Process Optimization Of MOCA Synthesis Over Solid Acids And Studies On The Mechanism Of Catalyst Deactivation

In this work, the catalytic synthesis of 3,3’-dichloro-4,4’-diamino diphenylmethane (MOCA) from o-chloroaniline and formaldehyde over solid acids in a FCF-1L autoclave has been studied.Firstly, the catalytic performances of four silica-alumina zeolites Hβ (Si/Al=40),Hp (Si/Al=100),HZSM-5 (Si/Al=300) and HY (Si/Al=7.5) in MOCA synthesis by condensation between o-chloroaniline and formaldehyde were comparetively investigated, indicating that HY(7.5) zeolite was an effective and suitable catalyst for this synthetic reaction. Then, under various reaction conditions and with using HY (7.5) zeolite as catalyst, the selectivity of product MOCA and the conversion of reactant HCHO were systematically evaluated and the process conditions optimized. Furthermore, based on careful analysis of experimental data, the micro reaction mechanism of MOCA synthesis by and the mechanism of intermediates as well as by-products formation in condensation of o-chloroaniline and formaldehyde over acidic zeolites were described and discussed. Finally, by experimental tests of reusing the catalyst and regenerating the deactivated catalyst in combination with the characterization of the fresh,deactivated and regenerated catalysts, the cause and mechanism of catalyst deactivation were explored.The experimental data of reaction showed that under the conditions of w(cataly st):w(formaldehyde)= 1:1, n(o-chloroaniline):n(formaldehyde)=8:1,160 ℃ temperature and 4h reaction time, the reaction of o-chloroaniline and formaldehyde condensation over HY(7.5) zeolite gave MOCA selectivity of 80.30% and HCHO conversion of 81.7%, respectively. The optimized process of two-step reaction running at 140 ℃ for 0.5h and then 160℃ for 0.5h gave the HCHO conversion of 85.43% and enhanced the MOCA selectivity up to78.99%.By combining the apparent data of experiments with the understanding of organic reaction chemistry, the micro reaction mechanism of MOCA synthesized from o-chloroaniline and formaldehyde over zeolite solid acid was suggested to go through three main steps:firstly, the formation of the reaction intermediates N,N’-bis(2-chlorophenyl)methanediamine and N-(4-amino-3-chlorobenzyl)-2-chloroaniline by condensation of o-chloroaniline and formaldehyde; then, the rearrangement of N,N’-bis(2-chlorophenyl)methanediamine into N-(4-amino-3-chlorobenzyl)-2-chloroaniline and N-(2-amino-3-chlorobenzyl)-2-chloroaniline; finally, the rearrangement of N-(4-amino-3-chlorobenzyl)-2-chloroaniline over Bronsted acid into MOCA.The experimental tests of HY(7.5) zeolite deactivation and regeneration showed that after repeatedly used for 4 times, the selectivity of main product MOCA decreased from 78.99% to 24.15% while the conversion of reactant HCHO still remained in higher level, probably because the formation of key intermediate by condensation of o-chloroaniline and formaldehyde could be carried out without the catalyst. In order to regenerate the deactivated catalyst, the spent catalyst after reusing 4 times was treated by two methods of calcination and benzene washing., The calcination recovered activity of the catalyst significantly to give MOCA selectivity of 79.94% and HCHO conversion of 92.19%, respectively.The characterization of surface and structure of the catalyst showed that the pore volume and surface area of the spent HY (7.5) regenerated by calcination were decreased, but the crystal structure and the acidity were almost unchanged, indicating that the HY (7.5) zeolite had good thermal stability, and therefore could be reused after calcination to extend service time and to reduce the production cost.