Study On Noble Metal Catalysts For Oxidation Of PMIDA To PMG

Glyphosate is widely used herbicide due to its high efficiency,broad-spectrum and low residue characteristics.The production of by the catalytic oxidation of n-phosphonomethylglycine（PMIDA）is one of the key processes in the production of glyphosate.However,when glyphosate is produced,equimolar formaldehyde will be yielded as a byproduct.The presence of formaldehyde will not only aggravate the formation of other by-products,leading to lower the yield of glyphosate but also accompanying high-concentration formaldehyde wastewater incurred the increase of production costs.Pt/C catalyst can be used catalytical oxidizing PMIDA to gain glyphosate,and at the same time,completely decomposit formaldehyde to CO₂and H₂O in situ,which can not only improve the quality of glyphosate but also effectively reduce the generation of aldehyde-containing wastewater.However,the existing Pt/C catalysts cannot be used in industrial production due to insufficient activity and poor stability.In this paper,preparation of active and stable Pt/C catalyst is selected as the research object.Based on optimizing the synthesis method and preparation process,a Pt/C catalyst with high activity and high stability is developed through subsequent thermal treatment.The main results are as follows:Based on optimizing the reaction process of the catalytic oxidation of PMIDA to glyphosate,the effects of preparation processes such as the type of reductants,reduction temperature,reduction time,and reduction pH on the yield of glyphosate and formaldehyde conversion of the Pt/C catalyst were studied.Influence,the preferred Pt/C catalyst preparation process is:first use NaOH solution to modify the carrier,and then use formaldehyde as a reductant to reduce the Pt in situ on the NaOH modified carbon carrier.The optimal conditions for in-situ reduction of Pt loading are:pH 10,reduction temperature 130℃,and reduction time 3.5 h.The as prepared Pt/C catalyst has excellent catalytic activity,the conversion of PMIDA is greater than 99%,the yield of glyphosate is greater than 97%,and the decoposition of formaldehyde is greater than 95%.The characterization results show that NaOH modification can effectively increase the specific surface area of the carrier,facilitate the loading and dispersion of Pt active components,and facilitate the mass transfer during the reaction.Besides,NaOH modification can increase the content of hydroxyl groups on the catalyst surface.The hydroxyl groups can not only disperse precious metals and stabilize precious metal nanoparticles,but also facilitate the catalytic oxidation of formaldehyde.The effect of thermal treatment on the stability of the Pt/C catalyst was studied in detail.The stability test results show that thermal treatment can effectively improve the stability of the Pt/C catalyst in the catalytic oxidation of formaldehyde removal while retaining a high yield of glyphosate.However,the improvement effect depends on the thermal treatment temperature and atmosphere.The Pt/C catalyst heat-treated with H₂/Ar mixed gas at 600℃has excellent stability.In 12 consecutive reactions,the high glyphosate yield（>99%）and formaldehyde conversion（>88%）can be retained.The characterization results show that high-temperature thermal treatment can strengthen the interaction between Pt and activated carbon support.The activated carbon support transfers electrons to the metal Pt.The interaction between Pt and the support inhibits the loss of Pt,thereby,improve the stability of the catalyst in the catalytic oxidation of formaldehyde.