Direct oxidation of methane to methanol using copper phthalocyanines as precursors

Abstract

The direct oxidation of methane to methanol (DOMTM) in the liquid phase was investigated using a copper α-3,5-(di-tert-butyl)phenyl phthalocyanine (CuPc) supported on hierarchical ZSM-5 zeolites prepared by alkaline desilication, with H2O2 as an oxidant (50 ºC, 30 bar CH4, 0.5 M H2O2). Catalysts with varying Cu loadings (0.5–1.0 wt%), framework compositions (SiO2/Al2O3 = 23 and 30), and thermal treatment (intact CuPc vs. calcined) were evaluated. Qualitative kinetic analysis was performed through the quantification of the products over time (0–4 h) by HPLC, 1H NMR, and potentiometric titration. X-ray absorption spectroscopy (XAS) established that the CuPc macrocycle remains structurally intact after incorporation into the zeolite (Cu-N distance 1.94 Å, coordination number∼4.4), while calcination leads to complete macrocycle decomposition and formation of isolated Cu2+-2Z framework species, as confirmed by XANES, UV-Vis-diffuse reflectance spectroscopy, and H2-TPR. Elemental mapping by HAADF-STEM demonstrated that copper is homogeneously dispersed throughout the zeolite. Calcination significantly improved catalytic performance. The catalyst 1CuPc-ZSM-5–30-DS-Calc (Cu/Al molar ratio = 0.17) achieved a CH3OH productivity of 553 μmol gcat−1 h−1 and 47% selectivity at isoconversion (0.1%), corresponding to a methanol yield of 4.3 molCH3OH molCu−1 h−1, surpassing many recent reported phthalocyanine-based systems for DOMTM. An even higher methanol yield of 8.0 molCH3OH molCu−1 h−1 was obtained over 0.5CuPc-ZSM-5–30-DS-Calc (Cu/Al molar ratio = 0.09). The Brønsted to Lewis acid site (BAS/LAS) ratio and Brønsted acid site density were identified as key descriptors of CH3OH selectivity and productivity. These findings establish that CuPc functions primarily as a precursor to well-dispersed Cu²⁺ active sites, and that copper speciation, zeolite acidity, and mesoporosity jointly govern methanol selectivity in this reaction system.