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C catalyst for glycerol hydrogenolysis in combination with an ion-exchange resin
Applied Catalysis A: General 318 (2007) 244–251 www.elsevier.com/locate/apcata
Development of a Ru/C catalyst for glycerol hydrogenolysis in combination with an ion-exchange resin Tomohisa Miyazawa, Shuichi Koso, Kimio Kunimori, Keiichi Tomishige * Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan Received 3 August 2006; received in revised form 8 November 2006; accepted 9 November 2006 Available online 12 December 2006
Abstract The combination of Ru/C and Amberlyst ion-exchange resin is effective for the dehydration and hydrogenation (denoted as hydrogenolysis) of glycerol to 1,2-propanediol under mild reaction conditions (393 K). A Ru/C catalyst prepared by using active carbon with a low surface area (250 m2/g) showed better performance than that prepared by using active carbon with a high surface area. In addition, treatment of Ru/C catalysts prepared from Ru(NO)(NO3)3 with Ar flowing at the appropriate temperature enhanced the performance compared to that of the commercially available Ru/C catalysts. This temperature treatment can be influenced by the decomposition of Ru precursor salt and aggregation of Ru metal particles. In addition, the degradation reaction as a side-reaction to C1 and C2 compounds of glycerol hydrogenolysis was more structure-sensitive than the hydrogenolysis reaction, and the selectivity of hydrogenolysis was lower on smaller Ru particles. The combination of Ru/C with the Amberlyst resin enhanced the turnover frequency of 1,2-propanediol formation drastically, and this indicates that 1,2-propanediol can be formed mainly by dehydration of glycerol to acetol catalyzed by Amberlyst and subsequent hydrogenation of acetol to 1,2-propanediol catalyzed by Ru/C. # 2006 Elsevier B.V. All rights reserved. Keywords: Glycerol; Hydrogenolysis; Ruthenium; Ion-exchange resin; Propanediol
1. Introduction Interest in the catalytic conversion of renewable feedstocks and chemicals has been increasing. Such conversion to hydrogen can contribute to the utilization of renewable energy sources [1–5], and conversion to petrochemicals can facilitate the replacement of petroleum by renewable resources [6,7]. Recently, it has been proposed that commodity chemicals that are used to produce pharmaceuticals, agricultural adjuvants, plastics and transportation fuel that are derived from fossil resources might be producible in future biorefineries from renewable resources, such as plant-derived sugar and other compounds [8]. Glycerol is a building block that might serve as an important biorefinery feedstock [8]. In addition, it is a byproduct from the production of biodiesel from vegetable oils [9]. Glycerol can be converted to hydrogen and synthesis gas by a reforming reaction [10,11]. One of the methods is conversion of glycerol
* Corresponding author. Tel.: +81 29 853 5030; fax: +81 29 853 5030. E-mail address: [email protected] (K. Tomishige). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.11.006
to 1,2-propanediol and 1,3-propanediol, which have been produced from petroleum derivatives [12]. Several routes to the formation of propanediol can be traced from renewable feedstocks. The commonest route is the conversion of sugar or sugar alcohols at high temperatures and pressures in the presence of a metal catalyst to produce propanediol and other lower polyols [13]. It has been reported that propanediol is producible through catalytic conversion of polyols [14] and glycerol. Supported metal catalysts, hydrogen pressure of 6– 10 MPa and reaction temperatures of 453–513 K have been applied [15–18]. Recently, it was reported that the reaction of glycerol proceeded at a hydrogen pressure of 1.4 MPa and a temperature of 473 K [13]. Judging from these various reports, it seems to be difficult to use milder reaction conditions, especially reduced reaction temperatures. Our group reported recently that the addition of solid acid catalysts to Ru/C enhanced conversion and selectivity in glycerol hydrogenolysis [7,19]. Our results suggest that the conversion of glycerol to propanediols proceeds by the combination of dehydration over acid catalysts with subsequent hydrogenation over metal catalysts [7,19]. In this study, it is found that Ru/C catalysts prepared by using Ru(NO)(NO3)3 and active carbon with a low
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surface area and treated under Ar flowing at the optimum temperature exhibited much higher performance in the glycerol reaction under H2 in combination with Amberlyst as a solid acid catalyst than the commercial Ru/C. In addition, from the comparison of data obtained in the optimization process of Ru/ C catalysts, the structure-sensitivity of the glycerol hydrogenolysis and degradation reaction is discussed, and the reaction route to various products is discussed on the basis of the turnover frequency. 2. Experimental 2.1. Catalyst Active carbon-supported Ru catalysts were prepared by using various active carbon materials. Vulcun-XC72 was supplied by Cabot Corporation Ltd., Shirasagi DO-2, Shirasagi M and Carboraffin were supplied by Japan EnviroChemicals Ltd. Vulcun-XC72, Shirasagi DO-2, Shirasagi M and Carboraffin are denoted as C(I), C(II), C(III) and C(IV), respectively. Ru was loaded by impregnating carbon supports with an aqueous solution of Ru(NO)(NO3)3 and RuCl3, and an acetone solution of Ru(acac)3 as a precursor. After impregnation and solvent removal by evaporation, the catalysts were dried for 12 h at 393 K. The amount of Ru loaded was in the range of 3– 10 wt%. The amount of Ru loaded is denoted as Ru5/C(I) in the case of 5 wt% Ru supported on active carbon(I). These catalysts were used also after treatment under Ar flowing at a temperature of 473–773 K. In addition, 5 wt% Ru/C was purchased from Wako Pure Chemical Industries Ltd., and this catalyst is denoted as Ru5/C(V). The cation-exchange resin Amberlyst 15 (4.7 equiv./kg resin dried, particle size 0.4– 1.2 mm, highest operating temperature 393 K; MP Biomedicals), which consists of highly cross-linked styrene–divinyl benzene copolymer beads that are functionalized with sulfonic groups, was used as the solid acid catalyst. All catalysts were in powder form, with less than 0.1 mm. 2.2. Activity test Glycerol hydrogenolysis was carried out with a 20 ml aqueous solution of glycerol in a 70 ml stainless-steel autoclave. The standard reaction was conducted under the following conditions: 393 K reaction temperature, 8.0 MPa initial hydrogen pressure, 10 h reaction time, 20 wt% glycerol aqueous solution, 150 mg Ru/C catalysts, and 300 mg Amberlyst. Reaction conditions were varied for investigation of the effects of different conditions. Details of the reaction conditions are described for each result. In addition, to elucidate the mechanism of the glycerol reaction, we used an aqueous solution of 2 wt%. 1,3-Propanediol (1,3-PD), and 1,2propanediol (1,2-PD) as a reactant. In all experiments, the aqueous solution of the reactant, the catalyst powder, and the spinner were put into the autoclave; then the reactor was purged with H2 (99.99%; Takachiho Trading Co. Ltd.). After purging, the reactor was heated to the required temperature and the H2 pressure was increased to
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8.0 MPa in standard experiments. The temperature was monitored with a thermocouple that was inserted into the autoclave and connected to the thermo-controller. The reaction consumed hydrogen, and the total pressure decreased. However, the decrease in hydrogen pressure was, at most, 1/10 of the initial pressure. After the reaction, the gas phase products were collected in a gasbag and the liquid phase products were separated from the used catalyst by filtration. These products were analyzed using a gas chromatograph (GC-353; GL Sciences Inc.) equipped with a flame ionization detector (FID). A TC-WAX capillary column (diameter 0.25 mm, length 20 m) was used for separation and the column temperature was 493 K. Products were identified also using GC–MS (GCMS-QP5050, column Stabilwax; Shimadzu Corp.) and the products detected were: 1,3propanediol (1,3-PD), 1,2-propanediol (1,2-PD), 1-propanol (1-PO), and 2-propanol (2-PO) are hydrogenolysis products, and ethylene glycol (EG), ethanol, methanol and methane are degradation products.Conversion of the reactants in all reaction tests were calculated on the basis of the following equation: conversion of reactant ð%Þ ¼
sum of C-based mol of all products 100 sum of C-based mol of reactant and all products
The conversion of a reactant is usually defined as: reactant before reactant afterwards reactant before but we have to determine the conversion and the selectivity even when the level of conversion is very low. Considering the error of the analysis procedure, we applied the above method of calculation. It should be noted that the conversions calculated by our method and the method based on mass balance agreed well when the conversion was greater than 5%. The selectivity of the products in all reaction tests was calculated with the following equation, considering that the degradation byproducts (ethylene glycol, ethanol, methanol, and methane) were always formed: selectivity ð%Þ ¼
C-based mol of the product 100 sum of C-based mol of all products
Here, we simply assume that the degradation products are formed directly from glycerol and other reactants in terms of carbon number in each molecule. For example, when one molecule of glycerol is converted to one molecule of ethylene glycol and one molecule of methane, the selectivity of ethylene glycol and methane is calculated to be 66.7% and 33.3%, respectively. Here, it is interpreted that two-thirds of a glycerol molecule is converted to one molecule of ethylene glycol molecule, and at the same time one-third of the glycerol molecule is converted to one molecule of methane. The yield is calculated as: conversion ð%Þ selectivity ð%Þ : 100
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Table 1 Properties of various active carbon-supported Ru catalysts Catalysts a
Ru5/C(I) Ru5/C(II)a Ru5/C(in)a Ru5/C(IV)a Ru5/C(V)
Surface area (m2/g)
Pretreatment temperature
Dispersion
254 521 629 1046 485
573 K, 573 K, 573 K, 573 K, None
0.60 0.88 0.90 0.70 0.41
Ar Ar Ar Ar
a Ru were impregnated from RuNO(NO3)3 and loading amount of Ru is 5 wt%. C(I), Vulcun-XC72; C(II), Shirasagi DO-2; C(III), Shirasagi M; C(IV), Carboraffin; Ru/C(V), Ru/C (Wako).
2.3. Characterization The surface area of the supported metal catalysts was measured using the BET method (N2 adsorption) with a Gemini apparatus (Micromeritics Instrument Corporation). The sizes of metal particles and the dispersion of the carbon-supported Ru catalyst were estimated from irreversible CO adsorption measurements performed at room temperature. The gas pressure at the adsorption equilibrium was about 1.1 kPa and the sample weight was about 0.2 g. The dead-volume of the apparatus was about 60 cm3. The results of the characterization of the fresh metal catalysts are given in Table 1. Before the CO adsorption, the catalyst was treated without evaporating to the atmosphere under Ar flowing at a temperature of 473–773 K for 2 h, and reduced under flowing hydrogen at 393 K for 1 h. X-ray diffraction (XRD) spectra recorded with a Philips X’pert diffractometer was used in order to confirm the graphite phase of the carbon supports. Transmission electron microscope (TEM) images were taken for determination of the particle size with a JEM 2010 instrument (JEOL) operated at 200 kV. The samples after the glycerol reaction were dispersed by supersonic waves in 2propanol; they were placed on Cu grids under air atmosphere. Temperature-programmed desorption (TPD) of the carbon supports was done in a closed circulating vacuum system equipped with a variable leak valve, by which the gas was introduced into a differentially pumped quadrupole mass spectrometer (Balzers QMS 200F). Without any pretreatment, the sample support (10 mg) was heated under vacuum from room temperature to 1273 K at 10 K/min. Desorbed CO2 and CO were analyzed by QMS. In order to characterize the effect of treatment with Ar, the thermal stability of impregnated Ru/C catalyst was evaluated using the TPD profile of Ru5/C(I). Fresh catalyst (25 mg) was used without pretreatment. The sample was heated under vacuum from room temperature to 773 K at 10 K/min.
Fig. 1. Glycerol reaction on various active carbon-supported Ru catalysts + Amberlyst. Reaction conditions: 20 mass% glycerol aqueous solution 20 ml, 393 K reaction temperature, 8.0 MPa initial H2 pressure, 10 h reaction time, 150 mg of Ru catalyst + 300 mg of Amberlyst. PD, propanediol; PO, propanol; others, ethylene glycol + ethanol + methanol + methane. *Ru was impregnated from RuNO(NO3)3 and treated with Ar at 573 K. The loading amount of Ru is 5 wt%.
Fig. 1 and the characterization results are given in Table 1. Except for Ru5/C(V), all the carbon-supported Ru catalysts were pretreated under 30 cm3/min Ar flow at 573 K. This treatment with Ar is the optimum activation condition, as shown later. It is found that Ru5/C(I) + Amberlyst exhibited a higher level of glycerol conversion and selectivity toward 1,2propanediol than the commercially available Ru5/C(V) + Amberlyst, which was shown to be an effective catalyst in our earlier studies [7,19]. On the other hand, Ru5/C(II), Ru5/C(III) and Ru5/C(IV) catalysts showed a much lower level of activity than the commercial Ru5/(V). In addition, we tested Ru5/C(V) after treatment with Ar and found that the performance was almost the same as that without the treatment with Ar. From a comparison between the catalytic activity and characterization results, such as BET surface area and metal dispersion, it seems that a lower surface area is more suitable, and the metal dispersion seems to be unrelated. Fig. 2 shows the XRD
3. Results and discussion 3.1. Glycerol reaction on various active carbon-supported Ru catalysts Results of the activity test with various combinations of Ru/ C and Amberlyst in the reaction of glycerol are described in
Fig. 2. XRD patterns of various Ru/C catalysts. C(I), Vulcun-XC72; C(II), Shirasagi DO-2; C(III), Shirasagi M; C(IV), Carboraffin, All samples were used without treatment after impregnation.
T. Miyazawa et al. / Applied Catalysis A: General 318 (2007) 244–251
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Fig. 3. TPD profiles of various carbon supports. (a) CO2 desorption; (b) CO desorption; C(I), Vulcun-XC72; C(II), Shirasagi DO-2; C(III), Shirasagi M; C(IV), Carboraffin. Sample weight 10 mg, heating rate 10 K/min. All samples were used without pretreatment.
patterns of various Ru/C catalysts. The peak around 25.28 is assigned to diffraction of the graphite phase (JCPDS File, no. 41–1487). It is characteristic that the graphite phase was observed much more clearly for Ru/C(I) than for other catalysts. This suggests that Ru metal particles on the graphite phase have a high level of catalytic activity. Fig. 3 shows the TPD profiles of CO and CO2 on the various carbon supports. It is clear that the desorption of CO and CO2 on C(I) was much less than it was on other supports. The desorption of CO2 and CO is due to the decomposition of surface functional groups, such as carboxyl and carbonyl groups [20–22]. This result indicates also that C(I) has a much fewer surface functional groups than other carbon supports. The order of the total amount of desorbed CO and CO2 (C(IV) C(III) > C(II) C(I)) can be related to the order of the catalytic activity in the glycerol reaction (Ru/C(I) Ru/ C(II) > Ru/C(III) > Ru/C(IV)). This suggests that the carbon surface with less oxygen-containing functional groups, such as carbonyl and carboxyl groups, is more suitable as a support for Ru metal particles. This can be related to the suitability of the graphite phase. At present, the reason for the suitability of a graphite phase with less surface carboxylic and carbonyl
functional groups is not clear and further investigation is necessary. The Ru5/C(I) catalysts were used for subsequent experiments. The effect of the Ru precursor, the loading amount and the catalyst treatment over Ru/C(I) + Amberlyst in the glycerol reaction at 393 K are given in Table 2. Except for Ru5/C(I) from Ru(acac)3, it is clear that pretreatment with Ar enhanced the catalyst activity. In the case of Ru5/C(I) from Ru(acac)3 after treatment with Ar, the conversion of glycerol was very low (<0.2%) (data not shown). This can be related to the sublimation of Ru(acac)3 during the treatment. On the other hand, regarding RuCl3, the sublimation was negligible. This is based on the thermogravimetric analysis (TGA) of Ru5/C(I) from RuCl3 under Ar flow from room temperature to 573 K. Except the vaporization of water in the catalysts at room temperature to 373 K, the weight loss corresponds to almost 5%, and this could be due to the amount of Cl in the Ru precursor (RuCl3), where the decomposition of HCl was detected by MS. This is supported by the fact that the treatment with Ar improved the catalytic activity of Ru5/C(I) from RuCl3 + Amberlyst, and it is interpreted that the precursor salt decomposed during the treatment. Ru5/C(I) from Ru(NO)(NO3)3 + Amberlyst exhibited the greatest
Table 2 Effect of Ru precursor, loading amount and catalyst treatment over Ru/C + Amberlyst Ru catalyst
b
Pretreatment temperature
Ru3 /C(I) Ru5/C(I) RulO/C(I) Ru5/C(I) Ru5/C(I)c
Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(acac)3 RuCl3
None None None None None
Ru3/C(I) Ru5/C(I) RulO/C(I) Ru5/C(I)
Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(NO)(NO3)3 RuCl3
573 K, 573 K, 573 K, 573 K,
Ar Ar Ar Ar
Conversion (%)
Selectivity of each product (%)a 1,2-PD
1,3-PD
1-PO
2-PO
Others
2.4 4.7 3.6 3.7 0.3
69.6 59.4 65.5 32.9 24.0
3.4 6.7 3.1 11.8 0.0
14.1 13.5 10.5 17.6 0.0
1.6 0.8 1.3 1.1 15.2
11.4 19.6 19.6 36.5 34.7
9.5 21.3 18.1 5.2
75.8 76.7 46.8 73.8
4.6 1.5 5.2 4.9
8.1 2.5 16.3 13.9
1.3 0.5 0.6 1.7
10.2 18.8 31.1 5.7
Reaction conditions: 20 mass% glycerol aqueous solution 20 ml, 393 K reaction temperature, 8.0 MPa initial H2 pressure, 10 h reaction time, 150 mg Ru catalyst + 300 mg Amberlyst, PD, propanediol; PO, propanol; others, ethylene glycol + ethanol + methanol + methane. a C-based selectivity. b loading amount of Ru(wt%). c Rest 26% product is acetol.
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Fig. 4. Effect of temperature of pretreatment of catalysts on glycerol reaction over Ru5/C(I) + Amberlyst at 393 K. The reaction conditions are the same as those given in the legend to Fig. 1. *Ru5/C(V) + Amberlyst was used as a reference.
Fig. 5. Effect of pretreatment temperature of Ru5/C(I) on the glycerol reaction at 393 K. The reaction conditions are the same as those given in the legend to Fig. 1, except the amount of Amberlyst. *Ru5/C(V) was used as a reference.
conversion of glycerol both with and without treatment with Ar. In addition, regarding the effect of the loading amount of Ru, the activity of Ru10/C(I) + Amberlyst with Ar treatment was comparable to the case of Ru5/C(I); however, it showed higher degradation selectivity. These results indicate that the amount of Ru on the carbon support must be optimized for high selectivity of hydrogenolysis reactions, and too much Ru promotes degradation reactions. This tendency is compatible with earlier results [7]. On the basis of the results presented here, we focused on the Ru5/C(I) catalyst in subsequent studies.
pretreatment temperature-dependence is similar to that of Ru5/ C(IV) + Amberlyst. From comparison of the results illustrated by Figs. 4 and 5, it is clear that the addition of Amberlyst enhanced the activity of glycerol hydrogenolysis remarkably, compared to the degradation activity. The selectivity for hydrogenolysis products was promoted drastically. In addition, without pretreatment with Ar, Ru5/C(I) and Ru5/C(I) + Amberlyst exhibited a lower level of activity than the corresponding commercial Ru5/C(IV) catalysts. In order to investigate the structural change of the Ru catalyst during treatment with Ar, we measured the CO uptake and the results are given in Table 3. The CO uptake and dispersion decreased monotonously with increasing pretreatment temperature. This phenomenon is explained easily as the aggregation of Ru by the thermal treatment. Here, an important point is that Ru5/C(I) treated below 523 K showed a lower level of activity in the glycerol reaction, in spite of the large amount of CO adsorption. In order to investigate what happens on Ru5/C(I) during the treatment, the TPD profile was obtained, and the result is described in Fig. 6. A large amount of NO desorption was observed in the temperature range 400–600 K. The total amount of desorbed NO corresponded to a molar ratio of NO/Ru of nearly 4. Since the precursor is Ru(NO)(NO3)3, it is suggested that the
3.2. Effect of catalyst pretreatment temperature on glycerol reaction over Ru/C(I) The effect of catalyst pretreatment temperature on the glycerol reaction over Ru5/C(I) + Amberlyst is shown in Fig. 4. The activity was maximum on the catalyst treated at 573 K, and this showed the highest level of selectivity for 1,2-propanediol (1,2-PD). It should be noted that Ru5/C(I) + Amberlyst treated at 573 K is highly active in glycerol hydrogenolysis compared to Ru5/(V) + Amberlyst. As a reference, the effect of pretreatment temperature on the glycerol reaction over Ru5/ C(I) in the absence of Amberlyst is shown in Fig. 5. The Table 3 Characterization results of Ru5/C Catalysts
Ar pretreatment temperature (K)
CO uptake amount (mmol)
Dispersion (CO/Ru)
Acetol hydrogenation Conversion (%)
TOF (1/100 h1)
Ru5/C(I)
473 523 573 623 673 773
0.40 0.41 0.30 0.27 0.23 0.19
0.81 0.82 0.60 0.55 0.47 0.38
22.7 29.6 41.3 36.5 30.6 22.1
2.1 2.6 5.0 4.8 4.7 4.3
Ru5/C(V)
None
0.20
0.41
24.6
4.3
Acetol hydrogenation reaction conditions: 2 mass% acetol aqueous solution 20 ml, 393 K reaction temperature, 1.0 MPa initial H2 pressure, 1 h reaction time, 1.5 mg Ru catalyst.
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Fig. 6. Temperature-programmed desorption profile of NO on Ru5/C(I). Sample weight 25 mg, heating rate 10 K/min.
precursor can be maintained during the drying process in the catalyst preparation procedure, and the decomposition of the precursor starts at 400 K. At higher temperatures, NO and NO3 species can be desorbed as NO. In addition, the result means that NO molecules from the precursor exist even after treatment with Ar at 473–523 K. On the other hand, considering the result of the large amount of CO adsorption, the precursor salt before the decomposition can also adsorb the CO molecule. However, the activity in the glycerol hydrogenolysis of this species is thought to be very low, probably because adsorbed NO can suppress the hydrogenolysis activity. In contrast, it is possible to interpret the behavior of the catalysts treated at temperatures higher than 573 K as simply being influenced by the aggregation of Ru metal particles. Fig. 7 shows the TEM images of the Ru/C(I) and Ru/C(V) used in the glycerol reaction. The average particle size of Ru is calculated as: P 3 ni d mean diameter ðds Þ ¼ P i2 ni di where ni is the number of particles having a characteristic diameter di (within a given diameter range) [23]. The metal
Fig. 8. Effect of the pretreatment temperature on the TOF in the glycerol reaction. TOFs were calculated from the data shown in Figs. 3 and 4.
particle size on Ru/C(I) is calculated as 1.7 0.3 nm, and that on Ru/C(V) is 2.5 0.3 nm. This indicates that Ru metal particles have greater dispersion on Ru/C(I). This tendency agreed with that from the CO adsorption measurement. On the basis of the relationship D = 1.32/d between particle size d (in nm) and D (in %) [24], the dispersion is calculated to be 78 12% on Ru/C(I) and 53 7% on Ru/C(V). The dispersion estimated from TEM is a little higher than that from CO adsorption (Table 3) for both catalysts. This is probably due to the error in the assumption of CO/Rus = 1, because a CO bridge can be present. This tendency in terms of metal dispersion can be related to the higher level of activity of Ru/C(I) compared to that of Ru/C(V). 3.3. Reaction scheme On the basis of CO uptake, we calculated the turnover frequency (TOF) of hydrogenolysis and degradation reactions
Fig. 7. TEM image of Ru/C catalysts after the glycerol reaction at 393 K for 10 h. (a) Ru5/C(I); (b) Ru5/C(V).
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Fig. 9. Reaction scheme of glycerol hydrogenolysis and degradation.
In order to confirm this expectation, we carried out an activity test of acetol hydrogenation, and the results are summarized in Table 3. The behavior of 1,2-PD formation in the glycerol reaction agrees well with that of acetol hydrogenation, and this supports the expected reaction route to 1,2-PD. In addition, the TOF of degradation was increased by the presence of Amberlyst. However, the effect is not so significant as that in the TOF of 1,2-PD formation. This can be explained by the degradation via 1,2-PD. In order to investigate the consecutive reactions, the reaction tests of 1,2-PD and 1,3-PD were carried out (Table 4). The conversion in the glycerol reaction was comparable to that of the 1,3-PD reaction; in contrast, the conversion of 1,2-PD was much lower than that of glycerol. The low reactivity of 1,2-PD is related to the high yield of 1,2-PD in the glycerol reaction, and the high reactivity of 1,3-PD causes the very low yield of 1,3-PD in the glycerol reaction. In addition, the selectivity ratio of 1-PO to 2-PO is characteristic. In the reaction of 1,3-PD, mainly 1-PO was formed; in contrast, the formation of 1-PO was comparable to that of 2-PO in the 1,2PD reaction. The selectivity ratio of 1-PO to 2-PO in the glycerol reaction was more similar to that in the 1,3-PD reaction, and this suggests that 1-PO and 2-PO can be formed via 1,3-PD in the glycerol reaction. Furthermore, the degradation reaction proceeded in the reactions of both 1,2-PD and 1,3-PD, and this can be related to the tendency that the TOF of degradation was increased slightly by the presence of Amberlyst, as shown in Fig. 8.
in the glycerol reaction, and the results are shown in Fig. 8. In particular, TOFs of hydrogenolysis are represented as 1,2propanediol (1,2-PD) and 1,3-propanediol (1,3-PD) + 1-propanpol (1-PO) + 2-propanol (2-PO) separately. This is because 1-PO and 2-PO are formed mainly via 1,3-PD, as discussed later. The TOF of glycerol hydrogenolysis over Ru5(I)/C in the treatment temperature range 573–773 K was not changed, although the dispersion was changed from 0.38 to 0.60 (Table 3). On the other hand, the TOF of the degradation reaction over the Ru5/C(I) catalysts was changed remarkably. This indicates that the reaction to degradation products (EG + C2H5OH + CH3OH + CH4) is more influenced than the hydrogenolysis reaction over Ru5/C(I) catalysts. This suggests that smaller metal particles can decrease the hydrogenolysis selectivity. From the comparison of Ru5/C(I) with and without Amberlyst, it was found that the TOF of 1,3PD + 1-PO + 2-PO was not influenced by the presence of Amberlyst, and this suggests that the formation of 1,3-PD + 1PO + 2-PO can be catalyzed by only Ru5/C(I). In contrast, it is interesting that the TOF of the 1,2-PD formation can be enhanced remarkably by the presence of Amberlyst. This result suggests that 1,2-PD formation can be catalyzed by Amberlyst and Ru5/C(I). From the combination of the present discussion and that reported previously [7], the reaction route from glycerol to 1,2-PD is thought to be as follows: dehydration of glycerol to acetol is catalyzed by Amberlyst and consecutive hydrogenation of acetol to 1,2-PD is catalyzed by Ru/C. The possible reaction route is shown in Fig. 9.
Table 4 Results of the reaction test of 2 mass% aqueous solution of various compounds over Ru5/C(I) + Amberlyst under H2 Catalysts
Reactant
Conversion (%)
Selectivity of each product (%) a 1,2-PD
1,3-PD
1-PO
2-PO
EG
C2H5OH
CH3OH
CH4
Ru5/C(I) + Amberlyst
Glycerol 1,2-PD 1.3-PD
79.3 20.1 81.0
74.7 – 0.0
0.0 0.0 –
7.7 35.7 23.5
1.6 30.9 0.1
6.8 0.3 0.0
3.8 21.9 50.9
0.4 0.0 0.0
4.9 11.1 25.4
Reaction conditions: 2 mass% glycerol aqueous solution 20 ml, 393 K reaction temperature, 8.0 MPa initial H2 pressure, 10 h reaction time, 150 mg Ru catalyst + 300 mg Amberlyst. PD, propanediol; PO, propanol; EG, ethylene glycol. a C-based selectivity.
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4. Conclusions
References
(1) The combination of Ru/C + Amberlyst is effective for the glycerol reaction under mild reaction conditions (393 K), and the conversion and selectivity were strongly dependent on the catalytic performance of Ru/C. (2) Ru/C prepared by using active carbon with a low surface area, such as 250 m2/g, was more suitable than that with higher surface area. It is suggested that Ru species supported on the graphite phase can be active. (3) Treatment of Ru/C catalysts prepared by using Ru(NO)(NO3)3 as a precursor with Ar flowing influenced the performance of Ru/C catalysts. The decomposition of the Ru precursor is necessary for high-level activity. On the other hand, treatment at too high a temperature decreased the level of activity due to aggregation of Ru metal particles. Treatment at 573 K was optimal. (4) In the glycerol reaction, the degradation reaction over Ru5/ (I) is more structure-sensitive than the hydrogenolysis reaction, and the selectivity of hydrogenolysis was lower on smaller Ru particles. (5) The combination of Ru/C with Amberlyst enhanced the TOF of 1,2-PD formation drastically; in contrast, it did not affect the TOF of 1,3-PD + 1-PO + 2-PO. This indicates that 1,2-PD can be formed mainly in dehydration of glycerol to acetol catalyzed by Amberlyst and subsequent hydrogenation of acetol to 1,2-PD catalyzed by Ru/C. In contrast, 1-PO and 2-PO can be formed via 1,3-PD catalyzed by Ru/C.
[1] R.D. Cortright, R.R. Davda, J.A. Dumesic, Nature 418 (2002) 964. [2] G.W. Huber, J.W. Shabaker, J.A. Dumesic, Science 300 (2003) 2735. [3] G.A. Deluga, J.R. Salge, L.D. Schmidt, X.E. Verykios, Science 303 (2004) 993. [4] M. Asadullah, S. Ito, K. Kunimori, M. Yamada, K. Tomishige, J. Catal. 208 (2002) 255. [5] K. Tomishige, M. Asadullah, K. Kunimori, Catal. Today 89 (2004) 389. [6] G.W. Huber, J.N. Chheda, C.J. Barrett, J.A. Dumesic, Science 308 (2005) 1446. [7] T. Miyazawa, Y. Kusunoki, K. Kunimori, K. Tomishige, J. Catal. 240 (2006) 213. [8] S. Fernando, S. Adhikari, C. Chandrapal, N. Murali, Energy Fuels 20 (2006) 1727. [9] G.W. Huber, S. Iborra, A. Corma, Chem. Rev. 106 (2006) 4044. [10] R.R. Soares, D.A. Simonetti, J.A. Dumesic, Angew. Chem. Int. Ed. 45 (2006) 3982. [11] T. Hirai, N. Ikenaga, T. Miyake, T. Suzuki, Energy Fuels 19 (2005) 1761. [12] T. Haas, B. Jaeger, R. Weber, S.F. Mitchell, Appl. Catal. A 280 (2005) 83. [13] M.A. Dasari, P.P. Kiatsimkul, W.R. Sutterlin, G.J. Suppes, Appl. Catal. A 281 (2005) 225. [14] D.K. Sohounloue, C. Montassier, J. Barbier, React. Kinet. Catal. Lett. 22 (1983) 391. [15] J. Chaminand, L. Djakovitch, P. Gallezot, P. Marion, C. Pinel, C. Rosier, Green Chem. 6 (2004) 359. [16] C. Montassier, J.C. Me´ne´zo, L.C. Hoang, C. Renaud, J. Barbier, J. Mol. Catal. 70 (1991) 99. [17] D.G. Lahr, B.H. Shanks, Ind. Eng. Chem. Res. 42 (2003) 5467. [18] D.G. Lahr, B.H. Shanks, J. Catal. 232 (2005) 386. [19] Y. Kusunoki, T. Miyazawa, K. Kunimori, K. Tomishige, Catal. Commun. 6 (2005) 645. [20] M.J. Bleda-Martı´nez, D. Lozano-Castello´, E. Morallo´n, D. CazorlaAmoro´s, A. Linares-Solano, Carbon 44 (2006) 2642. ´ rfa˜o, Carbon 37 [21] J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. O (1999) 1379. [22] U. Zielke, K.J. Hu¨ttinger, W.P. Hoffman, Carbon 34 (1996) 983. [23] D.G. Mustard, C.H. Bartholomew, J. Catal. 67 (1981) 186. [24] J.R. Anderson, Structure of Metallic Catalysts, Academic Press Inc., New York, 1975, p. 295.
Acknowledgement A part of this study was supported by a Grant in Aid for Exploratory Research under the Japan Society for the Promotion of Science (JSPS).
Development of a Ru/C catalyst for glycerol hydrogenolysis in combination with an ion-exchange resin Tomohisa Miyazawa, Shuichi Koso, Kimio Kunimori, Keiichi Tomishige * Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan Received 3 August 2006; received in revised form 8 November 2006; accepted 9 November 2006 Available online 12 December 2006
Abstract The combination of Ru/C and Amberlyst ion-exchange resin is effective for the dehydration and hydrogenation (denoted as hydrogenolysis) of glycerol to 1,2-propanediol under mild reaction conditions (393 K). A Ru/C catalyst prepared by using active carbon with a low surface area (250 m2/g) showed better performance than that prepared by using active carbon with a high surface area. In addition, treatment of Ru/C catalysts prepared from Ru(NO)(NO3)3 with Ar flowing at the appropriate temperature enhanced the performance compared to that of the commercially available Ru/C catalysts. This temperature treatment can be influenced by the decomposition of Ru precursor salt and aggregation of Ru metal particles. In addition, the degradation reaction as a side-reaction to C1 and C2 compounds of glycerol hydrogenolysis was more structure-sensitive than the hydrogenolysis reaction, and the selectivity of hydrogenolysis was lower on smaller Ru particles. The combination of Ru/C with the Amberlyst resin enhanced the turnover frequency of 1,2-propanediol formation drastically, and this indicates that 1,2-propanediol can be formed mainly by dehydration of glycerol to acetol catalyzed by Amberlyst and subsequent hydrogenation of acetol to 1,2-propanediol catalyzed by Ru/C. # 2006 Elsevier B.V. All rights reserved. Keywords: Glycerol; Hydrogenolysis; Ruthenium; Ion-exchange resin; Propanediol
1. Introduction Interest in the catalytic conversion of renewable feedstocks and chemicals has been increasing. Such conversion to hydrogen can contribute to the utilization of renewable energy sources [1–5], and conversion to petrochemicals can facilitate the replacement of petroleum by renewable resources [6,7]. Recently, it has been proposed that commodity chemicals that are used to produce pharmaceuticals, agricultural adjuvants, plastics and transportation fuel that are derived from fossil resources might be producible in future biorefineries from renewable resources, such as plant-derived sugar and other compounds [8]. Glycerol is a building block that might serve as an important biorefinery feedstock [8]. In addition, it is a byproduct from the production of biodiesel from vegetable oils [9]. Glycerol can be converted to hydrogen and synthesis gas by a reforming reaction [10,11]. One of the methods is conversion of glycerol
* Corresponding author. Tel.: +81 29 853 5030; fax: +81 29 853 5030. E-mail address: [email protected] (K. Tomishige). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.11.006
to 1,2-propanediol and 1,3-propanediol, which have been produced from petroleum derivatives [12]. Several routes to the formation of propanediol can be traced from renewable feedstocks. The commonest route is the conversion of sugar or sugar alcohols at high temperatures and pressures in the presence of a metal catalyst to produce propanediol and other lower polyols [13]. It has been reported that propanediol is producible through catalytic conversion of polyols [14] and glycerol. Supported metal catalysts, hydrogen pressure of 6– 10 MPa and reaction temperatures of 453–513 K have been applied [15–18]. Recently, it was reported that the reaction of glycerol proceeded at a hydrogen pressure of 1.4 MPa and a temperature of 473 K [13]. Judging from these various reports, it seems to be difficult to use milder reaction conditions, especially reduced reaction temperatures. Our group reported recently that the addition of solid acid catalysts to Ru/C enhanced conversion and selectivity in glycerol hydrogenolysis [7,19]. Our results suggest that the conversion of glycerol to propanediols proceeds by the combination of dehydration over acid catalysts with subsequent hydrogenation over metal catalysts [7,19]. In this study, it is found that Ru/C catalysts prepared by using Ru(NO)(NO3)3 and active carbon with a low
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surface area and treated under Ar flowing at the optimum temperature exhibited much higher performance in the glycerol reaction under H2 in combination with Amberlyst as a solid acid catalyst than the commercial Ru/C. In addition, from the comparison of data obtained in the optimization process of Ru/ C catalysts, the structure-sensitivity of the glycerol hydrogenolysis and degradation reaction is discussed, and the reaction route to various products is discussed on the basis of the turnover frequency. 2. Experimental 2.1. Catalyst Active carbon-supported Ru catalysts were prepared by using various active carbon materials. Vulcun-XC72 was supplied by Cabot Corporation Ltd., Shirasagi DO-2, Shirasagi M and Carboraffin were supplied by Japan EnviroChemicals Ltd. Vulcun-XC72, Shirasagi DO-2, Shirasagi M and Carboraffin are denoted as C(I), C(II), C(III) and C(IV), respectively. Ru was loaded by impregnating carbon supports with an aqueous solution of Ru(NO)(NO3)3 and RuCl3, and an acetone solution of Ru(acac)3 as a precursor. After impregnation and solvent removal by evaporation, the catalysts were dried for 12 h at 393 K. The amount of Ru loaded was in the range of 3– 10 wt%. The amount of Ru loaded is denoted as Ru5/C(I) in the case of 5 wt% Ru supported on active carbon(I). These catalysts were used also after treatment under Ar flowing at a temperature of 473–773 K. In addition, 5 wt% Ru/C was purchased from Wako Pure Chemical Industries Ltd., and this catalyst is denoted as Ru5/C(V). The cation-exchange resin Amberlyst 15 (4.7 equiv./kg resin dried, particle size 0.4– 1.2 mm, highest operating temperature 393 K; MP Biomedicals), which consists of highly cross-linked styrene–divinyl benzene copolymer beads that are functionalized with sulfonic groups, was used as the solid acid catalyst. All catalysts were in powder form, with less than 0.1 mm. 2.2. Activity test Glycerol hydrogenolysis was carried out with a 20 ml aqueous solution of glycerol in a 70 ml stainless-steel autoclave. The standard reaction was conducted under the following conditions: 393 K reaction temperature, 8.0 MPa initial hydrogen pressure, 10 h reaction time, 20 wt% glycerol aqueous solution, 150 mg Ru/C catalysts, and 300 mg Amberlyst. Reaction conditions were varied for investigation of the effects of different conditions. Details of the reaction conditions are described for each result. In addition, to elucidate the mechanism of the glycerol reaction, we used an aqueous solution of 2 wt%. 1,3-Propanediol (1,3-PD), and 1,2propanediol (1,2-PD) as a reactant. In all experiments, the aqueous solution of the reactant, the catalyst powder, and the spinner were put into the autoclave; then the reactor was purged with H2 (99.99%; Takachiho Trading Co. Ltd.). After purging, the reactor was heated to the required temperature and the H2 pressure was increased to
245
8.0 MPa in standard experiments. The temperature was monitored with a thermocouple that was inserted into the autoclave and connected to the thermo-controller. The reaction consumed hydrogen, and the total pressure decreased. However, the decrease in hydrogen pressure was, at most, 1/10 of the initial pressure. After the reaction, the gas phase products were collected in a gasbag and the liquid phase products were separated from the used catalyst by filtration. These products were analyzed using a gas chromatograph (GC-353; GL Sciences Inc.) equipped with a flame ionization detector (FID). A TC-WAX capillary column (diameter 0.25 mm, length 20 m) was used for separation and the column temperature was 493 K. Products were identified also using GC–MS (GCMS-QP5050, column Stabilwax; Shimadzu Corp.) and the products detected were: 1,3propanediol (1,3-PD), 1,2-propanediol (1,2-PD), 1-propanol (1-PO), and 2-propanol (2-PO) are hydrogenolysis products, and ethylene glycol (EG), ethanol, methanol and methane are degradation products.Conversion of the reactants in all reaction tests were calculated on the basis of the following equation: conversion of reactant ð%Þ ¼
sum of C-based mol of all products 100 sum of C-based mol of reactant and all products
The conversion of a reactant is usually defined as: reactant before reactant afterwards reactant before but we have to determine the conversion and the selectivity even when the level of conversion is very low. Considering the error of the analysis procedure, we applied the above method of calculation. It should be noted that the conversions calculated by our method and the method based on mass balance agreed well when the conversion was greater than 5%. The selectivity of the products in all reaction tests was calculated with the following equation, considering that the degradation byproducts (ethylene glycol, ethanol, methanol, and methane) were always formed: selectivity ð%Þ ¼
C-based mol of the product 100 sum of C-based mol of all products
Here, we simply assume that the degradation products are formed directly from glycerol and other reactants in terms of carbon number in each molecule. For example, when one molecule of glycerol is converted to one molecule of ethylene glycol and one molecule of methane, the selectivity of ethylene glycol and methane is calculated to be 66.7% and 33.3%, respectively. Here, it is interpreted that two-thirds of a glycerol molecule is converted to one molecule of ethylene glycol molecule, and at the same time one-third of the glycerol molecule is converted to one molecule of methane. The yield is calculated as: conversion ð%Þ selectivity ð%Þ : 100
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Table 1 Properties of various active carbon-supported Ru catalysts Catalysts a
Ru5/C(I) Ru5/C(II)a Ru5/C(in)a Ru5/C(IV)a Ru5/C(V)
Surface area (m2/g)
Pretreatment temperature
Dispersion
254 521 629 1046 485
573 K, 573 K, 573 K, 573 K, None
0.60 0.88 0.90 0.70 0.41
Ar Ar Ar Ar
a Ru were impregnated from RuNO(NO3)3 and loading amount of Ru is 5 wt%. C(I), Vulcun-XC72; C(II), Shirasagi DO-2; C(III), Shirasagi M; C(IV), Carboraffin; Ru/C(V), Ru/C (Wako).
2.3. Characterization The surface area of the supported metal catalysts was measured using the BET method (N2 adsorption) with a Gemini apparatus (Micromeritics Instrument Corporation). The sizes of metal particles and the dispersion of the carbon-supported Ru catalyst were estimated from irreversible CO adsorption measurements performed at room temperature. The gas pressure at the adsorption equilibrium was about 1.1 kPa and the sample weight was about 0.2 g. The dead-volume of the apparatus was about 60 cm3. The results of the characterization of the fresh metal catalysts are given in Table 1. Before the CO adsorption, the catalyst was treated without evaporating to the atmosphere under Ar flowing at a temperature of 473–773 K for 2 h, and reduced under flowing hydrogen at 393 K for 1 h. X-ray diffraction (XRD) spectra recorded with a Philips X’pert diffractometer was used in order to confirm the graphite phase of the carbon supports. Transmission electron microscope (TEM) images were taken for determination of the particle size with a JEM 2010 instrument (JEOL) operated at 200 kV. The samples after the glycerol reaction were dispersed by supersonic waves in 2propanol; they were placed on Cu grids under air atmosphere. Temperature-programmed desorption (TPD) of the carbon supports was done in a closed circulating vacuum system equipped with a variable leak valve, by which the gas was introduced into a differentially pumped quadrupole mass spectrometer (Balzers QMS 200F). Without any pretreatment, the sample support (10 mg) was heated under vacuum from room temperature to 1273 K at 10 K/min. Desorbed CO2 and CO were analyzed by QMS. In order to characterize the effect of treatment with Ar, the thermal stability of impregnated Ru/C catalyst was evaluated using the TPD profile of Ru5/C(I). Fresh catalyst (25 mg) was used without pretreatment. The sample was heated under vacuum from room temperature to 773 K at 10 K/min.
Fig. 1. Glycerol reaction on various active carbon-supported Ru catalysts + Amberlyst. Reaction conditions: 20 mass% glycerol aqueous solution 20 ml, 393 K reaction temperature, 8.0 MPa initial H2 pressure, 10 h reaction time, 150 mg of Ru catalyst + 300 mg of Amberlyst. PD, propanediol; PO, propanol; others, ethylene glycol + ethanol + methanol + methane. *Ru was impregnated from RuNO(NO3)3 and treated with Ar at 573 K. The loading amount of Ru is 5 wt%.
Fig. 1 and the characterization results are given in Table 1. Except for Ru5/C(V), all the carbon-supported Ru catalysts were pretreated under 30 cm3/min Ar flow at 573 K. This treatment with Ar is the optimum activation condition, as shown later. It is found that Ru5/C(I) + Amberlyst exhibited a higher level of glycerol conversion and selectivity toward 1,2propanediol than the commercially available Ru5/C(V) + Amberlyst, which was shown to be an effective catalyst in our earlier studies [7,19]. On the other hand, Ru5/C(II), Ru5/C(III) and Ru5/C(IV) catalysts showed a much lower level of activity than the commercial Ru5/(V). In addition, we tested Ru5/C(V) after treatment with Ar and found that the performance was almost the same as that without the treatment with Ar. From a comparison between the catalytic activity and characterization results, such as BET surface area and metal dispersion, it seems that a lower surface area is more suitable, and the metal dispersion seems to be unrelated. Fig. 2 shows the XRD
3. Results and discussion 3.1. Glycerol reaction on various active carbon-supported Ru catalysts Results of the activity test with various combinations of Ru/ C and Amberlyst in the reaction of glycerol are described in
Fig. 2. XRD patterns of various Ru/C catalysts. C(I), Vulcun-XC72; C(II), Shirasagi DO-2; C(III), Shirasagi M; C(IV), Carboraffin, All samples were used without treatment after impregnation.
T. Miyazawa et al. / Applied Catalysis A: General 318 (2007) 244–251
247
Fig. 3. TPD profiles of various carbon supports. (a) CO2 desorption; (b) CO desorption; C(I), Vulcun-XC72; C(II), Shirasagi DO-2; C(III), Shirasagi M; C(IV), Carboraffin. Sample weight 10 mg, heating rate 10 K/min. All samples were used without pretreatment.
patterns of various Ru/C catalysts. The peak around 25.28 is assigned to diffraction of the graphite phase (JCPDS File, no. 41–1487). It is characteristic that the graphite phase was observed much more clearly for Ru/C(I) than for other catalysts. This suggests that Ru metal particles on the graphite phase have a high level of catalytic activity. Fig. 3 shows the TPD profiles of CO and CO2 on the various carbon supports. It is clear that the desorption of CO and CO2 on C(I) was much less than it was on other supports. The desorption of CO2 and CO is due to the decomposition of surface functional groups, such as carboxyl and carbonyl groups [20–22]. This result indicates also that C(I) has a much fewer surface functional groups than other carbon supports. The order of the total amount of desorbed CO and CO2 (C(IV) C(III) > C(II) C(I)) can be related to the order of the catalytic activity in the glycerol reaction (Ru/C(I) Ru/ C(II) > Ru/C(III) > Ru/C(IV)). This suggests that the carbon surface with less oxygen-containing functional groups, such as carbonyl and carboxyl groups, is more suitable as a support for Ru metal particles. This can be related to the suitability of the graphite phase. At present, the reason for the suitability of a graphite phase with less surface carboxylic and carbonyl
functional groups is not clear and further investigation is necessary. The Ru5/C(I) catalysts were used for subsequent experiments. The effect of the Ru precursor, the loading amount and the catalyst treatment over Ru/C(I) + Amberlyst in the glycerol reaction at 393 K are given in Table 2. Except for Ru5/C(I) from Ru(acac)3, it is clear that pretreatment with Ar enhanced the catalyst activity. In the case of Ru5/C(I) from Ru(acac)3 after treatment with Ar, the conversion of glycerol was very low (<0.2%) (data not shown). This can be related to the sublimation of Ru(acac)3 during the treatment. On the other hand, regarding RuCl3, the sublimation was negligible. This is based on the thermogravimetric analysis (TGA) of Ru5/C(I) from RuCl3 under Ar flow from room temperature to 573 K. Except the vaporization of water in the catalysts at room temperature to 373 K, the weight loss corresponds to almost 5%, and this could be due to the amount of Cl in the Ru precursor (RuCl3), where the decomposition of HCl was detected by MS. This is supported by the fact that the treatment with Ar improved the catalytic activity of Ru5/C(I) from RuCl3 + Amberlyst, and it is interpreted that the precursor salt decomposed during the treatment. Ru5/C(I) from Ru(NO)(NO3)3 + Amberlyst exhibited the greatest
Table 2 Effect of Ru precursor, loading amount and catalyst treatment over Ru/C + Amberlyst Ru catalyst
b
Pretreatment temperature
Ru3 /C(I) Ru5/C(I) RulO/C(I) Ru5/C(I) Ru5/C(I)c
Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(acac)3 RuCl3
None None None None None
Ru3/C(I) Ru5/C(I) RulO/C(I) Ru5/C(I)
Ru(NO)(NO3)3 Ru(NO)(NO3)3 Ru(NO)(NO3)3 RuCl3
573 K, 573 K, 573 K, 573 K,
Ar Ar Ar Ar
Conversion (%)
Selectivity of each product (%)a 1,2-PD
1,3-PD
1-PO
2-PO
Others
2.4 4.7 3.6 3.7 0.3
69.6 59.4 65.5 32.9 24.0
3.4 6.7 3.1 11.8 0.0
14.1 13.5 10.5 17.6 0.0
1.6 0.8 1.3 1.1 15.2
11.4 19.6 19.6 36.5 34.7
9.5 21.3 18.1 5.2
75.8 76.7 46.8 73.8
4.6 1.5 5.2 4.9
8.1 2.5 16.3 13.9
1.3 0.5 0.6 1.7
10.2 18.8 31.1 5.7
Reaction conditions: 20 mass% glycerol aqueous solution 20 ml, 393 K reaction temperature, 8.0 MPa initial H2 pressure, 10 h reaction time, 150 mg Ru catalyst + 300 mg Amberlyst, PD, propanediol; PO, propanol; others, ethylene glycol + ethanol + methanol + methane. a C-based selectivity. b loading amount of Ru(wt%). c Rest 26% product is acetol.
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Fig. 4. Effect of temperature of pretreatment of catalysts on glycerol reaction over Ru5/C(I) + Amberlyst at 393 K. The reaction conditions are the same as those given in the legend to Fig. 1. *Ru5/C(V) + Amberlyst was used as a reference.
Fig. 5. Effect of pretreatment temperature of Ru5/C(I) on the glycerol reaction at 393 K. The reaction conditions are the same as those given in the legend to Fig. 1, except the amount of Amberlyst. *Ru5/C(V) was used as a reference.
conversion of glycerol both with and without treatment with Ar. In addition, regarding the effect of the loading amount of Ru, the activity of Ru10/C(I) + Amberlyst with Ar treatment was comparable to the case of Ru5/C(I); however, it showed higher degradation selectivity. These results indicate that the amount of Ru on the carbon support must be optimized for high selectivity of hydrogenolysis reactions, and too much Ru promotes degradation reactions. This tendency is compatible with earlier results [7]. On the basis of the results presented here, we focused on the Ru5/C(I) catalyst in subsequent studies.
pretreatment temperature-dependence is similar to that of Ru5/ C(IV) + Amberlyst. From comparison of the results illustrated by Figs. 4 and 5, it is clear that the addition of Amberlyst enhanced the activity of glycerol hydrogenolysis remarkably, compared to the degradation activity. The selectivity for hydrogenolysis products was promoted drastically. In addition, without pretreatment with Ar, Ru5/C(I) and Ru5/C(I) + Amberlyst exhibited a lower level of activity than the corresponding commercial Ru5/C(IV) catalysts. In order to investigate the structural change of the Ru catalyst during treatment with Ar, we measured the CO uptake and the results are given in Table 3. The CO uptake and dispersion decreased monotonously with increasing pretreatment temperature. This phenomenon is explained easily as the aggregation of Ru by the thermal treatment. Here, an important point is that Ru5/C(I) treated below 523 K showed a lower level of activity in the glycerol reaction, in spite of the large amount of CO adsorption. In order to investigate what happens on Ru5/C(I) during the treatment, the TPD profile was obtained, and the result is described in Fig. 6. A large amount of NO desorption was observed in the temperature range 400–600 K. The total amount of desorbed NO corresponded to a molar ratio of NO/Ru of nearly 4. Since the precursor is Ru(NO)(NO3)3, it is suggested that the
3.2. Effect of catalyst pretreatment temperature on glycerol reaction over Ru/C(I) The effect of catalyst pretreatment temperature on the glycerol reaction over Ru5/C(I) + Amberlyst is shown in Fig. 4. The activity was maximum on the catalyst treated at 573 K, and this showed the highest level of selectivity for 1,2-propanediol (1,2-PD). It should be noted that Ru5/C(I) + Amberlyst treated at 573 K is highly active in glycerol hydrogenolysis compared to Ru5/(V) + Amberlyst. As a reference, the effect of pretreatment temperature on the glycerol reaction over Ru5/ C(I) in the absence of Amberlyst is shown in Fig. 5. The Table 3 Characterization results of Ru5/C Catalysts
Ar pretreatment temperature (K)
CO uptake amount (mmol)
Dispersion (CO/Ru)
Acetol hydrogenation Conversion (%)
TOF (1/100 h1)
Ru5/C(I)
473 523 573 623 673 773
0.40 0.41 0.30 0.27 0.23 0.19
0.81 0.82 0.60 0.55 0.47 0.38
22.7 29.6 41.3 36.5 30.6 22.1
2.1 2.6 5.0 4.8 4.7 4.3
Ru5/C(V)
None
0.20
0.41
24.6
4.3
Acetol hydrogenation reaction conditions: 2 mass% acetol aqueous solution 20 ml, 393 K reaction temperature, 1.0 MPa initial H2 pressure, 1 h reaction time, 1.5 mg Ru catalyst.
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Fig. 6. Temperature-programmed desorption profile of NO on Ru5/C(I). Sample weight 25 mg, heating rate 10 K/min.
precursor can be maintained during the drying process in the catalyst preparation procedure, and the decomposition of the precursor starts at 400 K. At higher temperatures, NO and NO3 species can be desorbed as NO. In addition, the result means that NO molecules from the precursor exist even after treatment with Ar at 473–523 K. On the other hand, considering the result of the large amount of CO adsorption, the precursor salt before the decomposition can also adsorb the CO molecule. However, the activity in the glycerol hydrogenolysis of this species is thought to be very low, probably because adsorbed NO can suppress the hydrogenolysis activity. In contrast, it is possible to interpret the behavior of the catalysts treated at temperatures higher than 573 K as simply being influenced by the aggregation of Ru metal particles. Fig. 7 shows the TEM images of the Ru/C(I) and Ru/C(V) used in the glycerol reaction. The average particle size of Ru is calculated as: P 3 ni d mean diameter ðds Þ ¼ P i2 ni di where ni is the number of particles having a characteristic diameter di (within a given diameter range) [23]. The metal
Fig. 8. Effect of the pretreatment temperature on the TOF in the glycerol reaction. TOFs were calculated from the data shown in Figs. 3 and 4.
particle size on Ru/C(I) is calculated as 1.7 0.3 nm, and that on Ru/C(V) is 2.5 0.3 nm. This indicates that Ru metal particles have greater dispersion on Ru/C(I). This tendency agreed with that from the CO adsorption measurement. On the basis of the relationship D = 1.32/d between particle size d (in nm) and D (in %) [24], the dispersion is calculated to be 78 12% on Ru/C(I) and 53 7% on Ru/C(V). The dispersion estimated from TEM is a little higher than that from CO adsorption (Table 3) for both catalysts. This is probably due to the error in the assumption of CO/Rus = 1, because a CO bridge can be present. This tendency in terms of metal dispersion can be related to the higher level of activity of Ru/C(I) compared to that of Ru/C(V). 3.3. Reaction scheme On the basis of CO uptake, we calculated the turnover frequency (TOF) of hydrogenolysis and degradation reactions
Fig. 7. TEM image of Ru/C catalysts after the glycerol reaction at 393 K for 10 h. (a) Ru5/C(I); (b) Ru5/C(V).
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Fig. 9. Reaction scheme of glycerol hydrogenolysis and degradation.
In order to confirm this expectation, we carried out an activity test of acetol hydrogenation, and the results are summarized in Table 3. The behavior of 1,2-PD formation in the glycerol reaction agrees well with that of acetol hydrogenation, and this supports the expected reaction route to 1,2-PD. In addition, the TOF of degradation was increased by the presence of Amberlyst. However, the effect is not so significant as that in the TOF of 1,2-PD formation. This can be explained by the degradation via 1,2-PD. In order to investigate the consecutive reactions, the reaction tests of 1,2-PD and 1,3-PD were carried out (Table 4). The conversion in the glycerol reaction was comparable to that of the 1,3-PD reaction; in contrast, the conversion of 1,2-PD was much lower than that of glycerol. The low reactivity of 1,2-PD is related to the high yield of 1,2-PD in the glycerol reaction, and the high reactivity of 1,3-PD causes the very low yield of 1,3-PD in the glycerol reaction. In addition, the selectivity ratio of 1-PO to 2-PO is characteristic. In the reaction of 1,3-PD, mainly 1-PO was formed; in contrast, the formation of 1-PO was comparable to that of 2-PO in the 1,2PD reaction. The selectivity ratio of 1-PO to 2-PO in the glycerol reaction was more similar to that in the 1,3-PD reaction, and this suggests that 1-PO and 2-PO can be formed via 1,3-PD in the glycerol reaction. Furthermore, the degradation reaction proceeded in the reactions of both 1,2-PD and 1,3-PD, and this can be related to the tendency that the TOF of degradation was increased slightly by the presence of Amberlyst, as shown in Fig. 8.
in the glycerol reaction, and the results are shown in Fig. 8. In particular, TOFs of hydrogenolysis are represented as 1,2propanediol (1,2-PD) and 1,3-propanediol (1,3-PD) + 1-propanpol (1-PO) + 2-propanol (2-PO) separately. This is because 1-PO and 2-PO are formed mainly via 1,3-PD, as discussed later. The TOF of glycerol hydrogenolysis over Ru5(I)/C in the treatment temperature range 573–773 K was not changed, although the dispersion was changed from 0.38 to 0.60 (Table 3). On the other hand, the TOF of the degradation reaction over the Ru5/C(I) catalysts was changed remarkably. This indicates that the reaction to degradation products (EG + C2H5OH + CH3OH + CH4) is more influenced than the hydrogenolysis reaction over Ru5/C(I) catalysts. This suggests that smaller metal particles can decrease the hydrogenolysis selectivity. From the comparison of Ru5/C(I) with and without Amberlyst, it was found that the TOF of 1,3PD + 1-PO + 2-PO was not influenced by the presence of Amberlyst, and this suggests that the formation of 1,3-PD + 1PO + 2-PO can be catalyzed by only Ru5/C(I). In contrast, it is interesting that the TOF of the 1,2-PD formation can be enhanced remarkably by the presence of Amberlyst. This result suggests that 1,2-PD formation can be catalyzed by Amberlyst and Ru5/C(I). From the combination of the present discussion and that reported previously [7], the reaction route from glycerol to 1,2-PD is thought to be as follows: dehydration of glycerol to acetol is catalyzed by Amberlyst and consecutive hydrogenation of acetol to 1,2-PD is catalyzed by Ru/C. The possible reaction route is shown in Fig. 9.
Table 4 Results of the reaction test of 2 mass% aqueous solution of various compounds over Ru5/C(I) + Amberlyst under H2 Catalysts
Reactant
Conversion (%)
Selectivity of each product (%) a 1,2-PD
1,3-PD
1-PO
2-PO
EG
C2H5OH
CH3OH
CH4
Ru5/C(I) + Amberlyst
Glycerol 1,2-PD 1.3-PD
79.3 20.1 81.0
74.7 – 0.0
0.0 0.0 –
7.7 35.7 23.5
1.6 30.9 0.1
6.8 0.3 0.0
3.8 21.9 50.9
0.4 0.0 0.0
4.9 11.1 25.4
Reaction conditions: 2 mass% glycerol aqueous solution 20 ml, 393 K reaction temperature, 8.0 MPa initial H2 pressure, 10 h reaction time, 150 mg Ru catalyst + 300 mg Amberlyst. PD, propanediol; PO, propanol; EG, ethylene glycol. a C-based selectivity.
T. Miyazawa et al. / Applied Catalysis A: General 318 (2007) 244–251
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4. Conclusions
References
(1) The combination of Ru/C + Amberlyst is effective for the glycerol reaction under mild reaction conditions (393 K), and the conversion and selectivity were strongly dependent on the catalytic performance of Ru/C. (2) Ru/C prepared by using active carbon with a low surface area, such as 250 m2/g, was more suitable than that with higher surface area. It is suggested that Ru species supported on the graphite phase can be active. (3) Treatment of Ru/C catalysts prepared by using Ru(NO)(NO3)3 as a precursor with Ar flowing influenced the performance of Ru/C catalysts. The decomposition of the Ru precursor is necessary for high-level activity. On the other hand, treatment at too high a temperature decreased the level of activity due to aggregation of Ru metal particles. Treatment at 573 K was optimal. (4) In the glycerol reaction, the degradation reaction over Ru5/ (I) is more structure-sensitive than the hydrogenolysis reaction, and the selectivity of hydrogenolysis was lower on smaller Ru particles. (5) The combination of Ru/C with Amberlyst enhanced the TOF of 1,2-PD formation drastically; in contrast, it did not affect the TOF of 1,3-PD + 1-PO + 2-PO. This indicates that 1,2-PD can be formed mainly in dehydration of glycerol to acetol catalyzed by Amberlyst and subsequent hydrogenation of acetol to 1,2-PD catalyzed by Ru/C. In contrast, 1-PO and 2-PO can be formed via 1,3-PD catalyzed by Ru/C.
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Acknowledgement A part of this study was supported by a Grant in Aid for Exploratory Research under the Japan Society for the Promotion of Science (JSPS).