Nano Res

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A versatile cooperative template-directed coating method to synthesize hollow and yolk-shell mesoporous zirconium titanium oxide nanospheres as catalytic reactors Buyuan Guan, Tao Wang, Shangjing Zeng, Xue Wang, Dong An, Dongmei Wang, Yu Cao, Dingxuan Ma,

Yunling Liu, and Qisheng Huo () Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-013-0392-9

http://www.thenanoresearch.com on November 22 2013

© Tsinghua University Press 2013

Just Accepted

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Nano Research DOI 10.1007/s12274-013-0392-9

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TABLE OF CONTENTS (TOC)

A Versatile Cooperative Template-Directed Coating

Method to Synthesize Hollow and Yolk-Shell

Mesoporous Zirconium Titanium Oxide Nanospheres

as Catalytic Reactors

Buyuan Guan, Tao Wang, Shangjing Zeng, Xue

Wang, Dong An, Dongmei Wang, Yu Cao, Dingxuan

Ma, Yunling Liu and Qisheng Huo*

Jilin University, China

Page Numbers. The font is

ArialMT 16 (automatically

inserted by the publisher)

The yolk-shell structured mesoporous zirconium titanium oxide

spheres containing Pd yolk exhibit high catalytic activity and

recyclability in a one-pot two-step synthesis involving an acid

catalysis and subsequent catalytic hydrogenation for desired

benzimidazole derivative.

Provide the authors’ webside if possible.

Author 1, webside 1

Author 2, webside 2

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A Versatile Cooperative Template-Directed Coating Method to Synthesize Hollow and Yolk-Shell Mesoporous Zirconium Titanium Oxide Nanospheres as Catalytic Reactors

Buyuan Guan, Tao Wang, Shangjing Zeng, Xue Wang, Dong An, Dongmei Wang, Yu Cao, Dingxuan Ma, Yunling Liu and Qisheng Huo () State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, China.

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011

ABSTRACT The design of hollow mesoporous nanostructures for cascade catalytic reactions can inject new vitality into the development of the attractive nanostructures. In the study, we report a versatile cooperative template-directed coating method for the synthesis of hollow and yolk-shell mesoporous zirconium titanium oxide nanospheres with varying compositions (ZrO2 content from 0 to 100%), high surface areas (465 m2 g-1) and uniform mesopores. Particularly, the hexadecylamine (HDA) used in the coating procedure serves as the soft template for silica@mesostructured metal oxide core-shell nanosphere formation. By a facile solvothermal treatment route with an ammonia solution and calcination under air, the silica@mesostructured zirconium titanium oxide spheres can be converted into highly uniform hollow zirconium titanium oxide spheres. By simply replacing hard template silica nanospheres with core-shell silica nanocomposites, the synthesis appoach can be further used to prepare yolk-shell mesoporous structures through the coating and

etching process. The approach is similar to the preparation of mesoporous silica nanocomposites from the self-assembly of the core, the soft template CTAB and silica precursor and can be extended as a general method to coat mesoporous zirconium titanium oxide on other commonly used hard templates (e.g., mesoporous silica spheres, mesoporous organosilica ellipsoids, polymer spheres, and carbon nanospheres). The highly permeable mesoporous channels of zirconium titanium oxide shells has been demonstrated by the reduction of 4-nitrophenol with yolk-shell Au@mesoporous zirconium titanium oxide as the catalyst. Moreover, a cascade catalytic reaction including an acid catalysis and a catalytic hydrogenation for a desired benzimidazole derivative can effectively carry out by using the yolk-shell structured mesoporous zirconium titanium oxide spheres with accessible acidity containing Pd core as the bifunctional catalyst, which makes the hollow zirconium titanium oxide spheres a practicable candidate for advanced catalytic systems.

Nano Res DOI (automatically inserted by the publisher) Research Article Please choose one

———————————— Address correspondence to Qisheng Huo, huoqisheng@jlu.edu.cn.

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KEYWORDS

hollow structure, yolk-shell structure, mesoporous zirconium titanium oxide, self-assembly, multiple catalyst

1 Introduction

Significant research efforts in recent years have been devoted to the development of hollow mesoporous nanostructures for applications in many important research fields due to their outstanding properties such as low density, high surface areas, and well-defined mesoporous wall structures that allow high permeability for controlled mass transport [1-8]. Hence, they can be used in optics and electronics [3-4], controlled release [5, 9], catalysis [6], energy storage [7], as well as confined synthesis [8]. Among the hollow mesoporous structures, the rattle-type or yolk-shell nanostructure, in which a movable core is encapsulated inside a mesoporous shell, has attracted tremendous interest in recent years [5, 10-27]. With the mesoporous shells and structures of functional cores, yolk-shell mesoporous structures can provide advanced applications of hollow mesoporous materials [5, 10-27]. For each core in the yolk-shell structure is isolated by a mesoporous shell and has a homogeneous environment, coalescence between functional cores can be prevented under harsh conditions. Hollow mesoporous structures incorporated with catalytically active cores have therefore presented promising applications as nanoreactors for catalysis.

In the past decade, significant efforts have been made in the design and synthesis of hollow and yolk-shell mesoporous structures with desired components [5, 10-27]. Several synthetic strategies been developed, including hard or soft-templating method [28-32], Kirkendall [33] or Ostwald ripening effect [34], ship-in-bottle methods [35], and selective etching [36-39]. Among these strategies, hard-templating is an effective method through which the cavity sizes and shapes of hollow or yolk-shell mesoporous nanostructure can be precisely adjusted by preprepared templates [40]. The hard templates coated with the mesoporous silica shells [41-49] derived from cooperative self-assembly of structure directing agents, silica

precursors and hard templates are the most attractive because they possess the outstanding properties of silica: high surface area and large porosity of mesoporous framework. In particular, mesoporous silica coating employs a simple synthetic protocol of self-assembly of surfactants (e.g., cetyltrimethylammonium bromide) and silica precursors (e.g., tetraethyl orthosilicate) in ethanol/water mixtures under alkaline conditions (e.g., ammonia) [50-52]. More importantly, it is a general method to precisely control silica shells with tunable thickness, mesoporosity, and functionality [53-54]. However, in addition to hollow structures with mesoporous silica shells, there are rarely any reports on the simple and effective preparation of uniform hollow structures with other metal oxides shells [55], especially uniform mesoporous binary zirconium titanium oxide shells via the cooperative self-assembly of structure directing agents and corresponding precursors.

Zirconia, titania and their binary oxides have been widely employed in fields including catalysis, photocatalysis, dye-sensitized solar cells, photoluminescence, capillary electrophoresis, chromatography, inorganic pigments, dielectric ceramics, heavy metal ion and radioactive waste sequestration, for their specific importance and characteristics in a wide variety of technical fields [56-65]. Compared to the individual zirconia or titania, the zirconium titanium binary oxide shows enhanced stability as the presence of -Zr-O-Ti-O-Zr- networks can retard the nucleation and crystallization of the individual components, therefore giving rise to an amorphous binary metal oxide framework with adjustable porosity and high surface area [66-67]. Moreover, such zirconium titanium oxides also exhibit modifiable surface properties, which could promote the applicability of the binary oxide as a high performance catalyst. Until now, however, there have rarely been any examples of both hollow and yolk-shell mesoporous zirconium titanium oxide nanostructures with high

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surface areas [68]. It remains a great challenge to develop general and effective strategies for preparing these hollow mesoporous zirconium titanium oxide nanostructures via commonly used hard templates.

Although yolk-shell nanostructures with different compositions have been prepared, most reports focus on preparations of desired catalytically active cores and there are rare works that succeed in designing and utilizing catalytically active mesoporous shells. Zheng et al. reported a hollow mesoporous aluminosilica spheres with a catalytically active metal core as a catalytic nanoreactor[26]. Liu and Yang et al. have developed a yolk–shell nanoreactor with a basic core and an acidic shell for cascade reactions [69]. In synthetic chemistry, syntheses of complex molecules are in general multistep reactions. The design of multifunctional nanoreactor systems employing yolk-shell mesoporous structures with catalytically active mesoporous shells would expand new applications in one-pot cascade catalysis.

Herein, we report a versatile cooperative template-directed coating method to prepare catalytically active hollow and yolk-shell mesoporous zirconium titanium oxide spheres with uniform pore channels. Simply by alkaline etching of silica@zirconium titanium oxide core-shell precursors, the cavity sizes and shell thicknesses are controllable to produce hollow mesoporous zirconium titanium oxide spheres with tailored shell compositions, surface areas, and crystallinity. Since silica coated nanocomposites can be used as hard templates, the synthetic approach in this study can be used as an effective route to produce yolk-shell mesoporous structures. Moreover, other commonly used hard templates, such as mesoporous silica nanoparticles (MSNs), mesoporous organosilica nanoellipsoids (MONs), polymer nanospheres (PNs), and carbon nanospheres (CNs), can be perfectly coated with the binary metal oxide shells, which may further expand the applications of this method. To the best of our knowledge, it is the first time that yolk-shell mesoporous structures are prepared with catalytically active zirconium titanium oxide shells with uniform mesopores. The hollow and yolk-shell mesoporous zirconium titanium oxide spheres were applied in heterogeneous acidic catalysis and

confined catalysis, respectively. On the basis of acid activity, the yolk-shell nanostructures reported can be used as multifunctional catalysts for one-pot cascade reactions.

2 Experimental Section

Chemicals. Titanium(IV) isopropoxide (TIP, 97%), zirconium(IV) butanoxide (ZrB, 80 wt.% in 1-butanol), tetraethyl orthosilicate (TEOS, 99 %), and hexadecylamine (HDA, 90%) were purchased from Sigma-Aldrich. Benzaldehyde dimethyl acetal, benzaldehyde, 4-nitrobenzaldehyde, 1,2-phenylenediamine, and benzimidazole were purchased from Alfa Aesar. Ammonia solution (25-28%), sulfuric acid (98 wt. %), ethanol, 2-propanol, L-ascorbic acid, and 4-nitrophenol were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). All reagents were used without further purification. Deionized water was used in all experiments. Preparation of the SiO2 Spheres. SiO2 spheres were fabricated via a modified Stöber reaction [70]. In a typical reaction, 23.5 mL of water, 63.3 mL of 2-propanol and 13 mL of ammonia (27%, aqueous solution) were mixed and heated in an oil bath to 35 °C. Then, 0.6 mL of TEOS was added dropwise into this solution and reacted for 30 min under vigorous stirring to form the silica seeds. Then, 5 mL of TEOS were added dropwise into the reaction mixture. The reaction mixture was kept for 2 hours at 35 °C. The SiO2 spheres were isolated by centrifugation and washed with ethanol and water repeatedly and lastly dried in the air. Preparation of the SiO2@Zr0.5Ti0.5O2 and Hollow Zr0.5Ti0.5O2 Spheres. For zirconium titanium oxide coating, 0.8 g of as-obtained SiO2 spheres was homogeneously dispersed in ethanol (97.4 mL) by ultrasonication, followed by the addition of 1.0 g of HDA, and 2 mL of ammonia, and stirring at room temperature for 30 min to form a uniform dispersion. Then, 1.1 mL of titanium isopropoxide and 1.7 mL of zirconium butoxide was added to the dispersion under stirring; the milky white mixture was kept static after 2 h

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and then aged at room temperature overnight. The product SiO2@Zr0.5Ti0.5O2 precursor was collected by centrifugation then washed with water and ethanol several times. To prepare hollow mesoporous zirconium titanium oxide spheres with amorphous framework, a solvothermal treatment of the precursor beads was performed. A 1.6 g of the SiO2@Zr0.5Ti0.5O2

precursor spheres was dispersed in a 20 mL ethanol and 10 mL water mixture with an ammonia concentration of 0.5 M. Then the resulting mixtures were sealed within a Teflon-lined autoclave (50 mL) and heated at 160 °C for 16 h. After centrifugation and ethanol washing, the air-dried powders were calcined at 600 °C for 4 h in air to remove HDA templates for characterization. More details are described in the Supporting Information. Preparation of Sulfated Hollow Zr0.5Ti0.5O2 Spheres. After the solvothermal treatment of the core-shell SiO2@Zr0.5Ti0.5O2 precursor spheres was performed. The organic templates HDA were removed by extraction in ethanol three times (0.1 g of precursor in 50 mL of ethanol, 2 h), and then the solid was collected by centrifugation, washed with ethanol and dried in air at 100 °C. The solid product was further immersed into 1.0 M H2SO4 solution at room temperature for 6h, followed by drying at 100 °C overnight and calcining at 550 °C for 3 h in air. Preparation of the Core@Zr0.5Ti0.5O2 Shell Nanocomposites with Other Commonly Used Hard Templates. Core–shell MSN@Zr0.5Ti0.5O2, core–shell MON@Zr0.5Ti0.5O2, core–shell PN@Zr0.5Ti0.5O2, and core–shell CN@Zr0.5Ti0.5O2 nanospheres were synthesized under similar reaction conditions as for SiO2@Zr0.5Ti0.5O2 nanospheres except replacing SiO2 nanospheres with MSNs, MONs, PNs, and CNs. The detailed synthesis

procedures of MSN@Zr0.5Ti0.5O2, MON@Zr0.5Ti0.5O2, PN@Zr0.5Ti0.5O2, and CN@Zr0.5Ti0.5O2 nanospheres are provided in the supporting information. Preparation of Yolk-Shell Au@Zr0.5Ti0.5O2 Nanospheres. The Au@SiO2 core–shell nanoparticles were prepared according to the method reported by Sch¨uth et al.[71] For zirconium titanium oxide coating, 0.8 g of as-obtained Au@SiO2 spheres was homogeneously dispersed in ethanol (97.4 mL) by ultrasonication, followed by the addition of 1.0 g of HDA, and 2 mL of ammonia, and stirring at room temperature for 30 min to form a uniform dispersion. Then, 0.22 mL of titanium isopropoxide and 0.34 mL of zirconium butanoxide was added to the dispersion under stirring; the pink mixture was kept static after 2 h and then aged at room temperature overnight. The product Au@SiO2@Zr0.5Ti0.5O2 precursor was collected by centrifugation then washed with water and ethanol several times. To prepare yolk-shell Au@Zr0.5Ti0.5O2 nanospheres, a solvothermal treatment of the precursor beads was performed. A 0.8 g of the Au@SiO2@Zr0.5Ti0.5O2 precursor spheres was dispersed in a 20 mL ethanol and 10 mL water mixture with an ammonia concentration of 0.5 M. Then the resulting mixtures were sealed within a Teflon-lined autoclave (50 mL) and heated at 160 °C for 16 h. After centrifugation and ethanol washing, the air-dried powders were calcined at 550 °C for 4 h in air to remove HDA templates. A 0.1g of the obtained pink powder was dispersed in a 30 mL water mixture with a NaOH concentration of 0.05 M. Then the resulting mixtures were sealed within a Teflon-lined autoclave (50 mL) and heated at 85 °C for 1.5 h. The Au@Zr0.5Ti0.5O2 nanospheres were isolated by centrifugation and washed with ethanol and water repeatedly and lastly dried in the air for characterization.

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Synthesis of Pd/Au@Zr0.5Ti0.5O2 Yolk-Shell Structures. 40 mg of Au@Zr0.5Ti0.5O2 nanospheres and 140 mg of L-ascorbic acid were mixed in 9 mL of deionized water. 0.5 mL (30 mmol L-1) of K2PdCl4 aqueous solution was then added to the mixed suspension and stirred at room temperature for 10 min. The resulting solid Pd/Au@Zr0.5Ti0.5O2 nanospheres were collected by centrifugation and washed with deionized water and then dried at room temperature under vacuum overnight. Reduction of 4-Nitrophenol with Yolk-Shell Au@Zr0.5Ti0.5O2 Nanospheres. Ten milligrams of yolk-shell Au@Zr0.5Ti0.5O2 nanospheres was homogeneously dispersed in 5 mL of deionized water by ultrasonication, followed by the addition of 0.5 mL of NaBH4

aqueous solution (0.5 M), and the mixture was stirred for 10 min at room temperature. Then, 0.25 mL of 4-nitrophenol (0.12 M) was added to the mixture, which was stirred until the deep yellow solution became colorless. During the course of reaction, the reaction progress was monitored by measuring UV-vis absorption spectra of the mixture. Acid-Catalyzed Deprotection of Benzaldehyde Dimethyl Acetal. Benzaldehyde dimethyl acetal (1mmol) and water (2.3 mmol) were mixed in 5 mL of toluene at room temperature, followed by the addition of 20 mg of the catalyst ( hollow Zr0.5Ti0.5O2 or sulfated hollow Zr0.5Ti0.5O2). The resulting mixture was then stirred at reflux temperature for 3h. After reaction, the catalyst was separated by centrifugation. The product and the internal standard substance were detected by UV spectrum equipped on HPLC. Separation occurred on a Alltima C18 reversed-phase column (4.6×100mm), 5m. The mobile phase was the mixed solution of methanol and water with the volume ratio of 8: 2 and the flow-rate

was 0.8 mL min-1. The sample was injected with 5l every time detected by UV absorbance at 254 nm. Synthesis of the 2-(4-Amidophenyl)-1H-benzimidazole Using Pd/Au@Zr0.5Ti0.5O2 Catalyst. Sixty milligrams of Pd/Au@Zr0.5Ti0.5O2 nanospheres was dispersed in 5 mL of methanol, followed by the addition of 0.5 mmol of 4-nitrobenzaldehyde and 0.6 mmol of 1,2-phenylenediamine. The resulting mixture was stirred at reflux temperature under air atmosphere for 5 h and then transferred into a glass pressure vessel without any separation or extraction. The vessel was then charged with H2 to 2 bar reacting for 8 h. After reaction, the reaction mixture was diluted with methanol and the catalyst was separated by centrifugation. The yield of the product was analyzed by HPLC with an internal standard substance (benzimidazole). The product and the internal standard substance were detected by UV spectrum equipped on HPLC. Separation occurred on a Alltima C18 reversed-phase column (4.6×100mm), 5m. The mobile phase was the mixed solution of methanol and water with the volume ratio of 8: 2 and the flow-rate was 0.8 mL min-1. The sample was injected with 5l every time detected by UV absorbance at 254 nm. Characterization. TEM images were obtained on FEI Tecnai G2 F20 s-twin D573 field emission transmission electron microscope with an accelerating voltage of 200 kV. SEM images were obtained on a JEOL JSM-6700F field-emission scanning electron microscope. The particle size was measured by photon correlation spectroscopy employing a Nano ZS90 laser particle analyzer (Malvern Instruments, UK) at 25 °C. Powder XRD patterns were obtained by using a Rigaku 2550 diffractometer with Cu K radiation ( =

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1.5418 Å). N2 adsorption–desorption isotherms were obtained at -196 °C on a Micromeritics ASAP 2010 sorptometer. Samples were degassed at 150 °C for a minimum of 12 h prior to analysis. Brunauer–Emmett–Teller (BET) surface areas were calculated from the linear part of the BET plot. UV-Vis spectra were recorded with a Shimadzu UV-2450 spectrometer. The adsorption of ammonia over the products was characterized by temperature-programmed desorption (NH3-TPD, Micromeritics AUTOCHEM II 2920). The mass spectra were recorded in positive-ion mode on a Bruker-Esquire HCT instrument interfaced by an electrospray ionization source. High-performance liquid chromatograms (HPLC) were performed using the UltiMateTM 3000 system, including autosampler ASI-100, degasser DG-1210, pump P680 and Detector VWD-3400. The column was Acclaim TM 120, C18 (4.6×100mm), 5m. The mobile phase was the mixed solution of methanol and water with the volume ratio of 8: 2 and the flow-rate was 0.8ml/min. The sample was injected with 5l every time detected by UV absorbance at 254 nm.

3 Results and discussion

3.1 Synthesis and Characterization of Hollow Mesoporous Zirconium Titanium Oxide Nanospheres. Hollow mesoporous zirconia spheres with robust mechanical and thermal stability have been successfully used as mesoporous shells for hollow and yolk-shell structures [5, 72-75].

Scheme 1 Schematic illustration of the synthesis of core-shell

silica@zirconium titanium oxide spheres and their

transformation into hollow zirconium titanium oxide spheres

with uniform mesopores. Catalytically active mesoporous sulfated zirconia shells are a benefit for upgrading and applications of hollow and yolk-shell mesoporous structures, for incorporation of SO42- into the zirconia network can provide mesoporous zirconia with preferable properties in superacidity in various chemical and physical environments [76-77]. However, hollow and yolk-shell mesoporous structures with zirconia shells have received only limited success due to the relatively low surface area and the lack of an effective route for controlling formation of uniform mesostructured zirconia shell. In previous studies, conventional mesoporous zirconia shell was usually obtained by coating pure zirconia on the specific hard template [5, 71]. Although these approaches are able to coat the zirconia on the hard template core, the mesopores of as-synthesized zirconia shell are always irregular and surface areas are low due to the formation of zirconia nanocrystals after calcination treatments. In contrast to the individual zirconia, the zirconium titanium binary oxide with the mixed metal oxide

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networks can retard the nucleation and crystallization of the ZrO2 component, which

may result in an amorphous binary metal oxide shell with high surface area. In this study, we

Figure 1 SEM images and TEM images of the core-shell SiO2@Zr0.5Ti0.5O2 and hollow Zr0.5Ti0.5O2 spheres: a), b) and c)

as-prepared SiO2@Zr0.5Ti0.5O2 precursors, d), e) and f) solvothermally-treated hollow Zr0.5Ti0.5O2 spheres, g), h) and i)

calcined hollow Zr0.5Ti0.5O2 samples. Inset in c), f) and i) is a higher magnification TEM image showing the shell of a

single sphere. The core-shell and hollow zirconium titanium oxide spheres with other Zr compositions are provided in the

supporting information (Figure S3).

have now revealed that it is also feasible to use Zr and Ti precursors, soft template HDA and hard templates to fabricate hollow and yolk-shell mesoporous zirconium titanium oxide spheres with uniform mesopores and high surface area.

The overall synthetic procedure is shown in Scheme 1. The core is a silica sphere synthesized by the Stöber method and the shell was a uniform layer formed from the self-assembly of hexadecylamine (HDA) and amorphous zirconium titanium oxide. For all shell compositions studied the sol–gel self-assembly in the presence of soft template HDA resulted in uniform core-shell nanospheres. The

compositions of the binary oxide shells can be varied from 0 to 100% ZrO2. In a typical synthesis, the silica nanospheres with diameters of 210 nm, HDA, Zr, and Ti precursors self-assemble into core-shell structures by using a 1:1 molar ratio of starting Zr and Ti precursors during synthesis. The precursor core-shell spheres are significantly monodisperse, smooth and uniform (Fig. 1a). Fig. 1b shows a transmission electron microscopy (TEM) image of the core-shell SiO2@Zr0.5Ti0.5O2 nanospheres with diameters of ca. 210 nm and shell thicknesses of ca. 50 nm. The dynamic light scattering (DLS) curves of SiO2 and SiO2@Zr0.5Ti0.5O2 nanospheres (Fig. S1) show that the hydrodynamic

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diameter of the zirconium titanium oxide-coated particles is 60 nm larger than that of the SiO2 nanoparticles, which are very close to those measured by TEM. The wormlike mesostructure were observed from TEM image of the samples (Fig. 1c). The mesoporous Zr0.5Ti0.5O2 coated silica nanospheres were transformed into hollow zirconium titanium oxide nanospheres through the solvothermal treatment of as-prepared core-shell SiO2@Zr0.5Ti0.5O2 precursors at 160 °C in ammonia solution (0.5 M) for 16 hours. The hollow spheres obtained were smooth and uniform without any cracks in the shell, which is in agreement with the SEM and TEM observation (Fig. 1d and e). From the TEM image (Fig. 1f), the wormlike uniform mesopores with diameters of around 3 nm in the zirconium titanium oxide shells were similar to that of MCM-41-type mesoporous silica (2-4 nm), because both organic templates HDA and CTAB possess the same carbon chain length. After calcination treatment of these hollow spheres at 600 °C for 6h in the air, the monodispersity and smooth surface of the spheres were retained (Fig. 1g). The diameter of the spheres was not altered significantly during the heating processes (Fig. 1h). The original worm-hole like mesostructure in the precursor spheres is retained in the calcined sample (Fig. 1i). This indicates that the Ti and Zr species were well distributed within the nanospheres, preventing sufficient size domains of either species to occur and therefore crystallize during calcination. The corresponding chemical mappings are displayed in Fig. 2. As can be seen, Zr, Ti, O, and C components are evenly distributed within the shell, and Si and O components are evenly distributed in the core. This further confirms that self-assembly of soft template HDA and zirconium titanium oxide and the even distribution of Zr and Ti in the shell. An energy dispersive X-ray spectroscopy (EDX) analysis further reveals the existence of five elements Ti, Zr, Si, O, and C (Fig. S2).

Figure 2 Elemental mappings of core-shell SiO2@Zr0.5Ti0.5O2 spheres.

Figure 3 a) Low angle and b) wide angle XRD patterns of the solvothermally-treated and calcined hollow zirconium titanium oxide microspheres prepared with varying Zr/Ti molar ratios. A = anatase titania, M = monoclinic zirconia and T = tetragonal zirconia.

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The low angle XRD patterns of the samples with different Zr and Ti compositions after solvothermal treatment and calcination at 600 °C show a broad reflection (Fig. 3a), indicating the presence of a short-range ordered mesoporous structure, which is in agreement with the TEM observation. The corresponding wide angle XRD patterns are shown in Fig. 3b. The samples prepared using 100% ZrO2 contained both tetragonal phase zirconia and monoclinic nanocrystals. These crystals were small (< 5 nm) as determined by the peak broadening in the wide angle XRD patterns (Fig. 3b) and as observed in the TEM images (Fig. S4). Interestingly, even with the growth of these crystals, the short-range ordered mesopores are still observed in the final hollow nanospheres as indicated by low angle patterns (Fig. 3a) and TEM images (Fig. S4). The TEM images (Fig. S4a and b) of the pure zirconia hollow spheres after solvothermal treatment and calcination show a granular nature due to the presence of the zirconia nanocrystals. High resolution TEM images (for example, Fig. S4c) and calculations from the wide angle XRD patterns confirm the nanocrystals are < 5 nm in diameter. Such a small crystal size is essential to prevent disruption to the mesostructure [78-79]. The similar results were obtained in 100% TiO2 sample, in which anatase crystals were observed as there was no ZrO2 present to prevent crystal growth (Fig. S5). As the amount of titania in the sample increased, the samples maintained amorphous phase even after calcination at 600 °C, suggesting a relatively high thermal stability of the resulting amorphous sphere [80]. This would indicate that the Zr and Ti species were well distributed within the spheres, preventing sufficient size domains of either species to occur and therefore crystallize during calcination [80-81].

Figure 4. a) Nitrogen sorption isotherms of the calcined hollow

zirconium titanium oxide spheres synthesized with varying Zr/Ti

molar ratio and b) the corresponding pore size distribution

determined by using the BJH method based on the oxide

cylindrical pore model for N2 at 77 K. The Zr0.25Ti0.75O2,

Zr0.5Ti0.5O2, Zr0.75Ti0.25O2 and ZrO2 curves are shifted in vertical

axis by 25, 84, 175 and 210 cm3/g in a) and 0.51, 1.02, 2.02 and

2.52 cm3 g-1 nm-1 in b) for clarity.

Table 1 Physical properties of the calcined hollow mesoporous zirconium titanium oxide spheres.

Sample name SBETa

(m2/g) PSDb (nm)

VSPc

(cm3/g) Crystal phase

TiO2 264 3.5 0.219 Anatase TiO2

Zr0.25Ti0.75O2 295 3.4 0.280 Amorphous

Zr0.50Ti0.50O2 465 2.8 0.319 Amorphous

Zr0.75Ti0.25O2 329 3.3 0.208 Amorphous

ZrO2 235 3.4 0.214 Monoclinic &

Tetragonal ZrO2

From gas sorption studies, Fig. 4, type IV

isotherms were observed for all the calcined hollow nanospheres indicating the presence of mesopores. This result is in good agreement with those derived from both low angle XRD and TEM characterizations. For the calcined Zr0.5Ti0.5O2 hollow spheres, a very high specific surface area of 465 m2 g-1 was attained due to

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the small subunit particles ( 3 nm determined

by TEM, Supporting Information Fig. S4). From the pore size distribution curves shown in Fig. 4b, the peak in pore size can be seen to decrease substantially from the pure TiO2 sample to the samples prepared with 25, 50 and 75% ZrO2, and increase again for the pure ZrO2 sample. The presence of crystals within the pure TiO2 and ZrO2 samples obviously vary the mesostructure relative to the amorphous samples. As shown in Table 1, the calcined mixed zirconium titanium oxide samples possess high specific surface areas, which may be due to the interconnected nature of the worm like mesopores. This result is in agreement with that from the TEM observation.

Figure 5 Characterization results of the series of hollow Ti0.5Zr0.5O2 spheres with tunable hollow sizes and shell thicknesses. TEM images (a-c) show hollow Ti0.5Zr0.5O2 spheres with identical hollow size of ca. 210 nm but varied shell thicknesses: a) 10 nm, b) 30 and c) 50 nm. TEM images (d-f) of hollow Ti0.5Zr0.5O2 spheres with average diameters of d) 340 nm, e) 480 nm, and f) 650 nm; with average hollow sizes of 190 nm, 350 nm, and 440 nm.

We further demonstrated that the diameter, hollow core size, and shell thickness are also tailorable by tuning the synthesis conditions. By changing the amount of Zr and Ti precursors or the particle size of the hard templates silica spheres, we synthesized core-shell zirconium titanium oxide nanospheres with different

hollow core sizes (190–440 nm) and shell thicknesses (10–50 nm). The corresponding hollow structure could be obtained easily by templates removal. TEM was used to determine the hollow core sizes and the shell thicknesses of these nanospheres. As shown in Fig. 5a-f, the shell thicknesses are 10, 30, 50 nm, and the hollow core sizes are 190, 350, and 440 nm, respectively. Hence, the versatility of our synthesis towards uniform hollow nanospheres has been successfully demonstrated.

Figure 6 TEM images of yolk-shell Zr0.5Ti0.5O2 spheres and core–shell Zr0.5Ti0.5O2 spheres with different hard templates. a) SiO2@Zr0.5Ti0.5O2, b) Au@Zr0.5Ti0.5O2, c) MSN@Zr0.5Ti0.5O2, d) MON@Zr0.5Ti0.5O2, e) PN@Zr0.5Ti0.5O2, f) CN@Zr0.5Ti0.5 O2 nanospheres.

3.2 Yolk-Shell Structures with Zirconium Titanium Oxide Shells and Core-Shell Structure with Different Hard Template Cores. In addition to the synthesis of hollow zirconium titanium oxide structures, yolk-shell mesoporous structures can be easily and precisely controlled by partial etching silica core of SiO2@Ti0.5Zr0.5O2 spheres (Fig. 6a) or choosing core@SiO2 spheres as hard templates. Silica is well-known for its versatility in coating many nanostructures to form core−shell structures [82]. Therefore, we have also demonstrated that uniform mesoporous zirconium titanium oxide shells can be constructed with a silica interlayer. For example, monodisperse sandwich-like core−shell structures can be obtained with Au

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nanoparticles as the inner core, silica as the interlayer, and uniform mesoporous zirconium titanium oxide as the outer shells (Fig. S6). The yolk-shell Au@zirconium titanium oxide spheres can also be obtained by etching silica layer with an appropriate amount of NaOH (Fig. 6b). The versatility and capability of this strategy in coating other commonly used hard templates has also been demonstrated. We show that mesoporous silica nanoparticles (MSNs), mesoporous organosilica nanoellipsoids (MONs), polymer nanospheres (PNs), and carbon nanospheres (CNs) can be uniformly coated by mesoporous zirconium titanium oxide shells via the cooperative template-directed coating method; all the samples obviously show the monodisperse core−shell structures (Fig. 6c-f). These observations illustrate that uniform mesoporous zirconium titanium oxide shells can be coated on surface of various cores via the cooperative template-directed coating method independent of composition and morphologies, which further broaden the applications of this method.

Figure 7 (a) UV-vis spectra showing gradual reduction of 4-nitrophenol with yolk-shell Au@Zr0.5Ti0.5O2 nanospheres. (b) Plot of ln(Ct/C0) versus time for yolk-shell Au@Ti0.5Zr0.5O2 and core-shell Au@SiO2 nanospheres. (c) Conversion of 4-nitrophenol in 10 successive cycles of reduction with yolk-shell Au@Zr0.5Ti0.5O2 nanospheres. (d) TEM image of the finally retrieved yolk-shell Au@Zr0.5Ti0.5O2 nanospheres.

3.3 Catalytically Active Yolk Protected by Permeable Mesopore Channels. Yolk-shell nanomaterials have been recognized as ideal structures to stabilize metal nanoparticle cores owing to the structural feature that functional cores are isolated by a mesoporous shell and have homogeneous surrounding environments. Yolk-shell Au@Zr0.5Ti0.5O2 nanospheres were chosen to demonstrate the stable structures in our yolk-shell nanomaterials. The reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride was chosen as a model reaction to evaluate the catalytic ability and stability of yolk-shell Au@Zr0.5Ti0.5O2 nanospheres. The reduction process was monitored by UV-vis absorption spectroscopy (Fig. 7a). The reduction reaction did not proceed in the absence of yolk-shell Au@Zr0.5Ti0.5O2 nanospheres, which was evidenced by a constant absorption peak at 400 nm. When yolk-shell Au@Zr0.5Ti0.5O2 nanospheres were introduced as catalysts into the solution, the intensity of the characteristic absorption peak at 400 nm corresponding to 4-nitrophenol quickly decreased and the characteristic absorption of 4-aminophenol at 295 nm appeared accordingly. The reduction of 4-nitrophenol into 4-aminophenol was completely finished in 12 min, and the color change of bright yellow to colorless was observed. The ratio of Ct and C0, where Ct and C0 are 4-nitrophenol concentrations at time t and 0, was measured from the relative intensity of the respective absorbances, At/A0. The linear relations of ln(Ct/C0) versus time indicates that the reactions followed first-order kinetics (Fig. 7b), and the rate constant is estimated to be 0.32 min-1. In comparison, as nonporous SiO2 shells are impermeable to 4-nitrophenol, no measurable catalytic activity of core-shell Au@SiO2 structure can be detected in the same reduction reaction. These results clearly reveal that mesoporous zirconium titanium oxide

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shells in our yolk-shell Au@Zr0.5Ti0.5O2 nanospheres are highly permeable to the reaction substances. In addition to catalytic activity, the structural stability of yolk-shell Au@Zr0.5Ti0.5O2 nanosphere was also examined. The catalytic recyclability was investigated by conducting the catalytic reduction of 4-nitrophenol to demonstrate the excellent stability of yolk-shell Au@Zr0.5Ti0.5O2 nanospheres. As shown in Fig. 7c, yolk-shell Au@Zr0.5Ti0.5O2 nanospheres were still highly active with a conversion over 99% at 12 min even after 10 successive cycles of reactions. The TEM images of the final catalysts reveal that the mesoporous channels and the Au yolk were well retained after 10 repeating catalytic cycles (Fig. 7d). The high catalytic performance and favorable stability of yolk-shell Au@Zr0.5Ti0.5O2 nanospheres make the developed synthesis strategy useful for the design of active and stable catalytic reactors.

Figure 8 (a)Elemental mappings of sulfated hollow Zr0.5Ti0.5O2 nanospheres. (b) NH3-TPD of hollow Ti0.5Zr0.5O2 and sulfated hollow Zr0.5Ti0.5O2 nanospheres. (c) Scheme of acidcatalyzed deprotection of benzaldehyde dimethyl acetal to benzaldehyde.

3.4 Acid Activity of Mesoporous Sulfated Zirconium Titanium Oxide Shells. It is known that sulfated ZrO2 have been well demonstrated as versatile solid acid catalysts owing to its superacidity properties [3, 76-77]. In general, the

acidity of sulfated ZrO2 depends on the amount of sulfates in frameworks as well as their structures. The BET surface area of the hollow Zr0.5Ti0.5O2 spheres is ~465 m2 g-1, which is much larger than that of traditional hollow mesoporous ZrO2 materials. This mesostructure provides advantages in mass diffusion and transport. The chemical mappings of hollow Zr0.5Ti0.5O2 spheres are also displayed in Fig. 8a. As can be seen, the elements Ti, Zr, and S are evenly distributed within the spheres. The acid strength of sulfated hollow Zr0.5Ti0.5O2 spheres was experimentally investigated by temperature-programmed ammonia desorption (NH3-TPD). Hollow Zr0.5Ti0.5O2 spheres exhibit weak Lewis acids property [83-87]. Two broad peaks on the NH3-TPD curve with a maximum in the 500-700 K range characterize the Lewis acid sites of this oxide (Fig. 8b). On the TPD curves of sulfated oxide, more distinct peaks appeared. The sulfate groups generate strong

Lewis and Br nsted acidity in hollow

Zr0.5Ti0.5O2 spheres. The strongest acid sites release NH3 at temperatures as high as about 950 K. These sites were shown to be of Lewis acid nature [86]. The broad peak at temperatures below 900 K was treated as an envelope consisting of overlapping component peaks, which may include ammonia released from weak Lewis acid sites, such as NH4+ ions and coordinately unsaturated zirconium and titanium ions, desorption of ammonia strongly bound to the strong Lewis acid sites and decomposition of the NH4+ ions. The sulfated hollow Zr0.5Ti0.5O2 nanospheres are hypothesized to be more acidic than hollow Zr0.5Ti0.5O2 nanospheres by considering their available protons for donation. To validate the acid catalytic activity of sulfated hollow

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Zr0.5Ti0.5O2 nanospheres and hollow Zr0.5Ti0.5O2 nanospheres, the acid-catalyzed deprotection of benzaldehyde dimethyl acetal to benzaldehyde was chosen as a probe reaction (Fig. 6c) [26]. As the acetal is stable in neutral and alkaline conditions, the deprotection reaction does not proceed without acid catalysts. When hollow Zr0.5Ti0.5O2 nanospheres were used as the acid catalysts, nearly 15% of the benzaldehyde dimethyl acetal was converted into benzaldehyde after a 3 h reaction. In contrast, a conversion (>90%) of the benzaldehyde dimethyl acetal was achieved within 3 h when the same dosage of sulfated hollow Zr0.5Ti0.5O2 nanospheres was used as the catalyst. The higher acid-catalyzed activity of sulfated hollow Zr0.5Ti0.5O2 nanospheres can therefore be nicely explained by the superior acidity of sulfated hollow Zr0.5Ti0.5O2 nanospheres over hollow Zr0.5Ti0.5O2 nanospheres. On the basis of these results, sulfated hollow Zr0.5Ti0.5O2 nanospheres obtained in this work are promising candidates for solid acid catalysts.

Figure 9 (a) Schematic illustration of the multistep reaction sequence involving an acid catalysis and subsequent catalytic hydrogenation for the synthesis of 2-(4-aminophenyl)-1H-benzimidazole. (b) Elemental mappings of yolk-shell Pd/Au@ Zr0.5Ti0.5O2 nanospheres.

3.5 Yolk-Shell Structures for Multistep Reaction Sequences. Recently, great efforts have been made to use acid- and base- functionalized silica as functional catalysts for multistep reactions [88-91]. However, the design of new multifunctional catalysts with either cooperative or independent catalytic performances is still a challenge.

Benzimidazole derivatives have attracted increasing interest over recent decades due to their anticancer, antiviral, and antiulcer properties [92-94]. Recently, Zheng’s group has reported a recyclable yolk-shell Pd@ordered mesoporous aluminosilica as bifunctional catalysts for multistep reactions to the syntheses of benzimidazole derivatives [26]. To demonstrate that the yolk-shell Pd/Au@Zr0.5Ti0.5O2 structures in this work can also be used as high performance catalyst for multistep reactions, the same synthesis route including an acid catalysis and catalytic hydrogenation was used as a model one-pot reaction for the synthesis of benzimidazole derivatives (Fig. 9a). Biologically active compound 2-(4-aminophenyl)-1H-benzimidazole was proposed as the target compound.

As the mesoporous zirconium titanium oxide shells would serve as acid catalysts, we therefore employed Pd nanoparticles for catalytic hydrogenation. In our studies, yolk-shell Au@Zr0.5Ti0.5O2 nanospheres were used as the precursors to Pd incorporation into hollow zirconium titanium oxide spheres for Au nanoparticles in Au@Zr0.5Ti0.5O2 nanospheres can serve as the seeds to facilitate the Pd deposition only inside the hollow spheres.

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Typically, a potassium tetrachloropallidate (K2PdCl4) solution was added to an aqueous suspension containing Au@Zr0.5Ti0.5O2 nanospheres and ascorbic acid under constant stirring at room temperature. During the reaction, the change of the reddish suspension into a final black one can be observed. The elemental mappings and TEM images of the isolated final black suspension confirmed the confined growth of fine Pd nanoparticles on Au nanoparticles inside the hollow cavity (Fig. 9b and Fig. S7). The as-synthesized Pd/Au@Zr0.5Ti0.5O2 nanospheres were then used as the catalysts for the one-pot two-step synthesis of 2-(4-aminophenyl)-1H-benzimidazole from 4-nitrobenzaldehyde and 1,2-phenylenediamine. The intermediate 2-(4-aminophenyl)-1H-benzimidazole and the final product 2-(4-aminophenyl)-1H-benzimidazole were identified by mass spectrometry and 1H NMR, respectively, in the Supporting Information. After both acid catalysis and hydrogenation steps, nearly 100% conversion of the reactant 4-nitrobenzaldehyde and 96% yield of the desired product 2-(4-aminophenyl)-1H-benzimidazole were obtained. In the previous report on the two-step synthesis of 2-(4-aminophenyl)-1H-benzimidazole, the total

yield was 92% using recyclable catalysts [26].

In a control experiment, no conversion of reactants was detected in the absence of the catalysts. Although ordered mesoporous zirconium titanium oxide shells could catalyze the first step of the two-step reaction sequence, neither Zr0.5Ti0.5O2 nanospheres nor Au@Zr0.5Ti0.5O2 nanospheres gave the

conversion of the reactants into the final product 2-(4-aminophenyl)-1H-benzimidazole. In the absence of Pd components in the catalysts, the intermediate 2-(4-nitrophenyl)-1H-benzimidazole was obtained as the main product. Pd/Au@Zr0.5Ti0.5O2 nanospheres are hypothesized to have high catalytic activity and chemically stable structures after catalysis cycles as Au@Zr0.5Ti0.5O2 nanospheres. After the synthesis of 2-(4-aminophenyl)-1H-benzimidazole, Pd/Au@Zr0.5Ti0.5O2 nanospheres were separated by centrifugation, washed with methanol, and redispersed into methanol for the next catalysis cycle. The catalyst Pd/Au@Zr0.5Ti0.5O2 nanospheres owed high activity in the ten successive cycles under the same reaction conditions (Fig. S8). The yield of the products in the tenth cycle was 84%, confirming that the catalytic sites of Pd and mesoporous zirconium titanium oxide shell were stable and recyclable.

4 Conclusions

In summary, we have developed a general coating method for the preparation of hollow and yolk-shell zirconium titanium oxide nanospheres with uniform mesopores. A key feature of the synthetic strategy is that HDA serves as the soft template for mesostructured metal oxides formation, which allows for uniform mesoporous zirconium titanium oxide shell coating on commonly used hard templates with different compositions and morphologies. The hollow and yolk-shell mesoporous nanostructures are obtained by solvothermal treatment and selectively etching the hard templates. The hollow core sizes and shell thicknesses of hollow structures can be tailored by adjusting the diameters of silica spheres and the concentration of metal oxide precursors. The excellent catalytic performance and stability of the reduction of 4-nitrophenol with yolk-shell

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Au@Zr0.5Ti0.5O2 nanospheres demonstrate that mesoporous shells are permeable and yolk-shell structures with mesoporous zirconium titanium oxide shells are ideal candidates for nanoreactors. More importantly, on the basis of the available acidity and adjustable yolk, Pd/Au@Zr0.5Ti0.5O2 nanospheres show high catalytic performances and recyclability in two-step reaction sequences of synthesizing benzimidazole derivatives. The hollow mesoporous zirconium titanium oxide structures reported in this work are powerful platforms for nanoreactors and we believe that the synthesis strategy can be extended to produce new composite materials with enhanced properties for advanced applications.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 21171064 and 21071059).

Electronic Supplementary Material: Supplementary material (Figures showing the dynamic light scattering results, EDS result, TEM images, photograph, NMR and HRMS results of the resultant samples) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher). References 1. Lou, X. W.; Archer, L. A.; Yang, Z. Hollow

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