A General Synthetic Approach for Integrated Nanocatalysts of Metal

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A General Synthetic Approach for Integrated Nanocatalysts of Metal-Silica@ZIFs Baojuan Xi, Ying Chuan Tan, and Hua Chun Zeng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04147 • Publication Date (Web): 15 Dec 2015 Downloaded from http://pubs.acs.org on December 24, 2015

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Chemistry of Materials

Baojuan Xi, Ying Chuan Tan, and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

ABSTRACT: Integration of different nano-components into a greater assemblage or object for applications poses a significant challenge to materials chemists. At present, it still remains extremely difficult to achieve high monodispersivity for such assembled products. In order to gain better synthetic controllability, ideally, an integration of this type should be done in a stepwise manner. Herein, we report a versatile stepwise approach for preparation of integrated nanocatalysts of metal-mSiO2@ZIFs (metal = Pt, Pd, Ru, Ag, and Pt53Ru47; mSiO2= mesoporous silica; and ZIFs = ZIF-8 and ZIF-67). Starting with uniform solid Stöber silica spheres in submicron scale, mesoporous channels with desired length and diameter can be created for silica which serves as a support. With measurements of amino-modification of mesopores and selection of metal precursors applied, subsequently, ultrafine metal nanoparticles (25 nm) can be deposited evenly onto the inner walls of silica channels. Resultant metal-mSiO2 spheres are then modified by a layer of anionic polymer which imparts negative charges around and facilitates coating of ZIF-8 shell and thus formation of metal-mSiO2@ZIF-8. Through coordination interaction between polyvinylpyrrolidone (PVP; as surfactant molecules) and unsaturated Zn2+ ions exposed on the ZIF-8 shell, uniform metal-mSiO2@ZIF-8 spheres with desired shape and size can be obtained and simultaneously well-dispersed. Fundamental study and optimization are also carried out, aiming at a greater generality of this synthetic approach. The workability of these catalysts is demonstrated with hydrogenation of different alkenes using asproduced Pd-mSiO2@ZIF-8 catalyst. Indeed, reactant-selective hydrogenation is achieved based on different interactions of the alkene molecules with the shell structure of ZIF-8, possibly influencing the flexible gate opening of ZIF-8.

1 Introduction Over the past years, metal-organic frameworks (MOFs) have emerged as a new class of nanoporous materials in which coordination bonds between metal cations and organic linkers lead to facile construction of three-dimensional supramolecular solids.1−7 Diverse metal ions and linker molecules result in a huge collection of MOFs in terms of their crystalline structure and chemical composition. Relying on the geometric dimensions of metal and linkers, the network topology can also be predicted theoretically.8,9 In particular, the outstanding advantages of uniform micropore structure and large specific surface area of this class of supramolecular materials, promote extensive research across many technological fields such as catalysis,10,11 gas separation,12,13 drug carriers,14,15 hydrogen storage,16 and so on. Furthermore, the hybridization of MOF materials presents enormous possibilities for rational construction of functional structures and elegant regulation of advanced properties.17,18 Novel architectures based on MOFs can be attained to exploit new functionalities of hybrid nanocomposites which often offer superior capabilities17,18 and even synergetic properties,19−23 in comparison to their pristine MOFs counterparts. In the above research, for example, encapsulation of different nanoparticles (NPs) in MOFs has become an attractive area concerning design and construction of multifunctional

nanocomposites. In recent years, myriad types of NPs have been integrated with different MOFs materials, including metal and metal-oxide NPs.24−29 As reported previously, loading of NPs inside MOFs can be accomplished either by embedding in intrinsic cavities or incorporating preformed particles into MOFs matrices. The first method is to generate metal NPs, mostly in-situ, where intrinsic cavities serve as confined space to limit particle growth. Generally, exposure or infiltration of gaseous or liquid metal precursors is followed by on-site oxidation, reduction or decomposition to attain targeted NPs. 30−36 In this way, the size, shape and channel arrangement of a MOF structure strictly determine the dimension of resultant NPs. The second method is to grow MOFs or coordination polymers around prefabricated host NPs to form architectures such as core-shell or yolk-shell structures.37−39 Concerning the applications of these nanocomposites, metal@MOFs have been extensively investigated for heterogeneous catalysis.32,40−44 By taking advantage of the active sites and/or controllable interior cavities, for example, Pt@UiO-66 exhibited the enhanced catalytic conversion of ring enlargement for methylcyclopentane, suggesting the synergetic effect between metal nanocrystals as the core and MOF structure as the shell.24 As a notable member, zeolitic imidazolate frameworks (ZIFs) have similar topotactic structure to zeolite. Its excellent chemical and thermal stability makes broad applica-

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tions realized. For instance, by utilizing the merit of suitable window size, ZIF-8 is often used an exterior shell surrounding other particles to form a selective layer for desired catalysis reactions. Fabrication of different particles@ZIF-8 composite polyhedrons has been reported, wherein Au@ZIF-8 was used to selectively reduce nitrobenzene instead of p-nitrophenol depending on selective gate restriction of ZIF-8.45 Furthermore, Pd NPs have also been successfully immobilized in a composite ZIF-8/mesoporous silica spheres; the catalysts offered excellent selectivity and antipoisoning properties for the hydrogenation of alkenes.46 Despite large research endeavors devoting to the fabrication and application of metal@MOFs composites, some challenges still obstruct the progress of the emerging field. First, as for metal nanoparticles, the size distribution and dispersivity exert vital influence on the catalysis performance.47 In this regard, how to protect small-sized nanoparticles from aggregation still remain as an urgent research subject. To tackle this challenge, a stronger catalyst support with large surface area should be considered; ideally, one should also take good control over the mesoporosity such as in the MCM-41 or SPA-15 supports. Second, designable modulation over the shell configuration and spatial uniformity of such hybrid nanocomposites also require immediate attention. Third, general strategies are lacking with regard to a holistic integration of all catalyst components such as tailorable mesoporous support, stable metal NPs and integral MOF shell. For example, preparation of mesoporous support, incorporation of different metal NPs, and encapsulation of MOF shell should be executed in a stepwise manner in order to gain process controllability. Some existing methods, nevertheless, cannot separate such steps and therefore process generality is not high. Finally, to initiate the deposition of a new material phase on a previous one, as proposed in this stepwise integration of nanocatalysts, fundamental aspects on surface modification, chemical formulation of reaction precursor, bonding situation and crystal nucleation and growth need to be addressed. With the above issues in mind, in this work, we describe a stepwise approach which allows good control over the mesoporosity of monodisperse silica spheres in terms of geometric shape, and one-dimensional (1D) pore length and diameter. This approach also provides great flexibility for in-situ synthesis of ultrafine metal nanoparticles (only 2−5 nm; serve as nanocatalysts) evenly distributed within the 1D mesopores of silica. Subsequently, a shell of ZIFs (e.g., ZIF-8) can be further added, which serves as a membrane to allow only certain types of molecules to access the interior nanocatalysts. It has been well established that the immobilization of metal NPs inside mesoporous channels can effectively retard the aggregation and retain the active sites.48,49 Figure 1 illustratively outlines the process scheme of this work. More specifically, 1D channel length and mesopore size of spherical SiO2 support can be tuned facilely by virtue of varying aging duration or employing co-surfactant system of hexadecyltrimethylammonium bromide (CTAB) and dimethyldecylamine (DMDA). Excellent dispersion and stabilization of catalytic noble metal NPs inside the mesopores are attained for the first time for the Metal-mSiO2 spheres (where Metal = Pt, Pd, Ag, Ru and even bimetallic alloy Pt53Ru47) with chemical modification of pore surfaces; after which heterogeneous nucleation evokes a smooth deposition of ZIF-8 phase by altering surface charge

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of metal-mSiO2, giving rise to the formation of final metalmSiO2@ZIF-8. Our experimental results also demonstrate that the interaction of polyvinylpyrrolidone (PVP) molecules and ZIF-8 is crucial to production of non-agglomerated catalysts. As a case of verification, our as-prepared Pd-mSiO2@ZIF-8 catalyst shows selective-catalysis activity in hydrogenation of alkenes. In addition to ZIF-8, with more study in future, this synthetic strategy is sufficiently versatile for making other ZIF-encapsulated catalysis such as metal-mSiO2@ZIF-67.

SiO2

i

SiO2 mSiO2

mSiO2

ii

iii

iv

v ZIF-8 shell

Metal NPs

vi

vii

viii

2-MeIM

Zn

2+

PVP

Figure 1. A schematic illustration of this stepwise synthetic approach: (i) preparation of monodisperse Stöber solid silica spheres (SiO2), (ii−iv) formations of MCM-41-like mesoporous SiO2 phase (mSiO2) on solid SiO2 with different channel lengths, (v) expansion of channel diameter of mSiO2 phase (i.e., pore enlargement), (vi) deposition of catalytic metal NPs into aminodecorated 1D-channels of mSiO2 to produce Metal-mSiO2 composites, (vii) surface charge modification with anionic polyelectrolyte, and (viii) encapsulation of the Metal-mSiO2 core with a ZIF8 shell to obtain integrated nanocatalysts (Metal-mSiO2@ZIF-8). Enlarged circle describes the bonding situation in the interfacial region between ZIF-8 and PVP.

2 Experimental Section 2.1 Materials. The following provides information on the chemicals used in this work: absolute ethanol (Fisher, analytical reagent grade), tetraethyl orthosilicate (TEOS, Aldrich, ≥99.0%), ammonia solution (Merck, ~35%), hexadecyltrimethylammonium bromide (CTAB, Aldrich, ≥96.0%), 2methylimidazole (2-MeIM) (Aldrich, 99.0%), zinc nitrate hexahydrate (Zn(NO3)26H2O, Sigma-Aldrich, ≥99.0%), toluene (Fisher, analytical reagent grade), 3-aminopropylethoxylsilane (APTES, Sigma-Aldrich, ≥98%), Pd(OAc)2 (Aldrich, ≥ 99.9%), acetone (Fisher, analytical reagent grade), dimethyl formamide (DMF, Fisher, HPLC grade), potassium tetrachlo-


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roplatinate (K2PtCl4, Merck, 99+%), ruthenium chloride hydrate (RuCl3xH2O, Aldrich, reagentplus), chloroplatinic acid hydrate (H2PtCl6xH2O, Sigma-Aldrich), ruthenium (III) acetylacetone (Ru(acac)3, Merck, for synthesis), platinum (II) acetylacetone (Pt(acac)2, Strem Chemicals, 98%), poly(sodium-4styrene sulfonate) (PSS, Aldrich, average MW: 200 k), polyvinylpyrrolidone (PVP, Aldrich, MW: 40 k), Cobalt nitrate hexahydrate (Co(NO3)26H2O, Acros Organics, ACS Reagent), N,N-dimethyldecylamine (DMDA, Sigma-Aldrich, 98%), ethyl acetate (Merck, AR for analysis), n-heptane (Merck, Reag. Ph Eur), and styrene (Alfa Aesar, 99%). All the chemicals and solvents were used as received without further purification. 2.2 Preparation of solid Stöber SiO2 spheres. In this work, SiO2 spheres were fabricated according to a modified Stöber method. Briefly, 46 mL of absolute ethanol and 2.5 mL of TEOS were mixed in a normal beaker under stirring, followed by adding 5 mL of 20% ammonia solution immediately and then continuing with stirring magnetically for 4 h. Once the reaction was finished, the mixture was centrifuged and washed by ethanol twice. The final SiO2 sample was dried at 80°C oven and collected for the reactions below. 2.3 Synthesis of amino-functionalized MCM-41-like SiO2 spheres. 30 mg of prefabricated Stöber SiO2 was dispersed in 10 mL of deionized water and ultrasonicated for 1 h, which was then transferred to a Teflon-lined reactor. Aqueous solutions of 0.8 mL 0.2 M CTAB and 20 mL 0.548 M 2-MeIM were introduced. The final concentration of CTAB was 5.2 mM in the solution. After sealing in the stainless steel autoclave, hydrothermal treatment was carried out at 120180°C for 12 h. The white SiO2 sample was collected and isolated from solution via centrifuging. After drying at 80°C, the sample was heated in an electric furnace at 550°C in laboratory air for 5 h at a ramping rate of 3°Cmin1 to remove CTAB fully from the pores. To functionalize the mesoporous spheres with amino group, in a typical synthesis, a mixture of 0.2 g of the above mesoporous silica and 15 mL of toluene was ultrasonicated. Then, 0.5 mL of APTES was added and refluxed overnight. After that, the final sample was harvested by centrifugation, washed by ethanol twice and dried at 80°C overnight. 2.4 Synthesis of pore-expanded monodisperse mesoporous SiO2 spheres. Briefly, 15 mg of solid SiO2 powder was first dispersed in 5.0 mL of deionized water and treated by ultrasonication for 1 h. 0.4 mL of 0.2 M CTAB, 1.0 mL of DMDA, and 10 mL of 0.548 M 2-MeIM aqueous solution were introduced and stirred for 15 min magnetically. After sealing in an autoclave, the mixture was treated hydrothermally at 140°C for 12 h. The final sample was centrifuged and washed with ethanol in order to remove organic additives. 2.5 Synthesis of Pd-loaded mesoporous SiO2 (denoted as Pd-mSiO2). 10 mM Pd(OAc)2 solution was prepared by dissolving a certain amount of Pd(OAc)2 in its corresponding volume of acetone or DMF. 20 mg of amino-SiO2 (produced in Section 2.3) was added in the mixture containing 1.0 mL of 10 mM Pd(OAc)2 solution, 1.0 mL of ethanol and 1.0 mL of deionized water by ultrasonication for 30 min to ensure good dispersion and stirred magnetically for 4 h. Before the final sample was dried overnight at 80°C, the suspension was centrifuged and washed with ethanol twice. Subsequently, the dried sample was annealed in H2 flow of 50 mLmin1 at

300°C for 3 h with a ramping rate of 3°Cmin1. Likewise, the supporting matrix used for incorporation of other metal nanoparticles (the sections below) was also mesoporous SiO 2, synthesized according to Section 2.3 unless otherwise specified. 2.6 Synthesis of Ru-loaded mesoporous SiO2 (denoted as Ru-mSiO2). The procedures were similar to Section 2.5 except that 3.0 mL of 5 mM RuCl3 aqueous solution were used to mix with the mesoporous SiO2 and final sample was washed twice by deionized water. 2.7 Synthesis of Pt-loaded mesoporous SiO2 (denoted as PtmSiO2). The procedures were similar to Section 2.6 except that 3.0 mL of 5 mM H2PtCl6 aqueous solution was used instead. 2.8 Synthesis of Ag-loaded mesoporous SiO2 (denoted as Ag-mSiO2). The procedures were similar to Section 2.7 except that 3.0 mL of 5 mM AgNO3 aqueous solution was used as metal precursor instead; the resultant powder was annealed in a H2 flow of 50 mLmin1 at 200°C for 1 h with a ramping rate of 3oCmin1. 2.9 Synthesis of Pt53Ru47-loaded mesoporous SiO2 (denoted as Pt53Ru47-mSiO2). 20 mg of the amino-SiO2 was dispersed in 3.17 mL of deionized water by ultrasonication, followed by addition of 1.5 mL of 5 mM H2PtCl6 and 1.33 mL of 5 mM RuCl3 aqueous solution. After continuous ultrasonication for 30 min, the final mixture was stirred for 4 h. The sediment was recovered by centrifugation and washed twice with deionized water. The dried sample was annealed in a H2 flow of 50 mLmin1 at 300°C for 3 h with a ramping rate of 3°Cmin1. 2.10 Preparation of Metal-mSiO2@ZIF-8 (Metal = Pt, Pd). Typically, 10 mg of the above pre-fabricated metal-mSiO2 was dispersed in 10 mL of deionized water by ultrasonication for 1 h and then 130 L of APTES was added. The final mixture was stirred magnetically overnight to obtain amino-modified metal-mSiO2 spheres, followed by centrifugation and washing with absolute ethanol for three times to remove residues. The obtained sample was redispersed in 0.3 wt% PSS aqueous solution by sonication for 30 min. After centrifugation, the sediment was transferred to a glass vial containing 10 mL of deionized water mixed with a certain amount of PVP (0.01−0.04 g), and the mixture was stirred for a short time. 1.0 mL of 0.0672 M Zn(NO3)2 aqueous solution was added into the above mixture, followed by addition of 4.0 mL of 0.1096 M 2-MeIM aqueous solution dropwise. After stirring for several hours, the sample was collected by centrifugation and washed with methanol three times. Alternatively, methanol can be used as solvent to prepare Zn(NO3)2 and 2-MeIM solution for synthesis of PdmSiO2@ZIF-8 to replace DIW. The obtained Pd-mSiO2@ZIF8 sample was activated at 300°C under Ar flow rate of 50 mL/min for 24 h with a ramping rate of 1°C/min and collected for catalysis reactions. 2.11 Preparation of Metal-mSiO2@ZIF-67 (Metal = Pt, Pd). The preparation procedures of ZIF-67 coated M-SiO2 were identical to Section 2.9, except that 40 L of 1.0 M Co(NO3)2 aqueous solution and 5.0 mL of 0.0667 M 2-MeIM replaced the counterpart reagent in ZIF-8. 2.12 Catalytic hydrogenation of alkene. The selective hydrogenation of alkenes (e.g., cyclohexene and styrene) was done in a batch mode. In a round-bottom flask, 20 mg Pd-



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mSiO2 powder or 30 mg Pd-mSiO2@ZIF-8 was added in 5.0 mL ethyl acetate containing a certain amount of different alkene as substrate and 0.10 mL n-heptane as internal standard. The reaction concentration of alkene/ Pd was 2.17 mmol/ mg Pd. The air of the system was evacuated by bubbling for 1 min and then the reactor was connected to a balloon filled with hydrogen. After that, the hydrogenation reaction proceeded at 308 K. In addition, simpler mSiO2@ZIF-8 core-shell structure was fabricated as a control sample. With the MCM-41-like mesoporous SiO2 as a core material, the coating procedure of ZIF-8 shell was similar to Subsection 2.10. 2.13 Materials characterization. The size and crystalline structure of colloidal composite nanoparticles were investigated by transmission electron microscopy (TEM, JEM-2010, FETEM-2100F, accelerating voltage: 200 kV). The crystallographic microstructure was obtained by X-ray diffractometer (XRD, Bruker D8 Advance) equipped with Cu Kα radiation source. The bulk and surface chemical compositions of samples were analyzed with energy-dispersive X-ray spectroscopy (EDX/FESEM, JEM-6700F, accelerating voltage: 15 kV, working distance: 15 mm) and X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical), respectively. The working pressure of XPS detection was 5109 torr and the Xray source was monochromatic Al K (h = 1486.71 eV, 5 mA, 15 kV). Specific surface area and pore size distribution of samples were defined by N2 physisorption isotherms at 77 K (Quantachrome NOVA-3000 system). Metal content of catalyst was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 7300DV, Perkin Elmer, USA). 2.14 Product quantification of catalysis. Analysis of catalytic reaction was done as follows. After the reaction, samples were centrifuged to separate the catalyst from the mixture. The resulting supernatant was analyzed by a gas chromatography (GC, Agilent-7890A; with Agilent HP-5 capillary column) using helium as a carrier gas. Temperatures of massspectrometer (MS) detector and injector were both set to be 280°C. The quantitative analysis of conversion and selectivity was calculated using the measured amount of reagent substrate consumed and products formed based on the internal standard calibration.

3 Results and Discussion 3.1 Creating mSiO2 phase and engineering pore structure. Highly dispersed solid spheres of SiO2 can be readily obtained by a modified Stöber method based on the hydrolysiscondensation mechanism, catalyzed by ammonia.50 Figure S1 shows a panoramic view of the product SiO2 spheres with an average diameter of about 280 nm. These monodisperse SiO2 spheres (Figure 1i) could be further engineered by adding onedimensional (1D) mesopore channels perpendicular to the surface of spheres with desired channel length and pore diameter via pseudomorphic transformation of parental solid SiO2 (Figure 1ii−iii)51. In this respect, CTAB, the structuredirecting agent, can empower the formation of mesoporous SiO2 spheres with MCM-41-like ordered pores, retaining the matrix morphology,51 as shown in Figure 2. In this process, both temperature and CTAB concentration play a vital role in adjusting pore channel length. When the treatment temperature

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increased from 120 to 140°C, the average channel length could be tuned from ~54 to ~86 nm, analyzed from Figures S2 and 2. The channel length remained quite similar for the sample prepared at 160°C, compared to that obtained at 140°C, as revealed in Figure S3. On the other hand, when the CTAB concentration was increased from initial 5.2 to 18.2 mM, the 1Dchannels could be deepened to ~100 nm, as shown in Figure S4. Here, CTAB concentrations used were all above its critical micelle concentration of 0.81.1 mM,52,53 which could make micelle form and result in longer cylindrical channel under a higher concentration. Remarkable porous structural configuration observed shows no major difference, elucidating good manipulation of mesoporous channels formation. The average diameter of mesoporous silica is around 370 nm in Figure 2, which is bigger than the starting solid microspheres of 280 nm. The size increment of mesoporous SiO2 verifies the experience of dissolution-reconstruction process in structural transformation.54 At the temperature range of 120160°C, amorphous solid SiO2 could be retained partially, providing matrix for deposition of SiO2-CTA+ to form the mesopore shell. At 180°C, however, the size of resultant spheres became less uniform, ranging from 230 to 600 nm (Figures S5-S6). In this case, 1D-porous channels were formed through the entire SiO2 spheres (Figure 1iv). Actually, as studied earlier,55 postcooling speed also can influence the size of resulted mesoporous silica spheres. In our system, obviously, aging temperature is a significant factor dictating the kinetics of dissolution of silica and assembly rate of silica- CTA+. Understandably, a lower process temperature limits the dissolution of silica as well as the concentration of dissolved silica species-CTA+. As a result, redeposition of SiO2-CTA+ hybrid could be kept at a self-consistent pace, maintaining the narrow size distribution. On the contrary, at higher temperature, the dissolution of parental SiO2 is faster than the redeposition of SiO2-CTA+ hybrid. Decoupling dissolution-redeposition results in the random reconstruction of silica and thus leads to the formation of nonuniform product spheres.55 Furthermore, mesopores of silica spheres could be enlarged (Figure 1v) by using a co-structure directing agent, namely N,N-dimethyl-decylamine (DMDA). TEM images of Figure 3 describe the morphological profile of such pore-expanded SiO2 spheres. Partial cleavage is observed in some spheres, likely due to the stress generated during the pore expansion. Driven by the hydrophobic interaction, DMDA molecules, as a kind of neutral amine with long hydrocarbon chains, assemble with CTAB in a parallel manner spontaneously with respective heads opposite. 56 The diameter of micelle formed, thus, is augmented, resulting in larger pores of SiO2 spheres (Figure 1v). Under the condition of less DMDA used, only part of SiO2 solid spheres were transformed into mesoporous ones. Insufficient DMDA molecules conjugated with CTAB in an orderly motif, causing the presence of more disordered pore channels (See Figures S7 and S8). The porous texture can be obtained by analyzing N 2 absorption and desorption isotherm behavior. Curve i in Figure S9a is the isotherm of MCM-41-like SiO2 with a sharp condensation step at a relative pressure P/Po = 0.3, typical of orderly mesoporous structures with narrow pore size distribution. After the synergistic treatment of DMDA and CTAB, the characteristic capillary adsorption step was retarded to higher relative pressure and the hysteresis loop was more remarkable, as shown in curve b, confirming the emergence of pore expansion. Esti-


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mated by multi-point BET method, surface areas are 664.6 and 615 m2/g for the normal and pore-expanded mSiO2 spheres, respectively. The corresponding pore size distribution profiles indicate the mode BJH pore sizes of 3.0 and 4.2 nm for samples in Figures 2 and 3, respectively, further testifying the accomplishment of pore expansion.

a

400 nm

c

100 nm

b

200 nm

d

40 nm

Figure 2. TEM images (a−d; at different magnifications) of MCM-41-like mesoporous SiO2 spheres with normal pore size synthesized at 140°C for 12 h.

a

400 nm

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100 nm

b

300 nm

d

60 nm

Figure 3. TEM images (a−d; at different magnifications) of poreenlarged mesoporous SiO2 spheres synthesized at 140°C with addition of 1.0 mL of N,N-dimethyldecylamine.

3.2 Anchoring metal NPs onto mSiO2 channels. The introduction of metal NPs into mesoporous silica phase is depicted in Figure 1vi. Figure 4a describes a panoramic TEM image of Pt NPs immobilized in mesopores of SiO2, displaying a high yield of monodisperse Pt-mSiO2 composite spheres. It is noted that no large Pt aggregates appear in or out of pores from the magnified TEM image of Figure 4b, demonstrating the even fixation of Pt NPs. The addition of Pt NPs in this stepwise route does not cause any morphological changes of pristine mSiO2. High-angle annular dark-field (HAADF) scanning TEM (STEM) image is recorded (Figure 4c) from several representative Pt-mSiO2 spheres. The corresponding elemental mapping image of platinum is displayed in Figure 4d, revealing the homogeneous distribution of Pt NPs inside the mesopores instead of accumulation on the spherical surface of silica. By grafting of NH2 groups onto mSiO2 channels, the adsorption of PtCl62 ions could be enhanced and at the same time the metal precursor could be distributed uniformly via ion exchange.57 Under a strongly acidic environment, after adding H2PtCl6, NH2 groups are subjected to generation of NH3+ cationic groups grafted on the inner pore surface of mSiO2. Thus, full ion exchange with PtCl62 anions takes place, causing the attachment of these anions onto the surface through the strong electrostatic interaction; this process could also be confirmed by the colorless supernatant after centrifugation of H2PtCl6-amino-mSiO2 suspension. To further solidify this analysis, K2PtCl6 was also used as platinum precursor to Pt NPs in our study. Rather expected, we still could observe the yellowish color for the supernatant after centrifugation of mixture of K2PtCl6-amino-mSiO2. In such a low acidic solution, the ratio of –NH3+ to –NH2 would be lower, triggering the detachment of PtCl62 from the mSiO2 substrate; this control experiment further strengthens the pivotal role of electrostatic interaction between PtCl62 and the –NH3+ surface groups on the implantation of Pt NPs in mSiO2 spheres. In addition, when K2PtCl4 (instead of H2PtCl6 and K2PtCl6) was used, however, the color of reaction suspension turned into grey after 4 h, indicating that metallic Pt NPs were also formed insitu (see Figure S10). This observation is consistent with the well-known findings that the –NH2 surface groups possess certain reductive ability to metal ions.58,59 In fact, PtCl42 is an intermediate complex ions when PtCl62 is reduced (the standard reduction potential for this half reaction: 0.68 V) and therefore PtCl42 can be reduced more readily to metallic Pt (the standard reduction potential for this half reaction: 0.73 V) by the unprotonated surface groups of –NH2. Apart from Pt NPs, the above process was also applied as a general strategy for incorporation of other metals, and it displayed high process versatility for immobilization of Ag, Pd, Ru, and alloy Pt53Ru47. As described in Figure 4e, final monodisperse Ag-mSiO2 spheres essentially took on the same shape as their structural precursor (mSiO2). Moreover, Figure 4f−h exhibit resembled uniform distribution profile of small-sized Ag NPs (see HRTEM image of Figure 4r) without undesired metal aggregates. Similar to simple ions, Ag+ can bind with – NH2 groups via coordination interaction, wherein -NH2containing agents are well-established surface modifiers due to the excellent coordination functionality.60 With respect to the formation of Pd NPs fixed in mSiO2, two kinds of polar organic solvents, acetone and DMF, are employed to dissolve the



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metal precursor Pd(OAc)2. As shown in Figure 4i−l and Figure S11, in both events, similar spherical composites of Pd-mSiO2 are acquired. Figure 4i and j displays the constructed PdmSiO2 composite, revealing that indeed Pd NPs are evenly dispersed inside mesoporous channel matrix with small average size restricted by the pore dimensions (see HRTEM image in Figure 4s). The HAADF-STEM and elemental mapping images in Figure 4k and l are recorded from several Pd-mSiO2 microspheres, also affirming the similar confinement of evenly fixed metal NPs. Analogously, the morphological and structural characterizations of as-obtained Ru-mSiO2, using RuCl3 as a metal source, are reported in Figure 4m-p. Similar structural configuration of Ru-mSiO2 spheres exhibits excellent metal dispersion in the mesopores. Meanwhile, HAADF-STEM image and the corresponding elemental mapping profile, in Figure 4o and 4p respectively, reveal the uniform distribution of Ru NPs, resembling the above-mentioned metal-mSiO2 case. Herein, as analyzed in the case of Ag+ mentioned above, Ru3+ ions analogously are immobilized by NH2 surface groups through coordination interaction, which largely serve as chelating agent.60 It should be pointed out that chemical of Ru(acac)3 dissolved in DMF was also used as stock solution in this study. Ru(acac)3, like Pd(OAc)2, is a coordination complex comprised of Ru3+ centers and acac ligands. The competition between Ru3+ coordination with –NH2 surface groups and with acac in liquid defines the final loading of Ru NPs on the mSiO2. For instance, the stronger bonding with acac ions holds Ru3+ back in liquid and thus brings about the failure of Ru NPs loading in mesopores of mSiO2, which also explains another aborted trial using Pt(acac)2 dissolved in DMF as the platinum precursor to incorporate Pt NPs in mSiO2. Likewise, Pt2+ ions were retained in liquid phase and could not be anchored onto the channels of mSiO2. At last, the alloy Pt53Ru47 NPs were also immobilized in mesopores of mSiO2 spheres. TEM and STEM images in Figure S12 unveil the morphology feature and the uniform coexistence of Ru and Pt elements. Due to the confinement of mesoporous channels of mSiO2 supports, those metal NPs are ranging from 2 to 5 nm in size, demonstrated by the HRTEM images of corresponding metallic nanocrystals in Figure 4q−t. The ultrafine sizes make it hard to define the metallic phase through the XRD technique (See Figure S13). Taking Pd-mSiO2 spheres as instance, BET surface area is analyzed to be 450 m2g1 and BJH pore size becomes smaller than pristine mSiO2 spheres, as shown in Figure S9b. The final metal loading and distribution depend on the adsorption of metal ions on the amino-modified mSiO2 surface, via electrostatic force as well as dative covalent bonding, in competing with the metal reservation in the solution phase. By choosing appropriate metal precursors, monodisperse metal-mSiO2 spheres have been obtained with well dispersed metals NPs inside their 1D-channels. The surface composition of metal NPs was also investigated by ex-situ XPS analysis to gain an insight into surface elemental profile and valence state. In Figure 5, the binding energies (BEs) of all studied elements in XPS spectra are referred to C 1s photoelectron peak originating from adventitious carbon (its BE was set at 284.6 eV, Figure 5d,g). The spectrum in Figure 5a has two sets of prominent peaks at 71.4 eV (4f7/2) and 74.6 eV (4f5/2), resulting from platinum in the metallic

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state. Besides, two shoulder peaks centered at 72.7 and 76.4 eV are attributable to 4f7/2 and 4f5/2 orbit states of platinum oxide form,61 whose presence is confirmation of oxidation of platinum surface. Ag 3d photoelectron spectrum in Figure 5b are fitted into two sets of peaks, whereupon 368.2 and 374.3 eV are prominently assigned to the binding energies of Ag 3d5/2 and 3d3/2 photoelectrons of zero-valent state.62,63 It is understandable that silver on the mesopores of Ag-mSiO2 is mainly in its metallic form, considering the large peak area of Ag(0). Another pair of weak peaks at 367.8 and 373.8 eV are corresponding to Ag(I) of oxidized silver 62,63 because of the contact with air during sample handling. The XPS spectrum of Pd 3d core level in Figure 5c shows two pronounced bands at 335.8 eV and 340.9 eV which can readily be assigned to Pd(0) 3d5/2 and 3d3/2 photoelectrons,64 respectively. The presence of other two satellite peaks at 337.6 and 342.7 eV, attributed to the binding energy of 3d3/2 and 3d5/2 of its oxide, gives us a hint that the surface of Pd nanoparticles is partially oxidized. In Figure 5d,e, Ru 3d5/2, Ru 3p3/2 and Ru 3p1/2 have BEs located at 280.6, 462.1 and 484.7 eV, characteristic of metallic Ru.65 Their respective shoulder peaks appear at 282.6, 464.6 and 486.7 eV, indicating the presence of ruthenium oxides. 66 As shown in Figure 5f−h, the spectra of Pt 4f7/2 and Pt 4f5/2 as well as Ru 3d5/2 and Ru 3p3/2 core level photoelectrons of Pt53Ru47-mSiO2 sample are in excellent agreement with those analyzed for the Ru-mSiO2 and Pt-mSiO2 samples.

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Figure 4. Different-magnification TEM, HAADF-STEM, mapping images of metal-mSiO2 microspheres: (a−d,q) Pt-mSiO2, (e−h,r) Ag-mSiO2, (i−l,s) Pd-mSiO2, and (m−p,t) Ru-mSiO2.


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Figure 5. XPS spectra of various metal photoelectrons of (a) Pt 4f of Pt-mSiO2; (b) Ag 3d of Ag-mSiO2; (c) Pd 3d of Pd-mSiO2; (d) Ru 3d and (e) Ru 3p of Ru-mSiO2; (f) Pt 4f, (g) Ru 3d and (h) Ru 3p of Pt53Ru47-mSiO2.

3.3 Encapsulating Metal-mSiO2 with ZIFs. In order to coat ZIF-8 on the surface of metal-mSiO2 spheres, here taking PtmSiO2 for example, we first modify them with amino groups of APTES and subsequently with PSS molecules which offer a uniform layer of negative charges (Figure 1vii).46,67 Zn2+ cations are then attracted onto the negatively charged surfaces due to electrostatic interaction, contributing to the nucleation of ZIF-8. In order to encapsulate nanomaterials with MOFs, certain organic modifiers are normally required to functionalize the surface of a material matrix, such as 3-mercaptopropionic acid (MPA) for Au nanoparticles,68 and carboxyl termination for SiO2.69 As a result, the affinity between the NPs and MOFs can be reinforced. Figure 6a−d displays the TEM images of our as-prepared Pt-mSiO2@ZIF-8 core-shell structures (Figure 1viii), taking on good dispersion and uniform shell thickness. From magnified Figure 6c−d, the shell region is recognized to have an average thickness of about 34 nm. This structural configuration is visualized more clearly by STEM technique in Figure 6e−j, in which elemental mappings of Zn and N describe a uniform encapsulation of ZIF-8. The line scan of a

typical core-shell sphere in Figure 6e also gives the compositional and structural details on the Pt-mSiO2 core and ZIF-8 shell. We found that the amount of PVP did not affect the formation much and thus the final morphology of ZIF-8 coating. Nevertheless, the actual function of PVP was still exploited in detail in our synthesis; its quantity was varied in the range of 0.01, 0.02, 0.03 and 0.04 g (see TEM images of Figure S14). Similar core-shell profiles of products verify the little effect of PVP amount on the formation of core-shell structures and thickness of coating layer. However, this soluble polymer serves as a reaction medium and it is indispensable in synthesis, which will be discussed below. The investigation over the effect of ZIF-8 precursor and PVP on the synthesis of ZIF-8 shell could shed light on its formation mechanism. As moles of Zn(NO3)2 and 2-MeIM were increased to 0.168 mmol and 1.1 mmol, respectively, in the presence of 0.02 g PVP, the product was comprised of the desired core-shell structures and other undesired freestanding ZIF-8 nanoparticles (see Figure S15a,b). However, when the amount of Zn(NO3)2 and 2-MeIM were simultaneously increased to 0.252 mmol and 1.65 mmol, no coating of ZIF-8 was formed on the surface of Pt-mSiO2 spheres and only independent platelet-like ZIF-8 crystals were present (see Figure S15c,d). From the standpoint of crystallization, free particles and coating layers of ZIF-8 are dominated by homogeneous nucleation and heterogeneous nucleation respectively. At a lower precursor concentration, just as the synthetic condition of the sample in Figure 6, heterogeneous nucleation dominates and thus ZIF-8 coating can be achieved. During the synthesis, we found that the mixture became more opaque within 3 min after addition of precursor agents. Theoretically, heterogeneous nucleation happens faster than homogeneous nucleation due to the lower energy barrier required. In this case, nucleation of ZIF-8 took place more easily on the Pt-mSiO2 surface. When the concentrations of precursor were increased, the two kinds of structures coexisted, obviously originating from the contemporary occurrence of the two different nucleation mechanisms. Under the condition of a much higher concentration, as observed in Figure S15c,d, however, homogeneous nucleation controls the formation reaction of ZIF-8. Hitherto, the crystallization of ZIF-8 coating could be modulated by tuning the precursor concentration in our system to optimize the synthesis of core-shell structures. Due to the heterogeneous nature in our core-shell system, the above encapsulation process is obviously different from the report in literature,70 in which ZIF-8 phase encapsulated various nanoparticles involving metal, metal oxide nanoparticles and so on. In that study,70 PVP-protected NPs had been successively adsorbed on fresh surfaces of growing ZIF-8 spheres until all NPs were depleted from the solution. In comparison, the overall size of our core matrix is much bigger. Bigger matrices expose higher surface area to liquid, which is favorable for surface-induced heterogeneous nucleation. Hence, in our system, it is reasonable that heterogeneous nucleation initiates the encapsulation of ZIF-8. In view of its wide application, the mechanistic role of PVP in preparations of metals, semiconductors has been investigated.71−73 As we have known, PVP is widely considered as an amphiphilic polymer, arising from its special structure conformation which consists of a highly polar amide group within the pyrrolidone ring and apolar alkyl chain.74,75 As depicted in Figure 1, oxygen atoms of carbonyl groups in pyrrolidone



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rings of PVP are subject to binding with partially coordinated Zn2+ ions sitting on the surface of ZIF-8. Simultaneously, a portion of the PVP molecules will be hydrated and a bigger hydration shell will thus be formed, therefore sterically stabilizing and preventing agglomeration of the composite particles. To further solidify the analysis, we performed a control experiment in absence of PVP. As expected, we also observed the successful coating of ZIF-8 with smooth coating shells (Figure S16). Nevertheless, these as-obtained spheres aggregated severely, which implies that the interaction between ZIF-8 surface phase and PVP surfactant is indeed responsible for the dispersion of freestanding products. In order to demonstrate the general deposition of ZIF-8 on metal-mSiO2 spheres by the present method, Pd-mSiO2 spheres were also used as another catalyst core for coating study. As expected, they are encapsulated by ZIF-8 with similar thickness. TEM image of Figure 7a show panoramic views of as-obtained Pd-mSiO2@ZIF-8 sample. The clear contrast difference in Figure 7b−d obviously shows us the successful integration of Pd-mSiO2 core and ZIF-8 shell. Moreover, the thickness of ZIF-8 layer is also around 34 nm, similar to that of Pt-mSiO2@ZIF-8. The conclusion can also be drawn from the elemental mapping presence in Figure 7e−h. In addition, an intermediate of Pt-mSiO2@ZIF-8 after an initial growth of 1.5 h was also collected for TEM study (Figure S17). At this stage, the layer already became as thick as that of final sample. Considering the fact, during the process of ZIF-8 crystallization, the thickness of layer mainly depends on the initial nucleation rate controlled by the deprotonation of 2-MeIM, on the basis of the theory previously conveyed. 76 Likewise, we found that Pd-mSiO2@ZIF-8 spheres could also be prepared with methanol as a solvent. As indicated in Figure S18, the product shows no prominent morphological difference from that in Figure 7. The phasic information of Pd-mSiO2@ZIF-8 prepared with methanol was revealed by XRD technique (curve g in Figure S13), confirming the formation of ZIF-8 phase. Even via calcination, the structure was not destroyed, demonstrating the excellent thermal stability (see curve g in Figure S13). According to the comments of adsorption behaviors of MOFs,77 the BET equation must be applied within the relative pressure below 0.04, and then a linear plot with positive C constant can be derived, which is different from classical method. Nitrogen sorption isotherm model is observed for Pd-mSiO2@ZIF-8 (curve iv of Figure S9a), which indeed reveals its microporous and mesoporous natures. The BET surface area of composite sample in the case of methanol is 730 m2/g, as analyzed from Figure S9. Hence, ZIF-8 coating on either Pt-mSiO2 or Pd-mSiO2 spheres is of similar thickness. The adsorption of Zn2+ cations on metal-mSiO2 spheres is responsible for the nucleation of ZIF-8. In principle, other ZIF species with the same ligands as ZIF-8 also can be prepared here because of low selective ability of electrostatic force. As being predicted, Pt-mSiO2@ZIF67 and Pd-mSiO2@ZIF-67 have been fabricated in this work, which has the same zeolite SOD topology structure except the metal ions to be Co2+ instead.78 Figure 8 exhibits TEM images of resultant Pt-mSiO2@ZIF-67 and Pd-mSiO2@ZIF-67. Although the ZIF-67 is not formed as a compact shell at this stage, the successful introduction of ZIF-67 on the metalmSiO2 surfaces suggests the viability of this stepwise ap-

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proach. We believe a more condensed shell of ZIF-67 can be similarly attained with additional process optimization.

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Figure 6. Different magnification images of typical PtmSiO2@ZIF-8 core-shell spheres: (a-d) TEM, (e,h) STEM: line scan: purple-Si, yellow-Zn, red-N, cyan-Pt, elemental mappings of (f,i) Zn and (g,j) N.

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Figure 7. (a−d) typical TEM images of monodisperse PdmSiO2@ZIF-8 core-shell microspheres. (e) TEM and STEM mapping images of two microspheres: (f) Pd, (g) Zn. (h) line scan for a single microsphere: purple-Si, red-Zn, yellow-N, and cyanPd.


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Figure 8. TEM images of Pt-mSiO2@ZIF-67 core-shell spheres (a,b; at two different magnifications), and Pd-mSiO2@ZIF-67 core-shell spheres (c,d; at two different magnifications).

3.4 Enhancing selectivity in alkene hydrogenation reaction. To explore the potential advantages of metal-mSiO2@ZIFs in catalytic performance, selective hydrogenation of alkenes was chosen as a reference reaction, as summarized in Figure 9.46,79 For Pd-mSiO2 (metal content determined by ICP: Pt-4.61%), the reaction concentration of alkene/Pd was 2.17 mmol/mg Pd. As shown in Figure 9a, the kinetic profiles of the hydrogenation reactions of styrene and cyclohexene were tested. Styrene was totally converted into ethylbenzene, while no reduction of the phenyl rings was observed. The product yield reached maximum (100% conversion) within 1 h. Similarly, cyclohexene also reached full conversion (100%) within 1 h at room temperature. Samples collected at different catalysis stages were analyzed and no methylcyclopentane was detected, suggesting that the isomerization of cyclohexene did not take place either. The Pd-mSiO2 catalyst exhibited TOFs of 7.04 and 5.53 min1 for the hydrogenation of styrene and cyclohexene after 20 min respectively. These TOF values were comparable in magnitude, thus the product selectivity attained was low. The reaction rate achieved was more rapid compared to other similar reports, 46,79 which highlights the excellent catalytic activity of mSiO2-supported Pd nanoparticles coupled with efficient mass diffusion rate through the support matrix. After the reaction, the morphology of Pd-mSiO2 spheres were maintained at pristine condition without obvious change (Figure S19a,b), proving that the structure configuration of PdmSiO2 spheres prepared in our system is indeed stable and immobilization of NPs in pore channels can effectively prevent agglomeration which often occurs in catalysis reactions. After coating of ZIF-8, the resultant core−shell Pd-mSiO2@ ZIF-8 still delivered a good catalytic activity (71.1% conversion) for styrene after its hydrogenation reaction, as reported in Figure 9b. The lowering in conversion from 100% is rather expected because of the addition of ZIF-8 shell. It has been well known that the diffusion rate of reactant is limited, due to

the sensitivity of mass transportation to pore size. 67,80 That is to say, since intrinsic window size of ZIF-8 is small, the time for styrene to approach Pd NPs is increased and thus decreases the catalytic reaction rate.46,47,78 On the contrary, the conversion of cyclohexene was only 5.7%. The conversion was largely determined by the access of cyclohexene to the Pd NPs, which was affected by the pore size distribution of and the interaction of reagent substrates with the catalysts. Quite interestingly, styrene and cyclohexene have comparable kinetic diameter of around ~6.0 Å, while their conversions varied so significantly (71.1% vs 5.7%, Figure 9b). Therefore, it is rationally speculated that the higher reactivity for styrene is ascribable mainly to the π−π stacking interaction between styrene and aromatic framework of ZIF-8.81 Because MOFs have flexible frameworks, the gate opening can also be tuned under external stimulation. As a result, the flexible ZIF-8 shell drove the gate opening to accommodate the relatively large styrene. Morphological profiles of the core−shell Pd-mSiO2@ ZIF-8 spheres after the catalysis reaction show no observable variation (see Figure S19c,d) and thus confirm their good structural integrity. Furthermore, post-reaction ICP analytical results reveal a 6.5% loss of Pd content for the unprotected Pd-mSiO2 but only 0.3% for the Pd-mSiO2@ZIF-8, testifying that the outer ZIF-8 coating can effectively protect catalytically active Pd from leaching during the catalysis reactions.

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Figure 9. (a) Kinetic behaviors of Pd-mSiO2 spheres towards alkene hydrogenation: (i) styrene, and (ii) cyclohexene. (b) Catalytic performance of Pd-mSiO2 spheres before and after ZIF-8 coating for liquid-phase hydrogenation of styrene and cyclohexene with pure mSiO2@ZIF-8 as controls for comparison.



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4 Conclusion In summary, the general synthesis of monodisperse metalmSiO2@ZIF-8 core-shell composite spheres has been devised, containing catalytic Pt, Ag, Ru, Pd and Pt53Ru47 nanoparticles as the metal phase. Small-sized metal nanoparticles were formed and anchored in the mesopores of silica support with high dispersity owing to interaction between metal precursors and amino-modified SiO2. In addition, mesoporous SiO2 spheres were engineered with respect to the mesopore channel length by controlled solvothermal treatment parameters and to the pore expansion by conjugation of CTAB and DMDA. Detailed investigation on the growth of ZIF-8 shell unveiled that there are two types of nucleation mechanism (homogenous and heterogeneous) and the latter which initiates the ZIF-8 formation on metal-SiO2 can be promoted at lower precursor concentrations. In principle, this synthetic approach is applicable to other ZIF material coating; in particular, ZIF-67 coated core-shell structures has also been fabricated though the study is only in a preliminary stage. On the basis of this work, we have also demonstrated that the metal-mSiO2@ZIF-8 composite spheres prepared with methanol as solvent can serve as molecule-selective reactors for controlled hydrogenation of alkenes and exclusion of cyclohexene. On the contrary, styrene molecules can go through the windows of ZIF-8 driven by the synergistic effects of π−π stacking interaction between the substrate and linkers of the framework as well as flexible gate opening of ZIF-8. It is anticipated that other complex assemblages with similar structural architectures and material components can also be developed via this stepwise synthetic approach.

Additional results of SEM, TEM, and XRD. This material is available free of charge via the Internet at http://pubs.acs.org.

*E-mail: [email protected].

The authors declare no competing financial interest.

The authors gratefully acknowledge the financial support provided by the Ministry of Education, Singapore, NUS, and GSK Singapore. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

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A General Synthetic Approach for Integrated Nanocatalysts of Metal

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