Heterometallic Calcium–Alkali Metal Aryloxides as Catalysts for the Solvothermal Alcoholysis of Nylon-6 Waste

In this study, we report the synthesis of homometallic and heterometallic calcium aryloxides: [Ca(sal-Me)2(MeOH)]n (1), [Ca3(sal-Me)6(MeOH)2] (2), [Ca2Li2(sal-Me)6(THF)2] (3), [Ca2Na2(sal-Me)6(MeOH)4] (4), and [Ca2K2(sal-Me)6(MeOH)4] (5). Compounds 1 and 2 were obtained via direct reaction of metallic calcium with methyl salicylate (Hsal-Me) in methanol (MeOH). A similar synthetic route, incorporating one equivalent of an alkali metal (M′ = Li, Na, K) and tetrahydrofuran (THF), was used to obtain the heterometallic derivatives 3-5. A central focus of this work is the synthesis of heterometallic Ca²⁺–M′⁺ complexes, of which only 13 examples have been previously documented. A detailed investigation of the reaction pathways led to the identification of intermediate compounds: [Ca3(sal-Me)6(THF)4] (2a), [CaLi6(sal-Me)8]‧(6), and [Ca3Na4(sal-Et)10(Hsal-Et)2] (7) (Hsal-Et = ethyl salicylate). Additionally, reactions conducted under atmospheric moisture conditions yielded the uncommon pentanuclear complexes [Ca4Na(μ5-OH)(sal-Et)8(MeOH)] (8) and [Ca4K(μ5-OH)(sal-Et)8(EtOH)] (9). Compounds 1-9, along with previously reported [M′₆(sal-Me)₆] (M′⁺ = Li⁺(10), Na⁺(11), K⁺(12)), [Mg₂(sal-Et)₄(EtOH)₂] (13), [Zn₄(sal-Me)₈] (14), [M₂M′₂(sal-Me)₆(THF)x] (M²⁺ = Mg²⁺; M′⁺ = Li⁺(15), Na⁺(16), K⁺(17); M²⁺ = Zn²⁺; M′⁺ = Li⁺(18), Na⁺(19), K⁺(20); x = 0, 2, 4), and [Mg₄Na₂(sal-Me)₆(sal)₂(THF)₄] (21) were evaluated as catalysts for the chemical recycling of polyamide waste. Among them, the heterometallic catalysts 2, 6, and 18–20 exhibited the highest catalytic activity in the methanolysis of nylon-6 at 220 °C. Particular emphasis is placed on the reaction conditions and the high efficiency of the catalysts. Key factors influencing catalytic activity and the reaction mechanism are also analyzed.