Chinese Journal of Chemical Physics  2019, Vol. 32 Issue (6): 747-752

The article information

Han-bao Chong, Gui-qi Gao, Guang Li
崇汉宝, 高贵琪, 李广
Selective Oxidation of Aldehyde over Hydroxymethyl Group Catalyzed by Gold Nanoparticles in Aqueous Phase
水相中金纳米颗粒催化的醛基选择性氧化
Chinese Journal of Chemical Physics, 2019, 32(6): 747-752
化学物理学报, 2019, 32(6): 747-752
http://dx.doi.org/10.1063/1674-0068/cjcp1905101

Article history

Received on: May 20, 2019
Accepted on: August 2, 2019
Selective Oxidation of Aldehyde over Hydroxymethyl Group Catalyzed by Gold Nanoparticles in Aqueous Phase
Han-bao Chonga,b , Gui-qi Gaob , Guang Lia     
Dated: Received on May 20, 2019; Accepted on August 2, 2019
a. School of Physics and Material Science, Anhui University, Hefei 230601, China;
b. Institute of Physical Science and Information Technology, Anhui University, Hefei 230601, China
Abstract: A protocol for selectively oxidizing aldehyde over hydroxymethyl group is developed, using biomass starch protected gold nanoparticles (NPs) as catalyst. The Au NPs show high selectivity that aldehyde is oxidized into carboxylic acid while alcoholic hydroxyl group stays intact in selective oxidation of 4-(hydroxymethyl)-benzaldehyde. The heterogeneous catalysis system is composed of soluble catalysts and insoluble substrate. The gold catalyst is prepared, preserved and applied for catalytic oxidation all in water. After reaction conditions are optimized, H$_2$O$_2$ is found to be the best oxidizing agent with complete conversion. Besides, the gold catalyst displays good versitility for aldehyde derivatives. After reaction completes, organic components are extracted by organic solvent and gold NPs in water are separated and recycled.
Key words: Au nanoparticle    Water-solubility    Selective oxidation    Starch    
Ⅰ. INTRODUCTION

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 [34]. Wang group developed a catalyst-free aerobic oxidation of aldehydes method in water, but with low aldehyde concentration [35]. 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$ _2 $O$ _2 $. The gold NPs are generated in water under oxygen atmosphere at 363 K, no reducing agent like NaBH$ _4 $ or urea is used except the use of naturally abundant starch as both capping agent and reducing reagent. The oxidation process occurs in water as H$ _2 $O$ _2 $ is also stored in aqueous, and the product is isolated by extraction with organic solvents as well as the residual aldehydes after reaction completes. In the past decades, the growing awareness of the environment has evoked a demand for efficient oxidation process with environmentally friendly oxidants. Hence, the quest for more sustainable and selective oxidation systems is a current hot research field [36].

In this work, water, H$ _2 $O$ _2 $ and starch are cheap and facile in industry. No base, buffer or co-catalyst are required, which all consent with the idea of "sustainable development". Starch is a qualifiedly reducing agent and protecting ligand [37] which is innoxious to operater. The facile synthesis and catalytic reaction protocol are presented in FIG. 1. Briefly, gold salt is mixed with starch in water under oxygen atmosphere at 363 K for 4 h. The colloid is preserved in fridge prior to use. After catalytic reaction completes, dichloromethane is used to extract organic components for several times while the aqueous layer containing catalyst is separated. The gold NPs are characterized by transmission electron microscope (TEM), infrared radiation (IR), and X-ray photoelectron spectroscopy (XPS). And Au loading on supported NPs and in water is determined by ICP AES as 16 mg/g and 160 mg/L, respectively.

FIG. 1 Schematic illustrating of catalyst synthesis and recycling.
Ⅱ. METHODS A. Experimental methods

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$ _4\cdot $4H$ _2 $O (0.05 mmol) solution was injected into 100 mL water containing 1 g starch under stirring. The mixture was heated to 363 K for 4 h under oxygen atmosphere. After reaction completed, the colloid was cooled down and then preserved in fridge at 277 K prior to use.

2. Typical procedure for the catalytic reaction

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$ _2 $O$ _2 $ (30% $ V/V $). After cooled down, the organic layer was extracted by dichloromethane (3 mL for 6 times). Finally, the purification process was carried out by column chromatograph.

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.

FIG. 2 TEM images (a, b) and size distribution (inset of (b)) of starch-caped gold NPs, the scale bars in (a) and (b) are both 100 nm; (c) UV-Vis spectrum of original Au NPs.

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$ ^{-1} $ which are ascribed to OH stretch vibration in IR spectra is consistent with the XPS result (FIG. S3 in supplementary materials).

FIG. 3 XPS spectrum of Au 4f in gold NPs.
B. Results of catalytic evaluation for selective oxidation

Starch stabilized Pt/Au NPs supported on hydrotalcite are characteristic for polyols oxidation with good activity [38]. 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$ _2 $Cr$ _2 $O$ _7 $ and NaClO exhibit similar selectivity preference with or without catalyst, while less K$ _2 $Cr$ _2 $O$ _7 $ is required to achieve analogous conversion as NaClO does (Entry 3-6). These results indicate, in the presence of metal salt oxidizing agents, gold NPs don't function as catalyst species. H$ _2 $O$ _2 $ is readily degradable which decomposes into water and oxygen making it environmentally benign oxidizing agent. TBHP (tert-butyl hydroperoxide) and H$ _2 $O$ _2 $ are both peroxides, and they show much better activity and selectivity, especially the latter with very high selectivity (Entry 7-10). Meanwhile, in the absence of NPs, slight conversion of substrate is acquired compared with excellent conversion when catalysts are added. Sole starch added in the system doesn't lay any influence on the outcome (Entry 11), suggesting gold NPs are true active species. No matter what kind of oxidation agents are employed, no over-oxidized product such as terephthalic acid is detected. In this work, environment-friendly reagent H$ _2 $O$ _2 $ utilized as the oxidizing agent which gives complete conversion and absolute selectivity is in line with "sustainable development".

Table Ⅰ Gold NPs catalyzed oxidation of 4-(hydroxymethyl)-benzaldehyde in water with different oxidizing agent$ ^{\rm{a}} $.

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.

FIG. 4 Converison-time plot of 4-(hydroxymethyl)-benzaldehyde selective oxidation. Reaction condition: 5 mL catalyst storage solution (water, 0.004 mmol), 0.1 g substrate (0.735 mmol, 363 K).

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.

FIG. 5 Recyclability of gold NPs for catalytic oxidation of aldehydes. Reaction condition: 5 mL catalyst storage solution (water), 2 mmol benzaldehyde, O$ _2 $ balloon, 363 K, 6 h.

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$ _2 $O$ _2 $ in the presence of Au NPs. Afterwards, 4-(hydroxymethyl)-benzaldehyde collides with peracid into two molecules of carboxylic acids. The hydroxyl group is relatively stable when blended with H$ _2 $O$ _2 $ and Au NPs, besides, it doesn't react with peracid. As a result, 4-(hydroxymethyl)-benzaldehyde is selectively oxidized into 4-hydroxythylbenzoic acid.

FIG. 6 Schematic illustrating of possible mechanism.

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$ ^{-1} $. The derived substrates with electro-withdrawing groups give rise to more product while the others with electron-donating groups show worse results. Interestingly, the o-positioned substrate (Entry 6, Table Ⅱ) does not present as expected probably due to the hindrance from the adjacent position. In a word, the gold NPs are qualified catalysts for synthesis of carboxylic acid.

Table Ⅱ Gold NPs catalyzed oxidation of 4-(hydroxymethyl)-benzaldehyde in water with different oxidizing agent$ ^{\rm{a}} $.
Ⅳ. CONCLUSION

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$ _2 $O$ _2 $. After reaction completes, slurry is washed by dichloromethane and aqueous layer containing gold NPs is separated, which doesn't contaminate the product. Biomass starch is used as protecting ligand and reducing agent, water is employed in synthesis and catalysis of protocol, both make the gold catalysts good candidates for "sustainable development".

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

4-(hydroxymethyl)benzoic acid

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.

Terephthalaldehyde

1H NMR (400 MHz, DMSO) δ10.14 (s, 1H), 8.12 (s, 2H).13C NMR (100 MHz, DMSO) δ193.58, 140.20, 130.48.

Benzoic acid

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.

Cinnamic acid

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.

4-(2-bromoethyl)benzoic acid

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.

4-hydroxybenzoic acid

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.

4-nitrobenzoic acid

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.

4-methoxybenzoic acid

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.

4-bromobenzoic acid

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.

2-nitrobenzoic acid

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.

FIG. S1 TEM images (a, b) and size distribution (inset) of starch-caped gold NPs after 5th round catalysos, the scale bars in a and b are both 50 nm; UV-vis spectrum of original Au NPs is presented in c.
FIG. S2 XPS spectrum of O1s in pure starch and NPs.
FIG. S3 IR spectra of ligand starch and starch-capped gold NPs.
FIG. S4 Conversion at different temperature. Reaction conditions: 5 ml catalyst storage solution (water, 0.004 mmol), 0.1 g substrate (0.735 mmol), overnight.

NMR spectra of products

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水相中金纳米颗粒催化的醛基选择性氧化
崇汉宝a,b , 高贵琪b , 李广a     
a. 安徽大学物理与材料科学学院,合肥 230601;
b. 安徽大学物质科学与信息技术研究院,合肥 230601
摘要: 本文开发了一种新型的用生物质淀粉保护的金纳米颗粒作为催化剂,选择性氧化醛基得到羧酸的方法.在4-羟甲基苯甲醛的催化氧化中,金纳米颗粒对醛基表现出了压倒性的选择性,而醇羟基则保持不变.该非均相催化体系由可溶解的催化剂和不溶解的底物构成.金催化剂的制备、储存、和使用都在水相中.反应条件优化之后,双氧水被证明是最佳的氧化剂,可以得到100%转化率.此外,在不同官能团取代的醛衍生物中,金纳米颗粒也表现出了很好的普适性.反应结束之后,有机组分被有机溶剂萃取,而金颗粒被保留在水中通过分液分离以回收使用.
关键词: 金纳米颗粒    水溶性    选择性氧化    淀粉