Aerosol-Assisted Rapid Fabrication of Heterogeneous

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Aerosol-Assisted Rapid Fabrication of Heterogeneous Organopalladium Catalyst with Hierarchically Bimodal Pores Yongyi Wei, Zhan Mao, Zhengzhong Li, Fang Zhang, and Hexing Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04543 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Materials & Interfaces

Aerosol-Assisted Rapid Fabrication of Heterogeneous Organopalladium Catalyst with Hierarchically Bimodal Pores

Yongyi Wei, Zhan Mao, Zhenzhong Li, Fang Zhang*, Hexing Li* The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, China

*

(F. Z.) Email: [email protected]; Telephone: +86-21-64321673.

(H. L.) E-mail: [email protected]; Telephone: +86-21-64322272.

Keywords. Aerosol synthesis, Heterogeneous organometallic catalyst, Well-defined active sites, Hierarchically bimodal pores, Carbon-carbon cross-coupling reaction

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ABSTRACT. Heterogeneous organometallic catalysts with well-defined active sites and hierarchical pores hold tremendous promise for efficient and eco-friendly chemical processes. However, the simple and scalable preparation of these materials remained difficult to date, which has hampered a more board application scope. Herein we reported a low cost, rapid and scalable aerosol-assisted assembly approach for the synthesis of well-defined PdDPP (PdCl 2 (PPh 2 (CH 2 ) 2 ) complex-containing benzene-bridged organosilica-based catalyst with hierarchically bimodal micro-macroporous structure. This novel material was realized by using Pd(II) organometallic silane (Pd[PPh 2 (CH 2 ) 2 Si(OEt) 3 ] 2 Cl 2 ) as the active species, organosilane 1,4-bis(triethoxysilyl)benzene (Ph[Si(OEt) 3 ] 2 ) as the silicate scaffold and surfactant cetyltrimethylammonium bromide (CTAB) and inorganic salt NaCl as the dual templates on a home-built aerosol spraying instrument. Multiple techniques including XPS and solid NMR spectra reavealed that the organopalladium complex with well-defined molecular configuration of major trans model and minor cis model existed in the phenylfunctionalized silica material. As expected, it efficiently promoted a variety of important carbon-carbon cross-coupling transformations including Tsuji-Trost, Sonogashira and Suzuki reactions in pure water without the assistance of any additives. In comparison with homogeneous catalyst PdCl 2 (PPh 2 CH 3 ) 2 , it even exhibited the enhanced activity and selectivity in some cases owing to the unique confinement effect and the shape-selectivity generated from the hierarchical pore structure. Meanwhile, it was easily recycled and reused for eight times without the loss of the activity.


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1. Introduction Homogeneous organometallic catalysts are widely used for the production of high-value fine chemicals, pharmaceuticals and molecular organic materials.1 Despite their high activity and selectivity, these catalysts are relatively expensive and cannot be recycled and reused.2 These drawbacks inevitably add cost and also cause the pollutions in both the environment and the products by metal ions and/or ligands.3 In addition, homogeneous catalysts are usually not designed to fit into continuous-flow reactors, which are economically attractive to the chemical industries.4 Accordingly, heterogeneous organometallic catalysts are believed as an effective way to address these problems.5 Their obvious advantage is the ease of recovery and recycling and is readily amenable to continuous processing, which substantially decrease the production cost and chemical waste. However, they usually exhibited the inferior catalytic performances comparing to their corresponding homogeneous ones. The major reasons could be attributed to the decreased accessibility of the active sites caused from the increased mass transfer limitation and the changed molecular configuration of the active sites derived from the multistep preparation process.6 Thus, the development of heterogeneous organometallic catalysts with well-defined active species has recently attracted considerable attention because this unique character facilitates the improved catalytic activity and selectivity as well as the detailed mechanism studies.7-18 To date, a variety of catalysts with different chemical compositions has been synthesized by surface organometallic chemistry, encapsulation or physical adsorption. Also, these catalysts were successfully used in olefin metathesis,19 olefin polymerization,20 alkane homologation,21 hydrogenation,22 oxidation23 and carbon-carbon coupling reactions.24 Despite these remarkable achievements, the synthetic protocols of the previously reported heterogeneous organometallic catalysts with well-defined active sites were typically conducted in the tedious and time-consuming solution-based technique.25 As a result, the major difficulty in the implementation of these laboratory-designed catalysts is the scale-up into technically demanding process.26 In addition, heterogeneous organmetallic ACS Paragon Plus Environment


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catalysts with hierarchical porosity provide en efficient approach to address the mass transfer limitation by the combined advantages of large surface area and big pore size.27-32 Therefore, the design of the low-cost, facile and scalable approach for the fabrication of heterogeneous organometallic catalysts with well-defined active sites and hierarchical pores is urgently needed. Aerosol process is a fast and simple method to synthesize porous materials because it is comprised of a limited number of preparation steps and produces materials continuously with very low waste.33,

34

Recently, the coupling of template-directed assembly with aerosol

process was used for the synthesis of hierarchically porous materials with controllable pore structure, size and tunable surface properties35-37, providing the great potential for the scalable fabrication of sophisticated porous catalysts in an environmentally benign process. However, up to now, the synthesis and application of well-defined heterogenoeus organometallic catalysts with hierarchically porous structure by the aerosol protocol are very rare.38 In this contribution, we reported an aerosol-assisted assembly technique that can be used as a low cost, rapid and scalable approach for the preparation of well-defined PdDPP (PdCl 2 (PPh 2 CH 2 ) 2 ) complex-containing silica-based catalyst with hierarchical bimodal porous structure (PdDPP-SHCs-HP). It was achieved by using Pd(II) organometallic silane (Pd[PPh 2 (CH 2 ) 2 Si(OEt) 3 ] 2 Cl 2 ) bis(triethoxysilyl)benzene

as

well-defined

(Ph[Si(OEt) 3 ] 2 )

as

the

active

species,

silicate

organosilane

scaffold

with

1,4-

surfactant

cetyltrimethylammonium bromide (CTAB) and inorganic salt NaCl as the dual pore templates on a home-built aerosol spraying instrument. The as-prepared solid catalyst contains the micropores and macropores with 1.0-2.0 nm and 40-80 nm sizes, respectively. Also, the welldefined PdCl 2 (PPh 2 CH 2 CH 2 ) 2 -complex was accommodated in the channel of the hydrophobic phenyl-bridged silica without the alternation of its molecular configuration. As expected, it exhibited good activity and excellent selectivity in a series of carbon-carbon coupling transformations including Tsuji-Trost, Sonogashira and Suzuki reactions by using ACS Paragon Plus Environment

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water as the sole medium. Moreover, it could be used repetitively for at least eight times without the significant activity loss, showing a good potential in the industrial application.

2. Experimental Section 2.1 PdDPP organometallic silane synthesis In a typical synthesis, PdCl 2 (COD) (0.50 g, 1.75 mmol) was suspended in toluene (25 ml) and then 2-(Diphenylphosphino)ethyltriethoxysilane (1.35 g, 3.50 mmol) was added dropwise and stirred for 2.0 h at 25oC. A clear orange solution was formed while it was concentrated to 5.0 ml. After adding petroleum ether, the yellow precipitate was obtained and then washed three times by petroleum ether, resulting in PdDPP organometallic silane (PdCl 2 [PPh 2 (CH 2 ) 2 Si(OEt) 3 ] 2 , see 1H and

31

P NMR spectra in Figure S1-2). 1H NMR

(CDCl 3 ) δ 7.2-7.5, 7.6-7.7 (2m, 20 H, ArH), 3.71 (q, JD7.0 Hz, 12 H), 2.47 (m, 4H, CH 2 P), 1.13 (t, JD7.0 Hz, 18 H), 0.79 (m, 4 H, SiCH 2 ).

P{1H} NMR (CDCl 3 ) δ 32.3 (cis), 22.0

31

(trans). 2.2 Catalyst preparation 1.60 g CTAB (Cetyltrimethylammonium bromide), 58 ml ethanol, 10 ml H 2 O, 0.22 ml 2.0 mol/l HCl aqueous solution, a certain amount of NaCl and 1.81 g BTEB (Ph[Si(OEt) 3 ] 2 ) were mixed and allowed to pre-hydrolysis for 1.0 h at 25oC. Then, 2.0 ml THF solution containing 0.465 g PdDPP organometallic silane (PdCl 2 [PPh 2 (CH 2 ) 2 Si(OEt) 3 ] 2 ) was added, followed by stirring for another 1.0 h. Then, the solution was atomized by an aerosol instrument (Scheme 1), followed by passing through tube furnace at 300oC with 0.15 MPa N 2 as the carrier gas. The solid product was refluxed in 500 mL 0.50 mol/l HCl/ethanol solution at 80°C for 24 h to remove the CTAB and NaCl templates. The obtained sample was dried in a vacuum at 80°C for 8.0 h. The as-received samples were designated as PdDPP-HHOCs, PdDPP-SHCs-HP-1, PdDPP-SHCs-HP-2 and PdDPP-SHCs-HP-3, corresponding to 0, 0.190, 0.380, and 0.570 g NaCl amount in the initial solution, respectively.



2.3 Catalyst characterization The PdDPP loadings were determined by inductively coupled plasma optical emission spectrometer (ICP, Varian VISTA-MPX). The chemical compositions of the heterogeneous catalysts were characterized by thermogravimetric and differential thermal analysis (TG/DTA, Shimadzu DTG-60) and solid nuclear magnetic resonance spectra (NMR, Bruker AV-400). The surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C ESCA). All the binding energy values were calibrated by using C 1S = 284.6 eV as a reference. The morphologies and porous structures were examined by transmission electron microscopy (TEM, JEOL JEM2011), field emission scanning electron microscopy (SEM, HITA-CHI S4800) and X-ray powder diffraction patterns (XRD, Rigaku D/Max-RB). The surface area, pore size and pore volume were calculated by N 2 sorption isotherms, which were measured at 77 K on a Quantachrome NOVA 4000 e analyzer. The surface hydrophobicity tests were carried out on an intelligent gravimetric analyse (Hiden Isochema IGA-002/3) by introducing a dosed amount of high-purity vapor directly into the sample chamber and recording the weight change after stable equilibrium pressure was reached. 2.4 Catalytic testing Tsuji-Trost reaction: Generally, each run of organic reactions was carried out in a 25 ml Schlenk flask. The reproducibility was checked by repeating experiments for at least three times and was found to be within acceptable limits (± 5 %). In a typical run, 2.0 mmol allyl acetate, 1.0 mmol ethyl benzoylacetate, 2.0 mmol triethylamine, 100 μl diphenylmethane, 5.0 ml water and a catalyst containing 5.0 mol% Pd were mixed and stirred at 60oC for 12 h. After being cooled to room temperature, the mixture was extracted by ethyl acetate for three times. The organic layers were collected and then diluted with ethyl acetate to 10 ml, followed


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by GC analysis. The reaction conversion was calculated based on ethyl benzoylacetate since allyl acetate was excess. Sonogashira reaction: 1.0 mmol iodobenzene, 2.0 mmol benzylacetylene, 2.0 mmol triethylamine, 5.0 mL water, 100 μl decane as internal standard, and a catalyst containing 5.0 mol% Pd were mixed and stirred at 60oC for 16 h, followed by extraction and GC analysis as described above. The reaction conversion was calculated based on iodobenzene since benzylacetylene was excess. Suzuki reaction: 1.0 mmol iodobenzene, 1.5 mmol 4-methylbenzylboroic acid, 2.0 mmol K 2 CO 3 , 100 μl decane, 5.0 ml water and a catalyst containing 5.0 mol% Pd were mixed and stirred at 50oC for 6.0 h, followed by extraction and GC analysis as described above. The reaction conversion was calculated based on iodobenzene since 4-methylbenzylboroic acid was excess. 2.5 Adsorption testing To test the adsorption behaviours of phenylacetylene, 50 mg catalyst after being soaked in 200 mL deionic water at 25oC overnight was added into 50.0 ml aqueous solution containing 50 mg/L phenylacetylene and 0.10% n-butanol, followed by oscillating at 25oC. The solution was sampled at given intervals, followed by measuring phenylacetylene concentration (C) on LC-MS spectrophotometer (UV 210 nm, 10 mg/L phenol as internal standard), from which the adsorption kinetics was studied based on the adsorption equation dC/dt = kCn.

3. Results and Discussion The fabrication procedure of PdDPP-SHCs-HP samples was depicted in Scheme 1. The PdDPP organometallic silane (PdCl 2 [PPh 2 (CH 2 ) 2 Si(OEt) 3 ] 2 ) was firstly synthesized by ACS Paragon Plus Environment


corrdination

reaction

between

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dichloro(1,5-cyclooctadiene)palladium(II)

(Diphenylphosphino)ethyltriethoxysilane.

The

PdDPP

organometallic

with

silane,

2-

BTEB

(Ph[Si(OEt) 3 ] 2 ), surfactant CTAB and NaCl salt were added into HCl ethanol/water solution. The droplets of this mixture were generated by aerosol atomizer with N 2 as the carrier gas. Solvent evaporation at the air/liquid interface enriched the aerosol droplets in CTAB, NaCl, pheyl-bridged silicate and Pd organometallic silicate. This process promoted their cooperative assembly into liquid-crystalline phases. Next, these assembling particles passed through the heating zone of the process, further drying and silicate co-condensation reaction result in the formation of solid particles containing PdDPP organometallic complex. Finally, the surfactant and salt removal resulted in the formation of heterogeneous organopalladium catalyst with bimodal pores. Figure 1a-d displayed the scanning electron microscopy (SEM) images of four products by our aerosol synthesis, namely, PdDPP-SHCs-HP, PdDPP-SHCs-HP-1, PdDPP-SHCs-HP-2 and PdDPP-SHCs-HP-3 corresponding to the use of 0, 0.19, 0.38, 0.57 g NaCl, repectively. All the samples had a spherical shape with the size ranging from 50 nm to 500 nm, which were primarily determined by the size of the aerosol droplets. By counting the certain amount of the particles, the size distributions of these samples were shown in Figure 1e-h. The diameters of these particles were mainly around 100 to 300 nm. These results revealed that this areosol approach can rapidly produce the spherical nanoparticles with the relatively uniform sizes and meanwhile the amount of NaCl has the little effect on their morphologies. Transmission electron microscopy (TEM) images (Figure 2) further revealed that the cubic macropores were observed in the PdDPP-SHCs-HP-1 and PdDPP-SHCs-HP-2. Meanwhile, their measured macropore sizes were about 50 nm. The increased NaCl amount from 0.19 to 0.38 g caused the increased macropore number in the PdDPP-SHCs-HP-2 while the use of 0.58 g NaCl led to the obviously increased pore size around 80 nm in the PdDPP-SHCs-HP-3. Interestingly, these macropores were surrounded by the abundant pores with relatively smaller ACS Paragon Plus Environment

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size. However, the cubic hollow macropores were disappeared in PdDPP-SHCs-HP without the addition of NaCl while the porous structure was still remained, which indicated that the macropores was generated by NaCl template in the prepartion process. Owing to the large amount of the organic moieties in these samples, the strong electron beam irradiation in the TEM analysis inevitably destroyed these small pores. We used N 2 sorption test to further investigate the detailed pore structure (Figure 3a). The typical type-IV isotherms with a sharp uptake at low relative pressure indicative of the micropores in PdDPPSHCs-HP-1, PdDPP-SHCs-HP-2 and PdDPP-SHCs-HP-3 samples were observed.39 At the high pressure area, two other adorption steps can be obtained in these isotherms. At the relative pressure range P/P 0 =0.20~0.40, a H 1 -type hysteresis loop was identified due to the capillary condensation, revealing the existence of large micropores. Moreover, in the higher pressure stege, there was an additional adsorption at P/P 0 =0.42~0.95 with a large hysteresis loop, which was due to the emptying of the large internal macropore,40 in accordance with TEM observations. Also, the loop sizes derived from the macropores were increasing with the added amount of NaCl, revealing its template effect in the aerosol assembly process. Furthermore, the pore size distributions (PSD) of these samples (Figure 3b) calculated by the BJH method displayed two peaks centered at 0.9 nm and 1.9 nm, respectively, which further confirmed the simultaneous existence of the micropores. Moreover, the amounts of micropores with 1.9 nm were increased from PdDPP-SHCs-HP-1 to PdDPP-SHCs-HP-3, which was mayde due to these micropores existed as the entrances beteween the radial arranged micropores and the central cubic macropores. For a comparison, PdDPP-SHCs-HP displayed type I N 2 sorption isotherm with the increase in the adsorption branch at low relative pressure (P/P 0 =0.05~0.15), indicating that it only had the micropores (Figure 3a). These results could be deduced that CTAB surfactant was responsible for the generation of the micropores. Compared to the previously reported CTAB-template inorganic silane assembly process with around 2.0 nm pore size, the decreased pore size in the PdDPP-SHCs ACS Paragon Plus Environment


and PdDPP-SHCs-HP samples could be attributed to the large amount of the organometallic complex and the phenyl-groups in the surface of the pore channels. Moreover, the absence of the hysteresis loop in the high pressure area in PdDPP-SHCs-HP confirmed that the disappearance of the macropores. The pore size distribution curve further demonstrated that only one peak centred at 0.90 nm was found (Figure 3b), confirming that the addition of NaCl salt resulted in the formation of macropore. Owing to the unique tetramodal micromacroporous structure, PdDPP-SHCs-HP-1, PdDPP-SHCs-HP-2 and PdDPP-SHCs-HP-3 samples showed the high specific surface areas ranging from 1202 to 1340 cm2/g (Table S1). The increasing macropores in PdDPP-SHCs-HP-3 led to the lowest surface area, confirming that the micropores are mainly responsible for the high surface area. Meanwhile, the lowangle X-ray diffraction (XRD) patterns showed that all the samples displayed a well-resolved diffraction peak at 2.5~2.7o indicative of the ordered porous structure (Figure S3).41 The relatively weak peak intensities and the disappearnce of the other characteric peaks were due to the PdDPP-organometallic silica and the phenyl-bridged organosilica partially disturbed the silica species/CTAB assembly and thus reduced the ordering degree.42 On basis of these results, the postulated mechanism of the hierarchical pores formation of PdDPP-SHCs-HP was proposed (Scheme 1).43 In our aerosol process, each droplet that generated by the atomizer was composed of PdDPP organometallic silane, BTEB organosilane, surfactant CTAB, inorganic salt NaCl and acid catalyst HCl as well as water and ethanol as the solvents. Evaporation of the solvents in the drying zone created within each droplet radial gradients for silanes, surfactant, salt and solvent concentrations, which steepened with time and maintain maximum for silanes, surfactant and salt at the droplet surface. The addition of ethanol in the mixture greatly reduced the solubility of NaCl in water and meanwhile the dissolution/crystallization of NaCl is a very quick process and is controlled by diffusion process. During the continuous evaporation in the heating zone, the strong ionic force between the ions and the hydrophilic NaCl crystalline repelled the ACS Paragon Plus Environment

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interactions of the organic silica acid molecules that generated from HCl catalyzed hydrolysis/condensation reactions, proceeding NaCl diffused down the droplet center and grew up to cubic crystals. Meanwhile, the CTAB and the hydrophobic organometallic and organic silica acid molecules in the interface were arranged by the electrostatic force to induce silicate-surfactant self-assembly into micelles and their further organization into liquid-crystalline phases. Also, the radial concentration gradient and the present of the vaporliquid interface served as a nucleating surface for silica to grow radically inward, resulting in the silica/surfactant/salt nanocomposite. Finally, the hierarchical bimodal micro-macroporous structure was obtained after removing the surfactant and salt templates. ICP analysis revealed that the organopalladium contents in all the PdDPP-containing samples by using different NaCl amounts in the initial solutions were around 1.4 wt.% (Table S1). TG/DTA curves of the representative PdDPP-SHCs-HP-2 under air atomsphere (Figure S4) showed that an endothermic peak at 80oC with the weight loss around 8.6%, which could be assigned to the desorption of residual water and solvents. The large exothermic peak from 450 to 650oC with weight loss around 30% was attributed to the oxidation of the phenyl groups inside the silica framework.44 Meanwhile, the two exothermic peaks with weight loss about 10% were observed at the temperature from 250 to 400oC, which could be ascribed to the removal of Pd-PPh 2 CH 2 CH 2 - moieties. These data indicated that the PdDPP-complex and the phenyl fucntional groups were successfully incooperated at the same time in this one-step areosol process. To prove the construction of well-defined PdDPP active sites, we firstly characterized the the electronic states of palladium and phosphorus elements by using X-ray photoelectron spectroscopy (XPS). As shown in Figure 4a, the XPS spectra revealed that all the Pd species in the PdDPP-SHCs-HP-2 were present in +2 oxidation state, corresponding to the binding energy (BE) of 337.7 eV and 343.0 eV in the Pd 3d 5/2 and 3d 3/2 levels, respectively.45 The Pd(PPh 2 CH 3 ) 2 Cl 2 complex as a comparsion exhibited the binding energy of 337.8 eV and ACS Paragon Plus Environment


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343.1 eV. Almost the same binding energies between Pd(PPh 2 CH 3 ) 2 Cl 2 and PdDPP-SHCsHP-2 demonstrated the unchanged molecular configuration of PdDPP complex after this aerosol process. Meanwhile, no other peaks was obtained in Pd XPS spectrum, indicating that the defined form of PdDPP active sites.46 Accordingly, P XPS spectrum (Figure 4b) of PdDPP-SHCs-HP-2 clearly exhibited alomost the same binding energy with that of Pd(PPh 2 CH 3 ) 2 Cl 2 , which further confirmed the above explanation. Figure 5 showed

29

Si,

13

C and

31

P solid-state NMR spectra of PdDPP-SHCs-HP-2. The

peaks marked with asterisks were resulted from the rotational sidebands which were confirmed by changing the rotational speed. 29Si NMR spectrum (Figure 5a) of PdDPP-SHCsHP-2 displayed three distinct resonance peaks at -64, -72 and -81 ppm, which corresponded to T1 [R(HO) 2 Si(OSi)], T2 [RHOSi(OSi) 2 ] and T3 [RSi(OSi) 3 ] silica species, respectively. The absence of Q peaks (Qn= Si(OSi) n (OH) 4-n , n=2-4) indicative of inorganic silica species revealed that all the Si species were covalently bonded with carbon atoms, which also confirmed that the Si-C bond remained intact during the co-condensation between organosilane BTEB and PdDPP organometallic silane.47 Meanwhile, in the

13

C CP/MAS

NMR spectrum (Figure 5b), two peaks at 15 and 58 ppm could be assigned to two carbon atoms in the PPh 2 -CH 2 -CH 2 -ligands of PdDPP complex while a broad peak around 130 ppm was derived from the carbon atoms in the phenyl functional groups embedded in the silica framework and the benzene ring connecting with the phosphorus ligands.48 Furthermore, 31P CP/MAS NMR spectrum exhibited one large major peak around 20 ppm and the other very small peak around 31 ppmm, which could be attributed to the P atom that coordinated with Pd2+ ions in the trans and cis model, respectivley (Figure 5c).49 It may be safely concluded from these results that our aerosol-assisted assembly approach could successfully incorporate the organometallic complex with defined configuration into the phenyl-bridged functional silica while the molecular structure of organometallic complex could be perfectly retained.


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We attempted to investigate the performances of PdDPP-containing samples that acts as the green heterogeneous catalyst for carbon-carbon cross-coupling reactions in pure water. Indeed, we found that three PdDPP-SHCs-HP-1, PdDPP-SHCs-HP-2 and PdDPP-SHCs-HP-3 samples efficiently accelerated the Tsuji-Trost reaction with ethyl benzoylacetate and allyl acetate as the reactants. Noted that they also exhibited the high selectivity to the monoallyated product with the negelible diallylated product (Table 1, entry 1-3). Time-conversion curves showed that PdDPP-SHCs-HP-2 showed the highest yield of 88% after 12 h with 5.0 mol% catalyst loading (Figure S5). The inferior catalytic activities of the PdDPP-SHCs-HP-1 and PdDPP-SHCs-HP-3 were maybe attributed to their porous structure with different macropore amounts and morphologies owing to their the similar PdDPP complex content and the identical active species. In comparsion with PdDPP-SHCs-HP-2, PdDPP-SHCs-HP-1 had the smaller macropore size while PdDPP-SHCs-HP-3 had the less macroporous amount, which probably resulted in the enhanced diffusion limitation in water. Also, the control PdDPPSHCs catalyst without the internal macropores displayed the lowest yield (Table 1, entry 4), further confirming the advantage of the hierarchical bimodal porous structure. Interestingly, homogeneous PdCl 2 (PPh 2 CH 3 ) 2 complex (Table 1, entry 5) obtained the decreased coversion (86%) and selectivity (94%). This result indicated that the hierarchical porous structure with hydrophobic phenyl groups in the pore surface could effectively stabilize and concentrate the hydrophobic reactants in pure water.50 Meanwhile, the existence of the large amounts of micropores enhanced the monoallyated product selectivity due to the relatively big molecular size of diallylated product. In addition, the possible reason was that ethyl benzoylacetate was more hydrophobic than allyl acetate. Thus, the surface hydrophobility of the optimized PdDPP-SHCs-HP-2 was more favorable for the adsorption and the activation of ethyl benzoylacetate and also it led to the diminished adsorption of excess allyl acetate, preventing the secondary addition to the diallylated product.51 Furthermore, other allylic substitution reactions were also examined by the optimized PdDPP-SHCs-HP-2 catalyst. It ACS Paragon Plus Environment


still showed excellent activity and selectivity to the corresponding products with 89% to 92% yields by using 1,3 diester and 1,3-diketone as the reactants (Table 2), demonstrating the universal advantage in the water-medium Tsuji-Trost reaction. To determine whether the PdDPP organometallic complex bonded to the hierarchical porous support or the dissolved Pd organometallic leached from the hierarchical porous support, the hot-filtration test was carried out. The Tsuji-Trost reaction between ethyl benzoylacetate and allyl acetate was allowed to react for 6.0 h by the PdDPP-SHCs-HP-2 catalyst until iodobenzene conversion exceeded 65%. Then, the mixture was immediately separated by centrifugation to remove the solid catalyst. Next, the mother liquor was continued to react for another 16 h under the same reaction conditions. No significant change in either the conversion or the yield was obtained, indicating that the present catalysis indeed was heterogeneous in nature rather than the dissolved PdDPP species leached from the PdDPP-SHCs-HP-2 catalyst.52 To further explore the synthetic scope of PdDPP-SHCs-HP-2 catalyst, other important carbon-carbon coupling reactions including Sonogashira and Suzuki reactions were investigated in pure water (Table 3-4). As expected, in the Sonogashira reaction between iodobenzene and acetylenbenzene and Suzuki reaction between iodobenzene and benzylboric acid, PdDPP-SHCs-HP-2 gave the absolute cross-coupling products with the highest catalytic reactivities compared to other heterogeneous PdDPP complex-containing heterogeneous catalysts. Also, it showed the comparable efficiencies with the homogeneous catalyst PdCl 2 (PPh 2 CH 3 ) 2 . It was found that the use of either bromobenzene or chlorobenzene instead of iodobenzene caused remarkable decrease in activity, obviously due to the difficult activation of Br-C or Cl-C bond. On the other hand, PdCl 2 (PPh 2 CH 3 ) 2 also exhibited the same trend in these reactions. However, by simply extending the reaction time, PdDPP-SHCsHP-2 obtained the significant improvements in both bromobenzene and chlorobenzene substances in the Suzuki reaction. Meanwhile, the use of NO 2 -substituted iodobenzene instead of the iodobenzene slightly promoted the Suzuki reaction while the use of CH 3 OACS Paragon Plus Environment

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substituted iodobenzene slightly suppressed the Suzuki reactions since the NO 2 was a typical electron withdrawing group, which could reduce electron density on the I-C bond, making the I-C bond easily activated (Table 4). These results demonstrated that PdDPP-SHCs-HP-2 efficiently maintained the intrinsic property of the PdCl 2 (PPh 2 CH 3 ) 2 catalytic system, showing a good potential in the industrial applications. An important merit of heterogeneous organometallic catalyst is the convenient recycle and reuse. As shown in Figure 6, PdDPP-SHCs-HP-2 could be reused at least 8 times without any remarkable reduction in the catalytic efficiency. ICP analysis showed that after being used for eight repetitions, PdDPP content in the recycled PdDPP-SHCs-HP-2 remained almost the same (1.3 wt.%). TEM image (Figure S6) further revealed that the hierarchically bimodal micro-macroporous structure could be well retained after the stability tests. Meanwhile, Pd(0) nanoparticles was not observed by TEM analysis. On basis of these reports and our results, we supposed that the active sites was maybe the Pd(II)/Pd(0) composites.53-55 This phenomenon could be explained by the strong surface hydrophobility to prevent the attack of silica walls from water, leading to the enhanced hydrothermal stability of porous structure. Meanwhile, the periodically arranged PdDPP active sites in the silica framework could effectively inhibited Pd speices leaching in the reaction process.56 To gain precise insight into the unique properties of PdDPP-SHCs-HP-2, the vapor adsorptions of toluene and xylene tests were first conducted, which is used to confirm the hydrophobic pore surface that helps it to absorb the hydrophobic reactants from water solvent. As shown in Figure 7, the isotherms were of type V, indicative of weak absorbent-absorbate interaction. Meanwhile, hierarchical bimodal micro-macroporous structure was also confirmed by three obvious hysteresis loops from the relatively low to high pressures. In addition, the high adsorption capacities of PdDPP-SHCs-HP-2 for toluene and xylene were 59.4 wt.% and 55.5 wt.%, respectively. These results demonstrated that the pore surface was hydrophobic, resulting in the efficient diffusion of the organic reactants in water. Furthermore, ACS Paragon Plus Environment


the reactants adsorption test was also investigated by using phenylacetylene in the Sonogashira reaction (Figure 8a). PdDPP-SHCs-HP-2 displayed much higher saturated adsorption capacity (53.7 wt.%) than that of PdDPP-SHCs-HP (32.8 wt.%), even the latter with the large surface area. Straight lines (Figure 8b) were achieved by plotting 1/C vs t, where C refers to the concentration of phenylacetylene in water after t hour adsorption, indicating the second order adsorption process. As a result, the adsorption rate of the reactant molecules possibly played a critical role in determining the catalytic reactivity of these PdDPP-containing solid catalysts. Acorrding to the tangent slopes, the adsorption constants (k) of PdDPP-SHCs-HP-2 and PdDPP-SHCs-HP were calculated as 0.037 and 0.025, respectively. It indicated that PdDPP-SHCs-HP-2 adsorbed the hydrophobic substances more rapidly than PdDPP-SHCs-HP, which revealed the presence of hierarchically micromacroporous structure promoted the mass transport and therefore enhanced the catalytic efficiency.57

4. Conclusions In summary, we demonstrated that varing the amount of NaCl inorganic salt template in the aerosol process is an efficient approach to develop highly active and stable well-defined heterogeneous PdCl 2 (PPh 2 CH 2 CH 2 ) 2 complex (PdDPP)-containing silica-based catalyst with hierarchical bimodal porous structure. By using multiple techniques including SEM, TEM, N 2 sorption, XRD, TG, XPS and solid NMR, we found that the PdDPP active sites with the major trans model and minor cis model was embedded in the hydrophobic phenyl-bridged silica framework with the radial micropores and the cubic macropores. This unique multicomponent assembly process efficiently overcomes the tedious, multistep process and the uncontrollable chemical configurations by the traditional methods. We identified that it exhibited high activity and excellent selectivity in a variety of carbon-carbon cross-coupling reactions by using water as sole reaction medium and remarkable robustness in stability tests, ACS Paragon Plus Environment

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making it a highly attractive candidate for future applications. This low cost, rapid and scalable synthetic strategy could be further extended to develop more highly efficient welldefined heterogeneous organometallic catalysts for green chemical transformations.

Acknowledgments Financial support for this work provided by the National Natural Science Foundation of China (51273112), PCSIRT (IRT-16R49) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (TP2016034). Supporting

Information

Available.

1

H

and

31

P

NMR

spectra

of

PdCl 2 [(PPh 2 (CH 2 ) 2 Si(OEt) 3 ] 2 compound, Chemical composition, structural parameters and XRD patterns, reaction profiles of different PdDPP-containing catalysts, TG/DTA data of PdDPP-SHCs-HP-2 and TEM image of the recylced PdDPP-SHCs-HP-2. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Table 1. Catalytic performances of heterogeneous and homogeneous PdDPP organometallic catalysts in water-medium Tsuji-Trost reaction.a O O

O

O O

O

O

O

O

O

O

a

Entry

Sample

Conversion (%)

Selectivity (%)

1

PdDPP-SHCs-HP-1

84

99

2

PdDPP-SHCs-HP-2

89

99

3

PdDPP-SHCs-HP-3

81

99

4

PdDPP-SHCs

74

99

5

PdCl 2 (PPh 2 CH 3 ) 2

86

94

Reaction conditions: 2.0 mmol allyl acetate, 1.0 mmol ethyl benzoylacetate, 2.0 mmol triethylamine, 0.10

ml diphenylmethane, 5.0 ml water and a catalyst containing 5.0 mol% Pd, 60oC, 12 h.


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Table 2. The Tsuji-Trost reaction of various 1,3-dicarbonyl compounds with allyl acetate in water catalyzed by PdDPP-SHCs-HP-2 and PdCl 2 (PPh 3 ) 2 catalysts.a

Entry

Sample

1

PdDPP-SHCs-HP-2

2


3

PdDPP-SHCs-HP-2

4


5

PdDPP-SHCs-HP-2

6


7

PdDPP-SHCs-HP-2

1,3-dicarbonyl

Conversion (%)

Selectivity (%)

93

91

89

80

90

99

87

94

91

99

86

93

90

99

86

93

O

O

O

O

O O O

O O O

8 a


O

O

Reaction conditions were shown in Table 1.



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Table 3. Catalytic performances of heterogeneous and homogeneous PdDPP organometallic catalysts in water-medium Sonogashira reaction.a

+ Sample PdDPP-SHCs-HP-1 PdDPP-SHCs-HP-2 PdDPP-SHCs-HP-3 PdDPP-SHCs PdCl 2 (PPh 2 CH 3 ) 2 PdDPP-SHCs-HP-2 PdCl 2 (PPh 2 CH 3 ) 2 a

X X I I I I I Br Br

Conversion (%) 89 93 88 85 96 21 28

Selectivity (%) 99 99 99 99 99 99 99

Reaction conditions: 1.0 mmol iodobenzene, 2.0 mmol benzylacetylene, 2.0 mmol triethylamine, a catalyst

containing 5.0 mol% Pd, 0.10 ml decane, and 10 ml water, 60oC, 16 h.


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Table 4. Catalytic performances of heterogeneous and homogeneous PdDPP organometallic catalysts in water-medium Suzuki reaction.a B(OH)2

a

+ X

Y

Y

Sample

X

Y

PdDPP-SHCs-HP-1 PdDPP-SHCs-HP-2 PdDPP-SHCs-HP-3 PdDPP-SHCs PdCl 2 (PPh 2 CH 3 ) 2 PdDPP-SHCs-HP-2 PdCl 2 (PPh 2 CH 3 ) 2 PdDPP-SHCs-HP-2 PdCl 2 (PPh 2 CH 3 ) 2 PdDPP-SHCs-HP-2 PdDPP-SHCs-HP-2

I I I I I Br Br Cl Cl I I

H H H H H H H H H NO 2 CH 3 O

Conversion (%) 90 95 88 86 97 56 (96b) 63 13 (51c) 21 99 87

Selectivity (%) 99 99 99 99 99 99 99 99 99 99 99

Reaction conditions: 1.0 mmol iodobenzene, 1.5 mmol 4-methylbenzylboroic acid, 2.0 mmol K 2 CO 3 , a

catalyst containing 5.0 mol%, 0.10 ml decane, and 10 ml water, 50oC, 6.0 h. b reacting at 50oC for 24 h. c reacting at 100oC for 24 h.



Scheme 1. Scheme for the preparation of PdDPP-containing heterogeneous organometallic catalysts by an aerosol-assisted assembly method.


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Figure 1. SEM images (a-d) and particle size distribution diagrams (e-h) of PdDPP-SHCsHP-1, PdDPP-SHCs-HP-2, PdDPP-SHCs-HP-3 and PdDPP-SHCs samples.



Figure 2. TEM images of PdDPP-SHCs-HP-1 (a), PdDPP-SHCs-HP-2 (b), PdDPP-SHCsHP-3 (c) and PdDPP-SHCs (d) samples.


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Figure 3. N 2 sorption isotherms (a) and pore size distribution curves (b) of PdDPP-SHCs-HP1, PdDPP-SHCs-HP-2, PdDPP-SHCs-HP-3 and PdDPP-SHCs samples.



Figure 4. XPS spectra of PdDPP-SHCs-HP-2 and Pd(PPh 2 CH 3 ) 2 Cl 2 samples (a. Pd element; b. P element).


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Figure 5. Solid NMR spectra of PdDPP-SHCs-HP-2 (a. 29Si NMR; b. 13C NMR, *: rotational sideband; c. 31P NMR).



Figure 6. Recycle tests of PdDPP-SHCs-HP-2 catalyst in the Tsuji-Trost reaction with ethyl benzoylacetate and allyl acetate as the reactants.


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Figure 7. Vapor adsorption tests of toluene (a) and xylene (b) of PdDPP-SHCs-HP-2 catalyst.



Figure 8. Phenylacetylene substance adsorption profiles of PdDPP-SHCs-HP-2 and PdDPPSHCs-HP samples in water (a. adsorption amount curve, b. adsorption rate curve).


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Aerosol-Assisted Rapid Fabrication of Heterogeneous

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