组合型Pt3Sn_Al2O3催化剂用于芳香硝基化合物一锅法合成N-烷基芳胺

[Article]物理化学学报(Wuli Huaxue Xuebao )Acta Phys.⁃Chim.Sin .2012,28(9),2141-2147September Received:April 6,2012;Revised:June 20,2012;Published on Web:June 20,2012.∗Corresponding author.Email:xinhuanyan139@;Tel:+86-571-88320791.The project was supported by the National Natural Science Foundation of China (21076197),Natural Science Foundation of Zhejiang Province,China (Y4090440),and Qianjiang Talent Program of Zhejiang Province,China (2010R10038).国家自然科学基金(21076197),浙江省自然科学基金(Y4090440)和浙江省钱江人才项目(2010R10038)资助ⒸEditorial office of Acta Physico ⁃Chimica Sinicadoi:10.3866/PKU.WHXB201206201组合型Pt 3Sn/Al 2O 3催化剂用于芳香硝基化合物一锅法合成N -烷基芳胺杨芳许响生顾辉子陈傲昂严新焕*(浙江工业大学绿色化学合成技术国家重点实验室培育基地,杭州310014)摘要:采用吸附法制备了组合型Pt 3Sn/Al 2O 3双金属催化剂,将该催化剂用于芳香硝基化合物原位液相加氢一锅法合成N -烷基芳胺.研究表明,在503K,空速为7.5h -1,水体积分数为5%时,1%(质量分数)Pt 3Sn/Al 2O 3催化剂具有较高的催化性能,硝基苯的转化率为100%,N -乙基苯胺和N ,N -二乙基苯胺的总选择性为98.2%.同时,该催化剂对原位液相加氢烷基化反应具有一定普适性,本文研究的14种芳香硝基化合物与低级脂肪醇反应,均具有较高的N -烷基化产率.关键词:N -烷基化;原位液相加氢;芳香硝基化合物;N -烷基芳胺;铂锡双金属催化剂中图分类号:O643Heterogeneously Catalyzed One-Pot Synthesis of N -alkyl Anilines fromNitroaromatics by Assembled Pt 3Sn/Al 2O 3CatalystYANG FangXU Xiang-ShengGU Hui-ZiCHEN Ao-AngYAN Xin-Huan *(State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology,Zhejiang University ofTechnology,Hangzhou 310014,P .R.China )Abstract:N -alkyl anilines were obtained from nitroaromatics by a one-pot method using assembled Pt 3Sn/Al 2O 3catalyst for heterogeneous in situ hydrogenation in a continuous-flow fixed-bed reactor.At the optimum reaction conditions (503K,liquid hourly space velocity (LHSV)of 7.5h -1,5%(volume fraction)water,1%(mass fraction)Pt 3Sn/Al 2O 3catalyst),nitrobenzene conversion was 100%,with a total N -ethyl and N ,N -diethyl aniline yield of 98.2%.Moreover,the Pt 3Sn/Al 2O 3catalyst had a great conjoint effect for all in situ hydrogenation reactions for N -alkylation.High yields of N -alkylation products were obtained in aliphatic alcohol/water systems for 14selected nitroaromatics.Key Words:N -alkylation;In situ liquid hydrogenation;Nitroaromatic;N -alkyl aniline;Pt-Sn bimetallic catalyst1IntroductionN -alkyl anilines are widely used as synthetic intermediates for pharmaceuticals,agrochemicals,fine chemicals,bioactive compounds,and dye chemicals.1,2The most commonly used method for N -alkylation is the coupling of amines with alkyl halides.3-5This procedure,however,can be problematic be-cause of the toxic nature of many alkyl halides,as well as the concomitant formation of large quantities of undesired waste.The reductive amination of aldehydes and ketones is another well-known method.6-8Unfortunately,this method requires the usage of strong reducing reagents.In comparison with the methodologies mentioned above,transition metal-catalyzed2141Acta Phys.⁃Chim.Sin .2012V ol.28amine alkylation with alcohols has attracted much interest thanking to the ubiquitous availability of alcohols,high atom efficiency,and formation of water as the sole byproduct.In the former reports,extensive attention has been focused on Pd,9,10Ru,11,12Au,13Ir,14,15Ni,16and Zn 17,18as effective catalysts.But,an-ilines need to be firstly synthesized by the reduction of ni-troaromatics.Therefore,direct synthesis of N -alkyl anilines from nitroaromatics and alcohols as starting materials in one-pot reactions is an important issue in chemistry owing to several inherent advantages such as simplifying separation steps,reducing the use of reagents,and increasing yields.Re-cently,it was reported that N -alkyl anilines could be synthe-sized in one-pot using nitroaromatics as starting materials with alcohol as alkylation agent.19-24But,most of the reports focused on homogeneous catalytic systems,which are not practically useful due to the problem of wax-catalyst separation,the indis-pensable use of large amounts of addictives or co-catalysts,highly demanding scaling-up,and catalyst deactivation.25Therefore,it would be highly desirable if a heterogeneous cata-lyst could be used in one-pot synthesis of N -alkyl anilines from nitroarenes without additional hydrogen gas and organic ligand or base,simplifying separation steps,enhancing the life time of the catalyst.In this study,we report one-pot synthesis of N -alkyl anilines from nitroaromatic catalyzed by a simple and versatile Pt x Sn/Al 2O 3(molar ratio of Pt and Sn is x :1)heterogeneous catalyst in the presence of water in a continuous fixed-bed reactor (Scheme 1).2Experimental2.1Catalyst preparationA γ-alumina (i.d.,2-3mm,surface area,238m 2·g -1,Zheji-ang jingjing alumina Ltd.)was used as support,which was pre-viously calcined in flowing air at 773K for 5h.The assembled Pt x Sn/Al 2O 3catalyst was prepared by the method of absorp-tion as following.(1)the γ-alumina was impregnated with an appropriate concentration of SnCl 2·2H 2O (CP,Shanghai Exper-imental Reagent Ltd.)to get a precursor.The precursor was stirred for 1h and kept for 24h,then the excess solvent was re-moved by heating at 333K.Then the precursor was calcined in air at 623K for 3h,signed as Sn/Al 2O 3.(2)The procedure to prepare Pt 2(dba)3(dba=dibenzylideneacetone)was described in the literature.26,27Pt 2(dba)3(34mg)in 200mL of propylene car-bonate was placed in a 500mL stainless autoclave.And the system purged three times with hydrogen.Hydrogenation was carried out under 4.0MPa hydrogen pressure at ambient tem-perature for about 2h under vigorous stirring to obtain brownPt precursor.(3)For depositing the sol on the support,the ob-tained Sn/Al 2O 3was added to Pt precursor solution under stir-ring for 24h,and then the solid catalyst was separated by filtra-tion and purification by acetone to get Pt x Sn/Al 2O 3catalyst.The actual metal content of the catalyst was determined by Shi-madzu inductively coupled plasma mass spectrometry (ICP-MS)measurements.2.2Catalyst characterizationThe Brunauer-Emmett-Teller (BET)specific surface area was measured using nitrogen volumetric adsorption (BET,Mi-cromeritics ASAP 2010)at 77K.Prior to measurement,the samples were degassed to 0.1Pa at 373K.The specific surface areas were calculated in a relative pressure range (0.05<p /p 0<0.2)assuming a cross-sectional area of 0.162nm 2for the N 2molecules.Transmission electron microcopy (TEM)photographs were taken by using a JEOL JEM-200C electron microscope.The catalyst powders were dispersed in ethanol by ultrasonic vibra-tion.One drop of the solution was then deposited onto a thin holey-carbon film supported on lacey-carbon/Cu grid (300Mesh)and left to dry.The powder grains based the metal parti-cles were well separated and supported on the thin holey-car-bon film and then characterized by TEM.TEM was operated at an accelerating voltage of 200kV .X-ray photoelectron spectroscopy (XPS)was acquired with ESCALab220i-XL.A resolution of 0.1eV was used to deter-mine the metal atomic ratio of the surface region and the metal oxidation state of the selected catalysts.Samples were in pow-der form and were pressed on a double-side adhesive copper tape.All measurements were carried out at room temperature without any sample pre-treatment.An Al K αX-ray source was used in this work.To compensate for surface charge effects binding energies (E B )were calibrated using C 1s hydrocarbon peak at 284.6eV .2.3Catalytic reactionsThe catalytic activity of the Pt x Sn/Al 2O 3was tested by the hy-drogenation and further N -alkylation of nitrobenzene.The cata-lytic tests were performed in a continuous fixed-bed reactor de-signed by our group.28The catalyst (4g)was loaded in the iso-thermal region of the reactor.The reaction temperature was kept constant by CHB 702thermometer within ±0.1°C,and the corresponding pressure was controlled by GO BP-60Back Pressure control regulator.The appropriate pressure was to en-sure that the reactant was in liquid phase.The corresponding concentration of nitrobenzene was dissolved in ethanol/water solution.A HPLC pump (Model 501style,Syltech Corpora-tion)was adopted to pump the reactant into the fixed-bed reac-tor with internal diameter of 0.52cm at liquid hourly space ve-locity (LHSV)of 7.5h -1.After reacting in the fixed-bed reac-tor,the liquid and gas were separated in the vapor-liquid sepa-rator.The reaction products were identified by GC-MS (Agi-lent-6890GC-5973MS equipped with 30m HP-5capillary),and quantified by GC (GC-9790equipped with a flame ioniza-Scheme 1Synthesis of N -alkylanilineYANG Fang et al.:One-Pot Synthesis of N-alkyl Anilines from Nitroaromatics by Assembled Pt3Sn/Al2O3 No.9tion detector and SE-30capillary column)every2h.The oven temperature of both was453K,injector temperature was523 K,and detector temperature was533K.3Results and discussion3.1Catalyst characterizationThe BET specific surface area of the Pt3Sn/Al2O3is222.4 m2·g-1,and the corresponding pore volume and pore diameter are0.5cm3·g-1and8.6nm,respectively.The TEM micrograph of the fresh catalyst is shown in Fig.1a.It is obvious that the Pt particles are highly dispersed onγ-alumina,and the mean size of Pt particles is1.9nm(Fig.1b).XPS data are shown in Fig.2.No indication can be drawn re-garding alloy formation on our samples from XPS data due to limitations of this technique.Because the energy region of Pt 4f levels was overshadowed by the presence of a very strong Al2p peak,29,30thus Pt3Sn/C was analyzed instead.In this re-gion,the spectra show two peaks.The one at lower binding en-ergy corresponds to Pt4f7/2level and at the higher energy to the Pt4f5/2.The binding energy of71.0eV can be ascribed to Pt(0), and the binding energy of72.6eV can be assigned to the pres-ence of Pt(II)species.The Sn3d spectra of the Pt3Sn/C catalyst show two peaks at about488.2and485.3eV.On the basis of literature indication,31the peak of lower intensity at485.3eV can be characteristic of tin in metallic whereas that at488.2eV can be assigned to the oxidized tin species(Sn(II)and/or Sn(IV)).Unfortunately,distinguishing oxide forms of tin is dif-ficult because their binding energies are too close.The Pt4f7/2 and Sn3d5/2at binding energies of71.0and485.3eV,respec-tively are characteristic of Pt and Sn atoms in the Pt3Sn alloy, which is agreement with the results of Dupont et al.323.2Catalytic performance of different Pt-basedcatalystsFirstly,the catalytic activity and selectivity for the N-alkyla-tion of nitrobenzene to the corresponding N-alkyl anilines were compared among various catalysts(Table1).As for entry1, 45.9%aniline was observed if using Pt/Al2O3as the catalyst al-though58.8%nitrobenzene was reduced.In entry2,the intro-duction of tin could remarkably improve the formation of N-alkyl aniline,i.e.,the conversion of aniline to N-alkyl aniline. With the presence of Sn,Sn would dilute Pt sites and increase the electron density of Pt.When the molar ratio of Pt and Sn al-ters from5to3,the conversion of nitrobenzene increases from 99.0%to100%,and the total yields of N-ethyl aniline and N, N-diethyl aniline increase from70.0%to98.2%(entries2-4). The conversion is100%and the selectivity to N-alkyl anilines is98.2%using1%Pt3Sn/Al2O3catalyst(the mass ratio of Pt and Al2O3is1%)(entry4).When the molar ratio of Pt and Sn is2(entry6),the conversion does not change,but the selectivi-ty of N-alkyl anilines decreases compared to the1%Pt3Sn/ Al2O3.This could be explained from the results of XPS.When the molar ratio of Pt and Sn is3:1,the metallic Pt and Sn would exist in the formation of Pt3Sn alloy,which would be fa-vorable for hydrogenation of N＝O.We also investigate theef-Fig.1(a)TEM image of fresh Pt3Sn/Al2O3and(b)size distribution of Pt particles on the freshcatalystFig.2XPS spectra of(a)Pt4f and(b)Sn3d of the fresh Pt3Sn/C catalyst2143Acta Phys.⁃Chim.Sin.2012V ol.28fect of the loading of Pt increasing from0.5%to2%,the re-sults show that there is only little distinction on the conversion and selectivity(entries4,6,7).This illustrates that0.5%Pt has sufficient active sites in the catalyst.However,0.5%Pt is easi-ly leaching with the reaction.3.3Effect of reaction conditions of the N-alkylationof nitrobenzeneThe effects of the reaction conditions on the conversion and yield are gathered in Table2.In the reaction,water plays an im-portant role,therefore,confirming that the optimum percentage of water is necessary.The effect of water on the reaction is shown in entries8-11.When the volume ratio of ethanol/wa-ter increases from70/30to95/5,no obvious changes on con-version are observed,but the yield of the N-alkyl anilines in-creases significantly.Besides,the yield of N,N-diethyl aniline also increases with the increasing of the volume ratio of etha-nol/water,while that of the quinaldine decreases.This could be explained that with increasing water volume,the efficient of steam reforming would be higher.28However,in the reaction, higher active hydrogen(H*)production is unfavourable to the yield of aniline and further N-alkylation to corresponding N-alkyl aniline because it would cause over-hydrogenation.In ad-dition,reaction temperature would affect catalytic activity and selectivity significantly.It is obvious that with increasing the reaction temperature from443to503K,the conversion of ni-trobenzene raises from81.2%to100%,and the yield of the to-tal N-alkyl aniline raises from80.9%to98.2%(entries11-14). The result suggests that higher reaction temperature produces higher N-alkyliniline,whereas it would produce higher quanti-ties of C-alkylated products if the reaction temperature is high-er according to previously reported results.33Besides,the con-centration and LHSV of nitrobenzene are also investigated(en-tries15-20).With the increase of the concentration and LHSV of nitrobenzene,the conversion and the yield decrease.This could be explained by the following:with the increase of the concentration and LHSV of nitrobenzene,the contact time be-tween the reactant and the catalyst decreases.3.4N-alkylation of nitroaromatics with alcohols tothe corresponding N-alkyl anilinesThen the scope of the reaction with respect to nitrobenzene with aliphatic alcohols was investigated(Table3).Methanol and nitrobenzene proceeded efficiently with85.5%yield(entry 21).When using ethanol as alkylation agent,the yields of N-alkyl anilines are up to98.2%(entry22).Compared to metha-Table1Results of different catalysts on nitrobenzeneprepared by the method of adsorption.The overall loading of Pt and the relative constitution of Pt/Sn in the catalyst were measured by ICP-MS.AN:aniline, NEA:N-ethyl aniline,DEA:N,N-diethyl aniline,QL:quinaldine,others:C-alkylaniline and intermediates,NAA:N-ethyl aniline and N,N-diethyl aniline3232144YANG Fang et al .:One-Pot Synthesis of N -alkyl Anilines from Nitroaromatics by Assembled Pt 3Sn/Al 2O 3No.9nol,ethanol used as alkylated agent gives the higher yields of target compounds,especially higher conversion of anilines,thanking to high hydrogen atom utilization.Isopropanol,buta-nol,and cyclohexanol used as alkylated reagents react smooth-ly with nitrobenzene,giving the corresponding product in mod-erate yields (entries 23-25),and higher selectivity of di-alkyl aniline than mono-alkyl aniline.This could be attributed to the steric effect.However,the benzyl alcohols do not react due toTable 3Alkylation of nitrobenzene with different aliphatic alcohols323reaction pressure,5MPa;LHSV ,7.5h -1323reaction pressure,5MPa;LHSV ,7.5h -13Possible mechanism of one-pot synthesis of N -ethyl anilne and N ,N -diethyl aniline from nitrobenzene2145Acta Phys.⁃Chim.Sin.2012V ol.28the poor solubility.This indicates that hydrogen produced by aliphatic alcohol/water reforming is feasible.The results for the alkylation of nitroarenes with ethanol are shown in Table4.With1%Pt3Sn/Al2O3as a catalyst,the ethyl-ation of different nitroarenes containing electron-donating sub-stituents in ethanol goes smoothly and produces the corre-sponding N-ethyl and N,N-diethyl anilines,more than90% yields are obtained(entries26-32).This could be explained by the electronic effect and synergetic effect of Sn,which would decrease the electronic density of N atom.The electron abun-dant Pt atom would absorb the electron deficienct N atom, strengthening the adsorption of reactant,which is benefit for the hydrogenation reaction.However,the substituents at differ-ent positions on the nitroarenes affect the reaction yield slight-ly(entries27-29).The yield of para position is better than that of ortho position,which could be explained by the stereo-scopic effect.The reaction tolerates the presence of substitu-ents(entries33-37).Meanwhile,the yields of products de-crease slightly due to the electron-withdraw group,which agrees with the result pointed out by Kawahara et al.14Interest-ingly,two halogenated atoms presented in the substrate exhibit high selectivity of N-alkyl aniline with the yield larger than 94%(entries38,39).3.5Possible mechanism of in situ hydrogenationN-alkylationAlthough the mechanism for the present reaction is not com-pletely clear yet,a possible mechanism is shown in Fig.3.First-ly,the nitro group is reduced to amine by H*produced by etha-nol/water reforming.And ethanol dehydrogenates to corre-sponding acetaldehyde.In the next step,two different routes are proposed.On the one hand,the amine reacts with the alco-hol to give N-ethyl aniline.The produced N-ethyl aniline fur-ther undergoes substitution of N-ethylation with ethanol as al-kylation reagent,getting N,N-diethyl aniline.On the other hand,aldol condensation would happen between ethanol and acetaldehyde,and further react with amine to2-methyl-quino-line.Besides,the amine reacts with H*to corresponding cyclo-hexanamine,therefore,rich H*and water conditions should be avoid.4ConclusionsIn summary,the one-pot synthesis of N-alkyl anilines from nitroaromatics was successfully realized with assembled Pt3Sn/ Al2O3as catalyst in a continuous fixed-bed reactor.The conver-sion of nitrobenzene was100%,with the total yield of N-ethyl and N,N-diethyl anilines of98.2%at the optimum reaction con-ditions(503K,7.5h-1,5%water).This novel reaction contains the advantages such as highly hydrogen atom utilization,sim-plification procedure,and environment-friendly.References(1)Xia,J.H.;Zhang,X.;Matyjaszewski,K.ACS Symp.Ser.2000,760,207.doi:10.1021/bk-2000-0760.ch013(2)Clardy,J.M.;Fischbach,A.;Walsh,C.T.Nat.Biotechnol.2006,24,1541.doi:10.1038/nbt1266(3)Oku,T.;Arita,Y.;Tsuneki,H.;Ikariya,T.J.Am.Chem.Soc.2004,126,7368.doi:10.1021/ja048557s(4)Salvatore,R.N.;Nagle,A.S.;Jung,.Chem.2002,67,674.doi:10.1021/jo010643c(5)Basu,B.;Paul,S.;Nanda,A.K.Green Chem.2009,11,1115.doi:10.1039/b905878h(6)Tripathi,R.P.;Verma,S.S.;Pandey,J.;Tiwari,.Chem.2008,10,1093.(7)Byun,E.;Hong,B.;De Castro,K.A.;Lim,M.;Rhee,.Chem.2007,72,9815.doi:10.1021/jo701503q(8)Abdel-Magid,A.F.;Mehrman,.Process Res.Dev.2006,10,971.doi:10.1021/op0601013(9)Shimizu,K.;Shimura,K.;Ohshima,K.;Tamura,M.;Satsuma,A.Green Chem.2011,13,3096.doi:10.1039/c1gc15835j(10)Xu,C.;Xiao,Z.;Zhou,B.;Wang,Y.;Huang,P.Chem.Commun.2010,46,7834.doi:10.1039/c0cc01487g(11)Yamaguchi,K.;He,J.;Oishi,T.;Mizuno,N.Chem.Eur.J.2010,16,7199.(12)Hamid,M.H.S.;Allen,A.C.L.;Lamb,G.W.;Maxwell,A.C.;Maytum,H.C.;Waston,A.J.A.;Williams,J.M.J.J.Am.Chem.Soc.2009,131,1766.doi:10.1021/ja807323a(13)He,L.;Lou,X.;Ni,J.;Liu,Y.;Cao,Y.;He,H.;Fan,K.Chem.Eur.J.2010,16,13965.doi:10.1002/chem.201001848(14)Kawahara,R.;Fujita,K.;Yamaguchi,R.Adv.Synth.Catal.2011,353,1161.doi:10.1002/adsc.201000962(15)Saidi,O.;Blacker,A.J.;Farah,M.M.;Marsden,S.P.;Williams,mun.2010,46,1541.doi:10.1039/b923083a(16)Ruano,J.L.G.;Parra,A.;Aleman,J.;Yuste,F.;Mastranzo,V.mun.2009,404.(17)Zhu,A.;Li,L.;Wang,J.;Zhuo,K.Green Chem.2011,13,1244.doi:10.1039/c0gc00763c(18)Enthaler,S.Catal.Lett.2011,141,55.doi:10.1007/s10562-010-0463-4(19)Peng,Q.;Zhang,Y.;Shi,F.;Deng,mun.2011,47,6476.doi:10.1039/c1cc11057h(20)Pandarus,V.;Ciriminna,R.;Beland,F.;Pagliaro,M.Catal.Sci.Technol.2011,1,1616.doi:10.1039/c1cy00097g(21)Lee,C.C.;Liu,mun.2011,47,6981.doi:10.1039/c1cc11609f(22)Xiang,Y.Z.;Li,X.N.;Lu,C.;Ma,L.;Zhang,Q.AlppliedCatal.A:Gen.2010,375,289.doi:10.1016/j.apcata.2010.01.004(23)Feng,C.;Liu,Y.;Peng,S.;Shuai,Q.;Deng,G.;Li,.Lett.2010,12,4888.doi:10.1021/ol1020527(24)Bea,J.W.;Cho,Y.J.;Lee,S.H.;Yoon,C.M.;Yoon,C.M.mun.2000,1857.(25)Reddy,R.C.;Vijeender,K.,Bhusan,B.P.;Madhavi,P.P.;Chandrasekhar,S.Tetrahedron Lett.2007,48,2765.doi:2146YANG Fang et al.:One-Pot Synthesis of N-alkyl Anilines from Nitroaromatics by Assembled Pt3Sn/Al2O3 No.910.1016/j.tetlet.2007.02.050(26)Ely,T.O.;Pan,C.;Amiens,C.;Chaudret,B.;Dassenoy,F.;Lecante,P.;Casanove,M.J.;Mosset,A.;Respaud,M.;Broto,J.M.J.Phys.Chem.B2000,104,695.doi:10.1021/jp9924427 (27)Mao,J.Z.;Yan,X.H.;Gu,H.Z.;Jiang,L.C.Chin.J.Catal.2009,30,182.[毛建忠,严新焕,顾辉子,江玲超.催化学报,2009,30,182.]doi:10.1016/S1872-2067(08)60095-9(28)Zhou,L.;Gu,H.Z.;Yan,X.H.Catal.Lett.2009,132,16.doi:10.1007/s10562-009-0036-6(29)Riguetto,B.A.;Damyanova,S.;Goluliev,G.;Marques,C.M.P.;Petrov,L.;Bueno,J.M.C.J.Phys.Chem.B2004,108,5349.(30)Navarro,R.M.;Álvarez-Galván,M.C.;Sánchez-Sánchez,M.C.;Rosa,F.;Fierro,J.L.G.Appl.Catal.B:Environ.2005,55,229.doi:10.1016/j.apcatb.2004.09.002(31)Merlen,E.;Beccat,P.;Bertolini,J.C.;Delichère,P.;Zanier,N.;Didillon,B.J.Catal.1996,159,178.doi:10.1006/jcat.1996.0077(32)Dupont,C.;Delbecq,F.;Loffreda,D.;Jugnet,Y.J.Catal.2011,278,239.doi:10.1016/j.jcat.2010.12.012(33)Narayanan,S.;Deshpande,K.Appl.Catal.A1996,135,125.doi:10.1016/0926-860X(95)00220-02147。