Selective Lanthanide‐Organic Catalyzed Depolymerization of Nylon‐6 to ϵ‐Caprolactam

Abstract

Nylon-6 is selectively depolymerized to the parent monomer ɛ-caprolactam by the readily accessible and commercially available lanthanide trisamido catalysts Ln(N(TMS) 2 ) 3 (Ln = lanthanide). The depolymerization process is solvent-free, near quantitative, highly selective, and operates at the lowest Nylon-6 to ɛcaprolactam depolymerization temperature reported to date. The catalytic activity of the different lanthanide trisamides scales with the Ln 3 + ionic radius, and this process is effective with post-consumer Nylon-6 as well as with Nylon-6 + polyethylene, polypropylene or polyethylene terephthalate mixtures. Experimental kinetic data and theoretical (DFT) mechanistic analyses suggest initial deprotonation of a Nylon terminal amido NÀ H bond, which covalently binds the catalyst to the polymer, followed by a chain-end back-biting process in which ɛcaprolactam units are sequentially extruded from the chain end.

Catalyst Design to Address Nylon Plastics Recycling

“…Chemical recycling of commodity polymers typically utilizes high reaction temperatures and therefore, is energy-intensive and may be accompanied by undesired thermal decomposition. 2,36 Although the present 240 °C reaction temperature is lower than typically reported for Nylon-6 depolymerization to -caprolactam (> 300 °C) 25,26,28,30 , it is above the Nylon-6 (218.3 °C) melting point. 37 In an effort to lower the reaction temperature, catalytic studies were next conducted at 220 °C with 1 mol% catalyst 3 loading for 2 h with a respectable yield (Table 1, entry 9) and 95% yield for 4 h and 0.2 mol% catalyst loading (Table 1, entry 10).…”

“…Cp*2LaN(TMS)2 (2), Cp*2LaCH(TMS)2 (3), Cp*2LuCH(TMS)2 (4), Cp*2YCH(TMS)2 (5), Me2SiCp''2YCH(TMS)2 (6) were synthesized according to published reports. 33,48,57 All polymers were washed with 1 M KOH solution overnight, 30 filtered, washed with deionized H2O, and dried under a high vacuum at 100 °C for at least 24 h prior to use. Pristine Nylon-6 powder with a mean particle size of 15-20 µm and a molecular mass (Mn) of 14,800 g/mol (as determined by GPC) was obtained from Goodfellow Inc. All manipulations in catalyst synthesis were carried out with the exclusion of O2 and moisture in oven-dried Schlenk-type glassware on either a dual-manifold Schlenk line, interfaced to a high-vacuum manifold (10 −6 Torr), or in an Argon-filled MBraun glovebox with a high-capacity recirculatory (< 1 ppm O2).…”

“…of TMS2NH are released at the onset of reaction). 30 Based on this finding, it seems unlikely that catalyst 1 can readily interchain "hop" following the consumption/deconstruction of a polymer chain, reflecting the aforementioned La-polymer macromolecule cross-linkages arising from the homoleptic tris-amido structure (Figure 5, left). Instead, deactivation of catalyst 1 by a carboxylate chain end (see below for details) can occur.…”

“…27 Recently, we reported in an initial screening study that tris[bis(trimethylsilyl)amido] lanthanide catalysts Ln NTMS selectively depolymerize Nylon-6 powder to monomeric caprolactam in a solvent-free process at 240-280 °C (Figure 1B). 30 Although >90 % yields of the -caprolactam are achieved, a substantial 5 mol% of the lanthanum catalyst (La NTMS , 1) is required. Furthermore, catalyst 1 is volatile under the reaction conditions and sublimes from the reaction mixture if the Nylon-6 powder and the catalyst are not well-mixed before and during the polymer melting stage at reaction onset.…”

Silica Gel Supported Chlorosulfonic Acid Catalyzed Beckmann Rearrangement of Cyclohexanone Oxime in Liquid Phase

“…It has a brilliant application in industry, which is cyclohexanone oxime Beckmann rearrangement to prepare caprolactam. Caprolactam is almost always used to synthesize nylon 6 fibers [1,2], and nylon 6 fibers are commonly used to produce civilian silk, carpets, and industrial silk, in transportation [3] and computers. In addition, caprolactam is widely used as a pharmaceutical raw material in the production of antiplatelet drugs such as 6-aminohexanoic acid [4], antimalarial drugs, and the production of laurylazone.…”