(735a) Evaluating the Mixed Homogeneous/ Heterogeneous Mechanism of Hydroxyapatite in HTL

To make food waste a viable energy source, oil yields need to increase. Hydroxyapatite (HAP) was evaluated for improved oil yield from hydrothermal liquefaction of food waste at 300 °C. HAP is a crystalline mineral found in bone and has shown to form during microalgae HTL, indicating its potential as a hydrothermally stable catalyst.¹

HAP tripled the oil yield from food waste (13.6 wt.% to 40.6 wt.%). The drastic increase in oil yield was seen with a specific HAP, coined HAP-1.86 due to its Ca/P ratio. To ascertain the effects of the hydrothermal conditions, HAP was treated in hot liquid water at 300 °C for over 100 hours and shown to maintain crystallinity within 5%, as verified through X-ray diffraction. Despite maintaining its crystal structure, HAP activity decreased from 40.6 wt.% to 32.4% after one use, leading to the idea of mixed homogeneous and heterogeneous activity.

To understand this phenomenon, we analyzed acid-base properties and pH before and after HTL. The pH of HAP-1.86 in DI water was 12 before use, and 8 after one run in HTL. These tests revealed that fresh hydroxyapatite acts as a homogeneous base to neutralize the acid found in food waste and promote oil formation. Figure 1 shows that HAP contains a heterogeneous mechanism indicated by the effectiveness of the neutralized, washed HAP. The effects on oil yield of the supernatant and washed solid combined produced the same yield, within error, as fresh hydroxyapatite.

Additionally, the findings presented here are a direct result of the Ca/P ratio of the catalyst. Stoichiometric HAP with a Ca/P ratio of 1.67 has a pH in DI water of 8, a stark difference from 12 seen with non-stoichiometric, calcium-rich HAP-1.86 variant presented above. This work reveals the catalytic stability and performance of hydroxyapatite in hydrothermal systems.