Hydrothermal liquefaction of algae and bio-oil upgrading into liquid fuels: Role of heterogeneous catalysts

Renewable and Sustainable Energy Reviews 81 (2018) 1037–1048

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Hydrothermal liquefaction of algae and bio-oil upgrading into liquid fuels: Role of heterogeneous catalysts Ahmad Galadimaa, Oki Murazaa,b, a b

MARK

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Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

A R T I C L E I N F O

A BS T RAC T

Keywords: Algae Hydrothermal liquefaction Liquid fuels Heterogeneous catalysts Prospects

The numerous challenges attributed to fossil fuels and the transformation of food crops for energy production necessitate the search for better energy options. Algae valorization via liquefaction under hydrothermal conditions is one of the re-ignited areas of interest for liquid fuels production. Their cultivation and processing feasibilities could infer good benefits for future energy sustainability. The paper explored the role of catalytic systems, especially the heterogeneous catalysts during the liquefaction process. Solid catalysts such as supported metals, zeolites and silica-alumina are so far given preference as catalytic materials for improving bio-oil and associated hydrocarbon fuels yields. The paper therefore analyzed critical literature on the process catalysis and simultaneously discussed new directions for further investigations.

1. Introduction The fossil fuels are composed of those categories of fuels derived from decayed plants and animals over many centuries (usually many million years). There are indications that these non-renewable fuels being used globally today have their initial formation stages traced to the geological Carboniferous Period [1–3]. Since many decades, the world population relied on these fossil fuels for the production of transportation, industrial and household fuels and petrochemicals. In fact, there are indications that up to 90% of global energy relied on the fossil fuels [4,5]. However, the numerous problems associated with the fossil fuels exploration and utilization have accounted for the search for new, reliable and better options [6,7]. Among the major identified challenges, environmental pollution is a forefront issue. Right from the production fields (i.e. oil and gas fields), spillage of the crude oil products into the marine habitats have caused serious concerns over the years [8,9]. Thousands of tonnes of oil have been spilled into the marine and nearby terrestrial areas, damaging agricultural land (see Fig. 1), killing aquatic habitats and in some instances rendering the fresh water unsuitable for human consumption [10–12]. Gas flaring and combustion of the refined crude oil fuels on the other hand emit serious environmental pollutants. The emissions of CO2 and CH4 as well as hydrogen sulfide, nitrous oxide and polyaromatic hydrocarbons are very dangerous for the planet (i.e. earth) [13]. The global warming problems are associated with these emissions and are with continuous effect of global degradation [14,15].

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The agricultural lands are poorly yielding, plant and animal species are going into extinction, the level of sea ice is declining and the marine organisms are suffering [16,17]. Human exposures to some of the pollutant gases are associated with serious health degradation including the development of cancer and psychological disorders [18–20]. In addition to the environmental pollution problems, the fossil fuels are declining in reserves in most depositing areas (see Fig. 2) [21,22]. They are similarly unevenly distributed (i.e. large deposits in some regions with little or no deposits in other regions). According to the statistical information recently published by British Petroleum (BP), the total world reserves of fossil fuels are 1688 bbl, 186tcm and 892 bt of crude oil, natural gas and coal, respectively [23]. The crude oil in particular is expected to be completely exhausted by 2067, whereas the natural gas by 2069. In fact, the disadvantages of fossil fuels are too numerous and very dangerous for global sustenance. The shift to biofuel alternatives have in the recent years became an important issue of interest in the world [24,25]. Their environmental sustainability, abundance, distribution pattern and processing feasibilities have accounted for their strong potentials as fuels for the future [26]. Like the fossil fuels, biofuels can be derived in the liquid, solid and gaseous forms for various household and industrial applications [27,28]. They can be utilized in existing infrastructure and automobiles without the need for re-configuration [29]. They are not associated with destruction to marine organisms because biomass would be produced and processed into fuels using 100% onshore infrastructure. Similarly, the issue of gas flaring can be eradicated completely. However, the used

Corresponding author at: Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia. E-mail address: [email protected] (O. Muraza).

http://dx.doi.org/10.1016/j.rser.2017.07.034 Received 8 January 2017; Received in revised form 6 April 2017; Accepted 9 July 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

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preferentially due to their advantages that include availability, reusability and established prospects in biogas, biodiesel and bioethanol production from biomass substrates. Post-liquefaction valorization of algae-based bio-oil was also substantially covered. The ability of the heterogeneous catalyst systems to produce hydrocarbon compounds (including the gasoline scale BTEX aromatics) that are in the fuels range was the central focus of the paper. The paper simultaneously identified and discussed areas requiring further investigations for the benefit of other researchers in the field. 2. Biofuels potential of algae Algae are commonly described as non-flowering plants that are mostly found in aquatic environment. However, they can be cultivated in almost all the types on environments suitable and no-suitable for agricultural crops production [43]. The algae species, which could be multi- or singled-cell organisms are characterized by chlorophyll like other plant species, but are lacking in some features like true stems and vascular tissues. The diversity of the algae species (i.e. millions of species) is a forefront issue in realizing their potentials in addressing the challenges of environment and fuels production facing the world today. The marine algae are accounting for the fixation of at least 40% of CO2 gas through the carbon fixation cycle [44,45]. They are similarly very fast growing plants [46]. In fact, there are evidences that the average growth rate for algae is more than twice that of most other biofuel-crops. They possessed 20–50% of extractable oil that can be upgraded into biodiesel with less-difficulty [47,48]. Some dry algae biomass can possess more than 50% of long-chained hydrocarbons that can be processed into gasoline, diesel or even the jet fuels [49]. Fig. 3 provides a schematic representation of some of the fundamental routes for algae valorization. After development of the suitable algae strains for production and harvesting, the derived algae species are subjected to extraction and refining processes for the production of liquid fuels. The residues can also be transformed into biogas or utilized as feed for animals. Recent studies have documented a good prospect for the algae species (i.e. both macro- and micro-alage) as biodiesel feedstock, especially due to high lipid content and economics of production. A model study by Speranza et al. [50], demonstrated that algae cultivation for biodiesel production had been economically very profitable for countries in the Europe, USA and Brazil. They found the production efficiency to range between 90% and 100%. Similarly, the production was projected to be successful without recourse to the food land. Similar findings were also reported by other authors from many parts of the world, including the African countries [51– 56]. An acid-catalyzed upgrading of oil extracted from algae species (Spirulinasp. and Chlorellasp.) with H2SO4 as catalyst at 60 °C for a period of 1 h by Nautiyal et al. [57] showed that up to 80% yield of biodiesel could be achieved. Chen and co-workers [58] found 62% yield of oil from Scenedesmus sp. when transesterified with KOH as catalyst at 65 °C. The biodiesel yield reached 100% within 30 min of reaction period. Infact, the

Fig. 1. Thousands of tons of oil spill into the environment over the years 1970–2015). Data source from ITOPF. Ref: [19].

Fig. 2. Estimated period (in years) for exhausting the remaining reserves of fossil fuels from 2014. Data sourced from BP. Ref: [21].

of edible biomass feedstock (e.g. vegetable oils, corns and cereals) for the biofuels production had been classified as unsustainable due to possibility of hunger and price control challenges [30,31]. Therefore, a shift to non-edible sources would be the most appropriate choice. There is in the recent times a special consideration for algae and algaebiomass as the affordable sources of biofuels. This can be attributed certain considerable advantages [32]. Algae species could be cultivated throughout the year, and are similarly higher yielding in terms of oil composition. For example, the yielding biodiesel potential of algae oil is 12,000 l/ha compared to 1190 l/ha for the best vegetable oil crop [33]. The ability of the algae species to be grown, requiring lesser water supply, implies a positive advantage for freshwater conservations when compared to the situation involving seed crops [34]. Similarly, algae species can be cultivated even in brackish water (i.e. salty waters) producing commercially good yields. Another important environmental benefit is the ability of the algae species to fix CO2 gas and prevent it from escalating the problems of global warning [35,36]. The advantages identified above are certainly crucial for the current and future world generations that are already into energy and environmental crisis. To this scale, authors have studied the upgrading of algae and algae-biomass into different fuel products under different conditions, especially for the production of biodiesel and biogas [37– 42]. However, catalytic thermal liquefaction is a recent issue of interest. 1.1. Objectives of the review The main objective of the current paper was to review a wide range of literature on the progress made regarding the exploitation of algae as sources of fuels (i.e. both liquid and gaseous fuels). Therefore, details on the biofuels potentials of algae were first presented in Section 2, and in the subsequent sections emphasis was given to the role of catalysts and catalytic parameters during algae liquefaction into liquid fuels (i.e. gasoline, diesel and jet) via hydrothermal process. We have carefully identified and analyses literature on heterogeneous catalyst systems

Fig. 3. Routes for algae valorization into biofuels and useful residues.

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process of biodiesel production from algae feedstock had been comprehensively documented in some of our published works. It had been practically established that the production of oil from algae and its subsequent upgrading into biodiesel of internationally approved compositions and properties could be more efficient than for other feedstock from soybean, sunflower and palm. Transesterification of the derived algae oil with heterogeneous bases and acids like the group II oxides, composites of CaTiO3, CaCeO3, CaZrO3, WO3/ZrO2, H4SiW12O40, H3PMo12O40, H3PW12 O40 and related compounds had been found the most economically and environmentally sustainable approach. Mechanistically, the transestrification process is an equilibrium-mediated system through which the triglyceride esters in the algae oil are reacted with monohydric-alcohols (usually C1 to C3) to produce biodiesel and glycerine. Initially, the first –OH group gets introduced into the triglyceride ester network yielding a monoalkyl ester. The three steps sequential process finally eliminates the glycerine byproduct and three molecules of biodiesel esters (i.e. monoalkyl esters). In the past, the derived glycerine (i.e. glycerol) was considered as a waste product by the Companies. However, technologies are now available for glycerol purification and upgrading into numerous other raw-materials of valuable industrial importance. These include lighter alcohols, glycerol carbonates, glyceraldehydes and its derivatives and many medicinallybased products. These opportunities have certainly escalated the prospects of algae as biofuels feedstock. The potentials of algae-biomass for valorization into fuel-grade biogas had also been substantially studied [59]. The algae species are characterized by numerous and specific advantages that favor their preference for this purpose. They possessed large quantities of polysaccharides and lipids with insignificant concentrations of non-biodegradable compounds [60]. Their rapid growth rates entailed a yearround production possibility. Fermentation process had been identified as the most suitable degradation option for the biogas (i.e. methane) production from algae. According to Harun et al. [61], the entire biomass-compositions of algae (including the lipids and proteins) are very susceptible to biodegradation and therefore are environmental friendly feedstock. Wiley et al. [62] showed the whole production chain (i.e. from algae cultivation to biogas usage) as economically viable and sustainable. According to Vergara-Fernendez et al. [60], the biogas yield of most known algae species could be as high as 60–70%, which is economically sustainable. Similar observations were also corroborated by different authors who examined a range of algae species for biogas production [63–67].

Fig. 4. Schematic approach to hydrothermal liquefaction of algae with or without catalyst.

hydrocarbons from other products. Their variable compositions are usually within the range suitable for gasoline, diesel and jet fuels. The rising degree of publications in the area of algae liquefaction in the last couple of years is an indication of the raising interest for the process. The studies have mostly targeted the influence of reaction conditions such as temperature, pressure, feedstock composition and the liquefaction reaction time on the bio-oil yield. Some of these key studies would be presented here. Singh et al. [72] conducted the upgrading of different algae species at 280 °C under hydrothermal conditions (algae/water = 1:6) for a short period of 15 min. A correlation had been established between the feedstock compositions and the bio-oil yield. The algal specie (Ulva fasciata sp.) with characteristic high carbohydrate composition produced the highest conversion of 81% compared to other species such as Sargassum and Enteromorpha Sp. These species produced 77% and 67% conversions, respectively. Their reaction products were majorly solid residues. As evident from the FT-IR characterization data, the bio-oil derived from the former algal-specie comprised mainly of hydrocarbon compounds whereas oxygenates dominated the bio-oils of the algae species. A similar study was reported by Biller and Ross [73], who employed four different algae species (i.e. Chlorella Sp., Nannochloropsis Sp., Phorphyridium and Spirulina Sp.) The liquefaction process was performed at 350 °C by utilizing a water/feedstock ratio of 9:1. In this case, the bio-oil yield linearly correlated with the protein content of the algae species. The Spirulina Sp. with 65% protein composition produced the highest bio-oil yield of 29% compared to the other species with protein content < 60%. In fact, different studies have shown that without catalyst the bio-oil is protein content dependent, whereas the composition of carbohydrates and lipids usually determines the yield of bio-oil when catalyst is employed during the liquefaction process [72–77]. The work of Anastasika and Ross [78] investigated how the reaction parameters influenced the compositional products from Laminaria saccharina Sp.liquefaction. For the reaction carried out between 250 and 370 °C, the bio-oil generally increased with increasing reaction temperature whereas the production of residual materials declined. A similar trend was observed when the ratio of feedstock/water was raised from 1:15 to 1:10 for the reaction period of 1 h. On the other hand, increasing the reaction time favored bio-oil yield because enough time was usually required to ensure complete feedstock transformation. Jena et al. [79] varied the same parameters for Spirulina platensis upgrading into bio-oil. Increasing the temperature from 200 to 380 °C enhanced the production of bio-oil progressively up to 350 °C, beyond which no further activity was observed. The yield of bio-oil at temperatures exceeding 300 °C reached between 50% and 63%. Increasing the liquefaction time from 30 min to 2 h raised both the feedstock conversion (i.e. carbon conversion) and bio-oil yield, as similarly observed with improving the feedstock concentration from 10% to 50% in excess of water. A number of other studies were

3. Hydrothermal liquefaction of algae Hydrothermal liquefaction of algae biomass is a recently considered alternative for upgrading the algae-biomass into more useful liquid fuels. The process normally proceeds by subjecting of water containing algae-biomass to heating at temperatures and pressures in the range of 200–380 °C and 10–20 MPa, respectively (see Fig. 4) [68,69]. The process degrades and upgrade the building compounds in the algaebiomass into a bio-crude oil. The presence of water in the reaction media initiates the cleavages of chemical bonds in the substrates leading to the bio-oil production. Another important role of water during the reaction is to allow efficient separation of the derived bio-oil after the reaction. It is a known fact that the algae species are composed of polymeric substances such as carbohydrates and proteins. At the early stage of the hydrothermal process, these polymers are degraded into their monomeric derivatives [70,71]. These are then further cracked into fragmented molecules like simple sugars, alcohols, ketones and aldehydes (i.e. oxygenates). The fragmentation (i.e. cracking) process removed the oxygenates and heterogeneous derivatives containing N-, S- and Pspecies, to produce mainly hydrocarbon compounds. Although the crude bio-oil derived can be used directly for energy applications due to high heating value in the range of 30–40 MJ/Kg, refining process would be necessary to separate the high quality form of the fuels-range 1039

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hydrocarbons (see Fig. 6). On the other hand, the protein compounds were known as polymeric-derivatives of amino acids coupled together by peptide linkages. They are susceptible to rapid hydrolysis in hydrothermal conditions, forming amino acid monomers and subsequently the nitrogenous compounds found in the final bio-oil product (see Fig. 6). They are also responsible for the production of ammonia and nitrogenous gases during the upgrading process. There are indications that poly-hydrocarbons called algaenans undergo direct degradation into aromatics and lighter n-alkanes and alkanes [91-93], that could considered suitable for gasoline, diesel and jet fuel usage. Therefore, their conversion is critical for the bio-oil hydrocarbons compositions. Although the above detailed processes were believed to occur during algae biomass liquefaction in aqueous media, the overall mechanism of the process is not yet established. Recent attempts by some authors such as Torri et al. [93], and Faeth et al. [76], gave emphasis to the rupture of bonds associated with N-containing compounds, although some other cleavages based on the C–O, C–C and C–S were also identified. They argued that at temperatures exceeding 300 °C, polymeric compounds like proteins and cellulose underwent fragmentation into amino acids and simple sugars as well as other oxygenates. However, these observations are still not fully resolved. The incorporation of catalysts (i.e. homogeneous or heterogeneous) can promote or retard the occurrence of these intermediate reactions. Therefore, their activity could include the modification of bio-oil yield and its compositions.

reported to show parameters such as the composition of the algaefeedstock, operating conditions and reactor configuration as influential in enhancing the yield of bio-oil. Such studies have indicated the optimal bio-oil yields to fall within the range of 50–60% [78–83]. Therefore, new strategies for improving these yields would be very important for industrial considerations. There are some recent studies that captured new improvements in terms of algae liquefaction for biofuels production. According to Shanmugam et al. [84], in addition to excellent bio-oil yield, the aqueous phase of the derived bio-oil could be used for the production of phosphorous and nitrogen nutrients. The fertilizer-based feedstocks were recovered with efficiency of 99% and 100%, respectively. The biooil on the other hand was also rich in oxygenates that could be upgraded into aromatics and other hydrocarbon fuel products. Similarly, the gaseous compositions of the liquefaction reaction produced high methane yield (i.e. 182 ml/g), indicating a good prospect for biogas fuel generation. According to Biswas et al. [85], production of the bio-oil and associated gaseous fuels (e.g. biogas) is dependent on the reaction temperature and the nature of the liquefaction solvent employed. In the presence of water as, the Sargassum tenerrimum algae employed for the studies yielded high bio-oil content at the moderate temperature of 280 °C. On the other hand, organic solvents such as methanol and ethanol produced higher respective bio-oil yields of 22.8 and 23.8 wt% compared to 16.33 wt% found with water. Majority of the bio-oil compounds are soluble in organic solvents and therefore this factor accounted for their higher production. At comparable temperatures, water interacts with these compounds to mediate a series of reactions including hydration, chain reactions and disproportionation and therefore reducing the overall bio-oil yield [84–86].

4.1. Homogeneous catalysts Homogeneous catalysts otherwise called soluble catalysts are catalyst systems that exist in the same phase with the reactants. Such catalysts are usually soluble in the solvent media through which the reaction is taking place. The application of these catalysts for algae liquefaction is an earlier approach compared to the heterogeneous catalysts. The common forms of the homogeneous catalysts under exploitation for the liquefaction process are the mineral and organic acids, their corresponding salts, metallic cations of transition elements (e.g. Zn2+ and Co3+) and the alkali compounds. Their mechanism of action involved aiding the cleavages of C–C bonds in the algaefeedstock, enhancing hydrolysis as well as promote dehydration. These series of processes are responsible for the production of a wide range of oxygenates, hydrocarbons, gases as well as solid residues after the liquefaction process. However, there are literature evidences that the homogeneous catalysts do not easily produce hydrocarbons that are suitable for gasoline, diesel and/or jet fuels. The work of Jena et al. [94] was one of those critical studies involving such categories of catalysts. They compared the influence of selected catalysts such as Na2CO3, Ca3(PO4)2 and NiO for upgrading Spirulina platensis Sp. Liquefaction process was performed utilizing algae/water ratio of 1:4 for a period of 1 hand at 300–350 °C temperatures. Consistent with some previous studies [93–98], the Na2CO3 produced a bio-oil yield of 52% (i.e. 29% higher than for the uncatalysed process). On the other hand, Ca3(PO4)2 and NiO systems produced a negative effect on the bio-oil yield. They reduced the yield by more than 14%. The higher activity of the Na2CO3 catalyst was attributed to its reactivity in the formation of CH3CONa at the intermediate stages, which later participated in other reactions of interest like decarbonylation, dehydration as well as hydroxylation. These in turn generated the oxygenates and hydrocarbon derivatives in the bio-oil. The incorporation of Na2CO3 enhanced the production of BTEX (i.e. benzene, toluene, ethylbenzenes and xylenes) and C5 to C18 aliphatic hydrocarbons to 45%. These compounds were identified as very important components of the gasoline and diesel fuels. According to Shakya et al. [99], the activity of Na2CO3 catalyst was dependent on the composition of the algae feedstock and the associated reaction conditions. For the reactions conducted at 300–350 °C, the micro-algae

4. Hydrothermal liquefaction catalysts The catalysts employed for algae liquefaction under hydrothermal conditions can be categorized into the homogeneous (i.e. water soluble) and the heterogeneous (i.e. non-water soluble) catalysts. However, understanding the chemistry of the conversion processes occurring would provide a baseline for adequately exploring the role of the catalyst systems. There are different opinions on the chemistry involved during the liquefaction process but a unique agreement among authors so far is that any factor that enhances the degradation of algae-biomass into the desired bio-oil is critical for the reaction chemistry. The incorporation of hot water could be attributed to its unique properties such as promoting the solubility of less-soluble organic compounds (i.e. hydrophobic compounds) and the availability of ions (i.e. H+ and OH-) which are vital for initiating and catalyzing the intermediate reactions of interest like hydride transfer, nucleophilic addition versus cleavage and cracking [78]. On the contrary, when supercritical water (SCW) is employed for the process, radical species are commonly generated and consequently the production of gaseous reaction products (especially CH4 and C2 –C4 as gaseous hydrocarbons). Irrespective of whether the reaction proceed with supercritical water or not, the early stages of the reaction involved rapid depolymerization of polymeric compounds in the algae biomass. The depolymerization or degradation process depends on the species involved [87]. There were some literature arguments that the depolymerization of carbohydrates occur very fast and does not normally produce bio-oil but rather generates oxygenates such as aldehydes and ketones (see Fig. 5) [88,89]. However, these derivatives can underwent further conversion into hydrocarbons including aliphatics and aromatics, especially in the presence of selective catalyst systems. The lipid components are usually degraded into triglycerol derivatives and consequently glycerol. The process also emit gaseous products like hydrogen, CO and CO2 [90]. However, the fatty acid compounds due to their unique resistance to degradation can undergo decarboxylation to produce corresponding 1040

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Fig. 5. Oxygenates compounds obtainable from carbohydrates degradation during algae liquefaction.

Fig. 6. Typical reaction products obtainable from fatty acids/proteins degradation during liquefaction process.

recently reported by Zhuang et al. [102]. Bio-oil yield was found to increase with increasing the concentration of the catalyst (i.e. H2SO4). With 1% catalyst the yield was < 20%, but increased to > 70% when the acid concentration reached 6%. However, the GC-MS data revealed oxygenates as the dominant products in the bio-oil (i.e. produced to ~ 67%). Therefore, H2SO4 was very active for depolymerization, but does not effectively facilitated reactions such as cracking, hydride transfer and decarbonylation suitable for the production of fuels range hydrocarbon derivatives. Similarly, the results indicated that the microalgae specie (i.e. Ulva prolifera) employed could be considered rich in the polymers of oxygen-based molecules. Yang et al. [103] conducted similar studies using H2SO4 and CH3COOH as catalysts for Enteromorpha Sp. (another macroalgae specie) upgrading at 290 °C for a period of 20 min. Their results reported 28% as the maximum obtainable bio-oil yield, whose compositions were mainly oxygenates including carboxylic acids and glycerol derivatives. The production of aliphatics, especially alkenes was suppressed over time whereas the H2SO4 catalyst further promoted the esterification of glycerol into ester compounds. According to these results, the production of fuel range hydrocarbon compounds, especially the gasoline (including BTEX) and diesel was very difficult with macroalgae species when H2SO4 was incorporated as the homogeneous catalysts. According to Zou et al. [104], H2SO4-catalyzed liquefaction can predominantly produce derivatives of ketones and organic acids as the

species with high lipid and carbohydrate contents (i.e. Pavlova and Isochrysis sp.) produced higher bio-oil yields (50–60%) when the catalyst was employed. Therefore, these compounds were primarily degraded into monomers of aldehydes, ketones, phenols and associated oxygenate compounds which were found as important components of the bio-oil [97–101]. On the other hand, the bio-oil yield was significantly reduced for Nannochloropsis sp.of comparable strength having higher protein content. The content of the hydrocarbon species in the bio-oil was dependent of the reaction temperature irrespective of the algae specie. At temperatures of 300–350 °C, BTEX, long-chained aliphatics and cyclics showed an increased yields with increasing temperature. However, their concentrations declined at higher reaction temperatures due to subsequent cracking. It was therefore established that the production of lighter fuel hydrocarbons (i.e. gasoline range) requires the effect of low to moderate temperatures (i.e. like the 300– 350 °C employed) even when the Na2CO3 catalyst was incorporated. At higher temperatures, the process mainly produced gaseous products. Under the hydrothermal condition, the catalyst became splitted into Na2+ and CO32- both of which interacted with the substrate to initiate C–C and C–O bond breaking, but slightly catalyzed secondary cracking, isomerization and hydride transfer reactions due to the absence of active Brønsted acid sites. Acid-catalyzed upgrading of Ulva prolifera Sp. (a macro-algae) was 1041

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Table 1 Influence of homogeneous catalysts on the liquefaction of algae under hydrothermal conditions. Catalyst

Algae specie

Liquefaction Conditions

Bio-oil Yield, %

Hydrocarbon (i.e. BTEX, fuels-range), %

Ref.

KOH Na2CO3 CH3COOH HCOOH Na2CO3 KOH

Spirulina Spirulina Spirulina Spirulina E. prolifera Laminariasaccharina

350 °C, 350 °C, 350 °C, 350 °C, 300 °C, 350 °C,

15.2 20.0 16.6 14.2 23.0 67

Mainly oxygenates Mainly oxygenates Mainly oxygenates Mainly oxygenates 20.0 Not reported

[95] [95] [95] [95] [105] [106]

3 g algae + 27 ml of catalyst solution 3 g algae + 27 ml of catalyst solution 3 g algae + 27 ml of catalyst solution 3 g algae + 27 ml of catalyst solution 30 min, 20 g algae + 150 ml water + 5 wt% catalyst. algae/water = 1:10, 15 min.

were were were were

produced. produced. produced. produced.

4.2. Heterogeneous catalysts

main components of the bio-oil but this also depends on the nature of the algae specie (i.e. macro-algae or the microalgae). They upgraded Dunaliellatertiolecta Sp.(a micro-algae) using variable acid concentrations and temperatures (120–200 °C). The bio-oil produced composed of more than 90% of esters, carboxylic acids and ketones with insignificant yield of hydrocarbon compounds. Chemically, the production fuel grade hydrocarbon compounds requires a catalyst that can catalyze reaction such as deoxygenation of the oxygenates, cracking, hydrogen transfer and isomerization to a high degree. Therefore, according these results, achieving these could be difficult for both the micro- and macro-algae species when H2SO4 was utilized as the homogeneous catalyst system. Some additional studies on the role of homogeneous catalysts during algae upgrading to bio-oil via the liquefaction process were presented in Table 1. Generally, the incorporation of these catalysts do not yield large quantities of hydrocarbons suitable for liquid fuels applications [93,103,104]. Similarly, irrespective of the strength of the algae specie, the yield of bio-oil can be as low as 15%, leading to the formation of mainly residues and gaseous reaction products. Oxygenate compounds were mainly produced by these catalytic materials. Therefore, the produced bio-oil must be subjected to ex-situ upgrading via hydrotreating to generate the hydrocarbons that fit gasoline, diesel and jet fuels compositions. It could be observed that KOH and Na2CO3 were usually the most effective homogeneous catalysts for the liquefaction process due to higher yield of bio-oil compared to other homogeneous catalyst counterparts. However, the nature of the reaction products obtained was closely the same. Similar observations were corroborated by some other authors [105–107]. There are indications that a lot of homogeneous catalyst systems were successful. According to a recent study by Muppaneni et al. [107], homogeneous catalysts such as KOH can produce additional 5–10% of biooil than for the non catalytic process under similar reaction conditions. They performed the Liquefaction of microalgae (Cyanidioschyzon merolae) with 10 wt% solid loading for 30 min at temperatures in the range of 180–300 °C. Without the incorporation of any catalyst, the maximum bio-oil yield was 16.98% but increased significantly to 22.67% when the KOH catalyst was incorporated under comparable reaction conditions. Fully evaluated for this application, especially due to low accessing and/or production costs. They can produce bio-oil with limited coke formation during the liquefaction reaction. Similarly, their solubility promotes the interaction between algae feedstock and the active ions in solutions. However, there are certain identified challenges that could significantly hinder their prospects for this application, especially in the situation where fuel range hydrocarbons are required to dominate the bio-oil product. They hardly catalyze intermediate reactions such as decarboxylation of organic acids, isomerization and hydride transfer as well as aromatization, which are very critical for the production of hydrocarbons (including the BTEX). They are non-recyclable and therefore must be disposed at the end of the reaction. This is associated with environmental pollution problem. Similarly, the bio-oil yield could be low (i.e. < 20%) in some instances, which is not so economical for industrial considerations.

The incorporation of heterogeneous catalysts for algae-biomass liquefaction could be attributed to an attempt to mitigate the challenges encountered with their homogeneous counterparts. The heterogeneous catalyst systems, which are solids, exist in different phase with the reaction media [108-110]. Their primary benefits include the feasibility of separation at the end of the process and the possibility of reuse for several cycles [111,112]. Such catalyst systems could be more resistant to harsh operating conditions that normally destroy the homogeneous catalyst materials [113–115]. There modes of action are also entirely different [116]. The catalysis involved adsorption-desorption of reactant species over the catalyst surfaces. This consequently initiates sequential elementary steps at the intermediate stage of the reaction [117,118]. In algae-biomass liquefaction, the action of the heterogeneous catalysts should be to promote rapid bonds cleavages and facilitate the production of bio-oil and consequently fuel-range hydrocarbons. Although their exploitation for this application started recently, several catalytic materials were evaluated. They include supported metals (e.g. Pd and Pt supported over carbon), zeolites and SiO2 and Al2O3 materials. The work of Biller et al. [118] examined the catalytic activity of metals (i.e. Pt, Ni and CoMo) supported Al2O3 for the upgrading of two algae species (i.e. Chlorella vulgaris sp. and Nannochloropsisocculta sp.). The active metals were deposited onto the alumina support via incipient impregnation method involving 20 wt% metal-loading, except in the case of Co which was only 6 wt%. Algae valorization was later performed at 350 °C using a 1:9 (catalyst: water) for a period of 1 h. The yield of bio-oil was dependent on both the nature of the algae specie and the catalyst composition. Without the addition of any catalyst, the yields of bio-oil were 36% and 34% for the two algae species, respectively. The Pt/Al2O3 and CoMo/ Al2O3 raised the bio-oil yield from Chlorella sp. to 39% (indicating a 3% increased), whereas the Ni/Al2O3 catalyst decreased the yield to 30% (i.e. 6% reduction). On the other hand, all the catalysts caused an overall reduction of bio-oil yield with the Nannochloropsisocculta sp. In fact, the yield reduced to only 18% for the Ni/Al2O3 catalyst. Chromatographic analysis of the bio-oils showed the presence of paraffinic and aromatic hydrocarbons following the catalyst incorporation, especially with the CoMo/Al2O3 and Pt/Al2O3 catalysts. This was attributed to their high degree of C–C and C–O cleavages as well as susceptibility to deoxygenation of oxygenates than the Ni/Al2O3 catalyst. Similarly, the CoMo/Al2O3 could be very tolerant to impurities like S- and N-species present in the reaction media [120–125]. Yang et al. [126] employed Ni/REHY for the conversion of Dunaliella Sp. The Ni-particles were incorporated onto the REHY support via wetness impregnation with nickel nitrate solution at ambient temperature. The upgrading process was conducted at 200 °C for a period of 1 h, using 2 g of catalyst in 90 ml of solvent at 2.0 MPa pressure. Without the addition of any catalyst, the bio-oil yield was 35% but increased to 52% and 72% when REHY and Ni/REHY were incorporated, respectively. It was therefore established that both the REHY support and the Ni-particles synergically improved the algae conversion by catalyzing bond cleavages and depolymerization process. All the bio-oils were composed of oxygenates such as ketones and

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the C. pyrenoidosa specie produced primarily C6-hydrocarbons 5– 10%) with H-ZSM-5 and C6 to C16 hydrocarbons (> 20%) with the Ce/ H-ZSM-5 catalyst. Therefore, the presence of Ce-species in the zeolite promoted both bio-oil and hydrocarbons yields. The study by Gao and co-workers [128], demonstrated that the incorporation of a heterogeneous catalyst system can produce high biooil yield with very low nitrogen content. They employed MgAl-LDO/HZSM-5 as catalyst for the valorization of natural algae (Cyanobacteria). The optimal reaction conditions of 550 °C, heating rate of 10 K/min and catalyst/substrate ratio of 0.75 produced > 41% of bio-oil with basically negligible nitrogen content. In their recent review, Dimitriadis and Bezergianni [129], reported microalgae species hydrothermal liquefaction with heterogeneous catalysts of the oxides and zeolites category as an important approach for improving the bio-oil yield, reducing reaction byproducts and the considered the process as environmentally sustainable than when homogeneous catalysts were employed. Similarly, the chance of producing fuel-grade hydrocarbons (including BTEX aromatics) could be enhanced. The different published works reported herein have indicated heterogeneous catalysts as good candidates for improving the yield of bio-oil and the required hydrocarbon species that can fit the requirements for liquid fuels applications. Catalysts based on Pd, CoMo and H-ZSM-5 zeolite have so far demonstrated good efficiency in this regard. However, there is still a long way to consider industrial application of the liquefaction process. Development of excellent catalysts that can produce up to 90% bio-oil yield and corresponding hydrocarbon compositions would be more economical for the industry. The evaluation of important parameters such as effect of active metal loading, zeolite topological versus acidity properties and algae/water ratio can be critical to achieving optimal catalytic properties. Similarly, extending the reaction period to several hours can provide an insight into catalyst stability with time. Detail investigations on the mechanisms of actions of the various catalysts would also be very helpful. There are other heterogeneous catalysts such as heteropoly acids and promoted-zirconias that have not been evaluated. Further studies can explore with these catalyst systems, especially due to their records in acidity-dependent reactions suitable for hydrocarbon formation and/or upgrading [130–133].

carboxylic acids. However, the composition of hydrocarbons was much higher for the Ni/REHY catalyst whereas the composition of O- and Sbearing compounds was very negligible. Therefore, the catalytic processes such as deoxygenation and desulfurization were favored by this catalyst. The uncatalyzed reaction produced up to 33% of glycerols and their derivatives due to low bonds cleavages and depolymerization activity. Zhang et al. [124] conducted the liquefaction of Chlorella sp. using Raney-Ni and H-ZSM-5 as catalyst systems and ethanol as the solvent unlike the water utilized by all the other authors. Their reaction conditions included variable temperatures (200–300 °C), pressures (2.8–2.9 MPa) and 0.5 h reaction period. They established that the incorporation of either catalysts does not improve the yield of bio-oil for the conditions studied. However, there were indications that the catalysts generated different reaction products when compared with the non-catalytic process due to improvement in the degree of bonds rupture, depolymerization and hydrogenation. With the catalysts, light fuel-range (i.e. gasoline range) hydrocarbons were produced as a result of cracking reactions, especially when hydrogen was introduced into the reaction media. Its presence promotes the hydrogenation of C˭C bonds in the molecules, especially when the Raney-Ni catalyst was used. In Table 2, recent studies were documented on the influence of some heterogeneous catalysts during liquefaction of selected algae species. According to Duan et al. [125], the incorporation of Pd/C and CoMo/Al2O3 catalysts can improve the bio-oil yield from 35% for the non-catalytic process to 57% and 55%, respectively. Similarly, the concentration of oxygenates can be significantly reduced due to the influence of catalysts. Without catalyst, the Nannochloropsis sp. produced mainly oxygenates whereas up to 20% hydrocarbons were obtained from the catalytic reactions. An important issue is that the hydrocarbons are within the ranges desired for fuel (i.e. diesel or jet) requirements. The catalytic behavior was also similar when Pt/C and Ru/C were employed as catalyst systems. However, their bio-oil yields were generally lower than for the former catalysts. Yang et al. [126] employed a similar algae specie and Pd/C catalyst under hydrogen pressure. Unlike the commonly employed single reactor system, the authors utilized a two-chamber batch reactor that physically separated the 5% Pt/C catalyst from the algae and any solid material by a porous metal frit. About 35% of bio-oil was derived from the non-catalytic reaction. However, the value increased to 40% and 38% for the Pd/C catalyst when 10 and 30 bar of hydrogen were applied, respectively. Therefore, moderate hydrogen pressure was sufficient enough to facilitate depolymerization and other intermediate reactions suitable for bio-oil production. Under both pressure conditions, the formation of hydrocarbons was nearly constant. Aromatics and C5 to C20 alkanes were dominantly produced. Xu et al. [127] found the incorporation of H-ZSM-5 and Ce/H-ZSM-5 catalysts to raise the yield of bio-oil from 32% for the non-catalytic process to 38% and 52% for these catalysts, respectively. While only oxygenates were produced in the former case,

4.2.1. Bio-oil upgrading with heterogeneous catalysts Another important approach for producing fuels range hydrocarbons is the post-liquefaction upgrading of the derived bio-oil. Usually uncatalyzed liquefaction of the algae-biomass is initially performed and then followed by the bio-oil upgrading. The process involved the application of suitable heterogeneous catalyst to convert the oxygenate compounds into hydrocarbons [134]. Although the application of hydrotreating catalysts like Pt/C and Co-MoS2/Al2O3 had been reported [135], zeolite systems are the main heterogeneous catalysts exploited for the upgrading process. This can be attributed to their recorded performance during ex-situ upgrading of bio-oil derived from

Table 2 Influence of some heterogeneous catalysts during algae upgrading to liquid fuels. Catalyst

Algae specie

Non-catalytic Pd/C Pt/C Ru/C Ni/SiO2-Al2O3 CoMo/Al2O3 Zeolite Non-catalytic Pd/C Pd/C Non-catalytic H-ZSM−5 Ce/H-ZSM−5

Nannochloropsis Nannochloropsis Nannochloropsis Nannochloropsis Nannochloropsis Nannochloropsis Nannochloropsis Nannochloropsis Nannochloropsis Nannochloropsis C. pyrenoidosa C. pyrenoidosa C. pyrenoidosa

sp. sp. sp. sp. sp. sp. sp. sp. sp. sp.

Liquefaction conditions

Bio-oil yield, %

Hydrocarbons composition, %

Ref.

350 °C, 1 h, 0.384 g of catalyst, 95% water volume. 350 °C, 1 h, 0.384 g of catalyst, 95% water volume. 350 °C, 1 h, 0.384 g of catalyst, 95% water volume. 350 °C, 1 h, 0.384 g of catalyst, 95% water volume. 350 °C, 1 h, 0.384 g of catalyst, 95% water volume. 350 °C, 1 h, 0.384 g of catalyst, 95% water volume. 350 °C, 1 h, 0.384 g of catalyst, 95% water volume. 350 °C, 1 h, 15 mg of catalyst, 87.5% water volume 10 bar H2. 350 °C, 1 h, 15 mg of catalyst, 87.5% water volume 10 bar H2. 350 °C, 1 h, 15 mg of catalyst, 87.5% water volume 30 bar H2. 7 g algae, 70 ml water, 0.35 g catalyst, 300 °C, 20 min 7 g algae, 70 ml water, 0.35 g catalyst, 300 °C, 20 min 7 g algae, 70 ml water, 0.35 g catalyst, 300 °C, 20 min

35 57 49 50 50 55 48 35 40 38 32 38 52

Mainly oxygenates. Mainly C15 to C18, 20%. Mainly C15 to C18. Mainly C15 to C18. Mainly C15 to C18. Mainly C15 to C18. Mainly C15 to C18. Oxygenates as dominant products. C5 to C20 alkanes, Aromatics. C5 to C20 alkanes, Aromatics. Oxygenates Mainly C6, 5–10% C6 to C16, > 20%

[125] [125] [125] [125] [125] [125] [125] [126] [126] [126] [127] [127] [127]

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The resonance stability of the aromatic rings limited their hydrogenation during the process. Duan and co-workers [147] evaluated the effect of zeolite topology and acidity characteristics at 400 °C and 0.126 mol H2 for a period 4 h. Irrespective of the zeolite structure, the same type of hydrocarbons (i.e. C9-C21) were produced. However, the selectivity was structure dependent. The H-MCM-41 catalyst produced the highest hydrocarbons yield of 83%, whereas the least yield was obtained with H-ZSM-5 (57%). Similarly, a model study with SAPO-11 material produced a lower yield of 54% under similar conditions. The results inferred that zeolites with more open-pore structure were more selective because of diffusion limitation issues. On the other hand, HZSM-5 (SiAl = 25) of higher acidity produced 61% yield of hydrocarbons compared to H-ZSM-5 (SiAl = 50) of lower acidity. High acidity is more susceptible to promote deoxygenation as well as hydride transfer for hydrocarbons formation. According to Zhang et al. [148], although both Ru/C and Pt/C produced aromatics as dominant hydrocarbons at 350 °C and 6 MPa H2 pressure, the Ru/C catalyst was more active producing a yield of 65%. A least activity was observed when the catalysts were replaced with Mo2C which produced 50% yield. Therefore, the metallic functions can also behave differently depending on the overall catalyst compositions. Bai et al. [149] found a Raney-Ni catalyst as more effective than either Ru/C or mixture of the two catalysts. The Raney-Ni material produced 87% of alkanes compared to < 60% found with the other catalysts. Recently, Duan et al. [150], conducted a valorization of algaederived and pre-treated bio-oil using a range of zeolite and SAPO-x catalysts with various acidity and structural properties. The reaction conditions employed include 400 °C, 6 MPa hydrogen pressure and 10 wt% catalyst in supercritical water (i.e. density = 0.025 9/ml). The overall yield of the reaction products and the upgrading efficiency were strongly dependent on the catalyst nature. The catalyst H-SAPO-11 yielded the lowest upgrading efficiency of 42.4% and produced 80% of hydrocarbon compounds. On the other hand, the H-MCM-41 catalyst showed the highest upgrading efficiency of 54.5% and 95.6% yield of fuel-range hydrocarbon compounds. Interestingly, irrespective of the catalyst nature, hydrotreating reactions such as hydrodenitrogenation, hydrodesulfurization and hydrodeoxygenation were favored by increased in the number of catalytically active acidic sites. In their recent review, Arvindnarayan et al. [151], corroborated that in addition to the hydrotreating reactions, processes such as decarbonylation, decarboxylation, hydride transfer and inter-mediate species rearrangements could be very influential during the upgrading of algae derived bio-oil and lipids. However, the reported tungsten based catalysts as the most effective materials for the decarbonylation and decarboxylation reactions. Similarly, the overall process was considered more cost-effective than when noble metal catalysts were employed. According to the various works, zeolites are so far more effective for the production of alkanes and aromatics (i.e. fuel range hydrocarbons) [146,148]. Their activities were dependent on structure, temperature and catalyst loading [148]. However, there are issues that need further investigations. Little attention was paid to metals (e.g. Pd, Pt, Ce etc.) modified zeolites. These catalysts can improve both the yield of bio-oil and quantity of hydrocarbons due to associated hydrogenation and isomerization selectivity properties. For gasoline fuel, the presence of large isomers concentration is very important for achieving optimal octane properties [152-153]. Studies on catalyst acidity should be extended to include the H-MCM-41 and H-Y catalysts. Similarly, the nature of the acid sites (i.e. Brønsted or Lewis) that are more favorable for the reaction should be fully evaluated. Addressing these issues can provide insights into the process prospect for large scale applications. Ru and Pt supported with C are also good candidates for the reaction [149,150], but the production of mainly aromatics implied a lesser prospect for diesel and jet fuels application. Therefore, varying catalyst loadings, incorporating different support materials like SO4/ZrO2 and nano-scale catalyst design could be evaluated for possibility of achieving optimal alkanes yield. Other areas requiring further investigations

biomass pyrolysis [136–138]. The alumino-silicate nature of the zeolite materials characterized their structural and acidity properties as well as their potential roles in particular reactions of interest [139–141]. In algae bio-oil valorization, processes such as cracking, decarbonylation and decarboxylation of O-bearing compounds in the bio-oil are the main intermediate reactions that produce the desired hydrocarbons. The process is usually conducted at temperatures in the range of 300–500 °C and elevated reaction pressures [142-144]. The reaction products could also include gaseous alkanes, COx (x = 1 or 2) and organic derivatives in the liquid phase [66,144]. Water vapor can similarly be generated due to dehydration process. Therefore, the selection of appropriate zeolite topology and acidity properties are very critical to achieving successful bio-oil valorization. The incorporation of metallic species such as Pd, Pt, Ce and Zn can facilitate hydrogenation process. This in turn converts aromatics and alkenes into corresponding alkanes that can be considered suitable for diesel, gasoline and/or jet fuels applications. The work of Barreiro et al. [144], compared the activity of Pt/Al2O3 and a H-ZSM-5 zeolite as catalysts for valorization of bio-oil derived from some algae species (i.e. Scenedesmusalmeriensis and Nannochloropsisgaditana sp.). The catalytic studies were carried out at 400 °C for a period of 4 h, employing 0.3 g of catalyst in 0.55 ml of de-ionized water. The gaseous reaction products composed of COx (x= 1 or 2) and C1-C4 alkanes, and that irrespective of the catalyst nature, methane dominated the composition with selectivities in the range of 35–45%. The two catalysts produced comparable hydrocarbon yields under the studied conditions. Pentadecane and hexadecane were the dominant alkanes produced, possibly due to hydrogenation of corresponding alkenes or the decarboxylation of the corresponding carboxylic (i.e. fatty) acids. The detection of phenols to significant concentration showed the valorization process to be incomplete. The product distribution mainly comprised of alkanes 50–70%), untransformed fatty acids, phenols and gaseous cracking species. Li and Savage [145] conducted a related study using H-ZSM-5 zeolite at high hydrogen pressure. Reactions were performed under various conditions of temperature and catalyst loadings of 400–500 °C and 5–50 wt%, respectively. Generally, the valorization process eliminated S-, O- and N-bearing derivatives by converting them into corresponding hydrocarbons with the elimination of gaseous species. In fact, the concentration of S-derivatives was reduced to < 0.1%. Reaction at 400 °C produced the optimal alkanes’ yield of 95%, indicating a good prospect for fuel purpose. However, when the temperature was raised to 500 °C, the composition of aromatics reached 44%, of which BTEX are the dominant species. Therefore, prospective for gasoline fuel purpose. The overall products distribution could be attributed to the occurrence of reactions such as hydrotreating (hydrodeoxygenation, hydrodenitrogenation and hydrodesulfurization), hydrogenation, hydrocracking and aromatization [146]. This can also be confirmed from the evolution of N-, O- and S-bearing gases as well as lighter alkanes. Duan and Savage [146] compared the effect of employing H-ZSM-5 with other catalysts (i.e. Pt/C and Mo2C). Their results also showed that at 430 °C, the amounts of S-bearing compounds declined to below the detectable limit. Similarly, O- and N-species greatly reduced due to associated decarboxylation, hydrodeoxygenation and hydrodenitrogenation. Among the three catalysts, the Mo2C material produced the highest yield of saturated fuel range hydrocarbons of 76%. At higher temperature of 530 °C, the Pt/C and H-ZSM-5 catalysts produced 98% each of aromatics whereas the former catalyst produced 88% of the aromatics. However, since BTEX hydrocarbons were the dominant compounds in all the cases, all the catalysts could be considered very prospective under the studied conditions, but the aromatization property was more favored for the Pt/C and H-ZSM-5 catalysts. The hydrocarbons production potentials of some catalytic materials were demonstrated in Table 3. A general observation was that the incorporation of a heterogeneous catalyst have a positive effect on the yield of hydrocarbons, which could be aromatics or alkanes. Similarly, the presence of hydrogen in the reaction stream hydrogenates compounds with C=C bonds (i.e. alkenes) into their corresponding alkanes. 1044

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Table 3 Effect of some heterogeneous catalysts on bio-oil valorization activity. Catalyst system

Bio-oil upgrading conditions

Hydrocarbons yield, %

H-BEA H-ZSM−5 (SiAl = 25) H-ZSM−5 (SiAl = 50) H-Y H-MCM−41 SAPO−11 Non-catalytic Ru/C Pt/C Mo2C Non-catalytic Ru/C Raney-Ni Ru/C + Raney-Ni

3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h. 3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h. 3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h. 3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h. 3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h. 3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h. 1.5 g bio-oil, 6 MPa H2, 350 °C, 4 h. 1.5 g bio-oil, 0.15 g catalyst, 6 MPa H2, 350 °C, 4 h. 1.5 g bio-oil, 0.15 g catalyst, 6 MPa H2, 350 °C, 4 h. 1.5 g bio-oil, 0.15 g catalyst, 6 MPa H2, 350 °C, 4 h. 3 g bio-oil, 0.126 mol H2, 400 °C, 4 h. 3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h. 3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h. 3 g bio-oil, 0.3 g catalyst, 0.126 mol H2, 400 °C, 4 h.

Mainly Mainly Mainly Mainly Mainly Mainly Mainly Mainly Mainly Mainly Mainly Mainly Mainly Mainly

C9-C21 hydrocarbons, C9-C21 hydrocarbons, C9-C21 hydrocarbons, C9-C21 hydrocarbons, C9-C21 hydrocarbons, C9-C21 hydrocarbons, aromatics, 52% aromatics, 65% aromatics, 59% aromatics, 50% alkanes, 56% alkanes, 59% alkanes, 87% alkanes, 53%

Ref. 75% 61% 57% 80% 83% 54%

[147] [147] [147] [147] [147] [147] [148] [148] [148] [148] [149] [149] [149] [149]

stability under the reaction conditions [162-164]. However, the modified H-BEA catalyst showed complete stability and retarded coke formation. This feature was therefore responsible to a very high stability for the 2 h reaction period. The incorporation of fluoride ions can usually be achieved using aqueous solutions of HF, NH4F, NaF or KF as the fluoride source. The Fions perform mineralization role similar to that observed in OH- media [165-167]. They are also good for the formation of large crystalline materials. According to Kim and co-workers [166], modification of HBEA with F- ions using a F/Si ratio of 0.1 promoted complete crystallization in 4 h compared to 14 h required for the non-fluoride media. Similarly, the fluoride catalyst exhibited higher stability properties. Recently, dos Santos et al. [167] showed H-FER zeolites modified with F- ions to improve both the surface area and crystallinity of the ferrierite zeolites. It also completely retarded the chance of coke formation. The modified catalysts were stable for 10 h during glycerol valorization in water at 300 °C and 2 h1. On the other hand, the pure ferrierite zeolites deactivated over time due to the presence of Brønsted acid sites with high susceptibility to coke formation. The role of P-modification on the stability of the zeolite catalysts have been reported by some authors. Recently, Yamasaki et al. [168], synthesized a range of P-modified CHA zeolite systems using P/Al ratios in the range of 0.02–0.15. Their characterization data demonstrated that the modified catalysts could be obtained without any destruction to the zeolite pore-structure or the active catalytic sites. The catalysts exhibited high thermal stability due to complete retention of structural features even when calcined at 1050 °C for 1 h period. They similarly yielded an optimal activity of 90% conversion even after subjection to severe hydrothermal conditions (i.e. 900 °C for 8 h). The behavior was therefore an indication of high hydrothermal stability. According to the results of an NMR study by Damodaran et al. [169], the P species can modify the zeolite frameworks through the formation of P-O-P and P-O-Al as well as polyphosphates. These features coupled with the homogeneous distribution of the P species even at low P/Al ratios were responsible for the improved zeolite stability under hydrothermal conditions. Song et al. [170], demonstrated that Pmodified H-ZSM-5 catalysts could be subjected to hydrothermal treatment at 600 °C without destruction to the activity/stability properties. They compared the activity of a P-ZSM-5 (PAl = 0.5) with an unmodified HZSM-5 catalyst during ethanol upgrading at 600 °C and 0.01 g ml−1 min. The pure H-ZSM-5 catalyst was completely inactive following the steaming process due to dealumination and coke deposition. On the other hand, the P-ZSM-5 (PAl = 0.5) catalyst exhibited a retention of activity (i.e. ~ 32% yield of propylene) both before and after the steaming/hydrothermal treatment. For the pure H-ZSM-5 catalyst, the Si–O–Al bonds are ruptured by the hydrothermal treatment [171].

include the evaluation of catalyst stability with time and the effect of impurities in the bio-oil feedstock on the hydrocarbons yield at the end of the reaction. 5. Hydrophobic zeolites as prospective catalysts Zeolitic catalyst systems have so far been given priority as the hydrothermal liquefaction and bio-oil upgrading catalysts due to their enhanced activity and selectivity properties. The hydrothermal liquefaction process involved the incorporation of water into the reaction media. Similarly, water is generated in situ during the bio-oil valorization into hydrocarbons. Therefore, tolerance of the catalyst systems to water or steam would be very important for long time application. According to Yeh et al. [154], zeolite catalysts could be deactivated under during hydrothermal processing of alage due to the reduction in the number of exposed metal atoms associated with the zeolite frameworks. Zhao et al. [155], have reported the combined effect of steam from oxygenates dehydration and bulky unsaturated hydrocarbons from oligomerization reactions to be responsible for rapid catalyst deactivation during the upgrading of microalgae oil into fuels grade hydrocarbon compounds. The solvation and consequent degradation of zeolitic frameworks by combined effect of water and heat and the pore blockage by carbonaceous deposits must be properly tackled to ensure improved catalyst efficiency. An important way to promote zeolite stability under hydrothermal conditions is to improve the hydrophobic properties of the zeolite. This can be achieved in a number of ways that include silane modification, P or metals (e.g. La, Ga, Zn) incorporation or the preparation of the zeolite catalysts in fluoride media using a suitable F- source [156-159]. The modification with these species can also reduce the possibility of coke formation during the reaction [160]. According to Xu et al. [160], modification of H-ZSM-5 zeolite with organosilane compounds (e.g. dimethyl dimethoxy silane and nhexadecyltrimethoxysilane) as well as Ru particles improved the water tolerance of the parent H-ZSM-5 catalyst during alcohols upgrading via dehydration process. The modified catalysts were stable for 20 h, producing 99% selectivity to hydrocarbons with up to 71% conversion during dehydration of cyclohexanol, cyclopentanol and hexan-2-ol at 150 °C in the presence of 5 ml of water. On the other hand, the pure HZSM-5 catalyst produced < 20% conversion that decayed rapidly. Therefore, the modification does not only improve catalyst stability but also catalytic activity. This behavior was a consequence of the hydrophobic surface that eliminate water from interacting with the active acid sites. In our previous work [161], we have successfully demonstrated the modification of H-BEA zeolite surface with triphenylsilane to robustly promoted the steam-stability of the parent H-BEA catalyst and also improved the production of light hydrocarbons during heavy oil cracking. Without any modification the steaming process caused serious zeolite dealumination and consequently reduced its

6. Conclusions and further works The review presented herein clearly established that algae biomass 1045

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can be upgraded into biogas, bio-oil and other biofuel products of industrial and household importance. There is in fact a renewed interest in the recent times for the valorization of algae-biomass into liquid fuels via catalytic hydrothermal liquefaction. In addition to the production of bio-oil with high heating value, hydrocarbon compounds with suitability for applications in diesel, gasoline and jet engines could be derived. Another alternative approach is the catalytic bio-oil upgrading after the liquefaction process. The yield and compositions of the bio-oil in the former case is dependent on the algae specie and the selected catalyst material. The review showed that homogeneous catalysts based on carbonates, hydroxides and simple carboxylic acids have been evaluated. However, their low susceptibility to catalyze decarboxylation of fatty acids, isomerization and aromatization coupled to their non-recyclable nature accounted for their low prospect for this application. As captured in the paper objectives, heterogeneous catalysts were also explored for the algae upgrading processes. Among the heterogeneous catalysts, zeolites, SiO2, Al2O3 and supported metals were the most extensively studied for direct hydrothermal liquefaction of the algae-biomass. Similarly, the CoMo based catalyst systems. The production of bio-oil yields in the range of 20–60% with these materials indicated that further efficiency improvement studies are necessary. Catalysts that can produce up to 90% of bio-oil and corresponding hydrocarbons would be very attractive for industrial applications. There are certain parameters that could be evaluated in this direction. These include the role of replacing Co or Mo in CoMo catalysts with other metals like Ni, Ru or Pd. Incorporation of acidic support like SO4/ZrO2, effect of zeolite topology-acidity properties and the role of metal loading on zeolites. There are clear indications that the mechanism of the process is only partly resolved. Therefore, further investigations with the view of establishing the full process chemistry would be critical for the optimal catalyst design and consequently industrial consideration. Bio-oil upgrading with zeolite catalysts presents another opportunity for generating hydrocarbon compounds (including the BTEXs) with good potentials for fuels application. However, there are certain issues that should be further evaluated. These include the nature of acid sites (i.e. Brønsted or Lewis) that catalyze hydrocarbons formation, effect of zeolite topological and textural properties on the selectivity to particular category of hydrocarbons (i.e. light alkanes, long chained-alkanes, BTEX or olefins) and the overall stability of the zeolites under hydrothermal conditions. Hydrophobic zeolites developed via silylation, through the incorporation of metallic particles, P or F-ions have indicated strong water and steam tolerance, resistance to deactivation by coking and improved activities in reactions like dehydration, isomerization and alkylation. As these reactions do occur during bio-oil upgrading, full evaluation of the prospects of the hydrophobic zeolites could be positive for process industrialization. Acknowledgement The authors acknowledge the funding provided by Saudi Aramco for supporting this work through Project Contract 6600011900 as part of the Oil Upgrading theme at King Fahd University of Petroleum and Minerals. References [1] Ourisson G, Albrecht P, Rohmer M. Microbial origin of fossil fuels. Sci Am 1984:251, (United States). [2] Abas N, Kalair A, Khan N. Review of fossil fuels and future energy technologies. Futures 2015;69:31–49. [3] Tegelaar E, De Leeuw J, Derenne S, Largeau C. A reappraisal of kerogen formation. Geochim Cosmochim Acta 1989;53:3103–6. [4] Höök M, Tang X. Depletion of fossil fuels and anthropogenic climate change—a review. Energy Policy 2013;52:797–809. [5] Le Page M. World without fossil fuels. N Sci 2014;224:34–9. [6] Pousa GPAG, Santos ALF, Suarez PAZ. History and policy of biodiesel in Brazil. Energy Policy 2007;35:5393–8.

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