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Palladium nanoparticles supported on a two-dimensional layered zeolitic imidazolate framework-L as an efficient size-selective catalyst
Microporous and Mesoporous Materials 221 (2016) 220e227
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Palladium nanoparticles supported on a two-dimensional layered zeolitic imidazolate framework-L as an efficient size-selective catalyst Songlin Xue a, Hong Jiang a, Zhaoxiang Zhong a, Ze-Xian Low b, Rizhi Chen a, *, Weihong Xing a, ** a b
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, China Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2015 Received in revised form 28 September 2015 Accepted 29 September 2015 Available online 8 October 2015
A new Pd@ZIF-L hybrid catalyst having a unique crosshair-star shape with a size of about 20 mm was synthesized, in which a two-dimensional layered zeolitic imidazolate framework-L (ZIF-L) was used as a support to immobilize palladium nanoparticles via an assembly method. The hybrid catalyst with unique cushion-shaped cavities between layers with a size of 6.64 Å can be used for the diffusion of specific reactants not achievable by other ZIF systems such as ZIF-8. The crystal structure and morphology of Pd@ZIF-L are strongly dependent on the synthesis conditions. Palladium nanoparticles with an average size of 3 nm are homogeneously encapsulated in the ZIF-L crystals. The as-synthesized Pd@ZIF-L catalyst favors the conversion of alkenes with larger molecular sizes, and exhibits excellent molecule-sizeselectivity and anti-leaching performance. © 2015 Elsevier Inc. All rights reserved.
Keywords: Layered zeolitic imidazolate frameworks Two-dimensional Pd nanoparticles Alkene hydrogenation Size-selective catalysis
1. Introduction Metal nanoparticles (MNPs) have been widely investigated for their potential applications in catalysis [1e3]. Due to the small grain size, a significant volume of the microstructure is composed of interfaces/grain boundaries and hence a large fraction of the atoms resides in grain boundaries, leading to more catalytic sites and enhanced catalytic performance [4]. Furthermore, recent advances in controlling the particle size and shape have opened up the possibility to optimize the particle geometry, providing the optimum size and surface properties for specific reactions [5]. Due to high surface energies and large surface areas, these MNPs are thermodynamically unstable, and therefore tend to aggregate during reactions, resulting in a significant decrease in the catalytic activity [6e8]. In this regard, solid materials with confined void spaces such as zeolites, metal organic frameworks (MOFs) and polymers are used as supports for MNPs immobilization. By utilizing the interaction or space limitations between the metal and the support, the agglomeration of MNPs can be prevented [9,10].
* Corresponding author. Tel.: þ86 25 83172286; fax: þ86 25 83172292. ** Corresponding author. Tel.: þ86 25 83172286; fax: þ86 25 83172292. E-mail addresses: [email protected] (R. Chen), [email protected] (W. Xing). http://dx.doi.org/10.1016/j.micromeso.2015.09.053 1387-1811/© 2015 Elsevier Inc. All rights reserved.
Zeolitic imidazolate frameworks (ZIFs), a sub-family of metalorganic frameworks (MOFs) [11,12], are highly ordered porous solids that consist of inorganic metal ions bridged by imidazolate ligands. Compared to other types of MOFs, ZIFs exhibit better thermal, hydrothermal and chemical stability [13]. Owing to their high porosity, large surface area and better stability, ZIFs provide great potential as hosts for various MNPs. Loading MNPs into the pores of ZIFs could limit the growth of MNPs within the confined cavities and also the migration and aggregation of MNPs during the reaction [14e16], thus increasing their catalytic activity and stability. There are two main strategies to obtain ZIF-immobilized MNPs d (1) the loading of ZIFs with molecular precursors followed by their subsequent decomposition inside the pores of ZIFs [17e19]. (2) The encapsulation of pre-synthesized MNPs inside the growing host frameworks [20]. The latter is more desirable as it avoids the formation of MNPs on the external surface and damaging the ZIF frameworks during the after-treatment processes which may occur in the former strategy. Selection of ZIFs as supports for MNPs immobilization is one of the most important steps because the physicalechemical properties of ZIFs such as the pore structure can significantly affect the corresponding catalytic performance of MNPs@ZIFs. Each ZIF has a definite window size, such as ZIF-8 has small-pore apertures of 3.4 Å, which allow reactants having smaller molecule sizes than the
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window size to easily diffuse into the pores to reach the active site or MNPs, thus showing outstanding molecular-size selectivity. For example, Pd@ZIF-8 showed high activity for ethylene hydrogenation, but no detectable activity was observed for cyclooctene hydrogenation [21]. These results were clearly due to the moleculesize-selective property of the ZIF-8, because the ethylene molecules (2.5 Å) are small enough to diffuse through the pore apertures of the ZIF-8 (3.4 Å), but not the cyclooctene molecules with a size of 5.5 Å. Recently, we have successfully synthesized a new twodimensional zeolitic imidazolate framework (ZIF-L) in zinc salt and 2-methylimidazole aqueous solution at room temperature [22]. ZIF-L, comprised of identical building blocks of ZIF-8, has twodimensional crystal lattices stacked layer-by-layer forming cushion-shaped cavities between layers. The two neighboring twodimensional sod layers in ZIF-L are bridged by hydrogen bonds. They are parallel to each other and 3.97 Å apart, and the cushionshaped cavities are 6.64 Å [23]. The special pore structure makes ZIF-L a good candidate for MNPs immobilization. In this work, Pd@ZIF-L was synthesized via an assembly method that involves the successive adsorption of palladium nanoparticles onto the continuously forming surfaces of the growing ZIF-L crystals. The obtained Pd@ZIF-L was characterized by X-ray diffraction spectroscopy (XRD), inductively coupled plasma emission spectroscopy (ICP-AES), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and nitrogen sorption studies. Liquid-phase hydrogenation of alkenes with different molecular sizes namely 1-hexene (1.7 Å), cyclohexene (4.2 Å), cyclooctene (5.5 Å) and tetraphenylethylene (6.7 Å) were used to evaluate the molecular-size-selectivity of the as-prepared Pd@ZIF-L. 2. Experimental 2.1. Chemicals Zn(NO3)2$6H2O (99%) and 2-methylimidazole (2-MeIM) (99%) were purchased from SigmaeAldrich. Pd(OAc)2 was acquired from Sin-platinum Metals Co., Ltd., China. p-Nitrophenol (99.5%) was supplied by Aladdin. Hydrazine hydrate (N2H4$H2O) was obtained from Lingfeng Chemical Reagent Co., Ltd., China. KBH4 and polyvinylpyrrolidone (PVP) were supplied by Sinopharm Chemical Reagent Co., Ltd., China. NaOH was provided by Xilong Chemical Co., Ltd., Guangxi, China. Ethanol was supplied by Yasheng Chemical Co., Ltd., Wuxi, China. Deionized water (electrical conductivity <12 ms cm1) was produced in-house. All materials were used as received without further purification. 2.2. Preparation of Pd@ZIF-L The procedure for immobilization of palladium nanoparticles in ZIF-L is presented in Scheme 1. Firstly, Pd NPs colloid was prepared by a reduction method of Pd(OAc)2. Typically, 0.5 mM of Pd(OAc)2 and 2.83 mM of PVP were dissolved in 40 ml of methylene dichloride to yield the precursor solution. 11 ml of a reduction solution composed of hydrazine hydrate, potassium borohydride and sodium hydroxide with the molar ratio of 3:20:1 was added into the precursor solution under vigorous stirring, and the reaction was stirred at room temperature for 3 h. The obtained miscible liquid was washed with methylene dichloride for several times to remove the excess PVP in the solution, and then dispersed in 100 ml of deionized water to produce the Pd NPs colloid. Secondly, Pd@ZIF-L was prepared by an assembly method according to the literature [20]. The pre-
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Scheme 1. Schematic of the preparation of Pd@ZIF-L catalyst.
synthesized Pd NPs colloid (10 ml) was mixed with a solution of 2-MeIM in deionized water (100 ml, 50 mM) at room temperature. Afterward a solution of zinc nitrate hexahydrate in deionized water (100 ml, 2.5 mM) was added quickly, and the mixture was stirred briefly at room temperature for 20 s and left for 48 h without stirring. The resulting black powder was collected by centrifugation, washed by deionized water for several times, and dried at 80 C for 12 h to produce the ZIF-L immobilized palladium catalyst, denoted as Pd@ZIF-L-I. ZIF-L was prepared under the same conditions as the Pd@ZIF-L-I except for the addition of Pd NPs colloid. For comparison, the hybrid catalyst was synthesized under other conditions by changing the amount of Pd NPs colloid, the reaction time and the stirring time. Pd@ZIF-L-II was achieved with a reaction time of 24 h. The catalyst obtained by adding 15 ml of Pd NPs colloid is marked as dia(Zn)-I, while the one obtained by stirring the reaction solution for 48 h is denoted as dia(Zn)-II. 2.3. Characterization of Pd@ZIF-L X-ray diffraction (XRD) patterns were obtained in the 2q range of 5e50 on a Rigaku MiniFlex 600 diffractometer using Cu Ka radiation performed at 40 kV and 15 mA with a 2q step of 0.02 . The microscopic morphology of the catalysts was examined using a field emission scanning electron microscope (FESEM, Hitachi S4800II) and high resolution transmission electron microscopy (HRTEM, Jeol JEM-200CX). The HRTEM samples were prepared by sonication in ethanol for 10 min and then deposited onto carboncoated copper grids. The content of palladium was determined by inductively coupled plasma emission spectroscopy (ICP-AES, Optima 2000DV). The measurements were performed at the Pd standard (340.458 nm). For ICP analyses, the samples were digested in 10% (v/v) nitric acid solution at 60 C for 1 h. Thermal gravimetric analysis (TGA) measurements were performed on a NETZSCH STA 449 F3 Thermoanalyzer, and about 15 mg of each sample was used for the TGA analysis. Fourier-transform infrared (FT-IR) spectra were obtained using a Nicolet 8700 spectrometer with KBr pellets. Nitrogen physisorption isotherms were measured at 77 K on an automatic volumetric adsorption apparatus (Micromertics ASAP 2020). The samples were filled into glass ampoules and outgassed in high vacuum at 120 C for 12 h before the start of the sorption measurements. For sorption analyses, the samples were immersed in methanol at 60 C for 2 h. X-ray photoelectron spectrometry (XPS) was performed on Thermo ESCALAB 250 equipped with a
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monochromatized Al Ka radiation (hn ¼ 1486.6 eV). The C1s peak (284.8 eV) was used for the calibration of binding energy. 2.4. Catalytic hydrogenation of alkenes Liquid-phase hydrogenation of alkenes with different molecular sizes such as 1-hexene (1.7 Å), cyclohexene (4.2 Å), cyclooctene (5.5 Å) and tetraphenylethylene (6.7 Å) were carried out in ethyl acetate solution at a static hydrogen atmosphere (1 bar). In each experiment, 0.3 g of the catalyst was placed in a reactor, and residual air in the reactor was removed by flushing with 1 atm of hydrogen for several times [20,24,25]. Then, 15.0 ml of ethyl acetate and a certain amount of alkene were added to the reactor, and the alkene concentration was controlled as 16 mM of alkene/mg of Pd. After the reactor was again flushed one time with hydrogen, the reaction was carried out at 1 atm of hydrogen and 35 C for 48 h, and hydrogen was provided by a balloon. Similar reaction processes were also reported [26,27]. After the reaction, the resulting mixture was removed from the reactor and centrifuged. For the hydrogenation of 1-hexene, cyclohexene or cyclooctene, the filtrate was analyzed by a gas chromatograph (Shimadzu, GC-2014) equipped with a 30-m capillary column (Rtx-5, 30 m 0.25 mm 0.25 mm) with heptane as an internal standard. With respect to the hydrogenation of tetraphenylethylene, the filtrate was evaluated using a high-performance liquid chromatography (HPLC) system (Agilent 1200 Series, USA) equipped with a diode array detector (DAD) and an auto-sampler. For the reusability tests, the catalyst was separated from the reaction system by centrifugation and washed with ethyl acetate for several times, and then reused in the next run under the same reaction conditions. 3. Results and discussion 3.1. Physicalechemical properties of Pd@ZIF-L catalysts Fig. 1 shows the XRD patterns of ZIF-L and Pd@ZIF-L catalysts. The peak positions of the parent ZIF-L are in good agreement with our previous work [22]. The results in Fig. 1 indicate that the synthesis of Pd@ZIF-L is strongly dependent on the preparation conditions. The XRD pattern of Pd@ZIF-L-I (Fig. 1b) shows similar peaks of ZIF-L, demonstrating that the framework of ZIF-L is well maintained during the synthesis under the preparation conditions as presented in Section 2.2. According to the literature [28,29], the main characteristic diffraction peaks of Pd nanoparticles are at around 40 and 46 in the 2q range of 5e50 . However, no obvious
Fig. 1. XRD patterns of ZIF-L (a), Pd@ZIF-L-I (b), dia(Zn)-I (c), Pd@ZIF-L-II (d) and dia(Zn)-II (e).
characteristic Pd peaks are observed in the XRD pattern of Pd@ZIFL-I, presumably because of the low concentration (0.5 wt%) and/or small size (3 nm) of Pd nanoparticles as discussed in the following TEM characterization. In addition, some peaks between 15 and 20 and 25 and 30 miss by comparing Fig. 1a to b, which might be caused by the peak overlap and/or lower intensity. When more Pd NPs colloid (15 ml) was added into the synthesis solution, the product is a dense dia(Zn) structure (Fig. 1c) [30]. When the synthesis time was shorten (24 h), the framework of ZIF-L is maintained as indicated by the diffraction pattern of ZIF-L, but with lower intensities (Fig. 1d), suggesting that longer synthesis time is beneficial to obtain higher crystallinity. When the synthesis solution was continuously stirred for 48 h, a dense dia(Zn) structure is also obtained (Fig. 1e). These results indicate that the framework of ZIF-L is very sensitive to the synthesis environment and it is necessary to carefully control the preparation conditions. SEM was performed to analyze the morphology and particle size of ZIF-L and Pd@ZIF-L catalysts. It is interesting to note from Fig. 2a that the as-prepared ZIF-L crystals exhibit a unique crosshair-star shape (a shape formed by connecting opposing vertices of an octahedron) with a size of about 20 mm, significantly different from our previous work [22] where the ZIF-L crystals with a unique leaflike shape were obtained. The difference may be caused by the synthesis conditions. In this study, the molar ratio of 2-MeIM/zinc ion, a key synthesis parameter, was 20, while the molar ratio was 8 in our previous work [22]. The morphology of ZIF-L is strongly dependent on the molar ratio of 2-MeIM/zinc ion. Comparing Pd@ZIF-L-I (Fig. 2b) to ZIF-L (Fig. 2a), the Pd@ZIF-L-I crystals are larger and thicker but retain the crosshair-star morphology of ZIF-L. This result indicates that encapsulating PVP-stabilized Pd NPs during the synthesis of ZIF-L can promote the formation of ZIF-L crystals. In the present work, PVP was added during the synthesis of Pd NPs to form the stable Pd NPs colloid, and then the Pd NPs colloid was introduced into the synthesis solution of ZIF-L to prepare the Pd@ZIF-L catalysts. The coordination interactions between pyrrolidone rings (C]O) in PVP adsorbed onto the surfaces of Pd NPs and zinc atoms in ZIF nodes can enhance the adsorption of Zn2þ and 2-MeIM on the formed ZIF-L crystals [20], resulting in the formation of larger and thicker ZIF-L crystals. A high magnification FESEM image shown in Fig. 2c indicates that a Pd@ZIF-L-I particle is likely composed of many flakes similar to the staking of leaf-like ZIF-L to a certain extent. The dense dia(Zn) (Fig. 2d) on the other hand, shows a typical plate-like structure with a size of about 20 mm which is in agreement with that reported by Yao et al. [30]. The morphology and size of Pd@ZIF-L-II (Fig. 2e) are similar with those of the Pd@ZIF-L-I, but the Pd@ZIF-L-I crystals have more uniform morphology and higher crystallinity, which is consistent with the XRD result. The dia(Zn)-II crystals (Fig. 2f) also show platelike structure similar with dia(Zn)-I, however the particles are smaller, possibly due to continuous stirring which promotes the nucleation process and formation of smaller particles [31]. TEM was carried out for the size and morphology analyses of palladium nanoparticles in the colloid and ZIF-L. Fig. 3a shows that the palladium nanoparticles are well separated with no apparent aggregation, and the average particle size is about 3 nm, based on TEM observation (Fig. 3b). The TEM image of Pd@ZIF-L-I catalyst with a Pd loading of 0.5 wt% (Fig. 3c) indicates that the palladium nanoparticles are homogeneously encapsulated in the ZIF-L crystals, further verified by the XRD result that no large palladium particles are formed in the as-prepared Pd@ZIF-L-I (Fig. 1b). The HRTEM image of Pd@ZIF-L-I (insert in Fig. 3c) indicates the highly crystalline feature of the nanoparticles with a crystalline spacing of 2.2 Å, which agrees with the (110) lattice spacing of Pd [32]. Furthermore, the particle size of palladium nanoparticles confined in the ZIF-L framework is also about 3 nm (Fig. 3d), same as that of
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Fig. 2. SEM images of ZIF-L (a), Pd@ZIF-L-I (b, c), dia(Zn)-I (d), Pd@ZIF-L-II (e) and dia(Zn)-II (f).
Fig. 3. TEM images of palladium nanoparticle colloid (a) and Pd@ZIF-L-I (insert shows the enlarged image of a single Pd NP along the [110] zone axis orientation with a proximate size of 2.2 Å) (c). Size distributions of palladium nanoparticle in the colloid (b) and Pd@ZIF-L-I (d).
palladium nanoparticle colloid, suggesting that the synthesis process has no obvious influence on the particle size of palladium nanoparticles. TGA was performed to analyze the thermal stability of ZIF-L and Pd@ZIF-L catalysts. Pd@ZIF-L-I and ZIF-L show similar behavior during the TGA measurement, and both materials exhibit a gradual weight-loss to 260 C and then a steep weight-loss up to 285 C (Fig. 4a and b), corresponding to the remove of the weakly linked 2MeIM and guest water molecules [22]. A long plateau is shown in the temperature range of 285e550 C, indicating the high thermal stability of the two samples in the absence of guest molecules and unreacted species. After 550 C, the Pd@ZIF-L-I and ZIF-L crystals begin to decompose and transform to ZnO. Compared to ZIF-L,
small mass loss is observed for Pd@ZIF-L-I, due to its palladium nanoparticle counterpart. The TGA curve of dia(Zn)-I (Fig. 4c) shows a significantly different trend as compared to Pd@ZIF-L-I, and there is only a gradual weight-loss of 10% up to 550 C. According to the XRD characterization (Fig. 1c), dia(Zn)-I is a dense structure, and thus only little H2O can adsorb on its outer surfaces and be removed during the TGA analysis. FT-IR characterization was used to verify the integrity of the structure of ZIF-L and Pd@ZIF-L-I (Fig. 5). For both samples, the peak positions of the FT-IR spectra are almost the same. The absorption bands at 3133 and 2927 cm1 are for the stretching vibrations of CeH bonds in the imidazole ring and the methyl group, respectively. The peak at 1581 cm1 can be attributed to the C]N
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Fig. 4. Thermogravimetric analysis curves of ZIF-L (a), Pd@ZIF-L-I (b) and dia(Zn)-I (c).
Fig. 5. FT-IR spectra of ZIF-L (a) and Pd@ZIF-L-I (b).
stretch. The bands in the spectral region of 1350e1500 cm1 are associated with the entire ring stretching. The bands at 900e1350 cm1 are for the in-plane bending of the ring while those at 600e800 cm1 are assigned as the out-of-plane bending [33]. However, for Pd@ZIF-L-I, the intensities of the absorption bands decrease as compared to ZIF-L, which might be caused by the encapsulation of Pd nanoparticles inside the framework. Similar phenomenon was reported by Wang et al. [34]. Pd@ZIF-L-I exhibits a small band at approximately 1675 cm1, which can be assigned to the C]O stretching vibration of the incorporated PVP [32]. Microporosity of the parent ZIF-L and Pd@ZIF-L-I were demonstrated by sorption isotherm measurements performed at 77 K using liquid nitrogen, as shown in Fig. 6. Both sample exhibit type I isotherms. The increase in the amounts adsorbed at very low relative pressures is due to the presence of micropores; while a second uptake at high relative pressures indicates the existence of textural meso/macroporosity formed by particles packing. The BET surface area of ZIF-L is 118 m2/g, much lower than that of ZIF-8, because of the smaller pore size and higher density of ZIF-L [22]. After the incorporation of palladium nanoparticles, the BET surface area of Pd@ZIF-L-I decreases to 65 m2/g, which could be due to the contributions of non-porous palladium nanoparticles and PVP to the masses of the composite [20]. The surface composition of ZIF-L and Pd@ZIF-L-I were analyzed by XPS. C, N, O and Zn were detected in the as-synthesized ZIF-L (Fig. 7a). Element C, N and Zn were originated from ZIF-L while element O could be from the absorbed oxygen, water molecules, or
Fig. 6. Nitrogen sorption isotherms of ZIF-L (a) and Pd@ZIF-L-I (b). Solid symbols indicate gas adsorption and open symbols express gas desorption.
both [22]. Compared to ZIF-L, there is no obvious difference in the spectrum of Pd@ZIF-L-I, and a Pd signal cannot be identified in the XPS spectrum, possibly due to the lower loading of Pd or the encapsulation of Pd nanoparticles within the frameworks of ZIF-L. Vu et al. also did not observe the rhodium signal in the XPS spectrum of Rh@IRMOF-3 [35]. However, from the high resolution Pd 3d XPS spectrum (Fig. 7b), we can confirm the presence of Pd in the asprepared Pd@ZIF-L-I catalyst. Fig. 7b gives two prominent peaks at 335.03 and 340.32 eV that can be readily assigned to Pd(0) 3d5/2 and Pd 3d3/2. The peaks at 337.65 and 342.9 eV reveal the presence of Pd(II), which may due to the incomplete reduction of Pd (II) in the preparation of palladium nanoparticle colloid or the partial oxidation of metallic Pd to PdO species on the surface of Pd@ZIF-L-I [36]. Fig. 7c shows the high resolution Zn 2p XPS spectra of the parent ZIF-L and Pd@ZIF-L-I. The Zn 2p2/3 binding energy for ZIF-L is 1021.41 eV, which is 0.15 eV lower than that of Pd@ZIF-L-I, indicating a positive binding energy shift. However, a negative binding energy shift (about 0.19 eV) is observed for the O 1s spectrum of Pd@ZIF-L-I (Fig. 7d). The results suggest that the Pd loading has a severe impact on the appearance and location of the Zn 2p and O 1s XPS signals, respectively. The binding energy shifts of Zn 2p and O 1s might be due to the strong adsorption between the PVP and ZIF-L frameworks [20,37]. The formation process of Pd@ZIF-L is based on the successive adsorption of PVP-modified Pd nanoparticles on the continuously forming fresh surfaces of the growing ZIF-L crystals [20]. In the encapsulation process, the PVP adsorbed on the surfaces of Pd nanoparticle not only stabilizes the Pd nanoparticles in the reaction solution, but also provides the Pd nanoparticles with an enhanced affinity to ZIF-L crystals through weak coordination interactions between pyrrolidone rings (C]O) and zinc atoms in ZIF nodes, which might result in the electron transfer between Zn and O. 3.2. Catalytic properties of Pd@ZIF-L catalysts ZIF-L has a 2D layer network along the ab plane and these layers are then stacked along the c direction, and these layers are part of the sodalite (SOD) topology found in the 3D structure of ZIF-8 [22]. The unique pore structure may provide the ability of size-selective catalysis. As a proof of concept, liquid-phase hydrogenation of alkenes with different molecular sizes including 1-hexene, cyclohexene, cyclooctene and tetraphenylethylene were selected as the probe reactions to demonstrate the structural advantage of the asprepared Pd@ZIF-L catalysts in selective catalysis. The reasons for selecting these alkenes are as follows. The molecular sizes of the
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Fig. 7. (a) Survey scan XPS spectra of ZIF-L and Pd@ZIF-L-I, (b) high resolution Pd 3d XPS spectrum of Pd@ZIF-L-I, (c) high resolution Zn 2p XPS spectra of ZIF-L and Pd@ZIF-L-I, (d) high resolution O 1s XPS spectra of ZIF-L and Pd@ZIF-L-I.
selected alkenes are around the pore size of ZIF-L. In addition, these alkenes are often studied for the size-selective catalysis in the literature [20,38,39]. Thus, the obtained reaction performance in this work can be compared to those reported in the literature.
Fig. 8 shows the conversions of alkenes hydrogenation over the Pd@ZIF-L-I, and the results of ZIF-L are also presented for comparison. As expected, ZIF-L has no catalytic property and exhibits zero conversion in all the alkenes hydrogenation, but the Pd@ZIF-L-
Fig. 8. Catalytic performance of Pd@ZIF-L-I for liquid-phase hydrogenation of 1hexene, cyclohexene, cyclooctene and tetraphenylethylene. Pure ZIF-L was used as the control. Reaction conditions: catalyst concentration 20 g/L, alkene concentration 16 mM of alkene/mg of Pd, reaction temperature 35 C, reaction pressure 1 bar, reaction time 48 h.
Fig. 9. Recycling of Pd@ZIF-L-I for alkene hydrogenation. Reaction conditions were as shown in Fig. 8.
Scheme 2. Schematic illustration of the size-selective effect of Pd@ZIF-L.
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Fig. 10. (a) XRD patterns of fresh and recovered Pd@ZIF-L-I and (b) SEM image of recovered Pd@ZIF-L-I.
I can effectively convert the alkenes, indicating the successful incorporation of Pd active sites for the hydrogenation of alkenes [38]. Interestingly, the as-prepared Pd@ZIF-L-I catalyst displays different conversions for these alkenes under the same reaction conditions, and the conversion follows the order: 1hexene > cyclohexene > cyclooctene > tetraphenylethylene. The difference in the conversion can be caused by the molecular sieving function of ZIF-L, as illustrated in Scheme 2. As mentioned earlier, ZIF-L has unique cushion-shaped cavities between layers with a size of 6.64 Å [23]. Thus, the alkenes with kinetic diameter smaller than 6.64 Å can pass through the cavities of ZIF-L, reaching the Pd active sites and undergo reaction with hydrogen. The kinetic diameters of 1-hexene, cyclohexene and cyclooctene are 1.7, 4.2 and 5.5 Å, respectively. Therefore, the three alkenes all can be converted over the Pd@ZIF-L-I catalyst, and the conversion rate is consistent with the kinetic diameters of the alkenes. For example, after 48 h of reaction, Pd@ZIF-L-I converts 78% of 1-hexene. The observation of a lower 1-hexene conversion in comparison with the literature may be due to the lower Pd loading (0.5 wt%, analyzed by ICP-AES) [39]. The conversions decrease to 32.6% and 7.4% when it comes to the hydrogenation of cyclohexene and cyclooctene respectively during the same reaction period, presumably because of a lower diffusion rate through the ZIF-L layer [39]. It is worth noting that the conversion of cyclohexene or cyclooctene is higher than the reported results in spite of the lower 1-hexene conversion in the present work. Lin et al. found that the conversion of 1-hexene could reach 100% over the as-prepared Pd/SiO2@ZIF-8 catalyst, but only 7.5% for cyclohexene and no conversion for cyclooctene [38]. Such observations should be due to the topological difference between ZIF-L and ZIF-8. Compared to ZIF-8, ZIF-L has a larger aperture size that is beneficial for the diffusion of reactants, leading to higher conversions of cyclohexene and cyclooctene. These results indicate that ZIF-L as a support to load MNPs will be favorable to the conversion of reactants having larger molecular sizes in comparison with ZIF-8. Tetraphenylethylene has a larger molecular size (6.7 Å) than the pore size of ZIF-L (6.64 Å), and therefore they cannot enter freely the cavities of ZIF-L and access the catalytically active palladium, resulting in no activity in the tetraphenylethylene hydrogenation. The result also suggests that all the pre-synthesized Pd NPs can be efficiently confined within the ZIF-L framework, and not on the surface of ZIF-L. The molecular-size selectivity of ZIF-L was clearly demonstrated by the conversion of the alkenes of increasing size. The reusability of the Pd@ZIF-L-I as a catalyst for the hydrogenation of an equimolar two-component mixture (1-hexene and tetraphenylethylene) was investigated in terms of activity and sizeselectivity. The conversions of 1-hexene and tetraphenylethylene in the recycling experiments are shown in Fig. 9. The Pd@ZIF-L-I catalyst retains high catalytic activity, with an above 60%
conversion in the 1-hexene hydrogenation after three cycles. The slight drop in the activity of Pd@ZIF-L-I might be explained by the loss of catalyst during its after-treatments after the reaction. In addition, the difference of activity between cycle 1 and cycle 2 is higher than that between cycle 2 and cycle 3, which might be caused by the change of ZIF-L microstructure during the reaction cycles as discussed below. No activity in the hydrogenation of tetraphenylethylene was observed during the three cycles, indicating the stability of ZIF-L. However, the structure and morphology of Pd@ZIF-L-I catalyst cannot be well retained after three cycles (Fig. 10), possibly due to Ostwald ripening effect [40]. In addition, Low et al. found that ZIF-L could transform to ZIF-8 in organic solvents and its morphology also changed [23]. In the present work, the hydrogenation reaction over Pd@ZIF-L-I was performed with ethyl acetate as a solvent as presented in the Experimental Section, and thus the ZIF-L could transform to ZIF-8 during the reaction cycles, resulting in the changes of particle morphology and crystal structure of Pd@ZIF-L-I. Furthermore, negligible amount of Pd is leached according to the ICP-AES analysis of the filtered reaction solution. Thus, it can be concluded that the palladium nanoparticles can be retained within the ZIF-L framework during continuous hydrogenation cycles, and the as-prepared Pd@ZIF-L-I can act as a reusable catalyst in the hydrogenation of alkenes. 4. Conclusions In summary, we have demonstrated the successful preparation of ZIF-immobilized palladium nanoparticles with a recently discovered two-dimensional layered zeolitic imidazolate framework-L as the support. The microstructures of Pd@ZIF-L hybrid materials are very sensitive to the synthesis environmental and it is necessary to carefully control the preparation conditions. The as-prepared Pd@ZIF-L catalyst shows the highly size-selective catalytic performance in the hydrogenation of 1hexene, cyclohexene, cyclooctene and tetraphenylethylene due to the unique pore structure of ZIF-L. Our study provides new insights into the synthesis and applications of ZIF-immobilized MNPs. Acknowledgments Financial supports from the Jiangsu Natural Science Foundation for Distinguished Young Scholars (BK20150044), the National Natural Science Foundation (21306081, 21125629), the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (14KJB530004), the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201402, ZK201407), and the Technology Innovation Foundation for Science and Technology Enterprises in Jiangsu Province (BC2015008) of China are gratefully acknowledged.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Palladium nanoparticles supported on a two-dimensional layered zeolitic imidazolate framework-L as an efficient size-selective catalyst Songlin Xue a, Hong Jiang a, Zhaoxiang Zhong a, Ze-Xian Low b, Rizhi Chen a, *, Weihong Xing a, ** a b
State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, China Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 August 2015 Received in revised form 28 September 2015 Accepted 29 September 2015 Available online 8 October 2015
A new Pd@ZIF-L hybrid catalyst having a unique crosshair-star shape with a size of about 20 mm was synthesized, in which a two-dimensional layered zeolitic imidazolate framework-L (ZIF-L) was used as a support to immobilize palladium nanoparticles via an assembly method. The hybrid catalyst with unique cushion-shaped cavities between layers with a size of 6.64 Å can be used for the diffusion of specific reactants not achievable by other ZIF systems such as ZIF-8. The crystal structure and morphology of Pd@ZIF-L are strongly dependent on the synthesis conditions. Palladium nanoparticles with an average size of 3 nm are homogeneously encapsulated in the ZIF-L crystals. The as-synthesized Pd@ZIF-L catalyst favors the conversion of alkenes with larger molecular sizes, and exhibits excellent molecule-sizeselectivity and anti-leaching performance. © 2015 Elsevier Inc. All rights reserved.
Keywords: Layered zeolitic imidazolate frameworks Two-dimensional Pd nanoparticles Alkene hydrogenation Size-selective catalysis
1. Introduction Metal nanoparticles (MNPs) have been widely investigated for their potential applications in catalysis [1e3]. Due to the small grain size, a significant volume of the microstructure is composed of interfaces/grain boundaries and hence a large fraction of the atoms resides in grain boundaries, leading to more catalytic sites and enhanced catalytic performance [4]. Furthermore, recent advances in controlling the particle size and shape have opened up the possibility to optimize the particle geometry, providing the optimum size and surface properties for specific reactions [5]. Due to high surface energies and large surface areas, these MNPs are thermodynamically unstable, and therefore tend to aggregate during reactions, resulting in a significant decrease in the catalytic activity [6e8]. In this regard, solid materials with confined void spaces such as zeolites, metal organic frameworks (MOFs) and polymers are used as supports for MNPs immobilization. By utilizing the interaction or space limitations between the metal and the support, the agglomeration of MNPs can be prevented [9,10].
* Corresponding author. Tel.: þ86 25 83172286; fax: þ86 25 83172292. ** Corresponding author. Tel.: þ86 25 83172286; fax: þ86 25 83172292. E-mail addresses: [email protected] (R. Chen), [email protected] (W. Xing). http://dx.doi.org/10.1016/j.micromeso.2015.09.053 1387-1811/© 2015 Elsevier Inc. All rights reserved.
Zeolitic imidazolate frameworks (ZIFs), a sub-family of metalorganic frameworks (MOFs) [11,12], are highly ordered porous solids that consist of inorganic metal ions bridged by imidazolate ligands. Compared to other types of MOFs, ZIFs exhibit better thermal, hydrothermal and chemical stability [13]. Owing to their high porosity, large surface area and better stability, ZIFs provide great potential as hosts for various MNPs. Loading MNPs into the pores of ZIFs could limit the growth of MNPs within the confined cavities and also the migration and aggregation of MNPs during the reaction [14e16], thus increasing their catalytic activity and stability. There are two main strategies to obtain ZIF-immobilized MNPs d (1) the loading of ZIFs with molecular precursors followed by their subsequent decomposition inside the pores of ZIFs [17e19]. (2) The encapsulation of pre-synthesized MNPs inside the growing host frameworks [20]. The latter is more desirable as it avoids the formation of MNPs on the external surface and damaging the ZIF frameworks during the after-treatment processes which may occur in the former strategy. Selection of ZIFs as supports for MNPs immobilization is one of the most important steps because the physicalechemical properties of ZIFs such as the pore structure can significantly affect the corresponding catalytic performance of MNPs@ZIFs. Each ZIF has a definite window size, such as ZIF-8 has small-pore apertures of 3.4 Å, which allow reactants having smaller molecule sizes than the
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window size to easily diffuse into the pores to reach the active site or MNPs, thus showing outstanding molecular-size selectivity. For example, Pd@ZIF-8 showed high activity for ethylene hydrogenation, but no detectable activity was observed for cyclooctene hydrogenation [21]. These results were clearly due to the moleculesize-selective property of the ZIF-8, because the ethylene molecules (2.5 Å) are small enough to diffuse through the pore apertures of the ZIF-8 (3.4 Å), but not the cyclooctene molecules with a size of 5.5 Å. Recently, we have successfully synthesized a new twodimensional zeolitic imidazolate framework (ZIF-L) in zinc salt and 2-methylimidazole aqueous solution at room temperature [22]. ZIF-L, comprised of identical building blocks of ZIF-8, has twodimensional crystal lattices stacked layer-by-layer forming cushion-shaped cavities between layers. The two neighboring twodimensional sod layers in ZIF-L are bridged by hydrogen bonds. They are parallel to each other and 3.97 Å apart, and the cushionshaped cavities are 6.64 Å [23]. The special pore structure makes ZIF-L a good candidate for MNPs immobilization. In this work, Pd@ZIF-L was synthesized via an assembly method that involves the successive adsorption of palladium nanoparticles onto the continuously forming surfaces of the growing ZIF-L crystals. The obtained Pd@ZIF-L was characterized by X-ray diffraction spectroscopy (XRD), inductively coupled plasma emission spectroscopy (ICP-AES), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR) and nitrogen sorption studies. Liquid-phase hydrogenation of alkenes with different molecular sizes namely 1-hexene (1.7 Å), cyclohexene (4.2 Å), cyclooctene (5.5 Å) and tetraphenylethylene (6.7 Å) were used to evaluate the molecular-size-selectivity of the as-prepared Pd@ZIF-L. 2. Experimental 2.1. Chemicals Zn(NO3)2$6H2O (99%) and 2-methylimidazole (2-MeIM) (99%) were purchased from SigmaeAldrich. Pd(OAc)2 was acquired from Sin-platinum Metals Co., Ltd., China. p-Nitrophenol (99.5%) was supplied by Aladdin. Hydrazine hydrate (N2H4$H2O) was obtained from Lingfeng Chemical Reagent Co., Ltd., China. KBH4 and polyvinylpyrrolidone (PVP) were supplied by Sinopharm Chemical Reagent Co., Ltd., China. NaOH was provided by Xilong Chemical Co., Ltd., Guangxi, China. Ethanol was supplied by Yasheng Chemical Co., Ltd., Wuxi, China. Deionized water (electrical conductivity <12 ms cm1) was produced in-house. All materials were used as received without further purification. 2.2. Preparation of Pd@ZIF-L The procedure for immobilization of palladium nanoparticles in ZIF-L is presented in Scheme 1. Firstly, Pd NPs colloid was prepared by a reduction method of Pd(OAc)2. Typically, 0.5 mM of Pd(OAc)2 and 2.83 mM of PVP were dissolved in 40 ml of methylene dichloride to yield the precursor solution. 11 ml of a reduction solution composed of hydrazine hydrate, potassium borohydride and sodium hydroxide with the molar ratio of 3:20:1 was added into the precursor solution under vigorous stirring, and the reaction was stirred at room temperature for 3 h. The obtained miscible liquid was washed with methylene dichloride for several times to remove the excess PVP in the solution, and then dispersed in 100 ml of deionized water to produce the Pd NPs colloid. Secondly, Pd@ZIF-L was prepared by an assembly method according to the literature [20]. The pre-
221
Scheme 1. Schematic of the preparation of Pd@ZIF-L catalyst.
synthesized Pd NPs colloid (10 ml) was mixed with a solution of 2-MeIM in deionized water (100 ml, 50 mM) at room temperature. Afterward a solution of zinc nitrate hexahydrate in deionized water (100 ml, 2.5 mM) was added quickly, and the mixture was stirred briefly at room temperature for 20 s and left for 48 h without stirring. The resulting black powder was collected by centrifugation, washed by deionized water for several times, and dried at 80 C for 12 h to produce the ZIF-L immobilized palladium catalyst, denoted as Pd@ZIF-L-I. ZIF-L was prepared under the same conditions as the Pd@ZIF-L-I except for the addition of Pd NPs colloid. For comparison, the hybrid catalyst was synthesized under other conditions by changing the amount of Pd NPs colloid, the reaction time and the stirring time. Pd@ZIF-L-II was achieved with a reaction time of 24 h. The catalyst obtained by adding 15 ml of Pd NPs colloid is marked as dia(Zn)-I, while the one obtained by stirring the reaction solution for 48 h is denoted as dia(Zn)-II. 2.3. Characterization of Pd@ZIF-L X-ray diffraction (XRD) patterns were obtained in the 2q range of 5e50 on a Rigaku MiniFlex 600 diffractometer using Cu Ka radiation performed at 40 kV and 15 mA with a 2q step of 0.02 . The microscopic morphology of the catalysts was examined using a field emission scanning electron microscope (FESEM, Hitachi S4800II) and high resolution transmission electron microscopy (HRTEM, Jeol JEM-200CX). The HRTEM samples were prepared by sonication in ethanol for 10 min and then deposited onto carboncoated copper grids. The content of palladium was determined by inductively coupled plasma emission spectroscopy (ICP-AES, Optima 2000DV). The measurements were performed at the Pd standard (340.458 nm). For ICP analyses, the samples were digested in 10% (v/v) nitric acid solution at 60 C for 1 h. Thermal gravimetric analysis (TGA) measurements were performed on a NETZSCH STA 449 F3 Thermoanalyzer, and about 15 mg of each sample was used for the TGA analysis. Fourier-transform infrared (FT-IR) spectra were obtained using a Nicolet 8700 spectrometer with KBr pellets. Nitrogen physisorption isotherms were measured at 77 K on an automatic volumetric adsorption apparatus (Micromertics ASAP 2020). The samples were filled into glass ampoules and outgassed in high vacuum at 120 C for 12 h before the start of the sorption measurements. For sorption analyses, the samples were immersed in methanol at 60 C for 2 h. X-ray photoelectron spectrometry (XPS) was performed on Thermo ESCALAB 250 equipped with a
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monochromatized Al Ka radiation (hn ¼ 1486.6 eV). The C1s peak (284.8 eV) was used for the calibration of binding energy. 2.4. Catalytic hydrogenation of alkenes Liquid-phase hydrogenation of alkenes with different molecular sizes such as 1-hexene (1.7 Å), cyclohexene (4.2 Å), cyclooctene (5.5 Å) and tetraphenylethylene (6.7 Å) were carried out in ethyl acetate solution at a static hydrogen atmosphere (1 bar). In each experiment, 0.3 g of the catalyst was placed in a reactor, and residual air in the reactor was removed by flushing with 1 atm of hydrogen for several times [20,24,25]. Then, 15.0 ml of ethyl acetate and a certain amount of alkene were added to the reactor, and the alkene concentration was controlled as 16 mM of alkene/mg of Pd. After the reactor was again flushed one time with hydrogen, the reaction was carried out at 1 atm of hydrogen and 35 C for 48 h, and hydrogen was provided by a balloon. Similar reaction processes were also reported [26,27]. After the reaction, the resulting mixture was removed from the reactor and centrifuged. For the hydrogenation of 1-hexene, cyclohexene or cyclooctene, the filtrate was analyzed by a gas chromatograph (Shimadzu, GC-2014) equipped with a 30-m capillary column (Rtx-5, 30 m 0.25 mm 0.25 mm) with heptane as an internal standard. With respect to the hydrogenation of tetraphenylethylene, the filtrate was evaluated using a high-performance liquid chromatography (HPLC) system (Agilent 1200 Series, USA) equipped with a diode array detector (DAD) and an auto-sampler. For the reusability tests, the catalyst was separated from the reaction system by centrifugation and washed with ethyl acetate for several times, and then reused in the next run under the same reaction conditions. 3. Results and discussion 3.1. Physicalechemical properties of Pd@ZIF-L catalysts Fig. 1 shows the XRD patterns of ZIF-L and Pd@ZIF-L catalysts. The peak positions of the parent ZIF-L are in good agreement with our previous work [22]. The results in Fig. 1 indicate that the synthesis of Pd@ZIF-L is strongly dependent on the preparation conditions. The XRD pattern of Pd@ZIF-L-I (Fig. 1b) shows similar peaks of ZIF-L, demonstrating that the framework of ZIF-L is well maintained during the synthesis under the preparation conditions as presented in Section 2.2. According to the literature [28,29], the main characteristic diffraction peaks of Pd nanoparticles are at around 40 and 46 in the 2q range of 5e50 . However, no obvious
Fig. 1. XRD patterns of ZIF-L (a), Pd@ZIF-L-I (b), dia(Zn)-I (c), Pd@ZIF-L-II (d) and dia(Zn)-II (e).
characteristic Pd peaks are observed in the XRD pattern of Pd@ZIFL-I, presumably because of the low concentration (0.5 wt%) and/or small size (3 nm) of Pd nanoparticles as discussed in the following TEM characterization. In addition, some peaks between 15 and 20 and 25 and 30 miss by comparing Fig. 1a to b, which might be caused by the peak overlap and/or lower intensity. When more Pd NPs colloid (15 ml) was added into the synthesis solution, the product is a dense dia(Zn) structure (Fig. 1c) [30]. When the synthesis time was shorten (24 h), the framework of ZIF-L is maintained as indicated by the diffraction pattern of ZIF-L, but with lower intensities (Fig. 1d), suggesting that longer synthesis time is beneficial to obtain higher crystallinity. When the synthesis solution was continuously stirred for 48 h, a dense dia(Zn) structure is also obtained (Fig. 1e). These results indicate that the framework of ZIF-L is very sensitive to the synthesis environment and it is necessary to carefully control the preparation conditions. SEM was performed to analyze the morphology and particle size of ZIF-L and Pd@ZIF-L catalysts. It is interesting to note from Fig. 2a that the as-prepared ZIF-L crystals exhibit a unique crosshair-star shape (a shape formed by connecting opposing vertices of an octahedron) with a size of about 20 mm, significantly different from our previous work [22] where the ZIF-L crystals with a unique leaflike shape were obtained. The difference may be caused by the synthesis conditions. In this study, the molar ratio of 2-MeIM/zinc ion, a key synthesis parameter, was 20, while the molar ratio was 8 in our previous work [22]. The morphology of ZIF-L is strongly dependent on the molar ratio of 2-MeIM/zinc ion. Comparing Pd@ZIF-L-I (Fig. 2b) to ZIF-L (Fig. 2a), the Pd@ZIF-L-I crystals are larger and thicker but retain the crosshair-star morphology of ZIF-L. This result indicates that encapsulating PVP-stabilized Pd NPs during the synthesis of ZIF-L can promote the formation of ZIF-L crystals. In the present work, PVP was added during the synthesis of Pd NPs to form the stable Pd NPs colloid, and then the Pd NPs colloid was introduced into the synthesis solution of ZIF-L to prepare the Pd@ZIF-L catalysts. The coordination interactions between pyrrolidone rings (C]O) in PVP adsorbed onto the surfaces of Pd NPs and zinc atoms in ZIF nodes can enhance the adsorption of Zn2þ and 2-MeIM on the formed ZIF-L crystals [20], resulting in the formation of larger and thicker ZIF-L crystals. A high magnification FESEM image shown in Fig. 2c indicates that a Pd@ZIF-L-I particle is likely composed of many flakes similar to the staking of leaf-like ZIF-L to a certain extent. The dense dia(Zn) (Fig. 2d) on the other hand, shows a typical plate-like structure with a size of about 20 mm which is in agreement with that reported by Yao et al. [30]. The morphology and size of Pd@ZIF-L-II (Fig. 2e) are similar with those of the Pd@ZIF-L-I, but the Pd@ZIF-L-I crystals have more uniform morphology and higher crystallinity, which is consistent with the XRD result. The dia(Zn)-II crystals (Fig. 2f) also show platelike structure similar with dia(Zn)-I, however the particles are smaller, possibly due to continuous stirring which promotes the nucleation process and formation of smaller particles [31]. TEM was carried out for the size and morphology analyses of palladium nanoparticles in the colloid and ZIF-L. Fig. 3a shows that the palladium nanoparticles are well separated with no apparent aggregation, and the average particle size is about 3 nm, based on TEM observation (Fig. 3b). The TEM image of Pd@ZIF-L-I catalyst with a Pd loading of 0.5 wt% (Fig. 3c) indicates that the palladium nanoparticles are homogeneously encapsulated in the ZIF-L crystals, further verified by the XRD result that no large palladium particles are formed in the as-prepared Pd@ZIF-L-I (Fig. 1b). The HRTEM image of Pd@ZIF-L-I (insert in Fig. 3c) indicates the highly crystalline feature of the nanoparticles with a crystalline spacing of 2.2 Å, which agrees with the (110) lattice spacing of Pd [32]. Furthermore, the particle size of palladium nanoparticles confined in the ZIF-L framework is also about 3 nm (Fig. 3d), same as that of
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Fig. 2. SEM images of ZIF-L (a), Pd@ZIF-L-I (b, c), dia(Zn)-I (d), Pd@ZIF-L-II (e) and dia(Zn)-II (f).
Fig. 3. TEM images of palladium nanoparticle colloid (a) and Pd@ZIF-L-I (insert shows the enlarged image of a single Pd NP along the [110] zone axis orientation with a proximate size of 2.2 Å) (c). Size distributions of palladium nanoparticle in the colloid (b) and Pd@ZIF-L-I (d).
palladium nanoparticle colloid, suggesting that the synthesis process has no obvious influence on the particle size of palladium nanoparticles. TGA was performed to analyze the thermal stability of ZIF-L and Pd@ZIF-L catalysts. Pd@ZIF-L-I and ZIF-L show similar behavior during the TGA measurement, and both materials exhibit a gradual weight-loss to 260 C and then a steep weight-loss up to 285 C (Fig. 4a and b), corresponding to the remove of the weakly linked 2MeIM and guest water molecules [22]. A long plateau is shown in the temperature range of 285e550 C, indicating the high thermal stability of the two samples in the absence of guest molecules and unreacted species. After 550 C, the Pd@ZIF-L-I and ZIF-L crystals begin to decompose and transform to ZnO. Compared to ZIF-L,
small mass loss is observed for Pd@ZIF-L-I, due to its palladium nanoparticle counterpart. The TGA curve of dia(Zn)-I (Fig. 4c) shows a significantly different trend as compared to Pd@ZIF-L-I, and there is only a gradual weight-loss of 10% up to 550 C. According to the XRD characterization (Fig. 1c), dia(Zn)-I is a dense structure, and thus only little H2O can adsorb on its outer surfaces and be removed during the TGA analysis. FT-IR characterization was used to verify the integrity of the structure of ZIF-L and Pd@ZIF-L-I (Fig. 5). For both samples, the peak positions of the FT-IR spectra are almost the same. The absorption bands at 3133 and 2927 cm1 are for the stretching vibrations of CeH bonds in the imidazole ring and the methyl group, respectively. The peak at 1581 cm1 can be attributed to the C]N
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Fig. 4. Thermogravimetric analysis curves of ZIF-L (a), Pd@ZIF-L-I (b) and dia(Zn)-I (c).
Fig. 5. FT-IR spectra of ZIF-L (a) and Pd@ZIF-L-I (b).
stretch. The bands in the spectral region of 1350e1500 cm1 are associated with the entire ring stretching. The bands at 900e1350 cm1 are for the in-plane bending of the ring while those at 600e800 cm1 are assigned as the out-of-plane bending [33]. However, for Pd@ZIF-L-I, the intensities of the absorption bands decrease as compared to ZIF-L, which might be caused by the encapsulation of Pd nanoparticles inside the framework. Similar phenomenon was reported by Wang et al. [34]. Pd@ZIF-L-I exhibits a small band at approximately 1675 cm1, which can be assigned to the C]O stretching vibration of the incorporated PVP [32]. Microporosity of the parent ZIF-L and Pd@ZIF-L-I were demonstrated by sorption isotherm measurements performed at 77 K using liquid nitrogen, as shown in Fig. 6. Both sample exhibit type I isotherms. The increase in the amounts adsorbed at very low relative pressures is due to the presence of micropores; while a second uptake at high relative pressures indicates the existence of textural meso/macroporosity formed by particles packing. The BET surface area of ZIF-L is 118 m2/g, much lower than that of ZIF-8, because of the smaller pore size and higher density of ZIF-L [22]. After the incorporation of palladium nanoparticles, the BET surface area of Pd@ZIF-L-I decreases to 65 m2/g, which could be due to the contributions of non-porous palladium nanoparticles and PVP to the masses of the composite [20]. The surface composition of ZIF-L and Pd@ZIF-L-I were analyzed by XPS. C, N, O and Zn were detected in the as-synthesized ZIF-L (Fig. 7a). Element C, N and Zn were originated from ZIF-L while element O could be from the absorbed oxygen, water molecules, or
Fig. 6. Nitrogen sorption isotherms of ZIF-L (a) and Pd@ZIF-L-I (b). Solid symbols indicate gas adsorption and open symbols express gas desorption.
both [22]. Compared to ZIF-L, there is no obvious difference in the spectrum of Pd@ZIF-L-I, and a Pd signal cannot be identified in the XPS spectrum, possibly due to the lower loading of Pd or the encapsulation of Pd nanoparticles within the frameworks of ZIF-L. Vu et al. also did not observe the rhodium signal in the XPS spectrum of Rh@IRMOF-3 [35]. However, from the high resolution Pd 3d XPS spectrum (Fig. 7b), we can confirm the presence of Pd in the asprepared Pd@ZIF-L-I catalyst. Fig. 7b gives two prominent peaks at 335.03 and 340.32 eV that can be readily assigned to Pd(0) 3d5/2 and Pd 3d3/2. The peaks at 337.65 and 342.9 eV reveal the presence of Pd(II), which may due to the incomplete reduction of Pd (II) in the preparation of palladium nanoparticle colloid or the partial oxidation of metallic Pd to PdO species on the surface of Pd@ZIF-L-I [36]. Fig. 7c shows the high resolution Zn 2p XPS spectra of the parent ZIF-L and Pd@ZIF-L-I. The Zn 2p2/3 binding energy for ZIF-L is 1021.41 eV, which is 0.15 eV lower than that of Pd@ZIF-L-I, indicating a positive binding energy shift. However, a negative binding energy shift (about 0.19 eV) is observed for the O 1s spectrum of Pd@ZIF-L-I (Fig. 7d). The results suggest that the Pd loading has a severe impact on the appearance and location of the Zn 2p and O 1s XPS signals, respectively. The binding energy shifts of Zn 2p and O 1s might be due to the strong adsorption between the PVP and ZIF-L frameworks [20,37]. The formation process of Pd@ZIF-L is based on the successive adsorption of PVP-modified Pd nanoparticles on the continuously forming fresh surfaces of the growing ZIF-L crystals [20]. In the encapsulation process, the PVP adsorbed on the surfaces of Pd nanoparticle not only stabilizes the Pd nanoparticles in the reaction solution, but also provides the Pd nanoparticles with an enhanced affinity to ZIF-L crystals through weak coordination interactions between pyrrolidone rings (C]O) and zinc atoms in ZIF nodes, which might result in the electron transfer between Zn and O. 3.2. Catalytic properties of Pd@ZIF-L catalysts ZIF-L has a 2D layer network along the ab plane and these layers are then stacked along the c direction, and these layers are part of the sodalite (SOD) topology found in the 3D structure of ZIF-8 [22]. The unique pore structure may provide the ability of size-selective catalysis. As a proof of concept, liquid-phase hydrogenation of alkenes with different molecular sizes including 1-hexene, cyclohexene, cyclooctene and tetraphenylethylene were selected as the probe reactions to demonstrate the structural advantage of the asprepared Pd@ZIF-L catalysts in selective catalysis. The reasons for selecting these alkenes are as follows. The molecular sizes of the
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Fig. 7. (a) Survey scan XPS spectra of ZIF-L and Pd@ZIF-L-I, (b) high resolution Pd 3d XPS spectrum of Pd@ZIF-L-I, (c) high resolution Zn 2p XPS spectra of ZIF-L and Pd@ZIF-L-I, (d) high resolution O 1s XPS spectra of ZIF-L and Pd@ZIF-L-I.
selected alkenes are around the pore size of ZIF-L. In addition, these alkenes are often studied for the size-selective catalysis in the literature [20,38,39]. Thus, the obtained reaction performance in this work can be compared to those reported in the literature.
Fig. 8 shows the conversions of alkenes hydrogenation over the Pd@ZIF-L-I, and the results of ZIF-L are also presented for comparison. As expected, ZIF-L has no catalytic property and exhibits zero conversion in all the alkenes hydrogenation, but the Pd@ZIF-L-
Fig. 8. Catalytic performance of Pd@ZIF-L-I for liquid-phase hydrogenation of 1hexene, cyclohexene, cyclooctene and tetraphenylethylene. Pure ZIF-L was used as the control. Reaction conditions: catalyst concentration 20 g/L, alkene concentration 16 mM of alkene/mg of Pd, reaction temperature 35 C, reaction pressure 1 bar, reaction time 48 h.
Fig. 9. Recycling of Pd@ZIF-L-I for alkene hydrogenation. Reaction conditions were as shown in Fig. 8.
Scheme 2. Schematic illustration of the size-selective effect of Pd@ZIF-L.
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Fig. 10. (a) XRD patterns of fresh and recovered Pd@ZIF-L-I and (b) SEM image of recovered Pd@ZIF-L-I.
I can effectively convert the alkenes, indicating the successful incorporation of Pd active sites for the hydrogenation of alkenes [38]. Interestingly, the as-prepared Pd@ZIF-L-I catalyst displays different conversions for these alkenes under the same reaction conditions, and the conversion follows the order: 1hexene > cyclohexene > cyclooctene > tetraphenylethylene. The difference in the conversion can be caused by the molecular sieving function of ZIF-L, as illustrated in Scheme 2. As mentioned earlier, ZIF-L has unique cushion-shaped cavities between layers with a size of 6.64 Å [23]. Thus, the alkenes with kinetic diameter smaller than 6.64 Å can pass through the cavities of ZIF-L, reaching the Pd active sites and undergo reaction with hydrogen. The kinetic diameters of 1-hexene, cyclohexene and cyclooctene are 1.7, 4.2 and 5.5 Å, respectively. Therefore, the three alkenes all can be converted over the Pd@ZIF-L-I catalyst, and the conversion rate is consistent with the kinetic diameters of the alkenes. For example, after 48 h of reaction, Pd@ZIF-L-I converts 78% of 1-hexene. The observation of a lower 1-hexene conversion in comparison with the literature may be due to the lower Pd loading (0.5 wt%, analyzed by ICP-AES) [39]. The conversions decrease to 32.6% and 7.4% when it comes to the hydrogenation of cyclohexene and cyclooctene respectively during the same reaction period, presumably because of a lower diffusion rate through the ZIF-L layer [39]. It is worth noting that the conversion of cyclohexene or cyclooctene is higher than the reported results in spite of the lower 1-hexene conversion in the present work. Lin et al. found that the conversion of 1-hexene could reach 100% over the as-prepared Pd/SiO2@ZIF-8 catalyst, but only 7.5% for cyclohexene and no conversion for cyclooctene [38]. Such observations should be due to the topological difference between ZIF-L and ZIF-8. Compared to ZIF-8, ZIF-L has a larger aperture size that is beneficial for the diffusion of reactants, leading to higher conversions of cyclohexene and cyclooctene. These results indicate that ZIF-L as a support to load MNPs will be favorable to the conversion of reactants having larger molecular sizes in comparison with ZIF-8. Tetraphenylethylene has a larger molecular size (6.7 Å) than the pore size of ZIF-L (6.64 Å), and therefore they cannot enter freely the cavities of ZIF-L and access the catalytically active palladium, resulting in no activity in the tetraphenylethylene hydrogenation. The result also suggests that all the pre-synthesized Pd NPs can be efficiently confined within the ZIF-L framework, and not on the surface of ZIF-L. The molecular-size selectivity of ZIF-L was clearly demonstrated by the conversion of the alkenes of increasing size. The reusability of the Pd@ZIF-L-I as a catalyst for the hydrogenation of an equimolar two-component mixture (1-hexene and tetraphenylethylene) was investigated in terms of activity and sizeselectivity. The conversions of 1-hexene and tetraphenylethylene in the recycling experiments are shown in Fig. 9. The Pd@ZIF-L-I catalyst retains high catalytic activity, with an above 60%
conversion in the 1-hexene hydrogenation after three cycles. The slight drop in the activity of Pd@ZIF-L-I might be explained by the loss of catalyst during its after-treatments after the reaction. In addition, the difference of activity between cycle 1 and cycle 2 is higher than that between cycle 2 and cycle 3, which might be caused by the change of ZIF-L microstructure during the reaction cycles as discussed below. No activity in the hydrogenation of tetraphenylethylene was observed during the three cycles, indicating the stability of ZIF-L. However, the structure and morphology of Pd@ZIF-L-I catalyst cannot be well retained after three cycles (Fig. 10), possibly due to Ostwald ripening effect [40]. In addition, Low et al. found that ZIF-L could transform to ZIF-8 in organic solvents and its morphology also changed [23]. In the present work, the hydrogenation reaction over Pd@ZIF-L-I was performed with ethyl acetate as a solvent as presented in the Experimental Section, and thus the ZIF-L could transform to ZIF-8 during the reaction cycles, resulting in the changes of particle morphology and crystal structure of Pd@ZIF-L-I. Furthermore, negligible amount of Pd is leached according to the ICP-AES analysis of the filtered reaction solution. Thus, it can be concluded that the palladium nanoparticles can be retained within the ZIF-L framework during continuous hydrogenation cycles, and the as-prepared Pd@ZIF-L-I can act as a reusable catalyst in the hydrogenation of alkenes. 4. Conclusions In summary, we have demonstrated the successful preparation of ZIF-immobilized palladium nanoparticles with a recently discovered two-dimensional layered zeolitic imidazolate framework-L as the support. The microstructures of Pd@ZIF-L hybrid materials are very sensitive to the synthesis environmental and it is necessary to carefully control the preparation conditions. The as-prepared Pd@ZIF-L catalyst shows the highly size-selective catalytic performance in the hydrogenation of 1hexene, cyclohexene, cyclooctene and tetraphenylethylene due to the unique pore structure of ZIF-L. Our study provides new insights into the synthesis and applications of ZIF-immobilized MNPs. Acknowledgments Financial supports from the Jiangsu Natural Science Foundation for Distinguished Young Scholars (BK20150044), the National Natural Science Foundation (21306081, 21125629), the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (14KJB530004), the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201402, ZK201407), and the Technology Innovation Foundation for Science and Technology Enterprises in Jiangsu Province (BC2015008) of China are gratefully acknowledged.
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