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catalysts

Article Hydrogenolysis of Glycerol to 1,2-Propanediol and over Ru-Co/ZrO2 Catalysts

Jian Feng 1,*, Youquan Zhang 2, Wei Xiong 1, Hao Ding 1 and Bai He 1

1 College of Chemistry and Chemical Engineering, Chongqing University of Science & Technology, Chongqing 401331, China; [email protected] (W.X.); [email protected] (H.D.); [email protected] (B.H.) 2 Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China; [email protected] * Correspondence: [email protected]; Tel.: +86-23-6502-3739

Academic Editor: Keith Hohn Received: 2 February 2016; Accepted: 22 March 2016; Published: 25 March 2016

Abstract: A series of ZrO2 supported Ru-Co bimetallic catalysts were prepared and evaluated for the hydrogenolysis of glycerol. The Ru-Co/ZrO2 bimetallic catalyst combines the advantages of both Ru and Co, exhibiting high activity and good selectivity to 1,2-propanediol. The X-ray diffraction (XRD) and TEM results show that higher calcination temperature leads to lower reducibility of cobalt oxides and larger metal particle size, which is responsible for the decrease of glycerol conversion. Increasing the reduction temperature causes an inhibition effect on the catalytic activity, but it is beneficial to promote the 1,2-propanediol selectivity. The low temperature (<300 ˝C) reduction can prevent the growth of metal particles, resulting in higher activity. Co oxide is an important component for the good performance of Ru-Co/ZrO2. The reaction temperature, pressure, and glycerol concentration have significant effects on the catalytic performance of the Ru-Co/ZrO2 catalyst.

Keywords: glycerol; hydrogenolysis; ruthenium; cobalt; 1,2-propanediol; ethylene glycol

1. Introduction In the past few decades, because of the growing shortage of fossil fuels, the utilization of renewable fuels has become a fundamental way to implement sustainable development. is such a widely-used renewable fuel that can be produced from biomass-derived plant oils or animal . The rapid development of forms large quantities of glycerol as a byproduct. Therefore, the conversion of glycerol to other high value-added products has received increased attention [1]. One attractive route for the transformation of glycerol involves the catalytic hydrogenolysis to 1,2-propanediol (1,2-PDO). This diol is an important chemical for the production of polyesters, resins, and . Although cost-competitiveness is yet a key barrier, the hydrogenolysis of glycerol to 1,2-PDO is indisputably a promising green technology. Hydrogenolysis of glycerol can be achieved over various heterogeneous metal catalysts. These catalysts and their catalytic performances have been summarized and discussed in several recent reviews [2–5]. Among the reported catalysts, Ru is usually found to be very active even under relatively low temperature [3,5]. Nevertheless, Ru is also active for the excessive hydrogenolysis reaction, resulting in some undesired degradation products, such as ethylene glycol (EG), 1-propanol, 2-propanol, , , and even methane. With the purpose of improving the performance of Ru, especially the selectivity to 1,2-PDO, efforts are devoted to Ru-based bimetallic catalysts. The synergistic effect between Ru and the other metal usually make the catalytic performance of the bimetallic catalysts better than that of the

Catalysts 2016, 6, 51; doi:10.3390/catal6040051 www.mdpi.com/journal/catalysts Catalysts 2016, 6, 51 2 of 13 monometallic catalysts. Ru-Cu [6–9] and Ru-Re [10–12] were two of the mainly reported bimetallic catalysts. Cu is well known to suppress C–C bond cleavage while still showing high rates of C–O bond cleavage in hydrogenolysis reactions. However, the Cu component usually leads to lower activity and particle agglomeration during the reaction [7,9,13]. In some cases, Ru and Cu in the bimetallic catalyst even work independently without obvious synergistic effect [6]. Using Ru-Cu/NaY and Ru-Cu/HY as catalyst, the dominating product is 1-propanol rather than 1,2-PDO [8]. The research group of He [10,11] studied the hydrogenolysis of glycerol over a series of bimetallic Ru-Re catalysts, including Ru-Re/SiO2, Ru-Re/ZrO2, Ru-Re/TiO2, Ru-Re/H-β, Ru-Re/H-ZSM5. The addition of the Re component can improve the dispersion of Ru and generate ReOx species with acidic sites. Ru-Re bimetallic catalysts exhibited higher hydrogenolysis activity as compared to the monometallic Ru catalyst, while the monometallic Re catalyst showed almost no catalytic activity for the reaction [11,12]. Unfortunately, the selectivity to 1,2-PDO over Ru-Re bimetallic catalysts was not satisfactory (<50%), which would restrict its practical application. Recently, Co [14–16] has been reported effective for the glycerol hydrogenolysis reaction. Co species can interact with MgO, layered double hydroxides, or ZnO, thereby enhancing the stability of the catalysts. More importantly, Co exhibits very low activity for the cleavage of the C–C bond, resulting in high selectivity to 1,2-PDO [16]. However, the catalytic activity of Co is much lower than that of Ru in most cases. Mauriello and co-workers have demonstrated that the interaction between Pd and Co can modify the electronic properties of Pd, which enhances the catalytic performance for both the conversion of glycerol and the selectivity to 1,2-PDO [17,18]. This effect may also work for the Ru-Co bimetallic catalyst. On the basis of our previous works on monometallic Ru catalysts [19–22], we envisage combining the advantages of Ru and Co to obtain a more efficient Ru-Co bimetallic catalyst. Our preliminary study [23] screened out ZrO2 as a good support for the selective hydrogenolysis of glycerol. Therefore, in this work, a series of ZrO2 supported Ru-Co bimetallic catalysts were prepared and evaluated for the hydrogenolysis of glycerol. The calcination and reduction treatments remarkably influenced the catalyst structure and the catalytic performance. Ru-Co/ZrO2 was found to be an efficient catalyst for the hydrogenolysis of glycerol to 1,2-PDO under optimized conditions.

2. Results and Discussion

2.1. Catalytic Performance of Monometallic and Bimetallic Catalysts

Firstly, the glycerol hydrogenolysis reaction was performed using the Ru/ZrO2 and Co/ZrO2 monometallic catalysts, as well as the Ru-Co/ZrO2 bimetallic catalyst. The results are comparatively presented in Table1. A blank reaction was also conducted with neat ZrO 2; however, almost no conversion was observed in the absence of a metal catalyst, indicating that ZrO2 is unable to catalyze the hydrogenolysis of glycerol independently and that the metal catalyst is necessary for the reaction. As shown in Table1, Ru/ZrO 2 exhibited high activity and low selectivity to 1,2-PDO, resulting in large amounts of degradation products (EG and monohydroxy-). The results are consistent with other monometallic Ru-based catalysts [9,20]. In contrast, Co/ZrO2 showed low activity, but the selectivity to 1,2-PDO was high, up to 83.2%. The addition of Co metal to the Ru-based catalyst remarkably enhanced the selectivity to 1,2-PDO, although the activity was slightly decreased in comparison with that of the monometallic Ru catalyst. Some physical properties of the as-prepared catalysts are listed in Table2. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) results show that the metal weight loadings are near to the set value described in the experimental section. The Brunauer-Emmett-Teller (BET) surface areas of all the catalysts were decreased after the preparation procedures. The bimetallic catalyst possesses bigger metal particles as compared to the monometallic catalysts. Because the catalytic performance of different metals is associated with their intrinsic property [4,5], no obvious relations can be found between the determined physical properties and the catalytic performance. In spite of this, our preliminary study [23] on the effect of Ru/Co molar ratio has demonstrated that the catalytic activity decreased with increasing Co loading, while a contrary trend was found for the selectivity Catalysts 2016, 6, 51 3 of 13 to 1,2-PDO. The best performance can be obtained by the catalyst with a Ru/Co molar ratio of 1:2; thus, this catalyst was used in the present work. It was also reported that Cu has a similar effect on the catalytic performance of Ru-Cu/ZrO2 [8].

Table 1. Catalytic performance of the prepared catalysts in glycerol hydrogenolysis a.

Selectivity (%) c Catalyst b Conversion (%) LCB (%) 1,2-PDO EG

ZrO2 <1.0 - - - Ru/ZrO2 67.5 41.1 29.8 18.2 Ru-Co/ZrO2 [23] 56.2 70.3 18.0 10.7 Co/ZrO2 22.7 83.2 7.3 4.1 a Reaction conditions: 3 mL glycerol aqueous solution (20 wt. %), 180 ˝C, 5 MPa, 10 h, 50 mg catalyst. 1,2-PDO: 1,2-propanediol, EG: ethylene glycol, LCB: loss of carbon balance; b The catalysts were calcined at 350 ˝C and reduced at 250 ˝C; c Other products: 1-propanol, 2-propanol, ethanol, methanol and 1,3-propanediol.

Table 2. Some characterization results from inductively coupled plasma-atomic emission spectrometry

(ICP-AES), N2 adsorption, and TEM.

Metal Loading BET Surface Area TEM Particle Size Catalyst a (wt. %) (m2/g) (nm)

ZrO2 - 65 - Ru/ZrO2 5.06 57 8.1 b Ru-Co/ZrO2 Ru: 4.97, Co: 5.75 48 9.8 Co/ZrO2 5.12 52 6.3 a The catalysts were calcined at 350 ˝C and reduced at 250 ˝C; b Molar ratio of Ru:Co = 1:2 (this is the best ratio according to our previous study [23]).

Combination of the results in Table1 and our study on the effect of Ru/Co molar ratio reveals that the product selectivity is remarkably influenced by the Co component in the Ru-Co/ZrO2 bimetallic catalyst. Co can promote the cleavage of C–O bond, which is responsible for the increase of 1,2-PDO selectivity in the glycerol hydrogenolysis reaction. In addition, the Co content in the Ru-Co/ZrO2 bimetallic catalyst differs slightly with that in the Co/ZrO2 monometallic catalyst (5.75 wt. % and 5.12 wt. % respectively, Table2), but the activity of Ru-Co/ZrO 2 was much higher than that of Co/ZrO2. This indicates that the Ru-Co bimetallic catalyst functions more like monometallic Ru in terms of catalytic activity. The Ru-Co/ZrO2 bimetallic catalyst combines the high activity of Ru and the good selectivity of Co.

2.2. Effect of Calcination Temperature The calcination temperature is of importance for the decomposition of metal precursors and the formation of various phases [24]. Table3 presents the catalytic performance of Ru-Co/ZrO 2 calcined at different temperatures. These catalysts were all reduced at 250 ˝C. As the calcination temperature elevated from 250 ˝C to 550 ˝C, there was a uniform decrease in the conversion of glycerol, from 58.7% to 29.1%. The selectivity to 1,2-PDO and EG changed slightly when the calcination temperatures were above 350 ˝C.

a Table 3. Effect of calcination temperature on the catalytic performance of Ru-Co/ZrO2 .

Calcination Conversion Selectivity (%) LCB (%) Temperature (˝C) (%) 1, 2-PDO EG 250 58.7 64.8 24.4 15.0 350 56.2 70.3 18.0 10.7 450 43.5 68.9 15.3 7.9 550 29.1 71.8 16.9 5.2 a Reaction conditions: 3 mL glycerol aqueous solution (20 wt. %), 180 ˝C, 5 MPa, 10 h, 50 mg catalyst (reduced at 250 ˝C). Catalysts 2016, 6, 51 4 of 13

FigureCatalysts 1 shows2016, 6, 51 the XRD patterns of the Ru‐Co/ZrO2 catalysts calcined at different temperatures.4 of 13 The support, ZrO2, is in the crystalline phase of baddeleyite (JCPDS Card #86‐1450). The peak intensity increasesFigure1 shows with the increasing XRD patterns calcination of the Ru-Co/ZrO temperature,2 catalysts calcinedsuggesting at different that temperatures.higher calcination temperatureThe support, leads to ZrO higher2, is in the crystallinity crystalline phase and of larger baddeleyite particle (JCPDS size. Card The #86-1450). diffraction The peak peaks intensity at 2θ = 38.5° increases with increasing calcination temperature, suggesting that higher calcination temperature leads and 44.2° (partially overlapped with a peak of ZrO2 at 2θ = 44.8°) are attributed to metallic Ru (JCPDS to higher crystallinity and larger particle size. The diffraction peaks at 2θ = 38.5˝ and 44.2˝ (partially Card #89‐3942). The diffraction peaks at 2θ = ˝34.0° (mostly overlapped with a peak of ZrO2 at 2θ = overlapped with a peak of ZrO2 at 2θ = 44.8 ) are attributed to metallic Ru (JCPDS Card #89-3942). 34.2°) and 39.5° can be assigned to˝ CoO (JCPDS Card #75‐0419). As can be seen˝ from Figure˝ 1, the The diffraction peaks at 2θ = 34.0 (mostly overlapped with a peak of ZrO2 at 2θ = 34.2 ) and 39.5 can phases ofbe metallic assigned toRu CoO and (JCPDS CoO Cardwere #75-0419). present Asin all can the be seen catalyst from Figuresamples.1, the It phases is interesting of metallic to Ru note that Co3O4 (theand strongest CoO were presentdiffraction in all thepeak catalyst at 2 samples.θ = 36.9°, It is JCPDS interesting Card to note #74 that‐1656) Co3 Oexisted4 (the strongest in the sample ˝ ˝ calcined diffractionat 550 °C, peak which at 2 θindicates= 36.9 , JCPDS higher Card calcination #74-1656) existed temperature in the sample is unfavorable calcined at 550 to theC, which reduction of indicates higher calcination temperature is unfavorable to the reduction of cobalt oxides. Previous cobalt oxides. Previous works [14,25] also demonstrate that increasing the calcination temperature works [14,25] also demonstrate that increasing the calcination temperature can result in decreased can resultreducibility in decreased of the cobalt reducibility oxides. This of may the be cobalt one of oxides. the reasons This for themay decreased be one glycerol of the conversion reasons for the decreasedat higherglycerol calcination conversion temperature at higher (Table calcination3). temperature (Table 3).

Ru # Co O 3 4 * CoO

CT = 550 oC # *

CT = 450 oC Intensity (a.u.)

CT = 350 oC

CT = 250 oC

10 20 30 40 50 60 70 80 2 Theta (degree)

Figure 1.Figure X‐ray 1. DiffractionX-ray Diffraction (XRD) (XRD) patterns patterns of Ru of Ru-Co/ZrO‐Co/ZrO2 calcined2 calcined at at different temperatures. temperatures. The ˝ catalystsThe were catalysts reduced were at reduced 250 °C at after 250 Ccalcination. after calcination. CT: CT:calcination calcination temperature. temperature.

Figure2 illustrates the TEM images of the Ru-Co/ZrO catalysts calcined at different temperatures. Figure 2 illustrates the TEM images of the Ru2 ‐Co/ZrO2 catalysts calcined at different As can be clearly seen from the images, the nanosized particles (including metal and metal oxide) are temperatures. As can be clearly seen from the images, the nanosized particles˝ (including ˝metal and well dispersed on the ZrO2 support when the calcination temperature is 250 C (Figure2a) or 350 C metal oxide)(Figure are2b), well whereas dispersed the dispersion on the is not ZrO good2 support when the when calcination the temperature calcination increases temperature to 450 ˝ Cis 250 °C (Figure (Figure2a) or2 c)350 or 550°C ˝C(Figure (Figure 22b),d). This whereas indicates the that dispersion higher temperature is not treatment good when will lead the to thecalcination temperatureaggregation increases of metal to and450 metal °C (Figure oxide particles. 2c) or In 550 fact, the°C average(Figure particle 2d). sizeThis in indicates Figure2a,b wasthat higher temperaturedetermined treatment to be aboutwill lead 8.3 nm to and the 9.8 aggregation nm, respectively. of metal However, and it ismetal hard tooxide estimate particles. the particle In fact, the size in Figure2c,d because the TEM image clarity is not very good. These results are consistent with average particle size in Figure 2a and 2b was determined to be about 8.3 nm and 9.8 nm, respectively. the XRD experiments (Figure1) as discussed above. The growth in the metal particle size may be However,another it is hard reason to for estimate the decreased the particle glycerol conversion size in Figure at higher 2c,d calcination because temperature the TEM (Tableimage3). clarity is not very good. These results are consistent with the XRD experiments (Figure 1) as discussed above. The growth in the metal particle size may be another reason for the decreased glycerol conversion at higher calcination temperature (Table 3).

Catalysts 2016, 6, 51 5 of 13 Catalysts 2016, 6, 51 5 of 13

Figure 2. TEM images of Ru‐Co/ZrO2 calcined at: (a) 250 °C;˝ (b) 350 °C; ˝(c) 450 °C and˝ (d) 550 °C. The˝ Figure 2. TEM images of Ru-Co/ZrO2 calcined at: (a) 250 C; (b) 350 C; (c) 450 C and (d) 550 C. Thecatalysts catalysts were were reduced reduced at 250 at °C 250 after˝C after calcination. calcination.

2.3. Effect of Reduction Temperature 2.3. Effect of Reduction Temperature Reduction is a crucial step in the catalyst preparation process. The reduction temperature can Reduction is a crucial step in the catalyst preparation process. The reduction temperature can influence the catalyst reducibility, thereby influencing the catalytic performance [18,26]. The reaction influence the catalyst reducibility, thereby influencing the catalytic performance [18,26]. The reaction results of glycerol hydrogenolysis using the Ru‐Co/ZrO2 catalysts reduced at different temperatures results of glycerol hydrogenolysis using the Ru-Co/ZrO catalysts reduced at different temperatures are presented in Table 4. These catalysts were all calcined2 at 350 °C. Obviously, increasing the are presented in Table4. These catalysts were all calcined at 350 ˝C. Obviously, increasing the reduction reduction temperature causes an inhibition effect on the catalytic activity, but it is beneficial to temperature causes an inhibition effect on the catalytic activity, but it is beneficial to promote the promote the 1,2‐PDO selectivity and to restrain the production of EG. To be specific, as the reduction 1,2-PDO selectivity and to restrain the production of EG. To be specific, as the reduction temperature temperature was raised from 250 °C to 500 °C, the conversion of glycerol decreased from 56.2% to was raised from 250 ˝C to 500 ˝C, the conversion of glycerol decreased from 56.2% to 14.8%, while 14.8%, while the 1,2‐PDO selectivity increased from 70.3% to 78.5%. It is worth noting that the the 1,2-PDO selectivity increased from 70.3% to 78.5%. It is worth noting that the unreduced catalyst unreduced catalyst exhibited even higher activity than that of the catalysts reduced at 400 °C and exhibited even higher activity than that of the catalysts reduced at 400 ˝C and 500 ˝C. One reasonable 500 °C. One reasonable explanation for this is that the catalyst can be reduced in situ in the glycerol explanation for this is that the catalyst can be reduced in situ in the glycerol hydrogenolysis reaction hydrogenolysis reaction under hydrogen atmosphere. However, the unreduced catalyst obtained the under hydrogen atmosphere. However, the unreduced catalyst obtained the lowest selectivity to lowest selectivity to 1,2‐PDO, which may be related to the chloride ions contained in the catalyst (the 1,2-PDO, which may be related to the chloride ions contained in the catalyst (the unreduced catalyst unreduced catalyst was not washed with deionized , see the Experimental section). The chloride was not washed with deionized water, see the Experimental section). The chloride ion was testified to ion was testified to go against the formation of 1,2‐PDO by Vasiliadou and co‐workers [27]. go against the formation of 1,2-PDO by Vasiliadou and co-workers [27].

a Table 4. Effect of reduction temperature on the catalytic performance of Ru‐Co/ZrO2 a. Table 4. Effect of reduction temperature on the catalytic performance of Ru-Co/ZrO2 . Reduction Selectivity (%) Reduction ConversionConversion (%) Selectivity (%) LCB (%) Temperature (°C) 1, 2‐PDO EG LCB (%) Temperature (˝C) (%) Unreduced 43.0 1, 2-PDO62.2 26.7 EG 5.9 Unreduced250 56.2 43.0 62.270.3 18.0 26.7 10.7 5.9 300250 42.6 56.2 70.371.8 16.5 18.0 10.78.4 400300 28.7 42.6 71.874.0 10.3 16.5 5.5 8.4 400 28.7 74.0 10.3 5.5 500500 14.8 14.8 78.578.5 11.6 11.6 3.8 a Reaction conditions: 3 mL glycerol aqueous solution (20 wt. %), 180 °C, 5 MPa, 10 h, 50 mg catalyst a Reaction conditions: 3 mL glycerol aqueous solution (20 wt. %), 180 ˝C, 5 MPa, 10 h, 50 mg catalyst (calcined (calcinedat 350 ˝C). at 350 °C).

Figure 3 depicts the XRD patterns of the Ru‐Co/ZrO2 catalysts reduced at different temperatures. As has been discussed above, the diffraction peaks at 2θ = 38.5° and 39.5° are attributed to metallic Ru and CoO, respectively (other characteristic peaks are overlapped with the peaks of ZrO2). Due to

Catalysts 2016, 6, 51 6 of 13

Figure3 depicts the XRD patterns of the Ru-Co/ZrO 2 catalysts reduced at different temperatures. ˝ ˝ CatalystsAs has 2016 been, 6, discussed 51 above, the diffraction peaks at 2θ = 38.5 and 39.5 are attributed to metallic6 of 13 Ru and CoO, respectively (other characteristic peaks are overlapped with the peaks of ZrO2). Due to thethe weak weak intensity intensity of of corresponding corresponding diffraction diffraction peaks, peaks, we we are are unable unable to tocalculate calculate the the particle particle size size by theby theScherrer Scherrer equation. equation. Even Evenso, it can so, be it canseen be from seen Figure from 3 Figure that the3 thatpeak the intensity peak intensitygradually gradually enhanced withenhanced increasing with increasingreduction temperature, reduction temperature, implying that implying the crystal that the particles crystal became particles larger became at higher larger reductionat higher reductiontemperatures. temperatures. This changing This changing trend trendis similar is similar to the to thecatalysts catalysts calcined calcined at at different different temperaturestemperatures (Figure (Figure 11).). OurOur previousprevious workwork [[20]20] on thethe hydrogenolysishydrogenolysis ofof glycerolglycerol usingusing Ru/TiORu/TiO2 catalystcatalyst has has elucidated elucidated that that raising raising the the reduction reduction temperature temperature will will cause cause the the growth growth of of Ru particle size,size, thereby thereby decreasing decreasing the the catalyst catalyst activity. activity. As a a matter matter of of fact, fact, high high temperature temperature treatments treatments usually usually leadlead to the growth ofof metalmetal oror metal metal oxide oxide particles particles [ 28[28].]. Combination Combination of of the the catalytic catalytic performances performances in inTable Table4 shows 4 shows that that the the growth growth of metalof metal particle particle size size is a is logical a logical explanation explanation for for the the lower lower activity activity at athigher higher reduction reduction temperature. temperature. Moreover, Moreover, the the low low temperature temperature (<300 (<300˝ °C)C) reductionreduction can prevent the growthgrowth of metal particles, resulting in relatively higher activity.

Ru # Co O 3 4 * CoO RT = 500 oC

RT = 400 oC

RT = 300 oC Intensity (a.u.) Intensity *

RT = 250 oC *

# Unreduced

10 20 30 40 50 60 70 80 2 Theta (degree)

Figure 3. XRD patterns of Ru‐Co/ZrO2 reduced at different temperatures. The catalysts were calcined Figure 3. XRD patterns of Ru-Co/ZrO2 reduced at different temperatures. The catalysts were calcined at 350 °C before reduction. RT: reduction temperature. at 350 ˝C before reduction. RT: reduction temperature.

In addition, the phase of metallic Ru (2θ = 38.5°) appeared in all the reduced catalysts, which In addition, the phase of metallic Ru (2θ = 38.5˝) appeared in all the reduced catalysts, which indicates that ruthenium oxides are more easily reduced. The Co3O4 phase (2θ = 36.9°) only existed indicates that ruthenium oxides are more easily reduced. The Co O phase (2θ = 36.9˝) only existed in in the unreduced sample, but CoO (2θ = 39.5°) presented in the samples3 4 reduced at 250 °C and 300 °C. the unreduced sample, but CoO (2θ = 39.5˝) presented in the samples reduced at 250 ˝C and 300 ˝C. This implies the reduction of Co3O4 is a two‐step process, which corresponds to the reduction of This implies the reduction of Co3O4 is a two-step process, which corresponds to the reduction of Co3O4 to CoO, and then the reduction of CoO to metallic Co. No obvious diffraction peaks of metallic Co O to CoO, and then the reduction of CoO to metallic Co. No obvious diffraction peaks of metallic Co 3(the4 strongest diffraction peak at 2θ = 47.4°, JCPDS Card #89‐7373) can be observed in the XRD Co (the strongest diffraction peak at 2θ = 47.4˝, JCPDS Card #89-7373) can be observed in the XRD patterns, probably because the Co particles are too small to be detected. patterns, probably because the Co particles are too small to be detected. Figure 4 presents the temperature programmed reduction (TPR) profiles of the Ru/ZrO2, Figure4 presents the temperature programmed reduction (TPR) profiles of the Ru/ZrO 2, Co/ZrO2, and Ru‐Co/ZrO2 catalysts. The Ru/ZrO2 sample showed a single reduction peak at around Co/ZrO2, and Ru-Co/ZrO2 catalysts. The Ru/ZrO2 sample showed a single reduction peak at 217 °C, which can be assigned to the reduction of RuO2 to Ru metal [10,29]. The Co/ZrO2 catalyst around 217 ˝C, which can be assigned to the reduction of RuO to Ru metal [10,29]. The Co/ZrO exhibited two distinct reduction maxima with one at 334 °C and2 another at 460 °C. The former2 catalyst exhibited two distinct reduction maxima with one at 334 ˝C and another at 460 ˝C. The former reduction peak is attributed to the reduction of Co3O4 to CoO, and the second one corresponds to the reduction peak is attributed to the reduction of Co O to CoO, and the second one corresponds to the reduction of CoO to metallic Co. This reduction characteristic3 4 is in good agreement with other studies reduction of CoO to metallic Co. This reduction characteristic is in good agreement with other studies using SiO2 [30,31] and ZrO2 [32] as support. As for the Ru‐Co/ZrO2 bimetallic catalyst, the TPR pattern using SiO [30,31] and ZrO [32] as support. As for the Ru-Co/ZrO bimetallic catalyst, the TPR presented 2a maximum at 2452 °C and two shoulder peaks at 196 °C2 and 312 °C. Obviously, the pattern presented a maximum at 245 ˝C and two shoulder peaks at 196 ˝C and 312 ˝C. Obviously, reducibility of ruthenium and cobalt species was enhanced, suggesting that there is a synergistic the reducibility of ruthenium and cobalt species was enhanced, suggesting that there is a synergistic effect between Ru and Co in the Ru‐Co/ZrO2 bimetallic catalyst [30,31]. This phenomenon can be explained by a mechanism commonly known as “intraparticle hydrogen spillover” [31,33]. According to the TPR result of Ru‐Co/ZrO2, the reduction of Co3O4 to CoO started from about 210 °C, and this process was accompanied by the reduction of CoO to metallic Co. The cobalt oxides can be reduced to metallic Co at around 330 °C. This reduction characteristic can reasonably explain the

Catalysts 2016, 6, 51 7 of 13

effect between Ru and Co in the Ru-Co/ZrO2 bimetallic catalyst [30,31]. This phenomenon can be explained by a mechanism commonly known as “intraparticle hydrogen spillover” [31,33]. According ˝ to the TPR result of Ru-Co/ZrO2, the reduction of Co3O4 to CoO started from about 210 C, and this process was accompanied by the reduction of CoO to metallic Co. The cobalt oxides can be reduced to metallicCatalysts Co 2016 at, 6, around51 330 ˝C. This reduction characteristic can reasonably explain the XRD7 of 13 result ˝ ˝ in FigureXRD3 result: CoO in appeared Figure 3: CoO in the appeared samples in reduced the samples at 250 reducedC and at 250 300 °CC and because 300 °C of because the reduction of the of ˝ ˝ Co3Oreduction4, and then of Co CoO3O4, disappeared and then CoO in disappeared the samples in reduced the samples at 400 reducedC and at 400 500 °CC and due 500 to °C the due reduction to of CoOthe to reduction metallic of Co. CoO to metallic Co.

460 oC

334 oC

Co/ZrO 2 245 oC

210 oC o 196 C 312 oC Ru-Co/ZrO 2 o

Intensity (a.u.) 217 C 330 oC

Ru/ZrO 2

0 1020304050 Time (min) Figure 4. Temperature programmed reduction (TPR) profiles of the monometallic and bimetallic Figure 4. Temperature programmed reduction (TPR) profiles of the monometallic and bimetallic catalysts. The catalysts were calcined at 350 °C before TPR experiments. catalysts. The catalysts were calcined at 350 ˝C before TPR experiments.

A typical Co 2p X‐ray photoelectron spectroscopy (XPS) spectrum of Ru‐Co/ZrO2 reduced at A250 typical °C is shown Co 2p in X-rayFigure photoelectron5. For reference, spectroscopythe Co 2p XPS spectra (XPS) spectrumof Ru‐Co/ZrO of2 Ru-Co/ZrO reduced at 3002 reduced°C, 400˝ °C, and 500 °C are also provided in Figure S1, Figure S2, and Figure S3 (Supplementary Material), at 250 C is shown in Figure5. For reference, the Co 2p XPS spectra of Ru-Co/ZrO 2 reduced at 300 ˝respectively.C, 400 ˝C, and As 500can ˝beC seen are also in Figure provided 5, there in Figureare two S1, doublets Figure in S2, the and XPS Figure pattern. S3 One (Supplementary doublet (dotted line) with binding energy at 786.9 eV and 802.4 eV is attributed to the satellite peaks. Another Material), respectively. As can be seen in Figure5, there are two doublets in the XPS pattern. One doublet (solid line) can be assigned to CoO (binding energy: Co 2p3/2 = 780.8 eV, Co 2p1/2 = 796.2 eV) doublet (dotted line) with binding energy at 786.9 eV and 802.4 eV is attributed to the satellite [34,35]. The presence of satellite peaks is a typical feature for Co2+, and they cannot be observed for peaks. Another doublet (solid line) can be assigned to CoO (binding energy: Co 2p = 780.8 eV, Co3+ [16]. Almost the same XPS results were obtained for Ru‐Co/ZrO2 reduced at 300 °C (Figure3/2 S1), 2+ Co 2psuggesting1/2 = 796.2 that eV )[CoO34 ,is35 the]. The main presence cobalt phase of satellite in the catalysts peaks is reduced a typical at 250 feature °C and for 300 Co °C., andThis they 3+ cannotagrees be observedwell with the for XRD Co and[16 TPR]. Almostresults. Nevertheless, the same XPS the binding results energy were obtained only shifts for a little Ru-Co/ZrO lower 2 ˝ reduced(Co 2p at3/2 300 = 780.5C (Figure eV) for S1),the catalysts suggesting reduced that at CoO 400 °C is the (Figure main S2) cobalt and 500 phase °C (Figure in the S3), catalysts indicating reduced at 250that˝C the and cobalt 300 element˝C. This is agreesalso in oxidation well with state. the XRDThis may and be TPR owing results. to the Nevertheless,re‐oxidization of the metal binding surface that occurred during the catalyst storage before the XPS experiments [16]. The Ru˝ 3d XPS energy only shifts a little lower (Co 2p3/2 = 780.5 eV) for the catalysts reduced at 400 C (Figure S2) and 500spectra˝C of (Figure Ru/ZrO S3),2 and indicating Ru‐Co/ZrO that2 (reduced the cobalt at 250 element °C) are shown is also in inFigure oxidation S4 and state.Figure S5 This in the may be Supplementary Material, respectively. The Ru 3d5/2 peak of Ru/ZrO2 and Ru‐Co/ZrO2 is almost at the owing to the re-oxidization of metal surface that occurred during the catalyst storage before the XPS same binding energy position (about 280.3 eV), indicating that there is no obvious change in the experiments [16]. The Ru 3d XPS spectra of Ru/ZrO and Ru-Co/ZrO (reduced at 250 ˝C) are shown electronic properties of Ru after the addition of Co. 2This may be the reason2 that the bimetallic Ru‐Co in Figurecatalyst S4 andworks Figure more S5like in a themonometallic Supplementary Ru catalyst Material, in terms respectively. of catalytic The activity Ru 3d(Table5/2 peak 1). of Ru/ZrO2 and Ru-Co/ZrOBased on2 is the almost catalytic at the reaction same results binding in energy Table 4, position the optimal (about tradeoff 280.3 eV),between indicating the glycerol that there is noconversion obvious change and the in 1,2 the‐PDO electronic selectivity properties can be obtained of Ru after by the the catalyst addition calcined of Co. at This350 °C may and be reduced the reason that theat 250 bimetallic °C. From the Ru-Co XRD, catalyst TPR, and works XPS results, more it like can a be monometallic seen that the Co Ru component catalyst is in present terms mostly of catalytic activityas the (Table phase1). of CoO. This Co oxide may be beneficial for the formation and transformation of Basedintermediate, on the resulting catalytic in good reaction performance results of in the Table corresponding4, the optimal Ru‐Co/ZrO tradeoff2 catalyst. between However, the glycerol the conversionexact role and of the Co 1,2-PDOoxide needs selectivity further investigation. can be obtained by the catalyst calcined at 350 ˝C and reduced at 250 ˝C. From the XRD, TPR, and XPS results, it can be seen that the Co component is present mostly as the phase of CoO. This Co oxide may be beneficial for the formation and transformation of intermediate, resulting in good performance of the corresponding Ru-Co/ZrO2 catalyst. However, the exact role of Co oxide needs further investigation.

Catalysts 2016, 6, 51 8 of 13 Catalysts 2016, 6, 51 8 of 13

Catalysts 2016, 6, 51 8 of 13 Intensity (a.u.) Intensity (a.u.)

satellites

satellites 810 805 800 795 790 785 780 775 770 Binding Energy (eV) 810 805 800 795 790 785 780 775 770 Figure 5. Co 2p X‐ray photoelectron spectroscopyBinding (XPS) Energy spectrum (eV) of Ru‐Co/ZrO2 reduced at 250 °C. ˝ Figure 5. Co 2p X-ray photoelectron spectroscopy (XPS) spectrum of Ru-Co/ZrO2 reduced at 250 C.

Figure 5. Co 2p X‐ray photoelectron spectroscopy (XPS) spectrum of Ru‐Co/ZrO2 reduced at 250 °C. 2.4. Effect of Reaction Conditions 2.4. Effect of Reaction Conditions 2.4.According Effect of Reaction to the Conditionsabove results and discussions, Ru‐Co/ZrO2 calcined at 350 °C and reduced at According to the above results and discussions, Ru-Co/ZrO calcined at 350 ˝C and reduced 250 °C exhibited the best catalytic performance in the glycerol hydrogenolysis2 reaction, giving satisfying at 250 ˝AccordingC exhibited to the above best catalytic results and performance discussions, in Ru the‐Co/ZrO glycerol2 calcined hydrogenolysis at 350 °C and reaction, reduced giving at activity and good 1,2‐PDO selectivity. Therefore, this catalyst was selected for further study. satisfying250 °C exhibited activity andthe best good catalytic 1,2-PDO performance selectivity. in Therefore,the glycerol this hydrogenolysis catalyst was reaction, selected giving for further satisfying study. activityThe effect and good of reaction 1,2‐PDO temperatureselectivity. Therefore, on the glycerolthis catalyst conversion was selected and for product further study.selectivity was The effect of reaction temperature on the glycerol conversion and product selectivity was studied studiedThe in the effect range of ofreaction 140–200 temperature °C. The results on the are glycerol depicted conversion in Figure 6.and As productexpected, selectivity the conversion was in the range of 140–200 ˝C. The results are depicted in Figure6. As expected, the conversion of glycerol of glycerolstudied in increased the range remarkably of 140–200 °C. with The increasing results are reactiondepicted temperature.in Figure 6. As The expected, selectivity the conversion to 1,2‐PDO increased remarkably with increasing reaction temperature. The selectivity to 1,2-PDO gradually graduallyof glycerol increased increased when remarkably the reaction with temperatureincreasing reaction was raised temperature. from 140 The °C selectivity to 180 °C, to after 1,2‐ PDOthat it increased when the reaction temperature was raised from 140 ˝C to 180 ˝C, after that it began to begangradually to decrease. increased Correspondingly, when the reaction the temperatureselectivity to was EG raisedchanged from slightly 140 °C before to 180 180 °C, °C, after but that then it it decrease. Correspondingly, the selectivity to EG changed slightly before 180 ˝C, but then it became becamebegan higher. to decrease. This indicates Correspondingly, that overhigh the selectivity reaction temperatureto EG changed favors slightly the before cleavage 180 of°C, the but C–C then bond, it higher. This indicates that overhigh reaction temperature favors the cleavage of the C–C bond, leading leadingbecame to higher.the formation This indicates of degradation that overhigh products. reaction Therefore, temperature high favors selectivity the cleavage to 1,2 ‐ofPDO the C–Crequires bond, an to the formation of degradation products. Therefore, high selectivity to 1,2-PDO requires an optimal leading to the formation of degradation products. Therefore, high selectivity to 1,2‐PDO requires an optimal reaction temperature, as was found˝ at 180 °C in Figure 6. reactionoptimal temperature, reaction temperature, as was found as was at 180foundC at in 180 Figure °C in6 .Figure 6. 80 80 70 70 ) ) % 60

% 60

50 50 Conversion of Glycerol Conversion of Glycerol 40 Selectivity to 1,2-PDO 40 Selectivity to 1,2-PDO Selectivity to EG Selectivity to EG 30 30

2020 Conversion or selectivity ( Conversionselectivity or Conversion or selectivity ( Conversionselectivity or

1010

0 0 140140 150 150 160 160 170 170 180 190 190 200 200 ReactionReaction temperaturetemperature (ooC)

FigureFigure 6. 6.Effect Effect of ofreaction reaction temperature temperature onon glycerolglycerol hydrogenolysis overover RuRu‐Co/ZrO‐Co/ZrO2. 2.Reaction Reaction Figure 6. Effect of reaction temperature on glycerol hydrogenolysis over Ru-Co/ZrO2. Reaction conditions: 3 mL glycerol aqueous solution (20 wt. %), 5 MPa, 10 h, 50 mg catalyst (calcined at 350 °C, conditions:conditions: 3 3 mL mL glycerol glycerol aqueous aqueous solution solution (20 wt. %), %), 5 5 MPa, MPa, 10 10 h, h, 50 50 mg mg catalyst catalyst (calcined (calcined at at 350 350 °C,˝C, reduced at 250 °C). reducedreduced at 250 °C).˝C).

Figure 7 shows the effect of hydrogen pressure on the catalytic performance of Ru‐Co/ZrO2. The Figure 7 shows the effect of hydrogen pressure on the catalytic performance of Ru‐Co/ZrO2. The Figure7 shows the effect of hydrogen pressure on the catalytic performance of Ru-Co/ZrO . hydrogenhydrogen pressure pressure dramatically dramatically influenced influenced the the glycerol glycerol conversion, while while it it only only slightly slightly affected affected the the 2 Theproduct hydrogen selectivity. pressure The dramatically conversion influenced of glycerol the increased glycerol monotonously conversion, while with itincreasing only slightly hydrogen affected product selectivity. The conversion of glycerol increased monotonously with increasing hydrogen

CatalystsCatalysts2016 2016, 6,, 516, 51 9 of9 13of 13

pressure. The solubility of hydrogen in water is proportional to the hydrogen pressure [36]. Therefore, the product selectivity. The conversion of glycerol increased monotonously with increasing hydrogen theCatalysts increasing 2016, 6, 51 glycerol conversion may be caused by the enhanced concentration of hydrogen9 of 13in the pressure. The solubility of hydrogen in water is proportional to the hydrogen pressure [36]. Therefore, aqueous solution with increasing pressure. Similar results have been obtained over other catalysts the increasing glycerol conversion may be caused by the enhanced concentration of hydrogen in the suchpressure. as Co/MgO The solubility [14]. of hydrogen in water is proportional to the hydrogen pressure [36]. Therefore, aqueousthe increasing solution glycerol with increasing conversion pressure. may be caused Similar by results the enhanced have been concentration obtained of over hydrogen other catalysts in the suchaqueous as Co/MgO solution [14 with]. increasing pressure. Similar results have been obtained over other catalysts such as Co/MgO [14]. 80

80 70 )

% 70 Conversion of Glycerol ( 60

) Selectivity to 1,2-PDO

% Conversion of Glycerol ( 60 Selectivity to EG 50 Selectivity to 1,2-PDO Selectivity to EG 50 40

4030

3020 Conversion or selectivity selectivity or Conversion

2010 Conversion or selectivity

100 23456

0 H2 pressure (MPa) 23456 H pressure (MPa) Figure 7. Effect of hydrogen pressure on glycerol2 hydrogenolysis over Ru-Co/ZrO2. Reaction conditions: Figure 7. Effect of hydrogen pressure on˝ glycerol hydrogenolysis over Ru‐Co/ZrO˝ 2. Reaction 3 mLFigure glycerol 7. Effect aqueous of hydrogen solution (20 pressure wt. %), on 180 glycerolC, 10 h, hydrogenolysis 50 mg catalyst (calcinedover Ru‐Co/ZrO at 350 2C,. Reaction reduced at conditions: 3 mL glycerol aqueous solution (20 wt. %), 180 °C, 10 h, 50 mg catalyst (calcined at 350 °C, 250conditions:˝C). 3 mL glycerol aqueous solution (20 wt. %), 180 °C, 10 h, 50 mg catalyst (calcined at 350 °C, reduced at 250 °C). reduced at 250 °C). As we know, glycerol is a with high . Overhigh concentration of glycerol will AsAs we know, glycerolglycerol is is a a liquid liquid with with high high viscosity. viscosity. Overhigh Overhigh concentration concentration of glycerol of glycerol will will cause substrate diffusion problems because of the mass transfer limitation [21]. The effect of glycerol causecause substratesubstrate diffusion problems problems because because of of the the mass mass transfer transfer limitation limitation [21]. [21]. The The effect effect of glycerol of glycerol concentration on the catalytic performance of Ru-Co/ZrO2 2is presented in Figure8. As the glycerol concentrationconcentration on thethe catalyticcatalytic performance performance of of Ru Ru‐Co/ZrO‐Co/ZrO2 is presentedis presented in Figurein Figure 8. As 8. theAs glycerolthe glycerol concentration increased, the conversion of glycerol decreased. This changing tendency is easy to concentrationconcentration increased,increased, thethe conversionconversion of of glycerol glycerol decreased. decreased. This This changing changing tendency tendency is easy is easy to to understand: the substrate amount increases with increasing glycerol concentration, while the available understand:understand: the substratesubstrate amountamount increasesincreases with with increasing increasing glycerol glycerol concentration, concentration, while while the the numberavailableavailable of catalyticnumber sitesofof catalyticcatalytic (i.e., the sitessites amount ( i.e.(i.e., , ofthe the catalyst) amount amount isof constant. ofcatalyst) catalyst) Interestingly,is constant.is constant. Interestingly, the Interestingly, selectivities the tothe 1,2-PDOselectivitiesselectivities and EG to 1,2 were‐‐PDOPDO almost andand unchangedEG EG were were almost almost with unchanged the unchanged change inwith with glycerol the the change concentration,change in glycerolin glycerol which concentration, concentration, is in accord withwhichwhich previous isis inin accord findings withwith over previous previous monometallic findings findings over Ru over [37 monometallic ,monometallic38] and Co [Ru14 Ru ][37,38] catalysts. [37,38] and and Co [14]Co [14] catalysts. catalysts.

8080

7070 Conversion of Glycerol 60 Conversion of Glycerol 60 Selectivity to 1,2-PDO Selectivity to 1,2-PDO Selectivity to EG Selectivity to EG 50 50

40 40

30 30

20 20 Conversion or selectivity (%) selectivity or Conversion

Conversion or selectivity (%) selectivity or Conversion 10 10 0 0 10 20 30 40 50 60 10 20Glycerol concentration 30 40 (wt%) 50 60 Glycerol concentration (wt%) Figure 8. Effect of glycerol concentration on glycerol hydrogenolysis over Ru‐Co/ZrO2. Reaction Figure 8. Effect of glycerol concentration on glycerol hydrogenolysis over Ru-Co/ZrO2. Reaction Figureconditions: 8. Effect 3 mL ofglycerol glycerol aqueous concentration solution, 180on °C,glycerol 5 MPa, hydrogenolysis 10 h, 50 mg catalyst over (calcinedRu‐Co/ZrO at 3502. Reaction °C, conditions: 3 mL glycerol aqueous solution, 180 ˝C, 5 MPa, 10 h, 50 mg catalyst (calcined at 350 ˝C, conditions:reduced at 250 3 mL °C). glycerol aqueous solution, 180 °C, 5 MPa, 10 h, 50 mg catalyst (calcined at 350 °C, reduced at 250 ˝C). reduced at 250 °C).

Catalysts 2016, 6, 51 10 of 13

3. Experimental Section

3.1. Catalyst Preparation

The support, ZrO2, was purchased as analytical reagent (AR) sample from KeLong chemical (Chengdu, China). The Ru-Co bimetallic catalysts were prepared by co-impregnation method. The loading of Ru was kept at 5 wt. %, based on the amount of RuCl3¨ 3H2O (Kunming Institute of Precious Metals, Kunming, China). Typical procedures for the preparation of Ru-Co/ZrO2 (5 wt. % Ru, 5.8 wt. % Co) catalyst with a molar ratio of Ru:Co = 1:2 are described as follows. Weighed amounts of ZrO2 were impregnated with the aqueous solution of RuCl3¨ 3H2O and Co(NO3)2¨ 6H2O (KeLong chemical, Chengdu, China). After impregnation, the was removed under reduced pressure on a rotary evaporator. The resulting powder was dried in vacuum at 110 ˝C for 10 h. The dried samples were calcined in a muffle furnace at different temperatures (250 ˝C, 350 ˝C, 450 ˝C, and 550 ˝C) for 4 h. Subsequently, the calcined samples were reduced with flowing hydrogen in a fixed-bed quartz reactor at different temperatures (250 ˝C, 300 ˝C, 400 ˝C, and 500 ˝C) for 4 h. The reduced samples were washed repeatedly with deionized water to remove chloride ions, and then dried at 60 ˝C in vacuum for 10 h to obtain the final catalysts. For comparison, the monometallic Ru/ZrO2 (5 wt. %) and Co/ZrO2 (5 wt. %) catalysts were prepared in the same way.

3.2. Catalyst Characterization The weight loading of Ru or Co was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES) (IRIS Intrepid, Thermal Elemental, Boston, MA, USA). BET surface areas of the catalysts were measured on an auto adsorption/desorption analyzer (ZXF-6, Northwest , Xi’an, China) using nitrogen as adsorbate. The X-ray diffraction (XRD) measurements were performed on an X’Pert Pro MPD X-ray diffractometer (Philips, Amsterdam, Netherlands) equipped with a Cu Kα radiation source (λ = 0.15406 nm). About 50 mg of catalyst samples were pressed in the sample holder. The operating voltage and current were 40 kV and 25 mA, respectively. The XRD patterns were obtained with a scan rate of 2 s/step and a step size of 0.05˝ (2θ). The particle size of corresponding crystalline phase was determined by XRD using Scherrer equation (d = 0.9λ/βcosθ) if the peaks were intense enough. Transmission electron microscopy (TEM) was used to determine the particle size and the morphology of the catalyst samples. The TEM images were obtained by JEM-1200EX at 100 kV (JEOL, Tokyo, Japan). The sample was ultrasonically suspended in ethanol and deposited on a Cu grid recovered with a thin layer of carbon. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on an X-ray photoelectron spectrometer (XSAM800, Kratos, Manchester, UK), employing Mg Kα radiation. The X-ray source was operated at 12 kV and 12 mA. The reduction characteristics of the catalysts were studied by temperature programmed reduction (TPR). The TPR experiments were carried out on a homemade apparatus equipped with a U-shaped quartz reactor and a thermal conductivity detector (Haixin, Shanghai, China). To remove the adsorbed water and other contaminants, 100 mg of the calcined catalyst sample in the reactor was pretreated in flowing He at 350 ˝C for 1 h, followed by cooling to 100 ˝C. After the pretreatment, the was ˝ switched to 10% H2/He at a flow rate of 30 mL/min, and the temperature was raised to 750 C (10 ˝C/min).

3.3. Catalytic Experiments Hydrogenolysis of glycerol was carried out in a 60 mL stainless steel autoclave equipped with a magnetic stirrer and an electric temperature controller. The catalyst and glycerol aqueous solution were charged into the autoclave. The reactor was sealed and purged repeatedly with hydrogen to eliminate air. Then, the reactor was pressurized to the necessary hydrogen pressure and heated to the desired reaction temperature. Details of the reaction conditions are described for each experiment in the text. A full list of products and selectivities are shown in Tables S1–S3 in the Supplementary Material. Catalysts 2016, 6, 51 11 of 13

After the reaction, the reactor was cooled to room temperature. The liquid-phase products were analyzed by HPLC (SHIMADZU, Tokyo, Japan) using an Aminex HPX-87H (Bio-Rad, Hercules, CA, USA) column and a detector. The mobile phase was sulphuric acid with a concentration of 5 mM. The gas products were analyzed by GC (GC-9790, Wenling, China) equipped with a 601 molecular sieve packed column and a thermal conductivity detector. The liquid products detected were 1,2-propanediol (1,2-PDO), ethylene glycol (EG), 1-propanol, 2-propanol, ethanol, methanol, and trace amounts of 1,3-propanediol. Gas products contained small amounts of CH4 and CO2. The selectivity denotes the selectivity in liquid products [10,11]. The loss of carbon balance is calculated as the percentage of missing carbon (the carbon except the liquid phase) accounted for at the end of the reaction. Because the reactor was not equipped to analyze the gas and liquid products simultaneously, a certain amount of gas phase products were inevitably lost during liquid sampling [39]. To be specific, the glycerol conversion and product selectivity were calculated on carbon basis according to the following equations:

moles of glycerol consumed Conversion of glycerol p%q “ ˆ 100 moles of glycerol initially charged

moles of carbon in specific product Selectivity p%q “ ˆ 100 moles of carbon in all detected products

4. Conclusions The addition of Co component to the Ru-based catalyst remarkably enhanced the selectivity to 1,2-PDO for the glycerol hydrogenolysis reaction. The bimetallic Ru-Co catalyst functions more ˝ like monometallic Ru in terms of catalytic activity. Ru-Co/ZrO2 calcined at 350 C and reduced at 250 ˝C exhibited the best catalytic performance. The XRD and TEM results show that the growth in the metal particle size may be the reason for the lower activity at higher calcination or reduction temperature. The TPR results indicate that a synergistic effect between Ru and Co exists in the Ru-Co/ZrO2 bimetallic catalyst. Co oxide is an important component for the good performance of Ru-Co/ZrO2 catalyst. The reaction temperature, hydrogen pressure, and glycerol concentration have significant effects on the catalytic performance of the catalyst. The reaction conditions used in this work are mild, but the 1,2-PDO selectivity is not very satisfying as compared to some Ru-Cu catalysts [8,9]. More investigations are under progress to further improve the catalytic activity and selectivity.

Supplementary Materials: The following are available online at www.mdpi.com/2073-4344/6/4/51/s1, ˝ Figure S1: Co 2p XPS spectrum of Ru-Co/ZrO2 reduced at 300 C; Figure S2: Co 2p XPS spectrum of Ru-Co/ZrO2 ˝ ˝ reduced at 400 C; Figure S3: Co 2p XPS spectrum of Ru-Co/ZrO2 reduced at 500 C; Figure S4: Ru 3d XPS ˝ ˝ spectrum of Ru/ZrO2 reduced at 250 C; Figure S5: Ru 3d XPS spectrum of Ru-Co/ZrO2 reduced at 250 C; Table S1: Catalytic performance of the prepared catalysts in glycerol hydrogenolysis; Table S2: Effect of calcination temperature on the catalytic performance of Ru-Co/ZrO2; Table S3: Effect of reduction temperature on the catalytic performance of Ru-Co/ZrO2. Acknowledgments: This work was supported by the National Natural Science Foundation of China (No. 21302237), the Basic and Frontier Research Program of Chongqing (cstc2014jcyjA90004), and the Opening Project of Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology (2013K006). Author Contributions: Jian Feng and Youquan Zhang conceived and designed the experiments; Jian Feng and Hao Ding performed the experiments; Wei Xiong and Bai He analyzed the data; Jian Feng wrote the paper. All authors reviewed the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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