Sustainable synthesis of nylon intermediates from γ-valerolactone: Influence of key reaction parameters for improving the selectivity of methyl 4-pentenoate

Gamma-valerolactone (GVL) has been converted into a mixture of methyl pentenoate isomers (MPs) in presence of methanol using Zr/SiO₂ catalysts. These isomers are valuable precursors in fine chemistry and for polymer production. The terminal alkene methyl 4-pentenoate (M4P) is bio-based intermediate to produce caprolactam in excellent yield via hydroformylation to methyl 5-formylvalerate (M5FV) with subsequent reductive amination and ring-closure. For an economic process, a high selectivity of M4P is desired. In this paper, the dependency of the product selectivities on various key reaction parameters (e.g. Zr loading, temperature, GVL/MeOH ratio etc.) is presented. In addition, a set of control experiments were performed using the products (MPs) as educts under similar reaction conditions with the aim to investigate CC double bond isomerisation. The initial ring opening of GVL leads to a mixture of M4P and methyl 3-pentenoate (M3P) with small amounts of M2P. Surprisingly, M4P can be converted back to GVL and also to M3P. However, M3P can be converted mainly to M2P and vice-versa. GVL formation is not observed from the conversion of M3P and M2P. The underlying isomerizations limit the selectivity of M4P to ≤80 %, whereas the space time yield could be remarkably improved to 35.3 kg_MP/kg_Cath⁻¹ under optimum conditions. Interestingly, the best catalyst (25Zr/SiO₂) developed through the present study exhibited an excellent long-term stability for a period of >340 h-on-stream.