Heterometallic calcium–alkali metal aryloxides as catalysts for the solvothermal alcoholysis of nylon-6 waste

Abstract

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 3, 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.