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An Ab Initio Investigation of the 4, 4′-Methlylene Diphenyl Diamine (4

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An Ab Initio Investigation of the 4, 4′-Methlylene Diphenyl Diamine (4 polymers Article An Ab Initio Investigation of the 4,40-Methlylene Diphenyl Diamine (4,40-MDA) Formation from the Reaction of Aniline with Formaldehyde R. Zsanett Boros 1,2,László Farkas 1,Károly Nehéz 3,Béla Viskolcz 2,* and Milán Sz˝ori 2,* 1 Wanhua-BorsodChem Zrt, Bolyai tér 1., H-3700 Kazincbarcika, Hungary; [email protected] (R.Z.B.); [email protected] (L.F.) 2 Institute of Chemistry, University of Miskolc, Miskolc-Egyetemváros A/2, H-3515 Miskolc, Hungary 3 Department of Information Engineering, University of Miskolc, Miskolc-Egyetemváros Informatics Building, H-3515 Miskolc, Hungary; [email protected] * Correspondence: [email protected] (B.V.); [email protected] (M.S.); Tel.: +36-46-565-373 (B.V.); +36-46-565-111/1337 (M.S.) Received: 30 January 2019; Accepted: 27 February 2019; Published: 1 March 2019 Abstract: The most commonly applied industrial synthesis of 4,40-methylene diphenyl diamine (4,40-MDA), an important polyurethane intermediate, is the reaction of aniline and formaldehyde. Molecular understanding of the 4,40-MDA formation can provide strategy to prevent from side reactions. In this work, a molecular mechanism consisted of eight consecutive, elementary reaction steps from anilines and formaldehyde to the formation of 4,40-MDA in acidic media is proposed using accurate G3MP2B3 composite quantum chemical method. Then G3MP2B3-SMD results in aqueous and aniline solutions were compared to the gas phase mechanism. Based on the gas phase calculations standard enthalpy of formation, entropy and heat capacity values were evaluated using G3MP2B3 results for intermediates The proposed mechanism was critically evaluated and important side reactions are considered: the competition of formation of protonated p-aminobenzylaniline (PABAH+), protonated aminal (AMH+) and o-aminobenzylaniline (OABAH+). Competing reactions of the 4,40-MDA formation is also thermodynamically analyzed such as the formation of 2,4-MDAH+, 3,4-MDAH+. AMH+ can be formed through loose transition state, but it becomes kinetic dead-end, while formation of significant amount of 2,4-MDA is plausible through low-lying transition state. The acid strength of the key intermediates such as N-methylenebenzeneanilium, PABAH+, + 4-methylidenecyclohexa-2,5-diene-1-iminium, and AMH was estimated by relative pKa calculation. Keywords: bis(4-aminophenyl)methane; MDA; PABA; aniline; water; reaction mechanism; ab initio; G3MP2B3; transition state; pKa; standard enthalpy of formation 1. Introduction Polyurethane industry requires large amount of methylene diphenyl diisocyanate (MDI) as raw material and global market of MDI reached 6 Mt in 2016 [1]. The MDI production is mostly based on methylene diphenyl diamine (methylenedianiline, MDA). MDA can be also applied as ingredient of epoxy resins, intermediate for pigments, organic dyes, coatings, plastic fibers, and insulation materials [2] making it an important intermediate for chemical industry. The most commonly applied industrial MDA synthesis is the reaction of aniline with formalin in the presence of hydrochloric acid under mild (60–110 ◦C) reaction conditions [3]. To avoid the use of corrosive hydrochloric acid and the formation of large amount of salt solution as waste, several attempts [4–11] had been made to replace the current technology with catalytic process using solid acids, zeolites, delaminated materials, ionic liquids, or ion exchange resins as catalysts. However, neither of Polymers 2019, 11, 398; doi:10.3390/polym11030398 www.mdpi.com/journal/polymers Polymers 2019, 11, 398 2 of 14 Polymers 2019, 11 FOR PEER REVIEW 2 theneither catalytic of the MDA catalytic productions MDA haveproductions gone beyond have the gone laboratory beyond stage. the Although,laboratory proposedstage. Although, reaction mechanismproposed reaction for current mechanism MDA synthesis for current is presented MDA synthesis in Ref. [12 is] (seepresented left side in of Ref. Figure [12]1), (see it has left not side been of clarifiedFigure 1), entirely. it has not According been clarified to this mechanism,entirely. According aniline (A) to this is reacted mechanism, with formaldehyde aniline (A) is (F) reacted producing with N-hydroxymethylformaldehyde (F) anilineproducing as the N-hydroxymethyl initial reaction step. aniline In acidic as the medium initial reaction this product step. loses In acidic water medium rapidly tothis form product N-methylidene loses water anilinium rapidly to which form reacts N-methylidene with aniline anilinium to form N-(p-aminobenzyl)anilinewhich reacts with aniline (PABA).to form PABAN-(p-aminobenzyl)aniline is then decomposed (PABA). to 4-aminobenzylium PABA is then anddecomposed aniline. In to the4-aminobenzylium last step of this rearrangementand aniline. In 0 0 4,4the -methylenelast step of diphenylthis rearrangem diamineent (4,4 4,4'-methylene-MDA) is formed diphenyl as final diamin product.e (4,4’-MDA) Beside missing is formed elementary as final steps, this early mechanism also cannot clarify the detected side products of MDA. FigureFigure 1.1. OverallOverall reactionreaction mechanismmechanism forfor methylenemethylene diphenyldiphenyl diaminediamine (methylenedianiline,(methylenedianiline, MDA) synthesissynthesis accordingaccording Kirk–OthmerKirk–Othmer Encyclopedia of Chemical Technology [12] [12] as well as Wang [[13]13] comparedcompared withwith ourour proposedproposed mechanismmechanism basedbased onon G3MP2B3G3MP2B3 computation.computation. MoreMore recently Wang et al. al. [13] [13] suggested suggested an an alternative alternative mechanism mechanism in inwhich which the the reaction reaction of N- of N-hydroxymethylaminehydroxymethylamine and and aniline aniline (A) (A) gives gives N,N’-diphenylmethylenediamine N,N’-diphenylmethylenediamine first first (so-called aminal notednoted asas AMAM inin FigureFigure1 )1) before before the the formation formation ofof PABA.PABA. Due Due to to the the protonation protonation ofof one one of of the the + secondarysecondary amineamine groupgroup inin AMAM (AMH(AMH+), the C-N bondbond isis activatedactivated forfor thethe rearrangementrearrangement makingmaking PABA.PABA. AccordingAccording to to Wang, Wang, same same activation activation is supposed is supposed to happen to happen to the to other the other secondary secondary amine amine group + togroup form to MDA: form PABAMDA: is PABA protonated is protonated (PABAH (PABAH) which+ then) which initiates then anilineinitiates rearrangement. aniline rearrangement. Wang et al.Wang found et al. some found evidences some evidences to support to thesupport latter the mechanism latter mechanism by isolation by isolation and identification and identification of aminal of usingaminal the using combination the combination of isotope of isotope labeling labeling and HPLC-MS and HPLC-MS [13]. Albeit[13]. Albeit other other intermediates intermediates has nothas beennot been characterized characterized at all. at all. In theIn the same same study study by-products by-products including including oligomers oligomers of of MDA MDA (e.g., (e.g. 3-3- andand 4-ring4-ring MDA)MDA) were were also also suggested. suggested. As As a continuationa continuation of thisof this work work stabilities, stabilities, potential potential protonation protonation sites andsites structural and structural characterization characterization of these of these MDA MDA oligomers oligomers were were determined determined using using ion mobility-mass ion mobility- spectrometrymass spectrometry (IM-MS) (IM-MS) and tandem and tandem mass mass spectrometry spectrometry (MS/MS) (MS/MS) techniques techniques [14]. [14]. This This resulted resulted in twoin two partially partially overlapping, overlapping, but but still still gappy gappy mechanism mechanism for MDAfor MDA production production as seen as seen from from the left the side left andside theand center the center of Figure of Figure1. Furthermore, 1. Furthermore, initial init stepial step of both of both mechanisms mechanisms requires requires close close contact contact of of aniline and aqueous formaldehyde. Although aniline and water have similar density [15], but they are only slightly soluble in each other and aniline has large viscosity (3.770 cP) due to strong Polymers 2019, 11, 398 3 of 14 aniline and aqueous formaldehyde. Although aniline and water have similar density [15], but they are only slightly soluble in each other and aniline has large viscosity (3.770 cP) due to strong hydrogen bond between amine groups, [16] therefore, solvent effect can be crucial in the MDA production for these solvents. Although, the MDA synthesis is essential to MDI production, the thermochemical properties of the participated species are also poorly characterized [17]. To the best of our knowledge, only the standard reaction enthalpy of formation for MDA, PABA, and AM is estimated using Benson group additivity method [18]. Only the heat of formation of 4,40-MDA had been reported very recently using G3MP2B3 quantum chemistry protocol and group additivity increments for OCN and NHCOCl groups has been recommended [19] based on protocols mentioned in Ref. [20,21]. The aim of this study is to establish an ab initio-based molecular mechanism consisting of consecutive elementary reaction steps from anilines and formaldehyde to the formation of 4,40-MDA. These elementary reactions can include short living species with low steady state concentrations which makes them inaccessible for most of the current detection techniques. Furthermore, we
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