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Preparation of Janus-type catalysts and their catalytic performance at emulsion interface
Accepted Manuscript Preparation of Janus-type catalysts and their catalytic performance at emulsion interface Yijiang Liu, Jiankang Hu, Xiaotian Yu, Xinyu Xu, Yong Gao, Huaming Li, Fuxin Liang PII: DOI: Reference:
S0021-9797(16)30930-4 http://dx.doi.org/10.1016/j.jcis.2016.11.053 YJCIS 21781
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
26 October 2016 14 November 2016 15 November 2016
Please cite this article as: Y. Liu, J. Hu, X. Yu, X. Xu, Y. Gao, H. Li, F. Liang, Preparation of Janus-type catalysts and their catalytic performance at emulsion interface, Journal of Colloid and Interface Science (2016), doi: http:// dx.doi.org/10.1016/j.jcis.2016.11.053
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Preparation of Janus-type catalysts performance at emulsion interface
and their
catalytic
Yijiang Liua, b*, Jiankang Hua, Xiaotian Yuc, Xinyu Xua, Yong Gaoa, Huaming Lia, Fuxin Liangb, c* a
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province,
China b
Beijing National Laboratory for Molecular Sciences, Zhongguancun North First
Street 2, Beijing100190, China c
State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China *Corresponding author. Tel.: +86 731 58298572; Fax: +86 731 58293264. E-mail address: [email protected]; [email protected]
Abstract: Two Janus-type catalysts were synthesized by selective modification and further functionalization with metal nanoparticles on one or both beads of snowmanlike Janus particles. The catalytic performance of Janus-type catalysts both in homogeneous and interfacial reaction systems was systematically investigated using the reduction of nitro-compound as model reaction. The results showed that Janustype catalysts have excellent catalytic activity in homogeneous reaction system and they are easy to recycle. Further, the Janus-type catalysts exhibited more efficient catalytic activity at emulsion interface than that of oil-water biphasic interface due to the exposed Au nanoparticles on snowman-like Janus particles offer high accessibility to reactants.
Keywords: Janus-type catalyst; selective modification; metal nanoparticles loading; recyclable; emulsion interfacial catalysis.
1. Introduction “Janus” named from the two-faced Roman god and was firstly proposed by PierreGilles de Gennes in his speech for the Nobel Prize in 1991[1]. Since then, Janus materials have attracted widespread attention for both academic and industrial considerations, which include those who have two different compositions and properties compartmentalized onto the same surface [2-5]. Now, various morphology and compositions of Janus materials have been reported, and they have potential applications in solid surfactants, self-propelled motors, display, self-assembly and so on [6-12]. Among various Janus materials, the two-headed snowman-like Janus particles are especially interesting for their tunable chemical composition, shape, and further for their tunable wettability and Janus balance [13-15]. Seed emulsion polymerization is the most typical and powerful methods to synthesize snowman-like Janus particles with tunable Janus balance, which is important to determine the stability and types of the emulsion when they serve as solid surfactants [16-18]. Recently, noble metal nanoparticles are widely used as nanocatalysts, but the aggregation can reduce the catalytic efficiency [19-20]. Consequently, the Janus geometry can be applied to the nanocatalysts to address the aggregation and facilitate the recovery and reuse [21-22]. There are already some reports about Janus geometries as efficient solid catalysts. For example, Pradhan et al. have prepared AuTiO2 snowman-like heterodimers and used as the catalyst in the oxidation of methanol to formaldehyde in the presence of light. The enhanced photocatalytic performance, as compared to that of pure TiO2 nanoparticles, was ascribed to the convenient charge separation of the photogenerated electrons and holes at the metal-TiO2 interface [23]. Z. W. Seh also demonstrated the highest catalytic activity of Janus nanostructured AuTiO2 owing to the Janus geometry offers high accessibility to reactants [24]. A. Kirillova reported the hairy hybrid Janus catalyst comprised of a silica core and two types of polymer shells decorated with Au or Ag nanoparticle, which could catalyze the degradation of dyes and the reduction of the nitro-compound with extremely low amounts of catalyst [25]. Considering the high interfacial energy and excellent emulsion-stabilizing properties of Janus particles, they should have unique advantage
in the interfacial catalysis when used as solid catalyst. Resasco and coworkers have firstly demonstrated the interfacial catalysis of Janus particles in 2010 [26]. They prepared a Janus solid catalyst by depositing palladium onto hydrophilic silica of carbon nanotube-silica hybrid nanoparticles, which could stabilize water-oil emulsions and catalyze biomass refining at the liquid/liquid interface. Further, they reported the Janus silica particles with selectively deposited metal nanoparticles on one side or two sides and catalyzed the reactions in biphasic systems with controlled “phase selectivity” [27-28]. Recently, Ji and coworkers reported polymeric ionic liquid (PIL) functionalized Janus nanosheets and introduced catalytic PW12O403- anion onto the PIL side. The as-synthesized Janus nanosheets exhibit excellent catalytic degradation of water-soluble formethyl orange dyes at the emulsion interface [29]. In the field of interfacial catalysis, Janus-type solid catalyst can avoid the use of phase transfer agent and facilitate the purification of the products and the reuse of the catalyst. However, the research of Janus particles in emulsion interfacial catalysis is rare, and it is still highly desirable to prepare novel Janus-type catalyst for effective emulsion interfacial catalysis. Therefore, we designed and prepared two Janus-type catalysts containing a snowman-like SiO2@PDVB/PS Janus particles as support and Au nanoparticles loaded onto one head or two heads as catalytic active species (Scheme 1), which was noted as Janus catalyst 1 or 2. The reduction of 4-nitrophenol in the aqueous and 4nitroanisole in oil/water interface were selected as the model reactions, and the catalytic properties of the two Janus-type catalysts were investigated in detail.
Scheme 1. Schematic of the preparation of Janus-type catalysts.
2. Experimental 2.1 Materials Styrene (St) and divinylbenzene (DVB) were purchased from Aldrich and destabilized over a neutral Al2O3 column. 3-Methacryloxypropyltrimethoxysilane (MPS) and 3-aminopropyltriethoxysilane (APTES) were purchased from Alfa Aesar. Chloroauric Acid (HAuCl4) was purchased from Adamas. Potassium peroxydisulfate (KPS), sodium dodecyl benzenesulfonate (SDS), azobisisbutyronitrile (AIBN), silver nitrate (AgNO3), n-butylamine, 4-nitrophenol, 4-nitroanisole and NaBH4 were purchased from Sinopharm Chemical Reagent. Aqueous ammonia (28 wt %), N, Ndimethylformamide (DMF), toluene and ethanol were purchased from Beijing Chemical Works. All reagents were used as received unless otherwise stated. 2.2 Synthesis of silica@PDVB/PS Janus particles The silica@PDVB/PS Janus particles were prepared by seed emulsion polymerization as described in the recent literature [30]. The typical procedure is as follows: In 160.0 mL of water, 5.0 g of polystyrene (PS) hollow particles, 3.0 g of DVB, 0.03 g of AIBN and 0.048 g of SDS were dispersed and emulsified. The emulsion was firstly stirred at room temperature for 8 h to swell the PS shell with the monomer/initiator mixture and then heated to 70 ºC to initiate the polymerization for 12 h. The obtained PDVB/PS hollow particles (1.0 g) was further dispersed in 20.0 mL of water at 70 ºC. Then a monomer emulsion containing 0.6 g of MPS, 0.6 g of 1 wt % aqueous KPS, 0.02 g of SDS, and 10.0 mL of water was dropped into the dispersion of PDVB/PS hollow particles to initiate the polymerization for another 12 h. After the polymerization, a desired amount of aqueous ammonia (28 wt %) was added under stirring at 70 ºC for 1 h to induce a further sol-gel process of PMPS. The silica@ PDVB/PS Janus particles were obtained after centrifugation and washing with water and ethanol. 2.3 Synthesis of NH2− modified silica@PDVB/PS Janus particles The silica@ PDVB/PS Janus particle (300 mg) was dispersed in 35.0 mL of ethanol under ultrasonication. Then, 1 mL of APTES added and the mixture was refluxed at 80 ºC for 12 h. After centrifugation and washing with ethanol several times, the APTES modified silica@ PDVB/PS (noted as NH2−silica@ PDVB/PS)
Janus particles were obtained. With the similar approach, the second APTES modified silica@ PDVB Janus particles were gained with toluene as the solvent, linear PS was removed during this process ( noted as NH2−silica@ PDVB Janus particles). 2.4 Synthesis of Janus-type catalysts The NH2−silica@ PDVB/PS Janus particles (200 mg) were dispersed in 40.0 mL of ethanol under ultrasonication. 15 mg of HAuCl4 was added and the mixture was stirred at room temperature for 12 h. Then, 50 mg of NaBH4 was added and the mixture was stirred for another 2 h. The product was centrifuged and washed with ethanol several times, and the Janus-type catalyst of Au-NH2−silica@ PDVB/PS Janus particles (marked as Janus-type catalyst 1) was obtain after dried at vacuum oven. With the analogous method, the Janus-type catalyst 2 of Au-NH2−silica@ PDVB Janus particles was achieved. Moreover, Janus-type catalyst of Ag nanoparticles composited silica@ PDVB/PS Janus particles can be directly prepared by the following process: the silica@ PDVB/PS Janus particles (50 mg) were dispersed in 50.0 mL of ethanol under ultrasonication, then 45 mg of AgNO3 and 18 μL of n-butylamine were added. The mixture was heated to 80 ºC for 1 h. After cooling down, Ag@silica@ PDVB/PS Janus particles were received by centrifuging and washing with ethanol several times. 2.5 Emulsification with Janus-type catalysts 5 mg of Janus-type catalyst 1 was added to the mixed solvent of 0.5 mL of water and 4.5 mL of toluene at 25 ºC. Then, a water-in-oil emulsion was formed under vigorously stirring for 2 min. Similarly, Janus-type catalyst 2 was used, and another water-in-oil emulsion was obtained. 2.6 General procedure for the catalytic reduction of 4-nitrophenol 5.0 mg of Janus-type catalyst 1 particles were added to 4-nitrophenol (4.5 mL, 0.1 mM) aqueous solution under ultrasonication. Then, freshly prepared aqueous of NaBH4 (0.5 mL, 0.2 M) was added. After that, interval samples were taken and analyzed by UV−vis spectroscopy immediately until the absorption intensity keep unchanged. 2.7 General procedure for the catalytic reduction of 4-nitroanisole
5.0 mg of Janus-type catalyst 2 particles were added to the mixture of 4nitroanisole (4.5 mL, 0.1 mM) and toluene under ultrasonication. Then, freshly prepared aqueous of NaBH4 (0.5 mL, 0.2 M) was added and an emulsion was formed under violently stirring. The reaction was proceeded at 25 ºC, interval samples were taken by centrifugation and analyzed by UV-Vis spectroscopy immediately until the absorption intensity keep unchanged. As comparison, the reduction of 4-nitroanisole at oil-water biphasic was performed as follow: 5.0 mg of Janus-type catalyst 2 particles were added to the solution of 4nitroanisole and toluene (4.5 mL, 0.1 mM) under ultrasonication. Then, freshly prepared aqueous of NaBH4 (0.5 mL, 0.2 M) was added. The mixture was stirred at 25 ºC. Interval samples were taken by centrifugation and analyzed by UV-Vis spectroscopy immediately until the absorption intensity keep unchanged. 2.8 Characterization Morphology of the as-synthesized Janus particles was characterized with scanning electron microscopy (Hitachi S-4800 at 15KV) equipped with an energy dispersive Xray (EDX) analyzer and transmission electron microscopy (JEOL1011 at 100 kV). Zeta Sizer 3000 HS (Malvern Instruments) was used to measure Zeta potential. X-ray diffraction (XRD) was measured on Rigaku D/max-2500 X-ray diffractometer with Cuka radiation (λ=0.154 nm) as the X-ray source. UV-Vis spectra were recorded with an Agilent Cary 100 UV-Vis spectrometer. 3. Results and discussion 3.1 Synthesis and characterization of NH2− modified silica@PDVB/PS Janus particles Silica@PDVB/PS Janus particles with a hollow PDVB/PS sphere (about 400 nm) and a SiO2 hemisphere (about 300 nm) were prepared by seed emulsion polymerization as described in the literature (Fig. 1a) [30]. The Si-OH groups on SiO2 hemisphere are reactive, which can be selectively modified by silanes. For example, the NH2− modified silica@PDVB/PS Janus particles were obtained by APTES to introduce functional NH2− groups on SiO2 hemisphere. The presence of the NH2− groups was confirmed by Zeta potential, EDX element analysis and FT-IR. The Zeta potential enhanced from –28.2 mV to 12.1 mV, indicating that Si-OH groups on the surface were mainly terminated by NH2− groups (Fig. S1a). The new peak at 3271
cm−1 was assigned to the stretching vibration of –NH2 (Fig. S1b). The N element in EDX spectrum further verified the successful modification of Janus particles (Fig. S1d). The morphology of silica@PDVB/PS Janus particles was not influenced by the introduced NH2− groups when the reaction proceed in ethanol (Fig. 1b). However, when toluene was used as the solvent, the surface of polymer sphere became coarse (Fig. 1c) and the porous hollow polymer sphere was observed (Fig. 1d). This can be explained that the linear PS portion was dissolved in toluene.
a)
b)
c)
d)
Fig. 1. SEM and inset TEM images of (a) silica@PDVB/PS Janus particles; (b) NH2−modified silica@PDVB/PS Janus particles prepared in ethanol; (c) SEM and (d) TEM image of NH2− modified silica@PDVB Janus particles prepared in toluene. 3.2 Synthesis and characterization of Janus-type catalysts After introducing NH2− groups on the surface of silica hemisphere, Au nanoparticles could grow on the surface of silica hemisphere because of the coordination interaction between NH2− and HAuCl4. Fig. 2a shows the SEM image of Janus-type catalyst 1 with Au nanoparticles loaded on silica sphere of silica@PDVB/PS Janus particles. It is clearly seen that the silica surface became coarse. Au nanoparticles with about 5 nm in diameter were selectively grown on silica hemisphere (Fig. 2b). At the same time, Janus-type catalyst 2 was prepared with Au
nanoparticles grown on both beads of the NH2− modified silica@PDVB Janus particles due to the coordination interaction between NH2− and HAuCl4 on silica hemisphere and capillary action on PDVB sphere. The surface Janus-type catalyst 2 also became coarser (Fig. 2c), TEM image shows that Au nanoparticles were regularly distributed on two beads (Fig. 2d). The new Au element in EDX spectrum together with the characteristic diffraction peaks in XRD spectrum confirmed the Au nanoparticles were successfully loaded (Fig. S2). More interesting, Ag nanoparticles can be directly grown on silica hemisphere of silica@PDVB/PS Janus particles for the interaction between Ag+ and Si-OH groups on silica head (Fig. S3). Ag nanoparticles with about 50 nm in diameter were selectively grown on silica hemisphere. EDX and XRD spectra confirmed that Ag nanoparticles were successfully loaded. This method for preparation of Janus-type catalysts on silica@PDVB/PS Janus particles is general and applicable to many other metal nanoparticles.
a)
b)
c)
d)
Fig. 2. (a) SEM and (b) TEM image of Janus-type catalyst 1; (c) SEM and (d) TEM image of Janus-type catalyst 2. 3.3 Emulsification performance of the Janus-type catalysts
The snowman-like Janus silica@PDVB/PS particles is an excellent kind of solid emulsifier, which can stabilize a water-in-oil emulsion with an average diameter about 10-40 μm (Fig. 3a). After functionalizing with metal nanoparticles, there is no obvious change of the dispersibility of the obtained Janus-type catalysts, both of which can disperse in water and organic solvent such as toluene (Fig. 3b). In other word, metal nanoparticles have a negligible effect on the amphiphilicity of the Janustype catalysts. Therefore, the Janus-type catalysts can be used as a solid surfactant to emulsify the incompatible water and toluene. After vigorously stirring, a stable waterin-oil emulsion was formed at the bottom phase for Janus-type catalyst 1 and 2, and the color of the emulsion phase became brown and light black because of the loaded metal nanoparticles, respectively (inset Fig. 3c and 3d). The average diameter of the emulsion droplets is in the range of 50-100 μm and 100-200 μm (Fig. 3c and 3d). The slight bigger emulsion droplets for Janus-type catalyst 2 may attribute to both beads of Janus-type catalyst 2 were loaded with metal nanoparticles.
a)
c)
b)
d)
Fig. 3. (a) Optical microscopy image and inset photograph of the water-in-oil emulsion emulsified by silica@PDVB/PS; (b) Photographs of Janus-type catalyst 1 and 2 dispersed in water (left) and toluene (right); Optical microscopy images and inset photographs of the water-in-oil emulsion emulsified by (c) Janus-type catalyst 1; (d) Janus-type catalyst 2.
3.4 Catalytic reduction of 4-nitrophenol in aqueous system To investigate the catalytic performance of the immobilized Au nanoparticles on the snowman Janus particles, the reduction of 4-nitrophenol was selected as the model reaction. The reaction started immediately after NaBH4 aqueous solution was added to the mixture of 4-nitrophenol and Janus-type catalyst 1. The samples were taken and analyzed by UV-Vis spectroscopy after 1 min, 2 min, 20 min and 30 min. The lightyellow aqueous solution of 4-nitrophenol gradually became colorless with the reaction proceeding (inset, Fig. 4a). It can be preliminarily ascertained that 4-nitrophenol was almost completely changed to 4-aminophenol about 30 min from the color of the reaction mixture. In UV-Vis spectra, 4-nitrophenol showed a typical absorption peak at 317 nm, and which shifted to 400 nm for the ionization of 4-nitrophenol after the addition of NaBH4. With increasing the reaction time, the absorption peak at 400 nm was gradually decreased and accompanied by the appearance of a new absorption band at 300 nm (Fig. 4a). The enhanced new absorption band was confirmed to be the characteristic band of the reduction product of 4-aminophenol from the UV-Vis spectrum (Fig. S4). The recycle of Janus-type catalyst 1 was achieved by centrifuging and washing with ethanol. There was no obvious change of the catalytic efficiency after five times recycles (Fig. 4b). Under the same condition, Janus-type catalyst 2 showed the similar catalytic performance (Fig. 4c).
a)
b)
c) Fig. 4. (a) UV-Vis spectra of the samples collected after the reaction was proceeding for 1 min, 2 min, 20 min, and 30 min, inset photographs; (b) The UV-Vis spectra of the reduction product with the reused Janus-type catalyst 1 after the reaction was proceeding for 30 min; (c) UV-Vis spectra of the samples collected after the reaction was proceeding for 30 min with Janus-type catalyst 2 as catalyst. 3.5 Catalytic reduction of 4-nitroanisole in emulsion interface The Au nanoparticles functionalized snowman-like Janus particles could be used as solid emulsifier and catalyst at the same time, and the catalytic performance at emulsion interface was systematically studied. An oil-in-water emulsion containing 4nitroanisole and toluene as oil phase and NaBH4 aqueous solution as aqueous phase was stabilized by Janus-type catalyst 2 (Fig. S5), the reduction of 4-nitroanisole was proceeding at emulsion interface as illustrated in Scheme 2 at 25 ºC. Fig. 5a shows the UV-Vis spectra of 4-nitroanisole, 4-aminoanisole and the samples taken from the reaction system at different reaction time. The absorption peak of 4-nitroanisole centered at 304 nm was gradually red-shifted to 309 nm with the reaction proceeding. The conversion rate of 4-nitroanisole to 4-aminoanisole was estimated to be 90.76% at 2 h according to the standard curve based on concentration and absorbance of 4nitroanisole (Fig. S6a) and 4-aminoanisole (Fig. S6b). There was only 3.76% enhancement after prolong the reaction time to 10 h (Fig. S7), thus 2 h was selected as the reaction time in the next steps. Further, Janus-type catalyst 1 was used as catalyst for the reduction of 4-nitroanisole under the same condition. The UV-Vis spectra of the samples taken from different reaction time are shown Fig. 5b. The conversion rate at 2 h with Janus-type catalyst 1 was 89.8%, which was a litter lower than that of 2 at the emulsion interface.
Scheme 2. Schematic of the emulsion interfacial catalysis. For comparison, the reduction of 4-nitroanisole was performed at the oil-water biphasic interface and the UV-Vis spectra of the samples taken at 2 h are shown in Fig. 5c, and the conversion rate was also calculated. Fig. 5d shows the relationship between the conversion rate and the two reaction systems. It is clearly seen that the conversion of 4-nitroanisole at emulsion interface was significantly higher than that of oil-water biphasic interface for the two Janus-type catalysts. The excellent catalytic performance of Janus-type catalysts on emulsion interface was attributed to the larger reaction area of the emulsion interface compared to biphasic interface. At the same time, the conversion rate of Janus-type catalyst 2 was higher than that of Janus-type catalyst 1 on oil-water biphasic interfacial catalysis, which was consist with the emulsion interface catalysis. Although the relative bigger emulsion droplets of Janustype catalyst 2, it is understandable that the higher conversion rate result from the more loaded Au nanoparticles on both beads of Janus-type catalyst 2.
a)
b)
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Fig. 5. UV-Vis spectra of the samples collected at different reaction time using (a) Janus-type catalyst 2; (b) Janus-type catalyst 1; (c) UV-Vis spectra of the samples taken at 2 h using Janus-type catalyst 1 and 2 in biphasic system; (d) Conversion rate of 4-nitroanisole in emulsion interfacial catalysis (A) and biphasic interfacial catalysis (B) using the two Janus-type catalysts. 4. Conclusions In summary, two Janus-type catalysts were prepared with snowman-like Janus particles as a support and Au nanoparticles on one or two beads as catalytic species through selective modification. The Janus-type catalysts showed excellent catalytic efficiency in homogeneous reaction systems and could be reused with similar catalytic activity. What’s more, Janus-type catalysts could serve as emulsifier and catalysis at the same time. Compared to water-oil biphasic interfacial catalysis, a higher conversion rate of nitro-compound was obtained at water-in-oil emulsion interfacial catalysis due to the good emulsification property of Janus-type catalysts provided a high accessibility for the reactants. The conception of emulsion interfacial catalysis using Janus-type catalyst provide a new way for conventional interfacial catalysis, which may be widely used in future.
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Graphical abstract
The Janus-type catalyst can stabilize a water-in-oil emulsion and exhibit high catalytic activity for the reduction of nitro-compound at emulsion interface.
S0021-9797(16)30930-4 http://dx.doi.org/10.1016/j.jcis.2016.11.053 YJCIS 21781
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
26 October 2016 14 November 2016 15 November 2016
Please cite this article as: Y. Liu, J. Hu, X. Yu, X. Xu, Y. Gao, H. Li, F. Liang, Preparation of Janus-type catalysts and their catalytic performance at emulsion interface, Journal of Colloid and Interface Science (2016), doi: http:// dx.doi.org/10.1016/j.jcis.2016.11.053
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of Janus-type catalysts performance at emulsion interface
and their
catalytic
Yijiang Liua, b*, Jiankang Hua, Xiaotian Yuc, Xinyu Xua, Yong Gaoa, Huaming Lia, Fuxin Liangb, c* a
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province,
China b
Beijing National Laboratory for Molecular Sciences, Zhongguancun North First
Street 2, Beijing100190, China c
State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China *Corresponding author. Tel.: +86 731 58298572; Fax: +86 731 58293264. E-mail address: [email protected]; [email protected]
Abstract: Two Janus-type catalysts were synthesized by selective modification and further functionalization with metal nanoparticles on one or both beads of snowmanlike Janus particles. The catalytic performance of Janus-type catalysts both in homogeneous and interfacial reaction systems was systematically investigated using the reduction of nitro-compound as model reaction. The results showed that Janustype catalysts have excellent catalytic activity in homogeneous reaction system and they are easy to recycle. Further, the Janus-type catalysts exhibited more efficient catalytic activity at emulsion interface than that of oil-water biphasic interface due to the exposed Au nanoparticles on snowman-like Janus particles offer high accessibility to reactants.
Keywords: Janus-type catalyst; selective modification; metal nanoparticles loading; recyclable; emulsion interfacial catalysis.
1. Introduction “Janus” named from the two-faced Roman god and was firstly proposed by PierreGilles de Gennes in his speech for the Nobel Prize in 1991[1]. Since then, Janus materials have attracted widespread attention for both academic and industrial considerations, which include those who have two different compositions and properties compartmentalized onto the same surface [2-5]. Now, various morphology and compositions of Janus materials have been reported, and they have potential applications in solid surfactants, self-propelled motors, display, self-assembly and so on [6-12]. Among various Janus materials, the two-headed snowman-like Janus particles are especially interesting for their tunable chemical composition, shape, and further for their tunable wettability and Janus balance [13-15]. Seed emulsion polymerization is the most typical and powerful methods to synthesize snowman-like Janus particles with tunable Janus balance, which is important to determine the stability and types of the emulsion when they serve as solid surfactants [16-18]. Recently, noble metal nanoparticles are widely used as nanocatalysts, but the aggregation can reduce the catalytic efficiency [19-20]. Consequently, the Janus geometry can be applied to the nanocatalysts to address the aggregation and facilitate the recovery and reuse [21-22]. There are already some reports about Janus geometries as efficient solid catalysts. For example, Pradhan et al. have prepared AuTiO2 snowman-like heterodimers and used as the catalyst in the oxidation of methanol to formaldehyde in the presence of light. The enhanced photocatalytic performance, as compared to that of pure TiO2 nanoparticles, was ascribed to the convenient charge separation of the photogenerated electrons and holes at the metal-TiO2 interface [23]. Z. W. Seh also demonstrated the highest catalytic activity of Janus nanostructured AuTiO2 owing to the Janus geometry offers high accessibility to reactants [24]. A. Kirillova reported the hairy hybrid Janus catalyst comprised of a silica core and two types of polymer shells decorated with Au or Ag nanoparticle, which could catalyze the degradation of dyes and the reduction of the nitro-compound with extremely low amounts of catalyst [25]. Considering the high interfacial energy and excellent emulsion-stabilizing properties of Janus particles, they should have unique advantage
in the interfacial catalysis when used as solid catalyst. Resasco and coworkers have firstly demonstrated the interfacial catalysis of Janus particles in 2010 [26]. They prepared a Janus solid catalyst by depositing palladium onto hydrophilic silica of carbon nanotube-silica hybrid nanoparticles, which could stabilize water-oil emulsions and catalyze biomass refining at the liquid/liquid interface. Further, they reported the Janus silica particles with selectively deposited metal nanoparticles on one side or two sides and catalyzed the reactions in biphasic systems with controlled “phase selectivity” [27-28]. Recently, Ji and coworkers reported polymeric ionic liquid (PIL) functionalized Janus nanosheets and introduced catalytic PW12O403- anion onto the PIL side. The as-synthesized Janus nanosheets exhibit excellent catalytic degradation of water-soluble formethyl orange dyes at the emulsion interface [29]. In the field of interfacial catalysis, Janus-type solid catalyst can avoid the use of phase transfer agent and facilitate the purification of the products and the reuse of the catalyst. However, the research of Janus particles in emulsion interfacial catalysis is rare, and it is still highly desirable to prepare novel Janus-type catalyst for effective emulsion interfacial catalysis. Therefore, we designed and prepared two Janus-type catalysts containing a snowman-like SiO2@PDVB/PS Janus particles as support and Au nanoparticles loaded onto one head or two heads as catalytic active species (Scheme 1), which was noted as Janus catalyst 1 or 2. The reduction of 4-nitrophenol in the aqueous and 4nitroanisole in oil/water interface were selected as the model reactions, and the catalytic properties of the two Janus-type catalysts were investigated in detail.
Scheme 1. Schematic of the preparation of Janus-type catalysts.
2. Experimental 2.1 Materials Styrene (St) and divinylbenzene (DVB) were purchased from Aldrich and destabilized over a neutral Al2O3 column. 3-Methacryloxypropyltrimethoxysilane (MPS) and 3-aminopropyltriethoxysilane (APTES) were purchased from Alfa Aesar. Chloroauric Acid (HAuCl4) was purchased from Adamas. Potassium peroxydisulfate (KPS), sodium dodecyl benzenesulfonate (SDS), azobisisbutyronitrile (AIBN), silver nitrate (AgNO3), n-butylamine, 4-nitrophenol, 4-nitroanisole and NaBH4 were purchased from Sinopharm Chemical Reagent. Aqueous ammonia (28 wt %), N, Ndimethylformamide (DMF), toluene and ethanol were purchased from Beijing Chemical Works. All reagents were used as received unless otherwise stated. 2.2 Synthesis of silica@PDVB/PS Janus particles The silica@PDVB/PS Janus particles were prepared by seed emulsion polymerization as described in the recent literature [30]. The typical procedure is as follows: In 160.0 mL of water, 5.0 g of polystyrene (PS) hollow particles, 3.0 g of DVB, 0.03 g of AIBN and 0.048 g of SDS were dispersed and emulsified. The emulsion was firstly stirred at room temperature for 8 h to swell the PS shell with the monomer/initiator mixture and then heated to 70 ºC to initiate the polymerization for 12 h. The obtained PDVB/PS hollow particles (1.0 g) was further dispersed in 20.0 mL of water at 70 ºC. Then a monomer emulsion containing 0.6 g of MPS, 0.6 g of 1 wt % aqueous KPS, 0.02 g of SDS, and 10.0 mL of water was dropped into the dispersion of PDVB/PS hollow particles to initiate the polymerization for another 12 h. After the polymerization, a desired amount of aqueous ammonia (28 wt %) was added under stirring at 70 ºC for 1 h to induce a further sol-gel process of PMPS. The silica@ PDVB/PS Janus particles were obtained after centrifugation and washing with water and ethanol. 2.3 Synthesis of NH2− modified silica@PDVB/PS Janus particles The silica@ PDVB/PS Janus particle (300 mg) was dispersed in 35.0 mL of ethanol under ultrasonication. Then, 1 mL of APTES added and the mixture was refluxed at 80 ºC for 12 h. After centrifugation and washing with ethanol several times, the APTES modified silica@ PDVB/PS (noted as NH2−silica@ PDVB/PS)
Janus particles were obtained. With the similar approach, the second APTES modified silica@ PDVB Janus particles were gained with toluene as the solvent, linear PS was removed during this process ( noted as NH2−silica@ PDVB Janus particles). 2.4 Synthesis of Janus-type catalysts The NH2−silica@ PDVB/PS Janus particles (200 mg) were dispersed in 40.0 mL of ethanol under ultrasonication. 15 mg of HAuCl4 was added and the mixture was stirred at room temperature for 12 h. Then, 50 mg of NaBH4 was added and the mixture was stirred for another 2 h. The product was centrifuged and washed with ethanol several times, and the Janus-type catalyst of Au-NH2−silica@ PDVB/PS Janus particles (marked as Janus-type catalyst 1) was obtain after dried at vacuum oven. With the analogous method, the Janus-type catalyst 2 of Au-NH2−silica@ PDVB Janus particles was achieved. Moreover, Janus-type catalyst of Ag nanoparticles composited silica@ PDVB/PS Janus particles can be directly prepared by the following process: the silica@ PDVB/PS Janus particles (50 mg) were dispersed in 50.0 mL of ethanol under ultrasonication, then 45 mg of AgNO3 and 18 μL of n-butylamine were added. The mixture was heated to 80 ºC for 1 h. After cooling down, Ag@silica@ PDVB/PS Janus particles were received by centrifuging and washing with ethanol several times. 2.5 Emulsification with Janus-type catalysts 5 mg of Janus-type catalyst 1 was added to the mixed solvent of 0.5 mL of water and 4.5 mL of toluene at 25 ºC. Then, a water-in-oil emulsion was formed under vigorously stirring for 2 min. Similarly, Janus-type catalyst 2 was used, and another water-in-oil emulsion was obtained. 2.6 General procedure for the catalytic reduction of 4-nitrophenol 5.0 mg of Janus-type catalyst 1 particles were added to 4-nitrophenol (4.5 mL, 0.1 mM) aqueous solution under ultrasonication. Then, freshly prepared aqueous of NaBH4 (0.5 mL, 0.2 M) was added. After that, interval samples were taken and analyzed by UV−vis spectroscopy immediately until the absorption intensity keep unchanged. 2.7 General procedure for the catalytic reduction of 4-nitroanisole
5.0 mg of Janus-type catalyst 2 particles were added to the mixture of 4nitroanisole (4.5 mL, 0.1 mM) and toluene under ultrasonication. Then, freshly prepared aqueous of NaBH4 (0.5 mL, 0.2 M) was added and an emulsion was formed under violently stirring. The reaction was proceeded at 25 ºC, interval samples were taken by centrifugation and analyzed by UV-Vis spectroscopy immediately until the absorption intensity keep unchanged. As comparison, the reduction of 4-nitroanisole at oil-water biphasic was performed as follow: 5.0 mg of Janus-type catalyst 2 particles were added to the solution of 4nitroanisole and toluene (4.5 mL, 0.1 mM) under ultrasonication. Then, freshly prepared aqueous of NaBH4 (0.5 mL, 0.2 M) was added. The mixture was stirred at 25 ºC. Interval samples were taken by centrifugation and analyzed by UV-Vis spectroscopy immediately until the absorption intensity keep unchanged. 2.8 Characterization Morphology of the as-synthesized Janus particles was characterized with scanning electron microscopy (Hitachi S-4800 at 15KV) equipped with an energy dispersive Xray (EDX) analyzer and transmission electron microscopy (JEOL1011 at 100 kV). Zeta Sizer 3000 HS (Malvern Instruments) was used to measure Zeta potential. X-ray diffraction (XRD) was measured on Rigaku D/max-2500 X-ray diffractometer with Cuka radiation (λ=0.154 nm) as the X-ray source. UV-Vis spectra were recorded with an Agilent Cary 100 UV-Vis spectrometer. 3. Results and discussion 3.1 Synthesis and characterization of NH2− modified silica@PDVB/PS Janus particles Silica@PDVB/PS Janus particles with a hollow PDVB/PS sphere (about 400 nm) and a SiO2 hemisphere (about 300 nm) were prepared by seed emulsion polymerization as described in the literature (Fig. 1a) [30]. The Si-OH groups on SiO2 hemisphere are reactive, which can be selectively modified by silanes. For example, the NH2− modified silica@PDVB/PS Janus particles were obtained by APTES to introduce functional NH2− groups on SiO2 hemisphere. The presence of the NH2− groups was confirmed by Zeta potential, EDX element analysis and FT-IR. The Zeta potential enhanced from –28.2 mV to 12.1 mV, indicating that Si-OH groups on the surface were mainly terminated by NH2− groups (Fig. S1a). The new peak at 3271
cm−1 was assigned to the stretching vibration of –NH2 (Fig. S1b). The N element in EDX spectrum further verified the successful modification of Janus particles (Fig. S1d). The morphology of silica@PDVB/PS Janus particles was not influenced by the introduced NH2− groups when the reaction proceed in ethanol (Fig. 1b). However, when toluene was used as the solvent, the surface of polymer sphere became coarse (Fig. 1c) and the porous hollow polymer sphere was observed (Fig. 1d). This can be explained that the linear PS portion was dissolved in toluene.
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Fig. 1. SEM and inset TEM images of (a) silica@PDVB/PS Janus particles; (b) NH2−modified silica@PDVB/PS Janus particles prepared in ethanol; (c) SEM and (d) TEM image of NH2− modified silica@PDVB Janus particles prepared in toluene. 3.2 Synthesis and characterization of Janus-type catalysts After introducing NH2− groups on the surface of silica hemisphere, Au nanoparticles could grow on the surface of silica hemisphere because of the coordination interaction between NH2− and HAuCl4. Fig. 2a shows the SEM image of Janus-type catalyst 1 with Au nanoparticles loaded on silica sphere of silica@PDVB/PS Janus particles. It is clearly seen that the silica surface became coarse. Au nanoparticles with about 5 nm in diameter were selectively grown on silica hemisphere (Fig. 2b). At the same time, Janus-type catalyst 2 was prepared with Au
nanoparticles grown on both beads of the NH2− modified silica@PDVB Janus particles due to the coordination interaction between NH2− and HAuCl4 on silica hemisphere and capillary action on PDVB sphere. The surface Janus-type catalyst 2 also became coarser (Fig. 2c), TEM image shows that Au nanoparticles were regularly distributed on two beads (Fig. 2d). The new Au element in EDX spectrum together with the characteristic diffraction peaks in XRD spectrum confirmed the Au nanoparticles were successfully loaded (Fig. S2). More interesting, Ag nanoparticles can be directly grown on silica hemisphere of silica@PDVB/PS Janus particles for the interaction between Ag+ and Si-OH groups on silica head (Fig. S3). Ag nanoparticles with about 50 nm in diameter were selectively grown on silica hemisphere. EDX and XRD spectra confirmed that Ag nanoparticles were successfully loaded. This method for preparation of Janus-type catalysts on silica@PDVB/PS Janus particles is general and applicable to many other metal nanoparticles.
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Fig. 2. (a) SEM and (b) TEM image of Janus-type catalyst 1; (c) SEM and (d) TEM image of Janus-type catalyst 2. 3.3 Emulsification performance of the Janus-type catalysts
The snowman-like Janus silica@PDVB/PS particles is an excellent kind of solid emulsifier, which can stabilize a water-in-oil emulsion with an average diameter about 10-40 μm (Fig. 3a). After functionalizing with metal nanoparticles, there is no obvious change of the dispersibility of the obtained Janus-type catalysts, both of which can disperse in water and organic solvent such as toluene (Fig. 3b). In other word, metal nanoparticles have a negligible effect on the amphiphilicity of the Janustype catalysts. Therefore, the Janus-type catalysts can be used as a solid surfactant to emulsify the incompatible water and toluene. After vigorously stirring, a stable waterin-oil emulsion was formed at the bottom phase for Janus-type catalyst 1 and 2, and the color of the emulsion phase became brown and light black because of the loaded metal nanoparticles, respectively (inset Fig. 3c and 3d). The average diameter of the emulsion droplets is in the range of 50-100 μm and 100-200 μm (Fig. 3c and 3d). The slight bigger emulsion droplets for Janus-type catalyst 2 may attribute to both beads of Janus-type catalyst 2 were loaded with metal nanoparticles.
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Fig. 3. (a) Optical microscopy image and inset photograph of the water-in-oil emulsion emulsified by silica@PDVB/PS; (b) Photographs of Janus-type catalyst 1 and 2 dispersed in water (left) and toluene (right); Optical microscopy images and inset photographs of the water-in-oil emulsion emulsified by (c) Janus-type catalyst 1; (d) Janus-type catalyst 2.
3.4 Catalytic reduction of 4-nitrophenol in aqueous system To investigate the catalytic performance of the immobilized Au nanoparticles on the snowman Janus particles, the reduction of 4-nitrophenol was selected as the model reaction. The reaction started immediately after NaBH4 aqueous solution was added to the mixture of 4-nitrophenol and Janus-type catalyst 1. The samples were taken and analyzed by UV-Vis spectroscopy after 1 min, 2 min, 20 min and 30 min. The lightyellow aqueous solution of 4-nitrophenol gradually became colorless with the reaction proceeding (inset, Fig. 4a). It can be preliminarily ascertained that 4-nitrophenol was almost completely changed to 4-aminophenol about 30 min from the color of the reaction mixture. In UV-Vis spectra, 4-nitrophenol showed a typical absorption peak at 317 nm, and which shifted to 400 nm for the ionization of 4-nitrophenol after the addition of NaBH4. With increasing the reaction time, the absorption peak at 400 nm was gradually decreased and accompanied by the appearance of a new absorption band at 300 nm (Fig. 4a). The enhanced new absorption band was confirmed to be the characteristic band of the reduction product of 4-aminophenol from the UV-Vis spectrum (Fig. S4). The recycle of Janus-type catalyst 1 was achieved by centrifuging and washing with ethanol. There was no obvious change of the catalytic efficiency after five times recycles (Fig. 4b). Under the same condition, Janus-type catalyst 2 showed the similar catalytic performance (Fig. 4c).
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c) Fig. 4. (a) UV-Vis spectra of the samples collected after the reaction was proceeding for 1 min, 2 min, 20 min, and 30 min, inset photographs; (b) The UV-Vis spectra of the reduction product with the reused Janus-type catalyst 1 after the reaction was proceeding for 30 min; (c) UV-Vis spectra of the samples collected after the reaction was proceeding for 30 min with Janus-type catalyst 2 as catalyst. 3.5 Catalytic reduction of 4-nitroanisole in emulsion interface The Au nanoparticles functionalized snowman-like Janus particles could be used as solid emulsifier and catalyst at the same time, and the catalytic performance at emulsion interface was systematically studied. An oil-in-water emulsion containing 4nitroanisole and toluene as oil phase and NaBH4 aqueous solution as aqueous phase was stabilized by Janus-type catalyst 2 (Fig. S5), the reduction of 4-nitroanisole was proceeding at emulsion interface as illustrated in Scheme 2 at 25 ºC. Fig. 5a shows the UV-Vis spectra of 4-nitroanisole, 4-aminoanisole and the samples taken from the reaction system at different reaction time. The absorption peak of 4-nitroanisole centered at 304 nm was gradually red-shifted to 309 nm with the reaction proceeding. The conversion rate of 4-nitroanisole to 4-aminoanisole was estimated to be 90.76% at 2 h according to the standard curve based on concentration and absorbance of 4nitroanisole (Fig. S6a) and 4-aminoanisole (Fig. S6b). There was only 3.76% enhancement after prolong the reaction time to 10 h (Fig. S7), thus 2 h was selected as the reaction time in the next steps. Further, Janus-type catalyst 1 was used as catalyst for the reduction of 4-nitroanisole under the same condition. The UV-Vis spectra of the samples taken from different reaction time are shown Fig. 5b. The conversion rate at 2 h with Janus-type catalyst 1 was 89.8%, which was a litter lower than that of 2 at the emulsion interface.
Scheme 2. Schematic of the emulsion interfacial catalysis. For comparison, the reduction of 4-nitroanisole was performed at the oil-water biphasic interface and the UV-Vis spectra of the samples taken at 2 h are shown in Fig. 5c, and the conversion rate was also calculated. Fig. 5d shows the relationship between the conversion rate and the two reaction systems. It is clearly seen that the conversion of 4-nitroanisole at emulsion interface was significantly higher than that of oil-water biphasic interface for the two Janus-type catalysts. The excellent catalytic performance of Janus-type catalysts on emulsion interface was attributed to the larger reaction area of the emulsion interface compared to biphasic interface. At the same time, the conversion rate of Janus-type catalyst 2 was higher than that of Janus-type catalyst 1 on oil-water biphasic interfacial catalysis, which was consist with the emulsion interface catalysis. Although the relative bigger emulsion droplets of Janustype catalyst 2, it is understandable that the higher conversion rate result from the more loaded Au nanoparticles on both beads of Janus-type catalyst 2.
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Fig. 5. UV-Vis spectra of the samples collected at different reaction time using (a) Janus-type catalyst 2; (b) Janus-type catalyst 1; (c) UV-Vis spectra of the samples taken at 2 h using Janus-type catalyst 1 and 2 in biphasic system; (d) Conversion rate of 4-nitroanisole in emulsion interfacial catalysis (A) and biphasic interfacial catalysis (B) using the two Janus-type catalysts. 4. Conclusions In summary, two Janus-type catalysts were prepared with snowman-like Janus particles as a support and Au nanoparticles on one or two beads as catalytic species through selective modification. The Janus-type catalysts showed excellent catalytic efficiency in homogeneous reaction systems and could be reused with similar catalytic activity. What’s more, Janus-type catalysts could serve as emulsifier and catalysis at the same time. Compared to water-oil biphasic interfacial catalysis, a higher conversion rate of nitro-compound was obtained at water-in-oil emulsion interfacial catalysis due to the good emulsification property of Janus-type catalysts provided a high accessibility for the reactants. The conception of emulsion interfacial catalysis using Janus-type catalyst provide a new way for conventional interfacial catalysis, which may be widely used in future.
Acknowledgement This study was financially supported by National Natural Science Foundation of China (NSFC 51603177) and the Opening Foundation of Beijing National Laboratory for Molecular Sciences. Notes and references [1] P. G. de Gennes, soft mater, Rev. Mod. Phys. 64 (1992) 645-648.
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Graphical abstract
The Janus-type catalyst can stabilize a water-in-oil emulsion and exhibit high catalytic activity for the reduction of nitro-compound at emulsion interface.