Vapor-phase hydrogenolysis of biomass-derived lactate to 1,2-propanediol over supported metal catalysts

Applied Catalysis A: General 349 (2008) 204–211

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Vapor-phase hydrogenolysis of biomass-derived lactate to 1,2-propanediol over supported metal catalysts Long Huang a,b, Yulei Zhu a,c,*, Hongyan Zheng c, Mingxian Du a, Yongwang Li a,c a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, PR China Graduate University of the Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100039, PR China c Synfuels CHINA Co. Ltd., Taiyuan 030001, PR China b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 March 2008 Received in revised form 22 July 2008 Accepted 28 July 2008 Available online 3 August 2008

Vapor-phase hydrogenolysis of ethyl lactate to 1,2-propanediol was performed over a series of SiO2 supported metal (Fe, Co, Ni, Ru, and Pd) catalysts in a fixed-bed reactor. Among them, the Co/SiO2 and Cu/ SiO2 catalysts exhibit promising performance, and the Co/SiO2 were more active than the traditional copper catalysts under mild conditions. Effects of support, metal loading and preparation method were investigated to optimize the performance of the Co-based catalysts. Over the optimal catalyst (a 10 wt.% Co/SiO2 catalyst prepared via rotary evaporation drying method), the 1,2-propanediol selectivity was in excess of 98% at 90.2% lactate conversion, 2.5 MPa and 160 8C. The cobalt catalysts were characterized by X-ray diffraction (XRD) and temperature programmed reduction by H2 (H2-TPR) and temperature programmed desorption of H2 (H2-TPD). Interestingly, a quasi-linear correlation is observed between the average reaction rate and the percentage of bulk-like Co3O4 phase precursors, suggesting that the metallic cobalt from the bulk-like Co3O4 phase precursor is more active than that from the cobalt surface support species. The emerging technologies for production of low-cost lactate ester, make this high-yield route a sustainable benign process for 1,2-propanediol. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Ethyl lactate 1,2-Propendiol Co/SiO2 Hydrogenolysis

1. Introduction 1,2-Propanediol is a nontoxic and high-demand chemical extensively used for polyester resins, food products, antifreeze, liquid detergents, pharmaceuticals, etc. Currently, the industrial production of 1,2-propanediol is mainly by the hydration of propylene oxide derived from petrochemical resource, while toxic hypochlorous acid and relative high-cost H2O2 are usually used in the production of propylene oxide [1]. Accordingly, alternative routes are needed to produce 1,2-propanediol environmentally. With the rapid growth in biotechnology, many raw chemicals are expected to effectively produce from renewable resource via platform molecules. It supply a green and vital challenge for fine chemical industry based on traditional fossil-derived material [1,2]. Lactic acid and its ester are of such important biomassderived platform molecules, and they have been commercially

* Corresponding author at: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, PR China. Tel.: +86 351 4124899; fax: +86 351 4124899. E-mail address: [email protected] (Y. Zhu). 0926-860X/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2008.07.031

produced on a large scale (120 000 ton/year) through fermentation of renewable feedstock, e.g. sugars, starches and xylose [3]. Much attention has focused on converting them into high-value raw chemicals [3,4–11]. Among the research efforts, lactic acid hydrogenolysis is regarded as a promising and environmentalfriendly route for 1,2-propanediol production [4–9]. The reaction equations are illustrated in Scheme 1. Several processes have been developed as following: (i) Zhang et al. [4,5] and Luo et al. [6,7] reported hydrogenolysis of lactic acid or ethyl lactate to 1,2propanediol over Ru-based catalysts in aqueous or solvent phase. (ii) Cortright et al. [8] studied vapor-phase hydrogenolysis of lactic acid over Cu/SiO2, and 88% selectivity of 1,2-propanediol was obtained at optimal conditions (200 8C, 0.72 MPa and WHSV of 0.07). (iii) Recently, Dalavoy et al. [9] described a new process of electrocatalytic hydrogenolysis in aqueous electrolyte at ambient pressure and 70 8C, but the major product was 2-hydroxy propanaldehyde. Direct hydrogenolysis of lactic acid seems to be attractive, but our preliminary experiments show that the fouling and plugging problems are serious. Moreover, the corruption of catalyst and sets is severe under the reaction conditions. In fact, the commercial processes of conversion carboxylic acid to alcohols are generally

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Scheme 1. Reaction scheme of 1,2-propanediol preparation from lactic acid or lactate.

obtained through hydrogenolysis of its corresponding ester to avoid the above problems [8,12,13]. Furthermore, the commercial production of lactic acid is usually through an esterification column, in which lactate ester is produced as an intermediate [3]. Thus, lactate ester can be mass-produced as available raw chemical. Luo et al. [6,7] have done a pioneer work on hydrogenolysis of ethyl lactate in solvent phase, however, lots of solvent was needed and the selectivity was reduced by the hydrolyzation and trans-esterification reactions. These side reactions also exist in hydrogenolysis of maleate to 1,4-butanediol, and they can be restrained by preventing the condensation of reactants, i.e. complete vapor-phase reaction [12,13]. Accordingly, the vaporphase hydrogenolysis of lactate esters shows a promising process with several advantages: no corruption of catalyst and sets, improved 1,2-propanediol yield and sustainable benign process. Up to now, few research efforts have been done on this potentially important process. It is necessary to study this promising route through catalyst screening and reaction condition optimization. Traditional practices of ester hydrogenolysis mostly rely on Cu-based catalyst, e.g. hydrogenolysis of maleate to 1,4-butanediol [12,13], formate to methanol [14,17]. The Group VIII metals are also possible catalysts for ester hydrogenolysis. The Co- and Ru-based catalysts were active in hydrogenolysis of ester to alcohols [6,7,15–17]. The Pd/ SiO2 also worked as catalyst for ester hydrogenolysis [17–19]. Rachmady and Vannice [20,21] reported acetic acid hydrogenolysis over the Fe- and Pt-based catalysts. However, few studies are provided to systematically compare the reaction performance of copper and Group VIII metal catalysts for ester hydrogenolysis [17]. In this work, in order to identify the most active metal component, copper and Group VIII metal (Cu, Co, Ni, Pd, Ru and Fe) supported catalysts are systematically prepared, characterized and investigated for vapor-phase ethyl lactate hydrogenolysis. The results show that the Co/SiO2 and Cu/SiO2 catalysts have promising reaction performances. Then, further studies reveal that the Co/ SiO2 catalyst is more active than the Cu/SiO2 catalyst under mild temperature and pressure. Effects of support, metal loading and preparation method are examined to optimize the performance of Co/SiO2, and the correlation is also explored between the reaction performance and catalyst structure 2. Experimental 2.1. Catalyst preparation Silica (SiO2, Surface area 424 m2/g, Pore volume 0.95 cm3/g, Qingdao Haiyang Co., Ltd.) and Al2O3 (Gamma type, Surface

area196 m2/g, Pore volume 0.62 cm3/g, Shandong Aluminum Corp.) were dried at 120 8C for 12 h prior to impregnation. Active carbon (AC) was soaked in 12N HNO3 for 36 h at 40 8C, and then it was washed with deionized water and dried at 120 8C prior to impregnation. The catalysts were prepared by conventional incipient-wetness impregnation technique with the following precursors in aqueous phase: Co(NO3)23H2O, Cu(NO3)23H2O, Ni(NO3)23H2O, Fe(NO3)33H2O, Pd(NO3)2, RuCl3xH2O. The impregnation was performed at atmosphere temperature for 12 h. Then, the catalysts were dried at 120 8C over night and calcined at 450 8C for 3 h (after a 4 h ramp). The catalysts were generally labeled as (n)Metal/Support, in which the number n stood for the nominal metal loading. For example, the 10Ni/SiO2 stood for a Ni catalyst supported on SiO2 with 10 wt.% metal loading. A 10 wt.% Co catalyst was dried via rotary evaporation at 70 8C, and then the dried catalyst was calcined at 450 8C for 3 h. This catalyst was named as 10Co/SiO2 (RE). In one case of catalyst preparation, the impregnating solutions also contained sucrose with the Co/sucrose molar ratio of 10, and this catalyst was named as 10Co/SiO2 (S). 2.2. Catalysts characterization The metal loading of the SiO2 supported cobalt catalysts was determined by atomic absorption spectroscopy (AAS) on a TJA AtomScan16 absorption spectro-photometer. Power X-ray diffraction (XRD) was performed in a Rigaku (Japan) with Cu Ka radiation (l = 0.154 nm) operated at 40 kV and 100 mA. In order to obtain the average crystalline size, the XRD profiles were collected at a step width of 0.028 and a step time of 0.58/min. The average size of metal oxide was calculated according to the Scherrer equation [22]: d = 0.9l/(B cos u), where d is crystallite size, l is wavelength of the electromagnetic radiation applied (0.154 nm), u is Bragg angle, and B is full-width at half maximum (FWHM) of the most intense peak. A highly crystalline standard (SiO2) was used to determine the instrumental broadening correction. As for the Co-based catalysts, the most intense reflection is at 2u of 36.78. The diameter of a given Co3O4 particle could be used to calculate the diameter of metallic Co crystallite by the formula as follows: dv(Co0) = 0.75d(Co3O4) [23]. The Brunauer–Emmett–Teller (BET) surface area (SBET) and pore volumes of the catalysts and support were measured by N2 physisorption at 196 8C using an ASAP 2420 (Micrometrics, USA). The temperature programmed reduction by H2 (H2-TPR) was performed in a conventional apparatus Micromeritics Auto Chem. 2920, USA) with a thermal conductivity detector (TCD). The sample loading was 60 mg. The samples were dried under Argon flow at

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150 8C for 60 min, and then cooled to 40 8C. Then, the Argon flow was switched to 10 vol% H2/Ar mixed gas, and a cold trap with isopropanol–liquid nitrogen slurry was added to condense the water vapor. After the TCD signal returned to the baseline, the reduction was carried out from 40 up to 900 8C with a ramp of 10 8C min1. The amount of chemisorbed hydrogen was measured using the Micromeritics Auto Chem. 2920 by temperature program desorption (TPD) technique. The sample weight was always 200 mg. The catalyst was activated using hydrogen at 450 8C for 8 h. Then it was cooled under flowing hydrogen to the adsorption temperature 100 8C, which was more reliable for SiO2 supported cobalt catalysts [23–25]. The sample was held at 100 8C under flowing argon to remove physisorbed and/or weakly bound species until the TCD signal returned to the baseline. The hydrogen desorption was carried out at a temperature ramp 10 8C/min up to 450 8C in argon gas. The dispersion percentage (D%) was calculated according to the equation [23]: (D%) = 1.179X/W, where X is the total H2 uptake in micromoles per gram of catalyst, W the weight percentage of cobalt and the reduction degree is not considered. Average crystallite sizes were also calculated from D% assuming spherical metal crystallites of uniform diameter d with a site density of 14.6 atoms/nm2. Thus, dp = 96/D%, where D% is the percentage dispersion [23]. 2.3. Catalytic test Generally, the hydrogenolysis reaction was performed in a conventional stainless steel fix-bed reactor (i.d. 13 mm and length 1300 mm), in which 4.7 g (about 10 ml) of catalyst was packed. With the exception of the 10Cu/SiO2 which was in situ reduced in a flowing of 5 vol% H2/N2 at 300 8C, other catalysts were in situ reduction in a flowing of H2 at 450 8C for 8 h. Ethyl lactate was introduced into the reactor via a syringe pump, mixed with pure hydrogen and then vaporized in a preheated line. The liquid simples were collected in a 10 m trap immersed in ice-water. The liquid products analyzed using a GC-950 gas-chromatograph (GC) equipped with flame ionization detector and a capillary column (OV-101). The tail gas products were analyzed by GC (models 6890N and 4890D; Agilent). The liquid products were identified by gaschromatograph (6890N, Agilent) with mass spectrometer (5973, Agilent). The by-products mainly include methanol, ethanol, 1-/2propanol, acetol, ethyl propionate, ethyl acetate. The selectivity is calculated based on the mol of products observed to be formed per the mol of lactate actually reacted. During the experiment process, no deactivation was observed over the tested catalysts. 3. Results and discussion 3.1. Metal component screening The metal loading is generally 10 wt.%, in exception of 1.8Pd/ SiO2 (1.8 wt.%). Table 1 lists the reaction performance and

characterization results of SiO2 supported metal catalysts. The metal incorporation leads to a decrease in surface area and pore volume, and the lower loading of the 1.8Pd/SiO2 results in the slighter decrease of surface area. The XRD profiles show that only crystalline peaks of metal oxide phase are found in the calcined supported metal catalysts, and no peak of metal-silicate species can be detected. The H2 consumption peaks of SiO2 supported metal catalyst are also summarized in Table 1. The 10Cu/SiO2 shows two peaks with approximately equal area, which indicates a two-step reduction (Cu2+ ! Cu+ ! Cu0) [26]. The H2-TPR profile of the 10Ru/SiO2 has only one peak, corresponding to direct reduction Ru4+ to Ru0 [27]. As shown in Table 1, the reduction peaks of 10Ni/ SiO2 and 10Co/SiO2 are similar with that in reference [28], in which the Ni and Co ions were mainly reduced at 400 8C. Thus the metal ions on 10Ni/SiO2 and 10Co/SiO2 should be mostly converted to metallic phase after reduction at 450 8C. Otherwise, the XRD profile shows that metal oxide phase disappears on the reduced 10Co/SiO2 and 10Ni/SiO2, which give further evidence for sufficient reduction of 10Co/SiO2 and 10Ni/SiO2. The H2-TPR of 10Fe/SiO2 is not measured, but the reduced Fe/SiO2 only contains a-Fe0 phase after reduction at 400 8C [20,21]. Considering the H2-TPR results, on all of the SiO2 supported catalysts, the metal fraction can be mostly reduced into metallic phase under the standard reduce conditions. The metal component screening is performed at 180 8C, 5.0 MPa and H2/ethyl lactate molar ratio of 110, in which complete conversion of ethyl lactate is thermodynamically possible (Fig. S1, in supporting information). As shown in Table 1, the conversion of ethyl lactate over SiO2 supported metal catalysts follows the order of 10Co/SiO2 = 10Cu/SiO2  10Ru/SiO2 > 1.8Pd/SiO2 > 10Ni/ SiO2 > 10Fe/SiO2. The 10Co/SiO2 and 10Cu/SiO2 show complete conversion with high selectivity, while other metal catalysts show negligibly low activity. The high activity of 10Cu/SiO2 is consistent with the previous reports [14,17,20,21,25], in which it is regarded as active catalyst for ester hydrogenolysis. The promising performance of 10Co/SiO2 seems to be unexpected, since there are only few reports on cobalt-based catalyst for ester hydrogenolysis [15,17]. According to the promising results of the 10Co/ SiO2 and 10Cu/SiO2, the Cu- and Co-based catalysts are further investigated at different conditions. 3.2. Effects of temperature and pressure on the Co- and Cu-based catalysts A further investigation was carried out at different pressure and temperature on the Co- and Cu-based catalysts, and the results were summarized in Figs. 1 and 2, respectively. Fig. 1(a) lists the effect of temperature on ethyl lactate hydrogenolysis at 5.0 MPa over the 10Cu/SiO2 catalyst. The conversion of ethyl lactate increases from 3.3% to essential 100% when the reaction temperature increases from 140 to 180 8C. The 1,2-propanediol selectivity is high (>98%) in the range of 140–180 8C, but it reduces greatly from 98.5% to 70.0% as the temperature increases from 180

Table 1 The activity performance and characterization of SiO2 supported metal catalystsa Catalysts

Conv. (%)

Sel. (%)

Metal loadingb (wt.%)

SBET (m2/g)

Pore volume (cm3/g)

Average pore size (nm)

Average oxide particle sizec (nm)

TPR Tmax (8C)

TPR Tmax in reference (8C)

10Cu/SiO2 10Co/SiO2 10Ni/SiO2 10Fe/SiO2 1.8Pd/SiO2 10Ru/SiO2

100.0 100.0 3.3 1.1 8.2 10.1

98.5 97.4 96.0 40.2 92.6 95.7

10 10 10 10 1.8 10

356 332 354 357 411 372

0.82 0.78 0.82 0.78 0.93 0.81

7.1 7.2 7.3 6.9 7.0 6.8

22.7(CuO) 17.9(Co3O4) 21.4 (NiO) 18.3 (Fe2O3) 8.6 (PdO) 11.9 (RuO2)

216, 260 290, 338 375 N.D. 95 204

250 [28] 305, 380 [28] 390 [28] 120 [28] 182 [27]

a b c

Reaction conditions: 180 8C, 5.0 MPa, WSHV = 0.125 h1, and H2/ethyl lactate = 110/1 (molar ratio). Nominal metal loading. Determined from XRD in the Scherrer equation.

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Fig. 1. Ethyl lactate hydrogenolysis over 10Cu/SiO2. (*) Ethyl lactate conversion; selectivity towards: (*) 1,2-propanediol, (~) isopropanol, (") n-propanol (a) as a function of temperature, reaction conditions: 5.0 MPa, WHSV = 0.125 h1, H2/ethyl lactate = 110 (molar ratio) (b) as a function of pressure, reaction conditions: 180 8C, WHSV = 0.125 h1, H2/ethyl lactate = 110 (molar ratio).

to 240 8C. Fig. 1(b) shows that the ethyl lactate conversion increases gradually from 10.7% to be complete when the reaction pressure increases from 0.3 to 7.0 MPa. The selectivity to 1,2propanediol is over 96.0% at 1.0–7.0 MPa, and only a slight decrease is observed when pressure decreases to 0.3 MPa. Fig. 2(a) shows the effect of temperature on reaction performance over the 10Co/SiO2. The conversion of 98.2% is obtained at 160 8C, and the selectivity to 1,2-propanediol is above 95.0% in the range of 140– 180 8C at 5.0 MPa. However, the undesired 1-/2-propanol increase

greatly from 180 to 240 8C due to excessive hydrogenolysis. Fig. 2(b) presents the effect of pressure on 10Co/SiO2. High conversion can be gained at relative low pressure, for example, the complete conversion of ethyl lactate can be gained at 1.0 MPa and 180 8C. However, the 1,2-propanediol selectivity reduces from 95.4% to 68.5% as the pressure decreases from 7.0 to 0.3 MPa. In comparison with the 10Cu/SiO2, the Co/SiO2 is more active at milder temperature and pressure. For example, at 5.0 MPa and 160 8C, the conversion of ethyl lactate is 21.9% and 99.6% over the

Fig. 2. Ethyl lactate hydrogenolysis over 10Co/SiO2. (*) Ethyl lactate conversion; selectivity towards: (*) 1,2-propanediol, (~) isopropanol, (") n-propanol (a) as a function of temperature, reaction conditions: 5.0 MPa, WHSV = 0.125 h1, H2/ethyl lactate = 110 (molar ratio) (b) as a function of pressure, reaction conditions: 180 8C, WHSV = 0.125 h1, H2/ethyl lactate = 110 (molar ratio).

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Table 2 Effect of support on ethyl lactate hydrogenolysis over 10 wt.% Co-based catalystsa Support

SiO2 AC Al2O3 Al2O3d

Sel.b (%)

Conv. (%)

65.3 29.3 2.3 8.1

1,2-PDO

1-PO

2-PO

Othersc

98.3 98.4 96.3 95.8

0.7 0.4 1.7 2.0

0.4 0.3 1.3 1.3

0.6 0.9 0.7 0.9

a Reaction conditions: 160 8C, 2.5 MPa, WSHV = 0.25 h1, and H2/ethyl lactate = 50/1 (molar ratio). b 1,2-PDO = 1,2-propanediol, 1-PO = 1-propanol, 2-PO = 2-propanol c Other products: mainly containing acetol, and ethyl propionate. d Reduction under H2 flow at 550 8C for 8 h.

10Cu/SiO2 and 10Co/SiO2, respectively. In addition, the 10Co/SiO2 catalyst is highly selective at suitable conditions, e.g., the 1,2propanediol selectivity is observed to be 98.2% at 5.0 MPa and 160 8C. Accordingly, the Co-based catalyst may be a promising substitution for the traditional Cu-based catalysts, especially at milder conditions. In addition, little research efforts have been reported on ester hydrogenolysis over Co-based catalysts [15]. The Co-based catalysts were further investigated on effect of support, metal loading and preparation method. On the basis of above reaction results, we selected milder reaction conditions for further catalyst development as follows: 2.5 MPa, 160 8C and H2/ethyl lactate molar 50. Luo et al. [6,7] found that lactic acid and 2-hydroxyl propyl lactate were produced in the hydrogenolysis of ethyl lactate in solvent phase. These two byproducts are not detected via GC and acid titration in this work. This may be attributed to the vaporphase hydrogenolysis process, in which the hydrolyzation and trans-esterification reactions can be restrained [12,13]. The evidence for complete vapor-phase of lactate was listed in Table S2 of supporting information. 2-Hydroxy propanaldehyde is the main by-product in lactic acid hydrogenolysis [8,9], and it is suggested to be in equilibrium with hydrogen and 1,2-propanediol [8]. However, it is not found in the vapor-phase hydrogenolysis of ethyl lactate, even at lower pressure (0.3 MPa). In brief, the vaporphase hydrogenolysis of ethyl lactate shows a highly selective process. 3.3. Investigation of Co-based catalysts 3.3.1. Effect of support Table 2 summarizes the reaction performance over 10 wt.% cobalt supported on SiO2, Al2O3 and AC. With respect to support, the activity sequences is SiO2 > AC  Al2O3. It is well known that the cobalt interacts strongly with Al2O3 [25,30], and this is further demonstrated by the H2-TPR profiles in Fig. 3. The reducibility of the cobalt is hindered by metal-support interaction on Co/Al2O3, usually leading to inferior activity [25,30]. In

Fig. 3. H2-TPR profiles of 10Co/SiO2 and 10Co/Al2O3.

this work, a significant fraction of cobalt ions should be reduced into metallic cobalt at 450 8C for 8 h from H2-TPR of 10Co/Al2O3. In addition, a complementary experiment is performed at reduction temperature of 550 8C, and the result is listed in Table 2. The higher reduction temperature increases the conversion by 5.8%. However, the conversion is still much lower than that on 10Co/SiO2 and 10Co/AC. From the above results, it is clearly indicated that the reduction percentage is not the exclusive reason for the low activity of 10Co/Al2O3 for ethyl lactate hydrogenolysis. As will be discussed later, the low activity of 10Co/Al2O3 should be mainly ascribed to the low percentage of bulk-like Co3O4. In brief, the catalytic activity is dependent on the nature of support, and the SiO2 shows the superior performance. Thus, the studies were further focused on cobalt supported on SiO2. 3.3.2. Effect of metal loading and preparation method Table 3 shows the effect of cobalt loading and preparation method on reaction performance. The conversion of ethyl lactate increases with the increasing metal loading, and the 1,2propanediol selectivity decreases slightly. In order to study the efficiency of cobalt metal, the average reaction rate (mmol gCobalt1 s1) is further inspected on the SiO2 supported cobalt catalysts [30,31]. Table 3 indicates that the average reaction rate show a maximum at 10% cobalt loading. Other two well-known preparation methods were also tried to obtain the catalysts with well dispersion. One is an impregnated catalyst dried via rotary evaporation technique at 70 8C. The other one is that sucrose is added during impregnation process, which is reported by Girardon et al. [32]. As shown in Table 3, the ethyl lactate conversion increases in the order of 10Co/SiO2 (RE) > 10Co/SiO2 (S) > 10Co/ SiO2. Over the optimal 10Co/SiO2 (RE), the conversion of ethyl lactate is attained at 90.2%, with the 1,2-propanediol selectivity of 98.8%.

Table 3 Effect of metal loading and preparation method on ethyl lactate hydrogenolysis over Co based catalysta Catalysts

3Co/SiO2 7Co/SiO2 10Co/SiO2 10Co/SiO2(RE) 10Co/SiO2(S) 15Co/SiO2 a b c

Conv. (%)

7.2 37.6 65.3 90.2 80.5 85.8

Sel.b (%)

Average Reaction rate (mmol gCobalt1 s1)

1,2-PDO

1-PO

2-PO

Others

98.7 98.5 98.3 98.8 98.4 97.4

0.3 0.6 0.7 0.7 0.8 1.2

0.1 0.2 0.4 0.2 0.1 0.4

0.9 0.7 0.6 0.3 0.7 1.0

Reaction conditions in Table 2. 1,2-PDO = 1,2-propanediol, 1-PO = 1-propanol, 2-PO = 2-propanol. Other products: mainly acetol and ethyl propionate.

c

1.55 3.42 4.35 5.93 5.27 3.82

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Table 4 The characterization of Co/SiO2 with different metal loading and preparation method Catalysts

3Co/SiO2 7Co/SiO2 10Co/SiO2 10Co/SiO2(RE) 10Co/SiO2(S) 15Co/SiO2 a b c

Metal loadinga (wt.%)

SBET (m2/g)

Pore volume (cm3/g)

XRD Average d of Co3O4b (nm)

Average dv of Co0 b (nm)

Desorption amount (mmol gCatalyst s1)

Dispersionc (%)

Average dp of Co0 c (nm)

2.98 7.06 9.65 9.82 9.78 14.45

380 360 332 353 343 307

0.89 0.82 0.78 0.80 0.82 0.72

14.1 16.1 17.9 14.2 15.9 18.7

10.6 12.1 13.4 10.7 11.9 14.0

– 42.9 45.0 62.7 50.2 58.8

– 7.2 5.5 7.7 6.1 4.9

– 13.3 17.5 12.5 15.7 19.5

H2 chemisorption

Determined from atomic absorption spectroscopy (AAS) on a TJA AtomScan16 absorption spectro-photometer. Determined from the line half broadening of the diffraction lines using the Scherrer equation. Determined from the amount of H2 desorption in the H2-TPD, in which the H2 adsorption was performed at 100 8C.

Table 4 summarizes the characterization results with different metal loading and preparation method. The average cobalt particle size from the XRD is generally in accordance with that from H2 chemisorption, and this is well consistent with the work by Sun et al. [23]. From Table 4, it is shown that increasing the cobalt loading results in inferior metal dispersion, which is consistent with previous reports [25,30,31]. As expected [25,32], two cobalt catalysts prepared by alterative methods have superior metal dispersion. It is proved that the average reaction rate is directly proportional to the cobalt dispersion in Fischer–Tropsch reactions [23,25,33] and citral hydrogenation [30]. However, no direct proportional correlation is observed on SiO2 supported cobalt catalysts, when the average reaction rate in Table 3 is correlated with the cobalt dispersion in Table 4. This indicates that the metal dispersion is not sufficient to explain the overall reaction results. The reduce percentage and the nature of cobalt species are important to reaction performance of Co/SiO2 in the Fischer– Tropsch synthesis and unsaturated aldehyde hydrogenation [23,25,30]. The H2-TPR was powerful to determine these two key factors [25,30–32], thus it was carried out on the SiO2

supported cobalt catalysts. Jacobs et al. [25] firstly proposed the supported cobalt catalysts include two kinds of cobalt species: the bulk-like Co3O4 phase and Co surface support species, and the cobalt species can be differentiated via H2-TPR. Later, Rodrigues and Bueno [29] further demonstrated the presence of three type d cobalt ion species: the bulk-like Co3O4 phase, Con+ and Co + d+ n+ species. Both the Co and Co species are the Co surface support species, and they have intermediate or strong interaction with silica, respectively. Rodrigues and Bueno [31] assigned the H2 consumption peaks of the bulk-like Co3O4 phase at 300 and 360 8C (a two-step reduction process Co3O4 ! CoO ! Co0), the Con+ species at 430 8C d and the Co + species at 550–800 8C. Very recently, Jacobs et al. [34] proved that the first peak only includes Co3O4–CoO transformation and no Co0 phase is formed. This gives powerful evidence for the peak assignment of H2-TPR in the references [25,31]. In agreement with these prior works [25,31,34], the H2-TPR curves are divided into four peaks at 290, 330, 390 and 625 8C (rang of 450–850 8C) using Lorentz multi-fit method and the results are shown in Figs. 4 and 5. The first and second peaks, with a maximum at 290 and

Fig. 4. H2-TPR profiles of Co/SiO2 with different metal loading.

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Fig. 5. H2-TPR profiles of Co/SiO2 with different preparation method.

330 8C respectively, can be identified as the stepwise reduction of bulk-like Co3O4 phase species. As shown in Table 5, the ratio is nearly three between the second peaks and first one, giving further evidence for a two-step reduction process of the bulk-like Co3O4 phase. The peaks at 390 and 625 8C are attributed to the Con+ d species and Co + species, respectively. Table 5 summarizes the relative amount of H2 consumption of different reduction peaks. According to the total H2 consumption, the cobalt ions are completely reduced according to the amount of H2 consumption in the H2-TPR procedure. That is, the amounts of the three kinds of Co species are all presented in the H2-TPR

profiles. Thus, it is reasonable to compare the amount of different d cobalt species via the H2-TPR profiles. The Co + species has been suggested to be cobalt silicate by Ming and Baker via XPS [35], and it is generally considered to be correlated with the reduction percentage for its high reduce temperature [25,31,32]. Based on d the percentage of the Co + species, it is investigated whether the reduction percentage can explain the reaction performance of the d SiO2 supported cobalt catalysts. The amount of Co + species decreases with the increase of cobalt loading. This behavior is similar with the previous reports [25,31,36], in which the reduction percentage improved when the average cluster size increased by increasing the metal loading. In terms of the 10Co/ SiO2 and 10Co/SiO2 (RE), the same correlation is observed between d the average size and the amount of Co + species. This further indicates that the amount of cobalt silicate is correlative with the average cluster size. The percentage of cobalt silicate in 10Co/SiO2 (S) is higher than that determined from the correlation between d average size and percentage of Co + species. It is possibly ascribed to formation of chelate agent on silica surface, which increases the interaction between cobalt and silica [32]. Considering the average d reaction rate and the percentage of Co + species on the three SiO2 supported 10 wt.% cobalt catalysts, it is insufficient to correlate the specific activity with the reduction percentage. Furthermore, this d can be further demonstrated by analysis of the amount of Co + d+ species. As shown in Table 5, the percentage of the Co species is the highest at 3Co/SiO2 (27%) and the lowest at 15Co/SiO2 (17%), respectively. This difference is too little to correlate the great difference of the average reaction rate on SiO2 supported catalysts. When the cobalt species are inspected from the view of the bulk-like Co3O4 species in Table 5, a correlation is found between the average reaction rate and the percentage of bulk-like Co3O4 species. The catalysts with higher activity, 10Co/SiO2 (S) and 10Co/ SiO2 (RE), show higher percentage of bulk-like Co3O4 phase precursors. Increasing metal loading, the average reaction rate increases with bulk-like Co3O4 phase with the exception of 15Co/ SiO2. It is likely that the percentage of the metallic cobalt from bulk-like Co3O4 phase precursors influence the activity of ester hydrogenolysis. In order to confirm this hypothesis, a quantificational analysis was carried out. As shown in Fig. 6, a quasi-linear correlation is observed between average reaction rate and the percentage of the bulk-like Co3O4 phase precursors. It indicates that the average reaction rate of ester hydrogenolysis is influenced by the percentage of the bulk-like Co3O4 phase. Therefore, it is

Table 5 The percentage of cobalt precursors identified by H2-TPR Catalyst

3Co/SiO2 7Co/SiO2 10Co/SiO2 10Co/SiO2 (RE) 10Co/SiO2 (S) 15Co/SiO2

Ratio (2/1)a

3.0 2.9 2.3 2.7 3.3 2.3

Co reduced

108 106 106 100 106 99

b

(%)

Cobalt precursor species (%) d+

Bulk-like Co3O4

Con+

Co

29 42 46 67 65 55

44 38 36 10 10 28

27 20 18 23 25 17

a

Ratio of the area of Peak 2 to that of Peak 1. Determined by H2 consumption in the H2-TPR, which is based on the H2-TPR profile of pure CuO (>99%, Beijing Chem.), with the suppose that all the cobalt ions are trivalent. b

Fig. 6. Correlation between the specific activity and the percentage of bulk-like Co3O4 phase in Co/SiO2 with different metal loading and preparation method.

L. Huang et al. / Applied Catalysis A: General 349 (2008) 204–211

proposed that the metallic cobalt from the bulk-like Co3O4 phase precursor is more active, probably suggesting that the ethyl lactate hydrogenolysis is structure-sensitive over cobalt catalysts. A closer inspection of Fig. 3 shows that the percentage of bulk-like Co3O4 on 10Co/Al2O3 is much less than that on 10Co/SiO2. Based on the proposal of structure-sensitive reaction, the low activity of 10Co/ Al2O3 should be partially ascribed to its low percentage of bulk-like Co3O4. Very recently, Venezia et al. [37] established the relation between the activity of hydrodesulfurization reaction and the percentage of bulk-like Co3O4 phase, and they present the possibility of improve the reaction performance through increasing the percentage of the bulk-like Co3O4 phase. Therefore, it is possible to further improve the cobalt based catalyst for ester hydrogenolysis. An 85 h stability tests was performed over 10Co/SiO2 (RE), there was no loss of activity or selectivity under the reaction conditions of 160 8C, 2.5 MPa, 0.25 h1 and H2/ethyl lactate of 50. 4. Conclusion

(2007YQNRC19). This work is also financially supported by Synfuel CHINA Co. Ltd.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcata.2008.07.031. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

After metal component screening, it is revealed that both Coand Cu-based catalysts are active for ethyl lactate hydrogenolysis. A further investigation on reaction conditions shows that the Cobased catalyst is more active, especially at mild conditions. Using Al2O3 and active carbon instead of SiO2, it is found that SiO2 is the optimal support. The cobalt supported Al2O3 has especially inferior performance, which is attributed to both its poor reduction percentage and low percentage of bulk-like Co3O4. The cobalt loading and preparation method greatly affect the activity of SiO2 supported cobalt catalysts. Base on the H2-TPR, H2-TPD and XRD, it is found that the average reaction rate is correlated with the amount of the bulk-like Co3O4 phase precursor instead of the cobalt dispersion. This indicates that the metallic cobalt from bulklike Co3O4 precursor is more active for lactate hydrogenolysis. Over the optimal catalyst (a 10 wt.% Co/SiO2 via rotary evaporation drying method), the ethyl lactate conversion is 90.2% and the 1,2propanediol selectivity is 98.8%. In comparison with other process of converting lactic acid or its ester to 1,2-propanediol, several byproducts are obviated, e.g. 2-hydroxy propionaldehyde and transesterification products. In summary, a highly effective and stable route is developed for 1,2-propanediol production via vapor-phase hydrogenolysis of ethyl lactate.

[31] [32]

Acknowledgements

[33] [34]

The authors thank Dr. Zhiyong Zeng and Dr. Chenghua Zhang for their helpful discussion. This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences

211

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[35] [36] [37]

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