光催化有机合成

Practical synthesis of aromatic amines by photocatalytic reduction of aromatic nitro compounds on nanoparticles N-doped TiO 2

Huqun Wang a ,Junping Yan a ,Wenfu Chang b ,Zhimin Zhang a,*

a School of Chemistry and Chemical Engineering,Shanxi University,Wucheng Road,Taiyuan 030006,PR China b

Institute of Molecular Science,Shanxi University Taiyuan 030006,PR China

a r t i c l e i n f o Article history:

Received 9September 2008

Received in revised form 15December 2008Accepted 17December 2008

Available online 25December 2008Keywords:Reduction

Aromatic amines N-doped TiO 2

Potassium iodide

a b s t r a c t

A novel efficient method for the catalytic reduction of aromatic nitro compounds to the corresponding amines was reported.Aromatic nitro compounds were chemoselectively reduced to the corresponding amines by using N-doped TiO 2and potassium iodide as photocatalysts in the presence of methanol.The novel method is highly efficient with very short reaction time (<20min),excellent yields (>90%)and wide functional group tolerance such as carbonyl,halogen,amino,hydroxyl and carboxylic acid groups.And N-doped TiO 2was prepared by a modified sol-gel method using urea as nitrogen source and had higher photocatalytic activity comparing with pure TiO 2.The catalysts were characterized by XRD,XPS,TEM and UV–Vis DRS.

Ó2009Published by Elsevier B.V.

1.Introduction

Aromatic amines are widely used key intermediates in the industrial synthesis of dyes,pharmaceuticals and agrochemicals [1].A variety of methods for the direct reduction of aromatic nitro compounds to the corresponding amines has been well docu-mented [2–4].However,development of new methodology espe-cially the environmentally benign process still attracts the great interests in the chemistry community [5–8].In comparison to the commonly used methods which involve hydrogenation,elec-tron transfer and hydride reduction,photocatalytic reduction emerges as cost-effective,highly selective,rapid and environmen-tally friendly.Li and co-workers first reported a photoinduced reduction of nitro compounds to the corresponding amines using TiO 2semiconductor as a catalyst [9].Ferry and co-workers further investigated the mechanism of photocatalytic reduction of nitro aromatics at the surface of titanium dioxide slurries in the pres-ence of the sacrificial electron donor methanol or isopropanol [10].Heterogeneous photocatalysis has been rapidly becoming an exciting and growing area of research due to its direct application for synthetic chemistry,such as Photo–Kolbe oxidation [11],reduc-tion [12],amino acid [13],Diels–Alder [14]and Friedel–Crafts alkylation [15]reactions.However,so far these reactions are still hardly applied to the industrial field.Research in our laboratory has focused in the late few years on new active-TiO 2based reduc-

ing systems.The reaction for synthesis of amines from nitro com-pounds was catalyzed by irradiating N-doped TiO 2(N-TiO 2)and potassium iodide in solution of methanol (Scheme 1).The proce-dure of synthesis of aromatic amines was much simpler and more efficient than those in any other literature.In addition,the photo-catalyst could be reused and remained sufficient catalytic activity.2.Experimental

2.1.Catalyst preparation and characterization

All reagents were analytical reagent grade and were used with-out any further purification.A solution of tetrabutyltitanate (8.5mL)in absolute ethanol (30mL)was mixed with glacial acetic acid (1.5mL)as constraining reagent to prevent the precipitation of oxides and stabilize the solution and an ethanol solution of dis-tilled water and urea (EtOH:H 2O:CH 4ON 2=3:48:1)was added to above solution under vigorous stirring.The pH of solution was ad-justed about three by nitric acid.After 3h,the gel so obtained had been left ageing overnight at room temperature to ensure the com-pletion of the hydrolysis,subsequently evaporation of the solvent,drying at 100°C for 8h and finally calcination at 450°C for 4h.The anatase crystal phase was determined from the X-ray diffraction (XRD)patterns obtained by using an X-ray diffractometer (Model D/Max 2550V)with a Cu target Ka-ray (k =1.544178Å).The mor-phology of the N-TiO 2powders was examined by using a Hitachi-600-2transmission electron microscope (TEM)and UV–Vis diffuse reflectance spectrophotometer (Cary 300,Varian,US)was employed to determine the optical properties of N-TiO 2and pure

1566-7367/$-see front matter Ó2009Published by Elsevier B.V.doi:10.1016/j.catcom.2008.12.045

*Corresponding author.Tel.:+863517010588;fax:+863517011688.E-mail address:mqz1003@ (Z.Zhang).Catalysis Communications 10(2009)

989–994

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