| N-methyl amines have great applications in the production of medicines,pesticides,dyes,surfactants and so on.The current N-methyl industrial process which include chloromethane method,dimethyl sulfate method and formaldehyde-formic acid method have lots of problems such as low atom economy,high toxicity,serious corrosion,severe environmental pollution and so on.With the realization of the importance of environmental protection,the development of environment protecting,economical and efficient N-methyl industrial process is of vital importance.In this article,the N-methyl process catalyzed by the combined Al2O3-mordenite catalyst(Al2O3-HM)with amine and methanol as raw materials was developed.Its byproduct is mainly water,which belongs to the green production technology.Firstly,the Al2O3-HM catalysts were prepared through deposition-precipitation and the catalysts evaluation experiments were carried out based on the reaction of n-butylamine with methanol in fixed bed reactor.Results show that the catalyst shows best performance when the mass fraction of Al2O3 is 40%and it keeps stable within 300 hours.XRD,BET,NH3-TPD and infrared analysis of the catalysts were performed.Results show that the addition of Al2O3 causes changes in the acidic concentration,superficial area and pore structure.The main reason for the catalyst deactivation is the coking material attached to the catalyst surface,which can be burned with oxygen to achieve regeneration.Compared to the supported metal catalysts,the reaction process using Al2O3-HM catalyst of low manufacture cost does not involve hydrogen and the disproportionation is not preferred.Secondly,the N-methyl technology in hydrogen free system were developed using Al2O3-HM catalyst.The optimal conditions were determined using N-methylation of n-butylamine with methanol as model reaction.When reaction temperature is 270?,the reaction pressure is 1.0MPa,the mole ratio of methanol/n-butylamine is 4:1,the space velocity is 0.15h-1,the conversion of n-butylamine is 99.0%and the selectivity of N,N-dimethylbutylamine is 87.7%.Compared to the N-methyl technology in hydrogen system,the N-methyl technology in this article is safe and simple with high target product yields and less disproportionation reaction,which is more appropriate for the production of unsymmetrical tertiary amines.Based on the N-methyl technology in hydrogen free system above,the reactions involving other amines and methanol are also investigated.The yields of N,N-dimethylpiperazine and N-methylmorpholine are over 87%,the yields of N,N-dimethylcyclohexanamine and N,N-diisopropylmethylamine are nearly 70%while the yields of N-methylation of aniline,benzylamine and ethanediamine is lower than 60%.The main reasons for the differences in yields could be the electronic effect and steric effect.Finally,the reaction mechanism of N-methylation in hydrogen free system with Al2O3-HM catalyst was studied using in situ FTIR technology.The characteristic IR adsorption peaks 1812cm-1 and 1921cm-1 of intermediate imine and imine cations,which appeared in N-methylation process with Co-No/y-Al2O3 catalyst,weren't detected during the reaction process,which indicated that the mechanism is the dehydration reaction mechanism rather than the supported metal catalyst's dehydrogenation/hydrogenation mechanism.To better discuss the dehydration reaction process,the adsorption process of methanol on the surface of the Al2O3-HM catalyst was studied using in situ FTIR technology.The characteristic adsorption peaks 2952cm-1 and 2846cm-1 of methoxyl species was detected at the temperature of 250? and the N-methylation mechanism of n-butylamine with methanol on Al2O3-HM catalyst was proposed:the adsorbed methanol was formed with the catalyst to give six-membered cyclic transition state and the transition state dehydrated to form methoxyl species,then the N-methyl products were generated through the six-membered cyclic transition state which was formed by the methoxyl species and the absorbed n-butylamine or N-methylbutylamine. |
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