Microfibrous-Structured Pd/AlOOH/Al-Fiber for CO Coupling to Dimethyl Oxalate: Effect of Morphology of AlOOH Nanosheet Endogenously Grown on Al-Fiber

We report a green, template-free, and general one-pot method of endogenous growth of free-standing boehmite (AlOOH) nanosheets on a 3D-network 60 μm-Al-fiber felt through water-only hydrothermal oxidation reaction between Al metal and H₂O (2Al + 4H₂O → 2AlOOH + 3H₂). Content and morphology of AlOOH nanosheets can be finely tuned by adjusting the hydrothermal oxidation time length and temperature. Palladium is highly dispersed on such AlOOH endogenously formed on Al-fiber felt via incipient wetness impregnation method and as-obtained Pd/AlOOH/Al-fiber catalysts are checked in the CO coupling to dimethyl oxalate (DMO) reaction. Interestingly, Pd dispersion is very sensitive to the thickness (26-68 nm) of AlOOH nanosheet, and therefore the conversion shows strong AlOOH-nanosheet-thickness dependence whereas the intrinsic activity (TOF) is AlOOH-nanosheet-thickness independence. The most promising structured catalyst is the one using a microfibrous-structured composite with the thinnest AlOOH nanosheet (26 nm) to support a small amount of Pd of only 0.26 wt %. This catalyst, with high thermal-conductivity and satisfying structural robustness, delivers 67% CO conversion and 96% DMO selectivity at 150 °C using a feed of CH₃ONO/CO/N₂ (1/1.4/7.6, vol) and a gas hourly space velocity of 3000 L kg^-1 h^-1, and particularly, is very stable for at least 150 h without deactivation sign.