Study On Green Process For The Liquid Phase Oxidation Of Methyl Aromatics Hydrocarbon To Aldehydes

Aromatic aldehydes are widely used as important intermediates,synthetic flavoring agent and perfume additives,in the fields of medicine,pesticides and dyes manufacture,etc.Methyl aromatic hydrocarbons which are usually obtained from petroleum refining or tar separation,are cheap,and easily available for the chemical processes.Therefore,conversion of methyl aromatic hydrocarbons to value-added aromatic aldehydes and the relative derivatives has attracted global interests.Nowadays,most of the aromatic aldehydes are produced by the hydrolysis of benzal chloride.However,using Cl2 results in several probems,including complicated process,serious environmental problems and possible chlorine residual which bans the products to be applied as food additives.In China,aromatic aldehydes free from chlorine are in short supply while the demand grows at high speed every year.Aromactic aldehydes of high quality which cost twice as much as the ordinary aromatic aldehydes,are mainly obtained as byproduct of direct liquid oxidation of methyl aromatic hydrocarbons to aromatic acids.In general,liquid aromatic hydrocarbons are oxidized in the bubbling reactor by using high-pressure air as oxidant.However,this process suffers much from limited gas-liquid contact area,long reaction time,higher reaction temperature,low oxygen utilization,exhaust emissions and the like.Therefore,development of new,economical and environmental process for the manufacture of high quality aromatic aldehydes is required.The thesis focused on development of green process for the liquid-phase oxidation of methyl aromatics hydrocarbon to aldehydes.Firstly,the characteristics of oxidation were carefully investigated with Co/Mn/Br(MC)catalytic system being optimized to improve reaction efficiency.With MC catalyst system,adding acetic anhydride in acetic acid solvent could increase aldehyde selectivity and weaken solvent corrosion by consuming the produced water.Secondly,solubility performance of oxygen in toluene and acetic acid was obtained to help finding a superior way for the oxidation of methyl aromatic hydrocarbons.Oxidation with pure oxygen in the jet reactor could significantly shorten the reaction time,improve usage of oxygen and avoid the gas emissions.Besides application of jet reactor,recovery of MC catalyst makes the whole process more"green" for the industrial application.Main research contents are listed as follows.(1)The optimization for liquid-phase oxidation system of tolueneBy using acteic acid as solvent and oxygen as oxidant at the atmospheric and higher pressure,the effects of MC catalyst system and operating conditions on liquid oxidation of toluene were investigated to obtain higher selectivity and yield of benzaldehyde.Under atmospheric pressure,the optimum reaction system are listed as:nco/ntoluene=3.0%,nBr/nco=0.24,nco/nMn=3,n(MnSO4):n(Mn(OAc)2)=1:1,reaction temperature 98?,and reaction time 9 h.Accordingly,48.69%of toluene are converted with the yield of benzaldehyde reaching 21.02%.Under higher pressure,the optimum reaction conditions are:nCo/ntoluene=1.5%,nBr/nCo=1/3?1/4,nco/nMn=2?6,n(MnSO4):n(Mn(OAc)2)=1:1,oxygen pressure 0.3 MPa,reaction temperature 98?,and reaction time 4 h,leading to 33.97%toluene conversion and 16.66%benzaldehyde yield.It was revealed that higher pressure could accelerate the oxidation but decrease the selectivity of benzaldehyde.(2)Improved way for preparation of benzaldehydeDue to the serious corrosion of acetic acid,total or partly replacement of acetic acid with acetic anhydride,acetic ester derivatives,acid ionic liquids for the preparation of aromatic aldehyde,was attempted.It was revealed that adding a certain amount of acetic anhydride in acetic acid could greatly increase both the selectivity and yield of benzaldydehy.A possible explanation is that acetic anhydride could consume water produced in the oxidation and accelerate the transfer of the electron from Br-1 to metal cation,reinforcing the oxidation.Acetic anhydride can also enhance the formation of inertial benzyl bromide which would reduce the activity of Co/Mn/Br catalyst system.However,due to the low activity,C2H4Br4 which is also bromide source could strongly limit the generation of benzyl bromide.Acetic anhydride could intensify the formation of benzyl acetate and benzyl alcohol.The former would be oxidized to benzaldehyde,and the latter could inhibit further oxidation of benzaldehyde.(3)Solubility of oxygen in toluene and acetic acidBy using the dual vessel equilibrium method,the solubility of oxygen in toluene and acetic acid was measured with pressure ranging from 0.1 MPa to 1.0 MPa and temperature varying in the range of 293.1 K and 383.1 K.The results show that the solubility of oxygen in toluene or acetic acid both increases with the temperature rising from 293.1 K to 383.1 K.Moreover,the solubility of oxygen in toluene is higher than that in acetic acid.Based on the experimental data,Henry coefficients for oxygen-toluene and oxygen-acetic acid systems were expressed as functions of temperature.Accordingly,Gibbs energy,partial molar enthalpy,and the partial molar entropy of the solution were estimated.(4)Liquid-phase oxidation of other aromatic hydrocarbons to aldehydesLiquid-phase oxidation of aromatic hydrocarbons,including p-chlorotoluene,p-tert-butyltoluene and p-methylanisole were also investigated by using MC catalyst system.It was found that with electronic ability of the p-groups increasing in the order:-Cl,-H,C(CH3)3 and-OCH3,the oxidation activity of the aromatic hydrocarbons grew up accordingly.Therefore,optimal temperatures for the oxidation of p-chlorotoluene,p-tert-butyltoluene and p-methylanisole were 105?,80? and 75?,and the reaction time was 9 h,9 h and 8 h,respectively.Moreover,a simple and effective way for MC catalyst recovery was developed for oxidation process of aromatic hydrocarbons.(5)Oxidation strengthened in the jet reactorUsing water as liquid medium,transfer performance of our jet reactor was investigated to determine the appropriate structure and operation parameters.It was found that the entrainment rate and gas holdup in the jet reactor increased with the rise of circulation flow rate,while the higher liquid temperature leaded to smaller entrainment rate.Conversion of p-tert-butyltoluene in the jet reactor grew up with the increase of the liquid circulation flow rate.However,further increase of liquid circulation flow rate slightly affects the conversion when the rate reached a certain value.Compared with the cases of other reactors,oxidation time of p-tert-butyltoluene in the jet reactor could be shortened significantly with oxygen consumed close to the theoretical value.And there is no exhaust emission for the jet reactor.