Sharp size-selective catalysis in a liquid solution over Pd nanoparticles encapsulated in hollow silicalite-1 zeolite crystals

Abstract

A lab-in-a-shell method was designed to prepare Pd nanoparticles encapsulated in hollow silicalite-1 zeolite crystals for size-selective catalysis in liquid solution. Due to the pore size limitation of the microporous silicalite-1 shell (0.53 × 0.56 nm), the catalyst showed high activity in the hydrogenation of 3-methyl-2-butenal (0.38 × 0.62 nm) and cinnamaldehyde (0.54 × 0.92 nm), but no activity for 3,3-diphenylacrylaldehyde (0.81 × 1.0 nm).

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Size-selective catalysis is important in catalysis.^1–3 To achieve such a goal, precise control of the pore sizes and rational structural design of catalysts are needed to control the diffusion of reaction species with different dynamic sizes. One famous example is zeolites, which are widely used in petrochemical processes with their size-selectivity.^4–12 Making use of the pore-size limitations of zeolites, active metal nanoparticles encapsulated in zeolites were also fabricated for size-selective catalysis.^13–17 However, direct encapsulation of metal nanoparticles is usually against the growth habit of the zeolite, and core–shell structure lacks the necessary void space around the metal nanoparticles. A better catalyst design is a yolk–shell structure in which the metal nanoparticles are in a confined void space.^18–21 Through such design, the zeolite shell provides size-selectivity and the cavity facilitates the reaction rates. For example, Li et al. reported the synthesis of metal nanoparticles encapsulated in hollow silicalite-1 through impregnation and then etching and reduction processes.^22–26 However, almost all these zeolites catalysts were used in gas-phase reactions, very few of them were investigated in liquid-phase solution, which is often the case in fine chemical industry such as pharmaceutical and fragrance industries.^27,28

Complexing metal ions using polymers, silane couplers with ligands or other agents was an effective strategy to capture massive metal nanoparticles and limit the migration and the growth of metal particles in nanoreactors.^29,30 Qiao et al. described a lab-in-a-shell strategy for the preparation of core–shell nanospheres consisting of a core of metal clusters and an outer microporous silica shell.³¹ Metal clusters (e.g., Pd and Pt) were encapsulated and confined in the cavity by the entrapped polymer dots inside hollow silica nanospheres, acting as a complexing agent for metal ions and then as a encapsulator for metal clusters. This strategy may be promising for fabricating yolk–shell hollow zeolite nanoreactors with massive entrapped metal nanoparticles and excellent catalytic size-selectivity using different molecule sizes of reactants or products.

Herein, we used this efficient lab-in-a-shell strategy to fabricate a yolk–shell nanoreactor with Pd nanoparticles encapsulated in hollow silicalite-1 shell. Such a catalyst showed high activity and sharp size-selectivity in the hydrogenation of olefin aldehydes in aqueous solution. Due to the pore size limitation of silicalite-1 shell (0.53 × 0.56 nm), 3-methyl-2-butenal (0.38 × 0.62 nm) and cinnamaldehyde (0.54 × 0.92 nm) can be hydrogenated, while no conversion for 3,3-diphenylacrylaldehyde (0.81 × 1.0 nm).

The synthesis procedures of Pd nanoparticles encapsulated in hollow silicalite-1 zeolite yolk–shell nanoreactor (denoted as Pd@HS-1) were illustrated in Scheme 1. Hollow silicalite-1 zeolite crystal was first prepared through etching of solid silicalite-1 zeolite by tetrapropylammonium hydroxide (TPAOH) solution, an interesting method reported by Guo et al.³² This hollow zeolite crystal has a wall of about 20 nm in thickness. The key of this preparation design was the following two step lab-in-a-shell method. The first step, the cross-linked polymerization of ethylene diamine (EDA) and carbon tetrachloride (CTC) was introduced to form the poly(ethylene diamine-carbon tetrachloride) nanoparticles inside the hollow silicalite-1 zeolite crystal, forming the yolk–shell composite (denoted as PEC@HS-1). This lab-in-a-shell method allowed the EDA and CTC molecules, which are smaller than the pore size of silicalite-1 to diffuse through the HS-1 shell and into the cavity. After polymerization, the cross-linked polymer particles are too large to diffuse out of the silicalite-1 shell. The PEC polymer particles become the carrier for Pd nanoparticles, which are also introduced into the cavity by the second step. The presence of large amounts of amine groups on the polymers particles enables the Pd²⁺ ions to be stabilized on the polymer surface. Pd nanoparticles were then produced by reduction of Pd²⁺ ions using sodium formate as reducing agent. Thus, the formed composite is a yolk–shell structured material, with zeolite shells and Pd/PEC cores.

Scheme 1 Synthesis route to Pd@HS-1 yolk–shell nanoreactor.

Typical SEM and TEM images of original solid silicalite-1 and hollow silicalite-1 zeolite are shown in Fig. S1.† It can be seen that nearly all the hollow silicalite-1 zeolites retained integral hexagonal crystal structures with a large cavity inside. The size of the cavity is about 300 nm and the shell thickness is about 20 nm. XRD pattern results (Fig. S6†) showed that hollow silicalite-1 retained crystalline structure of solid silicalite-1 zeolite. After polymerization and Pd nanoparticles loading, TEM images revealed that Pd nanoparticles were successfully encapsulated inside the silicalite-1 shells (Fig. 1a–c). No Pd nanoparticle was found at the edge or outside of silicalite-1 crystals. The size distribution of Pd nanoparticles (Fig. 1d) indicated that the average diameter of Pd nanoparticles was 2.8 nm. The small size of Pd nanoparticles can be ascribed to the large amount of residual amine groups on the polymers surface, which acts as a coordinating agent and a support for Pd²⁺ ions, limiting the growth and aggregation of Pd nanoparticles during the reduction process. EDX spectrum and element analysis results (Fig. S3 and Table S1†) showed that Pd@HS-1 was composed of Pd, Cl, C, H and N. FTIR spectrum of Pd@HS-1 (Fig. S4†) exhibited several peaks from the CH₂ groups on the interior cross-linked PEC polymer chains, indicating successful polymerization of EDA and CTC. The weight percentage of polymers in the Pd@HS-1 was about 10.9 wt% according to the TGA analysis (Fig. S5†). TEM energy dispersive X-ray spectroscopy (EDX) elemental mapping images (Fig. 1e–h) further confirmed that Pd nanoparticles and polymers were uniformly dispersed in the cavity.

Fig. 1 (a, b) Bright field TEM images of the composite; (c, e) dark field STEM images of the composite; (d) size distribution of Pd nanoparticles, (f) Si Kα₁, (g) Pd Lα₁ and (h) Cl Kα₁ EDX elemental mappings of Pd@HS-1 yolk–shell nanoreactor.

Then XPS spectrum was acquired to confirm that Pd nanoparticles were located in the cavity. Due to the thickness detection limit f of X-ray photoelectron (less than 10 nm), only Pd nanoparticles located on the outside surface of silicalite-1 could be detected. However, no Pd signal was detected by XPS, indicating that all Pd species were indeed resided inside the zeolite void space. This is the key feature of the zeolite for size selectivity.

Pd/S-1 with nearly the same Pd weight content, but all Pd clusters bedding loaded on the external surface of silicate-1 zeolite was prepared for comparison. As can be seen in Fig. 2 and Table 1, although the total Pd content was very similar, surface Pd content of Pd@HS-1 was much lower than that of Pd/S-1. These results further suggested that Pd nanoparticles were existed in the cavity of silicate-1 shells for Pd/HS-1 sample. The low Pd signal on Pd@HS-1 by XPS spectrum may come from the damaged shells during the grinding and pressing for XPS sample preparation.

Fig. 2 Pd 3d XPS spectra of (a) Pd@HS-1 and (b) Pd/S-1.

Table 1 Pd contents in Pd@HS-1 and Pd/S-1

Sample	Pd loading^a (wt%)	Pd 3d atomic^b (%)
a Determined by ICP-AES.b Obtained from XPS data.
Pd@HS-1	0.15	0.09
Pd/S-1	0.12	1.72

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N₂ adsorption–desorption isotherms of Pd@HS-1 yolk–shell nanoreactor were shown in Fig. 3. It displayed typical type I sorption isotherm, indicating the loading of polymers and Pd nanoparticles did not change the microporous structure of silicalite-1 zeolite shell. Brunauer–Emmett–Teller (BET) surface area analysis demonstrated that the specific surface area of Pd@HS-1 was 191 m² g⁻¹, which was apparently lower than that of hollow silicalite-1. The lowered specific surface area was likely due to the presence of dense cross-linked polymers that occupied the inner void space.

Fig. 3 Nitrogen adsorption–desorption isotherms of S-1, HS-1, Pd@HS-1 and Pd/S-1.

All the above results suggested that Pd nanoparticles were totally entrapped in the cavity. Combined with the molecular sieving capability of silicalite-1 shell, we chose hydrogenation of olefin aldehyde with different molecular sizes to investigate the size-selectivity in the liquid-phase solution (see Scheme S1†). As shown in Table 2 and Fig. S7a,† using Pd@HS-1 as catalyst, 3-methyl-2-butenal (0.38 × 0.62 nm) and cinnamaldehyde (0.54 × 0.92 nm) were converted to corresponding products very quickly. However, nearly no conversion and no hydrogenation products of 3,3-diphenylacrylaldehyde (0.81 × 1.0 nm) were observed even after 6 h.

Table 2 Hydrogenation of olefin aldehyde with different molecular sizes on Pd@HS-1 and Pd/S-1^a

	Pd@HS-1	Pd/S-1
a Reaction condition: catalyst 20 mg, substrate 0.5 mmol, H2O 7.5 mL, H2 pressure 1.0 MPa, temperature 100 °C, time 2 h.
	∼100%	∼100%
	∼100%	∼100%
	-ND	∼100%

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For comparison, the hydrogenation reactions can be fulfilled completely for all three substrates in short time with Pd/S-1 (Table 2 and Fig. S7b†) with Pd nanoparticles being loaded outside the zeolite shell. For Pd@HS-1 catalyst, all three substrates showed lower reactivity in the initial stage, probably due to the slower diffusion rate in the zeolite shell. However, the sharp difference is that the two small substrates (3-methyl-2-butenal and cinnamaldehyde) can be converted to corresponding products in short time, while nearly no conversion of 3,3-diphenylacrylaldehyde were observed even after 6 h, indicating that its inherently lower reactivity is not the reason for the near zero conversion with Pd@HS-1 catalyst. Thus the shape selectivity of the Pd@HS-1 catalyst is accredited for the observation.

The pore size of silicalite-1 shell is 0.53 × 0.56 nm, which is bigger than sizes of 3-methyl-2-butenal (0.38 × 0.62 nm) and cinnamaldehyde (0.54 × 0.92 nm), but smaller than that of 3,3-diphenylacrylaldehyde (0.81 × 1.0 nm), therefore 3-methyl-2-butenal and cinnamaldehyde can pass through the shell for conversion, while 3,3-diphenylacrylaldehyde cannot diffuse through the shell and enter the cavity to reach active sites of Pd nanoparticles. No conversion of 3,3-diphenylacrylaldehyde was observed. In addition, the mass transfer of 3-methyl-2-butenal in liquid-phase solution was more facile than that of cinnamaldehyde, resulting in faster reaction rate on Pd@HS-1 catalyst. These results demonstrated that the selective hydrogenation for different size of reactant molecules can be adjusted effectively by the micropore in the shell of hollow silicalite-1 nanoreactor.

For size-selective catalysis, although metal–organic frameworks (MOFs), covalent–organic frameworks (COFs) and micro- or meso-SiO₂ shells have been reported for selective reactions in liquid-phase solution,^1,31,33–35 reports on crystalline zeolite shells in liquid-phase is rare. Zeolites are highly crystalline porous materials with uniform and well-defined micropores. In general, zeolites have higher chemical stability than that of MOFs, COFs and amorphous silica. On the basis of these results, it is likely that other reactions can also be catalyzed in liquid-phase solution by Pd@HS-1 catalyst. Other metal nanoparticles can also be loaded into the cavity for size-selective catalysis. In addition, the nanoreactor configuration, i.e. the size of the PEC nanoparticles, the size of the void space inside the zeolite, and the size of the hollow zeolite can all be tuned for optimal size-selective catalyst.

In summary, we designed a lab-in-a-shell method to produce Pd@HS-1 composite, in which Pd nanoparticles were all encapsulated in void space of hollow silicalite-1 zeolite. Making use of the pore sizes limitation of silicalite-1, size-selective catalysis of hydrogenation of olefin aldehydes was achieved in the liquid-phase solution. The synthesis route presented in this work is expected to produce other metal nanoparticles for size-selective catalysis.

Acknowledgements

We gratefully thank the National Natural Science Foundation of China (NSFC 21333009, 21573245), Anshun University (Asxybsjj201511), Guizhou Science and Technology Department (NZ[2013]3029) and the Chinese Academy of Sciences for financial support.

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Footnote

† Electronic supplementary information (ESI) available: SEM and TEM images of S-1, HS-1 and Pd/S-1, EDX spectrum of Pd@HS-1, FTIR spectrum of interior polymers, TGA curve of Pd@HS-1, XRD patterns of S-1, HS-1, Pd@S-1 and Pd/S-1, and scheme of hydrogenation reactions. See DOI: 10.1039/c6ra20789h

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