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Porous MOFs supported palladium catalysts for phenol hydrogenation: A comparative study on MIL-101 and MIL-53
Catalysis Communications 41 (2013) 47–51
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Porous MOFs supported palladium catalysts for phenol hydrogenation: A comparative study on MIL-101 and MIL-53 Damin Zhang a, Yejun Guan a,⁎, Emiel J.M. Hensen b, Li Chen a, Yimeng Wang a a b
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, North Zhongshan Road 3663, 200062 Shanghai, China Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5612AZ Eindhoven, The Netherlands
a r t i c l e
i n f o
Article history: Received 3 April 2013 Received in revised form 11 June 2013 Accepted 28 June 2013 Available online 4 July 2013 Keywords: Palladium Chromium Benzenedicarboxylate Hydrogenation Phenol Cyclohexanone
a b s t r a c t Two metal organic frameworks (MOFs), chromium benzenedicarboxylates MIL-101 and MIL-53, have been synthesized and used as the support of palladium catalysts. The palladium catalysts were characterized by XRD, TEM, and CO chemisorption. MIL-101 is highly hydrophilic and beneficial as support for fine Pd nanoparticles with an average size of 2.3 nm. Microporous MIL-53 is relatively hydrophobic and larger Pd particles with an average size of 4.3 nm were formed on the external surface. Pd/MIL-101 showed better phenol selective hydrogenation activity to cyclohexanone (N 98%) under mild reaction conditions because of its smaller particle size. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Cyclohexanone is of great industrial interest for the production of caprolactam and adipic acid [1]. The industrial production of cylcohexanone commonly involves the oxidation of cyclohexane or the hydrogenation of phenol [2]. The gas phase phenol hydrogenation is usually performed at high temperature leading to high energy cost and coke tendency [3]. Liquid phase phenol hydrogenation at relatively low temperatures is therefore a more attractive process [4,5]. The main challenge is to develop a catalyst which allows high selectivity at high phenol conversion [6–8]. Some newly synthesized materials such as carbon nitride [9], polyaniline-functionalized carbon-nanotube [10], ionic liquid-like copolymer [11], and metal organic frameworks (MOFs) [12] have been reported to show better catalytic activity than conventional supports for phenol hydrogenation. In particular, a MIL-101 supported palladium catalyst has been reported to show very high activity under ambient conditions for selective hydrogenation of phenol [12]. An important feature of MOFs is their large surface area and robust structure compared to other micro- and mesoporous materials, which renders them promising candidate materials in a number of applications in gas storage, separation, and especially catalysis [13,14]. However, an inherent drawback in the application as catalyst (support) may be mass transfer limitations of certain reactants as a result of the molecular sieving properties of MOFs. These materials show specific structure dependent acidity and surface polarity. Such properties may be fine-tuned in order ⁎ Corresponding author. Tel.: +86 21 32530334; fax: +86 2132530334. E-mail address: [email protected] (Y. Guan). 1566-7367/$ – see front matter. © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.06.035
to bring about synergy with metal nanoparticles (NPs) [15–17]. Understanding of the interaction between NPs and the MOF is of great importance for developing such highly active and selective MOF based nanoparticle catalysts [18–22]. This inspired us to study the structural effect of MOFs in supporting metal NPs and their catalytic properties by using MIL-101 and MIL-53 as model materials, which have similar chemical compositions but distinctly different structures [23,24]. Herein we report the loading of palladium nanoparticles on MILs and their catalytic activities for phenol hydrogenation. The results show that MIL-101 is superior to MIL-53 as a support when aqueous solution of palladium chloride is used as a precursor. 2. Experimental MIL-101 and MIL-53 were prepared according to references [23,24]. Supported Pd catalysts were prepared by a deposition–reduction method, and the obtained catalysts were subjected to XRD, SEM, TEM, ICP-AES, and CO chemisorption measurements (see Supplementary materials). The phenol adsorption behaviors on MILs were studied with different initial phenol concentrations (0.05, 0.1, 0.15, 0.2, and 0.25 M) at 20 °C to compare their surface hydrophobicity. Before adsorption, MILs were dried in vacuum at 140 °C for 2 h. The dried powder (50 mg) was added to the aqueous phenol solutions (50 mL) and vigorously stirred for 6 h. After adsorption, the solution was recovered by centrifugation and was diluted, and the equilibrium phenol concentration was determined using the absorbance at 270 nm with Shimadzu UV-2550 ultraviolet visible spectrophotometer. The calibration curve was obtained from the UV spectra of standard solutions (10–125 μM).
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D. Zhang et al. / Catalysis Communications 41 (2013) 47–51
A Teflon-lined (120 mL) steel batch reactor was used to carry out the liquid phase hydrogenation [25]. No pretreatment on the catalyst was conducted prior to reaction. The reactor was charged with 100 mg of catalyst and 10 mL of aqueous phenol solution (0.25 M). Then the reactor was purged five times with H2 and pressurized (0.5 MPa H2). The mixture was heated up to 50 °C and held for 2 h. For the recycle test of Pd/MIL-53, the reaction temperature and reaction time were 60 °C and 2 h, respectively. The products were analyzed on a Shimadzu GC 2014 instrument equipped with a DB-Wax capillary column (30 m length). Only cyclohexanone and cyclohexanol were detected in all cases. 3. Results and discussion 3.1. Characterization of MILs Fig. 1 displays the thermo gravimetric curves of activated MIL-101 and MIL-53 after exposure to saturated water vapor for 1 h. Neither solvent nor free BDC was observed from TG curves. MIL-101 started decomposing at 390 °C (Td). MIL-53 showed much higher thermal stability (Td: 490 °C) than MIL-101 under otherwise identical conditions. A clear weight loss was observed below 100 °C for both materials, which is assigned to water adsorption. MIL-101 showed larger amount of water (12 wt.%) adsorbed than MIL-53 (7 wt.%). It has been shown that the water uptake on MIL-101 can be as high as 1.6 g/g via a continual adsorption on the coordinatively unsaturated Cr3+ sites and capillary condensation in the mesopores [26]. In contrast, stronger adsorption of organic phenolic compound on MIL-53 was observed (Fig. 2). When choosing MIL-101 as adsorbent, the phenol adsorbed on MIL-101 was about 220 mg/g at an equilibrium concentration of 0.2 mol/L. The adsorption amount of phenol on MIL-53 reached a 100 o
Td=490 C
80
o
60
3.2. Characterization of Pd/MILs The supported palladium catalysts were characterized by XRD, TEM, and CO chemisorption. XRD patterns of supported palladium catalysts (Fig. S3) did not show characteristic diffractions of palladium nanoparticles in Pd/MIL-101 and Pd/MIL-53, suggesting that the palladium nanoparticles are finely dispersed. An alternative explanation is that the signal is below the detection limit because of the relatively low Pd loading [21]. To determine the particle size distribution, we have conducted TEM analysis on both samples. Fig. 3 shows typical TEM images of Pd/MIL-101. The low magnification image (Fig. 3a) clearly shows the morphology of MIL-101, without any aggregation of palladium particles. HRTEM (Fig. 3b) evidences the presence of nanoparticles in the range of 1–3 nm (dav = 2.5 ± 0.5 nm). The particle size distribution is given in Fig. 3c. Fig. 4a shows the low magnification image of Pd/MIL-53. Aggregated palladium particles are clearly seen on the external surface of the support. Some aggregated particles even do not contact the surface (Fig. 4b). HRTEM shows that these palladium particles are in the range of 2–6 nm (dav = 4.3 ± 0.9 nm). According to pulse CO chemisorption, the metal dispersion of Pd/MIL-101 and Pd/MIL-53 is 44% and 35% (Table 1), respectively. Clearly, the mesoporous structure of MIL-101 favors formation of small palladium nanoparticles, probably due to a high dispersion of the precursor Pd2+ ions through weak π interaction with the benzene 700
a
MIL-53 MIL-101
40
Phenol uptake (mg/g)
Weight Loss (%)
Td=390 C
maximum of 663 mg/g at an equilibrium concentration of 0.18 mol/L. This result is in good agreement with a previous report [27]. It is worth noting that the phenol uptake on MIL-53 was always higher than that on MIL-101. Moreover, a substantial amount of phenol (20 wt.%) was retained on the surface of MIL-53 after two times of washing with water for 5 min, while on MIL-101 phenol was completely removed after one washing step. This result suggests that the narrower pore size of MIL-53 leads to a stronger hydrophobic moiety in its pores than in MIL-101 [27]. For comparison, the adsorption behavior of cyclohexanone on MILs was also evaluated. The results (not shown) show that the maximum cyclohexanone uptake for MIL-53 was 236 mg/g for a cyclohexanone equilibrium concentration of 0.18 M. In contrast, MIL-101 showed little affinity to cyclohexanone at a concentration lower than 0.2 M and the cyclohexanone uptake was only 98 mg/g at a cyclohexanone equilibrium concentration of 0.24 M. This result is in line with the hydrophobicity of MIL-53. It is worth mentioning that both MILs show stronger affinity to phenol than cyclohexanone. Similar finding has been reported previously on ionic liquid-like copolymers [11], which can be explained by the presence of a polar hydroxyl group of phenol.
20
MIL-101 MIL-53
b
600
600
500
500
400
400
300
300
200
200
100
100 0
0 400
500
600
700
800
Ce (mol/L)
0.25
g
Fig. 1. Thermogravimetric (TG) curves of activated MIL-101 (solid) and MIL-53 (dash-dot): ramp rate of 10 °C/min in N2 with a flow rate of 50 mL/min.
0.20
n hi
Temperature (oC)
0.15
ng hi as w
300
0.10
d 2n
200
0.05
as tw
100
0.00
1s
0
700
Fig. 2. Phenol uptakes on MILs at different equilibrium phenol concentrations (a) and phenol desorption results (b).
D. Zhang et al. / Catalysis Communications 41 (2013) 47–51
49
Fig. 3. TEM images of Pd/MIL-101: a. low resolution; b. high resolution; c. particle size distribution.
linkers [15,16]. Sun et al. [22] also found that very fine Au nanoparticles (1.8 nm) can be confined in the cages of MIL-101. We consider that the formation of larger palladium nanoparticle may result from a weak interaction between [PdCl4]2 − and the hydrophobic surface of MIL-53. A related result has been reported for Al-MIL-53 [28]. Introducing −NH2 groups in the framework structure may enhance the uptake of [PdCl4]2 − and therefore leading to highly dispersed metal nanoparticles [28]. 3.3. Phenol hydrogenation activity of Pd/MILs We compared the phenol hydrogenation activity of palladium supported on MIL-101 and MIL-53. Fig. 5 (left) shows the phenol
conversion at 50 °C on Pd/MIL-101 and Pd/MIL-53 as a function of the amount of catalyst used. Cyclohexanone was the dominant product, with selectivity above 98%, which can be explained by the stronger adsorption ability of phenol compared with cyclohexanone on the surface of both materials. Chen et al. also found that phenol was enriched in the hydrophilic cages of an ionic liquid-like copolymer, which resulted in high selectivity to cyclohexanone [11]. Pd/MIL-101 always outperformed Pd/MIL-53 under identical reaction conditions. The turnover frequency of phenol converted per surface Pd atom (based on CO chemisorption) is calculated to be 52 and 40 h−1 for Pd/MIL-101 and Pd/MIL-53 (Table 1, Pd/phenol molar ratio of 1.9 mol%), respectively. Previous experimental and theoretical results have suggested that the hydrogenation of phenyl group compounds
Fig. 4. TEM images of Pd/MIL-53: a. low resolution; b and c. high resolution; d. particle size distribution.
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D. Zhang et al. / Catalysis Communications 41 (2013) 47–51
Table 1 Structural parameters and catalytic results of Pd/MILs. Sample
Pd (wt.%)
Pd/MIL-53 Pd/MIL-101
DCO (%)a
dTEM (nm)
4.3 4.9
4.7 2.5
TOF (h−1)c
Xph (%)b
35 44
45.7 85
Selectivity (%)d
40 52
C_O
\OH
98.6 98.8
1.4 1.2
Reaction condition: 100 mg Pd/MILs, 10 mL 0.25 M phenol solution, 0.5 MPa H2, 50 °C, 2 h. a Measured by pulse injection of CO. b Phenol conversion. c Based on DCO. d Selectivity to cyclohexanone (C_O) and cyclohexanol (\OH).
4. Conclusion
likely takes place on the palladium surface [29]. Pd/MIL-101 (2.5 nm) contains smaller particles compared with Pd/MIL-53 (4.3 nm). Moreover, some palladium nanoparticles supported on MIL-101 may exist inside the pores and cavities [17], giving rise to the so called metal-π interactions between particles and framework [30], possibly enhancing the catalytic activity. The recyclability of the supported palladium catalysts was also investigated (Fig. 5, right). It can be seen that the phenol conversion decreased after three runs and then remained constant. ICP-AES analysis did not provide evidence for Pd leaching during the reaction. However, the particle size of Pd/MIL-101 and Pd/MIL-53 after 5 runs increased to 4.1 and 5.3 nm (Fig. S4), respectively. This result is in line with previous studies [11,21]. It is speculated that the particle aggregation may be one of the main reasons responsible for deactivation [11].
In summary, the effect of framework structure and surface hydrophilicity/hydrophobicity of chromium benzenedicarboxylates, MIL-101 and MIL-53, on palladium nanoparticles encapsulation and catalytic hydrogenation activity has been explored. MIL-101 acts better than MIL-53 as a support of palladium catalysts, probably because of its mesoporosity and hydrophilicity, which benefit to the adsorption of [PdCl4]2− species in aqueous solution into the framework and the subsequent reduction in the pores. On MIL-53, larger palladium nanoparticles are formed mainly on the external surface. Under the reaction conditions investigated (50 °C and 0.5 MPa H2), the turnover frequency of Pd/MIL-101 and Pd/MIL-53 is 52 and 40 h−1, respectively. Both catalysts give cyclohexanone selectivity above 98%. The catalysts can be recycled at least 5 times without palladium leaching. However,
Conversion Selectivity
100
60
Pd
/M
IL
-1
01
80
100
40
Pd/MIL-101 Pd/MIL-53
20
80
60 80
3
40
60
Pd
/M
IL
-5
Xphenol (%)
0 100
40
20 20
0
0 0
20
40
60
80
Catalyst Amount (mg)
100
1
2
3
4
5
Recycle (run)
Fig. 5. Phenol conversion as the function of catalyst used in liquid phase hydrogenation (left, 0.5 MPa H2, 50 °C, 2 h) and reusability of Pd/MILs (right).
D. Zhang et al. / Catalysis Communications 41 (2013) 47–51
particle size growth is observed for both catalysts and therefore the activity slightly decreases after three runs. Acknowledgment We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21203065) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.catcom.2013.06.035. References [1] A.C. Dimian, C.S. Bildea, Chemical Process Design: Computer-Aided Case Studies. Chapter 5 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2008. 129–172. [2] G. Bellussi, C. Perego, CATTECH 4 (2000) 4–16. [3] I. Dodgson, K. Griffin, G. Barberis, F. Pignataro, G. Tauszik, Chem. Ind. (1989) 830–833. [4] J.R. Gonzalez-Velasco, M.P. Gonzalez-Marcos, S. Arnaiz, J.I. Gutierrez-Ortiz, M.A. Gutierrez-Ortiz, Ind. Eng. Chem. Res. 34 (1995) 1031–1036. [5] P. Makowski, R.D. Cakan, M. Antonietti, F. Goettmann, M.M. Titirici, Chem. Commun. (2008) 999–1001. [6] S.T. Srinivas, L.J. Lakshmi, P.K. Rao, Appl. Catal. A Gen. 110 (1994) 167–172. [7] J. Matos, A. Corma, Appl. Catal. A Gen. 404 (2011) 103–112. [8] H. Liu, T. Jiang, B. Han, S. Liang, Y. Zhou, Science 326 (2009) 1250–1252.
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Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Porous MOFs supported palladium catalysts for phenol hydrogenation: A comparative study on MIL-101 and MIL-53 Damin Zhang a, Yejun Guan a,⁎, Emiel J.M. Hensen b, Li Chen a, Yimeng Wang a a b
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, North Zhongshan Road 3663, 200062 Shanghai, China Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5612AZ Eindhoven, The Netherlands
a r t i c l e
i n f o
Article history: Received 3 April 2013 Received in revised form 11 June 2013 Accepted 28 June 2013 Available online 4 July 2013 Keywords: Palladium Chromium Benzenedicarboxylate Hydrogenation Phenol Cyclohexanone
a b s t r a c t Two metal organic frameworks (MOFs), chromium benzenedicarboxylates MIL-101 and MIL-53, have been synthesized and used as the support of palladium catalysts. The palladium catalysts were characterized by XRD, TEM, and CO chemisorption. MIL-101 is highly hydrophilic and beneficial as support for fine Pd nanoparticles with an average size of 2.3 nm. Microporous MIL-53 is relatively hydrophobic and larger Pd particles with an average size of 4.3 nm were formed on the external surface. Pd/MIL-101 showed better phenol selective hydrogenation activity to cyclohexanone (N 98%) under mild reaction conditions because of its smaller particle size. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Cyclohexanone is of great industrial interest for the production of caprolactam and adipic acid [1]. The industrial production of cylcohexanone commonly involves the oxidation of cyclohexane or the hydrogenation of phenol [2]. The gas phase phenol hydrogenation is usually performed at high temperature leading to high energy cost and coke tendency [3]. Liquid phase phenol hydrogenation at relatively low temperatures is therefore a more attractive process [4,5]. The main challenge is to develop a catalyst which allows high selectivity at high phenol conversion [6–8]. Some newly synthesized materials such as carbon nitride [9], polyaniline-functionalized carbon-nanotube [10], ionic liquid-like copolymer [11], and metal organic frameworks (MOFs) [12] have been reported to show better catalytic activity than conventional supports for phenol hydrogenation. In particular, a MIL-101 supported palladium catalyst has been reported to show very high activity under ambient conditions for selective hydrogenation of phenol [12]. An important feature of MOFs is their large surface area and robust structure compared to other micro- and mesoporous materials, which renders them promising candidate materials in a number of applications in gas storage, separation, and especially catalysis [13,14]. However, an inherent drawback in the application as catalyst (support) may be mass transfer limitations of certain reactants as a result of the molecular sieving properties of MOFs. These materials show specific structure dependent acidity and surface polarity. Such properties may be fine-tuned in order ⁎ Corresponding author. Tel.: +86 21 32530334; fax: +86 2132530334. E-mail address: [email protected] (Y. Guan). 1566-7367/$ – see front matter. © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.06.035
to bring about synergy with metal nanoparticles (NPs) [15–17]. Understanding of the interaction between NPs and the MOF is of great importance for developing such highly active and selective MOF based nanoparticle catalysts [18–22]. This inspired us to study the structural effect of MOFs in supporting metal NPs and their catalytic properties by using MIL-101 and MIL-53 as model materials, which have similar chemical compositions but distinctly different structures [23,24]. Herein we report the loading of palladium nanoparticles on MILs and their catalytic activities for phenol hydrogenation. The results show that MIL-101 is superior to MIL-53 as a support when aqueous solution of palladium chloride is used as a precursor. 2. Experimental MIL-101 and MIL-53 were prepared according to references [23,24]. Supported Pd catalysts were prepared by a deposition–reduction method, and the obtained catalysts were subjected to XRD, SEM, TEM, ICP-AES, and CO chemisorption measurements (see Supplementary materials). The phenol adsorption behaviors on MILs were studied with different initial phenol concentrations (0.05, 0.1, 0.15, 0.2, and 0.25 M) at 20 °C to compare their surface hydrophobicity. Before adsorption, MILs were dried in vacuum at 140 °C for 2 h. The dried powder (50 mg) was added to the aqueous phenol solutions (50 mL) and vigorously stirred for 6 h. After adsorption, the solution was recovered by centrifugation and was diluted, and the equilibrium phenol concentration was determined using the absorbance at 270 nm with Shimadzu UV-2550 ultraviolet visible spectrophotometer. The calibration curve was obtained from the UV spectra of standard solutions (10–125 μM).
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D. Zhang et al. / Catalysis Communications 41 (2013) 47–51
A Teflon-lined (120 mL) steel batch reactor was used to carry out the liquid phase hydrogenation [25]. No pretreatment on the catalyst was conducted prior to reaction. The reactor was charged with 100 mg of catalyst and 10 mL of aqueous phenol solution (0.25 M). Then the reactor was purged five times with H2 and pressurized (0.5 MPa H2). The mixture was heated up to 50 °C and held for 2 h. For the recycle test of Pd/MIL-53, the reaction temperature and reaction time were 60 °C and 2 h, respectively. The products were analyzed on a Shimadzu GC 2014 instrument equipped with a DB-Wax capillary column (30 m length). Only cyclohexanone and cyclohexanol were detected in all cases. 3. Results and discussion 3.1. Characterization of MILs Fig. 1 displays the thermo gravimetric curves of activated MIL-101 and MIL-53 after exposure to saturated water vapor for 1 h. Neither solvent nor free BDC was observed from TG curves. MIL-101 started decomposing at 390 °C (Td). MIL-53 showed much higher thermal stability (Td: 490 °C) than MIL-101 under otherwise identical conditions. A clear weight loss was observed below 100 °C for both materials, which is assigned to water adsorption. MIL-101 showed larger amount of water (12 wt.%) adsorbed than MIL-53 (7 wt.%). It has been shown that the water uptake on MIL-101 can be as high as 1.6 g/g via a continual adsorption on the coordinatively unsaturated Cr3+ sites and capillary condensation in the mesopores [26]. In contrast, stronger adsorption of organic phenolic compound on MIL-53 was observed (Fig. 2). When choosing MIL-101 as adsorbent, the phenol adsorbed on MIL-101 was about 220 mg/g at an equilibrium concentration of 0.2 mol/L. The adsorption amount of phenol on MIL-53 reached a 100 o
Td=490 C
80
o
60
3.2. Characterization of Pd/MILs The supported palladium catalysts were characterized by XRD, TEM, and CO chemisorption. XRD patterns of supported palladium catalysts (Fig. S3) did not show characteristic diffractions of palladium nanoparticles in Pd/MIL-101 and Pd/MIL-53, suggesting that the palladium nanoparticles are finely dispersed. An alternative explanation is that the signal is below the detection limit because of the relatively low Pd loading [21]. To determine the particle size distribution, we have conducted TEM analysis on both samples. Fig. 3 shows typical TEM images of Pd/MIL-101. The low magnification image (Fig. 3a) clearly shows the morphology of MIL-101, without any aggregation of palladium particles. HRTEM (Fig. 3b) evidences the presence of nanoparticles in the range of 1–3 nm (dav = 2.5 ± 0.5 nm). The particle size distribution is given in Fig. 3c. Fig. 4a shows the low magnification image of Pd/MIL-53. Aggregated palladium particles are clearly seen on the external surface of the support. Some aggregated particles even do not contact the surface (Fig. 4b). HRTEM shows that these palladium particles are in the range of 2–6 nm (dav = 4.3 ± 0.9 nm). According to pulse CO chemisorption, the metal dispersion of Pd/MIL-101 and Pd/MIL-53 is 44% and 35% (Table 1), respectively. Clearly, the mesoporous structure of MIL-101 favors formation of small palladium nanoparticles, probably due to a high dispersion of the precursor Pd2+ ions through weak π interaction with the benzene 700
a
MIL-53 MIL-101
40
Phenol uptake (mg/g)
Weight Loss (%)
Td=390 C
maximum of 663 mg/g at an equilibrium concentration of 0.18 mol/L. This result is in good agreement with a previous report [27]. It is worth noting that the phenol uptake on MIL-53 was always higher than that on MIL-101. Moreover, a substantial amount of phenol (20 wt.%) was retained on the surface of MIL-53 after two times of washing with water for 5 min, while on MIL-101 phenol was completely removed after one washing step. This result suggests that the narrower pore size of MIL-53 leads to a stronger hydrophobic moiety in its pores than in MIL-101 [27]. For comparison, the adsorption behavior of cyclohexanone on MILs was also evaluated. The results (not shown) show that the maximum cyclohexanone uptake for MIL-53 was 236 mg/g for a cyclohexanone equilibrium concentration of 0.18 M. In contrast, MIL-101 showed little affinity to cyclohexanone at a concentration lower than 0.2 M and the cyclohexanone uptake was only 98 mg/g at a cyclohexanone equilibrium concentration of 0.24 M. This result is in line with the hydrophobicity of MIL-53. It is worth mentioning that both MILs show stronger affinity to phenol than cyclohexanone. Similar finding has been reported previously on ionic liquid-like copolymers [11], which can be explained by the presence of a polar hydroxyl group of phenol.
20
MIL-101 MIL-53
b
600
600
500
500
400
400
300
300
200
200
100
100 0
0 400
500
600
700
800
Ce (mol/L)
0.25
g
Fig. 1. Thermogravimetric (TG) curves of activated MIL-101 (solid) and MIL-53 (dash-dot): ramp rate of 10 °C/min in N2 with a flow rate of 50 mL/min.
0.20
n hi
Temperature (oC)
0.15
ng hi as w
300
0.10
d 2n
200
0.05
as tw
100
0.00
1s
0
700
Fig. 2. Phenol uptakes on MILs at different equilibrium phenol concentrations (a) and phenol desorption results (b).
D. Zhang et al. / Catalysis Communications 41 (2013) 47–51
49
Fig. 3. TEM images of Pd/MIL-101: a. low resolution; b. high resolution; c. particle size distribution.
linkers [15,16]. Sun et al. [22] also found that very fine Au nanoparticles (1.8 nm) can be confined in the cages of MIL-101. We consider that the formation of larger palladium nanoparticle may result from a weak interaction between [PdCl4]2 − and the hydrophobic surface of MIL-53. A related result has been reported for Al-MIL-53 [28]. Introducing −NH2 groups in the framework structure may enhance the uptake of [PdCl4]2 − and therefore leading to highly dispersed metal nanoparticles [28]. 3.3. Phenol hydrogenation activity of Pd/MILs We compared the phenol hydrogenation activity of palladium supported on MIL-101 and MIL-53. Fig. 5 (left) shows the phenol
conversion at 50 °C on Pd/MIL-101 and Pd/MIL-53 as a function of the amount of catalyst used. Cyclohexanone was the dominant product, with selectivity above 98%, which can be explained by the stronger adsorption ability of phenol compared with cyclohexanone on the surface of both materials. Chen et al. also found that phenol was enriched in the hydrophilic cages of an ionic liquid-like copolymer, which resulted in high selectivity to cyclohexanone [11]. Pd/MIL-101 always outperformed Pd/MIL-53 under identical reaction conditions. The turnover frequency of phenol converted per surface Pd atom (based on CO chemisorption) is calculated to be 52 and 40 h−1 for Pd/MIL-101 and Pd/MIL-53 (Table 1, Pd/phenol molar ratio of 1.9 mol%), respectively. Previous experimental and theoretical results have suggested that the hydrogenation of phenyl group compounds
Fig. 4. TEM images of Pd/MIL-53: a. low resolution; b and c. high resolution; d. particle size distribution.
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D. Zhang et al. / Catalysis Communications 41 (2013) 47–51
Table 1 Structural parameters and catalytic results of Pd/MILs. Sample
Pd (wt.%)
Pd/MIL-53 Pd/MIL-101
DCO (%)a
dTEM (nm)
4.3 4.9
4.7 2.5
TOF (h−1)c
Xph (%)b
35 44
45.7 85
Selectivity (%)d
40 52
C_O
\OH
98.6 98.8
1.4 1.2
Reaction condition: 100 mg Pd/MILs, 10 mL 0.25 M phenol solution, 0.5 MPa H2, 50 °C, 2 h. a Measured by pulse injection of CO. b Phenol conversion. c Based on DCO. d Selectivity to cyclohexanone (C_O) and cyclohexanol (\OH).
4. Conclusion
likely takes place on the palladium surface [29]. Pd/MIL-101 (2.5 nm) contains smaller particles compared with Pd/MIL-53 (4.3 nm). Moreover, some palladium nanoparticles supported on MIL-101 may exist inside the pores and cavities [17], giving rise to the so called metal-π interactions between particles and framework [30], possibly enhancing the catalytic activity. The recyclability of the supported palladium catalysts was also investigated (Fig. 5, right). It can be seen that the phenol conversion decreased after three runs and then remained constant. ICP-AES analysis did not provide evidence for Pd leaching during the reaction. However, the particle size of Pd/MIL-101 and Pd/MIL-53 after 5 runs increased to 4.1 and 5.3 nm (Fig. S4), respectively. This result is in line with previous studies [11,21]. It is speculated that the particle aggregation may be one of the main reasons responsible for deactivation [11].
In summary, the effect of framework structure and surface hydrophilicity/hydrophobicity of chromium benzenedicarboxylates, MIL-101 and MIL-53, on palladium nanoparticles encapsulation and catalytic hydrogenation activity has been explored. MIL-101 acts better than MIL-53 as a support of palladium catalysts, probably because of its mesoporosity and hydrophilicity, which benefit to the adsorption of [PdCl4]2− species in aqueous solution into the framework and the subsequent reduction in the pores. On MIL-53, larger palladium nanoparticles are formed mainly on the external surface. Under the reaction conditions investigated (50 °C and 0.5 MPa H2), the turnover frequency of Pd/MIL-101 and Pd/MIL-53 is 52 and 40 h−1, respectively. Both catalysts give cyclohexanone selectivity above 98%. The catalysts can be recycled at least 5 times without palladium leaching. However,
Conversion Selectivity
100
60
Pd
/M
IL
-1
01
80
100
40
Pd/MIL-101 Pd/MIL-53
20
80
60 80
3
40
60
Pd
/M
IL
-5
Xphenol (%)
0 100
40
20 20
0
0 0
20
40
60
80
Catalyst Amount (mg)
100
1
2
3
4
5
Recycle (run)
Fig. 5. Phenol conversion as the function of catalyst used in liquid phase hydrogenation (left, 0.5 MPa H2, 50 °C, 2 h) and reusability of Pd/MILs (right).
D. Zhang et al. / Catalysis Communications 41 (2013) 47–51
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