Zhentao Chen,Yaxin Liu,Jian Chen,Yang Zhao,Tao Jiang,Fangyu Zhao,Jiahuan Yu,Haoxuan Yang,Fan Yang,Chunming Xu

State Key Laboratory of Heavy Oil Proceeing,China University of Petroleum,Changping,Beijing 102249,China

Keywords:Nitrogen-doped carbon materials Catalyst hydrodesulfurization (HDS)Diesel Dibenzothiophene (DBT)

ABSTRACT More stringent environmental legislation imposes severe requirements to reduce the sulfur content in diesel to ultra-low levels with high efficient catalysts.In this paper,a series of CoMo/NDC@alumina catalysts were synthesized by combination of the chemical vapor deposition of nitrogen-doped carbon(NDC)using 1,10-phenanthroline and co-impregnation of Mo and Co active components.The optimal catalyst with additive of 25%1,10-phenanthroline was screened by a series of property characterization and the hydrodesulfrization (HDS) active test.The amount of “CoMoS” active phase of the optimal CoMo/C3 catalyst increased 5.3% as compared with the CoMo/γ-Al2O3.The introduction of NDC improved the sulfidation degree of Mo by 21.8% as compared to the CoMo/γ-Al2O3 catalyst,which was beneficial to form more active sites.The HDS conversion of the NDC supported catalysts are higher than CoMo/γ-Al2O3 whether for the dibenzothiophene (DBT) or 4,6-dimethyl dibenzothiophene (4,6-DMDBT).Further hydroprocessing evaluation with Dagang diesel revealed that the CoMo/C3 catalyst possessed higher HDS property and the removal rate of DBTs in the diesel increased by 4%–11% as compared to the CoMo/γ-Al2O3 catalyst.

1.Introduction

In recent years,more stringent environmental legislation has been adopted to limit the sulfur content of diesel to an extremely low concentration.Catalysts play an important role in the purification of diesel feedstocks by hydrotreatment to produce clean fuels.Alumina and transition metals(Co(Ni)Mo(W))are usually used as the support and active metal in traditional hydrodesulfurization(HDS)catalysts,respectively.The nature of the support plays a vital role in the HDS activity of the catalyst.Jm et al.[1]had found that the dispersion and morphology of MoS2are affected by catalyst support.The γ-Al2O3provides a high surface area to maximize active phase dispersion,whereas the strong interaction between the active phase and support will retard their reducibility and sulfidability and then constrain the HDS activity [2,3].Therefore,extensive research has been devoted to improving the performance of the traditional HDS catalysts by regulating the interaction.

Except for the introduction of additives to catalysts,many new supports,such as SiO2[1],TiO2[4],carbon material [5–10],and composite support[11]have been introduced to modify the acidity of the support and promote the dispersion of the active phase.Carbon materials,including active carbon[12],carbon nanotubes[13],graphene[14],etc.have aroused great interest of researchers since their excellent pore structure and huge specific surface area.Recently,carbon-supported catalysts have been investigated and showed high HDS activity.However,fewer functional groups on the surface of carbon materials have been found to limit their application.Further study revealed that the surface of carbon materials can be functionalized by doping of heteroatoms,which gifts them special properties [15].Among them,nitrogen-doped carbon(NDC)supports exhibit great potential due to their extraordinary electronic properties.The introduction of N to carbon materials endows them with electron-donating capability,which has an attractive effect on the electrons to the metal cations [16–19] and reactants and then can improve the dispersion property of metal particles [19,20] owing to the high charge and spin density of nitrogen atom.

At present,NDC materials have been successfully used in organic synthesis,electrochemical catalysis,and other fields[21–23].However,there are relatively few studies on NDC materials for hydrodesulfurization.Hu et al.[24,25] prepared MoS2/NMC(nitrogen-doped carbon support with a mesoporous structure)catalysts,which through an in-situ self-assembly process and showed a great advantage on HDS of thiophene than pure carbon supported catalyst.Li et al.[26]synthesized a Fe based NDC@γ-Al2O3composite support catalyst.By comparison to the catalyst barely using γ-Al2O3as support,the Fe-based catalyst possessed much higher HDS activity on DBT.Although the above studies have shown an optimistic perspective of NDC-based HDS catalysts,the related research is still far immature and needs to be further developed.

In the present study,a series of CoMo/NDC@γ-Al2O3catalysts were synthesized through changing the mass ratio of 1,10-phenanthroline to γ-Al2O3from 1:2 to 1:5.The optimum content of NDC materials for the CoMo based catalyst was obtained from the HDS activity test by using model sulfur compounds of dibenzothiophene (DBT) and 4,6-dimethyl dibenzothiophene(4,6-DMDBT).Then,the HDS activity of the optimal catalyst was evaluated by using the industrial diesel as feedstock.

2.Materials and Methods

2.1.Preparation of γ-Al2O3 and NDC-coated γ-Al2O3 supports

γ-Al2O3was obtained by the calcination(550°C,6 h under static air) of commercial pseudo-boehmite with the heating rate at 2 °C·min-1.Then the prepared γ-Al2O3was crushed and sieved to obtain support particles in the range of 40–60 mesh.

NDC-coated γ-Al2O3supports were synthetized through the chemical vapor deposition(CVD)method from the previously prepared γ-Al2O3,which was impregnated by 1,10-phenanthroline(99.7% (mass),J&K Scientific Co.,Ltd.) with a mass ratio of 1,10-phenanthroline to γ-Al2O3at 1:2,1:3,1:4 and 1:5.The composite supports were sonicated for 30 minutes and dried at 110 °C for 4 h,which was followed by calcination in flowing Ar(10°C·min-1)at 800°C for 2 h.The supports with the mass ratio at 1:2,1:3,1:4,and 1:5 were denoted as S-1,S-2,S-3 and S-4,respectively.

2.2.Preparation of the catalysts

The oxide catalysts were prepared by the equal volume impregnation method with 18%(mass)MoO3and 4%(mass)CoO.A certain amount of ammonium heptamolybdate ((NH4)6Mo7O24·4H2O,Shandong West Asia Chemical Industry Co.,Ltd.)and cobalt nitrate hexahydrate (Co (NO3)2·6H2O,Shanghai Wokai Biotechnology Co.,Ltd.) with deionized water were mixed to form a clear solution.The solution was dropped on the surface of the previously prepared supports.After impregnation,all catalysts were ultrasonicated for 30 min and dried at 100 °C for 4 h,and then calcination at 550°C for 6 h under Ar(200 ml·min-1)atmosphere with a heating rate of 5 °C·min-1.In this way,the CoMo-based catalysts were obtained and abbreviated to CoMo/γ-Al2O3and CoMo/Cx(x=1–4),where x(1-4)corresponds to the S-1–S-4 obtained in the previous step.

2.3.Characterization of the catalysts

2.3.1.XRD

The D8 advance series X-ray powder diffractometer produced by Bruker in Germany was used to characterize the phase structure of the oxide catalysts and supports.The scanning range used in the experiment is 10°-90° with a scanning speed of 2 (°)·min-1,and the step size is 0.2°.

2.3.2.Nitrogen adsorption-desorption

The specific area,pore distribution,and pore structure parameters were determined by the nitrogen adsorption-desorption method using an SSA 4200 specific surface area analyzer.All samples were degassed at 200 °C for 3 h under a vacuum atmosphere and analyzed by static adsorption method under N2atmosphere at-196 °C.Pore diameter and distribution were determined by BJH method.Pore volume of micropores and mesopores were obtained by BJH adsorption cumulative curve.

2.3.3.Py-IR

Pyridine adsorption infrared spectroscopy (Py-IR) was measured using an FT3000 infrared spectrometer to obtain the acid strength and amount data of the oxide catalysts.After dried at 100 °C for 4 h,the weighed samples were pressed into tablets and sealed in the in-situ pool.Raise the vacuum to 400 °C for 4 h,and collect the spectrum when the temperature is lowered to 100 °C with the resolution at 4 cm-1.The total acid amount was obtained from the amount of pyridine desorbed above 200 °C,the amount of strong acid was obtained from the amount of pyridine desorbed above 350 °C,and the difference between the two data is the amount of weak acid.

2.3.4.H2-TPR

H2-temperature programmed reduction (H2-TPR) was carried out to measure the reduction temperature of active metals on the catalyst surface using Autosorb iQ type multifunctional physical-chemical adsorption instrument.The sample with mass of 0.1 g was pretreated at 300°C for 1 h under Ar atmosphere with flow rate of 40 ml·min-1and then cooled to room temperature,then the atmosphere was switched to H2/Ar mixture(10%Ar)with a flow rate of 40 ml·min-1.The temperature is programmed(10°C·min-1)to 1000°C and kept for 30 min.The signal is recorded by a TCD detector.

2.3.5.XPS

The metal element valence state and relative content of the active component of the catalyst were studied by X-ray photoelectron spectroscopy measurements (XPS) utilizing a Thermo Fisher K-alpha X-ray Photoelectron Spectrometer.The XPS PEAK software is used to perform peak splitting of the sulfide catalyst according to the different chemical state binding energy of Mo and Co.

2.3.6.SEM

The surface morphology of NDC-coated support and catalyst was observed by SU 8010 cold field scanning electron microscope(SEM) produced by Hitachi.

2.3.7.HRTEM

HRTEM images of sulfide catalysts were obtained on a Tecnai G2 F20 high magnification transmission electron microscope made by FEI Netherlands.The samples firstly were ground into powder and dispersed by ultrasonication in ethanol for 20 min,and a drop of supernatant liquid was placed on a micro-grid,the HRTEM analysis was performed after dried naturally.More than 300 crystallites were taken from different parts of the chosen catalysts to determine the average length (Lav) and average stacking layer number (Nav),which are calculated according to the following two equations

where liis the stripe length of MoS2,niis the number of MoS2slabs with length liand Niis the number of stacked layers of the active phase.

2.4.HDS activity test

HDS experiments of model sulfur compounds were performed in a batch stirred autoclave.The catalysts were pre-sulfided ex-situ by a solution containing a mixture of elemental sulfur and dimethyl disulfide with a mass ratio of 2:3.The catalysts were immersed in the autoclave at 140°C for 1 hour and then heated to 350°C for 1 hour under 3 MPa.After presulfidation,the solutions of DBT/4,6-DMDBT in decalin containing 500 μg·g-1S were introduced to replace the pre-sulfided feedstocks.The HDS reaction conditions were kept at 300 ℃and 6 MPa for DBT,while 320 ℃and 7 MPa for 4,6-DMDBT.

HDS evaluation of Dagang diesel (the properties of the diesel feedstock were summarized in Table S1 in supporting information)was performed in a fixed-bed microreactor with an inner diameter of 10 mm and a length of 550 mm.Firstly,a volume of 6 ml catalyst with 250–380 μm in size was loaded in the middle zone of the reactor and sandwiched by quartz sand of the same size.Secondly,the oxide catalysts were presulfided by the solution of 2.5%(mass)CS2in hydrogenated diesel under the following two stages.After reaching 230 °C,the presulfided reaction maintained for 10 h under 4 MPa,1 h-1liquid hourly space velocity(LHSV),H2/Oil(volume ratio of hydrogen to feed) of 200,and then the temperature was changed to 320 °C to perform the second stage of sulfidation for 10 h with the same pressure and feeding conditions.After sulfidation,Dagang diesel was pumped into the reactor and then the activity of the catalysts was tested under 6 MPa,H2/Oil=200;1,1 h-1and with temperature shifting from 300 °C to 360 °C.After the reaction,the sulfur content of the HDS product was measured by the ANTEK7000NS sulfur and nitrogen analyzer (America).Duplicate samples of the HDS experiment under each condition were collected and subjected to sulfur determinations to ensure the accuracy of the results.To deeply analyze the transformation of various sulfur compounds in diesel,the products were analyzed by GC-SCD to obtain the compositions.

The HDS conversion was defined as:

where Sfand Spare the sulfur content of the diesel in feedstock and product,respectively.

3.Results and Discussion

3.1.XRD

The XRD results of supports and oxide catalysts are shown in Fig.1 (a),(b).The supports exhibit three well-resolved diffraction peaks indexed to (311),(400),and (440) planes,respectively,which attributes to γ-Al2O3.After being coated with NDC,the existence of the aforementioned diffraction peaks but no diffraction peaks of carbon materials indicates that the addition of NDC does not change the crystal structure of γ-Al2O3[26].By comparison with the supports,the oxide catalysts have an extra diffraction peak at 26.5°,which is attributed to CoMoO4.And the peak intensity of CoMoO4increases with the NDC content.In addition,the MoO2peaks of CoMo/C1 and CoMo/C2 at both 2θ=26.5° and 53°represent NDC can reduce Mo6+to Mo4+at high temperatures.MoO3peak is not found in the spectra for all the NDC-coated catalysts,indicating that MoO3does not agglomerate to form large particles.

Fig.1.XRD spectra of supports (a) and oxide catalysts (b).

3.2.Nitrogen adsorption-desorption

The N2physisorption isotherms of the supports and oxide catalysts are depicted in Fig.2.All the samples exhibit type IV isotherms with H4type hysteresis loops at 0.7 ＜P/P0＜1,which corresponds to the mesopore structure.After the active components are loaded,the adsorption and desorption isotherm tend to flatten,and the hysteresis loop shifts to the direction of decreasing P/P0,which indicates the loss of pore volume due to the active metal deposited and the collapse of the pores by multiple calcinations.

Fig.2.N2 adsorption-desorption isotherms of the supports (a) and oxide catalysts (b).

The detailed textural properties of the supports and oxide catalysts calculated using the BJH method (dBJH) are summarized in Table 1.The results show that the specific surface area of γ-Al2O3decreases after NDC coating.The slight variation on total pore volume indicates that the NDC is coated on the surface of porous γ-Al2O3instead of entering the tunnels.The generally increase in the average pore size of the NDC coated γ-Al2O3might result from the collapse of the pore structure by calcination.The decrease in the average pore diameter of CoMo/C1 might due to the excessive carbon deposition.

After metal impregnation,both the CoMo/γ-Al2O3and NDCcoated catalysts represent a similar downward trend in the specific area,total volume,and average pore diameter.The NDC-coated catalyst has a greater reduction in most pore properties than the CoMo/γ-Al2O3catalyst,which is also consistent with the conclusion obtained from the N2-adsorption desorption isotherms.

3.3.Py-IR

The acidities and types of acid sites can be detected by the Pyridine-IR method,and the corresponding spectra are shown in Fig.3.The bands centered at～1540 cm-1and 1635 cm-1are the characteristic signals of the Br?nsted acid sites (BAS),while the adsorption peaks at～1448 cm-1,1575 cm-1,1610 cm-1,and 1622 cm-1are assigned to the Lewis acid sites(LAS).Furthermore,the band at 1492 cm-1is attributed to the pyridine molecules bound to both of the Br?nsted and Lewis acid sites [27].The contents of the Br?nsted and Lewis acid are summarized in Table 2.It showed that the amount of LAS increases significantly after the NDC material is introduced.

Table 1 Textural properties of supports and oxide catalysts

Table 2 Amounts of acid sites of oxide catalysts obtained from Py-FTIR

3.4.SEM

The surface morphology of the supports and oxide catalysts observed by means of SEM are displayed in Fig.4 (a)-(d).The filamentous structure marked by red circle (c) and (d) is N-doped carbon layer coating in γ-Al2O3,which is consistent with the results in literature [26].The surface of γ-Al2O3is clearly changed after being coated NDC,which evidences the NDC materials were successfully deposited.After loading the metal oxides,the filamentous NDC materials still existed on the surface of the catalyst,indicating that the loading of metal oxides do not damage the structure of NDC materials.

3.5.H2-TPR

H2-TPR is used to characterize the reducibility of metal oxide precursors and determine the interacted strength between the active phase and the support.As shown in Fig.5,there are two strong H2reduction peaks in the range of 450–500 °C and 750–800 °C for the pure γ-Al2O3supported catalyst.The first reduction peak is located at around 463°C,which is attributed to the octahedral Mo6+being reduced to Mo4+.The second reduction peak is located at around 793 °C,which is attributed to the secondary reduction peak of polymer octahedron,tetrahedron,and massive molybdate species(Mo4+)being deeply reduced to Mo0[6].In addition,no reduction peaks belonging to large particles of MoO3are found in the range of 600–630 °C,which is consistent with the results of XRD spectra.All the catalysts with coating NDC have a strong H2reduction peak around 410°C attributable to Mo6+being reduced to Mo4+.Compared with the CoMo/γ-Al2O3,the peak position of the NDC-coated catalysts is shifted to a lower temperature by about 53 °C together with greatly reduced peak intensity,indicating that the reduction temperature of Mo6+is decreased significantly.This phenomenon results from a weaker interaction between the Mo oxide species and the composite support.In addition,the secondary reduction peak of Mo4+being reduced to Mo0in CoMo/Cxat high temperature is almost negligible,and a negative peak is generated around 870–890 °C.As Nikulshin [28] pointed out,the negative peak could appear as a result of the carbon material pyrolysis at temperatures higher than the synthesis temperature of carbon-coating supports.

Fig.3.Py-IR spectra of oxide catalysts.

3.6.XPS

3.6.1.Oxide catalyst

The structure and electron interactions of the active phase of N sites in the oxide catalyst are characterized by XPS,in which the content of N is determined to be 0.347%.As shown in Fig.6,three types of nitrogen components in CoMo/C3 can be deconvoluted from the spectrum,namely,pyridine nitrogen (binding energy of 398.0 eV),pyrrole nitrogen(binding energy of 399.7 eV),and nitrogen oxide(binding energy of 404.0 eV)[29,30],which accounts for 77%,18%,and 5%,respectively.The results reveal that nitrogen atoms are successfully incorporated into carbon materials,which is consistent with the EDS and Mapping drawn by SEM (Fig.S1 and Fig.S2 in the Supplementary Material).

Fig.4.SEM images of supports and oxide catalysts.(a)γ-Al2O3,(b) CoMo/γ-Al2O3,(c) S-3,(d) CoMo/C3.

Fig.5.H2-TPR profiles of oxide catalysts.

Fig.6.XPS N 1s spectra of the oxide CoMo/C3 catalyst.

3.6.2.Sulfide catalysts

The chemical species represent on the surface of the sulfide catalysts are evaluated and the Mo 3d and Co 2p XPS spectra of all the samples were shown in Fig.7.In Mo 3d spectra,the binding energy of Mo 3d5/2and Mo 3d3/2of Mo4+are around(228.4±0.1)eV and(232.0 ± 0.1) eV,respectively;the binding energy of Mo 3d5/2and Mo 3d3/2of Mo5+are around(230.5±0.1)eV and(233.6±0.1)eV,respectively;the binding energy of Mo 3d5/2and Mo 3d3/2of Mo6+are around(232.5± 0.1) eV and (235.6± 0.1) eV,respectively.In S 2 s spectra,the binding energy at (226.1 ± 0.1) eV is assigned to sulfides in the form of S2-[31].For Co 2p spectra,the peak of the binding energy at 778.3–778.9 eV is assigned to “CoMoS”,which is regarded as the active phase in HDS reaction [32,33];the peak at a binding energy of 777.6–778.1 eV is characteristic of Co9S8,which is an inert species produced during catalyst sulfurization;the peak at 780.8–781.5 eV corresponds to the un-sulfide oxidized species (mainly CoO) [34].

Fig.7.Mo 3d and Co 2p XPS spectra of sulfide catalysts.(a)Mo 3d spectra of CoMo/γ-Al2O3;(b)Mo 3d spectra of CoMo/C1;(c)Mo 3d spectra of CoMo/C2;(d)Mo 3d spectra of CoMo/C3;(e)Mo 3d spectra of CoMo/C4.(f)Co 2p spectra of CoMo/γ-Al2O3;(g)Co 2p spectra of CoMo/C1;(h)Co 2p spectra of CoMo/C2;(i)Co 2p spectra of CoMo/C3;(j)Co 2p spectra of CoMo/C4.

The calculation results of spilt peak fitting of Mo 3d and Co 2p spectra are shown in Table 3.It reveals that the SMo(the sulfidation degree of Mo)increases first and then decreases with the reduction of NDC content and follows the order of CoMo/C3 ＞CoMo/C2 ＞Co Mo/C4 ＞CoMo/C1 ＞CoMo/γ-Al2O3.The SMoof the CoMo/C3 catalyst has reached 70.3%,which is higher than that of CoMo/γ-Al2O3by 21.8%.Similarly,from the results of Co 2p spectra,the amount of “CoMoS” in CoMo/C3 is 5.3%higher than CoMo/γ-Al2O3.

Table 3 XPS results of series of sulfide catalysts

The exact mechanisms of electronic effect for different N site types in NDC materials on the active metal are not fully understood.The electron donating behavior between the nitrogen atom and the platinum atom have been found to greatly improve the catalytic performance of the noble metal catalyst [35,36].Compared with the Pt-N bond(0.228 nm and 1.70 eV),the shorter bond distance and higher bonding energy of the Co-N bond (0.181 nm and 6.9 eV) indicates nitrogen atoms have the potential of electron-donating-accepting between Co and N atoms[37].Therefore,the introduction of NDC to CoMo/γ-Al2O3catalysts increases the sulfidation degree of Mo and the amount of “CoMoS”,indicating that the addition of NDC can weaken the interaction between the support and the active components,and tends to increase the sulfidation degree of the active metals [26].At the same time,the electron-donating effect of the nitrogen atoms facilitates the sulfidation of Co atoms to form more “CoMoS” active phase.

Fig.8.Representative HRTEM images of the sulfide catalysts.(a) CoMo/γ-Al2O3;(b) CoMo/C1;(c) CoMo/C2;(d) CoMo/C3;(e) CoMo/C4.

3.7.HRTEM

The morphology and dispersion state of MoS2on the surface of the sulfide catalysts is obtained by HRTEM.And the TEM images and calculated statistical results are shown in Fig.8 (a)-(e),Fig.9 and Table 4,respectively.It can be found that after loading NDC materials,the number of stacked layers of MoS2on the catalyst surface increased significantly.Comparison the results shows the number of stacked layers increase firstly and then decrease with the reduction of carbon content.The proportion of MoS2stripe with large length increases firstly and then decreases among the NDC materials,in which CoMo/C3 possesses the maximum number of long stripes of MoS2,which is consistent with the result of the number of stacking layers.

Table 4 Average stacking layer number and average layer length of sulfide catalysts obtained from HRTEM

Table 5 Effect of temperature on HDS of representative sulfur compounds in diesel

3.8.HDS activity

The catalysts are all performed in HDS reaction by using DBT or 4,6-DMDBT model compounds and the results are shown in Fig.10.It exhibits that the HDS reactivity of DBT and 4,6-DMDBT in the catalysts follows the same order of CoMo/C3 ＞CoMo/C4 ＞CoMo/C2 ＞CoMo/C1 ＞CoMo/γ-Al2O3.Obviously,the HDS conversions of DBT or 4,6-DMDBT in NDC-coated catalysts are all higher than that in pure γ-Al2O3supported catalyst.The NDC coated catalysts enhances the HDS conversion of DBT by 5.7%-11.1%.Comparing with CoMo/γ-Al2O3,NDC-coated catalysts have the higher sulfidation degree and more amount of “CoMoS”,which is believed to be the active phase in HDS process.By comparison,it reveals CoMo/C3 has the highest conversion for HDS reaction.The introduction of NDC enlarges the pore diameter,which is conducive to the HDS reaction [27,38].Li et al.[26] synthesized the composite catalysts by using Fe and Zn as the active metal and nitrogen-doped carbon(NDC) coated alumina as the support.Comparison the data shows that the NDC base catalysts of our study possess much larger HDS activities on DBT,which can be deduced from the lower reaction temperature adopted therein.

4,6-DMDBT is known to be more difficult to remove than DBT because of steric hindrance caused by methyl groups.After partially hydrogenated to saturate the aromatic ring,4,6-DMDBT can achieve sulfur removal by increasing the accessibility to the active center of the catalyst.It is known that BAS is good for removing sulfur,whereas LAS is good for hydrogenation[38].Therefore,better sulfur removal performance of 4,6-DMDBT in NDC materials partly results from the increased of the LAS amounts,which is beneficial for hydrogenation.The appearance of negative peaks in H2-TPR spectra indicates that NDC material has the function of storinghydrogen [39,40].The NDC coated catalysts can provide part of hydrogen for the reaction and then further improve the HDS conversion when the reaction takes place in a hydrogen-deficient atmosphere.

Fig.9.HRTEM results of the sulfide catalysts.

Fig.10.Model compound hydrodesulfurization results.

To further investigate the catalytic activity of the CoMo/C3 and CoMo/γ-Al2O3catalysts,the HDS reactions are performed by using Dagang diesel as feedstock.As shown in Fig.11,the CoMo/C3 catalyst also represents more HDS active on the diesel feedstock than that on the CoMo/γ-Al2O3catalyst.Comparison the results shows that the HDS conversion of diesel in the CoMo/C3 catalyst at 320°C and 340°C approached to that in the CoMo/γ-Al2O3catalyst at 340°C and 360°C,respectively.It indicates that the introduction of NDC to the conventional catalyst can lower the reaction temperature for the similar HDS conversion.

Fig.11.HDS results of Dagang diesel.

In order to obtain the transformation of sulfur compounds,Dagang diesel feedstocks and the HDS products are analyzed by GC-SCD and the chromatograms are illustrated in Fig.12 and Fig.13,respectively.As shown,benzothiophene (BT) and dibenzothiophene (DBT) as well as their alkyl-substituted derivatives are dominant in the diesel.According to the qualitative method of previous studies[41,42],the BTs and DBTs in diesel are classified into four and three categories,which are C1,C2,C3,C4+and C1,C2,C3+,respectively.Comparing the GC-SCD results of different products,it reveals that the HDS conversion of various sulfur compounds (BTs,DBTs) increases with the temperature.And most of the BTs compounds are removed when the temperature reached 360°C.However,part of C2+-DBT compounds behaves less reactive and more difficult to be removed.

Fig.12.GC-SCD spectra of diesel.

Fig.13.GC-SCD spectra at 300 ℃and 360 °C.

Comparison the HDS results of the two catalysts in Table 5,it shows that the removal rate of various sulfur compounds in CoMo/C3 is higher than that in CoMo/γ-Al2O3.Especially,the CoMo/C3 catalyst increases HDS conversion of DBTs compounds in diesel by～11%as compared to the CoMo/γ-Al2O3catalyst,which indicates that the former favors to remove the less active DBTs compounds.By combination of the catalyst characterization mentioned above,it indicates that the introduction of NDC materials to the catalyst can improve the HDS reactivity by adjusting the electron-donating interactions between the support and the metals,which leads to more efficient active phase provided by the NDC-coated catalysts.

4.Conclusions

A series of NDC coated CoMo catalysts are prepared by chemical vapor deposition of nitrogen-doped carbon (NDC) using 1,10-phenanthroline.The characterization results of XRD,N2physisorption,Py-FTIR,XPS,H2-TPR,SEM,and HRTEM reveals that the introduction of NDC can reduce the interaction between the support and active components,which improves the dispersion of the active phase and increases the sulfidation degree of Mo by 21.8%.The results of HDS tests show that γ-Al2O3-NDC composite catalysts exhibit more activity on both model compounds and diesel fuel than the γ-Al2O3supported catalyst.Especially,the CoMo/C3 catalyst with the optimum carbon content increases HDS conversion of DBTs compounds in diesel by～11% as compared to the CoMo/γ-Al2O3catalyst,indicating the former favors to remove the less active DBTs compounds.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the supports by National Natural Science Foundation of China (NSFC) (Nos.21878329 and 21476257),the National Key Research and Development Program Nanotechnology Specific Project (No.2020YFA0210900) and Science Foundation of China University of Petroleum,Beijing (No.2462018QZDX04).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.09.015.