Upgrading of Oils from Biomass and Waste: Catalytic Hydrodeoxygenation

: The continuous demand for fossil fuels has directed significant attention to developing new fuel sources to replace nonrenewable fossil fuels. Biomass and waste are suitable resources to produce proper alternative fuels instead of nonrenewable fuels. Upgrading bio-oil produced from biomass and waste pyrolysis is essential to be used as an alternative to nonrenewable fuel. The high oxygen content in the biomass and waste pyrolysis oil creates several undesirable properties in the oil, such as low energy density, instability that leads to polymerization, high viscosity, and corrosion on contact surfaces during storage and transportation. Therefore, various upgrading techniques have been developed for bio-oil upgrading, and several are introduced herein, with a focus on the hydrodeoxygenation (HDO) technique. Different oxygenated compounds were collected in this review, and the main issue caused by the high oxygen contents is discussed. Different groups of catalysts that have been applied in the literature for the HDO are presented. The HDO of various lignin-derived oxygenates and carbohydrate-derived oxygenates from the literature is summarized, and their mechanisms are presented. The catalyst ’ s deactivation and coke formation are discussed, and the techno-economic analysis of HDO is summarized. A promising technique for the HDO process using the microwave heating technique is proposed. A comparison between microwave heating versus conventional heating shows the benefits of applying the microwave heating technique. Finally, how the microwave can work to enhance the HDO process is presented.


Introduction
In light of the fast-growing global energy demand, depletion of fossil fuels, and associated detrimental environmental issues, it is of the utmost significance to find alternative renewal energy resources to meet the energy demand. The environmental challenge caused by global warming has focused most of the research in recent years to convert biomass and waste into chemicals and transportation fuels [1]. Although there are several forms of renewable energy, such as wind and solar power, that contribute increasingly to the energy supply, liquid fuel remains the essential source of energy for various sectors and, in particular, the transportation sector.
Biofuel can be produced from several feedstocks that are widely available in all countries, especially industrialized ones, and the most important feedstocks are biomass and west. Biomass plants mainly consist of cellulose, hemicellulose, and lignin [2][3][4]. Several factors affect the characteristics and the percentage of the biomass components into the plants, such as the age of the plant, the place where the plant was growing, etc., [4]. It was demonstrated that cellulose exists in the biomass with 35-45 wt.%, hemicellulose with 25-30 wt.%, and lignin with 20-35 wt.% [4][5][6][7]. Hemicellulose is the weakest component if exposed to degradation because of the large numbers of branches compared to other components. Cellulose is much stronger than hemicellulose because the long polymers in cellulose with side changes enhance the bonds' strength. The complex structure of lignin, which mainly consists of a large number of aromatics with side chains, makes it the most substantial component and has a high resistance to decomposition if compared with cellulose and hemicellulose [4,[8][9][10][11]. Biomass can produce first and second generations of biofuels. The firstgeneration biofuels or the original biofuels are produced from palatable food crops (for example, sugarcane, wheat, grain, corn, potato, soybean, sunflower, and coconut). The second-generation biofuels are produced from various agriculture and forest feedstocks (mainly the lignocellulosic materials), building waste, industrial waste, and municipal waste [12,13]. Third-generation biofuel is usually derived from alga. Indeed, biomass and waste pose a significant danger to the environment, and their disposal creates a serious issue. Therefore, converting them into a useful biofuel is a promising proposition to lessen their dangerous effect and help solve their disposal problem [14].
Liquid fuels can be produced from biomass and waste through gasification to produce syngas, pyrolysis to produce bio-oil, and hydrolysis to produce aqueous sugar [14], Figure 1. Owing to the high bio-oil yield and economic efficiency, pyrolysis has been regarded as the most effective conversion technology to produce liquid biofuel from biomass and waste. Pyrolysis-produced biooil usually contains various chemical compounds, including carboxylic acids, aldehydes, ketones, furans, aliphatic and aromatic hydrocarbons, phenolics, and sugars.  [15].
Pyrolysis is the thermal cracking of carbonaceous material into lower density components in the absence of oxygen. The thermal degradation of the biomass leads to the production of charcoal (solid), bio-oil (liquid) and syngas (gaseous) energy products. Pyrolysis, based on the operating conditions, can be classified under three main categories [16]: • Conventional Pyrolysis: Pyrolysis at the slow heating rate, which permits the formation of the solid, liquid and gaseous products in significant portions.
• Fast Pyrolysis: If the procedure aims to produce a liquid or gaseous product, fast pyrolysis is employed. In this case, the products are considered marginal with a low yield. The operating conditions are declared: High operating temperature, short residence time and very fine particles.

•
Flash Pyrolysis: In this method, the feedstock is directly fed into the high-temperature zone of the reactor, which subsequently undergoes flash pyrolysis where the final products are in the gaseous phase with a short contact time inside the reactor.
Pyrolysis in the forms of flash or fast pyrolysis is employed during the gasification process correspondingly. Three methods are employed to raise the wanted products' yield: Steam reforming of the pyrolysis liquids, tar removal and maintaining the operating temperature around 700 °C and using a catalyst at temperatures below 750 °C. Table 1 compares the operating conditions of different pyrolysis methods. It is worth mentioning that great attention is directed to the fast pyrolysis process for bio-oil production from biomass. This aspect's main reason is the high liquid yield produced with specific characterizations that make it has unique advantages in transport, storage, combustion, retrofitting, and flexibility in production and marketing [17].
Unfortunately, upgrading bio-oil produced from the pyrolysis process is essential due to the high content of hazardous materials and bio-oil properties if compared with the fuel oil. Bio-oil has a high water content (15-30 wt.%), a low corrosive pH (<3), a higher oxygen content (35-40 wt.%), and a higher heating value (HHV) of 16-19 MJ/kg [18], and hazard materials such as S-compounds, metals, N-compounds, and a high content of oxygenated compounds [19]. Sulfur is emitting to the atmosphere in the form of SOx, causing acid rain formation through its reaction with water, oxygen, and other chemicals in the atmosphere. The presence of sulfur in oil increases corrosion issues during the refinery process but helps in the deactivation of the catalyst in minimal time. The removal of dibenzothiophene and its alkyl derivatives is a big challenge, as the compounds cannot be transferred into H2S due to the steric hindrance adsorption on the surface of the catalyst [20]. Also, NOx emits into the atmosphere and poisons the catalyst during the refinery process. Metals in the oil enhance the production of high volumes of coke and dry gases and decrease the liquid output [21]. In addition, the metals enhance the fast catalyst deactivation [21,22], which is mainly due to the deposition of metal oxides formed on the pore of the catalyst and blocking the active site. The presence of oxygenates in the bio-oil results in several technical challenges when using bio-oil as the "drop-in fuel," like low energy density, instability, high viscosity, and corrosion post-processing to remove oxygen atoms from bio-oil must be carried out [23]. Table 2 shows the elemental analysis of different biomass feedstocks.
Even though there is a wide range of upgrading techniques that have been investigated for the fast pyrolysis-produced bio-oil, hydrotreatment (e.g., hydrodeoxygenation, hydrodesulfurization, hydrodemetallization, and hydrodenitrogenation) has been employed in oil refineries for many years, and the process is well-established [24]. Among the hydrotreatment techniques, catalytic hydrodeoxygenation (HDO) is a compelling upgrading approach to produce petroleum-like hydrocarbon fuels or chemical building blocks from fast pyrolysis-produced bio-oil, during which oxygen present in the bio-oil is removed through water formation [25].
In this review article, the different upgrading techniques of bio-oil were discussed, catalytic hydrodeoxygenation of different compounds derived from the pyrolysis oil of biomass and waste, the mechanism of the HDO, the various families of catalysts used for HDO, different catalysts that have been applied for the HDO process, the issue of the deactivation of catalysts, and the technoeconomic aspects of the HDO process. Different deoxygenation approaches for bio-oil.

Oxygenated Compounds Exist in Biomass Pyrolysis Oil
The bio-oil compositions produced from the biomass pyrolysis are very complex and contain predominantly water and hundreds of different chemical compounds, which include various aldehydic compounds, esters, phenols, sugar, acids, ketones, lignin-derived compounds, alcohols, and more compounds. Other compounds that have been detected in the rice husk-based biomass pyrolysis oil can be seen in Table 3.

Issues Related to the Presence of Oxygen in the Feedstock or Products
The bio-oil properties obtained from fast pyrolysis are presented in Table 4, as shown below [27]. As indicated in the table, bio-oil often contains around 30-45 wt.% of oxygen, thereby leading to a significantly higher heating value (HHV) of 16-19 MJ/kg compared to that of heavy fuel (40 MJ/kg). Another problem relating to the high oxygen content in bio-oil is the low pH value (i.e., 2-4), which makes bio-oil an undesirable corrosive property. Besides, bio-oil is highly viscous, and it is not chemically-stable upon storage, which is attributed to the series of reactions, like oligomerization and polymerization, occurring between the different oxygenated functionalities [28].

Different Deoxygenation Approaches for Bio-Oil
A wide range of upgrading methods has been developed for fast pyrolysis bio-oil. Such techniques can be generally categorized into physical and chemical techniques, as depicted in Figure  2 [29].

Solvent Addition
Solvent extraction is a simple upgrading approach for fast pyrolysis bio-oil, in which polar solvents (e.g., ethanol, methanol, ethyl acetate, and acetone) are added to crude bio-oil to reduce its viscosity and improve the homogeneity and energy density. As indicated by Khosravanipour et al. [30], the use of methanol is reported to lower the bio-oil's viscosity by 20-fold. Liu et al. [31] explored the effect of acetone addition on the physicochemical properties of bio-oil from the fast pyrolysis of pinewood sawdust in a fluidized bed reactor, and it was found that the pH value of bio-oil increased, and the water content and viscosity decreased. The authors also stated that acetone's presence showed an inhibiting impact on the aging reaction of bio-oil and introducing new chemical compounds. However, this upgrading technique does not help eliminate undesirable compounds, such as oxygenates, from the bio-oil.

Emulsion
Emulsion, as another easy-operation upgrading method, has been investigated for the bio-oil obtained from fast pyrolysis. In the emulsion treatment, fast pyrolysis bio-oil is blended with other diesel fuels or biodiesel with the addition of surfactant to enhance its ignition properties. For instance, Jiang and Ellis [ [32] performed a parametric study for upgrading bio-oil from softwood by biodieselassisted emulsion and the use of octanol as the surfactant, and the optimal reaction conditions were 4 vol.% of surfactant dosage, a bio-oil/biodiesel ratio of 4/6 vol.% a stirring speed of 1200 rpm, a processing time of 15 min, and a temperature of 30 °C. At these optimized conditions, the viscosity, acidity, and water content of softwood-derived bio-oil were considerably reduced after the emulsion treatment. Even though emulsion is a simple upgrading technique, the high-cost of surfactant, high energy consumption, and the inability to remove unfavourable chemical substances of bio-oil limit its large-scale application [33].

Esterification
Esterification is a relatively simple chemical method often carried out at moderate conditions to upgrade bio-oil where the addition of alcohol is needed. Among all the investigated alcohols, methanol is the most commonly used solvent. The main reactions occurring between bio-oil from fast pyrolysis and alcohol are shown in Figure 3 [34]. Reprinted from reference [34].
Previously, Weerachanchai et al. [35] investigated the upgrading of palm shell-derived bio-oil by esterification under varying reaction conditions, and the authors reported that the use of Amberlyst 15 as the catalyst was helpful to promote the conversion of carboxylic acids in the bio-oil to their corresponding alcohols and simultaneously reduce the concentration of aldehydes in the biooil. However, the heating value of upgraded bio-oil ranged from 23 to 25 MJ/kg and thus, further boosting treatment is still required. In summary, esterification offers several advantages of mild reaction conditions, cheap alcohol price, and easy-operation; however, its ineffectiveness in removing nitrogen-containing compounds makes it impractical to upgrade algae-derived bio-oil owing to the presence of highly concentrated nitrogenates in the bio-oil [24].

Supercritical Fluid
In recent years, supercritical fluid has been widely applied as an alternative upgrading approach for fast pyrolysis bio-oil, which accounts for the inherent benefits of supercritical fluid, such as liquidlike density, gas-like diffusivity and viscosity, and high mass and heat transfer rates [36]. For example, Lee et al. [37] catalytically upgrade the bio-oil obtained from the fast pyrolysis of woody biomass by the supercritical catalytic fluid at 250-350 °C with the use of a Ni-based catalyst. It was found that the highest upgraded bio-oil yield of 70.5 wt.% was obtained at 350 °C with MgNiMo/AC, which was accompanied by the lowest O/C molar ratio of 0.19, total acid number (TAN) of 6.2 mg KOH/g, and higher heating value (HHV) of 33.4 MJ/kg. According to the Gas chromatography Mass Spectrometry GC-MS analysis results, the undesirable oxygen-containing compounds, such as carboxylic acids, aldehydes, and levoglucosan, were largely removed from the bio-oil after upgrading. Despite that, the acidity and viscosity can be reduced, and the supercritical fluid can improve the energy content of bio-oil; a high-pressure resistant reactor is needed, and the corrosion of material at supercritical conditions cannot be neglected.

Steam Reforming
Steam reforming of fast pyrolysis bio-oil is an effective way to produce H2-rich syngas [38]. This approach is usually used at high temperatures of 700-1000 °C with the use of a Ni-based catalyst. Lan P et al. [39] catalytically upgraded the bio-oil obtained from the fast pyrolysis of a rice husk by steam reforming, and it was reported that the highest H2 yield of 75.88 was achieved at 700-800 °C, a steam/carbon molar ratio of 15-20, and a liquid hourly space velocity of 0.5-1.0 h −1 . Although highly energy-dense H2 can be generated from the steam reforming of bio-oil, there is a requirement for using a high-temperature resistant reactor.

Hydrocracking
The hydrocracking process is one of the upgrading processes that involves cracking the heavy molecular feeds into smaller valuable products. The process involves a two-step reaction: (1) the first step is the catalytic cracking of the high molecular weight compounds, and (2) the second is the hydrogenation reaction of the cracked molecules. A higher temperature of 300-500 °C and pressure of 10-20 MPa is often applied for hydrocracking over a heterogeneous catalyst, in which an organic liquid, a water phase, gas, and coke are commonly generated as the main products. The general reaction pathway for hydrocracking is shown in Figure 4 [40]. Tanneru and Steele [41] have studied the influences of reaction temperature, H2 pressure, residence time, and catalyst type on the hydrocracking of bio-oil obtained from the fast pyrolysis of pinewood chips to produce hydrocarbons. The resulting hydrocarbon mixture contained an HHV of 43.6 MJ/kg, the oxygen content of 0.5 wt.%, and TAN of 0.3 mg KOH/g, along with the density and viscosity of 0.9 g/mL and 1.8 cSt, respectively. Although the light components can be produced by hydrocracking, the need for a high operating temperature and high-pressure H2 could significantly increase the entire process's cost.
In conclusion, due to the hydrotreatment upgrading approach's maturity, the catalyst, technical challenges, reactor configuration, and economics of HDO are discussed in the following sections.

Hydrotreatment
Hydrotreatment is a well-established process in oil refineries that are often carried out at moderate temperatures of 300-450 °C and H2 pressure up to 20 Mpa. The hydrotreating process includes hydrodemetallization for metal removal, hydrodesulfurization for S removal, hydrodenitrogenation for N removal, and hydrodeoxygenation for oxygen removal. Hydrotreating processes usually occur in the presence of a catalyst, and the most traditional catalysts used for hydrotreatment are NiMo, NiW, and CoMo [42]. Owing to the complexity of bio-oil's chemical composition, a series of reactions possibly occur in bio-oil upgrading, as depicted in Figure 5. The main advantages and disadvantages of each upgrading method are summarized in Table 5.  Cannot remove nitrogenates and thus cannot be used for algae-derived bio-oil

Hydrodeoxygenation
As one of the hydrotreatment approaches, HDO is highly effective in removing oxygen from bio-oil through water formation. The yield and properties of upgraded bio-oil obtained from HDO are dependent on the temperature, residence time, pressure, solvent, catalyst type, and reactor configuration. The selection of a suitable catalyst is a critical aspect of the HDO process.

Catalytic Hydrodeoxygenation
Hydrodeoxygenation HDO is an upgrading process applied to produce a high-quality oil yield with higher carbon content. The process involves removing oxygen from a hydrocarbon by applying different catalytic reactions at pressures up to 200 bar and temperatures up to 400 °C [46]. The HDO process occurs through different reactions, including the hydrogenation of C-O, C=O and C=C bonds, the dehydration of the C-OH group, condensation and the decarbonylation of the C-C bond cleavage using retro-aldol and the hydrogenolysis of C-O-C bonds [47,48]. Hydrodeoxygenation of biomass can occur through different biomass conversion techniques, which include the hydrotreating of biooils [49], hydropyrolysis [50], hydrogenolysis of biomass into oxygenated chemicals [51], aqueousphase reforming of carbohydrates into fuel [52] and hydrotreating of organics acids. The main reactions that take place during the HDO involve the separation of water that exists with oil, then the oil being exposed to a dehydration reaction as a result of a condensation-polymerization reaction, followed by a decarboxylation reaction where oxygen is eliminated in the form of H2O, then the oil is exposed to a hydrogenation reaction to transfer the unsaturated bonds formed during the reaction into saturated bonds, and a hydrogenolysis reaction, which breaks down the C-O bond and liberates the oxygen in the form of H2O. A hydrocracking reaction occurs to crack the high molecular weight compounds into low molecular weight compounds in the last step.
Attributed to the series of reactions, like oligomerization and polymerization, occurring between the different oxygenated functionalities [28].
Elkasabi et al. [53] upgraded the various bio-oil samples obtained from the fast pyrolysis of Switchgrass, Eucalyptus benthamii, and horse manure by HDO at 320 °C and 145 bar for 4 h over a Pt, Ru, and Pd-based catalyst, and HDO using Pt/C as the catalyst of switchgrass-derived bio-oil, which led to the highest oxygen reduction rate and carbon retention. The authors also reported that the biooil containing high syringe content greatly influenced H2 consumption. However, there are two main drawbacks to the HDO process, including the requirement for high-pressure H2 and a short catalyst lifetime caused by the coke formation [54].
Elliott and his co-worker developed a two-stage hydrodeoxygenation process for pyrolysis biooil [55,56]. The first step of the process is the catalytic hydrogenation of the thermally unstable biooil at a temperature of 270 °C and pressure of 136 atm using Co-Mo/Al2O3 or Ni-Mo/Al2O3 catalysts that were sulfides firstly in the reactor. The thermally unstable bio-oil compounds usually decompose, forming cock that directly plugs the reactors. The main hydrodeoxygenation reaction primarily occurs in the second step, which involves a catalytic hydrogenation reaction but at a higher temperature, reaching up to 400 °C, and at the same pressure using the same catalyst. The main findings of the process are the production of 0.4 L of treated oil for every 1 L of bio-oil and converting 20-30% of the C existing in the bio-oil into gas-phase carbon, which decreased the oil yield.
The two-stage hydrodeoxygenation process was also performed by Furimsky, (2000) for the conversion of biomass pyrolysis oil into high-quality oil [48]. The first stage involves removing the unstable oxygenated compounds to produce a stable crude at a low temperature below 300 °C with the sulfide NiMo or CoMo catalyst. The second stage of the process is mainly performed to convert the low HDO reactivity compounds, such as furans and phenols, into deoxygenated compounds at 350 °C and 13.8 Mpa. The main disadvantage of the process is the short lifetime of the catalyst due to the support breakdown. Ly et al., (2019) [57]hydrotreated bio-oil obtained from the fast pyrolysis of Saccharina japonica in a fluidized bed reactor at 350 °C and 3-15 bar of H2 over HZSM-5, Co/γ-Al2O3, CoP/γ-Al2O3, CoMoP/γ-Al2O3, Fe/γ-Al2O3, and Fe2P/γ-Al2O3. The oxygen content in the bio-oil was removed through the formation of water, CO, and CO2.
Meanwhile, an increase in the kerosene-diesel fraction of bio-oil was found. Schmitt et al., (2018) [58] performed catalytic HDO experiments on beech wood-derived bio-oil at 175-325 °C and 80-100 bar of H2, and the results indicated that around 42% of oxygen content in the crude bio-oil was eliminated at 325 °C and 80 bar and the ketones that cause the chemical instability of bio-oil were observed to be removed entirely. Delmon and his co-workers have studied the hydrodeoxygenation of a mixture of 4-methyl acetophenone, ethyl decanoate, and guaiacol as a model compound [59][60][61][62][63], Figure 6. The ketone group is highly reactive toward the HDO process; therefore, it is easily hydrogenated into a methylene group at a low temperature of 200 °C [61]. The hydrodeoxygenation of the carboxylic and guaiacol groups requires a temperature higher than 300 °C to be converted into deoxygenated compounds because of the lower reactivity of the two groups than the ketonic group. It was noticed that there is a decarboxylation pathway parallel to the hydrogenation of the carboxylic acid at comparable rates [61]. Guaiacol was converted into catechol then phenol, and the catalyst was deactivated during the high-temperature reactions due to coke formation. Increasing the acidity of the catalyst support does not affect the hydrogenation of 4-methyl acetophenone but raises the rate of decarboxylation and coke formation from the guaiacol group. It was found that the presence of water strongly affects the catalyst activity and decreases it by one-third of its initial activity.

HDO Catalysts
Until this point, a great effort has been made to study the formation of different catalysts using noble and non-noble metals for the HDO process. Recently, a novel, multifunctional red mudsupported nickel (Ni/red mud) was synthesized for the hydrodeoxygenation of aqueous-phase, pinyon-juniper, biomass chips. The process was performed using a 30 wt.% of the Ni/red mud catalyst at a temperature of 350 °C. The process's main findings are the production of 47.8 wt.% of the organic liquid yield with an oxygen content of 1.14 wt.%. The organic liquid yield is mainly composed of aliphatics, aromatics, and alkylated aromatic hydrocarbons with small oxygen content. In comparison, the commercial Ni/SiO2−Al2O3 catalyst was used as well in the HDO process. The 65 wt.% of the catalyst was gasified in the feed's organic phase and did not produce liquid hydrocarbons [64].
Different groups of catalysts have been investigated for the HDO of fast pyrolysis bio-oil, which can be classified into sulphides, transition metals, phosphides, and other catalysts (carbides and nitrides) [43].

Sulphide Catalysts
CoMoS2 and NiMoS2 have been widely applied not only in conventional hydrotreatments but also in HDO reactions. For CoMoS2 and NiMoS2, Co or Ni acts as the promoter and donates an electron to the Mo atom, thereby weakening the bond between Mo and S and creating an S vacancy site is reported to be an active site during the HDO process [40]. During the upgrading treatment of petroleum crude oil, CoMoS2 and NiMoS2 can be maintained in the form of sulfide due to the presence of H2 and H2S in the reactor. These components are generally directly produced from the thiols in the petroleum oil. On the contrary, due to a limited or zero sulfur content in the biomass, the sulfide in the form of CoMoS2 and NiMoS2 cannot be maintained, and they are often transformed into the corresponding oxide forms if no sulfur source is added to the system. However, the addition of an extra sulfur source could result in the catalyst being poisoned in the post-processing and SOx emission upon combustion [65]. Zhang et al. [66] reported that a 90% oxygen removal efficiency from the biooil was achieved using a dispersed unsupported MoS2 catalyst, and the coke formation was in the range of 0.8/100 g of bio-oil to 1.8/100 g of bio-oil. Kanhoinnon et al. [33] high catalytic activity of molybdenum sulphide in the catalytic HDO of furan and 2-methyl furan at 400 °C and 70 bar H2. The reaction mechanism for the HDO of 4-methyl phenol over MoS2 was proposed by Wang et al. [67], as shown in Figure 7. Table 6 presents the HDO of various feedstocks over sulphide catalysts.

Oxide Catalysts
It had been reported previously in the literature that oxide catalysts, such as Mo, Ni, W, and V, exhibit significant catalytic activity in the HDO reaction. Based on the basic mechanism of oxide catalysts, a low H2 pressure is needed to limit the transformation of active species into inactive species; however, a high pressure of H2 is a necessity to avoid coke formation during the HDO process. Generally, the catalytic activity of oxides in the HDO mainly depends on the acidic sites. At the initial chemisorption stage, the Lewis acidity is the key factor and the oxygen lone pair of the oxygenated compounds can be chemisorbed, and the availability of acidic sites on the oxide catalyst is affected by the Brønsted acidity [43]. Previously, Prasomsri et al. [76] found that the use of MoO3 effectively deoxygenated the linear and cyclic ketones, phenolics, and cyclic ethers at low H2 pressure of ≤1 bar.

Transition Metal Catalysts
Transition metal catalysts, including Ni, Pt, Pd, Ru, and Rh, promote the HDO and hydrogenation reactions, and their reaction rates are proportional to H2 pressure. Compared to sulfide catalysts, there is no requirement for the additional sulfur source to maintain the active form. The main drawback for the transition metal catalysts is related to their high sensitivity to sulfur; hence, it is necessary to remove sulfur-containing compounds from bio-oil before HDO treatment [43]. In a previous study conducted by [77], the catalytic activity in the HDO of guaiacol in hexadecane at 100 °C for 80 bar of H2 was as follows: Rh/ZrO2 > CoMoS2/Al2O3 > Pd/ZrO2 > Pt/ZrO2. Until now, though, the basic mechanism for transition metal in the HDO reaction is still not clear. It is accepted that the metal plays a role in the hydrogen donation; however, there is no conclusion on the mechanism for the activation of oxygenates.

Phosphide, Carbide, and Nitride Catalysts
Phosphide catalysts have been broadly studied in hydrotreatment for the petroleum industry and have thus drawn attention recently in the HDO reaction of bio-oil, which accounts for the presence of acidic sites and H2-activating sites [78]. Moreover, phosphide catalysts are characterized by the low activation energy required and highly active [66]. Mendes et al. [79] prepared a nickelphosphide supported on a carbon-covered alumina catalyst (Ni2P/CCA) to upgrade bio-oil by conducting the HDO process at 250 °C and 75 bar of H2 in a batch reactor. The results indicated that similar catalytic activity was provided by Ni2P/CCA when compared to a commercially-available catalyst (Ru/C). Gutierrez-Rubio et al. [80] investigated the catalytic HDO reaction of the mixture of guaiacol and acetic acid over Ni2P/ZSM-5 at 260-300 °C and 40-100 bar of H2. The results showed that low deoxygenation efficiency resulted from the use of Ni2P/ZSM-5 as the catalyst since the presence of acetic acid could partially block the active sites of the catalyst. Thus, it can be speculated that the removal of carboxylic acid from bio-oil before the HDO reaction might be useful to ensure a high catalytic activity.
Except for phosphide catalysts, as discussed above, carbides and nitrides have recently attracted a great deal of attention, owing to their low-cost and comparable properties to common HDO catalysts. Previously, Lopez et al. [81] used a molybdenum carbide catalyst in the HDO process to upgrade acacia wood or empty fruit bunches-derived bio-oil at 350 °C and 50 bar of H2 for 4 h, and it was observed that the oxygen content of bio-oil was greatly reduced and thus improved the HHV of bio-oil from 29.1 MJ/kg to 36.9 MJ/kg due to the formation of the hexagonal β-Mo2C phase that demonstrates a strong ability for the deoxygenation. In the catalyst recycling studies, the high catalytic activity of NiMoS2 was maintained after being recycled 5 times. Vasilevich et al. [1]. carried out the catalytic HDO experiments for guaiacol over Mo2C/SBA-15 at 320 °C and 40 bar of H2 for 180 min in a batch reactor, and a 70-73% conversion of guaiacol was attained. Even though few studies so far have investigated the influence of nitride catalysts on the HDO, it is obvious that they are more favorable than water-sensitive carbide catalysts. The presence of water, however, is normal during the HDO treatment [43]. Thus, future work must be focused on the effects of nitride catalysts on the bio-oil HDO in terms of product properties, reaction mechanism, kinetics, and deactivation.
Despite the fact that phosphide, nitride, and carbide catalysts demonstrate effective HDO efficiency for the bio-oil, they remain at the initial development stage and thus cannot be replaced with the commercial sulfide catalysts in terms of industrial applications.

Phenol and Alkylated Phenol
Phenolic monomers mainly consisting of phenols, guaiacols, and syringols as the model compounds are the simplest product obtained from lignin degradation. Phenol and alkylated phenols (such as cresol and 2-ethylphenol) are the basic lignin-derived phenolic monomers. The cleavage of the C-OH bond is the key element for the HDO process. As shown in Figure 8 there are two common reaction routes to produce cycloalkanes and arenes: (i) hydrogenation of the aromatic ring and then alcohols undergo deoxygenation to generate cycloalkanes; and (ii) the direct deoxygenation into arenes through the C-OH bond breakage [82].

Guaiacol
The conversion of guaiacol to phenol undergoes a series of reactions, such as deoxygenation, demethoxylation, and demethylation as shown in Figure 9. During the process of demethylation and demethoxylation, guaiacol is converted to form catechol and phenol, respectively. On the other hand, catechol is further converted to phenol and its derivatives methylcatechol via hydrogenation. Following this, these intermediates further deoxygenate to form benzene, cyclohexane, methylcyclohexane, and other hydrocarbons [83].

Phenolic Dimers
Phenolic dimers share a similar chemical structure to that of lignin. The most important reaction in the HDO of phenolic dimers, as presented in Figure 10, is the breakage of the ether linkage. β-O-4, α-O-4 and 4-O-5 are the typical linkages in the lignin molecular, as presented in Figure 10. After the cleavage of ether bonds, phenolic monomers are created, and they follow the same reaction as discussed above. Figure 10. The reaction mechanism for the HDO of β-O-4 dimers. Reprinted from reference [84].

HDO of Carbohydrates-Derived Oxygenates
In general, lower deoxygenation temperatures are needed for the carbohydrates-derived oxygenates, such as organic acid and ketone, compared to lignin-derived oxygenates, and the same trend can be observed for the activation energy.

Levulinic Acid
Levulinic acid is one of the most reactive biomass compounds that can be produced from lignocellulosic waste at a low cost [85]. Due to the high reactivity of the functional groups existing in the composition of the Levulinic acid, the acid can be converted into an upgraded high-quality biofuel [86].
The main reaction pathways for the HDO of levulinic acid, as shown in Figure 11, include: (1) the HDO of levulinic acid into α-or β-angelica lactone via dehydrative cyclization and then hydrogenated into γ-valerolactone, (2) the hydrogenation of levulinic acid to form 4hydroxypentanoic acid, followed by the dehydrative cyclisation into γ-valerolactone, (3) the direct decarboxylation of levulinic acid to produce 2-butanone and (4) the C-C coupling reaction by aldol addition [87].

Acrylic Acid
As illustrated in Figure 12, initially, acrylic acid adsorbs parallel to the surface, followed by deprotonation to form a propenoate species and dehydration to produce H2C=CH-CO on the Mo2C/Al2O3 catalyst surface. Subsequently, hydrogen attacks the carboxylic group of H2C=CH-CO and thus results in the formation of 2-propenal, which can be further converted into propane by hydrogenation [88].

Furfurals
A new pathway for furfural HDO over a Cu-based catalyst was proposed by Zheng et al. [89], Figure 13. In the proposed pathway, the furfural is first exposed to hydrogenation to produce furfuryl alcohol and C-C cleavage yielding furan. With successive hydrogenations of furan, it has two pathways: the first pathway produces n-butanal that can give n-butanol; the second pathway is the production of tetrahydrofuran. There are three pathways for converting furfuryl alcohol: The first pathway can occur through the C-C bond cleavage between the methyl alcohol side and the furan groups, which gives rise to furan formation. The second pathway is the hydrogenation of furfuryl alcohol into tetrahydrofurfuryl alcohol. Tetrahydrofurfuryl alcohol can produce δ-valerolactone. Through the C-O bond cleavage or tetrahydrofuran through C-C cleavage, 2-methyl furan can be produced via the C-O bond cleavage. Different products, such as 1-pentanol, 2-pentanone, and 2methyl tetrahydrofuran, can be produced from 3-methyl furan through the C-O bond cleavage [89]. Figure 13. HDO pathways of furfural. Reprinted from reference [89].
Other several feedstocks that were exposed to a treatment process for oxygen removal using the HDO process are shown in Table 7. Although HDO is highly effective for removing oxygenates from bio-oil, one of the main challenges regarding this technology is the demand for high-pressure H2. Hence, it poses safety concerns and involves high costs [25]. In addition, the formation of coke, which then leads to shortening the lifetime of the catalyst, is another challenge facing the catalytic HDO that must be solved prior to its industrial-scale application.

Coke Formation
One of the problems faced by the HDO reaction is the generally very low yield of upgraded biooil, which is caused by the formation of char, coke, and tar. The occurrence of coke becomes severe in the case of a fixed-bed reactor where coke formation can lead to the shut-down of the reactor. A possible solution to avoid coke formation in the HDO process is to co-feed bio-oil with a hydrogen donor solvent, like methanol, tetralin, and decalin. The hydrogen donor solvents can not only promote the cracking and hydrogenation reaction but are also capable of diluting lignin and thus preventing the polymerization that forms coke at a high reaction temperature [98]. Patil et al. [83] applied tetralin as the hydrogen donor solvent in the HDO reaction of guaiacol (a model compound of lignin) and the upgrading treatment was conducted at 330 °C for 15-600 min and 30 bar of H2. It was observed that a 100% conversion of guaiacol was achieved for 600 min, accompanied by a yield of 45.3% phenol and 11.1% cyclohexane.
Coke formation is dependent on the acidity of the catalyst and the type of feedstock. In general, a higher acidic catalyst tends to promote coke formation; however, the acidity is a key element ensuring the catalytic activity. In addition, the formation of coke often occurs when using unsaturated hydrocarbons (e.g., alkenes and aromatics) as the feedstock, which could be resulting from the interaction between C=C bonds/aromatic rings with the active sites of the catalyst. Oxygenates containing >2 oxygen atoms are another feedstock that have a strong tendency towards coke formation, due to the promoted polymerization reaction [99].
As suggested by Cheng et al. [99], there are several solutions to deter coke formation during HDO treatment, including (i) the use of a low acidity but high stability catalyst support, such as activated carbon and silica; (ii) carrying out HDO at moderate reaction conditions, like a relatively lower reaction condition and a higher pressure of H2; (iii) a two-stage HDO process; and (iv) co-processing of bio-oil and hydrogen donor solvent (e.g., methanol, ethanol, acetone, and tetralin) in the HDO reaction.

Catalyst Deactivation
Catalyst deactivation and its short lifetime are the bottleneck for the industrialization of the biooil HDO process. The primary causes of the catalyst deactivation include the following: (i) the presence of water; (ii) sintering of the active sites; (iii) poisoning by nitrogen, sulfur and chlorine; (iv) metal deposition, and (v) coke formation. Among them, the formation of coke and water exposure are the most significant factors causing the deactivation of the catalyst [100].
Cordero-Lanzac et al. [101] explored the underlying mechanism for the catalytic HDO of bio-oil obtained from the fast pyrolysis of black poplar sawdust at 400-450 °C and 65 bar of H2 over Pt-Pd/C and Pt-Pd/Al2O3. The authors stated that there were two main reaction pathways appearing in the coke formation: (i) at 400 °C, the presence of unstable alkyl phenol and alkylmethoxy phenols resulted in the decomposition of thermal lignin on the external surface and mesopores of the catalyst, thereby affecting the catalytic activity and possibly causing reactor blockage; and (ii) as the temperature increased until 450 °C, alkyl phenol and alkylmethoxy phenols converted into aromatics and the deposition rate of aromatic coke by condensation increased, which was further promoted by the acidic sites and high reaction temperatures. Based on these results, a possible reaction scheme for coke formation during the HDO of bio-oil was developed and illustrated in Figure 14.

Techno-Economic Analysis of HDO
In recent years, several researchers have performed the techno-economic analysis for the bio-oil upgrading by HDO reaction. Bagnato and Sanna [102] reported that the highest economic potential was estimated to be $38,234 MM/yr based on the capacity for upgrading bio-oil of 10 Mt/yr, which was accompanied by a return on investment and pay-off time of 69.18% and 2.48 years, respectively. Carrasco et al. [103] obtained a minimum fuel selling price of $1.27 per liter from the pyrolysis of woody biomass and HDO of bio-oil and the simultaneously produced H2 in a 2000 ton/day plant. They found that the high CAPEX, feedstock cost and short catalyst lifetime were the major economic concerns.

Microwave Heating
Electromagnetic radiation behaves like photons carrying radiated energy and waves moving at the speed of light. Electromagnetic waves are comprised of an alternating electric field orthogonal to an alternating magnetic field.
Due to the conversion of electromagnetic energy to heat taking place within the entire volume of the target material, respecting the penetration limits of the applied waves and the material, microwave heating is defined as a volumetric energy conversion mechanism [10,11,104,105]. This mechanism is fundamentally different from the superficial heat transfer of conventional heating. Therefore, noticeable advantages of microwave heating have been highlighted during the last few years.

Applying the Microwave in the Case of the HDO Processes
The unique advantages of MWH establish this heating mechanism as an alternative to conventional heating [CH] in several applications. The mechanism of MWH relies on direct volumetric energy conversion, i.e., direct conversion of electromagnetic energy to heat energy within the volume of the heated material. This mechanism is fundamentally different from the superficial heat transfer of CH and, thus, can avoid most of the issues and limitations associated with CH, which most importantly include the temperature gradient inside and outside the heated material. The interaction between microwaves and the heated material, mainly based on the material's dielectric properties, leads to selectively heating the target. This reduces the heat energy required to achieve a particular end, which consequently results in decreasing the operating costs as well as the potential of thermal hazards. In addition, it can restrict secondary reactions and might lead to (1) producing material with a novel microstructure, (2) initiating reactions that cannot be initiated when CH is applied, and (3) achieving the existing reactions under conditions that are different from those of traditional processing. Furthermore, numerous investigations have reported that MWH can save more in energy consumption and enhance product quantity and quality compared to the conventional processed materials [106][107][108][109][110][111][112].
Shekara C. et al. have documented that MWH achieves a better conversion compared to CH [106]. MWH modifies the porosity of the processed materials and leads to a faster reaction rate with less energy consumption compared to CH, as claimed by Paixão and Monteiro et al. [107]. Patil, Gude et al. agree with Paixão and Monteiro et al. on the impacts of microwaves on the reaction rate [113]. A higher product yield in less reaction time has been reported by Zhang and Zhao [108]. Budarin and Clark et al. have investigated the influence of microwaves on product quality compared with conventional processes, which led to documenting that the obtained oil contains few impurities and is rich in aromatics [110]. Guiotoku and Rambo et al. tend to believe that MWH increases the carbonization yield [114]. MWH decreases catalyst loading and liquefaction according to Krzan and Zagar [115]. Dogan and Hilmioglu found that MWH significantly shortens the reaction time compared to the traditional methods [116]. Increasing the conversion was reported by Sithambaram, Nyutu et al. [117]. Orozco, Ahmad et al. are in agreement with the above-mentioned claims regarding the higher product yields, shorter reaction time, and higher reaction rate at a moderate temperature [118]. The same conclusion was reached by Lucchesi [104,120]. Further details about the nature of MWH are demonstrated in the references [11,121].
In summary, it is believed that applying MWH impacts reaction kinetics in addition to the noticeable effects on the product yield, product quality, and energy consumption. It should be noted that these key conclusions highly depend on the dielectric properties of each component of the target(s) and other aspects.
In the oil industry, most common feedstocks-namely, crude oils and gaseous components-are not active microwave receptors and, in turn, fail to establish substantial interaction with electromagnetic microwaves. On the other hand, most metals, metal oxides and catalytic agents are highly efficient microwave-to-heat converters. This aspect, which is a rather complicated matter in several applications, could revolutionize the industry. Having a high local temperature on the active site promotes catalytic and noncatalytic reactions while the undesirable secondary reactions was restricted due to the relatively low temperature of the bulk material(s). As a result, it is believed that the applications of the outstanding temperature gradient of microwave heating ameliorate not only the productivity but also the selectivity of various reactions in oil processing.
Coupling microwave heating with HDO technology would take advantage of this heating mechanism. Most importantly, it includes most of the catalysts that are usually applied in HDO processes, such as the transition metal catalysts, phosphide, carbide, and nitride catalysts, oxide catalysts, and sulphide catalysts, and the oxygenated compounds that exist in the oil are polar materials. This would act as a strong microwave-to-heat converter. On the other hand, petroleum oils have a relatively low level of interaction with microwaves. In contrast, the polar site containing oxygen in the oxygenated compounds inside the oil have a high level of interaction, which means the generation of heat energy is performed locally where the C-O bond cleavage takes place. Polar compounds that exist in pyrolysis oil are strong microwave receptors.