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Understanding of the Oxidation Behavior of Benzyl by Peroxymonosulfate via Nanotubes Activation Jiaquan Li, Mengting Li, Hongqi Sun, Zhimin Ao,* Shaobin Wang,* and Shaomin Liu*

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ABSTRACT: Selective oxidation of benzyl alcohol (BzOH) into benzaldehyde (BzH) is very important in synthetic chemistry. Peroxymonosulfate (PMS) is a cheap, stable, and soluble solid oxidant, holding promise for organic oxidation reactions. Herein, we report the catalytic PMS activation via carbon nanotubes (CNTs) for the selective oxidation of BzOH under mild conditions without other additives. A remarkable promotion of BzH yield with a selectivity over 80% was achieved on modified CNTs, i.e., O-CNTs via the radical oxidation process, and the oxygen functionalities for were comprehensively investigated by experimental study and theoretical exploration. To understand the different surface oxygen species on CNTs for the activation of PMS, density functional theory (DFT) calculations were performed to investigate the adsorption behavior of PMS on various CNTs. The electrophilic oxygen was identified as the electron captor to activate PMS by O−O bond cleavage to •− •− • •− form SO5 and SO4 radicals. The nucleophilic carbonyl groups can also induce a cycle to generate OH and SO4 radicals, but phenolic hydroxyl groups impede the radical process with antioxidative functionality. The carbocatalysis-assisted PMS activation may provide a cheap process for the selective oxidation of into or . The insight achieved from this fundamental study may be further applied to other organic syntheses via selective oxidation. KEYWORDS: carbon nanotubes, peroxymonosulfate, selective oxidation, radical process, oxygen functionalities

■ INTRODUCTION attention in the oxidation of alcohols and can afford high 11 Selective oxidation of alcohols into aldehydes or ketones is one / yields. Nevertheless, PMS in these cases of the most important reactions in the synthesis of organic only serves as the precursor to generate the actual oxidizing 1 agents through the reaction between PMS and some additives compounds. Benzyl alcohol (BzOH) oxidation has attracted − such as ketones,12 o-iodoxybenzoic acid,13 15 2-iodoxybenzen- extensive attention because the produced benzaldehyde (BzH) 16,17 − −18 can serve as a versatile intermediate in the manufacture of sulfonic acid, and Cl /Br instead of using as an oxidant many fine chemicals.2 A variety of reaction routes have been in the alcohol oxidation. Recently, it has been reported that Downloaded via GUANGDONG UNIV OF TECHNOLOGY on June 1, 2020 at 00:31:57 (UTC).

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. developed based on different catalysts and oxidants. Stoichio- PMS can be activated by carbon catalysts and present superior metric metal oxides (chromates and permanganates) and activity in advanced oxidation processes (AOPs) for aqueous contaminant degradation. Metal-free carbon materials such as peroxides (hydrogen peroxide and t-butylhydroperoxide) are 3−5 carbon nanotubes (CNTs), reduced oxides, and frequently employed as oxidants, where the former oxidants nanodiamonds can afford high activity in phenol degradation cause pollution by the nature of heavy metals and the latter under mild conditions, and the carbonyl groups would suffer from the poor stability in storage and transportation. In facilitate the electron transfer from an sp2 carbon network to view of green chemistry, molecular oxygen as an oxidant has − PMS to generate highly reactive radicals.19 24 However, there economical and environmental advantages over other oxidants. is still a lack of systematic investigation on the catalytic roles of Noble metals such as Au and Pd can catalyze BzOH oxidation other oxygenated functional groups in PMS activation with with high efficiency under moderate conditions, but large-scale applications are hampered by the scarcity and high cost of the catalysts.6,7 Transitional-metal-8,9 and nanocarbon10-based Received: December 6, 2019 catalysts are developed as substitutes for noble metals, but Revised: January 20, 2020 high reaction temperatures or extra additives/co-catalysts are Published: February 11, 2020 the major issues impeding the application of O2 as an oxidant. − Peroxymonosulfate (PMS, HSO5 ), as a chemically stable and inexpensive solid oxidant, has received continuous

© 2020 American Chemical Society https://dx.doi.org/10.1021/acscatal.9b05273 3516 ACS Catal. 2020, 10, 3516−3525 ACS Catalysis pubs.acs.org/acscatalysis Research Article solid experimental evidence. In addition, to the best of our Table 1. Selective Oxidation of Benzyl Alcohol by Various a b knowledge, the feasibility and efficiency of carbocatalyst- Catalysts , activated PMS in the selective oxidation of BzOH have never BzOH conversion BzH selectivity BzH yield been reported. entry catalyst (%) (%) (%) In this paper, we present a novel process of selective 1 14.7 6.8 1.0 oxidation of BzOH with CNT-activated PMS as the terminal 2 MWCNT 22.2 62.6 13.9 oxidant. PMS was activated by annealed O-CNTs under 3 O-CNT 39.1 58.3 22.8 benign conditions without any additives for BzOH oxidation. 4 OCNT-400 47.0 80.9 38.1 Experimental and theoretical studies on the catalytic roles of 5 OCNT-600 57.9 72.2 41.8 different surface oxygen species on CNTs, including carbonyl, 6 OCNT-800 57.6 80.1 46.2 phenolic hydroxyl, and carboxylic acids and electrophilic 7b OCNT-800 59.2 79.1 46.9 peroxide/superoxide, were comprehensively conducted. The fi 8 OCNT- 60.3 75.3 45.4 oxidizing species responsible for BzH formation were identi ed 1000 in detail. 9 Pt@Al2O3 17.9 71.2 12.8

10 Fe3O4 10.9 21.7 2.4 ■ RESULTS AND DISCUSSION a Reaction conditions: 10 mg of catalyst, 0.2 mmol BzOH, 0.22 mmol Commercial multiwalled carbon nanotubes (MWCNTs) were PMS, 10 mL of acetonitrile/water (1:1, volume ratio), 50 °C, 5 h. first sonicated in a mixture of nitric acid and sulfuric acid to bThe dissolved oxygen in the solvent was removed before reaction by remove metal residuals and introduce oxygen species onto the continuous N2 bubbling, and the reaction was carried out under N2 carbon surface to obtain O-CNT. Then, the O-CNT was protection. ff annealed in N2 at di erent temperatures between 400 and ° 2,26 1000 C for the removal and redistribution of surface oxygen typical selective oxidation by O2. Without catalyst addition, groups. The samples obtained are noted as OCNT-400, only 14.7% conversion of BzOH and a very minimal BzH yield OCNT-600, OCNT-800, and OCNT-1000. According to (1%) were observed (entry 1) through the self-activation of transmission electron microscopy (TEM) images (Figure S1), PMS. After adding MWCNT, BzOH conversion and BzH yield both O-CNT and OCNT-800 are rich in crooked tubular were increased, but to a very limited degree. The presence of segments and amorphous carbon fragments on the outer layer oxygen species on the carbon surface (O-CNT) immediately of the tubes, contributing to the structural and topological conferred the catalytic system a higher reaction efficiency up to fl defects. The ratio ID/IG from Raman spectra (Figure 1)re ects 22.8% BzH yield, due to the generation of active oxygen species on the CNT surface. After annealing of O-CNT, the catalytic activity of the O-CNT was further enhanced and the BzH selectivity/yield was significantly increased with the highest values up to 80.1/46.2% for OCNT-800 (at an annealing temperature of 800 °C). To shed some light on the reaction pathway, an oxygen-free process was conducted by fl removing the dissolved oxygen from the solvent via owing N2 through the reaction solution (entry 7). There was no decline in BzH yield in O2-free solution, indicating that the activation of PMS and the selective oxidation process are independent of the dissolved O2. In comparison to metal-free CNT catalysts, a noble-metal material (commercial Pt@Al2O3, 1 wt % of Pt, entry 9) and a transitional-metal oxide (commercial Fe3O4, entry 10) were tested under the similar reaction conditions, but their oxidation efficiencies were much lower than the annealed CNTs. Notably, neither benzoic acid nor other byproducts were found in the above cases according to gas chromatography−mass spectrometry (GC−MS) results, in- Figure 1. Raman spectra of pristine MWCNT, O-CNT, and various dicating that BzOH was either partially oxidized into BzH or annealed O-CNTs. fully oxidized into CO2/H2O in this reaction system. Previous studies reveal that carbon-based catalysts can the defectiveness of CNT samples,25 and the I /I value activate PMS to generate hydroxyl radicals (•OH) and sulfate D G •− increased from 0.97 to 1.57 after the oxidation of original radicals (SO4 ) for the degradation of aqueous organics. A MWCNT due to acid etching and tube shortening. The ID/IG nonradical process also exists in AOPs through adsorption/ value further increased as the annealing temperature increased, chemical bonding between the organic substrates/O−O bond 2 and ID/IG of OCNT-1000 can reach 1.91. The results indicate of PMS and sp -hybridized carbon network after N heteroatom that the high-temperature annealing treatment introduces doping, where the carbon catalyst acts as a bridge to enable the numerous defect sites from the decomposition of oxygen electron transfer from the organic compounds to PMS.27 The • •− •− groups. radical generation routes of OH, SO4 , and SO5 from PMS − •− Catalyst performance tests were conducted in aqueous are presented in eqs 1 4. The SO4 radicals are reported to acetonitrile to afford adequate solubility for both aromatic have a higher oxidation potential (2.5−3.1 V) than •OH (2.7 •− 28 •− compounds and PMS. Table 1 lists the catalytic performances V) and SO5 (1.1 V). In addition, SO5 radicals can •− 1 of various catalysts in BzOH oxidation by PMS. The reaction produce SO4 radicals and singlet oxygen ( O2) by self- ° 28−30 • •− temperature was controlled at 50 C, much lower than the decomposition (eq 5). The evolution of OH/SO4

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• ● •−⧫ Figure 2. EPR spectra of O-CNT- and OCNT-800-catalyzed PMS activation in the presence of (a) DMPO (DMPO- OH , DMPO-SO4 ) and (b) TMP. (c, d) Quenching effects on the selective oxidation of benzyl alcohol. Reaction conditions: 10 mg of OCNT-800, 0.2 mmol BzOH, 0.22 mmol PMS, 10 mL of acetonitrile/water (1:1, volume ratio), 50 °C, 5 h.

1 radicals (Figure 2a) and O2 (Figure 2b) during the reactions when ethanol was added, with only 2% BzH yield remained at was recorded by in situ electron paramagnetic resonance 100 times of ethanol dosage. The quenching results reveal that •− fi (EPR) using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and SO4 radicals with a high oxidative potential signi cantly 2,2,6,6-tetramethyl-4-piperidinol (TMP) to trap the radicals contribute to the selective oxidation of BzOH into BzH − and singlet oxygen, respectively.31 34 The peak intensities of whereas the radical formation processes via eqs 1−4 suggest • •− the OH/SO4 radicals and singlet oxygen induced by CNTs that the electron transfer between PMS and carbocatalysts followed the sequence of OCNT-800 > O-CNT > free PMS, plays a pivotal role in radical generation by either electron gain demonstrating that the annealed O-CNT produced more or loss. The oxygenated functional groups and defect sites on 1 2 radicals and O2 species than the O-CNT without annealing. CNTs can break the inertness of the highly stable sp graphitic −−• 2− structure and act as catalytic active sites. A wide range of HSO54+→ e OH + SO (1) oxygen species, including ketonic carbonyl (CO), phenolic −− − •− hydroxyl (C−OH), and carboxylic acid (COOH) groups, HSO+→ e OH + SO 54 (2) coexist on CNTs.37 In the view of electroaffinity, there are •−−•2 − electrophilic (peroxides and superoxides) and nucleophilic OH+↔+ SO OH SO 4 4 (3) (e.g., CO) oxygen species on carbocatalysts.38,39 The − − −− + •− electrophilic peroxide (O 2 ) and superoxide (O ) species HSO−→ e H + SO (4) 2 2 55 are electron-deficient and tend to attack electron-rich •− •− •−1 •− molecules. The electron-rich CO groups can serve as an 2SO→→+ SO O SO O 2SO (5) 542424 electron donor. In general, both nucleophilic CO and To identify the intrinsic oxidizing components that are electrophilic groups can activate PMS to generate reactive responsible for the conversion of BzOH and the formation of radicals through electron transfer between PMS and oxygen BzH, we further conducted a series of quenching experiments. functional groups. Meanwhile, in the field of polymer materials, Ethanol and tert-butanol (TBA) were adopted as quenching it is known that hindered phenolic antioxidants are able to agents owing to their different scavenging abilities. Ethanol is prevent the degradation of polymers since the phenolic groups • •− 40,41 used to quench OH and SO4 radicals with high reaction can give H atoms to stop the free-radical chain reaction. rates.35,36 tert-Butanol (TBA) is an effective •OH scavenger Therefore, phenolic hydroxyl groups on a carbocatalyst may ff •− 36 but less e ective in scavenging SO4 . The quenching possess similar functionality to capture the generated radicals reaction rate constants for the two scavengers are summarized and impede the selective oxidation of BzOH. in Table S1. A comparison of quenching effects in low (1 equiv In this study, the iodometric titration method was used to of PMS) and high (100 times of PMS) dosages of the quantitatively determine the amount of surface electrophilic quenching agents on the oxidation efficiency with OCNT-800 oxygen species from the peroxides or superoxides on the CNT 42,43 is displayed in Figure 2c,d. The dosage of TBA did not cause a catalysts. The derived amount of I2 by Na2S2O3 titration severe decrease of the oxidation efficiency, and there is only can quantitatively reflect the presence of the electrophilic 10% loss of BzH yield with 100 times of TBA loading, oxygen species (Supporting Information). Such an iodometric indicating that the •OH radicals play a minor role in the BzH oxidation method can be specifically used to deactivate the formation. A substantial decrease in BzH yield was observed electrophilic oxygen groups, which will be discussed in the

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Table 2. Concentrations of Surface Oxygen Groups Determined by XPS Survey and Electrophilic Oxygen Determined by Iodometric Titration

oxygen species (% of total oxygen content) catalyst total oxygen (fresh, atom %) C−OOC−OCO total oxygen (used, atom %) electrophilic oxygen (10−4 mol/g cat.) MWCNT 0.83 32.4 47.9 19.7 1.82 0.9 O-CNT 7.43 42.8 45.8 11.4 7.33 1.3 OCNT-400 4.96 49.3 36.7 14.0 6.30 2.8 OCNT-600 3.06 51.3 35.9 12.7 4.90 5.0 OCNT-800 2.23 47.6 41.1 11.3 4.88 8.7 OCNT-1000 2.58 49.2 44.2 6.5 4.75 3.5 reaction mechanism. The contents of the surface oxygen before and after the catalytic reaction and the amount of electrophilic oxygen derived from pristine MWCNT and O-CNTs annealed at different temperatures are presented in Table 2. After acid etching/oxidation, a remarkable increase of the total oxygen content and electrophilic oxygen was observed on MWCNT. As the annealing temperature increased, the total oxygen content reduced due to the decomposition of the oxygen species with different thermal stabilities. Meanwhile, the amount of electrophilic oxygen increased with rising annealing temperature because the desorption of oxygen groups (carboxyl, lactone, ether, etc.) in thermal treatment leads to the exposure of highly active defect sites, on which the oxygen species including electrophilic oxygen can be formed in an oxygen-containing atmosphere. Notably, the content of the electrophilic oxygen (8.7 × 10−4 mol/g cat.) was observed to be the highest on OCNT-800 and at a lower amount on Figure 3. Selective oxidation of benzyl alcohol on CNT catalysts OCNT-1000. At the temperature exceeding 1000 °C, the before and after the elimination of surface electrophilic oxygen species by KI. Reaction conditions: 10 mg of catalyst, 0.2 mmol BzOH, 0.22 phenolic hydroxyl and carbonyl groups started to decompose ° 44 mmol PMS, 10 mL of acetonitrile/water (1:1, volume ratio), 50 C, 5 accompanied by the vanishing of some structural defects. h. The deconvolution of O 1s X-ray photoelectron spectroscopy (XPS) (Table 2 and Figure S2)ofdifferent annealed O-CNT  −   − − group. The three target oxygen groups (C O, C OH, and shows the coexistence of C O, O C O, and C O groups COOH) on O-CNT were deactivated by phenylhydrazine 37,42  on CNTs. OCNT-1000 contained less C O groups but (PH), benzoic anhydride (BA), and 2-bromo-1-phenyl-  − − more O C O and C O groups than OCNT-800, indicating ethanone (BrPE), respectively, to obtain the relevant that the defect sites on OCNT-1000 were more likely to derivatives through highly specific reactions between the  − −  generate O C O/C O instead of C O. As a result, the titrants and oxygen groups (Scheme S1).37 The derived  regeneration of C O and electrophilic oxygen are impeded samples were referred to as OCNT-PH, OCNT-BA, and on OCNT-1000. OCNT-BrPE. Meanwhile, p-benzoquinone, 1,2-dihydroxyben- The electrophilic oxygen groups on MWCNT, O-CNT, and zene, and 1,4-dicarboxybenzene were used to mimic CO, OCNT-800 were then selectively deactivated (MWCNT-KI, C−OH, and COOH in the absence of catalysts. The selective OCNT-KI, and OCNT-800-KI) by iodometric oxidation to oxidation results are listed in Table 3. After the CO groups unveil the catalytic role of the electrophilic oxygen in PMS on O-CNT were deactivated (entry 2), both the BzOH activation. The catalytic performances of those samples before conversion and BzH yield decreased evidently. Meanwhile, and after the titration are displayed in Figure 3. Though the adding p-benzoquinone as the mimic of CO leads to a activity of MWCNT, O-CNT, and OCNT-800 varied greatly considerable increase in conversion/yield (entries 5 and 6), in PMS activation, the samples showed consistent perform- confirming that CO played an essential role as the electron ances after the electrophilic oxygen groups were eliminated. donor in PMS activation. The removal of C−OH on O-CNT The BzOH conversion and BzH yield dramatically decreased slightly enhanced the conversion/yield (entry 3), while the to approximately the same level after the titration treatment for additional C−OH mimics lead to a reduced BzH yield (entry the three samples. Notably, OCNT-800-KI gave even slightly 7). To further confirm the role of C−OH groups, a larger lower activity than OCNT-KI, agreeing well with the peak amount of C−OH was produced on O-CNT by wet chemical  − − intensity variation in EPR (Figure S3). We can infer that the reduction using NaBH4. The conversion of O C O into C enhanced catalytic performance after annealing of O-CNT OH groups (XPS, Figure S4) leads to a significant decrease in mainly originates from the generation of electrophilic oxygen the BzH yield from 22.8 to 6.9% on O-CNT (Figure S5). ff •− species, which contribute to the e ective production of SO5 Thus, it can be inferred that the phenolic hydroxyl groups are •− and SO4 radicals. impediments during the radical evolution. As for carboxyl The catalytic behavior of the CO, C−OH, and COOH groups, entries 4 and 8 showed a subtle but still beneficial groups in the oxidation of BzOH was further explored by effect of COOH on the reactions, which might be owing to the selective deactivation and small-molecule mimics of each CO in COOH groups. The EPR tests showed that the

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Table 3. Selective Oxidation of Benzyl Alcohol by O-CNT using PMS, and our catalyst showed better recovery ability.21,45 a Derivatives and Small-Molecule Mimics The XPS survey spectra and O 1s deconvolution demonstrate that the surface oxygen content on the catalyst increased after BzOH conversion BzH selectivity BzH yield entry catalyst (%) (%) (%) each run (Figure S7b). The oxidative environment during the reaction mainly induced the regeneration of C−O and OC− 1b OCNT-c 42.6 52.3 22.3 O groups on the carbon surface, while the CO groups 2 OCNT-PH 22.2 25.4 5.6 showed no obvious change (Figure S8). The determination of 3 OCNT-BA 48.2 55.6 26.8 electrophilic oxygen on the catalysts after each run by the 4 OCNT- 42.5 49.2 20.9 BrPE iodometric titration (Figure S7b) reveals almost a total c consumption of the electrophilic oxygen from the fresh catalyst 5 14.7 6.8 1.0 − − d × 4 × 4 6 CO 24.6 50.6 12.5 (8.7 10 mol/g cat.) to the fourth run (0.7 10 mol/g e − cat.). After the annealing recovery treatment, the amount of 7 C OH 14.1 7.0 0.8 − f × 4 8 COOH 15.4 9.1 1.4 electrophilic oxygen increased back to 5.2 10 mol/g cat. It aReaction conditions: 10 mg of catalyst, 0.2 mmol BzOH, 0.22 mmol can be inferred that the electrophilic oxygen groups were PMS, 10 mL of acetonitrile/water (1:1, volume ratio), 50 °C, 5 h. bO- consumed during the reaction, which is responsible for the fast CNT underwent the solvent-only treatment (OCNT-c) to control deactivation of the catalyst. This consumption also confirms variables with the derivatives. cNo catalyst was loaded. dp- the occurrence of electron transfer from PMS to electrophilic Benzoquinone (0.4 mmol) was used to mimic carbonyl group. e1,2- oxygen. As a result, the electrophilic oxygen is reduced to Dihydroxybenzene (0.4 mmol) was used to mimic hydroxyl group. enable the radical process for the BzOH oxidation instead of f 1,4-Dicarboxybenzene (0.4 mmol) was used to mimic carboxyl the nonradical process in which the carbon catalyst serves as an group. electron bridge. The CO groups stay unchanged after the catalytic reaction because of a reversible conversion between radical signals induced by OCNT-PH/PMS were remarkably CO and C−O groups, thus forming a redox cycle.21,38,46 weakened compared to O-CNT/PMS (Figure S6). The other ff ff To fully understand the roles of di erent oxygen-containing two derivates showed no apparent di erence in radical functional groups in promoting the PMS activation, all possible generation compared to O-CNT, which also agrees with the − adsorption of PMS molecules (HSO5 ) on various CNT sites results in Table 3 that the catalytic activity of O-CNT was not with the lowest energy configuration is described in Figure 4. profoundly altered after C−OH/COOH deactivation. The results provide convincing experimental verifications to the above-mentioned hypothesis, identifying the roles of carbonyl and phenolic hydroxyl groups in the PMS activation and the selective oxidation processes. The proposed mechanism of CNT-facilitated PMS activation to transform BzOH to BzH is − summarized in Scheme 1. HSO5 can attract electrons from

Scheme 1. Proposed Mechanism of PMS Activation on OCNT-800 and the Oxidation of Benzyl Alcohol into Benzaldehyde

Figure 4. Structure of PMS molecules adsorbed on pristine CNT and CNT with different oxygen functional groups: (a) PMS on pristine CNT, (b) PMS on pristine CNT edge, (c) PMS on O-CNT with −OH, (d) PMS on O-CNT with −COOH, (e) PMS on O-CNT with −CO, (f) PMS on O-CNT with −OO, and (g) PMS on O-CNT • − − − electron-rich CO groups on CNTs to generate OH and with O O . The gray, white, red, and yellow atoms are C, H, O, •− and S, respectively. SO4 radicals. The electrophilic peroxide and superoxide species on CNTs are able to gain electrons from HSO − and •− 5 produce SO5 radicals, which can be further transformed into The corresponding data of the adsorption energy and bond •− − − SO4 radicals. Thus, the selective oxidation of BzOH to BzH length (O O: lO−O and O H: lO−H) are listed in Table 4. For is significantly enabled by the radical process. −OH, −COOH, and −CO groups, their behavior in ff − The stability of OCNT-800 in successive tests was a ecting the O ObondinPMS(HO-OSO3)was investigated, and both the BzOH conversion and BzH yield investigated. For electrophilic −O−O−/−OO groups on decreased after each run (Figure S7a). The BzH yield CNTs, we focused on their abilities to stretch and cleave the fi − decreased from 47.4% in the rst run to 28.1% in the second O H in PMS (H-OOSO3) to echo the experimental results. run and further reduced to 5.6% after the fourth run. Then, the As summarized in Table 4, the bond length lO−O of free PMS ° 20 catalyst after the fourth run was annealed at 800 C molecules is 1.326 Å, whereas lO−O was remarkably stretched (Recovered OCNT-800) and a full recovery was observed when PMS was adsorbed on pristine CNT and nucleophilic O- (47.6% BzH yield). The deactivation of the carbocatalyst was CNT, implying the potential to be transformed into HO and also found in aqueous contaminant degradation processes SO4. The adsorption energies of PMS on CNTs ranged from

3520 https://dx.doi.org/10.1021/acscatal.9b05273 ACS Catal. 2020, 10, 3516−3525 ACS Catalysis pubs.acs.org/acscatalysis Research Article E − − Table 4. Adsorption Energy ( ads) of PMS on CNT, the O PMS activated by nucleophilic O-CNT, the O O bond l − O Bond Length ( O−O) of PMS (HO-OSO3), and the O H cleavage and the energy barrier Eb follow the order of OH > l ff  Bond Length ( O−H) of PMS (H-OOSO3)inDi erent COOH > C O. The highest Eb of 0.91 eV in the HO-CNT@ Density Functional Theory (DFT) Models Shown in Figure PMS system demonstrates the ultralow degradation rate of 4 PMS for radical formation. Meanwhile, it was found that in all • fi of these three systems, the generated OH was directly bonded con guration Eads (eV) lO−O (Å) lO−H (Å) to the carbon atoms on the CNT, implying the possible free PMS 1.326 1.037 quenching of •OH by CNT itself. The binding between •OH (a) −1.17 1.434 1.004 and CNT results in the decrease of the yield of free •OH (b) −1.13 1.434 1.008 radicals, which explains the ultrashort lifetime of •OH in (c) −1.66 1.466 0.997 •− 49 comparison to SO4 . Particularly, in the case of the HOOC- − •− (d) 1.07 1.458 1.006 CNT@PMS system, we found that SO can be feasibly − 4 (e) 1.12 1.426 1.005 bonded to the α-C (the C atom directly bonded to −COOH) − (f) 1.13 1.425 1.014 of the COOH function to form C−SO in geometry − 4 (g) 1.25 1.438 0.998 optimization. Thus, two pathways of PMS activation by COOH function should be taken into consideration: pathway − − − − 1.07 to 1.66 eV, among which the maximum Eads of PMS → − − 1, HSO5 + CNT C OH + C SO4; pathway 2, HSO5 + − •− on HO-CNT was 1.66 eV with the longest lO−O of 1.466 Å. It → − CNT C OH + SO4 . The calculated Eb values for should be noted that in the case of HO-CNT, the H atom in − − breaking the O O bond are 0.35 and 0.40 eV in pathways 1 the OH group was separated from the O atom and was and 2, respectively. The energy barriers of the two pathways bonded to the PMS molecule, while the −OH group on CNT ff  are slightly di erent, manifesting that they can occur was transferred to the C O group, which helps to explain the simultaneously on the HOOC-CNT. In addition, the highest Eads and longest lO−O in the HO-CNT system. This − − desorption of the SO4 group from C SO4 generated in process implies that the PMS activation on OH is not feasible pathway 1 was investigated to evaluate the possibility of the from the thermodynamic perspective according to a previous 22 regeneration of reactive SO4 radicals (MS to FS for the case of report. − COOH in Figure 5). The extremely high Eb of 1.48 eV for Particularly, there is no obvious change in lO−O of PMS •− this process implies that the SO4 regeneration is impossible. when adsorbed by −O−O−/−OO on the CNTs with •− Thus, the generated SO4 radicals from PMS is easily comparable adsorption energy to pristine CNT, implying a α ff − − inactivated by the -C of the COOH function. Therefore, the di erent activation mechanism for PMS activation in O COOH groups on CNT do not play a dominant role in O−/−OO systems. Thus, based on the experimental results,  − − activating PMS. The rather low Eb of 0.53 eV in O C-CNT@ the cleavage of O O and O H bonds of PMS on nucleophilic PMS system manifests that the activation of PMS is efficient to and electrophilic O-CNTs was investigated using transition- − fi break the O O bond and to produce free radicals, in good state searching method. The possible energy potential pro les consistency with the experimental observation. For electro- of these reactions are shown in Figure 5. The reaction energy philic −O−O− and −C−OO, the H−O bond is broken to •− produce SO5 , and the energy barrier was calculated to be 0.39 and 0.11 eV respectively, implying that the nucleophilic −CO and electrophilic groups can promote the activation of ff PMS but in di erent mechanisms. The ultralow Eb value in electrophilic groups manifests the ultrafast formation rate of •− 1 SO5 , which can subsequently transfer into O2. These simulation results fit perfectly with our experimental observation. Finally, the effects of different reaction conditions (reaction time, temperature, PMS dosage, catalyst dosage, and solvent) on catalytic efficiency were studied using OCNT-800 as a representative catalyst (Table 5). At increased reaction time (entries 1, 2, 3), temperature (entries 2, 4, 5), and PMS loading (entries 2, 9, 10), both the BzOH conversion and BzH yield increased but the BzH selectivity decreased due to the Figure 5. Reaction pathway of the activation of PMS on pristine CNT overoxidation. Notably, the BzH yield reached 56.9% with and CNT with different oxygen functional groups, where IS, TS, MS, 92.2% BzOH conversion when the PMS loading was doubled. and FS represent the initial structure, transition structure, On the other hand, doubling the catalyst dosage (entry 11) intermediate product structure, and final structure, respectively. slightly increased the conversion, selectivity, and BzH yield. The reaction is also strongly influenced by the composition of ff barriers Eb corresponding to di erent paths are displayed in the solvent. Acetonitrile/water turned out to be the best Figure S9. For the pristine CNT, despite the O−O bond solvent (Table S2), and the catalytic efficiency was enhanced stretching, the E value of the O−O bond cleavage to produce with a higher proportion of acetonitrile in the solvent (entries •− b SO4 was 1.64 and 1.72 eV at bulk carbon and edge sites, 2, 6, 7), which is possibly due to the limited aqueous solubility respectively. As previously reported, the critical energy barrier of BzOH and the hydrophobic nature of CNTs.50 A higher of 0.9 eV would determine the possibility of a chemical acetonitrile/water ratio facilitates the adsorption of BzOH and reaction at room temperature.47,48 Thus, it is difficult to PMS on CNTs’ surface for further reactions, whereas the generate radicals through PMS activation by pristine CNT. For overhigh acetonitrile/water ratio (entry 8) leads to poor

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a Table 5. Selective Oxidation of BzOH under Various Reaction Conditions

entry temperature (°C) time (h) PMS/BzOHb AN/Wc BzOH conversion (%) BzH selectivity (%) BzH yield (%) 1 50 2 1.1 1 44.3 73.4 32.5 2 50 5 1.1 1 57.6 80.1 46.2 3 50 10 1.1 1 64.5 70.5 45.5 4 30 5 1.1 1 33.4 70.5 23.5 5 70 5 1.1 1 69.7 72.6 50.6 6 50 5 1.1 0.3 60.1 58.4 35.1 7 50 5 1.1 3 57.1 84.3 48.1 8 50 5 1.1 5 24.8 55.3 13.7 9 50 5 0 1 3.5 13.1 0.4 10 50 5 2 1 92.2 52.5 56.9 11d 50 5 1.1 1 57.9 83.2 48.2 aOCNT-800 (10 mg) was employed as catalyst and 0.2 mmol BzOH was added in 5 mL of solvent. bMolar ratio of PMS and BzOH. cThe volume ratio of acetonitrile (AN)/water (W). d20 mg of catalyst was loaded. solubility of PMS and hinders the reaction. Comparing the (CNTs/PMS) for free-radical generation can also be applied to results in entry 7 with reported work via different reaction other organic syntheses via the selective oxidation scheme. systems (summarized in Table S3),18,26,51,52 BzOH can be However, awareness should be raised on the correlation oxidized by O2 with precious metals or nanocarbon catalysts to between the oxidation potential of the radicals and the energy obtain over 90% BzOH conversion/BzH selectivity at barrier of different oxidative reactions to ensure the selective temperatures larger than 100 °C. Peroxide oxidants, including transformation without overoxidizing. ff H2O2 and PMS, can a ord a high BzH yield (>87%) in a homogeneous environment with extra additives at low ■ EXPERIMENTAL SECTION temperatures. Herein, we report a much more favorable reaction route to be performed under benign conditions Preparation of Annealed CNT Samples. Multiwalled without any additives, for which the reaction system can still be carbon nanotubes (MWCNTs) were purchased from Time- optimized to further improve the BzH yield. snano, Chengdu, China. The length, inner diameter, and purity of the MWCNTs are 10−30 μm, 10−20 nm, and 95%, ■ CONCLUSIONS respectively. The other chemicals used in this study were supplied by Sigma-Aldrich. To prepare the annealed CNTs, We presented a novel approach for the selective oxidation of first, 5 g of MWCNT was mixed with 250 mL of an acid BzOH to BzH using CNTs/PMS and explored the reaction − mixture composed of 1:3 volume ratio of HNO3 (65 68%) pathways. The catalytic roles played by surface carbonyl, and H SO (95−98%), which was sonicated at 50 °C for 5 h. phenolic hydroxyl, carboxylic acid, and electrophilic peroxide/ 2 4 Then, the acid mixture was diluted and cooled down to room superoxide groups on CNTs were fundamentally investigated temperature. The oxidized MWCNT was filtered and washed by selective deactivation and small-molecule mimics. Electro- with ultrapure water until the pH of the filtrate reached 7. The philic oxygen species on CNTs were quantitatively determined precipitate was dried at 60 °C for 48 h and ground to obtain and correlated to be the consumable active sites for the •− •− the oxidized carbon nanotubes (O-CNT). Then, O-CNT (0.5 generation of SO5 and SO4 radicals. The thermal annealing g) was annealed in a N2 atmosphere at 400, 600, 800, or 1000 treatment conferred highly active defect sites on CNTs for the ° ° attachment of electrophilic oxygen. The carbonyl groups C for 1.5 h at a heating rate of 5 C/min to obtain OCNT- served as electron donors to activate PMS into •OH and 400, OCNT-600, OCNT-800, and OCNT-1000, respectively. •− Iodometric Titration of Electrophilic Oxygen on SO4 radicals. The phenolic hydroxyl groups on CNTs impeded the BzOH oxidation due to the antioxidation nature . A CNT sample (50 mg) was added to a mixture that inhibits the radical process. Density functional theory containing 5 mL of KI (40 g/L), 5 mL of H2SO4 (0.05 mol/L), (DFT) calculations confirm that the nucleophilic −CO and two drops of (NH4)6Mo7O24 (30 g/L). The reactions groups can cleave O−O bonds in PMS molecules, while the between electrophilic oxygen on the CNT surface and KI are − electrophilic peroxide/superoxide groups would generate shown in eqs 6 7. For convenience, the amount of detected I2 •− − was designated as the total amount of electrophilic oxygen. SO5 by the O H-bond cleavage from PMS. However, •− − After 5 min sonication followed by 1 h stirring at room SO4 generated by the O O-bond cleavage is easily − quenched by the α-C atom on the carboxylic acid group, temperature in the dark, I was oxidized to I2. Then, the resulting in a nonsignificant effect of the carboxylic acid groups precipitate was filtered out and washed with ultrapure water to on the PMS activation. The highly active radicals generated remove the adsorbed ion and dried at 60 °C for 12 h. All of the from PMS dominantly contributed to the selective oxidation of filtrate was collected and then 0.3 g of the soluble starch fi BzOH to BzH with over 80% selectivity. The annealed O-CNT indicator was added to the solution. The I2 in the ltrate was × −4 at 800 °C possessed the largest amount of electrophilic oxygen titrated by Na2S2O3 (2 10 mol/L), as demonstrated in eq groups and thus afforded the highest BzH yield in this work. In 8, until the blue color disappeared. The total concentration of contrast to the conventional synthesis using precious metals as the electrophilic oxygen (mol/g cat.) on the sample was the catalysts, this study provides a new perspective for the calculated by eq 9, where c (mol/g), V (mL), and m (g) selective oxidation of organics with carbon-based catalysts and represent the electrophilic oxygen concentration, volume PMS oxidant under benign conditions without other consumption of the Na2S2O3 solution, and mass loading of promoters or expensive additives. The current catalytic system the CNT sample, respectively.

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2−−2 O2 ++ 2KI 2H24 SO → O + 2KHSO42 ++ I H 2O instrument to record the evolution of free radicals during the (6) selective oxidation, and the data were analyzed by Xeon software. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) and − 2O224++ 6KI 6H SO 2,2,6,6-tetramethyl-4-piperidinol (TMP) were used as spin- 2− trapping agents. →+O 6KHSO42 ++ 3I 3H 2O (7) Computational Methods. To understand the role of different surface oxygen species on CNTs, including ketonic +→+ I2223 2Na S O 2NaI Na 246 S O (8) carbonyl (−CO), phenolic hydroxyl (C−OH), carboxylic − − − − −7 acid ( COOH), and electrophilic peroxide ( O O )/ c (electrophilic oxygen)=× 1 10 V/m (9) superoxide (C−OO), in the activation of PMS, density Selective Deactivation of Oxygenated Groups on O- functional theory (DFT) calculations were performed to CNT. In the preparation of OCNT-PH, 0.2 g of O-CNT, 0.5 g investigate the adsorption behavior of PMS on CNTs of phenylhydrazine (PH), and 100 μL of HCl (38%) were (including pristine CNT and O-CNTs). The spin-unrestricted ° DFT calculations were conducted in Dmol3 package using mixed in 10 mL of CHCl3 solvent and stirred at 60 C for 72 h fi general gradient approximation (GGA) with the Perdew− in a N2 atmosphere in the dark. The precipitate was ltered ° Burke−Ernzerhof (PBE) exchange−correlation functional.53 out, washed with CHCl3 several times, and then dried at 60 C An all-electron double numerical plus polarization (DNP) was for 24 h to obtain OCNT-PH. 54 For the synthesis of OCNT-BA, 0.2 g of O-CNT and 1 g of employed as the basis set. The convergence tolerance of energy was assumed at 10−5 Hartree (1 Hartree = 27.21 eV), benzoic anhydride (BA) were mixed in 10 mL of CHCl3 ° and the maximal allowed force and displacement were 0.002 solvent and stirred at 60 C for 24 h with N2 protection. The fi Hartree/Å and 0.005 Å, respectively. The DFT-D method precipitate was ltered out, washed with CHCl3 several times, ° within the TS scheme was used in all calculations to take the and then dried at 60 C for 24 h to obtain OCNT-BA. 55 OCNT-BrPE was prepared as follows: 0.2 g of O-CNT and van der Waals forces into account. In the simulations, a 0.5 g of 2-bromo-1-phenylethanone (BrPE) were mixed in 10 nonperiodic cluster model was used for (5, 5) single-walled carbon nanotubes (SWCNTs) with dangling bonds saturated mL of CHCl3 solvent and stirred at room temperature for 10 h fi by H atoms. It is noteworthy that SWCNT was adopted to in a N2 atmosphere in the dark. The precipitate was ltered ° simplify the model structure because the electronic properties out, washed with CHCl3 several times, and then dried at 60 C for 24 h to obtain OCNT-BrPE. of perfect MWCNTs are rather similar to these perfect SWCNTs and the number of CNT walls slightly affected the The wet chemical reduction of O-CNT was conducted using −  − adsorption.56 59 For the PMS molecule adsorbed on CNTs, NaBH4 to transform the surface C O into C OH groups. the adsorption energy E is defined as For the reduction, 0.1 g of O-CNT and 1 g of NaBH4 were ads dissolved in 10 mL of ethanol and stirred in a N atmosphere 2 EE=−+() E E at 50 °C for 12 h. The precipitate was filtered out, washed with ads PMS/CNT PMS CNT HCl and water, and then dried at 60 °C for 24 h to obtain where E , E , and E are the total energies of the OCNT-NaBH . PMS/CNT PMS CNT 4 PMS/CNT system, the isolated PMS molecule, and CNT in Catalytic Selective Oxidation of BzOH. In a typical the same slab, respectively. To further investigate the activation reaction process for the selective oxidation of benzyl alcohol, mechanism of PMS on different CNTs, linear synchronous 10 mg of catalyst was added into 10 mL of the solvent mixture transit/quadratic synchronous transit (LST/QST) tools in (acetonitrile/water = 1:1, volume ratio) in a 25 mL flask and Dmol3 package were used, which have been well validated to sonicated for 5 min. Then, 0.2 mmol BzOH and 0.22 mmol determine the structure of the transition state and the PMS (oxone, KHSO ·0.5KHSO ·0.5K SO ) were added to 5 4 2 4 minimum-energy reaction pathway.60 initiate the oxidation. The solution was stirred at 50 °C for 5 h. After the flask was cooled down, 0.2 mmol anisole was injected into the flask as an internal standard. A certain volume (1 mL) ■ ASSOCIATED CONTENT of the solution was withdrawn by a syringe, and the precipitate *sı Supporting Information was filtered by a Millipore film. The organic compounds in the The Supporting Information is available free of charge at solution were extracted by 6 mL of toluene for three times, and https://pubs.acs.org/doi/10.1021/acscatal.9b05273. the organic phase was collected. The composition of the products was analyzed by GC/MS using a 30 m × 0.25 mm × Experimental material, characterizations, and catalytic ff 0.25 μm HP-5MS capillary column and FID detector. results; TEM images; reaction rate constants of di erent Characterization Measurements. Transmission electron quenching agents and radicals; EPR spectra of OCNT- microscopy (TEM) imaging was conducted on a JEOL 2100 800 and OCNT-800-KI; and selective deactivation microscope. Raman spectra were obtained under ambient pathways (PDF) conditions on a Renishaw Raman spectrometer with a 785 nm laser beam. X-ray photoelectron spectroscopy (XPS) data were ■ AUTHOR INFORMATION obtained with a Kratos AXIS Ultra DLD system using Al Kα radiation. The base pressure was about 1 × 10−8 torr. The Corresponding Authors binding energy value was referenced to the C 1s line at 284.6 Zhimin Ao − Guangdong Key Laboratory of Environmental eV from defect-free graphite. Deconvolution of the O 1s Catalysis and Health Risk Control, School of Environmental spectra was performed using mixed Gaussian−Lorentzian Science and Engineering, Institute of Environmental Health and component profiles after subtraction of a Shirley background Pollution Control, Guangdong University of Technology, using XPSPEAK41 software. The electron paramagnetic Guangzhou 51006, China; orcid.org/0000-0003-0333- resonance (EPR) was conducted on a Bruker EMS-plus 3727; Email: [email protected]

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