalyst (Figure 3c) there is an important predominance of weak and moderate acid sites (37 and 43%, respectively), confirming a weaker acidity of NiW in comparison with CoMo catalysts.These results and proportions of acidity strength between the catalysts are also presented in Table 2. Fonte: Authors.

Transformation of 3-methylthiophene
The performance of these materials was compared for the transformation of 3- CoMo/Nb2O5 calcined support.This could be explained by the fact that increasing the calcination temperature of the niobium solid, a superficial rearrangement in the hydroxyl group occurs, and water molecules could leave the molecular structure (Santos et al., 2017).It is important to notice that even though a lower activity of the CoMo/Nb2O5 non-calcined catalyst (activity for the non-calcined and calcined CoMo/Nb2O5 equal to 0.12 and 0.22 mmol/g.h,respectively), the product distribution has shown a lower pentane/pentene ratio (iC5/=C5), which means a lower tendency to produce hydrogenated products during the hydrotreatment reactions (Santos et al., 2017).3.This difference could be attributed to the interaction metalsupport as well as the lower available surface area provided by the Nb2O5 support, which was confirmed by the comparison of BET results presented in Table 2. Indeed, the specific surface area of the alumina support is about 20 times higher in comparison with the Nb2O5 support, which leads to a better available surface area to distribute the active phase and thus enhance the catalytic activity and selectivity.Taking into account the difference in specific surface area of alumina and niobium supports, Table 3  process with H2S, prior to the hydrodesulfurization reaction.Comparing the activity of NiW/Nb2O5 and CoMo/Nb2O5 catalysts it is possible to notice an important difference which, in the first case, the activity is equal to 0.43 mmol/g.hand the second case equal to 0.22 mmol/g.h,representing 2 times higher for the NiW than CoMo catalyst.Comparing the catalytic activity as a function of specific surface area, it is possible to notice a difference about 5 times between calcined niobium supported NiW and CoMo catalysts (0.21 and 0.04 mmol/m 2 .h,respectively).These results suggest that the effect of Ni as a promoter of the WS2 phase would favor the hydrogen transfer reactions more than the promotion of Co for the MoS2.This fact could facilitate the C-S scission through a direct desulfurization reaction and thus there is a higher activity of NiW over CoMo catalyst for the hydrodesulfurization of 3MT.These results are in line with the literature where a similar behavior was observed when was studied the effect of different tungsten catalysts for the hydrodesulfurization of thiophene (Bendezú et al., 2000;González-Cortés et al., 2014).Thus, these results suggest a more efficient Ni-W-S interaction in comparison with the Co-Mo-S interaction.
The supported CoMo and NiW over Nb2O5 were prepared by wet impregnation with an aqueous solution of the precursor of CA, CoN and AHM for CoMo and CA, AM and NiN for NiW.The pH of each sample was 2.70 for the non-calcinated support, 1.38 for the CoMo/Nb2O5 calcined support and 1.15 for the CoMo/Nb2O5 calcined support.

Table 2 .
NH3-TPD desorption and BET surface area determined for different catalysts.

Nb 2 O 5 -calcined support CoMo/ Nb 2 O 5 -calcined support
through the results obtained in the transformation of 3MT over NiW/Nb2O5 and CoMo/Nb2O5 catalysts since the Ni:W and Co:Mo molar ratios are similar as indicated in Table1.Indeed, the HDS reaction occurs over the MoS2 and WS2 catalytic phase formed during the sulfiding NiW/