Smart pH-Responsive Polymer-Tethered and Pd NP-Loaded NMOF

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Smart pH-Responsive Polymer-Tethered and Pd NPs-loaded NMOF as the Pickering Interfacial Catalyst for One-Pot Cascade Biphasic Reaction Wei-Ling Jiang, Qi-Juan Fu, Bing-Jian Yao, Luo-Gang Ding, Cong-Xue Liu, and Yu-Bin Dong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12166 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Applied Materials & Interfaces

Smart pH-Responsive Polymer-Tethered and Pd NPs-loaded NMOF as the Pickering Interfacial Catalyst for One-Pot Cascade Biphasic Reaction Wei-Ling Jiang, ‡ Qi-Juan Fu,‡ Bing-Jian Yao,* Luo-Gang Ding, Cong-Xue Liu, and Yu-Bin Dong* College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, P. R. China.

ABSTRACT. A Pd nano particles (NPs)-loaded and nano metal-organic framework (NMOF)based Pickering emulsifier is reported. The poly[2-(diethylamino)ethyl methacrylate)] (PDEAEMA) chains were grafted onto UiO-66-type NPs via post-synthetic approach to generate PDEAEMA-g-UiO-66 NMOF (termed as MOF-3). The Pd NPs-loaded Pd@MOF-3 was synthesized via solution impregnation. Stable toluene-in-water Pickering emulsion was prepared with emulsifier Pd@MOF-3. Notably, the obtained Pd@MOF-3 is pH-responsive, and it is able to trigger the emulsification (at neutral condition) and demulsification (at acidic condition) of toluene droplets. Furthermore, it can be a highly active interfacial catalyst to effectively promote one-pot Knoevenagel condensation-hydrogenation cascade reaction at ambient conditions. The

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pH-responsive property allowed it to be in situ separated and recycled by demulsifying via simply tuning pH value at the end of the reaction. This smart Pickering emulsion catalytic system is robust, and it can be recycled at least five times without loss of its catalytic activity.

KEYWORDS: MOFs, Pickering emulsion, biphasic interface, cascade catalysis, pH response

INTRODUCTION Pickering emulsions involve the self-assembly of solid particles at the interface between two immiscible liquids (typically oil vs water) to prevent the coalescence of the droplet phase.1-4 In Pickering emulsions, the oil and water phases are sufficiently mixed to form a high area of oilwater interface. The recent studies revealed that such a micro-mixing enables the organicaqueous biphasic reactions to proceed efficiently through the auto-diffusion of reactant molecules.5-7 As a rapid emerging class of biphasic catalytic reactions, Pickering interfacial catalysis, where solid particles can simultaneously stabilize an emulsion and catalyze reactions at the interface of two immiscible reagents, has become an attractive topic. More interestingly, if the particles surface is switchable between hydrophilicity and hydrophobicity, Pickering emulsions could be inverted or destroyed.8-11 In this view, Pickering interfacial catalysis might have some unique advantages for the biphasic catalytic reactions, particularly for converting poorly oil-soluble substrates into oil-soluble products, and the catalysts and products could be simply separated based on the stimuli-responsive behavior of the Pickering emulsifiers. So far, various inorganic and organic solid particles such as palygorskite,12 clay,13 silica,14-15 cellulose,16 chitosan,17 lignin,18 lipid NPs,19 zein20 and so on have been used as Pickering emulsion stabilizers. On the other hand, the involved solid particles would affect not only the

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emulsion stability, but also the emulsion functionality. Although Pickering emulsions have been academically investigated for many years, their further applications might be largely limited by the unfunctionalized particle emulsifiers. In principle, the development of functionalized solid particle stabilizers would lead to the new type of multifunctional Pickering emulsions.21 Metal-organic frameworks (MOFs), as an emerging of crystalline porous materials, with high surface area, structural diversity, and tailorability, attract extensive interest and exhibit a variety of potential applications, especially in catalysis. Their permanent porosity enables their inherent superiority in confining guest species such as metal nanoparticles (MNPs) for improved catalytic performance and/or the expansion of reaction scope.22-24 Notably, the size of MOFs can be readily scale down to nanoregimes. The generated nanoscale metal-organic frameworks (NMOFs)25-26 which have well-defined shape and relatively narrow size distribution would expected to be the ideal Pickering emulsifiers. On the other hand, MOFs have been demonstrated to be the ideal supports to upload active catalytic species such as precious metal NPs to result in MOF-supported composite solid catalysts.27-35 Furthermore, MOF NPs with switchable interfacial properties, such as pH-responsive behavior, could be logically realized by tuning the surface decorated functional groups or polymers36-38 via post-synthetic chemical modification.3941

In this way, stimuli responsive and catalytic functionalities would be integrated in Pickering

emulsion, and the new types of catalytic species-loaded and MOF NPs-based stimuli-responsive smart emulsion catalytic systems would be developed. Herein, a Pd NPs-loaded and pH-switchable polymer-grafted UiO-66-type NMOF-based Pickering interfacial catalyst Pd@PDEAEMA-g-UiO-66 (termed as Pd@MOF-3) is reported. It can highly promote one-pot biphasic Knoevenagel condensation-hydrogenation cascade reaction under ambient conditions. The UiO-66 NMOF surface decorated poly[(2-

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diethylamino)ethyl methacrylate)] (termed as PDEAEMA) polymer is pH-responsive, and its hydrophilic-hydrophobic balance can be adjusted by simply tuning pH values. Therefore, the obtained Pd@MOF-3 can transfer a traditional biphasic system to an oil-in-water (O/W) Pickering emulsion system, furthermore, catalyze the biphasic interfacial reaction in the Pickering “micro-reactor” droplets. Once the reaction finished, this Pickering emulsion system can be demulsified upon addition of acid, and then the macroscopic phase separation can be realized. In this way, the organic phase is commodiously isolated via simple liquid transfer, meanwhile the aqueous phase with Pd@MOF-3 catalyst is recycled and used directly for the next catalytic after tuning the system pH value. This pH-triggered emulsifying and demulsifying processes are of economical and environmental interest since it avoids costly separation and purification of intermediate products, while involving less investment and energy consumption. EXPERIMENTAL SECTION

Materials and Instrumentation. All the chemicals were obtained from commercial sources and used without further purification. Infrared (IR) spectra were obtained in the 4004000 cm-1 range with a Bruker ALPHA FTIR Spectrometer. 1H NMR data were collected with a Bruker Avance-400 spectrometer. Chemical shifts are reported in δ relative to TMS. Thermogravimetric analysis was carried out with a TA Instruments Q5 simultaneous TGA under flowing nitrogen of 60 mL min-1 at a heating rate of 10 ℃ min-1. SEM micrographs were recorded on a Gemini Zeiss Supra TM scanning electron microscope equipped with energydispersive X-ray detector (EDS). The XRD pattern was obtained on D8 ADVANCE X-ray powder diffractometer with Cu Kα radiation (λ = 1.5405 Å). The contact angle (CA) measurement was performed with a Data Physics OCA-20 contact angle analyzer that had a CCD camera equipped for image capture. The deionized water droplet volume was 4 µL, and the

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measurement were taken under ambient atmospheric conditions. Gel permeation chromatography (GPC) equipped with a Waters 515 system, a 2410 refractive index detector and three Styragel gel columns was calibrated with narrow-molecular-weight polystyrene (PS) standards for molecular weight characterization. Transmission electron microscopy (TEM) was performed on a JEM-1400 electron microscope (JEOL) operated at an accelerating voltage of 120 KV. Confocal image of the specimen was obtained using Leica Microsystems CMS GmbH (TCS SP5 II type) confocal microscopy. The water phase was stained with FITC prior to the test. The emulsion was excited by laser with wavelength of 488 nm. The DLS measurements were performed using the Malvern Zetasizer Nano ZS90 instrument. Hydrodynamic diameter of the emulsion droplets was calculated from autocorrelation function which were analyzed with the CONTIN method. X-ray photoelectron spectroscopy (XPS) data were obtained with an PHI 5000 Versaprobe II (VP-II) electron spectrometer from Ulvac-Phi using 300W Al Kα radiation. The base pressure for the measurements was approximately 3×10-9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. XPS results were obtained by analyzing the elements present in the air-facing side of the specimen films with a 90° to the electron beam. Preparation of UiO-66-NH2 NPs (MOF-1). A mixture of ZrCl4 (93.2 mg, 0.4 mmol), 2aminoterephthalic acid (72.5 mg, 0.4 mmol) and acetic acid (1200 mg, 20 mmol) in DMF (16 mL) was heated in a sealed tube at 120 °C for 24 h under static conditions. The product was isolated by centrifugation and washed with DMF and ethanol. The resulted solids were dried in vacuum to afford 120 mg of the as-synthesized MOF-1 NPs. Preparation of UiO-66-NH-Met NPs (MOF-2). MOF-2 nanoparticle was prepared according to our reported method.42 A mixture of MOF-1 (600 mg, 2 mmol), trimethylamine

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(TEM, 1.2 equiv) and methacrylic anhydride (2.36 mL, 16 mmol, 8 equiv) was stirred at 55 °C for 24 h. The resulting powder was washed with CHCl3 and dried under reduced pressure to generate the MOF-2 NPs. The modification yield was determined to be ca. 94 % based on the 1H NMR measurement. (Supporting Information). Preparation of PDEAEMA-g-UiO-66 NPs (MOF-3). A mixture of MOF-2 (640 mg, 2 mmol), 2-[(diethylamino)ethyl methacrylate] (1.0 mL, 0.5 mol) and potassium persulfate (KPS) (18 mg, 0.005 equiv) was stirred in water (30 mL) at 70 °C for 48 h in N2 atmosphere. The crude product was washed with THF and then dried in vacuum to generate MOF-3 NPs. Preparation of Pd@PDEAEMA-g-UiO-66 NPs (Pd@MOF-3). A mixture of MOF-3 (100 mg) and palladium acetate (65 mg, 0.29 mmol) in toluene (30 mL) was allowed to stand at room temperature for 4 h. The resulting NPs were washed with toluene (3 times) and dried in air. The obtained Pd(II) loaded NMOF was soaked in an aqueous solution (20 mL) of NaBH4 (20 mg, 0.52 mmol) for 0.5 h. The resulting powder was washed with water (3 times) and dried in air to afford Pd@MOF-3 as dark brown powder. Inductively coupled plasma (ICP) measurement indicated that the encapsulated amount of Pd NPs in Pd@MOF-3 is 3.2 wt %. Emulsification and Demulsification. To prepare Pickering emulsion, toluene and deionized water are used as the incompatible oil and water phase, respectively. The Pickering emulsion was fabricated by an emulsification. Typically, Pd@MOF-3 NPs (28 mg) were added into a glass vial containing toluene-water (2 mL/1 mL) mixed liquid. The mixtures were sonicated with a Model: FJ-150 Sonicator (145 W) at 6000 rpm for 80 s to generate a 1.0 wt % Pd@MOF-3based Pickering emulsion. The other types of emulsions were prepared in the same way. Stability of the emulsions was assessed by visual inspection and hydrodynamic diameter measurements. To test the pH-triggered emulsification and demulsification behavior of the emulsions, pH of the

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obtained emulsion was adjusted using 1.5 M HCl or 1.5 M NaOH aqueous solution. The effect of pH variation, oil-water ratio, and the content of emulsifiers on the phase separation behavior was characterized by confocal microscopy. Pickering Interfacial Catalytic Performance. The one-pot Knoevenagel condensationhydrogenation cascade reaction was carried out in the Pd@MOF-3-stabilized Pickering emulsion. Nitrobenzaldehydes and dicyanopropane were chosen as the starting materials. In a typical procedure, the prepared Pickering emulsion containing 28 mg of Pd@MOF-3 (0.42 mol % Pd with respect to nitrobenzaldehyde), 0.2 mmol of nitrobenzaldehyde, 0.4 mmol of dicyanopropane, 2.0 mL of toluene, and 1.0 mL of deionized water was placed in a 25mL reaction vessel. The first condensation step was performed at room temperature in air (monitored by TLC), and the following hydrogenation step was carried out under ordinary hydrogen pressure (balloon). Once the reaction finished, pH value of the reaction system was adjusted to 2.0 by diluted HCl aqueous solution (1.5 M). After complete phase separation, the target organic product in the upper toluene phase was isolated, while the bottom aqueous phase containing Pd@MOF-3 was collected and used for next catalytic run after adjusting pH value to ca.7 using aqueous NaOH (1.5 M) solution. Leaching Test. The solid catalyst Pd@MOF-3 was separated from the catalytic emulsion system right after reaction for a given time (50 min for condensation reaction, and 15 min for hydrogenation). The reaction was continued with the filtrate in the absence of Pd@MOF-3. No further increase in the yield of the aimed product was detected, which confirmed that the active sites for the reaction was located on Pd@MOF-3. RESULT AND DISCUSSION

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Synthesis and Characterization of Pd@MOF-3. UiO-66-type NMOF was chosen by us as the model nanoscale secondary building units (SBUs) to construct the MOF-based pH-sensitive catalytic composite owing to following reasons. First, UiO-type MOFs are known to be very stable in common organic and aqueous media,43 and they possess high potential in numerous applications. Second, their size can be readily scaled down to nano regime and their surface can be expediently modified via post-synthetic reactions.44-46 Third, UiO-type MOFs are highly porous, and it can accommodate and stabilize the catalytic species through attached functional moieties on the organic linkers.47-48

Scheme 1. Schematic description of the fabrication of Pd@MOF-3 via post-synthetic approach. The encapsulated Pd NPs are shown as orange balls. The samples pictures of MOF-3 and Pd@MOF-3 are inserted. As shown in Scheme 1, the Pd NPs loaded catalytic active and pH-responsive MOF-based Pickering emulsifier was prepared via four-step reactions. MOF-1 NPs was synthesized from zirconium (IV) salt and the amido-attached dicarboxylic acid in DMF under solvothermal conditions.48 MOF-1 NPs reacted with excess of methacrylic anhydride in the presence of triethylamine (TEA) at 55 °C for 24 h to generate methacrylamide group decorated MOF-2 via a covalent post-synthetic modification in high yield (94 %, determined by 1H NMR spectrum).49

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By surface grafting polymerization of MOF-2 via the free radical process, the polymer brushes were introduced onto the surface of MOF NPs to provide 2-[(diethylamino)ethyl methacrylate] decorated MOF-3 as the light-yellow solids. Pd NPs loaded Pd@MOF-3 was obtained as the dark-brown solids via solution impregnation of MOF-3 by palladium acetate and subsequent reduction with NaBH4.

Figure 1. a) TEM image of MOF-1. b) Dynamic light scattering (DLS) measurement of MOF1. c) IR spectra of MOF-2 and MOF-3. d) 1H NMR spectra of the poly(DEAEMA) homopolymer and the freed polymer from digested MOF-3. As shown in Figure 1a, the transmission electron microscopy (TEM) and scanning electron microscopy (SEM, Supporting Information) micrographs showed that the obtained MOF-1 NPs were uniformly distributed and their size was centered at ca. 220 nm in diameter, which is further supported by the dynamic light scattering (DLS) measurement (Figure 1b). FTIR spectra of MOF-2 and MOF-3 are shown in Figure 1c. Compared to MOF-2, the new characteristic adsorption bands for poly(DEAEMA) linkage in MOF-3, such as -CH3, ˃CH2 and ˃C=O

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appeared at 2977, 2819 and 1722 cm-1, respectively. In contrast to PDEAEMA homopolymer, the 1H NMR spectrum performed on the digested MOF-3 shows that PDEAEMA was unambiguously grafted on the NMOF surface (Figure 1d), which can be a direct evidence for the formation of polymer brushes. To further ensure this successful polymer chain grafting in MOF3, gel permeation chromatography (GPC) measurement was performed on the digested MOF-3. The freed polymer was obtained by the digesting MOF-3 by certain amount of concentrated H2SO4 and DMF followed by precipitation in large amount of methanol. GPC analysis of the freed polymer gave a moderate number average molecular weight and a relatively broad polymer molecular weight distribution (Mn = 6216 g mol-1, PDI = 1.45), which features a typical of polymers generated by free radical polymerization (Supporting Information).

Figure 2. a) XPS spectra of Pd(II)@MOF-3 and Pd@MOF-3 and it after use. b) HRTEM images of Pd@MOF-3. c) SEM image and SEM-EDX elemental mapping of Pd@MOF-3. Pd@MOF-3 formation was accompanied with a distinct visual color change from light yellow to dark brown (Scheme 1). As determined by Inductively coupled plasma (ICP), the encapsulated amount of Pd species in Pd@MOF-3 was ca. 3.2 wt % (Supporting Information). The oxidation

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state of the encapsulated Pd species before and after reduction was determined by the X-ray photoelectron spectroscopy (XPS, Figure 2a). The observation of the Pd d5/2 and d3/2 peaks at 341 and 334.5 eV demonstrated that the Pd species was reduced from Pd(II) to Pd(0) in [email protected] High resolution transmission electron microscopy (HRTEM) analysis revealed that the most of crystalline Pd NPs highly dispersed in porous MOF matrix (tiny amount of Pd NPs were stabilized in polymer layer

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) with an average particle size of ca. 8 nm (Figure 2b).

The lattice fringes had an interplanar spacing of 0.24 nm, corresponding to 1/3 (4 2 2) fringes of face-centered cubic (fcc) Pd.52 The SEM-EDX elemental mapping of Pd@MOF-3 was also measured (Figure 2c). It shows the elemental maps of the Pd and Zr images, respectively, further indicating that Pd NPs were highly dispersed in Zr(IV)-MOF matrix.

Figure 3. PXRD patterns of MOF-1, MOF-2, MOF-3 and Pd@MOF-3, and their SEM images. As shown in Figure 3, the XRPD patterns of MOF-2, MOF-3 and Pd@MOF-3 are in good agreement with that of pristine MOF-1, demonstrating that the crystalline and structural integrity of the UiO-66-NH2 are well maintained during the post-synthetic organic modification and the Pd loading and the following reduction processes. In addition, the contour and size of MOF-1

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NPs were also maintained, demonstrating that the covalent polymerization between MOF NPs should be a surface post-synthetic modification. Fabrication and Characterization of Pd@MOF-3-Stabilized Pickering Emulsion. All Pickering emulsions were prepared using a toluene-water system under neutral conditions. Each contains a given amount of Pd@MOF-3 in a fixed total volume of two phases. The obtained emulsions were then examined by visual inspection and confocal microscopy. For easy observation, the aqueous phase was stained with FTIC dye in advance.

Figure 4. Optical images and Confocal microscopy images of Pd@MOF-3 stabilized Pickering emulsions with different toluene to water ratios. The aqueous phase was stained with FITC prior to the test. The emulsion was excited by laser with wavelength of 488 nm. We firstly investigated the effect of toluene-to-water ratios on the Pickering emulsion. The toluene to water ratio was altered from 5:1 to 1:5. After homogenized at 6000 rpm for 80 s, a highly viscous toothpaste-like gel emulsion was obtained, suggesting the formation of Pickering emulsion. After homogenization, sample images of the emulsions with different toluene to water ratios were shown in Figure 4. The type of toluene-in-water emulsion was firstly confirmed by the drop test, where a drop of emulsion was added to pure water, and for the emulsion with toluene to water ratio from 2:1 to 1:5, a quick dispersibility was observed. As a more direct evidence, it was clear that the FITC-labeled water phase became a continuous green phase when the emulsion was excited by laser with wavelength of 488 nm (Figure 4). Thus, all the emulsions with toluene to water ratio from 2:1 to 1:5 showed an oil-in-water type emulsion. However, as

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the internal phase exceeded the volume fraction of the critical packing for an oil-in-water system, the emulsion type underwent catastrophic phase inversion from oil-in-water (o/w) to water-in-oil (w/o) with the toluene-to-water ratio of 5:1.53 Furthermore, it can be see that only the sample with the toluene to water ratio of 2:1 is suitable to form the most stable and complete emulsion. Thus, the toluene-to-water ratio was fixed as 2:1 for the further investigation.

Figure 5. DLS result, optical images, and Confocal microscopy images of the Pickering emulsions with different amounts of Pd@MOF-3. The effect of the mass fraction of Pd@MOF-3 on the emulsion (oil-to-water ratio, 2:1) was also explored. As shown in Figure 5, the emulsion with 0.5 wt % of Pd@MOF-3 could be prepared, but the system was unstable (ca. 5373 nm). In this regime, the smaller droplets coalesced to produce large ones, it is most likely due to the insufficient amount of Pd@MOF-3 that cannot effectively stabilize the emulsion system. When Pd@MOF-3 mass fraction was up to 1.0 wt %, coalescence was suppressed as the toluene emulsions were coated with sufficient Pd@MOF-3 to stabilize toluene droplets. Therein, homogeneous and stable emulsions were obtained with a narrow droplet size distribution of ca. 3876 nm. The average emulsion droplet sizes were controlled by the solid particles to toluene mass ratio, which decreased to 1815 nm and 640 nm when the Pd@MOF-3 mass fraction increased from 2.0 to 5.0 wt %, respectively.

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The corresponding emulsions were stable but too viscous to use. The decrease of droplet size with the increase of MOF content indicated that the emulsion was stabilized by MOF NPs. The observed trend well agrees with the results of other general Pickering emulsions.54-55 The emulsion with 1.0 wt % of Pd@MOF-3 is very stable and no any phase separation was observed. The droplet size distribution remains unchanged after 7 days of storage based on The DLS measurement, indicating that Pd@MOF-3 is an effective emulsifier to stabilize the toluene-in-water emulsions (Supporting Information). pH-Responsive Behavior of Pd@MOF-3-Stabilized Pickering Emulsion. Figure 6 shows

Figure 6. Schematic illustration of the pH-responsive Pd@MOF-3-stabilized Pickering emulsion. Photographs and Confocal micrographs of the emulsions (toluene-water ratio, 2:1) stabilized with Pd@MOF-3 (1.0 wt %) under different pH values are inserted. that the pH-switchable nature of Pd@MOF-3-stabilized Pickering emulsion. All Pickering emulsions were prepared by sonicating toluene-water (2 mL/1 mL) mixed solvent system containing 1.0 wt % Pd@MOF-3. Then the pH of the aqueous phase was adjusted to the desired

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value using 1.5 M HCl or 1.5 M NaOH aqueous solution. As indicated in the Figure 6, the Pd@MOF-3-stabilized emulsion system is very stable under neutral conditions, and became unstable with the pH value decrease under gentle stirring. Confocal microscopy image indicated that the small droplets began to coalesce at pH = 5, and the complete phase separation occurred at pH = 2. As shown in Figure 6, the resultant separated toluene and aqueous phases are clear, and the Pd@MOF-3 NPs are located at the bottom of the vial. Obviously, the PDEAEMA chains attached to MOF NPs are highly protonated at the terminal diethylamino groups under acidic conditions,56 and they are more compatible with the aqueous phase due to increased hydrophily, which is further demonstrated by the air-water contact angle measurement (Supporting Information). When the pH value increased, Pd@MOF-3 NPs became more hydrophobic and were incompatible with the aqueous phase due to the highly deprotonated of the diethylamino groups on the PDEAEMA chains under alkaline conditions. As a result, the stable emulsion was formed again at neutral conditions (pH = 6.0-7.0). The emulsificationdemulsification cycle can be repeated five times without any loss in reversibility. This result suggests the nature of Pd@MOF-3-stabilized emulsion can be effectively controlled by simply tuning the pH values. In addition, the stability of Pd@MOF-3 at different acidic pH values was examined. The powder X-ray diffraction (PXRD) patterns measured at pH = 1-5 are the same as that of as-prepared Pd@MOF-3 (Supporting Information), indicating that Pd@MOF-3 is stable under the given acidic conditions. Catalytic Performance. Owing to the loaded Pd NPs, we propose that the Pd@MOF-3stabilized toluene-in-water Pickering emulsion could be used as the multifunctional catalytic system for the biphasic organic reactions. The one-pot condensation-hydrogenation cascade reaction based on p-nitrobenzaldehyde and malononitrile (Table 1, entry 1) was used as a model

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reaction to evaluate the performance of this smart catalytic system. The cascade reaction was performed at room temperature under ambient pressure in the presence of Pd@MOF-3 (0.4 mol % Pd with respect to p-nitrobenzaldehyde). The first-step Knoevenagel condensation proceeded under air atmosphere, and the following hydrogenation reaction was carried out in hydrogen atmosphere. Table 1. Results of Knoevenagel Condensation-Hydrogenation Cascade Reactions for Nitrobenzaldehydes with Malononitrile Promoted by Pd@MOF-3-Stabilized Pickering Emulsion a

entry

Substrate

T (°C)

t (h)

Product

Yield (%)b

CN

CHO CN

1

25

3.5

NO2

95 NH2

CN

CHO NO2

2

25

3.5

CN NH2

91

CN

CHO

3

25

3.5

CN

62

NO2 NH2

a

Reaction conditions: Pickering emulsion (1mL water, 2 mL toluene, and 28 mg Pd@MOF-3) with nitrobenzaldehydes (0.2 mmol) and malononitrile (0.4 mmol). For Knoevenagel condensation, the mixture was stirred in air for 3h. For the following hydrogenation reaction, the mixture was stirred in an atmospheric hydrogen balloon for 0.5 h. b The isolated yields. The characterization data for the products were provided in Supporting Information.

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Figure 7. Reaction time examination and leaching test for (a) Knoevenagel condensation and (b) hydrogenation, respectively. (c) Yield of product C in repeated runs for the cascade Knoevenagel condensation-hydrogenation reaction catalyzed by the Pickering emulsion stabilized by Pd@MOF-3 NPs. (d) The XRPD patterns of Pd@MOF-3 after each catalytic run. For first condensation step, the relationship between conversion and reaction time is shown in Figure 7a. The initial conversion of 4-nitrobenzaldehyde (A) is continuously increased, and the maximum yield of 99 % for B was observed at ca. 3 h. In order to gain insight into the heterogeneous nature of Pd@MOF-3, the hot leaching test was carried out. The solid catalyst was separated from the emulsion during the reaction, where the filtrate was transferred to a new

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vial and reaction was carried out under the same conditions for additional ca. 3h. As indicated in Figure 7a, no further reaction took place without catalyst after ignition of the condensation reaction at 45 min. This finding demonstrated that Pd@MOF-3 exhibits a typical heterogeneous catalyst nature for the first condensation step. For the subsequent hydrogenation step, the highest conversion of 98 % was reached at ca. 30 min. It is similar to the first step, the Pd@MOF-3 catalyzed reduction is also a typical heterogeneous catalytic process. As shown in Figure 7b, no further reaction took place without Pd@MOF-3 after ignition of the hydrogenation at 15 min. For this whole one pot cascade reaction, the total reaction time was fixed as 3.5 h, and the isolated yield for the final product of 2-(4-aminobenzylidene) malononitrile (C) was 95 % (Supporting Information). The control experiment demonstrated that the first Knoevenagel condensation step was catalyzed by the UiO-MOF.30 The intermediate product of 2-(4-nitrobenzylidene)malononitrile (B) was obtained quantitatively in the presence of MOF-3 (Supporting Information). MOF-3, however, showed no catalytic activity for the second hydrogenation step. Thus, composite Pd@MOF-3 herein is bifunctional, and the UiO-66 MOF support and the encapsulated Pd NPs are responsible for the Knoevenagel condensation and the following hydrogenation reaction,57 respectively. Impressively, when this one-pot reaction finished, the reaction emulsion system could be easily demulsified by decreasing the pH value. Indeed, the toluene and aqueous phase were clearly separated once the pH value was adjusted to 2 upon addition of 1.5 M HCl aqueous solution with gently stirring. Meanwhile, the product was extracted into toluene phase, and the Pd@MOF-3 settled at the bottom of the reaction vial during the demulsifying process. In this way, the product-catalyst separation was facilely realized based on this smart catalytic system.

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In addition, the recyclability of this smart catalytic emulsion was examined. After each catalytic run, the pH value of the recovered aqueous solution with Pd@MOF-3 was adjusted to ca. 7 by NaOH (1.5 M). Upon addition of malononitrile and the toluene solution of 4nitrobenzaldehyde (A), the reaction was carried out under the same conditions after homogenization. As shown in Figure 7c, after five consecutive catalytic runs, the yield of 4-(4aminobenzylidene)-malononitrile (C) is still up to 92 %, indicating no obvious degradation in the catalytic activity of Pd@MOF-3 during the recycling processes. Moreover, Pd@MOF-3 was demonstrated to be highly crystalline and integrated from the PXRD patterns even after five catalytic runs (Figure 7d). On the other hand, HRTEM image of the used MOF shows no obvious aggregation of Pd NPs after five catalytic runs (Supporting Information). The ICP measurement showed that the Pd content in the MOF particle is basically unchanged (3.0 wt %) after five catalytic runs (Supporting Information), indicating no Pd species leaching occurred during the consecutive catalytic processes. After that, we investigated the catalytic activity of Pd@MOF-3 for one-pot condensationreduction of o-nitrobenzaldehyde and m-nitrobenzaldehyde with malononitrile, respectively (Table 1, entries 2 and 3). For the Knoevenagel condensation, the Pickering interfacial catalyst shows good activity for all the two substrates, and the intermediate products (B) were generated quantitatively. However, the following hydrogenation of the intermediate products (B) is somehow complicated. As shown in Table 1, for the ortho-substituted o-nitrobenzaldehyde, 2-(2aminobenzylidene)-malononitrile (C) was obtained in good isolated yield (91%). For the metasubstituted m-nitrobenzaldehyde, the desired 2-(3-aminobenzylidene)-malononitrile product (C) was generated in only a moderate isolated yield (62%) with some unidentified byproducts. Thus the -NO2 substitution position on the phenyl ring herein has great impact on the final products.

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CONCLUSION In summary, we developed a facile method to prepare a smart pH-responsive oil-in-water Pickering emulsion catalytic system based on a UiO-66 type NMOF. The PDEAEMA polymeranchored and Pd NPs-loaded NMOF-based Pickering emulsifier was synthesized by a free radical polymerization on the MOF NPs surface and a following solution impregnation process. The resultant Pd@MOF-3 is able to stabilize toluene-water Pickering emulsion under natural condition, furthermore, displayed a smart pH-induced emulsifying and demulsifying behavior caused by the alternative protonation-deprotonation of the amino groups on PDEAEMA. This NMOF-based Pickering emulsion catalytic system can highly promote one-pot Knoevenagel condensation-hydrogenation cascade reactions in the biphasic oil/water interface. We expect the obtained results herein might provide some guidance on the design and fabrication of new MOFbased Pickering emulsion catalytic systems for other types of organic reactions. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional details, such as additional characterization data of Pd@MOF-3, SEM image, stability at different acidic conditions, GPC measurement, ICP results, control experiment for catalysis, and characterization of the products generated from Knoevenagel condensation-hydrogenation cascade reactions. AUTHOR INFORMATION Corresponding Author *[email protected]

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*[email protected] Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for financial support from NSFC (Grant Nos. 21671122, 21604049 and 21475078), Shandong Provincial Natural Science Foundation (BS2015CL015), China, A Project of Shandong Province Higher Educational Science and Technology Program (J15LC05) and the Taishan Scholar’s Construction Project. REFERENCES (1) Chevalier, Y.; Bolzinger, M.-A. Emulsions Stabilized with Solid Nanoparticles: Pickering Emulsions. Colloids Surf., A 2013, 439 (2), 23-34. (2) Ramsden, W. Separation of Solids in the Surface-Layers of Solutions and “Suspensions” (Observations on Surface-Membranes, Bubbles, Emulsions, and Mechanical Coagulation).Preliminary Account. Proc. R. Soc. London 1903, 72 (477-486), 156-164. (3) Pickering S. U. CXCVI.-Emulsions. J. Chem. Soc., Trans. 1907, 91 (0), 2001-2021. (4) Tcholakova, S.; Denkov, N. D.; Lips, A. Comparison of Solid Particles, Globular Proteins and Surfactants as Emulsifiers. Phys. Chem. Chem. Phys. 2008, 10 (12), 1608-1627.

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For Table Content

A pH-responsive NMOF-based Pickering emulsion catalytic system for one-pot cascade biphasic reaction is reported

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Smart pH-Responsive Polymer-Tethered and Pd NP-Loaded NMOF

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