Self-assembled nanostructures of Ag₆[PV₃Mo₉O₄₀] with N-donor ligands and their catalytic activity

Abstract

Four types of nanostructures were obtained by self-assembly of Ag₆[PV₃Mo₉O₄₀] with N-donor ligands. Monodentate ligands lead to zero-dimensional nanostructures (nanoparticles). Linear didentate bridging ligands result in linear nanorods. Planar didentate chelating ligands develop two-dimensional nanostructures (nanosheets). Nonplanar didentate chelating ligands produce a transition morphology between nanorods and nanosheets. Kinetically driven (U_total(x) < 0) self-assembly processes lead to structures with less defined shapes. Thermodynamically driven self-assembly (U_total(x) is close to zero) produces ordered nanostructures. It is demonstrated that these nanocomposites are active catalysts for activating molecular oxygen under mild conditions.

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Introduction

Self-assembly is defined as the spontaneous association of two or more molecules or ions to create a larger, aggregate species through the formation of reversible (generally supramolecular) interactions.¹ The concepts of self-assembly are based on molecules, but components of any size can self-assemble in a permissive environment.² With expanding contact of chemistry with biology and materials science, there are three important ranges of self-assembly component size: molecular, nanoscale (colloids, nanoparticles, nanorods, nanowires and related structures), and meso- to macroscopic. Self-assembly provides a simple and controllable strategy for fabrication of useful structures of nano- and macroscale components, which have great potential applications in microelectronics,^3,4 photonics,^5,6 near-field optics,⁷ and the emerging field of nanoscience.⁸ Solution-based self-assembly of inorganic nanoclusters can generate a variety of structures, including chains,⁹ sheets,¹⁰ vesicles,¹¹ three-dimensional (3D) crystals¹² or more complex 3D architectures.¹³ Such assembly is directed by the balance of attractive and repulsive forces.

Polyoxometalates (POM) are polymeric oxoanions of early transition metals and can be formulated as [M_mO_y]^p− (isopolyoxometalates) and [X_xM_mO_y]^q− (x < m) (heteropolyoxometalates), where M is the addenda atom and X is the heteroatom. When selecting certain constituent elements and counter-cations, they exhibit various chemical properties such as acidity, redox potentials, and solubility in many media. These properties stimulate broad research in their applications in catalysis, materials, and medicine.¹⁴ The spherical surface and the terminal bridging oxygen atoms of POM anions give a better opportunity to be used as inorganic building units for the aesthetic construction of supramolecular arrays.¹⁵

Supramolecular structures of giant polyoxometalate clusters have been reviewed by Müller et al.¹⁶ Supramolecular structures from anionic POM clusters and organic cations–coordination complex cations have been extensively explored.¹⁷ Supramolecular self-assembly features of polyoxometalate with transition metal complexes were investigated in the work.

Many polyoxometalates are effective catalysts for green oxidation using H₂O₂ or O₂ as oxidants.¹⁸ However, most of these oxidations are homogeneous and problematic in recycling the catalysts and separating the products. In practical applications, there is a quest for the development of recoverable and recyclable heterogeneous oxidation catalysts. There are two strategies for heterogenizing: (1) solidification of polyoxometalates through the formation of insoluble solid ionic materials with selected countercations;¹⁹ (2) immobilization of polyoxometalates through adsorption, covalent linkage and ion-exchange.²⁰ Polyoxometalates can be used as building blocks for many novel functional materials. This is a new horizon for the development of heterogeneous polyoxometalate-based oxidation catalysts.²¹ Self-assembly is an efficient and low-cost method for exploring novel nanocatalysts based on POM building units.

Phosphovanadomolybdate Keggin-type polyoxometalates (H_3+xPV_xMo_12−xO₄₀ (x = 1–6)) have a redox potential of about 0.7 V vs. SHE, which is lower than that of O₂. These compounds can activate molecular oxygen for homogeneous oxidation under mild conditions.²² Silver salt of phosphovanadomolybdate shows high thermal stability (>800 °C) and has great potential in acting as a heterogeneous catalyst for controllable oxidation with molecular oxygen.^23,24 In this work, Ag₆[PV₃Mo₉O₄₀] was prepared with the same procedure as its analogy (Ag₅[PV₂Mo₁₀O₄₀]).²³ Polymeric coordinate materials were formed through self-assembly with nitrogen ligands in CH₃CN solution. Different N-ligands lead to different self-assembled nanostructures, and several of them exhibit high activity in catalyzing C–H direct oxidation with 1.0 atm oxygen.

Experimental

Materials

All chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. Acetonitrile was distilled and purified before using. All the reagents were used as received.

Ag₆PV₃Mo₉O₄₀. To 40 mL of 0.214 M aqueous Na₆PV₃Mo₉O₄₀ (8.55 mmol) at 25 °C was added AgNO₃ (8.71 g, 51.2 mmol). The resulting red-orange amorphous powder was washed three times with water (25 °C) and three times with ether and dried to afford 5.0 g (26.3% yield based on Na₆PV₃Mo₉O₄₀).

Self-assembled nanostructures. Nanostructures were obtained through self-assembly of Ag₆[PV₃Mo₉O₄₀] with N-donor ligands in CH₃CN solution. Ag₆[PV₃Mo₉O₄₀] was dissolved in extra pure and dry CH₃CN (100 ml, 1.0 × 10⁻³ M). CH₃CN solution of a N-donor ligand (10 ml, 0.1 M) was added by automatic microinjector over 6.0 h with gentle stirring. The mixture was stored at room temperature for 24 h. The product was separated by high-speed centrifugation and washed with ethanol three times.

Characterization methods. X-ray photoelectron spectroscopy data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al-Kα radiation. The base pressure was about 3 × 10⁻⁹ mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. Eclipse V2.1 data analysis software supplied by the VG ESCA-Lab200I-XL instrument manufacturer was applied in the manipulation of the acquired spectra. Transmission electron microscopy (TEM) was obtained by a JEOL 2010 TEM with an accelerating voltage of 200 kV. The leaching amount of silver species was detected by inductively coupled plasma atomic emission spectroscopy (ICP-OES) using Perkin Elmer Optima 5300 dv. Reaction products were analyzed by GC and GC-MS. GC was carried out over GC-2014 (SHIMADZU) with a high temperature capillary column (MXT-1, 30 m, 0.25 mm ID) and FID detector. GC-MS was carried out over GCT Premier/Waters with capillary column (DB-5MS/J&W Scientific, 30 m, 0.25 mm ID).

Results and discussion

Self-assembled nanostructures of Ag₆[PV₃Mo₉O₄₀] with N-donor ligands

Monovalent silver (Ag⁺) exhibits a high coordinative flexibility in coordination numbers between 2 and 6 and in typical closed-shell ion polyhedra.²⁴ Strong N-donor ligands coordinate Ag⁺ linearly in cationic complexes, with trigonal or tetrahedral environments, depending on the strength of the competing ligands and stoichiometry.^25,26 For Ag₆[PV₃Mo₉O₄₀], Ag⁺ is bonded to the phosphovanadomolybdate polyanion with strong static electronic interaction. The Ag⁺ is polarizable and has a high tendency to coordinate soft Pearson bases (aromatic N-donor ligands) with covalent bonding contributions. When introducing N-donor ligands into a CH₃CN solution of Ag₆[PV₃Mo₉O₄₀] (1.0 × 10⁻³ M), the self-assembly of the POM cluster with N-donor ligands occurs through [Ag^I←N] coordinate interaction. In our experiments, twelve aromatic N-donor ligands were attempted. Four typical self-assembled nanostructures were obtained (as shown in Fig. 1).

Fig. 1 (a) Zero-dimensional nanostructures, nanoparticles (1a), from the self-assembly of Ag₆[PV₃Mo₉O₄₀] with mononitrogen donor benzoimidazole. (b) One-dimensional nanostructures, nanorods (1b), from the self-assembly of Ag₆[PV₃Mo₉O₄₀] with 4,4′-dipyridine. (c) Self-aggregate balance of Ag₆[PV₃Mo₉O₄₀] clusters in CH₃CN solution. (d) Self-assembled nanostructures (1d) from Ag₆[PV₃Mo₉O₄₀] and 2,2′-dipyridine. (e) Two-dimensional nanostructures, nanosheets (1e), from the self-assembly of Ag₆[PV₃Mo₉O₄₀] with 1,10-phenanthroline.

It is a generally accepted fact that soluble inorganic ions should distribute homogeneously in dilute solutions. But soluble POM macroanions in a polar solvent usually do not exist as single ions. The Tyndall effect was observed from the POM solutions.²⁷ The Tyndall effect is defined as the visible light scattering from suspended particles of colloidal size. In the solution of Ag₆[PV₃Mo₉O₄₀], the balance of self-aggregate of primary units is the existing pattern of POM clusters (Fig. 1c).

Zero-dimensional nanostructures, nanoparticles (1a), result from the self-assembly of Ag₆[PV₃Mo₉O₄₀] with mononitrogen donor benzoimidazole (Fig. 1a). Nanoparticles are dispersed well and their average size is about 10 nm. One-dimensional nanostructures, nanorods (1b), were prepared by the self-assembly of Ag₆[PV₃Mo₉O₄₀] with 4,4′-dipyridine (Fig. 1b). Two-dimensional nanostructures, nanosheets (1e), were prepared by introducing 1,10-phenanthroline into Ag₆[PV₃Mo₉O₄₀] CH₃CN solution (Fig. 1e). Fig. 1d shows nanostructures (1d) between nanorods and nanosheets, which were prepared through the procedure involving 2, 2′-dipyridine. It is revealed that the morphology of self-assembled nanostructures has a subtle relation to the molecular structure of N-donor ligands. Monodentate ligands lead to zero-dimensional nanostructures, nanoparticles. Linear didentate bridging ligands result in linear nanorods. Planar didentate chelating ligands develop two- dimensional nanostructures, nanosheets.

Nonplanar didentate chelating ligands produce a transition morphology between nanorods and nanosheets. Sub-structures of self-assembled nanostructures were further characterized by TEM with higher magnification. Fig. 2b–d shows closer views of the sub-structures of nanorods (1b). It is revealed that nanorods are built up by arranging sub-nanostructures, nanoclusters with size of about 1.0 nm, which meet the scale of a Ag₆[PV₃Mo₉O₄₀] cluster.^23,24 The nanoparticles (10 nm) in Fig. 1a are self-assembled structures of Ag₆[PV₃Mo₉O₄₀] with mononitrogen donor benzoimidazole. The silver 3d_5/2 XPS peak of Ag₆[PV₃Mo₉O₄₀] has a binding energy of 368.2 eV, which is indexed to a Ag^I ion chelated to oxygen.²⁸ In nanorods from 4,4′-dipyridine and Ag₆[PV₃Mo₉O₄₀], the binding energy of Ag 3d_5/2 shows a marked shift and the peak is broadened (Fig. 3). The [Ag^I←N] coordinate interaction can increase the electron density of the Ag^I ion and lower the binding energy of Ag 3d_5/2. The involvement of the N-donor complicates the chemical state of the Ag^I ion and expands the half peak width of Ag 3d_5/2.

Fig. 2 (a) Nanorods from the self-assembly of Ag₆[PV₃Mo₉O₄₀] with 4,4′-dipyridine. (b), (c), (d) TEM images of sub-structures of nanorods.

Fig. 3 Ag 3d_5/2, Ag 3d_3/2 XPS peaks of Ag₆[PV₃Mo₉O₄₀] and nanorods (1b).

The correlation between nanoscale nanostructures and self-assembly conditions in solution

Fig. 1c illustrates the self-assembly of Ag₆[PV₃Mo₉O₄₀] clusters with N-ligands in CH₃CN solution. Assembly is controlled by the balance of attractive and repulsive forces.²⁹ Suppose that U_total(x) is the total net potential of the entire self-assembly process with AgPOM clusters. The U_total(x) can be formulated as the net total of all of the attractive and repulsive potentials involved in each step of the self-assembly:³⁰

U_total(x) = f_P|U_A,P(x) + U_R,P(x)| + f_S|U_A,S(x) + U_R,S(x)| + f_H|U_A,H(x) + U_R,H(x)| + ··· + U_ext(x)

with

U _A,P(x) is the attractive potential of the self-assembly of primary building units and U_R,P(x) represents the repulsive potentials. U_A,S(x) and U_R,S(x) are for the self-assembly of secondary building units, and U_A,H(x) and U_R,H(x) are for the self-assembly of higher-order building units. f_P, f_S, and f_H are the fraction coefficients of each self-assembly step to the total net potential. U_ext(x) is the potential contribution from the external forces.

For the self-assembly of Ag₆[PV₃Mo₉O₄₀] clusters with N-ligands in CH₃CN solution, U_ext(x) is zero. When Ag₆[PV₃Mo₉O₄₀] is dissolved in CH₃CN, U_total(x) is equal to zero for the dissolution balance. The presence of strong chelating N-ligands for silver ion can change the chemical environment around the Ag₆[PV₃Mo₉O₄₀] clusters. This affects the balance of attractive and repulsive forces, which govern the self-assembly processes.

There are three possibilities:³¹ when U_total(x) is positive for the new system, the self-assembly is not possible in most cases. When U_total(x) is negative, the self-assembly is kinetically driven. The self-assembly occurs until most of the primary units are exhausted. The resulting structures show indefinite sizes and poor physical shapes. The higher the value of |U_total(x)|, the faster the self-aggregates form. When U_total(x) is equal to or close to zero, the self-assembly is thermodynamically driven. The self-assembly steps are in or close to the equilibrium with the self-aggregates. The resulting structures have finite sizes and well-defined shapes.

In our experiments, the self-assembly of Ag₆[PV₃Mo₉O₄₀] with N-donor ligands was carried out in diluted CH₃CN solution. Ag₆[PV₃Mo₉O₄₀] was dissolved in extra pure and dry CH₃CN (100 ml, 1.0 × 10⁻³ M). CH₃CN solution of an N-donor ligand (10 ml, 0.1 M) was added by automatic microinjector with gentle stirring. The rate of injection of an N-ligand solution is a key parameter governing the final self-assembled nanostructures. In the self-assembly of Ag₆[PV₃Mo₉O₄₀] with benzoimidazole, the structures shown in Fig. 4a were obtained by the injection of 10 ml CH₃CN solution containing 1.0 mmol benzoimidazole in 5.0 min. The structure in Fig. 4b was synthesized in 1.0 h and the injection time of Fig. 4c was 3.0 h. Well-defined nanoparticles were achieved through 6.0 h injection of 10 ml CH₃CN solution containing 1 mmol benzoimidazole (Fig. 4d). It is clearly revealed that the fast introduction of N-ligands leads to structures with indefinite sizes and poor physical shapes. The controlled and gradual introduction of N-ligands results in well-defined nanostructures. The same phenomena were observed in the self-assembly processes of Ag₆ [PV₃Mo₉O₄₀] with 4,4′-dipyridine (Fig. 5) and 1,10-phenanthroline (Fig. 6).

Fig. 4 TEM images of self-assembled structures of Ag₆[PV₃Mo₉O₄₀] clusters with benzoimidazole in CH₃CN solution. Self-assembly time: (a) 5.0 min; (b) 1.0 h; (c) 3.0 h; (d) 6.0 h.

Fig. 5 Images of self-assembled structures of Ag₆[PV₃Mo₉O₄₀] clusters with 4,4′-dipyridine in CH₃CN solution. Self-assembly time: (a) 5.0 min; (b) 1.0 h; (c) 3.0 h; (d) 6.0 h.

Fig. 6 TEM images of self-assembled structures of Ag₆[PV₃Mo₉O₄₀] clusters with 1,10-phenanthroline in CH₃CN solution. Self-assembly time: (a) 5.0 min; (b) 1.0 h; (c) 3.0 h; (d) 6.0 h.

If the disturbance of N-ligands cannot be buffered (with a high rate of injection of N-ligands), the self-assembly is kinetically driven (U_total(x) < 0). Less-defined structures were obtained (Fig. 4a, Fig. 4b, Fig. 5a, Fig. 5b, Fig. 6a, and Fig. 6b). When the introduction of N-ligands was in a controlled and gradual way, the self-assembly is thermodynamically driven (U_total(x) is close to zero). Well-defined nanostructures were achieved (Fig. 4d, Fig. 5d, and Fig. 6 d).

As discussed above, self-assembled nanostructures have subtle relations with the molecular structure of N-ligands. Chemical heterogeneity around Ag₆[PV₃Mo₉O₄₀] clusters induces site-specific interactions with N-ligands in the solution-based self-assembly.³² Coordinative flexibility of silver ions and N-ligands generates a variety of structures. Monodentate ligands lead to zero-dimensional nanostructures (nanoparticles, Fig. 4d). Linear didentate bridging ligands result in linear nanorods (Fig. 5d). Planar didentate chelating ligands develop two-dimensional nanostructures (nanosheets, Fig. 6d). Nonplanar didentate chelating ligands produce transition morphology between nanorods and nanosheets (Fig. 1c).

Catalytic activity

Self-assembly has been recognized as a powerful strategy for manipulating the structure and properties of ensembles of inorganic nanoparticles.³³ N-ligand directed ensembles of Ag₆[PV₃Mo₉O₄₀] nanoclusters exhibit collective properties, which are different from those of individual polyoxometalate cluster and bulk materials. H_3+xPV_xMo_12−xO₄₀ (x = 1–6) phosphovanadomolybdate Keggin-type polyoxometalates are active catalysts for oxygen activation.²² Silver salt of phosphovanadomolybdate shows a high thermal stability (>800 °C) and has great potential in acting as a heterogeneous catalyst for controllable oxidation with molecular oxygen.^23,24 The catalytic activity of self-assembled nanostructures was tested for the direct oxidation of the C–H bond adjacent to aromatic cycle. The analogous reactions over homogeneous polyoxometalate systems have been explored through well-designed methods, as listed in Table S1, ESI.† In our experimental observation, these as-synthesized nanostructures have improved catalytic activity.

Oxidation of diphenylmethane to benzophenone with oxygen was selected as a model reaction. The catalyst 1b shows the highest activity in solvent of benzonitrile at 100 °C (as shown in Fig. S1, ESI†). Table 1 lists the experimental results of various heterogeneous silver catalysts and control tests without silver polyoxometalates. It is demonstrated that 1a, 1b, 1d, and 1e are efficient catalysts for activation of molecular oxygen. The conversions of the substrate under 1b and 1e are near 100%. The substrates for catalytic oxidation are expanded, as shown in Table 2. Nanocatalyst 1b show highly catalytic activity for diphenylmethane, fluorene, and 9H-xanthene to produce corresponding ketones. Oxidation of triphenylmethane leads to a complex mixture and shows only 36% selectivity to triphenylmethanol. Oxyfunctionalization of alkyl aromatics is an important industrial process for producing intermediates for plastic, alkyd and polyester resins, dyes, and numerous fine chemicals. General catalytic processes are carried out over V₂O₅ catalysts at 350–550 °C with excess air. Homogeneous liquid-phase oxidation (Rhône-Progil process) can proceed at a low temperature of 150–200 °C, but requires high radical catalyst concentration with cocatalysts containing bromine. In our experimental observation, heterogeneous nanocatalyst 1b, shows medium catalytic activity for selective oxidation of 1,2,3,4-tetrahydronaphthalene, 1,2,4,5-tetramethylbenzene, and 1,3,5-trimethylbenzene at 100 °C with 1.0 atm oxygen. Unexpectedly, phthalic anhydride is formed from oxidation of o-xylene under such mild reaction conditions. Nanocatalysts from the self-assembly of twelve N-donor ligands and Ag₆[PV₃Mo₉O₄₀] were investigated and the results were listed in Table S2.† There are two possible reaction mechanisms: (1) the direct activation of molecular oxygen by the catalyst, (2) a Fenton-type radical chain mechanism. N-ligand directed ensembles of Ag₆[PV₃Mo₉O₄₀] nanoclusters exhibit collective properties, which include these two catalytic properties.

Table 1 Catalytic activity tests of different silver heterogeneous catalysts in oxidizing diphenylmethane with molecular oxygen^a

Catalyst	Conversion/selectivity (%)
a Reaction conditions: 2.0 mmol diphenylmethane, 1.0 ml PhCN, 100 °C, oxygen balloon, 50 mg catalyst, reaction time 10 h.
Blank	<5.0
AgCl	11.3/78.3
Ag2MoO4	13.4/72.2
AgVO3	21.4/83.5
Ag6[PV3Mo9O40]	48.6/90.1
Na6PV3Mo9O40	29.7/87.5
H6PV3Mo9O40	34.1/91.2
Benzoimidazole	<5.0
1a	98.2/95.2
4,4′-Dipyridine	<5.0
1b	>99.9/99.2
2,2′-Dipyridine	<5.0
1d	97.8/94.3
1,10-Phenanthroline	6.7
1e	>99.9/98.7

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Table 2 Catalytic oxidation of alkyl aromatics over 1b with molecular oxygen^a

Substrate	Product/Selectivity (%)	Conversion (%)
a Reaction conditions: 2.0 mmol substrate, 1.0 ml PhCN, 100 °C, oxygen balloon, 50 mg catalyst (1b), reaction time 10h.
	99.2	>99.9
	99.0	>99.9
	99.0	96.7
	36	82.0
	95.4	65.2
	89.6	47.6
	36	32.3
	47
	75	36.8
	25
	90.4	19.7
	86.7	30.9
	62.3	28.5

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Phthalic anhydride (PA) is an important chemical for the manufacture of phthalate plasticizers, alkyd and polyester resins, and phthalocyanine dyes. The word capacity for PA was about 4.5 × 10⁶ t/a in 2005.³⁴ BASF has developed commercial gas-phase oxidation of o-xylene to PA.³⁵ The process is carried out at 375–410 °C over V₂O₅–TiO₂ catalysts in multitube reactors with about 10 000 tubes. The Air/feed ratio is 18 : 1 and the contact time of feed with the catalyst is <0.5 s. The oxidation of o-xylene to PA by 1b was further investigated in a one-batch nonsolvent test (Fig. 7). The conversion of o-xylene increases with the pressure of oxygen in the sealed autoclave and the selectivity to PA reaches 40% at 100 °C under 3.0 MPa oxygen.

Fig. 7 Nonsolvent oxidation of o-xylene to phthalic anhydride catalyzed by 1b with O₂ and N₂O. Reaction conditions: 2.0 ml o-xylene, 100 °C, O₂ or N₂O, 50 mg catalyst, 25 ml autoclave with a magnetic stir, reaction time 10 h.

Under the same reaction conditions, there are no oxidation reactions over the V₂O₅–TiO₂ catalyst.³⁶ Heterogeneous Co and Mn catalysts for liquid phase o-xylene oxidation show the selectivity (5 mol %) to PA at the o-xylene conversion of 20 mol% under 172 °C.³⁷ N₂O was found to be the active oxidant for converting o-xylene to PA over nanocatalyst 1b. PA is the main product and its selectivity reaches 85% at the conversion of 25% at 100 °C under 3.0 MPa N₂O (Fig. 7). N₂O is a safer oxidant than oxygen. Higher reaction pressure can be attempted to increase the conversion of o-xylene. This shows a possibility of developing a mild oxidation process of o-xylene to PA based on nanocatalyst 1b. The mild oxidation process can abate the byproducts and avoid the danger of explosion and CO₂ from total oxidation at a high reaction temperature. However, the converting efficiency must be improved. There are two solutions: (1) improving the catalytic activity of the catalysts, (2) improving feed recycling and contact with catalyst in the reactors, like the multitube reactor of the BASF process. Further investigation on this topic is under way.

The recyclability of these nanocomposites was investigated by a five-run test of the catalytic oxidation of diphenylmethane over 1b. After each test run, the catalyst was separated by filtration and washed three times with acetone, then dried for the next run. The catalytic activity was denoted by TOF. Fig. 8a shows the results of the five-run test. After the fifth test, there is a nearly 25% decrease in the catalytic activity. Fig. 8b is the TEM image of the used catalyst. The one-dimensional nanostructure (nanorod), can be clearly discriminated. The nanorod is built up by arranging sub-nanostructures (Ag₆[PV₃Mo₉O₄₀] clusters), as observed in the TEM image of the fresh catalyst (1b) (Fig. 2b). This proves that the nanostructure of the used catalyst remains constant. During the catalytic process, the leaching out of some active species may take place. The decomposition of edge sites is the main reason that causes leaching of the catalyst. The concentration of silver ions in the reaction solvent can be used as a quantitative index to the degree of catalytic stability. If a high concentration of silver ions were detected after the reaction, serious decomposition of the catalyst might happen. Fig. 8c shows the silver ion concentrations in the reaction solvent after each test run. After the first test run, the leaching silver concentration is 124 ppb. In the following test runs, the value is in the range 20 to 40 ppb. The relatively high concentration of leaching silver in the first run is attributed to loosely chelated Ag₆[PV₃Mo₉O₄₀] clusters in the surface edge defects. However, the leaching amount on such a negligible scale shows no marked influence on the catalytic behavior. This has been confirmed by Fig. 8a and Fig. 8b. In summary, self-assembled nanostructures of Ag₆[PV₃Mo₉O₄₀] with N-donor ligands can tolerate the operation conditions of the liquid-phase oxidation process. They can be recycled efficiently.

Fig. 8 a) Five-run test of recyclability of 1b. Reaction conditions: 2.0 mmol diphenylmethane, 1.0 ml PhCN, 100 °C, oxygen balloon, 50 mg catalyst, reaction time 10 h. (b) TEM image of the used catalyst. (c) The silver ion concentrations in the reaction solvent after each test run.

Conclusion

In summary, four types of nanostructures were obtained by self-assembly of Ag₆[PV₃Mo₉O₄₀] with N-donor ligands. Monodentate ligands lead to zero-dimensional nanostructures, nanoparticles. Linear didentate bridging ligands result in linear nanorods. Planar didentate chelating ligands develop two-dimensional nanostructures, nanosheets. Nonplanar didentate chelating ligands produce a transition morphology between nanorods and nanosheets. Kinetically driven self-assembly (U_total(x) < 0) leads to less-defined structures. When the introduction of N-ligands took place in a controlled and gradual way, the self-assembly is thermodynamically driven (U_total(x) is close to zero). Well-defined nanostructures were achieved.

It is demonstrated that these nanocomposites are active catalysts for activating molecular oxygen under mild conditions. In oxyfunctionalization of alkyl aromatics, nanorods (1b) show highly catalytic activity for diphenylmethane, fluorene, and 9H-xanthene to produce the corresponding ketones, and medium catalytic activity for selective oxidation of 1,2,3,4-tetrahydronaphthalene, 1,2,4,5-tetramethylbenzene, and 1,3,5-trimethylbenzene at 100 °C with 1.0 atm oxygen. Phthalic anhydride is formed through 1b catalyzed oxidation of o-xylene under such mild reaction conditions. Under nonsolvent reaction with 3.0 MPa O₂ in a sealed autoclave, the selectivity to PA increases to 40% at 33.1% conversion. Under the same conditions, N₂O shows 85% selectivity to PA at 25.3% conversion of o-xylene.

Acknowledgements

This work was financially supported by the National Natural Sciences Foundation of China (no. 21101161, 21174148).

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Table S1, Table S2, and Fig. S1. See DOI: 10.1039/c2ra21858e

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