# Review of Studies on Joint Recovery of Macroalgae and Marine Debris by Hydrothermal Liquefaction
## 2 Experience of Hydrothermal Liquefaction of Macroalgae
### Using Macroalgae for Biorfied Production in a One-Step Process
The use of catalytic systems reduces the pressure of the process and temperature (which in turn causes a decrease in capital and operating costs), increases the yield of biofuel, and improves its quality. The existing catalysts can be divided into two \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Algae Type** & **Conditions** & **Fuel Yield Efficiency** & **Reference** \\ \hline \multirow{8}{*}{_Sargassum tencrrimum_} & Temperature 260, 280, and 300 \({}^{\circ}\)C, pressure 4.5–12 MPa, catalysts: CaO, applied on CeO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\) and ZrO\({}_{2}\) in doses of 5 to 25 wt\%. Contact time 15 min. Solvent: water, ethanol, and a mixture of water: ethanol & The fuel yield in the non-catalytic process was 3.3 wt\%, 23.3 wt\%, and 32.0 wt\% when using water, ethanol, and their mixture, respectively. In the catalytic process, the maximum yield when using CaO/ZrO\({}_{2}\) as a catalyst was 25.2 wt\% and 33.0 wt\% when using ethanol and water, respectively & [68] \\ \hline \multicolumn{4}{c}{Homogeneous acid/alkaline catalysts with water as solvent} \\ \hline \multirow{2}{*}{_Spirulina platensis_} & Maximum fuel yield 29.7\% without catalyst, with CH\({}_{3}\)COOH with \({}_{3}\)COOH 28\% & [69] \\  & Temperature 270, 290, and 310 \({}^{\circ}\)C) and reaction time (10, 20, and 30 min), catalysts: KOH, NaOH and Na\({}_{2}\)CO\({}_{3}\) & Maximum yield was 12.0 wt\% at 290 \({}^{\circ}\)C without catalysis. In the catalytic reaction, the maximum yield was 26.7 wt\% when using KOH & [70] \\ \hline \multirow{2}{*}{_Sargassum petters_} & Temperature 340 \({}^{\circ}\)C, time 15 min, catalyst: 5\% Na\({}_{2}\)CO\({}_{3}\) & Maximum yield was without catalyst 32\%, with Na\({}_{2}\)CO\({}_{3}\) yield was 28\% & [71] \\ \hline \multirow{2}{*}{_Cladophora sociali_} & Temperature 350 \({}^{\circ}\)C, catalysts: KOH, K\({}_{2}\)CO\({}_{3}\)H\({}_{3}\)PO\({}_{4}\), HCOOH, zeolites, Ni-Re, Ru/C, Fe & Maximum yield was 36.2\% in the presence of Fe & [72] \\ \hline \multirow{2}{*}{_Ulva prolifera_} & Temperature 260–300 \({}^{\circ}\)C, time 15–45 min, pressure 4.3–7 MPa, catalyst: zeolites & & \\ \hline \multirow{2}{*}{_Spirulina platensis_} & Temperature 320 \({}^{\circ}\)C, 37 min, catalysts 0.15\% Fe\({}_{3}\)O\({}_{4}\) & & \\  & Temperature 320 \({}^{\circ}\)C, 37 min, catalysts & & \\ \hline \multirow{2}{*}{_Spirulina platensis_} & Temperature 250 \({}^{\circ}\)C, 30 min, catalysts 0.4\% CeO\({}_{2}\) & Fuel yield without catalyst 16\%, with CeO\({}_{2}\)—26\% (+\)62.5\% & [75] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._groups: heterogeneous and homogeneous catalysts. Alkaline catalysts and acids are more commonly used as homogeneous catalysts. Vietnamese researchers processed _Cladophora sociali_ macroalgae in the presence of homogeneous (KOH, K\({}_{2}\)CO\({}_{3}\), H\({}_{3}\)PO\({}_{4}\), HCOOH) and heterogeneous (zeolite, Ni-Re, Ru/C, Fe) catalysts at 350 \({}^{\circ}\)C. The maximum yield (36.2%) and the minimum content of heteroatoms were achieved using metallic iron as a catalyst [72]. As previously stated, the use of catalytic systems in the hydrothermal process allows for increased fuel yield and quality. Macroalgae _Ulva prolifera_ were treated at different temperatures (270 \({}^{\circ}\)C, 290 \({}^{\circ}\)C, and 310 \({}^{\circ}\)C) and reaction times (10, 20, and 30 min). The process was carried out in the presence of basic homogeneous catalysts (KOH, NaOH, and Na\({}_{2}\)CO\({}_{3}\)). The maximum bio-oil yield during non-catalytic liquefaction was 12.0 wt% at 290 \({}^{\circ}\)C. In the catalytic reaction, the maximum bio-oil yield (26.7 wt%) was observed when KOH was used as a catalyst. The catalytic process allowed for an increasing in the calorific value of the fuel (HHV) by 58.5% [70]. The peculiarity of using homogeneous catalysis is that their regeneration is practically impossible. Effectiveness of alkaline catalysis is quite moderate [70] or even negative [71].