Controllable synthesis of Pd@ZIF-L catalysts by an assembly method

Abstract

The Pd@ZIF-L catalysts with uniform crosshair-star shape and a size of about 20 μm were fabricated by an assembly method that enables the crystallization of ZIF-L in water with the presence of polyvinylpyrrolidone (PVP)-stabilized Pd nanoparticles (NPs). The physical and catalytic properties of the Pd@ZIF-L catalysts are greatly affected by the molar ratios of 2-methylimidazole (2-MeIM)/Zn²⁺ and PVP/Zn²⁺. Their catalytic activities were evaluated by the catalytic reduction of p-nitrophenol to p-aminophenol. The results indicate that a lower molar ratio of 2-MeIM/Zn²⁺ is beneficial for the increase of Pd loading and p-nitrophenol conversion, but a suitable molar ratio (≥20) of 2-MeIM/Zn²⁺ is helpful to the crystallization of ZIF-L and the encapsulation of the Pd NPs within the ZIF-L framework. The Pd@ZIF-L catalyst synthesized at a lower PVP/Zn²⁺ molar ratio has higher Pd loading and p-nitrophenol conversion due to the easy adsorption of PVP–Pd NPs onto the ZIF-L surfaces. This work would aid the development of high-performance Pd@ZIF-L catalysts.

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1. Introduction

Palladium nanoparticles (Pd NPs) play a significant role in a variety of catalytic reactions, e.g., hydrogenation, dehydrogenation and C–C coupling reactions.^1–4 To achieve the ease in catalyst separation and recovery, Pd NPs are often immobilized on solid materials to synthesize the supported catalysts.^5–7 To date, a lot of supports, e.g., carbon,^8–10 membranes,¹¹ silica,^12,13 hydroxyapatite,¹⁴ zeolites^15,16 and organic polymers,^17,18 are available for Pd NP immobilization. The as-synthesized Pd catalysts have often exhibited excellent catalytic properties in some catalytic reactions. For example, significant catalytic activity of the Pd NPs supported on nano-zeolite was found in the synthesis of aryl alkynes and diaryl ethers with high yields.¹⁹ However, some challenges are still met in the synthesis of supported Pd catalysts. The morphology, size and Pd distribution should be carefully controlled. Restraining the leaching of Pd NPs and avoiding the aggregation of Pd NPs during the reactions are also very challengeable.²⁰

Zeolitic imidazolate frameworks (ZIFs) are the sub-family of metal–organic frameworks (MOFs), which have zeolitic porous structures with hybrid frameworks. In these frameworks, the inorganic metal ions are coordinated with organic imidazolate ligands.^21,22 ZIFs have better thermal, hydrothermal and chemical stability than other types of MOFs.²³ Owing to their high porosity, large surface area and better stability, great potential is found for ZIFs to be the supports for various metal nanoparticles (MNPs). Loading MNPs into the ZIFs pores could limit the MNPs growth within the confined cavities, and the migration and aggregation of MNPs are also weakened during the reactions.^24,25 Since the excellent catalytic properties are presented by the ZIFs-supported catalysts, the synthesis of these catalysts has attracted much more attention nowadays.

Two main strategies are reported to obtain the ZIF-supported MNPs. (1) The loading of ZIFs with molecular precursors followed by their subsequent decomposition inside the pores of ZIFs.^26–28 (2) An assembly method, i.e., the encapsulation of pre-synthesized MNPs inside of the growing ZIF matrixes where the MNPs are surrounded.²⁹ The assembly method is very desirable, since it avoids the formation of MNPs on the external surface and the ZIF frameworks are protected during the after-treatment processes. However, parts of MNPs can deposit on the external surface of ZIFs and the ZIF frameworks are readily damaged in the first strategy. The pre-synthesized MNPs are usually stabilized with certain surfactants, capping agents or even ions during their synthesis. The surfactants such as PVP can control the morphology of MNPs and limit the MNPs aggregation. More importantly, it is very beneficial for the synthesis of ZIFs to use the surfactants, since the weak coordination interactions would provide an enhanced affinity between pyrrolidone rings (C=O) in PVP and zinc atoms in ZIF nodes.²⁹ Meanwhile, the crystallization process of ZIFs is strongly dependent on the molar ratio of precursors,^30,31 which would affect the microstructure and the corresponding catalytic performance of MNPs@ZIF composites. Thus, the surfactants and the molar ratio of precursors are key parameters for the synthesis of MNPs@ZIF composites by the assembly method. However, to the best of our knowledge, there are few reports on the influence of the above parameters on the textural properties of MNPs@ZIF composites and their catalytic performance.

In this work, Pd@ZIF-L catalysts were fabricated by an assembly method that involves the successive adsorption of Pd NPs onto the surfaces of the growing ZIF-L crystals.²⁹ The ZIF-L was selected to demonstrate our study considering the following facts. ZIF-L is made up of the same building blocks as ZIF-8, and has two-dimensional crystal lattices stacked layer-by-layer. There are cushion-shaped cavities with a size of 6.64 Å between the layers.^32,33 The special pore structure makes ZIF-L a good candidate for the immobilizing the Pd NPs. In our previous work,³⁴ a Pd@ZIF-L catalyst was synthesized by an assembly method and its molecular-size-selectivity in liquid-phase hydrogenation of alkenes with different molecular sizes was evaluated. The aim of the present work was to achieve the controllable synthesis of Pd@ZIF-L by investigating the effects of the molar ratios of 2-MeIM/Zn²⁺ and PVP/Zn²⁺ on the physical properties and catalytic activities of the Pd@ZIF-L catalysts. The structure and morphology of these Pd@ZIF-L catalysts were characterized in detail by X-ray diffraction (XRD), Field-emission Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM) and Inductively Coupled Plasma Emission Spectroscopy (ICP-AES), and their corresponding catalytic activities were tested in the catalytic reduction of p-nitrophenol to p-aminophenol.

2. Experimental

2.1. Chemicals

Zn(NO₃)₂·6H₂O (≥99%) and 2-methylimidazole (2-MeIM) (≥99%) were purchased from Sigma-Aldrich. Pd(OAc)₂ (Pd content ≥ 47.0%) was acquired from Sino-platinum Metals Co., Ltd., China. Hydrazine hydrate (N₂H₄·H₂O) was obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd., China. KBH₄ and polyvinylpyrrolidone (PVP) were supplied by Sinopharm Chemical Reagent Co., Ltd., China. NaOH was provided by Guangxi Xilong Chemical Co., Ltd., China. Methanol (chromatography grade) was acquired by Shandong Yuwang Group Co., Ltd., China. p-Nitrophenol and p-aminophenol were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd., China. Pure water was provided by Hangzhou Wahaha Group Co., Ltd., China. Deionized water (electrical conductivity < 12 μs cm⁻¹) was homemade. All materials were used as received without further purification.

2.2. Preparation of Pd@ZIF-L catalysts

Pd@ZIF-L catalysts were synthesized via an assembly method, as presented in Scheme 1, according to the reported procedure³⁴ that involves the pre-synthesis of PVP-stabilized Pd NPs and the construction of ZIF-L crystals around the Pd NPs.

Scheme 1 Schematic illustration of the preparation of Pd@ZIF-L by an assembly method.

Preparation of PVP-stabilized Pd NPs colloid. The Pd NPs colloid was prepared by the reduction of Pd(OAc)₂. Typically, 0.5 mmol of Pd(OAc)₂ and a certain amount of PVP were separately dissolved in 20 ml of methylene dichloride, and then the PVP solution was added into the Pd(OAc)₂ solution under stirring to yield the precursor solution. 11 ml of a reduction solution composed of hydrazine hydrate, potassium borohydride and sodium hydroxide with a molar ratio of 3 : 20 : 1 was added into the precursor solution and the mixture was stirred vigorously at 30 °C for 4 h. The obtained miscible liquid was washed with methylene dichloride for three times, and then dispersed in 100 ml of deionized water to produce the colloid of PVP-stabilized Pd NPs (PVP–Pd).

Preparation of Pd@ZIF-L. Typically, 10 ml of PVP–Pd colloid was mixed with 100 ml of 2-MeIM aqueous solution. Subsequently, an aqueous solution of zinc nitrate hexahydrate (100 ml, 2.5 mM) was added to the above solution under stirring for 1 min. Then the mixture was allowed to react at 30 °C for 48 h without stirring. The product was collected by centrifugation and washed by deionized water for two times, and then dried in an oven at 80 °C overnight.

2.3. Characterization of Pd@ZIF-L catalysts

The X-ray diffraction (XRD) patterns were recorded on a Rigaku MiniFlex 600 diffractometer using Cu Kα radiation performed at 40 kV and 15 mA in the 2θ range of 5–50° at a scan rate of 0.02° s⁻¹. The field emission scanning electron microscopy (FESEM) images were taken using a Hitachi S-4800 microscope. The transmission electron microscopy (TEM) images were taken using a JEOL JEM 2100 microscope. The inductively coupled plasma emission spectroscopy (ICP-AES) was obtained on a Perkin-Elmer Optima 2000DV optical emission spectrometer to measure the content of Pd in Pd@ZIF-L. The samples were digested efficiently using 10% (v/v) diluted nitric acid solution at 60 °C for 1 h before ICP analyses. Thermal gravimetric analysis (TGA) measurements were performed on a NETZSCH STA 449 F3 Thermoanalyzer.

2.4. Catalytic evaluation of Pd@ZIF-L catalysts

The catalytic reduction of p-nitrophenol to p-aminophenol with NaBH₄ as the hydrogen source was chosen as a model reaction to test the catalytic activities of Pd@ZIF-L catalysts. The reaction was performed in a 50 ml centrifuge tube equipped a stirrer, and a water bath was used to control the reaction temperature. Typically, 0.0974 g of p-nitrophenol and 2.648 g of NaBH₄ were dissolved in 20 ml of solvent consisting of ethanol/deionized water (50 : 50 v/v) at 30 °C. Then, 0.03 g of Pd@ZIF-L catalyst was added into the reactor, and the reduction reaction was performed at 30 °C with stirring for 120 min. The stirring speed was maintained at 500 rpm to eliminate the diffusion. Samples were taken from the reactor at specified time intervals and analyzed by high performance liquid chromatography (HPLC, Agilent 1200 Series, USA) equipped with a diode array detector (DAD), an auto-sampler and a ZORBAX Eclipse XDB-C18 column (5 μm, 4.6 mm × 250 mm). The analyses were performed at 30 °C by a UV detector at 310 nm using a mobile phase consisting of methanol/pure water (80 : 20 v/v) at a flow rate of 1.0 ml min⁻¹. The automatic injection volume was 2 μl per sample.

3. Results and discussion

3.1. Effect of molar ratio of 2-MeIM/Zn²⁺

The design and synthesis of ZIFs are mainly based on the coordination chemistry of metal centers and ligands, and ZIF-L is formed by self-assembly of divalent metal cations (Zn²⁺) and anionic 2-MeIM linkers in aqueous solutions.^32,35 It has been found that the molar ratio of 2-MeIM/Zn²⁺ has a significant influence on the crystal, size and topology of ZIFs.^30,36–38 In this study, the Pd@ZIF-L was synthesized via the assembly process which is similar to ZIF-L except for the addition of PVP-stabilized Pd NPs. Thus, the molar ratio of 2-MeIM/Zn²⁺ also might affect the microstructure of Pd@ZIF-L, such as the crystallinity, particle size, Pd loading and Pd spatial distribution. Therefore, the influence of 2-MeIM/Zn²⁺ molar ratio on the formation of Pd@ZIF-L was investigated. In the present work, the 2-MeIM/Zn²⁺ molar ratio ranged from 8 to 25 with a fixed PVP/Zn²⁺ molar ratio of 4.

The resulting reaction mixture was treated by centrifugation and the state in the centrifuge tube was recorded by a digital camera (Fig. S1 in the ESI†). When the molar ratio of 2-MeIM/Zn²⁺ is set to 8, the supernatant liquid is colorless and transparent. The as-synthesized dark catalyst particles almost deposit at the bottom of centrifuge tube, because of the bigger particle size and higher Pd loading. Similar phenomenon except for the gray darkness of catalyst is observed as the molar ratio of 2-MeIM/Zn²⁺ is set to 15. When the molar ratio of 2-MeIM/Zn²⁺ is increased to 20 or 25, the catalyst exhibits gray, which may be caused by the lower Pd loading. In addition, some catalyst particles are suspended in the supernatant liquid, because the particle size is smaller and parts of catalyst particles are again dispersed in the supernatant liquid when moving the centrifuge tube.

The crystallinity of Pd@ZIF-L was evaluated by XRD measurements as depicted in Fig. 1. The 2-MeIM/Zn²⁺ molar ratio can significantly affect the crystallinity of Pd@ZIF-L. As the molar ratio of 2-MeIM/Zn²⁺ is set to 8, only several peaks are observed in the XRD spectrum of the as-prepared catalyst, which are significantly different from those of ZIF-L.³² This phenomenon indicates that the ZIF-L structure is not easy to form at a lower 2-MeIM/Zn²⁺ molar ratio and the framework of ZIF-L is very sensitive to the synthesis conditions.³⁴ When the molar ratio of 2-MeIM/Zn²⁺ is larger than 8, each sample shows the same main peaks as those of the pristine ZIF-L, suggesting that as-synthesized Pd@ZIF-L has the same crystal structure as the pristine ZIF-L.³² Furthermore, the intensities of the characteristic peaks increase with the increasing amount of 2-MeIM, indicating more 2-MeIM is beneficial for the formation of Pd@ZIF-L. According to the literature,^39,40 the main characteristic diffraction peaks of Pd NPs are at around 40° and 46° in the 2θ range of 5–50°. However, no obvious characteristic Pd peaks are observed in the XRD patterns, presumably because of the low concentration and/or small size (3–5 nm) of Pd NPs as discussed in the following TEM characterization.

Fig. 1 XRD patters of Pd@ZIF-L catalysts synthesized under different 2-MeIM/Zn²⁺ molar ratios: (a) 8, (b) 15, (c) 20 and (d) 25.

The FESEM was performed to analyze the morphology and particle size of the as-synthesized Pd@ZIF-L catalysts. Interestingly, the morphology and particle size is strongly dependent on the molar ratio of 2-MeIM/Zn²⁺. When the molar ratio of 2-MeIM/Zn²⁺ is set to 8, non-uniform morphology is found and the aggregation is serious, while the flake-like structure with a larger particle size of about 45 μm can be observed (Fig. 2a). For the catalyst synthesized with the 2-MeIM/Zn²⁺ molar ratio of 15, except for the flake-like structure with a smaller particle size, the rod-like and crosshair-star structures are also observed. When the molar ratio of 2-MeIM/Zn²⁺ is increased to 20, the as-prepared Pd@ZIF-L catalyst exhibits a unique crosshair-star shape, in good agreement with our previous work.³⁴ Further increasing the molar ratio of 2-MeIM/Zn²⁺ to 25 also produces the uniform crosshair-star shape. However, the particle size decreases. The significant influence of 2-MeIM/Zn²⁺ molar ratio on the structure and morphology of Pd@ZIF-L should be related with formation mechanism of ZIF-L.^32,33 When the amount of 2-MeIM is less, the concentration of Zn²⁺ is higher, a dense dia(Zn) structure is easy to form.⁴¹ At the same time, the nucleation rate is slower due to the lower reactant concentration. As a result, the crystals with a larger particle size are formed and easily deposit, which are in consistent with the results of Fig. S1a† and 1a. As the amount of 2-MeIM increases, the nucleation rate increases, resulting in the formation of flake-like structure with a smaller particle size. Furthermore, the rod-like or crosshair-star structure is formed, likely due to the staking of flake-like ZIF-L to a certain extent.³⁴ When the amount of 2-MeIM further increases, such as the molar ratio of 2-MeIM/Zn²⁺ is 20, a uniform crosshair-star shape can be formed (Fig. 2c). At the molar ratio of 2-MeIM/Zn²⁺ of 25, excess 2-MeIM can enhance the nucleation rate of ZIF-L, resulting in the smaller particle size (Fig. 2d). Due to the smaller particle size, some parts of particles suspend in the reaction medium, in agreement with the phenomenon in Fig. S1d.†

Fig. 2 FESEM images of Pd@ZIF-L catalysts synthesized under different 2-MeIM/Zn²⁺ molar ratios: (a) 8, (b) 15, (c) 20 and (d) 25.

The TGA was applied to analyze the thermal stability of the as-prepared Pd@ZIF-L catalysts (Fig. 3). The TG curves are significantly different for the two catalysts. When the molar ratio of 2-MeIM/Zn²⁺ is set to 8, the catalyst has a gradual weight loss. The weight loss at low temperature corresponds to the remove of water molecules, and the decomposition of PVP and 2-MeIM is responsible for the weight loss at high temperature. The weight loss curve is similar to that of the dense dia(Zn) structure in our previous work.³⁴ However, in this work, a weight-loss of 25% up to 550 °C is much higher than the value of 10% for the dia(Zn) structure. In the present work, the sample is not fully crystallized and only parts generate the dense dia(Zn) structure at the molar ratio of 2-MeIM/Zn²⁺ of 8 as presented by the XRD and FESEM characterizations (Fig. 1 and 2). As the molar ratio of 2-MeIM/Zn²⁺ is set to 20, the plateau up to 260 °C and the steep weight-loss up to 285 °C are found for the as-synthesized Pd@ZIF-L, showing the remove of the weakly linked 2-MeIM and guest water molecules. A long plateau is presented at the temperature range of 285–550 °C, indicating the high thermal stability of the sample in the absence of guest molecules and unreacted species. When the temperature is higher than 550 °C, the Pd@ZIF-L begins to decompose and is converted to ZnO, which is in agreement with our previous work.³⁴

Fig. 3 TG curves of Pd@ZIF-L catalysts synthesized under different 2-MeIM/Zn²⁺ molar ratios: (a) 8 and (b) 20.

The ICP was carried out to analyze the Pd loading of the as-prepared Pd@ZIF-L catalysts. As shown in Table 1, the Pd loading of Pd@ZIF-L decreases with the 2-MeIM/Zn²⁺ molar ratio, due to the obvious influence of the 2-MeIM amount on the yield of catalyst.³⁵ The yield of catalyst can increase with the increase of 2-MeIM amount. However, as presented in the Experimental section, the addition of PVP–Pd during the preparation of Pd@ZIF-L is fixed, thus the Pd proportion in the total catalyst, i.e., the Pd loading of Pd@ZIF-L, will decrease. The results are consistent with the phenomena in Fig. S1.†

Table 1 ICP analyses of Pd@ZIF-L catalysts prepared with different 2-MeIM/Zn²⁺ molar ratios

2-MeIM/Zn^2+ molar ratio	8	15	20	25
Pd loading (wt%)	0.67	0.56	0.32	0.30

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The TEM was used to analyze the size and morphology of Pd NPs in the ZIF-L. Fig. 4 indicates that the molar ratio of 2-MeIM/Zn²⁺ can significantly affect the morphology of ZIF-L and the distribution of Pd NPs. When the molar ratio of 2-MeIM/Zn²⁺ is set to 8, the sample is not fully crystallized, in consistent with the XRD and FESEM results (Fig. 1 and 2), and the Pd NPs are randomly distributed on the aggregated support. As the molar ratio of 2-MeIM/Zn²⁺ is increased to 15, most of Pd NPs are confined within the ZIF-L framework with uniform distribution, but few free NPs can be observed. Further increasing the 2-MeIM/Zn²⁺ molar ratio to 20, the Pd NPs have been completely encapsulated inside the ZIF-L framework. Similar phenomenon is observed for the Pd@ZIF-L catalyst synthesized at the 2-MeIM/Zn²⁺ molar ratio of 25. According to the formation mechanism of Pd@ZIF-L by an assembly method,^29,34 the Pd NPs may be encapsulated into the ZIF-L crystals during the crystal growth. Of course, TEM results are not enough to evaluate the exact position of Pd NPs in the ZIF-L. In the further work, other analysis technologies such as electron tomography will be attempted to confirm the exact position of Pd NPs in the ZIF-L.²⁶ It is also found from Fig. 4 that the particle size of Pd NPs in each sample is 3–5 nm, in agreement with our previous work.³⁴ However, as the 2-MeIM/Zn²⁺ molar ratio is higher (20, 25), the Pd NPs tend to aggregate, which may be related with the encapsulated states. Although the Pd loading is lower for the catalyst synthesized at a higher 2-MeIM/Zn²⁺ molar ratio (Table 1), all the Pd NPs can be confined in the ZIF-L framework. Thus, the amount of Pd NPs in the ZIF-L framework might be higher, leading to the aggregation of Pd NPs.

Fig. 4 TEM images of Pd@ZIF-L catalysts synthesized under different 2-MeIM/Zn²⁺ molar ratios: (a) 8, (b) 15, (c) 20 and (d) 25.

The differences for the Pd@ZIF-L due to the variation of 2-MeIM/Zn²⁺ molar ratio can be indirectly proven by the reduction of p-nitrophenol to p-aminophenol. In this work, the Pd NPs are encapsulated inside the ZIF-L framework, and the contract of p-nitrophenol with Pd NPs might be caused by the flexibility of the ZIF-L framework.^29,34 It is clear to see from Fig. 5 that the molar ratio of 2-MeIM/Zn²⁺ has an important effect on the catalytic activity of Pd@ZIF-L. As the 2-MeIM/Zn²⁺ molar ratio increases, the catalytic activity first increases and then decreases. Together with the above characterization results, it can be concluded that the loading of Pd NPs and catalyst microstructure are responsible for the influence of 2-MeIM/Zn²⁺ molar ratio on the catalytic activity of Pd@ZIF-L. The larger molar ratio of 2-MeIM/Zn²⁺ results in the lower Pd loading (Table 1) and then reduces the active sites and p-nitrophenol conversion. When the molar ratio of 2-MeIM/Zn²⁺ is set to 8, the as-synthesized Pd@ZIF-L has the highest Pd loading as shown in Table 1. But the catalytic activity is lower than that of the one prepared at the 2-MeIM/Zn²⁺ molar ratio of 15, mainly due to the different microstructures of the catalysts. The catalyst synthesized at the 2-MeIM/Zn²⁺ molar ratio of 8 has a part of dense dia(Zn) structure (Fig. 1–3), and some Pd NPs may been capsulated inside the dense dia(Zn) structure, leading to the decrease of Pd loading, then to lower catalytic activity. For the catalyst synthesized at the 2-MeIM/Zn²⁺ molar ratio of 15, the catalytic activity is the highest. But the sample is a mixture of flake-like, rod-like and crosshair-star shape (Fig. 2b). In addition, some Pd NPs are free (Fig. 4b), which is not favorable for the recycling of Pd@ZIF-L. The catalyst synthesized at the 2-MeIM/Zn²⁺ molar ratio of 20 has a uniform crosshair-star shape (Fig. 2c), and all Pd NPs can be confined in the ZIF-L framework (Fig. 4c). Furthermore, the catalyst also shows high catalytic activity, and the p-nitrophenol conversion can reach 100% after 90 min of reaction. Therefore, based on the microstructure and catalytic activity, the 2-MeIM/Zn²⁺ molar ratio of 20 is feasible for the synthesis of Pd@ZIF-L.

Fig. 5 Catalytic activities of Pd@ZIF-L catalysts synthesized under different 2-MeIM/Zn²⁺ molar ratios: (a) 8, (b) 15, (c) 20 and (d) 25.

3.2. Effect of molar ratio of PVP/Zn²⁺

The Pd@ZIF-L catalysts were synthesized via an assembly method, in which the coordination interactions between the introduced pyrrolidone rings (C=O) in PVP and zinc atoms in ZIF nodes benefit the successive adsorption of PVP-stabilized Pd NPs onto the growing ZIF-L crystals.³⁴ Thus, the molar ratio of PVP/Zn²⁺ should be one of the key parameters affecting the encapsulation of Pd nanoparticles into the ZIF-L matrixes and the corresponding catalytic activity. In this study, the PVP/Zn²⁺ molar ratio on the formation of Pd@ZIF-L was investigated by changing the amount of PVP with a fixed 2-MeIM/Zn²⁺ molar ratio of 20.

As revealed by the photographs shown in Fig. S2 in the ESI,† when the molar ratios of PVP/Zn²⁺ are set to 2 and 4, the as-synthesized gray black catalysts are observed. Increasing the molar ratio of PVP/Zn²⁺ to 6 or 8, the catalyst exhibits gray. The phenomena might be associated with the Pd loading, and the higher Pd content is responsible for preparing the gray black catalyst. For each sample, some catalyst particles are suspended in the supernatant liquid due to the smaller particle size, which is coincident with the experimental phenomena as presented in Fig. S1.†

Fig. 6 shows the influence of PVP/Zn²⁺ molar ratio on the XRD pattern of Pd@ZIF-L. The main peaks of the Pd@ZIF-L samples are almost the same as those of the pristine ZIF-L,³² suggesting that the addition of PVP has no significant impact on the formation of ZIF-L. Similar results were reported in our previous work.⁴² There are no obvious characteristic Pd peaks in the XRD patterns due to the low concentration and/or small size (3–5 nm) of Pd NPs, as discussed in Fig. 1.

Fig. 6 XRD patters of Pd@ZIF-L catalysts synthesized under different PVP/Zn²⁺ molar ratios: (a) 2, (b) 4, (c) 6 and (d) 8.

It is seen from Fig. 7 that all of the samples show the uniform crosshair-star shape, and the average particle size is about 20 μm. The results indicate that the addition of PVP during the catalyst preparation process has no significant influence on the morphology and particle size of the as-synthesized Pd@ZIF-L catalyst. For different PVP/Zn²⁺ molar ratios, the Pd@ZIF-L catalysts are small and well dispersed, resulting in the suspension of some particles in the supernatant liquid (Fig. S2†).

Fig. 7 FESEM images of Pd@ZIF-L catalysts synthesized under different PVP/Zn²⁺ molar ratios: (a) 2, (b) 4, (c) 6 and (d) 8.

Table 2 shows the Pd loading of the as-prepared Pd@ZIF-L catalysts according to the ICP analyses. When the molar ratio of PVP/Zn²⁺ is set to 2 or 4, the Pd loading is almost the same. By increasing the molar ratio of PVP/Zn²⁺ to 6 or 8, the Pd loading significantly decreases. The results highlight the significant influence of PVP on the amount of Pd NPs encapsulated inside the ZIF-L, which should be concerned with the formation mechanism of Pd@ZIF-L catalyst.³⁴ In this work, the Pd@ZIF-L was synthesized via an assembly method (Scheme 1). As presented in the Experimental section, the PVP was added during the synthesis of Pd NPs to form the stable Pd NPs colloid, and then the PVP–Pd colloid was introduced into the synthesis solution of ZIF-L. The pyrrolidone rings (C=O) in PVP can coordinate with the zinc atoms in ZIF nodes,²⁹ and thus the PVP–Pd NPs will be gradually adsorbed onto the growing ZIF-L crystals to obtain the Pd@ZIF-L catalysts.³⁴ Some free PVP exists in the PVP–Pd colloid. As a result, the interaction between the PVP–Pd and Zn²⁺ is inhibited by the competitive coordination of free PVP and Zn²⁺. As the amount of PVP increases, more free PVP will exist in the PVP–Pd colloid and reduces the coordination probability between the PVP–Pd and Zn²⁺, resulting in a lower Pd loading in the Pd@ZIF-L sample.

Table 2 ICP analyses of Pd@ZIF-L catalysts synthesized under different PVP/Zn²⁺ molar ratios

PVP/Zn^2+ molar ratio	2	4	6	8
Pd loading (wt%)	0.31	0.32	0.23	0.24

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The TEM images of Pd@ZIF-L catalysts prepared under different PVP/Zn²⁺ molar ratios are given in Fig. 8. Irrespective of the PVP/Zn²⁺ molar ratio, the ZIF-L is fully crystallized and almost all Pd NPs can be encapsulated inside the ZIF-L framework. Therefore, the introduction of PVP has no obvious influence on the formation of ZIF-L and the encapsulation of Pd NPs in the ZIF-L. In addition, compared to the samples synthesized at the PVP/Zn²⁺ molar ratios of 6 and 8, more Pd NPs are encapsulated inside the ZIF-L when the molar ratios of PVP/Zn²⁺ are 2 and 4, in good agreement with the ICP analyses (Table 2). At the same time, some parts of Pd NPs tend to aggregate due to the higher Pd loading as shown in Fig. 8a and b. The particle size of Pd NPs is about 3–5 nm, which is similar to the results in Fig. 4.

Fig. 8 TEM images of Pd@ZIF-L catalysts synthesized under different PVP/Zn²⁺ molar ratios: (a) 2, (b) 4, (c) 6 and (d) 8.

Fig. 9 shows the conversions of p-nitrophenol to p-aminophenol over the Pd@ZIF-L catalysts. It is clear to see that the molar ratio of PVP/Zn²⁺ has an important effect on the p-nitrophenol conversion in the experimental range. When the molar ratio of PVP/Zn²⁺ is low (2 or 4), the Pd@ZIF-L catalyst exhibits higher catalytic activity, and the conversion of p-nitrophenol can reach 100% after 90 min of reaction. In our previous work,⁴³ only 97.5% of p-nitrophenol conversion could be obtained over Pd@ZIF-8 after 90 min of reaction, even the amount of Pd NPs (about 0.26 mg) was more than the one (about 0.09 mg) in this study. The higher catalytic activity of Pd@ZIF-L may be related with the special pore structure of ZIF-L.³⁴ With the increase of PVP/Zn²⁺ molar ratio (6 or 8), the catalytic activity of Pd@ZIF-L significantly reduces, and the conversion of p-nitrophenol only reaches about 90% at after 120 min of reaction. Correlating the results with the previous characterization results, it is found that the Pd loading is a key factor for the important effect of PVP/Zn²⁺ molar ratio on the catalytic activity of Pd@ZIF-L. When the molar ratio of PVP/Zn²⁺ is lower, the Pd loading is higher (Table 2 and Fig. 8), leading to more active sites and higher catalytic activity. As the molar ratios of PVP/Zn²⁺ are 2 and 4, the catalytic activity is almost the same, but the amount of PVP is less for the PVP/Zn²⁺ molar ratio of 2. Therefore, a feasible PVP/Zn²⁺ molar ratio for the preparation of Pd@ZIF-L is chosen to 2.

Fig. 9 Catalytic activities of Pd@ZIF-L catalysts synthesized under different PVP/Zn²⁺ molar ratios: (a) 2, (b) 4, (c) 6 and (d) 8.

4. Conclusions

The Pd@ZIF-L catalysts with uniform size and morphology have been successfully fabricated by an assembly method. The molar ratios of 2-MeIM/Zn²⁺ and PVP/Zn²⁺ can significantly affect the morphology, size, Pd loading and distribution and catalytic activity of the Pd@ZIF-L catalysts. Lower molar ratios of 2-MeIM/Zn²⁺ and PVP/Zn²⁺ favor the synthesis of Pd@ZIF-L catalyst with higher Pd loading. Higher 2-MeIM/Zn²⁺ molar ratio is beneficial for the encapsulation of Pd nanoparticles within the ZIF-L framework. This work opens a way for the controllable synthesis of nanoparticle/MOF composites.

Acknowledgements

Financial supports from the Jiangsu Natural Science Foundation for Distinguished Young Scholars (BK20150044), the National Natural Science Foundation (91534110, 21306081), 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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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00909c

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