The Development Of A New Catalyst For The Oxidation Of N-Phosphonomethyliminodiacetic Acid

Along with the extensive cultivation of genetically modified crops,the demand for the glyphosate(PMG)which is an organophosphorus herbicide is increasing gradually.Currently,the catalytic oxidation of N-phosphonomethyl-aminodiacetic acid(PMIDA)is the main industrial pro-duction process of PMG.Since it's a reaction in series,the activity and selectivity of the catalyst effect significantly on the yield of glyphosate which is the intermediate product.And among all the oxidation catalysts,ammonia or nitrogen supported activated carbon catalyst calcined at high tem-perature is the best performance catalyst with low cost.It is difficult to further improve the catalyst performance because both the catalytic active components and the reaction mechanism of the cata-lyst have not been understood clearly.In this paper,the surface species of the traditional ammo-nia-supported catalyst will be explored to find out the active component of catalysts.Then the new catalyst with more active species is developed by direct loaded with the active species under dif-ferent calcination conditions,which will prepare the better performance catalysts.For the purpose of applying this newly developed catalyst to industrial practice,it is necessary to establish kinetic equations of synthesizing of PMG from PMIDA over new catalyst by experimental investigation.The detailed tasks of study are as follows:1.Exploring on the Active C.omponents and Structures of Catalysts:The content of Nitrogen and nitrogen-containing structures on surface of activated carbon(The specific surface area is 1437.137 m2/g)which is supported by ammonia at a temperature of 250?850?,is detected by XPS.The Performance of the catalyst prepared at different temperatures is also tested by oxidation reaction of PMIDA.The catalyst surface contains four structures of nitrogen-containing material including pyridine nitrogen type,pyrrole nitrogen type,quaternary nitrogen type and oxidized pyridine nitrogen type.The average content of nitrogen of activated carbon loaded with ammonia at the temperature of 250-500?is 0.4704%,and 0.1451%,0.1063%,0.1818%,0.0373%is for each nitrogen-containing structures respectively,which is consistent with the original activated carbon.The results show that the nitrogen element is not supported on the activated carbon surface.With the increase of the load temperature,the nitrogen content of the catalyst surface increases greatly and the nitrogen-containing structures gradually changed in orders of pyridine nitrogen type,pyrrole nitrogen type,quaternary nitrogen type and oxidized pyridine nitrogen type.The re-sults of catalytic experiments show that the catalytic activity of ammonia-supported activated car-bon below 500?doesn't change with the non-ammonia-supported activated carbon.The perfor-mance of the catalysts is improved rapidly once they are prepared above 750?.When the loading temperature is 800?,the total nitrogen content is 4.0436%and the pyridine nitrogen type content is 1.7088%according to the result of XPS,the catalytic performance is the best that the maximum yield of PMG is 85.6%(The conversion of PMIDA is 91.6%)after 25 min.The total nitrogen con-tent on the surface of catalyst is 4.5020%when loading temperature is 850?,but the content of pyridine nitrogen is reduced to 1.4402%and catalytic activity and selectivity decreased about 2.5%at the same time.The above results fully explain that the total nitrogen content is not the decisive factor for the catalytic oxidation reaction of PMIDA to PMG.The main active component of the catalyst is pyridine nitrogen type material.And the catalytic reaction mechanism is deduced based on the literature,too.2.Reach on the performance and the development of the new catalyst:Based on the above re-sults,the active nitrogen-containing pyridine is directly supported on activated carbon(The specific surface area is 388.835 m2/g)by impregnation at different loading temperatures of 500?850?.The transformation of the nitrogen-containing material and the catalytic performance of the catalyst are both investigated.The experimental results show that pyridine can be well supported on acti-vated carbon above700?,and the catalyst has good catalytic performance.Especially,the catalyst has the best catalytic performance when the loading temperature is 750?that the maximum yield of PMG is 86.0%(The conversion of PMIDA is 92.8%)after 25 min and the content of pyridine nitrogen type of activated carbon is 1.7450%.Compared with nitrogen-loaded activated carbon catalyst,the pyridine-loaded temperature is obviously reduced and the content of pyridine nitrogen type on the surface of catalyst increased obviously at the same loading temperature so that in the case of a catalyst with a specific surface area of 2/3,the maximum yield is still increased.The con-tinuous operation of 100 h shows that the catalyst has good catalytic stability.3.Reach for kinetics of chemical reaction:A series of experiment was carried out in a self-made stainless steel batch reactor with a new type of pyridine loaded activated carbon as cata-lyst and oxygen as oxidant with mass ratio of PMIDA and catalyst from 0.25 to 1.0,reaction tem-perature from 313.15 K to 343.15 K and reaction temperature pressure from 0.1 MPa to 0.6 MPa to investigate the change law of the concentrations of PMIDA and PMG with time and establish the kinetic equation of the reaction which shows the catalytic oxidation reaction of PMIDA to PMG is fitted to a 1-1 cascade.And the power law indexes of dissolved oxygen concentration in the macro kinetic equations are 0.7 and 0.5.The macro kinetic model is verified by experimental results and the mean relative error of PMIDA and PMG concentrations is 7.029%and 3.532%respectively which proves the reliability of the model.