Catalytic pyrolysis of biomass to produce bio‐oil using layered double hydroxides (LDH)‐derived materials

Owing to the enormous consumption of petroleum products and their environmental polluting nature, attention has been given to seeking alternative resources for the development of sustainable products. Biomass is a renewable source that can be converted to a variety of fuels and chemicals by different approaches, which are the best replacements for traditional petroleum‐derived products. Pyrolysis is a process in which chemical bonds of biomass macromolecules such as cellulose, hemicellulose, and lignin, are fractured into small molecular intermediates under high pressure, and results bio‐oil, biochar, and fuel gases as desired products. Of these pyrolysis products, bio‐oil is the primary product that usually contains large amounts of oxygen and nitrogen compounds that hinder its application potential. Catalytic pyrolysis is a beneficial method that is reported to alter the constituents and quality of bio‐oil and to upgrade them for diverse applications. Catalytic hydropyrolysis and copyrolysis of biomass are an alternative approaches to overcome the drawbacks raised toward product formation in the pyrolysis process. Layered double hydroxides (LDH) and their derived forms are well‐known catalytic/catalytic support materials for various chemical reactions due to their superior properties, such as easy preparation, thermal stability, and tuneable acid/base properties. This review summarizes the progress in the utilization of as‐synthesized LDH and their modified forms such as mixed metal oxides and functionalized/composite materials as active catalysts for the pyrolysis of various biomass sources.

replacements for on-going petroleum derivatives (Ahn et al., 2023;Jeong et al., 2023;Oh et al., 2022).Owing to its vast availability and structural diversity, rather than satisfying food requirements, biomass has been targeted for essential applications in the production of fuels, chemicals, and materials (Choe et al., 2020;Kim et al., 2020;Oh et al., 2023).From an environmental perspective, biomass has advantages such as zero generation of additional carbon dioxide and less release of nitrogen oxides and sulfur oxides, which makes biomass a "carbon neutralization" resource (Kim et al., 2022;Moroń & Rybak, 2015).Biorefining is the sustainable process of converting biomass to a various spectrum of bio-based marketable products (Choe et al., 2021).Within this task, a broad range of different biorefinery systems is currently under evaluation, some of which are already competitive in the market, while others are still in progress (Gong et al., 2023;Ubando et al., 2020).
Biomass energy conversion methods are separated into two categories: (i) biochemical methods (e.g., methane fermentation) and (ii) thermochemical methods (e.g., pyrolysis) (Kang et al., 2023;Won & Maravelias, 2017).Of these, thermochemical methods are preferred over biochemical methods because of their advantages such as simple handling, less use of chemicals, wide coverage of feedstocks, time saving, and higher efficiency (Jha et al., 2022).Mainly, the amount of oxygen (O 2 ) in the system changes the thermochemical process into different classes consisting of (i) combustion: an excess amount of O 2 , (ii) gasification (high temperature), or liquefaction (low temperature and high pressure): a limited amount of O 2 and (iii) pyrolysis: absence of O 2 (Ram & Mondal, 2022).The pyrolysis process decomposes biomass at high temperature into liquid (bio-oil and water), solid (biochar), and gaseous (fuel gas) products that are used for energy applications (Christopher et al., 2023).Depending on the type and composition of biomass, as well as process conditions (e.g., heating rate and biomass feed rate), the fractions of the end products might vary.Based on the temperature, heating rates, processing times, and particle sizes, pyrolysis processes can be categorized as (i) flash/fast pyrolysis, (ii) slow pyrolysis, and (iii) medium/intermediate pyrolysis (Fahmy et al., 2020).
Bio-oils (pyrolysis oil) are mainly brown, free-flowing liquid with a distinctive smoky odor that have different chemical compositions, such as acids, aldehydes, esters, alcohols, ketones, phenols, hydrocarbons, and ligninderived oligomers (Wang, Akbarzadeh, et al., 2022).Biooil is a homogeneous mixture of (i) an aqueous phase consisting of low-molecular-weight, oxygenated organic compounds and (ii) a oil phase consisting of highmolecular-weight oxygenates, aromatics, and polycyclic aromatic hydrocarbons (Conrad et al., 2022).Generally, whole or fractionated bio-oil or extracted and/or modified/upgraded chemicals from bio-oil can be utilized for wide potential applications, which are summarized in Table 1.Compared with petroleum fuel oil, bio-oils have limitations, such as high water content, viscosity, ash content, oxygen content (low heating value), corrosiveness and surface tension, thermal/chemical instability, low pH values, and poor ignition and combustion properties, which restrict their direct use as transportation fuels (Mishra et al., 2022;Zhang, Yang, et al., 2022).To overcome these drawbacks, bio-oil should be upgraded with improved properties for exploration as liquid fuel.
Biomass pyrolysis can be explored in the presence or absence of catalysts, in which catalytic pyrolysis has gained more research interest because of its potential to enhance the yield and quality of bio-oil.In catalytic pyrolysis, the catalyst could be either directly mixed with biomass (in situ approach) or only contacted with the pyrolysis vapors (ex situ approach) (Yildiz et al., 2013).The catalyst can enhance the secondary reactions of pyrolysis intermediates toward certain products, which significantly improve the conversion and selectivity to desirable components to upgrade the bio-oil fraction (Wang, Duo, et al., 2022).Similarly, tremendous efforts have been made for the catalytic copyrolysis of biomass, in which different sources are combined prior to the pyrolysis process to accelerate the degradation rates and shift the reaction temperature to the lower temperature range (Kumar et al., 2023).Likewise, catalytic pyrolysis of biomass sources in the presence of hydrogen (H 2 ) which is known as hydropyrolysis is also considered as an remarkable approach to produce the high-quality bio-oil (Wu et al., 2023).In recent years, emerging technologies such as microwave-assisted (Ravikumar et al., 2017), solarassisted (Weldekidan et al., 2019), and plasma-assisted (Xiao et al., 2022) catalytic pyrolysis of biomass have been explored.Figure 1 discusses the outlook of catalytic pyrolysis of biomass for the production of bio-oil and the molecular distillation process of bio-oil to value-added products.
The pyrolysis of biomass can be performed by using different catalysts, such as sodium hydroxide, metal salts, zeolites, metal oxides, and carbon catalysts.All the catalysts have unique properties and different influences on the pyrolysis process.In catalytic pyrolysis, some important parameters, such as the reaction conditions, type/nature of catalysts, and composition of feedstocks, have vital roles in achieving the desired products.In general, zeolite catalysts are highly investigated due to their acidic sites, which enhance different reactions to achieve high quality/yield bio-oil (Yuan et al., 2022).Recently, carbon-based materials have also been evolved as an important catalytic material for the pyrolysis of biomass (Zhang, Duan, et al., 2019).Alternatively, basic metal oxides (e.g., CaO) (Zhang, Xiong, et al., 2022), transition metal oxides (e.g., CuO) (Zhang, Zhang, et al., 2019) and transition metalbased carbides, nitrides, and phosphides (e.g., Mo 2 C) (Shafizadeh et al., 2023) were also reported as catalysts for the pyrolysis of biomass.Because of the easy and rapid synthesis, comparatively low-cost, possibility to produce in high scale, enhanced/flexible physicochemical properties, metal-based catalysts are in high demand for the catalytic pyrolysis of biomass to high quality bio-oil.
Layered double hydroxides (LDH) or hydrotalcite-like (HT-like) materials are well-known layered materials and have advantages such as variable metal ions in layers, variable anions in the interlayer, tuneable basic/acidic/ redox properties, thermal stability, interlinked porous structure, easy modification, and rapid high scalability (Sharma et al., 2021;Xu & Wei, 2018).In general, both as-synthesized (pristine) LDH and their derived materials are highly utilized as catalysts/catalyst supports for various chemical transformations (Debecker et al., 2009;Xu & Wei, 2018) that includes valorization of biomass (Yan et al., 2017).In recent years, LDH-based materials have gained more research interest for the catalytic pyrolysis of biomass in which LDH-derived mixed metal oxides are largely reported due to their high surface area and thermal stability, and good dispersion of metal oxide species (Hao et al., 2021).As an interesting scenario, LDH-based functionalized/composite catalysts (e.g., LDH-zeolites) have also been reported for the catalytic pyrolysis of biomass due to their bifunctional nature.This review aims to discuss the existing literature on the recent development on the LDH-based materials as catalysts for the pyrolysis/ copyrolysis/hydropyrolysis of biomass by using thermal and solar-assisted technologies.Further, the challenges and advancements related to catalytic biomass pyrolysis technologies using LDH-derived materials are also outlined in this study.In addition to that we envisioned a segment to aid in the future development of LDH-based catalytic materials systems for the biomass pyrolysis technologies.
T A B L E 1 Potential applications of bio-oil.

Catalytic hydrogenation followed by cracking
Transportation fuels (Tanneru & Steele, 2015) Super critical fluids (SCFs) Diverse applications (Gharib et al., 2020;Hu & Gholizadeh, 2020;Yadykova & Ilyin, 2023) In general, the term biomass is defined for all plant or animal-derived organic materials that are not fossilized and fall under the category of renewable resources (Loppinet-Serani et al., 2008).Biomass is divided into different categories, such as (i) agro or forestry residues, (ii) woody or herbaceous crops, (iii) aquatic biomass or products, (iv) industrial coproducts, and (v) municipal wastes (Ephraim et al., 2020;Sankaranarayanan et al., 2021).However, almost any kind of biomass ranging from agro/ forestry residues, energy crops, industrial coproducts, aquatic biomass, and biodegradable parts of municipal solid wastes with animal wastes (e.g., cattle manure) has been subjected to pyrolysis processes (Dhyani & Bhaskar, 2018) either by noncatalytic or by catalytic approaches.

| Catalytic pyrolysis process
The noncatalytic (direct) pyrolysis of biomass has limitations due to the formation of bio-oil with high oxygen contents that result in relatively low heating values, stability, volatility and pH and high viscosity, corrosive and chemical complexity (Balasundram et al., 2017).To overcome these issues, catalytic pyrolysis is highly preferred, which can tune the quality and yield of bio-oil by inducing catalytic reactions (Ma et al., 2020).Catalytic pyrolysis has advantages over noncatalytic pyrolysis approaches, such as reducing the pyrolysis temperature by lowering the reaction activation energy, reducing the reaction time, increasing the selectivity toward desired liquid products and favoring narrower and enhanced control over the distribution of hydrocarbon products (Hafeez et al., 2019).
During the catalytic pyrolysis of biomass, a combination of multiple reactions such as dehydration, decarboxylation, decarbonylation, deoxygenation, cracking, aromatization, ketonization, aldol condensation, hydrotreating, and reforming are expected to amend the properties of bio-oil (Shurong et al., 2017).Figure S1 shows a schematic representation of the major reaction pathways involved in the catalytic biomass pyrolysis process.
Hydropyrolysis is similar to pyrolysis process in which thermal decomposition of biomass takes place in a highpressure hydrogen (H 2 ) atmosphere to obtain high-quality liquid fuels while increasing the efficiency of hydrogen utilization (Oh et al., 2021).Through catalytic hydropyrolysis approach, bio-oil with less oxygen content and higher calorific value can be achieved due to the occurrence of various reactions such as hydrodeoxygenation, dehydration, decarboxylation, and direct deoxygenation (Jindal et al., 2023).In an another approach, cofeeding of biomass and other feedstock with a high effective hydrogen-to-carbon ratio (H/C eff ) in the presence of catalysts is referred to as catalytic copyrolysis, which can overcome drawbacks such as the lower carbon yield of aromatics and coking issues created by the low effective H/C eff of biomasses in the pyrolysis process.In general, numerous H 2 -enriched feedstocks, such as polymers, waste plastics, methane, alcohols, and waste oils, have been applied as hydrogen donors in the catalytic copyrolysis of biomass (Ahmed et al., 2020;Ryu et al., 2020) to yield higher amounts of high-quality bio-oil.In general, cofeeding of hydrogen-enriched feedstocks with biomass typically results in new reaction pathways (Figure S2) for the formation of bio-oil compared with catalytic pyrolysis (Chen, Che, et al., 2019).The following section covers the different types of catalysts utilized for the catalytic pyrolysis of biomass.

| Catalysts for pyrolysis of biomass
Generally, the selection of catalyst has a vital role in the pyrolysis of biomass, which preferably should have criteria such as low cost, high activity/selectivity, resistance to deactivation and easy recyclability.

| Inorganic salt additives
In general, alkali and alkaline earth metals such as potassium, calcium, sodium, and magnesium have different effects on the bio-oil yield depending on the biomass feedstock type, reaction condition, and salt type, for example, (i) potassium oxalate lowered the oxygen contents with high content of hydrocarbons compared with potassium hydroxide and potassium carbonate (Shen et al., 2020), and (ii) alkali and alkaline earth metal chloride (e.g., KCl) and acetate salts (e.g., CH 3 COOK) decreased the yield of bio-oil (Wang et al., 2023).Iron salts such as FeCl 2 and FeCl 3 produced ketone-rich bio-oil, whereas Fe 2 (SO 4 ) 3 produced acid-rich bio-oil (Xia et al., 2023).It was identified that basic salts (e.g., K 3 PO 4 ) supported the formation of phenolic compounds (Lu et al., 2018), whereas acidic salts (e.g., ZnCl 2 ) resulted in the formation of a high yield of furfural (Hu et al., 2021).When inorganic salts are used for the catalytic pyrolysis of biomass, drawbacks such as complex pre-treatment processes and complications in catalyst recycling need to be considered.

| Zeolites
Owing to their acidic nature and shape selectivity, zeolite catalysts are highly investigated for the pyrolysis of various biomass and are found to efficiently convert unwanted oxygen-containing compounds in bio-oil to aromatics, such as benzene, toluene, and xylene.The physicochemical properties of zeolites, such as pore size and shape, have a significant influence on their catalytic activity, for example, catalysts with small pores cannot produce any aromatics, catalysts with medium pores produce higher aromatic yields, and catalysts with large pores generate lower aromatic, high coke, and low oxygenated compound yields (Cai et al., 2020).Furthermore, properties such as medium crystallite size and low Si/Al ratio (Hernández-Giménez et al., 2021) in ZSM-5 also supported the formation of higher aromatics.It is very important to tune the physicochemical properties of zeolite catalysts (e.g., pore structure, surface, and metal modifications) to enhance the yield of aromatics in bio-oil.In the case of pore structure modifications, alkali treatment (Tang et al., 2019) and mesoporous introduction (Zhang et al., 2018) resulted in high yields of aromatics and low yields of coke.Surface modification such as chemical vapor deposition (Zhang, Tan, et al., 2017) and acid dealumination (Zhang, Shao, et al., 2017) reduces the number of acidic sites on the surface of zeolite (e.g., ZSM-5) catalysts, which inhibits the coke formation and promotes the aromatics formation in bio-oil.Another approach to improve the physicochemical properties of the catalyst is the loading of metals on the zeolite materials to create new catalytic active sites that facilitate various reactions and hydrogen transfer.In catalytic copyrolysis of wheat straw and high-density polyethylene, 1%Zn incorporated HZSM-5 produced a higher yield of aromatics (25.1%) followed by 5% Ni, Zn, and Mn (15.6%-16.2%)incorporated HZSM-5 (Nandakumar et al., 2023).Although noticeable efforts have been made to prepare modified ZSM-5 catalysts, limitations such as low yields of aromatics and heavy coke formation still need to be overcome.

| Metal oxides
Owing to their redox and acid-base properties, various metal oxides have been explored for the catalytic pyrolysis of biomass to produce high-quality bio-oil (Shafizadeh et al., 2023).In general, acidic metal oxides (e.g., Al 2 O 3 and SiO 2 ) reduce the oxygen content with reduced organic compounds in the liquid phase.To some extent, acidic catalysts (e.g., Al 2 O 3 ) enhance the dehydration reaction that results in a high water content in the product (Zhou et al., 2019).Similarly, basic metal oxides (e.g., CaO and MgO) reduced the oxygen content with a drop in the yield of bio-oil.It was identified that at low temperature (400-600°C), CaO reacted with H 2 O, CO 2 , acids and phenols, whereas at high temperature, CaO enhanced the decarbonylation of ketones (Chen, Li, et al., 2019).Apart from acidic and basic metal oxides, various transition metal oxides have also been used for the catalytic pyrolysis of biomass.It was identified that V 2 O 5 , Mn 2 O 3 , TiO 2 and CoO supported the formation of heavy bio-oil whereas CeO 2 , Cr 2 O 3 , CuO and Fe 2 O 3 resulted lighter bio-oil because of disturbance of the evolution of individual components by metal oxides (Zhang, Zhang, et al., 2019).Transition metal oxides (e.g., NiO, CoO, ZnO and CeO) loaded on ZSM-5 were explored as catalytic pyrolysis of neem dust, which resulted the increase in hydrocarbon selectivity with a drop in bio-oil yield compared to ZSM-5 catalysts (Liu et al., 2023).The deoxygenation reactions of biomass intermediates (e.g., furans and alkoxy phenols) resulted the formation of hydrocarbons which were subsequently produced H 2 O, CO, and CO 2 and hence the yield of bio-oil was reduced.Further, modified forms such as functionalized metal oxides (e.g., SO 4 2− /ZrO 2 and WO x -ZrO) (Guda & Toghiani, 2017), mixed metal oxides (e.g., CaO-MgO and Nb x M y O z ) (Li, Hu, et al., 2022;Locatel et al., 2021) were also used to improve the quality of the bio-oil.In the case of metal oxide-based catalytic pyrolysis of biomass, the choice of catalyst, its properties, and the reaction conditions have a significant role in determining the end product quality and yield.

| Carbon-based catalysts
In recent years, owing to their advantages, such as low cost, functional groups, characteristic pore natures, flexible surface modifications and high inorganic minerals, carbon-based materials (e.g., biochar) have gained more research interest for the catalytic pyrolysis of biomass (Zhang, Duan, et al., 2019).It was reported that biochar and activated carbons show promising behaviors in biooils, such as (i) resulting in high phenol concentrations (Yang et al., 2018), (ii) reducing the oxygen content (Chen et al., 2018), and (iii) favoring the cracking of heavy compounds into lighter compounds (Dong et al., 2018).For carbon-based catalysts, a few hurdles, such as (i) no proper studies on the relationship between the catalytic effect of biochar and its physicochemical properties, (ii) the stability, inactivation mechanism and regeneration process of carbon-based catalysts are ambiguous, and (iii) the properties of biochar are highly dependent upon biomass composition, pyrolysis conditions, and pre-/post-treatment methods, need to be overcome.
2.2.5 | Layered double hydroxide-derived materials LDH and their modified forms are well-known catalytic material for the conversion of biomass resources to value-added products through various reactions (Hernández et al., 2017;Yang et al., 2021).In the case of pyrolysis of biomass to produce bio-oil, mixed metal oxides (layered double oxide [LDO]) derived from LDH are known are predominantly utilized followed by LDO/zeolite composites rather than as-synthesized LDH.This finding gives tremendous scope for the design of unique LDH-derived materials for the catalytic pyrolysis of biomass.The following section covers the reported literature for the catalytic pyrolysis of biomass using LDH-derived materials.

HYDROXIDES (LDH)
LDH are naturally occurring hydroxycarbonate of magnesium-aluminum (Mg 6 Al 2 (OH) 16 CO 3 •4H 2 O), which can also be artificially synthesized by changing the metal cations.LDH have the general formula is the interlayer anion, m is the moles of water and x is the ratio of M(III)/(M(III) + M(II)) (generally ranges between 0.20 and 0.40) (Figure 2a).Structurally, LDH exhibit a brucitelike (Mg(OH) 2 ) layered network wherein a partial swap of divalent ions (M 2+ ) by trivalent ions (M 3+ ), for example, Al 3+ , occurs, and the resulting excess positive charge is compensated by anions that occupy between the sheets along with water molecules.By changing not only the ratio of divalent metal cations to trivalent metal cations (e.g., Mg, Ca, Ni, Al, and Fe) but also the interlayer anions (e.g., organic and complex anions), a series of LDH materials can be prepared.

| Synthesis methods of LDH
In general, LDH materials are synthesized by coprecipitation methods, for example, MgAl-LDH are synthesized using Mg and Al salts (nitrates/chlorides) in aqueous alkaline solutions at a fixed pH under stirring in an inert atmosphere followed by sequential aging, filtration, washing and drying processes (Jiang et al., 2021;Theiss et al., 2016).Apart from coprecipitation, methods such as urea and hexamine hydrolysis, ion exchange, rehydration, hydrothermal/solvothermal, self-assembly, in situ chemical reduction, mechanochemical, electrochemical, microwave-assisted and sol-gel methods were also explored for the synthesis of LDH materials (Karim et al., 2022).Figure 2b shows some of the common methods employed for the synthesis of LDH materials.In general, LDH-based catalysts have various advantages, such as (i) cation tuneability in layers by changing the metal ions, (ii) anion exchangeability in interlayers by in situ/ post-modification processes, (iii) the possibility of obtaining highly dispersed, stable, metal-supported catalysts because of uniform dispersion of metal cations in the layers and favored alignment of anions in the interlayer, (iv) unique acid-base/redox properties that can be tuned by altering both the layer and the interlayer, (v) easy structural modifications, and (vi) support materials to host other active catalysts (e.g., metals, nanomaterials, polymers), which favor their role as active catalytic materials (Xu & Wei, 2018;Yang et al., 2020).

| Modifications of LDH for catalytic applications
Owing to the electrostatic forces between the layers, the sheet-like morphology of as-synthesized LDH likely to undergo stacking process which fades their functional properties by the decrease in specific surface area and active sites (Tang et al., 2020).These issues are limiting the catalytic application of as-synthesized LDH materials and to overcome these drawbacks modification of LDH is highly preferred.In general, numerous modification approaches of as-synthesized LDHs, such calcined LDH, supported LDH, intercalated LDH, functionalized/composite LDH, exfoliated LDH and LDH with irregular morphologies (e.g., coreshell, flower-like, and hollow-sphere LDH structures) have been attempted.The complete details about the different modification approaches of as-synthesized LDH materials are discussed in the following sections.

| Mixed metal oxides
Thermal calcination (300-600°C) of LDH (Figure 3a) results in 30%-45% weight loss, which leads to the formation of nonstoichiometric LDO (Figure 3b) which are well-known active base catalysts for various organic transformations (Bharali et al., 2015;Sankaranarayanan et al., 2012).As-synthesized LDH exhibit relatively poor basic properties because the adsorbed water hinders access to basic sites on the surface (Turco et al., 2004).In addition to the removal of water and carbon dioxide, rearrangement of surface and bulk atoms occurs during pretreatment/calcination, which changes the number and nature of the basic sites with increasing pre-treatment temperature.Therefore, the optimum pre-treatment temperature varies with the type of reaction (Hattori, 2001).In general, metal oxides derived from LDH materials are showing improved thermal stability, higher surface area and active sites compared to the parent as-synthesized LDH materials as well as metal oxides prepared by other processes.Though LDO possess superior properties than LDH, the drawbacks such as complication to establish a relationship between the structure, surface basicity and metal composition are considering as a limiting factor for catalytic applications.

| Reconstructed LDH
LDO can be reconstructed to a layered structure if put in contact with water or steam environment (Figure 3c) which is known as the "memory effect" and this can be flexibly controlled by calcination temperature, pH, reconstruction, coexisting anions, and metal composition (Ye et al., 2022).In general, not all the LDO can be constructed back LDH material because of different chemical composition or structure of LDO than LDH, stability/reactivity of LDO, lack of suitable interlayer space.Reconstructed LDH are actively utilized as catalytic materials for various chemical reactions (Prakruthi et al., 2015;Tajuddin et al., 2022).The reconstructed LDH possess some disadvantages such as poor cycling performance, less surface area and defected structure and metal oxide loading (Ye et al., 2022) that hinders their catalytic activities.

| Functionalized/composite LDH
In general, functionalized/composite LDH can be prepared by introducing guest materials in both the outer and/or inter layer of the LDH materials through various approaches (Sankaranarayanan et al., 2015;Thao et al., 2016;Vasseghian et al., 2023) and the resulted materials overcome the problems such as requirements for high temperatures, additives, and high catalyst loading compared with reported catalysts.Surface functionalization of LDH materials with active component creates a supported LDH material (Figure 3d) with unique properties.Mostly, metal/metal oxide nanoparticles/nanomaterials are loaded on the surface of the LDH materials to form supported LDHs.Functionalization of LDH with other organic/inorganic/bio materials (e.g., polymer, zeolite, and biochar) can create a functionalized material in which the size and nature of the counter material plays a vital role in the morphology of the end material.The resulting material is composite (Figure 3e) in nature in which combined functions of both the materials promote the efficiency to overcome the obstacles associated with the individual materials.In general, the composite of LDH can be divided into three types based on the morphological properties as follows, (i) LDH as shell, (ii) LDH as core, and (iii) layer by layer assembly of LDH (Khan et al., 2023) new active sites (Figure 3f).Because of the introduction of larger sized guest molecules, the stacking structure of the original LDH are improved.Intercalated LDH materials are used for various catalytic applications because of their stable structure and recyclable nature.Further disadvantages such as variation in synthesis strategies from one system to another, high costs, tedious operation, nongreen solvents, and possible contamination are needs to be considered for the development of intercalated LDH materials (Chen et al., 2023).

| Exfoliated LDH
Exfoliation is the process of converting bulk LDH materials to single-or few-layered nanosheets (Figure 3g), which increases the surface area and rich defects; thus, the electronic structure and physicochemical properties of LDH are changed (Chen et al., 2020).The exfoliation approach will transform delaminated LDH into positively charged, 2D nanosheets, which can be assembled with numerous negatively charged species, such as polymers, metal complexes, and carbon-based materials, with well-defined architectures that results composite materials (Karim et al., 2022).Recently, exfoliated LDH and their derived nanocomposites have gained more research interest for catalytic studies due to their improved surface activity compared with that of bulk LDH (Munonde et al., 2019;Thiensuwan et al., 2023).Still more studies need to be done to understand the structure-activity relationship, effect of synthesis methods, and methods to improve the regeneracy of the exfoliated LDH for the catalytic studies.

| LDH structures with irregular morphologies
The LDH-based functionalized/composite material can also be prepared as a LDH structures with irregular morphologies.In general, LDH materials prepared by conventional coprecipitation synthesis method, shows the hexagonal plate morphology and their application potential can be improved by achieving LDH material with irregular morphologies.By exploring different synthesis methods, different structured (micro/nano) as well as different dimensional LDH materials were reported (Kuang et al., 2010).The coreshell structure of LDH composite materials (Figure 3h) are spherical in nature and are the combination of inner core materials and outer layer materials in which LDH can be either as core or shell material (Arshad et al., 2023;Das et al., 2023).Further, flower-like (Figure 3i) and hollowsphere LDH structures (Figure 3j), were also prepared by adopting different synthesis approaches (Ding et al., 2023;Feng et al., 2021;Xu et al., 2017;Zhang, Lu, et al., 2022) were.Specific synthesis methods, difficulty in retaining the structure and regeneracy for the next cycle are the limited factor for the catalytic applications of LDH structures with irregular morphologies.

BIOMASS USING LDH-DERIVED MATERIALS
As previously stated, in recent years, LDH and their modified forms have been explored for the catalytic valorisation of biomass/derivatives to numerous value-added products.This section covers the catalytic activity of different LDH-derived materials for the pyrolysis of various biomasses.Tables 2 and 3 summarizes the catalytic activities of as-synthesized LDH, LDO and LDH-based functionalized/composite materials toward pyrolysis of biomass for the production of bio-oil.

| As-synthesized LDH
Hai et al. ( 2022) reported the as-synthesized LDH as catalysts for the pyrolysis of date seeds (DS), using MgFe-LDH, NiFe-LDH, and CoFe-LDH synthesized by the coprecipitation method.Prior to the study, the prepared catalyst was mixed with DS at a ratio of 1:10 and subjected to pyrolysis at an operating temperature of 500°C.Under the optimized conditions, DS/LDH exhibited a bio-oil yield between 65 and 67 wt% (Table 2; No. 1-3).Schematic representation of the in situ production and upgrading of DS bio-oil via the pyrolysis and preparation of LDH are shown in Figure S3a,b.The FT-IR analysis of DS-and DS/ LDH-derived bio-oils indicated that catalytic activities of the as-synthesized LDHs led to the deoxygenation and cracking of the macromolecules which result in the formation of straight-chain compounds (Figure S3c-f).This clearly shows that presence of LDH catalysts significantly modifies the composition of bio-oil in DS pyrolysis compared to noncatalytic pyrolysis.

| LDH-derived mixed metal oxides
LDO are investigated for the catalytic pyrolysis of biomass by thermal and solar-assisted processes as well as solar-assisted hydropyrolysis due to their superior physicochemical properties.Andrade, Barrozo, et al. (2018) explored MgAl-LDO derived from MgAl-LDH by calcination at 550°C for 2 h as catalysts for the solar pyrolysis of Chlamydomonas reinhardtii.Surface response methodology was applied to evaluate the effects of biomass loading rate, reaction time, and catalyst on the product distribution and liquid composition.Catalytic fast pyrolysis studies were performed by solar assisted in situ approach, in which the maximum liquid yield of 57% was achieved (Table 2; No. 6) and authors identified that LDO as a catalyst decreased the nitrogenated compounds with increasing relative hydrocarbon percentage.
The same group compared the catalytic pyrolysis of Chlamydomonas reinhardtii using MgAl-LDO with a noncatalytic pyrolysis (Andrade, Batista, et al., 2018).For the studies, a catalyst-to-biomass ratio of 1:2 was selected and the pyrolysis experiments were carried out in the temperature range of 450-750°C (Table 2; No. 7).In the presence of catalysts, the selectivity of aromatic hydrocarbons and some alkenes was improved with a drop in the selectivity of oxygenated compounds at higher temperatures.In the noncatalytic pyrolysis, a wide range of aromatic and nonaromatic hydrocarbons were formed at different temperatures, exceeding the nitrogenated and oxygenated compounds.The authors identified that LDO can improve the bio-oil composition by decreasing the nitrogenated compounds due to the formation of ammonia or hydrogen cyanide.Navarro et al. (2018) synthesized MgAl-LDH with different Mg/Al ratios, calcined at 500°C for 15 h and T A B L E 2 As-synthesized LDHs and LDH derived mixed oxides toward catalytic pyrolysis of biomass for the production of bio-oil.

No. Catalyst (C) Catalyst preparation Biomass (B) C:B ratio
As-synthesized LDHs the influence of the Mg/Al ratio in MgAl-LDO catalysts for the fast pyrolysis of wheat straw.The reactor for the experiment (ex situ) contains two independently controlled heating areas: (i) the pyrolysis zone maintained at 550°C, where the pyrolysis reaction occurs and (ii) the catalytic section at 450°C, where the pyrolysis vapors make contact with the catalyst bed.The yield of bio-oil was higher for MgAl-LDO catalysts (45%, 43%, and 41% for Mg/Al ratios of 2, 3, and 4, respectively) (Table 2; No. 8-10) than for the ZSM-5 catalyst (40%), especially for the catalyst with a lower Mg/ Al ratio, which may be attributed to less effective basic sites of LDO for the cracking reactions of the pyrolysis vapors.In bio-oil composition, levoglucosan is identified as the main component with organic acids and oxygenated aromatics.
Various oxides such as ZnAl-LDO, Zn 2 Al-LDO, MgAl-LDO, and Mg 2 Al-LDO were prepared by calcining the respective LDH precursor at 500°C for 3 h and were used as catalysts for the ex situ fast pyrolysis of pine wood (Edmunds et al., 2019) (Table 2;(12)(13)(14)(15).Compared to non-catalyzed process, the LDOs showed a drop in the relative yield of acetic acid, methoxyphenols, and total phenols with the enhancement in the relative yield of aromatics, H 2 O, CO, and CO 2 .The authors identified that MgAl-LDO catalysts produced higher amount of H 2 O, CO, and nonoxygenated aromatics, toluene and compared with ZnAl-LDO materials.They also reported that MgAl-LDO materials showed higher yield of deoxygenated aromatics and ZnAl-LDO materials were prominent in decarboxylation reaction because of the enhanced yield of CO 2 .Barbosa et al. (2020) investigated the solar-assisted in situ pyrolysis of Spirulina platensis using MgAl-LDO catalysts derived from MgAl-LDH by calcination at 550°C for 4 h.The pyrolysis was carried out in a solar reactor under a specific range of solar flux radiation (700-850 W/m 2 ), which led to a temperature of ~500 ± 20°C.Owing to their higher composition of carbohydrates, which are less thermostable and more degradable at lower temperatures, Spirulina platensis showed an improved yield of liquid product at intermediate reaction times.Low biomass with a high catalyst percentage supported liquid formation, whereas high biomass with a high catalyst amount exhibited a high concentration of coke formation (Table 2; No. 19).The authors revealed that Maillard reactions caused the formation of nitrogenated compounds in bio-oil and observed that LDO contributed to increasing the hydrocarbon formation in the bio-oil at lower reaction times.Under the optimized reaction conditions, LDO catalysts exhibited a higher bio-oil yield of 35.9%.
Solar catalytic pyrolysis of Chlamydomonas reinhardtii and Spirulina platensis using MgAl-LDO catalysts exhibited liquid yields of 50.6% and 40.4%, respectively (Andrade et al., 2020) (Table 2; No. 20 and 21).Solar pyrolysis performed with low biomass resulted in improved liquid yields in both microalgal biomasses.The study reveals that bio-oil obtained from Chlamydomonas reinhardtii and Spirulina platensis has different compositions of chemical compounds (Table S1).Interestingly, Spirulina platensis-derived bio-oil showed higher oxygenated compounds due to its higher carbohydrate composition, whereas Chlamydomonas reinhardtii-derived bio-oil showed higher nitrogenated compounds due to its higher protein composition.LDO catalysts supported different reaction pathways for both microalgal species in solar catalytic pyrolysis, for example, deoxygenation reactions were preferred in Spirulina platensis bio-oil than in Chlamydomonas reinhardtii.Barbosa et al. (2021) investigated the ex situ catalytic solar pyrolysis of Chlamydomonas reinhardtii using a MgAl-LDO derived from MgAl-LDH by calcination at 550°C for 4 h.A schematic representation of the solar equipment used for the pyrolysis process is shown in Figure S4a.Under solar irradiation of 750 ± 30 W/m 2 (~500 ± 20°C), the volatile products, removed by the vacuum pump, were passed through the catalytic fixed bed and then through a condensation system.The structure and surface area of fresh and regenerated catalysts were similar, which supports the possibility of reusing the catalyst for subsequent cycles.Figure S4b summarizes the scheme of the catalyst reuse study performed in this work.A high yield of bio-oil (38.55%) with a high hydrocarbon content (32.65%) was achieved under optimal conditions (Table 2; No. 22). Figure S4c shows the compositions of oxygenated compounds derived from carbohydrates and lipids, nitrogenated compounds derived from protein and hydrocarbons in bio-oil.Although a drop in bio-oil yield was observed during recycling, the hydrocarbon content was not affected (Figure S3d,e).Rossi et al. (2021a) studied the ex situ catalytic solar hydropyrolysis of Chlamydomonas reinhardtii using MgAl-LDO by feeding the H 2 gas produced from alkaline electrolysis.Prior to the experiment, the system was purged with H 2 to maintain an oxygen-free environment.The maximum bio-oil yield reached as 48.8% under optimal experimental conditions (Table 2; No. 24).Interestingly, a drop in the yield of nitrogenated compounds with an increase in hydrocarbons was observed because of the interaction between the vapors and the catalyst precursors.The authors further observed that the use of a H 2 atmosphere resulted in a liquid product with fewer oxygenated compounds.
The same group compared the catalytic activity of MgAl-LDO (LDH calcined at 550°C/4 h) for ex situ catalytic solar pyrolysis (CSP) and ex situ catalytic solar hydropyrolysis (CSH) of Chlamydomonas reinhardtii (Rossi et al., 2021b) (Table 2;No. 25 and 26).Although both the CSP and CSH processes were similar using the ex situ setup, the experimental apparatus was slightly changed for the H 2 feed in the latter.Of the analyzed approaches, CSH resulted in a higher liquid yield of 48.8%, whereas CSP resulted 47.6% yield of liquid.The H 2 in the system favors the deoxygenation and hydrogenation reactions that resulted lower oxygenated compounds (40.8%) and higher hydrocarbons (21.7%) in bio-oil.Prabhakara et al. (2022) utilized MG70 HT pellets (LDH precalcined at 550°C) for in situ catalytic fast pyrolysis of beechwood (Table 2; No. 27).Noncatalytic and catalytic pyrolysis experiments were carried out on analytical Py-GC/MS (pyroprobe) and a bench-scale fluidized bed reactor at 500°C.The physicochemical properties of bio-oils obtained by noncatalytic and catalytic pyrolysis processes are compared in Table S2.Compared to the noncatalytic approach, the pH and carbon content of the bio-oil increased from 3.2 to 6.0 and from 45.7 to 70.0 wt%, respectively, with a decrease in oxygen content from 46.3 to 20.5 wt% in the catalytic approach.Interestingly, the high heating value (HHV) for the bio-oil obtained from catalytic studies was evaluated as 29.3 MJ/kg (as received), and that obtained from the noncatalytic studies was 18.1 MJ/kg, which reveals that the catalyst and operating conditions are key in obtaining high-quality bio-oil.Moreover, the water content of the bio-oil decreased to 6.2% in the organic phase in the catalytic approach, whereas the same was 20.1% in the noncatalytic approach.At a catalyst/ biomass ratio of 4, the MG70 HT catalyst promoted the formation of alkylated cyclopentenones, cresols, methyl furans and hydrocarbons.This study clearly indicates that the MG70 HT catalyst has an important role in reducing the oxygen which is the main purpose of the catalytic pyrolysis process.
4.3 | LDH-based functionalized/ composite catalysts Gao et al. (2017) reported the catalytic pyrolysis of Cyanobacteria using MgAl-LDO/ZSM-5 composites derived from the calcination of MgAl-LDH/ZSM-5 precursor (prepared by loading LDH on HZSM-5 with hydrothermal synthesis) at 550°C for 4 h.The study at 550°C showed superior activity for the MgAl-LDO/ZSM-5 composite toward the highest liquid yield (41.1%) compared to their individual counterparts (MgAl 4 -LDO = ~35% (Table 2; No. 4); ZSM-5 = 25.7%) and the mechanically mixed MgAl 4 -LDO and ZSM-5 catalyst (34.0%) (Table 3;No. 4).Studies on the effect of the Mg/Al ratio in the MgAl-LDO/ZSM-5 composite revealed that an increase in the Mg/Al ratio caused in an increase in the liquid yield from 30.7% to 41.1% (Table 3; No. 1-3).The main compounds in bio-oil obtained by the noncatalytic and catalytic pyrolysis are divided into four types: (i) hydrocarbons, (ii) aromatic compounds, (iii) nitrogenous compounds and (iv) oxygenic compounds (Table S3).The nitrogenous compounds of the bio-oil from noncatalytic and catalytic pyrolysis were 50.9% and 45.3%, respectively, which clearly indicates the reduction in the nitrogen compounds by the catalytic approach.
The same research group further reported the catalytic pyrolysis of cyanobacteria with methanol using MgAl-LDO/ZSM-5 as a catalyst (Bai et al., 2018).They found that the inclusion of methanol resulted in an increase in liquid yield from 37% to 67.7% compared to the reaction in a nitrogen atmosphere for the MgAl 3 -LDO/ZSM-5 catalyst (Table 3;No. 5 and 6).These results may be attributed to the promotion of water production by methanol and the impeded formation of COx.Additionally, the liquid yield obtained for MgAl 3 -LDO/ZSM-5 (67.7%) was higher than that obtained from individual counter parts (MgAl 3 -LDO = 60.6% (Table 2; No. 5); ZSM-5 = 63.2%).Although MgO/ZSM-5 resulted in a higher yield of liquid products (75.5%), the yield of bio-oil was lower (~16%) than that of the MgAl 3 -LDO/ZSM-5 catalyst (~22%).The resulting bio-oil using a methanol atmosphere is rich in aromatic compounds with less oxygenated and nitrogenated compounds.Yang et al. (2019) reported the catalytic pyrolysis of cyanobacteria using metal-loaded MCM-41 catalysts with vaporized methanol at a temperature of 550°C.Of the analyzed catalysts, NiAl-LDO/MCM-41 showed a higher yield of liquid products (higher yield of bio-oil: 26%) (Table 3; No. 7) (Figure S5a) compared to NiAl-LDO (yield of bio-oil: 24.0%) (Table 2; No. 11).The relative contents of hydrocarbon and nitrogenous, oxygenated and aromatic compounds in the bio-oil composition for the investigated catalysts are shown in Figure S5b.The feasible pathway for the formation of nitriles and nitrogen transformation in bio-oils during catalytic pyrolysis of microalgae is shown in Figure S5c.At high temperature, inorganic protein release NH 3 which reacts with fatty acid and results amides.On another hand, cracking of protein nitrogen result pyrrolic-N, quaternary-N and pyridinic-N which are subsequently convert into N-Heterocyclic compounds.The formed N-Heterocyclic compounds were converted into HCN and mononitriles which also results amides in presence of methanol.The metal loaded MCM-41 provided an active site where the Ritter reaction could take place to produce amides and then convert into nitriles.The structural effect of MCM-41 has significant effect in the preferred formation of long-chained nitriles such as pentadecanenitrile.
NiAl-LDO/MCM-41 catalyst was reported for the pyrolysis of cyanobacteria under nitrogen and methanol atmospheres at 450°C (Xu, Gao, Xiao, et al., 2020).The prepared NiAl-LDH/MCM-41 material was calcined at 500°C for 3 h to obtain the NiAl-LDO/MCM-41 catalyst.It was determined that all the catalysts (NiAl-LDO, MCM-41 and NiAl-LDO/MCM-41) improved the yield of liquid products (Table 2; No. 16 and 17, Table 3; No. 8 and 9) and bio-oil due to the inhibition of gas products by the alkaline nature of the catalysts.The authors identified that a methanol atmosphere promoted a higher yield of bio-oil with lower hydrocarbon/aromatic compound compositions than a N 2 atmosphere, which clearly indicates the participation of methanol in the pyrolysis process.The results revealed that the amounts of nitrogenous compounds increased with increasing catalyst/cyanobacteria ratio and exhibited a higher yield of liquid products (50.5%) with ~25% yield of bio-oil.
The same group utilized NiAl-LDO/MCM-41 catalysts for the pyrolysis of distilled lemon grass.The initial experiment was carried out in the absence of catalysts, revealing that a methanol atmosphere and a reaction temperature of 450°C are suitable conditions for the study (Xu, Gao, Yang, et al., 2020) (Table 3;No. 10-13).The addition of catalyst significantly decreased the biochar yield with a significant increase in the yield of liquid products because of the easy dehydration of methanol to dimethyl which recombined with the pyrolysis intermediates at the acid sites of the catalysts.The highest bio-oil yield (16.7%) with >90% aromatic substances was obtained for 20% loading of NiAl-LDO on the MCM-41 catalyst whereas NiAl-LDO catalyst resulted ~10% yield of bio-oil (Table 2; No. 18).
Recently, Hao et al. (2021) investigated the copyrolysis of rice straw and Ulva prolifera macroalgae using a series of LDO supported on activated biochar catalysts at 500°C (Table 3; No. 14-16).The authors found that copyrolysis produced a higher yield of bio-oil compared to individual pyrolysis and resulted in a maximum bio-oil yield of 46.7% in the absence of catalysts.It was observed that compared to noncatalytic studies, the efficiencies of biooil were significantly decreased in the catalytic studies due to coke formation.Although NiFe-LDO catalysts resulted in a higher yield of bio-oil (43.2%) (Table 2; No. 23) than prepared composite catalysts (38.1% to 40.2%) (Table 3; No. 14-16), the quality of the bio-oil was improved with a higher composition of aromatic and aliphatic compounds when using 5%Ga/NiFe-LDO catalysts, which may be attributable to acidic sites.This finding reveals that Ni-Ga would be a promising phase for the deoxygenation of fatty acids, esters, aldehydes, and phenols.

LDH-based catalysts
In the case of catalytic pyrolysis of biomass, deactivation of the catalysts mainly because of coke/ash deposition, metal sintering, agglomeration, blocking of active sites/ poisoning, and structural transformations due to series of reactions (Li, Nishu et al., 2022), which limits their recyclability for further cycles.The pyrolysis of biomass at high temperature in the presence of as-synthesized LDH catalysts lead to the decomposition of LDH to LDO, which hinders the recyclability of the catalysts because of the structural transformation.In the case of LDO catalysts coke formation may be the main reason for the deactivation of the catalysts.For both as-synthesized LDH and LDO catalysts for the pyrolysis reactions, recalcination is the possible regeneration approach to convert the used catalysts into active catalysts.The biomass pyrolysis using LDO/zeolite as catalysts may obey the deactivation behavior as same like their individual counterparts.In common, the regenerated catalysts may not have similar activity that of fresh catalysts due to the change in physicochemical properties and loss of active sites.In general, ex-situ catalytic approach may have better recyclability nature compared to the in-situ catalytic studies.At this stage, detailed studies are highly recommended for the used LDH-based catalysts and that will give further directions to design the proper regeneration approach for the biomass pyrolysis reactions.

| CONCLUSION
Pyrolysis of biomass is an effective way to produce biooil, and the process can be performed in the presence of catalysts to improve the quality/yield of bio-oil.In this way, the oxygen contents of bio-oils are decreased and become more suitable for fuel applications.Mainly agro/ forestry residues and microalgal biomass were explored as the feedstock for the catalytic pyrolysis process.In recent years, layered double hydroxides (LDH)-derived materials have gained more research interest for the catalytic pyrolysis of biomass and studies were explored for thermal and solar pyrolysis processes whereas no studies are available for microwave and plasma pyrolysis.Though the concept of catalytic pyrolysis process along with hydrogen (hydropyrolysis) or hydrogen-enrich feedstocks (copyrolysis) are well-known to improve the yield/quality of bio-oil, only limited studies have been performed using LDH-derived materials.It has been reported that changing the transition metal in catalytic materials has a substantial influence on the composition of the pyrolysis products.In the case of LDH-based composite catalysts, all the reported materials used only zeolites as a counterpart with LDO phases.LDO/ zeolite composite catalysts exhibited improved activity toward the catalytic pyrolysis compared to their individual counter parts as well as their physically mixed catalysts.It was identified that methanol, as a hydrogen donor, can promote the catalytic pyrolysis process to overcome the low hydrogen/carbon ratio in bio-oil, which limits the hydrocarbon yield.It has been reported that in LDH-based catalytic pyrolysis of biomass, hydropyrolysis is more promising than copyrolysis.In most catalytic pyrolysis processes, in situ mixing of biomass with catalysts hinders the reuse of the catalysts and makes the processes expensive.Studies revealed that the ex situ pyrolysis of biomass using LDH-derived catalysts almost retains its activity for consecutive cycles, which gives a scope for the recyclability of the materials.LDO catalysts have a more positive impact on solar-assisted hydropyrolysis than solar-assisted pyrolysis.

| FUTURE PERSPECTIVES
In the case of feedstocks, industrial coproducts and municipal wastes can also be considered for catalytic pyrolysis using LDH-derived materials.By using wastes as alternative feedstock for catalytic pyrolysis, assessment is possible, which can create a vast circular economy market.It will be interesting to use the of modified (e.g., ball-milling) biomass for the catalytic pyrolysis process because of their alteration in properties can have huge influence on the pyrolysis products.It is noteworthy to focus on hydropyrolysis of different biomass sources to high-quality biooil in which the process can be designed by using the in situ generated biohydrogen source.It will be interesting to perform the catalytic copyrolysis of biomass with hydrogen-enriched feedstocks, such as polymers, waste plastics and waste oils, in the presence of LDH-derived materials, which can provide a better understanding of the copyrolysis process for the production of high-quality bio-oil.In catalytic pyrolysis of biomass, as-synthesized LDH have not been extensively explored and so exploration of ternary LDH with numerous di/tri valent cations as catalysts will be an interesting task.In most studies, only MgAl-LDO were used as catalysts due to their easy synthesis method and it will be exciting to extend the study with different alkali/alkaline metals containing LDO for the catalytic pyrolysis of biomass.Altering the transition metal ions in the LDH-derived catalysts will change the acidic/basic/redox properties, which will have a direct connection to the yield of the bio-oil.The fabrication of composite materials consisting of LDH and/or LDO with biocarbon will be a unique design for pyrolysis approaches where the synergistic effects of individual materials can have a vital role in achieving high-quality/yield bio-oil.It will be interesting to design LDH/LDO-based composites with metal-containing materials, such as metal organic frameworks (MOFs), metal chalcogenides and MXenes, as an option to synthesize novel LDH-derived composite materials for the catalytic pyrolysis of biomass.
Further studies can be performed by varying the methanol concentration in the system to improve the quality and yield of bio-oil.It is in high demand to design the proper reactor setup to carry out solar-assisted pyrolysis to produce bio-oil in large quantities.Owing to the high activity of LDO catalysts, plans can be made to perform catalytic hydropyrolysis/copyrolysis of biomass with hydrogen-enriched feedstocks in solar-assisted systems.It will be interesting to explore microwave and plasma pyrolysis of biomass using LDH-derived catalysts.The catalyst performance drop with time is the main limitation for the pyrolysis process, which creates interest in checking the lifetime and regenerating nature of the catalysts to maximize the economic value.It is very important to give special attention to the efficient recovery of biochar obtained as the by-product during the pyrolysis process, which has noticeable market value in recent years.Additionally, some components of pyrolysis gases can be used as cofeed for the catalytic pyrolysis process to stimulate the quality of bio-oil, whereas the remaining components can be used for the industrial applications.To the best of our knowledge, no studies on the technoeconomic and environmental analyses for the catalytic pyrolysis of biomass using LDH-derived materials are available.At this stage, it is very important to perform technoeconomic and environmental analyses for the production of bio-oil and other products by catalytic pyrolysis of biomass/waste using LDH-derived catalysts which will provide guidance for the opportunities and challenges present in better process development.
. Precise synthesis methods, complication in characterizing the active sites in atomic level and degeneracy of the catalyst are the factors of consideration for the catalytic exploration of supported LDH and functionalized/composite LDH materials.In case of intercalated LDH materials, the anions present in the inter layer of the parent LDH are replaced by other anions (e.g., polymers and biomolecules) to form F I G U R E 3 (a) LDH, (b) mixed metal oxides (LDO prepared from LDH by calcination), (c) reconstructed LDH (prepared from LDO), (d) supported LDH, (e) composite LDH, (f) intercalated LDH, (g) LDH nanosheets, (h) core-shell LDH, (i) flower-like LDH and (j) hollowsphere LDH.

Atmosphere Pyrolysis approach Pyrolysis temp. Yield References
A B L E 3 LDH-based functionalized/composite materials toward catalytic pyrolysis of biomass for the production of bio-oil.No.Catalyst (C) a Copyrolysis.| 13f 22SANKARANARAYANAN and WON