Ying Xu,Pengru Chen,Wei Lv,2,3,Chenguang Wang,2,3,Longlong Ma,2,3,Qi Zhang,2,3,

1 Guangzhou Institute of Energy Conversion,Chinese Academy of Sciences,Guangzhou 510640,China

2 Key Laboratory of Renewable Energy,Chinese Academy of Sciences,Guangzhou 510640,China

3 Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development,Guangzhou 510640,China

4 Jiangsu Key Laboratory for Biomass Energy and Material,Nanjing 210042,China

Keywords:Lignin Hydrogenolysis depolymerization High-dispersion Ni-based catalyst Al-SBA-15

ABSTRACT Efficiency and recycling of catalysts are important for the lignin hydrogenolysis to obtain phenolic monomers.In this work,a series of high-dispersion Ni/Al-SBA-15 catalysts were prepared by a direct and effective preparation method,and then used in the hydrogenolysis of diphenyl ether (DE) and organosolv hydrolyzed lignin (OHL) for phenolic monomers.The universality of as-made catalysts in different solvents and cyclic performance were investigated.Results showed that the addition of ethylene glycol(EG) during the loading process of Ni promoted the dispersion of metal efficiently.High dispersion of Ni species could highly enhance the conversion of DE and the OHL which Ni/Al-SBA-15(1EG) exhibited the excellent catalytic performance.Decalin was found to be most effective solvent on the conversion of DE(99.16%).84.77%liquefaction ratio and 21.36%monomer yield were achieved,and no obvious char was observed after the depolymerization of OHL in ethanol solvent at 280°C for 4 h over the Ni/Al-SBA-15(1EG) catalyst.

1.Introduction

Lignin,one of the most abundant natural polymers in plants,counts for 15%-30% of lignocellulosic biomass [1,2].As the only natural polymer made of aromatics,many researchers have explored many potential methods to convert lignin into aromatic chemicals [3–6].Lignin is a kind of complex three-dimensional polymer which consists of three principal building blocks,p-hydroxyphenyl(H),guaiacyl(G) and syringyl(S),which differ from each other by the number of methoxyl groups attached on the aromatic ring [7,8].The key steps to promote the depolymerization of lignin is the cleavage of the high content C-O-C bond[9–11].Many compounds with typical lignin structure (e.g.β-O-4,ɑ-O-4,4-O-5 and etc.) have been selected as model compounds to investigate the activity of catalysts and the reaction mechanism[12–15].Ruiz et al.[16] used phenethyl phenyl ether to study the depolymerization reaction over 3.8wt% Ru/C and 3.9wt% Ru/Al2O3catalysts.They found that both Cβ-O and Caryl-O bond were broken during the reaction,but the cleavage of Cβ-O occupied the dominant position.Zhang et al.[17] prepared MIL-100(Fe) supported Pd-Ni BMNP catalyst and used 2-phenoxy-1-phenethanol to test the hydrogenolysis activity of the catalyst,which exhibited excellent catalytic stability during the reaction process.Yan et al.[18] applied bimetallic catalysts of Ni and M(M=Ru,Rh and Pd)to hydrodepolymerize the β-O-4,α-O-4,and 4-O-5 model compounds.NiRh and NiPd catalysts were found to be more active and selective during the hydrogenolysis reaction.The model compounds were more specific to determine the activity and selectivity of the catalyst and more accurate to interpret the reaction mechanism owing to its single lignin structure bond.

Among various hydrogenolysis catalysts,Ni-based catalysts have aroused a lot of interest due to their low cost and considerable catalytic activity [19–21].It has been demonstrated that the dispersion of nickel has played a significant role in the depolymerisation[22].However,the supported catalyst obtained by common impregnation method possesses low dispersion degree of active metal particles,large metal particle size and relatively weak binding force between the active site and the surface of the support,thus affecting the catalytic activity[23].Wang et al.[14]found that only a small amount of Ni particles (<6 nm) were detected inside the channels of the Ni/Al-SBA-15,while most Ni particles were distributed on the surface of the carrier and the particle size was distributed in 30 nm–50 nm,demonstrating that the increase in metal dispersion and decrease in metal particle size can enhance the activity of the catalyst.Qiu et al.[21] studied the activity of Ni/MCM-41 catalysts with different Ni dispersion on the hydrogenation of naphthalene.The results indicated that the hydrogenation activity of the catalysts was improved when the Ni particles dispersion was higher.Using Ni/MCM-41 as catalysts,the naphthalene converted completely(100%)at 55°C when the Ni particles dispersion was higher,while,at 135 °C when the Ni particles dispersion was poor.Large Ni particles easily undergo severe sintering and coking,and result in losing their initial activity[22].Therefore,fabricating catalysts with high dispersion of Ni metal particle through a simple and efficient method is highly desirable for high yield lignin depolymerization.In the previous report,many methods including Atomic Layer Deposition (ALD) [24],solvents methods [25],impregnation [26],precipitation [27] were applied to improve the dispersion of Ni species in the mesoporous silica.Traditionally,as-prepared SBA-15 is calcinated to remove the template and then modified support surface by some ligands (βcyclodextrin and poly(N-vinyl-2-pyrrolidone))and lastly the metal loading is employed.However,there exists special nanoconfined spaces between silica walls and template in SBA-15.Such nanoconfined spaces might hinder the loading of Ni species during the preparation of samples [28].

Organosolv pretreatment affords sulfur-free lignin with high purity,which makes it a suitable precursor for different applications[8].The properties of the organosolv lignin are largely dependent on the biomass substrate as well as the specific solvent pretreatment method [7].It has been shown to retain significant native ether linkages compared to the native lignin in biomass.Lignin with abundant ether linkages are easier to depolymerization and upgrading [29,30].Therefore,the key way to improve organosolv lignin conversion is achieving efficient reaction systems and inhibiting lignin repolymerization.In our previous work,we have found that the mesoporous structure of Al-SBA-15 could inhibit the repolymerization during the depolymerization process of lignin.It was evidenced that the mesopore structure and large pore size of SBA-15 could prevent the repolymerization of the highly reactive intermediates based on the steric constraints.In addition,the present of Al and Ni in SBA-15 could improve the cleavage of ether bond and stabilize the highly reactive intermediates [4].Herein,the purpose of this work is to further improve Ni dispersion in the Al-SBA-15 support by a direct and simple method,then explore the practicability of the as-made highly dispersive catalyst in both the model compounds and the organicsolv lignin.The conversion pathway of the model compound was speculated.The catalytic performance in different solvents and the cyclic test were also studied.It was found that the high-dispersion Ni/Al-SBA-15 catalysts could enhance the conversion and resistance the coke during the model compound and the organosolv hydrolyzed lignin depolymerization.

2.Materials and Methods

2.1.Chemicals

Ethanol (>99.9%),ethylene glycol(EG) (>99.9%) and Ni(NO3)2-﹒6H2O(AR) were purchased from Tianjin Fu Yu Fine Chemical Co.,Ltd (Tianjin China).Diphenyl ether(DE) were purchased from TCI Co.Ltd.All the reagents were analytic grade and used directly without further purification treatment.

Organosolv hydrolyzed lignin(OHL) was isolated from the Hydrolyzed Lignin (HL),which was provided by Guangzhou Institute of Energy conversion,CAS[4].The treatment process consisted on the digestion of the HL in a mixture of ethanol–water(65%)and sulfuric acid (98%) at 170 °C for 30 min in a pressure reactor.The liquid fraction was separated from the solid fraction by filtration.Dissolved lignin was isolated by precipitation with adding deionized water.After that,the suspension was filtrated to recover solid which was then freeze dried under vacuum overnight to obtain OHL.

2.2.Catalyst

2.2.1.Synthesis of the catalysts

As described in our previous study [4],mesoporous Al-SBA-15 was synthesized according to the method reported by Zhao [31].Under acidic conditions,the triblock copolymer surfactant P123(EO20PO70EO20) was used as a template,tetraethyl orthosilicate(TEOS)and aluminum isopropoxide were used as the silicon source and aluminum source,respectively.The synthetic procedures began with the dissolving P123 (2.0 g) in HCl aqueous solution(75 ml,2.0 mol﹒L-1) at 55 °C under 200 rpm stirring rates.When a homogenous solution formed,then aluminum isopropoxide was added into the mixture followed by 4.3g of TEOS added dropwise and further maintained at static condition for 24 h.Subsequently,the resultant mixture was transferred into a Teflon bottle and hydrothermally aged for 24 h without stirring at 110°C.The products were recovered by filtering and washing with distilled water.After drying at 110 °C for 12 h in oven,the solid was calcined with a heating rate of 1.0oC﹒min-1to 550 °C and kept at this temperature for 6 h to provide the pure mesoporous Al-SBA-15.The molar ratio of Si/Al was 20 and the load of Ni was 20%.To obtain the Ni/Al-SBA-15 catalysts with different dispersion,EG was added in the process of impregnation treatment.Take Ni/Al-SBA-15(0.5EG) for example,Ni was deposited on the support by impregnation 4 ml of aqueous solutions of Ni(NO3)2·6H2O within EG(with the Ni/EG mole radio of 1:0.5) followed by evaporation to dryness at 100°C for 12 h and calcination at 550°C for 4 h under the air atmosphere.In the same method,we got Ni/Al-SBA-15(1EG)and Ni/Al-SBA-15(3EG).

2.2.2.Catalyst characterization

The specific surface area and pore size distribution of the catalysts were carried out over a Quantachrome chemical adsorption instrument and the surface area was calculated by the Brunauer Emmett Teller (BET) method.Powder X-ray diffraction (XRD) patterns of the catalysts were measured with X-ray Diffraction Radiation (X Pert Pro MPD with CuKa),Philip.The scanning angle was ranged from 1° to 5° and 10° to 80°,respectively.Transmission electron microscopy (TEM) was measured on a Jeol TEM-100CX instrument at 200 kV accelerating voltage.

Temperature-programmed desorption of ammonia (NH3-TPD)was performed on an apparatus PX200(Tianjin Golden Eagle Technology Limited Corporation).The sample(100 mg)was pretreated at 300 °C for 1 h and then cooled to 120 °C under a He flow.Pure NH3was injected until adsorption saturation was reached,followed by a flow of He for 1 h.Then the temperature was raised from 120 to 850 °C with a heating rate of 10 °C﹒min-1and the amount of desorbed ammonia was detected by using thermal conductivity detector (TCD).

2.3.Hydrogenolysis procedure

2.3.1.Model compound

The catalytic hydrogenolysis of the model compound was running in a 50 ml stainless autoclave.0.50 g model compound,0.15 g catalyst,and 20 ml solvent were put into the reactor.After purging with H2three times,the reactor was pressurized to 2 MPa with H2.Then it was heated up to 150 °C (with the rate of 5 °C﹒min-1) and the reaction started under vigorous stirring of 400 rpm.When the specific time of 2 hours was reached,circulation water was used for cooling.

2.3.2.Organosolv hydrolyzed lignin

The catalytic hydrogenolysis of organosolv hydrolyzed lignin(OHL) was carried out in a 100 ml high pressure autoclave equipped with an overhead stirrer.Typically,the autoclave was charged with a suspension of 0.50 g OHL,0.20 g catalyst in 30 ml solvent.The reactor was sealed and purged with H2three times to remove oxygen.After gas leak testing,the pressure was set to 1 MPa and the reaction mixture was heated to the desired temperature under continuous stirring at 450 r﹒min-1for 90 min.After reaction,the reactor was cooled to room temperature rapidly in a water bath.The solid catalyst was separated out by filtration and washed by decalin and ethanol.After drying at 60 °C under vacuum,the recovered catalyst was collected for the next time to investigate the catalyst recyclability without further treatment.

2.4.Product analysis

The separation of OHL was shown in Fig.1.The conversion of OHL,the degree of OHL liquefication,the yields of monomers and the yield of residues were calculated (in %) by Eqs.(1)–(4) respectively and the conversion of model compound was calculated by Eq.(5).In order to present the efficiency of dimer C-O-C model compound conversion to monomer product,we defined the mole ratio of products to substrate to describe the product yields in Eq.(6).

WL:the mass of pure dry lignin;WU:the mass of un-reacted lignin;WF:the mass of feed lignin;WC:the mass of char;WM:the mass of phenolic monomer.

Fig.1.Procedure for product separation.

3.Results and Discussion

3.1.Characterization of the catalysts

The porosity of all the catalysts was verified by N2adsorption–desorption isotherms.As shown in Fig.2(A),typical IV type isotherms with a H1 hysteresis loop at relative pressure (p/p0)=0.52–0.78 were observed for all the catalysts,which was the characteristic of mesoporous structure [32–34].Compared with the N2adsorption–desorption isotherm plots of Al-SBA-15,the hysteresis loop became slightly small after loading with Ni,which indicated the mesorporous structure of all the catalysts were preserved after the addition of EG and the introduction of Ni specie.Meanwhile,all the catalysts exhibited small mesoporous pore diameter distribution ranging from 4 nm to 6 nm.As shown in Table 1,the BET surface area and pore volume were determined to be 714.86 m2﹒g-1and 0.85 m3﹒g-1for Al-SBA-15.After loading,the BET surface area and pore volume dropped to 499.95 m2﹒g-1and 0.85 m3﹒g-1for Ni/Al-SBA-15(0EG).It was worth to note that no change in the surface area and pore volume of the catalysts adding with EG during the loading process.This meant that the loading of Nickel changed the specific surface area and pore volume of the carrier,but the addition of EG did not affect the characteristics of the carrier itself.

The acidic properties of Al-SBA-15 and Ni-Al-SBA-15(xEG)catalysts were investigated by temperature-programmed desorption of ammonia and the results were concluded in Table 1.It could be seen that after impregnation with Ni metal salts,the catalysts gave significantly improved acidities,while the addition of the EG would lead to the decrease of the acidities.When the addition ratio of Ni:EG was 1,Ni/Al-SBA-15(1EG) possessed less acidity compared with other catalysts.The acidity of the catalysts was changed after addition of EG,which might not be the main reason for the catalyst activity.

Table 1 The properties of Ni/Al-SBA-15 with different Ni dispersion

The XRD patterns of Ni/Al-SBA-15(xEG)catalysts were shown in Fig.3.From Fig.3.(A),the three characterized peaks detected at 0.9° and 1.5°–2° were corresponding to (100),(110) and (200)planes.This indicated that the well-order mesoporous structure of SBA-15 was still maintained after loading Nickel species and the addition of EG [33,35].Fig.3(B) and (C) present the XRD patterns of Ni/Al-SBA-15 (xEG) before and after reduction.For all the catalysts,the broad peak at 2θ=15°–30° were related to the SiO2amorphous phase.The three additional XRD peaks (Fig.3B)at 36°,42°,64°were attributed to the(111),(200),and(220)facets of NiO species [36,37].The XRD results (Fig.3C) for Ni/Al-SBA-15(xEG)showed a large peak at 45°that could be identified as metallic Ni (111) and the two smaller peaks at 52°and 76°related to Ni phases [38].As shown in Fig.3 (B) and (C),the diffraction peaks of NiO and Ni were sharp and narrow without the addition of EG,while they became weak and wide after adding EG.Obviously,the addition of EG during the loading process of Ni highly improved the dispersion of NiO and Ni.

The TEM images of Ni/Al-SBA-15 catalysts with and without EG were shown in Fig.4.The particles observed in the TEM image were mainly NiO particles because the Ni metal was highly active and easily oxidized in the air.As shown in Fig.4(A),the porous structure of the carrier was visible and NiO particles located closely to each other in larger agglomerates[38].The NiO particle size varies from 25 nm to 175 nm,with the average particle size of 109.8 nm.The particles size significantly decreased when adding EG.Moreover,the agglomeration of NiO particles was weakened and the dispersibility was greatly improved.The change in the addition amount of EG did affect the NiO particle size.When Ni:EG was 1:0.5,the NiO particles size became significantly smaller and distributed evenly on the carrier.The average particle size of Ni/Al-SBA-15(0.5EG) decreased from 109.8 nm to 3.5 nm,but it could be seen that the agglomeration still existed in the catalysts from Fig.4 (B).With the increasing of Ni:EG to 1:1 and 1:3,the NiO particles were evenly distributed on the catalyst surface,and the particle size was significantly reduced.A higher and more homogeneous dispersion of NiO clusters was obtained.The results showed that the addition of EG in the preparation process improved the dispersion of NiO on the carrier,which were in good accordance with the XRD results above.

Fig.2.(A) N2 adsorption–desorption isotherm.(B) Pore size distribution curves of the catalysts.

Fig.3.Powder X-ray Diffraction patterns of Ni/Al-SBA-15(xEG) catalysts.(A) Small-angle XRD patterns of Ni/Al-SBA-15(xEG);(B) Wide-angle XRD patterns of Ni/Al-SBA-15(xEG) before reduction;(C) Wide-angle XRD patterns of Ni/Al-SBA-15(xEG) after reduction.

Fig.4.TEM micrograph and particle size of Ni/Al-SBA-15(xEG) catalysts.(A) Ni/Al-SBA-15(0 EG);(B) Ni/Al-SBA-15(0.5EG);(C) Ni/Al-SBA-15(1EG);(D) Ni/Al-SBA-15(3EG).

3.2.Hydrogenolysis of model compound over Ni/Al-SBA-15(xEG)

The catalytic activities of the catalysts were tested by DE at 150 °C and 2 MPa.Table 2 showed the DE conversion radio and the product distributions.It could be seen that Ni/Al-SBA-15(xEG) catalysts with different Ni metal dispersion demonstrated quite difference in activity.As the addition amount of of EG increased,the catalytic activity increased first and then decreased.That meant the addition of EG and the smaller Ni particle size could increase the conversion of DE dramatically,but further increasing the amount of EG would not optimistic for the conversion of DE.Cyclohexane,cyclohexanol,phenol,cyclohexyl phenyl ether and cyclohexyl ether were the main products in the reaction systems over the series catalysts.

As for product distribution,the selectivity of cyclohexane increased with the addition and the increasing of EG.The selectivity of cyclohexanol and phenol increased first and then decreased,which reached the highest point (35.38%) over the Ni/Al-SBA-15-15(1EG) catalyst.The selectivity of cyclohexyl phenyl ether was on the opposite trend compared with the selectivity of cyclohexanol and phenol,which increased first,then decreased and reached its lowest points over Ni/Al-SBA-15-15(3EG).Furthermore,the sum of the selectivity of cyclohexyl phenyl ether and the cyclohexyl ether decreased with the increasing amount of EG.It could be deduced that the addition of EG could enhance the ability of C-O-C breaking and hydrogenation of the catalysts,especially when the amount of EG added was 1.

3.3.Hydrogenolysis of model compound in different solvents over Ni/Al-SBA-15(1EG)

Ethanol,tertiary butanol,ethyl acetate and decalin were chosen as the solvents to investigate the effect on the hydrogenolysis of DE.The results were shown in Table 3.

Table 2 Hydrogenolysis of diphenyl ether over Ni/Al-SBA-15(xEG) catalysts

Table 3 Hydrogenolysis of diphenyl ether in different solvents over Ni/Al-SBA-15(1EG)

As shown in Table 3,cyclohexane,cyclohexanol,phenol,cyclohexyl phenyl ether and cyclohexyl ether were the products in different solvent systems.Among the four solvents,DE exhibited the highest conversion in decalin(99.16%),whereas the lowest conversion in ethanol (36.49%).DE had different selectivity for depolymerization products in different solvents.When hydrogenolysis reaction happened in decalin,the main products were cyclohexane,cyclohexanol and cyclohexyl ether,whose selectivity were the highest.Nevertheless,cyclohexane and cyclohexanol possessed the lowest selectivity in ethanol solvent system,with 12.29% and 14.69% respectively.Moreover,ethanol,tertiary butanol and ethyl acetate solvent system facilitated the production of cyclohexyl phenyl ether,while the selectivity of cyclohexyl ether decreased.From the results of the product distribution,the selectivity of each product was quite different from each other even over the same catalyst.The selectivity of hydrolysis products was the highest in decalin among the solvents used in the paper which was 85.69%.In addition,the summary selectivity of cyclohexyl phenyl ether and cyclohexyl ether showed a maximum of 68.71%,suggesting that hydrogenation was the primary reaction process in tertiary butanol.

3.4.The pathway of diphenyl ether during the hydrogenolysis reaction

According to the discussion above,we further investigated the relationship between diphenyl ether conversion,product selectivity and reaction time.It could be speculated from Figs.5 and 6 that there were two conversion pathways for the hydrogenolysis reaction of DE over Ni/Al-SBA-15(1EG).(I) Diphenyl ether was hydrogenated to cyclohexyl phenyl ether,cyclohexyl phenyl ether was then broken by hydrogenation depolymerization to phenol and cyclohexane,and phenol was further hydrogenated to form cyclohexanol.(II) Diphenyl ether was hydrogenated to cyclohexyl phenyl ether,and the cyclohexyl phenyl ether was further hydrogenated to cyclohexyl ether.Finally,the hydrogenolysis of cyclohexyl ether happened,and the ether bond was broken to form cyclohexane and cyclohexanol.Through the conversion of diphenyl ether at different reaction times and the change in product selectivity,it can be speculated that these two reaction pathways proceed simultaneously.

Fig.5.The conversion yield of diphenyl ether and selectivity of products under different reaction times.

Fig.6.Possible pathway of diphenyl ether during the hydrogenolysis reaction.

3.5.Hydrogenolysis of OHL over Ni/Al-SBA-15 catalysts

Organosolv hydrolyzed lignin was used as the reactant for further investigation.Fig.7 showed the yield of liquefication,phenolic monomer and char of hydrogenolysis of OHL over Ni/Al-SBA-15(xEG).When the addition ratio of Ni:EG was 1:1,the liquefication rate of OHL significantly increased to 84.77%,the yield of phenolic monomer also increased to 21.36%and the coke content decreased from 7.3% to below 2%.These results indicated that the increasing dispersion of Ni activity could effectively improve the depolymerization of OHL,which was consistent with the results of the model compound conversion above.

Fig.7.Effect of different Ni dispersion on the OHL depolymerisation.Conditions:0.5 g OHL,0.2 g Ni/Al-SBA-15(xEG),30 ml ethanol,1 MPa H2,280 °C.

The GPC analysis of oligomers also showed that the molecular weight of oligomers decreased with the increasing of Ni particle dispersion.In the absence of EG during the preparation of catalyst(Ni/Al-SBA-15),the molecular weight of oligomer was 759 g﹒mol-1after lignin depolymerization.However,the value of the molecular weight decreased to 656 g﹒mol-1when Ni/Al-SBA-15(1EG) was used as catalyst for the depolymerization.That meant that higher dispersion of Ni particle could enhance the activity of the catalysts.The result also indicated that under the set catalyst preparation protocol,Ni:EG=1:1 was the optimal ratio for lignin depolymerization.

Ni/Al-SBA-15(1EG) catalyst was used to further investigate the catalytic activity in different solvents for lignin depolymerization.The results of OHL hydrogenolysis in different solvents over Ni/Al-SBA-15(1EG) were shown in Fig.8.As shown Fig.8,the liquefication ratio of OHL in decalin was 77.32%,while increased to 81.80%in ethyl acetate and 84.77%in ethanol.The yield of phenolic monomer in decalin was 15.90% and increased to 17.43% in ethyl acetate and 21.36% in ethanol.It could be concluded that among three solvents,both the liquefication rate and the yield of phenolic monomer in the hydrogenolysis of the OHL were the highest in ethanol,which might be ascribed to the better solubility of lignin.And the coke content in ethyl acetate was lower (1.03%) than that in decalin (1.75%) and ethanol (2%).

Fig.8.Effect of different solvents on the OHL depolymerisation.Conditions:0.5 g OHL,0.2 g Ni/Al-SBA-15(1EG),30 ml solvent,1 MPa H2,280 °C.

Fig.9.Cycling performance of Ni/Al-SBA-15(0EG) (a) and Ni/Al-SBA-15(1EG) (b).Conditions:0.5 g OHL,0.1 g catalyst,30 ml ethanol,1 MPa H2,280 °C.

The catalysts of Ni/Al-SBA-15(0EG) and Ni/Al-SBA-15 (1EG)were separated from the liquid products by filtration,and were reused in the successive runs.Fig.9 showed the results of reusability tests of the catalyst.For Ni/Al-SBA-15(0EG) catalysts,the yield of phenolic monomer decreased and char content increased as the cycling number increased.To be more specific,the yield of phenolic monomer decreased from 21.36% to 12.21% after four runs.However,Ni/Al-SBA-15(1EG) catalyst exhibited a relatively stable catalytic activity.The phenol yield was almost unchanged after 2 runs and the char content was only 1.45%.With the increase of cycling runs,the phenol monomer yield decreased and the char yield increased slightly.Even after 5 runs recycling,the yield was slightly reduced to 20.10% and the char yield was only 5.77%.Hence,the Ni/Al-SBA-15(1EG)catalyst can be used at least for five successive runs.The excellent recycle performance for OHL depolymerization might be ascribed to the high dispersion of Ni particles and smaller particle size.

4.Conclusions

A series of highly dispersed catalysts were prepared using ethylene glycol and used in the hydrodepolymerization for 4-O-5 model compound diphenyl ether and organosolv hydrolyzed lignin.In this work,we demonstrated that the addition of EG during the preparation process of Ni/Al-SBA-15 catalysts effectively improved the dispersion of metal particle and the NiO particle size became significantly smaller.The Ni/Al-SBA-15(1EG) catalyst exhibited the best and more stable catalytic activity.The highest conversion yield(99.16%)of 4-O-5 model compound was achieved in decalin solvent,while for real lignin (OHL),the highest yield(21.36%) of phenolic monomer was obtained in ethanol solvent.The catalyst could be used at least five successive runs with a 1.26%decrease in the yield of phenolic monomer.It can be concluded that the improvement of metal particle dispersion and smaller metal particle sizes effectively enhance the catalytic activity of the catalysts.

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.

Acknowledgments

The authors gratefully acknowledge the National Natural Science Foundation of China (No.51676191&5181101221),Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No.2017BT01N092),‘‘Transformational Technologies for Clean Energy and Demonstration”,Strategic Priority Research Program of the Chinese Academy of Sciences(No.XDA 21060102) and Jiangsu Key Laboratory for Biomass Energy and Material(JSBEM201905).