Atomic proximity-controlled Pd-Ti bifunctional catalyst for the sustainable direct ammoximation of cyclohexanone in water
The industrial production of ε-caprolactam, the essential precursor for nylon-6, is a cornerstone of the modern polymer industry. Historically, this process evolved from energy-intensive non-catalytic routes to the more atom-economical ammoximation of cyclohexanone over titanosilicate catalysts using H2O2 as a green oxidant. Despite this progress, the reliance on concentrated H2O2 presents a significant sustainability bottleneck, as its commercial production via the anthraquinone process is energy-intensive, waste-prone, and involves hazardous transportation. A more sustainable ideal reaction involves the direct use of H2 and O2 to generate active oxygen species in situ. However, implementing this bifunctional route has long been thwarted by high noble metal loadings, poor H2 efficiency due to the rapid decomposition of intermediate H2O2, and the inherent instability of catalysts in the alkaline aqueous media required for ammoximation. In the January 2026 issue of Nature Catalysis, Wu and colleagues report a breakthrough by engineering a titanium-mordenite-confined, low-loaded Pd catalyst (0.055 wt% Pd@A-Ti-MOR-R) that achieves exceptional efficiency and industrial-grade longevity for direct ammoximation in water. The researchers proposed a “structured” solution to spatial confinement by utilizing an acid-treated Ti-MOR (A-Ti-MOR) featuring specific Ti-OH defect sites adjacent to silanol nests. These defects act as precise anchors to stabilize subnanometric Pd2 clusters, ensuring that the Pd and Ti active sites remain in “atomic proximity” within the 8-ring side pockets of the zeolite. This atomic-level configuration was rigorously verified using spherical-aberration-corrected annular dark field scanning transmission electron microscopy (ADF-STEM), which identifies bright contrasts from subnanometric Pd clusters with diameters below 0.5 nm near the framework pores. Furthermore, Pd K-edge extended X-ray-absorption fine-structure (EXAFS) analysis confirms the formation of Pd–O–Ti bridges through the identification of a specific scattering path at 3.67 Å, proving that the Pd clusters are chemically bonded to the framework Ti sites.