*3.5. Catalyst Recycling*

Catalyst recycling experiments were carried out with 5Fe-ACs catalyst with a3hreaction time and a reaction temperature of 170 ◦C. After the reaction, liquid samples were taken normally for furfural and xylose analyses. In addition, metal leaching (Fe and Zn) was monitored by measuring the metal content of the water phase by AAS (see Section 3.6) and the catalyst was collected using a PALL Easy Pressure Syringe Filter Holder and hydrophilic polypropylene membrane (GHP). The catalyst was first washed with methanol (4 + 10 mL) and water (3\*10 mL) and then dried and weighed. Used catalysts were analyzed with SEM and TEM (see Section 3.3).

#### *3.6. Analytical Methods for Conversion Studies*

Two different HPLC analyses were used to detect furfural and xylose in the samples. In the analyses, calibrations were performed with commercial furfural or xylose. HPLC analysis for furfural was carried out using a Waters 2695 separation module fitted with an Atlantis T3 (3 μm, 4.6 × 150 mm) column and a Waters 996 photodiode array (PDA) detector (Waters Corp., Milford, MA, USA). A mixture of water (0.1% TFA) and methanol (0.1% TFA) (90:10) was used as the mobile phase, with a flow rate of 1 mL/min. The column temperature was kept constant at 30 ◦C and the UV detection for furfural was performed at 277 nm. HPLC analysis for xylose was carried out using a Shimadzu LC-20AT liquid chromatograph instrument fitted with an SIL-20A TH autosampler, RID-20A refractive index detector, SUGAR SH-G pre-column and Shodex SUGAR SH1821 column (8.0 × 300 mm). Sulfuric acid (5 mM) was used as a mobile phase with a flow rate of 0.8 mL/min and the column temperature was kept constant at 60 ◦C.

Atomic absorption spectroscopy (AAS) was used to determine iron and zinc leaching from 5Fe-ACs and 5Fe-ACz catalysts. First, water phase samples of the reactions were diluted with water to a minimum 10 mL. Then, determinations were made using Varian AA240FS equipment (Varian Inc., Palo Alto, CA, USA), air-acetylene fuel, a Varian SpectrAA lamp (Cu/Mn/Zn/Fe) and flame emission wavelengths of 372.0 nm for Fe and 213.9 nm for Zn.

*Catalysts* **2020**, *10*, 821
