Lihua Qian,Guojun Lan,Xiaoyan Liu,Zhenqing Li,Ying Li

Institute of Industrial Catalysis,Zhejiang University of Technology,Hangzhou 310014,China

Keywords:Levulinic acid Hydrogenation Cobalt-ruthenium Synergistic effect Carbon coating

ABSTRACT The hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL) by using water as solvent is a crucial process in the production of fine chemicals from biomass.An ultrathin carbon layer coating CoRu bimetallic catalyst supported on silica (CoRu@C/SiO2) is prepared by using tannis-ligated cobalt-ruthenium complex on silica as precursors,and applied for catalyzed synthesis of GVL from LA.Because of the synergistic effect between cobalt and ruthenium,the addition of small amounts of Ru to Co catalysts can increase the catalytic activity in the aqueous hydrogenation of LA.The ultrathin carbon layer covered on the CoRu bimetallic catalyst can greatly reduce the leaching of active metals.The CoRu@C/SiO2 catalyst achieves high stability and is reused up to 5 runs without significant loss of performance in aqueous hydrogenation of levulinic acid.

1.Introduction

The renewable biomass is of increasing interest as a source of fuel and chemicals,due to depletion of fossil fuel reserves and growing environmental concerns about greenhouse gases.In 2004,the US Department of Energy selected 12 biomass platform chemicals with the most potential for development,including succinic acid,levulinic acid,itaconic acid and so on [1].Among them,levulinic acid(LA)is a vital platform molecule used as an intermediate to produce fine chemicals such as medicines,pesticides and fragrances,and the hydrogenation of biogenic LA and its esters to γ-valerolactone(GVL)is a key reaction,because GVL can be widely used as fuel additives,food ingredients and intermediates in fine chemical production [2–4].Noble metal catalysts (Rh,Pd,Ir,Au,Ru) are efficient catalysts for hydrogenation of LA [5],while the expensive cost usually limits their application.Yet,non-noble metals such as Ni [6–8],Cu [9–10],Co [11–13] and Fe [14] have been widely studied in the aqueous hydrogenation of LA in recent years due to its low cost.

Cobalt catalyst is one of the most promising hydrogenation catalysts for replacing noble metal catalysts because of the relatively inexpensive and acceptable catalytic performance.It is found that unsupported cobalt catalyst prepared from commercially available Co3O4could catalyze the reaction efficiently without solvent,which can make the yield of the product GVL as high as 94%[15].Since metallic cobalt has been confirmed to be the active constituent in the hydrogenation of ethyl levulinate,the performance of Co catalysts is affected by the proportion of metallic cobalt on the surface.The polymer-stabilized Co nanoparticles,such as PVP-Co-NPs and it-PMMA-Co-NPs,were also proved to be much active to catalyze hydrogenation of LA into GVL up to 93%isolated yield by utilizing FA or ethanol as the hydrogen source [16].A cobalt nano-catalyst supported on commercial silica,which was generated from levulinic acid ligated cobalt complex,has also been proved to be highly efficient for the hydrogenation of LA [17].

Choosing the suitable solvent is very important from the point of view of catalytic performance,catalytic environment and catalytic upgrading.The solvent can significantly affect the selectivity of GVL.The most of LA hydrogenation catalyzed by various Co catalysts in organic solvents has low GVL selectivity.The main product of LA hydrogenation in methanol was methyl levulinate and GVL is 43%.Interestingly,hydrogenation in water gives GVL almost complete selectivity [11].With water as the solvent,the conversion rate of ethyl levulinate hydrogenation to GVL was as high as 97.4%and the yield of GVL was as high as 91.8%on Co/ZrO2catalyst[18].Michel et al.found that the presence of a water molecule strongly changed the energetic span values and the alkoxy route was significantly superior to the alkyl one for Co,Ru and Ni metals[19].In addition,because biomass derived levulinic acid generally produced by acid-catalyzed hydrolysis of high-oxygen biomass feedstocks,a mixture of biobased chemicals and water is always accompanied by levulinic acid production.Therefore,water is the best solvent in the process of converting LA to downstream chemicals.

However,there are seriously corrosive effects of water which cause degradation of Co-based catalyst for hydrogenation of LA in aqueous.If water is used instead of hydrocarbon as the solvent,poisoning and destruction of Co catalyst becomes more challenging.Long et al.[13] reported an excellent active Co catalyst for the hydrogenation of LA in 1,4-dioxane.The core–shell structure and the strong interaction between Co and Al species made Co particles had better stability in organic solvents.However,when water was used as a solvent,the conversion of levulinic acid was only 6%.And an above 200°C reaction temperature or highly active H donors,such as isopropanol and formic acid,were required to achieve high activity and selectivity of GVL [20].In contrast to the enhancement of the catalytic activity of the ruthenium catalyst in the aqueous-phase hydrogenation of LA,the activity of the cobalt catalyst was dramatically inhibited.The catalytic performance of Co@HZSM-5 catalyst decreased with the increasing of water content from 10 to 100 %(mass),although cobalt nanoparticles were carefully protecting the inside of HZSM-5 zeolite crystal[21].Therefore,it is of great concern to design highly efficient and robust catalysts for aqueous hydrogenation of LA.In the process of aqueous-phase hydrogenation of LA,the water negative effect is related to the formation of inactive cobalt oxide.

It is well documented that the combination of noble metals and non-noble metals may largely enhanced the activity of the nonnoble metal catalysts with the significant electronic configuration synergy.As reported by Chaudhari et al.[22],the doping of ruthenium in cobalt catalysts increased the hydrogenation activity by 3–4 times,showing a strong synergistic effect.Tao et al.[23] studied Co-Ru bimetallic catalysts for the carbon dioxide methanation.Compared with cobalt catalyst,the cobalt-ruthenium bimetal ultra-thin film formed on the surface of ruthenium-doped cobalt oxide significantly enhanced its activity and selectivity for CH4formation.Moreover,the addition of a small amount of platinum[24],ruthenium[25-26]and rare earth [27]could significantly improve the performance of Co-based catalyst.It is believed that the addition of noble metals can help the reduction of cobalt oxide to the active metal state,or stabilize the cobalt species through hydrogen spillover,thus speeding up the rate of Fischer-Tropsch synthesis[26].In addition to the reported reduction-improvement effect,these bimetallic catalysts can activate reactants/intermediates more efficiently by changing their electronic properties and geometrical structures.

Herein,we demonstrate that the introduction of trace quantity(～0.5%(mass))of ruthenium in cobalt-based catalysts has a significant improvement of activity for the hydrogenation of LA to GVL in aqueous.It is turn out that the key to obtain high GVL yields is the higher dispersion of Co0and the Co-Ru synergistic effect from the results of XRD,H2-TPR,TEM,and XPS.The conversion of LA to GVL can reach 57%at 70°C with 4 MPa hydrogen,which is higher than the activity of their corresponding monometallic counterpart.Although the addition of noble metals can improve the reduction of cobalt,the metallic cobalt exposed to the reaction medium cannot resist the hydrothermal erosion of water.The numbers of active sites will be inevitably lost in the aqueous phase.Encapsulating Co nanoparticles in ultrathin carbon layer should be an efficient strategy to prevent the metal catalyst from leaching and sintering in acidic water solution because of good hydrothermal stability of carbon [28].Therefore,the ultrathin carbon layercoated Co NPs supported on silica were prepared by pyrolysis the silica supported tannis-ligated cobalt-ruthenium complex under inert atmosphere to reduce the leaching of Co species.Significantly,due to the protection of a well-designed carbon shell,the CoRu@C/SiO2sample showed remarkable stability under highly acidic conditions and could be reused at least 5 times with comparable performance to fresh catalyst.

2.Experimental

2.1.Materials

Cobaltous nitrate hexahydrate (Co(NO3)2.6H2O) and other metal precursors (Ni(NO3)2.6H2O,Cu(NO3)2.6H2O,PdCl2,HAuCl4.4H2O,H2PtCl6.6H2O) were purchased from Aladdin Co.Ltd.Ruthenium chloride hydrate (RuCl3.xH2O) was obtained from Sino-Platinum Metals Co.Ltd.The nano-silica used in the manuscript was commercially obtained from Hangzhou Wanjing New Material Co.Ltd.Levulinic acid (99%) was obtained from Aladdin Co.Ltd.

2.2.Synthesis of catalysts

2.2.1.Preparation of CoM/SiO2(M=Ni,Cu,Pd,Au,Pt,Ru) bimetallic catalysts

The Co-Ru nanoparticles supported over Nano-silica (denominated as CoRu/SiO2) were prepared through an incipient-wetness impregnation method.6.0 g of SiO2was impregnated with 17 ml of a Co(NO3)2.6H2O (1.56 g) and RuCl3.xH2O (0.09 g) mixed aqueous solution.After being dried at 100 °C for 6 h,the obtained catalysts were reduced for 2 h at 400 °C under a H2flow of 30 ml.min-1H2flow.The nominal Co and Ru loadings of the catalyst were 5.0%(mass)and 0.5%(mass),respectively.The CoM/SiO2(M=Ni,Cu,Pd,Au,Pt)bimetallic catalysts were prepared in a similar way.The nominal Co and M loadings of the catalyst were 5.0%(mass) and 0.5 %(mass),respectively.In addition,for the convenience of comparison,SiO2supported cobalt and ruthenium catalysts with the same catalyst loading as above were prepared by impregnation method.

2.2.2.Preparation of CoRu@C/SiO2

First,Co(NO3)2.6H2O(1.56 g)and RuCl3.xH2O(0.09 g)were dissolved in 17 ml of aqueous solution containing 1.13 g tannic acid.The mixture solution was sonicated for 10 min.Then the prepared solution was impregnated onto the SiO2support for 12 h.After being dried at 100°C for 6 h,the obtained samples were pyrolyzed at 700 °C for 3 h in a nitrogen flow and then reduced under a H2flow of 30 ml.min-1at 400 °C for 2 h.The as-prepared materials were denoted as CoRu@C/SiO2.

2.3.Catalysts characterization

Nitrogen adsorption isotherms were obtained by a Quantachrome Autosorb-IQ apparatus at -196 °C.Before the test,the samples were degassed at 300°C for 8 h.The surface area was calculated in the relative pressure range of 0.05–0.30 using the Multi-Point BET method.The pore size distribution was obtained by the BJH method based on the desorption branches of the isotherms.The total pore volume was acquired at a relative pressure of 0.99.X-ray powder diffraction (XRD) measurements was carried out with a Rigaku D/Max-2500/pc powder diffraction system,using CuKα radiation(λ=0.1541 nm)over the range 10°≤2θ ≤80°with a step width of 0.05°.The operating voltage and current is 40 kV and 100 mA,respectively.

The temperature-programmed reduction (TPR) experiments were carried out with a self-made TPR instrument.About 50 mg of the catalysts was placed in a U-shaped quartz tube to remove the water under an Ar atmosphere at 110 °C for 1 h.Then,under a 5% H2/Ar flow,the temperature was raised from 100 °C to 850°C with a rate of 10°C.min-1.And the exhaust was simultaneously monitored by an on-line Hiden gas analyzer (QIC 20).

Transmission electron microscope (TEM) was carried out on TECNAI G2 F30 S-Twin FEI TECNAI electron microscope working at 300 kV.The sample was ultrasonic dispersed in ethanol,and then a small amount of suspended liquid was placed on a Cu mesh coated with porous C film,and then dried under ambient conditions.X-ray Photoelectron Spectroscopy (XPS) data was recorded with a Thermo ESCALAB 250XI with monochromatic AlKα(1486.6 eV) radiation.The sample analysis area is approximately 400 μm2.The spectra were averaged over five sweeps for high resolution,with step size 0.05 eV,dwell time 0.2 s,and pass energy of 50 eV.The pressure of the sample analysis chamber was lower than 5.0 × 10-9MPa during data acquisition.Data analysis was achieved using the XPSPEAK41 software pack using mixed Gaussian (20%)–Lorentzian (80%) product function and Shirley background.The binding energy is modified by charge to the C 1s signal (284.6 eV).

The Co content of the catalysts was analyzed by the inductively coupled plasma optical emission spectrometer(ICP-OES).The analysis of Ru content in the sample was conducted on ICP-MS(NexION 300X).The samples were dissolved with hydrochloric acid and hydrofluoric acid before testing.Thermogravimetric analysis(TGA) was carried out in TA Instruments SDTQ600.Analysis was done in the presence of air with flow of 30 ml.min-1and heating rate of 10 °C.min-1,heating from room temperature to 850 °C.Raman spectra of the samples were obtained from a Renishaw in Via RM1000 Raman spectrometer with a laser excitation at 514 nm,resolution is 1 cm-1.

2.4.Catalytic performance evaluation

The catalytic performance of the catalysts was measured using a 50 ml stainless-steel autoclave.A certain amount of catalysts,LA and deionized water were introduced into the autoclave.Before the reaction,the air in the autoclave needs to be discharged by pumping in hydrogen 3 times.Then,when the required temperature was preheated,hydrogen was introduced to start the reaction,and the stirring speed was 1000 r.min-1.After the completion of the reaction,the autoclave was quickly transferred to an ice bath to cool down.The product was analyzed using a SHIMADZU GC-2014 equipped with an FID detector and an AT-FFAP chromatographic column.Finally,the relevant calculation formula is as follows:

Conversion=(mol LA consumed/mol LA fed)×100%

GVL Selectivity=(mol GVL formed/mol LA consumed)×100%

For the recyclability test,LA (17.8 mmol) and deionized water(20 ml)were introduced into the autoclave containing 0.2 g of catalyst.At the end of the reaction,the catalyst was collected by centrifugation and washed with deionized water,and then the reaction was carried out under the same reaction conditions.

3.Results and Discussion

3.1.Improvement of the hydrogenation activity of the Co catalyst in aqueous phase

A series of bimetallic catalysts supported on SiO2was tested hydrogenation of LA at 70 °C in water and compared with that of Co/SiO2with a LA/Co molar ratio of 10 (Table 1,Entries 1–6,8).In the bimetallic catalysts,the loading of Co was fixed at 5.0%(mass),the amount of additive metal was fixed at 0.5%(mass).The Co/SiO2showed only 2.0 % conversion of LA within 1 hour.Unfortunately,the addition of non-noble metal metals (i.e.,Cu,and Ni) resulted in a slight improvement of conversion (2.3 % and 4.5 %,respectively)than Co/SiO2.Noble metals(i.e.,Pd,Pt,Au and Ru)were subsequently added to the Co/SiO2.CoPd/SiO2and CoAu/SiO2exhibited low conversion of LA (3.1% and 5.3%,respectively),CoPt/SiO2showed a slightly higher conversion of LA (21.2%),the corresponding monometallic counterpart Pt/SiO2shown 19.4%conversion of LA (Table 1,Entry 7).Notably,CoRu/SiO2showed the highest conversion.The introduction of ruthenium greatly improves the hydrogenation activity of the cobalt-based catalyst.With increasing of LA/Co molar ratio from 10 to 290,the CoRu/SiO2catalyst still can achieve 57.1 % conversion of LA(Table 1,Entry 10).In addition,except for GVL,there is almost no other byproducts is detected for LA hydrogenation in aqueous-phase.We carried out the kinetic profile of LA hydrogenation reaction.With increasing of reaction time to 2 hours,the conversion of LA can achieve over 90%(Fig.S1).In addition,we have summarized the activity of cobalt-based catalysts reported in some literatures for the hydrogenation of levulinic acid/ethyl levulinate.From Table S2,the cobalt-based catalysts achieved the desired activity and selectivity,not only the more severe reaction conditions,the lower ratio of reactant to catalyst,but also the choice of solvent was extremely important,water as a solvent mostly had a negative effect on its activity.However,the introduction of ruthenium can greatly improve the activity of cobalt catalysts for aqueous hydrogenation of levulinic acid.

The isotherms of Co/SiO2,Ru/SiO2and CoRu/SiO2catalysts are typical type IV with an H1 hysteresis loop(Fig.S2-a),which is typical isotherms curves for mesoporous materials.The pore size of Co/SiO2,Ru/SiO2and CoRu/SiO2catalysts is centered at 31 nm,which is calculated from desorption branch of the isotherm(Fig.S2-b).The specific surfaces area of Co/SiO2,Ru/SiO2and CoRu/SiO2is 188,204 and 186 m2.g-1,the pore volume is 1.33,1.31 and 1.21 cm3.g-1(Table S1),respectively.The textural properties similarity of the above catalysts indicates that the difference of their catalytic performance is not directly related to their porous structure.

Table 1Catalytic performance for LA hydrogenation of various catalysts

Table 2The binding energy and relative content of Co and Ru species in various catalysts,determined by XPS.

The TEM image of Co/SiO2in Fig.1a shows Co particles are randomly distributed on the surface of SiO2.As observed from Fig.1b,Ru/SiO2has good dispersion due to its low loading.The brighter regions in image of CoRu/SiO2(Fig.1c and d) are metal particles,predominantly Co.Elemental analysis of the metal particles shows that Ru appears along with Co within the large metal particles in the tested region.This phenomenon can also be proved from the elemental line scanning profiles of Co and Ru that the detection signal of cobalt and ruthenium changes synchronously,confirming the uniform distribution of Co and Ru atoms within a nanoparticle.The overlap of Co and Ru spatial distribution indicates that Co particles could contact with Ru promoter directly or indirectly.It can be concluded that there is interaction between cobalt and ruthenium.The particle size distribution of the Co nanoparticles for the CoRu/SiO2is in the range of 10–30 nm and centered at 16.7 nm.

The XRD pattern in Fig.2a shows that no diffraction peaks corresponding to Ru is observed for Ru/SiO2due to the high dispersion of the Ru or the low Ru content(0.5%(mass)).While the diffraction peaks at 2θ=41.5° and 47.3° in the XRD pattern of Co/SiO2are assigned to the reflection planes of(1 0 0)and(1 0 1)of hcp metallic Co(JCPDS 05-0727).Besides the diffraction peaks for the hcp-Co phase,the diffraction peaks at 2θ=44.3°,51.4° and 75.6° are indexed to the reflection planes of (1 1 1),(2 0 0),and (2 2 0) of the fcc-Co phase (JCPDS 15-0806).Compared with Co/SiO2,the diffraction peak intensity of CoRu/SiO2is significantly weakened,indicating the higher dispersion or smaller particle size of Co NPs in this sample.The Co particle size calculated Scherrer equation in Co/SiO2and CoRu/SiO2is 12 and 24 nm,respectively.This is larger than the average particle size calculated from TEM images.Thisis to be expected,since it has been suggested that XRD estimates always underestimate the size.Because it is based only on the diffracted surface rather than the entire particle [29].

Fig.1.STEM images materials of(a)Co/SiO2,(b)Ru/SiO2,(c,d)CoRu/SiO2 and particle size distribution of the CoRu/SiO2,(e)linear distributions of Co and Ru along the line on(d),and (f) corresponding maps of elemental distribution of the CoRu/SiO2 sample.

H2-TPR curves of CoRu/SiO2,Ru/SiO2and Co/SiO2catalysts are shown in Fig.2b.For Ru/SiO2,there is only one peak of hydrogen consumption at 240 °C,assigned to the reduction of RuCl3or Ru(OH)x [30].There are three hydrogen consumption peaks observed for Co/SiO2and CoRu/SiO2.For Co/SiO2,the first peak at 205°C is assigned to the decomposition of cobalt nitrate,the peaks around 280 and 394°C are attributed to the reduction of Co3O4taking place in two steps(Co3O4+H2→3CoO+H2O and CoO+H2→Co+H2O) [31].Notably,Co/SiO2exhibits an extremely broad reduction peaks around 394 °C,this may be related to the cobalt silicate component formed with silica support.Unlike Co/SiO2exhibits a broad peak,CoRu/SiO2exhibits sharp and symmetrical peaks,implying the different reduction behavior.And the three hydrogen consumption peaks of CoRu/SiO2are obviously shifted to lower temperature compared with that of Co/SiO2.This is because the free active H at the formed Ru0site spills over into the adjacent CoOxparticles to accelerate their reduction [32,33].The Ru0sites can dissociate hydrogen molecules into H?radicals,which are then transferred to adjacent Ni atoms,activating the inert Ni center in the aqueous phase,as demonstrated by Zhao et al.[34].This hydrogen spillover may be beneficial to hydrogenation performance of CoRu/SiO2catalyst in water.

Fig.2.(a) XRD patterns (b) H2-TPR curves of the as-prepared CoRu/SiO2,Co/SiO2 and Ru/SiO2 catalysts.

Fig.3.(a) XPS Co 2p spectra of Co/SiO2 and CoRu/SiO2 catalysts;(b) XPS Ru 3d spectra of Ru/SiO2 and CoRu/SiO2 catalysts.

The deconvolutions of Co 2p,Ru 3d and C 1s XPS spectra for the various catalysts are also conducted(Fig.3 and Table 2).As can be seen in Fig.3a,the Co/SiO2catalyst exhibits three peaks at about 779.6,780.9 and 782.8 eV,corresponding to Co0,Co3+and Co2+[35],respectively.This may because that surface of metallic cobalt particles is particularly susceptible to oxidation in the air,which is also consistent with its poor hydrogenation activity.Similar oxidation phenomena are often reported[36].There are three Co species are appearing in the CoRu/SiO2catalyst.The lower energy peak at about 780.0 eV can be assigned to Co0.The Co0atomic ratio of CoRu/SiO2become higher compared with Co/SiO2.The Ru species present near the Co species will promote the reduction of Co species through hydrogen spillover.In addition,in comparison with Co/SiO2,the CoRu/SiO2exhibits a shifting to high binding energy,such as Co3+species at about 781.5 eV for CoRu/SiO2.This is largely due to the transfer of charge from Co to Ru.

The weak peak signal of the high resolution of Ru 3p spectrum indicates the low content of Ru.As shown in Fig.3b,the combined peaks of Ru 3d and C 1s is deconvoluted into the following peaks:the C 1s spectrum is deconvoluted into four peaks at 284.4,285.5,286.6,and 289.2 eV,corresponding to C==C,C-O,C==O and O-C=O,respectively [37].Because the catalysts were reduced under a H2flow of 30 ml.min-1at 400°C for 2 hours,the Ru species should be reduced completely,according to the results of the H2-TPR curves.And the full XPS derived elemental concentration for all catalysts is shown in Table S3,almost no chlorine is observed on the surface of the catalysts(Fig.S3).For the Ru 3d5/2 core level of Ru/SiO2,the two peaks at 280.6 and 282.3 eV are assigned to metallic Ru0and cationic Run+(the Ru species at the interface of Ru nanoparticles and the SiO2support) [38–41].Compared with the corresponding Ru/SiO2catalyst,the C 1s XPS spectrum is basically the same as CoRu/SiO2,the binding energy of Ru 3d has a tendency to shift to the lower binding energy (Fig.3b).While the binding energy of Co 2p tends to shift to the higher binding energy,which indicates there should be an electron transfer from less electronegative Co to more electronegative Ru,resulting in a strong interaction between the Co and Ru.These results are in accordance with the easier discussion of the TPR,the XRD and the TEM.

Fig.4.The stability test of hydrogenation of levulinic acid to γ-valerolactone by CoRu/SiO2 and CoRu@C/SiO2 catalyst.Reaction conditions:LA,17.2 mmol,LA/Co molar ratio=100;H2O,20 ml;70 °C,1 h,H2 pressure,4.0 MPa,1000 r.min-1.

3.2.Improvement of the stability of the Co catalyst in aqueous phase

The stability of the active bimetallic CoRu/SiO2catalyst was subsequently investigated at 70 °C in water with a LA/Co molar ratio of 100 (Fig.4).Unfortunately,for CoRu/SiO2catalyst,there is a considerable decrease in catalytic activity during these recycling tests.The conversion of LA decreases from 90% to 50% for the five runs,demonstrating that the CoRu/SiO2catalyst showed a significant deactivation in this aqueous phase hydrogenation reaction.The common reasons of metal catalyst deactivation in aqueous-phase hydrogenation include leaching of active components and collapse of support structure.Therefore,the loading of metal components was detected.As revealed by ICP-OES results in Table S1,the cobalt content decreases from 5.61 %(mass) to 1.80 %(mass) after five consecutive runs,indicating that the phenomenon of the loss of metal active components is relatively severe.Therefore,the main reason for the deactivation of CoRu/SiO2catalyst is the leaching of the cobalt.

The leaching of active phase is fatal for catalytic process in liquid media [42],which can pollute the products and make recycle meaningless.Encapsulating the CoRu/SiO2catalyst in a carbon shell is an effective strategy.With the help of this protective layer,the metal active components are less prone to oxidize and detach from the support under hydrothermal conditions.The CoRu@C/SiO2catalysts were prepared by pyrolyzing the tannisligated cobalt-ruthenium complex on silica at different temperatures (500,600,700,and 900 °C) under N2atmosphere.The activity of the catalysts gradually increases with the pyrolysis temperature increasing from 500 to 700 °C,while the activity of the CoRu@C/SiO2prepared at 900 °C decreases sharply (Fig.S4).The CoRu@C/SiO2prepared at 700°C is the most active and shows a LA conversion of 51%,which is slightly lower catalytic activity than CoRu/SiO2under identical conditions (Entry 12 vs Entry 10,Table 1).It should be noted that the CoRu@C/SiO2shows high stability and can be easily recycled and reused five times without significant loss of activity or selectivity (Fig.4).The high stability of the CoRu@C/SiO2catalyst may largely depend on the confined effect of the carbon shell,which can suppress the leaching of the cobalt.The nitrogen adsorption–desorption isotherms of the CoRu@C/SiO2are type-IV adsorption isotherm with an H1 hysteresis loop (Fig.S5),which is typical for mesoporous materials,and the textural properties are summarized in Table S1.The specific surface area of CoRu@C/SiO2is 177 m2.g-1,and pore size is 31 nm.This result indicates that the physical properties of the CoRu@C/SiO2is similar with the CoRu/SiO2.The XRD patterns of the CoRu@C/SiO2is presented in Fig.5a.There are four diffraction peaks are observed,which are attributed to the metallic Co phase.However,compared with CoRu/SiO2,the diffraction peak intensity of CoRu@C/SiO2is significantly enhanced.The Co particle size in CoRu@C/SiO2is 24 nm calculated by using Scherrer equation.This result may be due to the agglomeration of metals during pyrolysis.Consequently,the stability of CoRu@C/SiO2is significantly improved and the activity remains almost the same,although Co particle size CoRu@C/SiO2is larger than that of CoRu/SiO2.In order to study the positive effect of carbon coating on metal leaching,we detected the metal content in the samples before and after the five consecutive runs.As revealed by ICP-OES (Table S1),we can find that the loss of metallic cobalt has been greatly inhibited,from 5.65 %(mass) to 4.04 %(mass) (28.3% Co leaching).However,67.9% of cobalt in CoRu/SiO2are leached (Table S1).The above results suggest that the CoRu@C/SiO2has better hydrothermal stability and resistance to metal leaching,and achieves higher catalytic stability.At the same time,from Table S1 and Fig.S6,we can see that the specific surface area of CoRu@C/SiO2is 177 m2.g-1,and the specific surface area of CoRu@C/SiO2after five consecutive runs becomes 179 m2.g-1,which shows little change compared with fresh catalyst.

Fig.5.(a) XRD patterns;(b) H2-TPR curves;(c)XPS Co 2p spectra and (d) XPS Ru 3d spectra of CoRu@C/SiO2 catalysts.

Fig.6.A series of characterizations for the CoRu@C/SiO2 (a,b) TEM images,(c) STEM image and (d) STEM-mapping.

Fig.5b shows the H2-TPR curve of CoRu@C/SiO2catalyst.The reduction behavior of the catalyst presents two main reduction peaks,one is the low temperature peak (130 °C),the other is the high temperature peak (300 °C).Since the CoRu@C/SiO2catalyst undergoes a pyrolysis process,the metal component is reduced by carbon,the signal of the hydrogen consumption is weak.The first peak corresponds to the reduction of Co3O4to CoO and the second peak is mainly attributed to the reduction of CoO to cobalt in the second step.Fig.5c shows the Co 2p spectra of CoRu@C/SiO2,the peak at 780.2 eV (satellite peak at 786.8 eV) is ascribed to Co0specie,the peak at 783.2 and 781.9 eV are assigned to Co2+and Co3+,respectively.Fig.5d shows the Ru 3d and C 1s spectra of CoRu@C/SiO2,the C 1s spectra in CoRu@C/SiO2at binding energies of 288.9,286.8,285.3 and 284.5 eV are assigned to C=C,C-O,C=O and O-C=O,respectively(Fig.5d).The Ru 3d spectrum of CoRu@C/SiO2exhibits two predominant peaks located at 280.5 and 282.2 eV,which move to higher binding energy compared with CoRu/SiO2.This may result from an electron transfer might occur at the interface of carbon and the adjacent Ru species,thus leading to the enhancement of surface electron density of active Ru species.Moreover,the relative content of different cobalt species(excluding cobalt satellite peaks)is obtained through the ratio of peak areas.Compared to CoRu/SiO2,the content of Co0in the CoRu@C/SiO2increases from 20.5%to 25.7%,indicating that the carbon shell can further inhibit the oxidation of cobalt on the surface.

A representative TEM image of CoRu@C/SiO2(Fig.6a) reveals that some agglomeration of Co particles after pyrolysis step,in line with the XRD results.Fig.6b showed that it is difficult to observe the coated carbon layer.It is speculated that the carbon layer may not be observed clearly because of the thin carbon layer.Fig.6c and d showed the STEM image and element mapping of CoRu@C/SiO2.However,no higher magnification image can be given due to the configuration of the instrument.Even so,we can notice that C appears along with Si in the tested region,and Co and Ru NPs are uniformly dispersed in the tested region throughout the SiO2support.It is worth noting that the distribution of C and Si is basically the same,and carbon may cover the entire material uniformly.From the Raman spectra,CoRu@C/SiO2showed absorption peaks at 1340 cm-1and 1570 cm-1attributed to the D and G bands (Fig.S7),which correspond to the defects and the graphite structure in the carbon material,respectively.From the TGA-DTG curves of CoRu@C/SiO2,the carbon content is approximately 2% (mass) (Fig.S8).And a weight increase phenomenon occurred at about 200 °C.This phenomenon may be caused by the oxidation of cobalt metal to CoOxby air.Oxidation of cobalt can be significantly inhibited by the carbon shell from the weight increment at about 200 °C.In conclusion,the thin carbon layer coated on the catalyst surface can improve oxidation resistance and inhibit leaching of cobalt.

4.Conclusions

In summary,an ultrathin carbon layer coated CoRu bimetallic catalysts (CoRu@C/SiO2) dispersed on silica has been successfully achieved by pyrolysis the silica supported tannis-ligated cobaltruthenium complex under inert atmospheres.The ruthenium can greatly improve the activity of cobalt-based catalyst in the process of aqueous hydrogenation of levulinic acid.The intimate contact between Ru with CoOxfacilitates the reduction of CoOx.The high dispersion of Co0and Co-Ru synergistic effect improves GVL yield.The CoRu@C/SiO2shows high catalytic activity and stability for the catalytic hydrogenation of LA to GVL,which benefits from the protection of the ultrathin carbon layer structure.The CoRu@C/SiO2is reused up to 5 runs without obvious loss of its catalytic activity in aqueous hydrogenation of levulinic acid.

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

This work was supported by the National Natural Science Foundation of China (21908197) and Natural Science Foundation of Zhejiang Province (LY17B030010).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.07.004.