Catalysis Communications 41 (2013) 47–51

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Catalysis Communications

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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 Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, North Zhongshan Road 3663, 200062 Shanghai, Chinab Schuit Institute of Catalysis, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5612AZ Eindhoven, The Netherlands

⁎ Corresponding author. Tel.: +86 21 32530334; fax:E-mail address: yjguan@chem.ecnu.edu.cn (Y. Guan

1566-7367/$ – see front matter. © 2013 Elsevier B.V. Alhttp://dx.doi.org/10.1016/j.catcom.2013.06.035

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 April 2013Received in revised form 11 June 2013Accepted 28 June 2013Available online 4 July 2013

Keywords:PalladiumChromiumBenzenedicarboxylateHydrogenationPhenolCyclohexanone

Two metal organic frameworks (MOFs), chromium benzenedicarboxylates MIL-101 and MIL-53, have beensynthesized and used as the support of palladium catalysts. The palladium catalysts were characterizedby XRD, TEM, and CO chemisorption. MIL-101 is highly hydrophilic and beneficial as support for fine Pdnanoparticles with an average size of 2.3 nm. Microporous MIL-53 is relatively hydrophobic and larger Pdparticles with an average size of 4.3 nmwere formed on the external surface. Pd/MIL-101 showed better phenolselective hydrogenation activity to cyclohexanone (N98%) under mild reaction conditions because of its smallerparticle size.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Cyclohexanone is of great industrial interest for the production of cap-rolactam and adipic acid [1]. The industrial production of cylcohexanonecommonly involves the oxidation of cyclohexane or the hydrogenation ofphenol [2]. The gas phase phenol hydrogenation is usually performed athigh temperature leading to high energy cost and coke tendency [3].Liquid phase phenol hydrogenation at relatively low temperatures istherefore amore attractive process [4,5]. Themain challenge is to devel-op 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 co-polymer [11], and metal organic frameworks (MOFs) [12] have beenreported to show better catalytic activity than conventional supportsfor phenol hydrogenation. In particular, aMIL-101 supported palladiumcatalyst has been reported to show very high activity under ambientconditions for selective hydrogenation of phenol [12].

An important feature of MOFs is their large surface area and robuststructure compared to other micro- and mesoporous materials, whichrenders them promising candidate materials in a number of applicationsin gas storage, separation, and especially catalysis [13,14]. However, aninherent drawback in the application as catalyst (support) may be masstransfer limitations of certain reactants as a result of themolecular sievingproperties of MOFs. These materials show specific structure dependentacidity and surface polarity. Such properties may be fine-tuned in order

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to bring about synergy with metal nanoparticles (NPs) [15–17]. Under-standing of the interaction between NPs and the MOF is of great impor-tance for developing such highly active and selective MOF basednanoparticle catalysts [18–22]. This inspired us to study the structuraleffect of MOFs in supporting metal NPs and their catalytic properties byusingMIL-101 andMIL-53 asmodelmaterials, which have similar chem-ical compositions but distinctly different structures [23,24]. Herein wereport the loading of palladium nanoparticles on MILs and their catalyticactivities for phenol hydrogenation. The results show that MIL-101 issuperior to MIL-53 as a support when aqueous solution of palladiumchloride is used as a precursor.

2. Experimental

MIL-101 and MIL-53 were prepared according to references [23,24].Supported Pd catalystswere prepared by a deposition–reductionmethod,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 dif-ferent 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 vigor-ously stirred for 6 h. After adsorption, the solution was recovered bycentrifugation and was diluted, and the equilibrium phenol concentra-tion was determined using the absorbance at 270 nm with ShimadzuUV-2550 ultraviolet visible spectrophotometer. The calibration curvewas obtained from the UV spectra of standard solutions (10–125 μM).

48 D. Zhang et al. / Catalysis Communications 41 (2013) 47–51

A Teflon-lined (120 mL) steel batch reactor was used to carry outthe liquid phase hydrogenation [25]. No pretreatment on the catalystwas conducted prior to reaction. The reactor was charged with100 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 reactiontime were 60 °C and 2 h, respectively. The products were analyzed ona Shimadzu GC 2014 instrument equipped with a DB-Wax capillarycolumn (30 m length). Only cyclohexanone and cyclohexanol weredetected in all cases.

3. Results and discussion

3.1. Characterization of MILs

Fig. 1 displays the thermo gravimetric curves of activated MIL-101and MIL-53 after exposure to saturated water vapor for 1 h. Neithersolvent nor free BDC was observed from TG curves. MIL-101 starteddecomposing at 390 °C (Td). MIL-53 showed much higher thermal sta-bility (Td: 490 °C) thanMIL-101 under otherwise identical conditions. Aclear weight loss was observed below 100 °C for both materials, whichis assigned to water adsorption. MIL-101 showed larger amount ofwater (12 wt.%) adsorbed than MIL-53 (7 wt.%). It has been shownthat the water uptake on MIL-101 can be as high as 1.6 g/g via acontinual adsorption on the coordinatively unsaturated Cr3+ sitesand capillary condensation in the mesopores [26]. In contrast, strongeradsorption of organic phenolic compound on MIL-53 was observed(Fig. 2). When choosing MIL-101 as adsorbent, the phenol adsorbedon MIL-101 was about 220 mg/g at an equilibrium concentrationof 0.2 mol/L. The adsorption amount of phenol on MIL-53 reached a

100 200 300 400 500 600 700 8000

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80

100

Td=390 oC

Wei

gh

t L

oss

(%

)

Temperature (oC)

MIL-101

MIL-53

Td=490 oC

Fig. 1. Thermogravimetric (TG) curves of activatedMIL-101 (solid) andMIL-53 (dash-dot):ramp rate of 10 °C/min in N2 with a flow rate of 50 mL/min.

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 isworth noting that the phenol uptake on MIL-53 was always higherthan that on MIL-101. Moreover, a substantial amount of phenol(20 wt.%) was retained on the surface of MIL-53 after two times ofwashingwithwater for 5 min, while onMIL-101 phenol was completelyremoved after one washing step. This result suggests that the narrowerpore size of MIL-53 leads to a stronger hydrophobic moiety in its poresthan in MIL-101 [27]. For comparison, the adsorption behavior of cyclo-hexanone on MILs was also evaluated. The results (not shown) showthat the maximum cyclohexanone uptake for MIL-53 was 236 mg/gfor a cyclohexanone equilibrium concentration of 0.18 M. In contrast,MIL-101 showed little affinity to cyclohexanone at a concentrationlower than 0.2 M and the cyclohexanone uptake was only 98 mg/g at acyclohexanone equilibrium concentration of 0.24 M. This result is inline with the hydrophobicity of MIL-53. It is worth mentioning thatboth MILs show stronger affinity to phenol than cyclohexanone. Similarfinding has been reported previously on ionic liquid-like copolymers[11], which can be explained by the presence of a polar hydroxyl groupof phenol.

3.2. Characterization of Pd/MILs

The supported palladium catalysts were characterized by XRD,TEM, and CO chemisorption. XRD patterns of supported palladiumcatalysts (Fig. S3) did not show characteristic diffractions of palladiumnanoparticles in Pd/MIL-101 and Pd/MIL-53, suggesting that the palla-dium nanoparticles are finely dispersed. An alternative explanationis that the signal is below the detection limit because of the relativelylow Pd loading [21]. To determine the particle size distribution, wehave conducted TEM analysis on both samples. Fig. 3 shows typicalTEM images of Pd/MIL-101. The low magnification image (Fig. 3a)clearly shows the morphology of MIL-101, without any aggregationof palladium particles. HRTEM (Fig. 3b) evidences the presence ofnanoparticles in the range of 1–3 nm (dav = 2.5 ± 0.5 nm). The parti-cle size distribution is given in Fig. 3c. Fig. 4a shows the low magnifica-tion image of Pd/MIL-53. Aggregated palladium particles are clearlyseen on the external surface of the support. Some aggregated particleseven do not contact the surface (Fig. 4b). HRTEM shows that these pal-ladium particles are in the range of 2–6 nm (dav = 4.3 ± 0.9 nm).According to pulse CO chemisorption, the metal dispersion ofPd/MIL-101 and Pd/MIL-53 is 44% and 35% (Table 1), respectively.Clearly, the mesoporous structure of MIL-101 favors formation ofsmall palladium nanoparticles, probably due to a high dispersion ofthe precursor Pd2+ ions through weak π interaction with the benzene

0.00 0.05 0.10 0.15 0.20 0.25

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500

600

700 MIL-53 MIL-101

Ph

eno

l up

take

(m

g/g

)

Ce (mol/L)

a

0

100

200

300

400

500

600

700b

2nd washing

1st washing

Fig. 2. Phenol uptakes on MILs at different equilibrium phenol concentrations (a) andphenol desorption results (b).

Fig. 3. TEM images of Pd/MIL-101: a. low resolution; b. high resolution; c. particle size distribution.

49D. Zhang et al. / Catalysis Communications 41 (2013) 47–51

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 thatthe formation of larger palladium nanoparticle may result from aweak interaction between [PdCl4]2− and the hydrophobic surfaceof MIL-53. A related result has been reported for Al-MIL-53 [28]. In-troducing−NH2 groups in the framework structure may enhancethe uptake of [PdCl4]2− and therefore leading to highly dispersedmetal nanoparticles [28].

3.3. Phenol hydrogenation activity of Pd/MILs

We compared the phenol hydrogenation activity of palladiumsupported on MIL-101 and MIL-53. Fig. 5 (left) shows the phenol

Fig. 4. TEM images of Pd/MIL-53: a. low resolution; b a

conversion at 50 °C on Pd/MIL-101 and Pd/MIL-53 as a function ofthe amount of catalyst used. Cyclohexanone was the dominant prod-uct, with selectivity above 98%, which can be explained by the stron-ger adsorption ability of phenol compared with cyclohexanone onthe surface of both materials. Chen et al. also found that phenol wasenriched in the hydrophilic cages of an ionic liquid-like copolymer,which resulted in high selectivity to cyclohexanone [11]. Pd/MIL-101always 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 forPd/MIL-101 and Pd/MIL-53 (Table 1, Pd/phenol molar ratio of1.9 mol%), respectively. Previous experimental and theoretical resultshave suggested that the hydrogenation of phenyl group compounds

nd c. high resolution; d. particle size distribution.

Table 1Structural parameters and catalytic results of Pd/MILs.

Sample Pd (wt.%) dTEM (nm) DCO (%)a Xph (%)b TOF (h−1)c Selectivity (%)d

C_O \OH

Pd/MIL-53 4.3 4.7 35 45.7 40 98.6 1.4Pd/MIL-101 4.9 2.5 44 85 52 98.8 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).

50 D. Zhang et al. / Catalysis Communications 41 (2013) 47–51

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). More-over, some palladium nanoparticles supported on MIL-101 may existinside the pores and cavities [17], giving rise to the so called metal-πinteractions between particles and framework [30], possibly enhancingthe catalytic activity.

The recyclability of the supported palladium catalysts was alsoinvestigated (Fig. 5, right). It can be seen that the phenol conversiondecreased after three runs and then remained constant. ICP-AES anal-ysis did not provide evidence for Pd leaching during the reaction.However, the particle size of Pd/MIL-101 and Pd/MIL-53 after 5 runsincreased to 4.1 and 5.3 nm (Fig. S4), respectively. This result is inline with previous studies [11,21]. It is speculated that the particleaggregation may be one of the main reasons responsible for deactiva-tion [11].

0 20 40 60 80 1000

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40

60

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100

Pd/MIL-101 Pd/MIL-53

Xp

hen

ol (

%)

Catalyst Amount (mg)

Pd

Pd

Fig. 5. Phenol conversion as the function of catalyst used in liquid phase hydro

4. Conclusion

In summary, the effect of framework structure and surfacehydrophilicity/hydrophobicity of chromium benzenedicarboxylates,MIL-101 and MIL-53, on palladium nanoparticles encapsulation andcatalytic hydrogenation activity has been explored. MIL-101 acts betterthan MIL-53 as a support of palladium catalysts, probably because ofits mesoporosity and hydrophilicity, which benefit to the adsorptionof [PdCl4]2− species in aqueous solution into the framework andthe subsequent reduction in the pores. On MIL-53, larger palladiumnanoparticles are formed mainly on the external surface. Under thereaction conditions investigated (50 °C and 0.5 MPa H2), the turnoverfrequency of Pd/MIL-101 and Pd/MIL-53 is 52 and 40 h−1, respectively.Both catalysts give cyclohexanone selectivity above 98%. The catalystscan be recycled at least 5 times without palladium leaching. However,

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/MIL

-53

Conversion Selectivity

/MIL

-101

Recycle (run)1 2 3 4 5

0

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genation (left, 0.5 MPa H2, 50 °C, 2 h) and reusability of Pd/MILs (right).

51D. Zhang et al. / Catalysis Communications 41 (2013) 47–51

particle size growth is observed for both catalysts and therefore theactivity slightly decreases after three runs.

Acknowledgment

We acknowledge the financial support from the National NaturalScience Foundation of China (Grant No. 21203065) and the FundamentalResearch 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.

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