A General, Green Chemistry Approach for Immobilization of Inorganic

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A general, green-chemistry approach for the immobilization of inorganic catalysts in monolithic porous flow-reactors Yuchao Wang, Da Shi, Shengyang Tao, Wentong Song, Hongmin Wang, Xinkui Wang, Guangtao Li, Jieshan Qiu, and Min Ji ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01541 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 22, 2015

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ACS Sustainable Chemistry & Engineering

A general, green-chemistry approach for the immobilization of inorganic catalysts in monolithic porous flow-reactors Yuchao Wang‡§, Da Shi†‡, Shengyang Tao*†, Wentong Song†, Hongmin Wang†, Xinkui Wang†, Guangtao Li∥, Jieshan Qiu⊥, and Min Ji†

AUTHOR ADDRESS Shengyang Tao* E-mail: [email protected] † Department of Chemistry, Dalian University of Technology, Dalian 116024, Liaoning, P.R. China. § Water Desalination and Reuse Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ∥Department of Chemistry, Key Lab of Organic Optoelectronics & Molecular Engineering, Tsinghua University, Beijing 100084, P.R. China ⊥Carbon Research Laboratory, Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning, P.R. China.

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KEYWORDS: bio-inspired; polyphenols; flow-reactors; porous; catalysts.

ABSTRACT

A general, green-chemistry approach is developed for the immobilization of inorganic catalysts in monolithic porous flow-reactors for continuous flow reactions, in which pyrogallol (PG) acts as an assistant layer for the in situ immobilization of various catalysts on the inner pore surface of the flow-reactor. Au nanoparticles (AuNPs), Al2O3 and phosphotungstic acid (HPW) are selected as examples of noble metals, metal oxides and polyoxometalate (POM) type catalysts, respectively, and the corresponding flow-reactors show high catalytic performance in continuous flow hydrogenation reactions, esterification reactions and Knoevenagel reactions, respectively. These reactions are important in the treatment of environmental contaminants and the chemical industry. In addition, multiple monolithic flow-reactors can be assembled together for efficient performance of multi-step cascade reactions.

Introduction Chemical conversion within a confined flow-reactor has attracted significant interest (1-3) because of its advantages over large-scale batch chemistry, such as continuous production, high automation and easy handling for recycling.(4-5) Several types of flow-reactors have been widely investigated, such as porous monolith type, powder-packed type, and membrane type. Specifically, the porous monolith-based flow-reactors overcome several physical-chemical disadvantages, such as low contact efficiency, poor heat and mass transfer, pressure drops and low thermal stability.(6-7) With the benefits of large surface area, interconnected porous


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structure, multi-scaled pore size and block shape, hierarchically porous monoliths can greatly facilitate mass transport and molecule diffusion.(8-10) During the design and construction of porous monolith type flow-reactors, catalyst immobilization is a key step for the fabrication of high efficiency flow-reactors. Conventionally, inorganic catalysts are loaded into the pore channels via the traditional wet impregnation– calcination process. This process is easy but not suitable for the continuous flow flow-reactors for the three following reasons: (i) during high-temperature calcination, it is difficult to maintain the structure of certain types of porous monolithic materials without cracking; (ii) without other additional chemicals or treatment, the inorganic catalysts tend to aggregate inside the complicated pore channels, making uniform dispersal across the surface difficult;(11) and (iii) during the flow reactions, nano catalysts that are simply deposited, gradually leach out with the flow of reagents in solution, which causes a decrease in catalytic activity.(12) Until now, there have been several strategies developed to immobilize catalysts on the surface of the porous materials, including mineralization of the surfaces,(5, 13) organic silane modification,(14-15) copolymerization(16) and layer-by-layer assembly.(17) Using these established methods, inorganic catalysts can be immobilized in porous flow-reactors. However, most of these techniques are tedious and usually valid only for specific types of substrates or catalysts. In particular, the organic groups anchored on a modified surface can only have strong interactions with a certain type of inorganic catalysts. In addition, the modification process often involves the use of organic solvents and expensive chemicals, which are not eco-friendly or costeffective. Therefore, developing a general, low-cost, green approach for the immobilization of common inorganic catalysts in hierarchically porous monoliths is still a challenge for flowreactor research. Lately, Messersmith et al. successfully introduced inexpensive plant


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polyphenols onto solid surfaces, and the polyphenol film showed adsorption and reduction properties with metal ions.(18)

Figure 1. Schematic representation of the fabrication of flow-reactors. Pyrogallol can polymerize in glycine buffer at pH=7.0 and coat the surface of porous silica-based flow-reactors. The pyrogallol layer, as an assistant, can reduce and immobilize inorganic catalysts (such as AuNPs) on flow-reactor surfaces. Inspired by the unique adhesion properties of plant polyphenols in nature,(19-20) we developed a general, green-chemistry approach for the immobilization of inorganic catalysts, including noble metals, metal oxides and polyoxometalates (POMs) in porous flow-reactors. A hierarchically porous silica (HPS) monolith was prepared via the sol-gel method and utilized as the substrate for the flow-reactor. The HPS surface was covered by a multifunctional PG coating via polymerization in glycine buffer (pH=7.0). With the assistance of the PG membrane, noble metal nanoparticles, metal oxides and POMs were immobilized on the surface of flow-reactors. The PG membrane could also act as a reducing agent for the formation of noble metal nanoparticles by converting precursor metal ions into a crystal structure (Figure 1).(21) To


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evaluate the catalytic properties of our flow-reactors, several reactions, including hydrogenation, esterification, Knoevenagel and cascade reactions, were studied as easy-to-follow models. The constructed monolithic reactors showed excellent flow catalytic performance for the treatment of environmental contaminants and the synthesis of fine chemicals under mild conditions. Experimental Section Reagents and Materials. Tetramethoxysilane (TMOS) was purchased from the Chemical Factory of Wuhan University. Phosphotungstic acid (HPW), silicotungstic acid (HSiW), phosphomolybdic acid (HPMo) and PEG (polyethylene glycol, Mw=10,000 Da) were purchased from the Sinopharm Chemical Reagent Co., Ltd. Hydrogen tetrachloroaurate, N,N-Bis(2hydroxyethyl)glycine, 4-nitrophenol (4-NP), 4-aminophenol (4-AP), nitrobenzene, o-/m-/pnitrotoluene, isobutyraldehyde, benzaldehyde, 1,2-propanediol, pyrogallol and sodium borohydride were purchased from Aladdin Chemical Co., Ltd. Acetic acid, nitric acid, hydrofluoric acid, sodium chloride, anhydrous sodium sulfate, ethyl acetate, 25 % ammonia (w/v), anhydrous ethanol, and hydrochloric acid were purchased from Fuyu Fine Chemical of Tianjin Co., Ltd. Aluminum nitrate, magnesium nitrate and ferric nitrate were purchased from Xilong Chemical Co., Ltd. Malononitrile was purchased from Shanghai Kefeng Chemical Reagents Co., Ltd. Benzaldehyde, n-propyl alcohol and n-butyl alcohol was purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Ethyl cyanoacetate was purchased from J&K Scientific Ltd. All reagents were used without further purification. Synthesis of Hierarchically Porous Silica Monolith (HPS). First, PEG (1.2 g) and acetic acid (10 mM, 10 mL) were mixed together and stirred to obtain a homogeneous solution. Under vigorous stirring at 0 °C, TMOS (4.0 g) was added to initiate hydrolysis. Approximately 20 min


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later, the semitransparent sol was transferred into polythene (PE) tubes, sealed for gelation and aged (36 h) at 40 °C. The resulting gels were treated with 1 M ammonium hydroxide solution at 100 °C for 8 h. Subsequently, the wet silica gels were washed to pH=7.0 and dried at 80 °C. The monolith in centimeter scale was then obtained via calcination at 650 °C for 5 h in air. Fabrication of Pyrogallol Layer-Covered HPS Monolith (PG-HPS). The solution was freshly prepared by dissolving pyrogallol (10 mg) in glycine buffer (pH=7.0, 10 mL) and sonicating for 10 sec at room temperature (RT). Then, HPS (0.5 g) was immersed in the solution under vacuum conditions for 3 h. Subsequently, the monoliths were washed with deionized water and dried overnight. In situ Generation and Fabrication of Noble Metal Nanoparticle-Modified Flowreactors. PG-coated HPS (0.5 g) was immersed in cooled HAuCl4 (2 mM, 10 mL), AgNO3 (2 mM, 10 mL), H2PtCl6 (2 mM, 10 mL) or PdCl2 (2 mM, 10 mL) for 3 h. The modified material was washed with deionized water and ethanol and dried under vacuum. The resulting monoliths are referred to as Au/PG-HPS, Ag/PG-HPS, Pt/PG-HPS and Pd/PG-HPS, respectively. To enhance the stability of flow-reactors, the noble metal-loaded monoliths were calcined at 500 °C for 5 h under nitrogen. As the PG converted to carbon layer, the final materials were named as Au-HPS, Ag-HPS, Pt-HPS and Pd-HPS. Fabrication of Metal Oxide-Modified Flow-Reactors. PG-coated HPS (0.5 g) was immersed in an aqueous solutions of Al(NO3)3 (0.1 M, 10 mL), Mg(NO3)2 (0.1 M, 10 mL) or Fe(NO3)3 (0.1 M, 10 mL) for 24 h. The modified material was washed with deionized water and dried under vacuum. The resulting modified monoliths are referred to as Al2O3/PG-HPS, MgO/PG-HPS and Fe2O3/PG-HPS, respectively. The modified HPS monoliths were calcined at


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600 °C for 5 h under nitrogen. The final materials are referred to as Al2O3-HPS, MgO-HPS and Fe2O3-HPS, respectively. The metal nitrates decompose to metal oxides as follows: ∆

2( ) → 2 + 4 +

∆

4( ) → 2 + 12 + 3

(1) (2)

Fabrication of Polyoxometalate (POM)-Loaded Flow-Reactors. PG-coated HPS (0.5 g) was immersed in aqueous solutions of HPW (0.1 M, 10 mL), HSiW (0.1 M, 10 mL) or HPMo (0.1 M, 10 mL) for 24 h. The modified monoliths were washed with deionized water and dried under vacuum. The modified monoliths are referred to as HPW-HPS, HSiW-HPS and HPMoHPS, respectively. Characterization. The scanning electron microscopy (SEM) images were taken with a QUANTA 450 environmental scanning electron microscope at 20 kV (FEI Co. Ltd, USA). Transmission electron microscope (TEM) images were obtained with a Tecnai G220S-Twin electron microscope equipped with a cold field emission gun (200 kV). The nitrogen adsorption and desorption isotherms were measured at 77 K using an ASAP 2010 analyzer (Micromeritics Co. Ltd). FT-IR spectra (4000-400 cm-1) were collected on a Nicolet Avatar 360 FT-IR spectrometer. The surface elements were analyzed using X-ray photoelectron spectroscopy (XPS, Shimadzu KROTAS AMICUS spectrometer). Thermal gravimetric analysis (TGA) was performed using a TGA/SDTA851e from Mettler Co. Ltd, Switzerland. The samples were heated progressively from 25 to 800 °C at a heating rate of 10 °C min-1 in air. UV-Vis spectra were obtained on a UV1000 spectrophotometer from TECHCOMP Co. Ltd, China. The hydrogenation reaction products were analyzed using gas chromatography (GC7900 with an HP-5 column from TECHCOMP Co. Ltd). X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAX-


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2400 X-ray powder diffraction (Japan) using Cu Kα radiation, operating at 40 kV and 10 mA. The pore size distribution were obtained using an AutoPore IV 9500 V1.05 mercury intrusion porosimeter. The residence time, a measure of the total time the fluid molecules spent within the reactor, was calculated by the species transport model using 4-NP solution as the tracer. Catalytic Reduction of Nitroarenes. A piece of AuNP-modified flow-reactor (1.1 cm in diameter and 1.8 cm in length) was inserted into a heat-shrinkable tube (1.5 cm in diameter). Both ends of the heat-shrinkable tube were blocked by thick-walled quartz tubes. The whole device was put into an oven and heated at 120 °C for three hours. The heat-shrinkable tube shrank and wrapped the flow-reactor and quartz tubes tightly. Then, a reactor cell based on the AuNP-modified flow-reactor was formed and was directly connected with a liquid chromatography (LC) pump for catalytic reaction. Typically, a defined amount of NaBH4 (7.6 mg) was mixed with aqueous 4-NP solution (0.5 mM, 10 mL). Then, the yellow reaction solution was forced to flow through the flow-reactor using the LC pump at a flow rate of 2.0 mL min-1. 4AP in NaBH4 aqueous solution was used as a standard to ensure the band position in the gas chromatogram, which was used for comparison with the chromatograms of catalytic reaction products. The GC samples were prepared as follows: the solution was neutralized with HCl (0.1 M) and extracted using a comparable amount of ethyl acetate three times. The extract liquor was dehydrated with anhydrous Na2SO4 overnight. The other aromatic nitro compounds, such as nitrobenzene and o-/m-/p-nitrotoluene, were dissolved in a mixed aqueous solution (5 v/v % of ethanol). GC test conditions were as follows: the temperatures of the injector port and detector were 220 and 230 °C, respectively. The temperature of the column oven was kept at 60 °C for 5 min and increased to 230 °C at 12 °C min-1 for another 20 min. The injection volume was kept at 0.2 µL.


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Catalytic Esterification Reaction. A piece of HPW-HPS flow-reactor (1.1 cm in diameter and 1.8 cm in length) was fixed in a heat-shrinkable tube to form a catalytic reactor cell. Eight milliliters of acetic acid and 8 mL of alcohol (ethanol, n-propanol and n-butyl alcohol) were mixed and forced to flow through the HPW-HPS monolith using an LC pump. The monolith was heated to 110 °C, and the flow rate was 0.05 mL min-1. The conversions of reactions were determined using GC. GC test conditions were as follows: the temperatures of the injector port and detector were 180 and 200 °C, respectively. The temperature of the column oven was kept at 40 °C for 3 min and increased to 150 °C at 10 °C min-1 for another 3 min. The injection volume was kept at 0.2 µL, and the partial pressure was 0.14 MPa. Catalytic Knoevenagel Reaction. A piece of Al2O3-HPS flow-reactor (1.1 cm in diameter and 1.8 cm in length) was fixed in a heat-shrinkable tube to form a catalytic reactor cell. For reaction of malononitrile and benzaldehyde, a solution of 6.5 g of malononitrile and 10 mL of benzaldehyde was prepared. For reaction of benzaldehyde and ethyl cyanoacetate, a solution of 8 mL of benzaldehyde and 8 mL of ethyl cyanoacetate was prepared. For reaction between malononitrile and isobutyraldehyde, a solution of 7.2 g of malononitrile and 10 mL of isobutyraldehyde was prepared. All reaction solutions were forced to flow through the Al2O3HPS monolith using an LC pump. The reactions were all heated to 80 °C, and the flow rates were 0.10 mL min-1. The conversion of reactions were determined using GC. GC test conditions were as follows: the temperatures of the injector port and detector were both 240 °C. The temperature of the column oven was kept at 40 °C for 3 min and increased to 180 °C at 10 °C min-1 for another 3 min. The injection volume was kept at 0.2 µL, and the partial pressure was 0.8 MPa.


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Catalytic Cascade Reaction. 20 mL of benzaldehyde 1,2-propanediol was mixed with 2.3 mL of deionized water and forced to flow through the HPW-HPS-based catalytic reactor cell at 80 °C using LC pump I at a flow rate of 0.05 mL min-1. The product of benzaldehyde 1,2propanediol after hydrolysis was flowed into a T-junction. The internal diameter of the Tjunction is approximately 6 mm. Then, a mixture of 20 mL of ethyl cyanoacetate and 20 mL of DMF was forced using LC pump II to flow into the T-junction through the other end. As the chemical reagents from two ends were mixed together, the mixture was flowed out of the Tjunction and through the Al2O3-HPS based catalytic reactor cell at 80 °C at a flow rate of 0.05 mL min-1. The conversion of reactions were determined using GC. GC test conditions were as follows: the temperatures of injector port and detector were both 240 °C. The temperature of the column oven was kept at 40 °C for 3 min and increased to 200 °C at 10 °C min-1 for another 14 min. The injection volume was kept at 0.2 µL. Results and Discussion The Structural Characteristics of the Monolith. An HPS monolith was first prepared via the sol-gel method using polyethylene glycol as the structure-directing agent and tetramethoxysilane (TMOS) as the precursor, which was later used as the matrix material for the flow-reactor. The HPS matrix exhibited highly developed interconnected three-dimensional (3D) macroporous structure as shown in its SEM image (Figure 2a). The size of the worm-like macropores was approximately 2.2 µm, estimated using the mercury intrusion method (Figure S1a in the Supporting Information). In addition to the macroporous structure, nitrogen sorption analysis revealed that the silica skeleton of this monolith also exhibited mesoporosity with a high surface area of 266 m2 g-1 and a mesopore volume of 0.83 cm3 g-1 (Figure S2a and Table S1). The mesoporous structure can be clearly observed using transmission electron microscopy


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(TEM) (Figure 2c-d). The pore size was disordered, the wall thickness of the silica skeleton was approximately 8 nm. The mesopore size was mainly larger than 15 nm, which is larger than most organic molecules and low molecular weight polymers. Such co-continuous porous structure with both macro- and mesopores is convenient for the diffusion and transfer of fluid.(22) The monolithic HPS was at the centimeter scale and was mechanically resilient, allowing convenient further handling. One piece of HPS could be considered a reactor cell after packaging in heatshrinkable tube. Several cells could be connected together for different reaction conditions, such as the cascade reaction.

Figure 2. SEM images of the HPS monolith (a) and the PG covered monolith (b). The TEM image of HPS (c), and the same image in high magnification (d). PG modification was achieved by soaking the HPS in a PG aqueous solution without any organic solvent and subsequent simple vacuum drying treatment. After modification with PG, the PG-HPS maintained its regular and smooth morphology in centimeter scale and monolith form


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without surface fissures or cracks (Figure 3a). The macroporous structure of PG-HPS (Figure 2b) was similar to that of naked HPS, demonstrating the high stability of the porous silica framework. The silica skeleton became slightly rougher with the PG layer coating on the HPS surface.

Figure 3. (a) Optical images of the HPS monolith, PG-covered monolith, and the catalyst-loaded monoliths (Au, Al2O3 and HPW). (b) An example reactor cell: The catalyst-loaded monolith was cut to a suitable size and cladded in heat-shrinkable casing ended by two quartz tubes to ensure the connections to the pump for flow reactions. The diameter of the samples we used is approximately 1.1 cm, and the length is approximately 1.8 cm. The presence of the polyphenol coating derived from PG was confirmed from the Fourier transform infrared (FTIR) spectra (Figure S3). The bands at approximately 3433, 2987, 16251400 and 670 cm-1 were assigned to the stretching vibrations of O-H, C-H and C=C in the benzene ring, and the out-of-plane bending vibrations of C-O from the PG, respectively.(23) The deposition of PG on HPS was confirmed using X-ray photoelectron spectroscopy (XPS). Two


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high-intensity peaks (Figure 4a-d) representing the Si were observed from the naked HPS at binding energies of 150.5 eV (Si2s) and 100.2 eV (Si2p). Another two weak peaks at 532.7 eV and 284.8 eV were attributed to O1s and C1s, respectively.(24) After modification with PG, the Si2p peak from the silica substrate became weak. Instead, the intensity of the O1s and C1s peaks were enhanced relative to Si2p, indicating the formation of the PG coating on the HPS surface. The loading amount of PG was estimated using thermogravimetric analysis (TGA). The weight loss up to 275 °C in the TG curves indicated the oxidation of PG.(25) The content of PG was ca. 2.67 %, equaling a monolayer of plant polyphenol molecules.

Figure 4. (a-d) XPS patterns of HPS, PG-HPS, Au/PG-HPS and Au-HPS, respectively. (e-f) O1s spectra of PG-HPS and Au/PG-HPS, respectively. A scanning electron microscope (SEM) examination revealed that the catalyst-loaded HPS (C-HPS) had an interconnected three-dimensional macroporous structure similar to that of naked HPS (Figure S4-S6 in the Supporting Information). The macroporous framework was stable and showed no collapse after PG loading and immobilization of various catalysts. The pore size


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distribution provided by the mercury intrusion porosimeter further showed that the diameter of the macropores in C-HPS ranged from 302 to 676 nm, smaller than those in HPS (Figure S1). The reduction in pore size is thought to be caused by the modification process. Nevertheless, this size was large enough to ensure the transfer of substances during the flow catalytic reaction. The nitrogen sorption isotherms of the modified HPSs are shown in Figure S2. The materials all showed broad H1-type hysteresis loops, indicative of the existence of uniform mesopores with a narrow pore-size distribution.(26) The mesopore diameters of different C-HPSs were larger than 10 nm, smaller than those of HPS (Table S1). However, they were still large enough to allow the diffusion of most organic molecules. The SBET of the C-HPS was approximately 200 m2g-1. The hierarchically porous scaffold provided a micro/nano-scale confined space for sufficient contact between the reactants and the catalysts. Polyphenolic compounds, such as PG in this work, are typically multifunctional molecules. Such compounds, which contain abundant orthophenolic hydroxyls, possess adsorption and reduction properties with metal ions.(18) There is no need to add extra reducing agent with a certain of toxic for the formation of catalytic nanomaterials, such as noble metals. However, polyphenols also strongly bind to inorganic/silica surfaces through covalent and noncovalent interactions, including hydrogen bonds, π-π interactions and electrostatic interactions.(27-29) Therefore, various biomolecules containing polyphenolic compounds combine strongly with inorganic substrates, such as mussels on a reef.(30) We used these properties to immobilize three types of inorganic catalysts, including noble metals, metal oxides and POMs. The polyphenol, PG, used in this study has three adjacent hydroxyl groups that can combine with metal ions and POMs with coordinate bonds and hydrogen bonds, respectively.(31-32)


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For immobilization of noble metals, such as Au, the process is more interesting. The PG molecule is strongly reducible. For example, di- and tri-hydroxyphenols have been reacted with HAuCl4 to form nanoparticles in previous studies.(33) Further surface modification was conducted through the in situ reduction of AuIII ions due to the unique reductive properties of PG. The reduction process did not require any exogenous reducing agents. Under mild reaction conditions, hydroxyphenols release protons and electrons though rapid oxidative self-conversion into their quinone forms. As the FTIR spectra of Au/PG-HPS shows (Figure S3a), there was an extra absorption band at 1730 cm-1, which can be ascribed to the stretching vibrations of the C=O bond, indicating that part of the PG had turned into the quinone form during the reduction of gold.(34) More details were found in the XPS spectra. For PG-coated materials, three O1s photoelectron peaks were detected from the SiO2 (532.7 eV), hydroxy group (533.5 eV) and the quinone oxygen generated by oxidation (530.9 eV).(35-36) After the in situ reduction, the signal assigned to quinone oxygen at 530.9 eV was enhanced (Figure 4e-f). Meanwhile, the Au 4f7/2 signal centered at 84.0 eV had a binding energy characteristic of bulk neutral gold (Figure 4c-d) (37-38), confirming the generation of metallic Au0 particles and the reductive properties of polyphenol. The resultant nanoparticles were immediately immobilized on the silica support by the multifunctional coating. To monitor the growth of AuNPs, SEM was conducted on Au/PGHPS (Figure S4a). Nanoparticles (the bright spots confirmed from the Au mapping image in Figure S4b) were widely distributed over the surface of the PG-HPS. A typical high-resolution TEM image (Figure S7a) of individual AuNPs showed that the nanoparticle had a well-defined crystalline structure with regular lattice spacing of d=0.24 nm, consistent with the (111) plane in the face-centered cubic (fcc) AuNPs, just as the XRD patterns showed in Figure S7b.(39) Only 0.02 % (wt) of Au was loaded on the HPS. Similar to Au, Ag, Pd and Pt were immobilized on


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HPS using the same process. The XPS and element mapping images are shown in Figure S4c-h and Figure S8. For metal oxides, the metal ions were first combined with the plant polyphenol film via coordinate bonds.(40-41) Then, the adsorbed metal nitrates were easily transformed to metal oxides through calcination. According to the FTIR spectra (Figure S3c), a new sharp absorption band appeared at 1384 cm-1 after the Al3+, Fe3+ and Mg2+ ions were adsorbed, due to the C-O stretching mode of the M-O-C, confirming that the metal ions had coordinated with the hydroxyl groups.(42) After calcination, that peak disappeared, indicating that the coordination complex was transferred to the metal oxides. The XPS spectra further proved the existence of metal elements in the hybrid porous silica (Figure S9). The Al, Fe and Mg photoelectron peaks were clearly observed in the relevant HPS materials.(43-44) The element content was measured using X-ray fluorescence (XRF). The Al, Fe and Mg content in different C-HPSs was 4.4 %, 5.1 % and 2.1 % (wt), respectively. These results showed that the PP film effectively immobilized the catalysts. The PG-HPS also had a good affinity for POMs. The PG-HPS could load 72.3 % (wt) phosphotungstic acid (HPW), while the bare HPS could load only 38.93 %. As shown in the SEM images (Figure S6), the pore structure of the POM-HPS was well preserved, while the POM formed blockages in the bare HPS pores. For silicotungstic acid (HSiW) and phosphomolybdic acid (HPMo), the situations were similar (Figures S6 and S10). However, the mechanism of the loading process was unclear. It is possible that the hydrogen bond in the PP film might promote the deposition of POMs onto the pore surface of the PG-HPS.(45) As the thermal stability of POM is not high, the POM-HPS was used as synthesized, without thermal treatment. As mentioned above, the polymerization and coating of PG take place in aqueous


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solution without any other organic solvents. The universal adhesion of the PG offers the possibility to develop a general, eco-friendly method to immobilize catalysts on the pore surface of monolithic flow-reactors. The Catalytic Performances of Monolithic Flow-reactors. The porous monolith with chosen inorganic catalysts (AuNPs, Al2O3 and HPW) as a flow-reactor for catalytic reduction, which was 1.0 cm in length and 0.8 cm in diameter, was sealed in a heat-shrinkable PTEF tube with a quartz tube and connected to a HPLC pumps (Figure S11). The reactants flowed through the monolith and formed products. Before the reactants were pumped into the monolith, the pure solvents used in the reactions were allowed to flow through the reactor to assess the leaching of the catalyst. For comparison, Au/PG-HPS and Au-HPS were washed with weakly alkaline liquor (7.6 mg of NaBH4 mixed with 10 mL deionized water). The resulting filtrates were tested using UV-Vis spectroscopy. There was a weak peak at approximately 530 nm (ascribed to AuNPs) in the UV-Vis spectrum (Figure S12) of filtrate from the Au/PG-HPS. However, there was no distinct peak in the curve of Au-HPS, which was cured by thermal treatment under N2. Such result meant no catalyst leached from the Au-HPS. We advanced a hypothesis that the slow etching of the silica monolith in basic conditions was the main reason for the catalysts leaching from the PG-covered flow-reactor. In contrast, the carbon layer was more stable in weak alkaline liquor and protected the silica matrix. To confirm the hypothesis, we grew a layer of PG on a polystyrene (PS) petri dish. AuNPs were loaded on the surface following the same protocol for Au/PG-HPS. The resultant PS-based surface was stable in basic conditions. After washing with a NaBH4 aqueous solution, almost no leaching of AuNPs was observed. Therefore, the carbon layer, formed by the calcination of PG, protected the silica monolith from the etching of alkaline reactants, and inorganic catalysts (such as AuNPs) were not washed away from the reactor.


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Three types of reactions, hydrogenation, Knoevenagel and esterification reactions, were performed in the monolithic flow-reactor to evaluate its efficiency. Each of these reactions is important in academia and in the chemical industry for the manufacture of fine chemicals and pharmaceuticals. In the field of environmental contaminant treatment, Au-HPS was used as a flow-reactor to perform the hydrogenation reaction. The catalytic performance and efficiency of Au-HPS were investigated in the catalytic reaction of p-nitrophenol (4-NP) to p-aminophenol (4-AP) with NaBH4. As shown in Table 1, the flow-reactor gave complete conversion (100 %). Moreover, the flow-reactor exhibited good catalytic activity with outstanding conversions and yields toward other nitrobenzene compounds, regardless of the position and types of the substituents, at room temperature (RT). These nitro-compounds were transformed to the corresponding aminocompounds with over 99 % conversion within a short time (less than 1 min). This result produced without extra heating is ideal compared with those in the existing literature.(46-48) Table 1. The model catalytic reduction of nitroarenes (entries 1-5, Au-HPS as catalytic flowreactor), Knoevenagel reactions (entries 6-8, Al2O3-HPS as catalytic flow-reactor) and esterification reactions (entries 9-11, HPW-HPS as catalytic flow-reactor). a) Conversionb) (%)

Yield (%)

1

100

>99

2

>99

82

3

>99

81

4

>99

83

Entry

Substrate

Product


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5

>99

78

6

100

>99

7

92

>99

8

90

>99

9

92

>99

10

98

>99

11

95

>99

a)

Reaction conditions for entries 1-5: 9.5 mL of 20 mM NaBH4 aqueous solution, 10 mL of 0.5 mM nitrobenzene ethanol solution at RT. The residence times of entries 1-5 were approximately 40 s. Entry 6: 6.5 g malononitrile and 10 mL of benzaldehyde at 80 °C; entry 7: 8 mL of benzaldehyde and 8 mL of ethyl cyanoacetate at 80 °C; entry 8: 7.2 g of malononitrile and 10 mL of isobutyraldehyde at 80 °C. The residence times of entries 6-8 were approximately 26 min. Entries 9-11: 8 mL of acetic acid and 8 mL of alcohol at 110 °C. The residence times of entries 9-11 were approximately 13 min. b) Conversion was determined by GC-MS (gas chromatography–mass spectrometry). The average of three independent runs in the same flow-reactor is given in the table. As a common reaction for the preparation of fine chemical intermediates, the Knoevenagel reaction was chosen to test the performance of the Al2O3-HPS flow-reactor. Benzaldehyde and ethyl malononitrile were used as reactants to synthesize the electrophilic alkenes. The yield of the reaction in the Al2O3-HPS flow-reactor neared 100 % at 80 °C with a flow of 0.10 mL min-1. The content of Al in Al2O3-HPS was 4.3 % (wt). With a relatively low catalyst loading amount, Al2O3-HPS still performed the reaction efficiently. As shown in Table 1, the average yields from the other two Knoevenagel reactions of various active methylene compounds and carbonyl compounds (including isobutyraldehyde and ethyl cyanoacetate), were both greater than 90 %.


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The HPW-HPS catalytic flow-reactor was used on fine chemicals to perform esterification reactions. During the test, acetic acid was reacted with ethanol, n-propanol and n-butyl alcohol. The conversions were 92 %, 98 % and 95 %, respectively (Table 1). The reaction temperature was 110 °C with a flow of 0.05 mL min-1. HPW is well-known as an acid catalyst for esterification. The catalytic property of HPW persisted after immobilization on the HPS monolith.

Figure 5. (a) The target cascade reaction requires different catalysts at different stages. (b) A schematic representation of cascade reaction. (c) Optical image of the catalytic reaction system (the core flow-reactors can be connected for continuous multi-step reactions).


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To clarify the function of the PG layer in catalyst loading, we used the bare HPS as substrate for the fabrication of flow-reactors. The catalysts, including noble metals, metal oxides and POM, were immobilized via a simple impregnation process. The resulting flow-reactors showed lower performance on the all three types of catalytic reactions. For example, without the assistance in immobilization and protection, the size of the AuNPs was larger than 200 nm. For comparison, the size of AuNPs in Au-HPS was approximately 60 nm. The flow-reactor based on bare HPS showed obvious leaching of AuNPs (Figure S12) and undesirable catalytic performance (Figure S13). Additionally, the Al2O3 and HPW directly immobilized on bare HPS also revealed disappointing catalytic performances. The conversions in the Knoevenagel (entry 6 in Table 1) and esterification reactions (entry 9 in Table 1) were approximately 60 % and less than 10 %, respectively. The flow-reactors thus relied on the PG layer for catalyst immobilization for high catalytic performance. Two or more monoliths were connected and used for cascade reactions. As POM could catalyze the hydrolysis of the acetal bond, a cascade reaction was designed to further explore the catalytic performance of the monolith reactors. The HPW-HPS and Al2O3-HPS were connected with a T-joint and pumps. Various reactants were injected from different connectors. The acetal bond hydrolysis and the Knoevenagel reaction were integrated. The final yield of the two-step cascade reactions reached 99 % (Figure 5). This result indicated that it is possible to assemble flow-reactors with the desired catalytic HPS monoliths for multistep complex reactions. In most of the reactions mentioned in this article, the flow-reactor could continuously work at high conversion levels for more than 8 hours. The flow-reactors (such as Au-HPS) showed a high stability for catalytic reactions even in alkaline reaction media. Furthermore, easy recycling and reusability are also important characteristics of a durable flow-reactor. When treated with 20


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mL of solvent, such as ethanol or acetone, the flow-reactors again showed nearly identical conversions. The Au-HPS-based flow-reactor revealed a complete conversion for 4-NP to 4-AP in first cycle (Figure 6). After eight cycles, the conversion of Au-HPS was still higher than 99 %. The Al2O3-HPS flow-reactor could also keep a high conversion rate of approximately 90 % during several cycles. However, HPW-loaded flow-reactor (HPW-HPS) was only able to maintain conversions at 90 % in the first three cycles. Then, the conversion in the formation of ethyl acetate fell to 80 %, where it remained for another four cycles. The decrease in catalytic performance could be attributed to the leaching of POM because of the weak interaction with the organic coating (Figure S14).(49) Despite this problem, the yield of the esterification reaction was still greater than 80 %. In addition, the pressure drop provided by the pump was about than 2 psi, indicating that the worm-like macroporous channels and disordered mesopores were convenient for the diffusion and transfer of reactant molecules.(22) The pressure drops of the samples at different stages of preparation were all 2 psi, indicating that the interconnected porous channels were not blocked during the surface modification and immobilization.


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Figure 6. Recycling of the catalytic flow-reactors. The reaction using Au-HPS was conducted on 4-NP; the reaction using Al2O3-HPS was between benzaldehyde and ethyl cyanoacetate; and the reaction using HPW-HPS was between acetic acid and ethanol. Conclusions In summary, we have demonstrated a general, green-chemistry approach to immobilize inorganic catalysts on hierarchically porous monoliths for the fabrication of highly active and easily recyclable flow-reactors using continuous-flow catalytic reactions. The monolithic scaffold has a large SBET and stable macro-mesoporous structure. PG, a plant polyphenol, can immobilize inorganic catalysts on the HPS. The resultant hybrid HPS monoliths show a high catalytic activity for hydrogenation, Knoevenagel and esterification reactions in environmental contaminant treatment and the production of fine chemicals. Several monolithic flow-reactors can be assembled with various catalysts for multistep cascade reactions, and a 99 % yield can still be achieved. This general, green-chemistry approach can be widely extended to other inorganic-catalyzed systems. Based on this strategy, modular monoliths offer opportunities to integrate different catalytic units into one set for complex reactions.

ASSOCIATED CONTENT Supporting Information Additional results include data on pore size distributions, N2 adsorption-desorption isotherms, FT-IR spectra, images of SEM and element mapping, HRTEM images of Au-HPS, XRD patterns


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of Au-loading flow-reactors, XPS patterns, and a table of structural properties. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Phone: 86-411-84986035; E-mail: [email protected] Author Contributions Y. Wang and S. Tao conceived the project and planned the experiments. Y. Wang and D. Shi prepared the samples and performed most of the experiments. H. Wang performed the SEM and SEM-EDS elemental mapping measurements. Y. Wang and S. Tao analyzed the data and wrote the paper. All authors discussed the results and commented on the paper. ‡These authors contributed equally. Funding Sources National Natural Science Foundation of China (21473019, 51273030, 21176037) and Fundamental Research Funds for the Central Universities (DUT14ZD217, DUT15LK29). Notes The authors declare no competing financial interest ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21473019, 51273030, 21176037) and the Fundamental Research Funds for the Central Universities (DUT14ZD217, DUT15LK29).


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For Table of Contents Use Only

A general, green-chemistry approach for the immobilization of inorganic catalysts in monolithic porous flow-reactors Yuchao Wang, Da Shi, Shengyang Tao*, Wentong Song, Hongmin Wang, Xinkui Wang, Guangtao Li, Jieshan Qiu, and Min Ji Synopsis A general, green-chemistry approach is developed to fabricate sustainable flow-reactors by immobilization of inorganic catalysts with the assistance of pyrogallol.


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A General, Green Chemistry Approach for Immobilization of Inorganic

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