Zi-Zheng Xie,Meng Zhang,Xing-Bao Wang*,Liang Guo,Zhen-Yi Du,Wen-Ying Li*

State Key Laboratory of Clean and Efficient Coal Utilization,Taiyuan University of Technology,Taiyuan 030024,China

Key Laboratory of Coal Science and Technology (Taiyuan University of Technology),Ministry of Education,Taiyuan 030024,China

Keywords:Oxygen-containing compound Hydrogenation Deoxygenation Catalysis Computational chemistry

ABSTRACT The low-temperature coal tar contains a considerable number of oxygen-containing compounds,which results in poor quality.The catalytic hydrodeoxygenation of oxygen-containing compound to an added-value chemical compound is one of the most efficient methods to upgrade coal tar.In this study,density functional theory calculations are employed to assess and analyze in detail the hydrodeoxygenation of dibenzofuran,as a model compound of coal tar,on the Ni(1 1 1)surface.The obtained results indicate that dibenzofuran can be firstly hydrogenated to tetrahydrodibenzofuran and hexahydrodibenzofuran.The five-membered-ring opening reaction of tetrahydrodibenzofuran is more straightforward than that of hexahydrodibenzofuran (Ea=0.71 eV vs.1.66 eV).Then,both pathways generate an intermediate 2-cyclohexylphenoxy compound.One part of 2-cyclohexylphenoxy is hydrogenated to 2-cyclohexylphenol and consecutively hydrogenated to cyclohexylcyclohexanol,and another part is directly hydrogenated to cyclohexylcyclohexanone.The hydrogenated intermediates of 2-cyclohexylphenol have higher deoxygenation barriers than 2-cyclohexylphenol and cyclohexylcyclohexanol.During the hydrogenation process of cyclohexylcyclohexanone to cyclohexylcyclohexanol,the intermediate 26,formed by adding H to O atom of cyclohexylcyclohexanone,exhibits the lowest deoxygenation barrier of 1.08 eV.High hydrogen coverage may promote the hydrogenation of tetrahydrodibenzofuran,hexahydrodibenzofuran,and intermediate 26 to generate dodecahydrodibenzofuran and cyclohexylcyclohexanol.This dibenzofuran hydrodeoxygenation reaction mechanism corroborates well with previous experimental results and provides a theoretical basis for further optimization of the design of nickel-based catalysts.

1.Introduction

Low-temperature coal tar is a by-product of coal pyrolysis at 450–700 °C [1].The oxygen-containing compounds are relatively abundant in the coal tar [2,3],making it highly corrosive,with a low calorific value and poor stability.Hydrodeoxygenation can be used to refine the low-temperature coal tar[4],preserving its cyclic structure and yielding high value-added organic chemicals.The hydrodeoxygenation of the low-temperature coal tar requires the use of catalysts.The low-temperature coal tar contains dibenzofuran(DBF),a compound that does not deoxidize readily.Thus,it can be used as a typical model compound to evaluate the catalyst hydrodeoxygenation performance [5].

Pt is one of the most commonly used noble metal catalysts for the hydrodeoxygenation [6–8],but its deoxidation ability is hampered by poisoning with sulfur-containing compounds [9].Ni catalysts have recently attracted increasing attention [10,11].We have previously explored the effect of Ni/SBA-15 catalyst on DBF hydrodeoxygenation [12,13].The main reaction products are dodecahydrodibenzofuran (DHDBF),cyclohexylcyclohexanol(CHCHOH),and bicyclohexane(BCH);however,the yield of deoxygenation product (BCH) is low.Doping of the Ni/SBA-15 catalyst with Fe enhances its oxygen affinity,which increases the BCH yield up to 83%.These results indicate that the Ni-based catalyst has excellent potential for hydrodeoxygenation.However,since separation and detection of radical intermediates during reaction are challenging,the reaction mechanism is still illusive,which adversely influences the optimization of Ni catalysts.Thus,it is very important to explore the hydrodeoxygenation of DBF on the Ni surface further.

The hydrodeoxygenation mechanism on the metal surface can be investigated by density functional theory (DFT) calculations from the atomic level.Yanget al.[14] studied phenol deoxidation on different Ni surfaces,finding the lowest activation barrier on the defect Ni(2 1 1)surface.This result indicated that unsaturated Ni atoms promote the C-O bond cleavage.The smaller the Ni particle size,the more favorable the phenol deoxygenation.Zhanget al.investigated the influence of doping of Ni with different single-metal atoms on acidic support on the phenol deoxygenation[15].Ni atoms can both activate the transformation of hydrogen molecules to radicals and strongly adsorb phenol.This promotes the protonation of the phenol hydroxyl group,and thus,the deoxygenation.Zhouet al.[16] carried out a comprehensive theoretical investigation on the deoxygenation effect of Ni(1 1 1)surface doping with different oxyphilic single atoms (M).The M-doping changes the local electronic structure and increases the d-band center of a Ni-M-Ni site,which enhances the phenol adsorption and promotes the deoxygenation.

The hydrogenation and deoxygenation studies of monocyclic benzene [17],phenol [18,19],and guaiacol [20] on the Ni surface was performed by DFT calculations.However,there are only a few reports about the hydrodeoxygenation mechanism of polycyclic DBF.The hydrodeoxygenation mechanism of DBF on Ni(1 1 1)surface will be explored by DFT calculations and combined with the previous experimental results [12,13].The dominant pathways and rate-determining steps in the hydrodeoxygenation network will be clarified.This study is expected to provide a theoretical basis for the Ni catalyst design.

2.Computational Models and Methods

The Ni(1 1 1)surface was modeled on ap(6×6)supercell.The model was made as three-layer slabs,and the height of the vacuum layer was 2.5 nm.Its bottom layer was fixed into bulk positions,the upper two layers and molecules were allowed to relax.As shown in Fig.1,carbon atoms in DBF are numbered clockwise.

According to the previous experiments [12,13,21],DBF can easily convert to tetrahydrodibenzofuran (THDBF) under high hydrogen pressure.Thus,in this study,the conversion of DBF to THDBF was not additionally explored.The previous studies[22,23] showed a low barrier for the H diffusion across the Ni(1 1 1) surface.This enables the fast H diffusion,so H atoms do not affect the DBF hydrodeoxygenation.

The Dmol3package was used to obtain perform the calculations[24,25].The spin-unrestricted method was used in this study.The calculation parameters were similar to those used in the previous research[26],as shown in supplementary material S1.The calculation formulas of adsorption energies (Eads),reaction energies(ΔErxn) and activation energies (Ea) were also in supplementary material S1.In all potential energy profiles,the energies of intermediates reference to THDBF and H2in gas.A*in all figures representsthe adsorption state of species A on the surface.

3.Results and Discussions

3.1.Ring-opening and hydrogenation pathways of THDBF and hexahydrodibenzofuran (HHDBF) on the Ni (1 1 1) surface to form 2-cyclohexylphenol (CHPOH)

During the reaction,THDBF may undergo a ring-opening reaction or hydrogenation to HHDBF.Finally,CHPOH was generated.Two possible pathways are shown in Fig.2.

Fig.1.The enumeration of C atoms (according to IUPAC nomenclature) in DBF.

The THDBF adsorption configurations on the Ni (1 1 1) surface were constructed and optimized.Four adsorption configurations of THDBF are listed in Table S1.When the benzene ring is adsorbed on the Fcc or Bri0 site[27],their C5a and C9a atoms cannot adsorb on the Ni atom,and it is not conducive to the subsequent ringopening reaction.The configuration of THDBF adsorbed on the Bri30 site is selected (Fig.3),with an adsorption energy of-0.79 eV.C1 and C4 form σ-bonds with metal atoms.C2 and C3 share a Ni atom in the π-bond mode,as well as C4a and C9b.The distance between C and corresponding Ni atoms is about 0.205–0.214 nm,similar to the Ni-C bond length of the phenol adsorption configuration on the Ni (1 1 1) surface (0.203–0.221 nm)[14].Along the reaction coordinate,the distance of C5a-O gradually increases from 0.145 to 0.201 nm,while the C4a-O bond decreases from 0.139 to 0.136 nm.The O atom slowly moves down to the surface.In the final state (2),the distance of C5a to O is 0.295 nm,and the O atom finally adsorbs on the nearby Ni atom.The reaction shows an exothermic nature (ΔErxn=-0.21 eV),with a barrier of 0.71 eV.

The hydrogenated intermediates HHDBF still adsorb on the Bri30 site,which is the most stable configuration (Table S2).Its adsorption energy is-1.02 eV,higher than that of THDBF.We used the energetic method developed by Hammer [28].The HHDBF adsorption energy can be divided into three parts:the change of deforming energy of catalyst (0.28 eV),the change of deforming energy of HHDBF (1.22 eV),and the change of interaction energy between catalyst and HHDBF (-2.52 eV) from 7 to TS(7–4).For THDBF,they are 0.36,1.88,and -2.95 eV,respectively.Although the interaction energy change between catalyst and HHDBF is low,the deforming energy change of HHDBF is little.Therefore,the HHDBF adsorption energy on the Ni (1 1 1) surface is higher.In the ring-opening transition state(TS(7–4)),C5a-O bond is elongated,and the hydrogenated ring moves down to the surface.In the final state (4),the hydrogenated ring is vertically adsorbed on the surface via C5a,while O is adsorbed on the Ni atom.The reaction is almost thermally neutral (ΔErxn=0.02 eV),with a barrier of 1.66 eV.

The HHDBF ring-opening barrier is higher than that of THDBF,which may originate from two factors.First,the bond length of C5a-O in THDBF is closer to that of HHBDF (0.145 vs.0.147 nm).This means the C5a-O bond is activated in 1.Second,when the THDBF ring opens,the C5a=C9a bond in the partially hydrogenated ring can be adsorbed on the Ni atom,and the transition state can be stabilized.

If O atoms of intermediates 2,3,and 4,formed by the ringopening of THDBF and HHDBF,are hydrogenated first,a higher energy will be required.Their energy barriers are 1.91,1.86,and 1.58 eV,respectively.It is more favorable if C atoms of these intermediates are hydrogenated first,since their energy barriers are 1.11,1.46,and 1.01 eV,respectively,as shown in Fig.4.We calculated the Mulliken charge of partial atoms of these intermediates.In Table S3,all Ni atoms that interact with O atoms are positive.When O atoms are hydrogenated,positively charged H atoms could be repelled by Ni atoms.This yields unstable transition states and high energy barriers.

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Potential energy profiles for the ring-opening and hydrogenation pathways of THDBF and HHDBF to CHPOH are compared in Fig.4.Their rate-determining steps are hydrogenation (3 →4,Ea=1.46 eV)and ring-opening(7 →4,Ea=1.66 eV)steps.Thus,the ring-opening and hydrogenation pathways of THDBF are dynamically more favorable.However,the barrier of THDBF hydrogenation is 0.92 eV,slightly higher than that of the ring-opening(0.71 eV) reaction.If H coverage is high enough,HHDBF might be generated,but its ring-opening will be difficult.The result is consistent with the experimentally observed phenomenon that the HHDBF content was high in the initial reaction stage [21].

Fig.2.The proposed reaction scheme for the ring-opening and hydrogenation pathways of THDBF and HHDBF on the Ni (1 1 1) surface.

Fig.3.Side and top views of the (a) THDBF and (b) HHDBF ring-opening reactions on the Ni (1 1 1) surface.

3.2.Hydrogenation of HHDBF to DHDBF and ring-opening of DHDBF

HHDBF can be hydrogenated to DHDBF (Fig.5).The relative energies of intermediates with different hydrogenation degrees are calculated as described in Table S4.The results showed that the phenyl ring is consecutively hydrogenated.The order is C3C4C2C4aCC9bC1 (Fig.5 and Fig.S1),unlike the nonconsecutive hydrogenation of phenol and benzene on the Pt (1 1 1) surface[29,30].The benzene hydrogenation on the Ni (1 1 1) surface was studied by Mittendorfer [17].The 1,3-cyclohexadiene but not 1,4-cyclohexadiene was the main product after the second hydrogenation step,which indicates that only a consecutive hydrogenation path for HHDBF on the Ni(1 1 1)surface occurred.Potential energy profiles of hydrogenation of HHDBF to DHDBF are shown in Fig.6.The rate-determining step is the first step(7 →9),with a barrier of 1.78 eV,which is slightly higher than that for the HHDBF ring-opening(1.66 eV).The results show that high H coverage can also promote the hydrogenation of HHDBF.The energy barrier of rate-determining step (7 →9) reduces form 1.78 eV to 1.53 eV(Fig.S2).This is similar to the previous studies[31,32].

DHDBF formed in above hydrogenation pathway is horizontally adsorbed on the surface with an adsorption energy of-0.22 eV.Its O atom is positioned far away from surface metal atoms,so the ring-opening reaction is impossible.The vertical configuration of DHDBF was chosen,and the distance of O to the nearest surface Ni atom was 0.284 nm,with its adsorption energy of -0.30 eV.The DHDBF ring-opening is shown in Fig.S3,being endothermic(ΔErxn=0.11 eV) with an activation energy of 1.93 eV.Thus,the high activation energy of the DHDBF of the ring-opening reaction indicates its low probability,which is consistent with the previous experiment [12].

Fig.4.Potential energy profiles of the ring-opening and hydrogenation pathways of THDBF (blue) and HHDBF (black) to CHPOH on the Ni (1 1 1) surface.

Fig.5.The consecutive hydrogenation scheme of HHDBF to DHDBF and ring-opening of DHDBF on the Ni (1 1 1) surface.

Fig.6.Potential energy profiles of hydrogenation of HHDBF to DHDBF (black line) and ring-opening of DHDBF (blue line) on the Ni (1 1 1) surface.

3.3.Hydrogenation of cyclohexylphenoxy to cyclohexylcyclohexanol(CHCHOH) and 2-cyclohexylcyclohexanone (CHCHO)

Cyclohexylphenoxy is stably adsorbed on the Ni(1 1 1)surface,with adsorption energy of -2.03 eV.It may be hydrogenated to CHPOH with a barrier of 1.25 eV,being slightly endothermic(ΔErxn=0.05 eV).CHPOH is adsorbed in the Bri30 configuration,but its adsorption is weaker(ΔEads=-0.77 eV).Therefore,the phenyl ring of cyclohexylphenoxy might be directly hydrogenated,similar to the process of phenoxy hydrogenation to ketone [27].The hydrogenation of cyclohexylphenoxy on O and C3 (Fig.S4) is represented by two different pathways.

Cyclohexylphenoxy can be hydrogenated firstly to CHPOH and then to CHCHOH(Fig.7,the path I).The potential energy and barrier of intermediates formed by incorporating hydrogen atoms to the phenyl ring of CHPOH were calculated (Table S5).The adsorption energy of the C3 hydrogenated product of CHPOH is 0.06 eV lower than that for C4,and the corresponding barrier is 0.41 eV.Yoonet al.studied the first hydrogenation step of phenol by DFT theory[33],showing that the ortho C of hydroxyl group was easier hydrogenated.This difference might be caused by the steric hindrance between the cyclohexyl of CHPOH and Ni atom.CHPOH was hydrogenated via a consecutive pathway,similar to that for HHDBF.

Cyclohexylphenoxy can be also consecutively hydrogenated to CHCHO (Fig.7,II),and the hydrogenation order of phenyl ring is C3C4C2C1C9b.After C3C4C2 is hydrogenated,the O atom is still adsorbed on the Ni atom.If C4a is hydrogenated,the steric hindrance will be considerable.Thus,the C1 is hydrogenated(Fig.S5a),which is different from the hydrogenation order of HHDBF and CHPOH.Comparing the potential energy profiles of these two pathways (Fig.8),the rate-determining steps are the hydrogenation of 16 →18 and 22 →24,respectively.Their barriers were 1.35 and 1.51 eV,respectively.The barrier of the ratedetermining step of path II is slightly higher.However,the potential energy of intermediate 19 is higher than that of CHCHO.When CHCHO is generated,a higher amount of reaction energy will be released,so both paths are possible.Moreover,CHCHO might be hydrogenated to CHCHOH under high hydrogen coverage.The hydrogenation order is OC4a,and the potential energy profile is also shown in Fig.8 (green lines).Their barriers are 0.97 and 1.07 eV,respectively.The structures of C4a hydrogenation are shown in Fig.S5b.

3.4.Deoxygenation of CHPOH and related hydrogenation intermediates

In the reaction-network,CHPOH and its hydrogenated intermediatesmight be deoxygenated.The related deoxygenation barriers and structures are listed in Table 1 and Fig.S6.The deoxygenation of CHPOH is endothermic(ΔErxn=0.63 eV)with an activation barrier of 1.71 eV.All consecutive hydrogenation intermediates,16,17,and 18,have deoxygenation were about 2 eV,which is higher than that of CHPOH.Therefore,the consecutive hydrogenation degree of CHPOH has a little effect on the deoxygenation of intermediates.

In order to compare with the previous results [29],we studied the deoxygenation of nonconsecutive hydrogenation intermediates,28,29,30,and 31.Their deoxygenation barriers and reaction energies are reduced with the increase of hydrogenation degree.However,only the energy barrier of intermediate 31 is lower than that of CHPOH.The partially hydrogenated intermediate 26 exhibitsthe lowest deoxygenation barrier (1.08 eV),being slightly endothermic (ΔErxn=0.13 eV).The CHCHOH barrier is increased to 1.84 eV,and the corresponding reaction energy is 0.23 eV.Its deoxygenation barrier is lower than that of most of the intermediates,but closer to CHPOH.These results indicate that if the conjugation effect of the benzene ring of CHPOH is wholly destroyed,the deoxygenation barrier will be reduced.However,as shown in Table S5,the barriers of nonconsecutive hydrogenation are relatively high,which may be caused by the unstable adsorption of intermediates on the Ni (1 1 1) surface.

Table 1Reaction energies (ΔErxn) and activation barriers (Ea) for deoxygenation of CHPOH and its hydrogenated intermediates on the Ni (1 1 1) surface

3.5.HDO pathway of DBF on the Ni (1 1 1) surface

As shown in Fig.9,a possible DBF hydrodeoxygenation mechanism on the Ni (1 1 1) surface is proposed based on DFT calculations.DBF is firstly hydrogenated to THDBF and HHDBF.The ring-opening and hydrogenation reactions of both THDBF and HHDBF are competitive processes,and H coverage can affect these equilibria.Enhancing the H coverage may promote further hydrogenation of THDBF and HHDBF.HHDBF is further hydrogenated to DHDBFviaa consecutive hydrogenation pathway.DHDBF cannot be stably adsorbed on the Ni(1 1 1)surface.Consequently,the corresponding barrier for the ring-opening reaction is the highest in the reaction network.The yield of DHDBF experimentally obtained is high (40.0%) [12,13],so these two results can be mutually verified.

Cyclohexylphenoxy and CHPOH are generated after the ringopening of THDBF and HHDBF.Cyclohexylphenoxy can be stably adsorbed on the Ni(1 1 1)surface because of the strong interaction between Ni and O atom.Cyclohexylphenoxy can be consecutive hydrogenated to CHCHO and then hydrogenated to CHCHOH.In this process,the partially hydrogenated intermediate 26 will be generated,which deoxygenation barrier is the lowest.The deoxygenation barriers of CHPOH hydrogenation intermediates are higher than those of CHPOH and CHCHOH.

If cyclohexylphenoxy is firstly hydrogenated to CHPOH and then hydrogenated to CHCHOH,only CHPOH and CHCHOH can be deoxygenated in this pathway.However,their deoxygenation reactions are difficult.If the cyclohexylphenoxy is directly hydrogenated to CHCHO,the deoxygenation can be carried out via the intermediate 26.However,the hydrogenation of cyclohexylphenoxy is hard to control on the Ni(1 1 1)surface,so the experimental yield of CHCHOH was also high (24.2%) [12,13].Enhancing the oxygen affinity of the Ni surface can promote the ring-opening ofTHDBF and HHDBF and can also promote the hydrogenation of cyclohexylphenoxy to CHCHO,increasing the BCH yield.

Fig.7.The proposed reaction scheme for hydrogenation of the intermediate (5) to CHCHOH (I) and CHCHO (II) on the Ni (1 1 1) surface.The dashed line represents the possible hydrogenation of CHCHO to CHCHOH.

Fig.8.Potential energy profiles of hydrogenation of the intermediate 5 to CHCHOH(blue lines)and CHCHO(black lines)on the Ni(1 1 1)surface.Green lines represent the hydrogenation of CHCHO to CHCHOH.

Fig.9.The proposed reaction mechanism for hydrodeoxygenation of DBF to BCH on the Ni (1 1 1) surface.

Compared with previous results of the DBF hydrodeoxygenation on the Pt (1 1 1) surface [26],there are two main differences:1)between the Ni and O atoms exists a strong interaction so that cyclohexylphenoxy can be stably adsorbed on the Ni(1 1 1)surface and further hydrogenated to intermediate 26.Moreover,the reaction barriers of DHDBF and CHCHOH are relatively low on the Ni(1 1 1) surface;and 2) multiple-radical intermediates produced by nonconsecutive hydrogenation of CHPOH cannot be stably adsorbed on the Ni (1 1 1) surface.Therefore,the nonconsecutive hydrogenation path does not exist.The intermediate 26 cannot be formed in the consecutive hydrogenation pathway.

4.Conclusions

We investigated the hydrodeoxygenation mechanism of DBF on the Ni (1 1 1) surface by DFT calculations.There is a competitive relationship between the ring-opening and hydrogenation reactions of THDBF and HHDBF.Decreasing the hydrogen coverage can reduce the yield of DHDBF.Cyclohexylphenoxy is generated after the ring-opening of THDBF and HHDBF.One part of cyclohexylphenoxy is directly consecutively hydrogenated to CHCHO,and then hydrogenated on O atom to form intermidiate 26.The intermidiate 26 has the lowest deoxygenation barrier and possibly deoxygenated to BCH.The ring-opening reactions of DHDBF and deoxygenation of CHCHOH are challenging due to weak adsorption.Therefore,DHDBF and CHCHOH are the main products of DBF hydrodeoxygenation on the Ni (1 1 1) surface.

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 gratefully acknowledge financial support from the National Key Research and Development Program of China(2016YFB0600305) and National Natural Science Foundation of China (21808153,22078220).

Supplementary Material

Supplementary data to this article,the material S1,Fig.S1–S6 and Table S1–S5 can be found online at https://doi.org/10.1016/j.cjche.2021.02.005.