Direct Oxidation of Methane to Methanol Using Copper Phthalocyanines as Precursors

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 H 2 O 2 as an oxidant (50 ºC, 30 bar CH 4 , 0.5 M H 2 O 2 ). Catalysts with varying Cu loadings (0.5 - 1.0 wt.%), framework compositions (SiO 2 /Al 2 O 3 = 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, 1 H 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 Cu 2+ -2Z framework species, as confirmed by XANES, UV-Vis-diffuse reflectance spectroscopy, and H 2 -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 CH 3 OH productivity of 553 μmol g cat -1 h -1 and 47% selectivity at isoconversion (0.1%), corresponding to a methanol yield of 4.3 mol CH3OH mol Cu -1 h -1 , surpassing many recent reported phthalocyanine-based systems for DOMTM. An even higher methanol yield of 8.0 mol CH3OH mol Cu -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 CH 3 OH 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. • Copper α-3,5-(di- tert -butyl)phenyl phthalocyanine (CuPc) acts as a Cu 2+ -2Z precursor, not an intact biomimetic catalyst. • Isolated Cu 2+ -2Z framework species identified as the selective active sites by XAS, UV-Vis-DRS, and H 2 -TPR. • BAS/LAS ratio and Brønsted acid site density govern CH 3 OH selectivity and productivity. • CH 3 OH yield of 8.0 mol CH3OH mol Cu ⁻¹ h⁻¹ surpasses comparable metallophthalocyanines-based systems. • CO 2 quantified by potentiometric titration: selectivities reach up to 45%.