SiO2 catalyst

Catalysis Communications 43 (2014) 38–41

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Short Communication

Aqueous phase hydrogenation of acetic acid to ethanol over Ir-MoOx/SiO2 catalyst Zhiqiang Wang a,b,1, Guangyi Li a,c,1, Xiaoyan Liu a, Yanqiang Huang a, Aiqin Wang a, Wei Chu b, Xiaodong Wang a, Ning Li a,⁎ a b c

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, CAS, Dalian 116023, PR China Department of Chemical Engineering, Sichuan University, Chengdu 610065, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e

i n f o

Article history: Received 27 April 2013 Received in revised form 15 July 2013 Accepted 5 September 2013 Available online 13 September 2013 Keywords: Aqueous-phase Hydrogenation Acetic acid Ethanol Ir Mo

a b s t r a c t Mo modified Ir/SiO2 (Ir-MoOx/SiO2) was firstly used for the aqueous-phase hydrogenation of acetic acid to ethanol and exhibited the best performance among the investigated catalysts. The synergy effect between the closely contacted Ir and Mo species is responsible for the excellent performance of Ir-MoOx/SiO2. Ir-MoOx/SiO2 is also effective for the hydrogenation of other biomass derived carboxylic acids. Especially when levulinic acid was used as the feedstock, high yields of 2-pentanol (39.0%) and 1,4-pentanediol (42.3%) were obtained simultaneously. To our knowledge, this is the first report about the direct hydrogenation of levulinic acid to 2-pentanol and 1,4-pentanediol over heterogeneous catalyst. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Recently, with the increasing social concern about the energy and environmental problems, the conversion of biomass to fuels and chemicals has attracted a lot of attention [1–4]. Carboxylic acids are a family of important platform chemicals which can be produced by biological and chemical pretreatments of biomass. The catalytic transformation of the biomass derived carboxylic acids to fuels and chemicals is a promising field [5–9]. Hydrogenation is a very important technology for the catalytic conversion of carboxylic acids. The hydrogenation of biomass derived mono-functional carboxylic acids can make alcohols which can be used as fuel, solvent and nonionic surfactant [10–13]. The hydrogenation of biomass derived multifunctional carboxylic acids can produce polyols (such as 1,2-propanediol [14], 1,4-butanediol [15–18] and 1,4-pentanediol [19,20]) which can be used as the monomers for polymer production [4]. Moreover, the hydrogenation of carboxylic acids to alcohols was considered as one of the slowest steps in the upgrading of bio-oil by hydrogenation [21]. To fulfill the need of real application, some more effective catalysts should be developed. Compared to the hydrogenation of other carbonyl compounds, the hydrogenation of carboxylic acids is more difficult due to the weaker polarizability of carboxyl group. To get higher conversion of carboxylic ⁎ Corresponding author. Tel.: +86 411 84379738. E-mail address: [email protected] (N. Li). 1 These authors contributed equally to this work. 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.09.007

acid, the doping of the second metal is necessary to increase the adsorption of carboxyl group. In this work, Mo modified Ir/SiO2 (Ir-MoOx/SiO2) was used, for the first time, in the aqueous-phase hydrogenation of acetic acid to ethanol and exhibited the best performance among the investigated catalysts. The feasibility for the application of Ir-MoOx/SiO2 to the hydrogenation of other biomass derived carboxylic acids was also explored. 2. Experimental 2.1. Catalyst preparation M/SiO2 (M = Ir, Pd, Ru) catalysts were prepared by the incipient wetness impregnation of SiO2 (Qingdao Ocean Chemical Ltd., BET surface area = 509 m2 g−1) with the aqueous solutions of H2IrCl6·6H2O, PdCl2 and RuCl3·3H2O respectively, then dried at 393 K for 12 h. The noble metal contents in the catalysts are denoted by their weight percentages. Mo, W, and Co modified M/SiO2 catalysts were prepared by the successive incipient wetness impregnation of M/SiO2 (after the drying procedure) with the aqueous solutions of (NH4)6Mo7O24·4H2O, (NH4)10W12O41·5H2O and Co(NO3)2·6H2O respectively. For comparison, MoOx/SiO2 was also prepared by impregnation of SiO2 with the aqueous solution of (NH4)6Mo7O24·4H2O. The content of Mo in MoOx/SiO2 was controlled at 0.26% which is same as that in the 4% Ir-MoOx/SiO2 (Mo/Ir atomic ratio = 0.13). All catalysts were dried at 393 K overnight then calcined in air at 773 K for 3 h.

Z. Wang et al. / Catalysis Communications 43 (2014) 38–41

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2.2. Activity test

3. Results and discussion

The reaction was carried out in a 316 L stainless steel tubular flow reactor described in literature [22]. Before the reaction, the catalysts were reduced in-situ in the reactor by hydrogen (160 mL min−1) at 473 K and 6 MPa for 2 h. After cooling down to reaction temperature (373 K), the aqueous solution of carboxylic acid was feed into the reactor from the bottom by a HPLC pump along with hydrogen at a flow rate of 60 mL min−1. The products from the reactor passed through a gas– liquid separator, and became two phases. The gaseous products flowed through a back pressure regulator to maintain the pressure in reaction system at 6 MPa and were analyzed on-line by an Agilent 6890N GC. CO2 in the gaseous products was analyzed by a Thermal Conductivity Detector (TCD) equipped with an Alltech HAYESEP DB 100/120 packed column (30 ft, 1/8 in. O.D., 2.0 mm I.D.). Alkanes in the gaseous products were analyzed by a Flame Ionized Detector (FID) equipped with an Rt®-Q-BOND capillary column (30 m, 0.32 mm I.D., 10 μm film). Liquid products were drained from the gas–liquid separator after reaction for 6 h and analyzed by another Agilent 6890N GC equipped with a HP-INNOWAX capillary column (30 m, 0.25 mm I.D., 0.5 μm film) and a FID detector.

As shown in Fig. 1, the 4% Ir/SiO2 exhibited low activity for the hydrogenation of acetic acid at 373 K and 6 MPa. Over it, low acetic acid conversion (5.6%) and ethanol yield (1.2%) were achieved. After the modification of 4% Ir/SiO2 with small amount of Mo, evident promotion effect was observed. Over the 4% Ir-MoOx/SiO2 with Mo/Ir atomic ratio of 0.13, high acetic acid conversion (75.9%) and ethanol yield (47.2%) were achieved. We also investigated the catalytic performances of MoOx/SiO2 and the physical mixture of MoOx/SiO2 and 4% Ir/SiO2. To facilitate the comparison, the Ir and/or Mo content in these catalysts was same as the one in 4% Ir-MoOx/SiO2 (Mo/Ir atomic ratio = 0.13). From Fig. 1, we can see that both catalysts have low activity for the hydrogenation of acetic acid. From these results, there should be a synergy effect between the closely contacted Ir and Mo species, which is responsible for the excellent catalytic performance of Ir-MoOx/SiO2. According to literature [6,23], the modification of Ir catalysts with Mo may promote the adsorption of carboxyl acid (by the interaction between Mo cations and the lone electron pair of hydroxyl or carbonyl group oxygen), which is very important for the hydrogenation of carboxylic acid. To understand the interaction between Ir and Mo species, the Ir-MoOx/SiO2 catalysts were characterized by a series of techniques (the detail information was offered in Support information). According to the XRD, TEM and CO-chemisorption results shown in Figs. S2, S3 and Table S1 of Support information, the Ir species was totally reduced to metallic state after being pretreated in H2 at 473 K. The modification of Ir/SiO2 with Mo has no evident effect on the dispersion (or average particle sizes) of Ir species. Therefore, we can't attribute the promotion of Mo to the change of Ir dispersion. From the HAADF/STEM, H2-TPR, EXFAS and DRIFTS for CO-adsorption illustrated in Figs. S4 to S8 and Table S2 of Support information, Mo species are attached on the Ir particle in the form of partially reduced isolated MoOx. The close contacting of Mo species with Ir particles promotes its reduction by the spillover of hydrogen. The synergism effect of Ir particles and the partially reduced isolated MoOx species attached on them may be the intrinsic reason for the excellent performance of Ir-MoOx/SiO2 catalyst. Fig. 2 shows the influence of Mo/Ir atomic ratio on the catalytic performance of 4% Ir-MoOx/SiO2. From Fig. 2, we can see that the activity of 4% Ir-MoOx/SiO2 increased with Mo content initially, reached the maximum when Mo/Ir atomic ratio was about 0.13, and then decreased with the further increase of Mo content. To prove the beneficial role of Ir in Mo-based systems, we tested the catalytic performance of MoOx/SiO2 and 4% Ir-MoOx/SiO2 at the Mo content of 4%. From the results shown in Fig. S1, the MoOx/SiO2 is inactive for the hydrogenation of acetic acid. In contrast, the 4% Ir-MoOx/SiO2 catalyst with Mo content of 4% is active for the hydrogenation of acetic acid. The conversions of acetic acid (42.0%) and ethanol yield (25.5%) over this catalyst are evidently lower than the ones (75.9% and 47.2%) observed over the 4% IrMoOx/SiO2(Mo/Ir = 0.13) catalyst. This result further proved that Ir is the active sites for the hydrogenation of acetic acid under the investigated conditions. In previous work of Chaudhari et al. [18], evident promotion effect of Co was observed for the hydrogenation of succinic acid over Ru/C catalyst. Moreover, considering the similar chemical properties of Mo and W (same group in periodic table), we think that W may also have promotion effect on the hydrogenation of acetic acid over Ir/SiO2 catalyst. Therefore, we studied the performances of the W and Co doped 4% Ir/SiO2 catalysts and compared them with that of 4% Ir-MoOx/SiO2. To facilitate the comparison, the atomic ratios of promoter to Ir in all these catalysts were fixed at 0.13. From the results illustrated in Fig. 3, W also has promotion effect on the hydrogenation of acetic acid over Ir/SiO2 catalyst. However, such a promotion effect is less evident than that of Mo. In contrast to Mo and W, the modification of Co only led to a slight increase of ethanol yield (from 1.2% to 3.0%) over Ir/SiO2 catalyst.

Conversion or yield (%)

80

a)

Acetic acid conversion (%) Ethanol yield (%)

60

40

20

0 iO 2

4%

O2

/Si Ox

Ir/S

4%

o Ir-M

O2

/Si Ox

Mo

iO 2 Ir/S iO 2 4% O x/S o M +

b)

TOF (h-1)

10

5

0 O2

iO 2

4%

/Si Ox

Ir/S

4%

o Ir-M

iO 2

O2

/Si Ox

Mo

4%

Ir/S

O2

/Si Ox

o +M

Fig. 1. Acetic acid conversion, ethanol yield (a) and the turnover frequency (TOF) of acetic acid to ethanol (b) over the 4% Ir/SiO2, 4% Ir-MoOx/SiO2 (Mo/Ir atomic ratio = 0.13), 0.26% Mo/SiO2, the physical mixture of 4% Ir/SiO2 and 0.26% Mo/SiO2 (at the mass ratio of 1:1). Reaction conditions: 373 K, 6 MPa, 2.0 g catalyst, 10% acetic acid aqueous solution flow rate = 0.04 mL min−1, and H2 flow rate = 60 mL min−1. The TOFs of acetic acid to ethanol were deduced according to the ethanol yields and the Ir dispersions of catalysts (see Table S1).

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Z. Wang et al. / Catalysis Communications 43 (2014) 38–41

Acetic acid conversion (%) Ethanol yield (%)

a)

80

Conversion or yield (%)

Conversion or yield (%)

80

60

40

20

Acetic acid conversion (%) Ethanol yield (%)

60

40

20

0

0 0

0.05

0.13

0.25

Mo/Ir atomic ratio

4%

b)

O2

O2

o d-M

P

O2

/Si Ox

/Si Ox

0.5

4%

o u-M

R

/Si Ox

4%

o Ir-M

Fig. 4. Acetic acid conversion and ethanol yield over 4% M-MoOx/SiO2 (M = Pd, Ru, Ir; Mo/M atomic ratio = 0.13). Reaction conditions: 373 K, 6 MPa, 2.0 g catalyst, 10% acetic acid aqueous solution flow rate = 0.04 mL min−1, and H2 flow rate = 60 mL min−1.

TOF(h-1)

10

5

0 0

0.05

0.13

0.25

0.5

Mo/Ir atomic ratio Fig. 2. Acetic acid conversion, ethanol yield (a) and the TOF of acetic acid to ethanol (b) over 4% Ir-MoOx/SiO2 as the function of Mo/Ir atomic ratio. Reaction conditions: 373 K, 6 MPa, 2.0 g catalyst, 10% acetic acid aqueous solution flow rate = 0.04 mL min−1, and H2 flow rate = 60 mL min−1.

The nature of noble metal is also very important for the hydrogenation of carboxylic acid. Huber group [24] investigated the aqueous phase hydrogenation of acetic acid over a series of transition metal catalysts. Among the investigated candidates, Ru catalyst exhibited the highest activity and selectivity to ethanol. In the recent work of Besson group

Conversion or yield (%)

80

[16,17] and Tomishige et al. [11], Re modified Pd catalysts were found to be active for the hydrogenation of carboxylic acids. In this work, we compared the catalytic performances of Mo doped Ir, Ru and Pd catalysts. To facilitate the comparison, the noble metal content and Mo/M (M = Ir, Ru, Pd) atomic ratio in these catalysts were fixed at 4% and 0.13 respectively. As shown in Fig. 4, the 4% Ru-MoOx/SiO2 catalyst is also active for the hydrogenation of acetic acid, but its activity is lower than that of the 4% Ir-MoOx/SiO2. On the contrary, low acetic acid conversion (8.2%) and ethanol yield (1.0%) were obtained over the 4% Pd-MoOx/SiO2 catalyst. Finally, we explored the activities of Ir-MoOx/SiO2 for the aqueousphase hydrogenation of other biomass derived carboxylic acids under the similar reaction conditions. As shown in Table 1, high carboxylic acid conversions and good selectivities to alcohols (or diols) were achieved by the hydrogenation of different carboxylic acids. Considering the excellent performances of Ir-MoOx/SiO2 for the low-temperature hydrogenation of various carboxylic acids, we think that it may be a promising catalyst for the hydrotreatment of bio-oil. In our future work, the reaction conditions will be tuned to maximize the yield of alcohol (or polyols) from the hydrogenation of biomass derived

Table 1 Results for the hydrogenation of different biomass derived carboxylic acids over the 4% Ir-MoOx/SiO2 (Mo/Ir atomic ratio = 0.13). Reaction conditions: 373 K, 6 MPa, 2.0 g catalyst, 10% carboxylic acid aqueous solution flow rate = 0.04 mL min−1, and H2 flow rate = 60 mL min−1.

Acetic acid conversion (%) Ethanol yield (%)

Reactant

60

Conversion (%)

Acetic acid b

75.9

Lactic acid

61.3

Succinic acidc

86.8

40

20

Levulinic acid

0 Ir/S

4%

/Si Ox

/Si Ox

o Ir-C

O2

O2

O2

iO 2

4%

4

r-W %I

4%

/Si Ox

o Ir-M

Fig. 3. Acetic acid conversion and ethanol yield over 4% Ir-POx/SiO2 (P = Co, W, Mo; P/Ir atomic ratio = 0.13). Reaction conditions: 373 K, 6 MPa, 2.0 g catalyst, 10% acetic acid aqueous solution flow rate = 0.04 mL min−1, and H2 flow rate = 60 mL min−1.

a b c

100

Product

Selectivity (%)

Ethanol Othersa 1,2-Propanediol 1-Propanol 2-Propanol Othersa 1,4-Butanediol 1-Butanol Othersa 1,4-Pentanediol 2-Pentanol 1-Pentanol γ-Valerolactone Methyl-tetrahydrofuran Othersa

62.2 37.8 44.0 12.3 7.1 36.6 30.8 24.8 44.4 42.3 39.0 1.2 4.1 0.8 12.6

Others means light alkanes and some unidentified products. The hydrogenation of lactic acid was carried out at 353 K. 5% succinic acid aqueous solution flow rate of 0.06 mL min−1.

Z. Wang et al. / Catalysis Communications 43 (2014) 38–41

carboxylic acids. Furthermore, it was very interesting that high yields of 2-pentanol (39.0%) and 1,2-pentanediol (42.3%) were obtained by the aqueous phase hydrogenation of levulinic acid over Ir-MoOx/SiO2. The levulinic acid can be produced by the chemical treatment of lignocellulose which is the main component of agricultural wastes and forest residues [25]. The 2-pentanol can be used as bio-gasoline [26]. The 1,4pentanediol can be used as a potential monomer for the synthesis of polyester [19,20]. Compared with the homogenous organometallic catalysts (based on Ru(acac)3 and triphos ligand) which have been reported for the hydrogenation of levulinic acid to 1,4-pentanediol [19,20], the Ir-MoOx/SiO2 catalyst is less toxic and reusable, which is advantageous in real application.

4. Conclusions Ir-MoOx/SiO2 is an effective catalyst for the low-temperature aqueous-phase hydrogenation of acetic acid to ethanol. The synergy effect between the closely contacted Ir and Mo species is responsible for the excellent performance of Ir-MoOx/SiO2. Ir-MoOx/SiO2 catalyst has high activity for the low-temperature hydrogenation of different biomass derived carboxylic acids, which makes it a prospective catalyst for bio-oil upgrading. As another potential application, Ir-MoOx/SiO2 is a very promising catalyst for the simultaneous production of 2-pentanol and 1,4-pentanediol by the direct hydrogenation of levulinic acid from lignocellulose.

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2013.09.007. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

Acknowledgments This work was supported by the Natural Science Foundation of China (No. 21106143; No. 21277140) and, 100-talent project of Dalian Institute of Chemical Physics (DICP). The authors thank the staffs in beamline 14W1 of the Shanghai Synchrotron Radiation Facility (SSRF) for their great help on measurements and data analyses of EXAFS.

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