Unraveling catalytic conversion of spent coffee grounds through alkaline and alkaline earth metal phosphates in hydrothermal carbonization

Abstract

Hydrothermal carbonization (HTC) offers a promising pathway for sustainable energy production, with spent coffee grounds (CG) emerging as an eco-friendly feedstock for hydrochar generation. However, the potential applications of alkaline and alkaline earth metal phosphates as catalysts for novel carbonaceous materials remain largely unexplored. This study investigates the impact of alkaline types on the characteristics, morphology, and heating value of hydrochar derived from CG. Incorporating Mg3(PO4)2, K3PO4, and Na3PO4 into HTC processes resulted in significant reductions in carbon content, leading to decreased hydrochar yield, higher heating value (HHV), and energy yield (EY). Notably, HHV reduction ranged from 2 % to 15 %, depending on catalyst type and temperature. The catalysts contributed to increasing ash content in hydrochar due to their inorganic nature. The presence of catalysts minimally impacted the hydrochar surface functional groups, with a more amorphous structure observed at 200 °C across all samples. A higher abundance of microspheres was observed in hydrochar catalyzed by K3PO4 and Na3PO4 at 200 °C compared to Mg3(PO4)2. Interestingly, K3PO4 and Na3PO4 reduced nitrogen content of the hydrochar rather than Mg3(PO4)2. The proposed catalytic mechanism involved metal phosphate catalyzing hydrolysis reactions, thereby reducing hydrochar yields while augmenting volatile fatty acid (VFA) and furan contents in the process water. Alkaline and alkaline earth metal phosphates catalysts exhibited trade-offs between hydrochar properties and hydrolysis byproducts in the HTC of CG.