Ce2(MoO4)3 as an efficient catalyst for aerobic oxidative desulfurization of fuels

Xiao-Yi Liu，Xiu-Ping Li，Rong-Xiang Zhao

School of Petrochemical Engineering，Liaoning Petrochemical University，Fushun，113001，Liaoning，China

Keywords:Ce2(MoO4)3 Aerobic oxidative desulfurization Dibenzothiophene Reflux Renewability

ABSTRACT Ce2(MoO4)3 was synthesized by a simple reflux method using cerium nitrate hexahydrate and ammonium molybdate as reactants.The as-prepared Ce2(MoO4)3 was characterized by Fourier transform infrared spectroscopy (FT-IR)，X-ray diffraction (XRD)，Scanning electron microscope (SEM)，and X-ray photoelectron spectroscopy (XPS).The removal of dibenzothiophene (DBT) in model oil was studied using Ce2(MoO4)3 as catalyst and oxygen as oxidant.The reaction factors such as reaction temperature，amount of catalyst，and sulfide type on sulfur removal were researched.The results prove that both Ce3+and play significant role in the conversion from DBT to DBTO2.The Ce2(MoO4)3 catalyst has an excellent performance for the sulfur removal of DBT.Under the optimum reaction conditions，sulfur removal of 99.6% was obtained.After recycling five times，no significant loss in catalyst activity of Ce2(MoO4)3.Mechanism of aerobic oxidative desulfurization was proposed based on the experiment of free radical capture and infrared characterization.

1.Introduction

The emission of sulfur oxides(SOX)in vehicle exhaust can cause serious environmental problems and threaten human health.Thus，countries around the world have set up strict limits on the sulfide content in fuels，and it is very urgent to develop efficient deep desulfurization technology.In recent years，hydrodesulfurization(HDS) has been widely used to remove organic sulfur compounds，which is conducted at harsh reaction conditions (300-350°C，2-10 MPa) with expensive hydrogenation catalysts.HDS can effectively remove aliphatic sulfur compounds (Sampieri et al.，2005).Furthermore，removal of aromatic sulfur compounds is difficult because steric hindrance.Some technologies have been developed to overcoming technical defects of HDS，these desulfurization technologies are adsorption desulfurization (ADS) (Luo et al.，2021)，extractive desulfurization (EDS) (Li et al.，2013) and oxidative desulfurization (ODS) (Wang et al.，2018)，etc.Among them，the ODS technology has been widely researched due to its advantages such as mild operating conditions，low energy consumption，and high removal efficiency of aromatic sulfides.In the ODS process，organic sulfides can be oxidized into sulfones with strong polarity，and then removed by extraction or adsorption(Zhen et al.，2019).Various oxidants，including H2O2，oxygen and organic peroxides have been applied to the ODS process.H2O2is widely used due to its lower cost，higher oxidation performance and environmental protection.However，the production and storage of hydrogen peroxide may cause safety and cost issues (Liu et al.，2021).Oxygen is not only environmentally friendly，moreover it is easy to get.Thus，oxygen as the oxidant in ODS，it has attracted the attention of researchers (Wang et al.，2021).

In ODS，the reaction between oxygen and sulfide is difficult to be carried out under mild conditions.Therefore，it is very important to develop high activity catalysts.In recent years，some catalysts，such as inert metal-based material(Nakagawa et al.，2019)，metal organic frameworks(MOFs)(Tang et al.，2020)，polyoxometalate(POMs)(Chi et al.，2019)，transition metal oxide(TMO)(Wang et al.，2017)etc.，are applied to the field of aerobic oxidative desulfurization(AODS).TMO has been widely concerned in ODS due to their higher activity and lower cost(Wang et al.，2020).For example，Wang et al.(Wang et al.，2020)synthesized V2O5nanosheets with oxygen vacancies by rapid gas drive stripping method.The sulfur removal can reach 99.7%under the optimum reaction conditions.Shi et al.，(2016)proposed a simple sol-gel method to prepare Ce-Mo-O catalysts.The catalyst can completely remove BT and 4，6-DMDBT，the removal rate of BT is 97%，and the oxidation process does not need to add sacrificial agent.Dong (Dong et al.，2019) et al.has developed ultra-thin a-CO(OH)2nanosheets with molybdate intercalation，its derived Co-Mo-O mixed metal oxide has shown excellent sulfur removal performance.Wu et al.，(2020) found that the interaction of strong metal edges between Pt and h-BN can improve the aerobic oxygen oxidation performance in fuel oil.Liu et al.，(2020)established Co-Ni-Mo-O mixed metal oxide nanotubes with a hollow structure preferred to application of AODS.The preparation of the above catalysts often requires harsh preparation conditions such as calcination or the addition of organic reagents to control the structure of the catalyst.These defects are not conducive to the industrialization process of aerobic oxidative desulfurization.

Cerium molybdate has been widely applied in the fields of inorganic pigments(Dargahi et al.，2020)and photocatalysts(Xing et al.，2016).At present，there is no report of cerium molybdate used as a catalyst for AODS.In this paper，Ce2(MoO4)3was synthesized and characterized by FT-IR，XRD，SEM，XPS.Compared with the previous TMO，the synthesis of the Ce2(MoO4)3can be carried out under mild reaction conditions (lower temperature and shorter reaction time)，and the reaction process does not need to add structure promoter and high-temperature calcination.The oxidative desulfurization of model oil was determined by using Ce2(MoO4)3as the catalyst and oxygen as the oxidant.Furthermore，based on the experimental conclusion，the functions of Ce and MoO4are illustrated in ODS，and proposed possible reaction mechanism.

2.Experimental

2.1.Materials and instruments

Ammonium molybdate (98 wt%)，Cerium Nitrate Hexahydrate(AR)，Decahydronaphthalene (AR)，Dibenzothiophene (AR)，4，6-Dimethyldibenzothiophene (AR)，Benzothiophene (AR)，All above reagents were purchased from Aladdin Reagent Co.，Ltd..Oxygen(99 wt%，Hubei Guangao Biological Technology Co.，Ltd.).

FT-IR of the synthesized catalysts was characterized by a Nicolet FT-IR spectrophotometer (Nexus 470，Thermo Electron Corporation) with spectral range of 4000-400cm-1 and resolution better than 1.5 cm-1.XRD graphics of the Ce2(MoO4)3sample were obtained using the Philips diffractometer utilizing high-intensity Cu Kα radiation in X’Pert MPD model (40 kV;100 mA;1.5406 Å)，and the step scan technique at 2 theta angles range between 10°and 70°.Surface morphology of the Ce2(MoO4)3was characterized by SEM (ZEISS Gemini SEM 500，Germany).The elemental composition of the Ce2(MoO4)3was studied by energy-dispersive X-ray spectroscopy(EDS).XPS(PHI5000 Versaprobe II，Japan)was used to survey and evaluate the elemental composition and surface chemical state of Ce2(MoO4)3.

2.2.Synthesis of Ce2(MoO4)3

In this work，the Ce2(MoO4)3catalyst was synthesized by the reflux method.0.5296g ammonium molybdate was dissolved in 40 mL distilled water，marked as solution-A，then，solution-A was added to a triangular flask with a cooling condenser.After that，aqueous solution (40 mL) of 0.8744g cerium nitrate hexahydrate was added dropwise to Solution-A.The triangular flask containing the mixed solution was transferred to an oil bath and stirred at 80°C for 1 h.The pale yellow precipitation was obtained.The products after centrifugal separation were washed three times with absolute ethanol and distilled water，and dried at 90°C for 5 h.

2.3.Aerobic oxidation desulfurization experiments

The model oil with S-content of 250 μg/g was prepared by dissolving 0.718g dibenzothiophene in 500 mL decahydronaphthalene.The AODS reaction was carried out in a three-neck flask.Firstly，20 mL simulated oil and a certain amount of Ce2(MoO4)3were added to three-necked flask with reflux device.Then，the three-neck flask was placed in a preheated oil bath.oxygen was injected at a flow rate of 0.2L/min.The AODS reaction begins at a certain temperature and agitation speed.A small amount of upper oil phase is taken as sample every 20 min and the sulfur content of the sample was measured by WK-2D microcoulomb analyzer，then calculation of sulfur removal by formula(1).The reaction device is shown in Fig.1.

3.Results and discussion

3.1.Characterization

3.1.1.FT-IR and XRD analysis

Fig.1.Device diagram of aerobic oxidative desulfurization.

Fig.2.(a) FT-IR and (b) XRD spectra of Ce2(MoO4)3.

The FT-IR and XRD characterization results of the as-synthesized Ce2(MoO4)3are revealed in Fig.2.As shown in Fig.2(a)，the infrared absorption peaks at 3382 cm-1and 1616 cm-1are attributed to stretching and bending mode of O-H from water adsorbed on the surface of samples(Xing et al.，2016)，the smaller peak at 1384 cm-1corresponds to the bending vibration of the Ce-O-H bond.The narrow peaks between 1070 and 1150 cm-1，the peaks around 870，712 and 634 cm-1associate to the stretching vibration peaks of the Mo-O bond (Yousefi et al.，2012).Crystallinity of Ce2(MoO4)3was determined by XRD analysis.The XRD patterns are shown in Fig.2(b).From the results，typical characteristic peaks of samples associate to Ce2(MoO4)3crystals (JCPDS:00-057-0952) and the structure is amorphous nanocrystal(Kartsonakis and Kordas，2010).The individual peaks at 2θ angles are obtained to 21.19°，24.47°，27.61°，28.33°，29.99°，36.39°and 46.34°，which are assigned to(112)，(004)，(200)，(204)，(220)，(116)，(312) planes of monoclinic Ce2(MoO4)3，respectively.No peak of other crystal phases is observed in the spectrum.

3.1.2.SEM analysis

The morphology of the as-prepared Ce2(MoO4)3was investigated by SEM.As shown in Fig.3(a)，the sample is composed of nanocrystals with rod-like structure and particles.The particle size of nanocrystals is uneven due to lower reaction temperature and shorter reaction time.Moreover，nanocrystals were revealed high dispersibility and low packing density.According to relevant reports (Xing et al.，2016)，the morphology of Ce2(MoO4)3can be changed according to different synthesis conditions.The EDS measurement of the sample is shown in Fig.3(b)，indicates that the sample is composed of Ce，Mo and O elements，and its molar ratio is approximately n(Ce):n(Mo):n(O)=2:3:12.It is consistent with the composition of cerium molybdate.

3.1.3.XPS analysis

For the benefit of obtaining the composition and chemical state of catalyst，XPS analysis of Ce2(MoO4)3catalysts was performed.As shown in Fig.4(a)，the survey curves of catalysts further demonstrate the coexistence of Ce，Mo and O elements，each element has a spin-orbit core energy level.Among them，the spin-orbit energy spectrum of Ce elements can be divided into two multiple energy levels (U and V)，they correspond to the core-level spin-orbit splitting of Ce 3d3/2and Ce 3d5/2(Kanai et al.，2017).The highresolution XPS spectrum of Ce 3d was shown in Fig.4(b)，the U0，V0，U1，V1peaks corresponding to Ce3+state and the remaining 5 peaks corresponding to Ce4+state (Sakthivel et al.，2015).The presence of Ce3+and Ce4+indicates that the Ce2(MoO4)3catalyst has redox properties.

The high-resolution XPS spectrum of the spin-orbit core energy level of O1s was displayed Fig.4(c).The sharp peak observed at 530.51 eV assigned to lattice oxygen in the Mo-O bond.And the side peak at 532.55 eV can be corresponded the O2was adsorbed on Ce2(MoO4)3surface (Sakthivel et al.，2015).

Fig.3.(a) SEM and (b) EDS images of Ce2(MoO4)3.

Fig.4.XPS spectrum of (a) Ce2(MoO4)3，high resolution spectra of (b) Ce3d，(c) O1s and (d) Mo3d.

The XPS spectrum of the spin-orbit core energy levels of Mo 3d5/2 and Mo 3d3/2，the binding energy at 232.68 eV related to the Mo6+oxidation state of the Mo 3d5/2spin-orbit core energy level in Fig.4(d).The other peak appears at 235.87 eV is the Mo 3d3/2spin-orbit core energy level of Mo6+oxidation state，it can be attributed to the existence of Mo=O or Mo-O bond，but no additional peaks of Mo4+or Mo5+are observed.These peaks are consistent with the reported XPS spectra of Ce 3d and Mo 3d (Karthik et al.，2017).

3.2.Effect of various catalysts for AODS

In order to study the effect of catalyst structure in ADOS，oxidative desulfurization activity of different catalysts such as Ce2(MoO4)3，CeVO4，Ce2(WO4)3and Na2MoO4were investigated under the same experimental conditions.As shown in Fig.5，Thehas the highest sulfur removal in these catalysts owning the same cation.Efficiencies change follows the sequenceSimilarly，based on the fact that the sulfur removal performance of Ce2(MoO4)3is much higher than that of Na2MoO4.It can conclude that Ce3+also play a significant role in ODS.The above results indicate that the synergistic effect between Ce3+and MoO42-leads to higher sulfur removal.

3.3.Effect of reaction temperature

The reaction temperature is an important factor affecting the desulfurization rate.In AODS，the catalyst can show high sulfur removal at high temperature (＞100°C).The oxidative desulfurization activity of Ce2(MoO4)3at different reaction temperatures was shown the Fig.6，sulfur removal increases from 10.8% at 80°C to 99.6% at 100°C in 120 min.This is because increased the temperature can accelerate the collision rate between reactant molecules(Zhao et al.，2007).When the reaction temperature was increased from 100°C to 110°C，the sulfur removal rate increased to 99.6% in 80min.However，high temperature will also cause side reactions of hydrocarbons oxidation and the increase of energy consumption(Eseva et al.，2021).In conclusion，100°C as the optimum reaction temperature.

3.4.Effect of catalyst dosage

Fig.5.Effect of different catalysts for sulfur removal (Reaction conditions:20 mL model oil，O2 airflow rate=0.2L/min，T=100°C，catalyst dosage=0.05g).

The catalyst dosage is one of the most important parameters for the industrialization of ODS.The effect of Ce2(MoO4)3dosage on sulfur removal was investigated.The results are shown in Fig.7，catalyst dosage increased from 0.02g to 0.05g results in the sulfur removal of DBT increasing from 58.8% to 99.6%.Nevertheless，the catalyst dosage was increased from 0.05g to 0.06g，the sulfur removal decreased from 99.6% to 90.8%.The results show that increasing the amount of catalyst is beneficial to increasing the number of active sites(Qiu et al.，2016).However，excessive catalyst will cause agglomeration and limit the contact area with DBT，which will affect the diffusion of reactants and products，so reduce the sulfur removal (Eseva et al.，2021).Therefore，the optimal catalyst dosage of 0.05g was selected to oxidative desulfurization process.

3.5.Removal of different types of sulfide

Fig.6.Effect of reaction temperature on sulfur removal (Reaction conditions:20 mL model oil，O2 airflow rate=0.2L/min，Ce2(MoO4)3=0.05g).

Fig.7.Effect of the Ce2(MoO4)3 dosage on sulfur removal(Reaction conditions:20 mL model oil，O2 airflow rate=0.2L/min，T=100°C).

In this experiment，the Ce2(MoO4)3has a high oxidation desulfurization activity on DBT in AODS.However，it is very necessary to study sulfur removal performance of catalysts for other sulfides such as BT and 4，6-DMDBT due to the diversity of sulfides in actual fuel.As shown in Fig.8，the removal rate of 4，6-DMDBT and BT are 94% and 26%，they are lower than that of DBT.According to the literature (Zhao et al.，2017)，the difference of removal efficiency can be attributed to the electron cloud density of sulfur atoms and steric hindrance effect.The methyl groups in 4，6-DMDBT would hinder process of oxidative desulfurization reaction，resulting in a lower removal rate of DBT.The higher electron cloud density results the higher the oxidation desulfurization capacity.The electron densities of the S atom in DBT，4，6-DMDBT and BT are 5.758，5.760 and 5.739，respectively (Mao et al.，2017).Therefore，sulfur removal of DBT，4，6-DMDBT and BT are affected by the steric hindrance and electron density (Otsuki et al.，2000).

3.6.Effects of olefins and aromatics

In ODS process，the presence of olefins and aromatics will affect the removal of organic sulfide.Here，toluene and cyclohexene are selected as the models of aromatics and olefins，effects of their addition on sulfur removal are explored in Fig.9.Under optimal reaction conditions，when 5 wt%-toluene and 5 wt%-cyclohexene were added to 20 mL model oil，removal efficiency slightly decreased to 97.6%and 94.1%，respectively.It can be seen that both cyclohexene and toluene have a certain influence on the sulfur removal，which may be caused by the competitive reaction among toluene，cyclohexene and DBT (Liu et al.，2021).The results show that the sulfur removal of DBT in the AODS system is less affected by olefins and aromatics.

3.7.Recycling performance of Ce2(MoO4)3

After the AODS reaction，the catalyst was filtered from the reaction system，and then washed with CCl4under a magnetic stirrer at 25°C for 30min.Finally，the catalyst was dried at 80°C for 8 h.Thereafter，the recovered Ce2(MoO4)3catalyst was used in the next AODS under the optimal conditions.The exhibited results in Fig.10 illustrates that the sulfur removal of DBT reduced to 94.7%after the five cycles.The recovered Ce2(MoO4)3catalyst was analyzed by FTIR analysis measurement.The FT-IR spectrum was shown in Fig.11.It can be seen that the fresh catalyst and the recovered catalyst have similar absorption peaks，Therefore，the catalyst has high stability.

Fig.8.Removal efficiency of different sulfides(Reaction conditions:20 mL model oil，O2 airflow rate=0.2L/min，T=100°C，Ce2(MoO4)3=0.05g).

Fig.9.Effect of olefins and aromatics on sulfur removal(Reaction conditions:20 mL model oil，O2 airflow rate=0.2L/min，T=100°C，Ce2(MoO4)3=0.05g).

3.8.AODS mechanism in the presence of Ce2(MoO4)3

In order to investigate aerobic oxidation desulfurization mechanism，a free radical capture experiment was designed.P-benzoquinone (·O2-trapping agents) and isopropanol (·OH trapping agents)were added to AODS system，respectively.The experiment results are shown in Fig.12.Sulfur removal after adding isopropanol can reach 97.2%，while the sulfur removal after adding pbenzoquinone is only 12%，this indicates that a large number of superoxide radicals [] generated during the reaction are captured by p-benzoquinone，leads to reduce sulfur removal.The results show that[]radical is the intermediate activated product of oxidation reaction.

Fig.10.Recycling performance of Ce2(MoO4)3.

Fig.11.FT-IR spectra of Ce2(MoO4)3 before and after the reaction.

After oxidative desulfurization reaction，The Ce2(MoO4)3was washed by CCl4.The CCl4solution was evaporated by rotary evaporator to obtain oxidation products of the sulfur compounds，FT-IR characterization results of oxidation products are shown in Fig.13(a).Two infrared absorption peaks at 1292 and 1166 cm-1，which correspond to the characteristic absorption peaks of dibenzothiophene sulfone (DBTO2).In addition，the obtained CCl4solution was analyzed by GC-MS measurement.The results are shown in Fig.13(b)，the strong peak corresponding to DBTO2(m/z=216.0)at 17.604 min was found.The FT-IR and GC-MS analysis proved that DBT was oxidized to DBTO2.

In the research of aerobic oxidation of molybdenum-based catalysts (Lü et al.，2013;Ma et al.，2020;Xun et al.，2019)，molybdenum sites have mixed valence states，the conversion between different valence states leads to the production of active molybdenum species.This experiment is different from these literatures.Through the XPS characterization，it can be seen that there is only Mo6+in the Ce2(MoO4)3catalyst.Mo peroxides may be formed in the presence of oxygen.This phenomenon is very common in the oxidative desulfurization system with molybdenum-based catalyst(Jiang et al.，2019;Zhang et al.，2019).Refer to previous research(Shi et al.，2016，Zhang et al.，2019)，the mechanism of oxidative desulfurization was shown in Fig.14.First，oxygen molecules are adsorbed on the Ce3+site of the catalyst to form Ce3+-O2，and some of the Ce3+-O2further formation Ce4+-[]superoxide，then these Ce4+-[] interact with Mo sites on the catalyst to produce some active molybdenum species to further oxidize DBT compounds to DBTO2(Shi et al.，2016;Zhu et al.，2007).

Fig.12.Free radical capture experiment(Reaction conditions:20 mL model oil，O2 airflow rate=0.2L/min，T=100°C，Ce2(MoO4)3=0.05g).

Fig.13.(a) FT-IR spectrum and (b) GC-MS of oxidation products.

Fig.14.AODS mechanism in the presence of Ce2(MoO4)3.

Fig.15.AODS of diesel fuel.

3.9.AODS of diesel fuel

The aerobic desulfurization performance of Ce2(MoO4)3for diesel fuel with S-content of 150 μg/g was also investigated.It can be seen from Fig.15，under optimal conditions，the sulfur removal of 21.1% was obtained.Compared with the model oil，the sulfur removal is too low due to the complexity of components in actual diesel.After the AODS，1 mL of acetonitrile was added to the AODS system to extraction of oxidation products of sulfur，removal of sulfur compounds from diesel was reached 74.8%.When extraction desulfurization in diesel fuel was carried out by acetonitrile as extractant，sulfur removal is only 8.2%.The experimental results show that the AODS-extraction desulfurization system under the action of Ce2(MoO4)3can still remove most of the sulfide in diesel fuel.

4.Conclusions

Ce2(MoO4)3was synthesized by a simple reflux method using ammonium molybdate and cerium nitrate hexahydrate as raw materials.Ce2(MoO4)3was used as the catalyst in AODS systems.Sulfur removal of 99.6%for DBT，94%for 4，6-DMDBT and 26%for BT were obtained under the optimized conditions of 20 mL model oil，catalyst dosage of 0.05g，oxygen flow rate of 0.2L/min，100°C，respectively.The introduction of both olefin and aromatic hydrocarbon cannot significantly change the oxidative desulfurization activity of the catalyst.The superoxide radical generated by oxygen under the action of catalyst is the key factor for the oxidation of sulfide.The Ce2(MoO4)3has a strong regenerative capacity and the sulfur removal reached 94.7% after five cycles.

Acknowledgements

The authors also acknowledge the financial support of the Natural Science Foundation of Liaoning Province (2019-ZD-0064);Doctoral Fund of Liaoning Province (201501105).