A theoretical study on the phosgenation of methylene diphenyl diamine (MDA)

Searching for the Achilles' Heel of Urethane Linkage-An Energetic Perspective

A sudden increase in polyurethane (PU) production necessitates viable recycling methods for the waste generated. PU is one of the most important plastic materials with a wide range of applications; however, the stability of the urethane linkage is a major issue in chemical recycling. In this work, termination reactions of a model urethane molecule, namely methyl N-phenyl carbamate (MPCate), are investigated using G3MP2B3 composite quantum chemical method. Our main goal was to gain insights into the energetic profile of urethane bond termination and find an applicable chemical recycling method. Hydrogenation, hydrolysis, methanolysis, peroxidation, glycolysis, ammonolysis, reduction with methylamine and termination by dimethyl phosphite were explored in both gas and condensed phases. Out of these chemicals, degradation by H2, H2O2 and CH3NH2 revealed promising results with lower activation barriers and exergonic pathways, especially in water solvation. Implementing these effective PU recycling methods can also have significant economic benefits since the obtained products from the reactions are industrially relevant substances. For example, aniline and dimethyl carbonate could be reusable in polymer technologies serving as potential methods for circular economy. As further potential transformations, several ionizations of MPCate were also examined including electron capture and detachment, protonation/deprotonation and reaction with OH-. Alkaline digestion against the model urethane MPCate was found to be promising due to the relatively low activation energy. In an ideal case, the transformation of the urethane bond could be an enzymatic process; therefore, potential enzymes, such as lipoxygenase, were also considered for the catalysis of peroxidation, and lipases for methanolysis.

How To Get Isocyanate?

Isocyanate, a pivotal chemical intermediate to synthesize polyurethane with widespread applications in household appliances, automobiles, and construction, is predominantly produced via the phosgene process, which currently holds a paramount status in industrial isocyanate production. Nonetheless, concerns arise from the toxicity of phosgene and the corrosiveness of hydrogen chloride, posing safety hazards. The synthesis of isocyanate using nonphosgene methods represents a promising avenue for future development. This article primarily focuses on the nonphosgene approach, which involves the formation of carbamate through the reaction of nitro-amino compounds with carbon monoxide, dimethyl carbonate, and urea, among other reagents, subsequently leading to the thermal decomposition of carbamate to get isocyanate. This paper emphasizes the progress in catalyst development during the carbamate decomposition process. Single-component metal catalysts, particularly zinc, exhibit advantages such as high activity, cost-effectiveness, and compatibility with a wide range of substrates. Composite catalysts enhance isocyanate yield by introducing a second component to adjust the active metal composition. The central research direction aims to optimize catalyst adaptation to reaction conditions, including temperature, pressure, time, and solvent, to achieve high raw material conversion and product yield.

A Theoretical Study on the Phosgenation of 2,4-Toluenediamine (2,4-TDA)

Industrially relevant phosgenation mechanisms of 2,4-toluenediamine (2,4-TDA) were investigated using G3MP2B3 model chemistry. Six reaction pathways had been explored, which resulted in the formation of toluene diisocyanate (2,4-TDI) including different scenarios of the ‘phosgenations first’ and ‘consecutive phosgenations’ mechanisms in both gas and condensed phases. Two possible ‘phosgenations first’ mechanisms show superior to the others in terms of energy, regardless of which phases are considered. Due to the o-dichlorobenzene (ODCB) solvation, the reaction barriers are dramatically reduced compared to the gas-phase reaction mechanism and the solvent effect can be described by linear relationship. Standard enthalpy of formation value was also recommended for 2,4-TDA (59.3 kJ/mol) and 2,4-TDI (−94.1 kJ/mol), as well as for the gas-phase intermediates (IM).

Optimized Operating Conditions for a Biological Treatment Process of Industrial Residual Process Brine Using a Halophilic Mixed Culture

Residual process brine is a sustainable raw material for chlor-alkali electrolysis processes. This study investigates the influence of critical process parameters on the performance of a continuous treatment process for residual process brine using halophilic microorganisms. The goal of the bioprocess is an efficient degradation of the organic impurities formate, aniline, phenol, and 4,4′-methylenedianline from this residual stream. It was shown that formate could be degraded with high efficiencies (89–98%) during the treatment process. It was observed that formate degradation was influenced by the co-substrate glycerol. The lowest residual formate concentrations were achieved with specific glycerol uptake rates of 8.0–16.0 × 10−3 g L−1 h−1 OD600−1. Moreover, a triple-nutrient limitation for glycerol, ammonium, and phosphate was successfully applied for continuous cultivations. Furthermore, it was shown that all aromatic impurities were degraded with an efficiency of 100%. Ultimately, this study proposed optimized operating conditions, allowing the efficient degradation of organics in the residual process brine under various process conditions. Future optimization steps will require a strategy to prevent the accumulation of potential intermediate degradation products formed at high aniline feed concentrations and increase the liquid dilution rates of the system to achieve a higher throughput of brines.

Urethane Formation with an Excess of Isocyanate or Alcohol: Experimental and Ab Initio Study

A kinetic and mechanistic investigation of the alcoholysis of phenyl isocyanate using 1-propanol as the alcohol was undertaken. A molecular mechanism of urethane formation in both alcohol and isocyanate excess is explored using a combination of an accurate fourth generation Gaussian thermochemistry (G4MP2) with the Solvent Model Density (SMD) implicit solvent model. These mechanisms were analyzed from an energetic point of view. According to the newly proposed two-step mechanism for isocyanate excess, allophanate is an intermediate towards urethane formation via six-centered transition state (TS) with a reaction barrier of 62.6 kJ/mol in the THF model. In the next step, synchronous 1,3-H shift between the nitrogens of allophanate and the cleavage of the C–N bond resulted in the release of the isocyanate and the formation of a urethane bond via a low-lying TS with 49.0 kJ/mol energy relative to the reactants. Arrhenius activation energies of the stoichiometric, alcohol excess and the isocyanate excess reactions were experimentally determined by means of HPLC technique. The activation energies for both the alcohol (measured in our recent work) and the isocyanate excess reactions were lower compared to that of the stoichiometric ratio, in agreement with the theoretical calculations.

An Ab Initio Investigation of the 4,4'-Methlylene Diphenyl Diamine (4,4'-MDA) Formation from the Reaction of Aniline with Formaldehyde

The most commonly applied industrial synthesis of 4,4'-methylene diphenyl diamine (4,4'-MDA), an important polyurethane intermediate, is the reaction of aniline and formaldehyde. Molecular understanding of the 4,4'-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 4,4'-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 4,4'-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 pK_a calculation.