*2.2. Experimental*

The co-pyrolysis experiments were carried out on a temperature-controlled fixed-bed reactor system, which includes a heating, cooling and gas adsorption section, as illustrated in Figure 1. The coal was mixed with 1, 3 and 5 wt.% of WT-2 powders by ball milling, corresponding to 0.02, 0.06 and 0.1 wt.% of Zn in the hybrid, respectively. To further expand the scope of the study, the 1 and 5 wt.% of ZnO powders were also blended with the coal, related to 0.8 and 4 wt.% of Zn in the mixture, respectively. Then the blends were put into the quartz tube of fixed-bed reactor and heated to 700, 900 and 1050 ◦C and kept for 2 h with a heating rate of 5 ◦C/min in nitrogen, respectively. For comparison, the separate pyrolysis of coal or WT-2 was also conducted under the same condition. The

produced tar was found to be condensed on the wall of the quartz tube and in the tar collector. To analyze the Zn content in tar, the quartz tube and tar collector were calcined in muffle furnace at 800 ◦C for 2 h and the obtained ash was dissolved in HCl aqueous solution (37.5 wt.%), which was subjected to inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement. Additionally, the HCl aqueous solution in the gas absorber was also analyzed by ICP-AES to determine the amount of Zn vaporized into gas. Zn content in coke was determined by subtraction method.

**Figure 1.** Schematic diagram of fixed-bed experimental device. 1—N2 cylinder, 2—flowmeter, 3—electric heating furnace, 4—quartz tube, 5—program temperature controller, 6—coal tar collector, 7—ice water, 8—HCl solution.

#### *2.3. Materials Characterization*

The phase compositions of samples were analyzed by X-ray diffraction (XRD) on a Rigaku Smartlab 9 kw diffractometer with a Cu Kα radiation (λ = 0.15406 nm) in the 2θ range of 5–90◦ with a scanning rate of 10◦/min. The thermal decomposition behaviors of samples were analyzed with thermogravimetric and differential scanning calorimetry (TG-DSC) on a Chi 449F3 thermogravimeter from room temperature to 1100 ◦C at a heating rate of 10 ◦C/min under a nitrogen flow.
