3.1.2. Other Catalysts

Except zeolites, other catalysts such as metal oxides and supported metal catalysts are applied in catalytic pyrolysis. Transition metal such as nickel has high activity in C-C and C-O bonds cleavage, resulting in high performance for decarbonylation and decarboxylation [12]. Furthermore, reducible metal oxides are considered as a favorable support or catalyst because of their superior redox properties and low carbon deposition rate [104]. It was found that the pyrolysis of *Tetraselmis* sp. and *Isochrysis* sp. over Ni-Ce/Al2O3 and Ni-Ce/ZrO2 produced a higher yield of bio-oil (26 wt %) [105]. The catalysts exhibited strong deoxygenation and denitrogenation ability, with only 9–15% oxygen remained and removal of 15–20% nitrogen from bio-oil. In addition, pyrolysis of Pavlova sp. over Ce/Al2O3-based catalysts produced bio-oil with a low O/C ratio (0.1–0.15). MgCe/Al2O3 exhibited the best performance on the reduction of oxygen content from 14.1 to 9.8 wt %, while NiCe/Al2O3 produced the highest hydrocarbon fraction [106]. Catalytic fast pyrolysis of *Nannochloropsis oculate* over Co-Mo/γ-Al2O3 was carried out in an analytical micropyrolyzer coupled with a gas chromatograph/mass spectrometer (py-GC-MS) [74]. It was found that aliphatic alkanes and alkenes, aromatic hydrocarbons, and long-chain nitriles were the main products in bio-oil. Co-Mo/γ-Al2O3 catalyst could promote the formation of 1-isocyanobutane and dimethylketene with 35% selectivity. In addition, catalytic pyrolysis over Co-Mo/γ-Al2O3 produced pyrolysates with higher calorific value (33–39 MJ·kg−1) compared with that of algal feedstock (18 MJ·kg−1).

### *3.2. Catalytic HTL of Algae*

### 3.2.1. Catalytic HTL with Homogeneous Catalysts

Because HTL is conducted in aqueous phase, water-soluble homogeneous catalysts can be used. They commonly have the ability to improve biocrude yield or produce target compounds with high selectivity. Generally, the homogeneous catalysts include acid catalysts (HCl, H2SO4 and other organic acids), alkali catalysts (Na2CO3), or other inorganic salts. These catalysts usually promote bonds cleavage, so the degradation of components in algae to small molecular compounds is facilitated. In the presence of homogeneous catalysts, the dissolution of components from algae is facilitated and hence the biocrude yield enhances. However, the homogeneous catalysts can hardly improve the quality of biocrude. In other words, they have a weak influence on deoxygenation and denitrogenation of algal biocrude [3]. A summary of recent works on catalytic HTL on homogeneous catalysts is displayed in Table 4.

In general, the homogeneous catalysts used in HTL are catalysts with proper acidity or basicity. Koley et al. studied the catalytic and non-catalytic HTL of wet *Scenedesmus obliquus* (Table 4) [107]. The optimizations of HTL temperature, pressure, and residence time were conducted on both catalytic and non-catalytic HTL. They found that 300 ◦C, 200 bar, and 60 min was the optimum condition for obtaining the maximum yield of biocrude (35.7 wt %). By adding catalysts, the biocrude yield increased and followed the order: acidic catalyst CH3COOH (45%) > HCOOH (40%) > HCl (39%) > H2SO4 (38%) > H3BO3 (37%), and basic catalyst Na2CO3 (40%) > NaOH (38%) > Ca(OH)2 (37%) > KOH (37%) > K2CO3 (36%). In addition, the acetic acid had the ability to reduce the oxygen content of biocrude, resulting in a considerable HHV of 40.2 MJ·kg−1. However, the composition of biocrude in the presence of CH3COOH was still complex, containing fatty acids, phenols, indoles, monoaromatics, and N-heterocycles.

The effect of acidic, neutral and basic catalysts on the conversion of microalgae (*Spirulina platensis*) by HTL at various temperatures was studied by Zhang et al. [108]. HCl and acetic acid were used to create acidic condition, while KCl for neutral, and K2CO3 and KOH for basic conditions. Among these catalysts, only acetic acid and KOH are found to positively influence the biocrude yield, which was more obvious at lower temperatures. The acid and base catalysts promoted the degradation of components in microalgae and suppressed the condensation reaction, resulting in lower average molecular weight of biocrude. However, the distribution of compounds detected by GC-MS showed little change even with the aid of acid or base catalysts.

Typically, Na2CO3 is a commonly-used homogeneous catalyst for HTL because of its ability to enhance the biocrude yield. Shakya et al. studied the effect of temperature on the HTL of three kinds of algal species *Nannochloropsis*, *Pavlova*, and *Isochrysis* over Na2CO3 [109]. It can be concluded that the biocrude yield increased with the rise of temperature from 250 to 350 ◦C. The maximum biocrude yields from HTL of three algae species followed the order: *Nannochloropsis* (48.67 wt %) > *Isochrysis* (40.69 wt %) > *Pavlova* (39.96 wt %). When using Na2CO3 as the catalyst, the biocrude yield changed to the order of *Pavlova* > *Isochrysis* > *Nannochloropsis*. The biocrude yields of algae with high carbohydrates content (*Pavlova* and *Isochrysis*) increased at higher temperatures (300–350 ◦C) with the aid of Na2CO3, whereas the high-protein-containing algae (*Nannochloropsis*) showed higher yield of biocrude at lower temperature (i.e., 250 ◦C). However, the conversion of algae with Na2CO3 did not significantly improve biocrude properties. The biocrudes obtained were still not suitable for application in transportation.

Overall, homogeneous catalysts have a strong impact on the products yields, especially enhancing the biocrude yields. This is probably ascribed to the degradation ability of these catalysts to create an acid or basic condition. There were few reports about the deoxygenation and denitrogenation ability of homogeneous catalysts. Thus, the quality of biocrude from HTL can hardly be improved by homogeneous catalysts. The biocrude obtained cannot meet the standard for transportation fuel. Furthermore, the acidity and basicity of catalysts might influence the pH value of biocrude and lead to corrosion to the equipment. The homogeneous catalysts can hardly be recovered after reaction, and this leads to further expense [3,82]. Therefore, finding catalysts with good reusability and highly efficient for deoxygenation and denitrogenation is pressingly required.


**Table 4.** Catalytic HTL of algae with homogeneous catalysts.

### 3.2.2. Catalytic HTL with Heterogeneous Catalysts

The heterogeneous catalysts, which exist in different phases with the reaction media, are usually solid catalysts [114]. Heterogeneous catalysts can be easily recovered after reaction, therefore reducing the cost of the process [115,116]. Conversion of algae in the presence of heterogeneous catalysts might cover the deficiency of homogeneous catalysts. Commonly, heterogeneous catalysts include zeolites (e.g., H-ZSM-5), supported metal catalysts (Pt/C), and other metal oxide supported catalysts (e.g., sulfide CoMo/Al2O3 and Ni/TiO2) [117–119]. These materials have strong activity for bonds cleavage, resulting in facilitation of macromolecule degradation and conversion of oxygenates and nitrogenates to high-grade hydrocarbons. As a result, biocrude with low viscosity, high HHV, and low N content is produced in the presence of heterogeneous catalysts.

The effects of heterogeneous catalysts on the yield and quality of biocrude are summarized in Table 5. After screening, the majority of the catalysts listed in the table are supported metal catalysts and can be recycled several times. For example, magnetic nanoparticles (MNPs) were synthesized for microalgae separation and catalytic HTL by Egesa et al. [115]. Firstly, the MNPs were used for separation of algae from the culture medium, with a separation efficiency of 99% achieved. Then, the MNPs were applied in catalytic HTL for the production of biocrude from microalgae. The biocrude yield significantly increased from 23.2% (without catalyst) to 37.1% in the presence of Zn/Mg-ferrite MNPs. Moreover, the percentage of hydrocarbons increased by 26.4%, and the percentage of heptadecane increased by 27.8%, while the percentage of oxygenates and N-containing compounds decreased. This indicated the catalysts had activity in deoxygenation and denitrogenation. In addition, the MNPs could be easily recovered and recycled several times.

Additionally, noble metal catalysts such as commercial Pd/C, Ru/C and Pt/C are widely used in algae conversion and show excellent catalytic performance. Liu et al. reported a two-step catalytic conversion of algae (*Spirulina*) via solvent extraction followed by catalytic HTL of the extracted

residue [120]. In the extraction process, ethanol was found to be the best solvent with the highest extraction e fficiency, and the introduction of MgSO4 could produce ethyl esters from fatty acids. The lipid extracted residue was treated by HTL in the presence of commercial Pd/C, Pt/C, Ru/C, Rh/C, and Pd/HZSM-5. Among all catalysts, Rh/C exhibited the best performance in catalytic conversion of algae, producing 50.98% yield of biocrude with 30.7 MJ·kg−<sup>1</sup> HHV. The O and N content of biocrude obtained from HTL over Rh/C decreased significantly from 32.2% and 7.1% to 23.6% and 4.4%, respectively. In addition, the percentage of hydrocarbons in biocrude obtained over Rh/C based on GC-MS results was 55.7%. Xu et al. studied the catalytic e ffects of Pt/C, Ru/C and Pt/C + Ru/C on the HTL of *Chlorella* in the presence of H2 (Table 5) [117]. They divided the biocrude into water-soluble biocrude (WSB) and water-insoluble biocrude (WISB). The addition of catalysts could decrease the fraction of WSB but increase WISB fraction. At optimized conditions, Pt/C and Ru/C led to the highest carbon (63.6% and 74.2%) and hydrogen (7.3% and 8.4%) contents but lowest oxygen (14.1% and 9.2%) and nitrogen (12.2% and 7.1%) contents, and the highest HHV (29.7 and 35.6 MJ·kg−1) for WSB and WISB fraction, respectively. The water insoluble biocrude obtained from HTL of algae over Pt/C contained amides (48.2%), hydrocarbons (17.7%), acids (12.8%), and phenols (7.7%). In addition, catalytic HTL produced biocrude with more low boiling point fractions.

Apart from noble metal catalysts, the application of non-noble metal catalysts in algae HTL has drawn lots of attention due to their low cost and high activity in bonds cleavage. Among the non-noble metals, nickel, cobalt, iron, and molybdenum are proved to be active in deoxygenation and denitrogenation [2]. Kohansal et al. conducted the HTL of *Scenedesmus obliquus* in the presence of Ni-based catalysts (Ni/AC, Ni/AC-CeO2 nanorods and Ni/CeO2 nanorods) [121]. The optimum condition for the catalytic HTL of microalgae over the catalysts was to set at 324.12 ◦C, 43.52 min, and 19.90 wt % feedstock. With the addition of heterogeneous catalysts, the biocrude yields over three catalysts were higher than that from a non-catalytic process. The highest biocrude yield of 41.87% was achieved over Ni/AC-CeO2 nanorods, with the HHV of 38.57 MJ·kg−1. In the presence of Ni-based catalysts, the percentage of hydrocarbons in biocrude was higher than that of the non-catalytic biocrude, but the content of nitrogen-containing compounds was also higher.

Overall, heterogeneous catalysts perform better than homogeneous catalysts in terms of the improvement of algal biocrude quality. In addition, the yield of biocrude can be improved in the presence of heterogeneous catalysts. The catalysts can be easily recovered and reused after HTL, but the coke formation is still the major problem during catalytic HTL. However, in the previous literature, the contents of oxygen and nitrogen in the obtained biocrude are still too high to satisfy the standard of transportation fuel. Therefore, the technologies for production of high-quality liquid fuel with extremely low O and N content from algae need to be further developed.



### **4. Catalytic Conversion of Oil Derived from Algae**

### *4.1. Catalytic Hydroprocessing of Extracted Algal Oil*

Generally, the oil recovered from algal cell consists of different types of triglycerides. The fatty acids fraction of triglycerides usually contains palmitic, palmitoleic acid, stearic acid, and oleic acid. Some algal species also contain polyunsaturated fatty acids (PUFA) such as eicosapentaenoic acid (EPA), arachidonic acid (AA), and docosahexaenoic acid (DHA), which are value-added health care products [12,127]. The algae derived triglycerides can be hydrotreated by catalysts to fuel-like hydrocarbons with the aid of a proper catalyst. Since hydrogen could be obtained from a wealth of sources, including water splitting, especially by electrolysis of water with renewable electricity such as wind power or solar power, the consumption of hydrogen in the hydroprocessing can be ignored. A summary of recent works on hydroprocessing of algal oil is listed in Table 6.

Conventional NiMo sulfide catalyst has been widely used in hydrogenation of natural oil into fuel-ranged hydrocarbons. Liu et al. investigated the hydrocracking of algal oil from *Botryococcus braunii* (CnH2n−10, n = 29–34) with sulfide NiMo into fuel-ranged hydrocarbons [128]. The support effect on the selectivity of products was studied under the conditions of 300 ◦C for 6 h under 4 MPa H2. For the hydrotreating of the model compound (squalene C30H50), the main product was squalane (C30H62) over NiMo/SiO2, C1-C4 gas hydrocarbons over NiMo/HZSM-5, C5-C9 gasoline-ranged hydrocarbons over NiMo/HY and NiMo/SiO2-Al2O3, and C10-C15 aviation fuel-ranged hydrocarbons over NiMo/Al13-Mont, respectively. The hydrocracking of algal oil over NiMo/Al13-Mont gave aviation fuel-ranged hydrocarbons (C10-C15) with a yield of 52%. The sulfide NiMo catalyst acted as a

bifunctional catalyst for hydrogenation of squalene to squalane followed by cracking of the formed squalane to shorter-chain hydrocarbons. Zhao et al. explored the hydrotreating of extracted algal lipids from *Nannochloropsis* for the production of aviation fuel [129]. The effect of hydrotreating temperature (270–350 ◦C) and catalyst loading (10-30%) was investigated. The optimum condition for hydrotreating reaction was 350 ◦C and 30% catalyst loading. The main components of biofuel were C8-C16 hydrocarbons and aromatics. The two-step hydrotreating process obtained biofuels with oxygen content below 0.3% and nitrogen content below 0.007% and HHV of 46.24 MJ·kg−1.

In addition, noble metal catalysts such as Pt, Pd and Ru, which have high activity in hydrodeoxygenation, are also suitable for conversion of algal oil into high-grade hydrocarbons [130,131]. Xu et al. explored a technology for selective extraction of neutral lipid from algae *Scendesmus dimorphus* and subsequently conversion into jet fuel [132]. Hexane and ethanol solvent mixture was used for selective extraction of neutral lipids. Then, the extracted lipid was hydrogenated over the Pt/Meso-ZSM-5 catalyst. The obtained product oil (38%) mainly contained branched paraffin with C9-C15 chain length. The jet fuel product satisfied the ASTM 7566 standard with the desired freeze point (−57 ◦C), flash point (42 ◦C), heating value (45 MJ·kg−1), and aromatics content (<1%).

Although metal sulfides and noble metals are highly active in deoxygenation of algal lipid/oil, the sulfur leaching of metal sulfides and high-cost noble metal make these process not environmentally and economically friendly [47,133]. Except for metal sulfides and noble metal catalysts, the sulfur-free non-noble metal catalysts are promising in heterogeneous catalysis. It is found that non-noble metals (e.g., Ni, Co, Cu) are active in deoxygenation of fatty acids and natural oil to hydrocarbons [47,134,135]. Santillan-Jimenez et al. investigated the continuous catalytic hydrogenation of model compound and algal lipids to fuel-like hydrocarbons using Ni-Al layered double hydroxide [136]. In addition, Ni/Al2O3, Ni/ZrO2, and Ni/La-CeO2 were applied for the comparison experiments. Of all Ni-based catalysts, Ni-Al LDH showed the best results for conversion of tristearin to C10-C17 hydrocarbons at 260 ◦C. Higher temperatures favored the cracking reaction to form lighter alkanes, while lower H2 pressure favored the formation of heavier hydrocarbons. For the hydrogenation of algal oil, ~50% yield of hydrocarbons was obtained over Ni-Al LDH.


**Table 6.** Catalytic hydroprocessing of algal oil.

Generally, extracted algal oil/lipids have the potential to be converted to high-grade, fuel-like hydrocarbons. The nitrogen and oxygen content of algal-lipid derived fuel are low enough to satisfy the standard for transportation fuel due to the low nitrogen content and easily editable oxygen of algal lipids. However, due to the limited lipid content of algae, the yield of algal lipids derived green fuel based on the whole algal cell is also low. Based on the concept of "waste-free biorefinery", the utilization of other components of algae except for lipids needs to be explored.

### *4.2. Catalytic Upgrading of Biocrude Oil from Thermochemical Conversion of Algae*

Another way for production of fuel-ranged hydrocarbons from algae is the upgrading of the biocrude oil from thermochemical conversion. The biocrude oil obtained at high temperatures contains the components derived from saccharides and proteins apart from lipid. Some of the oxygenates and nitrogenates cause the undesired properties of biocrude oil [141]. Therefore, the large proportion of oxygen and nitrogen of bio-oil needs to be removed for obtaining high-quality biofuel. This process involved the use of a proper heterogeneous catalyst with high activity in deoxygenation and denitrogenation. Generally, the upgrading process also needs H2 to remove the heteroatoms (O, N, and S), for improving the heating value and reducing the O, N, and S content of product oil. The resulted biofuel should have low viscosity, high stability, and high HHV [68,82]. A summary of recent works on catalytic upgrading of algal biocrude oil is presented in Table 7.

### 4.2.1. Catalytic Upgrading of Pyrolysis Bio-Oil

As mentioned in the previous section, the bio-oil from direct pyrolysis of algae contains high proportion of O and N due to the degradation of the whole algae cell at high temperatures. The quality of pyrolytic bio-oil needs to be upgraded via catalytic process before utilization as transportation fuel. Elkasabi et al. investigated one-step hydrotreating and aqueous extraction of O and N-containing compounds, for the production of fuel-range hydrocarbons from *Spirulina* pyrolysis bio-oil [142]. The catalytic hydrodeoxygenation (HDO) and hydrodenitrogenation (HDN) were conducted over commercial Ru/C catalyst. The upgrading at 385 ◦C resulted in organic oil with low N and O content (<1 wt %). The selectivity of products could be controlled by varying reaction conditions. More paraffins were obtained at higher temperature (~400 ◦C), while lower temperature (350 ◦C) resulted in more phenolics. The remaining oxygen and nitrogen-containing compounds in the upgraded oil could be removed through aqueous extraction with HCl.

Guo et al. reported an approach to ge<sup>t</sup> high-quality liquid fuel from catalytic HDO of pyrolysis oil from *Chlorella* and *Nannochloropsis* by using Ni-Cu/ZrO2 bimetallic catalysts [143]. The highest HDO activity was obtained over 15.71 wt % Ni 6.29 wt % Cu supported on ZrO2, with the HDO efficiency of 82% for the upgrading of bio-oil from *Chlorella*. The Ni-Cu/ZrO2 catalyst showed excellent stability after reaction, with low sintering and coking. In addition, the heating value, viscosity, and the water content of the upgraded oil were improved. Particularly, the cetane number of the product oil from *Nannochloropsis* satisfied the standard of EN 590-09.

Overall, catalytic upgrading of pyrolytic oil from algae can successfully remove the O and N content to a low level. In addition, the remaining N and O can possibly be removed using physical adsorption or the extraction method. The resulting liquid biofuel can meet the standard of transportation fuel. However, the mentioned works did not study much on the recyclability and regeneration of the catalysts, and the mechanism of upgrading process for a better understanding of catalyst design.

### 4.2.2. Catalytic Upgrading of HTL Biocrude

Like pyrolysis, direct HTL of algae produces biocrude oil with poor quality, especially for its high O and N content, and the physical properties that do not suit the fuel standards [141]. Patel et al. explored a method for catalytic upgrading of biocrude from fast HTL of algae over Pt, Pd, Ru supported on C and Al2O3, and sulfide NiMo/Al2O3 [144]. The highest oil yield (60 wt %) and highest denitrogenation ability (2.05 wt %) were obtained over NiMo/Al2O3, but the effect of deoxygenation was poor. The oxygen content of the upgraded biocrude ranged from 1.60-6.07 wt %, while the nitrogen content ranged from 2.05-3.47 wt %. The decrease of O content resulted in the increase of HHV to 38.36-45.40 MJ·kg−1. The boiling point distribution of upgraded biocrude decreased from the gas oil fraction (271–343 ◦C) to the kerosene fraction (<271 ◦C). In addition, the abundant components in upgraded biocrude were branched alkanes and straight-chain alkanes.

Shakya et al. studied the catalytic upgrading of biocrude oil from HTL of *Nannochloropsis* [145]. Five di fferent catalysts (Pt/C, Ru/C, Ni/C, ZSM-5 and Ni/ZSM-5) were used as the upgrading catalysts. Upgrading at 300 ◦C showed higher oil yield, while higher temperature at 350 ◦C resulted in bio-oil with higher quality. The maximum upgraded oil yield was obtained over Ni/C at 350 ◦C, whereas upgraded oil with higher HHV, lower acidity, and nitrogen content was achieved over Ru/C and Pt/C. The HHVs of upgraded biocrude ranged from 40–44 MJ·kg−1, which were highly improved compared with biocrude feedstock (36.44 MJ·kg−1). The catalytic upgrading produced upgraded oil with a 65–75% decrease in nitrogen content, and 95-98% decrease in oil acidity.

Biller et al. investigated the hydroprocessing of biocrude on sulfide CoMo and NiMo catalysts from continuous HTL of Chlorella [146]. In the non-catalytic HTL step, 40 wt % yield of biocrude was obtained with 6% nitrogen, 11% oxygen, and HHV of 35 MJ·kg−1. The upgrading of biocrude over both NiMo and CoMo catalysts was conducted at 405 ◦C and 350 ◦C. The two catalysts showed similar performance on the improvement of hydroprocessed oil. The upgraded oil with the highest HHV (45.4 MJ·kg−1) was achieved over CoMo catalyst at 405 ◦C, with the oil yield of 69.4 wt %, nitrogen content of 2.7%, and oxygen content of 1.0%. Hydroprocessing at high temperature (405 ◦C) resulted in upgraded oil with higher gasoline and diesel #1 fractions. Moreover, hydrocarbons (C9-C26) were the main products in upgraded bio-oil.


**Table 7.** Catalytic upgrading of biocrude oil from thermochemical conversion.

In the previous section, catalytic HTL can improve the quality of biocrude, but the characteristics of biocrude still cannot meet the ideal standard. In comparison to one-step catalytic HTL, the upgrading of biocrude from HTL obtained biofuel with higher quality, with low boiling point, low viscosity, high HHV over 40 MJ·kg−1, and low oxygen and nitrogen content, which is more preferable than the one-step catalytic HTL.
