Vardon et al. (2016) demonstrated the first end-to-end bio-based route to nylon-6,6: biological production of cis,cis-muconic acid, crystallization-based separation, and continuous catalytic hydrogenation to adipic acid over carbon-supported rhodium, with catalyst deactivation observed but mechanistically unresolved. Several strands of that deactivation problem have since been advanced — protective overcoatings (atomic-layer-deposition Al2O3 on Pd/TiO2, Settle et al. 2019), non-platinum-group catalysts (Ni/Al2O3 in continuous micropacked-bed operation, Liang et al. 2025), fed-batch biological optimization (Ling et al. 2022 and later work), integrated real-lignin processing via a muconolactone route (Mains et al. 2026), and techno-economic and life-cycle assessments — while the mechanistic core remains open: no study has yet identified the adsorbed foulants, determined the chemical speciation of the residual nitrogen impurities (inorganic ammonium versus amino acids and proteins) and their poisoning contributions, partitioned reversible from irreversible deactivation, or demonstrated regeneration of the spent catalyst, and all follow-up catalytic studies have used purified or model substrates. The open problem is therefore a species-resolved, quantitative understanding of catalyst deactivation during hydrogenation of biologically derived muconic acid, together with a validated countermeasure.

Background

Adipic acid (hexanedioic acid) is a commodity dicarboxylic acid manufactured at multi-million-tonne scale, principally as the monomer that reacts with hexamethylenediamine to form nylon-6,6; its incumbent production by nitric-acid oxidation of cyclohexanol/cyclohexanone is a major industrial source of nitrous oxide, a potent greenhouse gas. A biological alternative routes through cis,cis-muconic acid, the unsaturated six-carbon diacid (2,4-hexadienedioic acid) that bacteria such as the soil bacterium Pseudomonas putida accumulate when engineered to funnel aromatics through the β-ketoadipate pathway, in which the ring-cleavage enzyme catechol 1,2-dioxygenase converts catechol into cis,cis-muconate. Because glucose, benzoate (a model aromatic), and depolymerized-lignin aromatics can all serve as feedstocks, cis,cis-muconic acid is regarded as a bioprivileged intermediate: complete hydrogenation of its two carbon–carbon double bonds yields adipic acid chemically identical to the petrochemical product.

Vardon et al. (2016) provided the first integrated demonstration of this route: muconic acid was produced biologically (P. putida grown on glucose plus the model aromatic benzoate), recovered from the broth by pH-controlled crystallization, hydrogenated to adipic acid over a carbon-supported rhodium catalyst (Rh/AC) in continuous operation, and the product was polymerized to nylon-6,6 of verified quality. The catalytic step, however, lost activity with time-on-stream (cumulative operating time of the continuous reactor). The authors' characterization attributed the primary initial activity loss to organic fouling — the accumulation of unidentified adsorbed material on the catalyst — while the purified muconic acid still carried roughly 90 ppm of residual nitrogen-containing material of undetermined chemical form. This distinction matters because amino acids and proteins carried over from fermentation broth are classical poisons of hydrogenation catalysts even at parts-per-million levels, whereas inorganic ammonium salts are comparatively benign; whether the residual nitrogen was poison or bystander could not be decided. The relative contributions of reversible fouling and irreversible deactivation (poisoning, metal leaching, metal restructuring) were unresolved, and no procedure existed for regenerating the spent catalyst. Earlier and parallel catalyst studies — rhenium on titania (She et al. 2011), platinum on carbon (Pt/C; Scelfo et al. 2016), and palladium on carbon (Pd/C) under mild aqueous conditions (Capelli et al. 2017, 2019) — established activity and intrinsic kinetics on clean substrates but did not address deactivation by biogenic impurities. Published techno-economic and life-cycle work on the bio-adipic route identifies catalyst lifetime as a cost-sensitive parameter of the overall biorefinery, which is what makes the deactivation question an engineering question and not only a mechanistic one.

Problem Statement

Establish the mechanistic basis of catalyst deactivation during continuous hydrogenation of biologically derived cis,cis-muconic acid (or muconate) to adipic acid over supported metal catalysts, and demonstrate a countermeasure. Concretely: (i) identify the chemical nature of the adsorbed species responsible for the organic fouling observed on carbon-supported noble-metal catalysts during this reaction; (ii) determine the speciation of the residual nitrogen-containing impurities present in purified bio-muconic acid — distinguishing inorganic ammonium salts from organic amino acids and proteins — and quantify how each impurity class poisons the hydrogenation catalyst at trace (parts-per-million) levels; (iii) quantify the relative contributions of reversible and irreversible deactivation as a function of time-on-stream, disentangling fouling, nitrogen-species poisoning, metal leaching, and metal restructuring; and (iv) on that mechanistic basis, develop and validate a regeneration protocol for the spent catalyst that restores activity and adipic-acid selectivity over repeated deactivation–regeneration cycles — or an impurity-tolerant catalyst formulation that retains activity and adipic-acid selectivity over repeated exposures to the biogenic feed. These four parts jointly form one scientific objective: a predictive, species-resolved account of why the catalyst loses activity on real bio-derived feed and what restores it.

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