Silica Composite CoreâShell Microspheres

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An efficient chitosan/silica composite core-shell microspheres supported Pd catalyst for aryl iodides Sonogashira coupling reactions Qing Cui, Hong Zhao, Guangsheng Luo, and Jianhong Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04077 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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Industrial & Engineering Chemistry Research

An efficient chitosan/silica composite core-shell microspheres supported Pd catalyst for aryl iodides Sonogashira coupling reactions Qing Cui,a,b Hong Zhao,a Guangsheng Luo,a and Jianhong Xu*a a

The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua

University, Beijing 100084, China. b School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. * Corresponding author. Email: [email protected] KEYWORDS: porous microspheres, Pd nanoparticles, heterogeneous catalyst, microfluidics, Sonogashira

ABSTRACT: Monodispersed chitosan/silica core-shell microspheres supported Pd catalysts were prepared by a simple and affordable microfluidic device. Merely 5 mg supported Pd catalyst (1 wt. %) performed superior capability in Sonogashira coupling reactions between various aryl iodides and terminal alkynes in a mild environment (ligand-free, copper-free, amine-free, water-ethanol as solvent at 65 oC) even after 8 times’ reusing. These novel porous composite microspheres provide a new carrier to reduce the loading, enhance the activity, and increase the stability of noble metal catalyst.

INTRODUCTION Palladium, a noble metal, with the empty d electron orbit, smaller energy gap and diversity of coordination, has become a research focus in multiple fields, such as electronic, agricultural, pharmaceutical and chemical industries, particularly in organic catalysis process. However, high price, low utilization and rare storage have severely restricted its excellent catalytic application.1-3 Heterogeneous catalyst has become a hot catalytic field during the last decade due to its easy separation from the reaction system and reuse for many times,4-6 so supported Pd catalysts show advantages on solving the bottleneck problem of

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Pd utilization. Many supports have been used to load noble metal for catalysis. Bao et al. 7 used γ-Al2O3 to support Pd which succeeded in catalyzing admiration of esters. However, average particle size of the catalyst was increased by 33% to 4 nm, and the loading capacity of Pd was also decreased by 20% just after 5 times’ usage. Meanwhile, isolated yield recorded a drop from less than 90% to 70%. Yang et al.8 synthesized nanocomposites of Fe3O4, graphene oxide and carbon nanotubes as carriers, which were easily magnetically separated from reaction mixture. The loading content of Pd was over 10% which means there was a pretty large consumption of noble metal, and after recycling for 4 times, the reaction yield declined by 10%. The decrease was attributed to the loss of Pd during the recycling as the interaction between Pd and carriers are pretty weak. Other carriers in literatures, such as carbon nitrides/silica9, silica gel confined ionic liquid and zirconium phosphate carboxyphosphonate10 are also have the similar problems. Therefore, issues of synthesizing a suitable carrier supported Pd catalyst to minimize Pd consumption and achieve better stability should be addressed. Chitosan, from deacetylation of chitin, contains abundant amino and hydroxyl groups,11-13 which contribute to its chemical interaction (chelation) and electrostatic interaction (ion exchange) with different noble metal ions by adjusting pH, etc.14-18 Zhou et al.19 prepared magnetic chitosan nanoparticles modified by ethlenediamine which were good at loading noble metal; however, the wide particle size distribution from 5 nm to 40 nm was definitely detrimental to the adsorption performance. Moucel et al.20 bonded ionic liquid containing Pd catalyst with chitosan beads and finally achieved 5 times efficiently recycling of the catalyst during which chitosan showed marked advantages as it can be easily adjusted to different structures like ball, sheet, frame in the weak acid solution,17, 21 but ionic liquid with high price was used in the process. So porous chitosan is an ideal support material for Pd; however, the widely used chemical crosslinking method for porous chitosan carriers has a problem in obtaining similar pore structures inside and outside pure chitosan microspheres due to the excessive crosslinking of the chitosan on the outside surface, so silica gel can be introduced into shell system to address the issure and at the same time enhance the mechanical strength of the composite. Sonogashira coupling reaction of sp2-carbon in aryl halide and sp-carbon in terminal alkynes under the co-catalysts of Pd (II) and Cu (I), has been widely applied in the synthesis of substituted alkynes and large conjugated alkynes22-24, so it plays a key role in the synthesis of many natural compounds, pesticides, pharmaceuticals, new materials and nano molecular devices. With the development of green chemistry, this reaction is gradually catalyzed by the Pd catalyst alone.25-27 Sarmah et al.28 developed a mild palladium catalyst system where nitro-iodobenzene and phenyl acetylene catalyzed by Pd acetate can achieve 98% yield within 3 hours. However, this system is difficult for the effective recovery of Pd acetate cata-


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lyst, resulting in waste of noble metal. Frindy et al.29 used glutaraldehyde cross-linked chitosan to support Pd (II). Reaction achieved high yield at 65 oC water /ethanol solvent after 6 h, and catalysts achieved more than 6 times efficient recycling. The amount of supported catalyst for 1 mmol equivalent of reaction was 54 mg (3 wt. % Pd loaded), which was pretty large consumption of noble metal. Therefore, it is necessary to prepare a heterogeneous catalyst in which Pd is loaded on chitosan/silica microspheres to achieve efficient use of the noble metal and efficient completion of Sonogashira coupling reaction. In this work, we prepared novel monodispersed chitosan/silica porous microspheres supported Pd catalyst in a simple and affordable microfluidic device, and 5 mg Pd catalyst (1 wt.%) obtained outstanding catalytic activity in the copper-free, amine-free and ligand-free Sonogashira coupling reactions of various aryl iodides in ultrapure water/ethanol solvent even after being reused for 8 times. Highlights of this kind of catalyst distinctly performed on its excellent efficiency and stability.

EXPERIMENTAL Materials and Chemicals Chitosan (2 wt.%, deacetylation degree below 95%, Sinopharm Chemical Reagent Beijing Co., Ltd., P.R. China) acetic aqueous solution (2 wt.%, VAS Chemical Co., Ltd., Tianjin, P.R. China) was used as the inner fluid. TEOS (2 wt.%) was dissolved in the acetic aqueous solution (2 wt.%), used as the middle fluid, and the TEOS solution need to be stirred over 8 hours to generate silica sol. N-octanol (VAS Chemical Co., Ltd., Tianjin, P.R. China) with 2 wt.% Span 80 (2.0 g, VAS Chemical Co., Ltd., Tianjin, P.R. China), was the outer fluid. Mixture of N-octane (100.0 g), cross-linking reagent - glutaraldehyde (1.03 g) and surfactant - Span 80 (2.0 g) was used as the solidification bath. Pd chloride, potassium carbonate, aryl iodides and terminal alkynes, used in the catalysis reactions were all purchased from Alfa Asar (Tianjin, P.R. China).

Microfluidic device A circular glass capillary, with 0.3 mm inner diameter is tapered to approximately 200 μm in tip by a micropipette puller (P-97, SUTTER Co.Ltd., USA) to be an inner channel. This capillary is inserted into the middle channel - another glass capillary (0.7 mm×1.0 mm, tip tapered to around 330 μm). Next, both two capillaries are subsequently inserted into a third glass capillary (1.05 mm×1.5 mm) to form the dual coaxial structure, as shown in Figure 1(a). Teflon tube was used as multiphase flow collected channel. Three


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microsyringe pumps were used as power unit. Droplets collected in the solidification bath would be placed on a shaker.

Preparation of monodispersed chitosan/silica porous microspheres chitosan and silica sol were pumped into the micro-chip and then broken into monodispersed droplets by the shearing force of the continuous flow at the intersection. Glutaraldehyde and protonated chitosan cross-linked in the solidification bath and the gelation of silica dioxide in n-octanol were used to solidify the droplets. Through adjusting time of crosslinking and curing, the structure microspheres can be regulated.. In a typical experiment, microspheres were washed and submerged in n-octane before being freeze-dried. Design of the synthetic process was shown as Figure 1(b). (a)

O

O

Figure 1. (a) Dual co-axial loop microchannel. (b) Design of the catalyst synthesis process.


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Adsorption kinetics and thermodynamic experiments of Pd (II) A conical flask with 0.02 g microspheres and 50 ml aqueous solution of different contents of Pd (II) was shaked in a water-bath at 25 oC. Raman Atomic Absorption Spectrometer (AAS) was used to test Pd (II) concentration. Based on the mass balance equation, adsorbing capacity of Pd (II) in microspheres at time ti, q ti (mg/g), can be calculated as:

where C0 (mg/l) is the initial Pd (II) concentration, Ci is the concentration at time ti, Vi is the volume of the remaining solution at time ti (samples for AAS were not added back to the conical flask), and m is the weight of the microspheres in the beaker. Pd (II) adsorption process involves three steps: first, Pd (II) diffuse to the surface of the sorbent, also known as membrane diffusion; second, Pd (II) diffuse along the channel to the adsorption sites from the surface of sorbent, known as intraparticle diffusion; third, Pd (II) adsorb at the adsorption sites of the sorbent by ion exchange. We use three kinds of model to simulate the dynamic data fitting analysis. Pseudo-second-order kinetic model (for ion exchange process) expressed as:

where t is contact time, qt, adsorption capacity at time t and balance time, while qet means equilibrium adsorption capacity. k2 is kinetic constant. Membrane diffusion kinetic model (for membrane diffusion process) expressed as:

where F is equilibrium index, qt, qet and t are in accordance with the definition above, k is adsorption rate constant. Intraparticle diffusion kinetic model (for intraparticle diffusion process) expressed as:


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where qt and t are in accordance with the definition above; k is rate constant. In the adsorption thermodynamics research, adsorption thermodynamic behavior of Pd (II) under 25oC has been studied using Langmuir model. The basic assumption of this model is that the adsorbate on the adsorbent surface forms monolayer. Linear model expressed as:

where Ce is equilibrium concentration, its unit is mg/L; qe and qm are equilibrium absorption capacity and saturated adsorption capacity, respectively, their units are mg/g; KL is adsorption equilibrium constant.

Preparation of Pd-loaded chitosan/silica core-shell microspheres There are three different loading protocols: Protocol 1: A conical flask with different amounts of hybrid microspheres, PdCl 2 and 50 g aqueous solution were put into a water-bath shaker at room temperature for a fixed time. Next, the resulting microspheres were separated and mixed with NaBH4 in 50 g aqueous solution. The mixture was put into a water-bath shaker at room temperature for another 2 h and the resulting microspheres were separated and dried by freeze-drying. Protocol 2: A conical flask with different amounts of hybrid microspheres, PdCl2 and 50 g ethyl alcohol were put into a water-bath shaker at room temperature for the fixed time during which ethanol one-step reduction happened.31 The resulting microspheres were separated and dried by freeze-drying. Protocol 3: A conical flask with different amounts of hybrid microspheres, PdCl2 and 50 g aqueous solution were put into a water-bath shaker at room temperature for a fixed time. Next, the resulting microspheres were dissolved in 50 g ethyl alcohol. The mixture was put into a water-bath shaker at room temperature for another 2 h and the resulting microspheres were separated and dried by freeze-drying.

Catalysis of the Sonogashira coupling reactions A classic reaction process could be described as: 5 mg supported catalyst (1 wt.% Pd loaded), terminal alkyne (2.4 mmol, 0.2448 g), aryl iodide (2 mmol, 0.496 g) and potassium carbonate (6 mmol, 0.828 g) were added into a conical flask where solvent is the mixture of ultrapure water and ethyl alcohol (1:5, 2 ml/10 ml). The reaction was performed at 65oC under nitrogen atmosphere for 5 h. Finally, the product was separated by dissolving in diethyl ether and purified by column chromatography. The catalyst was


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simply separated by sedimentation and subsequently reused after drying. The yield was obtained by dividing the quality of product by the quality calculated in theory. The space-time yield (STY) was defined as the quality of product obtained under one gram catalyst in one hour.

Analysis and characterization The preparation process and overall morphology of the droplets and microspheres were observed using an optical microscope (Type BX-61, Olympus, Japan) and an online CCD (Pixelink, Canada). The detailed structure of the microspheres and element analysis were obtained with Scanning Electron Microscopy (SEM, tTM3000 and F6301, Hitachi, Japan) and Transmission Electron Microscopy (TEM, HT7700, Japan). Pore structure was characterized using N2 Stripping Absorption Isotherm Curve (Quanta chrome autosorb-1-C) and Mercury Injection Apparatus (Autopore IV 9510, Allied Domecq PLC). The element valence of the catalyst was obtained using X-ray Photo-electron Spectroscopy (XPS). The crystal structure of Pd inside and outside microspheres was characterized by X-ray Diffractometer (XRD, S2, RIGAKU, Japan). The content of Pd during kinetics characterization was tested by Atomic Adsorption Spectrometer (172-8035, Hitachi Chem) and the content of Pd in the waste and product was obtained by ICP-OES (IRIS Intrepid II, Thermo).The product was analyzed by 600 M Nuclear Magnetic Resonance Spectrometer (JNMECA600, Japan) and Gas Chromatograph-Mass Spectrometer (GC-MS, TRACE DSQ, Thermo).

RESULTS AND DISCUSSIONS Characterization of the core-shell porous microspheres. Specific influence on the morphology of microspheres from crosslinking and gelation time could be found in Figure S1&S2. In our previous work, core-shell structure of these microspheres has been confirmed.30 After being calcinated at 800 oC, chitosan/silica microspheres almost turned be hollow as chitosan degraded totally ((Figure S2 (f)). Meanwhile, mechanical propoties also were proved to be strengthened.30 (Figure S2 (g)) After optimization, typical uniform (Figure S2 (h)) chitosan/silica core-shell porous microspheres (cross-linked for 17 min and gelated for 16 h) were characterized. The ratio of chitosan and silica was confirmed through Thermogravimetric Analysis in which chitosan started to degrade at 240 oC (Figure S3). And it was observed that the microspheres had good sphericity and a uniform size distribution. Furthermore, both core and shell of these microspheres were porous, which means there were abundant adsorption sites for catalyst to load, as shown in Figure 2(a~c). Next, to characterize the pore structure of hybrid microspheres, N2 adsorption and desorption experiments were conducted and distribution


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curves of micropores and mesopores were obtained, as shown in Figure 2 (d~f). The average diameter of micropores was approximately at 1.75 nm, while that of mesopores reached 2.5 nm. And mercury injection apparatus was used to characterize their large pore distribution, also shown in Table 1. Average large pore diameter was 5.99 m, and porosity was up to 81.15%.

(a)

(c)

(b)

(d)

(e)

(f)

Figure 2. SEM images and BET results. (a)(b)(c) SEM analysis of chitosan/silica microspheres(after optimization). (d)(e)(f) BET analysis of porous chitosan/silica microspheres. (d) N2 stripping absorption isotherm curve. (e) The diameter of mesopores (BJH method). (f) The diameter distribution of micropores (BJH method). Table 1. More information on pore structure of porous chitosan/silica microspheres

chitosan/silica microspheres a

Surface Area (Multipoint BET) (m2/g)a

Pore Volume (BJH method) (ml/g)a

Pore Diameter (BJH method ) (nm)a

Porosity (%)b

Total Pore Area (m2/g)b

Average Pore Diameter (m)b

13.40

0.029

1.65

81.15

3.44

5.99

measured by nitrogen sorption analysis. b measured by mercury intrusion method.

Dynamic adsorption results of different initial concentration of Pd (II) at 25 oC was shown in Figure 3(a). Concentration gradient had been set to investigate whether initial Pd (II) concentration influenced adsorption process to some extent. All six groups (details can be seen in Table S1) with different initial Pd concentration could finish the whole adsorption process within 60 min, and up to 90% adsorption amounts finished within 10 minutes. The loading capacity on the novel microspheres could be controlled under 30-


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100 mg/g (calculated by atomic absorption spectrum) through adjusting initial Pd (II) concentration, as shown in Figure 3 (b) and Table S1. Then three kinds of model had been used to fit these dynamic data to confirm which step is rate-determining step. Two sets of data, initial concentration of 10.92 ppm and 29.04 ppm, had been used for fitting. As shown in Figure 3 (c~e) and Figure S4, two sets of data both fitted pseudo-second-order kinetic model well, which means that the chelation reaction is the ratedetermining step of the whole adsorption process and the influence of diffusion on the adsorption process is negligible because of the porous structure. Furthermore, the thermodynamic data fitted Langmuir model well, as shown in Figure 3(f). This means the adsorption of Pd (II) on the chitosan/silica porous functional carrier was monolayer adsorption. The maximum adsorption capacity is up to 130.04 mg/g according to the fitting model.

(b)

(a)

(c)


(d)

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(f)

(e)

Figure 3. Adsorption results of Pd (II). (a) Dynamic adsorption process of different initial concentration (10.92-45.98 ppm) of Pd (II) at 25 oC. (b) Adsorption capacity of different initial concentration (10.9245.98 ppm) of Pd (II) at 25 oC. (c)(d)(e) kinetic data from where initial Pd (II) concentration is 10.92 ppm. (c) is pseudo-second-order kinetic model fitting results. (d) is intraparticle diffusion kinetic model fitting results. (e) is membrane diffusion kinetic model fitting results. (f) Langmuir thermodynamic fitting model. 5/2

(a) Pd3d3/2

5/2

(b) Pd3d 3/2


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5/2

(c) Pd3d 3/2

(d)

(e)

Figure 4. (a) (b) (c) XPS analysis of element Pd on chitosan/silica microspheres after three loading patterns. (d) (e) XPS analysis of element N on chitosan/silica microspheres before and after loading Pd.

Preparation and characterization of supported Pd catalysts. Details of three loading patterns could be found in supplementary information (Figure S5). XPS spectra of microspheres appeared two obvious peaks after loading, and binding energy of Pd 3d5/2 and Pd 3d3/2 after the first two loading patterns (Figure 4 (a)(b)) were 335 eV and 341 eV, respectively. The results were in accordance with binding energy of Pd (0). However, binding energy of Pd 3d5/2 and Pd 3d3/2 after the third pattern (Figure 4(c)) were 337 eV and 343 eV, which means element Pd existed as Pd (II). So at the follow-up experiments, the third pattern was abandoned. Ion chelation between Amino group and Pd (II) had also been illustrated by XPS. Binding energy of element N in pure chitosan (Figure 4(d)) was 399.23 eV, while it turned to 399.73 eV (Figure 4(e)) after chelating with Pd (II). As element Pd had a free electron


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orbital and element nitrogen had a pair electrons, binding energy of element nitrogen increased, and XPS spectra peak moved right. TEM images of supported Pd on microspheres which were loaded by the first two patterns showed different loading status (Figure 5). Supported Pd prepared by ethanol one-step reduction pattern had obvious particles’ coalescence on the surface (Figure 5 (A)(B)), and the particle size distribution (Figure 5 (G)) was relatively wide. Meanwhile, particle size of supported microspheres prepared by loading Pd (II) in aqueous solution and then reduced by NaBH4 concentrated at 3.4 nm, as shown in Figure 5 (C)(D) (H). So in the following experiments, the second pattern was abandoned. However, the lattices of Pd nanoparticles loaded in the microspheres by these two patterns are all clearly, with spacing almost 0.24 nm (Figure 5 (E)(F)). The lattice spacing shown in TEM analysis is in accordance with spacing of the main crystal [1, 1, 1] in XRD characterization, as shown in Figure 5 (I). Pd (0)(PDF:65-2867) reduced by NaBH4 has a more obvious peak than that reduced by ethyl alcohol which means a more clear crystal structure. However, main exposure crystal of both of them was [1, 1, 1], which approximately has a lattice spacing of 0.24 nm and was proved to be the main catalyst crystal. The first peak in two patterns indicates the existence of non crystalline structure, like silica (PDF: 29-0085) in the shell. Furthermore, Pd nanoparticles loaded in chitosan/silica composite microspheres with different loading capacity from 0.58 wt.% to 6.88 wt.% are all relatively uniform, and the particle size distribution is relatively narrow, as shown in Figure S6 in the supplementary information. So chitosan/silica microspheres are excellent carriers for loading Pd nanoparticles with good monodispersity and stability.


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H G 0.24nm

0.24nm 5n m

E

F


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Figure 5. TEM analysis of nanoparticles Pd on chitosan/silica micro-spheres and their size distribution. (A)(B) TEM images of nanoparticles Pd loaded by Pattern 2 (C2H5OH) and their initial Pd (II) concentration are 10.92 ppm and 29.04 ppm, respectively. (C)(D) TEM images of nanoparticles Pd loaded by Pattern 1 (NaBH4) and their initial Pd (II) concentration are 10.92 ppm and 29.04 ppm, respectively. (E)(F) Size distribution of nanoparticles Pd under two loading patterns ( initial Pd (II) concentration 29.04 ppm). (G)(H) TEM analysis of lattice spacing of nanoparticles Pd on chitosan/silica microspheres after loading Pattern 1((B), NaBH4) and Pattern 2((A), C2H5OH). (I) XRD analysis of nanoparticles Pd on chitosan/silica microspheres after loading Pattern 1 (NaBH4) and Pattern 2 (C2H5OH).

Performance of chitosan/silica microspheres supported Pd catalyst in Sonogashira coupling reactions. Before studying the stability of chitosan/silica microspheres supported catalyst, the reaction condition should be optimized first in order to avoid the waste of noble metal and maximize the benefit. The supported Pd catalyst was tested in the reaction of 4-iodoacetophenone and phenyl acetylene, as shown in Figure 6 (a). Content of optimization includes catalyst Pd loading capacity, reaction time and amount of catalyst, as shown in Figure 6 (b) (c) (d) and Table S2. For the first optimization condition, loading capacity from 1 wt.% to 5 wt.% obtained similar yields, therefore 1 wt. % was selected for maximizing Pd efficiency. Then, for reaction time, intersection of yield and space-time yield (STY) was the optimal reaction time. So reaction time was optimized to 5 h. Furthermore, isolated yield was proportional to the amount of catalyst which was consistent with theoretical reports. Taking STY into consideration, amount of supported catalyst was optimized as 5 mg. Details of the optimization process and its results are shown in the supplementary information. The product was yellowish-brown powder, and its isolated yield and STY could be as high as 98.9% and 17.4 respectively under the optimized experimental condition of 1 wt.% loading capacity, 5 h reaction time and 5 mg supported catalyst with the amount of Pd used is only 50 g.


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Based on the optimized conditions, catalytic capacity and stability of supported catalyst were studied. High yield up to 99% was obtained and the catalyst could be reused for more than 8 times (Figure 6 (e)). And TEM images (Figure 6(f)) of the catalyst after being recycled for 8 times showed there was almost no aggregation of Pd (0) nanoparticles, and little amount of coalescence of the metal particles has negligible influence on the catalyst performance. Meanwhile, ICP-OES analysis of product was used to detect the amount of leached Pd element during the reaction especially in the first recycle process, as shown in Table 2. Leaching Pd ratio in organic phase is only 0.006%, while it is even negligible in aqueous phase (LOD of the method was 0.0957 ppm). So the ratio of leached Pd to microsphere supported Pd was

only 0.006%. Trivial loss of element indicated that thesecomposite microspheres have a strong interaction with the catalyst, which could avoid the problem of Pd pollution in the product.

supported Pd

(a)

+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


65 ℃，K2CO3 H2O/EtOH=2/10ml

O

(b)

(c)


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(d)

(f)

(e)

Figure 6. (a) Sonogashira coupling reaction between 4-iodoacetophenone and phenyl acetylene. (b) (c) (d) Optimization of the reaction conditions. Amount of catalyst, reaction time and amount of loaded Palladium were studied. a Isolated yield. b space-time yield (STY) is the quality of product obtained under one gram supported catalyst in one hour. (e) Capacity and stability of chitosan/silica core-shell microspheres supported catalyst over the reaction between 4-iodoacetophenone and phenyl acetylene. (f) TEM images of chitosan/silica core-shell microspheres supported catalyst after 8 times’ recycle.

Table 2. ICP-OES analysis of the product catalyzed by supported catalyst used for the first time. Product a Organic liquid Aqueous phasephase

Supported-Pd b (wt.%) 1.2 ()(wt.%) 1.2

Leached Pd c (ppm) 0. 7205 ND d

a

Product is naturally divided into aqueous and organic liquid phase after the reaction. b Measured by Raman atomic absorption spectrophotometer. c Residual Pd in solution after Sonogashira cross-coupling of 4-iodoacetophenone to phenyl acetylene, measured by ICP-OES analysis. d ND means below the detection limit.

Table 3. Sonogashira coupling reactions of various aryl iodides and terminal alkynes. R1

X

+

R2

Supported Pd R1

R2

K2CO3, H2O/EtOH


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Entry

Aryl iodide a

1

Time

Yield b

TONc

I

5h

>99%

>4209

I

5h

>99%

>4214

12h

3.03%

129

12h

11.08%

472

I

5h

>99%

>4214

I

5h

>99%

>4214

5h

49.93%

2125

5h

>99%

>4214

Terminal alkyne

O

2

O2N

3

I

4

I

5 O

6

O2N

7 8

I

I

a

aryl iodide (2.0 mmol), terminal alkyne (2.4 mmol), K2CO3 (3 mmol), chitosan/silica core-shell supportedPd (1 wt.%, 5 mg), ultrapure water/EtOH (2 ml/10 ml), in nitrogen atmosphere, 65 oC. b Isolated yield after column chromatography. c TON = ratio of moles of product formed to moles of catalyst used.

The novel chitosan/silica core-shell microspheres supported Pd catalyst could also apply well to other Sonogashira coupling reactions. Some reactions of aryl iodides and terminal alkynes had been studied, as shown in Table 3. For instance, electron withdrawing groups such as nitro, ketone carbonyl (Table 3, entries 1-2, 5-6) provide excellent yield using 1 wt.% of Pd catalyst. This can be attributed to the reduction of the electron density around carbon-iodine bond by electron withdrawing groups, hence Pd is easier to insert into carbon and iodine. In contrast, there was a sharp decrease in the yield for iodobenzene and electron donating groups like methyl; even if reaction time was prolonged to 12 h, their yields still performed undesirably. Furthermore, terminal alkynes with electron donating groups like tertbutyl, provide almost quantitative conversion as well as selectivity for the desired products (Table 3, entries 5-8), which had a better performance than phenyl acetylene. The catalytic activity per catalytic sites, that is the turn-over-number TON (number of moles of products formed per moles of Pd) of this supported catalyst, could reach over 4200 for systems of aryl iodides with electron withdrawing groups such as nitro, ketone carbonyl (Table 3, entries 1-2, 5-6).


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The high activities of chitosan/silica microspheres supported catalysts could be attributed to uniformly distributed Pd nanoparticles which had good catalytic performance. The Mass spectrum, 13C NMR and 1H NMR spectrums of the products of different Sonogashira coupling reactions could be seen in Figures S7S16 in the supplementary information. According to the results of these Sonogashira coupling reactions, when R1 is a electron-withdrawing group, electron cloud density between carbon-iodine key decreased and Pd is easier to insert into it. So stronger electron-withdrawing groups led to easier and more efficient catalytic performance. Meanwhile, stronger electron-donating ability of R2 led to better reactivity, which enables substituent groups to have a strong influence on the Sonogashira coupling reaction and the silica/chitosan core-shell hybridmicrosphere-supported Pd catalyst is suitable for the serial reactions.

CONCLUSIONS Chitosan/silica core-shell porous microspheres with uniform particle size were prepared by a dual coaxial microfluidic device, and performed excellent capacity at the adsorption of noble metal Pd. Chelation between chitosan and Pd (II) was the rate-determining step of the adsorption process. And chitosan/silica microspheres with Pd loaded from 1 wt.% to 10 wt.% could be prepared through simply adjusting initial Pd (II) concentration. After reduction by NaBH4, Pd nanoparticles loaded in the microspheres was wellcrystallized and main catalytic crystal exposed successfully. With the optimization of experimental conditions, chitosan/silica core-shell microspheres supported Pd catalyst (1 wt.%) obtained outstanding catalytic activity in Sonogashira coupling reactions of various aryl iodides with electron withdrawing groups or terminal alkyne with electron donating groups at 65oC water-ethanol environment under free ligand, copper and amine conditions. TONs were more than 4200 in several cases. To the best of our knowledge, this is the least amount of the noble metal Pd we used to obtain a similar catalytic performance in the Sonogashira coupling reactions compared to the references. The novel supported catalyst could be simply separated from the experimental system by filtering, and it also maintained an excellent stability as Pd leaching was almost negligible during the recycling and there was barely Pd nanoparticles’ aggregation after being reused for 8 times.

ASSOCIATED CONTENT Supporting Information. Detailed synthetic protocols of supported catalyst, GC-MS, NMR results of the products are in supplementary figures. This material is available free of charge ACS Publications website.


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ACKNOWLEDGMENT The authors gratefully acknowledge the supports of the National Natural Science Foundation of China (21322604, 21476121) and Beijing Natural Science Foundation (2162020). This invited contribution is part of the I&EC Research special issue for the 2017 Class of Influential Researchers.

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Prof. Dr. Jianhong XU received his B.Sc. and Ph.D. at Tsinghua University in 2002 and 2007 respectively. He continued his research in Tsinghua University as a postdoctor after graduation. He finished the postdoctoral program in May 2009, and became a formal faculty of the Department of Chemical Engineering, Tsinghua University. He had studied as a visiting scholar at Prof. David Weitz lab in Harvard University during 2012.7~2013.6. At present, his research areas are focusing on the multiphase microfluidics and functional materials synthesis. He has more than 100 peer-reviewed publications. He got the “Excellent Young Scientists Fund” from the National Natural Science Foundation of China (NSFC) in 2013.

Prof. Guangsheng LUO received his B.Sc. Degree in applied chemistry in 1988 and his Ph.D. in chemical engineering in 1993 at Tsinghua University. He worked at Caen University, France, as a Postdoctoral Research fellow from 1995 to 1996 and at Massachusetts Institute of Technology, USA, as a visiting scientist from 2001 to 2002. His research interests include microstructured chemical systems, separation science and technology, mass transfer phenomena, and controllable preparation of functional Materials. He has published more than 300 peer reviewed papers and obtained more than 70 invention patents in China. He obtained the “National Out-


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standing Young Scientist Award” from the National Natural Science Foundation of China in 2005 and he was appointed as a Chungkung Professor by the Ministry of Education of China in 2009. He is now the director of the State Key Lab of Chemical Engineering.

Miss. Qing CUI received her B.Sc. at Shandong University in 2014 and continued to pursue her Master at Shandong University. She started her Joint Training Program in 2015 at the State Key Lab of Tsinghua University, supervised by Prof. Jianhong XU.

Dr. Hong ZHAO received her B.Sc. at Sichuan University in 2011 and Ph.D. at Tsinghua University in 2016, respectively. She had gone to Harvard University as a visiting student at Prof. David Weitz lab during 2014.9~2015.2. Now she continues her research in Sinopec Research Institute of Petroleum Processing as an engineer after graduation.


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For Table of Contents Only porous microspheres + Pd nanoparticles

Sonogashira coupling reactions


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Silica Composite CoreâShell Microspheres

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