*2.2. Synthesis of Magnetic Carbon-Doped CuO/Fe3O4 Nanocomposite Catalyst*

Fe3O4/cellulose microfibril (CMF) composites were prepared using chemical co-precipitation of aqueous ferrous and ferric ions, and the procedures were similar to those described in the literature [35]. CMF (7.5 g, ~4%, *w*/*w*) was diluted to 0.3 wt.% with distilled water and stirred for 5 min at 1000 rpm. The diluted CMF suspension was further treated in a sonicator (QSON-ICA) for 5 min to disperse the individual nanocellulose, followed by heating at 65 ◦C and bubbling with nitrogen for

10 min under magnetic stirring (500 rpm) to remove the dissolved oxygen from the CMF suspension. Then, citric acid (1 mg) and diluted HCl solution (1 mL, 2 mol/L) were added to the above solution, followed by the successive additions of 1.3 mmol Fe(NO3)3·9H2O and 0.65 mmol FeSO4·7H2O solid samples with nitrogen bubbling and magnetic stirring. The function of citric acid was to protect the as-prepared Fe3O4 NPs from being oxidized by the dissolving oxygen in water. A dilute NaOH solution (10 mL, 2 mol/L) was added drop-wise into the above system, followed by bubbling with nitrogen and magnetic stirring (500 rpm) for 120 min at room temperature. The obtained Fe3O4/CMF composites (using a magnet) were washed three times with distilled water in a centrifugation step (4000 rpm, 10 min).

Copper (II) acetate monohydrate was dissolved in distilled water. Fe3O4/CMF nanocomposites and H3BTC were dispersed in ethanol and then treated in a sonicator (QSON-ICA) for 5 min. The ethanol solution containing Fe3O4/CMF composite and H3BTC was added to the aqueous system. The color of the solution changed from light blue to blue-black immediately, and the system was stirred continuously for 4 h. The final HKUST-1/Fe3O4/CMF composite was washed in a centrifugation step (4000 rpm, 10 min) with distilled water and absolute ethanol three times. Then, the drying was done at 50 ◦C under vacuum.

The synthesized HKUST-1/Fe3O4/CMF composites were placed in a ceramic boat, then transferred to a muffle roaster, which was operated under the conditions of 500 ◦C, air atmosphere, and a heating rate of 20 ◦C/min, before being held at 500 ◦C for 5 h; the procedures were similar to those described in the literature [36]. After cooling to room temperature, the sample was washed several times with distilled water and absolute ethanol in a centrifugation step (4000 rpm, 10 min), and finally dried in a vacuum at 60 ◦C for 5 h.
