Process Optimization Of MDT Catalytic Synthesis In Fixed Bed Reactor And Studies On The Mechanism Of Catalyst Deactivation

In this work, the green production technology for catalytic synthesis of 3,3’-dimethyl-4,4’-diaminodiphenylmethane (MDT) from o-toluidine and formaldehyde over zeolites in fixed bed reactor has been developed.The catalytic activities of zeolites TS-1, Hβ-40 and Hβ-100 in synthesis of MDT have been compared, and the results indicated that Bronsted acid on zeolite played a dominant role in catalyzing synthesis of MDT, while Lewis acid showed insignificant effect on this reaction. Also within certain range of acid strength, the higher acid content, the better catalytic activity.In the case of Hβ-40 as a catalyst, and under the conditions of LHSV=5h-1, nitrogen flow rate=10mL·min-1, n(o-toluidine):n(formaldehyde)=5:1,170 ℃ and 0.50MPa, this synthetic reaction gave final product MDT selectivity of 65.3%, intermediate product ITD selectivity of 8.5%, and reactant formaldehyde conversion of 96.7%, respectively. After the synthetic reaction was continually proceeded for 84h under the optimal conditions as above, the selectivities of MDT and ITD changed from 65.3% and 8.5% to 52.2% and 19.0%, respectively. And per gram of the catalyst could produce 170 grams of MDT product.Based on the identification of chemical structures of intermediate ITD and by-product BITD, the mechanism of MDT synthesis from formaldehyde and o-toluidine over zeolite catalysts has been established. Which mainly involved in the following reaction steps:(1) the formation of N,N’-Di-o-tolyl-methanediamine and 2-methyl-4-(o-tolylamino-methyl)-phenylamine by condensation of formaldehyde with o-toluidine; (2) the structural rearrangement of N,N’-Di-o-tolyl-methanediamine to 2-methyl-4-(o-tolylamino-methyl)-phenylamine; (3) the rearrangement of 2-methyl-4-(o-tolylamino-methyl)-phenylamine into MDT under catalysis of Bronsted acid. According to the mechanism of MDT formation, the mechanisms of various by-products formation were also explored.The fresh, deactivated and regenerated catalysts were characterized by various analytic methods, such as BET, Py-IR, NH3-TPD, TG-DTA and XRD. The results showed that surface area and pore volume of the deactivated catalysts were much lower than those of the fresh catalysts, but the crystal structure of deactivated catalysts was not destroyed significantly. Therefore, it was suggested that the deactivation of catalyst was mainly caused by plugging of pore channel due to the deposition of by-products on catalysts.After burning most of the sediments by calcination at 550℃ for 3h, the catalytic activity of spent Hβ-40 zeolites could be approximately recovered to the original level. However, the lifetime of regenerated catalysts was shortened, probably because a small amout of residual deposits on catalyst might hinder diffusion of reactant and products so as to accelerate the deactivation.