Confinement and synergy effect of bimetallic Cu-Ni clusters encapsulated in Beta zeolite for methyl acetate formation from methanol alone

Abstract

The direct synthesis of methyl acetate (MA) from methanol is an appealing and green approach, but an efficient catalyst is urgently needed. Herein, Cu-Ni bimetallic clusters, with a diameter of ∼1.2 nm, have been successfully encapsulated within BEA zeolite by an in-situ two-step encapsulation method (Cu-Ni@Beta), which significantly favors the dispersion of metals compared to the ion-exchanging (Cu/Ni@Beta) and wet impregnation approach (Cu-Ni/Beta). The as-synthesized Cu-Ni@Beta catalyst shows a MA formation rate of 1.46 mmol s⁻¹ g_(Cu+Ni)⁻¹, which is much higher than Cu/Ni@Beta of 0.02 mmol s⁻¹ g_(Cu+Ni)⁻¹ and Cu-Ni/Beta of 0.12 mmol s⁻¹ g_(Cu+Ni)⁻¹. Characterization studies reveal that zeolite constraints could not only provide the spatial confinement for metal clusters but also induce an electronic interaction between confined Cu and Ni species in Cu-Ni@Beta. This interaction can increase and stabilize Cu⁺ sites as well as Lewis acid sites, which are crucial for improving catalytic performance. In-situ FTIR experiments indicate that the formation of formaldehyde (CH₂O*) is the critical step for methanol to MA, and the CH₂O* further couples with methoxy (CH₃O*) to form acetate species (CH₃CHO*), which subsequently couples with CH₃O* to produce MA.