Chinese Chemical Letters  2016, Vol. 27 Issue (11): 1679-1682   PDF    
Effect of organic moieties (phenyl, naphthalene, and biphenyl) in Zr-MIL-140 on the hydrogenation activity of Pd nanoparticles
Jie Yanga, Jian-Jun Maa, Da-Min Zhangb, Teng Xueb, Ye-Jun Guanb     
a School of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, China ;
b Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200062, China
Abstract: 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 different accessibility to CO is observed.The hydrogenation performance in C=C saturation with respect 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-π interaction between Pd and frameworks consisting of mono-phenyl groups (C6H4).
Key words: Metal-organic-framework     Palladium     Selective hydrogenation     Zirconium    
1. Introduction

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 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) withspecific porous structures, metalnodes, and organic framework moietiesshow various surface propertiesasmentioned 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 showssuperiorcatalyticpropertiesundermildconditionscompared to conventional supports in the liquid phase selectivehydrogenation 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 biphenyl groups, respectively, were synthesized and used as supports of Pd NPs. Their catalytic activitiesinthe hydrogenationof phenoland alkene werecompared. The results show that mono-phenyl groups greatly favor the catalytic performance of Pd NPs.

2. Experimental

The preparation and characterization of MIL-140A, MIL-140B, andMIL-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 °C were obtained using a BELSORP-Max instrument. Prior to each measurement, the sample was outgassed at 150 °C 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 Kα radiation (λ=1.5405 Å) operated at 35 kV and 25 mA. Thermogravimetric analysis (TG) was performed using a NET2SCH STA449F3 TGA analyzer with a ramp rate of 10 °C/min from 25 °C to 800 °C under N2 flow. Transmission electronmicroscopy (TEM) images weretaken ona 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 °C for 3 h and then flushed with He for 1 h. After cooling to 40 °C with He, the CO gas pulses (5 vol% in He) were introduced at 100 mL/min and the signal was recorded using a TCD.

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Scheme. 1. The typical structures of MIL-140-type analogs with different "π" systems.

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 °C 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 °C) 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

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 °C. Their hydrophobic nature is revealed by the minute amount of water desorbed, which is clearly shown on the TG curves at temperature below 100 °C (Fig. 1). The weight losses of the three materials are all below 2 wt% at T < 400 °C, with MIL-140B having the lowest amount of water desorbed.

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Figure 1. The TG curves of MIL-140 analogs with different organic moieties.

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.

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Figure 2. XRD patterns (a) and N2 adsorption-desorption isotherms (b) of Pd/Zr-MOFs.

Table 1
The physical properties of supported Pd/MIL-140 catalysts and their activities in hydrogenation of phenol.

Fig. 3 shows the TEM images and particle size distributions of Pd/ Zr-MOFs.ThePdparticlesizeofPd/Zr-MOFisinrangeof1-6 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.

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Figure 3. TEM images of Pd NPs supported on (a) Pd/MIL-140A: (b) Pd/MIL-140B; (c) Pd/MIL-140C.

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 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 "π 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 C=C 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 "π" electrons, which deserves further study for the purpose of designing robust supported Pd catalyst.

Table 2
The catalytic activities of Pd/MIL-140 catalysts in hydrogenation of cyclohexene.

4. Conclusion

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 C=C hydrogenation using cyclohexene as a model substrate.

Acknowledgments

This work was supported by the Science and Technology Commission of Shanghai Municipality (No. 13ZR1417900) and the National Natural Science Foundation of China (No. 21203065).

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