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Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles
G Model
CCLET 3678 1–4 Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet 1 2 3 4
Original article
Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles
5
Q1 Jie
6 7
a b
Yang a,*, Jian-Jun Ma a, Da-Min Zhang b, Teng Xue b, Ye-Jun Guan b,*
School of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, China Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200062, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 February 2016 Received in revised form 23 March 2016 Accepted 14 April 2016 Available online xxx
MIL-140-type metal organic frameworks (isoreticular zirconium oxide MOFs) with different aromatic moieties (phenyl, naphthalene, and biphenyl) have been synthesized and employed as the supports of palladium nanoparticles (Pd NPs). The catalysts were characterized by XRD, BET, TEM and CO chemisorption. The results reveal that Pd NPs are homogeneously dispersed on all materials whereas 5C saturation with respect different accessibility to CO is observed. The hydrogenation performance in C5 to the effect of the aromatic moiety is compared. The Pd/MIL-140A MOF with the highest hydrogenation activity among the three catalysts comprised of different aromatic rings points to a unique Pd–p interaction between Pd and frameworks consisting of mono-phenyl groups (C6H4). ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords: Metal–organic-framework Palladium Selective hydrogenation Zirconium
8 9
1. Introduction
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Pd nanoparticles (NPs) have found wide applications in chemical industry including hydrogenation, oxidation, carbon–carbon bond formation, and etc. [1,2]. The Pd NPs can be used either as unsupported NPs capped with small organic ligands, or supported on the surfaces of solids [3–6]. The functionalized organic ligands may act as a shell on the particle’s surface that enhance or resist the access of particular substrates on catalytic sites [7]. The choice of supports, namely silica, alumina, metal oxides, carbon based materials, or their mixtures also plays an essential role because the properties of the supports affect the metal states, the adsorption and diffusion of reactants particularly in liquid phase reactions. Therefore to achieve desired catalytic properties regarding both conversion and selectivity, the rational design of supports with suitable acidity, porous structure and hydrophilic-hydrophobic properties is required and has been widely investigated [8–11]. For instance, a simple, efficient and recoverable Pd-based catalyst was successfully prepared by immobilizing Pd NPs on structured mesoporous silica microspheres with hydrophobic cores and hydrophilic shells which show excellent catalytic performance in the liquid phase hydrogenation of phenol. The success was attributed to the enhanced synergistic effect between highly
* Corresponding author.
dispersed Pd NPs and significantly decreased phenol mass transfer resistance [9]. On the other hand, nitrogen modified carbon materials are found to be superior to their carbon counterparts by electronically interacting with Pd NPs in the liquid phase hydrogenation of phenol [11,12]. Metal organic frameworks (MOFs) with specific porous structures, metal nodes, and organic framework moieties show various surface properties as mentioned above [13]. It is well known that the textural properties of MOFs can be simply adjusted by choosing different precursors or synthetic conditions, rendering it an ideal support for metal NPs [14]. The tunable framework structures allow for efficient incorporation of metal NPs showing a synergistic effect between metal-frameworks and metal in catalysis. One such example of Pd loaded on Cr (or Al) based MOFs shows superior catalytic properties under mild conditions compared to conventional supports in the liquid phase selective hydrogenation of phenol resulting in a high yield of cyclohexanone [15]. We have previously found that the physicochemical properties of chromium and aluminum based MIL-101(53), i.e., framework structure, surface hydrophilicity, organic functional groups, can significantly affect the catalytic properties of Pd NPs in phenol hydrogenation [16,17]. In this continuing study, we investigated the effect of aromatic moiety of the zirconium based MOFs on the catalytic activity of Pd NPs, which has been shown to significantly influence the hydrogenation activity of Ru NPs in an earlier report [18]. To this end, a series of isoreticular zirconium oxide based MOFs, namely, MIL-140A, B, and C containing phenyl, naphthalene, and
http://dx.doi.org/10.1016/j.cclet.2016.04.015 1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Yang, et al., Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.015
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
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57 58 59 60
biphenyl groups, respectively, were synthesized and used as supports of Pd NPs. Their catalytic activities in the hydrogenation of phenol and alkene were compared. The results show that monophenyl groups greatly favor the catalytic performance of Pd NPs.
61
2. Experimental
62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102
The preparation and characterization of MIL-140A, MIL-140B, and MIL-140C, comprising different organic linkers (Scheme 1), have been reported previously [18,19]. The supported Pd catalysts were prepared by a deposition-reduction method with a desirable amount of aqueous H2PdCl4 solution (Pd: 29.5 mg/mL) [16,17]. Nitrogen adsorption–desorption isotherms at 196 8C were obtained using a BELSORP-Max instrument. Prior to each measurement, the sample was outgassed at 150 8C under vacuum for 6 h. Specific surface areas were calculated according to the BET-method using five relative pressure points in the interval of 0.05–0.3. The powder X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV X-ray diffractometer using Cu Ka radiation (l = 1.5405 A˚) operated at 35 kV and 25 mA. Thermogravimetric analysis (TG) was performed using a NET2SCH STA449F3 TGA analyzer with a ramp rate of 10 8C/min from 25 8C to 800 8C under N2 flow. Transmission electron microscopy (TEM) images were taken on a FEI Tecnai G2 F30 microscope operating at 300 kV. The Pd loading was determined by a Thermo Elemental IRIS Intrepid II XSP inductively coupled plasma emission spectrometer (ICP-AES). Pulse CO chemisorption was performed on a Micromeritics AutoChem 2910 to determine the metal dispersion of the reduced catalysts. Prior to the measurement, the catalyst (ca. 100 mg) was reduced by 80 mL/min of 10 vol% H2 in Ar at 150 8C for 3 h and then flushed with He for 1 h. After cooling to 40 8C with He, the CO gas pulses (5 vol% in He) were introduced at 100 mL/min and the signal was recorded using a TCD. A Teflon-lined (120 mL) steel batch reactor was used to carry out the liquid phase hydrogenation of phenol and cyclohexene. No specific pretreatment was conducted prior to reaction. For phenol hydrogenation, 100 mg of catalyst and 10 mL of aqueous phenol solution (0.25 mol/L) was charged into the reactor. The reactor was purged five times with H2 and then pressurized with 0.5 MPa H2. The reaction mixture was heated to 50 8C and held for 3 h. For comparison, the hydrogenation was also tested in the organic phase with toluene as the solvent. For cyclohexene hydrogenation, 16 mg of catalyst and 1 mL of cyclohexene was mixed in 5 mL of toluene. The mixture was purged five times with H2 and then pressurized with 5 bar H2. The reaction mixture was placed at r.t. (20 8C) and held for 40 min. The products were analyzed on a Shimadzu GC 2014 equipped with a DB-Wax capillary column (30 m length and 0.25 mm i.d.).
3. Results and discussion
103
The results with detailed characterization of MIL-140s can be found in our published report [18]. These materials show typical microporous textural properties with a pore size about 0.7 nm and the surface areas of MIL-140A, B, and C measuring 426, 427, 387 m2/g, respectively. It is worthy of mentioning that all MIL-140s are thermally stable at temperature up to 450 8C. Their hydrophobic nature is revealed by the minute amount of water desorbed, which is clearly shown on the TG curves at temperature below 100 8C (Fig. 1). The weight losses of the three materials are all below 2 wt% at T < 400 8C, with MIL-140B having the lowest amount of water desorbed. Fig. 2a shows the XRD patterns of Pd/Zr-MOFs. Pd/MIL-140A and Pd/MIL-140B retain their crystalline structure, whereas Pd/MIL-140C lost the long-range order after loading Pd. We have followed the changes of XRD patterns of Pd/MIL-140C during the preparation procedure. The results (not shown) clearly demonstrated that the framework structure of MIL-140C collapsed in the precipitation step when alkali was introduced. Fig. 2b shows the N2 adsorption and desorption isotherms of Pd/Zr-MOFs. The BET surface areas from the adsorption isotherms of Pd/MIL-140A, B, and C were 401, 352, and 74 m2/g (Table 1), respectively. The surface areas of MIL-140A and B slightly decreased. In contrast, a substantial decrease in the surface area of MIL-140C was noticed, which is likely caused by the collapse of the framework structure as suggested by XRD. No diffraction lines attributable to Pd nanoparticles were observed in the XRD patterns, suggesting high dispersion of the Pd particles. Fig. 3 shows the TEM images and particle size distributions of Pd/Zr-MOFs. The Pd particle size of Pd/Zr-MOF is in range of 1–6 nm. The average particle size of 2.8 0.6, 3.8 0.9, and 4.4 1.1 nm were observed for Pd/MIL-140A, B, and C, respectively. The TEM images suggest that the Pd NPs on MIL-140A are more homogeneously dispersed than on MIL-140B and C. Another finding can be concluded is that on MIL-140B most of the Pd NPs locate on the external surface (Fig. 2b), while some of the Pd particles are partially covered by the framework surface for Pd/MIL-140A and Pd/MIL-140C materials. These Pd NPs were not observed by XRD (Fig. 2a) probably due to the low Pd loading (2.5 wt%) and small particle size. The total population of accessible metal sites and the Pd dispersion on MIL-140A, B, and C was estimated to be 17%, 15%, and 11%, respectively, by the pulse CO chemisorption, which is in line with the size distribution from TEM. Table 1 shows the performance of Pd/Zr-MOFs in the liquid phase hydrogenation of phenol by using water or toluene as solvents. The reactions were carried out under 0.5 MPa H2 with phenol/Pd mole ratio about 100. When water was used as solvent, the phenol conversion after 3 h reaction follows the trend of
104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150
100
Weight percent (%)
MIL-140B 99
MIL-140A 98
MIL-140C
97
96
95 100
200
300
400
500
o
Temperature (C) Scheme 1. The typical structures of MIL-140-type analogs with different ‘‘p’’ systems.
Fig. 1. The TG curves of MIL-140 analogs with different organic moieties.
Please cite this article in press as: J. Yang, et al., Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.015
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CCLET 3678 1–4 J. Yang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
Pd/MIL-140B
b Volume adsorbed (mL/g)
Intensity (a.u.)
Pd/MIL-140A
Table 2 The catalytic activities of Pd/MIL-140 catalysts in hydrogenation of cyclohexene.
250
a
200
Pd/MIL-140A
150
100
Pd/MIL-140B
50
20
30
40
0 0.0
2θ (degree)
Pd/MIL-140C
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P )
Fig. 2. XRD patterns (a) and N2 adsorption–desorption isotherms (b) of Pd/Zr-MOFs.
151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170
Catalyst
Conversion (%)
TOFa
Pd/MIL-140A Pd/MIL-140B Pd/MIL-140C
63 28 15
2273 1127 462
Reaction conditions: 15 mg of catalysts, 1 mL (9.87 mmol) cyclohexene, 5 mL toluene, 20 8C, 40 min, 0.5 MPa H2. a The TOF was based on molcyclohexene molPd 1 h 1.
Pd/MIL-140C
10
3
Pd/MIL-140A (50%) > Pd/MIL-140B (28%) > Pd/MIL-140C (5.7%), with cyclohexanone selectivity higher than 95%. The turnover frequencies based on Pd loading are calculated to be 15, 9.5, and 1.5 h 1 for Pd/MIL-140A, B, and C, respectively. These results are the first examples to show the hydrogenation activities of Pd NPs supported on MOFs comprising various aromatic rings. As aforementioned, the organic part of the MIL-140A, B, and C materials is composed of phenyl, naphthalene, and biphenyl rings, respectively. Since the particle size and porous structures of these materials are quite similar to each other, the differences in reactivity is likely associated to the interaction between Pd and the various ‘‘p stacks’’. It can be clearly seen that the Pd-phenyl combination shows the best performance in this case. This result may point to an electronic effect of supports on the Pd catalysis, as shown in our previous study that some electron donating organic substitutes also act as promoters of Pd catalysis [17]. It was proposed that the enhanced activity might be ascribed to low overall-activation energy of phenol hydrogenation on Pd supported on MOFs with H or –OCH3 groups [17]. The support–Pd interaction has also been shown to play a vital role in promoting
the hydrogenation activity of Pd catalysts with specific hybrid structures [20,21]. On the other hand, the framework collapse may be also one of the reasons to explain the lowest activity of the Pd/ MIL-140C catalyst because the low surface area may limit the adsorption and diffusion of substrates. Early studies [9,20] have shown that the catalytic activity can be enhanced by strong adsorption of the organic substrate from solution onto the surface of the catalyst. When the reaction medium turned to organic solvent, such as toluene, the same trend in phenol conversion was noticed: Pd/MIL-140A (17%) > Pd/MIL-140B (6.0%) > Pd/MIL-140C (0.9%). Accordingly, the TOFs of Pd/MIL-140A, B, and C in toluene were 5.0, 2.0, and 0.24 h 1, respectively, which again showed that Pd-phenyl combination was the most active one. The higher hydrogenation activity of Pd in water compared with toluene has been explained by the water promoted H2 activation with a very low energy barrier [22]. The hydrogenation of alkenes is also one of the well-known industrial processes catalyzed by supported Pd catalysts. We herein compared the effect of the organic moiety on the hydrogenation activity of C5 5C by using cyclohexene as a model compound. From Table 2, one can clearly see that the cyclohexene conversions over Pd/MIL-140A, B, and C are 63%, 28%, and 15%, respectively. These results also point to a pronounced organicmoiety dependent activity, with phenyl rings again favoring the hydrogenation. Pd/MIL-140A gave the highest activity in cyclohexene hydrogenation and a TOF of 2273 h 1 was achieved. At this stage, we still do not have clear explanation on this interesting behavior of Pd catalyst affected by the ‘‘p’’ electrons, which deserves further study for the purpose of designing robust supported Pd catalyst.
Table 1 The physical properties of supported Pd/MIL-140 catalysts and their activities in hydrogenation of phenol. Catalyst
BET surface area (m2/g)
Loading (%)
Diameter (nm)
Pd/MIL-140A
401
2.9
2.8
Pd/MIL-140B
352
2.6
3.8
Pd/MIL-140C
74
3.4
4.4
Solvent
Water Toluene Water Toluene Water Toluene
Xphenol (%)
50 17 28 6 5.7 0.9
TOFa
Sel. (%) C5 5O
C–OH
95 93 100 97 100 100
5.0 7.0 0 3.0 0 0
15 5.0 9.5 2.0 1.5 0.24
Reaction condition: 100 mg catalyst, 2.5 mmol phenol, 10 mL solvent, 50 8C, 3 h, 0.5 MPa H2. a The TOF was based on molphenol molPd 1 h 1.
Fig. 3. TEM images of Pd NPs supported on (a) Pd/MIL-140A: (b) Pd/MIL-140B; (c) Pd/MIL-140C.
Please cite this article in press as: J. Yang, et al., Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.015
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J. Yang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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4. Conclusion
202 203 204 205 206 207 208 209 210 211 212 213 214
In summary, a series of isoreticular zirconium oxide based MOFs, namely, Zr-MIL-140A, B, and C containing phenyl, naphthalene, and biphenyl groups, respectively, were synthesized and used as supports of Pd NPs. The Pd NPs were finely dispersed thanks to the microporous structures of MIL-140s. These supported Pd catalysts showed very high selectivity in hydrogenating phenol to cyclohexanone with the activity depended on the organic moiety of the metal organic frameworks and the textural properties. Pd loaded on frameworks containing phenyl groups showed superior performance to that having isoreticular structures but with different aromatic frameworks (naphthalene and biphenyl groups). Similar reactivity trends were noted in the C5 5C hydrogenation using cyclohexene as a model substrate.
215
Acknowledgments
216 Q2 This work was supported by the Science and Technology 217 Commission of Shanghai Municipality (no. 13ZR1417900) and the 218 National Natural Science Foundation of China (no. 21203065). 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233
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[7] J. Seth, C.N. Kona, S. Das, B.L.V. Prasad, A simple method for the preparation of ultra-small palladium nanoparticles and their utilization for the hydrogenation of terminal alkyne groups to alkanes, Nanoscale 7 (2015) 872–876. [8] A. Tungler, G. Fogassy, Catalysis with supported palladium metal, selectivity in the hydrogenation of C5 5C, C5 5O and C5 5N bonds, from chemo- to enantioselectivity, J. Mol. Catal. A: Chem. 173 (2001) 231–247. [9] F.W. Zhang, H.Q. Yang, Multifunctional mesoporous silica-supported palladium nanoparticles for selective phenol hydrogenation in the aqueous phase, Catal. Sci. Technol. 5 (2015) 572–577. [10] Y. Yabi, Y. Sawama, Y. Monguchi, H. Sajiki, New aspect of chemoselective hydrogenation utilizing heterogeneous palladium catalysts supported by nitrogen- and oxygen-containing macromolecules, Catal. Sci. Technol. 4 (2014) 260–271. [11] Y. Wang, J. Yao, H.R. Li, D.S. Su, M. Antonietti, Highly selective hydrogenation of phenol and derivatives over a Pd@carbon nitride catalyst in aqueous media, J. Am. Chem. Soc. 133 (2011) 2362–2365. [12] J.Z. Chen, W. Zhang, L.M. Chen, et al., Direct selective hydrogenation of phenol and derivatives over polyaniline-functionalized carbon-nanotube-supported palladium, ChemPlusChem 78 (2013) 142–148. [13] A. Corma, H. Garcı´a, F.X. Llabre´s i Xamena, Engineering metal organic frameworks for heterogeneous catalysis, Chem. Rev. 110 (2010) 4606–4655. [14] A. Dhakshinamoorthy, H. Garcia, Catalysis by metal nanoparticles embedded on metal-organic frameworks, Chem. Soc. Rev. 41 (2012) 5262–5284. [15] H.L. Liu, Y.W. Li, R. Luque, H.F. Jiang, A tuneable bifunctional water-compatible heterogeneous catalyst for the selective aqueous hydrogenation of phenols, Adv. Synth. Catal. 353 (2011) 3107–3113. [16] D.M. Zhang, Y.J. Guan, E.J.M. Hensen, L. Chen, Y.M. Wang, Porous MOFs supported palladium catalysts for phenol hydrogenation: a comparative study on MIL-101 and MIL-53, Catal. Commun. 41 (2013) 47–51. [17] D.M. Zhang, Y.J. Guan, E.J.M. Hensen, T. Xue, Y.M. Wang, Tuning the hydrogenation activity of Pd NPs on Al-MIL-53 by linker modification, Catal. Sci. Technol. 4 (2014) 795–802. [18] Q.Q. Yuan, D.M. Zhang, L. van Haandel, et al., Selective liquid phase hydrogenation of furfural to furfuryl alcohol by Ru/Zr-MOFs, J. Mol. Catal. A: Chem. 406 (2015) 58–64. [19] V. Guillerm, F. Ragon, M. Dan-Hardi, et al., A series of isoreticular, highly stable, porous zirconium oxide based metal-organic frameworks, Angew. Chem. Int. Ed. 51 (2012) 9267–9271. [20] R.F. Nie, M. Miao, W.C. Du, et al., Selective hydrogenation of C5 5C bond over N-doped reduced graphene oxides supported Pd catalyst, Appl. Catal. B: Environ. 180 (2016) 607–613. [21] J.W. Zhong, J.Z. Chen, L.M. Chen, Selective hydrogenation of phenol and related derivatives, Catal. Sci. Technol. 4 (2014) 3555–3569. [22] Y. Li, X. Xu, P.F. Zhang, et al., Highly selective Pd@mpg-C3N4 catalyst for phenol hydrogenation in aqueous phase, RSC Adv. 3 (2013) 10973–10982.
Please cite this article in press as: J. Yang, et al., Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.015
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CCLET 3678 1–4 Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters journal homepage: www.elsevier.com/locate/cclet 1 2 3 4
Original article
Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles
5
Q1 Jie
6 7
a b
Yang a,*, Jian-Jun Ma a, Da-Min Zhang b, Teng Xue b, Ye-Jun Guan b,*
School of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, China Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200062, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 23 February 2016 Received in revised form 23 March 2016 Accepted 14 April 2016 Available online xxx
MIL-140-type metal organic frameworks (isoreticular zirconium oxide MOFs) with different aromatic moieties (phenyl, naphthalene, and biphenyl) have been synthesized and employed as the supports of palladium nanoparticles (Pd NPs). The catalysts were characterized by XRD, BET, TEM and CO chemisorption. The results reveal that Pd NPs are homogeneously dispersed on all materials whereas 5C saturation with respect different accessibility to CO is observed. The hydrogenation performance in C5 to the effect of the aromatic moiety is compared. The Pd/MIL-140A MOF with the highest hydrogenation activity among the three catalysts comprised of different aromatic rings points to a unique Pd–p interaction between Pd and frameworks consisting of mono-phenyl groups (C6H4). ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords: Metal–organic-framework Palladium Selective hydrogenation Zirconium
8 9
1. Introduction
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Pd nanoparticles (NPs) have found wide applications in chemical industry including hydrogenation, oxidation, carbon–carbon bond formation, and etc. [1,2]. The Pd NPs can be used either as unsupported NPs capped with small organic ligands, or supported on the surfaces of solids [3–6]. The functionalized organic ligands may act as a shell on the particle’s surface that enhance or resist the access of particular substrates on catalytic sites [7]. The choice of supports, namely silica, alumina, metal oxides, carbon based materials, or their mixtures also plays an essential role because the properties of the supports affect the metal states, the adsorption and diffusion of reactants particularly in liquid phase reactions. Therefore to achieve desired catalytic properties regarding both conversion and selectivity, the rational design of supports with suitable acidity, porous structure and hydrophilic-hydrophobic properties is required and has been widely investigated [8–11]. For instance, a simple, efficient and recoverable Pd-based catalyst was successfully prepared by immobilizing Pd NPs on structured mesoporous silica microspheres with hydrophobic cores and hydrophilic shells which show excellent catalytic performance in the liquid phase hydrogenation of phenol. The success was attributed to the enhanced synergistic effect between highly
* Corresponding author.
dispersed Pd NPs and significantly decreased phenol mass transfer resistance [9]. On the other hand, nitrogen modified carbon materials are found to be superior to their carbon counterparts by electronically interacting with Pd NPs in the liquid phase hydrogenation of phenol [11,12]. Metal organic frameworks (MOFs) with specific porous structures, metal nodes, and organic framework moieties show various surface properties as mentioned above [13]. It is well known that the textural properties of MOFs can be simply adjusted by choosing different precursors or synthetic conditions, rendering it an ideal support for metal NPs [14]. The tunable framework structures allow for efficient incorporation of metal NPs showing a synergistic effect between metal-frameworks and metal in catalysis. One such example of Pd loaded on Cr (or Al) based MOFs shows superior catalytic properties under mild conditions compared to conventional supports in the liquid phase selective hydrogenation of phenol resulting in a high yield of cyclohexanone [15]. We have previously found that the physicochemical properties of chromium and aluminum based MIL-101(53), i.e., framework structure, surface hydrophilicity, organic functional groups, can significantly affect the catalytic properties of Pd NPs in phenol hydrogenation [16,17]. In this continuing study, we investigated the effect of aromatic moiety of the zirconium based MOFs on the catalytic activity of Pd NPs, which has been shown to significantly influence the hydrogenation activity of Ru NPs in an earlier report [18]. To this end, a series of isoreticular zirconium oxide based MOFs, namely, MIL-140A, B, and C containing phenyl, naphthalene, and
http://dx.doi.org/10.1016/j.cclet.2016.04.015 1001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: J. Yang, et al., Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.015
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
G Model
CCLET 3678 1–4 J. Yang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
2
57 58 59 60
biphenyl groups, respectively, were synthesized and used as supports of Pd NPs. Their catalytic activities in the hydrogenation of phenol and alkene were compared. The results show that monophenyl groups greatly favor the catalytic performance of Pd NPs.
61
2. Experimental
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The preparation and characterization of MIL-140A, MIL-140B, and MIL-140C, comprising different organic linkers (Scheme 1), have been reported previously [18,19]. The supported Pd catalysts were prepared by a deposition-reduction method with a desirable amount of aqueous H2PdCl4 solution (Pd: 29.5 mg/mL) [16,17]. Nitrogen adsorption–desorption isotherms at 196 8C were obtained using a BELSORP-Max instrument. Prior to each measurement, the sample was outgassed at 150 8C under vacuum for 6 h. Specific surface areas were calculated according to the BET-method using five relative pressure points in the interval of 0.05–0.3. The powder X-ray diffraction (XRD) patterns were collected on a Rigaku Ultima IV X-ray diffractometer using Cu Ka radiation (l = 1.5405 A˚) operated at 35 kV and 25 mA. Thermogravimetric analysis (TG) was performed using a NET2SCH STA449F3 TGA analyzer with a ramp rate of 10 8C/min from 25 8C to 800 8C under N2 flow. Transmission electron microscopy (TEM) images were taken on a FEI Tecnai G2 F30 microscope operating at 300 kV. The Pd loading was determined by a Thermo Elemental IRIS Intrepid II XSP inductively coupled plasma emission spectrometer (ICP-AES). Pulse CO chemisorption was performed on a Micromeritics AutoChem 2910 to determine the metal dispersion of the reduced catalysts. Prior to the measurement, the catalyst (ca. 100 mg) was reduced by 80 mL/min of 10 vol% H2 in Ar at 150 8C for 3 h and then flushed with He for 1 h. After cooling to 40 8C with He, the CO gas pulses (5 vol% in He) were introduced at 100 mL/min and the signal was recorded using a TCD. A Teflon-lined (120 mL) steel batch reactor was used to carry out the liquid phase hydrogenation of phenol and cyclohexene. No specific pretreatment was conducted prior to reaction. For phenol hydrogenation, 100 mg of catalyst and 10 mL of aqueous phenol solution (0.25 mol/L) was charged into the reactor. The reactor was purged five times with H2 and then pressurized with 0.5 MPa H2. The reaction mixture was heated to 50 8C and held for 3 h. For comparison, the hydrogenation was also tested in the organic phase with toluene as the solvent. For cyclohexene hydrogenation, 16 mg of catalyst and 1 mL of cyclohexene was mixed in 5 mL of toluene. The mixture was purged five times with H2 and then pressurized with 5 bar H2. The reaction mixture was placed at r.t. (20 8C) and held for 40 min. The products were analyzed on a Shimadzu GC 2014 equipped with a DB-Wax capillary column (30 m length and 0.25 mm i.d.).
3. Results and discussion
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The results with detailed characterization of MIL-140s can be found in our published report [18]. These materials show typical microporous textural properties with a pore size about 0.7 nm and the surface areas of MIL-140A, B, and C measuring 426, 427, 387 m2/g, respectively. It is worthy of mentioning that all MIL-140s are thermally stable at temperature up to 450 8C. Their hydrophobic nature is revealed by the minute amount of water desorbed, which is clearly shown on the TG curves at temperature below 100 8C (Fig. 1). The weight losses of the three materials are all below 2 wt% at T < 400 8C, with MIL-140B having the lowest amount of water desorbed. Fig. 2a shows the XRD patterns of Pd/Zr-MOFs. Pd/MIL-140A and Pd/MIL-140B retain their crystalline structure, whereas Pd/MIL-140C lost the long-range order after loading Pd. We have followed the changes of XRD patterns of Pd/MIL-140C during the preparation procedure. The results (not shown) clearly demonstrated that the framework structure of MIL-140C collapsed in the precipitation step when alkali was introduced. Fig. 2b shows the N2 adsorption and desorption isotherms of Pd/Zr-MOFs. The BET surface areas from the adsorption isotherms of Pd/MIL-140A, B, and C were 401, 352, and 74 m2/g (Table 1), respectively. The surface areas of MIL-140A and B slightly decreased. In contrast, a substantial decrease in the surface area of MIL-140C was noticed, which is likely caused by the collapse of the framework structure as suggested by XRD. No diffraction lines attributable to Pd nanoparticles were observed in the XRD patterns, suggesting high dispersion of the Pd particles. Fig. 3 shows the TEM images and particle size distributions of Pd/Zr-MOFs. The Pd particle size of Pd/Zr-MOF is in range of 1–6 nm. The average particle size of 2.8 0.6, 3.8 0.9, and 4.4 1.1 nm were observed for Pd/MIL-140A, B, and C, respectively. The TEM images suggest that the Pd NPs on MIL-140A are more homogeneously dispersed than on MIL-140B and C. Another finding can be concluded is that on MIL-140B most of the Pd NPs locate on the external surface (Fig. 2b), while some of the Pd particles are partially covered by the framework surface for Pd/MIL-140A and Pd/MIL-140C materials. These Pd NPs were not observed by XRD (Fig. 2a) probably due to the low Pd loading (2.5 wt%) and small particle size. The total population of accessible metal sites and the Pd dispersion on MIL-140A, B, and C was estimated to be 17%, 15%, and 11%, respectively, by the pulse CO chemisorption, which is in line with the size distribution from TEM. Table 1 shows the performance of Pd/Zr-MOFs in the liquid phase hydrogenation of phenol by using water or toluene as solvents. The reactions were carried out under 0.5 MPa H2 with phenol/Pd mole ratio about 100. When water was used as solvent, the phenol conversion after 3 h reaction follows the trend of
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100
Weight percent (%)
MIL-140B 99
MIL-140A 98
MIL-140C
97
96
95 100
200
300
400
500
o
Temperature (C) Scheme 1. The typical structures of MIL-140-type analogs with different ‘‘p’’ systems.
Fig. 1. The TG curves of MIL-140 analogs with different organic moieties.
Please cite this article in press as: J. Yang, et al., Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.015
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Pd/MIL-140B
b Volume adsorbed (mL/g)
Intensity (a.u.)
Pd/MIL-140A
Table 2 The catalytic activities of Pd/MIL-140 catalysts in hydrogenation of cyclohexene.
250
a
200
Pd/MIL-140A
150
100
Pd/MIL-140B
50
20
30
40
0 0.0
2θ (degree)
Pd/MIL-140C
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P )
Fig. 2. XRD patterns (a) and N2 adsorption–desorption isotherms (b) of Pd/Zr-MOFs.
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Catalyst
Conversion (%)
TOFa
Pd/MIL-140A Pd/MIL-140B Pd/MIL-140C
63 28 15
2273 1127 462
Reaction conditions: 15 mg of catalysts, 1 mL (9.87 mmol) cyclohexene, 5 mL toluene, 20 8C, 40 min, 0.5 MPa H2. a The TOF was based on molcyclohexene molPd 1 h 1.
Pd/MIL-140C
10
3
Pd/MIL-140A (50%) > Pd/MIL-140B (28%) > Pd/MIL-140C (5.7%), with cyclohexanone selectivity higher than 95%. The turnover frequencies based on Pd loading are calculated to be 15, 9.5, and 1.5 h 1 for Pd/MIL-140A, B, and C, respectively. These results are the first examples to show the hydrogenation activities of Pd NPs supported on MOFs comprising various aromatic rings. As aforementioned, the organic part of the MIL-140A, B, and C materials is composed of phenyl, naphthalene, and biphenyl rings, respectively. Since the particle size and porous structures of these materials are quite similar to each other, the differences in reactivity is likely associated to the interaction between Pd and the various ‘‘p stacks’’. It can be clearly seen that the Pd-phenyl combination shows the best performance in this case. This result may point to an electronic effect of supports on the Pd catalysis, as shown in our previous study that some electron donating organic substitutes also act as promoters of Pd catalysis [17]. It was proposed that the enhanced activity might be ascribed to low overall-activation energy of phenol hydrogenation on Pd supported on MOFs with H or –OCH3 groups [17]. The support–Pd interaction has also been shown to play a vital role in promoting
the hydrogenation activity of Pd catalysts with specific hybrid structures [20,21]. On the other hand, the framework collapse may be also one of the reasons to explain the lowest activity of the Pd/ MIL-140C catalyst because the low surface area may limit the adsorption and diffusion of substrates. Early studies [9,20] have shown that the catalytic activity can be enhanced by strong adsorption of the organic substrate from solution onto the surface of the catalyst. When the reaction medium turned to organic solvent, such as toluene, the same trend in phenol conversion was noticed: Pd/MIL-140A (17%) > Pd/MIL-140B (6.0%) > Pd/MIL-140C (0.9%). Accordingly, the TOFs of Pd/MIL-140A, B, and C in toluene were 5.0, 2.0, and 0.24 h 1, respectively, which again showed that Pd-phenyl combination was the most active one. The higher hydrogenation activity of Pd in water compared with toluene has been explained by the water promoted H2 activation with a very low energy barrier [22]. The hydrogenation of alkenes is also one of the well-known industrial processes catalyzed by supported Pd catalysts. We herein compared the effect of the organic moiety on the hydrogenation activity of C5 5C by using cyclohexene as a model compound. From Table 2, one can clearly see that the cyclohexene conversions over Pd/MIL-140A, B, and C are 63%, 28%, and 15%, respectively. These results also point to a pronounced organicmoiety dependent activity, with phenyl rings again favoring the hydrogenation. Pd/MIL-140A gave the highest activity in cyclohexene hydrogenation and a TOF of 2273 h 1 was achieved. At this stage, we still do not have clear explanation on this interesting behavior of Pd catalyst affected by the ‘‘p’’ electrons, which deserves further study for the purpose of designing robust supported Pd catalyst.
Table 1 The physical properties of supported Pd/MIL-140 catalysts and their activities in hydrogenation of phenol. Catalyst
BET surface area (m2/g)
Loading (%)
Diameter (nm)
Pd/MIL-140A
401
2.9
2.8
Pd/MIL-140B
352
2.6
3.8
Pd/MIL-140C
74
3.4
4.4
Solvent
Water Toluene Water Toluene Water Toluene
Xphenol (%)
50 17 28 6 5.7 0.9
TOFa
Sel. (%) C5 5O
C–OH
95 93 100 97 100 100
5.0 7.0 0 3.0 0 0
15 5.0 9.5 2.0 1.5 0.24
Reaction condition: 100 mg catalyst, 2.5 mmol phenol, 10 mL solvent, 50 8C, 3 h, 0.5 MPa H2. a The TOF was based on molphenol molPd 1 h 1.
Fig. 3. TEM images of Pd NPs supported on (a) Pd/MIL-140A: (b) Pd/MIL-140B; (c) Pd/MIL-140C.
Please cite this article in press as: J. Yang, et al., Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.015
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4. Conclusion
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In summary, a series of isoreticular zirconium oxide based MOFs, namely, Zr-MIL-140A, B, and C containing phenyl, naphthalene, and biphenyl groups, respectively, were synthesized and used as supports of Pd NPs. The Pd NPs were finely dispersed thanks to the microporous structures of MIL-140s. These supported Pd catalysts showed very high selectivity in hydrogenating phenol to cyclohexanone with the activity depended on the organic moiety of the metal organic frameworks and the textural properties. Pd loaded on frameworks containing phenyl groups showed superior performance to that having isoreticular structures but with different aromatic frameworks (naphthalene and biphenyl groups). Similar reactivity trends were noted in the C5 5C hydrogenation using cyclohexene as a model substrate.
215
Acknowledgments
216 Q2 This work was supported by the Science and Technology 217 Commission of Shanghai Municipality (no. 13ZR1417900) and the 218 National Natural Science Foundation of China (no. 21203065). 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233
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Please cite this article in press as: J. Yang, et al., Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.015
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