Study On The Synthesis Of PMG By Pt/C Catalytic Oxidation Of PMIDA

Glyphosate(PMG)is an efficient and low toxic organophosphorus herbicide.Recently it has become the most productive and widely applied pesticide in the world because it is biological degradable and environmental friendly.Among all the methods the iminodiacetic acid route(IDA)has gained more and more attention with the advantage of simple,easy and cost-effective,of which the catalytic oxidation of N-phosphonomethyliminodiacetic acid(PMIDA)into PMG is the critical step.The oxidation reaction forms equimolar side-product formaldehyde.When recycling mother liquid in the process of continuous production,the accumulation of formaldehyde will increase the side reaction and decrease the yield of PMG.The Pt/C catalyst used here can catalyze the formaldehyde oxidation reaction while assist the oxidation of PMIDA,which contributes to less wastewater,higher recycle efficiency of mother liquid and higher yield of PMG.The effects of various activation and modification factors on Pt/C catalyst activity and formaldehyde removal rate were investigated through single factor experiments.Combined physical characterization such as TEM,BET,XRD together with reaction evaluation,the optimum activation and modification conditions were established:temperature 1050?,pressure 0.8 MPa,time 2.5 h and chemicals ratio m(NaHCO3)/m(NH4HCO3)=1:1.The Pt/C catalyst prepared under above conditions have good catalytic performance with 98.5%PMG yield,0.0145 mol/L formaldehyde concentration in the reaction solution and a formaldehyde removal rate of more than 91.0%at the end of the reaction.The effects of different factors on the catalytic oxidation of PMIDA by Pt/C catalyst prepared under above conditions were investigated,and the optimum reaction conditions were established:temperature 90?,pressure 0.5 MPa,stirring speed 500 rpm,m(Pt/C)/m(PMIDA)=12.5%.Under such condition the yield of PMG can be as high as 98.7%while the formaldehyde concentration in the reaction solution was only 0.008 mol/L.The removal ratio of formaldehyde can reach more than 95.0%.Besides,the research on mother liquid showed that after 5 recycling runs of mother liquid,PMG yield did not change significantly,formaldehyde removal ratio remained at a high level and Pt loss rate remained stable at 0.014%under the optimized condition.After eliminating the influence of internal and external diffusion,catalytic oxidization kinetic experiments were conducted at 75-90? under the pressure of 0.6 MPa,and the stirring speed was 750 rpm,oxygen flow rate was kept at 140 mL/min,the mass of Pt/C catalyst was 0.56 g.The kinetic model concerning the adsorption of PMIDA and PMG was proposed and the parameters were numerically obtained by non-linear data fitting of the experimental data.The activation energy for oxidation of PMIDA is 28.41 kJ/mol and the adsorption heats of PMIDA and PMG are 18.09 kJ/mol and 18.57 kJ/mol,respectively.