CaMoO ₄ –Hydrochar: Super-Amphiphilic and Oxygen-Deficient Catalyst with Nanopits for Oxidative Desulfurization Using Stoichiometric H ₂ O ₂

Engineering valuable materials like hydrochar from biomass waste is one of the key concepts in environmental sustainability. In this work, besides being used for preparing hydrochar, tamarind tree bark provides calcium to prepare hydrochar-supported calcium molybdate (CaMoO 4 ) catalysts (cat- x, where x is the theoretical weight percentage of molybdenum). Calcium available in tamarind tree bark initially becomes calcium carbonate and then reacts with ammonium heptamolybdate to yield CaMoO 4 under hydrothermal conditions, as evidenced by PXRD analysis. CaMoO 4 formation over hydrochar is further confirmed by Raman, FT-IR, and HR-TEM analyses. XPS and EPR data indicate the presence of oxygen vacancies in the prepared catalysts. CaMoO 4 particles with nanopits are highly dispersed on hydrochar according to the FE-SEM analysis. Advantageously, the catalysts are superamphiphilic, as confirmed by the contact angle measurements using water and n -heptane. Nanopits of CaMoO 4 particles might increase the contacting surface and wettability with solvents and thus the mass transfer. With these intriguing properties, the optimized catalyst (cat-5) achieves 100% efficiency in the oxidative desulfurization of dibenzothiophene (1000 ppmw S) using only a stoichiometric amount of H 2 O 2 and also maintains 80% of the original activity in the sixth catalytic cycle. This outstanding performance is attributed to (i) superamphiphilicity that accelerates mass transfer across the liquid phases of different polarity and (ii) oxygen vacancies that promote the activation of H 2 O 2 . Therefore, CaMoO 4 -hydrochar would be an attractive catalyst for biphasic reactions. Also, we believe that the present work would provide key insights into preparing valuable materials from biomass waste.