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One-Step Synthesis of Aniline From Benzene With Hydroxylamine Over V-MCM-41 Mesoporous Molecular Sieves.

Byline: Chunhua Yang and Gang Chen

Summary: V-MCM-41 catalysts were prepared using five concentrations of VOSO4. The resulting catalysts were characterized by FT-IR, XRD and nitrogen adsorption-desorption measurements. They were used for the direct amination of benzene to aniline. Experimental results showed that the catalytic activities of these catalysts were related to the vanadium content. The highest conversion of benzene was 68.7% over V-MCM-41(60), with a benzene-to-hydroxylamine ratio of 1:1, benzene 11.25 mmol, NH2OH 11.25 mmol, temperature 70 C, catalyst amounts 0.05 g, acetic acid 7.5 mL (70 vol.%), and reaction time 2 h.

Keywords: V-MCM-41, Direct amination, Benzene, Catalyst, Aniline.

Introduction

Aniline is widely used in dyes, pharmaceuticals, and pesticides etc.[1] At present, the production method of aniline is multiple reaction in industry. However, the method has many disadvantages, for example, high temperature, high pressure and unfriendly for the environment, and disobeys the purposes of the green chemistry.[2] Therefore, the development of single-step synthetic method for aniline has great practical significance in industry.

This reaction has been attracting numerous attentions, [3-5] in which amination agent usually used ammonia, and the oxidant used gaseous oxygen. The yield and selectivity to aniline of the process were both low, and suffered from critical operation conditions. Mantegazza et al. [6] studied the direct oxidation of ammonia to hydroxylamine with high yield at low temperature and atmospheric pressure, in which hydrogen peroxide was used as oxidant, Ti-silicalite was used as catalyst. The ammonia wasn't deeply oxidized, thus no byproduct generated during this process. At present, although the use of hydroxylamine is uneconomical, the method provides the idea for direct synthesis of aniline with hydroxylamine as amination agent. Thus this will provide an idea for one-step synthesis of aniline with high yield with ammonia and hydrogen peroxide in the future.

It has been reported that MCM-41 by transition metal-modified is great catalysts for a variety of organic chemical reactions in the industrial production.[7,8] For example, Samanta, et al.[9] synthesized Cr-MCM-41 materials, which showed high selectivity and great catalytic activity in the direct oxidation of various cycloalkanes to cyclic ketones under mild liquid phase reaction conditions, respectively, using dilute aqueous or tert-butyl hydroperoxide as oxidant; Debraj Chandra, et al.[10] prepared mesoporous titanium silicate, which catalyzed epoxidation of styrene using dilute aqueous H2O2 as oxidant with excellent catalytic activity and selectivity; Nabanita et al.[11] reviewed the syntheses and application of functionalized mesoporous solids in liquid phase catalytic reactions. Especially, the catalytic activity of the modification of MCM-41 by vanadium is more excellent in some oxidation reaction.[12,13]

In this paper, MCM-41 and a series of ordered mesoporous composite materials V-MCM-41 have been prepared for direct amination of benzene to aniline using NH2OH as aminating agent under mild conditions. The composite materials were characterized by FT-IR, X-ray diffraction analysis, and nitrogen adsorption-desorption analysis.

Experimental

Catalyst Preparation

V-MCM-41 catalysts were perpared with VOSO4 as the vanadium source, tetraethyl silicate (TEOS) as silica source and cetyltrimethylammonium bromide(CTAB)as a template, respectively. CTAB and NaOH were well dissolved in distilled water, and were stirred for 30 mins to become a clear solution, which was mixed with TEOS and VOSO4 to form a gel with mole ratio of 0.l2CTAB:1.0TEOS:0.6NaOH:l00H2O:X VOSO4, where X represents the ratio of Si/V from 10 to 100.

The gels were stirred for 1.0 h, then left for 24 h at room temperature. After filtering, the products were washed with deionized water, then heated for 2-3 h at 90 C. After calcined in furnace at 540 C for 6.0 h , the catalysts with different amounts of vanadium were obtained. The catalysts are denoted as V-MCM-41(10), V-MCM-41(20), V-MCM-41(40), V-MCM-41(60), V-MCM-41(100), MCM-41(without V).

Catalyst Test

The catalytic amination occurred in a three-flask under atmospheric pressure. The catalyst (0.05 g ) and NH2OH*HCl (11.25 mmol) were added into the flask containing 7.5 mL CH3COOH (70 vol.%). After stirring at 40 C for 20 mins, 1 mL of 11.25 mmol benzene was added, then heating the mixture at 70 C. After two hours, the products were cooled at room temperature and neutralized by alkali solution. The final products extracted with ether were analyzed by gas chromatography through an HP-5 column.

The conversion of benzene was calculated according to the following formula: conversion of benzene =(initial quantity of benzene - residual quantity of benzene) / (initial quantity of benzene) ; the selectivity of aniline was obtained according to the following formula: selectivity = (aniline produced)/(benzene consumed).

Results and Discussion

The XRD patterns of samples with different amounts of vanadium species are depicted in Fig. 1 Several different diffraction peaks are observed, and the strong diffraction peak between 2th=2~3 is shown corresponding to d100. The two weak diffraction peaks (110) and (200) are also observed in these samples with Si/V[greater than or equal to]20 around 2th=3.5~4.5, which are attributed to the characteristic peak of ordered MCM-41 materials. [12,13]

It can be illustrated that the materials also maintain the MCM-41 mesostructure after the addition of vanadium. However, the strength of diffraction peaks decrease with the increasing of the loaded vanadium. Because all the samples were treated at the same conditions, the decreasing of the diffraction peak can be assigned to the introduction of vanadium into the MCM-41, which indicates that the introduction of vanadium inhibited the growth of the crystal.

The FT-IR spectras of the samples are shown in Fig. 2. The bands at 800 cm-1 and 1090 cm-1 belong to asymmetric stretching vibration of Si-O-Si band.[14] These peaks shift to lower wave number for V-MCM-41 because the length of V-O bond is longer than that of Si-O bond after Si in the silica framework is replaced by vanadium. The band at 462 cm-1 is the stretching vibration of V-O bonds.[15] which attributes to the increase of Si-O distance caused by substitution of the small ionic radii of silicon by the larger ionic radii of vanadium. The bands at 670 cm belong to the characteristic peak of V2O5.[16] Bands at 800 cm are observed in the samples of V-MCM-41, and the intensity becomes stronger as vanadium contents increase in the samples.

The band at 800 cm belongs to a stretching vibration of Si-O-V . This shows that the vanadium is introduced into the framework of MCM-41. The broad band around 3500 cm may be attributed to surface silanols and water molecules adsorbed, and the band at 1630 cm-1 is deformational vibration peak of it.

N2 adsorption-desorption isotherms of V-MCM-41(60) are given in Fig. 3. It shows isotherms of type IV which has a hystersis loop, and this is the typical nature of the mesoporous materials. Three obvious stages are observed in Fig. 3: the initial stage (P/P0=0.00-0.35) is monolayer adsorption on the pore walls, resulting in increasing in nitrogen adsorption; the intermediate stage (P/P0 =0.35-0.55) is hysteresis, which shows the capillary condensation in the mesopores; the high stage is a plateau portion, which is due to multilayer adsorption on the external surface of the sample.

The data of BET specific surface area, pore size and pore volume are summarized in Table-1.The surface area of the sample without vanadium is 1178 m2/g; this value gradually decreased as the amount of vanadium increased. However, from 1115 m2/g for V-MCM-41(100) to 830 m2/g for VMCM-41(40) the BET surface area still remains relatively high, the changes of the pore volume are in agreement with that of surface area. More amount of vanadium added into the pores will occupy more space, resulting in decreasing in pore size and pore volume.

Table-1: The data of BET specific surface area, pore size and pore volume.

Entry###Catalyst sample###SBET/m2g-1###Dp/A###Vp/cm3g-1

1###MCM-41###1178###29.4###0.89

2###V- MCM-41(100)###1115###26.4###0.88

3###V- MCM-41(60)###1112###26.0###0.87

4###V- MCM-41(40)###830###26.0###0.66

5###V- MCM-41(20)###104###25.2###0.18

6###V- MCM-41(10)###31###23.3###0.13

The pore size distribution of the catalyst samples is illustrated in Fig. 4. The pore size distribution of both MCM-41(100) and V-MCM-41(60) is narrow. However, with the increasing of vanadium content in the sample, the degree of non-uniform pore size distribution increases. This may be due to that a certain amount of vanadium did not enter into the skeleton of MCM-41, but supported on the inner surface of the zeolite, resulting in non-uniform pore size.

MCM-41 and MCM-41with vanadium were used for the one-step synthesis of aniline from benzene under atmospheric pressure at 70 C in acidic medium; effects of vanadium loading on amination of benzene are shown in Table-2. Benzene conversion was only 8.6%, and the selectivity to aniline was 62.0% with MCM-41 as catalyst for the reaction. However, compared with MCM-41, all the catalysts with vanadium showed better catalytic activity, which maybe due to the addition of vanadium ions in the structure of MCM-41. The conversion of benzene increased from 8.6% to 68.7% with the decreasing of Si/V ratio from 100 to 60. Further increasing in vanadium loading, the conversion of benzene decreases. The increasing of vanadium may make some pores smaller in MCM-41, leading to the decreasing of the surface area. When V-MCM-41 (Si/V = 60) was used as the catalyst, aniline selectivity also increases to the maximum and then decreases.

Table-2: Effect of V loading on amination of benzene.

###Entry###Catalyst###Conv(%)###Sel(%) Yield(%)

###1###MCM-41###8.6###62.0###5.3

###2###V- MCM-41(100)###28.3###87.2###24.7

###3###V- MCM-41(60)###68.7###98.6###67.7

###4###V- MCM-41(40)###52.5###95.1###50.0

###5###V- MCM-41(20)###39.5###90.6###35.8

###6###V- MCM-41(10)###38.9###86.0###33.5

Free-radical mechanisms have been proposed for amination reactions using N-chloroalkylamines, hydroxylamine-o-sulfonic acid, and alkylhydroxylamines as the aminating agents catalyzed by redox metal ions such as Ti3+, Fe2+, and Cu+ [17]. Kuznetsova et al. [18] proposed an idea that the amination of benzene and toluene based on the free-radical mechanism by an initio quantum mechanical calculations. In the present work, a similar free-radical mechanism can be put forward for the V-MCM-41 catalyzed amination of benzene to aniline. The catalyst interacts with hydroxylamine in acidic medium and generates protonated amino (*NH3+) radical, which was the active aminating species, by reduction of hydroxylamine. Then the protonated amino radicals react with benzene to give protonated aminocyclohexadienyl intermediates, which is unstable. Then catalyst can oxidize it to aniline, meanwhile, the catalyst was regenerated. The detailed mechanism of amination reaction is given in Scheme-1.

V-MCM-41 (Si/V = 60) was used for recycling study. In order to reuse the catalyst after 2 h reaction, the catalyst was separated by ltration, washed with distilled water several times, dried at 110 C and used in the amination reaction with a fresh reaction mixture. With the regenerated sample after five cycles, the percentage of conversion decreased from 67.7% to 49.8.7%, and selectivity decreased from 98.6% to 80.23%. The results were given in Fig. 5.

(Equations)

Conclusions

A series of ordered mesoporous composite materials V-MCM-41 was prepared. FT-IR studies showed that the vanadium was introduced into the framework of MCM-41; N2 adsorption-desorption isotherms indicated that the catalysts were mesoporous materials; Surface area, pore volume and pore diameter decreased with increasing vanadium content, for vanadium added into the pores occupied more space. V-MCM-41(60) has been used as the heterogeneous catalyst for one-step synthesis of aniline from benzene with high yield under mild reaction conditions. Therefore, the one-step amination was occurred at low temperature and atmospheric pressure, which brings new idea for one-step synthesis of aniline with high yield with ammonia and hydrogen peroxide in the future.

Acknowledgments

The authors gratefully acknowledge the financial support from Scientific Research Staring Foundation for PhDs of Northeast Dianli University(BSJXM-201322) and Department of Education of Jilin Province for the "the Twelfth Five" Scientific and Technological Research Projects (2015-247).

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Author:Chen, Gang; Yang, Chunhua
Publication:Journal of the Chemical Society of Pakistan
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Date:Apr 30, 2016
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