Tunable and Eco-Friendly Production of Polyurethane Precursors in a Three-Stage Reactor System Employing Distinct Solid Acids

Extensive industrial efforts have been made to replace corrosive HCl with eco-friendly solid acid catalysts for producing 4,4′-methylenedianiline (4,4′-MDA) and oligomeric MDAs, key intermediates in polyurethane production. However, these efforts have achieved limited success due to low product selectivity, poor process tunability, and rapid catalyst deactivation. Here, based on a comprehensive understanding of the reaction and catalyst deactivation mechanisms, we develop a versatile three-stage reaction employing distinct solid acids. First, aminal is selectively converted to para-aminobenzylaniline (PABA) using amorphous silica–alumina (ASA) at a low temperature via thermodynamic control. Second, PABA is transformed into 4,4′-MDA over hierarchical LTL zeolite. The unique one-dimensional, cage-like micropores facilitate the intermolecular rearrangement of PABA into 4,4′-MDA without forming oligomeric MDAs, while the intracrystalline mesopores significantly enhance mass transfer. Third, controlled amounts of oligomeric MDAs are produced over ASA at varied temperatures. Despite the formation of bulky oligomers, the purely mesoporous ASA exhibits no signs of deactivation due to fouling. The rational selection of catalysts and conditions for each reaction step enables the selective and stable production of 4,4′-MDA and oligomeric MDAs in tunable ratios, offering an industrially viable and eco-friendly process.