Current Challenges and Perspectives for the Catalytic Pyrolysis of Lignocellulosic Biomass to High-Value Products

Abstract

Lignocellulosic biomass is an excellent alternative of fossil source because it is low-cost, plentiful and environmentally friendly, and it can be transformed into biogas, bio-oil and biochar through pyrolysis; thereby, the three types of pyrolytic products can be upgraded or improved to satisfy the standard of biofuel, chemicals and energy materials for industries. The bio-oil derived from direct pyrolysis shows some disadvantages: high contents of oxygenates, water and acids, easy-aging and so forth, which restrict the large-scale application and commercialization of bio-oil. Catalytic pyrolysis favors the refinement of bio-oil through deoxygenation, cracking, decarboxylation, decarbonylation reactions and so on, which could occur on the specified reaction sites. Therefore, the catalytic pyrolysis of lignocellulosic biomass is a promising approach for the production of high quality and renewable biofuels. This review gives information about the factors which might determine the catalytic pyrolysis output, including the properties of biomass, operational parameters of catalytic pyrolysis and different types of pyrolysis equipment. Catalysts used in recent research studies aiming to explore the catalytic pyrolysis conversion of biomass to high quality bio-oil or chemicals are discussed, and the current challenges and future perspectives for biomass catalytic pyrolysis are highlighted for further comprehension.

1. Introduction

The modern industrial development and considerable increase in global population resulted in the substantial growth in worldwide energy consumption. However, the large-scale consumption of fossil resources has brought a series of eco-environmental crises, such as global warming, destruction of the ozone layer, acid rain, land desertification and so forth, which challenged the sustainable development of all walks of the world. These related questions necessitate the development and use of renewable resources as alternatives, such as solar, wind, tidal, geothermal and biomass. Among these choices, biomass, the only renewable carbon resource, has attained a large amount of attention due to its abundant reservation and low cost [1]. Approximately 1700 billion tons of biomass can be harvested each year, which is equivalent to 850 billion tons of standard coal or 600 billion tons of oil [2]. According to the data published by the International Energy Agency, 10% of global basic energy consumption will be supplied by biomass by 2023, and approximately 27% of worldwide transportation fuel will be supplied by biomass by 2050 [3]. Since the development and utilization of biomass have the potential and feasibility in handling sustainable development and environmental problems, effective policies promoting the use of biomass have been issued in many countries. For example, the Dutch Ministry of Economic Affairs requires that approximately 30% of transportation fuel should be derived from biomass and 20–45% of non-renewable resource are replaced by biomass until 2040 [4]. The U.S. Department of Agriculture and U.S. Department of Energy set goals that biofuels must take the place of one-fifth of transportation fuel and one quarter of oil-based platform chemicals by 2030 [2]. Thus, there is an urgent demand to integrate renewable biomass energy into contemporary energy systems for modern society towards a sustainable process.

The conversion methods of biomass consist of direct combustion, liquefaction, fermentation, pyrolysis, gasification and supercritical fluid conversion [5,6]. Among these ways, pyrolysis is one of the most promising methods due to its feasibility in industrialization, low cost in installation and convenience in operation [2,5,7]. Pyrolysis of biomass refers to the process where biomass is converted to bio-oil, biogas and charcoal in an inert atmosphere at high temperatures of approximately 400–600 °C. Among these products, bio-oil has been widely used to produce transportation fuel or platform chemicals through refinery technology, for example, the hydrodeoxygenation process [8]. However, pyrolysis of lignocellulosic biomass is a quite complex process, because a range of parameters contribute to the quality of bio-oil in the meantime, such as biomass species, pyrolysis conditions and so on [9]. The raw bio-oil derived from direct pyrolysis of biomass has several drawbacks: high corrosivity and viscosity, low heating value, high oxygen and water contents [10,11,12], which hinder the direct utilization as transportation fuels. During the past several decades, research studies have mainly focused on three strategies for the optimal harnessing of biomass from pyrolysis, as shown in Figure 1. The first one is the pretreatment of raw biomass. It is reported that the characteristics of biomass can be influenced by different kinds of pretreatment methods [13,14,15], such as grinding, torrefaction, chemical or biological pretreatment. Therefore, the pretreated biomass may yield high quality bio-oil. Secondly, refinement of bio-oil is another choice for further utilization [16,17]. After the liquid products are obtained from direct pyrolysis of biomass, some oriented catalysts can be used to improve the quality. Or some separation and purification approaches can be applied to obtain the single chemicals. Thirdly, catalysts are introduced to the pyrolysis process to change the distribution of chemicals in bio-oil products directly [18,19,20]. Compared to the former two methods, catalytic pyrolysis outstands as the form of one-pot reaction, which simplifies significantly the technological process, especially avoiding the cooling and reheating of the obtained liquid products.

Numerous studies have focused on the catalysts used for the catalytic conversion of lignocellulosic biomass, for example, metal oxides [21,22,23], inorganic salts [24,25,26], and zeolites [27,28,29]. The addition of catalyst is expected to decrease the reaction activation energy and change the reaction pathway, yielding high-quality bio-oil and high-value platform chemicals. Therefore, it is pivotal to appreciate and design the catalytic pyrolysis process of lignocellulosic biomass. There have many rave reviews about biomass catalytic pyrolysis from different viewpoints [30,31,32,33]. Huang et al. [30] reviewed the pyrolysis mechanism and pathway of some value-added products from cellulose, and Vuppaladadiyam et al. [31] emphasized the importance of techno-economic analysis and life-cycle assessment on the assessment for the performance of the conversion technology of biomass and associated constraints. The present work highlights the main challenges and future development of catalytic pyrolysis of lignocellulosic biomass. In addition, several factors affecting the catalytic pyrolysis process are discussed, for instance, categories of reactor and catalyst; the latest studies about biomass pyrolysis to obtain value-added fuels are commented in detail.

2. Factors Affecting the Catalytic Pyrolysis of Biomass

2.1. Lignocellulosic Biomass

Lignocellulosic biomass mainly includes lumber (softwood and hardwood), crop and agricultural waste (wheat, straw, rice hull, bagasse, etc.) and a large amount of energy plants [18,34]. Around the world, a large amount of biomass waste is either burned directly or discarded in open fields, although billions of tons of these are generated each year. These disposal methods do harm to both the environment and economy. As is mentioned in the literature, it is an urgent mission to protect or utilize the forest before it becomes detrimental to the environment [35,36]. Therefore, it is necessary to comprehend the all-round properties of lignocellulosic biomass in order to utilize this kind of renewable resource totally.

As known, lignocellulose mainly consists of cellulose, hemicellulose and lignin, as shown in Figure 2, with a small amount of colloid, protein, lipid and inorganic substances [9,18]. Due to the differences in physical–chemical properties of the three main ingredients, the initial temperature of thermal degradation in the pyrolysis process differs [37], 200–250 °C for hemicellulose, 240–350 °C for cellulose and 280–500 °C for lignin, respectively. As shown in

Table 1. The different contents of the three main components for common biomass.

Type of Biomass	Cellulose/(wt.%)	Hemicellulose/(wt.%)	Lignin/(wt.%)	Ref.
Willow	53.0	7.5	19.3	[41]
Cypress	52.9	7.3	23.9	[41]
Pinewood	39.0	27.5	26.9	[42]
Sawdust	41.9	19.3	29.6	[43]
Apple branch	35.6	17.7	32.5	[44]
Mulberry wood	44.5	15.1	22.2	[44]
Sugarcane bagasse	36.3	20.7	22.9	[42]
Sugarcane bagasse	43.8	26.2	21.8	[43]
Chili stem	27.1	44.2	18.4	[45]
Banana peel	40.2	10.5	24.3	[46]
Wheat straw	59.7	5.9	20.4	[44]
Wheat straw	40.3	20.7	20.6	[47]
Sorghum straw	36.4	26.4	18.6	[47]
Corn stover	42.9	21.3	20.5	[47]
Corncob residues	62.7	3.6	25.2	[48]
Corn straw	45.9	19.8	16.6	[44]
Rice straw	35.3	17.1	24.6	[47]
Rice husk	44.1	21.9	25.7	[43]
Pubescens	39.8	14.3	20.6	[41]
Azolla.f	29.0	16.5	14.7	[49]

, different kinds of biomass feedstocks contain different amounts of cellulose, hemicellulose and lignin, and the interactions between the three major components also varies. In addition, the polymer structure, cross-linkages, density, thermal conductivity, airflow permeability or specific heat capacity of different biomass also have large differences, which may lead to different pyrolysis performance [38]. Chen et al. [39] studied the pyrolysis process of different biomass, such as cotton straw, rapeseed straw, tobacco straw, rice husk and bamboo, and they found that the main products in liquid phase are water (48–65%) and some organic oxygenates, such as acids, ketones and phenols. Biswas et al. [40] explored the pyrolysis of corncob, wheat straw, rice straw and rice husk, and the highest bio-oil yield of 47.3 wt% was obtained from the pyrolysis of corncob. Therefore, the choice of different types of biomass as raw materials has a greater impact on the composition of obtained bio-oil, where the small-molecular products (furans, phenols, small molecular acids) and oligomers (sugars, phenols and humin precursors) are all different in terms of type and structure.

In general, the three main ingredients, namely cellulose, hemicellulose and lignin, are considered as the main source of value-added chemicals or biofuels from pyrolysis. Cellulose is unbranched linear homopolymer of D-glucose linked together through β-1,4 glycosidic bond, including amorphous and crystalline structures [50,51]. Hemicellulose, an amorphous branched polymer, is made up of hexose (D-Glucose, D-Mannose, D-Galactose), pentose (D-xylose, L-arabinose) and glucuronic acid [52]. The abundance and detailed structures of these polysaccharides of building blocks vary widely, depending on the biomass species. As for lignin, it is a highly branched amorphous aromatic natural polymer [53], and mainly composed of phenyl-propane monomers linked by C-O-C bonds (β-O-4, α-O-4, 4-O-5) and C-C bonds (β-β, β-5, β-1) [54]. Lignin monomer units can be classified into three types, p-hydroxyphenyl type (H), guaiacyl type (G) and syringyl type (S), according to the number and position of the -OCH₃ group next to the phenolic hydroxyl group [55,56]. In the lignocellulose matrix, cellulose homopolymers are tightly combined with each other through intramolecular and intermolecular hydrogen bonds, as well as van der Waals forces generated by the stacking of pyranose rings, forming crystalline microfibrils and acting as the structure skeleton [57]. Plus, hemicellulose and lignin are dispersed in the vacant space among the skeleton to maintain the structural tenacity, while carbohydrates and lignin are bound up with hydrogen and covalent bonds. In order to make full use of lignocellulosic biomass, it is necessary to understand the pyrolysis behavior and product distribution of each single component. More information about pyrolysis rules of the three primary components are shown in

Table 2. The comparison of the three main components in pyrolysis behaviors.

	Cellulose	Hemicellulose	Lignin
Temperature for decomposition	240–350 °C	200–250 °C	280–500 °C
Structure features	amorphous and crystalline	unstable polysaccharide	disordered polymer rich in phenols
Characteristic that impacted pyrolysis	crystallinity index and degree of polymerization	-	three structure units (G/H/S) and abundance of C-C and C-O-C linkage
Reaction mechanisms	free radical mechanism, ionic mechanism and concerted mechanism	similar with the mechanism of cellulose, except for the unstable intermediate from xylosyl cation	free radical mechanism
Main products	anhydrosugars, furans and humins	furan or pyran ring derivatives, anhydro sugars and acids	phenol, guaiacol, syringol and their derivatives

.

2.1.1. Pyrolysis of Cellulose

The cellulosic structure could be divided into two categories in terms of the arrangement approach of chain molecular, amorphous and crystalline zones, respectively. As reported, the glass transition temperature of amorphous cellulose is between 243–307 °C [58], while the crystalline cellulose initiates thermal degradation over 300 °C [59], since the amorphous regions of cellulose lies in a disordered condition. During the pyrolysis process, it has been a consensus that the amorphous zone decomposes firstly, then followed by the crystalline zone. Therefore, the crystallinity index, which refers to the relative content of crystalline proportion in cellulose, could play a significant role in the pyrolysis behaviors of cellulose. Trache et al. [60] compared the thermal stability of three kinds of microcrystalline cellulose extracted from Alfa fibers and found the cellulose with higher crystallinity index difficult to decompose. A similar result was obtained from the research of Wang et al. [61], where two samples of cellulose, with the crystallinity index of 60.5% and 6.5%, were selected as pyrolysis feedstocks. Furthermore, they also compared the pyrolytic products. The cellulose with higher crystallinity yielded more anhydrosugars at high temperatures, while the yield of levoglucosan was higher at low temperatures from the one with lower crystallinity. The significant difference between the pyrolysis behaviors of the two kinds of cellulose samples might be owing to the formation of a liquid intermediate in the cellulose sample with high crystallinity [62].

The glycosidic linkage in cellulose is fragile under harsh conditions, such as acidic environment and high temperature. Thereby, cellulose initiates decomposition via the cleavage of β-1,4-glycosidic bonds after the breakdown of hydrogen bonds, leading to the reduction in the degree of polymerization (DP). The DP do have a strong impact on the pyrolysis behaviors and product distribution of cellulose. According to Chen et al. [63], the greater the polymerization degree is, the higher the temperature is that initiates the depolymerization reaction. The conclusion was obtained based on the real-time in-situ infrared experiments of microcrystalline cellulose, cellobiose and β-d-glucopyranose, where the initial pyrolysis temperatures were 325, 255 and 230 °C, respectively. Similarly, the temperature of the maximum weight loss rate for the extracted cellulose was higher than that for the bought microcrystalline cellulose in the study of Wang et al. [64], which was mainly due to the fact that the extracted cellulose holds a greater degree of polymerization. Furthermore, Zhou et al. [65] analyzed the distribution of pyrolytic products derived from four cellulosic model compounds, namely cellobiose, maltohexaose, glucose as well as cellulose, and the yield of levoglucosan rose sharply from 8.10% to 54.50% with the increment of DP. The same intensive effect of DP on the levoglucosan yield was verified by Mettler et al. [66] and Patwardhan et al. [67].

To date, three reaction mechanisms of cellulose pyrolysis have been put forward, including free radicals mechanism [68], ionic mechanism [69] and concerted mechanism [70]. Through these reaction mechanisms, cellulose can be converted into active intermediates, and then degraded to small-molecular compounds, non-condensable gas, even coked to form bio-coal. Huang et al. [30] analyzed and summarized the optimal pyrolysis conditions of specific products from catalytic pyrolysis of cellulose, including levoglucosan, 1-hydroxy-3,6-dioxabicyclo[3.2.1]octan-2-one, levoglucosenone, furans, aromatics and olefins. Generally speaking, the basic pyrolytic products from cellulose pyrolysis is anhydrosugars, especially levoglucosan [71]. It is proposed that the multitudes of competing reactions occur at the same time, which could decrease the yield of levoglucosan, and Lindstrom et al. [72] verified this point by the corresponding exploration. In addition, the secondary reactions of these intermediates are inevitable, such as isomerization, oligomerization and retro-aldol condensation, which could impair the yield of primary products. In this case, the adjustment and optimization of pyrolysis parameters could ease this and produce more desirable products, such as vapor residence time and heating rate.

2.1.2. Pyrolysis of Hemicellulose

Hemicellulose is a complex and unstable polysaccharide, and it is tough to extract the innate hemicellulose from lignocellulose biomass, making the study of the pyrolysis behaviors of hemicellulose difficult. Allowing for xylan as the richest unit in hemicellulose, most scholars selected xylan as the alternative for further study [38,73,74]. Galactoglucomannan was also chosen as the alternative for hemicellulose in the study of Aho et al. [75] since it is a typical unit form from softwood. Compared to cellulose, the decomposition of hemicellulose occurs at lower temperatures. For example, the decomposition of hemicellulose separated from corn stalk easily occurred at the range of 206–349 °C [76], and a great deal of acetic acid, furfural and ketones were generated around 300 °C. In terms of pyrolytic products from hemicellulose, the obtained compounds could be sorted into three kinds [77]: (1) low-molecular compounds, which refer to C1, C2 and C3 species, for example, CO₂, CO, formic acid, acetic acid, acetol and acetaldehyde; (2) furan or pyran ring derivatives, such as 2-methy furan, 2-fural-dehyd and dianhydro xylopyranose; and (3) anhydro sugars, for instance, anhydroxylopyranose. Patwardhan et al. [77] obtained CO, CO₂, formic acid, acetol and furfural as primary products from the pyrolysis of extracted hemicellulose from switchgrass, and they proposed that there still existed competing reactions in the pyrolysis of hemicellulose, which is similar to the pyrolysis of cellulose. When it comes to the difference of the reaction mechanism between cellulose and hemicellulose [77], the dominant point was that a glucosyl cation generated from the cleavage of a glycosidic bond between pyranose rings of cellulose could be stabilized by the formation of 1,6-anhydride, while the xylosyl cation from hemicellulose could not produce a stable form of anhydride. The proposed mechanism was similar to the one from Ponder et al. [69]. Recently, Zhou et al. [78] established a mechanistic model of hemicellulose pyrolysis on the basis of reaction family approach. The mechanism model gave details about the depolymerization of hemicellulose chains, reactions of active intermediates, the generation of low-molecular compounds, as well as specified rate constants of all the related reaction. In addition, Yang et al. [79] proposed the possible pyrolysis mechanism of hemicellulose through the combination of TG-FTIR, Py-GC/MS and DFT calculation, and they concluded that the reaction network for hemicellulose pyrolysis contained primary glycosidic bond cleavage and the formation of volatiles via ring-opening, dehydration, retro-aldol, retro Diels−Alder and enol−keto tautomerization reactions. These proposed mechanisms for hemicellulose pyrolysis could provide us significant information and ideas to deal with the raw hemicellulose in lignocellulose biomass.

2.1.3. Pyrolysis of Lignin

H/C_eff is an important index to assess the relative hydrogen content of the resource [80], where a higher H/C_eff ratio means that less carbon and hydrogen are required for deoxygenation to obtain value-added biofuels [81]. The H/C_eff of lignin is much higher than that of cellulose and hemicellulose under the same conditions, so it is urgent to explore and obtain the science of lignin comprehensively.

Compared to cellulose and hemicellulose, lignin is a polymer with a more complex and disordered structure. Furthermore, the decomposition of lignin occurred over a wide temperature range since various linkage in lignin structure showed different thermal stability. Generally speaking, the ether linkages (α-O-4, β-O-4) in lignin are easier to be broken down than C-C bonds (β-1, β-5, β-β’), where the former one tends to decompose at 200–250 °C [82], and the cleavage of the latter occurred at approximately 400 °C. In addition, the cleavage of aliphatic side chains would happen around 300 °C [83]. Therefore, lignin samples derived from different kinds of biomass might hold various content of ether and C-C linkage, which contributed to the difference of their thermal decomposition behaviors. The studies of Zhao et al. [84] and Jiang et al. [85] showed similar results: that hardwood lignin started to decompose at lower temperature, which might be owing to the higher concentration of β-O-4 and the lower concentration of C₅-C₅ than lignin samples from softwood. It was speculated that the high content of C-C in softwood lignin could contribute to the high-yield of char products owing to its inferior reactivity. In addition, the content of functional groups in side chain, such as hydroxyl and methoxyl groups could influence the distribution of pyrolytic products. Yuan et al. [86] proposed that the char yield was negatively correlated with the content of the methoxyl group through comparing the TG-DTG analysis of four lignin samples isolated from different biomass species. The methoxyl groups might depolymerize into small-molecular radicals, and then acted as stabilizer for the large-molecular weight radicals derived from lignin structure to inhibit the further polymerization reaction, thereby reducing the production of biochar [87]. On the other hand, the cleavage of methoxyl in the syringyl unit could convert it to guaiacyl, even p-hydroxyphenyl, achieving the transformation of these primary products. The methoxyl groups could be released in the form of methane and methanol [84]. The aliphatic hydroxyl is another dominant functional group affecting the reactivity of lignin structure. According to Kawamoto et al. [88], the -OH at the position of C_γ or C_β provided hydrogen during lignin pyrolysis. Additionally, there contains other oxygenated functional groups in the lignin structure, including carbonyl and carboxyl groups, which might be derived into the lightweight products (CO, CO₂ and formaldehyde, etc.) [84,88].

Overall, the pyrolysis process of lignin undergoes three different stages [89,90]: the early stage decomposition below 375 °C, where the dehydration reaction and the cleavage of low-stability linkages occurs; the extensive decomposition stage between 375 and 450 °C, where the bulk degradation happens; and the charring stage, where the polymerization reaction starts dominantly. The dominant pyrolytic products from lignin are phenolic compounds and its derivatives, as well as other low-molecular weight chemicals, such as acetic acid, methanol, hydrocarbons and some non-condensable gas (CO, CO₂, CH₄, etc.). For the generation of these products, some possible pathways have been proposed [91,92], and it is widely accepted that radical reaction is widely regarded as one of the most dominant mechanisms of lignin pyrolysis [82]. During the pyrolysis process, the cleavage of the β-O-4 bonds in the lignin structure forms free radicals, namely the initial step of free radical chain reaction [82]. Free radicals can capture protons from other species (C₆H₅-OH) with weak C-H or O-H bonds, and then form decomposed products, such as vanillin. Thereafter, free radicals can be passed to other species for chain propagation, and the chain reaction terminates once the two radicals collide to form a stable compound.

2.2. The Reactor Design and Process Control

2.2.1. Diversity of Reactor

As shown in Figure 3, miscellaneous reactors have been used and explored for catalytic pyrolysis in a small scale, such as fluidized bed, circulating fluidized bed, fixed bed, ablative, rotative, auger, vacuum and microwave reactors [10,83,93,94,95,96], and the details of these reactors were reviewed by Vamvuka [97] and Vuppaladadiyam et al. [31] in terms of the scale, the yield of bio-oil, the operation complexity, the feed size, the inert gas needed and the ease of scaling up. More information is provided in

Table 3. Advantages and disadvantages of different pyrolysis reactors.

	Advantages	Disadvantages
Micro py-GC/MS	beneficial for pyrolysis mechanism investigation	difficulties in the calculation of macrolevel indexes of the process performance
Fluidized bed	ease in design and operation, high heat transfer rates, scale-up possibility and excellent temperature control	small biomass particles needed
Rotating cone	ease in solid mixing and up-scaling, no carrier gas needed, small investment cost	large amount of energy needed for bed heat transfer small biomass particles needed
Auger reactors	ease in design and operation, effective temperature control, efficient heat transfer and well mixing of biomass and catalyst	comparatively higher residence and poor mixing at the radical direction in large scale applications
Fixed bed	easy design and operation	poor heat transfer, difficulties in a continuous operation and char removal

.

Micro-pyroprobe reactors are widely used in a lab scale for catalytic pyrolysis of biomass [98], but some drawbacks impede commercialization, such as inefficient biomass/catalyst mixing, low loading and small particle size of biomass needed. However, it is inevitable to clarify the component of pyrolytic volatiles in order to optimize the quality and the yield of bio-oil. The gas chromatography/mass spectrometry is attached to the micro-pyroprobe to form a promising technique, namely py-GC/MS. During recent decades, py-GC/MS has been applied in catalytic pyrolysis of various biomass: the likes of softwood sawdust [99], pinewood [100,101] and poplar sawdust [102].

Fluidized bed reactors are massively utilized in industrial processes in that this mode shows many merits, such as scalability, excellent properties of mass and heat transfer, even uniform catalyst distribution and the ability to operate continuously [103,104]. However, the major disadvantages of fluidized bed reactors are the demands for small biomass particle size, high flow rate of gas and the complexity of heat addition to the bed, which is achieved through either a circulating heat carrier or an indirect transfer approach [93]. To optimize the pyrolysis process, different types of fluidized bed reactors are designed, including bubbling fluidized beds [104], spouting fluidized beds [105] and circulating fluidized beds [106].

The rotating cone reactor is another demo-scale device for biomass pyrolysis, and a significant trait of the rotating cone reactor is that the bio-oil yield is relatively high. The small gas volume reduces the volatiles residence time (VRT), and then the secondary cracking reactions of volatiles are reduced [93]. However, the rotating cone reactor needs a very small feed size, which makes scaling up hard [104]. The basic operation mode of the rotating cone reactor refers to the fact that biomass feedstock and heat carrier (for example, sand) are added into the bottom of the rotating cone reactor, where they are mixed evenly and conveyed upwards by centrifugal forces performed by the cone [107]. A rotating cone reactor was modified and developed by Hu et al. [108] for coal pyrolysis, where the weight loss of coal was found to be correlated with the cone rotational frequency.

Auger reactors are appealing for versatility of biomass conversion and have been recognized as one of the promising techniques with better strengths, not only for fast pyrolysis, but also for slow or intermediate pyrolysis [109]. The capital investment is relatively low owing to its simple design, and the questions of mass and heat transfer can be overcome. Furthermore, it is the key factor that the mechanical forces from auger reactors strengthen the process of both biomass mixing and heat transfer. Yildiz et al. [110] built a mini plant based on auger reactor technology, which was fully controlled and continuously operated. This set-up allows variation of the catalyst loading and reaction times while yielding suitable samples for full characterization in continuous operation.

2.2.2. The Process Parameters

Pyrolysis temperature significantly influences the distribution of pyrolysis products, which is a key factor in providing the energy needed for bonds cleavage. The thermogravimetry–differential thermal analyses (TG-DTG) is an important and widely used approach for the clarification of lignocellulosic biomass in different pyrolysis stages, as well as activation energy. The final pyrolysis temperature for various biomass could be decided based on the TG-DTG curves, avoiding wasting energy. The rise of pyrolysis temperature could contribute to the increase in bio-oil yield, while this rule could be adverse at extremely high temperatures. This phenomenon is owing to the fact that higher temperature can trigger the secondary cracking reaction of pyrolytic vapor, thereby yielding more gaseous products in the sacrifice of bio-oil [111]. Jung et al. [112] confirmed this principle in the pyrolysis of rice straw and bamboo. They observed that the bio-oil yield changed from 56 to 72, and then 61 wt%, while the corresponding pyrolysis temperature is 350, 405 and 510 °C, respectively. Similar results could also be obtained from the pyrolysis process of wheat straw, flax straw and sawdust [113]. Typically, the yield of bio-oil peaks over the temperature range of 400 to 550 °C, while it differs for different biomass species. In the reviews of Hoang et al. [9], the optimal pyrolysis temperature to obtain high yield of bio-oil for different biomass species were summarized. For example, 400 °C for neem de-oiled cake (40.2 wt%) and Cynara cardunculus L. (56.23 wt%), 450 °C for rice husk (70 wt%), 455 °C for poplar wood (69 wt%), 500 °C for residues from palm tree (72.4 wt%), 550 °C for pistachio shell (20.5 wt%), and 600 °C for olive bagasse (72.4 wt%). Furthermore, the content of polar aliphatic and aromatic compounds in bio-oil increased when the temperature increased from 300 °C to 500 °C. More compounds containing carbon in bio-oil might be converted to polycyclic aromatic hydrocarbons (PAHs), such as pyrene and phenanthrene, while the temperature is above 700 °C since decarboxylation and dehydration reactions are easier to occur at high temperature [114]. Thereby, the pyrolysis temperature highly affects the pyrolysis behaviors of biomass.

Generally speaking, heating rate is another important process parameter that has a remarkable effect on the composition and properties of pyrolytic products. A higher heating rate contributes to the thermal decomposition of lignocellulose biomass [9], which is due to the reason that the limitation of mass and heat transfer could be alleviated [19,114]. Thereby, the secondary reaction could be inhibited [111], yielding more bio-oil products and less biochar. Onay et al. [115] ascertained that the optimal heating rate for rape seed in a well-swept fixed-bed reactor is 300 °C/min. In the case of 300 °C/min, the highest yield of bio-oil is about 68 wt%, which was higher than that of 100 °C/min. Salehi et al. [116] reported that the bio-oil yield increased by 8 wt% as the heating rate increased from 500 °C/min⁻¹ to 700 °C/min⁻¹. Ozbay et al. [117] displayed a similar result in their study where a rapid increase in liquid yield was observed (from 26 to 35 wt%) when the heating rate was increased from 5 °C/min⁻¹ to 300 °C/min⁻¹. However, when the heating rate was further increased from 300 °C/min⁻¹ to 700 °C/min⁻¹, the bio-oil yield did not change apparently. Li et al. [118] studied the effect of heating rate on both the yields and distribution of the liquid phase products obtained from the slow pyrolysis of pubescens, and they found that the increment of the heating rate promoted the cleavage of C₁-C_α bonds, C_β-C_γ bonds in lignin, and the dehydration of C_α-OH units of lignin into C_α-C_β. Debdoubi et al. [119] observed that a high heating rate could improve the quality of obtained bio-oil from esparto, showing a decrease in the oxygen and water content of bio-oil. This was mainly due to the reason that a faster heating rate may restrict the secondary polymerization and dehydration reaction.

VRT also has a significant influence on the proportion and properties of pyrolytic products. In general, the residence time of pyrolytic vapor mainly depends on the rate of carrier-gas flow. It is well-accepted that a short VRT is beneficial for the formation of bio-oil, since the organic vapor can be quickly swept out of the reactor, and then the secondary cracking reactions can be minimized [18,83,120]. With a longer VRT, more active volatiles can be repolymerized on the surface of coke in the sacrifice of bio-oil products [121,122]. Scott et al. [121] observed that the bio-oil yield reduced from 75 to 57 wt% when the VRT was prolonged from 0.2 to 0.9 s. Qureshi et al. [123] increased the VRT of tenera palm shell pyrolysis from 0.25 s to 20 s, and then the bio-oil yield decreased from 73.86 to 57.06 wt%. In addition, the density and viscosity of bio-oil samples also decreased. Islam et al. [124] observed the decrease in liquid and coke products, and an increase in gaseous products, when the steam residence time increased from 5 s to 20 s, which may be caused by the enhancement of the secondary cracking reaction.

Overall, pyrolysis of biomass is an endothermic process where a large amount of heat is necessary to maintain the reaction condition during the whole period. Heat transfer from the pyrolysis reactor to the biomass feedstock is a key factor affecting biomass pyrolysis reactions and product distribution, which is over the control of the above operation parameters. Based on the three main factors, the thermal process can be classified to three types, slow pyrolysis, fast pyrolysis and flash pyrolysis. The main characteristics of the three types of pyrolysis are shown in

Table 4. The typical type of pyrolysis technology. Reprinted with permission from ref. [125]. Copyright Year Copyright Liu et al.

Type of Pyrolysis	Operating Conditions			The Main Pyrolytic Products
	Residence Time	Temperature/°C	Heating Rate	Coke/%	Bio-Oil/%	Gas/%
Slow pyrolysis	5–30 min	400–600	<50 °C/min	<35	<30	<40
Fast pyrolysis	<5 s	400–600	~1000 °C/s	<25	<75	<20
Flash pyrolysis	<0.1 s	650–900	~1000 °C/s	<20	<20	<70

. Taking these pyrolysis parameters into consideration, low temperature, low heating rate and long VRT are conducive to the formation of biochar; high temperature, high heating rate and long VRT are beneficial for the generation of gas products; and medium temperature, high heating rate and short VRT favor the production of bio-oil with a higher yield. Regulating the corresponding parameters in pyrolysis process in order to achieve high conversion of biomass as possible and yield high-grade bio-fuel is the heart of the matter in the field of bio-oil production technology.

2.2.3. The Installation of Catalysts

In terms of the location where the catalyst stands in the pyrolysis reactor, the mode of catalytic pyrolysis can be sorted into two categories, namely ex-situ and in-situ catalytic pyrolysis, as presented in Figure 4 [20]. Both reaction modes of biomass pyrolysis have been widely studied during the last decades.

The in-situ catalytic pyrolysis refers to the mode where catalyst and biomass are mixed evenly, and then the mixture is introduced in a reactor so that the generated pyrolytic vapor of biomass diffuses directly and quickly into the catalyst pores for catalytic reactions. Noticeably, the requirement of capital investment is relatively low owing to just one single reactor needed. Furthermore, it is easier and quicker to perform the catalytic reaction and vapor upgrading. However, the mode of in-situ catalytic pyrolysis has some unsatisfactory points [19]. The first one is that the yield of undesirable polyaromatics is higher than expected. The second one refers to the fact that catalyst deactivation happened rapidly due to the coke formation, deposition and block of pores of the catalyst. Obviously, it is a tough task to guarantee that catalysts and biomass powder could be mixed evenly.

The ex-situ process means that catalyst and feedstock are placed separately at different positions in one reactor, or directly in two different reactors, which means that pyrolytic volatiles are generated in the first reactor and then transferred to a secondary reactor for further catalytic reactions. The position of catalyst in the pyrolytic reactor does influence the distribution of pyrolytic production. Liu et al. [126] adjusted the relative position of HY zeolite catalyst and pubescens biomass and found that the yield of acetic acid and biochar increased while the yield of bio-oil and organics (such as aldehyde, phenols, etc.) decreased with the increment of distance between HY zeolite catalyst and pubescens biomass. According to Qiu et al. [19], the ex-situ mode of catalytic pyrolysis is beneficial to the generation of olefin products, namely it can be depicted as the highly-selective process. Furthermore, individual control of operating conditions and catalytic reactions in the initial and secondary reactors are allowed. Nevertheless, some drawbacks still restrict the wide utilization of ex-situ catalysis mode. Compared to in-situ catalytic pyrolysis, the configuration complexity of ex-situ mode contributes to an increase in the whole project investment. In addition, the hot-gas filter installed between the first and secondary reaction zone to mitigate metal accumulation causes carbon loss.

Overall, both catalytic pyrolysis modes have advantages and disadvantages. Compared with the ex-situ, the cost of in-situ mode is comparatively low, and the catalysts loaded not only participate in the catalytic process but also act as heat carriers. In addition, it is much more direct and quick for pyrolytic volatiles to undergo the further reaction. However, the coke formation and difficulties in catalyst recovery make the in-situ mode less competitive. Ex-situ is beneficial to control the secondary upgrading process flexibly, including catalyst species, reaction temperature, reaction gas atmosphere and so on. Before the catalytic pyrolysis technique is determined or improved, all aspects, for instance, the cost of device and catalysts, are needed to be taken into consideration.

2.3. The Types of Catalysts

Catalysts act as a key in biomass pyrolysis [127], such as decreasing the pyrolytic temperature, increasing the yield of bio-oil and changing the product proportion. For example, components with oxygen functionalities in bio-oil could be reduced by dehydration, decarbonylation and decarboxylation reactions according to Yildiz et al. [72]. Many researchers now focus on increasing the content of hydrocarbons, thereby improving the quality of bio-oil, for example the low heating value. As listed in

Table 5. The zeolites used in biomass catalytic pyrolysis.

	Feedstocks	Reactor	Catalyst	Pyrolysis Mode	Ref.
Zeolites	Pinewood sawdust	Py-GC/MS	ZSM-5	in-situ	[100]
	Pinewood, rice straw and wheat straw	Conical spouted bed reactor	ZSM-5	in-situ	[103]
	Cellulose	Bubbling fluidized bed reactor	ZSM-5	in-situ	[104]
	Sugarcane bagasse and pinewood	Circulating fluidized bed	ZSM-5	in-situ	[42]
	Maize straw	Fixed-bed reactor	ZSM-11	in-situ	[28]
	Beechwood	Bench-scale fixed-bed tubular reactor	Co/ZSM-5	ex-situ	[106]
	Rice husk, sawdust, sugarcane bagasse, cellulose, hemicellulose and lignin	Flowing fixed bed	La/HZSM-5	in-situ	[43]
	Douglas fir sawdust	A packed-bed catalysis closely coupled with microwave pyrolysis	Zn/ZSM-5	in-situ	[131]
	Pubescens	Fixed-bed reactor	HY	ex-situ	[126]
	Pine sawdust	Micro-pyrolyzer	Mo/HZSM-5, Ga/HZSM-5, W/HZSM-5	ex-situ	[27]
	Hemicellulose and plastic	Py-GC/MS	HZSM-5	ex-situ	[73]
	Pinyon−juniper	Fluidized bed reactor	HZSM-5	in-situ	[132]
	Glucose	Py-GC/MS	HZSM-5	in-situ	[133]
	Banana peel	Fixed-bed reactor	Al/SBA-15	in-situ	[46]
	Glucose	Py-GC/MS	Small pore ZK-5, SAPO-34, medium pore Ferrierite, ZSM-23, MCM-22, SSZ-20, ZSM-11, ZSM-5, IM-5, TNU-9, and large pore SSZ-55, Beta zeolite, Y zeolite	in-situ	[134]
	Poplar sawdust	Py-GC/MS	Fe-modified hierarchical ZSM-5	in-situ	[102]
	Glucose, xylitol, cellobiose and cellulose	Model 2000 pyroprobe analytical pyrolizer	ZSM-5, silicalite, beta, Y-zeolite and silica–alumina	in-situ	[135]
	Lignin	Fixed-bed reactor	Fe/ZSM-5	ex-situ	[136]
	Lignin	A quartz fixed-bed reactor	H-Ferrierite, H-Mordenite, H-ZSM-5, H-Beta and H-USY	in-situ	[137]
	Poplar	Py-GC/MS	HZSM-5 (SAR 23, 30, 55, 80 and 280), SO42− ZrO2/MCM-41	in-situ	[138]
	Yellow pine	Py-GC/MS	β-zeolite	in-situ	[139]
Metal oxides	Waste navel orange peels	Fixed-bed reactor	Cu2O, CaO, V2O5, Fe2O3, and ZnO	in-situ	[22]
	Poplar wood	Fixed-bed reactor	CoO, Cr2O3, CuO, Fe2O3, Mn2O3, NiO, TiO2, V2O5, and CeO2	ex-situ	[23]
	Hemicellulose and plastic	Py-GC/MS	CaO	ex-situ	[73]
	Pinewood sawdust	Py-GC/MS	CaO	in-situ	[100]
	Pine sawdust	Py-GC/MS	Pt-Ni/γ-Al2O3		[101]
	Chili stem	Fixed-bed reactor	Ni–Ca/SiO2	in-situ	[45]
	Cellulose	Py-GC/MS	WO3/γ-Al2O3	ex-situ	[21]
	Wood fibers	Fixed-bed reactor	Na2CO3/γ-Al2O3	ex-situ	[12]
Soluble inorganic salts	Pine wood	TGA	CH3COOK	in-situ	[140]
	Cellulose	Py-GC/MS	NaCl, KCl, MgCl2, CaCl2, Ca(OH)2, Ca(NO3)2, CaCO3 and CaHPO4	in-situ	[141]
	Pubescens	Fixed-bed reactor	MgCl2	in-situ	[25]
	Lignin	Fixed-bed reactor	Ni(NO3)2	in-situ	[24]
	Avicennia marina biomass	Fixed-bed horizontal furnace	FeCl3 and Fe(NO3)3	in-situ	[142]
	Cellulose	A horizontal oven with infrared heating system and dynamic cooling system	CuCl2 and AlCl3	in-situ	[143]
	Corncob, fir wood, bagasse and rice husk	Lab-scale fixed bed	ZnCl2	in-situ	[144]
	Kraft lignin	A horizontal furnace	FeSO4	in-situ	[145]
	Eucalyptus	fixed-bed reactor	Ce, Mn, Fe, Co, Ni, Cu and Zn (nitrates)	in-situ	[146]
	Cellulose	Fixed-bed reactor	NaCl, KCl, MgCl2 and CaCl2	in-situ	[147]
	Birch wood	Py-GC/MS	MgCl2, NiCl2	in-situ	[148]
	Sweet sorghum	Py-GC/MS	ZnCl2 and MgCl2	in-situ	[149]
Low-cost materials	Pine woodchips	Auger reactor	Sepiolite, bentonite, attapulgite and red mud	in-situ	[130]
	Kraft lignin	Modified fixed bed	Ilmenite (FeTiO3), bentonite (Al-Si-OH), activated carbon (AC) and red mud (RM),	in-situ	[150]
	Pinyon−juniper	Fluidized bed reactor	Red mud	in-situ	[132]
	Beechwood	Py-GC/MS	Red mud	in-situ	[151]
	Corn cob lignin	Vertical fixed-bed microreactor	Red mud	ex-situ	[152]
	Alkaline lignin	Down-flow fixed-bed quartz reactor	Acid-activated bentonite clay	in-situ	[153]
	Almond shell	Oil cracking	Bentonite clay	-	[154]
	Castor oil and jatropha oil	Oil cracking	Acidic bentonite clay	-	[155]
	Azolla.f wastes	Fixed-bed reactor	Mg-Ni-Mo/modified pyro-char	ex-situ	[49]
	Pyrolytic tar	A bench-scale combined fixed-bed reactor	Biochar of corn stalks, reed and Sargassum horneri	ex-situ	[156]
	Wheat straw	Two-stage fixed-bed reactor	Ni/char (char from wheat straw, rice husk and cotton stalk)	ex-situ	[157]
	Pyrolytic tar	A downdraft fixed-bed pyrolysis-reforming facility	Nano Ni/rice husk char	in-situ	[158]
	Douglas fir sawdust	Batch microwave oven	Corn stover biochar	in-situ	[159]
	Bio-syngas	Fixed-bed reactor	Fe0/biochar (from pine)	in-situ	[160]
	Toluene	Continuous flow packed bed reactor system	Fe/biochar (from pine)	in-situ	[161]

, various types of catalysts have been developed and applied to improve the bio-oil properties, hopefully resulting in liquid fuel, which is comparable to that derived from fossil fuels, such as zeolites [28], metal-doping zeolites [128], metal oxides [23] and the immobilized catalysts [129], as well as low-cost materials [130]. Insights into the novel catalysts and the corresponding chemical mechanism facilitate our comprehension for catalytic pyrolysis of biomass.

2.3.1. Zeolites

Zeolite is one of the most widely used heterogeneous catalysts, for example, for petroleum processing and refinement [162] and catalytic pyrolysis of lignocellulosic biomass [28]. Zeolites are crystalline aluminosilicate with 3-dimensional porous networks, and the corner-sharing [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedron form the main structure via an oxygen atom, where the molecular dimensions of these interconnected channels and cages could be up to 10 Å [163]. Some properties of zeolites, such as pore size and acidity, play a significant role in the catalytic performance. Therefore, comprehensive knowledge about zeolite and oriented regulation of zeolite’s properties is vital to improve the yield and quality of biofuel, which is comparable to refined petroleum fuel.

Since the catalytic reactions of pyrolytic volatiles happen at the active site of the internal channels or cages of the catalyst [94], it should be guaranteed that these pyrolytic volatiles could enter and sweep out from the internal structure of zeolite catalysts. So, the pore size of zeolite is a crucial factor for catalytic pyrolysis. To be more detailed, there exist multitudes of apertures, which are defined in sizes on zeolites. Only when pyrolytic vapor reactants, active intermediates and products, have a molecular size smaller than the pore or channels of zeolite, could they diffuse into or out of the internal space of the zeolite catalyst freely. However, the reactant species with larger sizes could not enter the internal channel to encounter with active sites to yield desirable products. The generated active intermediates and products that hold the larger size could block the zeolite pores, leading to coke formation and catalyst deactivation. According to the pore size, zeolites could be divided into three categories, including small pore zeolites with diameters of 0.30–0.45 nm (e.g., zeolite A), medium pore zeolites with diameters of 0.45–0.60 nm (e.g., ZSM-5), large pore zeolites with 0.6–0.8 nm in average diameters (e.g., zeolite X and Y), and extra-large pore zeolites with diameters of 0.7–1.0 nm (e.g., zeolite UTD-1). Jae et al. [134] synthesized and characterized a range of zeolites, including small pore ZK-5, SAPO-34, medium pore Ferrierite, ZSM-23, MCM-22, SSZ-20, ZSM-11, ZSM-5, IM-5, TNU-9, and large pore SSZ-55, Beta zeolite, Y-zeolite, and their catalytic performance for glucose conversion to aromatics in terms of pore size and shape was assessed. In this case, it was found that the aromatic yield was a function of pore size of the catalyst [164], while internal pore space and steric hindrance also played a significant role for upgrading bio-oil. In detail, the use of zeolites with small pores in the cellulose pyrolysis yielded less oxygenated aromatics, and aromatic yields were the highest in the medium pore zeolites with pore sizes in the range of 5.2–5.9 Å. Kurnia et al. [137] compared the catalytic performance of five kinds of high aluminum zeolites on lignin pyrolysis in ex-situ mode in terms of channel structure and pore sizes on products distribution, coke formation and deoxygenation. According to the literature, HZSM-5 zeolite with ten-membered rings of channel system and 5.8 Å of pore size generated the highest yield of aromatic hydrocarbons. H-Beta zeolite with twelve-membered rings of channel system and 6.5 Å of pore size achieved the highest selectivity towards monoaromatic hydrocarbons. Thus, studying the effect of pore size or shape selectivity of zeolites for biomass conversion is the key for high selectivity of aimed products.

The acidity of zeolites shows a strong effect on determining the yield of bio-oil products since multitudes of biomass catalytic conversion reaction occur at the active acidic sites by carbonium ion mechanism [135]. Normally, the zeolite acidity refers to the comprehensive information, including the type of acidic sites, the density of acidic sites (amount) and the acidity strength distribution. The acidity of zeolite is mainly derived from the Brønsted and Lewis acid sites where the former one is originated from Si (OH)Al hydroxyl groups of zeolites [165], and the latter one is due to the extra-framework aluminum, which is not tetrahedrally bound in the zeolite network [166]. Both Brønsted and Lewis acid sites are catalytically active. According to Tan et al. [167], Brønsted acid sites favor aromatics while Lewis acid sites produce more alkanes. For the Brønsted acid sites, they provide an acidic proton to those pyrolytic intermediate volatiles, which could arrive at the internal structure of zeolites, forming carbocation intermediates. These carbocation intermediates could undergo the β-scission process to form olefin species, which could generate aromatic hydrocarbons via a range of reactions, such as oligomerization, aromatization, hydrogenation and isomerization [133]. Desilication or dealumination is the common method to redistribute the content of Brønsted and Lewis acid sites; the catalytic performance and selectivity of zeolites could be changed consequently. In addition, the introduction of metal could change the ratio of Brønsted and Lewis acid sites, such as Zn [131]. Normally, the density or amount of acid sites in zeolite catalysts is negatively correlated with the SiO₂/Al₂O₃ or Si/Al ratio (SAR) [168]. The lower the ratio is, the better the catalytic ability is to perform. Mukarakate et al. [139] used several β-zeolites with Si/Al range from 21 to 250 for catalytic pyrolysis of yellow pine, where the obtained bio-oil from the β-zeolite with low SAR consisted of predominantly aromatic hydrocarbons and olefins, hardly oxygenated compounds. Kelkar et al. [138] studied the catalytic performance of HZSM-5 with various Si/Al ratios (23, 30, 55, 80 and 280) on the pyrolysis conversion of poplar and found that the HZSM-5 with SAR of 23 and 30 obtained the highest aromatic yields. In the study of kraft lignin pyrolysis by Li et al. [169], the one with SAR of 25 showed higher selectivity to aromatic hydrocarbons and lower reactivity towards phenols and oxygenated compounds in comparison with HZSM-5 (SAR 200). These results both demonstrated that the higher density of zeolite acidic sites (low SAR) was beneficial to removal of methoxyl groups, dehydration of the aliphatic hydroxyl groups and cleavage of the C–C bonds for aliphatic and ether during the pyrolysis of lignocellulosic biomass. However, the coke formation was heavy with a lower SAR [139]. Except the density or number of acidic sites, the acidity strength also heavily influences the products distribution from biomass catalytic pyrolysis. Acidity strength is on behalf of the binding energy of a basic molecular probe with acidic sites [2]. Stephanidis et al. [170] synthesized Al-MCM-41 mesoporous material, which holds the low ratio of Si/Al (30) but weak acidity strength, and found that Al-MCM-41 mesoporous catalyst performed worse in the selective formation of aromatics in the pyrolysis of beechwood. Overall, tuning the acidity of zeolites could favor the oriented products from biomass pyrolysis and be significant in regulating the product distribution.

On the other hand, the doping of metal into zeolite structure is also a promising approach to modify the acidity of zeolites, thereby improving the catalytic performance of zeolites and modulating the pyrolytic products. To some extent, the modification of Fe on zeolite could increase the total acidity, according to Yang et al. [136], and the modified ZSM-5 with 3% of Fe showed the best selectivity towards light olefins and aromatic compounds. Dai et al. [171] discovered that the doping of nickel improved the framework of ZSM-5, thereby contributing to the generation of aromatic hydrocarbons in the pyrolysis of corncob. NaY zeolites could improve the production of bio-oil in the pyrolysis of four kinds of bamboo biomass [172]. Similarly, the Y zeolites (USY) modified with Mg enhanced the yield of the liquid phase in the pyrolysis of bamboo [173]. Zheng et al. [174] modified ZSM-5 catalysts with a range of metals including Zn, Ga, Ni, Co, Mg and Cu, and then the catalytic properties of these modified zeolites in pyrolysis of pine were assessed. In this study, Ga-ZSM-5 yielded the lowest biochar; the monoaromatic hydrocarbon from Zn- ZSM-5 was the highest; the content of polyaromatics from Ni-ZSM-5 catalyst was the highest, and the Co-based catalyst had the highest selectivity for ninhydrin. Some research studies focused on two different metals, which were expected to exert synergistic effects in the catalytic pyrolysis. For instance, the co-doping of Ni and Ce on HZSM-5 zeolite remarkably enhanced deoxygenation reaction in the pyrolysis of algae, where the obtained bio-oil from pyrolysis over HZSM-5, Ni/HZSM-5 and NiCe/HZSM-5 contained 15.52%, 12.54% and 8.77% of oxygen, respectively [175]. Shahsavari and Sadrameli [176] found that the pyrolysis of beechwood could selectively lead to aromatic or heterocyclic products by changing the content of Sn and Re on ZSM-5. A similar idea was performed in the work of Hao et al. [177], where the catalytic performance of Al₂O₃ catalyst was improved by the doping of both nickel and molybdenum for some typical biomass, such as pine, Alaskan spruce, tropical lauan and rice husks, for the production of hydrogen-rich gas where the yield of H₂ and CO was 33.6 and 326.3 g/kg biomass (def), respectively, with NiMo/Al₂O₃ catalyst at 723 K from pine, and the H₂ yield of woody species was higher than that of rice husks. To sum up, metal-doping zeolites act as bifunctional catalysts in biomass pyrolysis. The catalytic reactivity results from many parameters, such as zeolite porosity and acidity, the type, loading content and dispersion condition of the selected metal species. However, the interplays between zeolite supports and metal species are complex, and there needs to be a balance between catalytic performance and cost.

2.3.2. Metal Oxide Catalysts

Metal oxides have been widely used as heterogenous catalysts for biomass pyrolysis. According to Liu et al. [125], metal oxides may be preferable to generate stable products during biomass pyrolysis, which is owing to the fact that metal oxides are equipped with multivalent, acid-base and redox properties. Zhang et al. [22] found that the yield of 3-furaldehyde compounds from the waste navel orange peels pyrolysis could be enhanced almost 5.69 and 4.82 times with the addition of Cu₂O and Fe₂O₃ as catalyst, respectively. Cu₂O catalyst promoted the generation of bio-oil, while the addition of Fe₂O₃, V₂O₅, CaO and ZnO decreased the yield of bio-oil. In addition, CeO₂ was found to reduce the oxygen-contained compounds from 29.9% to 6.87% [175]. Zhang et al. [23] used eight types of transition metal oxides in the pyrolysis of poplar wood. Amongst them, TiO₂ and NiO could inhibit the further decomposition of the primary products, yielding more bio-oil products and less gaseous products; however, the formation of coke was heavy over these catalysts, including V₂O₅, Mn₂O₃, CuO and CoO; when it comes to the particular compounds in bio-oil product, the production of hydroxyl acetone was enhanced by all these metal oxides while the generation of hydroxyl aldehyde was inhibited; the addition of Fe₂O₃ led to a decrease in formation of the cellulose- and hemicellulose-based derivatives, except hydroxyl acetone; furthermore, CaO and La₂O₃ were found to hold the ability of chlorine removal during the catalytic pyrolysis of sewage sludge. Chen et al. [178] put up with the idea that less dehydration reaction and more decarboxylation pathway may be the best method to balance the quality and yield of bio-oil derived from biomass pyrolysis, and they discovered that CaO represents the outstanding catalytic performance on the balance between the quality and yield of the obtained bio-oil after comprehensive consideration of the composition, pH and water content of biofuels. Furthermore, metal doping on metal oxides have been widely applied for the upgrading and refinement of bio-based platform chemicals. A 5 wt% Cu/SiO₂ catalyst was tested during the conversion of γ-valerolactone (GVL) to 2-Methyltetrahydrofuran (MTHF), and a 97.2% MTHF selectivity with 71.9% GVL conversion were obtained [179].Noble metal is another choice for doping on metal oxides. Pothu et al. [180] prepared Pt/STA-ZrO₂ catalyst, and the catalyst contributed to the conversion of levulinic acid (89% of conversion) to ethyl levulinate (93% of selectively).

2.3.3. Soluble Inorganic Salts

Many studies have been exerted to explore the effect of different metal elements on pyrolysis behaviors of biomass. It can be divided into three categories: alkaline metals (K and Na), alkaline earth metals (Ca and Mg) and transition metals (Ni, Cu, Zn and so forth) in terms of metal ion, while sulfate, carbonate, chloride and nitrate were used. According to Shimada et al. [147], alkaline earth metal (MgCl₂ and CaCl₂) strongly decreased the weight loss temperature of cellulose while alkali metal (NaCl and KCl) did not show much influence. Eibner et al. [146] compared the catalytic effect of seven metal nitrates on the pyrolysis of eucalyptus, namely Ce, Mn, Fe, Co, Ni, Cu and Zn. The outstanding performance on LAC (1-hydroxy-(1R)-3,6-dioxabicyclo [3.2.1] octan-2-one) production was observed in the following order: Zn > Co > Ni, Mn > Ce > Fe. In addition, Ni(NO₃)₂ was found to inhibit the thermal conversion of organosolv lignin, yielding more biochar and less bio-oil products [24]. Wang et al. [181] used four kinds of nickel salts to improve the foaming and swelling behavior during the pyrolysis of alkali lignin, and the ability was shown in the following sequence: Ni(NO₃)₂ > (CH₃COO)₂Ni > NiCl₂ > NiSO₄. These data indicated that the cations and anions are vital for the catalytic performance. According to the study of Patwardhan et al. [141], in which the pyrolysis of cellulose was catalyzed by various inorganic salts (NaCl, KCl, MgCl₂, CaCl₂, Ca(OH)₂, Ca(NO₃)₂, CaCO₃ and CaHPO₄), the yields of low-molecular weight products, especially formic acid, glycolaldehyde and acetol, were favored while that of levoglucosan decreased. These results might be owing to the fact that the secondary cracking reaction was enhanced with the addition of these inorganic salts. The impact from these salts on the yield of levoglucosan from cellulose pyrolysis had been sorted in the following sequence: (a) cations-K⁺ > Na⁺ > Ca²⁺ > Mg²⁺; (b) anions-Cl⁻ > NO₃⁻~OH⁻ > CO₃²⁻~PO₄³⁻. These results gave an approach or thought to figure out the anions’ catalytic activity, too. Furthermore, the presence of inorganic salt could enhance the carbonization reaction in biomass pyrolysis, thereby yielding more char products, and this rule is suitable to CH₃COOK [140], MgCl₂ [25], ZnCl₂ [149] on different kinds of biomass species. The effect of impregnation of MgCl₂ on the pyrolysis of bamboo was investigated on a fixed bed reactor [25], and the temperature of thermal degradation could be decreased over 100 K after MgCl₂ impregnation. Furthermore, the yield of furfural showed a growth resulting from the catalytic effect on hemicellulose. ZnCl₂ was verified to have the same ability to enhance the production of furfural by Lu et al. [144]. According to Rutkowski [143], the addition of CuCl₂ and AlCl₃ during the pyrolysis of cellulose enhanced the generation of gaseous products in the sacrifice of bio-oil. At the same time, CuCl₂ is usually a more efficient catalyst in the levoglucosenone and 1,4:3,6-Dianhydro-α-d-glucopyranose formation than AlCl₃. The impregnation of ferric salts increased the fixed carbon content in solid product and the proportion of hydrocarbon fractions in liquid product and changed the evolution pathway of CO and H₂ in gaseous product [142].

2.3.4. Other Low-Cost Materials

Other low-cost materials as shown in Figure 5, including red mud, ilmenite, several clay minerals (sepiolite, bentonite and attapulgite) and carbon materials (activated carbon and biochar), were used as catalysts in biomass pyrolysis process [20,130,150].

Red mud is an alkaline by-product with alkaline characteristic (pH > 12) generated by the alumina industry, which has been widely used as infrastructure material, environmental remediation material, adsorbent for waste water and catalyst, etc. [182]. Red mud consists of various metal oxides, namely Fe₂O₃, Al₂O₃, SiO₂, CaO₂, TiO₂, NaO, and MgO, etc. Amongst these oxides, Fe₂O₃ accounts for over a half [132], and that is the reason why the red mud is red. Additionally, the high content of ferric leads to the catalytic performance of red mud. It was reported by Gupta et al. [151] that red mud inhibited the generation of phenolic compounds and promoted the formation of cellulose- and hemicellulose-derived furfurals and hemicellulose-derived acetic acid in the co-pyrolysis of beechwood and red mud. Generally, many pretreatment methods could be exerted to improve the catalytic ability, such as calcination, alkali activation and acid treatment [152]. Through these modification approaches, the catalyst possessed a high specific surface area, a porous structure, acidic sites and active metal oxides [183,184]. Wang et al. [152] modified red mud through a feasible digestion–precipitation method to obtain a hierarchical porous structure, and the modified red mud was applied to catalyze the pyrolysis of lignin, yielding more alkylphenols and hydrocarbons. In the catalytic pyrolysis of pine [130], red mud exhibited several remarkable properties, such as reducing the viscosity and density of the organic phase and enhancing the stability of bio-oil. Meantime, red mud could enhance the catalytic cracking of large-molecular compounds in the bio-oil, and this was mainly due to the fact that red mud consisted of some metal species, such as Fe, Ni and Mg [148].

Bentonite clay, namely a natural lamellar aluminosilicate mesoporous material, is mainly made up of montmorillonite mineral, and has a crystalline structure where an alumina octahedral sheet was caught between two sheets of tetrahedral silica [20]. In addition, the layered structure formed by the montmorillonite cells holds some cations, such as Cu²⁺, Mg²⁺, Na⁺, K⁺, etc. However, the bonding between these cations and cells is pretty unstable so that these cations are easy to be replaced, leading to an outstanding property of ion exchange. Acid treatment of bentonite clay could contribute to the replacement of the exchangeable cations with H⁺ ions, resulting in a great deal of Brønsted acid sites in the bentonite clay [153]. Meanwhile, acid treatment also leads to dealumination of the structure and improves the specific surface area and adsorption ability [185]. After being activated by HCl, the bentonite clay could enhance the selectivity for o- and p-xylenes, naphthalene and methyl naphthalene in lignin pyrolysis in the temperature range of 550–650 °C [153]. In addition, the modified bentonite clay could be applied to biofuels upgrading. After calcination, the bentonite clay decreased the viscosity of pyrolytic oil from almond shell [154]. According to Rabie et al. [155], HCl-modified bentonite clay showed an excellent catalytic cracking performance in converting castor and jatropha oils into biofuel which satisfied the standard specifications.

Another type of low-cost material is biochar, which is generated through the thermochemical conversion of biomass in an anoxic atmosphere [186]. In addition to the carbon element, biochar also holds many other element species that affect and determine the corresponding property and function of this material, such as H, O, S and many trace metals [187]. Biochar from biomass pyrolysis or hydrothermal carbonization has a particular structure with rich functional groups and a high surface area [188]. Furthermore, biochar acts as a bank of electron receivers and donors, and it holds the capacity of cation exchange, as well as the buffering ability between acid and base [189]. These characteristics give biochar a high reactivity, which is determined mainly by the biomass feedstock and preparation methods. Biochar is widely applied to many areas, for example, water and soil management, composting, electrochemical energy storage, as well as catalysis [187,188], owing to its special physical and chemical properties. The application of biochar as a catalyst has been explored by many scholars. Han et al. [150] found that active carbon could favor the generation of monophenols. Li et al. [156] assessed the tar removal ability of three types of biochar, where the tar yield reduced by 94.6%, and the contents of H₂ and CO in gaseous products were significantly enhanced. Similar result in tar removal ability of biochar was further verified in the research of Luo et al. [190]. Corn stover biochar was prepared through microwave pyrolysis by Ren et al. [159] and then used as the catalyst to the catalytic pyrolysis of Douglas fir sawdust. The result showed that the biogas generation was enhanced, and the quality of bio-oil was improved. Apart from directly acting as catalyst, the biochar could be modified by doping metals, such as Fe [145,160,161] and Ni [157,158,191,192], to improve the catalytic performance.

Overall, the variations in these mentioned factors all lead to different pyrolysis behaviors of biomass and product distribution, as shown in Figure 6. Understanding comprehensively the pyrolysis mechanism of lignocellulosic biomass and developing efficient technology, including pyrolysis characteristic of various biomass species, impact of different catalysts and the process parameters and reactor science is beneficial to its integrated utilization. According to these factors, the following viewpoints have been proposed for further development of catalytic pyrolysis in the future.

3. Challenges and Future Perspectives

Catalytic pyrolysis of lignocellulosic biomass is a promising technology for the generation of biofuels, and it is still in the initial stage, which is far from commercialization. Although huge progress in catalyst modification and catalytic pyrolysis has been made, the yield and quality of fuel-grade bio-oil still faces significant challenges. The following should be investigated, in order of priority.

(1) Since lignocellulosic biomass varies in composition and structure in terms of species, it is necessary to develop analytical methods and technology to characterize raw biomass or its single component at the molecular level. Then, these biomass species can be classified into particular categories, which is beneficial to design oriented catalysts, thereby obtaining the high conversion rate of biomass and high-quality biofuels. For example, for the efficient conversion of woody biomass species, one catalytic system might be designed and developed, while for algae, it is needed to develop another one. Furthermore, it should be estimated whether the raw biomass or the isolated component from biomass is more conductive to catalytic pyrolysis for desirable products, achieving the effective conversion and utilization of biomass to the greatest extent.

(2) How to fully exert the role of solid catalysts to solid biomass is another tough task. When the solid catalyst is mixed with biomass evenly, the pyrolytic volatiles can arrive at the active sites on catalyst quickly for the following secondary reactions towards aromatic hydrocarbons or deoxygenated compounds. However, the solid catalysts only have an impact on the secondary reactions, while the initial thermal decomposition stage of biomass is not affected. The introduction of soluble salts might be a promising approach, which can be mixed evenly with biomass and attached to the biomass surface, thereby changing the intrinsic structure and boosting the thermal conversion rate of biomass. Nevertheless, soluble salts as catalysts are accompanied with the introduction of anions, such as Cl⁻, SO₄²⁻, NO₃⁻, CO₃²⁻ and other organic acid ions. These anions pose a threat on the safety and stability of bio-products. For example, it can be difficult to remove the S_xO_y, N_xO_y or other undesirable gas products from pyrolytic gas, and some toxic chemicals might be generated during the pyrolysis of biomass, such as chlorinated hydrocarbons and dioxins.

(3) The accurate analytical technology for the reaction intermediates in pyrolysis needs to be urgently developed. These intermediates could not be identified clearly and the information about the real catalytic pyrolysis process is still scarce. Therefore, the development of analytical technology that holds the ability to qualify and quantify the large-molecular weight fragments in-situ is quite promising for the overall comprehension of the reaction mechanism. Only when the structure and properties of pyrolytic volatiles or reaction intermediates are understood clearly, could the oriented catalysts be conceived and developed. Furthermore, the comprehensive analysis and characterization of bio-oil derived from catalytic pyrolysis is of great importance. Precisely figuring out the chemical composition of the liquid phase is the prerequisite of further upgrading and utilization.

(4) The stability and reutilization of catalyst is another significant issue for catalytic pyrolysis technique feasibility and economy, and it is vital to develop the ability of coke resistance. To achieve this goal, the first step is to clarify the detailed catalytic conversion mechanism of pyrolysis vapor that occurred on the active site of the surface or channels of catalysts. On the other hand, the design of pyrolysis reactor that allows for the in-situ regeneration of catalysts may help reduce the capital invested in the technique routes of biomass pyrolysis.

(5) The cost or investment of catalyst determines the overall economics of the biomass pyrolysis process, so it is necessary to exert techno-economic analysis and life cycle analysis of the selected catalyst. Furthermore, whether the corresponding technique route is suitable for the environment, or allowed with national policies, should be taken into account first before scaling up.

(6) Industrialization of biomass catalytic pyrolysis technology still faces many challenges. Firstly, the mixing of catalysts and biomass particles is tough to be guaranteed in large scale, which makes catalysts invalid; at the same time, the recovery of the used catalysts could not be easily realized, which increases the cost. Secondly, the heat and mass transfer become significantly difficult owing to the properties of lignocellulosic biomass, so large amounts of energy need to be supplied to the large-scale pyrolysis device. Last but not least, the diversity of biomass species and wide distribution of biomass both make the biomass pyrolysis industry tough to develop towards a large scale.