Microwave-Hydrochar in Synergy with CO ₂ for More Effective Etherification of Glycerol

The production of glycerol tert-butyl ethers (GTBEs) via the etherification of glycerol is limited by the low efficiency and poor sustainability of conventional solid catalysts. This highlights the need for more environmentally friendly, carbon-based materials that are more active. This study presents a new catalyst derived from sago waste via microwave-assisted hydrothermal carbonization (MWHTC), followed by pressurization with CO2 to increase porosity and the number of active surface sites. The resulting sago hydrochar had an increased surface area of 51.07 m2 g–1 and a pore volume of 0.69 cm3 g–1, enabling highly efficient, microwave-assisted etherification of glycerol with tert-butyl alcohol. Under optimized reaction conditions, the catalyst achieved 99% glycerol conversion within 60 min at 110 °C (TBA/Gly = 6:1), 0.2 g of catalyst, producing 35% mono-, 40% di-, and 25% tri-tert-butyl ether. CO2 intensification further increased the selectivity from 46% to 57.9%, demonstrating a strong synergistic effect with the microwave-activation hydrochar (M-H200 + CO2). This catalyst significantly outperforms commercial catalysts such as Amberlyst-15 and activated carbon, which require higher temperatures and longer reaction times and yield lower selectivity. Excellent stability was observed across reuse cycles, with 99% conversion maintained after three cycles and 91% after five cycles. Overall, this study shows that integrating sago-derived hydrochar, MWHTC processing and CO2 activation provides a scalable, environmentally friendly catalytic process for producing GTBE at high yield.