1 Introduction

Biomass is widely used as a renewable source for substitution of fossil fuel and a precursor for the production of chemicals [1]. For example, the conversion of lignocellulosic biomass (i.e., sugarcane, corn) into bioethanol has been extensively investigated [2, 3]. Biomass is a carbon-rich biological material widely used due to its availability, lower processing cost, and higher conversion [4]. In the course of biomass conversion, there is no overall increase in carbon footprint, making biomass a potential sustainable renewable energy source, and having a critical role in environmental mitigation and energy supply. Biomass can be grouped into four subgroups: (1) agricultural and forestry residues, (2) municipal and industrial solid waste, (3) herbaceous crops: Napier grass and weeds, and (4) aquatic and marine biomass [5]. Biomass is converted into bioenergy and chemicals via biological and thermochemical processes [6]. The thermochemical processes are carried out at high temperatures, between 300 and 1400°C [7, 8]. Amongst the thermochemical processes, pyrolysis is widely used, with biomass conversion by high heat energy (207–434 kJ/kg) in the absence of oxygen [9, 10].

Pyrolysis enhances the energy density of biomass with the flexibility to be carried out at a small scale or remote location setups [11]. Pyrolysis is a flexible and attractive process to converting biomass into bio-oil, chemicals, and heating energy. Slow pyrolysis is usually performed in batch mode for long periods of residence time (5–30 min) at low temperatures and heating rates [12]. The decomposition of biomass gives rise to three main products: biogas, bio-oil, and biochar. Bio-oil is the main product of pyrolysis, with a higher heating value than the raw material, which can be converted into different chemicals [13]. Catalysts have been used to improve the efficiency of pyrolysis process and to upgrade the bio-oil quality [14].

Catalytic pyrolysis operates in in-situ and ex-situ modes [15]. In the in-situ catalytic pyrolysis process, the biomass and catalyst are mixed before the pyrolysis process. In the case of the ex-situ catalytic pyrolysis process, the biomass is separated from the catalyst, and pyrolytic vapour from the pyrolysis process reacts with the catalyst in a secondary reactor [15]. Nevertheless, there are not many comparative studies in the literature to understand the catalytic mechanisms and kinetic pathways of in-situ and ex-situ catalytic pyrolysis. The most important catalyst groups used in catalytic pyrolysis are zeolites, mesoporous catalysts, and biomass-derived catalysts.

Zeolites with distinctive pore structure and acidity (Lewis and Brønsted acid sites), are used in bio-oil upgrading [16, 17]. The zeolites predominantly used in pyrolysis are ZSM-5, Beta-zeolites, and Y-zeolites [18,19,20]. ZSM-5 has demonstrated excellent efficiency in deoxygenation reactions for aromatic compounds, producing bio-oil with low oxygen content and high calorific value [21]. Metals have been loaded into zeolites to enhance bio-oil quality due to their high resistance to coke deposition and high acidity [22]. However, mass transfer limitations, catalyst deactivation over coke deposition, and sintering should be optimized for zeolite catalytic pyrolysis. Mesoporous catalysts, with a pore size range of 20–30 Å, are used in catalytic pyrolysis due to their unique porosity and high surface area (900–1100 m2/g) [23, 24]. Mesoporous silica catalysts, such as SBA-15, MCM-41, and MUS-S, are widely used in catalytic pyrolysis of biomass due to their supramolecular structure, and their propensity to synthesize different crystalline structures [25]. Biochar produced during biomass pyrolysis is also utilized as biomass-derived catalyst for bio-oil upgrading [26].

In this review article, the impact of biomass composition on the quality and yield of bio-oil produced via pyrolysis has been discussed. Different types of pyrolysis, product distribution, and key factors on the process performance are also discussed in the subsequent sections. In the review article more stress has being laid on bio-oil physicochemical properties and its upgrading by catalytic pyrolysis. The reaction mechanisms and application of heterogeneous catalysts to produce high quality bio-oil are explained in detail. In the last section, advanced analytical techniques used for bio-oil characterisation are also reviewed. The main objectives of this review article are to (1) summarise the basic features of catalytic pyrolysis to produce high quality bio-oil, (2) recommend different types of catalysts for specific products/chemicals production, and (3) summarise bio-oil advanced characterisation techniques.

2 Biomass composition

Biomass comprises hemicellulose, cellulose, lignin, and a small number of other extractives [27]. Agricultural and forestry residues have a high energy content, which mainly consists of cellulose [28]. However, herbaceous plants are generally continuous, with loosely bonded fibres, which contains a small lignin percentage that connects the cellulose fibres [29]. Lignin has a higher resistance to heat and chemical degradation than cellulose and hemicellulose [27].

As shown in Fig. 1, different biomass feedstocks comprise different amounts of hemicellulose, cellulose, and lignin. The total amount of lignin and cellulose is one of the determinant factors for subsequent energy and chemical conversion processing. Numerous herbaceous crop families, such as elephant grass, Bermuda grass, esparto grass, alfalfa-full flower, contain 10–25% hemicellulose, 20–40% cellulose and 10–30% lignin [29, 37, 38]. Typical switch grasses contain 32% cellulose, 19.2% hemicellulose, and 15–30% lignin [30]. Generally, biomass with lower lignin content and higher cellulose/hemicellulose content is desired for activated carbon production [39]. High lignin content gives the lowest aromatic yield and the highest coke yield while high hemicellulose content contributes to low coke yield and high noncondensable gas production [40].

Fig. 1
figure 1

Composition of different biomass [30,31,32,33,34,35,36]

Full size image

The elemental composition of different biomass groups, pyrolysis conditions, bio-oil yield and composition obtained by conventional pyrolysis are summarized in Table 1. Herbaceous crop biomass has an overall elemental composition of 41–49% carbon, 44–47% oxygen, with bio-yield in the range of 44–60%. The energy per unit mass increases with decreasing oxygen content in the feedstock. For example, agricultural residues have a high oxygen percentage (38–47%), which reduces the calorific value of the bio-oil [61]. Municipal and industrial wastes are primary sources of nitrogenous compounds, having a nitrogen content of 3–8% [62].

Table 1 Elemental composition of different biomass groups, and bio-oil yield and composition obtained by conventional pyrolysis
Full size table

The quality and yield of bio-oil produced by thermochemical conversion is strongly affected by the elemental composition of biomass. The bio-oil constituents typically depend on the carbon and hydrogen content of biomass. As shown in Table 1, spruce wood contains a high carbon and hydrogen content, 49.11% and 6.14 %, respectively, which enhance the bio-oil phenolic content [47]. The hydrogen amount seems to increase the heating value and aromatic compounds of bio-oil, but slightly varies over biomass types. Amongst the agricultural residues, oak showed the highest aromatic content due to its high hydrogen content (7.16%).

Similarly, municipal and industrial wastes have a high conversion into bio-oil with a yield in the range of 30.1–65%. The coffee husk showed higher conversion into phenolic and aromatic compounds due to high carbon to hydrogen ratio (7.33) [57]. The average C, O, and H percentages of the aquatic and marine biomass are 41.62, 5.90, and 44.26%, respectively. The variation in the elemental composition of biomass results in a high variation in the bio-oil yield and composition. The high carbon to hydrogen ratio (8.2) of P. indicus results in a high aromatic yield in the bio-oil [58]. In contrast, Nannochloropsis showed a lower aromatic yield due to lower carbon to hydrogen ratio of 6.3, but the pyrolysis conditions were different which makes this comparison difficult. Understanding the degree of biomass composition variation helps to design an effective thermochemical process for biomass conversion [63]. The bio-oil properties such as viscosity, pH, and chemical composition depend on the feedstock biomass type, pyrolysis conditions and reactor design. The effect of pyrolysis process parameters on bio-oil quality is discussed in detail in Section 3.2.2.

3 Biomass processing methods

3.1 General overview

Biomass consists of various precursors to produce green chemicals and fuels [64]. In general, biomass conversion is undertaken by two types of processes: biological and thermochemical processes [65]. Thermochemical methods are preferred over the biological processes due to the short reaction times and high degradation efficiency [66,67,68,69].

The product distribution and bio-oil quality from thermochemical processes depend on the residence time, heating rate, temperature, degree of oxidation, the feedstock particle size, and moisture content. The thermochemical processes can be classified into three primary processes: pyrolysis, gasification, and liquefaction [65]. The main products of pyrolysis and gasification are biogas, bio-oil, and biochar, whereas bio-crude and sugars are the main products of the liquefaction process. The intermediate products obtained from thermochemical processes such as sugars, bio-crude, and biogas can be further converted into bioenergy and chemicals via catalytic pyrolysis, steam reforming, fermentation, water–gas shift reaction, and hydro-processing [70, 71].

Liquefaction is an alternative thermochemical process, primarily designed for producing liquid fuel from biomass [72]. The process is carried out in an aqueous medium at a pressure of between 5 and 20 MPa, and temperatures between 250 and 370°C. These are subcritical conditions in which complex biomass structures decompose by hydrolysis and repolymerize into smaller molecules such as levoglucosan, hydroxyacetaldehyde, hydroxyacetone, pyruvic aldehyde, glyceraldehyde, and furfural [73].

Gasification is a thermochemical process to converting biomass into gaseous fuel with the presence of a gasifying agent [74]. The gasification process is carried out at high temperatures, between 500 and 1400°C, and at a range of pressures, from atmospheric pressure to 33 bar [7]. The gaseous products during gasification are CO2, H2, CH4, CO, and N2 [71]. Biomass moisture content varies between 30 and 60% while gasification process requires biomass with a moisture content between 10 and 15%. Therefore, drying biomass is a fundamental pretreatment process to meet the moisture content criteria for gasification, which significantly increases the overall processing costs [75]. Complex operation, high energy costs because of low moisture content requirement for the biomass and relatively high processing temperature make gasification process unsuitable for biomass conversion [76]. On the other hand, pyrolysis is a versatile process to efficiently convert biomass into bio-oil, which is suitable for all types of biomass [77].

3.2 Pyrolysis

Pyrolysis is a thermochemical degradation of biomass by high heat energy (207–434 kJ/kg) in the absence of oxygen [9, 10]. The pyrolysis process is ascribed as the sum of three main routes: char formation, depolymerization, and fragmentation [78]. The char formation pathway results in producing solid residue with a high amount of polycyclic aromatic hydrocarbons [79]. The primary steps in this path are the production and incorporation of benzene rings in a polycyclic structure [80, 81]. Depolymerization consists of degradation of polymeric structures and at low temperatures the degraded monomers condense into a liquid fraction [82]. Fragmentation results in incondensable gas formation and a variety of organic compounds that are condensable at ambient temperature [80, 83].

3.2.1 Types of pyrolysis processes

Based on processing pyrolysis parameters, the conversion of biomass is divided into three classes: slow, fast, and flash. Operation conditions for various types of pyrolysis are summarised in Table 2. Slow pyrolysis is mainly applicable for charcoal production and chemicals like acetic acid, furfural, and phenols. Most slow pyrolysis literature is primarily focused on biochar production and its applications [89, 90]. Of the slow processes, carbonization, with a low heating rate of 0.1–0.4°C/s, is a widely applicable process carried out without condensation of the pyrolysis products [91]. Carbonization is a biomass conversion technique for charcoal production and performed when biochar is the desired product. The ideal feedstock moisture content for slow pyrolysis is between 15 and 20% [92].

Table 2 Operation conditions for bio-oil production via pyrolysis
Full size table

Flash pyrolysis produces high bio-oil yield (70%) and low gas and tar amounts in comparison with slow pyrolysis [93]. Flash pyrolysis takes place at high temperatures (650–1000°C) and requires short residence time (max. 2 s). During the process, the feedstock is heated rapidly to be vaporized and then condensed into bio-oil [94, 95]. On the other hand, fast pyrolysis takes place at moderately low temperature (500–800°C) and short residence vapour time (5 s) [96]. During fast pyrolysis, approximately 60–75% of the biomass is converted into bio-oil [97]. However, fast pyrolysis temperatures higher than 650°C favour biogas production [65]. Fast pyrolysis is a flexible and desired process to transform biomass into a liquid that is easily stored and transported for biofuel and chemical production [98, 99].

Vacuum pyrolysis is the decomposition of biomass in a pyrolysis reactor under vacuum to reduce vapours residence time [34]. Vacuum pyrolysis is characterised by a slow heating rate and takes place at temperature between 350 and 520°C resulting in low bio-oil yield (35–50 wt.%) [100]. Hydropyrolysis is the decomposition of biomass in the presence of hydrogen gas [101]. During hydropyrolysis hydrogen gas is reduced to form hydrogen radical, which reacts with pyrolytic vapour [99, 102]. The amount of aromatic hydrocarbons produced via catalytic pyrolysis is much lower than via catalytic hydropyrolysis [88].

3.2.2 Pyrolysis process parameters

The pyrolysis processing parameters affect the composition and yield of the desirable products. The main processing parameters include the heating rate, temperature, gas flow rate, reactor design, and particle size [103]. Any of the three pyrolysis products, such as bio-oil, biogas, or biochar can be improved by optimizing pyrolysis conditions [104,105,106,107,108]. The impact of operational conditions on the quality and yield of the pyrolysis products is summarized in the next paragraphs.

Temperature plays a predominant role in the degradation of high molecular weight components of biomass into smaller molecular fragments. Partial degradation of the biomass structure at the molecular level occurs at a temperature below 300°C that produces heavy residual tar. In contrast, large molecular weight biomass degradation occurs at a temperature higher than 550°C, enhancing the composition of bio-oil [109]. Some studies suggest that the temperature to achieve the highest bio-oil yield is 450–550°C. However, optimum processing temperature to maximise bio-oil yield depends on biomass composition and pyrolysis conditions such as heating rate and gas flow rate [105, 110, 111]. Ji-lu et al. [112] conducted rice husk pyrolysis in a fluidized bed at a temperature between 420 and 540°C, and obtained a maximum bio-oil yield of 56 wt.% at 465°C. This study demonstrated that a further increase of the pyrolysis temperature decreased bio-oil yield to 45 wt.%.

Biomass particle size affects mass and heat transfer rates during pyrolysis, which have an impact on bio-oil yield. The bio-oil yield is usually higher for biomass particles with a size lower than 2 mm [113]. Small biomass particles enhance the biomass decomposition rate due to a better and faster mass and heat transfer rates [114].The fast decomposition of small particles favours high bio-oil yields, however the larger particles cause slow decomposition and favour the production of char [115]. Biomass particles lower than 0.6 mm reduce bio-oil yield due to quick decomposition and participation in secondary reactions leading to an increase in biogas yield [116]. The mass and heat transfer rates are also depending on the types of reactors used in the pyrolysis process.

Many reactor types such as, fixed bed, fluidized bed, rotative, vacuum, plasma, and microwave have been used for pyrolysis. Fluidized bed reactor (bubbling) is frequently used to achieve high heating transfer rates (uniform temperature distribution) resulting in a high bio-oil yield of 70–75% [84]. However, it requires small biomass particles and is difficult to remove the biochar. Microwave reactor is another option that is mainly used due to high heating rates, high temperatures and short residence times resulting in a bio-oil yield of 60–70% [117]. However, the high processing costs, high power consumption and the need to use microwave absorbers limit its application [118]. Several researchers used plasma reactor for biomass pyrolysis, but despite of its high operating costs, high energy and small biomass particles requirements, the bio-oil yield was still low between 30 and 40% [119].

Pyrolysis process produces a significant amount of vapour during biomass conversion, which can promote side reactions, giving rise to thermal cracking, repolymerization, and recondensation into biochar, resulting in a reduction of bio-oil production [95, 103]. Nitrogen gas is preferably used to remove vapours from the pyrolysis reactor because is chemically stable, inexpensive, and abundant. Increasing the nitrogen gas flow rate reduces the residence time of vapour in the pyrolysis reactor [115]. Choi.et.al. [120] showed that an increase in nitrogen flow rate increased the noncondensable gas percentage, from 22.2 to 31.9%. The increased nitrogen flow rate enhanced the vigorous bubbling motion and improved both mixing and heat transfer rates. For example, Mohamed. A.R. et al. [121] showed that an increase of nitrogen gas flow rate from 150 ml/min to 500 ml/min, in empty fruit bunch pyrolysis fluidized the bed reactor decreased bio-oil yield from 45.7 to 37.8%. However, the noncondensable gas percentage increased from 28.4 to 35.1%.

Heating rate is also another key pyrolysis variable that influences the extent of degradation during pyrolysis. The abundance of volatile matter during the degradation process increases with an increase of the heating rate due to the endothermic decomposition of feedstock [122]. An increase of the heating rate also impacts on the optimal pyrolysis temperature for bio-oil production. For example, Debdoubi et al. [123] conducted pyrolysis of esparto by varying the pyrolysis temperatures from 400 to 700°C and using different heating rates of 50°C/min, 150°C/min, and 250°C/min. The researchers found the optimum heating rate for 57% of bio-oil yield was 150°C/min at 500°C. However, higher bio-oil yield was achieved for a heating rate of 250°C/min at 550°C. In general, a comprehensive ANOVA analysis of all parameters is necessary to optimize the pyrolysis process to obtain high bio-oil quality and yield.

3.3 Bio-oil from pyrolysis: composition and properties

Bio-oil is a dark brown colour liquid that can be used for power generation or extraction of various chemicals. Huber et al. [124] stated that typical pyrolysis bio-oil contains acids (propionic and acetic), alcohols (ethanol, methanol and ethylene glycol), phenols, aldehydes (acetaldehyde, formaldehyde and ethanediol), ketones, aromatics and furans, regardless of the type of feedstock. Table 1 presents the major chemical groups present in bio-oil for different biomass feedstock processed via pyrolysis.

Biomass with higher lignin content gives a higher bio-oil yield. Coffee husk pyrolysis at higher temperatures produces bio-oil with low molecular weight compounds of ketones, acids, and aromatic hydrocarbons [125]. Bio-oil obtained from herbaceous crops, for example Para grass and Arundo donax, contains a high amount of phenolic and high molecular weight aromatic compounds, making this feedstock desirable for phenol extraction. However, bio-oil produced from the pyrolysis process exhibits high viscosity, is corrosive, and thermally unstable. These properties make bio-oil undesirable for the synthesis of fuel and chemicals [126].

Different physicochemical properties of bio-oil produced via biomass pyrolysis and typical crude oil properties are summarized in Table 3. The concentration of elemental oxygen and moisture content in bio-oil are much higher than in crude oil and explain the low heating value of bio-oil. Several studies have reported that the quality of bio-oil is affected by physicochemical properties, such as pH, elemental composition, oxygen content, char, suspended solid, and ash content [123, 131, 132].

Table 3 Physical characteristics of bio-oil and crude oil [127,128,129,130]
Full size table

Kinematic viscosity of bio-oil ranges from 35 to 100 cP, which depends on the types of biomass and pyrolysis processing parameters. Bio-oil viscosity tends to increase over time during storage due to further chemical reactions between the bio-oil components [29]. Boucher et al. [133] reported the effect of adding a stabilizing agent (alcohol) on the viscosity of bio-oil. When bio-oil was stored in 10% methanol, the viscosity only increased from 20 to 22 cP over four months at 20°C. Similarly, 20% of ethanol showed a marginal increment, from 13 cP to 15 cP, on the viscosity of bio-oil at 40°C [134]. High viscosity of bio-oil causes incomplete combustion and poor atomisation during applications; however, adding organic solvents could enhance physicochemical properties and storage stability of bio-oil [135].

The presence of acetic and formic acids in the bio-oil increases acidity (pH <3). Reactive oxygenated compounds in bio-oil causes a change in viscosity, which alters thermal and storage stability [136]. Thereby these acids make the bio-oil corrosive and unsuitable for handling storage vessels and equipment [137]. Ash content in bio-oil arises from the different inorganic compounds such as sodium, magnesium, and potassium (predominantly) in the feedstock. Thangalazhy-Gopakumar et al. [138] reported that bio-oil synthesis from wood biomass showed 0.09 to 0.2% ash content. Moisture content in bio-oil results from dehydration reactions during pyrolysis and moisture in the feedstock [55]. In general, bio-oil can have 15–30% moisture content depending on the type of biomass [139]. Heo et al. [55] reported bio-oil with moisture content ranging from 40 to 60% obtained by pyrolysis of sawdust with 9.1% moisture content The rise in bio-oil moisture content is due to esterification reactions taking place between bio-oil constituents.

The complexity of bio-oil composition limits its application as an alternative energy source. The separation of bio-oil fractions has been employed to improve the calorific value and recover valuable chemicals from bio-oil. Several methods such as solvent extraction, distillation, centrifugation, and column chromatography have been employed to recover and separate of bio-oil fractions [140, 141]. Amongst the bio-oil fractions, phenolic compounds are most suitable for various applications including pharmaceuticals, resin manufacturing, fine chemicals, and food processing [142]. Solvent extraction of phenolic compounds from bio-oil is mainly performed by hexane, chloroform dichloromethane, and toluene [141]. However, the requirement of large volumes of solvent makes the solvent extraction undesirable.

Direct application of bio-oil without upgrading is giving undesired results due to high oxygen content, high viscosity, thermal instability, and low calorific value. Bio-oil can be upgraded via different techniques such as hydrotreating, steam reforming, emulsification, and catalytic pyrolysis [143, 144]. Amongst bio-oil upgrading techniques, catalytic pyrolysis can lower decomposition temperature and requires low energy and hydrogen cracking throughout [145, 146]. Catalytic pyrolysis enhances the quality of bio-oil by removing oxygenated compounds in the form of CO, CO2, and H2O [147]. Catalytic pyrolysis is a potential process for high quality bio-oil production.

4 Biomass catalytic pyrolysis and reaction mechanism

4.1 Catalytic pyrolysis

Catalysts play a critical role in promoting process efficiency, targeting specific reactions and reducing processing temperature and time. Catalysts affect chemical composition and distribution of pyrolysis products. Catalytic pyrolysis has shown potential for converting oxygenated compounds in bio-oil mixture and consequently enhancing bio-oil quality. Catalysts have been used in the bio-oil upgrading process through various approaches [148, 149]. The process configuration of catalytic pyrolysis are grouped into in-situ and ex-situ modes, based on how pyrolytic vapour contacts with catalyst [150].

In-situ catalytic pyrolysis consists of mixing catalyst with biomass directly in the pyrolysis reactor [151]. For ex-situ, catalytic reaction occurs in a secondary independent reactor instead of the pyrolysis reactor [152]. Nevertheless, it is possible to convert oxygenated compounds effectively into hydrocarbons by either mode. However, during the in-situ process, the pyrolytic vapours could not react with substantial quantities of catalyst, which requires a higher biomass to catalyst ratio (i.e., 2:1) for adequate reaction [153]. Also, the optimum pyrolysis temperature is insufficient for in-situ upgrading, requiring a separate ex-situ reactor. Char formed during the in-situ catalytic pyrolysis can also lead to deactivation of the catalyst due to pores blockage [154]. The secondary reactor in ex-situ mode gives an advantage over in-situ mode such as, easy recovery of biochar without catalyst contamination and versatile temperature controls [155].

Synthesis of catalyst can be tailored to the final product requirements. Understanding the reaction mechanisms in the catalytic pyrolysis of bio-oil upgrading is fundamental. The mechanisms of catalyst pyrolysis depend on the reaction pathways of the catalytic system and specific compositions of biomass. The complexity of biomass matrix, inadequate mass transfer phenomena, and immobilisation of catalysts challenge the understanding of the catalyst pyrolysis mechanisms [156].

The major reaction pathways during catalytic pyrolysis are deoxygenation, ketonization, cracking, aldol condensation, and aromatization (Eqs. 1–5) [157, 158]. Hydrodeoxygenation (HDO) is a promising route to enhance the quality of bio-oil by removing oxygenated compounds in the form of CO, CO2, and H2O in the presence of H2 and catalyst [147]. The primary renewable fuel products from HDO include gasoline and diesel hydrocarbons. Various catalysts have been used during HDO including, noble metals, metal oxides, microporous (zeolites), and mesoporous.

$$ \mathrm{Hydrodeoxygenation}:\kern0.5em \mathrm{R}-\mathrm{OH}+{\mathrm{H}}_2\to \mathrm{R}-\mathrm{H}+{\mathrm{H}}_2\mathrm{O} $$
(1)
$$ \mathrm{Hydrocracking}:{\mathrm{R}}_1-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2-{\mathrm{R}}_2+{\mathrm{H}}_2\to {\mathrm{R}}_1-\mathrm{C}{\mathrm{H}}_3+{\mathrm{R}}_2-\mathrm{C}{\mathrm{H}}_3 $$
(2)
$$ \mathrm{Ketonization}\kern0.5em :\kern0.5em {\mathrm{R}}_1-\mathrm{CO}-\mathrm{OH}+{\mathrm{R}}_2-\mathrm{CO}-\mathrm{OH}\to {\mathrm{R}}_1\mathrm{CO}{\mathrm{R}}_2+\mathrm{CO}2+{\mathrm{H}}_2\mathrm{O} $$
(3)
$$ \mathrm{Aldol}\ \mathrm{condensation}\kern0.5em :{\mathrm{R}}_1-\mathrm{CO}-{\mathrm{R}}_2\to {\mathrm{R}}_1-{\mathrm{C}}_2{\mathrm{H}}_2-\mathrm{CO}-{\mathrm{R}}_2+{\mathrm{H}}_2\mathrm{O} $$
(4)
$$ \mathrm{Decarboxylation}\kern0.5em :\mathrm{R}-\mathrm{CO}-\mathrm{OH}\to \mathrm{R}-\mathrm{H}+\mathrm{C}{\mathrm{O}}_2 $$
(5)

HDO has significant benefits, such as high effectiveness on removing oxygen atoms, low reaction temperatures, and preserves the number of carbons in the products [159]. Various types of reactions are taking place during the hydrodeoxygenation process, including hydrogenation, decarboxylation, hydrogenolysis, dehydration, and hydrocracking [160]. Apart from phenolic molecules, aromatic compounds like guaiacols and syringol are also hydrogenated into a wide range of products, including cycloketones, cycloalcohols, arenes, methanol, and cycloalkanes [161, 162].

Conversion of phenols via HDO, as shown in Fig. 2, can be carried out through three different reaction paths: the first is the removal of oxygen by the cleavage of the C=O bond from the aromatic compound. Then cyclohexane and cyclohexene are formed after forming benzene in the presence of hydrogen. The second path is hydrogenation of phenol into cyclohexanol, followed by the removal of oxygen to produce cyclohexene and cyclohexane. The third path is the combination of both hydrodeoxygenation and hydrogenation to convert phenol compounds into cyclohexanone, which is immediately followed by hydrogenation to form cyclohexene, cyclohexanol, and cyclohexane [164, 165]. Eventually, all three paths lead to cyclohexane formation, which can also isomerize into methyl cyclopentane. The selection of one of the three paths to convert phenol into methyl cyclopentane depends on different parameters of catalysts such as metal composition, surface properties, reaction temperature, and required intermediate products.

Fig. 2
figure 2

Reaction mechanisms for phenols, adapted from [163]

Full size image

4.2 Bio-oil quality: catalytic reactions and mechanisms

Zeolite catalysts have received much attention due to its relatively low cost, availability, and its potential to yield high quality bio-oil. Amongst zeolite catalysts, ZSM-5 (exhibiting high acidity and pore size) demonstrated excellent efficiency for bio-oil upgrading, producing less viscous, less acid, and high energy value bio-oil [166]. ZSM-5 also increased the concentration of aromatic hydrocarbons, organics, and gaseous compounds in bio-oil caused by aromatization, decarbonization, and cracking reactions [167, 168]. Zhang et al. [169] utilized ZSM-5 for ex-situ mode catalytic pyrolysis of corncobs using a fluidized bed reactor. The bio-oil obtained from the reactor showed a reduction of oxygenated compounds by 25% with a high heating value (HHV) of 34.6 MJ/kg, which is similar to heavy fuel oil and diesel values.

Many transition metals, such as cobalt, nickel, iron, cerium, and gallium, have been used to fine-tune ZSM-5 acidity to enhance bio-oil yields and decrease coke formation on catalysts [168, 170, 171]. Zeolite supports are frequently used to support metal-based catalysts because of the need to have metals and acidic sites to support the H2 and O-containing compounds activations. Zeolite supports with high Lewis and Brønsted acid site density favour high dehydration reaction. Kumar et.al. [172] prepared metal-based catalysts over zeolite support catalysts (Cu/zeolite, Ni/zeolite, and Cu–Ni/zeolite) to investigate their synergy effect on the deoxygenation reaction of pinewood. The authors found Cu–Ni/zeolite catalyst produced 34% of aliphatic hydrocarbons; however, monometallic combination favoured the production of aromatic hydrocarbons, Cu/zeolite: Ni/zeolite (1:1) generated 18.87% of aromatic hydrocarbons. Also, Cu/zeolite: Ni/zeolite (1:3) significantly reduced in comparison to noncatalytic pyrolysis, with 1.81% of acids, 6.42% of phenols, and 0.4% of ketones in the oxygenated compounds of bio-oil. Table 4 describes the key findings from modification of zeolites using transition metals and pyrolysis conditions. Selectivity and yield of catalysts depend on the catalyst to feedstock ratio, types, and percentage of metals. For example, incorporation of metals in the zeolite framework increases the production and composition of polycyclic aromatics while decreases bio-oil yield.

Table 4 Analysis of the impact of metal-ZSM-5 catalysts and pyrolysis condition on bio-oil production
Full size table

Metal catalyst activates hydrogenation of C▬O or C〓O to produce polycyclic aromatic hydrocarbons. Noble metal catalysts show better hydrogenation performance due to its stability and selectivity [176]. Metal electronic configuration and band structure also contribute to high hydrogenation performance. The binding capacity of substrate to metal catalyst surface depends on the availability of d orbital in spd hybrid bonding orbit. Therefore the higher d orbital percentage of noble metal the stronger interaction between substrate and catalyst [177]. The overall reaction pathways, advantages, and disadvantages of the use of metal-based catalysts in bio-oil production are summarized in Table 5.

Table 5 Summary of reaction mechanisms and main process challenges for metal-based catalysts
Full size table

Metal oxides are also viable for deoxygenation of the pyrolysis vapour to form aromatic hydrocarbons. Metal oxides are widely used in biomass pyrolysis because of their higher degree of active sites during reaction [186]. Additionally, they are highly temperature-stable and resistant against relatively nonpolar compounds under different pH conditions [112, 187]. Kaewpengkrow et al. [188] upgraded fast pyrolysis vapours from Jatropha curcas waste residue produced at 600°C using metal oxide/activated carbon catalysts prepared by wet impregnation. These metal oxide/activated carbon catalysts promoted aromatics formation and produced 86.56% hydrocarbon yield, considerably higher than 11.32% yield without catalysts.

The small size of micropores in the zeolite structure hinders the mass transfer of reactant and formation of polycyclic aromatic hydrocarbons [189]. Therefore to overcome this problem, hierarchically structured zeolites have been developed [190]. Hierarchical zeolites are vastly utilized in biomass catalysis because of their high surface area, better mass transfer, high selectivity, and yield [157]. As shown in Table 6 hierarchically structured zeolites can be achieved by creating zeolite materials with multiple porosity levels, i.e., mesoporous and microporous structures. Mesoporosity on zeolite materials is obtained by alkaline treatment (to remove Si atom) and acid treatment (to remove Al atom). The dealumination process increases the Si/Al ratio and enhances the formation of mesoporosity in the zeolite framework [196, 197]. Desilication of zeolites provides a well-controlled mesoporous formation with an optimal Si/Al ratio between 25 and 50 [195].

Table 6 Properties of hierarchical zeolites used on catalytic pyrolysis
Full size table

A wide variety of mesoporous silica has also been used for bio-oil upgrading such as MCM-41 (Mesoporous molecular sieve) and SBA (Santa Barbara Amorphous). MCM-14 exhibits a high surface area (1000 m2/g), narrow pore size distribution (20–30 Å), and a hexagonal arrangement [198]. However, due to weak acidity compared to aluminosilicate, MCM-14 is only applicable to a narrow range of processes. Acidic properties of mesoporous silica were enhanced by loading metals into the silica structure [23, 24]. Aluminium is the principal metal-doped into the structure of mesoporous silica to enhance catalytic cracking [199]. For instance, by optimizing the Al/Si ratio, the new mesoporous alumina–silica catalyst is created with high acid properties and high surface area. Similarly, different metals including Co, Sn, and Zr are used to prepare high-performing mesoporous silica catalysts [200].

Jeon et.al. [201] studied the application of mesoporous Pt and Al within SBA-15 support catalysts for catalytic pyrolysis of cellulose, hemicellulose, and lignin. AlSBA-15 and Pt/AlSBA-15 showed better catalytic performance than SBA-15 and Pt/SBA-15. In particular, Pt/AlSBA-15 showed a high yield (65 wt.%) for aromatics and furans. The presence of both acid sites and Pt are responsible for the conversion of levoglucosan into aromatics and furans during catalytic upgrading. Pd/SBA-15 revealed a better selectivity for the production of phenol from lignin-derived oligomers [202]. As shown in Fig. 3, the lignin depolymerized into monomeric phenols that were further converted to phenols without the side chain and unsaturated C▬C bond [203, 204]. The incorporation of acidity or alkalinity in the structure of mesoporous silica is likewise a promising methodology to duplicate its applications in catalysis [205]. The pore volume of mesoporous silica gives sufficient space to accommodate these species [206]. Table 7 presents a concise conclusion for the advantages and disadvantages of using mesoporous catalysts for bio-oil synthesis.

Fig. 3
figure 3

Reaction mechanism of lignin depolymerization for phenolic monomers production

Full size image
Table 7 Main advantages and disadvantages of using mesoporous catalysts for bio-oil production
Full size table

Biomass waste (sawdust) was also used to produce a highly efficient magnetic solid-acid catalyst through a fast pyrolysis–sulphonation process [213]. First, the Fe3+ ions were adsorbed into the biomass waste to achieve Fe-loaded biomass, then pyrolysis to produce biochar. Finally, solid-acid magnetic porous catalyst was prepared via sulfonation method from the biochar. The fast pyrolysis method induced reduction of Fe3+ to Fe3O4 and incorporated magnetism into the material, which was kept after sulfonation. The catalyst exhibits a surface area of 296.4 m2/g and acidity of 2.57 mmol/g. The catalyst had notable catalytic activity, including dehydration, esterification, and hydrolysis for distinct acid catalytic reactions. A furfural yield of 6% in dimethyl sulfoxide (DMSO) was obtained at 150°C with a xylose conversion of 96%. The sulfonated catalyst was less active, producing only 45% furfural under the same conditions, due to its lower acidity of 1.26 mmol/g than the iron catalyst. The catalyst was also extremely efficient in producing 94% glucose and fructose [213].

5 Advanced analytical techniques for bio-oil characterisation

Evaluating bio-oil chemical and physical characteristics is a significant process to decide future applications as well as upgrading techniques to improve the composition. Bio-oil physical characteristics, such as viscosity, pH, ash content, moisture content, cetane index, refractive index, heating values, and elemental composition, can be performed accurately by the existing standards procedure. However, qualitative and quantitative analysis of chemical properties remains challenging. The complexity and number of compounds in bio-oil require multiple analytical methods for its chemical characterisation. Therefore, the spectroscopic and chromatographic techniques are implemented and interpreted as complementary. Nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), gel permeation chromatography (GPC), gas chromatography (GC), and Fourier-transform infrared spectroscopy (FTIR) have been used to elucidate the chemical characteristics of bio-oils at distinct levels [214]. Amongst the analytical technique, NMR spectroscopy and GC are the most comprehensive techniques to characterise bio-oil components. The next section will discuss the application of NMR, GC, and TGA for bio-oil chemical analysis.

5.1 Gas chromatography

Gas chromatography (GC) is a widely used separation method to identify thermally stable volatile compounds. The flame ionisation detector (FID) and thermal conductivity detector (TCD) have been used for GC detector because of highly sensitive, rapid response, and a wide range of linear dynamics. GC-FID can be used for bio-oil composition characterisation such as phenols, aldehydes, alcohols organic acids, sugars, and ketones [215]. Additionally, GC-FID has been used to estimate the concentrations of compounds in biochar from the noncondensate stream during pyrolysis [216]. A conventional 1-D GC is usually used to separate bio-oil in a nonpolar or a weak-polar column depending on the boiling point or vapour pressure [217,218,219].

5.1.1 Conventional gas chromatography (1-D GC)

Conventional 1-D GC techniques employed in bio-oil characterisation are based on bio-oil compounds boiling point and volatility [220, 221]. Also, the type of detector, polarity difference between the molecules and their interaction with column material are an important consideration. Types of solvent used during sample preparation should not affect the early eluting of bio-oil fractions. However, most solvents except acetone hinder separation, such as chloroform, tetrahydrofuran, and ethyl acetate elute along with low molecular weight fraction of bio-oil [222]. Co-elutions of solvent and bio-oil fractions hinder absolute quantification while using FID and TCD. Therefore, combining GC with mass spectrometry (GC-MS) will help to accurately identify peaks, which are not detected by FID such as alkanes, C5–C15 hydrocarbon, 2-methoxy and phenols [223].

1-D GC mostly uses nonpolar or slightly polar column for boil-oil characterisation, which oversight polar fraction [218, 219]. Therefore, it requires additional GC columns to quantify both nonpolar and polar compounds. 1-D GC identifies a small portion of high molecular weight or nonvolatile polar fractions of bio-oil due to the low volatility nature of the compounds [224, 225]. Derivatization enhances the detectability of nonvolatile polar fractions by converting into volatile low polarity derivatives using derivatization reagents [226]. Silylation and N-methyl N-(trimethylsilyl) trifluoroacetamide are amongst the most used derivation agents for quantification of hydroxyl and carboxyl groups compounds [227, 228].

5.1.2 Two-dimensional gas chromatography (2-D GC)

2-D GC (GC × GC) analytical technique employs two independent columns with different polarity for GC separation, superior peak detection and resolution [229]. Bio-oil analysis conducted by 2-D GC identified 70% of chromatograph; however, 1-D GC only identified about 47% [222]. Quantification of bio-oil components with 2-D GC depends on the type of columns and the modulator. Typically, the first column is nonpolar or slightly polar, while the second column is polar [230]. Bio-oil fractions are separated by their volatility in the first column, while the second column separates via hydrogen-bonding, π–π interactions, and steric effects [231]. The chromatographic resolution of 2-D GC technique can be improved by increasing resolution in the first column and optimizing split-flow [232]. Analysing all bio-oil fractions using a single analytical is almost impossible, therefore there is a need to combine GC with other technique such as NMR to better understand the chemical and molecular weight properties of bio-oil samples. Gas chromatography is an effective technique to analysis volatile components in bio-oil; however, analysing higher molecular weight molecules of bio-oil is still a challenge.

5.2 Nuclear magnetic resonance (NMR) spectroscopy

NMR spectroscopy provides structural information of high molecular weight compounds in bio-oil. NMR is a powerful technique to analyse bio-oil functional groups such as aromatic, carbonyl, olefin, aliphatic, and methoxy/hydroxyl from the integration of appropriate chemical shift regions [233, 234]. The advantages of NMR over other spectroscopy are its simplicity, short analysis time, and ability to acquire information about the bio-oil composition from a single spectrum [235]. Hydrogen (1H) and carbon (13C) NMR techniques are widely used to analyse the hydrogen-carbon framework of bio-oil. Accuracy and repeatability of NMR analysis depend on solvent, baseline compensation, selection of chemical-shift regions, and longitudinal relaxation [236]. Polar solvents are mainly used for analysis of bio-oil components such as furan, ketones, phenols, and organic acids [235]. During sample preparation dried bio-oil is dissolved in polar deuterated solvents such as dimethylsulfoxide (DMSO-d6), deuterated dichloromethane (DCM-d2), deuterium oxide (D2O), and ethanol-d2. The hydrogen bond strength of polar solvent affects NMR analysis of bio-oil, solvents such as ethanol, dichloromethane, and water, exhibit strong hydrogen bonding, but DMSO-d6 exhibits much less hydrogen proton shifts. Also, solvent signals such as CDCl3 (13C NMR 77.00 ppm and 1H NMR 7.25 ppm) overlap with aromatic group chemical shift region and interfere with quantification of bio-oil fractions. Therefore, the use of DMSO-d6 as solvent allows the collection of chemical structure information from both 13C and 1H NMR [218]. In the following section, the application of 1H and 13C NMR for bio-oil analysis with chemical shift assignments will be discussed.

5.2.1 1H NMR

1H NMR is the most extensively and convenient spectrometric technique used to quantify the oxygenated compounds in bio-oil [237]. The abundance of hydrogen atoms (major isotope 1H) in an organic compound makes 1H NMR spectroscopy analysis sensitive to identifying bio-oil constituents. This technique is characterised by fast analysis and high sensitivity [238]. Table 8 summarizes the major chemical shifts of bio-oil components and the hydrogen percentage of bio-oil obtained from noncatalytic and ZSM-5 catalytic pyrolysis of pinewood. Bio-oil produced with ZSM-5 catalyst contained more hydrogen from ethers (3.0–4.2 ppm) than the noncatalyst pyrolysis bio-oil. The chemical shift range of 9.5–11.0 ppm is assigned to aldehydes and phenols while carboxylic acid proton is assigned to the range of 11.0–12.5 ppm. The spectral overlap of aldehydes and phenols in the region from 9.5 to 11.0 ppm made the quantification of phenols difficult because of low resolution and chemical shift overlaps of 1H NMR. Therefore, it is required to use several characterisation techniques simultaneously to obtain a full insight into bio-oil composition. However, the chemical shift of the hydrogen atom on alkanes and aromatic groups shows clear signals making 1H NMR spectroscopy suitable for the analysis of aromatic ring rich bio-oil.

Table 8 1H NMR chemical shifts for common compounds presented in bio-oil and hydrogen percentage obtained from pinewood pyrolysis [214, 218, 234, 239]
Full size table

1H NMR spectroscopy can also explain the effect of biomass types in the overall chemical composition of bio-oil. Mullen et al. [234] used 1H NMR to characterise bio-oil from different energy crops and categorised bio-oil composition based on the hydrogen atom percentage. 1H NMR is an essential and sensitive technique for determining hydrogen distributions in bio-oil; however, chemical shift ranges are not well-known because of several overlaps. Therefore, to obtain distinguished chemical shift range, the 1H NMR spectrum should complement additional NMR techniques such as 13C NMR spectroscopy.

5.2.2 13C NMR

13C NMR provides a quantitative analysis of carbon atoms in the different functional groups, which can be used as complementary information for bio-oil characterisation [240]. The low natural abundance of 13C atom makes 13C NMR spectroscopy less sensitive, therefore it provides a better signal to noise ratio by accumulating large numbers of transient [241]. The 13C NMR chemical shift of carbon atom from various compounds in bio-oil is summarized in Table 9. The region between 1 and 60 ppm corresponds to alkyl hydrocarbons, which enhance the energy content of bio-oil [232]. The region between 50 and 65 ppm provides information about hydroxyl or methoxy functional groups in bio-oil while the region from 65 to 105 ppm explains carbohydrate (levoglucosan) in bio-oil. 13C NMR spectra between 150 and 215 ppm resonates with the presence of acid, ketones, esters, and aldehydes. 13C NMR spectroscopy techniques provide valuable qualitative analysis; however, spectra overlap occurs due to bio-oil complexity, limiting its application. Therefore, it is required to correlate both 13C NMR and 1H NMR spectra information to obtain a better insight into the overlapping regions.

Table 9 13C NMR chemical shift of bio-oil [218, 242]
Full size table

5.3 Thermal analysis

Thermal properties of bio-oil are studied using thermogravimetry (TGA) and its derivatives (DTG). TGA measures weight losses based on the volatility of molar fractions against the temperature or time at a specific heating rate. Thermal degradation takes place in three stages: the first stage corresponds to carbon dioxide, carbon monoxide, and water removal from the feedstock at a temperature less than 200°C; the primary degradation occurs in the second stage at a temperature between 460°C and 680°C; and the final stage of decomposition occurs at slow reaction rate at a temperature higher than 680°C [243].

TGA analysis helps on the characterisation of evaporation, combustion, and thermal degradation of bio-oil. Also, TGA analysis of biomass generates information about carbon, water, ash, cellulose, hemicellulose, and lignin content that can be used to enhance the quality and composition of bio-oil. The percentage of weight loss in a region during the thermal analysis of biomass provides information about the reactivity; for example, biomass containing high lignin content showed low reactivity resulting in high biochar production. TGA data is also used to optimize bio-oil yield by analysing the ash content in different biomass, where higher ash content corresponds to lower bio-oil yield [244]. TGA has further been used to determine the amount of coke deposited in porous catalysts such as zeolites. The formation of coke on internal and external surfaces of catalysts causes catalyst deactivation and reduces catalyst activity for bio-oil upgrading [245].

TGA analysis has frequently been used to determine chemical kinetic parameters such as a preexponential factor (A) and activation energy (E) using different modelling methods [246]. Modelling of chemical kinetics uses TGA analysis conducted via nonisothermal and isothermal with multiple and single heating rates [247, 248]. However, more than one reaction pathway is considered to study kinetic parameters of thermal decomposition of biomass [249, 250].

6 Conclusions

Biomass is a renewable source and potential fossil fuel replacement due to its availability, lower processing cost, higher conversion, and lower carbon emissions. Pyrolysis is an attractive and flexible process of converting biomass into bio-oil, which can be utilized for the production of energy and chemicals. However, bio-oil obtained from biomass pyrolysis process is not suitable for fossil fuel substitution due to the high amount of oxygenate compounds (i.e., phenols, ketones, aromatic hydrocarbons, sugars, alcohols). Therefore, there is a need to upgrade bio-oil properties by converting the oxygenated compounds into aromatic hydrocarbons. Catalysts have been used to upgrade bio-oil properties, but not all the desired properties of a fuel have been achieved yet. According to our literature review, most catalytic upgrading of bio-oil has been carried out via a monocatalytic system (acid or base catalysts), which is unable to address all oxygenated compounds available in the bio-oil. Also, catalyst deactivation over coke deposition and sintering promote lower catalytic activity. Future research should focus on synthesising robust bifunctional catalysts to address both acidic and alkaline bio-oil fractions.

In addition, hierarchical zeolites have been used to enhance the bio-oil quality. The sequential dealumination–desilication process is used to create additional mesoporosity in the zeolite framework. However, the optimum amount of mesoporosity for high bio-oil quality is still unknown, so we recommend that future research should be focused on studying the effect of both mesoporosity and loading of metal oxides on bio-oil quality and yield. Analysing chemical composition of bio-oil is fundamental for the optimisation of the pyrolysis process and its application as alternative energy source. NMR spectroscopy and GC techniques are mainly used to obtain structural and molecular weight information. To obtain a comprehensive understanding of bio-oil molecular fractions, combining both analytical techniques is required. Further work should be carried out to better understand the impact of pyrolysis processing parameters on bio-oil composition using statistical techniques.