b. Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, China
Having been used as coinage, jewelry and arts for thousands of years, gold was considered inert in organic synthesis until it was discovered efficient for CO oxidation and chlorination of ethylene [1-4]. Afterwards, gold nanoparticles witnessed an increasing popularity as catalysts and they have been identified as the most active catalyst in a variety of reactions [5-8]. Usually the gold nanoparticles are anchored on metal oxide [9-13], carbon [14-16], graphene [17, 18], chitosan [19, 20], polymer [21-24], and silica [25-27], etc., to serve as heterogeneous catalysts. The insolubility of heterogeneous catalysts assists isolation and recovery of catalysts from the slurry, which makes them more popular [28, 29]. Utilization of these catalysts in aqueous phase leads to highly efficient and environmentally benign catalytic systems [30-33]. Usually the supported catalysts are heterogeneous with substrates that are soluble in organic solvents. In this work, we reverse the logic to prepare soluble catalyst while the substrate and product are dispersed in media.
Oxidation of aldehydes to corresponding carboxylic acids is of great significance for industrial manufacturing due to its great potential in organic synthesis . Wang group developed a catalyst-free aerobic oxidation of aldehydes method in water, but with low aldehyde concentration . Both aldehyde and alcohol groups can be oxidized into carboxylic acid, while some alcohol groups can be first oxidized into aldehyde and then to carboxylic acid. As a result, controllable oxidation of alcohol and aldehyde is a technical issue of synthesis. Herein, we develop a strategy to prepare an efficient catalytic system to selectively oxidize 4-(hydroxymethyl)-benzaldehyde in aqueous phase by H
In this work, water, H
All the chemicals and reagents were purchased from Aladdin corporation. All the organic products were analyzed by Bruker AM 400 MHz NMR to confirm the structure. The UV tests of NPs were operated with the help from Hewlett-Packard (HP) 8453 diode array spectrophotometer. TEM images were acquired by JEM 2100. TLC plates (Merck Silica Gel 60 F254) were used for analytical thin layer chromatography. Merck Kieselgel 200-300 was used for preparative column chromatography. The Au loading was determined by ICP AES (iCAP 7400 Duo) from Thermo Fisher. XPS data are collected by ESCALAB 250Xi from Thermo Fisher.B. Sample preparation 1. Synthesis of gold NPs
100 μL HAuCl
0.1 g 4-(hydroxymethyl)-benzaldehyde (0.735 mmol) was added into 5 mL catalyst storage solution (Au loading 0.004 mmol). Then the slurry was heated to 363 K for 6 h before adding 1215 μL H
After reaction completed, the organic components were washed with 3 mL dichloromethane for 5 times, and the water phase was collected after skimming. The water containing gold NPs could be used for next round of catalysis.Ⅲ. RESULTS AND DISCUSSION A. Material characterization
The size of polydisperse Au NPs is characterized by TEM ranging from 3 nm to 60 nm, and the results are illustrated in FIG. 2 (a) and (b). The gold NPs size mostly distributes between 3-20 nm based on 500 counted particles. UV-Vis spectrum further confirms with TEM result: a broad absorption band centered at 540 nm (FIG. 2(c)). The gold NPs are relatively stable after 5 runs of catalysis, which is verified by TEM (FIG. S1 (a) and (b) in supplementary materials) and UV-Vis (FIG. S1(c) in supplementary materials), probably due to identical reaction conditions of synthesis and catalysis. UV-Vis spectrum shows 3 nm bathochromic-shift and TEM image exhibits slight aggregation.
XPS spectrum of Au (FIG. 3) indicates binding energy of Ar 4f is 84.0 and 87.6 eV, implying the elementary state of metal. Lewis acid of Au (Ⅰ and Ⅲ) is available for various kinds of catalytic reactions, the absence of these species eradicates the possibilities of Lewis acid catalysis. Besides, it means starch is an excellent reducing agent to reduce gold salt and extra reducing agent is absolutely unnecessary. The binding energy of O 1s from ligand starch which is found lower after stabilizing gold NPs interprets that starch serves as the capping agent (FIG. S2 in supplementary materials). Meanwhile, deflating of absorption band centered at 3400 and 1644 cm
Starch stabilized Pt/Au NPs supported on hydrotalcite are characteristic for polyols oxidation with good activity . In this work, water-dispersed gold NPs are employed to catalyze selective oxidation of 4-(hydroxymethyl)-benzaldehyde into 4-hydroxythylbenzoic acid while terephthalaldehyde and terephthalic acid are possible byproducts. Several oxidation agents are examined to test selectivity of starch capped gold NPs. Oxygen is the weakest agent which gives rise to nothing while the catalyst is absent (Entry 1 in Table Ⅰ). When the NPs are present, negligible enhancement is observed and the catalyst prefers acid as the product (Entry 2). Metal salts K
The role that temperature plays in the catalysis performance is evaluated (FIG. S4 in supplementary materials). The temperature is set from 303 K to the water boiling point with 10 K increment and reaction time is overnight. It is obvious that high temperature is helpful for high conversion, however, it needs more energy. The target molecule with too low content is generated at room temperature. The substrate is completely transformed at 363 K, so 373 K is unnecessary for more yield.
The transformation process of 4-(hydroxymethyl)-benzaldehyde is monitored by GC-MS (FIG. 4). From the very beginning, only trace byproduct terephthalaldehyde is detected and the pattern is kept till the end of reaction. In the first 3 h, 84% substrate is consumed. To totally convert the starting material, another 3 h is requested. After 6 h stirring, almost all the substrate is switched into target product with exclusive selectivity.
The water-dispersed gold catalysts are capable of recycling use (FIG. 5). Unreacted aldehyde and carboxylic acid are washed by organic solvent, hence, the aqueous phase could be used directly for next run of catalysis. However, some activity loss is observed in first use which drops to 82%. It's possibly ascribed to catalyst aggregation. In the subsequent 4 tests, the yields are stable between 63% and 69%. The lower activity may be due to slight catalyst aggregation. The results demonstrate starch-capped gold NPs are highly active and recyclable up to the 5th run with little loss of activity, which entitles them "green" catalysts.
Oxidation mechanism of aldehyde group and hydroxyl group are different, aldehyde oxidation adopts racial path while hydroxyl oxidation can't. As shown in FIG. 6, the possible mechanism is speculated to elucidate the transforming process. The aldehyde group could be oxidized into peracid by H
The versatility of the oxidation catalytic system has been demonstrated by the oxidation of various aldehydes. The product selectivity is exclusive regardless of substrate structure that was used. The conversion varies from goodness to excellence with different substituted groups. Catalytic oxidation of aldehydes into acid is performed under identical conditions operated in catalytic synthesis process. Substrate is dispersed in 5 mL catalyst storage solution to generate acid product under vigorous stirring. After 1 h heating, 100% benzaldehyde is transformed into benzoic acid, the TON (turnover numbers) value is 183.8 mol(aldehyde)/mol(Au) while TOF (turnover frequency) value reaches the top as 183.8 h
A new heterogeneous catalysis system is established, water-dispersed gold NPs are prepared and evaluated for their excellent catalytic ability for selective oxidation of 4-(hydroxymethyl)-benzaldehyde and versatility for aldehyde derivatives. The catalyst displays complete conversion and excellentselectivity in the presence of H
Supplementary materials: TEM images and UV-Vis spectrum of gold nanocatalyst after 5th round catalysis, IR and XPS characterization of NPs, temperature tests along with spectroscopic data and NMR spectra of catalytic products can be found in the supplementary materials.Ⅴ. ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (No.11674001), the Ministry of Education, Anhui Provincial Natural Science Foundation (No.1708085MA07 and No.1608085QB39), and Doctoral Startup Foundation of Anhui University (No.10113190077).Spectroscopic properties of products
1H NMR (400 MHz, DMSO) δ 7.90 (d, J = 8.2 Hz, 1H), 7.43 (d, J = 8.2 Hz, 1H), 5.34 (s, 1H), 4.57 (s, 1H).13C NMR (100 MHz, DMSO) δ7.91, 7.89, 7.44, 7.42, 5.34, 4.57.
1H NMR (400 MHz, DMSO) δ10.14 (s, 1H), 8.12 (s, 2H).13C NMR (100 MHz, DMSO) δ193.58, 140.20, 130.48.
1H NMR (400 MHz, CDCl3) δ 8.20 – 8.13 (m, 1H), 7.70 – 7.61 (m, 1H), 7.55 – 7.47 (m, 1H).13C NMR (100 MHz, CDCl3) δ 172.31, 133.84, 130.24, 129.33, 128.52.
1H NMR (400 MHz, CDCl3) δ 7.83 (d, J = 16.0 Hz, 1H), 7.59 (dd, J = 6.8, 2.9 Hz, 2H), 7.47 – 7.38 (m, 3H), 6.49 (d, J = 16.0 Hz, 1H).13C NMR (100 MHz, CDCl3) δ 172.42, 147.13, 134.04, 130.79, 128.99, 128.40, 117.31.
1H NMR (400 MHz, DMSO) δ 7.91 (d, J = 8.1 Hz, 1H), 7.35 (d, J = 8.1 Hz, 1H), 3.69 (t, J = 7.1 Hz, 1H), 3.22 (t, J = 7.1 Hz, 1H).13C NMR (100 MHz, DMSO) δ 172.40, 148.85, 134.59, 134.47, 133.81, 43.50, 38.26.
1H NMR (400 MHz, DMSO) δ 7.90 – 7.72 (m, 1H), 6.90 – 6.68 (m, 1H).13C NMR (100 MHz, DMSO) δ 172.65, 166.64, 136.52, 126.49, 120.01.
1H NMR (400 MHz, CDCl3) δ 8.28 (d, J = 8.5 Hz, 1H), 8.21 (d, J = 8.5 Hz, 1H).13C NMR (100 MHz, CDCl3) δ 171.01, 154.99, 141.57, 135.60, 128.18.
1H NMR (400 MHz, CDCl3) δ 8.09 (d, J = 9.0 Hz, 1H), 6.97 (d, J = 9.0 Hz, 1H), 3.91 (s, 1H).13C NMR (100 MHz, CDCl3) δ 171.50, 164.04, 132.37, 121.65, 113.76.
1H NMR (400 MHz, DMSO) δ 8.01 – 7.86 (m, 1H), 7.58 (t, J = 5.5 Hz, 1H).1H NMR (400 MHz, CDCl3) δ 7.91, 7.90, 7.89, 7.60, 7.59, 7.57.13C NMR (100 MHz, DMSO) δ 172.25, 136.26, 136.08, 134.81, 132.20.
1H NMR (400 MHz, CDCl3) δ 7.98 – 7.83 (m, 1H), 7.77 – 7.67 (m, 1H).13C NMR (100 MHz, CDCl3) δ 169.04, 132.73, 132.62, 130.47, 125.86, 123.92.
NMR spectra of products
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b. 安徽大学物质科学与信息技术研究院，合肥 230601