A multi-functional Ru Mo bimetallic catalyst for ultra-efficient C3 alcohols production from liquid phase hydrogenolysis of glycerol

Guoxiao Cai ,Wei Xiong ,Susu Zhou ,Pingle Liu,*,Yang Lv ,Fang Hao,*,Hean Luo,ChangYi Kong

1 College of Chemical Engineering,National &Local United Engineering Research Centre for Chemical Process Simulation and Intensification,Xiangtan University,Xiangtan 411105,China

2 Department of Environment and Energy System,Graduate School of Science and Technology,Shizuoka University,3-5-1 Johoku,Naka-ku,Hamamatsu 432-8561,Japan

Keywords:Heterogeneous catalysis Bimetallic catalyst Glycerol hydrogenolysis Ruthenium Molybdenum Phosphotungstic acid

ABSTRACT Ru and Mo bimetallic catalysts supported on active carbon modified by phosphotungstic acid(PW)were designed and applied in glycerol hydrogenolysis reaction.The physicochemical properties of the catalysts were characterized and the presence of active sites was investigated from the perspective of the glycerol hydrogenolysis performance.The MoOx is highly selective for the C-O bond cleavage of glycerol molecules,which can reasonably regulate the strong C-C bond cleavage activity of Ru nanoparticles.By using sequential deposition of Ru and Mo supported on mesoporous PW-C,the characterization results show that the combination of isolated low-valence MoOx with metal Ru particles can form ‘‘MoOx-Ru-PW”,which provides highly catalytic activity toward C-O bond cleavage,selectively producing more C3 alcohols (mainly 1,2(3)-propanediol).The glycerol conversion of 1% Mo/Ru/PW-C catalyst was 59.6%,the selectivity of C3 alcohol was 96.1%,and the selectivity of propanediol (1,2(3)-propanediol) was 94.9%.It is noteworthy that the selectivity of 1,3-propanediol reached 20.7% when the PW was 21.07% (mass).This study provides experimental evidence for the tandem dehydration and hydrogenation mechanism of the multifunctional Mo/Ru/PW-C catalyst.

1.Introduction

Since the fossil fuels are threatened by depletion,the full utilization of renewable energy is very essential for the sustainable development of society.Biodiesel,a kind of renewable energy,is formed by the transesterification of triglycerides in biomass.The utilization of green catalytic technology to convert bio-renewable raw materials into commercial chemicals and clean fuels have attracted widespread attention [1].With the substantial increase in the annual output of biodiesel,the main by-product glycerol has become more and more abundant [2] and has been reported as one of the top 12 cornerstone chemical platform compounds in renewable biomass products[3].Therefore,products of glycerol hydrogenolysis have become a research hotspot in recent years.Especially the 1,2(3)-propanediol (1,2(3)-PDO) is the basic raw stock,severs as significant production of cosmetics,antifreeze,pharmaceuticals,paints,polyester resins,and food additives [4].1,3-PDO is a kind of chemical substance and one of the six new petrochemical products with great utilization value.Its polycondensation monomer can be used in plastics production,typically as poly terephthalate,polyurethane,methyl ester polyester,monomer ring compound,and polyether,etc.[1-4].At present,how to develop efficient hydrogenation catalysts with outstanding catalytic activity (Include: high selectivity and stability) has become the key to promote the production and application of the glycerol hydrogenolysis technique.Meanwhile,the proportion of C-C/ CO in glycerol molecules,which requires a thorough and complete understanding of bonding and bond cleavage mechanisms in biodiesel derivatives.

Currently,selective hydrogenolysis of glycerol to propylene glycol(PDO)requires both metal active sites and acid sites(Lewis acid or Br?nsted acid)[5].Pt nano-metal is perceived as an effective catalyst (e.g.,Pt-WOxbased catalysts) for glycerol hydrogenolysis,whose performance is closely related to the type of oxide carriers.By now,this catalytic system of PGM (precious group metal)/oxides are one of the main ways for the preparation of 1,3-PDO by glycerol hydrogenolysis [6-10].On the premise of maintaining the stability of catalytic activity,ruthenium (Ru) as the catalytic active center,instead of expensive platinum,will greatly help to control the production cost of catalyst preparation.Therefore,the preparation of Ru-based catalysts equivalent to the hydrogenolysis reaction of Pt-based catalysts will help to reduce the cost of glycerol hydrogenolysis industry and ensure the low cost of upstream and downstream industries.Additionally,Ru has been extensively reported to catalyze the selective hydrolysis of glycerol to PDOs in the past few decades [11].The hydrocracking ability by metal Ru toward the bond of C-C in glycerol molecules is evidently more intense than that toward C-O under the condition of neutral or weak acid[12].Accurately,the catalytic selectivity of Ru in glycerol molecules (C-C or C-O) is also highly dependent on the physicochemical structure and the mean nanoparticles size of Ru.Nakagawa [13] found that Ru supported catalysts with high surface area(BET)and specific physicochemical structure exhibited admirable activity(Sel.1,2-PDO=44.8%;Sel.1,3-PDO=4.2%;Conv.Gly=51.7%)after comparing the catalytic activity of Ru loaded into the pore structure or merely on the surface of different supports.As a matter of fact,ruthenium nanoparticles can be uniformly dispersed on mesoporous materials or microporous materials to form active sites with unique physicochemical characteristics [13-15].Wuet al.[16] developed another Ru-based heterogeneous catalytic supported on active carbon (AC) and added with levulinic acid(LA).At 200 ℃and 3 MPa,the conversion of glycerol was 64.5%,and the selectivity of 1,3-PDO was 35.8%.Our previous work combined Ru-Mo/CNTs catalyst and phosphotungstic acid (PW) in the reaction system to carry out glycerol hydrogenolysis.It was found that the selectivity of 1,3-propanediol(PDO)could be improved by increasing the dosage of Br?nsted acid.The optimal catalyst was Ru-1% Mo/CNTs with glycerol conversion of 55.2% and C3 alcohol selectivity of 96.8%.In addition,26.4%of 1,3-PDO was obtained by adding 0.12 g phosphotungstic acid [17].The main reason is that tertiary carbon cations are more stable and more easily formed than that secondary carbon cation on account of their stable hyperconjugation.Besides,the protonation could be eliminated by Br?nsted acid.The 2-OH of the glycerol molecule may be dehydrated,which is of great help in the hydrogenation of glycerol to 1,3-PDO [18].However,this is under a heterogeneous catalytic.The catalytic reaction is extremely complicated and the cost of subsequent separation of phosphotungstic acid is also extremely unfavourable for industrial production.

As in our previous work,the development of bimetallic catalysts by the metal promoter modification is another effective way to promote the activity of hydrogenolysis.Ru-based bimetallic catalysts such as Ru-Cu [19],Ru-Mo [20],and Ru-Fe [21]) all demonstrate good catalysis performance towards C-O bond in complex selective hydrogenolysis reactions (e.g.,selective hydrogenolysis of carbonyl and carboxyl) [22].Specifically,Ru-Cu bimetallic catalyst was prepared by adding ionic liquid on clay[19],and Ru-Fe/CNT[21]was a kind of efficient bimetallic catalyst for hydrolysis of glycerol to ethylene glycol (EG).In addition,the effects of metal oxides on hydrolysis are also imparity,which is attributed to different structural characteristics of Ru formed with different metal oxides.These results also indicated that the bimetal had a great advantage in catalytic selectivity compared with mono metal [23].However,Ru-based catalysts could excessively stimulate C-C bond cleavage for glycerol molecules than most metals according to the article description: The activity intensity: Ru ≈Rh ＞Ni ＞Ir ＞Co ＞Pt ＞Pd ＞Fe [24].Therefore,the mechanism of glycerol hydrogenolysis is extremely complicated but full of challenges and exploration of application value.The possible proposed mechanisms of glycerol hydrogenolysis are shown in Fig.1 [19-28].

Fig.1.Possible mechanism of glycerol hydrogenolysis [19-28].

In the present work,we explored the possibility of improving selective glycerol hydrogenolysis to prepare 1,3-PDO by introducing molybdenum oxide,which could effectively regulate the strength of Mo=O bond adsorption with -OH of glycerol.The active center of the multi-component catalyst was designed effectively,and the specific function of each component was clarified respectively.In our previous work,the addition of PW has exhibited excellent catalytic performance in the bimetallic Ru-1% Mo/CNTs catalyst system,thus an effort to combine PW in the catalytic structure to form heterogeneous catalyst and replaces more expensive CNTs with active carbon were carried out for developing more effective but low-cost catalyst with high hydrogenolysis activity.

A series of phosphotungstic acid modified active carbon supported Ru and Mo bimetallic catalysts were designed by using the sequential metal deposition of Ru and Mo metals.And these catalysts were applied to glycerol hydrogenolysis reaction.The correlation between the oxidation valance state of Mo in Ru and Mo bimetallic catalysts,different supported,loading and deposition methods (co-impregnation and sequential deposition),as well as their catalytic performance were studied.The hydrocracking mechanism of glycerol on precious metal Ru catalysts (especially after metal molybdenum modification) and the affecting factors of C-C or C-O bonds cleavage were simultaneously investigated.The potential catalytic active sites and mechanisms were also discussed.

2.Experimental

2.1.Materials

Glycerol,1,2(3)-propanediol and 1,4-butanediol (The internal standard substance of GC)were acquired from Macklin Biochemical Co.,Ltd(Shanghai).Active carbon from Nanotech Port Co.,Ltd(Shenzhen).Ruthenium trichloride hydrate (RuCl3·xH2O) from Xilong Chemical Co.,Ltd (Sichuan).Ammonium molybdate tetrahydrate(H8MoN2O4)and phosphotungstic acid(H3O40PW12·xH2O,PW)from Kemiou Chemical Co.,Ltd(Tianjin).

2.2.Catalyst preparation

The preparation method of PW-C is as follows:For the first,1 g active carbon (AC) and 0.5 g phosphotungstic acid were weighed accurately and dispersed in deionized water (DIW,solid to liquid:1:10;10 g for AC and 5 g for PW) for 30 min by ultrasonic apparatus,respectively.After that the mixed solution of AC and deionized water was continuously stirred at the speed of 300 r·min-1and the mixed solution of PW was added drop by drop into the mixed solution of AC.Finally,the mixture was dried overnight at 80 °C,and calcined at 400 °C for 2 h with a nitrogen flow rate of approximately 8 cubic feet per minute.

The preparation process ofy% Mo/Ru/PW-C catalyst is as follows: For the first,PW-C was dispersed in DIW (solid to liquid ratio:1:10)at 30°C for 30 min by ultrasonic apparatus.RuCl3·xH2-O (0.098 mmol) and H8MoN2O4(0.0105 mmol,0.0210 mmol,0.0630 mmol,0.105 mmol,respectively) were dissolved with the appropriate amount of DIW (Solid to liquid of 1:10) at ambient temperature by ultrasonic dispersion.After that the PW-C solution was continuously stirred at the speed of 300 r·min-1by the dropwise addition of Ru precursor aq.which was labeled of Ru/PW-C.Then Mo precursor aq.was added into the stirring solution of Ru/PW-C drop by drop.Subsequently,stirring overnight at 110 °C.Finally,calcined with about 8 cubic feet per minute nitrogen flow at 400 °C for 2 h which was labeled asy% Mo/Ru/PW-C.The Ru loading content was fixed to 2% (mass).

For comparison,the catalysts of Ru-y%Mo/PW-C were prepared by the method of co-impregnation.RuCl3·xH2O (0.098 mmol) and H8MoN2O4(0.0150 mmol,0.0210 mmol,0.0630 mmol,0.105 mmol,respectively) were simultaneously dissolved to each other consistent with the foregoing.Finally,the same process as other catalysts include mixing,stirring,drying overnight (110 ℃),grinding,and calcining as well as other catalysts.

2.3.Characterization

The material structure was visually analyzed and imaged on a transmission electron microscope (TEM) of TecnaiG2-TF30(Thermo Fisher Scientific,USA)with 300 kV electron beam acceleration voltage.Energy dispersive spectrometer (EDS-mapping,Thermo Fisher Scientific,USA) was used to analyze the types and contents of elements in the micro-region of the material with scanning electron microscopes and transmission electron microscopes.The catalysts were accurately weighted and meticulously placed in the tube of FLEL for combustion,then collected by CuO mesh and the element composition was finally calculated.The crystalline of each sample was measured by the X-ray powder diffractometer(XRD)spectra of X‘Pert-Pro MPD(Holland analytical,Netherlands).The catalyst was placed on the aluminium tube,the voltage of 40 kV,the angle range of 5° -90°,the current of 40 mA,and the scan speed of 10(o)·min-1.NH3-temperature programmed desorption(NH3-TPD)was carried out on automatic chemisorption apparatus of AutoChem II 2920 (Micromeritics,USA) to trace the acid properties of different catalysts.The sample was purged with flowing He at 400°C for 1 h.After air-cooling down to 100°C,continue to purge the mixed gas(10%NH3-90%He)until different positions of the catalyst surface were adsorbed to saturation by NH3.Subsequently,the NH3desorption was carried out with a heating rate of 30 °C·min-1range from 100 °C to 1000 °C.X-ray photoelectron spectroscopy (XPS) of different reduction catalysts was carried out on K-Alpha XPS (Thermo Fisher Science,USA).Highresolution spectra were obtained under the excitation of monochromatic Al-Kα source of 1253.6 eV.The binding energy scans spanned from 0 to 1100 eV at the interval of 100 eV to 1.00 eV and the measurement accuracy is±0.1 eV.The isothermal equation of N2adsorption-desorption was calculated with a 2020 ASAP Plus physical adsorption (Micromeritic,USA).Before testing,the sample was pre-treated under 150 ℃nitrogen atmosphere for 2 h,and the ad(de)sorption isothermals were developed at temperature-196 ℃fluid nitrogen.The surface area was obtained by the BET method calculation.The mean pore diameter (DP) of mesopores were calculated by the BJH method.The total pore volume(VP) was determined at a relative pressure (P/P0) of 0.99.The acid properties were analyzed by a ThermoNicolet 380 spectrometer(Thermo Fisher Scientific,USA).The contents of different metals in the catalyst were quantitatively analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES) (Analytik Jena AG,Germany).0.05 g samples were weighed (accurate to 0.0001 g) and placed into Teflon crucible.In the fume hood,1 ml of experimental water were added to the crucible for samples of wetting,and 5 ml concentrated hydrochloric acid is placed on an electric heating plate to dry at 180 ℃and removed to cool slightly.Then add 5 ml of concentrated nitric acid,5 ml of hydrofluoric acid,and 3 ml of perchloric acid.After the lid is closed,the crucible is heated on the electric plate at 180 ℃until the residual liquid is 2 ml.When heated to a thick white smoke,the lid breaks down the black organic carbides.Then,3 ml of concentrated nitric acid,3 ml of hydrofluoric acid and 1 ml of perchloric acid were added to repeat the digestion process.After cooling,transfer to a 100 ml volumetric flask and wash the crucible with an appropriate amount of nitric acid solution,then transfer the leaching solution to the volumetric flask,and use nitric acid solution to constant volume to the marking line,mix and wait for measurement.The chemisorption technique of H2programmed temperature reduction (H2-TPR) was used by Auto Chem II 2920 (Micromeritics,USA).According to the operating steps described in the literature,the dispersion of Ru nano-metals was calculated by hydrogen chemisorption [29].First,the sample was pre-reduced at 300 ℃last 1 h then evacuated by the mixed gas (5% H2/95% Ar).The furnace was air-cooled to room temperature secondly and then Rus-O species was formed under vacuum for He (99.999%) adsorption.Finally,the hydrogen adsorption was carried out at 373 K for the amount of Rus-O species precise quantification.According to the stoichiometric ratio of H/Rus=3,the metal Ru dispersity was calculated as following Equations.

2.4.Catalytic test procedure

A 50 ml autoclave (Yanzheng instrument,304 stainless steel made in China)lined with PTFE was used to minimize the influence of the metallic wall in glycerol hydrogenolysis.The glycerol substrate,DIW,and different catalysts were placed into the autoclave one by one,respectively.Then the reactor was flushed several times with H2,aiming to replace the air in the autoclave before the reaction.After that,this reactor was heated to a specific temperature and maintained at different H2atmospheres for different reaction times with constantly speeding stirring at 800 r·min-1.Finally,cooled to 30 ℃by water.The catalysts after reaction were separated by centrifuge with 2000 r·min-1.The analytical method was used by an H2flame ionization detector (FID) of gas chromatography (GC,Shimadzu,Japan) with a capillary column (KBWAX of Kromat,USA).

The conversion and products selectivity of glycerol were calculated by following Eqs.(3) and (4):

The turnover frequency (TOF) was defined as the molecules of glycerol converted to different products of C3 alcohols (1,2(3)-PDO,1(2)-PO) by per Ru atom at per hour,which counted on the quantity of reactive Ru metals exposed on the surface determined from H2-TPD (Eq.(5)).According to the results of qualitative and quantitative analysis,the carbon mass balance,and its mass loss of the glycerol hydrogenolysis were carried out.

3.Results and Discussion

3.1.Ruthenium on active carbon modified by phosphotungstic acid(PW-C)

As shown in Fig.2,the modified supported PW-C,Ru/C and Ru/PW-C catalysts were characterized by TEM.The pore structure of AC was clearly visible without obvious blockage after loading a large amount of PW,as shown in Fig.2(a),indicating that loading of PW was successful.It is consistent with the results of Ning’s study [30].Meanwhile,the elemental analysis spectra in Fig.2(e)also show that the special polymer structure of PW molecules was uniformly dispersed on the activated carbon [31].After Ru metal was loaded on PW-C or AC,plenty of Ru nanoparticles were accumulated and agglomerated (Fig.2(b)-(d)).According to the statistical calculation of average particle size distribution,Ru/C nanoparticles were mainly distributed in around 2.5 nm,which was related to the sintering of Ru nanoparticles in the process of high temperature reduction.However,no obvious particle sintering was observed on Ru/PW-C,the Ru nanoparticles were uniformly dispersed,and the size of nanoparticles were mainly distributed in 2.2 nm (Fig.2(c)).

To further analyze the sample porosity,nitrogen adsorption isotherm was used to characterize the carbon-based supported.It shows that the different catalysts have microporous-mesoporous structure (Fig.S1).The mesopore sizes of PW-C were calculated to be 2.0 nm.Its specific surface area was 1455.14 m2·g-1and total pore volume was 0.192 cm3·g-1(Table S1).All catalysts did not show significant changes in pore structure,even after a large amount of PW was loaded on AC.The adsorption amount rises sharply whenP/P0is very low.This is because the interaction between the adsorbent and the adsorbate enhances in narrow micropores (molecular-sized micropores),as a result,micropores are filled under extremely low relative pressure.However,when approaching the saturation pressure,the adsorbate is condensed,leading to an increase in the slope of the curve.On the other hand,an H4 type hysteresis loop appeared in the high-pressure region(P/P0＞0.4).The PW-C,a typical curve of micro-mesoporous carbon materials,with narrow fissure holes.The carbon-based support PW-C still has larger specific surface area of 1455.14 m2·g-1to make Ru and Mo fully dispersed.These decreases in specific surface area and pore volume are consistent with the literature,indicating that HPW is deposited in mesopore and well dispersed on AC surface [31-33].

To accurately measure the content of phosphotungstic acid(PW),the PW-C tested by ICP-OES and the elemental analysis.In fact,phosphotungstic acid loses all water of crystallization between 250 °C and 400 °C and becomes anhydrous(H3PW12O40).At temperatures above 600 °C,the phosphotungstic acid structure begins to decompose in the nitrogen atmosphere(Keggin structure decomposition with the crystallization of oxides,resulting P2O5and WO3).Moreover,HPW/AC is stable and can be used at temperatures as high as 450 °C [34,35].Therefore,using the anhydrous phosphotungstic acid formula of H3PW12O40combined with the ICP test results,the load of phosphotungstic acid of PW-C can be calculated as 23.65% (mass).

Meanwhile,the doping of 15%heteropoly acid molecules with a cross-section of 144 ?2(1 ?=0.1 nm)was equivalent to the monolayer coverage of 45 m2·g-1[36].The surface area of AC was 2227.63 m2·g-1and that of PW-C was 1455.14 m2·g-1(Table S1),suggesting that the dispersion of PW in activated carbon was sub-monolayer coverage.Moreover,this coverage of submonolayer PW could ensure a strong interaction between PW and active carbon,thus improving the thermal stability and avoiding loss in the process of hydrogenolysis [31].

The catalytic supported PW-C,active carbon,Ru/PW-C,and Ru/C were analyzed by X-ray diffraction (XRD),as shown in Fig.3(a).The broad shoulder peaks of all catalysts located at around 25.0°and 45° were attributed to amorphous carbon [37].The peaks of PW could also be seen on the surface of activated carbon located at 20.0°,and the intensity of amorphous carbon peak decreased on the surface of each catalyst,which indicated that the doping of PW was successful.Surprisingly,the metal dispersion on the surface of catalysts was more uniformity by doping of PW in Ru/PW-C.Additionally,no diffraction peaks were assigned to Ru(Fig.3(a)),indicating that Ru nanoparticles were highly dispersed,which is consistent with TEM characterization results in Fig.2.It is worth noting that the peak of phosphotungstic acid become obvious when the ruthenium metal was loaded,which might be due to a special interaction between Ru and PW.

The hydrogen temperature reduction (H2-TPR) of different Rubased catalysts was studied (Fig.3(b)).The initial reduction peak of PW-C appeared at 435 °C,which was assigned to the methanation of carbon under the presence of hydrogen with high temperature [38].According to the literature description,the phosphotungstic acid begins to decompose at 200 ℃and loses its crystal water,and the Keggin structure begins to decompose into WO3and P2O5at 470 ℃.The peaks after 470 ℃were assigned to the reduction peak of tungsten oxide (WOx).Therefore,the situation of phosphotungstic acid after 470 ℃is not discussed in this experiment [17].For the catalyst Ru/C,the reduction peak at 211℃was the RuOxspecies,and the broad shoulder reduction peak after 400 °C was carbon methanation.After the introduction of PW on the AC,the reduction peak of Ru/PW-C had an obvious reduction peak and a negative peak.The first peak was assigned to the weak interaction between the RuOxand PW-C,and the second negative peak was assigned to the strong metal-supported interaction(SMSI).Considering the strong activity of Ru nanoparticles on the cleavage of the C-C bond,the methanation peak of Ru/PW-C was more advanced than that of Ru/C,because the SMSI between Ru and PW-C has been enhanced by doping of PW.Comparing Ru/PW-C,Ru/C and PW-C,it was found that the reduction peak of PW shifted forward and its density decreased after doping with Ru,which also indicated that the interaction between Ru metal and PW-C has been improved significantly by the doping of PW.Compared with Ru/C,the catalyst of Ru/PW-C showed an obvious hydrogen reduction peak at 80 ℃ascribed to RuOx,which may be due to that the doping of PW is favourable for the dispersion of Ru metals and promotes the ability of ruthenium to dissociate hydrogen.

Fig.2.TEM images of different Ru-based catalysts: (a) PW-C,(b) Ru/C,(c-d) Ru/PW-C,(e) EDS-mapping of PW-C.

Surface acidity (NH3-TPD) patterns in Fig.3(c) show that the surface acidity was all promoted by PW doping.To reveal the role of PW,different catalysts of PW-C,Ru/C,and Ru/PW-C were used for glycerol hydrogenolysis in the liquid phase,and their performances are shown in Fig.3(d).In the temperature range of 0-400 ℃,the doping of phosphotungstic acid can significantly increase the surface acidity of the catalyst.Meanwhile,the acid intensity of Ru/PW-C was significantly increased,indicating that a strong interaction between Ru and phosphotungstic acid was generated.

The catalyst Ru/C performed extremely active towards the bond of C-C cleavage(direct hydrogenolysis of C-C bond),whose products were widely distributed under the conversion of 59.6%,most of the products were others (CH4,CO,methanol,etc.).However,the conversion increased from 59.6% to 60.5% and the selectivity of ethylene glycol (EG) significantly decreased from 35.7% to 3.7%,because the cleavage activity of the C-C bond was reduced when PW was introduced,and the path of reaction transformed from direct hydrogenation by Ru to acid dehydrationhydrogenation.It is worth noting that the selectivity of C3 alcohols increased from 40.3% to 78.9%,in which the selectivity of 1,3-PDO significantly increased from 0% to 7.9%.To verify the function of PW,Ru/C was used for glycerol hydrogenolysis with the addition of 0.1 g PW,which demonstrated that the cleavage activity of the C-C bond was also reduced,and the performance results show that the selectivity of EG decreased from 35.7%to 4.2%.Meanwhile,the selectivity of C3 alcohols increased from 40.3% to 76.1%,in which the selectivity of 1,3-PDO was notably increased from 0% to 5.8%after the addition of PW.It was found that the single PW-C did not show catalytic activity in the glycerol hydrogenolysis,indicating that Ru played a necessary role as the active metal.Wanet al.[26,39-41]point out that PW can produce large amounts of Lewis acids,that hydroxy-acetone is always formed by hydrogenation and dehydration,and that further hydrogenation of hydroxyacetone can also yield 1,2-PDO.However,the PDOs could be excessively hydrogenated to produce POs.Meanwhile,it also shows that PW can not only provide a lot of acidities,but the strong interaction with precious metal (Ru) can anchor the precious metal to form higher dispersion of the metal particles [42,43].

3.2.Different bimetallic catalysts of Mo/Ru/PW-C and Ru-Mo/PW-C prepared by co-impregnation and sequential deposition of metal Ru and Mo,respectively

After the advantages of PW-C was revealed,metal molybdenum(MoOx) was introduced as a promoter to modify the catalyst Ru/PW-C by co-impregnation and sequential deposition of metal Ru and Mo,which were labelled as Ru-5% Mo/PW-C and 5% Mo/Ru/PW-C,respectively.The mean nanoparticles size of Ru in Ru-5%Mo/PW-C prepared by co-impregnation method(Fig.4(a))slightly decreased from 2.2 nm of Ru-5% Mo/PW-C (Fig.2(c)) to 2.0 nm(Fig.4(a1)).Meanwhile,the metal dispersion of 5% Mo/Ru/PW-C prepared by sequential deposition also performed well,and Ru mean nanoparticles size were almost 2.0 nm(Fig.4(b1)).However,HR-TEM shows that the metal dispersion on the catalyst surface was better than that of the catalyst Ru-5% Mo/PW-C,because the surface of 5% Mo/Ru/PW-C has no obvious particle agglomeration and sintering.As shown in Fig.4(a2) and Fig.4(b2),the particle agglomeration on the catalyst surface of Ru-5% Mo/PW-C is obvious than that of 5% Mo/Ru/PW-C.At the same time,the particle size statistics showed that the surface particles of Ru-5% Mo/PWC were distributed between 1.6-3.6 nm,its maximum reached 3.6 nm,while the 5% Mo/Ru/PW-C was between 1.2-3.0 nm,its maximum 3.0 nm.All of evidence suggests that the sequential deposition of two metal could further promote the metal dispersion.

The Ru-5 %Mo/C,Ru-5% Mo/PW-C,5% Mo/Ru/PW-C,and PW-C were analyzed by X-ray diffraction (XRD),as shown in Fig.4(c).There are multiple molybdenum oxide (MoOx) diffraction peaks on the surface of Ru-5%Mo/C,while the diffraction peaks disappear in Ru-5% Mo/PW-C and 5% Mo/Ru/PW-C,which also shows that PW can promote the dispersion of Mo.It is consistent with our previous characterization.The elemental analysis mapping shows that Mo metals were distributed in the whole Ru region and in close contact with Ru.

Fig.4.(a)Ru-5%Mo/PW-C TEM images(a1 particle size statistical of a,a2 partial aggregation of particle on a);(b)5%Mo/Ru/PW-C TEM images(b1 particle size statistical of b,b2 partial aggregation of particle on b);(c) XRD patterns of different Ru-catalysts;(d) EDS-mapping of PW-C.

The interaction between Ru and Mo was investigated by XPS characterization,as shown in Fig.5(a).The Ru 3d peaks of Ru/PW-C,Ru-5% Mo/PW-C,and 5% Mo/Ru/PW-C all appeared at Ru 3d3/2(～280.2±0.8)eV and Ru 3d5/2(～285.2±0.8)eV[44-46].Virtually,the Ru 3d3/2optoelectronic emission line overlapped with C1s at the bind of 284.8 eV,Ru 3d5/2was used to analyze the surface composition.As shown in Fig.5(a),the binding energy of Ru 3d5/2orbital of catalyst Ru/PW-C was located at 281.3 eV.Interestingly,the Ru 3d5/2orbitals of both Ru-5%Mo/PW-C and 5%Mo/Ru/PW-C were shifted to lower binding energy after Mo doping.For Mo 3d5/2spectrograms,binding energies individually located in 232.9 eV,231.6 eV,229.2 eV and 228.2 eV were corresponded to Mo6+,Mo5+,Mo4+and Mo0of Mo in 5% Mo/PW-C,5% Mo/Ru/PWC and Ru-5% Mo/PW-C catalysts,respectively (Fig.5(b)) [47].As can be seen from the semi-quantitative determination of Mo in Table 1,with the difference of Mo and Ru deposition,the valence state of Mo6+and the low price of Mo4+and Mo0in Ru-5% Mo/PW-C and 5% Mo/Ru/PW-C catalysts showed different dispersion.The percentage of Mo6+increased from 17%to 62%and 42%,that of Mo4+changed from 26%to 14%and 39%,while that Mo0decreased from 36% to 9% and even disappeared in the catalyst 5% Mo/Ru/PW-C.This phenomenon may be ascribed to the different interaction intensities and migration patterns between the metal Ru and Mo by different deposition,which was mainly caused by the differences in dispersion modes between Ru and Mo.Compared with the mono-metal 5%Mo/PW-C,the oxidation orbitals of Mo6+,Mo5+and Mo4+in bimetallic Ru Mo catalysts all shifted to higher binding energy,while Ru orbitals shifted to lower binding energy,indicating that the electron transformation of MoOxto Ru which led to the reducibility of Ru.Compared with 5% Mo/Ru/PW-C catalyst,the binding energy of Mo4+and Mo0were different from that catalyst of Ru-5% Mo/PW-C.In addition,this abnormal phenomenon of MoOxwas coated on the surface of Ru which weakened the coreduction of metal Mo and Ru,making it more difficult for MoOxto form a low-valence state.

Table 1Content diagram of different valence states of Mo

H2-TPR was further explained,as shown in Fig.5(c).All catalysts exhibited obvious broad shoulder peaks located at around 435 °C,which was attributed to the methanation of AC.As mentioned before,all the peaks shifted forward after the doping with metal Mo.Meanwhile,compared with the Ru/C and Ru/PW-C (Section 3.1),the profiles of Ru-5% Mo/C,Ru-5% Mo/PW-C,and 5%Mo/Ru/PW-C all showed appeared broader shoulder peaks located in the range of 450-550°C,corresponding to the reduction of octahedral coordinated MoO3to MoO2,while the H2consumption peak at 550°C was defined as the MoO3tetrahedral coordinated species were reduced to lower Mo oxidized [17,48,49].Additionally,compared with Ru-1 %Mo/C,the Ru reduction of Ru-5% Mo/C was shifted from 178 °C to 136 °C after Mo doping.Furthermore,comparing Ru-1% Mo/PW-C with Ru-5% Mo/PW-C was also observed(the peak shifted from 211 °C to 143 °C).Surprisingly,the reduction peak of the Ru in 5%Mo/Ru/PW-C was lagged,which indicated that a large amount of Mo was accumulated on the surface of Ru,weakening the Ru hydrogen dissociation ability been weakened after Mo doping.All evidence above suggests the formation of the ‘‘Ru-MoOx”.

Fig.5.(a) Ru 3d spectrograms: Ru/PW-C,5%Mo/Ru/PW-C,Ru-5%Mo/PW-C;(b) Mo 3d spectrograms: 5%Mo/PW-C,5% Mo/Ru/PW-C,Ru-5%Mo/PW-C;(c)H2-TPR of Ru-1%Mo/C,Ru-5% Mo/C,5% Mo/Ru/PW-C,Ru-5% Mo/PW-C,Ru-1%Mo/PW-C;(d) NH3-TPD 5% Mo/Ru/ PW-C,Ru-5% Mo/PW-C,PW-C.

NH3-TPD results (Fig.5(d)) indicate that the different preparation methods of co-impregnation and sequential deposition of MoOxand Ru resulted in different coverage forms of MoOxand Ru and different acid intensities.The catalyst 5% Mo/Ru/PW-C exhibited higher NH3adsorption capacity and stronger acid strength than others.In the temperature of 98 ℃,5% Mo/Ru/PWC demonstrated better acid density than that of Ru-5% Mo/PW-C.Meanwhile,different sequences deposition of metals resulted in difference acid intensity.Compared Ru-5% Mo/PW-C,the acid intensity of 5%Mo/Ru/PW-C was significantly enhanced which also indicated that different interactions could be generated by different deposition sequences.

A large amount of Lewis acid was assembled with a small amount of Br?nsted acid to form PW.In our previous study,we found that the density of Lewis acid could be enhanced by doping of Ru [17].Py-FTIR spectra of different Ru-based catalysts also show that the Lewis acid density gradually decreased with the increase of Mo modification amount (Fig.S2).In addition,the hydrogenolysis of hydroxyl groups could be promoted by lowvalent MoOxsuch as (Mo4+/ Mo0) as shown in Fig.6.

To verify the function of MoOx,catalytic performances in glycerol hydrogenolysis by Ru-5% Mo/C,Ru-5% Mo/PW-C,and 5% Mo/Ru/PW-C were explored,as shown in Fig.6.Compared with Ru-5%Mo/C with Ru/C in Fig.3(d),the former showed higher selectivity to C3 alcohols,especially that of 1,2-PDO increased from 14.2%to 52.4%after Mo doping.It might be attributed to the formation of specific MoOxwhich was conducive to the 1-OH adsorption of glycerol molecules [19].

Fig.6.Catalytic performances in glycerol hydrogenolysis by Ru-5% Mo/C,Ru-5%Mo/PW-C and 5% Mo/Ru/PW-C.Reaction condition: 4 MPa,200 °C,6 h;Glycerol solution(10%(mass))=12 ml(Products:1,2(3)-PDO,1(2)-PO,EG,Others=CH4,CO,methanol, etc.).

As shown in Fig.6,the Ru-5% Mo/PW-C shows higher selectivity to C3 alcohol than Ru-5%Mo/C,especially the selectivity of 1,3-PDO increased from 0% to 12.5% after the support active carbon was modified by PW.Moreover,the selectivity to other products significantly decreased,which may be caused by the formation of Bronsted acid after adding PW and the conversion of glycerol to 1,3-PDO [50].The performances of Ru-5% Mo/PW-C and 5% Mo/Ru/PW-C prepared by different methods shows that difference sequential deposition methods of two metal have an obvious advantage.The C3 selectivity of 5% Mo/Ru/PW-C was increased by 93.0%,and that of 1,3-PDO was increased from 12.5% to 12.9%.However,the conversion of 5% Mo/Ru/PW-C was increased to 56.2% slightly after using different method of metal deposition.The main differences between them were the distribution of the product.Furthermore,the contact between ruthenium and molybdenum is different due to the different dispersion of metals.Meanwhile,the increased dispersion of Ru-Mo affects the valence state of molybdenum and the contact mode of Ru-Mo bimetals.The selectivity of Ru-5% Mo/PW-C to 2-PO was 5.1% and that to EG was 1.6%.Interestingly,these two products of 2-PO and EG were not detected in the reaction catalyzed by 5% Mo/Ru/PW-C.To verify the effect of ‘‘Ru-MoOx-PW” on the hydrogenolysis of glycerol,we conducted a quantitative study through experiments,and the results are shown in the Supporting information.In fact,this special interface of ‘‘Ru-MoOx-PW” has strong catalytic activity for glycerol hydrogenolysis,as shown in Table S2,the selectivity of Ru-5% Mo/PW bulk to C3 alcohols was 76.0% and that to EG was 11.5%.Meanwhile,we continued to choose the neutral catalytic supports of SiO2and MCM-41,both of which showed excellent catalytic performance.Among them,the catalytic selectivity of 5%Mo/Ru/PW-MCM-41 for C3 alcohol was 81.1%,and the selectivity of EG was higher.In addition,the selectivity distribution of 2-PO in Fig.6 is quite different,with a selectivity of 5.1% only when Ru-5% Mo/PW-C catalyst is used.The propanol (2-PO) is mainly formed by dehydration of 1,2-propanediol (1,2-PDO) in the presence of catalyst to produce acetone and then hydrogenation in acid catalytic system,which can be considered as the excessive hydrogenation of 1,2-propanediol(1,2-PDO)[51,52].Our previous study found that low valence state of MoOx(Mo4+and Mo0) has higher adsorption ability for 2-OH,and the protons dissociated by H2are more likely to attack 2-OH and hydrogenolysis to 1,3-propanediol.While Mo6+is more likely to attack -C-C-C-OH to produce 1,2-propanediol.The excessively high electron density of MoOxon the Ru surface led to the more difficult desorption of 1,2-propanediol,followed by excessive hydrogenation to form 2-PO,which is consistent with experimental conclusion in Table S2.The above evidence indicated that ‘‘Ru-MoOx-PW” is effective for liquid phase hydrogenolysis of glycerol in this catalytic system.The difference in product distribution indicates the transformation of the catalytic reaction path,as shown in the mechanism diagram in Fig.7.

Fig.7.Possible mechanism of glycerol hydrogenolysis catalyzed by 1%Mo/Ru/PW-C.

3.3.Regulation of Mo doping amount in Mo/Ru/PW-C

In order to further reveal the interaction between MoOxand Ru,a series of Mo/Ru/PW-C catalysts were prepared by adjusting the doping amount of Mo (0.5%,1%,3% and 5%).TEM (Fig.8(a),(a2))and XRD(Fig.8(b))show the formation of bimetallic nanoparticles(MoOx-Ru).The reduced Ru nanoparticle were able to be protected by monolayer MoOxspecies.As shown in Fig.8(a),(a1),the Ru metal distribution on the surface of 1% Mo/Ru/PW-C was uniform and the Ru mean nanoparticle size were all around 1.8 nm,which indicated that the sintering of Ru was suppressed,and the dispersion of nanoparticles was significantly improved after Mo metal doping.Compared with the catalyst 1% Mo/Ru/PW-C (Fig.8(a),(a2)) with 5% Mo/Ru/PW-C (Fig.4(b)),the metal distribution of the former was more uniform than that of the latter,implying that the interaction between Mo and Ru was different.Meanwhile,the metal sintering of 5% Mo/Ru/PW-C was more significant than that of 1%Mo/Ru/PW-C,which was ascribed to the formation of‘‘MoOx-Ru” species (The Ru 002 of 0.21 nm and the MoOx200 of 0.2 nm measured by the application of image J were closed to each other as shown in Fig.8(a2)).The greater the amount of molybdenum metal,the more obvious species agglomeration[53,54].Moreover,XRD patterns in Fig.8(b)show that no diffraction peak belonged to Ru in different samples,indicating that Ru was highly dispersed on the Mo/Ru/PW-C surface.Additionally,the characteristic diffraction peaks of MoOxwere inconspicuous,indicating that the metals Ru and Mo were well dispersed on the PW-C surface and simultaneously promoted by the interaction between the two metals[55].

Fig.8.(a)1%Mo/Ru/PW-C TEM images(a1 particle size statistical of a,a2 partial aggregation of particle on a);(b)XRD patterns of(0.5%,1%,3%and 5%,by mass)Mo/Ru/PW-C.

The formation of the ‘‘MoOx-Ru” species was attributed to charge transformation from MoOxto Ru surface,which resulted in change of the valence state of Mo and a lower valence state of molybdenum (Mo4+/ Mo0) were formed.NH3-TPD in Fig.9(b)shows that the NH3desorption temperature and the acid intensity on the surface of the catalysts increased with the increase of Mo doping amount,which indicates that Mo is beneficial to regulate the surface acidity of the catalyst.Besides,Py-FTIR(Fig.S2)spectra show that there were abundant acidic sites on the surface of 1%Mo/Ru/PW-C.As XPS spectra are shown in Fig.9(c) and Fig.9(d)suggest that 0.5%Mo/Ru/PW-C and 1%Mo/Ru/PW-C displayed four different Mo valence states (Mo6+,Mo5+,Mo4+,and Mo0,respectively).However,3% Mo/Ru/PW-C and 5% Mo/Ru/PW-C have only displayed three valence states of Mo6+,Mo5+,and Mo4+,which is due to the difference in the strength of co-reduction between different amounts of MoOxand metal Ru.As a matter of fact,different amounts of Mo doping led to different dispersion of valence states of MoOxin the action of H2.The valence state of Mo6+and lower valence of Mo4+and Mo0in these four catalysts show regular variation with the increase of Mo loading amount from 0.5%to 5%.The percentage of Mo6+varied from 18%to 42%,while that of Mo4+decreased from 46% to 39%,and Mo0decreased as well and even disappeared in the catalyst 3% Mo/Ru/PW-C and 5% Mo/Ru/PW-C.This phenomenon may be perceived as the ongoing interaction between these two metals of MoOxand Ru,in which the ratio of low-valence MoOxhas decreased with the increase of Mo doping.The charge transformation from MoOxto Ru surface was also observed in the orbit of Ru 3p,as shown in Fig.9(d).Because C1 orbital has certain influence on Ru electron migration,Ru 3p orbital was used.With the increase of Mo loading,the Ru 3p3/2orbital peak decreased from 463.2 eV to 462.2 eV which was attributed to RuO2.The reduction of Ru could be protected partially by the accumulation of MoOx,and then the detection intensity of Ru was improved.In addition,the concentration of surface nanoparticle measured by XPS is consistent with the HR-TEM,showing good metal dispersion in all bimetallic catalysts.Boufadenet al.[56,57]proposed that the Mo loading was excessive (3% (mass) and 5%(mass)) in all Mo/Ru catalysts,and that MoOx-Ru core-shell nanoparticles could be formed,which led to decline in the activity of MoOx-Ru.The oxidation resistance of 1% Mo/Ru/PW-C catalyst was significantly improved and the average particle size was reduced,but the formation of MoOx-Ru core-shell structure was not confirmed by XPS.The results of XPS clearly stated that with the increase amount of Mo doping,the redistribution and transformation of the metal MoOxto Ru was magnified.The surface species of Ru were modified by the MoOxspecies following other metal/metal oxide catalysts (e.g.,SiO2supported Rh-Mo),which were defined as metal to ligand charge transfer (MLCT).Kosoet al.[58]used the catalyst Rh-MoOx/SiO2to catalyze the substrate containing CH2OH groups,such as tetrahydrofurfuryl alcohol,various diols,and their ethers,and found that the reactivity of the C-O bond in the -O-C-C-CH2OH is higher than that in the -O-C-CH2OH over Rh-MoOx/SiO2.To continue to investigate the strong metal interaction (SMI),H2-TPR characterization of fresh catalyst was conducted,as shown in Fig.9(b).The H2consumption spectra of different Mo/Ru/PW-C were similar.The H2consumption peaks at 200°C were defined as the reduction of RuO2to Ru,the H2consumption peaks after 400°C was the initial reduction peak of metal Mo,while the peak at 600 °C was attributed to the reduction of Mo4+to Mo0[59].With the increase of the doping amount of Mo,the peak of Ru reduction has been decreased gradually,specifically,the Ru reduction temperature of 5% Mo/Ru/PW-C decreased from 224 °C to 101 °C when compared with that of 0.5% Mo/Ru/PW-C[60-62].A large amount of Mo doping (Ru: Mo=2:5% (mass))changed the peak shape of the Ru monometallic catalyst in regions of low-temperature regions,while the initial reduction peak of Ru shifted to 101 °C,indicating that Mo doping promotes the reduction of Ru.

Fig.9.(a) NH3-TPD;(b) H2-TPR;(c) XPS Mo 3d;(d) Ru 3p spectrograms of different samples.

Based on the XPS results of this transformation,it is proposed that the existence of a large amount of MoOxcan indeed promote the reduction of Ru.In other words,this bimetallic initiator was likely to involve H2dissociation on the formed Ru metal site and migrate to MoOxspecies to promote its co-reduction as the spillover mechanism proposal [57,63,64].In addition,the generation of oxygen vacancies is also important as an alternative explanation for this transformation.In addition,the generation of oxygen vacancies is also important as an alternative explanation for this transformation.The presence of oxygen vacancy will increase the local electron state density.The presence of Mo oxygen vacancies will result in the presence of the corresponding Moδ+defects for charge balance.As the electron cloud density increases,the valence electron energy of XPS decreases correspondingly.This is consistent with our XPS results.For MoOxcatalyst,the presence of surface oxygen vacancies is also considered to be a good explanation for the high selectivity of C-O bond cleavage [65].Moreover,the reduced molybdenum species with oxygen vacancies(MoOx)can be used as Lewis acid sites[66],and the production of Lewis acid sites can effectively promote the dehydration and hydrogenation of glycerol,and it can interact with oxygen electron pairs in 2-OH.The transition metal interacts with oxygen vacancy to form Mo-O bond,and the C-O cleavage on oxygen vacancy to form diol product.Meanwhile,the surface oxygen vacancies can be regenerated cycle by the reduction of Ru.The strong interaction between metallic Ru and partially reduced Mo particles profoundly affects the adsorption and desorption of reactive molecules and the pathway of C-O bond cleavage.In conclusion,the electron transfer(or oxygen vacancy)between MoOxspecies and Ru effectively promotes the selective cleavage of C-O bond.

Fig.10.Catalytic performances of catalyst Mo/Ru/PW-C with different amount of Mo loading y (0%,0.5%,1%,3%,and 5%) Mo/Ru/PW-C in glycerol hydrogenolysis.Reaction conditions: 4 MPa,200 °C,6 h;Glycerol solution (10% (mass))=12 ml(Products: 1,2(3)-PDO,1(2)-PO,EG,Others=CH4,CO,methanol, etc.).

As Fig.10 shows,the catalytic performances of 0.5% Mo/Ru/PW-C,1% Mo/Ru/PW-C,3% Mo/Ru/PW-C,and 5% Mo/Ru/PW-C were investigated,respectively.On the 1% Mo/Ru/PW-C catalyst,the conversion of glycerol was 59.6% and the total C3 alcohol selectivity was 96.1%,among which the selectivity for 1,2-PDO and 1,3-PDO were 74.2% and 20.7% respectively.The conversion of Mo-doping catalysts was a little lower than that of Ru/PW-C,which was caused by the promotion of Ru reduction in the process of Ru-Mo migration process and the enhancement of the catalytic hydrogenolysis performance of Ru.With the increase of metal doping amount from 0% to 0.5%,the catalytic activity of Mo/Ru/PW-C increased significantly,the conversion slightly decreased from 60.5% to 60.1%,the PDOs selectivity increased from 67.4% to 84.0%,and others decreased significantly.Meanwhile,the selectivity of 2-OH group adsorption and the selectivity of PDOs was also improved by the modification of MoOx[67].Interestingly,the catalysts 0.5% Mo/Ru/PW-C,1% Mo/Ru/PW-C,3% Mo/Ru/PW-C and 5% Mo/Ru/PW-C also exhibited different products distributions in glycerol hydrogenolysis.The selectivity of EG decreased from 7.3% to 0% when the doping amount of Mo increased from 0.5% to 1%,which indicated that the catalytic reaction path varied from the priority of C-C cleavage to dehydration-hydrogenation of glycerol.However,with the continuous increase of doping amount of Mo,the conversion gradually decreased from 60.1% to 56.2%,and the POs selectivity and others significantly increased.Reyeset al.[68] enhanced the activity and selectivity of cinnamol in the hydrogenation of cinnamaldehyde in the liquid phase by adding MoO3to Rh/SiO2catalyst.The presence of MoO3-xand Rhδ+contributes to the hydrogenation of C=O bonds.All the above results showed that MoOxpair could selectively adsorb C-O due to the strong interaction between unsaturated coordination metal oxides (lowpriced oxides) and precious metals and there is an electron transfer phenomenon between Ru and MoOx[69-71].Moreover,it was verified that the hydrodeoxygenation performance was significantly improved by Mo4+and carbon mass was balanced without carbon atom loss [72,73].The turnover frequency(TOF) and H2-TPD were shown in Table S3 and Table S4,respectively.The glycerol conversion of Ru/C to C3 alcohols was 40.3%and the TOF was 40.49 h-1.Moreover,Ru/PW-C performed well with the conversion of 60.5% and C3 alcohols selectivity of 78.9%,TOF was 81.74 h-1.The foremost reason for higher catalytic activity over Ru/PW-C is that the PW was conducive to glycerol dehydration,and beneficial to selective glycerol hydrogenolysis to 3-HPA and could serve as an intermediate of 1,3-PDO.Moreover,the C3 alcohols selectivity increased from 78.9% to 96.1%after the introduction of Mo,and the TOF increased from 81.74 h-1to 87.71 h-1over 1% Mo/Ru/PW-C.Surprisingly,the EG selectivity of direct hydrogenation products was significantly decreased to trace.Besides,compared with the present works on the hydrogenolysis of glycerol with Ru-based catalysts shown in the Table S5,our catalyst is effective and the selectivity to C3 alcohols could reach to 96.1%,and the selectivity of 1,3-PDO was 20.7% with the conversion of 59.6%.The active selectivity between C-C and C-O of glycerol is effectively regulated by the introduction of metal Mo.Compared with our previous work,the selectivity of 1,2-PDO was greatly enhanced from 41.5% to 74.2%,while the selectivity of 1-PO decline from 28.9% to 1.2%significantly.The over hydrogenation of glycerol was suppressed,which ascribed to the different addition ways of phosphotungstic acid.These results indicated that the reaction path must be changed from glycerol direct hydrogenation to dehydration,as the possible mechanism shown in Fig.7.

3.4.Proposal of glycerol hydrogenolysis mechanism

Given the performances of glycerol hydrogenolysis and the different characterization of the above catalysts,the strong interaction between Ru of active metal,Mo of metal promoter and PW of the acid promoter,the dispersion of PW species and MoOxspecies,the interface of PW-C,and the microstructure of Mo-Ru all determine the different of conversion and products dispersion by Mo/Ru/PW-C bimetallic catalyst in the liquid phase glycerol hydrogenolysis[58,74].The results showed that when the concentration of glycerol was 10%(mass),the adsorption and transformation process of glycerol molecules on the catalyst reached saturation,the influence of external diffusion was also reduced to the minimum as shown in Fig.11.As can be seen from Fig.7,there are both paths of direct hydrogenation and dehydration hydrogenation catalyzed by the Mo/Ru/PW-C catalyst,and these two reaction mechanisms were independent but also closely related to each other.Meanwhile,the difference in product distribution was mainly caused by the competitive relationship between two simultaneous paths when glycerol molecule was adsorbed on the interfaces of ‘‘MoOx-Ru” and PW-C in the catalytic system,respectively.

As the strong adsorption ability of -OH of low-valent molybdenum oxide could contribute to direct hydrogenation,the H2dissociation and selectively hydrogenolysis of C-O bond to 1,3-PDO or 1,2-PDO were generated on the surface of ‘‘MoOx-Ru”,where Mo4+had a strong selectivity for secondary hydroxyl groups and is beneficial to the 1,3-PDO obtained.Additionally,in the absence of glyceraldehyde,the dehydrogenation of glycerol on the metal surface was found to be irreversible and out of equilibrium.In the process of the direct hydrogenation,the selectivity between C-C or C-O of Mo/Ru/PW-C catalyst was still difficulty,in which trace amounts of EG,methanol,and other products were formed by C-C bond cleavage,while the EG could also be further hydrogenated to form lower alcohols and other products (CH3OH,CO2,etc.).Therefore,the function of the lower valence state of MoOxis significant,especially Mo4+.We believe that the C-C bond cleavage strength was weakened by the formation of a low-valence state of MoOxand that C-O was strengthened.As the results are shown in Fig.10.The yield of EG significantly decreased and the selectivity of C3 alcohols reached its maximum of 96.1% (Sel.1,3-PDO=20.7%;Sel.1,2-PDO=74.2%,respectively) when 1% Mo/Ru/PW-C was used as a catalyst.After all,the migration of molybdenum oxide could produce a large amount of molybdenum trioxide [75].We speculate that molybdenum trioxide will cover up the active Ru sites,reducing the triatomic alcohols.The decline in the content of low-valence state molybdenum oxide (Table 2) resulted in significant differences in the adsorption of C-O bond.Meanwhile,the H2dissociation was strengthened on the surface of active ruthenium which yielded to hydrogenolysis of 1,2-PDO excessively to 1-PO.

Table 2Content diagram of different valence states of Mo

The PW with a large number of acidic centers were counted,followed by a dehydration-hydrogenation reaction.Py-FTIR and NH3-TPD analysis showed that 1% Mo/Ru/PW-C had abundant Lewis acid and reasonable Br?nsted acid.Modification of Mo metal further improved the density of acid center.The Lewis acid was beneficial to dehydration of the 1-OH of glycerol molecules to acetone alcohol,while the Br?nsted acid promoted the elimination of 2-OH to form 3-hydroxypropionaldehyde(3-HPA)[76-78].The intermediate products of glycerol hydrogenolysis,pyruvate alcohol (PA),and 3-hydroxypropionaldehyde (3-HPA) could be further hydrogenated to 1,2-PDO and 1,3-PDO aid of ‘‘MoOx-Ru”.However,excessive hydrogenolysis of 1,2-PDO would produce a large amount of 1-PO and a little bit 2-PO.During reaction,2-PO was not detected,which indicated that this reaction system was not just consistent with the dehydration process.The kinetics of dehydrogenation is always related to glycerol hydrogenolysis under neutral and alkaline conditions,which has been confirmed in the noble metal catalytic reaction mechanism [39,79].Above all,it is worth mentioning that 3-hydroxypropionaldehyde was not detected in the experiment,which might be caused by its universal conversion in this catalytic process [80].

Fig.11.(a) The investigation of substrate conversion with different concentrations;(b) Carbon mass balance of substrate conversion with different concentrations and its loss.Reaction conditions: 0.1 g 1% Mo/Ru/PW-C catalyst,4 MPa,200 °C,6 h and glycerol solution (5% (mass),10% (mass),15% (mass),20% (mass),50% (mass))=12 ml.

3.5.The investigation of reaction conditions and catalyst stability

Fig.12 shows that the glycerol conversion and selectivity of C3 alcohols changed as temperature,hydrogen pressure,and reaction time changed.However,the hydrogen pressure had slight influences on the selectivity of C3 alcohols,indicating that the effect of hydrogen pressure could be eliminated under the present reaction condition (ρH2=4 MPa).With the extension of time,the substrate molecules of glycerol were hydrogenated excessively,while the selectivity of other products increased continuously.Undoubtedly,the reaction temperature is always the critical factor affecting reaction rates.The reaction rate (Vt) increases exponentially with temperature (T) defined by the Arrhenius equation.Practically,most exothermic reactions tend to accelerate as the reaction proceeds and the reaction velocity could be doubled or trebled by an increase every 10 °C.As temperature increased from 180 °C to 220 °C,the conversion also increased while the selectivity of C3 alcohols decreased,mainly due to excessive hydrogenolysis of glycerol.The selectivity of C3 alcohols was the highest while the others were the lowest at 200 ℃.We suggested that the diffusion of substrate glycerol molecules during the reaction was the most effective at such temperature.

Fig.12.The investigation of reaction conditions of 1% Mo/Ru/PW-C.Reaction conditions: 0.1 g 1% Mo/Ru/PW-C catalyst and glycerol solution (10%(mass))=12 ml.

The mass action law indicates that the concentration and ratio of the reactants have a great influence on the reaction rate,considering that the increase of the substrate concentration may have a great influence on the reaction rate.The substrate conversion with different concentrations is shown in Fig.11(a),and its carbon mass balance and loss are shown in Fig.11(b).The glycerol conversion decreased sharply from 62.0% to 8.8% when concentration increased from 5% (mass) to 50% (mass),while the selectivity of C3 alcohols reached its maximum when concentration was 10%(mass).Furthermore,the selectivity of C3 alcohols decreased when the glycerol concentration exceeded 10% (mass) (96.1%),which may because the glycerol concentration was too high and a large number of substrates-CH2OH groups were adsorbed on the surface of the ‘‘MoOx-Ru-PW”,thus leading to saturation of mass transfer and reducing the hydrogenolysis conversion of glycerol [81,82].Moreover,the 1,3-PDO selectivity was also influenced by the glycerol concentration,and 10% (mass) was suitable for this hydrogenolysis reaction.In addition,based on the experimental conditions and time constraints,it can be inferred that the external diffusion in the mass transfer can be eliminated in the catalytic system.Therefore,the carbon mass balance of glycerol hydrogenolysis was calculated by the selectivity of each product,as shown in Fig.11(b).The products of C3 alcohols mainly include 1,(2)3-PDO,1(2)-PO,C2 were defined as EG and a little bit of ethanol,the C1 was mainly methanol,and the gas by-products were mainly CO,CH4,and methanol (assayed by GC-MS Agilent 8860-5977B,USA).First,the C3 transformation increased and then decreased when the substrate concentration increased from 5%(mass) to 50% (mass),but it reached its maximum of 96.8% when the glycerol concentration was 15% (mass).Moreover,C1 reached its minimum of 3.2%.According to the analysis of the calculation results of carbon mass balance,the variation trend of carbon loss was opposite to that of C3 products,which decreased first,then increased,and reached its minimum of 2.1%when the glycerol concentration was 15% (mass).

Fig.13.The catalyst stability investigation of 1%Mo/Ru/PW-C.Reaction conditions:0.1 g 1% Mo/Ru/PW-C catalyst,4 MPa,200 ℃,6 h and glycerol solution (10%(mass))=12 ml.

All the evidence suggested that glycerol molecules were adsorbed firstly then converted to be saturated when the glycerol concentration was 15% (mass).In fact,results of glycerol hydrogenolysis with different substrate concentrations of 10%(mass) demonstrated the best performance.Therefore,the effect of external diffusion was also reduced to its minimum,which was consistent with our performance of glycerol hydrogenolysis and its possible mechanism.

To investigate the stability of catalyst 1%Mo/Ru/PW-C,the recycling experiments were carried out (Fig.13).This conversion progressively decreased from 60.1% (1st) to 56.8% (5th).The byproducts (C1 and C2 alcohols) increased from 1.2% (1st) to 7.5%(5th) and the total C3 decreased from 98.8% (1st) to 92.5% (5th).Among them,1,2-PDO decreased from 80.3% (1st) to 65.1% (5th),while 1,3-PDO increased from 16.4% (1st) to 22.7% (5th),and 1-PO also decreased significantly.It is worth noting that 1,2-PDO selectivity decreased significantly after the 3rd cycle,while 1-PO and others increased gradually,and EG emerged for the first time,which increased from 3.5%(4th)to 4.2%(5th).All these data above indicate that after five cycles,Ru slightly enhanced the cleavage activity of the C-C bond.

To explore the cause of decreased catalytic activity and its variable selectivity,the catalyst characterization of 1% Mo/Ru/PW-C(Fresh and 5th used) was analyzed by XPS,XRD,TEM,ICP,and BET (Recovering method in Supporting Information).

The XPS characterization results of fresh 1%Mo/Ru/PW-C and it was used for 5th(Fig.14(a)).It was observed that the valence state of molybdenum oxide did not change obviously after five cycles,which is consistent with the stable catalytic performance.The XRD of the spent 1% Mo/Ru/PW-C indicated the characteristic diffraction peak of metallic ruthenium and molybdenum oxide.

TEM results(Fig.15(b))show that the metal particles exhibited obvious agglomeration on the recycled catalysts.ICP data (shown in Fig.15(a)) also indicate the loss of ruthenium (1.519% (mass)to 1.319% (mass)) and molybdenum (0.919% (mass) to 0.732%(mass)) in the recovered catalyst.The loading of phosphotungstic acid of 1% Mo/Ru/PW-C (Fresh) can be calculated as 21.07% and decreased slightly to 19.87% of used catalyst.Base this basis,this article utilizes qualitative and quantitative tools to demonstrate the primary reason for the decline in the catalytic activity.The agglomeration and accumulation of Ru and MoOxfrequently occured with active carbon,which led to the destruction of the‘‘MoOx-Ru”structure.The greater the exposure of Ru nanoparticles,the higher cleavage selectivity toward C-C bond in glycerol resulted in the higher by-products(C1,C2) selectivity.N2physical adsorption-desorption data in Table 3 show that surface area of 1%Mo/Ru/PW-C decreased significantly,which was ascribed to glycerol molecules adsorption of carbonaceous species such as humus,resulting in the loss of active groups covered by carbonaceous,as noted in previous reports [81-84].

Fig.15.TEM of 1% Mo/Ru/PW-C catalyst: (a) Fresh;(b)-(b1) Used.

Table 3ICP of 1% Mo/Ru/PW-C catalyst: (a) fresh;(b) used

4.Conclusions

To sum up,this current study proposed a multi-functional Ru-Mo bimetallic catalyst for ultra-efficient C3 alcohols production from liquid phase glycerol hydrogenolysis.A series of bimetallic catalysts were prepared by loading Ru and Mo on PW-C by different deposition methods.In addition,the optimal catalyst 1% Mo/Ru/PW-C prepared by sequential deposition of Ru and Mo metals shows excellent hydrogenolysis performance.The conversion of glycerol reached 59.6% with 96.1% in selectivity to C3 alcohols,including 74.2%for 1,2-PDO and 20.7%for 1,3-PDO.After the modification of Mo,the mean nanoparticle size of metal nanoparticles on PW-C decreased,and the selective adsorption capacity of 2-OH in glycerol molecule could be enhanced by the formation of lower valence state MoOx(especially the Mo4+),which could promote CO bond cleavage for 1,3-PDO produced.Meanwhile,the PW modified could effectively provide acidic sites(Br?nsted and Lewis acid)necessary for dehydration of glycerol molecules.Glycerol molecules could produce more 3-HPA under the action of Br?nsted acid,which was sequentially but instantaneously hydrogenated by Ru to form 1,3-PDO.Moreover,the problem of the recycling and reusing of phosphotungstic acid is solved,which is loaded on activated carbon to form a heterogeneous system.This unique‘‘MoOx-Ru-PW”interface provides a high activity for glycerol hydrogenolysis to PDOs,which is ascribed to the critical step in determining the overall reaction.The reaction conditions and catalytic stability were also investigated.This article suggests that the catalyst deactivation is ascribed to the aggregation of the metal nanoparticles and adsorption of carbonaceous during this reaction.Specifically,the metal nanoparticles aggregation could lead to destruction of‘‘MoOx-Ru” and more Ru exposure,resulting in higher activity toward the C-C bond of glycerol for higher selectivity to byproducts.And a possible mechanism for the loss of carbon mass is also proposed through additional research.All in all,a promising method for rationally designing selective hydrogenolysis catalysts was presented in this work.With the further improvement of catalyst efficiency and robustness,the catalytic system prepared by this deposition method will show great potential in the actual production of biomass hydrogenolysis.

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(21908185,22178294),Project of Hunan Provincial Natural Science Foundation of China (2021JJ30663),Project of Hunan Provincial Education Department(19B572,20B547),Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization,and National Department of Education Engineering Research Centre for Chemical Process Simulation and Optimization.