CO2-Triggered Recoverable Metal Catalyst ... - ACS Publications

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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CO2‑Triggered Recoverable Metal Catalyst Nanoreactors using Unimolecular Core−Shell Star Copolymers as Carriers Yuchen Zhang,† Pingwei Liu,*,‡ Bo-Geng Li,† and Wen-Jun Wang*,† †

State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China ‡ Institute of Polymer and Polymerization Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Metallic nanocatalysts are highly active in a variety of organic transformations, but their durable and facile reuse without comprising activity remains a great challenge. Herein, we describe the use of a CO2-triggered unimolecular core−shell star copolymer as carrier for immobilizing gold (Au) and silver (Ag) nanocatalysts for preserving high activity, while achieving facile recoverability. The core−shell star polymers comprising an inert hyperbranched polyethylene (HBPE) core and multiple CO2-responsive arms with tertiary amine groups were synthesized by palladium-catalyzed chain walking polymerization of ethylene and 2-(2bromoisobutyryloxy)ethyl acrylate, followed by atom-transfer radical polymerization of dimethylaminoethyl methacrylate (DMAEMA) and diethylaminoethyl methacrylate (DEAEMA) using HBPE as a macroinitiator. The star copolymers were synergistically engineered via chain topology, composition, and functionality control. They form well-dispersed unimolecular micelles in aqueous solution with CO2 treatment and immobile Au or Ag nanoparticles on the arms via electrostatic interaction. The supported Au nanoparticle nanoreactors were shown to possess high activity in catalyzing the reduction of 4-nitrophenol. They could be readily precipitated with N2 bubbling after reaction and redispersed into the aqueous solution upon purging CO2 for successive reactions. The catalytic reaction process could be paused or resumed by controlling CO2/N2 bubbling steps without scarifying the catalytic activity. The catalysts had an average catalytic activity of apparent reaction rate constant (kapp) of 8.4 × 10−2 s−1 in 15 cycles of reduction of 4-nitrophenol, in comparison with the reported highest kapp = 1.2 × 10−2 s−1 in 10 cycles previously reported. Further experimental and model studies indicate that larger Au/star polymer ratio, higher grafting density, and longer arm length provide for higher catalytic activity. It is demonstrated here that CO2-triggered star copolymer carrier is a promising approach for providing metallic nanocatalysts with high activity and durability. KEYWORDS: unimolecular nanoreactors, metal nanoparticle, CO2-responsive core−shell star copolymer, hyperbranched polyethylene, catalysis, recoverability

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INTRODUCTION Large surface-to-volume ratios and concentrated (active) electron density provide metallic nanoparticles (NPs) with high efficiencies in catalyzing various organic transformations. Examples of NP applications include gold (Au) for nitrocompound reductions,1 silver (Ag) for hydrations,2 and palladium (Pd) for C−C coupling reactions.3,4 Sustainable use of NPs with regard to economics, safety, and environment requires their recovery and reuse subsequent to chemical reactions. One potential approach to meet these requirements is the immobilization of NPs on polymeric supports. In compatible solvents, dispersion of polymer-supported NPs forms homogeneous-like reaction systems, which provides for substantially increased catalytic activity. Furthermore, catalytic activity and selectivity of the NPs can be tuned through polymer functionality and structural designs.5,6 Specifically, tailoring polymer chains to promote the assembly of nanostructures © XXXX American Chemical Society

with tunable nano- and microenvironment leads to the formation of hydrophobic pockets or isolated reaction space for metallic NPs.7,8 Nanogel particles,9 dendrimers,4 hyperbranched/starshaped block copolymers,10,11 and their assembled micelles12,13 or vesicles11,14−16 have been reported as nanocarriers for supporting metallic NPs that enabled high catalytic efficiency. However, the complete recovery of these supported NPs from their homogeneous-like reaction systems without compromising activities remains elusive. Thus, the recovery and reuse of the supported NPs with minimal solvent use, waste generation, and energy input is an essential research issue.17,18 Star-shaped block copolymers, such as that having a βcyclodextrin core and up to 21 poly(styrene-b-acrylic acid) (PSReceived: January 8, 2018 Accepted: February 21, 2018

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DOI: 10.1021/acsanm.8b00031 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials b-PAA) arms, form unimolecular micelles and have served as templates for the preparation of metallic,19 ferroelectric,20 magnetic,21 semiconductor,22 and luminescent nanocrystals23 and imaging materials.24 Size, composition, and architecture of nanocrystals can be controlled by tailoring polymer core size and chain length of each arm block.23 The unimolecular star block copolymers have also been found to possess unique characteristics making them well-suited for use as nanoreactors10 and drug carriers.25,26 Solubility (or dispersibility) of the nanoreactors in media or solvents is governed by the outer layer of arms, which also provide compartments for supporting NPs or catalytic motifs to minimize aggregation and interactions.14 The inner core regulates access and diffusion of reactants, isolates the interior nanocatalysts or catalytic groups,13 and possibly concentrates the reactants due to compatibility.15 Dong and Ye developed a cross-linked hyperbranched poly(phenylacetylene) with ppm-level catalysts for multiple cross-coupling reactions.27,28 Fréchet et al.11,29 investigated PS-based, non-interpenetrating star polymers with isolated iminium, enamine, and hydrogen-bonding catalytic sites for one-pot multicomponent asymmetric cascade reactions. Kimura et al.30 investigated a poly(propyleneimine) based dendrimer as a nanoreactor, where the catalytic activity for thiol oxidation was tuned with temperature-controlled polymer conformation. Ballauff et al.31 developed polystyrene/poly(N-isopropylacrylamide) (PNIPAM) core−shell particles as a carrier for Ag NPs, which can modulate catalytic activity for the reduction of 4-nitrophenol with phase transitions in cross-linked PNIPAM networks modulated using temperature. Although such unimolecular nanoreactors are highly active for catalysis, their recovery and reuse subsequent to reactions is difficult. Normally, extraction and precipitation using solvents is applied to recover the nanoreactors from their dispersion systems, but a large amount of solvent is required and the recovery efficiency is low.32 The introduction of stimuliresponsive polymers as nanocarriers, such as those possessing CO2/N2 switchability, provides a potential approach for overcoming these issues.32−34 The CO2/N2 system works via a pH trigger, which provides a highly efficient and rapid response. Such switchable systems are typically low-cost, environmentally friendly, and feasible for large-scale applications. Cunningham and Jessop’s,35−39 as well the authors’ group,40−45 have developed CO2/N2-triggered coagulatable and redispersible polymeric nanoparticles. Yuan’s46−48 and Zhao’s groups49−52 have achieved CO2 induced self-assembly transformation of nanoparticle morphology. Unlike other pH triggers, the CO2/N2 trigger has the advantage of not introducing contaminants (e.g., salts) to the system during their use and recovery process. Desset and Cole-Hamilton53 developed a biphasic catalysis system where the CO2/N2 could provide for the transfer of a rhodium complex between an aqueous solution and toluene for the product separation, albeit at a modest mass transfer rate. Zhao et al. synthesized Au NPs coated with CO2-switchable poly(diethylaminoethyl methacrylate) (PDEAEMA) used for the reduction of 4-nitrophenol in water.54 Hence, the introduction of CO2/N2 switchability to unimolecular nanoreactors should offer them unique characteristics that allow for recovery and reuse. However, the challenge lies in how to construct CO2/N2 switchable nanoreactors for supporting metallic NPs without sacrificing reactivity. Here we demonstrate the synergetic engineering of unimolecular nanoreactors via the tailoring of chain functionality, comonomer composition, and topology of CO2-resposive star

polymer supporters. Amphiphilic star copolymers with CO2/N2 switchable arms and hyperbranched polyethylene (HBPE) core were synthesized and used as unimolecular carriers for supporting Au or silver (Ag) NPs in water. The aliphatic nature of HBPE makes it an ideal core material with good chemical stability and adequate hydrophobicity to facilitate the reuse of the unimolecular nanoreactors, due to easy aggregation in aqueous solution. Ease of tuning chain topology and functionality through the chain walking copolymerization (CWP) of ethylene and comonomers possessing functional groups offers additional benefits to using HBPE as the core. Reduction or oxidation reactions and CO2/N2 triggered recovery for multiple cycles were examined for the Au and Ag NP nanoreactors. The catalysts were found to have a higher activity and better recyclability than previously reported values.55,56 Also, a model was developed and validated with experimental results, which quantifies the influence of Au NP loadings, grafting density, and star polymer arm length on catalytic activities of the unimolecular nanoreactors.

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EXPERIMENTAL SECTION

Materials. The preparation of Pd-diimine catalyst [(ArN C(Me)(Me)CNAr)Pd(CH2)3C(O)OMe]+SbF6− (1) and functional monomer 2-(2-bromoisobutyryloxy)ethyl acrylate (BIEA) were described in previous publications.57,58 Operations involving air- or moisture-sensitive chemicals were carried out in a glovebox. Ultrahigh purity N2 (Jinggong Gas) and polymerization-grade ethylene (Sinopec) were purified by passing them through CuO and a 3-Å molecular sieve column prior to use. Dimethylaminoethyl methacrylate (DMAEMA) and diethylaminoethyl methacrylate, DEAEMA, (>99%, J&K Chemical) were passed through an alkaline aluminum oxide column for purification. Dichloromethane (>99%, Sinopharm) was refluxed over CaH2 (98%, Sinopharm) for more than 12 h and then distilled. The CuCl (99%, Alladin), toluene (HPLC, >99%, TEDIA), methanol (>99.8%, Sinopharm), CuCl2 (Sinopharm), 1,1,4,7,10,10-hexamethyltriethylene-tetramine (HMTETA, 97%, J&K Chemical), ethyl 2bromoisobutyrate (EBIB, 98%, J&K Chemical), HAuCl4 (Sinopharm), NaBH4 (Sinopharm), and 4-nitrophenol (Sinopharm) were all used as received. Synthesis of HBPE Macroinitiators. Synthesis of hyperbranched polyethylene (HBPE) macroinitiators (MIs) followed procedures reported previously.59 The synthesis procedure for MI1 is outlined here as an example. A 50 mL Schlenk flask was flame-dried and then purged with ethylene multiple times. The comonomer solution (0.4 g of BIEA in 15 mL of CH2Cl2) was injected, followed by the catalyst solution (0.167 g of 1 in 15 mL of CH2Cl2), and finally the ethylene (1 atm) was charged. After 24 h at 25 °C, the solution was poured into 300 mL of methanol to precipitate HBPE MI1. The collected polymer was then washed three times with 100 mL of methanol, dissolved in 100 mL of tetrahydrofuran (THF), and passed through a 220 nm syringe filter to remove Pd residues. The polymer solution containing the HBPE MI was concentrated using rotary evaporation, washed three times with 300 mL of methanol, and dried under vacuum at 50 °C. The experimental conditions are provided in Table S1 of the Supporting Information. Synthesis of Star HBPE−Poly(DMAEMA-co-DEAEMA)s and Linear Copolymer by Atom-Transfer Radical Polymerization (ATRP). A series of star polymers HBPE−poly(DMAEMA-coDEAEMA)s (SCs) and a linear poly(DMAEMA-co-DEAEMA) were synthesized with different MIs, DMAEMA, and DEAEMA compositions. Synthesis of run 1 SC based on MI1 with a recipe of [MI1]0/ [CuCl] 0 /[CuCl 2 ] 0 /[HMTETA] 0 /[DMAEMA] 0 /[DEAEMA] 0 = 1:1:0.1:2:100:100 is outlined here as an example. MI1 (0.3 g), DMAEMA (1.28 g, 1.38 mL), DEAEMA (1.52 g, 1.65 mL), HMTETA (0.025 mL), and toluene (3.06 mL) were mixed under magnetic stirring in a 50 mL Schlenk reactor tube sealed with a rubber septum. The mixture was stirred and purged with N2 for 40 min to remove oxygen. CuCl (8 mg) and CuCl2 (1.1 mg) were then added under N2 protection and mixed at room temperature for 40 min. The polymerization was B


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ACS Applied Nano Materials

Figure 1. (a) Synthetic route for SCs and Au/Ag SNRs, (b) CO2/N2 trigged tunable reactivity of SNRs, (c) switchable precipitation/redispersion of SNRs in water with bubbling CO2/N2, (d) cryo-TEM image of SNR5 NPs after CO2 bubbling, and (e) cryo-TEM image of SNR5 particle showing supported Au NPs. carried out at 60 °C for 1 h. Reactions were terminated by charging 15 mL of ethanol and bubbling air. The resulting polymer solution was dialyzed against ethanol for 1 day and deionized water for another 3 days to remove the residual solvent, Cu2+, and monomers, followed by freezedrying to obtain the SC. In the preparation of the linear copolymer, the same approach was used for the preparation of linear poly(DMAEMAco-DEAEMA) except EBIB was used as the initiator. Support of Au/Ag NPs on Star HBPE−Poly(DMAEMA-coDEAEMA) Copolymers. The Au or Ag NP catalysts were immobilized on SC carriers to produce supported NP catalyst nanoreactors (SNRs) using an in situ reduction method. For the Au SNR, the SC (15−45 mg) was added into 5 mL of deionized water followed by purging with CO2 for approximately 10 min, to completely dissolve the polymer and form a light blue aqueous solution. The HAuCl4 aqueous solution (0.1 mL, 10

mg/mL) was then added under stirring. After 30 min, NaBH4 aqueous solution (0.5 mL, 10 mg/mL) was introduced dropwise under vigorous stirring. The solution turned wine red within seconds, indicating the formation of Au SNRs. In the reduction of 4-nitrophenol, the size of Au NPs with high catalytic activity ranges from 4 to 12 nm, and rate constant, normalized to total NP mass, decreases with increasing NP size.60 High DMAEMA/metal ion ratio61 and excess amounts of reductant NaBH462 are beneficial to ion reduction, to form metal NPs of smaller size. The same procedure was applied for preparation of the Ag SNR producing a transparent yellow solution. Reduction of 4-Nitrophenol with Au SNRs in Water. The Au SNRs (5−10 mg) were dissolved in water (6 mL) under sonication by bubbling CO2 for approximately 5 min, which adjusted the pH to 7. The resulting transparent wine red Au SNR solution ([Au]0 = 15.1−16.5 C


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ACS Applied Nano Materials Table 1. Characterization Results for the Star HBPE−poly(DMAEMA-co-DEAEMA) run

MI

Mn,MIa (kg/mol)

ĐMIa

Narmb

f P(DMAEMA‑co‑DEAEMA)c

Mn,NMRd (kg/mol)

Mn,GPCe (kg/mol)

ĐGPCe

DParmf

SC1 SC2 SC3 SC4 SC5 SC6 Lg

MI1

72.9

1.2

8.0

MI2

104.4

1.5

28.6

0.10 0.14 0.21 0.08 0.23 0.33 1.0

123 143 189 156 282 405 13.1

131 148 208 152 299 429 12.8

1.2 1.3 1.2 1.4 1.3 1.2 1.2

36.4 50.8 84.6 10.2 36.2 61.2 76.8

EBIB

Mn,MI and ĐMI are number-average molecular weight and distribution of HBPE macroinitiator (MI) determined by triple-detection GPC using THF as eluent, respectively. bNarm is number of arms on each SC estimated from 1H NMR. cf P(DMAEMA‑co‑DEAEMA) is the mole fraction of P(DMAEMA-coDEAEMA) in the SC measured with 1H NMR. For all samples, each arm comprised random copolymer of DMAEMA and DEAEMA at a molar ratio of 1:1. dMn,NMR is number-average molecular weight of SC estimated from 1H NMR. eMn,GPC and ĐGPC are number-average molecular weight and distribution of SC1−6 and linear copolymer L measured by triple-detection GPC using THF with 3.0 vol % triethylamine as eluent, respectively. f DParm is degree of polymerization of DMAEMA and DEAEMA on each arm estimated from 1H NMR. gLinear DMAEMA and DEAEMA random copolymer synthesized with EBIB as initiator. a

ppm in 6 mL) was then combined with a mixture of 2 mL of 4nitrophenol (0.5 mM) and 8 mL of NaBH4 (5 mM), producing a green color. The ratio of AuNPs/4-nitrophenol was determined according to the literature55,66−69 so that we could easily monitor the reduction process and compare our results with previously reported values. The UV−vis absorption of the solution at 400 nm was monitored during the reduction process. Upon completion of the reaction, the Au SNR was precipitated using N2 bubbling and collected for reuse. Oxidation of Triethylsilane with Ag SNRs in Water. The Ag SNR6 (5 mg) was dissolved in water (6 mL) by bubbling CO2 for approximately 5 min to form a transparent yellow solution. Triethylsilane (1 mmol) was then added into the solution and mixed for 30 min. After the reaction, the Ag SNR6 was precipitated with N2 bubbling. The conversion of triethysilane was determined by 1H NMR, and the recovered Ag SNR6 was redissolved for further oxidation upon purging with CO2. Characterization. The 1H NMR spectra were determined using a Bruker Advance 400 spectrometer. Molecular weights (MW) and distributions (Đ) were determined by a Waters−Wyatt gel permeation chromatography (GPC) system equipped with differential refractive index, four-bridge capillary viscometer (IV), and light scattering detectors (45° and 90°). THF containing 3 vol % of triethylamine was used as eluent at a flow rate of 1 mL/min at 35 °C. Poly(methyl methacrylate) standards having a MW range of 0.875−625.5 kg/mol were used for universal calibration. The Au concentrations of all aqueous solutions were determined by a Thermo X series 2 inductively coupled plasma mass spectrometer (ICP-MS). Particle size distributions of samples were determined using a Malvern Zetasizer NanoZS model ZEN 3690 at 25 °C. The instrument is equipped with an argon ion laser with a wavelength of 633 nm at a detection angle of 90°. Transmission electron microscopy (TEM) images of samples were acquired on a Tecnai G2 Spirit Cryo-TEM operated at 120 kV. Copper grids coated with carbon film stabilizing Formvar were used for sample preparation by sample solution dipping and liquid nitrogen freezing prior to characterization. UV−vis spectra were obtained using a Persee TU1901 ultraviolet and visible spectrophotometer (UV−vis). The change of absorbance at 400 nm versus reaction time was used to monitor the reduction process.

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(DMAEMA-co-DEAEMA) was controlled by tuning the average number of Br groups per HBPE polymer chain (NBr), which was tailored by changing BIEA concentrations and polymerization times during the CWP.59 The NBr, which is equal to the number of grafted arms per HBPE (Narm), can be determined by 1H NMR, while the HBPE particle sizes can be measured by dynamic light scattering. The grafted density was estimated by dividing the surface area of each HBPE by Narm. At BIEA feed ratios of 0.1 and 0.2 M, the CWP step produced two macroinitiators, MI1 and MI2, with number-average molecular weights (Mn) = 72.9 and 104.4 kg/mol, branching density (BD) values = 94.8 and 98.8 C/1000 C, and Narm = 8 and 28, respectively. The GPC traces and NMR spectra of the MIs are shown in Figures S1 and S2, respectively, of the Supporting Information. Macroinitiator MI1 was used for initiating ATRP of DMAEMA and DEAEMA to produce SC1−SC3 having an average degree of polymerization (DP) = 36, 51, and 85 per arm estimated from 1H NMR, while SC4−SC6 with DP of 10, 36, and 61 per arm were generated with MI2 at 1, 2, and 4 h polymerization times, respectively. Each arm in the SCs comprised random copolymer of DMAEMA and DEAEMA at molar ratio of 1:1. The SCs were well-controlled with narrow Đ values of 1.23−1.37 and Mn of 131.0−429.3 kg/mol. The characterization results of the SCs are summarized in Table 1. The 1H NMR spectra for SC1−SC6 are shown in Figure S1 of the Supporting Information. The six as-prepared SCs would not dissolve in water. However, after CO2 purging for 10 min, the SCs were readily dispersed, owing to quaternary amination of tertiary amine groups in the arms of SCs.40,47 The pH values of the solutions dropped from 7.4 to 6.4 after CO2 bubbling, corresponding to an amine group protonation degree of 88.9%.52 These protonated SCs formed stable unimolecular micelles in water, as evidenced by the narrow Đ values as shown in Table 1, narrow particle size distributions (close number- and volume-particle size distributions) from DLS results in Figure S3 of the Supporting Information, and TEM images in Figure 1d. The SCs had zeta potentials of 28.2−34.3 mV and volume-average particle diameter (DV) of 29.7−63.7 nm (Figure S3). Since the particle size of MIs is approximately 10 nm54 and statistical segment length of PDMAEMA is 0.309 nm,65 the particle sizes of the SCs are estimated to be approximately 16−61.8 nm, which are the same as the measured values supporting the existence of unimolecular micelles for SCs. The HAuCl4 and reductant NaBH4 were charged into the SC solutions to immobilize Au NPs in situ to produce SNRs. The Au

RESULTS AND DISCUSSION

Preparation of Supported Au NP Nanoreactors. The star HBPE−poly(DMAEMA-co-DEAEMA)s, SCs, having a HBPE core and multiple poly(DMAEMA-co-DEAEMA) arms were synthesized via controlled “living” polymerization technique as shown in Figure 1a. The synthesis technique comprises a sequential palladium-catalyzed chain walking polymerization (CWP) of ethylene and BIEA58,59,63,64 and atom-transfer radical polymerization (ATRP) of DMAEMA and DEAEMA using HBPE as MI. The density of grated polyD


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SNR3 possessed the highest activity, while the SNR4 had the lowest. In the reduction of 4-nitrophenol, the reaction can be paused and restarted and the catalytic activity of the SNRs can be manipulated simply by controlling the N2/CO2 bubbling. The dissolved Au SNRs were easily precipitated from the reaction systems with N2 bubbling for 5 min, which deprotonates bicarbonate to amine groups in the poly(DMAEMA-coDEAEMA) arms increasing the hydrophobicity of the SNRs. This destabilized the Au SNRs leading to aggregation and coagulation.43,44,52 An interesting observation was that all of the SNR precipitants deposited on the walls of the vials with N2 bubbling. This operation could achieve complete separation of the Au SNRs from the reaction systems. The evidence for this can be seen in Figure S5 of the Supporting Information. The bubbling N2 in the middle of the reaction halted changes in UV absorbance values as shown in Figure 2b. Further bubbling of CO2 for 10 s could fully redissolve the aggregated Au SNRs, and reactions resumed at the same rates as those prior to pausing. During the reaction process, the duration of CO2 bubbling could also control catalyst activity as presented in Figure 2c. Longer bubbling time increased the protonation degree of the poly(DMAEMA-co-DEAEMA) arms, resulting in Au SNRs with a more extended spatial conformation, which is indicated by the increased particle size of the Au SNRs along with CO2 bubbling from 1 to 10 s as shown in Figure 2d. In general, longer CO2 bubbling period produced higher Au SNRs catalytic activity. It is worth pointing out that the above CO2/N2 bubbling process is scalable and does not introduce contamination into the reaction system. The reusability of the Au SNR5 for the reduction of 4nitrophenol at [SC]0 = 0.83 mg/mL (equivalent to [Au]0 = 15.1 ppm) over 15 cycles of N2/CO2-triggered precipitation and redispersion was examined. After each SNR precipitation, the reaction solution was completely discharged, and fresh reactants and water were added for use in subsequent reactions with the aggregated SNR (see Figure S5 of the Supporting Information). The SNR preserved high activity with little decrease over the 15 cycles as shown in Figure 2e. The kapp values varied from 6.5 × 10−2 to 1.5 × 10−1 s−1, and the average kapp value was 8.4 × 10−2 s−1, which is higher than (0.2−1.4) × 10−2 s−1 reported for the same reaction system catalyzed with micelle, dendrimer, and polymer brush supported Au NPs for a maximum of 6 reuse cycles.55,67−71 Higher reaction rate constants might be due to good dispersion of the Au SNRs assembled from the core−shell star carriers allowing better access of substrates to Au NPs. The [Au] was determined as 14.9 ppm after 15 reuse runs, indicating only 1.3 wt % Au was lost during multiple recycle processes. The unimolecular Au SNR size distribution changed little during recycles after each dispersion as shown in Figure 2f, with original DV = 53.6 to 55.6 nm after 15 reuse runs. For comparison, a supported Au NP with a linear random poly(DEAEMA-coDMAEMA) (without HBPE core) as carrier was used for catalyzing the reduction of 4-nitrophenol following the same coagulation and redispersion procedure. The kapp value changed from 2.3 × 10−1 to 5.3 × 10−3 s−1 after 5 cycles. (Table S2 of the Supporting Information lists the kapp values.) The kapp values of the catalyst with a continuous decrease are comparable to those of the Au SNRs in the first 4 runs, but substantially lower than that of the SNRs in fifth run. This is due to the anchoring of multiple poly(DEAEMA-co-DMAEMA) arms to the HBPE core limiting the chain mobility and constraining Au NPs in a relatively confined space. This prevents the leak of Au NPs and

loadings were 95.1−103.8 ppm determined by ICP-MS, and the SC concentrations ([SC]0) were 3−9 mg/mL. After reduction and purification processes, the SCs comprised approximately 1.7−3.3 wt % Au NPs determined by the ICP-MS as summarized in Table 2. The Au NPs had particle sizes in the range of a few Table 2. Summary of Supported Au/Ag Nanoreactors (SNRs) run

SC

[SC]0a (mg/mL)

[M]0a (mg/mL)

Eb (%)

Wa (wt %)

DV (nm)

ζc (mV)

SNR1 SNR2 SNR3-1 SNR3-2 SNR4 SNR5-1 SNR5-2 SNR6 SNR6-1 SNR6-2 SNR6-3 SNR6Ag

SC1 SC2 SC3 SC3 SC4 SC5 SC5 SC6 SC6 SC6 SC6 SC6

5.0 5.0 5.0 5.7 5.0 5.0 5.5 5.0 3.0 6.0 9.0 5.0

95.1 102.0 103.8 101.3 101.1 92.6 100.8 101.5 100.3 99.8 101.2 127.0

95.8 98.0 97.8 97.3 95.5 97.8 97.7 97.5 98.1 97.2 96.8 97.9

1.8 2.0 2.0 1.7 1.9 1.7 1.8 2.0 3.3 1.6 1.1 2.5

36.6 45.8 58.9

30.5 33.6 35.4

36.5 53.6

29.3 35.7

71.5

38.8

70.3

36.3

a

[M]0 and [SC]0 are concentrations of metal precursors (Au3+ or Ag+) and star copolymers, SCs, added in the solution measured, and W is the weight percentage of metal NPs immobilized in the SCs determined by ICP-MS. HAuCl4 was applied in all SNRs except for SNR6-Ag in which AgNO3 was added. bE is the percentage of HAuCl4 or AgNO3 to be supported in the SC. cζ is the zeta potential of the SNR.

nanometers (95%. All SNRs readily redispersed in water after CO2 bubbling with zeta potential (ζ) of 29.3−38.8 mV and DV of 32.6−65.5 nm. The immobilization of Au NPs did not change the unimolecular structures of the SCs, which was confirmed from the particle size distribution of the SCs before and after loading Au NPs as shown in Figure S3 of the Supporting Information. The as-prepared solutions were ready for catalyzing reactions. In addition, Ag SNR (SNR6-Ag) was prepared by loading silver NPs onto SC6 via an in situ reduction of AgNO3. The SNR6-Ag comprised 2.5 wt % Ag NPs in SC6. Reduction or Oxidation with Au or Ag SNRs. The catalytic performance of Au SNRs in the reduction of 4nitrophenol with NaBH4 (Figure. 1b) was examined.55,56,66−74 Upon mixing 4-nitrophenol and NaBH4 with the Au SNRs, the color of the reaction systems readily changed from (transparent) green to wine red, indicating the reduction had occurred. The intensity of the UV−vis absorption peak at 400 nm, which is proportional to 4-nitrophenol concentration in the solution system,75 was monitored as the reaction proceeded as shown in Figure S4 of the Supporting Information. Figure 2a shows the ratio of 4-nitrophenol concentration in the system to its initial concentration (C/C0) plotted against reaction times using Au SNR5 as catalyst with [SC5]0 = 0.17 mg/mL (equivalent to [Au]0 = 3.0 ppm). The regression curve of ln(C/C0) is linear, suggesting first-order kinetics66−73 with an apparent reaction rate constant, kapp, of 1.4 × 10−2 s−1 (Figure. 2a). Further reductions were carried out with SNR1−SNR6 at [SC]0 = 0.83 mg/mL (equivalent to [Au]0 = 14.3−16.9 ppm), which were also found to follow first-order kinetics with kapp values ranging from 2.6 × 10−2 to 1.3 × 10−1 s−1 as shown in Figure 3. It appears that the E


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Figure 2. Reduction of 4-nitrophenol using Au SNR5 with N2-triggered coagulation and CO2-assisted redispersion in 15 cycles. (a) The ratio of 4nitrophenol concentration to its initial concentration (C/C0) and ln(C/C0) as a function of reaction time using Au SNR5 at [SC5]0 = 0.17 mg/mL (equivalent to [Au]0 = 3 ppm), (b) N2/CO2-triggered pause/restart of reduction and precipitation/redispersion of SNR5 at [Au]0 = 3 ppm, (c) plots of ln kapp of SNR5 with different CO2 bubbling time during redispersion at [Au]0 = 3 ppm, (d) volume-based size distribution of SNR5 with different CO2 bubbling times during redispersion, (e) variation of ln(kapp) of SNR5 at [Au]0 = 15.1 ppm in different cycles, and (f) volume-based size distribution of SNR5 in different cycles.

Table 2, were synthesized by fixing the same HAuCl 4 concentration at 0.2 mg/mL and changing the addition of SC6 from 3.0 to 9.0 mg/mL. Since Au NPs were immobilized by interaction with tertiary amine (N) in the polymer arms but not the inert HBPE core, the [Au]0/[N]0 values for SNR6 to SNR6-3 were estimated as 1.39−4.16 mol %. It can be seen that higher [Au]0/[N]0 values led to higher kapp when the same amount of SNR was used for catalyzing the reduction as shown in Figure 4a. The shell thickness of SCs also influenced the kapp values of the SNRs. The SC3, SC5, and SC6 were used as supports to study the chain structure effect with a fixed [Au]0 = 16.5 ppm. The SNR3-2, SNR5-2, and SNR6 had [Au]0/[SC]0 of 1.73−1.98 wt % and the same [Au]0/[N]0 = 2.48 mol %. The catalytic activities of the Au SNRs increased with the shell thickness of the SNRs as shown in Figure 4b. The arm density (δ) and DP of the SCs determine shell thicknesses of the SNRs. For further understanding of the performance of Au SNR, a physical model for the catalytic activity of the SNRs was constructed. The kapp for the Au SNRs can be broken down into two contributions,76

aggregation of Au SNRs, preserving better catalytic performance. In addition, the poly(DEAEMA-co-DMAEMA) aggregates caused by the N2 bubbling were suspended in the solution as shown in Figure S6 of the Supporting Information and needed to be centrifuged for separation. This is in stark contrast to the Au SNRs, which were easily precipitated and removed on their own. The SC carriers clearly provide better separation from the aqueous phase due to the hydrophobic HBPE core. The study also examined use of SC as the nanocarrier for Ag NPs. The catalytic performance of the supported Ag NP SNR6Ag was used to catalyze the oxidation of triethylsilane using water as the oxidant. Upon charging the substrates into the SNR6-Ag solution at [SC]0 = 0.83 mg/mL (equivalent to [Ag]0 = 20.7 ppm), gas bubbles were released from the reaction, indicating the formation of hydrogen. The conversion of triethylsilane in the first run was greater than 99%. The SNR6-Ag could also be readily precipitated by bubbling N2 and redispersed by purging CO2 for further oxidation. This is demonstrated for 5 cycles in Figure S7 of the Supporting Information. The individual run conversions were greater than 99% for all 5 cycles. Understanding SNR Performance. The metal NP catalyst loading and chain structure of SC have great influence on the catalytic activities of the SNRs. By using SC6 as the carrier, SNR6 to SNR6-3 having [Au]0/[SC]0 of 1.09−3.28 wt %, as listed in

kapp−1 = ksurf −1 + kD−1C −1

(1)

where ksurf is a surface chemical reaction rate constant mainly determined by the ratio of [Au]0/[N]0, kD is defined as a F


Article


Figure 3. Ratio of 4-nitrophenol concentration to its initial concentration C/C0 and ln(C/C0) as functions of reaction time of the 4-nitrophenol reduction using SNR1−6: (a) SNR1; (b) SNR2; (c) SNR3; (d) SNR4; (e) SNR5; (f) SNR6.

Figure 4. (a) kapp−1 as a function of ln([Au]0/[N]0) using the same SC (SC6) for [Au]0/[N]0 ratios of SC6 to SC6-3 of 2.48, 1.39, 2.08, and 4.16 mol %, (b) kapp−1 as a function of shell thickness at a fixed [Au]0/[N]0 ratio of 2.48 mol % for SC3 (δ = 0.011, DP = 84.6), SC5 (δ = 0.023, DP = 36.2), and SC6 (δ = 0.023, DP = 84.6) (lines are model fits), and (c) Schematic representation of the model assumption used for the analysis of the kinetic behavior of SNRs.

Considering that a higher [N]0 results in more binding with the Au NPs and thus a smaller S,77 we further define

diffusion rate constant, which is affected by the composition and structure of the SC, and C is the reactant concentration. For the surface chemical reaction rate constant,64 ksurf = kSθNipθBH4

⎧ [Au]0 ⎫ p S ⎬ = a⎨ S′ ⎩ [N]0 ⎭

(2)

where k is the molar rate constant per square meter of the Au NPs, S is the free surface area of the Au NPs that are not covered by tertiary amine groups of the arms, and θNip and θBH4 are the surface coverage degree of the reactants 4-nitrophenol and NaBH4 in the Au SNRs, respectively. The θNip and θBH4 are constants for a given reaction system.75 Assuming all the Au NPs are spheres having the same radius r, we calculate the total surface area (S′) of the Au NPs as

3m 4πr 2 = S′ = 4π r 3 ρr ρ

where a is a front factor used for the correction and p gauges the extent to which tertiary amine groups stabilize Au NPs, which is outlined elsewhere (dendrimer−metal nanocomposites consisting of surface amino groups).77 Combining these expressions results in ksurf

m

3

(4)

−1

⎛ 3m ⎧ [Au] ⎫ p ⎞−1 0 ⎜ ⎬ θNipθBH4⎟⎟ = ⎜k a⎨ ⎝ ρr ⎩ [N]0 ⎭ ⎠

(5)

The diffusion-controlled rate constant kD for Au NPs embedded in the shell of the SCs with a distance R from the center of core−shell spherical nanoreactors, where spherical nanoreactors are assumed for simplicity, can be estimated by the

(3)

where m and ρ are the mass and density of the Au NPs, respectively. G


Article

ACS Applied Nano Materials Smoluchowski diffusion equation. In the steady-state, it can be expressed as kD−1 = =

∫R ∫R

∞

exp[Gr /kBT ] 4πDr r

L

2

exp[Gr /kBT ] 4πDr r 2

kapp−1 = ksurf −1 + c −1

dr dr +

∫L

∞

exp[Gr /kBT ] 4πDr r 2

L = ls DPδ n

dr

CONCLUSIONS Core−shell star copolymers, SCs, were synthesized for use as unimolecular carriers in the immobilization of Au and Ag NPs. The unique star topology and characteristic composition of the SC carriers, consisting of a hydrophobic HBPE core and multiple CO2/N2-responsive poly(DMAEMA-co-DEAEMA) arms, were shown to provide highly active and durable catalysis in the Au NP-catalyzed reduction and Ag NP-catalyzed oxidation reactions. The catalytic reaction process and reaction rates could be precisely controlled with the bubbling CO2/N2. Furthermore, N2/CO2 switching was shown to readily precipitate and redisperse the SNRs in aqueous solutions. Little NP loss or activity decrease existed with the SNRs in the 15-reuse cycles, suggesting excellent stability of the SNRs. A physical model was developed to describe SNR performance. The experimental findings combined with modeling studies illustrated that the high [Au]/[SC] ratio and shell thickness of the SCs were beneficial to high catalytic activity. In summary, a platform of CO2-swichable supported nanoparticle nanoreactors was designed, which can manipulate the reaction process and rate. This technology can likely be adapted for a variety of chemical reactions.

where, ls is the length of the monomeric units and n is a positive number constant, which increases with the increase of arm density.78 If it is assumed that the reactants have the same diffusion constant DL inside the arms (shell) and D0 in the bulk solution, with DL ≪ D0 according to ref 76, then we have (8)

The immobilization of the Au NPs should occur close to the surface of the SCs, not through the entire shell, in consideration of strong electrostatic binding between the quaternary amine and AuCl4−.79 Assuming two amine groups bind one AuCl4− ion,

⎛ 2[Au]0 ⎞ R = ⎜1 − ⎟L [N]0 ⎠ ⎝ 2[Au]0 exp[Gr /kBT ] 1 [N]0 − 2[Au]0 4πDL R L

■

kapp

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00031. Summary of HBPE MIs, 1H NMR spectra and GPC traces of HBPE MIs, 1H NMR spectra and particle size distributions of the SCs and SNRs, UV−vis absorption spectra in reduction, images of the reaction system after N2-triggered coagulation and CO2-assisted redispersion of 4-nitrophenol with Au SNRs or oxidation of triethylsilane with Ag SNR, and the kapp values of a supported Au NPs with a linear random poly(DEAEMA-co-DMAEMA) carrier (PDF)

(10)

⎛ 3m ⎧ [Au] ⎫ p ⎞−1 2[Au]0 0 ⎬ θNipθBH4⎟⎟ + = ⎜⎜k a⎨ [N]0 − 2[Au]0 ⎝ ρr ⎩ [N]0 ⎭ ⎠ exp[Gr /kBT ] 1 c −1 4πDL R ls DPδ n

■

(11)

At the same [Au]0 loaded at the same SC but different SC amounts ([N]0), the difference of diffusion term is negligible since [N]0 ≫ [Au]0 ([N]0/[Au]0 ratios greater than 25 were maintained for all runs). Equation 11 can be rewritten as, kapp

−1

⎛ 3m ⎧ [Au] ⎫ p ⎞−1 0 ⎬ θNipθBH4⎟⎟ + kD−1c −1 = ⎜⎜k a⎨ ⎝ ρr ⎩ [N]0 ⎭ ⎠

ASSOCIATED CONTENT

S Supporting Information *

(9)

Combining eqs 5 and 10, we have −1

j

■

(7)

exp[Gr /kBT ] exp[Gr /kBT ] + 4πDL L 4πDL R

n −1

Hence, kapp should have a linear relationship with (DP ·δ ) . Since the grafting density of the SCs < 0.1, the shell thickness can be scaled as L ≈ DPδ1/3.80−83 Equation 13 can be used to fit our experimental results, as shown in Figure 4b. The physical model, eq 11, developed in this work provides an accurate description of the Au SNR performance.

Here, the first item describes the diffusion inside the shell layer (L) as shown in Figure 4c, and the second item designates the diffusion in solution. Dr is the distance-dependent diffusion constant for the reactants diffusing toward the NPs, and Gr is the solvation free energy of the reactants diffusing from solution toward Au NPs embedded in the shell of the SNR. L is the thickness of PDMAEMA−PDEAEMA arms (Figure 4c) determined by arm density (δ) and arm length or degree of polymerization of arm (DP). For polymer arms anchored to a spherical particle core, the arm thickness L scales as L ≈ DPδn,65

kD−1 =

(13)

−1

(6)

kD−1 = −

2[Au]0 exp[Gr /kBT ] 1 [N]0 − 2[Au]0 4πDL R ls DPδ n

AUTHOR INFORMATION

Corresponding Authors

*P.L. Tel: +86-571-8795-2772. Fax: +86-571-8795-2772. E-mail: [email protected]. *W.-J.W. Tel: +86-571-8795-2772. Fax: +86-571-8795-2772. Email: [email protected]. ORCID

(12)

Wen-Jun Wang: 0000-0002-9740-2924

−1

Therefore, kapp should possess a linear relationship with {([Au]0)/([N]0)}−p. The p value of 0.6 was used for eq 12, which is estimated from the literature values for dendrimer−metal nanocomposites consisting of surface amino groups.66 Equation 12 fits well with the experimental results as shown in Figure 4a. If the same [Au]0/[N]0 is used while the shell thickness of the SC is different, eq 11 can be rewritten as,

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21376211, 21536011, and H


Article


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21420102008) and the Chinese State Key Laboratory of Chemical Engineering at Zhejiang University (Grants SKLChE-15D03 and SKL-ChE-14D01).

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