Highly dispersed platinum and phosphomolybdic acid (PMo) on the UiO-66 metal–organic framework (MOF) for highly efficient and selective hydrogenation of nitroaromatics

Abstract

Pt-PMo@UiO-66 catalysts synthesized via a solvothermal method utilizing phosphomolybdic acid as a stabilizing agent exhibit exceptional catalytic performance in the selective hydrogenation of nitroarenes. The nanostructure of the Pt-PMo@UiO-66 catalysts is demonstrated by ICP-OES, XRD, XPS, TEM, HRTEM, STEM-EDS and other characterization studies. The Pt nanoparticles (NPs) or clusters are mainly supported on the surface of UiO-66. There is a synergistic catalytic effect between Pt, phosphomolybdic acid and UiO-66, improving their catalytic hydrogenation performance (activity, selectivity and stability) for nitroarene hydrogenation. It is found that 0.32%Pt-PMo@UiO-66 has the best catalytic properties for nitroacetophenone hydrogenation, with 98.8% conversion and 99.6% selectivity to aminoacetophenone under relatively mild reaction conditions (1.75 h, 40 °C, 3.0 MPa H₂). The turnover frequency (TOF) reaches 1498.0 h⁻¹. The catalyst also shows high selectivity for the hydrogenation of halogen-containing nitroarenes without dehalogenation. The appropriate size of the Pt NPs (4.44 nm) and the electronic effect (electron transfer from Pt to Mo) improves the catalytic hydrogenation activity and selectivity of Pt-PMo@UiO-66 for the selective hydrogenation of nitroarenes under moderate reaction conditions.

---

1 Introduction

As important industrial chemicals, aromatic amines can be widely applied in the production of pharmaceuticals, pigments, dyes and agricultural chemicals.^1–6 However, chemical engineers predominantly often synthesize aromatic amines by reducing nitroarenes using chemical reducing agents in the chemical industry.^7,8 Nevertheless, these production methods exhibit various drawbacks such as low efficiency, significant environmental pollution, and challenges in achieving the objectives of green chemistry.^9,10 In addition, the selectivity of the functional groups undergoing reduction cannot be guaranteed when there are two or more reducible functional groups in nitroarenes and it is hard to achieve high yields of the ideal target products.^11,12 Therefore, it remains a challenge to obtain a recoverable heterogeneous catalyst with excellent catalytic properties and selectivity for one –NO₂ group hydrogenation at room temperature.^13,14

The catalysts for nitroarene hydrogenation reported in the literature primarily consist of non-noble metals, such as Cu-,^15,16 Co-,^17,18 Ni-^19,20 based catalysts, etc. and noble metals, such as Au-,^21,22 Pd-,^23–25 Pt-,^26–28 Rh-^29,30 based catalysts. Although non-noble metals are cheaper, the reaction conditions are harsh, and their catalytic performance is often poor.^31,32 The cost of the noble metal-based catalysts is relatively high, but their catalytic performance is relatively excellent. Extremely high aromatic amine yields can be obtained at room temperature at low reaction pressure, so noble metal-based catalysts are widely used. Noble metal-catalysts such as Pd and Pt are widely used in the catalytic reduction of nitroarenes, because the outermost electron orbitals are not filled and there are vacancies that easily form active intermediates.^33–36 Pt is extensively applied in the field of catalysis due to its high activity and low hydrogen dissociation temperature. However, the excellent catalytic hydrogenation activity of Pt-based catalysts results in poor selectivity for the hydrogenation of specific groups, therefore improving the selectivity of the Pt-based monometallic catalysts for specific group hydrogenation remains a challenge.

As an excellent three-dimensional framework structure, metal–organic frameworks (MOFs) have the characteristics of porosity and structural diversity.^37–39 These exceptional properties facilitate the efficient loading of the precious metals onto the MOF support. Guo⁴⁰et al. added chloroplatinic acid and hydrochloric acid solutions into UiO-66, forming a mixture; chloroplatinate ions were introduced into the UiO-66 structure by the impregnation method and reduced to obtain the Pt@UiO-66-X catalyst. In addition, phosphomolybdic acid, as a type of polyoxometalate, has the characteristics of high anionic charge and oxygen-rich surface.^41,42 These characteristics allow phosphomolybdic acid to combine with noble metal elements (e.g., Pt), making the noble metals stably supported and highly dispersed by the interaction of noble metal–phosphomolybdic acid. UiO-66 was chosen as the support due to its outstanding thermal stability and moderate pore size.⁴³ The pore size of UiO-66 is similar to that of phosphomolybdic acid, which allows the active components of the proposed catalyst to be well and highly dispersed.⁴⁴

Herein, Pt-PMo@UiO-66 catalysts with different Pt loadings were prepared using PMo@UiO-66 as the support. The influence of Pt loading on the catalytic performance of Pt-PMo@UiO-66 for the selective hydrogenation of nitroarenes was investigated, and the corresponding reasons were explored.

2 Experimental

2.1 Materials

Zirconium chloride (ZrCl₄), phosphomolybdic acid (H₃PMo₁₂O₄₀·xH₂O), terephthalic acid (C₈H₆O₄), platinum acetyl acetone (C₁₀H₁₄O₄Pt), N,N-dimethylformamide (DMF, C₃H₇NO), acetic acid solution (CH₃COOH), ethanol (C₂H₅OH) and methanol (CH₃OH) were purchased from Aladdin. The above reagents were used directly without further treatment.

2.2 Preparation of catalysts

The Pt-PMo@UiO-66 catalysts were synthesized by the solvothermal method. 0.70 g ZrCl₄ and 0.50 g terephthalic acid were weighed and put into a conical flask, then 60 mL N,N-dimethylformamide (DMF) was injected into the above mixture. After 30 min ultrasound, 0.05 g phosphomolybdate and a certain amount of acetyl acetone platinum (Pt(acac)₂) was weighed and placed into a conical flask, then 3.0 mL acetic acid solution was added. After another 30 min ultrasound, it was stirred again for 30 min at room temperature to mix them well.

After stirring, the mixture was transferred to a Teflon-lined hydrothermal kettle, and then the kettle was placed in an oven and maintained at 120 °C for 12 h. After natural cooling to room temperature, the colloid was added into the centrifuge tube and centrifuged. Then the solid was washed several times with solvents (methanol and DMF), and the as-obtained sample was dried in vacuum at 80 °C for 12 h to ensure complete removal of solvents. Then it was ground into a powder and put into a tube furnace. It was heated in a mixed atmosphere (90%N₂ + 10%H₂) at 150 °C for 2 h with the heating speed of 5 °C min⁻¹ and then it was cooled to room temperature naturally. The powder is denoted as Pt-PMo@UiO-66, the Pt content was regulated by changing the amount of acetyl acetone platinum. The preparation process of PMo@UiO-66 was consistent with the above steps except that Pt(acac)₂ was not added.

2.3 Catalyst characterization

Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to reveal the metal contents in the catalysts (Agilent 725). The crystal structures were determined by X-ray diffraction (XRD), which was conducted on a Rigaku D/MAX 2000 PC X-ray diffractometer equipped with Cu Kα (λ = 0.15406 nm, 40 kV, 30 mA). The Fourier transform infrared spectroscopy (FT-IR) profiles were collected at 400–2000 cm⁻¹ (Bruker ALPHA FT-IR spectrometer). The specific surface areas and pore structures of the samples were obtained using an ASAP 2020 Micromeritics (BET). In order to analyze the thermal stability of the samples, the thermo-gravimetry (TG) test was performed in N₂ on an SDT650 thermal analyzer with the heating speed of 10 °C min⁻¹. For obtaining the composition and chemical states of the surface species, X-ray photoelectron spectroscopy (XPS) spectra were collected using a PHI 5000 Versa Probe XPS spectrometer. Transmission electron microscopy (TEM) measurements were taken on a JEM-F200 microscope to obtain the nanostructure of the catalysts. High resolution TEM (HRTEM) and scanning transmission electron microscopy (STEM) images, and STEM-energy dispersive X-ray spectroscopy (EDS) elemental mapping of the samples were collected using a TECNAI F30.

2.4 Catalytic hydrogenation of nitroarenes

Catalytic hydrogenation of nitroarenes was performed in an autoclave. Before the reaction, the catalyst (0.05 g) and nitroaromatic solution (10 mL) were added to the autoclave liner and mixed, purging the autoclave with hydrogen for a few minutes and then sealing it. The hydrogen pressure was raised to the target pressure (3.0 MPa). The reaction was conducted at 40 °C with a stirring speed of 870 rpm. After the reaction was finished, the quantitative and qualitative analysis of the liquid was performed by gas chromatography (Agilent GC 7820A) and gas chromatography-mass spectrometry (Agilent GC-MS 7890B 5977), respectively. And the conversion and selectivity of the catalysts could be calculated from the above analysis results. The catalytic activity can be evaluated from turnover frequency (TOF), as given by the following formula.³⁴

3 Results and discussion

3.1 Catalyst characterization

As given in the ICP-OES results, it can be seen that the loading of Pt in the Pt-PMo@UiO-66 catalysts is 0.11 wt%, 0.32 wt%, and 0.71 wt% (labeled 0.11%Pt-PMo@UiO-66, 0.32%Pt-PMo@UiO-66, and 0.71%Pt-PMo@UiO-66, respectively). The Mo content shows little difference among various catalysts. The metal dispersion of Pt (D) in the samples obtained by CO chemical adsorption was 0.57, 0.46, and 0.41, respectively (Table 1).

Table 1 Contents of the metal elements in the catalysts

Catalyst	Elemental content (wt%)
	Pt/%	P/%	Mo/%
0.11%Pt-PMo@UiO-66	0.11	0.06	2.00
0.32%Pt-PMo@UiO-66	0.32	0.06	2.09
0.71%Pt-PMo@UiO-66	0.71	0.06	1.99

---

To determine the crystallite structure of the catalysts and support, XRD was applied to characterize the catalysts, the results are presented in Fig. 1a. The overall structure of UiO-66 showed no change after the doping of phosphomolybdic acid, and the loading of the precious metal (Pt) did not modify the phase structure of UiO-66. No characteristic peak of Pt is found in the XRD patterns of the catalysts, it is probably because the Pt-related species are relatively evenly dispersed or exist in the form of tiny clusters, single atoms or in an amorphous form.

Fig. 1 (a) XRD patterns, (b) FT-IR spectra, (c) N₂ adsorption–desorption isotherms and (d) pore size distributions of different samples.

The catalysts and support were investigated by FT-IR to explore the structures of the samples (Fig. 1b), the FT-IR spectra of the catalyst precursor (Pt(acac)₂-PMo@UiO-66), support (UiO-66) and catalyst (0.32%Pt-PMo@UiO-66) could be observed. The peaks at 1396 and 1590 cm⁻¹ correspond to the symmetric and asymmetric tensile vibrations of terephthalic acid (O=C=O). The peaks at 1510 cm⁻¹ are attributed to the vibration of the benzene ring, and the peaks at 1000–1200 cm⁻¹ are assigned to the tensile vibration of Zr–O. The tensile vibrations of Zr–O_μ3-O and Zr–O_μ3-OH correspond to the peaks located at 660 and 480 cm⁻¹,⁴⁵ indicating that the as-synthesized catalysts retain the structure of UiO-66. The peak of 960 cm⁻¹ is due to the tensile vibration of Mo–O_d,⁴⁶ which is relatively weak. But it still remains the Keggin structure of phosphomolybdic acid, which further indicates that UiO-66 is successfully doped with phosphomolybdic acid.

The results of nitrogen adsorption–desorption are presented in Fig. 1c, d and Table 2. In Fig. 1c, the catalysts and support exhibit type IV isotherms with obvious adsorption hysteresis, indicating that the Pt-PMo@UiO-66 catalysts have an ordered three-dimensional mesoporous structure. But the pore size of Pt-PMo@UiO-66 is smaller than that of PMo@UiO-66 as some pores are occupied; it can be found that their specific surface area decreases after Pt is loaded on PMo@UiO-66. Besides, the specific surface area decreases with the enhancement of the Pt content. The possible reason is that the specific surface area decreases owing to the Pt particles entering the pore. Fig. 1d shows the pore size distribution of the catalysts and support. All samples are mesoporous materials, and the mesoporous structure of the support does not change with the Pt doping. The average pore size of the samples ranges from 30 to 40 nm. When the loading of Pt increased to 0.71%, the average pore size is smallest because a larger number of Pt NPs enter the pore of the support.

Table 2 Specific surface area, pore volume and average pore diameter of different samples

Samples	Specific surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore diameter (nm)
PMo@UiO-66	616.9	0.54	39.6
0.11%Pt-PMo@UiO-66	542.7	0.48	38.5
0.32%Pt-PMo@UiO-66	468.7	0.40	36.6
0.71%Pt-PMo@UiO-66	461.4	0.34	33.3

---

To investigate the structural changes of the catalysts and support during heat treatment, thermogravimetric (TG) analysis was conducted. Fig. 2 manifests the TG curves of PMo@UiO-66 and Pt-PMo@UiO-66 with different Pt loadings. Their thermogravimetric curves are basically the same, but the temperature of the weight loss peak and residual mass is slightly different, which is probably due to various Pt contents. The pyrolysis residue of the catalyst also increases with the enhancement of the Pt loading. TG-DTG curves were acquired after analyzing the thermogravimetric data (Fig. 2). The TG-DTG curve of 0.11%Pt-PMo@UiO-66 (Fig. 2b) can be used to observe the weight loss at three temperatures in various steps: 60 °C, 168.3 °C, 574.3 °C, respectively. The first weight loss may be caused by the loss of surface “free-water”. The removal of the “crystal-water” results in the second weight loss. The collapse of the support structure leads to the third weight loss at 574.3 °C. It is obvious that the weight loss peaks of the 0.32%Pt-PMo@UiO-66 and 0.71%Pt-PMo@UiO-66 catalysts (Fig. 2c and d) are basically similar to those of 0.11%Pt-PMo@UiO-66. Three weight loss events of 0.32%Pt-PMo@UiO-66 occur at 60.2 °C, 185.7 °C, and 565.0 °C, and those of E-mail: 0.71%Pt-PMo@UiO-66 occur at54.3 °C, 218.3 °C, and 566.4 °C. It can be found that the Pt doping has little influence on the support and the thermal stability is basically the same. The basic framework structures of the catalysts are not changed after being reduced in 90%N₂ + 10%H₂ at 150 °C. The thermogravimetric testing indicates that the catalysts exhibit relative stability during the reduction process and catalytic reactions.

Fig. 2 TG and DTG curves of (a) PMo@UiO-66, (b) 0.11%Pt-PMo@UiO-66, (c) 0.32%Pt-PMo@UiO-66, (d) 0.71%Pt-PMo@UiO-66.

The XPS characterization results are displayed in Fig. 3 to further explore the chemical states of surface species of the catalysts and analyze them qualitatively. The XPS survey spectra of Pt-PMo@UiO-66 with different Pt loadings are presented in Fig. 3A, which indicate that the catalysts contain Pt, C, Mo, Zr, and O elements. It can be seen from Fig. 3B that there are three peaks of C–C, C–O, and O–C=O species, corresponding to the fitting peaks with the binding energies of 284.8 eV, 286.2 eV and 288.7 eV (C 1s), respectively.⁴⁷ However, the binding energy of C–O species in the PMo@UiO-66 support shows a positive shift of 0.1 eV compared with that of the Pt-PMo@UiO-66 catalysts, which may be caused by some carbon residue in the pore channel. In the Zr 3d spectra of different samples (Fig. 3C), the Zr 3d peak positions of PMo@UiO-66 and Pt-PMo@UiO-66 are identical, and the binding energies are 182.85 eV and 185.2 eV.⁴⁸ It shows that the Pt loading does not change the binding energy of the Zr 3d XPS spectra.

Fig. 3 (A) XPS survey spectra, (B) C 1s, (C) Zr 3d, (D) Mo 3d and (E) Pt 4f XPS spectra of the support and different catalysts: (a) 0.11%Pt-PMo@UiO-66, (b) 0.32%Pt-PMo@UiO-66, (c) 0.71%Pt-PMo@UiO-66, (d) PMo@UiO-66.

The Mo 3d XPS results are shown in Fig. 3D. In the Mo 3d spectrum of the support, the binding energies at 231.1 eV and 234.4 eV correspond to Mo²⁺ species, and the peaks at 232.3 eV and 235.4 eV can be assigned to Mo⁶⁺ species.⁴⁹ In the comparison spectra of the Mo 3d XPS spectra, the binding energies of the Mo⁶⁺ species of PMo@UiO-66 and Pt-PMo@UiO-66 catalysts are nearly identical. However, the peak binding energy of Mo²⁺ is different. The Mo²⁺ peaks of 0.11%Pt-PMo@UiO-66 are located at 228.1 eV and 231.4 eV, the peaks at 228.0 eV and 231.3 eV correspond to Mo²⁺ for E-mail: 0; .32%Pt-PMo@UiO-66, and 227.8 eV and 231.1 eV to (0.71%Pt-PMo@UiO-66)-Mo²⁺. As mentioned above, with the increase of the Pt loading, the binding energy of Mo²⁺ species gradually shifts negatively, indicating that the Mo²⁺ species gain electrons.

The Pt 4f spectra of 0.11%Pt-PMo@UiO-66, 0.32%Pt-PMo@UiO-66 and 0.71%Pt-PMo@UiO-66 are depicted in Fig. 3E. The Pt species can be divided into Pt⁰ and Ptⁿ⁺ species, wherein the binding energy of the Ptⁿ⁺ species is at 70.95 eV and 74.30 eV in Pt 4f_7/2 and Pt 4f_5/2. The binding energy of the Ptⁿ⁺ species is the same for all Pt-PMo@UiO-66 catalysts. By comparing the changes of binding energy of the Pt⁰ species in Pt-PMo@UiO-66 and Pt/C mentioned in the literature,⁵⁰ it is found that the binding energy of the Pt⁰ species shows a partial positive shift. 0.32%Pt-PMo@UiO-66 shows the largest positive shift among the Pt-PMo@UiO-66 catalysts. This is probably caused by moderate Pt loading, leading to its stronger interaction with Pt⁰–Mo²⁺ and a larger number of electrons transferring from Pt⁰ to Mo²⁺.

The above characterization results explain the difference in the catalytic performance of Pt-PMo@UiO-66 for the hydrogenation of nitroaromatics. The electron interaction of Pt–Mo elements is present. Mo gets electrons from Pt, and Pt has much more positive charge, which favors the adsorption of nitro groups by the interaction of Pt^δ+–O₂N (Pt^δ+ probably from tiny Pt clusters or Pt single atoms), improving the selectivity for –NO₂ hydrogenation. With moderate electron transfer in 0.32%Pt-PMo@UiO-66, the adsorption of nitro groups and desorption of amino groups are fast, and the activated hydrogen species form by H₂ dissociation at the Pt sites of the Pt nanoparticles, reacting with the adsorbed nitro group at Pt^δ+, making the catalytic performance of 0.32%Pt-PMo@UiO-66 much more remarkable than that of 0.11%Pt-PMo@UiO-66 and 0.71%Pt-PMo@UiO-66 (including activity and selectivity).

The morphology and nanostructure of the catalysts were characterized through TEM and HRTEM characterization. The TEM images (Fig. 4a–c: 0.11%Pt-PMo@UiO-66) show that Pt mainly exists in the form of tiny clusters. The low Pt loading results in particles being too small to be detected by conventional TEM techniques. Nonetheless, it is evident that the Pt clusters are uniformly dispersed.

Fig. 4 (a–c) TEM images of 0.11%Pt-PMo@UiO-66; (d) TEM image, (e) particle size distribution and (f) HRTEM image of 0.32%Pt-PMo@UiO-66; (g) TEM image, (h) particle size distribution and (i) HRTEM image of 0.71%Pt-PMo@UiO-66.

The nanostructure of 0.32%Pt-PMo@UiO-66 is also illustrated, and the results are demonstrated in Fig. 4d. The figure shows that some Pt exists in the form of particles, and the particle size distribution of the Pt NPs is given in Fig. 4e. The average particle size of the Pt particles in 0.32%Pt-PMo@UiO-66 is 4.44 nm. But the Pt clusters and Pt single atoms cannot be distinguished by the common TEM. Fig. 4f demonstrates that Pt is highly dispersed on UiO-66. A subsequent observation of one Pt NP at high resolution (Fig. 4f) clearly displays the lattice fringe of Pt(111) with a distance of 0.232 nm, which is consistent with the reported literature.⁵¹

The TEM and HRTEM results of 0.71%Pt-PMo@UiO-66 are shown in Fig. 4g–i. The Pt particles in 0.71%Pt-PMo@UiO-66 exist as large NPs, and the particle size distribution is given in Fig. 4h. The average particle size of Pt NPs is 23.8 nm. The HRTEM image of the catalyst is shown in Fig. 4i, which clearly shows the lattice fringes with a spacing of 0.224 nm, which is attributed to the Pt(111) plane. From the TEM characterization of different catalysts, the size of the Pt NPs gradually increases with the increase of the Pt loading. Phosphomolybdate is well integrated into the UiO-66 support, which is in accord with the expected results of the synthesis process.

In the STEM image of 0.32%Pt-PMo@UiO-66 (Fig. 5), the Pt NPs are uniformly dispersed. STEM-EDS mapping of the selected region shows that C, O, P and Mo are evenly distributed on the UiO-66 support, and Pt is highly dispersed. C, O, P and Mo are uniformly distributed in the catalyst. Some Pt exists in the form of nanoparticles.

Fig. 5 (a) STEM image of 0.32%Pt-PMo@UiO-66, (b–f) STEM-EDS mapping (red: C, orange: O, pink: P, blue: Mo, yellow: Pt).

3.2 Catalytic performance

The catalytic performances of Pt-PMo@UiO-66 with different Pt loadings were tested by the selective hydrogenation reactions of nitroarenes (Table 3). All catalysts showed good selectivity for the catalytic hydrogenation of nitrobenzene, while PMo@UiO-66 did not show catalytic activity for nitrobenzene hydrogenation. The Pt-PMo@UiO-66 catalysts showed excellent catalytic properties and selectivity to aniline, especially for 0.32%Pt-PMo@UiO-66 (entries 1–4). For other nitroarenes containing reducible groups (entries 5–16), the catalytic performance of 0.11%Pt-PMo@UiO-66 was the worst compared with other Pt-PMo@UiO-66 catalysts. The 0.32%Pt-PMo@UiO-66 catalyst showed excellent selectivity to m-nitroaniline (100%) in the 1,3-dinitrobenzene hydrogenation and had the highest TOF (1708.8 h⁻¹). The 0.71%Pt-PMo@UiO-66 catalyst also exhibited outstanding performance for highly active and selective hydrogenation of p-nitroacetophenone, but its TOF was lower than that of 0.32%Pt-PMo@UiO-66. The 0.32%Pt-PMo@UiO-66 catalyst demonstrated the highest activity for the hydrogenation of all nitroarenes. It showed relatively excellent catalytic properties relative to reported catalysts in the literature under comparable reaction conditions (Table S1†). The results show that moderate Pt loading is advantageous to improve the catalytic hydrogenation performance and selectivity of nitrobenzene and its derivatives. The possible reason is that the catalyst with the appropriate Pt loading can provide much more active sites. There is electron interaction between Pt and Mo, while Mo gains electrons from Pt. Moreover, there is much more electron transfer between Pt and Mo in 0.32%Pt-PMo@UiO-66 due to the appropriate interaction of Pt and Mo, and Pt has more positive charge, improving the adsorption of nitro groups and thus increasing the catalytic performance.

Table 3 Catalytic performance of the Pt-PMo@UiO-66 catalysts and PMo@UiO-66 for the selective hydrogenation of nitroarenes^a

Entry	Substrate	Catalyst	t (h)	Product	C (%)	S (%)	TOF (h^−1)
a Reaction conditions: 1 mmol of substrate, 0.05 g catalyst, 3.0 MPa H2, 40 °C; C – conversion of nitroarenes, S – selectivity to one –NO2 group hydrogenation.
1		PMo@UiO-66	3		0	—	0
2		0.11%Pt-PMo@UiO-66	12		76.1	95.0	394.6
3		0.32%Pt-PMo@UiO-66	2.1		97.9	98.9	1235.7
4		0.71%Pt-PMo@UiO-66	1.5		93.1	90.0	831.9
5		0.11%Pt-PMo@UiO-66	12		11.6	83.8	60.2
6		0.32%Pt-PMo@UiO-66	3.1		94.6	85.2	808.9
7		0.71%Pt-PMo@UiO-66	2.0		92.6	86.8	620.6
8		0.11%Pt-PMo@UiO-66	12		56.6	65.4	293.5
9		0.32%Pt-PMo@UiO-66	3.0		97.6	92.1	862.3
10		0.71%Pt-PMo@UiO-66	1.5		96.2	97.7	859.6
11		0.11%Pt-PMo@UiO-66	7		95.0	98.4	844.5
12		0.32%Pt-PMo@UiO-66	1.75		98.8	99.6	1498.0
13		0.71%Pt-PMo@UiO-66	1.5		99	98.5	884.6
14		0.11%Pt-PMo@UiO-66	12		83.7	83.4	434.0
15		0.32%Pt-PMo@UiO-66	1.5		96.7	100	1708.8
16		0.71%Pt-PMo@UiO-66	1.5		96.1	90.0	858.7

---

3.3 Stability of 0.32%Pt-PMo@UiO-66

Stability tests were conducted on 0.32%Pt-PMo@UiO-66 for 1,3-dinitrobenzene hydrogenation. The 1,3-dinitrobenzene hydrogenation reaction was performed at 40 °C within 1.5 h. As shown in Fig. 6, both the conversion of 1,3-dinitrobenzene and selectivity of m-nitroaniline were preserved after five reaction runs. It further demonstrates the excellent catalytic stability of the Pt-PMo@UiO-66 catalysts. The XRD and XPS characterization results indicate that the recycled 0.32%Pt-PMo@UiO-66 is relatively stable (Fig. S1 and S2†).

Fig. 6 Stability of 0.32%Pt-PMo@UiO-66 for 1,3-dinitrobenzene hydrogenation (reaction conditions: 1,3-dinitrobenzene-1 mmol, reaction time-1.5 h, reaction temperature-40 °C, reaction pressure-3.0 MPa).

4 Conclusions

In conclusion, Pt-PMo@UiO-66 catalysts with different Pt loadings (0.11%, 0.32%, 0.71%) were successfully synthesized by a solvothermal method. The catalytic properties of the Pt-PMo@UiO-66 catalysts were investigated by the selective hydrogenation of nitroaromatics. Highly dispersed Pt particles were loaded on the surface of PMo@UiO-66 and highly dispersed through the limitation effect of the support. Besides, there was a synergistic effect between Pt and phosphomolybdic acid, which improved the catalytic performance and selectivity of the hydrogenation of nitroaromatics. The 0.32%Pt-PMo@UiO-66 catalyst showed the best catalytic performance and selectivity for one –NO₂ hydrogenation in nitroarene hydrogenation. The conversion of p-nitroacetophenone was 98.8%, and the selectivity of p-aminophenone was 99.6% under mild reaction conditions (40 °C). When other groups (bromine and chlorine) or 1, 3-dinitrobenzene was present, the selectivity was also excellent and no dehalogenation occurred. The results showed that the appropriate size of the Pt particles was better for selective catalytic hydrogenation of nitroarenes. There is electron transfer between Pt and phosphomolybdic acid (electron transfer from Pt to Mo), improving the catalytic properties and selectivity of one –NO₂ hydrogenation.

Data availability

The data are available from the corresponding author on reasonable request. Requests may be sent to: E-mail: zhulihua@jxust.edu.cn.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22162012 and 22202089), Youth Jinggang Scholars Program in Jiangxi Province ([2019]57), Thousand Talents Plan of Jiangxi Province (jxsq2019201083), Natural Science Foundation of Jiangxi Province for Distinguished Young Scholars (Grant No. 20224ACB213005), Key R&D Program of Ganzhou (2023PGX16983), Program of Qingjiang Excellent Young Talents, Jiangxi University of Science and Technology (JXUSTQJBJ2019002), Research Foundation of Education Bureau of Jiangxi Province of China (GJJ210833) and Jiangxi Provincial Key Laboratory of Functional Crystalline Materials Chemistry (20212BCD42018).

Notes and references

1. J. Song, Z.-F. Huang, L. Pan, K. Li, X. Zhang, L. Wang and J.-J. Zou, Appl. Catal., B, 2018, 227, 386–408.
2. K. Rajendran, N. Pandurangan, C. P. Vinod, T. S. Khan, S. Gupta, M. A. Haider and D. Jagadeesan, Appl. Catal., B, 2021, 297, 120417.
3. F. Yang, M. Wang, W. Liu, B. Yang, Y. Wang, J. Luo, Y. Tang, L. Hou, Y. Li, B. Zhang, W. Yang and Y. Li, Green Chem., 2019, 21, 704–711.
4. M. Li, S. Chen, Q. Jiang, Q. Chen, X. Wang, Y. Yan, J. Liu, C. Lv, W. Ding and X. Guo, ACS Catal., 2021, 11, 3026–3039.
5. X. Zhang, Q. Gu, Y. Ma, Q. Guan, R. Jin, H. Wang, B. Yang and J. Lu, J. Catal., 2021, 400, 173–183.
6. S. Wu, X. Huang, H. Zhang, Z. Wei and M. Wang, ACS Catal., 2022, 12, 58–65.
7. K. Shanmugaraj, T. M. Bustamante, J. N. D. de León, R. Aepuru, R. V. Mangalaraja, C. C. Torres and C. H. Campos, Catal. Today, 2022, 392, 93–104.
8. H. Yang, L. Wang, S. Xu, X. Hui, Y. Cao, P. He, Y. Li and H. Li, Chem. Eng. J., 2022, 431, 133863.
9. W. C. Cheong, W. Yang, J. Zhang, Y. Li, D. Zhao, S. Liu, K. Wu, Q. Liu, C. Zhang, D. Wang, Q. Peng, C. Chen and Y. Li, ACS Appl. Mater. Interfaces, 2019, 11, 33819–33824.
10. S. Sun, M. Du, R. Zhao, X. Jv, P. Hu, Q. Zhang and B. Wang, Green Chem., 2020, 22, 4640–4644.
11. H. Fu, H. Zhang, G. Yang, J. Liu, J. Xu, P. Wang, N. Zhao, L. Zhu and B. Chen, New J. Chem., 2022, 46, 1158–1167.
12. W. Shi, B. Zhang, Y. Lin, Q. Wang, Q. Zhang and D. Su, ACS Catal., 2016, 6, 7844–7854.
13. Q. Zhao, W. Ni, X. Tan, F. Cao, T. Liu, H. Huang, Z. Cheng, Y. Li, S. He, H. Ning and M. Wu, J. Mater. Chem. A, 2022, 10, 9435–9444.
14. H. Ishikawa, N. Nakatani, S. Yamaguchi, T. Mizugaki and T. Mitsudome, ACS Catal., 2023, 13, 5744–5751.
15. H. Liu, X. Li, Z. Ma, M. Sun, M. Li, Z. Zhang, L. Zhang, Z. Tang, Y. Yao, B. Huang and S. Guo, Nano Lett., 2021, 21, 10284–10291.
16. Y. Li, H. Li, S. Le, X. Bai and X. Wang, J. Clean. Prod., 2020, 245, 118919.
17. W. Li, J. Artz, C. Broicher, K. Junge, H. Hartmann, A. Besmehn, R. Palkovits and M. Beller, Catal. Sci. Technol., 2019, 9, 157–162.
18. F. J. Anaya-Castro, M. Beltrán-Gastélum, O. M. Soto, S. Pérez-Sicairos, S. W. Lin, B. Trujillo-Navarrete, F. Paraguay-Delgado, L. J. Salazar-Gastélum and M. I. Salazar-Gastélum, Nanomaterials, 2021, 11, 3156.
19. H. Pan, Y. Peng, X. Lu, J. He, L. He, C. Wang, F. Yue, H. Zhang, D. Zhou and Q. Xia, Mol. Catal., 2020, 485, 110838.
20. L. Zhao, H. Liu, Y. Liu, X. Han, J. Xu, W. Xing and W. Guo, ACS Appl. Mater. Interfaces, 2020, 12, 40248–40260.
21. L. Qin, G. Zeng, C. Lai, D. Huang, C. Zhang, M. Cheng, H. Yi, X. Liu, C. Zhou, W. Xiong, F. Huang and W. Cao, Sci. Total Environ., 2019, 652, 93–116.
22. L. Lu, L. Peng, L. Li, J. Li, X. Huang and Z. Wei, J. Energy Chem., 2020, 40, 52–56.
23. J. Xu, F. Chen, X. Xu and G. P. Lu, Mol. Catal., 2020, 495, 111157.
24. J. Kou, W. D. Wang, J. Fang, F. Li, H. Zhao, J. Li, H. Zhu, B. Li and Z. Dong, Appl. Catal., B, 2022, 315, 121487.
25. Z. Zhang, H. Gai, Q. Li, B. Feng, M. Xiao, T. Huang and H. Song, Chem. Eng. J., 2022, 429, 132224.
26. Z. Wang, C. Wang, S. Mao, B. Lu, Y. Chen, X. Zhang, Z. Chen and Y. Wang, Nat. Commun., 2022, 13, 3561.
27. T. Sheng, Y. J. Qi, X. Lin, P. Hu, S. G. Sun and W. F. Lin, Chem. Eng. J., 2016, 293, 337–344.
28. F. Zhang, H. Guo, M. Liu, Y. Zhao, F. Hong, J. Li, Z. Dong and B. Qiao, Chin. J. Catal., 2023, 48, 195–204.
29. W. C. Chang, H. Randel, T. Weyhermüller, A. A. Auer, C. Farès and C. Werlé, Angew. Chem., Int. Ed., 2023, 62, e202219127.
30. Y. Sheng, Y. Liu, Y. Yin, X. Zou, J. Ren, B. Wu, X. Wang and X. Lu, Chem. Eng. J., 2023, 452, 139448.
31. Q. Liu, Y. Zhong, H. Fu, R. Wang and L. Zhu, Appl. Catal., A, 2023, 665, 119373.
32. J. Ding, F. Li, J. Zhang, H. Qi, Z. Wei, C. Su, H. Yang, Y. Zhai and B. Liu, Adv. Mater., 2023, 36, 2306480.
33. H. Wei, Y. Ren, A. Wang, X. Liu, X. Liu, L. Zhang, S. Miao, L. Li, J. Liu, J. Wang, G. Wang, D. Su and T. Zhang, Chem. Sci., 2017, 8, 5126–5131.
34. Y. Zhang and J. Zhou, J. Catal., 2021, 395, 445–456.
35. J. D. Kim, M. Y. Choi and H. C. Choi, Mater. Chem. Phys., 2016, 173, 404–411.
36. Q. Yan, X. Wu, H. Jiang, H. Wang, F. Xu, H. Li, H. Zhang and S. Yang, Coord. Chem. Rev., 2024, 502, 215622.
37. R. Du, Y. Wu, Y. Yang, T. Zhai, T. Zhou, Q. Shang, L. Zhu, C. Shang and Z. Guo, Adv. Energy Mater., 2021, 11, 2100154.
38. R. Freund, O. Zaremba, G. Arnauts, R. Ameloot, G. Skorupskii, M. Dincă, A. Bavykina, J. Gascon, A. Ejsmont, J. Goscianska, M. Kalmutzki, U. Lächelt, E. Ploetz, C. S. Diercks and S. Wuttke, Angew. Chem., Int. Ed., 2021, 60, 23975–24001.
39. H. Zhou, H. Wang, C. Yue, L. He, H. Li, H. Zhang, S. Yang and T. Ma, Appl. Catal., B, 2024, 344, 123605.
40. M. Guo, F. Wang, M. Zhang, L. Wang, X. Zhang and G. Li, J. Catal., 2023, 424, 221–235.
41. D. L. Long, R. Tsunashima and L. Cronin, Angew. Chem., Int. Ed., 2010, 49, 1736–1758.
42. J. Li, S. Zhao, Z. Li, D. Liu, Y. Chi and C. Hu, Inorg. Chem., 2021, 60, 7785–7793.
43. L. Valenzano, B. Civalleri, S. Chavan, S. Bordiga, M. H. Nilsen, S. Jakobsen, K. P. Lillerud and C. Lamberti, Chem. Mater., 2011, 23, 1700–1718.
44. X. Yang, L. Qiao and W. Dai, Microporous Mesoporous Mater., 2015, 211, 73–81.
45. C. Wang, A. R. Li and Y. L. Ma, Fuel Process. Technol., 2021, 212, 106629.
46. Y. Shan, D. Liu, C. Xu, P. Zhan, H. Wang, J. Wang, R. He and W. Wang, New J. Chem., 2021, 45, 7344–7352.
47. C. Chen, D. Chen, S. Xie, H. Quan, X. Luo and L. Guo, ACS Appl. Mater. Interfaces, 2017, 9, 41043–41054.
48. J. Li, Z. Xu, T. Wang, X. Xie, D. Li, J. Wang, H. Huang and Z. Ao, Chem. Eng. J., 2022, 448, 136900.
49. C. Chen, A. Wu, H. Yan, Y. Xiao, C. Tian and H. Fu, Chem. Sci., 2018, 9, 4746–4755.
50. Z. Paál, P. Tétényi, D. Prigge, X. Z. Wang and G. Ertl, Appl. Surf. Sci., 1983, 14, 307–320.
51. M. Liu, W. Tang, Z. Xie, H. Yu, H. Yin, Y. Xu, S. Zhao and S. Zhou, ACS Catal., 2017, 7, 1583–1591.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta03635b

‡ These authors contributed equally to this work.

---