Agricultural Waste Liquefied Hydrothermally using Heterogeneous Catalyst

Due to its abundance and sustainability, lignocellulosic biomass is a possible replacement for petroleum oil in the production of energy and chemicals. Numerous thermochemical processes have been used in significant study to turn biomass into products with added value. One of the best methods for creating bio-fuels and bio-based compounds among them is hydrothermal liquefaction (HTL). However, a number of technological obstacles still need to be removed before HTL technology can be widely used in industry. Hydrothermal liquefaction is now thought to be amongst the most popular effective processes to converting moist biomass for bio crude, but it requires costly renovation procedures to be utilized as biofuel. It is crucial to employ catalysts that may straightforwardly improve the bio crude yield as well as the efficiency of the reaction process; the benefit of raising the operation’s overall production; the impacts of adding heterogeneous catalysts and how they affect the bio-crude yield. In lignocellulosic biomass hydrothermal liquefaction, a typical catalytic activity was discovered, dividing the various catalysts into four separate groups (transition metal, lanthanide oxide, alkaline metal oxide, and zeolite). The purpose study is to objectively evaluate the hydrothermal liquefaction of lignocellulosic biomass and know effecting of adding a zeolite catalyst on it, with a focus on increasing the production and efficiency of the biofuel. In addition, it has drawn attention to the natural stimulatory effects associated with zeolite catalysts.


Introduction
The scientists and researchers notice that the supply of fossil fuels is limited; they become very concerned about the concentration of greenhouse gases in the atmosphere and the rising global demand for energy.At the same time, they search for environmentally safe renewable energy infrastructure.Researchers recently discovered biomass to be a readily available and sustainable alternative energy source.Biomass is used less frequently because of its low energy density and high oxygen and water content (high_heating-value: 10 to 20 MJ/kg) [1].The characteristics of renewable fuel are influenced by the kind of biomass.The most efficient way to manipulate biomass is to turn it to make it more helpful and convenient fuels such as syn-gas , bio_oil.[2][3][4].Although the principal biomass source for the initial start waving for biofuels is really agricultural crops like wheat, maize, and sugar cane, their widespread usage is not advised due to their detrimental consequences on bio -diversity as well as food availability [5].
The initial generation of biofuels has certain problems, also in the interest of dealing with this problems, it is necessary to study and successfully implement the use of biomass.This will also help with the creation of a second of bio-fuels generation, which will be produced primarily from sources like agricultural waste, paper, manure, and sawmills are examples of forests or refuse.[6][7][8][9].But then again, the molecular structures of the biomass used to produce Biofuels for the initial generation are still more difficult and complicated than those of lignocellulosic biomass, and the energy density is constrained.As a result, problems arise with the conversion processes used to create biofuels, and also for circulation of feedstock's.Besides a significant research initiative, as well as the activities of the industry's research and development segments, Further research must be conducted in order to reduce the cost of the second gen of lignocellulosic bio-mass-based bio-fuels and make their production feasible [10].To address the issues of low energy density for biomass, difficulties with the distribution system and inefficient biofuel conversion processes, it is crucial to reduce the cost of biofuel production [11].Processes that convert biomass to biofuel come in two flavors: biochemical and thermochemical [12][13][14][15][16] .One of the best methods for lignocellulosic biochemical conversion is thermochemical, where this degradation occurs.Due to the great crystalline density in the matrix, lignin is more difficult to ferment than fermentable biomass and organic matter from food crops.Among these thermo_chemical processes are hydro_thermal liquefaction, gasification, as well as pyrolysis, which operates under high P,T or both [17].These methods have greater reaction rates, which results in smaller reactor dimensions [18].An especially crucial processes creating liquid-fuels known as )bio-crude ( is thermal liquefaction.A geological formation of fossil fuels is comparable to this process.Although biomass is subjected to long-term exposure to extreme temperature and pressure underground to create fossil fuels, hydrothermal liquefaction produces liquid fuel in a matter of hours or even minutes [19].During this process, Water with an organic-solution are mixed, the wet-biomass is converted into liquid-fuel in a high-P reactor (5 to 30 MPa) and mid-T [250.0 to 400.0] ℃ environment.The most popular organic solutions are (ethyl, methyl) alcohol , 1-propanol as well as phenolic acid [20][21][22].Benefits from utilizing H 2 O as an absolver nowadays are as follows: It is friendly to the ecosystem, within an installation biomass, and under HTL conditions, where temperatures are lower and pressures are higher than at its critical-point [374.00 °C for T, 22.0640 MPa for P], water's specific properties, such as a lower dielectric constant and viscosity, increase the solubillity of hydrophobic-organic compound and increase the product of ionic by acting as a catalyst in acid-based reaction.There is no need to first dry the biomass.And as a result uses less energy than traditional pyrolysis methods.In order to carry out the HTL processes at less T so as less utilizes energy than the traditional pyrolysis processes, the dielectric constant and viscosity are reduced.This increases the solubility of the organic hydro-phobic substances and increases the ionic product through giving the acid-base reaction catalyzed process [23,24].Because there is a lower concentration of oxygenated chemicals in the biofuel generated by the HTL method, it has a greater heating value (HHV) than biofuel produced by pyrolysis as shown in table (1) [25,26].During HTL, water is produced, as well as CO 2 , CO when oxygen(O) moleculs have been eliminated in part but despite the greater quality of this method, a significant quantity of (O) is still present, resulting in an unstable biofuel with a low heating-value (HHV), and (viscosity, acidity) is high.[27,28].The bio crude undergoes many development procedures, such as catalytic-cracking, hydrotreating for future as a replacement to fuels for transport.Its characteristics are heavily influenced by the biomass utilized and the working circumstances [29,30].Studies are currently being conducted to find Sustainable energy sources for H 2 production required to advance and promote the use of bio-fuel.Fossil fuels production is continues primarily dependent on the transformation of natural gas steam [31].Homogeneous acid or alkaline catalysts (H 3 PO 4 , HCl, Na 2 CO 3 , K 2 CO 3 ,etc.)have been the subject of substantial investigation [32][33][34].According to research, adding these catalysts increases the quantity and quality of biofuels but is expensive These catalysts' homogenous structure calls for corrosion-resistant machinery as well as separation procedures intended to separate catalysts at the conclusion of the reaction [35,36].In fact, it is difficult and expensive to recover the homogenous catalytic reaction, so it is best to discharge the catalyst request the required neutralizing treatments at the conclusion of the operation using water phase.For example, depending on the type of catalyst chosen and adding acidic or alkaline substances would make the procedure more expensive.[37].
Because of their intense activity as well as simplicity in recovering using liquid substances, heterogeneous catalysts have recently received a lot of attention.As a consequence, large-scale processing is now made possible and the enormous expenses incurred across the chain of supplies for bio-crude are reduced.They are also non-corrosive and have improved thermal stability [38].
Regardless of the benefits of hetero-catalysts, these substances are often fewer efficient than homogeneous ones because of the restrictions on external and internal diffusion brought on by liquid-solid also gas-solid interactions [39].

Sources of Biomass and Its Component
Biomasses are living or recently living organic materials mostly made of (C, H, O) that have molecular bonds that store solar energy.The highest usage potential of all renewable energy sources is found in biomass.Trees, algae, maize-wheat-rye straw, gras, and fruits are examples of plants.are examples of biomass, as are vegetable-wastes, plants-based trashes, urban wastes, and agroindustrial wastes [40][41][42][43].Lignin, cellulose, hemicellulose, and extractives make up the majority of lignocellulosic materials.
Cellulose typically makes up the biggest part (35 -55 percent), followed by hemicellulose (20 -40 percent), and a negligible quantity of lignin protein (15 -25 percent) [44].Various techniques can be used to prepare biomass for liquid phase catalytic processing.First, a liquid phase catalytic reactor may be fed directly with raw biomass fuel.In this situation, often fairly straightforward feeds like vegetable oils, starch, or cellulose are used.Often, the biomass is treated to provide a liquid stream good for catalytic upgrading.when processing more complicated lignocellulosic feeds.Additionally, liquid-phase processes can use bio-oil and its constituent parts as feedstock's.Aqueous acidic, basic, or ionic liquid-mediated mechanisms can all be used to dissolve cellulose.For liquid phase upgrading, cellulose can be a source of glucose monosaccharides and breakdown products.Mild aqueous acid treatment makes it simple to separate hemicellulose from lignocellulose, producing a mixture of C5 and C6 sugars as well as breakdown products.It is challenging to find pure hemicellulose as a beginning feedstock.The holocellulose portion of biomass can be processed using a variety of methods, such as (Krafts, organosolve) pulping, or aqueous acid-hydrolysis, to separate the lignin from the sugar fractions.Different processes leave the percentage of solid-lignin with varying levels of condensation and crosslinking.The Kraft pulping process produces black liquor as a byproduct, which also contains lignin and some inorganic substances.A potential feedstock for further catalytic upgrading is the aromaticrich stream.Starch and sucrose are typical food ingredients but they can also be used as fuel feedstocks.Starch (glucose-polymer) that can be processed of producing glucos as well as other byproducts of cellular breakdown.A straightforward disaccharide with units of glucose and fructose is sucrose.To create range fuels like gasoline and diesel, liquid phase catalytic processing of plant and animal glycerides is a viable option.Virgin and used oils can both be used as feedstock in these procedures.However, these proportion amounts of feeds are insufficient to make a major dent in the transportation fuel requirements.Another category of sustainable components needed for liquid-phase catalysis is proteins.Although they might be beneficial for the manufacturing of chemicals, they do not constitute a feasible feedstock for fuels.Unfortunately, performance investigation has shown the influence the kind of reactor on HTL biocrude almost infrequently circumstances, and future research on this topic will be anticipated in research projects.Food processing wastes are the organic byproduct of the food producing process.High protein and fat content (up to 50%) is usual in industries like restaurants and food production.An appropriate technique that may convert when used as a feedstock, crude bio-oil with a high protein to lipid percentage ratio produces relatively less crude bio-oil, a high oil yield is hydrothermal liquefaction condition reaction at (280.0 °C) in contrast [45][46][47].As an illustration, subcritical water has been used to liquefy (SCG) wasted coffee grounds, which contain (15.0 percent)-lipid and (17.40 percent)-protein, N 2 environment to produce bio-crude oil at a reaction temperature between (200.0 °C and 300.0 °C) and a response time of (5.0 min to 25.0 min) [45].Limit at 275 °C in 10 minutes, acetone-recovered bio crude yields were 47.3 percent, with SCG's (20.2 MJ/kg) HHV being substantially lower at 31.0 MJ/kg.The both prevalent animal wastes that examined the synthesis of crude bio-oil by HTL are swine and cattle manure [48,49].
The material used was swine dung, which had high crude-protein about (17.100 %) as well as low levels of lignin, saccharide, and ash (22.3 percent).Additionally, the swine-manure HTL was done in nitrogen for 15 minutes and temperatures about (260.0 °C to 340.0 °C).Maximum output of bio-crude (24.20 wt%) was obtained at (340 °C) temperature with an HHV of (36.050MJ/kg).The important substances that were found in the Bio crude were phenols, carboxylic acids, and carbonyls.Protein content in swine manure can be used to get GC-MS, and several nitrogenous chemicals were also found.The GC-MS is also used to measure the concentrations of various compounds; an example is DBPs resulting from chlorination of water sterilization [50].Microalgae have been selected as the most viable third-generation raw material for biofuel production due to its dominance over upland lignocellulosic biomass, climatological conditions, shorter development cyclical, great adaptation, and variety of aquatic environment.[51,52].Nanochloropsis-sp.(Green-marine-algae) often has a high protein (52.0 percent) and fat (28.0 percent) content [53].It liquefied in the water-medium from 200 °C to 500 °C at a temperature where the reaction-time extends 60 minutes.At 350 °C (43.0 percent wt), with a higher value of (39 MJ/kg) for heating, the limit bio crude yield was attained.
In addition to having a high percentage of crude fat (20.50%) and crude protein (63.6%), seaweed green also underwent HTL for 5 min and 60 min at T ranging from 50 °C to 340 °C [54].Maximum yield, a 43.80 % of oil, which achieved at 300 °C, a calorificevalue of 34.0 MJ/Kg for 5 minutes, and a calorificevalue off 34.00 MJ/Kg for five minutes of reactionetime.Lowest degree of oil and viscosity were produced under the reaction conditions of 340 °C temperature and 60 mint periods.
Long-chainefatty acids, ketoneseproduced from the HTL of microalgae, fattyamides, and other N 2containing chemicals make up the majority of the resulting bio-crude [55].biodegradable waste with a lot of wet is known as sewage or sludge.Material produced during the waste water treatment procedure.A sizable amount of this garbage has been disposed of through incineration, ocean dumping, and landfilling [56].In addition, they reported that the highesteenergy densityeof (35.40 MJ/Kg) wassobtained at temperature (280.0 °C), which was significantly higherethan that of (18.300MJ/Kg) crudeysludge powder.Theehighesteyield of oil (24.0 percenteof weight) waseobtained at temperaturee(350.0 °C) and time offreaction (60 min).Plastics like polypropylenee, polyvinylechloride , polyethylenee, and polystyreneeare examples of semisyntheticeor synthetic polymers withehigh molecular masses generated from petrochemicals.Because of theireresistance to deteriration, lowerecycling rates 10.0 percent and low-grade uses, waste plastics account for roughly 10.0 percent of landfill-waste byemass, and theiredisposal has led to serious environmntal issues across the globe.Alternative methods for the disposal of used plastics have been thoroughly investigated for energy recovery and reuse.Research on the liquefction of plastic-garbag in sub-critical water is scant.[57] Reported that by hydro-thermally liquefyingeplastic waste at a temperature of 300 °C for 30 minutes, an 8.0% bio crudeeyield was produced.It has been noted that plastice bio-crude waserecovered after hexane using the solvent-benzene.
In short, HTL's single feedstock has a very high bio-crude yield.Depending on the ratio of different chemical components in the feedstock.Bio-crude yields are often low when using forest debris and growing methods high in carbohydrates.Algalbiomass (protein , lipid-rich micro-algae) is referred to as a preferred feedstock because its bio-crude yield is typically higher than that of other feedstock.Bio-crude, on the other hand which made from microalgae contains a lot of N 2 -containing compounds and should be expanded denitrogenated.HTL might also be a way to deal with using food industry waste, sewag sludge, animal manure, and even plastic-trash.So according to what has been shown above, the composition of the feedstock affects the composition of the bio-oil, as shown in the table 2 [58].

Steps of Feedstock Hydrothermal De-Composition
The primary processes used to transform feedstock originating from biomass into fuel as well as chemicals areehydrolysis, iisomerization, reforminge, C-C couplinge hydro-genation, selective oxidatione, hydro-genolysis, dehydration/hydrogenation, olefin oligomerization, and metathesis.During the HTL process, lignocellulosic, biomass macromolecules are first hydrolyzed and subsequently broken down into low molecular weight chemicals.Decomposition of cellulose and hemicellulose yields water-soluble molecules such dihydroxyaceton, hydroxymethylfurfurale, and glyceraldehyde.However, any of these materials are unstable and take part in condensation reactions that provide water-insoluble compounds like bio-crude and chare [59].On the other hand, lignin is a structure built of polymer made of aromatic-ring contains monomers usually located in mono-lignols [52].Different chemical groups and types of bonds hold the monomers together.Different chemicals that exist in solids, liquids, and gases phase will be produced as lignin HTL decomposes.The chemistry and mechanics of biomass liquefaction are similarly complicated since biomass is a complex combination of lipid, protein, lignin, and carbohydrate.[60].In HTL circumstances, the general pathway of cellulose and hemicellulose degradation as shown in figure (1) [34]: Decomposition in HTL conditions.

Hydrolysis
Among the primary processes steps for poly-saccharides is hydrolysis, which involves the cleavage of sugar units glycosidic-bonds to produce simple sugar like glucos, fructos, and xylos as well as partially hydrolyzed-dimers, trimer, and others oligomer.Finding the right reaction conditions and catalysts to transform a range of polysaccharides (such cellulose, hemicellulose, starch, inulin, and xylan) derived from various biomass sources is the problem.Based on the configuration and makeup of the polysaccharides, hydrolysis reactions are commonly carried out using acid or base catalysts at temperatures ranging from 370 to 570 K.Because base hydrolysis results in more side reactions and lower yields, acid hydrolysis is more frequently used.By cleaving the (C-O-C) bonds at an intermediary oxygene atom between the two molecules of sugar, acid hydrolysis is carried out.The circumstances of the reaction frequently cause carbohydrates to degrade further and produce potentially harmful compounds including furfural and HMF.Due to its high crystallinity, cellulosethe most prevalent polysaccharide with a glyosidic linkage is the substance that is hardest to hydrolyze.Enzymatic catalysts are more selective than mineral acids, yet both can be employed to hydrolyze cellulose.Cellulose can be hydrolyzed by enzymes to release glucose that are almost hundred percent, whereas the maximum glucose yields that can be reached generally less than 70% for cellulose hydrolysis with concentrated mineralacids [61].Hemicelluose is more vulnerable to assault at intermediate locations, which allows for the breakdown of the oligomers into single sugar molecules while only necessitating low temperatures and diluted acid concentrations, preventing further simple sugar degradation.At low temperatures (340-420 K), soluble-starch, a poly-glucan with glycol-sidic connections derived from corn anderice, and inullin, a poly-fructan derived from chicory, can be digested to produce glucose and fructose, respectively [62].

Dehydration
An important class of processes in the study of sugar chemistry is the dehydration of molecules produced from carbohydrates and carbohydrates.Sugars can be dehydrated to create furan compounds like furfural and HMF, which can then be transformed into additives for diesel-fuel [63].Solvents for industry such as furan, tetra-hydro-furfuryl alcohola and furfuryl-alcohol [64].Using the Quaker Oats technique with furfural is produced industrially using pentosan-rich biomass and mineral acid as a catalyst.mineral acid as a catalyst, furfural is generated industrially from biomass rich in pentosane [65] .Despite numerous researchers' encouraging findings across a wide range of potential applications, HMF is not currently a high-volume chemical due to manufacturing challenges [66,67].
The difficulty in processing highly functionalized carbohydrate molecules with selectivity is demonstrated by the problem of producing HMF from sugars.Hexoses' dehydration in water, organic solvents, biphasic systems, ionic liquids, and near-or super-critical water have all been examined, and other media utilizing a range of catalysts, including salts, solid acid catalysts such ion-exchang resins and zeolites, mineral and organic acids, organocatalysts, and salts.[68].Even though evidence suggests that both open-chain and cyclic fructofuransyl intermediate pathways are involved, it is clear that the reaction intermediates and the HMF product degrade through a variety of processes [69,70].Acrolein, a major industrial chemical, can be produced in a similar manner by dehydrating glycerol.
Acrolein can be converted into a number of different chemicales, including acrylic-acid, which is used in supereadsorbents, and possibly 1,3-propandiol, which is used in Sorona TM poly-ester.Also attained up to 80% acrolein yields using sub-and supercritical water and zinc sulfate salts as catalysts, and their results are promising.However, due to the corrosion caused by the water and salt, the reaction necessitates the use of expensive corrosion-resistant materials [71].

Isomerization
In the presence of base catalysts, carbohydrates are frequently isomerized at low temperatures and in a variety of solvents.Making high fructose corn syrup involves a typical procedure that changes glucose into fructose.The selectivity of HMF from glucoses can also be improved by isomerizing glucoses to fructoses.Temperatures between 310 and 350 K are suitable for isomerization in the presence of a base catalyst, such as magnesium-aluminum hydrotalcite.Variable amounts of open chain (acyclic) and ring structures, such as a furanose, a furanose, a pyranose, and a pyranose, are present as carbohydrates in solutions [72].To convert aldo-hexoses to keto-hexoses, the isomerization reaction creates intermediate enolate species through open chain forms.Thus, the proportion of glucose molecules in the open chain state, which is controlled by the solvent medium and temperature, determines the rate of glucose isomerization.In aprotic solvents like dimethylsulfoxide (DMSO), where the abundance of the acyclic form for fructose is roughly 3% as opposed to water, where it is less than 0.8%, the reaction rates are higher as a result.Additionally, a 350 K temperature increase causes the open chain form to expand, speeding up the isomerization process [73,74].
In fact, the aromatic part of lignin serves as a concentrated precursor to oligomers and phenolic compounds to produce charcoal and chemicals that are insoluble in water.In addition to producing water-soluble molecules (as acids) by dissolving lateral alkene or alcohol groups, this process can also yield methane and methanol simultaneously [53].
During thermal processing, the lignin portion of plant biomass decomposes into chemical species with less oxygen than cellulose or hemicelluloses.Thus, it is expected that plant feedstock's with greater lignin concentration and lower hemicellulose content will be chosen for selective thermal treatment.The feedstock's requirement with a higher lignin content conflicts with the desire for biological deconstruction plants with a lower lignin concentration.Minimizing the amount of water in the biomass feedstock is desired for quick pyrolysis-type processing.Reduced minerals present in plant matter could make a discriminating mineral extraction process easier.

Product Characteristics
As liquefaction conditions result in a significant quantity of decarboxylation and dehydration, the biooil products have a substantially compared to bio-oils from rapid-pyrolysis, has a superior heating value and lower oxygen content.Decarboxylation is responsible for more than 50% of the oxygen eliminated during liquefaction [75][76][77].Product Analysis for Liquids Compared to rapid pyrolysis, considerably the biofuel process produces a heavy organic liquid with an H/C ratio of 1.1 that is insoluble in water and solidifies at 80 degrees Celsius.Liquefaction bio-oil has a lower oxygen content than fast pyrolysis bio oil, making it a more appealing fuel.Besides relation to energy density, but its high viscosity limits its utility without some sort of upgrading [78].As shown in figure (2) the biomass HTL process yields four primary products: bio -crude, products that are water soluble, and solid product (bio char) [79], pressure (5 -35 MPa), high temperature (250 -500) ℃ and co-solvents (water or alcohol-water combination), with homogeneous or heterogeneous catalyst or without [10,11].

The Bio-Crude
In general, biomass consists of 30.0-50.0%O 2 , HTL intends to create bio crude having less oxygen in them, it occurs via both of the pathways: dehydration (removal of O 2 in H 2 O form) and decarboxylation (removal of O 2 in CO 2 form).High temperatures as well as pressure are primarily responsible forethe removal ofooxygen from biomass via de-hydration, while de-carboxylation and breaking of long-chain carboxylic acids cause oxygencontent to decrease in the shape of CO 2 [80].Biocrude's carbon content has been slightly increased from (70.770 -74.210) % with an increase in temperature.Even as temperature is raised, the concentration of oxygen decreased from (19.650 to 16.270) %. [81].Reported the oxygen content of pine saw-dust HTL as (33.990 ± 0.140 , 29.320 ± 0.070) for non-catalyst and with K 2 CO 3 were between.Discovered an oxygen content of 24.2 wt.% in HTL of oak wood at subcritical conditions, which is substantially more than what was discovered in the study of treating with lignocellulosic-biomass [82].The (H2, N 2 ) ranges were found to be nearly identical across all experimental runs.Furthermore, HHV was found in all biocrude with only a slight variation in range among ( 35.250 -35.970 (MJ/kg)).The biocrude's greater heating value suggests that a significant portion of the oxygen transferred to other products, such as gas or the aqueous-phase.Despite the fact that the catalyst increased the biocrude's yield, it had little effect on the biocrude quality.As well as with total biocrude yield, optimum power recovery of 69.530% at catalytic 350 ℃ and least 48.380% at super -critical condition were also observed.Finally, it was discovered that the catalyst had no significant effect on the quality of bio-crude or H.H.V. Additionally, it was discovered that regardless of process conditions, the nitrogen content range (1.140-1.640)%.Mass of carbon in biocrude was found to be between (44.240 and 55.720) % of the carbon available in the biomass.In comparison to the original biomass, the biocrude had a lower oxygen concentration.The oxygen content is reduced as a result of reactions of de-hydration and decarboxylation.That true when it was water-phase and carbon-dioxide.Increasing in content of (C) and decreasing in (O) result in higher H.H.V. of bio-crude by comparing to biomass.Biocrudes atomic ratios of both (H/C and O/C) were also in the (1.310-1.390),(0.160-0.210) ranges, by respect.Also range of bio-crudes ratios of (O/C) were lower than in feedstock, that might improve viscosity of biocrude.Moreover, that reduce the amount of (H) required to upgrade bio-crude.However, HTL biocrude required upgrading to achieve a higher H/C ratio when compared to petroleumefuel.Because of its highly oxygenated branches, which are easier to decompose, hemicellulose is the first of the carbohydrates to decompose.Acetic acid and other organic acids are produced during the breakdown of hemicellulose [83].All reaction conditions yielded acids, including short-chain acids and long-chain faty acid.[n-Hexade-canoic acid] plays an important role in all four conditions.Acetic and octade-canoic acid were reduced during the catalyst.Absence of a high acid-content is detrimental to the superiority of bio-crude.
The main objective of HTL is to increase the yield, bio crude characteristics, and physicochemical qualities of big chemical compounds.Bio-crude is a chemical mixture that is frequently classified by functional groups as alcohols, aromatics, ketones, aldehydes, carboxylic-acids, and (straight hydrocarbons or cyclic) [57].This wide range of compounds points to the origins of bio-crude as a natural biofuel or bio-based additive [84].High-cost refining and the requirement for updates are significant in contrast.The characteristics and yield of bio-crude are influenced by HTL operational factors, including temperature and residence time [3].Even though, there is no universal reaction (T or t) of residence is required for each HTL treatment.The temperature ranges between 250 -500 C, and the time of residence varies between zero (simultaneously) and a few minutes [85].
All parameters are chosen in accordance with the facilitation of bio-crude yield or a chemical component of interest [86].Catalysts are frequently used to increase biomass hydrothermal liquefaction efficiency and yield of bio-crude.

Solid Residue Analysis
Another of the HTL process's outputs is solid residue, also known as bio char.The char produced by the HTL process is high in inorganics and could be improved to use as a fertilizer in the farming industry [3,87,88] or as a raw-material in the gasification process [89].It was discovered that increasing the temperature from 350.0 to 400.0 C increased (C) content in solids from 54.560 to 58.660%.Furthermore, (O) concentration decreasedeas the temperature increased from low to high.Itewas discovered thatethe addition of theecatalyst reduced the carbonecontent of the solid residue.Temperature has less of an impact on the heatingevalues ofesolids.Lower H/C ratios in solideresidue indicate hydrogen consumptions, This might be the cause of unsaturated and aromatic compound production in solid-residue [90].Moreover, (O/C) large ranging as 0.470 to 0.710, which could attribute to a large degreeeof de-oxygenation.
Coke and char formation is common in continuous scale up HTL plants.Increased char and coke formation are uninvited for the HTL plant for it reduces bio-crudes yield or might start causing plugflow reactor blockage.Even so, while catalyst using for reducing char-formation in overall, recovering catalysts are difficult commission in the HTLeprocess, that may be addressed by increasing the biocrude yield via fast heating systems, as discussed by researchers [58,91] .That is possible to conclude that the temperature takes fewer an impacteon the quality of solid-product during in the HTL process.
The catalyst, on the other hand, prevented the transfer of carbon to solid residue.The mechanism of bio char production is currently unknown.According to the majority of experts, it is composed of extra oil fraction polymerization processes with extended residence durations.[92].

Aqueous Phase Analysis
HTL well-known for using water as a reaction-medium, which is obtained at the HTL process's outlet end and is known as the aqueous phase.Temperature and catalyst both have an effect on the pH of the aqueous phase.The acidicenature of the aqueousephase observed under all conditions from 350.0 -400.0 degrees Celsius, the pH increased from 3.870 to 4.190, while the addition of catalyst increased it from 5.430 -6.820.Acidicerange of the resulting aqueous phase could be due to the formation of mono-meric sugars as a result of the hydrolysis of available sugar compounds, which degraded and converted into organic acids.pH varies primarily as a result of process operating conditions and  [93,94].Temperature and catalyst both had an effect on total organic carbon (TOC).The addition of catalyst increased the concentration of TOC from 25.940 to 30.670 g/L at both subcritical conditions, while it increased slightly at both supercritical conditions from 28.620 to 33.520 g/L.Shah et al. and other researchers [26,95,96] also observed an increase in TOC with the addition of an alkali catalyst.Pedersen et al. found a higher concentration of organic carbon in the water phase by co-liquefying lignocellulosic biomass with glycerol for a continuous HTL plant [97].
Even though it is a byproduct of the aqueous process, it is significant.Production portion of HTL by depended on the feedstock as well as reaction circumstances, the water phase's composition differs, but water and any additional solvents (whether any), light organic molecules that are water soluble are frequently present [36,98].
The most significant functional chemical group in the aqueous phase is made up of the following organic compounds: (acetic, formic, glycollic) acids, ethylen-glycol, phenol, methanol, ethanol, and lactone [99].
The remaining HTL products are in the gaseous state, which is primarily composed of CO 2 with minor amounts of CO, H 2 , and CH 4 .These substances are created during the liquefaction process by the decarboxylation and biomass cracking [36].

Zeolite as a Heterogeneous Catalyst for the HTL Phase
The use of catalysts in hydrothermal liquefaction processes aims to improve process efficiency by reduces formation coal and tar.There are two types of catalysts, homogeneous and heterogeneous.Homogeneous catalysts include alkali salts such as Sodium carbonate, Potassium carbonate, and Potassium bicarbonate [100][101][102][103][104] .
Although there are some advantages to using homogeneous catalysts such as lower solids yield, higher biological stock yield, higher pH and thus less dehydration reactions that normally lead to instability of unsaturated particles.Homogeneous catalyst recovery is expensive due to the cost intensive separation process and consumes a great deal of energy.Therefore, heterogeneous catalyst is preferred in hydrothermal liquefaction processes for its easy separation and recovery and reduction of energy and cost [105].
Although heterogeneous catalysts are most commonly used in hydrothermal gasification, they are also used to improve the quality of lignocellulosic biomass in hydrothermal liquefaction processes.While some gasification is required to remove the oxygen, extending it can reduce vital oil production.Platinum, nickel, and palladium are examples of heterogeneous catalysts.Because these metals are scarce, attention has shifted to metal oxides, specifically zirconium dioxide (ZrO2) [91,[106][107][108].
Aside from these catalysts, alkaline catalysts have been used in studies on catalytic hydrothermal liquefaction to improve biooil yield.MnO, MgO, NiO, ZnO, CeO2, La2O3, and other well-known metal oxide catalysts include MnO, MgO, NiO, ZnO, CeO2, La2O3, and others [36,46,109].Nickelbased nanocatalysts have been explored because they may increase biooil yield at lower temperatures, which might aid in the commercialization of HTL [110].Due to the high cost of reductive noble metal catalysts like Pt and Ru, an attempt was made to use zeolite as a catalyst.The use of zeolite as a catalyst in the hydrothermal liquefaction of biomass has been suggested [111].However, high hydrothermal stability catalysts are important for avoiding catalyst disruption during hydrothermal operation.Carbonaceous materials, such as carbon nanotubes (CNTs) that use activated carbon as a support for metal catalysts, are suitable for industrial applications because they can provide a large surface area and recycle noble metals [112,113].Zeolites are porous alumino-silicate minerals that are frequently utilized as a catalyst and reinforcement in a variety of chemical conversion processes.The SEM picture shows that the ZSM-5 are crusty, random sharp and edges shape as shown in figure (3).Zeolites of numerous sorts have been researched and found to improve products for converting biomass to liquid.These results in hydrothermal biomass liquefaction were attained using ZSM-5.As a result of their acidity and form selectivity, zeolite is an effective catalyst frequently employed in deoxygenating procedures of biocrude upgrading treatments [114].The study's findings suggested that using these materials somewhat increased the efficiency and yield of bio-crude, demonstrating the strong behavior of zeolite cracking, hydration, and cyclization reactions.On the other hand, the amount of zeolite used grows as the  While solid reside decreases with the use of a catalyst, HZSM-5, the yields of bio crude and gas improve.Due to the addition of zeolites, a GC-MS research discovered a decrease in hydrocarbon concentration and an increase in oxygenated chemicals.Hydrocarbons have been created from oxygenated molecules by the processes of dehydration, cracking, and oligomerization [116], encouraging the production of more abundant bio-crude hydrocarbons.Yan et al. investigated the effects of incorporating ZSM-5 into sugarcane-derived HTL bagasse.The best yield was produced using [46] and water as the solvent without the use of a catalyst at a temperature of 285 ℃ with 200 ml of water, 10.0 g of biomass, and a 30-minute reaction time.By reducing acidic components while increasing yields of bio-crude, the catalyst ZSM-5 was added to boost production of bio-crude.
A rise in chare production was observed when more zeolite was added to the bio-crude yield, which enhanced the yield of bio-crude.Reverse trend in tests with ZSM-5-catalyst additions ranging from (10 -90) wt.percent based on the weight of biomass.When zeolite was present, the majority of the water-soluble compounds were esters, while the amount of reduced acidic compounds was significantly higher.This demonstrates that the zeolite possesses catalytic activity in the esterification processes.Important fluctuations in furfural concentrations were also seen; in the presence of zeolite, the amount of furfural reduced because zeolites catalyzed the conversion of furfural to phenolic compounds, reducing the carbon content.The bio-crude made from zeolite has better consistency and higher HHVs.Although a larger percentage of CH 4 was discovered in H 2 , the presence of elevated zeolite concentrations also had an impact on the gas' composition.These results show that zeolites favor the hydrogination , methanation reactions.

Challenges for Continuous HTL Processing
The primary criteria used to assess the process efficiency include bio crude yield, solid residue, and the quality of the generated bio crude.According to lab experimentation, increased bio crude yields result in bio crude of higher quality that is connected with HHVs that promote the use of water medium for continuous HTL process.As seen, aqueous phase recirculation also helps to increase the production of bio crude while using less freshwater.

Conclusion
Lignocellulosic biofuels' hydrothermal liquefaction is a simple method of producing liquid.The purpose of this inquiry is to inform readers about recent advancements regarding HTL zeolite catalysts.The use of heterogeneous catalysts in the HTL technology appears to be a practical replacement for homogeneous catalysts due to the possibility of recovering and reusing the catalyst at the completion of the reaction, which will considerably lower the overall costs connected with this technology.Reality shows that lanthanide oxide and transition metals are efficient catalyst groups for enhancing the yield and quality of bio-crude, with a 38 percent yield increase and a 19.0 percent rise in HHV compared to the control test.Maximum bio-crude yield near (40.0 percent) was produced with the help of the alkaline earth metals catalyst, but it was 10 percent less than what would have been produced without it.
.1088/1755-1315/1259/1/012032 11 amount of zeolite used grows because the char formed by higher determined catalyst degradation obstructs active pores.In order to clarify how the Ni / HZSM-5 catalyst affects the HTL of pine sawdust, Cheng et al.[115] conducted experiments using simply HZSM-5 in the aforementioned working conditions.

Table 1 .
Comparison of Hydrothermal Liquefaction and Pyrolysis processes for biomass conversion.

Table 2 .
Effect of feedstock types on the composition of bio-oil by hydrothermal process.