Dual structure Brønsted sites activated carbon beads solid acid enhances the highly efficient synthesis of HMF: From batch to continuous

In this work, ACB₁₈-SO₃H-Cl with dual structure Brønsted sites was prepared via chemical grafting method, and promoted the efficient production of HMF from batch to continuous reactors. The optimal catalyst was obtained by adjusting the acid etching time and the grafting amount of acidic functional groups. Through BET and NH₃-TPD, it was found that the specific surface area of the 0.3 mm spherical activated carbon support reached 1179 m²/g, and the acid concentration of prepared ACB₁₈-SO₃H-Cl catalyst reached 7.86 mmol/g. The fructose conversion of 99.2 % and HMF yield of 96.2 % were achieved in stirred tank reactors (STRs) within 2 h, and 90.6 % HMF yield was obtained in micropacked bed reactors (μPBRs) within 86.4 s. Meanwhile, Py-IR and selective poisoning experiments confirmed that the Brønsted acid site is the catalytic active center, and there is a synergistic catalytic effect between the two structural Brønsted sites. The space–time yield (STY) of HMF in μPBRs can reach 2.96 kg/L/h/g, which is at least one order of magnitude larger than reported values in batch and continuous reactors. Moreover, based on the synergistic strengthening effect of μPBRs and ACB₁₈-SO₃H-Cl, the apparent activation energy for synthesizing HMF in μPBRs is lower than that of STRs.