H₃PW₁₂O₄₀ synergized with MCM-41 for the catalytic nitration of benzene with NO₂ to nitrobenzene

Abstract

Developing a new environmentally friendly process for benzene nitration to nitrobenzene has been highly desirable for a long time. In this work, NO₂ was used as a nitration agent to replace traditional nitric acid, and different mesoporous SiO₂ and their supported heteropoly acid (salt) were employed to catalyze benzene nitration to nitrobenzene. Several typical catalysts were characterized using XRD, BET and FT-IR, and the acid amounts of the various catalysts were determined. The effects of various factors such as different catalysts, the molar ratio of benzene to NO₂, reaction temperature, reaction time, HPW loading, the acid amounts of the catalyst and the reuse of the catalyst on the nitration reaction have also been systematically examined. The results indicate that the supported HPW/MCM-41 catalysts exhibit a remarkably synergistic catalytic performance on the nitration reaction of benzene to nitrobenzene. In particular, the 50%HPW/MCM-41 catalyst gives the best results with 73.4% benzene conversion and 98.8% selectivity to nitrobenzene under the optimal reaction conditions. Moreover, the mesoporous structure of MCM-41 was retained under the high loading of HPW. The possible reaction mechanism for the nitration reaction of benzene with NO₂ over HPW/MCM-41 is suggested in the present work. This method provides a promising strategy for the preparation of nitro-aromatic compounds from a catalytic nitration reaction by using NO₂ as the nitration reagent.

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Introduction

Nitration of aromatic compounds is one of the most important reactions in the chemical industry, which provides key organic intermediates or energetic materials. For example, nitrobenzene is known to be a chemical intermediate for producing useful substances such as aniline, benzidine and metanilic acid,¹ and is a starting material for the manufacture of various industrial products such as pharmaceuticals, dyes, explosives and plastics.² Traditionally, the nitration of aromatic compounds is performed with a mixture of nitric and sulfuric acids.^3,4 However, this method often deteriorates with poor selectivity for the desired products and a high environmental cost. Furthermore, it is very energy consuming to separate the aqueous waste from the nitrating agent and to recycle it for further use.⁵ Therefore, the development of a new environmentally friendly nitration process is highly desirable in the chemical industry.

From the separation of the catalyst and environmental points of view, the use of solid acidic catalysts to replace sulfuric acids in this nitration process seems more attractive.⁶ In the past few decades, lots of effort has been made to develop efficient solid acidic catalysts, such as modified Y-zeolite,⁷ modified mordenite zeolite,⁸ Al³⁺-montmorillonite,⁹ mixed metal oxides (TiO₂–MoO₃) and oxides treated with sulfuric acid (SO₄²⁻/TiO₂–MoO₃) at 500 °C,¹⁰ and sulfuric acid supported on silica.¹¹ Brei and coworkers¹² reported solid superacid WO₃/ZrO₂ catalysts in the vapor-phase nitration of benzene, the yield of nitrobenzene was 65–80% at 170 °C. These research results revealed that the catalysts showed good catalytic performance.

Recently, the use of relatively stable and strongly acidic heteropoly acids in acid-catalyzed reactions has attracted wide attention. However, pure heteropoly acids have very low specific surface areas (<30 m² g⁻¹) and are easily dissolved in polar solvents resulting in difficult recovery and reuse. One of the best ways to resolve these problems is to support heteropoly acids on a carrier. At present, the main kinds of supports are SiO₂,^13–18 Al₂O₃,¹⁹ ZrO₂,^20–22 activated carbon,²³ SiO₂–Al₂O₃,^24,25 and MCM-41.^26–28 Among these supports, MCM-41 mesoporous materials have attracted much attention due to their regular pore structure, uniform pore diameter and high surface area, which can act as an excellent support for expanding the catalytic capability of traditional acidic materials for some applications.²⁹

In the above-mentioned catalytic nitration processes, the nitrating reagent used is nitric acid. As everybody knows, commercial nitric acid is manufactured from lower nitrogen oxides via an energy consuming multi-step process. Therefore, aromatic nitration using nitrogen dioxide (NO₂) in place of the traditional nitric acid can be taken into account as one of the promising alternatives,^30–35 especially in large scale production. First, neutral reaction conditions can provide a safe and non-corrosive operation and also minimize the irritating oxidative side products. Second, the use of NO₂ can save costs in manufacturing nitric acid. In particular, not using sulfuric acid which causes a large amount of wastewater, can construct an environmentally benign process.

In this work, we developed a simple and efficient nitration approach for the highly selective preparation of nitrobenzene from benzene and NO₂, catalyzed by H₃PW₁₂O₄₀ synergized with MCM-41. The synergistic catalysts were prepared by an incipient wetness impregnation method and characterized using N₂ physi-sorption, XRD, and FT-IR. The effects of various factors on the catalytic nitration reaction were systematically investigated. And the detailed results obtained from the catalytic nitration process will be reported in this paper.

Results and discussion

Characterization of catalysts

The XRD patterns of HPW, 50%HPW/MCM-41 and MCM-41 are shown in Fig. 1. From Fig. 1(a), it is clearly shown that MCM-41 has a typical amorphous structure, and HPW has a highly crystalline nature. In particular, 50%HPW/MCM-41 shows high intensity peaks at 2θ values of 6.8°, 25.2°, 36.4°, 53.5° and 60.8°, which are attributed to the characteristics of the HPW phase. In Fig. 1(b), low angle XRD patterns of MCM-41 and 50%HPW/MCM-41 show an evident diffraction peak at a 2θ value of 2.08°, which is characteristic of the mesoporous structure of MCM-41. These results indicate that HPW was successfully supported on MCM-41, the HPW particles were well dispersed on the MCM-41 support, and the mesoporous structure of MCM-41 under the high loading of HPW was retained.

Fig. 1 XRD patterns of MCM-41, HPW and 50%HPW/MCM-41 catalysts.

BET surface areas, pore volumes and pore diameters of several typical catalysts for this reaction are summarized in Table 1. It can be seen that HPW presents the lowest surface area, MCM-41 and other supports have a higher surface area, pore volume and pore diameter, these values coincide with the reported values.²⁴ The surface areas and pore volumes of the supported catalysts decreased when HPW was loaded onto the supports. The results indicate that the active component, HPW, entered the pores of the supports.

Table 1 Textural properties of various catalysts

Catalysts	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
HPW	25.3	—	—
SBA-15	700.2	1.0	5.9
SiO2	525.5	1.0	7.9
MCM-41	871.9	0.9	2.8
50%HPW/SBA-15	648.7	0.9	5.8
50%HPW/SiO2	467.3	0.9	7.8
50%HPW/MCM-41	823.9	0.8	2.7

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The N₂ adsorption–desorption isotherms of HPW, MCM-41 and 50%HPW/MCM-41 are depicted in Fig. 2. It can be seen that HPW showed a type III adsorption isotherm and hysteresis loop of type H3, which shows HPW is a non-porous and flake particulate material. MCM-41 showed a type IV adsorption isotherm and hysteresis loop of type H2, this type has the characteristics of a drastic increase of the adsorption capacity caused by capillary condensation after multilayer adsorption and it is typical of mesoporous structured materials. Well-defined mesoporosity results in a high surface area of 871.9 m² g⁻¹ and narrow pore size distribution with a mean diameter of 2.8 nm, as shown in Table 1.

Fig. 2 N₂ adsorption and desorption isotherms of samples: (a) 50%HPW/MCM-41, (b) MCM-41, (c) HPW.

The pyridine adsorbed FT-IR spectra of the MCM-41 and 50%HPW/MCM-41 samples are shown in Fig. 3. The IR band at 1450 cm⁻¹ is the adsorption of pyridine on Lewis acidic centers, the band at 1490 cm⁻¹ is the interaction of pyridine with both Lewis and Brønsted acid sites, and the bands at 1540 cm⁻¹ and 1600 cm⁻¹ are the adsorption of pyridine on Brønsted acid centers. The results of the pyridine FT-IR spectra indicate that the catalysts possess both Lewis and Brønsted acid sites.

Fig. 3 Pyridine adsorbed FT-IR spectra of samples: (a) MCM-41 and (b) 50%HPW/MCM-41.

The FT-IR spectra of MCM-41 and 50%HPW/MCM-41 are shown in Fig. 4. In the FT-IR spectra of the samples, the broad band around 3444 cm⁻¹ due to an –OH stretch and the corresponding H–O–H bending vibration mode around 1637 cm⁻¹ correlate very well with the water adsorption property (hydrophilic property) of the catalysts. The framework bands of MCM-41 are at 1236, 800, and 465 cm⁻¹.³⁶ Pure HPW with five strong bands at 1082 (P–O), 988 (W=O), 800 (W–O–W), 965 (P–O) and 525 (W–O–P) cm⁻¹ (ref. 37) can be observed in Fig. 4(b). The FT-IR spectra of the samples indicated that HPW was successfully supported on MCM-41.

Fig. 4 FT-IR spectra of samples: (a) MCM-41 and (b) 50%HPW/MCM-41.

Effects of various catalysts on the nitration reaction

The representative results for the nitration of benzene with NO₂ over various catalysts are summarized in Table 2. Only 37.1% conversion with 90.4% selectivity to nitrobenzene was obtained in the absence of any catalysts. The selectivity to nitrobenzene was obviously improved as the MCM-41, SiO₂ and SBA-15 supports, as catalysts, were introduced to this nitration process. The possible reason is that these supports are mesoporous structured materials and they have a certain amount of acid. These structural characteristics and surface properties of the supports may play an important role in improving the selectivity to nitrobenzene by shape-selective catalysis in this nitration reaction. Therefore, the benzene conversion and selectivity to nitrobenzene were efficiently improved. While HPW, HPMo, AMPW and AMPMo were respectively introduced to the nitration reaction, the conversion of benzene was increased in different degrees. This demonstrates that the heteropoly acid (salt) is the active component for the nitration reaction of benzene and NO₂. In particular, the conversion of benzene and the selectivity to nitrobenzene were further enhanced as HPW was supported on these supports. In this catalytic system, the catalytic performances of the present catalysts were different. Among these supported catalysts, the 50%HPW/MCM-41 catalyst shows superior activity and gives better results with 55.7% benzene conversion and 98.6% selectivity to nitrobenzene. However, the conversion of benzene was only 42.3% with 95.7% selectivity to nitrobenzene when equivalent amounts of HPW and MCM-41 were added to the system and physically mixed. This fully shows that there is a synergistic effect between HPW and MCM-41 in the nitration. These results demonstrate that the acidity and acid amounts of the catalyst are also important factors for conversion and selectivity. Among these catalysts, the 50%HPW/MCM-41 catalyst exhibits good synergistic effects on the nitration of benzene with NO₂.

Table 2 Comparison of the catalytic performance of various catalysts in the nitration reaction of benzene with NO₂^a

Catalyst	Acid amount (mmol g^−1)	Conversion (%)	Selectivity of nitrobenzene^b (%)
a Reaction conditions: reaction temperature: 90 °C, benzene (5 g) : NO2 = 1 : 5 (molar ratio), catalyst: 0.2 g, reaction time: 6 h.b The other product is dinitrobenzene, the major product of dinitrobenzene is meta-dinitrobenzene.c 0.1 g HPW and 0.1 g MCM-41 were physically mixed.
None	0	37.1	90.4
MCM-41	0.564	40.6	96.8
SiO2	0.217	38.7	96.0
SBA-15	0.482	38.4	94.9
HPW	4.845	46.4	96.5
HPMo	4.156	43.5	92.2
AMPW	1.129	41.6	93.6
AMPMo	1.047	40.2	93.8
50%HPW/MCM-41	2.398	55.7	98.6
50%HPW/SiO2	2.151	40.2	97.3
50%HPW/SBA-15	2.193	50.7	97.3
HPW + MCM-41^c	2.639	42.3	95.7

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Effects of reaction temperature on the nitration reaction

The nitration of benzene with NO₂ was carried out at different temperatures ranging from 60 to 150 °C. The effects of the reaction temperature on the conversion of benzene and selectivity to nitrobenzene were examined, and the obtained results are depicted in Fig. 5. It is clearly found that there is a significant influence of the reaction temperature on the conversion and selectivity. The conversion of benzene rapidly increased from 13.4% to 69.2% while the selectivity to nitrobenzene gradually decreased from 98.0% to 79.8% with the increased reaction temperature from 60 °C to 150 °C. The possible reason is that elevating the reaction temperature favors the performance of the nitration reaction from benzene to nitrobenzene, meanwhile, it also accelerated the side reaction from nitrobenzene to dinitrobenzene; the products m-dinitrobenzene and p-dinitrobenzene were detected in our experimental results when the reaction temperature was raised to 150 °C. These results indicate that the reaction temperature is a significant factor to regulate the selectivity to nitrobenzene in the nitration reaction of benzene with NO₂. For optimal conversion and selectivity, a reaction temperature of 90 °C was selected.

Fig. 5 Effects of the reaction temperature on the nitration reaction. Reaction conditions: reaction time is 6 h, the molar ratio of benzene (5 g) to NO₂ is 1 : 5, and the amount of catalyst (50%HPW/MCM-41) is 0.2 g.

Effects of the molar ratio of benzene to NO₂ on the nitration reaction

Table 3 summarizes the representative results of different molar ratios of benzene to NO₂ on the nitration reaction. It can be seen that the conversion of benzene increases rapidly with the reducing of the molar ratio or raising of the concentration of NO₂. The conversion of benzene increases from 22.7% to 55.7% with reducing of the molar ratio from 1 : 1 to 1 : 5. While the selectivity of nitrobenzene was not obviously affected by the molar ratio of benzene to NO₂, and remained in the range of 97–98%. When the molar ratio of benzene to NO₂ continued to decrease from 1 : 5 to 1 : 6, the conversion of benzene only increased a little and the selectivity to nitrobenzene was maintained at 98%. This demonstrates that further elevating the concentration of NO₂ has little contribution to the conversion of benzene and selectivity to nitrobenzene. These results show that changes to the molar ratio only affect the conversion, while they have little effect on the selectivity to nitrobenzene. For optimal conversion and selectivity, it is clearly found that the favourable molar ratio of benzene to NO₂ is 1 : 5 under a reaction time of 6 h and a reaction temperature of 90 °C.

Table 3 Effects of the molar ratio of benzene to NO₂ on the nitration reaction^a

Benzene : NO2 (molar ratio)	Conversion (%)	Selectivity to nitrobenzene (%)
a Reaction conditions: reaction temperature is 90 °C, reaction time is 6 h, the mass of benzene is 5 g and the amount of 50% HPW/MCM-41 catalyst is 0.2 g.
1 : 1	22.7	97.3
1 : 2	30.3	97.0
1 : 3	41.8	97.4
1 : 4	45.2	97.3
1 : 5	55.7	98.6
1 : 6	56.0	97.9

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Effect of reaction time on the nitration reaction

Fig. 6 depicts the effects of the reaction time on the nitration reaction of benzene with NO₂ at 90 °C. It can be seen that the conversion of benzene rapidly increases from 45.5% to 73.4% with a prolonged reaction time from 3 h to 8 h, while the selectivity to nitrobenzene is not obviously changed. Thereafter, a small increase of the conversion is evident when the reaction time is further elevated from 8 h to 24 h. These results indicate that the reaction time mainly affects the conversion of benzene, while the selectivity of nitrobenzene was almost unchanged. From the point of conversion and selectivity, a reasonable reaction time is 8 h.

Fig. 6 Effects of the reaction time on the nitration reaction. Reaction conditions: 90 °C, benzene (5 g) : NO₂ = 1 : 5 (molar ratio), catalyst: 50%HPW/MCM-41 (0.2 g).

Effects of the loading of HPW supported on MCM-41 on the nitration reaction

The effects of the loading of HPW supported on MCM-41 on the nitration reaction of benzene and NO₂ with the molar ratio of 1 : 5 were tested at 90 °C for 8 h. Table 4 shows the effects of the loading of HPW supported on MCM-41 on the nitration reaction. Only 40.6% conversion with 97.0% selectivity to nitrobenzene was obtained in the absence of HPW. The conversion of benzene was obviously improved when pure HPW was added to this reaction system. The conversion of benzene was 50.5% when the catalyst loading is 10%. When increasing the loading of the catalyst from 10% to 50%, the conversion of benzene increased rapidly from 50.5% to 73.4%, and the selectivity of nitrobenzene hardly changed. The possible reason for this is that the elevated loading of the catalyst leads to the increase of active components on the support. Thus, the conversion of benzene is evidently raised. However, the conversion of benzene showed an evident decline from 73.4% to 55.3% as the catalyst loading was further elevated from 50% to 75%. Maybe, the reason is due to too much HPW accumulating on the surface of MCM-41, thus the active components are congested and not fully dispersed on the support, which leads to a decline in the conversion of benzene. However, even with changing the acid amount of the catalyst and retaining the same total acid content as 50%HPW/MCM-41, the conversion of benzene was still lower than that of 50%HPW/MCM-41. This shows that with different loadings of HPW supported on MCM-41, the synergistic catalytic performance is different. Therefore, suitable acidity and the amount of acid in the catalyst can adequately and efficiently catalyze the nitration reaction of benzene with NO₂. The suitable loading of HPW supported on MCM-41 was 50% in the present nitration reaction.

Table 4 Effects of the HPW loading on the nitration reaction^a

Catalyst	Acid amount (mmol g^−1)	Total acid content (mmol)	Conversion (%)	Selectivity (%)
a Reaction conditions: reaction temperature: 90 °C, reaction time: 8 h, benzene (5 g) : NO2 = 1 : 5 (molar ratio), catalyst: 0.2 g.b The mass of catalyst is 0.35 g.c The mass of catalyst is 0.24 g.
MCM-41	0.564	0.11	40.6	97.0
HPW	4.845	0.97	53.5	96.4
10%HPW/MCM-41	0.884	0.18	50.5	98.2
30%HPW/MCM-41	1.385	0.28	57.2	98.6
40%HPW/MCM-41	1.979	0.40	64.5	98.4
50%HPW/MCM-41	2.398	0.48	73.4	98.8
75%HPW/MCM-41	3.015	0.60	55.3	97.8
30%HPW/MCM-41^b	1.385	0.48	61.5	98.4
40%HPW/MCM-41^c	1.979	0.48	68.7	98.3

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Effects of the amount of catalyst on the nitration reaction

The experimental results for the nitration reaction of benzene and NO₂ catalyzed by HPW/MCM-41 with different acid amounts are listed in Table 5. The conversion of benzene increased gradually, whereas the selectivity to nitrobenzene was almost unchanged with the elevated total acid content of the catalyst. The conversion of benzene reached 73.4% with 98.8% selectivity to nitrobenzene when 0.2 g of 50%HPW/MCM-41 was added to the nitration reaction. However, the conversion of benzene was not obviously improved when the amount of catalyst exceeded 0.2 g. A possible reason is that too much catalyst was difficult to disperse in the reaction solution, and the catalyst was easily agglomerated, thus the catalytic activity of the catalyst was not further promoted. Therefore, a suitable amount of catalyst can adequately and efficiently catalyze the nitration reaction of benzene with NO₂. The optimal amount of 50%HPW/MCM-41 was 0.2 g in the present nitration reaction.

Table 5 Effects of the acid amount of the catalyst on the nitration reaction^a

Amount of catalyst (g)	Acid amount (mmol g^−1)	Total acid content (mmol)	Conversion (%)	Selectivity (%)
a Reaction conditions: reaction temperature: 90 °C, reaction time: 8 h, benzene (5 g) : NO2 = 1 : 5 (molar ratio).
50%HPW/MCM-41 (0.05)	2.398	0.12	40.9	98.3
50%HPW/MCM-41 (0.1)	2.398	0.24	45.9	97.8
50%HPW/MCM-41 (0.2)	2.398	0.48	73.4	98.8
50%HPW/MCM-41 (0.3)	2.398	0.72	74.0	98.7
30%HPW/MCM-41 (0.35)	1.385	0.48	61.5	98.4
40%HPW/MCM-41 (0.24)	1.979	0.48	68.7	98.3

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Recycling and reuse of the HPW/MCM-41 catalyst in the nitration reaction of benzene with NO₂

Based on its properties, HPW/MCM-41 can be easily recovered by filtrating and then washing three times with benzene. The results of the recovered HPW/MCM-41 after five runs are shown in Table 6. It can be seen that, compared to the fresh catalyst, a decrease in the conversion was observed after the second and third runs, while the conversion became stable after five runs, showing a considerable value of about 40%. Interestingly, almost no change in the selectivity to nitrobenzene was found whatever the recycling number. This could be due to the decrease in the catalyst acidity by leaching of HPW from the MCM-41 support. It is worth mentioning that the conversion is still only 63.1% and far lower than the catalytic performance of the fresh catalyst when 0.02 g of HPW is supplemented into the recycled catalyst after the first run in order to maintain the acid content in the reaction system. This fully demonstrates the synergistic catalytic effects on the nitration reaction of benzene to nitrobenzene with NO₂ between HPW and MCM-41.

Table 6 Results for the recycling and reuse of HPW/MCM-41 in the nitration reaction of benzene with NO₂^a

Recycling times	Acid amount (mmol g^−1)	Conversion (%)	Selectivity to nitrobenzene (%)
a Reaction conditions: 90 °C, benzene (5 g) : NO2 = 1 : 5 (molar ratio), catalyst: 50%HPW/MCM-41 (0.2 g).b After the first run, 0.02 g of HPW was supplemented in order to maintain the acid content in the reaction system.
Fresh	2.40	73.4	98.8
1	1.97	68.5	98.3
2	1.27	55.4	98.0
3	0.89	49.6	97.8
4	0.56	41.2	98.4
5	0.56	41.0	97.9
1^b	2.40	63.1	98.3

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In addition, the catalyst after recycling five times was also characterized using FT-IR, the spectra are shown in Fig. 7. The results indicate that the FT-IR spectra of the used and fresh catalyst are similar, which shows that the structure of MCM-41 is retained. However, the characteristic peak of HPW disappeared after recycling the catalyst five times. This also showed the leaching of HPW from the MCM-41 support. Hence, further studies on the stability of HPW/MCM-41 in the nitration reaction are in progress.

Fig. 7 FT-IR spectra of samples: (a) MCM-41, (b) fresh HPW/MCM-41, (c) after recycling one time, (d) after recycling five times.

The possible reaction mechanism for the nitration reaction of benzene with NO₂ over HPW/MCM-41

Generally, the reaction mechanism for the liquid phase nitration of aromatic hydrocarbon with NO₂ (or N₂O₄) over a solid-acid catalyst was regarded as the electrophilic attack mechanism.³⁸ H-complexes were formed by hydrogen bonding between nitrogen tetraoxide (N₂O₄) and the surface OH group of the solid acid. And then the H-complexes are likely to coexist in equilibrium with nitronium ions (NO₂⁺), which easily attack the aromatic ring via electrophilic reaction. The generated nitrous acid (HNO₂) easily decomposed to NO, NO₂ and water. Therefore, N₂O₄ seems to be the most probable reactive species of the nitrating agent on the surface of solid acids.^39,40 In our present system, the NO₂ used for nitration is also regarded as an equilibrium mixture between N₂O₄ (b.p. 21.3 °C at 760 mm Hg) and NO₂, and the dimer predominates at low temperatures.⁴¹ Therefore, on the basis of the results in this work, a similar reaction mechanism in the liquid phase nitration reaction of benzene with NO₂ over HPW/MCM-41 is also proposed in Scheme 1. Firstly, a solid-acid catalyst (HPW/MCM-41) activated N₂O₄ as a result of its interaction with Brønsted sites forming NO₂⁺ and nitrous acid (HNO₂). Then, the aromatic hydrocarbon (benzene, ArH) was subsequently converted to ArNO₂ (nitrobenzene) by classical electrophilic attack of NO₂⁺ on the aromatic ring. It should be noted that the aromatic hydrocarbon (ArH) did not need to be activated on the catalyst surface in this nitration process.

Scheme 1 The possible reaction mechanism for the nitration reaction of benzene with NO₂ over HPW/MCM-41.

Conclusions

A simple and efficient approach for highly selective preparation of nitrobenzene from the nitration of benzene with NO₂ catalyzed by HPW/MCM-41 has been developed successfully in the present work. The synergistic effects of H₃PW₁₂O₄₀ and MCM-41 on the nitration reaction were investigated under different conditions. And the optimal reaction conditions for the nitration of benzene with NO₂ were obtained. The results indicated that 50%HPW/MCM-41 exhibited a good synergistic catalytic performance with a higher conversion of benzene and excellent selectivity to nitrobenzene. Further studies on the stability of HPW/MCM-41 in the nitration reaction are in progress. If the stability of HPW/MCM-41 could be effectively maintained, this study may provide a promising approach for the highly selective preparation of nitro-aromatic compounds from such catalytic nitration reactions by using NO₂ as the nitration reagent, with potential industrial applications.

Experimental

Reagents and instruments

NO₂ (purity > 99.9%) was purchased from Beijing Chemical Co., Ltd., China. Benzene (AR), pure-silica MCM-41, SiO₂, and SBA-15 were obtained commercially. H₃PW₁₂O₄₀ (phosphotungstic acid, HPW), H₃PMo₁₂O₄₀ (phosphomolybdic acid, HPMo), ammonium phosphotungstate (AMPW) and ammonium phosphomolybdate (AMPMo) were purchased from Aladdin Industrial Corporation. Except where specified, all chemicals were analytical grade. Gas chromatography (GC) was performed on a Shimadzu GC-2010 plus equipped with hydrogen flame ion detector (FID) and an RTX®-5 (30 m × 0.25 mm × 0.25 um) column for quantitative analysis by using the internal standard method (chlorobenzene as internal standard substance). Gas chromatography-mass spectrometry (GC-MS) was run on a Shimadzu GCMS-QP2010 PLUS for qualitative analysis of the products.

The preparation and characterization of the catalysts

A series of supported HPW/MCM-41 catalysts with different HPW loading (10–75 wt%) were prepared by the incipient wetness impregnation method. Firstly, HPW and MCM-41 were pre-treated at 300 °C for 8 h and then cooled in a dry box before use. Then, a certain amount of dried MCM-41 was impregnated in the desired amount of HPW solution with vigorous stirring at room temperature for 12 h. The resulting catalyst was dried at 100 °C for 12 h in an oven, calcined at 300 °C for 3 h, sealed and then stored. Four different loading (10%, 30%, 40%, 50% and 75%) HPW/MCM-41 catalysts, which were marked as 10%HPW/MCM-41, 30%HPW/MCM-41, 40%HPW/MCM-41, 50%HPW/MCM-41 and 75%HPW/MCM-41 respectively were prepared according to the above procedure.

The X-ray diffraction (XRD) analysis of HPW, MCM-41 and HPW/MCM-41 was performed on a Rigaku D/max2550 18KW Rotating Anode X-ray Diffractometer with monochromatic Cu Kα radiation (λ = 1.5418 Å) radiation at a voltage and current of 40 kV and 300 mA, respectively. Fourier transform infrared (FT-IR) spectra of the samples were recorded using a Nicolet380 instrument in a KBr matrix in the range of 400–4000 cm⁻¹. Pyridine adsorbed FT-IR was used to distinguish the nature of the acid sites. The textural properties of the MCM-41, MCM-41 supported HPW, SiO₂ and SBA-15 samples were characterized using N₂ physi-sorption on a NOVA-2200e (Quantachrome, USA) at 77 K after out-gassing the samples at 150 °C and 1 mmHg for 6 h.

Typical experimental procedure

The nitration reaction of benzene with NO₂ was performed in a 100 ml reaction autoclave with a magnetic stirrer and temperature-controlling device. Benzene (5 g), 0.2 g catalyst and a certain amount of cold liquid NO₂ (<5 °C) were added in an autoclave under low temperature recirculation cooling conditions (retaining the temperature between 5–8 °C). And then the sealed autoclave was immersed in an oil bath maintained at a preselected temperature. Finally, the reaction mixture was stirred in a closed autoclave at 90 °C for 6 h. The whole reaction system is closed. When the reaction was finished and the reaction mixture cooled to room temperature, the catalyst was separated and recovered by filtration. The resulting products were analyzed using GC and GC-MS.

Acknowledgements

The authors are grateful for the financial support for this work by the National Natural Science Foundation of China (21276216), Specialized Research Fund for the Doctoral Program of Higher Education (20124301130001), Project of Technological Innovation & Entrepreneurship Platform for Hunan Youth (2014) and Major Science and Technology Projects of Hunan Province (2012FJ1001).

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