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Effect of the ZrO2 phase on Pd-based bifunctional catalysts for the hydrogenolysis of glucose
Catalysis Communications 128 (2019) 105688
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Effect of the ZrO2 phase on Pd-based bifunctional catalysts for the hydrogenolysis of glucose
T
⁎
Chengwei Liua, Yaning Shanga, Huan Qia, Xianzhou Wangb,c, Jianzhou Guia,d, , ⁎⁎ Chenghua Zhangb,e, , Yulei Zhub,e, Yongwang Lib,e a
State Key Laboratory of Separation Membranes & Membrane Processes, College of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, People's Republic of China b State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People's Republic of China c University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China d School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, People's Republic of China e National Energy Center for Coal to Liquids, Synfuels China Technology Co., Ltd., Huairou District, Beijing 101407, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Zirconia Crystal phase Bifunctional catalyst Glucose Hydrogenolysis Value-added chemicals
ZrO2 species with different crystal phases (monoclinic, amorphous and tetragonal) were synthesized and used as supports for Pd-based bifunctional catalysts. ZrO2 species could affect the acidic and basic properties of the catalysts and the dispersion of Pd nanoparticles. The hydrogenolysis of glucose has been performed on the Pd/ ZrO2 catalysts. The synergistic effect between Pd nanoparticles and ZrO2 species could catalyze the selective hydrogenolysis of glucose. For example, the highest selectivity of ethanol was obtained on the Pd/t-ZrO2 catalyst, with the value of 39.7%.
1. Introduction The catalytic conversion of biomass, which is the only renewable carbon resource, has attracted increasing attention due to the depletion of fossil resources and growing concerns about environmental issues [1–3]. In this context, glucose, with a simple molecular structure, is a key platform molecule from the decomposition of cellulose by hydrolysis of acids or enzyme [4]. The rich hydroxyl groups in its structure make it an ideal feedstock to produce the oxygen-containing chemicals (oxygenates) such as ethylene glycol (EG), 1,2-propanediol (1,2-PDO) and butanediol (BDO), which can be obtained through the hydrogenolysis reaction. Furthermore, the hydrothermal catalytic hydrogenolysis of glucose is an efficient, energy saving and clean reaction process, which can be operated in a continuous system with high concentration [5]. The key of the hydrogenolysis of biomass to oxygenates is the selective elimination of the excess oxygen atoms and breaking CeC bonds in biomass molecule. In this respect, bifunctional catalysts, which contain active metals and mesoporous supports, have been reported to be effective for such multistep reactions. Furthermore, metal supported
catalysts have been used for glucose hydrogenolysis [6–9]. Especially, the noble metal-oxide bifunctional catalysts showed superior performance and stability in the hydrothermal condition. For example, Hirano reported that the synergistic effect of ZnO and Ru/C could promote the selective hydrogenolysis of glucose to 1,2-PDO, with 38% yield at 100% conversion of glucose [9]. A Pd/C and ZnO catalytic system was used for the transformation of glucose to 1,2-PDO, with the yield of 33.3% [10]. Liu et al. investigated the Ru-WOx catalysts with different supports including mesoporous SiO2, activated carbon and carbon nanofibers [5]. They found that the product distribution was dependent on the supports and the highest diols selectivity were 87.3% (33.8% of EG, 29.2% of 1,2-PDO and 24.2% of BDO) on the Ru-W/SiO2 catalyst. In summary, the product distribution of glucose hydrogenolysis was strongly related with the properties of noble metals and the supports. As pointed out previously, Pd has high activity for the cleavage of C-C/C-O bonds in biomass [11]. Meanwhile, ZrO2, with an amphoteric surface, is suitable for the complex reaction [12]. Furthermore, the ZrO2 species with different morphology such as monoclinic, amorphous and tetragonal phases show great discrepancy in structural and electronic
⁎ Correspondence to: J. Gui, State Key Laboratory of Separation Membranes & Membrane Processes, College of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, People's Republic of China. ⁎⁎ Correspondence to: C. Zhang, State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People's Republic of China. E-mail addresses: [email protected] (J. Gui), [email protected] (C. Zhang).
https://doi.org/10.1016/j.catcom.2019.04.020 Received 17 December 2018; Received in revised form 20 March 2019; Accepted 24 April 2019 Available online 04 May 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 128 (2019) 105688
C. Liu, et al.
XRD patterns of the three catalysts are almost the same as that of the corresponding ZrO2 supports. Moreover, no diffraction peak of PdO was observed in the XRD patterns of the three catalysts, which could be due to the low content of PdO species. The morphology and structure of the prepared catalysts were also characterized by SEM and HRTEM (Fig. 1S in Supplementary information). As shown in SEM images, all the ZrO2 species in the calcined samples were composed of nanoparticles, which provided the high surface areas of the catalysts. The HRTEM patterns confirmed the tetragonal and the monoclinic structure of ZrO2 in the Pd/t-ZrO2 and Pd/m-ZrO2 catalysts, respectively. Furthermore, monoclinic structure of ZrO2 was also observed on the Pd/am-ZrO2 catalyst. The HRTEM images also displayed the (101) planes of dispersed PdO nanoparticles. H2-TPR were performed to study the reduction of PdO (Fig. 2S in Supplementary information). Pd/m-ZrO2 shows a sharp reduction peak at around 61 °C, which is ascribed to the reduction of high dispersed PdO species on m-ZrO2. The reduction temperature of PdO on Pd/amZrO2 was higher than that of Pd/m-ZrO2, which is around 310 °C. The results suggest that large nanoparticles were formed, which is difficult to be reduced. No reduction peak was observed on the image of Pd/tZrO2. However, a negative peak at about 65 °C was present, which is due to the H2 desorption of PdHx formed by the reduction of PdO at a lower temperature [16]. It indicates that t-ZrO2 could promote the reduction of PdO. The metallic surface areas of Pd were calculated by the pulse chemisorption of CO, with the sequence as follows: Pd/t-ZrO2 (2.83 m2/ g) > Pd/m-ZrO2 (1.12 m2/g) > Pd/am-ZrO2 (4.14 m2/g). It indicates that t-ZrO2 has the best dispersion effect for Pd nanoparticles among the three supports. CO-DRIFT spectra were also performed to confirm the electronic state of Pd (Fig. 3S in Supplementary information). The results turned out that only Pd0 nanoparticles were present on the three reduced Pd/ZrO2 catalysts. Furthermore, the band of CO adsorbed on Pd0 in the Pd/t-ZrO2 catalyst showed the highest intensity after the purge of He for 15 min, indicating the large quantity of Pd0 sites. NH3-TPD and CO2-TPD patterns were obtained to study the acidic and basic properties of the catalyst surface. As shown in Fig. 2A, the amount of NH3 desorbed from the three catalysts varied greatly with each other and followed a sequence of Pd/m-ZrO2 > Pd/amZrO2 > Pd/t-ZrO2. Fig. 2B shows the CO2-TPD profiles of the three Pd/ ZrO2 catalysts, with a different order of Pd/am-ZrO2 > Pd/mZrO2 > Pd/t-ZrO2. The results indicate that Pd/t-ZrO2 has the lowest quantity of acidic and basic sites among the three catalysts. This is consistent with the report that the quantities of acidic and basic sites on t-ZrO2 were always lower than that on m-ZrO2 [17,18]. The acidity of the catalysts was also detected by Py-FTIR (Table 1). Large quantities of Lewis acid sites exist on the Pd/m-ZrO2 catalyst. Only a small quantity of Lewis acid sites is present on the Pd/am-ZrO2 catalyst. However, no band of the coordination of pyridine molecules to Lewis acid sites was observed on the Pd/t-ZrO2 catalyst. Furthermore, no signal of Brønsted acid sites was found on the three catalysts. The Py-FTIR results show that the quantity of Lewis acid sites of Pd/amZrO2 is only 10% of Pd/m-ZrO2, which is different with the NH3-TPD results, where the total acidity of Pd/am-ZrO2 is a little lower than that of Pd/m-ZrO2. The discrepancy is related with the molecular sizes of pyridine and ammonia and the pore structure of ZrO2 species. The size of pyridine is larger than that of ammonia, which makes it difficult to diffuse into the interior of smaller pores. Furthermore, the average pore size of Pd/am-ZrO2 is much smaller than that of Pd/m-ZrO2, which could inhibit the diffusion and adsorption of pyridine molecules. The catalytic conversion of glucose was conducted on the ZrO2 supports, as shown in Table 2. Small quantities of polyols were detected in the products, indicating that the hydrogenolysis of glucose to polyols could happen in the hydrothermal condition with ZrO2. Fructose and humins were the main products, which confirmed that the ZrO2 species could promote the isomerization of glucose to fructose and the polymerization of glucose and the unsaturated intermediates. The m-ZrO2
properties [13]. Therefore, the ZrO2 species with different morphology were synthesized and used for the Pd-based bifunctional catalysts. The effect of ZrO2 species on the Pd/ZrO2 bifunctional catalysts for the hydrogenolysis of glucose was studied. 2. Experiment Pure monoclinic (m-) ZrO2 and tetragonal (t-) ZrO2 species were synthesized using a hydrothermal method reported previously [8,14]. The amorphous ZrO2 was prepared using the same method as that of mZrO2 without the calcined procedure. The Pd/ZrO2 samples were prepared by an incipient wetness impregnation method with an aqueous solution of Pd(NO3)2 (Shanxi Kaida Chemical Engineering Co., Ltd.). The precursor was dried at 100 °C overnight and calcined at 350 °C for 5 h. The theoretical amount of Pd was about 1 wt%. The prepared samples are denoted as Pd/m-ZrO2, Pd/am-ZrO2 and Pd/t-ZrO2, respectively. The Pd/t-ZrO2 catalyst with different metal loading were also prepared with the same method. The catalytic tests were carried out in a tubular fixed-bed reactor (i.d. 12 mm, length 600 mm). Typical reaction conditions are as follows: 180 °C, 4 MPa, 5 wt% glucose aqueous solution, WHSV = 0.24 h−1. The conversion of glucose and the selectivity of products were quantified based on the carbon by the following equation:
Conversion (%) = 100 −
Selectivity (%) =
carbon mol of reactant after reaction × 100 carbon mol of reactant in feedstock
carbon mol of each product × 100 sum of carbon mol for all prodcts
More detailed information about materials, characterization and reaction process is provided in Supplementary information. 3. Results and discussion The monoclinic (m-), amorphous (am-), and tetragonal (t-) ZrO2 species were synthesized by hydrothermal method and used as supports for Pd catalysts. The BET results show that all the three catalysts have high surface area, with the value of 100, 123, and 153 m2/g for the Pd/ m-ZrO2, Pd/am-ZrO2, and Pd/t-ZrO2 catalysts, respectively. XRD patterns of ZrO2 and the prepared Pd/ZrO2 catalysts are shown in Fig. 1. Four typical diffraction peaks at 24.2, 28.2, 31.5, and 34.3 with a shoulder at 35.3° of monoclinic ZrO2 phase (JCPDS 37-1484) are present in m-ZrO2. A broad X-ray diffraction peak is observed for am-ZrO2, pointing to an amorphous nature of zirconia as expected [15]. For tZrO2, only two peaks were present at 30.3 and 34.8°, which is ascribed to the tetragonal ZrO2 (JCPDS 17–0923) [8]. The diffraction peaks in
Fig. 1. XRD patterns of ZrO2 and the calcined Pd/ZrO2 catalysts. 2
Catalysis Communications 128 (2019) 105688
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Fig. 2. (A) NH3-TPD and (B) CO2-TPD patterns of the prepared catalysts.
could not act as hydrogen donor in the reaction system. Meanwhile, there are large amounts of humins in the liquid products on the Pd/mZrO2 catalysts, which could be due to the polymerization of glucose and the intermediates on the acid sites. Combined with the characterization, it turns out that highly dispersed Pd nanoparticles with large surface area and t-ZrO2 with lower quantities of acid and basic sites are favorable for the enhancement of the ethanol selectivity. In order to study the effect of Pd metal sites on the selective hydrogenolysis of glucose, The Pd/t-ZrO2 catalysts with different Pd contents were also tested at 160 °C and 180 °C (Table 3). The selectivity of 1,2-PDO and ethanol decreased gradually with the decrease of Pd contents. However, the selectivity of fructose increased, which is due to the deficiency of Pd metal sites. The Pd surface areas were also decreased with the sequence of 0.5Pd/t-ZrO2 (3.0 m2/g) > 0.3Pd/t-ZrO2 (2.3 m2/g) > 0.1Pd/t-ZrO2 (1.52 m2/g), as shown in Table 1S in Supplementary information. It is reported that Pd has high CeC and CeO bond cleavage activity [20]. Therefore, it is inferred that the surface area of Pd has important effect on the hydrogenolysis of glucose. The main pathway of glucose hydrogenolysis is displayed in Scheme 1 [21–24]. As suggested previously, the hydrogenation and the retroaldol condensation could be performed on the metal sites. The basic and Lewis acid sites could promote the isomerization, retro-aldol condensation, dehydration and polymerization reactions [25–28]. The reaction results turned out that the selectivity of C3 polyols (glycerol +1,2-PDO) on Pd/am-ZrO2, with the value of 49.0%, were higher than that on other catalysts. Therefore, it is inferred that the largest quantity of basic sites and the middle number of acid sites on the catalyst surface could selectively promote the isomerization of glucose and the dehydration of intermediates, resulting in more C3 polyols such as 1,2-PDO. However, the excessive acid sites on the Pd/m-ZrO2 catalyst could
Table 1 The physicochemical properties of the prepared Pd/ZrO2 catalysts. Catalyst
Pd/m-ZrO2 Pd/am-ZrO2 Pd/t-ZrO2
SBET (m2/ g)
100 123 153
VP (cm3/ g)
0.25 0.09 0.32
DP (nm)
8.04 3.41 6.21
Chemisorption (μmol CO gCat.−1)
59.8 23.7 87.3
Pd surface area (m2/g)
2.83 1.12 4.14
Acid amount (mmol Py gCat.−1) B
L
0 0 0
0.221 0.023 0
support showed the highest selectivity of humins, which is related with the large quantity of acid sites. Furthermore, it has been pointed out that the Lewis acid and basic sites could promote the isomerization of glucose to fructose [19]. Moreover, Fructose is more unstable than glucose in the hydrothermal condition. All the three Pd/ZrO2 catalysts showed high activity for the hydrogenolysis of glucose, with the conversions of 95.9%, 94.8%, and 98.5%, respectively. A variety of oxygenates were detected in the liquid products, such as sorbitol, ethylene glycol, glycerol, 1,2-PDO, ethanol, etc. Among the oxygenates, 1,2-PDO and ethanol are the main products. The selectivities of 1,2-PDO are 35.0%, 37.7%, and 33.4% on the Pd/mZrO2, Pd/am-ZrO2, and Pd/t-ZrO2 catalysts, respectively, which show little discrepancy on the three catalysts. However, the selectivities of ethanol show an obvious decrease with the following sequence: Pd/tZrO2 (39.7%) > Pd/am-ZrO2 (21.2%) > Pd/m-ZrO2 (10.5%). The catalytic reaction of glucose was also conducted in the nitrogen atmosphere, as shown in Table 2. The result turned out that glucose was mainly converted to fructose and humins, which suggests that ethanol Table 2 The hydrogenolysis of glucose on Pd/ZrO2 catalysts.a Sample
m-ZrO2 am-ZrO2 t-ZrO2 Pd/m-ZrO2 Pd/am-ZrO2 Pd/t-ZrO2 Pd/t-ZrO2c Pd/SiO2 Pd/AC a b c
Conv. (%)
83.3 34.9 82.4 95.9 94.8 98.5 89.1 40.2 87.2
Selectivity (%) Fructose
Sorbitol
Erythritol
EG
Glycerol
1,2-PDO
1,2-BDO
Ethanol
Othersb
0.0 0.0 0.0 2.6 2.5 1.4 20.1 0.0 0.0
11.8 40.1 20.7 3.8 1.2 1.8 0.8 53.1 44.3
4.7 6.2 4.4 2.7 2.7 0.8 2.9 8.4 6.8
5.3 0.0 3.7 5.5 10.1 6.4 0.7 9.7 8.2
0.0 4.1 1.1 8.1 11.3 5.1 3.2 8.3 6.7
6.4 6.1 8.2 35.0 37.7 33.4 7.1 17.8 25.6
0.0 7.4 4.8 10.2 10.7 8.2 0.8 0.0 0.0
0.0 0.0 0.0 10.5 21.2 39.7 0.0 0.0 5.3
71.8 36.1 57.1 21.6 2.6 3.2 64.4 2.7 3.1
Reaction conditions: 180 °C, 4 MPa H2, 5 wt% glucose aqueous solution, WHSV = 0.24 h−1. Others: humins, etc. Reaction conditions: 180 °C, 4 MPa N2, 5 wt% glucose aqueous solution, WHSV = 0.24 h−1. 3
Catalysis Communications 128 (2019) 105688
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Table 3 The hydrogenolysis of glucose on the Pd/t-ZrO2 catalysts.a Sample
1Pd/t-ZrO2 0.5Pd/t-ZrO2 0.3Pd/t-ZrO2 0.1Pd/t-ZrO2
a b
T (°C)
160 180 160 180 160 180 160 180
Conv. (%)
95.8 98.5 73.7 92.7 72.8 89.5 62.5 82.7
Selectivity (%) Sorbitol
Fructose
Erythritol
EG
Glycerol
1,2-PDO
1,2-BDO
Ethanol
Othersb
3.5 1.8 5.2 2.3 3.8 2.6 3.9 2.6
4.0 1.4 15.5 5.3 17.8 6.7 24.8 11.9
1.1 0.8 4.3 3.2 4.4 3.4 4.4 4.6
7.9 6.4 8.3 8.8 8.1 6.4 7.5 7.3
11.8 5.1 13.1 10.6 13.0 8.1 11.5 9.2
36.9 33.4 25.3 29.6 25.4 23.5 20.5 21.4
6.8 8.2 5.8 9.1 7.0 8.7 4.9 7.4
25.5 39.7 11.4 22.4 10.1 15.2 9.8 14.9
2.5 3.2 11.1 8.7 10.4 25.4 12.7 20.7
Reaction conditions: 4 MPa H2, 5 wt% glucose aqueous solution, WHSV = 0.24 h−1. Others: humins, etc.
hydrogenolysis of glucose on Pd catalysts supported on neutral SiO2 and activated carbon was also performed to confirm the synergistic effect between Pd and ZrO2. The results turned out that the neutral catalysts could mainly promote the hydrogenation of glucose to sorbitol. Meanwhile, no ethanol was detected on Pd/SiO2 catalyst. The selectivity of ethanol was only 5.3% on the Pd/C catalyst.
catalyze the polymerization of glucose, resulting in the highest selectivity of humins among the three catalysts. Meanwhile, the selective hydrogenolysis of glucose to C2 chemicals such as ethanol needs the synergy between highly dispersed Pd nanoparticles and t-ZrO2 with proper quantities of acid and basic sites through the retro-aldol condensation, dehydration and hydrogenation reactions. Furthermore, the
Scheme 1. The main pathway of the hydrogenolysis of glucose. 4
Catalysis Communications 128 (2019) 105688
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4. Conclusion
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ZrO2 species with different crystal phase (monoclinic, amorphous and tetragonal) were synthesized and used as supports for Pd-based bifunctional catalysts. ZrO2 species with different morphology have great effect on the reduction and dispersion of Pd nanoparticles. Pd/amZrO2 with the largest quantity of basic sites and middle number of acid sites showed superior performance on the hydrogenolysis of glucose to C3 polyols, with the selectivity of 49% (11.3% of glycerol and 37% of 1,2-PDO). Meanwhile, the synergy between highly dispersed Pd and tZrO2 with proper quantities of acidic and basic sites could promote the hydrogenolysis of glucose to ethanol, with the highest selective of 39.7%. Acknowledgements This work was financially supported by the Major State Basic Research Development Program of China (973 Program) (No. 2012CB215305), the Natural Science Foundation of Tianjin (No. 18JCQNJC06600) and the Science & Technology Development Fund of Tianjin Education Commission for Higher Education (2018KJ203). This work was also supported by Synfuels China Co., Ltd. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.04.020. References [1] P. Sudarsanam, R. Zhong, S.V. den Bosch, S.M. Coman, V.I. Parvulescu, B.F. Sels,
5
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Effect of the ZrO2 phase on Pd-based bifunctional catalysts for the hydrogenolysis of glucose
T
⁎
Chengwei Liua, Yaning Shanga, Huan Qia, Xianzhou Wangb,c, Jianzhou Guia,d, , ⁎⁎ Chenghua Zhangb,e, , Yulei Zhub,e, Yongwang Lib,e a
State Key Laboratory of Separation Membranes & Membrane Processes, College of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, People's Republic of China b State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People's Republic of China c University of Chinese Academy of Sciences, Beijing 100049, People's Republic of China d School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, People's Republic of China e National Energy Center for Coal to Liquids, Synfuels China Technology Co., Ltd., Huairou District, Beijing 101407, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Zirconia Crystal phase Bifunctional catalyst Glucose Hydrogenolysis Value-added chemicals
ZrO2 species with different crystal phases (monoclinic, amorphous and tetragonal) were synthesized and used as supports for Pd-based bifunctional catalysts. ZrO2 species could affect the acidic and basic properties of the catalysts and the dispersion of Pd nanoparticles. The hydrogenolysis of glucose has been performed on the Pd/ ZrO2 catalysts. The synergistic effect between Pd nanoparticles and ZrO2 species could catalyze the selective hydrogenolysis of glucose. For example, the highest selectivity of ethanol was obtained on the Pd/t-ZrO2 catalyst, with the value of 39.7%.
1. Introduction The catalytic conversion of biomass, which is the only renewable carbon resource, has attracted increasing attention due to the depletion of fossil resources and growing concerns about environmental issues [1–3]. In this context, glucose, with a simple molecular structure, is a key platform molecule from the decomposition of cellulose by hydrolysis of acids or enzyme [4]. The rich hydroxyl groups in its structure make it an ideal feedstock to produce the oxygen-containing chemicals (oxygenates) such as ethylene glycol (EG), 1,2-propanediol (1,2-PDO) and butanediol (BDO), which can be obtained through the hydrogenolysis reaction. Furthermore, the hydrothermal catalytic hydrogenolysis of glucose is an efficient, energy saving and clean reaction process, which can be operated in a continuous system with high concentration [5]. The key of the hydrogenolysis of biomass to oxygenates is the selective elimination of the excess oxygen atoms and breaking CeC bonds in biomass molecule. In this respect, bifunctional catalysts, which contain active metals and mesoporous supports, have been reported to be effective for such multistep reactions. Furthermore, metal supported
catalysts have been used for glucose hydrogenolysis [6–9]. Especially, the noble metal-oxide bifunctional catalysts showed superior performance and stability in the hydrothermal condition. For example, Hirano reported that the synergistic effect of ZnO and Ru/C could promote the selective hydrogenolysis of glucose to 1,2-PDO, with 38% yield at 100% conversion of glucose [9]. A Pd/C and ZnO catalytic system was used for the transformation of glucose to 1,2-PDO, with the yield of 33.3% [10]. Liu et al. investigated the Ru-WOx catalysts with different supports including mesoporous SiO2, activated carbon and carbon nanofibers [5]. They found that the product distribution was dependent on the supports and the highest diols selectivity were 87.3% (33.8% of EG, 29.2% of 1,2-PDO and 24.2% of BDO) on the Ru-W/SiO2 catalyst. In summary, the product distribution of glucose hydrogenolysis was strongly related with the properties of noble metals and the supports. As pointed out previously, Pd has high activity for the cleavage of C-C/C-O bonds in biomass [11]. Meanwhile, ZrO2, with an amphoteric surface, is suitable for the complex reaction [12]. Furthermore, the ZrO2 species with different morphology such as monoclinic, amorphous and tetragonal phases show great discrepancy in structural and electronic
⁎ Correspondence to: J. Gui, State Key Laboratory of Separation Membranes & Membrane Processes, College of Chemistry and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, People's Republic of China. ⁎⁎ Correspondence to: C. Zhang, State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, People's Republic of China. E-mail addresses: [email protected] (J. Gui), [email protected] (C. Zhang).
https://doi.org/10.1016/j.catcom.2019.04.020 Received 17 December 2018; Received in revised form 20 March 2019; Accepted 24 April 2019 Available online 04 May 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 128 (2019) 105688
C. Liu, et al.
XRD patterns of the three catalysts are almost the same as that of the corresponding ZrO2 supports. Moreover, no diffraction peak of PdO was observed in the XRD patterns of the three catalysts, which could be due to the low content of PdO species. The morphology and structure of the prepared catalysts were also characterized by SEM and HRTEM (Fig. 1S in Supplementary information). As shown in SEM images, all the ZrO2 species in the calcined samples were composed of nanoparticles, which provided the high surface areas of the catalysts. The HRTEM patterns confirmed the tetragonal and the monoclinic structure of ZrO2 in the Pd/t-ZrO2 and Pd/m-ZrO2 catalysts, respectively. Furthermore, monoclinic structure of ZrO2 was also observed on the Pd/am-ZrO2 catalyst. The HRTEM images also displayed the (101) planes of dispersed PdO nanoparticles. H2-TPR were performed to study the reduction of PdO (Fig. 2S in Supplementary information). Pd/m-ZrO2 shows a sharp reduction peak at around 61 °C, which is ascribed to the reduction of high dispersed PdO species on m-ZrO2. The reduction temperature of PdO on Pd/amZrO2 was higher than that of Pd/m-ZrO2, which is around 310 °C. The results suggest that large nanoparticles were formed, which is difficult to be reduced. No reduction peak was observed on the image of Pd/tZrO2. However, a negative peak at about 65 °C was present, which is due to the H2 desorption of PdHx formed by the reduction of PdO at a lower temperature [16]. It indicates that t-ZrO2 could promote the reduction of PdO. The metallic surface areas of Pd were calculated by the pulse chemisorption of CO, with the sequence as follows: Pd/t-ZrO2 (2.83 m2/ g) > Pd/m-ZrO2 (1.12 m2/g) > Pd/am-ZrO2 (4.14 m2/g). It indicates that t-ZrO2 has the best dispersion effect for Pd nanoparticles among the three supports. CO-DRIFT spectra were also performed to confirm the electronic state of Pd (Fig. 3S in Supplementary information). The results turned out that only Pd0 nanoparticles were present on the three reduced Pd/ZrO2 catalysts. Furthermore, the band of CO adsorbed on Pd0 in the Pd/t-ZrO2 catalyst showed the highest intensity after the purge of He for 15 min, indicating the large quantity of Pd0 sites. NH3-TPD and CO2-TPD patterns were obtained to study the acidic and basic properties of the catalyst surface. As shown in Fig. 2A, the amount of NH3 desorbed from the three catalysts varied greatly with each other and followed a sequence of Pd/m-ZrO2 > Pd/amZrO2 > Pd/t-ZrO2. Fig. 2B shows the CO2-TPD profiles of the three Pd/ ZrO2 catalysts, with a different order of Pd/am-ZrO2 > Pd/mZrO2 > Pd/t-ZrO2. The results indicate that Pd/t-ZrO2 has the lowest quantity of acidic and basic sites among the three catalysts. This is consistent with the report that the quantities of acidic and basic sites on t-ZrO2 were always lower than that on m-ZrO2 [17,18]. The acidity of the catalysts was also detected by Py-FTIR (Table 1). Large quantities of Lewis acid sites exist on the Pd/m-ZrO2 catalyst. Only a small quantity of Lewis acid sites is present on the Pd/am-ZrO2 catalyst. However, no band of the coordination of pyridine molecules to Lewis acid sites was observed on the Pd/t-ZrO2 catalyst. Furthermore, no signal of Brønsted acid sites was found on the three catalysts. The Py-FTIR results show that the quantity of Lewis acid sites of Pd/amZrO2 is only 10% of Pd/m-ZrO2, which is different with the NH3-TPD results, where the total acidity of Pd/am-ZrO2 is a little lower than that of Pd/m-ZrO2. The discrepancy is related with the molecular sizes of pyridine and ammonia and the pore structure of ZrO2 species. The size of pyridine is larger than that of ammonia, which makes it difficult to diffuse into the interior of smaller pores. Furthermore, the average pore size of Pd/am-ZrO2 is much smaller than that of Pd/m-ZrO2, which could inhibit the diffusion and adsorption of pyridine molecules. The catalytic conversion of glucose was conducted on the ZrO2 supports, as shown in Table 2. Small quantities of polyols were detected in the products, indicating that the hydrogenolysis of glucose to polyols could happen in the hydrothermal condition with ZrO2. Fructose and humins were the main products, which confirmed that the ZrO2 species could promote the isomerization of glucose to fructose and the polymerization of glucose and the unsaturated intermediates. The m-ZrO2
properties [13]. Therefore, the ZrO2 species with different morphology were synthesized and used for the Pd-based bifunctional catalysts. The effect of ZrO2 species on the Pd/ZrO2 bifunctional catalysts for the hydrogenolysis of glucose was studied. 2. Experiment Pure monoclinic (m-) ZrO2 and tetragonal (t-) ZrO2 species were synthesized using a hydrothermal method reported previously [8,14]. The amorphous ZrO2 was prepared using the same method as that of mZrO2 without the calcined procedure. The Pd/ZrO2 samples were prepared by an incipient wetness impregnation method with an aqueous solution of Pd(NO3)2 (Shanxi Kaida Chemical Engineering Co., Ltd.). The precursor was dried at 100 °C overnight and calcined at 350 °C for 5 h. The theoretical amount of Pd was about 1 wt%. The prepared samples are denoted as Pd/m-ZrO2, Pd/am-ZrO2 and Pd/t-ZrO2, respectively. The Pd/t-ZrO2 catalyst with different metal loading were also prepared with the same method. The catalytic tests were carried out in a tubular fixed-bed reactor (i.d. 12 mm, length 600 mm). Typical reaction conditions are as follows: 180 °C, 4 MPa, 5 wt% glucose aqueous solution, WHSV = 0.24 h−1. The conversion of glucose and the selectivity of products were quantified based on the carbon by the following equation:
Conversion (%) = 100 −
Selectivity (%) =
carbon mol of reactant after reaction × 100 carbon mol of reactant in feedstock
carbon mol of each product × 100 sum of carbon mol for all prodcts
More detailed information about materials, characterization and reaction process is provided in Supplementary information. 3. Results and discussion The monoclinic (m-), amorphous (am-), and tetragonal (t-) ZrO2 species were synthesized by hydrothermal method and used as supports for Pd catalysts. The BET results show that all the three catalysts have high surface area, with the value of 100, 123, and 153 m2/g for the Pd/ m-ZrO2, Pd/am-ZrO2, and Pd/t-ZrO2 catalysts, respectively. XRD patterns of ZrO2 and the prepared Pd/ZrO2 catalysts are shown in Fig. 1. Four typical diffraction peaks at 24.2, 28.2, 31.5, and 34.3 with a shoulder at 35.3° of monoclinic ZrO2 phase (JCPDS 37-1484) are present in m-ZrO2. A broad X-ray diffraction peak is observed for am-ZrO2, pointing to an amorphous nature of zirconia as expected [15]. For tZrO2, only two peaks were present at 30.3 and 34.8°, which is ascribed to the tetragonal ZrO2 (JCPDS 17–0923) [8]. The diffraction peaks in
Fig. 1. XRD patterns of ZrO2 and the calcined Pd/ZrO2 catalysts. 2
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Fig. 2. (A) NH3-TPD and (B) CO2-TPD patterns of the prepared catalysts.
could not act as hydrogen donor in the reaction system. Meanwhile, there are large amounts of humins in the liquid products on the Pd/mZrO2 catalysts, which could be due to the polymerization of glucose and the intermediates on the acid sites. Combined with the characterization, it turns out that highly dispersed Pd nanoparticles with large surface area and t-ZrO2 with lower quantities of acid and basic sites are favorable for the enhancement of the ethanol selectivity. In order to study the effect of Pd metal sites on the selective hydrogenolysis of glucose, The Pd/t-ZrO2 catalysts with different Pd contents were also tested at 160 °C and 180 °C (Table 3). The selectivity of 1,2-PDO and ethanol decreased gradually with the decrease of Pd contents. However, the selectivity of fructose increased, which is due to the deficiency of Pd metal sites. The Pd surface areas were also decreased with the sequence of 0.5Pd/t-ZrO2 (3.0 m2/g) > 0.3Pd/t-ZrO2 (2.3 m2/g) > 0.1Pd/t-ZrO2 (1.52 m2/g), as shown in Table 1S in Supplementary information. It is reported that Pd has high CeC and CeO bond cleavage activity [20]. Therefore, it is inferred that the surface area of Pd has important effect on the hydrogenolysis of glucose. The main pathway of glucose hydrogenolysis is displayed in Scheme 1 [21–24]. As suggested previously, the hydrogenation and the retroaldol condensation could be performed on the metal sites. The basic and Lewis acid sites could promote the isomerization, retro-aldol condensation, dehydration and polymerization reactions [25–28]. The reaction results turned out that the selectivity of C3 polyols (glycerol +1,2-PDO) on Pd/am-ZrO2, with the value of 49.0%, were higher than that on other catalysts. Therefore, it is inferred that the largest quantity of basic sites and the middle number of acid sites on the catalyst surface could selectively promote the isomerization of glucose and the dehydration of intermediates, resulting in more C3 polyols such as 1,2-PDO. However, the excessive acid sites on the Pd/m-ZrO2 catalyst could
Table 1 The physicochemical properties of the prepared Pd/ZrO2 catalysts. Catalyst
Pd/m-ZrO2 Pd/am-ZrO2 Pd/t-ZrO2
SBET (m2/ g)
100 123 153
VP (cm3/ g)
0.25 0.09 0.32
DP (nm)
8.04 3.41 6.21
Chemisorption (μmol CO gCat.−1)
59.8 23.7 87.3
Pd surface area (m2/g)
2.83 1.12 4.14
Acid amount (mmol Py gCat.−1) B
L
0 0 0
0.221 0.023 0
support showed the highest selectivity of humins, which is related with the large quantity of acid sites. Furthermore, it has been pointed out that the Lewis acid and basic sites could promote the isomerization of glucose to fructose [19]. Moreover, Fructose is more unstable than glucose in the hydrothermal condition. All the three Pd/ZrO2 catalysts showed high activity for the hydrogenolysis of glucose, with the conversions of 95.9%, 94.8%, and 98.5%, respectively. A variety of oxygenates were detected in the liquid products, such as sorbitol, ethylene glycol, glycerol, 1,2-PDO, ethanol, etc. Among the oxygenates, 1,2-PDO and ethanol are the main products. The selectivities of 1,2-PDO are 35.0%, 37.7%, and 33.4% on the Pd/mZrO2, Pd/am-ZrO2, and Pd/t-ZrO2 catalysts, respectively, which show little discrepancy on the three catalysts. However, the selectivities of ethanol show an obvious decrease with the following sequence: Pd/tZrO2 (39.7%) > Pd/am-ZrO2 (21.2%) > Pd/m-ZrO2 (10.5%). The catalytic reaction of glucose was also conducted in the nitrogen atmosphere, as shown in Table 2. The result turned out that glucose was mainly converted to fructose and humins, which suggests that ethanol Table 2 The hydrogenolysis of glucose on Pd/ZrO2 catalysts.a Sample
m-ZrO2 am-ZrO2 t-ZrO2 Pd/m-ZrO2 Pd/am-ZrO2 Pd/t-ZrO2 Pd/t-ZrO2c Pd/SiO2 Pd/AC a b c
Conv. (%)
83.3 34.9 82.4 95.9 94.8 98.5 89.1 40.2 87.2
Selectivity (%) Fructose
Sorbitol
Erythritol
EG
Glycerol
1,2-PDO
1,2-BDO
Ethanol
Othersb
0.0 0.0 0.0 2.6 2.5 1.4 20.1 0.0 0.0
11.8 40.1 20.7 3.8 1.2 1.8 0.8 53.1 44.3
4.7 6.2 4.4 2.7 2.7 0.8 2.9 8.4 6.8
5.3 0.0 3.7 5.5 10.1 6.4 0.7 9.7 8.2
0.0 4.1 1.1 8.1 11.3 5.1 3.2 8.3 6.7
6.4 6.1 8.2 35.0 37.7 33.4 7.1 17.8 25.6
0.0 7.4 4.8 10.2 10.7 8.2 0.8 0.0 0.0
0.0 0.0 0.0 10.5 21.2 39.7 0.0 0.0 5.3
71.8 36.1 57.1 21.6 2.6 3.2 64.4 2.7 3.1
Reaction conditions: 180 °C, 4 MPa H2, 5 wt% glucose aqueous solution, WHSV = 0.24 h−1. Others: humins, etc. Reaction conditions: 180 °C, 4 MPa N2, 5 wt% glucose aqueous solution, WHSV = 0.24 h−1. 3
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Table 3 The hydrogenolysis of glucose on the Pd/t-ZrO2 catalysts.a Sample
1Pd/t-ZrO2 0.5Pd/t-ZrO2 0.3Pd/t-ZrO2 0.1Pd/t-ZrO2
a b
T (°C)
160 180 160 180 160 180 160 180
Conv. (%)
95.8 98.5 73.7 92.7 72.8 89.5 62.5 82.7
Selectivity (%) Sorbitol
Fructose
Erythritol
EG
Glycerol
1,2-PDO
1,2-BDO
Ethanol
Othersb
3.5 1.8 5.2 2.3 3.8 2.6 3.9 2.6
4.0 1.4 15.5 5.3 17.8 6.7 24.8 11.9
1.1 0.8 4.3 3.2 4.4 3.4 4.4 4.6
7.9 6.4 8.3 8.8 8.1 6.4 7.5 7.3
11.8 5.1 13.1 10.6 13.0 8.1 11.5 9.2
36.9 33.4 25.3 29.6 25.4 23.5 20.5 21.4
6.8 8.2 5.8 9.1 7.0 8.7 4.9 7.4
25.5 39.7 11.4 22.4 10.1 15.2 9.8 14.9
2.5 3.2 11.1 8.7 10.4 25.4 12.7 20.7
Reaction conditions: 4 MPa H2, 5 wt% glucose aqueous solution, WHSV = 0.24 h−1. Others: humins, etc.
hydrogenolysis of glucose on Pd catalysts supported on neutral SiO2 and activated carbon was also performed to confirm the synergistic effect between Pd and ZrO2. The results turned out that the neutral catalysts could mainly promote the hydrogenation of glucose to sorbitol. Meanwhile, no ethanol was detected on Pd/SiO2 catalyst. The selectivity of ethanol was only 5.3% on the Pd/C catalyst.
catalyze the polymerization of glucose, resulting in the highest selectivity of humins among the three catalysts. Meanwhile, the selective hydrogenolysis of glucose to C2 chemicals such as ethanol needs the synergy between highly dispersed Pd nanoparticles and t-ZrO2 with proper quantities of acid and basic sites through the retro-aldol condensation, dehydration and hydrogenation reactions. Furthermore, the
Scheme 1. The main pathway of the hydrogenolysis of glucose. 4
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4. Conclusion
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ZrO2 species with different crystal phase (monoclinic, amorphous and tetragonal) were synthesized and used as supports for Pd-based bifunctional catalysts. ZrO2 species with different morphology have great effect on the reduction and dispersion of Pd nanoparticles. Pd/amZrO2 with the largest quantity of basic sites and middle number of acid sites showed superior performance on the hydrogenolysis of glucose to C3 polyols, with the selectivity of 49% (11.3% of glycerol and 37% of 1,2-PDO). Meanwhile, the synergy between highly dispersed Pd and tZrO2 with proper quantities of acidic and basic sites could promote the hydrogenolysis of glucose to ethanol, with the highest selective of 39.7%. Acknowledgements This work was financially supported by the Major State Basic Research Development Program of China (973 Program) (No. 2012CB215305), the Natural Science Foundation of Tianjin (No. 18JCQNJC06600) and the Science & Technology Development Fund of Tianjin Education Commission for Higher Education (2018KJ203). This work was also supported by Synfuels China Co., Ltd. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.04.020. References [1] P. Sudarsanam, R. Zhong, S.V. den Bosch, S.M. Coman, V.I. Parvulescu, B.F. Sels,
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