Construction of a Repairable Fixed Porous Catalytic Bed Loaded with

Publication Date (Web): July 20, 2017 ... Because of the reversible host–guest interactions between β-cyclodextrin and azobenzene, the catalytic sy...
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Research Article pubs.acs.org/journal/ascecg

Construction of a Repairable Fixed Porous Catalytic Bed Loaded with Gold Nanoparticles via Multivalent Host−Guest Interactions Da-Huan Zhou,† Cong-Cong Liang,† Jun Nie,†,‡ and Xiao-Qun Zhu*,†,‡ †

State Key Laboratory of Chemical Resource Engineering & Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, China ‡ Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Changzhou, Jiangsu 213164, China S Supporting Information *

ABSTRACT: The reversible combination between gold nanoparticles (AuNPs) and carriers is crucial for the preparation of a recycle system. Here, a repairable catalytic system was constructed on the basis of AuNPs and porous nickel (PNi) which were combined through the multivalent host−guest interactions between βCD-AuNPs and PNi@IPTS-Azo [β-CD, β-cyclodextrin; IPTS, (3-isocyanatopropyl) triethoxysilane; Azo, azobenzene]. The large specific surface area and connected porous structure of PNi provide a good opportunity to achieve the multivalent interactions between βCD-AuNPs and PNi@IPTS-Azo in the nickel. Additionally, the reaction solution could be catalyzed by flowing over the PNi@IPTS-Azo@βCD-AuNPs substrates. This catalytic model showed a high efficiency close to 95%. Because of the reversible host−guest interactions between β-cyclodextrin and azobenzene, the catalytic system could be regenerated by removing the deactivated AuNPs with UV-light irradiation and recombining new ones through multivalent interactions in situ. This type of catalytic system is regenerative, material-saving, and effective. This system could be expected to be constructed as catalytic fixed beds and applied in industry. KEYWORDS: Gold nanoparticle, Multivalent host−guest interactions, Catalytic bed, Continuous-flow, Repairability

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INTRODUCTION Because of their advantages over bulk materials in the application of catalysis, metal nanoparticles have been extensively explored in recent years. Being small, they have special features, such as large surface area and unusual facets, which could greatly improve catalytic performance. As an important member of the metal nanoparticles family, goldnanoparticle-based (AuNP-based) catalysts have been widely researched in many organic catalytic reactions, including hydrogenation1 and the reduction of nitro-containing pollutants.2,3 However, because of the large surface energy, AuNPs tend to aggregate, and the performance of the catalysis may decline. The postreaction recycling of the tiny AuNPs is also difficult. If they are recovered through centrifugation and filtration, a serious aggregation of AuNPs would be unavoidable. For avoidance of these problems, many solutions, such as the reversible transfer between the solvent and reaction solution,2 magnetic recovery,3,4 and a variety of scaffolds based on metal or covalent organic frameworks (MOFs or COFs),5,6 have been introduced into the catalyst system. Among these, anchoring the AuNPs on supporting materials is one of the most common methods.5,7,8 Magnetic Fe3O4 particles, MOFs, COFs, and some inorganic oxide particles are often used as carriers. The recyclability can be greatly improved by the in situ reduction of abundant AuNPs onto a support material.9,10 Such © 2017 American Chemical Society

catalyst carriers can accelerate the reaction process by directly dispersing the nanoparticles into the solution. Even so, recycling these particles after each catalytic reaction is still tedious and time-consuming, and is contrary to the high efficiency and automation of industry. For industrial applications, the high efficiency and maximum recyclability of catalysts could be achieved by the construction of a fixed bed. For their large surface area and sizable pores, nanoscale fibrous11,12 or porous structures13 are often chosen as the materials for the fixed bed. The catalysts are anchored on the fixed bed and catalyze in a continuous-flow mode. Chen et al.12 prepared a fixed-bed system using glass fibers loaded with Au nanowires as a catalyst through in situ reduction. The performance of the catalysis and the efficiency of recyclability were greatly improved. However, it is difficult to separate the AuNPs from the supporting materials. Once the AuNPs were poisoned or damaged, there would be a problem in either discarding the catalyst together with supporting materials, or removing the nanoparticles from the supporting materials through a blunt method if the bonding was not reversible. Received: March 22, 2017 Revised: June 29, 2017 Published: July 20, 2017 7587

DOI: 10.1021/acssuschemeng.7b00879 ACS Sustainable Chem. Eng. 2017, 5, 7587−7593

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Scheme 1. (A) Construction Process of the Fixed Porous Catalyst Bed Loaded with AuNPs and Its Photoresponse Property and (B) Model of the Catalytic Fixed Bed

and acetonitrile were purchased from TianJingFuChen Chemical Reagents. Sodium borohydride (NaBH4) and dimethyl sulfoxide (DMSO) were purchased from Tianjin Jinke Fine Chemical Research Institute. Thiolated β-cyclodextrin (SH-βCD) was supplied by Shandong BinzhouZhiyuan Bio-Technique Co., Ltd. Ammonia solution (NH3·H2O, 25%), ethanol, acetone, and ethyl acetate (EtAc) were donated by Beijing Chemical Works. EtAc was purified by reduced-pressure distillation. Other materials were used as received. Characterization. X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo Fisher Scientific) was employed to test the chemical composition of modified porous substrates. Transmission electron microscopy (TEM, Tecnai G2 20 S-TWIN FEI) was used to observe the morphology and size of the Au nanoparticles. The Fouriertransform infrared (FTIR) spectra were obtained on a Nicolet 50XC spectrometer (Nicolet). A 365 nm LED lamp (RX Firefly, SN 490454) was used as a light source. The light intensity was measured using a ferric oxalate actinometer, which was bought from the Photoelectric Instrument Factory of Beijing Normal University. UV−vis spectra of the solution were obtained using a Hitachi U-3010 UV−vis spectrophotometer (Hitachi High-Technologies Corp., Tokyo, Japan). The amount of Au loaded on porous nickel was recorded by inductively coupled plasma mass spectrometry (ICP-MS, 7700 Series, Agilent Technologies). Synthesis of SH-βCD-Functionalized Gold Nanoparticles (βCD-AuNPs). The βCD-AuNPs were prepared according to a previous report.24 Briefly, 1% HAuCl4·3H2O aqueous solution (w/w, 0.4726 g) was dissolved in 16 mL of DMSO under vigorous magnetic stirring, then quickly mixed with another 16 mL of DMSO containing SH-βCD (4 mg) and NaBH4 (60.4 mg). The reaction mixture continued to stir for 24 h at room temperature to ensure the full reduction of HAuCl4. After that, 32 mL of CH3CN was added to the reaction mixture, and the precipitate was collected by centrifugation (2000 rpm, 20 min). The precipitate was washed with 50 mL of CH3CN/DMSO (1:1, v/v) and 50 mL of ethanol by centrifugation (4500 rpm, 20 min), sequentially. The sample was dried under vacuum at 30 °C overnight, and stored at room temperature. The morphology and size of βCD-AuNPs are shown in Section S1 in the Supporting Information, and the catalytic performance of βCD-AuNPs to reduce the 4-NP solution is shown in Section S2. Synthesis of Silanized Azobenzene (IPTS-Azo). AAzo (10.5 mmol, 207.1 mg) was dissolved in anhydrous EtAc (100 mL). DBTL (7% w/w, 31.81 mg) was added to the solution as a catalyst. The mixed yellow solution was heated to 80 °C under vigorous magnetic stirring. Then, IPTS (10 mmol, 247.36 mg) was added. IPTS-Azo solution was obtained after 3 h. The FT-IR spectrum was measured every 1 h to monitor the reaction process (Section S3 in the Supporting Information). Preparation of AuNP-Loaded Porous Nickel (PNi@IPTSAzo@βCD-AuNPs). Prior to use, the porous nickel substrate was cut into small pieces (5 mm × 5 mm); ultrasonically cleaned with acetone, ethanol, and ultrapure water, sequentially, for 10 min each;

Thus, in the long term, this is not a material-saving or environmental-protecting way to deal with the catalysis system. Multivalency, defined as strong, reversible interactions of multiple receptors and ligands based on individually weak, noncovalent bonds, has received extensive attention.14,15 To date, multivalent host−guest interactions between β-cyclodextrin (β-CD) and azobenzene (Azo), which can be understood as the combination between a large number of βCD and Azo molecules, have been adopted in several research domains.16−18 This is because trans-Azo can form a stable inclusion complex with β-CD by matching size and hydrophobic interactions. Irradiating with UV light can extract Azo from the cavity of β-CD for photoisomerization from the transto the cis-form.19 Most reports have been aimed at constructing a stimuli−response supramolecular polymer,20,21 modifying the surface to achieve specially tunable functions,18,22 forming a reversible connection among several individuals,23 and so on. For an improvement in the recyclability of the fixed-bed catalyst, a repairable porous catalytic bed anchored with AuNPs based on multivalent interactions was proposed. Porous nickel (PNi) was chosen as the support rather than other porous materials because of the magnetism of nickel, which makes spatial transfer easy by magnetic force. The PNi substrate possesses a certain mechanical strength and can be used as a fixed bed for AuNPs. The multivalent host−guest interactions between β-CD and Azo were used to anchor AuNPs onto the PNi substrate to construct a repairable catalytic bed (Scheme 1A), in which the Azo molecules act as target points that could reversibly combine βCD-AuNPs, and provide enough space to prevent the aggregation of nanoparticles. The catalytic activity and stability of such a catalyst bed were verified by the conversion of 4-nitrophenol (4-NP) into 4-aminophenol (4AP). The PNi supports allow liquid to flow through it, which is the key to catalyze the reduction process of 4-NP in a continuous-flow model (Scheme 1B). Importantly, once AuNPs lose their catalytic activity, they could be recovered by removing old AuNPs under UV-light irradiation and then recombining with new ones.

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EXPERIMENT AND MATERIALS

Materials. Tetraethyl orthosilicate (TEOS, 99%) was purchased from XiLong Chemical Reagents Company. (3-Isocyanatopropyl) triethoxysilane (IPTS, 95%) and p-aminoazobenzene (AAzo, 98%) were supplied by Tianjin Heowns Biochem Technologies, LLC. HAuCl4·3H2O was obtained from Sinopharm Chemical Reagent Beijing Co., Ltd. Dibutyltindilaurate (DBTL), 4-nitrophenol (4-NP), 7588

DOI: 10.1021/acssuschemeng.7b00879 ACS Sustainable Chem. Eng. 2017, 5, 7587−7593

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Figure 1. (A) XPS spectra of the PNi substrate modified with SiO2, IPTS-Azo, and βCD-AuNPs step-by-step, and spectra of two types of contrasted substrates functionalized in the absence of SH-βCD or AAzo. (B) XPS survey spectra of PNi@IPTS-Azo@βCD-AuNPs after being irradiated with UV light (365 nm, 30 mW/cm2) and reimmersed in the βCD-AuNP aqueous solution overnight. large-scale use of the fluid-bed catalyst, another 80 mL of mixed reaction solution was continuously added to the syringe tube. Repair of the Broken Catalytic Bed. The repair process of the catalytic bed was divided into two parts: detachment and recombination. In the detachment part, acetone was kept flowing through the catalyst-filled tube with UV-light (365 nm, 30 mW/cm2) irradiation at the same time. After 10 min, the small catalyst parts in the tube were dried with ultrapure N2, reimmersed into an aqueous solution of βCD-AuNPs overnight, then washed with flowing ultrapure water for 10 min to remove redundant βCD-AuNPs, and then dried under a flow of ultrapure N2. The performance of the catalytic bed after both the detachment and recombination processes was monitored by UV−vis spectroscopy of the mixed solution of 4-NP and NaBH4.

and dried under an ultrapure nitrogen stream. SiO2 was coated onto the small PNi pieces according to a previously reported paper.25 The treated PNi’s were immersed in a mixture of 320 mL of ethanol and 80 mL of ultrapure water. Under continuous magnetic stirring, 12 mL of NH3·H2O (25%) was added to the mixture, and 1.2 mL of TEOS was added dropwise. The reaction was allowed to proceed at 40 °C for 3 h. PNi@SiO2 samples were washed with ultrapure water and dried under a flow of ultrapure N2. PNi@SiO2 samples were immersed into the solution of IPTS-Azo for 12 h. After that, the substrates were washed with EtAc to remove the surplus IPTS-Azo, then washed thoroughly with ethanol and dried under a flow of ultrapure N2. The obtained dry PNi@IPTS-Azo substrates were stored in a desiccator before use. βCD-AuNPs (10 mg) was dissolved in 4 g of ultrapure water with ultrasonic dispersion. The PNi@IPTS-Azo substrates were immersed in the solution overnight, then washed with ultrapure water at least three times to remove surplus βCD-AuNPs, and then dried under a flow of ultrapure N2 to afford the target substrates, PNi@IPTS-Azo@ βCD-AuNPs. The composition of the modified PNi substrates after every step was investigated by XPS, and the results are shown in Section S4 in the Supporting Information. The status of the βCDAuNPs on the surface of the PNi@IPTS-Azo@βCD-AuNPs was characterized with TEM, and the TEM image is shown in Section S5 in the Supporting Information. Catalytic Reduction of 4-Nitrophenol to 4-Aminophenol. In a typical reaction, an aqueous solution of 4-NP (0.05 mM, 10 mL) was mixed with a freshly prepared NaBH4 aqueous solution (10 mM, 10 mL), leading to a color change from light yellow to yellow-green marking the conversion of 4-NP to 4-nitrophenolate.26 First, PNi@IPTS-Azo@βCD-AuNPs were chosen as a solid catalyst. There were 10 pieces of the substrate added to the above-mentioned mixed solution (2 mL). The yellow-green solution gradually turned to colorless as the reaction proceeded. Then, the catalytic substrates were extracted by a magnet every 2 min to test the UV−vis spectra of the reaction solution. The yellow-green reaction solution became colorless within 12 min. In the recycle experiments, the substrates were removed from the reaction solution, washed with ultrapure water at least three times, and dried with ultrapure N2 before they were reused. The catalytic reaction was repeated five times. For comparison, 10 pieces of PNi@IPTS-Azo were added to the same mixed solution of 4NP and NaBH4, and the reaction process was monitored by UV−vis spectroscopy (Section S6). Second, the PNi@IPTS-Azo@βCD-AuNPs were used as a fluid bed to catalyze the reduction of 4-NP because of their porous structure. Briefly, 20 pieces of clean catalytic substrate were cut into small pieces and tightly packed into a syringe tube. Then, the prepared mixed solution of 4-NP and NaBH4 (20 mL) was added to the catalytic unit. The aqueous solution quickly flowed out dropwise from the pinhead. The solution clearly changed from yellow-green to colorless as it flowed through the syringe tube. For verification of the possibility of

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RESULTS AND DISCUSSION AuNPs, the surfaces of which were covered with β-CD (Figure S1D), and the average diameter of which was 3.27 nm (Figure S1B), were fabricated through a one-step method via the reduction of HAuCl4 by sodium borohydride (NaBH4) in the presence of perthiolated β-cyclodextrin (SH-βCD).24 The distribution and stability of the gold nanoparticles in the solution were good, as could be proven by the TEM images (Figure S1A). More importantly, as host, β-CD endowed the AuNPs with the possibility of multivalent interactions with other materials covered with Azo. PNi has a porous structure and large specific surface area which provide the possibility for fluid to pass through it and load a large number of functional groups on the surface. Also, because of its fairly good mechanical properties, PNi is a good carrier for fixed-bed catalysis in flow mode. For the construction of PNi with Azo on the surface, a series of surface modifications were conducted (Scheme 1A). After PNi was ultrasonically cleaned in water, ethanol, and acetone, sequentially, a silica layer was coated onto the substrate through a sol−gel process by hydrolysis of TEOS in a mixed solution of ultrapure water and ethanol. On one hand, silica could protect the substrate from corrosion and oxidation during operation. On the other hand, the silica layer contains Si−OH groups, which facilitate the further modification of the substrate. IPTS-Azo, which was synthesized through the reaction of IPTS and AAzo (the characterization of the synthesis is outlined in Section S3 in the Supporting Information), could be anchored onto the silica layer of the PNi through a sol−gel process. Thus, the surface of the PNi contained large quantities of Azo (PNi@IPTS-Azo), which 7589

DOI: 10.1021/acssuschemeng.7b00879 ACS Sustainable Chem. Eng. 2017, 5, 7587−7593

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Figure 2. (A) Time-evolution UV−vis absorption spectra of 4-nitrophenolate ions in aqueous solution. Inset picture shows the operational method. (B) Kinetic trace of the absorbance at 400 nm during the reduction process of 4-NP and the pseudo-first-order fitting for experimental data (red line). (C) Conversion of 4-NP versus the reaction cycle numbers.

β-CD. Changes of the chemical composition of the modified PNi surfaces are shown in Table S1. Since the catalyst, PNi@IPTS-Azo@βCD-AuNP, was constructed by reversible multivalent interaction, it is necessary to verify the performance of the catalysis. Catalysis by NaBH4 is usually employed for the conversion of 4-NP to 4-AP. This conversion could not proceed without catalysis by Au nanoparticles. Before the catalytic performance of the PNi@ IPTS-Azo@βCD-AuNPs was verified, the catalytic activity of PNi@IPTS-Azo was studied, and the results are shown in Figure S5. There was almost no change in the intensity at 400 nm after 12 min, confirming that PNi@IPTS-Azo alone has no ability to catalyze the reduction of 4-NP, while the βCD-AuNP aqueous solution showed a good catalytic activity. In this experiment, 10 pieces of PNi@IPTS-Azo@βCD-AuNPs (5 mm × 5 mm) were added to the mixed aqueous solution of 4-NP (0.05 mM, 1 mL) and NaBH4 (10 mM, 1 mL). As Figure 2A shows, the mixed solution of 4-NP and NaBH4 exhibits a strong maximum absorption at 400 nm. After the catalysts were added, the absorption peak at 400 nm declined gradually, and a new peak appeared at 293 nm, because of the formation of 4-AP. The UV−vis spectrum was tested every 2 min by taking PNi@ IPTS-Azo@βCD-AuNPs to the top of the cuvette with a magnet (inset picture of Figure 2A). After 12 min, the peak at 400 nm had almost disappeared, revealing the reduction of 4NP. Since there was a much higher concentration of NaBH4 than of 4-NP, and the absorbance of 4-NP is proportional to its concentration, the pseudo-first-order kinetics with respect to 4NP could be established to evaluate the catalytic activity of the PNi@IPTS-Azo@βCD-AuNPs, as shown in eqs 1 and 2:

provides the possibility of host−guest interaction with βCDAuNPs. For proof that the βCD-AuNPs and PNi@IPTS-Azo could be combined through a multivalent interaction between β-CD and Azo, PNi@IPTS-Azo was immersed in the solution of βCD-AuNPs for 12 h, and the change of chemical composition on the surface of the PNi after every step was examined by XPS analysis. As shown in the full spectra of the PNi (Figure 1A), the sample contained Si and N atoms after being coated with SiO2 and grafted with Azo in sequence. After being immersed in the solution of βCD-AuNPs and then washed, new peaks at 83.5 eV (Au 4f7/2) and 87.8 eV (Au 4f5/2), assigned to Au0,27 appeared. It turned out that Au nanoparticles were successfully connected to the surface of the PNi. For proof that these Au nanoparticles were anchored to the surface of the PNi through multivalent host−guest interactions, two contrasting experiments, similar to those performed for the multivalent interaction between PNi and AuNPs, were conducted. In one experiment, the PNi@IPTS-Azo was immersed in the solution of AuNPs without β-CD for 12 h. In the other experiment, the PNi without azobenzene was immersed in the solution of βCDAuNPs for 12 h. The change of chemical composition on the surface of the PNi was tested by XPS analysis. There was no peak contributing to the Au atom, which indicated that PNi and Au nanoparticles could not integrate with each other without Azo or β-CD. Therefore, the above experiments demonstrated that AuNPs could be anchored to the surface of the PNi through multivalent host−guest interactions between Azo and β-CD. It is well-known that the host−guest interaction between Azo and β-CD is reversible by control of the irradiation. Irradiated with UV light, azobenzene detaches from the cavity of β-CD for the photoisomerization from trans-form to cis-form. For verification of whether the multivalent host−guest interaction between βCD-AuNPs and PNi@IPTS-Azo is reversible, the PNi@IPTS-Azo@βCD-AuNP substrates were irradiated with UV light for 10 min in acetone. After being rinsed with acetone three times, these PNi@IPTS-Azo@βCD-AuNP-UV substrates were immersed in the solution of βCD-AuNPs to interact again. XPS analysis was again used to investigate the change in chemical composition of the surface of PNi. The result is shown in Figure 1B. Corresponding to the above step, the peak assigned to the Au atom disappeared after irradiation and appeared again after immersion, which indicated that gold nanoparticles were removed from the surface of the PNi after UV irradiation and connected to the surface again after immersion in the solution of βCD-AuNPs. This result proved that the Au nanoparticles could interact with PNi reversibly through multivalent host−guest interactions between Azo and

dA t dCt = = kCt dt dt

(1)

⎛A ⎞ ⎛C ⎞ ln⎜ t ⎟ = ln⎜ t ⎟ = −kt ⎝ A0 ⎠ ⎝ C0 ⎠

(2)

Here, the terms are as follows: At and Ct are the absorbance value and concentration value of 4-NP at time t, A0 and C0 are the initial absorbance value and concentration value of 4-NP, and k is the rate constant of reaction. Figure 2B describes the linear relationship between ln(Ct/C0) and reaction time t. The rate constant k was determined to be 0.318 min−1. Other than catalytic activity, the recovery and stability, which determine the service life of the catalyst, are other important factors to evaluate the PNi@IPTS-Azo@βCD-AuNPs as a catalyst. Because the PNi has a large volume and magnetic properties, it is very feasible to separate PNi@IPTS-Azo@βCD-AuNP substrates from the reaction solution and to re-employ them for 7590

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more homogeneous than that with the batch mode. Therefore, the catalytic efficiency is close to 95%, which is similar to that of the batch mode. In addition, from the UV−vis spectrum of the effluent, there is no absorption peak around 518 nm, which is the characteristic absorption peak of AuNPs (inset picture of Figure 3B). This demonstrates that the multivalent interactions are strong enough to anchor the AuNPs on the surface of the substrate to withstand solution rinsing. After the reduction of 100 mL of 4-NP solution (0.05 mM), this 2 mL fixed bed still showed high stability and catalytic activity (Figure 3C). It is inevitable that the catalytic activity of AuNPs will eventually be lost. Usually, once the catalytic activity is lost, the catalyst and carrier would be discarded together, which is material waste. There is no doubt that the performance of reactivation in situ of the catalyst in the fixed bed saves material and is highly effective. In our fixed bed, the interaction of AuNPs with PNi is reversible. Thus, it is easy to extract or recombine AuNPs with PNi. For verification of the reactivation of the catalyst, UV light was applied to the syringe, and the AuNPs were extracted from the PNi because of the transformation of trans-Azo to cis-Azo, which induced a reduction of the host−guest interaction between Azo and βCD (Figure 4A).28 From the ICP result, it could be calculated

the next reaction of the same reactants. As shown in Figure 2C, PNi@IPTS-Azo@βCD-AuNPs could be successfully reused at least five times with a catalytic efficiency higher than 95%. This result proves that the reversible catalyst prepared via multivalent interaction possessed good stability. Since the density of PNi is greater than that of the solution, the PNi@IPTS-Azo@βCD-AuNPs usually lie at the bottom of the solution. This is not beneficial for homogeneous catalysis. In addition, the batch-mode reaction could not avoid a recycling step, which is costly in time and money. Thus, batch-wise reactions are not the best platform for exploiting the advantages of our catalyst. Because of the connected porous structure and good mechanical properties of PNi, and the Au nanoparticles anchored on its surface, it is ideal for application in continuous-flow catalysis. Continuous-flow catalysis is automatic, which is beneficial for improving the processing rate. In addition, once the catalytic activity of the Au nanoparticles is lost, irradiation of the PNi with light of 365 nm and rinsing with acetone causes the Au nanoparticles to detach from the surface of the PNi, and the PNi@IPTS-Azo could be recombined with new Au nanoparticles in situ through reversible multivalent host−guest interactions. Thus, it is very suitable for industrial applications. As shown in Figure 3A, a simple device of a fixed-bed system was constructed for reducing the aqueous solution of 4-NP with

Figure 4. (A) Scheme to remove broken AuNPs from the surface of PNi. (B) UV−vis absorption spectra of the freshly prepared mixture of 4-NP and NaBH4, and the effluent reaction solution from the catalystfilled syringe tube after UV-light irradiation and reimmersion processes.

that the detachment efficiency was 93.7% (Section S7), which was consistent with the results of XPS (Table S1). For recombination with Au nanoparticles, the new solution of βCDAuNPs was added into the syringe and then rinsed with acetone. The catalytic activity of the fixed bed was checked, and the results are shown in Figure 4B. After the Au nanoparticles were extracted by UV irradiation, 4-NP could not be reduced any more, but the reduction would proceed again after recombination with new Au nanoparticles. This result proved our idea that multivalent interaction could be used to construct a reversible catalyst.

Figure 3. (A) Photograph showing the setup and the reduction of the mixture of 4-NP (0.05 mM, 10 mL) and NaBH4 (10 mM, 10 mL); the solutions in the right and left vials are the mixtures of 4-NP (0.05 mM, 10 mL) and NaBH4 (10 mM, 10 mL) before and after reduction, respectively. (B) UV−vis absorption spectra of the mixed solution and effluent in part A. (C) The conversion of 4-NP to 4-AP after a different volume of solution flows through the catalyst-filled syringe tube.

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CONCLUSIONS In summary, regeneration of the catalytic activity of metal nanoparticles anchored on a carrier is a challenge for catalytic systems, such as a catalytic fixed bed. We designed a repairable catalytic system based on gold nanoparticles and porous nickel, through the reversible multivalent host−guest interactions between β-CD and Azo. Because of the large specific surface area and connected porous structure of porous nickel, a large number of Au nanoparticles could be anchored on the surface through multivalent host−guest interactions, and the reactant

NaBH4. In short, PNi@IPTS-Azo@βCD-AuNPs were cut into small pieces and placed into a 2 mL syringe. The solution of 4NP flows through the pores of the PNi and contacts the Au nanoparticles on the surface of the PNi, reducing it to 4-AP. The color of the solution of 4-NP is bright yellow, turning to colorless through the fixed bed. The solution permeating the pores means that the opportunity for the 4-NP to contact the Au nanoparticles is uniform, or in other words, this catalysis is 7591

DOI: 10.1021/acssuschemeng.7b00879 ACS Sustainable Chem. Eng. 2017, 5, 7587−7593

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ACS Sustainable Chemistry & Engineering solution could be catalyzed by flowing through the pores. Because of the photoisomerization property of Azo, the multivalent host−guest interactions between β-CD and Azo could be removed by UV irradiation. Thus, the gold nanoparticles could be anchored on or removed from the surface of the porous nickel. This enabled the regeneration of the catalytic activity of the fixed-bed in situ. This approach could be expected to provide an effective and regenerative solution for industrial catalysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00879. Information concerning characterization of β-CD modified AuNPs (βCD−AuNPs), catalytic reduction of 4-nitrophenol (4-NP) with βCD−AuNPs, synthesis and characterization of silanized azobenzene (IPTS-Azo), XPS analysis, morphology of Au on the PNi, catalytic reduction of 4-NP with PNi@IPTS-Azo, and calculation of TOF and TON (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-010-64421310. ORCID

Jun Nie: 0000-0003-2698-1751 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the National Natural Science Foundation of China (51603007) and National Key R&D Plan 2017YFB0307800. The authors also appreciate the support of the Beijing Laboratory of Biomedical Materials.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b00879 ACS Sustainable Chem. Eng. 2017, 5, 7587−7593

Research Article

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DOI: 10.1021/acssuschemeng.7b00879 ACS Sustainable Chem. Eng. 2017, 5, 7587−7593