Improved Regioselective Mononitration of Naphthalene over Modified BEA Zeolite Catalysts

Abstract

HBEA zeolite modified with highly electronegative cations is a highly efficient and reusable catalyst for the nitration of naphthalene with nitric acid, which has been successfully prepared in this work. Catalytic selective mononitration of naphthalene was investigated. The ratio of 1-nitronaphthalene isomer to 2-nitronaphthalene could reach 19.2, with a moderate yield of 68.2%, when the reaction was carried out in 1,2-dichloroethane, with 1.0 mmol naphthalene, 0.22 mL nitric acid (95%), and 0.10 g HBEA-25 at −15 °C. The effects of reaction temperature and the quantity of zeolites on 1-nitronaphthalene were also studied. The catalyst is readily recyclable, and we believe this to be a major step forward in the area of clean technology for aromatic nitration.

1. Introduction

Aromatic nitration is one of the most significant organic reactions for the formation of aromatic nitro compounds, which are widely used in dyestuffs, explosive, pharmaceuticals, etc. [1,2]. The traditional nitration process is carried out with mixture of nitric and sulfuric acids [3]. This process has some defects, such as low selectivity, involving highly corrosive substances, and causing environmental pollution. Thus, there is an urgent need for new nitration methods to overcome these deficiencies. A lot of effort has been directed into the search for reusable and environmentally friendly catalysts, and much progress has been achieved [4]. Among them, Lewis acid catalysts have been extensively used in nitration reaction as these catalysts are inexpensive and effective, such as aluminium dihydrogen phosphate [5], ammonium nickel sulfate [6], Ferric (III) [7,8], Cupric nitrate [9], etc. However, traditional Lewis acid catalysts have several major disadvantages, such as difficulty in separation and recovery due to dissolution in the reaction mixture, serious corrosion of industrial equipment, and inevitably generating high-risk pollutants [10].

In recent years, alternative zoelite catalysts are widely used to replace sulfuric acid, due to easy separation from product, convenience for recycling, and environmental friendliness. These include ZSM-5 [11,12,13,14,15], Y [15,16], BEA [6,12,13,14,15,17], MCM-41 [18], SBA-15 [19], which have been studied in pursuit of overcoming the above-mentioned drawbacks. Nitronaphthalene, as an important chemical raw material, is widely utilized in the manufacture of aniline; 1-naphthylamine is useful, but 2-naphthylamine is carcinogenic and highly toxic. Therefore, the selective nitrification of naphthalene is of great significance. In the past, phosphate-impregnated titania [20], NBS/AgNO₃ [21], ZnCl₂ [22], and 3-methyl-1-sulfonic acid imidazolium nitrate [23] were used in the preparation of nitronaphthalene; the isolated yield was up to 80–84%, but there was no discussion of the yield of 2-nitronaphthalene and selectivity. In this paper, we have carried out the mononitration reaction of naphthalene under modest conditions with nitric acid over zeolites, where the region-selection of nitration can be improved.

2. Results and Discussion

2.1. Characterization of Catalysts

2.1.1. N₂ Adsorption-Desorption

It can be seen from the data in Figure 1 and

Table 1. Surface properties of the different HBEA-25.

Samples	Surface Area (m2/g)	Pore Volume (cm3/g)	Pore Diameter (nm)
Fresh HBEA-25	355	0.269	3.09
recovered HBEA-25	258	0.213	3.37
regenerated HBEA-25	351	0.265	3.09

that the specific surface area of the recovered HBEA-25 catalyst has decreased significantly, from 345 m²g⁻¹ to 252 m²g⁻¹, and the pore size has decreased from 0.265 cm³g⁻¹ to 0.211 cm³g⁻¹. It is proposed that the main reason for this may be that the catalyst used has adsorbed other substances during the nitrification reaction, and even after solvent washing, the substances entering the pore cannot be resolved, resulting in the reduction of the specific surface area and pore size of the blocked pore. After calcination and regeneration, the catalyst almost returned to its original level, but was slightly reduced, which may be caused by the skeleton collapse and silicon migration of the catalyst in a highly acidic reaction environment.

2.1.2. XRD

It can be seen from the XRD spectra (Figure 2) that the spectra of fresh catalyst (a), recovered catalyst (b) and regenerated catalyst (c) are similar, and their 2θ angles have two strong peaks at 22.4 and 7.4, showing a typical BEA zeolite structure peak. This indicates that the crystal structure of HBEA-25 zeolite has not been obviously damaged during its use. Meanwhile, the diffraction peak of recovered catalyst (b) 2θ angle at 7.4 is significantly weakened, indicating that the channels of HBEA-25 catalyst may be blocked by adsorption during use.

2.1.3. FT-IR Spectra

The nature of adsorbed species on the catalyst sites was ascertained from FT-IR spectra of the used HBEA-25 catalyst after nitration reaction in 1,2-dichloroethane solvent. The IR bands (Figure 3) at 1521 and 1347 cm⁻¹ are due to asymmetric and symmetric vibrations of nitro-product, respectively. The intense band at 1719 cm⁻¹ shows the presence of acetic acid formed during the reaction [24]. FT-IR spectrum, after activation at 550 °C for 6 h, is similar to fresh catalyst indicating the regeneration of the catalyst (Figure 3).

2.1.4. SEM

It can be seen from the SEM images (Figure 4) that with the increase in silicon–aluminium ratio, the particle size of the catalyst becomes smaller and smoother, which may also give HBEA-25 catalyst better catalytic performance. The surface of H-BEA-280 in Figure 4a is smoother than that of H-BEA modified by rare-earth ions (Figure 4b–d), and the average particle size is larger, indicating that the morphology of H-BEA catalyst modified by rare-earth ions has changed slightly. This may lead to changes in the strength and number of B-acid sites and L-acid sites on its surface, which is closely related to its catalytic performance.

2.1.5. EDS

It can be seen from Figure 5 that the surface of the modified BEA zeolite catalyst mainly contains Si, Al, O, Ce, etc., and Cu, Pd and Au are the elements brought in by the supported copper sheet and gold spraying, respectively. The content of oxygen is particularly high, which indicates that all Si, Al, O and Ce elements should exist in the form of oxides. The presence of a small amount of Ce indicates that BEA zeolite, Al, and rare-earth ion Ce have conducted ion exchange. Hydrothermal modification of rare-earth ions is successful, but the introduction of Ce is less so.

2.2. Effects of Various Catalysts on Nitration Reaction

Classical methodology for the preparative nitration of naphthalene involves mixtures of nitric and sulfuric acids. Unfortunately, this reaction was quite unselective, giving a mixture of nitronaphthalene: 1-nitronaphthalene 2, 2-nitronaphthalene 3, 1,5-dinitronaphthalene 4, and 1,8-dinitronaphthalene 5 (Scheme 1). 1-Nitronaphthalene 2 is an important fine chemical, with a constantly increasing world market owing to its usefulness as synthetic intermediate. Zeolites have three-dimensional channels, appropriate pore sizes, and high selectivity for nitration of aromatic compounds [25,26]. When the reaction was carried out in a system of nitric acid and zeolite catalysts, the yield was higher than with a system using nitric acid alone. Smith and his coworkers [27] have reported that simple aromatic compounds (benzene, alkybenzenes, halobenzenes) can be nitrated with an excellent yield using nitric acid and acetic anhydride. The traditional nitrating system has a low selectivity for 1-nitronaphthalene; we keep searching for an effective method for nitration of naphthalene. Therefore, the use of zeolite catalysts to enhance the yield and/or selectivity was expected. Zeolites possess both the Lewis and the Bronsted acid sites, which are catalytically active sites [28]. The acetic anhydride and nitric acid system, catalyzed by zeolites, is used to improve the selectivity of 1-nitronaphthalene, and the results are shown in

Table 2. The effect of zeolite type on the nitration of naphthalene with HNO₃ and acetic anhydride ^a.

Entry	Zeolite b	Si/Al2	Yield c	Isomer Proportion d (%)		2:3 Ratio
				2	3
1	none		80.1	78.2	21.8	3.58
2	HBEA-25	31	93.9	79.0	21.0	3.76
3	HBEA-280	256	40.5	78.3	21.7	3.59
4	HBEA-500	501	55.9	78.3	21.7	3.59
5	HZSM-5	30	73.0	78.8	21.9	3.72

^a All reactions were carried out in acetic anhydride (5.0 mL) using substrate 1 (4.0 mmol), fuming nitric acid (95%, 6.0 mL) and catalyst (0.10 g). The reaction was stirred at 0 °C. ^b Zeolites were calcined at 550 °C for 2 h in air prior to use. ^c Combined yield of 2 and 3 based on consumed 1. ^d Proportions of products as determined by HPLC.

.

As shown in Table 2, when zeolite catalysts are used, the selectivity of 1-nitronaphthalene was higher than in the absence of any catalyst. The advantage of this method was that there was no dinitration product generated. Acetic anhydride has two actions: action 1, as a water absorbent to accelerate the nitrification reaction, and action 2, the ability to react with nitric acid to form nitrated acetyl nitrate with lower oxidation. With the Si/Al ratio increasing, the selectivity of 1-nitronaphathalene decreased (entries 2, 3 in Table 2), and a lower yield was achieved. This may be due to dealumination, which would modify the distribution of the size and shape of pores in the zeolites, resulting in a decrease in the number of acid sites with catalytic properties. It can be seen from SEM that with the increase of Si/Al ratio, the particle size decreases and the surface area increases, meanwhile the absorption of substrate and products increases, resulting in a decrease in the yield of mononitration.

Unfortunately, the 2:3 isomer ratio was not obvious improved. Pure acetyl nitrate can be distilled under reduced pressure, but explosively decomposes at atmospheric pressure at 60 °C. When fuming nitric acid was used as the nitrate agent, the nitration reactions have higher yield and improved 1-nitro/2-nitro isomer ratio, as fuming nitric acid has a higher capacity for nitration (entries 1–11 in

Table 3. The effect of zeolite type on the nitration of naphthalene with HNO₃ ^a.

Entry	Zeolite b	Yield c	lsomer Proportion d (%)				2:3 Ratio
			2	3	4	5
1	none	71.7	83.3	15.8	0.43	0.47	5.26
2	HBEA-25	70.5	84.6	8.90	3.10	3.40	9.51
3	HBEA-500	74.0	80.4	14.9	2.28	2.42	5.39
4	CdBEA-25	72.8	79.3	13.8	3.59	3.31	5.76
5	CuBEA-25	80.9	81.9	13.7	2.74	1.66	5.96
6	FeBEA-25	74.0	82.6	14.8	1.26	1.34	5.58
7	CeBEA-25	69.4	75.1	22.0	1.31	1.59	3.43
8	LaBEA-25	65.9	80.0	13.4	3.39	3.21	5.97
9	HZSM-5	68.2	76.8	18.6	2.46	2.14	4.12
10	CdZSM-5	77.4	78.0	16.2	2.25	3.55	4.86
11	CuZSM-5	72.8	72.9	14.1	3.28	9.72	5.19

^a All reactions were carried out in 1,2-dichloroethane (5.0 mL) using substrate 1 (5.0 mmol), nitric acid (95%, 0.40 mL) and catalyst (0.13 g). The reaction was under reflux. ^b Zeolites were calcined at 550 °C for 2 h in air prior to use. ^c Combined yield of 2 and 3 based on consumed 1. ^d Proportions of products as determined by HPLC.

).

Zeolite BEA was used as the catalyst because preliminary screening suggested that it was quite active. In order to find a better catalyst to improve reaction selectivity, several cation-exchanged forms of zeolite were also tested (entries 4–8 in Table 3). With the dimensional channels and pore sizes changing, the catalysts modified by different cations showed different selectivity for mononitration of naphthalene. Among all the metal ion-exchanged zeolites, CuBEA-25 was observed to give the maximum yield of 80.9%, with 5.96 1-nitronaphthalene selectivity (entry 5 in Table 3). It may be that cation-exchanged zeolite would display both Lewis and Brφnsted types of acidity, due to high charge density generating acidic hydroxyl groups inside the zeolite cavities. Compared with CdZSM-5 and CuZSM-5 (entries 10–11 in Table 3), BEA zeolite catalysts (entries 4–5 in Table 3) show better selectivity of 1-nitronaphthalene, as ZSM-5 zeolite has a medium pore-size structure, placing more restriction on the transport of 1-nitronaphthalene through the pore.

H-form BEA-zeolite was superior to the metal-exchanged types in regioselectivity. This is probably due to the interaction of high density effective sites via hydrogen bonding. Due to the large diversity of composition and structure, HBEA-25 zeolites (entry 2 in Table 3) displays outstanding catalytic capacity, even in the absence of shape selectivity. HBEA-25 was chosen for more detailed research. The average particle size of the zeolites decrease with Si/Al₂ ratio, which was detected from the SEM images. The XRD spectra of catalysts showed the reflections at 2θ 7.6 and 22.5, exhibiting the presence of similar structures to the BEA catalyst.

The effect of temperature on the reaction was investigated. The results were shown in

Table 4. Effect of reaction temperature ^a.

Temp./℃	Yield b	lsomer Proportion c (%)				2:3 Ratio
		2	3	4	5
−15	56.8	91.7	7.29	0.26	0.75	12.6
0	68.2	88.9	9.17	0.86	1.07	9.69
25	79.6	86.8	11	0.98	1.22	7.89
45	73.9	84.7	13.5	0.58	1.22	6.27
65	68.2	86.1	11.6	0.75	1.55	7.42
80	79.6	76.0	8.33	4.29	11.4	9.12

^a All reactions were carried out in 1,2-dichloroethane (5.0 mL) using substrate 1 (1.0 mmol), fuming nitric acid (95%, 0.22 mL) and catalyst (0.02 g). ^b Combined yield of 2 and 3 based on consumed 1. ^c Proportion of products was determined by HPLC with nitrobenzene as internal standard.

. Table 4 shows that the selectivity for 1-nitronaphthalene is quite good at a low temperature. With increasing temperature, the yield of 2-nitronaphthalene and dinitronaphthalene increased. Therefore, a lower temperature is preferable for good selectivity.

The effect of the amount of zeolite for the nitration reaction was then investigated at the optimal temperature, which was found as above. The results were shown in

Table 5. Effect of different amounts of HBEA-25 ^a.

Amount of Zeolite/g	Yield b	lsomer Proportion c (%)				2:3 Ratio
		2	3	4	5
0	55.2	89.6	9.15	0.21	1.04	9.79
0.02	56.8	91.3	7.26	0.25	1.19	12.6
0.05	56.8	93.3	5.88	0.35	0.47	15.9
0.08	59.6	93.4	5.02	0.96	0.62	18.6
0.1	68.2	93.5	4.86	0.87	0.77	19.2
0.12	62.4	93.7	4.80	0.95	0.55	19.5

^a All reactions were carried out in 1,2-dichloroethane (5.0 mL) using substrate 1 (1.0 mmol), fuming nitric acid (95%, 0.22 mL) and an amount of catalyst at −15 °C. ^b Combined yield of 2 and 3 based on consumed 1. ^c Proportion of products was determined by HPLC, with nitrobenzene as internal standard.

. Increasing the amount of catalyst favored the reaction further toward the 2:3 ratio up to 19.5. This probably because of the higher internal/external surface area, and effective catalytic sites. However, the use of a large amount of catalyst would cause the absorption of substrate and products, resulting in a small decrease in the yield of mononitration. HBEA-25 zeolite was easily recovered from the reaction mixture by simple filtration. Even when HBEA-25 zeolite was used four times, only a slight decrease was observed in selectivity and yield (

Table 6. Efficiency of recycled HBEA-25 in the preparation of nitro- naphthalene ^a.

Recycle Number	Yield b	lsomer Proportion c (%)				2:3 Ratio
		2	3	4	5
0	68.2	93.5	4.86	0.87	0.77	19.2
1	66.8	93.3	4.88	0.90	0.92	19.1
2	66.2	93.4	4.89	0.89	0.82	19.1
3	66.4	93.4	4.92	0.85	0.83	19.0

^a All reactions were carried out in 1,2-dichloroethane (5.0 mL) using substrate 1 (1.0 mmol), fuming nitric acid (95%, 0.22 mL) and catalyst (0.10 g) at −15 °C. ^b Combined yield of 2 and 3 based on consumed 1. ^c Proportion of products was determined by HPLC, with nitrobenzene as internal standard.

). The XRD spectrum of fresh and regenerated catalyst displayed two typical peaks located at 2θ 22.5 and 7.4, exhibiting the presence of structures similar to HBEA-25 zeolite. SEM and FT-IR showed the same results. Surface properties of used HBEA-25 zeolite were also recovered after regeneration (Table 1).

3. Experimental

3.1. Reagents and Instruments

Naphthalene (AR) and Nitric acid (purity > 99.9%) were purchased from Tianjin Kermel Chemical Co., Ltd. (Tianjin, China) and Beijing Chemical Co., Ltd. (Beijing, China), respectively. Anhydrous FeCl₃, AlCl₃, and ZnCl₂ were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Mesoporous silica gel (100 mesh) was obtained commercially. Gas chromatography (GC) was performed on a Shimadzu GC-2010 (Kyoto, Japan) Plus equipped with hydrogen flame ion detector (FID) and an RTX-5 column (30 m × 0.25 mm × 0.25 um) for quantitative analysis using internal standard method. ¹H NMR spectra of products were determined in CDCl₃ on a Bruker 400 MHz spectrometer (Billerica, MA, USA).

3.2. Catalyst Preparation

The standard procedure for rare-earth cation-exchange involves stirring a supplied commercial zeolite (5.00 g) in an aqueous solution of ammonium chloride and corresponding metal chloride (0.2 mol/L, 100 mL) at reflux for 3 h; then the solid is filtrated. The exchange operation was repeated three times, then the catalyst is washed with deionized water until chloride ions are removed (examined by silver nitrate solution), and dried at 110 °C for 1 h, then calcined in air at 550 °C for 6 h. The catalysts of H-form, modified with highly electronegative cations, were achieved.

3.3. Nitration with Nitric Acid and Acetic Anhydride

Naphthalene (0.51 g, 4.0 mmol), zeolite (0.10 g), nitric acid (0.27 mL, 6.0 mmol, 95%), and acetic anhydride (5.0 mL) were placed in a three-necked flask. The mixture was stirred at 0 °C. The reaction was monitored by TLC. When the reaction was completed, the zeolite was removed by filtration, and the filter liquor was washed with water, 5% aqueous solution of sodium bicarbonate, then followed again with water. The organic layer was separated, dried with anhydrous sodium sulfate, and concentrated under reduced pressure to give a solid yellow residue. The product composition was analyzed by HPLC. The zeolite was recovered by washing and calcination.

3.4. Catalyst Characterization

The textural properties of the samples were acquired on a Quantachrome Instruments NOVA 1000e analyzer (Boynton Beach, FL, USA) by N₂ adsorption at −70 °C, using BET and BJH method for calculation. The X-ray diffraction analysis (XRD) of the catalysts were carried out on a Japan Rigaku (Tokyo, Japan) D-Max-2550VB⁺ 18 kW X-ray diffractometer using Cu Kα radiation (λ = 0.15418 nm) at a voltage and current of 40 kV and 20 mA, respectively. FT-IR of the samples were recorded on a Nicolet380 instrument (Thermo Electron Corp., Waltham, MA, USA) using KBr pellets in the range of 400–4000 cm⁻¹. The morphology of the samples and EDS analysis was examined using scanning electron microscopy (SEM) on a JSM-6610LV spectrometer (JEOL Ltd., Tokyo, Japan).

4. Conclusions

In conclusion, nitration of naphthalene in the presence of HBEA-25 zeolite with fuming nitric acid shows improved 1-nitronaphthalene selectivity and moderate yield. Among the zeolite catalysts studied, HBEA-25 gave maximum selectivity: 19.2, with a moderate yield of 68.2% in 1,2-dichloroethane. In addition, one major advantage of this process is the elimination of inorganic acids as catalysts, which are essential in the conventional process, thus avoiding hazardous waste disposal and reducing environmental pollution. Zeolite catalysts can easily be separated from products and can be regenerated by simple calcination for aromatic nitration reactions; making an important contribution to green chemistry.