Metal-Free Mesoporous SiO2 Nanorods as a Highly Efficient Catalyst

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Metal-Free Mesoporous SiO2 Nanorods as a Highly Efficient Catalyst for the Baeyer−Villiger Oxidation under Mild Conditions Xueyao Zhang, Honglei Yang, Guangxue Yang, Shuwen Li, Xiang Wang, and Jiantai Ma* State Key Laboratory of Applied Organic Chemistry (SKLAOC), Gansu Provincial Engineering Laboratory for Chemical Catalysis, College of Chemistry and Chemical Engineering, Lanzhou University, Tianshui South Road 222, Lanzhou, Gansu 730000, PR China S Supporting Information *

ABSTRACT: The development of efficient and environmentally friendly catalysts for oxidative reaction is of great importance in applied catalysis. In this work, a simple and environmentally benign approach for highly selective preparation of ε-caprolactone by oxidation of cyclohexanone has been carried out, which employed metal-free mesoporous silica (mSiO2) nanorods as catalyst under atmospheric pressure. The metal-free silica catalyst was applied for the first time in the Baeyer−Villiger (B−V) oxidation reaction. It showed efficient catalytic performance for the B−V oxidation of various cyclic ketones and aliphatic ketones with O2/benzaldehyde as oxidant. The catalyst could be easily separated from the reaction system by filtration and reused several times without significance loss of activity. Moreover, electron paramagnetic resonance (EPR) spectra of the reaction were obtained, indicating the existence of benzoyloxyl radical. The mechanism study of the reaction demonstrated that the super large surface area diluted the concentration of radicals and the adsorption of radicals could protect the radical species from inhibition. KEYWORDS: Metal-free, mSiO2 nanorods, Baeyer−Villiger reaction, EPR spectra·of mechanism study

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metal oxides except for stannide.10 While in the aldehydes/O2 system, it usually employs different metallic catalysts, including metal complexes,11 hydrotalcite,12 metallic oxides,13 and supported metal and metal oxides,14−16 because metal active sites could speed up the initiation of the radical chain reaction. However, these catalysts need a large amount of consumption of metal and are harmful to the environment due to metal pollution. On the other hand, the metal active sites would run off through the strong interaction with reactants. To avoid the drawbacks of metal, various substitute materials have been designed and applied to the B−V oxidation reaction. Carbon material (Ketjen Black) is one of the successful attempts,17 opening up the research on carbon materials for B−V oxidation.18 However, their poor dispersion in common solvents largely limits their further application. Therefore, the development of novel green and effective metal-free catalysts for the B−V oxidation reaction is still highly required. In the past several decades, silica materials have been excellent candidates as supports in many heterogeneous catalysts, on account of their large specific surface area, great dispersion in the majority of solvents, and favorable hydrothermal stability. The catalysts employing silica as supports have successfully catalyzed many oxidation reactions, such as ethylbenzene oxidation, alcohol oxidation,19 and oxidation of cyclic olefins.20 Silica materials have various morphologies in different dimensions, including one-dimensional (1D) nanowires and nanorods, 2D hexagonal MCM-41, and 3D cage-like

INTRODUCTION Baeyer−Villiger (B−V) oxidation reaction plays a significant role in synthesizing lactones or esters and has been extensively applied in synthesis of bioactive compounds, pharmaceuticals, and various fine chemicals.1,2 In previous reports, there are three kinds of oxidative systems used for B−V oxidation. A former, convenient method is conducted with organic peracidbased oxidants. The peracids directly oxidize the cyclohexanone to ε-caprolactone along with generating an equivalent of benzoic acid.3 The peracids as oxidants are usually expensive, hazardous, and shock sensitive, which limits their wide application in industry. To overcome these shortcomings, two protocols have been developed to provide greener and less costly oxidants for B−V oxidation. One of the preferred benign agents is hydrogen peroxide (H2O2). Regrettably, H2O2 is of poor stability and the generated water leads to the hydrolysis of the product. Besides, H2O2 possesses weak oxidizing capacity at low concentration, whereas it is dangerous at high concentration. The other strategy employs aldehydes and molecular oxygen as green oxidants, known as the Mukaiyama method. This strategy will be quite attractive for industrial application, due to the usage of air or O2 meeting both environmental and cost concerns. However, the system usually uses pure oxygen and sacrifices agents more over the stoichiometric ratio, and thus, energy-extensive consumption is unavoidable. Overall, considering the environment, safety, and cost, the aldehyde/O2 system shows more promising application potential for industry. B−V oxidation employed H2O2 always makes use of Sn containing mesoporous silica (mSiO2) or zeolites,4−7 silicates,8 and biological approaches9 as catalysts. There are seldom other © XXXX American Chemical Society

Received: November 10, 2017 Revised: March 16, 2018 Published: March 28, 2018 A

DOI: 10.1021/acssuschemeng.7b04167 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Table 1. Different Catalysts for the B−V Reactiona

SBA-16. Compared with the multidimension silica materials, the 1D silica materials own a larger contact area and large amount of pores, which can mitigate the volume effect and improve the cycling property.21 In brief, the 1D nanorods not only provide more active positions but also improve the transmission rate of electrons.22 Hence, the metal-free 1D silica materials are the perfect candidates to catalyze several environmentally friendly processes. Usually, silica is employed to be a catalytically inactive material and just to be a support. Silica materials that directly catalyze reactions, not used as supports, may avoid the lengthy synthetic steps and prevent the consumption of needless energy. Besides, silica materials usually can remain stable during the hydrothermal reaction process, and it is unnecessary to consider the loss of active sites. Up to now, direct catalysis of B−V oxidation by the silica materials has not been investigated. In this work, the metal-free mesoporous silica (mSiO2) nanorods catalysts were utilized for B−V oxidation with a green and efficient catalytic approach in the aldehydes/O2 oxidative system. The catalytic activity and reusability of the silica nanorods were systematically evaluated. Furthermore, the mechanism of the oxidation of cyclohexanone was proposed, and the controlled experiments were employed to verify the mechanism.

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entry 1 2 3 4 5 6 7 8 9 10 11

yieldb (sel.) (%)

catalyst MCM-41 SBA-15 SBA-16 HMS mSiO2-248 mSiO2-372 mSiO2-500 mSiO2-619 mSiO2-991 mSiO2-1486

77 73 81 74 36 84 93 >99 97 91 87

(>99) (>99) (>99) (>99) (>99) (>99) (>99) (>99) (>99) (>99) (>99)

a

Reaction conditions: cyclohexanone (2 mmol), benzaldehyde (2.0 equiv), dodecane (1 mmol), and catalyst (25 mg) in DCE (5 mL), 50 °C, 6 h, pumping air. bThe yield and selectivity were determined by GC-MS on the basis of the internal standard method (dodecane).

and selectivity reached over 99%. Notably, it was spontaneous for the B−V oxidation without catalyst in the oxidative system of O2/aldehyde, and the yield was 36% (entry 5). When the mSiO2-500 was selected as the best catalyst, amount of catalyst, temperature, solvent, and reaction time were explored (Table 2). To investigate the different effects of various amount of catalysts, the reactions with 15, 20, 25, 30, and 35 mg of catalysts were conducted (Table 2, entries 1−5). When the amount was 25 mg, the yield was the highest. As the amount went up, the yields remained. According to the reported results, the polarity of solvents made great effects on the reaction activity.24 In this work, reactions with most polar solvents, such as 1,4-dioxane, DMF, THF, DMSO, and CH3CN, gave low yields except those employing ethyl acetate (EtOAc) (entries 10−12, 15−17). In contrast, the reactions with nonpolar solvents like 1, 2-dichloroethane (DCE), chloroform, and n-hexane were conducted in high yields (entries 1, 14, and 18). Water was also employed, while the result was unpromising (entry 13). During the detection of products, 6-hydroxyhexanoic acid was found. While the low conversion did not matter with the hydrolysis of εcaprolactone, the selectivity reached 95%. It is supposed that water may prevent the effective reaction of substrates’ interaction with catalysts and oxygen to restrict the catalytic performance. Furthermore, water is prone to form hydrogen bonding with the Si−OH on the surface of mSiO2 nanorods, which could significantly influence the cooperation of reagents.5,25,26 Solvent-free reaction proceeded well (entry 19), indicating a possibility of solvent-free attempt for application. Furthermore, the effects of time on product were investigated (entries 21−25). With the time increasing to 5 h, the yields went up to the best. After 5 h, the yields had no change with the time increase. With extensive screening of temperature, solvent, and time, the optimum conditions of B− V oxidation with mSiO2-500 were 25 mg of catalyst in DCE for 5 h at 50 °C. Nevertheless, the solvent DCE is toxic, which is harmful to the environment. Therefore, it is necessary to employ a solvent meeting the requirements of sustainability. As shown in Table 2, the reaction employing ethyl acetate exhibited good catalytic activity of 82% yield. Thus, EtOAc was

EXPERIMENTAL SECTION

Materials. The information for the related materials is found in the Supporting Information. Synthesis of mSiO2 Rods. The synthetic method was reported in previous literature.23 The detailed synthetic strategy was shown in the Supporting Information. The molar ratios of reactants in the resulted mesoporous silica materials were 1 TEOS/x H2O/20 EtOH/10.3 NH3·H2O/0.3 CTAB. x ranged from 248 to 1486. The resulted mSiO2 materials were defined as mSiO2-x. Characterization of the mSiO2 Rods Catalyst. Characterization methods of the mSiO2 rods catalysts were listed in the Supporting Information. Catalytic Performance of B−V Reaction and Recyclability. Cyclohexanone (2 mmol, 196 mg), benzaldehyde (4 mmol, 424.5 mg), dodecane (1 mmol) as internal standard, 5 mL of solvent, and a certain amount of catalysts reacted in a 25 mL two-neck flask. The reaction mixture was stirred for several hours with the air pumping into the system. Different kinds of catalysts, the ratio of cyclohexanone and benzaldehyde, temperature, reaction time, and solvents were tested. After the reactions, the catalysts were filtered and washed with ethanol for several times and then dried under vacuum before being devoted to the next catalytic cycle. In the recirculated experiments, reaction conditions were the same as the optimal condition except for reaction time, which was shorten to 0.5 h. The reaction was analyzed by GCMS on the basis of the internal standard method.

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RESULTS AND DISCUSSION Catalytic Activity. The B−V oxidation reaction was proceeded by employing benzaldehyde and air as oxidant. In order to choose the most suitable catalyst and optimal reaction conditions, the oxidation of cyclohexanone was used as model reaction with dodecane as the internal standard. The catalytic activities of various synthetic mesoporous silica materials were listed in Table 1. Various catalysts applied in B−V oxidation exhibited excellent selectivity. Compared to the high yields of mesoporous silica nanorods, MCM-41, SBA-15, SBA-16, and HMS were borne and provided moderate yields (Table 1, entries 1−4). Among the nanorods with different water ratios ranging from 248 to 1486, the reaction with water ratio in 500 presented the optimum activity (entry 8). Both the conversion B


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influence the yields. From an economical view, 2 equiv of benzaldehyde was the preferred amount (entry 3). Meanwhile, kinds of aldehydes were applied to the B−V oxidation. When the carbon atoms of aldehydes were lower than four, the reactivities were relatively poor (entries 6−11). The reaction employing n-butanal and n-heptanal gave yields of 21% and 45%, respectively (entries 12−13). It was reported that the mechanism of the B−V oxidation involved free radical species. The above results indicated that the longer alkyl chain and the extended π bond of benzene ring were good stabilizers for free radicals. For multisubstituted aromatic aldehydes, the reaction with p-phthalaldehyde gave 4-formylbenzoic acid as the major product (entry 14). A possible reason was that it is hard to form two free radicals in the same molecule at the same time. Among the tested aldehydes, benzaldehyde exhibited the best performance. With the optimized reaction conditions, the scope of various ketones employing DCE and EtOAc as solvent toward the B−V oxidation was explored, respectively, and listed in Table 3. It is delightful that various cyclic ketones could be smoothly transferred into corresponding lactones in both solvents, with excellent selectivity and conversion except for cycloheptanone (Table 3, entries 2, 4, and 5). For cycloheptanone, it was supposed that the seven-membered ring was difficult to cleave, leading to the extremely poor yield. When 1-indanone was employed in the present reaction, the desired products were not obtained with DCE as solvent. The reaction with EtOAc exhibited a yield of 5%, while the reaction selectivity was low (71%). The byproduct of 1H-indene-1,3(2H)-dione was found, leading to the low selectivity (entry 6). Among the cyclic ketones, the six-membered ring was the most efficient. Although the aliphatic ketones showed lower conversion than the cyclic ones, 3-pentanone and 4-heptanone obtained the desired product in 17% and 21% with EtOAc (entries 8 and 10). Nevertheless, the reaction with 2-heptanone gave product below detection with DCE (entry 9), illustrating that symmetric ketones were more active and C−C cleavage would easily occur in the side of longer alkyl chains. For asymmetrical ketones, the desired product was likely to prefer the side with more carbon atoms. Moreover, due to the limitation of pore size in the catalyst, the longer chain alkane with large steric hindrance was difficult to react. When the reaction of aliphatic ketones with two solvents was compared, EtOAc exhibited better reaction affinity. p-Methylacetophenone was more likely to transform into corresponding ester than acetophenone, because methyl owned the ability to stabilize the positive charge.27 For cyclic ketones, the reactions using the two solvents had good yields. For aliphatic ketones, the reaction with EtOAc processed higher yields, whereas the reactions with DCE were significantly better for acetophenone. In spite of the reaction rate depending on the structure of ketones, it can be inferred that various ketones could be oxidized to corresponding lactones or esters by the simple mSiO2 nanorods under such mild reaction conditions. The comparison of reaction conditions of the B−V oxidation reaction employing mSiO2 nanorods and other catalysts including carbon materials and metallic catalysts was listed in Table 4. The nonmetallic carbon catalysts could give product in moderate to good yields with low catalysts dosage, but they need pure O2, indicating the low utilization of catalyst for oxygen. Other reports employed nonprecious metal for catalyzing the oxidation of cyclohexanone under mild conditions, while the consumption of metal and the lengthy

Table 2. Optimization Conditions of the B−V Reaction

entry

amount of catalyst (mg)

temperature (°C)

time (h)

1 2 3 4 5 6 7 8 9 10 11

15 20 25 30 35 25 25 25 25 25 25

50 50 50 50 50 20 30 40 60 50 50

5 5 5 5 5 5 5 5 5 5 5

12 13 14 15 16 17 18 19 20 21 22 23 24 25

25 25 25 25 25 25 25 25 25 25 25 25 25 25

50 50 50 50 50 50 50 50 50 50 50 50 50 50

5 5 5 5 5 5 5 5 0.5 1 2 3 4 6

solvent DCE DCE DCE DCE DCE DCE DCE DCE DCE CH3CN 1,4dioxane EtOAc H2O n-hexane DMF THF DMSO CHCl3 DCE DCE DCE DCE DCE DCE

yielda (sel.) (%) 64 (>99) 75 (>99) >99 (>99) >99 (>99) >99 (>99) 58 (>99) 77 (>99) 92 (>99) 89 (>99) 74 (>99) 46 (>99) 82 (>99) 37 (95) 61 (>99) trace 13 (>99) trace 97 (>99) 65 (>99) 28 (>99) 56 (>99) 72 (>99) 94 (>99) 98 (>99) >99 (>99)

a

The yield and selectivity were determined by GC-MS on the basis of the internal standard method (dodecane).

employed to investigate the optimum reaction conditions (In Table S1, Supporting Information). With EtOAc as solvent, the optimum conditions for B−V oxidation were 20 mg of catalyst for 5 h at 50 °C. At the optimum conditions, the B−V reaction could reach a high yield of 92% (Table S1, entry 3), indicating the feasibility of EtOAc as solvent. For aerobic reaction, it is necessary to investigate the source of oxygen (Table S2). When O2 was employed or air was pumped into the reaction system, the yields of the reaction were beyond 99% (Table S2, entries 1 and 3), while the reaction system under open air only achieved 74% yield (entry 2). No desired products were detected with the N2 balloon (entry 4). When 2 mmol of cyclohexanone, approximately 12 mmol of H2O2, and 25 mg of mSiO2 nanorods were added to the reaction system and stirred at 50 °C for 5 h under argon atmosphere, the reaction reached a yield of 9%, with 26% selectivity (entry 5). H2O2 was added dropwise to the reaction system. From the above results, it is suggest that the catalysts were efficient by making use of O2 in air. In view of the amount and structure of aldehydes making key influences on the reactivity of B−V oxidation, various aldehydes and the consumption amount of benzaldehyde were investigated (Table S3). When the amount of benzaldehyde was 1 equiv, the yield of ε-caprolactone was 61% (Table S3, entry 1). With the amount of benzaldehyde increasing to 2 equiv, the yields increased. When the amount of benzaldehyde was beyond 2 equiv, the increase of benzaldehyde could not C


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ACS Sustainable Chemistry & Engineering Table 3. B−V Oxidation Reaction of Various Ketones

a

Reaction conditions: ketones (2 mmol), aldehyde (2 equiv), dodecane (1 mmol), and the catalyst (25 mg), DCE (5 mL), air (20 mL/min), 5 h, 50 °C. bReaction conditions: ketones (2 mmol), aldehyde (2 equiv), dodecane (1 mmol), and the catalyst (20 mg), EtOAc (5 mL), air (20 mL/min), 5 h, 50 °C. cThe yield and selectivity were determined by GC-MS on the basis of the internal standard method (dodecane).

Table 4. Comparison of Catalytic Performance of Different Catalysts Applied in the B−V Oxidation Reaction entry

catalyst and dosage (mg)

subtrate/aldehyde (mmol)

1 2 3 4 5

Ketjen Black/5 graphite/20 c-MWCNTs/20 Cu-MCM-41/50 Fe−Cu bimetal oxide/25

4/4 10/20 2/4 2/4 2/5

6

Fe3O4-l-dopa-CuII/SnIV@ m,mSiO2/50

2/4

7

mSiO2 nanorods/25

2/4

active site loading

Cu 4.34% CuFe2O4 100% Cu 1.0% Sn 4% Fe 14%

T (°C)

time (h)

yield (%)

resource of oxygen

recycle of catalyst

50 rt 50 50 50

4 3 8 3 4

65 92.5 >99 99 >99.9

O2 balloon pumping O2 O2 balloon pumping O2 pumping air

practicable practicable practicable practicable practicable

17 18 34 14 13

50

6

>99.9

pumping air

practicable

16

50

5

>99

pumping air

practicable

this work

ref.

yield and selectivity. Our results implied that the metal-free catalyst exhibited the same catalytic performance as the metallic catalysts.

synthetic process were undesired. Thus, the simply synthetic, economical, and eco-friendly mSiO2 nanorods exhibited excellent catalytic activity for the B−V reaction with 99% D


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Figure 1. SEM images of (a) mSiO2-248, (b) mSiO2-372, (c) mSiO2-500, (d) mSiO2-619, (e) mSiO2-991, and (f) mSiO2-1486 and TEM images of (g, i) mSiO2-500 and (h, j) the used mSiO2-500.

Figure 2. (A) Small angle XRD patterns. (B) N2 sorption isotherms of mSiO2 nanoparticles: (a) mSiO2-248, (b) mSiO2-372, (c) mSiO2-500, (d) mSiO2-619, (e) mSiO2-991, and (f) mSiO2-1486.

Characterizations of the Catalyst mSiO2 Nanorods. Figure 1a−f showed the SEM images of the mesoporous silica materials with different water ratios. Six different sizes of mSiO2 nanoparticles were successfully fabricated. The mSiO2-248 displayed a morphology of microspheres with a diameter of ca. 310 nm. In the sample of mSiO2-372, a mixture of microspheres, cylinders, and rods was present simultaneously. From mSiO2-500 to mSiO2-1486, nanorods were dominant, with an average diameter of 150, 80, 70, and 60 nm and length of 1 μm, 800 nm, 350 nm, and 250 nm, respectively. Each of the four nanorods revealed uniform morphology with a smooth surface and good dispersion. With the water ratios increasing, the sizes of nanorods decreased, in which the length decreased faster than the diameter. The TEM patterns of mSiO2-500 before and after reaction were shown in Figure 1g−j, in accordance with the results of SEM. The nanorods were in good dispersion and nearly uniform in dimension. The mSiO2500 (Figure 1g,i) revealed parts of the mesopores were in highly ordered arrangements, while some parts were out of order. The resulted mSiO2 nanorods were in noncrystal structure and in long-range order and short-range disorder, revealing more surface defects compared with highly crystalline silica. When the B−V oxidation finished, the structure of mSiO2 nanorods was well maintained (Figure 1h,j). Small-angle X-ray diffraction patterns of mSiO2 nanoparticles with different water ratios were depicted in Figure 2A. All the samples showed a single weak and broad peak at 2θ around

2.7°, indicating that the resulted nanoparticles were in amorphous state. With the water ratio increasing, the intensity and width of diffraction peaks became lower and broader, demonstrating the nanorods were less ordered in higher water ratios. Meanwhile, the shifts of the curves reflected the changes of pore diameter. When the curve shifted to the left side, the pore diameter became larger. Reversely, the pore diameter became smaller. Among the different nanoparticles, the mSiO2500 possessed the smallest pore diameter, consistent with that in nitrogen physisorption studies. The nitrogen physisorption isotherms of various mSiO2 materials were plotted in Figure 2B, and the data of BET surface area, BJH adsorption average pore size, and total pore volume were exhibited in Table 5. All the curves exhibited type Table 5. BET Surface Area, Pore Size, and Volume of Various mSiO2 Materials

E

sample

BET surface area (m2 g−1)

BJH adsorption average pore size (nm)

total pore volume (cm3 g−1)

mSiO2-248 mSiO2-372 mSiO2-500 mSiO2-619 mSiO2-991 mSiO2-1486

1603 1270 1256 1231 1353 1461

2.60 2.61 2.57 2.67 3.27 3.24

0.82 0.63 0.66 0.66 0.81 1.01


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Reusability of the mSiO2 Nanorods. The recovery and reusability of the catalyst were examined. When the reaction finished, mSiO2-500 could be easily separated through simple filtration. Then, the used catalyst was washed several times with ethanol and dried under air at 60 °C. Figure S2 showed the performance of mSiO2-500 in the reused cycles of cyclohexanone oxidation. In order to conveniently compare the catalyst activity before and after reaction, the conversion of the reaction was kept fairly low by reducing the reaction period to 0.5 h. During the five runs of recycle, the yield of εcaprolactone decreased from 29% to 16%, indicating the catalyst mSiO2 nanorods showed considerable stability and reusability. Plausible Mechanism for the Cyclohexanone Oxidation with Benzaldehyde and O2 as Oxidant. On the basis of the previous reports,11,18 a possible mechanism of cyclohexanone oxidation with benzaldehyde/O2 was proposed in Scheme 1. First, the benzaldehyde interacted with the active

IV isotherm patterns according to the IUPAC classification, which is characteristic of mesoporous materials. Type H3 hysteresis loops were found in the mSiO2-991 and mSiO2-1486, presumably arising from the interparticle voids. Compared with other silica materials, the BET specific surface areas of our samples were as high as 1200 m2/g. The catalytic activities of the B−V oxidation showed close correlation with the BET surface area.17 The pore size distributions measured by the BJH method and the density functional theory (DFT) method were displayed in Figure S1. The pore size distributions of a series of mSiO2 nanorods were sharply distributed in a narrow range. When the water ratio was 500, the BET surface area, pore size, and pore volume were 1256 m2/g, 2.12 nm, and 0.66 cm3/g, revealing the best catalytic activity. It is supposed that suitable pore volume and pore size could provide excellent selectivity for the reagent in a similar BET surface area. The small pore size could allow the transition of a smaller size molecule and restrict the transition of large steric hindrance molecule, benefiting the reaction selectivity.28,29 To obtain deep insights for the organic species deposited on the mSiO2 nanorods, FT-IR spectra of fresh and spent mSiO2 nanorods within 6 h were displayed in Figure 3. The two bands

Scheme 1. Plausible Mechanism of Oxidation of Cyclohexanone

sites in the mSiO2 nanorods, with the generation of an acyl radical. Then, the O2 in the air was quickly captured by the acyl radical to form benzoyloxyl radical. Meanwhile, the generated radicals were adsorbed on the surface of mSiO2 nanorods, and the super large specific surface area diluted the concentration of radicals, improving the reaction efficiency. After that, another molecule of benzaldehyde joined and perbenzoic acid was formed, which was the key process in the reaction. Followed by the attack of cyclohexanone, Criegee adduct was generated. As a consequence, Criegee adduct decomposed to form εcaprolactone and the byproduct benzoic acid. In order to clarify the role of the catalyst and verify the mechanism of B−V oxidation catalyzed by silica nanorods, a series of controlled experiments was conducted. Plenty of reports proposed that the mechanism of the B−V oxidation involved free radical species. Hence, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) was introduced into the reaction system as a radical scavenger. When the catalytic amount of TEMPO and the equivalent amount with benzaldehyde of TEMPO were used in the reaction, no desired products were detected. It is obvious that the reaction involved a radical pathway. In order to further study the radical species, electron paramagnetic resonance (EPR) spectra of the reaction were

Figure 3. FT-IR spectra of fresh and used mSiO2 nanorods within 6 h.

at 3442 and 1631 cm−1 were attributed to the O−H stretching vibration from both silanol groups and the water absorbed on the solid surface. The three bands at 1076, 809, and 455 cm−1 were related to the asymmetric stretching for Si−O−Si bridges, Si−O−Si symmetric stretching, and bending vibration, respectively.30 The band at 958 cm−1 was assigned to the typical Si−OH stretching vibration. Compared with the fresh catalyst, several new peaks were observed in the spent catalysts. The band at 2946 cm−1 was attached to the C−H stretching bands of cyclohexanone. The bands at 1704 and 1392 cm−1 were assigned to carboxylic CO and C−O stretching of carboxylic acid. Similarly, the two bands at 1457 and 1401 cm−1 were typical peaks of O−C−O symmetric stretching.31,32 Consequently, the organic species including CH2, CO, and C−O groups were absorbed on the surface of mSiO2 during the reaction process. F


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Figure 4. EPR spectra of (A) oxidation of cyclohexanone over mSiO2 nanorods. (B) The EPR of the reaction at different times.

analyzed. Figure 4 showed the EPR spectra of cyclohexanone oxidation employing N-tert-butyl-α-phenylnitrone (PBN) as radical spin trapping agent. In Figure 4A, there were two types of PBN adducts. The one with symbol of ⧫ was assigned to benzoyloxyl (C6H5COO·) adduct of PBN, consisting of a triplet of doublets. The other one with ▼ was attributed to acyl radical (C6H5CO·) adduct of PBN.33−36 In Figure 4B, the reaction of benzaldehyde and perbenzoic acid was due to autoxidation, because the reaction at 0 min showed the same radical signal with the signals of other time. The PBN was added to the reaction system in the beginning. The response intensity was relatively low in the starting stage (0−3 min), and then, the intensity of the radical rapidly became stronger in 5 min. The results suggested that the mSiO2 nanorods could not only gather radicals quickly but also protect the radicals from inhibition. The inhibition directly transferred benzaldehyde to benzoic acid rather than to perbenzoic acid. The intensity remained stable after 30 min. EPR measurement showed that acyl radical and benzoyloxyl radical were successfully captured. Meanwhile, the mechanism proposed was confirmed. Generally, there are two reaction steps for the O2/aldehyde oxidation system of the B−V oxidation reaction: (a) aldehyde and O2 generating peracid; (b) reactant oxidation by peracid.37,38 It is meaningful to figure out either stage a or stage b the catalysts promoted. To clarify whether the mSiO2 nanorods sped up the generation of peracid, EPR was carried out with and without catalyst by adding the PBN in 10 min. In the comparison of the EPR spectra with and without catalysts, the intensity of the two spectra showed no obvious difference. Then, the reactions employing m-chlorobenzaldehyde were conducted. Reactions of m-chlorobenzaldehyde and O2 were proceeded with and without mSiO2 nanorods. When the reaction finished, the reaction systems with and without catalysts were evaporated to remove solvents, and then, the residue was treated with saturated NaHCO3 solution. The resulted samples were analyzed by an FT-IR analyzer, and the FT-IR spectra were displayed in Figure 5. In samples a and b, m-chlorobenzoic acid could be found in the two curves, showing related peaks. The O−H stretch appeared in 2100− 3300 cm−1 and CO stretch, in 1691 cm−1. In order to verify the existence of m-chloroperoxybenzoic acid, sample a and sample b were treated with saturated NaHCO3 solution. Sample c, resulting from sample b, exhibited several peaks corresponding to m-chloroperoxybenzoic acid, showing CO stretch in 1712 cm−1, while the treated sample a found no

Figure 5. FT-IR spectra of (a) m-chlorobenzaldehyde reacted with O2 without catalyst, (b) m-chlorobenzaldehyde reacted with O2 over mSiO2 nanorods, and (c) sample b treated with NaHCO3. Reaction conditions: m-chlorobenzaldehyde, 4.0 mmol; mSiO2 nanorods, 25 mg; 1,2-dichloroethane, 5 mL; pumping air; 50 °C, 5 h.

residue. In conclusion, the mSiO2 nanorods could catalyze the reaction of benzaldehyde and O2 to generate peracid. Whether the mSiO2 materials accelerated the cyclohexanone oxidation, m-chloroperbenzoic acid (m-CPBA) was used to test the catalytic performance with and without mSiO2 nanorods. The experimental results shown in Table S4 exhibited similar yields for above two reactions, demonstrat that the mSiO2 nanorods give little contribution to the cyclohexanone oxidation when catalyst existed. As a result, the catalyst made a contribution in accelerating the first stage of the reaction. FTIR analysis of pyridine adsorption was carried out to evaluate the surface acidic sites of the SiO2 nanorods (Figure S3). The two bands at 1445 and 1596 cm−1 were associated with Hbonded pyridine corresponding to the weak acid sites arising from the surface hydroxyl groups attached to Si. The two bands were observed after adsorption and desorption of pyridine at low temperature and disappeared after the desorption temperature increased to 200 °C. The bands at 1486 and 1577 cm−1 were attributed to pyridine adsorbed on Lewis acid sites in accordance with the literature,6,39,40 while the strength of the peaks decreased with the increase of desorption temperature, indicating the weak Lewis acidity of the sites. The weak Lewis G


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ACS Sustainable Chemistry & Engineering acid sites may act as active sites in the first stage for catalyzing B−V oxidation. Furthermore, the enhancement of catalytic performance was due to the sustained release effect of the super large area of mSiO2 nanorods as well, which adsorbed and protected the radical species from the inhibition. It can be concluded that the catalyst surface of mSiO2 nanorods could serve as a buffer for the radical species. In conclusion, a simple heterogeneous catalyst, mSiO2 nanorods, has been successfully prepared and is the first to directly catalyze the B−V oxidation via a metal-free process under open air. The nonmetallic mesoporous silica nanorods were a green, effective, and economically friendly catalyst for the B−V reaction with both high selectivity and yields. For most ketones, the application of EtOAc could completely replace the DCE to avoid toxic solvent. The catalyst could be reused several times without significance loss of activity. In addition, a plausible mechanism was proposed and confirmed by EPR measurement. The strategy making use of green mSiO2 nanorods as catalyst and employing air as oxidant under facile conditions is a promising process, which has high potential in industrial applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04167. Information on related materials, procedures for preparation of mSiO2 rods, characterization of the catalyst mSiO2 rods, optimization conditions of the B− V reaction with EtOAc, source of oxygen, the B−V oxidation reaction of various aldehydes, oxidation of cyclohexanone by m-chloroperoxybenzoic acid, pore size distribution of mSiO2 nanoparticles, recycling experiments, and FT-IR spectra of mSiO2 nanorods after pyridine adsorption (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-931-8912582. Tel: +86-931-8912577. ORCID

Shuwen Li: 0000-0001-9511-2555 Jiantai Ma: 0000-0003-3133-7696 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 51602138) and the Fundamental Research Funds for the Central Universities (lzujbky-2017kb11 and lzujbky-2017-100).

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Metal-Free Mesoporous SiO2 Nanorods as a Highly Efficient Catalyst

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