The Role of Catalysts in Biomass Hydrothermal Liquefaction and Biocrude Upgrading

: Hydrothermal liquefaction (HTL) of biomass is establishing itself as one of the leading technologies for the conversion of virtually any type of biomass feedstock into drop-in biofuels and renewable materials. Several catalysis strategies have been proposed for this process to increase the yields of the product (biocrude) and/or to obtain a product with better properties in light of the ﬁnal use. A number of different studies are available in the literature nowadays, where different catalysts are utilized within HTL including both homogeneous and heterogeneous approaches. Additionally, catalysis plays a major role in the upgrading of HTL biocrude into ﬁnal products, in which ﬁeld signiﬁcant developments have been observed in recent times. This review has the ambition to summarize the different available information to draw an updated overall picture of catalysis applied to HTL. The different catalysis strategies are reviewed, highlighting the speciﬁc effect of each kind of catalyst on the yields and properties of the HTL products, by comparing them with the non-catalyzed case. This allows for drawing quantitative conclusions on the actual effectiveness of each catalyst, in relation to the different biomass processed. Additionally, the pros and cons of each different catalysis approach are discussed critically, identifying new challenges and future directions of research. derivative of hexose and pentose sugars present in lignocellulosic biomass, to 1,5-pentanediol. Figure 3 shows the mechanistic pathway for the hydrogenation of the furfural formation of hydride species over the Pd–Ir–ReO x /SiO 2 catalyst. The ReO x species with modiﬁed Pd and Ir sites decreased the metal particle size and increased dispersion of Pd and Ir metals, therefore enhancing the catalytic activity for the hydrogenation of furfural into 1,5-pentanediol.


Introduction
Fossil fuels are declining rapidly due to their excessive use and increase in global industrialization. Therefore, it is necessary to develop not only renewable and environmental friendly but also economical energy sources for the production of sustainable fuels and chemicals [1,2]. Due to the absence of competition for land use and water resources, low value feedstocks such as forest residues, agricultural residues, sewage sludge, and municipal solid waste are promising and the most widely available resources for the production of green fuels. Currently, biofuels derived from organic matters or plant biomass are the only sustainable carbon source for the production of liquid fuels and well-suited to the existing transport infrastructure [3,4]. Through the implementation of effective production processes, biofuels can generate less greenhouse gases (GHGs) compared to fossil fuels [5][6][7].
Direct combustion, gasification, pyrolysis, and hydrothermal liquefaction (HTL) are the most developed thermochemical technologies, which can produce heat, syngas (H 2 and CO), and biocrude oil, respectively, through the conversion of various biomass. However, combustion, gasification, and pyrolysis are energy intensive processes due to pre-drying of feedstocks with higher water content [2,8]. Therefore, hydrothermal liquefaction (HTL) is a promising technology, which is potentially able to convert higher water content feedstocks into considerably lower oxygen content and higher calorific value biocrude oil without demanding a preliminary drying step of the biomass. In the HTL process, wet biomass is processed at high temperatures (250-450 • C) and high pressures (100-350 bar) in the presence of water. Water as a solvent presents several advantages close to its critical point such as lower viscosity and dielectric constant, leading to better solubility of organic compounds, while catalytic activity for acid-base reactions increases due to the increase in its ionic product. During HTL, the oxygen present in the feedstock is partly removed by dehydration and decarboxylation reactions, producing CO 2 , CO, and H 2 O. Despite the better quality of biocrude, oxygen content is still high and results in a highly viscous and unstable biocrude. For the replacement of transportation fuel, successful hydrotreating is required for the upgrading of biocrude [9,10]. Many informative reviews are accessible in the literature in which many research efforts have been dedicated to the production of biocrude through the HTL process and investigated the influence of various process variables such as temperature, pressure, catalyst, retention time, and biomass:water ratio on HTL products [10][11][12][13][14][15][16][17][18].
Research activities on HTL are highly focused on the use of various homogeneous and heterogeneous catalysts to enhance biocrude yield and quality simultaneously. This review paper aims to provide a dedicated review on the effects of homogeneous and heterogeneous catalysts on HTL products as well as biocrude upgrading. This study will increase awareness regarding the impact of these catalysts during the HTL conversion of various feedstocks and biocrude upgrading. Several catalysts have been employed on various biomass feedstocks. The catalyst role is highly linked with several process parameters such as temperature, retention time, and the chemical composition of feedstock and biocrude. In this review paper, alkali salts, organic acids, transition metals, metal oxides, and activated carbon were investigated in the category of homogeneous and heterogeneous catalysts, respectively. For the upgrading of biocrude, some commercial catalysts were also studied. In this work, potential solutions are also proposed to identify the challenges and research needs.
Moderate temperatures (300−350 • C) facilitate the hydrolysis of biomass, condensation, and repolymerization of reactive substances to form biocrude [30,42]. However, temperatures above the critical point (373.94 • C) improve the degree of deoxygenation and offer higher HHVs [36]. Retention time (RT) is another important parameter, longer residence time, higher than 10 min, mostly increases the biocrude yield, however, above the threshold level, biocrude yield decreases on account of higher organic loss in the form of water-soluble organics to the aqueous phase or gases by cracking reactions [14]. Xu et al. stated the possible reasons behind the leveling off or decreasing the biocrude yield at prolonging retention times, which include cracking of biocrude components to gases, repolymerization to form char, and condensation to aqueous products [43]. Malins et al. used a catalyst (FeSO 4 ) for sewage sludge at 300 • C in an autoclave under reaction times of 10 to 100 mins and reported a maximum biocrude yield of 48% at 40 min. On the other hand, prolonged RTs enhance the gaseous products, and the biocrude quality is improved through intermingling tar substances with biocrude that could positively affect the HHV [26]. Seehar et al. derived a different conclusion by conducting a catalytic (K 2 CO 3 ) reaction time study on eucalyptus at 350 • C from 10 min to 25 mins and reported that 15 min is the best reaction time for the eucalyptus conversion [27]. Another study indicated that 10 min is the optimum RT for the HTL of lignocellulosic biomass (Cunninghamia lanceolata) at 320 • C [44].
The reactor system also influences the overall energy recovery of the HTL system, typically, longer RTs are selected for autoclave-based reactors that give slightly lower yields due to lower heating rates [23,24,26]. Alternatively, improved biocrude productivity has been observed by many studies adopting micro-batch reactors. Even so, shorter RTs in the range of 10 to 20 min are declared as ideal for biomass liquefaction in all micro-batch reactor-oriented systems [14,27,28,45].

Methodology
Understanding the influence of the catalyst on the HTL products may be complicated. Indeed, catalysts affect several outputs of the process, resulting in variations in both the quantity and quality of the produced biocrude. Indeed, an important aim of catalysis is that of maximizing the yield of biocrude, reducing the production of other reaction products such as char, gases, and water-soluble compounds. Maximizing the amount of produced biocrude is a fundamental point as it determines the economy of the process.
On the other hand, another scope of catalysis is that of improving the "quality" of biocrude by obtaining a product that presents a reduced amount of oxygen, nitrogen, and other heteroatoms, which could therefore be upgraded with reduced efforts. This can be quantified by the higher heating value (HHV): an increased value of HHV is directly related to a reduced amount of heteroatoms.
These two tendencies (i.e., yield improvement and HHV improvement) do not necessarily take place at the same time, which makes it difficult to evaluate whether catalysis is advantageous or not. However, it could be stated that a successful catalytic process should be able to increase the share of biomass energy that is transferred into biocrude. This can be quantified by the so-called "energy ratio" (ER), defined as: Bi et al. [70] processed pretreated sorghum bagasse at 300 to 350 • C with alkali (K 2 CO 3 , KOH) and compared it with different heterogeneous catalysts (Ni/Si-Al, Ni 2 P, and zeolite). K 2 CO 3 turned out to be the most efficient catalyst, being able to improve the biocrude yield to 61%: three times higher compared to the non-catalytic run [71]. In alkaline conditions, the macromolecules of lignocellulosic biomass (i.e., cellulose, hemicellulose and lignin) undergo different chemical reactions. The first is hydrolysis, which produces monomers and oligomers such as glucose, guaiacol, fructose, etc. These monomers are further broken down into lower molecular weight intermediates, which may become part of the biocrude or aqueous phase. During HTL, free radicals are formed. If the hydrogen content is not sufficient enough to stabilize the free radicals, then these free radicals tend to associate with each other and form char (e.g., via repolymerization of 5-hydroxymethyl-furfural (HMF)) [70]. Detailed information on the effects of different homogenous catalysts on biocrude properties is given in Table 1. Conti et al. processed cow manure, swine manure, and fish sludge at temperatures of 350 and 400 • C with potassium salts. The negative effect of the catalyst was noticed on biocrude yield in the range of −4 to −8% from all the feedstocks at both temperatures. Nonetheless, catalyst promoted deoxygenation reactions and higher carbon contents were transferred to biocrude at 400 • C [36]. At supercritical conditions, it was noticed that a higher amount of gase, that resulting in a higher removal of oxygen in the form of CO 2 . This infers that the release of oxygen with a loss of organic carbon to the gas phase ultimately affects the magnitude of biocrude yield [36].
For high protein-containing feedstocks such as sewage sludge and microalgae, the catalytic effect of the alkalis depends upon the amount of carbohydrates and ash content [28,48,72,73]. Sometimes the intrinsic content of ash-alkaline inorganic compounds (carbonates, oxides, etc.) in the feedstock may deactivate the employed catalyst, after becoming dissolved in the water phase [65]. However, this intrinsic ash content may exert a catalytic activity by itself. This was observed by Suzuki et al., who performed the HTL of sewage sludge with the addition of sodium carbonate (Na 2 CO 3 ) They concluded that the addition of Na 2 CO 3 could play a positive role in enhancing biocrude yield in the case only a low amount of ash is present in feedstock [22] Indeed, catalyst addition to high ash-containing feedstock was unnecessary, as HTL yields were already high due to the intrinsic inorganic (mostly Ca) content in sewage sludge. On the other hand, Shah et al. [28] reported a positive impact of K 2 CO 3 on both biocrude yield (+3 to +5%) and HHV from high ash-containing (ash: 40%) sewage sludge with a high fraction of carbohydrates (48% daf) at 350 and 400 • C. This implies that, irrespective of the intrinsic ash content, the addition of potassium carbonate plays an active role in the conversion of carbohydrates, which improves biocrude productivity. Therefore, the addition of an alkali catalyst is still meaningful when a high amount of carbohydrates is present in the feedstock. However, in the same study [28], lower nitrogen was recorded at 400 • C in the catalytic run (−1.43% by value, compared with nitrogen in the non-catalyzed biocrude). The carbonates generated by the dissolution of K 2 CO 3 react with ammonium ions derived from amino acids (via deamination) and form ammonium carbonate, which can easily be dissolved in the aqueous phase [74,75]. This phenomenon cuts off the reaction pathway for the formation of amides and induces lower nitrogen in catalytic biocrude, as given in Equations (4) and (5) [28].
Chen et al. utilized a calcium carbonate with high ash containing microalgae, and experienced an inverse relationship between nitrogen content in the biocrude and concentration of calcium carbonate in the feedstock [76]. However, the actual mechanism through which these carbonates react with N-containing compounds within HTL requires further research [77]. Bio-pulp from food waste was processed at 350 and 400 • C with and without K 2 CO 3 catalysts. The best performance was achieved at sub-critical conditions (ER: 61.7% and biocrude yield: 36.6%), better than the results obtained at 400 • C. Subsequently, recycling of concentrated organics from the aqueous phase was applied and this enhanced the biocrude yield up to 49.3% after four recirculation cycles [62].
Aquatic biomasses, especially micro-and macroalgae, have been widely used for a long time as promising candidates for biofuel production [34,37,55,64,[78][79][80]. Generally, low or moderate temperatures (280 to 350 • C) are preferred for the liquefaction of microalgae [30,42]. Numerous studies show that alkali catalysts negatively affect the conversion of microalgae by depleting the yield and quality of biocrude. The main factor behind the inefficiency of the alkali catalyst is that as these are incompatible for the cleavage of peptide linkage bonds of protein-containing biomass [29,30,42,81]. Shakya et al. processed microalgae (Pavlova, Isochrysis, and Nannochloropsis) containing variable range of carbohydrate contents in subcritical water. Biocrude yields followed the percentage of carbohydrates (higher to lower) in the feedstock, according to the order: Pavlova > Isochrysis > Nannochloropsis [30]. It is reported that the Maillard reaction plays a vital role during the conversion of high protein-containing feedstock, which occurs between the amino acids and polysaccharides [74].
In order to investigate lipid conversion, researchers have often used model compounds. Ding et al. liquefied model compounds, particularly soya bean oil (reference component for triglycerides or lipids), protein, starch, and cellulose with alkali phosphates (KH 2 PO 4 , K 2 HPO 4, and K 3 PO 4 ), and alkalis (Na 2 CO 3 , and KOH). Biocrude yield was increased more than two times by using K 2 HPO 4 and K 3 PO 4 from starch, and cellulose at 320 • C, higher than the Na 2 CO 3 . The HHVs were not enhanced significantly by alkali catalysts [60].
It is a fact that biocrude yield and quality both determine the efficiency of the catalyst. Therefore, the energy recovery must be taken as a decisive value to rate the effectiveness of the catalyst for the biomass conversion. Figure 1 depicts that alkali catalysts increase energy recovery in all lignocellulosic feedstocks, whereas alkali salts along with organic acids show a reverse or negative effect on energy recovery for high protein-containing feedstocks.

Acid Catalysts
Acid catalysts such as formic acid (HCOOH), acetic acid (CH3COOH), hydrochloric acid (HCl), and sulfuric acid (H2SO4) have been employed in many cases of biomass conversion [29,34,42,50]. Acid catalysts are not considered as promising as alkali salts, especially for lignocellulosic biomass. For example, biomass (desert shrubs) was treated in an acetic acid medium that increased the biocrude yield from 26 to 30% with no effect on calorific values [49]. Yin et al. [70] presented an interesting study on cellulose under acidic, neutral, and alkaline conditions by developing variable pH values of 3, 7, and 14 from HCl, water, and NaOH, respectively, at temperatures of 275-320 °C. The reaction mechanism of the conversion of cellulose was different according to the pH of the solution. NaOH directed the conversion process towards the formation of C2-5 carboxylic acids. In contrast, 5-HMF was found to be the most dominant compound in acidic conditions, which tends to polymerize and resulted in higher char formation. Neutral conditions showed both 5-HMF and carboxylic acids due to the self-dissociation of H2O to H + and OH − at high temperatures. The conversion pathways of cellulose under different conditions are illustrated in Figure 2.

Acid Catalysts
Acid catalysts such as formic acid (HCOOH), acetic acid (CH 3 COOH), hydrochloric acid (HCl), and sulfuric acid (H 2 SO 4 ) have been employed in many cases of biomass conversion [29,34,42,50]. Acid catalysts are not considered as promising as alkali salts, especially for lignocellulosic biomass. For example, biomass (desert shrubs) was treated in an acetic acid medium that increased the biocrude yield from 26 to 30% with no effect on calorific values [49]. Yin et al. [70] presented an interesting study on cellulose under acidic, neutral, and alkaline conditions by developing variable pH values of 3, 7, and 14 from HCl, water, and NaOH, respectively, at temperatures of 275-320 • C. The reaction mechanism of the conversion of cellulose was different according to the pH of the solution. NaOH directed the conversion process towards the formation of C 2-5 carboxylic acids. In contrast, 5-HMF was found to be the most dominant compound in acidic conditions, which tends to polymerize and resulted in higher char formation. Neutral conditions showed both 5-HMF and carboxylic acids due to the self-dissociation of H 2 O to H + and OH − at high temperatures. The conversion pathways of cellulose under different conditions are illustrated in Figure 2. Yang et al. investigated the effect of acid catalysts (H2SO4 and CH3COOH) on macroalgae (Enteromorpha prolifera) and observed negative impacts on biocrude.. However, acid catalysts considerably increased the concentration of ketones in biocrudes [50]. Similarly, Shah et al. reported the insignificant effect of acetic acid on biocrude yield and energy recovery from sewage sludge, also finding that a slightly lower amount of carbon was transferred to the biocrude compared to non-catalytic samples [50].
Biller et al. performed a comprehensive catalytic study by using catalysts CH3COOH and HCOOH on microalgae (high protein-containing Spirulina and high lipid-containing Chlorella) and compared those with alkali catalysts (KOH and Na2CO3). The results revealed that both acids improved the biocrude yield, with a slightly higher amount of gases. During HTL, the added acids were found to be consumed, suggesting their behavior more as reactants than as catalysts. Improved biocrude HHVs were determined with alkali catalysts with a difference of 2 to 3 MJ/kg compared to acid catalysts [42]. In another study, the same authors explored the catalytic action of formic acid and alkali carbonate on glucose and soya protein. For soya protein, the negative effect of formic acid was noticed on energy recovery, while greater energy recoveries were obtained from glucose under both alkali and formic acid mediums [29]. It was also mentioned by Kruse et al. that lower production of N-heterocyclic compounds in the biocrude, occurring in an acidic environment, resulted in the production of higher amount of char [82].
Hu et al. [83] treated microalgae (Chlorella vulgaris) under acidic and alkaline conditions with HCOOH and Na2CO3 at 275 °C for 50 mins. The biocrude yield obtained from Na2CO3 (11%) was lower than the non-catalyzed conditions, while HCOOH led to almost the same biocrude yield from HTL (29%). Aqueous phase recycling with alkali salt improved the biocrude yield to a larger extent. The activation of Na2CO3 with aqueous phase recycling might be occurred due to a decrease in solubility of the organic compounds due to the ''salting out" effect, which drives them into the biocrude stream, thus contributing to an increase in biocrude yields. Yang et al. investigated the effect of acid catalysts (H 2 SO 4 and CH 3 COOH) on macroalgae (Enteromorpha prolifera) and observed negative impacts on biocrude.. However, acid catalysts considerably increased the concentration of ketones in biocrudes [50]. Similarly, Shah et al. reported the insignificant effect of acetic acid on biocrude yield and energy recovery from sewage sludge, also finding that a slightly lower amount of carbon was transferred to the biocrude compared to non-catalytic samples [50].
Biller et al. performed a comprehensive catalytic study by using catalysts CH 3 COOH and HCOOH on microalgae (high protein-containing Spirulina and high lipid-containing Chlorella) and compared those with alkali catalysts (KOH and Na 2 CO 3 ). The results revealed that both acids improved the biocrude yield, with a slightly higher amount of gases. During HTL, the added acids were found to be consumed, suggesting their behavior more as reactants than as catalysts. Improved biocrude HHVs were determined with alkali catalysts with a difference of 2 to 3 MJ/kg compared to acid catalysts [42]. In another study, the same authors explored the catalytic action of formic acid and alkali carbonate on glucose and soya protein. For soya protein, the negative effect of formic acid was noticed on energy recovery, while greater energy recoveries were obtained from glucose under both alkali and formic acid mediums [29]. It was also mentioned by Kruse et al. that lower production of N-heterocyclic compounds in the biocrude, occurring in an acidic environment, resulted in the production of higher amount of char [82].
Hu et al. [83] treated microalgae (Chlorella vulgaris) under acidic and alkaline conditions with HCOOH and Na 2 CO 3 at 275 • C for 50 mins. The biocrude yield obtained from Na 2 CO 3 (11%) was lower than the non-catalyzed conditions, while HCOOH led to almost the same biocrude yield from HTL (29%). Aqueous phase recycling with alkali salt improved the biocrude yield to a larger extent. The activation of Na 2 CO 3 with aqueous phase recycling might be occurred due to a decrease in solubility of the organic compounds due to the "salting out" effect, which drives them into the biocrude stream, thus contributing to an increase in biocrude yields.

Further Studies on Homogenous Catalysts
A number of HTL studies concerning homogenous catalysts are present in the literature, some of which could not be included in the discussion. In Table 2, the major findings from some recently published catalytic HTL studies are listed, in order to provide a more complete picture of the state-of-the-art. Kraft lignin 280-350 K 2 CO 3 The catalyst increased the yield of liquid products and reduced char formation. The catalyst improved the yield of monomeric aromatics. [87] Oil palm shell 210-330 K 2 CO 3 , Na 2 CO 3 , and NaOH K 2 CO 3 , Na 2 CO 3 , and NaOH showed maximum liquid product yields of 48, 47, and 53, respectively. [88] Bamboo chopsticks 290-380 K 2 CO 3 At 290 • C, biocrude yield improved up to 21.2 wt% compared to 3.8 wt% in a non-catalytic run. HHV increased to 31.6 MJ/kg. [89] Blackcurrant pomace 290-335 NaOH The catalyst increased bio-oil yield and reduced char formation. [90] Water hyacinth 250-300 KOH and K 2 CO 3 The use of alkaline catalysts increased bio-oil yield. Maximum bio-oil yield (23 wt%) was observed with KOH at 280 • C with 15 min of residence time. [56] Pretreated sorghum bagasse 300 and 350 K 2 CO 3 , KOH, formic acid, Ni/Si-Al, Ni 2 P and zeolite K 2 CO 3 , KOH, and Ni/Si-Al were identified as the best catalysts, which led to biocrude yields of 61.8%, 42.3%, and 45.0% at 300 • C, respectively. K 2 CO 3 resulted in the highest value for HHV 33.1 MJ/kg. [71] Microalga (Spirulina platensis) 300-350 Na 2 CO 3 , Ca 3 (PO 4 ) 2 , and NiO Na 2 CO 3 increased biocrude yield 52%, higher than non-catalytic (29%) conditions. NiO and Ca 3 (PO 4 ) 2 increased yield of gaseous products. [91] Pulp and paper sludge 250-380 K 2 CO 3 , Ca(OH) 2 , and Ba(OH) 2 , K 2 CO 3 significantly accelerated the organic conversion resulting in a lower amount of char. The alkali-earth catalysts, Ca(OH) 2 and Ba(OH) 2 shifted a greater amount of organics to the water phase. [

Heterogeneous Catalysts
Heterogeneous catalysts for the catalytic thermochemical conversion of different biomass have been used widely due to their unique advantages such as high catalytic activity, selectivity, recyclability, and reusability over homogeneous catalysts. Furthermore, properties such as tunable structure, shape selectivity, and sustained nature of heterogeneous catalysts are important considerations for their employability in industrial-scale biomass conversion processes. The development of high-performance heterogeneous catalysts with substantial amounts of catalytic active sites is important in achieving higher reaction rates, especially in cascade biomass conversion processes. A few reports have discussed the role of heterogeneous catalysts in the improvement of yield and the properties of biocrude obtained through the HTL of different biomass.
Among the different types of heterogeneous catalysts, three broad categories such as redox metals or acidic metal oxides (e.g., CeO 2 , ZrO 2 , Fe and Cu based), noble metals (e.g., Pd, Pt, Ru based), and non-noble metals (e.g., all other transition metals) have been used widely for the catalytic HTL of different biomass. Among the above-mentioned categories, catalysts based on redox metals possess unique abilities for easy regeneration due to their fast oxidation-reduction kinetics. Furthermore, these catalysts are suggested to promote the formation and hydrogenation of light organic compounds in the water-soluble fraction [94]. On the other hand, noble metal-based catalysts possess high catalytic activity toward the reduction in a wide range of oxygen-containing compounds to hydrocarbons present in the varieties of biomass [95]; whereas the heterogeneous catalysts containing base transition metals have been proven to be proficient in improving the quality of the bio-oil through the deoxygenation and denitrification of biomass containing oxygen and nitrogen compounds [52]. The main categories of biomass studied for the catalytic HTL are lignocellulosics, algae as well as lipid and protein-containing feedstocks such as sewage sludge and food waste. The nature of biomass feedstock also influences the catalytic action of heterogeneous catalysts, thus overall, affects the bio-oil yield and quality due to different biomass compositions. Almost all biomass materials include inorganic nutrients present in the form of precipitates or salts of carboxylic and/or phenolic groups, which may accumulate on the catalyst surface and affect catalytic activity [96]. Different categories of heterogeneous catalysts used previously for the catalytic HTL of different biomass are listed in Table 3 and discussed in detail in the following sections.

Redox Catalysts
Redox catalysis involves the most fundamental chemical reactions and transformations in which the catalyst undergoes both oxidation and reduction reactions (i.e., loss and gain of electrons), resulting in a change in its oxidation state. Among the vast majority of redox catalysts, the oxides and salts of redox-active transition metals such as iron (Fe), copper (Cu), zirconium (Zr), titanium (Ti), magnesium (Mg), and a few lanthanides (e.g., La and Ce) have been used repeatedly for the catalytic HTL of different biomass due to their redox-active catalytic sites, easy regeneration as well as high activity and selectivity.
Redox catalysts, especially Fe-based, have an affinity for producing in situ hydrogen and for the subsequent hydrogenation of reactive chemical species when used in aqueous media (i.e., water) [102]. Previously, the effect of the addition of 10% iron in different oxidation states (Fe, Fe 3 O 4 , and Fe 2 O 3 ) as redox catalysts was observed in the HTL of oak wood at different temperatures in a range of 260-320 • C [64]. The increment in HTL biocrude yields was about 30% and 13% with Fe and Fe 3 O 4 catalysts, respectively, in comparison to that obtained without catalysts. However, no effect of the Fe 2 O 3 catalyst on the bio-oil yield was observed. Furthermore, the enhanced H/C ratio of bio-oil of about 15% with respect to the non-catalytic HTL run indicated the hydrogenation of organic compounds in biomass with in situ generated hydrogen from water through the oxidation of Fe to Fe 3 O 4 .
In another report, Fe(0) was used as a redox agent for in situ hydrogen generation in catalytic HTL of lignocellulosic palm oil fruit bunch, producing bio-oil containing higher yields of water-soluble and water-insoluble fractions [103]. Upon increasing the H 2 O/biomass ratio from 0:1 to 5:1 in the presence of Fe, the HTL bio-oil yield was significantly enhanced from 25% to 79%, whereas the yield of solid residue was decreased. A similar trend in the yield of gaseous products was observed upon the addition of Fe catalyst. After completion of the HTL test, the oxidized catalyst mixed with char was regenerated by heating 1000 • C at a rate of 10 • C/min under a N 2 (100 mL/min) atmosphere for 2 h. The resulting reduced zerovalent Fe catalyst was reused in the HTL experiment of the palm oil fruit bunch.
Recently, Xu et al. explored the role of metallic Fe (10 wt%) in the production of in situ hydrogen during the catalytic HTL of cornstalk biomass through the controlled experiments carried out with Fe 3 O 4 [104]. HTL of cornstalk biomass was conducted in ethanol-water solvent mixture (50/50, 30 v/v) at 300 • C for 30 min in a 100 mL autoclave reactor. Fe led to the hydrogenation and deoxygenation of oxygenates present in biomass, consequently, improvement in the biocrude yield and quality in terms of higher H/C and HHV was noticed. However, the Fe 3 O 4 showed a negligible catalytic effect in the HTL process.
The role of the heterogeneous CeZrO x catalyst containing the redox Ce metal for promoting the in situ deoxygenation of water-soluble compounds to oil-soluble hydrocarbon products during catalytic HTL of food waste was investigated [105]. It was found that liquefaction of food waste at 300 • C for 1 h in the presence of the CeZrO x catalyst improved both biocrude HHV and energy recovery compared to non-catalytic experiments. Comparatively, the HTL aqueous phase and biocrude obtained in the presence of CeZrO x contained around half of the total organic carbon and less water-soluble organics, respectively. The catalyst was sustained after approximately 16 h of hydrothermal processing of food waste at 300 • C, which was reused three times for the conversion of model compounds.
The effect of a monoclinic ZrO 2 catalyst on the yield and properties of HTL biocrude obtained from dried distiller grains with solubles (DDGS) at a temperature of 300 • C, 250 bar pressure, and 15 min in a stop-flow reactor was investigated [106]. The detailed investigation of biocrude yield and its characteristic properties such as ash content, elemental composition, heating value, and chemical composition revealed the poor catalytic activity of ZrO 2 for the HTL of biomass.
Zhang et al. investigated the role of Ni-Tm/TiO 2 catalysts for the removal of heteroatoms and saw an increase in biocrude yield in the HTL of human feces [99]. The yield of HTL biocrude, obtained at a temperature of 330 • C and a reaction time of 30 min, increased from 41.57% to 46.09% upon the addition of the Ni-Tm/TiO 2 catalyst. Furthermore, the high conversion rate of human feces (89.61%) and high-energy recovery of the resulting biocrude (87.42%) showed the potential of Ni-Tm/TiO 2 for the catalytic HTL of high heteroatoms containing biomass. The synergistic effect of Ni led to superior catalytic activity of the Ni-Tm/TiO 2 catalyst for the desulfurization of biocrude and resulted in 22.58% reduction in sulfur contents.
Catalytic HTL of microalga Ulva prolifera was carried out over various metal oxides such as MgO, CaO, Al 2 O 3 , ZrO 2 , and CeO 2 and metal salts (MgCl 2 , FeCl 3 , and CuCl 2 ) as catalysts at different operating temperatures (260, 280, and 300 • C) using water, methanol, and ethanol as solvents [97]. Among the above-mentioned metal oxides, the highest biocrude yield of 50.6 wt% containing a higher amount of ester groups was achieved using the MgO catalyst in ethanol solvent. The regeneration of the MgO catalyst was carried out by treating the recovered spent catalyst from the bio-char at 550 • C for 3 h under an oxygen atmosphere, which was then reused three times in the liquefaction reaction with macroalgae under both ethanol and water solvents. This study verifies the high reproduction ability of the MgO catalyst for the HTL of macroalgae.

Noble Metal Catalysts
Noble metals are considered as highly valuable metals because of their low abundance and high corrosion resistant properties. Noble metal catalysts have been found to be effective in the key catalytic reaction steps such as hydrogenation, hydrodeoxygenation, dehydration, and oxidation for the formation of high-value products from different biomass [107]. Previously, a heterogeneous Pd-Ir-ReO x /SiO 2 catalyst containing ReO x deposited on noble Pd and Ir metal particles was used for the catalytic hydrogenation of furfural, a main derivative of hexose and pentose sugars present in lignocellulosic biomass, to 1,5-pentanediol. Figure 3 shows the mechanistic pathway for the hydrogenation of the furfural formation of hydride species over the Pd-Ir-ReO x /SiO 2 catalyst. The ReO x species with modified Pd and Ir sites decreased the metal particle size and increased dispersion of Pd and Ir metals, therefore enhancing the catalytic activity for the hydrogenation of furfural into 1,5-pentanediol.
high conversion rate of human feces (89.61%) and high-energy recovery of the resulting biocrude (87.42%) showed the potential of Ni-Tm/TiO2 for the catalytic HTL of high heteroatoms containing biomass. The synergistic effect of Ni led to superior catalytic activity of the Ni-Tm/TiO2 catalyst for the desulfurization of biocrude and resulted in 22.58% reduction in sulfur contents.
Catalytic HTL of microalga Ulva prolifera was carried out over various metal oxides such as MgO, CaO, Al2O3, ZrO2, and CeO2 and metal salts (MgCl2, FeCl3, and CuCl2) as catalysts at different operating temperatures (260, 280, and 300 °C) using water, methanol, and ethanol as solvents [97]. Among the above-mentioned metal oxides, the highest biocrude yield of 50.6 wt% containing a higher amount of ester groups was achieved using the MgO catalyst in ethanol solvent. The regeneration of the MgO catalyst was carried out by treating the recovered spent catalyst from the bio-char at 550 °C for 3 h under an oxygen atmosphere, which was then reused three times in the liquefaction reaction with macroalgae under both ethanol and water solvents. This study verifies the high reproduction ability of the MgO catalyst for the HTL of macroalgae.

Noble Metal Catalysts
Noble metals are considered as highly valuable metals because of their low abundance and high corrosion resistant properties. Noble metal catalysts have been found to be effective in the key catalytic reaction steps such as hydrogenation, hydrodeoxygenation, dehydration, and oxidation for the formation of high-value products from different biomass [107]. Previously, a heterogeneous Pd-Ir-ReOx/SiO2 catalyst containing ReOx deposited on noble Pd and Ir metal particles was used for the catalytic hydrogenation of furfural, a main derivative of hexose and pentose sugars present in lignocellulosic biomass, to 1,5-pentanediol. Figure 3 shows the mechanistic pathway for the hydrogenation of the furfural formation of hydride species over the Pd-Ir-ReOx/SiO2 catalyst. The ReOx species with modified Pd and Ir sites decreased the metal particle size and increased dispersion of Pd and Ir metals, therefore enhancing the catalytic activity for the hydrogenation of furfural into 1,5-pentanediol. Despite the high cost of noble metal-based catalysts, a few reports exist that have critically assessed and summarized the contribution of noble metal catalysts for catalytic HTL of cellulosic, microalgae, and soy protein-based biomass feedstocks. Noble metalbased catalysts mainly encounter problems of limited reserve and poor stability; therefore, support materials in heterogeneous catalysis are critical for the stability and efficiency of these catalysts [109]. Duan et al. used a variety of carbon-supported noble metal-based heterogeneous catalysts such as Pt/C, Pd/C, and Ru/C for the catalytic HTL of Nannochloropsis sp. microalga at 350 • C temperature in the absence and presence of H 2 [95]. The above-supported catalysts were selected due to their abilities for reduction, hydrogenation, and deoxygenation of heteroatom (e.g., S, O, and N) containing compounds. Normally, HTL bio-oil produced from microalgae is an enormously complex mixture of organic compounds with different functional groups and typically contains about 30-40% of light oil and 60-70% of heavy asphaltene-based constituents [110]. It was observed that all the catalysts resulted in higher HTL biocrude yields, but none of them had any influence on the HHV and elemental composition of the biocrude in the absence of H 2 . Carbon-supported Ru catalyst formed a higher amount of methane gas during the catalytic HTL process. Interestingly, the yield of gaseous products was decreased by applying an initial H 2 pressure or, in general, by running the system at higher pressures.
Catalytic HTL of a model protein (i.e., soy protein) was carried out using noble metals-based catalysts such as Pd, Pt, and Ru supported on porous carbon and Al 2 O 3 support [99,111]. The ideal conditions for the catalytic HTL of soy protein in the presence of Ru/C catalyst were found to be for 2 h retention time at a temperature of 350 • C with a 50 wt% loading of the catalyst. However, none of the catalysts had any significant influence on the biocrude yield, although Ru/C (50 wt%) led to less than half of the heteroatom content (including sulfur and nitrogen) of the biocrude and a 16% increase in higher heating value in comparison to the heating value of biocrude obtained in the non-catalytic test.
A metal co-catalyst system results in the synergistic acceleration of the catalytic activity of the main catalyst and lowers the activation energy of a chemical reaction by promoting charge separation [112]. Hirano et al. used Fe metal as a co-catalyst to accelerate the catalytic activity of noble metal-based Pd/Al 2 O 3 , Pt/Al 2 O 3 , and Ru/Al 2 O 3 catalysts for the selective hydrogenation of oxygenates during the catalytic HTL of lignocellulosic feedstock [94]. In comparison to the Fe-assisted catalytic HTL of cellulose, the presence of the Fe and Pd/Al 2 O 3 catalyst led to an approximately 8-9% increase in yield of a watersoluble organic fraction under the same conditions. Comparatively, a lower O/C ratio and a higher effective H/C ratio (H/C eff = 1.33) of the bio-oil obtained in the presence of Fe and Pd/Al 2 O 3 catalyst were achieved than that obtained during Fe-assisted HTL. The enhanced deoxygenation of oxygen-containing compounds present in cellulose was attributed to the synergistic acceleration of the catalytic activity of the Pd/Al 2 O 3 catalyst by metallic Fe. It was confirmed that in situ generated H 2 by Fe from water under hydrothermal conditions decreased the reducing capacity of Fe in the case of Fe-assisted HTL. Therefore, no effect on the yield of HTL bio-oil was observed. The added noble metal catalyst activates gaseous H 2 to form hydrogenated compounds in the water-soluble organic fraction, which would not be possible with only Fe as a catalyst. However, the efficiency of the catalytic system needs to be improved for biomass HTL to be economically feasible and a plausible HTL mechanism entails to be elucidated. Figure 4 shows a possible mechanism explaining the catalytic action of hydrogenation catalysts on the production of different HTL products [94].  Reproduced from [94] with permission from Elsevier, © Elsevier 2020.

Non-Noble Metal Catalysts
Apart from noble metal-based catalysts, the usage of non-noble metal catalysts in catalytic HTL has attracted significant research interest due to their high catalytic activity in bond cleavage, easy recovery, and low cost. Among the non-noble metals, nickel, cobalt, and molybdenum proved to be active in deoxygenation and denitrogenation [12,64].
Recently, the effect of carbon nanotube-supported transition metals such as Co and Ni was evaluated for the catalytic HTL of Spirulina microalgae in the presence of water, methanol, and ethanol solvents [98]. Among all the catalysts, Co/CNT led to a significant enhancement of 13 wt% in the biocrude yield in comparison to the non-catalytic test. The highest biocrude yield was found to be 43.6% with Co/CNT catalysts whereas the minimum yield in the non-catalytic test was 26.8 wt%. The significant enhancement in microalgae conversion and resulting biocrude yield was due to the formation of hydrogen radicals influenced by the Co/CNT catalyst. It was observed that Co/CNT participated in the reaction mechanism and led to the selective production of heptadecane over other catalysts. However, the gas yield was higher (27.3 wt%) in the case of Ni/CNT than that observed in the presence of Co/CNT.
Catalytic HTL of sewage sludge and Chlorella Vulgaris was carried out at 325 °C for a 30 min holding time to demonstrate the influence of NiMo/Al2O3 and CoMo/Al2O3 catalysts on the yield and quality of biocrude [51]. However, both catalysts did not show any effect on the biocrude yield but showed high catalytic activity to enhance its quality significantly. The O/C molar ratio of biocrude was decreased from 0.42 in the non-catalytic run to 0.21 and 0.19 in the presence of CoMo and NiMo based catalysts, respectively, whereas the biocrude yield was decreased by 3% with NiMo and increased by 3% with CoMo. The decrease in biocrude yield in the case of NiMo compared to the non-catalytic test was probably due to the formation of light water-soluble compounds. Quantitative desulfurization and lower oxygen contents in HTL biocrude produced from C. vulgaris using NiMo and CoMo catalysts were observed, whereas both catalysts were also active in decreasing the O/C and S/C ratio of biocrude obtained from sewage sludge. Catalytic HTL of oak wood was carried out using commercial Ni powder and nanostructured Ni

Non-Noble Metal Catalysts
Apart from noble metal-based catalysts, the usage of non-noble metal catalysts in catalytic HTL has attracted significant research interest due to their high catalytic activity in bond cleavage, easy recovery, and low cost. Among the non-noble metals, nickel, cobalt, and molybdenum proved to be active in deoxygenation and denitrogenation [12,64].
Recently, the effect of carbon nanotube-supported transition metals such as Co and Ni was evaluated for the catalytic HTL of Spirulina microalgae in the presence of water, methanol, and ethanol solvents [98]. Among all the catalysts, Co/CNT led to a significant enhancement of 13 wt% in the biocrude yield in comparison to the non-catalytic test. The highest biocrude yield was found to be 43.6% with Co/CNT catalysts whereas the minimum yield in the non-catalytic test was 26.8 wt%. The significant enhancement in microalgae conversion and resulting biocrude yield was due to the formation of hydrogen radicals influenced by the Co/CNT catalyst. It was observed that Co/CNT participated in the reaction mechanism and led to the selective production of heptadecane over other catalysts. However, the gas yield was higher (27.3 wt%) in the case of Ni/CNT than that observed in the presence of Co/CNT. Catalytic HTL of sewage sludge and Chlorella Vulgaris was carried out at 325 • C for a 30 min holding time to demonstrate the influence of NiMo/Al 2 O 3 and CoMo/Al 2 O 3 catalysts on the yield and quality of biocrude [51]. However, both catalysts did not show any effect on the biocrude yield but showed high catalytic activity to enhance its quality significantly. The O/C molar ratio of biocrude was decreased from 0.42 in the non-catalytic run to 0.21 and 0.19 in the presence of CoMo and NiMo based catalysts, respectively, whereas the biocrude yield was decreased by 3% with NiMo and increased by 3% with CoMo. The decrease in biocrude yield in the case of NiMo compared to the non-catalytic test was probably due to the formation of light water-soluble compounds. Quantitative desulfurization and lower oxygen contents in HTL biocrude produced from C. vulgaris using NiMo and CoMo catalysts were observed, whereas both catalysts were also active in decreasing the O/C and S/C ratio of biocrude obtained from sewage sludge. Catalytic HTL of oak wood was carried out using commercial Ni powder and nanostructured Ni particles at different temperatures (280-330 • C), reaction times (10-30 min), and catalyst loadings (10-50 wt%), in order to enhance the yield and quality of the biocrude [100]. Flower-like morphology of the nanostructured Ni catalyst resulted in a higher biocrude yield of 36.63% and inhibited char formation, which was attributed to the Ni induced hydrogenation of biomass. However, both catalysts showed high catalytic activity to improve biocrude quality in terms of increasing HHV and H/C ratio. The magnetic characteristics of both catalysts helped in easy recovery with an average recovery rate of 90%.
For a broader view, the change in the energy recovery of the biocrude via heterogeneous catalysts from different feedstocks is illustrated in Figure 5. loadings (10-50 wt%), in order to enhance the yield and quality of the biocrude [100]. Flower-like morphology of the nanostructured Ni catalyst resulted in a higher biocrude yield of 36.63% and inhibited char formation, which was attributed to the Ni induced hydrogenation of biomass. However, both catalysts showed high catalytic activity to improve biocrude quality in terms of increasing HHV and H/C ratio. The magnetic characteristics of both catalysts helped in easy recovery with an average recovery rate of 90%. For a broader view, the change in the energy recovery of the biocrude via heterogeneous catalysts from different feedstocks is illustrated in Figure 5. The deactivation of the catalyst mainly occurs due to the blocking of catalyst cavities via coking. The recovery of heterogeneous catalysts is very important for the sustainability of the HTL process. The recovery of heterogeneous catalysts is a tiresome process. Saber et al. utilized nano-Ni/SiO2 and nano-zeolites for the HTL of microalgae and experienced a maximum of three times use for the HTL cycling process, as the nickel catalyst was recovered back about 62 and 18% for the first and second recycle, respectively, and complete loss was recorded at the third cycle [113]. In conclusion, the usage of robust catalysts with efficient recycling/recovery approaches should be adopted to circumvent the catalytic deactivation, which could escalate the standards of HTL processing.

Further Studies on Heterogeneous Catalysts
Similar to homogenous catalysts (Section 3.3), many other studies are also available on heterogeneous catalysts. An account on them is presented in Table 4, where the most relevant aspects of some of the most recent studies are presented. The deactivation of the catalyst mainly occurs due to the blocking of catalyst cavities via coking. The recovery of heterogeneous catalysts is very important for the sustainability of the HTL process. The recovery of heterogeneous catalysts is a tiresome process. Saber et al. utilized nano-Ni/SiO 2 and nano-zeolites for the HTL of microalgae and experienced a maximum of three times use for the HTL cycling process, as the nickel catalyst was recovered back about 62 and 18% for the first and second recycle, respectively, and complete loss was recorded at the third cycle [113]. In conclusion, the usage of robust catalysts with efficient recycling/recovery approaches should be adopted to circumvent the catalytic deactivation, which could escalate the standards of HTL processing.

Further Studies on Heterogeneous Catalysts
Similar to homogenous catalysts (Section 3.3), many other studies are also available on heterogeneous catalysts. An account on them is presented in Table 4, where the most relevant aspects of some of the most recent studies are presented. The 0.20 g of CeO 2 was found to be an effective option at 250 • C, bio-oil yield 26%, as 10% higher than the non-catalytic run. However, mono-aromatic compounds and organic acids were improved with the addition of the catalyst. [120] Dunaliella tertiolecta 320 Co/CNTs The highest bio-oil yield of 40.25% was achieved by Co/CNTs along with lower O/C ratios. [

Hydrothermal Stability of Heterogeneous Catalysts
Catalyst stability is an important parameter regarding the process operation and economic performance. Stability of heterogeneous catalysts depends mainly on the surface functionalities, nature of support, and metal-support interactions. Under severe hydrothermal conditions, the concentration of the H + and OH − ions increases, which can affect the catalytic performance by attacking the surface of porous and hydrophilic supports (e.g., Al 2 O 3 , SiO 2 , TiO 2 , and carbon). These materials possess high surface area, thus they have been widely used in biomass conversion reactions. Chen et al. synthesized SBA-15 embedded with Ni, Pd, Co, and Ru metals for catalytic HTL of microalgae at the different temperatures of 573, 593, and 613 K for 1 h [126]. Among all the transition metals, Co enhances the hydrothermal stability of SBA-15 due to the higher pore wall thickness of Co-SBA-15, which exhibited a high catalytic HTL performance with 78.78% of conversion and 24.11 wt% of bio-oil yield. However, the Pd-SAB-15 catalyst exhibited the poor hydrothermal stability for the catalytic HTL of microalgae. Previously, a series of Ni/CeO 2 catalysts were synthesized to evaluate their catalytic activity and hydrothermal stability for the HTL of rice straw [127]. The results indicated a higher thermal stability of CeO 2 catalysts deposited with Ni nanoparticles than the pure CeO 2 catalyst, as the Ni/CeO 2 catalyst possessed excellent cycling stability after the recovery. Furthermore, the catalyst showed higher biomass conversion rate and biocrude yield due to the high dispersion of Ni nanoparticles and stronger interactions between Ni and CeO 2 .
Additionally, carbon as a catalyst support has several advantages such as a high resistance to basic and acidic media, a high stability in aqueous media at high temperature as well as a high surface area and the possibility of enhancing its chemical surface properties by adding anchoring groups. The hydrothermal stability of Ru/C catalyst was checked under supercritical water (SCW) conditions during the catalytic conversion of isopropanol [128]. It was found that a higher Ru dispersion was beneficial for the improvement in the catalytic activity. Furthermore, the carbon support appeared to be more thermally resistant after the SCWG treatment of isopropanol. However, the loss of catalytic activity of Ru/C catalyst was caused by the coke deposition. Huo et al. synthesized carbon-coated SBA-15 materials deposited with Pd nanoparticles for the hydrothermal treatment and aqueous-phase hydrogenation of furfural [129]. The Pd C/SBA-15 catalyst exhibited improved stability after the two separate treatments under hydrothermal conditions at 170 • C for 24 h as the structure of SBA-15 was unchanged and Pd sintering was successfully reduced.
Therefore, a basic understanding of the metal-support interactions and deactivation mechanisms will help to develop sustained catalytic materials for large-scale biomass conversion processes under hydrothermal conditions.

Catalysis for HTL Biocrude Upgrading
HTL biocrude is a diverse pool of unsaturated organics containing significant amounts of contaminants such as oxygen, nitrogen, and inorganics in higher amounts and sulfur in lower amounts. Inevitably, the presence of these organic contaminants makes HTL biocrude an intermediate product with high TAN, high viscosity/density, low H/C, and poor thermal stability. Therefore, a downstream refining step is essential before HTL biocrudes can be utilized for the production of drop-in fuels. To date, the removal of organic contaminants via catalytic hydrotreatment has been a widely explored research area. During catalytic hydrotreatment, the removal of inorganics, O, N, and S takes place with reactions involving hydrodemetallization (HDM), hydrodeoxygenation (HDO), decarboxylation, decarbonylation, hydrodenitrogenation (HDN), hydrodesulfurization (HDS), and hydrogenation (HYD) [54,130,131].
In the literature, researchers have largely utilized both non-sulfided and sulfided catalysts. Most of these studies have been carried out in batch units. The main purpose of these efforts was to demonstrate the practicability of hydroprocessing for the treatment of HTL biocrudes from different feedstocks (such as lignocellulosic residues, algae, sewage sludge etc.) toward the production of drop-in fuels. Moreover, these batch hydrotreating studies also documented the effect of different sulfided/non-sulfided catalysts and operating conditions. Likewise, both families of catalysts have also been tested to some extent in continuous units.
Hereafter, this section will comprehensively discuss and compare the effect of both non-sulfided and sulfided catalysts on the properties of hydrotreated oils (i.e., H/C, O/C, and N/C atomic ratios). Details of batch hydrotreating studies that utilized different nonsulfided and sulfided catalysts are listed in Tables 5 and 6, respectively. The documented results of these batch hydrotreating studies are discussed and compared based on the fuel properties (such as H/C, O/C, and N/C atomic ratios) in this section.

Non-Sulfided Catalysts in Batch Hydrotreating
Bai et al. [132] carried out an extensive catalytic screening study by employing a wide range of different non-sulfided catalysts (such as Pt/C, Ru/C, Pd/C, activated carbon, Raney-Ni, and Ru/C + Raney-Ni) to HTL biocrude from Chlorella pyrenoidosa algae under hydrotreating conditions. Their results showed that Pt/C has the highest HYD activity (resulting in an increase in H/C in the biocrude), Ru/C has the highest HDO, and Raney-Ni has the highest HDN. However, Ru/C + Raney-Ni (two-component catalyst) exhibited optimal HYD, HDO, and HDN [132].
Duan et al. [134] studied the influence of catalyst loading on the properties of hydrotreated oils. They found out that with 40% catalyst loading, high HDN (low N/C) and high HDO (low O/C) were achieved [134]. Moreover, Barreiro et al. [135] reported the hydrotreatment of two different microalgae HTL biocrudes with Pt/γ-Al 2 O 3 catalyst. They noticed a reduction in the heteroatom content and an increase in volatility of both microalgae HTL biocrudes [135]. Shakya et al. [136] also reported the hydrotreatment of Nannochloropsis sp. algae with several non-sulfided catalysts (Ni/C, Ru/C and Pt/C). Ru/C and Pt/C resulted in a better oil quality in terms of HHV, HDN, and TAN. However, Ni/C showed the highest upgraded oil yields. They also observed a significant decrease in the pore volume and surface area of the catalysts (Ni/C, Ru/C, and Pt/C), primarily because of coke formation [136].
Patel et al. [137] carried out the hydrotreatment of algae biocrude in the presence of noble metal catalysts (Pt, Pd, and Ru) with both carbon and γ-Al 2 O 3 supports. They documented an improvement in HDO when the γ-Al 2 O 3 support was added to Pt and Ru [137]. Xu et al. [138] investigated the hydrotreatment of algae biocrude with the Ni-Ru/CeO 2 and Ni/CeO 2 catalysts. They recorded higher HDS for Ni-Ru/CeO 2 and considered it as an optimal catalyst for the hydrotreatment of algal biocrude [138]. Xu et al. [139] explored the applicability of multi-metallic catalysts (NiMoW/γ-Al 2 O 3 , CoMoW/γ-Al 2 O 3 , and CoNiMoW/γ-Al 2 O 3 ) during the hydrotreatment of Chlorella microalgae HTL biocrude. They noted that both CoMoW/γ-Al 2 O 3 and CoNiMoW/γ-Al 2 O 3 effectively reduced both the molecular weight distribution and boiling point distribution. Guo et al. [140] investigated the hydrotreatment of Chlorella vulgaris and Nannochloropsis gaditana HTL biocrudes in the NiW/γ-Al 2 O 3 catalyst and reported higher HDS activity in comparison to the conventional hydrotreating catalyst. Yu et al. [141] also explored the NiW/γ-Al 2 O 3 catalyst during the hydrotreatment of aspen wood HTL biocrude and recorded an increase in H/C, HHV, and HDO activity. Yue et al. [142] presented the hydrotreatment of sweet sorghum bagasse by utilizing Ru/C as a HDO catalyst under mild operating conditions (350 • C and 3.5 MPa). Furthermore, Duan et al. [143] utilized Ru on activated carbon (Ru/C) and successfully upgraded the duckweed HTL biocrude by reducing the heteroatom content and increasing the overall H/C and HHV.

Sulfided Catalysts in Batch Hydrotreating
Sulfided catalysts represent the state-of-the-art in hydrotreating and have been widely employed in fossil oil refineries for the desulfurization of oil fractions [144]. Sulfided catalysts are often represented by supported CoMo and NiMo. Although sulfur removal is generally not the main issue in biocrude hydrotreating, sulfided catalysts have also proven to be effective for the removal of other heteroatoms such as O and N as well as for hydrogenation.
Bai et al. [132] investigated sulfided CoMo/γ-Al 2 O 3 and MoS 2 catalysts during the hydrotreatment of Chlorella pyrenoidosa algae HTL biocrude. Both sulfided catalysts reduced the heteroatom content and increased the H/C and HHV of hydrotreated oils. During their investigation, they found that CoMo/γ-Al 2 O 3 tends to reduce coke formation compared to other non-sulfided catalysts [132]. Biller et al. [145] reported the hydrotreatment of HTL biocrude from Chlorella microalgae with conventional sulfided catalysts (CoMo/γ-Al 2 O 3 and NiMo/γ-Al 2 O 3 ). They achieved higher HDN activity with sulfided NiMo and higher HDO activity with sulfided CoMo at given hydrotreating conditions (405 • C and 6.6 MPa) [145]. Jensen et al. [146] carried-out the hydrotreatment of hardwood biocrude with a commercial NiMo/γ-Al 2 O 3 catalyst. They found that the operating temperature and hydrogen to oil ratio had a positive influence on overall HYD and HDO. However, operating pressure mostly affects the HYD and HDO of low reactivity oxygenates [146]. Guo et al. [140] and Yu et al. [141] explored both non-sulfided NiW/γ-Al 2 O 3 and commercial sulfided NiMo/γ-Al 2 O 3 catalysts with Chlorella vulgaris/Nannochloropsis gaditana and aspen wood HTL biocrudes, respectively. Both of these separate studies showed higher HDN and HDO when the sulfided NiMo/γ-Al 2 O 3 catalyst was employed [140,141]. Zhao et al. [147,148] extensively investigated sulfided NiMo/γ-Al 2 O 3 , both alone [147] and combined with the guard bed NiMo catalyst [148]. They evaluated a two-stage approach for effective catalytic hydrotreatment and successfully achieved higher HDY, HDO, and HDN.
Haider et al. [149] employed a sulfided NiMo/γ-Al 2 O 3 catalyst during the hydrotreatment of Spirulina microalgae biocrude and carried out a statistical analysis to evaluate the significance of the different process conditions. It was revealed that, up to 350 • C, HDO is mainly temperature driven, while HDN is affected by both initial H 2 pressure and pressure-temperature interaction. They also documented complete HDO at 350 • C and 8 MPa [149]. Castello et al. [150] utilized sulfided NiMo/γ-Al 2 O 3 catalyst and studied the effect of different operating parameters on three different HTL biocrudes (Spirulina algae, sewage sludge, and Miscanthus). They achieved complete HDO for Spirulina algae and sewage sludge HTL biocrudes and reported a high extent of HDN. They found that higher hydrogen pressure is needed to prevent extensive coking and undesired decarboxylation reactions [150].
Rathsack et al. [151] and Zuber et al. [152] reported the hydrotreating of Chlorella vulgaris with sulfided NiMo/γ-Al 2 O 3 catalyst. They also achieved complete HDO, lower N/C (0.003), and higher H/C (1.91). Thanks to FT-ICR MS (Fourier-transform ion cyclotron resonance with mass spectrometry), they were able to conclude that N 1 species are difficult to remove compared to N 2 species [152]. Subagyono et al. [153] investigated sulfided NiMo/Al-SBA-15 as a catalyst for the hydrotreatment of microalgae HTL biocrude. During catalytic hydrotreatment with NiMo/Al-SBA-15, they attained high HYD, HDO, and HDN. In addition, they also realized that the acidity of the support material is directly related to product yield, while product quality is assured when NiMo is incorporated in the support material [153].
An important aspect is represented by the potential thermal instability of biocrude, which can seriously affect hydrotreating operations. Haider et al. [154] showed that HTL biocrudes are thermally unstable at high temperatures (400 • C) and they reported extensive coke formation upon directly subjecting these HTL biocrudes at these temperatures. They utilized a sulfided NiMo/γ-Al 2 O 3 catalyst in two-stages and ensured higher oil yields, higher HDN, complete HDO, and remarkable fuel properties with respect to H/C and HHV [154]. Figure 6 illustrates the effect of non-sulfided and sulfided catalysts on the properties of the hydrotreated HTL biocrudes by means of van Krevelen-like plots. Sulfided catalysts perform better compared to non-sulfided ones under given hydrotreated conditions. HTL biocrudes treated with sulfided catalysts are on the extreme left side of both diagrams, meaning that the upgraded oil possesses a higher degree of HYD (highest H/C atomic ratio) along with the highest HDO (lowest O/C atomic ratio) and HDN (lowest N/C atomic ratio) activity. In contrast, non-sulfided noble metal catalysts were comparatively not conducive to enhanced HYD, HDO, and HDN activity. Thereby, non-sulfided noble metal catalysts retain a lower drop-in fuel properties. The lower efficiency of non-sulfided noble metal catalysts is probably due to the rapid catalyst deactivation due to sulfur molecules [95]. Similar concerns regarding sulfur poisoning of precious noble metal catalysts (Pt, Pd, Rh, Ru, etc.) are also suggested during the catalytic HDO of pyrolysis bio-oils where sulfur concentrations up to a few hundred ppm are found [155]. However, the short-term nature of batch hydrotreating HTL experiments does not allow for a correct evaluation of the deactivation mechanism by sulfur poisoning.

Catalysis in Continuous Hydrotreating
Only a handful of continuous hydrotreating studies on HTL biocrudes are present in the open literature. Continuous processing indeed requires more complex facilities than batch units and, normally, also higher volumes of catalysts and biocrude feed. Continuous operations are, however, more significant toward the scale up of the process. Processing in a continuous unit may be substantially different from the batch, and results are often difficult to compare. Indeed, batch units often experience significant equilibrium and mass transfer limitations, which lead to lower performance compared to continuous operations in fixed beds or trickle beds.
All of the available continuous studies are carried-out in the presence of sulfided CoMo/γ-Al2O3 and NiMo/γ-Al2O3 hydrotreating catalysts (Table 5). However, only one continuous hydrotreating study [156] based on two-stage non-sulfided noble metal catalysts (NiW/SiO2/Al2O3 and Pd/Al2O3) is available in the open literature. In his work, Jensen [156] carried out the catalytic hydrotreatment of HTL biocrude from forestry residue and employed two individual reactors, the former filled with NiW/SiO2/Al2O3 and the latter with the Pd/Al2O3 catalyst. Overall results showed poor HDO, large exothermicity, and

Catalysis in Continuous Hydrotreating
Only a handful of continuous hydrotreating studies on HTL biocrudes are present in the open literature. Continuous processing indeed requires more complex facilities than batch units and, normally, also higher volumes of catalysts and biocrude feed. Continuous operations are, however, more significant toward the scale up of the process. Processing in a continuous unit may be substantially different from the batch, and results are often difficult to compare. Indeed, batch units often experience significant equilibrium and mass transfer limitations, which lead to lower performance compared to continuous operations in fixed beds or trickle beds.
All of the available continuous studies are carried-out in the presence of sulfided CoMo/γ-Al 2 O 3 and NiMo/γ-Al 2 O 3 hydrotreating catalysts (Table 5). However, only one continuous hydrotreating study [156] [160], and Collet et al. [161] employed CoMo as both the guard-bed HDM and bulk catalyst during the continuous hydrotreatment of algae, wastewater solids, and corn stover HTL biocrudes, respectively. They all achieved remarkable fuel properties with good HDO (0.008-0.015 O/C) and high HYD (1.93-2.02 H/C) and HDN (0.0003-0.002 N/C). Recently, PNNL investigated the continuous hydrotreatment of pine wood HTL biocrude [162] in a sulfided NiMo catalyst and sewage sludge HTL biocrude [163] in CoMo as the guard-bed HDM and NiMo as the bulk catalyst. By doing so, they obtained impressive drop-in fuel properties in the hydrotreated oils (Table 7).  [164] i WHSV = Weight hourly space velocity; LHSV = Liquid hourly space velocity.

Perspectives and Conclusions
Catalysis is a fundamental aspect across the whole process chain related to biofuel production via HTL and hydrotreating. As was observed in the previous treatise, the presence of a catalyst turns out to be decisive in order to increase both the yield and the quality of the produced biocrude. The choice of whether to use a catalyst or not is still an option during HTL, although for several kinds of biomass, it represents an important aspect. On the other hand, hydrotreating intrinsically needs a proper catalyst to be carried out. In this latter case, it is important to define what kind of catalyst to adopt.
As far as HTL is concerned, the available results in the literature show that the performance of a catalyst is strongly affected by the type of biomass feedstock that is involved. This is especially true for homogeneous catalysis with alkali metals, for which there is an appreciable effect only for lignocellulosic feedstock. For other types of biomasses, the effects are negligible or even negative, with a reduction in the amounts of product. The composition of the biomass feed therefore plays an utmost role and this is a piece of information that needs proper consideration in light of establishing the process. In this regard, it is worth mentioning that each biomass feedstock contains a certain amount of inorganics, often involving different metal species. This is especially true for many residual biomasses (e.g., sewage sludge or agricultural residues), whereas woody biomass is often relatively poor in inorganics. The catalytic effect of the intrinsic metal content of biomass is a potentially interesting aspect for future studies. Moreover, in order to come to a rational design of the catalyst, it is necessary to perform fundamental mechanistic studies to obtain a better understanding of the different reaction pathways.
Due to the utilization of relatively economical catalysts and simplicity of implementation, homogeneous catalysis represents a viable and effective solution for HTL. However, an important aspect to be considered is that of catalyst recovery, in order to reduce the overall consumption. After the reaction, homogenous alkali catalysts are usually found in the aqueous phase, which should therefore be recirculated. However, metals and ions, in general, could also distribute among the other reaction products (char and biocrude). Knowledge about the amount and the form in which ions are found in each phase is very important. Moreover, the effect of residual amounts of inorganics in the biocrude should be considered, with attention to the possible consequences on downstream upgrading operations.
The utilization of heterogeneous catalysts can simplify the issue of catalyst recovery. However, this also strongly depends on the reactor technology that is adopted and on the catalyst itself. For the conditions typical of HTL, it is more likely to deal with the catalyst added to the slurry feed, therefore in the form of dispersed particles. In this case, recovery by gravity (settling) is strongly affected by viscosity and other rheological properties of the product mixture, which need to be properly investigated. Recovery of heterogeneous catalysts can be greatly enhanced by some properties of the catalyst itself (e.g., magnetic properties). In general, due to the cost of these materials, catalyst recovery plays a fundamental role in the economy of the process and therefore needs proper attention.
An interesting perspective is that of utilizing catalyst systems able to produce in situ hydrogen from water and make it available in the reaction. This can be achieved with zero-valent metals (ZVMs) that can be oxidized by water, generating hydrogen. Then, appropriate catalysts with specific activity for hydrogenation could greatly improve the quality of biocrude. The drawback of this approach resides in the energy needed to again reduce the metal oxides to ZVMs in order to restart the cycle. The availability of a cheap energy source, or the possibility of using side streams of the process (e.g., char) for the reduction process, is vital for this kind of approach.
Heterogeneous catalysts are able to show interesting results even for feedstocks for which alkali catalysts are not very effective. On the other hand, heterogeneous catalysts are often expensive and they can impact negatively on the economy of the process. An interesting perspective could be that of utilizing by-products of other industrial processes, for instance, metal-rich residues from metallurgic industries such as the so-called "red mud" from aluminum production.
As far as biocrude downstream processing is concerned, the utilization of a catalyst is necessary to conduct the process. As has been shown, traditional sulfided catalysts appear to be better performing than non-sulfided ones. It should, however, be pointed out that sulfided catalysts result from an almost century-long development process and therefore have reached a high level of technological maturity. One of the most important aspects of sulfided hydrotreating catalysts is their relatively high robustness, which makes them able to better tolerate impurities such as sulfur and nitrogen with respect to other catalysts. For this reason, sulfided catalysts seem to be a good choice when processing biomass-derived feedstocks, especially from non-lignocellulosic sources, where considerable amounts of sulfur can be present. However, the effect of biocrude origin (i.e., of the type of biomass used for its production) on the effectiveness of the upgrading catalyst needs to be analyzed and discussed in more depth.
On the other hand, sulfided catalysts can also be very negatively affected by the metals that are typically present in HTL biocrude. Levels usually recommended by manufacturers (i.e., below 50 ppm of metals) cannot be normally achieved in biocrude, which requires the adoption of a strategy of demetallization to be achieved through pretreatment and/or the adoption of a hydrodemetallization (HDM) catalyst prior to the reactor bed. Demetallization is critical to prevent catalyst deactivation.
Even though non-sulfided catalysts appear less effective than sulfided ones in deoxygenation and denitrogenation, their usage could still be important for other upgrading treatments to improve certain properties of the final fuels such as isomerization or deep denitrogenation. Sulfided catalysts could therefore be utilized in the initial stages to remove the largest part of the contaminants, while non-sulfided catalysts might be beneficial in subsequent upgrading stages. This perspective is of high interest in the field.
In general, catalysis in the field of HTL and biocrude upgrading is receiving growing consideration. Progresses in this field will increase as far as more knowledge on the fundamental mechanisms of biomass and biocrude conversion is obtained. Therefore, obtaining better information on the fundamental mechanisms, for instance, by means of testing with model compounds, is important in order to advance the state-of-the-art and to find better catalysis strategies.